COMPUTATIONAL CHEMISTRY CALCULATIONS OF STRUCTURE EFFECTS AND DIFFUSION OF ETHANE AND NITROGEN IN ZEOLITIC IMIDAZOLATE FRAMEWORK-8

Abstract

Zeolitic Imidazolate Framework-8 (ZIF-8), a porous material, has attracted great attention due to its high chemical and thermal stability. In this work the applicability of parameter sets for the molecular interactions of ZIF-8 including guest molecules was investigated by comparing results of MD simulations with experiments. Using suitable parameter sets, the diffusion and the effect of the diffusing molecules on the window size were investigated for ethane and nitrogen. For ethane, two interesting effects were found at higher ethane loadings: reduction of the window size and, forces that push a given molecule toward the window. These two effects balance each other leading to a self-diffusivity nearly independent upon the concentration of ethane. For nitrogen a gate-opening effect could be found. This result is in agreement with experiments. The size of the windows increases at high loadings while this does not speed-up the nitrogen self-diffusivity. The complex interactions of host-host and host-guest molecules cause mutual hindrance of the nitrogen molecules and balance the speed of the mobility due to the larger window size. Moreover, the energy barrier for rotation of imidazolate linkers was studied by quantum mechanics (QM). The results show a very large activation energy about 200 kcal/mol for gate-opening phenomena.