| Summary: | Gas separation membranes were fabricated with varying trimethylmethoxysilane (TMMOS)/tetraethoxy orthosilicate (TEOS) ratios by a chemical vapor deposition (CVD) method at 650 °C and atmospheric pressure. The membrane had a high H<sub>2</sub> permeance of 8.3 × 10<sup>−7</sup> mol m<sup>−2</sup> s<sup>−1</sup> Pa<sup>−1</sup> with H<sub>2</sub>/CH<sub>4</sub> selectivity of 140 and H<sub>2</sub>/C<sub>2</sub>H<sub>6</sub> selectivity of 180 at 300 °C. Fourier transform infrared (FTIR) measurements indicated existence of methyl groups at high preparation temperature (650 °C), which led to a higher hydrothermal stability of the TMMOS-derived membranes than of a pure TEOS-derived membrane. Temperature-dependence measurements of the permeance of various gas species were used to establish a permeation mechanism. It was found that smaller species (He, H<sub>2</sub>, and Ne) followed a solid-state diffusion model while larger species (N<sub>2</sub>, CO<sub>2</sub>, and CH<sub>4</sub>) followed a gas translational diffusion model.
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