Vanadium based catalysts prepared by grafting: preparation, properties and performances in the ODH of butane

In the present work, many different vanadium based catalysts have been prepared and tested in the oxidative dehydrogenation (ODH) of butane. The catalysts were prepared both by impregnation and grafting for a useful comparison. In particular, the grafting procedure was studied by contacting solutions of increasing concentrations of vanadyl tri-isopropoxide, dissolved in dioxane, with two different supports: SiO2 and TiO2-SiO2. Grafting adsorption behaviour was studied using the isotherms obtained in the two cases. On silica we obtained an isotherm with two different recognisable plateaux. On the contrary, in the adsorption on TiO2-SiO2 the system followed the Langmuir adsorption law with a saturation value at about 7% of adsorbed V2O5. By greatly increasing the vanadyl concentration multi-layer adsorption seems to occur. In this latter case, catalysts obtained after calcination resulted less dispersed. Each point of both the mentioned isotherms corresponds to a catalyst with more or less dispersed vanadium. All these catalysts were characterised by using many different techniques (X-ray diffraction (XRD), diffuse reflectance spectroscopic analysis (DRIFT and DRUV) and laser-Raman spectroscopy (LRS) and XPS) and tested in the ODH of butane. In particular, we observed that vanadium dispersed catalysts are more active and selective. From the observation of the adsorption isotherms it is possible to decide the best conditions for preparing well dispersed catalysts with a relatively high vanadium load that have higher activities and selectivities. (C) 2004 Elsevier B.V. All rights reserved.