Titania-Silica Mixed Oxides: I. Influence of Sol-Gel and Drying Conditions on Structural Properties

Abstract

Mesoporous titania-silica aerogels with highly dispersed titanium have been prepared by an alkoxide-sol-gel process with ensuing semicontinuous extraction using supercritical CO 2. An acidic hydrolysant was added to a solution of tetraisopropoxytitanium(IV) modified by acetylacetonate and tetramethoxysilicon(IV) in isopropanol. The resulting titania-silica gels were dried by different methods, including conventional drying, high-temperature supercritical drying, and extraction with supercritical CO 2 (low-temperature aerogel). The influence of preparation parameters (the hydrolysis route, Ti-content, drying method, and calcination temperature) on the structural and chemical properties of the aero- and xerogels was studied by means of N 2-physisorption, X-ray diffraction, thermal analysis, and vibrational spectroscopy (FTIR, FTRaman). Prehydrolysis of the silicon alkoxide generally led to lower porosity, but did not influence the Si-O-Ti connectivity up to a calcination temperature of 1073 K. The conventionally dried xerogels contained titanium well-dispersed in the silica matrix and were predominantly microporous. High-temperature supercritical drying afforded meso- to macroporous aerogels with negligible microporosity, but undesired segregation of anatase. With the low-temperature aerogels, an increase of the nominal TiO 2 content from 2 to 20 wt% resulted in lower microporosity, higher BET surface area (up to ca. 700 m 2/g), and a rise in the contribution of Si-O-Ti species. Concomitantly the Ti-O-Ti connectivity increased without any indication of long-range ordering from X-ray analysis. The low-temperature aerogel with 20 wt% TiO 2 combined mesoporosity with high Ti dispersion and structural stability up to 873 K in air, properties desirable for epoxidation of olefins.