In-situ X-Ray Photoelectron Spectroscopy Detects Multiple Active Sites Involved in the Selective Anaerobic Oxidation of Methane in Copper-Exchanged Zeolites

A direct route to convert methane into high value commodities, such as methanol, with high selectivity is one of the primary challenges in modern chemistry. Copper-exchanged zeolites show remarkable selectivity in chemical looping process. Although multiple copper species have been proposed as active, an in-situ spectroscopic investigation is difficult, because of their similar fingerprints. We used ambient pressure x-ray photoelectron spectroscopy to investigate an actual powder sample. We could discriminate between different types of active species involved in the conversion of methane to methanol over two different copper-exchanged zeolites, namely mordenite and mazzite. After activation at 400°C in oxygen, we followed the reaction in-situ at 200°C, switching from methane to water and followed by a second cycle with anaerobic activation. Our experimental results, combined with theoretical calculations, prove that Cu(II) sites bound to extra-framework oxygen are involved in the reaction, and that their ...