Improvement of Catalytic Performance in Isobutane Oxidation to Methacrylic Acid of Keggin-Type Phosphomolybdates by Preparation via Lacunary Precursors: Nature of the Active Sites

A solvent-free method of preparation of a vanadium(V) phosphate is described and discussed. Reaction of V₂O₅ with H₃PO₄ in the absence of water at 150°C leads to the formation of a new catalytic material that is designated as anhydrous VOPO₄. The material readily hydrates to form VOPO₄2H₂O and has been characterised using powder X-ray diffraction, in situ Raman spectroscopy and ³¹P MAS NMR spectrometry. On activation in dry N₂ followed by reaction with butane/air another novel material is formed that has an intrinsic activity for maleic anhydride that is similar to catalysts derived from VOHPO₄0.5H₂O under comparable conditions. Activation of VOPO₄2H₂O under comparable conditions leads to the formation of _I-VOPO₄ which exhibits no partial oxidation activity. Reaction of anhydrous VOPO₄ with alcohols leads to the exclusive formation of VO(H₂PO₄)₂ in further contrast to VOPO₄2H₂O which under similar conditions leads to the synthesis of VOHPO₄0.5H₂O.