Flexible Cyclotriphosphonitrile-Based Covalent Organic Frameworks for the Capture of Iodine

The development of highly efficient materials for trapping radioactive iodine is essential for the safe use of nuclear energy. Herein, we present a strategy to synergistically enhance iodine capture by exploiting the flexible building blocks with the affinity properties of electrically rich heteroatoms by combining the cyclic triphosphonitrile derivative with amine linkers of different molecular lengths and constructing triangular pore-structured flexible cyclotriphosphonitrile-based covalent organic frameworks. A series of cyclotriphosphonitrile-based COFs (CTP-X-COF, X = PDA, ODA, BPY, DPT) with triangular pore topology were synthesized using CTP-6 as the node and connection point of p-phenylenediamine (PDA), 4,4-diaminodiphenyl ether (ODA), 5,5-diamino2,2-bipyridine (BPY), and 4,4-diaminotribiphenyl (DPT) with different molecular lengths. It was found that the adsorption capacities of CTP-PDA-COF, CTP-ODA-COF, CTP-BPY-COF, and CTP-DPT-COF for iodine vapor were 4.65 g g^-1, 4.18 g g^-1, 2.38 g g^-1, and 3.57 g g^-1, respectively. The results showed that the adsorption capacities of CTP-PDA-COF, CTP-ODA-COF, CTP-BPY-COF, and CTP-DPT-COF for iodine in cyclohexane were 281.50 mg g^-1, 221.67 mg g^-1, 161.14 mg g^-1, and 222.67 mg g^-1, respectively. It was found that the four COFs adsorbed iodine in cyclohexane solution was a mixed adsorption process with chemical adsorption playing a predominant role and physical adsorption as an auxiliary mechanism.