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 Nb_xW_{6 –x}O₁₉^{(x+ 2)–}(x= 2–4), [V₂W₄O₁₉]^4–, [V₁₀O₂₈]^6–, [Ti₂W₁₀PO₄₀]^7– and [NaP₅W₃₀O₁₁₀]^14–; MgAl[V₂W₄O₁₉^4–] was also formed via MgAl[Br^–] in dichloromethane–ethanol. The hexametalate ions in MgAl[M₆O₁₉^n–] orient themselves with their shortest dimensions (C₃ axis) perpendicular to the host layers (d₀₀₃ 12.0–12.5 Å) whereas in ZnAl[NaP₅W₃₀O₁₁₀^14–] 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 L_III-edge and niobium K-edge EXAFS data for the pillared derivatives and pure salts. The layered double hydroxide intercalates MgAl[Nb₄W₂O₁₉^6–], ZnAl[Ti₂W₁₀PO₄₀^7–] and ZnAl[NaP₅W₃₀O₁₁₀^14–] were also prepared by a direct coprecipitation procedure. In the third synthetic strategy, ZnCr[V₁₀O₂₈^6–], NiAl[V₂W₄O₁₉^4–] and ZnAl[XW₁₂O₄₀^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.