Homogeneity of Titania-Silica Mixed Oxides: On UV-DRS Studies as a Function of Titania Content

Abstract

The extraordinary catalytic activity and selectivity of Ti-containing crystalline and amorphous silicates for oxidation reactions with peroxides is attributed to isolated tetrahedral Ti sites in the silica framework. Diffuse reflectance UV-VIS spectroscopy (DRS) is commonly used to establish the presence of isolated Ti-sites. The charge transfer (CT) transitions between the oxide ions and the empty d-orbital of Ti{sup 4+} are strongly shifted from 26,000 cm{sup -1} in anatase towards higher wavenumbers with decreasing Ti-content. In two recent publications a new class of mesoporous silica-titania aerogels has been described, where the presence of highly dispersed titania in silica matrixes is emphasized. The authors attempted to balance the hydrolysis and condensation tendencies of the precursors and favour the use of the highly reactive tetramethoxysilane, together with acetylacetonate modified, chelated titanium tetraisopropoxide, in order to optimize the molecular mixing of the constituents. The materials were prepared from alkoxides by a sol-gel procedure. A rather monomodal mesoporosity is obtained by semicontinuous extraction with supercritical CO{sub 2}. The catalytic potential of these materials in the alkylhydroperoxide mediated epoxidation of olefins is attributed to the high titania dispersion in the silica matrix. These materials are not only important extensions of the available selective oxidation catalysts, butmore » their spectroscopic behaviour is an important contribution to fundamental understanding of site-isolated Ti-silicates. 16 refs., 1 fig.« less