Separation of Light Gas Mixtures Using SAPO-34 Membranes Joseph C. Poshusta, Vu A. Tuan, Eric A. Pape, Richard D. Noble, and John L. Falconer Dept. of Chemical Engineering, University of Colorado, Boulder, CO 80309 Continuous SAPO-34 membranes were prepared on porous alumina tubular sup- ports, and shown to be useful for light gas separations at low and high temperatures. Single-gas permeances of CO , N and CH decreased with increasing kinetic diame- 2 2 4 ter. For the best membrane at 300 K, the He and H permeances were less than that of 2 CO , because He, H , and CO were small compared to the SAPO-34 pore, and 2 2 2 differences in the heat of adsorption determined the permeance order. The smaller com - ponent permeated the fastest in CO r CH , CO r N,N r CH , H r CH and H r N 2 4 2 2 2 4 2 4 2 2 mixtures between 300 and 470 K. For H r CO mixtures, which were separated by com - 2 2 petiti®e adsorption at room temperature, the larger component permeated faster below 400 K. The CO r CH selecti®ity at room temperature was 36 and decreased with tem - 2 4 perature. The H r CH mixture selecti®ity was 8 and constant with temperature up to 2 4 480 K. Calcination, slow temperature cycles, and exposure to water ®apor had no per - manent effect on membrane performance, but temperature changes of approximately 30 Kr min decreased the membrane’s effecti®eness. Introduction Zeolite and other inorganic molecular sieve membranes have shown potential for separations based on molecular size and shape because of their small pore sizes, typically less than 1 nm, and their narrow pore-size distribution. The high ther- mal and chemical stability of these inorganic crystals make them ideal materials for use in high-temperature applications such as catalytic membrane reactors. Most of the progress with zeolite membranes has been with MFI zeolite prepared Ž on porous disks Geus et al., 1993; 1992; Jansen et al., 1994; Lovallo et al., 1998; Matsukata and Kikuchi, 1997; Vroon, . Ž 1995; Yan et al., 1995 and tubes Bai et al., 1995; Coronas et al., 1997; Giroir-Fendler et al., 1996; Jia et al., 1994; Kusak- . abe et al., 1996; Oh et al., 1997 . The MFI zeolite is a medium pore-size structure having nearly circular pores with diame- ters between 0.53 and 0.56 nm. Separation experiments through MFI membranes indicate that competitive adsorp- Ž tion separates light gas mixtures Bakker et al., 1996, 1993; Jia, et al., 1994; Kapteijn et al., 1995; Lovallo et al., 1998; Poshusta et al., 1999; van den Broeke et al., 1999; Vroon, . 1995 . Light gas selectivities are typically small, however, ow- ing to small differences in adsorption strengths and their small Correspondence concerning this article should be addressed to J. L. Falconer. sizes relative to the MFI pore opening. Furthermore, com- petitive adsorption does not work well at high temperature where zeolite membranes are stable and have potential appli- cation. Separation by differences in size has a greater potential to work at high temperature than competitive adsorption, but pores smaller than those in MFI zeolites are required. There- fore, some studies focused on the synthesis of small, 8-mem- Ž bered-pore structures such as zeolite A 0.41-nm pore diame- .Ž . ter Aoki et al., 1998; Masuda et al., 1995 and SAPO-34 Ž . Ž Lixiong et al., 1997; Poshusta et al., 1998 , a chabazite about . 0.4-nm pore diameter with 1.4 nm cages analog. The SAPO-34 structure is a silicoaluminophosphate having the Ž . composition Si Al P O where x s 0.01  0.98, y s x y z 2 Ž . 0.01  0.60, z s 0.01  0.52, and x q z s y Szostak, 1989 . The crystal structure of SAPO-34 crystals has not been ana- lyzed to determine the pore size, but adsorption experiments Ž . Ž . using n-C H 0.43 nm and i-C H 0.50 nm have shown 4 10 4 10 that the SAPO-34 pore diameter is between 0.43 and 0.50 nm Ž . Lok et al., 1984 . The smaller pore size of the zeolite A and SAPO-34 struc- tures made the separation of smaller molecules by differ- Ž . ences in size possible. Aoki et al. 1998 reported H rN se- 2 2 April 2000 Vol. 46, No. 4 AIChE Journal 779