Abstract
Vanadium phosphate catalysts were prepared by reducing square VOPO4 center dot 2H(2)O 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)(2)P2O7 catalyst. Without the addition of n-octane the product consists of typical rosette-type VOHPO4 center dot 0.5H(2)O 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 center dot 2H(2)O platelets which are then subsequently reduced by 1-butanol to form nanoscale rhomboidal VOHPO4 center dot 0.5H(2)O platelets. The co-addition of n-octane and 1-butanol, on the other hand, can reduce and reform the square VOPO4 center dot 2H(2)O crystals into rosette-type aggregates of angular VOHPO4 center dot 0.5H(2)O 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 center dot 2H(2)O crystal and the VOHPO4 center dot 0.5H(2)O precursor phase has been indentified in this study as [001](VOPO4 center dot 2H2O)//[001](VOHPO4 center dot 0.5H2O) and [100](VOPO4 center dot 2H2O)//[110](VOHPO4 center dot 0.5H2O). This information leads us to propose a two-step mechanism by which the topotactic transformation of VOPO4 center dot 2H(2)O to VOHPO4 center dot 0.5H(2)O occurs.