Gas permeation properties through Al-doped organosilica membranes with controlled network size

The sol–gel method was applied to the fabrication of Al-doped bis (triethoxysilyl) methane (BTESM)-derived membranes. The single-gas permeation properties for Al-doped BTESM-derived membranes were examined to evaluate the effect of aluminum concentration on amorphous silica network sizes. Each permeance was decreased with an increase in the Al concentration, and the H2/CH4 and H2/C3H8 permeance ratios increased with an increase in Al concentration. For example, an Al-doped BTESM (Si/Al=8/2) membrane fabricated at 200 °C showed a H2 permeance of 4.4×10−7 mol m−2 s−1 Pa−1, which was approximately 1/10th that of a BTESM membrane fabricated at 200 °C. The H2/CH4 and H2/C3H8 permeance ratios were 60 and 2700 with Al doping, but 30 and 1000 without Al-doping, respectively. The activation energy of He, H2, N2, and CH4 permeation was increased with an increase in the Al concentration, indicating that the pore size of BTESM-derived networks was decreased with an increase in Al concentration. The decrease in BTESM-derived network sizes that resulted from an increase in the Al concentration can be ascribed to the absolute amount of Al incorporated into BTESM-derived networks and/or coordinated with Si-OH groups, as suggested by 27Al MAS NMR. High C3H6/C3H8 permeance ratios of approximately 40 for Al-doped BTESM (Si/Al=9/1) membranes fabricated at 200 °C were achieved through the precise control of the silica network size via a spacer method using Si–C–Si units as well as the incorporation of Al in BTESM-derived networks.