Molecular Simulation of Adsorption and Diffusion of CH4 and H2 in ZIF-8 Material

Metal-organic frameworks (MOFs) with flexible organic linkers and bridging tetrahedral metal ions were extensively investigated in order to improve the selectivity and storage properties of sorbents. The so-called zeolitic imidazolate frameworks (ZIFs) are a particularly interesting class of MOFs due to their exceptional chemical and thermal stability. Adsorption and diffusion characteristics of CH4 and H-2 in porous zeolitic imidazolate framework-8 (ZIF-8) material were comparatively studied using the methods of equilibrium molecular dynamics (EMD) and the grand canonical Monte Carlo (GCMC) with the same force field. It was shown that using the force field of framework flexibility not only can reproduce the crystal structure of ZIF-8 at different temperatures and pressures well, but also can calculate the diffusion coefficients of CH4 and H-2 in ZIF-8 at different temperatures accurately. Especially, adsorbed CH4 at high temperature can run away the space constraint at the entrance of framework cage in ZIF-8, which results in a sharp increase of its diffusion coefficient. At the same time, using the force field can also simulate the adsorption isotherms of CH4 and H-2 molecules in ZIF-8 roughly. Thus, their probability density distribution data in the unit cell of ZIF-8 under the conditions of adsorption and diffusion equilibrium can be calculated via the self-compiled program and further visualized via the software of VMD in order to analyze the adsorption and diffusion behavior of the gases. The results indicate that the priority locations for adsorption of CH4 and H-2 molecules in ZIF-8 are regions close to the imidazole rings in the center of big pores. But there are two different levels of the preferential adsorption sites for CH4 and only one level of that for H-2, indicating that different adsorption mechanism of CH4 and H-2 exists in ZIF-8. The difference of the preferential adsorption sites for CH4 and H-2 in ZIF-8 is possibly resulting from the different size of the two molecules.