Controlling vanadium phosphate catalyst precursor morphology by adding alkane solvents in the reduction step of VOPO4·2H2O to VOHPO4·0.5H2O
Vanadium phosphate catalysts were prepared by reducing square VOPO4·2H2O platelets with n-octane and 1-butanol and the resultant materials were characterized using a combination of techniques including powder X-ray diffraction and electron microscopy. The specific order in which the alkane and alcohol are added during the reduction step was found to have a remarkable influence on the precursor morphology and eventual catalytic activity of the final (VO)2P2O7 catalyst. Without the addition of n-octane the product consists of typical rosette-type VOHPO4·0.5H2O and n-octane addition after the reduction step does not significantly change the precursor morphology. By way of contrast, the addition of the n-octane before the reduction step leads to the formation of octagonal VOPO4·2H2O platelets which are then subsequently reduced by 1-butanol to form nanoscale rhomboidal VOHPO4·0.5H2O platelets. The co-addition of n-octane and 1-butanol, on the other hand, can reduce and reform the square VOPO4·2H2O crystals into rosette-type aggregates of angular VOHPO4·0.5H2O platelets. Nearly all of the catalysts generated exhibit a high activity and selectivity to maleic anhydride. The only exception is the catalyst produced by activating the nanoscale rhomboidal morphology precursor, which is attributed to its poorer crystallinity and more limited exposure of the active (100) (VO)2P2O7 plane. Furthermore, the spatial orientation relationship between the starting VOPO4·2H2O crystal and the VOHPO4·0.5H2O precursor phase has been indentified in this study as VOPO4·2H2O//VOHPO4·0.5H2O and VOPO4·2H2O//VOHPO4·0.5H2O. This information leads us to propose a two-step mechanism by which the topotactic transformation of VOPO4·2H2O to VOHPO4·0.5H2O occurs.