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.     

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.     

Production of acetylene by a microwave catalytic reaction of water and carbon

The feasibility of producing hydrocarbons in a microwave induced catalytic reaction of carbon and water was successfully demonstrated. The major reaction products are acetylene, methane, ethylene and ethane. Other significant products include propylene, propyne, cyclopropane, carbon dioxide and carbon monoxide. Relative product yields and their distribution depend on a number of experimental variables, such as irradiation time, incident microwave power, water/carbon ratio and the characteristics of the microwave pulse train. At short irradiation times and low incident power only C₁ — C₂ products were observed, their rates of formation being an exponential function of the incident microwave power. High incident power led to the formation of C₃ to C₆ hydrocarbons at the expense of acetylene. Initial addition of methane and carbon dioxide to the reaction mixture increased the yield of acetylene, whereas addition of methanol to water resulted in a sharp increase in the amounts of both methane and acetylene. Mechanisms are considered to account for these observations.     

Methane Conversion Over Sr2+/La2O3 Catalyst Modified with Nickel and Copper

Methane transformation over Ni and Cu modified Sr²⁺/La₂O₃ catalysts has been studied. These species favor formation of reducible mixed oxides and change the surface reactivity of the Sr²⁺/La₂O₃ system, modifying the reaction mechanism, since Sr²⁺/La₂O₃ favors methane oxidative coupling but with copper methane combustion is favored and nickel favors partial oxidation.     

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.     

Gas-phase oxidation of propylene into acetone on a V<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript> catalyst: Effect of pressure and role of water

Positive effect of raising the pressure and water content in the reaction mixture in the oxidative conversion of propylene into acetone on a V₂O₅/TiO₂ catalyst was observed. It was shown that the reaction occurs via intermediate formation of isopropanol, which id produced in situ as a result of the acid-catalyzed hydration of propylene and, under certain conditions, may be the main product.     

The Catalytic Oxidative Coupling of Methane

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 radicals in the gas phase. The yield of C₂H₄ and C₂H₆ is limited by secondary reactions of CH 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 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.     

Ethylene oxidation under conditions of the oxidative coupling of methane

Ethylene conversion under conditions of the oxidative coupling of methane has been investigated. In an empty reactor above 740°C, ethylene oxidation occurs at a higher rate and its main product is carbon monoxide. Filling the reactor with an inert material (quartz) or a NaWMn/SiO₂ catalyst leads to a marked decrease in the ethylene conversion rate. Addition of methane to the reaction mixture dramatically slows down ethylene conversion rate and increases the C₃ hydrocarbon content of the reaction products. The kinetics of ethylene oxidation in the presence of methane over the NaWMn/SiO₂ catalyst is reported.     

Reaction of methane and sulfur: Oxidative coupling and carbon disulfide formation

Reaction between methane and sulfur vapor (mainly S₂) has been studied at 773–1073 K in the presence of possible catalysts and in the absence of a catalyst. Reaction was carried out in a silica tube reactor at a total pressure of 1 atm with excess methane, with prereaction sulfidation using H₂S/H₂. Sm₂O₃ had significant catalytic effect on CH₄ conversion, but all other potential catalysts (including Li/MgO) gave CH₄ conversion close to that in the absence of catalyst. CS₂ was the major product, accompanied by C₂ coupling product which in most cases was predominantly C₂H₄ with some C₂H₆. In some cases, small amounts of C₃, C₄ hydrocarbons and CH₃SH, (CH₃)₂S, (CH₃)₂S₂ were also found. Experiments with varying space velocity indicated that C₂H₆ was the primary coupling product. Possible reaction pathways are discussed in terms of homogeneous CH₃·generation, augmented by CH₃. generated heterogeneously at the surface of sulfided Sm₂O₃. Sulfidation of Li/MgO is considered to be the reason for the inactivity of this potential catalyst.     

Synthesis of a β-cyclodextrin derivative bearing an azobenzene group on the secondary face

An oxidative coupling Sonogashira-type reaction has been used to synthesize a β-cyclodextrin derivative bearing an azobenzene group on the secondary face for the first time starting from a β-cyclodextrin propargylated at one of its C-2 positions. The de-O-propargylation reaction and the formation of an oxidative homocoupling dimer were found to compete with the desired product under several Sonogashira-type reaction conditions. However, the use of a diluted reductive atmosphere of H₂ avoided the former and diminished the latter.     

Lithium Promotes Acetylide Formation on MgO During Methane Coupling Under Non-Oxidative Conditions

A prototypical material for the oxidative coupling of methane (OCM) is Li/MgO, for which Li is known to be essential as a dopant to obtain high C₂ selectivities. Herein, Li/MgO is demonstrated to be an effective catalyst for non-oxidative coupling of methane (NOCM). Moreover, the presence of Li is shown to favor the formation of magnesium acetylide (MgC₂), while pure MgO promotes coke formation as evidenced by solid-state ¹³C NMR, thus indicating that Li promotes C−C bond formation. Metadynamic simulations of the carbon mobility in MgC₂ and Li₂C₂ at the density functional theory (DFT) level show that carbon easily diffuses as a C₂ unit at 1000 °C. These insights suggest that the enhanced C₂ selectivity for Li-doped MgO is related to the formation of Li and Mg acetylides.     

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.     

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.     

The influence of La2O3 and TiO2 on NiO/MgO/α-Al2O3

Summary The effect of La₂O₃ and TiO₂ on product selectivity, methane conversion and coke formation over NiO/MgO/ α -Al₂O₃ catalyst were studied in a simultaneous steam and CO₂ reforming of methane to syngas. La₂O₃ and TiO₂ were added to the catalyst via incipient wetness impregnation and bulk precipitation techniques and catalyst activity was tested in a fixed bed quartz reactor. Results reveal that although the addition of these oxides has no effect on the product selectivity and methane conversion, but can reduce coke formation on the surface of the catalysts as it can enhance the mobility of lattice oxygen anions. The results further show that the catalysts prepared by bulk precipitation technique decrease the coke formation more effectively.     

Control of selectivity in methane conversion reactions in RF plasma: the influence of reaction conditions

RF plasma excitation of methane has been studied in an effort to optimize the reaction conditions for a selective partial oxidation of methane. The reaction products of RF-excited methane are C2 hydrocarbons such as ethane and acetylene when O₂ is not used. The introduction of a few percent of O₂, however, is found to switch the selectivity in favor of CO while CO₂ formation is suppressed down to a level below a few percent. Interestingly, in the low O₂ ratio regime (0–0.6), the selectivity between CO and C2 hydrocarbons is observed to vary systematically in response to the detailed reaction conditions, including flow rate, pressure and applied RF power, which are explained by the competition between coupling and partial oxidation reactions. Variation in the density and the residence time of the active species in the plasma is suggested to determine the overall reaction pathways. The present results suggest a possibility of a selective production of the partial oxidation products of methane such as CO with a high selectivity and a high conversion efficiency using controlled RF plasma from methane and O₂.     

Effect of HF Modification on the Catalytic Properties of Ferrospheres in the Oxidative Coupling of Methane

The effect of the HF modification of ferrospheres separated from fly ash after the combustion of brown coal on their chemical, phase compositions and catalytic properties in the oxidative coupling of methane was studied. The modification led to a change in the phase composition in comparison with that of the initial ferrospheres: a CaF₂ phase appeared, the hematite phase content increased, and the ferrospinel content decreased. The yield of C₂ hydrocarbons at 750°C increased by a factor of 1.5–2.0, and the fraction of ethylene in them increased to 30 or 65% at 750 or 850°C, respectively. It was assumed that an increase in the efficiency of HF-modified ferrospheres in the formation of ethane and its dehydrogenation into ethylene was due to the formation of oxyfluoride-type active sites. The pyrohydrolysis of fluorine-containing catalyst components at 850°C due to interaction with water vapor in a reaction atmosphere led to the formation of systems active in deep oxidation; this manifested itself in a sharp decrease in selectivity for the formation of C₂ hydrocarbons and an increase in selectivity for CO₂.

     

Mechanism of the formation of carbon oxides under conditions of the oxidative chlorination of methane: IV. Kinetics of the reaction of CCl<Subscript>4</Subscript> with oxygen on copper-containing salt catalysts for methane oxychlorination at reduced partial pressures of Cl<Subscript>2</Subscript> in the reaction mixture

The kinetics of the oxidative dechlorination of CCl₄ on two supported salt catalysts for methane oxychlorination (CuCl₂-KCl and CuCl₂-KCl-LaCl₃) in the presence of methane at 350–450°C was studied using a gradientless method. It was found that, at \(P_{Cl_2 } \) > 0.15 kPa, the presence of methane in the reaction mixture had no effect on the kinetics of CCl₄ oxidation but decreased the activation energy E to ～22 kcal/mol. At \(P_{Cl_2 } \) < 0.15 kPa, the kinetic orders with respect to oxygen and chlorine changed and E decreased to ～17 kcal/mol. The rate of formation of CO₂ as a by-product of the reaction of methane oxychlorination was described by an exponential equation in combination with rate equations for the oxidation of other chloromethanes and methane. An equation consistent with the observed rate laws of this reaction in the presence of methane was derived from the previously proposed reaction scheme for the mechanism of the oxidative dechlorination of CCl₄.     

Methane conversion under cold plasma over Pd-containing ionic liquids immobilized on γ-Al2O3

Pd-containing ionic liquid (IL) l-hexyl-3-methylimidazolium tetrafluoroborate (C₆MIMBF₄) immobilized on γ-Al₂O₃ (Pd-IL/γ-Al₂O₃) was prepared and characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis. The influences of C₆MIMBF₄ loading and Pd on methane conversion to C₂ hydrocarbons under cold plasma were investigated. FTIR and SEM analyses indicated that C₆MIMBF₄ had been successfully immobilized on γ-Al₂O₃ and the C₆MIMBF₄ showed excellent stability under cold plasma. The results of BET and methane conversion showed that with the increase in immobilization amount of C₆MIMBF₄ onto γ-Al₂O₃, the specific surface area and pore volume of IL/γ-Al₂O₃ decreased, while the selectivity and yield of C₂ hydrocarbons increased. The selectivity of C₂ hydrocarbons was 94.6% when the loading of C₆MIMBF₄ was 40%, and the percentage of C₂H₄ in C₂ hydrocarbons was as high as 64% when using Pd-IL/γ-Al₂O₃ as a catalyst with no conventional thermal reduction treatment. Optical emission spectra (OES) from the cold plasma reactor during methane conversion were also studied. The results indicated that the intensity of the C₂, CH, H, and C active species from methane and hydrogen decomposition increased when IL/γ-Al₂O₃ or Pd-IL/γ-Al₂O₃ was introduced into the plasma system. Based on the analyses of the gas product and OES spectra, it can be concluded that the surface catalyzed reactions between plasma and ionic liquid were very important for the reduction of Pd²⁺ and the formation of C₂H₄     

Photocatalytic Oxidative Coupling of Methane over Au1Ag Single-Atom Alloy Modified ZnO with Oxygen and Water Vapor: Synergy of Gold and Silver

C−H dissociation and C−C coupling are two key steps in converting CH₄ into multi-carbon compounds. Here we report a synergy of Au and Ag to greatly promote C₂H₆ formation over Au₁Ag single-atom alloy nanoparticles (Au₁Ag NPs)-modified ZnO catalyst via photocatalytic oxidative coupling of methane (POCM) with O₂ and H₂O. Atomically dispersed Au in Au₁Ag NPs effectively promotes the dissociation of O₂ and H₂O into *OOH, promoting C−H activation of CH₄ on the photogenerated O⁻ to form *CH₃. Electron-deficient Au single atoms, as hopping ladders, also facilitate the migration of electron donor *CH₃ from ZnO to Au₁Ag NPs. Finally, *CH₃ coupling can readily occur on Ag atoms of Au₁Ag NPs. An excellent C₂H₆ yield of 14.0 mmol g⁻¹ h⁻¹ with a selectivity of 79 % and an apparent quantum yield of 14.6 % at 350 nm is obtained via POCM with O₂ and H₂O, which is at least two times that of the photocatalytic system. The bimetallic synergistic strategy offers guidance for future catalyst design for POCM with O₂ and H₂O.     

Effect of CH4/Air Ratio on Selectivity of Partial Oxidation of Methane on V2O5/SiO2 Catalyst

We have studied partial oxidation of methane on V₂O₅/SiO₂ (0.8 mass % V) in a flow-through catalytic fixed-bed reactor. We found that the methane/air ratio in the starting reaction mixture has practically no effect on the selectivity of the process. The dependence of the selectivity on the methane conversion can be described by a model with such reaction parameters as the initial selectivity and the relative reactivity (with respect to methane) of the reaction products.