Organosilica membranes for gas separation were prepared by plasma-enhanced chemical vapor deposition (PECVD) using three different types of silicon precursors: hexamethyldisiloxane (HMDSO), trimethylmethoxysilane (TMMOS), and methyltrimethoxysilane (MTMOS). Based on gas permeation measurement, the MTMOS-derived membrane showed the highest He/N 2 selectivity, followed by the TMMOS-derived and HMDSO-derived membranes. FT-IR characterization indicated that the HMDSO-derived membrane had the highest content of methyl group and the lowest Si-O-Si, while the methyl group content for the MTMOS-derived membrane was the lowest and Si-O-Si was the highest. These results suggest that the pore size of organosilica membranes could be tuned by changing the chemical structure of the silicon precursor. The MTMOS-derived membrane was further heat-treated to determine the effect of thermal annealing on gas-permeation properties. The gas permeances were drastically improved by the thermal annealing. After heat-treatment at 500C, the membrane showed a high H 2 permeance of 6.5  10 -7 mol/(m 2 s Pa) with a H 2 /SF 6 selectivity of 410 at 200C, and 5.6  10 -7 mol/(m 2 s Pa) with a H 2 /SF 6 selectivity of 360 at 50C.

Organosilica membranes for gas separation were prepared on porous substratesby means of plasma-enhanced chemical vapor deposition (PECVD) using hexamehyldisiloxane as a precursor.Before the deposition of organosilica membranes, two types of intermediate layers with different pore sizes, aTiO 2 layer (d p ≈ 5 nm) and a SiO 2-ZrO 2 layer (1 nm), were prepared on α-alumina poroussupportsvia a sol-gel method.The stability at high temperatureswas evaluated by measuring the gas permeation characteristics before and after heat treatment. PECVD membranes deposited ona SiO 2-ZrO 2 intermediate layer were found to be morethermallystable than those deposited ona TiO 2 intermediate layer. After the treatment at 500°C, the membranes deposited ona SiO 2-ZrO 2 intermediate layer had a selectivity of ~1040 for He against SF 6 with a He permeance of 1.41× 10-6 mol/(m 2 s Pa) at 50°C, although the organosilica membranes were prepared at room temperature. On the other hand, the membrane deposited on a TiO 2 intermediate layer became non-selective after heattreatment at 400°C. The temperature dependence of single gas permeance for the membrane deposited ona SiO 2-ZrO 2 intermediate layer was measured after the high-temperature stability test. The permeation of He, H 2 and N 2 for the 200°C-treated membrane showed an activated diffusion, while those for the 500°C-treated membrane followed a Knudsen permeation, suggesting that heat treatment at 500°C formed pores that allowed the permeation of molecules that were the size of N 2 (0.364 nm) and smaller. In addition, since the permeation of SF 6 for the 500°C-treated membrane showed an activated diffusion, the membrane should have a uniformed structure with a small number of pinholes.