Understanding Nanopore Window Distortions in the Reversible Molecular Valve Zeolite RHO

TL;DR

This study uses atomistic simulations to explore how structural distortions in Zeolite RHO, influenced by hydration and cation type, affect its function as a molecular valve with potential biomedical and environmental applications.

Contribution

It provides detailed atomistic insights into the distortion mechanisms of Zeolite RHO, highlighting the role of cation type and hydration in controlling pore flexibility and valve behavior.

Findings

Distortions of zeolite rings are coupled and highly flexible.

Cation polarizing power and water loading significantly affect pore aperture.

High energy barriers explain slow phase transition kinetics.

Abstract

Molecular valves are becoming popular for potential biomedical applications. However, little is known concerning their performance in energy and environmental areas. Zeolite RHO shows unique pore deformations upon changes in hydration, cation siting, cation type, or temperature-pressure conditions. By varying the level of distortion of double eight-rings, it is possible to control the adsorption properties, which confer a molecular valve behavior to this material. We have employed interatomic potentials-based simulations to obtain a detailed atomistic view of the structural distortion mechanisms of zeolite RHO, in contrast with the averaged and space group restricted information provided by diffraction studies. We have modeled four aluminosilicate structures, containing Li$^+$, Na$^+$, K$^+$, Ca$^{2+}$, and Sr$^{2+}$ cations. The distortions of the three different zeolite rings are coupled, and the six- and eight-membered rings are largely flexible. A large dependence on the polarizing power of the extra-framework cations and with the loading of water has been found for the minimum aperture of the eight-membered rings that control the nanovalve effect. The calculated energy barriers for moving the cations across the eight-membered rings are very high, which explains the experimentally observed slow kinetics of the phase transition as well as the appearance of metastable phases.