Effects of alkali and alkaline-earth metal dopants on magnesium oxide supported rare-earth oxide catalysts in the oxidative coupling of methane

The oxidative coupling of methane (OCM) to ethane and ethylene was studied over alkali metal (Li and Na) and alkaline earth metal (Mg and Ca) doped rare earth oxides (Sm2O3,TbOx, PrOy and CeO2) catalysts supported on nanoparticle magnesium oxide (n-MgO). It was found that the Li-TbOx/n-MgO catalyst outperformed all other combinations of rare earth oxide and dopant, in terms of initial activity and selectivity, except at temperatures below 600 degrees C where the undoped Sm2O3/n-MgO, Ca-Sm2O3/n-MgO and Ca-CeO2/n-MgO catalysts resulted in higher C2+ yields. Li doping was most efficient in yielding C2+ products over all rare earth oxides, followed by doping with Na. These dopants result in the most basic sites on the surface according to CO2 desorption experiments. However, these sites are blocked at lower reaction temperatures, which can explain why most Li- and Na-doped catalysts are not active below 650 degrees C. Below reaction temperatures of 650 degrees C moderately basic sites, which bind CO2 up to similar to 350 degrees C, are important and correlate well with OCM activity. Li doping also resulted in the highest ethylene yields, which is a more desirable product than ethane, and the highest ethylene yield (9.7%) was obtained over the Li-CeO2/n-MgO (at 800 degrees C). However, Li-doped catalysts are not very stable, and the Na-TbOx/n-MgO catalyst therefore outperformed the Li-TbOx/n-MgO catalyst after only a few hours on stream. Removal of both Li and Na during reaction was also confirmed and is probably the reason for the loss in selectivity with time on stream. Tb2O3 is the only TbOx phase observed with XRD on the Na- and Li-doped TbOx/n-MgO catalysts after reaction and this is important since Tb2O3 is likely more selective to C2+ products than TbO1.81 in the OCM reaction. (C) 2016 Elsevier B.V. All rights reserved.