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Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation
Science, 57 (1991) 289-296 Elsevier Science Publishers B.V., Amsterdam
Journal of Membrane
289
Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation Mengdong Jia, Klaus-Viktor Peinemann and Rolf-Dieter Behling GKSS-Research 2054 Geesthacht
Center Geesthacht (Germany)
GmbH, Max-Planck-Strasse,
Postfach
1160,
(Received February 28,199O; accepted in revised form October 19,1991)
Abstract Composite membranes, consisting of a highly hydrophobic zeolite, silicalite-1, and PDMS polymer were prepared. Distinct gas permeation effects have been found for these membrane types. A new parameter, the facilitation ratio of zeolite, was introduced to characterize the function of silicalite in the membrane. Using this parameter it was confirmed that silicalite played an important role in the molecular transport and that the altered permeabilities and selectivities were a result of the molecular sieving effect of the silicalite. gas separations; facilitated transport; zeolite; polydimethylsiloxane membrane; composite membrane
Keywords:
Introduction
Because of their high surface area (up to 1000 m”/g), high void volume (ca. 30% of the total volume of zeolite) and uniform pore size distribution, zeolites have been used widely in chemical and physical processes such as shape-selective catalysis and gas separation [l-3]. In order to explore the possibility whether zeolites can be used as membrane materials for continuous separation processes, pioneer work has been carried out by some authors [ 4-81. From their results with the zeolite-polymer composite membranes Paul and Kemp proposed that filling zeolite into the membranes could cause very large increases in the diffusion time lag, but would have only minor effects on the steady-state permeation [ 41. Kulprathipanja et al. [ 51 found that membranes composed of silicalite-1 and cellulose acetate have better separating characteristics. The separation factor, e.g. cwO,/N,, was increased from 2.99 to 3.47, 3.36 and 4.06 by increasing the silicalite content from 0 to 25% [ 51. Hennepe et al. showed much improved results for the separation of various alcohols from water via pervaporation using silicalite-filled silicone rubber membranes [ 61.
0376-7388/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
290
Until now, however, no systematic research has been reported on gas permeation of the zeolite-filled membrane. It is obvious that further investigations into these types of membranes are of great interest. In the present work some of the gas permeation results of silicalite-filled membranes are presented. The main research aims were to explore the potential of these membranes for gas separation and to grasp the key function of zeolite in the membranes. Experimental
1. Zeolite: silicalite-1 Silicalite-1 was obtained from UOP. The silicalite sample was calcinated at 550°C for 4-5 hr and then washed twice with deionized water. After washing, it was dried at 110’ C overnight. 2. Membrane preparation
The silicone rubber (polydimethylsiloxane PDMS) membrane was prepared with VP7660A and VP7660B from Wacker. For the preparation of composite membrane VP7660A, VP7660B and silicalite powder were put together and thoroughly mixed until a homogenous paste was formed. The paste was then mechanically cast on a teflon plate, casting thickness from 100 to 500 pm. In order to ensure complete crosslinking the film was cured at R.T. overnight and further at 80°C for 4-5 hr. The thickness of the finished membranes was measured with a screw gauge. 3. Gas permeation measurement A schematic representation of the gas permeation apparatus is shown in Fig. 1. The test cell and a calibrated volume was placed in a thermostatized container controlled to ? 0.1 “C. The gas pressure on the upstream face of the membrane was essentially atmospheric pressure during the experiments. The downstream pressure was kept at less than 2% of the upstream pressure and permeation
ce,,
Fig. 1. Equipment for the gas permeation and separation measurements.
291
recorded as a function of time. The gas permeability was determined by the following equation: P=qll(p,
-Pl)At
where P= gas permeability [ cm3 (STP)cm/cm’-see-cmHg], q/t =volume flow rate of gas permeate [cm” (STP) /set] , Z= membrane thickness (cm), pz -pl =pressure difference across the membrane (cmHg) and A = effective membrane area, which was in this work 13.20 cm’. The ideal selectivity, cy, is defined as the ratio of the pure component permeabilities:
During the experiments it was observed that at the beginning of permeation, the gas permeabilities decreased with time while the selectivities increased. To avoid this influence newly prepared membranes were evacuated over night, with N, sweeping on the upstream face. Permeabilities of NP, 02, He, H,, CH, and CO, were measured after the membrane had been swept with each gas for about 15 min. Repeated measurements gave the same values (within the experimental error of 5% ), showing that the time is long enough to achieve the steady state. Adsorptions of n-butane, 1-butene and iso-butane in silicalite were noticed during the experiments, therefore membranes were swept with the gas for about 30 min before measurements and after that evacuated for more than 2 hr in order to desorb the hydrocarbons. Separation measurement with a gas mixture, i.e. synthetic air, was carried out separately. An on line mass-spectrometer was used to analyse the composition of feed and permeate. The tests were run mostly over night, with an automatic printer recording the analysis results every half hour. The separation factor is defined as the quotient of the molar ratio in the permeate and feed respectively:
The maximum and minimum values during the total processes are given directly. Results and discussion
1. Permeabilities and selectivities of the zeolite-filled PDMS membranes for gases The permeabilities and selectivities of zeolite-free and zeolite filled membranes are summarized in Tables 1 and 2. For reference purpose, data measured by Robb [9] are also cited. Both values of the zeolite-free membranes are in quite a good agreement, except for the relatively high permeability of butane and the high selectivity of butane to nitrogen measured by us. The reason may
292
TABLE 1 Permeabilities of PDMS and silicalite-filled PDMS membranes at 30°C Membrane
PDMS+33% silicab VP-7660 VP+Si-50 VPSSi-64 VP + Si-70
(pm)
sil. (wt.%)
0 0 50 64 70
150 88 95 130
permeabilities x 10” (cm3-cm/cm’-see-cmHg) N,
0,
He
H,
280 266 225 266 224
600 350 650 571 382 625 563 667 1050 730 1091 1530 655 1178 1635
CH,
CO,
*nC4
iC,
c,=
950 838 556 500 433
3250 2890 3150 4260 3835
9000 14400 8910 5920 -
6040 3460 2480 -
13200 5420 -
“silicalite contents in the membranes. bdata cited from [ 91. *nC, = n-butane; iC, = i-butane; C,= = 1-butene. TABLE 2 Selectivities of PDMS and silicalite-filled PDMS membranes at 30” C membrane
selectivity (Y(X/N,)
(pm) X=
PDMS+33% silica VP-7660 VP + Si-50 VP + Si-64 VP+Si-70
150 88 95 130
0,
He
H,
CH,
CO,
nC,
iC,
c,=
2.14 2.15 2.50 2.74 2.92
1.25 1.44 2.96 4.10 5.26
2.32 2.35 4.67 5.73 7.30
3.39 3.15 2.47 1.88 1.94
11.6 10.9 14.0 16.0 17.1
32.1 54.0 39.6 22.3 -
22.7 15.4 9.32 -
49.6 20.4 -
be the differences in experimental conditions, for the permeability of butane is strongly dependent on its partial pressure on the feed side. The effect of filling silicalite into the PDMS membranes were clearly seen. Permeabilities of He, H,, 0, and COz increased, while the permeabilities of Nz, CH4, n-butane, iso-butane and 1-butene decreased. With increased silicate content in the composite membranes, the differences of the permeability and selectivity became more obvious. The best selectivity for 03/N2 was 2.92. The permeability for oxygen, on the other hand, increased from 571 to 655 barrer. This indicates that a better selectivity was realized without the expense of good permeability. When the results are plotted as in the “oxygen permeability and 02-N2 selectivity” diagram [ 10 1, they are to be found above the curve of various silicone polymers and organic polymers, indicating that silicalite-filled membranes possess better permeation and separation properties for 0,/N, than any other unfilled silicone rubber or organic membrane.
293
2. Molecular sieving effect of silicalite in the membranes In order to clarify how silicalite changed the permeabilities of the membrane for various gases, a new parameter, (P,,, - P,) /P,, is introduced here as “facilitation ratio of zeolite”. The term Pz+p- P, is the permeability difference between the zeolite-filled membrane and the zeolite-free polymer membrane, which represents the permeability contributed by zeolite. P,, is the permeability of the polymer membrane. This parameter is in fact a measurement of the zeolite contribution to permeation as compared with the polymer. Figure 2 illustrates the correlation between the facilitation ratio and the kinetic diameter of the gas molecules. Curves, which are composed of two linear relationships, were obtained as in the diagram. Linear regions are found on the left-hand side, i.e. when the kinetic diameter, a, is smaller than 3.8 A. Horizontal lines are found on the right-hand side, i.e. when o greater than 4.3 A, which lie between - 0.4 and - 0.6. The oblique lines on the left side indicate that the efficiency of silicalite for facilitation permeation depends on the dimension of the molecules to be separated. The smaller the molecule, the higher the efficiency of permeation through silicalite and vice versa. More simply, silicalite in the membrane facilitates the permeation of smaller molecules, while it hinders the passage of bigger molecules. The horizontal lines on the right side can easily be understood by the definition of the facilitation ratio. In the extreme case of hinderance, the molecules are too slow to pass through the pores of silicalite and permeation takes place only in the polymer part of the membrane. Silicalite has a density of 1.76 g-cme3 [ 121, so 64 wt.% silicalite in
2.0
_
1.5 _
CL” \
. 50 wtY 0 silicalite
1.0 _
e
I 4
0.5
_
0
_
c
- 0.5
-
_
1.0
He 2.0
l-t? 3.0
C&
1 O2 I N2I CH, I
n-C4t-&ol-C~Hs
4.0 Kinetic
i-C&o 5.0
diameter
6 ( i)
Fig. 2. Correlation between the kinetic diameter 6 of various gases and the facilitation ratio.
294
the membrane corresponds to about 50 vol.%, thus a permeability of 40-50% of the unfilled polymers is to be expected. The dependence of the facilitation ratio on the molecule size could no longer be observed in this region. Figure 2 can also be interpreted in the following manner: In the case of unfilled membrane, Pz+P - Pp is equal to “O”, only a horizontal line through zero could be found. As the silicalite is filled into the membranes, the horizontal line divides into two parts. On the left side are linear regions, whose slopes become steeper when the silicalite content is higher, showing the facilitating effect of silicalite in the permeation of small molecules. The steepest slope could be expected if the membrane were composed of silicalite with the polymer only filling the voids between the silicalite crystallite. Because of the impeding effect of silicalite for larger molecules, the horizontal lines on the right side of the graph move to lower levels. However, the facilitation ratio (P,+,-- P,,) /P,, can not be smaller than - 1.0. The horizontal line through zero is a dividing line, above which is the facilitated zone for gases and below which is the hindered zone. The kinetic diameter, a, was used in Fig. 2 for the correlation [ 11. This is based on the Lennard-Jones approach, but calculated from the minimum equilibrium cross-section diameter [ 1,111. Adsorption phenomena on zeolites have revealed that the minimum kinetic diameter should be used in order to obtain a reasonable interpretation [ 11. Our present results show that the kinetic diameter correlates better with the facilitation ratio than with other measures of molecular dimension. The pore structure of silicalite is shown in Fig. 3 [ 12,131. Both openings are larger than the size of the permeating molecules. However, the molecular sieving effect is still found. It implies that the shape-selective effect [ 141 is not only inherent in the equilibrium adsorption, but also in the kinetic adsorption and diffusion. Chen and Garwood first proposed this novel type of shape selectivity of ZSM-5 zeolite according to the diagnostic catalytic reactions [ 151. They concluded that ZSM-5 exhibited molecular sieving effect among large molecules, such as alkylbenzens and alkyltoluenes, as well as among smaller molecules, such as n-hexane, 3-methylpentane and 2,3_dimethylbutane. Our
Fig. 3. The pore structure
of silicalite
[ 131.
295 TABLE
3
Separation
factors of membranes
membranes
(Pm)
VP-7660
150
VP + Si-64 VP + Si-70
95 130
“silicalite contents
for 0,/N,
measured with synthetic
sil.” (wt.%)
separation
0 64 70
2.0-2.1
OJ(0,/N,
air at 30°C
factor
1
2.6-2.8 2.7-2.9
in the membranes.
present permeation results confirm this sieving effect of silicalite for even smaller molecules such as He, O2 and Nz. We attribute the effect to configurational diffusion restriction. Based on the differences in the molecular size, which determines the adsorption and diffusion rate, separation purposes may be achieved. Figure 2 confirms the important role of silicalite for gas transport and explains the real function of silicalite in the permeation properties of the membranes, i.e. the molecular sieving effect. From Fig. 2 it can also be predicted for what kind of separation problem a silicalite-filled membrane may be used effectively. 3. Separation factor measurements with gas mixture In order to check if the ideal selectivities which were calculated from the permeability of single gases agree with the real separation factor of a gas mixture, permeation experiments were also performed using synthetic air as a gas mixture. The measurements were run mostly overnight. Compositions of the feed and permeate were analyzed on line by a mass-spectrometer and the results were printed automatically every half hour. The separation factors were calculated. The maximum and minimum values recorded during the total run are summarized in Table 3. The results show good agreement of selectivities with those of single gas permeations. Conclusions (1) Silicalite plays the role of a molecular sieve in the silicalite filled membranes. It facilitates the permeation of smaller molecules, but hinders that of larger molecules. In this manner, it changes the selectivities as well as the permeabilities of the membranes. (2 ) Silicalite filled PDMS membranes possess higher 0, permeabilities and better 0,/N, selectivities than unfilled membrane. In case of 70 wt.% filling, the separation factor increases from 2.15 to 2.92. (3) Permeation measurements with a gas mixture, i.e. air, show that the ideal separation factor for OJN, agrees well with that of the gas mixture.
296
(4) As a measurement, a new parameter - the facilitation ratio of zeolite P,) /P, - has been introduced to characterize the efficiency of the zeoR+,lite in permeation. A positive ( Pz+p - Pp ) /P, implies a facilitated permeation of the zeolite and a negative (P,,, - P,) /P, means a hindered permeation. Acknowledgments
The authors would like to thank E.M. Flanigen of Union Carbide and M.T. Staliunis of UOP for providing the silicalite-1 sample. We would also like to thank D. Fritsch of our research center for the helpful discussions.
References 1
D.W. Breck, Zeolite Molecule
2
C.V. McDaniel and P.K. Mayer, Molecular Sieves, Sot. Chem. Ind. (London), Monograph, 1968, p. 186. E.M. Flanigen, Molecular sieve zeolite technology: The first twenty-five years, Pure Appl.
3 4 5
6 7
8
9 10
Sieves, John Wiley, New York, NY, 1964, Chaps. 5,6,8.
Chem., 52 (1980) 2191. D.R. Paul and D.R. Kemp, The diffusion time lag in polymer membranes containing adsorp tive fillers, J. Polym. Sci., Polym. Symp., 41 (1973) 79. Santi Kulprathipanja, Hoffman Estates; Richard W. Neuzil, Downers Grove; Normal N. Li, Arlington Heights, Separation of fluids by means of mixed matrix membranes, U.S. patent 4,740,219,1988. H.J.C. te Hennepe, M.H.V.
Mulder, C.A. Smolders, D. Bargeman and G.A.T. Schriider, Per-
vaporation process and membrane, Eur. patent 0,254,758,1988. Hiroshi Suzuki, Composite having a zeolite, a layered compound or a crystalline-lattice material in the pores of a porous support and processes for production thereof, Eur. patent 0,180,200,1985. Alexandre Rojey, Andre Deschamps, Alain Grehier and Eric Robert, Pro&de de separation des constituants dun melange en phase gazeuse au moyen dune membrane composite, Eur. patent Appl. 0,324,675,1989. W.L. Robb, Thin silicone membranes - Their permeation properties and some applications, Ann. N. Y. Acad. Sci., 146 (1968) 119. Chi-Long Lee, Harry L. Chapman, Martin E. Cifuentes, Kenneth M. Lee, Linda D. Merrill, Katherine L. Ulman and Krishnaswamy gas permeability of silicone membranes,
Venkataraman, Effects of polymer structure on the J. Membrane Sci., 38 (1988) 55.
11
L. Pauling, Nature of the Chemical York, NY, 1960, Chaps. 7,13.
12
E.M. Flanigen, J.M. Bennett et al., Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature, 271 (1978) 512. G.T. Kokotailo, S.L. Lawton, and D.H. Olson, Structure of synthetic zeolite ZSM-5, Nature, 272 (1978) 437.
13 14 15
bond, 3rd edn., Cornell University
Press, Ithaca, New
P.B. Weisz and V.J. Frilette, Intracrystalline and molecular-shape-selective catalysis by zeolite salts, J. Phys. Chem., 64 (1960) 382. N.Y. Chen and W.E. Garwood, Some catalytic properties of ZSM-5, a new shape selective zeolite, J. Catal., 52 (1978) 453.
Journal of Membrane
289
Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation Mengdong Jia, Klaus-Viktor Peinemann and Rolf-Dieter Behling GKSS-Research 2054 Geesthacht
Center Geesthacht (Germany)
GmbH, Max-Planck-Strasse,
Postfach
1160,
(Received February 28,199O; accepted in revised form October 19,1991)
Abstract Composite membranes, consisting of a highly hydrophobic zeolite, silicalite-1, and PDMS polymer were prepared. Distinct gas permeation effects have been found for these membrane types. A new parameter, the facilitation ratio of zeolite, was introduced to characterize the function of silicalite in the membrane. Using this parameter it was confirmed that silicalite played an important role in the molecular transport and that the altered permeabilities and selectivities were a result of the molecular sieving effect of the silicalite. gas separations; facilitated transport; zeolite; polydimethylsiloxane membrane; composite membrane
Keywords:
Introduction
Because of their high surface area (up to 1000 m”/g), high void volume (ca. 30% of the total volume of zeolite) and uniform pore size distribution, zeolites have been used widely in chemical and physical processes such as shape-selective catalysis and gas separation [l-3]. In order to explore the possibility whether zeolites can be used as membrane materials for continuous separation processes, pioneer work has been carried out by some authors [ 4-81. From their results with the zeolite-polymer composite membranes Paul and Kemp proposed that filling zeolite into the membranes could cause very large increases in the diffusion time lag, but would have only minor effects on the steady-state permeation [ 41. Kulprathipanja et al. [ 51 found that membranes composed of silicalite-1 and cellulose acetate have better separating characteristics. The separation factor, e.g. cwO,/N,, was increased from 2.99 to 3.47, 3.36 and 4.06 by increasing the silicalite content from 0 to 25% [ 51. Hennepe et al. showed much improved results for the separation of various alcohols from water via pervaporation using silicalite-filled silicone rubber membranes [ 61.
0376-7388/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
290
Until now, however, no systematic research has been reported on gas permeation of the zeolite-filled membrane. It is obvious that further investigations into these types of membranes are of great interest. In the present work some of the gas permeation results of silicalite-filled membranes are presented. The main research aims were to explore the potential of these membranes for gas separation and to grasp the key function of zeolite in the membranes. Experimental
1. Zeolite: silicalite-1 Silicalite-1 was obtained from UOP. The silicalite sample was calcinated at 550°C for 4-5 hr and then washed twice with deionized water. After washing, it was dried at 110’ C overnight. 2. Membrane preparation
The silicone rubber (polydimethylsiloxane PDMS) membrane was prepared with VP7660A and VP7660B from Wacker. For the preparation of composite membrane VP7660A, VP7660B and silicalite powder were put together and thoroughly mixed until a homogenous paste was formed. The paste was then mechanically cast on a teflon plate, casting thickness from 100 to 500 pm. In order to ensure complete crosslinking the film was cured at R.T. overnight and further at 80°C for 4-5 hr. The thickness of the finished membranes was measured with a screw gauge. 3. Gas permeation measurement A schematic representation of the gas permeation apparatus is shown in Fig. 1. The test cell and a calibrated volume was placed in a thermostatized container controlled to ? 0.1 “C. The gas pressure on the upstream face of the membrane was essentially atmospheric pressure during the experiments. The downstream pressure was kept at less than 2% of the upstream pressure and permeation
ce,,
Fig. 1. Equipment for the gas permeation and separation measurements.
291
recorded as a function of time. The gas permeability was determined by the following equation: P=qll(p,
-Pl)At
where P= gas permeability [ cm3 (STP)cm/cm’-see-cmHg], q/t =volume flow rate of gas permeate [cm” (STP) /set] , Z= membrane thickness (cm), pz -pl =pressure difference across the membrane (cmHg) and A = effective membrane area, which was in this work 13.20 cm’. The ideal selectivity, cy, is defined as the ratio of the pure component permeabilities:
During the experiments it was observed that at the beginning of permeation, the gas permeabilities decreased with time while the selectivities increased. To avoid this influence newly prepared membranes were evacuated over night, with N, sweeping on the upstream face. Permeabilities of NP, 02, He, H,, CH, and CO, were measured after the membrane had been swept with each gas for about 15 min. Repeated measurements gave the same values (within the experimental error of 5% ), showing that the time is long enough to achieve the steady state. Adsorptions of n-butane, 1-butene and iso-butane in silicalite were noticed during the experiments, therefore membranes were swept with the gas for about 30 min before measurements and after that evacuated for more than 2 hr in order to desorb the hydrocarbons. Separation measurement with a gas mixture, i.e. synthetic air, was carried out separately. An on line mass-spectrometer was used to analyse the composition of feed and permeate. The tests were run mostly over night, with an automatic printer recording the analysis results every half hour. The separation factor is defined as the quotient of the molar ratio in the permeate and feed respectively:
The maximum and minimum values during the total processes are given directly. Results and discussion
1. Permeabilities and selectivities of the zeolite-filled PDMS membranes for gases The permeabilities and selectivities of zeolite-free and zeolite filled membranes are summarized in Tables 1 and 2. For reference purpose, data measured by Robb [9] are also cited. Both values of the zeolite-free membranes are in quite a good agreement, except for the relatively high permeability of butane and the high selectivity of butane to nitrogen measured by us. The reason may
292
TABLE 1 Permeabilities of PDMS and silicalite-filled PDMS membranes at 30°C Membrane
PDMS+33% silicab VP-7660 VP+Si-50 VPSSi-64 VP + Si-70
(pm)
sil. (wt.%)
0 0 50 64 70
150 88 95 130
permeabilities x 10” (cm3-cm/cm’-see-cmHg) N,
0,
He
H,
280 266 225 266 224
600 350 650 571 382 625 563 667 1050 730 1091 1530 655 1178 1635
CH,
CO,
*nC4
iC,
c,=
950 838 556 500 433
3250 2890 3150 4260 3835
9000 14400 8910 5920 -
6040 3460 2480 -
13200 5420 -
“silicalite contents in the membranes. bdata cited from [ 91. *nC, = n-butane; iC, = i-butane; C,= = 1-butene. TABLE 2 Selectivities of PDMS and silicalite-filled PDMS membranes at 30” C membrane
selectivity (Y(X/N,)
(pm) X=
PDMS+33% silica VP-7660 VP + Si-50 VP + Si-64 VP+Si-70
150 88 95 130
0,
He
H,
CH,
CO,
nC,
iC,
c,=
2.14 2.15 2.50 2.74 2.92
1.25 1.44 2.96 4.10 5.26
2.32 2.35 4.67 5.73 7.30
3.39 3.15 2.47 1.88 1.94
11.6 10.9 14.0 16.0 17.1
32.1 54.0 39.6 22.3 -
22.7 15.4 9.32 -
49.6 20.4 -
be the differences in experimental conditions, for the permeability of butane is strongly dependent on its partial pressure on the feed side. The effect of filling silicalite into the PDMS membranes were clearly seen. Permeabilities of He, H,, 0, and COz increased, while the permeabilities of Nz, CH4, n-butane, iso-butane and 1-butene decreased. With increased silicate content in the composite membranes, the differences of the permeability and selectivity became more obvious. The best selectivity for 03/N2 was 2.92. The permeability for oxygen, on the other hand, increased from 571 to 655 barrer. This indicates that a better selectivity was realized without the expense of good permeability. When the results are plotted as in the “oxygen permeability and 02-N2 selectivity” diagram [ 10 1, they are to be found above the curve of various silicone polymers and organic polymers, indicating that silicalite-filled membranes possess better permeation and separation properties for 0,/N, than any other unfilled silicone rubber or organic membrane.
293
2. Molecular sieving effect of silicalite in the membranes In order to clarify how silicalite changed the permeabilities of the membrane for various gases, a new parameter, (P,,, - P,) /P,, is introduced here as “facilitation ratio of zeolite”. The term Pz+p- P, is the permeability difference between the zeolite-filled membrane and the zeolite-free polymer membrane, which represents the permeability contributed by zeolite. P,, is the permeability of the polymer membrane. This parameter is in fact a measurement of the zeolite contribution to permeation as compared with the polymer. Figure 2 illustrates the correlation between the facilitation ratio and the kinetic diameter of the gas molecules. Curves, which are composed of two linear relationships, were obtained as in the diagram. Linear regions are found on the left-hand side, i.e. when the kinetic diameter, a, is smaller than 3.8 A. Horizontal lines are found on the right-hand side, i.e. when o greater than 4.3 A, which lie between - 0.4 and - 0.6. The oblique lines on the left side indicate that the efficiency of silicalite for facilitation permeation depends on the dimension of the molecules to be separated. The smaller the molecule, the higher the efficiency of permeation through silicalite and vice versa. More simply, silicalite in the membrane facilitates the permeation of smaller molecules, while it hinders the passage of bigger molecules. The horizontal lines on the right side can easily be understood by the definition of the facilitation ratio. In the extreme case of hinderance, the molecules are too slow to pass through the pores of silicalite and permeation takes place only in the polymer part of the membrane. Silicalite has a density of 1.76 g-cme3 [ 121, so 64 wt.% silicalite in
2.0
_
1.5 _
CL” \
. 50 wtY 0 silicalite
1.0 _
e
I 4
0.5
_
0
_
c
- 0.5
-
_
1.0
He 2.0
l-t? 3.0
C&
1 O2 I N2I CH, I
n-C4t-&ol-C~Hs
4.0 Kinetic
i-C&o 5.0
diameter
6 ( i)
Fig. 2. Correlation between the kinetic diameter 6 of various gases and the facilitation ratio.
294
the membrane corresponds to about 50 vol.%, thus a permeability of 40-50% of the unfilled polymers is to be expected. The dependence of the facilitation ratio on the molecule size could no longer be observed in this region. Figure 2 can also be interpreted in the following manner: In the case of unfilled membrane, Pz+P - Pp is equal to “O”, only a horizontal line through zero could be found. As the silicalite is filled into the membranes, the horizontal line divides into two parts. On the left side are linear regions, whose slopes become steeper when the silicalite content is higher, showing the facilitating effect of silicalite in the permeation of small molecules. The steepest slope could be expected if the membrane were composed of silicalite with the polymer only filling the voids between the silicalite crystallite. Because of the impeding effect of silicalite for larger molecules, the horizontal lines on the right side of the graph move to lower levels. However, the facilitation ratio (P,+,-- P,,) /P,, can not be smaller than - 1.0. The horizontal line through zero is a dividing line, above which is the facilitated zone for gases and below which is the hindered zone. The kinetic diameter, a, was used in Fig. 2 for the correlation [ 11. This is based on the Lennard-Jones approach, but calculated from the minimum equilibrium cross-section diameter [ 1,111. Adsorption phenomena on zeolites have revealed that the minimum kinetic diameter should be used in order to obtain a reasonable interpretation [ 11. Our present results show that the kinetic diameter correlates better with the facilitation ratio than with other measures of molecular dimension. The pore structure of silicalite is shown in Fig. 3 [ 12,131. Both openings are larger than the size of the permeating molecules. However, the molecular sieving effect is still found. It implies that the shape-selective effect [ 141 is not only inherent in the equilibrium adsorption, but also in the kinetic adsorption and diffusion. Chen and Garwood first proposed this novel type of shape selectivity of ZSM-5 zeolite according to the diagnostic catalytic reactions [ 151. They concluded that ZSM-5 exhibited molecular sieving effect among large molecules, such as alkylbenzens and alkyltoluenes, as well as among smaller molecules, such as n-hexane, 3-methylpentane and 2,3_dimethylbutane. Our
Fig. 3. The pore structure
of silicalite
[ 131.
295 TABLE
3
Separation
factors of membranes
membranes
(Pm)
VP-7660
150
VP + Si-64 VP + Si-70
95 130
“silicalite contents
for 0,/N,
measured with synthetic
sil.” (wt.%)
separation
0 64 70
2.0-2.1
OJ(0,/N,
air at 30°C
factor
1
2.6-2.8 2.7-2.9
in the membranes.
present permeation results confirm this sieving effect of silicalite for even smaller molecules such as He, O2 and Nz. We attribute the effect to configurational diffusion restriction. Based on the differences in the molecular size, which determines the adsorption and diffusion rate, separation purposes may be achieved. Figure 2 confirms the important role of silicalite for gas transport and explains the real function of silicalite in the permeation properties of the membranes, i.e. the molecular sieving effect. From Fig. 2 it can also be predicted for what kind of separation problem a silicalite-filled membrane may be used effectively. 3. Separation factor measurements with gas mixture In order to check if the ideal selectivities which were calculated from the permeability of single gases agree with the real separation factor of a gas mixture, permeation experiments were also performed using synthetic air as a gas mixture. The measurements were run mostly overnight. Compositions of the feed and permeate were analyzed on line by a mass-spectrometer and the results were printed automatically every half hour. The separation factors were calculated. The maximum and minimum values recorded during the total run are summarized in Table 3. The results show good agreement of selectivities with those of single gas permeations. Conclusions (1) Silicalite plays the role of a molecular sieve in the silicalite filled membranes. It facilitates the permeation of smaller molecules, but hinders that of larger molecules. In this manner, it changes the selectivities as well as the permeabilities of the membranes. (2 ) Silicalite filled PDMS membranes possess higher 0, permeabilities and better 0,/N, selectivities than unfilled membrane. In case of 70 wt.% filling, the separation factor increases from 2.15 to 2.92. (3) Permeation measurements with a gas mixture, i.e. air, show that the ideal separation factor for OJN, agrees well with that of the gas mixture.
296
(4) As a measurement, a new parameter - the facilitation ratio of zeolite P,) /P, - has been introduced to characterize the efficiency of the zeoR+,lite in permeation. A positive ( Pz+p - Pp ) /P, implies a facilitated permeation of the zeolite and a negative (P,,, - P,) /P, means a hindered permeation. Acknowledgments
The authors would like to thank E.M. Flanigen of Union Carbide and M.T. Staliunis of UOP for providing the silicalite-1 sample. We would also like to thank D. Fritsch of our research center for the helpful discussions.
References 1
D.W. Breck, Zeolite Molecule
2
C.V. McDaniel and P.K. Mayer, Molecular Sieves, Sot. Chem. Ind. (London), Monograph, 1968, p. 186. E.M. Flanigen, Molecular sieve zeolite technology: The first twenty-five years, Pure Appl.
3 4 5
6 7
8
9 10
Sieves, John Wiley, New York, NY, 1964, Chaps. 5,6,8.
Chem., 52 (1980) 2191. D.R. Paul and D.R. Kemp, The diffusion time lag in polymer membranes containing adsorp tive fillers, J. Polym. Sci., Polym. Symp., 41 (1973) 79. Santi Kulprathipanja, Hoffman Estates; Richard W. Neuzil, Downers Grove; Normal N. Li, Arlington Heights, Separation of fluids by means of mixed matrix membranes, U.S. patent 4,740,219,1988. H.J.C. te Hennepe, M.H.V.
Mulder, C.A. Smolders, D. Bargeman and G.A.T. Schriider, Per-
vaporation process and membrane, Eur. patent 0,254,758,1988. Hiroshi Suzuki, Composite having a zeolite, a layered compound or a crystalline-lattice material in the pores of a porous support and processes for production thereof, Eur. patent 0,180,200,1985. Alexandre Rojey, Andre Deschamps, Alain Grehier and Eric Robert, Pro&de de separation des constituants dun melange en phase gazeuse au moyen dune membrane composite, Eur. patent Appl. 0,324,675,1989. W.L. Robb, Thin silicone membranes - Their permeation properties and some applications, Ann. N. Y. Acad. Sci., 146 (1968) 119. Chi-Long Lee, Harry L. Chapman, Martin E. Cifuentes, Kenneth M. Lee, Linda D. Merrill, Katherine L. Ulman and Krishnaswamy gas permeability of silicone membranes,
Venkataraman, Effects of polymer structure on the J. Membrane Sci., 38 (1988) 55.
11
L. Pauling, Nature of the Chemical York, NY, 1960, Chaps. 7,13.
12
E.M. Flanigen, J.M. Bennett et al., Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature, 271 (1978) 512. G.T. Kokotailo, S.L. Lawton, and D.H. Olson, Structure of synthetic zeolite ZSM-5, Nature, 272 (1978) 437.
13 14 15
bond, 3rd edn., Cornell University
Press, Ithaca, New
P.B. Weisz and V.J. Frilette, Intracrystalline and molecular-shape-selective catalysis by zeolite salts, J. Phys. Chem., 64 (1960) 382. N.Y. Chen and W.E. Garwood, Some catalytic properties of ZSM-5, a new shape selective zeolite, J. Catal., 52 (1978) 453.