Coupling Of Methane Reaction Research Articles

ChemInform Abstract: Unusual Deuterium Isotope Effect in the Oxidative Coupling Reaction of Methane Over a Samarium Oxide Catalyst.

The oxidative coupling reaction of methane over various metal oxide catalysts has attracted much interest and speculation on plausible reaction mechanisms. We have recently shown that the addition of small amounts of C{sub 2}H{sub 6} to the reactants using Sm{sub 2}O{sub 3} catalysts at 700{degree}C resulted in no significant change in the total CO{sub x} formation, but in a substantial reduction in the net CH{sub 4} conversion. Use of {sup 13}C{sub 2}H{sub 6} also showed that under these conditions a large part of the total CO{sub x} formed was derived from the added {sup 13}C{sub 2}H{sub 6}. We now report that the replacement of {sup 13}C{sub 2}H{sub 6} by {sup 12}C{sub 2}D{sub 6} in similar experiments results in an unusually large effect on the net CH{sub 4} conversion and on the products formed from the CH{sub 4} and the additives. The data confirm the conclusions reached in our previous work, illustrate the dramatic effect that small disturbances in reaction rates have on the product distribution, and suggest that the interpretation of isotope effects in these systems may be more complex than previously thought.

The role of halides in selective oxidation reactions

The role of chlorine, in various chemical forms, in the methane activation reaction has been investigated. It has been found that although there is a release of methyl chloride and HCl from the surface of chloride-containing catalysts during the methane coupling reaction very little ethene is produced from ethane in the post catalyst volume. However, depending on the experimental conditions used, it is possible to envisage an important contribution from gas phase reactions in the catalyst bed.

Characterization of catalysts for methane-coupling by means of temperature programmed desorption

The technique of temperature programmed desorption has been employed to study the adsorptive behaviour of CaO and Na2O/CaO towards the reactants and products of the methane-coupling reaction. Two forms of adsorbed O2 can exist on CaO, and they desorb at intermediate temperatures: the first corresponds to the sites for adsorption and desorption without reaction of CH4, while the second to sites for desorption with total oxidation of CH4 itself. At the temperatures of the activity runs for methane coupling (750–800 °C), no adsorbed species is therefore stable on the surface of CaO. 7% Na2O/CaO shows different features, because the desorption of O2 produces only a large broad peak at very high temperatures, corresponding to sites for adsorption and desorption with total oxidation of CH4. (On the other hand a desorption peak of CH4 at intermediates temperatures is also present, which seems to be due to a species, adsorbed undissociatively and desorbed without reaction). At the temperatures of the activity experiments, adsorbed species are therefore stable on the surface of Na2O/CaO. A conclusion is therefore drawn on the necessity for the intervention of the gas phase O2 during the methane coupling reaction in order to produce C2-hydrocarbons, and on the greater importance of the surface reactions in the presence of Na2O, in comparison to the case of CaO.

Selective oxidation of methane in the presence of nitric oxide: comments on the reaction mechanism

The effect of addition of NO, a stable π radical, to CH4/oxidant reactants over Li/MgO and MgO is reported. Addition of NO with a Li/MgO catalyst, when both O2 and N2O are used as oxidants, causes an immediate inhibition in the methane coupling reaction. Removal of NO effects an immediate increase in C2 levels to values much higher than those observed prior to NO addition. Addition of CO2 in an analogous manner also causes an inhibition in the C2 production, but on removal of CO2 a slow increase in C2 levels is observed and no enhancement is apparent. The possible significance of these results with respect to the reaction mechanism are discussed in detail.

Structural aspects of the oxidative coupling of methane

A comparative study of the structure and catalytic performance for methane oxidation is reported for two samples of MgO. MgO prepared by thermal decomposition of the basic carbonate is found to give higher selectivities to C2 hydrocarbons in contrast to MgO obtained from burning metal ribbon in air. Additionally, the two oxides were found to exhibit different morphologies; MgO (ribbon residue) consisted mainly of single MgO crystallites with regular cubic structure, whereas MgO (ex basic carbonate) comprises of agglomerates of smaller MgO crystallites. The combination of the catalytic and structural studies demonstrate that the morphology of MgO is an important factor in the methane coupling reaction.

Kinetic and mechanistic study of the methane coupling reaction

Detailed kinetic measurements have been made for the methane coupling reaction over a reducible and an irreducible oxide catalyst for a range of temperatures and methane and oxygen partial pressures. The results show a correlation with methane partial pressure which suggests that the reaction is close to second order with respect to methane. The results are analysed in terms of Rideal–redox and Langmuir–Hinshelwood models, and it is shown that the results can be explained by a Langmuir–Hinshelwood model in which the rate determining step is assumed to be surface coupling of adsorbed methyl moieties. However, the existence of a kinetic isotope effect in the formation of ethane from methane is not consistent with this model. An alternative model is presented in which the regeneration of the active centres on the surface of the oxide catalysts is included in the kinetic analysis. This model correctly predicts a ‘pseudo’-second-order reaction with respect to the methane partial pressure, but is also consistent with the observed kinetic isotope effect.

Oxidative coupling of methane over manganese oxide catalysts

A series of Mn oxide catalysts both promoted by K and unpromoted have been prepared using high-temperature methods and investigated in the methane coupling reaction at 750 °C. Characterisation of quenched samples of the catalysts by XPS and XRD has allowed changes in the surface and bulk properties to be followed as a function of treatment conditions. The influence of injecting a single pulse of dichloromethane on the catalytic and structural properties has been investigated. Chlorine greatly enhances the selectivity of the Mn oxide catalysts for the formation of C2 products. Structural analysis has shown that the enhanced selectivity is associated with a Mn3O4-like phase. This phase is unstable under reaction conditions in the absence of chlorine and the chlorine-modified phase is unstable in the absence of potassium. The relationship between the structure of the various Mn oxides and their catalytic selectivities is discussed.

Oxidative Coupling Reaction of Methane

Direct conversion of methane to desirable fuels and chemicals by its partial oxidation is an attractive subject from both academic and industrial points of view. In the last decade oxidative coupling of methane has been extensively investigated and the recent progress on refinement of catalyst for this reaction brings about a possibility of establishment of a economical process for production of gasoline from natural gas through oxidative coupling. In this paper a current R & D of catalytic chemistry for direct conversion of methane to ethylene and higher hydrocarbons is reviewed as well as the cross-coupling between methane and other hydrocarbons to valuable hydrocarbons using methane as C1 source with examples taken from our recent studies.

Differences in catalytic and gas-phase reactions in methane coupling

Examination of the coupling reaction of methane in the gas-phase and over Pb–Mg–O catalysts reveals significant differences in certain kinetic parameters which are useful in determining the extent of homogeneous gas-phase contribution to the final product.

Measurements of kinetic isotope effects and hydrogen/deuterium distributions over methane oxidative coupling catalysts

The kinetic isotope effect for CH 4 compared to that for CD 4 has been measured for the oxidative coupling reaction of methane over Li MgO , SrCO 3, and Sm 2O 3 catalysts in a flow reactor. Each catalyst gave results consistent with CH bond breaking being the slow step. For temperatures between 680–780 °C over Li MgO , k H K D decreased slightly with temperature. The isotope effect for ethane production was more sensitive to the level of conversion and declined from 1.8 at low conversion to near unity under conditions where the ethylene to ethane ratio was high (~1). Selectivities to hydrocarbons were lower with CD 4 and did not change with decreased flow rates, implying that either CO x and C 2 products arise by totally separate slow steps or, if a common step with CH 3 radicals is involved, then CO x formation occurs on the catalyst. Experiments with CH 4 CD 4 mixtures showed that CH 3CD 3 and CH 2CD 2 were the dominant mixed products. The distribution of the ethanes always reflected the relative concentrations of CH 3 and CD 3 determined by the kinetic isotope effect. At low ethylene to total C 2 ratios (~0.2) this was also true for ethylene; but at higher ratios substantial exchange to produce ethylenes other than C 2H 4, CH 2CD 2, and C 2D 4 occurred. The concentration of the exchanged methanes correlated with total methane conversion but was dependent on the surface. Exchange in the ethylenes also correlated with exchange in the methanes and purely gas phase processes appear at least partially responsible. H 2: HD: D 2 ratios are always at equilibrium and exchange also occurs between CD 4 and H 2.

An unusual deuterium isotope effect in the oxidative coupling reaction of methane over a samarium oxide catalyst

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAn unusual deuterium isotope effect in the oxidative coupling reaction of methane over a samarium oxide catalystAlfred Ekstrom and Jacek A. LapszewiczCite this: J. Am. Chem. Soc. 1989, 111, 22, 8515–8516Publication Date (Print):October 1, 1989Publication History Published online1 May 2002Published inissue 1 October 1989https://doi.org/10.1021/ja00204a032RIGHTS & PERMISSIONSArticle Views85Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Utilization of CO 2 in the oxidative coupling reaction of methane over CaO based catalysts

Reactions by CH 4/CO 2/O 2 (100/100/1) gave higher C 2-compounds and ethene) yield than that by CH 4/O 2 (100/1) over CaO and BaOCaO catalysts at 1073 K. C 2-yield by CH 4CO 2O 2 reaction is even higher than the maximum theoretical yield by CH 4O 2 reaction, which suggests that one of the oxygen atoms of CO 2 is used for the hydrogen abstraction from methane. CO 2 is turned to CO. The reaction mechanism is proposed and the reactivity of the catalyst is discussed with relation to the catalyst properties.

Effect of transport limitations on C 2+ selectivity in the oxidative methane coupling reaction using a NaOH/CaO catalyst

The dependence of C 2+ selectivity on oxygen conversion (20 to 95%) and on particle size (0.2 mm, 1.2 mm, 4×4 mm) of the catalyst, i.e. under conditions of mass transport limitation during the oxidative coupling of methane on a NaOH/CaO catalyst was experimentally studied at three temperatures (953, 983, 1013 K) and at a partial pressure of oxygen of 0.075 bar and of methane of 0.7 bar. A kinetic reaction scheme was derived for which the overall kinetic parameters were reported. Reaction engineering calculations with respect to the particle size effect, taking into account transport phenomena, are presented and compared with experimental evidence.

The synthesis of ethene during the oxidative coupling of methane

The effect of residence time on the selectivity of a MgO catalyst for the formation of ethene in the methane coupling reaction has been studied. It is found that the variation in ethene selectivity with this time follows opposite trends depending on the experimental conditions. In both cases the results show a positive intercept at zero residence time which may indicate that ethene is a primary product. However, calculations show that gas phase radical reactions just above the catalyst surface could account for the ethene observed even at very short residence times. It is concluded that experiments such as these cannot reliably distinguish between primary and secondary product formation. A SmOCl catalyst was studied and it was found that HCl was released when wet gases were passed over the catalyst at relatively low temperatures. It is concluded that much of the ethene produced over oxychloride catalysts may be produced by gas phase reactions involving chlorine radicals.

Catalytic properties of perovskite oxide, BaPb1?x Bi x O3, for oxidative coupling reaction of methane

Etude de l'activite et de la selectivite du catalyseur BaPb 1−x Bi x O 3 en fonction de x. La vitesse de la reaction et l'energie d'activation de l'ethane et du dioxyde de carbone ainsi formes sont examinees

Origin of the low limits in the higher hydrocarbon yields in the oxidative coupling reaction of methane

A constant C 2+ concentration is reached in the catalysed oxidative coupling of methane when the rate of C 2+ formation from CH 4 is equal to its rate of conversion to CO x .

Mechanistic aspects of oxidative coupling of methane over LaAlO3

Mechanistic aspects of the oxidative coupling reaction of methane over an LaAlO3 catalyst (La:Al = 1 : 1) prepared by the mist decomposition method have been studied, using both continuous-flow and pulsed-flow techniques. The delayed pulse technique together with temperature-programmed desorption reveal that an adsorbed oxygen species is effective in the formation of the C2 compounds, while a gaseous or weakly adsorbed oxygen species is involved in the combustion reaction. Methane cannot stay on the surface stably. Comparing these results with those obtained using the continuous flow reactor, mechanistic aspects are considered from the viewpoint of oxygen activity. The stability of this catalyst is also discussed.