Thiophene-based MOFs for iodine capture: Effect of pore structures and interaction mechanism

Abstract

The effective capture of radioiodine is vital to the development of the nuclear industry and environmental protection. Metal-organic frameworks (MOFs) are promising adsorbents for iodine vapor due to the versatile metal nodes and linkers, as well as tunable pore structures. Here, five thiophene-based metal–organic frameworks with different pore geometries were investigated for their iodine adsorption properties, and the relationship between the pore structures and iodine adsorption capacities is examined. DUT-68 adsorbs a large amount of iodine (1081 mg/g), which is 28% higher than that of the homologous DUT-67 (843 mg/g), because its large cage could provide more space for the packing of molecular iodine in the middle of the pore. The MIL-53-TDC(In) with 1D straight-through pore channels only has an iodine capacity of 660 mg/g, due to its small pore volume and little confinement effect. Meanwhile, the In-DTC and Ho-DTC, with smaller pore sizes than the kinetic diameter of iodine molecules, exhibit almost no iodine adsorption properties. Raman, fluorescence, XPS spectra and DFT calculation confirm the strong charge transfer effect from thiophene linkers to trapped iodine. DFT calculation also provides quantitative adsorption energy of −44.5 kJ/mol between one iodine molecule and one thiophene group linked with Zr nodes, higher than that of thiophene groups linked with In nodes. This work may motivate the topology-directed design of MOFs for iodine capture with optimized pore structures, metal nodes and linkers to enhance the adsorption performance.