Preparation of silica hybrid membranes for high temperature gas separation

A molecular sieve silica hybrid membrane was successfully prepared by using a counter diffusion chemical vapor deposition (CVD) method. Propyltrimethoxysilane (PrTMOS) was employed for a silica precursor. O-3 was used as an oxidizer. Effects of deposition temperatures on permeation properties through the silica membranes were investigated. H-2 (0.29 nm), N-2 (0.36 nm) and SF6 (0.55 nm) permeances were measured at the deposited temperatures through the membranes. H-2/N-2 permeances ratios decreased sharply with increasing the deposition temperatures from 200 degrees C to 400 degrees C. In addition, H-2 permeance through the membrane deposited at 400 degrees C was very high (4.6 x 10(-7) mol m(-2) s(-1) Pa-1) with a low H-2/N-2 permeances ratio (3.4). This shows that silica was not deposited at 400 degrees C due to O-3 decomposition at the high temperature deposition. On the other hand, N-2/SF6 permeances ratios increased with increasing the deposition temperatures upto 320 degrees C. The maximum value of N-2/SF6 permeances ratio was 110 at 320 degrees C deposition. This is much larger than that of Knudsen diffusion separation (2.3). According to the kinetic diameters of N-2 and SF6, the pore sizes of the membrane were estimated at about 0.5 nm. These results show that the pore size of silica membranes can be controlled by changing the deposition temperatures. The decomposition reactions of propyl groups on silica hybrid materials were also investigated by TG and IR measurements. Propyl groups on silica hybrid materials decomposed at around 300 degrees C and 400 degrees C. Decomposition at around 300 degrees C was C2H4 removal from the surface to remain methyl groups on the surface. Thus, we conclude that the high selective membrane deposited at 320 degrees C was a silica hybrid membrane having methyl groups in the silica deposition.