Vanadium phosphate

Vanadium phosphate are inorganic compounds with the formula VO_xPO₄ as well related hydrates with the formula VO_xPO₄(H₂O)_n. Some of these compounds are used commercially as catalysts for oxidations.

A common vanadium phosphate is VOPO₄^.2H₂O. Four polymorphs are known for anhydrous VOPO₄, denoted α, β, γ, and δ. These materials are composed of the vanadyl group (VO) and phosphate (PO₄³⁻). They are yellow, diamagnetic solids, although when contaminated with vanadium(IV) derivatives, samples exhibit EPR signals and have bluish cast. For these materials, vanadyl refers to both vanadium(V) oxo and vanadium(IV) oxo centers, although conventionally vanadyl is reserved for derivatives of VO²⁺.

Several vanadium(IV) phosphates are known. These materiearls are typically blue. In these species, the phosphate anion is singly or doubly protonated. Examples include the monohydrogenphosphates, VOHPO₄^.4H₂O and VO(HPO₄)^.0.5H₂O, and the dihydrogen phosphate VO(H₂PO₄)₂).

Vanadium(III) phosphates lacking the oxo ligand]] have the formula VPO₄.H₂O and VPO₄^.2H₂O. The monohydrate is isostructural with MgSO₄^.H₂O. The dihydrate occurs naturally as the mineral metavariscite. It adopts the structure of the corresponding hydrated aluminium phosphate.

Heating a suspension of vanadium pentoxide and phosphoric acid gives VOPO₄^.2H₂O, isolated as a bright yellow solid. According to X-ray crystallography, the V(V) centers are octahedral, with a long, weak bond to an aquo ligand.

Reduction of this compound with alcohols gives the vanadium(IV) phosphates.

These compounds are catalysts for the oxidation of butane to maleic anhydride. A key step in the activation of these catalysts is the conversion of VO(HPO₄)^.0.5H₂O to the pyrophosphate (VO)₂(P₂O₇). This material (CAS#58834-75-6) is called vanadyl pyrophosphate as well as vanadium oxide pyrophosphate.