PECULIARITIES OF STEAM-PHASE HYDROTHERMAL MODIFICATION OF PYROGENIC SILICIUM DIOXIDE

This paper presents the investigations on the mechanism of high pressu re hydrothermal treatment (HTT) of pyrogenic silicas as well as titani a-and alumina-aerosils. The aerosils were modified by using different sizes of primary particles (A-175, A-300 and A-380) as well as by cont aining 8% and 9% w/w of the Al2O3 and TiO2 respectively. The HTT proce ss was performed in the gas phase (overheated steam with and without a ddition of ammonia) in the temperature range 100-350 degrees C. The ch anges of surface properties of the adsorbents tested caused by HTT wer e determined on the basis of adsorption isotherms of water, methanol, carbon tetrachloride, n-hexane and benzene, and using the data relatin g to chemisorption of hexaamethyldisilazane as well as on the basis of specific surface area determined by the method of thermal desorption of argon. It was stated that the hydrothermal treatment of aerosil lea ds to the increase of hydroxylation degree of their surface. In the HT T mechanism the processes of mass transfer of silica in the gas (steam ) phase play a very important role. The intensity of these processes d epends on the size of primary particles (globules) of modified adsorbe nt, on its chemical composition, and on the presence of ammonia in ste am. The possibilities of utilizing the processes occurring in the vapo ur (steam) phase to prepare porous coverages (layers) were demonstrate d. (C) 1998 Elsevier Science B.V.