TITANIA-SILICA MIXED OXIDES .4. INFLUENCE OF TI CONTENT AND AGING ON STRUCTURAL AND CATALYTIC PROPERTIES OF AEROGELS

Titania-silica mixed oxide aerogels semicontinuously extracted with su percritical CO2 have been shown to be promising catalysts for the epox idation reaction of various bulky reactants. In this study, the amount of titania in the silica matrix and the aging time of the wet sol-gel product were varied in order to optimize the structural and chemical properties of this material for the use as epoxidation catalysts. Addi tional preparation parameters involved are the prehydrolysis of the si licon alkoxide and the amount of acid added during We wet sol-gel step , The samples were characterized by means of nitrogen physisorption, e lemental analysis, FTIR, Raman and UV-Vis spectroscopies, and XRD. The catalytic activity was tested using cyclohexene and alpha-isophorone epoxidation. The highest contribution of Si-O-Ti entities, as derived from FTIR measurements, was found for samples with 30 wt% nominal TiO2 , whereas the optimum Ti-dispersion is achieved with a content of 20 w t% TiO2. Optimum textural properties were obtained after aging the sol -gel batch for 240 h before the supercritical CO2 extraction procedure . Reducing the amount of acid in the hydrolysant lowered the catalytic activity. The optimum aerogel preparation parameters derived from thi s study are 20 wt% nominal TiO2, an aging period of 240 h, at least an acid amount with a molar ratio water:alkoxide:acid of 5:1:0.09, and n o prehydrolysis. (C) 1996 Academic Press, Inc.