Preparation And Hydrogen Separation Of Perfluorodecyl-Modified Silica Membranes

Microporous silica membranes are considered as promising candidates for gas separation due to their high porosity, tunable pore size in the level of gas molecules, strong resistance to corrosion and high temperature, as well as high mechanical strength. However, pure silica membranes exhibit poor hydrothermal stability in humid atmosphere since their pore structure is easy to be destroyed by the physically adsorbed water owing to the presence of a large amount of hydrophilic hydroxyl groups on the membrane surface. Therefore, the preparation of hydrothermally stable microporous membranes has attracted great attention in the last two decades.In the present paper, perfluorodecyl, a hydrophobic group, was employed to replace the hydroxyl group via surface modification approach, aiming to improve the hydrothermal stability of silica membranes. A sol-gel technique was used to synthesize perfluorodecyl-modified silica sols with tetraethyl orthosilicate (TEOS) and1H,1H,2H,2H-Perflouorodecyltriethoxysilane (PFDTES) as precursors under acidic condition. Supported silica membranes were obtained by dip-coating y-Al2O3/α-Al2O3ceramic substrates with the modified silica sols under clean room condition. The particle size distribution of silica sols and the morphology, pore structure and hydrophobic properties of the modified silica membranes were characterized by means of dynamic light scattering (DLS), scanning electron microscopy (SEM), nitrogen adsorption, water contact angle measurement, fourier translation infrared spectroscopy (FT-IR), thermo gravimetric analysis (TG) and solid state29Si magic-angle spinning nuclear magnetic resonance (29Si MAS NMR), respectively. The hydrogen permeation and separation performance and the hydrothermal stability of the modified silica membranes were also investigated with home-made setups in detail.The results show that perfluorodecyl groups have been successfully incorporated, resulting in a transformation from hydrophility to hydrophobicity for the silica membranes. When the molar ratio of PFDTES/TEOS reaches0.2, the modified silica sols have a particle size centered at3.90nm and the modified silica membranes exhibit a water contact angle of112.6°±0.5°. The water contact angle of supported membranes increases with the amount of PFDTES in the mixture. The introduction of perfluorodecyl groups does not have much impact on the pore structure of the silica membranes and the silica membranes still maintain a desirable microporous structure, with a pore size ranging from0.45to0.9nm.The hydrogen permeance increases with increasing temperatures, leading to a considerably high H2permeance of9.71×10-7mol·m·s-1·Pa-1, a H2/CO2perm selectivity of7.19, and a H2/CO2binary gas (molar ratio=1:2) separation factor of12.11at300℃, higher than the corresponding Knudsen value (H2/CO2=4.69) and showing a good molecular sieving mechanism. Under a humid condition with a temperature of250℃and a water vapor molar ratio of5%, the single H2permeance and H2/CO2separation factor remain almost constant for at least200hours, indicating that the modified membranes possess an outstanding hydrothermal stability.