STRUCTURE AND PROPERTIES OF TUNGSTATES FORMED IN W-MG-O SYSTEMS

New W-Mg-O systems have been obtained by impregnation of MgO with aque ous ammonium paratungstate solutions. The systems have been characteri zed by X-ray diffraction, Raman and W L-3 X-ray absorption (XAS) spect roscopies, FT-IR monitoring of pyridine adsorption for surface acidity , nitrogen adsorption at 77 K, and electron microscopy. Results obtain ed indicate that amorphous Mg tungstates are formed for low (723 K) ca lcination temperatures, but crystalline compounds are formed after cal cination above 973 K. W6+ coordination in these solids depends on W co ntent and calcination temperature. So, tetrahedral [WO4] species exist in amorphous species with low W content, and the crystalline ones obt ained after calcination at 973 or 1073 K, while octahedral [WO6] speci es are present in amorphous solids with large (24 wt%) W content and i n the crystalline phases formed after calcination at 1273 K, Formation of tungstates at rather low calcination temperature arises from the s trong acid/base interaction between both reactants. Incorporation of t ungsten leads to development of surface Lewis acid sites. The tungsten -containing species are less reducible in all cases than bulk WO3. (C) 1997 Academic Press.