Non-oxidative Methane Coupling Research Articles

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.

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.

Oxygen vacancy self-doped single crystal-like TiO 2 nanotube arrays for efficient light-driven methane non-oxidative coupling

Photocatalytic non-oxidative coupling of methane (PNOCM) is a mild and cost-effective method for the production of multicarbon compounds.&nbsp;However, the separation of photogenerated charges and activation of methane are the main challenges for this reaction. Here, single crystal-like TiO₂&nbsp;nanotubes (V_O-p-TNT) with oxygen vacancies (V_O) and preferential orientation were prepared and applied to PNOCM. The results demonstrate that the&nbsp;significantly enhanced photocatalytic performance is mainly related to the strong synergistic effect between preferential orientation and V_O. The preferential orientation of V_O-p-TNT along [001] direction reduces the formation of complex centers at grain boundaries as the form of interfacial states and potential barriers, which improves the separation and transport of photogenerated carriers. Meanwhile, V_O&nbsp;provides abundant coordination unsaturated sites for methane chemisorption and also acts as electron traps to hinder the recombination of electrons and holes, establishing an effective electron transfer channel between adsorbed CH₄&nbsp;molecule and photocatalyst, thus weakening C–H bond. In addition, the introduction of V_O&nbsp;broadens the light absorption range. As a result, V_O-p-TNT exhibits excellent PNOCM performance and provides new insights into catalyst design for CH₄&nbsp;conversion.

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.

Elucidation of the CH4 coupling mechanism to C2H4 over GaN catalysts under non-oxidative conditions

The second C−H bond cleavage in methane to form methylene (CH2*) from methyl (CH3*) is the rate-limiting step for the direct non-oxidative methane coupling to ethylene over gallium-nitride catalysts. Herein, we use isotope labelled experiments, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), 13C solid state-NMR, as well as kinetic modelling to elucidate the reaction mechanism. Our experiments showed that supported GaN/SBA-15 had more aliphatic surface intermediates (CH3* and CH2*) than unsupported GaN catalysts, which had higher amounts of aromatics intermediates and coke deposition. Isotope labelling experiments and the kinetic study confirmed that the most likely pathway is via the fast abstraction of the first H from CH4 to form methyl (CH3*) surface intermediates, followed by the slow second C−H bond cleavage to form methylene (CH2*) and subsequent coupling to ethylene. The intrinsic activation energy predicted by kinetic modelling was rather low at around 20 kJ mol−1.

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.