Heteropoly acid 3036956 215477901 2008-05-28T11:21:35Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. 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>nāˆ’</sup></small> | <small>Dawson structure, X<sub>2</sub>M<sub>18</sub>O<sub>62</sub><sup>nāˆ’</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,<ref name=Kozhevnikov1998>{{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 }}</ref> 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]] ==Notes== {{reflist}} ==References== {{Cotton&Wilkinson6th}} [[Category:Heteropoly acids| ]]