Doctoral Dissertation

Development of metal-coordinatedaminosilica networks for highly permeable
and selective membrane separation(㔠ᒓ㓄఩䜰䝭䝜䝅䝸䜹⭷䛾㛤Ⓨ䛸㧗㏱㐣ᛶ䞉㧗㑅ᢥᛶศ㞳)

UFAFA ANGGARINI

Department of Chemical Engineering,
Graduate school of Engineering, Hiroshima Universityᗈᓥ኱Ꮫ኱Ꮫ㝔ᕤᏛ◊✲⛉໬ᏛᕤᏛᑓᨷ 
September 2022

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --iHiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Abstract of dissertation
Membrane processes and technology are considered energy efficient for the
separation of gas and liquid systems. Over the past decades, organosilica membranes haveattracted high attention due to high thermal and chemical stability with no swelling, highflux, high separation performance, and adjustable pore sizes. The primary goal of thisdissertation was to fabricate and optimize a hybrid thin film of metal-coordinatedaminosilica membranes based on sol-gel processing followed by coordination reaction
and investigate their performance in the separation of gas and liquid mixtures via
pervaporation. The best performance of metal coordinated-organosilica based membranes
was achieved by optimizing four parameters including the affinity of transition metal as
Lewis-acid molecules, the affinity of the ligand as Lewis-base molecules, the mole ratioof metal/amine groups, and the existence of metal in a different phase to facilitate thetransport of permeating molecules. At each step of optimization, the fixed variable wasselected and applied, and the investigation proceeded to the next step.
In the first stage of modification of metal coordinated-organosilica membranes,
the effect of metal doping was evaluated using a single gas permeation system. Values
for surface area and pore volume of the metal-doped bis[3-(trimethoxysilyl) propyl]
amine (BTPA) at metal/amine mole ratio of 0.25 mol mol-1 were both expanded byincreasing metal coordination affinity on the order of Ni-BTPA (214 m2 g-1) > Cu-BTPA(75.7 m2 g-1) > Ag-BTPA (20.1 m2 g-1) > BTPA (2.36 m2 g-1). In line with material
characterization, gas permeation was also increased in the same order, which corresponds
to a higher affinity for metal coordination. Ni-BTPA membrane with the highest affinity
for coordination achieved high levels of both permeance of N2 and N2/SF6 at 3.75 × 10-7
mol m-2 s-1 Pa-1 and 1,900, respectively.
In the second stage, the optimization of coordination was examined by employing
the incorporation of different amounts of nickel dopant (mole ratio of nickel/amine) into
BTPA structure that ranged from 0.125 to 0.50 mol mol-1. Higher nickel content
increased the rigidity and resulted in a porous membrane structure with a high specificsurface area that beneficially showed high performance for the pervaporation organic
mixture of methanol/toluene on the order of Ni-BTPA 0.50 > Ni-BTPA 0.25 > Ni-BTPA
0.125 > BTPA. Ni-BTPA 0.50 has been found to exhibit superior performance with aii

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --high level of flux at 2.8 kg m-2 h-1 and a separation factor higher than 900 in a 10 wt%
methanol feed solution at 50 ⁰C. In that work, Ni-BTPA 0.50 shows an attractive
performance due to the balance between the formation of microporosity that was inducedby nickel-amine coordination and the excessive amount of nickel ions that facilitated thefast transport of methanol while restraining the transport of toluene.
To prove the concept of metal-coordinated aminosilica membranes, in the third
investigation, the affinity of amine-functionalized organosilica as a ligand was evaluated
via different types of precursors. Nickel doping restructured the aminosilica network via
a coordination bond, which then increased both the rigidity of the organic chain and thesurface area or resultant material on the order of nickel doped bis [3-(trimethoxysilyl)
propyl] amine (Ni-BTPA) > nickel-doped 3-aminopropyl triethoxy silane (Ni-APTES) >nickel-doped trimethoxy[3-(methyl amino) propyl silane (Ni-MAPTS). The preparedhybrid nickel-composite membranes were used for the pervaporation of various types oforganic mixtures with methanol 10 wt% at 50 ⁰C in the feed solution. Ni-BTPA with thestrongest affinity for coordination recorded the highest separation performance for theflux that reached as high as 1.42 kg m-2 h-1 with an optimum separation factor formethanol/toluene of 5,000. The enhancement for both membrane flux and the separation
factor verifies the balance between higher levels of interconnected micropores fromnickel-amine coordination and the existence of uncoordinated nickel acting as active sites
for the diffusion of methanol.
Based on the previous investigation, two phases of metal can facilitate a specifictransport of components while the coordinated network generates a high flux ofmembrane. In the fourth evaluation, we examine the fabrication of silver-doped BTPA
for facilitating the transport of H2. The evolution of silver in two phases as silver ions andnanoparticles successfully modify the membrane structure from dense into microporous,and silver-modified aminosilica membranes showed hydrogen permeance of 1.46 10-6
mol m-2 s-1 Pa-1 with excellent H2/C3H8 selectivity of 1,500. Based on this result, it isevident that the proposed modification via the two-phase structural evolution of silver
reorganized the organosilica framework and improve the separation performance of themembranes.

iii

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --CONTENTS

CHAPTER 1 GENERAL INTRODUCTION .......................................................................................... 1
1.1
MEMBRANE SEPARATION TECHNOLOGY ................................................................................................ 1
1.2
MEMBRANE GAS SEPARATION............................................................................................................. 3
1.2.2 Membrane transport mechanism through different structures ................................................. 6
1.2.2.1
1.2.2.2
1.2.2.3
1.2.2.4

Dense membrane ......................................................................................................................... 7
Porous membrane ...................................................................................................................... 10
Gas Translation model (GT) ........................................................................................................ 17
Modified-Gas Translation model (m-GT) .................................................................................... 18

1.2.3 A recent application and development in gas separation by membrane technology ............... 24
1.3
LIQUID PHASE SEPARATION PROCESSES ............................................................................................... 28
1.3.1 Organic solvent nanofiltration (OSN) ........................................................................................ 28
1.3.2 Organic solvent reverse osmosis (OSRO) .................................................................................. 33
1.3.3 Pervaporation (PV) ............................................................................................................... 35
1.3.3.1 Introduction: membrane transport properties of pervaporation ...................................................... 35
1.3.3.2
Application of the pervaporation ............................................................................................... 41a) Dehydration of alcohol ......................................................................................................................... 41b) Isomer separation ................................................................................................................................. 44c) Polar/non-polar organic mixtures separation ....................................................................................... 46

1.4
MEMBRANE FABRICATION AND MATERIALS ......................................................................................... 47
1.4.1 Membrane fabrication: introduction ........................................................................................ 47
1.4.2 Permeation and physicochemical properties of the membranes .............................................. 49
1.4.3 Membrane Materials ................................................................................................................ 51
1.4.3.1 Metal membranes ............................................................................................................................. 51
1.4.3.2 Polymeric membranes ....................................................................................................................... 52
1.4.3.3 Carbon molecular sieving membranes .............................................................................................. 55
1.4.3.4 Zeolite membranes ............................................................................................................................ 57
1.4.3.5 Porous coordination polymer or metal-organic frameworks (MOFs) membranes ............................ 60
1.4.3.6 Mixed matrix membranes.................................................................................................................. 61
1.4.3.7 Silica and organosilica-based membranes ......................................................................................... 63
1.4.3.7.1 Preparation technique for silica and organosilica membranes .................................................. 63
1.4.3.7.2 Silica and types of organosilica membranes .............................................................................. 66
1.4.3.7.3 Tailoring pore structure of organosilica membranes for improved performance ..................... 69

1.5
SCOPE OF THIS THESIS ..................................................................................................................... 76
1.5.1 Proposal of organosilica-based coordination polymer ............................................................. 76
1.5.2 Objectives and organization of this study ................................................................................. 82
CHAPTER 2 METAL-INDUCED MICROPOROUS AMINOSILICA CREATES A HIGHLYPERMEABLE GAS-SEPARATION MEMBRANE ........................................................................... 101
2.1 INTRODUCTION ................................................................................................................................... 101
2.2 EXPERIMENTAL ................................................................................................................................... 103
2.2.1 Synthesis of bis [3-(trimethoxysilyl) propyl] amine (BTPA) and bis-(trimethoxysilyl)-hexane(BTMSH) ........................................................................................................................................... 103
2.2.2 Synthesis of metal-doped BTPA and BTMSH ........................................................................... 104
2.2.4 Fabrication of metal-doped BTPA membranes and gas permeation measurement ............... 105
2.3
RESULTS AND DISCUSSION ............................................................................................................. 106
2.3.1 Metal addition and the effect on the physical and microstructure properties of aminosilica 106iv

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --2.3.2 Pore textures induced by metal ion coordinated with amine sites ......................................... 111
2.3.3 CO2 adsorption of metal-doped BTPA ..................................................................................... 113
2.3.4 Metal modification of amine- and non-functionalized organosilica precursors ..................... 115
2.3.5 Gas permeation properties of metal-doped BTPA membranes .............................................. 116
2.3.6 H2/N2 and N2/SF6 separation through metal-doped aminosilica membranes compared withother types of membranes ............................................................................................................... 123
2.4 CONCLUSIONS .................................................................................................................................... 125
REFERENCES ............................................................................................................................................. 125
SUPPORTING INFORMATION ........................................................................................................................ 130
CHAPTER 3 MICROPOROUS NICKEL-COORDINATED AMINOSILICA MEMBRANES FOR
IMPROVED PERVAPORATION PERFORMANCE OF METHANOL/TOLUENESEPARATION ........................................................................................................................................ 135
3.1 INTRODUCTION ................................................................................................................................... 135
3.2
EXPERIMENTAL ............................................................................................................................ 137
3.2.1 Preparation of a nickel-doped bis [3-(trimethoxysilyl) propyl] amine (Ni-BTPA) sol ............... 137
3.2.2 Characterization of nickel-doped aminosilica films and xerogels ........................................... 137
3.2.3 Fabrication of Ni-BTPA membranes and single-gas permeation measurements ................... 138
3.2.4 Pervaporation experiment ...................................................................................................... 139
3.3
RESULTS AND DISCUSSIONS ............................................................................................................ 140
3.3.1 Optimization of the coordination driven by the nickel-metal mole ratio ................................ 140
3.3.2 Metal-induced microporosity via nickel-amino interaction .................................................... 143
3.3.3 Tuning a membrane pore-structure via alterations in the nickel content .............................. 146
3.3.4 Ni-BTPA hybrid membranes for azeotropic organic separation by pervaporation ................. 149
3.4 CONCLUSIONS .................................................................................................................................... 154
REFERENCES ............................................................................................................................................. 155
SUPPORTING INFORMATION ........................................................................................................................ 159
CHAPTER 4 STRUCTURAL TRANSFORMATION OF THE NICKEL COORDINATIONINDUCED SUBNANOPOROSITY OF AMINOSILICA MEMBRANES FOR METHANOLSELECTIVE, HIGH-FLUX PERVAPORATION .............................................................................. 169
4.1 INTRODUCTION ................................................................................................................................... 169
4.2
EXPERIMENTAL ............................................................................................................................ 172
4.2.1 Material synthesis: aminosilica and nickel-modified aminosilica sols .................................... 172
4.2.2 Characterization of nickel-doped aminosilica films and xerogels ........................................... 173
4.2.3 Fabrication of nickel-doped aminosilica membranes and single-gas permeation measurement......................................................................................................................................................... 174
4.2.4 Pervaporation (PV) measurement .......................................................................................... 175
4.3 RESULTS AND DISCUSSION ..................................................................................................................... 177
4.3.1 The three-dimensional micropore network assembly of nickel-aminosilica via coordinated solgel reaction ...................................................................................................................................... 177
4.3.2 Structural transformation, textural properties, and porosity induced via nickel-amine
coordination: The effect of calcination temperature and nickel mole ratio .................................... 184
4.3.3 Evaluating the establishment of microporosity in modified nickel-aminosilica membranes via
single-gas permeation ..................................................................................................................... 190
4.3.4 Selective methanol removal from organic solvents via pervaporation using nickel-doped
aminosilica membranes ................................................................................................................... 198
4.3.5 Permeation and separation mechanism through nickel-doped aminosilica membranes ....... 202v

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --4.4 CONCLUSIONS .................................................................................................................................... 204
REFERENCES ............................................................................................................................................. 205
SUPPORTING INFORMATION ........................................................................................................................ 212
CHAPTER 5 STRUCTURAL TWO-PHASE EVOLUTION OF AMINOSILICA-BASED SILVERCOORDINATED MEMBRANES FOR INCREASED HYDROGEN SEPARATION ................... 221
5.1 INTRODUCTION ................................................................................................................................... 221
5.2
EXPERIMENTAL ............................................................................................................................ 225
5.2.1 Preparation of bis[3-(trimethoxysilyl)propyl] amine (BTPA) and silver-modified bis[3(trimethoxysilyl)propyl] amine (Ag-BTPA)........................................................................................ 225
5.2.2 Structural characterization of Ag-BTPA films and xerogels .................................................... 225
5.2.3 Preparation of Ag-BTPA membranes and the evaluation of membrane performance ........... 226
5.3
RESULTS AND DISCUSSION .............................................................................................................. 227
5.3.1 Structural analysis of silver-modified amine- and non-functionalized organosilica ............... 227
5.3.2 The effect of preparation temperature on the Ag-BTPA structure ......................................... 230
5.3.3 The silver doping ratio and its effect on the construction of the Ag-BTPA network ............... 232
5.3.4 The effect of membrane preparation temperature ................................................................ 237
5.3.5 The effect of the silver doping ratio ........................................................................................ 238
5.3.6 H2 transport through Ag-BTPA membranes: the temperature dependence of gas permeance......................................................................................................................................................... 241
5.3.7 H2 separation for Ag-BTPA membranes .................................................................................. 244
5.4 CONCLUSIONS .................................................................................................................................... 248
REFERENCES ............................................................................................................................................. 249
SUPPORTING INFORMATION ........................................................................................................................ 254
CHAPTER 6 CONCLUSIONS ............................................................................................................. 255
6.1
6.2

CONCLUSIONS ............................................................................................................................. 255
RECOMMENDATIONS .................................................................................................................... 257

LIST OF PUBLICATIONS ................................................................................................................... 259
ACKNOWLEDGMENTS ...................................................................................................................... 261

vi

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --vii

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Chapter 1
General introduction
1.1 Membrane separation technology
Membranes were defined as the semipermeable barriers that allow the separation of
one or more components with a higher permeation for one compound but hinder thetransport of the other as illustrated in Fig. 1-1(a). The membrane separation barrier
divided the feed stream into the retentate stream and permeate stream. The retentate
stream is the stream that has been retained by the membrane containing both the materials
that have been rejected by the membrane barrier and the material that has not been
rejected by the semipermeable barrier but has not been allowed to pass through themembrane. On contrary, permeate stream was defined as a stream that contains smallermolecules than membrane pores that allow the compound to pass through.
(a)

(b)

Feed

Membrane barrier

Permeate

Fig. 1-1. The schematic image of membrane separation process (a), and the schematic
representative of membrane technology reduce the energy cost in separation (b)Starting at the end of the 1960s, membrane separation technology is considered to bea proficient competitive technique for industrial applications compared with conventional
separation processes such as filtration, evaporation, crystallization, solvent extraction, or
distillation, albeit with the consequence of high operational demand on energy [1].
Membrane separation processes have been proven to be clean, faster, more efficient, andmore economical than conventional separation techniques (Fig. 1-1(b)) [2, 3]. In addition,1

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the separation processes could be performed at ambient temperatures, thus offering
temperature-sensitive compounds to be treated without being chemically modified. The
separation processes of components can be classified into several types, based on thephase species of components in both feed and permeate sides. In the case of liquid-phaseseparation, pressure-driven membrane processes were classified into four major
categories based on the pore size of the membranes and the required transmembranepressure as described in Fig. 1-2: microfiltration (MF), ultrafiltration (UF), nanofiltration(NF), and reverse osmosis (RO).

MF (0.1-5 μM, 1-10 bar)

Suspended particles

UF (500-100,000 Da, 1-1,000 nm, 1-10 bar)
 Macromolecules
NF (100-500 Da, 0.5-10 nm, 10-30 bar)Divalent ions, organic molecules
RO (<0.5 nm, 35-100 bar)
 Monovalent ions

Water

Fig. 1-2. Schematic images of different separation processes based on sizes

In addition, the pervaporation process (PV) is the membrane separation process witha typical pore size < 0.5 nm in which compounds are selectively transported from theliquid phase in the upper stream to the vapor phase in the lower stream, which enables
better separation performance and lower levels of energy consumption [4]. The driving
force for pervaporation is the vapor pressure difference between the feed and permeate
sides; the pressure of the feed side is maintained at atmospheric levels while that of thepermeate side is evacuated. On the other hand, the gas separation process (GS)corresponds to the separation where both feed and permeate streams are in the gaseous
phase through the microporous or dense membrane network.
Pressure-driven membrane process could be operated in two different modes of deadend and cross-flow operations as shown in Fig. 1-3 [5]. In the case of a dead-end module,
the feed streams enter one side of the module and flow vertically towards the membrane2

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --surface with the other streams for the permeate components leaving the module.


Dead-end modeCross-flow mode

Feed

Feed

Retentate

Membrane barrier

Membrane barrier

Permeate

Permeate

Fig. 1-3. Schematic diagrams of dead-end and cross-flow mode in pressure-driven
membrane processes

However, the accumulation of the rejected particles is severe which results in impracticalissues for this type of module. The other type of module was a cross-flow mode where
the tangential flow could minimize the effect of accumulation of rejected species, limit
the height of separated layered particles, and maintain the permeate flux.

1.2 Membrane gas separation
Membrane engineering has been growing during the last decades with several
membrane operations that are dominant technologies in various areas such as indesalination of seawater, wastewater treatment, the concentration of vegetable and fruit
juice, artificial organs, and gas separation. Membrane technology demonstrated lower
energy consumption about 10 times lower compared with the thermal process. The use ofa membrane in gas separation technology competes with the traditional thermal-based
separation technologies, thus this method is recognized as the best available technology.
However, several factors need to be considered to get the high performance of membranes
for the real industrial application to be realized, which includes but is not limited to thedesign and development of new membrane materials with tailored structures for improved
mass transport properties, the membrane's robustness in aggressive environments andability of membrane materials to be operated in a wide range of operating condition. In

3Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --this section, an overview of the transport mechanism through different membrane
structures and the interaction between permeating gases with the pore wall of themembrane, together with possible application in gas separation membrane technology isprovided. Therefore, the important information from this discussion can be used toanalyze and facilitate further optimization of materials design.

1.2.1 Transport mechanism: general introduction
Recently, an increased interest has been focused on the gas separation process, where
the membrane performance can be tailor-made via the construction of the microstructureof the materials and the selection of membrane materials. Gas permeation (GP) is aseparation process where membranes were used to separate gas mixtures without a phasechange [6]. The separation performance of membrane-based gas separation is commonlycorrelated with permeability and selectivity. The permeation rate or flux of gas
transported through the membrane was defined as the number of molecules that passthrough in a unit area per unit time and it's proportional to the concentration gradientthrough the membrane as expressed in equation (1-1).
‫ ܬ‬ൌ െ‫ܦ‬

ௗ௖

(1-1)

ௗ௫

஼ ି஼

‫ ܬ‬ൌ ‫ ܦ‬మ௅ భ(1-2)

Where ‫ ܦ‬is diffusivity, c is the concentration, and x is the position across the membrane.
C1 and C2 are the downstream and upstream concentrations, respectively, and L ismembrane thickness. Therefore, the permeability (P) of the membrane which represents
the thickness-independent property that is related to flux (J) can be expressed in equation(1-3).
ܲൌ௃௅௣మ ି௣భ஼ ି஼ൌ ቀ௣మ ି௣భ ቁ ‫ܦ‬
మ

భ

(1-3)

When the upstream pressure is much larger than the downstream pressure, thepermeability can be expressed as follows.
஼ܲ ൌ ቀ మቁ ‫ܦ‬
௣మ

(1-4)

The ratio of concentration over the pressure can be defined as the solubility coefficient(S) so that the permeability can be expressed.
ܲ ൌ ܵ‫ܦ‬

4

(1-5)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Permeance (P) is the product of the diffusivity (D) and solubility (S) of permeatedmolecules across the membrane barrier, where the diffusivity value is affected by the poredimensions, and the solubility is affected by the dynamic parameter which describes themobility of the compound and the interaction between membrane surface and permeatedmolecules [7-9]. The ideal separation factor of the mixture that contains A and B
molecules can be expressed as the ratio of permeability of molecules A over molecule B,
or express the separation factor as the ratio of diffusivity or solubility selectivity as
follows.
ߙ஺Ȁ஻ ൌ

௉ሺ஺ሻ
௉ሺ஻ሻ

ൌ

஽ሺ஺ሻ
஽ሺ஻ሻ

ൌ

ௌሺ஺ሻ
ௌሺ஻ሻ

(1-6)

This equation is known as a solution-diffusion mechanism. Since the limiting stage of themass transfer is overcoming the diffusion energy, the mechanism implies the activateddiffusion and the temperature dependency of the diffusion coefficient and permeability
coefficient can be expressed by the Arrhenius equation as follows.
‫ܦ‬஺ ൌ ‫ܦ‬஺‫ כ‬ሺെܵ஺ ൌ ܵ஺‫ כ‬ሺെ

୼ாೌ

ோ்
୼ுೌ
ோ்

ሻ

(1-7)

ሻ

(1-8)

Where ȟ‫ܪ‬௔ < 0 is the enthalpy of sorption, the equation (1-5) can be expressed as follows.
୼ாܲ஺ ൌ ܲ஺‫ כ‬ሺെ ோ்ು ሻ(1-9)

Where ȟ‫ܧ‬௉ ൌ  ȟ‫ܧ‬௔ +ȟ‫ܪ‬௔ . The transport mechanism of gases that diffuse within dense or
porous membranes was different, depending on the pore size distribution of themembranes as shown in Fig. 1-4.
Upstream

Downstream
Non-porous membrane

Activated diffusion

Microporous membrane

Surface diffusion

Large-sized pore membrane

Knudsen diffusion

Fig. 1-4. The transport mechanism for the permeation of gases through membrane pore wall5

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --In the case of dense membrane structure with narrow pore size, the molecular sieving
mechanism was considered as the activated diffusion process, while the larger pore sizeof the membrane facilitates the surface diffusion and Knudsen diffusion mechanism. The
pore size modification of the membrane should be chosen based on the pore aperture sizeof molecules that need to be separated, as shown in equation (1-10) [10].
(1-10)

݀௜  ൏  ݀௣  ൏  ݀௝

Where ݀௜ and ݀௝ are the molecular size of the i and j gases, and ݀௣ is the pore aperturesize of the membrane. Table 1-1 summarized the Lennard-Jones constant, molecularweight, and the average velocities of several compounds.
Table 1-1 Molecular size, molecular weight, and the average velocities of permeated gases
at room temperature [11]࣌ሺ%ሻMW (g mol-1)

ࣇത (m s-1)

He

2.60

4.00

1277

H2

2.89

2.02

1800

CO2

3.30

44.01

385

O2

3.46

32.00

452

N2

3.64

28.01

483

CO

3.69

25.01

476

Ar

3.54

39.95

399

CH4

3.87

16.04

638

n-C5H12

5.78

72.15

297

C2H6

4.44

30.07

455

SF6

5.13

146.06209

Gas

1.2.2 Membrane transport mechanism through different structures
Solid membranes were classified into 2 main categories based on morphology or
structure: isotropic (symmetric) and anisotropic (asymmetric) membranes. Symmetricmembranes are further divided into two subclasses dense (non-porous) and porous

6

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membranes, while the asymmetric membranes can be divided into anisotropic asymmetricand composites membranes [12]. The schematic images of membrane structure
morphology as shown in Fig. 1-5.

Fig. 1-5. Schematic representative of different membrane structures [13]1.2.2.1

Dense membrane

A dense or non-porous membrane consists of a dense thin film layer through whichpermeated gases are transported via the solution-diffusion mechanism under the driving
force of pressure, electrical, and concentration potential gradient. The solution-diffusion
model assumes the pressure within the membrane is uniform and the chemical potential
gradient across the membrane is only expressed as a concentration gradient [14]. As
consequence, when pressure is applied to the system, the pressure everywhere within themembrane is constant at a high-pressure value. In this membrane structure, thepenetrating molecules dissolve in the membrane matrix from the feed side, then diffuse
through the membrane down a concentration gradient. The gas transport through a dense
membrane involves three processes: the adsorption at the upstream boundaries, activateddiffusion through the membrane, and desorption on the permeate stream. The separationof the different components in a mixture was directly related to the relative transport rate
within the membrane, which was determined by the diffusivity (‫ )ܦ‬and the solubility (ܵ)
in the membrane material.

7

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ܲ ൌ ܵൈ‫ܦ‬(1-11)The permeability coefficient (ܲ௜ீ ) can be expressed as follows [14].
ܲ௜ீ ൌ

஽೔ ఊ೔
ఊ೔ሺ೘ሻ ௉೔ೞೌ೟

(1-12)

Where ‫ܦ‬௜ is diffusion coefficient, ߛ௜ is permeating gas activity coefficient, ߛ௜ሺ௠ሻ is theaffinity of membrane material, and ܲ௜௦௔௧ is the saturation vapor pressure. The value of
permeant’s saturation vapor pressure and diffusion coefficient decreased with increasing
molecular weight, competing effects on permeability coefficient. This means thepermeability of the component decreased significantly as the molecular weight of thecomponent increased.
Dense membranes structure such as those found in perovskite, metal, polymeric,glassy, and organosilica ceramic materials were used in membrane reactors for different
applications such as methane conversion, hydrogen production, toxic gas treatment, andCO2 separations [15]. However, the commercialization of dense membranes is seriously
hindered by several challenges, which show a low permeating flux with high selectivity
due to the thickness and small pore sizes of the membranes. Yu, et al [9] developed aminesilica membranes that were synthesized from different organosilica precursors of 3(triethoxysilyl) propylamine (PA-Si), 3-(triethoxysilyl)-N-methyl propane-1-amine (SASi), and 3-(triethoxysilyl)-N, N-dimethyl propane-1-amine (TA-Si). Based on N2 sorption
characterization as shown in Fig. 1-6(a), all amine-silica membranes show low surface
area ( < 1 m2 g-1) with negligible pore volume, which suggested the formation of denseor non-porous structure. Moreover, all amine-silica membrane shows low permeance forbig-sized gases such as N2 and exhibit high separation performance for CO2/N2 as highas 22 at 35 ⁰C (Fig. 1-6(b)). Better membrane structure will continue to be a focus researcharea for improving the separation performance of targeted compounds in a mixture.

8

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

(a)

(b)

Fig. 1-6. N2 sorption isotherms at -196 ⁰C (a), and the permeance of permeatinggases as a function of temperature dependence (b) for PA-Si, SA-Si, and Ta-Si
xerogel powder calcined at 250 ⁰C [9].
Plisko et al. [16] reported the surface morphology modification of polysulfone (PSF)
based membranes modified with different content of poly(ethylene glycol)-b-poly(propylene glycol)-b- poly(ethylene glycol) (Pluronic F127) as shown in Fig. 1-7. The
SEM images of PSF-Pluronic F127 hybrid membranes show the transformation from adense membrane structure to a porous sponge-like (the pore size, pore quantity, and thepolydispersity were increased) by increasing the Pluronic F127 content. At zero loadingof Pluronic F127, the selective membrane barrier shows no pores that were detected (Fig.
1-7(a)). Higher porosity on PSF-Pluronic membranes was expected to improve the flux
and the separation ability of the ethyl acetate-water mixture.

9

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 1-7. The SEM images of the membrane surface for hybrid PSF-Pluronic F127 flat
sheet membrane at different Pluronic content (wt.%) of 0 (a), 5 (b), 7 (c), and 10 (d) [16]1.2.2.2

Porous membrane
Porous membranes consist of a solid matrix with defined pores diameters

ranging from less than 10 nm to more than 50 nm [3]. Generally, these membranes havea rigid structure with highly pores that were randomly distributed and interconnected
pores. The gas transport mechanism through porous membranes was achieved via amolecular sieving mechanism with the pores and molecular size of penetrating molecules
being the determination parameter. Based on IUPAC classification, the pore size of themembranes could be classified based on the diameter size and the gas transport that
dominated that structure, as shown in Table 1-2 [17].
Table 1-2. Membrane classification is based on pore size and gas transport mechanism
inside pores
Type ofPore size
Transport mechanism
membrane[nm]
Viscous flow
Macroporous
membranes> 50 nm
Molecular diffusion

10

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Knudsen diffusion
Mesoporous
membranes

2 – 50 nm
Surface diffusion

Microporous
membranes< 2 nmCapillary condensationand pore filling
Micropore diffusion

Viscous flow is predominant occurs for the porous membrane with a pore size
larger compared to the mean free path of molecules or on the pore size larger than 50 nm[18]. The overall gas transport is mainly decided by the collisions of the gas species inthe bulk phase and non-selective among different types of components. The permeance(ܲ) of gases through a macroporous membrane can be calculated using equation (1-13).
(1-13)ܲ ൌ ܲ௄ ൅ ߚܲ௠

Where ܲ௄ represent the contribution of Knudsen flow and the second term related to theviscous flow contribution. The constant ߚ is related to the area of porosity (ࢿ), tortuosity
factor (߬), thickness (L), the average pore sizes of the membranes (r), the viscosity ofpermeating molecule (ߤ), and temperature (T).
ଵ

ఌ

଼

ఛ௅

ߚ ൌ ቀ ቁ ቀ ቁ ‫ ݎ‬ଶ ሺߤܴܶሻିଵ

(1-14)The Knudsen permeance related to the gas diffusivity in the membrane pores can be
estimated as follows.
ఌ

ଵ

ଵ

ఛ

௅

ோ்

ܲ௄ ൌ ቀ ቁ ቀ ቁ ቀ ቁ ‫ܦ‬(1-15)In this membrane structure, the gas diffusion (‫ )ܦ‬is governed by the Knudsen mechanism,
so that ‫ܦ‬௄ can be obtained from the data of pore structure with an average radius of andthe molecular weight of permeating molecule (M) as follows.
ଶ଼ோ் ଵȀଶଷగெ‫ܦ‬௄ ൌ  ቀ ቁ ቀቁ‫ݎ‬(1-16)

By combining equation (1-15) and (1-16), gives:
ଵ

ఌ

௅

ఛ

ܲ௄ ൌ ͳǤͲ͸ ቀ ቁ ሺ ሻ‫ݎ‬ሺܴܶ‫ܯ‬ሻିଵȀଶ

(1-17)Mi et al. [19] investigated the permeation mechanism of helium (He), hydrogen (H2), and11

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --nitrogen (N2) through a multi-walled carbon nanotube (CNT) membrane with porous αalumina support as shown in Fig. 1-8.

Fig. 1-8. Gas permeance of He, H2, and N2 through alumina support and CNT separation
layer at 30 ⁰C [19]Gas permeance through the porous support suggested an inversely proportional value tothe squared root of the molecular weight of permeating molecule. Moreover, thepermeance of gases increases with the mean transmembrane pressure (Fig. 1-8) whichindicated a significant contribution to the viscous flow with a large pore size of
approximately around 0.3 μm.
Knudsen diffusion occurs depending on the gas pressure system and the mean
free path in the gas phase (λ) applies to the intermediate pore size (r) between 10 Å and500 Å, specifically, ࢘ൗࣅ ‫ ا‬૚. In this region, the collision was dominated between gas
species and the pore wall, rather than between gas molecules in the bulk phase [18].It's
common to use the Knudsen number (ࡷ࢔ ) to characterize the regime of permeation

12

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --through membrane pore structure (݀) as follows.
ఒ‫ܭ‬௡ ൌ

(1-18)

ௗ

When ‫ܭ‬௡ ‫ͳ ا‬, the viscous flow mechanism is operated, ‫ܭ‬௡ ‫ ͳ ب‬the Knudsen diffusion
was operative, and an intermediate regime is realized when ‫ܭ‬௡ ൎ ͳ.
The Knudsen diffusion coefficient (‫ܦ‬௞ ) can be expressed as follows.
‫ܦ‬௞ ൌ

ௗଷఛ(1-19)‫ݑ‬ത

Where ߬ is pore tortuosity and ‫ݑ‬ത is the average velocity of permeating molecules which
depends on the molecular mass of the component (m). The average velocity of thecomponent can be calculated using the Maxwell speed distribution, as follows.
଼ோ்‫ݑ‬ത ൌ ට(1-20)

గ௠Hence the Knudsen diffusion coefficient (‫ܦ‬௞ ) and the flux (J) of the membranes in the
Knudsen regimes can be calculated as follows.
ଵȀଶ‫ܦ‬௞ ൌ ቀ݀ൗ͵߬ ቁ൫ͺܴܶൗߨ݉൯

(1-21)‫ ܬ‬ൌ ݊ߨ݀ଶ ο‫ܦ݌‬௞ ȀͶܴܶ‫ܮ‬(1-22)

Where n is the surface concentration of the pores with the diameter d, ο‫ ݌‬is the pressure
drop across the membrane and L is the membrane thickness. By substituting equation (121) to (1-22), the flux and the permeability coefficient can be expressed as follows [20].
ଵȀଶ‫ ܬ‬ൌ ൫݊ߨଵȀଶ ݀ ଷ ο‫݌‬Ȁ͸߬‫ܮ‬൯൫ʹൗܴ݉ܶ൯

(1-23)ଵȀଶܲ ൌ ൫݊ߨଵȀଶ ݀ଷ Ȁ͸߬൯൫ʹൗܴ݉ܶ൯

(1-24)The gas transport via Knudsen diffusion occurs in the gaseous phase where the adsorptionof components and the interaction between permeating molecules and membrane porewall is negligible, thus the Knudsen permeance (ܲ௞ ) equation can be simplified as follows
[18].
ఌௗ

଼

ଵȀଶ

ܲ௞ ൌ ఛ௅ ቀଽగ௠ோ்ቁ

(1-25)

Where ࢿ is the porosity of the membrane, R is the gas constant, and T is the absolutetemperature. The ideal selectivity of two components in the Knudsen regime is expressedin the square root of molecular weight as shown in the following equation.
ଵȀଶ‫ܯ‬ߙ௜௝ ൌ ൬ ௝൘‫ ܯ‬൰௜

13(1-26)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The separation factor between two components of Knudsen diffusion is small due to theback diffusion, concentration polarization on the feed or permeate side, the occurrence ofviscous flow on larger pores, and non-separative diffusion [21] as shown in Table 1-3.
Table 1-3. The calculated value for Knudsen selectivity
Gas mixture system

Knudsen selectivity [-]
3.74
4.70
4.11
5.64
0.80
1.07
0.60
1.17
8.50

H2/N2
H2/CO2
H2/H2S
H2/SO2
CO2/N2
N2/O2
CO2/CH4
O2/CO2
H2/SF6Ren et al. [22] reported the gas permeation mechanism for bis(triethoxysilyl)ethane(BTESE) and bis(triethoxysilyl)octane (BTESO) membranes coated on silica zirconia(SiO2-ZrO2) and methyltrimethoxysilane-tetraethoxysilane (Me-SiO2) intermediate layer
at 200 ⁰C under dry conditions as shown in Fig. 1-9. Both SiO2-ZrO2 and Me-SiO2 layers
show high gas permeability, but low selectivity. The large pore size from the intermediate
layers failed to separate specific gas pairs, and the gas transport mechanism wasdominated by Knudsen diffusion. After the deposition of the selective membrane layer of
BTESE and BTESO, the permeance of the membrane decrease with an improvement inmembrane selectivity.

Fig. 1-9. The permeance of several gases as a function of the kinetic diameter ofpermeating molecules through SiO2-ZrO2 and Me-SiO2 intermediate layer together with
BTESE and BTESO membrane top layer [22].
14Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Surface diffusion is the diffusion mechanism that occurs when the membrane pore sizein the region between activated diffusion and Knudsen diffusion [20]. The surface
diffusion mechanism occurred especially at low temperatures when the permeating
molecules failed to escape from the surface potential field due to the interaction between
the inner membrane pore wall and the gas species becoming strong compared to theirkinetic energy [18]. Gas molecules adsorb on the surface of the membrane at the poreentrance, diffuse through the membrane, and desorp at the pore exit. This process
generally takes place below monolayer adsorption, which is related to the Langmuiradsorption model. Since surface diffusion is also a form of activated diffusion, the energy
barrier of ο‫ܧ‬ௌ is required for the penetrating molecules to jump from one adsorption site
to another across the surface of pores. The energy barrier is proportional to the enthalpyof adsorption so the equation for the surface diffusion coefficient was expressed in (1-27).
‫ܦ‬ௌ ൌ ‫ܦ‬ௌ‫  כ‬ቀି௔௤ோ்

(1-27)

ቁWhere ‫ܦ‬ௌ‫ כ‬is a pre-exponential factor and ‫ ݍ( ݍ‬൐ Ͳሻ is the heat of adsorption and ܽ is a
proportionally constant (0< ܽ൏ͳ ). It should be noted that more strongly adsorbed
components are less mobile than weakly adsorbed ones. In this transport, theconcentration of gas was described by Henry’s law.
(1-28)ܿ ൌ ‫݌ܭ‬Where ‫ ܭ‬is a temperature-dependent Henry law coefficient of ‫ ܭ‬ൌ ‫ܭ‬଴ ሺ‫ݍ‬Ȁܴܶሻ .
Since solubility is the ratio of the equilibrium concentration over the pressure, thesolubility can be expressed as follows.
ܵௌ ൌ ‫ܭ‬଴  ቀି௔௤ோ்

(1-29)

ቁThe permeance is the product of diffusivity and the solubility as shown in equation (130).
ܲௌ ൌ ܲௌ‫  כ‬ቀି௔௤ோ்

(1-30)

ቁܲௌ‫ כ‬is a constant and since the value of ܽ is in the range of 0 < ܽ < 1, the total permeability
will decrease with increased temperature which means that the increase of diffusivity iscorrelated with a decrease in surface concentration. The permeance in the surface
diffusion model can also be expressed as follows.
ିοுೌ ିοாೄܲௌ ൌ ܲ଴ ݁‫ ݌ݔ‬ቀ

15

ோ்

ቁ

(1-31)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Where the value of െο‫ܪ‬௔ െ ο‫ܧ‬ௌ being the energy barrier for diffusing molecules topermeate through the membrane, which shows a simple Arrhenius equation. Since theheat of adsorption is a positive quantity, and if this value is larger than the activationenergy, the overall component becomes positive. Thus, the permeance of component
decrease with increasing temperature.
Wei et al. prepared Pd-doped organosilica (POS) membranes by controlling thecalcination atmosphere (Air, N2, H2, and H2/N2) to tailor and improve membrane
separation performance. Fig. 1-10 shows the Arrhenius plot of H2 and CO2 permeance asa function of temperature dependency, where the slopes of each graph were equal to െοࡱ(kJ mol-1). Based on the result, H2 molecules show positive activation energy, whichmeans the transport of activated diffusion. On contrary to that, CO2 permeance shows adecreased value at higher temperatures and exhibits negative apparent activation energies,
which suggested the transport of CO2 is dominated by the surface diffusion mechanismand exhibited strong adsorption capacity for CO2.

Fig. 1-10. Temperature dependence on H2 (a) and CO2 (b) permeance for POS-Air, POSN2, Pos-H2, and POS-H2/N2 membranes.
The molecular sieving mechanism occurs when the pore apertures of themembranes approach the molecular dimension of the diffusing gas molecules. In thismechanism, the interaction energy between the molecules and membrane materials wascomprised of both dispersive and repulsive interaction. When the pore opening was
sufficiently small relative to the size of permeating molecules, the repulsive force wasdominated, and the molecules require activation energy to pass through this constriction.
In this region of activated diffusion, molecules with a slight kinetic diameter size can be

16

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --effectively separated through a molecular sieving mechanism [23]. The pore size of themembrane in the range of 0.7-2.0 nm provides the adsorption sites that would responsible
for the surface diffusion, while the narrow ultra micropores (pores smaller than 0.7 nm)
are enabled for molecular sieving mechanism, which makes the membrane show highpermeance and selectivity for small-sized gases and restricts the diffusion of larger
molecules [24]. Rungta et al. [25] reported the pore structure and bimodal distribution ofcarbon molecular sieve (CMS) which consist of micropores and ultra micropores asshown in Fig. 1-11. The critical ultra micropores window restricts diffusion, allowing
effective discrimination between similar-sized gases via a molecular sieving effect. The
existence of ultra micropores provides the separation ability under different conditions.

Fig. 1-11. The pore structure of CMS with a bimodal distribution of micropores and ultra
micropores [25]1.2.2.3

Gas Translation model (GT)The gas translational (GT) model or best known as activated Knudsen diffusion
was proposed based on the combination of the Knudsen diffusion model and the surface
diffusion when the diffusing gas species have enough kinetic energy to escape the surface
potential but cannot readily do because of the presence of membrane pore wall on theother side. The permeance can be expressed as follows [26].
ܲீ் ൌ

ఌௗು ఘ
ఛ௅

ቀ

଼

గெோ்

ଵȀଶ

ቁ

(1-32)

Where ߝ is the porosity of the membrane, ݀௉ is the pore size of the membrane, ߬ is
tortuosity, ‫ ܮ‬is membrane thickness, ‫ ܯ‬is the molecular weight of permeating molecules,
R is a gas constant, T is the absolute temperature and ߩ is the probability that consists ofpre-exponential factor, ߩ௚ , and the kinetic energy to overcome the diffusion barrier.

17

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ߩ ൌ ߩ௚ ݁‫ ݌ݔ‬ቀെοாோ்

(1-33)

ቁ

By substituting equation (1-33) into (1-32), the permeance of single gases can be
expressed as follows.
ܲீ் ൌఌௗು ఘ೒
ఛ௅

ଵȀଶ

଼

ቀ

గெோ்

ቁ

݁‫ ݌ݔ‬ቀെοாோ்

(1-34)

ቁThe permeation equation on the GT model assumes the diameter of permeating molecules
was smaller compared to the membrane pore diameter, and there is no physical pore
blockage effect [18].

1.2.2.4

Modified-Gas Translation model (m-GT)

The Gas translation (GT) model was originally proposed by Shelekhin et al in
1995 [26]. In 2011, the Tsuru research group proposed the modified GT (m-GT) models
to predict the gas permeation behavior through microporous membranes [8, 27, 28]. In
the m-GT model, micropore permeation of a molecule with molecular size dk through acylindrical pore (pore size: dp, length: L, tortuosity: Wi ) was formulated as follows. Using
probability, U, which indicates the probability of diffusion through the micropore, andfurther expressed with the pre-exponential, Ug, and the kinetic energy, Ep, to overcome
the diffusion barrier.
ܲ௜ ൌ ߝ௜ ݀௣ǡ௜ ߩ௜ ට଼ோ்

ଵ

(1-35)

గெ೔ ோ்ఛ೔ ௅೔ܲ௜ ൌ ߝ௜ ݀௣ǡ௜ ߩ௚ǡ௜ ට

଼ோ்

ா

ଵ

గெ೔ ோ்ఛ೔ ௅೔೛ǡ೔ ቀെ ோ்ቁ

(1-36)

By considering the molecular size of a permeating gas (dk,i) and the effective area for
permeation, the modified GT model was obtained as shown in Equation (1-37).
ܲ௠ିீ்ǡ௜ ൌ

ଵ ఌ
ଷ ఛ௅

൫݀௣ െ ݀௜ ൯ൌ  ݇଴ǡ௜ ට

ଵ
ெ೔ ோ்

൫ௗ೛ ିௗ೔ ൯
మ
ௗ೛

మ

଼
೔

ா

ாುǡ೔

ටగெ ோ் ݁‫ ݌ݔ‬ቀെ ோ் ቁುǡ೔݁‫ ݌ݔ‬ቀെ ோ்ቁ

(1-37)(1-38)

where k0 is the pre-exponential factor expressed by the configuration factors of a18

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membrane (porosity, tortuosity, membrane thickness, mean pore diameter) andpermeating molecule (molecular size).
య݇଴ǡ௜ ൌఌ ൫ௗ೛ ିௗ೔ ൯
ଷఛ௅

ට

మ
ௗ೛

଼
గ

(1-39)

The activation energy of permeating molecules ‫ܧ‬௉ǡ௜ and the pre-exponential factor ofk0 can be calculated via regression using equation (1-38) from the experimental
measurement of temperature dependency of the single gas permeance result. The
activation energy (‫ܧ‬௉ǡ௜ ) can be expressed as the sum of the activation energy ofdiffusion (‫ܧ‬஽ǡ௜ ) for the permeated molecules to surpass the potential energy barrier,and the energy of interaction or enthalpy of sorption (െο‫ܪ‬௜ ). In the case of smallsized micropores, the diffusion barrier act as the dominant factor for thepermeation, the value of activation energy of permeating molecules is ‫ܧ‬௉ǡ௜ > 0 andreferred to as the activated diffusion (permeation of compounds increases withincreasing temperature). For the permeation of molecules through large-sized
micropores where the interactions between membrane matrix and permeatedmolecules are more dominant than the geometrical barrier, the apparent activationenergy is ‫ܧ‬௉ǡ௜ < 0 and referred to as surface diffusion (permeation of gases
decreases with increasing temperature). On the other hand, when the permeation
occurs through much larger micropores in which the Knudsen diffusion mechanism
is more dominant, the value of activation energy is ‫ܧ‬௉ǡ௜ = 0 (permeation ofcomponents constant with increasing temperatures).
The ideal selectivity for ith and jth components that need to be separated
could be estimated from the ratio of permeances as follows.
௉ெ ൫ௗ೛ ିௗ೔ ൯ߙ௜Ȁ௝ ൌ ௉ ೔ ൌ ට ெೕೕయ೔ ൫ௗ೛ ିௗೕ ൯

య

ா

ିா

݁‫ ݌ݔ‬ቀെ ುǡ೔ோ் ುǡೕቁ

(1-40)

To simplify the equation (1-40), several assumptions were made such as all gases
permeated through an identical size of pores, the micropores size of the membrane
were justified in the range from several angstroms to 1 nm so that the difference in
activation energy for permeation is negligible and independent from each gas
species.
19

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ߙ௜Ȁ௝ ൌ௉೔௉ೕெ ൫ௗ೛ ିௗ೔ ൯

య

ൎට ೕ

(1-41)

ெ೔ ൫ௗ೛ ିௗೕ ൯య

Based on equation (1-41), an ideal selectivity can be approximately calculated as afunction of the mean pore diameter of the membrane (݀௣ ) and the size of permeating
molecules (݀௜ and ݀௝ ).
Pore size determination: normalized Knudsen permeance (NKP) and ࢑૙ǡ࢏ method
The normalized Knudsen-based permeance (NKP) method was
calculated the ratio of an experimentally observed permeance of ith component to
the ideal Knudsen permeance for each permeated gas (ith component) from thepermeance of a standard gas (s component) such as Helium (He). NKP suggested
the degree of permeance decrement from the ideal Knudsen permeance. The meaneffective pore size of the membrane (݀௉ ) less than 1 nm was qualitatively estimatedbased on the molecular size that caused the permeation mechanism to deviate from
Knudsen to molecular sieving. The plot of NKP obtained from experimental
research against the molecular size together with the NKP equation derived fromthe modified-gas translation (m-GT) model is as follows.
݂ே௄௉ ൌ

௉೔

ெ

ೄ

ೄ

ሺௗು ିௗ ሻయටெ ೔ ൌ ሺௗ ିௗ ೔ ሻయ  ቀെ௉ುೄாುǡ೔ ିாುǡೄோ்ቁ

(1-42)In the case of gas permeation by the ideal Knudsen diffusion, ݀ௌ ൌ ݀௜ ൌ Ͳ; ‫ܧ‬௉ǡௌ ൌ‫ܧ‬௉ǡ௜ ൌ Ͳ, and ݂ே௄௉ is equal to unity. The equation in (1-42) can be simplified asfollows.
య݂ே௄௉ ൌ௉೔௉ೄெටெ ೔ ൎೄ೏൬ଵି ೔ ൰೏ು೏ య೏ು൬ଵି ೄ ൰(1-43)The experimentally obtained ݂ே௄௉ at a certain permeation temperature of T
(generally at the highest temperature is preferred) as a function of the kinetic
diameter is fitted by equation (1-43), with ݀௉ as a fitting parameter [8, 27, 28].
The pore sizes of the membranes can also be determined via the correlation
between the cubic root of ݇଴ǡ௜ and the kinetic diameter of the ith component (݀௜ ).
20

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Meng et al. [29] reported the mixing of bis(triethoxysilyl) ethanetetraethoxysilane (BTESE-TEOS) precursors to control the pore sizes oforganosilica networks in hybrid membrane structure. The single-gas permeation
results and the normalized Knudsen-based permeance (NKP) show the averagepore size of the membrane (calculated based on equation (1-43)) was reduce in theorder of BTESE (݀௉ = 0.51 nm) > BTESE-TEOS (݀௉ = 0.43 nm) > TEOS (݀௉ = 0.34nm) as shown in Fig. 1-12. The modified BTESE-TEOS structure exhibited highO2 permeance of 2.87 × 10-8 mol m-2 s-1 Pa-1, with a high level of O2/SO2 permeanceratio of 7.3.

Fig. 1-12. NKP plot as a function of the kinetic diameter of permeating molecules
at 200 ⁰C for BTESE-, BTESE-TEOS-, and TEOS-derived membranes [29]࢑૙ǡ࢏ method was used for the determination of membrane pore size
according to the equation (1-39). If ܽ is defined as a constant that depends only on
the structure of the membrane pores and is independent of the permeation
molecules, as given by the following equation.
ܽൌ

ఌ
మ
ଷఛ௅ௗು

ට

଼

(1-44)

గThus, the following equation can be obtained.
݇଴ǡ௜ ൌ ܽ൫݀௣ െ ݀௜ ൯21

ଷ(1-45)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The cubic root of ݇଴ǡ௜ gives the linear correlation with ݀௜ , with a slope of െܽଵȀଷand an intercept of െܽଵȀଷ ݀௣ as shown below.
యయయඥ݇଴ǡ௜ ൌ ඥܽ݀௣ െ ඥܽ݀௜(1-46)

ଵȀଷBased on that equation, from the plot of ݇଴ǡ௜ against ݀௜ , the value of the meaneffective pore size of the membrane can be estimated [28].
Nagasawa et al. [8] have shown that the cubic root of ݇଴ǡ௜ could be used forthe pore sizes determination of BTESE-derived membranes synthesized withdifferent water molar ratios as shown in Fig. 1-13. The plot between the cubic rootof ݇଴ǡ௜ and kinetic diameter of permeated molecules giving a linear correlation, sothat the pore size of the membrane (݀௣ ) precisely estimated from the intercept ofthe x-axis based on equation (1-46). The pore size of the membrane was reduced
as the water molar ratio increased in the order of BTESE-WR6 (݀௣ = 0.47 nm) >BTESE-WR120 (݀௣ = 0.44 nm) > BTESE-WR240 (݀௣ = 0.45 nm). The change inwater molar ratio was sucessfully control the pore size of BTESE-derived
membranes.

Fig. 1-13. The plot of ݇଴ǡ௜ ଵȀଷ as a function of kinetic diameter permeated gases for
BTESE-derived membranes at different water molar ratios [8]In another strategy to estimate the membrane pore size, the equation of (138) can be converted as follows.

22

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ଵȀଷ൫ඥ‫ܯ‬௜ ܲ௜ ൯

ൌ൬

ாುǡ೔

௞బ
ξோ்

ଵȀଷ

݁‫ ݌ݔ‬ቀെ ோ் ቁ൰൫݀௣ െ ݀௜ ൯

(1-47)

If the ‫ܧ‬௉ǡ௜ assumed to be constant to simplify the equation, the pore size of theଵȀଷ

membrane (݀௣ ) of the membrane can be calculated by regressing ൫ඥ‫ܯ‬௜ ܲ௜ ൯ଵȀଷequation (1-47). The value of ൫ඥ‫ܯ‬௜ ܲ௜ ൯tofor each permeated gas is plotted as afunction of the kinetic diameter of permeating molecules (݀௜ ), and the intercept ofଵȀଷ

molecular size at ൫ඥ‫ܯ‬௜ ܲ௜ ൯

ൌ Ͳ reflects the pore size of the membrane.

Moriyama et al. [30] reported the pore size of BTESE-derived membranes
that synthesized with different water and acid molar ratio could be determined viaଵȀଷ൫ඥ‫ܯ‬௜ ܲ௜ ൯

as a function of molecular size permeating gases as shown in Fig. 1-

14. Two membranes refer to as M1 (WR=120, AR=1) and M2 (WR=240, AR=0.1)
show different pore sizes of 0.58 and 0.42 nm, respectively. By changing the waterand acid molar ratio, the H2/N2 permeance ratio was increased from 13 to 59, whichthen confirm the structure of the membranes was successfully controlled.

ଵȀଷFig. 1-14. The correlation between ൫ඥ‫ܯ‬௜ ܲ௜ ൯
gas for BTESE-derived membranes [30].

23

and molecular size of permeatingHiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --1.2.3 A recent application and development in gas separation by
membrane technology
Nowadays, Gas permeation membrane technology gaining large attention
in the chemical and petrochemical industry such as hydrogen recovery, airseparation, CO2 separation, recovery of organic from gas streams, natural gas
dehydration, etc [31]. Table 1-4 presents the summary of industrial application formembrane gas separation-based process.
Table 1-4. Main industrial application of membrane gas separation
Separation
H2/N2
H2/CO2H2/hydrocarbon
CO2/N2
O2/N2
CO2/CH4
CO2/Hydrocarbon
N2/CH4
N2/SF6
H2O/CH4
He/Hydrocarbon
He/N2
Hydrocarbon/air
Volatile organic
species (e.g. ethylene,
propylene)/ light gases(e.g. N2)

Process
Ammonia purge gas
Syngas ratio adjustment
Hydrogen recovery in refineries
Post-combustion carbon dioxide capture
Nitrogen generation, oxygen-enriched air
production
Natural gas sweetening, landfill gas
upgradingNatural gas liquid removal
Nitrogen removal
Gas-insulated equipment
Natural gas dehydration
Helium separation
Helium recovery
Hydrocarbon recovery, pollution control
Polyolefin purge gas purification,
adsorption, refigeration andturboexpander plants

Ref.
[31]
[31]
[31], [1][32][31][31], [1]
[20]
[20]
[33]
[1]
[31]
[31]
[31]
[20], [31]The increased use of membrane technology is expected to reduce theenvironmental impact with a small footprint and costs of industrial processes. Gas
separation properties largely depend on the membrane materials, membrane
structure, thickness, membrane configuration, and membrane system design.
Therefore, much research work is being addressed to evaluate and investigate newmaterial that can develop a membrane with high performance including high24Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --selectivity and permeability to specific gases.
Most membranes perform a trade-off between permeance (the ability topermeate molecules passing through the membrane barrier) and selectivity (thedesired molecules are separated from the rest). Key design criteria for membranes
have emerged with properly sized free-volume elements (pores), narrow freevolume or pore size distribution, a thin active layer, and highly tuned interaction
between permeants of interest and the membrane [34]. Robeson in the 1990s
reported the permeability data on several common gases such as He, H2, O2, N2,
CO2, and CH4 on polymeric membranes, and the trade-off relationship between
permeability and selectivity was analyzed [35].
Several polymeric membranes show the highest selectivity at a certain
permeability lay near a line, which is called the upper bound. More permeable
polymers tend to have lower selectivity and vice versa. The result of the upperbound in 1991 was revisited in 2008 with a much larger database and notableadvance in the search for more permeable and selective membranes [36]. Fig. 1-15
shows the trade-off for the different gas systems based on Robeson upper bound[36].

Fig. 1-15. Upper bound correlation for H2/N2 (a), H2/CO2 (b), CO2/N2 (c), andCO2/CH4 (d)25Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --In general trend, most membranes show modest shifts in the upper bound that werestudied between 1991 and 2008.
Various approaches have been explored to develop membrane materials thatcan exceed the upper bound, including surface modification, facilitated transport,
hybrid membrane structure, and mix-matrix membrane. In principle, designing
membrane structure to overcome the trade-off curve always encounter threecommon trends, as shown in Fig. 1-16.

Fig. 1-16. Modification of membrane structure to overcome the upper bound line
encountered with three common trends [34].
In the first case, the modification of the membrane with molecular sieving fillers
such as carbon molecular sieving (CMS) and zeolites lead to enhances theselectivity of the membrane but decreases the permeability. The modification ofpore structure for increasing the permeation rate often enhances the permeabilityof large-sized gases more than small-sized gases, resulting in increased permeancebut decreased selectivity. In the second trend, the modification of molecular sieving
filler with nano size or nanosheet shape can improve both permeability andselectivity. Membranes with very narrow pore size distribution or uniform poresizes known as isoporous membranes could show both high permeability andselectivity due to the contribution of isoporous geometry with no tortuosity,
precise, and uniform pore size. However, in the practical development of themembrane, isoporous structure was difficult to achieve and there is always a pore26

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --size distribution in membrane structure. On the contrary, in the third case, themodification of the membrane with interfacial voids resulted in high permeability
and lower selectivity.
Many studies focus on the development of membrane materials that show
better permeability and/or better selectivity. Generally, high flux membranes canbe obtained by using high-permeability materials and modifying the thickness ofthe membranes. Therefore, most of the current separating layers of the membranes
are often less than 100 nm thick. Nagasawa et al. [37]reported the synthesis of
microporous silica membrane with hexamethyldisiloxane (HMDSO) precursor via
atmospheric-pressure, low-temperature plasma deposition. In that work, thethickness of the topmost layer was uniformly formed on the porous substrate witha thickness of 13 nm (Fig. 1-17). Plasma deposited layer simultaneously improves
the membrane permeance for small-sized gas of >10-6 mol m-2 s-1 Pa-1 with highselectivity for H2/SF6 of 6,300 which surpasses the trade-off line (Fig. 1-17(b)).
(a)
AP-PECVD+SiO2-ZrO2
Intermediate layerα-Alα
Al2O3 support(b)Fig. 1-17. The TEM cross-sectional images of the AP-PECVD membrane preparedat 200 ⁰C (a), and the trade-off curve between H2 permeance with H2/SF6
permeance ratio for different types of membranes.
27

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --In addition, the excellent transport properties must meet the criteria of
mechanically robust, chemically, and thermally tolerant to operation conditions sothat the membrane can survive manufacturing, installation, and long-term use.

1.3 Liquid phase separation processes
At present, membrane technology has been widely used for liquid phaseseparation including water treatment such as water production from seawater and surface
water, and wastewater treatment. The common industrial application of membrane
technology has shown an impressive growth in the wastewater treatment, biotechnology,
pharmaceutics, chemical, and food industry which are categorized for the separation ofliquid mixture. The use of membrane technology for the separation of liquid phasemixture has been recognized for the following advantages simplicity for the operational,
low energy, high stability, easy control, large flexibility, and scale-up. In this section, theauthors focus on the separation of liquid mixtures with microporous and dense membrane
structures with a pore size smaller than 2 nm. Thus, the main feature of this section is anelaborate discussion of important and promising liquid phase separation techniques suchas organic solvent nanofiltration (OSN), organic solvent reverse osmosis (OSRO), andpervaporation (PV). The general background of membrane separation for the liquid
mixture containing organic solvents is provided and integrated with the application via
different types of membrane materials for a comprehensive study. The interaction
between solvent and membrane materials is then explained and related to process
performance, i.e. permeability, flux, and separation factor.

1.3.1 Organic solvent nanofiltration (OSN)
Organic solvent nanofiltration (OSN) is a molecular separation technology basedon membranes that allow the discrimination of dissolved species in an organic solvent on
basis of size, shape, and the charge of molecules [38]. Based on the pressure required forthe separation process, Mulder [39] classified the OSN as a process in which pressure
from 5 to 20 or extended to 40 bar is applied. Most OSN membranes have a big pore sizein the range of 0.7-2 nm. Stable OSN membranes with molecular weight cut-offs

28

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(MWCOs) ranging from 200-1,000 g mol-1 have emerged for the separation of peptides,
dyes, catalysts, and chemical intermediates [40]. However, the separation capability can
not only be described by the MWCO only but the steric and electrostatic separation
mechanism, typical environment of the solvent, structure, and properties of the solvent
can be significantly affected. To predict the fluxes and the rejection through the porous
and/or non-porous membranes, the transport mechanism between solutes and solvents has
to be understood. Two main assumptions for the nanofiltration transport model were thepore-flow models and the solution-diffusion approach.
In the pore-flow model, stable porosity (ࢿ) is assumed to be present inside themembrane with the tortuosity of ߬ . The driving force for the transport is the pressure
gradient across the membrane (‫׏‬௉ ), and the system was assumed at a constant temperature(Fig. (1-18)), so the equation can be written from the Stefan-Maxwell model as follows.
௫ே೔ ି௫೔ሺ೘ሻ ேೖെ‫ݔ‬௜ሺ௠ሻ ‫்׏‬ǡ௉ ߤ௜ െ ‫ݔ‬௜ሺ௠ሻ ܸത௜ ‫׏‬௉ ൌ σ௞ ߞ௜ǡ௞ ೖሺ೘ሻ൅ ߞ௜ǡ௠ ௖ሺఌ Τఛሻ
௖೟ሺ೘ሻே೔೟ሺ೘ሻ ሺఌ Τఛሻ൅ ߙ௜ᇱ఍೔ǡೖ ௫೔ሺ೘ሻ ఉబఎ‫׏‬௉

(1-48)

Where ‫ݔ‬௜ሺ௠ሻ is the molar fraction of ith component, ‫் ׏‬ǡ௉ is the gradient at constanttemperature and pressure, ߤ௜ is the chemical potential, ܸത௜ is partial molar volume, ߞ௜ǡ௞ isfriction coefficient, ܿ௧ሺ௠ሻ is the molar concentration of component i, ܰ௞ is molar flux, ߙ௜ᇱis viscous flow parameter, ߟ is viscosity, and ߚ଴ is specific permeability and depends onthe membrane structure. Since the contribution of the pressure gradient is much moresignificant than the activity gradient, equation (1-48) can be simplified as follows.

Fig. 1-18. The profile of chemical potential, pressure, and solvent activity characteristics
of an organic component through a membrane barrier according to pore-flow transport

௫
ே ି௫
ேೖ
఍೔ǡ೘ ௫೔ሺ೘ሻ ఉబσ௞ ߞ௜ǡ௞ ೖሺ೘ሻ ೔ ሺఌ೔ሺ೘ሻ
െ‫ݔ‬௜ሺ௠ሻ ܸത௜ ‫׏‬௉ െ ߙ௜ᇱ‫׏‬ൌ൅ ߞ௜ǡ௠ ௖
௉
ሻ
Τఛ
ఎ
௖
೟ሺ೘ሻ

29

ே೔
೟ሺ೘ሻ ሺఌ Τఛ ሻ

(1-49)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Considering the friction of chemical species with the membrane is much higher than thefriction between species, the equation can be written as follows.
ఉబ఍ ௫‫ݔ‬௜ሺ௠ሻ ܸത௜ ‫׏‬௉ ൅ ߙ௜ᇱ ೔ǡ೘ ೔ሺ೘ሻ ‫׏‬௉ ൌ െߞ௜ǡ௠ఎே೔௖೟ሺ೘ሻ ሺఌ Τఛሻ

(1-50)

The first term on the left side is much lower compared to the second term, due to the smallvalue of the species' partial molar volume, so the equation can be expressed as follows.
௫ܰ௜ ൌ െߙ௜ᇱ ೔ሺ೘ሻ
ఎ

ఉబ ఌ
ఛ

‫׏‬௉

(1-51)For small-sized permeating molecules concerning the pore size, the viscous selectivity isone, and the total volume flux (ܰ௩ ) is expressed by.
ఉ ఌܰ௩ ൌ െ బ ‫׏‬௉ఎ
ఛ(1-52)

If the structure of the membranes contains more or less cylindrical pores, the HagenPoiseulle equation can be used.
ܰ௩ ൌ െ

మ
ௗ೛೚ೝ೐
ఌ

ଷଶఎ ఛ

‫׏‬௉

(1-53)

If the membrane consists of packed particles, the Carman-Kozeny can be used as follows.
ௗమఌ೛ೌೝ೟೔೎೗೐ܰ௩ ൌ െ ଵ଼଴ሺଵିఌሻమ ఎ ‫׏‬௉

ఛ

(1-54)In the solution-diffusion model, it is assumed that each permeating molecule
dissolved in the membrane matrix and diffuse through the membrane concentration
gradient. In this model, there is no pressure gradient inside the membrane as shown in
Fig. 1-19. The equation of Stefan-Maxwell can be expressed as follows.

Fig. 1-19. The profile of chemical potential, pressure, and solvent activity characteristics
of one component through a membrane via a solution-diffusion model

30

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --௫ே ି௫೔ሺ೘ሻ ேೖെ‫ݔ‬௜ሺ௠ሻ ‫் ׏‬ǡ௉ ߤ௜ ൌ σ௞ ߞ௜ǡ௞ ೖሺ೘ሻ ௖೔

೟ሺ೘ሻ

௫ ே

൅ ߞ௜ǡ௠ ௖೘ ೔

(1-55)

೟ሺ೘ሻConsidering the friction between components is smaller than the friction of species withthe membrane matrix, the equation can be expressed as follows.
௫ ேെ‫ݔ‬௜ሺ௠ሻ ‫் ׏‬ǡ௉ ߤ௜ ൌ ߞ௜ǡ௠ ೘ ೔௖

(1-56)

೟ሺ೘ሻ

Kinetic coupling and the gradient of chemical potential with a molar fraction gradientwere assumed to be negligible, the equation (1-56) can be written as follows.
ܰ௜ ൌ െ

௖೔ሺ೘ሻ௫೘ ఍೔ǡ೘(1-57)‫ݔ׏‬௜ሺ௠ሻ

By assuming a constant value of ࢞࢓ , and the activity coefficient for each penetrant inside
the membrane remains constant, the equation (1-57) can be expressed as follows.
ఊ

ഥ ୼௉
௏

ఊ೔ǡ೑

ோ்

௠௢௟௔௥
ܰ௜ ൌ ܲ௜ǡ௠ቆ‫ݔ‬௜ǡ௙ െ ೔ǡ೛ ‫ݔ‬௜ǡ௣ ݁‫ ݌ݔ‬ቀെ భ ቁቇ

(1-58)The OSN process is much more complex compared to the aqueous filtration due
to the contribution of all relevant component interactions such as solvent, solute, andmembranes. Table 1-5 shows the viscosity (ߟ), Hansen solubility parameter (δ), and the
kinetic diameter (dkin) of each type of solvent to analyze the effect of the physicalproperties of the component on the OSN performance. The viscosity of components wasmeasured at 25 ⁰C, and the total Hansen solubility parameter was used to reflect thepolarity and the interaction strength between the solvent and membrane materials. The
Hansen solubility parameter consists of dispersion, polarity, and hydrogen bonding
parameter, which has been used to explain the membrane performance. The kinetic
diameter of permeating molecules describes the size of molecules when passing througha membrane pore and reflects the smallest effective dimension of the molecules.
Table 1-5. The physical properties summary of selected solvents [41-43]
Solvent
Water (H2O)
Methanol (MeOH)
Ethanol (EtOH)
Isopropanol (IPA)
Acetone (Ace)
Tetrahydrofuran(THF)ࣁ [mPa s]
0.916
0.539
1.081
2.058
0.316

δ [Mpa1/2]
47.8
29.6
26.5
23.5
19.94

dkin [nm]
0.265
0.38
0.44
0.47
0.47

0.457

19.4

0.48

31

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Dichloromethane(DMC)
Dimethylformamide(DMF)
Acetonitrile (AcN)
Toluene (Tol)Methyl acetate(MA)
Methyl ethyl ketone(MEK)
Ethyl acetate (EtAc)
Isopropyl acetate (iPAc)
Butyl acetate (BA)
Heptane (n-Hep)
Hexane (n-Hex)
Butanol (BuOH)
Tertiary butanol(Tert-BuOH)

0.414

20.3

0.49

0.816

24.8

0.5

0.342
0.555

24.4
18.2

0.55
0.55

0.385

18.7

0.478

0.410

19.05

0.504

0.440

18.1

0.52

0.490

17.6

0.57

0.400
0.294
2.60

17.41
15.3
14.9
23.2

0.585
N.a.
0.51
0.505

2.57

14.3

0.51

The industrial application of OSN can be found in the oil- and petrochemical, food,bioproduct industry, bulk chemistry, and the pharmaceutical industry. Table 1-6 gives an
overview of OSN membrane application together with membrane materials. Higher OSN
separation performance requires a membrane structure with high interconnected
micropores and a thin membrane layer that shows higher permeance and permeating flux.
Table 1-6. OSN membrane application [38]
Solvent
Acetone
Chloroform/ toluene
Cyclopentane
Dimethylformamide(DMF)
Ethanoln-heptane
HexaneMethanolApplication
Vegetable oil deacidification
Removing metals and carbon residue
from heavy oil
Fractionation of hydrocarbon oils
Pharmaceuticals,
purificationofpolymers
Nutraceuticals, flavors, pigments,
vegetable oil, protein, amino acids
Aromatics from heavy oil
Vegetable oil refining, regeneration ofused lube oils
Refining
vegetable
pharmaceuticals

32

Membrane
Polyamide (PA)Poly(vinylidene
fluoride) (PVDF)
Polycarbonate
Polydimethylsiloxane(PDMS)
Polysulfone(PSf),
polyimide (PI)Polypropylene
PDMS, PI, polyvinyl
alcohol(PVA),
ceramic membranes
oil, Composites, ceramic
membranes

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Methyl esters

Biodiesel, vitamins

Composites
membranes
Methyl ethyl ketone Removal of dewaxing solvents
Cellulose
acetate(MEK)(CA), PIn-paraffins
Lube oil dewaxing
Polycarbonate
PropaneOils from organic solvents
PI
Toluene/DMF/xylene Removal of low molecular weight PI
compounds in polymer manufacture

1.3.2 Organic solvent reverse osmosis (OSRO)
Organic solvent reverse osmosis (OSRO) was developed for the separation of thesmaller size of solute molecules in the different kinetic diameter range of 0.1-0.3 nm inliquid mixtures with the gradient of chemical potential across the membrane [44]. The
membrane pore size that could be used for the OSRO process primarily provides a small
size below 0.7 nm, and pressurizes the membrane feed to levels that exceed thetransmembrane osmotic pressure is required before the permeation can occur [45].
Furthermore, OSRO membranes were responsive to the existence of defects that could
induce pressure-driven diffusion and reduce the separation ability of the membranes.
Properly high pressure for membrane operation was required to overcome the highosmotic pressure (generally higher than 30 bar) and enhance productivity, hence highstability of the membrane in organic media, high-pressure resistance, and favorable
separation characteristics must be performed.
The transport mechanism of the permeated component through OSRO membranes
generally follows the solution-diffusion mechanism where the molecules dissolved inside
the membrane matrix and then diffuse through it as a permeate [46]. The separation oforganic mixtures strongly depends on the sorption and diffusion behaviors of eachcomponent inside the membrane. The permeating molar flux (J) of one species is equal
to its chemical potential gradient.
‫ܬ‬ൌെ஽஼೘ ௫೘ ௗఓ
ோ்

ௗ௭

ൌെ

஽஼೘
ோ்

ቀܴܶ

ௗ௫೘
ௗ௭

൅ ‫ݔ‬௠ ‫ݒ‬ௗ௣೘ௗ௭ቁ ൌ െ‫ܥܦ‬௠ௗ௫೘ௗ௭

(1-59)

Where ‫ ݒ‬is the molar volume of one species, D is the diffusion coefficient, ‫ܥ‬௠ is the total
molar concentration, R is the gas constant, and T is the absolute temperature. The
integration equation (1-59) is based on the membrane thickness from 0 to L, the flux of33

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the membrane can be expressed as follows.
‫ܬ‬ൌ

஽஼೘
௅

ሺ‫ݔ‬ଵ௠ െ ‫ݔ‬ଶ௠ ሻ(1-60)

The chemical potential equilibrium between the membrane surface and the solution was
assumed to get the equation (1-61) and (1-62). On the feed side, it was assumed that ߤଵ௦ ൌଵߤ௠, while on the permeate side ߤଶ௦ ൌ ߤଶ௠ .

ߤ଴ ሺ‫݌‬ଵ ǡ ܶሻ ൅ ܴ݈ܶ݊‫ݎ‬ଵ௦ ‫ݔ‬ଵ௦ ൌ ߤ଴ ሺ‫݌‬ଵ ǡ ܶሻ ൅ ܴ݈ܶ݊‫ ݎ‬௠ ‫ݔ‬ଵ௠

Feed side:(1-61)

Permeate side:ߤ଴ ሺ‫݌‬ଶ ǡ ܶሻ ൅ ܴ݈ܶ݊‫ݎ‬ଶ௦ ‫ݔ‬ଶ௦ ൌ ߤ଴ ሺ‫݌‬ଶ ǡ ܶሻ ൅ ܴ݈ܶ݊‫ ݎ‬௠ ‫ݔ‬ଶ௠ ൅ ‫ݒ‬ሺ‫݌‬ଵ െ ‫݌‬ଶ ሻሺͳǦ͸ʹሻ
By substituting ‫ݔ‬ଵ௠ and ‫ݔ‬ଶ௠ into equation (1-60), and change the

஽஼೘
௅௥ ೘

ൌ ‫ܭ‬, so that the flux(J) of the membrane can be expressed as follows.
‫ ܬ‬ൌ ‫ ܭ‬ቂ‫ݎ‬ଵ௦ ‫ݔ‬ଵ௦ െ ‫ݎ‬ଶ௦ ‫ݔ‬ଶ௦ ݁‫݌ݔ‬௩ሺ௣మ ି௣భ ሻோ்ቃ

(1-63)The osmotic pressure can be defined as follows.
ோ்ߨ ൌ െ ௩ ሺ‫ݔݎ‬ሻ

(1-64)

By substituting equation (1-64), the flux of the membrane can be transformed into
equation (1-64).
‫ݒ‬‫ݒ‬‫ ܬ‬ൌ ‫ݎܭ‬ଵ௦ ‫ݔ‬ଵ௦ ቈͳ െ ݁‫ ݌ݔ‬ቆ ሺ‫݌‬ଶ െ ‫݌‬ଵ ሻቇ ൈ ݁‫ ݌ݔ‬ቆ ሺߨଶ െ ߨଵ ሻቇ቉ܴܴܶܶ‫ ܬ‬ൌ ‫ݎܭ‬ଵ௦ ‫ݔ‬ଵ௦ ቂͳ െ ݁‫ ݌ݔ‬൬
If we assumed

ି࢜
ࡾࢀ

ି௩
ோ்

ሺο‫ ݌‬െ οߨሻ൰ቃ(1-65)ሺο‫ ݌‬െ οߨሻ ൎ Ͳ, the flux can be stated in equation (1-66)‫ܬ‬ൌ௄௥భೞ ௫భೞ ௩ோ்ሺο‫ ݌‬െ οߨሻ

(1-66)

The flux in the RO condition can be equally expressed to the molar fraction ratio in thepermeate of components M and N, as shown below.
௃ಾ௃ಿሺ௫మೞ ሻಾൌ ሺ௫ ೞ ሻ ൌమ ಿೡሺ೛మ ష೛భ ሻቃೃ೅ಾೡሺ೛ ష೛ ሻ௄ಿ ቂ௥భೞ ௫భೞ ି௥మೞ ௫మೞ ௘௫௣ మ భ ቃೃ೅ಿ௄ಾ ቂ௥భೞ ௫భೞ ି௥మೞ ௫మೞ ௘௫௣(1-67)

Thus, the permeation flux and the molar fraction of each component in the permeate canbe calculated based on equations (1-63) and (1-67), respectively. The total flux (JT) andthe rejection (R) can be expressed below.
‫ ்ܬ‬ൌ ‫ܬ‬ெ ൅ ‫ܬ‬ே

(1-68)

ሺ௫మೞ ሻಿ
ሺ௫భೞ ሻಿ

(1-69)ܴ ൌ ቂͳ െቃ ൈ ͳͲͲΨWhere ሺ࢙࢞૚ ሻࡺ and ሺ࢙࢞૛ ሻࡺ is the molar fraction of component N in the feed and on the34

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --permeate side, respectively [47].
Presently, the industrial OSRO development is in the beginning stage, so thedevelopment of materials and the separation mechanism need to be further examined.
Table 1-7 shows common separation systems of various binary mixtures and theappropriate membrane materials for each application.
Table 1-7. Organic mixtures separation performance via OSRO process
MembranePressure
[Mpa]
3.0
3.0

System [molar ratio]

SWC5
PK-PA
BTESA

MeOH/Tol (96.3/3.7)
MeOH/Tol (90/10)
MeOH/Tol (95/5)

BTESA
BTESA
BTESA
CAB
CA
Cellulose

MeOH/MA (95/5)
MeOH/DMC (95/5)
MeOH/MTBE (95/5)
MeOH/Pentane (15/85)
MeOH/Pentane (15/85)
MeOH/iso-BuOH(60/40)EtOH/Xylene (48.5/51.5)

PE
CA
PA
CA
CAB
PA
AF2400/polyketone
Glass
PE
CA

Separation
factor [-]
4.8
45

12
12
12
3
3

Flux [kgm-2 h-1]
7.60
2.40
0.243.98
2.20
1.85
1.70
N.a.
N.a.

15.5

3.20

2-18

EtOH/Xylene (49/51)
EtOH/P-Xylene (40/60)
EtOH/N-Heptane (67/33)
EtOH/N-Heptane (40/60)
EtOH/N-Heptane (40/60)
EtOH/N-Hexane (40/60)
EtOH/Cyclohexane(45/55)N-heptane/Xylene(49.5/50.5)
N-heptane/Xylene(49.5/50.5)

Ref.
[48]
[46]

19.7-240

[49]

8
10
1,100
5.7
15

[49]
[49]
[49]
[50]
[50]

1.7

[51]

36.7-61.8

[52]

2.7
2.3
7.7
2.3
8.5
9.5

[52]
[53]
[52]
[53]
[53]
[54]

6.9
1.7
6.9
5.2
5.2
4

0.150.23
0.54
N.a.
0.65
0.42
0.10
1.60

1.3

0.04

1.3

[55]

6.9

0.59

1.1

[52]

6.9

0.23

5.2

[52]

6.9

1.3.3 Pervaporation (PV)
1.3.3.1 Introduction: membrane transport properties of pervaporation
Pervaporation (PV) is a membrane-based separation process where the transportof components occurs through a dense membrane [56]. Membrane performance wasdetermined by the chemical properties and the structure of the membrane matrix so that35

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the mass transport in pervaporation is based on permeated component-membraneinteraction. Compounds are selectively transported from the liquid phase in the upperstream to the vapor phase in the lower stream, which enables better separation
performance and lower levels of energy consumption [4, 57, 58]. These downstream
operating conditions ensure the efficient removal of the component from the membrane
downstream surface when maintaining the partial pressure of permeating component
close to zero. The higher partial pressure of the feed stream can be achieved via apreheated process of the mixtures on the feed side (generally at a temperature close tothat of its saturation temperature) [59]. In the pervaporation process, there is a masstransport and heat transfer across the membrane, including the change of physical statefrom the liquid in the feed stream to a vapor solution on permeate side with the enthalpy
of vaporization. In most cases, the permeating compounds are recovered via thecondensation process and the composition of permeate was evaluated with gas
chromatography characterization as schematically shown in Fig. 1-20.

Fig. 1-20. Schematic diagram of vacuum pervaporation process [60]The solution-diffusion model is frequently used for explaining mass transport
through dense or non-porous membrane processes, particularly pervaporation (PV). The
transport of components through a membrane consists of three main processes, including
the sorption of the permeating component from the feed liquid into the membrane matrix,
the diffusion of permeating component through the membrane, and the desorption of
permeating component to the vapor phase on the downstream of the membrane as
schematically describe in Fig. 1-21. In solution diffusion mode, several assumptions weremade such as the membrane matrix is a homogeneous medium, an isothermal system,
steady-state, no convection where the flux of the membrane is only diffusive, the first law

36

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --of Fick is valid, no coupling mass transport effect, interfacial equilibria where thechemical potential of separated compounds at the feed are the same and at the samepressure, and the chemical potentials of the component at the membrane or downstream
interface are equal [56].
The driving force for the pervaporation was evaluated from the difference between
the partial pressure in the feed and permeate streams so that the permeance of the i-th (ܲ௜ )component can be expressed as follows.
‫ܬ‬ܲ௜ ൌ ௜൘
൫‫݌‬ଵǡ௜ െ ‫݌‬ଶǡ௜ ൯(1-70)

Fig. 1-21. Schematic image of the solution-diffusion process [60].
Where ‫ܬ‬௜ is the molar flux of the i-th component, ‫݌‬ଵǡ௜ and ‫݌‬ଶǡ௜ are the feed partial pressure
and the permeate partial pressure, respectively. The feed streams partial pressure ofcomponent i can be calculated as follows.
‫݌‬ଵǡ௜ ൌ ߛ௜ ‫ݔ‬௜ ‫݌‬௜ǡ଴

(1-71)

 ‫݌‬௜ǡ଴ ൌ ‫ ܣ‬െ ‫ܤ‬Τሺ‫ݐ‬ሾԨሿ ൅ ሾ‫ܥ‬ሿሻ

(1-72)

Where ‫݌‬௜ǡ଴ is partial vapor pressure that is calculated based on the Antoine equation (Eq.
1-72), t is the temperature of the pervaporation system, ‫ݔ‬௜ is the mole fraction ofcomponent i, and ߛ௜ is activity coefficient that was determined by the Wilson equation. In
this system, the permeate pressure (‫݌‬ଶǡ௜ ) was assumed to be zero. The activity coefficientof each component can be calculated with Wilson parameters (Ȧଵଶ and Ȧ ଶଵ ) based onequations (1-73) and (1-74) [4].
 ߛଵ ൌ െ ሺ‫ݔ‬ଵ ൅ Ȧଵଶ ‫ݔ‬ଶ ሻ ൅ ‫ݔ‬ଶ ቂ37ஃభమ
௫భ ାஃభమ ௫మെஃభమஃభమ ௫భ ା௫మቃ

(1-73)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ஃ ߛଶ ൌ െ ሺ‫ݔ‬ଶ ൅ Ȧ ଶଵ ‫ݔ‬ଵ ሻ ൅ ‫ݔ‬ଵ ቂ௫ ାஃభమ ௫ െ ஃ
భ

భమ మ

ஃభమ

మభ ௫భ ା௫మ(1-74)

ቃThe Wilson parameters for several methanol organic mixtures were summarized in Table
1-8.
In addition, a facilitated transport mechanism is possibly applied for the separationof liquid mixtures through a dense membrane structure where the accelerated transfer oftargeted molecules occurs in addition to the normal Fickian diffusion.
Table 1-8. Kinetic diameter, Antoine coefficient, and Wilson parameter several organic
compounds with methanol (MeOH) as compound 2 [61-63]
Compound 1
Ethanol (EtOH)
Isopropyl alcohol(IPA)
Tertiary butanol(Tert-BuOH)
Methyl acetate(MA)
Ethyl acetate(EA)
Butyl acetate(BA)
Dimethylcarbonate(DMC)
Isopropyl acetate(i-PAc)Toluene (Tol)0.43

Antoine coefficient(Compound 1)
A
BC
8.044941554.3222.65

Antoine coefficient(Compound 2)
A
BC
7.87863 1473.11
230

઩૚૛
0.20397઩૛૚
2.307560.47

6.6604813.055

132.937.878631473.11230

1.24972

0.566210.51

7.60203

1285.067

189.318

7.878631473.11230

0.4825

2.2131

0.478

7.065

1157.63-53.42

7.878631473.11230-99.919452.060.52

7.202111232.83228

7.878631473.11230

0.48274

0.50375

0.585

7.02845

1368.5204

7.878631473.11230

287.874182.56

0.53

6.43381413-44.25

7.19736

1574.99-34.3

0.77430.28360.57

7.202111232.83228

7.878631473.11230

0.48274

0.50375

0.59

6.95464

1344.8219.482

7.878631473.11230

0.28838

0.121

Molecular
size [nm]Wilson parameters

The mobility of permeated components through facilitated transport can be classified into
three classes, including a mobile carrier membrane, semi-mobile carrier membrane, andfixed-site carrier membrane as shown in Fig. 1-22 [64]. In mobile carrier membranes, thecarriers can diffuse freely, while in semi-mobile carrier membranes the carrier can migrateotherwise it gains high energy to reach the state of activation. In the case of a fixed-site
carrier membrane, the carrier can only oscillate within a nanoconfined space. The
facilitated transport phenomena in pervaporation is dominated by the fixed-site carrier
membrane, where a reversible reaction of A + B ֖ AB can take place between the carrier
B and the permeating molecule A and lead to a preferential diffusion of component A

38

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --through the membrane matrix.

Fig. 1-22. Schematic image of the facilitated transport through mobile carrier membrane(a), semi-mobile carrier membrane (b), and fixed-site carrier membrane (c) [64].
Facilitated transport based on π-complexation has gained great attention in hybrid
membranes for pervaporation. Some transition metal ions such as Ni2+, Co2+, Cu2+, Ag+,and Pd2+ can interact with specific components including pyrroles, thiophenes, pyridine,
and aromatic species via π-complexation. Zhang et al. [65] reported the modification ofCo(HCOO)2-based hybrid membranes as schematically shown in Fig. 1-23 for the
separation of benzene (aromatic) over cyclohexane and n-heptane (nonaromatic andaliphatic) via pervaporation. In that work, Cobalt formate has demonstrated a stableperformance for facilitating the transport of aromatic compounds, resulting highpermeating flux of 0.771 kg m-2 h-1 with a separation factor of 5.1.

Fig. 1-23. Schematic synthesis concept of Co(HCOO)2-PEBA membrane for facilitatedtransport of aromatic compound [65].
The separation factor on pervaporation performance was affected by the operating
condition such as temperature, feed flow rate, and the feed concentration. Ma et al. [66]reported with an increase in temperature, the flux of permeated components such as waterand ethanol was increased due to the increase of the vapor pressure that acts as the driving
force of the permeation. Fig. 1-24 shows the correlation between the membrane
permeance and the temperature dependence of water and ethanol. Permeation of thecomponent was assumed to occur only by the diffusion following three consecutive steps:(1) the sorption onto the membrane, (2) diffusion of the adsorbed molecules through the39

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membrane, and (3) the desorption from the permeated side of the membrane.

(c)Fig. 1-24. Temperature dependence for permeance (a), the separation factor (b), andcorrelation between ethanol feed concentration with permeance for silica membranes (c)[66].
If it was assumed that the rate of adsorption and desorption are similar, adsorption is ܵ ൌܵ଴ ݁‫݌ݔ‬ሺെο‫ܪ‬ௌ Ȁܴܶሻ, and the diffusivity is ‫ ܦ‬ൌ ‫ܦ‬଴ ݁‫݌ݔ‬ሺെ‫ܧ‬஽ Ȁܴܶሻ, the permeance can becalculated as follows.
௉೔௟௃ൌ ο௉೔ ൌ

ௌ஽
௟

ௌ ஽

ିா೛

ൌ బ௟ బ ݁‫ ݌ݔ‬ቂ ோ் ቃ

(1-75)The value of οܲ is equal to the feed partial vapor pressure for each component, ܵ଴ǡ ‫ܦ‬଴ are
pre-exponential factors, ‫ܧ‬஽ is the activation energy for diffusion, and ο‫ܪ‬ௌ is the heat ofadsorption. Therefore, ‫ܧ‬௣ is the total activation energy of the permeating molecule.
‫ܧ‬௣ ൌ ‫ܧ‬஽ െ ο‫ܪ‬ௌ

(1-76)

A straight line in Fig. 1-24 was fitted based on the Arrhenius equation (1-75). The total
activation energy of permeating component can be positive or negative, depending on theactivation energy of diffusion exceeding the heat of adsorption. If ‫ܧ‬௣ ൐ Ͳ, the energy ofactivated diffusion is greater than the heat of adsorption, hence, permeance increase withtemperature, while the value of ‫ܧ‬௣ ൏ Ͳ , the heat of adsorption is greater than theactivation energy for diffusion, resulting in permeance that decreases as the temperature
increases. Fig. 1-24(c) shows concentration dependence of the membrane, where water
permeance remained nearly constant, while the ethanol permeance shows a different trend
depending on the pore size of the membrane. Based on these results, the difference inpartial vapor pressure as the main driving force and the interaction between membranesurface and permeating molecules could be taken into consideration.
The history of pervaporation was started in the 1910s by Kober who defined
pervaporation as the permeation and evaporation after he evaluated water selective
permeation through the membrane [67]. The first industrial pervaporation membrane was40

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --launched by Gesellschaft fur Trenntechnik (GFT) using a composite membrane ofcrosslinked polyvinyl alcohol (PVA) on a porous poly(acrylonitrile) on non-woven
support [68], followed by the installation of pervaporation plant for the dehydration of
ethanol in Brazil [69]. Thereafter, the development of membrane material for thepervaporation process grew fast over the decades. Today, membrane researchsignificantly increased from the aspect of the search for competitive membrane materials,
the process, and its application in dehydration of organics and recovery of organic-organic
separation.

1.3.3.2

Application of the pervaporationa) Dehydration of alcohol
The separation of low molecular weight components such as alcohol andwater has been one of the most challenging tasks due to the formation of azeotropic
mixtures. The conventional separation process such as distillation requires high costs andenergy consumption with low separation ability. In the case of solvent dehydration, wherea small concentration of water content needs to be removed, hydrophilic membranes are
generally used. The hydrophilic membrane can preferentially allow water to penetrate,
resulting in high water content on permeate side and dehydrating the solvent in theretentate streams [70]. On contrary to that, hydrophobic membranes can be used toremove a small amount of the organic solvent from the systems that are dominated bywater, leaving the water on the retentate side while admitting the solvent to penetrate.
Three main classes of membranes have recently been used for the dehydrationof alcohol mixtures, including polymeric, inorganic membranes such as ceramic, zeolite,
and organosilica, and composite membranes. Polymeric membranes are widely used in
the solvent dehydration process due to their high-selective sorption and high-selective
diffusion, which is capable to promote ion-dipole interaction, hydrogen bonding withwater molecules, and preferential sorb water molecules. However, most polymeric
membrane faces swelling problem during the operation process. At that time, the polymer
chain is stretched and the pore size of the membranes was enlarged, resulting in higher
membrane flux while decreasing selectivity as the large-sized molecules find it easier topass through [59]. Ceramic membranes demonstrated high thermal and chemical stability
41

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --which could be able to operate in a wide range of pH conditions in any type of solvent.
Ceramic class membranes such as silica-based membranes and zeolite were commonlyused for this application. Zeolite materials with highly ordered well-defined structures
exhibit micropores structure that can be varied ranging from 3 to 8 Å depending on theratio of aluminum, oxygen, and silicon. The pervaporation of water and alcohol mixtures
through zeolite membranes occurs by the capillary condensation of water molecules
through the zeolitic and non-zeolitic pores, which reflect the formation of defect from thehighly ordered crystalline structure was unavoidable [71]. The existence of defects gives
rise to enhancing the greater flux while reducing the separation factor of the membranes.
To date, the development of mixed matrix membranes has been reported byseveral researchers for the application of pervaporation. The structure of mixed matrix
membranes consists of inorganic materials that were dispersed into a polymeric basemembrane as a matrix. Generally, the presence of inorganic filler could effectively
strengthen the mechanical stability of the membrane and reduce the free volume in whichlarge-sized molecules can diffuse. However, the combination of two different materials
often generates interfacial voids that lead to reduced separation performance. Uragami etal. [72] reported the mixing of chitosan membrane with tetraethoxysilane (TEOS) up to
45 wt% effectively improved the selectivity of the chitosan membrane for the separationof water over ethanol. Despite that, a higher concentration of TEOS was found to further
increase the swelling degree of the membrane during operation. The pervaporation
performance of different types of membranes for the dehydration of alcohols issummarized in Table 1-9.
Table 1-9. Dehydration alcohol mixture via pervaporation through different types ofmembranes
Membrane
PVA, PSStSA-coMA
PVA
Sericin
PVA
Sericin/PVA(50/50)Chitosan/HEC
Alginate
PVA/Na-Alg

TPV
[⁰C]

H2O
feed
conc
[wt%]

Total
flux [kgm-2 h-1]

Sep.
factor[-]Ref.

H2O/EtOH

30

4

0.43

190

[73]

H2O/EtOH
H2O/EtOH
H2O/EtOH

60
60
60

4
10
10

0.12
0.07
0.12

10
90
115

[74]

H2O/EtOH

60

10

0.07

130

H2O/EtOH
H2O/EtOH
H2O/EtOH

60
50
45

10
4
10

0.424
0.095
0.384

5,469
202
384

System

42

[75]
[76]
[77]
[78]

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(80/20)Na-Alg/PVP (3/1)
PSF/PEGPSF
Sodium sulfonate
PSF
TGN/PSF(12.5/87.5)
Poly(amic methyl
ester)PSF hollow fiber
BAPP-Polyimide
PVA/SSA-Na+
PVA/SSA-Li+
PVA/SSA-K+
PAA
PAA-Li+
PAA-Na+
PAA-K+
PAA-Ca2+
PAA-Al3+
PEI/PAA
PEI/PAA
PVA
PVA/Pani(0.78/0.22)
PVA/Pani(0.73/0.27)
PVA/Pani(0.52/0.48)
PMDA-ODA
PMDA-MDA
BTDA-PDA
BTDA-ODA
BTDA-MDA
PAN/SstSA,
HEMA
Zeolite A
Zeolite T
Zeolite A
Zeolite A
Zeolite X
Zeolite Y
Zeolite T
ECN silica
membrane
PAN
PAN-Zeolite X(25 wt%)
PAN-Zeolite X(32 wt%)
PAN-Zeolite X

H2O/EtOH
H2O/EtOH
H2O/EtOH

30
25
25

4.6
10
10

0.09
0.6
0.7-0.9

364
325
600

[79]
[80]
[81]

H2O/EtOH

25

10

0.75

1,900

[82]

H2O/EtOH

25

10

0.44

680

[82]

H2O/EtOH

40

10

1.7

240

[83]

H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH

25
25
50
50
50
24
24
24
24
24
24
70
40
30

10
10
10
10
10
10
10
10
10
10
10
5
5
10

0.173
0.27
0.06
0.07
0.06
0.175
0.160
0.215
0.380
0.220
1.010
0.314
0.140
0.095

23.9
22
44
39
42
4.9
8
6.3
5.6
5.5
5
604
1,207
77.3

[84]
[85]

H2O/EtOH

30

10

0.035

18.6

H2O/EtOH

30

10

0.061

514.3

H2O/EtOH

30

10

0.069

564.2

H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH

45
45
75
45
45

11.1
11.1
11.1
11.1
11.1

0.014
0.023
0.005
0.022
0.035

346
47
1594
562
478

[91]

H2O/EtOH

30

50

0.65

212

[92]

H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH

70
70
45
75
75
75
75

10.3
10.1
5
10
10
10
10

1.12
0.91
0.86
2.15
0.89
1.59
0.81

18,0001,000
54,00010,000360
130830

H2O/EtOH

70

3.6

1.6

350

H2O/EtOH

50

8

0.007

281

H2O/EtOH

50

8.7

0.054

35.9

H2O/EtOH

50

9

0.088

51.9

H2O/EtOH

50

7

0.369

3.2

[86]

[87]

[88]
[89]

[90]

[93]
[94]
[95]
[96]

[97]

43

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(40 wt%)
PAN-Zeolite X(50 wt%)ChitosanChitosan/Carboxyl
Chitosan/Carboxyl
PVA/Chitosan(80/20)PSSA-gPTFE/Chitosan
Na-Alginate
PVA
Na-Alg/PVA(25/75)Na-Alg/PVA(50/50)Na-Alg/PVA(75/25)
PVA/Na-Alg(80/20)P84-Polyimide
P84-copolyimide
Zeolite A
Zeolite T
ECN silica
membrane
SilicaSilicaSilica-Zr 10 mol%
Silica-Zr 30 mol%
Silica-Ti 10 mol%
Silica-Al 10 mol%
Silica-Al, Mg 10
mol%Pervap SMS
PVA-ZSM-5 (2mass%)
PVA-ZSM-5 (4mass%)
PVA-ZSM-5 (6mass%)
SilicaSilicaSMART NaAPervap SMS

H2O/EtOH

50

8.5

0.277

7.1

H2O/EtOH
H2O/EtOH
H2O/IPA

50
60
50

10
10
10

0.201
0.300
0.178

127
634
491

[98]

H2O/IPA

60

10

0.644

900,000[99]

H2O/IPA

25

10

0.409

1,490

[100]

H2O/IPAH2O/IPA

30
30

10
10

0.012
0.041

356
21

H2O/IPA

30

10

0.024

195.5

H2O/IPA

30

10

0.034

119.6

H2O/IPA

30

10

0.039

91

H2O/IPA

45

10

0.414

1,727

[78]

H2O/IPAH2O/IPAH2O/IPAH2O/IPA

60
60
75
75

15
15
10.4
10.2

2.578
0.432
1.58
2,10

5
3,508
30,000
9,000

[102]
[103]

H2O/IPA

80

4.5

1.9

1,150

[96]

H2O/IPAH2O/IPAH2O/IPAH2O/IPAH2O/IPAH2O/IPA

70
80
80
80
80
80

5
10
10
10
10
10

1.0
0.65
0.86
0.67
0.78
0.08

100
73
300
27
400
210

[104]

H2O/IPA

80

10

0.31

90

H2O/IPA

70

8.2

1.9

53

H2O/IPA

30

10

0.005

91.1

H2O/IPA

30

10

0.003

141.1

H2O/IPA

30

10

0.003

216.2

H2O/n-BuOH
H2O/n-BuOH
H2O/Tert-BuOH
H2O/Tert-BuOH

75
80
60
60

5
5
10
10

3.0
2.9
1.5
3.5

250
1,200
1,600
144

[101]

[93]

[105]

[106]

[107]

[104]
[108]
[109]

b) Isomer separation
The separation of isomer molecules offers several difficulties and is still very

44

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --challenging due to the similarity in size [110], similarity in physical and chemical
properties, and similar volatility of the mixture pairs [111]. Compared to the conventional
separation

techniques

such

as

distillation,

low-temperature

crystallization,

chromatography, and extraction, the membrane-based process via pervaporation
demonstrated numerous advantages including clean technology, environmentally friendly,
and attractive energy efficiency. The Pervaporation process has become a necessary
technology for the separation of azeotropic mixture with a close boiling point, where theseparation performance of the membranes depends on the differences in solubility and the
diffusivity of the permeated component [112]. However, the separation of organic mixtureisomers based on the diffusivity through pervaporation is remain a challenging task.
Therefore, the development of membrane materials that could determine the slightdifference in the physical and chemical properties of the mixture must be considered.
Table 1-10 shows the pervaporation performance of different types of membranes for theisomers separation.
Table 1-10. Pervaporation separation of organic liquid isomer via different types ofmembranes
Membrane
Polyamide-imide(PAI)
Polyamide-imide(PAI)/ αcyclodextrin (α-CD)
Polyamide-imide(PAI)/ βcyclodextrin (β-CD)
Polyamide-imide(PAI)/ βcyclodextrin (β-CD)
CD-grafted PAIMFI ZSM-5
6FDA6FpDA/4MPD
Polyurethane
Polyurethane/zeolite
Silicalite (SS1)
Silicalite (SS2)

SystemFeed
TPVcomposition
[⁰C]
[wt%]

Total
flux
[kg m2 -1
h ]Sep.
factor[-]n-BuOH/tert-BuOH

50/50

60

0.008

1.11

n-BuOH/tert-BuOH

50/50

60

0.003

1.34

Ref.

[113]
n-BuOH/tert-BuOH

50/50

60

0.010

1.30

n-BuOH/tert-BuOH

50/50

60

0.022

1.13

n-BuOH/tert-BuOH

50/50

0.002750/50

2.30
0.940.96[114]

p-xylene/o-xylene

60
2675

p-xylene/o-xylene

50/50

65

24.9

1.29

[110]

p-xylene/o-xylene
p-xylene/o-xylene
p-xylene/o-xylene
p-xylene/m-xylene
p-xylene/o-xylene
p-xylene/m-xylene

50/50
50/50
50/50
50/50
50/50
50/50

25
25
50
50
50
50

0.30
0.31
0.271
0.272
0.253
0.258

1.37
1.89
21
24
16
19

45

0.16

[115]

[111]
[116]

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --MFI zeolite(templated)
MFI zeolite(template-free)
Co(HCOO)2/PEBA
APAF/3MPD6FDA
MOP-SO3NanHm
PU(0)-650

p-dichlorobenzene(DCB)/o-DCB
p-dichlorobenzene(DCB)/o-DCB
Toluene/iso-octane
Benzene/cyclohexane
Toluene/cyclohexane
Toluene/n-heptane

50/50

60

0.40

2.3

50/50

60

0.106

16.7

10/90
10/90
10/90
10/90

40
40
40
40

0.826
0.760
0.685
0.771

7.2
4.6
4
5.1

[65]

Toluene/n-heptane

40/60

80

0.036

5.1

[118]

Benzene/cyclohexane
Benzene/cyclohexane

50/50
54/46

40
25

0.540
2.284

8.4
2.7

[119]
[120]

[117]

c) Polar/non-polar organic mixtures separation
The pervaporation of organic mixtures has been focused on polar/non-polar
mixtures such as methanol/ methyl tert-butyl ethers (MeOH/ MTBE – azeotropic mixture
with a ratio of 14/ 86 wt%), methanol/ dimethyl carbonate (MeOH/ DMC) – 70/30 wt%,
and methanol/ toluene (MeOH/ Tol) – 73/27 wt% [121-123]. MeOH forms azeotropic
mixtures during the separation process, which necessitates further methanol removal inorder to acquire a highly pure solvent. The pervaporation for the separation of organicfrom organic mixtures performs a challenging task since most organic mixtures show
similar physicochemical properties which enhance the difficulties to break them apart[45] and a strong interaction between molecules which leads to generating a high swelling
degree for the membranes and rapidly reducing the pervaporation performance [124]. One
approach to reducing the membrane swelling degree when the pervaporation of organic
mixtures is to use a rigid backbone polymer or to use a copolymer consisting of the rigid
segment that provides a strong network and softer segments to facilitate the transport ofpermeant [125]. Two main criteria for identifying the suitable membrane materials forpervaporation separation of given mixtures were affinity and the size sieving properties.
The affinity is generally related to the interaction that is created between the membrane
matrix and the penetrating solvent, while the size sieving effect could promote thediffusion selectivity of the small-sized component over a large-sized component. Table
1-11 shows the pervaporation performance of different types of membranes for the
separation of the organic mixture.

46

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Table 1-11. Pervaporation performance for different types of membranes in organic
mixture separations
Membrane

System

Ppy-PF
LPS/pDMAEMA/PAIHEMAPEGMA306
MePEG200MA/
PEGMA306
Polyelectrolyte
surfactant complex
NaY
NaY
NaY
NaY
NaX
ZIF-67BTESA
PA-6/Al2O3
PA-6/TiO2
PA-6/ZrO2
GA-STA-CS
PAA/PVASTA/CS
NafionBTESA
BTESE
Ppy-PF
TiPCS
ChitosanCellulose acetate
PA/UiO-66(NH2)EDTA/cellulose
SPI
TEOS-CS

MeOH/IPA
MeOH/IPA
MeOH/cyclohexane
MeOH/cyclohexane

10/90
15/85
3.2/96.8
3.2/96.8

57.5
50
50
50

Total
flux
[kg
m-2 h1
]
0.50
3.39
3.5
0.8

MeOH/cyclohexane

3.2/96.8

50

2.2

1,986

MeOH/cyclohexane
EtOH/cyclohexane
MeOH/benzene
EtOH/benzene
EtOH/cyclohexane
MeOH/MTBE
MeOH/MTBE
MeOH/MTBE
MeOH/MTBE
MeOH/MTBE
MeOH/MTBE
MeOH/MTBE
MeOH/DMC
MeOH/DMC
MeOH/DMC
MeOH/DMC
MeOH/DMC
MeOH/DMC
MeOH/Tol
MeOH/Tol
MeOH/Tol
MeOH/Tol

12.1/87.9
12.8/87.2
10/90
10/90
10/90
10/90
10/90
15/85
6/94
50/50
50/50
50/50
50/50
60/40
50/50
40/60
50/50
50/50
10/90
10/90
10/90
10/90

50
50
50
60
60
50
50
40
40
30
30
30
10
10
10
10
10
10
50
50
35
30

1.08
0.87
1.02
0.22
0.27
1.70
0.46
1.84
1.54
0.476
0.327
0.400
1.03
0.61
1.16
0.93
3.13
0.45
0.28
0.64
0.80
1.80

2,415
106
7,000
930
1,000
5,300
10,000
3,186
8,050
20
11
46
91
0.8
67
11
118
235
441
410
162
80

MeOH/Tol

72/28

22

1.55

5

[141]

MeOH/Tol
MeOH/Tol

10/90
10/90

50
30

0.79
0.18

49
396

[142]
[143]

Feed
TPV
composition
[⁰C]
[wt%]

Sep.
factor
[-]
2
9
121
1,482

Ref.
[126]
[127]
[128]
[129]

[130]
[131]
[132]
[133]
[134]
[135]
[136]
[137]
[132]
[132]
[126]
[138]
[139]
[140]

1.4 Membrane fabrication and materials
1.4.1 Membrane fabrication: introduction
The selection technique for membrane fabrication depends on the choice ofmaterials, desired structure of membranes, and the final membrane application. The
membrane fabrication technique can greatly affect the performance of the membranes.

47

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Membranes can be classified into two classes based on material synthesis, includingorganic and inorganic membranes. Organic membranes generally include polymer
materials such as polysulfone (PS), cellulose acetate (CA), polyethersulfone (PES), andpolyvinylidene fluoride (PVDF), while the inorganic membrane includes ceramicmaterials such as zeolite, carbon molecular sieves (CMS), amorphous silica, etc. Table 112 presents the summary of the membrane fabrication method for different types ofmaterials.
Table 1-12. Summary of commonly used membrane materials and the fabrication
methods
Membrane
Membrane
Fabrication method
Application Ref.
typematerial
Phase inversion
MF, UF,[144]
NF, RO
Organic
SinteringMF[144]
Organic polymer
membranes
StretchingMF[144]
Track etchingMF[144]
ElectrospinningMD[145]
Vacuum sputtering
GS[146]
Pyrolysis
GS[147]
Metal membrane Electroless platingGS[148]
Bi-metalmulti-layerGS[149]
deposition
CarbonPyrolysisofpolymermolecular sieve precursors (500 – 1000 ⁰C) GS
[145]
(CMS)
under vacuum or inert gas
Slip-castingUF[145]
Metal oxide
Atomic layer deposition
MF, UF, NF [145](TiO2)(ALD)
Hydrothermal
GS[145]
Inorganic
HydrothermalPV[150]
membranes Zeolite
Hydrothermal-seeded
NF[151]
growthIn situ growthGS
Seeded-assisted (secondary)
Metal-organicGSgrowth[145]
framework(MOF)
Rapid thermal deposition
GS
Contra-diffusion
GS
Silica,Sol-gelGS, PV[145]
Thermal chemical vapor
organosilica,GS[145]
templated silica, deposition (CVD)metal oxide silica Plasma-enhanced chemical
GS, PV[145]
vapor deposition (PECVD)*MF = microfiltration, UF = ultrafiltration, NF = nanofiltration, RO = reverse osmosis, PV =pervaporation, MD = membrane distillation48

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --1.4.2 Permeation and physicochemical properties of the membranes
The separation performance of the membrane was determined by the selectivity andpermeability of the membrane materials. The available range of membrane materials
includes polymeric, carbon, organosilica, zeolite, MOF, and other ceramic materials. The
porosity of the membrane is an important characteristic for determining separation
efficiency. Based on permeating flux and selectivity, membranes can be classified into
three categories porous, non-porous, and asymmetric [21]. A porous membrane wascharacterized by rigid, highly voided with randomly distributed interconnected pore
structures. Porous membranes generally show very high levels of flux but low separation
performance. Each type of membrane material has a different structure of pores as shownin Fig. 1-25. A membrane with permanent porosity has an immovable network of poresin which permeating molecules can travel, while most polymeric membranes with flexible
porosity demonstrated a dynamic fluctuating pore that can move, shrink, expand anddisappear. Non-porous membranes exhibited high separation performance but commonly
show low permeability, where most permeated components are transported by thesolution-diffusion mechanism. On the other hand, asymmetric membranes consist of two
structurally distinct layers, one is a thin, dense selective layer, and the other is a porous
substructure layer that provides support for the thin-topmost separation layer.

Fig. 1-25. Pore structure from different types of membranes [20]49Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The selection of membrane materials was expected to enhance the transport of moleculeson the membrane surface, thus a microporous membrane material with a high surface area
and interconnected pore structure or free volumes is more desirable. However, theadjustment of pore sizes must be made to improve the gas selectivity. Besides that, thechemical properties, the ease of fabrication, and the real-life performance can be used in
the consideration for selecting membrane materials. For this reason, it is important tocharacterize the physicochemical properties of the material before evaluating themembrane performance. To be properly analyzed, Table 1-13 provides a summary of theexperimental technique that can be used for membrane material characterization.
Table 1-13 Membrane physicochemical properties and corresponding characterization
technique
Membrane property
Characterization technique
Ref.
Physical properties
Hydrophilicity and hydrophobicity

Contact angle[42]

Size distribution

Dynamic light scattering (DLS)[152]

Porosity, surface area

N2 sorption isotherms[42]

Chemical properties
Surface chemistry, ligand binding Fourier transform infrared[42,(surface-sensitive),

152]

electronic spectroscopy (FT-IR), X-ray

structure
Opticalphotoelectron spectroscopy (XPS)
properties,

electronic Ultraviolet-visible spectroscopytransition,(UV-Vis)Mass, composition, and thermal
stabilities
Order-disorder[152]

Thermogravimetric analysis (TGA)transformation,

[152]
[152]

crystal structure, crystalline grain X-ray diffraction (XRD)
size
Structural properties
Cross-section, surface roughness, Scanning electron microscope
the thickness of the topmost layer(SEM), transmission electron
microscope (TEM)

50

[42]

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --1.4.3 Membrane Materials
1.4.3.1 Metal membranes
Dense metal-based membranes made of palladium, and its alloys such as silver,
nickel, tantalum, vanadium, niobium, and zirconium show primary high selective
separation for gas mixtures. Pd and its alloys membrane show specific characteristic toallow only hydrogen molecules to permeate, thus making these membranes possible toproduce a pure stream up to 99.99% [153]. The development of metallic membrane wasstarted in 1866 by Thomas Graham who observe unusually high adsorption capacity fromthe palladium-based membrane. The industrial commercialization of metallic membranes
was started in the early 1960s by Johnson Matthey via palladium-silver alloy membrane
for hydrogen purification [154].
The transport of metallic membranes occurs via charged particles or either
surface-limited or diffusion-limited. Besides that, alloy-based membranes gained anincrease of interest due to their improved performance including higher resistance,stability, decrease embrittlement tendency, and enhance hydrogen permeability. The
abundance for each metal was plotted in Fig. 1-26 as a function of cost for the fabricationof metallic-based membranes.

Fig. 1-26. The abundance and the cost of most commonly used metals for the preparationof Pd-based membranes (each price is referred to 2019) [155].
Pd77%Ag23% membrane shows 1.5-2 times higher permeability compared to pure Pd
membrane and higher resistance against hydrogen embrittlement [156]. Pd-Cu and Pd-Pt

51

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --alloys have been found to have higher resistance against sulfur poisoning, thus to achieve
higher permeability and resistance Pd alloy membranes were developed with more than
two elements such as Pd-Ag-Au, Pd-Ag-Cu, and Pd-Au-Pt [157-159]. Other metals such
as V, Nb, Ta, Zr, and Ni have been studied for highly permeable H2 membranes as analternative way to reduce the use of precious metals. Zr-Ni alloys have a reactive surface
which can allow hydrogen dissociation and increase hydrogen solubility [160]. However,
the high solubility of hydrogen led to generating hydrogen embrittlement and resulted in
the precipitation of metal hydride. However, metal membranes have limitations on thevery dense structure which led to low permeability. In addition, dense metallic-based
membranes relatively show a high cost to be one to three orders of magnitude more
expensive per unit surface area. The Pd-based membrane was reported to be incapable toproduce high flow rates of hydrogen as high as 10,000-100,000 m3 h-1 on an industrial
large scale [161].

1.4.3.2 Polymeric membranes
Polymeric membranes are increasingly deal with interest for industrial
applications due to their low-cost investment material, good processability, economic
competitiveness, scalability, tuneability, and the easiness of large-scale manufacture. To
achieve highly separation performance of the membrane, attention has been focused on
modifications to the solution-diffusion properties in polymers. The modification ofpolymer structure could be conducted via crosslinking, grafting, blending, andcopolymerization processes as presented in Fig. 1-27 [122]. The crosslinking method was
generally performed to reduce the swelling degree and enhance the resistance of thepolymeric membrane matrix in the feed mixture in order to derive higher stability andselectivity of the membranes. The crosslinking of polymer membranes was carried out bya chemical reaction, irradiation, and physical crosslinking [162]. On contrary to that, theoptimize crosslinking process must be considered to avoid the formation of a brittle
polymer with a loss in the dimensional stability. The grafting technique on polymer
modification uses the oligomer chains that were attached as side chain branches
irregularly to the polymer main chain by chemical reaction or irradiation. The moleculesthat are desired to be grafted must contain reactive functional groups which be able to52

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --react with the functional groups of the polymer. The blending process of two polymers
that are not covalently bonded can be done for creating hydrophilicity in the hydrophobic
membranes. Two classes of polymer blends including homogeneous and heterogeneous
blends, in which two polymers are miscible for all compositions and the other one is nottotally miscible, respectively. However, the heterogeneous blends of polymers will notgive enough mechanical strength to the thin membrane structure. Copolymerization canbe implemented for two different polymers structure which is covalently bonded toincrease the mechanical strength of the membrane.
(a) Crosslinking(b) Grafting(c) Blending(d) Copolymerization

Fig. 1-27. Schematic concept of polymer membrane modification via crosslinking [163](a), grafting [164] (b), blending [165] (c), and copolymerization [166] (d).
The gas separation process using polymeric membranes was first commerciallyapplied in 1979 with prism separators from Monsanto with a modular design for the
separation of hydrogen from the product stream of ammonia synthesis [167]. Polymericmaterials are commonly used because it has film-forming ability, processability, and goodmechanical properties. Most polymeric membranes show a trade-off effect, i.e. highly
permeable membranes generate lower selectivity and vice versa [36]. Moreover,membrane plasticization and aging effects are commonly present which led to unstableand deteriorated separation performance in industrial applications. Plasticization refers tomembrane dilating in the presence of condensable gas such as CO2, which enhances53

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --polymer chain mobility, and gives rise to higher permeability and lower separation factor.
The dilation polymer matrix by CO2 results in higher permeability for other gases andreduces overall separation performance. Consequently, concerted efforts have been
devoted to modifying the properties of polymer membrane which can exhibit both highpermeability and selectivity such as creating a hybrid polymer-inorganic structure,fabricating an active thin separation layer, and facilitating transport mechanism throughthe incorporation of appropriate carrier fillers with specific functional groups into thepolymer matrix [168].
The utilization of polymeric membranes for pervaporation application isparticularly based on the preferential absorption of polymer towards the components andthe diffusion through the free volumes or pore structure of the polymers. The affinity
between membrane materials and the permeating component becomes the most importantparameter to select the appropriate membrane materials for pervaporation. The size
determination effect from the free volume cavities provides an additional contribution to
the permeation of the small-sized component over the large-sized ones [169]. In
accordance with affinity, hydrophilic polymers were used to develop membranes forwater or polar selective permeation, while hydrophobic (organophilic) polymers wereoperated for organic or non-polar selective permeation. Examples of hydrophilic
membranes include a large number of polymers such as poly (vinyl alcohol) (PVA),
polyamide (PA), Polyimide (PI), and polyelectrolyte complex (PEC). By contrast, alimitednumberofhydrophobicpolymer

membraneisavailable

such

as

polydimethylsiloxane (PDMS) and poly (ether-block-amide) (PEBA). PEBA is anorganophilic polymer consisting of polyamide (PA) and polyether (PE). The rigid
properties of PA provide mechanical strength whereas the PE provides affinity for organic
compounds. PEBA membranes generally show higher hydrophobicity compared to
PDMS membranes which are favorable for the phenol/water separation or selective
permeation of organics with large molecular size [170].
The other challenge to using polymeric membranes in organic/organic mixtures is
the much larger swelling of the polymer matrix in the pure organic solvent which could
reduce the size determination ability and stability of the membrane. Thus membranesurface modifications were used to solve this problem such as by introducing bulky
fluorine groups into the PI structure to increase the free volume of PI membranes and54

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --crosslinking is proposed to enhance the swelling resistance of PI membrane towards theorganic solvents [171]. Copolymerization modification such as using PEBA andpolyurethane (PU) to tune the soft/hard segments for achieving comprehensive separation
performance and reducing the swelling degree of the membranes [120, 172].

1.4.3.3 Carbon molecular sieving membranes
Carbon membranes were fabricated via thermal conversion of polymer layers with
pyrolysis or carbonization at high temperatures (generally 400-900 ⁰C) in an inert
atmosphere (Fig. 1-28(a)) [173]. At high temperatures, many polymer structures will
thermally rearrange, crosslinked, and begin to carbonize. The predominant transportmechanism for most carbon membranes is the molecular sieving mechanism. Fig. 1-28(b)shows the comparison of surface morphology for carbon hollow fiber membrane withpolymeric hollow fiber membrane. CMS membranes are considered porous materials
where the permeates could penetrate through the pore system, while polymeric
membranes consist of a dense active layer where the permeation is dominated by thesolution-diffusion process [173].
(a)

(b)

Fig. 1-28. Pyrolysis protocol of polymeric precursor for the development of CMS
membranes [174] (a), and membrane surface comparison for CMS and polymeric hollow
fiber membranes (b) [173].

55

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Carbon membranes contain constrictions in the matrix that approach the size of permeatedmolecules, thus these materials can effectively determine the gas species of similar size.
The separation occurred via size discrimination, where small-sized molecules can passthrough the membrane pore structure while larger-sized molecules are obstructed.
Permeation through the carbon matrix is related to the pore structure of the membrane.
The pore structure of the carbon membrane consists of a relatively wide opening withnarrow constrictions, where the opening pore contributed to the free volume and is
responsible for the adsorption capacity. On the other hand, the constrictions are
responsible for the discrimination of penetrated molecules, hence the diffusivity of acomponent depends on the size and shape of the molecule because the carbon molecularsieving membrane has a pore size that is close to the dimension of penetrated molecules.
In these constrictions, the interaction energy between molecules and carbon matrix is
involved both in dispersive and repulsive interaction. If the pore opening of themembranes becomes small relative to the size of penetrating molecules, the repulsive
interaction was predominant and the molecules require higher activation energy to passthrough the constriction. Therefore, molecules with slight differences in size can beeffectively separated through molecular sieving. The permeation mechanism on carbon
molecular sieve membranes is determined by the internal surface area, the dimension ofpores, and the surface properties of the membrane matrix [23].
The permeation characteristics of CMS membranes can be varied by changing
several synthesis parameters such as organic precursors, carbonization temperatures, andthe chemistry of pyrolysis. CMS membranes can be obtained from the pyrolysis ofdifferent types of thermosetting polymers such as poly (vinylidene chloride) (PVDC),
poly (furfuryl alcohol) (PFA), cellulose, polyacrylonitrile (PAN), phenol-formaldehyde
and various coal. Xu et al. [175] reported the fabrication of CMS membrane fromphenolphthalein-based cardo poly(arylene ether ketone) (PEK-C) that were carbonized at
different temperatures with ultrahigh permeability for hydrogen purification. The resultof that work shows that the H2 permeability was decreased as the pyrolysis temperature
increased due to the formation of dense and ordered carbon structure and the separation
performance of H2/CH4 reach 1,1152 for CMS membrane calcined at 900 ⁰C. In addition,
CMS membrane could be applied for liquid-phase separation via reverse osmosis. Koh etal. [176] reported the fabrication of CMS membrane (Fig. 1-29) for the separation of56

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --xylene isomers via organic solvent reverse osmosis (OSRO). CMS hollow fiber
membranes calcined at 450 C exhibited the highest p-xylene permeance of 1.6 × 10-8 mol
m-2 s-1 Pa-1 with the selectivity of p-xylene/o-xylene of 7, while at higher calcination
temperature (550 ⁰C) the membrane shows the highest separation of 30 but with low
permeance of 3.8 × 10-10 mol m-2 s-1 Pa-1.

Fig. 1-29. The schematic image for the fabrication of CMS hollow fiber membranes atthe pyrolysis temperature of 450-550 ⁰C [176].
In contrast to the high separation performance of CMS membranes, most carbon
membrane is brittle and fragile, and thus requires careful handling. In addition, thefabrication process of CMS membranes requires high thermal energy which led togenerating difficulty and high-cost production. Pre-purifier step was required before
membrane fabrication for removing traces of strongly adsorbing vapors which can clog
up the pore system.

1.4.3.4 Zeolite membranes
Zeolite membranes are microporous aluminosilicate crystals with a pore size thatwas constructed by the TO4 (T = Si, Al) tetrahedral unit framework with an oxygen atomthat is shared between two adjacent atoms. Zeolites with 8- (e.g. LTA, CHA, DDR), 10(MFI, FAU), and 12- (MOR) framework rings have different sizes of pore structure in therange of 0.38 to 0.74 nm as shown in Fig. 1-30.

57

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 1-30. Different zeolite structures are derived from oligomeric silicate andaluminosilicate species [177].
Zeolite membranes are commonly prepared by hydrothermal method via in-situ andsecondary growth methods. In the case of in-situ growth, the surface of the substrate canbe functionalized to enhance its interaction with zeolite crystals to generate a continuous
membrane layer, while in the secondary growth method zeolite crystals were introduced
as seeds on the substrate prior to hydrothermal synthesis. The secondary growth method
shows a better process for controlling membrane integrity and thickness [178]. In
addition, the properties of the seed layer including crystal size, thickness, and particle
distribution are important to determine the properties of zeolite membranes. The
membrane properties of zeolites are dependent on the ratio of Si/Al (Fig. 1-31), where alow Si/Al ratio exhibits high hydrophilicity but low acid resistance, while a higher Si/Al
ratio generates a membrane with high acid stability but the pore size of the membranes
were ranged from 0.56 to 0.74 nm which was too large for the separation of small-sizedmolecules [179].

58

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 1-31. The pore size, hydrophilicity, and the acid resistance properties of zeolite
membranes as a function of the Si/Al ratio [180]Several zeolite membranes are widely used in pervaporation applications,
including water dehydration, organic/organic, and isomers separation. LTA (NaA) zeolitethat was synthesized from a Si/Al ratio of 1 has an 8-membered oxygen ring that shows
the pore diameter in the range of 0.23 and 0.42 nm [179]. The low Si/Al ratio of thismembrane shows an extremely hydrophilic property, and the pore size of the membrane
is close to the size of water molecules (kinetic diameter-KD = 0.29 nm), thus making
LTA membranes appropriate for the application of dehydration of alcohol or othersolvents. CHA zeolite membrane with a pore size of 0.38 nm shows a high flux of 13.3
kg m-2 h-1 with excellent separation of 6,000 for dehydration of ethanol/water mixture[181]. MFI membrane that was synthesized with a high Si/Al ratio showed defect-freequality with a high flux of 2.9 kg m-2 h-1 and achieved a separation factor of 160 forethanol/water mixture at 25 ⁰C [182]. Despite that, it was found that the performance ofthe MFI membrane declined after 177 hours of pervaporation at 100 C due to the loss of
Silicon atoms from the zeolite framework and the further formation of silanol groups in
the aqueous mixture [183, 184]. Moreover, the formation of a continuous and defect-freezeolite layer on porous support is quite challenging due to weak adhesion between
microporous zeolite particles and macroporous support.

59

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --1.4.3.5 Porous coordination polymer or metal-organic frameworks(MOFs) membranes
Metal-organic frameworks (MOFs) have become an interest over the past decades
due to their high crystallinity, porosity, modularity, and diverse functionality. MOFs aredefined as hybrid materials where metal ions or small inorganic nanoclusters are linked
into one-, two-, or three-dimensional networks via multifunctional organic linkers [185].
The crystallization of MOF structure in solution is based on the nucleation of secondary
building units (SBUs) which are then joined together to form the final crystal structure as
schematically depicted in Fig. 1-32. MOF as microporous crystalline materials shows animpressive performance for molecular sieving membranes with high permeability andselectivity compared to polymeric membranes due to their high microporosity and tunablechannel structure. MOFs membrane was recently used for hydrogen purification,hydrocarbon separation, and CO2 capture [186].

Fig. 1-32. Schematic images of inorganic secondary building units (SBUs) (a), organic
linkers (b), and different structures of MOFs materials (c-e) [186]60

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Continuous

MOF

membraneisgenerally

synthesizedvia

precursors

crystallization through hydrothermal, solvothermal, interfacial, and contra-diffusionsynthesis. However, the different solubility between solvent and MOF precursors
becomes the limitation for membrane synthesis. Li et al. [187] reported the sol-gel
technique to synthesize ZIF-8 membranes with controllable thickness resulting in the
precise synthesis of molecular sieves membrane. The sol-gel process was executed by gel
coating for the dispersion of the precursor, then followed by thermal treatment for the
crystallization process. The prepared ZIF-8 membrane shows impressive performance for
C3H6/C3H8 separation with C3H6 permeance of 0.8 × 10-8 mol m-2 s-1 Pa-1 and selectivityof 200.
The other advantage of MOF structure after exceptional porosity is open metalsites (OMS) which could represent a major step forward in functional materials withspecial properties. HKUST-1 is one of the OMS structures which shows a high surface
area of 1,800 m2 g-1 after fully desolvated and is suggested used for new methane gas
storage materials [188]. MOF-505 also demonstrated an extremely high surface area of
1830 m2 g-1 and high hydrogen uptake of 2.47 wt% due to the existence of Cu metals as
OMS in that structure [189]. The combination of high surface area that reflects permanentporosity and the occurrence of OMS contribute to the superior performance with respect
to hydrogen, methane, and carbon dioxide sorption. Liu et al. [190] reported the effective
enhancement of the structure of Cu-BTC via nitrogen-doping and lithium-doping as theopen metal site (OMS). In that reported work, 3N-CuBTC shows higher methanol
capacity due to the increased dispersive interaction caused by the lone pair electron of
nitrogen atoms, while 3Li-3N-CuBTC exhibited higher methanol capacity due to thesynergy effect from the lone pair electron of nitrogen atoms and the existence of metal in
the cage structure of MOF that can significantly affect to the polarity and the preferential
adsorption sites of the framework.

1.4.3.6 Mixed matrix membranes
The ceramic membrane such as CMS and zeolite are good in selectivity but are
far too expensive for use on an industrial scale. To address this problem, membranes could
be prepared from a material consisting of inorganic particles (e.g. zeolites, carbon

61

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --molecular sieves, silica) dispersed in a continuous polymer matrix, referred to as a mixedmatrix membrane as schematically shown in Fig. 1-33(a). Mixed-matrix membranes aresupposed to combine the desired selectivity of inorganic membranes with low cost andthe easiness of fabrication of polymer membranes.
(a)

(b)

Fig. 1-33. The schematic representation of an ideal mixed-matrix membrane (a), andnonideal mixed matrix membrane morphology including interfacial voids, rigidified
polymer chain layer, and pore blockage (b) [191]The membrane properties of mixed-matrix structure depend on the particle size, particle
pore size, dispersed load, polymer type, and polymer characteristics. The fabrication ofideal mixed-matrix membranes is very difficult because of the difference between thepolymer and inorganic phase properties and weak polymer-particle adhesion, which
results in the formation of organic-inorganic interface defects. Interface defects of mixedmatrix membranes are classified into three categories, including interface voids, rigidified
polymer layer around particles filler, and particle pore blockage (Fig. 1-33(b)) [191]. The
formation of interface voids is attributed to stresses after solvent evaporation and could
occur from the nucleation of non-solvent or polymer lean phase around the inorganic
phase during the phase separation process. Rigidified polymer layer related to uniform
stress around particles filler after the removal of solvent, while pore blockage occurs forporous filler where particles clogged with the sorbent, solvent, contaminant, or minor

62

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --component in the feed gas or polymer chain in the fabrication step of the mixed-matrix
membrane.
The inorganic materials used for the filler in mixed-matrix membranes could beclassified into porous and non-porous types. Porous filler acts as a molecular sieving
agent in the polymer matrix and discriminates permeated molecules based on their shapeand sizes [192]. On the other hand, non-porous filler could improve the separation
properties of the resultant membrane via increasing matrix tortuous pattern and limiting
the diffusion of the large-sized component [193]. Vu et al. [194] reported the effect ofcarbon molecular sieves (CMS) as porous filler that was added into Matrimide 5218 as
the polymer matrix. The addition of CMS 800-2 into Matrimide 5218 increased thepermeability of the membrane from 10 to 44 barrier and the CO2/CH4 selectivity wasincreased from 35.3 to 200. In other work, Ahn et al. [195] reported the addition of 20
vol% nonporous silica particles into the polysulfone matrix led to an increase in thesolubility and diffusivity coefficient, increasing the free volume of polymer membraneand enhancing CO2 permeability from 6.3 to 19.7 Barrer. However, low permeances
become one problem due to the difficulty of preparing a thin layer of the mixed-matrix
membrane. In addition, the fabrication of mixed-matrix membranes usually involves aweak contact from the polymer particles matrix and poor distribution from the dispersed
inorganic phase in the continuous polymer matrix.

1.4.3.7 Silica and organosilica-based membranes
1.4.3.7.1 Preparation technique for silica and organosilica membranes
Amorphous silica and organosilica-based membranes have been studied
extensively for gas and liquid separation due to their chemical and physical stabilities.
Tetraethylorthosilicate (TEOS) was generally used as silica precursors for the fabricationof silica membranes. On the other hand, organosilica membranes are derived from organic
ligand silicon alkoxides which convert to organic/inorganic components after being fired
in a N2 atmosphere. In general, silica and organosilica-based membranes are prepared via
CVD and sol-gel processes as discussed in this chapter.

63

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --a) Chemical Vapor Deposition (CVD)
Chemical vapor deposition (CVD) was commonly used to precisely control thepore size of inorganic membranes via thermal decomposition of a silicon-based precursor
and the chemical reaction with an oxidant gas, resulting in the growth of the thin film
layer [17]. The silica precursors that are mainly used for CVD reaction are
tetraethylorthosilicate (TEOS) [196], tetramethoxysilane (TMOS) [197], silicontetrachloride (SiCl4) [198], and silane (SiH4) [199] with oxidant gases such as air, O2, O3,
N2O, and water vapor. To fabricate a membrane on the porous substrate via the CVD
method, two types of configurations can be used: counter-diffusion CVD where tworeactants were supplied from the opposite side of substrates, and co-current CVD where
precursors are supplied from one side of the substrate, while vacuumed on the other sideof the system to obtain a defect-free membrane quality.
On the other hand, low-temperature CVD methods have been developed tominimize the cost of the high-temperature CVD process. Nagasawa et al. [200] reported
a highly efficient gas separation membrane of hexamethyldisiloxane (HMDSO) was
successfully synthesized via atmospheric-pressure plasma-enhanced chemical vapor
deposition (AP-PECVD) with CO2 permeance of 1.9 × 10-7 mol m-2 s-1 Pa-1 and a highlevel of CO2/CH4 selectivity of 166 at 50 ⁰C. Plasma-enhanced CVD technique wascapable to control the decomposition of converted gaseous precursors as a deposit hybrid
thin films which have high thermal and chemical stability [17].

b) Sol-gel processes
Sol is a stable colloid suspension particle in a liquid, and a gel consists of a threedimensionally continuous solid network surrounding a continuous liquid phase (wet gel).
The formation of gels (gelation) is due to the formation of covalent bonds between thesol particles [201]. Gelation could induce by rapid evaporation of the solvent giving rise
to capillary forces that cause shrinkage of the gel network producing an aerogel. The solgel processing of silica-based materials is to transform Si-OR and Si-OH species tosiloxane (Si-O-Si) via a condensation reaction. A stable gel could be produced by
maximizing the number of siloxane bonds and consequently minimizing the number of64

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --silanol (Si-OH) and alkoxy (Si-OR) groups. The chemical reaction of silica-basedmaterials during the sol-gel process can be described in the following equation.
Hydrolysis:‫  ؠ‬െ  ൅ ଶ  ՜‫  ؠ‬െ ൅ (1-75)

Condensation:‫  ؠ‬െ  ൅  െ  ՜‫  ؠ‬െ  െ  ‫ ؠ‬൅

(1-76)

Condensation:‫  ؠ‬െ  ൅  െ   ՜‫  ؠ‬െ  െ  ‫ ؠ‬൅ ଶ (1-77)

Hydrolysis reaction initiated in alkoxide precursors dissolved in alcohol system via theaddition of water (hydrolysis agent) to generate Si-OH. Generally, the hydrolysis rate ofprecursors in water and alcohol is slow, therefore acid or base catalyst is necessary to
improve the hydrolysis and condensation reaction rate. The most important parameter forthe sol-gel reaction is the types of precursor, the pH condition that depends on acid orbase catalyst, the ratio of alkoxy group to water content, the kind of solvent, temperaturefor synthesis, and the concentration of precursor. The reaction rate for hydrolysis andcondensation has a pH dependence as shown in Fig. 1-34(a).

(a)

(b)

Fig. 1-34. The dependence of relative rates of Si(OR)4 hydrolysis and condensation
towards pH condition [201] (a), Polymerization behavior of silica under different pH
conditions [202]The minimum reaction rate for hydrolysis is pH 7, while for condensation at pH 4.5. thehydrolysis of silicon alkoxide is faster than condensation in acid conditions. Most species
are hydrolyzed at the beginning of the reaction and condensed to form a small oligomer.
65

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --On contrary to that, in basic conditions, the condensation rate is faster compared to thehydrolysis and the rate of hydrolytic cleavage of Si-O-Si bonds is higher than in acidconditions. Therefore, resulting in a network with a particulate character with big particle
size and large pores. The schematic images for the structural development of silica gels
under different pH as shown in Fig. 1-36(b).

1.4.3.7.2 Silica and types of organosilica membranes
Silica and organosilica membranes have been categorized as a class of inorganic
membranes. Most pure silica membranes were fabricated from tetraethoxysilane (TEOS)
showing a narrow pore size distribution in the range of 0.3-0.4 nm, which generally
performed high selectivity for hydrogen separation. However, silica-based membranes
presented low hydrothermal stability, small pore size distribution, and high transport
resistance [203, 204]. On contrary to that, a different type of material derived fromorganoalkoxysilanes contains various organic functional groups in the inorganic Si-O-Si
frameworks, which are known as organosilica material. Organosilica-based materials
were characterized via covalent bonding between oxygen and hydrocarbons to silicon[205]. Hybrid organosilica membranes were getting great interest by using pendant-type
precursors, then the appearance of bridged silica with stable and reliable performance
become available for large-scale industrial application. The difference between both thependant and bridge structure of organosilica precursors is shown in Fig. 1-35.

Fig. 1-35. The schematic images of the bridge- and pendant-type precursor of organosilica
and their structure after hydrolysis and condensation reaction [204].
66

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The bridge-type precursor contains organic groups that are located between two silicon
atoms (as the linking unit O-Si-R-Si-O), and the pendant structure consists of organicgroups that are connected with one silicon atom (Si-O-Si-R) [204]. A variety of organicgroups can be used such as an alkane, alkene, alkyne, aromatic, and other functionalmoieties. Organosilica membranes are typically fabricated via sol-gel reaction which isfinalized with a thermal treatment to consolidate the materials. A lower degree ofcalcination temperatures for the preparation of organosilica membranes results infunctional membranes with lower performance including the separation factor andpermeability.
These membranes exhibit high thermal and chemical stability with no swelling,high flux, high separation performance, and controllable pore size [206-208]. The
separation ability of organosilica membranes depends mostly on the pore size and on theinteraction between permeating molecules and the pore walls of membranes. Therefore,material modification is required to produce a highly porous material that can facilitatehigh permeability without sacrificing selectivity. However, the hydrothermal stability oforganosilica membranes can be varied or even change at a high temperature and in a water
vapor environment. Extreme operation conditions dramatically affect the microporous
structure of silica membranes, where hydrophobic silicon-oxygen bonds from thesiloxane network (Si-O-Si) tend to break and transform into hydrophilic silanol groups(Si-OH) [209].
ୌ୷ୢ୰୭୪୷ୱ୧ୱ

‫  ؠ‬െ  െ  ‫  ؠ‬൅ ଶ  ሱۛۛۛۛۛۛሮ ʹ ‫  ؠ‬െ (1-78)The existence of water molecules in hydrothermal conditions assists the breakage of thesiloxane framework and the rehydration occurs on the silica surface via physisorption of
H2O molecules to a hydroxyl group, followed by the reaction with a nearby siloxane. In
this condition, as more silanol groups were created, more sites for H2O adsorption andthe chain reaction of siloxane breakage occurs across the surface resulting in thelocalization of silanol groups and subsequently cross-pore condensation leading tosmaller pore size as shown in Fig. 1-36(a).

67Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

(a)

(b)

Fig. 1-36. The schematic mechanism of densification of the organosilica-based membraneat the hydrothermal condition [210] (a), and densification under different calcination
conditions [211].
Song et al. [211] reported that BTESE-derived membrane calcined at 400 ⁰C shows a highsurface area of 338 m2 g-1, while decreased at a higher calcination temperature of 600 ⁰C(SBET = 122 m2 g-1). This result indicated that the organosilica membranes show a denser
structure after calcination at higher temperatures as schematically depicted in Fig. 1-36(b).
At low calcination temperature, a low crosslinking degree, and a high amount of organic
linking unit can be retained. Slow calcination at high temperature resulting in a high crosslinking with the destruction of organic groups leads to the formation of a denser structure.
On contrary to that, expeditious calcination results in a high crosslinking degree and lessdecomposition of organic groups.
To solve this problem, several modifications to the organosilica structure can be68Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --made, especially for controlling the movement or regeneration of silanol groups including
modification of silica matrix with methyl (-CH3) groups [212], use of organically bridged
bi-silyl precursors [213], doping transition metals or metal oxides [210], and copolymerization of precursors [214].

1.4.3.7.3 Tailoring pore structure of organosilica membranes for
improved performancea) Organic template method
One approach for the development of a membrane that shows both highpermeability and separation performance is to introduce the organic ligand embedded ina dense inorganic matrix that serves as a template for the creation of porosity. Raman andBrinker [215] reported the organic template approach which shows the densification of
the inorganic matrix in the first stage and the establishment of micropores after theremoval of the organic template via pyrolysis. The schematic image of the synthesis
method of the inorganic membrane through the organic template method is presented inFig. 1-37.

Fig. 1-37. The schematic image of the organic template method for the fabrication of
porous inorganic membrane [215].
In general, the successful transcriptive synthesis of the inorganic membrane must be
followed the criteria of the use of organic ligand that can homogeneously be incorporated
into the inorganic matrix without the formation of phase separation, the synthesis
69

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --conditions give rise to the arrangement of the dense matrix so that pores will only becreated via organic template removal process, and high durability of the matrix must be
maintained without the collapse of the original network after the template removal
process [216]. Li et al. [217] prepared an organic-inorganic hybrid silica membrane by
using

two

different

precursorsofmethyltriethoxysilane(MTES)

andphenyltriethoxysilane (PhTES). In that work, both types of precursors show dense
structure at the calcination temperature of 400 ⁰C, but the N2 adsorption capacity showsa significant increase after 550 ⁰C which suggested the creation of pores after thepyrolysis of terminal organic groups in MTES-, and PhTES-derived xerogel powders (Fig.
1-38). At higher firing temperature, the H2 permeance increased from 4.1 × 10-7 to 5.8 ×
10-7 mol Pa-1 m-2 s-1 with a decrease in selectivity from 1300 to 77 due to the formation
more loose structure.

Fig. 1-38 N2 adsorption isothermal of MTES (a) and PhTES (b) -derived gel powders
calcined at different temperatures [217]b) Spacer method
A spacer method is frequently prescribed for tailoring the microporosity bychoosing bi-silyl precursors with different structures that produce organosilica-basedmaterials with an optimum membrane pore size [218]. The utilization of a short and rigid
linking chain between silicon atoms generates a porous structure, whereas the increase in
the carbon number and flexibility of the linking units tends to produce a denser structurethat is inappropriate for the construction of a high-flux membrane. Kanezashi et al. [219]reported the development of organosilica membranes via different types of spacer units
as shown in Fig. 1-39(a). A short and rigid structure of the spacer unit shows the formation

70

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --of porous structure (BTESM Si-CH2-Si, BTESE Si-C2H4-Si, and BTESB Si-Ph-Si), whilea long and flexible linking unit shows the formation of non-porous structure (BTESP SiC3H6-Si, BTMSH Si-C6H12-Si, and BTESO Si-C8H16-Si) as confirmed via N2 sorption
isotherms characterization (Fig. 1-39(b)). In line with material characterization,
membrane evaluation (Fig. 1- 39(c)) shows that short and rigid precursors such as
BTESM and BTESE exhibited higher permeance for small-sized gases such as He and H2compared with flexible-types precursor. However, the BTESB membrane with a rigid
linking unit shows smaller H2 permeance which is probably related to the free movementof the organic chain at the permeation temperature of 200 ⁰C.

(a)

(c)

(b)

Fig. 1-39 Chemical structures of different types organosilica precursor (a), N2 sorption
isotherms for resulting xerogel powders calcined at 350 ⁰C (b), and single-gas permeation
for different types of membranes at the measurement temperature of 200 ⁰C (c) [219]Table 1-14 compares the effect of organic chain flexibility to the formation ofporosity of the xerogel powder together with membrane performance. Organosilica
71Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membrane with a long organic linking unit (carbon number higher than 3) shows the nonporous structure (negligible N2 capacity) with low gas permeance and high selectivity.
Table 1-14 Organosilica-based membranes with different types of linking unit and theirgas performance
Precursor
BTESM
BTESP
BTMSH
BTESO
BTESE
BTESEthy
BTESA

Chemical structure

TC
[⁰C]

SBET
[m2g-1]

Permeance[10-7 mol
m-2 s-1 Pa-1]

Selectivity
[-]

350

̴ 600

H2 = 10

H2/N2 = 70

350

0H2 = 8

H2/N2 = 38

350

0H2 = 3

H2/N2 = 15350

0H2 = 6

H2/N2 = 10

300

552

H2 = 9.5

H2/N2 = 22

300

763

H2 = 16.1

H2/N2 = 40

300

661

H2 = 26.8

H2/N2 = 10

300

476

H2 = 18300

556

H2 = 10300

594

H2 = 32200

120

H2 = 12.9

200

77

H2 = 19.3

200

0

H2 = 23.2

300

2

BTESE
BTESEthy
BTESA
BTESE
BTESBPBTESOTTESPTWR60

H2 = 19.7

H2/N2 = 55
C3H6/C3H8 =5
H2/N2 = 68
C3H6/C3H8 =1.4
H2/N2 = 12
C3H6/C3H8 =13
H2/N2 = 16
CO2/H2 = 1
H2/N2 = 16
CO2/H2 = 2
H2/N2 = 5
CO2/H2 = 2

Ref.

[219]

[220]

[221]

[218]

H2/N2 = 9[222]

TTESPTWR240
300

80

72

H2 = 14.6
C3H6 = 0.59

H2/N2 = 13
C3H6/C3H8 =8Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --BTESABTESA/BTESBBTESA/BTESBP

/

/

300

594

CO2 = 25

CO2/N2 = 13
CO2/CH4 =
23

300

565

CO2 = 20

CO2/N2 = 25
CO2/CH4 =
35

300

783

CO2 = 30

CO2/N2 = 9
CO2/CH4 = 8

300

373

300

155

BTES-EDH2 = 8.3

H2/N2 = 11

[223]

[224]

BTES-MOU
H2 = 22.2

H2/N2 = 12
[225]

BTES-POU
BTMSN
BTPP-self12
BTPP-HCl40
BTPP-HCl240

300

0

H2 = 6

H2/N2 = 19

300

336

H2 = 8.6

H2/N2 = 12

300

0

300

0

H2 = 3.02

H2/N2 = 32.8

300

0H2 = 3.89
CO2 = 0.34

H2/N2 = 20.4
CO2/N2 = 25

H2 = 5.32[226]

H2/N2 = 19.8
[227]c) Metal doping
Silica and organosilica membranes could be doped with transition metals such as
Ag [228], Al [229], Co [230], Zr [231], Fe [232], Nb [233], and Ni [234] to increase thestability characteristics of the membranes. Doping metal into silica structure presents
functionalities that are not available for pure silica membranes. One of the specificfunctionalities that were developed via metal doping has been reported by Battersby et al.
[235]. In that reported work, the structure of cobalt-doped tetraethylorthosilicate (CoTEOS) was maintained microporous as H2 selectivity was not affected by steam exposurefor 200h of operation, while pure silica membrane shows the opening of larger pores
resulting in mesoporous structure. Another specific functionality after the incorporationof metal-doping was reported by Kanezashi et al. [229]. Al doped bis (triethoxysilyl)73

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ethane (Al-BTESM) shows the densification structure with a lower surface area (SBET =507 m2 g-1) compared to pure BTESM xerogel powder (SBET = 597 m2 g-1), but higher
performance for C3H6/C3H8 separation was achieved for Al-BTESM with the highest
permeance ratio of 40. Table 1-15 provides the summary metal-doped organosilica
membranes for the separation in gas and liquid system.
Table 1-15. Performance comparison of the pristine and metal-doped silica andorganosilica membranes
Si/Ni = 0
Si/Ni = 4/1
Si/Ni = 2/1
Si/Al = 0

TC
[⁰C]
550
550
550
200

SBET
[m2g-1]
n.a.
n.a.
n.a.
597

Permeance [10-7mol m-2 s-1 Pa-1]
H2 = 0.89
H2 = 2.23
H2 = 10.26
C3H6 = 0.5

Al-BTESM

Si/Al = 9.5/0.5

200

531

C3H6 = 0.1

Al-BTESM

Si/Al = 9/1

200

507

C3H6 = 0.08

Al-BTESM

200
400

387
n.a.

C3H6 = 0.01
H2 = 1.8

400

n.a.

H2 = 0.9

H2/CO2 = 17

400

n.a.

H2 = 1.0

H2/CO2 = 31

400

n.a.

H2 = 0.8

H2/CO2 = 35

BTESE

Si/Al = 8/2
Thydrothermal = Thydrothermal =100 ⁰C
Thydrothermal =200 ⁰C
Thydrothermal =300 ⁰C
Si/Nb = 0

H2/H2O = 13
H2/H2O = 19
H2/H2O = 25
C3H6/C3H8 = 9
C3H6/C3H8 =10
C3H6/C3H8 =20
C3H6/C3H8 = 6
H2/CO2 = 12

Nb-BTESE

Si/Nb = 3/1

400
300
350
400
450

120
119
126
88.4
26.8

H2 = 2H2 = 2.25
H2 = 1.85
H2 = 1.07
H2 = 0.97

BTESE

Metal/Si =
0/10

150

694

H2O = 13.4

Al-BTESE

Al/Si = 1/9

150

514

H2O = 10.8

Zr-BTESE

Zr/Si = 1/9

150

395

H2O = 17.0

H2/CO2 = 2.8
H2/CO2 = 10.2
H2/CO2 = 10.9
H2/CO2 = 56.3
H2/CO2 = 220
H2O/IPA =3,757
H2O/IPA =8,994
H2O/IPA =14,205

Pd-NbBTESE-0.05
Pd-NbBTESE-0.1
Pd-NbBTESE-0.2
Pd-NbBTESE-0.4
Pd-BTESE(Pd/Si =Pd/Nb/Si =0.05/0.33/1
Pd/Nb/Si =0.1/0.33/1
Pd/Nb/Si =0.2/0.33/1
Pd/Nb/Si =0.4/0.33/1

400

233

H2 = 1

H2/CO2 = 50

400

223

H2 = 0.5

H2/CO2 = 62.5

400

214

H2 = 0.4

H2/CO2 = 57.4

400

177

H2 = 1.5

H2/CO2 = 150

400

410

H2 = 3

H2/CO2 = 8.91
H2/CH4 = 8.4

Metal-doped
organosilica
TEOS
Ni-TEOS
Ni-TEOSBTESM

Zr-BTESE

Condition

Calc. atm.= air74

Selectivity [-]

Ref.
[234]

[229]

[231]

[233]

[236]

[237]

[238]

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --0.11/1)TEOS
Co-TEOSTEOS
Mg-TEOSBTESE
B-BTESE
Ta-BTESE
Nb-BTESE

Calc. atm.= N2

400

547

H2 = 1

Calc. atm.= H2

400

507

H2 = 4400

540

H2 = 2

500
500
500
500
600
600
600
600

390
420
200
195
n.a.
n.a.
n.a.
n.a.

H2 = 20
H2 = 50
H2 = 60
H2 = 30
H2 = 5.26
H2 =2.10
H2 =2.20
H2 =0.70

H2/CO2 =8.6
H2/CH4 = 10.7
H2/CO2 =8.0
H2/CH4 = 31.3
H2/CO2 =15.0
H2/CH4 = 173
H2/N2 = 80
H2/N2 = 200
H2/N2 = 300
H2/N2 = 50
H2/CO2 = 9
H2/CO2 =19
H2/CO2 =24
H2/CO2 = 350

300

n.a.

H2 = 3.12

H2/CH4 = 19.5

300

n.a.

H2 = 4.98

H2/CH4 = 12.7

300

n.a.

H2 = 5.65

H2/CH4 = 12.3

300

n.a.

H2 = 5.03

H2/CH4 = 12

Calc. atm.=
H2/N2
Co = 0 mol%
Co = 10 mol%
Co = 30 mol%
Co = 50 mol%
Mg = 0 mol%
Mg = 10 mol%
Mg = 15 mol%
Mg = 20 mol%
Metal = 0mol%
B/(B+Si) = 20
mol%
Ta/(Ta+Si) =20 mol%Nb/(Nb/Si) =20 mol%

[239]

[240]

[241]

Based on Table 1-15, most metal-doped organosilica membranes exhibit higher
separation performance with lower permeance of components. Song et al. [242] reported
that a pristine organosilica (1,2-bis(triethoxysilyl)ethane/BTESE) membrane has
relatively large intra-particle pores and a loose network, while a metal-doped organosilica
membrane shows a less porous structure as shown in Fig. 1-40.

Fig. 1-40. The comparison of pore structure from pristine and metal-doped organosilica
membranes [242].
75

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --1.5 Scope of this thesis
1.5.1 Proposal of organosilica-based coordination polymer
A coordination polymer composes of metal ions linked by the coordinated ligands
in an infinite direction. The interconnected structural units have the potential to house
nanometer-sized space. The infinite net structure is defined by the coordination bonds andmolecular species linked by hydrogen bonding. Hence, a coordination polymer structure
consists of metal and ligands or often includes guest and counterions [243]. The metal
ions that act as connectors are commonly transition metals and/or lanthanoids with 2-6
coordination numbers. The coordination number of coordinated polymer structures canbe varied depending on the metal and the oxidation state, which resulting different
geometries and porosity. A transition metal easily forms complex due to the existence ofempty d-orbitals, while alkali metal and rare earth ions tend to lose valence electrons andform cations with the positive charge which results in the ionic interaction. Ag(I) ionstend to form a linear chain with several linear linkers as a result of their preference for acoordination number of two [244]. The other metals such as Co(II) and Zn(II) show higher
coordination numbers with higher dimensional network connectivity generated withligands binding sites. The schematic presentation for the construction of coordinatedpolymers via different building blocks is shown in Fig. 1-41 [245].

Fig. 1-41. Schematic images for the construction of coordination polymers with different
building blocks [245].

76

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --A typical ligand or linker might have two or more coordination sites such as
pyridyl, imidazole, nitrile, or carboxylate. Yang et al. [246] reported the formation of
coordinated Cu2+ ions with the hydroxyl groups of cellulose polymer chain that could
open the spacing between the molecule chain and create channels in cellulose nanofiber(CNF) that serve for ions conducting pathways as shown in Fig. 1-42.

Fig. 1-42. The SEM image of cellulose nanofiber and the schematic structure of Cu2+
create coordination with hydroxyl groups opening channel structure for the transport of
Li+ ions [246].
In another work, Jia et al. [247] reported the formation of ZIF-8@cellulose nanofiber(ZIF-8@CNF) composite membranes that were prepared via electrostatic forces between
Zn2+ and -COO- from the CNF. After the formation of coordinated species (Zn2+-COO-),
the 2-methylimidazole solution was poured into CNF-Zn2+ to facilitate the in situ growthof the ZIF-8@CNF composite membrane. The schematic concept of coordinated metalpolymer structure is shown in Fig. 1-42. Pristine CNF membrane shows a very dense
structure where most permeating molecules are transported based on the solutiondiffusion mechanism. After the in-situ growing of porous ZIF-8, connected-MOF

77

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --channels were constructed, increasing both CO2 permeability and the selectivity for
CO2/N2 of 566 Barrer and 45.6, respectively.

Fig. 1-43. Schematic image of the synthesis procedure for ZIF-8@CNF composite
membranes [247].
Tanaka et al. [248] reported the hybridization of organosilica membranes connected with
ZIF-8 networks for enhanced separation performance of C3H6/C3H8. The preparation ofpristine ZIF-8 membrane shows a poor membrane adhesion with α-alumina support due
to ZIF-8 cannot form a covalent bond with the surface OH from the support. In that work,
they use 3-(2-imidazolin-1-yl) propyltriethoxysilane (IPTES) grafted on the surface of αalumina support and promote the nucleation of ZIF-8 via coordination between IPTESZn-2 methylimidazole as schematically shown in Fig. 1-44. After the grafting of IPTES
on the α-alumina support, the selectivity of C3H6/C3H8 increased to 36 with C3H6
permeance of 8.5 × 10-8 mol m-2 s-1 Pa-1.

Fig. 1-44. Schematic image of IPTES grafted on α-alumina support followed by the
nucleation of ZIF-8 structure [248]78Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Based on the reported works, it was possible to construct the coordination between
transition metal (can act as Lewis-acid molecules) with polymer structure with specificfunctional groups such as amine moieties (can act as Lewis-base molecules or ligand). By
following this concept, we firstly proposed the development of organosilica-based
coordination polymers by adding a transition metal to change the flexibility of organic
linking units of the precursors and improve the microporosity of the membrane while
maintaining the amorphous nature of the silica networks. The concept of functionalizedorganosilica-based coordination polymer is shown in Fig. 1-45.

(a)Metal (M) doping

Hydrolysis

coordination

CondensationonAmine functionalizedorganosilica
precursors

Network with flexible
linking unit (dense)

Network with rigid
linking unit (porous)(b)

Nickel-amine coordination

Excess amount of Ni forfacilitated transport
Amorphous phase from silica network

Fig. 1-45. Schematic image for the development of metal-coordinated aminefunctionalized organosilica framework via hydrolysis and condensation followed bycoordination (a), and the proposed membrane structure of hybrid organosilica-based
coordination polymer
The varied structure of ligands from rigid to completely flexible was important toproduce a definite structure of coordination polymer. When functional or non-functionalorganic groups are covalently linked to oxide networks, sol-gel coordinated materials are
obtained. The strong interaction between metal and ligand produces a robust porosity,
while weak coordination generates lining porosity. Based on the isothermal studies, thecoordination-based polymer could provide permanent porosity through the linking offlexible moieties with a strong bond (between organic and inorganic components)

79

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --together with weaker bonds that show physical interaction such as van der walls orhydrogen bonding. When a weak bond increases the dimensionality of the framework into
2D or 3D, a higher dimensional structure shows a more robust nature. A new strategy ofusing metal-coordinated amine-functionalized organosilica precursors could control the
microporosity of silica-based amorphous network. The addition of metal-generated
subnanopores via the coordinated covalent bonds increased the crosslinking between
network layers, which converted the texture of the membrane from a non-porous to a
microporous entity. Fig. 1-46 shows the chemical structure of different types of Nfunctionalized organosilica precursors for the preparation of coordinated polymers as
ligand connectors with a metal center. A hybrid network consisting of an ordered structurefrom coordinated polymers and an amorphous structurefrom the organosilica could be generating a membrane with both combinations of highpermeating flux and selectivity.

3-Aminopropyltriethoxysilane Trimethoxy[3- (MethylAmino) Propyl] Silane 3, N,N-Dimethylamino propyl trimethoxy
DAPTMS
MAPTS
silane
APTES

Bis [3-(trimethoxysilyl) propyl] amine
N-[3-(Trimethoxysilyl)propyl]ethylenediamine

TMSPED1- [3- (Trimethoxysilyl) propyl] urea

TMSPU

BTPA

Fig. 1-46. The chemical structure of possible functionalized-organosilica precursors as
the connector ligand for the formation of coordination polymer with metal center

Table 1-16 compares different types of membrane materials together with organosilicabased coordination polymer as one novel class of materials which provides a promisingchance to approach high membrane performance. Organosilica-based coordination
polymers can be constructed from coordination bonds (binding energy-BE = 20-50 kcal
mol-1), hydrogen and metal-metal bonds (BE = 1-30 kcal mol-1), π-π stacking (BE = 0-12
kcal mol-1), CH-π, electrostatic, and van der Waals interaction (BE = 0.1.5 kcal mol-1),
thus network with robust and rigid properties can be produced [244, 249].

80

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Table 1-16. Comparison between the present proposed organosilica-based coordination
polymer and conventional materials
Membrane materials

Building block

Fabrication
methodAdvantages

Disadvantages

Organic monomer
molecules

Phase inversion,interfacial
polymerization,
electrospinning

Low cost,
easy tofabricateLowpermeability,
swellingHighpermeability,
highperformance

Poor chemicalandhydrothermal
stabilities(lability of
coordinate
bonds)

Solvent-casting
method, spin
coating,

Significantly
enhanceselectivity

Poor adhesion
between thepolymer phaseand MOFparticles,
defects, and low
separation
capability

Seeding freeaqueoussynthesis

Grafting theorganosilica
precursors
can control
the grainsize of MOF
particles,
reduce theformation of
defects

Lower
permeability
after grafting theorganosilica
precursor

Sol-gelHighpermeability,highselectivity,
easy forfabrication

Low thermal
stability due tothe existence ofamine organic
linkerPolymer
1.

Metal-organic framework (MOF)

In situ
crystallization,
seeded-assisted(secondary)growth, rapid
thermaldeposition,
contra-diffusion

1. Metal ions or
inorganic
nanocluster
2. Organic linker

MOF-Polymer
1. Metal ions or
inorganic
nanocluster
2. Organic linker
3. Organic monomer
molecules

Organosilica-MOF
1.
2.
3.

Organosilica
precursor
Metal ions or
inorganic
nanocluster
Organic linker

Organosilica-based coordination
polymerAmine-functionalized organosilica precursors

Hydrolysis
Condensation

Metal-coordination (M)Rigid linking unit

Without metal addition

1. Organosilica
precursor
2. Metal ions

Flexible linking unit81Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --In addition, the sol-gel process for the synthesis of organosilica materials has the main
advantages of its allowing an easy-preparation method, lower energy consumption, andapplicability to a wide variety of materials with the different functional components inthe structure of organosilica precursors.

1.5.2 Objectives and organization of this study
The overall objective of this dissertation research is to develop metal-coordinatedamine-functionalizedorganosilica-induced

microporosity

forhighlypermeable

membranes for both gas and liquid separation. By following the concept of metal-organic
framework (MOF) where metal-linker interactions form nano spaces, in this dissertation,
we attempted a novel synthesis strategy to control the pore size of amorphous aminefunctionalized organosilica via sol-gel followed by coordination. Metal-doped
aminosilica membranes are expected to generate subnanopores via the coordination ofcovalent bonds between aminosilica precursors as Lewis-base molecules (ligand) and thetransition metal acted as Lewis-acid molecules. The crosslinking between metal andligand was proposed to convert membrane texture from the non-porous entity into
microporous ones via the development of more rigid linking units. The concept of metalcoordinated aminosilica membranes is schematically shown in Fig. 1-47. However, theresearch about organosilica-based coordination polymer was very limited, especially inthe viewpoint of optimized conditions for the affinity of metal types, metal concentration,and the affinity of ligand.

Metal-doped

Coordination

Rigid linking units - microporous

Flexible linking units – non-porous

Fig. 1-47. The schematic image for the coordination mechanism between metal acceptors
and amine donor electrons results in a rigid linking unit with a microporous structure
Therefore, this research has focused on the development of metal-coordinated aminosilica
for highly permeable membranes, and the evaluation of their gas permeation properties

82

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --for the separation of specific gas mixtures and/or liquid systems via pervaporation. The
main work of this study is as follows:(i) Synthesis, characterization, and membrane evaluation of different types of transition
metals including silver (Ag), copper (Cu), and nickel (Ni) coordinated with bridgetype aminosilica precursor of bis [3-trimethoxysilyl) propyl] amine (BTPA);(ii) Optimization of the formation microporosity of nickel-doped bis [3-trimethoxysilyl)
propyl] amine (Ni-BTPA) at different nickel/amine (Ni/NH) mole ratios and theevaluation of membrane performance for the separation of azeotrope mixture via
pervaporation;(iii) Comparative studies on the formation of the coordinated network via different typesof aminosilica ligand precursors including bis [3-(trimethoxysilyl) propyl] amine(BTPA), trimethoxy [3-(methylamino) propyl] silane (MAPTS) and 3-amino propyl
triethoxysilane (APTES) and the separation performance of organic/organic mixtures
via pervaporation;(iv) The evaluation of the evolution of silver in two phases for facilitated transport on
silver-doped BTPA membranes at different silver/amine mole ratios.
This thesis consists of 6 chapters, as follows:
Chapter 1 is the “General introduction”, which involves the researchbackground and the purpose of this study.
Chapter 2 is “Metal-induced microporous aminosilica creates a highly
permeable gas-separation membrane”. Hybrid microporous aminosilica membranes
have been successfully synthesized via doping with Ag-, Cu- and Ni- into dense bis [3(trimethoxysilyl) propyl] amine (BTPA) membranes, which creates micropores via thecrosslinking between donor pairs of electrons in the amine moiety and electron acceptors
in the empty “d” orbital of a transition metal. The formation of micropores within thecoordinated covalently bonded compound was investigated via Ultraviolet-Visible
spectroscopy (UV-Vis), Fourier Transform Infrared spectroscopy (FT-IR), X-ray
Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Transmission Electron
Microscopy (TEM), and the isotherms of N2 and CO2 sorption. Values for the surface
area and pore volume of the metal-doped BTPA were both expanded by increasing themetal coordination affinity to 214 m2 g-1 and 0.185 cm3 g-1, respectively. The effect ofmetal doping on the membrane separation performance was evaluated using a single-gas

83Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --permeation system and the activation energy of permeance was measured. Gas
permeation was increased following the doping process due to the formation of a
microporous structure on the order of Ni-BTPA > Cu-BTPA > Ag-BTPA > BTPA, whichcorresponds to a higher affinity for metal coordination. Permeation behavior was
dominated by the molecular sieving effect that showed a high level of H2 permeance at
4.45 × 10-6 mol m-2 s-1 Pa-1 with an H2/SF6 permeance ratio that reached 15,500, whichis indicative of a defect-free membrane. By comparison with Cu-BTPA (14.8 kJ mol-1),
Ag-BTPA (19.5 kJ mol-1), and BTPA (31.1 kJ mol-1) membranes, the Ni-BTPA
membrane showed the lowest value for CO2 activation energy (8.9 kJ mol-1), which canbe ascribed to the microporosity that was formed by a higher coordination interaction.
The nickel-doped BTPA achieved high levels of both permeance of N2 and selectivity for
N2/SF6 at 3.75 × 10-7 mol m-2 s-1 Pa-1 and 1,900, respectively.
Chapter 3 is “Microporous nickel-coordinated aminosilica membranes for
improved pervaporation performance of methanol/toluene separation”. The nickeldoped bis [3-(trimethoxysilyl) propyl] amine (BTPA) derived membrane has a
microporous coordinated network that offers high potential to be an ideal separation
barrier for methanol-toluene azeotropic mixtures via the pervaporation process. Ni-BTPA
membranes were modified by employing a nickel dopant over amine groups in mole ratios(mol mol-1) that ranged from 0.125 to 0.50. The incorporation of different amounts ofnickel dopant into flexible amine-rich organosilica precursors of BTPA increased therigidity and resulted in a porous structure with a large specific surface area (increased
from 2.36 up to 282 m2 g-1) and a high pore volume (from 0.024 up to 0.184 cm3 g-1).
Methanol-toluene separation performance by the nickel-doped BTPA (Ni-BTPA)
membranes was increased with increases in the nickel concentration. Ni-BTPA 0.50
showed separation performance that was superior to other types of membranes, alongwith a high level of flux at 2.8 kg m-2 h-1 and a separation factor higher than 900 in a 10
wt% methanol feed solution at 50 ⁰C. These results suggest that the balance between the
microporosity induced by amine-nickel coordination and an excessive amount of nickelion facilitates high levels of flux and separation of methanol.
Chapter 4 is “Structural transformation of the nickel-coordination-induced
subnanoporosity of aminosilica membranes for methanol-selective, high-flux
pervaporation”. A novel strategy to modify the dense framework of different types of84

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --amine-functionalized organosilica membranes has been successfully applied via nickel
coordination to induce the formation of microporous membranes. Prior to the nickelcoordination reaction, aminosilica sols were prepared via hydrolysis of several types ofamine precursors: bis [3-(trimethoxysilyl) propyl] amine (BTPA), trimethoxy [3-(methyl
amino) propyl] silane (MAPTS), and 3-aminopropyl triethoxy silane (APTES). The
optimal nickel/amine mole ratio was established within a range from 0-0.50 mol mol-1,
and calcinations of the membranes were performed at 250, 300, and 350 ⁰C. We foundthat nickel doping restructured the aminosilica network via a coordination bond, whichthen increased both the rigidity of the organic chain and the surface area of the resultant
materials on the order of nickel-doped bis [3-(trimethoxysilyl) propyl] amine (Ni-BTPA)> nickel-doped 3-aminopropyl triethoxy silane (Ni-APTES) > nickel-doped trimethoxy[3-(methyl amino) propyl] silane (Ni-MAPTS). Spectroscopy characterization studies
such as Ultraviolet-visible (UV-vis), Fourier Transform Infrared (FT-IR), and X-ray
Diffraction (XRD) along with micropore analysis of N2 sorption isotherms revealed thatthe formation of a coordinated network could be sterically hindered by the existence of
non-hydrolyzable methyl groups on the pendant chain. The prepared composite
membranes were utilized for the pervaporation of various types of organic mixtures for acomposition ratio of methanol/solvents of 10/90 wt% at 50 ⁰C. All nickel-composite
membranes showed high flux and outstanding performance for the pervaporation of
methanol/ dimethyl carbonate (MeOH/DMC) and methanol/ toluene (MeOH/Tol)mixtures. Ni-BTPA membranes with bridge-structured secondary-amine ligands recorded
values for the flux that reached as high as 1.42 kg m-2 h-1 with an optimum separation
factor for MeOH/Tol of 5,000.
Chapter 5 is “Structural two-phase evolution of aminosilica-based silvercoordinated membranes for increased hydrogen separation”. Hybrid organosilica
membranes of silver-doped bis [3-(trimethoxysilyl) propyl] amine (BTPA) were prepared
via sol-gel processing followed by coordination reactions to improve the permeability andseparation performance. The evolution of silver in two phases as silver ions and as
nanoparticles was observed during the modification of the aminosilica networks; thesilver ions coordinated with amine moieties while the silver nanoparticles developed
following reduction on the aminosilica surface. The silver/amine mole ratio was evaluatedagainst the formation of coordinated and particulate species during modification in the85

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --range of 0.1 to 0.5 mol mol-1. The formation of microporosity was successfully developed
from 2.36 to 115 m2 g-1 by increasing the silver mole ratio. Furthermore, silver-modified
aminosilica membranes showed hydrogen (H2) permeance of 1.46ൈ10-6 mol m-2 s-1 Pa-1,
which was 65-fold higher than pure BTPA with excellent selectivity for H2/propane(C3H8) separation of 1,500. It is evident that the proposed modification method via thetwo-phase structural evolution of silver coordination and nanoparticles reorganized theorganosilica framework and improved the separation in the dehydrogenation of propane.
Chapter 6 is “Conclusions and recommendations”. In this chapter, the main
conclusions of this thesis are summarized and the recommendations are provided forfurther study.
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100

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Chapter 2
Metal-induced microporous aminosilica creates a highly
permeable gas-separation membrane
2.1 Introduction
Microporous inorganic membrane technology has shown great potential for
producing molecular separation barriers that will be more economicallycompetitive than conventional techniques such as filtration, evaporation,
crystallization, solvent extraction, or distillation [1-3]. Over the past few years, gas
separation has become one of the most widely used industrial applications of
microporous membranes that offer high levels of permeability and separation. The
transport system for gas separation in microporous inorganic membranes isdominated by molecular sieving via affinity, which places a premium on the control
of pore size in order to create accessibility for guest molecules through the poresand determine the adsorptive properties of the membrane matrix that accomplishes
separation [4]. Microporous silica membranes are a type of inorganic membranewith advantages that include controllable pore size, excellent mechanical strength,
and high hydrothermal and chemical stability [5, 6].
Several methods such as the organic template and spacer method in the caseof SiO2-based polymers were developed to design as well as control the formationof micropores that would be suitable for gas separation [6-9]. The organic template
method is generally used to create changes in pore size through the removal of thetemplate during the calcination of organic functional groups [10]. Another method
for porosity control has been developed using bi-silyl precursors (with a linking
unit as a spacer) for the fabrication of microporous organosilica membranes, where
the formation of the membrane network is strongly influenced by the introductionof spacers into an ordinary silica network [11-13]. A notable example of pore size
control when using the spacer method was explained using several organosilica
precursors with different numbers of carbon elements between bi-silyl bridges.
Studies

have

shown

that

bis-(triethoxysilyl)-methane

101(BTESM),bis-

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(triethoxysilyl)-ethane (BTESE), and bis-(triethoxysilyl)-benzene (BTESB) have arigid linking chain (Si-CH2-Si, Si-C2H4-Si, and Si-Ph-Si). By contrast, bis(triethoxysilyl)-propane (BTESP), bis-(trimethoxysilyl)-hexane (BTMSH), andbis-(triethoxysilyl)-octane (BTESO) with carbon linking units of C3, C6, and C8,
respectively, form a dense structure related to network flexibility [14]. Spacer units
serve as the minimum unit in the construction of a loose organosilica framework,and the increased flexibility of the linking unit chain tends to generate a denser
structure that is not suitable for the creation of a highly permeable membrane.
Porous coordination polymers such as a metal organic framework (MOF)
have recently been developed and can be characterized by the repeating
coordination interactions that occur between metal receptors and ligands that form
1D, 2D, and 3D networks [7, 15, 16]. During the synthesis of coordination
polymers, metal ions create interactions with ligands as crosslinkers, and form
polymers with structures that feature a high-dimensional framework, a tailorable
pore structure, and a large specific surface area. On the other hand, the higher
dimensional structure of a porous coordinated polymer exhibits a more rigid
framework that tends to form interfacial defects between the crystal structures,
which results in a risk of reducing the selectivity of gas separation and makes it
difficult to produce uniform film [17]. By following the concept of a MOF (metal–
linker interactions form nanospaces), we attempted a novel process to control thepore size of amorphous amine-enriched silica via sol-gel processing followed bycoordination.
Several studies on metal-doped organosilica have been carried out toincrease membrane selectivity [18-21]. The mechanism for increasing selectivity
involves the formation of covalent bonding with a siloxane network, which results
in a denser membrane structure so that the separation factor between the different
sizes of gases becomes higher. This was exemplified in experimental work
undertaken by doping a porous BTESE precursor with zirconium, and theconclusion was that the addition of zirconium caused densification due to structural
relaxation via limited diffusion [20]. The same result was reported in a study where
the presence of doped cobalt caused decreased flux in a membrane when the metal
content had been increased [21]. Even though the selectivity was increased due to102

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the small pore size, side effects included a decrease in the permeating flux and anincrease in the activation energy. Apart from that, the amorphous nature oforganosilica membranes confers great advantages in flexibility and produces fewerof the defects that are inevitable in membranes composed of crystalline materials
such as zeolite and MOF. Therefore, organosilica membranes enable specific gas
molecules to pass through while maintaining selectivity for gas separation.
Based on these observations, a hybrid framework consisting of an orderedstructure from coordinated polymers and an amorphous structure from theorganosilica network could be an alternative way to obtain both a higher
permeating flux and selectivity for molecules with similar radii and properties [22].
In this study, we propose a new strategy of using metal-amine coordination tocontrol the microporosity of silica-based amorphous networks. To the best of ourknowledge, silica-based coordination polymers have not been reported. The aminegroups in the linking units of bis [3-(trimethoxysilyl) propyl] amine (BTPA) wereused as Lewis-base molecules (ligand) and transition metals acted as electron-pair
acceptors, or Lewis acids. The addition of metal generated subnanopores via thecoordination of covalent bonds increased the crosslinking between network layers,which converted the texture of the membrane from non-porous to a microporous
aminosilica via the formation of more rigid linking units. In this research, silver,
copper, and nickel were used as metal receptors with different coordination
affinities that could form interactions with the secondary amine groups on BTPA
molecules.

2.2 Experimental
2.2.1 Synthesis of bis [3-(trimethoxysilyl) propyl] amine (BTPA) and bis(trimethoxysilyl)-hexane (BTMSH)
Aminosilica sol was prepared via the hydrolysis and condensation ofaminosilica (BTPA) precursors in ethanol, and the reaction was catalyzed byexposure to nitric acid for 12 hours at room temperature. A mixture of water andnitric acid as a catalyst, was added into BTPA and ethanol under vigorous stirring.

103

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The final molar ratio of BTPA/H2O/HNO3 was 1/300/1 and pure ethanol was used
to adjust the BTPA concentration to 5 wt%. In this experiment, bis(trimethoxysilyl)hexane (BTMSH) was used as a control group to confirm theformation of uncoordinated metal doping. As shown in Fig. 2-1, BTPA and
BTMSH are similar in structure, and the only difference is the existence of theamine moieties in BTPA. BTMSH was synthezised in the manner of the
preparation of BTPA sol, in order to achieve a clear comparison for both types ofprecursors.

Fig. 2-1. The chemical structure of non-functionalized and amine-functionalizedorganoalkoxysilane precursors

2.2.2 Synthesis of metal-doped BTPA and BTMSHAgNO3, Cu(NO3)2٠3H2O, and Ni(NO3)2٠6H2O were used as metal dopant
to create different degrees of coordination with secondary aminosilica precursor.
Metal-doped colloidal sols were prepared by adding dissolved metal salts into a
BTPA sol with a stoichiometric metal/amine mole ratio of 0.25 (mol/mol) undervigorous stirring. The concentration of metal-doped BTPA sols was maintained at
5 wt% during preparation and was diluted at 0.1 wt% using pure water for
membrane fabrication. Ni-BTMSH was also synthesized using the same
parameters as metal-doped BTPA in order to obtain a comparative analysis of thecoordinated components.

2.2.3 Characterization of metal-doped BTPA
The electronic interaction between metal receptors and amine donor
electrons was examined using an Ultraviolet-visible spectrometer (UV-570,

104

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --JASCO, Japan). Functional groups of BTPA and metal-doped BTPA wereidentified using an FT-IR Spectrometer (FT/IR-4100, JASCO, Japan) with awavenumber that ranged from 4000 – 500 cm-1. Metal-doped aminosilica sols were
coated onto KBr plates and then calcined at various temperatures under a N2
atmosphere to evaluate the thermal stability of amines in coordination with metal
prior to the decomposition of these moieties. X-ray photoelectron spectroscopy(JEOL RE series JES-RE1X ESR spectrometer) was used to investigate thedifferent environments of amine sites after the addition of a transition metal, andto examine their bonding energy. All samples were prepared by coating the sols
onto the surface of a silicon wafer via spin-coating (2,500 rpm), followed bycalcination at 250 ⁰C under a N2 atmosphere. The samples were kept in a dry
vacuum oven overnight before the measurement.
Metal-doped aminosilica gel powder was prepared by drying at 50 ⁰C before
thermal calcination at a different temperature under an inert atmosphere. The
microstructure of the xerogel powder was characterized using an X-Ray diffraction
D2 PHASER, (Bruker, Germany) with Cu-Kα radiation at a scanning range of 5⁰
to 80⁰. Morphology and different microstructure formations between coordinatedand uncoordinated metal ions with amino moieties were characterized using a
Transmission Electron Microscope (TEM JEOL2010 Ltd., Japan). A small amountof the sample powder was pulverized and transferred into a Cu microgrid. BTPA
and metal-BTPA powder were calcined at 250 ⁰C under a N2 atmosphere andthereafter evacuated at 150 ⁰C under vacuum conditions for 12 hours before the N2
and CO2 sorption measurement (BELMAX JAPAN Inc., Japan). Membranes
cross-section morphology were characterized using Field Emission ScanningElectron Microscope (FE-SEM, JSM-7800F, JEOL).

2.2.4 Fabrication of metal-doped BTPA membranes and gas permeation
measurement
Metal-doped aminosilica membranes were fabricated in a manner similar tothat of organosilica membranes using porous α-alumina tubes as a membrane
support with 50% porosity and an average pore size of 1 μm (length, 100 mm; inner105

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --diameter, 8 mm; outer diameter, 10 mm), as supplied by Nikkato Corporation(Japan) [23, 24]. First, porous α-Al2O3 particle layers were developed from αalumina particles mixed with 2 wt% SiO2-ZrO2 sol as a binder and subsequently
calcined at 550 ⁰C under an air atmosphere for 15 minutes. The second porous layer
had an average pore size of about 0.6 nm made by coating a 0.5 wt% SiO 2-ZrO2
sol onto the surface of the particle layer and firing under an air atmosphere at 550
⁰C as an intermediate layer. The topmost separation layer of metal-doped BTPA
was formed with a wipe-coating of 0.1 wt% sol followed by calcination at 250 ⁰C
under a N2 atmosphere for 30-60 minutes.
Single-gas permeation measurement was conducted using the experimental
setup schematically depicted in Fig. S2-1. High-purity gas such as He (kinetic
diameter: 0.26 nm), H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm), CH4 (0.38 nm),
CF4 (0.48 nm), and SF6 (0.55 nm) were fed to the outside of the membrane using
an upstream pressure at 200- 400 kPa, while the permeate stream inside the system
was maintained at atmospheric pressure (the pressure drop across the membrane
was approximately 1 bar). The fresh membrane was pretreated at 200 ⁰C under a
He flow of 50 cm3 min-1 for at least 6 hours to remove any adsorbed molecules
such as water from the surface of the membrane. The permeation test was
conducted at temperatures ranging from 200 – 50 ⁰C and the permeation rate data
were recorded when the membrane reached a steady state.

2.3 Results and Discussion
2.3.1 Metal addition and the effect on the physical and microstructure
properties of aminosilica
In the present study, spectrometric data were collected and compiled toconfirm the formation of coordinated interactions between transition metals andfree electrons originating from the amine functionality of BTPA molecules. Fig. 22(a) shows that doping with metal ions shifted the highest absorption peak of
approximately 212 nm to a higher wavelength, which corresponded to the different
free-electron transitions of the amine groups (n → σ*) in the UV light range.

106Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

-CH2-

N=O

Si-O-Si
Si-OH

(a)

(b)

n → σ*(c

250qC

n → π*

d-d transition of d8 Ni(II) complex

Ni-BTPA

Ni-BTPA

Cu-BTPA

Cu-BTPAd-d transition of d9 Cu(II) complex

Cu-BTPA

BTPA

50qC
O-H

H-N

Black line : BTPA
Dash line : metal salt
Solid line : metal-doped BTPAn → σ*

Absorbance [a.u.]

Absorbance [-]n → π*

Absorbance [a.u.]

Ag-BTPAn → σ*

Ni-BTPAddashline = 50qC
solid line = 250qC

H-N

O-H

Ag-BTPA

Ni-BTPABTPA

Cu-BTPAn → π*

Ag-BTPA

Ag-BTPA
200

300

400

500

BTPA
600

700

800

4000

3500

3000

Cu 2p

Ni 2p

O 1s

2000

1500

1000

1700

1650

Wavenumber [cm-1]

Wavelength [nm]
Ni 2s

2500

N 1s

Si 2p

Ag 3d
C 1s

Si 2s

1600

1550

1500

Wavenumber [cm-1]

Cu 2p

3500
(e) 35

(d)

N 1s

3000

Intensity [cps]Intensity[a.u]Ni-BTPA

Cu-BTPA
Ag-BTPA

2500
2000
1500
1000

Ni-BTPA
Cu-BTPA
Ag-BTPA
BTPA

BTPA
500

1000

800

600

400

200

0

Binding Energy [eV]

0
406

404

402

400

398

396

Binding Energy (eV)Fig. 2-2. UV-Vis absorption spectra of BTPA, metal salt and metal-doped BTPA (a). Infrared-1
spectra in the range of 600 – 4000 cm for BTPA, Ag-BTPA, Cu-BTPA, Ni-BTPA at two stagesof the calcination (b), higher magnification for NH deformation band in the range 1700 – 1500
-1
cm (c). XPS spectra originating from BTPA and metal-doped BTPA wafer calcined at 250 ⁰C
under inert atmosphere, survey spectra (d), and N (1s) high-resolution spectra (e).

Transition peaks that reflect coordination interactions between metals andligands were observed in the range of visible light for Cu (d9) and Ni (d8)complexes through the presence of new peaks at a higher wavelength, but the Ag(d10 = fully filled d orbital) complex showed no absorption peak at this range,
because the resultant solution is colorless. NH (amine) groups are strong field
ligands due to strong σ bond electron donor properties, and, therefore, it is easy togenerate electrostatic interaction with metal-ion d orbitals [25, 26]. Cu and Ni
generate coordinated covalent bonding to NH via a lone pair of electrons in the πelectron system, and this type of electron delocalization weakens the donor
properties of the ligand, which results in an appearance of the new absorption peak107

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --at a higher wavelength ( ̴420 nm), which can be attributed to the d-d transition of
metal (II)-BTPA complexes [27, 28]. UV-Vis characterization has provided
evidence that the metal-doping process forms crosslinks between metal and amine
moieties that can be described as Lewis acid-base interactions.
Fig. 2-2(b) shows the FT-IR spectra of BTPA and metal-doped BTPA films
that were coated onto the surface of KBr plates and calcined at different
temperatures. All BTPA sols doped with different metals exhibited the vibrationthat is typical of functional groups that are similar to a pure BTPA sol. A higher
calcination temperature increased the formation of siloxane bands (1,025-1,110cm-1) and decreased the silanol bands (909 cm-1), which resulted in a higher levelof siloxane connectivity [29]. In addition, the NH deformation band (1,591 cm-1)of metal-doped BTPA was shifted to a higher wavenumber ( ̴1,617 – 1,623 cm-1)
and the peak shape was broadened after calcination at a higher temperature of 250
⁰C (Fig. 2-2(c)). The shifting of the N-H band after calcination at 250 ⁰C could be
an indication of the coordination interaction between amine nitrogen atoms andmetal ions, whereas the intensity of the N-H deformation band was changed after
the formation of a metal complex [26, 30]. The coordination between a doped metal
and the amine groups of BTPA molecules was observed more clearly after
calcination temperature was raised to 250 ⁰C probably due to reactions includinghydrolysis, condensation, and coordination, which involved dynamic processes.
Without calcination, reactions such as hydrolysis and condensation of thealkoxysilanes predominated because of the involvement of water and a nitric acidcatalyst. Nevertheless, following calcination 250 ⁰C, the coordination reaction
became more dominant due to evaporation of the water and catalyst.
XPS characterization was conducted to examine the covalence bonding to
the metal center in an aminosilica ligand binding environment [31]. Fig. 2-2(d)features the survey spectra for the sample before and after the metal doping process
under a calcination temperature of 250 ⁰C. The existence of metal peaks can bedetected in certain orbitals, whereas the interaction of BTPA molecules with metals
are apparent via the N (1s) orbital peak that reflects the contribution interaction of
the amine moiety against metal receptors (Fig. 2-2(e)). A higher metal affinity
generated an increase in the covalence bonding characteristics that led to an108

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --increase in the density of the electrons on metal atoms but a decrease in the donor
atoms of the ligand. Consequently, as the covalence interactions of donor-acceptor
components increased, the peak of N (1s) that was attributed to an increase in theshift of donor atoms (ligand) to a higher binding energy (eV) [32, 33]. The
electronic transition (UV-Vis), the upshifted frequency of NH deformation mode
(FT-IR), and nitrogen N(1s) line analysis (XPS) all were consistent, which suggests
coordination between organosilica-based NH (amino functional groups) and metal,
as summarized in Table 2-1.
Table 2-1. UV-Vis electronic charge-transfer, FT-IR frequency of N-H deformation band,and N (1s) photoemission data that verified the formation of metal-amine interactions
UV-Visible
Transition

FTIR νNH[cm-1]XPS N (1s)Binding Energy
[eV]

212

n → σ* (N-H)

1591

399.8

234

*

n → σ (N-H)

303

n → π* (N=O)

1617

400.1

235

n → σ * (N-H)

297

n → π* (N=O)

1618

400.3

420

d → d (d9Cu(II))

229

n → σ * (N-H)

309

n → π* (N=O)

1623

400.5

421

8

CompoundWavelength ofabsorption [nm]BTPA
Ag-BTPA

Cu-BTPA

Ni-BTPAd → d (d Ni(II))

In general, microporous silica membranes prepared using colloidal sols
consist of a bimodal pore size distribution of both intraparticle and interparticle
pores [9]. Intraparticle pores are formed as siloxane networks via interactions
between atoms such as those in the Si-O-Si skeleton, whereas interparticle pores
are the product of void spaces among the packed particles [34]. Therefore, it isnecessary to optimize the formation of stable metal-amine coordination that issuitable for gas separation by avoiding the establishment of metallic nanoparticles
that form gaps between BTPA-derived silica as interparticle spaces. Fig. 2-3(a)features X-Ray diffractograms of BTPA and metal-doped BTPA at a calcination

109

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --temperature of 250 ⁰C under a N2 atmosphere. Silver, copper, and nickel were used
as metal dopants with low, moderate, and high affinities, respectively. Following
the metal-doping process, calcination at higher temperatures can produce metal
nanoparticles, which generally determines the formation of large pores (i.e.,interparticle pores). At 250 ⁰C, the silver (II) nitrate began to decompose, which
resulted in the formation of silver nanoparticles and was confirmed by 2θ valuesof 38.5, 44.4, 64.6, and 77.6⁰.

(b)

(a)

Intensity [a.u]Ni-BTPAx Cu
Cu-BTPA

i Ag

Ag

i Ag

iAg i
Ag-BTPA

BTPA
10

20

30

40

50

60

70

80

2Tq [degree]

Fig. 2-3. XRD pattern of aminosilica xerogel powder calcined at 250 ⁰C in N 2 atmosphere (a), andTEM photographs of BTPA and metal composite (b) (upper: low magnification at a scale of 50 nm,
lower: higher magnification at a scale of 5 nm, insert: electron diffraction pattern).

The same result could be observed in Cu-BTPA, but the intensity of the copper
nanoparticles was very small. On the other hand, Ni-BTPA showed different
results—no nanoparticle peaks were observed, even though the calcination
temperature was increased, as shown in Fig. 2-6(b). That figure shows only a broad
peak that is similar to, or even broader than, that of a pure BTPA, which indicatesthe formation of a randomly oriented amorphous structure, suggesting that nickel
was fully incorporated into the BTPA network and did not segregate into acrystalline structure that could be detected using X-ray Diffraction [35]. As theaffinity of a metal dopant increases, a higher degree of coordination interaction
develops and induces the formation of a stable compound in the metal-ion state.
This result was observed in the amorphous Ni-BTPA structure when there is a highaffinity for the metal-dopant, whereas a silver dopant with low affinity showed a110

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --straightforward formation of nanoparticles, as characterized by a sharp crystallinepeak.
TEM photographs describe the microstructure, dispersity, and aggregation
characteristics [36]. As shown in Fig. 2-3(b), the TEM image of BTPA wastransparent with a uniform surface and a halo electron diffraction pattern thatreflected the formation of an amorphous phase in good agreement with the XRD
results. By contrast, the addition of silver with a weak affinity to the amine group
generated metal particles that were clearly visible as dark patches with a size of
approximately 5 nm. The existence of uncoordinated silver tends to agglomerate
and then generate a phase separation to form a crystalline structure following
calcination at 250 ⁰C, which was also confirmed by a bright spot pattern with anadditional white ring electron diffraction image. Cu-BTPA showed a homogenous
distribution with a small number of dark spots, whereas Ni-BTPA showed no dark
spots. In that case, a higher stabilization degree of Ni-doped BTPA sol was
produced via electrostatic interaction between the metal ion with a higher affinity
to the secondary amine group, which prohibited the formation of metal segregation.
The evidence presented thus far supports the suggestion that an amorphous silica
network had coordinated with metal via the amine moiety. Metals with a higher
coordination affinity tend to generate a greater number of interactions that induce
the reorganization of bridge amine chains so that the siloxane framework
structurally rearranges to allow maximum coordination. Copper and nickel
possibly form small clusters that give rise to a more rigid structure and higher levelsof drying stress.

2.3.2 Pore textures induced by metal ion coordinated with amine sites
To confirm the formation of the permanent porosity that is generated bymetal and amine coordination, gas sorption isotherms were performed. The
microstructure properties of BTPA and metal-doped gel powder were evaluated via
N2 sorption isothermal testing at -196 ⁰C, as illustrated in Fig. 2-4(a). According to
the IUPAC, BTPA powders are classified as an adsorption isotherm type II for nonporous material with a negligible amount of adsorbed N 2 (< 1 cc) [37]. These111

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --results further suggest that BTPA is one of the organoalkoxysilane precursors thatfeature extremely flexible organic segments that fill the voids in a rigid Si-O-Si
network following the sol-gel process, which consequently generates dense
structure [14]. Following calcination at 250 ⁰C, the BTPA and metal-doped BTPA
showed different types of adsorption curves. Following the addition of transition
metal salts, the adsorption curves demonstrated types I- and IV microporous andmesoporous structures, respectively [37]. At very low relative pressure, themicroporous structure exhibited a large capacity for nitrogen adsorption, while themesoporous structure can be evaluated by the existence of a hysteresis loop at ahigher relative pressure.
(a)
Absorbed volume [cm3(STP) g-1]

120

100

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA
Closed - adsorption
Open - desorption

SBET = 214 m2g-1

80

60

SBET = 75.7 m2g-1

40

20

SBET = 20.1 m2g-1
SBET = 2.36 m2g-1

0
0.0

0.2

0.4

0.6

0.8

1.0

p/p0 [-](b)

Fig. 2-4. N2 adsorption-desorption isotherms for BTPA and metal-doped BTPA at -196 ⁰C, thexerogel powder was obtained at a calcination temperature of 250 ⁰C (a), and schematic imageof the crosslink formation between metal receptor with a different coordination number andamine site as donor electron (b).

The Brunauer-Emmett-Teller (BET) surface area and total pore volume were
determined from the N2 sorption isothermal data, and t-plot analysis was used todetermine the micropore volume of metal-doped BTPA. Based on the data shownin Table 2-2, the higher level of coordination by the transition metal generated a112

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --greater amount of surface area and total pore volume. Based on XRD confirmation,
at calcination temperatures of 250 ⁰C silver nanoparticles began to form, whichsuggested that an increase in surface area is also associated with the formation ofinterparticle pores. As expected, the percentage of micropores was increased withmetal affinity, which confirmed an increase in the number of crosslinks and a
microporous network. Ni-BTPA showed the greatest levels of surface area and total
pore volume at 214 m2 g-1 and 0.185 cm3 g-1, respectively.

Table 2-2 BET surface area and pore volume for BTPA and metal-doped BTPA
Metal doped-BTPA

SBET a(m2 g-1)

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

2.36
20.1
75.7
214

a

Pore volume
Vtotalb
Vmicroc
3 -1(cm g )
(cm3 g-1)
0.024
1.1 × 10-4
0.038
8.7 × 10-4
0.112
6.31 × 10-2
0.185
0.12

%
Microporosity
0
2
56
83

The BET surface area; b the total pore volume; c the micropore volume

Porosity is believed to be created by the coordination between metal
receptors and the donor electron pairs that originate from the amine groups inside
BTPA molecules. As illustrated in Fig. 2-4(b), the higher metal affinity enables
crosslinking via either the amine bridges or coordination of the neutral Lewis bases,
which increases the number of interactions and generates arrangements that can beone-, two- or three-dimensional (Fig. 2-4(b)). Increased rigidity in the BTPA
structure gives rise to a porous structure and a high surface area, which switches
the gas transport mechanism from solution diffusion to molecular sieving, as will
be discussed in 2.3.5 [38].

2.3.3 CO2 adsorption of metal-doped BTPA
Fig. 2-5(a) compares the CO2 sorption isotherms of pure BTPA and metaldoped BTPA dried gel powders measured at 25 ⁰C. In this measurement, dried gel
powder was obtained at a calcination temperature of 250 ⁰C under an inert
atmosphere. BTPA as a secondary aminosilica material exhibited a clear hysteresis
113

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --loop that correlated with the irreversible adsorption and desorption mechanisms of
CO2 gases inside free amine sites [39]. With a free amine as an adsorption site, the
BTPA gel showed a higher level of CO2 adsorption, that facilitated adsorption via
a chemisorption mechanism by producing carbamate ions, as follows [40].
‫ܱܥ‬ଶ ൅  ʹܴଶ ܰ‫ ܪ‬՜ ܴଶ ܰ‫ ି ܱܱܥ‬൅ ܴܰ‫ܪ‬ଶା 

(a)

20

(2-1)

(b))

18

V CO2 adsorbed [cm3.g-1]

16
14
12
10
8
6
4

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

2
0
0

20

40

60

80

100

P [kPa]Fig. 2-5. CO2 adsorption-desorption isotherms curve at 25 ⁰C for BTPA and metal-doped BTPA
xerogel powder fired at 250 ⁰C under N2 atmosphere (a), and schematic evolution of CO2 transport
inside BTPA and metal modified BTPA precursors with possible crosslink formation and theiraction of capturing CO2 (b).

The stoichiometric mechanism of CO2 transport by sterically hindered secondaryamine groups is double that of the primary amine site, where one molecule of CO2
can interact with two moles of an amine [41]. The isotherm curves for metal-doped
BTPA show no saturation plateaus under high pressure and smaller loops or evenno hysteresis for the order of Ag-BTPA > Cu-BTPA > Ni-BTPA, which indicatesthe complete removal of CO2 as an adsorbed gas during the desorption process andtuning of the CO2 adsorption mechanism to produce physical adsorption such asthat seen in Henry sorption models [42, 43].
The CO2 adsorption capacity was not only decided by an affinity for thematerial, but also by the existence of microporosity (pore volume and specificsurface area) and the number of active adsorption sites (amine surface density).
Even though Ag-BTPA shows the existence of a free amine site, the CO2 adsorption
exhibited the lowest level of capacity, and was associated with a weak coordination
114

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --interaction leading to the partial formation of coordination and nanoparticles as thetemperature was increased to 250 ⁰C. As described, the CO 2 adsorption curve for
Ni-BTPA and Cu-BTPA shows reversible and linear sorption isotherms, while ahysteresis loop could be observed for Ag-BTPA, indicating an irreversible
adsorption-desorption mechanism on uncoordinated amines. So, it was reasonably
suggested that Ag+ partially existed in two different phases.
As schematically shown in Fig. 2-5(b), CO2 gas transport inside the aminemodified silica matrix networks shows a chemical interaction whereas metal-doped
BTPA exhibits physical adsorption. The coordination framework that was
generated between the metal and amine groups induces transformation from adense BTPA into a porous structure, which results in a CO2 adsorption mechanismthat is a reversible process. A large amount of energy is required for pure BTPA
since the nucleophilic CO2 gas attack via the free amine group leads to theformation of carbamate, which gives rise to a significant amount of additional
operating costs [44]. However, the regeneration process for removing CO2 at thesurface of metal-doped aminosilica materials requires less consumption of energy.

2.3.4 Metal modification of amine- and non-functionalized organosilica
precursors
Two types of organoalkoxysilane precursors were used to compare andanalyze the different forms of coordination between amine and metal centers.
BTMSH is a simple alkoxysilane that consists of a flexible organic bridge
precursor without an amine functional group; like BTPA it has a very small N2
adsorption capacity due to its flexible structure, as shown in Fig. 2-1 [14]. BothBTMSH and BTPA are type II, which indicates a non-porous structure (Fig.2-6(a)).
Following the addition of metal, however, the surface area of a metal composite isincreased, particularly for Ni-BTPA, while Ni-BTMSH displays only a small
increase in the surface area, which likely is due to the formation of nickel
nanoparticles. This argument was confirmed by the XRD pattern of Ni-BTMSH(Fig. 2-6(b)), which showed peaks for the formation of Ni(OH)2, NiO, and Ni
particles under higher calcination temperatures.
115

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(a) 150(b)
Closed : adsorption
Open : desorption
Ni-BTPA 500qC
NiO

Ni-BTPA-SBET = 214 m2g-1

100

Intensity [a.u]Va [cm3(STP) g-1]

Ni
Ni

Ni
Ni-BTMSH 500qC
Ni-BTPA 250qC

NiO

NiO

NiO

50

Ni-BTMSH 250qC

Ni-BTMSH-SBET = 35.4 m2g-1

BTPA

BTMSH

0
0.0

0.2

0.4

BTPA 250qC

0.6

0.8

BTMSH 250qC

1.0

10

20

p/p0 [-]

30

40

50

60

70

80

2Tq [degree]

Fig. 2-6 N2 sorption isotherms at -196 ⁰C for non-functionalized organosilica (BTMSH), aminefunctionalized organosilica precursors (BTPA) and Ni-doped silica gel fired at 250 ⁰C (a), and
XRD pattern of BTMSH, BTPA, and Nickel composite gel powder prepared at various calcination
temperature (b).

The decomposition reaction of a nickel-metal salt is described by the following
equations [45].
ܰ݅ሺܱܰଷ ሻଶ ή ͸‫ܪ‬ଶ ܱ ՜  ܰ݅ሺܱܰଷ ሻଶ ή ͵‫ܪ‬ଶ ܱ ൅  ͵‫ܪ‬ଶ ܱ(2-2)ܰ݅ሺܱܰଷ ሻଶ ή ͵‫ܪ‬ଶ ܱ ՜  ܰ݅ሺܱܰଷ ሻଶ ή ʹ‫ܪ‬ଶ ܱ ൅  ‫ܪ‬ଶ ܱ(2-3)͵ܰ݅ሺܱܰଷ ሻଶ ή ʹ‫ܪ‬ଶ ܱ ՜  ܰ݅ሺܱ‫ܪ‬ሻଶ ή ܰ݅ሺܱܰଷ ሻଶ ή ʹ‫ܪ‬ଶ ܱ ൅  ʹ‫ܪ‬ଶ ܱ ൅  ʹ‫ܱܰܪ‬ଷ ሺʹͲͲԨሻ(2-4)ܰ݅ሺܱ‫ܪ‬ሻଶ ή ܰ݅ሺܱܰଷ ሻଶ ή ʹ‫ܪ‬ଶ ܱ ՜ ͵ܱܰ݅ ൅  ͵‫ܪ‬ଶ ܱ ൅  ͵ܱଶ ൅  ܱܰଶ ሺʹͷͲԨሻ

(2-5)ʹܱܰ݅ ՜ ʹܰ݅ ൅ ܱଶ ሺͷͲͲԨሻ(2-6)

Ni-BTMSH is more likely to produce nickel nanoparticles, while the formation of
nanoparticles has yet to be observed for Ni-BTPA at the same, or even higher
temperature, which is further evidence of the coordination between BTPA andmetal ions.

2.3.5 Gas permeation properties of metal-doped BTPA membranes
Fig. 2-7 shows the SEM cross-sectional images of thin-layer BTPA and NiBTPA membranes. Both membranes present a typical asymmetric structure
consisting of an ultrathin top-most layer as a separation barrier conjoined with a
SiO2-ZrO2 intermediate layer prepared on α-Al2O3 particles layer. Based on thisSEM image, the thickness of the top-layer and intermediate layer were estimatedto be approximately 235 – 275 nm without clear boundaries between the layers.

116

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --These results are similar to those reported by our research group, where thethickness of the combined layers was approximately 200 – 300 nm, and theseparation layer was estimated to have a thickness that ranged from 13 to 50 nm[46, 47].

BTPA

Ni-BTPATop-most layer + SiO2-ZrO2

Top-most layer + SiO2-ZrO2

α – Al2O3 particle layer

α – Al2O3 particle layer

Fig. 2-7 The cross-sectional images for BTPA and Ni-BTPA consisting of a top-most layer as aseparation barrier conjoined with a SiO2-ZrO2 intermediate layer with α-Al2O3 particles as asupport layer.

Fig. 2-8 illustrates (a) single-gas permeance and (b) dimensionlesspermeance based on helium gas permeance at200 ⁰C as a function of the molecular
kinetic diameter. Fig. 2-8(c) illustrates the dependence of doped metals on gas
permeance. All aminosilica membranes were fabricated at 250 ⁰C under a N2
atmosphere to avoid oxidation of the metal phase [48]. Single-gas permeancedecreased as the molecular kinetic diameter increased, indicating a molecularsieving effect. Fig. 2-8(b) confirms that selectivity was much higher than that ofKnudsen diffusion. As summarized in Fig. 2-8(c), following metal doping, thepermeation of H2 was increased from 7.13 × 10-7 mol m-2 s-1 Pa-1 (pure BTPA) to
4.45 × 10-6 mol m-2 s-1 Pa-1 (Ni-BTPA). Similar results were found for thepermeance of gases such as He, CO2, CH4, and N2. The increase in gas permeancecan be ascribed to the formation of micropore structures on the order of Ni-BTPA> Cu-BTPA > Ag-BTPA > BTPA, which corresponds to the increased metal
coordination affinity. The increase in metal affinity resulted in a greater degree ofcrosslinking with amine groups, a higher surface area of the metal-doped BTPA
powders, and an enlarged network pore size that induce a higher rate of gas
permeation in metal-doped BTPA membranes.

117

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

10-5

(b)

200qC
10-6

10-7

-8

10

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

10-9

200qC

100

Dimensionless permeance [-]Permeance [mol m-2 s-1 Pa-1](a)Knudsen
10-1

10-2

10-3

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

10-4

10-5

10-10
0.25

0.30

0.35

0.40

0.45

0.50

0.25

0.55

Permeance [mol m-2 s-1 Pa-1]

(c)

0.30

0.35

0.40

0.45

0.50

0.55

Molecular diameter [nm]Molecular diameter [nm]
10-5

H2

200qC

He

10-6

CO2
10-7

CH4
N2

10-8

SF6

10-9

10-10

BTPA

Ag-

Cu-

Ni-

Metal doped-BTPA [-]Fig. 2-8 Single gas permeance as a function of molecular size dependency (a), dimensionlesspermeance based on He permeance for BTPA and metal-doped BTPA measured at 200 ⁰C (b),and Gas permeance at 200 ⁰C as a function of metal affinity (c).

These findings indicate, and spectroscopy studies consistently show, a correlation
between the structural properties of the materials and membrane performance. In
contrast to the permeance of small gases, the permeance of SF6 was sharply
decreased after the addition of nickel-metal ions. This result suggests that the poresize distribution of nickel-doped BTPA is narrowly defined to within the range ofthe molecular sizes of N2 (0.36 nm) and SF6 (0.55 nm), and likely approximatesthat of SF6 due to the high level of N2 permeance. Ni-BTPA showed the mostpromising performance of H2 and N2 permeance at 4.45 × 10-6 and 3.75 × 10-7 mol
m-2 s-1 Pa-1 with higher selectivity for both H2/SF6 and N2/SF6 at 15,500 and 1,900,
respectively. Ultrafine micropores are known to be induced by the crosslinking
reactions among dense polyimide, which has resulted in gas separation
performance that surpasses the trade-off lines [49]. The same concept was observedin this study as microporosity was created from a dense structure via crosslinking
induced by the coordination of metals and electron-donor amines, resulting inhigher porosity from the increase in membrane permeation.

118

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 2-9 shows the correlation between the permeance ratio of He/SF6 as afunction of He/N2 together with different types of microporous organosilica
membranes such as BTESE, BTESA, and BTESB [4, 50, 51]. It should be notedthat each plotted point corresponds to a different membrane indicating a reasonablereproducibility of permeation properties for metal-doped membranes. The solid
curve illustrates the theoretical value that was calculated based on the modified gas
translation (modified-GT model) equation for uniformly sized pores, as shown in
Eq. (2-7) [4].

He/SF6 permeance ratio [-]105
BTESE 6BTESE 60
BTESE 90
BTESE 120
BTESE 240
BTESA
BTESB
BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

Ni-BTPA

10

4

BTPA

103
Cu-BTPA
Ag-BTPA-BTPA

102

calculated curve based on m-GT model
10

1

100

101

102

103

He/N2 permeance ratio [-]Fig. 2-9. The permeance ratio of He/SF6 as a function of He/N2 permeance ratio for different typeof porous organosilica membranes (solid line : calculated curve that obtained based on modified
gas translation model)

Microporous organosilica membranes generally show a comparatively scatteredexperimental relationship between the permeance ratio of He/N2 and that of He/SF6
due to the existence of micropores formed as network pores and larger pores that
include defects from the grain boundaries. Ag-BTPA and Cu-BTPA both showeda lower permeance ratio of He/SF6 and the corresponding He/N2 ratio comparedwith the theoretical values. This can be ascribed to the possibility that after dopingwith low-affinity metals (Ag-, and Cu-), several larger pores were generated, whichallowed SF6 to permeate and resulted in a low permeance ratio for He/SF6 and abroad pore-size distribution. Different results were observed for Ni-BTPA

119

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membranes, which agreed well with the theoretical prediction curve at a high
He/SF6 ratio (He/SF6 > 10,000). This result suggests that the formation of anordered micropore framework from a fully coordinated amine-nickel interaction
and the amorphous natural properties of organosilica generate a more uniform porewith a defect-free quality, which was also confirmed via both the m-GT model andthe NLDFT analysis obtained by N2 sorption calculation (Fig. S2-7 and S2-6 in the
Supporting Information).
Ƚ୧Ȁ୨ ൌ
୫ିୋ୘ǡ୧ ൌ

ଵ க
ଷ த୐

ൌ  ට୔౟୔ౠ୑ౠ ሺୢ౦ ିୢ౟ ሻయൎට൫୮ െ ୧ ൯
୩బǡ౟

୑౟ ୖ୘

(2-7)



୑౟ ሺୢ౦ ିୢ౟ ሻయ൫ୢ౦ ିୢ౟ ൯

మ

ୢమ
౦
୉

 ቀെ ౌǡ౟ ቁ
ୖ୘

଼

୉ౌǡ౟

ට஠୑ ୖ୘  ቀെ ୖ୘ ቁ
౟

(2-8)
(2-9)

Fig. 2-10 shows the temperature dependence of single-gas permeance formetal-doped BTPA-derived membranes calcined at 250 ⁰C. The permeance of eachgas is affected by the pore size distribution of the amorphous metal-doped
aminosilica network [52]. The interaction between permeating molecules and thecylindrical microporous wall of the membranes was used to determine theactivation energy based on the permeance equation of the modified gas translation
model expressed in Eqs. (2-8) and (2-9) [53]. In that model, ݀௣ , ߝ, ߬, and ‫ ܮ‬aremembrane structural factors of the pore diameter, porosity, tortuosity, andthickness, respectively, while ݀௜ is the kinetic diameter of permeating
molecules,‫ܯ‬௜ is the molecular weight of the i component, R is the gas constant,and T is the absolute temperature. The activation energy for the permeating
molecules (‫ܧ‬௉ǡ௜ ) was obtained from the slope of the regression using Eq. 2-9 withthe results of single-gas permeance measured at different temperatures [52, 54].
Most gas permeance values were increased when the temperature was elevated toa higher degree via the activated permeation mechanism. If there are defects in the
structure of the membrane, the permeance of all gases larger than the defect sizes
becomes high and follows Knudsen selectivity. The largest-sized permeating
molecule of SF6 was generally used to assess the existence of defects in the
structure of the membrane. However, all metal-doped BTPA showed good120

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --separation performance, particularly the Ni-BTPA version with H2/SF6 and N2/SF6
selectivities of 15,500 and 1,900, respectively, as an appropriate indicator of adefect-free membrane.
10-5
11.4 kJ mol
12.2 kJ mol

10-6

10.4 kJ mol-1-1

H2 Permeance [mol m-2 s-1 Pa-1]

He Permeance [mol m-2 s-1 Pa-1]

10-5

-1

10.9 kJ mol-1

10-7

15.7 kJ mol-1

10-8

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA13.6 kJ mol-1

10-6
10.6 kJ mol-1

10-7

10-8

2.0

2.4

2.8

2.0

3.2

1000/T [K-1]
CH4 Permeance [mol m-2 s-1 Pa-1]

8.9 kJ mol-1

10-6
14.8 kJ mol-119.5 kJ mol-1

10-7

31.1 kJ mol-1

-8

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

2.8

3.2

1000/T [K-1]

10-6

8.7 kJ mol-1
12.6 kJ mol-1

10-712.9 kJ mol-1

10-8

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA21.3 kJ mol-1

10-9

10-9
2.0

2.4

10-5

10-5

CO2 Permeance [mol m-2 s-1 Pa-1]

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

10-9

10-9

10

16.1 kJ mol-1

2.4

2.8
-1

N2 Permeance [mol m-2 s-1 Pa-1]

1000/T [K ]

2.0

3.2

2.4

2.8

3.2

1000/T [K-1]

10-5

10-6

10 kJmol-1
13.2 kJmol-1

10-7

10-8

13.3 kJmol-1

BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA21.8 kJmol-1

10-9
2.0

2.4

2.8

3.2

1000/T [K-1]

Fig. 2-10. Temperature dependence of gas permeance for BTPA and metal-doped BTPA
membranes calcined at 250 ⁰C

What is interesting about the data in this experimental work is that a uniform andthe defect-free thin layer was successfully deposited onto a porous substrate via a
simple sol-gel process without sacrificing either high permeation or selectivity.

121Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Compared with the results of most reported MOF membranes, where the formationof interfacial defects between intergrown MOF facilitates the rapid diffusion ofmost permeating gases resulting a significant decrease in selectivity [55-57].
The activation energy of each gas in the metal-doped aminosilica membrane
is summarized in Table 2-3. The value of activation energy was obtained using themodified gas-translation approach to analyze the interaction between permeating
molecules and the pore wall of the aminosilica membrane [14]. After the metal
doping process, the permeance value for He, H2, CO2, N2, and CH4 were increased
but the activation energy was decreased, which is associated with the increased
pore sizes of metal-doped aminosilica membranes (estimated via NKP method, as
shown in the supporting information Fig. S2-2). This translates to a smaller
repulsive force being generated between gas molecules and the pore wall of themembrane. Moreover, when a metal has a higher coordination affinity, theconnectivity formed via the amine-metal crosslinking is higher and pore space isenlarged by a coordinated metal so that the activation energy of small-to-medium
sized gases becomes smaller and gas flux increases. The activation energies of He,
H2, N2 and CH4 were decreased with increases in the membrane pore size and inmembrane network connectivity on the order of Ni-BTPA < Cu-BTPA < AgBTPA < BTPA.

Table 2-3. The estimated value of activation energy for BTPA and metal-doped
BTPA derived aminosilica membrane
Gases

BTPA
He
15.7
H2
16.1
CO2 (200⁰-120⁰C) 20.2
CO2 (120⁰-50⁰C)
41.9
CO2 (200⁰-50⁰C)
31.1
N2
21.8
CH4
21.3

Ep [kJ mol-1]
Ag-BTPA Cu-BTPA
10.9
11.4
10.6
10.4
11.3
27.5
19.5
14.8
13.3
13.2
12.9
12.6

Ni-BTPA
12.2
13.6
8.9
10.0
8.7

Interestingly, the CO2 activation energy of BTPA and Ag-BTPA showed a verylarge value, particularly at low temperatures compared with other metal-doped
forms of BTPA. Ag-BTPA showed a lower CO2 activation energy than pure BTPA
122

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --but higher than Cu- and Ni-BTPA. This is because silver metals only established a
low coordination affinity to the amine moiety. Free amine groups in pure BTPA
and Ag-BTPA tend to form interactions with CO2 resulting in the formation of
carbamate, and the release of these species becomes more difficult, particularly at
lower temperatures [58]. This trend is similar to a report by Yu et al., where thebasicity of the secondary amine group in the organosilica precursor is higher than
the sterically hindered amine group, and a greater level of energy is generated forthe binding of CO2 with the surface of the material, but the transport of CO2 gas
across membranes is then limited [14, 59]. In the work undertaken by Kim et al.
and Wang et al., permeation behavior also showed that a strong interaction between
CO2 and amine moieties causes a decrease in the mobility and diffusivity of CO2that is associated with the formation of CO2-induced amine crosslinking, leading
to an enhancement of activation energy [60, 61].

2.3.6 H2/N2 and N2/SF6 separation through metal-doped aminosilica
membranes compared with other types of membranes
Fig. 2-11(a) shows the trade-off between H2 permeance and the selectivityof H2 over N2. Each point in the trade-off figures represents a different membrane,
which again validates the reproducibility of BTPA metal-doped membranes withdifferent metal variations. Membranes that are more gas-permeable show lower
selectivity, and vice versa. Therefore, trade-off analysis is important in order tosecure the best combination between permeability and selectivity. For comparison,
the permeability data for mixed-matrix and polymeric membranes were compiled
with an upper boundary that would reflect the highest selectivity of a polymer
membrane for a given level of permeance. The upper boundary created by Robeson(membrane thickness around 1 μm) could be used as a comparison against
permeance and selectivity for new and improved membrane materials [62, 63]. NiBTPA at 200 ⁰C exhibited H2 permeance of 1.14×10-6 ̴ 4.45×10-6 mol m-2 s-1 Pa-1
with a H2/N2 permeance ratio of 10 ̴ 28. Cu-BTPA also showed high permeance of
4.36 × 10-6 mol m-2 s-1 Pa-1 while demonstrating a moderate permeance ratio of 27.
The H2 permeance and H2/N2 permeance ratio for Ag-BTPA with a low metal
123

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --affinity are similar to those of BTPA. When compared with silica-based
microporous membranes such as BTESE and TEOS, metal-doped BTPA
membranes show a trend similar to that of porous membranes with high permeanceand moderate selectivity for H2/N2 separation.
(a)(b) 10000

Black symbol = 100°C < T [qC] < 200°C
Red symbol = 100°C > T [qC]

1000
1000

Robeson – 2008

100

10

1
10-10

BTESE
TEOS
CVD-Silica
Sol-gel silica
BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA
Polymer-1991
Polymer-2008
MOF
Mixed matrix membrane
10-9

10-8

Ni-BTPAA Cu-BTPA
BTPA

Ag-BTPAPN2/PSF6 [-]PH2/PN2 [-]Robeson – 1991
100

10

Polymeric membrane
Zeolite (MFI, ZSM-5, Chabazite)
Silica Membrane (BTESE)
Aminosilica membrane (BTPA)

1

10-7
-2

10-6
-1

10-5

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

N2 Permeance [mol m-2 s-1 Pa-1]-1

H2 Permeance [mol m s Pa ]Fig. 2-11 H2/N2 permeance ratio as a function of H2 permeance for a different type of hydrogen
separation membranes (a), and correlation of N2/SF6 gas separation from various type of membranes
as a function of N2 permeance (b). Lines in (a) are calculated via Robeson upper bound 2008 and 1991

Fig. 2-11(b) compares the separation performance between N2 and SF6,
which potentially is a larger contributor to global warming than CO 2 [64].
According to commercial applications in N2-enriched discharge streams, theconcentration of SF6 should be reduced to less than 800 ppm, but high N2
permeation must be maintained so that it can reach industrial separation levels
with a large-scale project capacity of as much as 50 N m3 h-1 [65, 66]. Several
types of membranes have shown N2/SF6 selectivity up to 476, such as polymeric
membranes, but with a low permeation rate for N2 of approximately 1.61 × 10-8
mol m-2 s-1 Pa-1 [67]. Ni-BTPA has shown a promising separation factor of N2/SF6
at 1,900 with high N2 permeance of 3.75 × 10-7 mol m-2 s-1 Pa-1 at 200 ⁰C. In orderto focus on the durability and stability of Ni-BTPA as prime criteria for industrial
applications, the time course of the single-gas permeation experiment was
performed under different operation temperatures (Fig. S2-5 in the Supporting
Information). The results showed stable performance where the permeance ofgases was completely recovered to the initial measurement level at 200 ⁰C even124

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --after the temperature of the operation was changed from 200 ⁰C to 50 ⁰C and thenreturned to 200 ⁰C. By comparison with other types of membranes, the modified
dense BTPA structure produces a microporous metal-doped aminosilica
membrane with both higher flux and selectivity, which represents a significant
improvement in performance and is suitable for industrial separation applications.

2.4 Conclusions
The present research was focused on examining the affinity of different metals forthe formation of coordinated compounds with the amine moiety of an amorphous silica
matrix. The characterization of metal-doped BTPA has shown evidence of a metal-amineinteraction that is induced by the reorganization of bridges in the amine-silsesquioxane
network that allow maximum coordination to generate a more rigid structure. In thisinvestigation, we found that the surface area and pore volume of a metal-doped BTPA
composite was increased by increasing the metal coordination affinity on the order of NiBTPA > Cu-BTPA > Ag-BTPA > BTPA. An evaluation of single-gas permeance showed
higher values for gas permeation, which corresponded to transformations in themembrane structure following the metal-doping process and simultaneously generated a
defect-free membrane. The activation energy of permeated gas was decreased byincreases in membrane pore size and in the network connectivity on the order of Ni-BTPA< Cu-BTPA < Ag-BTPA < BTPA. The Ni-BTPA membrane revealed excellent separation
performance with permeance and selectivity of N2 that reached 3.75 × 10-7 mol m-2 s-1
Pa-1 and 1,900, respectively.

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[66] K. Inami, Y. Maeda, Y. Habuchi, M. Yoshimura, S. Hamano, H. Hama, Problems of
the application of N2/SF6 mixtures to gasʘinsulated bus, Electr. Eng. Jpn., 137(2001) 25-31.
[67] A. Wolińska-Grabczyk, A. Jankowski, R. Sekuła, B. Kruczek, Separation of SF6
from binary mixtures with N2 using commercial poly (4-methyl-1-pentene) films,
Sep. Sci. Technol., 46 (2011) 1231-1240.

129

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Supporting Information

Fig. S2-1. Set up single gas permeation equipment

1.0
BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPANKP [-]

0.8

0.6
dP = 5.37 Å
dP = 5.83 Å

0.4

dP = 6.15 Å

dP = 5.29 Å
0.2

0.0
2.5

3.0

3.5

4.0

4.5

5.0

5.5

Kinetic diameter [Å]Fig. S2-2. Pore size prediction based on the NKP method of BTPA and metal-doped
BTPA with different metals at 200 ⁰C

130Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

10-5

10-6

10-7

10

BTPA (1)
BTPA (2)BTPA (3)Ag-BTPA (1)Ag-BTPA (2)Cu-BTPA (1)Cu-BTPA (2)Ni-BTPA (1)Ni-BTPA (2)Ni-BTPA (3)

-8

10-9

10-10

200qC

100

Dimensionless permeance [-]Permeance [mol m-2 s-1 Pa-1]

200qC

10-1

10

BTPA (1)
BTPA (2)BTPA (3)Ag-BTPA (1)Ag-BTPA (2)Cu-BTPA (1)Cu-BTPA (2)Ni-BTPA (1)Ni-BTPA (2)Ni-BTPA (3)

-2

10-3

10-4

10-11
0.25

0.30

0.35

0.40

0.45

0.50

0.25

0.55

0.30

0.35

0.40

0.45

0.50

Molecular diameter (nm)Molecular diameter (nm)Fig. S2-3. Reproducible data of single gas permeance for Ni-BTPA as a function of
kinetic diameter size of gases measured at 200 ⁰C

H2/N2 permeance ratio [-]

100

80

BTESM [1]60

BTESE [1]Ni-BTPA

BTESP

40

[1]

BTESB [1]Cu-BTPA

20

Ag-BTPA

BTMSH [1]BTPA

BTESO [1]

0
0

2

4

6

8

10

Carbon number in linking unit [-]Fig. S2-4. permeance ratio of H2/N2 as a function of carbon number in linking unit of bisilyl structure

Fig. S2-4. reveals that there has been a gradual decrease in the H2/N2 permeanceratio by increasing the number of carbon molecules between the bi-silyl bridges
that give rise to higher flexibility in the linking unit. BTPA has a structure that issimilar to that of BTMSH (six carbon atoms in a linking unit) and shows an H2/N2
permeance ratio of 15, which corresponds to the small pore size of the membranes.
By contrast, after the addition of metal doping into the BTPA structure, a higher
permeance ratio was achieved, which equates to an increase in membrane

131

0.55

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --connectivity and in the rigidity of the linking unit via amine-metal interactions.
The linking unit structure transforms from flexible into rigid with increases in themetal affinity.
Temperature of measurement [⁰C]
200 ⁰C150

50 ⁰C

100 ⁰C

200 ⁰C

10-5

Permeance [mol m-2 s-1 Pa-1]

H2
He

He

CH4
N2

He

He

CO2

CO2 He

10-6

H2

H2
H2

CO2

H2

CO2
He
CH4
CO2
CH4

N2

N2

10-7

CH4
N2

CH4

N2

10-8
0

100

200

300

400

500

600

Time [min]Fig. S2-5. Effective time courses for Ni-BTPA single-gas permeation performance at
different operation temperatures (200 ⁰C ื 150 ⁰C ื 100 ⁰C ื 50 ⁰C ื 200 ⁰C)

dVp [cc/nm/g]

0.015

NLDFT method
BTPA
Ag-BTPA
Cu-BTPA
Ni-BTPA

0.010

0.005

0.000
0

1

2

3

4

5

Pore size [nm]Fig. S2-6 NLDFT pore size distribution of amorphous aminosilica and metal-doped powder

According to the pore size distribution data in Fig. S2-6, as affinity for the metal-dopant
increased, the pore size distribution narrowed as evident by a decrease in the full width at
half maximum (FWHM) value in the NLDFT curve when fitted with a Gaussian function.
In this case, the Ni-BTPA showed the most uniform pore size distribution while the BTPA
and Ag-BTPA with highly maintained flexible amorphous material showed typicallynegligible and broad pore size distributions, respectively.

132

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --0.020

M Pi)1/3 [mol1/6 g1/6 m-2/3 s-1/3 Pa-1/3]

(a
)

(b)

H2

He
0.015

N2

0.010

BT
PA

Ag
-B
TP
A

Cu
-B
TP
A

0.005

Ni
-B
TP
A
SF6

T = 200 ⁰C
0.000
2.5

3.0

3.5

4.0

4.5

5.0

5.5

Molecular size [Å]ଵΤଷFig. S2-7 The correlation of ൫ඥ‫ܯ‬௜ ܲ௜ ൯
as a function of molecular size of permeating gases(points and lines are experimental and theoretically calculated, respectively) (a), and schematicgas transport for different gases in a cylindrical pore of Ni-BTPA (b).

The pore size of metal-doped BTPA-derived membranes was estimated by using
modified-GT models, as shown in Eq. S.2-1, where the functions of ൫ඥࡹ࢏ ࡼ࢏ ൯૚ൗ૜for different permeating gases were plotted as the sizes of the permeating gases (di). As
shown in Fig. S2-7(a), experimental permeation reveals that the membrane pore size wasenlarged on the order of Ni-BTPA (6.10 nm) > Cu-BTPA (5.51 nm) > Ag-BTPA (5.06nm) > BTPA (4.89 nm). Fig. S2-7(b) presents a schematic illustration of gas transport
within cylindrically shaped pores of Ni-BTPA. H2 and N2 were accepted within the NiBTPA pores, whereas a large gas such as SF6 was rejected by the repulsive forces between
neighboring membrane pore walls. H2 as a smaller molecule tends to perform at a higher
level of potential energy compared with that N2. On this occasion, the micropore diffusion
rate was determined by the slower passage rate of N2 through the membrane pore wall.
As reported for most microporous organosilica membranes, the gas separation factor istypically at a moderate level for H2/N2 separation with a relatively high rate of permeationଵൗthat relates to a thin topmost layer. More importantly, the ൫ඥ‫ܯ‬௜ ܲ௜ ൯ ଷ of most gases has
been the best fit for Ni-BTPA membranes, which indicate that Ni-BTPA consist of
uniform pores. On the other hand, other metal-doped BTPA showed points that deviatedfrom the calculated lines, which indicated that the pore sizes were not uniform.
൫ඥ‫ܯ‬௜ ܲ௜ ൯

ଵൗ
ଷ

ൌ൬

௞బ
ξோ்

ா೛ǡ೔

݁‫ ݌ݔ‬ቀെ ோ் ቁ൰

133

ଵൗ
ଷ

ሺ݀଴ െ ݀௜ ሻ

(S.2-1)

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --134

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Chapter 3
Microporous nickel-coordinated aminosilica membranes for
improved pervaporation performance of methanol/toluene
separation
3.1 Introduction
Methanol is one of the most commonly used solvents in industrial applications such as
petroleum, pharmaceutical, electrochemical, petrochemical, fertilizer, textile, paint, cosmetics,
etc [1-4]. One of the most challenging tasks in these industries is the separation of the organic
mixture of methanol from toluene, which is associated with the formation of an azeotropic
mixture with a composition of 27 wt% toluene under atmospheric pressure [5]. Extractive
distillation and azeotropic distillation are frequently applied to separate mixtures with closeboiling points [6-9]. In thermal-based separations such as distillation, the heat of evaporation
is provided from the reboiler at the bottom stream and the heat of condensation is removed
from the condenser at the upper stream and results in energy consumption estimated to bearound 45 – 50% of the separation process [10]. Overcoming this expense, in a sustainable
manner requires new technology.
Pervaporation has shown promise as a membrane-based separation technology for the
separation of organic compounds with close boiling points, and azeotropic mixtures [1, 11].
With this technique liquid mixtures are separated via partial vaporization throughout the
selective barrier of membranes. Compounds are selectively transported from the liquid phasein the upper stream to the vapor phase in the lower stream, which enables better separation
performance and lower levels of energy consumption [12-14]. The driving force forpervaporation is the vapor pressure difference between the feed and permeate sides; thepressure of the feed side is maintained at atmospheric levels while that of the permeate side isevacuated. Polymeric and inorganic membranes are used for a wide variety of pervaporation
applications [14, 15]. A considerable number of industrial pervaporation applications use
polymeric membrane materials due to their relatively high flux and economical investment [16,17]. Crosslinked poly(vinyl alcohol), poly(acrylic acid), highly hydrophilic chitosan, andpolyacrylonitrile are frequently used in the preparation of highly permeable membranes that135

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --are suitable for alcohol dehydration [18-23]. Nevertheless, a major problem of polymeric-based
membranes is swelling that leads to instability and lower levels of selectivity.
Inorganic membranes, which are better known as ceramic membranes, are synthesizedfrom organosilica-based materials. These membranes exhibit high thermal and chemical
stability with no swelling, high flux, high separation performance, and controllable pore size
[24-26]. The separation ability of organosilica membranes depends mostly on the pore size andon the interaction between permeating molecules and the pore walls of membranes. Therefore,material modification is required to produce a highly porous material that can facilitate highpermeability without sacrificing selectivity. A spacer method is frequently prescribed for
tailoring the microporosity by choosing bi-silyl precursors with different structures that
produce organosilica-based materials with an optimum membrane pore size [27]. The
utilization of a short and rigid linking chain between silicon atoms generates a porous structure,
whereas the increase in the carbon number and flexibility of the linking units tends to produce
a denser structure that is inappropriate for the construction of a high-flux membrane.
In the present study, we propose a new method to control the microporosity induced bycrosslinking of the amine moieties in the linking units of bis [3-trimethoxysilyl) propyl] amine(BTPA) as a strong field ligand with electron donor properties (Lewis base) and nickel fromtransition metal groups as electron-pair acceptors (Lewis acid). The intercalation of metaldeveloped subnanopores through the coordination of covalent bonds expanded the aminecrosslinking between network layers, which transformed the texture of the membrane from thatof a flexible non-porous entity into a porous structure by means of the formation of more rigid
linking units. We reported that hybrid microporous aminosilica membranes can be successfully
synthesizedvia

metal-amine

coordination-induced

microporosity

withhigh-level

improvements in both permeance and selectivity [28]. Nickel has a high coordination affinity
to amine and was found to be fully incorporated into the amorphous BTPA framework. The
nickel also induced the rearrangement of bridged amine chains into a more rigid structure witha high surface area and an increase in total pore volume. Therefore, it is necessary to investigate
the effect that concentrations of nickel-doped BTPA (Ni-BTPA) exert on the formation of the
microporosity of aminosilica-based coordination polymers.
The construction of silica-based amorphous microporous frameworks in Ni-BTPA
could facilitate the diffusion selectivity of a smaller molecule such as methanol (kinetic
diameter: 0.38 nm) rather than toluene (0.59 nm) [13, 29], while the nickel-amine coordinationsites tend to generate a rigid segment via crosslinking among network layers that gives rise toa porous structure and may create an affinity with methanol. A high degree of metal doping136

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --ensures the development of a high-flux membrane and a sorption selectivity that favors
methanol (polarity index: 12.3 MPa0.5) as a hydrophilic polar smaller-sized component, while
the sorption of toluene (1.4 MPa0.5) as a hydrophobic larger-sized compound would be limited[30]. In this study, an optimization experiment was performed via different mole ratios ofnickel-metal against amine groups between bi-silyl precursors, and the textural properties of
the resultant materials were characterized. The methanol/toluene pervaporation performanceof each Ni-BTPA membrane was evaluated and discussed with respect to any effects ofdifferent nickel phases in the form of amorphous and nanoparticulated nickel.

3.2 Experimental
3.2.1 Preparation of a nickel-doped bis [3-(trimethoxysilyl) propyl] amine(Ni-BTPA) sol
Ni-BTPA coordinated sols were prepared via the hydrolysis and condensation of bis [3(trimethoxysilyl) propyl] amine (BTPA) precursors with different mole ratios ofNi(NO3)2۰6H2O as metal dopants [28]. First, BTPA was dissolved into ethanol and a mixtureof water and nitric acid as catalysts was added to the solutions under vigorous stirring. The
mole ratio of BTPA/H2O/HNO3 was 1/300/1, and ethanol was used to adjust the concentrationof BTPA to 5 wt%. The hydrolysis and condensation reactions were established at room
temperature for 12 hours prior to the addition of a nickel-metal salt. Ni(NO3)2۰6H2O was added
into the aminosilica sol in mole ratios (nickel/amine = mol/mol) of 0 (denoted as BTPA), 0.125(Ni-BTPA 0.125), 0.25 (Ni-BTPA 0.25), and 0.50 (Ni-BTPA 0.50) to examine the effect thatmetal-doping exerts on the transformation of a dense BTPA structure. After the addition ofdissolved nickel-metal salt, the mixtures were further stirred (550 rpm) at room temperaturefor 1 hour. In this study, Ni-BTPA with a ratio of 1.00 was also synthesized as a control group
to facilitate analysis.

3.2.2 Characterization of nickel-doped aminosilica films and xerogels
Fourier transform infrared spectroscopy (FT/IR-4100, JASCO, Japan) was used toobserve the evolution of the amine deformation band during the formation of coordinatedinteractions with a nickel-metal center. BTPA sols with different metal ratios were coated onto
KBr plates and then calcined at 250 ⁰C under a N2 atmosphere. The binding energy and137

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --environment of the amine sites were investigated via X-ray photoelectron spectroscopy (JEOL
RE series JES-RE1X ESR spectrometer). Each nickel-doped BTPA film was coated onto asilicon wafer followed by calcination at 250 ⁰C.
The Ni-BTPA sols were dried at 50 ⁰C and then ground to obtain a fine powder, which
was then calcined at 250 ⁰C for 30 minutes under a nitrogen flow. The microstructures of thecomposites after the doping process were examined using X-ray diffraction (XRD D2
PHASER, Bruker, Germany) at a scanning range of 5⁰ - 80⁰. A transmission electron
microscope (TEM JEOL2010 Ltd., Japan) was used to confirm the microscopic morphology
of coordinated nickel ions and nanoparticles. Textural properties of BTPA and Ni-BTPA were
determined using a nitrogen sorption isotherm (BELMAX JAPAN Inc., Japan) at -196 ⁰C.
Methanol and toluene vapor sorption isotherms were measured to determine adsorption
capacity, and the heats of adsorption for each of the organic solvents were evaluated at threedifferent temperatures (25, 30, and 35 ⁰C). Ni-BTPA powder with different mole ratios waspretreated at 200 ⁰C under vacuum conditions for 12 hours before each measurement.

3.2.3 Fabrication of Ni-BTPA membranes and single-gas permeation
measurements
Porous α-alumina tubes were used as membrane supports with 50% porosity and an
average pore size of 1 μm (length 100 mm; inner diameter 8 mm; outer diameter 10 mm), as
supplied by Nikkato Corporation (Japan) [31]. A Ni-BTPA membrane consists of a porous
support layer (α-alumina particle and intermediate layer) with an ultrathin topmost layer acting
as a specific separation barrier. First, porous particle layers were developed by coating αalumina particles (average particle sizes 2 μm and 0.2 μm) that were dispersed into 2 wt%
SiO2-ZrO2 sol as a binder and subsequently calcined at 550 ⁰C under an air atmosphere for 15
minutes. The intermediate layer was constructed from 0.5 wt% SiO2-ZrO2 sol that was coatedonto the surface of the particle layer and fired under an air atmosphere at 550 ⁰C. These
procedures were repeated several times until the average pore size of the support reachedapproximately 1 nm. Finally, the topmost layer was obtained by twice coating nickel-doped
BTPA sols diluted to 0.1 wt% onto the intermediate layer followed by calcination at 250 ⁰C
under a flow of nitrogen for 30-60 minutes.

138

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Before the measurements of single-gas permeation, a fresh membrane was pretreatedat 200 ⁰C for 6 hours under a He flow to remove any adsorbed molecules. A single-gas
permeation experiment was performed using high-purity He, H2, CO2, N2, CH4, CF4, and SF6.
Each permeation gas was fed to the outside of the membrane with upstream pressures of 200 –400 kPa, and the pressure inside the system was maintained at atmospheric levels. The gas
permeation test was established at temperatures ranging from 200 – 50 ⁰C, and the flow rate of
the permeate side was measured using a film flow meter (Horiba, Co. Ltd., Japan).

3.2.4 Pervaporation experiment
The separation of an organic-azeotropic mixture of methanol and toluene (Table 3-1)
was performed using the pervaporation process. A schematic diagram of the pervaporation(PV) apparatus is shown in Fig. S3-2. The mixtures of methanol and toluene were circularizedusing a stirrer at 3,000 rpm to prevent a concentration polarization on the surface of themembrane. The pressure of the feed side was maintained at atmospheric levels, while thepermeate side was evacuated to 0 – 2 kPa via the use of a vacuum pump. The temperatures ofthe feed solutions were maintained within a range of 30 – 50 ⁰C via the use of a thermocouple
and a heating coil controller, while the concentrations of methanol in the feed solutions werevaried from 10 to 50 wt%.
Table 3-1. Physical properties of organic solvents as azeotropic mixture
Penetrant

MW-1[g mol ]Stokes diameter29[nm]

Boiling point[⁰C]

Polarity32 [MPa ]

MeOH

32.04

0.38

64.7

12.3

Toluene

92.14

0.59

110.6

1.40

1/2

The permeate vapor was collected using a glass trap cooled in a dewar vessel with liquid
nitrogen for each time interval. The concentration of the feed solution was maintained constantduring the measurement via the periodical addition of an appropriate amount of pure methanol.
Each composition of the feed and permeate streams was determined via gas chromatography(GC-14B Shimadzu Cor., Japan).
Membrane flux (‫ )ܬ‬and the flux of each component (‫ܬ‬௜ ) on the permeate side aredescribed by equations (3-1) and (3-2), respectively.
139

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --‫ ܬ‬ൌ ஺ൈ௧

௪

(3-1)

‫ܬ‬௜ ൌ ‫ ܬ‬ൈ  ‫ݕ‬௜ǡ௣

(3-2)‫ ܬ‬is the total permeated flux (kg m-2 h-1), ‫ ݓ‬is the permeate weight (kg), ‫ ܣ‬is the membranesurface area (m2), ‫ ݐ‬is the permeating time (h), and ‫ݕ‬௜ǡ௣ is the weight fraction of component ݅in the permeate. The permeance of component ݅ could be calculated from the molar flux (‫ܬ‬ெ௜ )of the ݅ component divided by the difference in the partial pressures of the feed (‫݌‬௜ǡ௙ ) andpermeate streams (‫݌‬௜ǡ௣ ), as determined by the following equation.
‫ܬ‬ܲ௜ ൌ ெ௜൘ሺ‫݌‬௜ǡ௙ െ ‫݌‬௜ǡ௣ ሻ(3-3)

The feed partial pressure of the ݅th component was calculated as ‫݌‬௜ǡ௙ ൌ ߛ௜ ‫ݔ‬௜ ‫݌‬௜଴ , where ߛ௜ is theactivity coefficient according to the Wilson parameters, ‫ݔ‬௜ is the mole fraction of the ݅ thcomponent, and ‫݌‬௜଴ is the vapor pressure according to the Antoine equation. On the other hand,the pressure in the permeate stream was presumed to be zero. The separation factor is definedas the ratios of the mole fractions in the permeate ( ‫ݕ‬ெ௘ைு ܽ݊݀‫்ݕ‬௢௟ ሻ and feed(‫ݔ‬ெ௘ைு ܽ݊݀‫்ݔ‬௢௟ ሻstreams, as follows.
௬ቀ ௬ಾ೐ೀಹ ቁ೅೚೗ߙ ൌ൙ ௫ಾ೐ೀಹ
ቀ
ቁ

(3-4)

௫೅೚೗

3.3 Results and discussions
3.3.1 Optimization of the coordination driven by the nickel-metal mole ratio
The introduction of nickel into amine-containing organosilica precursor (BTPA)
molecules was expected to increase the network dimensionality and convert the texturalproperties of the BTPA membrane from dense to porous. In order to clarify this idea, thespectroscopy characterization was collected for BTPA and Ni-BTPA sols and powders withdifferent nickel ratios obtained via thermal treatment at 250 ⁰C.
Fig. 3-1(a) compares the UV-Vis spectral results for three Ni-BTPA sol solutions ofdifferent nickel content together with pure aminosilica precursors of BTPA and nickel nitrate.
The absorption spectra of BTPA as an amine organosilica ligand (solid black line) presented
one peak at 212 nm due to the amine electron intraligand transition (n ՜ ߪ*).

140

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --BTPA
Ni(NO3)26H2O
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50

Absorbance [-]n → σ*

n → π*
8

d-d transition of d Ni(II) complex(b) 3500
3

400.5

N 1s

3
3000

Intensity [cps]

(a)

399.8

Ni-BTPA 0.25

2500

401.3

2000
1500

Ni-BTPA 1.00

1000

BTPA

uncoordinated Ni2+

500
0
410

200

300

400

500

600

700

408

406

800

404

402

400

398

396

Binding Energy (eV)Wavelength [nm]
-CH2-

N=O

250 ⁰C

H-N

Absorbance [-]O-H

Si-O-Si

Si-C

(d)

BTPA
Ni-BTPA 0.25

Ni-BTPA 0.125
Ni-BTPA 0.5

Si-OH

Absorbance [-](c)

Ni-BTPA 0.5

Ni-BTPA 0.25

250qC

Ni-BTPA 0.125

BTPA

50 qC
4000

3500

3000

2500

2000

1500

1800

1000

Wavenumber [cm-1]

1750

1700

1650

1600

1550

1500

Wavenumber [cm-1]

Fig. 3-1. UV-Vis spectra (200 – 800 nm) of BTPA, Ni(NO3)2٠6H2O, and nickel-doped BTPA
with different nickel content (a), N(1s) high-resolution spectra (b), infrared spectra in the range-1of 4,000 – 650 cm (c), and higher magnification for NH deformation band in the range 1,700 –-1
1,500 cm (d) originating from BTPA and Ni-BTPA wafer calcined at 250 ⁰C

The emitted absorbance for nickel-doped BTPA samples displayed three peaks at 227 - 241,
282 - 313, and 420 – 422 nm, which were related to the transitions of n ՜ ߪ* (N-H), n ՜ ߨ*(N=O), and d՜d (d8 Ni2+), respectively [28]. After the addition of a different amount of nickel,
the ̴212 nm peak gradually shifted to a higher wavelength, which was associated with a newenvironment of the free-electron transition of amine moieties [28]. Moreover, as an equivalentmole ratio (0.125 – 0.50) of nickel was added into the BTPA solution, peaks indicating
coordination (300 nm and 420 nm) increased almost linearly to the nickel/amine mole ratio(Fig. S3-4). On the other hand, the new absorbance peak at ̴395 nm, which was originallyobserved for Ni(NO3)2٠6H2O, was gradually shifted to a higher wavenumber that was
accompanied by a higher intensity due to a stronger d՜d transition from lone pairs of electrons
in the amine groups to the empty d orbitals of nickel [32, 33]. The high-resolution spectra of
N1s of the XPS spectra confirmed the environment of nitrogen and its covalence binding
interaction to nickel (Fig. 3-1(b)). The N1s binding energy for BTPA was centered at 399.8
eV, whereas Ni-BTPA containing nitrogen coordinated with nickel and showed a gradualshifting to a higher level of binding energy (400.5 to 401.3eV) that was ascribed to the highly
coordinated donor atoms of nitrogen [34, 35].
141

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 3-1(c) shows a wide range of FT-IR spectra, whereas Fig. 3-1(d) shows a narrow
range of FT-IR spectra and exhibits a characteristic band at 1,550 – 1,650 cm-1, which was
assigned to N-H deformation from amine groups in the linking units of BTPA molecules, assummarized in Table 3-2. The formation of a coordination compound between amine groups
and metal could be analyzed from this type of vibration. At low temperature, the N-H
deformation band was shifted to a lower wavenumber due to the formation of a covalentcoordination bond between the amine groups and nickel, which led to a stabilization of the NiBTPA structure [36]. Also, a broad N-H deformation band centered at 1,620 cm-1 was observed
for a high nickel ratio of 0.50, which suggested the appearance of unreacted nickel after
reaching the approximate saturation point, and led to the formation of a nickel agglomerate. In
addition, the N-H deformation band become broader (1,610 – 1,620 cm-1) and the intensity
gradually increased when the calcination temperature was elevated to 250 ⁰C, suggesting the
involvement of a dynamic process that include three different reactions: hydrolysis,
condensation, and coordination [28]. The hydrolysis and condensation were controlled as the
major reactions at low temperature due to the existence of water and nitric acid, while thecoordination reaction was more predominant at higher temperatures after the removal of thewater and the catalyst.
Table 3-2. FT-IR frequency of N-H deformation band
CompoundBTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50

50 ⁰C
ν N-H [cm-1]
1588
1576
1580
1583 & 1617

250 ⁰Cν N-H [cm-1]
1588
1579
1582 & 1620
1587 & 1620

Fig. 3-2(a) compares the X-Ray diffractograms (XRD) of BTPA and Ni-BTPA powders
at different amounts of nickel, which were fired under different calcination temperatures. Allof the samples showed the main characteristics of amorphous properties, indicating that nickel
ions were incorporated and dispersed well into the BTPA matrix. The formation of coordinatednickel and amine moieties generated coordination covalent bonds with lower energy levels,
thereby increasing the stability of nickel to remain in an ionic phase. The ordered structure of
the intermolecular periodic structure [37] at 2θ of 8⁰ could be maintained at a lower
concentration of nickel, but became more disordered and shifted to a lower degree of 2θ byincreasing the metal ratio. Moreover, a wide diffraction peak of siloxane of near 2θ at 20⁰
142

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --showed that the intensity was both decreased and broadened by increasing the nickel/aminemole ratio, indicating the formation of a more amorphous structure [37, 38]. The excesses ofnickel uncoordinated with amino groups easily formed metallic nickel and resulted in phaseseparation that was detected by the formation of crystalline nickel nanoparticles. Hightemperature phase separation or metal segregation could be occurring from inhomogeneously
distributed nickel. An optimized amount of nickel content generates a higher stabilization
degree of coordinated Ni-BTPA sol and prohibits metal segregation. As a comparison, Fig. 32(b) presents the XRD patterns of Ni-BTPA xerogel powders with metal ratios ranging from 0
to 1.0 obtained at a calcination temperature of 250 ⁰C. The slight conformation of metallic
nickel nanoparticles at a 2θ of 45⁰ was observed when the metal ratio was 0.50, which
obviously was higher than 0.25. The intensity of nickel nanoparticles became stronger withincreases in the metal concentrations. It should be noted that nickel is known to be theoreticallycoordinated with four amines in free solutions [39, 40], which seems appropriate fororganosilica-based sol and gels.
In Fig. 3-2(c), the low-resolution TEM photographs (upper figures) and representative
high resolution (lower figures) examples show a transparent surface for BTPA and Ni-BTPAwith a low concentration of nickel (Ni/NH = 0.125, and 0.25), which means a uniform
dispersion of nickel nanoparticles. Nickel nanoparticles could be observed at a metal ratio of
0.50, indicating that a nickel mole ratio higher than 0.25 tends to inhomogeneously distribute
nickel [41], resulting in non-coordinating nickel that leads to the formation of phase separation.
Similarly, diffraction pattern images also show diffusing halo patterns for pure materials andnickel-composites at low concentrations. The crystallization of nickel particles was observed
from a clear ring pattern.

3.3.2 Metal-induced microporosity via nickel-amino interaction
The nitrogen sorption isotherms of BTPA and various Ni-BTPA xerogel powders at 196 ⁰C are presented in Fig. 3-3(a), and the textural properties of each material are presented
in Fig. 3-3(b) as a function of the nickel/amine mole ratio. BTPA showed a negligible amountof adsorbed N2 indicating a non-porous structure. The addition of nickel-ion created a crosslink
between the metal electron receptors and the amine electron donors, which produced
intraparticle pores as a permanent porosity.

143

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(b) 300

100

50

0.20

0.15
200

0.10

100
0.05

0

0
0.0

0.2

0.4

0.6

0.8

0.00
0.0

1.0

Pore volume [cm3 g-1]

BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Ni-BTPA 1.00
Closed : adsorption
Open : desorption

Surface area [m2 g-1]

Absorbed volume [cm3(STP) g-1](a) 150

0.1

0.2

0.3

0.4

0.5

Nickel/Amine mol ratio [mol/mol]p/p0 [-]

Fig. 3-3. N2 isotherms (-196 ⁰C) for BTPA and different Ni-BTPA powders heat treatedat 250 ⁰C (a), and textural properties (surface area and pore volume) as a function ofnickel mole ratio for nickel doped aminosilica xerogel powder obtained at the thermal
treatment of 250 ⁰C (b)As summarized in Fig. 3-3(b), higher metal ratios produced higher surface areas on the orderof Ni-BTPA 0.50 > Ni-BTPA 0.25 > Ni-BTPA 0.125 > BTPA. Although the theoreticalcoordination in free solutions is 0.25, nickel/amine mole ratios higher than 0.25 are also
effective at forming a microstructure, as shown in Fig. 3-3(b). At an overly high nickel moleratio of 1.00, the excess of nickel ions that were uncoordinated with the amine groups reduced
the stability of coordination and led to the formation of nanoparticles resulting in a lowering of
the surface area due to phase separation, as confirmed by XRD and TEM characterization (Fig.
3-3(a)).
Vapor-phase sorption isotherms were used to predict the separation performance andpossible separation mechanism of Ni-BTPA. As shown in Fig. 3-4(a) and (b), the organicvapor adsorption isotherm curves of microporous Ni-BTPA powders are preferable for thepolar and smaller-sized methanol rather than the non-polar larger-sized toluene. The methanol
adsorption capacity for Ni-BTPA was increased by small hysteresis loops following increases
in the concentration of the nickel dopant, whereas BTPA showed a lower capacity and a large
hysteresis loop. This finding implies that the nitrogen in the BTPA structure and the nickeldopant both participate to enhance the methanol sorption capacity. The adsorbed organic vapor
per the Ni-BTPA repeating unit appears in Fig. 3-4(c), and the undoped-BTPA showed a 0.24
mol mol-1 adsorption of methanol that confirmed the preferential adsorption of methanol into
polar functional groups such as the amines in the BTPA structure.
144

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- -(a)

(b)

0.8

0.6

BTPA
Ni-BTPA 0.125
5Ni-BTPA 0.25
Ni-BTPA 0.50
Closed - adsorption
Open - desorption

MeOH - 25 ⁰C

0.4

0.2

0.0
0.0

0.2

0.4

0.6

1.0

Absorbed amount of Toluene [mol mol-1]

Absorbed amount of MeOH [mol mol-1]

1.0

0.8

BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Closed - adsorption
Open - desorption

0.8

0.6

0.4

0.2

0.0
0.0

1.0

0.2

Absorbed amount of organic vapor [mol mol -1]
at P/P0 of 0.5

0.4

0.6

0.8

1.0

p/p0 [-]

p/p0 [-](c)Toluene - 25

1.0
MeOH
Toluene

0.5

0.0
0.0

0.1

0.2

0.3

0.4

0.5

Nickel/Amine mol ratio [mol/mol]

Fig. 3-4. Vapor sorption isotherms of methanol (a) and toluene (b) on Ni-BTPA xerogel
powders with different metal mol ratio obtained at 25 ⁰C, and the absorbed amount of organicvapor as a function of nickel/amine mol ratio at 25 ⁰C (c)Thereafter, a significant enhancement of methanol sorption was observed approximately linear
to the nickel content, and a Ni-BTPA of 0.50 showed the highest adsorption capacity of 0.57
mol mol-1. In contrast, the low adsorption capacity of toluene as a less polar molecule provided
plausible evidence of no specific interaction between toluene and the surface of materials.
Toluene adsorption was slightly increased after the addition of nickel, which could be related
to the higher microporosity of Ni-BTPA.
The specific active site of nitrogen was facilitated due to strong polarization for methanol[42], and the addition of nickel into the BTPA framework created a porous structure that
contributed to higher methanol adsorption with a rapid diffusion rate. According to the workof Liu et al., the preferential adsorption of methanol onto a metal organic framework (MOF)
structure was simulated via the grand canonical Monte Carlo method (GCMC) combined withdensity functional theory [43]. That observation shows a lone pair of electrons from the outerorbital of nitrogen molecules induced higher polarization onto methanol molecules, while theexistence of a metal dopant firmly increased the specific interaction with methanol that led to145

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --an enhancement of network polarity and multiplied the adsorption affinity [43, 44]. The
favorable adsorption of methanol over toluene in Ni-BTPA and BTPA can be ascribed to anaffinity with polar amine groups as well as to doped nickel and indicated the enhanced affinity
associated to nickel filling pores. In this study, the modified framework was developed from a
metal ion-amine crosslink to bring out the hydrophilic surface tendency, resulting in highseparation for methanol as a polar component [45].

3.3.3 Tuning a membrane pore-structure via alterations in the nickel content
Single-gas permeance and dimensionless permeance based on He permeance arepresented as a function of molecular sizes of the permeating gases in Fig. 3-5. An increase in
the nickel mole ratio led to lower gas-transport resistance, and thus improved the permeanceof gases on the order of Ni-BTPA 0.50 > Ni-BTPA 0.25 > Ni-BTPA 0.125 > BTPA. Ni-BTPA
1.00, which showed a loose structure with a Knudsen diffusion mechanism. This result could
be explained as the generation of interparticle pores by nickel nanoparticles, which were
unsuitable for gas separation [25, 46]. In this experiment, single-gas permeation of SiO2-ZrO2
as the intermediate layer was measured to compare Knudsen diffusion with Ni-BTPA 1.00 andto analyze the contribution of nickel nanoparticles to the pervaporation performance, as will be
discussed in section 3.3.4.

10-5(b) 101He

H2

CO2 N2

10-6

CF4

SF6

CH4

10-7

BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Ni-BTPA 1.00
Intermediate layer
Knudsen

10-8

10-9

10-10
0.25

0.30

200qC

200qC

0.35

Dimensionless permeance [-]Permeance [mol m-2 s-1 Pa-1](a) 10-4

100

10

He H2

-1

CH4
CO2
N2

10-2

BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Ni-BTPA 1.00
Intermediate layer
Knudsen

10-3

10-4

CF4

SF6

10-5

0.40

0.45

0.50

0.25

0.55

0.30

0.35

0.40

0.45

0.50

0.55

Molecular diameter (nm)Molecular diameter (nm)Fig. 3-5. Single-gas permeance as a function of gas molecule sizes (a), and dimensionlesspermeance based on He permeance (b) for BTPA and Ni-BTPA measured at 200 ⁰C

Fig. 3-6 shows the permeance and permeance ratios of H2, N2, and SF6 as a function of
the nickel/amine mole ratio of Ni-BTPA-derived membranes. The reproducibility ofpermeation properties has been confirmed since each point in Fig. 3-6 corresponds to a different

146

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membrane. The permeance of H2 and N2 (Fig. 3-6a-lower figure) became higher with anincrease in the nickel mole ratio, while their activation energy (Fig. 3-6b-upper figure)decreased with an increase in the concentration of nickel. The activation energy reflects theinteraction between the pore wall of the membrane and permeating molecules [47, 48]; a lower
magnitude of activation energy was equated with the generation of larger the pore sizes in themembranes. A general framework structure of organosilica networks originated from thehydrolysis and condensation of pure aminosilica precursors (BTPA), which allowed theformation of a broad pore size distribution and/or defects from grain boundaries. The Ni-BTPA
0.125 and 0.25 membranes showed smaller SF6 permeance than pure BTPA (Fig. 3-6a-lower
figure), which can be explained by the formation of more uniform micro-sized network pores
and a defect-free thin layer through the involvement of metal-amine coordination.
Nickel/NH mol ratio [mol/mol]Permeance ratio [-]

0.0

0.1

0.2

0.4

Nickel/NH mol ratio [mol/mol]

(b)

0.5

N2/SF6

103

10

0.3

0.0
Activation energy [kJ mol-1]

(a)

2

H2/N2

0.2

0.3

0.4

0.5

0.4

0.5

20

H2
10

N2

0

10-5

10

H2

10-6
10-7

Membrane pore size [Å]Permeance [mol m-2 s-1 Pa-1]

101

0.1

N2

10-8
10-9

SF6

10-10
0.0

0.1

0.2

0.3

0.4

5

0

0.5

0.0

0.1

0.2

0.3

Nickel/NH mol ratio [mol/mol]Nickel/NH mol ratio [mol/mol]

Fig. 3-6. Gas permeance (lower figure) with permeance ratio (upper figure) as a function of nickel
mol ratio at 200 ⁰C (a), and membrane pore size (lower figure) with the activation energy (upper
figure) of permeating gases as a function of nickel content (b)Ni-doped BTPA-derived membranes showed moderate H2/N2 selectivity along with adecreasing trend (20 to 10) in the nickel/amine mole ratio, whereas the permeance ratios for
N2/SF6 (Fig. 3-6a-upper figure) were higher than 100 and 1,000 for nickel/amine mole ratios
of 0.50 and 0.25, respectively, which means the membrane pore size was controlled to less than147

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --0.55 nm [49]. This finding indicates that a higher level of microporosity could be produced byincreasing the nickel concentration, which would lead to an enlargement of the membrane poresize. The cross symbol (red color) in the lower portion of Fig. 3-6(b) shows the pore sizes ofmembranes that were calculated via a modified-gas translation (m-GT) model [50-52], which
also confirms this idea (detailed calculation results are summarized in Fig. S3-10) in the
Supporting Information). The pore sizes of the Ni-BTPA membrane gradually increased alongwith an increase in the nickel/amine mole ratio, which indicated that a higher nickel content
tended to generate more crosslinking in the framework. As shown in Fig. 3-6(b), Ni-BTPA
0.25 showed the highest selectivity, which revealed the formation of uniform pores (Fig. S310). In contrast, BTPA, Ni-BTPA 0.125, and Ni-BTPA 0.50 showed lower selectivity for SF6,
which indicated the pore sizes were not uniform. This result is reasonably explained by a low
concentration of nickel, which began the formation of a partially microporous structure thatreached a saturation point at 0.25. At a metal ratio of 0.50, the excess amount of nickel tended
to produce interparticle pores and gave rise to a wide distribution of pore sizes. Thus, it is clearthat the pore size of Ni-BTPA membranes could be controlled by the concentration of nickel
as an active site for coordination.
104
Black symbol = 100 °C < T [qC] < 200 °C
Red symbol = 100 °C > T [qC]
Ni-BTPA 0.25

103

PN2/PSF6 [-]Ni-BTPA 0.125
Ni-BTPA 0.50

102
BTPA
101

100

Polymeric membrane
Zeolite membrane
Silica Membrane (BTESE)
Aminosilica membrane (BTPA)

10-11 10-10

10-9

10-8

10-7

10-6

10-5

-2 -1

-1

10-4

10-3

N2 Permeance [mol m s Pa ]Fig. 3-7. Permeance ratio of N2/SF6 as a function of N2 permeance for various type ofmembranes. Data plotted in this figure can be found in SI-18.

Fig. 3-7 summarizes the separation performance of N2 over SF6 for various types ofmembranes, including BTPA and Ni-BTPA membranes. As shown in the figure, microporous
Ni-BTPA exhibited competitive performance with an excellent permeance ratio for N2/SF6 ofhigher than 100 – 1,000 and superior N2 permeance of larger than 1.0ൈ10-7 mol m-2 s-1 Pa-1.
Thus, Ni-BTPA membranes could be used for the prospective separation of molecules with a148

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --critical kinetic diameter that ranges from 0.364 nm (N2) ൏  ݀௣ಿ೔షಳ೅ುಲ < 0.55 nm (SF6). In aprevious study, Moriyama et al. evaluate and observed a consistent correlation between singlegas permeation performance at 200 ⁰C and pervaporation (PV) at 50 ⁰C [53]. Therefore, byfollowing a similar systematic approach, the tuning of membrane pore size in this study was
focused on the separation of the methanol (0.38 nm)/toluene (0.59 nm) azeotropic mixture
because both molecules have similar molecular sizes of N2 (0.36 nm)/SF6 (0.55 nm). Nickel
doping was used to tailor the microporosity with an optimized pore size, which would allow
methanol (0.38 nm) to pass through the membrane separation barrier, but would reject toluene
molecules (0.59 nm) [29]. Based on single-gas permeance data, it was reasonable to expect thepossible separation of methanol/toluene by Ni-BTPA membranes.

3.3.4 Ni-BTPA hybrid membranes for azeotropic organic separation by
pervaporation
Fig. 3-8 shows the time courses for methanol/toluene pervaporation (PV) of BTPA andvarious Ni-BTPA membranes coated onto a SiO2-ZrO2 intermediate layer under a constantmethanol feed concentration and temperature of 10 wt% and 50 ⁰C, respectively.

For

comparison, the PV performance of a SiO2-ZrO2 intermediate layer with an average pore sizeof 1 nm was also added to Fig. 3-8 [54]. Steady-state performance of membranes was observed
after the first 200 minutes, which corresponds to the stabilization step reported by Yu et al.,and this rate of performance continued for more than 10 hours [55]. The SiO2-ZrO2 membrane
showed high flux and permeance of toluene as a major component in the feed solution, but,due to the large pore size, this membrane failed to separate methanol from a rich toluene
mixture (separation factor = 1.9). After the deposition process of amino silica sol as a topseparation layer, the separation performance of the BTPA membrane improved (442), but themembrane flux declined to 0.04 kg m-2 h-1. Nickel doping considerably enhanced bothmethanol flux and permeance, while maintaining a small flux of toluene. A higher flux ofmethanol could be obtained by increasing the concentration of the nickel dopant. BTPA, NiBTPA 0.125, and Ni-BTPA 0.25 showed low values for flux of 0.04 kg m-2 h-1, 0.07 kg m-2 h1, and 0.14 kg m-2 h-1, respectively. By contrast, Ni-BTPA 0.50 exhibited excellent performancein a stable manner for as long as 7 hours while maintaining a high-flux of 2.8 kg m-2 h-1 andthe highest level of separation factor for methanol (larger than several hundred).

149

MeOH
10-8
10-9

20

Intermediate layer = SiO2-ZrO2

2

0

200

MeOH
0

100

200

300

Toluene

MeOH
400

600

800

1000

1200

1400

[mol m s Pa ]100

Toluene
10-9
10-10
3
2

Ni-BTPA 0.25

1
0

1600

[-]

MeOH

MeOH
0

200

400

600

800

1000

Toluene

1200

1400

1600

Operation time [min]

104

103

103

102

[-]

[mol m-2 s-1 Pa-1]

Separation factor

[-]

10-8

Operation time [min]Permeance

101

101

102
101

101

[-]

200

102

Permeance ratio

0

1000

103

10-7

Flux

Ni-BTPA 0.125

1

900

102

[kg m-2 h-1]

Toluene

10-10
3

0

Separation factor

Permeance

10-8

2

800

103

-1

0

MeOH

10-9

700

104

-1

10

Toluene

600

-2

101

[-]

102

Permeance ratio

103

[-]

Separation factor

Permeance[mol m-2 s-1 Pa-1]

103

101

[kg m-2 h-1]

Flux

104

10-7

500

Operation time [min]Operation time [min]

102

400

Permeance ratio

100

BTPA

1

MeOH
0

Toluene10-10
3[kg m-2 h-1]

10-8
Toluene

[-]

[-]

[-]

-1

10-7

0

100

10-7

-1

MeOH

10

101

-2

Permeance

10-6

102

101

[mol m s Pa ]

10

103

103
102

-1

Toluene104

Permeance ratio

100

10-1
10-5

Separation factor

1

Permeance ratio

10

[-]

10

0

102

Flux

Permeance

10

1[kg m-2 h-1]

Flux

102

[mol m-2 s-1 Pa-1]

Separation factor

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

10-5
MeOH

10-6
10-7

TolueneFlux[kg m-2 h-1]

3
MeOH

2

Ni-BTPA 0.50

1Toluene

0
0

100

200

300

400

500

Operation time [min]Fig. 3-8 Effective time courses for SiO2-ZrO2 layer, BTPA, and Ni-BTPA pervaporation
performance in methanol/toluene (10 wt%/ 90 wt%) mixture

The enhancement of both methanol flux and permeance ratio could be ascribed to the specificinteraction between uncoordinated nickel as a Lewis acid with a hydroxyl group from methanol
as a Lewis-base and increased diffusivity of methanol via larger pore sizes. When the nickel
mole ratio exceeds 1.00 (Fig. S3-13) the formation of phase separation tends to reduce theseparation factor of the membrane, thus confirming the optimum mole ratio of nickel dopant
at 0.50.

150

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Pervaporation performances are summarized as a function of the nickel/amine moleratio in Fig. 3-9(a), while Fig. 3-9(b) shows the trade-off relationship between total flux andmethanol separation performance for comparison with other types of membranes. Ni-BTPA
0.125 and Ni-BTPA 0.25 showed higher total flux than pure BTPA membranes but with lower
selectivity. This result revealed a trade-off where a more permeable membrane tends to show
a lower level of selectivity [56]. Contrary to expectations, Ni-BTPA 0.50 showed a significant
improvement in performance and reached methanol flux higher than 1 kg m-2 h-1, which isabout 70-fold higher than that of BTPA, while selectivity increased from 442 to 907. It should
be noted that each point in Fig. 3-9(a) corresponds to one membrane, which confirmed the
reproducibility of membrane fabrication. The Ni-BTPA 0.50 derived membrane showedattractive performance with a much-enhanced flux and separation factor by comparison withthe other types of membranes (Fig. 3-9(b)), due to the balance between the formation of
microporosity that was induced by amine-nickel coordination and the excessive amount ofnickel ions that facilitated the fast transport of methanol while restraining the transport of
toluene.
(a) 100

(b)

10
100
1

10
0.1

0.01

SiO2-ZrO2 intermediate layer
0.0

0.1

0.2

0.3

0.4

0.5

Separation factor [-]Total flux [kg m-2 h-1]

SiO2-ZrO2 intermediate layer

1000
1

Separation factor [-]

1000

100

10

1

0.1
0.01

1

Nickel/Amine mol ratio [mol/mol]

Ni-BTPA 0.50
BTPA

Ni-BTPA 0.125

Ni-BTPA 0.25

Inorganic membrane
PCS
TiPCS
SiO2-ZrO2
Ni-BTPA 1.00
TEOS
BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Ni-BTPA 1.00

Polymeric membrane
Cellulose acetate
Polypyrrole
PVA
SiO2-ZrO2
ChitosanPolyimide
Mix matrix membrane
PVA-HZSM5

0.1
1
Total Flux [kg m-2 h-1]

10Fig. 3-9. The pervaporation performance of SiO2-ZrO2 as intermediate layer, BTPA, and NiBTPA membranes for methanol/toluene separation (10 wt%/ 90 wt%) at 50 ⁰C (a), and the trade
off between methanol/toluene separation factor as a function of total flux for a different type ofmembranes (b).

The effect of the methanol feed concentration on Ni-BTPA 0.50 pervaporation
performance was studied for compositions ranging from 10 wt% to 50 wt% of methanol in feedsolutions at a constant temperature of 50 ⁰C, as presented in Fig. 3-10. The flux of methanol
was gradually increased, while the flux of toluene remained constant when the concentrationof methanol in the feed solution was increased. Methanol and toluene permeance wereapproximately constant from 10 – 50 wt%, indicating an almost constant permeance ratio. It
151

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --should be noted that toluene permeance in the pure system was lower than that in the mixture,
which could be ascribed to the preferential adsorption of methanol. The methanol permeancein a single component was higher than in the mixture, suggesting that toluene hindered themethanol permeation. This finding is consistent with that of Moriyama et al. who also foundthat, as a major component, an organic mixture blocks the permeation of minor compounds,
resulting in lower permeance in a binary system [53]. In addition, methanol preferentiallyadsorbs at the surface of the membrane due to hydrophilic surface properties via crosslinked
amine-nickel coordination, which leads to high performance in breaking the azeotropicbalance.

Feed MeOH concentration [wt %]Permeance

103

[-]

102

102

101

101

100
10-6

100

[-]

10 20 30 40 50 60 70 80 90 100
104
Permeance ratio

0103[mol m-2 s-1 Pa-1]

Separation factor

10

4

MeOH
10-7

Toluene-8Flux[kg m-2 h-1]

10
5
4

MeOH

3
2
1

Toluene00

10 20 30 40 50 60 70 80 90 100

Feed MeOH concentration [wt %]Fig. 3-10. The effect of methanol feed concentration (wt%) through Ni-BTPA 0.50
membrane pervaporation performance at 50 ⁰C
A competitive level of adsorption was also observed even at low concentrations of methanol
where the diffusion selectivity favors the smaller molecules (0.38 nm) of methanol comparedwith the larger size of toluene molecules (0.59 nm), which results in pore blockage for toluene.
Pervaporation performance for the Ni-BTPA 0.50 membrane was examined attemperatures ranging from 30 ⁰C to 50 ⁰C with a constant methanol concentration of 10 wt%,
as shown in Fig. 3-11. Methanol flux was increased by increasing the temperature ofmeasurement (0.76 to 1.52 kg m-2 h-1), while the flux of toluene remained constant or slightly
increased (0.04 to 0.05 kg m-2 h-1), resulting in a higher separation factor at high temperature.
152

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --A higher operating temperature of pervaporation enhances the feed-side vapor pressure and asa driving force for the smaller molecules of methanol, which results in the enrichment ofmethanol in the micropores and prevents the entrance of toluene.
Temperature [qC]

Permeance

40

45

50

104

103

103

102

102

101

101

100
10-6

100

[-]

[-]

[mol m-2 s-1 Pa-1]

Separation factor

104

35

Permeance ratio

30

MeOH
10-7

Toluene-8Flux[kg m-2 h-1]

10
3
2

MeOH

1

Toluene

0
30

35

40

45

50

Temperature [qC]

Fig. 3-11. Temperature dependency of Ni-BTPA 0.50 membrane on: flux, permeance, permeanceratio and separation factor for methanol/toluene (10/90 wt%)

Moreover, the higher operating temperature of the pervaporation process may lead to an
enhancement of the framework structural mobility, which would create more free volume to
be occupied by permeating molecules and would facilitate fast transport within the membrane
network.
To deepen the understanding of molecular transport in the pervaporation process, theactivation energy of permeance for each component was calculated using the Arrhenius
equation, as shown in Eqns. (3-5) and (3-6), and summarized in Table 3-3.
οா೛

ቁ

(3-5)

ο‫ܧ‬௣ ൌ ο‫ܧ‬ௗ െ ο‫ܪ‬௦

(3-6)

ܲ ൌ ܲ଴ ݁‫ ݌ݔ‬ቀെோ்In that equation, ܲ, ܲ଴ , ο‫ܧ‬௣ , ܴ , and ܶ are the permeance, the pre-exponential factor, theactivation energy of permeation, the gas constant, and the absolute temperature, respectively,
while ο‫ܪ‬௦ is the heat of adsorption and ߂‫ܧ‬ௗ is the activation energy for the diffusion process.

153

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Table 3-3. The activation energy for methanol and toluene according to the Arrhenius equation-1

CompoundEp [kJ mol ]

MeOH

-3.73

Toluene-18.6

As with the previously described effect of the methanol feed-solution concentration, theadsorption-diffusion mechanism dominates the molecular transport behavior of the Ni-BTPA
0.50 membrane. The permeation of methanol and toluene proceeded from the adsorption
process with the heat of ο‫ܪ‬௦ (positive), followed by diffusion of ‫ܧ‬஽ (positive), giving anactivation energy for the permeating molecules of ο‫ܧ‬௣ ൌ ο‫ܧ‬ௗ െ ο‫ܪ‬௦ . Larger ο‫ܪ‬௦ and ߂‫ܧ‬ௗ areachieved for more preferential adsorption and stronger steric hindrance, respectively. It isapparent from this table that both methanol and toluene have a negative activation energy (Ep< 0), which means the heat of adsorption is higher than the activation energy for diffusion [57].
The Ni-BTPA 0.50 membrane shows a negative value of ‫ܧ‬௣ for both methanol and toluene,
which means that the permeance of both molecules was decreased as the temperature ofmeasurement increased. Interestingly, methanol showed a large ‫ܧ‬௣ compared with toluene,
which revealed that methanol permeance increased faster as the temperature increased. As
discussed in Figure 3-4(c), ο‫ܪ‬௦ for methanol would be larger than that for toluene due to alarger affinity to Ni-BTPA. Therefore, the ο‫ܧ‬ௗ for methanol should be larger than that for
toluene. Although the ο‫ܧ‬ௗ of toluene should have been larger than that of methanol undermolecular sieving, a lower ο‫ܧ‬ௗ for toluene than that for methanol suggests the permeation of
methanol occurs primarily through small pores via activated diffusion while toluene could
permeate only through a large pore via the surface diffusion process.

3.4 Conclusions
Ordered microporous nickel-doped BTPA membranes were fabricated via different nickel
loadings to create coordination with nitrogen-enriched BTPA precursors. Spectroscopic
characterization consistently showed a linear correlation between material properties andmembrane performance, where a higher nickel content generated a higher surface area of NiBTPA powders on the order of Ni-BTPA 0.50 > Ni-BTPA 0.25 > Ni-BTPA 0.125 > BTPA.
The pore size of the Ni-BTPA membranes also increased with Ni-doping with a higher gas fluxof the membrane. Afterward, a higher nickel content (1.00) tended to generate phase separation
154

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --that reduced the separation factor of the membrane, thus further confirming that the optimal
point for a nickel mole ratio is 0.50. Ni-BTPA exhibited superior performance for gas
separation (N2/SF6 > 100 – 1,000, N2 permeance > 10-7 mol m-2 s-1 Pa-1) and methanol/toluene
separation with high flux and separation factors of 2.8 kg m-2 h-1 and 907, respectively, for a
10 wt% methanol concentration in the feed solution.

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158

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Supporting Information----------SI-(3-1)----------

Fig. S3-2. Experimental single-gas permeation apparatus set-up---------------------------------------SI-(3-2)----------

Fig. S3-3. Schematic design of pervaporation apparatus------------------------------

159

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(3-3)----------

Fig. S3-3. A schematic illustration for the formation of pore structure in Ni-BTPA-derived
N
membrane via hydrolysis and condensation of BTPA precursors, then coordination with nickel ion

Fig. S3-3. shows a possible mechanism for the formation of a coordinated network on NiBTPA-derived gels. Metal-doped amine-functionalized organosilica was synthesized via a
two-step process that includes (1) hydrolysis and condensation of BTPA precursors in ethanol
with nitric acid as an acid catalyst, followed by (2) the incorporation of metal ion (Ni2+) to
produce coordination that then induces the formation of a microporous framework. In thisstudy, we assumed that all the methyl groups in the alkoxy chain were fully hydrolyzed andcondensed, which was then followed by the conversion of the silanol groups into a siloxane
network with calcination under a temperature of 250 ⁰C (confirmed with FTIR
characterization). The incorporation of transition metals such as nickel produced coordination
with the amine groups on the linking unit of the BTPA chain so that it could control the rigidityof the BTPA chain to generate a three-dimensional porous network (marked with a coloredarea in the schematic Fig. S3-3.
----------------------------------------------------------------------

160

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(3-4)---------1.00
n → π* (N-H)
d-d transition of d8 Ni(II) complex
uncoordinated Ni2+

Absorbance [a.u]

0.75

0.50

0.25

0.00
0.0

0.1

0.2

0.3

0.4

0.5

Nickel/Amine mol ratio [mol/mol]

Fig. S3-4. UV-Vis spectral changes of Ni-BTPA as a function of nickel/amine mol ratio------------------------------------------------------------------------------------------------------------------------SI-(3-5)---------Ni 2p

Ni 2s

O 1s

N 1s

Si 2p
C 1s

Si 2s

Intensity [a.u]Ni-BTPA 1

Ni-BTPA 0.25

BTPA

1000

800

600

400

200

0

Binding Energy [eV]Fig. S3-5. The wide scan energy XPS spectra of BTPA and Ni-BTPA wafer calcined at 250 ⁰C----------------------------------------------------------------------------------------------------------------

161

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(3-6)----------CH2-(a)

Si-O-Si

N=O

(b)

BTPA
Si-C

Absorbance [a.u.]

O-H

350qC
250qC

3500

3000

Absorbance [a.u.]

H-N

Si-OH

350qC
250qC
150qC

50qC

50qC

2500

2000

1500

4000

1000

Si-C

Si-OH

150qC

-CH2-

(c)

3000

2000

1000

Wavenumber [cm-1]

N=O

Si-O-Si

-CH2-

(d)

N=O

Si-O-Si

Ni-BTPA 0.5

Ni-BTPA 0.25
Si-C

H-N

Si-OH
O-H

350qC

250qC

Absorbance [a.u.]

H-N

350qC
250qC
150qC
50qC

50qC
3000

2000

Si-C

Si-OH

O-H

150qC

4000

Si-O-Si

O-H

Wavenumber [cm-1]

Absorbance [a.u]N=O

Ni-BTPA 0.125
H-N

4000

-CH2-

1000

4000

3000

2000

1000

Wavenumber [cm-1]

Wavenumber [cm-1]-1Fig. S3-6. Infrared spectra in the range of 4000-650 cm for BTPA (a), Ni-BTPA 0.125 (b),Ni-BTPA 0.25 (c), and Ni-BTPA 0.50 (d) at different calcination temperatures-----------------------------------------------------------------------------------------------------------------------SI-(3-7)---------BTPA

Ni-BTPA 0.25

Ni-BTPA 0.125

Ni-BTPA 0.50
Top-most layer + SiO2-ZrO2
α-Al2O3 particle layer

BTPA

Ni-BTPA 0.25

Ni-BTPA 0.125

Ni-BTPA 0.50

Fig. S3-7. The cross-sectional SEM images for BTPA and Ni-BTPA at different nickel content(upper: low magnification, lower: higher magnification)--------------------------------------------------------------------------------------------

162

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(3-8)----------

Fig. S3-8. TEM microphotograph of Ni-BTPA 1.00 (insert figure : Ni-BTPA selected-areadiffraction pattern with additional white crystalline ring)-------------------------------------------------------------------------------SI-(3-9)---------0.125

dVp [cc nm-1 g-1]

0.100

NLDFT method
BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50

0.075

0.050

0.025

0.000
1

2

3

4

5

Pore size [nm]Fig. S3-9. NLDFT pore size distribution of BTPA and different Ni-BTPA powder calcined at 250 ⁰C
Fig. S3-9 reveals a gradual increase in the pore size of Ni-BTPA powder with increases in the metal
ratio on the order of Ni-BTPA 0.50 > Ni-BTPA 0.25 > Ni-BTPA 0.125 > BTPA. At a high metal
concentration of 0.50, an increasing number of BTPA molecules that act as amine ligands could becoordinated with nickel and result in higher porosity.
----------------------------------------------------------------------------------------------------

163

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- -----------SI-(3-10)----------The membrane pore size of BTPA and nickel-doped BTPA were estimated by the modified-gas
translation (m-GT) model, as defined by the following equation.
ଵൗଷ൫ඥ‫ܯ‬௜ ܲ௜ ൯ா೛ǡ೔௞బൌ ൬ ோ்݁‫ ݌ݔ‬ቀെ ோ் ቁ൰ξଵൗଷ

(S.3-1)ሺ݀଴ െ ݀௜ ሻܲ௜ is the permeance of i-th gas,‫ܯ‬௜ is the molecular weight of permeating gas, ݇଴ is a constantthat depends on the structure of the membrane and is independent of any permeating gases, ‫ܧ‬௣ǡ௜
is a constant for the sake of simplicity, ሺ݀଴ െ ݀௜ ሻ is the diffusion distance instead of d0 for thei-th component, R and T correspond to gas constant and temperature, respectively. The value ofଵൗଷ൫ඥ‫ܯ‬௜ ܲ௜ ൯of different gases was plotted as a function of the molecular size of permeatinggases (݀௜ ሻ, as presented in Fig. S3-10. The intercept of molecular size at ൫ඥ‫ܯ‬௜ ܲ௜ ൯

ଵൗ
ଷ

ൌͲ

M Pi)1/3 [mol1/6 g1/6 m-2/3 s-1/3 Pa-1/3]

corresponds to the pore size of the membrane ሺ݀଴ ሻ.
0.020
BTPA
Ni-BTPA 0.125
Ni-BTPA 0.50
Ni-BTPA 0.25

0.015

0.010

0.005

0.000
2.5

3.0

3.5

4.0

4.5

5.0

5.5

Molecular size [Å]ଵΤଷFig. S3-10. Relationship between molecular size of permeating gases and ൫ඥ‫ܯ‬௜ ܲ௜ ൯ for NiBTPA derived membranes (solid curves: theoritical value based on Eq (1), symbols:
experimental result)--------------------------------------------------

164

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- -----------SI-(3-11)---------10-5

10-5
BTPA

10

10

10-7

10-7

10-8

Permeance [mol m-2 s-1 Pa-1]

Ni-BTPA 0.125

-6

10-9

-6

He
H2
CO2

10-8

CH4
N2

10-9

CF4
SF6

10-10
2.0

2.5

10-10
2.0

3.0

10-5

2.5

3.0

10-5
Ni-BTPA 0.5

Ni-BTPA 0.25
10-6

10-6

10-7

10-7

10-8

10-8

10-9

10-9

10-10
2.0

2.5

10-10
2.0

3.0

2.5

3.0

1000/T [K-1]

Fig. S3-11. Temperature dependence of gas permeance for BTPA and Ni-BTPA with different
metal ratio at the temperature range between 50⁰ to 200⁰C----------------------------------------------------------------------------------------------------------------------------------SI-(3-12)----------

Permeance [mol m-2 s-1 Pa-1]

10-4

10-5

He

Ni-BTPA 0.50

H2
CO2

10-6

N2

CH O
3 H

CH4

10-7

CH
6

10-8

5 CH
3

CF4

10-9

Single-gas permeation = 200 qC
Pervaporation (PV) = 50 qC

SF6

10-10
0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Molecular diameter (nm)Fig. S3-12. Single gas permeance as a function of molecular diameter dependency (solid line) andpervaporation performance of methanol-toluene separation for Ni-BTPA 0.50------------------------------

165

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

102
101

101

[mol m-2 s-1 Pa-1]

Permeance

102

10-6
MeOH
10-7

TolueneFlux

3[kg m-2 h-1]

[-]

[-]

Separation factor

103

Permeance ratio----------SI-(3-13)----------

MeOH

2

Ni-BTPA 1.00

1Toluene

0
0

100

200

300

400

500

Operation time [min]Fig. S3-13. Effective time courses for Ni-BTPA 1.00 pervaporation performance inmethanol/toluene (10 wt%/ 90 wt%) mixture---------------------------------------------------------------------SI-(3-14)----------

50
5
0

100
1

Ni-BTPA
Ni-BTPPA
A 0.50
0

[-]

102

50

101
100
10-6

0

[-]

10

100

20

100

50

100

30

100

40

100

50

Permeance ratio

10[mol m-2 s-1 Pa-1]

Permeance

Separation factor

100
3

100

Feed MeOH concen
concentration
ration ((wt
wt %)

MeOH

10-7

Toluene
10-8

-2

Flux

-1

[kg m h ]

3

MeOH

2
1

Toluene
0

0

100

200

300

400

500

600

700

800

900

Operation time [min]Fig. S3-14. Time course of separation performance at different methanol feed concentration
for Ni-BTPA 0.50 membrane measured at 50 ⁰C------------------------------------------------------------------------------------------

166

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(3-15)---------Feed temperature [⁰C]
50

30

500

455

50

300

40

100

[-]

102

50

101
100
10-6

10

0
MeOH

Ni-BTPA 0.50

-7

[-]

45

Permeance ratio

400

103

[mol m-2 s-1 Pa-1]

Permeance

Separation factor

30

Toluene
10-8

-2

Flux

-1

[kg m h ]

3

MeOH

2
1

Toluene
0

0

100

200

300

400

500

600

700

800

Operation time [min]Fig. S3-15 Time course of pervaporation at different temperature through Ni-BTPA 0.50
membrane at feed composition of methanol/toluene mixture of 10 wt%/90 wt%-------------------------------------------------------------------------------------------------------------SI-16----------

Permeance ratio [-]100

Inorganic membrane
PCS
TiPCS
SiO2-ZrO2
10
TEOS
Ni-BTPA
Polymeric membrane
Cellulose acetate
1
Polypyrrole
PVA
ChitosanPolyimide
0.1 Mix matrix membrane
PVA-HZSM5

0.01

10-11

10-10

0.50
Ni/NH=0

0.125

0.25
1.00

10-9

10-8

10-7

10-6

MeOH permeance [mol m-2 s-1 Pa-1]

Fig. S3-16 The trade off between methanol/toluene permeance ratio as a function of methanol
permeance for a different type of membranes.
--------------------------------------------------------------------------------------------------------------

167

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(3-17)---------Table S3-1. Textural properties for BTPA and various Ni-BTPA xerogel powder obtained at thethermal treatment of 250 ⁰C

Metal doped-BTPA(Ni/NH=mol/mol)
BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Ni-BTPA 1.00

Surface area
2 -1
(m g )
2.36
74.6
162
282
38.7

----------------------------------------

168

Pore volume
3 -1(cm g )
0.024
0.086
0.124
0.184
0.064

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Chapter 4
Structural transformation of the nickel coordination-induced
subnanoporosity of aminosilica membranes for methanolselective, high-flux pervaporation
4.1 Introduction
Interest continues to grow for the use of pervaporation as an efficient membranebased separation technology for the separation of azeotropic compounds, mixtures withclose boiling points, mixtures with similar sizes of components, and heat-sensitive
organic mixtures [1-4]. Pervaporation is considered a proficient competitive techniquecompared with conventional separation processes such as extractive distillation,
azeotropic distillation, and pressure-swing distillation, albeit with the consequence ofhigh operational demand on energy [5-9]. With pervaporation, feed liquid mixtures are
selectively transported into the vapor phase at the permeate side after vaporization
through a semi-permeable membrane via preferential sorption and diffusion processing[10]. This technique shows great performance and cost-effectiveness for the removal of
trace volatile organic components from aqueous solutions, dehydration of aqueousorganic solvents, and the separation of organic-organic mixtures [11-16].
The pervaporation of organic mixtures has been focused on polar/non-polar mixtures
such as methanol/ methyl tert-butyl ethers (MeOH/ MTBE – azeotropic mixture with aratio of 14/ 86 wt%), methanol/ dimethyl carbonate (MeOH/ DMC) – 70/30 wt%, andmethanol/ toluene (MeOH/ Tol) – 73/27 wt% [2, 17, 18]. Ethyl acetate (EA) mixed with
MeOH is commonly used in the pharmaceutical industry to produce aminothioxime acid,gastrodin, etc. However, the separation of EA and MeOH is required in order to recycle
and reduce pollution [19]. MTBE as a lead-free octane enhancer in gasoline fuel has beenindustrially produced from MeOH and isobutylene, but this additive faces a challenge inpurification due to the formation of an azeotropic mixture [20]. DMC is an importantchemical that is widely used as a gasoline additive due to an oxygen content that is higher
than that of MTBE [21]. DMC is also used as a non-aqueous solvent for lithium batteries,

169

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --as a constituent in dimethyl sulfate, and as a phosphate substituent in methylation andcarbonylation reactions [22, 23]. Toluene is another important solvent that is used
extensively for many applications – particularly in the pharmaceutical, petrochemical,
and textile industries. In the manufacturing industry, toluene is usually produced throughthe alkylation of benzene with methanol. However, the product streams contain a mixtureof methanol and toluene that must be separated [24]. In addition, MeOH forms azeotropic
mixtures during the separation process, which necessitates further methanol removal inorder to acquire a highly pure solvent.
For industrial purposes, the development of pervaporation remains a challenging task
— membrane materials with high permeance and flux that nonetheless are able tomaintain adequate selectivity. The main polymeric membrane materials being considered
include cellulose polymers, polyvinyl alcohol, chitosan, polyimide, and polyacrylonitrile[25-31]. These materials are considered to be versatile with simple fabrication methods
that translate to a reasonably low-cost investment [32]. On the other hand, polymeric
membranes additionally have issues with selective layer swelling, which significantly
reduces the stability of membranes under harsh environments [2, 10]. Several studies haveused zeolite and metal-organic frameworks (MOFs), which have exhibited chemical,
thermal, and mechanical stability for the pervaporation of alcohol/organic solvents [33].
Chen et al. [34] prepared NaY zeolite membranes via fluoride-containing precursors,which provided high flux with a good separation factor for MeOH/DMC that reached1.02 kg m-2 h-1 and 100, respectively. Msahel et al. investigated the pervaporation
performance of Fe-MOF-MW with a total flux of 0.63 kg m-2 h-1 and a separation factorof 11 for the separation of MeOH/MTBE [35]. In contrast to such high-flux performance,
zeolite and MOF membranes have shown stiff and brittle properties that can lead to theformation of intercrystalline defects during membrane fabrication, which reduces
selectivity during operation [36].
An alternative to this has been the emergence of membranes that are robust,
microporous, inorganic, and silica-based membranes. These membranes have relatively
narrow pore sizes that are adjustable and tremendous levels of performance in terms ofboth permeance and selectivity; they do not swell in organic solvents; they are able tomaintain chemical and thermal stability; and, they have an amorphous structure that gives

170

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --them flexibility [37-40]. Moriyama et al. [41] have reported promising pervaporation
performance for microporous 1,2-bis(triethoxysilyl)ethane (BTESE –CH2–CH2–)derived membranes in dehydration systems with flux as high as 1.1 kg m-2 h-1. Dong etal. [14] also reported the utilization of microporous bis(triethoxysilyl)acetylene (BTESA–C≡C–) membranes with a separation factor as high as 120 and flux as high as 4 kg m-2
h-1 in the pervaporation of MeOH/DMC (10/90 wt%) mixtures at 50 ⁰C. Currently,
microporous organosilica membranes are commonly custom-tailored via the use oforganic templates and spacers [42-44]. Ordered microporous silica is easily formed after
the removal of the organic compounds that were introduced as a template into an
inorganic matrix via thermal processing. Unfortunately, capillary stress and shrinkage areoften involved during condensation reactions, which could result in network collapse andreduction in the pore volume of affected membranes [45]. The spacer method uses a
bridged bis-silyl structure and is frequently used to modify the textural properties oforganosilica precursors. Kanezashi et al. [46] have successfully developed loose
amorphous networks by using high-permeance, defect-free BTESE. Nevertheless, theformation of microporosity has been limited to the use of short and rigid precursors,
because an increase in the carbon number between silicon atoms creates high flexibility
and has failed to develop a highly permeable network [43, 47].
More recently, we proposed a novel concept for sol formation and the following
coordination reaction via the addition of metal ions to create crosslinking units withamine-functionalized organosilica precursors. In our previous studies, we examined theeffect that metal affinity and metal mole ratios can exert on the formation of a
microporous coordinated network [11, 48]. Nickel ion is known to generate outstanding
properties of high microporosity compared with other transition metals such as silver andcopper due to the high affinity of nickel ions, which theoretically creates four
coordination numbers with amine moieties. In other experimental work, we also foundthat the microporosity was increased with increases in the nickel/amine mole ratio, which
improved the surface area of materials from 2.36 to 282 m2 g-1 [49]. The metamorphosisof dense to tailorable versions of microporous aminosilica has successfully improved thepermeability of membranes, and the amorphous phase of the organosilica structure is
responsible for maintaining the high selectivity of the membranes due to flexibility.
Anggarini et al. [50] report that another attractive feature of this method is the evolution
171

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --of the use of metals in two different phases — as ions coordinated with amine moieties
and as nanoparticles that confer both high microporosity and facilitate the transport ofspecific components. A membrane structure that could selectively separate methanol fromorganic solvents would be desirable for industrial applications, particularly one that
preserves the high flux of methanol. A previous investigation by our group showed thatthe balance between the microporosity induced by amine-nickel coordination and theuniform distribution of an excessive amount of nickel ions in nickel-doped bis [3(trimethoxysilyl) propyl] amine (Ni-BTPA) facilitates flux that reaches as high as of 2.8
kg m-2 h-1 with a MeOH/Tol separation factor as high as 900 at 50 ⁰C [49].
In accordance with previous findings based on one type of aminosilica (BTPA), thepresent study provides new insights for predicting the construction of microporous nickel
coordinated aminosilica membranes via different types of amine precursors. Herein, we
discuss three types of nickel-doped aminosilica precursors that were synthesized frombis[3-(trimethoxysilyl) propyl] amine (BTPA-bridge-type/secondary amine), trimethoxy[3-(methylamino) propyl] silane (MAPTS-pendant-type/ secondary amine), and 3-amino
propyl triethoxy silane (APTES-pendant-type/ primary amine). The resultant materials
were systematically studied for their effect on the formation of microporosity, and thestructural phases of nickel were studied for their effect on the specific transport ofmethanol and on selective pervaporation under different types of solvents.

4.2Experimental
4.2.1 Material synthesis: aminosilica and nickel-modified aminosilica
sols
The set of pristine aminosilica precursors used in this experimental work consistedof bridge and pendant amines: bis [3-(trimethoxysilyl) propyl] amine (BTPA), trimethoxy[3-(methylamino) propyl] silane (MAPTS), and 3-amino propyl triethoxy silane(APTES). The chemical structures of these three types of aminosilica precursors appear
in Fig. 4-1. Briefly, the sols of aminosilica precursors were synthesized via hydrolysis
and condensation to achieve molar ratios of precursor/H2O/HNO3 of 1/300/1 and 1/150/1

172

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --for bridge and pendant types of precursors, respectively. Ethanol (C2H5OH) was used as
the solvent to adjust the concentration of precursors to 5 wt%, and then the hydrolysis
and condensation reactions were performed under vigorous stirring (550 rpm) at room
temperature for 12 h.
Another set of nickel-doped samples was obtained via the addition of nickel nitrate(Ni(NO3)2۰6H2O) as the metal dopant into aminosilica sols followed by a coordination
reaction. The coordination reaction was facilitated via the addition of a stoichiometricamount of nickel-metal salts dissolved into the aminosilica precursors in nickel/amine(Ni/NH) mole ratios of 0, 0.25, and 0.5 mol mol-1, followed by stirring for 1 h at ambient
temperature. The obtained nickel-doped aminosilica samples are referenced according to
the different amounts of nickel ions: Ni-BTPA 0.25, Ni-BTPA 0.50, Ni-MAPTS 0.25,
Ni-MAPTS 0.50, Ni-APTES 0.25, and Ni-APTES 0.50. For the membrane preparation,
the resultant nickel-doped aminosilica composite materials were diluted by 0.1 wt% using
pure water.
(a)

BTPA
BT

2⁰

(b)

MAPTS
M

2⁰

bis [3-(trimethoxysilyl) propyl] amine

Trimethoxy[3- (MethylAmino) Propyl] Silane

Secondary amine-Bridge type

Secondary amine-Pendant type

(c)

APTES
APT

1⁰

3-Aminopropyltriethoxysilane

Primary amine-Pendant typeFig. 4-1. The chemical structure of amine-functionalized alkoxysilanes utilized in this work:
BTPA (a), MAPTS (b), and APTES (c).

4.2.2 Characterization of nickel-doped aminosilica films and xerogels
Both pristine and nickel-doped aminosilica gel powders were prepared via a slow
solvent evaporation process at 50 ⁰C under an air atmosphere [47]. The chemical structureof nickel-modified aminosilica was examined using Fourier Transform Infrared
Spectroscopy (FT/IR-4100, JASCO, Japan) over a wavenumber range of from 500-4,000
cm-1. The nickel-doped aminosilica-derived films were coated onto a potassium bromide(KBr) plate and then calcined at 250, 300, and 350 ⁰C under a N2 flow for 30 min to assess
the thermal stability of amine moieties as the active site for coordination. The formationof the coordinated compound was verified using a UV-Vis diffuse reflectancespectrophotometer (Shimadzu UV-3600 plus) in a range of 200-1,000 nm. The thermal
properties of the nickel-modified aminosilica powder were determined using athermogravimetric mass spectrometer (TG-MS, TGA-DTA-PIMS 410/S, Rigaku, Japan).
173

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --A wide-angle powder D2 PHASER X-ray diffractometer (Bruker, Germany) that used Cu
Kα radiation in a 2θ range of from 5-80⁰ was used to evaluate the structuraltransformations of all composites, which included amorphous and crystalline structuresascribed to organosilica and uncoordinated nickel particles, respectively. The formationof nickel nanoparticles was confirmed using a transmission electron microscope (TEM
JEOL 2010 Ltd., Japan). Textural characterization was performed using N2 sorption
isotherms at -196 ⁰C and CO2 adsorption-desorption isotherms at 25 ⁰C in a BELMAX(BELJAPAN Inc.). The specific surface area was calculated based on the BrunauerEmmett-Teller (BET) technique in a P/P0 range of from 0.05-0.20. All samples werepretreated with heating at 200 ⁰C under vacuum conditions for 12 hours prior to theadsorption-desorption measurement.

4.2.3 Fabrication of nickel-doped aminosilica membranes and single-gas
permeation measurement
Porous α-alumina tubes with 50% porosity and average pore size of 1 μm (length,
100 mm; inner diameter, 8 mm; outer diameter, 10 mm) were used as membrane supports,
as supplied by the Nikkato Corporation (Japan) [51]. The α-alumina tubes were connected
to non-porous alumina (Al2O3) tubes before membrane preparation with glass joints via
a heat treatment process at 950 ⁰C under an air atmosphere [52]. The connection process
was repeated two or three times to prevent gas leakage. α-Al2O3 particles with average
particle sizes of 2 and 0.2 μm were dispersed into a silica-zirconia (SiO2-ZrO2) sol as abinder with a solid concentration of 10 wt% and were subsequently coated onto the outersurface of a porous α- Al2O3 support followed by calcination at 550 ⁰C under an air
atmosphere for 15 min. These procedures were repeated three times for each type ofparticle to smooth the surface of the membrane. The intermediate layer consisted of a 0.5
wt% SiO2-ZrO2 sol coated onto the surface of the particle layer with firing under an air
atmosphere at 550 ⁰C. These procedures were repeated several times until the averagepore size of the support reached approximately 1 nm. Finally, the topmost layer was
obtained by double coating 0.1 wt% nickel-doped aminosilica sols (Ni-BTPA, NiMAPTS, and Ni-APTES) onto the intermediate layer followed by calcination at 250-350
174Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --⁰C under a flow of N2 for 30-60 min. The nickel-doped aminosilica-derived membranes
are referenced by their calcination temperatures (250, 300, and 350 ⁰C under a nitrogen(N2) atmosphere): Ni-BTPA250, Ni-BTPA300, Ni-BTPA350, Ni-MAPTS250, NiMAPTS300, Ni-MAPTS350, Ni-APTES250, Ni-APTES300, and Ni-APTES350.
Single-gas

permeation

experiments

were

conductedusing

high-purity

components (He, H2, CO2, N2, CH4, CF4, and SF6) under conventional constant pressure
at 50-200 ⁰C. Each permeation gas was fed to the outside of the membrane with an
upstream pressure of 200 – 400 kPa. The pressure in the permeate was maintained at
atmospheric levels, and the flow rate of the permeate was measured using a film flow
meter (Horiba, Co. Ltd., Japan). The permeation measurement was recorded after themembrane reached a steady-state with respect to the pressure difference between the shelland the inner side of the membrane and flux. The permeate flow rate was evaluated witherrors of less than 5 and 20% for permeance values larger than 10-7 and smaller than 10-7
mol m-2 s-1 Pa-1, respectively. The permeance of each gas (ܲ௜ ) and the ideal selectivity forthe ݅ െ ‫ ݄ݐ‬and ݆ െ ‫ ݄ݐ‬components (ߙ௜Ȁ௝ ) were calculated using the following equations.
ܲ௜ ൌ

‫ܨ‬௜
ൗ‫ܣ‬ο‫݌‬
௜
௉

(4-1)
(4-2)

ߙ௜Ȁ௝ ൌ  ೔௉ೕwhere ܲ௜ is the permeance of the ݅ െ ‫ ݄ݐ‬component (mol m-2 s-1 Pa-1), ‫ܨ‬௜ is the permeate
molar flow rate of the ݅ ௧௛ component (mol s-1), A is the effective cross-sectional area ofthe membrane (m2), and ο‫݌‬௜ is the partial pressure difference of component ݅ between thefeed and the permeate of the membrane (Pa).

4.2.4 Pervaporation (PV) measurement
The separations of 7 types of binary organic mixtures containing methanol(MeOH) (10 wt%) in the feed solution were conducted via the pervaporation process at
50 ⁰C. A schematic diagram of the pervaporation (PV) apparatus appears in the
Supporting Information, Fig. S4-2. The physical properties of methanol and the 7 typesof organic solvents are summarized in Table 4-1. The mixtures of methanol and organic
solvents were circulated using a stirrer at 3,000 rpm to prevent concentration polarizationon the surface of the membrane. The pressure of the feed side was maintained at

175

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --atmospheric levels, while the permeate side was evacuated to 0 – 2 kPa (absolute) via theuse of a vacuum pump. The permeate vapor was collected using a glass trap cooled in a
dewar vessel with liquid nitrogen for each time interval. The concentration of the feedsolution was maintained at a constant level during the measurement via the periodical
addition of appropriate amounts of pure methanol.
Table 4-1. Physical properties of the selected organic solvents
Penetrant(abbreviation)MW-1[g mol ]Molecular size[nm]

Density[g cm-3]

Boiling
point [⁰C]

Hansensolubility
1/2[MPa ]

Methanol(MeOH)

32.04

0.38 [53]

0.791

64.7

29.6

Ethanol(EtOH)

46.07

0.43 [53]

0.789

78.37

26.5

Isopropyl
alcohol (IPA)

60.11

0.47 [53]

0.786

82.6

23.6

Tertiary
butanol (TertBuOH)

74.12

0.51 [53]

0.788

82.0

14.3

Ethyl acetate
(EA)

88.12

0.52 [53]

0.895

77.2

18.2

Dimethylcarbonate
(DMC)

90.07

0.53 [54]

1.069

90.0

18.7

Isopropyl
acetate(i-Pac)102.1

0.57 [55]

0.872

89.0

17.6

Toluene (Tol)92.14

0.59 [53]

0.867

110.6

18.0

The compositions of the feed and permeate streams were determined via gas
chromatography (GC-14B Shimadzu Cor., Japan). Membrane flux (‫ )ܬ‬and the flux of eachcomponent ( ‫ܬ‬௜ ) on the permeate side are described by equations (4-3) and (4-4),
respectively.
‫ ܬ‬ൌ ஺ൈ௧

௪

(4-3)

‫ܬ‬௜ ൌ ‫ ܬ‬ൈ  ‫ݕ‬௪ǡ௜

(4-4)

176

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --In equations (4-3) and (4-4), ‫ ܬ‬is the total permeate flux (kg m-2 h-1), ‫ ݓ‬is the permeate
weight (kg), ‫ ܣ‬is the membrane surface area (m2), ‫ ݐ‬is the permeating time (h), and ‫ݕ‬௪ǡ௜
is the weight fraction of component ݅ in the permeate. The permeance of component ݅
could be calculated from the molar flux (‫ܬ‬ெ௜ ) of the ݅ component divided by the differencein the partial pressures of the feed (‫݌‬௜ǡ௙ ) and permeate streams (‫݌‬௜ǡ௣ ), as determined using
equation (4-5).
‫ܬ‬ܲ௜ ൌ ெ௜൘ሺ‫݌‬௜ǡ௙ െ ‫݌‬௜ǡ௣ ሻ(4-5)

The feed partial pressure of the ݅th component was calculated as ‫݌‬௜ǡ௙ ൌ ߛ௜ ‫ݔ‬௜ ‫݌‬௜଴ , where ߛ௜is the activity coefficient according to the Wilson parameters, ‫ݔ‬௜ is the mole fraction ofthe ݅th component, and ‫݌‬௜଴ is the vapor pressure according to the Antoine equation. On
the other hand, the pressure in the permeate stream was presumed to be zero. The
separation factor is defined as the ratios of the mole fractions in the permeate(‫ݕ‬௜ ܽ݊݀‫ݕ‬௝ ሻand feed (‫ݔ‬௜ ܽ݊݀‫ݔ‬௝ ሻstreams, as shown by equation (4-6).
௬

൬ ೔൰
௬
ߙ ൌ ೕ ൙ ௫൬௫ ೔ ൰

(4-6)

ೕ

4.3 Results and discussion
4.3.1 The three-dimensional micropore network assembly of nickelaminosilica via coordinated sol-gel reaction
The reactivity of different types of aminosilica precursors that developed via thenickel-amine coordination was examined using the nickel dopant as the corresponding
metal connector at a nickel/amine (Ni/NH) mole ratio of 0.25 mol mol-1 with calcination
at 250 ⁰C. The UV-vis diffuse reflectance spectra of pristine materials and nickel-doped
aminosilica xerogel powders prepared with different types of amine precursors appears
in Fig. 4-2(a) along with the FT-IR of both materials.

177

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(a)Absorbance [-]2.0

Ni-BTPA
Ni-B

Ni-MAPTS

Ni(NO3)26H2O
BTPA
Ni-BTPA

1.5

Ni-APTES

Ni(NO3)26H2O
MAPTS
Ni-MAPTS

Ni(NO3)26H2O
APTES
Ni-APTES

1.0

0.5

0.0

200

400

600

1000 200800(b)

Si-O-Si

Tc = 250 qC

400

600

800

1000 200

400

Wavelength [nm]Wavelength [nm]Tc = 250 qC

600

800

1000

Wavelength [nm]Si-O-Si

Tc = 250 qC
Si-O-Si-CH2H-NO-H-CH2O-H

BTPA

H-N

Ni-APTES

Ni-MAPTS

Ni-BTPA

Si-OH

H-N

Si-OH

-CH2O-H

Si-OH

Absorbance [-]

N=O

APTES

MAPTS

N=O
N=O

4000

3000

2000

1000 4000

Wavenumber [cm-1]

3000

2000

1000 4000

Wavenumber [cm-1]

3000

2000

1000

Wavenumber [cm-1]

Fig. 4-2. UV-Vis diffuse reflectance absorption spectra (a), and wide range FT-IR spectra (b) for
pristine aminosilica (BTPA, MAPTS, APTES) and the resultant nickel-doped aminosilica (NiBTPA, Ni-MAPTS, Ni-APTES) xerogel powders obtained at calcination temperature of 250 ⁰C.

We observed the appearance of two absorption peaks that were detected at wavelengths
of approximately 200 (n ՜  ߪ ‫ ) כ‬and 300 nm (n ՜  ߨ ‫ ) כ‬for all three aminosilica xerogel
powders, which corresponds to the intraligand free-electron transition of amine groups(N-H) and the electron transfer between nitrate ions (N=O), respectively [48]. With theaddition of nickel ions at a Ni/NH mole ratio of 0.25 mol mol-1, the color of the mixture
turned to yellow, bright green, and dark green for Ni-BTPA, Ni-MAPTS, and Ni-APTES,
respectively. The n ՜ ߪ ‫ כ‬electronic transition was shifted to a higher wavelength,
confirming the ligand-to-metal charge transfer (LMCT) transition, which further
indicated the formation of a stable coordination of amine groups as a ligand to the nickelmetal ions with lower energy levels [56]. In addition, significant changes in the spectrumof nickel-doped aminosilica composites were discovered in the visible range with theappearance of two new bands, which are summarized in Table 4-2. Similar to thoseprevious reports, the formation of nickel-amine coordination was confirmed via the d-d178

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --transition and further demonstrated characteristics of the tetrahedral environment nearby
nickel ions [56, 57]. These results are consistent with our previous findings that nickel
ions can be coordinated with four amines in an organosilica solution [49].
The characteristics of the FT-IR peaks were examined in terms of the chemicalbonding environment, and the shift of peaks was an indication of the establishment of
coordination, as shown in Fig. 4-2(b). All samples showed intense absorption peaks at
1,000-1,100 cm-1, which is attributed to the vibration of the siloxane network (Si-O-Si),accompanied by an amine (N-H) deformation band at approximately 1,600 cm-1.
Table 4-2. The spectroscopy characterization results of UV-vis diffuse reflectance, FTIR, and powder XRD spectra
Periodic chain distancePeak transitionMembraneν N-H [cm-1]b
position [2θ/degree]cvisible range [nm]a
BTPA

-

1588

9.0

Ni-BTPA
MAPTS

391, 704-

1583 and 1623
1626

6.5
7.1

Ni-MAPTSAPTES
Ni-APTES

406, 735
554, 925

1622
1649
1593

6.8
7.1
5.8

Values in parentheses are collected from UV-vis a, FT-IR b, XRD c characterization.

A comparison of each nickel-doped amino composite shows that the relative intensity of
their Si-O-Si bonds is similar, while the intensity and wavenumber position of the aminedeformation band showed an increased absorbance that featured a red-shift, which islisted in Table 4-2. Wang et al. [58] reported a strong shift for the secondary amine bondafter the adsorption of metal ions, which indicated a robust coordination interaction
between both components. Behaviors such as intensification and red-shifting verify theincorporation of nickel ions that act as lewis-acids that are capable of inducing theformation of coordination bonds with amine groups as lewis-base molecules [48, 49].
Fig. 4-3(a) shows the X-ray diffraction patterns of pristine aminosilica xerogel
powder and nickel-composite materials calcined at 250 ⁰C. The details of the peak
positions are tabulated in Table 2. All three aminosilica compounds show two amorphousdiffractions patterns centered at 2θ of 7-9⁰ (peak A) and 20-22⁰ (peak B), which reflects
the distance of intermolecular periodic linking unit chains in polysisesquioxane and theaverage spacing in the siloxane framework, respectively (schematic images as shown in
Fig. S4-3) [59]. After doping with nickel ions, a distortion into a lower degree of 2θ was179

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --generated around the intermolecular periodic chain consisting of an amine moiety. Insteadof that, Ni-MAPTS showed only small structural changes at 2θ of 7⁰. This finding
suggests that the formation of partial nickel-amine coordination on Ni-MAPTS was dueto the existence of non-hydrolyzable methyl groups subsequent to the amine functionalgroups at the calcination temperature of 250 ⁰C. The methyl substituent in the pendant
structure of the secondary amine structure of MAPTS increased the steric hindrance forthe coordination between nickel ions and amine groups, which consequently prevented
the construction of a coordinated framework. In accordance with the present results,
previous studies demonstrated that the methylated-pendant chain of organosilica
precursors was able to aggregate within the silica network and produced a relatively large
volume of steric hindrance, which thereby enforced the nickel-metal center to move awayfrom amine ligands, and resulted in a weakened affinity for coordination [42, 60, 61].

(a)Intensity [a.u](b)A

Tc = 250 qC

Tc = 250 qC

Tc = 250 qC

B

iNi

Ni-BTPA

Ni-MAPTS

BTPA

MAPTS

i

Ni-APTES

APTES

10 20 30 40 50 60 70 80

10 20 30 40 50 60 70 80

10 20 30 40 50 60 70 80

2Tq [degree]

2Tq [degree]

2Tq [degree]

Ni-BTPA

Ni-MAPTS

Ni-APTES

Fig. 4-3. The XRD petterns
ttffor aminosilica
ili (BTPA,(BTPA MAPTS,MAAPTES) andd nickel-doped
i k laminosilica (Ni-BTPA, Ni-MAPTS, Ni-APTES) xerogel powders (a), and representative TEM
images for nickel-composites material (Ni-BTPA, Ni-MAPTS, Ni-APTES) calcined at 250 ⁰C

180

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The coordination of APTES with nickel exclusively resulted in a moderate affinity
because hydrogen substitution on the primary alkyl-amines structure reduced the sterichindrance of the metal center for the approach of ligand precursors, while the basicity of
APTES was lower than that of the secondary amines in both bridge and pendant
structures. Yu et al. [62] also reported a higher level of basicity secondary aminosilica
precursors compared with the primary amine structure due to the balance between theelectronic donating effect of alkyl groups, steric hindrance, and a solvation effect in the
aqueous phase. Hence, the moderate affinity of APTES is likely to induce a shifting of
the X-ray diffraction peak at 2θ from 7.1⁰ to 5.8⁰ while simultaneously generating theformation of nickel nanoparticles.
Moreover, the second diffraction peak at 2θ of 20-22⁰ showed a slight shift to ahigher degree for Ni-BTPA and Ni-APTES, while the opposite trend was observed for
Ni-TMAPS. The shifting of correlated peak positions corresponds to a change in dspacing values that reflect the estimated network distance of materials [63-65]. The peakshifts to a higher degree of 2θ for Ni-BTPA and Ni-APTES are likely to be related to adecrease in the interplane distance of the siloxane network (Si-O-Si), which was
confirmed by the peak broadening after doping with nickel ions. In contrast with NiBTPA and Ni-APTES, however, Ni-MAPTS showed a slight shift to a lower degree of
2θ, which meant that a larger level of steric hindrance from the non-hydrolyzable methylgroups had slightly enlarged the Si-O-Si network and restricted the nickel ion to modify
the framework via coordination.
In order to confirm the role of nickel-dopant in different types of aminosilica
precursors, a TEM microphotograph was examined for three different samples of nickelcomposites, as presented in Fig. 4-3(b). As the TEM images show, the nickel-dopant was
fully incorporated within the bridge structure of the BTPA matrix, which resulted in afiner dispersion of the dopant. Conversely, the incorporation of nickel into the pendant
structures of MAPTS and APTES shows an increased amount of metal dispersion (darker
colors and round shapes), which supports the evidence in the XRD results that nickel
nanoparticle reflection peaks increase with lower basicity and from an affinity for amine
precursors. The amine surface of different types of precursors represents an importantrole in controlling the growth and dispersion of metal via catalyzing partially coordinated181Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --networks and in the reduction of metal nitrate into well-dispersed nanoparticles, asreported by Anggarini et al. [66].
Fig. 4-4 shows the N2 and CO2 adsorption-desorption isotherms of bareaminosilica and nickel-doped aminosilica samples, and the corresponding texturalproperties are summarized in Table 3. All aminosilica samples (Fig. 4-4(a))-left side Fig.)
exhibited low surface area and pore volume, which suggests extremely dense materials.
The high flexibility of the organic chain incontrovertibly occupies the siloxane network,
thus reducing porosity [39, 47]. The doping of nickel ions into different types ofaminosilica precursors clearly shows the significant enhancement of both the surface area
and the pore volume (Fig. 4-4(b)-right side Fig.), with the exception of the Ni-MAPTS

(b)

200

150

BTPA
Tc = 250 qC
MAPTS
APTES
Closed: adsorption
Open: desorption

Ni-BTPA
Ni-MAPTS Tc = 250 qC
Ni-APTESClosed: adsorption
Open: desorption
Ni/NH = 0.25 mol mol-1

100

50

CO2 Absorbed volume [cmm3 g-1](a)Absorbed volume [cm3 g-1]

composite.

00.0

negligible N2
adsorption amount
0.2

0.4

0.6

0.8

1.0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 [-]

P/P0 [-]
25
20

BTPA
Tc = 250 qC
MAPTS
APTES
Closed: adsorption
Open: desorption

15

Ni-BTPA
Ni-MAPTS Tc = 250 qC
Ni-APTESClosed: adsorption
Open: desorption
Ni/NH = 0.25 mol mol-1

10
5
0
0

20

40

60

80

100 0

20

40

60

80

100

P [kPa]P [kPa]Fig. 4-4. N2 sorption isotherms at -196 ⁰C (a) and CO2 uptake isotherms at 25 ⁰C (b) of pristineaminosilica powders (left-side Fig.: BTPA, MAPTS, APTES) and Nickel-doped aminosilica
materials (right-side Fig.: Ni-BTPA, Ni-MAPTS, Ni-APTES) calcined at 250 ⁰C.

182

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Anggarini et al. [48] discovered that microporosity could be controlled via thecoordination affinity between a metal-dopant and an amine ligand, where a higher levelof affinity enables an increased number of cross-linking interactions and an arrangement
into a three-dimensional framework.
Table 4-3. Textural properties of pristine aminosilica and modified nickel-doped samples
obtained in this study
Pore volume
CO2 sorption
Membrane
SBET [m2 g-1]a[cm3 g-1]acapacity [cm3 g-1]b
BTPA

2.36

0.024

16.7

MAPTS
APTES

0.49
0.18

0.007
0.006

13.8
3.23

Ni-BTPA 0.25
Ni-MAPTS 0.25
Ni-APTES 0.25

162
1.18
173

0.124
0.026
0.271

8.14
3.91
7.53

a
b

Values in parentheses are calculated based on the results of N2 sorption isotherms
Values in parentheses are CO2 sorption capacities at 25 ⁰C and 100 kPa.

Moreover, to examine the Lewis basicity strength of the relevant aminosilica
precursors, CO2 sorption isotherms were measured, as shown in Fig. 4-4(b). Based on
sorption behavior, the CO2 capacity followed the order of BTPA > MAPTS > APTES,
which suggested the order of affinity for aminosilica materials. BTPA as a secondaryamine precursor with a bridge structure conferred the highest Lewis-base nature (thehighest CO2 capacity with pronounced hysteresis loop), which allowed strong
coordination with nickel ions, and resulted in a high surface area of 162 m2 g-1 with a porevolume of 0.124 cm3 g-1. The development of the microporosity of Ni-MAPTS was
influenced by the steric hindrance of non-hydrolyzable methyl groups and by the higher
flexibility of the pendant-organic chain compared with that of the bridge type. Instead,
Ni-APTES exhibited a high surface area (173 m2 g-1) and pore volume (0.271 cm3 g-1)that compared favorably with that of Ni-BTPA due to the unhindered pendant structureof the primary amine that allowed easy coordination with nickel ions. However, thelowest basicity properties of APTES (the lowest level of CO2 sorption, as shown in Fig.
4(b)) indicates that the increases in pore volume and surface area following the
introduction of nickel-dopant were due to the partially coordinated network and to theestablishment of nanoparticles.

183

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Compared with pristine materials, Ni-BTPA and Ni-APTES (Fig. 4-4(b)-rightside Fig.) show a lower capacity for CO2 with a reversible sorption trend and no saturation
plateaus, which reflects the amine saturation level as an active site for the adsorption of
CO2. These results prove that most nickel ions are covalently coordinated with the amine
ligands of BTPA and APTES precursors. On the other hand, Ni-MAPTS showed thelowest CO2 sorption capacity with a small hysteresis loop, confirming that the pendant
structure of the secondary amine precursor provided only partial coordination due to thesteric hindrance of non-hydrolyzable methyl groups, which markedly reduced thereactivity. Pitteri et al. [67] found that steric hindrance by alkyl-amines promotes aretardation effect that limits the free-rotation movement around metal-amine bonds.
Taken together, these findings suggest the roles of basicity, flexibility, and the sterichindrance of aminosilica precursors in promoting the formation of coordinatedcompounds in the following order: Ni-BTPA > Ni-APTES > Ni-MAPTS.

4.3.2 Structural transformation, textural properties, and porosity
induced via nickel-amine coordination: The effect of calcination
temperature and nickel mole ratio
Microporous networks were formed via the coordination between nickel ions andthe amines of the organosilica precursors. In previous studies, we established the criticaltemperature required for the metamorphosis of dense structures into microporous
versions. This temperature is also restricted to the decomposition temperature of aminesas organic linkers [50]. To investigate the transformation together with the thermal
stabilities of nickel-doped aminosilica membranes, FT-IR characterization was
performed, and the results are summarized in Fig. 4-5. The absorbance of the amine
deformation band for pristine materials (Fig. 4-5(a)) was observed at calcination
temperatures as high as 300 ⁰C, but the band almost disappeared after calcination at 350
⁰C. This result indicates the decomposition of amine groups could be minimized at alower calcination temperature of 300 ⁰C, while higher temperatures accelerated thecomplete degradation.

184

250 qC

1700
1600
Wavenumber [cm-1]

1500 1800

Absorbance [a.u.]

500 qC
350 qC
300 qC
250 qC

1700
1600
Wavenumber [cm-1]

300 qC
250 qC

Ni-BTPA

1800

350 qC

1500

1700
1600
Wavenumber [cm-1]

1500

Ni-MAPTS
500 qC

350 qC

300 qC
250 qC

1800

1700
1600
Wavenumber [cm-1]

1500

Absorbance [a.u.]

300 qC

MAPTS

500 qC

500 qC

APTES

350 qC
300 qC
250 qC

1800

Absorbance [a.u.]

350 qC

1800

(b)

BTPA

Absorbance [a.u.]

500 qC

Absorbance [a.u.](a)Absorbance [a.u.]
a.u.]

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

1700
1600
Wavenumber [cm-1]

1500

Ni-APTES

1800

500 qC
350 qC
300 qC
250 qC

1700
1600
Wavenumber [cm-1]

1500

Fig. 4-5. The narrow FT-IR spectra for pristine aminosilica films of BTPA, MAPTS, APTES(a) and Ni-BTPA 0.25, Ni-MAPTS 0.25, and Ni-APTES 0.25 (b) at different calcination
temperature

According to the narrow FT-IR spectra (Fig. 4-5(b)) for Ni-BTPA, Ni-MAPTS, and NiAPTES, moderate absorption peaks from the NH deformation band of amine moieties
were observed at a pyrolysis temperature of 250 ⁰C, while at higher calcination
temperatures the peak intensity was gradually decreased until complete thermal
degradation of amine groups was observed at 350 ⁰C.
In line with FT-IR characterizations, diffractograms of Ni-BTPA (Fig. 4-6.-leftside Fig.) and Ni-MAPTS (Fig. 4-6.-middle side Fig.) showed amorphous phases at 250
⁰C, and the formation of nickel nanoparticles was observed when the calcination
temperature was increased to 300 ⁰C. By contrast, the diffractogram of Ni-APTES (Fig.
4-6-right side Fig.) shows the formation of nickel nanoparticles at 250 ⁰C, which indicateslower stability of the metal-dopant to withstand the ion phase, which resulted in theformation of phase separation. Anggarini et al. [48] reported that a high degree of
coordination affinity induced the formation of a stable compound in the metal-ion state,while weak coordination led to the partial incorporation of metal into the aminosilica
network. Calcination at 350 ⁰C caused a complete decomposition of the amine groups,
which resulted in a reduction of nickel-amine coordination stability followed by theformation of nanoparticles.
185

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Intensity [a.u]Ni-BTPA

Ni-MAPTS

Ni-APTES

i 350 qCii

i

300 qC
250 qC
250 qC

350 qC
300 qC
250 qCi Ni

i i
350 qCi i
i i

300 qC
250 qC

250 qC

250 qC
APTES
10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80

BTPA

2Tq [degree]

MAPTS

2Tq [degree]

2Tq [degree]

Fig. 4-6. Wide angle XRD patterns for Ni-BTPA 0.25, Ni-MAPTS 0.25, and Ni-APTES 0.25
at different calcination temperature

N2 adsorption isotherms for nickel composites were performed to analyze theirtransformation and to determine the appropriate temperature for the conversion of the
microporous network. As shown in Fig. 4-7, the nickel loading at higher calcination
temperatures has an influence on increasing the N2 sorption capacity for Ni-BTPA and
Ni-MAPTS, while a slight decrease was observed for Ni-APTES (the pore size
distribution for related materials, as shown in SI-(4-4), Fig. S4-4). During thermal
treatment above 250 ⁰C, the Ni-BTPA composite showed an increase in N2 adsorption at
low relative pressure due to enhanced coordination, and at high relative pressure due toan increased number of mesopores that likely were formed via nanoparticles. That result
further indicates higher calcination temperatures encourage the formation of bimodal pore
structures of intraparticle and interparticle pores that originated from the coordinatedorganosilica network and the void spaces between packed nickel particles, respectively,
as was confirmed by XRD. Further examination of Ni-MAPTS suggested thatmicroporosity formed at the calcination temperature of 300 ⁰C, rather than at 250 ⁰C.
According to the thermogravimetric analysis curve and the differential
thermogravimetric profile of Ni-MAPTS (Fig. 4-7(b)), the sample showed weight loss
between 300 and 350 ⁰C due to the partial decomposition of the propylamine chain (basedon the mass signal at m/z of 30 – Fig. 4-7(c)), and the non-hydrolyzable methyl groups(m/z = 15 – Fig. 4-7(c)).

186

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

200

150

Intensity [a.u]Ni-APTES

Ni-MAPTS

Ni-BTPA
350 qC
300 qC100

250 qC

250 qC
350 qC5000.0

300 qC

300 qC
250 qC
0.2

0.4

0.6

0.8

1.00.0

0.2

0.4

0.6

350 qC0.8

1.0 0.0

0.2

P/P0 [-]

P/P0 [-]
0

Intensity [10-10 A]

50

200

600

30

m/z = 30

10m/z = 15

1.0

800

1000

Methyl groups
150

(b)

75

100

50

50

25

0

0

0.8

Propyl amine

20

0

0.6

P/P0 [-]

(c)

40

0
100

Weight [%]

400

0.4

200

400

600

800

Heat flow [PV]

(a)

-50

1000

Temperature [qC]

Fig. 4-7. The nitrogen adsorption-desorption isotherms (at -196 ⁰C) of Ni-BTPA 0.25 (left-side
figure), Ni-MAPTS 0.25 (middle-side figure), and Ni-APTES 0.25 (right-side figure) calcinedat different temperature (a), TGA-DTA curve of Ni-MAPTS composite in helium with a heating-1

rate 10 ⁰C min (b), and the mass signal of methyl groups (m/z = 15) together with propyl amine(m/z = 30) as a function of thermal treatment (c).

Therefore, after the progressive removal of methyl groups (at 300 ⁰C) from the MAPTS
structure, the reduction in steric hindrance allowed nickel ions to freely coordinate withamine, which yielded a higher level of network connectivity and surface area. In contrast
to secondary amine precursors, however, Ni-APTES with primary amine fragments
display a decrease in N2 adsorption capacity at higher calcination temperatures. The
viability of primary amines promoted a rapid degradation of organic linkers due to their
weaker basicity and lack of steric hindrance. This finding corroborates the FT-IR (Fig. 45) and XRD (Fig. 4-6) characterization that showed complete degradation of aminegroups and the formation of nickel particles at 300 ⁰C. Zeleňák et al. [68] found that the187

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --basicity of secondary amines is relatively higher than that of primary amines. Thisdifference was influenced by the steric constraints, number of amine centers, and amine
surface density. In general, therefore, the optimized calcination temperature for nickeldoped aminosilica membranes was determined to be 250 ⁰C for Ni-BTPA and Ni-APTES,
whereas a higher calcination temperature of 300 ⁰C was required for Ni-MAPTS.
According to our previous study evaluating the effect of the mole ratio of nickel,
the incorporation of different amounts of metal ions increased the specific surface areawith high pore volume [49]. Interestingly, the existence of metal in two-phase structures
as ions coordinated with amines and nanoparticles promoted a high level of flux with asuperior separation factor. Therefore, in this section, the effect of the metal ratio on the
different types of aminosilica precursors was discussed. The powder XRD patterns ofnickel-doped aminosilica composites prepared at various Ni/NH mole ratios are depicted
in Fig. 4-8(a) along with the diffractions of pristine materials. The diffractograms of
coordinated compounds were characterized by a shifting in the first peak around 2θ of 7⁰
into a lower degree on the order of Ni-BTPA > Ni-APTES >>Ni-MAPTS, whichcorresponds to the change in the periodic linking unit distance [48, 49, 66]. As expected,
the first peak shifted significantly with an increase in the nickel concentration due to astrong coordination level for all nickel composites. However, at a higher nickel mole ratioof 0.50, an observable amount of Ni nanoparticles was generated, particularly for NiAPTES, which clearly showed the high intensity of metal segregation. In addition,
another part of diffraction at a 2θ of 20⁰ showed a broader peak compared with that ofpure materials, which suggested that the randomly oriented amorphous structure of theorganosilica network could be maintained.

188

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(a)Ni-BTPA
Ni-BT

Ni-MAPTS

Intensity [a.u]TC=250 ⁰Cii Ni

Ni-APTES

TC=250 ⁰Ci

TC=250 ⁰C

0.50

0.50

0.25

0.25

ii

i0.50

i i
0.25

0.00

0.00
APTES
10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80
0.00

BTPA

Absorbed volume [cm3(STP) g-1]

(b)

MAPTS

2Tq [degree]

2Tq [degree]

2Tq [degree]

200

Ni-APTES

Ni-MAPTS

Ni-BTPA150

TC=250 ⁰C

TC=250 ⁰C

TC=250 ⁰C
0.50
0
50

100

0.50
0.25

50

0.00
0
0.0

0.25

0.50

0.2

0.4

0.6

P/P0 [-]

0.8

0.00
1.00.0

0.2

0.00

0.25
0.4

0.6

P/P0 [-]

0.8

1.0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 [-]

Fig. 4-8. XRD diffraction patterns (a), and N2 sorption isotherms at -196 ⁰C (b) for nickel-

doped aminosilica (Ni/NH mole ratio = 0-0.50 mol mol-1; calcination temperature = 250 ⁰C)
prepared from BTPA (left-side Fig.), MAPTS (middle-side Fig.), and APTES (right-side Fig.)
precursors.

In order to deepen our understanding of the previous observation, porosity
analysis of the nickel-doped aminosilica composites was performed. Fig. 4-8(b)summarizes the nitrogen sorption properties for tunable feeding mole ratios of Ni/NH ata calcination temperature of 250 ⁰C. Ni-BTPA and Ni-APTES displayed type I isotherms
after the addition of nickel in a ratio of 0.25 mol mol-1, which showed the microporous
structural ordering. Moreover, Ni-APTES also shows adsorption above P/P0 = 0.8 due tothe condensation of adsorbed gas into interparticle voids, which were possibly generated
from nickel nanoparticles, as observed via XRD characterization [69]. The preparation ofnickel-doped aminosilica composites with a higher mole ratio of Ni/NH shows anincrease in the N2 adsorption capacity, which suggested a higher metal content was also
effective in developing the microstructure, as reported by Anggarini et al. [49]. A
representative N2 sorption isotherm of Ni-MAPTS showed the smallest formation of a
189

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --micropore structure under similar conditions, which could be attributed to the existenceof non-hydrolyzable methyl groups that inhibited the coordination of the nickel metalcenter with amines. Nobakht et al. [70] reported the formation of a coordinated compoundwith a methyl substituent, which resulted in a lower-dimension structure (1D), whereas aligand without steric hindrance showed the formation of a 3D network. Li et al. [71]observed pendant groups of methyltriethoxysilane (MTES) occupying the free space ofan organosilica network and becoming a barrier among silica pores. In that study, after
the removal of the pendant chain via pyrolysis at high temperatures, more pore space wasavailable for N2 sorption. As a consequence, the occupation of methyl groups in thependant structure of the MAPTS ligand resulted in a lack of coordination ability, which
sterically reduced the free-volume surface area. It is plausible that modification of theaminosilica network via nickel-coordination exhibited a blended structure of partial
crystallization that surrounded with the amorphous phase of polysilsesquioxane
framework, which would be expected to display comparable properties as well as a metalorganic framework (MOF) structure.

4.3.3 Evaluating the establishment of microporosity in modified nickelaminosilica membranes via single-gas permeation
Fig. 4-9(a) shows the molecular-diameter dependence on single-gas permeancefor pristine aminosilica membranes and nickel composites with a Ni/NH mole ratio of
0.25 mol mol-1 prepared at 250 ⁰C. The decreasing order for the permeation of gases
through pristine aminosilica membranes was BTPA (dP = 0.46 nm) > MAPTS (dP = 0.44nm) > APTES (dP = 0.41 nm), which indicated that greater free volume was created byincreasing the organic chain in the dual flexible-rigid structure, as confirmed by N2 andCO2 adsorption [62]. More importantly, the membranes fabricated with a nickel dopant
possess higher permeance compared with that of pristine versions, and nickel doping
produced molecular sieving properties on the order of Ni-BTPA (dP = 0.57 nm) > NiAPTES (dP = 0.54 nm) > Ni-MAPTS (dP = 0.48 nm). It should be noted that membranepore size, dP, was determined via the normalized Knudsen-based permeance (NKP)
method [72-74], as shown in SI-7. It should be noted that there is no clear and constant190

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --relationship between N2 sorption isotherms at -196 ⁰C of the membrane materials and gas
permeation properties at 200 ⁰C for membrane films. Even though pristine aminosilica
materials with high flexible organic chains may result in a nonporous structure (Fig. 44(a)), the gas permeation mechanism of the membrane thin layer at 200 ⁰C (Fig. 4-9(a))
was influenced by the existence of free-volume pores (dual flexible-rigid network) insteadof rigid pores. Ni-BTPA showed increased H2 permeance that was approximately 5-foldgreater, from 8.6 × 10-7 to 4.5 × 10-6 mol m-2 s-1 Pa-1, while maintaining selectivity for
H2/SF6 of > 10,000 (Fig. 4-9(a)).
CH4 CF SF6
4

10-5

CH
H
SF
He 2CO2 4 CF4 6

Permeance [mol m-2 s-1 Pa-1]

N2

10-5

10-6
0.57 nm

10-6

10-8
10-9

0.46 nm

200 qCBTPA
Ni-BTPA

10-7

10-7

10-8 0.44 nm

10-8

10-9

10-9

200 qC
MAPTS
Ni-MAPTS-10

104

Polymer
MOF
BTPA
MAPTS
APTES

103

0.48 nm0.54 nm

10-7

(b)

N2

N2

10-6

CH
H
SF
He 2CO2 4 CF4 6

H2/N2 [-]

H2(a) 10-5 He CO2

Mix-matrix
BTESE
Ni-BTPA
Ni-MAPTS
Ni-APTES

102

101
0.41 nm

200 qCAPTES
Ni-APTES
-10

10-10
10
10
0.2 0.3 0.4 0.5 0.6
0.2 0.3 0.4 0.5 0.6
0.2 0.3 0.4 0.5 0.6

Molecular diameter [nm]100 -9
10

10-8

10-7

10-6
-2

10-5

H2 permeance [mol m s Pa-1]

BTPA

MAPTS

APTES

Ni-BTPA

Ni-MAPTS

Ni-APTES-1(c)Fig. 4-9. Kinetic diameter dependence of single-gas permeance at 200 ⁰C for pristine aminosilica
membrane along with nickel composites prepared from BTPA (left-side Fig.), MAPTS (middle-sideFig.), and APTES (right-side Fig.) (a), H2/N2 permeance ratio as a function of H2 permeation fordifferent types of membranes (b), and schematic images of the formation of a nickel-amine
coordinated network affected by different structure of aminosilica precursors (c).

191

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The observed higher permeance of gases could be ascribed to the formation of the
microporous network that was induced via nickel-amine coordination, which resulted inhigher levels of surface area and pore volume (Fig. 4-4(a)). A higher H2/SF6 permeanceratio also was observed for Ni-composites membranes, which suggested that thecontribution of nickel-amine coordination resulted in the formation of pore sizes that weremore uniform and narrower with defect-free quality. By contrast, the change in NiMAPTS was insignificant by comparison with that of Ni-BTPA and Ni-APTES, which
could be ascribed to the existence of non-hydrolyzable methyl groups as an ‘arm’ thatprevented coordination. The features of the methyl pendant arm on the amine moiety
occupied the free volume sites and further prohibited the formation of coordination [75].
In addition, we compared the corresponding network pore sizes of the membranes
following the coordination process, and the H2/N2 permeance ratio as a function of H2
permeance for different types of membranes is summarized in Fig. 4-9(b). Pristineaminosilica membranes exhibited low H2 permeance with moderate to high selectivity,
while nickel-doped aminosilica membranes showed higher permeance with moderate to
low selectivity of H2/N2. For example, in the case of nickel-doping into the BTPA
structure, the permeance of H2 was significantly increased from 8.6×10-7 to 4.5×10-6 mol
m-2 s-1 Pa, and the H2/N2 permeance ratio was decreased from 30 to 10 while maintaininga high H2/SF6 selectivity of more than 10,000. This result agreed well with thecharacterization results showing that flexible amine precursors tend to generate a dense
structure in membranes, whereas the incorporation of nickel ions tunes the rigidity of theamine segment and increases network connectivity, which results in membranes withgreater permeability. These results could be rationally interpreted from the viewpoint of
membrane performances, as represented in Fig. 4-9(c). The presence of a bridgedsecondary amine structure on BTPA with high basicity and low steric hindrance generated
a strong level of coordination. Likewise, a small hindrance from the primary amine
structure of APTES led to the facile development of a microporous network, while thenon-hydrolyzable methyl groups on MAPTS lowered the probability of coordination andprevented the nickel ions from migrating closer to the amine coordination sites.
In order to further elucidate the transport mechanism through the undoped andnickel-doped aminosilica membranes, the temperature dependence of each permeating

192

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --gas for different types of membranes was compared, and the results appear in Fig. 4-10.
Also, the quantitatively calculated activation energy values are tabulated in Table 4-4.
Table 4-4. The calculated values of activation energy, Ep (kJ mol-1), for pristineaminosilica- and nickel-doped aminosilica-derived membranes
Membrane

H2

N2

BTPA

16.1

21.8

Ni-BTPA 0.25
MAPTS

13.6
8.87

10.0
9.39

Ni-MAPTS 0.25
APTES
Ni-APTES 0.25

10.5
11.6
10.3

12.7
20.7
11.4

Generally, the permeance of H2 and N2 was increased at the higher permeation
temperature, which suggests an activated transport mechanism [76]. After coordination
with nickel ions, the activation energy was significantly decreased as an indication of theenlargement of the pore sizes and higher levels of membrane connectivity via nickelamine crosslinking that built a highly permeable sub-nanoporous framework [66].
Specifically, the activation energy of N2 followed the sequence of Ni-MAPTS >> NiAPTES > Ni-BTPA, which further confirms the higher ability of each amine for thedevelopment of a coordinated framework. Ni-MAPTS and MAPTS exhibited comparable
H2 permeance of 6.3 × 10-7 and 3.8 × 10-7 mol m-2 s-1 Pa-1, respectively. Moreover, NiMAPTS showed no significant change in the activation energy compared with that of thepure MAPTS membrane. The steric hindrance of methyl groups effectively weakens theability of secondary amine groups to interact with nickel ions. The consistent connection
between membrane performance and the microstructure of materials (Fig. 4-4(a))
suggests that the steric hindrance of non-hydrolyzable methyl groups has a substantial
effect to prevent further coordination of amine ligands with a metal center. In this case,
Ni-MAPTS failed to construct a microporous framework at the calcination temperatureof 250 ⁰C.

193

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

10-5

10-5

10-5

Ni-APTES

Permeance [mol m-2 s-1 Pa-1]

Ni-BTPA
10-6

10-6

10-7

10-7

BTPA

Ni-MAPTS

10-6

10-7

MAPTS

10-8

10-8

10-8

10-9

10-9

10-9

-10

10

2.0

H2
N2
2.4

(a)
-10

2.8

10
3.2 2.0

H2
N2
2.4

(b)
-10

2.8

10
3.2 2.0

APTES

H2
N2
2.4

(c)
2.8

3.2

-1

1000/T [K ]Fig. 4-10. Temperature dependence of single-gas permeance for pristine aminosilica
membranes compared with nickel-doped aminosilica membranes calcined at 250 ⁰C: BTPA
and Ni-BTPA (a), MAPTS and Ni-MAPTS (b), APTES and Ni-APTES (c).

A coordination bond was formed by reacting amine-functionalized organosilica
precursors as organic linkers with nickel ions in the liquid phase, and this was explicitly
confirmed via the microstructure characterization of gelled powders. A previous study
revealed that the thermal degradation of amine linkers could generate a network collapseof the coordinated framework [66]. Herein, the fabrication of different types of nickelcoordinated, amine-functionalized, organosilica precursors was also evaluated at
calcination temperatures of 250, 300, and 350 ⁰C. Fig. 4-11 shows the gas permeance at
200 ⁰C for all types of membranes as a function of the molecular diameters of permeatedgases. The gas permeance of pristine membranes was slightly increased at higher
calcination temperatures with maintained selectivity for pristine membranes (Fig. 411(a)). H2 permeance was slightly increased from 0.86×10-6 to 1.3×10-6 mol m-2 s-1 Pa-1
for BTPA, while the permeance ratio of H2/N2 was slightly decreased from 21 to 17 as
the temperature was increased from 250 ⁰C to 350 ⁰C. Generally, the permeance of nonfunctionalized silica membranes was decreased as the calcination temperature increased
because higher temperatures facilitated the further condensation of silanol groups (SiOH) and the hydrolyzable groups of Si-OR to form a siloxane (Si-O-Si) network.
However, the enhanced gas permeance on aminosilica membranes could be attributed to194

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the partial degradation of amine groups, the liberation effect of organic groups, and alower reaction rate for the condensation process [77, 78].
(a)Permeance [mol m-2 s-1 Pa-1]

10-5

200 qC

10-5

200 qC

10-5

10-6

10-6

10-6

10-7

10-7

10-7

10-8

10-8

10-8

BTPA
250 qC
300 qC
350 qC

MAPTS
250 qC
300 qC
350 qC-10

200 qC

APTES
250 qC
300 qC
350 qC
-10
-10
10
10
10
0.2 0.3 0.4 0.5 0.6
0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6
10-9

10-9

10-9

Molecular diameter [nm](b)Permeance [mol m-2 s-1 Pa-1]

10-5

200 qC

10-5

200 qC

10-5

10-6

10-6

10-6

10-7

10-7

10-7

10-8

10-8

10-8

10-9

Ni-BTPA
250 qC
300 qC
350 qC

Ni-MAPTS
250 qC
300 qC
350 qC-10

10-9

200 qC

Ni-APTES
250 qC
300 qC
350 qC
-10

10-9

10-10
10
10
0.2 0.3 0.4 0.5 0.6
0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6

Molecular diameter [nm]Fig. 4-11. Molecular diemeter dependency of permeance (measured at 200 ⁰C) for pristineaminosilica membrane of BTPA, MAPTS, and APTES (a), and nickel-doped aminosilica
membranes at Ni/NH mole ratio of 0.25 for Ni-BTPA, Ni-MAPTS, and Ni-APTES preparedat various calcination temperature

Nickel-composite

membranes

showedthatgaspermeances

increasedconsiderably as the preparation temperature was increased from 250 to 300 ⁰C (Fig. 411(b)). As the temperature of calcination was increased from 250 ⁰C to 300 ⁰C, the H2
permeance for Ni-APTES was increased from 2.4×10-6 to 4.9×10-6 mol m-2 s-1 Pa-1 with
the H2/N2 permeance ratio maintaining values that ranged from 17 to 13 and with asignificant decrease in the H2/SF6 permeance ratio ranging from 700 to 100, whichindicated the presence of defects. APTES is the simplest primary amine precursor withbasicity that is lower than that of the secondary amine precursors [79, 80]. Due to weak
coordination with Ni, higher calcination temperatures led to the formation of interparticle
195

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --pores that were associated with the formation of nickel particles (Fig. 4-6). Moreover, thecalcination of membranes at 350 ⁰C resulted in the formation of a loose network with adominant Knudsen diffusion transport, which is not feasible for the separation of bothgases and liquid mixtures. Anggarini et al. [66] reported that the hybrid structure of metalcoordinated aminosilica membranes demonstrated a partially crystallized structure thatwas consistent with that of MOF structures that are surrounded by an amorphous siloxane
network phase that derives from alkoxysilane precursors. The thermal degradation ofamine linkers causes the fragmentation of nickel-amine coordination, which results in thecollapse of the microporous framework [81]. On the other hand, Ni-MAPTS-300 showed
increased H2 permeance compared with that of Ni-MAPTS-250 from 0.63 × 10-6 to
1.7×10-6 mol m-2 s-1 Pa-1, respectively, while maintaining H2/SF6 selectivity of as high as
1,000. This finding confirms the textural properties of Ni-MAPTS that were explained inFig. 4-7(a) where a higher calcination temperature was required to get a microporous
network and remove non-hydrolyzable methyl groups (as confirmed by TG-DTA results– Fig. 4-7(b) and (c)). Based on membrane performances in line with the characterizationof the materials, we proposed a possible mechanism for microporous frameworktransformations under different calcination temperatures, as shown in Fig. 4-12. A
coordinated network was established at 250 ⁰C, while nickel-amine cleavage was detected
at an amine decomposition temperature of 350 ⁰C. Therefore, the optimized calcination
temperature for Ni-BTPA and Ni-APTES is 250 ⁰C, while for the Ni-MAPTS structureis 300 ⁰C.

250 ⁰C

Ni-BTPA

Ni-TMAPS
Ni-MAPTS

Ni-APTES

Ni-BTPA

Ni-MAPTS
Ni-TMAPS

Ni-APTES

350 ⁰C

Fig. 4-12. Schematic image of the effect of different stage of calcination temperature on
nickel-doped aminosilica framework structure196

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The synthesis strategy of a microporous coordinated nickel-amine framework can
be rationally controlled by modifying the stoichiometric nickel/amine mole ratios tomaximize the formation of the microporous network coupled with the formation of open
metal sites for preferential transport properties. Fig. 4-13(a) shows the gas permeance for
Ni-BTPA-, Ni-MAPTS-, and Ni-APTES-derived membranes at varying nickel ratios
prepared at a calcination temperature of 250 ⁰C. As the higher nickel content generated
higher microporosity and greater surface area of the materials (Fig. 4-8), enhanced gas
permeation with lower resistance from the enlarged membrane pore size was obtained.
The improvement in gas permeance followed the order of Ni-BTPA 0.5 > Ni-BTPA 0.25
> BTPA; Ni-MAPTS 0.5 > Ni-MAPTS 0.25 > MAPTS; and Ni-APTES 0.5 > Ni-APTES
0.25 > APTES.

Permeance [mol m-2 s-1 Pa-1]

(a)
10-5

200 qC

200 qC

200 qC

10-5

10-5

10-6

10-6

10-6

10-7

10-7

10-7

10-8

10-8

10-8

Ni-BTPA
0
0.25
0.50
-10

10-9

10

Ni-MAPTS
0
0.25
0.50
-10

10-9

Ni-APTES
0
0.25
0.50
-10

10-9

10
10
0.2 0.3 0.4 0.5 0.6
0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6

Molecular diameter [nm](b)

Ni-APTES 0.25
Ni-TMAPS 0.5
Ni-MAPTS
0.5
Fig. 4-13. Single-gas permeance at 200 ⁰C as a function of the permeated gas sizes for nickeldoped aminosilica membranes (Ni-BTPA, Ni-MAPTS, Ni-APTES) with different
nickel/amine mole ratio (a), and schematic image of nickel coordinated aminosilica network
jointly with an excess of nickel nanoparticles (b).
Ni-BTPA 0.5

In the case of Ni-APTES 0.5, the permeation of most gases was high due to thecontribution of the Knudsen diffusion mechanism through non-selective large pore sizes

197

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --or defect voids that originated from large-sized nickel particles (Fig. 4-8(a)). In addition,
the pendant structure of Ni-MAPTS 0.5 showed higher gas permeance with a reduced
H2/SF6 permeance ratio of from > 1,000 to 600. Schematic illustrations of the network
structure for different types of membranes appear in Fig. 4-13(b). The stability of thecoordinated network derived from the bridge-structured precursor (BTPA) was higher
compared with the pendant-structured versions (MAPTS, APTES) since lower basicity
and larger steric hindrance of the pendant chain significantly perturbed the coordinationbond. Accordingly, the higher metal content of the dopants in the pendant types ofprecursors allowed a larger phase separation for metal particles; thus, the optimal nickel
mole ratios for the bridge (Ni-BTPA) and pendant precursors (Ni-MAPTS and NiAPTES) were 0.5 and 0.25, respectively.

4.3.4

Selective

methanolremovalfromorganic

solvents

via

pervaporation using nickel-doped aminosilica membranes
Our past work showed that combining the microporosity with an excess of nickel
ions creates high-flux membranes and facilitates the transport of methanol [49]. Our
strategy for the development of hybrid metal-coordinated amine-functionalizedorganosilica membranes is to assemble their structures as organic linkers in ways that will
facilitate the removal of methanol from different types of solvents. In Fig.4-14, wecompare the pervaporation time course for different types of hybrid nickel-doped
aminosilica membranes under different types of mixtures at 50 ⁰C (permeance values,
permeance ratio, flux, and separation factors are shown in Table 4-5).

Ni-BTPA

composite membranes boost the transport of MeOH with a permeate concentration higher
than 99 wt% with respect to Toluene (Tol), which results in a high flux of 1.42 kg m-2 h1

with the highest separation factor of 5,000. Based on the pore size distribution, thepermeance of Ni-MAPTS membranes is largely affected by calcination temperatures andby the concentration of nickel ions due to the existence of non-hydrolyzable methylgroups (The gas permeance and pervaporation performances for Ni-MAPTS 0.25-300
and Ni-MAPTS 0.50-250, as shown in Figs. S(4-10) and S(4-11), respectively).

198(b) Ni-MAPTS 0.25 – 300
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
⁰C 1000
MeOH/EA MeOH/DMC MeOH/i-PAc MeOH/Tol
MeOH/IPA

1000

Permeance ratio
Separation factor

2000
1500

100

1000

10

500

1
10-6

0

Permeance Separation factor
Flux
-2 -1
-1
[-]
[kg m-2 h-1] [mol m s Pa ]

10000

MeOH
MeOH
MeOH
MeOH
M
OH
MeOH
M
OH
MeOH
M
OH
MeOH/EA MeOH/DMC MeOH/i-PAc MeOH/Tol
MeOH/IPA

Permeance ratio [-]Permeance Separation factor
Flux
-2 -1
-1
[-]
-2 -1 [mol m s Pa ][kg m h ](a) Ni-BTPA 0.5 – 250 ⁰C

100

10-7
10-8
-10

5
4
3
2
1
0
0

75
50

10

25

1

10

-6

10

-7

100

0

10-8

10-9

10

Permeance ratio
Separation factor

Permeance ratio [-]Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

MeOH

200

10-9
5
4
3
2
1
0
0

Solvent

400

600

800

1000

Operation time [min]

1200

1400

MeOH

200

Solvent

400

600

800

1000

Operation time [min]

1200

1400

1600

10000
1000

MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH/IPA MeOH/EA MeOH/DMC MeOH/i-PAc MeOH/Tol
Permeance ratio
Separation factor

100

100

10

10

10

1000

1

1

-5

Permeance ratio [-]Separation factor
Permeance
Flux
[-]
-2 -1
-1
[kg m-2 h-1] [mol m s Pa ](c) Ni-APTES 0.25 – 250

10-6
10-7
10-8
10-9
5
4
3
2
1
0
0

MeOH

200

Solvent

400

600

800

Operation time [min]

1000

1200

Fig. 4-14. The time courses for pervaporation performance of MeOH-solvents mixtures at 50
⁰C for Ni-BTPA 0.5 – 250 ⁰C (a), Ni-MAPTS 0.25 – 300 ⁰C (b), and Ni-APTES 0.25 – 250 ⁰C(c) (methanol/solvents = 10 wt%/ 90 wt%; solvents used were IPA, EA, DMC, i-Pac, and Tol;pure MeOH was examined to confirm th stable PV performance before each PV separation of
MeOH-solvent mixture.)

Ni-MAPTS 0.50-250 shows a lower flux and separation factor compared with that of NiBTPA 0.50 due to smaller pore size and the contribution of methyl groups that
dramatically reduced the transport of MeOH. Moreover, the calcination of Ni-MAPTS
0.5-300 at a higher temperature (300 ⁰C) generated a greater amount of nickel
nanoparticles instead of removing the non-hydrolyzable methyl groups, which degraded
the performance (Figs. S(4-10) and S(4-11)). A higher level of performance was attainedby the pervaporation of Ni-MAPTS 0.25-300 with a high flux of 1.24 kg m-2 h-1 and aseparation factor for MeOH/Tol of 258. This improvement happened because theappropriate calcination temperature removed the non-hydrolyzable methyl groups, andthe low concentration of nickel tended to control the formation of interparticle pores.

199

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --The low level of steric hindrance generated by the amine precursors of Ni-APTES
0.25 composite membranes showed an attractive performance with a high flux of 1 kg m2

h-1 and a separation factor of 1,000. A comparison with the pervaporation results for the

undoped-APTES membrane (Fig. S4-12(a)) shows a small flux of 0.35 kg m-2 h-1 withmoderate selectivity for MeOH/Tol of 360, which indicates preferential sorption for
MeOH into polar amine functional groups, as reported by Anggarini et al. [49]. With ahigher nickel mole ratio, Ni-APTES 0.5 (Fig. S4-12(c)) failed to separate MeOH fromdifferent types of solvents due to the generation of large interparticle pores by nickel
nanoparticles. The pore structure of the membrane exhibited the presence of a significant
number of defects between the nanoparticles, which enhanced the contribution by
Knudsen diffusion (Fig. S4-7). The separation performance for MeOH/solvents was in
the following order: Ni-BTPA 0.5-250 > Ni-APTES 0.25-250 > Ni-MAPTS 0.25-300. In
each case, the permeance of MeOH was comparable to the pure system and approximately
constant for different types of mixtures, with the exception of the mixture of the
MeOH/IPA (separation mechanism for alcohol mixture as explained in SI-(4-12)). These
results indicated the preferential sorption of methanol and a high level of rejection forsolvents even at high concentrations. The pervaporation of MeOH/IPA shows thehistorical effect proven by the gradual increase of MeOH flux in the pure system after the
separation of IPA. Partial pore blockages by adsorbed IPA molecules result in theinability of pure MeOH to quickly transport through the membrane pore wall. The
observed results can be explained by the fact that the hydrophilic surface of nickelcrosslinked amine membranes facilitates the sorption of polar alcohol components. In
addition, the separation performance of nickel-doped aminosilica membranes showed anincrease behavior when a non-polar solvent was applied to the mixture. The permeanceof MeOH in different types of solvents was decreased as follows: in Tol ≈ i-Pac > DMC> EA >>> IPA. The extremely low flux of non-polar solvents such as i-Pac and Tol
through nickel-coordinated aminosilica membranes was experimentally explained via thereluctance of nonpolar molecules to pass through the hydrophilic surface region, whichis responsible for the enhanced separation between MeOH over non-polar solvents suchas DMC, i-Pac, and Tol [27, 82].

200

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Table 4-5. Comparison of methanol separation performance via pervaporation between
nickel-doped aminosilica membranes and different types of membranes
Membrane

System

Ppy-PF
MeOH/IPA
LPS/pDMAEMA
MeOH/IPA/PAI
Ni-BTPA 0.50 MeOH/IPA
Ni-MAPTS 0.50 MeOH/IPA
Ni-APTES 0.25 MeOH/IPA
Ni-APTES 0.50 MeOH/IPA
Ni-BTPA 0.50 MeOH/EA
Ni-MAPTS 0.50 MeOH/EA
APTES
MeOH/EA
Ni-APTES 0.25 MeOH/EA
Ni-APTES 0.50 MeOH/EA
BTESA
MeOH/DMCBTESE
MeOH/DMC
GA-STA-CS
MeOH/DMC
PAA/PVA
MeOH/DMCSTA/CS
MeOH/DMC
Nafion
MeOH/DMC
Ni-BTPA 0.50 MeOH/DMC
Ni-MAPTS 0.25 MeOH/DMC
Ni-MAPTS 0.50 MeOH/DMC
Ni-APTES 0.25 MeOH/DMC
Ni-APTES 0.50 MeOH/DMC
Ni-BTPA 0.50 MeOH/i-PAc
Ni-MAPTS 0.50 MeOH/i-Pac
Ni-APTES 0.25 MeOH/i-Pac
Ni-APTES 0.50 MeOH/i-Pac
Ppy-PF
MeOH/Tol
TiPCS
MeOH/Tol
ChitosanMeOH/Tol
Cellulose acetate MeOH/Tol
PA/UiO66(NH2)MeOH/Tol
EDTA/cellulose
SPI
MeOH/Tol
TEOS-CS
MeOH/TolBTPA
MeOH/Tol
Ni-BTPA 0.50 MeOH/Tol
Ni-BTPA 0.50 MeOH/Tol
Ni-MAPTS 0.25 MeOH/Tol
Ni-MAPTS 0.50 MeOH/Tol
APTES
MeOH/Tol
Ni-APTES 0.25 MeOH/Tol
Ni-APTES 0.50 MeOH/Tol

MeOH perm.
MeOH
TPV
Total flux
Sep.
Perm.
×10-7
feed conc
Ref.
[⁰C][kg m2 h-1]
factor
[-]
ratio
[-]*
[wt%]
[mol m-2 s-1 Pa-1]*
57.5
10
0.50
0.51
2
0.7 [83]
50

15

3.39

11.4

9

50
50
50
50
50
50
50
50
50
50
50
50
60
50
40
50
50
50
50
50
50
50
50
50
50
50
35
30

10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10

0.15
0.25
0.28
5.00
1.22
0.61
0.17
0.86
25.3
3.13
0.45
1.03
0.61
1.16
0.93
0.75
1.75
0.57
0.98
24.1
1.00
0.55
0.81
15.5
0.28
0.64
0.80
1.80

0.92
0.87
1.47
0.64
4.32
1.60
0.49
2.63
1.69
11.3
1.40
3.23
0.12
3.53
2.67
2.36
3.6
0.99
2.82
1.88
3.28
1.67
2.55
1.31
0.59
1.35
3.22
8.79

300
32
35
2
415
45
38
51
2
118
235
91
0.8
67
11
280
13
11
39
2.57
1700
110
373
4
441
410
162
80

22

72

1.55

9.51

5

5

[90]

50

10

0.79

1.43

49

4

[91]

30
50
50
50
50
50
50
50
50

10
10
10
10
10
10
10
10
10

0.18
0.07
2.81
1.42
2.63
0.81
0.36
0.90
11.2

0.93
0.14
5.95
3.03
5.34
1.51
0.75
2.16
0.64

396
382
907
5000
420
70
368
919
5

35
34
80
410
37
6
30
73
0.3

[28]
[49]
[49]

*

3

100
10
12
1
This
300 work
36
28
40
2
26
[14]
59
[14]
21
[85]
0.2 [86]
16
[87]
2
[88]
66
3
3
9
This
0.6 work
1350
82
300
3
39
[83]
36
[89]
14
[26]
7
[25]

MeOH permeance and the permeance ratios were calculated based on Eqs. (3) and (4)

201

[84]

This
work

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --4.3.5 Permeation and separation mechanism through nickel-doped
aminosilica membranes
Fig. 4-15(a) gives an overview of the molecular sieving properties of nickel-doped
aminosilica membranes and the polarity of the organic mixtures that affected theseparation performance. Permeance (P) is the product of the diffusivity (D) and solubility(S) of permeated molecules across the membrane barrier, where the diffusivity value isaffected by the pore dimensions, and the solubility is affected by the critical temperatureof the solvent and the interaction between membrane surface (organic phase) andpermeated solvent [62, 73, 92].
i-PAc

(a)

EA

Tol Tert-BuOH

Permeance [mol m-2 s-1 Pa-1]

10-6

MeOH

EtOH

IPA

DMC

10-7

10-8

10-9
Ni-BTPA 0.50
Ni-APTES 0.25
Ni-TMAPS 0.50

10-10
15

20

25

30
1/2

Hansen solubility parameter G[MPa ]
10000
1000
Separation factor [-](c)

MeOH/DMC
Me100
Ni-APTES0.25

Ni-MAPTS0.50

10
1
0.1
0.01

Ni-MAPTS 0.25

Ni-BTPA 0.50
Ni-APTES 0.25
Ni-TMAPS 0.50
Ni-TMAPS 0.25
Ni-APTES 0.50

0.1

10000

Polymer
ZeoliteMix matrix
Organosilica

1

Ni-APTES0.50

Ni-APTES

Total Flux [kg m-2 h-1]

Ni-BTPA0.125
BTPA

100

0.25
Ni-BTPA

APTES

1.00
Ni-MAPTS 0.25

Ni-BTPA 0.25
Ni-MAPTS 0.50

10

BTPA
Ni-BTPA 0.125
Ni-BTPA 0.25
Ni-BTPA 0.50
Ni-BTPA 1.00
APTES
Ni-APTES 0.25
Ni-APTES 0.50
Ni-MAPTS 0.25
Ni-MAPTS 0.50

Ni-APTES0.50
PolymerMix matrix
TiPCS
i-SiO2ZrO2Organosilica

1
0.1
0.01

10

Ni-BTPA 0.50

MeOH/Tol

1000

Ni-BTPA 0.50

Separation factor [-]

(b)

0.1

1

SiO2-ZrO2

10

Total Flux [kg m-2 h-1]

Fig. 4-15. The correlation between Hansen solubility parameter and permeance for Ni-BTPA,
Ni-MAPTS, and Ni-APTES (a), comparison pervaporation performance for the separation of
methanol/dimetil carbonate (MeOH/DMC) (b) and methanol/toluene (MeOH/Tol) mixture (c)between nickel-doped aminosilica membranes and different types of membranes.

202

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Sridhar et al. [93] reported that the performance of pervaporation-based membranes for a
specified mixture would depend on the solubility parameters and the free volume, whichreflects the interactions between the permeated molecules, the membrane materials, andthe pore structure that is available for adsorption. In this study, the Hansen solubility
parameter including the partial parameters of dispersion, dipole, and hydrogen bonding
was used to describe the relationship between the structure and the interaction strength ofsolvents [93, 94]. Fig. 4-15(a) shows a clear correlation between the Hansen solubility
parameter and the permeance (P) values. MeOH as a polar solvent showed the highest
solubility parameter with the highest value of permeance, while DMC, Tol, and i-Pac areless-polar solvents that showed the lowest solubility parameters, as well as permeance,
which resulted in high separation performance for both mixtures. The results reported
here support the assumption that MeOH freely interacts with a large quantity of metalcrosslinked groups that makes the surface properties of these membranes veryhydrophilic.
The solubility parameters for DMC, Tol, and i-Pac through hydrophilic
membranes show much lower values compared with MeOH due to their relativelyhydrophobic nature. Permselectivity is determined by the combination of molecularsieving properties (diffusivity) and the affinity for separation (solubility) since theseparation performance of nickel-doped aminosilica membranes in organic mixtures
followed the order of Ni-BTPA > Ni-APTES > Ni-MAPTS, and the bridge amine
structure (BTPA) generated a stable and highly coordinated network. The pendant
structure (MAPTS and APTES), on the other hand, showed moderate-to-low coordinationlevels due to steric hindrance that worsened the coordination ability and affected themembrane performance.
Fig. 4-15(b) shows the trade-off curve between total flux and the separation factor
of MeOH/DMC mixtures for different types of membranes at 50 ⁰C. Each plotted point
represents the experimental data of a different membrane, as tabulated in Table S1.
Commercial polymeric membranes are simply indicative of a trade-off trend where amore permeable membrane shows a high flux of 8 kg m-2 h-1 with a low separation ability
for MeOH/DMC of 2, while another membrane could show an extremely small flux of
0.03 kg m-2 h-1 with a moderate selectivity of 100. The hybrid structure from nickel-

203

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --coordinated amine-functionalized organosilica precursors breaks through the original
trade-off where the flux of the membranes was increased to approximately 1 kg m-2 h-1
with a separation factor higher than 200. Each plotted point for several nickel-doped
aminosilica membranes prepared under similar conditions showed a comparable
performance, which confirmed the reproducibility of membrane fabrication and themeasurement accuracy with standard deviations of less than 20%.
Fig. 4-15(c) summarizes the pervaporation performance for MeOH/Tol separationby the hybrid nickel-doped aminosilica membranes fabricated in this study, along withdifferent types of membranes. Compared with membranes such as polymeric, MOFs, andTiPCS, the Ni-BTPA showed the highest separation performance with a high flux of 1.42kg m-2 h-2 and the highest separation factor of 5,000. Other nickel-composites of NiAPTES and Ni-MAPTS also exhibited high potential for the separation of MeOH/Tol,with a high flux that was approximately > 1 kg m-2 h-1 and a separation factor higher than300. The enhancement of membrane flux and the separation factor can be ascribed to thebalance between higher levels of interconnected micropores generated from nickel-amine
coordination, and the existence of uncoordinated nickel ions that acted as active sites forthe diffusion of methanol [11, 50]. The pervaporation findings are consistent withmaterials characterization obtained from instrumental interpretations and the pore
structure estimation from single-gas permeation results, which suggested beneficial
modification through the precise control of the pore structure and the hydrophilic surface.
As far as we could ascertain, this study introduces the first high-flux membrane that has
demonstrated a methanol-selective performance that could be applied to industrial-scale
purifications.

4.4 Conclusions
Microporous nickel-doped aminosilica membranes were successfully synthesized via
a sol-gel process that was followed by a coordination reaction. The nickel-composite
membranes show attractive separation performance with methanol-selective properties
and high flux.

204

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --(1) The optimal nickel/amine mole ratio and appropriate calcination temperature forthe development of high porosity, facilitated the high diffusion of methanol asfollows.
a. Bridge-structure amine precursor (Ni-BTPA) = 0.50 – 250 ⁰C
b. Pendant-structure amine precursors = Ni-APTES = 0.25 – 250 ⁰C; Ni-MAPTS= 0.25 – 300 ⁰C(2) The permeation mechanism for nickel-doped aminosilica membranes was
influenced by the pore size distribution of the membranes and by the affinity
between the hydrophilic surface of the membranes and the permeating molecules.
(3) Competitive adsorption was observed for alcohol mixtures (MeOH/EtOH,
MeOH/IPA, and MeOH/tert-BuOH), which then resulted in pore blockage via
hydrogen bonding interactions.
(4) The nickel-doped aminosilica membranes show high levels of flux and separation
factors on the order of Ni-BTPA > Ni-APTES > Ni-MAPTS. The Ni-BTPA
membrane recorded the most outstanding performance with a high flux of 1.42 kg
m-2 h-1 and a superior separation factor for MeOH/Tol of 5,000.

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Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Supporting Information----------SI-(4-1)----------

Fig. S4-4. Experimental set-up of single-gas permeation apparatus-----------------------------------------------------------------------------------------------------------------------------SI-(4-2)----------

Fig. S4-5. Schematic diagram of pervaporation experiment-----------------------------------------------------------------------------------------------------------------------------SI-(4-3)---------(a)

(c)

(b)

Fig. S4-3. Overview of the intermolecular periodic linking unit chain distance (A) and theaverage spacing in the siloxane network (B) for Ni-BTPA (a), Ni-MAPTS (b), and NiAPTES (c).
--------------------------------------------------------------------------------------------------------------------

212

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(4-4)---------0.20

NLDFT method

dVp [cc/nm/g]

0.15

0.10

0

2

4

6

8

250 qC
350qCqC
300

250 qC
300 qC
350 qC

250 qC
300 qC
350 qC

0.00

Ni-APTES
Ni-APTES

Ni-TMAPS
Ni-MAPTS

Ni-BTPA
Ni-BTPA

0.05

10 0

2

4

6

8

350 qC

10 0

2

Pore size [nm]Pore size [nm]

4

6

8

10

Pore size [nm]Fig. S4-4. Pore size distribution (calculated with NLDFT method) for Ni-BTPA, Ni-MAPTS,
and Ni-APTES calcined at different temperatures----------------------------------------------------------------------------------------------------------------

Absorbed amount [cm3 g-1]----------SI-(4-5)---------100

MeOH
IPA
EA
DMC
75
i-PAc
Tol
Closed: adsorption
50 Open: desorption

Ni-BTPA

(a)

MeOH
IPA
EA
DMC
i-PAc
Tol
Closed: adsorption
Open: desorption

MeOH
IPA
EA
DMC
i-PAc
Tol
Closed: adsorption
Open: desorption

Ni-MAPTS(b)

Ni-APTES

(c)

25

0
0.0

0.2

0.4

0.6

P/P0 [-]

0.8

1.00.0

0.2

0.4

0.6

P/P0 [-]

0.8

1.00.0

0.2

0.4

0.6

P/P0 [-]

0.8

1.0

Fig. S4-5. Vapor sorption isotherms at 25 ⁰C of methanol-MeOH, isopropyl alcohol-IPA, ethyl
acetate-EA, dimethyl carbonate-DMC, isopropyl acetate-i-Pac, and toluene-Tol for Ni-BTPA-1(a), Ni-MAPTS (b), and Ni-APTES (c) (Ni/NH = 0.25 mol mol ) composites calcined at 250
⁰C under inert atmosphere of N2.
----------------------------------------------------------------------------------------------------------------

213

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --M Pi)1/3 [mol1/6 g1/6 m-2/3 s-1/3 Pa-1/3]----------SI-(4-6)---------0.02

0.02
MAPTS
Ni-MAPTS

BTPA
Ni-BTPA

0.01

0.01

(a)
0.00
2.5

(b)

3.0

3.5

4.0

4.5

5.0

0.00
5.5
2.5

3.0

3.5


M Pi)1/3 [mol1/6 g1/6 m-2/3 s-1/3 Pa-1/3]

Molecular size [Å]

4.0

4.5

5.0

5.5

Molecular size [Å]

0.02
APTES
Ni-APTES

0.01

(c)
0.00
2.5

3.0

3.5

4.0

4.5

5.0

5.5

Molecular size [Å]ଵΤଷFig. S4-6. The correlation of ൫ඥ‫ܯ‬௜ ܲ௜ ൯ as a function of molecular size of permeating gases(points and lines are experimentally and theoritically calculated, respectively, for BTPA and
Ni-BTPA (a), MAPTS and Ni-MAPTS (b), APTES and Ni-APTES (c)--------------------------------------------------------------------------------------------------------------------------SI-(4-7)---------(a)(b)

Intensity [a.u]

i

i

0.50

i i
0.25
0.00
APTES
0 10 20 30 40 50 60 70 80

2Tq [degree]

Permeance [mol m-2 s-1 Pa-1]

i Ni

Ni-APTES
Ni-A

10-4

He H2

CO2 N CH4
2

CF4

SF6

200 qC

10-5
10-6
10-7
Ni-APTES 0.5 (1)Ni-APTES 0.5 (2)Ni-APTES 0.5 (3)

10-8
10-9

0.25 0.30 0.35 0.40 0.45 0.50 0.55
Molecular diameter (nm)( )Fig. S4-7. The XRD diffractogram for APTES and Ni-APTES at different amount of nickel
calcined at 250 ⁰C (a), and the reproducible data of single-gas permeance at 200 ⁰C for NiAPTES 0.50 (b)----------------------------------------------------------------------------------------------------------------

214

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(4-8)----------

Table S4-1. The calculated values of activation energy, Ep (kJ mol-1), for pristineaminosilica- and nickel-doped aminosilica-derived membranes
He

H2

CO2

N2

CH4

15.7

16.1

31.1

21.8

21.3

Ni-BTPA 0.25
MAPTS

12.2
10.2

13.6
8.87

8.92
4.47

10.0
9.39

8.73
8.67

Ni-MAPTS 0.25
APTES
Ni-APTES 0.25

11.4
12.6
9.70

10.5
11.6
10.3

9.50
14.7
0.47

12.7
20.7
11.4

13.2
17.8
10.7

10-5
12.2 kJ mol-1
-6

10-7

15.7 kJ m -1
ol

10.2 kJ mo -1
l

9.7 kJ m -1
ol

11.4 kJ mol -1

12.6 kJ m -1
ol
-8

10

10-9
10-10
2.0

BTPA
TMAPS
APTES

2.2

Ni-BTPA
Ni-TMAPS
Ni-APTES

2.4

2.6

2.8

3.0

3.2

13.6 kJ mol-1-616.1 kJ m -1
ol

10

10-7

11.6 kJ mo -1
l

-8

10

10-9

BTPA
MAPTS
APTES

10-10
2.0

2.2

Ni-BTPA
Ni-MAPTS
Ni-APTES

2.4

N2 Permeance [mol m-2 s-1 Pa-1]

1000/T [K-1]
10-5
10-6

10.5 kJ
mol -1 10.3 kJ mol -1

8.87 kJ mo -1
l

2.6

3.0

3.2

1000/T [K-1]
BTPA
MAPTS
APTES

Ni-BTPA
Ni-MAPTS
Ni-APTES
10.0 kJ mol-1
11.4 kJ mo -1
l

10-7

12.7 kJ mo -1
l-8

9.39 kJ mo -1
l

-9

10

20.7 k
J

10-10
2.0

2.2

10

2.8

21.8 kJ

mol -1

2.4

2.6

2.8

mol -13.0

3.2

1000/T [K-1]

CH4 Permeance [mol m-2 s-1 Pa-1]

10

10-5

10-5
10-6

CO2 Permeance [mol m-2 s-1 Pa-1]

BTPA

H2 Permeance [mol m-2 s-1 Pa-1]

He Permeance [mol m-2 s-1 Pa-1]

Membrane

10-5
8.92 kJ mo -1
l

10-6
9.50 kJ mol -1

10-7

0.47 kJ mo -1
l4.47 kJ mol-1

1014.71 kJ mol-1

10-9
10-10
2.0

BTPA
MAPTS
APTES

31.1 kJ mol-1

-8

BTPA
MAPTS
APTES

2.2

Ni-BTPA
Ni-MAPTS
Ni-APTES

2.4

2.6

2.8

3.0

3.2

1000/T [K-1]
Ni-BTPA
Ni-MAPTS
Ni-APTES

8.73 kJ mol -1

10-7

13.2 kJ mol -1

10-8

8.67 kJ m -1
ol

-917.8 kJ
mol -1

10.7 kJ m -1
ol
21.3 k
J

10

10-10
2.0

2.2

2.4

2.6

2.8

mol -13.0

3.2

1000/T [K-1]

Fig. S4-8. Temperature dependence of single-gas permeance for pristine aminosilica
membranes (BTPA, MAPTS, APTES) compared with nickel-doped aminosilica membranes(Ni-BTPA, Ni-MAPTS, Ni-APTES) calcined at 250 ⁰C.
----------------------------------------------------------------------------------------------------------------

215

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(4-9)---------(a)(b)

Intensity [a.u]iPermeance [mol m-2 s-1 Pa-1]

i Ni

TC = 300 qC
i

Ni-MAPTS 0.50i20

Ni-MAPTS 0.25

4060

He H2 CO CH4 CF SF
4
6
2

10-5

N2

10-6
10-7
10-8
10-9 Open: 250 qC

80

10Closed: 300 qC
Ni-MAPTS 0.25
Ni-MAPTS 0.50
-10

2Tq [degree]

0.2

0.3

0.4

0.5

0.6

Molecular diameter [nm]Fig. S4-9. The XRD diffractogram for Ni-MAPTS 0.25 and 0.50 calcined at 300 ⁰C (a), andkinetic diameter dependence of single-gas permeance at 200 ⁰C for Ni-MAPTS 0.25 and NiMAPTS 0.50 calcined at 250 ⁰C and 300 ⁰C (b)----------------------------------------------------------------------------------------------------------------

100

MeOH/IPA

EA

10

10

100
MeOH/
DMC MeOH/
i-PAc

MeOH/
Tol

50
25

1

0

-6

10-7

75

MeOH

MeOH

DMC
IPA

10-8

200

EA

MeOH
i-PAc

10

MeOH/
EA

400

600

MeOH/
MeOH/
DMC
i-PAc

800

1000

Operation time [min]

1200

1
10-6

MeOH/
Tol

50
25

MeOH/EA MeOH/DMC
MeOH

MeOH

EA

DMC

IPA

MeOH
MeOH

MeOH

i-PAc

Tol

-9

2

MeOH

1
0
0

1600

75
MeOH/i-PAc MeOH/Tol

0

10-8

Tol

1400

MeOH/IPA

10-7

MeOH

100

Ni-TMAPS
0.50
Ni-MAPTS0.50-250

100

10

10-9
5 MeOH
4
3
MeOH/
2
IPA
1
0
0

MeOH

1000

Permeance ratio [-]Ni-MAPTS0.25-300
Ni-TMAPS
0.25
MeOH/

Permeance Separation factor
Flux
-2 -1
-1
[-]
[kg m-2 h-1] [mol m s Pa ]1000

Permeance ratio [-]Permeance Separation factor
Flux
-2 -1
-1
[-]
[kg m-2 h-1] [mol m s Pa ]----------SI-(4-10)----------MeOH/
MeOH/ MeOH/EA DMC
IPA

MeOH/
i-PAc

MeOH/
Tol

100 200 300 400 500 600 700 800 900 1000 1100

Operation time [min]Fig. S4-10. Effective practical time courses of pervaporation performance through NiMAPTS0.25-300 and Ni-MAPTS0.50-250 for the methanol removal systems at 50 ⁰C----------------------------------------------------------------------------------------------------------------

216

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- ------------SI-(4-11)----------

1
10-6
10

MeOH

-7

10-8

50
25

MeOH/EA

0
MeOH

MeOH

MeOH
Tol

EA

10-9
2
1
00

MeOH

MeOH/EA

MeOH

MeOH/Tol

100 200 300 400 500 600 700 800 900 1000

Operation time [min]

10000
1000

Ni-APTES0.25
Ni-APTES
0.25

100 MeOH/IPA
MeOH/
EA

10

1
10-5
10

-6

10-7
10

-8

1000
MeOH/
DMC

MeOH

100

MeOH/
i-PAc

MeOH/
Tol

10
1

MeOH

MeOH

MeOH

IPA
10-9
5
4 MeOH
3
MeOH/
2
MeOH/
EA
IPA
1
0
0
200
400

DMC

i-PAc

MeOH/
DMC

MeOH/
i-PAc

600

800

Operation time [min]

Tol

10

Ni-APTES0.50

Ni-APTES 0.50
10

MeOH/
IPA

MeOH/
EA

IPA

MeOH

1
10-5
10-6

MeOH

EA

100
MeOH/
Tol

MeOH

Tol

i-PAc

MeOH

MeOH/
i-PAc

MeOH/
Tol

500

600

10-7
30

MeOH/
Tol
1000

1200

20 MeOH
10

MeOH/IPA

00100

MeOH/ MeOH/
EA
DMC

200

300

400

Operation time [min]Fig. S4-11. Effective practical time courses of pervaporation performance through APTES, NiAPTES 0.25, and Ni-APTES 0.50 for the methanol removal systems at 50 ⁰C (methanol/solvents =10 wt%/ 90 wt%)--------------------------------------------------------------------------------------------------------------------------SI-(4-12)---------Separation of alcohol mixtures through nickel-doped aminosilica membranes
In the previous section of 3.3 (main article), the pervaporation performance for
MeOH/IPA in all types of membranes showed a lower permeance and separation factor
compared with other types of solvents (Figure 14). This result might be explained by thefact that IPA (polarity index = 3.9) is a polar solvent with a polarity index comparable tothat of MeOH (polarity index = 5.1). To further ascertain the permeation mechanism ofalcohol mixtures through the hybrid membranes, the separation systems of MeOH/EtOH
and MeOH/tert-BuOH (10 wt%/90 wt%) were also performed at 50 ⁰C. Before measuring
the pervaporation of the alcohol mixtures, the membrane was soaked in pure methanol
overnight to remove the component that was adsorbed on the membrane pore wall andreduce the history effect. Separation of MeOH with different types of alcohol mixtures
had low performance, especially when the polarity indexes of the component weresimilar. The other reason for poor selectivity is related to the small difference in themolecular sizes between the separating molecules. As summarized in Figure S4-12, thepolarity of solvents directly affected the separation ability of crosslinked nickelaminosilica membranes.

217

5
0

DMC
MeOH

EA

MeOH

MeOH/
MeOH/
i-PAc
DMC

Permeance ratio [-]MeOH/Tol

1075]
Permeance ratio [-]

100

APTES

APTES

[

Flux
Permeance
Separation factor[kg m-2 h-1] [mol m-2 s-1 Pa-1]
[-]

100

Permeance ratio [-]1000

Separation factor
Permeance
Flux
[-]
-2 -1
-1
[kg m-2 h-1] [mol m s Pa ]Permeance Separation factor
Flux
-2 -1
-1
[-]
[kg m-2 h-1] [mol m s Pa ]p

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

50
25

1

10

MeOH
MeOH

-7 MeOH

EtOH

10-8
10

0
MeOH

MeOH/Tert-BuOH

MeOH/EtOH

00

100

200

300

400

500

Operation time [min]

600

700

1000
100

Ni-APTES
0.25
Ni-APTES0.25

10

MeOH/EtOH

10-7

100
75
MeOH/Tert-BuOH

50
0

MeOH

MeOH

-6

MeOH

MeOH
EtOH

500
400
300
200
100
0

MeOH

MeOH

Tert-BuOH

-10

5
MeOH
MeOH
4 MeOH
3
2
MeOH/EtOH
MeOH/Tert-BuOH
1
00
250
500
750
1000
1250

Operation time [min]

1500

(d)

MeOH

MeOH

EtOH

10-8
10

800

25

1
10-6

MeOH/EtOH

1

10

10

MeOH

2
1

10

10-9

MeOH

MeOH

100

10-8

Tert-BuOH

MeOH/Tert-BuOH

Ni-TMAPS
0.25
Ni-MAPTS0.25

1000

10-7

MeOH

-9

3

10000

Permeance ratio [-]75

MeOH/EtOH

10

10-6

Permeance Separation factor
Flux
-2 -1
-1
[-]
[kg m-2 h-1] [mol m s Pa ]MeOH/Tert-BuOH(b)

Permeance Separation factor
Flux[-][mol m-2 s-1 Pa-1][kg m-2 h-1]

Ni-BTPA 0.50

Ni-BTPA 0.50

100

(c)

100

Permeance ratio [-]1000

Permeance ratio [-](a)Permeance Separation factor
Flux
-2 -1
-1
[-]
[kg m-2 h-1] [mol m s Pa ]----------------- Fig. S(4-12). -----------------

MeOH

Tert-BuOH

-9

3

EtOH

MeOH

2 MeOH
1

MeOH/EtOH

00

250

MeOH/Tert-BuOH
500

750

Operation time [min]

1000

1250

Fig. S4-12. Time courses for the pervaporation performance of methanol/ alcohol mixture(ethanol and tert-butanol) at 50 ⁰C through the Ni-BTPA 0.50-250 (a), Ni-MAPTS 0.25-300(b), Ni-APTES 0.25-250 (c) membranes, and schematic representation of an alcoholcompetitive sorption on coordinated nickel-amine based silica membrane----------------------------------------The optimized nickel-doped aminosilica membranes such as Ni-BTPA 0.50 showed highflux and high separation factors for MeOH/Tol , but were found to be unselective for the
MeOH/alcohol mixture. The Ni-BTPA (Figure S4-12(a)) membrane exhibited a low fluxof 0.13 kg m-2 h-1 with a separation factor of less than 50 for MeOH/tert-BuOH. Similarly,
Ni-MAPTS (Figure S12(b)) and Ni-APTES(Figure S12(c)) exhibited low performancefor the separation of alcohol mixtures. The permeance of methanol always became lower
compared with the pure systems, and the flux of both components was largely decreased
as an indication of high competitive adsorption between alcohol mixtures.

218

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --A higher concentration of alcohol compounds with a larger size than MeOH could
have entered the pores of nickel-doped aminosilica membranes and interrupted the
diffusion of MeOH inside the pores structure as schematically shown in Figure S12(d).
Anggarini et al. reported the uncoordinated nickel nanoparticles acted as Lewis-acid andcould interact with hydroxyl groups from alcohol Lewis-base molecules which thenincreased the diffusivity of higher concentrations of alcohol components via larger pores.
In addition, most alcohol mixtures could interact with hydrophilic functional groups
presented in nickel-doped aminosilica composites via the physical interaction ofhydrogen bonding, which then blocks the available free volumes.

219

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --220

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Chapter 5
Structural two-phase evolution of aminosilica-based silvercoordinated membranes for increased hydrogen separation
5.1 Introduction
Gas separation membrane technology has grown significantly throughout the years
due to lowered costs, simpler operations, improvements in compactness, andimprovements in performance that include higher levels of permeability and selectivity[1]. Microporous organosilica is one of the inorganic membranes that has shown a highpotential for gas separation processes under chemically harsh environments. Other
advantages of microporous organosilica membranes include excellent mechanical andthermal stability and highly tunable pore sizes for specific gas-separation applications [24]. Adequately prepared organosilica membranes show high permeability for smaller
gases such as helium (He, kinetic diameter-KD = 0.26 nm), H2 (KD = 0.29 nm), andcarbon dioxide (CO2, KD = 0.33 nm) with significant selectivity of larger gases such ascarbon tetrafluoride (CF4, KD = 0.48 nm) and sulfur hexafluoride (SF6, KD = 0.55 nm),and of hydrocarbons such as propene (C3H6, KD = 0.45 nm) and C3H8 (KD = 0.48 nm).
Kanezashi et al. [5] evaluated the performance of silica membranes with tetraethoxysilane(TEOS) as the precursor at different calcination temperatures. A TEOS membrane
calcined at 550 ⁰C showed high H2 permeance of 1.3 × 10-7 mol m-2 s-1 Pa-1 with highH2/N2 selectivity of 80. Labropoulos et al. [6] also reported high performance oforganosilica membranes fabricated via chemical vapor deposition (CVD) with the highest
permeance for He of 5.2 × 10-7 mol m-2 s-1 Pa-1 and a selectivity for He over N2 thatreached 164.1. Moreover, Nagasawa et al. [7] developed a high-performance organosilica
membranevia

plasma-enhanced

chemicalvapor

deposition(PECVD)ofhexamethyldisiloxane (HMDSO)-derived membranes, and with it they reported Hepermeance of 1 × 10-7 mol m-2 s-1 Pa-1 with a permeance ratio for He/N2 that reached7,800. The molecular sieving properties of organosilica membranes with highpermeability and high selectivity represent the most valued qualities for applications in

221

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the industrial, scientific, and medical sectors [8].
The concept of tailoring the micropore structure was developed to enhance theseparation ability in order to produce higher levels of gas permeance. The highest
separation ability of organosilica membranes has been achieved when the characteristic
pore dimensions of the membrane are capable of separating penetrants that differ inkinetic diameter via molecular sieving [9, 10]. Hence, the critical pore size of themembrane must be determined to achieve a high level of molecular sieving performance.
The microporous structure of organosilica membranes can be tailor-made via various
techniques that include burning out the organic compounds during the post-synthesis
process and adjusting the structure of the linking units between bi-silyl precursors (spacer
method) [11, 12]. Raman and Brinker [12] introduced an organic template method tocreate a pore structure by using the organic component as a template that can beincorporated into an inorganic matrix, which then can be removed via thermal treatment
without collapsing the original matrix. Another process, the spacer method, refers to themodification of silsesquioxane with different lengths of linking units between two silicon
atoms [11, 13]. The pore size and structure of the resultant material were strongly
influenced by the organic functional groups. Most organosilica precursors with higher
carbon numbers between a bi-silyl structure, however, tend to produce a flexible structurethat fails to generate the microporosity of a highly permeable membrane.
The industrial application of H2 purification technology has developed rapidly overthe decades in the forms of water-gas shift processing, coal gasification, steam reforming,and the dehydrogenation of hydrocarbons [14-17]. Most H2 separation membranes
encounter obstacles to gas permeation accompanied by a desirable level of separation,and vice versa. Because it is the smallest molecule, H2 can be separated by solutiondiffusion in non-porous membranes or by molecular sieving in porous membranes [4].
Well-known examples of hydrogen-selective membranes originated from metal-organic
frameworks (MOFs) [18], two-dimensional lamellar nanomaterials [19], zeolites [20, 21],
covalent organic frameworks (COFs) [22], Pd and its composite alloys [23, 24], andcarbon molecular sieving membranes [25]. Ma et al. [26] reported that nano-sheets seeds
for the secondary growth of a Zeolite imidazolate framework-95 (ZIF-95) could control
the thickness of the membrane and improve the H2 permeance of 1.96 × 10-7 mol m-2 s-1

222

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Pa-1 with the highest selectivity of H2/CO2 of 49.4 via a narrow pore size of 0.37 nm andhuge cavities of 2.4 nm. Wen et al. [27] showed that graphene oxide (GO)-based
membranes make up a class that is derived from graphene with a laminar network andadjustable nanochannels that exhibit excellent molecular sieving features for the
separation of H2 isotopic water with high selectivity. However, the large oxygen content
in the GO structure degrades the selective diffusion properties of the membrane via
specific interactions with water (H2O) molecules, and this results in large pore sizes andswelling of the GO lamella. Carbon molecular sieving (CMS) membranes show a bimodal
pore size distribution of ultramicropores and micropores that provides high levels of gas
separation performance and permeability [28]. The pore structure of the membrane wasdeveloped via carbonization of polymeric precursors at high temperatures, which resulted
in a tightening of the micropores and a resultant higher level of selectivity. Moreover,energy consumption during the carbonization process continued to present an economical
problem for large-scale industrial applications.
High purity of H2 gases (99%) is attainable by separation via the dense structure ofmetallic membranes, in particular, this applies to composite palladium (Pd)-alloy types.
Several varieties of metals such as pure types (Pd, niobium-Nb, and titanium-Ti), binary
alloys (Pd-silver/Ag, Pd-copper/Cu, Pd-yttrium/Y), and complex alloys (Pd with morethan 3 types of metal alloys) have been investigated for their use in H2 separation [2932]. Most metallic alloy membranes, however, form thicknesses larger than 0.1 mm,
which makes them unattractive due to low permeance, low stability, and high cost. Porous
membranes, on the other hand, have generally demonstrated the promise of high flux withthermal stability at temperatures ranging from 200 to 600 ⁰C. The synthesis of MOF
materials has been widely elaborated due to their attractive properties for the separationand purification of gases [22, 29]. MOF membranes have shown high flux performancedue to ultra-high porosity with highly dimensional interconnected structures. A rigid
framework of porous coordinated materials, however, tends to generate interfacial defects
in their crystal structures, which results in a reduction in the separation performance ofthe membranes [33, 34].
Based on these findings, we first proposed a new concept for controlling the
microporosity of organosilica ceramic membranes via a sol-gel process that follows a223

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --coordination reaction to obtain significantly higher levels of permeance and selectivity.
Coordination is facilitated via cross-linking reactions between the amine groups in thelinking units of bis[3-trimethoxysilyl) propyl] amine (BTPA) as a Lewis base moleculewith a silver ion act as a Lewis acid. A pair of nonbonding electrons from an amine moiety
donated to the empty d orbitals of a silver ion induces the formation of a more rigid linking
unit structure of BTPA, which gives rise to a higher surface area and high permeating
flux of small molecules. In addition, the flexible amorphous phase of pristine BTPA
ensures a high level of separation ability.
Previous study by our group has shown that silver dopants with a weak affinity for
coordination easily generate metal segregation as a product of uncoordinated silver
nanoparticles following calcination at 250 ⁰C [35]. Ag is commonly used as metal alloyedwith Pd to increase the permeability of H2 and reduce the brittle effect of the membranes[36, 37]. We expected the existence of metallic Ag nanoparticles to facilitate the transportof H2 through an inorganic silica membrane and result in a higher permeance of H2. Yang
and Chen [38] showed that the introduction of metallic silver into TEOS andmethyltriethoxysilane (MTES) increased the flux of H2 by increasing the pressure
difference. This result was explained as Ag enhancing the surface diffusion of hydrogenthrough a silica network. The experimental work presented in this study is one of the firstinvestigations into how a two-phase structure that includes silver-amine coordination andsilver nanoparticles could facilitate the diffusion of H2 as a smaller-sized gas and how theamorphous properties of a silica-based framework tends to reject larger-sized gases suchas hydrocarbons. Therefore, the present research offers the first explanation of the effectthat a silver-dopant concentration exerts on the formation of microporous aminosilica
membranes for the separation of H2.

224

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --5.2Experimental
5.2.1 Preparation of bis[3-(trimethoxysilyl)propyl] amine (BTPA) andsilver-modified bis[3-(trimethoxysilyl)propyl] amine (Ag-BTPA)
BTPA was used as a precursor and hydrolyzed under a mole composition of
BTPA/H2O/HNO3 at 1/300/1. The weight percent of the precursor was maintained to 5%
by the addition of ethanol (C2H5OH) as a solvent agent. The hydrolysis and condensation
reactions were conducted at room temperature for 12 hours under vigorous stirring. Aftersynthesis of the BTPA sol, a certain amount of silver nitrate (AgNO3) was added into thesolution as a dopant with different mole ratios of silver/amine ranging from 0.1 to 0.5
mol/mol. Finally, the mixtures were further stirred continuously at room temperature for
1 hour. The resultant sols without and with silver content will herein be referred to as
BTPA, Ag-BTPA 0.1, Ag-BTPA 0.2, Ag-BTPA 0.3, and Ag-BTPA 0.5. The silverdoping process was also conducted using bis-(trimethoxysilyl)-hexane (BTMSH)
precursor with the same mole ratio for comparison between coordinated anduncoordinated species.

5.2.2 Structural characterization of Ag-BTPA films and xerogels
Silver-doped organosilica gel powders were prepared via a slow solvent
evaporation process at 50 ⁰C under an air atmosphere [39]. The coordinated (Ag-BTPA)
and non-coordinated (Ag-BTMSH) compounds were confirmed using a UV-Vis diffuse
reflectance spectrophotometer (Shimadzu UV-3600 plus). The thermal properties ofsilver-doped BTPA powder were determined using a thermogravimetric mass
spectrometer (TG-MS, TGA-DTA-PIMS 410/S, Rigaku, Japan). The formation of a twophase structure to produce silver-amine coordination and silver nanoparticles was studied
using powder X-ray diffraction (XRD D2 PHASER, Bruker, Germany) in the 2θ rangeof 5-80⁰. All the xerogel samples of Ag-BTPA with different contents of silver were
calcined at 250 ⁰C under a nitrogen (N2) atmosphere. The morphologies and the formationof the nanoparticle structure were confirmed using a transmission electron microscope(TEM JEOL2010 Ltd., Japan). The samples for TEM evaluation were prepared by225

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --suspending the silver-doped organosilica gel powder in a volatile solvent, which then was
deposited onto a copper mesh grid. Such pretreated samples were then submitted tovacuum conditions for 12 hours.
The chemical structure of Ag-doped aminosilica was examined using Fourier
Transform Infrared Spectroscopy (FT/IR-4100, JASCO, Japan) over a wavenumber rangeof 500-4000 cm-1. The Ag-BTPA-derived films were coated onto a potassium bromide(KBr) plate and then calcined at 250 ⁰C under a N2 flow for 30 minutes. The chemical
environment of pristine BTPA and Ag-BTPA was also studied using XPS elemental
analysis (Shimadzu ESCA-3400HSE spectrometer).
The textural characteristics of each powder were evaluated via N2 sorption
isotherms at -196 ⁰C and CO2 adsorption-desorption isotherms at 25 ⁰C in a BELMAX(BELJAPAN Inc.). The specific surface area was calculated based on the BrunauerEmmett-Teller (BET) technique in a P/P0 range of 0.05-0.20. All samples were pretreatedby heating at 200 ⁰C under vacuum conditions for 12 hours prior to measurement.

5.2.3 Preparation of Ag-BTPA membranes and the evaluation of
membrane performance
Porous α-alumina tubes were used as a membrane support with 50% porosity andan average pore size of 1 μm (length 100 mm; inner diameter 8 mm; outer diameter 10mm), as supplied by the Nikkato Corporation (Japan) [40]. The α-alumina tubes wereconnected with non-porous alumina (Al2O3) tubes via a highly thermal sintering process
before membrane preparation. α-Al2O3 particles with average particle sizes of 2 μm and0.2 μm were dispersed into a silica-zirconia (SiO2-ZrO2) sol as a binder with a solid
concentration of 10 wt% and subsequently coated onto the outer surface of a porous αAl2O3 support followed by calcination at 550 ⁰C under an air atmosphere for 15 minutes.
These procedures were repeated three times for each of the types of particles to smooththe surface of the membrane. The intermediate layer was made from a 0.5 wt% SiO2-ZrO2
sol that was coated onto the surface of the particle layer and fired under an air atmosphereat 550 ⁰C. These procedures were repeated several times until the average pore size of the226Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --support reached approximately 1 nm. Finally, the topmost layer was obtained by two
coatings of 0.1 wt% Ag-BTPA sols onto the intermediate layer followed by calcination
at 250 ⁰C under a flow of N2 for 30-60 minutes.
Single-gas

permeation

experiments

were

conductedusing

high-purity

components of He, H2, CO2, N2, CH4, CF4, and SF6 under conventional constant pressure
at temperatures ranging from 50-200 ⁰C. Each permeation gas was fed to the outside of
the membrane with an upstream pressure of 200 – 400 kPa. The pressure inside the system
was maintained at atmospheric levels, and the flow rate of the permeate was measured
using a film flow meter (Horiba, Co. Ltd., Japan). The permeation measurement wasrecorded after the membrane reached a steady state with respect to the pressure difference
between the shell and the inner side of the membrane and flux. The permeance of eachgas ( ܲ௜ ) and the ideal selectivity for the ݅ െ ‫ ݄ݐ‬and ݆ െ ‫ ݄ݐ‬components ( ߙ௜Ȁ௝ ) were
calculated using the following equations.
ܲ௜ ൌ

‫ܨ‬௜
ൗ‫ܣ‬ο‫݌‬
௜
௉

(5-1)
(5-2)

ߙ௜Ȁ௝ ൌ  ೔௉ೕwhere ܲ௜ is the permeance of the ݅ െ ‫ ݄ݐ‬component (mol m-2 s-1 Pa-1), ‫ܨ‬௜ is the molar flowrate of the ݅ ௧௛ component (mol s-1), A is the effective cross-sectional area of themembrane (m2), and ο‫݌‬௜ is the partial pressure difference of component ݅ between thefeed and the permeate of the membrane (Pa). The permeate flow rate was evaluated witherrors of less than 5 and 20% for the permeance value larger than 10-7 and smaller than
10-7 mol m-2 s-1 Pa-1, respectively.

5.3Results and discussion
5.3.1 Structural analysis of silver-modified amine- and nonfunctionalized organosilica
The amine groups in the linking units of BTPA molecules have the ability tocovalently bind to the transition metal of Ag(I) [35]. To compare the difference between

227

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --coordinated and uncoordinated compounds, Ag(I) was added into two types oforganoalkoxysilane:bis[3-(trimethoxysilyl) propyl] amine(BTPA) and bis(trimethoxysilyl) hexane (BTMSH). In our previous study, we discovered that thecoordinated compound of Ag-BTPA showed no absorption peak in the range of visible
light since the resultant solution was colorless. However, the UV-Vis diffuse reflectancespectra of powdered materials for pristine organosilica precursors, Ag-BTPA and AgBTMSH, were collected, as shown in Fig. 5-1. BTPA (Fig. 5-1(a)) showed thecharacteristic absorption peak at approximately 193 nm, which represents the freeelectron transition (n ื  ߪ ‫ כ‬ሻfrom the amine groups. After the coordination with Ag (Fig.
5-1(a)), the absorption peak shifted to a higher wavelength of 236 nm, which revealed ashifting into a lower energy level that was defined by a spectral red shift [41, 42]. The red
shift is thought to be due to the coordination that led to stabilization into a lower energy
level for the Ag-BTPA network. As a non-coordinated compound, however, Ag-BTMSH(Fig. 5-1(b)) displays an absorption peak that is similar to that of BTMSH.
2.0

2.0

(b)

1.5

Absorbance [-]Absorbance [-]

(a)

1.0

0.5

0.0

BTPA
500

750

1.0

BTMSH
Ag-BTMSH

0.5

Ag-BTPA
250

1.5

1000

0.0

250

Wavelength [nm]

500

750

1000

Wavelength [nm]Fig. 5-1. UV-vis diffuse reflectance absorption spectra of BTPA and Ag-BTPA (a) comparedwith non-functionalized BTMSH and Ag-BTMSH (b)Based on the UV-Vis spectra of the powders, a coordinated level of high density was later
confirmed in silver-doped aminosilica powder via XRD, FT-IR, and N2 sorption
isotherms. Manna et al. have explained how AgNO3 reacts with N-hexadecyl
ethylenediamine

togenerateacoordinatedcompoundofAg(hexadecyl

ethylenediamine)2NO3 that protects the metal cores of silver nanoparticles fromaggregation into floc particles [43].

228Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --In order to deepen understanding of the effect of the silver-amine modification,
the microstructural and adsorption isotherms properties were compared between aminefunctionalized (BTPA) and non-functionalized (BTMSH) alkoxysilane structures. Bothtypes of precursors have an extremely flexible chain with six carbons between the linking
units that definitely exhibit a textural structure of non-porosity (Fig. 5-2(a)). Yu et al. [44]reported that the intrinsic flexible properties of organic-rich alkoxysilane allow freemovement that creates free volume, but occupies the rigid siloxane skeleton, and reduces
the number of porosity. After the incorporation of silver metal, however, the amount ofadsorbed N2 gradually increased for Ag-BTPA, while a small capacity for adsorption wasobserved with Ag-BTMSH. This is due to the porosity created by the coordination
between the electron receptors of silver and the electron donors of the amine moieties,
which results in a higher level of rigidity for the linking units of the BTPA structure aswell as a greater amount of surface area [35].
(b)

30

20

BTPA
Ag-BTPA 0.3
BTMSH
Ag-BTMSH 0.3
Closed: adsorption
Open: desorptioni Ag nanoparticles iiiIntensity [a.u]Va [cm3(STP) g-1]

(a)

10

Ag-BTPA 0.3ii

250qC200qCiiii

150qC100qC
50qC

0

0.0

0.2

0.4

0.6

0.8

1.0

p/p0 [-]

(c)

i

10

(d)

i Ag0x Ag2O
Dash line = AgNO3

20

30

40

50

60

70

80

2Tq [degree]

Intensity [a.u]ixAg-BTMSH 250 qCiix

Ag-BTMSH 150 qC
Ag-BTMSH 50 qC
AgNO3

10

20

30

40

50

60

70

80

2Tq [degree]

Fig. 5-2. Comparison of N2 adsorption/desorption isotherms for both type of silver doped
organosilica precursors (a), powder XRD pattern of silver doped amino-silica precursors/Ag-BTPA(b) and amine-free silica precursor/Ag-BTMSH (c) xerogel powder at different calcination
temperature, and representatives TEM image of Ag-BTMSH at calcination of 250 ⁰C (d)229

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --As Fig. 5-2(b) illustrates, the XRD pattern of Ag-BTPA showed an amorphous
phase at lower calcination temperatures, which confirmed a full incorporation of silver
ions into the BTPA matrix. When the calcination temperature was raised to 200 ⁰C, theformation of a face-centered cubic (fcc) silver phase was detected. By contrast, theimpregnation of silver into non-functionalized alkoxysilane resulted in unreacted AgNO3,
Ag2O, and a heterogeneous distribution of metallic silver (Ag0) with crystal sizes larger
than 30 nm that were possibly deposited on the surface of BTMSH as confirmed by XRD
measurement (Fig. 5-2(c)) and TEM (Fig. 5-2(d)).
AgNO3 as a metal dopant was stable under acid conditions and promoted thermal
decomposition to produce Ag2O at 170 ⁰C followed by Ag0 at 440 ⁰C [45]. The
appearance of a silver metallic phase after the calcination temperature of 200 ⁰C on AgBTPA further confirmed that amine groups on the surface play an important role incatalyzing partially coordinated compounds and in the reduction of AgNO3 to produce
well-dispersed silver nanoparticles.

5.3.2 The effect of preparation temperature on the Ag-BTPA structure
Optimization of the calcination temperature is an essential parameter in the
fabrication of silver-modified aminosilica membranes. The thermal treatment oforganosilica-derived membranes generally enhances the condensation of silanol (Si-OH)
groups leading to the formation of a siloxane network via crosslinking of the Si-O-Si
backbone, which results in higher connectivity and the establishment of molecular sieving[46, 47]. Nevertheless, the calcination of aminosilica membranes at high temperatures
could be restricted to the degradation of amine moieties [48]. Fig. 5-3(a) shows the FTIR analysis for Ag-BTPA 0.5 films coated onto KBr plates at different calcination
temperatures: 150, 250, and 350 ⁰C. Ag-BTPA 0.5 samples displayed the characteristic
vibrational peaks of Si-O-Si at 1,025 – 1,110 cm-1, which was increased in intensity
according to increases in the calcination temperature. On the other hand, the existence of
Si-OH at a wavenumber of 909 cm-1 was observed at a calcination temperature of 150 ⁰C,
which was gradually decreased by increasing the temperature of heat treatment.

230

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Absorbance [a.u.]

(a)
H-N

Si-OH

Si-C

350 qC

250 qC

Absorbance [a.u.]

Si-O-Si

N=O

150 qC2000(b)
350 qC
250 qC
150 qC

1750

1500

1250

1000

Wavenumber [cm-1]

1800

750

1700
1600
Wavenumber [cm-1]

1500

Fig 5-3. FT-IR spectra for Ag-BTPA 0.5 in wavenumber range of 2000-700 cm-1 (a), and thedeformation NH mode range of 1800-1500 cm-1 (b)These findings suggested that higher thermal treatment induces the condensation of SiOH groups to establish the Si-O-Si framework [49]. The NH deformation band of
secondary amine groups in the linking units of BTPA (1600 cm-1) was shifted to a higher
wavenumber (1,616 cm-1) at 250 ⁰C as an indication of coordination (Fig. 5-3(b)).
Simultaneously, at a high calcination temperature of 350 ⁰C, Ag-BTPA 0.5 showed
reductions in the NH deformation band, indicating a complete decomposition of amine
moieties.
This result was validated by the TG-MS measurement shown in Fig. 5-4. AgBTPA 0.5 showed two stages of weight loss. The first weight loss of 14% occurred attemperatures ranging from 110-300 ⁰C, and was related to the condensation of -OH
groups (m/z = 18). The second weight loss was observed at approximately 350 ⁰C (20%),
and was ascribed to a significant removal of amine groups and organic chains. These
losses were based on the mass signals (Fig. 5-4-upper figure) at m/z = 30 (alkyl-amine
molecules) and m/z = 44 (CO2 molecules), respectively.

231

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --

50

(b)

4

m/z=44

3
2

30
20

=30

1

=18

0
100

(a)

10

0
150

75

100

50

50

25

0

0
0

200

400

600

800

Heat flow [PV]Weight [%]40

Intensity [10-9 A]

Intensity [10-10 A]

5

-50
1000

Temperaturep[qC]

Fig 5-4. Thermal properties of Ag-BTPA 0.5 confirmed by thermogravimetric (TG) and heatflow curve (a) accompanied by mass signal peaks (b)5.3.3 The silver doping ratio and its effect on the construction of the AgBTPA network
The microstructural properties of silver-doped BTPA xerogel powders wereanalyzed, and the results are presented in Fig. 5-5(a). Both pristine BTPA and Ag-BTPA
exhibited an XRD pattern with two amorphous peaks. The first sharp peak below 10⁰
reflected the distance from the intermolecular periodic chain to a chain in thepolysisesquioxane structure, and the second diffraction peak at around 20⁰ demonstrated
the average distance in the siloxane backbone [50]. The XRD patterns of Ag-BTPA
powders show broader peaks at 2θ of 8⁰ and 20⁰ with higher concentrations of silver,which represent the more disordered network structure that is induced by doping withsilver. The formation of face-centered cubic (fcc) planes of silver nanoparticles wereobserved at higher 2θ degrees of 38⁰ (1 1 1), 44⁰ (2 0 0), 64⁰ (2 2 0), and 78⁰ (3 1 1) [35,51, 52]. The intensity of the silver nanoparticles was gradually increased by increases inthe silver content in the matrix. This can be explained by silver acting as a metal dopantwith a weak affinity to the amine groups that are theoretically known to be coordinated232Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --with two amines creating a one-dimensional arrangement [53]. At a low concentration of
Ag/NH = 0.1, silver existed as an ion in an amorphous metallic phase. With a higher silver
ratio, however, the weak coordination between the silver and amine interaction tended to
produce an unstable complex, and phase separation was confirmed by the formation ofhigh-intensity fcc silver nanoparticles.
(a)Tc = 250 qC

i Ag0i

Intensity [a.u]

i
i
i

i

Ag-BTPA 0.3ii

Ag-BTPA 0.2
Ag-BTPA 0.1

BTPA
10

(b)

BTPA

20

30

40

50

60

70

80

2Tq [degree]

Ag-BTPA 0.1

Ag-BTPA 0.2

Ag-BTPA 0.3

Ag-BTPA 0.5

Fig. 5-5. XRD patterns (a) and TEM images (b) of BTPA and Ag-BTPA xerogel powders withdifferent silver content calcined at 250 ⁰C

In order to confirm the formation of a coordinated compound and the dispersionof ordered silver nanoparticles on the surface of the BTPA matrix, TEM imaging was
conducted for the xerogel powders obtained at a calcination temperature of 250 ⁰C (Fig.
5-5(b)). The amorphous nature of the aminosilica precursor was observed as a uniform
morphology with a diffuse electron-diffraction pattern (the insert shows the lower layer).
With greater amounts of metal doping the dispersion of the metallic silver phase (darker
color with a spherical shape) tended to uniformly increase. Anggarini et al. [54] foundthat amounts of metal doping that exceeded saturation conditions tended to generate
uncoordinated species that easily formed metallic nanoparticles. At low silver mole ratios
of 0.1 and 0.2, coordinated compounds were dominant, which resulted in the formation

233

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --of silver in the amorphous phase. The mean particle sizes of the silver nanoparticles in
the Ag-BTPA 0.3 and 0.5 samples agreed well with the Scherrer equation from the XRD
spectra, at 6.22, and 13.3 nm, respectively.
The transformation of the chemical environment of the silver element for AgBTPA films during the coordination was investigated using XPS, as shown in Fig. 5-6(a).
The Ag 3d3/2 and Ag 3d5/2 peaks for Ag-BTPA heat treatment at 250 ⁰C appeared at
approximately 375 and 369 eV, respectively. The first peak for a higher level of binding
energy indicates the photoelectron excitation probability in the inner orbital shell, and is
known to be influenced by the silver particle size.
(a)

-CH2-

(b)
375

370

365

Ag 3d
Ag-BTPA 0.5

Si-O-Si

H-N

Si-C

250qC

360
Absorbance [a.u.]

3380

N=O

Si-OH
O-H

Ag-BTPA 0.5
Ag-BTPA 0.3

Intensity [a.u]Ag-BTPA 0.2

BTPA

Ag-BTPA 0.3
4000

Ag-d3/2

Ag-d5/2

380

375

370

365

3000

2500

2000

1500

1000

Wavenumber [cm-1](c)

250 qC

BTPA
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5

Absorbance [a.u.]

Ag-BTPA 0.2

BTPA

3500

360

Binding energy [eV]
1800

1750

1700

1650

1600

1550

1500

Wavenumber [cm-1]

Fig. 5-6. XPS high-resolution spectra of Ag (3d) binding energy (a), FTIR wide-range spectra(b) and FT-IR narrow-range spectra (c) for BTPA and Ag-BTPA films heat treated at 250 ⁰C
under an inert atmosphere

The slight shift in the Ag 3d3/2 core level towards higher binding energy upon theincorporation of silver could be attributed to the formation of silver nanoparticles smaller
than 20 nm [52, 55]. Salido et al. [55] also found that the core levels of the energy forAg3d3/2 shifts to a higher level of binding energy for silver with a small particle crystal
size of 10 nm. Therefore, the establishment of the small-sized and well-dispersed silver

234

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --nanoparticles in Ag-BTPA is consistent with the XRD and TEM characterizations.
Another core level spectrum of Ag 3d5/2 was attributed to excitation of the photoelectrons
in the outer orbital shell, and indicated the existence of different oxidation states for silver[56]. We noted that at a low silver concentration of 0.2 (mol mol-1), the main characteristic
peak was centered at a lower binding energy of 369 nm, which meant that most of thesilver had remained in an ion phase (Ag+) rather than being reduced as in the silver
metallic phase (Ag0). As the covalence coordination interaction between the silver andamine groups increased, the binding energy of the core levels of the electrons weredecreased for the silver ions [57]. However, when the mole ratio of silver was increased
to higher than 0.3, this peak was shifted to a higher binding energy, which confirmed theformation of a high portion of silver nanoparticles (Ag0) as verified via XRD and TEM
characterization.
The establishment of coordinated silver-amine was also confirmed via FT-IR
measurement with high magnification in the range of 1,800-1,500 cm-1, which is related
to the N-H deformation vibrational mode (Fig. 5-6(c)). By comparing the FT-IR spectra,
it is clear that the monodentate amines coordinated to silver (I) ions could be observed
via a gradual shifting of the N-H deformation band into a higher wavenumber, whichsuggests the involvement of a dynamic process that includes three types of reactions:hydrolysis, condensation, and coordination [35, 58].
To assess the modification of silver into an aminosilica structure via coordination
leading to an increase in the surface area, textural analysis of Ag-BTPA xerogel powders
was performed via N2 adsorption/desorption isotherms at -196 ⁰C, as summarized in Fig.
5-7(a). As already discussed in Fig. 5-2(a), pristine BTPA with organic-rich properties
displayed a negligible amount of N2 adsorption, which indicates a nonporous structurethat is due to the high flexibility of the linking units that consequently occupy the siloxane
network. Kanezashi et al. [39] reported that the nonporous structure of organosilica
membranes could be ascribed to the increased flexibility of the linking units between thebi-silyl structure. Ag-BTPA 0.1 shows the comparable adsorption capacity of pristine
BTPA, which indicates that a low metal ratio of silver exhibits a weak effect to generatea transformation in the coordinated network of Ag-BTPA. During the silver modification,a gradual increase in N2 uptake was observed. This result suggested that the formation of235

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --microporosity could be achieved with a higher mole ratio of silver (Fig. 5-7(c)), where a
two-phase structural evolution from silver-amine coordination to silver nanoparticles wasdeveloped.
CO2 adsorption was measured to confirm the formation of coordinatedcompounds between silver-amine sites under different mole ratios of silver, because thechemisorption activity of CO2 on amine-functionalized organosilica materials has been
established [48, 59, 60].
(b)

150
15

100

BTPA
Ag-BTPA 0.1
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5
Closed: adsorption
Open: desorption

V CO2 adsorbed [cm3 g-1]

Va [cm3(STP) g-1]

(a)

50

0

0.0

0.2

0.4

0.6

0.8

25
20

Closed - adsorption
Open - desorption

15
10
5
0

1.0

BTPA
Ag-BTPA 0.1
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5

P/P0 [-]

0

20

40
60
P [kPa]

80

100

50

V adsorbed [cm3 g-1]

(c)

40
30
N2

20
10
0
0.0

CO2

0.1

0.2

0.3

0.4

0.5

Ag/NH mole ratio [mol/mol]Fig 5-7. N2 sorption isotherms curve (a), CO2 adsorption/desorption isotherms (b), and thegas sorption capacity (N2 at P/P0 = 0.5, and CO2 at P = 100 kPa) as a function ofsilver/amine mole ratio (c) originated from BTPA and Ag-BTPA xerogel powders at
calcination temperature of 250 ⁰C

Although BTPA shows a low surface area according to the Brunauer-Emmett-Teller(BET) calculation based on N2 sorption isotherms, its secondary amine structure showed
a greater CO2 uptake capacity than that of Ag-BTPA samples (Fig. 7(b)). A broadhysteresis loop was also observed in BTPA as an indication that the adsorbed moleculesof CO2 were trapped within the abundant active sites of amine moieties [61]. After the
addition of silver ions, the CO2 adsorption capacity was decreased with the increased236

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --silver content (Fig. 7(c)), and the hysteresis loops were narrowed to Ag/NH = 0.1-0.3 andeven linear sorption isotherms were achieved at Ag/NH = 0.5 in Ag-BTPA samples.
These results indicated that the saturation level of -NH groups for CO2 adsorption wasachieved after the formation of a coordinated compound from the silver-amine interaction
where fewer amine active sites were accessible for CO2 sorption.

5.3.4 The effect of membrane preparation temperature
To further explore the effect of calcination temperature on the pore structure
evolution of Ag-BTPA, three membranes were prepared by firing at 150, 250, and 350
⁰C, as presented in Fig. 5-8. The permeance of Ag-BTPA 0.5 was increased when
calcination temperature was increased from 150 to 250 ⁰C. Interestingly, the membrane
calcined at a higher temperature of 250 ⁰C showed higher H2 permeance of 1.46 ൈ 10-6
mol m-2 s-1 Pa-1 with high H2/CO2 selectivity of 12, while the membrane calcined at 150
⁰C showed lower H2 permeance and selectivity.
(b)

10-5

Ag-BTPA 0.5
10-6
10-7
10-8
10-9
10-10

Tc [qC]=
150
250
350
0.25

0.30

0.35

0.40

0.45

0.50

101
Dimensionless permeance [-]Permeance [mol m-2 s-1 Pa-1](a)

Ag-BTPA 0.5
100
10-1
10-2
10-3
10-4

0.55

Molecular diameter (nm)Tc [qC]=
150
250
350
0.25

0.30

0.35

0.40

0.45

0.50

0.55

Molecular diameter (nm)Fig 5-8. Molecular size dependence of single-gas permeance at the measurement test of 200 ⁰C(a), and dimensionless permeance based on He permeance for Ag-BTPA 0.5 calcined at atemperature range of 150 - 350 ⁰C

This result indicated that calcination at 250 ⁰C was required to produce coordination. As
reported by Anggarini et al., [35] a lower temperature for hydrolysis and condensation
was predominated by the existence of H2O and HNO3. After the evaporation of thosecomponents at a higher temperature, the formation of coordination was more dominant.
This finding is also supported by the FTIR characterization shown in Fig. 5-3(a). Si-OH
237

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --groups are known to reduce gas permeation properties due to low levels of
dehydroxylation of the silica network and to occupy the pore spaces in aminosilica
membranes [49, 62].
Furthermore, thermal treatment at 350 ⁰C of Ag-BTPA membranes promoted theformation of a loose framework similar to that of corresponding Knudsen-like permeation
properties (Fig. 5-8), which can be ascribed to a complete degradation of the amine groupsof Ag-BTPA 0.5 following calcination at 350 ⁰C (Fig. 5-3(a) and 5-3(b)). After thedecomposition of amine groups, the fragmentation of silver-amine coordination could
result in a collapse of the microporous network structure. Hussain et al. [63] have reported
thermal decomposition in a MOF following the decomposition of organic linkers, whichthen resulted in cleavage of the coordination between metal and organic linkers. In thecoordinated phase, Ag-BTPA exhibited a hybrid structure that could have been partiallycrystallized and would have surrounded the amorphous phase of the alkoxysilane network
where it would have been expected to show characteristics similar to that of a MOF.
Hence, the optimized calcination temperature for the development of Ag-BTPA was 250
⁰C, which allowed the development of a well-coordinated silver-amine network by
avoiding the thermal degradation of amine groups.

5.3.5 The effect of the silver doping ratio
The gas-permeation performance of hybrid Ag-BTPA membranes with different
mole ratios of silver are shown in Fig. 5-9(a). All Ag-BTPA membranes were developed
at 250 ⁰C under a N2 atmosphere. Single-gas permeation results revealed that increases
in the silver content resulted in slightly higher levels of gas permeance. Ag-BTPA 0.5
showed the highest H2 permeance, which was 65-fold higher than that of BTPA, with ahigh H2/SF6 permeance ratio of 1,700. The enhanced performance of metal-doped silica
membranes was also reported by Uhlmann et al. [64] via the existence of a cobalt (Co)phase that promoted a higher level of stability for the TEOS membrane and enabled thehigher permeation of smaller gases such as He while maintaining low permeance forlarger gases such as N2, which favors high selectivity for the membranes. In that work,
the Co dopant strongly inhibited the rate of the condensation process by creating a less-238

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --rigid and weakly branched silica matrix. In the same way, the condensation reaction rate
for Ag-BTPA membranes was controlled via coordination between the metal- andorganosilica-based amino functional groups. Fig. 5-9(b) shows that the permeance ofsmaller-sized gases such as He and H2 was slightly increased by increasing the silver
content, whereas the permeation of larger-sized gases was constant or even decreased
after the doping process. Based on the membrane performance and the results ofspectroscopy characterization, the formation of silver nanoparticles very possibly
generated interparticle pores as the product of voids among packed particles [65], whichthen affected the diffusion of larger-sized gases. On the other hand, the formation of acoordinated compound was also confirmed for all Ag-BTPA samples. Therefore, the twophases of the structural evolution of silver ions contributed to a transformation of the gaspermeation behavior of the membranes.
(a)

(b)

10-5

200 qC
Permeance [mol m-2 s-1 Pa-1]

Permeance [mol m-2 s-1 Pa-1]

10-5
10-6
10-7
10-8
10-9
10-10

BTPA
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5
Ag-BTPA 1.0

10-7

Molecular diameter (nm)

He

CO2

CH4
N2

10-8

SF6

10-9
10-10
0.0

0.25 0.30 0.35 0.40 0.45 0.50 0.55

H2

10-6

0.1

0.2

0.3

0.4

0.5

Silver/NH mol ratio [mol/mol]Fig 5-9. Single gas permeance as a function of permeating molecule-sizes at 200 ⁰C(a), thepermeance of gases as a function of silver/amine mole ratio (b)In accordance with the spectroscopy characterization results and membrane pore
structure evaluation, we proposed schematic illustrations for the silver-modified
aminosilica network structure as a result of changes in calcination temperature and in thesilver/amine mole ratio, and these appear in Figs. 5-10(a) and 5-10(b), respectively. Silver
facilitates a porosity transformation of the amine-functionalized alkoxysilane network via
sol-gel followed by coordination at different calcination temperatures (Fig. 5-10(a)). At a
low temperature of 150 ⁰C, low levels of gas permeance and selectivity of the membrane
were attributed to the existence of Si-OH groups and to the remaining nitrate ions(NO3-),

239

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --which was confirmed by FT-IR and TG-MS results (Figs. 5-3 and 5-4). An optimizedbalance between the coordinated network and the silver nanoparticles of Ag-BTPA wassuccessfully obtained at 250 ⁰C with higher gas permeation, while a disjointed network
was present after the decomposition (at 350 ⁰C) of amine groups, which facilitated alinkage to the silver ions.

(a)

150 ⁰C

250 ⁰C

350 ⁰C(b)

Ag loading increased
Fig. 5-10. Schematic illustration of the establishment of Ag-BTPA 0.5 network at different
calcination temperatures (a), and proposed mechanism for the formation of silver-amine
coordination and silver growth on the surface of aminosilica membranes (b)The effect that various silver/amine mole ratios exert on the formation of the AgBTPA network is shown in Fig. 5-10(b). The addition of silver dopant into the aminosilica precursor (BTPA) tended to generate the formation of silver nanoparticles, even at
the lowest concentration. The amine groups in the linking units of BTPA acted as colloidal
suspension stabilizers that modified the surface properties of AgNO3 by avoiding theagglomeration of metal particles [66]. Moreover, the amine groups possibly reduced theAg+ ions into Ag0 via the electronic transfer of lone pairs of electrons from the nitrogenatoms into the partially empty d orbitals of the silver ions. This process led to a decreasein the potential of Ag+/Ag which then promoted the reduction of Ag+ [66]. In addition,

240

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --the reduction of Ag+ into Ag0 was accelerated via heat treatment at 250 ⁰C [45], whichthen simultaneously resulted in silver growth to produce smaller-sized nanoparticles.

5.3.6 H2 transport through Ag-BTPA membranes: the temperaturedependence of gas permeanceH2 permeation enhancement of Ag-BTPA was achieved since the two-phasestructure of silver facilitated the molecular sieving and surface diffusion processes. To
further analyze the effect of silver on H2 permeation, the permeance of a particular gas
was compared with inert gases such as He and N2 in terms of temperature dependence, as
shown in Fig. 5-11. The activation energy value of each gas was calculated based on theslope of the regression curve obtained by using the modified gas translation (m-GT)
model as expressed in Eq. 5-3.
୩

୉

ౌǡ౟
୫ିୋ୘ǡ୧  ൌ  ට୑ బǡ౟ ቀെ ୖ୘
ቁ
ୖ୘
౟

(5-3)

The estimated activation energy values of He, H2, and N2 for BTPA and Ag-BTPA 0.5
are summarized in Table 5-1. BTPA with a flexible structure of organic linking units
between Si atoms showed a large value of activation energy for permeation that ranged
from 10 – 20 kJ mol-1, which can be ascribed to the dense structure of the membrane.
Meanwhile, the activation energies of He and N2 in Ag-BTPA were decreased with aslight increase in the gas permeance. This suggests that the formation of silver-amine
coordination improved the membrane connectivity and porosity as evidenced in Fig. 5-6
via the crosslinking between network layers that formed highly permeable subnanoporous networks. The permeance of all gases was decreased with decreasing
temperatures, indicating an activated permeation mechanism. The slopes of temperaturedependency for He and H2 in the pristine BTPA membrane showed identical trends,
suggesting similar transport for both types of gases. Conversely, the permeance of thesmaller-sized H2 in Ag-BTPA 0.5 was greatly decreased with decreasing temperature,
which is similar to the larger-sized N2 (Fig. 5-11(c)), which resulted in a high level ofactivation energy. The larger level of activation energy indicated that higher energy was241

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --required for H2 to travel through the Ag-BTPA membranes, particularly at a lower
temperature of 50 ⁰C.
(b)

10-6
13.7 kJ mol-1

10-7
BTPA
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5

10-8
2.0

15.7 kJ mol-1

2.4

2.8

Ep = 17.1 kJ mol-1

10-6

Ep = 16.6 kJ mol-1
Ep = 15.4 kJ mol-1

10-7

10-8
2.0

3.2

2.4

(d)

10-6
BTPA
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5-721.3 kJ mol-1

10-8
23.8 kJ mol-1
22.6 kJ mol-1

2.4

2.8

25

3.2

N2

20

H2

15

He

10
100

19.9 kJ mol-1

10-9
2.0

2.8
1000/T [K-1]

1000/T [K-1]

10

Ep = 15.1 kJ mol-1

BTPA
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5

Ep [kJ mol-1]

N2 Permeance [mol m-2 s-1 Pa-1](c)

H2 Permeance [mol m-2 s-1 Pa-1]

10.9 kJ mol-1

14.4 kJ mol-1

1 -5
10

Permeance ratio [-]He Permeance [mol m-2 s-1 Pa-1](a) 10-53.2

1000/T [K-1]

H2/N2
He/N2
10

1
0.0

H2/He
0.1

0.2

0.3

0.4

0.5

Silver/NH mole ratio [-]Fig. 5-11. Effect of the temperature measurement in the range of 50 – 200 ⁰C on He (a) and H2(b), N2 (c), and the effect of silver addition on the permeance ratio (lower figure) and theactivation energy (upper figure) of He, H2 and N2 for BTPA and Ag-BTPA membranes (d)Table 5-1. The estimated values of activation energy, Ep (kJ mol-1), for BTPA- and AgBTPA-derived membranes
Membrane

He

H2

N2

BTPA
Ag-BTPA 0.2
Ag-BTPA 0.3
Ag-BTPA 0.5

15.7
14.4
13.7
10.9

15.1
15.4
16.6
17.1

23.8
22.6
21.3
19.9

242

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --For organosilica membranes, permeation occurs through the adsorption process( ο‫ܪ‬ௌ ), which then accompanies the diffusion of permeating molecules ( ο‫ܧ‬஽ ) thatgenerates the activation energy of permeation (ο‫ܧ‬௣ ሻ, as shown by Equation (5-4).
ο‫ܧ‬௣ ൌ  ο‫ܪ‬ௌ ൅  ο‫ܧ‬஽

(5-4)

The heat of adsorption ( ο‫ܪ‬ௌ ) is related to the interaction between the permeating
molecules that pass through the membrane pore wall. In this analysis, He and H2 werecompared since both gases have similar smaller-sized kinetic diameters of 0.26 and 0.28
nm, respectively. Fig. 5-11(a) shows the temperature dependence of He for pristine BTPA
and Ag-BTPA membranes calcined at 250 ⁰C under an inert atmosphere. The activationenergy of He was decreased with increases in the silver content, which suggested larger
pore sizes in the membrane network layers. The activation energy of H2 for different
amounts of silver-doped BTPA membranes is compared in Fig. 5-11(b). The H2
permeation rate was increased with a silver mole ratio between 0.1 and 0.5. In line withthis finding, the activation energies were gradually increased with an increase in the silver
content. Ag-BTPA 0.5 exhibited the lowest He activation energy of 10.9 kJ mol-1 but
showed the highest H2 activation energy at 17.1 kJ mol-1. The existing silver nanoparticles
on the Ag-BTPA network facilitate intermolecular attractive interaction between H2 andthe membrane pore wall. For this reason, the heat of adsorption for H2 was increased witha higher silver mole ratio, which resulted in a higher level of activation energy for
diffusion. He showed a different trend, however, where the estimated heat of adsorption
equaled zero due to non-specific interactions that resulted in a smaller level of activationenergy for diffusion (ο‫ܧ‬஽ ). The permeation of He occurs mostly through the pores of themembrane network layers that are obtained at higher levels of the silver mole ratio. At anelevated temperature of 200 ⁰C, silver nanoparticles possess a specific surface interactionthat enhances the diffusion of H2 by substantially reducing its permeance at a lower
temperature. Zhao et al. [67] reported that silver modification through Pd-type
membranes changes the surface transport, which impacts the H2 permeation rate, thetemperature dependency, and increases the activation energy at low temperatures. The
higher diffusion rate of H2 at 200 ⁰C through the Ag-BTPA membranes could possibly
be correlated with the higher connectivity of a coordinated network and with the freemovement of molecules inside the crystalline face-centered (fcc) cubic silver lattice.

243

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 5-11(d) shows the permeance ratio for different pairs of gases as a functionof the silver/amine mole ratio. With an increase in the Ag/NH mole ratio, Ag-BTPA
membranes exhibit almost a constant, or at least a slightly decreased, permeance ratio of
30 – 20 for He/N2, which suggests that the average pore size of the membranes was
unchanged, or possibly only slightly increased, following modification with silver. On
the other hand, the permeance ratios of H2/He and H2/N2 showed a clear increase withincreases in silver doping. In addition, the activation energies of He and N2 as inert gases
were decreased with increases in the silver content while H2 activation energy showed an
opposite trend. These findings suggest that a higher silver content promotes specificdiffusion for H2 while maintaining the same level of rejection for other gases, which isinduced by specific interactions between the silver nanoparticles and H2. Yang and Chen[45] also reported that metallic silver accelerated the transport of H2 through silica
membranes, which resulted in high separation performance.

5.3.7 H2 separation for Ag-BTPA membranes
The commercialization of membranes for industrial application has gained muchattention during the past few decades in efforts to explore the opportunity for obtaining
more permeable and selective membranes [68]. The correlation between the selectivity
and permeability of polymer membranes is referred to as the upper boundary, as reported
by Roberson [69, 70]. Based on this correlation, more-permeable polymers have shown
lower levels of selectivity, and vice versa. Since doping a BTPA network with silver
enhanced the permeance of H2 (Fig. 5-9(a) and (b)) in this study, silver doping was
examined for its potential to improve the separation of H2 via specific interactions throughthe two-phase structural evolution of silver-amine coordination and silver nanoparticles.
Fig. 5-12 compares the trade-off between H2 permeance and the permeance ratio for H2/X
(X = CO2, and C3H8) in an effort to analyze the separation performance of Ag-BTPAderived membranes. In Fig. 5-12, the permeance ratio is plotted as a function of
permeance rather than permeability that was used for the Robeson plot, because levels of
permeance are primarily obtained from experiments, which makes them more practical
for real applications.

244

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Fig. 5-12(a) provides the experimental data on the separations of H2/CO2 fordifferent types of membranes. Fig. 5-12 confirms the reproducibility of Ag-BTPA
membranes since each plotted point represents a different membrane (details of the
reproducibility of Ag-BTPA appear in SI-(5-2), Fig. S5-2. Fig. S5-2 demonstrates theresults of the permeation measurements and the standard deviations for three different
Ag-BTPA 0.5 membranes prepared under identical conditions was less than 20%. AgBTPA membranes showed an improvement in both permeance and selectivity that
surpasses the trade-off lines. The permeance of H2 was increased by 1.46 ൈ 10-6 mol m-2
s-1 Pa-1 for Ag-BTPA 0.5, which is larger than the corresponding pristine BTPA (5.76 ൈ
10-7 mol m-2 s-1 Pa-1). Ag-BTPA showed separation performance that was higher than thatof polymer membranes such as polyimide, polyaniline, etc.

(a)

(b)

10
1000

10
10000

This work 0.5
1000This work

0.2

10

1

0.1
10-10

Polymer
ZeoliteCarbon
MOF
Organosilica
Ag-MTESAg-BTESM
Ag-BTPA

10-9

0.5
1.0

0

0.3

PH2/PC3H8 [-]H2/CO2 [-]

100

0.2

100

10

1

10-8

10-7

10-6
-2

-1

10-5

-1

H2 Permeance [mol m s Pa ]

0.1
10-10

0

0.3

Polymer
ZeoliteMOF
Organosilica
Ag-BTESM
Ag-BTPA
10-9

10-8

10-7

10-6

H2 Permeance [mol/(m2 s Pa)]

10-5

Fig 5-12. Gas permeation plots of H2/CO2 (a), and H2/C3H8 (b) for hybrid silver-doped
aminosilica membranes (Ag-BTPA) containing different metal in coordinated and metallic
crystals compared with various type of membranes

Yang and Chen [38] provided an in-depth analysis of the contribution of the methyl
ligands of methyltriethoxysilane (MTES), and doping with silver was investigated for its
effect on the enlargement of average pore size and total pore volume of the membranesfor H2/CO2 separation. As discussed in section 5.3.6, silver crystalline structures with anestimated average size of 28 nm promoted the surface diffusion of H2 through a silica
network, which resulted in a high permeance of H2 at 5.5 ൈ 10-6 mol m-2 s-1 Pa-1 with an

245

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --elevated value in the permeance ratio to 6 at an operating temperature of 200 ⁰C with apressure difference of 0.35 MPa. The increase in H2 permeance with increases in the silver
content was due to the formation of a rigid segment caused by the silver-amine
coordination, which led to an increase in microporosity, whereas a higher permeance ratioof H2/CO2 was generated by the dual-network structure of Ag-BTPA that reduced the freevolume for the diffusion of larger-sized gases. In addition, an increase in the silver content
tended to enhance the surface diffusion of H2, which is similar to reports by Xiao et al.
[71]. The results of that study indicated an enhancement of gas-pair selectivity as an
outcome of higher levels of interconnecting micropores that were formed by the migrationof the smaller-sized disintegrated silver particles and by growth from the larger-sizedcarbon particles during the pyrolysis process. This finding supports evidence that thecoordination following the sol-gel process possessed a tunable pore size of the membranewith increases in network connectivity, which is promising for hydrogen separation via
molecular sieving properties.
Moreover, the dual flexible-rigid network of Ag-BTPA demonstrated impressive
separation of H2 over C3H8, as shown in Fig. 5-12(b). The performance of the membrane
was remarkably enhanced with high H2 permeance at 1.46 ൈ 10-6 mol m-2 s-1 Pa-1 andH2/C3H8 selectivity of 1,500. The conversion yield of C3H6 from propane
dehydrogenation is strongly restricted by the equilibrium reaction, as demonstrated by
Eq. 5-5 [17]. The removal of coexistent H2 as a product of the reaction could purify C3H8.
The pore sizes of Ag-BTPA membranes averaged 0.45 nm, whereas the kinetic diameterof C3H8 (0.506 nm) was much larger than the membrane pore size. Hence, the design of
Ag-BTPA pore size allows H2 to pass through the network while rejecting C3H8
molecules.
ଷ଼  ֖  ଷ଺ ൅ଶ 

(5-5)

Fig. 5-13(a) shows the temperature dependencies of H2 and C3H8 permeance andthe permeance ratio of H2/C3H8 at temperatures ranging from 50-200 ⁰C for an Ag-BTPA
0.5 membrane calcined at 250 ⁰C under a N2 atmosphere. The activation energies of H2and C3H8 through an Ag-BTPA 0.5-derived membrane were 17.1 and 0.35 kJ mol-1,

246

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --respectively. The permeance of H2 was increased with increases in temperature, whichsuggests an activated diffusion mechanism.

H2
10

-6

1000
10-7

H2/C3H8

10-8

100

C3H8
10

-9

10-10
2.0

2.4

2.8

(b)

1 -5
10

H2

10-6

104

Dnetwork H2/C3H8

10-7

103

Dobserved H2/C3H8

10-8

C3H8 (observed)

10-9

102
101

C3H8 (network)
10

10
3.2

105
200 qC

-10

0.0

0.1

1000/T [K-1]

0.2

0.3

Permeance ratio H2/C3H8 [-]10000

Permeance [mol m-2 s-1 Pa-11]

1 -5
10

Permeance ratio [-]Permeance [mol m-2 s-1 Pa-1]

(a)

100
0.5

0.4

Silver/NH mole ratio [-]Fig 5-13. Temperature dependence of H2 and C3H8 permeation for Ag-BTPA 0.5 membrane
calcined at 250 ⁰C (a), and permeation performance as a function of silver/NH mole ratio of AgBTPA membranes for the separation of H2 and C3H8 at 200 ⁰C (b)Contrary to this, the permeation of C3H8 was increased at a lower temperature, whichindicated a surface diffusion mechanism. It should be noted that the surface diffusiontransport of C3H8 was different from the facilitated diffusion of H2 via silver
nanoparticles. Most of the H2 molecules permeated through the network of Ag-BTPA
membranes, while the larger-sized molecules of C3H8 could only permeate via surface
diffusion through an affinity with the membrane pore wall. This finding confirms thecontribution of silver ions and nanoparticles in facilitating the transport of H2 while
retaining C3H8 molecules through an affinity with the membrane pore wall, which has
resulted in a higher permeance ratio for H2/C3H8 at a higher temperature of 200 ⁰C.
To develop a better understanding of the permeation mechanism of C3H8 throughthe membrane pore structure of Ag-BTPA, the C3H8 permeance through the network is
defined by Eq. (5-6).
஼యுఴ ౤౛౪౭౥౨ౡ ൌ ஼యுఴ െ ൬ௌிల ට୑୛ೄಷల୑୛಴య ಹఴ

൰

(5-6)

A bulky molecule of SF6 was used as an indication of the permeation through cracks andthe extra crystalline pores or defects [72]. The practical development of organosilica

247

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --membranes has always favored the development of a distribution of pore sizes rather thana single-sized pore structure. Consequently, the permeation of C3H8 could be classified
into two different mechanisms: molecular sieving through network pores, and Knudsen
diffusion through the defects in a membrane. As Fig. 5-13(b) shows, the permeance of H2
was increased with an increase in the silver content, while the permeance of C3H8 throughnetwork pores revealed a sharp decrease in the higher content of silver. These findings
suggest that the increase in silver content resulted in a greater degree of crosslinking withthe amine moiety and a higher rate of smaller-sized silver nanoparticle development. The
formation of defects in Ag-BTPA 0.5 was minimized through the coordination of thelinking unit chain that led to the formation of more uniform network pores, thereby
reducing the permeance of C3H8 and increasing the selectivity of the H2/C3H8 that could
diffuse through the network pores of Ag-BTPA.

5.4 Conclusions
The present study was designed to determine the effect of a two-phase structural
evolution of silver: the Ag+ ion state with amine coordination and Ag0 nanoparticles thatdeveloped from reduction when exposed to the aminosilica surface. This investigationshows that a gradual increase in the formation of microporosity could be achieved with ahigh silver/amine mole ratio of 0.5 that produced a high surface area of 115 m2 g-1 and apore volume of 0.149 cm3 g-1. Spectroscopy study revealed that the dynamic processes ofhydrolysis and condensation were followed by coordination that generated silver
covalently bonded with monodentate-amine moieties. In addition, amine groups
promoted the reduction of Ag+ into Ag0 which then simultaneously became a smallersized nanoparticle. The two-phase structural evolution of silver was confirmed tocontribute to the transformation of gas permeation behavior. Ag-BTPA 0.5 showed highH2 permeance of 1.46 ൈ 10-6 mol m-2 s-1 Pa-1 and a high selectivity for H2/C3H8 separationof 1,500. The partial structure of the silver-amine coordination with respect to the silver
nanoparticles facilitated the diffusion of a smaller-sized gas such as H2 and the amorphousnature of a silica-based framework that rejected a larger-sized gas such as C3H8, resulting
in high selectivity by the membrane.

248

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stability and their applications to membrane reactors for steam reforming of
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Adsorption, 5 (1999) 369-372.

253

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Supporting Information----------SI-(5-1)---------Membrane preparation

Sol preparation
BTPA/water/HNO3=1/300/1

Porous Al2O3 glass tube
(pore size 500 nm)

EtOH +BTPA

Coating of α-Al2O3 particle (particle size
= 1-2, 0.2 μm), fired at 550 ⁰C for 15
minutes in air

H2O + HNO3
Hydrolysis and polycondensation at
25⁰C, 500 rpm, 12 hours.

BTPA sol

Hot coating of colloidal SiO2-ZrO2 sol,
fired at 550 ⁰C for 15 min in air

AgNO3Coordination 25⁰C for 1 hour

Hot coating of Ag-BTPA sol, fired at 250
⁰C for 30 min in N2

Ag-BTPA sol

Ag-BTPA membrane

Fig. S5-1. The flowchart of experimental study for the processes of sol preparationand membrane fabrication of Ag-BTPA-----------------------------------------------------------------------------------------------------------------------------SI-(5-2)----------

Permeance [mol m-2 s-1 Pa-1]

10-5
200 qC
-6

10

10-7
10-8
10-9
10-10

Ag-BTPA 0.5 (1)Ag-BTPA 0.5 (2)Ag-BTPA 0.5 (3)0.25 0.30 0.35 0.40 0.45 0.50 0.55
Molecular diameter (nm)Fig. S5-2. Reproducible data of single-gas permeance for Ag-BTPA 0.5 as a function of the
kinetic diameter size of gases measured at 200 ⁰C------------------------------------------------------------------------------254

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Chapter 6
Conclusions
This dissertation is dedicated to the development of metal-induced microporous
aminosilica via the crosslinking between donor pairs of electrons in the amine moiety andelectron acceptors in the empty “d” orbital of the transition metal for highly permeable
membranes and selective transport of specified components. The structuraltransformation of the coordinated network and surface chemistry have been clearly
confirmed and illustrated via a series of characterizations. To further improve themembrane performance, the microstructure of metal-doped aminosilica membranes was
tailored via the optimization of metal with different affinity, the effect of metal mole ratio,
and the evaluation of coordination affinity via different types of amine-functionalizedorganosilica precursors. The present research highlights the novel strategy to modify thedense framework of aminosilica network into microporous membranes, which favorable
for the development high flux and selective membranes for gas and liquid mixtures
separations. The main conclusions and outlook of this thesis were summarized as follows:

6.1Conclusions
The main conclusions in this thesis were summarized according to each chapter asfollows:[1]. The present research was focused on examining the affinity of different metals forthe formation of coordinated compounds with the amine moiety of an amorphoussilica matrix. The characterization of metal-doped BTPA has shown evidence of ametal-amine interaction that is induced by the reorganization of bridges in the aminesilsesquioxane network that allow maximum coordination to generate a more rigid
structure. In this investigation, we found that the surface area and pore volume of ametal-doped BTPA composite was increased by increasing the metal coordination
affinity on the order of Ni-BTPA > Cu-BTPA > Ag-BTPA > BTPA. An evaluation of
single-gas permeance showed higher values for gas permeation, which corresponded
to transformations in the membrane structure following the metal-doping process and255

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --simultaneously generated a defect-free membrane. The activation energy ofpermeated gas was decreased by increases in membrane pore size and in the network
connectivity on the order of Ni-BTPA < Cu-BTPA < Ag-BTPA < BTPA. The NiBTPA membrane revealed excellent separation performance with permeance andselectivity of N2 that reached 3.75 × 10-7 mol m-2 s-1 Pa-1 and 1,900, respectively.
[2]. Ordered microporous nickel-doped BTPA membranes were fabricated via different
nickel loadings to create coordination with nitrogen-enriched BTPA precursors.
Spectroscopic characterization consistently showed a linear correlation between
material properties and membrane performance, where a higher nickel content
generated a higher surface area of Ni-BTPA powders on the order of Ni-BTPA 0.50 >Ni-BTPA 0.25 > Ni-BTPA 0.125 > BTPA. The pore size of the Ni-BTPA membranes
also increased with Ni-doping with a higher gas flux of the membrane. Afterward, ahigher nickel content (1.00) tended to generate phase separation that reduced theseparation factor of the membrane, thus further confirming that the optimal point fora nickel mole ratio is 0.50. Ni-BTPA exhibited superior performance for gas
separation (N2/SF6 > 100 – 1,000, N2 permeance > 10-7 mol m-2 s-1 Pa-1) andmethanol/toluene separation with high flux and separation factors of 2.8 kg m-2 h-1
and 907, respectively, for a 10 wt% methanol concentration in the feed solution.
[3]. Microporous nickel-doped aminosilica membranes were successfully synthesized via
a sol-gel process that was followed by a coordination reaction. The nickel-composite
membranes show attractive separation performance with methanol-selective
properties and high flux. The optimal nickel/amine mole ratio and appropriate
calcination temperature for the development of high porosity, facilitated the highdiffusion of methanol as follows: Bridge-structure amine precursor (Ni-BTPA) =0.50 – 250 ⁰C; and Pendant-structure amine precursors = Ni-APTES = 0.25 – 250 ⁰C;
Ni-MAPTS = 0.25 – 300 ⁰C. The permeation mechanism for nickel-doped
aminosilica membranes was influenced by the pore size distribution of themembranes and by the affinity between the hydrophilic surface of the membranes
and the permeating molecules. Competitive adsorption was observed for alcohol
mixtures (MeOH/EtOH, MeOH/IPA, and MeOH/tert-BuOH), which then resulted inpore blockage via hydrogen bonding interactions. The nickel-doped aminosilica
membranes show high levels of flux and separation factors on the order of Ni-BTPA >256

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Ni-APTES > Ni-MAPTS. The Ni-BTPA membrane recorded the most outstanding
performance with a high flux of 1.42 kg m-2 h-1 and a superior separation factor for
MeOH/Tol of 5,000.
[4]. The present study was designed to determine the effect of a two-phase structural
evolution of silver: the Ag+ ion state with amine coordination and Ag0 nanoparticles
that developed from reduction when exposed to the aminosilica surface. This
investigation shows that a gradual increase in the formation of microporosity could
be achieved with a high silver/amine mole ratio of 0.5 that produced a high surface
area of 115 m2 g-1 and a pore volume of 0.149 cm3 g-1. Spectroscopy study revealed
that the dynamic processes of hydrolysis and condensation were followed bycoordination that generated silver covalently bonded with monodentate-amine
moieties. In addition, amine groups promoted the reduction of Ag+ into Ag0 whichthen simultaneously became a smaller-sized nanoparticle. The two-phase structural
evolution of silver was confirmed to contribute to the transformation of gas
permeation behavior. Ag-BTPA 0.5 showed high H2 permeance of 1.46 ൈ 10-6 mol
m-2 s-1 Pa-1 and a high selectivity for H2/C3H8 separation of 1,500. The partial
structure of the silver-amine coordination with respect to the silver nanoparticles
facilitated the diffusion of a smaller-sized gas such as H2 and the amorphous natureof a silica-based framework that rejected a larger-sized gas such as C3H8, resulting inhigh selectivity by the membrane.

6.2Recommendations
This thesis has contributed to an interesting topic about the formation of metalcoordinated amine-functionalized organosilica membranes through the coordination ofcovalent bonding between the transition metal and amine moieties. The formation of
coordinated networks with more rigid frameworks features a high-dimensionalframework, a tailorable pore structure, and a large specific surface area. Based on thestudies in Chapters 2 and 4, both ligand and metal affinity largely affected the formationof microporosity, thus resulting in membrane material with different properties comparedto the pristine aminosilica membranes. Pristine aminosilica membrane with a highly
flexible linking unit is commonly known to generate a dense structure with high activation

257

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --energy for permeating molecules, while metal-coordinated aminosilica membranes show
an interesting property of high permeating flux and high selectivity. This finding
suggested the existence of the partially crystalline structure from metal-amine
coordination and the amorphous nature of the silica networks could beneficially improve
membrane performance that surpasses the trade-off trend. In the other discussion in
Chapters 3 and 5, we found that the existence of metal in two different phases as ionsand nanoparticles can facilitate the formation of coordinated networks and act as open
metal sites for facilitated transport, respectively. However, some more in-depth studies
should also be done to further evaluate the role of the formation of coordinated networks
as well as its synergistic effect on the material structure and membrane performance inthe development of highly permeable membranes.
Based on the results and the conclusions derived from this thesis, the particular
recommendations were suggested for future works are given as follows:[1]. More studies should be devoted to the reaction rate estimation and the order ofreactions since the development of a coordinated network involves three steps
reaction of hydrolysis, condensation, and coordination. The optimized condition canbe used to predict the formation of a microporous structure in relation to themembrane separation performance.
[2]. Metal-coordinated aminosilica membranes contain a certain amount of metal ionsthat are connected to the amine moieties. The crosslinking degree of thesemembranes was high, thus increasing the hydrophilic properties of the membranes.
The existence of weak coordination bonding between metal and amine groups results
in a material that is stable in an organic system but unstable in an aqueous solution.
Therefore, an investigation for improving the stability of coordinated membranes in
an aqueous system can be considered.
[3]. Based on the discussion in chapter 3, the saturation point for the development of thecoordinated network was at a metal/amine mole ratio of 0.25 mol mol-1, which means
one metal ion can be coordinated with a maximum of four amine ligand molecules.
Studies on the utilization of extended linkers for increasing the coordinated network
as well as the microporosity on amorphous aminosilica membranes are alsosignificant to direct the preparation of highly permeable membranes.

258

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --List of Publications

Peer-reviewed journal articles:
1.

Ufafa Anggarini, Liang Yu, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori
Tsuru*, Metal-induced microporous aminosilica creates a highly permeable gasseparation membrane. Mat. Chem. Front. 2021: 5, 3029-3042.

2.

Ufafa Anggarini, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori Tsuru*,
Microporous

Nickel-CoordinatedAminosilica

Membranesfor

Improved

Pervaporation Performance of Methanol/Toluene Separation. ACS Appl. Mater.
2021: 13, 23247-23259.
3.

Ufafa Anggarini, Liang Yu, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori
Tsuru*,

Structuraltransformation

of

the

nickelcoordination-induced

subnanoporosity of aminosilica membranes for methanol-selective, high-flux
pervaporation. J. Membr. Sci. 2022:
4.

Ufafa Anggarini, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori Tsuru*,
Structural two-phase evolution of aminosilica-based silver-coordinated membranes
for increased hydrogen separation. J. Membr. Sci. 2022: 642, 119962.

Proceedings:
1.

Ufafa Anggarini, Liang Yu, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori
Tsuru*, A novel design of metal coordinated aminosilica for highly permeable
membrane, SCEJ 86th Annual Meeting 2021, March 21-22, 2021, Japan. (Oral)

259

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --2.

Ufafa Anggarini, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori Tsuru*,
Aminosilica-based metal coordinated membranes: Tailoring the microporosity foradvanced molecular separation, MRM2021 Materials Research Meeting, December
13-17, 2021, Yokohama, Japan. (Poster)3.

Ufafa Anggarini, Hiroki Nagasawa, Masakoto Kanezashi, Toshinori Tsuru*, Hybrid
nickel-coordinated aminosilica membranes for the selective pervaporation of
methanol: tuning coordinated structure by a variety of amine types, SCEJ 87th
Annual Meeting 2021, March 16-18, 2022, Kobe, Japan. (Oral)

260

Hiroshima University Doctoral Dissertation----------------------------------------------------------------------------------------------------------------------------- --Acknowledgments
First and foremost, all praise to Allah SWT, I praise and thank Allah the Almightyfor His grace, love, mercy, and guidance throughout my life. With Allah’s will and mercy,
I have been able to undertake this research study and complete it. Without the blessing
from Allah, this achievement would not have been possible.
I would like to express my sincere gratitude to my advisor Prof. Toshinori Tsuru,
for his continued guidance, inspiration, great support, and encouragement in every stageof my Ph.D. study. Prof. Toshinori Tsuru is a distinguished expert in both chemicalengineering and membrane separation technology. I am always touched and learn a lot
from his professional dedication, profound knowledge, and perseverance in scientificresearch which will provide me with lifetime benefits for my future research career.
I am very grateful to Prof. Masakoto Kanezashi and Prof. Hiroki Nagasawa for
their kind assistance in my experiment, insightful discussion of my research work, andvaluable suggestions for the organization of journal articles. I also would like to express
my sincere gratitude to Prof. Joji Ohshita and Prof. Toru Ishigami on my dissertationcommittee for reviewing my thesis in their busy schedule.
I would like to appreciate the help and guidance from Dr. Liang Yu who always
support and give kind assistance to my research work. He gave me constructive advice
when I started the experimental work and helped to understand through the discussion.
I am also grateful to my tutor, Mr. Keita Nakahiro, for his warm-hearted assistancein my early daily life in Japan. I would like to thank all other group members, past andpresent in the Separation Engineering Lab., for their sustained help and for all the good
moments we spent together.
I am also grateful to Japan Government for the financial support under
Monbukagakusho Scholarship that enabled this work to be successfully completed.
Lastly, I would like to thank my family member, especially my parents (Setyo
Suwarno and Sunarsih), my husband (Ndaru Candra Sukmana), and my son (AthalaDzaky Sukmana) for their endless support, understanding, and encouragement, which
motivated me to insist on my study. I cannot acknowledge them enough and I love themfor the sake of Allah.
July 2022
Ufafa Anggarini

261