Highest C2 Yield Research Articles

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.

Synthesis of Heterogeneous Catalysts in Catalyst Informatics to Bridge Experiment and High-Throughput Calculation.

The coupling of high-throughput calculations with catalyst informatics is proposed as an alternative way to design heterogeneous catalysts. High-throughput first-principles calculations for the oxidative coupling of methane (OCM) reaction are designed and performed where 1972 catalyst surface planes for the CH4 to CH3 reaction are calculated. Several catalysts for the OCM reaction are designed based on key elements that are unveiled via data visualization and network analysis. Among the designed catalysts, several active catalysts such as CoAg/TiO2, Mg/BaO, and Ti/BaO are found to result in high C2 yield. Results illustrate that designing catalysts using high-throughput calculations is achievable in principle if appropriate trends and patterns within the data generated via high-throughput calculations are identified. Thus, high-throughput calculations in combination with catalyst informatics offer a potential alternative method for catalyst design.

Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K2WO4/SiO2 Catalyst Revealed by In Situ Characterization

The oxidative coupling of methane (OCM) over a K2WO4/SiO2 catalyst was investigated using a CH4–O2 flow system. The addition of H2O to the reactant stream caused an increase in the CH4 conversion rate and C2 selectivity, consistent with the kinetics associated with the equilibrated OH radical generation, followed by H-abstraction from CH4 with OH radicals. The C2+ (a sum of C2 and higher hydrocarbons) yield reaches a maximum of 24.7% (C2H4 yield 17.0%) at a CH4 conversion of 47.1%, comparable to those over the previously reported Na2WO4/SiO2 catalyst. In situ near ambient pressure X-ray photoelectron spectroscopy uncovered K2O2 and KO2 species at and above 560 °C in the presence of gas-phase O2. These K2O2/KO2 species are considered to catalyze H2O activation for OH radical generation. By in situ high-temperature X-ray diffraction, K2WO4 maintained a crystalline phase in the bulk during the OCM reaction at 850 °C in contrast to the molten state of Na2WO4 previously reported. This indicates that the melting of the supported component and the covering of support are not essential to achieve a high C2 yield in supported alkali metal tungstate catalysts as long as inert support materials are used toward OCM.

Detailed Investigation of Sm2O3 Catalysts with Different Morphologies for Oxidative Coupling of Methane

AbstractSm2O3 catalysts with different morphologies (nanoparticle, nanorod, hollow nanosphere, nanofiber, flower‐like) were synthesized and tested for the oxidative coupling of methane. The results obtained showed that increasing specific surface area reduced the activity, C2 selectivity and, thus the yield. It was observed that the reaction initiation temperature was dependent on the total or partial oxidation of methane, which produced the heat for the initiation of the reaction. The effect of temperature on catalytic performance was relatively low at lower temperatures (<600 °C) than at higher temperatures (>700 °C). High reaction temperatures (>700 °C) decreased the C2 selectivity, lower temperatures were found beneficial to achieve higher C2 selectivity and yield as expected. Hollow nanosphere morphology showed the highest activity and selectivity at 450 °C and yielded 10.5 % C2 under high space velocity. The highest C2 yield (11.8 %) was obtained at 600 °C with nanoparticle morphology. Considering the results at the conditions that full oxygen conversion was attained, it was found that the C2 selectivity and yield decreased with the nanoparticle>flower‐like>nanorod and nanosphere>nanofiber sequence.

Improved Catalytic Activity and Stability of Ba Substituted SrTiO3 Perovskite for Oxidative Coupling of Methane

AbstractPerovskite oxides are widely used as the catalyst for oxidative coupling of methane (OCM). The partial substitution of A or B sites in ABO3 perovskites with heteroatoms was effective in improving the catalytic performance, but the promoting mechanism has not been thoroughly studied. In this work, the partial substitution of Sr with Ba in SrTiO3 perovskite was demonstrated to improve both the OCM reaction activity and stability. Compared with SrTiO3 and BaTiO3, the crystal size of Ba0.5Sr0.5TiO3 was greatly reduced (∼55 nm), the amount of surface chemisorbed oxygen species acting as the alkaline active centers for OCM was increased, and the electrophilic lattice oxygens with higher electronegativity rapidly replenishing the surface alkaline active sites were obtained. Thereby, a higher C2 yield (∼18 %) and C2H4/C2H6 ratio (∼1.7) were simultaneously achieved. Furthermore, the enhanced reaction stability was attributed to the better thermal resistance of nano‐sized Ba0.5Sr0.5TiO3 perovskite.

Effect of facile nitrogen doping on catalytic performance of NaW/Mn/SiO2 for oxidative coupling of methane

The oxidative coupling of methane (OCM) has great potential as a cost-effective pathway to directly convert methane to C2 species such as ethane and ethylene. Although NaW/Mn/SiO2 is one of the most promising catalysts for OCM, an improvement in its catalytic performance is required to achieve high C2 yield. Herein, we show that a relatively simple pyridine treatment of NaW/Mn/SiO2 leads to nitrogen-doping of the active sites, resulting in significantly high CH4 conversion rate while maintaining high C2 selectivity. Various characterisation techniques such as X-ray diffraction analysis, X-ray photoelectron spectroscopy, temperature-programmed reduction analysis, transmission electron microscopy, and density functional theory calculations are employed to understand the effect of N doping on the performance of NaW/Mn/SiO2 for OCM.

Direct Design of Catalysts in Oxidative Coupling of Methane via High‐Throughput Experiment and Deep Learning

AbstractThe combination of deep learning and high‐throughput experiments is proposed for the direct design of heterogeneous catalysts in the oxidative coupling of methane (OCM) reaction. Deep learning predicts 20 active catalysts from high‐throughput 12,708 OCM experimental data where 19 of the predicted 20 catalysts have not been previously reported. The predicted 20 catalysts are then evaluated through high‐throughput experiments where a highly active unreported catalyst Ti−Na2WO4/TiO2 is discovered. Ti−Na2WO4/TiO2 results in a high C2 yield of 18.8 % where the maximum C2 yield is reported to be approximately 20 % within the 12,708 OCM data. Furthermore, the experimental conditions predicted for Ti−Na2WO4/TiO2 are also reproduced by high‐throughput experiment. Thus, deep learning demonstrates that both catalysts and experimental conditions can be simultaneously explored for designing catalysts. More importantly, deep learning assisted catalysts search is found to dramatically increase the chances of finding active catalysts where 9 out of 20 predicted catalysts result in a C2 yield over 15 %. Therefore, the combination of deep learning with high‐throughput experiments is proposed to be an effective strategy for the direct design of catalysts.

Synthesis of Value-Added Chemicals via Oxidative Coupling of Methanes over Na2WO4-TiO2-MnO x /SiO2 Catalysts with Alkali or Alkali Earth Oxide Additives.

Na2WO4–TiO2–MnOx/SiO2 (SM) catalysts with alkali (Li, K, Rb, Cs) or alkali earth (Mg, Ca, Sr, Ba) oxide additives, which were prepared using incipient wetness impregnation, were investigated for oxidative coupling of methane (OCM) to value-added hydrocarbons (C2+). A screening test that was performed on the catalysts revealed that SM with Sr (SM–Sr) had the highest yield of C2+. X-ray photoelectron spectroscopy analyses indicated that the catalysts with a relatively low binding energy of W 4f7/2 facilitated a high CH4 conversion. A combination of crystalline MnTiO3, Mn2O3, α-cristobalite, Na2WO4, and TiO2 phases was identified as an essential component for a remarkable improvement in the activity of the catalysts in the OCM reaction. In attempts to optimize the C2+ yield, 0.25 wt % Sr onto SM–Sr achieved the highest C2+ yield at 22.9% with a 62.5% C2+ selectivity and a 36.6% CH4 conversion. A stability test of the optimal catalyst showed that after 24 h of testing, its activity decreased by 18.7%.

Special Features of Cyclohexane Cracking over Dual-Zeolite Catalysts in the Presence of Sulfur and Nitrogen Compounds

Conversions of cyclohexane and cyclohexane mixtures with 2-methylthiophene (5000 ppm S) and pyrrole (500 ppm N) over dual-zeolite cracking catalysts have been studied. The catalysts have been synthesized using zeolites Y in the HREE form and ZSM-5 modified with phosphorus. The effect of conditions (temperature in a range of 510–630°C, dilution ratio of the reaction medium with an inert gas up to 2 : 1 mol/mol) on the cracking product composition has been studied. It has been shown that an increase in temperature and the dilution ratio of the reaction medium contributes to an increase in the light olefin yield. The highest yield of C2–C4 olefins (40 wt %) has been achieved over a catalyst with an Y : ZSM-5 zeolite ratio of 1 : 3. The presence of sulfur and nitrogen compounds leads to a decrease in the feedstock conversion and an increase in the light olefin selectivity.

Zr-based metal–organic frameworks drived Rh–Mn catalysts for highly selective CO hydrogenation to C2 oxygenates

A series of Zr-based metal-organic frameworks (UiO-66) supported Rh–Mn catalysts with various metal loadings were prepared by using a co-impregnation method, and the catalytic activities were investigated for the direct synthesis of ethanol-based C2+ oxygenates from CO hydrogenation. The catalysts were comprehensively characterized by means of N2 sorption, XRD, TEM, XPS, H2-TPR, and DRIFT. The structural and textural properties of the catalysts show clearly that the overall framework structure of UiO-66 can be beneficial to the high dispersion of supported metals, but the synergistic effect of Rh and Mn would be influenced by the metal loading or Mn/Rh ratio. When the loadings of Rh and Mn reached 3 wt.% and 3 wt.%, the catalyst has the best catalytic activity with the highest yield of C2+ oxygenates of 200.1 g/(kg h), which can be attributed to the enhanced CO dissociation ability of Rh sites and weaker Rh−CO band strength.

Catalytic Conversion of Methane at Low Temperatures: A Critical Review

The current study reviews the recent development in the direct conversion of methane into syngas, methanol, light olefins, and aromatic compounds. For syngas production, nickel‐based catalysts are considered as a good choice. Methane conversion (84%) is achieved with nearly no coke formation when the 7% Ni‐1%Au/Al2O3 catalyst is used in the steam reforming of methane (SRM), whereas for dry reforming of methane (DRM), a methane conversion of 17.9% and CO2 conversion of 23.1% are found for 10%Ni/ZrOx MnOx/SiO2 operated at 500 °C. The progress of direct conversion of methane to methanol is also summarized with an insight into its selectivity and/or conversion, which shows that in liquid‐phase heterogeneous systems, high selectivity (>80%) can be achieved at 50 °C, but the conversion is low. The latest development of non‐oxidative coupling of methane (NOCM) and oxidative coupling of methane (OCM) for the production of olefins is also reviewed. The Mn2O3–TiO2–Na2WO4/SiO2 catalyst is reported to show the high C2 yield (22%) and a high selectivity toward C2 (62%) during the OCM at 650 °C. For NOCM, 98% selectivity of ethane can be achieved when a tantalum hydride catalyst supported on silica is used. In addition, the Mo‐based catalysts are the most suitable for the preparation of aromatic compounds from methane.

Effects of the preparation method on the crystallinity and catalytic activity of LaAlO3 perovskites for oxidative coupling of methane

Oxidative coupling of methane (OCM) was performed over three structurally different LaAlO3 perovskite catalysts prepared using the citrate sol-gel (LaAlO3_C), solid-state (LaAlO3_S), and hard-templating (LaAlO3_H) methods. Although three LaAlO3 perovskite catalysts were successfully prepared, the catalytic activity of the catalysts was significantly different depending on their preparation method. The preparation method, and thereby the crystallinity of the LaAlO3 perovskite catalysts, has a strong influence on the amount of surface lattice oxygen species, which play a key role in producing C2 compounds in OCM. Consequently, the C2 yield over LaAlO3 catalysts shows the same order as that shown by their relative crystallinity (LaAlO3_C > LaAlO3_S > LaAlO3_H). In other words, high crystallinity enabled LaAlO3 catalysts to retain a large amount of surface lattice oxygen species, resulting in a higher C2 yield. Among the tested catalysts, LaAlO3_C, prepared using the citrate sol-gel method, retained the highest amount of lattice oxygen species owing to its high crystallinity and exhibited the highest C2 yield (ca. 12% at 750°C) for OCM.

Oxidative Coupling of Methane Using Mg/Ti-Doped SiO2-Supported Na2WO4/Mn Catalysts

The oxidative coupling of methane (OCM) was performed using Na2WO4/Mn catalysts supported on Mg/Ti/Si mixed oxides. Na2WO4/Mn/Mg0.05Ti0.05Si0.90On exhibited a 23.1% C2 yield at 800 °C, which is a 35% higher C2 yield than that obtained using conventional Na2WO4/Mn/SiO2 in this study. The formation of Mg/Ti-doped α-cristobalite SiO2 was observed for supported Na2WO4/Mn catalysts, and their improved catalytic activity could be attributed to the suppressed incorporation of Na/W/Mn compounds into the α-cristobalite SiO2 structure and the existence of OCM-active Na/W/Mn compounds exposed on the support surface. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and O2-temperature-programmed desorption were performed in an effort to understand the improved catalytic OCM activity of Mg/Ti-doped SiO2-supported Na2WO4/Mn catalysts.

Oxidative Coupling of Methane in Small Scale Parallel Reactors

In this work the testing of high temperature reactions in small scale parallel reactors is explored using the oxidative coupling of methane as an example. The advantages of small scale reactors for this very exothermic and complex reaction are discussed. The data generated in this study is explored making use of response surface models derived from statistically designed experiments. Methane conversion has been explored over a Mn-promoted Na2WO4/SiO2 catalyst over the temperature range 755–875 °C, pressure range 0.2–8.4 barg and GHSV range 4,000–36,000 h−1 using air as oxidant. Methane to oxygen stoichiometries of 4–8 have been explored. The highest C2 yield (16 %) is obtained at at low pressure.

Synthesis of C2+ hydrocarbons by CO2 hydrogenation over the composite catalyst of Cu–Zn–Al oxide and HB zeolite using two-stage reactor system under low pressure

The combination of methanol synthesis and methanol-to-gasoline (MTG) reaction is a beneficial process to produce particular C2+ hydrocarbons from CO2 and H2, because MTG reaction can produce C2+ hydrocarbons selectively without the formation of methane. However, the composite catalysts consisting of methanol synthesis catalyst and zeolite previously reported have produced C2+ hydrocarbons in poor yields especially under low pressure conditions. This paper reports the high yield production of C2+ hydrocarbons by CO2 hydrogenation using a two-stage reaction system consisting of a Cu–Zn–Al oxide catalyst in the first reactor and a composite catalyst of the Cu–Zn–Al oxide and HB (zeolite beta) in the second reactor. The key points of this system are reversed water-gas-shift (RWGS) reaction in the first reactor and the following H2O removal before the second reactor. The gas mixture supplied to the second reactor containing much amount of CO and little H2O enhanced the methanol synthesis and suppressed the methanol decomposition to CO over the Cu–Zn–Al oxide catalyst in the composite catalyst, promoting the MTG reaction to produce C2+ hydrocarbons over HB zeolite. The highest yield of C2+ hydrocarbons reached to approximately 15C-mol% under 0.98MPa.

Methane Coupling Reaction in an Oxy‐Steam Stream through an OH Radical Pathway by using Supported Alkali Metal Catalysts

AbstractA universal reaction mechanism involved in the oxidative coupling of methane (OCM) is demonstrated under oxy‐steam conditions using alkali‐metal‐based catalysts. Rigorous kinetic measurements indicated a reaction mechanism that is consistent with OH radical formation from a H2O–O2 reaction followed by CH activation in CH4 with an OH radical. Thus, the presence of water enhances both the CH4 conversion rate and the C2 selectivity. This OH radical pathway that is selective for the OCM was observed for the catalyst without Mn, which suggests clearly that Mn is not the essential component in a selective OCM catalyst. The experiments with different catalyst compositions revealed that the OH.‐mediated pathway proceeded in the presence of catalysts with different alkali metals (Na, K) and different oxo anions (W, Mo). This difference in catalytic activity for OH radical generation accounts for the different OCM selectivities. As a result, a high C2 yield is achievable by using Na2WO4/SiO2, which catalyzes the OH.‐mediated pathway selectively.

Coupling catalysis and gas phase electrocatalysis for the simultaneous production and separation of pure H2 and C2 hydrocarbons from methane and natural gas

This study reports the simultaneous production of pure H2 and C2 hydrocarbons (ethane and ethylene) in a solid oxide electrolysis cell coupled with an active catalyst powder bed. For first time, an Ag/YSZ/Ag double chamber cell was used and high yields to both, hydrogen and C2 hydrocarbons were achieved. The separately feeding of H2O and CH4 to the inner and outer chamber of the reactor, respectively, allows to obtain both kinds of products streams. Thus, steam was electrolyzed on the inner Ag electrode of the electrochemical cell (cathode), producing pure H2 (as the first product) and O2− ions, which were electrochemically supplied through the YSZ solid electrolyte to the outer Ag electrode (anode). On this latter, the reaction of the O2− ions and the oxygen evolved with the methane stream led to the production of C2 hydrocarbons (together with CO and CO2 as secondary combustion products). The C2s yield of this outer stream was strongly enhanced by the addition of an active catalyst powder bed (Ce–Na2WO4/SiO2 or Mn–Ce–Na2WO4/SiO2) to obtain the second product stream (with a high C2 yield up to 15%). The influence of the oxidative coupling catalyst powder bed, the reaction temperature and the gas phase composition was studied. Finally, the optimized system was studied by feeding a synthetic natural gas stream (instead of CH4) in order to approach to a more practical system. The high performance of the system was again verified along with a durability test under polarization conditions in view of the possible practical application of this novel reactor configuration system.

Oxidative coupling of methane using catalysts synthesized by solution combustion method

Solution combustion synthesis (SCS) is a one-step technique for the preparation of nanostructured complex metal oxides. In the present study, SCS was used for the synthesis of several promising oxidative coupling of methane (OCM) catalyst series with varying metal ratios: (a) Sr–Al complex oxides, (b) La2O3, (c) La–Sr–Al complex oxides and (d) Na2WO4–Mn/SiO2. The C2 yield and ethylene/ethane ratio were measured for each catalyst over a wide range of temperatures. It was found that the double perovskite phase is active for OCM in the Sr–Al complex oxide series. The Na2WO4–Mn/SiO2 catalyst synthesized by SCS was stable and also showed high ethylene/ethane ratio ∼2 along with high C2 yield ∼25%, which is among the highest value reported in the literature, indicating that SCS is a promising method to prepare OCM catalysts.

Methane Oxidative Coupling: Synthesis of Membrane Reactor Networks

In this work, the performance of the methane oxidative coupling process based on different alternative reactor structures including fixed-bed reactor, two different feeding-structures of porous packed bed membrane reactor, and different conceptual network combinations of them was analyzed in a comprehensive model-based study. In this context, a contour-based graphical visualizing method accompanied by multiscenario generation and examination approach was exploited so as to systematically screen out the high achievable process performances and the corresponding reactor network specifications. As performance indicators, several objective measures including the highest achievable values for yield, selectivity and methane conversion were applied over a high number of design scenarios. Thereby, several sets of structural and operating parameters were evaluated. The investigated parameters are temperature, membrane thicknesses, types of catalysts, amount of inert packing in the catalyst bed, the flow rate of oxygen-rich stream entering the reactor system (total methane to oxygen ratio), distribution of the oxygen rich stream through the reactor blocks (local methane to oxygen ratio), distribution of the total methane-rich feed stream into the reactor blocks, and the contact time represented by the reactor length. A high C2-yield of 31% and 88% C2-selectivity were observed at the same time, and thus, highlighting the performance potential of the reactor network. Moreover, the proposed method enables one to consider operating aspects such as hot spot formation during the analysis and screening procedure.

Oxidative Coupling of Methane with High C2 Yield by using Chlorinated Perovskite Ba0.5Sr0.5Fe0.2Co0.8O3−δ as Catalyst and N2O as Oxidant

Assess the C2-ation: A novel chlorinated perovskite-type oxide is prepared as catalyst for oxidative coupling of methane in a packed-bed reactor. Chlorine doping significantly improves the selectivity towards C2 species (C2H6 and C2H4). Furthermore, using N2O instead of O2 as oxidant, to some extent, prevented the deep oxidation of C2. As a result, a high C2 yield was acquired.