Europium-Magnesium-Aluminum-Based Mixed-Metal Oxides as Highly Active Methane Oxychlorination Catalysts

Methane oxychlorination (MOC) is a promising reaction for the production of liquefied methane derivatives. Even though catalyst design is still in its early stages, the general trend is that benchmark catalyst materials have a redox-active site, with, e.g., Cu²⁺, Ce⁴⁺, and Pd²⁺ as prominent showcase examples. However, with the identification of nonreducible LaOCl moiety as an active center for MOC, it was demonstrated that a redox-active couple is not a requirement to establish a high activity. In this work, we show that Mg²⁺-Al³⁺-based mixed-metal oxide (MMO) materials are highly active and stable MOC catalysts. The synergistic interaction between Mg²⁺ and Al³⁺ could be exploited due to the fact that a homogeneous distribution of the chemical elements was achieved. This interaction was found to be crucial for the unexpectedly high MOC activity, as reference MgO and γ-Al₂O₃ materials did not show any significant activity. Operando Raman spectroscopy revealed that Mg²⁺ acted as a chlorine buffer and subsequently as a chlorinating agent for Al³⁺, which was the active metal center in the methane activation step. The addition of the redox-active Eu³⁺ to the nonreducible Mg²⁺-Al³⁺ MMO catalyst enabled further tuning of the catalytic performance and made the EuMg₃Al MMO catalyst one of the most active MOC catalyst materials reported so far. Combined operando Raman/luminescence spectroscopy revealed that the chlorination behavior of Mg²⁺ and Eu³⁺ was correlated, suggesting that Mg²⁺ also acted as a chlorinating agent for Eu³⁺. These results indicate that both redox activity and synergistic effects between Eu, Mg, and Al are required to obtain high catalytic performance. The importance of elemental synergy and redox properties is expected to be translatable to the oxychlorination of other hydrocarbons, such as light alkanes, due to large similarities in catalytic chemistry.