Heteropoly acid

A heteropoly acid is a class of acid made up of a combination of hydrogen and oxygen with particular metals and non-metals.

Structure of the phosphotungstate anion Keggin structure, XM12O40n−

To qualify as a heteropoly acid, the compound must contain:

an “addenda atom”
A metal such as tungsten, molybdenum, or vanadium;
a “hetero atom”
An element generally from the p-block of the periodic table, such as silicon, phosphorus, or arsenic;
oxygen
oxygen must link the metal atom(s);
acidic stems
acidic hydrogen atoms.

The metal addenda atoms linked by oxygen atoms form a cluster with the hetero-atom inside bonded via oxygen atoms. Examples with more than one type of metal addenda atom in the cluster are well known. The conjugate anion of a heteropoly acid is known as a polyoxometalate.

Possible combinations

Due to the possibilities of there being different combinations of addenda atoms and different types of hetero atoms there are a lot of heteropolyacids. Two of the better known groups of these are based on the Keggin, HnXM12O40, and Dawson, HnX2M18O62, structures.

Some examples are:

Generic structure     X “hetero atom”     M “addenda atom”     Chemical structure
(single example)
H4Xn+M12O40 Si, Ge Mo, W (tungsten) H4Ge4+Mo12O40
H3Xn+M12O40 P, As Mo, W (tungsten) H3As5+W12O40
H6X2M18O62 P, As Mo, W (tungsten) H6P2Mo18O62

Uses

This type of acid is a common re-usable acid catalyst in chemical reactions.[1]

Dawson ion Dawson structure, X2M18O62n−

The heteropolyacids are widely used as homogeneous and heterogeneous catalysts,[2] particularly those based on the Keggin structure as they can possess qualities such as good thermal stability, high acidity and high oxidising ability. Some examples of catalysis are:[3]

  • Homogeneous acid catalysis
  • Heterogeneous acid catalysis
  • Homogeneous oxidation
    • cyclohexene + H2O2 to adipic acid by the mixed addenda H3PMo6V6O40
    • ketone by O2 to acid and aldehyde by mixed addenda H5PMo10V2O40

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.

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See also

Citations
  1. Mizuno, Noritaka; Misono, Makoto (1998). "Heterogeneous Catalysis". Chemical Reviews. 98: 199–217. doi:10.1021/cr960401q.
  2. Kozhevnikov, I. V. (1998). "Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions". Chemical Reviews. 98 (1): 171–198. doi:10.1021/cr960400y. PMID 11851502.
  3. "Oxide catalysts in solid state chemistry". T Okuhara, M Misono. Encyclopedia of Inorganic Chemistry. Editor R Bruce King (1994). John Wiley and Sons. ISBN 0-471-93620-0

References
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