Mechanistic Insights into the Lanthanide-Catalyzed Oxychlorination of Methane as Revealed by Operando Spectroscopy

Abstract

Commercialization of CH₄ valorization processes is currently hampered by the lack of suitable catalysts, which should be active, selective, and stable. CH₄ oxychlorination is one of the promising routes to directly functionalize CH₄, and lanthanide-based catalysts show great potential for this reaction, although relatively little is known about their functioning. In this work, a set of lanthanide oxychlorides (i.e., LnOCl with Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) and Er- and Yb-based catalysts were synthesized, characterized, and tested. All lanthanide-based catalysts can directly activate CH₄ into chloromethanes, but their catalytic properties differ significantly. EuOCl shows the most promising catalytic activity and selectivity, as very high conversion levels (>30%) and chloromethane selectivity values (>50%) can be reached at moderate reaction temperatures (∼425 °C). Operando Raman spectroscopy revealed that the chlorination of the EuOCl catalyst surface is rate-limiting; hence, increasing the HCl concentration improves the catalytic performance. The CO selectivity could be suppressed from 30 to 15%, while the CH₄ conversion more than doubled from 11 to 24%, solely by increasing the HCl concentration from 10 to 60% at 450 °C. Even though more catalysts reported in this study and in the literature show a negative correlation between the S _CO and HCl concentration, this effect was never as substantial as observed for EuOCl. EuOCl has promising properties to bring the oxychlorination one step closer to an economically viable CH₄ valorization process.

Figure 1

(A) XRD patterns of the catalyst materials under study, including LaOCl, PrOCl, NdOCl, SmOCl, EuOCl, GdOCl, TbOCl, DyOCl, HoOCl, ErOCl, and YbOCl. For each of these materials, they were obtained in their LnOCl phase, except for ErOCl and YbOCl and (B) zoom-in of the XRD patterns, revealing lanthanide contraction in the LnOCl materials, as indicated by the shift of the [101] diffraction in the 27–33° region.

Figure 2

TEM image of the as-synthesized EuOCl where ill-defined particles with varying particle sizes are observed.

Figure 3

X_CH4 and corresponding selectivity plotted vs the temperature for LnOCl where Ln = (A) La, (B) Pr, (C) Nd, (D) Sm, (E) Eu, (F) Gd, (G) Tb, (H) Dy, (I) Ho, (J) Er, and (K) Yb. Conditions: CH₄/HCl/O₂/N₂/He of 2:2:1:1:14 (in mL/min) from 350 to 550 °C with a ramp rate of 1 °C/min. Selectivity is given when CH₃Cl and CO were above the detection limit of the GC.

Figure 4

(A) CH₄ conversion rate and (B) selectivity toward CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄, and CO for LnOCl (with Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Yb) at a reaction temperature of 480 °C. Note that for LaOCl, NdOCl, and YbOCl, the CO yield was below the detection limit, and therefore, the selectivity is not displayed. For all catalysts, the CO₂ levels were below the detection limit.

Figure 5

Schematic representation of relevant processes occurring during the MOC reaction over LnOCl materials. The oxychlorination cycle (green, arrows 1–4) and catalytic destruction cycle (red, arrows 5–8) are in competition with one another. The chlorination of terminal lattice oxygen (purple, arrow 9) is an important step because terminal lattice oxygen is held responsible for the catalytic destruction of higher chloromethanes. The surface composition is determined not only by the reactants but also by bulk diffusion of ions (gray, arrow 10).

Figure 6

X_CH4, S_CH3Cl, S_CH2Cl2, and S_CO over LnOCl materials (with Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Yb) plotted vs the HCl concentration for the four different categories of catalytic behavior under study. The elements with their corresponding symbol are displayed in the X_CH4–HCl concentration plot.

Figure 7

(A) X_CH4 plotted vs the temperature for EuOCl under 10% (black) and 80% (red) HCl. Selectivity toward CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄, CO, and CO₂ is plotted versus the temperature for EuOCl tested in (A) 10% HCl (CH₄/HCl/O₂/N₂/He 2:2:1:1:14) and (B) 80% HCl (CH₄/HCl/O₂/N₂/He 2:16:1:1:0).

Figure 8

Temperature ramp experiments where X_O2 is plotted vs the temperature for HCl oxidation reaction (black) and MOC (red) for (A) LaOCl with 10% HCl in the feed, (B) EuOCl with 10% HCl in the feed, and (C) EuOCl with 80% HCl in the feed.

Figure 9

Chlorination (minute 0–120, CH₄/HCl/O₂/N₂/He 0:20:0:1:19, T = 450 °C), dechlorination (minute 120–210, CH₄/HCl/O₂/N₂/He 2:0:1:1:16, T = 500 °C), and oxychlorination (minute 210–330, CH₄/HCl/O₂/N₂/He 2:2:1:1:14, T = 500 °C) steps were investigated with operando Raman spectroscopy for (A) LaOCl and (B) EuOCl materials. The Raman spectra are plotted as contour plots, and the intensity of key vibrations is plotted above. For 3-D plots and individual spectra, see Figure S6.

Figure 10

Temperature ramp experiments between 450 and 520 °C at 1 °C/min for EuOCl at the CH₄/HCl/O₂/N₂/He ratio of 2:2:1:1:14 with 2 h pretreatment of (A) CH₄/HCl/O₂/N₂/He 2:2:1:1:14 at 450 °C and (B) CH₄/HCl/O₂/N₂/He 0:4:0:1:15 at 450 °C. The photoluminescence spectra were collected, normalized to the highest peak, and integrated. The relative spectral area and X_CH4 were plotted vs the temperature.

Figure 11

(A) X_CH4 and S_CH3Cl, S_CH2Cl2, S_CHCl3, S_CCl4, S_CO, and S_CO2 plotted vs time on stream and (B) operando Raman spectra plotted vs time on stream between 38 and 42 h. The vertical lines in (B) are plotted individually in (C) to show the loss of Raman peak intensity of the vibrations corresponding to EuOCl.