Quantitatively Predicting Impact of Structural Flexibility on Molecular Diffusion in Small Pore Metal–Organic FrameworksA Molecular Dynamics Study of Hypothetical ZIF‑8 Polymorphs

Structural flexibility plays a critical role in the diffusivities of molecules in small pore metal–organic frameworks (MOFs). In this work, we use molecular dynamics simulations to study the diffusion of spherical models of methane, xenon, and sulfur hexafluoride through a set of 6-membered ring (6MR) windows in zeolitic imidazolate framework-8 (ZIF-8) polymorphs. Linear correlations among the effective molecular size, the adsorbate-loaded window size, and the hopping rate were found. These correlations are shown to be effective in predicting the hopping rate for other spherical molecules. These results suggest an efficient approach to quantify the flexibility of 6MR windows in ZIFs and the impact of this flexibility on molecular diffusion. Developing methods of this kind are crucial in progressing toward a situation where accurate large-scale screening of small pore MOFs for properties based on molecular diffusion is possible.