Constructing cationic flexible covalent organic frameworks through post-functionalization for enhancing the iodine adsorption capacity

[Display omitted] •Two cationic flexible COFs have been synthesized by post-functionalization method.•The I2 uptake capacities of cationic flexible COFs are higher that of neutral ones.•The segregative bromides and iodides can induce I2 to engender polyiodides.•The iodine adsorption amounts and the rates increase with temperature. With the incresing development of nuclear power plants, the radioactive iodine produced from nuclear waste and nuclear accidents poses increasing threat to the human health and environment. The iodine adsorption behavior of the neutral flexible covalent organic skeleton has been somewhat studied, but to our knowledge, there are no reports of iodine capture by ionized flexible COFs. Herein, we apply a post-functionalization process to construct two cationic flexible COFs (iCOFs-AB-1 and iCOFs-AB-2) for the first time. The iCOFs-AB-1 and iCOFs-AB-2 exhibit I2 uptake capacities of 3.36 and 4.26 g g−1 at 77 °C, which are improved 1.19 and 1.14 times compared to that of neutral flexible COFs. The promotion of ionic groups on iodine adsorption and the iodine valence states adsorbed at diverse sites have identified by experiments. The adsorption mechanism studies show that the segregative bromide and iodide anions in iCOFs-AB-1 and iCOFs-AB-2 can induce molecular iodine to engender polyiodide anions, and the quaternary ammonium salt cations in iCOFs-AB-1 and iCOFs-AB-2 can come into being the Coulomb interaction force with the aforementioned polyiodide anions. This study demonstrates an efficient method to synthesize cationic flexible COFs and further use them as adsorbents for capturing iodine.