Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

Mims, Charles A.; Hall, Richard B.; Rose, Kenneth D.; Myers, G. Rodney

doi:10.1007/bf00768178

Cited by 47 publications

(7 citation statements)

References 21 publications

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“…Therefore, an electron-acceptor catalyst (reducible, Lewis acid) would be required to activate adsorption of CH 4 to form CH 3 • , and an electron donor (oxidizable, Lewis base) would be needed to activate gas phase molecular O 2 on the catalyst surface, which is crucial for conducting the OCM reaction. It has been repeatedly demonstrated that the OCM mechanism involves gaseous CH 3 • radicals that recombine to form C 2 H 6 . ,, …”

Section: Proposed Catalytic Ocm Reaction Mechanismsmentioning

confidence: 99%

“…It has been repeatedly demonstrated that the OCM mechanism involves gaseous CH 3 • radicals that recombine to form C 2 H 6 . 29,109,110 Two proposed catalytic OCM reaction mechanisms have received the most attention in the catalysis literature. One of the very first and the most widely known OCM mechanism was proposed by Lunsford et al in 1995.…”

Section: Proposed Catalytic Ocm Reaction Mechanismsmentioning

confidence: 99%

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Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

et al. 2019

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The literature for the oxidative coupling of methane (OCM) on supported Mn/Na 2 WO 4 /SiO 2 catalysts is systematically and critically reviewed. The influence of the precursors, starting SiO 2 support crystallinity, synthesis method, calcination temperature, and OCM reaction conditions on the catalyst structure is examined. The supported Mn/Na 2 WO 4 /SiO 2 catalyst system is found to be dynamic with the catalyst structure quite dependent on the set of variables. Although almost all of the reported studies have determined the catalyst crystalline structures under ambient conditions (room temperature and air exposed), recent in situ/operando characterization study under OCM reaction conditions revealed that all previously detected crystalline phases of the active Mn−Na−W−O components are not present because the reaction temperature is above the melting points of their oxides. The presence of Na also induces the crystallization of the silica support to SiO 2 (cristobalite) at elevated temperatures. The nature of the surface active sites under OCM reaction conditions is still not known because of the absence of in situ/operando surface spectroscopy characterization studies under relevant reaction conditions. Consequently, the proposed structure−activity models in the literature are highly speculative since they are lacking supporting data. The rate-determining-step involves activation of the methane C−H bond by atomic surface O* as demonstrated by a kinetic isotope effect (KIE) between CH 4 and CD 4 . Although the reaction kinetics follow a Langmuir− Hinshelwood type mechanism, r = [CH 4 ] 1 [O 2 ] 1/2 , isotopic 18 O 2 − 16 O 2 studies have shown that the catalyst lattice also provides O* for the OCM reaction suggesting involvement of a Mars−van Krevelen mechanism. Recommendations are given regarding the experimental investigations that could establish the fundamental reaction aspects of OCM by supported Mn/Na 2 WO 4 /SiO 2 catalysts that would allow for the rational design of improved catalysts.

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Section: Proposed Catalytic Ocm Reaction Mechanismsmentioning

confidence: 99%

Section: Proposed Catalytic Ocm Reaction Mechanismsmentioning

confidence: 99%

Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

et al. 2019

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“…Increasing the dead volume only results in an overall increase in reaction rate. This is a result of the increased volume for the gas-phase recombination of methyl radicals to C 2 products 31,32 . No conductive heat transfer from the catalyst bed to the reactor walls was considered in the calculation.…”

Section: Resultsmentioning

confidence: 99%

“…It is suggested that molecular oxygen is activated at the silver surface where it forms atomically adsorbed oxygen (O α ). On the one hand, O α may react with CH 4 32 . At elevated temperatures (T>900 K) 44 , the Ag surface and bulk reconstructs to form the thermodynamically must (111) surfaces and the interstitialcy diffusion of oxygen through the silver lattice is activated by which oxygen substitutes for silver.…”

Section: The Irreversibility Of Reactionmentioning

confidence: 99%

“…Hydrogen abstraction has been proposed by a number of authors to be the reaction-limiting step of reaction for OCM over a variety of catalysts. 41,42,43 The gas-phase recombination of radicals occurs quickly with respect to the dehydrogenation step. 44 The suggestion that the formation of O γ is necessary for reaction implies that the reaction of methane is limited by the rate of bulk oxygen diffusion to the (111) surface via interstitialcy diffusion and not by the hydrogen abstraction step.…”

Section: The Modelmentioning

confidence: 99%

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The Role of Subsurface Oxygen in the Silver-Catalyzed, Oxidative Coupling of Methane

Nagy

Mestl

Schlögl

1999

Journal of Catalysis

107

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The silver-catalyzed, oxidative coupling of methane to C2 hydrocarbons (OCM) is shown to be an extremely structure-sensitive reaction. Reaction-induced changes in the silver morphology lead to changes in the nature and extent of formation of various bulk and surface -terminating crystal structures. This, in-turn, impacts the adsorption properties and diffusivity of oxygen in silver which is necessary to the formation of subsurface oxygen. A strongly-bound, Lewis-basic, oxygen species which is intercalated in the silver crystal structure is formed as a result of these diffusion processes. This species is referred to as O γ and acts as a catalytically active site for the direct dehydrogenation of a variety of organic reactants. It is found that the activation energy for methane coupling over silver of 138 kJ/mol is nearly identical to the value of 140 kJ/mol for oxygen diffusion in silver measured under similar conditions. This correlation between the diffusion kinetics of bulk -dissolved oxygen and the reaction kinetics of the oxidative coupling of methane to C2 hydrocarbons suggests that the reaction is limited by the formation of Oγ via surface segregation of bulk dissolved oxygen. Catalysis over fresh silver catalysts indicates an initially preferential oxidation of CH 4 to complete oxidation products. This is a result of the reaction of methane with surface bound atomic oxygen whic h forms preferentially on high-index terminating crystalline planes. Reaction-induced facetting of the silver results in a restructuring of the catalyst from one which initially catalyzes the co mplete oxidation of methane to COx and water to a catalyst which preferentially catalyzes the formation of coupling products. This represents an extremely dynamic situation in which a solid-state restructuring of the catalyst results in the formation of a Lewis-basic, silver-oxygen species which preferentially catalyzes the dehydrogenation of organic molecules.

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Synthesis from C₁Sources

2017

Hydrocarbon Chemistry, Two Volume Set

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Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

Cited by 47 publications

References 21 publications

Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

The Role of Subsurface Oxygen in the Silver-Catalyzed, Oxidative Coupling of Methane

Synthesis from C₁Sources

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Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

Cited by 47 publications

References 21 publications

Oxidative Coupling of Methane (OCM) by SiO2-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

Oxidative Coupling of Methane (OCM) by SiO2-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

The Role of Subsurface Oxygen in the Silver-Catalyzed, Oxidative Coupling of Methane

Synthesis from C1Sources

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Resources

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Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

Oxidative Coupling of Methane (OCM) by SiO₂-Supported Tungsten Oxide Catalysts Promoted with Mn and Na

Synthesis from C₁Sources