Gas Permeation Characteristics and Stability of Composite Silica-Metal Oxide Membranes
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Gas Permeation Characteristics and Stability of Composite Silica-Metal Oxide Membranes Masashi Asaeda, Masakoto Kanezashi, Tomohisa Yoshioka and Toshinori Tsuru Department of Chemical Engineering, Graduate School of Engineering Hiroshima University, Higashi-Hiroshima, 739-8527, Japan ABSTRACT In order to improve the stability of silica membranes against water (vapor) some metal oxides were added to silica to obtain composite silica-metal oxide membranes by the sol-gel techniques. A Ni-doped silica membrane (Ni/Si=1/2) fired at 500oC showed a relatively large permeance of 1.5x10-5 [m3(STP)/(m2skPa)] with selectivity of 350 (H2/CH4), 4200 (H2/SF6) at 200oC and 100 (CO2/CH4) at 35 oC. After leaving the membrane in humid air (RH: 60%, 40oC) for 70 days, the permeance of H2 decreased by about 50% but the selectivity was improved to 930 for H2/CH4. And little change was observed in the activation energy for H2 permeation, while under the same conditions a silica membrane showed a quite large change in the activation energy from 3.1kJ/mol to 14kJ/mol. There is a possibility that metal oxides added to silica help prevent the densification of silica networks through which hydrogen and helium molecules can permeate. INTRODUCTION Silica is one of the most attractive inorganic membrane materials for its highly porous structures in a wide range of temperature. Porous silica membranes have been found quite effective for separation of hydrocarbon gas mixtures [1], while at the end of 1980’s dense silica membranes were found to show some excellent separation performance for separation of hydrogen or helium at high temperature [2, 3]. Since then many researchers have been engaged in developments of H2-selective inorganic membranes. However, not only porous silica membranes but also dense silica membranes fabricated by the sol-gel or the CVD techniques have been found neither stable against water vapor at high temperature 500-600oC [4] nor even at room temperature [5]. The permeance of H2 or He in a silica membrane decreases and the activation energy of permeation increases after exposure of the membranes to water vapor or humid gases, resulting in a smaller permeance at lower temperature, 50-300oC, due to the large temperature dependency of permeance. A higher permeance with a high H2-or He-selectivity is preferably expected for a separation membrane in a relatively low temperature range of 50-300oC in practical separation and purification processes. In this work are added some metal oxides to silica in order to improve the membrane permeability and stability against water (vapor). The sol-gel techniques are applied to fabricate silica-metal oxide composite membranes for noble separation of hydrogen or helium in a temperature range of 50-300oC. EXPERIMENTAL Some silica-zirconia membranes of pore size of several nanometer were fabricated on porous α-alumina tubes by the sol-gel procedures proposed in the previous work [6]. The silica-zirconia membranes, which are quite stable against water [7], were used as the substrates f
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