(508a) Unravelling Catalysis By Small Metal Carbide Clusters for Methane Dehydroaromatization

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 intermediates¹. 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 rates². A relatively new paradigm of accessing the reaction rates is promulgated which is incorporating an ensemble average representation of all active species of Mo_xCy.

Theoretical calculations suggest a key factor in deciding experimental protocols to control catalyst reactivity and stability. This involves monitoring the evolution of the molybdenum carbide species which dictates the anchoring of the carbide cluster on the zeolite support. 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 acidity³ 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 to achieve desired reactivity with a potential for higher catalyst stability.

References

1 T. S. Khan, S. Balyan, S. Mishra, K. K. Pant and M. A. Haider, J. Phys. Chem. C, 2018, 122, 11754–11764.

2 S. Balyan, S. Saini, T. S. Khan, K. K. Pant, P. Gupta, S. Bhattacharya and M. A. Haider, Nanoscale, 2020

3 S. Balyan, M. A. Haider, T. S. Khan and K. K. Pant, Catal. Sci. Technol., 2020, 10, 3857–3867.