Direct C–C Coupling of CO2 and the Methyl Group from CH4 Activation through Facile Insertion of CO2 into Zn–CH3 σ-Bond

Methane Carboxylation Using Electrochemically Activated Carbon Dioxide

Direct synthesis of CH3 COOH from CH4 and CO2 is an appealing approach for the utilization of two potent greenhouse gases that are notoriously difficult to activate. In this Communication, we report an integrated route to enable this reaction. Recognizing the thermodynamic stability of CO2 , our strategy sought to first activate CO2 to produce CO (through electrochemical CO2 reduction) and O2 (through water oxidation), followed by oxidative CH4 carbonylation catalyzed by Rh single atom catalysts supported on zeolite. The net result was CH4 carboxylation with 100 % atom economy. CH3 COOH was obtained at a high selectivity (>80 %) and good yield (ca. 3.2 mmol g-1 cat in 3 h). Isotope labelling experiments confirmed that CH3 COOH is produced through the coupling of CH4 and CO2 . This work represents the first successful integration of CO/O2 production with oxidative carbonylation reaction. The result is expected to inspire more carboxylation reactions utilizing preactivated CO2 that take advantage of both products from the reduction and oxidation processes, thus achieving high atom efficiency in the synthesis.

Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations

Understanding molecular-scale reaction mechanisms is crucial for the design of modern catalysts with industrial prospect. Through joint experimental and computational studies, we investigate the direct coupling reaction of CH₄ and CO₂ , two abundant greenhouse gases, mediated by Ta_1,4 ⁺ ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH₄ dehydrogenation [Ta_1,4 CH₂ ]⁺ to CO₂ in a ring electrode ion trap. Product analysis and reaction kinetics indicate a predisposition of the tetramers for C-O coupling with a conversion to products of CH₂ O, whereas atomic cations enable C-C coupling yielding CH₂ CO. Selected experimental findings are supported by thermodynamic computations, connecting structure, electronic properties, and catalyst function. Moreover, the study of bare Ta_1,4 ⁺ compounds indicates that methane dehydrogenation is a significant initial step in the direct coupling reaction, enabling new, yet unknown reaction pathways.

Plasma-Assisted Reforming of Methane

Methane (CH₄ ) is inexpensive, high in heating value, relatively low in carbon footprint compared to coal, and thus a promising energy resource. However, the locations of natural gas production sites are typically far from industrial areas. Therefore, transportation is needed, which could considerably increase the sale price of natural gas. Thus, the development of distributed, clean, affordable processes for the efficient conversion of CH₄ has increasingly attracted people's attention. Among them are plasma technology with the advantages of mild operating conditions, low space need, and quick generation of energetic and chemically active species, which allows the reaction to occur far from the thermodynamic equilibrium and at a reasonable cost. Significant progress in plasma-assisted reforming of methane (PARM) is achieved and reviewed in this paper from the perspectives of reactor development, thermal and nonthermal PARM routes, and catalysis. The factors affecting the conversion of reactants and the selectivity of products are studied. The findings from the past works and the insight into the existing challenges in this work should benefit the further development of reactors, high-performance catalysts, and PARM routes.

Rational Design of Metal-Alkoxide-Functionalized Metal-Organic Frameworks for Synergistic Dual Activation of CH4 and CO2 toward Acetic Acid Synthesis

The concurrent conversion of CH₄ and CO₂ into acetic acid is an ideal route to migrate the two greenhouse gases and manufacture a high-value-added C₂ product with an atom economy of 100% but remains challenging due to the chemical inertness of both gases. By leveraging density functional theory (DFT) calculations, we report herein the computational design of metal-alkoxide-functionalized metal-organic framework (MOF) UiO-67 with well-defined dual sites that can activate CH₄ and CO₂ cooperatively to boost acetic acid synthesis. The dual sites are distributed on two adjacent functionalized organic linkers originating from the same node and feature a metal-metal distance of about 6-7 Å. Initially, a total of 13 single-site metal-alkoxide-functionalized UiO-67s (including three alkaline earth metals and 10 transition metals) are examined; then, favorable metal-alkoxides are identified and further used to design dual-site metal-alkoxide-functionalized UiO-67s for converting CH₄ and CO₂ into acetic acid. Detailed mechanistic investigation predicts that the dual-site UiO-67s functionalized with Mn-, Fe-, Co-, Ni-. and Zn-alkoxide are highly promising catalysts for this reaction. Compared to the single-site counterparts, the metal pair-site UiO-67s provide a subtle microenvironment for synergistic dual activation of CH₄ and CO₂, thus efficiently stabilizing the transition state and substantially reducing the reaction barrier for C-C coupling. The microscopic insights and design strategies in this work might advance the development of efficient MOF-based catalysts with built-in cooperative active sites toward direct acetic acid synthesis from CH₄ and CO₂.

Excess CO2 Reductions during CH3COOH Formation from CH4 and CO2 under Periodic Operation: Downhill Side Reactions in an Uphill Target Reaction under Unsteady Conditions

Acetic acid (CH3COOH) formation from methane (CH4) and carbon dioxide (CO2) is an ideal reaction for chemical production, whereas this reaction possesses a severe thermodynamic limitation. To address this issue, it has been reported that periodic operation allowing a non-equilibrium condition can overcome the thermodynamic limitation. However, although an intrinsic issue of uphill reactions in non-equilibrium conditions generally is occurrence of unfavorable downhill reactions, this issue has seldom been discussed for the CH3COOH formation under periodic operation. Herein, excess CO2 reductions were found to be the unfavorable downhill reactions possibly occurring in the reaction aiming at CH3COOH formation under periodically operated CH4 and CO2 feeds. The reaction using an isotopic reactant (i.e., 13CH4 ) unveiled that excess CO2 reductions to CO and even to CH3 moiety could occur, indicating importance of catalyst development. Furthermore, it was proposed that H2 O vapor introduction into the CO2 feed, which increased the CH3COOH product, most likely facilitated the reverse reaction of the excess CO2 reductions and thereby is effective to hamper the unfavorable side reaction.

Electron Regulation of Single Indium Atoms at the Active Oxygen Vacancy of In2 O3 (110) for Production of Acetic Acid and Acetone through Direct Coupling of CH4 with CO2

The production of acetic acid and acetone from the direct coupling of CO₂ and CH₄ on the doped In₂ O₃ (110) surface has been studied by extensive first-principles calculations, and the Ga or Al substitution for the single In atom at the active oxygen vacancy of In₂ O₃ (110) can stabilize the reaction species and reduce the free energy barrier of the rate-limiting C-H activation for the conversion of CO₂ and CH₄ to acetic acid. Herein, the metal doping lowers the energy level of partially empty s and p orbitals of In₁ at the oxygen vacancy site and manipulates its electronic properties, resulting in the activity improvement. The stable intermediate with the newly-formed CH₃ COO* has the available In₁ site for subsequent CH₄ activation, which may initiate the direct C-C coupling of CH₃ COO* and CH₃ * to yield C3 species on the doped In₂ O₃ (110). These findings suggest that the metal doping of the active oxygen vacancy opens an avenue for the carbon-chain growth through heterogeneously catalytic coupling of CO₂ and CH₄ .

Photocatalytic Conversion of Methane: Recent Advancements and Prospects

Abundant and affordable methane is not only a high-quality fossil fuel, it is also a raw material for the synthesis of value-added chemicals. Solar-energy-driven conversion of methane offers a promising approach to directly transform methane to valuable energy sources under mild conditions, but remains a great challenge at present. In this Review, recent advances in the photocatalytic conversion of methane are systematically summarized. Insights into the construction of effective semiconductor-based photocatalysts from the perspective of light-absorption units and active centers are highlighted and discussed in detail. The performance of various photocatalysts in the conversion of methane is presented, with the photooxidation classified according to the oxidant systems. Lastly, challenges and future perspectives in the photocatalytic oxidation of methane are described.

Efficient photoelectrochemical CO2 conversion for selective acetic acid production

Amidst the development of photoelectrochemical (PEC) CO₂ conversion toward practical application, the production of high-value chemicals beyond C₁ compounds under mild conditions is greatly desired yet challenging. Here, through rational PEC device design by combining Au-loaded and N-doped TiO₂ plate nanoarray photoanode with Zn-doped Cu₂O dark cathode, efficient conversion of CO₂ to CH₃COOH has been achieved with an outstanding Faradaic efficiency up to 58.1% (91.5% carbon selectivity) at 0.5 V vs. Ag/AgCl. Temperature programmed desorption and in situ Raman spectra reveal that the Zn-dopant in Cu₂O plays multiple roles in selective catalytic CO₂ conversion, including local electronic structure manipulation and active site modification, which together promote the formation of intermediate *CH₂/*CH₃ for C-C coupling. Apart from that, it is also unveiled that the sufficient electron density provided by the Au-loaded and N-doped TiO₂ plate nanoarray photoanode plays an equally important role by initiating multi-electron CO₂ reduction. This work provides fresh insights into the PEC system design to reach the multi-electron reduction reaction and facilitate the C-C coupling reaction toward high-value multicarbon (C₂₊) chemical production via CO₂ conversion.

Metalloradical complex Co-C˙Ph3 catalyzes the CO2 reduction in gas phase: a theoretical study

Metal-stabilized radicals have been increasingly exploited in modern organic synthesis. Here, we theoretically designed a metalloradical complex Co-C˙Ph3 with the triplet characters through the transition metal cobalt (Co0) coordinating a triphenylmethyl radical. The potential catalytic role of this novel metalloradical in the CO2 reduction with H2/CH4 in the gas phase was explored via density functional theory (DFT) calculations. For the CO2 reduction reaction with H2, there are two possible pathways: one (path A) is the activation of CO2 by Co-C˙Ph3, followed by the hydrogenation of CO2. The other (path B) starts from the splitting of the H-H bond by Co-C˙Ph3, leading to the transition-metal hydride complex CoH-H, which can reduce CO2. DFT computations show that path B is more favorable than path A as their rate-determining free energy barriers are 18.3 and 27.2 kcal mol-1, respectively. However, for the reduction of CO2 by CH4 two different products, CH3COOH and HCOOCH3, can be generated following different reaction routes. Both routes begin with one CH4 molecule approaching the metalloradical Co-C˙Ph3 to form the intermediate CoH-CH3. This intermediate can evolve following two different pathways, depending on whether the H bonded to Co is transferred to the O (pathway PO) or the C (pathway PC) of CO2. Comparing their rate-determining steps, we identified that the PO route is more favorable for the reduction of CO2 by CH4 to CH3COOH with the reaction barrier 24.5 kcal mol-1. Thus, the present Co0-based metalloradical system represents a viable catalytic protocol that can contribute to the effective utilization of small molecules (H2 and CH4) to reduce CO2, and provides an alternative strategy for the exploration of CO2 conversion.

Synthesizing High-Volume Chemicals from CO2 without Direct H2 Input

Decarbonizing the chemical industry will eventually entail using CO₂ as a feedstock for chemical synthesis. However, many chemical syntheses involve CO₂ reduction using inputs such as renewable hydrogen. In this review, chemical processes are discussed that use CO₂ as an oxidant for upgrading hydrocarbon feedstocks. The captured CO₂ is inherently reduced by the hydrocarbon co-reactants without consuming molecular hydrogen or renewable electricity. This CO₂ utilization approach can be potentially applied to synthesize eight emission-intensive molecules, including olefins and epoxides. Catalytic systems and reactor concepts are discussed that can overcome practical challenges, such as thermodynamic limitations, over-oxidation, coking, and heat management. Under the best-case scenario, these hydrogen-free CO₂ reduction processes have a combined CO₂ abatement potential of approximately 1 gigatons per year and avoid the consumption of 1.24 PWh renewable electricity, based on current market demand and supply.

Computational and experimental identification of strong synergy of the Fe/ZnO catalyst in promoting acetic acid synthesis from CH4 and CO2

DFT calculations have identified reaction pathways for acetic acid synthesis from CO₂ and CH₄ on ZnO, Cu/ZnO and Fe/ZnO surfaces. Fe/ZnO exhibits strong synergy in facilitating CH₄ activation, dissociation and C-C coupling. Thus, the surface acetate formation is significantly enhanced. The DFT predictions have been confirmed by in situ DRIFTS experiments.

Methane activation by heteronuclear diatomic AuRh+ cation: comparison with homonuclear Au2+ and Rh2. Phys Chem Chem Phys 2020;22:6231-6238. [PMID: 32129335 DOI: 10.1039/c9cp05699h]

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH₄ with Au₂⁺, AuRh⁺ and Rh₂⁺ cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au₂⁺ has the lowest reactivity, and it can activate methane but only produce H-Au₂-CH₃⁺ without H₂ release; Rh₂⁺ has the highest reactivity, and it can produce both carbene-type Rh₂-CH₂⁺ and carbyne-type H-Rh₂-CH⁺ with H₂ release; AuRh⁺ also has high reactivity to produce only AuRh-CH₂⁺ with H₂, avoiding the excessive dehydrogenation of CH₄. Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals.

In-Situ FT-IR Spectroscopy Investigation of CH4 and CO2 Reaction

An exclusive trace of CH4 direct carboxylation with CO2 by a stepwise technology was investigated using in-situ FT-IR spectroscopy. The results showed that CH4 was dissociated to atomic hydrogen and M-CHx species on catalyst surface when it was first introduced in the system, then CO2 was inserted into the intermediate to direct carboxylate. Finally, the subsequent adsorption of CH4 provided active hydrogen for the species of previous surface reaction, thus leading to the formation of the product. It was also found that the first introduction of CO2 on the surface of the “clean” catalyst might likely react with surface H species, which had an irreversible effect on the catalytic activity of CH4.

Direct Conversion of Methane with Carbon Dioxide Mediated by RhVO3 - Cluster Anions

Direct conversion of methane with carbon dioxide to value-added chemicals is attractive but extremely challenging because of the thermodynamic stability and kinetic inertness of both molecules. Herein, the first dinuclear cluster species, RhVO₃ ^- , has been designed to mediate the co-conversion of CH₄ and CO₂ to oxygenated products, CH₃ OH and CH₂ O, in the temperature range of 393-600 K. The resulting cluster ions RhVO₃ CO^- after CH₃ OH formation can further desorb the [CO] unit to regenerate the RhVO₃ ^- cluster, leading to the successful establishment of a catalytic cycle for methanol production from CH₄ and CO₂ (CH₄ +CO₂ →CH₃ OH+CO). The exceptional activity of Rh-V dinuclear oxide cluster (RhVO₃ ^- ) identified herein provides a new mechanism for co-conversion of two very stable molecules CH₄ and CO₂ .

C-H Bond Carboxylation with Carbon Dioxide

Carbon dioxide is a nontoxic, renewable, and abundant C₁ source, whereas C-H bond functionalization represents one of the most important approaches to the construction of carbon-carbon bonds and carbon-heteroatom bonds in an atom- and step-economical manner. Combining the chemical transformation of CO₂ with C-H bond functionalization is of great importance in the synthesis of carboxylic acids and their derivatives. The contents of this Review are organized according to the type of C-H bond involved in carboxylation. The primary types of C-H bonds are as follows: C(sp)-H bonds of terminal alkynes, C(sp² )-H bonds of (hetero)arenes, vinylic C(sp² )-H bonds, the ipso-C(sp² )-H bonds of the diazo group, aldehyde C(sp² )-H bonds, α-C(sp³ )-H bonds of the carbonyl group, γ-C(sp³ )-H bonds of the carbonyl group, C(sp³ )-H bonds adjacent to nitrogen atoms, C(sp³ )-H bonds of o-alkyl phenyl ketones, allylic C(sp³ )-H bonds, C(sp³ )-H bonds of methane, and C(sp³ )-H bonds of halogenated aliphatic hydrocarbons. In addition, multicomponent reactions, tandem reactions, and key theoretical studies related to the carboxylation of C-H bonds are briefly summarized. Transition-metal-free, organocatalytic, electrochemical, and light-driven methods are highlighted.

Active sites and mechanism of the direct conversion of methane and carbon dioxide to acetic acid over the zinc-modified H-ZSM-5 zeolite

The catalytic activity of the conversion of CH₄ and CO₂ on zinc modified H-ZSM-5 is strongly dependent on the structure of the active sites.

Direct synthesis of acetic acid from carbon dioxide and methane over Cu-modulated BEA, MFI, MOR and TON zeolites: a density functional theory study

Cu-Modulated zeolites can be promising candidate catalysts in the direct conversion of carbon dioxide and methane to acetic acid.

Coupling of Methane and Carbon Dioxide Mediated by Diatomic Copper Boride Cations

The use of CH₄ and CO₂ to produce value-added chemicals via direct C-C coupling is a challenging chemistry problem because of the inertness of these two molecules. Herein, mass spectrometric experiments and high-level quantum-chemical calculations have identified the first diatomic species (CuB⁺ ) that can couple CH₄ with CO₂ under thermal collision conditions to produce ketene (H₂ C=C=O), an important intermediate in synthetic chemistry. The order to feed the reactants (CH₄ and CO₂ ) is important and CH₄ should be firstly fed to produce the C₂ product. Molecular-level mechanisms including control of product selectivity have been revealed for coupling of CH₄ with CO₂ under mild conditions.

A review of plasma-assisted catalytic conversion of gaseous carbon dioxide and methane into value-added platform chemicals and fuels

CO2 and CH4 contribute to greenhouse gas emissions, while the production of industrial base chemicals from natural gas resources is emerging as well. Such conversion processes, however, are energy-intensive and introducing a renewable and sustainable electric activation seems optimal, at least for intermediate-scale modular operation. The review thus analyses such valorisation by plasma reactor technologies and heterogeneous catalysis application, largely into higher hydrocarbon molecules, that is ethane, ethylene, acetylene, propane, etc., and organic oxygenated compounds, i.e. methanol, formaldehyde, formic acid and dimethyl ether. Focus is given to reaction pathway mechanisms, related to the partial oxidation steps of CH4 with O2, H2O and CO2, CO2 reduction with H2, CH4 or other paraffin species, and to a lesser extent, to mixtures' dry reforming to syngas. Dielectric barrier discharge, corona, spark and gliding arc sources are considered, combined with (noble) metal materials. Carbon (C), silica (SiO2) and alumina (Al2O3) as well as various catalytic supports are examined as precious critical raw materials (e.g. platinum, palladium and rhodium) or transition metal (e.g. manganese, iron, cobalt, nickel and copper) substrates. These are applied for turnover, such as that pertinent to reformer, (reverse) water-gas shift (WGS or RWGS) and CH3OH synthesis. Time-on-stream catalyst deactivation or reactivation is also overviewed from the viewpoint of individual transient moieties and their adsorption or desorption characteristics, as well as reactivity.

One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis

The conversion of CO₂ with CH₄ into liquid fuels and chemicals in a single‐step catalytic process that bypasses the production of syngas remains a challenge. In this study, liquid fuels and chemicals (e.g., acetic acid, methanol, ethanol, and formaldehyde) were synthesized in a one‐step process from CO₂ and CH₄ at room temperature (30 °C) and atmospheric pressure for the first time by using a novel plasma reactor with a water electrode. The total selectivity to oxygenates was approximately 50–60 %, with acetic acid being the major component at 40.2 % selectivity, the highest value reported for acetic acid thus far. Interestingly, the direct plasma synthesis of acetic acid from CH₄ and CO₂ is an ideal reaction with 100 % atom economy, but it is almost impossible by thermal catalysis owing to the significant thermodynamic barrier. The combination of plasma and catalyst in this process shows great potential for manipulating the distribution of liquid chemical products in a given process.