High-Pressure oxidative coupling of methane on alkali metal catalyst – Microkinetic analysis and operando thermal visualization

Yu, Yuhang; Obata, Keisuke; Movick, William J.; Yoshida, Shintaro; Palomo Jiménez, J.; Lundin, Sean Thomas B.; Urakawa, A.; Sarathy, S. Mani; Takanabe, Kazuhiro

doi:10.1016/j.jcat.2024.115414

High-Pressure oxidative coupling of methane on alkali metal catalyst – Microkinetic analysis and operando thermal visualization

Title

High-Pressure oxidative coupling of methane on alkali metal catalyst – Microkinetic analysis and operando thermal visualization

Author

Yu, Yuhang (University of Tokyo)
Obata, Keisuke (University of Tokyo)
Movick, William J. (University of Tokyo)
Yoshida, Shintaro (University of Tokyo)
Palomo Jiménez, J. (TU Delft ChemE/Catalysis Engineering)
Lundin, Sean Thomas B. (National Institute of Advanced Industrial Science and Technology (AIST))
Urakawa, A. (TU Delft ChemE/Catalysis Engineering)
Sarathy, S. Mani (King Abdullah University of Science and Technology)
Takanabe, Kazuhiro (University of Tokyo; Japan Science and Technology Agency)

Date

2024

Abstract

To introduce promotional H₂O effects for both CH₄ rate and C₂ selectivity, the OH radical formation, catalyzed through H₂O activation with O₂ surface species, was critical for modeling selective Mn-K₂WO₄/SiO₂ catalysts. Based on our reported experimental evidence, which demonstrates the formation of H₂O₂ through surface alkali peroxide intermediate, the elementary reactions that account for the OH-mediated pathway were added into the microkinetic model. The advanced model adeptly replicated the promotional H₂O effects on both OCM rate and selectivity. The data from a low-pressure microkinetic study were treated isothermally, and extended for near-industrially relevant pressures up to 901 kPa. Thermal visualization using an infrared camera found substantial temperature increases at undiluted high-pressure conditions which caused C₂ selectivity to drop significantly. When the furnace temperatures were decreased after ignition, side reactions after O₂ depletion (e.g., hydrocarbon reforming) were suppressed, obtaining 13.7 (11.8) % yields at 19.9 % CH₄ conversion with 68.6 (59.1) % selectivities for C_2-4 (C₂) at 901 kPa. The temperature was found to be the determining factor of C₂ yield which was perturbed by varying space velocity or CH₄/O₂ ratios. The optimum temperature for high-pressure conditions was predicted as 885 °C at 901 kPa. The study provides mechanistic and industrially relevant understandings for further OCM catalyst design and system application.