(35f) Oxidative Coupling of Methane Over Samaria Catalysts

The oxidative coupling of methane (OCM) to C₂ products, such as ethane and ethylene, was studied over supported and unsupported Sm₂O₃ nanoparticles.  Two different methods of Sm₂O₃ nanoparticle preparation were investigated, one method based on the decomposition of an oleate complex and one more conventional microemulsion method, to investigate the effects on the activity and selectivity in the OCM.  The Sm₂O₃ nanoparticles prepared using these methods were also deposited onto a support and compared to catalysts prepared using incipient wetness impregnation of a samarium nitrate solution onto the same support.  Two supports were selected, a commonly used porous alumina support (p-Al₂O₃) and a nanoparticle magnesia (n-MgO) support, to determine the influence of the support on the activity and selectivity in the oxidative coupling of methane.  The effects of the CH₄:O₂feed ratio (between 4:1 and 9:1) and the temperature (between 740 and 800°C) were also investigated.   

Catalyst characterizations using BET surface area analysis and transmission electron microscopy (TEM) reveal that the oleate method results in smaller Sm₂O₃ nanoparticles compared with the microemulsion method.  In general, the lowest CH₄:O₂ ratio (4:1) gives the highest CH₄ conversion and the lowest C₂ selectivity, while the highest CH₄:O₂ ratio (9:1) results in the highest C₂ selectivity and lowest CH₄ conversion.  When supported on p-Al₂O₃, the Sm₂O₃ nanoparticles prepared using the microemulsion method exhibit the highest C₂ selectivities, while the Sm₂O₃ nanoparticles prepared using the oleate method yield the lowest C₂ selectivities.  The Sm₂O₃/p-Al₂O₃ catalysts prepared using incipient wetness impregnation gives an intermediate C₂ selectivity, but a slightly higher CH₄ conversion compared to the other methods.  Supporting the Sm₂O₃ nanoparticles onto n-MgO results in higher CH₄ conversions and C₂ selectivities compared with the Sm₂O₃/p-Al₂O₃ catalysts.  Catalyst characterizations are underway to provide insight into the properties of these catalysts and how they affect the activity and selectivity in the oxidative coupling of methane.