Structural studies of polyoxometalate-anion-pillared layered double hydroxides

New examples of polyoxometalate-anion-pillared layered double hydroxides have been prepared by ion exchange and direct synthesis methods and characterised by powder X-ray diffraction, extended X-ray absorption fine structure (EXAFS), IR and UV/VIS spectroscopy. Complete conversion into crystalline phases was achieved by reaction at pH 4-12 of wet MgAl[Cl-] or ZnAl[NO3-] layered double hydroxide precursors with the species NbxW6-xO19(x + 2-) (x = 2-4), [V2W4O19](4-), [V10O28](6-), [Ti2W10PO40](7-) and [NaP5W30O110](14-); MgAl[V2W4O194-] was also formed via MgAl[Br-] in dichloromethane-ethanol. The hexametalate ions in MgAl[M(6)O(19)(4-)] orient themselves with their shortest dimensions (C-3 axis) perpendicular to the host layers (d(003) 12.0-12.5 Angstrom) whereas in ZnAl[NaP5W30O11014-] the ion is oriented with its shortest dimension (C-5 axis) parallel to the host layers (d(003) 21 Angstrom). The structures of several of these incorporated clusters have been verified unequivocally by obtaining tungsten L(III)-edge and niobium K-edge EXAFS data for the pillared derivatives and pure salts. The layered double hydroxide intercalates MgAl[Nb4W2O196-], ZnAl[Ti2W10PO407-] and ZnAl[NaP5W30O11014-] were also prepared by a direct coprecipitation procedure. In the third synthetic strategy, ZnCr[V10O286-], NiAl[V2W4O194-] and ZnAl[XW(12)O(40)(n-)] (X = B or Si) layered double hydroxides were obtained via directly synthesised terephthalate-, 4-methylbenzoate- and 4-hydroxybenzoate-anion-pillared precursors respectively. 4-Methylbenzoate intercalates were found to be the best precursors out of a series of benzenecarboxylate anion-pillared layered double hydroxides investigated. Zinc, chromium and nickel K-edge EXAFS data in addition to UV/VIS spectroscopic data for the ZnCr and NiAl materials indicate that no significant disruption occurred in the layer Structures during ion exchange.