Constructing covalent organic frameworks with dense thiophene S sites for effective iodine capture†

Developing versatile sorption materials for radionuclides (e.g. iodine) capture has been a critical goal in nuclear energy and environmental science. At the same time, covalent organic frameworks (COFs), on account of their high porosity and functional scaffolds, have opened up a new way to develop adsorbents in recent years. Herein, two kinds of COF materials containing thiophene (TAPT-COF and TAB-COF), as iodine sorbents, are designed and synthesized by Schiff base reaction. Among them, TAB-COF has a higher surface area (TAPT-COF: 1141 m² g⁻¹, TAB-COF: 1378 m² g⁻¹), which is helpful for the physical iodine adsorption. More importantly, the COF backbone is rich in both N and S sites, which is advantageous to the chemical adsorption of iodine. These two features make the two COFs ideal iodine sorption materials. For example, TAB-COF has an excellent gaseous iodine adsorption capacity (2.81 g g⁻¹) and is one of the most efficient iodine adsorption materials. Meanwhile, TAB-COF has an excellent adsorption effect on iodine in the cyclohexane system, which can reach 200 mg g⁻¹. In addition, the DFT calculations proved that both imine N and thiophene S serve as active sites during the iodine adsorption. TAB-COF exposes more active sites on the premise of having a higher surface area, thereby leading to a higher iodine adsorption capacity. The results here indicate improved sorption efficacy by introducing thiophene in COFs for sorption applications in general and especially pave the way for developing stable and effective COF sorbents for iodine capture from various environments.