Force field for ZIF-8 flexible frameworks: atomistic simulation of adsorption, diffusion of pure gases as CH4, H2, CO2 and N2

A full set of flexible force field parameters for ZIF-8 is presented, based on the AMBER, UFF parameters and the partial charges computed by the density-derived electrostatic and chemical charge method (DDEC). The parameters for the 2-methyl imidazole (MeIM) ring are adopted from the AMBER force field, while the van der Waals (VDW) parameters for organic linkers and metal centers were determined by rescaling the UFF parameters as epsilon = 0.635(epsilon UFF) and sigma = 1.0 sigma(UFF) to fit the CH4 adsorption isotherms obtained by Grand Canonical Monte Carlo (GCMC) simulations with the force field parameters to the experimental ones. The CH4 adsorption isotherms on four different structures of ZIF-8 at 298 K obtained by GCMC simulations are compared with the experimental data. The results show that the simulated CH4 adsorption isotherms on the ZIF-8 structure reported from the Cambridge Crystallographic Data Centre (CCDC) are closest to the ones on the ZIF-8 structure from the report of Moggach et al. To test our wmodel, adsorption isotherms of CH4, H-2, CO2 and N-2 at different temperatures were computed using GCMC simulations, and the results were found to be in a good agreement with the experimental data. In the case of H-2, the equilibrium configurations obtained by GCMC simulations were statistically analyzed with ad hoc code to get probability density distribution profiles. These profiles were transformed to visual slice images, which indicate that the preferential adsorption sites of H-2 molecules in ZIF-8 are located close to the MeIM rings, where the host-guest VDW or electrostatic interactions are maximal, as revealed by the potential energy surfaces (PES). In addition, these force field parameters were confirmed to well reproduce the ZIF-8 structural properties including lattice constants, bond lengths and angles over a wide range of temperatures. The self-diffusivities at the specific loadings of adsorbed gases (CH4, H-2 and CO2) in ZIF-8 were calculated by the mean squared displacement (MSD) method. It was found that our self-diffusivities of H-2 are slightly higher than the ones in the literature, and our self-diffusivity of CO2 is as about three times as the one in the literature, due to the different partial charges and the effect of different force field parameters on framework shape and flexibility in our simulations.