Structural transformation sequences occurring during the activation of vanadium phosphorus oxide catalysts

Abstract

The structural transformations which occur when a VOHPO₄·0.5H₂O precursor is activated in a mixture of n-butane–air at 400 °C have been studied using a combination of XRD, TEM and ³¹P NMR spin–echo mapping. The VOHPO₄·0.5H₂O precursor was prepared by reducing V₂O₅ with isobutyl alcohol and had a plate-like morphology with a [001] normal. A systematic series of ‘activated’ catalysts were then prepared in which the activation time was varied between 0.1 and 132 h. Structural characterisation studies on these samples show the structural evolution of the catalyst during the activation procedure. At the periphery of the platelet a direct topotactic transformation from [001] VOHPO₄·0.5H₂O to [100](VO)₂P₂O₇ occurs. In the interior of the platelet a more complex indirect transformation sequence occurs. Regions are shown to exist where the VOHPO₄·0.5H₂O precursor initially transforms epitaxially into δ-VOPO₄. As the activation time increases the domains of δ-VOPO₄, which are embedded in a disordered matrix, shrink and further transform to the final (VO)₂P₂O₇ phase. An attempt has also been made to correlate measured catalytic performance data with the catalyst microstructure at various stages of the transformation process. It is found that there is a distinct parallel between improving catalytic performance and a decrease in the amount of V⁵⁺ phases present. A further sample of the same hemihydrate precursor was activated in a pure N₂ atmosphere for comparison. In this case, the whole VOHPO₄·0.5H₂O platelet was observed to undergo a direct topotactic transformation to the (VO)₂P₂O₇ phase. The transformation mechanism, however, involved the homogeneous nucleation of discrete epitaxial patches of crystalline (VO)₂P₂O₇ over the surface of the platelet. This is followed by the growth and eventual coalescence of the patches upon extended activation.