Correlation of hydrophilicity and active N sites with organic iodine adsorption in covalent organic frameworks

Covalent-organic frameworks (COFs) present a promising platform for the development of materials for gaseous iodine adsorption; however, achieving an optimal balance between stability, property, and functionality remains a significant challenge. In this study, we designed and synthesized a series of nitrogen-rich COF materials interconnected by sp²-C bonds. The active nitrogen sites within the porous COF backbone provide more accessible adsorption sites for organic iodine, while the sp²-C bond ensures exceptional radiation and acid-base stability of the materials. Notably, we discovered for the first time that the enhanced hydrophilicity of COFs leads to improved iodomethane adsorption. Among the synthesized COFs, COF-1, which exhibits the best hydrophilicity, shows the highest affinity for iodomethane, with a maximum adsorption capacity of 1.56 g g⁻¹ . XPS, solid-state NMR, GC-MS and UV spectroscopy characterizations revealed that the adsorption mechanism involves two sequential processes: quaternization of active nitrogen sites on the COF framework by methyl iodide, followed by oxidation and hydrolysis of methyl iodide driven by the hydrophilicity of the COF. In addition to identifying stable, efficient, and cost-effective adsorbents for organic iodine removal, this work offers valuable insights into the adsorption process of iodomethane in COFs, which undoubtedly benefits the future design and development of gaseous iodine adsorbents.