Tuning flexibility in MOFs: how stiffening and substitution control molecular intrusion and separation

Abstract

Zeolitic imidazolate frameworks, such as ZIF-8, possess high surface area and tunable porosity, making them attractive for many applications. However, their structural flexibility can hinder performance in pressure-dependent processes like gas separation or water intrusion for energy storage and conversion. Several strategies have been proposed to control the characteristics of these materials, but the general laws still remain elusive. In this study, we employ a combined theoretical/experimental approach to address this question. We focus on ZIF-8 and its derivatives to illustrate principles of endogenic and exogenic tuning with respect to processes related to the intrusion of gases and liquids into the porous system. Density functional theory and molecular dynamics simulations are used to investigate how linker swinging, the fundamental process controlling intrusion–extrusion/gas separation, depends on the endo-/exogenic modifications. These results will be tested against experimental pressure–volume–temperature liquid porosimetry data. Our results ultimately offer a design pathway for tailoring the optimization of ZIF-based MOFs and, possibly, MOFs and porous materials in general.