(724g) Evolution and Control of Active Sites in Methane DHA: Insights from Ab Initio Simulations Guiding the Choice of Catalyst Precursor and Pre-Treatment Protocols

Direct conversion of methane to aromatics in non-oxidative conditions generally utilizes a bifunctional catalyst having active molybdenum species supported on zeolite (Mo/HZSM-5). In this reaction, the nature of the active site for methane activation on Mo is suggested to be a carbide (or an oxycarbide) and the catalyst is known to deactivate from carbon deposition over time due to the formation of polyaromatic hydrocarbons. Herein, density functional theory (DFT) calculations are utilized to study the catalytically active carbide (Mo_xCy) clusters in C-H bond activation and C-C coupling reaction to form C₂ intermediates. A massively parallel cascade genetic algorithm (cGA) is applied to scan the potential energy surface (PES) for all possible low-energy structures (including the global minimum) of Mo_xCy. Catalytic activity of the global minimum and metastable clusters are accessed and compared for methane activation, highlighting the role of metastable clusters influencing the overall reaction rates. The evolution of the molybdenum carbide species from the precursor oxide and oxycarbide forms, dictating the anchoring of the carbide cluster on the zeolite support, is considered the key aspect in deciding the extent of coke formation on the catalyst. The mechanistic insights obtained from DFT calculations are thus providing a molecular level engineering approach, wherein the catalyst is rationally synthesized to obtain desired active sites anchored on the zeolite support, which are stable in providing high aromatic selectivity and overall reactivity. This is implemented in experiments at three stages; a) by altering the Mo precursors used in the synthesis of Mo/HZSM-5 catalyst, b) treating the zeolite support with the boric acid to alter support acidity and on c) changing the carburizing condition from pure methane to a mixture of hydrocarbon gases (mimicking the natural gas). In all three cases, the evolution and control of the active molybdenum carbide species, anchored on the zeolite support, is highlighted as a molecular level toolbox for obtaining desired reactivity and product selectivity, with a potential to achieve higher catalyst stability.

References:

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