Sol-gel bismuth-molybdenum-titanium mixed oxides - I. Preparation and structural properties

Bismuth molybdenum titanium oxides, potential catalysts for the partial oxi dation of olefins, were prepared via the sol-gel route. Either Bi(NO3)(3). 5H(2)O and (NH4)(6)Mo7O24. 4H(2)O, or BiCl3 and MoOCl4 were used as precurs ors together with Ti-isopropoxide. One sample was dried by semicontinuous e xtraction with supercritical CO2, affording an aerogel, the others were hea ted in vacuo resulting in xerogels. For comparison, two bismuth molybdenum oxides on a titania support, and a bismuth molybdate were prepared. The res ulting materials were characterized by ICP-AES, N-2 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 calcinati on in O-2 at 773 K, resulting in diminished surface area and a loss of micr opores. The bismuth molybdenum oxide phases of the materials containing mor e than 50 wt% titania remained X-ray amorphous after calcination. The Bi- a nd Mo-content, the type of Bi- and Mo-precursors and their prehydrolysis, a s well as the drying method had a major influence an the structural propert ies, and the surface and bulk composition of the aerogel and xerogels. Appl ication of Bi-chloride and Mo-chloride precursors can result in significant deviation from the desired bulk composition, likely due to incomplete hydr olysis and evaporation during the subsequent drying and calcination steps. (C) 1998 Elsevier Science B.V. All rights reserved.