Oxidative coupling of methane over unsupported and alumina-supported samaria catalysts

Being the shortest unsaturated hydrocarbon, ethylene is a valuable feedstock gas for synthesizing longer chain hydrocarbon products. Using the oxidative coupling of methane to produce ethylene and ethane (C-2 products) has been studied extensively over the past few decades. In this work, samaria nanoparticles (NPs) and alumina-supported samaria catalysts were prepared using different methods (water/toluene reverse microemulsion, metal-oleate high temperature decomposition, and incipient-wetness impregnation followed by calcination) and various types of alumina supports (high and low surface area nanoparticle alumina [n-Al2O3(+), n-Al2O3(-)] and a porous gamma alumina support [p-Al2O3]). The highest product yields were obtained over Sm2O3 NPs synthesized using the metal-oleate high temperature decomposition and a nitrate precursor. Using a chloride precursor in the preparation of the Sm2O3 NPs resulted in less active and selective catalysts, and should be avoided. While the C-2 selectivities were lower over the Sm2O3/Al2O3 catalysts, the yield per gram of Sm2O3 were higher compared with the Sm2O3 NPs. The best supported catalyst was the Sm2O3/n-Al2O3(-) prepared using incipient wetness impregnation of a nitrate precursor, since this leads to the smallest Sm2O3 particles as well as the largest coverage of the acidic Al2O3 support. XRD analysis revealed that high-temperature calcinations form SmAlO3 on the Al2O3-supported catalysts. While this reduced the near surface Sm2O3 concentration, it had beneficial effects as it also lowered the Al2O3 content. Therefore, Al2O3-supported Sm2O3 warrants further investigation, as surface modifications of the Al2O3 can reduce its acidity and lead to higher C-2 selectivities in the oxidative coupling of methane over these catalysts. Published by Elsevier B.V.