Organic–Inorganic Hybrid Silica Membranes with Controlled Silica Network Size for Propylene/Propane Separation

Bis(triethoxysilyl)methane (BTESM), which consists of Si–C–Si bonds, was used as a membrane precursor to control the size of amorphous silica networks. The single gas permeation characteristics of hybrid silica membranes were examined to determine the effect of silica precursors on amorphous networks. Pore size distribution, as determined by single gas permeation, suggested that average pore size was in the following order: bis(triethoxysilyl)ethane (BTESE)-derived silica > BTESM-derived silica > tetraethoxysilane (TEOS)-derived silica, due to differences in the minimum units of the silica precursor. The high C 3 H 6 /C 3 H 8 separation performance of BTESM-derived silica membranes in a wide temperature range (50–200 °C) can be due to the control of silica network size by the “spacer” method using a Si–C–Si unit. For example, a BTESM-derived silica membrane showed a high C 3 H 6 permeance of 6.32 × 10 –7 mol m –2 s –1 Pa –1 with a C 3 H 6 /C 3 H 8 permeance ratio of 8.8 at 50 °C. The order of C 3 H 6 and C 3 H 8 permeances of BTESM-derived silica membranes was C 3 H 6 > C 3 H 8, independent of the number of sol coats and temperature (50–200 °C), although the kinetic diameter of C 3 H 6 ( d k = 0.45 nm) was reported to be larger than that of C 3 H 8 ( d k = 0.43 nm). For permeation of hydrocarbons through amorphous silica membranes, it is suggested that the kinetic diameter, which is a minimum equilibrium cross-sectional diameter, is not applicable for effective molecular size, probably because diffusivity depends not only on the minimum cross section but also on molecular length.