Oxidative coupling of methane over mixed metal oxide catalysts: Steady state multiplicity and catalyst durability

Exothermic heat effects are a crucial factor in determining the performance and stability of catalysts for the oxidative coupling of methane (OCM). Fixed bed temperature rise, steady state multiplicity, and catalyst durability are investigated over a range of feed conditions for the mixed metal oxides Cs/Sr/MgO, Cs/Ba/MgO, Cs/Sr/La2O3, and Na2WO4-Mn/SiO2. A comparison with previous studies on doped metal oxides catalysts for OCM clearly indicates that doping not only improves the performance but also significantly improves the catalyst stability. We experimentally demonstrate for the first time hysteresis behavior for Cs/Sr/La2O3 powder catalyst. Our results show that the catalyst stability depends on the magnitude of temperature rise in the catalyst bed. At a lower space velocity of 3,600 cc/h/g, the catalysts exhibit moderate temperature rise (< 50 degrees C) at complete O-2 conversion and sustained activity for extended time-on-stream (50-72h). The performance of Cs/Sr/MgO and Cs/Ba/MgO are comparable to Na2WO4-Mn/SiO2 (similar to 19% C2+ yield). While Cs/Sr/La2O3 activates at lower temperature, its maximum C2+ yield (similar to 14%) is lower. At a higher space velocity of 14,400 cc/h/g, a significant temperature rise of similar to 300 degrees C and ignition-extinction behavior is encountered. Under these demanding conditions, improved OCM performance is observed for Na2WO4-Mn/SiO2 (35% methane conversion, C2+ selectivity 60%) but the catalyst deactivates due to the high bed temperature (930 degrees C). In comparison, methane conversion of similar to 25% and C2+ selectivity of similar to 38% is observed for Cs/Sr/La2O3 at a feed temperature of 395 degrees C. Despite a bed temperature of 830 degrees C, the Cs/Sr/La2O3 catalyst is stable for 50 h. The findings show the importance of heat effects in both promoting OCM catalyst performance and leading to deleterious catalyst deactivation.