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 CC 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 be lower on less reduced (higher positive charge) clusters. On comparing the two (Mo4C2 and Mo2C6) clusters with similar charges, the activation energies for the first methane dehydrogenation were observed to be of similar value on both clusters for the neutral charge. However, second methane dehydrogenation was calculated to show a significantly higher barrier on the Mo2C6 cluster for both neutral and +1 charges. In addition, the CH3 coupling reaction was facilitated with a relatively lower activation barrier on the Mo2C6 cluster as compared to that on the Mo4C2 cluster. Thus, Mo2C6 sites in the vicinity of the Bronsted acid sites of the zeolite are likely to be more active for the coupling of the two CH3 species and helpful in MDA. This alluded to the operando experimental findings by Lezcano-Gonzalez et al. [Angew. Chem., Int. Ed. 2016, 55, 5215-5219], wherein it was suggested that methane might be activated on carbide and oxycarbide species; however, formation of MoC3-type species on stream was linked directly to MDA.