Oxidative coupling of methane under microwave: core-shell catalysts for selective C₂ production and homogeneous temperature control

The oxidative coupling of methane (OCM) was investigated using a catalyst with a core@shell structure or a physical mixture comprised of MgO and SiC or Fe3O4, which was thermally activated via two different heating methods, namely, conventional resistive heating and microwave heating. The use of microwave radiation together with the catalyst structure was essential to achieve high reaction efficiency. The C2 selectivity and yield were correlated with the presence of temperature gradients in the catalytic bed under microwave radiation. These thermal gradients and their distribution were experimentally evaluated using operando thermal visualization. Hotspots and thermal gradients were beneficial to achieve a higher CH4 conversion; however, it was found that a uniform reactor temperature was crucial to attain a high C2 yield in OCM and the core@shell structure is beneficial. The hypothesis that an enhanced OCM performance can be achieved by keeping the catalyst material hot and the gas cold, using microwave to prevent uncontrolled gas-phase reactions was supported by a kinetic study and experimentally demonstrated.ChemE/Catalysis Engineerin

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

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.ChemE/Catalysis Engineerin