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Single-component adsorption isotherms for CO2, CH4, C3H8, and H2O on SAPO-34, SSZ-13, and zeolite T were measured, and isosteric heats of adsorption were estimated. Long equilibration times were required for C3H8 adsorption in SAPO-34 and SSZ-13 because C3H8 diffuses slowly in these zeolites, but C3H8 isotherms were obtained quickly on zeolite T. Thus, when CO2 and C3H8 coadsorption loadings were measured by temperature-programmed desorption on SSZ-13, CO2 loadings were higher than C3H8 loadings at short times, whereas at long times, C3H8 loadings were higher as C3H8 displaced adsorbed CO2. In contrast, adsorption of CO2/C3H8 mixtures equilibrated quickly in zeolite T because of the fast C3H8 diffusion. These adsorption measurements may explain previously reported zeolite membrane behavior where C3H8 decreased CO2 permeances for SAPO-34, SSZ-13, and zeolite T membranes under most conditions, and steady state was only slowly obtained for SAPO-34 and SSZ-13 membranes in the presence of C3H8. [ABSTRACT FROM AUTHOR]

Abstract: Previously, SAPO-34 membranes prepared using aluminum isopropoxide (Al(O–i–Pr)3) and two templates (tetraethylammoniumhydroxide (TEAOH); dipropylamine (DPA)) were shown to separate CO2/CH4 mixtures with high fluxes at high pressures and room temperature. In this study, the synthesis was simplified, made safer, and modified to create less toxic waste by using only one template (TEAOH), but at twice the concentration, and Al(OH)3 instead of Al(O–i–Pr)3. However, some membranes prepared with the new procedure had low fluxes because they had impermeable regions. The impermeable regions were determined to be due to synthesis gel that remained in the support following a 15-min, post-synthesis rinse with water. Rinsing the membranes for at least 24h in deionized water before calcination removed most of the residual gel and formed spatially-uniform membranes with high permeances (maximum of 1.2×10−6 mol/(m2 sPa)) and high CO2/CH4 separation selectivities (70) for 46-bar feed pressure. Additional benefits of these changes in membrane preparation include a 200-K increase in the temperature at which template could be removed, increased stability of calcined membranes contacted with liquid water, and better long-term stability during storage. [Copyright &y& Elsevier]

Abstract: SAPO-34 membranes had 95–100% higher CO2 permeances and 9–17% lower CO2/CH4 separation selectivities when their structure-directing templates were removed at 673K in flowing N2 or under vacuum instead of in flowing air. The average CO2 permeance was 8.2±2×10−7 mol/(m2 sPa) during CO2/CH4 separations at 4.6MPa feed pressure for five membranes whose templates were removed under vacuum. The average CO2/CH4 separation selectivity was 55±6. Membranes heated in N2 or under vacuum were white in color, whereas those heated in air were darker. Membrane preparation was reproducible. Temperature-programmed desorption (TPD) and oxidation (TPO) of SAPO-34 crystals showed that more of the templates decomposed and were removed at lower temperature in inert gas than in air or oxygen. Over 90% of the templates were removed by 773K during TPD in helium, and ethylene was the main decomposition product. In contrast, only 70% of the templates were removed by 773K during TPO in air or oxygen, and CO2 and water were the major products When the SAPO-34 crystals were heated in air or oxygen, some of the template, before it decomposed to form ethylene, was oxidized to more stable compounds that were only removed at higher temperatures. [Copyright &y& Elsevier]

Adding propane or n-butane in low concentrations to a H 2 /N 2 feed increases the H 2 /N 2 separation selectivity in SAPO-34 membranes by decreasing the N 2 permeance more than the H 2 permeance. More than two days were required to reach steady state because propane and n-butane diffuse slowly into the zeolite pores, but they adsorb strongly and thus decrease H 2 and N 2 loadings. Although the loading of the more-weakly adsorbed H 2 is expected to decrease more, the N 2 permeance decreased more than the H 2 permeance apparently because the diffusivity of the larger N 2 molecule decreased more. Propane more than doubled the H 2 /N 2 selectivity at 298 K, but its effect on selectivity decreased as the temperature increased because the propane loading in the SAPO-34 pores decreased. Ethane also decreased H 2 and N 2 permeances, but it did not change H 2 /N 2 selectivity. As with propane, ethane decreased H 2 loadings more than N 2 loadings, and decreased N 2 diffusivity more than H 2 diffusivity. However, the smaller ethane had less effect on the N 2 diffusivity than did propane, so the net effect was that the selectivity did not change. The effects of alkane adsorption in SAPO-34 membranes were reversed when the alkanes were removed from the feed. [ABSTRACT FROM AUTHOR]