Cylindrical silica membranes with dead-end structure were prepared by an extended counter-diffusion chemical vapor deposition (CVD) method, in which a tetramethylorthosilicate silica source was fed from the outside of a cylindrical membrane support with γ-alumina interlayer (the membrane side), and O gas was fed from the inside (the support side). Extended counter-diffusion CVD was a method of depositing silica films under highly pressurized conditions applied to the membrane side where tetramethylorthosilicate was supplied. Two silica membranes were deposited for 10h at 573K under differential pressures of 0.1MPa and 0.0MPa applied between the cylindrical membranes. The H permeances for these silica membranes were unaffected (5 x 10−8mol/m2sPa at 573K), although the methane and carbon dioxide permeances were greatly reduced for dense silica films prepared by high-pressure CVD (HPCVD). Therefore, the selectivity of H over methane and carbon dioxide was 24000, and 1200, respectively. It was suggested from energy dispersive X-ray microanalysis observations in scanning electron microscopy and scanning probe microscopy results that this high selectivity was due to the reduced number of defects and/or pinholes formed in the dense silica membranes by HPCVD.

Synthesis, Characterization and Gas Permeation Properties of a Silica Membrane Prepared by High-Pressure Chemical Vapor Deposition. S.Araki, N.Mohri, Y.Yoshimitsu, Y.Miyake: Journal of Membrane Science, 2007, 290[1-2], 138-45