Ni–Nb–O catalysts for ethane oxidative dehydrogenation

Ni–Nb–O catalysts with various Nb contents (0–19%) have been studied as catalysts for the oxidative dehydrogenation of ethane. An optimum ethane conversion of 33% with 78% of selectivity to ethylene was obtained for a Ni–Nb–O catalyst containing 15% of Nb at 350 °C with a weight to flow W/F ratio of 0.54 g s mL−1. The catalysts have been characterized using various techniques allowing to show that the efficiency of the catalysts was related to a surface interaction between the NiO active phase and an amorphous Nb2O5 phase. Because ammonium oxalato niobate was used as niobium precursor, oxalate ions were present; concomitantly to the surface interaction described, the thermal decomposition of these oxalate ions induced a reduction in NiO oxygen over-stoichiometry which also influences the catalytic properties. It was demonstrated that only a small percent of Nb was necessary to have these effects and the optimal activity observed for 15% of Nb had to be related to another process which corresponds to the decrease in the active phase particle size induced by the amorphous Nb2O5 phase; this phase prevents the sintering of the active phase particles during the thermal transformation of the catalysts precursors. It was also shown that the thermal decomposition of oxalates limits NiO crystal growth. Above 15% of Nb the amorphous phase reacted with the nickel oxide to form an inactive mixed niobium and nickel oxide phase NiNb2O6. The study of the catalytic properties over long periods of time showed that the Ni–Nb–O catalysts underwent a slow but systematic deactivation. This deactivation has been attributed both to the continuous reduction of the NiO oxygen over-stoichiometry in the reaction conditions and to the formation of the NiNb2O6 phase thermodynamically stable at the catalytic reaction temperature. Attempt to regenerate the catalysts has failed.

Figure optionsDownload high-quality image (137 K)Download as PowerPoint slideResearch highlights▶ High catalytic properties of NiO based catalysts in the oxidative dehydrogenation of ethane are reported. ▶ Catalytic properties are related to the reduction of the surface over-stoichiometry of NiO. ▶ The reduction of the surface over-stoichiometry of NiO can be obtained by addition of Nb or oxalic acid. ▶ The study of the catalysts over long periods of time showed that they underwent a slow but systematic deactivation.