Iodine Uptake by Zr-/Hf-Based UiO-66 Materials: The Influence of Metal Substitution on Iodine Evolution

Abstract

Many works reported the encapsulation of iodine in metal–organic frameworks as well as the I2 → I3 – chemical conversion. This transformation has been examined by adsorbing gaseous iodine on a series of UiO-66 materials and the different Hf/Zr metal ratios (0–100% Hf) were evaluated during the evolution of I2 into I3 –. The influence of the hafnium content on the UiO-66 structure was highlighted by PXRD, SEM images, and gas sorption tests. The UiO-66­(Hf) presented smaller lattice parameter (a = 20.7232 Å), higher crystallite size (0.18 ≤ Φ ≤ 3.33 μm), and smaller SSABET (818 m2·g–1) when compared to its parent UiO-66­(Zr)  a = 20.7696 Å, 100 ≤ Φ ≤ 250 nm, and SSABET = 1262 m2·g–1. The effect of replacing Zr atoms by Hf in the physical properties of the UiO-66 was deeply evaluated by a spectroscopic study using UV–vis, FTIR, and Raman characterizations. In this case, the Hf presence reduced the band gap of the UiO-66, from 4.07 eV in UiO-66­(Zr) to 3.98 eV in UiO-66­(Hf). Furthermore, the UiO-66­(Hf) showed a blue shift for several FTIR and Raman bands, indicating a stiffening on the implied interatomic bonds when comparing to UiO-66­(Zr) spectra. Hafnium was found to clearly favor the capture of iodine [285 g·mol–1, against 230 g·mol–1 for UiO-66­(Zr)] and the kinetic evolution of I2 into I3 – after 16 h of I2 filtration. Three iodine species were typically identified by Raman spectroscopy and chemometric analysis. These species are as follows: “free” I2 (206 cm–1), “perturbed” I2 (173 cm–1), and I3 – (115 and 141 cm–1). It was also verified, by FTIR spectroscopy, that the oxo and hydroxyl groups of the inorganic [M6O4(OH)4] (M = Zr, Hf) cluster were perturbed after the adsorption of I2 into UiO-66­(Hf), which was ascribed to the higher acid character of Hf. Finally, with that in mind and considering that the EPR results discard the possibility of a redox phenomenon involving the tetravalent cations (Hf4+ or Zr4+), a mechanism was proposed for the conversion of I2 into I3 – in UiO-66based on an electron donor–acceptor complex between the aromatic ring of the BDC linker and the I2 molecule.

Raman Mapping as a Tool for Evaluating I2 and I3– Diffusion Over Single-Crystal UiO-67_NH2(M) (M = Zr, Zr/Hf, or Hf)

“…However, recently, Leloire et al demonstrated that there is no redox phenomena related to the zirconium centers in the UiO framework during the I 3 – formation. This feature was later confirmed in another study that evaluated the influence of the metal content (zirconium of hafnium) on the iodine evolution . In fact, considering the isoreticular UiO-66 structure with different metals (Zr and Hf), it was demonstrated that the occurrence of a smaller band gap, coupled with more acidic elements (such as hafnium instead of zirconium), improves the I 2 uptake and the I 3 – stabilization in UiO materials .…”

Neutral and Cationic Phenothiazine‐Based Porous Organic Polymers via a Simple and Cost‐Effective Method for Iodine Capture

“…The PTA‐POP@I 2 (Figure 4a) and C‐PTA‐POP@I 2 (Figure 4b) complex showed both neutral I 2 and anion I 3 − information, which indicated some iodine molecules were transformed into I 3 − . These results revealed physical and chemical adsorption occurred in the process [4–6,37] . Compared to the PTA‐POP@I 2 , the C‐PTA‐POP@ I 2 complex indicated a high conversion rate from neutral I 2 to anion I 3 − .…”

“…These results revealed physical and chemical adsorption occurred in the process. [4][5][6]37] Compared to the PTA-POP@I 2 , the C-PTA-POP@ I 2 complex indicated a high conversion rate from neutral I 2 to anion I 3 À . We also check iodine anions of the C-PTA-POP@ I 2 complex via Raman analysis.…”

“…Recently, Metal-organic frameworks (MOFs) have been developed for iodine uptake to improve capture ability. [3][4][5][6] However, poor stability limited their practical application for the storage of iodine. Comparing traditional inorganic porous materials, porous organic polymers (POPs) are a new class of porous material to offer various advantages including high porosity, permanent pore structure, and special stability.…”

“…Their main problem is lower capacities because of their limited accessible surface areas and lower‐density functional units to interact with iodine molecules. Recently, Metal‐organic frameworks (MOFs) have been developed for iodine uptake to improve capture ability [3–6] . However, poor stability limited their practical application for the storage of iodine.…”