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

Abstract

Commercialization of CH 4 valorization processes is currently hampered by the lack of suitable catalysts, which should be active, selective, and stable. CH 4 oxychlorination is one of the promising routes to directly functionalize CH 4 , and lanthanidebased 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 CH 4 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 CH 4 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 CH 4 valorization process.

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

“…As previously reported, LaOCl and EuOCl are easily synthesized in the oxychloride phase without any noticeable contaminations from other crystalline phases. 33 Since LaOCl and EuOCl have the same space group, P 4/ nmm , and comparable ionic radii, 34 solid-state ion mixing of the two elements is expected to occur. 35 , 36 By deconvolution of the (110) XRD peaks of the as-synthesized LnOCl catalysts ( Figure 1 B–F) and applying Vegard’s law (see SI Section S1B for more details on the applied procedure), at least two mixed phases were distinguished with varying La 3+ /Eu 3+ ratios, referred to as phase 1 and phase 2 ( Table 1 ).…”

“…Individual activity and selectivity versus reaction temperature plots are given in Figure S1 . The catalytic performance of pure LaOCl and EuOCl are described elsewhere, 33 but the plots are given for facile comparison. The reaction temperature at which the catalyst becomes active, referred to as the onset temperature, is determined as the reaction temperature at which the X CH 4 > 2%.…”

“…Details on the experimental setup as well as definitions and calculations are reported elsewhere. 33 …”

“…We recently showed that EuOCl is a promising candidate for the MOC reaction as its performance is stable and, by varying the reaction temperature and feed mixture, also highly tunable. 33 EuOCl is suitable to be studied under working conditions with operando spectroscopy because of the Raman active modes of the material and the photoluminescent properties of Eu 3+ . Hence, we were able to conclude that the chlorination of the catalyst surface was rate limiting.…”

“…Operando Raman spectroscopy previously revealed that the chlorination of EuOCl to EuCl 3 is a slow process and can be rate limiting during the MOC reaction. 33 Based on thermodynamic calculations and experimental evidence, LaOCl was selected as a chlorine reservoir for Eu 3+ as the chlorination from LaOCl to LaCl 3 occurs readily at low HCl concentrations. La 1– x Eu x OCl (where x = 0, 0.25, 0.50, 0.75, or 1) solid solution catalysts were synthesized and characterized.…”