Oxidative Coupling Of Methane Reaction Research Articles

Oxidative Coupling of Methane in Membrane Reactors; A Techno-Economic Assessment

Oxidative coupling of methane (OCM) is a process to directly convert methane into ethylene. However, its ethylene yield is limited in conventional reactors by the nature of the reaction system. In this work, the integration of different membranes to increase the overall performance of the large-scale oxidative coupling of methane process has been investigated from a techno-economic point of view. A 1D membrane reactor model has been developed, and the results show that the OCM reactor yield is significantly improved when integrating either porous or dense membranes in packed bed reactors. These higher yields have a positive impact on the economics and performance of the downstream separation, resulting in a cost of ethylene production of 595–625 €/tonC2H4 depending on the type of membranes employed, 25–30% lower than the benchmark technology based on oil as feedstock (naphtha steam cracking). Despite the use of a cryogenic separation unit, the porous membranes configuration shows generally better results than dense ones because of the much larger membrane area required in the dense membranes case. In addition, the CO2 emissions of the OCM studied processes are also much lower than the benchmark technology (total CO2 emissions are reduced by 96% in the dense membranes case and by 88% in the porous membranes case, with respect to naphtha steam cracking), where the high direct CO2 emissions have a major impact on the process. However, the scalability and the issues associated with it seem to be the main constraints to the industrial application of the process, since experimental studies of these membrane reactor technologies have been carried out just on a very small scale.

Catalytic activity, water formation, and sintering: Methane activation over Co- and Fe-doped MgO nanocrystals.

Microstructure, structure, and compositional homogeneity of metal oxide nanoparticles can change dramatically during catalysis. Considering the different stabilities of cobalt and iron ions in the MgO host lattice [M. Niedermaier et al., J. Phys. Chem. C 123, 25991 (2019)], we employed MgO nanocube powders with or without transition metal admixtures for the oxidative coupling of methane (OCM) reaction to analyze characteristic differences in catalytic activity and sintering behavior. Undoped MgO nanocrystals exhibit the highest C2 selectivity and retain the nanocrystallinity of the starting material after 24 h time on stream. For the Co-Mg-O nanoparticle powder, which exhibits the highest activity and COx selectivity and where OCM-induced coarsening is strongest, we found that the Co2+ ions remain homogeneously distributed over the MgO lattice. Trivalent Fe ions migrate to the surface of Fe-Mg-O nanoparticles where they form a magnesioferrite phase (MgFe2O4) with a characteristic impact on catalytic performance: Fe-Mg-O is initially less selective than MgO despite its lower activity. An increase in C2 selectivity and a decrease in the CO2/CO ratio with time on stream are attributed to the increasing fraction of coarsened particles that become depleted in redox active Fe. Surface water is a by-product of the OCM reaction, favors mass transport across the particle surfaces, and serves as a sintering aid during catalysis. The characteristic changes in size and morphology of MgO, Co-doped, and Fe-doped MgO particles can be consistently explained by activity and C2 selectivity trends. The original morphology of the nanocubes as a starting material for the OCM reaction does not impact the catalytic activity.

Data-Driven Identification of the Reaction Network in Oxidative Coupling of the Methane Reaction via Experimental Data.

Identifying details of chemical reactions is a challenging matter for both experiments and computations. Here, the reaction pathway in oxidative coupling of methane (OCM) is investigated using a series of experimental data and data science techniques in which data are analyzed using a variety of visualization techniques. Data visualization, pairwise correlation, and machine learning unveil the relationships between experimental conditions and the selectivities of CO, CO2, C2H4, C2H6, and H2 in the OCM reaction. More importantly, the reaction network for the OCM reaction is constructed on the basis of the scores provided by machine learning and experimental data. In particular, the proposed reaction map not only contains the chemical compound but also contains experimental conditions. Thus, data-driven identification of chemical reactions can be achieved in principle via a series of experimental data, leading to more efficient experimental design and catalyst development.

In situ X-ray diffraction computed tomography studies examining the thermal and chemical stabilities of working Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes during oxidative coupling of methane.

In this study we present the results from two in situ X-ray diffraction computed tomography experiments of catalytic membrane reactors (CMRs) using Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) hollow fibre membranes and Na-Mn-W/SiO2 catalyst during the oxidative coupling of methane (OCM) reaction. The negative impact of CO2, when added to the inlet gas stream, is seen to be mainly related to the C2+ yield, while no evidence of carbonate phase(s) formation is found during the OCM experiments. The main degradation mechanism of the CMR is suggested to be primarily associated with the solid-state evolution of the BSCF phase rather than the presence of CO2. Specifically, in situ XRD-CT and post-mortem SEM/EDX measurements revealed a collapse of the cubic BSCF phase and subsequent formation of secondary phases, which include needle-like structures and hexagonal Ba6Co4O12 and formation of a BaWO4 layer, the latter being a result of chemical interaction between the membrane and catalyst materials at high temperatures.

Preparation of LaAlO3 perovskite catalysts by simple solid-state method for oxidative coupling of methane

Abstract The preparation of LaAlO3 perovskite catalysts for the oxidative coupling of methane (OCM) using a very simple solid-state method with good reproducibility is described. To obtain high performance LaAlO3 catalysts, preparation conditions such as time, temperature, and oxygen content, which have a strong effect on the catalytic activity, are controlled during the calcination process. As the calcination time and temperature increase, the catalytic activity of LaAlO3 improves due to the good homogeneity and high crystallinity of LaAlO3 perovskite. Especially, the LaAlO3 catalyst calcined at 1350 °C for 24 h shows the highest selectivity and yield of C2 hydrocarbons. LaAlO3 catalysts calcined at various oxygen fractions show a volcano-shaped curve with respect to oxygen content in calcination gas, and the LaAlO3 catalyst prepared with an oxygen content of 5 % exhibits the best catalytic performance in this reaction. The appropriate oxygen fraction provides the LaAlO3 catalyst with a large amount of electrophilic lattice oxygen species and a sufficient amount of adsorption oxygen species, which are known to play key roles in the OCM reaction. This study clearly reveals this simple, highly reproducible, and mass-productive solid-state method as a promising preparation method for high performance LaAlO3 perovskite catalysts for OCM.

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A combinatorial approach was applied to explore active binary catalysts for oxidative coupling of methane (OCM) to value-added hydrocarbons (C2+). A screening of 25 selected single components on SiO2 for OCM reaction identified the top-14 single active components as follows: La > Ce > Ga > Al > Ca > Cr > Ba > Na2WO4 > Mn > Cu > Ti > Zn > Rb > Ni. Binary catalyst screening was then performed and resulted in a combination of Na2WO4 and Mn producing the most active binary catalyst. X-ray powder diffraction measurement of the Na2WO4-Mn/SiO2 catalyst revealed that the presence of α-cristobalite phase was essential for the activation of methane. Moreover, the X-ray photoelectron spectroscopy spectrum of the Na2WO4-Mn/SiO2 catalyst showed that the binding energy of W4f and Mn 2p shifted toward a lower binding energy, thereby enhancing the catalytic activity. Optimization of C2+ production of the catalyst by varying Na2WO4:Mn weight ratios, total metal loadings, catalyst weights, and feeding gas compositions, achieved maximum C2+ yield of 23.54% with 60.5% selectivity and 39.67% methane conversion. Furthermore, the activity of the Na2WO4-Mn/SiO2 catalyst was monitored with time-on-steam for 50 h, revealing good catalyst stability.

Statistical Analysis and Discovery of Heterogeneous Catalysts Based on Machine Learning from Diverse Published Data

AbstractThe literature provides insights for catalyst design and discovery. Effective analysis of reported data using machine learning (ML) methods offers the ability to gain valuable information. However, utilizing the literature in this way has obstacles such as lack of compositional overlaps, bias from prior published data, and low sample counts for many elements. The present study describes an ML approach that considers elemental features as input representations instead of inputting catalyst compositions directly. This ML method has the potential for catalyst discovery, including catalytic reactions with limited catalyst composition overlap in the available data. Oxidative coupling of methane (OCM), water gas shift (WGS), and CO oxidation reactions were chosen to confirm the effectiveness of the proposed method by analysis using several state‐of‐the‐art ML methods. Among the ML methods tested, gradient boosting regression with XGBoost (XGB) provided the best results, and prediction accuracy was improved by the proposed approach for all three reaction types. In addition, a quantitative value of “feature importance score” was calculated to evaluate the most influential input variables on catalyst performance. Finally, catalyst optimization was explored using ML as a “surrogate” model, and the top 20 promising candidate catalysts were identified for the OCM reaction based on the optimization. The advantages of ML in catalysis analysis as well as the difficulties and limitations originating from the complexity of heterogeneous catalysis were explored.

Techno-economic analysis of oxidative coupling of methane: Current state of the art and future perspectives

The oxidative coupling of methane (OCM) process, which uses natural gas as a raw material for ethylene production and has been widely investigated since the 80’s, is an alternative to conventional processes based on naphtha cracking. However, many authors have shown that its viability is hampered by the low yields that can currently be achieved during the OCM reaction. In this work, the performance of the OCM reactor and its impact on the overall OCM plant have been quantified from a techno-economic point of view. It has been proven that a very high ethylene price (above 1500 €/ton C2H4) is obtained with the current OCM technology, which corresponds to a C2 reactor yield of around 15%. The sensitivity analysis of the OCM process has revealed that the best reactor performance does not necessarily imply the lowest ethylene price, but there are instead other factors affecting it. With the simulation of the overall plant it has also been demonstrated that a C2 reactor yield of at least 25–30% is the target to obtain an ethylene cost lower than 1000 €/ton C2H4. Finally, it has been shown that, based on the forecast of costs for natural gas and naphtha, the gap between the ethylene price obtained with conventional technologies and the one obtained with the current OCM state of the art is expected to progressively become smaller, forecasting OCM to be competitive with traditional technologies in around 20 years.

Oxidative Coupling of Methane in Solid Oxide Electrolysis Cell

Oxidative coupling of methane (OCM) is one of the most promising technologies for the effective use of methane. In this study, La0.3Sr0.7TiO3 was examined as an anode material for solid oxide electrolysis cell with OCM reaction. X-ray diffraction measurement was conducted for the material characterization, and alternating current impedance and current-voltage measurement were conducted for the cell characterization. The activity test was conducted at different current densities, and the anode gas compositions were analyzed. The electrochemical production rates of CO, CO2, C2H6, and C2H4 were increased linearly as the applied current voltages were increased, while the selectivities of all products were almost constant.

Electric Field and Mobile Oxygen Promote Low-Temperature Oxidative Coupling of Methane over La1-x Ca x AlO3-δ Perovskite Catalysts.

Oxidative coupling of methane (OCM) over La1–xMxAlO3−δ (M = Ca, Sr, Ba; x = 0, 0.1, 0.2, 0.3) in an electric field at low temperature (423 K) was investigated. Among the tested catalysts, the La0.7Ca0.3AlO3−δ catalyst showed the highest performance in terms of C2H6 + C2H4 yield (11.1%). Surface mobile oxygen species (O22– or O–), which were considered as active oxygen species for the OCM reaction, increased with increasing Ca doping amount, and thereby the La0.7Ca0.3AlO3−δ catalyst showed the best catalytic activity.

Plasma-Assisted Photocatalysis of CH4 and CO2 into Ethylene

Oxidative coupling of methane (OCM) using CO2 as the oxidant is a potential advancement for methane conversion and greenhouse gases reduction. However, the reaction is usually limited by both high energy consumption and strict reaction condition. Here, we report, for the first time, photoinduced efficient ethylene (C2H4) and CO production by OCM over TiO2-supported Ag nanoparticles at mild conditions. The success relies on a synergy coupling visible-light-induced strong surface plasma resonance (SPR) effect localized on Ag(0), ultraviolet-light-induced photoelectric effect on TiO2, and the separated adsorption of CO2 and CH4 on TiO2 and Ag. The yields of CO and C2H4 reach 1149 and 686 μmol·g–1·h–1, respectively, under simulated solar irradiation. The origination of carbon-based gas products from CO2 and CH4 is also certified by a 13C isotope-labeled experiment. This work presents a new and ideal route of photocatalytic OCM reaction for C2H4 and CO production by coupling both SPR and photoelectric effects.

A K2NiF4-type La2Li0.5Al0.5O4 catalyst for the oxidative coupling of methane (OCM)

In this study, the significant activity and noticeable C2-selectivity of a La2Li0.5Al0.5O4 catalyst with a K2NiF4-type structure in the oxidative coupling of methane (OCM) was reported for the first time. The high C2-selectivity of the La2Li0.5Al0.5O4 catalyst originated from its abundance of electrophilic lattice oxygen species, which facilitated the selective formation of C2-hydrocarbons from methane. Although the activity of the La2Li0.5Al0.5O4 catalyst changed slightly owing to its gradual structural change into LaAlO3 perovskite during OCM, it is highly significant that this catalyst, with a K2NiF4-type structure, appears as a potentially promising catalyst for the OCM reaction.

Oxidative Coupling of Methane over Mn2O3-Na2WO4/SiC Catalysts

The oxidative coupling of methane (OCM) is operated at high temperatures and is a highly exothermic reaction; thus, hotspots form on the catalyst surface during reaction unless the produced heat is removed. It is crucial to control the heat formed because surface hotspots can degrade catalytic performance. Herein, we report the preparation of Mn2O3-Na2WO4/SiC catalysts using SiC, which has high thermal conductivity and good stability at high temperatures, and the catalyst was applied to the OCM. Two Mn2O3-Na2WO4/SiC catalysts were prepared by wet-impregnation on SiC supports having different particle sizes. For comparison, the Mn2O3-Na2WO4/SiO2 catalyst was also prepared by the same method. The catalysts were analyzed by nitrogen adsorption–desorption, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The transformation of SiC into α-cristobalite was observed for the Mn2O3-Na2WO4/SiC catalysts. Because SiC was completely converted into α-cristobalite for the nano-sized SiC-supported Mn2O3-Na2WO4 catalyst, the catalytic performance for the OCM reaction of Mn2O3-Na2WO4/n-SiC was similar to that of Mn2O3-Na2WO4/SiO2. However, only the surface layer of SiC was transformed into α-cristobalite for the micro-sized SiC (m-SiC) in Mn2O3-Na2WO4/m-SiC, resulting in a SiC@α-cristobalite core–shell structure. The Mn2O3-Na2WO4/m-SiC showed higher methane conversion and C2+ yield at 800 and 850 °C than Mn2O3-Na2WO4/SiO2.

Screening of single and binary catalysts for oxidative coupling of methane to value-added chemicals

A combinatorial approach was applied to explore active binary catalysts for oxidative coupling of methane (OCM) to value-added hydrocarbons (C2+). A screening of 25 selected single components on SiO2 for OCM reaction identified the top-14 single active components as follows: La > Ce > Ga > Al > Ca > Cr > Ba > Na2WO4 > Mn > Cu > Ti > Zn > Rb > Ni. Binary catalyst screening was then performed and resulted in a combination of Na2WO4 and Mn producing the most active binary catalyst. X-ray powder diffraction measurement of the Na2WO4-Mn/SiO2 catalyst revealed that the presence of α-cristobalite phase was essential for the activation of methane. Moreover, the X-ray photoelectron spectroscopy spectrum of the Na2WO4-Mn/SiO2 catalyst showed that the binding energy of W4f and Mn 2p shifted toward a lower binding energy, thereby enhancing the catalytic activity. Optimization of C2+ production of the catalyst by varying Na2WO4:Mn weight ratios, total metal loadings, catalyst weights, and feeding gas compositions, achieved maximum C2+ yield of 23.54% with 60.5% selectivity and 39.67% methane conversion. Furthermore, the activity of the Na2WO4-Mn/SiO2 catalyst was monitored with time-on-steam for 50 h, revealing good catalyst stability.

Effect of Calcination Temperature on the Characteristics and Performance of Solid Acid WO3/TiO2‐Supported Lithium‐Manganese Catalysts for the Oxidative Coupling of Methane

A series of solid acid WO3/TiO2‐supported lithium‐manganese oxide (Li‐Mn/WO3/TiO2) catalysts for the oxidative coupling of methane (OCM) were prepared by the impregnation method. The XRD and XPS analyses showed that increasing the calcination temperature could promote the generation of Mn2+ and Mn3+ species and the anatase‐to‐rutile phase transformation of TiO2, which are beneficial to the OCM reaction. It was revealed by CO2‐TPD, O2‐TPD, H2‐TPR, XPS, and CH4‐TPSR that Li‐Mn/WO3/TiO2 with higher calcination temperature possessed much stronger surface basicity, lower lattice oxygen mobility, and more abundant lattice oxygen, which are favorable for the selectivity to C2 hydrocarbons. The catalytic, structural, and spectroscopic results showed that the catalytic performance, structure, and properties of catalysts were little influenced by the calcination temperature of the WO3/TiO2 support. The optimized calcination temperature of the support and the catalyst is 800 °C, on which the highest C2 yield of 16.3 % was achieved at a reaction temperature of 750 °C.

Gas-Phase Reaction Network of Li/MgO-Catalyzed Oxidative Coupling of Methane and Oxidative Dehydrogenation of Ethane

A gas-phase reaction network involving reactive intermediates such as alkyl and alkyl peroxide radicals and alkyl hydroperoxides have long remained a mystery in oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODHE) reactions. Herein we report direct observations of gas-phase reactive intermediates including CH2, CH3•, C2H5•, CH3OO•, C2H5OO•, CH3OOH, and C2H5OOH during OCM and ODHE reactions over Li/MgO catalysts with an online synchrotron VUV photoionization mass spectrometer connected to a catalytic reactor with adjustable distances between the catalyst bed and the sampling nozzle of the mass spectrometer. Secondary reactions of these reactive intermediates in the gas phase are elucidated, and the reaction network of OCM and ODHE reactions is established. These results greatly deepen the mechanistic understanding of OCM and ODHE reactions necessary for developing efficient catalysts and designing proper reactors.

A meta-analysis of catalytic literature data reveals property-performance correlations for the OCM reaction

Decades of catalysis research have created vast amounts of experimental data. Within these data, new insights into property-performance correlations are hidden. However, the incomplete nature and undefined structure of the data has so far prevented comprehensive knowledge extraction. We propose a meta-analysis method that identifies correlations between a catalyst’s physico-chemical properties and its performance in a particular reaction. The method unites literature data with textbook knowledge and statistical tools. Starting from a researcher’s chemical intuition, a hypothesis is formulated and tested against the data for statistical significance. Iterative hypothesis refinement yields simple, robust and interpretable chemical models. The derived insights can guide new fundamental research and the discovery of improved catalysts. We demonstrate and validate the method for the oxidative coupling of methane (OCM). The final model indicates that only well-performing catalysts provide under reaction conditions two independent functionalities, i.e. a thermodynamically stable carbonate and a thermally stable oxide support.

Operando and Postreaction Diffraction Imaging of the La–Sr/CaO Catalyst in the Oxidative Coupling of Methane Reaction

A La–Sr/CaO catalyst was studied operando during the oxidative coupling of methane (OCM) reaction using the X-ray diffraction computed tomography technique. Full-pattern Rietveld analysis was performed in order to track the evolving solid-state chemistry during the temperature ramp, OCM reaction, as well as after cooling to room temperature. We observed a uniform distribution of the catalyst main components: La2O3, CaO–SrO mixed oxide, and the high-temperature rhombohedral polymorph of SrCO3. These were stable initially in the reaction; however, doubling the gas hourly space velocity resulted in the decomposition of SrCO3 to SrO, which subsequently led to the formation of a second CaO–SrO mixed oxide. These two mixed CaO–SrO oxides differed in terms of the extent of Sr incorporation into their unit cell. By applying Vegard’s law during the Rietveld refinement, it was possible to create maps showing the spatial variation of Sr occupancy in the mixed CaO–SrO oxides. The formation of the Sr-doped CaO species is expected to have an important role in this system through the enhancement of the lattice oxygen diffusion as well as increased catalyst basicity.

Optimizing the Reaction Performance of La2Ce2O7‐Based Catalysts for Oxidative Coupling of Methane (OCM) at Lower Temperature by Lattice Doping with Ca Cations

To optimize the reaction performance of La2Ce2O7 for OCM, catalysts with varied La/Ce molar ratios and doped by Ca additive have been synthesized and characterized by different techniques. It is discovered that a La2Ce2O7 compound having a disordered cubic defective fluorite phase is predominantly formed in all the samples. Varying the La/Ce ratio influences the fine crystalline structure of the catalysts. As a result, the abundance of the surface facile oxygen and alkaline sites increase by increasing the La/Ce ratio. Moreover, the Ca additive exists on the surface of Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7 as dispersed CaCO3, which covers the active sites and is harmful to the OCM reaction. However, for La2Ce1.5Ca0.5O7, the major part of the Ca additive has entered into the lattice of La2Ce2O7 phase as Ca2+ cations, thus creating more oxygen vacancies and improving the surface alkalinity of the catalyst, which is favorable to the OCM reaction. Both the active surface oxygen and alkaline sites are vital for OCM, and their synergism determines the reactivity. By increasing La/Ce ratio and doping Ca2+ into La2Ce2O7 lattice, the quantities of both kinds of active sites can be significantly improved, thus obtaining catalysts with much improved reaction performance. La2Ce1.5Ca0.5O7 owns the largest amount of both types of active sites, hence depicting the best reaction performance, over which the highest C2 yield of 22.5 % can be achieved even at 750 °C.

Promoting Effect of Cerium Oxide on the Catalytic Performance of Yttrium Oxide for Oxidative Coupling of Methane.

The promoting effect of CeO2 on the catalytic performance of Y2O3, which is moderately active catalyst, for the oxidative coupling of methane (OCM) reaction was investigated. The addition of CeO2 into Y2O3 by coprecipitation method caused a significant increase in not only CH4 conversion but also C2 (C2H6/C2H4) selectivity in the OCM reaction. C2 yield at 750 °C was increased from 5.6% on Y2O3 to 10.2% on 3 mol% CeO2/Y2O3. Further increase in the CeO2 loading caused an increase in non-selective oxidation of CH4 to CO2. A good correlation between the catalytic activity for the OCM reaction and the amount of H2 consumption for the reduction of surface/subsurface oxygen species in the H2-TPR profile was observed, suggesting the possibility that highly dispersed CeO2 particles act as catalytically active sites in the OCM reaction. The 16O/18O isotopic exchange reaction suggested that the beneficial role of CeO2 in the OCM reaction is to promote the formation of active oxygen species via the simple hetero-exchange mechanism, resulting in the promotion of CH4 activation.