Favoring the Methane Oxychlorination Reaction over EuOCl by Synergistic Effects with Lanthanum

The direct conversion of CH4 into fuels and chemicals produces less waste, requires smaller capital investments, and has improved energy efficiency compared to multistep processes. While the methane oxychlorination (MOC) reaction has been given little attention, it offers the potential to achieve high CH4 conversion levels at high selectivities. In a continuing effort to design commercially interesting MOC catalysts, we have improved the catalyst design of EuOCl by the partial replacement of Eu3+ by La3+. A set of catalytic solid solutions of La3+ and Eu3+ (i.e., La x Eu1-x OCl, where x = 0, 0.25, 0.50, 0.75, and 1) were synthesized and tested in the MOC reaction. The La3+-Eu3+ catalysts exhibit an increased CH3Cl selectivity (i.e., 54-66 vs 41-52%), a lower CH2Cl2 selectivity (i.e., 8-24 vs 18-34%), and a comparable CO selectivity (i.e., 11-28 vs 14-28%) compared to EuOCl under the same reaction conditions and varying HCl concentrations in the feed. The La3+-Eu3+ catalysts possessed a higher CH4 conversion rate than when the individual activities of LaOCl and EuOCl are summed with a similar La3+/Eu3+ ratio (i.e., the linear combination). In the solid solution, La3+ is readily chlorinated and acts as a chlorine buffer that can transfer chlorine to the active Eu3+ phase, thereby enhancing the activity. The improved catalyst design enhances the CH3Cl yield and selectivity and reduces the catalyst cost and the separation cost of the unreacted HCl. These results showcase that, by matching intrinsic material properties, catalyst design can be altered to overcome reaction bottlenecks.