Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature

Water-participated mild oxidation of ethane to acetaldehyde

The direct conversion of low alkane such as ethane into high-value-added chemicals has remained a great challenge since the development of natural gas utilization. Herein, we achieve an efficient one-step conversion of ethane to C2 oxygenates on a Rh1/AC-SNI catalyst under a mild condition, which delivers a turnover frequency as high as 158.5 h-1. 18O isotope-GC-MS shows that the formation of ethanol and acetaldehyde follows two distinct pathways, where oxygen and water directly participate in the formation of ethanol and acetaldehyde, respectively. In situ formed intermediate species of oxygen radicals, hydroxyl radicals, vinyl groups, and ethyl groups are captured by laser desorption ionization/time of flight mass spectrometer. Density functional theory calculation shows that the activation barrier of the rate-determining step for acetaldehyde formation is much lower than that of ethanol, leading to the higher selectivity of acetaldehyde in all the products.

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.

Recent Advances in the Catalytic Conversion of Methane to Methanol: From the Challenges of Traditional Catalysts to the Use of Nanomaterials and Metal-Organic Frameworks

Methane and carbon dioxide are the main contributors to global warming, with the methane effect being 25 times more powerful than carbon dioxide. Although the sources of methane are diverse, it is a very volatile and explosive gas. One way to store the energy content of methane is through its conversion to methanol. Methanol is a liquid under ambient conditions, easy to transport, and, apart from its use as an energy source, it is a chemical platform that can serve as a starting material for the production of various higher-value products. Accordingly, the transformation of methane to methanol has been extensively studied in the literature, using traditional catalysts as different types of zeolites. However, in the last few years, a new generation of catalysts has emerged to carry out this transformation with higher conversion and selectivity, and more importantly, under mild temperature and pressure conditions. These new catalysts typically involve the use of a highly porous supporting material such as zeolite, or more recently, metal-organic frameworks (MOFs) and graphene, and metallic nanoparticles or a combination of different types of nanoparticles that are the core of the catalytic process. In this review, recent advances in the porous supports for nanoparticles used for methane oxidation to methanol under mild conditions are discussed.

Bioinspired Cu(II) Defect Sites in ZIF-8 for Selective Methane Oxidation

Activating the C-H bonds of alkanes without further oxidation to more thermodynamically stable products, CO and CO2, is a long-sought goal of catalytic chemistry. Inspired by the monocopper active site of methane monooxygenase, we synthesized a Cu-doped ZIF-8 metal-organic framework with 25% Cu and 75% Zn in the nodes and activated it by heating to 200 °C and dosing in a stepwise fashion with O2, methane, and steam. We found that it does oxidize methane to methanol and formaldehyde. The catalysis persists through at least five cycles, and beyond the third cycle, the selectivity improves to the extent that no CO2 can be detected. Experimental characterization and analysis were carried out by PXRD, DRUV-vis, SEM, and XAS (XANES and EXAFS). The reaction is postulated to proceed at open-coordination copper sites generated by defects, and the mechanism of methanol production was explicated by density functional calculations with the revMO6-L exchange-correlation functional. The calculations reveal a catalytic cycle of oxygen-activated CuI involving the conversion of two molecules of CH4 to two molecules of CH3OH by a sequence of hydrogen atom transfer reactions and rebound steps. For most steps in the cycle, the reaction is more favored by singlet species than by triplets.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Mechanistic insight into the effect of active site motif structures on direct oxidation of methane to methanol over Cu-ZSM-5

Direct oxidation of methane to methanol (DMTM), a highly challenging reaction in C1 chemistry, has attracted lots of attention. Herein, we investigate the continuous H2O-mediated N2O-DMTM over a series of Cu-ZSM-5-n zeolites prepared by a solid-state ion-exchange method. Excellent CH3OH productivity (194.8 μmol gcat-1 h-1) and selectivity (67.1%) can be achieved over Cu-ZSM-5-0.3%, which surpasses most recently reported zeolite catalysts. The effect of the active site motif structure on the reaction was systematically investigated by the combined experimental and theoretical studies. It has been revealed that both the monomeric [Cu]+ and binuclear [Cu]+-[Cu]+ sites function to produce CH3OH, following the radical rebound mechanism, wherein the latter one plays a dominant role due to the synergistic effect of neighboring [Cu]+ that can efficiently reduce the N2O dissociation barrier to generate active oxygen for CH4 oxidation. Microkinetic modeling results further show that the dicopper site possesses a much higher net reaction rate (1.23 × 105 s-1) than the monomeric Cu site (0.962 s-1); moreover, H2O can shift the rate determining step from the CH3OH desorption step to the N2O dissociation step over the dicopper site, thereby efficiently favoring CH3OH production and resisting carbon deposition. Generally, the study in the present work would substantially favor other highly efficient catalyst designs.

Speciation and Reactivity Control of Cu-Oxo Clusters via Extraframework Al in Mordenite for Methane Oxidation

The stoichiometric conversion of methane to methanol by Cu-exchanged zeolites can be brought to highest yields by the presence of extraframework Al and high CH4 chemical potentials. Combining theory and experiments, the differences in chemical reactivity of monometallic Cu-oxo and bimetallic Cu-Al-oxo nanoclusters stabilized in zeolite mordenite (MOR) are investigated. Cu-L3 edge X-ray absorption near-edge structure (XANES), infrared (IR), and ultraviolet-visible (UV-vis) spectroscopies, in combination with CH4 oxidation activity tests, support the presence of two types of active clusters in MOR and allow quantification of the relative proportions of each type in dependence of the Cu concentration. Ab initio molecular dynamics (MD) calculations and thermodynamic analyses indicate that the superior performance of materials enriched in Cu-Al-oxo clusters is related to the activity of two μ-oxo bridges in the cluster. Replacing H2O with ethanol in the product extraction step led to the formation of ethyl methyl ether, expanding this way the applicability of these materials for the activation and functionalization of CH4. We show that competition between different ion-exchanged metal-oxo structures during the synthesis of Cu-exchanged zeolites determines the formation of active species, and this provides guidelines for the synthesis of highly active materials for CH4 activation and functionalization.

Computational Screening of Supported Metal Oxide Nanoclusters for Methane Activation: Insights into Homolytic versus Heterolytic C-H Bond Dissociation

Since its discovery in zeolites, the [CuOCu]²⁺ motif has played an important role in our understanding of selective methane activation over supported metal oxide nanoclusters. Although there are two known C-H bond dissociation mechanisms, namely, homolytic and heterolytic cleavage, most computational studies on optimizing metal oxide nanoclusters for improved methane activation reactivity have focused only on the homolytic mechanism. In this work, both mechanisms were examined for a set of 21 mixed metal oxide complexes of the form of [M₁OM₂]²⁺ (M₁ and M₂ = Mn, Fe, Co, Ni, Cu, and Zn). Except for pure copper, heterolytic cleavage was found to be the dominant C-H bond activation pathway for all systems. Furthermore, mixed systems including [CuOMn]²⁺, [CuONi]²⁺, and [CuOZn]²⁺ are predicted to possess methane activation activity similar to pure [CuOCu]²⁺. These results suggest that both homolytic and heterolytic mechanisms should be considered in computing methane activation energies on supported metal oxide nanoclusters.

Recognizing the best catalyst for a reaction

Heterogeneous catalysis is immensely important, providing access to materials essential for the well-being of society, and improved catalysts are continuously required. New catalysts are frequently tested under different conditions making it difficult to determine the best catalyst. Here we describe a general approach to identify the best catalyst using a data set based on all reactions under kinetic control to calculate a set of key performance indicators (KPIs). These KPIs are normalized to take into account the variation in reaction conditions. Plots of the normalized KPIs are then used to demonstrate the best catalyst using two case studies: (i) acetylene hydrochlorination, a reaction of current interest for vinyl chloride manufacture, and (ii) the selective oxidation of methane to methanol using O₂ in water, a reaction that has attracted very recent attention in the academic literature.

In aqua dual selective photocatalytic conversion of methane to formic acid and methanol with oxygen and water as oxidants without overoxidation

The direct and selective transformation of naturally abundant methane (CH₄) into high-value-added oxygenates, e.g., methanol, ethanol, and formic acid, is one of the "Holy Grails" in chemistry and chemical productions. However, complex mixtures of products, often due to over-oxidations, make such transformations highly challenging. Herein, gallium nitride (GaN), a methane-active semiconductor, catalyzes the photooxidation of methane and empowers the fine-controlling of chemoselectivity toward methanol and formic acids, simply by regulating the O₂ content in water. In contrast to previous methods, no overoxidation products (CO₂ and CO) were observed in this process. Mechanistic investigations and the corresponding quantitative experiments indicated that the controllable generation of moderately reactive oxygen radicals (•OOH and •OH) in combination with the direct methane activation triggered by GaN is responsible for the highly selective reactivity and tunability through a photo-generated radical process.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

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.

One-step direct conversion of methane to methanol with water in non-thermal plasma

Achieving methane-to-methanol is challenging under mild conditions. In this study, methanol is synthesized by one-step direction conversion of CH₄ with H₂O at room temperature under atmospheric pressure in non-thermal plasma (NTP). This route is characterized by the use of methane and liquid water as the reactants, which enables the transfer of the methanol product to the liquid phase in time to inhibit its further decomposition and conversion. Therefore, the obtained product is free of carbon dioxide. The reaction products include gas and liquid-phase hydrocarbons, CO, CH₃OH, and C₂H₅OH. The combination of plasma and semiconductor materials increases the production rate of methanol. In addition, the addition of Ar or He considerably increases the production rate and selectivity of methanol. The highest production rate of methanol and selectivity in liquid phase can reach 56.7 mmol g_cat^-1 h^-1 and 93%, respectively. Compared with the absence of a catalyst and added gas, a more than 5-fold increase in the methanol production rate is achieved.

Efficient cell factories for the production of N-methylated amino acids and for methanol-based amino acid production

The growing world needs commodity amino acids such as L‐glutamate and L‐lysine for use as food and feed, and specialty amino acids for dedicated applications. To meet the supply a paradigm shift regarding their production is required. On the one hand, the use of sustainable and cheap raw materials is necessary to sustain low production cost and decrease detrimental effects of sugar‐based feedstock on soil health and food security caused by competing uses of crops in the feed and food industries. On the other hand, the biotechnological methods to produce functionalized amino acids need to be developed further, and titres enhanced to become competitive with chemical synthesis methods. In the current review, we present successful strain mutagenesis and rational metabolic engineering examples leading to the construction of recombinant bacterial strains for the production of amino acids such as L‐glutamate, L‐lysine, L‐threonine and their derivatives from methanol as sole carbon source. In addition, the fermentative routes for bioproduction of N‐methylated amino acids are highlighted, with focus on three strategies: partial transfer of methylamine catabolism, S‐adenosyl‐L‐methionine dependent alkylation and reductive methylamination of 2‐oxoacids.

Second-Sphere Lattice Effects in Copper and Iron Zeolite Catalysis

Transition-metal-exchanged zeolites perform remarkable chemical reactions from low-temperature methane to methanol oxidation to selective reduction of NOx pollutants. As with metalloenzymes, metallozeolites have impressive reactivities that are controlled in part by interactions outside the immediate coordination sphere. These second-sphere effects include activating a metal site through enforcing an "entatic" state, controlling binding and access to the metal site with pockets and channels, and directing radical rebound vs cage escape. This review explores these effects with emphasis placed on but not limited to the selective oxidation of methane to methanol with a focus on copper and iron active sites, although other transition-metal-ion zeolite reactions are also explored. While the actual active-site geometric and electronic structures are different in the copper and iron metallozeolites compared to the metalloenzymes, their second-sphere interactions with the lattice or the protein environments are found to have strong parallels that contribute to their high activity and selectivity.

Rational design of ZSM-5 zeolite containing a high concentration of single Fe sites capable of catalyzing the partial oxidation of methane with high turnover frequency

We successfully synthesized a Fe/ZSM-5 catalyst enabling conversion of methane to C1 oxygenates in record yields, and demonstrated that the fraction of the single Fe cation, as well as the Al distribution, are the powerful activity descriptors.

Activation and catalytic transformation of methane under mild conditions

In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH₄) under mild conditions for both chemical synthesis with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH₄ to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH₄ to value-added intermediate compounds including but not limited to CH₃OH, HCHO, HCOOH, and CH₃COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH₄ under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH₄ to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH₄ or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH₄ to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH₄ under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.

DFT Analysis of Methane C-H Activation and Over-Oxidation by [Cu2 O]2+ and [Cu2 O2 ]2+ Sites in Zeolite Mordenite: Intra- versus Inter-site Over-Oxidation

Methane over-oxidation by copper-exchanged zeolites prevents realization of high-yield catalytic conversion. However, there has been little description of the mechanism for methane over-oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu₃ O₃ ]²⁺ active sites can over-oxidize methane. However, the role of [Cu₃ O₃ ]²⁺ sites in methane-to-methanol conversion remains under debate. Here, we examine methane over-oxidation by dicopper [Cu₂ O]²⁺ and [Cu₂ O₂ ]²⁺ sites using DFT in zeolite mordenite (MOR). For [Cu₂ O₂ ]²⁺ , we considered the μ-(η² :η² ) peroxo-, and bis(μ-oxo) motifs. These sites were considered in the eight-membered (8MR) ring of MOR. μ-(η² :η² ) peroxo sites are unstable relative to the bis(μ-oxo) motif with a small interconversion barrier. Unlike [Cu₂ O]²⁺ which is active for methane C-H activation, [Cu₂ O₂ ]²⁺ has a very large methane C-H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over-oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH₃ group, followed by OH and can proceed near 200 °C. Thus, for [Cu₂ O]²⁺ and [Cu₂ O₂ ]²⁺ species, over-oxidation is an inter-site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra-site process for [Cu₃ O₃ ]²⁺ sites and the role of Brønsted acid sites.

Activation and conversion of alkanes in the confined space of zeolite-type materials

Microporous zeolite-type materials, with crystalline porous structures formed by well-defined channels and cages of molecular dimensions, have been widely employed as heterogeneous catalysts since the early 1960s, due to their wide variety of framework topologies, compositional flexibility and hydrothermal stability. The possible selection of the microporous structure and of the elements located in framework and extraframework positions enables the design of highly selective catalysts with well-defined active sites of acidic, basic or redox character, opening the path to their application in a wide range of catalytic processes. This versatility and high catalytic efficiency is the key factor enabling their use in the activation and conversion of different alkanes, ranging from methane to long chain n-paraffins. Alkanes are highly stable molecules, but their abundance and low cost have been two main driving forces for the development of processes directed to their upgrading over the last 50 years. However, the availability of advanced characterization tools combined with molecular modelling has enabled a more fundamental approach to the activation and conversion of alkanes, with most of the recent research being focused on the functionalization of methane and light alkanes, where their selective transformation at reasonable conversions remains, even nowadays, an important challenge. In this review, we will cover the use of microporous zeolite-type materials as components of mono- and bifunctional catalysts in the catalytic activation and conversion of C₁⁺ alkanes under non-oxidative or oxidative conditions. In each case, the alkane activation will be approached from a fundamental perspective, with the aim of understanding, at the molecular level, the role of the active sites involved in the activation and transformation of the different molecules and the contribution of shape-selective or confinement effects imposed by the microporous structure.

H2 O-Built Proton Transfer Bridge Enhances Continuous Methane Oxidation to Methanol over Cu-BEA Zeolite

Direct oxidation of methane to methanol (DMTM) is a big challenge in C₁ chemistry. We present a continuous N₂ O-DMTM investigation by simultaneously introducing 10 vol % H₂ O into the reaction system over Cu-BEA zeolites. Combining a D₂ O isotopic tracer technique and ab initio molecular dynamics (AIMD) simulation, we for the first time demonstrate that the H₂ O molecules can participate in the reaction through a proton transfer route, wherein the H₂ O molecules can build a high-speed proton transfer bridge between the generated moieties of CH₃ ^- and OH^- over the evolved mono(μ-oxo) dicopper ([Cu-O-Cu]²⁺ ) active site, thereby pronouncedly boosting the CH₃ OH selectivity (3.1→71.6 %), productivity (16.8→242.9 μmol g_cat ^-1  h^-1 ) and long-term reaction stability (10→70 h) relative to the scenario of absence of H₂ O. Unravelling the proton transfer of H₂ O over the dicopper [Cu-O-Cu]²⁺ site would substantially contribute to highly efficient catalyst designs for the continuous DMTM.

Harnessing of Diluted Methane Emissions by Direct Partial Oxidation of Methane to Methanol over Cu/Mordenite

The upgrading of diluted methane emissions into valuable products can be accomplished at low temperatures (200 °C) by the direct partial oxidation of methanol over copper-exchanged zeolite catalysts. The reaction has been studied in a continuous fixed-bed reactor loaded with a Cu–mordenite catalyst, according to a three-step cyclic process: adsorption of methane, desorption of methanol, and reactivation of the catalyst. The purpose of the work is the use of methane emissions as feedstocks, which is challenging due to their low methane concentration and the presence of oxygen. Methane concentration had a marked influence on methane adsorption and methanol production (decreased from 164 μmol/g Cu for pure methane to 19 μmol/g Cu for 5% methane). The presence of oxygen, even in low concentrations (2.5%), reduced methane adsorption drastically. However, methanol production was only affected slightly (average decrease of 9%), concluding that methane adsorbed on the active centers yielding methanol is not influenced by oxygen.

Critical Role of Al Pair Sites in Methane Oxidation to Methanol on Cu-Exchanged Mordenite Zeolites

Cu-exchanged aluminosilicate zeolites have been intensively studied for the selective oxidation of methane to methanol via a chemical looping manner, while the nature of active Cu-oxo species for these catalysts is still under debate. This study inquired into the effects of Al distribution on methane oxidation over Cu-exchanged aluminosilicate zeolites, which provided an effective way to discern the activity difference between mononuclear and polynuclear Cu-oxo species. Specifically, conventional Na+/Co2+ ion-exchange methods were applied to quantify isolated Al and Al pair (i.e., Al−OH−(Si−O)1–3−Al−OH) sites for three mordenite (MOR) zeolites, and a correlation was established between the reactivity of the resultant Cu-MOR catalysts and the portions of the accessible framework Al sites. These results indicated that the Cu-oxo clusters derived from the Al pair sites were more reactive than the CuOH species grafted at the isolated Al sites, which is consistent with in situ ultraviolet-visible spectroscopic characterization and density functional theory calculations. Further theoretical analysis of the first C–H bond cleavage in methane on these Cu-oxo species unveiled that stabilization of the formed methyl group was the predominant factor in determining the reactivity of methane oxidation.

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

Copper-exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time-resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono-μ-oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono-μ-oxo species, while the activation energy is two times lower.

Methane Oxidation to Methanol in Water

Methane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C-H activation and prevention of overoxidation to CO₂. Indirect methods such as steam reforming produce CO and H₂ chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years.The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N₂O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H₂O₂ as an oxidant (effectively O₂ + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H₂O₂ is an expensive oxidant to use in the production of methanol.This observation of activity under mild conditions led to new approaches to utilize O₂ as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C-H activation reactions using O₂ and H₂O₂, but the rapid decomposition of H₂O₂ over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H₂O₂ consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O₂ could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area.

Spectroscopic Definition of a Highly Reactive Site in Cu-CHA for Selective Methane Oxidation: Tuning a Mono-μ-Oxo Dicopper(II) Active Site for Reactivity

Using UV-vis and resonance Raman spectroscopy, we identify a [Cu₂O]²⁺ active site in O₂ and N₂O activated Cu-CHA that reacts with methane to form methanol at low temperature. The Cu-O-Cu angle (120°) is smaller than that for the [Cu₂O]²⁺ core on Cu-MFI (140°), and its coordination geometry to the zeolite lattice is different. Site-selective kinetics obtained by operando UV-vis show that the [Cu₂O]²⁺ core on Cu-CHA is more reactive than the [Cu₂O]²⁺ site in Cu-MFI. From DFT calculations, we find that the increased reactivity of Cu-CHA is a direct reflection of the strong [Cu₂OH]²⁺ bond formed along the H atom abstraction reaction coordinate. A systematic evaluation of these [Cu₂O]²⁺ cores reveals that the higher O-H bond strength in Cu-CHA is due to the relative orientation of the two planes of the coordinating bidentate O-Al-O T-sites that connect the [Cu₂O]²⁺ core to the zeolite lattice. This work along with our earlier study ( J. Am. Chem. Soc, 2018, 140, 9236-9243) elucidates how zeolite lattice constraints can influence active site reactivity.

Methane to Methanol through Heterogeneous Catalysis and Plasma Catalysis

Direct oxidation of methane to methanol (DOMTM) is attractive for the increasing industrial demand of feedstock. In this review, the latest advances in heterogeneous catalysis and plasma catalysis for DOMTM are summarized, with the aim to pinpoint the differences between both, and to provide some insights into their reaction mechanisms, as well as the implications for future development of highly selective catalysts for DOMTM.

A potential catalyst for continuous methane partial oxidation to methanol using N2O: Cu-SSZ-39

Continuous catalytic methanol production from methane is reported on Cu-SSZ-39 using N₂O as an oxidant. Through optimization of CH₄, N₂O and H₂O partial pressures, a methanol formation rate of 499 μmol_CH3OH g^-1 h^-1 and a methanol selectivity of 34% is achieved.

Data science assisted investigation of catalytically active copper hydrate in zeolites for direct oxidation of methane to methanol using H2O2

Dozens of Cu zeolites with MOR, FAU, BEA, FER, CHA and MFI frameworks are tested for direct oxidation of CH₄ to CH₃OH using H₂O₂ as oxidant. To investigate the active structures of the Cu zeolites, 15 structural variables, which describe the features of the zeolite framework and reflect the composition, the surface area and the local structure of the Cu zeolite active site, are collected from the Database of Zeolite Structures of the International Zeolite Association (IZA). Also analytical studies based on inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray fluorescence (XRF), N₂ adsorption specific surface area measurement and X-ray absorption fine structure (XAFS) spectral measurement are performed. The relationships between catalytic activity and the structural variables are subsequently revealed by data science techniques, specifically, classification using unsupervised and supervised machine learning and data visualization using pairwise correlation. Based on the unveiled relationships and a detailed analysis of the XAFS spectra, the local structures of the Cu zeolites with high activity are proposed.

Heterometallic [Cu–O–M]2+ active sites for methane C–H activation in zeolites: stability, reactivity, formation mechanism and relationship to other active sites

Formation energies and mechanisms, autoreduction and methane C–H reactivities were obtained for [Cu–O–M]²⁺ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in mordenite with DFT. These reveal that [Cu₂O]²⁺ is best suited for MMC.

Catalytic cycle of the partial oxidation of methane to methanol over Cu-ZSM-5 revealed using DFT calculations

Density functional theory (DFT) calculations were performed to investigate the catalytic cycle of methane conversion to methanol over both [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites in the Cu-ZSM-5 catalyst. The [Cu2(O2)]2+ site is found to be active for the partial oxidation of methane to methanol, and although it has a higher energy barrier in the methane activation step, it involves a very low energy barrier in the methanol formation step (36.3 kJ mol-1) as well as a lower methanol desorption energy (52.5 kJ mol-1). As the [Cu2(O2)]2+ active site is also thermodynamically stable, it may play an important role during methane conversion to methanol. Furthermore, the methane activation step follows the homolytic route and the heterolytic route for the [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites, respectively, whereas the methanol formation step follows the direct radical rebound mechanism and the indirect rebound mechanism, respectively. Our calculations further indicate that the electronic properties of the reactive O atoms in the active site influence their reactivity toward methane oxidation. More specifically, the higher the spin density and the more negative the charge of the reactive O atom at the active site are, the lower the energy barrier for methane activation will be; and the more negative the charge of the hydroxyl group in the reaction intermediate during the partial oxidation of methane to methanol is, the higher energy barrier of the methanol formation step will be in the triplet state. Furthermore, we used a larger cluster model to predict the mechanism of the methane activation step and the effect of atomic charge of the O atom at the [Cu2(μ-O)]2+ and [Cu2(O2)]2+ active sites on the energy barriers of partial oxidation of methane to methanol, and the conclusions drawn employing the larger cluster model are consistent with those drawn using the smaller double-5T-ring cluster model. In addition, different from the traditional mechanism for methane activation at [Cu2(O2)]2+, which consists of two transition states, we find that the partial oxidation of methane at [Cu2(O2)]2+ can also occur via a single step by direct insertion of one of the O atoms at the active site into the C-H bond of methane.

Applications of Zeolites to C1 Chemistry: Recent Advances, Challenges, and Opportunities

C1 chemistry, which is the catalytic transformation of C1 molecules including CO, CO₂ , CH₄ , CH₃ OH, and HCOOH, plays an important role in providing energy and chemical supplies while meeting environmental requirements. Zeolites are highly efficient solid catalysts used in the chemical industry. The design and development of zeolite-based mono-, bi-, and multifunctional catalysts has led to a booming application of zeolite-based catalysts to C1 chemistry. Combining the advantages of zeolites and metallic catalytic species has promoted the catalytic production of various hydrocarbons (e.g., methane, light olefins, aromatics, and liquid fuels) and oxygenates (e.g., methanol, dimethyl ether, formic acid, and higher alcohols) from C1 molecules. The key zeolite descriptors that influence catalytic performance, such as framework topologies, nanoconfinement effects, Brønsted acidities, secondary-pore systems, particle sizes, extraframework cations and atoms, hydrophobicity and hydrophilicity, and proximity between acid and metallic sites are discussed to provide a deep understanding of the significance of zeolites to C1 chemistry. An outlook regarding challenges and opportunities for the conversion of C1 resources using zeolite-based catalysts to meet emerging energy and environmental demands is also presented.