(257b) Dynamic Study of the Evolution of Metal Species in ZSM-5 during Activation and Reaction in Direct Methane Dehydroaromatization

Natural gas, mainly composed of methane, constitutes an available and cheap resource that can be used as a building block to produce chemical commodities. Methane dehydroaromatization (MDA) is a reaction capable of directly converting methane to value-added aromatics, without an intermediate syngas step. The reaction happens in non-oxidative conditions, producing mainly benzene and hydrogen, 6 CH₄(g) → C₆H₆(g) + 9H₂(g). Zeolite-supported Mo catalysts have so far been the most widely studied catalysts in MDA, but they do not fulfill the conversion and stability requirements for commercialization. During the reaction induction period, Mo oxide species gradually reduce to Mo carbides, which are responsible for methane activation and subsequent conversion to aromatics. Our work focuses on developing strategies to improve benzene yield and catalyst stability by (1) controlling the activation of the Mo species to optimize their reduction and dispersion before exposure to harsh reaction conditions, (2) adding a second transition metal (Fe, Co, or Ni) as a promoter.

Our results indicate that when activation of catalysts is performed by reduction in pure hydrogen under temperature-controlled conditions, the carbides formed (ex situ) lead to more selective catalysts that deactivate more slowly compared to carbides formed during reaction (in situ). To explain this difference, we studied the mechanism of formation of the carbide species under the different activation conditions by temperature programmed reduction and carburization experiments where the formation of H₂O and CO_x was monitored with mass spectrometry. The samples obtained at different stages of reduction were characterized by XRD, XPS, microscopy, and in situ X-ray absorption. Activity data of Mo-X (X = Fe, Co, Ni) catalysts suggest that precarburization leads to a synergy between Mo and X for optimum loadings of each metal with higher and more stable benzene yields due to a higher dispersion of the metal species.