Controlling vanadium phosphate catalyst precursor morphology by adding alkanesolvents in the reduction step of VOPO4·2H2O to VOHPO4·0.5H2O

Abstract

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 [001]^{VOPO₄·2H₂O}//[001]^{VOHPO₄·0.5H₂O} and [100]^{VOPO₄·2H₂O}//[110]^{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.