Zwitterionic Covalent Organic Frameworks: Attractive Porous Host for Gas Separation and Anhydrous Proton Conduction

Abstract

Ionic covalent organic frameworks (COFs) consisting of an anionic or cationic skeleton and corresponding counterions have demonstrated great potential in many application fields such as ion conduction, molecular separation, and catalysis. However, arranging anionic and cationic groups into the same COF to form zwitterionic materials is still unexplored. Herein we design the synthesis of three zwitterionic COFs as attractive porous hosts for SO2/CO2 separation and anhydrous proton conduction. The separated cationic and anionic groups in zwitterionic COFs' channels can act as two different polar sites for SO2 adsorption, allowing zwitterionic COFs to achieve a high SO2 adsorption capacity (216 mL/g, 298 K) and impressive SO2/CO2 selectivity (118, 298 K). Furthermore, after loading with triazole/imidazole, the zwitterionic groups in COFs' channels can induce complete proton carrier deprotonation, producing more freely migrating protons. The free protons migrate along a continuous hydrogen-bonding network in zwitterionic COFs' channels, leading to outstanding anhydrous proton conductivity up to 4.38 × 10-2 S/cm, which is much higher than other N-heterocyclic-doped porous materials under anhydrous conditions. Proton dissociation energy calculations combined with frequency-dependent dielectric analysis give insight into the role of zwitterionic COFs for proton conductivity. Our work provides the possibility to design well-defined zwitterionic frameworks for gas separation and ion conduction.