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)

Abstract

MnO x −Na 2 WO 4 /SiO 2 is one of the best-performing catalysts in the oxidative coupling of methane (OCM) to C 2 hydrocarbons (C 2 H 6 and C 2 H 4 ). The current mechanistic concepts related to the selectivity to the desired products are based on the involvement of crystalline Mn-containing phases, the molten Na 2 WO 4 phase, surface Na−WO x species, and the associated lattice oxygen. Using in situ X-ray diffraction, operando UV−vis spectroscopy, spatially resolved kinetic analysis of product formation in steady-state OCM tests, and temporal analysis of products with isotopic tracers, we show that these phases/species are not categorically required to ensure high selectivity to the desired products. M 2 WO 4 /SiO 2 (M = Na, K, Rb, Cs) materials were established to perform similarly to MnO x −Na 2 WO 4 /SiO 2 in terms of selectivity−conversion relationships. The unique role of the molten Na 2 WO 4 phase could not be confirmed in this regard. Our alternative concept is that the activity of M 2 WO 4 /SiO 2 and product selectivity are determined by the interplay between the lattice oxygen of M 2 WO 4 and adsorbed oxygen species formed from gas-phase O 2 . This lattice oxygen cannot convert CH 4 to C 2 H 6 but oxidizes CH 4 exclusively to CO and CO 2 . Adsorbed monoatomic oxygen species reveal significantly higher reactivity toward overall CH 4 conversion and efficiently generate CH 3 radicals from CH 4 . These reactive intermediates couple to C 2 H 6 in the gas phase and are oxidized, to a lesser extent, by the lattice oxygen of M 2 WO 4 to CO and CO 2 . Adsorbed diatomic oxygen is involved in the direct CH 4 oxidation to CO 2 . The electronegativity of alkali metal in M 2 WO 4 was established to affect the catalyst ability to generate adsorbed oxygen species from O 2 . This knowledge opens the possibility to influence product selectivity by controlling the coverage by adsorbed and lattice oxygen via reaction conditions or catalyst design.

Surface Oxygen Vacancies Induced by Calcium Substitution in Macroporous La2Ce2–xCaxO7−δ Catalysts for Boosting Low-Temperature Oxidative Coupling of Methane

“…The conversion of CH 4 into valuable chemical products has attracted much attention . CH 4 can be directly converted into C 2 H 4 and C 2 H 6 (C 2 hydrocarbons) products in the presence of O 2 via an oxidative coupling of methane (OCM) process . So far, Li/MgO, Mn/Na 2 WO 4 /SiO 2 , and rare earth oxide-based catalysts have been extensively studied for the OCM reaction. − Although the Li/MgO catalyst exhibited excellent catalytic performance, its deactivation occurs quickly because of the rapid vaporization of Li + cations at elevated temperatures .…”

Oxidative Dehydrogenation of Ethane to Ethylene over Potassium Tungstate Catalysts

“…Nevertheless, there are other proposals regarding this promotional water effect, one being that water changes the distribution of various surface active oxygen species. − Wachs and co-workers stated that two kinds of active oxygen species existed on the surface, namely, dissolved molecular O 2 species and lattice atomic O species, by utilizing the temporal analysis of products (TAP) technique. , The atomic O species contributed to CO formation, and molecular O 2 was associated with CO 2 formation. , Sinev et al explained the promotional water effect on C 2 selectivity as water suppressing the formation of surface molecular oxygen species . Kondratenko and co-workers suggested that water induced the transformation of molecularly adsorbed oxygen species (O 2,s ) into their atomic counterparts (O s ). − O 2,s participated in CH 4 combustion to CO 2 , and O s was rather responsible for C 2 and CO formation. A higher C 2 selectivity was achieved because the water-induced lower O 2,s concentration inhibited the formation of undesired CO 2 . − Such hypothesis was consistent with the less pronounced water promotional effect in the N 2 O-OCM system, which might be caused by the low ability of N 2 O to generate O 2,s species that is responsible for the promotional water effect .…”

“…Kondratenko and co-workers suggested that water induced the transformation of molecularly adsorbed oxygen species (O 2,s ) into their atomic counterparts (O s ). − O 2,s participated in CH 4 combustion to CO 2 , and O s was rather responsible for C 2 and CO formation. A higher C 2 selectivity was achieved because the water-induced lower O 2,s concentration inhibited the formation of undesired CO 2 . − Such hypothesis was consistent with the less pronounced water promotional effect in the N 2 O-OCM system, which might be caused by the low ability of N 2 O to generate O 2,s species that is responsible for the promotional water effect . However, the experimentally observed kinetic behavior of decreased CO 2 selectivity upon water addition − does not contradict the mechanism of OH radical-mediated hydrocarbon activation, since the CO to CO 2 conversion was inhibited in the gas phase radical chemistry.…”

Direct Injection of Hydrogen Peroxide for Selective Homogeneous Conversion of Methane into Higher Hydrocarbons

“…The mechanism of CH 3 radical formation on alkali metal tungstates, however, remains under debate, in large part due to differences in the interpretation of the H 2 O promotional effect. Several studies identified surface O species as the H-abstractor, ,, attributing the formation of CO x to either molecularly adsorbed O 2 or to molten Na 2 WO 4 . The role of H 2 O in this case is explained as driving surface O 2 species to dissociate, increasing the fraction of selective monatomic O. , The addition of MnO x has been used to increase the CH 4 conversion rate, and several mechanistic interpretations have been applied.…”