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

Radiocatalytic Synthesis of Acetic Acid from CH4 and CO2

The C-C coupling of methane (CH4) and carbon dioxide (CO2) to generate acetic acid (CH3COOH) represents a highly atom-efficient chemical conversion, fostering the comprehensive utilization of greenhouse gases. However, the inherent thermodynamic stability and kinetic inertness of CH4 and CO2 present obstacles to achieving efficient and selective conversion at room temperature. Our study reveals that hydroxyl radicals (⋅OH) and hydrated electrons (eaq -) produced by water radiolysis can effectively activate CH4 and CO2, yielding methyl radicals (⋅CH3) and carbon dioxide radical anions(⋅CO2 -) that facilitate the production of CH3COOH at ambient temperature. The introduction of radiation-synthesized CuO-anchored TiO2 bifunctional catalyst could further enhance reaction efficiency and selectivity remarkably by boosting radiation absorption and radical stability, resulting in a concentration of 7.1 mmol ⋅ L-1 of CH3COOH with near-unity selectivity (>95 %). These findings offer valuable insights for catalyst design and implementation in radiation-induced chemical conversion.

Dry Reforming of Methane to Syngas Mediated by Rhodium-Cobalt Oxide Cluster Anions Rh2CoO. J Phys Chem Lett 2024;15:9167-9174. [PMID: 39213481 DOI: 10.1021/acs.jpclett.4c01961] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Dry reforming of methane (DRM) to syngas is an important route to co-convert CH4 and CO2. However, the highly endothermic nature of DRM induces the thermocatalysis to commonly operate at high temperatures that inevitably causes coke deposition through pyrolysis of methane. Herein, benefiting from the mass spectrometric experiments complemented with quantum chemical calculations, we have discovered that the bimetallic oxide cluster Rh2CoO- can mediate the co-conversion of CH4 and CO2 at room temperature giving rise to two free H2 molecules and two adsorbed CO molecules (COads). The only elementary step requiring the input of external energy (e.g., high temperature) is desorption of COads from the reaction intermediate Rh2CoOC2O2-. The doping effect of Co has also been clarified that the Co could tune the charge distribution and orbital energy of the active metal Rh, enabling the enhancement of cluster reactivity toward C-H activation, which is essential to facilitating the DRM to syngas. This work not only underlines the importance of temperature control over elementary steps in practical thermocatalysis but also identifies a promising active species containing the late 3d transition metal to drive DRM to syngas. The findings could provide novel insights into design of bimetallic catalysts for co-conversion of CH4 and CO2 at low temperatures.

Conversion of Carbon Dioxide into a Series of CBxOy- Compounds Mediated by LaB3,4O2- Anions: Synergy of the Electron Transfer and Lewis Pair Mechanisms to Construct B-C Bonds

Converting CO2 into value-added products containing B-C bonds is a great challenge, especially for multiple B-C bonds, which are versatile building blocks for organoborane chemistry. In the condensed phase, the B-C bond is typically formed through transition metal-catalyzed direct borylation of hydrocarbons via C-H bond activation or transition metal-catalyzed insertion of carbenes into B-H bonds. However, excessive amounts of powerful boryl reagents are required, and products containing B-C bonds are complex. Herein, a novel method to construct multiple B-C bonds at room temperature is proposed by the gas-phase reactions of CO2 with LaBmOn- (m = 1-4, n = 1 or 2). Mass spectrometry and density functional theory calculations are applied to investigate these reactions, and a series of new compounds, CB2O2-, CB3O3-, and CB3O2-, which possess B-C bonds, are generated in the reactions of LaB3,4O2- with CO2. When the number of B atoms in the clusters is reduced to 2 or 1, there is only CO-releasing channel, and no CBxOy- compounds are released. Two major factors are responsible for this quite intriguing reactivity: (1) Synergy of electron transfer and boron-boron Lewis acid-base pair mechanisms facilitates the rupture of C═O double bond in CO2. (2) The boron sites in the clusters can efficiently capture the newly formed CO units in the course of reactions, favoring the formation of B-C bonds. This finding may provide fundamental insights into the CO2 transformation driven by clusters containing lanthanide atoms and how to efficiently build B-C bonds under room temperature.

Activation and Transformation of Methane on Boron-Doped Cobalt Oxide Cluster Cations CoBO2. Inorg Chem 2024;63:1537-1542. [PMID: 38181068 DOI: 10.1021/acs.inorgchem.3c03112] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The cleavage of inert C-H bonds in methane at room temperature and the subsequent conversion into value-added products are quite challenging. Herein, the reactivity of boron-doped cobalt oxide cluster cations CoBO2+ toward methane under thermal collision conditions was studied by mass spectrometry experiments and quantum-chemical calculations. In this reaction, one H atom and the CH3 unit of methane were transformed separately to generate the product metaboric acid (HBO2) and one CoCH3+ ion, respectively. Theoretical calculations strongly suggest that a catalytic cycle can be completed by the recovery of CoBO2+ through the reaction of CoCH3+ with sodium perborate (NaBO3), and this reaction generates sodium methoxide (CH3ONa) as the other value-added product. This study shows that boron-doped cobalt oxide species are highly reactive to facilitate thermal methane transformation and may open a way to develop more effective approaches for methane (CH4) activation and conversion under mild conditions.

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.

Ionization Energies and Cationic Bond Dissociation Energies of RuB, RhB, OsB, IrB, and PtB

Two-photon ionization thresholds of RuB, RhB, OsB, IrB, and PtB have been measured using resonant two-photon ionization spectroscopy in a jet-cooled molecular beam and have been used to derive the adiabatic ionization energies of these molecules. From the measured two-photon ionization thresholds, IE(RuB) = 7.879(9) eV, IE(RhB) = 8.234(10) eV, IE(OsB) = 7.955(9) eV, IE(IrB) = 8.301(15) eV, and IE(PtB) = 8.524(10) eV have been assigned. By employing a thermochemical cycle, cationic bond dissociation energies of these molecules have also been derived, giving D0(Ru+-B) = 4.297(9) eV, D0(Rh+-B) = 4.477(10) eV, D0(Os-B+) = 4.721(9) eV, D0(Ir-B+) = 4.925(18) eV, and D0(Pt-B+) = 5.009(10) eV. The electronic structure of the resulting cationic transition metal monoborides (MB+) have been elucidated using quantum chemical calculations. Periodic trends of the MB+ molecules and comparisons to their neutral counterparts are discussed. The possibility of quadruple chemical bonds in all of these cationic transition metal monoborides is also discussed.

Reverse water-gas shift reaction catalyzed by diatomic rhodium anions

The reverse water-gas shift (RWGS, CO₂ + H₂ → CO + H₂O, ΔH₂₉₈ = +0.44 eV) reaction mediated by the diatomic anion Rh₂^- was successfully constructed. The generation of a gas-phase H₂O molecule and ion product [Rh₂(CO)_ads]^- was identified unambiguously at room temperature and the only elementary step that requires extra energy to complete the catalysis is the desorption of CO from [Rh₂(CO)_ads]^-. This experimentally identified Rh₂^- anion represents the first gas-phase species that can drive the RWGS reaction because it is challenging to design effective routes to yield H₂O from CO₂ and H₂. The reactions were performed by using our newly developed double ion trap reactors and characterized by mass spectrometry, photoelectron spectroscopy, and high-level quantum-chemical calculations. We found that the order that the reactants (CO₂ or D₂) were fed into the reactor did not have a pronounced impact on the reactivity and the final product distribution (D₂O and Rh₂CO^-). The atomically precise insights into the key steps to guide the reaction toward the RWGS direction were provided.

Computational Studies of Coinage Metal Anion M- + CH3X (X = F, Cl, Br, I) Reactions in Gas Phase

We characterized the stationary points along the nucleophilic substitution (S_N2), oxidative insertion (OI), halogen abstraction (XA), and proton transfer (PT) product channels of M⁻ + CH₃X (M = Cu, Ag, Au; X = F, Cl, Br, I) reactions using the CCSD(T)/aug-cc-pVTZ level of theory. In general, the reaction energies follow the order of PT > XA > S_N2 > OI. The OI channel that results in oxidative insertion complex [CH₃–M–X]⁻ is most exothermic, and can be formed through a front-side attack of M on the C-X bond via a high transition state OxTS or through a S_N2-mediated halogen rearrangement path via a much lower transition state invTS. The order of OxTS > invTS is inverted when changing M⁻ to Pd, a d¹⁰ metal, because the symmetry of their HOMO orbital is different. The back-side attack S_N2 pathway proceeds via typical Walden-inversion transition state that connects to pre- and post-reaction complexes. For X = Cl/Br/I, the invS_N2-TS’s are, in general, submerged. The shape of this M⁻ + CH₃X S_N2 PES is flatter as compared to that of a main-group base like F⁻ + CH₃X, whose PES has a double-well shape. When X = Br/I, a linear halogen-bonded complex [CH₃−X∙··M]⁻ can be formed as an intermediate upon the front-side attachment of M on the halogen atom X, and it either dissociates to CH₃ + MX⁻ through halogen abstraction or bends the C-X-M angle to continue the back-side S_N2 path. Natural bond orbital analysis shows a polar covalent M−X bond is formed within oxidative insertion complex [CH₃–M–X]⁻, whereas a noncovalent M–X halogen-bond interaction exists for the [CH₃–X∙··M]⁻ complex. This work explores competing channels of the M⁻ + CH₃X reaction in the gas phase and the potential energy surface is useful in understanding the dynamic behavior of the title and analogous reactions.

Conversion of carbon dioxide to a novel molecule NCNBO- mediated by NbBN2- anions at room temperature

The activation of carbon dioxide (CO₂) mediated by NbBN₂^- cluster anions under the conditions of thermal collision has been investigated by time-of-flight mass spectrometry combined with density functional theory calculations. Two CO double bonds in the CO₂ molecule are completely broken and two C-N bonds are further generated to form the novel molecule NCNBO^-. To the best of our knowledge, this new molecule is synthesized and reported for the first time. In addition, one oxygen atom transfer channel produces another product, NbBN₂O^-. Both of the Nb and B atoms in NbBN₂^- donate electrons to reduce CO₂, and the carbon atom originating from CO₂ serves as an electron reservoir. The reaction of NbB^- with N₂ was also investigated theoretically, and the formation of NbBN₂^- from this reaction is thermodynamically and kinetically quite favorable, indicating that NCNBO^- might be produced from the coupling of N₂ and CO₂ mediated by NbB^- anions.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

Dinitrogen and Carbon Dioxide Activation to Form C-N Bonds at Room Temperature: A New Mechanism Revealed by Experimental and Theoretical Studies

In light of the current energy requirements, the conversion of CO₂ and N₂ into useful C-N bond-containing products under mild conditions has become an area of intense research. However, the inert nature of N₂ and CO₂ renders their coupling extremely challenging. Herein, nitrogen and carbon atoms originating from N₂ and CO₂, respectively, are fixed sequentially by NbH₂^- anions in the gas phase at room temperature. Isocyanate and NbO₂CN^- anions were formed under thermal collision conditions, thus achieving the formation of new C-N bonds directly from simple N₂ and CO₂. The anion structures and reaction details were studied by mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. A novel N₂ activation mode (metal-ligand activation, MLA) and a related mechanism for constructing C-N bonds mediated by a single non-noble metal atom are proposed. In this MLA mode, the C atom originating from CO₂ serves as an electron reservoir to accept and donate electrons.

The sequential activation of H2 and N2 mediated by the gas-phase Sc3N+ clusters: Formation of amido unit

The activation and hydrogenation of nitrogen are central in industry and in nature. Through a combination of mass spectrometry and quantum chemical calculations, this work reports an interesting result that scandium nitride cations Sc₃N⁺ can activate sequentially H₂ and N₂, and an amido unit (NH₂) is formed based on density functional theory calculations, which is one of the inevitable intermediates in the N₂ reduction reactions. If the activation step is reversed, i.e., sequential activation of first N₂ and then H₂, the reactivity decreases dramatically. An association mechanism, prevalent in some homogeneous catalysis and enzymatic mechanisms, is adopted in these gas-phase H₂ and N₂ activation reactions mediated by Sc₃N⁺ cations. The mechanistic insights are important to understand the mechanism of the conversion of H₂ and N₂ to NH₃ synthesis under ambient conditions.

Consecutive methane activation mediated by single metal boride cluster anions NbB4. Phys Chem Chem Phys 2021;23:12592-12599. [PMID: 34047332 DOI: 10.1039/d1cp01418h]

Cleavage of all C-H bonds in two methane molecules by gas-phase cluster ions at room temperature is a challenging task. Herein, mass spectrometry and quantum chemical calculations have been used to identify one single metal boride cluster anions NbB4- that can activate eight C-H bonds in two methane molecules and release one H2 molecule each time under thermal collision conditions. In these consecutive reactions, the loaded Nb atoms and the support B4 units play different roles but act synergistically to activate CH4, which is responsible for the interesting reactivity of NbB4-. Moreover, there are some mechanistic differences in these two reactions, including the mechanisms for the first C-H bond activation steps, dihydrogen desorption sites, and major electron donors. This study shows that non-noble metal boride species are reactive enough to facilitate thermal C-H bond cleavages, and boron-based materials may be one kind of potential support material facilitating surface hydrogen transport.

Simultaneous Functionalization of Methane and Carbon Dioxide Mediated by Single Platinum Atomic Anions

Mass spectrometric analysis of the anionic products of interaction among Pt^-, methane, and carbon dioxide shows that the methane activation complex, H₃C-Pt-H^-, reacts with CO₂ to form [H₃C-Pt-H(CO₂)]^-. Two hydrogenation and one C-C bond coupling products are identified as isomers of [H₃C-Pt-H(CO₂)]^- by a synergy between anion photoelectron spectroscopy and quantum chemical calculations. Mechanistic study reveals that both CH₄ and CO₂ are activated by the anionic Pt atom and that the successive depletion of the negative charge on Pt drives the CO₂ insertion into the Pt-H and Pt-C bonds of H₃C-Pt-H^-. This study represents the first example of the simultaneous functionalization of CH₄ and CO₂ mediated by single atomic anions.

C/C Exchange in Activation/Coupling Reaction of Acetylene and Methane Mediated by Os+: A Comparison with Ir+, Pt+, and Au. J Phys Chem Lett 2020;11:8346-8351. [PMID: 32885973 DOI: 10.1021/acs.jpclett.0c02068]

The activation and coupling reactions of methane and acetylene mediated by M⁺ (M = Os, Ir, Pt, and Au) have been comparatively studied at room temperature by the techniques of mass spectrometry in conjunction with theoretical calculations. Studies have shown that Os⁺ and Ir⁺ can mediate the activation/coupling reaction of CH₄ and C₂H₂, while Pt⁺ and Au⁺ cannot, which could be explained by the number of empty valence orbitals in the metal atom. In addition, there are different competition channels for the reaction mediated by Os⁺ and Ir⁺: an expected dehydrogenation and an unexpected C/C exchange. We find that if the rare C/C exchange reaction takes place, there are symmetric carbon atoms in the reaction intermediate and the C/C exchange reaction is favored kinetically. The C/C exchange reaction must be considered, which will affect the yield of the products in the primary reaction. This study shows the molecular-level mechanisms which include the C/C exchange reaction in the activation and coupling reaction of organic compounds mediated by different metals.

Photoassisted Selective Steam and Dry Reforming of Methane to Syngas Catalyzed by Rhodium-Vanadium Bimetallic Oxide Cluster Anions at Room Temperature

Photoassisted steam reforming and dry (CO₂ ) reforming of methane (SRM and DRM) at room temperature with high syngas selectivity have been achieved in the gas-phase catalysis for the first time. The catalysts used are bimetallic rhodium-vanadium oxide cluster anions of Rh₂ VO_1-3 ^- . Both the oxidation of methane and reduction of H₂ O/CO₂ can take place efficiently in the dark while the pivotal step to govern syngas selectivity is photo-excitation of the reaction intermediates Rh₂ VO_2,3 CH₂ ^- to specific electronically excited states that can selectively produce CO and H₂ . Electronic excitation over Rh₂ VO_2,3 CH₂ ^- to control the syngas selectivity is further confirmed from the comparison with the thermal excitation of Rh₂ VO_2,3 CH₂ ^- , which leads to diversity of products. The atomic-level mechanism obtained from the well-controlled cluster reactions provides insight into the process of selective syngas production from the photocatalytic SRM and DRM reactions over supported metal oxide catalysts.

Nb2BN2− cluster anions reduce four carbon dioxide molecules: reactivity enhancement by ligands

The thermal gas-phase reactions of Nb₂BN₂⁻ cluster anions with carbon dioxide have been explored by using the art of time-of-flight mass spectrometry and density functional theory calculations.

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 .

Size-Dependent Association of Cobalt Deuteride Cluster Anions Co3Dn- (n = 0-4) with Dinitrogen

Dinitrogen (N₂) activation by metal hydride species is of fundamental interest and practical importance while the role of hydrogen in N₂ activation is not well studied. Herein, the structures of Co₃D_n^- (n = 0-4) clusters and their reactions with N₂ have been studied by using a combined experimental and computational approach. The mass spectrometry experiments identified that the Co₃D_n^- (n = 2-4) clusters could adsorb N₂ while the Co₃D_n^- (n = 0 and 1) clusters were inert. The photoelectron imaging spectroscopy indicated that the electron detachment energies of Co₃D_2-4^- are smaller than those of Co₃D_0,1^-, which characterized that it is easier to transfer electrons from Co₃D_2-4^- than from Co₃D_0,1^- to activate N₂. The density functional theory calculations generally supported the experimental observations. Further analysis revealed that the H atoms in the Co₃H_n^- (n = 2-4) clusters generally result in higher energies of the Co 3d orbitals in comparison with the Co₃H_n^- (n = 0 and 1) systems. By forming chemical bonds with H atoms, the Co atoms of Co₃H_2-4^- are less negatively charged with respect to the naked Co₃^- system, which leads to higher N₂ binding energies of Co₃H_2-4N₂^- than that of Co₃N₂^-.

Selective Generation of Free Hydrogen Atoms in the Reaction of Methane with Diatomic Gold Boride Cations

The thermal reaction of diatomic gold boride cation AuB+ with methane has been studied by using state-of-the-art mass spectrometry in conjunction with density functional theory calculations. The AuB+ ion can activate a methane molecule to produce exclusively the free hydrogen atom, an important intermediate in hydrocarbon transformation. This result is different from the reactivity of AuC+ and CuB+ counterparts with methane in previous studies. The AuC+ cation mainly transforms methane into ethylene. The CuB+ reaction system principally generates the free hydrogen atoms, but it also gives rise a portion of ethylene-like product H2B−CH2. The B atom of AuB+ is the active site to activate methane. The strong relativistic effect on gold plays an important role for the product selectivity. The mechanistic insights obtained from this study provide guidance for rational design of active sites with high product selectivity toward methane activation.

Mechanistic understanding of catalysis by combining mass spectrometry and computation

The combination of mass spectrometry and computational chemistry has been proven to be powerful for exploring reaction mechanisms. The former provides information of reaction intermediates, while the latter gives detailed reaction energy profiles.