Synthesis and Properties of a Zwitterionic Covalent Organic Framework (COF): The Meltability Strategy and Processability

Covalent organic frameworks (COFs) are porous materials with well-defined structures and promising properties related to low-energy consumption technologies in industry. However, the lack of processability of powder COFs remains a great challenge and severely restricts their practical applications, such as thin membranes for industrial superpure species separation. The hot-melt process is widely applied in industrial manufacturing. Here, we synthesized the zwitterionic COF and demonstrated its easy melting at moderate temperatures with the assistance of buffer of Bro̷nsted acid (HA). The melting liquid COFdHA (<148 °C) enabled their process into a large-area thin film via a simple hot-casting knife method and crystalline membrane through a subsequent structure-recovery procedure. The crystalline membranes preserved the COF's pristine structure, and exhibited the highest nanofiltration performance compared to analogous COF/MOF obtained via traditional tedious processes, while the melt-quenched products can also be used as glassy solid electrolytes with fast ionic conduction. Therefore, such a zwitterionic COF can act as a porous and dense material for specific applications, showing two sides of a coin. Structure, structure–property relationship, and the melting mechanism for the new type of zwitterionic COF are evidenced via experiment and theoretical calculation.