Effect of quenching on the methane coupling reaction

The effect of reaction mixture quenching on C₂ product formation in the methane coupling reaction on La₂O₃/CaO is disclosed. For reaction with the mixture (vol. %): 54.5 CH₄, 9.1 O₂ and 36.4 N₂ at 973 K, quenching of products results in a two-fold decrease in C₂ product yield. The results give evidence that in methane oxidative coupling methyl radical formation can occur in the gas phase to an extent comparable with that on the catalyst surface. The effect described must be taken into account in the case of an industrial application of methane oxidative coupling, too, because quenching is a regular procedure in the high temperature oxidative processes.     

CO2选择氧化低碳烷烃的研究进展

本文以低碳烷烃的选择催化氧化反应为对象，对几种CO₂选择氧化低碳烷烃的反应工艺进行了归纳总结，重点分析讨论了催化氧化反应中CO₂作为氧化剂的作用机制，并提出了研究展望。     

Effect of the mobility of oxygen in perovskite catalyst on the dynamics of oxidative coupling of methane

The effect of the diffusion of oxygen from the volume of the catalyst to its surface on the dynamics of the oxidative coupling of methane was assessed on the basis of a mathematical model of the reaction of methane with the oxidized surface of KNaSrCoO_3–x perovskite. It was shown that the possible values of the diffusion coefficient lie in the range of 10^-18-10^-16 cm²/s characteristic of the diffusion of oxygen in oxide catalysts.     

Oxide catalysts for methane transformations in the absence of oxygen

The possibility of methane transformation to higher hydrocarbons in non-oxidizing atmosphere has been studied. It was found that in the presence of the MnO_x–Na/SiO₂-HZSM-5 catalytic system methane can be transformed into aromatics with a yield similar to that reported for oxidative coupling conditions.     

New approach to the preparation of catalysts for the oxidative coupling of methane

A new approach to the preparation of systems that exhibit catalytic activity in the oxidative coupling of methane (OCM) is considered. With the use of ferrospheres separated from power-generation ashes from different sources as an example, it was demonstrated that OCM catalysts can be prepared by the crystallization/solidification of oxide melts with the formation of microspherical particles. The dependence of activity and selectivity for the oxidative reforming of methane on the ferrospheres containing from 36.2 to 92.5 wt % Fe₂O₃ into the products of deep oxidation and OCM was studied. It was found that deep oxidation reactions on ferrospheres with Fe₂O₃ contents higher than 85% were suppressed, and the main reaction path of CH₄ conversion was its oxidative coupling with the formation of C₂ products (with selectivity to 60% at 750°C); moreover, the selectivity for C₂ formation in this region was proportional to the concentration of Fe₂O₃. Phases responsible for the catalytic conversion of methane into CO _x and OCM products were considered, and it was shown that the catalytic activity and selectivity of the oxidative transformation of CH₄ on ferrospheres is determined by the position of the point that corresponds to their composition on a phase diagram of CaO-Fe₂O₃-SiO₂.     

Formation of Acetylene in the Reaction of Methane with Iron Carbide Cluster Anions FeC3− under High‐Temperature Conditions

The underlying mechanism for non‐oxidative methane aromatization remains controversial owing to the lack of experimental evidence for the formation of the first C?C bond. For the first time, the elementary reaction of methane with atomic clusters (FeC₃^?) under high‐temperature conditions to produce C?C coupling products has been characterized by mass spectrometry. With the elevation of temperature from 300 K to 610 K, the production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization, was significantly enhanced, which can be well‐rationalized by quantum chemistry calculations. This study narrows the gap between gas‐phase and condensed‐phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non‐oxidative methane aromatization.     

Effect of lithium addition on the performance of proton-conducting catalysts for oxidative coupling of methane under microwave irradiation

The oxidative coupling of methane over lithium modified proton-conducting catalysts irradiated by microwaves has been studied. Compared with protonconducting catalysts, lithium addition to proton-conducting catalysts resulted in changes in product species and product selectivities, favoring the production of C₂ compounds.     

Influence of the composition of perovskites based on SrMnO<Subscript>3</Subscript> on their catalytic properties in the oxidative coupling of methane

The oxidative coupling of methane (OCM) in a periodic regime over the SrMnO₃ and its derivatives has been investigated. It has been established that partial replacement of the strontium ion by alkali metal ions leads to an increase in both catalytic activity and in selectivity with respect to higher hydrocarbons. Comparison of the results obtained in the present work with those obtained earlier for catalysts based on SrCoO₃ led to the conclusion that the Co-containing catalysts were preferable to the Mn-containing catalysts for carrying out oxidative coupling of methane in a periodic regime.__________Translated from Teoreticheskaya i Éksperimentalnaya Khimiya, Vol. 41, No. 1, pp. 30–34, January–February, 2005.     

Coupling of methane under pulse corona plasma (I)

At normal temperature and pressure, pulse corona plasma was used as a new method for the dehydrogenative coupling of methane in the absence of oxygen. The effects of voltage polarity and input energy on the dehydrogenative coupling of methane were investigated. The parameter “energy efficiency” was introduced to examine the coupling of the input energy and the dehydrogenative coupling of methane. The experimental results show that positive corona gives higher energy efficiency than negative corona. When the positive corona was chosen, C₂ yield per pass was 31.6% and acetylene yield per pass was 30.1% with 44.6% methane conversion at an input energy density of 1788kJ/mol and a pulse repetition frequency of 66Hz. The function of input energy density towards methane conversion may be expressed as a formula of-In(1-X) =k (PIF). In the range of input energy employed, C₂ yield is proportional to input energy density, but energy efficiency drops off with increasing input energy density.     

Kinetic limit of the ethane and ethylene yield in the gas phase condensation of methane

A kinetic simulation of the initiated oxidative condensation of methane in the gas phase showed that the additional generation of methyl radicalsvia the reaction CH₄ + O₂ CH₃ + HO₂ causes a nearly tenfold increase in the C₂ hydrocarbon yield. However, a kinetic limit of the yield exists that is close to that determined in experiments on the catalytic oxidative condensation of methane.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 380–382, February, 1995.     

The oxidative coupling of methane

Summary The oxidative coupling of methane to C₂ hydrocarbons was studied over a Bi₂O₃–P₂O₅–K₂O catalyst. Catalysts containing Bi and P are known to be active and selective catalysts for the oxidative dimerization of propylene to 1,5-hexadiene. This catalyst system was found to also be active and selective for the oxidative coupling of methane. The experimental results are interpreted in terms of a dual-site, redox model. Methane activation to form CH₃. is proposed to occur on Bi sites. The Bi site is subsequently reoxidized by bulk oxide ions and P becomes the oxygen inlet site. Adsorbed surface oxygen species reduce the selectivity to C₂ hydrocarbons.     

Decomposition of H2O2 by Fe Imidazole or Fe Histidine Complexes Immobilized in Porous Matrices

The smaller pore size of a ceramic membrane contributed to an increase in the selectivity of C2 hydrocarbons in the oxidative coupling of methane, with keeping high CH₄ conversions about 50 %.     

Oxidative coupling of methane in porous Vycor membrane reactors

Porous Vycor membrane tubes were used in shell-and-tube type membrane reactors to study the effect on the oxidative coupling of methane of metering the oxygen into the catalyst bed. Experimental studies showed that under conditions of complete oxygen conversion, Vycor membrane reactors packed with Sm₂O₃ catalyst exhibited enhanced hydrocarbon (C₂) selectivity. C₂ yields were comparable to those of the conventional co-feed packed bed reactors operated under the same conditions. The higher C₂ selectivity in the membrane reactors indicated that, for methane coupling, regulating the supply of oxygen along the length of the packed bed may be beneficial to C₂ formation.     

Oxidative coupling of methane over Sr?Ti, Sr?Sn perovskites and corresponding layered perovskites

The layered perovskites Sr₂TiO₄ and Sr₂SnO₄ are more active and selective for the oxidative coupling of methane at 1073 K than the corresponding perovskites SrTiO₃ and SrSnO₃. The role of the surface oxygen species for this reaction is discussed.     

Oxidative processing of light alkanes: State-of-the-art and prospects

Recent literature data on partial oxidation of light alkanes into syngas and oxidative coupling of methane into C₂ hydrocarbons are reviewed. The problems of these processes (high cost of pure oxygen; safety; activity, selectivity and stability of catalysts; temperature regime; coke formation and other by-products; insufficient level of methane transformation into ethane and ethylene) are considered. Possible solutions of these problems and prospects of practical use of light alkanes processing are discussed.     

Heterogeneous methane oxidative coupling using perovskites

Catalytic oxidative coupling of methane over perovskite CaTiO₃ prepared by modified ceramic method has been studied. The C₂ yield was 13% at 830 °C and promotion with Na₄P₂O₇ did not improve considerably the catalyst peformance. Regarding reactor test and mechanism studies it is believed that charge deficient, oxygen O^– generated by transforming oxygen adsorbed on surface defects and dissolved into bulk vacancies is responsible for activating methane (active site). Adsorbed oxygen generates the oxidizing sites for actived methane. Strict conditions are required for generation and regeneration of the activating sites in lattice.     

Kinetics of the oxidative coupling of methane in the presence of model catalysts

The kinetics of the oxidative coupling of methane (OCM) in the presence of La/MgO and NaWMn/SiO₂ catalysts in a flow reactor at low reactant conversions was studied. It was found that, in spite of different compositions and properties of the test catalysts, the formation of ethane from methane and ethylene from ethane can be described within the framework of the Mars-van Krevelen redox model in both cases. The rate laws of side reactions, which lead to the formation of carbon oxides, are different from the rate laws of the target reactions of the conversion of methane into ethane and ethane into ethylene. The kinetic parameters required for the numerical simulation of the OCM process were determined for either of the catalysts.     

Microwave effects on the oxidative coupling of methane over Bi2O3-WO3 oxygen ion conductive oxides

The oxidative coupling of methane over (Bi₂O₃)_1-x(WO₃)_x (x=0.2, 0.3, 0.4) oxygen ion conductive oxide catalysts irradiated by microwave has been studied. Compared with a conventional heating mode, the temperature of the catalytic bed is much lower with microwave irradiation and there is a change in selectivity favoring the production of C₂ products.     

Catalytic Activity and Morphological Properties of Ni/MgO Catalysts for the Oxidative Coupling of Methane

The catalytic activity of Ni/MgO catalysts was studied for the oxidative coupling of methane (OCM). The catalysts were characterized using transmission electron microscope (TEM) and XRD. The increase in C2+ selectivity of Ni/MgO was attributed to the presence of bulk dislocations and MgNiO₂ phase. This revised version was published online in June 2006 with corrections to the Cover Date.     

Polymorphic Mn/W/Na(K,Rb,Cs)/SiO<Subscript>2</Subscript> catalysts for oxidative coupling of methane

The phase composition of Mn/W/Na(K,Rb,Cs)/SiO₂ composites-catalysts was determined by X-ray diffraction. Polymorphic transformations involving tridymite (a SiO₂-based nonstoichiometric crystalline phase) were shown to be a reason for the high catalytic activity of Mn/W/Na(K,Rb,Cs)/SiO₂ in heterogeneous oxidative coupling of methane.