Effects of Novel Silane Modification of Zeolite Surface on Polymer Chain Rigidification and Partial Pore Blockage in Polyethersulfone PES Zeolite a Mi

7/24/2019 Effects of Novel Silane Modification of Zeolite Surface on Polymer Chain Rigidification and Partial Pore Blockage in

1/12

Journal of Membrane Science 275 (2006) 1728

Effects of novel silane modification of zeolite surface on polymer chainrigidification and partial pore blockage in polyethersulfone (PES)zeolite

A mixed matrix membranes

Yi Li a, Huai-Min Guan a, Tai-Shung Chung a,, Santi Kulprathipanja b

a Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singaporeb UOP LLC, 50 East Algonquin Road, Des Plaines, IL 60017-5016, USA

Received 1 July 2005; received in revised form 28 August 2005; accepted 31 August 2005

Available online 30 September 2005

Abstract

A novel silane coupling agent, (3-aminopropyl)-diethoxymethyl silane (APDEMS) was used in this work to modify zeolite surface for mixed

matrix membranes (MMMs). Both elementary analysis and XPS spectra confirm the chemical modification, while BET measurements show no

changes in zeolite surface area and total pore volume after the modification. Polyethersulfone (PES)zeolite 3A, 4A and 5A MMMs were fabricated

at high processing temperatures using unmodified and chemical modified zeolite. SEM images of these MMMs indicate the interface between

polymer and zeolite phases becomes better if modified zeolite is used. The effects of chemical modification of zeolite surface and zeolite loadings

on the gas separation performance of these MMMs were investigated. Both permeability and selectivity of MMMs made from APDEMS modified

zeolite are higher than those of MMMs made from unmodified zeolite at 20 wt% zeolite loading because of a decrease in the degree of partial pore

blockage of zeolites. The permeability of all studied gases decreases with increasing zeolite content for PESzeolite 4A-NH 2 MMMs, while for

PESzeolite 5A-NH2 MMMs, the gas permeability decreases and then increases with an increase in zeolite loadings. This unique phenomenon

implies that using large pore-size zeolite for MMMs would potentially offset the negative effects of partial pore blockage and polymer chain

rigidification on permeability. A modified Maxwell model with adjusted parameters was applied to study the PESzeolite 4A-NH2MMM system.

The predicted permeability and selectivity show very good agreement with experimental data, indicating the modified Maxwell model is fullycapable of predicting the gas separation performance of MMMs made from both unmodified and modified zeolite.

2005 Elsevier B.V. All rights reserved.

Keywords: Mixed matrix membrane; Silane modification of zeolite surface; Polymer chain rigidification; Partial pore blockage; Maxwell model

1. Introduction

During the last 2 decades, polymer-based organicinorganic

composites have received world-wide attention in the field of

material science. This is due to the fact that the resultant mate-

rials may offer superior performance in terms of mechanical

toughness for engineering resins, permeability and selectivity

for gas/liquid separation, and photoconductivity for electronics

[13]. This new concept has also been applied to the membrane-

based gas/liquidseparation by combining the easyprocessability

of organic polymers with the excellent separation properties

of inorganic materials [2,410]. However, it has been found

Corresponding author. Tel.: +65 6874 6645; fax: +65 6779 1936.

E-mail address:[email protected] (T.-S. Chung).

that there is an obstacle to the successful introduction of inor-

ganic molecular sieve materials into an organic polymer matrix

because of the poor compatibility between molecular sieve and

polymer matrix, especially for the case of rigid glassy polymer

materials[913].

Therefore, some methods have been proposed to improve

the interfacial strength in order to enhance the separation

performance [1318]. One of them is the chemical modi-

fication of the surface of molecular sieve with silane cou-

pling agents, such as (-aminopropyl)-triethoxy silane, N--

(aminoethyl)--aminopropyltrimethoxy silane, (-glycidyloxy-

propyl)-trimethoxy silane and (3-aminopropyl)-dimethylethoxy

silane[1417].By means of FTIR and 29 Si MAS NMR, Mat-

sumoto et al. have identified that there are three types of silanol

groups on the surface of zeolite [19], i.e., single ((SiO)3SiOH);

hydrogen-bonded ((SiO)3SiOH OHSi(SiO)3) and geminal

0376-7388/$ see front matter 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.memsci.2005.08.015


2/12

18 Y. Li et al. / Journal of Membrane Science 275 (2006) 1728

((SiO)2Si(OH)2). Among these silanol groups, free SiOH groups

are the most reactive groups and may provide the sites for the

physicaland chemical adsorptionof silane coupling agents. With

appropriate silane coupling agents, one may not only modify

surface properties of zeolite from hydrophilic to hydrophobic,

but also increase zeolite affinity to the functional groups of the

polymer matrix as illustrated in references[2022].

Duval et al. [16] demonstrated that the adhesion between

zeolite and polymer matrix phases could be improved by modi-

fyingzeolite surface with somesilane coupling agents. However,

there was no significant improvement on permselectivity though

SEM micrographs indicated good coupling between silane and

zeolite. Mahajan and Koros[14,23]also reported similar phe-

nomena. Although the improvement on interface was clearly

observed by SEM, their mixed matrix membranes (MMMs)

made of polymer-modified zeolite exhibited worse performance

(i.e., decreases in both permeability and selectivity) compared

with those made of polymer-unmodified zeolite. This maybe due

to the fact that the addition of silane coupling agents may reduce

the voids between unmodified zeolite and polymer phases, butcannot eliminated them completely. The size of these voids may

be in therangeof nanometeror sub-nanometer [14,15,23], which

is still larger than the sizes of gas molecules. As a consequence,

theadditionof silanecouplingagents on zeolite surface mayhelp

reduce voids and result in an increase in gas transport resistance

across the membrane, but there is almost no much improvement

on permselectivity. Unless a proper silane coupling agent is cho-

sen, it is very difficult to completely prevent the gas penetrants

from going through these voids.

Therefore, the first purpose of this study is to introduce a

novel silane coupling agent, (3-aminopropyl)-diethoxymethyl

silane (APDEMS),on zeolite surface andto investigateits effectson the separation performance of MMMs. To our best knowl-

edge, so far there is no academic literature available on using

APDEMS to modify the zeolite surface for MMMs. Zeolites

with different pore sizes are used in order to identify if the

effectiveness of silane modification is pore-size dependent. In

addition, although the interphase occupies an extremely small

volume fraction (i.e., less than 1010%), it appears to havea sig-

nificant effect on the separation performance of mixed matrix

membranes [10,1315,2325]. The origins of the imperfect

interphase are complicated. Poor compatibility between molec-

ular sieve and polymer matrix, uneven shrinkages and stresses of

these two components during the membrane formation may be

some of thepossiblecauses.As a result, polymer chain rigidifica-tion andpartial pore blockage of zeolites havebeen hypothesized

and partially confirmed from the lower gas permeation data of

mixed matrix membranes[10,13,15,2325].Therefore, the sec-

ond purpose of this paper is to investigate if and how the novel

silane coupling agent affects polymer chain rigidification and

partial pore blockage of zeolites and to study if we can reduce

the nanometric interphase.

For easy comparison with our previous work [25], zeo-

lite 3A, 4A and 5A were used as the dispersed phase in this

work, and polyethersulfone (PES) was chosen as the contin-

uous polymer matrix due to its appropriate Tg of 215C and

various applications in gas separation [26]. Zeolite and its

surface were characterized by elementary analysis, XPS and

BrunaerEmmettTeller (BET). A high processing temperature

was employed in order to maintain the flexibility of polymer

chains during membrane formation[1315,23,25].The gas per-

meation rates of the PESzeolite A MMMs were measured as a

function of zeolite loading and zeolite pore sizes. SEM and DSC

were utilized to study the morphology and Tgof these developed

MMMs, respectively.

2. Experimental

2.1. Materials

A commercial Radel A-300 PES, purchased from Amoco

Performance Products Inc., OH, USA, was selected as the con-

tinuous phase.It hasa weight-average molecular weight of about

15,000. N-Methyl-2-pyrrolidone (NMP) was purchased from

Merck; toluene was from Fisher Scientific UK Ltd.; methanol

was from J.T. Baker. NMP and toluene were dried using the

activated molecular sieve 4A beads with a diameter of 35 mmsupplied by Research Chemicals Ltd., and then filtered through

a 0.2m Teflon filter before use. The silane coupling agent,

APDEMS, was from SigmaAldrich and used without further

purification. The molecular sieves involved were zeolite 4A sup-

plied by UOP LLC, Des Plaines, IL, USA, zeolite 3A and 5A

resulted from K+ and Ca2+ exchange treatment of zeolite 4A,

respectively. Theion exchangetreatment wasdone by Dr. Huang

in our research group based on some related literatures[2729].

Their particle sizes vary from 1 to 2m tested by SEM. In order

to remove the adsorbed water vapor or other organic vapors,

all modified zeolites were dehydrated at 110 C for 2 h under

vacuum before use.

2.2. Chemical modification method of zeolite surface

Fig. 1shows the flowchart of chemical modification used in

this study. This procedure was derived with modifications from

the Plueddemanns method[20].It consists of three steps: (1)

stir the mixture of toluene (500 ml) + APDEMS (20 ml) + zeolite

(2.5 g) for 24 h at room temperature under N2; (2) filter and wash

the zeolite with 500 ml toluene and then 250 ml methanol to

remove the unreacted silane, respectively; (3) dry the modified

zeolite for 1 h at 110 C under vacuum.

2.3. Membrane preparation procedure

Because the fabrication method developed in our previous

paper achieves good properties for mixed matrix membranes

[25],the same method was employed in this work. We applied

high processing temperatures during membrane formation to

eliminate the void between polymer and zeolite phases. The

resultant dried MMMs have thicknesses varying from 60 to

90m for permeability measurements.

2.4. Gas permeability measurement

The gas permeation properties of every mixed matrix mem-

brane were measured using variable-pressure constant-volume


3/12

Y. Li et al. / Journal of Membrane Science 275 (2006) 1728 19

Fig. 1. Flowchart of the chemical modification of zeolite surface.

method with a pre-calibrated permeation cell described else-

where[30].Each pure gas was tested in the sequence of He, H 2,

O2, N2, CH4and CO2, and measured three times for each mem-

brane. The measurement of H2

gas for MMMs was performed

at 35 C and 3.546375 105 Pa (3.5 atm); the measurement

of other gases was conducted at 35 C and 10.1325 105 Pa

(10 atm).

2.5. Other characterizations

Two analysis methods were used to determine whether

the chemical modification had taken place. The first one

was elementary analysis carried out on a Perkin-Elmer PE

2400 Series II CHNS/O analyzer. The second analysis method

is X-ray photoelectron spectroscopy (XPS). XPS data were

obtained on a Shimadzu ESCAKL spectrometer using a non-

monochromatic Mg Kphoton source. Subsequently, nitrogen

adsorptiondesorption measurements were carried out at 77 K

on a Quantachrome Autosorb-1 apparatus to compare the BET

surface area and total pore volume between the unmodified and

modified zeolites.

The glass transition temperature (Tg) of MMMs was ana-

lyzed with differential scanning calorimetry (DSC) on a Mettler-

Toledo DSC 822e. Tg of the sample was determined as the

midpoint temperature of the transition region in the second heat-

ing cycle[31,32]. Tg provides a qualitative estimation of the

flexibility of polymer chains. It would be very useful to com-

pare the rigidity of polymer chains in MMMs at different zeolite

typesand loadings with that of neat polymer membrane [25]. Theelectron micrographs of MMM morphology were examined by

SEM on a JEOL JSM-5600LV and JSM-6700F to determine if

the zeolite particles were dispersed homogenously and if there

existed voids between polymer and zeolite phases.

3. Results and discussion

3.1. Characterization and comparison of unmodified and

modified zeolites

In order to determine if the chemical modification of zeolite

surface is actually successful, Table 1 lists the results of ele-

Table 1

Comparison of elementaryanalysis dataof zeolites before andafter the chemical

modification

Sample name N (%) C (%) H (%)

Zeolite 3 A 0.0 0.16 2.16

Zeolite 3A-NH2a 2.47 7.42 2.44

Zeolite 4A 0.0 0.16 2.12

Zeolite 4A-NH2a 2.42 7.30 2.30

Zeolite 5A 0.0 0.15 2.10

Zeolite 5A-NH2a 2.43 7.28 2.28

a NH2means the zeolite after the modification with APDEMS.

mentary analysis before and after the chemical modification.

Zeolite 3A, 4A and 5A mean the zeolite before the chemi-cal modification, while zeolite 3A-NH2, 4A-NH2 and 5A-NH2denote the zeolite after the chemical modification. Results show

that more nitrogen and carbon elements are detected after the

modification, thus confirming that the silane coupling agent has

been attached to the zeolite surface. XPS spectra of zeolite 3A

and 4A before and after the chemical modification are exhib-

ited in Fig. 2 . After modification, a peak clearly appears at

the electronic binding energy of about 397.2 eV that attributes

to the nitrogen atom of the silane coupling agent containing

amino group. This indicates that some molecules of the silane

coupling agent have been grafted onto the external surface of

zeolite, which is in good agreement with the elementary analysis

results.That APDEMS is selected as the silane coupling agent

because it has two ethoxy groups (CH3CH2O) and an addi-

tional hydrophobic group (CH3). Compared to 3-aminopropyl

triethoxy silane with three ethoxy groups, APDEMS may lower

the number of coupling points with zeolite surface during the

silanization reaction and thus avoid blocking the micropore of

zeolites.Table 2lists the results of the multipoint BET surface

area and total pore volume of zeolites before and after the chem-

ical modification. There are no sharp changes in the surface area

and total pore volume of zeolites after the modification. These

results imply that the addition of APDEMS does not alter the

micropore of zeolites.


4/12


Fig. 2. Comparison of XPS spectra of the zeolite before and after the chemical modification (A: zeolite 3A; B: zeolite 3A-NH 2; C: zeolite 4A; D: zeolite 4A-NH2).

Table 2

Comparison of total pore volume and multipoint BET surface area of zeolites before and after the chemical modification

Sample name Total pore volume (cm3/g) Multipoint BET surface area (m2/g)

Before modifying After modifying Before modifying After modifying

Zeolite 3 A 0.197 0.196 557.6 553.5

Zeolite 4A 0.200 0.198 567.0 561.6

Zeolite 5A 0.198 0.196 561.1 556.7

3.2. Effect of chemical modification of zeolite surface on

gas separation performance

PESzeolite 3A, PESzeolite 4A and PESzeolite 5A

MMMs before and after the chemical modification of zeolite

surface were fabricated with a zeolite loading of 20 wt%.Fig. 3

shows their permeability for four fast gases, while Fig. 4sum-

marizes their selectivity for four gas pairs. Interestingly, dif-

ferent from the previous studies [14,16], the permeability of

MMMs with modified zeolite is higher than those MMMs with

unmodified zeolite. This increasing trend in permeability seems

counter-intuitive at first because the contact between polymer

and zeolite phases becomes better as illustrated in Fig. 5andthe leakage through the interface becomes impossible after the

modification.

Perhaps we can explain this increasing trend from an oppo-

site perspective. Two possible mechanisms have been proposed

in the previous work[2325]in order to explain the significant

decrease in gas permeability for MMMs; namely, the inhibition

of polymer chain mobility near the polymerzeolite interface

and the partial pore blockage of zeolites induced by polymer

chains. The first mechanism can be easily detected by the Tgshift. Table 3 shows a comparison ofTgvalues of PESzeolite A

MMMs made of modified and unmodified zeolite. Their Tgval-

ues are almost the same, indicating the degree of polymer chain

rigidification for these MMMs are almost the same. Therefore,

we may be able to rule out a decrease in polymer chain rigidifi-

cation as a possible cause for the permeability increase.

Previous researchers have measured the chain length

of (3-aminopropyl)-triethoxy silane molecules of around

(59) 1010 m (59A) [33,34] by means of spectroscopic

ellipsometry. Because APDEMS and (3-aminopropyl)-triethoxy

silane have similar chemical structures, APDEMS may have

a molecular chain length of approximately (59) 1010 m

Table 3

Change of glass transition temperatures of MMMs over neat PES dense film

before and after the chemical modification of zeolite surface

Membrane name Glass transition temperature (C)

Before modifying After modifying

Neat PES dense film 215 1

PESzeolite 4A (20wt% loading) 216 1 216 1






PESzeolite 5A (40wt% loading) 220 l 220 l



5/12


Fig.3. Comparison of gaspermeabilityof PESzeolite A MMMs before andafter the chemicalmodificationof zeolite surface (A:He permeability;B: H2permeability;

C: O2permeability; D: CO2permeability).

Fig. 4. Comparison of gas pair selectivity of PESzeolite A MMMs before and after the chemical modification of zeolite surface (A: He/N2 selectivity; B: H2/N2

selectivity; C: O2/N2 selectivity; D: CO2/CH4 selectivity).


6/12


Fig. 5. Comparison of cross-section SEM images of MMMs before and after the chemical modification of zeolite surface (AC: PESzeolite 3A, 4A and 5A MMMs,

respectively; DF: PESzeolite 3A-NH2, 4A-NH2and 5A-NH2MMMs, respectively).

(59A). If the attachment of the polymer matrix upon zeolite

surface is the main cause of partial pore blockage of zeo-

lites [2325], the addition of (59) 1010 m (59A) thick

APDEMS on top of zeolite surface may effectively reduce the

direct partial pore blockage induced by the polymer matrix. Asa result, the gas permeability of MMMs increases if APDEMS

modified zeolite is used.

Thesamereasons maybe applicable to explain thepermselec-

tivity increase if MMMs are made of APDEMS modified zeolite

as illustrated inFig. 4.Previous works have demonstrated that,

owing to partial pore blockage, the selectivity obtained from the

experiments is far below from the prediction calculated from

the Maxwell model[2325].Because of the unique structure of

APDEMS, the thin APDEMS layer on zeolite surface not only

can reduce the pore blockage as discussed in the previous sec-

tion, but also can diminish the transport of gas penetrants via

the interphase due to better adhesion between APDEMS and the

polymer matrix and the existence of various functional groups

in this space (i.e., CH2and CH3). Therefore, the originally high

selectivity nature of zeolite remains almost intact and the resul-

tant mixed matrix membranes have a higher permselectivity.

3.3. Effect of zeolite loadings on gas permeability

PESzeolite 4A-NH2 and PESzeolite 5A-NH2 mixed

matrix membranes with different zeolite loadings were fab-

ricated. Neat PES dense films were also prepared with the

same procedure for comparison. The permeability of all gases

and selectivity of four gas pairs at different zeolite loadings

are shown in Figs. 6 and 7 , respectively. The permeability

of all gases decreases with an increase in zeolite content for

PESzeolite 4A-NH2 MMMs, while the relationship between

gas permeability and zeolite loading for PESzeolite 5A-NH2MMMs is not so straightforward. Their gas permeability shows

a decrease then an increase with increasing zeolite loading. Themaximum percentages of permeability increments of He, H2,

O2and N2for PESzeolite 5A-NH2MMMs are approximately

73%, 67%, 35% and 11%, respectively, at 50 wt% zeolite load-

ing.

The decreasing trend of gas permeability of PESzeolite 4A-

NH2MMMs with zeolite loading may be easily understandable

because it has been previously demonstrated that both poly-

mer chain rigidification and partial pore blockage of zeolites

may lead to a decrease in the permeability of MMMs [2325].

However, for PESzeolite 4A-NH2MMMs, the polymer chain

rigidification may play a much more important role than the

partial pore blockage. This is due to the fact that the surface

modification by APDEMShas lowered the degree of pore block-age in zeolite. In addition, the slight increase inTgwith zeolite

loading as shown in Table 3 implies greater chain rigidifica-

tion with increasing zeolite loading. As a result, permeability

of PESzeolite 4A-NH2 MMMs decreases with an increase

in zeolite loading. A modified Maxwell model [25] will be

used to quantitatively estimate the effect of both polymer chain

rigidification and partial pore blockage of zeolites on the O2per-

meability of PESzeolite 4A-NH2MMMs in the next section.

However, why the permeability of PESzeolite 5A-NH2MMMs decreasesand thenincreaseswith increasingzeolite con-

tent as shown in Fig. 6? This trend is different from our previous

results[25]where the permeability of PESzeolite 5A MMMs


7/12


Fig. 6. Effect of zeolite loadings on gas permeability of PESzeolite 4A-NH2and PESzeolite 5A-NH2 MMMs (A: He and H2permeability; B: CO2permeability;

C: O2permeability; D: N2and CH4permeability).

monotonously decreases with an increase in the zeolite loading.

This difference may arise from three factors: (1) the pore size of

zeolite 5A, 4.8 1010 m (4.8A), is larger than the molecular

kinetic diameter of all tested gases; therefore, the permeabil-

ity of PESzeolite 5A MMMs should increase with an increase

in zeolite loadings, (2) the effect of polymer chain rigidifica-

tion may still play an important role when the zeolite loading is

small. The benefit of using large pore-size zeolite only magni-

fies when its content is greater than approximately 30% and (3)

even though the APDEMS surface modification can reduce the

degree of partial pore blockage, the partial pore blockage can-

not be eliminated completely. A part of zeolite 5A pores may be

narrowed to approximately 4 1010 m (4 A), thus leading to a

permeability decrease in PESzeolite 5A MMMs. Once the zeo-lite loading is further increased, the positive effect of using large

pore-size zeolite gradually offsets the negative effect of partial

pore blockage on permeability. As a consequence, permeability

becomes increase with an increase in zeolite loading.

The above three factors compete one another and result in

different influence on the relationship between gas permeability

and zeolite loading for different gases. For He and H2gases with

smallest kinetic diameters, their permeability starts to surpass

that of neat PES dense film from 20 wt% zeolite loading, while

for O2gas, it begins from 40 wt% loading; for N2, it starts from

50 wt% loading; for CH4 gas, it is always lower than that of

neat PES dense film in our experimental range. The information

hints different gases exhibit different responses to polymer chain

rigidification and partial pore blockage of zeolites.

The above phenomena and arguments may not be applicable

to the case of PESzeolite 4A-NH2 MMMs. This arises from

the fact that the pore size of zeolite 4A is about 3.8 1010 m

(3.8A), which is very approximate with the kinetic diameter of

tested gases. Therefore, the APDEMS induced chemical modi-

fication cannot bring much positive effects on the permeability

versus zeolite content relationship.

3.4. Effect of zeolite loadings on gas permselectivity

Fig. 7 exhibits that the selectivity of both PESzeolite4A-NH2 and PESzeolite 5A-NH2 MMMs increases with an

increase in zeolite loadings. The maximum increment occurs

at 50 wt% zeolite loading. The maximum percentages of selec-

tivity increments of He/N2, H2/N2, O2/N2 and CO2/CH4 for

PESzeolite 4A-NH2 MMMs are approximately 33%, 34%,

32% and 44%, respectively, and for PESzeolite 5A-NH2MMMs are around 56%, 51%, 22% and 15%, respectively.

Amazingly, PESzeolite 5A-NH2MMMs not only increase the

permeability of He, H2 and O2, but also enhance the selectiv-

ity of He/N2, H2/N2 and O2/N2, especially at 50 wt% loading.

This simultaneous advance in both permeability and selectivity

is very attractive to potential industrial applications.


8/12


Fig. 7. Effect of zeolite loadings on gas pair selectivity of PESzeolite 4A-NH2 and PESzeolite 5A-NH2 MMMs (A: He/N2 selectivity; B: H2/N2 selectivity; C:

O2/N2selectivity; D: CO2/CH4selectivity).

The increasing trend in gas pair selectivity of both

PESzeolite 4A-NH2 and PESzeolite 5A-NH2 MMMs withan increase in zeolite loadings is easily explainable due to the

influence of molecular sieving mechanism via zeolite. Although

the magnitude of increment is still below from the calculation

of the Maxwell model, the gas separation performance of the

newly developed MMMs made from APDEMS modified zeo-

lite is much higher than those made from unmodified zeolite

[25].

3.5. Applicability of the modified Maxwell model to predict

gas separation performance

In our previous work[25],a modified Maxwell model was

proposed to predict the gas separation performance. It takes thecombined effects of the polymer chain rigidification and par-

tial pore blockage of zeolites into calculation. This modified

Maxwell modelneeds intrinsic gas permeationpropertiesof zeo-

lites to predict the gas separation performance of MMMs in the

calculation. So far only O2and N2 permeability data of zeolite

4A can be found in theliteratures and relatively consistent, while

there are no consistent CO2 and CH4 permeability data of zeo-

lite 4A available. For other zeolites, such as 5A, silicalite-1, no

consistent data on intrinsic gas permeability can be found in the

literatures. Therefore, the PESzeolite 4A-NH2MMMis chosen

as a sample to predict O2permeability and O2/N2selectivity by

using this modified Maxwell model in this study. This exercise

may provide convincing information to quantitatively estimate

the effect of chemical modification on the polymer chain rigid-ification and partial pore blockage of zeolites.

Firstly, we assume the whole system of mixed matrix mem-

branes as a pseudo three-phase composite, as illustrated in Fig. 8

. The first phase is the polymer matrix; the second phase is com-

posed of the third phase and rigidified polymer region near the

zeolite; and the third phase comprises the bulk of zeolite and the

zeolite skin affected by the partial pore blockage. Secondly, the

permeability of the third phase can be calculated with the aid of

the Maxwell equation[35]by assuming the bulk of zeolite as

the dispersed phase and the zeolite skin affected by the partial

pore blockage as the continuous phase

P3rd = PbloPD + 2Pblo 2v3(Pblo PD)PD + 2Pblo + v3(Pblo PD)

(1)

whereP3rdis the composite permeability of the third phase, PDthe permeability of the bulk of zeolite, Pblothe permeability of

the zeolite skin affected by the partial pore blockage and v3 is

the volume fraction of the bulk of zeolite in the third phase and

can be determined by the following equation:

v3 =vD

vD + vblo(2)

where vD is the volume fraction of the bulk of zeolite in the

total membrane and vblo is the volume fraction of zeolite skin

affected by the partial pore blockage in the total membrane.


9/12


Fig. 8. Schematic diagram for the modified Maxwell model.

Thirdly, the permeability of the second phase can be deter-

mined with the aid of Maxwell equation a second time by

regarding the rigidified polymer region as the continuous phase

and the third phase as the dispersed phase

P2nd = Prig

P3rd + 2Prig 2v2(Prig P3rd)

P3rd + 2Prig + v2(Prig P3rd)

(3)

whereP2ndis the composite permeability of the second phase,

Prig the permeability of the rigidified polymer region and v2 is

the volume fraction of the third phase in the second phase and

can be estimated by the following equation:

v2 =vD + vblo

vD + vblo + vrig(4)

where vrigis the volume fraction of the rigidified polymer region

in the total membrane.

Finally, one can obtain the permeability of the whole mixed

matrix membrane with the aid of Maxwell equation a third time

by considering the polymer matrix as the continuous phase (i.e.,

the first phase) and the second phase as the dispersed phase

PMMM= PC

P2nd + 2PC 2(vD + vblo + vrig)(PC P2nd)

P2nd + 2PC + (vD + vblo + vrig)(PC P2nd)

(5)

where PMMM is the composite permeability of mixed matrix

membranes and PC is the permeability of the polymer matrix

(i.e., the first phase).

Zeolite 4A Linde crystals have an O2 permeability

of around 5.775385 1018 m2 s1 Pa1 (0.77 Barrer) and

an O2/N2 selectivity of around 37 at 35C [36]. The

neat PES dense film has an O2 permeability of around

3.5927395 1018 m2 s1 Pa1 (0.479 Barrer) and an O2/N2selectivityof 5.81 at 35 C accordingto our experimental results.

Therefore,four unknowns in Eqs.(1)(5) needtobesettopredict

the resultant permeability of mixed matrix membranes. They are

Pblo,v3,Prigand v2, respectively.Prigand Pblocan be assumedto be the permeability of the polymer matrix divided by a chain

immobilization factorand the permeability of the bulk of zeo-

lite divided by an average decline factor induced by the partial

pore blockage, respectively[2325,37].v2and v3can be calcu-

lated if the volume fraction of the rigidified polymer region and

affected zeolite skin in the total membrane are known.

The polymer chain rigidification near the zeolite surface is

probably resulted from the inhibited isotropic contraction of the

polymer. Zeolite with a smaller particle size may have a less

effect on the inhibition of the polymer contraction, thus lead-

ing to a smaller thickness of rigidified polymer region[24].The

thickness of rigidified polymer region near the zeolite, r, has


10/12


Table 4

Change of O2 and N2permeability in different regions of the PESzeolite 4A-NH2MMM

Permeability in the

PES matrix (Pc)a

Permeability in the rigidified

PES region (Prig= Pc/b)

Permeability in the zeolite 4A skin affected

with partial pore blockage (Pblo= PD/c)

Permeability in the bulk

of zeolite 4 A (PD)d

O20.479 0.160 0.00308 0.770

N20.0825 0.0206 0.00208 0.0208

a Gas permeability data in the PES matrix come from experiment results of neat PES dense film.b is 3 for O2 gas, while is 4 for N2gas.c is 250 for O2gas, while

is 10 for N2 gas.d Gas permeability data in the bulk of zeolite 4A come from reference[36].

Table 5

Calculated volume fraction data of dispersed phase in different phases of PESzeolite 4A-NH 2 MMMs in the modified Maxwell model which simultaneously

considers both polymer chain rigidification and partial pore blockage of zeolites

Calculated volume fraction of the bulk of zeolite 4A (considered as the

dispersed phase) in the third phase (v3) (defined inFig. 8)

0.972

Calculated volume fraction of the third phase (considered as the dispersed

phase) in the second phase (v2

) (defined inFig. 8)

0.641

Calculated volume fraction of the second phase (considered as the

dispersed phase) in the whole mixed matrix membrane (defined inFig. 8)

20wt% Zeolite loading (18.6 vol% zeolite loading) 0.290




been assumed to be 0.66m[24], 0.3m[25]and 0.066m

[24]as the average particle size of zeolite 4A or carbon molec-

ular sieve is 5m[24],3m[25]and 1m[24],respectively.

Therefore,ris assumed to be 0.12m in this work because the

average particle size of used zeolite 4A is 1.5m. This inhibited

isotropic contraction is assumed to affect only the thickness of

rigidified polymer region, but no effect on the extent of polymer

chain rigidification[24]. Therefore, the chain immobilization

parameter is still assumed to be 3 for O2 gas and 4 for N2 in

this work which are the same as our previous work[25].The

partial pore blockage of zeolites is resulted from the attachment

of polymer chains on the zeolite surface. The thickness of this

affected zeolite skin, r, is assumed to be almost unchanged with

zeolite particle size[2325].However, the chemical modifica-

tion of zeolite surface may reduce the extent of the partial pore

blockage, so in this work, r is assumed to be 70 1010 m

(70A) lower than that (i.e., about 108 m (100A) or more) in

other related literatures[2325].To simplify the calculation, we

presume neither the particle size of zeolites nor the chemical

modification has any influence on the magnitude of permeabil-ity reduction in this zeolite skin affected by the partial pore

blockage; therefore, is still assumed to 250 for O2 and 10

for N2 in this work. The reason that and have different

values for different gases has been explained in our previous

work[25].O2 and N2 permeability data in different regions of

PESzeolite 4A-NH2 MMMs are shown inTable 4.All calcu-

lated volume fraction data of dispersed phase defined in Fig. 8 in

the third phase, second phase and whole MMM in this modified

Maxwell model are listed in Table 5. The calculation method

to obtain these volume fractions has been discussed extensively

by Moore et al.[24],so it is not necessary to duplicate it in this

work.

Fig. 9. Comparison of O2 permeability of PESzeolite 4A-NH2 MMMs

between experimental data and predictions from the Maxwell model and the

modified Maxwell model.

Fig. 10. Comparison of O2/N2 selectivity of PESzeolite 4A-NH2 MMMs

between experimental data and predictions from the Maxwell model and the

modified Maxwell model.


11/12


Figs. 9 and 10illustrate the ultimate prediction of O2perme-

ability and O2/N2 selectivity of PESzeolite 4A-NH2 MMMs,

respectively. It can be found that the prediction from the modi-

fiedMaxwellmodelis in very good agreementwith experimental

data. Therefore, this demonstrates our revised Maxwell model

canbe utilized for MMMs made from both unmodified and mod-

ified zeolite.

4. Conclusions

The following conclusions can be drawn from this work:

(1) Results from elementary analysis and XPS demonstrate that

the silane coupling agent (APDEMS) has been successfully

grafted onto theexternalsurface of zeolite after thechemical

modification. BET results show that the surface area and

total pore volume of zeolites before and after modification

are almost the same, indicating the micropore of zeolites is

almost not influenced by the addition of APDEMS. These

characterizations assure the zeolite modified by APDEMSas a good candidate which can be applied in the fabrication

of MMMs.

(2) Both gas permeability and gas pair selectivity of

PESzeolite A-NH2 MMMs are higher than those of

PESzeolite A MMMs at 20 wt% zeolite loading. This may

be mainly due to the fact that the presence of APDEMS

introduces a distance of around (59) 1010 m (59A)

between polymer chains and zeolite surface, thus reduces

the extent of the partial pore blockage of zeolites induced

by polymer chains.

(3) Thepermeability of allstudied gases decreaseswith increas-

ing zeolite content for PESzeolite 4A-NH2MMMs, whilefor PESzeolite 5A-NH2 MMMs, the gas permeability

decreasesand then increaseswith an increase in zeolite load-

ings. Clearly, a high loading of zeolite 5A-NH2offsets the

effects of partial pore blockage and polymer chain rigidifi-

cation on permeability. The selectivity of both PESzeolite

4A-NH2 and PESzeolite 5A-NH2 MMMs increases with

an increase in zeolite loadings due to the influence of molec-

ular sieving mechanism.

(4) A modified Maxwell model proposed in our previous work

[25]was applied to predict the gas separation performance

of the newly developed PESzeolite 4A-NH2 MMMs with

adjusted parameters. The predicted permeability and selec-

tivity show very good agreement with experimental data.

Acknowledgements

The authors would like to thank UOP LLC and NUS for

funding this research with the grant numbers of R-279-000-140-

592, R-279-000-140-112 and R-279-000-184-112. Dr. Huang is

thanked for aiding in the ion exchange treatment of zeolite 4A.

References

[1] G.W. Peng, F. Qiu, V.V. Ginzburg, D. Jasnow, A.C. Balazs, Forming

supramolecular networks from nanoscale rods in binary, phase-separating

mixtures, Science 288 (2000) 1802.

[2] T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin,

A.J. Hill, Ultrapermeable, reverse-selective nanocomposite membranes,

Science 296 (2002) 519.

[3] Y. Wang, N. Herron, X-ray photoconductive nanocomposites, Science

273 (1996) 632.

[4] S. Kulprathipanja, R.W. Neuzil, N.N. Li, Separation of fluids by means

of mixed matrix membranes in gas permeation, US Patent No. 4,740,219

(1988).

[5] H.J.C. Te Hennepe, Zeolite Filled Polymeric Membranes: A New Con-cept in Separation Science, Thesis, University of Twente, 1988.

[6] M. Jia, K.V. Peinemann, R.D. Behling, Molecular sieving effect of zeo-

lite filled silicone rubber membranes, J. Membr. Sci. 57 (1991) 289.

[7] J.M. Duval, B. Folkers, M.H.V. Mulder, G. Desgrandchamps, C.A. Smol-

ders, Adsorbent-filled membranes for gas separation. Part 1. Improve-

ment of the gas separation properties of polymeric membranes by

incorporation of microporous adsorbents, J. Membr. Sci. 80 (1993) 189.

[8] D.R. Paul, D.R. Kemp, The diffusion time lag in polymer membranes

containing adsorptive fillers, J. Polym. Sci. Symp. 41 (1973) 79.

[9] M.G. Suer, N. Bac, L. Yilmaz, Gas permeation characteristics of

polymerzeolite mixed matrix membranes, J. Membr. Sci. 91 (1994)

77.

[10] R. Mahajan, W.J. Koros, Factors controlling successful formation of

mixed-matrix gas separation materials, Ind. Eng. Chem. Res. 39 (2000)

2692.

[11] A. Erdem-Senatalar, M. Tatler, S.B. Tantekin-Ersolmaz, Estimation of

the interphase thickness and permeability in polymerzeolite mixed

matrix membrane, in: Proceedings of the 13th International Zeolite Con-

ference, Montpellier, France, 2001.

[12] S.B. Tantekin-Ersolmaz, L. Senorkyan, N. Kalaonra, M. Tatler, A.

Erdem-Senatalar,n-Pentane/i-pentane separation by using zeolitePDMS

mixed matrix membranes, J. Membr. Sci. 189 (2001) 59.

[13] R. Mahajan, R. Burns, M. Schaeffer, W.J. Koros, Challenges in forming

successful mixed matrix membranes with rigid polymeric materials, J.

Appl. Polym. Sci. 86 (2002) 881.

[14] R. Mahajan, W.J. Koros, Mixed matrix membrane materials with glassy

polymers. Part 1, Polym. Eng. Sci. 42 (7) (2002) 1420.

[15] R. Mahajan, W.J. Koros, Mixed matrix membrane materials with glassy

polymers. Part 2, Polym. Eng. Sci. 42 (7) (2002) 1432.

[16] J.M. Duval, A.J.B. Kemperman, B. Folkers, M.H.V. Mulder, G. Des-

grandchamps, C.A. Smolders, Preparation of zeolite filled glassy poly-

mer membrane, J. Appl. Polym. Sci. 54 (1994) 409.

[17] Y.L. Liu, Y.H. Su, K.R. Lee, J.Y. Lai, Crosslinked organicinorganic

hybrid chitosan membranes for pervaporation dehydration of

isopropanolwater mixtures with a long-term stability, J. Membr. Sci.

251 (2005) 233.

[18] H.H. Yong, H.C. Park, Y.S. Kang, J. Won, W.N. Kim, Zeolite-filled poly-

imide membrane containing 2,4,6-triaminopyrimidine, J. Membr. Sci.

188 (2001) 151.

[19] A. Matsumoto, K. Tsutsumi, K. Schumacher, K.K. Unger, Surface

functionalization and stabilization of mesoporous silica spheres by

silanization and their adsorption characteristics, Langmuir 18 (2002)

4014.

[20] E.P. Plueddemann, Silane Coupling Agents, second ed., Plenum Press,

New York, 1991.

[21] D.H. Flinn, D.A. Guzonas, R.H. Yoon, Characterization of silica surfaces

hydrophobized by octadecyltrichlorosilane, Colloids Surf. A: Physic-

ochem. Eng. Aspects 87 (1994) 163.

[22] K.C. Vrancken, K. Possemiers, P. Van Der Voort, E.F. Vansant, Surface

modification of silica gels with aminoorganosilanes, Colloids Surf. A:

Physicochem. Eng. Aspects 98 (1995) 235.

[23] R. Mahajan, Formation, characterization and modeling of mixed matrix

membrane materials, Ph.D. Dissertation, The University of Texas,

Austin, 2000.

[24] T.T. Moore, R. Mahajan, De Q. Vu, W.J. Koros, Hybrid membrane mate-

rials comprising organic polymers with rigid dispersed phases, AICHE

J. 50 (2) (2004) 311.

[25] Y. Li, T.S. Chung, C. Cao, S. Kulprathipanja, The effects of poly-

mer chain rigidification, zeolite pore size and pore blockage on


12/12


polyethersulfone (PES)zeolite A mixed matrix membranes, J. Membr.

Sci. 260 (2005) 45.

[26] J.S. Chiou, Y. Maeda, D.R. Paul, Gas permeation in polyethersulfone,

J. Appl. Polym. Sci. 33 (1987) 1823.

[27] H.M. Guan, T.S. Chung, Z. Huang, M.L. Chng, S. Kulprathipanja,

Poly(vinyl alcohol) multilayer mixed matrix membranes for the

dehydration of ethanolwater mixture, J. Membr. Sci., doi:10.1016/

j.memsci.2005.05.032.

[28] K. Taizo, N. Tooru, T. Isao, I. Wataru, Manufacture of zeolite 3A fromzeolite 4A, European Patent No. JP5147926 (1993).

[29] W. Heide, Production of acid-modified potassium zeolite 3A by cation

exchange of sodium zeolite 4A, European Patent No. DE10107819

(2002).

[30] W.H. Lin, R.H. Vora, T.S. Chung, The gas transport properties of

6FDA-durene/1,4-phenylenediamine (pPDA) copolyimides, J. Polym.

Sci. Polym. Phys. 38 (2000) 2703.

[31] R. Sirnha, R.F. Bayer, General relation involving the glass temperature

and coefficients of expansion of polymers, J. Chem. Phys. 37 (1962)

1003.

[32] A. Eisenberg, B. Hird, R.B. Moore, A new multiplet-cluster model

for the morphology of random ionomers, Macromolecules 23 (1990)

4098.

[33] V.V. Tsukruk, V.N. Bliznyuk, Adhesive and friction forces between

chemically modified silicon and silicon nitride surfaces, Langmuir 14

(1998) 446.

[34] D.F. Siqueira Petri, G. Wenz, P. Schunk, T. Schimmel, An improved

method for the assembly of amino-terminated monolayers on SiO2

and the vapor deposition of gold layers, Langmuir 15 (1999)4520.

[35] K. Mizoguchi, K. Terada, Y. Naito, Y. Kamiya, S. Tsuchida, S.

Yano, Miscibility blends and gas permeability of poly(ethylene-co-3,5,5-

trimethylhexyl methacrylate)polydimethylsiloxane, Colloid Polym. Sci.

86 (1997) 275.

[36] C.M. Zimmerman, A. Singh, W.J. Koros, Tailoring mixed matrix com-

posite membranes for gas separation, J. Membr. Sci. 137 (1997)

145.

[37] A.S. Michaels, R.B. Parker, Sorption and flow of gases in polyethylene,

J. Polym. Sci. 41 (138) (1959) 53.
http://dx.doi.org/10.1016/j.memsci.2005.05.032http://dx.doi.org/10.1016/j.memsci.2005.05.032http://dx.doi.org/10.1016/j.memsci.2005.05.032http://dx.doi.org/10.1016/j.memsci.2005.05.032

Documents

Effects of Novel Silane Modification of Zeolite Surface on Polymer Chain Rigidification and Partial Pore Blockage in Polyethersulfone PES Zeolite a Mi