Spiroborate-based three dimensional covalent organic framework for effective adsorption and separation of organic dyes

Covalent organic frameworks (COFs) are undergoing a period of rapid development of novel structures and diverse functions. Ionic COFs have been applied in environmental engineering due to their porous, robust and charged skeleton, making it necessary to expand the family of ionic frameworks. Based on an in-situ functionalization strategy, we report a new 3D anionic COF constructed by the linker 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP), linkage spiroborate (SPB) and counter cation [n-Bu2NH2]+ (DBA). Anionic spiroborate centers are distributed in a 4-fold interpenetrated network of srs topology, constituting a stable crystalline structure. The framework has been used for adsorption of charged dyes and present rapid cationic dyes removal(over 99 % within 10 min), effective adsorption capacities for methylene blue (MB), crystal violet (CV), and malachite green (MG) (maximum values of 892 mg g−1, 1632 mg g−1 and 457 mg g−1, respectively), impressive molecular sieving selectivity (little absorption of anionic dyes) and good recycle performance (no removal loss for at least 5 cycles). In addition, the adsorption behaviors of isotherm and kinetic are discussed. This study not only reveals a new strategy for constructing ionic 3D COFs, but also contributes to their development for environmental engineering related applications.