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

Abstract

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.

Effects of N2O and Water on Activity and Selectivity in the Oxidative Coupling of Methane over Mn–Na2WO4/SiO2: Role of Oxygen Species

“…This process also contributes to the acceleration of the rate of CH 4 conversion into C 2 through cofed water (Figure 2b). According to Takanabe and co-workers, 45,46 45,46 The monatomic oxygen species seem to reveal higher reactivity than biatomic oxygen species for heterogeneous CH 4 activation. The strength of the enhancing effect on the activity in N 2 O−OCM is, however, less pronounced (1.8 vs 5.3 at 775 °C) in comparison with O 2 −OCM (Figure 3b, Figure S6).…”

“…In addition to H 2 O 2 , a monatomic oxygen species is formed. H 2 O 2 decomposes to two OH radicals accelerating CH 4 conversion into a CH 3 radical in the gas phase. , The monatomic oxygen species seem to reveal higher reactivity than biatomic oxygen species for heterogeneous CH 4 activation. The strength of the enhancing effect on the activity in N 2 O–OCM is, however, less pronounced (1.8 vs 5.3 at 775 °C) in comparison with O 2 –OCM (Figure b, Figure S6).…”

“…This process also contributes to the acceleration of the rate of CH 4 conversion into C 2 hydrocarbons through cofed water (Figure b). According to Takanabe and co-workers, , sodium peroxide or potassium peroxides in Na 2 WO 4 /SiO 2 or K 2 WO 4 /SiO 2 are responsible for the conversion of H 2 O to H 2 O 2 . As we have previously established the enhancing water effect over catalysts free of such peroxides, e.g., MnO x /SiO 2 or PbO x /SiO 2 , we suggest that adsorbed biatomic oxygen species are involved in this reaction.…”

Machine Learning-Aided Catalyst Modification in Oxidative Coupling of Methane via Manganese Promoter

“…Moreover, the role of Mn still remains an open question in OCM studies. ,, For instance, there has been reports on not only the positive effect in Mn-Na 2 WO 4 /SiO 2 and K x MnO y /SiO 2 but also the negative effect in the La 2 O 3 - or MgO-based catalyst . Because under the OCM the following occur: (i) C 2 production is influenced not only via the surface of catalyst but also via the outer surfaces (gas-phase) reaction. − (ii) The reaction usually performs under a high reaction temperature above melting points of metal components. − (iii) Synergy of components might be different from kinds of support, ,, and the effect of Mn in OCM under operation conditions is still a controversial subject. In other words, it is demonstrated that the HTS data-driven ML approach can aid catalyst investigation without scientists’ experiences and discover the positive fourth element (viz.…”

The Role of Adsorbed and Lattice Oxygen Species in Product Formation in the Oxidative Coupling of Methane over M2WO4/SiO2 (M = Na, K, Rb, Cs)

“…Rb 2 WO 4 melts at 959 °C, while the OCM reaction was carried out at 850 °C. Recently, the K 2 WO 4 /SiO 2 catalyst was reported to show the same selectivity as its Na counterpart despite the presence of the crystalline K 2 WO 4 phase …”