Keggin anion

Lactones in rhodium catalysis, 32 365-366 Lacunary Keggin anion, heteropolyanions, 41 ... 

There are a few features relative to POMs that are necessary for obtaining the best performance. In all cases. Vanadium is present in the structure of the P/Mo Keggin anion, while the cations include different components, that is, protons, divalent transition metal ions (preferably either Fe " " or Cu " "), and alkali metal ions (preferably Cs" "). The role of Cu ions is to catalyze the reduction of molybdenum, thus increasing the activity of the catalyst it also affects the surface acidity. 

All of these teehniques eonfirmed the presence of a large amount of POM ions within the pores. Theoretieal estimation gave the value of 5 Keggin anions per nanoeage. 

Despite the structural equivalence of the twelve metal centers in the o -PWi204o Keggin anion, a stepwise reduction of the tungsten centers is observed [30f]. EPR experiments on this species show that the added electron is delocalized (type II in the scheme proposed hy Rohin and Day) [4]. Over all 12 tungsten centers suggesting that there is communication... 

Decomposition of the Keggin anion (PMoi204o) of the H3PM012O40 H20 is accompanied by the loss of water and the formation of the product, M0O3. K3 contains clean surfaces containing Keggin anions. The end member Ko is a water-soluble Bronsted acid, with a variable n-water concentration similar to that in as-prepared samples. 

Figure 3.30. Heteropoly molybdate catalysts (a) Keggin anion (PMoi204o) (the central PO4 tetrahedron is shaded and the M0O6 octahedra are unshaded) and (b) selectivity (S) against conversion (C) data for acrolein to acrylic acid over KjcH3 PMoi2O40 (with x = 1-3) at " 350 °C show high selectivities. (After Black et al 1987, Gai et al 1983.)...

Several deficit or lacunary derivatives of the Keggin structure exist as individual species or as fragments of other heteropolyanion structures. Removal of one M06 octahedron (stoichiometric loss of MO"+) from a- or /3-Keggin anions leads to isomers of [XM11O39]"-. Removal of three adjacent octahedra leads to A- and B-type XM9034 anions. The structures of these are shown in Figure 11. 

The extent of reduction of H3PM012O40 during the oxidation of methacrolein has also been investigated by application of ESR spectroscopy for detection of Mo5+ (103). The states of V in the mixed-valence Keggin anion and Cu countercation were also investigated. The results show that more reducible countercations or addenda atoms such as Cu2+ and V5+ are reduced first, and an electron is localized on them (104-106). 

Depending on the solvent, the acidity of the solution, and the charge of the polyanion, the reductions involve either single-electron or multi-electron steps, often accompanied by protonation. In protic solvents, the Keggin anions exhibit... 

Reduction of Keggin anions beyond 2e reduction leads to modest changes in electronic and molecular structure, although the reductions remain reversible. For example, in the case of molybdates, a 4e -reduced /(-Keggin structure is stabilized, with the bridging oxygen atoms being partially protonated [Eq. (27)] (3, 253, 254) ... 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

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