Water Gas Shift Reaction Of Organic-Inorganic Hybrid Silica Membranes Modified By Trifluoropropyl Group

Sol-gel-derived amorphous silica membrane is one of the candidates for gasseparation and purification due to its ease of fabrication, desirable pore structure, highphysical and chemical stability and high mechanical strength. However, pure silicamembranes have a poor hydrothermal stability so that their application in industry islimited. Hybrid organic-inorganic silica membranes with bridging organic groups inthe framework and hydrophobic organic groups on the pore surface are expected to bemore hydrothermally stable than pure silica membranes, if exposed to a humidatmosphere.In the present paper, ethyl-bridged hybrid organic-inorganic silica sols modifiedwith trifluoropropyl groups were synthesized by the acid-catalyzed co-hydrolysis andpolycondensation reaction of （3,3,3-trifluoropropyl）trimethoxysilane （TFPTMS） andbridged silsesquioxane1,2-bis（triethoxysilyl）ethane （BTESE）, and silica membranessupported on γ-Al₂O₃/α-Al₂O₃ceramic substrates were obtained by dip-coating underclean room conditions. The effect of trifluoropropyl groups on the particle size of soland the hydrophobic property of silica membranes was characterized by means ofdynamic light scattering （DLS）, water contact angle measurement, fourier transforminfrared spectroscopy （FT-IR）, solid state29Si magic-angle spinning nuclear magneticresonance （29Si MAS NMR） and thermogravimetry （TG）. The hydrogen permeationand separation behavior and the hydrothermal stability of the obtained membraneswere investigated on a home-made setup in detail. The water gas shift reaction wasalso conducted on a home-made membrane reactor setup.The results show that trifluoropropyl groups have been successfully incorporatedonto the surface of the hybrid organic-inorganic silica membranes. The sol particlesize decreases gradually and hydrophobic properties of the modified silica membranesare enhanced with the increasing amounts of TFPTMS in the mixture. When the molarratio of TFPTMS/BTESE increases to0.6, the organic-inorganic hybrid silicamembranes exhibit a water contact angle of111.6°±0.7°, and a narrow sol particlesize distribution centered at2.11nm.The results of hydrogen permeation and separation experiments demonstrate thatthe transport of hydrogen in the modified hybrid silica membranes complies with amicropore diffusion mechanism, with a high hydrogen permeance of8.86×10-7 mol·m-2·s-1·Pa-1, a H₂/CO₂permselectivity of5.4, and a H₂/CO₂binary gas （molarratio＝1:1） separation factor of4.82at300℃, higher than the corresponding Knudsenvalue （H₂/CO₂＝4.69）. The modified organic-inorganic silica membranes are hydroth-ermally stable while aged at a humid atmosphere with a temperature of250℃and asteam concentration of5%for more than300hours.The results of water gas shift reaction show that the CO conversion increaseswith operational temperatures, due to the enhancement of hydrogen separationperformance the elevated temperatures. The CO conversion of membrane reactors isup to98.15%with a H₂O/CO molar ratio of3:1at280℃, higher than that of fixed-bed reactors. Moreover, CO conversions at a constant operational temperature alsoincreases with increasing molar ratio of H₂O/CO reaching53.5%,94.94%and97.9%,respectively, when molar ratio of H₂O/CO is0.5:1,1.5:1and3:1at an operationaltemperature of240℃. This observation can be attributed to the increase of the partialpressure of hydrogen with increasing H₂O/CO molar ratio, which in turn provides alarger driving force for H₂diffusion through （0.6TFPTMS）Hybrid-SiO₂membranes.