Fluorinated metal-organic frameworks for enhanced stability and iodine adsorption selectivity under humid conditions

The selective removal of radioactive iodine from off-gases of reprocessing plants with high humidity is crucial for environmental security, and the metal-organic framework (MOF)-based porous material is considered as one of the promising adsorbents. However, MOFs usually suffer from low structural stability and the competitive adsorption of water. Herein, fluorinated groups are introduced into MOFs to improve the iodine adsorption performance under humid conditions. Two different strategies, the ligand exchange and direct synthesis, were employed to construct fluorinated Zr-based PCN-222 and UiO-66, respectively. Both the two fluorinated MOFs exhibit enhanced structural stability when interacting with iodine guests, and thus the iodine adsorption capacity is improved due to the more stable pore structure. Besides, after fluorination, the iodine adsorption selectivity is also enhanced in the presence of high humidity. Particularly, UiO-66-F exhibits an iodine adsorption capacity of 0.93 g/g under wet iodine vapor with 18% RH at 80 celcius, close to that in the dry iodine vapor. DFT calculation and XPS analysis suggest there is no F...I halogen bonding interaction between fluorinated groups and trapped iodine, and the Zr nodes transfer negative charges to iodine via O-containing groups in the Zr nodes.