Atomic Imaging of Ion-Triggered Flexibility and Local Electric Field Response in Zeolite Rings.

Zeolite flexibility triggered by ion-exchange tuning pore geometry impacts the responsive interactions with guest species in selective adsorption and industrial catalysis. However, direct observation of their local atomic structure deformation and subsequent fundamental flexibility correlation between ions and atomic microenvironment remains elusive. Here, we report atomic-resolution imaging of the microscopic ring flexibility in ion-exchanged MFI zeolites, revealing how ion-induced electric field alterations drive the framework distortions. Specifically, extra-framework Ba2+ ions confined within the 10-membered rings (10-MRs) trigger a 0.5 Å channel expansion, which exceeds the 0.2 Å expansion induced by Na+ ions while maintaining the framework's macroscopic rigidity. Using differential phase contrast imaging, we visualize the local coupling of the internal electric field between confined ions and the MFI framework. The combination of our theoretical analysis, which is based on the structure and the charge distribution, demonstrates that the electric field generated by the Ba2+ results in an 8° deformation in the nearby T-O-T bond angles, thereby distorting the 10-MR. The ion-exchange-driven 10-MR flexibility was further experimentally verified by the adsorption and separation of CO2, as this molecular size matches the expanded rings. This work resolves a long-term unsettled issue on the ion-exchange effect of pore size deformation in zeolites, offering insights into electric field-driven framework dynamics and introducing the intrinsic behavior of ions confined in zeolites.