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 a common re-usable acid catalyst in chemical reactions.^[1]

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_nXM₁₂O₄₀, and Dawson, H_nX₂M₁₈O₆₂, structures.

Keggin structure, XM12O40^n−	Dawson structure, X2M18O62^n−

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,^[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
  • 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.

See also

• Phosphotungstic acid
• Phosphomolybdic acid
• Silicotungstic acid

Notes

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

Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5