Direct Conversion Of Methane Research Articles

Ethylene production by partial oxidation of biogas using lime as catalyst

AbstractBACKGROUNDThe oxidative coupling of methane (OCM) is an interesting route for direct conversion of methane into ethylene and ethane. Nevertheless, COx is a secondary product of this route, which decreases the selectivity of hydrocarbons. In this work it is proposed to overcome this drawback by using biogas as source of methane. In addition to being a renewable source of methane, the presence of CO2 in biogas may shift the balance of the methane combustion. For this purpose, it was evaluated how process conditions such as O2 and CO2 flow rates and temperature affect biogas‐based OCM, in the temperature range of 650–800 °C, using lime as catalyst.RESULTSThe increase in CO2 flow from 0 to 15 mL.min−1 increased the ethylene selectivity from 25.9% to 44.5% at 800 °C. As consequence, CO2 formation dropped from 53.9% to 18.3%. Methane conversion was not affected significantly by CO2 flow rate, remaining in the range of 22–25%. O2 flow was essential to activate the OCM reaction and the presence of higher concentrations of O2 favored the selectivity of ethylene, but also increased the CO2 produced through methane combustion. In the OCM experiments with variable flow of CO2, the highest C2 yield of 18% was obtained for the highest flow of co‐fed CO2, whereas the experiments with biogas and variable O2 flow showed the highest yield of ethylene for the highest flow of air co‐fed.CONCLUSIONThus, biogas‐based OCM was shown to be a promising route to improve C2 selectivity, since the CO2 present in the biogas minimizes the formation of CO2 in the reaction. The use of lime as a low‐cost catalyst proved to be viable for the OCM reaction, presenting results compatible with those obtained for catalysts with more complex composition. © 2023 Society of Chemical Industry.

Direct Conversion of Methane to Methanol by CuN3 Supported on Graphene

Direct Conversion of Methane to Methanol by CuN3 Supported on Graphene

Rational Design of the Catalysts for the Direct Conversion of Methane to Methanol Based on a Descriptor Approach

The direct oxidation of methane to methanol as a liquid fuel and chemical feedstock is arguably the most desirable methane conversion pathway. Currently, constructing and understanding linear scaling relationships between the fundamental physical or chemical properties of catalysts and their catalytic performance to explore suitable descriptors is crucial for theoretical research on the direct conversion of methane to methanol. In this review, we summarize the energy, electronic, and structural descriptors used to predict catalytic activity. Fundamentally, these descriptors describe the redox properties of active sites from different dimensions. We further explain the moderate principle of descriptors in methane-to-methanol catalyst design and provide related application work. Simultaneously, the underlying activity limitation of methane activation and active species generation is revealed. Based on the selectivity descriptor, the inverse scaling relationship limitation between methane conversion and methanol selectivity is quantitatively understood. Finally, multiscale strategies are proposed to break the limitation and achieve the simultaneous enhancement of activity and selectivity. This descriptor-based review provides theoretical insights and guidance to accelerate the understanding, optimization, and design of efficient catalysts for direct methane-to-methanol conversion.

An efficient strategy for the partial oxidation of methane into methanol over POM-immobilized MOF catalysts under ambient conditions

One of the scientific obstacles in the direct conversion of methane to methanol lies in how to improve product selectivity. A dark photocatalysis mode that decouples solar energy in the dark may effectively alleviate this challenge. Herein, a range of composites had been prepared in which Fe-BTC was employed as the gas enrichment unit, while phosphotungstic acid (PTA) acted mainly as an electron storage within the framework. The modified reaction system allowed for the achievement of methanol yields up to 968 μmol·gcat−1·h−1 under ambient conditions. Besides extrinsic factors, such as a gentle reaction environment and avoidance of the oxidation reactions between photogenerated holes and alcohols, we also demonstrated through in situ EPR measurements that the introduced PTA effectively regulated •OH radicals and suppressed their further reaction with methanol. Moreover, the strategy demonstrated its universality by effectively enhancing the performance of other POM-immobilized MOF composites. This work opens a new path for the efficient and targeted conversion of inert methane to methanol.

Direct Conversion of Methane to Value-Added Hydrocarbons over Alumina-Supported Cobalt Modified by Alkaline Earth Catalysts

Direct Conversion of Methane to Value-Added Hydrocarbons over Alumina-Supported Cobalt Modified by Alkaline Earth Catalysts

Nonoxidative methane coupling in a micro‐DBD with enhanced secondary electron emission

AbstractThe conversion of methane into transportable and storable materials is crucial in the petrochemical sector. In this study, a specially constructed AC‐driven dielectric barrier discharge (DBD) that has charge injector pyramids on one of the electrodes and runs at ambient temperature and pressure was used to evaluate noncatalytic methane conversion. The obtained result was compared with the traditional flat electrode DBD. It was discovered that the product selectivity in direct nonoxidative methane conversion depended on the discharge conditions. Pyramid electrode plasma sources convert methane up to 50 more than flat electrode plasma due to the appearance of more microdischarges. Pyramid electrode plasma generally has a greater production efficiency than flat electrode plasma, while requiring more operational power. The turnkey solutions offered by the sustainable methane coupling method discussed here may be advantageous for the long‐term small‐scale ethylene exploration scenario.

Effective photocatalytic methane oxidation over the TiO2/methanotrophs system

The direct conversion of methane (CH4) to methanol (CH3OH) is a significant but challenging reaction. We present a photocatalytic system composed of brookite mesoporous titanium dioxide and methanotrophs (Methylosinus trichosporium OB3b) for the conversion of CH4 to CH3OH with high activity, selectivity and stability at ambient temperature and pressure. This system obtains the highest CH3OH production rate (15,761 ± 142 μmol g−1 h−1), which is dozens of times higher than most researches and 5 times of the optimal value of photcatalytic CH4 conversion under the same condition. The process achieved nearly 100% selectivity and excellent stability under simulated sunlight irradiation and without any cocatalysts, sacrificial agent or inhibitors. The biocompatibility of semiconductor materials and the contact mode with OB3b have significant impact on the process. Titanium dioxide is used as an exogenous electron donor to greatly promote the conversion activity of CH4 by OB3b under light irradiation. The work has significant pioneering implications for efficient CH4 selective conversion using natural gifts combined with artificial materials.

Theoretical Perspective of Promoting Direct Methane-to-Methanol Conversion at Complex Metal Oxide-Metal Interfaces.

Direct methane conversion to methanol has been considered as an effective and economic way to address greenhouse effects and the current high demand for methanol in industry. However, the process has long been challenging due to lack of viable catalysts to compromise the activation of methane that typically occurs at high temperatures and retaining of produced methanol that requires mild conditions. This Perspective demonstrates an effective strategy to promote direct methane to methanol conversion by engineering the active sites and chemical environments at complex metal oxide - copper oxide - copper interfaces. Such effort strongly depends on extensive theoretical studies by combining density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations to provide in-depth understanding of reaction mechanism and active sites, which build a strong basis to enable the identification of design principles and advance the catalyst optimization for selective CH4-to-CH3OH conversion.

Continuous gas-phase oxidation of methane into methanol over Cu-mordenite

The direct conversion of methane into methanol is an ideal, highly promising process; however, its application remains challenging. Herein, a systematic study was carried out on the continuous gas-phase oxidation of methane into methanol with steam in the absence and presence of O2 over Cu-mordenites (Cu-MOR) with various Cu loadings. The methanol productivity depended on the Cu/Al ratio of the Cu-MOR; it could be maximized by selecting the optimum Cu/Al ratio. Methanol productivity increased as the concentration of O2 in the feed increased from 0 to 100 ppm. However, methanol selectivity decreased when the O2 concentration in the feed was above 25 ppm. The stable methanol production rates based on the Cu content were 0.14 and 0.70 mol CH3OH/mol Cu/h at 400 °C over a Cu-MOR catalyst with 6.2 wt% Cu under anaerobic conditions and one with 3.0 wt% Cu in the presence of 25 ppm O2, respectively.

Computational Study of Single Metal Atom Anchored on Black Phosphorus for Methane Oxidation to Methanol by Nitrous Oxide.

Direct conversion of methane to high-value-added transportable methanol is a great challenge, which requires high energy input to break the strong C-H bond. Developing efficient catalysts for methane oxidation to methanol under mild conditions is of vital importance. In this work, single transition metal atoms (TM=Fe, Co, Ni, Cu) anchored on black phosphorus (TM@BP) were studied as catalysts to assist the methane oxidation to methanol by means of first-principles calculations. The results indicate that Cu@BP exhibits an outstanding catalytic activity through the radical reaction pathways and the formation of the Cu-O active site is rate-determining with an energy barrier of 0.48 eV. Meanwhile, electronic structure calculations and dynamic simulations show that Cu@BP offers excellent thermal stability. Our calculations provide a new approach for the rational design of single atom catalysts for methane oxidation to methanol.

Illustrating the Fate of Methyl Radical in Photocatalytic Methane Oxidation over Ag−ZnO by in situ Synchrotron Radiation Photoionization Mass Spectrometry

AbstractPhotocatalysis has emerged as an ideal method for the direct activation and conversion of methane under mild conditions. In this reaction, methyl radical (⋅CH3) was deemed a key intermediate that affected the yields and selectivity of the products. However, direct observation of ⋅CH3 and other intermediates is still challenging. Here, a rectangular photocatalytic reactor coupled with in situ synchrotron radiation photoionization mass spectrometry (SR‐PIMS) was developed to detect reactive intermediates within several hundred microseconds during photocatalytic methane oxidation over Ag−ZnO. Gas phase ⋅CH3 generated by photogenerated holes (O−) was directly observed, and its formation was demonstrated to be significantly enhanced by coadsorbed oxygen molecules. Methoxy radical (CH3O⋅) and formaldehyde (HCHO) were confirmed to be key C1 intermediates in photocatalytic methane overoxidation to CO2. The gas‐phase self‐coupling reaction of ⋅CH3 contributes to the formation of ethane, which indicates the key role of ⋅CH3 desorption in the highly selective synthesis of ethane. Based on the observed intermediates, the reaction network initiated from ⋅CH3 of photocatalytic methane oxidation could be clearly illustrated, which is helpful for studying the photocatalytic methane conversion processes.

Illustrating the Fate of Methyl Radical in Photocatalytic Methane Oxidation over Ag-ZnO by in situ Synchrotron Radiation Photoionization Mass Spectrometry.

Photocatalysis has emerged as an ideal method for the direct activation and conversion of methane under mild conditions. In this reaction, methyl radical (⋅CH3 ) was deemed a key intermediate that affected the yields and selectivity of the products. However, direct observation of ⋅CH3 and other intermediates is still challenging. Here, a rectangular photocatalytic reactor coupled with in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) was developed to detect reactive intermediates within several hundred microseconds during photocatalytic methane oxidation over Ag-ZnO. Gas phase ⋅CH3 generated by photogenerated holes (O- ) was directly observed, and its formation was demonstrated to be significantly enhanced by coadsorbed oxygen molecules. Methoxy radical (CH3 O⋅) and formaldehyde (HCHO) were confirmed to be key C1 intermediates in photocatalytic methane overoxidation to CO2 . The gas-phase self-coupling reaction of ⋅CH3 contributes to the formation of ethane, which indicates the key role of ⋅CH3 desorption in the highly selective synthesis of ethane. Based on the observed intermediates, the reaction network initiated from ⋅CH3 of photocatalytic methane oxidation could be clearly illustrated, which is helpful for studying the photocatalytic methane conversion processes.

Direct conversion of methane to heavier gaseous alkanes using an electron beam

The circulation treatment of methane with 0.5 MeV electron beam in a cyclone-type reactor, which protects the synthesized products from back degradation, has been studied. As a result of the conversion, the content of methane in the circulating gas can be reduced to 30–43 wt%. The main products are C2C5 alkanes, and the ethane content in the mixture can reach almost 40 wt%. By-products are hydrogen and C6C13 high-octane gasoline alkanes. Electron beam treatment can also be used to concentrate helium in natural hydrocarbon mixtures. The key control factors for conversion are the composition of the feedstock, the rate of beam energy deposition in the gas, temperature, and the residence time of the gas components in the beam penetration area. The features of the use of powerful technological DC accelerators are noted.

One-step conversion of methane and formaldehyde to ethanol over SA-FLP dual-active-site catalysts: A DFT study

One-step conversion of methane and formaldehyde into ethanol is a 100% atom-efficient process for carbon resources utilization and environment protection but still faces eminent challenges due to the lacking of efficient catalysts. Therefore, developing active and stable catalysts is crucial for the co-conversion of methane and formaldehyde. Herein, twelve kinds of “Single-Atom” - “Frustrated Lewis Pair” (SA-FLP) dual-active-site catalysts are designed for the direct conversion of methane and formaldehyde to ethanol based on density functional theory (DFT) calculations and microkinetic simulations. The results show that the SA-FLP dual active sites can simultaneously activate methane at the SA site and activate formaldehyde at the FLP site. Among the twelve designed SA-FLP catalysts, Fe1-FLP shows the best performance in the co-conversion of methane and formaldehyde to ethanol with the rate-determining barrier of 1.15 eV. Ethanol is proved as the main product with the turnover frequency of 1.32 × 10−4 s−1 at 573 K and 3 bar. This work provides a universal strategy to design dual active sites on metal oxide materials and offers new insights into the effective conversion of methane and formaldehyde to desired C2 chemicals.

Concepts of Computational Approach to Explore Heterogeneous Catalysts for Direct Methane Conversion

AbstractThe nonoxidative coupling of methane has attracted much attention because it yields two useful products: ethane and hydrogen. However, low conversion at low temperatures and carbon deposition at high temperatures are considered problematic. In this Concept article, a solution to these problems is presented. On the basis of the reaction enthalpy for the initial C−H bond cleavage of methane and the energy difference between C1 species (CH3 and CH) on the catalyst surface, a catalyst search guideline is provided to suppress carbon deposition while keeping the conversion rate as high as possible. Alloys are considered catalyst candidates. Few materials satisfy both of these requirements simultaneously; however, several alloys, including MgPt, are shown to be promising. The results of validation experiments for the catalytic performance of MgPt are also discussed.

Boosting Direct Oxidation of Methane with Molecular Oxygen at Low Temperature over Rh/ZSM‐5 Catalyst

AbstractAs a clean fossil energy and chemical feedstock, development of breakthrough strategies for direct conversion of methane (CH4) into various chemicals under mild reaction conditions is highly desired. Herein, Rh/ZSM‐5 catalyst was reported to convert CH4 to oxygenates with molecular oxygen (O2) in the presence of CO at low temperature with promising catalytic performance. The oxygenates productivity reached as high as 5638.0 μmol gcat.−1 h−1. Structure characterization confirmed the existence of highly dispersed Rh2O3 nanoparticles with an average diameter of 2.4 nm in the Rh/ZSM‐5 catalyst. Based on control experiments and mechanism study, it was suggested that CO plays a pivotal role over the Rh/ZSM‐5 catalyst to boost direct oxidation of methane. CO participates in water‐gas‐shift reaction to produce in‐situ H species, which enables to form *OH, *OOH and H2O2 active species by reacting with O2. The as‐obtained *OH, *OOH and H2O2 species will thus render high activity for methane oxidation.

Effect of silicon carbide-based iron catalyst on reactor optimization for non-oxidative direct conversion of methane

The conversion of methane to olefins, aromatics, and hydrogen (MTOAH) can be used to stably obtain hydrocarbons when the effect of the catalytic surface is optimized from the reaction engineering perspective. In this study, Fe/SiC catalysts were packed into a quartz tube reactor. The catalytic surfaces of SiC and the impregnated Fe species decreased the apparent activation energies (Ea) of methane consumption in the blank reactor between 965 and 1020 °C. Consequently, the hydrocarbon yield increased by 2.4 times at 1020 °C. Based on the model reactions of ethane, ethylene, and acetylene mixed with hydrogen in the range of 500–1020 °C, an excess amount of Fe in the reactor favored the C–C coupling reaction over the selective hydrogenation of acetylene; consequently, coke formation was favored over the hydrogenation reaction. The gas-phase reactions and catalyst properties were optimized to increase hydrocarbon yields while reducing coke selectivity. The 0.2Fe catalyst-packed reactor (0.26 wt% Fe) resulted in a hydrocarbon yield of 7.1% and a coke selectivity of <2% when the ratio of the void space of the post-catalyst zone to the catalyst space was adjusted to be ≥2. Based on these findings, the facile approach of decoupling the reaction zone between the catalyst surface and the gas-phase reaction can provide insights into catalytic reactor design, thereby facilitating the scale-up from the laboratory to the commercial scale.

Continuous partial oxidation of methane to methanol over Cu-SSZ-39 catalysts

The direct conversion of methane to methanol using molecular oxygen as the oxidant is a grand challenge in the chemical sciences. Here we describe our efforts in using copper-exchanged SSZ-39 for this chemistry and show it is a promising material for this reaction. Testing in a continuous flow reactor at 225 °C, Cu-SSZ-39 showed superior activity for methane partial oxidation compared to Cu-SSZ-13 (site time yields of 26.9 ± 0.8 molCH3OH·molCu−1·h−1 *10−3 versus 13.2 molCH3OH·molCu−1·h−1 *10−3 respectively). A clear maximum in methanol productivity, both in terms of a mass catalyst basis and copper atom basis is observed for Cu-SSZ-39 at a Cu/Al ∼ 0.2. Running this reaction in the absence of oxygen over samples with a Cu/Al > 0.1 the reactivity is markedly lower than that in the presence of oxygen unlike what is observed in SSZ-13.

Aqueous‐Phase Partial Oxidation of Methane over Pd−Fe/ZSM‐5 with O2 in the Presence of H2

AbstractThe selective oxidation of methane into methane oxygenates such as CH3OH and HCOOH with O2 is a promising technology for direct methane conversion. This study successfully demonstrates the partial oxidation of methane over Pd−Fe/ZSM‐5 catalysts in water, where H2O2 was generated in situ from H2 and O2. A low Pd content is essential for preventing agglomeration of Pd nanoparticles, and intimate contact between Pd and Fe species is beneficial for achieving high yields of methane oxygenates. This one‐body catalyst showed better catalytic performance than that of previously reported ones, especially at low temperatures. 0.28 %Pd–0.56 %Fe/ZSM‐5 exhibited a stable catalytic performance in a continuous flow reactor.

Selective Methane Oxidation to Acetic Acid Using Molecular Oxygen over a Mono-Copper Hydroxyl Catalyst.

Acetic acid is an industrially important chemical, produced mainly via carbonylation of methanol using precious metal-based homogeneous catalysts. As a low-cost feedstock, methane is commercially transformed to acetic acid via a multistep process involving energy-intensive methane steam reforming, methanol synthesis, and, subsequently, methanol carbonylation. Here, we report a direct single-step conversion of methane to acetic acid using molecular oxygen (O2) as the oxidant under mild conditions over a mono-copper hydroxyl site confined in a porous cerium metal-organic framework (MOF), Ce-UiO-Cu(OH). The Ce-UiO MOF-supported single-site copper hydroxyl catalyst gave exceptionally high acetic acid productivity of 335 mmolgcat-1 in 96% selectivity with a Cu TON up to 400 at 115 °C in water. Our spectroscopic and theoretical studies and controlled experiments reveal that the conversion of methane to acetic acid occurs via oxidative carbonylation, where methane is first activated at the copper hydroxyl site via σ-bond metathesis to afford Cu-methyl species, followed by carbonylation with in situ-generated carbon monoxide and subsequent hydrolysis by water. This work may guide the rational design of heterogeneous abundant metal catalysts for the activation and conversion of methane to acetic acid and other valuable chemicals under mild and environmentally friendly reaction conditions.