Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM‐5

Abstract

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 a 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. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be 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.

Implications of the Molybdenum Coordination Environment in MFI Zeolites on Methane Dehydroaromatisation Performance

“…These results are in agreement with Kosinov et al . who proposed that the conversion of methane to benzene might take place in a monofunctional manner on Mo carbides in the zeolite micropores, or mediated by carbonaceous species . Such mechanisms open the possibility to optimise active MDA catalysts based on non‐acidic zeolites.…”

“…[20] The same group has recently highlighted the involvement of confined carbon in the reaction mechanism instead. [21,22] Our previous synchrotron-based operando studies on Mo/H-ZSM-5 evidenced the relevance of Mo speciation in MDA product distribution. X-ray emission showed the selectivity to different hydrocarbon products to be dependent on Mo structure (i. e. partially carburised species were observed during the induction period when light hydrocarbons were produced, whereas fully carburised sites were selective to aromatics).…”

“…for Mo‐containing Silicalite‐1, the pure siliceous analogue of H‐ZSM‐5 zeolite, evidenced the formation of aromatics over this catalyst material, leading to the proposal that Brønsted acid sites may not be essential for the aromatisation of methane . The same group has recently highlighted the involvement of confined carbon in the reaction mechanism instead …”

Absorptive Hydrogen Scavenging for Enhanced Aromatics Yield During Non‐oxidative Methane Dehydroaromatization on Mo/H‐ZSM‐5 Catalysts

“…Mo/H-ZSM-5 ( Figure 1aii) showed anew band at 970 cm À1 as compared to MoO 3 and H-ZSM-5 physical mixture (Figure 1ai)a nd 973 Ka ir-treated H-ZSM-5 (Figure 1aiii). [3,12,20,21] Thei nduction (carburization) period and initial increase and subsequent reduction in aromatic formation rates with TOS( Figure 1e)i ss uggestive of ab ifunctional catalytic mechanism wherein CÀHb onds in CH 4 are activated on MoC x clusters formed during carburization to form C 2 H x species which subsequently undergo oligomerization/hydrogen transfer on residual Brønsted acid sites within zeolite channels to form aromatics.P revious reports showed that unsupported molybdenum carbide catalyzed CH 4 and C 2 H 6 conversion into C 2 products and ethylene,r espectively,b ut did not yield aromatics,w hereas MoC x /H-ZSM-5 catalyzed C 2 H 6 conversion into benzene,s uggesting that both carbidic MoC x sites and residual Brønsted acid sites are involved in CH 4 DHA. [3,12,20,21] Thei nduction (carburization) period and initial increase and subsequent reduction in aromatic formation rates with TOS( Figure 1e)i ss uggestive of ab ifunctional catalytic mechanism wherein CÀHb onds in CH 4 are activated on MoC x clusters formed during carburization to form C 2 H x species which subsequently undergo oligomerization/hydrogen transfer on residual Brønsted acid sites within zeolite channels to form aromatics.P revious reports showed that unsupported molybdenum carbide catalyzed CH 4 and C 2 H 6 conversion into C 2 products and ethylene,r espectively,b ut did not yield aromatics,w hereas MoC x /H-ZSM-5 catalyzed C 2 H 6 conversion into benzene,s uggesting that both carbidic MoC x sites and residual Brønsted acid sites are involved in CH 4 DHA.…”

“…[3][4][5][6][7][8][9] Although the stoichiometry and coordination of MoC x clusters is still atopic of debate,the proficiency and reduced nature of active Mo-centers is no longer debated. [8,[10][11][12][13][14] Four noteworthy recent publications examine evolution and speciation of MoC x species in CH 4 DHA, [10,12,15,16] however, we make no effort to probe identity of MoC x species in this study and instead report in situ absorptive-hydrogen removal as as trategy to overcome thermodynamic limitations that limit single-pass conversion in CH 4 DHA to about 10 %. Arecent report proposes that addition of Zr metal as ah ydrogen absorber to MoO x /H-ZSM-5 formulations leads to increased CH 4 conversion and aromatic yields.…”