Redox and Kinetic Properties of Composition‐Dependent Active Sites in Copper‐Exchanged Chabazite for Direct Methane‐to‐Methanol Oxidation

• Published in: Angewandte Chemie International Edition Vol. 63; no. 45; pp. e202411662 - n/a
• Main Authors: Brenig, Andreas, Fischer, Jörg W. A., Klose, Daniel, Jeschke, Gunnar, Bokhoven, Jeroen A., Sushkevich, Vitaly L.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 04.11.2024
• Edition: International ed. in English
• Subjects: Absorption spectroscopy; Aluminum; Copper; Electron paramagnetic resonance; Electron spin resonance; Fourier transforms; Heterogeneous Catalysis; High temperature; Infrared spectroscopy; Methane to Methanol; Operando Spectroscopy; Oxidation; Photoluminescence; Photons; Plug flow; Plug flow chemical reactors; Resonance; Silicon; Spectrum analysis; Zeolites; Copper, Heterogeneous Catalysis, Methane to Methanol, Operando Spectroscopy, Zeolites
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.202411662

The CH4 oxidation performance of Cu‐chabazite zeolites characterized by distinct Si/Al ratios and Cu loadings has been studied and the observed variations in reactivity have been correlated to the differences in the nature of the formed active centers. Plug flow reactor tests, in situ Fourier‐transform infrared, and X‐ray absorption spectroscopy demonstrate that a decrease in Cu loading shifts the reactivity/redox profile to higher temperatures and increases the CH3OH selectivity and Cu‐efficiency. In situ electron paramagnetic resonance, Raman, ultraviolet‐visible, Fourier‐transform infrared, and photoluminescence spectroscopies reveal that this behavior is associated with the presence of monomeric Cu active sites, including bare Cu2+ and [CuOH]+ present at low Si/Al ratio and Cu loading. Formation of two distinct [Cu2(μ‐O)]2+ moieties at higher Si/Al ratio or Cu loading forces these trends into the opposite direction. Operando electron paramagnetic resonance and ultraviolet‐visible spectroscopy show that the apparent activation energy of monomeric Cu active species decreases with increasing Si/Al ratio, whereas the one of dimeric centers is unaffected. Reactivity tests and advanced in situ/operando spectroscopic techniques are employed to elucidate the Cu speciation in Cu‐chabazite zeolites with varying Cu loading and Si/Al ratio and its influence on CH4 partial oxidation to CH3OH. An improved CH3OH selectivity and Cu‐efficiency of materials containing primarily monomeric Cu(II) active sites in contrast to zeolites with a significant fraction of dimeric Cu(II) centers has been found.