Catalyst Materials Based on Nb2O5 Supported on SiO2−Al2O3: Preparation and Structural Characterization

Supported niobium pentoxide materials have been effective catalysts for a variety of acid and redox reactions (e.g., dehydration of alcohols, esterifications, etc.). To the best of our knowledge, there are no reports about Nb2O5 supported on silica−alumina. Catalysts of Nb2O5/SiO2−Al2O3 were prepared with 2, 5, 10, 15, 20, and 25 wt % of Nb2O5 by aqueous solution impregnation using ammonium niobium oxalate on silica−alumina. The materials, after being dried at 100 °C, were calcined at 800 °C and characterized by several methods. Investigation through X-ray diffraction showed the typical patterns of crystalline Nb2O5, which were composed of mixtures of orthorhombic and monoclinic phases (T, M, and H, respectively) present in the materials with content higher than 10 wt %. DTA curves displayed an exothermic peak at 1356 °C (average) without mass loss (confirmed by TG), which may be ascribed to a phase transition (H phase formation) of Nb2O5 supported on silica−alumina. Pure Nb2O5·nH2O showed a transition from amorphous to hexagonal or orthorhombic phase (TT or T, respectively) at 567 °C. FTIR and DRIFTS results confirmed the reaction of the niobium oxide with the hydroxyl functionality of silica−alumina. The formation of surface niobium pentoxide species over the support through selective and progressive consumption of hydroxyl groups from the support and the appearance of characteristic niobium hydroxylated species on the surface were demonstrated. Raman spectra attested a two-dimensional overlayer of niobium pentoxide on silica−alumina at contents below 10 wt %. At higher concentrations, the absorptions characterize the formation of phases T and H of bulk Nb2O5.