Direct Oxidation Of Methane Research Articles

Roles of Acidic Proton for Fe-Containing Zeolite in Direct Oxidation of Methane

Roles of Acidic Proton for Fe-Containing Zeolite in Direct Oxidation of Methane

Tailored pore-confined single-site iron(III) catalyst for selective CH4 oxidation to CH3OH or CH3CO2H using O2.

Direct oxidation of methane to valuable oxygenates like alcohols and acetic acid under mild conditions poses a significant challenge due to high C‒H bond dissociation energy, facile overoxidation to CO and CO2 and the intricacy of C-H activation/C-C coupling. In this work, we develop a multifunctional iron(III) dihydroxyl catalytic species immobilized within a metal-organic framework (MOF) for selective methane oxidation into methanol or acetic acid at different reaction conditions using O2. The active-site isolation of monomeric FeIII(OH)2 species at the MOF nodes, their confinement within the porous framework, and their electron-deficient nature facilitate chemoselective C‒H oxidation, yielding methanol or acetic acid with high productivities of and , respectively. Experiments and theoretical calculations suggest that methanol formation occurs via a FeIII-FeI-FeIII catalytic cycle, whereas CH3CO2H is produced via hydrocarboxylation of in-situ generated CH3OH with CO2 and H2, and direct CH4 carboxylation with CO2.

Revealing Active Sites and Reaction Pathways in Direct Oxidation of Methane over Fe-Containing CHA Zeolites Affected by the Al Arrangement.

Fe-containing zeolites are effective catalysts in converting the greenhouse gases CH4 and N2O into valuable chemicals. However, the activities of Fe-containing zeolites in methane conversion and N2O decomposition are frequently conflated, and the activities of different Fe species are still controversial. Herein, Fe-containing aluminosilicate CHA zeolites with Fe species at different spatial distances affected by the arrangement of framework Al atoms were synthesized in a one-pot manner in the presence or absence of Na. The arrangement of framework Al atoms was identified by 27Al and 29Si MAS NMR spectra and thermogravimetry-differential thermal analysis (TG-DTA) curves. Ultraviolet (UV)-vis, X-ray absorption spectroscopy (XAS), and NO adsorption fourier transform infrared spectroscopy (FTIR) spectra were adopted to analyze Fe speciation. The higher proportion of Fe species in the 6 MR of Fe-CHA zeolites in the presence of Na was confirmed by the NO adsorption FTIR spectrum. The activities of proximal and distant Fe sites in reactions including direct oxidation of methane to methanol, methanol to hydrocarbon, and N2O decomposition were compared at different temperatures to provide the corresponding active sites and reaction pathways. The distant, isolated Fe and isolated proton were more active in the direct oxidation of methane to methanol and tandem conversion of methanol to hydrocarbon reactions than the proximal, isolated Fe and paired protons, respectively. Additionally, proximal, isolated Fe sites afforded higher activity in N2O decomposition. These findings guide the design of highly active catalysts in methane oxidation, methanol to hydrocarbon, and N2O decomposition reactions, addressing energy and environmental concerns.

Role of Hydroxyl Groups in Zn-containing Nanosized MFI Zeolite for the Photocatalytic Oxidation of Methane.

The effective conversion of methane to a mixture of more valuable hydrocarbons and hydrogen under mild conditions is a significant scientific and practical challenge. Here, we synthesized Zn-containing nanosized MFI zeolite for direct oxidation of methane in the presence of H2O and air. The presence of the surface hydroxyl groups on nanosized MFI-type zeolite and their significant reduction in the Zn-containing nanosized MFI zeolite were confirmed with Infrared Fourier Transform (FTIR) spectroscopy. Incorporation of zinc atoms into the framework of nanosized MFI zeolite is revealed by Nuclear Magnetic Resonance, X-ray Diffraction and UV-Vis Spectroscopy. Unexpectedly, pure silica MFI zeolite exhibited the highest photocatalytic performance. Our findings demonstrated that large number of isolated silanol groups and silanol nests increase the formation of ⋅OH, and enhance the productivity of oxygenate compounds and C2H6, while the Zn incorporated into the zeolite framework or attached to the silanol nests of the nanosized zeolites are less efficient. A mechanism of photocatalytic methane oxidation is proposed. These findings provide insights into the development of active nanosized zeolite photocatalysts with an extended amount of surface hydroxyl groups that can play a key role in photocatalytic methane conversion.

Monomeric metal species for low‐temperature methane oxidation

Direct oxidation of methane into value‐added chemicals is considered as one of the holy grails in chemistry and catalysis. Considerable efforts have been devoted to developing advanced catalyst systems operated under mild conditions, and mononuclear metal species, which are currently in the spotlight of methane oxidation, have shown great advantages and addressed significant attention. In this Concept, we highlight the recent breakthroughs and developments in monomeric metal species for low‐temperature (<100 °C) methane oxidation. Emphasis is put on the identification and validation of monomeric metal species as the intrinsic active sites. Promising characterization approaches for identifying catalytically relevant features of physicochemical properties of monomeric metal species are discussed, as well as the emerging insights on the development of in situ characterization technologies. Finally, the open challenges and outlooks for rational development of monomeric metal catalysts for methane valorization are examined for guiding further developments and inspiring the future work.

Direct Oxidation of Methane to C1 Oxygenates on Cu/ZSM‐5: Reaction Property and Configuration of Cu Species

AbstractDirect oxidation of methane (DOM) under mild conditions is arguably one of the holy grails in chemistry. Cu‐based zeolite catalysts have attracted much attention for their high yield and selectivity of C1 oxygenates. However, the structure‐activity relationships of DOM remain controversial due to the complex configurations of Cu on zeolites. Herein, we prepared Cu/ZSM‐5 catalysts with various Cu loading and investigated the effects of Cu configurations on the DOM reaction. By combining the characterization techniques and evaluating the catalytic performance, we found that the configurations of Cu species could significantly affect the yield and distribution of C1 oxygenates, which was strongly dependent on Cu loading. The highly dispersed mononuclear CuII in the channels of ZSM‐5 is proved to be highly active. While the multinuclear Cu species inhibit the formation of HCOOH, simultaneously suppressing the overall reaction yield. The agglomerated CuO species on the surface of ZSM‐5 show a relatively low correlation with DOM reaction. Thus, the relationship between the yield of C1 oxygenates and Cu loading exhibits a typical volcanic curve and 2.2 wt.% Cu/ZSM‐5 possess the best performance using H2O2 as oxidant (productivity of 17,000 μmol gcat−1 h−1 with 98.6 % selectivity at 50 °C).

Thermodynamic evaluation of solar energy-based methanol and hydrogen production and power generation pathways: A comparative study

This work presents a comparative evaluation of two distinct fuels, methanol and hydrogen, production and power generation routes via fuel cells. The first route includes the methanol production from direct partial oxidation of methane to methanol using solar energy, where the methanol is condensed, stored, and sent to a direct methanol fuel cell. The second route is hydrogen production from solar methane cracking (named as turquoise hydrogen), where heat is supplied from concentrated solar power, and hydrogen is stored and directed to a hydrogen fuel cell. This study aims to provide insights into these fuels' production conditions, storage methods, energy, and exergy efficiencies. The proposed system is simulated using the Engineering Equation Solver software, and a thermodynamic analysis of the entire system, including all the equipment and process streams, is performed. The methanol and hydrogen route’s overall energy and exergy efficiencies are 39.75 %, 38.35 %, 34.21 %, and 33 %, respectively. The highest exergy destruction rate of 1605 kW is observed for the partial oxidation of methane to methanol. The methanol and hydrogen routes generate 32.087 MWh and 11.582 MWh of electricity for 16-hour of fuel cell operation for the same amount of methane feedstock, respectively. Sensitivity analysis has been performed to observe the effects of different parameters, such as operating temperature and mass flow rate of fuels, on the electricity production and energy efficiencies of the systems.

Ultrathin PdAu Nanowires with High Alloying Degree for the Direct Oxidation of Methane to C1 Oxygenates

Ultrathin <scp>PdAu</scp> Nanowires with High Alloying Degree for the Direct Oxidation of Methane to <scp>C1</scp> Oxygenates

Unveiling Direct Electrochemical Oxidation of Methane at the Ceria/Gas Interface.

Solid oxide fuel cells (SOFCs) stand out in sustainable energy systems for their unique ability to efficiently utilize hydrocarbon fuels, particularly those from carbon-neutral sources. CeO2-δ (ceria) based oxides embedded in SOFCs are recognized for their critical role in managing hydrocarbon activation and carbon coking. However, even for the simplest hydrocarbon molecule, CH4, the mechanism of electrochemical oxidation at the ceria/gas interface is not well understood and the capability of ceria to electrochemically oxidize methane remains a topic of debate. This lack of clarity stems from the intricate design of standard metal/oxide composite electrodes and the complex nature of electrode reactions involving multiple chemical and electrochemical steps. This study presents a Sm-doped ceria thin-film model cell that selectively monitors CH4 direct-electro-oxidation on the ceria surface. Using impedance spectroscopy, operando X-ray photoelectron spectroscopy, and density functional theory, it is unveiled that ceria surfaces facilitate C─H bond cleavage and that H2O formation is key in determining the overall reaction rate at the electrode. These insights effectively address the longstanding debate regarding the direct utilization of CH4 in SOFCs. Moreover, these findings pave the way for an optimized electrode design strategy, essential for developing high-performance, environmentally sustainable fuel cells.

Plasma-catalytic direct oxidation of methane to methanol over Cu-MOR: Revealing the zeolite-confined Cu2+ active sites

Efficient methane conversion to methanol remains a significant challenge in chemical industry. This study investigates the direct oxidation of methane to methanol under mild conditions, employing a synergy of non-thermal plasma and Cu-MOR (Copper-Mordenite) catalysts. Catalytic tests demonstrate that the Cu-MOR IE-3 catalyst (i.e., prepared by three cycles of ion exchange) exhibits superior catalytic performance (with 51 % methanol selectivity and 7.9 % methane conversion). Conversely, the Cu-MOR catalysts prepared via wetness impregnation tend to over-oxidize CH4 to CO and CO2. Through systematic catalyst characterizations (XRD, TPR, UV–Vis, HRTEM, XPS), we elucidate that ion exchange mainly leads to the formation of zeolite-confined Cu2+ species, while wetness impregnation predominantly results in CuO particles. Based on the catalytic performance, catalyst characterizations and in-situ FTIR spectra, we conclude that zeolite-confined Cu2+ species serve as the active sites for plasma-catalytic direct oxidation of methane to methanol.

Electrons redistribution of palladium-copper nanoclusters boosting the direct oxidation of methane to methanol

The direct oxidation of methane to methanol (DOMM) at mild conditions is an enormous attractive yet highly challenging, which is known as the “Holy Grail” reaction. In this work, we prepare the PdxCuy NCs supported on mesoporous sulfur-carbon materials and investigate their catalytic DOMM performance. The Pd14Cu1 NCs exhibits the excellent DOMM performance with the productivity of 34.3 mmol g−1 h−1 and the selectivity of 92.3 % at 150 °C when H2/O2 as active oxygen species, which was obviously superior to other PdCu catalysts. The enhanced performance of Pd14Cu1 NCs in DOMM is attributed to the non-crystalline atomic arrangement of tiny cluster structures and the synergistic electronic effect between Pd and Cu atoms, which can reduce the desorption temperature for methane, improve the in-situ generation of H2O2, and provide more active oxygen. The time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) demonstrated that a radical-involved reaction pathway on Pd14Cu1 NC. This work provides the understanding for synergistic effect dual metallic and helps to design highly efficient and selective catalysts for the direct oxidation of CH4.

Direct partial oxidation of low-concentration methane to methanol with copper-based clay catalysts

Partial oxidation of methane to methanol (POMM) is an economic and environmental technology, especially for utilizing low-concentration methane resources in the explosive range. In this paper, mineral clays like sepiolite (SEP), attapulgite (ATP), montmorillonite (MMT), bentonite (BEN), kaolin (KAO) and diatomite (DIA) were used as supports to prepare copper-based clay catalysts by impregnation method and their performance in continuous direct POMM with O2 as oxygen source were tested. The methanol yields of these catalysts at 320 °C with feed ratio of CH4: O2: H2O = 2: 1: 8 exhibited the following order: Cu/KAO (145.26 μmol/gCu/h) > Cu/ATP (74.18 μmol/gCu/h) > Cu/SEP (63.99 μmol/gCu/h) > Cu/MMT (51.59 μmol/gCu/h) > Cu/DIA (42.29 μmol/gCu/h) > Cu/BEN (39.64 μmol/gCu/h). This demonstrated that copper-based clay catalysts were promising for use in POMM considering their economy and environmental acceptability. Additionally, these copper-based clay catalysts were characterized and it was found that different clays could tailor the interaction of Cu species with supports, alter the reducibility and surface acidity, thereby changing the methanol yield. What's more, it was confirmed that the superior methanol yield of Cu/KAO was attributed to its relatively more [Cu2O)]2+ sites with Cu–O, Cu–Si/Al and Cu–Cu coordination structures similar to that of copper-based zeolite catalysts.

Nitrogen Vacancy-Rich C3Nx-Confined Fe–Cu Diatomic Catalysts for the Direct Selective Oxidation of Methane at Low Temperature

Nitrogen Vacancy-Rich C₃N_<i>x</i>-Confined Fe–Cu Diatomic Catalysts for the Direct Selective Oxidation of Methane at Low Temperature

Direct Photocatalytic Methane Oxidation to Formaldehyde by N Doping Co-Decorated Mixed Crystal TiO2.

In this paper, N-doped TiO2 mixed crystals are prepared via direct calcination of TiN for highly selective oxidation of CH4 to HCHO at room temperature. The structures of the prepared TiO2 samples are characterized to be N-doped TiO2 of anatase and rutile mixed crystals. The crystal structures of TiO2 samples are determined by XRD spectra and Raman spectra, while N doping is demonstrated by TEM mapping, ONH inorganic element analysis, and high-resolution XPS results. Significantly, the production rate of HCHO is as high as 23.5 mmol·g-1·h-1 with a selectivity over 90%. Mechanism studies reveal that H2O is the main oxygen source and acts through the formation of ·OH. DFT calculations indicate that the construction of a mixed crystal structure and N-doping modification mainly act by increasing the adsorption capacity of H2O. An efficient photocatalyst was prepared by us to convert CH4 to HCHO with high yield and selectivity, greatly promoting the development of the photocatalytic CH4 conversion study.

Noble metal modified copper-exchanged mordenite zeolite (Cu-ex-MOR) catalysts for catalyzing the methane efficient gas-phase synthesis methanol

Copper mordenite catalysts are key for methane oxidation to methanol, yet lack sufficient activity. In this paper, noble metal (Au, Ru, Pt and Pd) modified copper ion-exchanged mordenite catalysts were prepared by the ion-exchange method for further improving the methanol yield in the gas-phase continuous catalytic direct partial oxidation of methane to methanol reaction, and the role of noble metal doping on Cu-ex-MOR catalysts was investigated. Experimental results showed that the doping of Ru resulted in a significant increase in the methanol yield of Ru/Cu-ex-MOR catalyst to 157.36 μmol/gcat/h compared to that of Cu-ex-MOR catalyst (12.89 μmol/gcat/h). SEM, XRD, FT-IR, N2 adsorption-desorption, XPS, NH3-TPD and H2-TPR results showed that Ru/Cu-ex-MOR had uniformly dispersed Ru elements and the largest number of surface acidic and oxidation sites, which facilitated the adsorption and activation of methane. Additionally, it was found by TEM, in situ FT-IR and DFT characterization that Ru played a role in stabilizing the Cu active sites, the adsorptive activation of water on the Ru site and the H-transfer process reduced the energy required for breaking C–H bond of CH4 at the Cu active site, which significantly improved the methane activation capacity of the Ru/Cu-ex-MOR catalysts, resulting in higher methanol yields.

CO-assisted partial oxidation of methane and selective C1 oxygenate formation using iridium-ZSM-5 catalyst

Abstract The direct partial oxidation of methane to oxygenates remains challenging. Here, CO-assisted methane conversion was investigated using an Ir-ZSM-5 catalyst, and it achieved higher C1 selectivity compared with that achieved using a model Rh-ZSM-5 catalyst. A plausible reaction sequence is proposed via kinetic analysis, and the differences between the two catalysts were revealed.

Solvent-Free Synthesis Enables Encapsulation of Subnanometric FeOx Clusters in Pure Siliceous Zeolites for Efficient Catalytic Oxidation Reactions.

Metal/metal oxide clusters possess a higher count of unsaturated coordination sites than nanoparticles, providing multiatomic sites that single atoms do not. Encapsulating metal/metal oxide clusters within zeolites is a promising approach for synthesizing and stabilizing these clusters. The unique feature endows the metal clusters with an exceptional catalytic performance in a broad range of catalytic reactions. However, the encapsulation of stable FeOx clusters in zeolite is still challenging, which limits the application of zeolite-encapsulated FeOx clusters in catalysis. Herein, we design a modified solvent-free method to encapsulate FeOx clusters in pure siliceous MFI zeolites (Fe@MFI). It is revealed that the 0.3-0.4 nm subnanometric FeOx clusters are stably encapsulated in the 5/6-membered rings intersectional voids of the pure siliceous MFI zeolites. The encapsulated Fe@MFI catalyst with a Fe loading of 1.4 wt % demonstrates remarkable catalytic activity and recycle stability in the direct oxidation of methane, while also promoting the direct oxidation of cyclohexane, surpassing the performance of conventional zeolite-supported Fe catalysts.

Copper Phosphate Nanostructures as Catalysts for the Direct Methane Oxidation

Copper Phosphate Nanostructures as Catalysts for the Direct Methane Oxidation

Contact-Electro-Catalysis for Direct Oxidation of Methane under Ambient Conditions.

The conversion of methane under ambient conditions has attracted significant attention. Although advancements have been made using active oxygen species from photo- and electro- chemical processes, challenges such as complex catalyst design, costly oxidants, and unwanted byproducts remain. This study exploits the concept of contact-electro-catalysis, initiating chemical reactions through charge exchange at a solid-liquid interface, to report a novel process for directly converting methane under ambient conditions. Utilizing the electrification of commercially available Fluorinated Ethylene Propylene (FEP) with water under ultrasound, we demonstrate how this interaction promote the activation of methane and oxygen molecules. Our results show that the yield of HCHO and CH3OH can reach 467.5 and 151.2 μmol ⋅ gcat -1, respectively. We utilized electron paramagnetic resonance (EPR) to confirm the evolution of hydroxyl radicals (⋅OH) and superoxide radicals (⋅OOH). Isotope mass spectrometry (MS) was employed to analyze the elemental origin of CH3OH, which can be further oxidized to HCHO. Additionally, we conducted density functional theory (DFT) simulations to assess the reaction energies of FEP with H2O, O2, and CH4 under these conditions. The implications of this methodology, with its potential applicability to a wider array of gas-phase catalytic reactions, underscore a significant advance in catalysis.

Contact‐Electro‐Catalysis for Direct Oxidation of Methane under Ambient Conditions

AbstractThe conversion of methane under ambient conditions has attracted significant attention. Although advancements have been made using active oxygen species from photo‐ and electro‐ chemical processes, challenges such as complex catalyst design, costly oxidants, and unwanted byproducts remain. This study exploits the concept of contact‐electro‐catalysis, initiating chemical reactions through charge exchange at a solid–liquid interface, to report a novel process for directly converting methane under ambient conditions. Utilizing the electrification of commercially available Fluorinated Ethylene Propylene (FEP) with water under ultrasound, we demonstrate how this interaction promote the activation of methane and oxygen molecules. Our results show that the yield of HCHO and CH3OH can reach 467.5 and 151.2 μmol ⋅ gcat−1, respectively. We utilized electron paramagnetic resonance (EPR) to confirm the evolution of hydroxyl radicals (⋅OH) and superoxide radicals (⋅OOH). Isotope mass spectrometry (MS) was employed to analyze the elemental origin of CH3OH, which can be further oxidized to HCHO. Additionally, we conducted density functional theory (DFT) simulations to assess the reaction energies of FEP with H2O, O2, and CH4 under these conditions. The implications of this methodology, with its potential applicability to a wider array of gas‐phase catalytic reactions, underscore a significant advance in catalysis.