Structural studies of polyoxometalate-anion-pillared layered double hydroxides

Abstract

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[M6O19n–] orient themselves with their shortest dimensions (C3 axis) perpendicular to the host layers (d003 12.0–12.5 Å) whereas in ZnAl[NaP5W30O11014–] the ion is oriented with its shortest dimension (C5 axis) parallel to the host layers (d003 21 Å). The structures of several of these incorporated clusters have been verified unequivocally by obtaining tungsten LIII-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[XW12O40n–](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.