The active site of low-temperature methane hydroxylation in iron-containing zeolites

Machine Learning a Simple Interpretable Short-Range Potential for Silica

A wide array of models, spanning from computationally expensive ab initio methods to a spectrum of force-field approaches, have been developed and employed to probe silica polymorphs and understand growth processes and atomic-level dynamical transitions in silica. However, the quest for a model capable of making accurate predictions with high computational efficiency for various silica polymorphs is still ongoing. Recent developments in short-range machine-learned models, such as GAP and NNPScan, have shown promise in providing reasonable descriptions of silica, but their computational cost remains high compared to force fields such as BKS which are based on simple interpretable functional forms. Here, we build on the recent success of our reinforcement learning (RL) workflow to derive a new set of optimal parameters for a promising short-range BKS-based model proposed by Soules. We use RL to navigate the eight-dimensional parameter space of the Soules potential using an experimental training data set that includes both local and global structural features from approximately 21 experimentally realized silica polymorphs, including high density phases and porous zeolites. We compare the performance of our machine-learned ML-Soules model with other high quality models including our recent machine-learned parametrization of BKS (ML-BKS), a machine-learned potential (GAP), as well as predictions of ab initio calculations with the highly fidelity SCAN functional. The ML-Soules accurately captures the relative energetic ordering of various polymorphs as well as their structural features at a significantly reduced computational expense. The ML-Soules model also reasonably captures the structure, density, and elastic constants of quartz, as well as metastable silica polymorphs. We further discuss the limitations of the Soules functional form and propose potential enhancements, including the incorporation of additional three-body terms and/or the utilization of different short-ranged functional forms to achieve greater accuracy for both global and local features in the modeling of silica while retaining low computational cost.

Advances and challenges in designing active site environments in zeolites for Brønsted acid catalysis

Zeolites contain proton active sites in diverse void environments that stabilize the reactive intermediates and transition states formed in converting hydrocarbons and oxygenates to chemicals and energy carriers. The catalytic diversity that exists among active sites in voids of varying sizes and shapes, even within a given zeolite topology, has motivated research efforts to position and quantify active sites within distinct voids (synthesis-structure) and to link active site environment to catalytic behavior (structure-reactivity). This Feature Article describes advances and challenges in controlling the position of framework Al centers and associated protons within distinct voids during zeolite synthesis or post-synthetic modification, in identifying and quantifying distinct active site environments using characterization techniques, and in determining the influence of active site environments on catalysis. During zeolite synthesis, organic structure directing agents (SDAs) influence Al substitution at distinct lattice positions via intermolecular interactions (e.g., electrostatics, hydrogen bonding) that depend on the size, structure, and charge distribution of organic SDAs and their mobility when confined within zeolitic voids. Complementary post-synthetic strategies to alter intrapore active site distributions include the selective removal of protons by differently-sized titrants or unreactive organic residues and the selective exchange of framework heteroatoms of different reactivities, but remain limited to certain zeolite frameworks. The ability to identify and quantify active sites within distinct intrapore environments depends on the resolution with which a given characterization technique can distinguish Al T-site positions or proton environments in a given zeolite framework. For proton sites in external unconfined environments, various (post-)synthetic strategies exist to control their amounts, with quantitative methods to distinguish them from internal sites that largely depend on using stoichiometric or catalytic probes that only interact with external sites. Protons in different environments influence reactivity by preferentially stabilizing larger transition states over smaller precursor states and influence selectivity by preferentially stabilizing or destabilizing competing transition states of varying sizes that share a common precursor state. We highlight opportunities to address challenges encountered in the design of active site environments in zeolites by closely integrating precise (post-)synthetic methods, validated characterization techniques, well-defined kinetic probes, and properly calibrated theoretical models. Further advances in understanding the molecular details that underlie synthesis-structure-reactivity relationships for active site environments in zeolite catalysis can accelerate the predictive design of tailored zeolites for desired catalytic transformations.

Advancing Heterogeneous Organic Synthesis With Coordination Chemistry-Empowered Single-Atom Catalysts

For traditional metal complexes, intricate chemistry is required to acquire appropriate ligands for controlling the electron and steric hindrance of metal active centers. Comparatively, the preparation of single-atom catalysts is much easier with more straightforward and effective accesses for the arrangement and control of metal active centers. The presence of coordination atoms or neighboring functional atoms on the supports' surface ensures the stability of metal single-atoms and their interactions with individual metal atoms substantially regulate the performance of metal active centers. Therefore, the collaborative interaction between metal and the surrounding coordination environment enhances the initiation of reaction substrates and the formation and transformation of crucial intermediate compounds, which imparts single-atom catalysts with significant catalytic efficacy, rendering them a valuable framework for investigating the correlation between structure and activity, as well as the reaction mechanism of catalysts in organic reactions. Herein, comprehensive overviews of the coordination interaction for both homogeneous metal complexes and single-atom catalysts in organic reactions are provided. Additionally, reflective conjectures about the advancement of single-atom catalysts in organic synthesis are also proposed to present as a reference for later development.

Contact-Electro-Catalysis for Direct Oxidation of Methane under Ambient Conditions

The conversion of methane under ambient conditions has attracted significant attention. Although advancements have been made using active oxygen species from photo- and electro- chemical processes, challenges such as complex catalyst design, costly oxidants, and unwanted byproducts remain. This study exploits the concept of contact-electro-catalysis, initiating chemical reactions through charge exchange at a solid-liquid interface, to report a novel process for directly converting methane under ambient conditions. Utilizing the electrification of commercially available Fluorinated Ethylene Propylene (FEP) with water under ultrasound, we demonstrate how this interaction promote the activation of methane and oxygen molecules. Our results show that the yield of HCHO and CH3OH can reach 467.5 and 151.2 μmol ⋅ gcat -1, respectively. We utilized electron paramagnetic resonance (EPR) to confirm the evolution of hydroxyl radicals (⋅OH) and superoxide radicals (⋅OOH). Isotope mass spectrometry (MS) was employed to analyze the elemental origin of CH3OH, which can be further oxidized to HCHO. Additionally, we conducted density functional theory (DFT) simulations to assess the reaction energies of FEP with H2O, O2, and CH4 under these conditions. The implications of this methodology, with its potential applicability to a wider array of gas-phase catalytic reactions, underscore a significant advance in catalysis.

Nature and Redox Properties of Iron Sites in Zeolites Revealed by Mössbauer Spectroscopy

Iron-containing zeolite-based catalysts play a pivotal role in environmental processes aimed at mitigating the release of harmful greenhouse gases, such as nitrous oxide (N2O) and methane (CH4). Despite the rich iron chemistry in zeolites, only a fraction of iron species that exhibit an open coordination sphere and possess the ability for electron transfer are responsible for activating reagents. In addition, the splitting of molecular oxygen is facilitated by bare iron cations embedded in zeolitic matrices. Mössbauer spectroscopy is the ideal tool for investigating the valency and geometry of iron species in zeolites because it leaves no iron forms silent and provides insights into in-situ processes. This review is dedicated to the utilization of Mössbauer spectroscopy to elucidate the nature of the extra-framework iron centers in ferrierite (FER), beta-structured (*BEA), and ZSM-5 zeolite (MFI) zeolites, which are active in N2O decomposition and CH4 oxidation through using the active oxygen derived from N2O and O2. In this work, a structured summary of the Mössbauer parameters established over the last two decades is presented, characterizing the specific iron active centers and intermediates formed upon iron's interaction with N2O/O2 and CH4. Additionally, the impact of preparation methods, iron loading, and the long-term stability on iron speciation and its redox behavior under reaction conditions is discussed.

Metal-Oxo Electronic Tuning via In Situ CO Decoration for Promoting Methane Conversion to Oxygenates over Single-Atom Catalysts

Direct methane conversion (DMC) to oxygenates at low temperature is of great value but remains challenging due to the high energy barrier for C-H bond activation. Here, we report that in situ decoration of Pd1-ZSM-5 single atom catalyst (SAC) by CO molecules significantly promoted the DMC reaction, giving the highest turnover frequency of 207 h-1 ever reported at room temperature and ~100 % oxygenates selectivity with H2O2 as oxidant. Combined characterizations and DFT calculations illustrate that the C-atom of CO prefers to coordinate with Pd1, which donates electrons to the Pd1-O active center (L-Pd1-O, L=CO) generated by H2O2 oxidation. The correspondingly improved electron density over Pd-O pair renders a favorable heterolytic dissociation of C-H bond with low energy barrier of 0.48 eV. Applying CO decoration strategy to M1-ZSM-5 (M=Pd, Rh, Ru, Fe) enables improvement of oxygenates productivity by 3.2-11.3 times, highlighting the generalizability of this method in tuning metal-oxo electronic structure of SACs for efficient DMC process.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Unraveling Ros Conversion Through Enhanced Enzyme-Like Activity with Copper-Doped Cerium Oxide for Tumor Nanocatalytic Therapy

Nanozyme catalytic therapy for cancer treatments has become one of the heated topics, and the therapeutic efficacy is highly correlated with their catalytic efficiency. In this work, three copper-doped CeO2 supports with various structures as well as crystal facets are developed to realize dual enzyme-mimic catalytic activities, that is superoxide dismutase (SOD) to reduce superoxide radicals to H2 O2 and peroxidase (POD) to transform H2 O2 to ∙OH. The wire-shaped CeO2 /Cu-W has the richest surface oxygen vacancies, and a low level of oxygen vacancy (Vo) formation energy, which allows for the elimination of intracellular reactive oxygen spieces (ROS) and continuous transformation to ∙OH with cascade reaction. Moreover, the wire-shaped CeO2 /Cu-W displays the highest toxic ∙OH production capacity in an acidic intracellular environment, inducing breast cancer cell death and pro-apoptotic autophagy. Therefore, wire-shaped CeO2 /Cu nanoparticles as an artificial enzyme system can have great potential in the intervention of intracellular ROS in cancer cells, achieving efficacious nanocatalytic therapy.

Two Allogons of an O2 -activating Bis(disiloxido)ferrate(II) Accessible Selectively just by Variation of the Crystallization Temperature

Deprotonation of O(iPr2 SiOH)2 (iPr LH2 ) with LiOtBu followed by reaction with FeCl2 in THF led to the complex [iPr L2 Fe][Li(THF)2 ]2 , 2, which represents a structural and spectroscopic model of the α-Fe sites of Fe/ZSM-5. Reaction with O2 in THF solution proceeds rather fast and is complete within 200 ms; an intermediate O2 adduct could not be identified by stopped-flow methods. Cooling blue solutions of 2 to -80 °C led to the growth of blue crystals of 2⋅THF, the analysis of which by XRD revealed a FeO4 core that is somewhat distorted from planarity towards a tetrahedral structure. By contrast, cooling such solutions to -30 °C led to pink crystals of an allogon featuring a perfectly square planar FeO4 entity. Hence, 2 represents a unique case where two different structural isomers (allogons) can be crystallized from the same solvent selectively, controlled by the temperature. DFT calculations were performed to understand this finding.

Role of H2O in Catalytic Conversion of C1 Molecules

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

Manipulating the Spin State of Co Sites in Metal-Organic Frameworks for Boosting CO2 Photoreduction

Photocatalytic CO2 reduction holds great potential for alleviating global energy and environmental issues, where the electronic structure of the catalytic center plays a crucial role. However, the spin state, a key descriptor of electronic properties, is largely overlooked. Herein, we present a simple strategy to regulate the spin states of catalytic Co centers by changing their coordination environment by exchanging the Co species into a stable Zn-based metal-organic framework (MOF) to afford Co-OAc, Co-Br, and Co-CN for CO2 photoreduction. Experimental and DFT calculation results suggest that the distinct spin states of the Co sites give rise to different charge separation abilities and energy barriers for CO2 adsorption/activation in photocatalysis. Consequently, the optimized Co-OAc with the highest spin-state Co sites presents an excellent photocatalytic CO2 activity of 2325.7 μmol·g-1·h-1 and selectivity of 99.1% to CO, which are among the best in all reported MOF photocatalysts, in the absence of a noble metal and additional photosensitizer. This work underlines the potential of MOFs as an ideal platform for spin-state manipulation toward improved photocatalysis.

Spectroscopic and Theoretical Investigation of High-Spin Square-Planar and Trigonal Fe(II) Complexes Supported by Fluorinated Alkoxides

The electronic structures and spectroscopic behavior of three high-spin FeII complexes of fluorinated alkoxides were studied: square-planar {K(DME)2}2[Fe(pinF)2] (S) and quasi square-planar {K(C222)}2[Fe(pinF)2] (S') and trigonal-planar {K(18C6)}[Fe(OC4F9)3] (T) where pinF = perfluoropinacolate and OC4F9 = tris-perfluoro-t-butoxide. The zero-field splitting (ZFS) and hyperfine structure parameters of the S = 2 ground states were determined using field-dependent 57Fe Mössbauer and high-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopies. The spin Hamiltonian parameters were analyzed with crystal field theory and corroborated by density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations. Whereas the ZFS tensor of S has a small rhombicity, E/D = 0.082, and a positive D = 15.17 cm-1, T exhibits a negative D = -9.16 cm-1 and a large rhombicity, E/D = 0.246. Computational investigation of the structural factors suggests that the ground-state electronic configuration and geometry of T's Fe site are determined by the interaction of [Fe(OC4F9)3]- with {K(18C6)}+. In contrast, two distinct countercations of S/S' have a negligible influence on their [Fe(pinF)2]2- moieties. Instead, the distortions in S' are likely induced by the chelate ring conformation change from δλ, observed for S, to the δδ conformation, determined for S'.

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

A key challenge for metal-exchanged zeolites is the determination of metal cation speciation and nuclearity under synthesis and reaction conditions. Copper-exchanged zeolites, which are widely used in automotive emissions control and potential catalysts for partial methane oxidation, have in particular evidenced a wide variety of Cu structures that are observed to change with exposure conditions, zeolite composition, and topology. Here, we develop predictive models for Cu cation speciation and nuclearity in CHA, MOR, BEA, AFX, and FER zeolite topologies using interatomic potentials, quantum chemical calculations, and Monte Carlo simulations to interrogate this vast configurational and compositional space. Model predictions are used to rationalize experimentally observed differences between Cu-zeolites in a wide-body of literature, including nuclearity populations, structural variations, and methanol per Cu yields. Our results show that both topological features and commonly observed Al-siting biases in MOR zeolites increase the population of binuclear Cu sites, explaining the small population of mononuclear Cu sites observed in these materials relative to other zeolites such as CHA and BEA. Finally, we used a machine learning classification model to determine the preference to form mononuclear or binuclear Cu sites at different Al configurations in 200 zeolites in the international zeolite database. Model results reveal several zeolite topologies at extreme ends of the mononuclear vs binuclear spectrum, highlighting synthetic options for realization of zeolites with strong Cu nuclearity preferences.

Atomic-level design of metalloenzyme-like active pockets in metal-organic frameworks for bioinspired catalysis

Natural metalloenzymes with astonishing reaction activity and specificity underpin essential life transformations. Nevertheless, enzymes only operate under mild conditions to keep sophisticated structures active, limiting their potential applications. Artificial metalloenzymes that recapitulate the catalytic activity of enzymes can not only circumvent the enzymatic fragility but also bring versatile functions into practice. Among them, metal-organic frameworks (MOFs) featuring diverse and site-isolated metal sites and supramolecular structures have emerged as promising candidates for metalloenzymes to move toward unparalleled properties and behaviour of enzymes. In this review, we systematically summarize the significant advances in MOF-based metalloenzyme mimics with a special emphasis on active pocket engineering at the atomic level, including primary catalytic sites and secondary coordination spheres. Then, the deep understanding of catalytic mechanisms and their advanced applications are discussed. Finally, a perspective on this emerging frontier research is provided to advance bioinspired catalysis.

Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kβ x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Fundamental Limitation in Electrochemical Methane Oxidation to Alcohol: A Review and Theoretical Perspective on Overcoming It

The direct conversion of gaseous methane to energy-dense liquid derivatives such as methanol and ethanol is of profound importance for the more efficient utilization of natural gas. However, the thermo-catalytic partial oxidation of this simple alkane has been a significant challenge due to the high C-H bond energy. Exploiting electrocatalysis for methane activation via active oxygen species generated on the catalyst surface through electrochemical water oxidation is generally considered as economically viable and environmentally benign compared to energy-intensive thermo-catalysis. Despite recent progress in electrochemical methane oxidation to alcohol, the competing oxygen evolution reaction (OER) still impedes achieving high faradaic efficiency and product selectivity. In this review, an overview of current progress in electrochemical methane oxidation, focusing on mechanistic insights on methane activation, catalyst design principles based on descriptors, and the effect of reaction conditions on catalytic performance are provided. Mechanistic requirements for high methanol selectivity, and limitations of using water as the oxidant are discussed, and present the perspective on how to overcome these limitations by employing carbonate ions as the oxidant.

Methane dehydroaromatization catalyzed by Mo/ZSM-5: location-steered activity and mechanism

This work examined the location-steered catalytic behavior of Mo/ZSM-5 catalyst for one-step methane dehydroaromatization to benzene reaction. The results indicated that α-site is the preferred location for the formation of ethylene, the main intermediate for aromatics production via the propagation pathway, while δ-site is favorable for the hydrocarbon pool aggregation reaction pathway.

Approaching High-Performance TS-1 Zeolites in the Presence of Alkali Metal Ions via Combination of Adjusting pH Value and Modulating Crystal Size

Lewis acid zeolites play an important role in industrially important green reactions closely related to fine chemical and biomass conversion. Titanium-doped TS-1 zeolite is a milestone Lewis acid zeolite widely used in industrially significant green oxidation processes with hydrogen peroxide as an oxidant under mild conditions. TS-1 zeolites are normally synthesized in basic conditions under hydrothermal treatment. Up to now, there has still been no success in synthesizing active TS-1 Lewis acid zeolites by using inorganic alkali, e.g., NaOH or KOH as base, which is cheaper and more stable compared to the quaternary ammonium hydroxide or organic amines used in traditional synthesis. Here, an inorganic base of NaOH was employed in synthesizing active TS-1 zeolites for the first time. The crucial factor was the control of adverse effects of sodium cations on the incorporation of active titanium cations. Higher catalytic activity was achieved by further reducing the size of the TS-1 crystal by using the seed-added strategy, which uses the catalytic activity of a commercial catalyst, the production cost being much lower than commercial TS-1 catalysts, indicating great commercial potential and the possibility of preparing other cheap Lewis acid catalysts by using inorganic alkali.

Zeolite-encaged mononuclear copper centers catalyze CO2 selective hydrogenation to methanol

The selective hydrogenation of CO2 to methanol by renewable hydrogen source represents an attractive route for CO2 recycling and is carbon neutral. Stable catalysts with high activity and methanol selectivity are being vigorously pursued, and current debates on the active site and reaction pathway need to be clarified. Here, we report a design of faujasite-encaged mononuclear Cu centers, namely Cu@FAU, for this challenging reaction. Stable methanol space-time-yield (STY) of 12.8 mmol gcat-1 h-1 and methanol selectivity of 89.5% are simultaneously achieved at a relatively low reaction temperature of 513 K, making Cu@FAU a potential methanol synthesis catalyst from CO2 hydrogenation. With zeolite-encaged mononuclear Cu centers as the destined active sites, the unique reaction pathway of stepwise CO2 hydrogenation over Cu@FAU is illustrated. This work provides a clear example of catalytic reaction with explicit structure-activity relationship and highlights the power of zeolite catalysis in complex chemical transformations.

Computational Discovery of Stable Metal-Organic Frameworks for Methane-to-Methanol Catalysis

The challenge of direct partial oxidation of methane to methanol has motivated the targeted search of metal-organic frameworks (MOFs) as a promising class of materials for this transformation because of their site-isolated metals with tunable ligand environments. Thousands of MOFs have been synthesized, yet relatively few have been screened for their promise in methane conversion. We developed a high-throughput virtual screening workflow that identifies MOFs from a diverse space of experimental MOFs that have not been studied for catalysis, yet are thermally stable, synthesizable, and have promising unsaturated metal sites for C-H activation via a terminal metal-oxo species. We carried out density functional theory calculations of the radical rebound mechanism for methane-to-methanol conversion on models of the secondary building units (SBUs) from 87 selected MOFs. While we showed that oxo formation favorability decreases with increasing 3d filling, consistent with prior work, previously observed scaling relations between oxo formation and hydrogen atom transfer (HAT) are disrupted by the greater diversity in our MOF set. Accordingly, we focused on Mn MOFs, which favor oxo intermediates without disfavoring HAT or leading to high methanol release energies─a key feature for methane hydroxylation activity. We identified three Mn MOFs comprising unsaturated Mn centers bound to weak-field carboxylate ligands in planar or bent geometries with promising methane-to-methanol kinetics and thermodynamics. The energetic spans of these MOFs are indicative of promising turnover frequencies for methane to methanol that warrant further experimental catalytic studies.

Selective Oxidation of Methane to Methanol over Au/H-MOR

Selective oxidation of methane to methanol by dioxygen (O₂) is an appealing route for upgrading abundant methane resource and represents one of the most challenging reactions in chemistry due to the overwhelmingly higher reactivity of the product (methanol) versus the reactant (methane). Here, we report that gold nanoparticles dispersed on mordenite efficiently catalyze the selective oxidation of methane to methanol by molecular oxygen in aqueous medium in the presence of carbon monoxide. The methanol productivity reaches 1300 μmol g_cat^-1 h^-1 or 280 mmol g_Au^-1 h^-1 with 75% selectivity at 150 °C, outperforming most catalysts reported under comparable conditions. Both hydroxyl radicals and hydroperoxide species participate in the activation and conversion of methane, while it is shown that the lower affinity of methanol on gold mainly accounts for higher methanol selectivity.

Methane Oxidation to Methanol

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Organization of Cooperating Aluminum Pairs in Ferrierite Evidenced by Luminescence Quenching

We show that four cooperating Al atoms located at the two neighboring six-membered (6-MR) rings in the ferrierite framework can be readily discerned by luminescence studies. Thus, luminescent Zn(II) cations accommodated by one aluminum pair of the 6-MR ring can be effectively quenched by neighboring Co(II) ions stabilized by the second ring. Quenching occurs via the energy transfer mechanism and allows estimation of the critical radius of Zn(II)-Co(II) interactions. This points to the appropriate geometry and distance of the transition metal ions accommodated within zeolite, providing direct evidence of the four-aluminum atom arrangement in the ferrierite framework.

Formation of Ag-Fe Bimetallic Nano-Species on Mordenite Depending on the Initial Ratio of Components

The formation and properties of silver and iron nanoscale components in the Ag-Fe bimetallic system deposited on mordenite depend on several parameters during their preparation. Previously, it was shown that an important condition for optimizing nano-center properties in a bimetallic catalyst is to change the order of sequential deposition of components; the order "first Ag⁺, then Fe²⁺" was chosen as optimal. In this work, the influence of exact Ag/Fe atomic proportion on the system's physicochemical properties was studied. This ratio has been confirmed to affect the stoichiometry of the reduction-oxidation processes involving Ag⁺ and Fe²⁺, as shown by XRD, DR UV-Vis, XPS, and XAFS data, while HRTEM, S_BET and TPD-NH₃ show little change. However, it was found the correlation between the occurrence and amount of the Fe³⁺ ions incorporated into the zeolite's framework and the experimentally determined catalytic activities towards the model de-NOx reaction along the series of nanomaterials elucidated in this present paper.

Selective methane oxidation by molecular iron catalysts in aqueous medium

Using natural gas as chemical feedstock requires efficient oxidation of the constituent alkanes-and primarily methane1,2. The current industrial process uses steam reforming at high temperatures and pressures3,4 to generate a gas mixture that is then further converted into products such as methanol. Molecular Pt catalysts5-7 have also been used to convert methane to methanol8, but their selectivity is generally low owing to overoxidation-the initial oxidation products tend to be easier to oxidize than methane itself. Here we show that N-heterocyclic carbene-ligated FeII complexes with a hydrophobic cavity capture hydrophobic methane substrate from an aqueous solution and, after oxidation by the Fe centre, release a hydrophilic methanol product back into the solution. We find that increasing the size of the hydrophobic cavities enhances this effect, giving a turnover number of 5.0 × 102 and a methanol selectivity of 83% during a 3-h methane oxidation reaction. If the transport limitations arising from the processing of methane in an aqueous medium can be overcome, this catch-and-release strategy provides an efficient and selective approach to using naturally abundant alkane resources.

Methane Oxidation over the Zeolites-Based Catalysts

Zeolites have ordered pore structures, good spatial constraints, and superior hydrothermal stability. In addition, the active metal elements inside and outside the zeolite framework provide the porous material with adjustable acid–base property and good redox performance. Thus, zeolites-based catalysts are more and more widely used in chemical industries. Combining the advantages of zeolites and active metal components, the zeolites-based materials are used to catalyze the oxidation of methane to produce various products, such as carbon dioxide, methanol, formaldehyde, formic acid, acetic acid, and etc. This multifunction, high selectivity, and good activity are the key factors that enable the zeolites-based catalysts to be used for methane activation and conversion. In this review article, we briefly introduce and discuss the effect of zeolite materials on the activation of C–H bonds in methane and the reaction mechanisms of complete methane oxidation and selective methane oxidation. Pd/zeolite is used for the complete oxidation of methane to carbon dioxide and water, and Fe- and Cu-zeolite catalysts are used for the partial oxidation of methane to methanol, formaldehyde, formic acid, and etc. The prospects and challenges of zeolite-based catalysts in the future research work and practical applications are also envisioned. We hope that the outcome of this review can stimulate more researchers to develop more effective zeolite-based catalysts for the complete or selective oxidation of methane.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Influence of the Valence of Iron on the NO Reduction by CO over Cu-Fe-Mordenite

A comprehensive study of the catalytic properties of the copper-iron binary system supported on mordenite, depending on the iron valence—CuFe2MOR and CuFe3MOR—was carried out, and redox ability has been considered as a decisive factor in determining catalytic efficiency. Acidity was studied by TPD-NH3, DRIFT-OH, and DRT methods. The total acidity of both samples was high. The Brönsted acidity is similar for both bimetallic samples and is explained by the acidity of zeolite; Lewis acidity varies greatly and depends on the exchange cations. A screening DRIFT study of CO and NO has shown redox capacity and demonstrated a potential for using these materials as catalysts for ambient protection. CuFe2MOR demonstrated stable Cu and Fe species, while CuFe3MOR showed redox dynamic species. As expected, CuFe3MOR displayed higher catalytic performance in NO reduction via CO oxidation, because of the easily reduced intermediate NO-complex adsorbed on the metallic Cu and Fe sites, which were observed through in situ DRIFT study.

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.

Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type

Selective CH₄ oxidation to CH₃OH or HCHO with O₂ in H₂O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH₃OH and HCHO in CH₄ photooxidation for the first time. Through tuning the band structure and the size of active sites (i.e., single atoms or nanoparticles) in our Au/In₂O₃ catalyst, we show alternative formation of two important radicals, ^•OOH and ^•OH, which leads to distinctly different reaction paths to the formation of CH₃OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g^-1 on In₂O₃-supported Au single atoms (Au₁/In₂O₃) and an exceptional CH₃OH selectivity and yield of 89.42% and 5.95 mmol g^-1 on In₂O₃-supported Au nanoparticles (Au_NPs/In₂O₃), respectively, upon photocatalytic CH₄ oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH₄ oxidation by delicate design of composite photocatalysts.

Cu and Si co-doping on TiO2 nanosheets to modulate reactive oxygen species for efficient photocatalytic methane conversion

In this study, we successfully construct Cu and Si co-doped ultrathin TiO₂ nanosheets. As confirmed by comprehensive characterizations, Cu and Si co-doping can rationally tailor the electronic structure of TiO₂ to maneuver reactive oxygen species for effective photocatalytic methane conversion. In addition, this co-doping greatly enhances the utilization efficiency of photogenerated charges. Furthermore, it is revealed that Cu and Si co-doping can significantly boost the adsorption and activation of methane on TiO₂ nanosheets. As a result, the optimized catalyst achieves a C₂H₆ production rate of 33.8 μmol g^-1 h^-1 with a selectivity of 88.4%. This work provides insights into nanocatalyst design toward efficient photocatalytic methane conversion into value-added compounds.

The Progress and Outlook of Metal Single-Atom-Site Catalysis

Single-atom-site catalysts (SASCs) featuring maximized atom utilization and isolated active sites have progressed tremendously in recent years as a highly prosperous branch of catalysis research. Varieties of SASCs have been developed that show excellent performance in many catalytic applications. The major goal of SASC research is to establish feasible synthetic strategies for the preparation of high-performance catalysts, to achieve an in-depth understanding of the active-site structures and catalytic mechanisms, and to develop practical catalysts with industrial value. This Perspective describes the up-to-date development of SASCs and related catalysts, such as dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs), analyzes the current challenges encountered by these catalysts for industrial applications, and proposes their possible future development path.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site

Natural gas, consisting mainly of methane (CH₄), has a relatively low energy density at ambient conditions (~36 kJ l^-1). Partial oxidation of CH₄ to methanol (CH₃OH) lifts the energy density to ~17 MJ l^-1 and drives the production of numerous chemicals. In nature, this is achieved by methane monooxygenase with di-iron sites, which is extremely challenging to mimic in artificial systems due to the high dissociation energy of the C-H bond in CH₄ (439 kJ mol^-1) and facile over-oxidation of CH₃OH to CO and CO₂. Here we report the direct photo-oxidation of CH₄ over mono-iron hydroxyl sites immobilized within a metal-organic framework, PMOF-RuFe(OH). Under ambient and flow conditions in the presence of H₂O and O₂, CH₄ is converted to CH₃OH with 100% selectivity and a time yield of 8.81 ± 0.34 mmol g_cat^-1 h^-1 (versus 5.05 mmol g_cat^-1 h^-1 for methane monooxygenase). By using operando spectroscopic and modelling techniques, we find that confined mono-iron hydroxyl sites bind CH₄ by forming an [Fe-OH···CH₄] intermediate, thus lowering the barrier for C-H bond activation. The confinement of mono-iron hydroxyl sites in a porous matrix demonstrates a strategy for C-H bond activation in CH₄ to drive the direct photosynthesis of CH₃OH.

Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O₂ at 700 °C, oxidative fragmentation occurs, and Pt²⁺ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt²⁺ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H₂ to Pt^δ+ monocarbonyls. This conversion, which weakens the platinum-zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).