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

## TL;DR

This study explores how the hydration level of ion-exchanged mordenite affects its compressibility under high pressure.

## Contribution

The study reveals that hydration and cation distribution influence the structural stability and compressibility of mordenite under pressure.

## Key findings

- Weakly hydrated cations like Cs-MOR and Na-MOR undergo a phase transition at about 1.6 GPa.
- Strongly hydrated cations like Sr-MOR and Eu-MOR show lower compressibility due to better structural buffering.
- PIH stabilizes the mordenite framework, preventing pore collapse under pressure.

## 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.

We systematically investigated structural evolution and compressibility of ion-exchanged mordenites under water-mediated high-pressure; intensely hydrated, centrally distributed cations (e.g. Sr- and Eu-MOR) facilitate higher levels of pressure-induced hydration.