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

Silica membranes have many advantages over other materials when they used at high temperatures and corrosive environments as material for gas separation because of their high mechanical strength, high thermal stability, and high resistance to various acid-base medium corrosion. However, silica membranes can not be used at humid environments due to their poor hydrothermal stability. Therefore, it is of great significance to prepare hydrothermally stable microporous membranes via surface modification.The present thesis focuses on the preparation of hydrophobic silica membranes and the investigation on their gas transport behavior and hydrothermal stability. Phenyl groups modified silica sols were prepared by the acid-catalyzed hydrolysis and condensation of phenyltriethoxysilane(PTES) and tetraethylorthosilicate (TEOS) in an ethanol (EtOH) medium. The supported silica membranes were prepared on the top ofγ-Al2O3/α-Al2O3 ceramic by the dip-coating technology under clean room condition. The pore structure, morphology and hydrophobic property of silica membranes were studied by means of AFM, N2 adsorption, SEM, water contact angle measurement, FT-IR,TG and solid state 29Si MAS NMR. Gas permeation experiment of the supported membranes was performed with a home-made setup.The results show that the modified membranes display a typical type I isotherms, indicating a microporous structure. The modified silica membranes have a narrow pore size distribution centered at 0.4-0.5nm. Such a microporous structure can be stabilized after exposured to humid atmosphere for 30d, in contrast to the collapse of micropores in the unmodified silica membranes. The successful modification with phenyl groups can be confirmed by FT-IR spectra and solid state 29Si MAS NMR. The water contact angle measurement indicates that a hydrophobic membrane is obtained upon the phenyl group modification and the hydrophobic property is gradually enhanced with increasing PTES concentration. It is reasonable that the presence of hydrophobic phenyl group on the pore surface and the decrease of the amount of hydrophilic silanols are responsible for the enhancement of the hydrophobicity of the modified membranes.The modified silica membranes conform to a combination of a surface diffusion mechanism associated with micropores and a Knudsen diffusion mechanism related to the larger pores or microcracks, with a H2 permeance of 1.49×10-6mol·m-2·Pa-1·s-1 and a permselectivity of 4.64 for H2/CO2 and 365.69 for H2/SF6.