Generalized Mechanistic Framework for Ethane Dehydrogenation and Oxidative Dehydrogenation on Molybdenum Oxide Catalysts

Ethane turnovers to ethylene in either oxidehydrogenation with diverse oxidants (O2, CO2, H2O) or dehydrogenation (without an oxidant) over two-dimensional MoOx dispersed on Al2O3 catalyst occur via a generalized mechanistic framework encompassing ternary catalytic cycles of C2H6 activation, oxidant activation, and carbon removal. This is confirmed from rate assessments, detailed kinetic analysis accounting for active site loss, isotopic tracer studies, and detailed spectroscopic characterization. Irrespective of the oxidant’s chemical identity, the C2H6 activation cycle occurs via the kinetically relevant C–H bond activation of C2H6 on lattice oxygen of MoOx, forming C2H4. The concomitant oxidant activation cycle either replenishes the oxygen vacancies or generates reactive oxygen species, which scavenge unwanted carbonaceous debris deposited on catalyst surfaces at contents dictated by the chemical identity of the oxidant and oxygen chemical potential that it exerts. Among the oxidants, O2 is the most effective, as it removes coke effectively, leading to essentially no rate decay. CO2 and H2O are alternate soft oxidants, and their use prevents the overoxidation of ethylene, thus resulting in higher ethylene selectivity (80–85%) than using O2. CO2 activation is, however, severely restricted kinetically, as evidenced from the reverse water–gas shift reaction that is far away from chemical equilibrium; thus, the generation of reactive oxygen species and their ability to concomitantly oxidize coke are much less effective than those with O2 oxidant. H2O dissociation is rapid and quasi-equilibrated, but its activation converts a portion of the active lattice oxygen to hydroxy species, reducing the active oxygen centers and lowering C2H6 turnovers. The rates of the C2H6 activation cycle dictate the intrinsic rates, whereas those of the concomitant oxidant activation and carbon removal cycles dictate the surface lattice oxygen density available for catalysis and in turn the extent of rate decay. Consolidating these findings within a generalized mechanistic framework leads to a universal rate expression containing two terms, one accounting for an intrinsic C2H6 activation rate and a second one for the time-dependent rate decay. This universal rate expression captures the kinetic properties of early transition-metal oxides in C2H6 catalysis, irrespective of the chemical identity of the oxidant.