Effect of fluorine doping on the network pore structure of non-porous organosilica bis(triethoxysilyl)propane (BTESP) membranes for use in molecular separation

Abstract

Long-chain organosilica bis(triethoxysilyl)propane (BTESP) membranes typically have a flexible non-porous structure. Fluorine was used to tune the network pore structure of BTESP membranes in an effort to improve the gas permeation properties. The network pore size was enlarged and the effect of calcination temperature on the network structure was evaluated based on gel and membrane characterizations. Fluorine-doped BTESP membranes calcined at 350 °C and 650 °C have shown H2 permeance on the orders of 1.2 × 10−6 mol m−2 s−1 Pa−1 and 1.5 × 10−6 mol m−2 s−1 Pa−1 with H2/N2 selectivities of 8 and 6, respectively, which indicates similar pore sizes with lower condensation effect at high temperature of 650 °C, that was suppressed due to the presence of Si–F and C–F bonds. Undoped BTESP membranes, on the other hand, showed H2/N2 selectivity that was significantly lower—from 24 to 11 at 650 °C. FT-IR and N2 adsorption isotherms clearly indicated that fluorine significantly decreased the Si–OH density and increased the surface area and micropore volume. Further water adsorption analysis revealed that fluorine significantly increased the hydrophobicity of the BTESP network structure. Overall, the results of this study endorse the effectiveness of fluorine to control the network pore structure in both wet and dry molecular separation systems.