Molecular simulation of adsorption and diffusion of CH4 and H2O in flexible metal-organic framework ZIF-8

Abstract

A quantitative understanding of adsorption and diffusion properties of CH4 and H2O in ZIF-8 is crucial for its application in adsorption-hydration coupling CH4 storage. Monte Carlo and molecular dynamics simulations are conducted to investigate these properties. Firstly, the spatial distribution of H2O in ZIF-8 cavity and intergranular space is discussed. H2O is mainly distributed in the intergranular space near the outer surface of the cavities. The growth rate of H2O adsorption amount in the intergranular space is about 6 times that in the cavities. Then, the effect of pre-adsorbed H2O on the CH4 adsorption in ZIF-8 at 275.15 K and 0–6 MPa is investigated. From the analysis of radial distribution functions and interaction energy, H2O occupies the CH4 preferential adsorption sites and reduces effective adsorption volume. Therefore, CH4 adsorption capacity decreases with increasing water content. Next, the mutual influence of H2O and CH4 on structural properties is analyzed. CH4 has no effect on the structure of H2O clusters, while H2O makes CH4 together more tightly. Finally, the self-diffusion coefficients of H2O, CH4, and CH4/H2O mixture are calculated. Compared with pure H2O, the presence of CH4 enhances the interaction between H2O and ZIF-8 and inhibits the diffusion of H2O. However, compared with pure CH4, the effect of H2O on the diffusion of CH4 is more complicated. At a low loading, the presence of H2O inhibits the diffusion of CH4 while it facilitates the diffusion of CH4 at a high loading. This study provides microscopic insight into adsorption and diffusion in ZIF-8.