A systematic examination of the impacts of MOF flexibility on intracrystalline molecular diffusivities

Abstract

Molecular diffusion calculations of metal–organic frameworks (MOFs) including structure flexibility are considerably more computationally expensive than more approximate calculations based on rigid frameworks. For this reason, efforts to probe diffusion in MOFs in a high-throughput manner have relied on rigid framework simulations. It is unclear, however, when this simplification is justified. We compare the self-diffusion coefficients of 12 adsorbates in a diverse set of MOFs with flexible and rigid frameworks to systematically study the influence of framework flexibility on molecular diffusion. In many cases framework flexibility has a strong impact on molecular diffusion. Descriptors based only on the properties of empty MOFs were not able to capture the effects of framework flexibility even qualitatively. In contrast, the average window size in the presence of a constrained molecule was found to be useful for judging the importance of framework flexibility. A transferable descriptor of adsorbate size in diffusion was introduced to efficiently estimate loaded window sizes for diverse collections of molecules. These results provide a practical approach to qualitatively establishing whether framework flexibility strongly influences molecular diffusion in MOFs, which in turn points to more reliable approaches to high throughput calculations of molecular diffusivities in these materials.