Mechanistic Insights into the Activity of Mo-Carbide Clusters for Methane Dehydrogenation and Carbon–Carbon Coupling Reactions To Form Ethylene in Methane Dehydroaromatization

Methane dehydrogenation and C–C coupling reactions to form ethylene on two different carbide clusters of molybdenum (Mo4C2 and Mo2C6) were studied. Density functional theory (DFT) calculations were performed to understand the reactivity of the two clusters, linking it to the overall methane dehydroaromatization (MDA) process. The electronic effect of catalyst reduction procedures and anchoring of the cluster on the zeolite framework was captured in simulations with varying positive charge on the cluster. In general, with one exception, DFT calculations suggested a reduction in dehydrogenation activation energies with more reduced (lesser positive charge) clusters. Similarly, activation barriers for the transfer of a H atom from the carbon to the neighboring Mo site were calculated to be lower on more reduced clusters. In contrast, the coupling reactions of the two CH3 and the two H atoms on the surface showed a reverse trend. The activation energies of the C–C and the H–H coupling steps were observed to b...