Heteropoly acid

Heteropoly acid

A heteropoly acid is a class of acid made up of a particular combination of hydrogen and oxygen with certain metals and non-metals. This type of acid is frequently used as a re-usable acid catalyst in chemical reactions.

To qualify as a heteropoly acid, the compound must contain:
*a metal such as tungsten, molybdenum or vanadium, termed the addenda atom;
*an element generally from the p-block of the periodic table, such as silicon, phosphorus or arsenic termed the hetero atom;
*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.

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:
*H4Xn+M12O40, X = Si, Ge; M = Mo, W
*H3Xn+M12O40, X = P, As; M = Mo, W
*H6X2M18O62, X=P, As;M = Mo, W

The heteropolyacids are widely used as homogeneous and heterogeneous catalysts,cite journal
author = Kozhevnikov, I.V.
year = 1998
title = Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions
journal = Chemical Reviews
volume = 98
pages = 171–198
doi = 10.1021/cr960400y
] 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 ["Oxide catalysts in solid state chemistry" T Okuhara, M Misono Encyclopedia of Inorganic chemistry Editor R Bruce King (1994) John Wiley and Sons ISBN 0471 93620 0 ] :
*Homogeneous acid catalysis
**hydrolysis of propene to give propan-2-ol by H3PMo12O40 and H3PW12O40
**Prins reaction by H3PW12O40
**polymerisation of THF by H3PW12O40
*Heterogeneous acid catalysis
**dehydration of propan-2-ol to propene and methanol to hydrocarbons by H3PW12O40
**reformation of hexane to 2-methylpentane (isohexane) by H3PW12O40 on SiO2
*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.

ee also

*Tungstosilicic acid
*Molybdosilicic acid
*Phosphotungstic acid



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