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.