610227 Transient Kinetic Characterization Surface Oxygen Species to Understand Steady-State Performance of Oxidative Coupling of Methane

Tuesday, November 17, 2020
Catalysis and Reaction Engineering Division (20) (PreRecorded+)
Yixiao Wang1, Bingwen Wang2, Sagar Sourav3, M. Ross Kunz1, Adam Yonge4, Zongtang Fang1, Rakesh Batchu1, Andrew Medford5, Eranda Nikolla2 and Rebecca Fushimi1, (1)Idaho National Laboratory, Idaho Falls, ID, (2)Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI, (3)Chemical & Biomolecular Engineering, Lehigh University, Bethlehem, PA, (4)Chemical Engineering, Georgia Institute of Technology, Atlanta, GA, (5)School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA

The Mn/Na2WO4/SiO2 catalyst is one of the most advanced catalysts for the OCM process but still shows comparatively low C2 yield that is below the threshold for industrial implementation. The adsorbed oxygen species on the catalyst surface are thought to play a major role in OCM performance [1]. The literature contains few reports regarding the correlation of the OCM reactivity with kinetic processes of surface oxygen [2]. An experimental methodology using isotopic O2 pump/probe transient kinetic experiments was described to characterize the surface oxygen transformation process. In addition, steady state flow experiments were applied to study OCM performance.

We performed pump/probe experiments over Mn/Na2WO4/SiO2 catalyst system between 16O2 and 18O2 at 700 ℃. Based on the kinetic analysis, monoxo species were suggested as the main oxygen species on Mn site. 2%Mn/SiO2 showed the highest oxygen activation ability while C2+ yield was less than 8%, which suggested that monoxo species was not active for OCM performance. The poor OCM performance of Na2WO4/SiO2 was due to the low gas-phase oxygen activation.

Dioxo species were indicated as the main oxygen species in the Mn and W complex system based on the kinetic analysis. Both 1.5%Mn and 4%Mn over Na2WO4/SiO2 showed best OCM performance with the highest ratio of Y(16O2)/Y(18O16O). The mean desorption time of dioxo species was longer than the monoxo species. These experiments implied that the dioxo species played determining roles in enhancing OCM performance and the monoxo species may act as unselective oxygen. An optimum Mn loading for C2 yield is correlated to an optimal surface residence time and concentration of dioxo and monoxo species. These transient kinetic experiments provide a new understanding for OCM catalyst operation at industrial conditions.

[1] Kim, I., et al., Molecular Catalysis, 2017. 435: 13-23.

[2] Kondratenko, E.V. and J. Pérez-Ramírez, Catalysis today, 2007. 119(1-4):243-246.


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