Catalysts For Coupling Of Methane Research Articles

Influence of Dopants on Pt/Al2O3-Based Monolithic Catalysts for Autothermal Oxidative Coupling of Methane

Autothermal oxidative coupling of methane (OCM) is a highly attractive approach for methane utilization. If platinum-based catalysts are operated in short-contact-time reactors with high space velocities, high methane conversion can be achieved. Using a 1 wt.% Pt/Al2O3 catalyst as a benchmark, the present study elucidates how different dopants, namely Ni, Sn, and V2O5, affect the OCM reaction. Kinetic catalyst tests reveal that acetylene (C2H2) is the predominant C2 product, irrespective of the catalyst formulation or operation conditions. Furthermore, the use of bimetallic catalysts allows either for the maintenance or even the improvement of the C2 selectivity during OCM, which is attributed to synergistic effects that occur when expensive Pt is partially replaced by cheaper dopants. In particular, the 1 wt.% Pt/Al2O3 reference catalyst yielded a maximum C2 selectivity of 8.2%, whereas the best-performing doped sample 0.25 wt.% Pt 0.75 wt.% V2O5/Al2O3 yielded a total C2 selectivity of 11.3%.

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.

Phosphate-containing alkali metal-doped TbOx/MgO catalysts for the oxidative coupling of methane

Addition of phosphate to alkali-metal (AM) doped terbium oxide (TbOx) supported on MgO was shown to improve both activity (methane conversion) and selectivity to C2+ products (ethane, ethylene and higher hydrocarbons) in the oxidative coupling of methane at reaction temperatures above 600 °C. The largest improvement upon phosphate addition was observed over the K-doped TbOx/MgO catalyst, and the K(PO4)y-TbOx/MgO catalyst is not very sensitive to K content (between K:Tb ratios of 1.2 and 2.0). Above 700 °C, the highest activity and C2+ selectivity were observed over the Li(PO4)y-TbOx/MgO catalysts. It was shown that TbOx is a necessary component and Tb2O3 is essential for a high activity and selectivity over these catalysts. Moreover, the C2H4 yield is significantly improved over the Li(PO4)y-TbOx/MgO catalysts, and the stability with time-on-stream higher, compared with the Li-TbOx/MgO catalyst, as long as the lithium and phosphate concentrations are not too high.

Effect of calcination temperature on the performance of K-Co/Al2O3 catalyst for oxidative coupling of methane

The oxidative coupling of methane (OCM) involves directly converting methane to C2+ hydrocarbons (such as ethylene and ethane) via a reaction with oxygen. This study elucidated the effect of the calcination temperature on the structure and catalytic performance of potassium-doped-cobalt oxide supported on an alumina (K-Co/Al2O3) catalyst for the OCM reaction. The catalyst was highly active at relatively low reactor temperatures (500–640 °C). Four calcination temperatures (400, 500, 600, and 700 °C) were investigated, with the results showing that the calcination temperature strongly affected catalytic properties, such as the crystalline phases, elemental distribution, physical properties, and catalytic basicity, leading to a wide range in catalytic performances. The catalyst calcined at 400 °C was superior among the catalysts, with 8.3 % C2+ yield, 24.8 % C2+ selectivity, and 33.6 % CH4 conversion at 640 °C. Furthermore, the catalyst was robust over 24 h of testing.

MonteCat: A Basin-Hopping-Inspired Catalyst Descriptor Search Algorithm for Machine Learning Models.

Proposing relevant catalyst descriptors that can relate the information on a catalyst's composition to its actual performance is an ongoing area in catalyst informatics, as it is a necessary step to improve our understanding on the target reactions. Herein, a small descriptor-engineered data set containing 3289 descriptor variables and the performance of 200 catalysts for the oxidative coupling of methane (OCM) is analyzed, and a descriptor search algorithm based on the workflow of the Basin-hopping optimization methodology is proposed to select the descriptors that better fit a predictive model. The algorithm, which can be considered wrapper in nature, consists of the successive generation of random-based modifications to the descriptor subset used in a regression model and adopting them depending on their effect on the model's score. The results are presented after being tested on linear and Support Vector Regression models with average cross-validation r2 scores of 0.8268 and 0.6875, respectively.

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.

Lithium carbonate-promoted mixed rare earth oxides as a generalized strategy for oxidative coupling of methane with exceptional yields

The oxidative coupling of methane to higher hydrocarbons offers a promising autothermal approach for direct methane conversion, but its progress has been hindered by yield limitations, high temperature requirements, and performance penalties at practical methane partial pressures (~1 atm). In this study, we report a class of Li2CO3-coated mixed rare earth oxides as highly effective redox catalysts for oxidative coupling of methane under a chemical looping scheme. This catalyst achieves a single-pass C2+ yield up to 30.6%, demonstrating stable performance at 700 °C and methane partial pressures up to 1.4 atm. In-situ characterizations and quantum chemistry calculations provide insights into the distinct roles of the mixed oxide core and Li2CO3 shell, as well as the interplay between the Pr oxidation state and active peroxide formation upon Li2CO3 coating. Furthermore, we establish a generalized correlation between Pr4+ content in the mixed lanthanide oxide and hydrocarbons yield, offering a valuable optimization strategy for this class of oxidative coupling of methane redox catalysts.

Robust data curation for improved kinetic modeling in oxidative coupling of methane using high-throughput reactors

High-throughput catalytic reactors are useful for developing large datasets of kinetic results and discovering faster new catalysts and models. However, these large datasets are often challenging to curate, and here we present a methodology for reinforced outlier detection using a robustified principal component analysis with a minimum covariance determinant. We estimate the kinetic parameters of three Mn-Na-W catalysts for the oxidative coupling of methane using datasets curated in different ways. These catalysts differ in their support composition (SiO2 and SiC), preparation method (impregnation or spray-dying), and performance. The kinetic model subsequently trained consists of six global reactions, and the obtained fit demonstrates that the proposed data curation method is a useful tool for enhancing the predictions of subsequent models and gaining further insight into underlying correlations within the data, especially when the aesthetic curation of data is too resource-demanding and/or biased.

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.

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.

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.

High‐Performance Mn2O3‐Na2WO4/SiO2‐TiO2 Catalyst for the Oxidative Coupling of Methane: TiO2‐Modulated MnTiO3 Formation for Enhanced Low‐Temperature Performance

AbstractAs a direct conversion route for the production of ethylene from methane, the oxidative coupling of methane (OCM) is highly promising. The Mn2O3‐Na2WO4/SiO2 catalyst doped with 44 μm anatase‐TiO2, consisting of 5 wt% TiO2, 5 wt% Mn2O3, 8 wt% Na2WO4 and balance SiO2 was prepared, achieving ∼22 % CH4 conversion with ∼72 % selectivity towards C2−C3 hydrocarbons even at the relatively low temperature of 700 °C. This catalyst was found to run stably for at least 300 h without signs of deactivation, which provides promising and comparable low‐temperature activity and selectivity but a more cost‐effective alternative to the previously‐reported Mn2O3‐Na2WO4/Ti‐MWW catalyst containing expensive Ti‐MWW zeolite. XRD and Raman analyses revealed that the in situ formation of MnTiO3 in the TiO2‐doped catalyst is responsible for the gains in low‐temperature OCM activity. In addition, TPR and TPO results also indicated that MnTiO3 activates O2 more readily at ∼650 °C, while three‐step switchover experiments clearly demonstrated that MnTiO3 could trigger facile redox behavior at low reaction temperatures. Thus, the MnTiO3‐induced redox cycle for O2 activation enables this TiO2‐doped catalyst to provide superior CH4 conversion at 700 °C.

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.

Lanthanum based catalysts for oxidative coupling of methane: Effect of morphology and structure

Lanthanum based catalysts for oxidative coupling of methane: Effect of morphology and structure

Synthesis of NaKCO3@(Mn-Na2 WO4/SiO2) core-shell catalyst for oxidative coupling of methane

Oxidative Coupling of Methane (OCM) is a process to produce ethylene as one of the most widely used petrochemicals from methane natural gas. The core–shell nano-catalyst of NaKCO3@(Mn-Na2WO4/SiO2) was prepared with different ratios of Na/K, and its activity in the OCM process was investigated and its performance was compared with the conventional Mn-Na2WO4/SiO2 nano-catalyst. The effect of temperature and methane-to-oxygen ratio on nano-catalyst performance was investigated. The performance criteria included methane conversion, ethylene selectivity, and C2+ yield. The formation of the core–shell structure has been deliberated using XRD and FE-SEM characterization tests. The results of characterization and reactor experiments indicated that the best performance was related to the nano-catalyst prepared with a ratio ofNa/K = 2/2.4, operating conditions: 850 °C, atmospheric pressure, and CH4/O2 ratio = 2 in the reaction feed. The formation of the core–shell structure does not reduce the nano-catalyst activity, but rather increases the selectivity of the main product, ethylene, by improving the nano-catalyst performance even at 850 °C. Stability tests were performed to examine the catalytic activity under high temperatures.

Enhancement of oxygen vacancy sites of La2-xMxCe2O7-δ (M = Ca, Ba, Sr) catalyst for the low temperature oxidative coupling of Methane: A combined DFT and experimental study

This study investigated the effect of different alkaline earth metals doped La2-xMxCe2O7-δ (M = Ca, Sr, Ba and x = 0.30) catalysts prepared by one-step hydrothermal method for the low-temperature methane oxidative coupling (OCM) to olefins. The presence of a disordered cubic fluorite phase of synthesized catalysts was detected by powder XRD and Raman analysis. The surface analysis by XPS and Raman shows the presence of oxygen vacancy sites, which are responsible for the formation of electrophilic superoxide O2– species in presence of molecular oxygen. We observed that the ratio of active oxygen species (O2− and O22−) to lattice oxygen (O2–) was linearly correlated with the number of basic sites. The catalytic activity of different catalysts for the OCM reactions follows the order: La1.7Ca0.3Ce2O7-δ > La1.7Sr0.3Ce2O7-δ > La1.7Ba0.3Ce2O7-δ > La2Ce2O7. The reactivity order follows the same trend as the amount of basic sites and reactive oxygen species. According to this study, the best catalyst is La1.7Ca0.3Ce2O7-δ, which gives the highest C2 yield and C2 selectivity of 23.6 % and 78.8 %, respectively, at 650 °C. DFT studies revealed that substitution of La by Ca in the La2Ce2O7 catalyst significantly reduces the oxygen vacancy formation energy by about 100 kJ/mol compared to the undoped LaCe-oxide. We believe that the superior activity of La1.7Ca0.3Ce2O7-δ catalyst is attributed to the higher amount of basicity and electrophilic oxygen content, which are the governing factors in OCM for CH4 conversion and C2 product selectivity as well.

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.

Prediction of suitable catalysts for the OCM reaction by combining an evolutionary approach and machine learning

A method to use the concept of directed evolution to synthesize new catalysts for the oxidative coupling of methane (OCM) in silico via a combination of a genetic algorithm and machine learning (ML) is described.

Indirect design of OCM catalysts through machine learning of catalyst surface oxygen species

Catalysts for oxidative coupling of methane (OCM) were designed through machine learning of a property of surface oxygen species on the basis of the knowledge that catalytic performance for the OCM is affected by catalyst surface oxygen species.

TiOx-supported Na-Mn-W oxides for the oxidative coupling of methane

Supported Na-Mn-W oxides are among the most studied catalysts for the oxidative coupling of methane (OCM) because of their superior thermal stability and relatively high C2+ product yields. However, because of the structural complexity, the roles of each component in these catalysts have been controversial. In this work, WOx and MnOx sites were supported on titanate nanowires and employed in OCM studies. Compared to the commonly studied silica support, which is subject to severe restructuring due to the Na-induced crystallization, titanate support not only serves as a reservoir for alkali metals (e.g., Na), but also stabilizes isolated MnOx species. The catalytic performance of the titanate-based catalyst is similar to that of reference catalyst, MnOx/Na2WO4/SiO2, with a synergistic effect between MnOx and WOx sites. Advanced electron microscopy, X-ray diffraction, infrared spectroscopy, and X-ray absorption near edge structure spectroscopy suggest that the basic NaOx and MnOx species have strong interactions with the acidic WOx and TiOx species, which might contribute to the high selectivity toward C2+ products and suppressed COx formation.