Investigation of the Effect of the Third Cation M (M = Mg, Al, Mn, and Fe) on the Properties and Catalytic Behavior in Ethane Oxidative Dehydrogenation of M-NiNbO Mixed Oxides

This work examines the effect of incorporating a third cation M (M = Mg, Al, Mn, or Fe) into Nb-promoted NiO catalysts (Ni0.85Nb0.15O) on their physicochemical properties and catalytic performance in the oxidative dehydrogenation (ODH) of ethane into ethylene. Therefore, a series of mixed oxides with the composition Ni0.765Nb0.135M0.1O was synthesized, characterized, and analyzed for catalytic behavior. The addition of the third cation markedly modified the redox and semiconductive properties of the catalysts, thus affecting their ODH performance. The selectivity toward ethylene was strongly dependent on the cation modifier, as it modulated the density of nonselective sites on the catalyst surface. In the low-temperature region, Mn-NiNbO demonstrated the highest conversion rates but with the lowest ethylene selectivity, whereas Fe-NiNbO, despite being less active than the undoped NiNbO, exhibited the highest ethylene selectivity. Overall, undoped NiNbO emerged as the best catalyst in terms of both activity and selectivity at isoconversion. All catalysts underwent partial reduction under reaction conditions, with the degree of reduction inversely correlated with the specific reaction rates. Although none of the catalysts remained stable at 400 °C, Al-NiNbO showed the least deactivation over time. The deactivation was linked to a reduction in p-type conductivity and a loss of redox functionality during operation.