Structural changes of iron active sites in zeolite FER during CO2 and CH4 conversion to oxygenates

The direct co-conversion of $CO_{2}$ and $CH_{4}$ into oxygenates offers an atom-efficient strategy for the simultaneous valorization of two major greenhouse gases. This work presents an integrated spectroscopic, diffraction, and computational investigation of Fe-containing ferrierite (Fe-FER) as a bifunctional catalyst capable of selectively transforming $CO_{2}$ and $CH_{4}$ into methanol and acetic acid, with methanol further converted into olefins. In situ FTIR–MS studies reveal sequential formation of methanol and acetic acid, indicating that under mild conditions, $CH_{4}$ is first oxidized to methanol on redox-active Fe centers, followed by methanol carbonylation on Brønsted acid sites. At elevated temperatures, methanol-to-olefin conversion becomes competitive. Mössbauer spectroscopy shows that the interaction of Fe(II) with $CO_{2}$ decreases the electron density around Fe nuclei, resulting in partial oxidation and coordination changes between tetrahedral and octahedral environments. Neutron powder diffraction confirms Fe displacement within the 6-membered ring plane upon $CO_{2}$ interaction, while subsequent $CH_{4}$ exposure restores Fe(II) species in tetrahedral coordination. Density functional theory, combined with machine-learning interatomic potentials, identifies hydrocarbonate species as key intermediates inducing Fe oxidation and local structural distortion, consistent with experimental observations. The cooperative interplay of redox-active and acidic sites enables coupled activation of $CO_{2}$ and oxidation of $CH_{4}$ with complete atom utilization. Overall, two types of Fe(II) species with distorted tetrahedral symmetry are involved in $CO_{2}/CH_{4}$ activation, whereas other Fe(II) species remain inactive, establishing Fe-FER as a structurally flexible and electronically adaptive catalyst for sustainable $C_{1}$ conversion chemistry.