Methane Activation Reaction Research Articles

Low-Temperature Methane Activation Reaction Pathways over Mechanochemically-Generated Ce4+/Cu+ Interfacial Sites.

Methane is a valuable resource and its valorization is an important challenge in heterogeneous catalysis. Here it is shown that CeO2/CuO composite prepared by ball milling activates methane at a temperature as low as 250°C. In contrast to conventionally prepared catalysts, the formation of partial oxidation products such as methanol and formaldehyde is also observed. Through an in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and operando Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS) approach, it can be established that this unusual reactivity can be attributed to the presence of Ce4+/Cu+ interfaces generated through a redox exchange between Ce3+ and Cu2+ atoms facilitated by the mechanical energy supplied during milling. DFT modeling of the electronic properties confirms the existence of a charge transfer mechanism. These results demonstrate the effectiveness and distinctiveness of the mechanical approach in creating unique and resilient interfaces thereby enabling the optimization and refining of CeO2/CuO catalysts in methane activation reactions.

Low temperature activation of methane to hydrogen depending on tailored electron transfer over Ni–Cr composite oxide

Current industrial use of methane depends mainly on high-temperature conversion, resulting in high energy consumption and catalyst sintering inactivation. In this paper, metal oxide catalysts with different Ni/Cr ratios were obtained by the sol-gel method for the methane activation reaction. The structure-property relationships of the catalysts were investigated by XRD, XPS, H2-TPR, TG, N2 adsorption-desorption, and CV, focusing on the interaction between Ni–Cr in the composite oxide catalysts. The results showed that Ni0–NiCrO3/Cr2O3 structure was formed after the Ni–Cr oxides underwent the reduction and calcination process, which provided a structural support for electron transfer between the two metals. The least prone to sintering nickel metal particles and the highest content of high-valent chromium ions were present in Ni1Cr2. XPS confirmed that this counterpart acts as the donor and acceptor for electron transfer in the catalysts, accelerating oxidation-reduction reaction over the catalyst. The catalyst exhibited the most excellent conversion and product selectivity in the methane activation reaction.

Methane C–H bond heterolysis versus homolysis on Cu-MFI and Au-MFI

The methane activation reaction is of fundamental importance for its transformation into high-value chemicals. Despite the fact that both homolysis and heterolysis are competitive mechanisms of C-H scission, experimental and DFT studies have revealed heterolytic scission of the C-H bond over metal-exchange zeolites. To rationalize the new catalysts, work must be done on the homolytic versus heterolytic scission of the C-H bond mechanism on these catalysts. We have performed the quantum mechanical calculations for the C-H bond homolysis versus heterolysis over Au-MFI and Cu-MFI catalysts. Calculations results showed that homolysis of the C-H bond is favorable both thermodynamically as well as kinetically over Au-MFI catalysts. However, over Cu-MFI, heterolytic scission is favorable. Both Cu(I) and Au(I) activate the CH4 via electronic density back donation from filled nd10 orbitals, according to NBO calculations. Cu(I) cation has a higher electronic density back donation than Au(I) cation. This is also supported by the charge on the C-atom of Methane. Additionally, a greater negative charge on the O-atom in the active site in case of Cu(I), where proton transfer occurs, promotes heterolytic scission. Because of the larger size of the Au-atom and the smaller negative charge of the O-atom in the active site, where proton transfer occurs, C-H bond homolytic fission is preferable over Au-MFI.

Rational design of nickel–borane complexes for methane activation and functionalization

For the methane activation reaction, the mono-phosphine ligand supported Ni–B complex is more active than the traditional ambiphilic ligand supported one.

Ion spectroscopy in methane activation.

This review is devoted to ion spectroscopy studies of complexes relevant for the understanding of methane activation with metal ions and clusters. Methane activation starts with the formation of a complex with a metal ion. The degree of the interaction between an intact methane molecule and the ion can be monitored by the perturbations of C–H stretch vibrations in the methane molecule. Binding mediated by the electrostatic interaction results in a η3 type coordination of methane. In contrast, binding governed by orbital interactions results in a η2 type coordination of methane. We further review the spectroscopic characterization of activation products of metal–methane reactions, such as the metal–carbene and carbyne products resulting from the interaction of selected 5d metals with methane. The focus of recent research in the field has shifted towards the investigation of interactions between methane and metal clusters. We show examples highlighting that metal clusters can be more reactive in methane activation reactions.

A computational study of CO oxidation on IrO2 (1 1 0) surface

Though, oxygen-rich iridium oxide (O-IrO2 (110)) surface exhibits excellent catalytic activity towards methane activation and oxygen evolution reactions, the CO oxidation mechanism on this surface remains elusive. Here, we used density functional theory calculations, ab initio molecular dynamics (AIMD), and microkinetic simulations to explore the CO oxidation reactions via Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R), and Mars-Van Krevelen (MvK) mechanisms on O-IrO2 (110) surface. We find that the C-O coupling and CO2 desorption are the rate-determining steps in L-H and E-R mechanisms, respectively. The CO oxidation via the MvK mechanism on the IrO2 (110) surface is more difficult to proceed. Both AIMD and microkinetic simulation results demonstrate that CO2 can desorb from the O-IrO2 (110) surface at 400 K. The production temperature of CO2 on the O-rich IrO2 (110) surface via E-R mechanism is lower than that via the L-H mechanism. Therefore, we predict that the O-rich IrO2 (110) surface can accelerate the CO oxidation reaction rate.

A Review of Methane Activation Reactions by Halogenation: Catalysis, Mechanism, Kinetics, Modeling, and Reactors

Methane is the central component of natural gas, which is globally one of the most abundant feedstocks. Due to its strong C–H bond, methane activation is difficult, and its conversion into value-added chemicals and fuels has therefore been the pot of gold in the industry and academia for many years. Industrially, halogenation of methane is one of the most promising methane conversion routes, which is why this paper presents a comprehensive review of the literature on methane activation by halogenation. Homogeneous gas phase reactions and their pertinent reaction mechanisms and kinetics are presented as well as microkinetic models for methane reaction with chlorine, bromine, and iodine. The catalysts for non-oxidative and oxidative catalytic halogenation were reviewed for their activity and selectivity as well as their catalytic action. The highly reactive products of methane halogenation reactions are often converted to other chemicals in the same process, and these multi-step processes were reviewed in a separate section. Recent advances in the available computational power have made the use of the ab initio calculations (such as density functional theory) routine, allowing for in silico calculations of energy profiles, which include all stable intermediates and the transition states linking them. The available literature on this subject is presented. Lastly, green processes and the production of fuels as well as some unconventional methods for methane activation using ultrasound, plasma, superacids, and light are also reviewed.

Theoretical study of methane adsorption and C─H bond activation over Fe‐embedded graphene: Effect of external electric field

The effect of an external electric field (EF) on the methane adsorption and its activation on iron-embedded graphene (Fe-GPs) are investigated by using the M06-L density functional method. The EF is applied in the perpendicular direction to the graphene in the range of -0.015 to +0.015 a.u. with the interval of 0.005 a.u. The effects of EF on the adsorption, transition state and product complexes of the methane activation reaction are revealed. The binding energies of methane on Fe site in Fe-GPs are increased from -12.9 to -15.3, -18.1 and -21.5 kcal/mol for the negative EF of -0.005, -0.010 and -0.015, respectively. By applying positive EF, the activation barriers for methane activation are reduced in range of 3-8 kcal/mol (around 12-31%) and the reaction energies are more exothermic. The positive EF kinetically favors the reaction compared to the system without EF. The adsorption and activation of methane on Fe-GPs can be easily tuned by adjusting the external electric field for various applications. © 2019 Wiley Periodicals, Inc.

Methane activation by alternant [N2O2][rad]+ and [N2S2][rad]+ cluster radical cations

Methane activation reaction by small alternant [N2Y2]+ (YO, S) radical cation rings is predicted by the methods of computational chemistry. Approximate density functional theory and coupled cluster singles and doubles with perturbative triples calculations are employed to investigate the potential energy surface of this putative reaction. On the contrary to previously reported methane activation processes by square planar four-membered rings following the proton coupled electron transfer mechanism, the [N2O2]+ and [N2S2]+ clusters, according to our results, may activate methane through the hydrogen atom transfer mechanism. On the contrary, the similar oxide cluster in which the nitrogen atoms were substituted by phosphorous atoms, [P2O2]+, does not perform methane activation.

Density Functional Theory Study of Cu-ZSM-5-Catalyzed C–H Bond Activation: The Importance of Active Centers

In the present contribution, we investigate the catalytic cycle of the methane activation reaction over several copper sites by density functional theory calculation. Our results demonstrate that t...

Enhanced Catalytic Activity of (DMSO)2PtCl2 for the Methane Oxidation in the SO3–H2SO4 System

Among the various methane activation reactions, oleum-mediated (SO3–H2SO4) methane oxidation to methyl bisulfate (MBS), a methanol precursor, is one of the best methods in terms of methane conversion and product selectivity. In this report, we investigated the effect of the catalyst concentration on the MBS yield using the catalyst systems (bpym)PtCl2, K2PtCl4, and (DMSO)2PtCl2 at 180 °C for 3 h. (bpym)PtCl2 showed a very high stability as well as a high MBS yield of over 84%, but its catalytic activity expressed in terms of turnovers for 3 h was in the range 50–500. K2PtCl4 showed very high catalyst turnovers of over 17 000 at a low catalyst concentration; however, it deactivated rapidly to PtCl2 which prevents achieving a high MBS yield that was achieved by (bpym)PtCl2. However, (DMSO)2PtCl2 showed an enhanced catalytic performance for both the MBS yield and the turnovers. An MBS yield of over 84% with a selectivity of 94% was obtained at a catalyst concentration of 3.0 mM, and its turnovers reached ove...

Methane C-H Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study.

This paper reports a density functional theory study of 3d transition-metal methoxide complexes with potentially redox-noninnocent pincer supporting ligands for methane C-H bond activation to form methanol (LnM-OMe + CH4 → LnM-Me + CH3OH). The three types of tridentate pincer ligands [terpyridine (NNN), bis(2-pyridyl)phenyl-C,N,N' (NCN), and 2,6-bis(2-phenyl)pyridine-N,C,C' (CNC)] and different first-row transition metals (M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the reaction mechanism as well as the effect of the metal identity on the thermodynamics and kinetics of a methane activation reaction. Spin-density analysis indicates that some of these systems, the NNN and NCN ligands, have redox-noninnocent character. A four-centered, kite-shaped transition state, σ-bond metathesis, or oxidative hydrogen migration has been found for methane activation for the complexes studied. Calculations suggest that the d electron count is a more significant factor than the metal formal charge in controlling the thermodynamics and kinetics of C-H activation and late 3d metal methoxides, with high d counts preferred. Notably, early-to-middle metals tend toward oxidative hydrogen migration and late metals undergo a pathway that is more akin to σ-bond metathesis, suggesting that metal methoxide complexes that favor σ-bond metathesis pathways for methane activation will yield lower barriers for C-H activation.

Methane Activations by Titanium Neopentylidene Complexes: Electronic Resilience and Steric Control.

The titanium neopentylidene complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2-) is capable of activating both sp2 and sp3 C-H bonds under mild conditions. In addition to methane C-H activation, competition between the initial hydrogen abstraction reaction to form the methane activation product and the tautomerization reaction of this product to form a terminal methylidene was also explored. Several modifications of the PNP and CHtBu ligands were explored to determine the effect of these changes on C-H bond activation. In general, on the one hand, the modifications involving electronic effects have small and inconsistent influence on the stability of the intermediates and products and on the reaction barriers. On the other hand, the use of bulky groups in the ligands favors the methane activation process. By replacing the iPr groups in the PNP ligand with tBu groups, both methane activation and tautomerization reactions become more energetically favorable than in the unmodified complex. On the one hand, the largest acceleration of the methane C-H activation occurs when tBu groups in the phosphine are combined with an extra CH2 linker between the aromatic ring and the phosphine. On the other hand, replacing the nitrogen in the PNP ligand by phosphorus results in lower barriers for the tautomerization reaction and the stabilization of the product of the tautomerization although it remains slightly less stable than product of methane C-H activation. While several ligand modifications related to the electronic effects were examined, it is interesting that most of them did not make a significant change on the barriers for either reaction, indicating a significant resilience of this titanium complex, which could be used to enhance the practical aspects of the complex without a significant loss of its activity.

High-temperature Shilov-type methane conversion reaction: Mechanistic and kinetic studies

Traditional Shilov reactions (performed in aqueous solution with a PtCl2 catalyst) for methane conversion suffer from catalyst deactivation at high temperatures (> 100 °C), therefore only very low conversion rates have been achieved. In this paper, we show that Shilov-type C–H activations are achievable at much higher temperatures (∼200 °C) by addition of concentrated aqueous solutions of Cl− to inhibit Pt catalyst precipitation. Various chloride-based ionic liquids also stabilized the Pt catalyst at mild reaction temperatures (∼140 °C). Under high-pressure conditions (> 25.5 MPa), achieved using a specially designed sealed gold-tube reactor, very high methane conversion rates (> 90%) were obtained; this is attributed to the improved methane solubility in aqueous solution. Deuterium isotope (H/D) exchange between methane and water was used to examine the reaction reactivity and selectivity. Multiply D-substituted products were observed, indicating that multiple C–H activations occurred. A comprehensive network reaction that included all the chain reactions was set up to clarify the reactivities and product selectivities of the methane activation reactions. The reaction network consisted of a series of parallel first-order reactions, which can be described by the Arrhenius equation. The kinetic parameters such as the frequency factor, activation energies, and stoichiometric coefficients were obtained by fitting the experimental data. Because all four C–H bonds in a methane molecule are equivalent, multiple substitutions during methane conversion cannot be avoided. Our studies indicate that mono-substituted and di-substituted methane isotopologue generations have similar activation energies, suggesting that the highest mono-substitution selectivity cannot be greater than 50%.

Kinetic modeling of methane dehydroaromatization chemistry on Mo/Zeolite catalysts in packed-bed reactors

This paper develops a detailed chemical kinetics reaction mechanism to represent methane dehydroaromatization (MDA) chemistry on bi-functional Mo/zeolite catalysts. The model is validated using a range of previously published results from packed-bed experiments. The reaction mechanism consists of four methane activation reactions on Mo2C sites. The resulting gas-phase hydrogen and ethylene continue to react on Brønsted acid sites within the zeolite structure using 46 reaction steps. In addition to the desired benzene formation, the model also predicts the formation of toluene, naphthalene, and other side products. In addition to reaction kinetics on the catalyst surfaces, the packed-bed model incorporates a Dusty-Gas model that accommodates ordinary and Knudsen diffusion as well as pressure-driven advection. Because the model represents essentially all published reports of MDA performance, it is reasonably expected that the model can be applied as a predictive tool to support reactor and process development.

All-Metal Aromatic Complexes Show High Reactivity in the Oxidation Reaction of Methane and Some Hydrocarbons

The C-H activations of methane, ethane, propane, and propene catalyzed by all-metal aromatic complexes Al(4)Fe were investigated. The results reveal that the rate-determining barrier of methane activation reaction with Al(4)Fe is lower than that of both some well-known inorganic catalysts and some metal organic catalysts. It was found that the all-metal aromatic complexes have high reactivity for the C-H activation of ethane, propane, and propene. Further research showed that the ability of all-metal aromatic complex to accept an electron and the degree of electron delocalization on its aromatic plane had obvious influences on the reactivity of Al(4)Fe. The present work predicts a new kind of catalyst for the alkyl C-H activation reaction: all-metal aromatic catalyst.

Methane Activation by Laser-Ablated Group 3 Metal Atoms: Infrared Spectra and Structures of the CH3−MH and CH2−MH2 Complexes

Methane activation reactions with group 3 metal atoms find that Sc, Y, and La are just as effective as other early transition metals. Two primary products, CH3−MH and CH2−MH2, are identified for each metal from the matrix IR spectra. The single insertion product becomes more favorable with increasing atomic mass of the metal, opposite the general trend observed from other early transition metal systems. The C−M bond lengths of CH3−MH and CH2−MH2 are comparable to each other, on the basis of electronic structure calculations, and the methylidene dihydride complex is symmetrical without agostic distortion. The number of valence electrons of the metal is evidently an important factor in the magnitude of the agostic interaction in these simple methylidene systems.

The Reactivity of All-Metal Aromatic Complexes: A Theoretical Investigation on the Methane Activation Reaction

The recent finding of all-metal aromaticity enlightens us to explore the reactivity of all-metal aromatic complexes. The methane activation reaction was chosen for this purpose. The whole catalysis process of all-metal aromatics was investigated by theoretical calculation. The results reveal that the reaction barriers with all-metal aromatic complexes are far lower than that without any catalysts. All-metal aromatic complexes are predicted to have potential to be used as catalysts for the first time.

Methane activation on superacidic catalysts based on oxoanion modified zirconium oxide

A series of metal promoted catalysts was prepared by impregnation of metal precursor solutions on oxoanion modified zirconias (sulfated, molybdated and tungstated), well recognized, in the literature, for their ability in catalyzing the isomerization of light alkanes at low temperatures. Structural characterization and nature of surface active sites were attempted to explain the methane activation reaction on these catalysts. Although Lewis and Brönsted acid sites were observed on the oxoanion modified zirconias, as detected by infrared of pyridine adsorption, the methane activation occurred only when promoted with specific metals, such as Fe-Mn, Co-Mn, Al, Ni and Cu, for sulfated zirconias, and Ni for molybdated and tugstated zirconias. The results obtained suggest different kinds of active surface sites, depending on the oxoanion present and thus different mechanisms could be proposed for the methane activation reaction. It was suggested that on metal promoted sulfated zirconias, methane was converted into ethane by an acid catalyzed mechanism, more specifically, by S OH groups as proton donors. On NiO promoted molybdated and tungstated zirconias, methane was not simultaneously converted into ethane but only after an induction period. The XRD patterns of tugstated zirconias after the reaction with methane, showed only reflections at 2 θ = 42.6°, 53.8° and 77.6°, characteristics of nickel on zero oxidation state. Although these reflections were absent for Ni promoted molybdated zirconias, these catalysts were black in color after reaction. TPR experiments using CH 4 as reduction gas gave support to the proposal for a mechanism catalyzed by metal for the methane conversion into ethane on Ni promoted molybdated and tungstated zirconias.

Gas-phase reactions of rhenium-oxo species ReOn+, n = 0, 2–6, 8, with O2, N2O, CO, H2O, H2, CH4 and C2H4

The reactions of ReOn+, n = 0, 2–6, 8, with the small molecules O2, N2O, CO, H2O, H2 , CH4 and C2H4, are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry under single collision conditions on the timescale of seconds. The ReOn+ species are produced by laser vaporization of solid rhenium and pulsed supersonic expansion in a helium–oxygen mixture into high vacuum. A wide variety of reactions are observed, including methane activation and epoxidation reactions. Re+ reacts with ethylene by sequential dehydrogenation. ReO2+ and ReO4+ exhibit the most diverse reaction pathways, while ReO5+ almost exclusively undergoes ligand exchange. ReO6+ and ReO8+ are largely unreactive, the only efficient reactions observed are with ethylene and water. Both molecules seem to be able to directly attack the dioxygen ligands. The observed chemistry is governed by a fine interplay between available coordination sites and thermochemistry.