Trifluoropropyl-modified Microporous Silica Membranes For Hydrogen Separation And Water Gas Shift Reaction

Microporous silica membranes have great advantages over other materials whenused at high temperatures and corrosive environments as material for gas separationdue to their large gas flux and selectivity, high mechanical strength, high thermalstability, and high resistance to various medium corrosion. However, pure silicamembrane can not be used in humid environments because of their poor hydrothermalstability. Therefore, preparing hydrothermally stable microporous membranes viasurface modification has become the current focus in the study of inorganicmembrane science.The present thesis focuses on the preparation of hydrophobic silica membranesand the investigation on their gas transport behaviors and hydrothermal stability.Trifluoropropyl groups modified silica sols were prepared by the acid-catalyzedhydrolysis and condensation of （trifluoropropyl）trimethoxysilane（TFPTMS） andtetraethylorthosilicate（TEOS） in an ethanol（EtOH） medium. The supported silicamembranes were prepared on the top of γ-Al₂O₃/α-Al₂O₃ceramic by the dip-coatingtechnology under clean room condition. The pore structure, morphology andhydrophobic property of silica membranes were studied by means of SEM, N₂adsorption, water contact angle measurement, FT-IR，TG and solid state29Si MASNMR. The gas permeation and serparation performance of supported silicamembranes was investigated with home-made setups. The water gas shift reactionwas conducted on the hydrophobic silica membrane reactor.The results show that the modified membranes retain a desirable microporousstructure with a pore size distributed from0.45to0.7nm. Such a microporousstructure can be stabilized after exposured to humid atmosphere for30d, in contrast tothe collapse of micropores in the unmodified silica membranes. The successfulmodification with trifluoropropyl groups can be confirmed by FT-IR spectra and solidstate29Si MAS NMR. The hydrophobic property of silica membranes has beenconsiderably enhanced with increasing of TFPTMS concentration, with a watercontact angle of102.7±0.1°at a TFPTMS/TEOS molar ratio of0.6.The hydrogen transport in the modified silica membranes complies with amicropore diffusion mechanism, with a single H₂permeance of4.77×10^-7mol·m^-2·s^-1·Pa^-1, a H₂/CO₂permselectivity of6.99and a H₂/CO₂binary gas mixtureseparation factor of6.93at300℃, higher than Knudsen permselectivity （4.69）. Under a humid condition with a temperature of200℃and a water vapor molar ratio of5%,the single H₂permeance slightly decrease in the first3hours and then almost remainconstant for at least220hours, incating that the modified silica membranes are highlyhydrothermally stable.For the water gas shift reaction, it is demonstrated that the CO conversion ofmembrane reactor is greater than that of fixed bed reactor and increases withincreasing temperatures and H₂O/CO molar ratio. The CO conversion also increaseswith increasing sweep gas flow due to the increase of hydrogen pressure differencein the system.