Confinement of Iodine Molecules into Triple-Helical Chains within Robust Metal–Organic Frameworks

Abstract

During nuclear waste disposal process, radioactive iodine as a fission product can be released. The widespread implementation of sustainable nuclear energy thus requires the development of efficient iodine stores that have simultaneously high capacity, stability and more importantly, storage density (and hence minimized system volume). Here, we report high I2 adsorption in a series of robust porous metal–organic materials, MFM-300(M) (M = Al, Sc, Fe, In). MFM-300(Sc) exhibits fully reversible I2 uptake of 1.54 g g–1, and its structure remains completely unperturbed upon inclusion/removal of I2. Direct observation and quantification of the adsorption, binding domains and dynamics of guest I2 molecules within these hosts have been achieved using XPS, TGA-MS, high resolution synchrotron X-ray diffraction, pair distribution function analysis, Raman, terahertz and neutron spectroscopy, coupled with density functional theory modeling. These complementary techniques reveal a comprehensive understanding of the host–I2 and I2–I2 binding interactions at a molecular level. The initial binding site of I2 in MFM-300(Sc), I2I, is located near the bridging hydroxyl group of the [ScO4(OH)2] moiety [I2I···H–O = 2.263(9) Å] with an occupancy of 0.268. I2II is located interstitially between two phenyl rings of neighboring ligand molecules [I2II···phenyl ring = 3.378(9) and 4.228(5) Å]. I2II is 4.565(2) Å from the hydroxyl group with an occupancy of 0.208. Significantly, at high I2 loading an unprecedented self-aggregation of I2 molecules into triple-helical chains within the confined nanovoids has been observed at crystallographic resolution, leading to a highly efficient packing of I2 molecules with an exceptional I2 storage density of 3.08 g cm–3 in MFM-300(Sc).

In Situ Assessment of Intrinsic Strength of X-I⋯OA-Type Halogen Bonds in Molecular Crystals with Periodic Local Vibrational Mode Theory

“…These peculiar interactions between diiodine molecules are the major factor which greatly weakens the I• • • O bonds. Similar interaction networks between diiodines were also found in recent work on metal-organic framework (MOF) structures designed for I 2 adsorption [134]. A more detailed and systematic investigation into this problem is currently in progress.…”

Exceptional Iodine Capture in 2D Covalent Organic Frameworks

“…Recently, ionic networks with complex cationic viologen skeletons exhibit the highest uptake capacity of only 1.40 g g −1 , driven by Lewis acid–base and electrostatic interactions between iodine and the viologen units . Very recently, a series of complex MOFs have been designed to reach the best capacity (MFM‐300 (Sc)) of 1.54 g g −1 , which is, however, only a quarter that of the TPB‐DMTP COF (6.2 g g −1 ) . These newly developed complex structures suggest that a rationalized physical picture of a porous structure that can efficiently uptake iodine is highly desired but it remains challenging.…”

Porous sorbents for the capture of radioactive iodine compounds: a review

“…Since then, MOFs have been particularly popular for the retention of iodine. 6,14,109,110,115,116,[118][119][120][121][122][123][124][125][126][127][129][130][131][132] Most of the studies were carried out on materials based on divalent metals such as Zn 2+ and Cu 2+ , as well as aluminium-based structures. In fact, the most commonly studied MOFs for the capture of iodine were the zeolitic imidazolate framework Zn(2methylimidazolate) 2 (ZIF-8) and HKUST-1 (Cu 3 (benzene-1,3,5tricarboxylate) 2 (H 2 O) 3 , Cu-BTC).…”