journal of MEMBRANE SCIENCE
Journal of Membrane Science 93 ( 1994) 283-289
Permselectivity of zeolite filled polysulfone gas separation membranes Turgut M. GiiP Centerfor Materials Research, Stanford University,Stanford, CA 943054045, USA Received 15 February 1994; accepted in revised form 2 1 April 1994
Abstract Gas permeabilities of molecular sieve 13X filled polysulfone and unblended polysulfone membranes fabricated by a melt extrusion process were measured by a constant volume technique at room temperature. The relative permeation rates of the industrially important gases were found to be in the order Hz > He > CO2 > O2 > CH., = N2. No pronounced effect of the filler was observed. Keywords: Polysulfone; Molecular sieve; Composite membrane; Diffusion; Gas separation
1. Introduction There is great interest in highly permselective gas separation membranes. For most polymers, however, selectivity and permeability seem to follow an adverse relationship. For instance, many elastomeric polymers, such as silicon rubber, have high gas permeabilities but poor selectivities. Conversely, most barrier type or glassy polymers, such as acrylics, are quite selective, but they have low permeabilities, which make them economically unattractive for many commercial applications. A materials approach was explored in the present study that involved incorporation of shape selective zeolites such as molecular sieves into a barrier type, glassy polymer matrix. The intent was to exploit the diffusion/sorption property of molecular sieve 13X and its effect on
gas permeation. The large pore size molecular sieve was chosen to exclude the possibility of separation by size exclusion. It was anticipated that suffkient increase in the population of amorphous regions in the polymeric matrix upon incorporation of molecular sieve particles would lead to an enhancement in the gas permeation rates. A similar enhancement in selectivity values would be possible for suffkient differences in the adsorption/desorption kinetics and surface diffisivities of gases in the micropores of the molecular sieve. It should be noted that this strategy is distinctly different from previous studies [ l-61 where various fillers were loaded into elastomeric, high permeability polymer matrices, such as silicon rubber, to alter their permeation properties.
*Corresponding author. Tel: (415) 723-6597; Fax: (415) 7233044; E mail: [email protected]
0376-7388/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO376-7388(94)00102-5
T.M. Giir /Journal of Membrane Science 93 (I 994) 283-289
2. Membrane fabrication
Udel P- 1700 polysulfone (Union Carbide ) , a glassy polymer with a rigid aromatic backbone and a glass transition temperature above 180” C, was chosen as the matrix material for the composite membrane. Molecular sieve 13X (Union Carbide), which has a nominal pore size of 10 8, and composition Nash [ (AlO,),,( Si02) 106]276H20, was chosen as the filler. The 13X powder and the polysulfone pellets were dried under vacuum at -200°C for 16 h. The powder-pellet mix was extruded twice in a ZSK twin screw extruder at 340 “C followed by vacuum drying at 200’ C for 16 h between each extrusion. Finally,
the dry mix was extruded into thin membranes through a slit die in a 4-heat zone 1 l/2” commercial extruder operating at 340 to 355 “C. The resulting membranes were defect-free, continuous films, 40 to 200 pm thick and N 13 cm wide. Membranes with three different loading compositions were fabricated, namely, 10, 20, and 40 ~01% molecular sieve 13X. The 40 ~01% tilled membranes were brittle and mechanically unsuitable for permeation measurements. Additionally, a control sample of unblended Udel P-l 700 polysulfone membrane was extruded under identical conditions. This control membrane sample was used for comparison purposes to determine the effect of the molecular sieve additive on the gas permeation properties.
3. Experimental aspects Gas permeability measurements were made in a constant volume apparatus connected to a vacuum system. Six industrially significant gases, namely, HZ, He, NZ, 02, COz, and CH4 were tested. The permeability cell was made of a stainless steel double flange with an exposed membrane area of 62 cm2. The inlet gas pressure was generally maintained at 7.8 atm ( 100 psig) while the permeate side usually was not allowed to exceed 0.066 atm (50torr). Permeation measurements, made at room temperature, typically involved pumping down the system and degassing the membrane from both sides overnight. The test gas was then brought in contact with the inlet side of the membrane at 7.8 atm ( 100 psig) for 2 to 8 h while the permeate side of the cell was still under dynamic vacuum. The vacuum was then cut off and the pressure-time data was recorded. Consequently, an incubation or “lag” period was not observed in most experiments because the membranes were pre-equilibrated with the test gas. The expression for permeability, P,at room temperature is given by Fig. 1. Scanning electron microscopy (SEM) pictures for the 10 ~01% molecular sieve 13X filled Udel polysulfone composite membrane in (a) plan view and (b) cross-sectional view.
T.M. Giir /Journal ofMembrane Science 93 (1994) 283-289
them are in direct contact with the test gas on the membrane surface and the interparticle spacing in the bulk (Fig. 1b ) lies in the submicron range. The composite membrane samples were tested individually for HZ, He, CO*, 02, CH4, and N2. As illustrated in Fig. 2, a linear pressure-time behavior was observed for all gases, except CO*, which showed an incubation period. The permeability values for N2 and CH., were indistinguishably close to each other and lower than all the other gases. As expected, H2 and He were the fastest gases, with CO2 and O2 exhibiting intermediate permeability values. Similar results were obtained for the unblended Udel control samples. The relative gas permeation rates followed the same trend, i.e., H2 > He > CO2 > O2> CH4 x N2. The permeabil-
where AZ is the time interval, pr, is permeate pressure, 1is the thickness, V, is the volume ( cm3 ) of the expansion chamber used, 63.0 is the dead volume of the gas handling system, and 1 barrer= lo-” cm3 (STP) cm/cm* s cmHg. It is apparent from Eq. ( 1) that the permeability can readily be obtained from the linear slope of the pressure-time plot. 4. Results and discussion The microstructure of the membranes are shown in the plan view and cross-sectional SEM pictures in Figs. la and 1b, respectively. The 13X particles, 2 to 8 pm in size, are distributed uniformly throughout the polymer matrix. Many of
13X Composite Membrane No. 8134/30-1C
Time (mid Fig. 2. Pressure-time plot for a 20 ~01% molecular sieve 13X filled Udel polysulfone composite membrane for Hz, He, COz, 02, CH.,, and N2 at room temperature and at an inlet pressure of 7.8 atm ( 100 psig) .
13.2 12.4 6.5 2.6 1.5 1.4
Control Udel P- 1700 (7856/29-1B) 203pm (8 mil)
HZ He co* 02 CH, N2
23.5 5.6 55.4
14.4 12.8 6.1 1.4 0.26 0.26
10 ~01% 13X/Udel (8134/34-3a) 89 pm (3.5 mil)
23.9 5.4 48.6
19.0 5.0 45.9
13.6 12.6 6.7 1.4 0.28 0.26
14.7a 12.5b 6.1’ 1.6b 0.32b 0.32b
Mohretal. [lo] (35°C) melt-extruded
20 ~01% 13X/Udel (8134/30-1B and 1C) 140pm (5.5 mil)
Mohretal. [lo] (25°C) solution cast
21.5 5.6 53.8
14.0 13.0 5.6 1.4 0.26 0.25
McHattieetal. (35°C) solution cast
Table 1 Permeability and selectivity values measured for the unblended and composite polysulfone membranes fabricated in this study. The literature values for UDEL polysulfone are also given for comparison
3 ; I?. 3 2 2 2 S &
T.M. Giir /Journal ofMembrane Science 93 (1994) 283-289
10 v% 13X Composite Membrane No. 8134/34-3B 2SO
200 c 0’ = s
D 2 L 9) ‘;;j
E” & a so
Time (hrs) Fig. 3. Long-term N2 permeation data for a 90 pm ( 3.5 mil ) thick, 10 ~01% molecular sieve 13X filled Udel polysulfone composite membrane at room temperature and at an inlet pressure of 7.8 atm ( 100 psig ) .
ities for the unblended, and the 10 and 20 ~01% molecular sieve 13X filled membranes were calculated from the slopes in accordance with Eq. ( 1) and summarized in Table 1, which also includes the literature values for comparison. Also included in this table are the selectivity values calculated for industrially important gas mixtures. The early time-pressure behavior for COz showed an incubation period in Fig. 2. Using the timelag method [ 3,7 1, the diffusivity of CO2 was calculated to be 3x lo-’ cm2/s in the 20 ~01% filled membrane at room temperature. A similar time-lag behavior for CO2 for the unblended polysulfone membrane gave a diffusion coefficient of 6x lo-’ cm’/s. These coefficients agree well with the literature value of 4.4x lo-’ cm2/s at
35 “C [ 8,9]. Similarly, the CO2 permeability of 6.5 barrer measured for the unblended Udel polysulfone also agrees well with the data of 6.7 barrer reported by Mohr et al. [ IO] for melt extruded polysulfone membranes. Several permeability measurements were carried out at substantially different gas inlet pressures in order to check the possibility of pinholes. The permeability values are expected to increase with increasing inlet pressure if membranes had pinholes. However, for the unblended UDEL membranes the permeability coefficients for Hz, 02, and N2 measured at 2.7 and 7.8 atm inlet pressures were found to be independent of the inlet pressure. Additionally, a long-term permeation test was undertaken using N2 in order to confirm that the
T.M. Giir /Journal ofMembrane Science 93 (1994) 283-289
permeability values observed for the filled membranes were not due to a transient phenomenon as suggested earlier [ 3 1. The permeation experiment, monitored over a period of 10 days, showed a linear pressure-time relationship as depicted in Fig. 3. Thus, the possibility of transient effects was ruled out. The permeability value of 0.29 barrer obtained in this long-term experiment was in good agreement with 0.26 barrer measured earlier (Table 1) . The permeability coefficients given in Table 1 for the six gases measured for the unblended and filled polysulfone membranes are generally in good agreement with the results reported in the literature [ lo- 141. Small differences in permeability values may be attributed to differences in the microstructure and texture of cast versus extruded membranes. The permeabilities of 02, CH4, and Nz in the unblended polysulfone membranes were found to be considerably higher than the literature values [ 10,111 as well as the filled membranes of this study. This was not the case for the other gases. SEM examination of these membranes did not show microscopic pinholes, and yet the permeation rates for these slower gases were deceptively high. Although the reason for this is unclear, a possible explanation may be related to the presence of a small leak in the permeation apparatus that was not detected. A leak rate of N 1.2 barrer would account for this discrepancy. Since the permeability values for the slower gases were suspect, no selectivity values were given for the unblended membrane in Table 1. The results of this study indicate that the addition of molecular sieve to polysulfone and the level of loading did not have a significant effect on gas permeabilities (Table 1). The nominal pore size of 10 A for molecular sieve 13X is considerably larger than the kinetic diameters of any of the six gases used in this study. Hence, separation due to size exclusion was not expected to be a dominant mechanism. However, it was anticipated that differences in the adsorption/desorption kinetics and surface difisivities of gases in the micropores might influence their permeation rates. Obviously, the role of the molecular sieve filler was insignificant for this system and
the separation process is governed predominantly by the polysulfone matrix [ 9 1. In fact, the agreement between the diffusion coefficients and permeability values for CO* measured for the unblended and filled polysulfone membranes seem to support this argument.
The author is grateful to Raychem Corporation of Menlo Park, CA, for supporting this study. This work has benefited from facilities and equipment made available to Stanford University by the NSF-MRL Program through the Center for Materials Research (CMR) at Stanford University. The author also wishes to thank Prof. W.J. Koros and L. Costello of The University of Texas at Austin for helpful discussions.
References [ I ] C.F. Most, Jr., Some tiller effects on diffusion in silicone rubber, J. Appl. Polym. Sci., 14 (1970) 1019.
[ 2lT.K. Kwei and W.M. Amheim, The diffusion of gases through filled polymers, J. Polym. Sci., Cl0 (1965) 103.
[ 31 D.R. Paul and D.R. Kemp, The diffusion time lag in polymer membranes containing adsorptive fillers, J. Polym. Sci. Symp., 41 (1973) 79. [ 41 C. Bartels-Casper, E. Tusel-Langer and R.N. Lichtenthaler, Sorption isotherms of alcohols in zeolite-tilled silicone rubber and in PVA-composite membranes, J. Membrane Sci., 70 ( 1992) 75. [ 51M. Jia, K.-V. Peinemann and R.-D. Behling, Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permation, J. Membrane Sci., 57 ( 199 1) 289. [ 6lH.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, Zeolite-filled silicone rubber membranes. Part I. Membrane preparation and pervaporation results, J. Membrane Sci., 35 ( 1987) 39. [ 71 A.S Michaels, W.R. Vieth and James A. Barrie, Diffusion of gases in polyethylene terephthalate, J. Appl. Phys., 34 ( 1963) 13. A.J. Erb and D.R. Paul, Gas sorption and transport in polysulfone, J. Membrane Sci., 8 ( 198 1) 11. [ 91 E. Sada, H. Kumazawa, P. Xu and M. Nishigaki, Mechanism of gas permeation through glassy polymer films, J. Membrane Sci., 37 (1988) 165. [ lo] J.M. Mohr, D.R. Paul, I. Pinnau and W.J. Koros, Surface fluorination of polysulfone asymmetric membranes and films, J. Membrane Sci., 56 ( 1991) 77.
T.M. Giir /Journal ofMembrane Science 93 (1994) 283-289 [ I 1 ] J.S. McHattie, W.J. Koros and D.R. Paul, Gas transport properties of polysulfones. 1. Role of symmetry of methyl group placement on bisphenol rings, Polymer, 32 ( 199 1) 840. [ 121J.M.S. Henis and M.K. Tripodi, Multicomponent membranes for gas separations, US Pat. 4,230,463 (Oct. 28, 1980).
[ 131J.M.S. Henis and M.K. Tripodi, Composite hollow fiber membranes for gas separation: the resistance model approach, J. Membrane Sci., 8 (1981) 233. [ 14lM.B. Moe, W.J. Koros and D.R. Paul, Effects of molecular structure and thermal annealing on gas transport in two tetramethyl bisphenol-A polymers, J. Polym. Sci., Part B, 26 (1988) 1931.