Sol–gel bismuth–molybdenum–titanium mixed oxides I. Preparation and structural properties Original Research Article

Bismuth molybdenum titanium oxides, potential catalysts for the partial oxidation of olefins, were prepared via the sol–gel route. Either Bi(NO3)3·5H2O and (NH4)6Mo7O24·4H2O, or BiCl3 and MoOCl4 were used as precursors together with Ti-isopropoxide. One sample was dried by semicontinuous extraction with supercritical CO2, affording an aerogel, the others were heated in vacuo resulting in xerogels. For comparison, two bismuth molybdenum oxides on a titania support, and a bismuth molybdate were prepared. The resulting materials were characterized by ICP-AES, N2 physisorption, XRD, XPS, FT Raman, and UV–vis spectroscopies. The sol–gel derived materials were X-ray amorphous and possessed a high surface area after drying, and a marked mesoporosity in case of the aerogel. The morphology changed upon calcination in O2 at 773 K, resulting in diminished surface area and a loss of micropores. The bismuth molybdenum oxide phases of the materials containing more than 50 wt% titania remained X-ray amorphous after calcination. The Bi- and Mo-content, the type of Bi- and Mo-precursors and their prehydrolysis, as well as the drying method had a major influence on the structural properties, and the surface and bulk composition of the aerogel and xerogels. Application of Bi-chloride and Mo-chloride precursors can result in significant deviation from the desired bulk composition, likely due to incomplete hydrolysis and evaporation during the subsequent drying and calcination steps.