Effects of Li Doping on MgO-Supported Sm2O3 and TbOx Catalysts in the Oxidative Coupling of Methane

Rare earth oxides (REOs), particularly the sesquioxides, such as Sm2O3 and La2O3, have been investigated as promising catalysts in the oxidative coupling of methane (OCM). Much less attention has been paid to the reducible REOs because they are expected to give oxidation products, such as CO and CO2 (COx), rather than the desirable ethane and ethylene (C2+). Because Li addition can improve the performance of Sm2O3 in the OCM reaction and Li/MgO is commonly used as a reference OCM catalyst, the effects of lithium addition to a reducible oxide, TbOx, were investigated in detail in this study and compared with a Sm2O3 catalyst, which is the best single component OCM catalyst. Because of the well-documented volatility of lithium under OCM conditions, particularly for the Li/MgO system, the stability of lithium-doped samaria and terbia catalysts was examined as a function of preparation methods in this study. As expected, terbia supported on nanoparticle magnesia (n-MgO) is not a very active or selective OCM catalyst, and most of the observed selectivity toward C2+ products is likely due to the n-MgO support. In contrast, Li-doped TbOx/n-MgO prepared using a coimpregnation method yields a highly active and selective catalyst. The Li-TbOx/n-MgO catalyst yields the same methane conversion as pure Sm2O3, and has a higher C2+ selectivity than the Li-Sm2O3/n-MgO catalyst. The stability of the Li-TbOx/n-MgO catalyst is also higher than that of the Li-Sm2O3/n-MgO catalyst, and the loss of activity for the lithium-doped terbia catalyst appears to be the same as for the undoped Sm2O3/n-MgO catalyst (and undoped TbOx/n-MgO). The characterization data indicate stronger interactions between Li and TbOx than between Li and Sm2O3, which may explain the higher stability of the Li-TbOx/n-MgO catalysts. There are also indications that Li enters the TbOx lattice and reduces TbO1.81, to Tb2O3 during reaction, which can explain the higher C2+ selectivity compared with undoped TbOx/n-MgO. Furthermore, the Li-TbOx/n-MgO catalyst in this study is active at lower temperatures (600-700 degrees C) than typically used in the OCM (around 800 degrees C). Therefore, the Li-TbOx/n-MgO catalysts have potential to be very effective OCM catalysts, even though undoped TbOx/n-MgO catalysts are more selective toward COx than C2+ products.