Ni–Me–O mixed metal oxides for the effective oxidative dehydrogenation of ethane to ethylene – Effect of promoting metal Me

This work examines the effect of Li, Mg, Al, Ga, Ti, Nb and Ta on the properties and catalytic behavior of Ni-based mixed metal oxides in the ethane oxidative dehydrogenation reaction. In all mixed oxides, with the exception of Ta, the dopants modify the NiO lattice, which contracts or expands depending on the radius of the cation, indication for the formation of solid solutions. This systematic study of doping metals varying from low (+1) to high valence (+5) elements clearly demonstrates the large effect of the dopants’ valence. O2-TPD experiments showed that increasing the valence of the foreign cation reduces the non-stoichiometric oxygen due to the cation deficient nature of the host oxide, in complete agreement with the principle of controlled valence. Temperature-programmed isotopic oxygen exchange measurements portray furthermore that not only the quantity but also the lability of the oxygen species reduces with increasing dopant valence. These electrophilic oxygen species catalyze the total oxidation reactions of ethane to COx, and thus their reduction/elimination favors the selective oxidation to ethylene. Ni–Nb–O exhibited the best catalytic performance with a 46% ethene yield at 400 °C, while Ni–Li–O demonstrated the worst results with a respective yield of 8.42%. Based on the results of the present work, it can be inferred that the catalytic properties of NiO in ethane ODH can be tuned with doping, based on the nature of the dopant in Me-promoted NiO catalysts. Depending on the valence of the foreign species, the dopants can increase or decrease the unselective electrophilic oxygen radicals of NiO, leading to, respectively, reduced or enhanced ethane ODH activity.