The Effect of C/Si Ratio and Fluorine Doping on the Gas Permeation Properties of Pendant-Type and Bridged-Type Organosilica Membranes

A series of pendant-type alkoxysilane structures with various carbon numbers (C₁-C₈) were used to fabricate sol-gel derived organosilica membranes to evaluate the effects of the C/Si ratio and fluorine doping. Initially, this investigation was focused on the effect that carbon-linking (pendant-type) units exert on a microporous structure and how this affects the gas-permeation properties of pendant-type organosilica membranes. Gas permeation results were compared with those of bridged-type organosilica membranes (C₁-C₈). Network pore size evaluation was conducted based on the selectivity of H₂/N₂ and the activation energy (E_p) of H₂ permeation. Consequently, E_p (H₂) was increased as the C/Si ratio increased from C₁ to C₈, which could have been due to the aggregation of pendant side chains that occupied the available micropore channel space and resulted in the reduced pore size. By comparison, these permeation results indicate that pendant-type organosilica membranes showed a somewhat loose network structure in comparison with bridged-type organosilica membranes by following the lower values of activation energies (E_p). Subsequently, we also evaluated the effect that fluorine doping (NH₄F) exerts on pendant-type [methytriethoxysilane (MTES), propyltrimethoxysilane (PTMS)] and bridged-type [1,2-bis(triethoxysilyl)methane (BTESM) bis(triethoxysilyl)propane (BTESP)] organosilica structures with similar carbon numbers (C₁ and C₃). The gas-permeation properties of F-doped pendant network structures revealed values for pore size, H₂/N₂ selectivity, and E_p (H₂) that were comparable to those of pristine organosilica membranes. This could be ascribed to the pendant side chains, which might have hindered the effectiveness of fluorine in pendant-type organosilica structures. The F-doped bridged-type organosilica (BTESM and BTESP) membranes, on the other hand, exhibited a looser network formation as the fluorine concentration increased.