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.