On the influence of the acid-base character of catalysts on the oxidative dehydrogenation of alkanes

Vanadium oxides supported on metal oxide, i.e. Al₂O₃, MgO and Mg-Al mixed oxide, and V-containing microporous materials (VAPO-5 and MgVAPO-5) have been tested in the oxidative dehydrogenation of C₂-C₄ alkanes. In all cases, tetrahedral vanadium species (isolated and/or associated) were mainly observed from⁵¹V-NMR and diffuse reflectance spectroscopies. The reducibility of V⁵⁺-species, determined from the onset-reduction temperature, decreases as follows: VO_x/AL > VAPO-5 > MgVAPO-5 =VO_x/MG > VO_x/MG + AL. The acid character of catalysts, determined from the FTIR spectra of pyridine adsorbed, decreases as: MgVAPO-5 > VO_x/AL > VAPO-5 > VO_x/MG + AL > VO_x/MG. A similar trend between V-reducibility of the catalyst and its catalytic activity for the alkane conversion was observed. However, the selectivity to olefins depends on the acid-base character of catalyst and the alkane fed. In the ODH ofn-butane, the higher the acid character of the catalyst the lower the selectivity to C₄-olefins, while in the ODH of ethane an opposite trend between the catalyst acidity and the selectivity to ethene was observed.On leave from the Department of Industrial Chemistry and Materials, V. le Risorgimento 4, 40136 Bologna, Italy.