Studies of heterogeneous oxidation catalysts. Part 2.—The vanadium (V) oxide + molybdenum (VI) oxide and other binary oxide systems

Abstract

Studies of vanadium (V) oxide + molybdenum (VI) oxide mixtures have shown that, in contrast to the V₂O₅+ TiO₂ system, molybdenum (VI) oxide can scarcely be described as a promoter for V₂O₅, since the molybdenum compound itself exerts a significant catalytic effect on oxidation reactions. Although 25–30 % w/w MoO₃ represents the optimum concentration for the evolution of reactive oxygen from binary mixtures of this compound with V₂O₅, and this constitutes the most selective catalyst for the oxidation of certain hydrocarbons, spectroscopic studies indicate the involvement of both components. The observed formation of compounds closely related to Mo₆V₉O₄₀ is thought to be of little significance catalytically.

In contrast, a study of vanadia + zirconia mixtures has revealed that, like titanium (IV) oxide, ZrO₂ acts as a true promoter. The preferred composition of catalyst for the selective oxidation of o-xylene also represents the optimum composition (5–10 mole % V₂O₅) for a high temperature solid-state reaction between V₂O₅ and ZrO₂. During this reaction a small proportion of V⁵⁺ ions (19 atom %) is reduced to a lower valency state.

Finally, thermal studies of other oxide pairs (V₂O₅+ ZnO, Nb₂O₅+ TiO₂(anatase), MnO₂+ MoO₃ and CuO + Cr₂O₃) have shown that the existence of a maximum in the weight loss against composition curves obtained at high temperatures constitutes a good diagnostic test of their catalytic activity. In isolation, however, such measurements give little information regarding the role in catalysis of the separate constituents.