Bis(triethoxysilyl)ethane (BTESE)-derived silica membranes: pore formation mechanism and gas permeation properties

Based on their high performance in gas and liquid-phase separations, 1,2-bis(triethoxysilyl)ethane (BTESE)-derived organosilica membranes have attracted much attention. To improve performance, we focused on the acid molar ratio (AR) in sol preparation and its effect on the pore formation mechanism during sol-gel processing. BTESE-derived sols with AR = 10−4–100 were prepared, and the effect of the AR on the gel structure was evaluated in detail via FT-IR, nuclear magnetic resonance (NMR), N2 adsorption, and positron annihilation lifetime (PAL) measurements. The chemical structure of the gels was confirmed by FT-IR and NMR and showed that sols with the largest number of silanol groups (AR = 10−2) experienced a significant increase in condensation during the firing process. The porous structures of fired gels characterized by N2 adsorption and PAL measurement showed that the AR = 10−2 fired gel consisted of a larger number of small pores that had formed during the firing process. Single-gas permeation experiments showed high H2 permeance (5–9 × 10−7 mol/(m2 Pa s)) and H2/CF4 selectivity (700–20,000). The gas permselectivity (He/H2, H2/N2, and H2/CF4) was highest for the intermediate AR (=10−2), which corresponded to the greatest amount of silanol groups in unfired gels and confirmed that small pores had formed from the condensation of silanol groups during firing.