Ab Initio Simulations and Experimental Investigations for Understanding the Evolution and Control of Mo Active Sites in Methane Dehydroaormatization Venue: May 18, 2020, at 3.00 P.M via
Abstract:
Single step methane conversion to value added product is an attempt to economically exploit the unlocked resources of the low cost stranded natural gas reservoirs. A bifunctional catalysthaving active molybdenum species supported on zeolite (Mo/HZSM-5) is well-studied for high aromatic selectivity in non-oxidative conversion of methane. However the catalyst is known to deactivate from carbon deposition over time due to the formation of polyaromatic hydrocarbons. One strategy to reduce carbon deposition is suggested via a molecular level engineering approach where active molybdenum carbide species anchored on the inner side of the zeolite framework are stabilized. 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 methane dehydroaromatization (MDA) process.
A theoretical finding on the evolution and anchoring of active molybdenum carbide species on the zeolite surface is providing a rational for implementing desirable experimental conditions for stabilizing the active species. This is achieved on tailoring the carburizing agent to convert the precursor molybdenum oxide species, so as to obtain higher amount of active carbide forms of the molybdenum. In order to improve catalyst stability and product selectivity, an attempt is made to stabilize these primary molybdenum oxide/carbide species over the zeolite after subsequent thermal and chemical treatment. Following which, a strategy is proposed to thermally treat the molybdenum precursors and convert orthorhombic α-MoO3 via hydrothermal topotactic transformation to metastable hexagonal h-MoO3. In this work, the role of Mo precursor interaction of with the zeolite in altering catalyst reactivity is studied. Furthermore, in order to improve upon catalyst stability and achieve higher aromatic (benzene) selectivity we propose here a strategy to control and moderate the BrØnsted acidity of the zeolite framework, which could possibly reduce the migration and agglomeration of active molybdenum carbide species and the formation of polyaromatic hydrocarbons. This is indirectly attempted by
pretreating the zeolite using boric acid. Instead of replacing the entire Al in HZSM-5 by preparing an isomorphous zeolite, herein an attempt is made to moderate the BrØnsted acidity of the HZSM-5 zeolite by pretreating it with boric acid. This is done with an aim to maintain high benzene selectivity. Overall the dissertation is elucidating the importance of developing a molecular level insights of the catalyst active site. In turn, this is providing a guiding tool for rational catalyst design by specifying the choice of Mo precursor, carburizing condition and zeolite treatment to achieve higher catalyst activity.