Gas permeation characteristics of polymer-zeolite mixed matrix membranes

Gas permeation characteristics of polymer-zeolite mixed matrix membranes

journal of MEMBRANE SCIENCE ELSEVIER Journal of Membrane Science 9 I,( 1994) 77-86 Gas permeation characteristics of polymer-zeolite membranes mixe...

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journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 9 I,( 1994) 77-86

Gas permeation characteristics of polymer-zeolite membranes

mixed matrix

Murat G. Siier, Nurcan BaC, Levent Yilmaz* Department of Chemical Engineering, Middle East Technical University, 06531 Ankara, Turkey

(Received February 15, 1993; accepted in revised form January IO, 1994)

Abstract Mixed matrix membranes of polyethersulfone (PES), a glassy polymer, and hydrophilic zeolites 13X and 4A were prepared by using different membrane preparation procedures. Using selected procedure (c), the permeation rates of N2, Oz, Ar, CO2 and Hz were measured with a variety of membranes prepared at different zeolite loadings. Significant differences in measured permeability and calculated selectivity values demonstrated the importance of the type and percentage of zeolite. For both zeolitic additives, permeabilities and selectivities are enhanced at high zeolite loadings. In order to understand the permeation mechanism taking into account the polymer-zeolite interactions, macropositioning of zeolites and matrix distribution, the heterogeneous membrane morphologies were investigated by scanning electron microscopy (SEM ). Significant changes in the membrane morphologies of PE- 13X and PES4A matrices were observed, implying the importance of zeolite type. Key words: Gas separations;

Mixed matrix membrane;

Polyethersulfone

1. Introduction

The development of membranes for the separation and purification of gases based on the selective permeation of one or more components of a mixture has attracted a great deal of interest during the last decade, and modifications in the physical and chemical structures of polymer films have been made to achieve better separation characteristics [ 11. The latest emerging membrane morphology with potential for the future is a matrix membrane composed of two interpenetrating matrices of different materials [ 2-61. Tsujita [ 71 investigated the effects of addi*Corresponding

membrane;

Zeolite; Membrane morphology

tives on polymeric membranes. His conclusion was that the polymer-filler system often has an elevated glass transition temperature ( Tg ) indicative of the restricted segmental motion of the polymer itself because of strong polymer-filler interaction. Therefore the coefficients of permeability and diffusion decrease with filler content. In the case of weak polymer-filler interaction, the filler may form a void in the interface between the polymer and the filler. The permeability coeffkient then increases considerably, indicating hydrodynamic permeation through the void or pore in the membrane matrix. These findings point to the need for the investigation of different polymer-filler combinations in order to cre-

author.

0376-7388/94/%01.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0376-7388(94)00018-T

et al. /Journal

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of Membrane Science 91 (I 994) 77-86

ate membranes with widely differing permeabilities and selectivities. Recently, interest has been focused on novel polymer-zeolite mixed matrix membranes, because the interaction of materials in the membrane matrix and the shape-selective catalytic properties of zeolites can favor permselective separations. Kulprathipanja et al. [ 21 found that membranes composed of cellulose acetate and silicalite- 1 have improved characteristics as the silicalite content increases. Te Hennepe [ 3 ] obtained improved results in the separation of various alcohols from water by pervaporation using silicalite-tilled silicone rubber membranes. Jia et al. [6] studied the permeation of various gases composed of membrane using a poly (dimethylsiloxane) (PDMS), which is a rubbery polymer, and silicalite-1, a hydrophobic zeolite. In their study only a couple of very high zeolite loadings were investigated, and they concluded that silicalite played the role of a molecular sieve in the membrane by facilitating the permeation of smaller molecules but hindering the permeation of larger ones. Okumus et al. [ 5 ] studied the separation of water-alcohol mixtures by pervaporation using a mixed matrix membrane composed of cellulose acetate and zeolite 13X or 3A. Their results showed that the addition of zeolite to the matrix improved the fluxes substantially, with some loss in selectivity. Gurkan et al. [ 4 ] focused on the separation of 02-Nz and HZ/N2 gas pairs using a zeolite 13Xfilled polysulfone membrane made by extrusion. They found substantial increases in selectivities

Table 1 Permeabilities

of various gases through homogeneous

Gas

Measurement (“C)

co*

35 35 35 25 25 25

He CH, 02

Ar N*

temp.

when compared with pure polysulfone. However, glassy polymer-hydrophilic zeolite pairs have not been investigated extensively in a systematic manner. As the resultant morphology is heterogeneous, the preparation and pretreatment of membranes may affect the structure and performance. In the membrane literature, this aspect is generally disregarded and very few details of membrane preparation and its effects are reported. The high performance engineering thermoplastic polyethersulfone (PES) is here selected as the membrane polymer. The repeat unit structure of polyethersulfone possesses a certain degree of rigidity. PES has a glass transition temperature of 225 “C [ 81 and is thus a glassy polymer at preparation and application temperatures. These properties have made PES a popular membrane material for various applications [ 8,9 1. The permeabilities of various gases measured under different conditions by using dense homogeneous PES films are summarized in Table 1. In this study, mixed matrix membranes of PES and zeolite 13X or 4A have been developed and the permeation rates of N2, 02, Ar, COZ and Hz have been measured, having regard to the potential effects of zeolite type and loading and membrane preparation methodology on permeability and separation factors. In order to understand the morphology of heterogeneous membranes, an investigation of the physical structure of the developed membranes has been carried out by scanning electron microscopy (SEM).

PES membranes

Pressure gradient (atm)

Permeability (barrer )

Membrane preparation

technique

10 10 10 7 7 7

2.8 8.0 0.1 0.629 0.229 0.0974

Compression molding Compression molding Compression molding Solution casting in DMF Solution casting in DMF Solution casting in DMF

Ref.

8 8 8 15 15 15

M.G. Siier et al. /Journal ofMembrane Science 91 (I 994) 77-86

2. Experimental 2.1. Materials

The polymeric material used was the commercial polyethersulfone Victrex 41OOP, which was kindly supplied by ICI. The solvent was dimethylformamide (DMF), which was purchased from Merck. As adsorptive fillers, zeolite 13X and zeolite 4A were used; they were supplied by Alfa Products. As the zeolites are highly hydrophilic, they were dried at 350-400°C for 2-3 h before each membrane preparation.

79

measurements. No change in membrane thickness was observed due to gas permeation . Measured thicknesses were in the range 70-80 pm. The measured thickness for each individual membrane was used in the permeability calculations. 2.3. Gas permeability measurements The permeability coefficients were determined in an apparatus conforming to ASTM D 1435-82, as shown in Fig. 1. Measurements were made by a constant vol-

2.2. Membrane preparation methodologies During this study, three different methods for membrane preparation were developed. The membranes were prepared by dissolving PES and dispersing zeolite 13X or 4A in DMF at the desired percentages with thorough mixing to ensure homogeneity. The mixture was then mechanically cast onto a glass plate as a film 500 pm thick. Procedure (a)

The cast film is allowed to dry at room temperature for one day. Further annealing is carried out at 80-100°C for 6-8 h. Procedure (b)

The cast film is allowed to dry at barometric pressure with nitrogen circulation at 60-80’ C for 8-10 h. It is then quenched by water, followed by annealing in a vacuum at 80-l 00°C for 1624 h. Procedure (c)

The cast film is allowed to dry under a partial vacuum with nitrogen circulation at 60-80’ C for 8-10 h. Subsequent quenching by water is followed by annealing at 80-100°C for 16-24 h at barometric pressure. All the membranes used in this study, unless otherwise specified in Table 5, were produced by procedure (c). The final thicknesses of the dry membranes were measured with a digital micrometer before and after the gas permeation

A: 0:

c: D: E: F :

G: Ii:

Membrane Pressure Transducer Digital Meter Gas Chamber High Pressure Purge Permeability Cell Vacuum Part Constant Temperature

Fig. 1. Permeability

Both

apparatus.

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M.G. Siier et al. /Journal of Membrane Science 9 I (1994) 77-86

ume, variable pressure technique in a permeation cell. These high pressure side was at 100 psig and the permeate side was initially at barometric pressure. The permeation rates of N2, 02, Ar, COZ and H2 were evaluated by measuring the increase in pressure on the permeate side using a pressure transducer (Data Instruments, Model SA). The individual gas permeations were evaluated using Eq. ( 1 ), which is a combined form of Fick’s law of diffusion and Henry’s law. Ji = PiAP/G

(1)

where Ji is the flux, Pi is the permeability, AP is the pressure difference across the membrane and 6 is the membrane thickness. For comparison purposes, the separation factor (Y is determined by ratioing the individual permeation rates: a=P,/Pj

(2)

2.4. Membrane characterization For the characterization of membrane morphology, electron micrographs of membranes, with different zeolite percentages were obtained by scanning electron microscopy on a Cambridge Stereo-Scan, Model S4-10 and a Leitz AMR 1000.

3. Results and discussion Reproducibility could be a serious problem in preparation and testing of membranes with complex heterogeneous structures [ 10 1. A series of reproducibility experiments was therefore carried out in order to check the gas permeation setup, the methodology and the membrane preparation techniques. The permeability of each gas through each membrane was measured three times, and the observed relative error between successive runs was ca. 5 l-3%. Further measurements were made with further portions from the same casting and the observed relative error was ca. 52-59/o. Finally, the permeabilities of membranes with the same zeolite composition but cast at different times were measured, and

the observed relative error was ca. + 3-6%. These results showed that our testing system and methodology were reliable, and that the membrane preparation system can reproducibly yield heterogeneous mixed matrix membranes with the same performance. It is believed that this reproducibility is achieved by using long preparation and annealing times to produce a stable membrane structure. The permeabilities of gases through PES-zeolite 13X and PES-zeolite 4A mixed matrix membranes prepared by procedure (c ) are given in Table 2. The maximum zeolite loading at which a workable membrane could be produced is 50%. The effects of zeolite type and percentage on the permeabilities of different gases can be evaluated with the help of Table 2 and Figs. 2-5. From the table and figures it can be observed that the permeabilities of N2, 02, Ar, CO2 and Hz decreased up to a loading of 8 wt% for zeolite 13X and 25 wt% for zeolite 4A. Above these loadings, a recovery in permeabilities started, favoring increasing permselectivity. Although the magnitude of the permeability Table 2 Permeabilities of gases through PES and zeolite 13X or 4Afilled PES membranes at T= 25”C, AP= 100 psig Zeolite type Zeolite content (wt%)

N,

0,

Ar

None

0.0

0.14

0.52

0.18

2.6

6.5

13x

4.0 8.3 16.6 33.3 42.0 50.0

NM 0.077 0.088 0.097 0.12 0.12

0.38 0.29 0.33 0.37 0.51 0.50

NM 0.11 0.13 0.15 0.19 0.20

2.4 1.5 1.8 2.7 4.2 5.2

4.9 3.8 4.7 6.0 7.5 8.5

4A

8.3 16.6 25.0 33.3 42.0 50.0

0.12 0.12 NM 0.097 0.17 0.25

0.47 0.47 NM 0.41 0.74 1.10

0.17 0.14 NM 0.12 0.26 0.35

2.3 2.3 1.6 2.0 5.8 10.7

6.0 5.8 4.5 4.8 8.9 14.1

CQ

Hz

Permeabilities are given in terms of barrer: 1 barrer = 1O-“’ cm3 (STP) cm/cm2 cmHg s. NM = not measured.

81

h4.G. Siier et al. /Journal of Membrane Science 91(1994) 77-86 0.6 L-

0

0

IO

30

20

40

F/, ZEOLITE

50

60

LOADING



Fig. 2. Effect of zeolite loading on permeabilities and Ar for PES- 13X mixed matrix membranes.

I

“1

1”

1”

20

o/uZEOLITE

of N2, O2

4”

50

611

LOADING

Fig. 4. Effect of zeolite loading on permeabilities and Ar for PES-4A mixed matrix membranes.

of NZ, 02,

D

1 CI

10

10

20

% ZEOLlTE

40

JO

60

04

LOADING

0

Fig. 3. Effect of zeolite loading on permeabilities HZ for PES-13X mixed matrix membranes.

of CO2 and

and its rate of change with zeolite loading were different for different gases, the general trend of permeability as a function of zeolite loading was the same for all the studied gases, suggesting a similar permeation mechanism for each. An important observation is that, at the maximum zeolite loading, gas permeabilities are either not altered or, in most cases, significantly improved. This effect is especially noticeable when zeolite 4A is used. The ideal separation factors of PES- 13X and PES-4A mixed matrix membranes for some gas pairs are tabulated in Table 3. Table 3 indicates that PES-13X membranes possess better separation characteristics than PES-4A membranes.

10

2”

30

R ZEOLITE

40

50

LOADING

Fig. 5. Effect of zeolite loading on permeabilities H2 for PES-4A mixed matrix membranes.

of CO2 and

On the other hand, gases permeate more readily through PES-4A membranes. The selectivities for the economically important gas pairs Hz/N*, C02/N2, and C02/02 [ 111 are substantially increased, owing to the faster permeation of H2 and COZ through the membrane matrix (Figs. 6 and 7 ) . However, for the pairs 0z/N2 and Ar/N2 the increase is only slight. Table 2 and Figs. 6 and 7 clearly show that at high zeolite loadings both gas permeabilities and selectivities increase, indicating the possible potential of this type of mixed matrix membrane for commercial applications. The increase in separation factors can not be

M.G. Siier et al. /Journal ofMembrane Science 91 (I 994) 77-86

82 Table 3 Selectivities membranes Zeolite type

of PES and zeolite

Zeolite content (wt%)

None

13X or 4A-filled

PES

X/N, O2

Ar

COz

Hz

3.71

1.29

18.8

46.4

5.6

CWO2

13x

8.3 16.6 33.3 42.0 50.0

3.70 3.75 3.81 4.29 4.18

1.43 1.47 1.54 1.61 1.68

19.5 20.5 21.9 35.7 43.1

49.4 53.4 61.3 63.1 70.8

5.26 5.45 7.32 8.30 10.4

4A

8.3 16.6 33.3 42.0 50.0

3.92 3.92 4.22 4.35 4.40

1.40 1.16 1.23 1.50 1.40

19.2 19.2 20.2 34.4 42.8

48.6 48.6 49.5 52.4 56.4

4.89 4.89 4.78 7.89 9.73

Table 4 Kinetic diameter of experimental

Kinetic diameter (A)

lites varies for the different gases, and this variation may contribute to the increase in permeabilities and selectivities. This chemical interaction effect plays an important role especially in CO2 permeability and selectivity at high zeolite loadings, because specific interactions with highly charged sites are most significant for gases (i.e. CO*) that have the ability to interact with the polar surfaces of zeolites A and X during permeation. The adsorption affinity of the studied gases on zeolite 4A is in the following sequence [ 131: COz>N2>CH4>02>Ar>H, For an uptake of one CO1 molecule per cavity the affinity sequence of zeolites is [ 14 ] A>X>Y 3. I. Comparison of membrane preparation procedures

gases

Nz

Ar

O2

CO2

H2

3.64

3.4

3.46

3.3

2.89

solely explained by using the molecular sieving effect of zeolites, because all the gas molecules are able to pass through the “windows” of zeolites 13X and 4A [ 121. On the other hand, shape selectivity may play a role if there is a homogeneous distribution in the matrix. The kinetic diameters of the gas molecules are given in Table 4

[121. The tables indicate that, although the argon molecule has a smaller kinetic diameter than oxygen, it has a lower permeability. Indeed, the permeability should have been greater if diffusional aspects are assumed to be significant. The reason for the result may be the non-spherical shape of the oxygen molecule. On the other hand, marked increases of CO2 permeability at high zeolite loading may be attributed to the polarity and adsorption of gases within the membrane matrix in addition to the diffusional aspects which imply faster permeation of smaller molecules (i.e., HZ). The chemical interaction of zeo-

Membranes prepared by different procedures are compared by measuring the permeation rates of gases through each at a specified zeolite loading. Membranes prepared by procedure (a) have a filter paper-like structure which is a highly permeable, non-selective film that allows passage of all gases. In terms of physical appearance, membranes prepared by procedure (b ) are much more brittle than those prepared by procedure (c), so higher zeolite loadings are not possible Table 5 Comparison

of membrane preparation

Method

Membrane

b

PES PES

C

Fr C

: C

Zeolite (wt%) 0 0

procedures

N2

O2

CO2

H2

0.14 0.14

0.50 0.52

2.6 2.6

6.5 6.5

PES-13X PES-13X PES-13X

33.3 33.3 33.3

AP AP 0.076 0.31 0.097 0.37

AP 1.0 2.7

AP 2.3 6.0

PES-4A PES-4A PES-4A

33.3 33.3 33.3

AP AP 0.11 0.38 0.097 0.41

AP 1.8 2.0

AP 3.5 4.8

AP: all pass.

83

M.G. Siier et al. /Journal ofMembrane Science 91(1994) 77-86

with membranes prepared by procedure (b). The results tabulated in Table 5 demonstrate that, for mixed matrix membranes, the membrane preparation procedure strongly affects the permeabilities of the gases. As the permeability values for homogeneous membranes prepared by two different procedures were almost the same, it can be concluded that the change in the procedure mainly affects the polymer-zeolite interactions. In our view, it can be claimed that the major difference between procedures (b) and (c) is the drying rate, and one can speculate on the effect of difference in drying rate as follows. In procedure (b ), initially a slow release of solvent takes place for the distribution of the matrix, and then rapid drying begins in the vacuum, which may inhibit the relaxation of intersegmental packing and cause substantial increases in the density of the matrix. This “densified” matrix may then show resistance to the permeation of gases and may have lost the homogeneous distribution of zeolite within the matrix. In contrast, in procedure (c) a quick release of solvent is followed by slow drying, which may allow relaxation of the packing and enhance homogeneity. The matrix is thus less dense and permeation is much higher, with an increase in separation factors. As shown in Table 5 permeability values for homogeneous membranes prepared by procedures (b) and (c) were almost the same, implying that both procedures produces stable and satisfactory structures. It can therefore be concluded that the change in procedure mainly affected the polymer-zeolite interactions and the resultant micromorphology of the heterogeneous membranes. 3.2. General character of membranes The SEM images of PES-13X and PES-4A membranes are shown in Figs. 8- 11.. These micrographs indicate a homogeneous distribution of zeolites in the matrix, where zeolite particles create cave-like porous structures into which they fit. The cave-like porous structure is believed to arise from the partial incompatibility of polymer chains and zeolite crystals, implying that the

‘51 -

I l&N21

P

s

F

p5

I

HUN2

60

I

I

.

I

m

0

P

•I

0

0

15

0

10

20 30 40 96 ZEOLITE LOADING

50

0

Fig. 6. Effect of zeolite loading on selectivities of C02/N2 and HZ/N2 for PES- 13X mixed matrix membranes.

0

CI

15 0

D

IO

P

20 30 40 cl0ZEOLITE LOADING

50

60

Fig. 7. Effect of zeolite loading on selectivities of C02/N2 and Hz/N2 for PES-4A mixed matrix membranes.

polymer interacts weakly with the zeolitic framework. As the filler content increases the void spaces that are formed around the zeolite crystals may combine to give a channel network, so increasing the permeation rates of gases (Fig. 9 ) . On the other hand the increase in the free volume at the zeolite locations with the increase in tiller content may cause an increase in the packing density of the unoccupied zones, which may restrict the diffusion of gases. Therefore, for the control of separation factors, it is difficult to consider a mechanism that is affected by a molecular sieving mechanism alone. At low zeolite loadings, the pores created by

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M.G. Siier et al. /Journal

ofMembrane

Fig. 8. SEM micrograph of PES-4A (50 wt%) mixed matrix membrane.

Science 91 (I 994) 77-86

Fig. 10. SEM micrograph of PES-4A (33.3 wtOY6)mixed matrix membrane.

Fig. 11. SEM micrograph matrix membrane.

Fig. 9. SEM micrograph trix membrane.

of PES-13X

(50 wt%) mixed ma-

of PES-13X

(33.3 wt%) mixed

zeolites cannot form a continuous channel network and gas molecules must cross polymer-void interfaces alternately, hindering gas permeation. Therefore the permeabilities of gases in both

M.G. Siier et al. /Journal of Membrane Science 91(1994) 77-86

PES-13X and PES-4A membranes are decreased. As the percentage of zeolite in the matrix increases, the channel network may mature and connect the separate voids that provide an alternate path for gas molecules. This may lead to increases in the permeation rates of all gases. In addition to diffusional aspects, which imply faster permeation of smaller molecules such as HZ, polar interaction of gases with zeolites may enhance the passage rates of polar gases such as COZ. Moreover, shape selectivity is expected to play a role by hindering the diffusion of large molecules, causing a further increase in the separation factor. The major difference between PES- 13X and PES-4A mixed matrix membranes is believed to be the macropositioning of zeolites in the matrix. Zeolite 13X crystals seemed to be more discrete (Fig. 9), whereas zeolite 4A crystals are partly aggregated, forming wider cavities. This could account for the different minima and recoveries in permeabilities in addition to the chemical interaction of gases with the zeolites [ 141. The agglomeration of 4A crystals in the membrane matrix increases the free volume at the zeolitic locations more markedly than is the case for 13X, resulting in a larger permeability increase. However, the separation factors are much higher in PES- 13X membranes because the positioning of zeolites is relatively separate with smaller cavities. This enhances the role of interaction of gas molecules with individual zeolite crystals.

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( 3 ) The type of zeolite in the matrix is very important. For PES-zeolite 13X membranes, permeabilities show a recovery above 8 wt% zeolite loading. However, for PES-zeolite 4A membranes, permeabilities show a recovery only above 25 wt% zeolite loading. The reason for this may be the different chemical interactions of polar gases with the different zeolites and the macropositioning of zeolites in the matrix, which imply the agglomeration of 4A crystals, whereas 13X crystals remain as discrete entities. (4) The addition of zeolite induces a microporous cavity and channeling system, demonstrating the polymer-zeolite interactions and incompatibility. (5 ) The increasing selectivities with increasing filler content cannot be due solely to a molecular sieving mechanism, considering the kinetic diameters of the gases. The shape-selective properties of zeolites, the polarity of gases and the microstructure of the membranes may play a role in increasing permselectivities. ( 6 ) Changes in membrane performance are not only due to zeolite crystals, but also depend on the complex heterogeneous micromorphology, including the lack or presence of zeolite particle aggregation, and the cave-like voids created around the zeolitic tiller. It should be noted that this micromorphology and the resultant membrane performance depend strongly on zeolite type and amount.

Acknowledgments

4. Conclusions ( 1) Permeabilities first decrease and then increase with increasing zeolite loading. At high zeolite loadings (42-50%) both permeabilities and selectivities increase, indicating the potential of PES-13X and PES4A membranes for commercial applications. (2) It was observed that, for heterogeneous membranes, the membrane preparation procedure strongly affects the performance of the membrane.

The authors acknowledge financial support from Turkish Scientific and Research Council (TUBITAK) through grant number TBAG 1166 and from METU Research Fund, project number AFP 92-03-04-o 1.

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[2]S. Kulprathipanja, R.W. Nousil and N.N. Li, Separation of fluids by means of mixed matrix membranes in gas permeation, US Pat. 4,740,219 (1988). [3]H.J.C. te Hennepe, Zeolite Filled Polymeric Membranes: A New Concept in Separation Science, Thesis, University of Twente, 1988. [4]T. Gurkan, N. Bat, G. Kiran and T. Gur, A new composite membrane for selective transport of gases, Proc. 6th Int. Symp. Synthetic Membranes in Science and Industry, Tiibingen, Germany, August 1989. [ 5 ] E. Okumus, T. Gurkan and L.Yilmaz, Development of a zeolite-polymer membrane for pervaporation, paper presented at Symp. Chromatographic and Membrane Processes in Biotechnology, Azores, Portugal, July 1990. [6]M. Jia, K.V. Peinemann and R.D. Behling, Molecular sieving effect of zeolite filled silicone rubber membranes, J. Membrane Sci., 57 ( 1991) 289. [ 7]Y. Tsujita, Physical Chemistry of membranes, in Y. Ostada and T. Nakagawa (Eds. ), Membrane Science and Technology, Marcel Dekker, New York, 1992, p. 3 [ 81 J.S. Chiou, Y. Maeda and D.R. Paul, Gas permeation in polyethersulfone, J. App. Polym. Sci., 33 (1987) 1823.

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[ 91 I. Pinnau, J. Wind and K.V. Peinemann, Ultrathin multicomponent polyethersulfone membranes for gas separation made by dry/wet phase inversion, Ind. Eng. Chem. Res., 29 (1990) 2028. [ 10 ] R.E. Kesting, Synthetic Polymeric Membranes: a Structural Perspective, 2nd ed., Wiley, New York, 1985. [ 111 R.R. Zolandz and G.K. Fleming, in: W.S.W. Ho and K.K. Sirkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, New York, 1992, pp. 17-95. [ 12 ] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. [ 13lR.J. Harper, G.R. Stifel and R.B. Anderson, Adsorption of gases on 4A synthetic zeolite, Can. J. Chem., 47 (1969) 4661. [ 14 ] B. Coughlan and S. Killmartin, Zeolites X, Y and A enriched with trivalent cations: sorption of carbon dioxide and ammonia, J. Chem. Sot. Faraday Trans. 1, 7 1 (1975) 1809. [ 151 K. Haraya and S.-T. Hwang, Permeation of oxygen, argon and nitrogen through polymer membranes, J. Membrane Sci., 71 (1992) 13.