New results on the oxidative dehydrogenation of ethane to ethylene

Abstract

Synergetic effects were observed in the oxidative dehydrogenation of ethane to ethylene, when Mo-V-O or Ni-V-O catalysts are mechanically mixed with α-Sb₂O₄. Synergy was observed on the conversion, the yield and the selectivity in ethylene. XRD and XPS analyses showed that a chemical reaction between these oxides occurred during the reaction. The role of this contamination was evaluated taking into account results presented in the literature (namely, formation of Mo-Sb-O and Ni₂Sb₂O₆) and experimentally, by preparing V-Sb-O and mixing it with Ni-V-O. It is concluded that the formation of a Mo-Sb-O phase is excluded and that contamination induced by V-Sb-O alone cannot explain the synergetic effect observed. The more plausible explanation of the synergy should be the existence of a cooperation between Mo-V-O or Ni-V-O with α-Sb₂O₄. It is proposed that the role of added α-Sb₂O₄ (and Ni₂Sb₂O₆, if formed) would be to reoxidize the surface of Mo-V-O and Ni-V-O during the reaction, thanks to a spillover of oxygen.

Introduction

Catalytic oxidative dehydrogenation is believed to become an important industrial technology for the effective utilization of alkanes. Many catalytic systems have been proposed so far ([1], [2], [3] and references therein) for oxidative dehydrogenation of ethane to ethylene. These can be classified into two groups, active at temperatures higher or lower than 600°C. The former group consists of basic oxides of alkaline, alkaline earth, and/or rare earth metals. The latter is composed of oxides of the group V and/or VI metals. Most of the relatively effective catalysts of the latter group contain several phases. A representative is a Mo-V-Nb mixed oxide catalyst which can give 37% ethylene yield at 350°C. The effect of adding various metals other than Nb (Sb, Ta, Si, Fe, W, Sn) to the Mo-V system has also been reported [4]. The addition of Nb to Mo-V oxide increases the selectivity to ethylene by about 20%. It was suggested that Nb had two roles, i.e., to eliminate total oxidation sites from MoO₃ or V₂O₅ and to form new mixed phases containing selective sites [5]. The Ni-V-Sb mixed oxide was also reported to be superior to Ni-V-O or Sb-V-O oxides. It was suggested that a cooperation between oxide phases (Ni₃V₂O₈, SbVO₄, NiSb₂O₆) and the absence of NiO and V₂O₅ contributed to the elevation of the selectivity [6]. We have previously demonstrated that synergetic effects were due to the concurrence of a remote control mechanism [7], [8], [9].

The objective of the present work is to contribute to the understanding of the catalytic performances of the oxide system reported in the literature. More specifically, we investigated the possibility that a cooperation between different phases does operate in the Mo-V-O and Ni-V-Sb catalysts and could explain literature results concerning Mo-V-Sb [4] and Ni-V-Sb [6] catalysts. We applied the following strategies: the starting hypothesis was that the Mo-V-O and Ni-V-O oxides were acceptors of spillover oxygen, and α-Sb₂O₄, a donor. We did not attempt to use pure Mo-V-O or Ni-V-O phases, but we used mixed oxide of the two metals, namely potentially bi- or multiphasic materials. These designated, respectively, Mo-V-O or Ni-V-O, were mixed mechanically with α-Sb₂O₄. The catalytic tests of each individual material and those of the mixtures were carried out in exactly the same conditions. Solid state modifications of the catalysts during the catalytic tests were detected thanks to characterization before and after the test, principally by XRD and XPS. The role of a possible chemical contamination which could occur due to (surface) reaction between the catalyst components in the Ni-V-O+α-Sb₂O₄ mixtures was evaluated by preparing a likely candidate product, namely V-Sb-O, and mixing it with Ni-V-O. We report on the synergetic effects introduced by mechanically mixing α-Sb₂O₄ with Mo-V-O or Ni-V-O catalysts. We propose a plausible explanation of the corresponding phenomena.