Separation of inorganic/organic gas mixtures by porous silica membranes

Abstract

Abstract The sol–gel procedures were applied to fabricate porous silica membranes coated on cylindrical porous α-alumina tubes by the hot coating methods. The pore diameters of the silica membranes were determined by the gas permeation methods to be less than 0.4 nm. The active layer thickness was about 1 μm, giving a large hydrogen permeance of around 3×10−5 m3 m−2 s−1 kPa−1 (1.3×10−6 mol m−2 s−1 Pa−1) at 300°C. The permeance of H2 and He were slightly dependent on temperature, while the observed CO2 permeance showed a large temperature dependency. The largest permeance ratios observed are 150 for H2/CH4, 1100 for H2/C2H6, and 6300 for H2/C3H8 at 300°C. As the CO2 permeance increases at a lower temperature, CO2 removal from organic gas mixtures can be preferably performed at a lower temperature near the room temperature. An example of observed CO2 permeance at 35°C was approximately 2×10−5 m3 m−2 s−1 kPa−1 (0.9×10−6 mol m−2 s−1 Pa−1). The permeance ratio of CO2/CH4 was around 80–110 at 35–50°C and the ratio became smaller at higher temperature. The porous silica membranes fabricated in this work were quite stable when used in dry conditions. In humid conditions, however, the gas permeances of CO2 and N2 decreased drastically, giving a larger permeance ratio for H2/CO2.