Identification of the Optimum Catalyst and Operating Conditions for Oxidative Coupling of Methane: Activity and Selectivity of Alkaline Earth-Doped Lanthanides

While numerous studies have been carried out using rare earth oxides as catalysts for the oxidative coupling of methane, there is substantial disagreement as to which catalyst is the best. More importantly, most of these prior evaluations were done under conditions that were not commercially viable. In the current work, 35 rare earth oxide catalysts were synthesized and tested under identical conditions (CH4/O2 = 7.4, no diluent, 5 ms residence time), and Sr-doped La2O3 was isolated as the optimum catalyst composition. However, the amount of Sr is important. As the bulk Sr/La ratio was increased, the activity of the catalyst decreased but the selectivity increased significantly. A bilayer catalyst structure is proposed comprising a thermally stable Sr-La2O3 base layer that controls activity covered at least in part by a layer of a partially amorphous mixture of SrO, Sr (OH)2, SrO2, and SrCO3 that controls selectivity. The importance of this catalyst in adiabatic autothermal operation of an oxidative coupling of methane reactor is discussed.