The selective oxidation of methane to ethane and ethylene over doped and un-doped rare earth oxides

Abstract

A comparison has been made of the behaviour in the oxidative coupling of methane of the oxides of Sm, Dy, Gd, La and Tb with that of a Li/MgO material. All but the Tb 4O 7 (which gave total oxidation) were found to give higher yields than the Li/MgO material at temperatures up to approaching 750°C but the Li/MgO system gave better results at higher temperatures. The cubic structure of Sm 2O 3 was found to be responsible for its good performance while the monoclinic structure was relatively inactive and unselective. The addition of Na or Ca to cubic Sm 2O 3 gives a higher optimum C 2 yield than that of unpromoted Sm 2O 3. Sm 2O 3 and Ca/Sm 2O 3 catalysts are more stable than Li/MgO, Li/Sm 2O 3 or Na/Sm 2O 3. The addition of Li or Na to Sm 2O 3 causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm 2O 3 catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm 2O 3.