Effect on the structure and morphology of vanadium phosphates of the addition of alkanes during the alcohol reduction of VOPO4·2H2O

Abstract

Vanadium phosphate catalysts were prepared by the reduction of VOPO₄·2H₂O 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(H₂PO₄)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(H₂PO₄)₂ is preferentially formed.