Preparation And H₂ Separation Research Of Ethylene-Modified Microporous Silica Membranes

Silica membrane is considered to be one of the most promising materials for hydrogen separation at elevated temperatures due to its high H2 permeance and selectivity. However, the hydrothermal stability of silica membranes should be improved because of their strong hydrophilicity.The present thesis focuses on the preparation of hydrophobic silica membranes modified with ethylene group and the hydrogen transport and separation behavior in the modified materials. The silica sol was prepared by acid catalysed co- hydrolysis and condensation reaction of tetraethylorthosilicate (TEOS) and ethylenetriethoxysilane (TEVS) in ethanol and coated onα-γ-Al₂O₃ support via dip-coating under clean room condition. The pore structure, morphology and hydrophobic property of silica membranes were studied by means of SEM, BET, water contact angle measurement, TG and solid state ²⁹Si MAS NMR.The pore structure and hydrophobic property of silica membranes are related to the molar ratio of TEVS in the initial mixture. Nitrogen adsorption measurement shows that the modified silica membranes display a typeâ… isotherm according to the IUPAC classification, indicative of a typical microporous material. The modified materials possess a microporous structure with a narrow pore size distribution centered at 0.5nm. Such microporous structure can be stabilized after exposured to humid atmosphere for 450hrs, in intense contrast to the collapse of micropores in unmodified silica membranes.The water contact angle measurement indicats that the hydrophobic property is gradually enhanced with increasing of TEVS molar ratio. The solid state ²⁹Si MAS NMR result proves that there are new groups (ethylene) incorporated into modified products and the condensation of–OH decreases, demonstrating that the hydrophobic ethylene groups have partly replaced the hydrophilic hydroxyl groups on the pore surface after surface modification. The modification by ethylene group is responsible for the enhancement of the hydrophobic property of silica membranes.The hydrogen permeance increases with increasing of operating temperature and reaches 1.79×10^-7 mol·m^-2s^-1Pa^-1 , with a H₂/CO₂ separation factor of 7 at 300℃. This indicates that the transport of hydrogen obeys the micropore diffusion mechanism.