Low-temperature Oxidative Coupling Of Methane Research Articles

Ca and Sr co-doping induced oxygen vacancies in 3DOM La2−xSrxCe2−yCayO7−δ catalysts for boosting low-temperature oxidative coupling of methane

It is urgent to develop catalysts with application potential for oxidative coupling of methane (OCM) at relatively lower temperature. Herein, three-dimensional ordered macroporous (3DOM) La2−xSrxCe2−yCayO7−δ (A2B2O7-type) catalysts with disordered defective cubic fluorite phased structure were successfully prepared by a colloidal crystal template method. 3DOM structure promotes the accessibility of the gaseous reactants (O2 and CH4) to the active sites. The co-doping of Ca and Sr ions in La2−xSrxCe2−yCayO7−δ catalysts improved the formation of oxygen vacancies, thereby leading to increased density of surface-active oxygen species (O2−) for the activation of CH4 and the formation of C2 products (C2H6 and C2H4). 3DOM La2−xSrxCe2−yCayO7−δ catalysts exhibit high catalytic activity for OCM at low temperature. 3DOM La1.7Sr0.3Ce1.7Ca0.3O7−δ catalyst with the highest density of O2− species exhibited the highest catalytic activity for low-temperature OCM, i.e., its CH4 conversion, selectivity and yield of C2 products at 650 °C are 32.2%, 66.1% and 21.3%, respectively. The mechanism was proposed that the increase in surface oxygen vacancies induced by the co-doping of Ca and Sr ions boosts the key step of CH bond breaking and CC bond coupling in catalyzing low-temperature OCM. It is meaningful for the development of the low-temperature and high-efficient catalysts for OCM reaction in practical application.

Enhancing Ethene Production through Low-Temperature Oxidative Coupling of Methane: Leveraging DFT and Data Analysis for Crafting Innovative and Efficient Catalyst Compositions

Enhancing Ethene Production through Low-Temperature Oxidative Coupling of Methane: Leveraging DFT and Data Analysis for Crafting Innovative and Efficient Catalyst Compositions

Low-temperature oxidative coupling of methane over LaCeZr ternary oxide supported Mn–Na2WO4

LaCeZr ternary oxides supported Mn–Na2WO4catalyst achieves 20.9% C2yield at 700 °C with at least 50 hour stability.

Atomically dispersed SrOx species on exposed {2 2 2} facets of pyrochlore La2Zr2O7 nanocrystals for boosting low-temperature oxidative coupling of methane

The efficient catalysts of strontium oxide (SrOx) supported on exposed {222} facets of pyrochlore La2Zr2O7 nanocrystals (SrOx/LZO-Y) were elaborately synthesized by incipient wetness impregnation method. The introduction of atomically dispersed SrOx species on La2Zr2O7 surface can increase the amount of surface oxygen vacancies induced by Sr2+ substitution of La3+ cations, and the increasing of moderate surface alkaline sites are beneficial to enhance catalytic performance for oxidative coupling of methane (OCM). The SrOx/LZO-Y catalysts have super catalytic performance during low-temperature OCM reaction. SrOx/LZO-12 catalyst with the highest concentrations of O2– species exhibits the highest activity and the lowest apparent activation energy (84 kJ mol−1) during low-temperature OCM into C2H4 and C2H6 (C2) products, i.e., its CH4 conversion, selectivity and yield of C2 products at 600 °C are 30.5 %, 47.4 % and 14.5 %, respectively, and its ratio of C2H4/C2H6 products is 0.77. Based on the results of multiple experimental characterizations, the mechanism of SrOx/LZO-Y catalysts for OCM reaction is proposed: the surface alkaline active centers (O2–) induced by doping of Sr ions can promote the critical step of CH4 activation significantly during low-temperature OCM reaction. It is a new strategy to develop the low-temperature and high-activity catalysts for OCM in practical application.

Synthesis and characterization of the flower-like LaxCe1−xO1.5+δ catalyst for low-temperature oxidative coupling of methane

Lanthanum-based oxides are promising candidates for low-temperature oxidative coupling of methane (OCM). To further lower the OCM reaction temperature, the Ce doped flower-like La2O2CO3 microsphere catalysts were synthesized, achieving a significantly low reaction temperature (375 °C) while maintaining high C2+ hydrocarbon selectivity (43.0%). Doping Ce into the lattice of La2O2CO3 created more surface oxygen vacancies and bulk lattice defects, which was in favor of the transformation and migration of oxygen species at 350–400 °C. The designed H2 temperature-programmed reduction (H2-TPR) experiments provided strong evidence that the low reaction temperature of LaxCe1−xO1.5+δ can be attributed to the transformation and migration of oxygen species, which dynamically generated surface oxygen vacancies for continuous oxygen activation to selectively convert methane. Moreover, designed temperature-programmed surface reaction (TPSR) clarified that two kinds of surface oxygen species in LaxCe1−xO1.5+δ catalysts were concerned with catalytic performance, that is, the surface chemisorbed oxygen species for the activation of CH4 and the formation of CH3• intermediates, surface La-Ce-O lattice oxygen species that caused the excessive oxidation of CH3• intermediates. Finally, the factors affecting the transformation and migration of oxygen species were explored.

In-depth understanding of the crystal-facet effect of La2O2CO3 for low-temperature oxidative coupling of methane

Facet effect of lanthanum-based oxides on low-temperature oxidative coupling of methane (OCM) reaction have attracted great attention. In the present work, La2O2CO3 nanoparticles in the morphologies of triangular nanoplate and triangular nanocube with different ratio of {001} and {100} facets were prepared by facile hydrothermal method. The {001} facets exhibited higher OCM activity than {100} facets at 410 °C due to larger number of La3+–O2− sites, which could adsorb CO2 to form bidentate carbonate, and then reacting with oxygen to form adsorbed oxygen species. However, the higher methyl radical adsorption energy and the lower free energy of methane successive dehydrogenation of La2O2CO3{001} surface caused the excessive dissociation and deep oxidation of methyl radical, thus {001} facets show poor selectivity than {100} facets. Our findings will get a better understanding the facet effect of lanthanum-based oxides and provide guidance for the design of efficient low-temperature OCM catalysts.

Hybrid catalysts containing Ba, Ti, Mn, Na, and W for the low-temperature oxidative coupling of methane

The oxidative coupling of methane (OCM) using hybrid catalysts containing BaTiO3 perovskite and Mn-Na2WO4 exhibited high activity and high selectivity at low temperature: 66.3 % C2+ (olefins and paraffins) selectivity and 25.9 % C2+ yield at 700 °C which is 100 °C lower than that used for Mn-Na2WO4 catalysts (800 °C). Upon the preparation of complex catalysts, the insertion of Mn into BaTiO3 (BaTi(Mn)O3) and the formation of NaxMn(Ti)O2 improved the oxygen-supplying ability of the catalysts. Additionally, strong interactions between the WO42− anions and BaTi(Mn)O3 or NaxMn(Ti)O2 stabilized the WO42− anions on the surface and improved the methane activation ability of the catalysts for the favorable production of hydrocarbons. The nanoscopic modification of catalysts was confirmed using H2–temperature-programmed reduction, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy results.

Mechanochemical Route for Preparation of MFI-Type Zeolites Containing Highly Dispersed and Small Ce Species and Catalytic Application to Low-Temperature Oxidative Coupling of Methane

Mechanochemical Route for Preparation of MFI-Type Zeolites Containing Highly Dispersed and Small Ce Species and Catalytic Application to Low-Temperature Oxidative Coupling of Methane

Surface coupling of methyl radicals for efficient low-temperature oxidative coupling of methane

Selective coupling of methyl radicals to produce C2 species (C2H4 and C2H6) is a key challenge for oxidative coupling of methane (OCM). In traditional OCM reaction systems, homogeneous transformation of methyl radicals in O2-containing gases are uncontrollable, resulting in limited C2 selectivity and yield. Herein, we demonstrate that methyl radicals generated by La2O3 at low reaction temperature can selectively couple on the surface of 5 wt% Na2WO4/SiO2. The controllable surface coupling against overoxidation barely changes the activity of La2O3 but boosts the C2 selectivity by three times and achieves a C2 yield as high as 10.9% at bed temperature of only 570 °C. Structure-property studies suggest that Na2WO4 nanoclusters are the active sites for methyl radical coupling. The strong CH3· affinity of these sites can even endow some methane combustion catalysts with OCM activity. The findings of the surface coupling of methyl radicals open a new direction to develop OCM catalyst. The bifunctional OCM catalyst system, which composes of a methane activation center and a CH3· coupling center, may deliver promising OCM performance at reaction temperatures below the ignition temperature of C2H6 and C2H4 (~600 °C) and is therefore more controllable, safer, and certainly more attractive as an actual process.

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.

Electron-Hopping Brings Lattice Strain and High Catalytic Activity in the Low-Temperature Oxidative Coupling of Methane in an Electric Field

Detailed reaction mechanisms for the oxidative coupling of methane (OCM) over Ce2(WO4)3 catalysts at low temperatures in an electric field were investigated. The influence of Ce cations in the Ce2(WO4)3 catalyst was evaluated by comparing the OCM activity over various Ln2(WO4)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) catalysts in an electric field. The electronic states of Ln and W cations and the relationship between the distorted Ce2(WO4)3 structure and methane activation were examined using X-ray absorption fine structure (XAFS) measurements and first-principles calculations. The results reveal that the Ln2(WO4)3 catalysts with redox-active Ln cations (Ce, Pr, Sm, Eu, and Tb) show OCM activity. First-principles calculations indicate that Ce3+ species in the Ce2(WO4)3 structure are oxidized to Ce4+ species in an electric field by extracting electrons from the Ce 4f orbitals near the Fermi level; as a result, its structure is distorted. The results indicate that the redox reaction of Ln cations in ...

MnTiO3-driven low-temperature oxidative coupling of methane over TiO2-doped Mn2O3-Na2WO4/SiO2 catalyst.

Oxidative coupling of methane (OCM) is a promising method for the direct conversion of methane to ethene and ethane (C2 products). Among the catalysts reported previously, Mn2O3-Na2WO4/SiO2 showed the highest conversion and selectivity, but only at 800° to 900°C, which represents a substantial challenge for commercialization. We report a TiO2-doped Mn2O3-Na2WO4/SiO2 catalyst by using Ti-MWW zeolite as TiO2 dopant as well as SiO2 support, enabling OCM with 26% conversion and 76% C2-C3 selectivity at 720°C because of MnTiO3 formation. MnTiO3 triggers the low-temperature Mn2+↔Mn3+ cycle for O2 activation while working synergistically with Na2WO4 to selectively convert methane to C2-C3. We also prepared a practical Mn2O3-TiO2-Na2WO4/SiO2 catalyst in a ball mill. This catalyst can be transformed in situ into MnTiO3-Na2WO4/SiO2, yielding 22% conversion and 62% selectivity at 650°C. Our results will stimulate attempts to understand more fully the chemistry of MnTiO3-governed low-temperature activity, which might lead to commercial exploitation of a low-temperature OCM process.

Low-temperature oxidative coupling of methane in an electric field using carbon dioxide over Ca-doped LaAlO3 perovskite oxide catalysts

Methane conversion was conducted over various perovskite oxide catalysts at 423K using carbon dioxide as an oxidant in an electric field. La0.7Ca0.3AlO3-δ catalyst showed high CO2-OCM activity. The highest CO2-OCM activity, C2 yield of 7.4%, was obtained even at 348K, that value was higher than those of other earlier reports. Dry reforming of methane to produce syngas (H2+CO) proceeded simultaneously, and no other byproduct was observed. Results of in-situ IR, XRD and TG measurements demonstrated that CO2 adsorption on the catalyst surface and formation of La2O2CO3 intermediate are important for progress of the CO2-OCM reaction, even at low temperature in an electric field. Ca atom incorporated in LaAlO3 perovskite played an important role in C2 hydrocarbon formation.

Review Low Temperature Catalysts for Oxidative Coupling of Methane

A simple review is given to the recent work of the oxidative coupling of methane at low temperature. Emphasis is laid on the different systems of low-temperature catalysts under conventional CH 4 /O 2 co-feed conditions, and on the investigations of low-temperature oxidative coupling of methane in the presence of steam in the feed. Other approaches, e.g. oxidative coupling of methane at elevated pressure and moderate temperature, preparing ethylene by oxidative coupling reaction of methane on laser-activated solid surface, are also included.

The low-temperature oxidative coupling of methane over zirconium oxide

Zirconium oxide is shown to be capable of catalyzing the conversion of methane to ethane at temperatures as low as 530 °C. The lowest temperature at which ethane is produced is found to be dependent upon the method employed for the preparation of the catalyst. The presence of surface hydroxyl groups appears to be necessary for the production of ethane at these low temperatures.