Hybrid TiO₂-SiO₂ Organic/Inorganic Gels: Preparation, Characterization, and Temperature-Programmed Pyrolysis and Oxidation Studies

Permethylated cyclooligosiloxane rings, tetraethylortosilicate (TEOS), and Ti(iPr)₂Cl₂ were used to prepare hybrid TiO₂-SiO₂ xerogels. A number of solvents were ineffective for leaching the organic silicon from the samples at 336 K after several hours. The nitrogen desorption isotherms of these materials reveal that microporous structures with broad pore size distributions are generated upon gelation. Characterization of the samples by means of ²⁹Si CP/MAS NMR shows incorporation of the "organic silicon" into the amorphous structure and also suggests a great extent of homogeneity in the proton distribution around all tetrahedral silicate centers. The thermal stability of these hybrid TiO₂-SiO₂ materials was studied under flowing nitrogen and air by means of temperature-programmed (TP) techniques, and in situ diffuse reflectance infrared spectroscopy (DRIFTS). The stability of the Si-C bond in the absence of oxygen is hardly influenced by the presence of Ti. All Ti-containing samples undergo high-temperature pyrolytic decomposition of the Si-C bond, even when oxygen was present. In the presence of oxygen, the Ti function catalyzes the complete oxidation of the remaining alkoxide groups, rather than Si-C bond oxidation. The thermal stability of the latter and the decomposition of alkoxide groups into olefins are confirmed by both TP methods and DRIFTS.