Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125_NH2

Two titanium-based MOFs MIL-125 and MIL-125_NH2 are synthesized and characterized using high-temperature powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), N-2 sorption, Fourier transformed infrared spectroscopy (FTIR), Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), and electron paramagnetic resonance (EPR). Stable up to 300 degrees C, both compounds exhibited similar specific surface areas (SSA) values (1207 and 1099 m(2) g(-1) for MIL-125 and MIL-125_NH2, respectively). EPR signals of Ti3+ are observed in both, whith MIL-125_NH2 also showing-NH2 center dot+ signatures. Both MOFs efficiently adsorbed iodine in continuous gas flow over five days, with MIL-125 trapping 1.9 g.g(-1) and MIL-125_NH2 trapping 1.6 g.g(-1). MIL-125_NH2 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-2 to "perturbed" I-2, and I-3(-). TGA and in situ Raman desorption experiments showed that-NH2 groups improved the stabilization of I-3(-) due to an electrostatic interaction with NH2 center dot+BDC radicals. The Albery model indicated longer lifetimes for iodine desorption in I-2@MIL-125_NH2, attributed to a rate-limiting step due to stronger interaction between the anionic iodine species and the-NH2 center dot+ radicals. This study underscores how MOFs with efficient charge separation and hole-stabilizer functional groups enhance iodine stability at higher temperatures.