AMORPHOUS MICROPOROUS TITANIA-SILICA MIXED OXIDES - PREPARATION, CHARACTERIZATION, AND CATALYTIC REDOX PROPERTIES

Microporous titania-silica mixed oxides with a narrow monomodal pore s ize distribution at pore diameters of 0.7 nm with highly dispersed tit anium in the silica matrix have been obtained by a simple acid-catalyz ed sol-gel process in the absence of chelating agents or the prehydrol ysis techniques. A mixture of titanium(IV)alkoxide and tetraethoxysila ne (TEOS) has been hydrolysed in alcoholic solution with aqueous hydro chloric acid followed by calcination, resulting in amorphous, micropor ous mixed oxides. There is no limitation on the chemical composition, which covers the whole range from microporous silica to microporous ti tania. The structural and chemical properties of the materials as a fu nction of preparation parameters (such as acid, titania, or water cont ent, respectively, nature of alcohol, gelation temperature, drying con ditions, and titanium source) have been studied by means of physisorpt ion (Ar and N-2), X-ray powder diffraction, spectroscopic techniques, high resolution TEM (in combination with EDX and electron diffraction) and catalytic test reactions (epoxidation of olefins, selective oxida tions of saturated hydrocarbons). The high Ti dispersion was negativel y affected by changes in the Ti source and/or the alcohol used for the sol-gel process. Increasing BET surface areas were detected as functi on of the Si/Ti ratio at an optimum in acid concentration. The Si-exce ss materials stayed X-ray amorphous up to 1173 K. FTIR studies after p yridine treatment showed the materials having weak acidity only. With increasing Ti content an increase in Ti-O-Ti connectivity is observed by DRIFT spectroscopy, resulting in a decrease in epoxidation activity . The glasses show not only catalytic activity for selective oxidation reactions with TBHP comparable with that of other titania containing materials, but size selective epoxidations of olefins are interpreted as shape selectivity resulting from their distinct microporosity. Comp etitive adsorption experiments of water and octane suggest the hydroph ilicity of the amorphous oxides to be the major difference in comparis on to their zeolitic analogues. (C) 1996 Academic Press, Inc.