Abstract
Silica-based membranes prepared by chemical vapor deposition of tetraethylorthosilicate (TEOS) on γ-alumina overlayers are known to be effective for hydrogen separation and are attractive for membrane reactor applications for hydrogen-producing reactions. In this study, the synthesis of the membranes was improved by simplifying the deposition of the intermediate γ-alumina layers and by using the precursor, dimethyldimethoxysilane (DMDMOS). In the placement of the γ-alumina layers, earlier work in our laboratory employed four to five dipping-calcining cycles of boehmite sol precursors to produce high H2 selectivities, but this took considerable time. In the present study, only two cycles were needed, even for a macro-porous support, through the use of finer boehmite precursor particle sizes. Using the simplified fabrication process, silica-alumina composite membranes with H2 permeance > 10−7 mol m−2 s−1 Pa−1 and H2/N2 selectivity >100 were successfully synthesized. In addition, the use of the silica precursor, DMDMOS, further improved the H2 permeance without compromising the H2/N2 selectivity. Pure DMDMOS membranes proved to be unstable against hydrothermal conditions, but the addition of aluminum tri-sec-butoxide (ATSB) improved the stability just like for conventional TEOS membranes.
Highlights
Silica-based membranes have been known to be effective for selective hydrogen permeation for more than 30 years, and are potentially less expensive than palladium-based membranes
Two membranes were synthesized in parallel under the same conditions conditions
The topmost silica layers were synthesized by chemical vapor deposition (CVD), which placed a thin (20~30 nm) silica layer on top of the γ-alumina substrate by the thermal decomposition of the silica precursor compound
Summary
Silica-based membranes have been known to be effective for selective hydrogen permeation for more than 30 years, and are potentially less expensive than palladium-based membranes. The H2 permeance will decrease due to the densification of the silica network, and the permeance of larger gas molecules will increase due to pinhole generation caused by the degradation of γ-alumina, when it is used as a support This was confirmed by Reference [17], where hydrothermal stability tests were conducted on both the inside and outside of the membrane tube. Resulted in a H2 permeance of the order of 10−6 mol m−2 s−1 Pa−1 and a H2 /SF6 selectivity of 6800 at Subsequent work [51,52] using this membrane confirmed that the H2/toluene separation ability was 300 ◦ C, which is comparable to the performance of palladium membranes.
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