Effect on the structure and morphology of vanadium phosphates of the addition of alkanes during the alcohol reduction of VOPO sub(4); 2H sub(2)O

Vanadium phosphate catalysts were prepared by the reduction of VOPO sub(4); 2H sub(2)O 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 C sub(4)-C sub(16) n-alkanes was observed to affect the structure of the vanadium phosphate products significantly. Without the alkane the product is VOHPO sub(4); 0.5H sub(2)O which is the precursor to the industrial catalyst. Addition of the alkane leads to the formation of VO(H sub(2)PO sub(4)) sub(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 V super(5+) to V super(4+) which then reacts with phosphoric acid to give either VOHPO sub(4); 0.5H sub(2)O or VO(H sub(2)PO sub(4)) sub(2). If the reduction step is fast then the local P: V super(4+) ratio is approximately 1 and VOHPO sub(4); 0.5H sub(2)O is the major product. However, if the reduction step is slow then the local P: V super(4+) ratio is much higher and VO(H sub(2)PO sub(4)) sub(2) is preferentially formed.