Construction of Cationic Covalent Organic Frameworks Based on Nitrogen Sites for Adsorption of Anionic Dyes

Limited by the structure and stability of monomers, it is hard to directly construct cationic covalent organic frameworks (COFs), so it is necessary to introduce cationic sites into the framework through the postsynthetic modifications (PSM) process. In this paper, BIPY-COF with different nitrogen sites in the skeleton is synthesized, and two different cationic COFs are constructed by the PSM process, named as HI/BIPY-COF and CH₃I/BIPY-COF, respectively. The adsorption properties of three materials for Acid Orange 10 (AO), methyl orange (MO), and eosin disodium (ED) are studied. It is found that the adsorption properties of cationic COFs are greatly improved. The adsorption capacity of MO increases from 12.2 mg/g to 122.7 mg/g and 174.3 mg/g, respectively. It can be inferred from theoretical calculations and instrument characterization that electrostatic interaction plays an important role in the adsorption process. This work reveals the possibility and great potential of constructing cationic COFs based on different nitrogen sites and provides a reference for enhancing the adsorption of anionic dyes by COFs.