Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125_NH

Two titanium-based MOFs MIL-125 and MIL-125_NH are synthesized and characterized using high-temperature powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), N sorption, Fourier transformed infrared spectroscopy (FTIR), Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), and electron paramagnetic resonance (EPR). Stable up to 300 °C, both compounds exhibited similar specific surface areas (SSA) values (1207 and 1099 m g for MIL-125 and MIL-125_NH, respectively). EPR signals of Ti are observed in both, whith MIL-125_NH also showing ─NH signatures. Both MOFs efficiently adsorbed iodine in continuous gas flow over five days, with MIL-125 trapping 1.9 g.g and MIL-125_NH trapping 1.6 g.g. MIL-125_NH exhibited faster adsorption kinetics due to its smaller band gap (2.5 against 3.6 eV). In situ Raman spectroscopy conducted during iodine adsorption revealed signal evolution from "free" I to "perturbed" I, and I . TGA and in situ Raman desorption experiments showed that ─NH groups improved the stabilization of I due to an electrostatic interaction with NH BDC radicals. The Albery model indicated longer lifetimes for iodine desorption in I@MIL-125_NH, attributed to a rate-limiting step due to stronger interaction between the anionic iodine species and the ─NH radicals. This study underscores how MOFs with efficient charge separation and hole-stabilizer functional groups enhance iodine stability at higher temperatures.