A Viewpoint on Direct Methane Conversion to Ethane and Ethylene Using Oxidative Coupling on Solid Catalysts

Promoting Effect of Cerium Oxide on the Catalytic Performance of Yttrium Oxide for Oxidative Coupling of Methane

The promoting effect of CeO₂ on the catalytic performance of Y₂O₃, which is moderately active catalyst, for the oxidative coupling of methane (OCM) reaction was investigated. The addition of CeO₂ into Y₂O₃ by coprecipitation method caused a significant increase in not only CH₄ conversion but also C₂ (C₂H₆/C₂H₄) selectivity in the OCM reaction. C₂ yield at 750 °C was increased from 5.6% on Y₂O₃ to 10.2% on 3 mol% CeO₂/Y₂O₃. Further increase in the CeO₂ loading caused an increase in non-selective oxidation of CH₄ to CO₂. A good correlation between the catalytic activity for the OCM reaction and the amount of H₂ consumption for the reduction of surface/subsurface oxygen species in the H₂-TPR profile was observed, suggesting the possibility that highly dispersed CeO₂ particles act as catalytically active sites in the OCM reaction. The ¹⁶O/¹⁸O isotopic exchange reaction suggested that the beneficial role of CeO₂ in the OCM reaction is to promote the formation of active oxygen species via the simple hetero-exchange mechanism, resulting in the promotion of CH₄ activation.

On the dynamic nature of Mo sites for methane dehydroaromatization

The dynamic catalytic site on Mo/HZSM-5 for methane dehydroaromatization is formed during the initial phases of the reaction. Labelling experiments show that carbon from the carbidic active site is incorporated into the final products.

The mechanism of methane activation on Mo/HZSM-5 is not yet fully understood, despite the great interest in methane dehydroaromatization (MDA) to replace aromatics production in oil refineries. It is difficult to assess the exact nature of the active site due to fast coking. By pre-carburizing Mo/HZSM-5 with carbon monoxide (CO), the MDA active site formation was isolated from coke formation. With this a clear ¹³C NMR signal solely from the active site and not obscured by coke was obtained, and it revealed two types of likely molecular Mo (oxy-)carbidic species in addition to the β-Mo₂C nanoparticles often mentioned in the literature. Furthermore, separating the active site formation from coking by pre-carburization helped us examine how methane is activated on the catalytic site by carrying out MDA using isotopically labelled methane (¹³CH₄). Carbon originating from the pre-formed carbide was incorporated into the main products of the reaction, ethylene and benzene, demonstrating the dynamic behavior of the (oxy-)carbidic active sites.

Carbothermal shock synthesis of high-entropy-alloy nanoparticles

The controllable incorporation of multiple immiscible elements into a single nanoparticle merits untold scientific and technological potential, yet remains a challenge using conventional synthetic techniques. We present a general route for alloying up to eight dissimilar elements into single-phase solid-solution nanoparticles, referred to as high-entropy-alloy nanoparticles (HEA-NPs), by thermally shocking precursor metal salt mixtures loaded onto carbon supports [temperature ~2000 kelvin (K), 55-millisecond duration, rate of ~105 K per second]. We synthesized a wide range of multicomponent nanoparticles with a desired chemistry (composition), size, and phase (solid solution, phase-separated) by controlling the carbothermal shock (CTS) parameters (substrate, temperature, shock duration, and heating/cooling rate). To prove utility, we synthesized quinary HEA-NPs as ammonia oxidation catalysts with ~100% conversion and >99% nitrogen oxide selectivity over prolonged operations.

Oxidative coupling of methane at elevated pressures: reactor concept and its validation

A novel quartz reactor has been developed for heterogeneously catalysed reactions at high pressures and temperatures.

Surface organometallic chemistry in heterogeneous catalysis

Surface organometallic chemistry has been reviewed with a special focus on environmentally relevant transformations (C–H activation, CO₂conversion, oxidation).

Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn-Na-W/SiO2 Catalyst

The work here presents a thorough evaluation of the effect of Mn-Na-W/SiO₂ catalyst surface parameters on its performance in the oxidative coupling of methane (OCM). To do so, we used microporous dealuminated β-zeolite (Zeo), or mesoporous SBA-15 (SBA), or macroporous fumed silica (Fum) as precursors for catalyst preparation, together with Mn nitrate, Mn acetate and Na₂WO₄. Characterizing the catalysts by inductively coupled plasma-optical emission spectroscopy, N₂ physisorption, X-ray diffraction, high-resolution scanning electron microscopy-energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and catalytic testing enabled us to identify critical surface parameters that govern the activity and C₂ selectivity of the Mn-Na-W/SiO₂ catalyst. Although the current paradigm views the phase transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right surface properties. Specifically, the SBA-based catalyst, characterized by relatively closed surface porosity, demonstrated low activity and low C₂ selectivity. By contrast, for the same composition, the Zeo-based catalyst showed an open surface pore structure, which translated up to fourfold higher activity and enhanced selectivity. By varying the overall composition of the Zeo catalysts, we show that reducing the overall W concentration reduces the size of the Na₂WO₄ species and increases the catalytic activity linearly as much as fivefold higher than the SBA catalyst. This linear dependence correlates well to the number of interfaces between the Na₂WO₄ and Mn₂O₃ species. Our results combined with prior studies lead us to single out the interface between Na₂WO₄ and Mn₂O₃ as the most probable active site for OCM using this catalyst. Synergistic interactions between the various precursors used and the phase transition are discussed in detail, and the conclusions are correlated to surface properties and catalysis.

First principles studies of CO2 and O2 chemisorption on La2O3 surfaces

Periodic density functional theory calculations were performed to study the surface structures and stabilities of the La₂O₃ catalyst in CO₂ and O₂ environments, relevant to the conditions of the oxidative coupling of methane (OCM) reaction. Thermodynamic stabilities of the clean surfaces were predicted to follow the order of (001) ≥ (011) ≫ (110) > (111) > (101) > (100), with their direct band gaps at the Γ point following the similar order of (001) > (011) > (110) > (111) > (100) > (101). Hubbard U corrections to the La 4f and 5d orbitals do not qualitatively change the predictions of surface energies and band gaps. For the most stable (001) surface, CO₂ chemisorption to form carbonate species is exothermic by -0.60 eV with a negligible energy barrier of 0.07 eV, whereas O₂ chemisorption to form peroxide species is endothermic by 0.64 eV with a considerable energy barrier of 1.29 eV. For the slightly less stable (011) surface, both CO₂ and O₂ chemisorption can occur at different surface sites, and the same applies to the other studied surfaces. Dissociation temperatures of surface carbonate species range from 300 to 1000 K at p_CO2 of 1 bar, which follow the order of (101) ≈ (110) > (111) ≈ (100) ≈ (011) ≫ (001), showing their strong sensitivity to the surface structure. Dissociation temperatures of surface peroxide species are mostly lower than the room temperature except for those of the (011) and (111) surfaces, although the significant kinetic barriers predicted should prevent their facile dissociation. Insights into the temperature-programmed desorption experiments and the methane reactivity of La₂O₃ in the OCM reaction were also given based on the results of our calculations.

Homogeneous Functionalization of Methane

One of the remaining "grand challenges" in chemistry is the development of a next generation, less expensive, cleaner process that can allow the vast reserves of methane from natural gas to augment or replace oil as the source of fuels and chemicals. Homogeneous (gas/liquid) systems that convert methane to functionalized products with emphasis on reports after 1995 are reviewed. Gas/solid, bioinorganic, biological, and reaction systems that do not specifically involve methane functionalization are excluded. The various reports are grouped under the main element involved in the direct reactions with methane. Central to the review is classification of the various reports into 12 categories based on both practical considerations and the mechanisms of the elementary reactions with methane. Practical considerations are based on whether or not the system reported can directly or indirectly utilize O₂ as the only net coreactant based only on thermodynamic potentials. Mechanistic classifications are based on whether the elementary reactions with methane proceed by chain or nonchain reactions and with stoichiometric reagents or catalytic species. The nonchain reactions are further classified as CH activation (CHA) or CH oxidation (CHO). The bases for these various classifications are defined. In particular, CHA reactions are defined as elementary reactions with methane that result in a discrete methyl intermediate where the formal oxidation state (FOS) on the carbon remains unchanged at -IV relative to that in methane. In contrast, CHO reactions are defined as elementary reactions with methane where the carbon atom of the product is oxidized and has a FOS less negative than -IV. This review reveals that the bulk of the work in the field is relatively evenly distributed across most of the various areas classified. However, a few areas are only marginally examined, or not examined at all. This review also shows that, while significant scientific progress has been made, greater advances, particularly in developing systems that can utilize O₂, will be required to develop a practical process that can replace the current energy and capital intensive natural gas conversion process. We believe that this classification scheme will provide the reader with a rapid way to identify systems of interest while providing a deeper appreciation and understanding, both practical and fundamental, of the extensive literature on methane functionalization. The hope is that this could accelerate progress toward meeting this "grand challenge."

Methane conversion into different hydrocarbons or oxygenates: current status and future perspectives in catalyst development and reactor operation

This Perspective highlights recent developments in methane conversion into different hydrocarbons and C₁-oxygenates. Our analysis identified possible directions for further research to bring the above approaches to a commercial level.