Experimental and theoretical verification of structure–adsorption relationships in cage-based covalent organic frameworks for iodine capture

To address the increasingly urgent problem of iodine pollution, we synthesized a nitrogen-rich cage-based covalent organic framework, Cage-COF-TP (T = bis-(tetraoxacalix[2]arene[2]triazine); P = piperazine), for iodine adsorption and release. Structural characterization revealed caged pores with a size of 26.847 Å and interconnected micropores with diameters of 4.527 Å, forming an ordered three-dimensional network of channels. This architecture, enriched with nitrogen and oxygen atoms, provides abundant adsorption sites and electron-rich channels. Owing to these features, Cage-COF-TP demonstrates exceptional iodine adsorption capacities of 1200 mg·g −1 from cyclohexane solution and 218% from vapor. Importantly, the adsorbed iodine rapidly desorbs in ethanol, underscoring the cyclability of the Cage-COF-TP framework. Density functional theory calculations revealed that the excellent physical adsorption performance of cage-based COFs arises from the unique electronic structure and van der Waals interactions associated with the polycyclic cage structure. The consistency between experimental and theoretical results establishes the structural origin of efficient and reversible iodine adsorption in cage-based COFs. These results highlight the potential of cage-based COFs for iodine removal and recovery and demonstrate how cage-based steric effects can be exploited to design high-performance physical adsorbents. • Cage-COF-TP features 26.8 Å cages and 4.5 Å micropores forming ordered channels. • Steric hindrance of the polycyclic cages enables physical confinement of iodine. • The structure–adsorption relationship was verified by experiments and DFT studies. • Cage-COF-TP achieves 1200 mg·g −1 iodine uptake from cyclohexane solution. • Iodine is rapidly desorbed in ethanol, demonstrating excellent cyclability.