Structural Control And Gas Separation Performance Of Nanoscale Hybrid Silica Membranes

The pore size of the microporous silica membrane prepared by silica can be precisely controlled by sol-gel method.So it is suitable for efficient separation of small molecule gases,and shows good application prospects in the fields such as hydrogen energy production,natural gas purification and CO₂ capture.However,due to the rich hydroxyl groups on the surface of traditional silica membranes,the hydrolysis of Si-O-Si in the structure of silica membranes is easy to occur in the hydrothermal environment,which limits its practical application.Therefore,the use of organic-inorganic hybrid alkoxides that contain hydrophobic groups such as methyltriethoxysilane?MTES?,1,2-bis?triethoxysilyl?ethane?BTESE?,1,8-bis?triethoxysilyl?octane?BTESO?,have been proposed by several research groups for the preparation of organic-inorganic hybrid silica membranes,which not only greatly improves the hydrophobicity of silica membranes,but also provides new inspiration for the structural control of hybrid silica membranes.In this paper,organic-inorganic hybrid silica sols were synthesized by sol-gel method using BTESE and MTES as precursors.The hybrid silica membranes were prepared by hot-coating method.The effects of BTESE/MTES ratio on the structure and gas separation properties of the membranes were investigated by Fourier transform infrared spectroscopy?FTIR?,N₂ sorption?BET?,scanning electron microscopy?SEM?and home-made gas permeation/separation device.The results showed that:with the increase of the proportion of MTES in the precursors,the porosity of the hybrid silica membrane increased.When BTESE/MTES=1/3,the hybrid silica membrane exhibited the largest gas permeation,the permeance of H₂ and CO₂ can reach 7.99×10^-7 mol·m^-2·s^-1·Pa^-1 and 3.53×10^-7 mol·m^-2·s^-1·Pa^-1,respectively.And when BTESE/MTES=1/2,the best gas separation selectivity was achieved on the membrane,the ideal selectivity of H₂/N₂,H₂/CH₄,CO₂/N₂ and CO₂/CH₄ were 64.4,73.6,28.5 and 32.6,respectively,the mixed gas separation selectivity of H₂/N₂ and CO₂/CH₄ were 19.0 and 14.6,respectively.Subsequently,the hybrid silica membranes were doped by metal-organic frameworks?MOFs?CuBDC crystals,and CuBDC/hybrid silica composite membranes were successfully prepared by dip-coating method.The effects of bulk-type and sheet CuBDC on the separation efficiency of CO₂/CH₄ were compared.The results showed that the CO₂ permeance for composite membranes prepared by bulk-type CuBDC doping was higher than that of pristine hybrid silica membranes,and it can reach 9.39×10^-7 mol·m^-2·s^-1·Pa^-1,but the ideal selectivity of CO₂/CH₄ was only19.0,while the composite membranes doped with sheet CuBDC can achieve 63.5,which was much higher than that of pristine hybrid silica membranes,and the CO₂permeance was 2.25×10^-7 mol·m^-2·s^-1·Pa^-1.The membrane still performed well in the mixed gas separation,the CO₂/CH₄ mixture selectivity can reach 33.4.