Experimental analysis by transient methods of the behaviour of Y2O3 and CaO based catalysts for methane coupling

Two catalysts for methane coupling, namely 3.5% Na2O/CaO and 2% Li2O/Y2O3, have been studied by means of transient techniques (transient response method and steady stare isotopic transient analysis) in the temperature range 710-770 degrees C, where both catalysts show an interesting activity and selectivity towards the formation of C-2-hydrocarbons. Transient response runs indicate that, at these temperatures, methane starts reacting almost immediately, with a very rapid consumption of gaseous oxygen, and therefore show that reaction occurs mainly in the gas phase. On the other hand the responses of C-2-hydrocarbons exhibit features similar to methane, demonstrating the poor adsorptive properties of these catalysts towards hydrocarbons at the very beginning of the reaction. These properties improve when the response of carbon dioxide reaches its steady level, and the surface is therefore carbonated, as confirmed by transient responses obtained at the end of the reaction. However the increased participation of the catalyst surface is accompanied by a decrease of the selectivity towards C-2-hydrocarbons. Steady stale isotopic transient runs confirm these adsorptive properties of the catalysts, but demonstrate, above all, that the formation of C-2- hydrocarbons is followed, up to a certain point, by their deep oxidation, or by their cracking to give methane again.