Methane dehydroaromatization is mediated by surface carbon

Methane dehydroaromatization is mediated by surface carbon
Nikolay Kosinov, Emiel J.M. Hensen
Inorganic Materials Chemistry, Eindhoven University of Technology, The Netherlands
Non-oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by lack of understanding about the mechanism and nature of the active sites in benchmark zeolite-based Mo/ZSM-5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. Here by applying spectroscopy and microscopy, we demonstrate that the active centers in Mo/ZSM-5 are partially reduced, single-atom Mo sites stabilized by the zeolite framework. Further, by combining a pulse reaction technique with isotope labeling studies, we demonstrate that MDA is governed by a hydrocarbon pool mechanism, in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation. Although this mechanism shares some similarities with the well-understood chemistry of methanol-to-hydrocarbons process, MDA was found to proceed via a radical pathway and not the carbocation one.
Important challenges in gaining insight into the mechanistic aspects of MDA were the high reaction temperature at which the reaction takes place and its transient nature, which involves rapid activation and deactivation stages when the fresh Mo/ZSM-5 catalyst is exposed to a methane feed. These factors complicate operando spectroscopy and kinetic investigations. A novel aspect of this work is the increased temporal resolution obtained by periodically pulsing small amounts of the reactant over the catalyst (Fig. 1) and the use of recovered samples at different stages of the reaction for spectroscopy (XPS, MAS NMR, EPR, etc.) characterization and operando XAS spectroscopy performed under exact reaction conditions.
Figure 1. Schematic representation of catalyst samples applied in this study, featuring exclusively atomically dispersed Mo(VI) species stabilized in the zeolite pores for the 1%Mo sample and a mixture of the dispersed species and larger Mo-oxo clusters on the external surfaces of 2%Mo and 5%Mo catalysts (a). Per-pulse profiles of methane conversion (b), CO/H2 ratio (c), and benzene yield (d), recorded while pulsing 5 mL methane every 200 sec with 30 mL/min flow of Ar carrier.
References
• N. Kosinov, A. S. G. Wijpkema, E. Uslamin, R. Rohling, F. J. A. G. Coumans, B. Mezari, A. Parastaev, A. S. Poryvaev, M. V. Fedin, E. A. Pidko and E. J. M. Hensen, Angew. Chemie Int. Ed., 2018, 57, 1016–1020.
• N. Kosinov, E. A. Uslamin, F. J. A. G. Coumans, A. S. G. Wijpkema, R. Rohling and E. J. M. Hensen, ACS Catal., 2018, 8, 8459-8467.
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