Transactions of the Materials Research Society of Japan, 29(7), 3299-3302(2004)

Gas and Vapor Permeation through Liquid Membrane Using Ionic Liquid
Nobuhiro Yamanouchi, Shuhong Duan and Akira Ito
Department of Chemistry and Chemical Engineering, Niigata University, Niigata 950-2181, Japan
Fax: 81-25-262-6789, e-mail: aito@eng.niigata-u.ac.jp

Ionic liquids are salts in liquid state at room temperature. This new material is expected to be applied toseparation processes. Ionic liquids are particularly promising material for supported liquid membrane process,because they are non-volatile and can solve other salts as separation agents. In this study, A liquid membraneusing an ionic liquid, 1-propyl-3-methylimidazolium iodide, was applied to gas and vapor separation of CO2,
water vapor from air and aromatic hydrocarbon vapors. A supported liquid membrane of the ionic liquid wasplaced on a hydrophobic microporous membrane. The thickness of the liquid membrane was 35-55 µm. Since
the ionic liquid is non-volatile, hydrophilic and high surface tension, the liquid membrane could be applied togas and vapor permeation with vacuum at permeate side. Overall permeation property, permeabilities, wassimilar to that of a triethylene glycol liquid membrane. The membrane of the ionic liquid showed selectivityof 20 for CO2 over CH4, 1000 for water vapor over air and 20 for benzene vapor over cyclohexane vapor. The
membrane of the ionic liquid is promising for dehumidification and the separation of CO2, VOC and aromatichydrocarbons. The high selectivity for water vapor may be caused by the hydrophilic property of the ionicliquid. By adding alkaline salt to the liquid membrane, the selectivity for CO2 was enhanced.

Key words: Membrane separation, Liquid membrane, Ionic liquid, Dehumidification, Permeability

1. INTRODUCTION
aromatic vapor separation [6].
Room temperature Ionic liquids(RTILs) are salts
In this study, we use a RTIL for liquid membrane
which melt near room temperature. The RTILs consist
material. The vacuum mode permeation experiments
large, organic cations and a variety of anions. This new
through the liquid membrane were conducted by feedmaterial has attracted for application to reaction solvents
gasorvapors
ofair/water-vapormixture
mixedgasand
as replacements for traditional organic solvents [1]. The(dehumidification),
CO2/CH4
aromatic/cyclohexane mixed vapor. The separation
RTILs are non-volatile and dissolve inorganic salts andperformances of the RTIL liquid membrane wereorganic compounds. These special features of RTILs
illustrated by concentration change between feed andsuggest new applications for separation processes, suchpermeate. The permeabilities of gases through the RTIL
as extraction, absorption and membrane separation. Theliquid membrane were evaluated from the permeation
research on a separation process using RTILs wasdata and were compared with that of other membranes.
recently started. Scovazzo et. al. [2] first applied asupported liquid membrane using RTIL to CO2
separation. Sorption and permeation properties of CO2
2. EXPERIMENTAL
and N2 were reported through a liquid membrane of theAn RTIL, 1-propyl-3-methylimidazolium iodide
most popular RTIL, [bmim][PF6]. They stated a([pmim][I]) (Shikoku Kasei Ltd., Japan), was used in
potentiality for RTIL liquid membrane
Hydrophilic treated PTFE
development for gas separation.
microporus membrane, δ=35 μm
We have reported gas and vapor
permeation through a triethyleneglycol
Ionic loqid(TEG) liquid membrane, which wasplaced on a hydrophobic microporous
membrane. Since TEG is non-volatile
Hydrophobic microporousand hydrophilic, the TEG liquidmembrane, δ=125 μm
membrane is stable on top of theDouble-layerhydrophobic microporous membraneliquid membraneundertransmembranepressure
Bubble flow meter
Flat typeconditions.
This
double-layer
permeation cell
construction enabled a vacuum-mode
101 kPa
permeation of gas and vapors throughGCthe TEG liquid membrane. The liquid
membranes using TEG and a mixture
Manometerof TEG with inorganic salts were CO2
Water saturator
applied to dehumidification [3],separation[4],
CO2/CH4CH4
1.3 kPa
Diaphragm vacuum pumppropane/propylene separation [5], andFigure 1. Membrane construction and experimental apparatus.

3. RESULTS AND DISCUSSION
3.1 Separation of carbon dioxide
Figure 2 shows the results of CO2/CH4 mixed gas
permeation, in which CO2 concentration in the permeateis plotted versus that in the feed. The feed gas was
saturated with water vapor. The RTIL Ionic liquid
membrane has CO2 selectivity of 20 over CH4. This
selectivity for CO2 is comparable to a TEG liquid
membrane [4] and a 1-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF6]) liquid membrane[2]. Water content in the RTIL liquid membrane may
increase the solubility and selectivity for CO2 [2]. In thisstudy, the liquid membrane of [pmim][I] can absorb 0.6
g-water/1 g-liquid in 95%RH air. The absorbed water inthe membrane play role to CO2 separation.
Ionic liquid can dissolve other salt. When K2CO3
was mixed to 40wt% with the ionic liquid membrane,

Ionic liquid δ= 55 μm
K 2 CO 3 50% δ=60 μm
Ionic liqid+K2CO3
1

CO2 in Permeate [mol farc.]

0.9
0.8

α= 60

0.7

α= 20

0.6
0.5
0.4
0.3
101 kPa

CO2/CH4CO2 /CH 40.2

1.3 kPaliquid membrane

0.1
000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

CO2 in feed [mol frac.]
Figure 2. Separation of CO 2 /CH 4 .

CO2 in Permeate [mol frac.]

1α=130.8
α=50.6Ionic liquid+K
K2CO32 CO 3 40% δ=60 μm0.4

Ionic liquid δ=55 μｍ0.2

CO 2 /CH 4CO2/H20Permeability
23[cm (STP)-cm/(cm -s-cmHg)]

this study. This RTIL has a hygroscopic property and ahigh surface tension (approximately 50 dyn/cm), which
are comparable to the properties of TEG.
The liquid membrane construction and experimental
setup for CO2/CH4 permeation are shown in Figure 1.
The liquid membrane had a double-layerconstruction. The top layer was a conventional
supported liquid membrane. A hydrophilic treatedpolytetrafluoroethylene (PTFE) microporous membraneof 1 µm pore size, 35 µm thickness and 83% porosity
was soaked in the ionic liquid, or its salt mixture. Thethickness of the liquid membrane was 20-60 µm, which
was comparable or more to the supporting membranethickness. The supported liquid membrane of the ionicliquid was placed on another microporous membrane, aDurapel membrane (Millipore). Durapel membrane issurface-treated microporous polyvinylidene fluoride(PVDF) membrane and has a highly-hydrophobic
surface. This membrane has developed for medical useof de-gassing from water. A Durapel membrane of 0.1µm pore size, 125 µm thickness and 70% porosity wasused throughout this study. Since the RTIL ishydrophilic and high surface tension, the liquid layer is
supported on the surface of the hydrophobic
microporous membrane under a transmembrane pressureof over 200 kPa.
The membrane cell was flat-type with a 24 cm2
permeation area. The feed side was atmospheric pressure.
The feed gas, CO2 and CH4 mixture, was saturated bywater vapor. The downstream pressure was maintained
1.3 kPa by a diaphragm vacuum pump. All
measurements were at room temperature.
The liquid membrane and membrane cell were samefor the dehumidification and the aromatic/aliphatic
vapor permeation. In the dehumidification experiment,
feed gas was humid air. The downstream pressure wasmaintained 0.1 kPa by a oil rotary vacuum pump. The
humidity change between feed air and retentate air wasmeasured using a humidity meter (HMP230,VAISALA).
In aromatic/aliphatic hydrocarbon mixed vapor
permeation, saturated vapors in carrier gas, N2, fed to themembrane cell. No water vapor was contained in thefeed. The downstream pressure was maintained 0.1 kPa
by a oil rotary vacuum pump. The permeate vapor andthe retentate vapor were collected in cold traps at 233 K.

-7

01x10

C02
0.2
0.4
CO2 permeability
HH2
2 permeability

0.6

0.8

1

Ionic liquid +K
K2CO32 CO 3 40%liquid membrane
-8

1x10

-9

1x10

00.2

0.4

0.6

0.8

1CO2 in feed [mol frac.]
Figure 3. Separation of CO 2 /H 2 .
the selectivity for CO2 improved to 60 at the lowconcentration range of CO2 as shown in the figure. It
may caused by a facilitated transport effect between thealkaline salt and CO2. In ionic liquid with water, a highcarrier concentration of alkaline salt and ionic field3.2 Dehumidification using RTIL liquid membrane
The dehumidification performance of the RTIL
liquid membrane was demonstrated by humidity changeof feed air through the membrane cell. Figure 4 shows
the dehumidification results, where the tested liquid
membrane were of TEG, RTIL and a mixture of RTIL
with hygroscopic salt, LiBr. The vertical axis shows thathumidity of the feed air decreases form 60%RH at theinlet to 20-30%RH at the outlet of the membrane cell.
Permeation rate of air was negligibly small. The driving
force of the water vapor permeation is the partialpressure difference between feed side and permeate side.
In the experimental condition, feed side partial pressureof water was corresponding to an average humidity
between inlet and outlet of 30-40%RH. Permeate side
partial pressure was much lower than 0.1 kPa. Since theabsolute permeation rate of water vapor was nearly
constant, the outlet humidity decreases with small feed
air rate. The RTIL liquid membrane has a comparable
dehumidification property to the TEG liquid membrane[3]. This results due to high hygroscopicity of the[pmim][I] ionic liquid.
To improve the dehumidification performance, ahygroscopic salt, LiBr, was mixed to the RTIL liquid
membrane. The water absorption of the ionic liquidincreases from 0.6 g-water/1 g-liquid to 1.5 g-water/1
g-liquid in 95%RH air with 25wt% mixing of LiBr.
Using this mixed liquid membrane, outlet air humidity
decreased compare to the pure ionic liquid membrane asshown in Figure 4. It may caused by a increased watercontent at the membrane surface faced to the feed air.
3.3 Separation of aromatic hydrocarbons
The RTIL is expected to have affinity to aromatichydrocarbons for its large polarity. Aromatic/aliphatic
mixed vapor permeation was tested thorough the RTIL
liquidmembrane.
Saturatedvaporsof
benzene/cyclohexane (C6), toluene/heptane (C7) orm-xylene/n-octane (C8), in carrier gas, N2, fed to themembrane cell. In Figure 5, the aromatic vapor mole
fractions in the permeate vapors are plotted versus thatin the feed vapors. Aromatic vapors passed
preferentiallythroughtheRTILmembrane.
Benzene(C6) selectivity was 20 over cyclohexane.
When the number of carbon increased from C6 to C8,
aromatic selectivity decreased. Vapor flux was about

Ionic liquid
Ionic liquid+LiBr 40%
TEG

100
101 kPa
0.1 kPa

Humidity [%RH]80

liquid membrane
20μm thickFeed air humidity

60

inlet
Dehumidificaiton

40outlet
20001002003004003Feed air flow rate [cm /min]Figure 4. Dehumidification of air.
Benzene(Ar)/Cyclohexane[C6] mixed vapor
Toluene(Ar)/n-Heptane[C7] mixed vapor
Aromatic hydrocarbon in permeate [mol frac.]

causes effective CO2 transport to the permeate side. This
results illustrate a potential of RTILs as base materialsof facilitated transport membranes for CO2 separation[2].
Figure 3 shows permeation result of CO2/H2 mixed
gas through the [pmim][I] liquid membrane contained
40wt% of K2CO3. CO2 concentration of permeate gas
and permeabilities of each components are plottedversus CO2 concentration of feed gas. CO2 alsopreferentially permeated in this process. The separation
factor for CO2 over H2 was 13. The lower permeabilityof light gas, H2, than CO2 means that the gas permeation
mechanism through the RTIL liquid membrane iscontrolled by solution process to the membrane material.
There also shown a increase of CO2 permeability at thelow concentration range due to a facilitated transporteffect for CO2 by alkaline salt.

m-Xylene(Ar)/n-Octane[C8] mixed vapor
10.9

α=20
α=9α=3.5

0.8
0.70.60.50.40.3

101 kPa

Mixed Vapor

0.20.1 kPaliquid membrane

0.1
000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.80.91

Aromatic hydrocarbon in feed [mol frac.]

Figure 5. Aromatic hydrocarbon separation.
0.02 kg/(m2-h). It was larger than that of polymers, andsmaller than triethylene glycol (TEG) liquid membraneor KI mixed TEG liquid membrane [6].
There has been no literature on selectivity or affinityof RTILs to aromatic hydrocarbon. It remains uncertain
on mechanism of preferentially permeation or facilitatedtransport for aromatics trough RTILs. In researches on
facilitated transport of liquid membrane, which arenormally aqueous solution, silver nitrate, AgNO3, is wellknown as a facilitating agent for permeation of olefin
and aromatic hydrocarbons [5, 6]. Propylene or benzene
vapor will preferentially permeate through a liquid
membrane of aqueous AgNO3 solution. In thisseparation process, the metal ion, Ag+, is considered to
be a carrier for the facilitated transport. The affinity ofIonic liquid
Ionic liquid+K2CO3
K2CO3 50%

3.4 Gas permeability through RTIL liquid
membrane
Figure 6 indicates the comparison of gas
permeability among the RTIL liquid membrane,
the TEG liquid membrane[3, 4] and silicone rubber
membrane. Permeability of liquid membranes wasevaluated from permeation rate, partial pressure
difference of the component across the liquid
membrane and liquid membrane thickness. The
permeation resistance of the Durapel microporous
membrane could be ignored. For the RTIL
membrane, the permeabilities of condensable gas,
CO2 and H2O, are larger than other gases. It means
the permeation through the RTIL membrane iscontrolled by the solution step of gas into themembrane material. This permeation property issimilar to TEG liquid membrane and silicone
rubber.
The gas permeabilities through the ionic liquid asa whole are lower than that through the TEG liquid
membrane. The difference in permeabilities due to thedifference in viscosity of the liquids, because diffusivity
in the liquid depends on viscosity. The viscosity of the[pmim][I] ionic liquid is grater than 100 mPa･s and theviscosity of TEG is 39 mPa･s at room temperature. Thisviscosity difference affects permeabilities for the twoliquid membranes.
From Figure 6, we can see advantages for a ionicliquid to be used as a liquid membrane material. One
advantage of the RTIL is low permeability of N2 andCH4. The RTIL liquid membrane is expected to apply tothe separation between air and condensable gas, e.g.
dehumidification and VOC removal from air. Anotheradvantage of the RTIL is that it dissolves other salt ororganic compounds as facilitation transport agent. As
shown in Figure 3, CO2 selectivity of the RTIL wasimproved by incorporation of alkaline salt.
Dehumidification performance was enhanced byincorporation of hygroscopic salt into the ionic liquid
membrane. The potential of RTIL liquid membranedevelopment exists in a facilitated transport membrane
for a special gas or vapor to be separated.
4. CONCLUSION
An application of the room temperature ionic liquid
was proposed and reported experimentally as a mew
material for separation process of gas and vapor. Aliquid
membraneusing
anionicliquid,
1-propyl-3-methylimidazolium iodide, was applied togas and vapor separation of CO2, water vapor from air
and aromatic hydrocarbon vapors. A supported liquid
membrane of the ionic liquid was placed on ahydrophobic microporous membrane. Since the ionicliquid is non-volatile, hydrophilic and high surface
tension, the liquid membrane could be applied to gas andvapor permeation with vacuum at permeate side. Overall

1x10

Triethylene glycol (TEG)-5

Silicone rubber

1x10

1x10

CO2-6

H2CH 4
N2-7

H 2O

32Gas Permeability [cm (STP)-cm/(cm -s-cmHg)]

an olefin with a metal cation result from theinteraction of the olefin π-orbital with the σ- andπ-orbitals of the metal. The selectivity of the RTIL
liquid membrane to aromatic vapor in this study
may due to a similar mechanism that the cation
part of the ionic liquid has an interaction toaromatic hydrocarbons.

1x10

1x10

1x10-8-9-1030

100CiilT

10[K]Figure 6. Comparison of gas permeabilities.
permeation property, permeabilities, was similar to thatof a triethylene glycol liquid membrane. The membraneof the ionic liquid showed selectivity of 20 for CO2 overCH4, 1000 for water vapor over air, and 20 for benzene
vapor over cyclohexane vapor. The membrane of theionic liquid is promising for dehumidification and theseparation of CO2, VOC and aromatic hydrocarbons.
The high selectivity for water vapor may be caused by
the hydrophilic property of the ionic liquid. By adding
alkaline salt to the liquid membrane, the selectivity for
CO2 was enhanced. The use of ionic liquids as a liquid
membrane material for gas and vapor separation has
advantage that ionic liquids have variety of sorption
properties by combination of anion and cation. Sinceionic liquids can dissolve other inorganic salts andorganic solvents, future applications of facilitatedtransport membrane are expected by mixing of afacilitation agent to an ionic liquid.
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Rogers, C. A. Koval, D. L. DuBois and R. D. Noble,
ACS Symp. Ser., 818, 69-87 (2003).
[3] A. Ito, J. Membrane Sci., 175, 35-42 (2000).
[4] A. Ito, S. Duan, Y. Ikenori and A. Ohkawa, Sep.
Rurif. Technol., 24, 235-242 (2001).
[5] S. Duan, A. Ito and A. Ohkawa, J. Membrane Sci.,
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