- Heteropoly acid
A heteropoly acid is a class of
acidmade up of a particular combination of hydrogenand oxygenwith certain metals and non-metals. This type of acid is frequently used as a re-usable acid catalystin chemical reactions.
To qualify as a heteropoly acid, the compound must contain:
*a metal such as
tungsten, molybdenumor vanadium, termed the addenda atom;
elementgenerally from the p-blockof the periodic table, such as silicon, phosphorusor arsenictermed the hetero atom;
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
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
volume = 98
pages = 171–198
doi = 10.1021/cr960400y] particularly those based on the
Keggin structureas 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
propeneto give propan-2-olby H3PMo12O40 and H3PW12O40
**Prins reaction by H3PW12O40
**polymerisation of THF by H3PW12O40
*Heterogeneous acid catalysis
propan-2-olto propeneand methanolto hydrocarbons by H3PW12O40
hexaneto 2-methylpentane (isohexane) by H3PW12O40 on SiO2
cyclohexene+ H2O2 to adipic acidby 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 assayand EPTA, ethanolic phosphotungstic acid.
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