Vanadium phosphates
Vanadium phosphates are inorganic compounds with the formula VOPO as well related hydrates with the formula VOPO(HO). Some of these compounds are used commercially as catalysts for oxidation reactions. Vanadium(V) phosphates. A common vanadium phosphate is VOPO•2HO. Seven polymorphs are known for anhydrous VOPO, denoted α, α, β, γ, δ, ω, and ε. These materials are composed of the vanadyl group (VO) and phosphate (PO). 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 VO. Preparation, reactions, and applications of VOPO•2HO. Heating a suspension of vanadium pentoxide and phosphoric acid gives VOPO•2HO, isolated as a bright yellow solid. According to X-ray crystallography, the V(V) centers are octahedral, with long, weak bonds to aquo ligands. 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(HPO)•0.5HO to the pyrophosphate (VO)(PO). This material (CAS#58834-75-6) is called vanadyl pyrophosphate as well as  vanadium oxide pyrophosphate. Vanadium(IV) phosphates. Several vanadium(IV) phosphates are known. These materials are typically blue. In these species, the phosphate anion is singly or doubly protonated. Examples include the hydrogenphosphates, VOHPO4HO and VO(HPO)0.5HO, as well as the dihydrogen phosphate VO(HPO). Vanadium(III) phosphates. Vanadium(III) phosphates lacking the oxo ligand have the formula VPO•HO and VPO•2HO. The monohydrate is isostructural with MgSO•HO It adopts the structure of the corresponding hydrated aluminium phosphate. Oxidation of VPO•HO yields the two-electron electroactive material ε-VOPO