Recent topics of research and development of catalysis by niobium and tantalum oxides

Current topics of catalysts containing niobium and tantalum, especially in the field of solid acid catalysis and selective oxidation of hydrocarbons a re reviewed. Hydrated niobium oxide and hydrated tantalum oxide are highly acidic. Hydrated niobium oxide is active for the hydration of ethene to eth anol, and Nb-W mixed metal oxide is more active for the reaction. Acid prop erties of tantalum oxide are changed by being supported on SiO2. Ta oxide/S iO2, prepared by the chemical reaction between tantalum alkoxide and surfac e hydroxyl groups of SiO2, is active and selective for the gas phase Beckma nn rearrangement of cyclohexanoneoxime to caprolactam. Niobium oxide and ta ntalum oxide easily react with many other oxides to form mixed metal oxide phases with complex structure. Mixed metal oxide catalysts, containing moly bdenum, vanadium, certain elements together with niobium are active for the selective oxidation of hydrocarbons. Especially, the selective oxidation o f propane by such mixed metal oxide catalysts has been paid attention. Addi tionally, recent progress of environmental catalysts, promoted by niobium a nd tantalum compounds, namely catalysts for the pollution abatement is revi ewed. (C)2000 Elsevier Science B.V. All rights reserved.