Aromatization of methane over zeolite supported molybdenum: active sites and reaction mechanism
Introduction
The direct catalytic conversion of methane into aromatics and hydrogen is an interesting route for upgrading natural gas. Recently, it has been shown that supported molybdenum, preferentially Mo supported on H-ZSM-5, are active and selective for benzene formation from methane [1], [2], [3], [4]. It was clearly established that at high temperature under CH4 molybdenum in Mo/HZSM-5 was reduced and formed Mo2C species [5] and further during the activation process of Mo/HZSM-5 molybdenum species migrated towards exchangeable sites in the zeolite framework [5], [6]. It is generally reported that the activation of methane occurs on molybdenum active species forming ethylene as primary product C2H4 being converted into benzene over H+ acid sites of the zeolite [1], [2], [3], [4], [5], [6]. In a very recent work it was shown by contrast that acetylene was the primary product of the reaction of methane over Mo/HZSM-5 [7], and the authors concluded that a possible route for the formation of benzene is the production of C2H2 on Mo sites, acetylene generating at high temperature benzene and naphthalene.
The aim of this work was to provide an integrated understanding on the nature of active sites operating during the aromatization of methane over Mo/H-ZSM-5 catalyst. The role of the zeolite will be examined by comparing H-ZSM-5 and H-MCM-22 having different topology. The role of protons in the zeolite, in which protons apparently improved the catalytic performances, will be examined, and a reaction mechanism proposed.
Section snippets
Catalysts
H-ZSM-5 zeolite (Si/Al=26 batch 1381 from Süd Chemie Germany) was impregnated with an aqueous solution of ammonium heptamolybdate. The sample was dried and then calcined in air at 773 K. The Mo loading was adjusted to 4 wt.%. In addition a series of Mo/H-ZSM-5 catalysts were prepared by the same procedure but using H-ZSM-5 zeolites with different Si/Al ratio synthesized in the laboratory. Mo loading was also 4 wt.%.
H-MCM-22 was synthesized as described in [8]. The structure of MCM-22 contains two
Results and discussion
When CH4 was reacted at 923 K on the oxidized Mo/HZSM-5 sample large amount of H2 evolved at the initial stage of the reaction. During this period almost no hydrocarbon was formed. Fig. 1 represents the conversion of methane, derived from H2 and hydrocarbons, with time on stream (TOS). The selectivities of the reaction to coke, benzene, naphthalene and ethylene are given in Fig. 2. Analysis of these two figures clearly indicates that at the early stage of the reaction while a significant amount
Conclusion
Mo loaded medium pore zeolites are active and selective for the conversion of methane to aromatics. At reaction temperature higher than 923 K the major product is benzene followed by naphthalene on 4 wt.% Mo loaded zeolites. The yields of aromatics formed on Mo/HZSM-5 and Mo/HMCM-22 were, respectively, 2 and 3.4% at 923 K. The higher efficiency of HMCM-22 support corresponds to a better dispersion of molybdenum and to a faster diffusion of molecules. The initial product of the methane reaction is
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Study on attrition of spherical-shaped Mo/HZSM-5 catalyst for methane dehydro-aromatization in a gas–solid fluidized bed
2021, Chinese Journal of Chemical EngineeringCarbidic Mo is the sole kinetically-relevant active site for catalytic methane dehydroaromatization on Mo/H-ZSM-5
2020, Journal of CatalysisCitation Excerpt :Ethylene aromatization rates at 923 K measured by Ha et al. [47] confirm C2H4 DHA is kinetically-controlled by Mo sites; benzene formation rates over H-ZSM-5 (25 μmol g−1 h−1) during C2H4 DHA sans methane feed are significantly lower than those on both Mo/H-ZSM-5 (97 μmol g−1 h−1) and Mo/SiO2 (50 μmol g−1 h−1), indicating Mo-mediated routes are more facile for aromatic formation, with or without Brønsted-acid sites. These reports [5,32,46,47], complemented by observed methane-ethane-ethylene equilibrium [12,31] and the analyses presented herein, unambiguously demonstrate that all rate-determining steps in methane DHA must occur after ethylene formation and be catalyzed by carbidic Mo sites, thereby falsifying acid-catalyzed olefin oligomerization (–enclosed route in Scheme 1) [10,14,25] and radical-mediated hydrocarbon pool catalysis [42–45] as kinetically-relevant routes of benzene formation. We surmise methane DHA is kinetically-controlled by acetylene formation or oligomerization catalyzed by Mo aggregates (C2H2-mediated route enclosed by in Scheme 1), plausibly in concert with equilibrated H+-catalyzed cyclization of unsaturated C2-4 intermediates; however, we cannot eliminate the possibility that benzene is formed via Mo-catalyzed C-C coupling of ethylene (direct route enclosed by in Scheme 1).
Supercritical solvothermal synthesis under reducing conditions to increase stability and durability of Mo/ZSM-5 catalysts in methane dehydroaromatization
2020, Applied Catalysis B: EnvironmentalNon-oxidative methane conversion in microwave-assisted structured reactors
2019, Chemical Engineering JournalCitation Excerpt :Although both coke types contribute to catalyst deactivation, the selectivity loss is mainly attributed to the soft coke, i.e. zeolite pore blocking, and the activity loss may be better explained with the hard coke on the Mo2C active sites. The actual strategies devoted to minimize coke deposits on the active sites focus on the improvement of metal dispersion [15,27–29]. The better the Mo dispersion, the lower clustering and coking probability.