Effect of Functionalized Linker on CO2 Binding in Zeolitic Imidazolate Frameworks: Density Functional Theory Study

Density functional theory (DFT) calculations are performed to investigate the effect of functionalized linkers on CO2 binding in zeolitic imidazolate frameworks (ZIFs). Three typical ZIFs with different functionalized linkers are considered: ZIF-68 with bIM linker (C6H6 group), ZIF-69 with cbIM linker (chlorine group), and ZIF-78 with nbIM linker (nitro group). Compared to bIM linker, the charge distributions of cbIM linker and nbIM linker have been significantly changed by the substitution of the functional groups and have been proved to have positive effects to enhance the binding capacity of CO,. The withdrawal of electronic charge from the linker leads to enhanced acidity for the hydrogen atoms on linker, which can form weak hydrogen bond interactions with the oxygen atoms of CO2. The binding capacity of nbIM linker for CO2 is much higher than those of bIM and cbIM linkers for CO2, which is due to the strong effect of nitro group on polarizing the CO2 molecule and thus enhances the interaction force between them. The binding energies of CO2 in the IM linkers are within the range of -1.64 to -18.54 kJ/mol, which suggests that the binding belongs to physisorption.