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
xPO
4 as well related hydrates with the formula VO
xPO
4(H
2O)
n. Some of these compounds are used commercially as catalysts for oxidation reactions.
Vanadium(V) phosphates
A common vanadium phosphate is VOPO
4•2H
2O.
Seven
polymorphs are known for anhydrous VOPO
4, 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 ...
(PO
43−). 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
2+.
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 VOPO
4•2H
2O, 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 ...
to
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(HPO
4)•0.5H
2O 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(P
2O
7). 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, VOHPO
4.4H
2O and VO(HPO
4)
.0.5H
2O, 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(H
2PO
4)
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 VPO
4•H
2O and VPO
4•2H
2O. 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 VPO
4•H
2O yields the two-electron electroactive material ε-VOPO
4
Notes
References
{{Vanadium compounds, state=expanded
Vanadium compounds
Catalysts
Phosphates