STURCTURAL FLEXIBILITY UNDER OXIDATIVE COUPLING OF METHANE; MAIN CHEMICAL ROLE OF ALKALI ION IN [MN+(LI, NA, K OR CS)+W]/SIO2 CATALYSTS

Molecular structure and catalytic promotional effect of Mn on supported Na2WO4/SiO2 catalysts for oxidative coupling of methane (OCM) reaction

Influences of cation and anion substitutions on oxidative coupling of methane over hydroxyapatite catalysts

Catalysts descriptors for representing catalytic activities have been challenging in regard to machine learning. Machine learning and catalyst big data generated from high-throughput experiments are combined to explore the catalyst descriptors. Catalyst descriptors are designed using the physical quantities from the periodic table in the oxidative coupling of methane (OCM) reaction. Machine learning unveils the five key physical quantities representing ethylene/ethane selectivity (C2s) in the OCM reaction, where machine learning predicted three catalysts to have high C2s values. Experiments confirm that the proposed three catalysts have high C2s values in the OCM reaction. Hence, the physical quantities can be used as alternative descriptors for designing heterogeneous catalysts.

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.

BACKGROUNDIn this work, the supported Mn‐Na2WO4/γ‐Al2O3, Mn‐Na2WO4/γ‐Al2O3‐TiO2 and Mn‐Na2WO4/ γ‐Al2O3‐SiO2 catalysts were prepared using the dry impregnation procedure and tested for suitability to the oxidative coupling of methane (OCM) reaction. The synthesized catalyst was characterized by various techniques such as X‐ray diffraction (XRD), Fourier transform infrared (FTIR), X‐ray photon spectroscopy (XPS), temperature‐programmed reduction (TPR), Brunauer–Emmett–Teller (BET) surface area and field emission scanning electron microscopy (FESEM), and the support effect of the prepared catalysts in the OCM reaction was evaluated.RESULTSThe XPS results showed that the tetrahedral WO42− phase is presented in all prepared catalysts and it is expected that the presence of this phase is correlated with high OCM activity. The XPS results show that the amount of O2− in the treated Mn‐Na2WO4/γ‐Al2O3‐TiO2 catalyst at 800 °C is higher than that in the Mn‐Na2WO4/γ‐Al2O3 and Mn‐Na2WO4/γ‐Al2O3‐SiO2 catalysts, but is reduced with increasing temperature. At 850 °C, the amount of O2− in the treated Mn‐Na2WO4/γ‐Al2O3 catalyst is higher than in the Mn‐Na2WO4/γ‐Al2O3‐TiO2 and Mn‐Na2WO4/γ‐Al2O3‐SiO2catalysts.CONCLUSIONThe results show that the MnSiO3 and MnTiO3 sites may deliver a much easier Mn2+↔Mn3+ cycle than the MnAl2O4 and MnWO4 sites at low temperatures, and may play a principal role in enhancing the catalyst activity in the OCM reaction at 800 °C. © 2023 Society of Chemical Industry (SCI).

Natural gas (NG), mainly composed of methane, is extremely resistant to autoignition. High efficiency premixed compression ignition (CI) combustion modes are therefore difficult to achieve in NG engines. One potential method for expanding the range of compression ignition operation in NG engines is to pretreat the fuel such that it becomes more reactive. This paper presents results from one-dimensional simulations of a medium duty multi-cylinder natural gas engine partially fueled by products from oxidative coupling of methane (OCM), a catalytic process by which methane is converted to C2 hydrocarbons like ethane and ethylene. First, the results of OCM reactor three-dimensional CFD simulations are presented to define the resulting product species distribution over a range of residence time. Using the reactor results, a one-dimensional model of an engine is implemented with and without OCM products to illustrate efficiency gains possible by operating in a compression ignition mode. Results of the modeling show that brake thermal efficiency improves by 1 to 7% with OCM-enabled CI combustion compared to spark ignition combustion of natural gas alone. The results also show that since the OCM reactor is exothermic, part of the incoming natural gas heating value is converted to sensible enthalpy. The OCM products therefore must be used to preheat the intake charge, thus avoiding an overall brake thermal efficiency (BTE) penalty. Overall, this study finds that OCM products have the potential to increase BTE of NG engines by allowing them to operate in premixed CI modes. Future work must look to improve OCM catalyst durability and reactor turndown ratio to provide a reliable high reactivity fuel stream for NG engines.

A direct method of converting natural gas into ethylene is the heterogeneously catalyzed oxidative coupling of methane (OCM), however, only with hydrocarbon yields limited to 30-35% despite enormous efforts to optimize the catalysts. By combining the exothermic OCM with a secondary process, namely steam reforming of methane (SRM), the methane conversion can be increased significantly while improving temperature control and simultaneously producing valuable synthesis gas. In this thesis, two different reactor concepts were developed to integrate the OCM and SRM reactions in an overall autothermal process, so that the OCM process is effectively cooled and the generated reaction energy is efficiently used to produce synthesis gas. The integration is most optimally achieved on the catalyst particle scale, which would eliminate the need for external heat exchange and opens up the possibility to use distributive oxygen dosing with which much higher product yields can be achieved. It is proposed to use a dual function catalyst particle in which the two chemical processes are physically separated by an inert, porous layer, such that additional diffusional resistances are intentionally created to control the reaction rates. This concept was studied with numerical simulations on the scale of a single catalyst particle and on reactor scale. It was found that the SRM and OCM reaction rates could be effectively tuned to achieve autothermal operation at the reactor scale, while the methane conversion was enhanced from 44% to 55%. An alternative integrated process can be achieved by combining OCM and SRM in a heat exchange reactor comprising of two separate reaction chambers which are thermally coupled. The OCM is carried out in packed bed reverse flow membrane reactor tubes submerged into a fluidized bed where the unconverted methane and byproducts from OCM are reformed, thus producing synthesis gas and consuming the reaction heat liberated by OCM. The feasibility of this concept is supported by experiments of OCM on a Mn/Na2WO4/SiO2 catalyst in a packed bed (porous Al2O3) membrane reactor. The results demonstrated that a C2 yield of 25-30 % can be achieved and that distributed feed of oxygen is optimal for the combined OCM/SRM reactor concept.

Performance and reaction mechanism of Mg-doped and Li-doped La2O3 catalysts in oxidative coupling of methane: DFT and experiment study

The effect of B site element in LaBO3(B = Al, Ga, In, Yb, Co, Mn, Fe) perovskites on the catalytic performance for the oxidative coupling of methane (OCM) reaction is investigated. Among the catalysts tested, LaAlO3, LaGaO3, LaInO3,and LaYbO3show higher activity for the formation of C2hydrocarbons in the OCM reaction. From the kinetic study on LaAlO3and LaCoO3, which are the highest and lowest active catalysts, the CH3• formation, as the first step in the OCM reaction, via the reaction of CH4with oxygen is strongly affected by the B site element. X‐ray photoelectron spectroscopy analysis and the16O/18O isotopic exchange reaction suggest that the role of the B site element in LaBO3perovskite is to create enough amount of surficial mobile oxygen species.

Effect of different oxygen species on the oxidative coupling of methane over TiO2 catalysts

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2 WO4 /SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2 -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na2 WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na-WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that (i) surface Na-WOx sites are responsible for selectively activating CH4 to C2 Hx and over-oxidizing CHy to CO and (ii) molten Na2 WO4 phase is mainly responsible for over-oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2 H6 to C2 H4 . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2 WO4 /SiO2 catalysts.

Isotopic Oxygen Exchange between Dioxygen and MgO Catalysts for Oxidative Coupling of Methane

The combination of electronic and catalytic features, in conjunction with empirical investigation, provides enriched perspectives on the analysis of catalysts, thus propelling progress and design. This study employs computational methods to deduce electronic characteristics, including properties such as bandgap, Fermi energy, and magnetic moment, for known catalysts involved in the oxidative coupling of methane (OCM) reaction. Through the comparison of these attributes with existing experimental OCM data, the ability to forecast the effectiveness of catalysis and subsequent reaction results is achieved, spanning CH4, C2H4, C2H6, and CO2 production. Transition metals, including Pt, Rh, Ru, and Ir, turn out to be promising catalyst promoters of OCM reactions. This study identified 58 innovative blends of metallic oxides and 3480 new catalytic configurations specifically designed for methane conversion at a moderately low temperature of 700 °C, placing them as effective catalysts for the OCM reaction. These emerging catalysts are projected to result in a rise in methane conversion extending from ±38.5% to ±95%, presenting a significant increase from the upper limit methane conversion of 36% reported in previous investigations.

Boosting the generation of key intermediate methyl radical (CH3•) in OCM reaction on magnesium oxide catalysts by regulating the electronic state of the active site

Statistical modeling applied to the oxidative coupling of methane reaction over porous (SrxLa1-x)CeO mixed oxides for optimization of C2 yield, C2 selectivity, and C2H4 selectivity