Positional Isomerism: A Novel Paradigm for Enhancing Iodine Adsorption in Functionalized Metal–Organic Frameworks

Porous metal-organic frameworks (MOFs) have shown great potential as adsorbents for capturing radioiodine, a major fission product generated during the reprocessing of nuclear fuel. However, studies exploring the correlation between the structure of MOFs and iodine uptake capacity remain notably rare. In this study, we introduce a new strategy for enhancing the iodine adsorption efficiency of MOFs by strategically varying the position of functional groups on the organic linkers. Employing ligand-functionalized UiO-67 MOFs, our findings reveal that <i>ortho</i>-amino substitution of UiO-67-<i>o</i>-NH₂, proximal to the node of the dicarboxylate linker, markedly accelerates adsorption kinetics of iodine vapor in comparison to <i>meta</i>-amino substitution of UiO-67-<i>m</i>-NH₂, where the amino groups are oriented away from the node. In contrast, UiO-67-<i>m</i>-NH₂ exhibits a higher adsorption capacity of 2.19 g/g, compared to 1.91 g/g for UiO-67-<i>o</i>-NH₂, attributable to its higher porosity. Furthermore, a competitive I₂/H₂O vapor adsorption study demonstrated that UiO-67-<i>o</i>-NH₂ exhibits faster adsorption kinetics and higher selectivity for iodine in the presence of water vapor compared to UiO-67-<i>m</i>-NH₂. Additionally, the crucial influence of positional isomerism on enhancing iodine adsorption has been corroborated through Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. These analyses reveal that the nitrogen atom positioned at the <i>ortho</i> site demonstrates a stronger affinity for iodine molecules compared to the nitrogen atom at the <i>meta</i> site, thereby improving adsorption kinetics.