Effect of the membrane quality on gas permeation and chemical vapor deposition modification of MFI-type zeolite membranes

Chemical vapor deposition (CVD) with methyldiethoxysilane (MDES) for MFI zeolitic pore size reduction is an effective way to improve the H ₂/CO₂ separation factor of zeolite membranes. However, only good membranes with few intercrystalline defects are able to show considerable improvement in H₂/CO₂ separation factor through CVD modification. It is unreliable to evaluate the membrane quality on the basis of the H₂/SF₆ or N₂/SF₆ ideal separation factor because the SF₆ adsorption-induced crystal swelling causes shrinkage of the intercrystalline defects in the MFI zeolite membranes. In this study, the quality of MFI zeolite membranes was evaluated on the basis of their performance in separating equimolar H₂/CO ₂ mixture gas at room temperature. High-quality membranes are more selective to CO₂ molecules rather than H₂ at room temperature due to the adsorption of CO₂ into the zeolitic pores and blocking of the pore channels for H₂ diffusion. The effectiveness of the quality evaluation method was confirmed by on-stream CVD modification of MFI zeolite membranes with different initial H₂/CO₂ separation factors/ideal separation factors at room temperature. Experimental results showed that only those membranes with low H₂/CO₂ separation factor at room temperature (good membranes) showed improvement in H₂/CO₂ separation factor through CVD modification. For the CVD modified zeolite membrane with good initial quality, the permeance of H₂ is lower than that of He with an activated diffusion behavior for small gas molecules in the modified zeolitic pores.