Hydration-level-driven buffering effects on the compressibility of ion-exchanged mordenite

Abstract

Understanding how large-pore zeolites respond to high-pressure conditions is essential for optimizing their structural stability and functional performance. In this study, we systematically investigated the compressibility and pressure-induced hydration (PIH) behavior of ion-exchanged mordenites using synchrotron X-ray powder diffraction under water-mediated conditions. The results reveal that the hydration level and spatial distribution of extra-framework cations (EFCs) at ambient conditions critically determine the initial number and arrangement of water molecules within the 12-membered ring (12MR) channels. Samples with weakly hydrated EFCs (e.g. Cs-MOR, Na-MOR) undergo a phase transition from Cmcm to Pbnm at about 1.6(1) GPa, because they fail to maintain the structural stability of the framework as compressed in water. In contrast, samples with EFCs strongly hydrated and uniformly distributed near the channel center (e.g. Sr-MOR, Eu-MOR) have lower compressibility, compared to cations aggregated near the channel wall (e.g. Pb-MOR, Cd-MOR). This study demonstrates that PIH acts as a structural buffer that stabilizes the framework by preventing pore collapse, thereby enhancing the compressibility in water. These findings underscore the critical role of the ambient EFC hydration state and PIH in governing the mechanical response of mordenite. The insights provide a basis for tailoring zeolite frameworks with optimized structural buffering effects for advanced industrial applications and geoscientific processes under extreme conditions.