Development of a force field for zeolitic imidazolate framework-8 with structural flexibility

Abstract

A force field is developed for zeolitic imidazolate framework-8 (ZIF-8) with structural flexibility by combining quantum chemical calculations and classical Amber force field. The predicted crystalline properties of ZIF-8 (lattice constants, bond lengths, angles, dihedrals, and x-ray diffraction patterns) agree well with experimental results. A structural transition from crystalline to amorphous as found in experiment is observed. The mechanical properties of ZIF-8 are also described fairly well by the force field, particularly the Youngs modulus predicted matches perfectly with measured value. Furthermore, the heat capacity of ZIF-8 as a typical thermophysical property is predicted and close to experimental data available for other metal-organic frameworks. It is revealed the structural flexibility of ZIF-8 exerts a significant effect on gas diffusion. In rigid ZIF-8, no diffusive behavior is observed for CH 4 within the simulation time scale of current study. With the structural flexibility, however, the predicted diffusivities of CH 4 and CO 2 are close to reported data in the literature. The density distributions and free energy profiles of CH 4 and CO 2 in the pore of ZIF-8 are estimated to analyze the mechanism of gas diffusion. © 2012 American Institute of Physics.