Lacunary Keggin anion, heteropolyanions

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

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. 

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. 

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