3924
Ind. Eng. Chem. Res. 1998, 37, 3924-3929
Synthesis and Permeation Properties of SAPO-34 Tubular Membranes Joseph C. Poshusta, Vu A. Tuan, John L. Falconer,* and Richard D. Noble Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
A SAPO-34 membrane was prepared on an alumina tubular support. This membrane appears to exhibit molecular sieving properties with permeances that decrease as the kinetic diameter increases. The room-temperature permeances of H2 and n-C4H10 were 2.4 × 10-8 and 1.9 × 10-10 mol/(m2 s Pa), respectively, and the permeances were in the order H2 > CO2 > N2 > CH4 > n-C4H10. As the temperature increased, the single gas permeances of H2 and N2 exhibited minima, whereas the permeance of CO2 decreased and that of CH4 increased. As the pressure increased with a constant pressure drop across the membrane, the permeances of H2, CO2, N2, and CH4 decreased. The H2/CH4, CO2/CH4, H2/N2, and CO2/N2 ideal selectivities at 300 K and 270 kPa feed pressure with a 138 kPa pressure drop were 25, 19, 7.4, and 5.7, respectively, and these selectivities decreased with increasing temperature and increased with increasing pressure. The ideal selectivity of N2/CH4 was 3.4 at the same conditions and decreased with increasing temperature and increasing pressure. The H2/CO2 ideal selectivity was 1.3 at the same conditions and increased with increasing temperature and pressure. At 270 kPa feed pressure and 138 kPa pressure drop, the CO2/CH4 mixture selectivity was 30 at 300 K and 3.4 at 470 K. Introduction Because the pore sizes of zeolites are of molecular dimensions, zeolites have the potential to continuously separate mixtures of molecules by molecular sieving if they can be prepared as continuous membranes. They have the additional advantages that they can be used at high temperatures and are resistant to chemical degradation. Most studies reported to date for gas separations with zeolite membranes have used MFItype zeolites such as silicalite-1 and ZSM-5 zeolites,1-7 which have pore dimensions of 0.53 × 0.56 nm, as determined by crystal structure. These zeolites can adsorb molecules whose kinetic diameters are larger than these dimensions, however. These membranes have been prepared on stainless steel and alumina supports, both flat disks and tubes. High separation selectivities in these membranes have mainly been reported for mixtures of organic molecules, and the differences in permeances have often been due to preferential adsorption rather than molecular sieving.8-10 Only a few studies have reported separations of light gases with MFI-type zeolite membranes,11-14 and the ideal selectivities (ratios of single gas permeances) have not been large. Membranes composed of zeolites with smaller diameters are thus desirable for gas separations. Zeolite A membranes (which have a pore diameter of 0.42 nm) have been reported in a number of studies,15-23 but these membranes have mainly been used for separations of liquid mixtures by pervaporation. Most zeolite A membranes were prepared by using a seeding method and by using TMAOH (tetramethylammonium hydroxide) as a template. Gas separations were only reported for some of the A membranes. Gas permeation in most of these membranes appears to be by Knudsen diffusion. Wang et al.,22 however, reported * To whom correspondence is addressed. E-mail:
[email protected]. Fax: (303) 492-4341. Phone: (303) 492-8005.
the order of the permeabilities was C2H4 > CO2 > CH4 > N2 > O2 for an A-type zeolite membrane. This suggests that permeation was dominated by surface diffusion through grain boundaries and not by molecular sieving for their membrane, because the order of permeances does not follow the order of kinetic diameter. Aoki et al.23 reported synthesis of a NaA zeolite (0.41nm pore size) membrane that exhibited improved performance for gas separations. At 308-393 K the permselectivity for H2/N2 was 4.5-4.8, which is higher than the Knudsen value of 3.7. The order of permeance was H2 > O2 > CH4 > CO2 ) N2 ) C3H8. Aoki et al.23 reported that this membrane contained defects, because the kinetic diameter of C3H8 is larger than the NaA pore size. A recent paper by Lixiong et al.24 reported synthesis of a membrane composed of SAPO-34 crystals supported on an alumina disk. This microporous material is composed of Si, Al, P, and O and is an analogue of chabazite, a zeolite with cylindrical pores with a diameter of about 0.40 nm based on the crystal structure. No measurements of SAPO-34 pore sizes have been made by analysis of crystal structure, but n-C6H14 (0.43 nm) and i-C4H10 (0.50 nm) adsorption experiments have shown that SAPO-34 pores are between 0.43 and 0.50 nm in diameter.29 SAPO-34 was found to be a selective catalyst for the conversion of methanol to light olefins25 and is a possible automotive exhaust catalyst.26 Also, SAPO-34 is effective for separation of CO2 from CO2/ N2 gaseous mixtures by pressure-swing adsorption.27 The structure of SAPO-34 was investigated by XRD and NMR and found to be thermally stable at temperatures as high as 1273 K. The SAPO-34 molecular sieve adsorbs water strongly. Hydration can cause a change in the crystal structure, but this change is reversed by dehydration at 573 K.26,28 Lixiong et al.24 reported single gas permeances of H2, N2, CO2, and n-C4H10 on two SAPO-34 membranes at 323 K with a feed pressure of 220 kPa and permeate
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Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3925
pressure of 101 kPa. For these two membranes, the H2 permeances were (1.3-3.3) × 10-7 mol/(m2 s Pa), the N2 permeances were (1.5-5.2) × 10-8 mol/(m2 s Pa), and the CO2 permeances were (6.4-8.9) × 10-8 mol/(m2 s Pa). For each membrane, the permeances decreased in the order H2 > CO2 > N2 > n-C4H10 and the n-C4H10 flux was reported to be zero. The H2/N2 and CO2/N2 ideal selectivities were 6.4 and 1.7 for one membrane and 8.8 and 4.2 for the other. Thus, SAPO-34 membranes appear to have promising properties for gas separations, but to date, only single gas permeances have been reported and only at 373 K and lower temperatures. In the present paper, we report synthesis of membranes with SAPO-34 deposited as a continuous layer on the inner wall of R-alumina tubular supports instead of on disk supports. Membranes on tubular supports required synthesis procedures different from those used for disk membranes. Single gas permeances were measured for the same gases as reported by Lixiong et al.,24 but measurements were made to higher temperatures and at higher pressures and methane was also used. The ideal selectivities of H2/N2 and CO2/N2 are similar to those observed by Lixiong et al.,24 and high ideal selectivities are also seen for CO2/CH4 and H2/CH4. The single gas permeances of H2, CO2, N2, CH4, and n-C4H10 decrease with increasing kinetic diameter. In addition, we show that these membranes can separate CO2/CH4 gas mixtures. The permeances of light gases (H2, N2, CO2, CH4) through our SAPO-34 membranes, and the ones reported by Lixiong et al.,24 are 2 orders of magnitude lower than permeances measured on silicate and ZSM-5 membranes, and higher permeances are desirable for applications. Experimental Procedures Membrane Preparation. SAPO-34 membranes were prepared by in situ synthesis from gels onto porous R-alumina tubes. The supports (U.S. Filter) had an inside diameter of 0.7 cm and were asymmetric with the inner layer, which had a pore diameter of 200 nm. To avoid bypass during permeance measurements, the ends of 4.7-cm-long, tubular supports were sealed with a glazing compound (GL 611A, Duncan), which was calcined at increasing temperatures with a final hold at 1193 K for 30 min. The supports were cleaned with an ultrasonic bath with water at room temperature for 5 min and then boiled in distilled water for 1 h. The hydrogel for synthesis was prepared using modification of a procedure described in a patent by Lok et al.29 In the patent, SAPO-34 was prepared with the molar gel composition Al2O3:P2O5:0.3SiO2:2TEAOH: 50H2O at 473 K for 120 h (TEAOH ) tetraethylammonium hydroxide). The composition of the hydrogel used by Lixiong et al.30 was Al2O3:P2O5:2SiO2:TEAOH: 170H2O. To reduce the crystallization time in our preparation, the template amount was decreased from that reported in the patent, and the molar composition Al2O3:P2O5:0.6SiO2:1.07TEAOH:56H2O was used at 468 K for 20 h. The membranes prepared by Lixiong et al.24 were synthesized at 453 K for 6-50 h. The procedure described by Jia et al.8 was used to synthesize the SAPO-34 membranes. One end of the wet support tube was wrapped with Teflon tape and plugged with a Teflon cap, and about 2 mL of synthesis gel was used to fill the inside of the tubular support. The other end was then wrapped with tape and plugged
with a Teflon cap. The tube was placed vertically in a Teflon-lined autoclave. Approximately 1 mL of water was also placed in the autoclave. After hydrothermal treatment, the membranes were washed twice with distilled water at 300 K and then dried at 373 K in a vacuum for 2 h. The room-temperature N2 permeance was measured to determine if the layer was continuous and defect-free before calcination. The template was then removed by calcination in air at 753 K for 8 h with a heating rate of 0.6 K/min, and the room-temperature permeance of n-C4H10 was measured. Before the N2 or n-C4H10 permeances were measured, the feed side was swept with the feed gas, but the permeate side was not swept and the membrane was not heated. In this study, single gas and mixture permeances were measured through two membranes, M1 and M2. A third membrane was prepared for SEM analysis. Membrane M1 and the membrane for SEM analysis were each prepared with four synthesis layers, and M2 was prepared with six layers. Permeation Measurements. Fluxes of H2, CO2, N2, CH4, and n-C4H10 through the SAPO-34 membrane were measured in an apparatus such as that used by Funke et al.31 but modified for the measurement of light gas permeances. The membranes were mounted in a stainless steel module, and silicone O-rings provided the seals at each end. The module was heated by a small electric oven, and the temperature was measured with thermocouples ((3 K) inside the feed and retentate ends of the membrane. The pressure on the retentate side of the membrane was controlled with a pressure regulator, and the permeate pressure was controlled with a backpressure regulator. Pressures were measured with Bourdon tube pressure gauges ((3 kPa), and the transmembrane pressure drop was measured with an electronic pressure transducer ((0.7 kPa). The gas flux was measured by a soap film bubble flowmeter ((0.01 mL) and a stopwatch ((0.1 s), with which the lowest practical flow rate measurable was estimated to be 7 × 10-11 mol/(m2 s Pa). Single gas permeances were measured with one end of the membrane tube blocked off so that the membrane operated in a dead-end mode. Before the permeance was measured, the feed and permeate sides of the membrane were swept with the gas to be studied and the membrane was heated to 473 K. Permeances were measured at 50 K intervals as the membrane was cooled to room temperature. For separation experiments, the retentate side of the membrane was not blocked, and both the retentate and permeate streams were analyzed with a Hewlett-Packard 5890/series II gas chromatograph equipped with a thermal conductivity detector. Only mixtures of CO2/ CH4 were studied, and a premixed gas cylinder of CO2 and CH4 (49 mol % CO2 and 51 mol % CH4) was used for the feed gas. Pressure drop provided the driving force across the membrane in both single gas and gas mixture experiments. No sweep gas was used. Single gas and gas mixture experiments were conducted between 290 and 470 K with a feed pressure of 270 kPa and a pressure drop of 138 kPa. The effects of pressure on permeance were measured with a constant pressure drop of 138 kPa between 270 and 770 kPa. Results Membrane Preparation. Table 1 shows the permeance of N2 through membranes M1 and M2 after
3926 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 1. N2 and n-C4H10 Permeances as a Function of the Synthesis Layers
membrane
no. of layers
N2 permeance before calcination (mol/(m2 s Pa))
n-C4H10 permeance after calcination (mol/(m2 s Pa))
M1 M1 M1 M2 M1 M2
0 1 2 3 4 6
7 × 10-6 2.5 × 10-7
