Electron-Donor Functional Groups, Band Gap Tailoring, and Efficient Charge Separation: Three Keys To Improve the Gaseous Iodine Uptake in MOF Materials

Abstract

Metal−organic frameworks (MOFs) have been largely investigated worldwide for their use in the capture of radioactive iodine due to its potential release during nuclear accident events and reprocessing of nuclear fuel. The present work deals with the capture of gaseous I 2 under a continuous flow and its subsequent transformation into I 3 − within the porous structures of three distinct, yet structurally related, terephthalate-based MOFs: MIL-125(Ti), MIL-125(Ti)_NH 2 , and CAU-1(Al)_NH 2 . The synthesized materials exhibited specific surface areas (SSAs) with similar order of magnitude: 1207, 1099, and 1110 m 2 g −1 for MIL-125(Ti), MIL-125(Ti)_NH 2 , and CAU-1(Al)_NH 2 , respectively. Because of that, it was possible to evaluate the influence of other variables over the iodine uptake capacity�such as band gap energies, functional groups, and charge transfer complexes (CTC). After 72 h of contact with the I 2 gas flow, MIL-125(Ti)_NH 2 was able to trap 11.0 mol mol −1 of I 2 , followed by MIL-125(Ti) (8.7 mol mol −1 ), and by CAU-1(Al)_NH 2 (4.2 mol mol −1 ). The enhanced ability to retain I 2 in the MIL-125(Ti)_NH 2 was associated with a combined effect between its amino group (which has a great affinity toward iodine), its smaller band gap (2.5 eV against 2.6 and 3.8 eV for CAU-1(Al)_NH 2 and MIL-125(Ti), respectively), and its efficient charge separation. In fact, the presence of a linkerto-metal charge transfer (LMCT) mechanism in MIL-125(Ti) compounds separates the photogenerated electrons and holes into the two distinct moieties of the MOF: the organic linker (which stabilizes the holes) and the oxy/hydroxy inorganic cluster (which stabilizes the electrons). This effect was observed using EPR spectroscopy, whereas the reduction of the Ti 4+ cations into the paramagnetic Ti 3+ species was evidenced after irradiation of the pristine Ti-based MOFs with UV light (<420 nm). In contrast, because CAU-1(Al)_NH 2 exhibits a purely linker-based transition (LBT)�with no EPR signals related to Al paramagnetic species�it tends to exhibit faster recombination of the photogenerated charge carriers as, in this case, both electrons and holes are located over the organic linker. Furthermore, the transformation of the gaseous I 2 into I n − [n = 5, 7, 9, ...] intermediates and then into I 3 − species was evaluated using Raman spectroscopy by following the evolution of their respective bands at about 198, 180, and 113 cm −1 . This conversion�which is favored by an effective charge separation and smaller band gaps�increases the I 2 uptake capacity of the compounds by creating specific adsorption sites for these anionic species. In fact, because the −NH 2 groups act as an antenna to stabilize the photogenerated holes, both I n − and I 3 − are adsorbed into the organic linker via an electrostatic interaction with these positively charged entities. Finally, changes regarding the EPR spectra before and after the iodine loading were considered to propose a mechanism for the electron transfer from the MOFs structure to the I 2 molecu...

Direct Observation of Enhanced Iodine Binding within a Series of Functionalized Metal–Organic Frameworks with Exceptional Irradiation Stability

“…Uptakes of I 2 were recorded as 1.79 (I 2 /Cu = 2.16), 2.91 (I 2 /Cu = 3.99), 3.37 (I 2 /Cu = 4.53), 3.47 (I 2 /Cu = 3.95), and 3.91 (I 2 /Cu = 5.37) g g –1 for NJU-Bai21, MFM-172, MFM-170, NJU-Bai20, and MFM-174, respectively (Figure a). These adsorption capacities compare favorably with state-of-the-art MOFs (Figure c), such as PCN-333­(Al) (4.42 g g –1 ), {Zr 6 O 4 (OH) 4 (peb) 6 } (peb 2– = 4,4′-[1,4-phenylenebis­(ethyne-2,1-diyl)]-dibenzoate) (2.79 g g –1 ), MOF-808 (2.18 g g –1 ), amino-functionalized MIL-125­(Ti)-NH 2 (1.7 g g –1 ), CAU-1­(Al)-NH 2 (1.3 g g –1 ), UiO-67-(NH 2 ) 2 (1.21 g g –1 ), TMU-16-NH 2 (1.28 g g –1 ), MIL-101-NH 2 (0.31 g g –1 ), and MIL-53-NH 2 (0.18 g g –1 ) . The high I 2 adsorption capacity of MFM-174 also results in an exceptional packing density of 4.83 g cm –3 , which is comparable with that for solid iodine (4.93 g cm –3 ), representing the most efficient MOF for I 2 storage (Figure c).…”

“…However, the direct visualization of host–guest interactions has revealed the key role of –NH 2 and –CC– sites in facilitating adsorption of I 2 . Although –NH 2 and –CC– sites have been previously reported to enhance the adsorption of I 2, ,,− to the best of our knowledge, this represents the first example of crystallographic observation of I 2 binding to these active sites.…”

Elucidating Structural Stability, Bandgap, and Photocatalytic Hydrogen Evolution of (H2O/DMF)@HKUST‐1 Host‐Guest Systems

“…[16] An efficient and simple strategy to tailor the physicochemical properties of MOFs is the infiltration of species; [18] e. g., in 2014, Talin et al, proposed the infiltration of TCNQ (7,7,8,8-tetracyanoquinodimethane) as an effective strategy to increase the electrical conductivity of HKUST-1, achieving an outstanding increment of six orders of magnitude. [19] The versatility in the modulation of the physicochemical properties of MOFs, [20] in particular HKUST-1 through infiltration [18] turns it into an interesting material for developing photocatalysts based on MOFs for water-splitting purposes. HKUST-1 is a material composed of trimesic acid (TMA) ligands (also known as benzene tricarboxylic acid, BTC) and Cu 2 + metallic centres, which by self-assembly produced a 3D lattice with paddlewheel secondary building units (SBUs), that also bear open metal sites.…”

Functionalized Metal–Organic Framework with Azo Double and Amide Groups for Highly Efficient Iodine Capture

“…Nevertheless, the development of MOFs with efficient I 2 capture properties remains a huge challenge. One efficient method is to introduce electron-pair donors or conjugated π-electron into the frameworks to functionalize MOFs, which could form steady halogen bonds or complexes with I 2 , thus improving the capture capability. − As Lewis base sites, azo double and amide groups are abundant in π-electron and exhibit intrinsic strong interactions with I 2 , highlighting their essential role as functional groups for I 2 capture. − …”