Oxidative coupling of methane (OCM) over supported the Na2WO4/SiO2 catalyst (prepared by incipient-wetness impregnation) was studied to identify the catalyst structure, roles of various oxide phases and catalytic active sites using in-situ Raman spectroscopic technique, along with transient kinetic analysis and steady-state (SS) OCM reaction performance. The in-situ Raman spectra of this catalyst under dehydrated conditions (400 °C, 10% O2/Ar) reveal the presence of surface Na-WOx species along with the crystalline Na2WO4 phase and the β-cristobalite phase of SiO2 support. Stability analysis of above metal oxide phases under OCM reaction conditions (900 °C, CH4+O2+N2 3.3:1:4), through in-situ Raman spectroscopy (see Figure 1 (left)), established that only surface Na-WOx sites, anchored to the SiO2 support, are thermally stable under OCM environment; whereas the crystalline Na2WO4 phase is unstable and melts. To further understand the nature and types of oxygen species associated with two different types of metal oxide phases (surface Na-WOx and molten Na2WO4), 16O2-18O2 pump-probe experiments were conducted in a Temporal Analysis of Products (TAP) reactor (see Figure 1 (Right)). The evolution of molecular 16O2* species (after 18O2 introduction) from the lattice of the supported 5%Na2WO4/SiO2 catalyst was observed at 800 °C. At lower temperature (650 °C) where the Na2WO4 phase crystallizes or when only dispersed Na-WOx sites are present (0.5%Na-5%WOx/SiO2 catalyst), the evolution of molecular 16O2* does not occur. This suggests that the evolved O2* species are associated with the molten Na2WO4 phase. Additional CH4 series pulsing experiment in the TAP reactor studies revealed the presence of an O* type species during anaerobic OCM. To determine the roles of these two oxygen species towards the catalytic OCM reaction, experimental protocols were undertaken with transient O2-CH4, O2-C2H6 and O2-C2H4 pump-probe experiments in the TAP reactor and SS-OCM reaction in a flow reactor. These studies showed that (i) the O2* species are mostly involved in the over-oxidation of CH4 to CO2 and in the catalytic dehydrogenation of C2H6 to C2H4 and (ii) the O* species selectively activate CH4 to form C2H6 and oxidize CHx to form CO.

New mechanistic and reaction pathway insitings into oxidative coupling of methane reaction over supported Na2WO4/SiO2 catalysts

ACS Fall 2021

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