Direct Conversion Of Methane Research Articles

Direct Conversion of Methane to C2Hydrocarbons in Solid-State Membrane Reactors at High Temperatures

Direct conversion of methane to C2 compounds by oxidative and nonoxidative coupling reactions has been intensively studied in the past four decades; however, because these reactions have intrinsic severe thermodynamic constraints, they have not become viable industrially. Recently, with the increasing availability of inexpensive "green electrons" coming from renewable sources, electrochemical technologies are gaining momentum for reactions that have been challenging for more conventional catalysis. Using solid-state membranes to control the reacting species and separate products in a single step is a crucial advantage. Devices using ionic or mixed ionic-electronic conductors can be explored for methane coupling reactions with great potential to increase selectivity. Although these technologies are still in the early scaling stages, they offer a sustainable path for the utilization of methane and benefit from the advances in both solid oxide fuel cells and electrolyzers. This review identifies promising developments for solid-state methane conversion reactors by assessing multifunctional layers with microstructural control; combining solid electrolytes (proton and oxygen ion conductors) with active and selective electrodes/catalysts; applying more efficient reactor designs; understanding the reaction/degradation mechanisms; defining standards for performance evaluation; and carrying techno-economic analysis.

Direct conversion of methane to methanol by electrochemical methods

A convenient method for methane (CH4) direct conversion to methanol (CH3OH) is of great significance to use methane-rich resources, especially clathrates and stranded shale gas resources located in remote regions. Theoretically, the activation of CH4 and the selectivity to the CH3OH product are challenging due to the extreme stability of CH4 and relatively high reactivity of CH3OH. The state-of-the-art ‘methane reforming - methanol synthesis’ process adopts a two-step strategy to avoid the further reaction of CH3OH under the harsh conditions required for CH4 activation. In the electrochemical field, researchers are trying to develop conversion pathways under mild conditions. They have found suitable catalysts to activate the C–H bonds in methane with the help of external charge and have designed the electrode reactions to continuously generate certain active oxygen species. These active oxygen species attack the activated methane and convert it to CH3OH, with the benefit of avoiding over-oxidation of CH3OH, and thus obtain a high conversion efficiency of CH4 to CH3OH. This mini-review focuses on the advantages and challenges of electrochemical conversion of CH4 to CH3OH, especially the strategies for supplying electro-generated active oxygen species in-situ to react with the activated methane.

Understanding the Preparation and Reactivity of Mo/ZSM-5 Methane Dehydroaromatization Catalysts.

Methane dehydroaromatization is a promising reaction for the direct conversion of methane to liquid hydrocarbons. The active sites and the mechanism of this reaction remain controversial. This work is focused on the operando X‐ray absorption near edge structure spectroscopy analysis of conventional Mo/ZSM‐5 catalysts during their whole lifetime. Complemented by other characterization techniques, we derived spectroscopic descriptors of molybdenum precursor decomposition and its exchange with zeolite Brønsted acid sites. We found that the reduction of Mo‐species proceeds in two steps and the active sites are of similar nature, regardless of the Mo content. Furthermore, the ZSM‐5 unit cell contracts at the beginning of the reaction, which coincides with benzene formation and it is likely related to the formation of hydrocarbon pool intermediates. Finally, although reductive regeneration of used catalysts via methanation is less effective as compared to combustion of coke, it does not affect the structure of the catalysts.

Power generation from a symmetric flat-tube solid oxide fuel cell using direct internal dry-reforming of methane

Direct conversion of methane to electricity in solid oxide fuel cells is challenging, especially for large-scale cells/stacks, due to coking on Ni-based anodes, which may cause an inhomogeneous temperature and/or stress distribution. In this work, we investigated for the first time direct internal dry reforming of CH 4 for power generation using a flat-tube SOFC (FT-SOFC) with symmetric double-sided cathodes with an active area of 70 cm 2 , and studied the effects of CO 2 :CH 4 ratio, temperature, and current density on the performance and durability of the cell. Our results indicate that the thick anode support and inner channels of the FT-SOFC function as an efficient micro-reactor for reforming of methane, resulting in high conversion rates of methane, and thus a durable cell operation for 500 h in CO 2 /CH 4 = 2 with a constant current density of 0.2 A cm −2 at 750 °C. Ni/YSZ catalysts granules placed in the internal fuel channels can further alleviate coking and improve the durability of cells. The unique cell configuration of FT-SOFC makes possible the direct utilization of methane for power generation. • Direct operation of flat-tube SOFC (FT-SOFC) with double-sided cathodes with methane. • Thick anode support and inner channels of FT-SOFC form an efficient microreactor. • The cell structure improved the methane conversion. • The degradation mechanism for varied CO 2 /CH 4 ratios were clarified. • Stable operation for 500 h was reached under a flow of CO 2 /CH 4 = 2 at 750 °C.

Direct methane conversion with oxygen and CO over hydrophobic dB-ZSM-5 supported Rh single-atom catalyst

Isolated Rh atoms are supported on hydrophobic dB-ZSM-5. Single-atom Rh on dB-ZSM-5 is characterized with Ac-HAADF/STEM and CO adsorption FTIR. Direct conversion of methane to oxygenates is carried out in an autoclave in water charged with CH4, O2 and CO at 150 °C. Hydrophobic Rh-dB-ZSM-5 is compared with hydrophilic Rh-ZSM-5 (Si/Al = 15) for its catalytic performance. Rh-dB-ZSM-5 catalyst exhibits higher CH3OH selectivity and lower CH3COOH and CO2 selectivity. The catalytic activity of Rh-dB-ZSM-5 is much higher than the activity of Rh-ZSM-5 when the reaction is carried out under low pressure.

Technology Readiness Level Assessment of Formalin Production Pathways

Formalin is among the highest production volume chemicals and can be obtained by several production pathways, though there are several potential production pathways in earlier development stages. To evaluate the maturity level of formalin production technologies, assessment is performed according to Technology Readiness Levels (TRL). TRL is a method used to describe the maturity of technologies, and assign them a TRL level or a range of levels from 1 to 9, with TRL 9 indicating the most mature technologic systems. Formalin is most synthesized from methanol produced via syngas which is obtained by steam reforming of natural gas. Other production pathways are formalin produced from biogas, through gasification of biomass, coal, black liquor and other, through methanol produced by direct hydrogenation of CO2 or via reverse water gas shift reaction, also known as the CAMERE process. Finally, emerging technologies such as direct conversion of methane, syngas or CO2 to formalin were considered. This work will contribute to a wider understanding of the various technologies used in the formalin production field.

Theoretical Insights into Dual-Metal-Site Catalysts for the Nonoxidative Coupling of Methane

Direct conversion of methane to C2 hydrocarbons under nonoxidative conditions is an attractive technology but is challenging due to high reaction temperature, severe coke deposition, and low selectivity. Here, we report three dual-metal-site catalysts (DMSCs) based on nitrogen-doped graphene (FeCo–N–C, Fe2–N–C, and Co2–N–C) for nonoxidative coupling of methane to C2 hydrocarbons from a theoretical perspective. Our calculated results reveal that DMSCs present universally better performance in methane activation than single-metal-site catalysts (Fe–N–C and Co–N–C). Among the three DMSCs, Co2–N–C exhibits the best catalytic activity and superior selectivity to ethane in the whole reaction pathway. Our microkinetic modeling reveals that the Co2–N–C catalyst can convert methane to CH3, C2H6, and H2 at 1200 K. The electronic structure analysis and ab initio molecular dynamics simulations demonstrate that Co2–N–C possesses both intrinsic stability and high-temperature stability. Moreover, Co2–N–C also manifests better coke resistance compared with larger Co clusters, indicated by the difficult kinetics of methane deep dehydrogenation to naked carbon. This work provides a potential catalyst prototype for the selective conversion of methane to C2 hydrocarbons under nonoxidative conditions.

Low-temperature conversion of methane to oxygenates by supported metal catalysts: From nanoparticles to single atoms

Direct cost-effective conversion of abundant methane to high value-added oxygenates (methanol, formic acid, acetic acid, etc.) under mild conditions is prospective for optimizing the structure of energy resources. However, the CH bond of products is more reactive than that of high thermodynamic stable methane. Exploring an appropriate approach to eliminate the “seesaw effect” between methane conversion and oxygenate selectivity is significant. In this review, we briefly summarize the research progress in the past decade on low-temperature direct conversion of methane to oxygenates in gas-solid-liquid phase over various transition metal (Fe, Cu, Rh, Pd, AuPd, etc.) based nanoparticle or single-atom catalyst. Furthermore, the prospects of catalyst design and catalysis process are also discussed.

Direct methane activation by atomically thin platinum nanolayers on two-dimensional metal carbides

Efficient and direct conversion of methane to value-added products has been a long-term challenge in shale gas applications. Here, we show that atomically thin nanolayers of Pt with a single or double atomic layer thickness, supported on a two-dimensional molybdenum titanium carbide (MXene), catalyse non-oxidative coupling of methane to ethane/ethylene (C2). Kinetic and theoretical studies, combined with in-situ spectroscopic and microscopic characterizations, demonstrate that Pt nanolayers anchored at the hexagonal close-packed sites of the MXene support can activate the first C–H bond of methane to form methyl radicals that favour desorption over further dehydrogenation and thus suppress coke deposition. At 750 °C and 7% methane conversion, the catalyst runs for 72 hours of continuous operation without deactivation and exhibits >98% selectivity towards C2 products, with a turnover frequency of 0.2–0.6 s−1. Our findings provide insights into the design of highly active and stable catalysts for methane activation and create a platform for developing atomically thin supported metal catalysts. The challenge in non-oxidative coupling of methane lies in the activation of the first C–H bond while avoiding further dehydrogenations, which lead to the formation of coke. Here, atomically thin platinum nanolayers on two-dimensional molybdenum titanium carbides are reported as a superior catalyst for this reaction owing to reduced coke formation.

Mechanism investigation and product selectivity control on CO-assisted direct conversion of methane into C1 and C2 oxygenates catalyzed by zeolite-supported Rh

Methane has emerged as an important energy source and chemical feedstock, and thus breakthrough strategies for the direct partial oxidation of CH4 into small oxygenates have been desired. Here, CO-assisted CH4 conversion towards C1 and C2 oxygenates over zeolite-supported single-atom Rh catalysts was demonstrated, and the key role of CO, as an indispensable additive, and the reaction mechanism were experimentally investigated. Step-by-step introduction of the reactants resulted in the formation of stoichiometric amounts of product. Replacement of O2 by H2O2 revealed that gaseous O2 acts as a true oxidant, and CO enhanced the reaction as a ligand. The critical effect of acid sites in the formation of the C2 product was also confirmed. Based on these results, a plausible reaction mechanism was proposed. Finally, a small-pore zeolite, SSZ-13, was found to be superior for the selective production of methanol.

Solar-energy-driven conversion of oxygen-bearing low-concentration coal mine methane into methanol on full-spectrum-responsive WO3−x catalysts

Sunlight-driven photocatalysis is regarded as a promising strategy for direct conversion of methane in low concentration coal mine methane (LC-CMM) to value-added methanol, yet remains a grand challenge to efficiently activate and convert methane. Herein, WO3−x nanosheets with gradient concentration of oxygen vacancy were synthesized and firstly served as full-spectrum responsive catalysts for transformation of LC-CMM to methanol at ambient conditions. Defect-rich WO3−x-N2.0 shows a methanol yield of 1475 μmol·g−1, roughly 4.5 times higher than WO3-A2.0 under simulated solar light irradiation. The selectivity of CH3OH on the optimal WO3−x-N2.0 is up to 76%. More importantly, WO3−x-N2.0 exhibits a methanol yield of 396 μmol·g−1 with the selectivity of 82% even under near infrared light irradiation while almost no CH3OH is detected over WO3-A2.0. Based on the results of energy-band structure, photoelectrochemical characterization, PLs and EPR tests, the significantly enhanced photocatalytic performance over WO3−x-N2.0 is ascribed to the synergistic effect caused by the formation of oxygen vacancies, including extending light absorption into NIR region, improving separation of photoinduced electron-hole pairs and boosting production of hydroxyl radicals (key active species that drive CH3OH production). This work will offer a sustainable pathway to broaden the utilization of LC-CMM via efficient coupling of solar energy.

Topological Transformation of Mg-Containing Layered Double Hydroxide Nanosheets for Efficient Photodriven CH4 Coupling.

Direct conversion of methane (CH4 ) to fuels and other high value-added chemicals is an attractive technology in the chemical industry; however, practical challenges to sustainable processes remain. Herein, we report the preparation of a heterostructured Co-doped MgO-based catalyst through topological transformation of a MgCo-layered double hydroxide (LDH) calcination from 200 to 1100 °C. Remarkably, the catalyst can activate CH4 coupling to produce C2 H6 with a selectivity of 41.60 % within 3 h under full-spectrum irradiation through calcination of LDH at 800 °C. Characterization studies and catalytic results suggest that the highly dispersed active sites and large interfaces amongst the Co-doped MgO-based catalysts enable surface activation of CH4 to methyl (CH3 *), in turn promoting coupling of CH3 * to C2 H6 . This study introduces a promising pathway for photodriven CH4 coupling to give high value-added chemicals by using layered double hydroxides as a precursor.

Light-Induced Nonoxidative Coupling of Methane Using Stable Solid Solutions.

Achieving efficient and direct conversion of methane under mild conditions is of great significance for innovations in the chemical industry. However, the efficiency and lifetime of most catalysts remain too far from practical requirements, since it is difficult to break the first C-H bond of methane as well as to suppress the following complete dehydrogenation (or overoxidation) and the resulting carbonaceous deposition (or CO2 ). Here, we report that wurtzite GaN:ZnO solid solutions exhibit unique and unprecedented photocatalytic performances for the nonoxidative coupling of methane at room temperature, exclusively generating ethane with nearly stoichiometric H2 . High conversion rate (>330 μmol g-1 h-1 ), long-term stability (>70 h), and superior coke-resistance were achieved. At 293 K, the methane conversion exceeds 7 %, comparable to the equilibrium conversion of thermal catalysis at 910 K. Mechanistic studies revealed that the N-ZnGa -ON units and the absence of acid sites on the surface played crucial roles in reactivity and coke resistance, respectively.

Light‐Induced Nonoxidative Coupling of Methane Using Stable Solid Solutions

AbstractAchieving efficient and direct conversion of methane under mild conditions is of great significance for innovations in the chemical industry. However, the efficiency and lifetime of most catalysts remain too far from practical requirements, since it is difficult to break the first C−H bond of methane as well as to suppress the following complete dehydrogenation (or overoxidation) and the resulting carbonaceous deposition (or CO2). Here, we report that wurtzite GaN:ZnO solid solutions exhibit unique and unprecedented photocatalytic performances for the nonoxidative coupling of methane at room temperature, exclusively generating ethane with nearly stoichiometric H2. High conversion rate (>330 μmol g−1 h−1), long‐term stability (>70 h), and superior coke‐resistance were achieved. At 293 K, the methane conversion exceeds 7 %, comparable to the equilibrium conversion of thermal catalysis at 910 K. Mechanistic studies revealed that the N‐ZnGa‐ON units and the absence of acid sites on the surface played crucial roles in reactivity and coke resistance, respectively.

Nature of the Active Sites on Ni/CeO2 Catalysts for Methane Conversions.

Effective catalysts for the direct conversion of methane to methanol and for methane’s dry reforming to syngas are Holy Grails of catalysis research toward clean energy technologies. It has recently been discovered that Ni at low loadings on CeO2(111) is very active for both of these reactions. Revealing the nature of the active sites in such systems is paramount to a rational design of improved catalysts. Here, we correlate experimental measurements on the CeO2(111) surface to show that the most active sites are cationic Ni atoms in clusters at step edges, with a small size wherein they have the highest Ni chemical potential. We clarify the reasons for this observation using density functional theory calculations. Focusing on the activation barrier for C–H bond cleavage during the dissociative adsorption of CH4 as an example, we show that the size and morphology of the supported Ni nanoparticles together with strong Ni-support bonding and charge transfer at the step edge are key to the high catalytic activity. We anticipate that this knowledge will inspire the development of more efficient catalysts for these reactions.

Plasma chemical conversion of methane by pulsed electron beams and non-self-sustained discharges

Investigations of methane conversion in atmospheric-pressure mixtures of various composition and temperature, irradiated by pulsed nanosecond electron beams and non-self-sustained discharges initiated by electron beams, were carried out. It was shown that during direct conversion at room temperature, the energy cost of the conversion of one CH4 molecule (5.6 eV molecule−1) was commensurate with that obtained in the experiments with the use of continuous electron beam. The main products of direct conversion were hydrogen and ethylene. If a small amount of oxygen was present in the mixture, methanol was added to the conversion products, which in turn decomposed into CO, CO2 and H2O. The results of numerical simulations and experiments were basically the same. The use of a non-self-sustained discharge increased the degree of direct conversion of methane, but the energy cost of conversion also increased. The main products of carbon dioxide conversion of CH4 (dry reforming) under the action of pulsed electron beam at room temperature were CO, H2, C2H6, and carbon. Energy cost of conversion (12 eV molecule−1) was about two times higher than in the case of direct conversion of methane. However, the use of pulsed electron beam for dry reforming turned out to be energetically more efficient than the use of the most types of discharges. The main products of CH4 oxidative conversion by pulsed electron beam were H2, C2H6, and C2H4. At room temperature, the energy cost of methane conversion without a catalyst was 9 eV molecule−1, whereas the use of Ni catalyst resulted in a seven-fold increase, and the use of NaOH/CaO one—in a 14-fold increase in the conversion efficiency compared to irradiation without a catalyst. When the temperature of the catalysts rised to 623 K, the conversion efficiency increased by another four times. The processes in electron-beam plasma that provide the implementation of the main mechanisms of methane conversion are also discussed in the paper.

Distinct Pathways in Visible-Light Driven Thermo-Photo Catalytic Methane Conversion.

The utilization of visible light for direct photocatalytic methane conversion remains a huge challenge. Here, we developed a thermo-photo catalytic process with a visible-light-responsive Pt/WO3 catalyst and realized highly efficient visible-light driven methane conversion for the first time. The conversion efficiency was enhanced by 4.6 and 14.7 times compared to room-temperature photocatalysis and thermal catalysis at 150 °C, respectively. Furthermore, the production of liquid oxygenates (mainly CH3OH) was found to proceed via photocatalysis with high apparent quantum efficiencies of 5.9%, 4.5%, and 1.9% at 350, 420, and 450 nm, respectively, while CO2 evolution was contributed by photoassisted thermal catalysis. Solid isotope evidence further confirmed that CH3OH, HCHO, and CO2 were produced via parallel rather than sequential reactions. These observations provide a valuable guide for designing a visible-light driven system for methane conversion with high efficiency and controllable selectivity.

Assessment of the current state of research and achievements in the field of catalytic processing of natural gas into valuable chemical products

The review examines the current state of the catalytic conversion of natural gas into valuable chemical products and fuel. The main component of natural gas is methane. Methane conversion processes are of great importance for society because natural gas, along with oil, supplies us with energy, fuel and chemical products. Direct and indirect methods of methane conversion are considered. Direct conversion of methane is often viewed as the holy grail of modern research, since methane molecules are very stable. The review considers the methods of obtaining such compounds as synthesis gas, methanol, ethylene, formaldehyde, benzene, etc. The greatest emphasis is placed on the direct processes of methane conversion, namely on the dehydroaromatization of methane. The catalysts and the conditions for their preparation are considered, the state of active centers is studied, and the mechanism of methane dehydroaromatization is proposed. The reasons for deactivation of the catalysts and methods of their regeneration are also described. This review will help to summarize the latest known achievements in the field of heterogeneous catalysis for natural gas processing.

One-step Hydrothermal Synthesis of Unsupported Nickel Phosphide Catalyst for Direct Dehydrogenative Conversion of Methane to Hydrocarbons

Abstract Unsupported Ni2P catalyst synthesized by the hydrothermal method was studied for the direct dehydrogenative conversion of methane to hydrocarbons. The unsupported Ni2P catalyst showed smaller crystallite size and porous surface structure due to the absence of calcination and reduction at higher temperatures and exhibited better catalytic activity at 1173 K. It has been found that the simple hydrothermal method was effective for the synthesis of active unsupported Ni2P catalyst.

Synthesis of Methanesulfonic Acid Directly from Methane: The Cation Mechanism or the Radical Mechanism?

In 2019, Diaz-Urrutia and Ott developed a high-yield method for direct conversion of methane to methanesulfonic acid and proposed a cationic chain reaction mechanism. However, Roytman and Singleton questioned this mechanism, and they favored a free-radical mechanism. In the present paper, we studied both the cationic chain and radical mechanisms and found the radical mechanism is more favorable, since it has a much lower energy barrier. However, the radical mechanism has not considered the effect of ions for the reaction taking place in oleum. Thus, we studied a simple model of a protonated radical mechanism, which further lowers the energy barrier. Although the true mechanism for the CH4 + SO3 reaction could be more complicated in electrolyte solutions, this model should be helpful for the further study of the mechanism of this reaction.