Niobium oxide based materials as catalysts for acidic and partial oxidation type reactions

Abstract

Niobic acid, H 8Nb 6O 19· xH 2O, was synthesized and studied for its acidic features as a function of its dehydroxylation extent. It was observed to be strongly acidic, using NH 3 adsorption calorimetry and isopropanol conversion reaction as probe techniques, and to be weakly acidic on its dehydrated form, Nb 2O 5. The mixed oxide Al 2o 3:Nb 2o 5in 1:1 molar ratio prepared from aluminum and niobium oxalates was shown to be amorphous up to 1023 K where it crystallized in the form of AINbO 4 and to exhibit higher acidity than Nb 2O 5 dehydrated, as expected from Tanabe's model but much less acidic than the niobic acid form. The single Nb 2o 5 and mixed A1 2O 3:Nb 2o 5 oxides were used as supports for grafting MoO x and VO x species, respectively. It was observed that for MoO x /Nb 2o 5 samples new strong acid sites were created while the redox properties were not as satisfactory as expected. For VO x /Al 2O 3:Nb 2O 5 samples, at relatively low coverage (ca. 15°10), acidic features were not appreciably modified (only slightly enhanced) while new redox features were generated resulting in a rather satisfactory mild oxidation catalyst. The degree of condensation of VO x species on the mixed oxide surface, including some possible epitaxial grafting, even if not clearly characterized, appeared to play a determining role in oxidative dehydrogenation properties. This constitutes a new example of structure sensitivity in partial oxidation reactions on oxides.