Methane Hydroxylation Research Articles

High pressure-derived nonsymmetrical [Cu2O]2+ core for room-temperature methane hydroxylation.

Nonsymmetrical oxygen-bridged binuclear copper centers have been proposed and modeled as intermediates and transition states in several C─H oxidation pathways, leading to the postulation that structural dissymmetry enhances the reactivity of the bridging oxygen. However, experimentally characterizing the structure and reactivity of these transient species is remarkably challenging. Here, we report the high-pressure synthesis of a metastable nonsymmetrical dicopper-μ-oxo compound with exceptional reactivity toward the mono-oxygenation of aliphatic C─H bonds. The nonequivalent coordination environment of copper stabilizes localized mixed valency and greatly enhances the hydrogen atom abstraction activity of the bridging oxygen, enabling room-temperature hydroxylation of methane under pressure. These findings highlight the role of dissymmetry in the reactivity of binuclear copper centers and demonstrate precise control of molecular structures by mechanical means.

Structural, electronic, and mechanical properties of transition metal porphyrin-like graphenes, and their application in hydroxylation of methane

Structural, electronic, and mechanical properties of transition metal porphyrin-like graphenes, and their application in hydroxylation of methane

Methane biohydroxylation into methanol by Methylosinus trichosporium OB3b: possible limitations and formate use during reaction.

Methane (CH4) hydroxylation into methanol (MeOH) by methanotrophic bacteria is an attractive and sustainable approach to producing MeOH. The model strain Methylosinus trichosporium OB3b has been reported to be an efficient hydroxylating biocatalyst. Previous works have shown that regardless of the bioreactor design or operation mode, MeOH concentration reaches a threshold after a few hours, but there are no investigations into the reasons behind this phenomenon. The present work entails monitoring both MeOH and formate concentrations during CH4 hydroxylation, where neither a gaseous substrate nor nutrient shortage was evidenced. Under the assayed reaction conditions, bacterial stress was shown to occur, but methanol was not responsible for this. Formate addition was necessary to start MeOH production. Nuclear magnetic resonance analyses with 13C-formate proved that the formate was instrumental in regenerating NADH; formate was exhausted during the reaction, but increased quantities of formate were unable to prevent MeOH production stop. The formate mass balance showed that the formate-to-methanol yield was around 50%, suggesting a cell regulation phenomenon. Hence, this study presents the possible physiological causes that need to be investigated further. Finally, to the best of our knowledge, this study shows that the reaction can be achieved in the native bacterial culture (i.e., culture medium containing added methanol dehydrogenase inhibitors) by avoiding the centrifugation steps while limiting the hands-on time and water consumption.

Heterogeneous catalysis of methane hydroxylation with nearly total selectivity under mild conditions.

The efficient utilization of methane, a vital component of natural gas, shale gas and methane hydrate, holds significant importance for global energy security and environmental sustainability. However, converting methane into value-added oxygenates presents a formidable challenge due to its inert nature. Direct selective oxidation of methane (DSOM) under mild conditions is essential for reducing energy consumption and carbon emissions compared with traditional indirect routes. Achieving total selectivity in methane hydroxylation remains elusive due to the competitive CO2 formation. This feature article highlights recent advancements in methane hydroxylation using thermo-, photo-, and electro-catalytic systems. Through strategically designing the structure of catalysts to control the reactive oxygen species and optimizing reaction parameters, significant progress has been made in enhancing oxygenate selectivity and minimizing overoxidation. A comprehensive understanding of the mechanisms underlying methane hydroxylation with total selectivity offers insights for improving catalyst design and reaction parameter optimization, promoting sustainable methane utilization.

Deciphering structural evolution of adsorbed ˙OH species on Zr-oxo nodes of UiO-66 to modulate methane hydroxylation

The ˙OH quantity and DSOM performance on Zr-oxo nodes in the UiO-66-H catalyst exhibit a volcano-like curve relationship due to the evolution of p-band centers of O atoms on adsorbed oxygenic species.

Transcriptional regulation of soluble methane monooxygenase via enhancer-binding protein derived from Methylosinus sporium 5.

Methane is a major greenhouse gas, and methanotrophs regulate the methane level in the carbon cycle. Soluble methane monooxygenase (sMMO) is expressed in various methanotroph genera, including Alphaproteobacteria and Gammaproteobacteria, and catalyzes the hydroxylation of methane to methanol. It has been proposed that MmoR regulates the expression of sMMO as an enhancer-binding protein under copper-limited conditions; however, details on this transcriptional regulation remain limited. Herein, we elucidate the transcriptional pathway of sMMO depending on copper ion concentration, which affects the interaction of MmoR and sigma factor. MmoR and sigma-54 (σ54) from Methylosinus sporium 5 were successfully overexpressed in Escherichia coli and purified to investigate sMMO transcription in methanotrophs. The results indicated that σ54 binds to a promoter positioned -24 (GG) and -12 (TGC) upstream between mmoG and mmoX1. The binding affinity and selectivity are lower (Kd = 184.6 ± 6.2 nM) than those of MmoR. MmoR interacts with the upstream activator sequence (UAS) with a strong binding affinity (Kd = 12.5 ± 0.5 nM). Mutational studies demonstrated that MmoR has high selectivity to its binding partner (ACA-xx-TGT). Titration assays have demonstrated that MmoR does not coordinate with copper ions directly; however, its binding affinity to UAS decreases in a low-copper-containing medium. MmoR strongly interacts with adenosine triphosphate (Kd = 62.8 ± 0.5 nM) to generate RNA polymerase complex. This study demonstrated that the binding events of both MmoR and σ54 that regulate transcription in M. sporium 5 depend on the copper ion concentration. IMPORTANCE This study provides biochemical evidence of transcriptional regulation of soluble methane monooxygenase (sMMO) in methanotrophs that control methane levels in ecological systems. Previous studies have proposed transcriptional regulation of MMOs, including sMMO and pMMO, while we provide further evidence to elucidate its mechanism using a purified enhancer-binding protein (MmoR) and transcription factor (σ54). The characterization studies of σ54 and MmoR identified the promoter binding sites and enhancer-binding sequences essential for sMMO expression. Our findings also demonstrate that MmoR functions as a trigger for sMMO expression due to the high specificity and selectivity for enhancer-binding sequences. The UV-visible spectrum of purified MmoR suggested an iron coordination like other GAF domain, and that ATP is essential for the initiation of enhancer elements. Binding assays indicated that these interactions are blocked by the copper ion. These results provide novel insights into gene regulation of methanotrophs.

Computational Discovery of Stable Metal-Organic Frameworks for Methane-to-Methanol Catalysis.

The challenge of direct partial oxidation of methane to methanol has motivated the targeted search of metal-organic frameworks (MOFs) as a promising class of materials for this transformation because of their site-isolated metals with tunable ligand environments. Thousands of MOFs have been synthesized, yet relatively few have been screened for their promise in methane conversion. We developed a high-throughput virtual screening workflow that identifies MOFs from a diverse space of experimental MOFs that have not been studied for catalysis, yet are thermally stable, synthesizable, and have promising unsaturated metal sites for C-H activation via a terminal metal-oxo species. We carried out density functional theory calculations of the radical rebound mechanism for methane-to-methanol conversion on models of the secondary building units (SBUs) from 87 selected MOFs. While we showed that oxo formation favorability decreases with increasing 3d filling, consistent with prior work, previously observed scaling relations between oxo formation and hydrogen atom transfer (HAT) are disrupted by the greater diversity in our MOF set. Accordingly, we focused on Mn MOFs, which favor oxo intermediates without disfavoring HAT or leading to high methanol release energies─a key feature for methane hydroxylation activity. We identified three Mn MOFs comprising unsaturated Mn centers bound to weak-field carboxylate ligands in planar or bent geometries with promising methane-to-methanol kinetics and thermodynamics. The energetic spans of these MOFs are indicative of promising turnover frequencies for methane to methanol that warrant further experimental catalytic studies.

Retrofitting Zr-Oxo Nodes of UiO-66 by Ru Single Atoms to Boost Methane Hydroxylation with Nearly Total Selectivity.

Direct selective oxidation of methane (DSOM) to high value-added oxygenates under mild conditions is attracting considerable interest. Although state-of-the-art supported metal catalysts can improve methane conversion, it is still challenging to avoid the deep oxidation of oxygenates. Here, we develop a highly efficient metal-organic frameworks (MOFs)-supported single-atom Ru catalyst (Ru1/UiO-66) for the DSOM reaction using H2O2 as an oxidant. It endows nearly 100% selectivity and an excellent turnover frequency of 185.4 h-1 for the production of oxygenates. The yield of oxygenates is an order of magnitude higher than that on UiO-66 alone and several times higher than that on supported Ru nanoparticles or other conventional Ru1 catalysts, which show severe CO2 formation. Detailed characterizations and density functional theory calculations reveal a synergistic effect between the electron-deficient Ru1 site and the electron-rich Zr-oxo nodes of UiO-66 on Ru1/UiO-66. The Ru1 site is responsible for the activation of CH4 via the resulting Ru1═O* species, while the Zr-oxo nodes undertake the formation of oxygenic radical species to produce oxygenates. In particular, the Zr-oxo nodes retrofitted by Ru1 can prune the excess H2O2 to inactive O2 more than •OH species, helping to suppress the over-oxidation of oxygenates.

Modulating the Energetics of C–H Bond Activation in Methane by Utilizing Metalated Porphyrinic Metal–Organic Frameworks

In recent years, much effort has been directed toward utilizing metal-organic frameworks (MOFs) for activating C-H bonds of light alkanes. The energy demanding steps involved in the catalytic pathway are the formation of metal-oxo species and the subsequent cleavage of the C-H bonds of alkanes. With the intention of exploring the tunability of the activation barriers involved in the catalytic pathway of methane hydroxylation, we have employed density functional theory to model metalated porphyrinic MOFs (MOF-525(M)). We find that the heavier congeners down a particular group have high exothermic oxo-formation enthalpies ΔHO and hence are associated with low N2O activation barriers. Independent analyses of activation barriers and structure-activity relationship leads to the conclusion that MOF-525(Ru) and MOF-525(Ir) can act as an effective catalysts for methane hydroxylation. Hence, ΔHO has been found to act as a guide, in the first place, in choosing the optimum catalyst for methane hydroxylation from a large set of available systems.

A computational mechanistic study of CH hydroxylation with mononuclear copper–oxygen complexes

A computational study of methane hydroxylation by oxygen-bound monocopper complexes.

Methanotrophy by a Mycobacterium species that dominates a cave microbial ecosystem.

So far, only members of the bacterial phyla Proteobacteria and Verrucomicrobia are known to grow methanotrophically under aerobic conditions. Here we report that this metabolic trait is also observed within the Actinobacteria. We enriched and cultivated a methanotrophic Mycobacterium from an extremely acidic biofilm growing on a cave wall at a gaseous chemocline interface between volcanic gases and the Earth's atmosphere. This Mycobacterium, for which we propose the name Candidatus Mycobacterium methanotrophicum, is closely related to well-known obligate pathogens such as M. tuberculosis and M. leprae. Genomic and proteomic analyses revealed that Candidatus M. methanotrophicum expresses a full suite of enzymes required for aerobic growth on methane, including a soluble methane monooxygenase that catalyses the hydroxylation of methane to methanol and enzymes involved in formaldehyde fixation via the ribulose monophosphate pathway. Growth experiments combined with stable isotope probing using 13C-labelled methane confirmed that Candidatus M. methanotrophicum can grow on methane as a sole carbon and energy source. A broader survey based on 16S metabarcoding suggests that species closely related to Candidatus M. methanotrophicum may be abundant in low-pH, high-methane environments.

Zirconium-oxo Nodes of MOFs with Tunable Electronic Properties Provide Effective ⋅OH Species for Enhanced Methane Hydroxylation.

Direct conversion of methane to high value-added oxygenates under mild conditions has attracted extensive interest. However, the over-oxidation of target products is usually unavoidable due to the easily excessive activation of C-H bond on the sites of supported metal species. Here, we identified the most efficient Zr-oxo nodes of UiO-66 metal-organic frameworks (MOFs) catalysts for the selective oxidation of methane with H2 O2 . These nodes were modified by three types of benzene 1, 4-dicarboxylates (NH2 -BDC, H2 BDC, and NO2 -BDC). Detailed characterizations and DFT calculations revealed that these ligands can effectively tune the electronic properties of Zr-oxo nodes and the H2 BDC ligand led to optimal electronic density of Zr-oxo nodes in UiO-66. Thus the UiO-66-H catalyst promoted the formation of ⋅OH species that adsorbed on Zr-oxo nodes, and facilitated the activation of methane with a lower energy barrier and subsequent conversion to hydroxylation oxygenates with 100 % selectivity.

Zirconium‐oxo Nodes of MOFs with Tunable Electronic Properties Provide Effective ⋅OH Species for Enhanced Methane Hydroxylation

Direct conversion of methane to high value-added oxygenates under mild conditions has attracted extensive interest. However, the over-oxidation of target products is usually unavoidable due to the easily excessive activation of C-H bond on the sites of supported metal species. Here, we identified the most efficient Zr-oxo nodes of UiO-66 Metal-Organic Frameworks (MOFs) catalysts for the selective oxidation of methane with H2O2 oxidant, which adopted the modification strategy by three types of benzene 1, 4-dicarboxylates (NH2-BDC, H2BDC, and NO2-BDC, respectively). Detailed characterizations and DFT calculations revealed that these ligands can effectively tune the electronic property of Zr-oxo nodes and the H2BDC ligand led to proper electronic density of Zr-oxo nodes in UiO-66. Consequently, this UiO-66-H catalyst can promote the formation of effective •OH species that adsorbed on Zr-oxo nodes, which facilitated the activation of methane with a lower energy barrier and subsequent conversion to hydroxylation oxygenates with 100% selectivity.

Electron Transfer to Hydroxylase through Component Interactions in Soluble Methane Monooxygenase.

The hydroxylation of methane (CH4) is crucial to the field of environmental microbiology, owing to the heat capacity of methane, which is much higher than that of carbon dioxide (CO2). Soluble methane monooxygenase (sMMO), a member of the bacterial multicomponent monooxygenase (BMM) superfamily, is essential for the hydroxylation of specific substrates, including hydroxylase (MMOH), regulatory component (MMOB), and reductase (MMOR). The diiron active site positioned in the MMOH α-subunit is reduced through the interaction of MMOR in the catalytic cycle. The electron transfer pathway, however, is not yet fully understood due to the absence of complex structures with reductases. A type II methanotroph, Methylosinus sporium 5, successfully expressed sMMO and hydroxylase, which were purified for the study of the mechanisms. Studies on the MMOH-MMOB interaction have demonstrated that Tyr76 and Trp78 induce hydrophobic interactions through π-π stacking. Structural analysis and sequencing of the ferredoxin domain in MMOR (MMOR-Fd) suggested that Tyr93 and Tyr95 could be key residues for electron transfer. Mutational studies of these residues have shown that the concentrations of flavin adenine dinucleotide (FAD) and iron ions are changed. The measurements of dissociation constants (Kds) between hydroxylase and mutated reductases confirmed that the binding affinities were not significantly changed, although the specific enzyme activities were significantly reduced by MMOR-Y93A. This result shows that Tyr93 could be a crucial residue for the electron transfer route at the interface between hydroxylase and reductase.

Explorations of the nonheme high-valent iron-oxo landscape: crystal structure of a synthetic complex with an [FeIV2(μ-O)2] diamond core relevant to the chemistry of sMMOH.

Methanotrophic bacteria utilize methane monooxygenase (MMO) to carry out the first step in metabolizing methane. The soluble enzymes employ a hydroxylase component (sMMOH) with a nonheme diiron active site that activates O2 and generates a powerful oxidant capable of converting methane to methanol. It is proposed that the diiron(II) center in the reduced enzyme reacts with O2 to generate a diferric-peroxo intermediate called P that then undergoes O-O cleavage to convert into a diiron(IV) derivative called Q, which carries out methane hydroxylation. Most (but not all) of the spectroscopic data of Q accumulated by various groups to date favor the presence of an FeIV2(μ-O)2 unit with a diamond core. The Que lab has had a long-term interest in making synthetic analogs of iron enzyme intermediates. To this end, the first crystal structure of a complex with a FeIIIFeIV(μ-O)2 diamond core was reported in 1999, which exhibited an Fe⋯Fe distance of 2.683(1) Å. Now more than 20 years later, a complex with an FeIV2(μ-O)2 diamond core has been synthesized in sufficient purity to allow diffraction-quality crystals to be grown. Its crystal structure has been solved, revealing an Fe⋯Fe distance of 2.711(4) Å for comparison with structural data for related complexes with lower iron oxidation states.

Cage effects control the mechanism of methane hydroxylation in zeolites.

Catalytic conversion of methane to methanol remains an economically tantalizing but fundamentally challenging goal. Current technologies based on zeolites deactivate too rapidly for practical application. We found that similar active sites hosted in different zeolite lattices can exhibit markedly different reactivity with methane, depending on the size of the zeolite pore apertures. Whereas zeolite with large pore apertures deactivates completely after a single turnover, 40% of active sites in zeolite with small pore apertures are regenerated, enabling a catalytic cycle. Detailed spectroscopic characterization of reaction intermediates and density functional theory calculations show that hindered diffusion through small pore apertures disfavors premature release of CH3 radicals from the active site after C-H activation, thereby promoting radical recombination to form methanol rather than deactivated Fe-OCH3 centers elsewhere in the lattice.

Copper-Oxo Active Sites for Methane C-H Activation in Zeolites: Molecular Understanding of Impact of Methane Hydroxylation on UV-Vis Spectra.

Here, we analyze changes in the optical spectra of activated copper-exchanged zeolites during methane activation with the Tamm-Dancoff time-dependent density functional theory, TDA-DFT, while using the ωB2PLYP functional. Two active sites, [Cu2O]2+ and [Cu3O3]2+, were studied. For [Cu2O]+, the 22 700 cm-1 peak is associated with μ-oxo 2p → Cu 3d/4s charge transfer. Of the [Cu2O]2+ methane C-H activation intermediates that we examined, only [Cu-O(H)(H)-Cu] and [Cu-O(H)(CH3)-Cu] have spectra that match experimental observations. After methane activation, the μ-oxo 2p orbitals lose two electrons and become hybridized with methanol C 2p orbitals and/or H 1s orbitals. The frontier unoccupied orbitals become more Cu 4s/4p Rydberg-like, reducing overlap with occupied orbitals. These effects cause the disappearance of the 22 700 cm-1 peak. For [Cu3O3]2+, the exact structures of the species formed after methane activation are unknown. Thus, we considered eight possible structures. Several of these provide a significant decrease in intensity near 23 000-38 000 cm-1, as seen experimentally. Notably, these species involve either rebound of the separated methyl to a μ-oxo atom or its remote stabilization at a Brønsted acid site in exchange for the acidic proton. These spectral changes are caused by the same mechanism seen in [Cu2O]2+ and are likely responsible for the observed reduced intensities near 23 000-38 000 cm-1. Thus, TDA-DFT calculations with ωB2PLYP provide a molecular-level understanding of the evolution of copper-oxo active sites during methane-to-methanol conversion.

Mechanistic Insights into the Dicopper-Complex-Catalyzed Hydroxylation of Methane and Benzene Using Nitric Oxide: A DFT Study.

Although hydrocarbons are known to act as reductants for the catalytic reduction of nitric oxides (NOx) over copper-based catalysts, the reaction mechanism requires clarification. Herein, density functional theory (DFT) calculations were carried out to investigate the reduction mechanisms of NOx to dinitrogen coupled to the hydroxylation of methane or benzene using the dicopper complex reported by Zhang and co-workers [ J. Am. Chem. Soc. 2019, 141, 10159-10164]. The B3LYP functional was used to optimize the (μ-oxo)(μ-nitrosyl)dicopper complex in the quartet state and the (μ-η2:η2-NO2)dicopper complex in the doublet state, the latter of which was found to be the ground state. Then, we investigated the reactivities of the (μ-η2:η2-NO2)dicopper complex toward methane and benzene by considering the conversions of N2O to N2 in the presence and the absence of methane or benzene. In the presence of methane and benzene, the calculated activation energies were 27.0 and 21.0 kcal/mol, respectively, whereas that with N2O alone was prohibitively high (61.9 kcal/mol). Thus, the (μ-η2:η2-NO2)dicopper complex prefers the reactions with methane and benzene to that with N2O. The reaction of the (μ-η2:η2-NO2)dicopper complex with methane or benzene generated the (μ-nitrosyl)dicopper complex. The (μ-nitrosyl)dicopper complex then reacted with N2O to regenerate the (μ-η2:η2-NO2)dicopper complex and N2 with an activation barrier of 31.5 kcal/mol. The overall reactions for methane and benzene hydroxylation were calculated to be exothermic by 41.7 and 54.1 kcal/mol, respectively. These results suggest that the catalytic reduction of NOx using hydrocarbons is feasible at certain operating temperatures. Thus, our calculations provide new insights into the design of catalysts for NOx purification.

A six-coordinate high-spin FeIVO species of cucurbit[5]uril: a highly potent catalyst for C-H hydroxylation of methane, if synthesised.

DFT and ab initio DLPNO-CCSD(T) calculations predict a stable S = 2 six-coordinate FeIVO species with cucurbit[5]uril (CB[5]) as a ligand ([(CB[5])FeIVO(H2O)]2+(1)). The strong oxidising capability of 1 far exceeds even that of metalloenzymes such as sMMOs in activating inert substrates such as methane, setting the stage for a new generation of biomimetic catalysts.

Direct Hydroxylation of Methane: Interplay Between Theory and Experiment

Direct Hydroxylation of Methane: Interplay Between Theory and Experiment