Preparation of high surface area vanadium phosphate catalysts using water as solvent

Abstract

The synthesis of vanadium phosphorus oxide catalysts using water as solvent is described and discussed. The use of H₃PO₃ as a reducing agent is contrasted with aqueous hydrochloric acid. Using H₃PO₃ as a reducing agent for V₂O₅ at 145 °C for 72 h, together with H₃PO₄ or H₄P₂O₇ as the additional phosphorus source, is found to produce VOHPO₄·0.5H₂O. Following activation in n-butane–air for 72 h, the catalysts derived from this method have surface areas (17–23 m² g⁻¹) that are comparable to those prepared using a standard non-aqueous solvent method. The specific (mol maleic anhydride per g catalyst per h) and intrinsic (mol maleic anhydride m⁻² h⁻¹) activities of the catalysts derived from using H₃PO₃ with water as solvent are also comparable to those for catalysts prepared using non-aqueous solvents. The activated catalysts comprise (VO)₂P₂O₇, together with α_II- and δ-VOPO₄. Refluxing the VOHPO₄·0.5H₂O catalyst precursors in water is found to decrease the crystallite size of the precursor, and this leads to higher area activated catalysts, but does not affect the specific or intrinsic activity significantly.