Understanding Catalysis by Molybdenum Carbide Nanoclusters in Mo/HZSM-5 Catalyst for Methane Dehydroaromatization
Date and Time: July 23rd, 2021 (Friday) at 9.30 AM.
Through video conferencing.
Abstract
Single step methane conversion to value added product is an economical route to exploit natural gas reservoirs from industrial perspective. A bifunctional catalyst having active molybdenum species supported on zeolite (Mo/HZSM-5) is well-studied for high aromatic selectivity in methane dehydroaromatization (MDA) process. However, the catalyst is known to deactivate due to carbon deposition over time-on-stream. The stabilization of active molybdenum carbide (Mo2C6) species anchored on the inner side of the zeolite framework can reduce carbon deposition. For this purpose, a massively parallel cascade genetic algorithm (cGA) search is employed to scan the potential energy surface for determining all possible low-energy structures of active Mo2C6 species. This search is further combined with density functional theory (DFT) calculations to study catalytic activity of the most stable and metastable Mo2C6 clusters for C-H bond activation and C-C coupling reactions to form ethylene and acetylene in MDA process.
Theoretical findings on the evolution and anchoring of active Mo2C6 species on HZSM-5 provides a rational for implementing desirable experimental conditions to improve the stability of the active species. This is achieved by obtaining a higher amount of Mo carbide by tailoring the carburizing agent. A strategy to stabilize these primary molybdenum oxide/carbide species over the zeolite after subsequent thermal and chemical treatment is proposed. It is achieved by thermal treatment of molybdenum precursors and conversion of orthorhombic α-MoO3 to metastable hexagonal h-MoO3 via hydrothermal topotactic transformation. In this work, the role of Mo precursor interaction with the zeolite in altering catalyst reactivity is studied.
In order to achieve higher aromatic (benzene) selectivity the BrØnsted acidity of HZSM-5 zeolite is moderated by pretreating it with boric acid. Moreover, this novel strategy of controlling the BrØnsted acidity of HZSM-5 zeolite enhances catalyst stability in MDA by reducing the agglomeration of active Mo2C6 species and the carbon deposition. Overall the dissertation provides a molecular level insights of active catalyst site of the reaction. In turn, this study provides a guiding tool for rational catalyst design by specifying the choice of Mo precursor, carburizing condition and zeolite treatment for MDA process.