Pore size control of a molecular sieve silica membrane prepared by a counter diffusion CVD method

A molecular sieve silica membrane was successfully prepared by using a counter diffusion chemical vapor deposition method. Effects of silica precursors on permeation properties through the silica membranes were investigated. Five types of silica alkoxides (tetramethyorthosilicate (TMOS), methyltrimethoxysilane (MTMOS), trimethylmethoxysilane (TMMOS), propyltrimethoxysilane (PrTMOS) and phenyltrimethoxysilane (PhTMOS)) were employed for the silica precursors. H₂, N₂ and SF₆ permeances were measured through the silica membranes at 300°C. H₂ permselective silica membranes were obtained from the smaller silica precursors such as TMOS, MTMOS or TMMOS. H₂/N₂ permecnces ratios were over 200, while N₂/SF₆ permeances ratios were around 1. According to the kinetic diameters of H₂ (0.26 nm), N₂ (0.36 nm) and SF₆ (0.55 nm), pore sizes of the membranes were about 0.3 nm. N₂ permselective silica membrane was obtained from the larger silica precursors such as PrTMOS or PhTMOS. N₂/SF₆ permeances ratios through the membranes prepared by PrTMOS or PhTMOS were 30 and 32, respectively. Those values are much larger than that of Knudsen diffusion (2.3). H₂/N₂ permeances ratios through the membranes were about 40 showing that the maximum pore sizes are around 0.5 nm. These results show that the pore size of silica membranes can be controlled by changing the silica precursors. The PhTMOS membrane was confirmed as a silica membrane without carbon remaining by the cross-sectional XPS measurements.