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

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 Tb₄O₇ (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 Sm₂O₃ 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 Sm₂O₃ gives a higher optimum C₂ yield than that of unpromoted Sm₂O₃. Sm₂O₃ and Ca/Sm₂O₃ catalysts are more stable than Li/MgO, Li/Sm₂O₃ or Na/Sm₂O₃. The addition of Li or Na to Sm₂O₃ causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm₂O₃ catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm₂O₃.