Development of Silica Membranes to Improve Dehydration Reactions

Silica membranes for use in a membrane reactor were developed by the counter diffusion chemical vapor deposition (CVD) method. Tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMOS), n-butyltrimethoxysilane (BTMOS) and 3-aminopropyltrimethoxysilane (APrTMOS) were used as the silica precursors and the high temperature separation performances were evaluated. The membranes prepared using TMOS and MTMOS at high temperature deposition showed H2/N2 selectivity of over 100. The membranes prepared using BTMOS and APrTMOS at 270 °C showed H2/N2 selectivity of about 100 and N2/SF6 selectivity about 100. Therefore, thermal stability of the organic group of the silica precursor must be important for the membrane performances. Decomposition of the organic group and deposition time were investigated. The membrane prepared by extending the deposition time of APrTMOS to 240 min showed high hydrogen permeance of 7.2 × 10−7 mol m−2 s−1 Pa−1 and H2/C3H8 selectivity of 21000. In addition, high temperature gas permeation tests at 500 °C were conducted using the membrane prepared using MTMOS. The mixed gas separation factor of about 100 for H2/C3H8 separation test was the same as the single gas permeance ratio of 98 calculated from the result of the single gas permeance. From the above results, in order to prepare a silica membrane having high separation performance in a binary system, it is necessary to develop a silica membrane that exhibits high separation performance in one-component permeation tests using a silica precursor having thermal stability of organic substituents.