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

Ethane turnovers to ethylene in either oxidehydrogenation with diverse oxidants (O 2, CO 2, H 2 O) or dehydrogenation (without an oxidant) over two-dimensional MoO x dispersed on Al 2 O 3 catalyst occur via a generalized mechanistic framework encompassing ternary catalytic cycles of C 2 H 6 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 C 2 H 6 activation cycle occurs via the kinetically relevant C–H bond activation of C 2 H 6 on lattice oxygen of MoO x, forming C 2 H 4 . 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, O 2 is the most effective, as it removes coke effectively, leading to essentially no rate decay. CO 2 and H 2 O are alternate soft oxidants, and their use prevents the overoxidation of ethylene, thus resulting in higher ethylene selectivity (80–85%) than using O 2 . CO 2 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 O 2 oxidant. H 2 O 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 C 2 H 6 turnovers. The rates of the C 2 H 6 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 C 2 H 6 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 C 2 H 6 catalysis, irrespective of the chemical identity of the oxidant.

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