Structure Sensitivity of La2O2CO3 Catalysts in the Oxidative Coupling of Methane

Comparing BaZrO3 Perovskite and La2Zr2O7 Pyrochlore for Oxidative Coupling of Methane (OCM): The Importance of Basic Sites vs Oxygen Vacancies

La2Zr2O7 pyrochlore with intrinsic oxygen vacancies and BaZrO3 perovskite without intrinsic oxygen vacancies were synthesized for the OCM reaction. It has been revealed that the OCM performance and surface selective oxygen species of BaZrO3 are higher than that of La2Zr2O7 under the reaction condition. This is because BaZrO3 possesses more basic sites than La2Zr2O7, and thus it can stabilize the OCM reactive oxygen species better at elevated temperature. In the structure, the A-O bond lattice oxygen of the two compounds mainly provides basic sites, but the B-O bond lattice oxygen mainly promotes deep oxidation of methane and the generated hydrocarbons. The types of the OCM reactive oxygen species are prone to be associated with the properties of the A-site metal oxides.

Capture gaseous arsenic in flue gas by amorphous iron manganese oxides with high SO2 resistance

In non-ferrous metal smelting, the problem of gaseous arsenic in high-sulfur flue gas is difficult to solve. Now we have developed oxygen-enriched amorphous iron manganese oxide (AFMBO) based on the unique superiority of iron-manganese oxide for arsenic capture to realize the effective control of gaseous arsenic in the non-ferrous smelting flue gas. The experimental results show that the arsenic adsorption capacity of AFMBO is up to 102.7 mg/g, which has surpassed most of the current adsorbents. In particular, AFMBO can effectively capture gaseous arsenic even at 12% v/v SO2 concentrations (88.45 mg/g). Moreover, the spent AFMBO possesses pronounced magnetic characteristics that make it easier to separate from dust, which is conducive to reducing the secondary environmental risk of arsenic. In terms of mechanism study, various characterization methods are used to explain the important role of lattice oxygen and adsorbed oxygen in the capture process of gaseous arsenic. Moreover, the reason for the efficient arsenic removal performance of AFMBO is also reasonably explained at the microscopic level. This study provides ideas and implications for gaseous arsenic pollution control research.

Plasma-Enhanced Chemical Looping Oxidative Coupling of Methane through Synergy between Metal-Loaded Dielectric Particles and Non-Thermal Plasma

A plasma–catalyst hybrid system has been developed for the direct conversion of methane to C2+ hydrocarbons in dielectric barrier discharge (DBD) plasma. TiO2 presented the highest C2+ yield of 11.63% among different dielectric materials when integrated with DBD plasma, which made us concentrate on the TiO2-based catalyst. It was demonstrated that MnTi catalyst showed the best methane coupling performance of 27.29% C2+ yield with 150 V applied voltage, without additional thermal input. The catalytic performance of MnTi catalyst under various operation parameters was further carried out, and different techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and H2-temperature-programmed reduction were used to explore the effect of Mn loading on methane oxidative coupling (OCM) performance. The results showed that applied voltage and flow rate had a significant effect on methane activation. The dielectric particles of TiO2 loaded with Mn not only synergistically affected the coupling reaction, but also facilitated charge deposition to generate a strong local electric field to activate methane. The synergy effects boosted the OCM performance and the C2+ yield became 1.25 times higher than that of the undoped TiO2 under identical operating conditions in plasma, which was almost impossible to occur even at 850 °C on the MnTi catalyst in the absence of plasma. Moreover, the reaction activity of the catalyst was fully recovered by plasma regeneration at 300 °C and maintained its stability in for at least 30 consecutive cyclic redox tests. This work presents a new opportunity for efficient methane conversion to produce C2+ at low temperatures by plasma assistance.

Constructing Y2B2O7 (B = Ti, Sn, Zr, Ce) Compounds to Disclose the Effect of Surface Acidity-Basicity on Product Selectivity for Oxidative Coupling of Methane (OCM)

To investigate the influence of surface acidity and basicity on the OCM reaction, four pure-phase Y₂B₂O₇ (B = Ti, Sn, Zr, Ce) compounds have been purposely constructed. The exquisite phase structure change results in the generation of different amounts of surface active O₂^- and O^- sites, which affects CH₄ molecule activation. Furthermore, both Lewis acidic sites and basic sites are formed on the catalysts in different amounts, which are related to the lattice disorder extent and the choice of A- and B-site elements. It is elucidated with strong evidence that the surface basic sites are favorable to C₂ product selectivity, but the surface acidic sites lead to deep oxidation of CH₄ and the coupling products to form CO_x. To design and fabricate Y₂B₂O₇ catalysts with better C₂ product selectivity for the reaction, a disordered defect fluorite structure should be engineered with A- and B- site elements having appropriate basicity.

Metal Ions (Li, Mg, Zn, Ce) Doped into La2O3 Nanorod for Boosting Catalytic Oxidative Coupling of Methane

A series of La2O3 nanorod catalysts with doping of active metal ions (Li, Mg, Zn and Ce) were synthesized successfully by the hydrothermal method. The La2O3 nanorods show a uniform size with the length of 50–200 nm and the width of 5–20 nm, and the {110} crystal facet is a preferentially exposed surface. The active metal ions (Li, Mg, Zn and Ce) doped into the lattice of La2O3 nanorods enhance the selectivity of the desired products during oxidative coupling of methane (OCM) and decrease the reaction temperature. Among these catalysts, the Mg-La2O3 catalyst exhibits the best catalytic performance during the OCM reaction, i.e., its selectivity and yield of C2 products at 780 °C is 73% and 21%, respectively. The effect of doped metal ions on catalytic activity for OCM was systematically investigated. Insight into the fabrication strategy and promoting factors of the OCM reaction indicates the potential to further design a high-efficient catalyst in the future.

Effects of Mg, Ca, Sr, and Ba Dopants on the Performance of La2O3 Catalysts for the Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) is a reaction to directly convert methane into high value-added hydrocarbons (C₂₊) such as ethylene and ethane using molecular oxygen and a catalyst. This work investigated lanthanum oxide catalysts for OCM, which were promoted with alkaline-earth metal oxides (Mg, Ca, Sr, and Ba) and prepared by the solution-mixing method. The synthesized catalysts were characterized using X-ray powder diffraction, CO₂-programmed desorption, and X-ray photoelectron spectroscopy. The comparative performance of each promoter showed that promising lanthanum-loaded alkaline-earth metal oxide catalysts were La-Sr and La-Ba. In contrast, the combination of La with Ca or Mg did not lead to a clear improvement of C₂₊ yield. The most promising LaSr50 catalyst exhibited the highest C₂₊ yield of 17.2%, with a 56.0% C₂₊ selectivity and a 30.9% CH₄ conversion. Catalyst characterization indicated that their activity was strongly associated with moderate basic sites and surface-adsorbed oxygen species of O₂ ^-. Moreover, the catalyst was stable over 25 h at a reactor temperature of 700 °C.

High-Throughput Screening and Literature Data Driven Machine Learning Assisting Investigation of Multi-component La2O3-based Catalysts for Oxidative Coupling of Methane

Multi-component La2O3-based catalysts for oxidative coupling of methane (OCM) were designed based on high-throughput screening (HTS) and literature datasets with multi-output machine learning (ML) approaches including random forest regression (RFR),...

Studies on activation factors for oxidative coupling of methane over lithium-based silicate/germanate catalysts

Activation factors for improving oxidative coupling of methane (OCM) catalytic performance have been identified. Potential OCM catalysts, Li4SiO4 and Li4GeO4, have been discovered.

Highly water-soluble dimeric and trimeric lanthanide carbonates with ethylenediaminetetraacetates as precursors of catalysts for the oxidative coupling reaction of methane

Highly water-soluble dimeric and trimeric lanthanide carbonates with ethylenediaminetetraacetates have been obtained. Their coordination modes provide a model for the oxidative coupling of methane of lanthanide carbonates.

Recent advances in non-noble metal-based oxide materials as heterogeneous catalysts for C–H activation

This perspective article summarizes the recent developments of non-noble metal-based oxides, as a new class of catalysts for C−H bond activation, focusing on their essential surface properties.

Noncatalytic Oxidative Coupling of Methane (OCM): Gas-Phase Reactions in a Jet Stirred Reactor (JSR)

Oxidative coupling of methane (OCM) is a promising technique for converting methane to higher hydrocarbons in a single reactor. Catalytic OCM is known to proceed via both gas-phase and surface chemical reactions. It is essential to first implement an accurate gas-phase model and then to further develop comprehensive homogeneous-heterogeneous OCM reaction networks. In this work, OCM gas-phase kinetics using a jet-stirred reactor are studied in the absence of a catalyst and simulated using a 0-D reactor model. Experiments were conducted in OCM-relevant operating conditions under various temperatures, residence times, and inlet CH₄/O₂ ratios. Simulations of different gas-phase models related to methane oxidation were implemented and compared against the experimental data. Quantities of interest (QoI) and rate of production analyses on hydrocarbon products were also performed to evaluate the models. The gas-phase models taken from catalytic reaction networks could not adequately describe the experimental gas-phase performances. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics; it is recommended for further use as the gas-phase model for constructing homogeneous-heterogeneous reaction networks.

An In Situ Temperature-Dependent Study of La2O3 Reactivation Process

Lanthanum-containing materials are widely used in oxidative catalytic and electrocatalytic reactions such as oxidative coupling of methane (OCM) and solid oxide fuel cells (SOFCs). However, many of these materials are highly susceptible to air contamination which means ex situ characterization results generally cannot be associated with their reactivity. In this study, the activation processes of an in situ–prepared bulk La₂O₂CO₃ sample and an ex situ as-prepared La(OH)₃ sample are in situ investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and online mass spectroscopy (MS). Results indicate that the La₂O₂CO₃ sample, during linear heating to 800°C, always contains some carbonates near the surface region, which supports a two-step model of bulk carbonate decomposition through surface sites. The La(OH)₃ sample structure evolution is more complex due to contaminations from air exposure. Together with TGA results, online mass analysis of water and CO₂ signal loss showed that three major catalyst structure phase change steps and a preheating up to 800°C are required for the as-prepared material to be transferred to La₂O₃. This process is carefully investigated combining the three in situ methodologies. XPS and XRD data further reveal transformations of variety of in situ surface structures and forms including hybrid phases with hydroxyl, carbonates, and oxide as the sample heated to different temperatures within the range from 200 to 800°C. The results provide useful insights on the activation and deactivation of La-contained materials.

Ratiometric Cataluminescence Sensor of Amine Vapors for Discriminating Meat Spoilage

The freshness of meat has always been the focus of attention from consumers and suppliers for health and economic reasons. Usually, amine vapors, as one of the main components of the gas produced in the process of meat spoilage, can be used to monitor meat spoilage. Here, a new ratiometric cataluminescence (CTL) sensor based on energy transfer was developed to identify amine vapors and monitor meat freshness. After Tb doping, amine vapors exhibit a dual-wavelength (490 and 555 nm) property of CTL signals when reacted on the surface of Tb-doped La₂O₂CO₃, and the ratio of I₅₅₅ to I₄₉₀ (R_555/490) is a unique value for a given analyte within a wide range of concentrations. To illustrate the new sensor, 15 amine vapors were successfully identified using R_555/490, including homologues and isomers. Besides, this sensor was used to monitor four meats, and the freshness of meats can be distinguished by cluster analysis successfully. Moreover, further discussion of energy-transfer phenomena and influence factors has facilitating effects on exploring the mechanism of energy transfer at the gas-solid interface.

Nanosheet-Like Ho2O3 and Sr-Ho2O3 Catalysts for Oxidative Coupling of Methane

In this work, Ho2O3 nanosheets were synthesized by a hydrothermal method. A series of Sr-modified Ho2O3 nanosheets (Sr-Ho2O3-NS) with a Sr/Ho molar ratio between 0.02 and 0.06 were prepared via an impregnation method. These catalysts were characterized by several techniques such as XRD, N2 adsorption, SEM, TEM, XPS, O2-TPD (temperature-programmed desorption), and CO2-TPD, and they were studied with respect to their performances in the oxidative coupling of methane (OCM). In contrast to Ho2O3 nanoparticles, Ho2O3 nanosheets display greater CH4 conversion and C2-C3 selectivity, which could be related to the preferentially exposed (222) facet on the surface of the latter catalyst. The incorporation of small amounts of Sr into Ho2O3 nanosheets leads to a higher ratio of (O− + O2−)/O2− as well as an enhanced amount of chemisorbed oxygen species and moderate basic sites, which in turn improves the OCM performance. The optimal catalytic behavior is achievable on the 0.04Sr-Ho2O3-NS catalyst with a Sr/Ho molar ratio of 0.04, which gives a 24.0% conversion of CH4 with 56.7% selectivity to C2-C3 at 650 °C. The C2-C3 yield is well correlated with the amount of moderate basic sites present on the catalysts.

Construction of a Pd(PdO)/Co3O4@SiO2 core-shell structure for efficient low-temperature methane combustion

Catalytic combustion is a promising way to remove trace amounts of CH4 to alleviate serious environmental concerns. However, the reactivity of a catalyst at low temperature is usually limited because of the difficulty to activate the C-H bond of methane. Herein, we design a Pd(PdO)/Co3O4@SiO2 bimetallic oxide core-shell catalyst which shows much higher activity in the methane combustion reaction compared with Pd(PdO)/SiO2 and Co3O4@SiO2 catalysts without a core-shell structure. The T50% and T90% of Pd(PdO)/Co3O4@SiO2 are 357 °C and 445 °C, respectively, which decrease by 67 °C and 55 °C in comparison with those of Pd(PdO)/SiO2. Extensive characterization demonstrates that the bimetallic oxide core-shell structure can effectively enhance the metal interaction between Pd and Co, which can weaken the strength of the Co-O bond in Pd(PdO)/Co3O4@SiO2. The weakening of the Co-O bond could promote the release of more lattice oxygen species to participate in the C-H breaking, resulting in superior catalytic performance in methane combustion at low temperature.

Exothermic methane coupling with ethylene as a hydrogen acceptor toward two-step methane conversion to ethylene

Exothermic methane coupling with ethylene (ethene) as a hydrogen acceptor (2CH4 + C2H4 → 2C2H6) was proposed as part of a two-step reaction that includes ethane cracking (C2H6 → C2H4 + H2), which is a common industrial process, to provide methane conversion into ethylene as the net reaction (2CH4 → C2H4 + 2H2).

Exploring the formation of carbonates on La2O3 catalysts with OCM activity

Two series of La2O3 samples with identical bulk structures but different morphologies indicate substantially different carbonate forming pathways, which provides insight into the related oxidative coupling of methane (OCM) reaction.

Exploiting oxidative coupling of methane performed over La2(Ce1−xMgx)2O7−δ catalysts with disordered defective cubic fluorite structure

The oxidative coupling of methane reaction to produce C₂ compounds was studied using La₂(Ce_1−xMg_x)₂O_7−δ catalysts with disordered defective cubic fluorite structures, varying the Mg content (0.0 ≤ x ≤ 1.0), CH₄/O₂ ratio, temperature, and WHSV.

Hexagonal and Monoclinic Phases of La2O2CO3 Nanoparticles and Their Phase-Related CO2 Behavior

In this study, we prepared hexagonal and monoclinic phases of La₂O₂CO₃ nanoparticles by different wet preparation methods and investigated their phase-related CO₂ behavior through field-emission scanning microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, CO₂-temperature programmed desorption, and linear sweeping voltammetry of CO₂ electrochemical reduction. The monoclinic La₂O₂CO₃ phase was synthesized by a conventional precipitation method via La(OH)CO₃ when the precipitation time was longer than 12 h. In contrast, the hydrothermal method produced only the hexagonal La₂O₂CO₃ phase, irrespective of the hydrothermal reaction time. The La(OH)₃ phase was determined to be the initial phase in both preparation methods. During the precipitation, the La(OH)₃ phase was transformed into La(OH)CO₃ owing to the continuous supply of CO₂ from air whereas the hydrothermal method of a closed system crystallized only the La(OH)₃ phase. Based on the CO₂-temperature programmed desorption and thermogravimetric analysis, the hexagonal La₂O₂CO₃ nanoparticles (HL-12h) showed a higher surface CO₂ adsorption and thermal stability than those of the monoclinic La₂O₂CO₃ (PL-12h). The crystalline structures of both La₂O₂CO₃ phases predicted by the density functional theory calculation explained the difference in the CO₂ behavior on each phase. Consequently, HL-12h showed a higher current density and a more positive onset potential than PL-12h in CO₂ electrochemical reduction.

Lower olefins from methane: recent advances

Modern methods for methane conversion to lower olefins having from 2 to 4 carbon atoms per molecule are generalized. Multistage processing of methane into ethylene and propylene via syngas or methyl chloride and methods for direct conversion of CH4 to ethylene are described. Direct conversion of syngas to olefins as well as indirect routes of the process via methanol or dimethyl ether are considered. Particular attention is paid to innovative methods of olefin synthesis. Recent achievements in the design of catalysts and development of new techniques for efficient implementation of oxidative coupling of methane and methanol conversion to olefins are analyzed and systematized. Advances in commercializing these processes are pointed out. Novel catalysts for Fischer – Tropsch synthesis of lower olefins from syngas and for innovative technique using oxide – zeolite hybrid catalytic systems are described. The promise of a new route to lower olefins by methane conversion via dimethyl ether is shown. Prospects for the synthesis of lower olefins via methyl chloride and using non-oxidative coupling of methane are discussed. The most efficient processes used for processing of methane to lower olefins are compared on the basis of degree of conversion of carbonaceous feed, possibility to integrate with available full-scale production, number of reaction stages and thermal load distribution.

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