The Role of the Nanoconfinement Effect in the Adsorption of Carbon Dioxide by Porous Liquids

Porous liquids based on porous organic cages (POCs) have attracted much attention recently due to their notable gas adsorption and separation ability. Compared with well-developed type II porous liquids, POC-based type III porous liquids were less explored and understood, especially those POCs solvated in ionic liquids. A previous experimental study showed that the POC cluster solvated in different ionic liquids presented different CO2 capacities, while the mechanism behind the molecular level is unknown. The CO2 diffusion mechanism needs to be further uncovered to guide solvent screening for the POC-based type III porous liquids. In this study, two porous liquid systems with the same POC cluster solvated in different ionic liquids were investigated through computational methods. The CO2 capacity of porous liquids composed of the POC cluster and [BPy]+[NTf2]− (BPy_NTf2) is higher than that of the consisted POC cluster and [BMIM]+[NTf2]− (BMIM_NTf2), which is supported by the experimental results. The detailed dynamical and structural analyses demonstrated that the CO2 diffusion rate was lowered by the interface between the solvent and the POC cluster in both systems. Moreover, in the BMIM_NTf2 system, the ring part of cation molecules concentrated above the cage window efficiently hinders the CO2 entering the POC cluster inside, which explains the difference in the CO2 capacity in the two systems. The observed nanoconfinement effect at the interface between the solvent and the POC cluster surface challenged the assumption that chemical groups of solvent molecules entered inside cages, leading to a steric effect on the CO2 diffusion dynamics. Our findings provide new insight into how the nanoconfined structure impacts the CO2 adsorption dynamics in porous liquids. This may contribute to the future screening or designing of solvents for POC-based type III porous liquids.