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

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 C2-4 (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.