Unlocking the Tertiary Nitrogen Effect in 3D Anisotropic Covalent Organic Frameworks to Achieve Record Iodine Capture under Industrial Conditions

Using porous adsorbents to achieve efficient capture of radioactive iodine from nuclear waste under industrial conditions remains a formidable challenge, since most adsorbents just exhibit a low iodine uptake under such conditions. We herein show two 3D [8 + 4]-assembled covalent organic frameworks (COFs, ECUT-COF-99 and ECUT-COF-100) for this challenging target. The two COFs feature a minimal-transitivity [3 4] flu-derived net, which allows anisotropic isoreticular expansion through a general sequential derived-net approach. ECUT-COF-100 was found to give an ultrahigh iodine uptake of 1.51 g/g under simulated industrial conditions (150 °C and 150 ppmv I2), 5.2-fold that of ECUT-COF-99 (0.29 g/g). Such performance also far exceeds all established organic adsorbents for this use. Mechanism studies reveal that nonaromatic tertiary nitrogen atoms allow for strong charge transfer from the lone-pair electrons of tertiary nitrogen to I2, leading to the formation of ionic iodides (I3- and I5-) and the N·+ radical cation and then fixing of ionic iodides by Coulomb interactions. This work not only provides a promising adsorbent with record iodine uptake for radioactive iodine capture from nuclear waste but also highlights a new insight into the tertiary nitrogen effect in 3D COFs for efficient iodine capture.