Gas Permeation and Diffusion Characteristics of MFI-Type Zeolite Membranes at High Temperatures

MFI-type zeolite membranes were prepared by the template-free secondary growth method followed by onstream counter-diffusion or one-side chemical vapor deposition (CVD) modification to eliminate intercrystalline pores. Gas permeation and separation experiments were conducted on unmodified and modified membranes at 25−500 °C. For unmodified MFI-type zeolite membranes, single-gas permeation of H 2, He, CO, and CO 2 exhibits characteristics of Knudsen diffusion up to 500 °C, and adsorption of CO 2 on MFI-type zeolite has a strong effect on ternary gas separation (H 2, CO, and CO 2 ) below 300 °C. Counter-diffusion CVD modification is effective in sealing the intercrystalline gaps resulting in defect-free MFI-type zeolite membranes. Permeation of nonadsorbing gases (He, H 2, and CO) through counter-diffusion CVD-modified zeolite membranes also exhibits Knudsen diffusion characteristics with very small activation energies for diffusion (0.1−3 kJ mol −1 ), with gas permeance (diffusivity) decreasing with increasing molecular weight. For one-side CVD-modified MFI-type zeolite membranes, gas permeance (diffusivity) decreases and activation energy for diffusion increases with increasing molecular size because of the formation of an amorphous microporous silica layer. High-temperature gas permeation data on defect-free MFI-type zeolite membranes confirm the translational gas diffusion model for zeolites.