Preparation, Characterization, and Iodine Adsorption Performance of Thorium-quinoline Metal-organic Framework

Abstract

Radioactive nuclides such as 129I are produced during the operation of nuclear facilities. 129I is volatile and can induce radiation sickness. Capture of radioactive iodine can protect public health and facilitate the sustainable development of the nuclear industry. Preliminary studies on the synthesis and iodine adsorption properties of thorium based MOFs have shown that thorium-based MOFs usually have different structures and iodine adsorption performance when they contain different ligands. Quinoline derivatives containing both hydroxyl and carboxyl groups are a class of rigid ligands with strong coordination ability. Although these ligands have been used to synthesize transition metal complexes, their application in the preparation of thorium MOFs remains unexplored. To further investigate the effect of ligands on the structure and iodine adsorption performance of thorium-based MOFs, in this paper thorium-quinoline MOF (Th-HQCA) was synthesized using 2-hydroxyquinoline-4-carboxylic acid (HQCA) as the ligand via solvothermal method, and its structure, composition, thermal stability, spectral properties, and iodine adsorption performance were investigated through various characterization techniques, including single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-Vis) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and fluorescence spectroscopy. Crystallographic analysis reveals that the coordination modes of thorium ions in the structure of Th-HQCA are the same, and each ion is coordinated with four carboxylate oxygen atoms from four HQCA and four hydroxyl oxygen atoms from another four HQCA. It is worth mentioning that thorium ions usually coordinate with carboxylate oxygen atoms in ligands, and rarely coordinate with hydroxyl group. The connections between thorium ions and HQCA ligands result in the formation of a novel 2,8-connected framework structure of Th-HQCA. The channels of the framework are filled with water molecules. Based on the crystallographic analysis, the ratio of thorium ions, HQCA, and water molecules is determined to be 1∶4∶1. However, further thermogravimetric analyses indicate that there may be twelve cocrystallized water molecules in the compound. Consequently, the chemical formula for Th-HQCA can be expressed as [Th(C9H5NOCOO)4]·12H2O. The solid-state fluorescence spectrum shows that Th-HQCA crystals emit blue light when they are excited by ultraviolet light. The adsorption experiments of Th-HQCA on gaseous iodine and iodine in cyclohexane solution show that their adsorption kinetic curves fit the pseudo-second-order kinetic equation, and the adsorption isotherm of the latter fits the Langmuir model. Through further calculations, the maximum adsorption capacity of Th-HQCA for iodine vapor is 535 mg/g, and the maximum adsorption capacity for iodine in cyclohexane solution is 295.63 mg/g. In summary, Th-HQCA is a thorium-quinoline MOF with a novel 2,8-connected topological structure, and it shows remarkable fluorescence properties and can effectively capture iodine in the gas phase and solution.

Keywords

• thorium-based mof
• crystal structure
• fluorescence property
• iodine adsorption