(446f) Doped Rare-Earth Oxides As Catalysts for the Oxidative Coupling of Methane

The oxidative coupling of methane (OCM) is a potential pathway to directly convert methane to highly valuable ethylene, as well as other hydrocarbon products.  However, it is very difficult to obtain high yields of C₂₊ products (i.e. products such as ethane, ethylene and higher hydrocarbons), due to the formation of higher oxidation products, such as CO and CO₂ (CO_x).  Li/MgO is a simple and active OCM catalyst with a relatively high C₂₊ product selectivity, but it suffers from severe deactivation due to the volatility of lithium under OCM reaction conditions.  Rare earth oxides (REOs), such as samaria (Sm₂O₃) and lanthana (La₂O₃), are amongst the most efficient single component oxide catalysts in the methane coupling reaction, while reducible REOs (e.g. ceria, terbia, and praseodymia) are typically more selective towards CO_x products than C₂₊ products.  In this study the effects of lithium on samaria and terbia catalysts supported on nanoparticle magnesia (n-MgO) were investigated in detail.  It was shown that addition of lithium to terbia supported on n-MgO is not only a more active and selective OCM catalyst compared with the reference Li/MgO, Sm₂O₃/MgO and Li-Sm₂O₃/MgO catalysts, it is also more stable with time on stream.  The higher stability appears to be due to a stronger interaction between lithium and terbia compared with lithium and samaria, as evidenced by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.  Terbia appears to be unique amongst the reducible rare earth oxides, as Li-doped praseodymium or cerium oxides are not as active as the Li-TbO_x/MgO catalyst.  Amongst Li, Na, Ca and Mg dopants, Li is the most effective, but due to its volatility Na-doped REO catalysts can outperform the Li-doped REO catalysts after extended time on stream.