Influence of the Nature of the Promoter in NiO Catalysts on the Selectivity to Olefin During the Oxidative Dehydrogenation of Propane and Ethane

A comparative study of the catalytic properties for the oxidation of C-2-C(3)alkanes and olefins has been carried out over unpromoted and M-promoted NiO catalysts (Me = K, La, Ce, al, Zr, Sn, Nb). The catalysts have been characterized by several physico-chemical techniques (UV Raman, Visible Raman, FTIR of adsorbed CO and XPS). The characteristics of promoter elements are of paramount importance, since they are able to modify both the nature of the active nickel and the concentration of electrophilic O-2(-)/O(-)oxygen species. Thus, a relatively high acidity and valence of the promoter oxide (with oxidation state higher than + 3) are necessary to achieve high selectivity to olefins during the oxidative dehydrogenation (ODH) of C-2-C(3)alkanes. In addition, an inverse correlation between the selectivity to the corresponding olefin and the concentration of electrophilic oxygen species has been observed, although the selectivity to propene during propane ODH is lower than the selectivity to ethylene achieved during ethane ODH. On the other hand, a very low influence of alkane conversion on the selectivity to the corresponding olefins is observed. This behaviour can be explained by considering that the reaction rate for olefin combustion is lower than the reaction rate for alkane oxidation. However, the comparative study of the oxidation of alkanes and olefins suggest that the differences observed between the ODH of propane and ethane are not related to the reactivity of olefins, but to the different number and reactivity of C-H bonds in both alkanes. A discussion on the importance of the concentration of active sites and the characteristics of the alkanes fed on the selectivity to olefin during the alkane ODH is also presented.