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

To introduce promotional H 2 O effects for both CH 4 rate and C 2 selectivity, the OH radical formation, catalyzed through H 2 O activation with O 2 surface species, was critical for modeling selective Mn-K 2 WO 4 /SiO 2 catalysts. Based on our reported experimental evidence, which demonstrates the formation of H 2 O 2 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 H 2 O effects on both OCM rate and selectivity. The data from a low-pressure microkinetic study were treated isothermally, and extended for nearindustrially relevant pressures up to 901 kPa. Thermal visualization using an infrared camera found substantial temperature increases at undiluted high-pressure conditions which caused C 2 selectivity to drop significantly. When the furnace temperatures were decreased after ignition, side reactions after O 2 depletion (e.g., hydrocarbon reforming) were suppressed, obtaining 13.7 (11.8) % yields at 19.9 % CH 4 conversion with 68.6 (59.1) % selectivities for C 2-4 (C 2 ) at 901 kPa. The temperature was found to be the determining factor of C 2 yield which was perturbed by varying space velocity or CH 4 /O 2 ratios. The optimum temperature for high-pressure conditions was predicted as 885 degrees C at 901 kPa. The study provides mechanistic and industrially relevant understandings for further OCM catalyst design and system application.