Characterization of Transition-Metal Oxides Promoted with Oxoanions by Means of Test Reactions

The catalytic activity of some transition-metal oxides known to develop strong acidity by promotion with oxoanions was compared with a set of acid-catalyzed test reactions (isomerization of n-butane, isobutene, n-hexane, and cyclohexane). They were further characterized by thermally programmed desorption of pyridine and 2,4,6-trimethylpyridine, X-ray diffraction, and sortometry. SO42--ZrO2, SO42--TiO2, SO42--Fe2O3, WO3-ZrO2, WO3-TiO2, WO3-Fe2O3, B2O3-ZrO2, B2O3-TiO2, and B2O3-Fe2O3 were synthesized by impregnation of the oxoanion over a fresh xerogel of the transition-metal hydroxide. Certain promoter-dependent orders, irrespective of the support, could be elucidated, mainly that the number of acid sites generated by promotion follows the order S, W > B. The proportion of strong acid sites is most important for S and B. The most active catalysts are always the zirconia ones, while the Fe2O3 catalysts are the least active.