Vanadium phosphates

Vanadium phosphates 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 oxidation reactions.

Vanadium phosphates

Identifiers
CAS Number	(anhydrous): 12359-27-2 (dihydrate): 12293-87-7
3D model (JSmol)	(anhydrous): Interactive image (dihydrate): Interactive image
ChemSpider	(anhydrous): 65999272 (dihydrate): 67027654
InChI (anhydrous): InChI=1S/H3O4P.O.V/c1-5(2,3)4;;/h(H3,1,2,3,4);;/q;;+3/p-3 Key: MEVGAPWXIYXSPM-UHFFFAOYSA-K (dihydrate): InChI=1S/H3O4P.2H2O.O.V/c1-5(2,3)4;;;;/h(H3,1,2,3,4);2*1H2;;/q;;;;+3/p-3 Key: IZKARBHCKXOZGC-UHFFFAOYSA-K
SMILES (anhydrous): [O-]P(=O)([O-])[O-].O=[V+3] (dihydrate): O.O.[O-]P(=O)([O-])[O-].O=[V+3]
Properties
Chemical formula	H4O7PV (dihydrate)
Molar mass	197.94 (dihydrate)
Appearance	yellow solid (dihydrate)
Solubility in water	insoluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Vanadium(V) phosphates

A common vanadium phosphate is VOPO₄•2H₂O.

Seven polymorphs are known for anhydrous VOPO₄, denoted α_I, α_II, β, γ, δ, ω, and ε.^[a][1][2][3] 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⁺.

Structure of the V-O-P framework of VOPO₄(H₂O)₂ (one lattice water and all hydrogen atoms are omitted).

Preparation, reactions, and applications of VOPO₄•2H₂O

Heating a suspension of vanadium pentoxide and phosphoric acid gives VOPO₄•2H₂O, isolated as a bright yellow solid.^[4] According to X-ray crystallography, the V(V) centers are octahedral, with long, weak bonds to aquo ligands.^[5]

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.^[6]

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, VOHPO₄^.4H₂O and VO(HPO₄)^.0.5H₂O, as well as the dihydrogen phosphate VO(H₂PO₄)₂.

Portion of the crystal structure of VO(HPO₄)^.0.5H₂O. The vanadium is octahedrally coordinated and phosphorus is tetrahedral. Water is a bridging ligand. Color code: red = O; gray = V, H; ochre = P.^[7]

Vanadium(III) phosphates

Vanadium(III) phosphates lacking the oxo ligand have the formula VPO₄•H₂O and VPO₄•2H₂O. The monohydrate is isostructural with MgSO₄•H₂O^[8] It adopts the structure of the corresponding hydrated aluminium phosphate. Oxidation of VPO₄•H₂O yields the two-electron electroactive^[9] material ε-VOPO₄.^[10]

Notes

1. The α_I and α_II polymorphs are sometimes called α₁ and α₂ instead.