Non-oxidative Methane Coupling Research Articles

Plasma-driven non-oxidative coupling of methane to ethylene and hydrogen at mild temperature over CuxO/CeO2 catalyst

We report one-step non-oxidative coupling of methane (CH4) to ethylene (C2H4) at atmospheric pressure and mild temperature (ca. 180–190 °C), by a combination of non-thermal plasma and a CuOx/CeO2 catalyst. The C2H4 selectivity gradually increases during an induction period. The corresponding spent catalysts at different stages were systematically characterized to disclose the evolution of the CuOx/CeO2 catalyst. During the induction period, the CuO/CeO2 catalyst was partially reduced to generate Cu+, Ce3+ and Ov species, which accompany the formation of Cu+-Ov-Ce3+ sites, as proven by XRD, HRTEM, XPS, Raman, EPR and H2-TPR. In addition, the C2H4 selectivity is proportional to the fraction of Cu+, Ce3+, Ov and Cu-O-Ce species, which indicates that Cu+-Ov-Ce3+ is the active site for non-oxidative coupling of CH4 to C2H4. Furthermore, in-situ FTIR results indicate that the Cu+-Ov-Ce3+ interface sites can promote dehydrogenation of CH3* (from CH4 plasma) to produce CH2* on the catalyst surface, which is the basic reason why CuOx/CeO2 acts as a catalyst in speeding up the non-oxidative coupling of CH4 for C2H4 production.

Vitreous silica supported metal catalysts for direct non-oxidative methane coupling

Direct non-oxidative methane coupling (NMC) transforms methane into valuable chemicals and hydrogen, which is a promising pathway to boost industrial decarbonization. The reaction, however, is challenged by high C-H bond activation temperature, catalyst coking, and low selectivity towards hydrocarbon products. Here we studied the efficacy of five vitreous silica supported 3d transition metal (Mn, Fe, Co, Ni, and Cu) catalysts (denoted as M/SiO2-v) for NMC. These catalysts were prepared by flame fusion of mixtures of quartz and metal silicate precursors. The metal species in the as-synthesized catalysts were fully or partially oxidized, with their stability following the order of Mn > Fe ≈ Co > Ni > Cu in the NMC reaction. The Cu species has low methane reaction rate, due to the high adsorption and dissociation energies of methane. The Mn and Ni species have high methane reaction and coke formation rates, which is supported by easy kinetics of methane deep dehydrogenation to carbon. In contrast, Fe and Co species showed the best hydrocarbon product selectivity. Particularly, the Co species emerged as the optimal catalyst for NMC, showcasing a balanced methane activation, hydrocarbon formation, and low coke yield. The relationship between the evolved metal species under reaction conditions and their NMC performance was discussed. The present study screens and provides effective catalyst formulations for NMC reactions.

Hybrid plasma catalysis-thermal system for non-oxidative coupling of methane to ethylene and hydrogen

We report a two-stage hybrid plasma catalysis-thermal system for non-oxidative coupling of methane (NOCM) to ethylene (C2H4) and hydrogen (H2), achieving 66 % C2H4 selectivity and 60 % H2 selectivity with 28 % CH4 conversion. This corresponds to a C2H4 yield of 18 %, which is one of the highest reported in literature. The system consists of the first plasma catalysis stage (stage 1) and the second thermal cracking stage (stage 2). Comprehensive analyses using in-situ mass spectrometry (MS) and gas chromatography (GC) reveal that the combination of plasma and Pt/ZrO2 catalyst in stage 1 predominantly facilitates the conversion of methane to ethane. Characterizations employing X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy-scanning transmission electron microscopy-energy dispersive X-ray (HRTEM-STEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR), indicate that the coordinatively unsaturated site of Pt could be the active sites for C–H bond cracking to generate abundant CH3 radicals (CH3·), enhancing the C2 selectivity by inhibiting the non-selective formation of coke and higher hydrocarbons. Subsequently, the products from stage 1 (especially C2H6) undergo pyrolysis reactions at 880 °C in stage 2, which further boosts the C2H4 selectivity and yield. Based on the reaction performance, catalyst characterization, optical emission spectroscopy (OES), and in-situ Fourier transform infrared spectroscopy (FTIR) results, we reveal the reaction mechanism within the hybrid two-stage plasma catalysis-thermal system for converting CH4 to C2H4 and H2.

Non-Oxidative Coupling of Methane via Plasma-Catalysis Over M/γ-Al2O3 Catalysts (M = Ni, Fe, Rh, Pt and Pd): Impact of Active Metal and Noble Gas Co-Feeding

Plasma-catalysis has attracted significant interest in recent years as an alternative for the direct upgrading of methane into higher-value products. Plasma-catalysis systems can enable the electrification of chemical processes; however, they are highly complex with many previous studies even reporting negative impacts on methane conversion. The present work focuses on the non-oxidative plasma-catalysis of pure methane in a Dielectric Barrier Discharge (DBD) reactor at atmospheric pressure and with no external heating. A range of transition and noble metals (Ni, Fe, Rh, Pt, Pd) supported on γ-Al2O3 are studied, complemented by plasma-only and support-only experiments. All reactor packings are investigated either with pure methane or co-feeding of helium or argon to assess the role of noble gases in enhancing methane activation via energy transfer mechanisms. Electrical diagnostics and charge characteristics from Lissajous plots, and electron temperature and collision rates calculations via BOLSIG+ are used to support the findings with the aim of elucidating the impact of both active metal and noble gas on the reaction pathways and activity. The optimal combination of Pd catalyst and Ar co-feeding achieves a substantial improvement over non-catalytic pure methane results, with C2+ yield rising from 30% to almost 45% at a concurrent reduction of energy cost from 2.4 to 1.7 \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\ ext{M}\ ext{J}\\:{\ ext{m}\ ext{o}\ ext{l}}_{\ ext{C}{\ ext{H}}_{4}}^{-1}$$\\end{document} and from 9 to 4.7 \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\ ext{M}\ ext{J}\\:\ ext{m}\ ext{o}{\ ext{l}}_{{\ ext{C}}_{2+}}^{-1}$$\\end{document}. Pd, along with Pt, further displayed the lowest coke deposition rates among all packings with overall stable product composition during testing.

Tandem dual-heterojunctions in Au/ZnGa2O4/ZnO for promoted photocatalytic nonoxidative coupling of methane

Rational design of photocatalysts to enhance the photocatalytic non-oxidative coupling of methane (NOCM) activity remains a great challenge. Herein, a composite photocatalyst Au/ZnGa2O4/ZnO (Au/ZGO/ZnO) with tandem heterojunctions was prepared by a hydrothermal method and impregnation reduction method for the photocatalytic NOCM reaction. The tandem heterojunctions effectively promote the migration of photogenerated electrons of ZGO and ZnO through the ZGO/ZnO S-scheme heterojunction and the Au/ZnO Schottky junction to Au active sites, significantly enhancing the photocatalytic NOCM activity. The C2H6 and H2 yields of the optimized photocatalyst Au0.75/ZGO/ZnO-15 are 12.2 and 10.3 μmol·g−1, with a ratio close to 1:1, within 4 hours under simulated solar light irradiation. The C2H6 production of Au0.75/ZGO/ZnO-15 is 4.5 and 21.4 times higher than that of Au0.75/ZnO and Au0.75/ZGO, respectively. This work provides a strategy for enhancing the photocatalytic NOCM of semiconductor photocatalysts and marks an important step toward the design and synthesis of dual-heterostructures composite photocatalysts.

Metaheuristics-guided active learning for optimizing reaction conditions of high-performance methane conversion

Converting greenhouse gases into value-added chemical compounds has been widely studied in chemical science and engineering for sustainable industry. In particular, nonoxidative coupling of methane (NOCM) that transforms methane into useful chemical compounds has received great interests because methane is a naturally abundant greenhouse gas. NOCM has received significant attention in sustainable industry for addressing the climate crisis. However, there are complex optimization problems in maximizing the efficiency of the nonoxidative methane conversion. Although conventional data-driven methods have been successfully applied to various engineering problems, a data-driven optimization of NOCM remains a challenging problem because expensive chemical experiments should be performed to collect prior data for model training. To avoid the expensive costs of chemical experiments, we propose an active learning method that performs training data augmentation and model re-training without pre-defined unlabeled experimental data. To this end, we combine active learning with metaheuristic algorithms to perform active learning with statistically augmented data. We applied the proposed method to a high-throughput screening task to discover new reaction conditions of high-performance NOCM, and the high-throughput screening error was significantly reduced by 69.11%.

Non-Oxidative Coupling of Methane Catalyzed by Heterogeneous Catalysts Containing Singly Dispersed Metal Sites

Direct upgrading of methane into value-added products is one of the most significant technologies for the effective transformation of hydrocarbon feedstocks in the chemical industry. Both oxidative and non-oxidative methane conversion are broadly useful approaches, though the two reaction pathways are quite distinguished. Oxidative coupling of methane (OCM) has been widely studied, but suffers from the low selectivity to C2 hydrocarbons because of the overoxidation leading to undesired byproducts. Therefore, non-oxidative coupling of methane is a worthy alternative approach to be developed for the efficient, direct utilization of methane. Recently, heterogeneous catalysts comprising singly dispersed metal sites, such as single-atom catalysts (SAC) and surface organometallic catalysts (SOMCat), have been proven to be effectively active for direct coupling of methane to product hydrogen and C2 products. In this context, this review summarizes recent discoveries of these novel catalysts and provides a perspective on promising catalytic processes for methane transformation via non-oxidative coupling.

Effect of Hydrogen Addition on Coke Formation and Product Distribution in Catalytic Coupling of Methane.

The effect of hydrogen addition on catalytic nonoxidative coupling of methane at 1000 °C was investigated. Experiments were performed at varying ratios between the catalyst and the postcatalytic volume to discern the effect of hydrogen on the catalytic reaction as well as on the gas-phase reaction. Adding 10% H2 decreases the methane conversion by a factor of 2, almost independent of the ratio between the catalyst and the postcatalytic residence time. The effect on the conversion is mostly determined by gas-phase chemistry. Hydrogen addition has no influence on the C2 hydrocarbon yield, whereas aromatic selectivity is significantly reduced. Changes in selectivity are attributed to changes in methane conversion. Quantitative determination of the amount of coke deposited on the catalyst reveals a decrease by 1 order of magnitude when dosing up to 10% H2, while carbon deposits-downstream of the catalyst bed are suppressed to a much lower extent. These results suggest a process in which the produced hydrogen is partly recycled, maximizing the carbon selectivity to C2 hydrocarbons while minimizing both aromatics and, most crucially, formation of coke on the catalyst as well as further deposits-downstream.

Unraveling the C–C coupling mechanism on Ni–O–Fe asymmetric sites for photocatalytic nonoxidative coupling of methane

Photocatalytic non-oxidative coupling of methane is an attractive strategy for CH4 to C2 conversion, how to facilitate the photogenerated charge separation and accelerate the C–C coupling to promote the reaction kinetics remains a great challenge. Herein, a symbiotic r-Fe2O3/NiO heterostructure with robust interfaces is constructed in situ. The coherent interface not only provides unique Ni–O–Fe asymmetric sites, but also achieves the separation and transport of photogenerated charge. The unbalanced charge density distribution of two adjacent methyl or methylene at the Ni–O–Fe asymmetric site alleviates the dipole repulsion of the C–C coupling, and promotes C–C coupling to form C2 hydrocarbons. Under visible light irradiation, the optimal r-Fe2O3/NiO performs a brilliant C2 activity up to 71.00 μmol g−1h−1, and the C2H4 yield can reach a high value of 40.46 μmol g−1h−1 and the selectivity of 56.98 %. This work provides new insights into the coupling of C–C to generate C2 hydrocarbons.

Wall-trapped acetylene tetramer in a metal-organic framework enables kinetic separation of C2H2/C2H4

Effective separation of acetylene/ethylene (C2H2/C2H4) mixtures by adsorption is a challenging but green method given their similar molecular characters. An increasing number of metal-organic frameworks (MOFs) have addressed the diffusion-related challenges to achieve the separation, yet almost all of them just focus on diffusion rates and disregard the geometrical arrangement of adsorbed molecules which exerts important effects in boosting the diffusion behaviors. Herein, we now present a paradigm using an ultramicroporous zinc-MOF (Zn-MOF, referred to as 1) with assessable size of 3.4 × 3.3 Å2, as demonstrated by single-crystal and theoretical analysis, which realizes the effective separation of C2H2 from its competitors. Static adsorption isotherms indicate that 1 possesses higher C2H2 uptake of 28.4 cm3 g−1 at 298 K and 0.1 bar, achieving a high IAST selectivity of 265 among the benchmark MOFs. Theoretical calculations reveal that C2H2 molecules are grasped through multiple binding interactions and adsorbed in the pore surface rather than the center of the pore, thus forming C2H2 tetramer through connecting four C2H2 motifs. Further, time-dependent kinetic tests coupled with molecular dynamics confirm the faster diffusion behavior for C2H2, mainly attributing to unimpeded diffusion channel. Column breakthrough tests demonstrate that 1 can easily accomplish exclusive separation of C2H2 from various binary ethane dehydrogenation (EDH) and senary non-oxidative coupling of methane (non-OCM) byproducts. Specially, structurally stable 1 can be readily synthesized utilizing inexpensive raw materials, yielding the cost of only $614 per kilogram. Accordingly, this work proposes a novel strategy of wall-trapped C2H2 tetramer in MOF that can effectively promote the kinetic behavior through available diffusion channel, expecting to offer a new perspective and provide guidance for future research.

Catalytic reactivity of Pt sites for non-oxidative coupling of methane (NOCM)

Recently, single atom catalysts have been suggested as stable and non-coking catalysts for non-oxidative coupling of methane (NOCM) to ethylene and higher hydrocarbons. In this work, reaction kinetics coupled with microscopy and spectroscopy technique were used to understand the nature of the Pt single atom active sites under reaction conditions relevant to industrial applications of NOCM. Under NOCM conditions (975C, Space velocity = 30 L hr-1 gcat-1), single atom Pt supported on CeO2 (Pt SAs) and Pt nanoparticles (∼2nm) supported on SiO2 (Pt NPs) show similar ethylene formation rates and show similar gas phase selectivity to major and minor products like ethylene (∼60 %), ethane (∼20 %) and higher hydrocarbons (propylene, butene, etc.), suggesting that the single sites do not offer a superior performance as claimed in the literature. Power rate law reaction orders with respect to methane, ethylene and hydrogen show similar results for the two catalytic systems (nCH4 = 1, nC2H4 = 0 and nH2 = −0.5), indicating a surface-initiated gas phase radical mechanism for NOCM reactions. Transmission electron microscopy (TEM) of the spent samples (CH4 treated at 975C for 3 h) reveals that materials containing Pt (Pt SAs and Pt NPs) are found to sinter to particles approximately 5–7 nm in size, suggesting that the single atoms do not survive NOCM reaction conditions. Ethylene hydrogenation and CO-IR spectroscopy on fresh and spent catalysts suggest loss of surface Pt sites by coking following NOCM. These results shed light on the nature of the active sites during non-oxidative coupling of methane at industrially relevant conditions and motivate further research on catalyst development for this reaction.

High photocatalytic yield in the non-oxidative coupling of methane using a Pd-TiO2 nanomembrane gas flow-through reactor.

The photocatalytic non-oxidative coupling of methane (NOCM) is a highly challenging and sustainable reaction to produce H2 and C2+ hydrocarbons under ambient conditions using sunlight. However, there is a lack of knowledge, particularly on how to achieve high photocatalytic yield in continuous-flow reactors. To address this, we have developed a novel flow-through photocatalytic reactor for NOCM as an alternative to the conventionally used batch reactors. Me/TiO2 photocatalysts, where Me = Au, Ag and Pd, are developed, but only those based on Pd are active. Interestingly, the preparation method significantly impacts performance, going from inactive samples (prepared by wet impregnation) to highly active samples (prepared by strong electrostatic adsorption - SEA). These photocatalysts are deposited on a nanomembrane, and the loading effect, which determines productivity, selectivity, and stability, is also analysed. Transient absorption spectroscopy (TAS) analysis reveals the involvement of holes and photoelectrons after charge separation on Pd/TiO2 (SEA) and their interaction with methane in ethane formation, reaching a production rate of about 1000 μmol g-1 h-1 and a selectivity of almost 95% after 5 hours of reaction. Stability tests involving 24 h of continuous irradiation are performed, showing changes in productivity and selectivity to ethane, ethylene and CO2. The effect of a mild oxidative treatment (80 °C) to extend the catalyst's lifetime is also reported.

Understanding Au facet effects in photocatalytic nonoxidative coupling of methane

The strong facet-dependent activity of Au co-catalysts on photocatalytic nonoxidative coupling of methane was studied. Au octahedra with preferentially exposed (111) facets showed the best performance for this reaction.

The Effect of Structural Change during the Activation Process on the Catalysis of In/SiO2 Nonoxidative Coupling of Methane: An Operando XAFS Study

The Effect of Structural Change during the Activation Process on the Catalysis of In/SiO₂ Nonoxidative Coupling of Methane: An <i>Operando</i> XAFS Study

Reduced CuWO4 photocatalysts for photocatalytic non-oxidative coupling of methane reaction

Thermal reduction of the sol-gel derived CuWO4 in an H2-containing atmosphere can drive the Cu phase segment and yield WO3 or oxygen-deficient WO3−x at the same time. The in-situ formed Cu-WO3−x composite owns the advantages of stronger visible light absorption and an in-situ built metal-metal oxide interface, which are beneficial for utilizing the surface plasmon resonance effect of the Cu nanoparticles and providing catalytic active sites. The photocatalytic activity of the non-oxidative coupling of methane is greatly enhanced compared to that of the pristine CuWO4 and physically mixed CuO+WO3, due to the abundant oxygen vacancies and the additional visible light absorbance brought by the Cu nanoparticles

Facilitating methane conversion and hydrogen evolution on platinized gallium oxide photocatalyst through liquid-like water nanofilm formation

The dehydrogenative coupling of methane, an approach aimed at converting methane into valuable chemicals like ethane and hydrogen, can be facilitated under ambient thermal conditions using semiconductor photocatalysts. Herein, we present an efficient continuous-flow photocatalytic system for methane conversion by inducing the formation of physically adsorbed water nanofilms on the surface of a Pt-loaded Ga2O3 photocatalyst. The microscopic properties of the adsorbed water were examined by diffuse reflectance infrared (IR) spectroscopy and ab initio molecular dynamics (AIMD) calculations. In comparison to the photocatalytic non-oxidative coupling of methane operated in the absence of water vapor, the presence of the interfacial water resulted in a 90-fold and 500-fold enhancement in the production rate of hydrogen and ethane, respectively. The rate of ethane formation significantly increased with increasing water vapor pressure (PH2O) under a high methane pressure (PCH4) of 200 kPa. The selectivity for ethane reached 65 % based on carbon content at PH2O = 3 kPa and PCH4 = 200 kPa. Through diffuse reflectance IR spectroscopy, volumetric water vapor adsorption analysis, and AIMD calculations conducted under the reaction conditions, we observed the formation of a liquid-like water nanolayer on the Ga2O3 surface at 300 K. Thus, this strategy of inducing water nanofilms affords a highly efficient photocatalytic system for hydrogen and ethane production.

Photocatalytic nonoxidative coupling of methane to ethylene over carbon-doped ZnO/Au catalysts

Photocatalytic nonoxidative coupling of methane to ethylene over carbon-doped ZnO/Au catalysts

Lithium Promotes Acetylide Formation on MgO During Methane Coupling Under Non-Oxidative Conditions.

A prototypical material for the oxidative coupling of methane (OCM) is Li/MgO, for which Li is known to be essential as a dopant to obtain high C2 selectivities. Herein, Li/MgO is demonstrated to be an effective catalyst for non-oxidative coupling of methane (NOCM). Moreover, the presence of Li is shown to favor the formation of magnesium acetylide (MgC2 ), while pure MgO promotes coke formation as evidenced by solid-state 13 C NMR, thus indicating that Li promotes C-C bond formation. Metadynamic simulations of the carbon mobility in MgC2 and Li2 C2 at the density functional theory (DFT) level show that carbon easily diffuses as a C2 unit at 1000 °C. These insights suggest that the enhanced C2 selectivity for Li-doped MgO is related to the formation of Li and Mg acetylides.

Lithium Promotes Acetylide Formation on MgO During Methane Coupling Under Non‐Oxidative Conditions

AbstractA prototypical material for the oxidative coupling of methane (OCM) is Li/MgO, for which Li is known to be essential as a dopant to obtain high C2 selectivities. Herein, Li/MgO is demonstrated to be an effective catalyst for non‐oxidative coupling of methane (NOCM). Moreover, the presence of Li is shown to favor the formation of magnesium acetylide (MgC2), while pure MgO promotes coke formation as evidenced by solid‐state 13C NMR, thus indicating that Li promotes C−C bond formation. Metadynamic simulations of the carbon mobility in MgC2 and Li2C2 at the density functional theory (DFT) level show that carbon easily diffuses as a C2 unit at 1000 °C. These insights suggest that the enhanced C2 selectivity for Li‐doped MgO is related to the formation of Li and Mg acetylides.

Nonoxidative methane coupling in a micro‐DBD with enhanced secondary electron emission

AbstractThe conversion of methane into transportable and storable materials is crucial in the petrochemical sector. In this study, a specially constructed AC‐driven dielectric barrier discharge (DBD) that has charge injector pyramids on one of the electrodes and runs at ambient temperature and pressure was used to evaluate noncatalytic methane conversion. The obtained result was compared with the traditional flat electrode DBD. It was discovered that the product selectivity in direct nonoxidative methane conversion depended on the discharge conditions. Pyramid electrode plasma sources convert methane up to 50 more than flat electrode plasma due to the appearance of more microdischarges. Pyramid electrode plasma generally has a greater production efficiency than flat electrode plasma, while requiring more operational power. The turnkey solutions offered by the sustainable methane coupling method discussed here may be advantageous for the long‐term small‐scale ethylene exploration scenario.