Titania–Silica Mixed Oxides: IV. Influence of Ti Content and Aging on Structural and Catalytic Properties of Aerogels

Abstract

Titania–silica mixed oxide aerogels semicontinuously extracted with supercritical CO2have been shown to be promising catalysts for the epoxidation 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. Additional preparation parameters involved are the prehydrolysis of the silicon alkoxide and the amount of acid added during the wet sol–gel step. The samples were characterized by means of nitrogen physisorption, elemental analysis, FTIR, Raman and UV–Vis spectroscopies, and XRD. The catalytic activity was tested using cyclohexene and α-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 wt% TiO2. Optimum textural properties were obtained after aging the sol–gel batch for 240 h before the supercritical CO2extraction procedure. Reducing the amount of acid in the hydrolysant lowered the catalytic activity. The optimum aerogel preparation parameters derived from this 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 no prehydrolysis.