Positional\nIsomerism: A Novel Paradigm for Enhancing\nIodine Adsorption in Functionalized Metal–Organic Frameworks

Porous metal–organic frameworks\n(MOFs) have shown\ngreat\npotential as adsorbents for capturing radioiodine, a major fission\nproduct generated during the reprocessing of nuclear fuel. However,\nstudies exploring the correlation between the structure of MOFs and\niodine uptake capacity remain notably rare. In this study, we introduce\na new strategy for enhancing the iodine adsorption efficiency of MOFs\nby strategically varying the position of functional groups on the\norganic linkers. Employing ligand-functionalized UiO-67 MOFs, our\nfindings reveal that <i>ortho</i>-amino substitution of\nUiO-67-<i>o</i>-NH<sub>2</sub>, proximal to the node of\nthe dicarboxylate linker, markedly accelerates adsorption kinetics\nof iodine vapor in comparison to <i>meta</i>-amino substitution\nof UiO-67-<i>m</i>-NH<sub>2</sub>, where the amino groups\nare oriented away from the node. In contrast, UiO-67-<i>m</i>-NH<sub>2</sub> exhibits a higher adsorption capacity of 2.19 g/g,\ncompared to 1.91 g/g for UiO-67-<i>o</i>-NH<sub>2</sub>,\nattributable to its higher porosity. Furthermore, a competitive I<sub>2</sub>/H<sub>2</sub>O vapor adsorption study demonstrated that UiO-67-<i>o</i>-NH<sub>2</sub> exhibits faster adsorption kinetics and\nhigher selectivity for iodine in the presence of water vapor compared\nto UiO-67-<i>m</i>-NH<sub>2</sub>. Additionally, the crucial\ninfluence of positional isomerism on enhancing iodine adsorption has\nbeen corroborated through Raman spectroscopy, X-ray photoelectron\nspectroscopy, and density functional theory calculations. These analyses\nreveal that the nitrogen atom positioned at the <i>ortho</i> site demonstrates a stronger affinity for iodine molecules compared\nto the nitrogen atom at the <i>meta</i> site, thereby improving\nadsorption kinetics.