(446f) Doped Rare-Earth Oxides As Catalysts for the Oxidative Coupling of Methane
AIChE Annual Meeting
2014 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Fundamentals of Oxide Catalysis I
Wednesday, November 19, 2014 - 10:10am to 10:30am
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 (COx). 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 COx 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-TbOx/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.