Transient Studies on Reaction Steps in the Oxidative Coupling of Methane over Catalytic Surfaces of MgO and Sm2O3

Buyevskaya, O.V.; Rothaemel, M.; Zanthoff, H.‐W.; Baerns, M.

doi:10.1006/jcat.1994.1073

Cited by 74 publications

(20 citation statements)

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“…Detailed descriptions of the equipment used and the operating conditions are given elsewhere (12,13). Therefore, only information specific to the present work are given below.…”

Section: Methodsmentioning

confidence: 99%

The Mechanism of Catalytic Ammoxidation of Propane and Propene over Vanadium Antimony Oxides

Buchholz

Zanthoff

1996

ACS Symposium Series

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The ammoxidation of propane and propene over V-Sb-O catalysts was studied using a vacuum transient technique. From the results gained it was concluded that the selective reaction to acrylonitrile occurs via the intermediate formation of propene, which is subsequently transformed into acrolein. Acrolein is converted into the nitrile via short-lived NH Xspecies (NH4 + or NH 3,ads ). These NH x -species, as well as NO x , are also intermediates in the non-selective N 2 formation pathway.The ammoxidation of C3 hydrocarbons produces acrylonitrile, an important raw material for the chemical industry. Acrylonitrile is presently produced from propene, mainly with the SOHIO/BP process which achieves acrylonitrile yields of about 80 % at a propene conversion of 98 % (7). However, in recent years much attention has been paid to the development of alternative processes which allow propane to be used as the feed gas. Such a new process could be economically viable due to the price difference between propane and propene (2). Conventional propene ammoxidation catalysts, Bi-Mo oxides or Fe-Sb oxides, show no or only little activity and/or selectivity to acrylonitrile in the conversion of propane (7,5). Therefore, new catalysts have had to be developed which offer active sites for both dehydrogenation and nitrogen insertion. Among the different catalytic systems investigated for the ammoxidation of propane into acrylonitrile, V-Sb oxides exhibited promising catalytic results (1,4), but maximum yields of around 40 % do not allow their use as industrial catalysts.In order to develop more selective and active catalysts much attention has been paid in the past to elucidating the structure and mode of operation of V-Sb-0 catalysts for the ammoxidation of propane. It is commonly suggested that propene is the primary intermediate in the reaction (5). However, the detailed reaction mechanism and the nature of the active sites of the nitrogen insertion step to acrylonitrile from propene is still a matter of discussion. Considering data from differential and integral kinetic investigations over V-Sb-Al-oxides (4) and V-Sb-W-Al-oxides (6), it has been concluded that direct formation of acrylonitrile from propene occurs, while acrolein is formed in a parallel reaction as an intermediate to CO x and HCN, which are undesired

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“…Detailed descriptions of the equipment used and the operating conditions are given elsewhere (12,13). Therefore, only information specific to the present work are given below.…”

Section: Methodsmentioning

confidence: 99%

The Mechanism of Catalytic Ammoxidation of Propane and Propene over Vanadium Antimony Oxides

Buchholz

Zanthoff

1996

ACS Symposium Series

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“…Moreover, there is both qualitative and quantitative evidence for the coupling of the methyl radicals in the gas phase, after activation of CH, on the catalyst. Using a matrix isolation electron spin resonance (MIESR) system, we have detected surfacegenerated, gas-phase radicals over Li/Mg0,[271 Na/CaO,['*I Na/Ce0,,[2y1 Sr/La,0,,[301 LiNi0,,[311 Li/ Zn0,[321 La,03,[331 Figure 2 are typical of the agreement between the CH; radical production rate and that of C, product formation.…”

Section: Methane Activationmentioning

confidence: 99%

The Catalytic Oxidative Coupling of Methane

Lunsford

1995

Angew. Chem. Int. Ed. Engl.

688

449

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One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas‐phase component, which includes free‐radical chemistry. Ethane is produced mainly by the coupling of the surface‐generated CH 3• radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH 3• radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O 22− ions. In addition to the oxidative coupling of CH4, cross‐coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross‐coupling reaction also appears to involve surface‐generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.

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“…The question of whether methane is adsorbed on the catalyst surface prior to hydrogen abstraction or whether hydrogen abstraction takes place through an Eley-Rideal step has been an important discussion topic in the literature. Both TAP experiments (Buyevskaya et al, 1994;Mallens et al, 1996) and SSTIKA experiments (Nibbelke et al, 1995) showed that ethane, the product of the coupling of two methyl radicals, leaves the reactor at the same time as the inert tracer, indicating that no surface intermediates are involved in the production of ethane. This can only be explained when methyl radicals are produced through an Eley-Rideal step.…”

Section: Kinetic Model Developmentmentioning

confidence: 99%

Kinetics of a Gas-Phase Chain Reaction Catalyzed by a Solid: The Oxidative Coupling of Methane over Li/MgO-Based Catalysts

Couwenberg

Chen

Marin

1996

Ind. Eng. Chem. Res.

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A kinetic model was developed for the heterogeneously catalyzed oxidative coupling of methane for temperatures between 923 and 1023 K, inlet methane-to-oxygen ratios of 2 to 12, and oxygen conversions between 10 and 100%. This model features 10 catalytic reactions coupled to 39 gas-phase chain reactions and accounts for irreducible mass-transport limitations for reactive intermediates. The observed conversions and selectivities are adequately described up to 200 kPa total pressure. The observed strong increase of the conversions between 200 and 1000 kPa is described qualitatively. The catalyst not only produces radicals but also acts as an important radical quencher. Regeneration of the active sites through water desorption was found to be a kinetically significant step in the catalytic sequence producing methyl radicals. At atmospheric pressure approximately 90% of the methane and oxygen is converted on the surface of Sn/Li/MgO, the balance being converted through branched-chain reactions in the pores of the catalyst and in the interstitial phase. For Li/MgO only one-third of the methane is converted on the catalyst surface. For Sn/Li/MgO the catalytic oxidation of methyl radicals, rather than gas-phase reactions, limits the selectivity toward C2 products.

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Transient Studies on Reaction Steps in the Oxidative Coupling of Methane over Catalytic Surfaces of MgO and Sm2O3

Cited by 74 publications

References 0 publications

The Mechanism of Catalytic Ammoxidation of Propane and Propene over Vanadium Antimony Oxides

The Mechanism of Catalytic Ammoxidation of Propane and Propene over Vanadium Antimony Oxides

The Catalytic Oxidative Coupling of Methane

Kinetics of a Gas-Phase Chain Reaction Catalyzed by a Solid: The Oxidative Coupling of Methane over Li/MgO-Based Catalysts

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