Heteropolyoxometalates

Heteropolyoxometalates, or heteropolyacids (HPAs) and their salts are materials which are finding widespread applications as acidic and/or redox catalysts. The most common examples are those with the Keggin structure, composed of a central hetero species, typically PO4~ or SiO4 , surrounded by 12 transition metal oxide octahedra, typically MoO6 or WO6, as depicted in Figure 10. The octahedra and central hetero species are linked via shared oxygens to yield materials with the formula [XM12O40]n~ where X = P(n = 3) or Si (n = 4) and M = MoorW. Many other structure types are known, with up to 40 transition metal octahedra per molecule. The negative charge is balanced by protons in an HPA and by certain cations in HPA salts. The charge-balancing cations are in many cases partially or wholly exchangeable, and physical properties such as solubility, surface area and porosity may vary widely depending on the nature of the cation (Table 4).

Heteropolyoxometalates are principally used as catalysts. Due to the high solubility of many of the cationic forms of heteropolyoxometalates in aqueous media, their application as ion exchangers has been limited. Apart from ammonium phosphomolybdate and ammonium phosphotungstate which possess low solubility and have been used to scavenge radioactive caesium, and [NaP5W30O110]14~, which has been shown to have high selectivity for lanthanide and certain multivalent ions, comparatively few data are

Figure 10 The structure of [XM12O40]"~ where X (P or Si) is located at the centre and is surrounded by 12 metal oxide oc-tahedra. (Reproduced with permission from Klemperer WG and Wall CG (1998) Polyoxoanion chemistry moves towards the future: from solids and solutions to surfaces. Chemical Reviews 98: 297-306.)

Figure 10 The structure of [XM12O40]"~ where X (P or Si) is located at the centre and is surrounded by 12 metal oxide oc-tahedra. (Reproduced with permission from Klemperer WG and Wall CG (1998) Polyoxoanion chemistry moves towards the future: from solids and solutions to surfaces. Chemical Reviews 98: 297-306.)

available concerning the ion exchange properties of the HPAs.

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