Activation and catalytic transformation of methane under mild conditions

Unidentical Twins: Geometrically Similar but Chemically Distinct Metal-Free Sites in Boron Oxide for Methane Oxidation to HCHO, CO and CO2

Metal-free boron-based catalysts such as boron oxide (B2O3) and boron nitride (h-BN) are promising catalysts for methane oxidation to HCHO and CO. The B2O3 catalyst contains various probable boron sites (B1 to B6), which may be responsible for methane oxidation. In this work, we utilized density functional theory to compare two relevant geometrically identical boron sites (B2 and B4) for their reactivities. The two sites are explored in-detail for the conversion of methane to formaldehyde (M2F), carbon monoxide and carbon dioxide. The B4 site activates the methane C-H bond easily as compared to the B2 site. In M2F conversion, the rate-determining step for the B2 site is the co-activation of dioxygen and methane, whereas over the B4 site, formaldehyde formation is the rate-determining step. The computationally-determined RDS for the B4 site coincides well with the reported experiments. It is further revealed that this site also prefers the formation of CO over CO2, which is in-line with the experiments in literature. It is also shown through orbital analysis that methanol formation does not occur during methane oxidation. We employed descriptors such as condensed Fukui functions and global electrophilicity index to chemically distinct these twin sites.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

Intergrowth Zeolites, Synthesis, Characterization, and Catalysis

Microporous zeolites that can act as heterogeneous catalysts have continued to attract a great deal of academic and industrial interest, but current progress in their synthesis and application is restricted to single-phase zeolites, severely underestimating the potential of intergrowth frameworks. Compared with single-phase zeolites, intergrowth zeolites possess unique properties, such as different diffusion pathways and molecular confinement, or special crystalline pore environments for binding metal active sites. This review first focuses on the structural features and synthetic details of all the intergrowth zeolites, especially providing some insightful discussion of several potential frameworks. Subsequently, characterization methods for intergrowth zeolites are introduced, and highlighting fundamental features of these crystals. Then, the applications of intergrowth zeolites in several of the most active areas of catalysis are presented, including selective catalytic reduction of NOx by ammonia (NH3-SCR), methanol to olefins (MTO), petrochemicals and refining, fine chemicals production, and biomass conversion on Beta, and the relationship between structure and catalytic activity was profiled from the perspective of intergrowth grain boundary structure. Finally, the synthesis, characterization, and catalysis of intergrowth zeolites are summarized in a comprehensive discussion, and a brief outlook on the current challenges and future directions of intergrowth zeolites is indicated.

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal-Organic Framework

Conversion of methane (CH4) to ethylene (C2H4) and/or acetylene (C2H2) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH4 controllably, and separating C2+ products from unreacted CH4 can be challenging. Here, we report the direct conversion of CH4 to C2H4 and C2H2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal-organic framework material, MFM-300(Fe). The selectivity for the formation of C2H4 and C2H2 reaches 96% with a high time yield of 334 μmol gcat-1 h-1. At a conversion of 10%, the selectivity to C2+ hydrocarbons and time yield exceed 98% and 2056 μmol gcat-1 h-1, respectively, representing a new benchmark for conversion of CH4. In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe-O(H)-Fe sites in activating CH4 and stabilizing reaction intermediates via the formation of an Fe-O(CH3)-Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C2H4 and C2H2 from unreacted CH4 for direct use. Integrating the processes of CH4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

Selective Oxidation of Methane to Oxygenates using Oxygen via Tandem Catalysis

Selective oxidation of methane to oxygenates using low-cost and environment-friendly molecular oxygen (O₂ ) under mild reaction conditions is a promising strategy but still remains grand challenge. It is of great importance to accelerate the activation of O₂ to generate highly active oxygen species, such as hydroxyl peroxide and hydroxyl species to improve catalytic performance for selective oxidation of methane. Selective oxidation of methane using O₂ by coupling with in situ generation of hydrogen peroxide via tandem catalysis ensures the easy formation of active oxygen species for methane activation, leading to high oxygenates productivity under mild conditions. In this concept, we summarized the recent progresses for selective oxidation of methane to oxygenates using O₂ based on tandem catalysis by coupling with in situ generation of hydrogen peroxide. The remaining challenges and future perspectives for selective oxidation of methane to oxygenates via tandem catalysis were also proposed.

Integrated in situ spectroscopic studies on syngas production from partial oxidation of methane catalyzed by atomically dispersed rhodium cations on ceria

Catalytic reforming of methane to produce syngas is an important strategy for producing value-added chemicals. The conventional reforming catalyst relies on supported nickel nanoparticles. In this work, we investigated singly dispersed Rh cations anchored on a CeO₂ catalyst (Rh₁/CeO₂) for high activity and selectivity towards the production of syngas via partial oxidation of methane (POM) in the temperature range of 600-700 °C. The yields of H₂ and CO at 700 °C are 83% and 91%, respectively. The anchored Rh₁ atoms on CeO₂ of Rh₁/CeO₂ are in the cationic state, and on an average each Rh₁ atom coordinates with 4-5 surface lattice oxygen atoms of CeO₂. Compared to inert CeO₂ for POM, via the incorporation of single-atom sites, Rh₁ modifies the electronic state of oxygen atoms proximal to the Rh₁ atoms and thus triggers the catalytic activity of CeO₂. The high activity of single-atom catalyst Rh₁/CeO₂ suggests that the incorporation of single atoms of transition metals to the surface of a reducible oxide can modulate the electronic state of proximal anions of the oxide support toward forming an electronic state favorable for the selective formation of ideal products.

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH₄ ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH₄ into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH₄ at low temperatures. In this review, the efficient catalysts designed to reduce the CH₄ oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH₄ in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH₄ at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH₄ at low temperatures.

Highly Efficient and Selective Photocatalytic Nonoxidative Coupling of Methane to Ethylene over Pd-Zn Synergistic Catalytic Sites

Photocatalytic nonoxidative coupling of CH4 to multicarbon (C2+) hydrocarbons (e.g., C2H4) and H2 under ambient conditions provides a promising energy-conserving approach for utilization of carbon resource. However, as the methyl intermediates prefer to undergo self-coupling to produce ethane, it is a challenging task to control the selective conversion of CH4 to higher value-added C2H4. Herein, we adopt a synergistic catalysis strategy by integrating Pd-Zn active sites on visible light-responsive defective WO3 nanosheets for synergizing the adsorption, activation, and dehydrogenation processes in CH4 to C2H4 conversion. Benefiting from the synergy, our model catalyst achieves a remarkable C2+ compounds yield of 31.85 μmol·g-1·h-1 with an exceptionally high C2H4 selectivity of 75.3% and a stoichiometric H2 evolution. In situ spectroscopic studies reveal that the Zn sites promote the adsorption and activation of CH4 molecules to generate methyl and methoxy intermediates with the assistance of lattice oxygen, while the Pd sites facilitate the dehydrogenation of methoxy to methylene radicals for producing C2H4 and suppress overoxidation. This work demonstrates a strategy for designing efficient photocatalysts toward selective coupling of CH4 to higher value-added chemicals and highlights the importance of synergistic active sites to the synergy of key steps in catalytic reactions.

Visible‐Light‐Induced, Single‐Metal‐Catalyzed, Directed C−H Functionalization: Metal‐Substrate‐Bound Complexes as Light‐Harvesting Agents

C−H functionalization represents one of the most rapidly advancing areas in organic synthesis and is regarded as one of the key concepts to minimize the ecological and economic footprint of organic synthesis. The ubiquity and low reactivity of C−H bonds in organic molecules, however, poses several challenges, and often necessitates harsh reaction conditions to achieve this goal, although it is highly desirable to achieve C−H functionalization reactions under mild conditions. Recently, several reports uncovered a conceptually new approach towards C−H functionalization, where a single transition‐metal complex can be used as both the photosensitizer and catalyst to promote C−H bond functionalization in the absence of an exogeneous photosensitizer. In this Minireview, we will provide an overview on recent achievements in C−H functionalization reactions, with an emphasis on the photochemical modulation of the reaction mechanism using such catalysts.

Selective oxidation of CH4 to valuable HCHO over a defective rTiO2/GO metal-free photocatalyst

A specially designed metal-free rTiO2/GO catalyst retarded the recombination of photo-generated electrons and holes and improved photocatalytic CH4 conversion performance.