(257h) Structure and Promotion Effect of Mn-Oxide during Oxidative Coupling of Methane (OCM) Reaction over Supported Mn-Na2WO4/SiO2 Catalysts
AIChE Annual Meeting
2021
2021 Annual Meeting
Catalysis and Reaction Engineering Division
Hydrocarbon Conversion I (Virtual)
Monday, November 15, 2021 - 8:40am to 9:00am
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-WOx and MnOx 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-WOx and MnOx 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 WOx species are active for OCM reaction and Mn only acts as a chemical promoter.