View source for Heteropoly acid

A '''heteropoly acid''' is a class of [[acid]] made up of a particular combination of [[hydrogen]] and [[oxygen]] with certain [[metal]]s and [[non-metal]]s.  This type of acid is frequently used as a re-usable acid [[catalyst]] in [[chemical reaction]]s.

To qualify as a heteropoly acid, the compound must contain:
*a metal such as [[tungsten]], [[molybdenum]] or [[vanadium]], termed the addenda atom;
*[[oxygen]];
*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, H<sub>n</sub>XM<sub>12</sub>O<sub>40</sub>, and Dawson, H<sub>n</sub>X<sub>2</sub>M<sub>18</sub>O<sub>62</sub>, structures.<br />
{| width="450"
| width="50%" |[[Image:Phosphotungstate-3D-polyhedra.png|200px|Structure of the phosphotungstate anion]]
|[[Image:Dawson_ion.png|200px|Dawson ion]]
|-
|<small>Keggin structure, XM<sub>12</sub>O<sub>40</sub><sup>n−</sup></small>
| <small>Dawson structure, X<sub>2</sub>M<sub>18</sub>O<sub>62</sub><sup>n−</sup></small>
|}

Some examples are:
*H<sub>4</sub>X<sup>n+</sup>M<sub>12</sub>O<sub>40</sub>, X = Si, Ge; M = Mo, W
*H<sub>3</sub>X<sup>n+</sup>M<sub>12</sub>O<sub>40</sub>, X = P, As; M = Mo, W 
*H<sub>6</sub>X<sub>2</sub>M<sub>18</sub>O<sub>62</sub>, X=P, As;M = Mo, W


The heteropolyacids are widely used as [[homogeneous catalysis|homogeneous]] and [[heterogeneous catalysis|heterogeneous]] catalysts, 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<ref>''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 </ref>:
*Homogeneous acid catalysis
**hydrolysis of [[propene]] to give [[propan-2-ol]] by H<sub>3</sub>PMo<sub>12</sub>O<sub>40</sub> and H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>
**Prins reaction by H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>
**polymerisation of THF by H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>
*Heterogeneous acid catalysis
**dehydration of [[propan-2-ol]] to [[propene]] and [[methanol]] to hydrocarbons by H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>
**reformation of [[hexane]] to 2-methylpentane (isohexane) by  H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub> on SiO2
*Homogeneous oxidation
**[[cyclohexene]] + [[hydrogen_peroxide|H<sub>2</sub>O<sub>2</sub>]] to [[adipic acid]] by the mixed addenda H<sub>3</sub>PMo<sub>6</sub>V<sub>6</sub>O<sub>40</sub>
**ketone by O<sub>2</sub> to acid and aldehyde by mixed addenda H<sub>5</sub>PMo<sub>10</sub>V<sub>2</sub>O<sub>40</sub>
<br />
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.

==See also==
*[[Tungstosilicic acid]]
*[[Molybdosilicic acid]]
*[[Phosphotungstic acid]]

==References==
{{Cotton&Wilkinson6th}}
{{reflist|2}}


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[[Category:Heteropoly acids| ]]
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