Improving the C2 Selectivity for the Oxidative Coupling of Methane Reaction By Doping Rare-Earth Oxide Based Catalysts With Lithium

The oxidative coupling of methane (OCM) has been extensively studied over the past few decades.  Ethylene (one of the two C₂ hydrocarbon products in OCM), as the shortest chain hydrocarbon, is an extremely valuable feedstock material for synthesizing longer chain hydrocarbons.  Early work in the OCM field discovered doping a magnesium oxide support with lithium produces an active catalyst under OCM conditions with a relatively high C₂ product selectivity.  This catalyst system has since been a reference system in many studies due to its activity and simplicity.  However, lithium is volatile under OCM reaction conditions.  Within the first few hours of reaction, the majority of the lithium will leave the surface of the catalyst resulting in a sharp decrease in activity.  In this study we have doped lithium onto supported samarium oxide and terbium oxide catalysts to observe the stability of these catalysts by monitoring the changes in activity during a time-on-stream study.  Samarium oxide, and other select rare-earth oxides (REOs), have been established as some of the highest performing single component OCM catalysts.  Surface sensitive characterization techniques, such as XPS and FTIR-DRIFTS, were utilized to identify surface species that could be important for enhancing the C₂ selectivity for this reaction.