Non-oxidative Coupling Research Articles

Defects influence photocatalytic NOCM product selectivity by controlling photogenerative carriers

Photocatalytic Non-oxidative coupling of methane (NOCM) is a green method to convert methane into high value-added products such as ethane and ethylene by using photogenerated electron-holes. Compared with oxidative coupling of methane (OCM), besides C2 products, H2 is also produced in NOCM process. However, the effect of the photogenerative carriers on the yield and selectivity of photocatalytic NOCM products is still unclear. We tested Au1-ZnO/TiO2 synthesized by NaBH4 reduction and photodeposition in a flowing gas reactor at atmospheric pressure without external heat source. The Au1-ZnO/TiO2-N sample showed a superior C2H6 yield of 0.015 mmol/h accompanied by a small amount of H2 production (1.32 μmol/h). Meanwhile, Au1-ZnO/TiO2-p120 sample shows much higher H2 yield (0.012 mmol/h) and a lower C2H6 yield (4.96 μmol/h). Through varies characterizations, we demonstrated that the selectivity of C2H6 and H2 are highly related to photogenerative carriers. We propose that defects affect the degree of peroxidation in photocatalytic NOCM by controlling the concentration of photogenerated holes.

In Situ Formation of Ru–Sn Bimetallic Particles for Non-Oxidative Coupling of Methane

In Situ Formation of Ru–Sn Bimetallic Particles for Non-Oxidative Coupling of Methane

Pt/CeO2 as Catalyst for Nonoxidative Coupling of Methane: Oxidative Regeneration.

Direct nonoxidative coupling is a promising route for methane upgrading, yet its commercialization is hindered by the lack of efficient catalysts. Pt/CeO2 catalysts with isolated Pt species have attracted an increasing amount of interest in recent years. Herein, we studied the catalytic role and evolution of isolated Pt centers on CeO2 prepared by flame spray pyrolysis under the harsh reaction conditions of nonoxidative methane coupling. During the reaction at 800 °C, the isolated Pt sites sinter, leading to a loss of the ethylene and ethane yield. The agglomerated Pt can be redispersed by using an in situ regeneration strategy in oxygen. We found that isolated Pt centers are able to activate methane only at the initial reaction stage, and the CePt5 alloy acts as the active phase in the prolonged reaction.

Revealing Active Sites and Reaction Pathways in Methane Non-Oxidative Coupling over Iron-Containing Zeolites.

Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe3+ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe2+ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products.

Unveiling the complexity of non-oxidative coupling of methane: A simplified kinetics approach

This study presents the generation and optimization of a detailed mechanism for the non-oxidative coupling of methane (NOCM), consisting of 1,112 species and 106,877 gas-phase reactions using Reaction Mechanism Generator. A mechanism reduction, performed in the Ansys Workbench, focused on four target products and employed various reduction methods, yielding an optimal skeletal mechanism of 178 species and 9,695 gas-phase reactions. Methane conversion and species concentration were evaluated using Chemkin-pro software, accounting for six independent variables, with temperature and pressure exhibiting the most significant impact. We observed that the formation rate of light hydrocarbons is higher at the initial position of the reaction, whereas aromatic hydrocarbons form more readily as reactor length increases and temperatures rise. Therefore, tailored reactor size and reaction conditions could improve the yield of C2 products under NOCM conditions. Further, a smaller reactor size at high temperatures and low pressure may optimize the formation of light hydrocarbons. Through sensitivity analyses, we identified critical reactions for product formation and aromatic coke precursors, thereby providing insights into gas-phase non-oxidative methane conversion. Further optimization studies are needed to investigate parameter interactions, aiming for optimal conversion with higher yields of target species and minimal aromatic coke precursors. These findings can inform effective reactor design and optimization strategies.

Non-oxidative coupling of methane over Mo-doped CeO2 catalysts: Understanding surface and gas-phase processes

Direct catalytic non-oxidative coupling is a promising route for the valorization of abundant methane. Understanding the mechanism is difficult because reactions at the surface of the catalyst and in the gas phase via radicals are important at the high temperatures employed. Herein, a series of Mo-doped CeO2 samples with isolated Mo sites were prepared by flame spray pyrolysis method and screened for their performance in non-oxidative coupling of methane. The selectivity to value-added C2 hydrocarbons (ethane and ethylene) among gas-phase products could reach 98%. During the reaction, the isolated Mo-oxo species in the as-prepared catalyst are reduced and convert into Mo (oxy-)carbide species, which act as the active sites for methane activation. By varying the available catalyst-free gas volume along the length of the reactor, we studied the contribution of gas-phase reactions in the formation of different products. Ethane is the primary product of non-oxidative methane coupling and, at least, a part of ethylene and most of benzene is formed through gas-phase chemistry. This work provides insights into the design of efficient catalysts for non-oxidative coupling of methane and highlights the importance of reducing the free volume in the reactor to limit secondary gas-phase reactions.

Built in electric-field active 2D β-BN/ZIS coated water-fed photoelectrode for methane conversion into hydrogen gas and VAPs through non-oxidative coupling

The photocatalytic conversion of methane into value added products with an add on power generation offers new sustainable solution for the GHG and commercial methane reforming. In present study photo-electrochemical conversion of CH4 gas into H2 gas and other value-added products like C2H6, C3H8, and C4H10 gas under ambient conditions through non-oxidative coupling of CH3* radicals at chain termination were investigated. Interfaces between two photocenters hinder the charge mobility therefore, built-in electric field induced VDW heterojunction of 2D/2D β-BiNbO4/ZnIn2S4 is constructed to enhance the spatial separation of excitons. Additionally, to remove the bubble diffusion challenge of gas to liquid phase 2D β-BiNbO4/ZnIn2S4 coated PES Air-photoanode is adopted. It presented gas phase photocatalysis redox reaction of alkylation and significant hydrogen generation. Presence of porous morphology and polar group in PES increased the ECSA and performance of photoanode by reducing the Rs, Rct and accumulation layer impedances. Electrochemical analysis also proven the potency of VDW 2D β-BN/ZIS for substantial hydrogen generation. 14.2% light-to-hydrogen efficiency with 61% selectivity was observed which is much higher than most of the reported LTH efficiency of photo fuel cell. The light-to fuel (LTF) efficiency is more than 17.3% and 49.7 mW/m2 of power production providing an opportunity to oxidise the methane under the ambient environment into energy-rich derivatives along with electricity generation.

Photo-Driven Iron-Induced Non-Oxidative Coupling of Methane to Ethane.

Photo-driven CH4 conversion to multi-carbon products and H2 is attractive but challenging, and the development of efficient catalytic systems is critical. Herein, we construct a solar energy-driven iron ions redox cycle for combining CH4 conversion and H2 production. A photo-driven iron-induced reaction system is developed, which is efficient at selective coupling of CH4 as well as conversion of benzene and cyclohexane under mild conditions. For CH4 conversion, 94% of C2 selectivity and 8.4 μmol h-1of C2H6 formation rate is achieved. Mechanism studies reveal that CH4 coupling is induced by hydroxyl radical, which is generated by photo-driven intermolecular charge migration of Fe3+ complex. The delicate coordination structure of [Fe(H2O)5OH]2+ complex ensures the selective C-H bond activation and C-C coupling of CH4. The produced Fe2+ can be used to reduce the potential for electrolytic H2 production, and then turns back into Fe3+, forming an energy-saving and sustainable recycle system.

Au–Pd Tandem Photocatalysis for Nonoxidative Coupling of Methane toward Ethylene

Au–Pd Tandem Photocatalysis for Nonoxidative Coupling of Methane toward Ethylene

Mechanistic and microkinetic study of nonoxidative coupling of methane on Pt-Cu alloy catalysts: From single-atom sites to single-cluster sites

Desirable catalysts possessing the ability to selectively break C–H bond and controllably catalyze C–C bond formation are highly demanded for the nonoxidative coupling of methane (NOCM). Herein, a series of Pt-Cu alloy catalysts including Pt1©Cu(111), Pt2©Cu(111) and Pt3©Cu(111) are deliberately designed and systematically studied for NOCM. Density functional theory calculations reveal that the Pt1, Pt2, and Pt3 sites on Cu(111) can selectively break the C–H bond to generate CH3, CH2, and CH species, respectively. However, direct coupling of corresponding CHx (x = 3, 2, 1) to form C2H6, C2H4, and C2H2 are favorable on Pt3, Pt1, and Pt2 sites on Cu(111), respectively. The different reactivity trends of the three Pt sites mainly originate from the varying bonding abilities of CHx species at the Pt sites. Microkinetic modeling manifests that the Pt1©Cu(111) is the most active for methane dissociation (TOF = 2.98 s–1 at 1000 K) and can selectively convert methane into ethylene with the highest selectivity up to 96.2% at 750 K. Moreover, Pt1©Cu(111) also shows superb stability under reaction conditions. Overall, our studies not only provide a comprehensive understanding of the reaction mechanism of NOCM on Pt single-atom sites (SASs) and Pt single-cluster sites (SCSs) but also predict that Pt SASs are advantageous over Pt SCSs for NOCM.

Non-Oxidative Coupling of Methane: Interplay of Catalyst Interface and Gas Phase Mechanisms.

Non-oxidative coupling of methane (NOCM) is a sought-after reaction that has been studied for decades. Harsh reaction conditions (T >800°C) in the face of limited catalyst stability lead to rapid catalyst deactivation and strong coking, preventing application thus far. Recent reports have shown the significance of an interplay of catalyst nature and reaction conditions, whereas metal carbides have prevailed to play a crucial role which involves incorporation of carbidic carbon in C2Hx and aromatic products. This perspective gives an overview of proposed mechanistic pathways and considerations about experiment conditions in order to foster a rational catalyst design platform for NOCM.

Concepts of Computational Approach to Explore Heterogeneous Catalysts for Direct Methane Conversion

AbstractThe nonoxidative coupling of methane has attracted much attention because it yields two useful products: ethane and hydrogen. However, low conversion at low temperatures and carbon deposition at high temperatures are considered problematic. In this Concept article, a solution to these problems is presented. On the basis of the reaction enthalpy for the initial C−H bond cleavage of methane and the energy difference between C1 species (CH3 and CH) on the catalyst surface, a catalyst search guideline is provided to suppress carbon deposition while keeping the conversion rate as high as possible. Alloys are considered catalyst candidates. Few materials satisfy both of these requirements simultaneously; however, several alloys, including MgPt, are shown to be promising. The results of validation experiments for the catalytic performance of MgPt are also discussed.

Recent Progress of Metal-Oxide-Based Catalysts for Non-Oxidative Coupling of Methane to Ethane and Hydrogen

Methane is the fundamental raw material of the C1 chemical industry, with abundant reserves. Its direct conversion into high-value-added chemicals has great scientific significance and broad commercial potential for the efficient use of methane resources. However, it is difficult to convert methane into more useful hydrocarbons and hydrogen, as the reaction usually requires external energy to overcome thermodynamic limitations. Non-oxidative coupling of methane to produce ethane and hydrogen is a promising supply technology. Catalysts which can be adapted to various energy sources are key to this technology. In recent years, considerable progress has been made in the design and application of these thermal and photocatalysts. This review outlines some typical catalysts, and reviews the progress in the understanding of reaction mechanisms. Finally, suggestions for the development of high-selectivity and high-stability catalysts for the future are presented.

Hybrid plasma-thermal system for methane conversion to ethylene and hydrogen

By combining dielectric barrier discharge plasma and external heating, we exploit a two-stage hybrid plasma-thermal system (HPTS), i.e., a plasma stage followed by a thermal stage, for direct non-oxidative coupling of CH4 to C2H4 and H2, yielding a CH4 conversion of ca. 17 %. In the two-stage HPTS, the plasma first converts CH4 into C2H6 and C3H8, which in the thermal stage leads to a high C2H4 selectivity of ca. 63 % by pyrolysis, with H2 selectivity of ca. 64 %.

Efficiently Light-Driven Nonoxidative Coupling of Methane on Ag/NaTaO3: A Case for Molecular-Level Understanding of the Coupling Mechanism

Metal-decorated oxide semiconductors are overwhelming photocatalysts for nonoxidative coupling of methane (NOCM). However, the overall NOCM mechanism remains an unopened black box, which hinders the design of high-performance catalysts. Herein, we systematically studied a series of noble metal (Ag, Au, Pt, Pd, Cu, and Ni)-decorated oxides (NaTaO3, CaTiO3, LiNbO3, and TiO2) for NOCM. We proposed that the active sites for H abstraction and C–C coupling of CH4 are spatially separated. Specifically, NaTaO3 only completes the initial H abstraction of CH4 activation, while metal nanoparticles are responsible for the final C–C coupling. Noble metals dominate NOCM by significantly decreasing the energy barrier of CH4 dissociation and promoting C–C coupling. Among various metals, Ag is preferential for the weak adsorption of ·CH3 intermediates and subsequent metal-induced C–C coupling. This contributes to Ag/NaTaO3 the highest C2H6 yield of 194 μmol g–1 h–1 and stoichiometric H2 with 11.2% quantum efficiency. This work provides a molecular-level insight into the CH4 coupling mechanism on metal-decorated photocatalysts.

Nonoxidative Coupling of Methane to Produce C2 Hydrocarbons on FLPs of an Albite Surface.

The characteristics of active sites on the surface of albite were theoretically analyzed by density functional theory, and the activation of the C-H bond of methane using an albite catalyst and the reaction mechanism of preparing C2 hydrocarbons by nonoxidative coupling were studied. There are two frustrated Lewis pairs (FLPs) on the (001) and (010) surfaces of albite; they can dissociate H2 under mild conditions and show high activity for the activation of methane C-H bonds. CH4 molecules can undergo direct dissociative adsorption on the (010) surface, whereas a 50.07 kJ/mol activation barrier is needed on the (001) surface. The prepared albite catalyst has a double combination function of the (001) and (010) surfaces; these surfaces produce a spillover phenomenon in the process of CH4 activation reactions, where CH3 overflows from the (001) surface with CH3 adsorbed on the (010) surface to achieve nonoxidative high efficiently C-C coupling with an activation energy of 18.51 kJ/mol. At the same time, this spillover phenomenon inhibits deep dehydrogenation, which is conducive to the selectivity of the C2 hydrocarbons. The experimental results confirm that the selectivity of the C2 hydrocarbons is maintained above 99% in the temperature range of 873 K to 1173 K.

Design of SA-FLP Dual Active Sites for Nonoxidative Coupling of Methane

Direct conversion of methane to value-added chemical products under nonoxidative conditions is one of the most effective routes but still faces eminent challenges due to thermodynamic constraints and the lack of efficient catalysts. Herein, we propose to construct “Single-Atom”-“Frustrated Lewis Pair” (SA-FLP) dual-active-site catalysts for nonoxidative coupling of methane (NOCM). The single-atom site is created by doping a Pt atom at the Ce site of the CeO2 surface. The FLP site is fabricated by removing oxygen atom(s) adjacent to Pt atoms. Density functional theory (DFT) calculations reveal that SA-FLP dual active sites can simultaneously activate two methane molecules and notably enhance the coupling of hydrocarbon species to generate C2 products. The SA-FLP sites with two oxygen vacancies show the best performance for methane activation with a low energy barrier of 0.32 and 0.71 eV at SA and FLP sites, respectively. The coupling of two methyl groups to further generate ethane and ethylene only needs to surpass the highest barrier of 1.31 eV. Microkinetic analysis demonstrates that on the designed SA-FLP sites, CH4 consumption can reach a high turnover frequency (TOF) of 0.3014 s–1 under the conditions of 1200 K and a CH3 partial pressure of 8.0 × 10–3 bar, which is nearly two orders of magnitude higher than the experimentally reported value (3.8 ∼ 5.5 × 10–3 s–1) on traditional Pt/CeO2 catalysts. Importantly, the main product on the SA-FLP sites is shown to be the desired ethane with a TOF of 0.2535 s–1 under the conditions mentioned above. This study not only provides a strategy for designing efficient catalysts for NOCM but also offers insights into C–C coupling to generate oriented C2 products.

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.

Oxygen vacancies stabilized Ag+to enhance the performance of an Ag/In2O3photocatalyst for non-oxidative coupling of methane

Introducing Ag+into In2O3can promote photocatalytic oxidative coupling of methane.

Role and dynamics of transition metal carbides in methane coupling.

Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to the popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the active participation of carbidic carbon in the formation of C2 products during methane coupling at high temperature that is associated with the dynamics of Mo and W carbides. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with methane (gas phase carbon). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study evidences the presence of a carbon equivalent to the Mars-Van Krevelen type mechanism for non-oxidative coupling of methane.