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 mi xed oxide, and V-containing microporous materials (VAPO-5 and MgVAPO-5 ) have been tested in the oxidative dehydrogenation of C-2-C-4 alkanes . In all cases, tetrahedral vanadium species (isolated and/or associat ed) were mainly observed from V-51-NMR and diffuse reflectance spectro scopies. The reducibility of V5+-species, determined from the onset-re duction temperature, decreases as follows: VOx/AL > VAPO-5 > MgVAPO-5 = VOx/MG > VOx/MG + AL. The acid character of catalysts, determined fr om the FTIR spectra of pyridine adsorbed, decreases as: MgVAPO-5 > VOx /AL > VAPO-5 > VOx/MG + AL > VOx/MG. A similar trend between V-reducib ility of the catalyst and its catalytic activity for the alkane conver sion was observed. However, the selectivity to olefins depends on the acid-base character of catalyst and the alkane fed. In the ODH of n-bu tane, the higher the acid character of the catalyst the lower the sele ctivity to C-4-olefins, while in the ODH of ethane an opposite trend b etween the catalyst acidity and the selectivity to ethene was observed .