Understanding the Preparation and Reactivity of Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

Abstract

Methane dehydroaromatization is a promising reaction for the direct conversion of methane to liquid hydrocarbons. The active sites and the mechanism of this reaction remain controversial. This work is focused on the operando X-ray absorption near edge structure spectroscopy analysis of conventional Mo/ZSM-5 catalysts during their whole lifetime. Complemented by other characterization techniques, we derived spectroscopic descriptors of molybdenum precursor decomposition and its exchange with zeolite Brønsted acid sites. We found that the reduction of Mospecies proceeds in two steps and the active sites are of similar nature, regardless of the Mo content. Furthermore, the ZSM-5 unit cell contracts at the beginning of the reaction, which coincides with benzene formation and it is likely related to the formation of hydrocarbon pool intermediates. Finally, although reductive regeneration of used catalysts via methanation is less effective as compared to combustion of coke, it does not affect the structure of the catalysts.

MoO3 Nanobelt-Modified HMCM-49 Zeolite with Enhanced Dispersion of Mo Species and Catalytic Performance for Methane Dehydro-Aromatization

“…XRD, FT-IR, N2 As shown in Figure 14, the weight loss below 220 • C is due to the evaporation of water. The weight loss between 220 and 400 • C is caused by the aromatics of physical absorption, and it is noteworthy that the carbon deposits on zeolite are burned and caused weight loss in the range of 400 to 800 • C [53]. Benzene is mainly formed in the channels of the zeolite, whereas naphthalene is produced on the external surface and/or the pore mouth of the zeolite [1,54].…”

Catalytic Routes for Direct Methane Conversion to Hydrocarbons and Hydrogen: Current State and Opportunities

“…In comparison, the coke removal under a reductive atmosphere occurs at higher temperatures (700–800 °C) but recarburization of the catalyst is not needed. ,,, Regeneration under H 2 partially removes the coke by methanation while allowing to recover part of the benzene yield and methane conversion. Contrarily to oxidative treatment, the H 2 coke-removal did not cause irreversible damage to the catalyst .…”

“…In comparison, the coke removal under a reductive atmosphere occurs at higher temperatures (700–800 °C) but recarburization of the catalyst is not needed. ,,, Regeneration under H 2 partially removes the coke by methanation while allowing to recover part of the benzene yield and methane conversion. Contrarily to oxidative treatment, the H 2 coke-removal did not cause irreversible damage to the catalyst . Indeed, molybdenum was preserved at a reduced state and no water was formed, while in the case of oxidative treatment, water from the coke combustion caused dealumination. , Despite these considerations, a reductive treatment is less effective than the low-temperature oxidative treatment in terms of catalyst deactivation after several MDA/regeneration cycles …”

“…Contrarily to oxidative treatment, the H 2 coke-removal did not cause irreversible damage to the catalyst. 258 Indeed, molybdenum was preserved at a reduced state and no water was formed, while in the case of oxidative treatment, water from the coke combustion caused dealumination. 179,260 Despite these considerations, a reductive treatment is less effective than the low-temperature oxidative treatment in terms of catalyst deactivation after several MDA/ regeneration cycles.…”

“…The coke as well molybdenum carbides to molybdenum oxides oxidations occur around 400−450 °C while above 500 °C, molybdenum oxides become volatile and migrate, leading to material damage through MoO x sintering or formation of extra-framework Al 2 (MoO 4 ) 3 species 64,153,179 that depend on the molybdenum content in the catalysts. 202,258 Different strategies for regeneration of MDA catalyst have been evaluated in order to remove efficiently the coke while minimizing structural damage of the catalyst structure. One strategy is based on burning the coke at lower temperatures.…”