Oxidative dehydrogenation of ethane and the coupling of methane over sodium containing cerium oxides

Abstract

The oxidation of ethane over a Ce 2(CO 3) 3 derived catalyst showing much better selectivity than pure ceria for methane coupling has been investigated. XPS and bulk analyses show that the presence of sodium is responsible for the improved selectivity. The effect can be duplicated by the addition of sodium to pure CeO 2. These catalysts also show better selectivity than pure CeO 2 for the oxidation of ethane to ethylene but the enhancement due to sodium is less dramatic than for methane coupling. Using a feed stream comprising 15% ethane and 4% oxygen a selectivity over 70% can be achieved at 775°C.A selectivity over 90% is possible at near complete oxygen conversion when using an ethane to oxygen ratio of fifteen. The improvement in selectivity relative to pure ceria is achieved at the expense of a three fold drop in activity. The overall kinetic order for sodium containing catalysts is higher in ethane than in oxygen indicating that the reaction is more likely to be limited by a reaction between the hydrocarbon and the catalyst than by resupply of oxygen to the catalyst. The kinetic order for ethylene production is higher in ethane and lower in oxygen than is carbon dioxide formation. Thus selectivity to ethylene improves with increase in ethane/oxygen ratio as is also true for the methane coupling reaction. It is suggested that for both reactions sodium acts by blocking sites which cause oxidation of hydrocarbon radicals liberated into the gas phase by the primary reaction.