Metal-free porphyrin porous organic cage for efficient iodine capture

Abstract

A large amount of radioactive iodine, which is harmful to the environment and humans, is produced during the operation of nuclear power plants, and the design of adsorbents for the efficient adsorption of radioactive iodine should be further studied. In this work, porphyrin-based porous organic cage (POC), denoted as PTC-2H, was synthesized using dynamic covalent chemistry, and PTC-Zn was obtained by the post-synthetic modification of PTC-2H with Zn²⁺. Subsequently, the iodine capture performance was studied. The iodine capture capacity of PTC-2H reached 5.46 g g⁻¹, exceeding the capacities reported for most POCs. Multi-spectral techniques confirmed that iodine adsorption occurs mainly through chemical adsorption, and porphyrin units together with imine bonds are the main active sites for iodine adsorption. Owing to the synergistic effect of the high specific surface area, large conjugated plane, and abundant N heteroatoms, PTC-2H exhibited a higher iodine capacity than other POCs. After the introduction of zinc ions into PTC-2H, PTC-Zn exhibited a lower iodine capacity (4.96 g g⁻¹), which could be attributed to the loss of the –NH bonds in the porphyrin units. Thus, POCs containing porphyrin units were used in this study for iodine adsorption, and the positive effects of –NH bonds in the N₄ cavity of the porphyrin plane, together with the higher specific surface area of PTC-2H, on the iodine adsorption capacity are demonstrated proved for the first time. The insights obtained in this study are of great significance for exploring new materials with more functional groups for improving their iodine adsorption performance.