Studies on Properties of Surface Group of Silica Gels Treated with Alcohols and the Change of their Surface Properties through Thermal Treatment

Abstract

Pyrolysis of silica gels treated with alcohols have been investigated in comparison with that of alcohols in gas phase or on silica gels. In the pyrolysis of alcohol on silica gel, the silica gel has selectively promoted dehydration reaction and the corresponding olefine and water have generated in mole fraction of 1. : 1. Pyrolysis of surface-treated silica gel has occurred in temperature range of 350 and 500°C and the pyrolysis products were found to be corresponding olefine and small amount of water ; they were different from that of alcohol in gas phase or on silica gel. Accordingly, the surface group was considered not to be chemisorbed alcohol, but to be alkoxy group combined chemically with the substrate. The surface group of the surface-treated silica gel after calcination at 450°C in vacuo was found to be removed completely by means of IR spectra, weight loss and dispersion into mixture of two immiscible media. Pore size distribution, total pore volume and surface area of the original silica gel substrate were found to be unaffected by the calcination at 450°C or lower temperature. By calcination at higher temperature, the surface area and the total pore volume became smaller, owing to sintering. Adsorption isotherms, surface areas and the total pore volumes of surface-treated silica gels, after calcination at 450°C, showed reasonable agreement with that of the original silica gel. Thus, the autoclave method for the surface treatment of silica gels with alcohols has been recognized as effective method not to change the submicro-structure of silica gel surface (pore size distribution, surface area etc.), but to change only the physicochemical surface properties (hydrophilic to hydrophobic).