Abstract

It has been observed that as a function of a time delay introduced between the onset of the reduction of V₂O₅ with hydroxylamine hydrochloride and the addition of phosphoric acid, in the aqueous route preparation of the catalytically important VOHPO₄·0.5H₂O phase, a series of NH₃ incorporated phases are obtained. Up to delay times of 3 min the crystalline VOHPO₄·0.5H₂O phase is obtained whereas a weakly crystalline hemihydrate is formed at a delay time of 4 min. Longer delay times result in X-ray amorphous phases until at delay time of 10 min and above a new crystalline phase containing mixed-valent vanadium emerges. All the phases obtained with delay times of 4 min and above have NH₃ incorporated into them. The weakly crystalline VOHPO₄·0.5H₂O phase as well as the X-ray amorphous phases undergo transformation to a crystalline vanadyl pyrophosphate phase on calcination at 723 K in spite of having NH₃ incorporated into them. The in-situ generation of NH₃ during the preparation procedure is a hitherto unreported aspect of VPO chemistry and provides a convenient method for the preparation of novel NH₃ incorporated VPO phases which could have strong potential as catalysts for alkane oxidation as well as for ammoxidation reactions.