Microwave-assisted Synthesis Of SAPO-34 Membranes And Their Applications In CO₂/CH₄ Separation

As a type of silicoaluminophosphate microporous zeolite, SAPO-34 has the chabazite structure with pore diameter of 0.38 nm. Due to its well defined pore structure and good thermal, mechanical, chemical stability, SAPO-34 crystals were widely used in catalysis fields, especially methanol to olefins(MTO) reaction. SAPO-34 membranes have shown potential for CO2/CH4 separations due to their molecular sieving and competitive adsorption.The cubic-shaped and uniformly distributed SAPO-34 crystals were synthesized without amorphous phase observed using pseudo-boehmite as aluminum source. The effect of synthesis procedure, heating method and synthesis time on SAPO-34 crystals was investigated. Synthesis procedure has a significant effect on SAPO-34 crystals and the well cubic-shaped SAPO-34 crystals were obtained when tetraethylammonium hydroxide(TEAOH) and fumed silica were added after mixture at room temperature. The cubic-shaped SAPO-34 crystals sized at ca. 240 nm were synthesized under microwave heating for 4 h while SAPO-34 crystals sized at ca. 470 nm in a narrower particle size distribution were synthesized under hydrothermal heating for 24 h. SAPO-34 crystals which were synthesized under hydrothermal heating for 12 h and 24 h were similar while SAPO-34 crystals which were synthesized under microwave heating for 2 h and 4 h were different in crystallinity. The sheet-like SAPO-34 crystals sized at ca. 480 nm were also synthesized without amorphous phase observed using aluminum isopropoxide as aluminum source and these crystals have the potential to prepare oriented SAPO-34 membranes.The well intergrown SAPO-34 membranes with the thickness of about 2.1 ?m were formed using microwave heating for 4 h by secondary growth on the porous ?-Al2O3 substrates. The effect of synthesis time, substrate setting, aging time and heating method on SAPO-34 crystals was investigated. SAPO-34 crystals were not well intergrown and membranes were not flat under microwave heating for 2 h while the well intergrown and dense SAPO-34 membranes were synthesized using microwave heating for 4 h. The continuous and compact SAPO-34 membranes were formed when the pretreated porous ?-Al2O3 disks were placed upward while the morphology of SAPO-34 membranes were not ideal when the pretreated porous ?-Al2O3 disks were placed downward. Aging time has an impact on the degree of crosslinking of SAPO-34 membranes and the well intergrown SAPO-34 membranes were synthesized under aging time for 24 h. Compared with hydrothermal heating, synthesis time was significantly shortened under microwave heating. The calcined membrane had CO2/CH4 separation as high as 83 with CO2 permeance of 3.77×10-7 mol·m-2·s-1·Pa-1 at 298 K and 138 kPa pressure drop.