In the current competitive scenario of looking out for alternate sources of fuel and chemical production, one of the potential alternates which are surfacing up is shale gas. MDA, the direct route for methane conversion is energy efficient as it eliminates the production of syngas and is a promising route to decrease the production cost of chemicals. The majority of studies highlight that Mo/HZSM-5 which is a promising catalyst in MDA shows a bifunctional mechanism. The activation of methane occurs on a metal carbide and it undergoes dimerization into ethylene/acetylene intermediate which gets aromatization to benzene over Brønsted acid sites (BAS) provided by the HZSM-5. A rational approach mechanism involves intermediate formation on Mo active species by bringing another methane molecule that is activated on the transition metal and undergoes C-C coupling with activated methane molecule earlier adsorbed to form intermediate. The conventional mechanism proposed in the literature did not consider carbon of Mo carbide to participate in the product formation. So, considering the dynamic nature of the active site in MDA will open a new path for the mechanism of the MDA reaction. In catalysis chemistry, the Mars Van Krevelen mechanism approach is encountered mostly and has been applied to many catalytic reactions and a similar mechanism has been extended in our study. Here in this work, by using density functional theory we are discussing how ethylene/acetylene intermediates are formed over stable Mo carbide active sites and reveal the reaction mechanism. As discussed earlier the catalyst is Mo/HZSM-5, where Mo is anchored over Al sites in the zeolite framework so the effect of support has also been studied. Reaction and activation energies over the Mo2C6 cluster were calculated using the generalized gradient approximation (GGA) revised Perdew–Burke–Ernzerhof (RPBE) based exchange-correlation functional. Two routes for C-C coupling on the stable Mo2C6 cluster have been explored. Incorporating the carbon from the active site results in a lower activation barrier for C-C coupling ~55 kJ/mol. Similar trends were observed for cluster anchored over 10 MR zeolite model.

Understanding methane activation and C-C coupling on molybdenum carbide anchored on ZSM-5 in methane dehydroaromatization (MDA) using DFT studies

ACS Fall 2021

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