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Improving the selectivity in the oxidative coupling of methane to ethane/ethylene poses a significant challenge for commercialization. The required improvements are hampered by the uncertainties associated with the reaction mechanism due to its complexity. Herein, we report about 90 % selectivity to the target products at 11 % methane conversion over Gd2O3‐based catalysts at 700 °C using N2O as the oxidant. Sophisticated kinetic studies have suggested the nature of adsorbed oxygen species and their binding strength as key parameters for undesired methane oxidation to carbon oxides. These descriptors can be controlled by a metal oxide promoter for Gd2O3. [ABSTRACT FROM AUTHOR]

The present study of oxidative coupling of methane (OCM) over MnOx−Na2WO4/support catalysts demonstrated that the selectivity to C2H6 and C2H4 (C2‐hydrocarbons) is affected by the kind of support, co‐fed water, and the kind of oxidant (O2 vs. N2O). In addition to previous studies with MnOx−Na2WO4/SiO2, an enhancing water effect was obtained using catalysts based on TiO2‐ or ZrO2‐containing supports. However, a negative effect on methane conversion was established for SiO2−Al2O3‐supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnOx−Na2WO4 to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnOx−Na2WO4. Among the prepared catalysts, MnOx−Na2WO4/TiO2 was found to be promising for H2O‐assisted OCM. The use of N2O instead of O2 further increases the selectivity to C2‐hydrocarbons to 84 % at 6.8 % CH4 conversion due to the formation of predominantly monoatomic oxygen species from N2O that selectively convert CH4 into C2H6. [ABSTRACT FROM AUTHOR]

[Display omitted] • Alkali metal in MnO x -M 2 WO 4 /SiO 2 affects activity under anaerobic conditions. • Lattice oxygen of MnO x -M 2 WO 4 /SiO 2 can convert CH 4 to hydrocarbons. • OCM over MnO x -M 2 WO 4 /SiO 2 and M 2 WO 4 /SiO 2 proceeds via adsorbed oxygen species. • Presence of MnO x does not create more active sites but changes their reactivity. This work unveils the fundamentals relevant for activity and product selectivity in the oxidative coupling of methane (OCM) over the (MnO x)-M 2 WO 4 /SiO 2 (M = Na, K, Rb, or Cs) catalysts. The presence of the molten phase of Na 2 WO 4 and the ability of the catalysts to release lattice oxygen were not found to be the performance-governing factors. As proven by oxygen isotopic exchange experiments combined with thorough catalytic tests, the performance of both tri- and bimetallic catalysts is defined by the involvement of adsorbed oxygen species formed from gas-phase O 2. Lattice oxygen of M 2 WO 4 /SiO 2 is not capable of converting CH 4 to C 2 H 6 , while that of MnO x -M 2 WO 4 /SiO 2 is active for this reaction. The OCM activity of M 2 WO 4 /SiO 2 decreases with the atomic weight of alkali metal, whereas there is no such effect in the presence of MnO x , which, however, increases the activity. The enhancing effect was explained by improving the active site turnover. [ABSTRACT FROM AUTHOR]

[Display omitted] • The performance of Mn-Na 2 WO 4 /SiO 2 catalysts in CL-OCM can be significantly improved by cofed water. • Water does not alter the scheme of product formation but influences their kinetics. • Water facilitates the releasability of lattice oxygen. • The role of MnO x and Na 2 WO 4 is shown by in situ XRD and UV–vis spectroscopy. The oxidative coupling of methane (OCM) to C 2 H 6 and C 2 H 4 (C 2 -hydrocarbons) is an industrially attractive reaction, which has not yet been commercialized. Apart from the low selectivity to C 2 -hydrocarbons at high degrees of CH 4 conversion, another substantial drawback is the necessity to use pure O 2 when this oxidant and methane are cofed. Original OCM studies used an approach called chemical looping OCM (CL-OCM) that consists of alternating feeding of methane and air for product formation and catalyst reoxidation, respectively. The developed catalysts, however, suffer from their low ability to provide lattice oxygen for the desired reaction. Herein, we demonstrate that this catalyst property can be significantly improved when performing CL-OCM over Mn-Na 2 WO 4 /SiO 2 catalysts in the presence of water. In comparison to water-free CL-OCM, both methane conversion and C 2 -hydrocarbons selectivity increase resulting in at least doubling of the yield of these products. The selectivity to ethylene of about 53 % was obtained at about 24 % CH 4 conversion. The total selectivity to C 2 -hydrocarbons was about 76 %. The analysis of selectivity-conversion relationships for C 2 H 4 , C 2 H 6 , CO and CO 2 proved that water does not alter their general formation pathways but influences the kinetics of their formation. In situ X-ray diffraction, in situ UV–vis spectroscopic, and temperature-programmed tests (H 2 -TPR and O 2 -TPD with or without cofed H 2 O) showed that Mn 2 O 3 acts as the oxygen carrier in CL-OCM. In-situ DRIFTS proved the presence of O 2 – and O 2 2–. They can interact with water yielding OH radicals that accelerate CH 4 conversion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mn-Na 2 WO 4 /SiO 2 oxidized CH 4 with 87.3% C 2+ -hydrocarbons selectivity at 675 °C. • H 2 O reduces the reaction order of C 2 H 6 formation with respect to O 2 near to zero. • Low oxygen partial pressure intensifies the enhancing H 2 O effect on C 2 H 6 formation. • The strength of this effect also increases with a decrease in reaction temperature. • Breaking the dilemma of low selectivity in the oxidative coupling of methane over Mn-Na 2 WO 4 /SiO 2 at low temperatures. Oxidative coupling of methane allows to directly produce higher hydrocarbons (C 2 -hydrocarbons, mainly C 2 H 6 and C 2 H 4) from cheap and available methane. Although various catalysts have been developed, they operate selectively above 750 °C that is detrimental in terms of reactor construction materials and safe operation. The present study demonstrates that the supported Mn-Na 2 WO 4 /SiO 2 catalyst achieves the selectivity to C 2+ -hydrocarbons of 87.3% at methane conversion of 6.6% at 675 °C only. With the aid of sophisticated kinetic measurements, the origins of this unexpectedly high performance have been elucidated. When operating with water-containing feeds, the reaction order of the rate of C 2 H 6 formation with respect to O 2 at 675 °C is practically zero in the presence of water, while the corresponding values for CO and CO 2 are 0.9 and 0.7. Moreover, O 2 partial pressure is also decisive for the strength of the enhancing water effect. The water-induced increase in the rate of CH 4 conversion into C 2 -hydrocarbons can be about 120 times using a feed with the ratio of CH 4 /O 2 of 24. The corresponding enhancement factors for the formation of CO and CO 2 are only 20 and 6, respectively. These new fundamentals provide hints for optimizing reactor operation for efficient conversion of methane into C 2+ -hydrocarbons at low temperatures. [ABSTRACT FROM AUTHOR]