Unraveling the High Uptake and Selectivity of CO2 in the Zeolitic Imidazolate Frameworks ZIF-68 and ZIF-69

Understanding the CO2 adsorption mechanism of zeolitic imidazole frameworks (ZIFs) is the key to improving the adsorption ability of new metal-organic framework materials for gas capture applications and designing novel frameworks. In this work, the ONIOM2(B3LYP/6-31G**:UFF) method is used to investigate the interactions between CO2 and CO molecules with ZIF-68 and ZIF-69, respectively. Calculation results indicate that a strong Lewis acid-base interaction exists between the gas molecules and nitro groups of 2-nitroimidazole linkers, which thus prevents the entry of gas molecules into the small pores of ZlFs. The simultaneous interaction of both oxygen atoms of CO2 with the H atoms of the benzimidazole (5-chlorobenzimidazole) rings in the large pores results in the high CO2 selectivity observed for both ZIFs. The presence of electron-withdrawing groups in the phenylimidazole linker of ZIP increases the interaction between the oxygen atoms of CO2 and hydrogen atoms of the phenyl rings in its large pores. This work elucidates the adsorption mechanism of CO2 and CO to ZIFs at the molecular level and provides useful information for the design of MOFs in gas purification and separation applications.