# Membrane-Based Separation of Phenol/Water  Mixtures Using Ionically and Covalently Cross-Linked Ethylene-Methacrylic Acid Copolymers

Authors: Alexander Mixa; Claudia Staudt
Journal: International Journal of Chemical Engineering
 (2008)

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## Abstract

Membrane-based separation of phenol/water mixtures with concentrations of phenol between 3 wt% and 8 wt% in the feed has been performed with nonmodified as well as  cross-linked ethylene-methacrylic acid (E-MAA) copolymers with different amounts of methacrylic acid. As cross-linking agents, aluminium acetyl acetonate, which leads to ionically cross-linked membranes, and 2,3,5,6-tetramethyl-1,4-phenylene diamine and glycerine digycidether, leading to covalently cross-linked membranes, have been used. Generally, it was found that with increasing phenol content in the feed, the total flux is increasing whereas the enrichment factor is decreasing. Using nonmodified membranes with higher methacrylic acid monomer content in the polymer, lower fluxes and higher enrichment factors were observed. Investigation of different cross-linked membranes showed that with high phenol concentration in the feed, ionic cross-linking seems to be very promising. Furthermore, variation of feed temperature shows that ionically cross-linked membranes reached higher fluxes as well as higher enrichment factors at elevated temperatures. The temperature-dependent data were fitted based on an Arrhenius-type equation, and activation energies for the permeation of phenol and water through the membrane were calculated.


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## Body

## 1. Introduction

In most
of the processes which are carried out in the chemical industry, the desired
purity of the products is not reached during the reaction process. Therefore,
technical separation methods are extremely important in order to reach high-quality
products in acceptable yields at low costs. Conventional methods of separating
mixtures like distillation, extraction, ad- or absorption as well as crystallization
processes and combinations of these methods are well established but in several
cases they are cost- and energy-consumptive so that alternative methods like
membrane-based separations are investigated. Membrane processes offer the
advantages of lower energy consumption compared to distillation as the
separation of mixtures can be performed at much lower temperatures than the
boiling temperatures of the components.Development
of suitable membrane materials and modules is necessary for successful
competition with conventional separation methods. Processes, well established
on the continuously growing membrane market, are dehydration of organic
solvents by pervaporation, enrichment of oxygen from air by gas permeation, and
gasoline vapour recovery by vapour permeation [1].
New fields of membrane applications are
the separation of olefin/paraffins [2],
the removal of di- and trinuclear aromatic components [3, 4] and sulphur containing aromatic components [5, 6] from gasoline as well as the removal of organic components as p-chloro- and dichlorophenols
[7], cresols, alkyl substituted phenols,
and aniline derivatives [8–10] from process or waste water streams.In particular, the removal of phenol is of
great interest in waste-water treatment. With a global production of 8 million
tons each year in 2001, phenol is one of the most important intermediates in
chemical industry. Phenol contaminated effluents arise, for example, during the
production processes of bisphenol A, phenol formaldehyde resins, and the Hock
process. The average concentration of phenol in these streams has been evaluated
to be between 3 wt% and 10 wt% depending on temperature. For the reduction of phenol from waste water,
several membrane extraction processes offer very promising separation
properties [9, 11, 12]. However, increasing prices for phenol as raw material
makes it worthwhile to recycle the valuable product from the stream, which is
difficult with membrane extraction processes. In order to realize an economically
feasible separation process, it is favorable to combine pervaporation with
other separation techniques, as shown in the hybrid process in Figure 1. Such
hybrid processes have been already proposed for the separation of other organic
compounds from aqueous streams. In this process, a one-step pervaporation
process is combined with a liquid-liquid phase separation where the condensed
permeate can be treated. The organic phase contains 72.5 wt% of phenol and can
be worked up by conventional methods, whereas the aqueous phase, containing 7-8 wt% phenol, can be combined with the feed stream. The retentate of the pervaporation
unit contains only a small amount of phenol, and can be reused as process
water.Figure 1
Process scheme for the removal of phenol from process water.Among all different kinds of
membrane polymers investigated for the phenol recovery from process and waste
water streams, poly dimethyl siloxanes and modified structures thereof are the
most frequently published ones [13–16]. For example, with a concentration of
approximately 5 wt% of phenol in feed, a flux of 0.3kg⋅m−2⋅h−1 for a PDMS membrane of 50 μm thickness was found with a
selectivity of 11.6 at 70°C [15]. With Polydimethylsiloxanimide, which is a siloxane
imide copolymer at the same feed concentration, much higher fluxes but lower
selectivities were found compared to pure PDMS at 60°C. Selectivities between 18–23
and fluxes between 12.4 and 27kg⋅μm⋅m−2⋅h−1 have
been obtained [17]. For polyimides as membrane material at a feed concentration
of 7.8 wt% phenol at 60°C, the selectivity reached is 7.5 and the fluxes are 14kg⋅μm⋅m−2⋅h−1 [18, 19]. For polyurethane, at the same phenol concentration in feed at 60°C, a flux
of 1.5kg⋅μm⋅m−2⋅h−1 was
reported with a selectivity of 74 [20, 21]. The ethylene methacrylic acid copolymer used
in this work showed separation data and fluxes comparable to those of the
polydimethylsiloxanimide.Since the permeate is phase separated after
condensation, and the aqueous phase can be recombined with the feed stream, for
the hybrid process as shown in Figure1, high flux membranes have much better application
potential than high selective membrane materials.The aim of this work was using a
commercially available rubbery polymer (ethylene methacrylic acid copolymer)
with functional groups, opening the opportunity of modifying the chemical structure
(e.g., performing ionical or covalent cross-linking). This has been shown before
for different polymer structures as a very successful method enhancing the
separation characteristics as well as the chemical and thermal stability of
membrane materials [22–25]. In the case of the ethylene methacrylic acid
copolymer used in this project it was even possible to use the basic polymeric
material for different separation problems (e.g., aromatics/aliphatics separation
as well as removal of phenol from aqueous streams applying suitable modifications)
[23, 24, 26].


## 2. Theoretical Background

### 2.1. The Pervaporation Process

Pervaporation processes are membrane-based
separations of liquid mixtures as principally shown in Figure2 [24]. Thereby a phase transfer from the liquid phase
to the vaporous phase through a polymeric membrane is occuring. In this process,
the liquid feed mixture to be separated flows along one side of the membrane
while the various feed components are permeating into and through the membrane
at different rates. Therefore, the liquid retentate leaving the unit on the
same side of the membrane as the feed enters is depleted in the components
permeating preferentially. Consequently, the vaporous permeate collected on the
back side of the membrane is enriched in the preferentially permeating
component. The driving force for the mass transport of each component, through
a membrane is the difference of the chemical potential Δμi (T, p, xi, where i=1,…,n) of the components present in the mixture between feed and permeate
side. In pervaporation, the difference in the chemical potential is mainly
achieved by keeping the permeate pressure much lower than the feed pressure.Figure 2
Principles of pervaporation: liquid feed, liquid retentate (depleted), solution-diffusion membrane, and vaporous permeate (enriched).


### 2.2. The Solution Diffusion Model

Often polymeric solution-diffusion membranes are
classified as “nonporous” membranes because no pores are existing, only
microvoids caused by free volume compartiments of the polymer chains. However,
porous membranes, which are separating mixtures based on a sieve mechanism, are
moving fast in the direction of nanoporous materials and the difference between
porous and nonporous membrane materials is disappearing. Therefore, it is
better to classifiy the polymeric membranes used in this work as
solution-diffusion-membranes instead of nonporous membranes.The main features of the so-called solution-diffusion model
widely used to describe the mass transport in selective polymeric layers are
given in Figure3. This model not only holds for pervaporation, but also for
vapor and gas permeation [27–29]. The basic assumptions of this model are the
existence of the thermodynamic phase equilibrium at both boundary surfaces of
the selective layer in contact with the upstream feed mixture and the
downstream permeate mixture.Figure 3
Solution-diffusion model describing mass transport through polymeric membranes.The diffusion process inside the selective layer is
described by Fick's law, which is assumed to be valid for each component,i=1,…,n, (see [30]) (1)Ji=−Di(wi,…,wn)ρMdwidz,where Ji is the mass flux, Di is the diffusion
coefficient, z is the coordinate perpendicular to the membrane surface, and ρM is the density of the selective
polymer layer. The concentration of each component in this polymer layer is
given by its weight fraction wi, defined as (2)wi=mimM+∑j=1nmj,where mi and mj are the masses of the various
components dissolved and mM is the mass of the polymer. Under
steady-state conditions, integration of (1) gives (3)Ji=ρMδM∫wiPwiFDi(wi,…,wn)dwi, with δM being the thickness of the
selective layer, while wiF and wiP are the weight fractions of the component i in the
selective layer in its boundary at the feed side and the permeate side,
respectively. These fractions depend on the composition of the corresponding
mixture in contact with the boundary. Generally, Di depends on the
weight fraction of each component permeating and consequently, the mass fluxes
Ji are mutually dependent.With (3), Ji of any component permeating can be calculated
for a given feed concentration if the solubilities wiF and wiP,
the
dependency of the diffusion coefficient Di on composition, the
thickness δM, and the density ρM are known at operating conditions.The weight fractionwiP of any component i in the permeate is given by the
ratio of partial molar flux Ji(partial) to the total molar flux Jtotal, (4)wiP=JiJtotal. In many cases, modeling of the mass transport through membrane materials
for better understanding of the factors influencing the transport is difficult
although the approaches have been strongly improved in the last ten years [31, 32].
The reason for this is that, for example, experimentally it is not possible to
measure concentration-dependent diffusion coefficients in mixtures and furthermore
the thickness and density of the membranes are strongly depending on operating
conditions, where in many cases swelling of the membrane is occurring. It has
been also shown that solubility measurements will not lead to better
understanding of the separation performance of different types of polymers since
according to Figure 3 only solubliliy experiments representing the steady-state
on the feed side and vapour sorption experiments representing the case on the
permeate side can be performed. From these experiments no information is obtained about how the
concentration gradient is changing in the polymer layer, if capillary
condensation, for example, occurs with high boiling components, or how swelling
is influencing the membrane microstructure.In order to obtain realistic
results about the membrane performances for different ethylene-methacrylic acid
copolymers, in this work concentration- and temperature-dependent pervaporation
experiments have been performed and discussed from the point of view of potential industrial application.


### 2.3. Membrane Separation Characteristics

The separation performance of a
membrane material is characterized by two parameters, the permeability which
gives an impression on the mass transport through the membrane material and the
selectivity which shows the quality of separation The permeability can be
measured by the mass flux through the membrane. The flux is normalized by the
membrane area, the membrane thickness, and the time, so that its unit iskg⋅μm⋅m−2⋅h−1.In order to compare the quality of
separation for the different membranes according to [33] either the enrichment
factor β which is the ratio of the weight
fraction in permeate and feed respectively of the preferred permeating
component can be used or the separation factor α, which is used in this work as defined in the following equation:(5)α=(wi/wj)permeate(wi/wj)feed.Here wi,wj are the weight fractions of the components (i
is the preferentially permeating component) in the permeate and feed,
respectively.If pervaporation experiments
are performed at different feed temperatures using the same feed mixture,
generally with increasing temperature increasing fluxes are obtained. In many
cases an exponential dependency of total as well as partial fluxes from the
temperature are observed. According to (6),
activation energies for the permeation of the single components as well as for the
mixture through the membrane material can be calculated, (6)J=J0⋅δM⋅exp(−EJRT),thereby J0 is a pre-exponential factor, δM is the thickness of the dry membrane
[μm], EJ is the activation energy [kJmol−1], R is the common gas constant [Jmol−1K−1], and T is the temperature [K].


## 2.1. The Pervaporation Process

Pervaporation processes are membrane-based
separations of liquid mixtures as principally shown in Figure2 [24]. Thereby a phase transfer from the liquid phase
to the vaporous phase through a polymeric membrane is occuring. In this process,
the liquid feed mixture to be separated flows along one side of the membrane
while the various feed components are permeating into and through the membrane
at different rates. Therefore, the liquid retentate leaving the unit on the
same side of the membrane as the feed enters is depleted in the components
permeating preferentially. Consequently, the vaporous permeate collected on the
back side of the membrane is enriched in the preferentially permeating
component. The driving force for the mass transport of each component, through
a membrane is the difference of the chemical potential Δμi (T, p, xi, where i=1,…,n) of the components present in the mixture between feed and permeate
side. In pervaporation, the difference in the chemical potential is mainly
achieved by keeping the permeate pressure much lower than the feed pressure.Figure 2
Principles of pervaporation: liquid feed, liquid retentate (depleted), solution-diffusion membrane, and vaporous permeate (enriched).


## 2.2. The Solution Diffusion Model

Often polymeric solution-diffusion membranes are
classified as “nonporous” membranes because no pores are existing, only
microvoids caused by free volume compartiments of the polymer chains. However,
porous membranes, which are separating mixtures based on a sieve mechanism, are
moving fast in the direction of nanoporous materials and the difference between
porous and nonporous membrane materials is disappearing. Therefore, it is
better to classifiy the polymeric membranes used in this work as
solution-diffusion-membranes instead of nonporous membranes.The main features of the so-called solution-diffusion model
widely used to describe the mass transport in selective polymeric layers are
given in Figure3. This model not only holds for pervaporation, but also for
vapor and gas permeation [27–29]. The basic assumptions of this model are the
existence of the thermodynamic phase equilibrium at both boundary surfaces of
the selective layer in contact with the upstream feed mixture and the
downstream permeate mixture.Figure 3
Solution-diffusion model describing mass transport through polymeric membranes.The diffusion process inside the selective layer is
described by Fick's law, which is assumed to be valid for each component,i=1,…,n, (see [30]) (1)Ji=−Di(wi,…,wn)ρMdwidz,where Ji is the mass flux, Di is the diffusion
coefficient, z is the coordinate perpendicular to the membrane surface, and ρM is the density of the selective
polymer layer. The concentration of each component in this polymer layer is
given by its weight fraction wi, defined as (2)wi=mimM+∑j=1nmj,where mi and mj are the masses of the various
components dissolved and mM is the mass of the polymer. Under
steady-state conditions, integration of (1) gives (3)Ji=ρMδM∫wiPwiFDi(wi,…,wn)dwi, with δM being the thickness of the
selective layer, while wiF and wiP are the weight fractions of the component i in the
selective layer in its boundary at the feed side and the permeate side,
respectively. These fractions depend on the composition of the corresponding
mixture in contact with the boundary. Generally, Di depends on the
weight fraction of each component permeating and consequently, the mass fluxes
Ji are mutually dependent.With (3), Ji of any component permeating can be calculated
for a given feed concentration if the solubilities wiF and wiP,
the
dependency of the diffusion coefficient Di on composition, the
thickness δM, and the density ρM are known at operating conditions.The weight fractionwiP of any component i in the permeate is given by the
ratio of partial molar flux Ji(partial) to the total molar flux Jtotal, (4)wiP=JiJtotal. In many cases, modeling of the mass transport through membrane materials
for better understanding of the factors influencing the transport is difficult
although the approaches have been strongly improved in the last ten years [31, 32].
The reason for this is that, for example, experimentally it is not possible to
measure concentration-dependent diffusion coefficients in mixtures and furthermore
the thickness and density of the membranes are strongly depending on operating
conditions, where in many cases swelling of the membrane is occurring. It has
been also shown that solubility measurements will not lead to better
understanding of the separation performance of different types of polymers since
according to Figure 3 only solubliliy experiments representing the steady-state
on the feed side and vapour sorption experiments representing the case on the
permeate side can be performed. From these experiments no information is obtained about how the
concentration gradient is changing in the polymer layer, if capillary
condensation, for example, occurs with high boiling components, or how swelling
is influencing the membrane microstructure.In order to obtain realistic
results about the membrane performances for different ethylene-methacrylic acid
copolymers, in this work concentration- and temperature-dependent pervaporation
experiments have been performed and discussed from the point of view of potential industrial application.


## 2.3. Membrane Separation Characteristics

The separation performance of a
membrane material is characterized by two parameters, the permeability which
gives an impression on the mass transport through the membrane material and the
selectivity which shows the quality of separation The permeability can be
measured by the mass flux through the membrane. The flux is normalized by the
membrane area, the membrane thickness, and the time, so that its unit iskg⋅μm⋅m−2⋅h−1.In order to compare the quality of
separation for the different membranes according to [33] either the enrichment
factor β which is the ratio of the weight
fraction in permeate and feed respectively of the preferred permeating
component can be used or the separation factor α, which is used in this work as defined in the following equation:(5)α=(wi/wj)permeate(wi/wj)feed.Here wi,wj are the weight fractions of the components (i
is the preferentially permeating component) in the permeate and feed,
respectively.If pervaporation experiments
are performed at different feed temperatures using the same feed mixture,
generally with increasing temperature increasing fluxes are obtained. In many
cases an exponential dependency of total as well as partial fluxes from the
temperature are observed. According to (6),
activation energies for the permeation of the single components as well as for the
mixture through the membrane material can be calculated, (6)J=J0⋅δM⋅exp(−EJRT),thereby J0 is a pre-exponential factor, δM is the thickness of the dry membrane
[μm], EJ is the activation energy [kJmol−1], R is the common gas constant [Jmol−1K−1], and T is the temperature [K].


## 3. Materials

In
order to understand the physical properties of ethylene-methacrylic acid
copolymer (Figure4), the properties of the homopolymers
should be discussed shortly.Figure 4
Ethylene-methacrylic acid copolymer (E-MAA).Whereas
the nonpolar hydrophobic polyethylene does not solve in any common solvent and therefore
has to be melt-extruded to give foils, tubes, isolations and containers for
food packages, the polar, hydrophilic, and in most cases the water soluble
polyacrylic acid and polymethacrylic acid are used as super absorbers (if
partially cross-linked) in cosmetic, textile, and paper additives as well as
for the production of industrial glues. The most important and interesting property
of ethylene methacrylic acid copolymer is the functional groups (–COOH) which can
be modified by polymer-analogous reactions.As
basic material for the preparation of cross-linked membranes, two different
types of ethylene-methacrylic acid copolymer have been investigated, which differ exclusively in the
content of methacrylic acid monomer. In Table1 the basic polymers used are
listed.Table 1
Basic copolymers used in this work.
CopolymerComposition [mol%]Composition [wt%]Titratableacid*Melting temperature [K]Recrystallization temperature[K]**EMASEMASmmol/gpolymerE-MAA 2789.110.972.827.23,16*349313–325E-MAA 3883.116.961.538.54,47*343—**Measured* by potentiometric titration according to DIN 53042;Measured** by DSC.


### 3.1. Preparation of Cross-Linked Membranes

Cross-linked membranes were obtained by using the two different basic copolymers
E-MAA 27 and E-MAA 38 as basic materials. As cross-linking agents, various
cross-linkers were applied as listed in Table2.Table 2
Cross-linking agents used.
Cross-linkerStructureMolecular weight [g/mol]Aluminium acetyl acetonate324.31Glycerine diglycidylether (GDE)204.222,3,5,6-Tetramethyl-1,4-phenylene diamine (4MPD)164.25


### 3.2. Ionical Cross-Linking

During the cross-linking reaction with aluminium acetyl acetonate, the
low volatile acetyl acetone is set free and an ionically cross-linked membrane
structure, as shown schematically in Figure5, is obtained.Figure 5
Schematic formation
of ionical cross-linked membranes [23].It has been found before that ionically cross-linked structures made
from rubbery or glassy carboxylic acid containing copolymers can be used very
successfully as membranes in different applications [34].


### 3.3. Covalently Cross-Linked Structures

In this work also covalent cross-linked structures were investigated. As
cross-linking agents, an aromatic diamine, 2,3,5,6-tetramethyl-1,4-phenylene
diamine, and different di- and triepoxides as shown in Table2 were used. As an
example for the formation of a covalently cross-linked copolymer structure in
Figure 6 the reaction of the carboxylic acids on the polymer backbone with
diexpoxides to di-β-hydroxyesters is shown.Figure 6
Schematic formation of covalent cross-linking.For the preparation of ionically and covalently cross-linked membranes,
0.9 g of the E-MAA copolymer was dissolved in 10 mL of tetrahydrofuran (THF). In all cases,
an equimolar amount of cross-linker calculated on the desired cross-linking
degree was dissolved in 5 mL of THF. After complete dissolution of the polymer,
the cross-linking solution was added and the clear solution was stirred for
another 30 minutes, filtered and casted on a polyethylene terephthalate (PET) foil
into a glass ring having a diameter of 12.4 cm. The membranes were covered with
glass funnels to keep away dust particles and dried for 12 hours at room
temperature inside the hood. For the cross-linking reaction, the membranes were
placed in the vacuum oven rising the temperature from 333 K to 393 K and lowering the vacuum from 500
to 50 mbar within two hours. The membrane samples were kept another 24 hours at
the same conditions inside the vacuum oven. After this procedure, clear,
noncolored ionically as well as covalently cross-linked membranes were
obtained. The membrane thickness of the different
samples determined with a micrometer was between 46μm and 69 μm.It was found that cross-linked membranes E-MAA 27 as well as E-MAA 38
cross-linked with aluminium acetyl acetonate, glycerine diglycidylether, and 2,3,5,6-tetramethyl-1,4-phenylene
diamine, respectively, were nonsoluble in THF which was used for the membrane
preparation. NMR spectroscopic investigations of the cross-linked polymers were
not possible as a result of the insolubility of the cross-linked materials. In
solid-state NMR, the protons of the cross-links tend to disappear in the
baseline. A qualitative or quantitative evaluation of the conducted cross-linking
reactions using IR spectroscopy was carried out but thereby only slight tendencies
but no quantitative correlation have been found. From other inquiries of
esterifications via the acid chloride of E-MAA 27, carboxylic groups with
isopropanol we found out that if less than 20% of the carboxyl groups are
esterified, it is not possible with an adequate reliability to detect the cross-links.
Over 20% nice qualitative correlations have been seen. However, it has been
shown that the cross-linking reaction is reproducible and leads to membranes
with reproducible separation characteristics. Experiments have been performed
with different membrane samples from the same batch, as well as membrane
samples prepared independently from each other.


## 3.1. Preparation of Cross-Linked Membranes

Cross-linked membranes were obtained by using the two different basic copolymers
E-MAA 27 and E-MAA 38 as basic materials. As cross-linking agents, various
cross-linkers were applied as listed in Table2.Table 2
Cross-linking agents used.
Cross-linkerStructureMolecular weight [g/mol]Aluminium acetyl acetonate324.31Glycerine diglycidylether (GDE)204.222,3,5,6-Tetramethyl-1,4-phenylene diamine (4MPD)164.25


## 3.2. Ionical Cross-Linking

During the cross-linking reaction with aluminium acetyl acetonate, the
low volatile acetyl acetone is set free and an ionically cross-linked membrane
structure, as shown schematically in Figure5, is obtained.Figure 5
Schematic formation
of ionical cross-linked membranes [23].It has been found before that ionically cross-linked structures made
from rubbery or glassy carboxylic acid containing copolymers can be used very
successfully as membranes in different applications [34].


## 3.3. Covalently Cross-Linked Structures

In this work also covalent cross-linked structures were investigated. As
cross-linking agents, an aromatic diamine, 2,3,5,6-tetramethyl-1,4-phenylene
diamine, and different di- and triepoxides as shown in Table2 were used. As an
example for the formation of a covalently cross-linked copolymer structure in
Figure 6 the reaction of the carboxylic acids on the polymer backbone with
diexpoxides to di-β-hydroxyesters is shown.Figure 6
Schematic formation of covalent cross-linking.For the preparation of ionically and covalently cross-linked membranes,
0.9 g of the E-MAA copolymer was dissolved in 10 mL of tetrahydrofuran (THF). In all cases,
an equimolar amount of cross-linker calculated on the desired cross-linking
degree was dissolved in 5 mL of THF. After complete dissolution of the polymer,
the cross-linking solution was added and the clear solution was stirred for
another 30 minutes, filtered and casted on a polyethylene terephthalate (PET) foil
into a glass ring having a diameter of 12.4 cm. The membranes were covered with
glass funnels to keep away dust particles and dried for 12 hours at room
temperature inside the hood. For the cross-linking reaction, the membranes were
placed in the vacuum oven rising the temperature from 333 K to 393 K and lowering the vacuum from 500
to 50 mbar within two hours. The membrane samples were kept another 24 hours at
the same conditions inside the vacuum oven. After this procedure, clear,
noncolored ionically as well as covalently cross-linked membranes were
obtained. The membrane thickness of the different
samples determined with a micrometer was between 46μm and 69 μm.It was found that cross-linked membranes E-MAA 27 as well as E-MAA 38
cross-linked with aluminium acetyl acetonate, glycerine diglycidylether, and 2,3,5,6-tetramethyl-1,4-phenylene
diamine, respectively, were nonsoluble in THF which was used for the membrane
preparation. NMR spectroscopic investigations of the cross-linked polymers were
not possible as a result of the insolubility of the cross-linked materials. In
solid-state NMR, the protons of the cross-links tend to disappear in the
baseline. A qualitative or quantitative evaluation of the conducted cross-linking
reactions using IR spectroscopy was carried out but thereby only slight tendencies
but no quantitative correlation have been found. From other inquiries of
esterifications via the acid chloride of E-MAA 27, carboxylic groups with
isopropanol we found out that if less than 20% of the carboxyl groups are
esterified, it is not possible with an adequate reliability to detect the cross-links.
Over 20% nice qualitative correlations have been seen. However, it has been
shown that the cross-linking reaction is reproducible and leads to membranes
with reproducible separation characteristics. Experiments have been performed
with different membrane samples from the same batch, as well as membrane
samples prepared independently from each other.


## 4. Pervaporation Apparatus and  Analytical Method

For the pervaporation measurements a
slightly modified standard apparatus, described elsewhere in detail, was used [24, 35].
The effective membrane area was 41.9 cm2 and the downstream pressure
was kept at 15–30 mbar. After steady state has been reached, feed and permeate
samples have been collected every 2 hours, the flux has been calculated by
weighing the cold traps and the enrichment factor was determined by cooling
the feed and permeate compositions by HPLC. For that purpose, the permeate
samples, which were phase-separated in most of the cases, had to be homogenized
with acetone. Afterwards, 4 μL of the samples have been mixed with 1 mL of water and injected into
the HPLC column. The solvent of the HPLC was 70/30 acetonitril/water, whereas
0.1 wt% of trifluoro acetic acid was added to the water to prevent the
formation of phenolate on the HPLC column. The flux on the column was 0.7 mL/min,
and the wave length of the detector was 215 nm. As HPLC column, a nonpolar C18 standard
column was used. With this method, the retention time of phenol was 4.14
minutes.In order to ensure reproducible
results, pervaporation data were measured at least with three different
membrane samples. For the E-MAA 27 and E-MAA 38, respectively, used in this
work as well as for other polymeric materials investigated in our group, it was
found that the deviation in flux and selectivtiy is ±10% [36–38] for
independently prepared membrane samples. This is in good agreement with
deviations of industrial produced membranes.


## 5. Results and Discussion

First of all, the two basic polymeric materials E-MAA 27 and E-MAA 38 should
be tested if they are in principle suitable materials for the removal of phenol
from aqueous solutions, and if cross-linking is indeed necessary. Figure7
shows the separation characteristics of E-MAA 27 with a methacrylic acid
monomer content of 27.2 wt% at 313 K. In Figure 7(a), the total flux, and the partial
phenol fluxes, and water fluxes are plotted versus the phenol concentration in the
feed mixture. With increasing phenol content in the feed, the flux shows a
linear increase due to swelling effects in the membrane material. Although the
concentration of phenol in the permeate did not change significantly with
increasing phenol concentration in the feed, the separation factor shows a significant decrease with increasing phenol concentration, as shown in Figure 7(b). The phenomenon of increasing flux with decreasing
selectivity is very common and has been observed before in many studies
concering pervaporation but also gas permeation using swelling feed components
(e.g., aromatics, sulphur-aromatics, phenol, ethylene oxid, and CO2
[39–41]. With increasing concentration of the swelling component), increasing
rearrangement of polymer chains is assumed with increasing chain mobility. As a result of this,
the flux increases. According to the increased chain mobility reached during
swelling, the diffusion coefficients of the different permeating components are
changing. In the case of water/phenol, the diffusion rate of the small water
molecules accelerates much more compared to the diffusion rate of phenol.(a) Total flux (•), partial water
flux (▿), and partial
phenol flux (◯) and (b) separation
factor with increasing phenol concentration in the feed mixture for the E-MAA
27 at 313 K and a permeate pressure of 15–30 mbar.
(a)(b)In
order to show that the swelling of the E-MAA 27 polymer is due to an increase
in phenol concentration sorption measurements have been performed exemplarily. Therby
binary mixtures of phenol/water have been prepared in different concentrations
excluding the miscibility gap. For each sorption experiment, three different
membrane samples with an average weight of 150 mg were used. The deviation of
the averaged mass uptake of each set of samples was ±10%.As
the data in Table3 show, the mass uptake is strongly dependent on phenol concentration. If pure water is used, the mass uptake is only 0.3 wt% for the
E-MAA 27 whereas the mass uptake in a 3 wt% phenol/water solution was three
times higher. If very high phenol concentrations (75 wt%) are applied, the
effect continues and the mass uptake is 6.2%.Table 3
Mass uptake of E-MAA 27 in different composed phenol/water mixtures.
CopolymerComposition phenol/water [wt%]Totaluptake* [wt%]wt% phenolwt% waterE-MAA 2701000.3——3970.983177933.8881275256.2919Compared* to dry polymer before exposure to phenol/water mixture at room temperature.In
order to investigate the phenol and water concentration inside of the membrane,
a photometric method was applied based on the oxidative coupling of phenol with
1-methyl-2-benzothiazonlinone hydrazon as already described in [42, 43], which
is a commercially applied test for phenol in aqueous solutions according to EPA
420.1, US Standard methods 5530 and ISO 6439. Further details are found in
[36].As
also shown in Table3, the basic membrane material shows strong affinity
towards phenol. For example, if the E-MAA 27 membrane is exposed to a 3 wt% a
phenol/water solution, a mass uptake of 83 wt% phenol in the membrane has been
found. At 7 wt% phenol/water solution an even higher mass uptake of 88 wt% was
observed. Based on the results of partial phenol and water mass uptakes, the solubility selectivity αsol is calculated
based on (5) as the ratio of weight fractions inside the membrane divided by
the ratio of weight fractions in solution. Analogous to (5), the weight fraction of the
preferred sorbed substance belongs
to the numerator. Thereby values of αsol between 98 and
157 have been determined. The overall selectivity of a membrane-based process is defined as the
product of solubility selectivity and diffusivity selectivity according to the
solution-diffusion mechanism. However, the kinetic diameter of the phenol is
much larger compared to the water molecule and so the diffusivity selectivity
has to be smaller than 1. Since experimentally phenol enrichment has been
observed it is obvious that the process is solubility controlled. This is in
good accordance with many other liquid/liquid membrane-based separation
processes as aromatic/aliphatic and water/ethanol separations. Due to the fact
that with cross-linking the chain mobility is reduced, the diffusion
coefficient of phenol is also lowered. Therefore, an increase of selectivity
with cross-linking has to be an effect of increased solubility selectivity.By
increasing the methacrylic acid content in the polymer structure, the copolymer
was expected to be more versatile to swelling effects in phenol/water mixtures. As shown in Figure8(a) with the E-MAA 38, containing a methacrylic acid content of 38.5 wt%, the
fluxes do not increase linearly but exponentially with increasing phenol
concentration in the feed solution. At phenol feed concentrations of 7 wt%, the
total flux of the E-MAA 38 is 3.4 times higher compared to the E-MAA 27
membrane. At the same time, the phenol flux of the E-MAA 38 is merely higher by
1.6 times compared to the E-MAA 27. This is due to the increase in
hydrophilicity, which is caused by the higher content of methacrylic acid
monomer in the E-MAA 38 compared to the E-MAA 27. Similar effects of increasing
water fluxes with increasing hydrophilicity of the polymer backbone have been
also observed for different polymer classes [44].(a) Total
flux (•), partial water
flux (▿), and partial
phenol flux (◯) and (b) selectivity with increasing phenol concentration in the
feed mixture for the E-MAA 38 at 313 K and a permeate pressure of 15–30 mbar.
(a)(b)


### 5.1. Conditioning Effects on Noncross-Linked Membranes

Conditioning effects have been
investigated in earlier work with polyimide membranes [4, 25, 45] and it was shown
that durable effects leading to increased fluxes and increased separation
factors have been observed. Investigating various conditioning experiments by
using different polymers led to the result that the membrane performance did
not change even after several measuring cycles if once a maximum feed
concentration of the swelling component was applied [38].The aim of this work was to
investigate if similar conditioning effects for the rubbery E-MAA 38 membrane
compared to the glassy polyimides can be reached or if undesirable changes of
the membrane performance (e.g., loss in selectivity) will occur.Therefore, pervaporation experiments
with the E-MAA 38 membrane were once performed with increasing phenol
concentration and after reaching the highest possible phenol concentration in
feed, the concentrations were lowered stepwise. The results are shown in Figure9. After reaching the highest phenol concentration, the total fluxes on the
back-measurements were slightly higher compared to the first measurements. It
is also obvious from Figure 9, that the separation factors are much higher when
the membrane once was exposed to high phenol concentration. So the observed
effects described here are conditioning effects which can be used to increase
the performance of a membrane material. Such effects have also been observed on
other membrane materials (i.e., copolyimides and carboxylic acid containing
copolyimides) [3, 4, 25, 38, 45].(a) Total flux in
pervaporation experiments performed with increasing phenol concentration in
feed (•) as well as
decreasing phenol concentration (□) and (b) separation
factor with increasing phenol concentration in the feed mixture (•) as well as
decreasing phenol concentration (□) for the E-MAA 38
at 313 K and a permeate pressure of 15–30 mbar.
(a)(b)


### 5.2. Comparison of Different Cross-Linking Agents

Using
E-MAA 38 as well as E-MAA 27 as a basic membrane material, different
cross-linking agents as well as different concentrations of cross-linking
agents have been investigated in order to figure out their influence on the
separation characteristics. Thereby all experiments have been performed at a
feed concentration of 8 wt%, the membrane temperature was at 313 K, and the
permeate pressure was kept at 15–30 mbar.In Figure10, the results for E-MAA 38
membranes with a methacrylic acid content of 38.5 wt%, noncross-linked, covalently
as well as ionically cross-linked are shown. It can be seen that ionical and covalent
cross-linking result in a reduction of fluxes. This is due to the fact
that cross-linking leads to a lower mobility of the polymer backbone and so the
permeation rate for the single components is lowered. At the same time, the
cross-linked membranes showed all higher separation factors compared to the noncross-linked
membrane.Figure 10
E-MAA 38
copolymer cross-linked covalently with 10% 4MPD
(2,3,5,6-tetramethyl-1,4-phenylene diamine), 10% GDE (glycerine diglycidylether)
and ionically using 10% and15%Al3+, feed concentration 8 wt% phenol, membrane temperature 313 K.From
all cross-linkers investigated, the ionic cross-linker aluminium acetyl
acetonate seemed to result in the highest separation factor and therefore it was
also investigated with the E-MAA 27 membrane having a lower methacrylic acid
content of 27.2 wt% compared to 38.5 wt% in the E-MAA 38. For the membranes
with the lower content of carboxylic acid groups, the effects on fluxes and separation
factor were much higher. In Figure11, it can be seen that separation factors
of up to 6.0 can be reached with the ionically cross-linked membrane.Figure 11
E-MAA 27 copolymer cross-linked ionically
using 10% and15%Al3+, feed concentration 8 wt% phenol, membrane
temperature 313 K.From
the solubility data in Table3, it is obvious that the diffusivity selectivity
has to be much lower than 1. During cross-linking, the chain mobility is
reduced and therefore the diffusivity selectivity for phenol in aqueous
solution is lowered. The experimental increase in selectivity can only be
explained by an increase of solubility selectivity in the cross-linked
material.


### 5.3. Temperature Dependency of the Separation Performance

In the following experiments, it was
investigated how changes of temperature affects the separation characteristics.
As a membrane material, E-MAA 27 cross-linked ionically (10%) with aluminium
acetyl acetonate was chosen. From Figure12, it is obvious that an exponential
increase in fluxes is obtained. It is also interesting that at low feed
temperatures, the partial phenol fluxes are significantly lower than the
partial water fluxes through the membrane. Whereas at feed temperatures higher
than 373 K, the phenol fluxes are slightly higher than the water fluxes,
resulting in very high separation factor as also shown in Figure 12.(a) Temperature-dependent
pervaporation experiments (8 wt% phenol in feed) with an E-MAA 27 membrane ionically cross-linked with10%Al3+, total flux (•), partial water
flux (▿), and partial
phenol flux (◯) and (b) separation
factor.
(a)(b)The
exponential increase of flux with increasing temperature is assumed to be
attributed to (i) increase in solubility with temperature, (ii) increase of
diffusivity with temperature, and (iii) increase in diffusivity of the
components due to increased swelling of the polymer material. Detailed
explanation on the theory and calculations of exponential increase of fluxes
with increasing temperatures can be found in [23]. According to the solution-diffusion model as
shown in Figure 3, inside of the polymer layer gradients of concentration, swelling
effects as well as pressure differences are occuring. It is assumed that only
in a minor part of the membrane layer, faced to the permeate side, low
pressures as those applied on the permeate side are existing. The major part of
the polymer layer is wet and/or swollen. Therefore, the exponential increase in
flux is not attributed to increase in vapor saturation, since phenol, for
example, has a boiling point of 181.9°C, whereas temperature-dependent
pervaporation measurements have been performed between 30°C and 100°C.


### 5.4. Activation Energy for Permeation

According to (6) and the Arrhenius
plot in Figure 13, the partial fluxes can be plotted versus 1/T and the
activation energies for the permeation of phenol and water can be calculated.Figure 13
Arrhenius
plot (8 wt% phenol in feed) with an E-MAA 27 membrane ionically cross-linked
with10%Al3+, total flux (•), partial water
flux (▿), and partial
phenol flux (◯).The activation energy for permeation
consists of two terms, the enthalpie of solution (phenol and water, resp., in
the membrane) and the activation energy for diffusion. Whereas the activation
energy for the diffusion is always positive, the enthalpie of solution can be
positve, or negative. For the system water/phenol investigated in this work, for the
activation energy of permeation positive values are obtained.The activation energy for diffusion
is considered as the energy that is necessary to form microvoids of adequate
size for the permeating molecules. Cohesion forces of the polymer chains are counteracting
against microvoid formation. High cohesion forces denote high activation energy
for the diffusion. If large molecules (e.g., phenol) are permeating, larger
microvoids are necessary, and therefore a higher activation energy for
diffusion occurs. It was found that for rubbery polymers the activation energy
for diffusion increases linearly with the size of the molecules [46].For the E-MAA 27, which is also a
rubbery polymer, activation energies for the permeation of phenol ofEJ=71.3 kJ/mol, water of 59.7 kJ/mol, and the mixture of 8 wt% of phenol in feed
64.49 kJ/mol were obtained. Thereby phenol has the highest activation energy
for permeation, due to the fact that it is the component with the largest
kinetic diameter. With the activation energy the experimental data of increasing
enrichment factors with increasing feed temperature can be explained.


## 5.1. Conditioning Effects on Noncross-Linked Membranes

Conditioning effects have been
investigated in earlier work with polyimide membranes [4, 25, 45] and it was shown
that durable effects leading to increased fluxes and increased separation
factors have been observed. Investigating various conditioning experiments by
using different polymers led to the result that the membrane performance did
not change even after several measuring cycles if once a maximum feed
concentration of the swelling component was applied [38].The aim of this work was to
investigate if similar conditioning effects for the rubbery E-MAA 38 membrane
compared to the glassy polyimides can be reached or if undesirable changes of
the membrane performance (e.g., loss in selectivity) will occur.Therefore, pervaporation experiments
with the E-MAA 38 membrane were once performed with increasing phenol
concentration and after reaching the highest possible phenol concentration in
feed, the concentrations were lowered stepwise. The results are shown in Figure9. After reaching the highest phenol concentration, the total fluxes on the
back-measurements were slightly higher compared to the first measurements. It
is also obvious from Figure 9, that the separation factors are much higher when
the membrane once was exposed to high phenol concentration. So the observed
effects described here are conditioning effects which can be used to increase
the performance of a membrane material. Such effects have also been observed on
other membrane materials (i.e., copolyimides and carboxylic acid containing
copolyimides) [3, 4, 25, 38, 45].(a) Total flux in
pervaporation experiments performed with increasing phenol concentration in
feed (•) as well as
decreasing phenol concentration (□) and (b) separation
factor with increasing phenol concentration in the feed mixture (•) as well as
decreasing phenol concentration (□) for the E-MAA 38
at 313 K and a permeate pressure of 15–30 mbar.
(a)(b)


## 5.2. Comparison of Different Cross-Linking Agents

Using
E-MAA 38 as well as E-MAA 27 as a basic membrane material, different
cross-linking agents as well as different concentrations of cross-linking
agents have been investigated in order to figure out their influence on the
separation characteristics. Thereby all experiments have been performed at a
feed concentration of 8 wt%, the membrane temperature was at 313 K, and the
permeate pressure was kept at 15–30 mbar.In Figure10, the results for E-MAA 38
membranes with a methacrylic acid content of 38.5 wt%, noncross-linked, covalently
as well as ionically cross-linked are shown. It can be seen that ionical and covalent
cross-linking result in a reduction of fluxes. This is due to the fact
that cross-linking leads to a lower mobility of the polymer backbone and so the
permeation rate for the single components is lowered. At the same time, the
cross-linked membranes showed all higher separation factors compared to the noncross-linked
membrane.Figure 10
E-MAA 38
copolymer cross-linked covalently with 10% 4MPD
(2,3,5,6-tetramethyl-1,4-phenylene diamine), 10% GDE (glycerine diglycidylether)
and ionically using 10% and15%Al3+, feed concentration 8 wt% phenol, membrane temperature 313 K.From
all cross-linkers investigated, the ionic cross-linker aluminium acetyl
acetonate seemed to result in the highest separation factor and therefore it was
also investigated with the E-MAA 27 membrane having a lower methacrylic acid
content of 27.2 wt% compared to 38.5 wt% in the E-MAA 38. For the membranes
with the lower content of carboxylic acid groups, the effects on fluxes and separation
factor were much higher. In Figure11, it can be seen that separation factors
of up to 6.0 can be reached with the ionically cross-linked membrane.Figure 11
E-MAA 27 copolymer cross-linked ionically
using 10% and15%Al3+, feed concentration 8 wt% phenol, membrane
temperature 313 K.From
the solubility data in Table3, it is obvious that the diffusivity selectivity
has to be much lower than 1. During cross-linking, the chain mobility is
reduced and therefore the diffusivity selectivity for phenol in aqueous
solution is lowered. The experimental increase in selectivity can only be
explained by an increase of solubility selectivity in the cross-linked
material.


## 5.3. Temperature Dependency of the Separation Performance

In the following experiments, it was
investigated how changes of temperature affects the separation characteristics.
As a membrane material, E-MAA 27 cross-linked ionically (10%) with aluminium
acetyl acetonate was chosen. From Figure12, it is obvious that an exponential
increase in fluxes is obtained. It is also interesting that at low feed
temperatures, the partial phenol fluxes are significantly lower than the
partial water fluxes through the membrane. Whereas at feed temperatures higher
than 373 K, the phenol fluxes are slightly higher than the water fluxes,
resulting in very high separation factor as also shown in Figure 12.(a) Temperature-dependent
pervaporation experiments (8 wt% phenol in feed) with an E-MAA 27 membrane ionically cross-linked with10%Al3+, total flux (•), partial water
flux (▿), and partial
phenol flux (◯) and (b) separation
factor.
(a)(b)The
exponential increase of flux with increasing temperature is assumed to be
attributed to (i) increase in solubility with temperature, (ii) increase of
diffusivity with temperature, and (iii) increase in diffusivity of the
components due to increased swelling of the polymer material. Detailed
explanation on the theory and calculations of exponential increase of fluxes
with increasing temperatures can be found in [23]. According to the solution-diffusion model as
shown in Figure 3, inside of the polymer layer gradients of concentration, swelling
effects as well as pressure differences are occuring. It is assumed that only
in a minor part of the membrane layer, faced to the permeate side, low
pressures as those applied on the permeate side are existing. The major part of
the polymer layer is wet and/or swollen. Therefore, the exponential increase in
flux is not attributed to increase in vapor saturation, since phenol, for
example, has a boiling point of 181.9°C, whereas temperature-dependent
pervaporation measurements have been performed between 30°C and 100°C.


## 5.4. Activation Energy for Permeation

According to (6) and the Arrhenius
plot in Figure 13, the partial fluxes can be plotted versus 1/T and the
activation energies for the permeation of phenol and water can be calculated.Figure 13
Arrhenius
plot (8 wt% phenol in feed) with an E-MAA 27 membrane ionically cross-linked
with10%Al3+, total flux (•), partial water
flux (▿), and partial
phenol flux (◯).The activation energy for permeation
consists of two terms, the enthalpie of solution (phenol and water, resp., in
the membrane) and the activation energy for diffusion. Whereas the activation
energy for the diffusion is always positive, the enthalpie of solution can be
positve, or negative. For the system water/phenol investigated in this work, for the
activation energy of permeation positive values are obtained.The activation energy for diffusion
is considered as the energy that is necessary to form microvoids of adequate
size for the permeating molecules. Cohesion forces of the polymer chains are counteracting
against microvoid formation. High cohesion forces denote high activation energy
for the diffusion. If large molecules (e.g., phenol) are permeating, larger
microvoids are necessary, and therefore a higher activation energy for
diffusion occurs. It was found that for rubbery polymers the activation energy
for diffusion increases linearly with the size of the molecules [46].For the E-MAA 27, which is also a
rubbery polymer, activation energies for the permeation of phenol ofEJ=71.3 kJ/mol, water of 59.7 kJ/mol, and the mixture of 8 wt% of phenol in feed
64.49 kJ/mol were obtained. Thereby phenol has the highest activation energy
for permeation, due to the fact that it is the component with the largest
kinetic diameter. With the activation energy the experimental data of increasing
enrichment factors with increasing feed temperature can be explained.


## 6. Conclusion

(i)
The nonmodified ethylene-methacrylic acid copolymer showed increasing fluxes and decreasing separation factors with increasing phenol content at 310 K.(ii)
With increasing methacrylic acid content in the polymers higher fluxes and lower enrichment factors were observed.(iii)
It has been shown that conditioning of ionically cross-linked ethylene-methacrylic acid copolymer membranes is possible leading to enhanced separation characteristics for the phenol/water separation.(iv)
Cross-linking generally leads to higher separation factors combined with lower fluxes due to the reduced mobility of the polymer backbone. This effect is stronger for the copolymer having the lower methacrylic acid content.(v)
For the ionically cross-linked ethylene-methacrylic acid copolymer membrane a strong increase in enrichment factor as well as an exponential increase in flux with increasing temperature is observed.(vi)
The activation energy of phenol was found to be higher by 11 kJ/mol compared to water which explains the enhanced separation characteristics at higher temperature.



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*Source: 319392-2009-02-04.xml*