Tailoring a Thermally Stable Amorphous SiOC Structure for the Separation of Large Molecules: The Effect of Calcination Temperature on SiOC Structures and Gas Permeation Properties

A SiOC membrane with high oxidative stability for gas separation was tailored by utilizing vinyltrimethoxysilane, triethoxysilane, and 1,1,3,3-tetramethyldisiloxane as Si precursors. Amorphous SiOC networks were formed via the condensation of Si–OH groups, the hydrosilylation of Si–H and Si–CHCH₂ groups, and a crosslinking reaction of Si–CH₃ groups, respectively. The crosslinking of Si–CH₃ groups at temperatures ranging from 600 to 700 °C under a N₂ atmosphere was quite effective in constructing a Si–CH₂–Si unit without the formation of mesopores, which was confirmed by the results of N₂ adsorption and by the gas permeation properties. The network pore size of the SiOC membrane calcined at 700 °C under N₂ showed high oxidative stability at 500 °C and was appropriate for the separation of large molecules (H₂/CF₄ selectivity: 640, H₂/SF₆: 2900, N₂/CF₄: 98). A SiOC membrane calcined at 800 °C showed H₂/N₂ selectivity of 62, which was approximately 10 times higher than that calcined at 700 °C because the SiOC networks were densified by the cleavage and redistribution reactions of Si–C and Si–O groups.