Preparation and Gas Permeation Properties of Fluorine–Silica Membranes with Controlled Amorphous Silica Structures: Effect of Fluorine Source and Calcination Temperature on Network Size

Triethoxyfluorosilane (TEFS), which is a pendant-type alkoxysilane with a Si-F bond, was utilized for the development of a molecular sieving membrane. The effect that a source of fluorine and calcination temperature exerted on gas permeation properties and network pore size was evaluated via single-gas permeation properties across a wide range of temperatures. A TEFS membrane calcined at 350 °C showed high H₂ permeance (2.0 × 10^-6 mol m^-2 s^-1 Pa^-1) and high selectivity for H₂ over larger molecules (H₂/CF₄: >300; H₂/SF₆: >18 000), indicating that this network pore size would be suitable for a H₂ permselective membrane that could promote the process of methylcyclohexane (MCH) dehydrogenation to produce toluene (TOL). Based on the gas permeation properties and the results of XPS and FTIR, network pore size depended on the fluorine concentration incorporated in SiO₂ that existed as Si-F bonds, irrespective of the fluorine source. A TEFS membrane showed approximately the same pore size distribution and level of gas permeance, irrespective of calcination temperature (350 and 550 °C), due to the low Si-OH density in the networks as suggested by the result of FTIR, which can prevent the densification caused by the condensation of Si-OH groups. The pair distribution function also suggested that densification of the network structure for TEFS was apparently suppressed compared with that of a tetraethoxysilane (TEOS)-derived structure.