Adsorption of C1-C4 Alcohols in Zeolitic Imidazolate Framework-8: Effects of Force Fields, Atomic Charges, and Framework Flexibility

A molecular simulation study is reported for the adsorption of normal alcohols (methanol, ethanol, propanol, and butanol) in zeolitic imidazolate framework-8 (ZIF-8). The effects of force fields, atomic charges, and framework flexibility are systematically examined and compared with experimental data. Among three force fields (UFF, AMBER, and DREIDING), DREIDING has the best agreement with experiment. The atomic charges and framework flexibility are found to have negligible effects. The four alcohols exhibit S-shaped isotherms without hysteresis loop, as attributed to adsorption at different preferential sites. At a low pressure, cluster formation is observed near the organic linker (2-methylimidazolate) in ZIF-8; with increasing pressure, cage-filling occurs in the large sodalite cage. The interaction between alcohol and ZIF-8 framework is enhanced as the chain length of alcohol increases; thus, the isosteric heat of adsorption rises with chain length. The simulation study provides microscopic insight into alcohol adsorption in ZIF-8, which is useful for quantitative understanding of adsorption mechanism in other ZIFs and nanoporous materials.