Effects of synthesis conditions on MFI zeolite membrane quality and catalytic cracking deposition modification results

MFI zeolite membranes were synthesized by the templated in-situ synthesis method on α-alumina porous supports under various conditions and subsequently modified by catalytic cracking deposition (CCD) of methyldiethoxysilane (MDES) to improve high temperature H₂/CO ₂ separation performance. The quality of the zeolite membranes was examined by helium permeation (before template removal) and room temperature H₂/CO₂ separation (after template removal). For membranes prepared through single hydrothermal synthesis, the helium permeance decreases and the room temperature H₂/CO₂ separation factor increases with increasing synthesis time (or membrane thickness). Helium flows through the defects of the as-prepared membranes before template removal decreases as membrane thickness increases, but the thicker membranes contain more defects formed during the template removal step. Twice-hydrothermal treatment with calcination in between is effective to improve the quality of MFI zeolite membranes. However, a minimal synthesis time for each hydrothermal cycle is needed to eliminate the intercrystalline defects. Catalytic cracking deposition of MDES improves the high temperature H₂/CO₂ separation performance of the MFI zeolite membranes and the extent of the improvements is directly correlated with the membrane synthesis conditions. After CCD modification, the H₂/CO₂ separation factor of the membrane synthesized twice with 4 h for each hydrothermal cycle was improved from 4.2 to 8.6 with around 40% reduction in H₂ permeance.