Direct Evidence on the Mechanism of Methane Conversion under Non‐oxidative Conditions over Iron‐modified Silica: The Role of Propargyl Radicals Unveiled

Puente‐Urbina, Allen; Pan, Zeyou; Paunović, Vladimir; Šot, Petr; Hemberger, Patrick; Bokhoven, Jeroen A. van

doi:10.1002/anie.202107553

Cited by 29 publications

(23 citation statements)

References 48 publications

(61 reference statements)

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“…It offers us an advantage in detecting elusive reaction intermediates sensitively and selectively because of its ability to soft-ionize the effluent gas beam (i.e., consists of unreacted reactants, in situ formed reaction intermediates, and desired products) from the reactor into the high vacuum. Owing to its sophisticated nature, this technique could spontaneously detect (gas-phase) carbon-based organic reaction intermediates with a short-lifetime, which could not be affordable by conventional methods (e.g., GC-MS) . Therefore, PEPICO recently emerged as a powerful technique for the identification of versatile and highly reactive (gas-phase) reaction intermediate (Figure d), specifically (isomer-specific) radical and ketene, in heterogeneous thermal catalysis, including catalytic pyrolysis, ,, combustion studies, , and oxyhalogenation of methane. ,,, Moreover, identifying species in an (isomer-)selective way is an additional benefit of this technique to investigate heterogeneously catalyzed reactions, both from the perspective of reaction mechanisms and kinetic analysis .…”

Section: Radical Chemistry In Zeolite Catalysismentioning

confidence: 99%

“…However, a DFT-based mechanistic analysis from Vlachos et al in 2021 suggested that ethylene formation on isolated iron sites of Fe@SiO 2 catalyst is much more favorable than gas phase dehydrocoupling of methyl radicals desorbed from iron sites . On the other side, a newly published research study from van Bokhoven, Hemberger, and colleagues presents strong spectroscopic evidence for the role of radical-mediated gas-phase reactions during high-temperature (945–1400 °C) conversion of methane into olefins and aromatics over iron modified silica catalysts . In this study, they investigated the gas-phase products at the reactor outlet using imaging photoelectron photoioncoincidence (iPEPICO) spectroscopy thanks to synchrotron radiation.…”

Section: Radical Chemistry In Zeolite Catalysismentioning

confidence: 99%

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First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Gong

Çağlayan

et al. 2022

Chem. Rev.

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Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical “technical bottleneck” that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such “short-lived and highly reactive” intermediates at the interface (gas–solid/solid–liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the “circular carbon economy” initiatives.

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Section: Radical Chemistry In Zeolite Catalysismentioning

confidence: 99%

Section: Radical Chemistry In Zeolite Catalysismentioning

confidence: 99%

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Gong

Çağlayan

et al. 2022

Chem. Rev.

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“…Such bimolecular reactions are thought to play an important role in PAH formation in the interstellar medium (ISM), because radical–neutral association processes can often proceed without an entrance barrier and are generally exothermic, making them viable at ultralow temperatures. SiC microreactors can be applied to study desorbed intermediates as well as gas-phase reaction steps in heterogeneous catalysis, such as C–H activation via oxyhalogenation − of alkanes or in methane coupling …”

Section: Introductionmentioning

confidence: 99%

“…SiC microreactors can be applied to study desorbed intermediates as well as gas-phase reaction steps in heterogeneous catalysis, such as C−H activation via oxyhalogenation 62−64 of alkanes or in methane coupling. 65 When investigating uni-, bimolecular or heterogeneous catalytic reactions, questions arise about the residence time, pressure, and temperature profile of the gaseous sample in the microreactor. Because of the small size (1 mm inner, 2 mm outer diameter; 35−50 mm length) of the reactor, these reaction conditions are difficult to measure directly but can be obtained via computational fluid dynamics (CFD) calculations.…”

Section: Introductionmentioning

confidence: 99%

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Hemberger

Pan

et al. 2022

J. Phys. Chem. A

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Resistively heated silicon carbide microreactors are widely applied as continuous sources to selectively prepare elusive and reactive intermediates with astrochemical, catalytic, or combustion relevance to measure their photoelectron spectrum. These reactors also provide deep mechanistic insights into uni-and bimolecular chemistry. However, the sampling conditions and effects have not been fully characterized. We use cation velocity map imaging to measure the velocity distribution of the molecular beam signal and to quantify the scattered, rethermalized background sample. Although translational cooling is efficient in the adiabatic expansion from the reactor, the breakdown diagrams of methane and chlorobenzene confirm that the molecular beam component exhibits a rovibrational temperature comparable with that of the reactor. Thus, rovibrational cooling is practically absent in the expansion from the microreactor. The high rovibrational temperature also affects the threshold photoelectron spectrum of both benzene and the allyl radical in the molecular beam, but to different degrees. While the extreme broadening of the benzene TPES suggests a complex ionization mechanism, the allyl TPES is in fact consistent with an internal temperature close to that of the reactor. The background, room-temperature spectra of both are superbly reproduced by Franck−Condon simulations at 300 K. On the one hand, this leads us to suggest that room-temperature reference spectra should be used in species identification. On the other hand, analysis of the allyl iodide pyrolysis data shows that iodine atoms often recombine to form molecular iodine on the chamber surfaces. Such sampling effects may distort the chemical composition of the scattered background with respect to the molecular beam signal emanating directly from the reactor. This must be considered in quantitative analyses and kinetic modeling.

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“…in the range of 800-1100°C, suggest the involvement of radical pathways in the gas phase and the catalyst activity is often ascribed to methyl radical formation which then form C2 products downstream of the catalyst bed. 17,18,19 Previous reports addressing the involvement of carbidic carbon in metal carbide catalysts 20,21 prompted us to investigate the role of carbon closer in bulk Mo and W carbides (Mo2C and WC) by applying a 13 C-labelling strategy combined with several spectroscopic techniques (XPS, ss-NMR, pXRD) and metadynamics simulations to illuminate the mechanistic aspects in methane coupling. Mo and W carbides were tested as catalysts for the non-oxidative methane coupling (NOCM).…”

mentioning

confidence: 99%

Role and Dynamics of Transition Metal Carbides in Methane Coupling

Zhang

Pessemesse

Lätsch

et al. 2022

Preprint

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Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to a popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the dynamics of Mo and W carbides and the active participation of carbidic carbon in the formation of C2 products during methane coupling. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with gas phase carbon (methane). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study supports the presence of a carbon equivalent to the Mars-Van Krevelen type mechanism for non-oxidative coupling of methane, thus introducing guiding principles to design and develop associated catalysts.

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Direct Evidence on the Mechanism of Methane Conversion under Non‐oxidative Conditions over Iron‐modified Silica: The Role of Propargyl Radicals Unveiled

Cited by 29 publications

References 48 publications

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Role and Dynamics of Transition Metal Carbides in Methane Coupling

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