(550d) Adsorption of Alcohols in Zeolitic Imidazolate Framework-8: Effects of Force Fields, Atomic Charges and Framework Flexibility

Emerging as a new family of nanoporous materials, metal-organic frameworks (MOFs) have attracted considerable interest in the past two decades. Most experimental and simulation studies for MOFs have been focused on gas storage and separation. With increasing interest in biofuel production, a number of studies have examined alcohol adsorption in MOFs towards biofuel purification. However, only few simulation studies have been conducted for alcohol adsorption in MOFs. For the development of novel MOFs for biofuel purification by adsorption technology, first, it is critical to better understand alcohol adsorption in MOFs. In this study, 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) examined, DREIDING has the best agreement with experiment. The atomic charges and framework flexibility are found to have negligible effects. The isotherms of all the four alcohols are S-shaped type V, indicating relatively weak adsorption in a microporous framework. With increasing pressure, three adsorption regimes are observed. At a low pressure, clusters are formed proximal to organic linker (2-methylimidazolate), and the C = C bond of organic linker is identified to be the most favorable site for adsorption. At an intermediate pressure, cage-filling occurs in sodalite cage with sharp increase in adsorption. Finally, saturation is approached at a high pressure. The isosteric heat of adsorption at infinite dilution rises linearly with the chain length of alcohol, as attributed to the enhanced interaction between aliphatic tail and hydrophobic ZIF-8. This simulation study systematically examines the effects of force fields, atomic charges and framework flexibility on alcohol adsorption in ZIF-8. The microscopic insight provided is helpful to better elucidate the adsorption behavior of alcohols in other ZIFs and MOFs, and would facilitate the development of new nanoporous materials for biofuel purification.