Selective Conversion Of Methane Research Articles

Low-Temperature Activation and Coupling of Methane on MgO Nanostructures Embedded in Cu2O/Cu(111).

The efficient conversion of methane into valuable hydrocarbons, such as ethane and ethylene, at relatively low temperatures without deactivation issues is crucial for advancing sustainable energy solutions. Herein, AP-XPS and STM studies show that MgO nanostructures (0.2-0.5 nm wide, 0.4-0.6 Å high) embedded in a Cu2O/Cu(111) substrate activate methane at room temperature, mainly dissociating it into CHx (x = 2 or 3) and H adatoms, with minimal conversion to C adatoms. These MgO nanostructures in contact with Cu2O/Cu(111) enable C-C coupling into ethane and ethylene at 500 K, a significantly lower temperature than that required for bulk MgO catalysts (>700 K), with negligible carbon deposition and no deactivation. DFT calculations corroborate these experimental findings. The CH4,gas → *CH3 + *H reaction is a downhill process on MgO/Cu2O/Cu(111) surfaces. The activation of methane is facilitated by electron transfer from copper to MgO and the existence of Mg and O atoms with a low coordination number in the oxide nanostructures. The formation of O-CH3 and O-H bonds overcomes the energy necessary for the cleavage of a C-H bond in methane. DFT studies reveal that smaller Mg2O2 model clusters provide stronger binding and lower activation barriers for C-H dissociation in CH4, while larger Mg3O3 clusters promote C-C coupling due to weaker *CH3 binding. All of these results emphasize the importance of size when optimizing the catalytic performance of MgO nanostructures in the selective conversion of methane.

From Methane to Methanol: Pd-iC-CeO2 Catalysts Engineered for High Selectivity via Mechanochemical Synthesis.

In the pursuit of selective conversion of methane directly to methanol in the liquid-phase, a common challenge is the concurrent formation of undesirable liquid oxygenates or combustion byproducts. However, we demonstrate that monometallic Pd-CeO2 catalysts, modified by carbon, created by a simple mechanochemical synthesis method exhibit 100% selectivity toward methanol at 75 °C, using hydrogen peroxide as oxidizing agent. The solvent free synthesis yields a distinctive Pd-iC-CeO2 interface, where interfacial carbon (iC) modulates metal-oxide interactions and facilitates tandem methane activation and peroxide decomposition, thus resulting in an exclusive methanol selectivity of 100% with a yield of 117 μmol/gcat at 75 °C. Notably, solvent interactions of H2O2 (aq) were found to be critical for methanol selectivity through a density functional theory (DFT)-simulated Eley-Rideal-like mechanism. This mechanism uniquely enables the direct conversion of methane into methanol via a solid-liquid-gas process.

Investigating the Metal-TiO2 Influence for Highly Selective Photocatalytic Oxidation of Methane to Methanol.

Methane conversion to valuable chemicals is a highly challenging and desirable reaction. Photocatalysis is a clean pathway to drive this chemical reaction, avoiding the high temperature and pressure of the syngas process. Titanium dioxide, being the most used photocatalyst, presents challenges in controlling the oxidation process, which is believed to depend on the metal sites on its surface that function as heterojunctions. Herein, we supported different metals on TiO2 and evaluated their activity in methane photooxidation reactions. We showed that Ni-TiO2 is the best photocatalyst for selective methane conversion, producing impressively high amounts of methanol (1.600 μmol·g-1) using H2O2 as an oxidant, with minimal CO2 evolution. This performance is attributed to the high efficiency of nickel species to produce hydroxyl radicals and enhance H2O2 utilization as well as to induce carrier traps (Ti3+ and SETOVs sites) on TiO2, which are crucial for C-H activation. This study sheds light on the role of catalyst structure in the proper control of CH4 photoconversion.

A Rational Design Strategy for Metal Exchanged Zeolites in the Conversion of Methane to Methanol – A Pathway Towards Commercial Viability

Methane is a potent greenhouse gas, and reducing methane released to the atmosphere is crucial in limiting global warming. A promising strategy is to convert methane to higher valuable chemicals right after emission. One class of materials active for this chemistry are metal exchanged zeolites. In this concept paper, we describe how it is possible to rationalize the performance of metal exchanged zeolites for the selective conversion of methane to methanol. We explore how phase diagrams can be used to identify condition dependent active site structures, how synthesis can be used to stir site distribution, and how the performance of single metal sites can be calculated. Combining these three aspects then allows to understand and predict experimentally measured quantities such as methane conversion and spectroscopic signals. In the end, we outline the most promising strategies to bring this technology to industrial feasibility.

Potential of Molecular Catalysts with Electron-Rich Transition Metal Centers for Addressing Long-Standing Chemistry Enigmas.

Molecular complexes with electron-rich metal centers are highlighted as potential catalysts for the following five important chemical transformations: selective conversion of methane to methanol, capture and utilization of carbon dioxide, fixation of molecular nitrogen, water splitting, and recycling of perfluorochemicals. Our initial focus lies on negatively charged metal centers and ligands that can stabilize anionic metal atoms. Catalysts with electron-rich metal atoms (CERMAs) can sustain catalytic cycles with a "ping-pong" mechanism, where one or more electrons are transferred from the metal center to the substrate and back. The donated electrons can activate the chemical bonds of the substrate by populating its antibonding orbitals. At the last step of the catalytic cycle, the electrons return to the metal and the product interacts only weakly with the formed anion, which enables the solvent molecules to remove the product fast from the catalytic cycle and prevent subsequent unfavorable reactions. This process resembles electrocatalysis, but the metal serves as both an anode and a cathode (molecular electrocatalysis). We also analyze the usage of CERMAs as the base of Frustrated Lewis pairs proposing a new type of bimetallic catalysts. This Featured Article aspires to initiate systematic experimental and theoretical studies on CERMAs and their reactivity, the potential of which has probably been underestimated in the literature.

100% selective methane conversion to C1 products over Ni/CeO2 nanorods

Selective CH4 upgrading to C1 products and avoiding over oxidation remains a key challenge. Here, we develop a highly efficient Ni/CeO2 nanorods containing both uniformly dispersed Ni-single-site and NiOx/CeO2 heterojunction for 100 % selectively CH4 conversion to C1 products (CH3OH, HCHO, CH3OOH and HCOOH). Under optimized photocatalytic experimental conditions, a high C1 product yield of 5.6 mmol g-1h−1 was obtained with 100 % selectivity with presence of H2O2. Mechanism study showed that the Ni-single-site together with formed oxygen vacancy (Ov) facilitated CH4 adsorption and activation. The terminal O atom of adsorbed ·OOH can fill the Ov, with the remaining *OH initiating *CH3 dehydrogenation and forming *CH2OH, which further reacts with ·OH to generate the main product HCHO. During the whole photocatalytic process, the formed NiOx/CeO2 heterojunction promoted carrier separation and enhanced catalytic performance.

Exploring the Impact of Active Site Structure on the Conversion of Methane to Methanol in Cu‐Exchanged Zeolites

AbstractIn the past, Cu‐oxo or ‐hydroxy clusters hosted in zeolites have been suggested to enable the selective conversion of methane to methanol, but the impact of the active site's stoichiometry and structure on methanol production is still poorly understood. Herein, we apply theoretical modeling in conjunction with experiments to study the impact of these two factors on partial methane oxidation in the Cu‐exchanged zeolite SSZ‐13. Phase diagrams developed from first‐principles suggest that Cu‐hydroxy or Cu‐oxo dimers are stabilized when O2 or N2O are used to activate the catalyst, respectively. We confirm these predictions experimentally and determine that in a stepwise conversion process, Cu‐oxo dimers can convert twice as much methane to methanol compared to Cu‐hydroxyl dimers. Our theoretical models rationalize how Cu‐di‐oxo dimers can convert up to two methane molecules to methanol, while Cu‐di‐hydroxyl dimers can convert only one methane molecule to methanol per catalytic cycle. These findings imply that in Cu clusters, at least one oxo group or two hydroxyl groups are needed to convert one methane molecule to methanol per cycle. This simple structure–activity relationship allows to intuitively understand the potential of small oxygenated or hydroxylated transition metal clusters to convert methane to methanol.

Exploring the Impact of Active Site Structure on the Conversion of Methane to Methanol in Cu-Exchanged Zeolites.

In the past, Cu-oxo or -hydroxy clusters hosted in zeolites have been suggested to enable the selective conversion of methane to methanol, but the impact of the active site's stoichiometry and structure on methanol production is still poorly understood. Herein, we apply theoretical modeling in conjunction with experiments to study the impact of these two factors on partial methane oxidation in the Cu-exchanged zeolite SSZ-13. Phase diagrams developed from first-principles suggest that Cu-hydroxy or Cu-oxo dimers are stabilized when O2 or N2O are used to activate the catalyst, respectively. We confirm these predictions experimentally and determine that in a stepwise conversion process, Cu-oxo dimers can convert twice as much methane to methanol compared to Cu-hydroxyl dimers. Our theoretical models rationalize how Cu-di-oxo dimers can convert up to two methane molecules to methanol, while Cu-di-hydroxyl dimers can convert only one methane molecule to methanol per catalytic cycle. These findings imply that in Cu clusters, at least one oxo group or two hydroxyl groups are needed to convert one methane molecule to methanol per cycle. This simple structure-activity relationship allows to intuitively understand the potential of small oxygenated or hydroxylated transition metal clusters to convert methane to methanol.

Selective conversion of methane to methanol facilitated by molecular metal-methoxy complexes via a self-correcting chemical cycle.

The controlled oxidation of methane to methanol has been an area of intense research over the past decades. Despite the efforts, the identification of an efficient catalyst with high selectivity is still elusive. Here we propose a thoroughly different strategy employing catalysts containing a metal methoxy unit. This family of catalysts has been used for the activation of C-H bonds but this is the first systematic investigation for the conversion of methane to methanol highlighting the advantages over the typically used metal oxides. Specifically, we start our investigation with an Fe(III) center coordinated by four ammonia ligands, (NH3)4FeOCH32+. Structures and energetics are reported for two mechanisms ([2+2] and proton coupled electrons transfer) and for different spin multiplicities via density functional theory, multi-reference, and coupled cluster quantum chemical calculations. The excited low-spin doublet state of this model system exhibits the best performance in terms of activation barriers and selectivity. Therefore, we then switched to the corresponding Ru(III) complex, which has a doublet ground state and manifests better performance than the doublet state of Fe(III). For both systems the activation barrier for methanol is larger than that of methane due to the interaction of the OH group of methanol with the coordinated NH3 ligands (hydrogen bonding) and/or the metal center. This observation suggests that the activation of methanol is slower, hindering its oxidation. In addition, we show that the metal-methoxy family of catalysts offers a potential mechanism that can prevent the oxidation of an activated methanol molecule (self-correcting chemical loop). This work aspires to induce experimental interest and pave the road for the development of high-performance high-selectivity methane to methanol direct conversion routes under mild conditions.

Continuous selective conversion of methane to methanol over a Cu-KFI zeolite catalyst using a water-O2 mixture as the oxygen source.

The continuous catalytic oxidation of methane to methanol on a Cu-KFI zeolite using water-O2 mixture as the oxidant is reported. A high methanol space-time yield of 880.3 mmol molCu-1 h-1 with 83% selectivity is achieved at 450 °C. Isotopic labelling experiments show that both H2O and O2 provide the oxygen source in this catalytic methane-to-methanol conversion reaction.

An alternative catalytic cycle for selective methane oxidation to methanol with Cu clusters in zeolites.

The partial oxidation of methane to methanol catalyzed by Cu-exchanged zeolites involves at present a three-step procedure that requires changing reaction conditions along the catalytic cycle. In this work we present an alternative catalytic cycle for selective methane conversion to methanol using as active species small Cu5 clusters supported on CHA zeolite. Periodic DFT calculations show that molecular O2 is easily activated on Cu5 clusters producing bi-coordinated O atoms able to dissociate homolytically a CH bond from CH4 and to react with the radical-like non-adsorbed methyl intermediate formed producing methanol, while competitive overoxidation to CO2 is energetically disfavored. The present mechanistic study opens a new avenue to design catalytic materials based on their ability to stabilize radical species.

Beyond Reduction Cocatalysts: Critical Role of Metal Cocatalysts in Photocatalytic Oxidation of Methane with Water**

AbstractEnvironmentally sustainable and selective conversion of methane to valuable chemicals under ambient conditions is pivotal for the development of next‐generation photocatalytic technology. However, due to the lack of microscopic knowledge about non‐thermal methane conversion, controlling and modulating photocatalytic oxidation processes driven by photogenerated holes remain a challenge. Here, we report novel function of metal cocatalysts to accept photogenerated holes and dominate selectivity of methane oxidation, which is clearly beyond the conventional concept in photocatalysis that the metal cocatalysts loaded on the surfaces of semiconductor photocatalysts mostly capture photogenerated electrons and dominate reduction reactions exclusively. The novel photocatalytic role of metal cocatalysts was verified by operando molecular spectroscopy combined with real‐time mass spectrometry for metal‐loaded Ga2O3 model photocatalysts under methane and water vapor at ambient temperature and pressure. Our concept of metal cocatalysts that work as active sites for both photocatalytic oxidation and reduction provides a new understanding of photocatalysis and a solid basis for controlling non‐thermal redox reactions by metal‐cocatalyst engineering.

Beyond Reduction Cocatalysts: Critical Role of Metal Cocatalysts in Photocatalytic Oxidation of Methane with Water.

Environmentally sustainable and selective conversion of methane to valuable chemicals under ambient conditions is pivotal for the development of next-generation photocatalytic technology. However, due to the lack of microscopic knowledge about non-thermal methane conversion, controlling and modulating photocatalytic oxidation processes driven by photogenerated holes remain a challenge. Here, we report novel function of metal cocatalysts to accept photogenerated holes and dominate selectivity of methane oxidation, which is clearly beyond the conventional concept in photocatalysis that the metal cocatalysts loaded on the surfaces of semiconductor photocatalysts mostly capture photogenerated electrons and dominate reduction reactions exclusively. The novel photocatalytic role of metal cocatalysts was verified by operando molecular spectroscopy combined with real-time mass spectrometry for metal-loaded Ga2O3 model photocatalysts under methane and water vapor at ambient temperature and pressure.Our concept of metal cocatalysts that work as active sites for both photocatalytic oxidation and reduction provides a new understanding of photocatalysis and a solid basis for controlling non-thermal redox reactions by metal-cocatalyst engineering.

Methane Activation and Coupling Pathways on Ni2P Catalyst

The direct catalytic conversion of methane (CH4) to higher hydrocarbons has attracted considerable attention in recent years because of the increasing supply of natural gas. Efficient and selective catalytic conversion of methane to value-added products, however, remains a major challenge. Recent studies have shown that the incorporation of phosphorus atoms in transition metals improves their selectivity and resistance to coke formation for many catalytic reactions. In this work, we report a density function theory-based investigation of methane activation and C2 product formation on Ni2P(001). Our results indicate that, despite the lower reactivity of Ni2P relative to Ni, the addition of phosphorus atoms hinders excessive dehydrogenation of methane to CH* and C* species, thus reducing carbon deposition on the surface. CH3* and CH2* moieties, instead, are more likely to be the most abundant surface intermediates once the initial C–H bond in methane is activated with a barrier of 246 kJ mol−1. The formation of ethylene from 2CH2* on Ni2P is facile with a barrier of 56 kJ mol−1, which is consistent with prior experimental studies. Collectively, these findings suggest that Ni2P may be an attractive catalyst for selective methane conversion to ethylene.

Pt overlayer for direct oxidation of CH4 to CH3OH

Highly selective conversion of methane (CH4) to methanol (CH3OH) is an emerging attractive but challenging process for future development of hydrogen economy, which requires efficient catalysts. Herein, we systematically explore the catalytic properties of Pt(111) overlayer on transition metal oxides (TMOs) for CH4 conversion by first principles calculations. The Pt(111) monolayer supported by Ce-terminated CeO2(111) substrate exhibits high activity and selectivity for CH4 conversion to CH3OH, with the kinetic barrier of rate-limiting step of 1.05 eV. Intriguingly, the surface activity of Pt overlayer is governed by its d-band center relative to the energy of bonding states of adsorbed molecules, which in turn depends on the number of charge transfer between Pt(111) monolayer and underlying TMOs substrates. These results provide useful insights in the design of metal overlayers as catalysts with high-ultra performance and atomic utilization.

Direct Conversion of Methane to Propylene.

Nonoxidative coupling of methane exhibits promising prospect in that it affords value-added hydrocarbons and hydrogen with high atom economy. However, challenge remains in direct, selective conversion of methane to more valuable hydrocarbons like olefins. The current work presents a catalyst with well-dispersed Ta atoms anchored by graphitic C3N4-supported phthalocyanine. Such a catalyst is able to convert methane selectively to ethylene and propylene at a relatively low temperature (350 °C). The conception of the active center and construction of the catalyst have been described, and the origins of the catalytic performance are discussed.

Selective conversion of methane to ethane and hydrogen over In/molecular-sieve-3A catalyst

The molecular-sieve-3A material functioned as a good support for the indium liquid metal catalyst for the dehydrogenative conversion of methane (DCM) to ethane, ethylene, and H2 with 100% selectivity at 873 K.

Visible-light-driven and selective methane conversion to oxygenates with air on a halide-perovskite-based photocatalyst under mild conditions

We have elaborately prepared perovskite-based heterojunctions to achieve high-efficiency photocatalytic CH4transformation to oxygenates under mild conditions.

Economical and Sustainable Synthesis of Small-Pore Chabazite Catalysts for NOx Abatement by Recycling Organic Structure-Directing Agents.

The application of small-pore chabazite-type SSZ-13 zeolites, key materials for the reduction of nitrogen oxides (NOx) in automotive exhausts and the selective conversion of methane, is limited by the use of expensive N,N,N-trimethyl-1-ammonium adamantine hydroxide (TMAdaOH) as an organic structure-directing agent (OSDA) during hydrothermal synthesis. Here, we report an economical and sustainable route for SSZ-13 synthesis by recycling and reusing the OSDA-containing waste liquids. The TMAdaOH concentration in waste liquids, determined by a bromocresol green colorimetric method, was found to be a key factor for SSZ-13 crystallization. The SSZ-13 zeolite synthesized under optimized conditions demonstrates similar physicochemical properties (surface area, porosity, crystallinity, Si/Al ratio, etc.) as that of the conventional synthetic approach. We then used the waste liquid-derived SSZ-13 as the parent zeolite to synthesize Cu ion-exchanged SSZ-13 (i.e., Cu-SSZ-13) for ammonia-mediated selective catalytic reduction of NOx (NH3-SCR) and observed a higher activity as well as better hydrothermal stability than Cu-SSZ-13 by conventional synthesis. In situ infrared and ultraviolet-visible spectroscopy investigations revealed that the superior NH3-SCR performance of waste liquid-derived Cu-SSZ-13 results from a higher density of Cu2+ sites coordinated to paired Al centers on the zeolite framework. The technoeconomic analysis highlights that recycling OSDA-containing waste liquids could reduce the raw material cost of SSZ-13 synthesis by 49.4% (mainly because of the higher utilization efficiency of TMAdaOH) and, meanwhile, the discharging of wastewater by 45.7%.

Selective conversion of methane to cyclohexane and hydrogen via efficient hydrogen transfer catalyzed by GaN supported platinum clusters

Non-oxidative liquefaction of methane at room temperature and ambient pressure has long been a scientific “holy grail” of chemical research. Herein, we exploit an unprecedented catalytic transformation of methane exclusively to cyclohexane and hydrogen evolution through effective surface-hydrogen-transfer (SHT) at the heterojunctions boundary consisting of electron-rich platinum cluster (Pt) loaded on methane-activating gallium nitride (GaN) host. The experimental analysis demonstrates that the interface-induced overall reaction starts with methane aromatization to benzene and surface-bound hydrogen initiated by the Ga–N pairs, followed by the hydrogenation of benzene to cyclohexane with surface-bound hydrogen. The in-situ activated hydrogen at electron-rich metal Pt cluster is crucial for the hydrogenation and enables an outstanding selectivity (up to 92%) and productivity (41 μmol g−1) towards cyclohexane and hydrogen evolution concurrently at 300 °C, which is well-delivered after 5 recycling runs.