(520d) Surface Modified SAPO-34 Membranes: Synthesis, Characterization and CO2/CH4 Separation

SAPO-34 membranes have been successfully employed to separate CO₂ from different gases owing to its chemical and thermal 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. Rational surface modification of SAPO-34 crystals will lead to membranes displaying preferential CO₂ adsorption and consequently to improved membranes for CO₂ separation over light gases.

Herein we present the successful synthesis of SAPO-34 seeds and membranes displaying small, uniform crystal size, high surface area and enhanced CO₂/CH₄ adsorption ratio employing crystal growth inhibitors (CGI) such as polyethylene glycol, Brij-35 and methylene blue. The resultant SAPO-34 crystals displayed BET surface areas up to 700 m²/g, enhanced CO₂/CH₄ adsorption ratios (>9), and crystal sizes in the ~0.6-0.9 μm range, with narrow particle size distribution. The relatively high N/H molar ratios observed in the SAPO-34 crystals prepared with CGI led to high CO₂/CH₄ adsorption capacities. Due to its small crystal size, narrow particle size distribution, higher surface area, and preferential CO₂ adsorption capacities over CH₄, these crystals represent ideal phases to prepare thin supported membranes for CO₂/CH₄ separations. SAPO-34 membranes were prepared on γ-Al₂O₃ supports by secondary seeded growth employing these high surface area and surface modified seeds. The separation performance for equimolar CO₂/CH₄ gas mixtures is presented.