Titania-silica mixed oxides .5. Effect of sol-gel and drying conditions on surface properties

Mesoporous titania-silica aerogels have been prepared by an alkoxide-sol-gel process with ensuing supercritical drying which involved either semicontinuous extraction using supercritical CO2 or transferring the solution-sol-gel liquid directly into the supercritical state. Surface properties of these materials were compared to those of conventionally dried titania-silica xerogel. The influence of several important preparation parameters (hydrolysis route, Ti-content, and drying method) on surface properties of the gels were investigated by temperature-programmed reaction and desorption (TPRD) of isopropanol and X-ray photoelectron spectroscopy (XPS). The information gained by these surface-sensitive methods was interpreted in the light of previous bulk structural investigations, i.e., X-ray diffraction, FTIR, FTRaman, and UV-vis spectroscopy, Up to 723 K, three TPRD peaks were identified and assigned to the different surface species in titania-silica mixed oxides found in previous structural investigations: inactive silica domains, titania domains, and highly dispersed Ti in silica (Si-O-Ti linkages). The Ti-containing surface species catalyzed dehydration of isopropanol to propene at different temperatures. Depending on the dispersion and nature of titanium oxo species on the surface of the titania-silica sol-gel mixed oxides (Si-O-Ti heteroconnectivity), significant differences in the reaction-desorption profiles were observed. Prehydrolysis of the silicon alkoxide had no influence on the relative abundance of Si-O-Ti connectivity inferred from FTIR. However, XPS and TPRD revealed that the Ti-concentration on the surface increased when prehydrolysis had been applied. An increase in the bulk titania content of the mixed oxides resulted in a concomitant rise in both the surface Ti/Si ratio and the relative contribution of Si-O-Ti linkages. All titania-silica gels showed enrichment of the surface with silica. This behavior strongly depended on the drying procedure applied. Low-temperature supercritical drying and evaporative drying yielded higher titania surface concentration (lower Si enrichment) compared to high-temperature supercritical drying. (C) 1996 Academic Press, Inc.