Bearing exceptionally high content pyridine-N into covalent organic frameworks to facilitate the adsorption of iodine and methyl iodide simultaneously

Radioactive iodine vapor (I2) and methyl iodide (CH3I) are released during nuclear fuel reprocessing and nuclear accidents. However, very few adsorbents can effectively adsorb I2 and CH3I simultaneously, resulting in the treatment processes ineffective and complicated. Covalent organic frameworks (COFs) have been identified as suitable materials for the adsorption of I2 and CH3I due to their ordered pore structures and design flexibility. Specifically, appropriate pore sizes, sufficient pore volumes, and a large specific surface area are beneficial for enhancing the physical adsorption performance of I2; while the incorporation of active sites can simultaneously improve the chemical adsorption performance of COFs for both I2 and CH3I. In this work, two pyridine-rich linkers were synthesized using Suzuki coupling reactions. Pyridine-N, acting as a Lewis base, can form charge transfer complexes with I2 and interact with CH3I via methylation reactions, making it an effective active group for adsorbing I2 and CH3I. Utilizing these pyridine-rich linkers, two well-crystallized PyN-COFs (PyN-COF-A/B) were successfully synthesized through Schiff base reactions. Notably, PyN-COF-B has never been reported previously, and exhibits an exceptionally high pyridine-N content (18.87 wt%). The large specific surface area not only enhances physical adsorption but also exposes more accessible active sites, enabling PyN-COFs to exhibit exceptional adsorption performance. The density functional theory (DFT) calculations reveal the presence of two active nitrogen sites in PyN-COFs, with pyridine-N demonstrating high binding energies for both I2 and CH3I. This study provides valuable insights for the development of porous adsorbents capable of simultaneously adsorbing I2 and CH3I.