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 a common 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.
|Keggin structure, XM12O40n−||Dawson structure, X2M18O62n−|
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, 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:
- Homogeneous acid catalysis
- Heterogeneous acid catalysis
- Homogeneous oxidation
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.
- Mizuno, Noritaka; Misono, Makoto (1998). "Heterogeneous Catalysis". Chemical Reviews. 98: 199–217. doi:10.1021/cr960401q.
- Kozhevnikov, I. V. (1998). "Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions". Chemical Reviews. 98 (1): 171–198. PMID 11851502. doi:10.1021/cr960400y.
- "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