Preparation of thin Li₄SiO₄ membranes by using a CVD method

CVD (chemical vapor deposition) procedures were investigated by using a counter diffusion CVD method. The effects of silica precursors on the hydrogen permeation properties of the silica membranes were discussed. 5 types of silica alkoxides (tetramethylorthosilicate (TMOS), methyltrimethoxysilane (MTMOS), propyltrimethoxysilane (PTMOS), dimethyldimethoxysilane (DMDMOS), and trimethylmethoxysilane (TMMOS)) were employed as the silica precursors; they have different numbers of methyl groups. Hydrogen permeance through the DMDMOS membrane prepared at 500 °C was 9.0 × 10^-7 mol m ^-2 s^-1 Pa^-1, and H₂/N₂ selectivity was 920. Activation energy of H₂ permeation through the silica membrane prepared from TMOS was 10.5 kJ mol^-1 that was the maximum among the 5 types of the silica precursor. This indicates dense silica layer can be obtained from TMOS. Thus, TMOS was employed for the further Li ₄SiO₄ preparation. High temperature CO₂ permselective membranes were successfully prepared by using Li ₄SiO₄ as a CO₂ selective layer. Pinholes of the Li₄SiO₄ layer was filled by the CVD post treatment at 600 °C. CO₂/N₂ permselectivity was 1.2 at the 600 °C permeation test. The CO₂ permeance ratio is higher than the Knudsen diffusion difference. Thus, this selectivity was explained by the CO₂ selective adsorption on the Li₄SiO₄ layer.