SAPO-34 membranes for CO2/CH4 separations: Effect of Si/Al ratio

Silicoaluminophosphate (SAPO) membranes with Si/Al gel ratios from 0.05 to 0.3 were synthesized by in situ crystallization onto porous, tubular stainless steel support. Pure SAPO-34 membranes were obtained when the Si/Al ratio was 0.15 or higher. The adsorbate polarizability correlated with the adsorption capacity on SAPO-34, and the amounts of gases adsorbed were in the order: CO2 > CH4 > N2 > H2. The Si/Al ratio did not affect the pore volume significantly, but it changed the CO2 and CH4 adsorption equilibrium constants. The SAPO-34 membranes effectively separated CO2 from CH4 for feed pressures up to 7 MPa. At 295 K, for a pressure drop of 138 kPa and a 50/50 feed, the CO2/CH4 selectivity was 170 for a membrane with a Si/Al gel ratio of 0.15. At 7 MPa, the CO2/CH4 selectivity was 100 and the CO2 permeance was 4 × 10−8 mol/(m2 · s · Pa) at 295 K. This membrane was also separated CO2/N2 (selectivity = 21) and H2/CH4 (selectivity = 32) mixtures at 295 K and a pressure drop of 138 kPa. Competitive adsorption and difference in diffusivities are responsible for CO2/CH4 and CO2/N2 separations, whereas the H2/CH4 separation was due to diffusivity differences. For a membrane with Si/Al gel ratio of 0.1, a mixture of SAPO-34 and SAPO-5 formed, and the CO2/CH4 selectivity was lower.