Abstract
AbstractStructural dynamics of a Mn‐Na2WO4/SiO2 catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na2WO4 with Mn7SiO12. The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.
Highlights
Results and DiscussionThe oxidative coupling of methane (OCM) to ethane and ethene represents an attractive alternative to current crudeoil-based processes in order to access value-added chemicals.[1]
According to Equation (1), active sodium oxide species, which are responsible for high activity and selectivity in the oxidative coupling of methane,[6o] are generated in the catalytically relevant temperature regime in small amounts
While the structural synergy of all phases is responsible for the high stability and activity of the catalyst, the supported Na2WO4 phase acts as storage phase responsible for transient generation of active sodium oxide species that would, in absence of the stabilizing Mn7SiO12– MnWO4 redox couple, suffer from steady sublimation,[16] leading to catalyst deactivation.[6b,g] As long as the oxygen partial pressure in the reactor is not too low, a small steadystate concentration of the active phase is formed according to Scheme 1
Summary
The oxidative coupling of methane (OCM) to ethane and ethene represents an attractive alternative to current crudeoil-based processes in order to access value-added chemicals.[1]. The band of Mn7SiO12 at 958 cmÀ1 is no longer distinguishable at higher temperatures due to shift or broadening of the band of Na2WO4 at 927 cmÀ1 The latter is most likely caused by phase transition and melting of Na2WO4 or by formation of tetrahedrally coordinated, silica-supported. At 226 8C, which is assigned to the phase transition of the acristobalite support to b-cristobalite.[12] Oxygen evolution from the catalyst is shifted to significantly lower temperatures when compared to the Mn7SiO12 reference, with the onset recorded at 653 8C (2) This could be associated with the phase transition of cubic Na2WO4 into unknown transient phases at 600 8C, the commencing formation of MnWO4 at 650 8C as well as the disappearance of Mn7SiO12, observed from 650 8C onwards (Figure 2 B). This, in turn, leads to the reduction of Mn3+ (in Mn7SiO12) to Mn2+ (in MnWO4), the release of molecular oxygen with its maximum at 927 8C ((3) and (4) in Figure 3 B), and the formation of amorphous or dispersed sodium oxide
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