Vanadium phosphates are

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

s 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(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. These materials are composed of the

vanadyl The vanadyl or oxovanadium(IV) cation, VO2+, is a functional group that is common in the coordination chemistry of vanadium. Complexes containing this functional group are characteristically blue and paramagnetic. A triple bond is proposed to ex ...

group (VO) and

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

(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⁺.

Preparation, reactions, and applications of VOPO₄•2H₂O

Heating a suspension of

vanadium pentoxide Vanadium(V) oxide (''vanadia'') is the inorganic compound with the formula V2 O5. Commonly known as vanadium pentoxide, it is a brown/yellow solid, although when freshly precipitated from aqueous solution, its colour is deep orange. Because o ...

and

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

gives VOPO₄•2H₂O, isolated as a bright yellow solid. According to X-ray crystallography, the V(V) centers are octahedral, with long, weak bonds to

aquo ligand In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...

s. Reduction of this compound with alcohols gives the vanadium(IV) phosphates. These compounds are

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s for the oxidation of

butane Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name but ...

maleic anhydride Maleic anhydride is an organic compound with the formula C2H2(CO)2O. It is the acid anhydride of maleic acid. It is a colorless or white solid with an acrid odor. It is produced industrially on a large scale for applications in coatings and poly ...

. A key step in the activation of these catalysts is the conversion of VO(HPO₄)•0.5H₂O to the

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P–O–P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), among other ...

(VO)₂(P₂O₇). 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, VOHPO₄^.4H₂O and VO(HPO₄)^.0.5H₂O, as well as the

dihydrogen phosphate Dihydrogen phosphate or dihydrogenphosphate ion is an inorganic ion with the formula 2PO4sup>−. Phosphates occur widely in natural systems. These sodium phosphates are artificially used in food processing and packaging as emulsifying agents, ...

VO(H₂PO₄)₂.

Vanadium(III) phosphates

Vanadium(III) phosphates lacking the

oxo ligand A transition metal oxo complex is a coordination complex containing an oxo ligand. Formally O2-, an oxo ligand can be bound to one or more metal centers, i.e. it can exist as a terminal or (most commonly) as bridging ligands (Fig. 1). Oxo ligan ...

have the formula VPO₄•H₂O and VPO₄•2H₂O. The monohydrate is isostructural with MgSO₄•H₂O It adopts the structure of the corresponding hydrated

aluminium phosphate Aluminium phosphate is a chemical compound. In nature it occurs as the mineral berlinite. Many synthetic forms of aluminium phosphate are known. They have framework structures similar to zeolites and some are used as catalysts, ion-exchangers or ...

. Oxidation of VPO₄•H₂O yields the two-electron electroactive material ε-VOPO₄

Notes

References