MFI-type zeolite membranes were prepared by using the template-free secondary growth method, followed by on-stream counter-diffusion or one-side chemical vapor deposition modification to eliminate intercrystalline pores. Gas permeation and separation experiments were conducted on unmodified and modified membranes at 25 to 500C (figure 11). For unmodified MFI-type zeolite membranes, single-gas permeation of H2, He, CO and CO2 exhibited the characteristics of Knudsen diffusion up to 500C, and adsorption of CO2 on MFI-type zeolite had a marked effect upon ternary gas separation (H2, CO, CO2) below 300C. Counter-diffusion chemical vapor deposition modification was effective in sealing the intercrystalline gaps, resulting in defect-free MFI-type zeolite membranes. Permeation of non-adsorbing gases (He, H2, CO) through counter-diffusion chemical vapor deposition-modified zeolite membranes also exhibited Knudsen diffusion characteristics, with very small activation energies for diffusion (0.1 to 3kJ/mol), and gas permeance (diffusivity) decreasing with increasing molecular weight. In one-side chemical vapor deposition-modified MFI-type zeolite membranes, gas permeance (diffusivity) decreased and the activation energy for diffusion increased 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 confirmed the translational gas diffusion model for zeolites.

Gas Permeation and Diffusion Characteristics of MFI-Type Zeolite Membranes at High Temperatures. M.Kanezashi, Y.S.Lin: Journal of Physical Chemistry C, 2009, 113[9], 3767–74