Reaction-path analysis of a homogeneous methane oxidative coupling mechanism

Tjatjopoulos, G.J.; Vasalos, I.A.

doi:10.1016/0926-860x(92)80216-y

Cited by 16 publications

(12 citation statements)

References 16 publications

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“…The modeling of the oxidative coupling of methane in the absence of catalyst has been reported in several articles (Geerts et al, 1990;Zanthoff and Baerns, 1990;Chen et al, 1991;Mackie, 1991;Tjatjopoulos and Vasalos, 1992). Such modeling is based on a radical mechanism of the reaction.…”

Section: Introductionmentioning

confidence: 99%

Effect of pressure on the oxidative coupling of methane in the absence of catalyst

1994

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Section: Introductionmentioning

confidence: 99%

Effect of pressure on the oxidative coupling of methane in the absence of catalyst

1994

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“…Our claim here is distinctive: we claim that such surface peroxide and superoxide species react more preferentially with H 2 O rather than CH 4 , and the formed OH radicals induce the substantial increment in the CH 4 conversion rate, as evidenced in Figure a,c. It is known that K 2 O 2 /KO 2 species easily react with H 2 O and form H 2 O 2 (Reactions and ). , Plausibly, OH radicals are formed through the decomposition of H 2 O 2 (Reaction ), where previous gas-phase reaction pathway analysis clearly showed that the rapid decomposition of H 2 O 2 forming OH radicals is the major destructive channel of the in situ formed H 2 O 2 during OCM . Two surface OH* (KOH) couple to form H 2 O and K 2 O (Reaction ), in which surface K 2 O can be oxidized to surface K 2 O 2 or KO 2 according to Reactions or , completing the catalytic cycle. …”

Section: Resultsmentioning

confidence: 99%

Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K₂WO₄/SiO₂ Catalyst Revealed by In Situ Characterization

Yoshida

Siritanaratkul

et al. 2021

ACS Catal.

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The oxidative coupling of methane (OCM) over a K2WO4/SiO2 catalyst was investigated using a CH4–O2 flow system. The addition of H2O to the reactant stream caused an increase in the CH4 conversion rate and C2 selectivity, consistent with the kinetics associated with the equilibrated OH radical generation, followed by H-abstraction from CH4 with OH radicals. The C2+ (a sum of C2 and higher hydrocarbons) yield reaches a maximum of 24.7% (C2H4 yield 17.0%) at a CH4 conversion of 47.1%, comparable to those over the previously reported Na2WO4/SiO2 catalyst. In situ near ambient pressure X-ray photoelectron spectroscopy uncovered K2O2 and KO2 species at and above 560 °C in the presence of gas-phase O2. These K2O2/KO2 species are considered to catalyze H2O activation for OH radical generation. By in situ high-temperature X-ray diffraction, K2WO4 maintained a crystalline phase in the bulk during the OCM reaction at 850 °C in contrast to the molten state of Na2WO4 previously reported. This indicates that the melting of the supported component and the covering of support are not essential to achieve a high C2 yield in supported alkali metal tungstate catalysts as long as inert support materials are used toward OCM.

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“…También se ha reportado que las velocidades de reacción entre especies radicales como *O, H* y *OH con las moléculas de combustible como CH 3 OH, CH 4 , C 2 H 6 , además de otras, son generalmente mayores que para las reacciones entre estas mismas especies con CO y H 2 ; por lo tanto, cuando aún permanecen o existen especies de hidrocarburos, la velocidad de reacción de las especies radicalarias (O*, H* y *OH) en la oxidación del CO y el H 2 disminuye. Como resultado, no existirá forma alguna para disminuir la concentración de radicales en regiones en las que aún hay cantidades de combustible (Westbrook & Dryer, 1981) (Tjatjopoulos & Vasalos, 1992); esto implicaría que al tener metano en la combustión de carbón con bajo contenidos de oxígeno tendría un efecto en cuanto a la formación de radicales (O*, H* y *OH), los cuales estarían reaccionando con el material carbonoso por reacciones de abstracción de H y escisión de enlaces C-C; este conjunto de reacciones modificaría la superficie del material alterando sus propiedades estructurales, la reactividad y la cinética de las reacciones químicas. En cuanto a esto último según este estudio se muestra la disminución de la energía de activación (Westbrook & Dryer, 1981).…”

Section: Resultsunclassified

Efecto del contenido de oxígeno y metano en la cinética de oxidación de un carbonizado de carbón

Laverde

Alvarado

Machado

2017

Rev. Acad. Colomb. Cienc. Ex. Fis. Nat.

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ResumenSe evaluó el efecto de la adición de metano en un carbonizado de carbón sometido a reacción con concentraciones de oxígeno bajas. Desde un punto de vista cinético, dicho efecto es determinando por la energía de activación (Ea) y por el factor preexponencial para diferentes concentraciones de oxígeno y mezclas de oxígeno y metano. Se preparó un carbonizado con un tamaño de partícula de 75 a 150 µm a partir de un carbón bituminoso altamente volátil, y se determinaron los parámetros cinéticos de la oxidación empleando el método termogravimétrico de velocidad máxima. Las variables evaluadas en la cinética de oxidación fueron la concentración de oxígeno (5, 8 y 21 %), y el efecto de la adición de metano en el medio de reacción (5, 10 y 14 % de metano mezclado con 5 % de oxígeno). Los resultados mostraron que la Ea disminuía al aumentar la concentración de oxígeno, en tanto que al agregar metano se observó un efecto promotor cuando este correspondía al 5 % de la mezcla total; sin embargo, cuando el porcentaje de metano aumentaba, la Ea también lo hacía debido a la rápida reacción del oxígeno con el metano en fase homogénea comparada con la reacción heterogénea del oxígeno con el carbonizado. La Ea obtenida con 5 % de oxígeno fue de 122,9 kJ/mol y con 21 %, de 90,69 kJ/mol; en presencia de metano al 5 % y 5 % de oxígeno, la Ea fue de 110,7 kJ/mol, y aumentó hasta 170,8 kJ/mol cuando el metano representaba el 14 % de la mezcla. © 2017. Acad. Colomb. Cienc. Ex. Fis. Nat. Palabras clave: Parámetros cinéticos; Carbonizado de carbón pulverizado; Contenido de metano; Contenido de oxígeno. Effect of oxygen and methane on the oxidation kinetics of coal char AbstractIn this work we evaluated the effect of adding methane during the oxidation process of a coal char with low oxygen concentration. We analyzed the effect from a kinetic point of view and we determined the activation energy (Ea) and the pre-exponential factor (A) with different oxygen concentrations and oxygen and methane mixtures. We used a coal char from a highly volatile bituminous coal with particle size of 75 to 150 µm; the kinetic parameters were determined by thermogravimetric method (TGA) with the high-velocity model; oxygen concentration variations were studied with 5, 8 and 21% of oxygen, as well as the effect of adding methane (5,10 and 14%) when oxidation occurred with 5% of oxygen. The results showed that the Ea decreased when oxygen concentration increased, and that adding 5% of methane to the total mixture enhanced the effect. Instead, when we increased the methane percentage, the Ea increased due to the rapid reaction of oxygen and methane in the homogeneous phase as compared with the heterogenous reaction of oxygen and coal char. With 5% oxygen the Ea value was 122.9 kJ/mol and with 21%, it was 90.69 kJ/mol; when methane was added at 5%, the Ea value was 110.7 kJ/mol, and it increased to 170.8 kJ/mol when there was 14% of methane in the mixture.

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Reaction-path analysis of a homogeneous methane oxidative coupling mechanism

Cited by 16 publications

References 16 publications

Effect of pressure on the oxidative coupling of methane in the absence of catalyst

Effect of pressure on the oxidative coupling of methane in the absence of catalyst

Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K₂WO₄/SiO₂ Catalyst Revealed by In Situ Characterization

Efecto del contenido de oxígeno y metano en la cinética de oxidación de un carbonizado de carbón

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Reaction-path analysis of a homogeneous methane oxidative coupling mechanism

Cited by 16 publications

References 16 publications

Effect of pressure on the oxidative coupling of methane in the absence of catalyst

Effect of pressure on the oxidative coupling of methane in the absence of catalyst

Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K2WO4/SiO2 Catalyst Revealed by In Situ Characterization

Efecto del contenido de oxígeno y metano en la cinética de oxidación de un carbonizado de carbón

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Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K₂WO₄/SiO₂ Catalyst Revealed by In Situ Characterization