(257h) Structure and Promotion Effect of Mn-Oxide during Oxidative Coupling of Methane (OCM) Reaction over Supported Mn-Na2WO4/SiO2 Catalysts

The addition of Mn to the supported Na₂WO₄/SiO₂ catalyst has been appreciated by many investigations towards improved catalytic activity for oxidative coupling of methane reaction. However, despite large number of characterization and kinetic studies, the stable structure of Mn-oxide phase and its clear promotion effects are still under debate in the literature, which is the focus of our current investigation.

The stable structure of the Mn-oxide phase present in 1.2Mn-5Na₂WO₄/SiO₂ catalyst was investigated by in-situ Raman spectroscopy (see Figure 1). The freshly calcined dehydrated catalyst possesses surface Na-WO_x and MnO_x species along with crystalline Na₂WO₄ and Mn₂O₃ nano-particles (NPs). Stability analysis of above metal oxide phases under OCM reaction conditions established that only surface Na-WO_x and MnO_x species are thermally stable in OCM environment, whereas Na₂WO₄ and Mn₂O₃ NPs are unstable due to melting and reduction of these oxide phases, respectively.

Further, multiple chemical probe experiments (H₂-TPR, O₂-TPD, ¹⁶O₂-¹⁸O₂ pump-probe experiment in TAP reactor) and transient (in TAP reactor) as well as stead-state (in fixed bed flow reactor) kinetic experiments were conducted in different environmental conditions to understand the promotion mechanism of Mn. The above experiments reveal that the Mn-oxide phase, in itself, is less active and highly unselective towards CH₄ oxidation. However, Mn addition (i) improves the reducibility and release rate of lattice oxygen species, (ii) helps in low-temperature activation of lattice oxygen species, and (iii) improves the exchange of gas-phase oxygen and lattice oxygen species present in supported Na₂WO₄/SiO₂ catalyst. These promotional effects of Mn are reflected in higher CH₄ activity and C₂H₄ to C₂H₆ ratio during steady-state OCM reaction. Moreover, the above conclusions confirm that the WO_x species are active for OCM reaction and Mn only acts as a chemical promoter.