Iodine Uptake by Zr-/Hf-Based UiO-66 Materials: The Influence of Metal Substitution on Iodine Evolution
Pedro H. M. Andrade, Natacha Henry, Christophe Volkringer, Thierry Loiseau, Hervé Vezin, Matthieu Hureau, Alain Moissette
Abstract
Many works reported the encapsulation of iodine in metal–organic frameworks as well as the I 2 → I 3 – 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 I 2 into I 3 – . 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 SSA BET (818 m 2 ·g –1 ) when compared to its parent UiO-66(Zr) ─ a = 20.7696 Å, 100 ≤ Φ ≤ 250 nm, and SSA BET = 1262 m 2 ·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 I 2 into I 3 – after 16 h of I 2 filtration. Three iodine species were typically identified by Raman spectroscopy and chemometric analysis. These species are as follows: “free” I 2 (206 cm –1 ), “perturbed” I 2 (173 cm –1 ), and I 3 – (115 and 141 cm –1 ). It was also verified, by FTIR spectroscopy, that the oxo and hydroxyl groups of the inorganic [M 6 O 4 (OH) 4 ] (M = Zr, Hf) cluster were perturbed after the adsorption of I 2 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 (Hf 4+ or Zr 4+ ), a mechanism was proposed for the conversion of I 2 into I 3 – in UiO-66─based on an electron donor–acceptor complex between the aromatic ring of the BDC linker and the I 2 molecule.