Effect on the structure and morphology of vanadium phosphates of the addition of alkanes during the alcohol reduction of VOPO4· 2H2O
Vanadium phosphate catalysts were prepared by the reduction of VOPO 4·2H2O with an alcohol and characterised using a combination of powder XRD, BET surface area measurement, scanning electron microscopy and transmission electron microscopy. The effect of the addition of an alkane co-solvent during the reflux stage of the preparation was investigated. The addition of C4-C16n-alkanes was observed to affect the structure of the vanadium phosphate products significantly. Without the alkane the product is VOHPO4·0.5H2O which is the precursor to the industrial catalyst. Addition of the alkane leads to the formation of VO(H2PO4)2, with its characteristic block-shaped crystallites, and the alkane/alcohol liquid phase solubilises the excess vanadium. The amount of alkane required to induce these changes decreased with increasing carbon number of the n-alkane. The effect of the addition of the alkane co-solvent is thought to effect the rate of reduction of V5+ to V4+ which then reacts with phosphoric acid to give either VOHPO4·0.5H2O or VO(H 2PO4)2. If the reduction step is fast then the local P: V4+ ratio is approximately 1 and VOHPO4·0. 5H2O is the major product. However, if the reduction step is slow then the local P: V4+ ratio is much higher and VO(H 2PO4)2 is preferentially formed.