Valorization Of Methane Research Articles

Elucidating the nature of Mo species on ZSM-5 and its role in the methane aromatization reaction

The valorization of methane is one of the most important goals during the transition period to the general use of renewable energies.

Direct piezocatalytic conversion of methane into alcohols over hydroxyapatite

The cascading of methane valorization and methanol upgrade, both of which are important in C1 chemistry, becomes a promising strategy to fabricate value-added chemicals from methane directly. Reconciling the contradiction between the C‒H bond activation of methane and the over-oxidation susceptibility of methanol is critical but challenging. Herein, we report the piezocatalytic methane conversion over hydroxyapatite (HAp) under ultrasonic vibration at ambient conditions, obtaining alcohols as liquid products without any sign of CO or CO2. Mechanistic studies reveal that the piezocatalytic effect of HAp drives H2O and methane activation and supplies hydroxyl radical (·OH) to attack the methane C‒H bond, while the ultrasonic-induced negative surface charge (SC) promotes methanol dehydration into methyl carbene as a fascinating C1 building block to facilitate product upgrade into C2 and C3 alcohols. Attributing to the enriched SC which tunes the surface acid-base property, piezocatalysis presents a new approach for C‒C coupling.

Selective activation of methane C[sbnd]H bond in the presence of methanol

Direct methane to methanol (MTM) conversion over heterogeneous catalysts is a promising route for valorization of methane. The methane CH bond activation is considered as the key step of the MTM and is the focus of considerable research activity. However, the formed methanol typically suffers from overoxidation largely due to the cleavage of a methanol CH bond, whose bond dissociation energy is ca. 0.5 eV lower than that of the methane CH bond, which usually translates to a transition state energy of the methanol CH bond cleavage that is ca. 0.55 eV lower than that of methane whenever the reactions proceed through a radical mechanism. Here, we propose a general approach for decreasing the transition state energy difference between the CH4 and CH3OH CH bond dissociation. When a metal-oxide supported cationic transition metal atom and a neighboring oxygen on the oxide surface serve as the active site, the transition state energy difference through a surface-stabilized pathway can be noticeably narrowed as compared with that of a radical pathway. For Ir, Pt and Rh-doped anatase TiO2(1 0 1), the CH4 CH bond activation can be preferred over that of CH3OH at significant methanol mole fraction. Also, for PdAu alloys containing adsorbed oxygen and positively charged Pd, calculations suggest in agreement with recent experiments (Science367 (6474), 193–197) that the CH4 CH bond can be selectively activated in the presence of CH3OH.

Direct functionalization of methane into ethanol over copper modified polymeric carbon nitride via photocatalysis

Direct valorization of methane to its alcohol derivative remains a great challenge. Photocatalysis arises as a promising green strategy which could exploit hydroxyl radical (·OH) to accomplish methane activation. However, both the excessive ·OH from direct H2O oxidation and the neglect of methane activation on the material would cause deep mineralization. Here we introduce Cu species into polymeric carbon nitride (PCN), accomplishing photocatalytic anaerobic methane conversion for the first time with an ethanol productivity of 106 μmol gcat−1 h−1. Cu modified PCN could manage generation and in situ decomposition of H2O2 to produce ·OH, of which Cu species are also active sites for methane adsorption and activation. These features avoid excess ·OH for overoxidation and facilitate methane conversion. Moreover, a hypothetic mechanism through a methane-methanol-ethanol pathway is proposed, emphasizing the synergy of Cu species and the adjacent C atom in PCN for obtaining C2 product.

Methanotrophy - Environmental, Industrial and Medical Applications.

Aerobic methanotrophs are an intriguing group of microbes with the singular ability to consume methane as their sole source of carbon and energy. As such, methanotrophs are receiving increased attention to control methane emissions to limit future climate change. Methanotrophs have a wide range of other applications, including pollutant remediation and methane valorization (e.g. conversion of methane to protein, bioplastics, and biodiesel amongst other products). Methanotrophs also produce a novel copper-binding compound, methanobactin, that has significant potential for the treatment of copper-related human pathologies. Here we provide an overview of aerobic methanotrophy, describe current and future applications of these unique microbes, as well as discuss various strategies one can consider to better realize the opportunities these microbes present.

Non-oxidative methane conversion in microwave-assisted structured reactors

The main problem to be addressed in the valorization of methane under non-oxidative conditions (MNOC) is to reduce or even avoid coke formation. In this work we report the use of microwave-assisted heating for MNOC. We have developed a system able to heat-up a Mo-ZSM5 catalyst coated on silicon carbide monolith that could operate stable for at least 19 h at reaction conditions, 700 °C. We demonstrate that under MW-heating the selectivity shifts to C2s and benzene. In contrast, the operation under conventional heating (CH) produces more coke and polyaromatics. The selective microwave heating has two effects in this reaction: i) during the activation of the catalyst the formation of the active catalytic species of Mo2C inside the microporous support is different affecting the selectivity and product distribution; ii) a gas-solid temperature gradient is established that prevents the formation of coke from PAHs in the gas phase. The MNOC process under controlled MW heating at high space velocity (3000 mL/gcat·h) gives a hydrocarbon yield of around 6% with a very low deactivation rate. These results open up new possibilities for process intensification using alternative sources of energy, as is the case of microwaves, for heating structured catalytic reactors.

Unraveling reaction networks behind the catalytic oxidation of methane with H2O2 over a mixed-metal MIL-53(Al,Fe) MOF catalyst.

Reaction paths underlying the catalytic oxidation of methane with H2O2 over an Fe containing MIL-53(Al) metal-organic framework were studied by periodic DFT calculations. Not only the activation of methane, but the full reaction network was considered, which includes the formation of the active site, the overoxidation of methane to CO2 and the decomposition of H2O2 to H2O and O2. Calculations indicate that the activation barrier for the initial activation of the Fe sites upon reaction with H2O2 is comparable to that of the subsequent C-H activation and also of the reaction steps involved in the undesirable overoxidation processes. The pronounced selectivity of the oxidation reaction over MIL-53(Al,Fe) towards the target mono-oxygenated CH3OH and CH3OOH products is attributed to the limited coordination freedom of the Fe species encapsulated in the extended octahedral [AlO6] structure-forming chains, which effectively prevents the direct overoxidation paths prior to product desorption from the active sites. Importantly, our computational analysis reveals that the active sites for the desired methane oxidation are able to much more efficiently promote the direct catalytic H2O2 decomposition reaction, rendering thus the current combination of the active site and the reactants undesirable for the prospective methane valorization process.

Reaction network of the chloromethane conversion into light olefins using a HZSM-5 zeolite catalyst

The second step of chlorine-mediated methane valorization into hydrocarbons has been investigated using a HZSM-5 zeolite catalyst. A parametric study has enabled to set the reaction network, which is dominated by the dual cycle mechanism and secondary reactions of light olefins. This network explains the formation of methane, light olefins, C5+ aliphatics, paraffins, aromatics and coke. Under the optimal conditions, the light olefin selectivity is >70%, of which >40% corresponds to propylene. Coke is originated in the zeolite micropores and then grows within the matrix meso- and macropores.

Copper and cerium-regulated gene expression in Methylosinus trichosporium OB3b.

In aerobic methanotrophs, copper and cerium control the expression and activity of different forms of methane monooxygenase and methanol dehydrogenase, respectively. To exploit methanotrophy for the valorization of methane, it is crucial to determine if these metals exert more global control on gene expression in methanotrophs. Using RNA-Seq analysis we compared the transcriptome of Methylosinus trichosporium OB3b grown in the presence of varying amounts of copper and cerium. When copper was added in the absence of cerium, expression of genes encoding for both soluble and particulate methane monooxygenases varied as expected. Genes encoding for copper uptake, storage, and efflux also increased, indicating that methanotrophs must carefully control copper homeostasis. When cerium was added in the absence of copper, expression of genes encoding for alternative methanol dehydrogenases varied as expected, but few other genes were found to have differential expression. When cerium concentrations were varied in the presence of copper, few genes were found to be either up- or downregulated, indicating that copper over rules any regulation by cerium. When copper was increased in the presence of cerium, however, many genes were upregulated, most notably multiple steps of the central methane oxidation pathway, the serine cycle, and the ethylmalonyl-CoA pathway. Many genes were also downregulated, including those encoding for nitrogenase and hydrogenase. Collectively, these data suggest that copper plays a larger role in regulating gene expression in methanotrophs, but that significant changes occur when both copper and cerium are present.

Characterization of the role of copCD in copper uptake and the 'copper-switch' in Methylosinus trichosporium OB3b.

Methanotrophs or methane-oxidizing bacteria exhibit a unique 'copper-switch' where expression of two forms of methane monooxygenase (MMO) is controlled by the availability of copper. In the absence of copper, a cytoplasmic or soluble methane monooxygenase (sMMO) is expressed. In the presence of copper, a membrane-bound or particulate methane monooxygenase (pMMO) is expressed. These two forms of MMO have very different properties, and elucidation of the basis of the copper-switch is of significant interest as methanotrophs are becoming increasingly popular for the valorization of methane. Recently, it was suggested via characterization of a mutant of Methylosinus trichosporium OB3b that expresses sMMO in the presence of copper (smmoC mutant) that the copper-switch may be based on copCD. These genes encode for a periplasmic copper-binding protein and an inner membrane protein, respectively, and are used by other bacteria for copper uptake. Specific knockouts of copCD in M. trichosporium OB3b wild type, however, show that these genes are not part of the copper-switch in methanotrophs, nor do they appear to be critical for copper uptake. Rather, it appears that the constitutive expression of sMMO in the smmoC mutant of M. trichosporium OB3b may be due to multiple lesions as smmoC was generated via random chemical mutagenesis.

Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous Catalysis: Challenges and Opportunities

In this Perspective, we highlight the main challenges to be addressed in the development of heterogeneous catalysts for the direct functionalization of methane. Along with our personal view on current developments in this field, we outline the main mechanistic, engineering, and catalyst design issues that have hampered implementation of new technologies and highlight possible paths to overcome these problems.

Methane dehydro-aromatization on Mo/ZSM-5: About the hidden role of Brønsted acid sites

Abstract Whereas the hegemony of the Fischer–Tropsch synthesis is well established for the valorization of methane in the case of important gas fields, the methane dehydro-aromatization reaction remains an interesting solution to convert methane obtained as by-product of oil extraction and thus fight against flaring and global warming. Until now, it was generally accepted that the molybdenum is anchored on the Bronsted acid sites of the zeolite as (Mo2O5)2+ species. The molybdenum performs the dehydrogenation and coupling of CH4 to ethylene which is consecutively oligomerized to benzene over the Bronsted acid sites. In the present work, we bring evidence that this picture is actually more complicated than it seems. The Si/Al ratio of the zeolite, and consequently the density of available Bronsted acid sites, plays a dramatic role on the anchoring mode of the molybdenum and on its catalytic activity.

Uses of transient kinetics for methane activation studies

The use of transient kinetics is reviewed for recent works performed in the wide area of methane valorization. The specific contribution of techniques such as steady-state isotopic transient kinetics, fast temperature transients, TAP reactor studies is illustrated for reactions involving different modes of methane activation and various types of catalysts.

Utilisation de la post-décharge d'un plasma micro-ondes d'air ou d'azote pour valoriser le méthane

Valorization of methane is obtained by means of the reaction with a plasma. The plasma is produced in a quartz tube (30 mm in diameter) crossing a wave guide. The energy is supplied by a generator (Thomson CSF, 2 450 MHz, 15 to 1 500 W). Directional couplers followed by an attenuator and thermistor detectors enable forward II i and reflected II r power to be measured. Methane is introduced in the post-discharge zone through five tubes symmetrically arranged around the reactor. Methane consumption α, selectivity S X and yield R X are measured as a function of the following parameters: β methane/air ratio, F gaz flow, d distance where methane is introduced in the plasma, P pressure, II microwave power absorbed by air or nitrogen. It appears that acetylene is the major hydrocarbon obtained. The corresponding selectivity is increased when II and P are increased or when d and F are decreased. The optimum value of β is 4/5. If the best experimental conditions are selected, selectivity of total C 2 reaches 44% with a conversion ratio of 80%. By decreasing methane/air ratio, carbon monoxide yield is increased. The ratio acetylene/ethylene can by varied without changing the conversion ratio, by introducing a catalyst in the post reaction zone

ChemInform Abstract: The Different Catalytic Routes for Methane Valorization: An Assessment of Processes for Liquid Fuels

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Different Catalytic Routes for Methane Valorization: An Assessment of Processes for Liquid Fuels

Abstract Scope of Review. This review is written from the point of view of the process design engineer trying to find the best catalytic options for upgrading methane. It is not intended to be a review of catalytic principles. In fact, actual catalysts used are only mentioned in passing. If the reader wishes to explore this aspect further, there are many recent review articles that can be used and these are referred to at an appropriate time and place.