chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

, the heteropolymetalates are a subset of the

polyoxometalate In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are ...

s, which consist of three or more

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...

s linked together by shared

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

atoms to form a closed 3-dimensional molecular framework. In contrast to isopolymetalates, which contain only one kind of metal atom, the heteropolymetalates contain differing

main group In chemistry and atomic physics, the main group is the group of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as ...

oxyanions. The metal atoms are usually

group 6 Group 6 may refer to: *Group 6 element, chemical element classification *Group 6 (racing) Group 6 was the official designation applied by the FIA to two motor racing classifications, the Prototype-Sports Car category from 1966 to 1971 and the Two ...

(Mo, W) or less commonly

group 5 Group 5 may refer to: *Group 5 element, chemical element classification *Group 5 (racing) Group 5 was an FIA motor racing classification which was applied to four distinct categories during the years 1966 to 1982. Initially Group 5 regulations def ...

(V, Nb, Ta) transition metals in their highest

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

s. They are usually colorless to orange,

diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...

anions. For most heteropolymetalates the W, Mo, or V, is complemented by main group oxyanions

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 phosph ...

and

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is a ...

. Many exceptions to these general statements exist, and the class of compounds includes hundreds of examples.

Structure

Certain structural motifs recur. The Keggin ion for example is common to both molybdates and tungstates with diverse central heteroatoms. The Keggin and Dawson structures have tetrahedrally-coordinated heteroatoms, such as P or Si, and the Anderson structure has an

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

central atom, such as

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

Heteropolyacids

Generally, the heteropolymetalates are more thermally robust than homopolymetalates. This trend reflects the stabilizing influence of the tetrahedral oxyanion that "glues" together the transition metal oxo framework. One reflection of their ruggedness, heteropolymetalates can be isolated in their acid form, whereas homopolymetalates typically cannot. Examples include: *

Silicotungstic acid Silicotungstic acid or tungstosilicic acid is a heteropoly acid with the chemical formula . It forms hydrates . In freshly prepared samples, ''n'' is approximately 29, but after prolonged desiccation, ''n'' = 6. It is a white solid although impu ...

, *

Phosphomolybdic acid Phosphomolybdic acid is the heteropolymetalate with the formula . It is a yellow solid, although even slightly impure samples have a greenish coloration. It is also known as dodeca molybdophosphoric acid or PMA, is a yellow-green chemical compound ...

, *

Phosphotungstic acid Phosphotungstic acid (PTA) or tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula . It forms hydrates . It is normally isolated as the ''n'' = 24 hydrate but can be desiccated to the hexahydrate (''n'' = 6). EPTA is the na ...

Isomerism

The Keggin structure has 5 isomers, which are obtained by (conceptually) rotating one or more of the four units through 60°.

Lacunary structures

The structure of some POMs are derived from a larger POM's structure by removing one or more addenda atoms and their attendant oxide ions, giving a defect structure called a ''lacunary structure''. An example of a compound with a Dawson lacunary structure is . In 2014, vanadate species with similar, selective metal-binding properties were reported.

Uses

This type of acid is a common re-usable acid

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 ...

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

The heteropolyacids are widely used as

homogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

and

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

catalysts, particularly those based on the

Keggin structure Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α-Keggin anions, which have a general formula of , where X is the heteroatom (most commonly are pentavalent phosphorus PV, tetravalent silico ...

as they can possess qualities such as good thermal stability, high acidity and high oxidising ability. Some examples of catalysis are:"Oxide catalysts in solid state chemistry". T Okuhara, M Misono. ''Encyclopedia of Inorganic Chemistry''. Editor R Bruce King (1994). John Wiley and Sons. *Homogeneous acid catalysis **hydrolysis of

propene Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petrol ...

to give

propan-2-ol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable organic compound with a pungent alcoholic odor. As an isopropyl group linked to a hydroxyl group (chemical formula ) it is the simpl ...

by and **

Prins reaction The Prins reaction is an organic reaction consisting of an electrophilic addition of an aldehyde or ketone to an alkene or alkyne followed by capture of a nucleophile or elimination of an H+ ion. The outcome of the reaction depends on reaction c ...

by **polymerisation of

THF Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...

by *Heterogeneous acid catalysis **dehydration of

and

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...

to hydrocarbons by **reformation of

hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relative ...

to 2-methylpentane (isohexane) by on *Homogeneous oxidation **

cyclohexene Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes. Cyclohexene is not very stable upon long term storage with exposure to light ...

+ to

adipic acid Adipic acid or hexanedioic acid is the organic compound with the formula (CH2)4(COOH)2. From an industrial perspective, it is the most important dicarboxylic acid: about 2.5 billion kilograms of this white crystalline powder are produced annuall ...

by the mixed addenda **ketone by to acid and aldehyde by mixed addenda Heteropolyacids have long been used in analysis and histology and are a component of many reagents e.g. the Folin-Ciocalteu reagent, folins phenol reagent used in the Lowry protein assay and EPTA, ethanolic phosphotungstic acid.

Citations

References

* {{Cotton&Wilkinson6th Acid catalysts