The importance of lattice flexibility for the migration of ethane in ZIF-8: Molecular dynamics simulations

Recent adsorption, diffusion and permeation studies give experimental evidence that ethane can easily adsorb and diffuse in the ZIF-8 framework. This finding is surprising since the pore window of ZIF-8 – derived from XRD data – is virtually too small for ethane migration. Therefore, the diffusion mechanism of ethane in ZIF-8 and the influence of the diffusing guest molecules on the flexible lattice are investigated in detail in this paper with focus on the diffusion limiting step, the passage of the ZIF-8 window between adjacent cavities. Contrary to zeolite research, well established interaction parameter sets are still not available for MOF simulations. Hence, a large part of this paper is devoted the comparison of several parameter sets for the molecular interactions. Also, the conservation not only of the unit box size in NPT simulations but also the conservation of the geometrical structure of the lattice (most important the “windows” between adjacent pores) has been examined. Employing a suitable parameter set, two interesting effects can be observed: (i) The window size of ZIF-8 is smaller at higher ethane loadings, while simultaneously (ii) at higher loadings the guest molecules within the cavity exert forces that press a given probe molecule toward the window. These two effects nearly compensate each other leading to a weak dependence of the self-diffusivity upon the concentration of guest molecules.However, no gate-opening mechanism by a torsional motion of the linker molecule methylimidazolate could be found in our MD simulations because of a too large activation energy for this motion.

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► Fluctuations of the ZIF-8 window size allow the passage of molecules like ethane.
► The window aperture of ZIF-8 becomes smaller with increasing ethane loading.
► Increasing guest loading (repulsions) ? decreasing barrier for jumps through window.
► Torsional motion of linkers requires huge activation energies ? no gate-opening.