Revealing Active Sites and Reaction Pathways in Direct Oxidation of Methane over Fe-Containing CHA Zeolites Affected by the Al Arrangement

Fe-containing zeolites are effective catalysts in converting the greenhouse gases CH₄ and N₂O into valuable chemicals. However, the activities of Fe-containing zeolites in methane conversion and N₂O decomposition are frequently conflated, and the activities of different Fe species are still controversial. Herein, Fe-containing aluminosilicate CHA zeolites with Fe species at different spatial distances affected by the arrangement of framework Al atoms were synthesized in a one-pot manner in the presence or absence of Na. The arrangement of framework Al atoms was identified by ²⁷Al and ²⁹Si MAS NMR spectra and thermogravimetry-differential thermal analysis (TG-DTA) curves. Ultraviolet (UV)-vis, X-ray absorption spectroscopy (XAS), and NO adsorption fourier transform infrared spectroscopy (FTIR) spectra were adopted to analyze Fe speciation. The higher proportion of Fe species in the 6 MR of Fe-CHA zeolites in the presence of Na was confirmed by the NO adsorption FTIR spectrum. The activities of proximal and distant Fe sites in reactions including direct oxidation of methane to methanol, methanol to hydrocarbon, and N₂O decomposition were compared at different temperatures to provide the corresponding active sites and reaction pathways. The distant, isolated Fe and isolated proton were more active in the direct oxidation of methane to methanol and tandem conversion of methanol to hydrocarbon reactions than the proximal, isolated Fe and paired protons, respectively. Additionally, proximal, isolated Fe sites afforded higher activity in N₂O decomposition. These findings guide the design of highly active catalysts in methane oxidation, methanol to hydrocarbon, and N₂O decomposition reactions, addressing energy and environmental concerns.