N2 in ZIF-8: Sorbate induced structural changes and self-diffusion

Abstract

In the field of nanoporous materials, guest-induced structural changes in metal–organic frameworks (MOFs) attracted great attention over the last years. One example concerns a gate-opening effect in MOF ZIF-8 which was found to occur upon sorption of N2. To mirror these structural changes in molecular simulations, suitable force fields for the proper description of framework flexibility are required.We demonstrate that our previously published force field is able to reproduce these structural changes in classical MD simulations. In particular, with our parameter set the diameter of the windows connecting adjacent cavities is found to match the X-ray values almost perfectly. We focus on investigating the impact of the structural changes on the mobility of N2 molecules in ZIF-8 framework and compare the results with those of another parameter set, which was published recently by Zhang and coworkers.The size of the windows increases notably, when the critical “gate-opening” loading is surpassed. Most surprisingly, this pronounced increase does not result in a speed-up of the N2 self-diffusivity. It appears, that a complex interplay of host-host and host-guest interactions increases the mutual hindrance of the N2 molecules and counter balance the acceleration of the mobility due to the larger window size.