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

Abstract

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.

Figure 1

Concept and experimental approach applied for the design of active MOC catalysts. (A) The synergistic concept reported for redox-active La³⁺–Eu³⁺, where La³⁺ acted as a chlorinating agent and Eu³⁺ as an active element, was adapted for catalyst materials based on nonreducible Mg²⁺ and Al³⁺. (B) Thermodynamic equilibrium calculations for the chlorination of the studied metal oxides and oxychlorides, performed with the HSC Chemistry 9.1 program, showing that Mg²⁺ can fulfill a similar role as La³⁺ as a chlorine buffer, while both Al³⁺ and Eu³⁺ are not easily chlorinated. (C–E) The synthesis approach applied here exploits the tunable character of the layered double hydroxide (LDH) composition to yield the mixed-metal oxide (MMO) catalyst material to obtain a homogeneous distribution of chemical elements. (E) The octahedral building units that compose the LDH can be readily modified without drastically changing the MMO physicochemical properties. This approach allows for a fair comparison of different catalyst compositions.

Figure 2

(A) X-ray diffraction (XRD) patterns of the as-synthesized MMO catalyst materials. The reflections of the MMO materials are indicated in the graph. (B) A zoom-in of the (200), where the diffractions are ordered based on the Mg²⁺/M³⁺ ratio where the Eu-MMO is plotted together with the corresponding MMO.

Figure 3

Overview of the catalytic performance of Mg_xAl mixed-metal oxide (MMO) catalyst materials and reference materials in the methane oxychlorination (MOC) reaction. A physical mixture with a 3:1 ratio of MgO/γ-Al₂O₃ was also tested, denoted as PM. (A) The CH₄ conversion (X_CH4) plotted vs the reaction temperature under 10 vol % of HCl in the feed. (B) The CH₄ conversion rate normalized to the amount of catalyst at 480 °C under 10% HCl. The selectivity at 480 °C is indicated in the bar. (C) The nonisothermal activity plotted versus the CO selectivity (S_CO, square) and the CH₃Cl selectivity (S_CH3Cl, circle) under 10% (filled) and 80% (open) HCl in the feed. (D) Stability tests for the Mg₄Al MMO catalyst over 100 h of reaction time-on-stream (TOS) at 480 °C, showing only slight deactivation in terms of X_CH4 and CH₃Cl yields (Y_CH3Cl).

Figure 4

Overview of the catalytic performance of EuMg_xAl mixed-metal oxide (MMO) catalyst materials in the methane oxychlorination (MOC) reaction. (A) The O₂ conversion (X_O2) plotted vs the temperature for the HCl oxidation under 10 and 80% HCl over Mg₃Al and EuMg₃Al MMOs. (B) The CH₄ conversion (X_CH4) plotted vs the temperature under 10% HCl in the feed. (C) The CH₄ conversion rate normalized to the amount of catalyst at 480 °C under 10 and 80% HCl in the feed. The selectivity at 480 °C is indicated in the bar. (D) The nonisothermal activity plotted vs the CO selectivity (S_CO, diamond) and the CH₃Cl selectivity (S_CH3Cl, triangle) under 10% (filled) and 80% (open) HCl in the feed.

Figure 5

Nonisothermal activity–selectivity (X–S) relation for the methane oxychlorination (MOC) reaction plotted for Mg₃Al, EuMg₃Al-2%, EuMg₃Al-4%, CeO₂ benchmark, and La_0.50Eu_0.50OCl from ref (23) under (A, B) 10% HCl and (C, D) 80% HCl in the feed. The selectivity toward (A, C) CH₃Cl and (B, D) CO is given. The X–S relation for CeO₂ under 80% is not plotted due to low activity over the entire tested temperature range.

Figure 6

Nonisothermal activity–selectivity (X–S) relation for the methane oxychlorination (MOC) plotted for La_0.50Eu_0.50OCl from ref (23), La_0.50Eu_0.50OCl + γ-Al₂O₃, and Mg₃Eu under 10% HCl (filled symbols) and 80% HCl (open symbols) in the feed. The La_0.50Eu_0.50OCl + γ-Al₂O₃ was a dual catalyst bed with 500 mg of La_0.50Eu_0.50OCl first contacting the gas feed and then 100 mg of γ-Al₂O₃ separated by quartz wool to prevent mixing. The selectivity toward (A) CH₃Cl (S_CH3Cl) and (B) CO (S_CO) is given.

Figure 7

Operando Raman spectroscopy measurements performed during chlorination, dechlorination, and oxychlorination where the Mg²–Cl Raman stretching vibration is probed. (A, B) The Mg–Cl Raman vibration during the chlorination step is plotted for (A) MgO and (B) Mg₃Al. (C) The height of the 253 cm^–1 Mg–Cl peak plotted versus the time-on-stream (TOS) for both catalysts, showing a lower growth rate for Mg₃Al. (D, E) The Mg–Cl Raman vibration is plotted versus the TOS during the dechlorination and oxychlorination steps for (D) MgO and (E) Mg₃Al. Furthermore, the height profile of the Mg–Cl vibration at 253 cm^–1 is given. Lastly, (F) methane conversion (X_CH4) was plotted versus the TOS during the dechlorination and oxychlorination steps for both catalysts.

Figure 8

Operando Raman and luminescence measurements performed on EuMg₃Al mixed-metal oxide (MMO) under MOC reaction conditions. (A) The Mg²⁺–Cl Raman vibration and the (B) ⁵D₁ → ⁷F₂ emission during the chlorination step. (C) Mg²⁺–Cl Raman vibration and the (D) ⁵D₁ → ⁷F₂ emission during the dechlorination and oxychlorination plotted as a function of time-on-stream (TOS). The height profiles of the Mg–Cl Raman vibration or the Eu³⁺ luminescence are given. (E) Methane conversion (X_CH4) plotted vs the TOS during the dechlorination and oxychlorination steps.