Methane Over-Oxidation by Extra-Framework Copper-Oxo Active Sites of Copper-Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

Copper-exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over-oxidation products like CO and CO₂. However, mechanistic pathways for methane over-oxidation by copper-oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over-oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over-oxidation by [Cu₃O₃]²⁺ active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ-oxo atom of the active site. This μ-(O−CH₃) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper-monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper-monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step must be stabilized away from copper-oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper-oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane-to-methanol conversion.     

Framework Effects on Activation and Functionalisation of Methane in Zinc-Exchanged Zeolites

The first selective oxidation of methane to methanol is reported herein for zinc-exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM-5 both form zinc formate and methanol. Selective methane activation to form [Zn-CH₃]⁺ species was confirmed by ¹³C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM-5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the ¹³C MAS NMR spectrum, resulting from two distinct [Zn-CH₃]⁺ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn-CH₃]⁺ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure–function correlations for methane conversion over zinc-exchanged zeolites.     

A non-radical mechanism for methane hydroxylation at the diiron active site of soluble methane monooxygenase

We propose a non‐radical mechanism for the conversion of methane into methanol by soluble methane monooxygenase (sMMO), the active site of which involves a diiron active center. We assume the active site of the MMOH_Q intermediate, exhibiting direct reactivity with the methane substrate, to be a bis(μ‐oxo)diiron(IV ) complex in which one of the iron atoms is coordinatively unsaturated (five‐coordinate). Is it reasonable for such a diiron complex to be formed in the catalytic reaction of sMMO? The answer to this important question is positive from the viewpoint of energetics in density functional theory (DFT) calculations. Our model thus has a vacant coordination site for substrate methane. If MMOH_Q involves a coordinatively unsaturated iron atom at the active center, methane is effectively converted into methanol in the broken‐symmetry singlet state by a non‐radical mechanism; in the first step a methane C? H bond is dissociated via a four‐centered transition state (TS1) resulting in an important intermediate involving a hydroxo ligand and a methyl ligand, and in the second step the binding of the methyl ligand and the hydroxo ligand through a three‐centered transition state (TS2) results in the formation of a methanol complex. This mechanism is essentially identical to that of the methane–methanol conversion by the bare FeO⁺ complex and relevant transition metal–oxo complexes in the gas phase. Neither radical species nor ionic species are involved in this mechanism. We look in detail at kinetic isotope effects (KIEs) for H atom abstraction from methane on the basis of transition state theory with Wigner tunneling corrections.     

Light‐Driven C−H Oxygenation of Methane into Methanol and Formic Acid by Molecular Oxygen Using a Perfluorinated Solvent

The chlorine dioxide radical (ClO₂^.) was found to act as an efficient oxidizing agent in the aerobic oxygenation of methane to methanol and formic acid under photoirradiation. Photochemical oxygenation of methane occurred in a two‐phase system comprising perfluorohexane and water under ambient conditions (298 K, 1 atm). The yields of methanol and formic acid were 14 and 85 %, respectively, with a methane conversion of 99 % without formation of the further oxygenated products such as CO₂ and CO. Ethane was also photochemically converted into ethanol (19 %) and acetic acid (80 %). The methane oxygenation is initiated by the photochemical Cl?O bond cleavage of ClO₂^. to generate Cl^. and O₂. The produced Cl^. reacts with CH₄ to form a methyl radical (CH₃^.). Finally, the oxygenated products such as methanol and formic acid were given by the radical chain reaction. A fluorous solvent plays an important role of inhibiting the deactivation of reactive radical species such as Cl^. and CH₃^..     

Effect of sodium cation on the electrochemical reduction of CO<Subscript>2</Subscript> at a copper electrode in methanol

The electrochemical reduction of CO₂ with a Cu electrode in methanol was investigated with sodium hydroxide supporting salt. A divided H-type cell was employed; the supporting electrolytes were 80 mmol dm⁻³ sodium hydroxide in methanol (catholyte) and 300 mmol dm⁻³ potassium hydroxide in methanol (anolyte). The main products from CO₂ were methane, ethylene, carbon monoxide, and formic acid. The maximum current efficiency for hydrocarbons (methane and ethylene) was 80.6%, at −4.0 V vs Ag/AgCl, saturated KCl. The ratio of current efficiency for methane/ethylene, r _f(CH₄)/r _f(C₂H₄), was similar to those obtained in LiOH/methanol-based electrolyte and larger relative to those in methanol using KOH, RbOH, and CsOH supporting salts. In NaOH/methanol-based electrolyte, the efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was suppressed to below 4%. The electrochemical CO₂ reduction to methane may be able to proceed efficiently in a hydrophilic environment near the electrode surface provided by sodium cation.     

The Fe Active Sites in FeZSM-5 Catalyst for Selective Oxidation of CH4 to CH3OH at Room Temperature

It was shown recently that iron complexes formed during the thermal treatment of FeZSM-5 zeolite perform single-turnover cycles of methane oxidation to methanol at ambient conditions when nitrous oxide is used as a source of oxygen. The long-living active intermediate is capable of transferring accepted O atom (called -oxygen) into C-H bond of methane to produce methanol at 100% selectivity. The present work is aimed to the identification of iron active sites through a comparison of in situ ⁵⁷Fe Mössbauer spectra of FeZSM-5 after various thermal treatments and reaction stages. It is established that vacuum activation at 900 °C accompanied by a manifold increase of -centers leads to the transformation of inactive Fe³⁺ to the active, coordinatively unsaturated Fe²⁺ states. After -oxygen loading, active Fe²⁺ states transform to a new Fe³⁺ states responsible for further methane oxidation. The latter reaction, as well as reaction with ²H₂, is not fully reversible: part of active Fe³⁺ transforms to other inactive Fe³⁺ form. On the contrary, reaction of -oxygen with CO leads to a complete restoration of the initial, vacuum activated Fe²⁺ states. On the base of joint Mössbauer and catalytic data, the structure and composition of iron active centers are suggested.     

Doped-ZrO2 Aerogels: Catalysts of Controlled Structure and Properties

Doped ZrO₂ aerogels (characterised by TEM, DTA and N₂ adsorption) have been prepared and catalytically tested in CO/CO₂ hydrogenation [1] and CH₄ oxidation [2]. The primary aerogels showed cross-linked clusters of (X-ray) amorphous particles smaller than 5 nm which led to well-developed mesoporous solids with an average pore size of about 10 nm and high surface area (up to 250 m²g⁻¹) [1]. Cu/ZrO₂ aerogels (known to be very active and selective towards methanol synthesis in CO hydrogenation without predominant formation of alkanes even at higher temperatures [1]) are now seen to show these effects even more clearly in CO₂ hydrogenation. In methane oxidation, both Rh/ZrO₂ and Y₂O₃/ZrO₂ were very active. Consideration is given to the nature of the active sites, the role of CO₂ and metal/oxide interfaces and how an understanding of this reactivity can lead to better dispersed ZrO₂.     

Methanol Decomposition on Fe2O3 /MCM-48 Silica Catalyst

Fe₂O₃/MCM-48 silica samples are characterized by high catalytic activity and methane selectivity in methanol decomposition. The catalytically active phase is substantially changed by the reaction medium and/or hydrogen pretreatment.     

Spin‐Selective Thermal Activation of Methane by Closed‐Shell [TaO3]+

Thermal reactions of the closed‐shell metal‐oxide cluster [TaO₃]⁺ with methane were investigated by using FTICR mass spectrometry complemented by high‐level quantum chemical calculations. While the generation of methanol and formaldehyde is somewhat expected, [TaO₃]⁺ remarkably also has the ability to abstract two hydrogen atoms from methane with the elimination of CH₂. Mechanistically, the generation of CH₂O and CH₃OH occurs on the singlet‐ground‐state surface, while for the liberation of ³CH₂, a two‐state reactivity scenario prevails.     

Selecting oxygen source for partial oxidation of methane to methanol using microwave plasmas

The methanol selectivity in partial oxidation of methane in microwave plasma reactors is improved by using H₂O in the presence or absence of O₂. The use of H₂O₂ as an oxygen source has a similar effect, although it is less effective than H₂O. The addition of H₂ to the system has little effect on selectivity. Two pathways are suggested for the formation of methanol. One involves a CH₃O^* or CH₃O₂ ^* intermediate, while the other involves a direct combination of CH₃ ^* and OH^* radicals. The first pathway is favored in the presence of O₂ while the latter is favored in the presence of H₂O or H₂O₂. The best results are obtained for the CH4-O₂-H₂O system when methanol is formed through both pathways.     

Tunable Diode Laser Diagnostic Studies of H2-Ar-O2 Microwave Plasmas Containing Methane or Methanol

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and ten stable molecules in H₂-Ar-O₂ microwave plasmas containing up to 7.2% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbons varied between 30 and 90% and the methyl radical concentration was found to be in the range 10 ¹⁰ –10 ¹² molecules cm ^–3 . The methyl radical concentration and the concentrations of the stable C-2 hydrocarbons C ₂ H ₂ , C ₂ H ₄ , and C ₂ H ₆ , produced in the plasma decayed exponentially when increasing amounts of O ₂ were added at fixed methane or methanol partial pressures. In addition to detecting the hydrocarbon species, the major products CO, CO ₂ , and H ₂ O were also monitored. For the first time, formaldehyde, formic acid, and methane were detected in methanol microwave plasmas, formaldehyde was detected in methane microwave plasmas. Chemical modeling with 57 reactions was used to successfully predict the concentrations in methane plasmas in the absence of oxygen and the trends for the major chemical product species as oxygen was added.     

Why do RuO2 electrodes catalyze electrochemical CO2 reduction to methanol rather than methane or perhaps neither of those?

The electrochemical CO₂ reduction reaction (CO₂RR) on RuO₂ and RuO₂-based electrodes has been shown experimentally to produce high yields of methanol, formic acid and/or hydrogen while methane formation is not detected. This CO₂RR selectivity on RuO₂ is in stark contrast to copper metal electrodes that produce methane and hydrogen in the highest yields whereas methanol is only formed in trace amounts. Density functional theory calculations on RuO₂(110) where only adsorption free energies of intermediate species are considered, i.e. solvent effects and energy barriers are not included, predict however, that the overpotential and the potential limiting step for both methanol and methane are the same. In this work, we use both ab initio molecular dynamics simulations at room temperature and total energy calculations to improve the model system and methodology by including both explicit solvation effects and calculations of proton–electron transfer energy barriers to elucidate the reaction mechanism towards several CO₂RR products: methanol, methane, formic acid, CO and methanediol, as well as for the competing H₂ evolution. We observe a significant difference in energy barriers towards methane and methanol, where a substantially larger energy barrier is calculated towards methane formation than towards methanol formation, explaining why methanol has been detected experimentally but not methane. Furthermore, the calculations show why RuO₂ also catalyzes the CO₂RR towards formic acid and not CO(g) and methanediol, in agreement with experimental results. However, our calculations predict RuO₂ to be much more selective towards H₂ formation than for the CO₂RR at any applied potential. Only when a large overpotential of around −1 V is applied, can both formic acid and methanol be evolved, but low faradaic efficiency is predicted because of the more facile H₂ formation.

Energy barriers are calculated for the electrochemical CO₂ reduction reaction on the RuO₂(110) surface towards methanol, methane, formic acid, methanediol, CO and the competing H₂ formation and compared with experimental literature.     

Catalytic and Mechanistic Insights of the Low‐Temperature Selective Oxidation of Methane over Cu‐Promoted Fe‐ZSM‐5

The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h^?1), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron‐ and copper‐containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h^?1 by using H₂O₂ as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by‐production of hydroxyl radicals. The catalytic system in question results in a low‐energy methane activation route, and allows selective C₁‐oxidation to proceed under intrinsically mild reaction conditions.     

Pentacoordinated Al3+‐Stabilized Active Pd Structures on Al2O3‐Coated Palladium Catalysts for Methane Combustion

Supported Pd catalysts are active in catalyzing the highly exothermic methane combustion reaction but tend to be deactivated owing to local hyperthermal environments. Herein we report an effective approach to stabilize Pd/SiO₂ catalysts with porous Al₂O₃ overlayers coated by atomic layer deposition (ALD). ²⁷Al magic angle spinning NMR analysis showed that Al₂O₃ overlayers on Pd particles coated by the ALD method are rich in pentacoordinated Al³⁺ sites capable of strongly interacting with adjacent surface PdO_x phases on supported Pd particles. Consequently, Al₂O₃‐decorated Pd/SiO₂ catalysts exhibit active and stable PdO_x and Pd–PdO_x structures to efficiently catalyze methane combustion between 200 and 850 °C. These results reveal the unique structural characteristics of Al₂O₃ overlayers on metal surfaces coated by the ALD method and provide a practical strategy to explore stable and efficient supported Pd catalysts for methane combustion.     

Stability of oxygen anions and hydrogen abstraction from methane on reduced SnO2 (110) surface

The stability of oxygen anions and the hydrogen abstraction from methane on a reduced SnO₂ (110) crystal surface have been studied theoretically using a point-charge model. The geometric and electronic structures for the present molecules are calculated by means of a hybrid Hartree–Fock/density functional method at the B3LYP/6-311+G(3df, 3pd) level of theory. The calculations of the energies on the point-charge model are performed using these optimized geometries. It is found that a low concentration of the active oxygen species O⁻ and O₂⁻ is expected on the reduced SnO₂ surface. The activation energies for the abstraction of hydrogen atom from methane on the reduced SnO₂ surface are obtained: 12 kcal/mol for O⁻ species and more than 48 kcal/mol for O₂⁻ species, indicating that O⁻ species on the surface is the main active center for the dissociation of a C(SINGLE BOND)H bond of methane, which is in agreement with the other oxide catalysts. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 669–678, 1998     

Biosynthesis of methanol from methane by <Emphasis Type="Italic">Methylosinus trichosporium</Emphasis> OB3b

Methanol has recently attracted significant interest in the energetic field. Current technology for the conversion of methane to methanol is based on energy intensive endothermic steam reforming followed by catalytic conversion into methanol. The one-step method performed at very low temperatures (35°C) is methane oxidation to methanol via bacteria. The aim of this work was to examine the role of copper in the one-step methane oxidation to methanol by utilizing whole cells of Methylosinus trichosporium OB3b bacteria. From the results obtained it was found that copper concentration in the medium influences the rate of bacterial biomass growth or methanol production during the process of methane oxidation to methanol. The presented results indicate that the process of methane oxidation to methanol by Methylosinus trichosporium OB3b bacteria is most efficient when the mineral medium contains 1.0 × 10⁻⁶ mol dm⁻³ of copper. Under these conditions, a satisfactory growth of biomass was also achieved. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

Partial Oxidation of Methane to Methanol with Oxygen or Air in a Nonequilibrium Discharge Plasma

The partial oxidation of methane to methanol with oxygen or air was investigated experimentally and theoretically in a dielectric-barrier discharge (DBD). The predominant parameters of specific electric energy, oxygen content, flow rate, temperature, and gas pressure were determined in CH ₄ /O ₂ and CH ₄ /air mixtures. Optimum selectivities toward methanol formation were found at an oxygen concentration of about 15% in both feed gas mixtures. Low specific energy favors the selectivity toward methanol and suppresses the formation of carbon oxides. The experiments indicate that high methanol selectivities can be obtained at high methane conversion. The highest methanol yield of 3% and the highest methanol selectivity of about 30% were achieved in CH ₄ /O ₂ mixtures. In CH ₄ /air mixtures, as high as 2% methanol yield was also obtained. In addition, other useful products, like ethylene, ethane, propane, and ethanol, were detected. Experiment and numerical simulations show that the formation of H ₂ O and CO has a strong negative influence on methanol formation.     

Identifying key mononuclear Fe species for low-temperature methane oxidation

The direct functionalization of methane into platform chemicals is arguably one of the holy grails in chemistry. The actual active sites for methane activation are intensively debated. By correlating a wide variety of characterization results with catalytic performance data we have been able to identify mononuclear Fe species as the active site in the Fe/ZSM-5 zeolites for the mild oxidation of methane with H₂O₂ at 50 °C. The 0.1% Fe/ZSM-5 catalyst with dominant mononuclear Fe species possess an excellent turnover rate (TOR) of 66 mol_MeOH mol_Fe⁻¹ h⁻¹, approximately 4 times higher compared to the state-of-the-art dimer-containing Fe/ZSM-5 catalysts. Based on a series of advanced in situ spectroscopic studies and ¹H- and ¹³C- nuclear magnetic resonance (NMR), we found that methane activation initially proceeds on the Fe site of mononuclear Fe species. With the aid of adjacent Brønsted acid sites (BAS), methane can be first oxidized to CH₃OOH and CH₃OH, and then subsequently converted into HOCH₂OOH and consecutively into HCOOH. These findings will facilitate the search towards new metal-zeolite combinations for the activation of C–H bonds in various hydrocarbons, for light alkanes and beyond.

The monomeric Fe species in Fe/ZSM-5 have been identified as the intrinsic active sites for the low-temperature methane oxidation.     

Methane content and isotopic composition of shallow groundwater: implications for environmental monitoring related to shale gas exploitation

The baseline methane for shallow groundwater can provide an important evidence to interpret possible methane stray associated with shale gas exploration. This study investigated and traced methane content and its origin of shallow groundwater in a karst aquifer in the Fuling shale gas block, SW China. The results show that methane contents of shallow groundwater are all less than 0.01 mg L^?1 and volumetric content in dissolved gas ranges from not detected to 0.0064%. The δ¹³C-CH₄ ranges from ?74.4 to ?49.1‰, suggesting biogenic origin. For the first time, the δ¹³C-CH₄ and ³He/⁴He end-numbers were determined.