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Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α—Keggin anions, which have a general formula of [XM12O40]n-, where X is the heteroatom (most commonly are P5+, Si4+, or B3+), M is the addenda atom (most common are molybdenum and tungsten), and O represents oxygen. The structure self assembles in acidic aqueous solution and is the most stable structure of polyoxometalate catalysts.
Additional recommended knowledge
The first α-Keggin anion, ammonium phosphomolybdate ((NH4)3[PMo12O40]), was first reported by Berzelius in 1826. In 1892, Blomstrand proposed the structure of phosphomolybdic acid and other poly-acids as a chain or ring configuration. Alfred Werner, using the coordination compounds ideas of Copaux, attempted to explain the structure of silicotungstic acid. He assumed a central group, [SiO4]4- ion, enclosed by four [RW2O6]+, where R is a unipositive ion. The [RW2O6]+ are linked to the central group by primary valences. Two more R2W2O7 groups were linked to the central group by secondary valences. This proposal accounted for the characteristics of most poly-acids, but not all.
In 1928, Linus Pauling proposed a structure for α-Keggin anions consisting of a tetrahedral central ion, [XO4]n-8, caged by twelve WO6 octahedral. In this proposed structure, three of the oxygen on each of the octahedral shared electrons with three neighboring octahedral. As a result, 18 oxygen atoms were used as bridging atoms between the metal atoms. The remaining oxygen atoms bonded to a proton. This structure explained many characteristics that were observed such as basicities of alkali metal salts and the hydrated of some of the salts. However the structure could not explain the structure of dehydrated acids.
J.F. Keggin with the use of X-ray diffraction experimentally determined the structure of α-Keggin anions in 1934. The Keggin structure accounts for both the hydrated and dehydrated α-Keggin anions without a need for significant structural change. The Keggin structure is the widely accepted structure for the α-Keggin anions. 
Structure and physical properties
Including the original Keggin structure there are 5 isomers, designated by the prefices α-, β-,γ-, δ- and ε-. The original Keggin structure is designated α- . These isomers are sometimes termed Baker, Baker-Figgis or rotational isomers , These involve different rotational orientations of the Mo3O13 units, which lowers the symmetry of the overall structure.
Lacunary Keggin structures
The term lacunary is applied to ions which have a fragment missing, sometimes called defect structures. Examples are the (XM11O39)n− and (XM9O34)n− formed by the removal from the Keggin structure of sufficient Mo and O atoms to eliminate 1 or 3 adjacent MO6 octahedra. The Dawson structure, X2M18O62n−, is made up of two Keggin lacunary fragment with 3 missing octahedra.
Group 13 cations with the Keggin structure
The cluster cation (Al13O4(OH)24(H2O)12)7+ has the Keggin structure with a tetrahedral Al atom in the centre of the cluster coordinated to 4 oxygen atoms. The formula can be expressed as (AlO4Al12(OH)24(H20)12)7+. This ion is generally called the Al13 ion. A Ga13 analogue is known an unusual ionic compound with an Al13 cation and a Keggin polyoxoanion has been characterised.
The stability of the Keggin structure allows the metals in the anion to be readily reduced. Depending on the solvent, acidity of the solution and the charge on the α-Keggin anion, it can be reversibly reduced in one- or multiple electron step. For example silicotungstate anion can be reduced to 20th state. Some anions such as silicotungstic acid are strong enough as an acid as sulfuric acid and can be used in its place as an acid catalyst.
In general α-Keggin anions are synthesized in acidic solutions. For example, 12-Phosphotungstic acid is formed by condensing phosphate ion with tungstate ions. The heteropolyacid that is formed has the Keggin structure.
[PO4]3- + 12[WO4]2- + 27H+ → H3PW12O40 + 12H2O
α-Keggin anions have been used as catalyst in the following reactions: hydration, polymerization and oxidation reaction as catalysts.. Japanese chemical companies have commercialized the use of the compounds in hydration of propene, oxidation of methacrolein, hydration isobutene, hydration of [[n—butene]], and polymerization of THF. 
12-Phosphotungstic acid the compound J.F. Keggin used to determine the structure can be purchased commercially. Other compounds that contain the α-Keggin anion such as silicotungstic acid and phosphomolybdic acid are also commercially available at Aldrich Chemicals, Fisher Chemicals, Alfa Aesar, VWR Chemical, etc.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Keggin_structure". A list of authors is available in Wikipedia.|