Effects of O₂Concentration on the Rate and Selectivity in Oxidative Dehydrogenation of Ethane Catalyzed by Vanadium Oxide:  Implications for O₂Staging and Membrane Reactors

Staged-O2 introduction and the effects of O2 concentration on primary and secondary reactions were examined during oxidative dehydrogenation on V2O5/γ-Al2O3 containing predominately isolated monovanadates. Cofeed and staged-O2 introduction modes led to similar ethane dehydrogenation and combustion rates, despite significant differences in the average O2 concentrations, as expected from their zero-order O2 dependences. The rate of ethene conversion to COx, however, was lower when O2 coreactants were introduced gradually as ethane conversion increased. These effects reflect inhibition of homogeneous ethene combustion pathways, which in contrast with their heterogeneous counterparts show a positive dependence in O2. Axial O2 distribution using multiple injectors or membranes will therefore influence alkene yields only by decreasing homogeneous alkene oxidation rates. Homogeneous contributions are much smaller in large reactors, because catalyst-to-volume ratios are greater than those in laboratory reactors. Other oxidation reactions occurring via redox cycles with lattice oxygens as the most abundant intermediates are expected to exhibit a response similar to that of O2 staging.