On the Diffusion of Ionic Liquids in ILs@ZIF-8 Composite Materials: A Density Functional Theory Study

Recently, composite materials consisting of ionic liquids (ILs) and metal-organic frameworks (MOFs) have attracted a great deal of attention due to their fantastic properties. Many theoretical studies have been performed on their special structures and gas separation applications. Yet, the mechanism for the diffusion of ILs inside MOF channels still remains unclear. Here, the DFT calculations (e.g., rigid and relaxed potential energy surface, PES, scan) together with frontier orbital analysis, natural charge analysis, and energy decomposition analysis were performed to investigate the diffusion behavior of a typical IL, [C₄mim][PF₆], into the ZIF-8 SOD cage. The PES profiles indicate that it is quite difficult for the cation [C₄min]⁺ to diffuse into the cage of ZIF-8 through the pristine pores because of the large imidazole steric hindrance, which results in a large energy barrier of ca. 40 kcal·mol^-1 at the least. Interestingly, the PES reveals that a successful diffusion could be obtained by thermal contributions, which enlarge the pore size through swing effects at higher temperatures. For example, both [C₄mim]⁺ and [PF₆]^- could easily diffuse through the channel of the ZIF-8 SOD cage when the pore size was increased to 6.9 Å. Subsequently, electronic structure analyses reveal that the main interactions between [PF₆]^- or [C₄mim]⁺ and ZIF-8 are the steric repulsion interactions. Finally, the effects of the amounts of [C₄mim][PF₆] on the ZIF-8 structures were investigated, and the results show that two pairs of [C₄mim][PF₆] per SOD cage are the most stable in terms of the interaction between energies and structural changes. With these findings, we propose that the high-temperature technique could be employed during the synthesis of IL@MOF membranes, to enrich their family members and their industrial applications.