Study On Low Temperature Oxidative Coupling Of Methane To Ethylene Over BaCO₃/La₂O₃ Catalysts

Oxidative coupling of methane is one of effective paths for direct conversion of natural gas to olefin, but most of oxidative coupling of methane could only take place at a high temperature over the catalysts. Oxidative coupling of methane is a powerful thermal explosion reaction, and deep oxidation of methane to CO_x is also a powerful thermal explosion reaction. If oxidative coupling of methane take place at a low temperature over the catalysts, it could reduce the reaction energy loss. Therefore, it has a remarkable prospect for low temperature oxidative coupling of methane.A series of 2.5wt%-15wt% BaCO₃/La₂O₃ catalysts with different lanthanum compound were prepared by dry and wet mixing method. The catalytic activity for the oxidative coupling of methane (OCM) was evaluated in a fixed-bed micro-reactor. Effect of preparation methods of catalysts, catalysts components and catalytic reaction conditions on the performance of catalysts had been investigated. The thermal effect of catalyst bed and stability of BaCO₃/La₂O₃ catalyst were also studied. Camparing the low temperature oxidative coupling of methane performance of other M_xCO₃/La₂O₃ catalysts was investigated. The structures of BaCO₃/La₂O3 catalysts were characterized using XRD and TPR techniques.In this study, results indicated BaCO₃/La₂O₃ catalysts within pure La₂O₃ prepared using wet mixing method showed the best catalytic activity. Results indicated that the BaCO₃/La₂O3 catalysts showed the better low-temperature catalytic activities for OCM. 10 wt% BaCO₃/La₂O₃ catalyst showed the best catalytic activity. When the OCM reaction was hold at the 300℃furnace temperature with ratio of CH₄/O₂ =2 and GHSV =36000 ml.g^-1.h^-1, the conversions of CH₄ and selectivity of C₂ were 37.4% and 39.3% over 10 wt% BaCO₃/La₂O₃ catalyst. The OCM reaction could be hold at the 150℃. The 10 wt% BaCO₃/La₂O₃ catalyst showed stable activities at 150℃. After reaction of 230 hours, the conversions of CH₄ and selectivity of C₂ decreased. The thermal effect of catalyst bed could be the reason of low temperature OCM reaction over the BaCO₃/La₂O₃ catalysts. Catalyst bed temperature of different 10% BaCO₃/La₂O₃ catalysts was measured at furnace temperature 300℃, and it was found that the highest temperature points higher than the furnace temperature near 400℃.XRD results indicated that surface phase structure of La₂O₃ existed in all catalysts, and there are BaCO₃ phase in catalysts prepared using dry mixing method. Surface phase structure of La(OH)₃ existed in catalysts within pure La₂O₃ prepared using wet mixing method, and La₂O₂CO₃ existed in the catalysts after the reaction. TPR results indicated that La₂O₃ component of catalysts within pure La₂O₃ prepared using wet mixing method was more stable, and the catalysts had the highest reduction temperature of BaCO₃ component.