Capture of Gaseous Iodine in Isoreticular Zirconium-Based UiO-n Metal-Organic Frameworks: Influence of Amino Functionalization, DFT Calculations, Raman and EPR Spectroscopic Investigation

A series of Zr-based UiO-n MOF materials (n=66, 67, 68) have been studied for iodine capture. Gaseous iodine adsorption was collected kinetically from a home-made set-up allowing the continuous measurement of iodine content trapped within UiO-n compounds, with organic functionalities (-H, -CH3, -Cl, -Br, -(OH)(2), -NO2, -NH2, (-NH2)(2), -CH2 NH2) by in-situ UV-Vis spectroscopy. This study emphasizes the role of the amino groups attached to the aromatic rings of the ligands connecting the {Zr6O4(OH)(4)} brick. In particular, the preferential interaction of iodine with lone-pair groups, such as amino functions, has been experimentally observed and is also based on DFT calculations. Indeed, higher iodine contents were systematically measured for amino-functionalized UiO-66 or UiO-67, compared to the pristine material (up to 1211 mg/g for UiO-67-(NH2)(2)). However, DFT calculations revealed the highest computed interaction energies for alkylamine groups (-CH2NH2) in UiO-67 (-128.5 kJ/mol for the octahedral cavity), and pointed out the influence of this specific functionality compared with that of an aromatic amine. The encapsulation of iodine within the pore system of UiO-n materials and their amino-derivatives has been analyzed by UV-Vis and Raman spectroscopy. We showed that a systematic conversion of molecular iodine (I-2) species into anionic I- ones, stabilized as (II2)-I-... or I-3(-) complexes within the MOF cavities, occurs when I-2@UiO-n samples are left in ambient light.

Automated summary

This study investigates the influence of amino-functionalization on Zr-based UiO-n MOF materials for iodine capture. Experimental results demonstrate the preferential interaction between amino functional groups and iodine, which is also supported by DFT calculations. UV-Vis and Raman spectroscopy analysis reveals the encapsulation of iodine within the pore system of UiO-n materials and their amino-derivatives. Moreover, molecular iodine (I-2) species are converted into anionic iodine complexes within the MOF cavities when exposed to ambient light.