Properties and deactivation of the active sites of an MoZSM-5 catalyst for methane dehydroaromatization: Electron microscopic and EPR studies

Зайковский, В. И.; Восмериков, А. В.; Anufrienko, V. F.; Коробицына, Л. Л.; Коденев, Е. Г.; Ечевский, Г. В.; Vasenin, N. T.; Журавков, С. П.; Матус, Е. В.; Ismagilov, Z. R.; Parmon, Valentin N.

doi:10.1134/s0023158406030104

Cited by 37 publications

(19 citation statements)

References 4 publications

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“…As such, these MoC 3 species are not stable and easily agglomerate with TOS. Ultimately, sintering of MoC 3 leads to the migration of the clusters to the outer zeolite surface and the formation of larger Mo 2 C‐like nanoparticles …”

Section: Figurementioning

confidence: 99%

Molybdenum Speciation and its Impact on Catalytic Activity during Methane Dehydroaromatization in Zeolite ZSM‐5 as Revealed by Operando X‐Ray Methods

et al. 2016

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Combined high-resolution fluorescence detection X-ray absorption near-edge spectroscopy, X-ray diffraction, and X-ray emission spectroscopyh ave been employed under operando conditions to obtain detailed new insight into the nature of the Mo species on zeoliteZ SM-5 during methane dehydroaromatization. The results showthat isolated Mo-oxo species present after calcination are converted by CH 4 into metastable MoC x O y species,w hich are primarily responsible for C 2 H x /C 3 H x formation. Further carburization leads to MoC 3 clusters,w hose presence coincides with benzene formation. Both sintering of MoC 3 and accumulation of large hydrocarbons on the external surface,e videnced by fluorescencelifetime imaging microscopy, are principally responsible for the decrease in catalytic performance.T hese results show the importance of controlling Mo speciation to achieve the desired product formation, which has important implications for realizing the impact of CH 4 as asource for platform chemicals.The increasing availability of cheap natural gas has attracted growing interest towards direct routes for the conversion of methane into high-value chemicals.[1] Catalytic routes that have been investigated include dehydroaromatization, oxidative coupling,and partial oxidation, but are currently not (yet) economically viable.[2] One of these routes,m ethane dehydroaromatization (MDA), is particularly promising for the direct conversion of CH 4 into aromatic compounds and H 2 using metal-exchanged zeolites such as Mo/H-ZSM-5, since it contains acid sites as well as Mo species possessing dehydrogenation and CÀCc oupling functionalities. [1][2][3] It is generally accepted that CH 4 is activated on the exchanged Mo species, forming C 2 H 4 .S ubsequently,C 2 H 4 reacts on the (remaining) Brønsted acid sites and is converted into aromatic compounds,a lso leading to coke formation by the consecutive reaction of aromatic derivatives with light olefins. [3c,4] Although active species are proposed to originate from either (MoO 2 ) 2+ monomers or (Mo 2 O 5 ) 2+ dimers, [3a,c, 5] there is also adebate as to whether the active sites are oxidic,carbidic (MoC x ), or oxycarbidic (MoC x O y )i nn ature. [3a,d, 4, 6] Recently, combined UV/Vis absorption and Raman spectroscopies and DFT calculations have shown the formation of monomeric species upon calcination, demonstrating that the debate over the active sites is still ongoing. [7] In addition, there is no clear understanding of the catalyst deactivation mode,c onsidered to be the main limitation for the commercialization of the process. [1,2] Herein, we present an operando time-resolved combined X-ray diffraction (XRD) and high energy resolution fluorescence detection (K a -detected) X-ray absorption near-edge spectroscopy (HERFD-XANES) study during the MDA reaction on Mo/H-ZSM-5. Thea dvantage of using these techniques in combination is that local structure information around the Mo ions can be considered alongside changes in long-range order,t hat is,t he zeolite fram...

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Section: Figurementioning

confidence: 99%

Molybdenum Speciation and its Impact on Catalytic Activity during Methane Dehydroaromatization in Zeolite ZSM‐5 as Revealed by Operando X‐Ray Methods

et al. 2016

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“…point out the instability of Mo carbide clusters with C/Mo ratio >1.5 inside the zeolite channels, considering them prone to migrate to the zeolite outer surface. In spite of the extensive investigations on the nature of the active Mo species, there is no consensus regarding the precise location and structure of active Mo sites,,,, and little is known about its impact on catalyst deactivation. Recent X‐ray emission studies carried out by our group, suggest that initial Mo‐oxo species are first transformed to MoC x O y at the early stages of reaction (also known as the induction period).…”

Section: Introductionmentioning

confidence: 99%

Determination of Molybdenum Species Evolution during Non‐Oxidative Dehydroaromatization of Methane and its Implications for Catalytic Performance

et al. 2018

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Mo/H‐ZSM‐5 has been studied using a combination of operando X‐ray absorption spectroscopy and High Resolution Powder Diffraction in order to study the evolution of Mo species and their location within the zeolite pores. The results indicate that after calcination the majority of the species present are isolated Mo‐oxo species, attached to the zeolite framework at the straight channels. During reaction, Mo is first partially carburized to intermediate MoCxOy species. At longer reaction times Mo fully carburizes detaching from the zeolite and aggregates forming initial Mo1.6C3 clusters; this is coincident with maximum benzene production. The Mo1.6C3 clusters are then observed to grow, predominantly on the outer zeolite surface and this appears to be the primary cause of catalyst deactivation. The deactivation is not only due to a decrease in the amount of active Mo surface but also due to a loss in shape‐selectivity which leads to an increased carbon deposition at the outer shell of the zeolite crystals and eventually to pore blockage.

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“…Differing from widely-held opinion about the active species for MDA being partially reduced molybdenum carbides, Zaikovskii et al [91,92] used HRTEM, EDXA, and EPR techniques, and concluded that methane is mainly acti-vated on oxidized molybdenum clusters located in the zeolite channels. They also concluded that Mo 2 C particles present on the external surface are deactivated in the early stages of the reaction due to graphite deposition.…”

Section: State and Location Of Mo In Zeolite Matrix And Active Mo Spmentioning

confidence: 99%

Recent progress in methane dehydroaromatization: From laboratory curiosities to promising technology

Guo

Zhao

et al. 2013

Journal of Energy Chemistry

217

183

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Properties and deactivation of the active sites of an MoZSM-5 catalyst for methane dehydroaromatization: Electron microscopic and EPR studies

Cited by 37 publications

References 4 publications

Molybdenum Speciation and its Impact on Catalytic Activity during Methane Dehydroaromatization in Zeolite ZSM‐5 as Revealed by Operando X‐Ray Methods

Molybdenum Speciation and its Impact on Catalytic Activity during Methane Dehydroaromatization in Zeolite ZSM‐5 as Revealed by Operando X‐Ray Methods

Determination of Molybdenum Species Evolution during Non‐Oxidative Dehydroaromatization of Methane and its Implications for Catalytic Performance

Recent progress in methane dehydroaromatization: From laboratory curiosities to promising technology

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