NOVEL OLIGOVANADATE-PILLARED HYDROTALCITES

Oligovanadate-pillared hydrotalcite-type layered materials are expected to be shape-selective oxidation catalysts with tunable acid-base character. The preparation and characterization of four oligovanadate-pillared clays are reported here. These are Mg5Al2(OH)(14)(V2O7)(0.5). 5H(2)O, Mg4Al2(OH)(12)(HV2O7)(0.67). xH(2)O, Mg5.3Al2(OH)(14.6)(V4O12)(0.5). 2H(2)O, and Mg2Zn2Al2(OH)(12) (V4O12)(0.5). 2H(2)O. The novel, one-step synthetic strategy used here was based on the fact that several oligovanadate anions are formed successively in aqueous solutions as a function of pH. For this particular study we focused our attention to the three anions, V4O124-,HV2O73- and V2O74-. The pH ranges where these anions exist in highest concentrations are: 6.5 to 8.5 for V4O124-, 8.5 to 10 for HV2O73-, and 10 to 13 for V2O74-. These di- and tetravanadate-pillared clays are prepared in one step by carrying out the synthesis at a pH where the desired anion exists in highest concentration. The isolated clays were characterized by elemental analysis, X-ray diffraction, thermal analyses (TGA, DTA), and surface studies.