Pore size tuning of bis(triethoxysilyl)propane (BTESP)-derived membrane for gas separation: Effects of the acid molar ratio in the sol and of the calcination temperature

Bis(triethoxysilyl)propane (BTESP) is a bridged-type organoalkoxysilane with a Si-C3H6-Si bond. It was utilized for membrane fabrication via a sol-gel method to achieve high permselectivity for large molecules. Membrane fabrication parameters such as the acid molar ratio (AR) in the sol and calcination temperature were evaluated for their effect on the network pore size and on gas permeation properties, as evaluated by the molecular size dependence (0.26–0.55 nm) and temperature dependence (50–200 °C) of gas permeance. BTESP membranes with different ARs (10−1, 100, and 10) showed H2/N2 and H2/CF4 selectivities of 20–30 and 640–32,000, respectively. As AR was increased, each gas permeance also increased, but H2 selectivity that corresponds to network pore size was decreased. FT-IR analysis indicated that the density of the Si-OH groups (Si-OH/Si-O-Si) of unfired gels was decreased with a higher AR, so that condensation of the Si-OH groups during the calcination process formed a dense network structure in the case of BTESP membranes with a low AR (10−1). Calcination temperature also affected the network structure of BTESP membranes. BTESP membranes calcined at different temperatures (350, 450, and 600 °C) showed H2/N2 and H2/CF4 selectivities of 10–30 and 410–32,000, respectively. A BTESP membrane calcined at high temperature (600 °C) showed loose networks since the linking units derived from BTESP were decomposed at temperatures above 500 °C, which resulted in the formation of methyl groups. In conclusion, the AR in a sol is suitable for tuning small pore sizes, while calcination temperature as a membrane fabrication parameter offers the advantage of controllability for loose network structures.