(543d) Amine-Functionalized SAPO-34 Membranes for CO2/CH4 Gas Separation

CO₂ separation from CH₄ is important in natural gas processing because CO₂ reduces the energy content of natural gas and acidic, corrosive in the presence of water. Zeolite membranes are ideal for functional gas separation applications because of their unimodal micropores close to the kinetic diameter of several gas molecules, superior thermal, mechanical, and chemical stability, good erosion resistance, and stability at high CO₂ pressures. SAPO-34 zeolite membranes have been successfully employed to separate CO₂ from different light gases owing to its stability, unique shape selectivity, molecular sieving properties, and atomically ordered microporous structure. The control of the crystal size, extent of crystallinity, homogeneity, and preferential CO₂ adsorption capacity are highly desirable properties to prepare improved SAPO-34 membranes for CO₂ separation. Smaller crystal size and narrow particle size distribution potentially lead to thinner membranes, larger-accessible surface area, reduced diffusion resistance, which impact positively the SAPO-34 membrane performance in functional gas separation applications.

Herein, we present surface modified SAPO-34 membranes prepared employing crystal growth inhibitors (CGI) inside stainless steel and alumina porous supports using secondary seeded hydrothermal treatment method. To better understand its structural, compositional and morphological properties, the membranes were characterized by SEM, XRD, TEM and CO₂, CH₄ adsorption capacities. The average thicknesses of the membranes were in the 5-8 µm range. Smaller crystal size with narrow particle size distribution led to thin SAPO-34 membranes and high CO₂/CH₄ sorption capacity resulting in high CO₂ permeances. The synthesized membranes were tested for the separation of CO₂/CH₄ by using equimolar mixtures at 295 K. The effect of different parameters like hydrothermal temperature, time, calcination temperature and crystal growth inhibitors on permeances and selectivity was studied. The gas separation ability of these membranes was tested over a range of pressures. High permeances up to 6.2 x 10^-7 mol/m² s Pa with CO₂/CH₄ separation selectivities up to 234 were obtained with single layered membranes. The high selectivities and permeances were attributed to the high CO₂ adsorption capacity of the SAPO-34 crystals resulted because of the relatively high concentration of nitrogen content in the crystals and the small closed-packed crystals. The separation performance of these membranes was improved by amino-carrier functionalization.