Preparation of High Surface Area VOHPO4·0.5H2O with the Alkoxide Method

Vanadium phosphorous oxides (VPO) catalyze the partial oxidation of butane to maleic anhydride. These VPO are commercially manufactured from solid vanadium oxides by treating them with a sequence of two processes. The first is the reflux of V2O5 with organic compounds, e.g. alcohols, to reduce the metal to the +4 oxidation state and also phosphoric acid as a phosphorous source to obtain the precursor (VOHPO4·0.5H2O). The second is the dehydration and calcination of the precursor at a high temperature to produce the active phase comprising vanadyl pyrophosphate ((VO)2P2O7). The purpose of the present work is to study the implementation of an alternative method, namely the alkoxide method, to synthesize the precursor (VOHPO4·0.5H2O) with an increased surface area, thus enhancing its characteristics as the raw material for the synthesis of (VO)2P2O7. The alkoxide method has been frequently adopted as a suitable route to obtain metal oxides with high surface areas. The method resorts to wet preparation, proceeding through hydrolysis and condensation reactions of the liquid metal alkoxides with water, or other acidic hydrolyzing agent, to yield metal oxides. Alternative approaches are available to synthesize VPO via the alkoxide method. For such approaches the starting materials are vanadium (V) alkoxides that are reacted with the orthophosphoric acid in different solvents; the reaction products are dried under atmospheric conditions. A procedure that uses aprotic solvents is modified by drying the product into an autoclave with controlled excess of solvent. Two important facts occur during the modified autoclave drying. The first is that high surface area material is obtained due to a sustainable gel structure allowed by low values of surface tension at the critical point. The right amount of solvent determines wether the liquid passes to the gas phase at or before the critical point influencing the surface area of the material. The second fact is the reduction process that is carried out inside the autoclave; the high temperature and the alcohol produced during the alkoxide hydrolysis reduce the vanadium form. Two simultaneous steps, reduction and drying are now yielding an improved precursor for the vanadyl pyrophosphate. The resultant oxides are characterized by diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), and X-ray diffraction (XRD). Moreover, their surface areas are measured with BET analysis.