Theoretical insights into CO2/N2 selectivity of the porous ionic liquids constructed by ion-dipole interactions

Porous liquids, a new class of materials, containing solid pores and liquid properties, have greatly aroused attention. In terms of gas absorption, porous liquids exhibit excellent advantages. Nevertheless, a variety of reports still lack the understanding of its absorption mechanism at the molecular level. Herein, we have figured out the factors contributing to the formation of porous liquids made of porous organic cages that can be dissolved in the crown ether, and the absorption mechanism of carbon dioxide, as well as CO2/N2 selectivity. Through charge and Wiberg index analysis, the results show that crown ethers can interact with alkali metals to form alkali metal complexes by ion–dipole interactions, the dominant driving force of which is electrostatic interaction rather than coordination effect. Besides, the metal complexes should be regarded as a whole entity, which increases the steric hindrance of the cations and greatly reduces the probability of the crown ether blocking the cavity. The porous organic cage does provide unoccupied pores for gas storage. Furthermore, compared with N2, cages prefer to absorb CO2 mainly through hydrogen bonding. It is hoped that this work can facilitate the design and synthesis of porous liquids for gas absorption and selectivity.