Formation of Acetylene in the Reaction of Methane with Iron Carbide Cluster Anions FeC3- under High-Temperature Conditions

Machine Learning Study of Methane Activation by Gas-Phase Species

The activation and transformation of methane have long posed significant challenges in scientific research. The quest for highly active species and a profound understanding of the mechanisms of methane activation are pivotal for the rational design of related catalysts. In this study, by assembling a data set encompassing a total of 134 gas-phase metal species documented in the literature for methane activation via the mechanism of oxidative addition, machine learning (ML) models based on the backpropagation artificial neural network algorithm have been established with a range of intrinsic electronic properties of these species as features and the experimental rate constants of the reactions with methane as the target variables. It turned out that the satisfactory ML models could be described in terms of four key features, including the vertical electron detachment energy (VDE), the absolute value of the energy gap between the highest occupied molecular orbital of CH4, and the lowest unoccupied molecular orbital of the metal species (|ΔEH'-L|), the maximum natural charge of metal atoms (Qmax), and the maximum electron occupancy of valence s orbitals on metal atoms (ns_max), based on the feature selection complemented with manual intervention. The stability and generalization ability of the constructed model was validated using a specially designed data-splitting strategy and newly incorporated data. This study proved the feasibility and discussed the limitations of the ML model, which is described by four key features to predict the reactivity of metal-containing species toward methane through oxidative addition mechanisms. Furthermore, a careful preparation of the training data set that covers the full expected range of target and feature values aiming to achieve good predictive accuracy is suggested as a practical guideline for future research.

Production of Acetylene from Viable Feedstock: Promising Recent Approaches

The potential of acetylene is extremely high both in chemical industry and synthetic applications due to unsaturated nature and the smallest active C≡C unit. The production of many essential necessities is originated from acetylene; however, the formation of acetylene molecule requires a lot of energy. Currently, the access to acetylene is based on coal processing, methane reforming and calcium carbide hydrolysis. Recently, extensive research has been done to decrease the cost of acetylene. In this review, the routes to acetylene were highlighted, considering the energy consumption in kW ⋅ h/t of the product to evaluate the best approach. Since energy prices depend on various regions, the cost of the product is complicated. The manufacturing of acetylene is usually accompanied by formation of by-products, which may be valuable or not. The review should help to identify current status and not overlook promising approaches.

Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Conversion of Methane at Room Temperature Mediated by the Ta-Ta σ-Bond

Metal-metal bonds constitute an important type of reactive centers for chemical transformation; however, the availability of active metal-metal bonds being capable of converting methane under mild conditions, the holy grail in catalysis, remains a serious challenge. Herein, benefiting from the systematic investigation of 36 metal clusters of tantalum by using mass spectrometric experiments complemented with quantum chemical calculations, the dehydrogenation of methane at room temperature was successfully achieved by 18 cluster species featuring σ-bonding electrons localized in single naked Ta-Ta centers. In sharp contrast, the other 18 remaining clusters, either without naked Ta-Ta σ-bond or with σ-bonding electrons delocalized over multiple Ta-Ta centers only exhibit molecular CH4-adsorption reactivity or inertness. Mechanistic studies revealed that changing cluster geometric configurations and tuning the number of simple inorganic ligands (e.g., oxygen) could flexibly manipulate the presence or absence of such a reactive Ta-Ta σ-bond. The discovery of Ta-Ta σ-type bond being able to exhibit outstanding activity toward methane conversion not only overturns the traditional recognition that only the metal-metal π- or δ-bonds of early transition metals could participate in bond activation but also opens up a new access to design of promising metal catalysts with dual-atom as reactive sites for chemical transformations.

Ligand effect on Ru-centered species toward methane activation

Ligands have been known to profoundly affect the chemical transformations of methane, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we demonstrate that the conversion of methane can be regulated by Ru centered cations with a series of ligands (C, CH, CNH, CHCNH). Gas-phase experiments complemented by theoretical dynamic analysis were performed to explore the essences and principles governing the ligand effect. In contrast to the inert Ru+, [RuC]+, and [RuCNH]+ toward CH4, the dehydrogenation dominates the reaction of ligand-regulated systems [RuCH]+/CH4 and [RuCHCNH]+/CH4. In active cases, CH acts as active sites, and regulates the activation of CH4 assisted by the "seemingly inert" CNH ligand.

Methane Activation by [OsC3]+: Implications for Catalyst Design

Gas-phase reactions of [OsC₃]⁺ with methane at ambient temperature have been studied by using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. The comparison of [OsC₃]⁺ with the product clusters revealed significant changes in cluster reactivity. In particular, with different ligands, the cluster may produce multiple products or, alternatively, just a single product. Theoretical calculations reveal the influence of electronic features such as molecular polarity index, charge and spin distribution, and HOMO-LUMO gap on the reactivity of the Os complexes. Fundamentally, it is the polarity of the clusters that leads to the cluster reactivity in the methane activation. Furthermore, reducing the local polarity of the catalyst active site may be one means of reducing the number of byproducts in the reaction.

High-temperature reactivity of vanadium oxide clusters in methane activation: Vibrational degrees of freedom matter

Understanding the properties of small particles working under high-temperature conditions at the atomistic scale is imperative for exact control of related processes, but it is quite challenging to achieve experimentally. Herein, benefitting from state-of-the-art mass spectrometry and by using our newly designed high-temperature reactor, the activity of atomically precise particles of negatively charged vanadium oxide clusters toward hydrogen atom abstraction (HAA) from methane, the most stable alkane molecule, has been measured at elevated temperatures up to 873 K. We discovered the positive correlation between the reaction rate and cluster size that larger clusters possessing greater vibrational degrees of freedom can carry more vibrational energies to enhance the HAA reactivity at high temperature, in contrast with the electronic and geometric issues that control the activity at room temperature. This finding opens up a new dimension, vibrational degrees of freedom, for the simulation or design of particle reactions under high-temperature conditions.

Room-Temperature Conversion of Methane to Methanediol by [FeO2]. J Phys Chem Lett 2023;14:1633-1640. [PMID: 36752636 DOI: 10.1021/acs.jpclett.2c03786] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Inspired by the activities of P-450 enzyme and Rieske oxygenases in nature, in which the high-valent Fe-oxo complexes play a key role for oxidation of alkanes, the oxidation process of methane by the high-valent iron oxide cation [FeO₂]⁺ has been explored by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. In contrast to the previously reported [FeO]⁺/CH₄ and [Fe(O)OH]⁺/CH₄ systems, which afford [FeOH]⁺ as the main product, the generation of Fe⁺ dominates the reaction of [FeO₂]⁺ with CH₄. Theoretical calculations suggest a novel "oxygen rebound" pathway for the liberation of methanediol. In particular, the inevitable valence increase of Fe prior to C-H activation is similar to the cytochrome P-450 mediated processes. To our best knowledge, this study provides the first example of methane activation by the high-valent Fe(V)-oxo species in the gas phase, which may thus bridge the gas-phase model and the condensed-phase biosystems.

Pyrolysis of Mass-Selected (V2O5)NO− (N = 1−6) Clusters in a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap that can stably run up to 873 K was newly designed and installed into a homemade reflectron time-of-flight mass spectrometer coupled with a laser ablation cluster source and a quadrupole mass filter. The instrument was used to study the pyrolysis behavior of mass-selected (V2O5) NO− ( N = 1−6) cluster anions and the dissociation channels were clarified with atomistic precision. Similar to the dissociation behavior of the heated metal oxide cluster cations reported in literature, the desorption of either atomic oxygen atom or molecular O2 prevailed for the (V2O5) NO− clusters with N = 2−5 at 873 K. However, novel dissociation channels involving fragmentation of (V2O5) NO− to small-sized V xO y− anions concurrent with the release of neutral vanadium oxide species were identified for the clusters with N = 3−6. Significant variations of branching ratios for different dissociation channels were observed as a function of cluster size. The kinetic studies indicate that the dissociation rates of (V2O5) NO− monotonically increased with the increase of cluster size. The internal energies carried by the (V2O5) NO− clusters at 873 K as well as the energetics data for dissociation channels have been theoretically calculated to rationalize the experimental observations. The decomposition behavior of vanadium oxide clusters from this study can provide new insights into the pyrolysis mechanism of metal oxide nanoparticles that are widely used in high temperature catalysis.

Role of Magnetization on Catalytic Pathways of Non-Oxidative Methane Activation on Neutral Iron Carbide Clusters

Methane has emerged as a promising fuel due to its abundance and clean combustion properties. It is also a raw material for various value added chemicals. However, the conversion of...

Quadruple C-H Bond Activations of Methane by Dinuclear Rhodium Carbide Cation [Rh2C3]. JACS AU 2021;1:1631-1638. [PMID: 34723266 PMCID: PMC8549038 DOI: 10.1021/jacsau.1c00265]

The structure of the [Rh₂C₃]⁺ ion and its reaction with CH₄ in the gas phase have been studied by infrared photodissociation spectroscopy and mass spectrometry in conjunction with quantum chemical calculations. The [Rh₂C₃]⁺ ion is characterized to have an unsymmetrical linear [Rh-C-C-C-Rh]⁺ structure existing in two nearly isoenergetic spin states. The [Rh₂C₃]⁺ ion reacts with CH₄ at room temperature to form [Rh₂C]⁺ + C₃H₄ and [Rh₂C₂H₂]⁺ + C₂H₂ as the major products. In addition to the [Rh₂C]⁺ ion, the [Rh₂ ¹³C]⁺ ion is formed at about one-half of the [Rh₂C]⁺ intensity when the isotopic-labeled ¹³CH₄ sample is used. The production of [Rh₂ ¹³C]⁺ indicates that the linear C₃ moiety of [Rh₂C₃]⁺ can be replaced by the bare carbon atom of methane with all four C-H bonds being activated. The calculations suggest that the overall reactions are thermodynamically exothermic, and that the two Rh centers are the reactive sites for C-H bond activation and hydrogen atom transfer reactions.

Subsurface-Carbon-Induced Local Charge of Copper for an On-Surface Displacement Reaction

Transition-metal carbides have sparked unprecedented enthusiasm as high-performance catalysts in recent years. Still, the catalytic properties of copper carbide remain unexplored. By introducing subsurface carbon to Cu(111), a displacement reaction of a proton in a carboxyl acid group with a single Cu atom is demonstrated at the atomic scale and room temperature. Its occurrence is attributed to the C-doping-induced local charge of surface Cu atoms (up to +0.30 e/atom), which accelerates the rate of on-surface deprotonation via reduction of the corresponding energy barrier, thus enabling the instant displacement of a proton with a Cu atom when the molecules adsorb on the surface. This well-defined and robust Cu^δ+ surface based on subsurface-carbon doping offers a novel catalytic platform for on-surface synthesis.

Recent advances in heterogeneous catalysis for the nonoxidative conversion of methane

The direct conversion of methane to high-value chemicals is an attractive process that efficiently uses abundant natural/shale gas to provide an energy supply. The direct conversion of methane to high-value chemicals is an attractive process that efficiently uses abundant natural/shale gas to provide an energy supply. Among all the routes used for methane transformation, nonoxidative conversion of methane is noteworthy owing to its highly economic selectivity to bulk chemicals such as aromatics and olefins. Innovations in catalysts for selective C-H activation and controllable C-C coupling thus play a key role in this process and have been intensively investigated in recent years. In this review, we briefly summarize the recent advances in conventional metal/zeolite catalysts in the nonoxidative coupling of methane to aromatics, as well as the newly emerging single-atom based catalysts for the conversion of methane to olefins. The emphasis is primarily the experimental findings and the theoretical understanding of the active sites and reaction mechanisms. We also present our perspectives on the design of catalysts for C-H activation and C-C coupling of methane, to shed some light on improving the potential industrial applications of the nonoxidative conversion of methane into chemicals.

The Reactive Sites of Methane Activation: A Comparison of IrC3+ with PtC3. Molecules 2021;26:molecules26196028. [PMID: 34641573 PMCID: PMC8512126 DOI: 10.3390/molecules26196028]

The activation reactions of methane mediated by metal carbide ions MC₃⁺ (M = Ir and Pt) were comparatively studied at room temperature using the techniques of mass spectrometry in conjunction with theoretical calculations. MC₃⁺ (M = Ir and Pt) ions reacted with CH₄ at room temperature forming MC₂H₂⁺/C₂H₂ and MC₄H₂⁺/H₂ as the major products for both systems. Besides that, PtC₃⁺ could abstract a hydrogen atom from CH₄ to generate PtC₃H⁺/CH₃, while IrC₃⁺ could not. Quantum chemical calculations showed that the MC₃⁺ (M = Ir and Pt) ions have a linear M-C-C-C structure. The first C-H activation took place on the Ir atom for IrC₃⁺. The terminal carbon atom was the reactive site for the first C-H bond activation of PtC₃⁺, which was beneficial to generate PtC₃H⁺/CH₃. The orbitals of the different metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. This study investigates the molecular-level mechanisms of the reactive sites of methane activation.

Catalytic Co-Conversion of CH4 and CO2 Mediated by Rhodium-Titanium Oxide Anions RhTiO2. Angew Chem Int Ed Engl 2021;60:13788-13792. [PMID: 33890352 PMCID: PMC8251526 DOI: 10.1002/anie.202103808]

Catalytic co‐conversion of methane with carbon dioxide to produce syngas (2 H₂+2 CO) involves complicated elementary steps and almost all the elementary reactions are performed at the same high temperature conditions in practical thermocatalysis. Here, we demonstrate by mass spectrometric experiments that RhTiO₂⁻ promotes the co‐conversion of CH₄ and CO₂ to free 2 H₂+CO and an adsorbed CO (CO_ads) at room temperature; the only elementary step that requires the input of external energy is desorption of CO_ads from the RhTiO₂CO⁻ to reform RhTiO₂⁻. This study not only identifies a promising active species for dry (CO₂) reforming of methane to syngas, but also emphasizes the importance of temperature control over elementary steps in practical catalysis, which may significantly alleviate the carbon deposition originating from the pyrolysis of methane.

Activation of Carbon Dioxide by CoCDn- (n = 0-4) Anions

Laser ablation generated CoCD_n^- (n = 0-4) anions were mass selected and then reacted with CO₂ in an ion trap reactor. The reactions were characterized by mass spectrometry and quantum chemical calculations. The experimental results demonstrated that the CoC^- anion can convert CO₂ into CO. In contrast, the bare Co^- anion is inert toward CO₂. Coordinated D ligands can modify the reactivity of CoCD_1-4^- in which CoCD_1-3^- can reduce CO₂ into CO selectively and CoCD₄^- can only adsorb CO₂. The crucial roles of the coordinated C and D ligands to tune the reactivity of CoCD_n^- (n = 0-4) toward CO₂ were rationalized by theoretical calculations. Note that the hydrogenation process that is usually observed in the reactions of gas-phase metal hydrides with CO₂ is completely suppressed for the reactions CoCD_n^- + CO₂. This study provides insights into the molecular-level origin for the observations that CO can be selectively generated from CO₂ catalyzed by cobalt-containing carbides in heterogeneous catalysis.

Unraveling the activity of iron carbide clusters embedded in silica for thermocatalytic conversion of methane

We report the detailed mechanism of direct nonoxidative CH4 conversion on iron carbide clusters embedded in silica, revealing that the FeC3 sites generated in situ from FeC2 are mainly responsible for CH4 conversion to CH3 and H2.

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.

The reactivity of Nbn+ clusters with acetylene and ethylene to produce a cubic aromatic metal carbide Nb4C4+

The reactions of niobium cationic clusters with acetylene and ethylene under sufficient gas collision conditions give rise to dominant dehydrogenation and produce a main metal carbide Nb4C4+ which is associated with cubic aromaticity.

Structures and bonding properties of CPt2 -/0 and CPt2H-/0: Anion photoelectron spectroscopy and quantum chemical calculations

We present a combined anion photoelectron spectroscopic and quantum chemical investigation on the structures and bonding properties of CPt₂ ^-/0 and CPt₂H^-/0. The experimental vertical detachment energies of CPt₂ ^- and CPt₂H^- are measured to be 1.91 ± 0.08 and 3.54 ± 0.08 eV, respectively. CPt₂ ^- is identified as a C_2v symmetric Pt-C-Pt bent structure, and CPt₂ has a D_∞h symmetric Pt-C-Pt linear structure. Both anionic and neutral CPt₂H adopt a Pt-C-Pt-H chain-shaped structure, in which the ∠PtCPt and ∠CPtH bond angles of CPt₂H^- are larger than those of CPt₂H. The Pt-C bonds in CPt₂ ^-/0 and CPt₂H^-/0 exhibit covalent double bonding characters. The Pt=C bonds are much stronger than the C-H bond that may explain why the C atom CPt₂H^-/0 prefers to form Pt=C bonds rather than C-H bonds. It may also explain why platinum can insert into the C-H bond to activate the C-H bond as reported in the literature.

Geometric and Electronic Structures of VB40/+ Clusters and Reactivity of the Cationic Cluster with Methane from Quantum Chemical Calculations

Quantum chemical methods have been employed to study the geometric and electronic structures of VB₄^0/+ clusters and the mechanism of the reaction of the cationic clusters with methane. It was found that the ground states of the neutral and cationic clusters were ⁴A' and ³A' of a planar isomer in C_s symmetry in which vanadium atom side-on binds to the rhombic B₄ moiety. The ionization energy of the neutral cluster was calculated to be 7.13 eV at the CCSD(T) level. The reaction pathways on the triplet and quintet potential energy profiles of the dehydrogenation and elimination of V⁺ in the reaction of VB₄⁺ cluster with methane were established based on the BPW91 functional calculations. Both of the dehydrogenation and elimination of V⁺ in the reaction of VB₄⁺ cluster with methane were initiated by the B₄ moiety of the VB₄⁺ cluster, and these two reaction channels were thermodynamically and kinetically favorable. The dehydrogenation and elimination of V⁺ in the reaction of VB₄⁺ cluster with methane were exothermic processes.

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.

Methane Activation by Gas Phase Atomic Clusters

The increasing supply of natural gas has created a strong demand for developing efficient catalytic processes to upgrade methane, the most stable alkane molecule, into value-added chemicals. Currently, methane conversion in laboratory and industry is mostly performed under high-temperature conditions. A lot of effort has been devoted to exploring chemical entities that are able to activate the C-H bond of methane at lower temperatures, preferably room temperature. Gas phase atomic clusters with limited numbers of atoms are ideal models of active sites on heterogeneous catalysts. The cluster systems are being actively studied to activate methane under room-temperature conditions. State-of-the-art mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations have been combined in our laboratory to reveal the molecular-level mechanisms of methane activation by atomic clusters. In this Account, we summarize our recent progress on thermal methane activation by metal oxide clusters doped with noble-metal atoms (Au, Pt, and Rh) as well as by oxygen-free species including carbides and borides of base metals (V, Ta, Mo, and Fe). In contrast to the generations of CH₃^• free radicals in many of the previously reported cluster reactions with methane, the generations of stable products such as formaldehyde, acetylene, and syngas as well as closed-shell species AuCH₃ and B₃CH₃ have been identified for the cluster reaction systems herein. Besides the well recognized mechanisms of methane activation by the O^-• radicals through hydrogen atom abstraction and by metal atoms through oxidative addition, the new mechanisms of synergistic methane activation by Lewis acid-base pairs (such as Au^δ+-O^δ- and B^δ+-B^δ-) and by dinuclear metal centers (such as Ta-Ta) have been recently revealed. In the reactions between methane and oxide clusters doped with noble-metal atoms, the oxide cluster "supports" can accept the H atoms and the CH _x species delivered through the noble-metal atoms and then transform methane into stable oxygenated compounds. The product selectivity (such as formaldehyde versus syngas) can be controlled by different noble-metal atoms (such as Pt versus Rh). The electronic structures of base metal centers can be engineered through carburization so that the low-spin states can be accessible to reduce the C-H bond of methane. Such active base metal centers in low-spin states resemble related noble-metal atoms in methane activation. The boron clusters (such as B₃ in VB₃⁺) can be polarized by the metal cations to form the Lewis acid-base pair B^δ+-B^δ- to cleave the C-H bond of methane very easily. These molecular-level mechanisms may well be operative in related heterogeneous catalysis and can be a fundamental basis to design efficient catalysts for activation and conversion of methane under mild conditions.

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.