# Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125\_NH 2.

Pedro Henrique Morais Andrade, Jeremy Dhainaut, Christophe Volkringer, Thierry Loiseau, Aurélien Moncomble, Matthieu Hureau, Alain Moissette

► To cite this version:

Pedro Henrique Morais Andrade, Jeremy Dhainaut, Christophe Volkringer, Thierry Loiseau, Aurélien Moncomble, et al.. Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125\_NH 2.. Small, 2024, Small, pp.e2400265. 10.1002/smll.202400265. hal-04582684

HAL Id: hal-04582684

https://hal.univ-lille.fr/hal-04582684

Submitted on 28 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125\_NH 2

Pedro Andrade, Jérémy Dhainaut, Christophe Volkringer, Thierry Loiseau, Aurélien Moncomble, Matthieu Hureau, Alain Moissette

► To cite this version:

Pedro Andrade, Jérémy Dhainaut, Christophe Volkringer, Thierry Loiseau, Aurélien Moncomble, et al. Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125\_NH 2. Small, 2024, Small, 20 (35), pp.e2400265. 10.1002/smll.202400265. hal-04582684

HAL Id: hal-04582684

https://lilloa.hal.science/hal-04582684v1

Submitted on 28 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Stability of Iodine Species Trapped in Titanium-Based MOFs: MIL-125 and MIL-125_NH₂

Pedro H. M. Andrade,* Jérémy Dhainaut, Christophe Volkringer, Thierry Loiseau, Aurélien Moncomble, Matthieu Hureau, and Alain Moissette*

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), N₂ 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 m² g⁻¹ for MIL-125 and MIL-125_NH₂, respectively). EPR signals of Ti³⁺ are observed in both, with MIL-125_NH₂ also showing —NH₂•⁺ 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 eV). In situ Raman spectroscopy conducted during iodine adsorption revealed signal evolution from “free” I₂ to “perturbed” I₂, and I₃⁻. TGA and in situ Raman desorption experiments showed that —NH₂ groups improved the stabilization of I₃⁻ due to an electrostatic interaction with NH₂•⁺BDC radicals. The Albery model indicated longer lifetimes for iodine desorption in I₂@MIL-125_NH₂, attributed to a rate-limiting step due to stronger interaction between the anionic iodine species and the —NH₂•⁺ radicals. This study underscores how MOFs with efficient charge separation and hole-stabilizer functional groups enhance iodine stability at higher temperatures.

## 1. Introduction

The immobilization of gaseous iodine in porous materials has been deeply studied particularly regarding nuclear accidents and Used Nuclear Fuel (UNF) reprocessing.[1,2] In this context, special attention is given to ¹²⁹I and ¹³¹I because of their ability to be retained on the thyroid, causing cancer, metabolic dysfunctions,

and a significant influence on growth.[3] For instance, ¹²⁹I exhibits a considerable long half-life time ($t_{1/2}$ = 1.6 × 10⁷ years) and significant mobility in most geological environments,[4] whereas ¹³¹I has a high activity (4.59 × 10¹⁵ Bq·g⁻¹) and is quickly absorbed into human metabolic systems.[3] Furthermore, iodine capture raises a great number of drawbacks since it is a fast-moving gas, with a high oxidation capacity, that may quickly dissolve in water and spread throughout many ecosystems.[2] In order to mitigate these problems, several materials such as activated carbon,[5] zeolites,[4] aerogels,[6] covalent organic frameworks (COFs),[7] and porous organic polymers (POPs)[8] have all been employed to trap gaseous iodine.

However, certain of these compounds are costly or amorphous, and some pose a risk of explosion under critical working circumstances, such as activated carbon and powdered samples.[4,5,9–12] To overcome these issues, Metal–Organic Frameworks (MOFs) have been extensively studied for their iodine adsorption properties.[13,14] These hybrid crystalline materials have

large Specific Surface Areas (SSAs) (up to 10,000 m² g⁻¹)[15] and are formed by the combination of various metal sites with organic linkers, resulting in a variety of structures with distinct characteristics.[16] Although iodine adsorption utilizing MOFs has been studied previously, there is still a dearth of knowledge concerning the interactions between iodine species and the crystalline network of these materials.[2,9,10] The I₂ kinetic adsorption

P. H. M. Andrade, A. Moncomble, M. Hureau, A. Moissette
Laboratoire de Spectroscopie pour les Interactions
la Réactivité et l'Environnement (LASIRE)
Université de Lille - Sciences et Technologies
Villeneuve d'Ascq 59655, France
E-mail: pedro.moraisandrade@univ-lille.fr; alain.moissette@univ-lille.fr

<figure><img src="image_2.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202400265

© 2024 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/smll.202400265

P. H. M. Andrade
Departamento de Engenharia Metalúrgica e de Materiais (DEMET)
Departamento de Física (DF)
Universidade Federal de Minas Gerais (UFMG)
Belo Horizonte Brazil

J. Dhainaut, C. Volkringer, T. Loiseau
Unité de Catalyse et Chimie du Solide (UCCS)
Univ. Lille
CNRS
Centrale Lille
Univ. Artois
UMR 8181
Lille F-59000, France
<figure><img src="image_2.png" /></figure>

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

Pedro H. M. Andrade,* Jérémy Dhainaut, Christophe Volkringer, Thierry Loiseau, Aurélien Moncomble, Matthieu Hureau, and Alain Moissette*

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), N₂ 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 m² g⁻¹ for MIL-125 and MIL-125_NH₂, respectively). EPR signals of Ti³⁺ are observed in both, with MIL-125_NH₂ also showing —NH₂•⁺ 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” I₂ to “perturbed” I₂, and I₃⁻. TGA and in situ Raman desorption experiments showed that —NH₂ groups improved the stabilization of I₃⁻ due to an electrostatic interaction with NH₂•⁺BDC radicals. The Albery model indicated longer lifetimes for iodine desorption in I₂@MIL-125_NH₂, attributed to a rate-limiting step due to stronger interaction between the anionic iodine species and the —NH₂•⁺ radicals. This study underscores how MOFs with efficient charge separation and hole-stabilizer functional groups enhance iodine stability at higher temperatures.

## 1. Introduction

The immobilization of gaseous iodine in porous materials has been deeply studied particularly regarding nuclear accidents and Used Nuclear Fuel (UNF) reprocessing.[1,2] In this context, special attention is given to ¹²⁹I and ¹³¹I because of their ability to be retained on the thyroid, causing cancer, metabolic dysfunctions,

and a significant influence on growth.[3] For instance, ¹²⁹I exhibits a considerable long half-life time ($t_{1/2}$ = 1.6 × 10⁷ years) and significant mobility in most geological environments,[4] whereas ¹³¹I has a high activity (4.59 × 10¹⁵ Bq·g⁻¹) and is quickly absorbed into human metabolic systems.[3] Furthermore, iodine capture raises a great number of drawbacks since it is a fast-moving gas, with a high oxidation capacity, that may quickly dissolve in water and spread throughout many ecosystems.[2] In order to mitigate these problems, several materials such as activated carbon,[5] zeolites,[4] aerogels,[6] covalent organic frameworks (COFs),[7] and porous organic polymers (POPs)[8] have all been employed to trap gaseous iodine.

However, certain of these compounds are costly or amorphous, and some pose a risk of explosion under critical working circumstances, such as activated carbon and powdered samples.[4,5,9-12] To overcome these issues, Metal–Organic Frameworks (MOFs) have been extensively studied for their iodine adsorption properties.[13,14] These hybrid crystalline materials have

large Specific Surface Areas (SSAs) (up to 10,000 m² g⁻¹)[15] and are formed by the combination of various metal sites with organic linkers, resulting in a variety of structures with distinct characteristics.[16] Although iodine adsorption utilizing MOFs has been studied previously, there is still a dearth of knowledge concerning the interactions between iodine species and the crystalline network of these materials.[2,9,10] The I₂ kinetic adsorption

P. H. M. Andrade, A. Moncomble, M. Hureau, A. Moissette
Laboratoire de Spectroscopie pour les Interactions
la Réactivité et l'Environnement (LASIRE)
Université de Lille - Sciences et Technologies
Villeneuve d'Ascq 59655, France
E-mail: pedro.moraisandrade@univ-lille.fr; alain.moissette@univ-lille.fr

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202400265

© 2024 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/smll.202400265

P. H. M. Andrade
Departamento de Engenharia Metalúrgica e de Materiais (DEMET)
Departamento de Física (DF)
Universidade Federal de Minas Gerais (UFMG)
Belo Horizonte Brazil

J. Dhainaut, C. Volkringer, T. Loiseau
Unité de Catalyse et Chimie du Solide (UCCS)
Univ. Lille
CNRS
Centrale Lille
Univ. Artois
UMR 8181
Lille F-59000, France
from a cyclohexane solution using the MIL-53(Al) series was sub-
ject of study in previous work.[17] The authors investigated the
ability of several functional groups to remove iodine [—H, —CH₃,
—NH₂, —(OH)₂, —COOH, —(COOH)₂, —NO₂, —Cl, —Br] and dis-
covered that I₂⁻ adsorption is not only related to the SSA of the
compounds. It is instead linked to the properties of the organic
pendant group. Indeed, among functions such as methyl, chlo-
ride, and carboxylic acid, the amino-group removal efficiency has
been shown to be advantageous. In this scenario, MIL-101_NH₂
uptakes seven times more iodine than MIL-100,[17] which was
mainly ascribed to the —NH₂ group's high electron donor prop-
erty, which allows it to form robust charge transfer complexes
(CTCs) with I₂. Later, Leloire et al.[9] observed that changing the
functionalization of the benzenedicarboxylate (BDC) ligand may
affect the quantity of gaseous iodine trapped inside the pores of
the UiO-6n(Zr) (n = 6, 7, or 8) MOF family. The authors also
discovered that when iodine was trapped within the pore sys-
tem of UiO-66 materials, it was converted into the unexpected
I₃⁻ species.

This I₃⁻ formation after I₂ adsorption in MOF ma-
terials was already reported for other systems, such
as I₂@UiO-66(Hf, Zr),[10] I₂@UiO-67(Hf, Zr)–NH₂,[18]
I₂@MIL-125(Ti),[19] I₂@MIL-125(Ti)–NH₂,[19] I₂@CAU-
1(Al)–NH₂,[19] I₂@BOF-1,[20] I₂@MFM-300(VIII),[21]
I₂@{[(ZnI₂)₃(TPT)₂]•5.5(C₆H₅NO₂)}n (TPT = 2,4,6-tris(4-
pyridyl)-1,3,5-triazine),[22] I₂@Th-BPYDC,[23] I₂@CAU-21-ODB
(ODB = 4,4'-oxydibenzoate),[24] I₂@Cu₃(HHTN)₂ (HHTN
= 2,3,8,9,14,15- hexahydroxytrinaphthylene),[25] I₂@Co₂(m-
DOBDC) (m-DOBDC = 4,6-dioxo-1,3-benzenedicarboxylate),[26]
I₂@HSB-W8 and I₂@HSB-W9.[27] In some of these circum-
stances, I₃⁻ production was associated with metal node oxidation,
but in others, the organic linker was responsible for forming
CTCs with the I₂ molecules, reducing them. For instance,
magnetic susceptibility measurements revealed that the I₂
into I₃⁻ transition in the cavities of the BOF-1 structure was
due to the oxidation of Ni²⁺ into Ni³⁺.[20] Similarly, XPS and
Raman spectroscopies confirmed that the oxidation of V³⁺ into
V⁴⁺ was responsible for the production of the I₃⁻ species in
MFM-300(VIII).[21] On the other hand, I₃⁻ formation on the I₂-
loaded Co₂(m-DOBDC) MOF, was attributed to an electrophilic
aromatic substitution mediated by activation of the C–H bond
at the electron-rich carbon (C5) of m-DOBDC⁴⁻.[26] In this case,
the reaction produces an aryl C–I bond as well as I⁻ or I₃⁻
ions, which coordinate to the unsaturated open Co sites. Also,
in HSB-W8 and HSB-W9 MOFs which are constructed using
hydrogenated Schiff linkers – 1,2-bis(4'-pyridylmethylamino)-
ethane (HSB-2) and trans-stilbene-4,4-dicarboxylate (TSBDC) –,
the CTC formed between the π-electron-rich stilbene moieties
and the I₂ was able to reduce the iodine species into polyiodide
anions.[27]

Surprisingly, a similar characteristic was identified in prior
experiments using I₂@UiO-66(Zr, Hf) and I₂@UiO-6n(Zr,
Hf)\_NH₂ systems (n = 6 or 7).[9,10,18] Using EPR spectroscopy,
these investigators exposed that the metal centers of UiO-6n ma-
terials (Zr¹⁰⁻ or Hf¹⁰⁻) cannot be oxidized or reduced, even when ex-
posed to UV light (λ < 420 nm).[9,10] Furthermore, no indication
of a C—I bond formation was observed in these compounds.[9,10]
Nonetheless, Raman spectroscopy confirmed the I₂ into I₃⁻ redox
conversion in both I₂@UiO-66(Zr, Hf)[10] and I₂@UiO-6n(Zr,

Hf)\_NH₂[9,18] materials. As a result, the transformation of I₂ to
I₃⁻ in the cavities of the UiO-66(Hf or Zr) porous structure was at-
tributed to a CTC between the BDC organic linker and the iodine
molecule. Interestingly, the lower band gap energies and greater
Lewis acidity of UiO-66(Hf) were found to increase I₂ adsorption
and evolution kinetics.[10] Finally, the I₃⁻ was discovered to be the
final and most stable species in these materials, owing to an elec-
trostatic interaction between the I₃⁻ and the generated cation rad-
ical BDC•⁺.[10] A spectroscopic analysis over single crystal UiO-
67\_NH₂ materials later confirmed this proposal, demonstrating
the lowest diffusion of I₃⁻ compared to I₂ in the pores of the MOF
following its synthesis.[18]

In this context, the use of titanium-based MOFs has been
proven to be particularly interesting for trapping iodine due to
their improved photocatalytic and electrochemical properties, a
high acidity of the titanium, low toxicity, and possibility of cre-
ating linker-to-metal charge transfer (LMCT) complexes.[19,28–31]
Dan-Hardi *et al*.[28] described for the first time in 2009 a
highly porous Ti-MOF synthesized under solvothermal con-
ditions from a combination of dimethylformamide (DMF),
methanol (MeOH), terephthalic acid (H₂BDC), and titanium
tetra-isopropoxide (TTIP). The MOF, named MIL-125 (MIL =
Material from Institute Lavoisier) or [Ti₈O₈(OH)₄](O₂C–C₆H₄–
CO₂)₆], is composed by a quasi-cubic tetragonal structure formed
by TiO₅(OH) octahedra that are connected to each other through
the 1,4-benzene dicarboxylate (BDC) linkers alternating edge-
sharing O with corner-sharing OH species, resulting in an oc-
tanuclear ring. It shows two types of cages with octahedral and
tetrahedral geometries, and diameters of ≈12 and 6 Å, respec-
tively. Free openings between 5 and 7 Å allow access to its porous
structure.

According to studies, microwave synthesis of MIL-125 can be
completed in reaction times lower than 1 h.[32] It has also been re-
ported in the literature that linker modification in MIL-125 com-
pounds can alter their band edges.[33] For example, the electron-
rich amine ligand NH₂-BDC helped to boost the aromatic linker's
electron density, which caused the valence band maximum
(VBM) of MIL-125\_NH₂ to shift upward relative to MIL-125.[33]
With these specific characteristics, Horiuchi et al.[34] reported
hydrogen generation utilizing MIL-125\_NH₂ as a visible-light
photocatalyst at irradiation wavelengths up to 500 nm. Addi-
tionally, MIL-125\_NH₂ performed less well than MIL-125 dur-
ing the oxidative desulfurization of dibenzothiophene, which
was attributed to less steric interference with the active sites.[35]
MIL-125 also showed better hydrophilicity during water vapor
adsorption.[35] In another study, Yang et al.[36] used butyric acid
as a modulator to improve the photocatalytic performance for
the oxidative desulfurization of 4,6-dimethyldibenzothiophene,
resulting in well-shaped truncated octahedron MIL-125 crystals.
Furthermore, MIL-125\_NH₂ has already been studied for extract-
ing iodine from waste water, removing around 85% of I₂ in
90 min.[37]

Clearly, MIL-125 and MIL-125\_NH₂ compounds have excel-
lent photocatalytic activity, which is primarily due to the pres-
ence of titanium cations in their oxo-hydroxo inorganic cluster.
In fact, the inclusion of Ti sites reduces the material's bandgap
when compared to other terephthalic-based MOFs such as UiO-
66(Zr, Hf).[10,33] According to prior research, the MOF's narrower
bandgap improves the CTC with I₂ molecules, which improves
from a cyclohexane solution using the MIL-53(Al) series was sub-
ject of study in previous work.[17] The authors investigated the
ability of several functional groups to remove iodine [—H, —CH₃,
—NH₂, —(OH)₂, —COOH, —(COOH)₂, —NO₂, —Cl, —Br] and dis-
covered that I₂⁻ adsorption is not only related to the SSA of the
compounds. It is instead linked to the properties of the organic
pendant group. Indeed, among functions such as methyl, chlo-
ride, and carboxylic acid, the amino-group removal efficiency has
been shown to be advantageous. In this scenario, MIL-101_NH₂
uptakes seven times more iodine than MIL-100,[17] which was
mainly ascribed to the —NH₂ group's high electron donor prop-
erty, which allows it to form robust charge transfer complexes
(CTCs) with I₂. Later, Leloire et al.[9] observed that changing the
functionalization of the benzenedicarboxylate (BDC) ligand may
affect the quantity of gaseous iodine trapped inside the pores of
the UiO-6n(Zr) (n = 6, 7, or 8) MOF family. The authors also
discovered that when iodine was trapped within the pore sys-
tem of UiO-66 materials, it was converted into the unexpected
I₃⁻ species.

This I₃⁻ formation after I₂ adsorption in MOF ma-
terials was already reported for other systems, such
as I₂@UiO-66(Hf, Zr),[10] I₂@UiO-67(Hf, Zr)-NH₂,[18]
I₂@MIL-125(Ti),[19] I₂@MIL-125(Ti)-NH₂,[19] I₂@CAU-
1(Al)-NH₂,[19] I₂@BOF-1,[20] I₂@MFM-300(VIII),[21]
I₂@{[(ZnI₂)₃(TPT)₂]•5.5(C₆H₅NO₂)}n (TPT = 2,4,6-tris(4-
pyridyl)-1,3,5-triazine),[22] I₂@Th-BPYDC,[23] I₂@CAU-21-ODB
(ODB = 4,4'-oxydibenzoate),[24] I₂@Cu₃(HHTN)₂ (HHTN
= 2,3,8,9,14,15- hexahydroxytrinaphthylene),[25] I₂@Co₂(m-
DOBDC) (m-DOBDC = 4,6-dioxo-1,3-benzenedicarboxylate),[26]
I₂@HSB-W8 and I₂@HSB-W9.[27] In some of these circum-
stances, I₃⁻ production was associated with metal node oxidation,
but in others, the organic linker was responsible for forming
CTCs with the I₂ molecules, reducing them. For instance,
magnetic susceptibility measurements revealed that the I₂
into I₃⁻ transition in the cavities of the BOF-1 structure was
due to the oxidation of Ni²⁺ into Ni³⁺.[20] Similarly, XPS and
Raman spectroscopies confirmed that the oxidation of V³⁺ into
V⁴⁺ was responsible for the production of the I₃⁻ species in
MFM-300(VIII).[21] On the other hand, I₃⁻ formation on the I₂-
loaded Co₂(m-DOBDC) MOF, was attributed to an electrophilic
aromatic substitution mediated by activation of the C—H bond
at the electron-rich carbon (C5) of m-DOBDC⁴⁻.[26] In this case,
the reaction produces an aryl C—I bond as well as I⁻ or I₃⁻
ions, which coordinate to the unsaturated open Co sites. Also,
in HSB-W8 and HSB-W9 MOFs which are constructed using
hydrogenated Schiff linkers – 1,2-bis(4'-pyridylmethylamino)-
ethane (HSB-2) and trans-stilbene-4,4-dicarboxylate (TSBDC) –,
the CTC formed between the π-electron-rich stilbene moieties
and the I₂ was able to reduce the iodine species into polyiodide
anions.[27]

Surprisingly, a similar characteristic was identified in prior
experiments using I₂@UiO-66(Zr, Hf) and I₂@UiO-6n(Zr,
Hf)\_NH₂ systems (n = 6 or 7).[9,10,18] Using EPR spectroscopy,
these investigators exposed that the metal centers of UiO-6n ma-
terials (Zr¹⁰⁻ or Hf¹⁰⁻) cannot be oxidized or reduced, even when ex-
posed to UV light (λ < 420 nm).[9,10] Furthermore, no indication
of a C—I bond formation was observed in these compounds.[9,10]
Nonetheless, Raman spectroscopy confirmed the I₂ into I₃⁻ redox
conversion in both I₂@UiO-66(Zr, Hf)[10] and I₂@UiO-6n(Zr,

Hf)\_NH₂[9,18] materials. As a result, the transformation of I₂ to
I₃⁻ in the cavities of the UiO-66(Hf or Zr) porous structure was at-
tributed to a CTC between the BDC organic linker and the iodine
molecule. Interestingly, the lower band gap energies and greater
Lewis acidity of UiO-66(Hf) were found to increase I₂ adsorption
and evolution kinetics.[10] Finally, the I₃⁻ was discovered to be the
final and most stable species in these materials, owing to an elec-
trostatic interaction between the I₃⁻ and the generated cation rad-
ical BDC⁺.[10] A spectroscopic analysis over single crystal UiO-
67\_NH₂ materials later confirmed this proposal, demonstrating
the lowest diffusion of I₃⁻ compared to I₂ in the pores of the MOF
following its synthesis.[18]

In this context, the use of titanium-based MOFs has been
proven to be particularly interesting for trapping iodine due to
their improved photocatalytic and electrochemical properties, a
high acidity of the titanium, low toxicity, and possibility of cre-
ating linker-to-metal charge transfer (LMCT) complexes.[19,28–31]
Dan-Hardi *et al*.[28] described for the first time in 2009 a
highly porous Ti-MOF synthesized under solvothermal con-
ditions from a combination of dimethylformamide (DMF),
methanol (MeOH), terephthalic acid (H₂BDC), and titanium
tetra-isopropoxide (TTIP). The MOF, named MIL-125 (MIL =
Material from Institute Lavoisier) or [Ti₈O₈(OH)₄](O₂C-C₆H₄-
CO₂)₆], is composed by a quasi-cubic tetragonal structure formed
by TiO₅(OH) octahedra that are connected to each other through
the 1,4-benzene dicarboxylate (BDC) linkers alternating edge-
sharing O with corner-sharing OH species, resulting in an oc-
tanuclear ring. It shows two types of cages with octahedral and
tetrahedral geometries, and diameters of ≈12 and 6 Å, respec-
tively. Free openings between 5 and 7 Å allow access to its porous
structure.

According to studies, microwave synthesis of MIL-125 can be
completed in reaction times lower than 1 h.[32] It has also been re-
ported in the literature that linker modification in MIL-125 com-
pounds can alter their band edges.[33] For example, the electron-
rich amine ligand NH₂-BDC helped to boost the aromatic linker's
electron density, which caused the valence band maximum
(VBM) of MIL-125\_NH₂ to shift upward relative to MIL-125.[33]
With these specific characteristics, Horiuchi et al.[34] reported
hydrogen generation utilizing MIL-125\_NH₂ as a visible-light
photocatalyst at irradiation wavelengths up to 500 nm. Addi-
tionally, MIL-125\_NH₂ performed less well than MIL-125 dur-
ing the oxidative desulfurization of dibenzothiophene, which
was attributed to less steric interference with the active sites.[35]
MIL-125 also showed better hydrophilicity during water vapor
adsorption.[35] In another study, Yang et al.[36] used butyric acid
as a modulator to improve the photocatalytic performance for
the oxidative desulfurization of 4,6-dimethyldibenzothiophene,
resulting in well-shaped truncated octahedron MIL-125 crystals.
Furthermore, MIL-125\_NH₂ has already been studied for extract-
ing iodine from waste water, removing around 85% of I₂ in
90 min.[37]

Clearly, MIL-125 and MIL-125\_NH₂ compounds have excel-
lent photocatalytic activity, which is primarily due to the pres-
ence of titanium cations in their oxo-hydroxo inorganic cluster.
In fact, the inclusion of Ti sites reduces the material's bandgap
when compared to other terephthalic-based MOFs such as UiO-
66(Zr, Hf).[10,33] According to prior research, the MOF's narrower
bandgap improves the CTC with I₂ molecules, which improves
<figure><img src="image_3.png" /><figcaption>**Figure 1.** High temperature PXRD pattern of a) MIL-125 and b) MIL-125_NH₂. λ<sub>CuKα</sub> = 1.5406 Å. The inset in (a) highlights the double Bragg peak that is observed between 320 and 420 °C in the MIL-125 diffrac-togram.</figcaption></figure>

I₂ immobilization and accelerates its transformation into I₃⁻.[¹⁰]
Also, the presence of titanium can alter the type of CTC from a
linker-based transition (LBT) in UiO-6n (n = 6, 7, or 8) materials
to a LMCT.[²⁹,³⁸] This difference was proven to modify the mecha-
nism of the iodine evolution in I₂@MIL-125 materials.[¹⁹] In fact,
the enhanced efficiency of gaseous I₂ removal using MIL-125 was
proven to be deeply related to the formation of Ti³⁺ and, conse-
quently, to the LMCT mechanism.[¹⁹] Moreover, in the amino-
functionalized version of MIL-125, it was observed that –NH₂
groups can act as a hole stabilizer in NH₂-BDC organic linker,
forming a cation radical NH₂•⁺-BDC.[³⁹⁻⁴¹] In light of this, it is
assumed that the I₃⁻ species could be further anchored to these
entities through an electrostatic interaction,[¹⁹] similar to that ver-
ified for UiO-66 and BDC•⁺ radicals[¹⁰] as well for UiO-67_NH₂
and NH₂•⁺-BPDC (BPDC = biphenyldicarboxylate) radicals.[¹⁸]
In this case, one should expect a higher energy of desorption for
I₃⁻ than for I₂. Therefore, this work evaluates the sorption phe-
nomena of I₂ and I₃⁻ via in situ Raman spectroscopy and thermo-
gravimetric analysis to better elucidate the mechanism of stabi-
lization of the different iodine species within MIL-125 and MIL-
125_NH₂.

## 2. Results and Discussion

2.1. Characterization of The Pristine MOFs

MIL-125 and MIL-125_NH₂ compounds were well crystallized according to their PXRD patterns, which are in agreement to those reported in the literature (Figure 1).[28,42]

No extra peaks associated to Ti⁴⁺ oxides or other titanium-
related phases were observed in the room-temperature powder
XRD patterns of both samples, suggesting that homogeneous
crystallites were obtained rather than a mixture of other titanium
phases and individual MIL-125 crystals.[⁴³] Furthermore, to eval-
uate the thermal stability of MIL-125 and MIL-125_NH₂, high-
temperature PXRD experiments were also applied (Figure 1).
In this case, MIL-125 was found to be completely stable up to
320 °C. However, another phase, which is characterized by a dou-
ble Bragg peak at 6.0° ≤ 2θ ≤ 8.0°, appears at temperatures be-
tween 340 and 420 °C (Figure 1a). Finally, after 420 °C the com-
plete degradation of MIL-125 is achieved, with the subsequent
formation of TiO₂ from 560 °C, as confirmed by the presence of
a peaks at 2θ about 25.3° and 27.4° –assigned to the (101) and
(110) planes of anatase and rutile TiO₂, respectively.[⁴³] On the
other hand, the Bragg peaks of MIL-125_NH₂ persist up to 300
°C (Figure 1b). Between 320 and 480 °C, two peaks from the orig-
inal structure can still be observed at 2θ = 25.6° and 37.7°. Then,
after 500 °C a broad peak is formed at 2θ = 24.9° and finally, four
new peaks are visible between 640 and 800 °C at 2θ = 27.0°, 35.5°,
40.7°, and 53.7° which are also related to the presence of a mixed
anatase/rutile TiO₂ phase.[⁴³] To complement the thermal analy-
sis, thermogravimetric experiments were also conducted for the
pristine MOF materials (Figure 2).

First, up to 280 °C, the weight loss is mostly attributed to
the presence of water (2.0 and 3.0 wt.% for MIL-125 and MIL-
125_NH₂, respectively). Then, the most significant mass loss was
observed between 280 and 500 °C, which is ascribed to the de-
composition of the organic linker and also from DMF residues
from the syntheses (45 wt.% for both compounds). Finally, for

<figure><img src="image_4.png" /><figcaption>**Figure 2.** TGA experiments showing the thermal decomposition of the MIL-125 (in black) and MIL-125_NH₂ (in red) compounds up to 800 °C under air atmosphere.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** High temperature PXRD pattern of a) MIL-125 and b) MIL-125_NH₂. λ<sub>CuKα</sub> = 1.5406 Å. The inset in (a) highlights the double Bragg peak that is observed between 320 and 420 °C in the MIL-125 diffractogram.</figcaption></figure>

I₂ immobilization and accelerates its transformation into I₃⁻.[¹⁰]
Also, the presence of titanium can alter the type of CTC from a
linker-based transition (LBT) in UiO-6n (n = 6, 7, or 8) materials
to a LMCT.[²⁹,³⁸] This difference was proven to modify the mecha-
nism of the iodine evolution in I₂@MIL-125 materials.[¹⁹] In fact,
the enhanced efficiency of gaseous I₂ removal using MIL-125 was
proven to be deeply related to the formation of Ti³⁺ and, conse-
quently, to the LMCT mechanism.[¹⁹] Moreover, in the amino-
functionalized version of MIL-125, it was observed that –NH₂
groups can act as a hole stabilizer in NH₂-BDC organic linker,
forming a cation radical NH₂•⁺-BDC.[³⁹⁻⁴¹] In light of this, it is
assumed that the I₃⁻ species could be further anchored to these
entities through an electrostatic interaction,[¹⁹] similar to that ver-
ified for UiO-66 and BDC•⁺ radicals[¹⁰] as well for UiO-67_NH₂
and NH₂•⁺-BPDC (BPDC = biphenyldicarboxylate) radicals.[¹⁸]
In this case, one should expect a higher energy of desorption for
I₃⁻ than for I₂. Therefore, this work evaluates the sorption phe-
nomena of I₂ and I₃⁻ via in situ Raman spectroscopy and thermo-
gravimetric analysis to better elucidate the mechanism of stabi-
lization of the different iodine species within MIL-125 and MIL-
125_NH₂.

## 2. Results and Discussion

2.1. Characterization of The Pristine MOFs

MIL-125 and MIL-125_NH₂ compounds were well crystallized according to their PXRD patterns, which are in agreement to those reported in the literature (Figure 1).[28,42]

No extra peaks associated to Ti⁴⁺ oxides or other titanium-
related phases were observed in the room-temperature powder
XRD patterns of both samples, suggesting that homogeneous
crystallites were obtained rather than a mixture of other titanium
phases and individual MIL-125 crystals.[⁴³] Furthermore, to eval-
uate the thermal stability of MIL-125 and MIL-125_NH₂, high-
temperature PXRD experiments were also applied (Figure 1).
In this case, MIL-125 was found to be completely stable up to
320 °C. However, another phase, which is characterized by a dou-
ble Bragg peak at 6.0° ≤ 2θ ≤ 8.0°, appears at temperatures be-
tween 340 and 420 °C (Figure 1a). Finally, after 420 °C the com-
plete degradation of MIL-125 is achieved, with the subsequent
formation of TiO₂ from 560 °C, as confirmed by the presence of
a peaks at 2θ about 25.3° and 27.4° –assigned to the (101) and
(110) planes of anatase and rutile TiO₂, respectively.[⁴³] On the
other hand, the Bragg peaks of MIL-125_NH₂ persist up to 300
°C (Figure 1b). Between 320 and 480 °C, two peaks from the orig-
inal structure can still be observed at 2θ = 25.6° and 37.7°. Then,
after 500 °C a broad peak is formed at 2θ = 24.9° and finally, four
new peaks are visible between 640 and 800 °C at 2θ = 27.0°, 35.5°,
40.7°, and 53.7° which are also related to the presence of a mixed
anatase/rutile TiO₂ phase.[⁴³] To complement the thermal analy-
sis, thermogravimetric experiments were also conducted for the
pristine MOF materials (Figure 2).

First, up to 280 °C, the weight loss is mostly attributed to
the presence of water (2.0 and 3.0 wt.% for MIL-125 and MIL-
125_NH₂, respectively). Then, the most significant mass loss was
observed between 280 and 500 °C, which is ascribed to the de-
composition of the organic linker and also from DMF residues
from the syntheses (45 wt.% for both compounds). Finally, for

<figure><img src="image_5.png" /><figcaption>**Figure 2.** TGA experiments showing the thermal decomposition of the MIL-125 (in black) and MIL-125_NH₂ (in red) compounds up to 800 °C under air atmosphere.</figcaption></figure>
temperatures higher than 500 °C only inorganic residues remain mainly related to titanium oxide phases (53 and 52 wt.% for MIL-125 and MIL-125_NH₂, respectively). Other characterization analyses, including N₂ sorption tests, SEM imaging, EDS chemical mapping, FTIR, Raman, UV-Vis, and EPR spectroscopies, have been reported in one of our recent works related to the MIL-125 and MIL-125_NH₂ compounds.[19]

## 2.2. Iodine Adsorption and Evolution

The MIL-125 and MIL-125_NH₂ powdered materials were exposed to the iodine gas flow for five days in order to obtain information about their adsorption kinetics. To limit the impact of the samples' molecular weights, the kinetic was evaluated using the mass of trapped iodine per mol of MOF (rather than a mass of iodine per mass of MOF), as shown in Figure 3. However, the values are also shown in g·g⁻¹ and mol·mol⁻¹ to facilitate the discussion (Table 1). The adsorption kinetic was evaluated using the Linear Drive Force model (LDF)[44] according to Equation (1), where F(t) is the ratio between the mass of iodine in a given time t and at t = 120 h, and k_LDF is the mass transfer coefficient (h⁻¹). It is worth mentioning that this model was intensively applied for studying the gas adsorption in microporous materials with remarkable correlation coefficients (R²).[9,10] The obtained values for k_LDF, the correlation coefficient (R²), and the equilibrium time to reach F(t) = 0.90 and 0.99 are also highlighted in Table 1 and Figure 3.

$$F(t) = 1 - e^{-k_{LDF}t} \quad (1)$$

After 120 h of iodine contact, the MIL-125 material exhibited a higher I₂ uptake of 1.9 g·g⁻¹, against 1.6 g·g⁻¹ for MIL-125_NH₂ in the same conditions. This behavior is related to its higher SSA (1207 against 1099 m²·g⁻¹)[19] coupled with the presence of a LMCT mechanism. In effect, improving the SSA creates more sites for iodine to adsorb, whereas the LMCT complex increases charge separation and facilitates the iodine adsorption.[19] However, it is clear that the adsorption kinetics of the amino-functionalized MOF is faster than that of MIL-125. In fact, MIL-125_NH₂ exhibited a mass transfer coefficient about twice the one of MIL-125: 0.0382 against 0.01991 h⁻¹. This feature is also visible in the Figure 3, as the MIL-125_NH₂ clearly reached a plateau of maximum adsorption after about 72 h, whereas MIL-125 appears to attain this plateau only after 120 h.

Previous studies conducted in I₂@UiO-6n (n = 6, 7, or 8) systems showed the opposite behavior, with the amino-functionalized UiO-66_NH₂ exhibiting lower iodine adsorption kinetics but with a higher adsorption capacity at the end of the process.[9] Leloire et al.[9] observed an I₂ uptake capacity of 210 mg·g⁻¹ in a UiO-66 material after 48 h of iodine exposure (SSA = 1387 m²·g⁻¹; k_LDF = 0.229 h⁻¹), whereas UiO-66_NH₂ was able to adsorb 565 mg·g⁻¹ in the same conditions (SSA = 825 m²·g⁻¹; k_LDF = 0.0909). A similar effect was observed for UiO-67 and UiO-67_NH₂, as the former captured 222 mg·g⁻¹ (SSA = 1599 m²·g⁻¹; k_LDF = 0.228 h⁻¹) against 1071 mg·g⁻¹ for the latter (SSA = 1565 m²·g⁻¹; k_LDF = 0.0320 h⁻¹).[9] In this case, the k_LDF reduction in the presence of -NH₂ groups should be related to the fact that UiO-6n (n = 6 or 7) does not have a great capacity for retaining I₂ when compared to MIL-125 compounds. In

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>Figure 3. a) Iodine adsorption kinetics evaluated by UV-Vis spectroscopy after 120 h of contact between the I₂ gas and the MIL-125 (black) and MIL-125_NH₂ (red) powdered samples. Adsorption kinetic was evaluated using the Linear Drive Force model (LDF) for b) I₂@MIL-125 and c) I₂@MIL-125_NH₂.</figcaption></figure>

fact, because the Zr⁴⁺ cations cannot reduce into Zr³⁺, the charge transfer mechanism in UiO-6n materials is mainly related to its organic linker, in a linker-based transfer (LBT) complex.[10] This effect attenuates the charge separation and increases recombination phenomena, which was proven to reduce the iodine up-
temperatures higher than 500 °C only inorganic residues remain mainly related to titanium oxide phases (53 and 52 wt.% for MIL-125 and MIL-125_NH₂, respectively). Other characterization analyses, including N₂ sorption tests, SEM imaging, EDS chemical mapping, FTIR, Raman, UV-Vis, and EPR spectroscopies, have been reported in one of our recent works related to the MIL-125 and MIL-125_NH₂ compounds.[19]

## 2.2. Iodine Adsorption and Evolution

The MIL-125 and MIL-125_NH₂ powdered materials were exposed to the iodine gas flow for five days in order to obtain information about their adsorption kinetics. To limit the impact of the samples' molecular weights, the kinetic was evaluated using the mass of trapped iodine per mol of MOF (rather than a mass of iodine per mass of MOF), as shown in Figure 3. However, the values are also shown in g·g⁻¹ and mol·mol⁻¹ to facilitate the discussion (Table 1). The adsorption kinetic was evaluated using the Linear Drive Force model (LDF)[44] according to Equation (1), where F(t) is the ratio between the mass of iodine in a given time t and at t = 120 h, and k_LDF is the mass transfer coefficient (h⁻¹). It is worth mentioning that this model was intensively applied for studying the gas adsorption in microporous materials with remarkable correlation coefficients (R²).[9,10] The obtained values for k_LDF, the correlation coefficient (R²), and the equilibrium time to reach F(t) = 0.90 and 0.99 are also highlighted in Table 1 and Figure 3.

$$F(t) = 1 - e^{-k_{LDF}t} \quad (1)$$

After 120 h of iodine contact, the MIL-125 material exhibited a higher I₂ uptake of 1.9 g·g⁻¹, against 1.6 g·g⁻¹ for MIL-125_NH₂ in the same conditions. This behavior is related to its higher SSA (1207 against 1099 m²·g⁻¹)[19] coupled with the presence of a LMCT mechanism. In effect, improving the SSA creates more sites for iodine to adsorb, whereas the LMCT complex increases charge separation and facilitates the iodine adsorption.[19] However, it is clear that the adsorption kinetics of the amino-functionalized MOF is faster than that of MIL-125. In fact, MIL-125_NH₂ exhibited a mass transfer coefficient about twice the one of MIL-125: 0.0382 against 0.01991 h⁻¹. This feature is also visible in the Figure 3, as the MIL-125_NH₂ clearly reached a plateau of maximum adsorption after about 72 h, whereas MIL-125 appears to attain this plateau only after 120 h.

Previous studies conducted in I₂@UiO-6n (n = 6, 7, or 8) systems showed the opposite behavior, with the amino-functionalized UiO-66_NH₂ exhibiting lower iodine adsorption kinetics but with a higher adsorption capacity at the end of the process.[9] Leloire et al.[9] observed an I₂ uptake capacity of 210 mg·g⁻¹ in a UiO-66 material after 48 h of iodine exposure (SSA = 1387 m²·g⁻¹; k_LDF = 0.229 h⁻¹), whereas UiO-66_NH₂ was able to adsorb 565 mg·g⁻¹ in the same conditions (SSA = 825 m²·g⁻¹; k_LDF = 0.0909). A similar effect was observed for UiO-67 and UiO-67_NH₂, as the former captured 222 mg·g⁻¹ (SSA = 1599 m²·g⁻¹; k_LDF = 0.228 h⁻¹) against 1071 mg·g⁻¹ for the latter (SSA = 1565 m²·g⁻¹; k_LDF = 0.0320 h⁻¹).[9] In this case, the k_LDF reduction in the presence of -NH₂ groups should be related to the fact that UiO-6n (n = 6 or 7) does not have a great capacity for retaining I₂ when compared to MIL-125 compounds. In

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 3.** a) Iodine adsorption kinetics evaluated by UV-Vis spectroscopy after 120 h of contact between the I₂ gas and the MIL-125 (black) and MIL-125_NH₂ (red) powdered samples. Adsorption kinetic was evaluated using the Linear Drive Force model (LDF) for b) I₂@MIL-125 and c) I₂@MIL-125_NH₂.</figcaption></figure>

fact, because the Zr⁴⁺ cations cannot reduce into Zr³⁺, the charge transfer mechanism in UiO-6n materials is mainly related to its organic linker, in a linker-based transfer (LBT) complex.[10] This effect attenuates the charge separation and increases recombination phenomena, which was proven to reduce the iodine up-
<table><caption>**Table 1.** Iodine uptake capacities and kinetic data obtained for MIL-125 and MIL-125_NH₂ materials.</caption><thead><tr><th rowspan="2">MOF</th><th colspan="3">Iodine uptake<sup>a)</sup></th><th rowspan="2">k<sub>LDF</sub></th><th rowspan="2">R<sup>2</sup></th><th rowspan="2">t [F = 0.90]<sup>b)</sup></th></tr><tr><th>[g.mol<sup>-1</sup>]</th><th>[g.g<sup>-1</sup>]</th><th>[mol.mol<sup>-1</sup>]</th></tr></thead><tbody><tr><td>MIL-125</td><td>2947</td><td>1.9</td><td>11.6</td><td>0.01991(7)</td><td>0.973</td><td>115 h, 39 min</td></tr><tr><td>MIL-125_NH<sub>2</sub></td><td>2699</td><td>1.6</td><td>10.6</td><td>0.0382(2)</td><td>0.979</td><td>60 h, 17 min</td></tr></tbody></table>

k<sub>LDF</sub>: mass transfer coefficient from the Linear Drive Force (LDF) model; <sup>a)</sup> Experiments carried for 120 h of iodine gas flow at room temperature; <sup>b)</sup> Equilibrium time to reach F = 0.90 from LDF fit.

take capacity of MOF materials.[19] On the other hand, it is important to note that MIL-125 exhibits significant iodine uptake even without the presence of -NH₂ groups, primarily attributed to its LMCT complex as discussed previously.[19] Consequently, the -NH₂ groups primarily function to accelerate the adsorption process, which enhances the k<sub>LDF</sub> value. However, it also introduces a steric effect that reduces the maximum uptake capacity. This particular characteristic is only noticeable in compounds exhibiting higher iodine uptakes and, therefore, was not observed in UiO-6n systems.[9] As a result, MIL-125_NH₂ exhibits a maximum adsorption plateau at shorter contact times (F(0.9) ≈ 60 h), leading to an increased mass transfer coefficient according to the LDF model. In contrast, MIL-125 requires more time to reach its maximum adsorption capacity (F(0.9) ≈ 115 h), even though it has the ability to adsorb a greater amount of iodine.

The band gap influence over the I₂ adsorption kinetics was also discussed before in different studies.[10,19] For instance, the smaller E_g value of a hafnium-based UiO-66(Hf) was found to improve its mass transfer coefficient compared to its zirconium parent UiO-66(Zr). This effect is in agreement with that observed between MIL-125 and MIL-125_NH₂, as the highest k<sub>LDF</sub> value was obtained for MIL-125_NH₂, which also shows a smaller band gap (2.5 against 3.6 eV).[19] Furthermore, the influence of the CTC on the I₂ uptake was also discussed, specially between MIL-125_NH₂ and a parent MOF showing an isoreticular arrangement named CAU-1(Al)_NH₂.[19,45] The authors observed that the presence of Al³⁺ in CAU-1(Al)_NH₂ hinders the formation of an LMCT inside the material. Instead, this MOF exhibits an LBT (similar to that observed for UiO compounds), which is detrimental to the iodine adsorption and evolution into I₃⁻ due to a higher recombination of charge carriers in this case.[19] Therefore, it is clear that the presence of titanium and -NH₂ groups have a synergistic effect on improving the I₂ adsorption kinetics in MIL-125 compounds.

After the contact with iodine, the MOF powdered samples exhibited a color change, from white to orange-brown in MIL-125 and from yellow to purple-brown in MIL-125_NH₂. Due to their color changes, the chemical behavior of iodine encapsulated in both MOF samples was first evaluated by UV-Vis spectroscopy (Figure 4).

Both I₂-loaded samples exhibited a similar response after exposure to iodine, with the appearance of one additional wide band peaking at around 355 nm. This new signal shed light on the production of the anionic I₃⁻ species.[46] Additionally, another band at ≈506 nm was clearly apparent in the I₂@MIL-125_NH₂ spectrum, while it was less noticeable in the I₂@MIL-125 compound. This signal was associated with the subsequent forms of the iodine species following adsorption, which includes additional con-

tributions in the visible range between 400 and 550 nm. The first species is predicted to be detected at 520 nm and corresponds to the iodine species in the gas phase as a result of the allowed π<sub>g</sub>* → σ<sub>u</sub>* transition.[47,48] Furthermore, spectral signatures at lower wavelengths (about 400–500 nm) were linked to the CTC mechanism between I₂ molecules and the aromatic ring of the BDC linker, since I₂ is known to develop strong interactions with high electron donor species, such as aromatic compounds.[49,50] These characteristics demonstrate the presence of several iodine species trapped simultaneously in both MOF materials upon interaction with I₂ gas: “free” I₂, “perturbed” I₂, and I₃⁻.

Because these iodine species have very characteristic vibrational modes that are active in Raman spectroscopy, the I₂ adsorption and evolution into I₃⁻ was evaluated in situ. For that, Raman spectra were collected each hour for five days under a continuous iodine gas flow. Then, the iodine supply was cut, and spectra were acquired each hour for three days with the sample open to the atmosphere (Figure 5). The main iodine vibrational modes were observed between 90 and 210 cm⁻¹ and because the band related to the carboxylate stretching vibration (≈1600 cm⁻¹) was not influenced by the adsorption process, it was chosen for the normalization of the spectra.

The first visible contributions were observed within the first hours of adsorption for both samples. I₂@MIL-125 showed a band at 200 cm⁻¹ after 1 h of adsorption. In the same conditions, I₂@MIL-125_NH₂ exhibited a broader band incorporating contributions at 200, 192, and 183 cm⁻¹. These signals are

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Kubelka-Munk UV-Vis spectra of the I₂@MIL-125 (black) and I₂@MIL-125_NH₂ (red) after 120 h of I₂ loading. Pictures of the powdered samples before and after the I₂ adsorption are also shown.</figcaption></figure>
<table><caption>**Table 1.** Iodine uptake capacities and kinetic data obtained for MIL-125 and MIL-125_NH₂ materials.</caption><thead><tr><th rowspan="2">MOF</th><th colspan="3">Iodine uptake<sup>a)</sup></th><th rowspan="2">k<sub>LDF</sub></th><th rowspan="2">R<sup>2</sup></th><th rowspan="2">t [F = 0.90]<sup>b)</sup></th></tr><tr><th>[g.mol<sup>-1</sup>]</th><th>[g.g<sup>-1</sup>]</th><th>[mol.mol<sup>-1</sup>]</th></tr></thead><tbody><tr><td>MIL-125</td><td>2947</td><td>1.9</td><td>11.6</td><td>0.01991(7)</td><td>0.973</td><td>115 h, 39 min</td></tr><tr><td>MIL-125_NH<sub>2</sub></td><td>2699</td><td>1.6</td><td>10.6</td><td>0.0382(2)</td><td>0.979</td><td>60 h, 17 min</td></tr></tbody></table>

k<sub>LDF</sub>: mass transfer coefficient from the Linear Drive Force (LDF) model; <sup>a)</sup> Experiments carried for 120 h of iodine gas flow at room temperature; <sup>b)</sup> Equilibrium time to reach F = 0.90 from LDF fit.

take capacity of MOF materials.[19] On the other hand, it is important to note that MIL-125 exhibits significant iodine uptake even without the presence of -NH₂ groups, primarily attributed to its LMCT complex as discussed previously.[19] Consequently, the -NH₂ groups primarily function to accelerate the adsorption process, which enhances the k<sub>LDF</sub> value. However, it also introduces a steric effect that reduces the maximum uptake capacity. This particular characteristic is only noticeable in compounds exhibiting higher iodine uptakes and, therefore, was not observed in UiO-6n systems.[9] As a result, MIL-125_NH₂ exhibits a maximum adsorption plateau at shorter contact times (F(0.9) ≈ 60 h), leading to an increased mass transfer coefficient according to the LDF model. In contrast, MIL-125 requires more time to reach its maximum adsorption capacity (F(0.9) ≈ 115 h), even though it has the ability to adsorb a greater amount of iodine.

The band gap influence over the I₂ adsorption kinetics was also discussed before in different studies.[10,19] For instance, the smaller E<sub>g</sub> value of a hafnium-based UiO-66(Hf) was found to improve its mass transfer coefficient compared to its zirconium parent UiO-66(Zr). This effect is in agreement with that observed between MIL-125 and MIL-125_NH₂, as the highest k<sub>LDF</sub> value was obtained for MIL-125_NH₂, which also shows a smaller band gap (2.5 against 3.6 eV).[19] Furthermore, the influence of the CTC on the I₂ uptake was also discussed, specially between MIL-125_NH₂ and a parent MOF showing an isoreticular arrangement named CAU-1(Al)_NH₂.[19,45] The authors observed that the presence of Al³⁺ in CAU-1(Al)_NH₂ hinders the formation of an LMCT inside the material. Instead, this MOF exhibits an LBT (similar to that observed for UiO compounds), which is detrimental to the iodine adsorption and evolution into I₃⁻ due to a higher recombination of charge carriers in this case.[19] Therefore, it is clear that the presence of titanium and -NH₂ groups have a synergistic effect on improving the I₂ adsorption kinetics in MIL-125 compounds.

After the contact with iodine, the MOF powdered samples exhibited a color change, from white to orange-brown in MIL-125 and from yellow to purple-brown in MIL-125_NH₂. Due to their color changes, the chemical behavior of iodine encapsulated in both MOF samples was first evaluated by UV-Vis spectroscopy (Figure 4).

Both I₂-loaded samples exhibited a similar response after exposure to iodine, with the appearance of one additional wide band peaking at around 355 nm. This new signal shed light on the production of the anionic I₃⁻ species.[46] Additionally, another band at ≈506 nm was clearly apparent in the I₂@MIL-125_NH₂ spectrum, while it was less noticeable in the I₂@MIL-125 compound. This signal was associated with the subsequent forms of the iodine species following adsorption, which includes additional con-

tributions in the visible range between 400 and 550 nm. The first species is predicted to be detected at 520 nm and corresponds to the iodine species in the gas phase as a result of the allowed π<sub>g</sub>* → σ<sub>u</sub>* transition.[47,48] Furthermore, spectral signatures at lower wavelengths (about 400–500 nm) were linked to the CTC mechanism between I₂ molecules and the aromatic ring of the BDC linker, since I₂ is known to develop strong interactions with high electron donor species, such as aromatic compounds.[49,50] These characteristics demonstrate the presence of several iodine species trapped simultaneously in both MOF materials upon interaction with I₂ gas: “free” I₂, “perturbed” I₂, and I₃⁻.

Because these iodine species have very characteristic vibrational modes that are active in Raman spectroscopy, the I₂ adsorption and evolution into I₃⁻ was evaluated in situ. For that, Raman spectra were collected each hour for five days under a continuous iodine gas flow. Then, the iodine supply was cut, and spectra were acquired each hour for three days with the sample open to the atmosphere (Figure 5). The main iodine vibrational modes were observed between 90 and 210 cm⁻¹ and because the band related to the carboxylate stretching vibration (≈1600 cm⁻¹) was not influenced by the adsorption process, it was chosen for the normalization of the spectra.

The first visible contributions were observed within the first hours of adsorption for both samples. I₂@MIL-125 showed a band at 200 cm⁻¹ after 1 h of adsorption. In the same conditions, I₂@MIL-125_NH₂ exhibited a broader band incorporating contributions at 200, 192, and 183 cm⁻¹. These signals are

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Kubelka-Munk UV-Vis spectra of the I₂@MIL-125 (black) and I₂@MIL-125_NH₂ (red) after 120 h of I₂ loading. Pictures of the powdered samples before and after the I₂ adsorption are also shown.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Raman spectra obtained for a) $I_2$@MIL-125 and (b) $I_2$@MIL-125\_NH_2$, each spectrum was recorded in an interval of 12 h from $t = 0$. On the left side: in situ Raman spectra showing the iodine adsorption process (inset: zoom on the spectra showing the first 4 h of adsorption and spectrum of the respective pristine MOF). On the right side: spectra collected after opening the sample to the atmosphere showing the evolution from $I_2$ into $I_3^-$ (the dotted spectrum represents the last data obtained before opening the quartz cell).</figcaption></figure>

related to the I–I stretching of “free” iodine molecules, which is basically the iodine that is weakly perturbed by the materials’ framework.[10] In fact, the I–I stretching mode of gaseous $I_2$ is expected at 210 cm⁻¹, and the observed redshift is ascribed to the interaction between $I_2$ and the aromatic ring of the BDC linker, similarly to what is observed in $I_2$-benzene complexes.[51–53] This species was also observed in $I_2$@UiO-66 materials, reinforcing the interaction between $I_2$ and the BDC organic linker.[10] These bands shifted toward lower wavenumber values within a few hours of analysis, down to 195 and 183 cm⁻¹ for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, respectively. This shift is again related to an increase in the interaction between the iodine molecules and the MOFs frameworks, and it is stronger in $I_2$@MIL-125\_NH_2$ due to the presence of the –NH₂ group that has a strong affinity for iodine.[9] These two initial signals reach their maximum intensity after 5 and 3 days for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, which is in agreement with the $I_2$ adsorption kinetics experiments (Figure 3 and Table 1). It is worth mentioning that two other contributions were observed in $I_2$@MIL-125, at 157 and 113 cm⁻¹. On the other hand, although $I_2$@MIL-125\_NH_2$ exhibited only the 113 cm⁻¹ signal, the 157 cm⁻¹ band is probably hidden as a contribution of its main band at 183 cm⁻¹. These signals intensified after the fifth day, when the iodine flow was interrupted, and the quartz cell was open to the atmosphere. At this point, the intensity of the “free” $I_2$ signal decreased, while other contributions developed at 178 cm⁻¹ for $I_2$@MIL-125 and 168 cm⁻¹ for $I_2$@MIL-125\_NH_2$. These signals (together with the band observed before opening the cell at 157 cm⁻¹) were related to molecular iodine in strong interaction with organic donor

molecules,[51] which can increase the length of the I–I bond[54] and form $I_n^-$ anions ($n = 5, 7, 9, ...$).[55] For instance, $I_7^-$ was found at 175 cm⁻¹[56] and an $I_5^-$ signature was reported at 161 and 175 cm⁻¹.[57,58] Those species are considered to be combinations of $I_2$ and I⁻ molecules such as $I_2 \bullet\bullet\bullet I^-$, $I_2 \bullet\bullet\bullet I_2$, indicating an intermediate state on the transformation between $I_2$ and $I_3^-$ . Finally, the signal observed at 113 cm⁻¹ for both compounds corresponds to the symmetric stretching vibration, and also increased its intensity under the atmosphere. In addition, the asymmetric stretching vibration of $I_3^-$ in interaction was also observed at 145 cm⁻¹ in $I_2$@MIL-125\_NH_2$.[54,57–62] Note that the $I_3^-$ signal is stronger in $I_2$@MIL-125\_NH_2$, which is related to the faster transformation of $I_2$ into $I_3^-$ in the presence of the amino group. This feature was observed in previous studies and is related to the electron donor character of the –NH₂ group and to the smaller band gap of the MIL-125\_NH₂ compound.[9,10,19] In fact, contributions related to $\bullet I_2^-$ species may also be present because its vibrational mode (at 114 cm⁻¹)[56] overlaps with that of symmetric $I_3^-$ stretching, suggesting a radical mechanism for the iodine reduction which would be favored by better harvesting of the visible light.[10] To sum it up, the $I_2$ into $I_n^-$ or $I_3^-$ transformation can be thought as a charge transfer between the organic linker and the $I_2$ molecule according to Equations (2) and (3) for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, respectively.

$$nI_2 + 2BDC \rightarrow 2I_n^- + 2BDC^+ \quad (n = 3, 5, 7, 9...) \quad (2)$$

$$nI_2 + 2NH_2BDC \rightarrow 2I_n^- + 2NH_2^+BDC \quad (n = 3, 5, 7, 9...) \quad (3)$$
<figure><img src="image_10.png" /><figcaption>**Figure 5.** Raman spectra obtained for a) $I_2$@MIL-125 and (b) $I_2$@MIL-125\_NH_2$, each spectrum was recorded in an interval of 12 h from $t = 0$. On the left side: in situ Raman spectra showing the iodine adsorption process (inset: zoom on the spectra showing the first 4 h of adsorption and spectrum of the respective pristine MOF). On the right side: spectra collected after opening the sample to the atmosphere showing the evolution from $I_2$ into $I_3^-$ (the dotted spectrum represents the last data obtained before opening the quartz cell).</figcaption></figure>

related to the I–I stretching of “free” iodine molecules, which is basically the iodine that is weakly perturbed by the materials’ framework.[10] In fact, the I–I stretching mode of gaseous $I_2$ is expected at 210 cm⁻¹, and the observed redshift is ascribed to the interaction between $I_2$ and the aromatic ring of the BDC linker, similarly to what is observed in $I_2$-benzene complexes.[51–53] This species was also observed in $I_2$@UiO-66 materials, reinforcing the interaction between $I_2$ and the BDC organic linker.[10] These bands shifted toward lower wavenumber values within a few hours of analysis, down to 195 and 183 cm⁻¹ for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, respectively. This shift is again related to an increase in the interaction between the iodine molecules and the MOFs frameworks, and it is stronger in $I_2$@MIL-125\_NH_2$ due to the presence of the $-NH_2$ group that has a strong affinity for iodine.[9] These two initial signals reach their maximum intensity after 5 and 3 days for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, which is in agreement with the $I_2$ adsorption kinetics experiments (Figure 3 and Table 1). It is worth mentioning that two other contributions were observed in $I_2$@MIL-125, at 157 and 113 cm⁻¹. On the other hand, although $I_2$@MIL-125\_NH_2$ exhibited only the 113 cm⁻¹ signal, the 157 cm⁻¹ band is probably hidden as a contribution of its main band at 183 cm⁻¹. These signals intensified after the fifth day, when the iodine flow was interrupted, and the quartz cell was open to the atmosphere. At this point, the intensity of the “free” $I_2$ signal decreased, while other contributions developed at 178 cm⁻¹ for $I_2$@MIL-125 and 168 cm⁻¹ for $I_2$@MIL-125\_NH_2$. These signals (together with the band observed before opening the cell at 157 cm⁻¹) were related to molecular iodine in strong interaction with organic donor

molecules,[51] which can increase the length of the I–I bond[54] and form $I_n^-$ anions ($n = 5, 7, 9, ...$).[55] For instance, $I_7^-$ was found at 175 cm⁻¹[56] and an $I_5^-$ signature was reported at 161 and 175 cm⁻¹.[57,58] Those species are considered to be combinations of $I_2$ and I⁻ molecules such as $I_2 \bullet\bullet\bullet I^-$, $I_2 \bullet\bullet\bullet I_2$, indicating an intermediate state on the transformation between $I_2$ and $I_3^-$.

Finally, the signal observed at 113 cm⁻¹ for both compounds corresponds to the symmetric stretching vibration, and also increased its intensity under the atmosphere. In addition, the asymmetric stretching vibration of $I_3^-$ in interaction was also observed at 145 cm⁻¹ in $I_2$@MIL-125\_NH_2$.[54,57–62] Note that the $I_3^-$ signal is stronger in $I_2$@MIL-125\_NH_2$, which is related to the faster transformation of $I_2$ into $I_3^-$ in the presence of the amino group. This feature was observed in previous studies and is related to the electron donor character of the $-NH_2$ group and to the smaller band gap of the MIL-125\_NH_2 compound.[9,10,19] In fact, contributions related to $\bullet I_2^-$ species may also be present because its vibrational mode (at 114 cm⁻¹)[56] overlaps with that of symmetric $I_3^-$ stretching, suggesting a radical mechanism for the iodine reduction which would be favored by better harvesting of the visible light.[10] To sum it up, the $I_2$ into $I_n^-$ or $I_3^-$ transformation can be thought as a charge transfer between the organic linker and the $I_2$ molecule according to Equations (2) and (3) for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, respectively.

$$ nI_2 + 2BDC \rightarrow 2I_n^- + 2BDC^+ \quad (n = 3, 5, 7, 9...) \quad (2) $$

$$ nI_2 + 2NH_2BDC \rightarrow 2I_n^- + 2NH_2^+BDC \quad (n = 3, 5, 7, 9...) \quad (3) $$
Raman spectra, and $I_2$@MIL-125 shows two stable bands at 113 and 178 cm$^{-1}$ ($I_3^-$ and “perturbed” $I_2$, respectively), whereas $I_2$@MIL-125\_NH_2$ main signals appear at 113 and 168 cm$^{-1}$ ($I_3^-$ and “perturbed” $I_2$, respectively). The amino functionalization had a clear influence over the stability of the remaining species within the pores of MIL-125 compounds. Indeed, in $I_2$@MIL-125, both $I_2$ and $I_3^-$ signals progressively disappear after a few hours at 40 °C, and after a few minutes at 80 and 120 °C. On the other hand, after 19 h the $I_3^-$ signal is still present at 120 °C in $I_2$@MIL-125\_NH_2$, while traces of $I_2$ remain at 80 °C for this material. To characterize thoroughly the difference between these two behaviors observed for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$ upon heating, the iodine desorption kinetics were studied using the Albery model,[69] which is one of the most appropriate approaches.

Briefly, Albery et al.[69] observed that the kinetics of several heterogeneous materials (semiconductor electrodes, colloidal particles, oxide surfaces) are frequently treated as semilogarithmic plots, normalized by the initial concentration, and then tested for first-order kinetics, in which the decay signal often lies on a common curve. However, they perceived that the data is better fitted when the heterogeneity of the system is also pondered. With that in mind and considering that the rate constant comes from the Gibbs free energy of the system, a Gaussian distribution of the system's free enthalpy of activation ($\Delta G^\ddagger$) around its mean value ($\overline{\Delta G^\ddagger}$) was proposed, according to Equation (4). Here, $\gamma$ is the distribution width, R is the gas constant (8.3144621 J K$^{-1}$ mol$^{-1}$), T is the temperature (K), and x is the gaussian distribution x-axis.[69]

$$ \Delta G^{\ddagger} = \overline{\Delta G^{\ddagger}} - \gamma xRT \quad (4) $$

Then, the authors considered that a Gaussian distribution around the rate constant logarithm average [$\ln(\bar{k})$] is more likely to be found than around its corresponding $\bar{k}$ data. Therefore, the former case was proposed, according to Equation (5).[69]

$$ \ln(k) = \ln(k) + \gamma^x \quad (5) $$

Finally, because the kinetics decay results from the sum of all the contributions of each microscopic state, the authors applied an integration on the Gaussian distribution in $\exp(-x^2)$. The result leads to the Equation (6), which describes the initial concentration ($C_0$) with time ($t$).[69] It is worth mentioning that when $\gamma = 0$, the system is considered homogeneous and therefore, the Equation (6) is reduced to the first-order exponential decay expression: $C/C_0 = e^{-kt}$.

$$ \frac{C(t)}{C_0} = \frac{\int_{-\infty}^{+\infty} e^{-x^2 - kt \cdot (e^{\gamma x} - 1)} dx}{\sqrt{\pi}} \quad (6) $$

It is worth mentioning that the Albery model was often applied for evaluating the dynamic of microporous systems such as zeolites with considerable correlation coefficients ($R^2$).[70] Therefore, to access the $\bar{k}$ values according to the Albery model for the “perturbed” $I_2$ and $I_3^-$ species, the intensity of the signals related to the $I_3^-$ (at 113 cm$^{-1}$) and $I_2^-$ (at 178 and 168 cm$^{-1}$ for $I_2$@MIL-125 and $I_2$@MIL-125\_NH_2$, respectively) species were considered. For that, the ratio between the Raman band's intensity at

a given time and the initial intensity ($I/I_0$) was plotted against the time in hours. The model showed a good correspondence to the experimental data points (Figure 8).

Moreover, the amino-group influence over the $I_3^-$ stabilization is now undeniable. In fact, after 19 h, $I_2$@MIL-125\_NH_2$ was able to retain 73%, 30%, and 5% of its initial $I_3^-$ signal at 40, 80, and 120 °C, respectively, whereas $I_2$@MIL-125 only retained 7%, 2%, and < 1% under the same conditions. Furthermore, the difference of the desorption kinetics for both species in the two materials is highlighted by their respective $\bar{k}$ values and lifetimes (Table 2 and Figure 9). Actually, the $I_2$@MIL-125 has systematically higher $\bar{k}$ values and smaller lifetimes than the $I_2$@MIL-125\_NH_2$ for both $I_3^-$ and “perturbed” $I_2$ species. This feature indicates that anionic iodine species are stabilized at higher temperatures by the presence of the amino group, which is consistent with their interaction with the trapped $-NH_2^{\bullet+}$ holes.[19] In fact, previous studies proposed the formation of a $\bullet$BDC$^+$ radical as a mechanism for the stabilization of the $I_3^-$ species in $I_2$@UiO-66 systems through electrostatic interactions.[10] This effect is then enhanced in amino-functionalized materials as the $-NH_2$ groups act as hole stabilizers in the $NH_2$-BDC organic linker.[39-41] Therefore, the analogous $NH_2^{\bullet+}$-BDC radical can further retain $I_3^-$ into the organic linker via an electrostatic interaction with these positively charged entities.

It is worth mentioning that when considering $I_2$@MIL-125, both iodine species have very similar lifetimes for temperatures higher than 40 °C, inferring that their desorption mechanism follows the same trend. On the other hand, a clear difference is observed when comparing the behavior of these species upon $I_2$@MIL-125\_NH_2$ heating. In this case, the lifetime of the $I_3^-$ species is about two-times longer than that of the “perturbed” $I_2$ in the entire temperature range (Table 2 and Figure 9). With that in mind and considering the Raman spectra (Figures 7 and 8) and the TGA experiments (Figure 6), it is undeniable that iodine is further stabilized in the presence of $-NH_2$ groups even at higher temperatures, reinforcing the influence of the amino group over the desorption mechanism. This feature suggests that a rate-limiting stage is present during the iodine desorption in $I_2$@MIL-125\_NH_2$. At this point, one must consider that anionic species cannot transform into gas without losing their charges first, and therefore, both “perturbed” $I_2$ (or $I_2^-$) and $I_3^-$ are oxidized back to $I_2$ before being desorbed (hence, the rate-limiting stage), as represented in Equations (7) and (8).

$$ 2\bar{I}_n^- + 2\text{BDC}^{\bullet+} \rightarrow n\text{I}_2 + 2\text{BDC} \quad (n = 3, 5, 7, 9 \dots) \quad (7) $$

$$ 2\bar{I}_n^- + 2\text{NH}_2^{\bullet+} \text{BDC} \rightarrow n\text{I}_2 + 2\text{NH}_2\text{BDC} \quad (n = 3, 5, 7, 9 \dots) \quad (8) $$

Therefore, the total amount of $I_2$ molecules that are available for the desorption depends on the initial amount of trapped iodine (which is higher in $I_2$@MIL-125), but also on the quantity of $I_2^-$ ($n = 3, 5, 7, 9 \dots$) entities that recombined according to Equations (7) and (8). In $I_2$@MIL-125\_NH_2$, the strong interaction between the anionic iodine species and the MOFs lattice attenuates the oxidation of these species. Indeed, the presence of $-NH_2$ groups induces a more efficient charge separation through the formation of the $NH_2^{\bullet+}$BDC cation radical.[19] This effect is related to the hole-stabilizer character of the amino-group, which allows a stronger interaction between the iodine
Raman spectra, and I₂@MIL-125 shows two stable bands at 113 and 178 cm⁻¹ (I₃⁻ and “perturbed” I₂, respectively), whereas I₂@MIL-125_NH₂ main signals appear at 113 and 168 cm⁻¹ (I₃⁻ and “perturbed” I₂, respectively). The amino functionalization had a clear influence over the stability of the remaining species within the pores of MIL-125 compounds. Indeed, in I₂@MIL-125, both I₂ and I₃⁻ signals progressively disappear after a few hours at 40 °C, and after a few minutes at 80 and 120 °C. On the other hand, after 19 h the I₃⁻ signal is still present at 120 °C in I₂@MIL-125_NH₂, while traces of I₂ remain at 80 °C for this material. To characterize thoroughly the difference between these two behaviors observed for I₂@MIL-125 and I₂@MIL-125_NH₂ upon heating, the iodine desorption kinetics were studied using the Albery model,[⁶⁹] which is one of the most appropriate approaches.

Briefly, Albery et al.[⁶⁹] observed that the kinetics of several heterogeneous materials (semiconductor electrodes, colloidal particles, oxide surfaces) are frequently treated as semilogarithmic plots, normalized by the initial concentration, and then tested for first-order kinetics, in which the decay signal often lies on a common curve. However, they perceived that the data is better fitted when the heterogeneity of the system is also pondered. With that in mind and considering that the rate constant comes from the Gibbs free energy of the system, a Gaussian distribution of the system's free enthalpy of activation (ΔG‡) around its mean value (ΔG‡) was proposed, according to Equation (4). Here, γ is the distribution width, R is the gas constant (8.3144621 J K⁻¹ mol⁻¹), T is the temperature (K), and x is the gaussian distribution x-axis.[⁶⁹]

$$ \Delta G^{\ddagger} = \overline{\Delta G^{\ddagger}} - \gamma xRT \quad (4) $$

Then, the authors considered that a Gaussian distribution around the rate constant logarithm average [ln(k̄)] is more likely to be found than around its corresponding k̄ data. Therefore, the former case was proposed, according to Equation (5).[⁶⁹]

$$ \ln(k) = \ln(k) + \gamma^x \quad (5) $$

Finally, because the kinetics decay results from the sum of all the contributions of each microscopic state, the authors applied an integration on the Gaussian distribution in exp(-x²). The result leads to the Equation (6), which describes the initial concentration (C₀) with time (t).[⁶⁹] It is worth mentioning that when γ = 0, the system is considered homogeneous and therefore, the Equation (6) is reduced to the first-order exponential decay expression: C/C₀ = e⁻⁴t.

$$ \frac{C(t)}{C_0} = \frac{\int_{-\infty}^{+\infty} e^{-x^2 - kt \cdot (e^x)^x} dx}{\sqrt{\pi}} \quad (6) $$

It is worth mentioning that the Albery model was often applied for evaluating the dynamic of microporous systems such as zeolites with considerable correlation coefficients (R²).[⁷⁰] Therefore, to access the k̄ values according to the Albery model for the “perturbed” I₂ and I₃⁻ species, the intensity of the signals related to the I₃⁻ (at 113 cm⁻¹) and I₃⁻ (at 178 and 168 cm⁻¹ for I₂@MIL-125 and I₂@MIL-125_NH₂, respectively) species were considered. For that, the ratio between the Raman band's intensity at

a given time and the initial intensity (I/I₀) was plotted against the time in hours. The model showed a good correspondence to the experimental data points (Figure 8).

Moreover, the amino-group influence over the I₃⁻ stabilization is now undeniable. In fact, after 19 h, I₂@MIL-125_NH₂ was able to retain 73%, 30%, and 5% of its initial I₃⁻ signal at 40, 80, and 120 °C, respectively, whereas I₂@MIL-125 only retained 7%, 2%, and < 1% under the same conditions. Furthermore, the difference of the desorption kinetics for both species in the two materials is highlighted by their respective k̄ values and lifetimes (Table 2 and Figure 9). Actually, the I₂@MIL-125 has systematically higher k̄ values and smaller lifetimes than the I₂@MIL-125_NH₂ for both I₃⁻ and “perturbed” I₂ species. This feature indicates that anionic iodine species are stabilized at higher temperatures by the presence of the amino group, which is consistent with their interaction with the trapped -NH₂•⁺ holes.[¹⁹] In fact, previous studies proposed the formation of a •BDC⁺ radical as a mechanism for the stabilization of the I₃⁻ species in I₂@UiO-66 systems through electrostatic interactions.[¹⁰] This effect is then enhanced in amino-functionalized materials as the -NH₂ groups act as hole stabilizers in the NH₂-BDC organic linker.[³⁹⁻⁴¹] Therefore, the analogous NH₂•⁺-BDC radical can further retain I₃⁻ into the organic linker via an electrostatic interaction with these positively charged entities.

It is worth mentioning that when considering I₂@MIL-125, both iodine species have very similar lifetimes for temperatures higher than 40 °C, inferring that their desorption mechanism follows the same trend. On the other hand, a clear difference is observed when comparing the behavior of these species upon I₂@MIL-125_NH₂ heating. In this case, the lifetime of the I₃⁻ species is about two-times longer than that of the “perturbed” I₂ in the entire temperature range (Table 2 and Figure 9). With that in mind and considering the Raman spectra (Figures 7 and 8) and the TGA experiments (Figure 6), it is undeniable that iodine is further stabilized in the presence of -NH₂ groups even at higher temperatures, reinforcing the influence of the amino group over the desorption mechanism. This feature suggests that a rate-limiting stage is present during the iodine desorption in I₂@MIL-125_NH₂. At this point, one must consider that anionic species cannot transform into gas without losing their charges first, and therefore, both “perturbed” I₂ (or Iₙ⁻) and I₃⁻ are oxidized back to I₂ before being desorbed (hence, the rate-limiting stage), as represented in Equations (7) and (8).

$$ 2\text{I}_n^- + 2\text{BDC}^{\bullet+} \rightarrow \text{nI}_2 + 2\text{BDC} \quad (\text{n} = 3, 5, 7, 9 \dots) \quad (7) $$

$$ 2\text{I}_n^- + 2\text{NH}_2^+ \text{BDC} \rightarrow \text{nI}_2 + 2\text{NH}_2\text{BDC} \quad (\text{n} = 3, 5, 7, 9 \dots) \quad (8) $$

Therefore, the total amount of I₂ molecules that are available for the desorption depends on the initial amount of trapped iodine (which is higher in I₂@MIL-125), but also on the quantity of Iₙ⁻ (n = 3, 5, 7, 9 ...) entities that recombined according to Equations (7) and (8). In I₂@MIL-125_NH₂, the strong interaction between the anionic iodine species and the MOFs lattice attenuates the oxidation of these species. Indeed, the presence of -NH₂ groups induces a more efficient charge separation through the formation of the NH₂•⁺BDC cation radical.[¹⁹] This effect is related to the hole-stabilizer character of the amino-group, which allows a stronger interaction between the iodine
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Raman intensities ratio I(t)/I₀ fitted according to the Albery method for the I₂ and I₃⁻ desorption in a) I₂@MIL-125 and b) I₂@MIL-125_NH₂.</figcaption></figure>

anionic species and the NH₂•⁺-BDC linker when compared to the BDC•⁺.[10,65–68] Therefore, less I₂ molecules are available for the desorption, as many of them are still stabilized in the form of anionic Iₙ⁻ species. On the other hand, I₂@MIL-125 exhibits similar lifetimes of both iodine species for temperatures higher than 40 °C (Table 2 and Figure 8). Hence, Equation (7) is not a rate-limiting step in this system and, consequently, most iodine anions are quickly transformed into I₂ in this case, which increases the number of molecules available for desorption. For this reason, there is no considerable difference over the kinetics of “perturbed” I₂ and I₃⁻ desorption in I₂@MIL-125.

## 2.4. Computational Modeling

DFT calculations were conducted for evaluating the theoretical desorption energies of both I₂ and I₃⁻ species. Due to the

complexity of the system, only the organic linkers were considered during the simulation and therefore, the effect of the metal was neglected. In this case, the protonated (H₂BDC) and deprotonated (BDC) versions of the organic linker were considered, as well as their respective radical (H₂BDC•⁺, BDC•⁺) and amino-functionalized versions (NH₂-BDC, NH₂•⁺-BDC, NH₂-H₂BDC, NH₂•⁺-H₂BDC). The adsorption energies were computed by subtracting the energies of the separated components to the energies of the associated molecules. Thus, a more negative value is associated with a larger stabilizing interaction between both species. To ensure a good comparison, the unrestricted formalism for DFT computations was used whatever the spin states. Due to the size of the basis set, and especially the presence of diffuse functions, no basis set superposition error (BSSE) correction was applied. Note that, due to the simplified system proposed during the computational modeling (without considering the entire bulk material), the absolute energy values should not
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Raman intensities ratio $I(t)/I_0$ fitted according to the Albery method for the $I_2$ and $I_3^-$ desorption in a) $I_2$@MIL-125 and b) $I_2$@MIL-125_NH_2.</figcaption></figure>

anionic species and the $NH_2^{●+}$-BDC linker when compared to the BDC$^{●+}$.[10,65–68] Therefore, less $I_2$ molecules are available for the desorption, as many of them are still stabilized in the form of anionic $I_n^-$ species. On the other hand, $I_2$@MIL-125 exhibits similar lifetimes of both iodine species for temperatures higher than 40 °C (Table 2 and Figure 8). Hence, Equation (7) is not a rate-limiting step in this system and, consequently, most iodine anions are quickly transformed into $I_2$ in this case, which increases the number of molecules available for desorption. For this reason, there is no considerable difference over the kinetics of “perturbed” $I_2$ and $I_3^-$ desorption in $I_2$@MIL-125.

## 2.4. Computational Modeling

DFT calculations were conducted for evaluating the theoretical desorption energies of both $I_2$ and $I_3^-$ species. Due to the

complexity of the system, only the organic linkers were considered during the simulation and therefore, the effect of the metal was neglected. In this case, the protonated ($H_2$BDC) and deprotonated (BDC) versions of the organic linker were considered, as well as their respective radical ($H_2$BDC$^{●+}$, BDC$^{●+}$) and amino-functionalized versions ($NH_2$-BDC, $NH_2^{●+}$-BDC, $NH_2$-$H_2$BDC, $NH_2^{●+}$-$H_2$BDC). The adsorption energies were computed by subtracting the energies of the separated components to the energies of the associated molecules. Thus, a more negative value is associated with a larger stabilizing interaction between both species. To ensure a good comparison, the unrestricted formalism for DFT computations was used whatever the spin states. Due to the size of the basis set, and especially the presence of diffuse functions, no basis set superposition error (BSSE) correction was applied. Note that, due to the simplified system proposed during the computational modeling (without considering the entire bulk material), the absolute energy values should not
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Lifetime of "perturbed" I₂ and I₃⁻ species obtained using the Albery method for a) I₂@MIL-125 and b) I₂@MIL-125_NH₂. Inset: zoom showing the behavior at temperatures above 80 °C.</figcaption></figure>

be considered. Instead, only comparisons between different systems must be studied. The obtained results are summarized in **Table 3**.

In the absence of the inorganic cluster moiety in the simulation, the use of both protonated and deprotonated versions of the organic linker is easily justified. Indeed, when considering the protonated organic linker, the hydrogen-oxygen interaction acts as a covalent bond. On the other hand, in the deprotonated case, there is no interaction between a positively charged element (either H⁺ or Ti⁴⁺) and the -COO⁻ entity. For this reason, metallic cations exhibiting intermediate covalent/ionic bonds with the -COO⁻ entity should stand in between these two cases. For this reason, any comparison between the experimental and theoretical values obtained here must be conducted carefully.

Considering first the deprotonated linkers, the I₂ species tend to be further stabilized in the presence of -NH₂, as verified by the more negative energies in this case (E, H, and G). This finding is related to the higher electron donor character of amines, which enhances the stabilization of iodine species, as discussed

before. It also corroborates the evidence that I₂ is harder to desorb in I₂@MIL-125(Ti)₂-NH₂ due to its strong interaction with -NH₂ groups, which leads to higher desorption lifetimes in all observed temperatures (Table 2 and Figure 9). In addition, I₂ has more negative Gibbs free energy in neutral organic linkers, which agrees with the mechanism proposed elsewhere.[10] Indeed, the first step of iodine adsorption is the interaction between the I₂ molecules and the organic linker, creating an electron donor-acceptor complex. Only then, the radical version of the organic linker is formed and, at this point, I₂ is transformed into I₃⁻. Note that, for the protonated organic linkers, these tendencies are also verified, with exception of the I₂@H₂BDC●⁺ system that has much smaller energies (E, H, and G). In this case (as in I₂@NH₂●⁺-H₂BDC), an interaction between iodine and the oxygen of the linkers seems to appear. This interaction could be an artifact of the model due to the fact that these oxygen atoms are not available for bonding when they interact with a metallic framework. Thus, the stability of these two complexes is probably overestimated.

Contrary to what is observed for I₂, the I₃⁻ species is clearly stabilized only in the presence of cation radicals, as shown by the impossibility to obtain energy values for its interaction with neutral organic linkers. This evidence also agrees with the mechanism proposed for the iodine evolution, as it states that I₃⁻ is likely observed alongside cation radical organic entities.[10] It is also noted that the energy values are negative for the entire protonated series, while they are positive for the deprotonated organic linkers. This indicates that introducing positively charged elements (such as H⁺ or Ti⁴⁺) bounded to the carboxylate group helps to stabilize iodine species, which is coherent to the fact that the MOF lattice exhibits mostly "intermediate states" between these two forms of organic linkers. Finally, it is clear that, in the case of protonated linkers, the order of magnitude from the I₃⁻ interactions with the linkers is higher than those of I₂ species. This finding suggests a stronger interaction between I₃⁻ and the MOF lattice, which agrees with the experimental results here. Indeed, the desorption lifetimes of I₃⁻ species are considerably higher than those of I₂ (Table 2 and Figure 9). Finally, contrary to what was experimentally observed, no great variation was detected when comparing the influence of -NH₂ over the I₃⁻ stabilization (i.e., I₃⁻@H₂BDC●⁺ against I₃⁻@NH₂●⁺-H₂BDC). This may be ascribed to the simplicity of the model. Indeed, the interaction of a neutral molecule (I₂) with the linker can be consistently described by a sorption model using only one molecule to simulate the framework. On the other hand, charged molecules introduce more complexity to the system. In this case, the attraction between a negatively (I₃⁻) and a positively charged (the linker) molecule leads to the formation of a strong bond: the energy values around 400-500 kJ.mol⁻¹ are closer to those associated to covalent bonds than to those of intermolecular interactions. Remarkably, this same reason explains why no value is obtained for the interaction of I₃⁻ with the deprotonated linker: the model implies the interaction of two negatively charged molecules, that prevents the stabilization of an aggregate.

### 3. Conclusion

The influence of the amino group over the iodine adsorption, evolution, and desorption in MIL-125 compounds was revealed
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9.** Lifetime of "perturbed" I₂ and I₃⁻ species obtained using the Albery method for a) I₂@MIL-125 and b) I₂@MIL-125_NH₂. Inset: zoom showing the behavior at temperatures above 80 °C.</figcaption></figure>

be considered. Instead, only comparisons between different systems must be studied. The obtained results are summarized in **Table 3**.

In the absence of the inorganic cluster moiety in the simulation, the use of both protonated and deprotonated versions of the organic linker is easily justified. Indeed, when considering the protonated organic linker, the hydrogen-oxygen interaction acts as a covalent bond. On the other hand, in the deprotonated case, there is no interaction between a positively charged element (either H⁺ or Ti⁴⁺) and the -COO⁻ entity. For this reason, metallic cations exhibiting intermediate covalent/ionic bonds with the -COO⁻ entity should stand in between these two cases. For this reason, any comparison between the experimental and theoretical values obtained here must be conducted carefully.

Considering first the deprotonated linkers, the I₂ species tend to be further stabilized in the presence of -NH₂, as verified by the more negative energies in this case (E, H, and G). This finding is related to the higher electron donor character of amines, which enhances the stabilization of iodine species, as discussed

before. It also corroborates the evidence that I₂ is harder to desorb in I₂@MIL-125(Ti)₂-NH₂ due to its strong interaction with -NH₂ groups, which leads to higher desorption lifetimes in all observed temperatures (Table 2 and Figure 9). In addition, I₂ has more negative Gibbs free energy in neutral organic linkers, which agrees with the mechanism proposed elsewhere.[10] Indeed, the first step of iodine adsorption is the interaction between the I₂ molecules and the organic linker, creating an electron donor-acceptor complex. Only then, the radical version of the organic linker is formed and, at this point, I₂ is transformed into I₃⁻. Note that, for the protonated organic linkers, these tendencies are also verified, with exception of the I₂@H₂BDC●⁺ system that has much smaller energies (E, H, and G). In this case (as in I₂@NH₂●⁺-H₂BDC), an interaction between iodine and the oxygen of the linkers seems to appear. This interaction could be an artifact of the model due to the fact that these oxygen atoms are not available for bonding when they interact with a metallic framework. Thus, the stability of these two complexes is probably overestimated.

Contrary to what is observed for I₂, the I₃⁻ species is clearly stabilized only in the presence of cation radicals, as shown by the impossibility to obtain energy values for its interaction with neutral organic linkers. This evidence also agrees with the mechanism proposed for the iodine evolution, as it states that I₃⁻ is likely observed alongside cation radical organic entities.[10] It is also noted that the energy values are negative for the entire protonated series, while they are positive for the deprotonated organic linkers. This indicates that introducing positively charged elements (such as H⁺ or Ti⁴⁺) bounded to the carboxylate group helps to stabilize iodine species, which is coherent to the fact that the MOF lattice exhibits mostly "intermediate states" between these two forms of organic linkers. Finally, it is clear that, in the case of protonated linkers, the order of magnitude from the I₃⁻ interactions with the linkers is higher than those of I₂ species. This finding suggests a stronger interaction between I₃⁻ and the MOF lattice, which agrees with the experimental results here. Indeed, the desorption lifetimes of I₃⁻ species are considerably higher than those of I₂ (Table 2 and Figure 9). Finally, contrary to what was experimentally observed, no great variation was detected when comparing the influence of -NH₂ over the I₃⁻ stabilization (i.e., I₃⁻@H₂BDC●⁺ against I₃⁻@NH₂●⁺-H₂BDC). This may be ascribed to the simplicity of the model. Indeed, the interaction of a neutral molecule (I₂) with the linker can be consistently described by a sorption model using only one molecule to simulate the framework. On the other hand, charged molecules introduce more complexity to the system. In this case, the attraction between a negatively (I₃⁻) and a positively charged (the linker) molecule leads to the formation of a strong bond: the energy values around 400-500 kJ.mol⁻¹ are closer to those associated to covalent bonds than to those of intermolecular interactions. Remarkably, this same reason explains why no value is obtained for the interaction of I₃⁻ with the deprotonated linker: the model implies the interaction of two negatively charged molecules, that prevents the stabilization of an aggregate.

### 3. Conclusion

The influence of the amino group over the iodine adsorption, evolution, and desorption in MIL-125 compounds was revealed
<table><caption>**Table 2.** Rate constant average (k) obtained by the Albery method and coefficient of determination (R²) for the desorption of I₂ and I₃⁻ species in I₂@MIL-125, and I₂@MIL-125_NH₂ at different temperatures.</caption><thead><tr><th rowspan="2">Temperature [°C]</th><th rowspan="2">MOF</th><th colspan="3">I₂</th><th colspan="3">I₃⁻</th></tr><tr><th>k [h⁻¹]</th><th>Lifetime (min)</th><th>R²</th><th>k [h⁻¹]</th><th>Lifetime [min]</th><th>R²</th></tr></thead><tbody><tr><td rowspan="2">40</td><td>I₂@MIL-125</td><td>0.219(3)</td><td>273</td><td>0.998</td><td>0.11(2)</td><td>545</td><td>0.975</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.15(2)</td><td>400</td><td>0.998</td><td>0.04(2)</td><td>1500</td><td>0.984</td></tr><tr><td rowspan="2">50</td><td>I₂@MIL-125</td><td>0.435(6)</td><td>137</td><td>0.983</td><td>0.350(6)</td><td>171</td><td>0.979</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.28(3)</td><td>214</td><td>0.997</td><td>0.071(7)</td><td>845</td><td>0.963</td></tr><tr><td rowspan="2">60</td><td>I₂@MIL-125</td><td>0.76(2)</td><td>78</td><td>0.931</td><td>0.77(2)</td><td>77</td><td>0.932</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.38(1)</td><td>157</td><td>0.991</td><td>0.13(6)</td><td>461</td><td>0.983</td></tr><tr><td rowspan="2">70</td><td>I₂@MIL-125</td><td>2.80(2)</td><td>21</td><td>0.998</td><td>1.71(3)</td><td>35</td><td>0.970</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.658(1)</td><td>91</td><td>0.993</td><td>0.29(5)</td><td>206</td><td>0.963</td></tr><tr><td rowspan="2">80</td><td>I₂@MIL-125</td><td>5.87(8)</td><td>10</td><td>0.989</td><td>4.6(1)</td><td>13</td><td>0.970</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.895(7)</td><td>67</td><td>0.999</td><td>0.52(1)</td><td>115</td><td>0.945</td></tr><tr><td rowspan="2">90</td><td>I₂@MIL-125</td><td>8.2(1)</td><td>7</td><td>0.989</td><td>6.3(1)</td><td>9</td><td>0.983</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>1.65(3)</td><td>36</td><td>0.997</td><td>0.87(1)</td><td>68</td><td>0.956</td></tr><tr><td rowspan="2">100</td><td>I₂@MIL-125</td><td>18.1(6)</td><td>3</td><td>0.981</td><td>15.4(6)</td><td>3</td><td>0.961</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>2.47(1)</td><td>24</td><td>0.998</td><td>1.38(8)</td><td>43</td><td>0.938</td></tr><tr><td rowspan="2">110</td><td>I₂@MIL-125</td><td>33.8(4)</td><td>2</td><td>0.984</td><td>29.7(3)</td><td>2</td><td>0.978</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>4.02(5)</td><td>14</td><td>0.997</td><td>1.85(4)</td><td>32</td><td>0.983</td></tr><tr><td rowspan="2">120</td><td>I₂@MIL-125</td><td>62.7(6)</td><td>1</td><td>0.988</td><td>57.4(6)</td><td>1</td><td>0.998</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>6.51(2)</td><td>9</td><td>0.994</td><td>2.87(4)</td><td>20</td><td>0.982</td></tr></tbody></table>

using UV–Vis, TGA, and in situ Raman spectroscopy. The I₂ uptake after five days of contact between the MOFs and the I₂ gas was slightly higher in MIL-125 (1.9 g g⁻¹) than in MIL-125_NH₂ (1.6 g g⁻¹), whereas the I₂ conversion into I₃⁻ was favored by the presence of the –NH₂ group. The interactions between the MOFs and the “perturbed” I₂ and I₃⁻ species were evaluated using TGA and in situ Raman desorption experiments in the 40–120 °C temperature range. These techniques revealed that iodine species have comparable desorption kinetics in MIL-125, whereas MIL-125_NH₂ material favored the stabilization of I₃⁻ thanks to an electrostatic interaction with the NH₂•⁺BDC organic linker. The desorption kinetics of both iodine species adsorbed on the MOFs were accessed using the Albery model. I₂@MIL-125_NH₂ consistently demonstrated longer lifetimes for both iodine species over the entire temperature range studied. This effect is assigned to a rate-limiting step for the iodine species desorption kinetics in MIL-125_NH₂. In essence, the stronger interaction between the anionic iodine species and –NH₂•⁺ radicals attenuate their oxidation into gaseous iodine, lowering the amount of I₂ available for desorption and, consequently, the energy required for the reaction. Computational modeling using DFT-based methods confirmed the higher stability of the I₃⁻. It also supported the presence of NH₂•⁺–BDC and BDC•⁺ radicals during the I₂ → I₃⁻ conversion, highlighting the stronger interaction between I₃⁻ and cationic radical organic linker species. This study demonstrates that materials with improved charge separation – as in the case of MOFs showing LMCT complexes coupled with hole-stabilized groups such as –NH₂– can stabilize iodine up to high temperatures by forming dipole–dipole interactions between the holes and the anionic iodine species, which is of great interest for applications in the nuclear industry field.

## 4. Experimental Section

**MIL-125 and MIL-125_NH₂ Synthesis:** The synthesis of MIL-125 and MIL-125_NH₂ followed procedures that were deeply described in previous studies.[19]

**X-Ray Diffraction:** High-temperature powder X-ray Diffraction (HT-PXRD) patterns of the titanium-based MOFs were registered in reflection mode using an HTK1200 Bruker diffractometer with a Bragg–Brentano theta–theta (θ–θ) geometry. The instrument is dedicated to the study of polycrystalline samples at high temperatures (up to 1200 °C), under different atmospheric conditions (in this case, the measurements were conducted under airflow), using a HT chamber of the Anton Paar HTK1200N type. The apparatus includes a copper anticathode (λ<sub>CuKα</sub> = 1.5406 Å) and is equipped with a fast LynxEye detector. During the data acquisition, the compounds were heated in an alumina support at 5° min⁻¹ up to 800 °C using temperature plateaus each 20 °C. Diffractograms were collected in the 6–80° 2θ-range with a step size of 0.02° and a speed of 2.4° min⁻¹.

**In Situ Raman Spectroscopy:** The in situ Raman adsorption experiments were conducted using a near-IR FT-Raman spectrometer (Bruker RFS 100/S instrument) with a CW Nd:YAG laser (λ<sub>ex</sub> = 1064 nm) as an excitation source. Briefly, a homemade filtration bench[9,10] supplied with non-radioactive iodine was coupled with the FT-Raman spectrometer by placing the quartz cell containing the MOF in the optical trajectory of the Raman laser. A laser power of 50 mW was used and the spectra were measured in the 50–4000 cm⁻¹ range, with a resolution of 2 cm⁻¹, and were accumulated over 1000 scans. The measurements were conducted every hour after launching the iodine gas flow for five days and then, after cutting the iodine supply, the acquisition continued for three days with the quartz cell open to the atmosphere. The in situ Raman desorption experiments were performed with a LabRam HR-Evolution (Horiba scientific) microspectrometer equipped with a 600 lines mm⁻¹ grating, using a 50 × 0.9 NA Olympus objective and a 785 nm excitation source. The samples were heated at 100 °C.min⁻¹ up to different temperatures (40, 50, 60, 70, 80, 90, 100, 110, and 120 °C) using a TMS 94 Ramp Temperature Programmer. The powdered samples were always replaced after one
<table><caption>**Table 2.** Rate constant average (k) obtained by the Albery method and coefficient of determination (R²) for the desorption of I₂ and I₃⁻ species in I₂@MIL-125, and I₂@MIL-125_NH₂ at different temperatures.</caption><thead><tr><th rowspan="2">Temperature [°C]</th><th rowspan="2">MOF</th><th colspan="3">I₂</th><th colspan="3">I₃⁻</th></tr><tr><th>k [h⁻¹]</th><th>Lifetime (min)</th><th>R²</th><th>k [h⁻¹]</th><th>Lifetime [min]</th><th>R²</th></tr></thead><tbody><tr><td rowspan="2">40</td><td>I₂@MIL-125</td><td>0.219(3)</td><td>273</td><td>0.998</td><td>0.11(2)</td><td>545</td><td>0.975</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.15(2)</td><td>400</td><td>0.998</td><td>0.04(2)</td><td>1500</td><td>0.984</td></tr><tr><td rowspan="2">50</td><td>I₂@MIL-125</td><td>0.435(6)</td><td>137</td><td>0.983</td><td>0.350(6)</td><td>171</td><td>0.979</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.28(3)</td><td>214</td><td>0.997</td><td>0.071(7)</td><td>845</td><td>0.963</td></tr><tr><td rowspan="2">60</td><td>I₂@MIL-125</td><td>0.76(2)</td><td>78</td><td>0.931</td><td>0.77(2)</td><td>77</td><td>0.932</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.38(1)</td><td>157</td><td>0.991</td><td>0.13(6)</td><td>461</td><td>0.983</td></tr><tr><td rowspan="2">70</td><td>I₂@MIL-125</td><td>2.80(2)</td><td>21</td><td>0.998</td><td>1.71(3)</td><td>35</td><td>0.970</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.658(1)</td><td>91</td><td>0.993</td><td>0.29(5)</td><td>206</td><td>0.963</td></tr><tr><td rowspan="2">80</td><td>I₂@MIL-125</td><td>5.87(8)</td><td>10</td><td>0.989</td><td>4.6(1)</td><td>13</td><td>0.970</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>0.895(7)</td><td>67</td><td>0.999</td><td>0.52(1)</td><td>115</td><td>0.945</td></tr><tr><td rowspan="2">90</td><td>I₂@MIL-125</td><td>8.2(1)</td><td>7</td><td>0.989</td><td>6.3(1)</td><td>9</td><td>0.983</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>1.65(3)</td><td>36</td><td>0.997</td><td>0.87(1)</td><td>68</td><td>0.956</td></tr><tr><td rowspan="2">100</td><td>I₂@MIL-125</td><td>18.1(6)</td><td>3</td><td>0.981</td><td>15.4(6)</td><td>3</td><td>0.961</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>2.47(1)</td><td>24</td><td>0.998</td><td>1.38(8)</td><td>43</td><td>0.938</td></tr><tr><td rowspan="2">110</td><td>I₂@MIL-125</td><td>33.8(4)</td><td>2</td><td>0.984</td><td>29.7(3)</td><td>2</td><td>0.978</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>4.02(5)</td><td>14</td><td>0.997</td><td>1.85(4)</td><td>32</td><td>0.983</td></tr><tr><td rowspan="2">120</td><td>I₂@MIL-125</td><td>62.7(6)</td><td>1</td><td>0.988</td><td>57.4(6)</td><td>1</td><td>0.998</td></tr><tr><td>I₂@MIL-125_NH₂</td><td>6.51(2)</td><td>9</td><td>0.994</td><td>2.87(4)</td><td>20</td><td>0.982</td></tr></tbody></table>

using UV-Vis, TGA, and in situ Raman spectroscopy. The I₂ uptake after five days of contact between the MOFs and the I₂ gas was slightly higher in MIL-125 (1.9 g g⁻¹) than in MIL-125_NH₂ (1.6 g g⁻¹), whereas the I₂ conversion into I₃⁻ was favored by the presence of the -NH₂ group. The interactions between the MOFs and the “perturbed” I₂ and I₃⁻ species were evaluated using TGA and in situ Raman desorption experiments in the 40–120 °C temperature range. These techniques revealed that iodine species have comparable desorption kinetics in MIL-125, whereas MIL-125_NH₂ material favored the stabilization of I₃⁻ thanks to an electrostatic interaction with the NH₂•⁺BDC organic linker. The desorption kinetics of both iodine species adsorbed on the MOFs were accessed using the Albery model. I₂@MIL-125_NH₂ consistently demonstrated longer lifetimes for both iodine species over the entire temperature range studied. This effect is assigned to a rate-limiting step for the iodine species desorption kinetics in MIL-125_NH₂. In essence, the stronger interaction between the anionic iodine species and -NH₂•⁺ radicals attenuate their oxidation into gaseous iodine, lowering the amount of I₂ available for desorption and, consequently, the energy required for the reaction. Computational modeling using DFT-based methods confirmed the higher stability of the I₃⁻. It also supported the presence of NH₂•⁺-BDC and BDC•⁺ radicals during the I₂ → I₃⁻ conversion, highlighting the stronger interaction between I₃⁻ and cationic radical organic linker species. This study demonstrates that materials with improved charge separation – as in the case of MOFs showing LMCT complexes coupled with hole-stabilized groups such as -NH₂- can stabilize iodine up to high temperatures by forming dipole-dipole interactions between the holes and the anionic iodine species, which is of great interest for applications in the nuclear industry field.

## 4. Experimental Section

**MIL-125 and MIL-125_NH₂ Synthesis:** The synthesis of MIL-125 and MIL-125_NH₂ followed procedures that were deeply described in previous studies.[19]

**X-Ray Diffraction:** High-temperature powder X-ray Diffraction (HT-PXRD) patterns of the titanium-based MOFs were registered in reflection mode using an HTK1200 Bruker diffractometer with a Bragg-Brentano theta-theta (θ-θ) geometry. The instrument is dedicated to the study of polycrystalline samples at high temperatures (up to 1200 °C), under different atmospheric conditions (in this case, the measurements were conducted under airflow), using a HT chamber of the Anton Paar HTK1200N type. The apparatus includes a copper anticathode (λ<sub>CuKα</sub> = 1.5406 Å) and is equipped with a fast LynxEye detector. During the data acquisition, the compounds were heated in an alumina support at 5° min⁻¹ up to 800 °C using temperature plateaus each 20 °C. Diffractograms were collected in the 6–80° 2θ-range with a step size of 0.02° and a speed of 2.4° min⁻¹.

**In Situ Raman Spectroscopy:** The in situ Raman adsorption experiments were conducted using a near-IR FT-Raman spectrometer (Bruker RFS 100/S instrument) with a CW Nd:YAG laser (λ<sub>ex</sub> = 1064 nm) as an excitation source. Briefly, a homemade filtration bench[9,10] supplied with non-radioactive iodine was coupled with the FT-Raman spectrometer by placing the quartz cell containing the MOF in the optical trajectory of the Raman laser. A laser power of 50 mW was used and the spectra were measured in the 50–4000 cm⁻¹ range, with a resolution of 2 cm⁻¹, and were accumulated over 1000 scans. The measurements were conducted every hour after launching the iodine gas flow for five days and then, after cutting the iodine supply, the acquisition continued for three days with the quartz cell open to the atmosphere. The in situ Raman desorption experiments were performed with a LabRam HR-Evolution (Horiba scientific) microspectrometer equipped with a 600 lines mm⁻¹ grating, using a 50 × 0.9 NA Olympus objective and a 785 nm excitation source. The samples were heated at 100 °C.min⁻¹ up to different temperatures (40, 50, 60, 70, 80, 90, 100, 110, and 120 °C) using a TMS 94 Ramp Temperature Programmer. The powdered samples were always replaced after one
<table><caption>**Table 3.** Adsorption energies (E), Helmholtz free energies (H), and Gibbs free energies (G) for the possible associations of organic linkers and iodine species.</caption><thead><tr><th>I<sub>2</sub>@</th><th>Energy (E)<br>[kJ.mol<sup>-1</sup>]</th><th>Helmholtz free<br>energy (H)<br>[kJ.mol<sup>-1</sup>]</th><th>Gibbs free energy<br>(G)<br>[kJ.mol<sup>-1</sup>]</th></tr></thead><tbody><tr><td colspan="4">Deprotonated<br>linkers</td></tr><tr><td>I<sub>2</sub>@BDC</td><td>-126.7</td><td>-123.3</td><td>-87.3</td></tr><tr><td>I<sub>2</sub>@BDC<sup>●+</sup></td><td>-117.6</td><td>-108.2</td><td>-67.6</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub>-BDC</td><td>-179.5</td><td>-178.1</td><td>-142.1</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub><sup>●+</sup>-BDC</td><td>-120.2</td><td>-115.0</td><td>-76.7</td></tr><tr><td colspan="4">Protonated linkers</td></tr><tr><td>I<sub>2</sub>@H<sub>2</sub>BDC</td><td>-25.8</td><td>-22.4</td><td>7.1</td></tr><tr><td>I<sub>2</sub>@H<sub>2</sub>BDC<sup>●+</sup></td><td>-99.2</td><td>-90.8</td><td>-50.4</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub>-H<sub>2</sub>BDC</td><td>-37.7</td><td>-33.7</td><td>1.4</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub><sup>●+</sup>-<br>H<sub>2</sub>BDC</td><td>-33.5</td><td>-28.7</td><td>5.2</td></tr><tr><td>I<sub>3</sub>-@</td><td>Energy (E)<br>[kJ.mol<sup>-1</sup>]</td><td>Helmholtz free<br>energy (H)<br>[kJ.mol<sup>-1</sup>]</td><td>Gibbs free energy<br>(G)<br>[kJ.mol<sup>-1</sup>]</td></tr><tr><td colspan="4">Deprotonated<br>linkers</td></tr><tr><td>I<sub>3</sub>-@BDC</td><td>—</td><td>—</td><td>—</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@BDC<sup>●+</sup></td><td>240.5</td><td>251.3</td><td>278.2</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@NH<sub>2</sub>-BDC</td><td>—</td><td>—</td><td>—</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@NH<sub>2</sub><sup>●+</sup>-BDC</td><td>246.1</td><td>249.7</td><td>273.3</td></tr><tr><td colspan="4">Protonated linkers</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@H<sub>2</sub>BDC</td><td>—</td><td>—</td><td>—</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@H<sub>2</sub>BDC<sup>●+</sup></td><td>-486.2</td><td>-475.5</td><td>-346.5</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@NH<sub>2</sub>-H<sub>2</sub>BDC</td><td>—</td><td>—</td><td>—</td></tr><tr><td>I<sub>3</sub><sup>—</sup>@NH<sub>2</sub><sup>●+</sup>-<br>H<sub>2</sub>BDC</td><td>-456.7</td><td>-340.4</td><td>-319.6</td></tr></tbody></table>

temperature cycle. Spectra were acquired each five minutes for 19 h. Data processing was performed using LabSpec6 software.

**Diffuse Reflectance UV-Vis Spectroscopy:** The UV-vis experiments were carried out on a Varian Cary 5000 spectrometer using Praying Mantis Diffuse Reflection Accessory adapted for the characterization of powder samples. The spectra were recorded in the 200-800 nm spectral range with a resolution of 1 nm. The diffuse reflectance spectra were plotted using the Kubelka-Munk function.

**Thermogravimetric Analysis:** Thermogravimetric analysis was conducted in a Mettler Toledo TGA/DSC 3+ instrument. The measurements were conducted under an 80 mL min⁻¹ air flow using alumina crucibles. Initially, the samples were heated at 5 °C min⁻¹ starting from room temperature up to 800 °C. Then, in a second step, the iodine-loaded compounds were heated at 100 °C min⁻¹ and kept 20 h at different temperatures (40, 60, 80, 100, and 120 °C).

**Iodine Adsorption:** The gaseous iodine adsorption was conducted under a continuous flow for five days using a filtration bench in the presence of the activated MOF samples.[9,10] The iodine adsorption was conducted in a home-made filtration bench. The experiment is supplied with non-radioactive iodine (¹²⁷I), which is generated in a permeation oven (VICI Metronics Dynacalibrator® Model 150) where three tubes containing iodine crystals are heated at 100 °C. The iodine vapor (0.8 mg h⁻¹) is carried by an Argon flow (10 L h⁻¹), controlled by a flowmeter (Bronkhorst High-tech E-Flow). The gas reaches an MOF powdered sample, which is kept over a frit of porosity 2 inside a glass cell containing a quartz window.

Here, the FT-Raman laser (λ<sub>ex</sub> = 1064 nm) is positioned facing the quartz window, and spectra are acquired each hour of the adsorption process during five days. The I<sub>2</sub> that is not trapped by the MOF sample goes to a two-liter bottle that contains a 0.1 M potassium iodide aqueous solution and is transformed into I<sub>3</sub><sup>—</sup> according to the Equation (9).

$$I_2 + I^- \leftrightarrow I_3^- \quad (9)$$

In order to guarantee the homogeneity of the solution and also to break the bubbles that are formed during the gas passage, a 900 rpm magnetic stirring is applied. This solution is pumped into a Shimadzu UV1800 UV-Vis spectrometer using a peristaltic pump (Cole-Parmer Masterflex L/S Model 77201-60). At the same time, in another bottle, a blank KI aqueous solution (0.1 M, 1 L) is also pumped for reference purposes. The wavelength is maintained at 352 nm during the analysis, which corresponds to the main adsorption band of the I<sub>3</sub><sup>—</sup> aqueous species. UV-Vis absorbance measurements are conducted each 4 min over a period of five days. For each sample, two measurements are conducted: one in the presence of the MOF sample and another one without any trapping material for the I<sub>2</sub> gas. In this way, the difference between these two measurements generates a third curve, which corresponds to the theoretical absorbance associated with the iodine trapped inside the pores of the materials. Thanks to a preliminary calibration, it is possible to quantify the mass of iodine trapped per gram of MOF.

**Computational Modeling:** All the computations were carried out using the Gaussian 16 suite of programs. The energies were computed out using DFT-based methods, more specifically the PBE0 global hybrid functional, which was augmented with an empirical correction for the dispersion (GD3). The Kohn-Sham wave-functions were described using the 6-311++G(d,p) basis set on all the atoms but iodine. This latter atom was described using the Stuttgart-Dresden ECP and the associated basis set. The geometry of organic molecules was optimized using the standard algorithms implemented in Gaussian 16 and the nature of the obtained was checked by the computations of the harmonic frequencies.

## Acknowledgements

The "Fonds Européen de Développement Régional" (FEDER), the "Centre National de la Recherche Scientifique" (CNRS), the "Région Hauts de France", and the "Ministère de l'Éducation Nationale de l'Enseignement Supérieur et de la Recherche" are acknowledged for the funding of X-ray diffractometers from the Chevreul Institute platform. The authors acknowledge the Characterization platform for the Raman facilities. P. H. M. A. would like to thank the University of Lille for the PhD grant.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

iodine, In situ Raman, MIL-125, MIL-125_NH<sub>2</sub>, sorption

Received: January 11, 2024

Revised: April 16, 2024

Published online:
<table><caption>**Table 3.** Adsorption energies (E), Helmholtz free energies (H), and Gibbs free energies (G) for the possible associations of organic linkers and iodine species.</caption><thead><tr><th>I<sub>2</sub>@</th><th>Energy (E)<br>[kJ.mol<sup>-1</sup>]</th><th>Helmholtz free<br>energy (H)<br>[kJ.mol<sup>-1</sup>]</th><th>Gibbs free energy<br>(G)<br>[kJ.mol<sup>-1</sup>]</th></tr></thead><tbody><tr><td colspan="4">Deprotonated<br>linkers</td></tr><tr><td>I<sub>2</sub>@BDC</td><td>-126.7</td><td>-123.3</td><td>-87.3</td></tr><tr><td>I<sub>2</sub>@BDC●+</td><td>-117.6</td><td>-108.2</td><td>-67.6</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub>-BDC</td><td>-179.5</td><td>-178.1</td><td>-142.1</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub>●+-BDC</td><td>-120.2</td><td>-115.0</td><td>-76.7</td></tr><tr><td colspan="4">Protonated linkers</td></tr><tr><td>I<sub>2</sub>@H<sub>2</sub>BDC</td><td>-25.8</td><td>-22.4</td><td>7.1</td></tr><tr><td>I<sub>2</sub>@H<sub>2</sub>BDC●+</td><td>-99.2</td><td>-90.8</td><td>-50.4</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub>-H<sub>2</sub>BDC</td><td>-37.7</td><td>-33.7</td><td>1.4</td></tr><tr><td>I<sub>2</sub>@NH<sub>2</sub>●+-<br>H<sub>2</sub>BDC</td><td>-33.5</td><td>-28.7</td><td>5.2</td></tr><tr><td>I<sub>3</sub>-@</td><td>Energy (E)<br>[kJ.mol<sup>-1</sup>]</td><td>Helmholtz free<br>energy (H)<br>[kJ.mol<sup>-1</sup>]</td><td>Gibbs free energy<br>(G)<br>[kJ.mol<sup>-1</sup>]</td></tr><tr><td colspan="4">Deprotonated<br>linkers</td></tr><tr><td>I<sub>3</sub>-@BDC</td><td>-</td><td>-</td><td>-</td></tr><tr><td>I<sub>3</sub>-@BDC●+</td><td>240.5</td><td>251.3</td><td>278.2</td></tr><tr><td>I<sub>3</sub>-@NH<sub>2</sub>-BDC</td><td>-</td><td>-</td><td>-</td></tr><tr><td>I<sub>3</sub>-@NH<sub>2</sub>●+-BDC</td><td>246.1</td><td>249.7</td><td>273.3</td></tr><tr><td colspan="4">Protonated linkers</td></tr><tr><td>I<sub>3</sub>-@H<sub>2</sub>BDC</td><td>-</td><td>-</td><td>-</td></tr><tr><td>I<sub>3</sub>-@H<sub>2</sub>BDC●+</td><td>-486.2</td><td>-475.5</td><td>-346.5</td></tr><tr><td>I<sub>3</sub>-@NH<sub>2</sub>-H<sub>2</sub>BDC</td><td>-</td><td>-</td><td>-</td></tr><tr><td>I<sub>3</sub>-@NH<sub>2</sub>●+-<br>H<sub>2</sub>BDC</td><td>-456.7</td><td>-340.4</td><td>-319.6</td></tr></tbody></table>

temperature cycle. Spectra were acquired each five minutes for 19 h. Data processing was performed using LabSpec6 software.

**Diffuse Reflectance UV-Vis Spectroscopy:** The UV-vis experiments were carried out on a Varian Cary 5000 spectrometer using Praying Mantis Diffuse Reflection Accessory adapted for the characterization of powder samples. The spectra were recorded in the 200–800 nm spectral range with a resolution of 1 nm. The diffuse reflectance spectra were plotted using the Kubelka-Munk function.

**Thermogravimetric Analysis:** Thermogravimetric analysis was conducted in a Mettler Toledo TGA/DSC 3+ instrument. The measurements were conducted under an 80 mL min⁻¹ air flow using alumina crucibles. Initially, the samples were heated at 5 °C min⁻¹ starting from room temperature up to 800 °C. Then, in a second step, the iodine-loaded compounds were heated at 100 °C min⁻¹ and kept 20 h at different temperatures (40, 60, 80, 100, and 120 °C).

**Iodine Adsorption:** The gaseous iodine adsorption was conducted under a continuous flow for five days using a filtration bench in the presence of the activated MOF samples.[9,10] The iodine adsorption was conducted in a home-made filtration bench. The experiment is supplied with non-radioactive iodine (¹²⁷I), which is generated in a permeation oven (VICI Metronics Dynacalibrator® Model 150) where three tubes containing iodine crystals are heated at 100 °C. The iodine vapor (0.8 mg h⁻¹) is carried by an Argon flow (10 L h⁻¹), controlled by a flowmeter (Bronkhorst High-tech E-Flow). The gas reaches an MOF powdered sample, which is kept over a frit of porosity 2 inside a glass cell containing a quartz window.

Here, the FT-Raman laser (λ<sub>ex</sub> = 1064 nm) is positioned facing the quartz window, and spectra are acquired each hour of the adsorption process during five days. The I<sub>2</sub> that is not trapped by the MOF sample goes to a two-liter bottle that contains a 0.1 M potassium iodide aqueous solution and is transformed into I<sub>3</sub><sup>-</sup> according to the Equation (9).

$$I_2 + I^- \leftrightarrow I_3^- \quad (9)$$

In order to guarantee the homogeneity of the solution and also to break the bubbles that are formed during the gas passage, a 900 rpm magnetic stirring is applied. This solution is pumped into a Shimadzu UV1800 UV-Vis spectrometer using a peristaltic pump (Cole-Parmer Masterflex L/S Model 77201–60). At the same time, in another bottle, a blank KI aqueous solution (0.1 M, 1 L) is also pumped for reference purposes. The wavelength is maintained at 352 nm during the analysis, which corresponds to the main adsorption band of the I<sub>3</sub><sup>-</sup> aqueous species. UV-Vis absorbance measurements are conducted each 4 min over a period of five days. For each sample, two measurements are conducted: one in the presence of the MOF sample and another one without any trapping material for the I<sub>2</sub> gas. In this way, the difference between these two measurements generates a third curve, which corresponds to the theoretical absorbance associated with the iodine trapped inside the pores of the materials. Thanks to a preliminary calibration, it is possible to quantify the mass of iodine trapped per gram of MOF.

**Computational Modeling:** All the computations were carried out using the Gaussian 16 suite of programs. The energies were computed out using DFT-based methods, more specifically the PBE0 global hybrid functional, which was augmented with an empirical correction for the dispersion (GD3). The Kohn-Sham wave-functions were described using the 6–311++G(d,p) basis set on all the atoms but iodine. This latter atom was described using the Stuttgart–Dresden ECP and the associated basis set. The geometry of organic molecules was optimized using the standard algorithms implemented in Gaussian 16 and the nature of the obtained was checked by the computations of the harmonic frequencies.

## Acknowledgements

The “Fonds Européen de Développement Régional” (FEDER), the “Centre National de la Recherche Scientifique” (CNRS), the “Région Hauts de France”, and the “Ministère de l'Éducation Nationale de l'Enseignement Supérieur et de la Recherche” are acknowledged for the funding of X-ray diffractometers from the Chevreul Institute platform. The authors acknowledge the Characterization platform for the Raman facilities. P. H. M. A. would like to thank the University of Lille for the PhD grant.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

iodine, In situ Raman, MIL-125, MIL-125_NH<sub>2</sub>, sorption

Received: January 11, 2024

Revised: April 16, 2024

Published online:
[1] A. P. Møller, T. A. Mousseau, *Biol. Conserv.* **2011**, *144*, 2787.

[2] W. Xie, D. Cui, S. R. Zhang, Y. H. Xu, D. L. Jiang, *Mater. Horiz.* **2019**, *6*, 1571.

[3] L. Pesce, P. Kopp, *Int J Pediatr Endocrinol* **2014**, *14*, 1.

[4] B. J. Riley, J. D. Vienna, D. M. Strachan, J. S. McCloy, J. L. Jerden, *J. Nucl. Mater.* **2016**, *470*, 307.

[5] S. U. Nandanwar, K. Coldsnow, V. Utgikar, P. Sabharwall, D. E Aston, *Chem. Eng. J.* **2016**, *306*, 369.

[6] B. J. Riley, J. Chun, J. V Ryan, J. Matyáš, X. S. Li, D. W. Matson, S. K. Sundaram, D. M. Strachan, J. D. Vienna, *RSC Adv.* **2011**, *1*, 1704.

[7] P. Wang, Q. Xu, Z. Li, W. Jiang, Q. Jiang, D. Jiang, *Adv. Mater.* **2018**, *30*, 1801991.

[8] T. Geng, S. Ye, Z. Zhu, W. Zhang, *J. Mater. Chem. A* **2018**, *6*, 2808.

[9] M. Leloire, C. Walshe, P. Devaux, R. Giovine, S. Duval, T. Bousquet, S. Chibani, J. Paul, A. Moissette, H. Vezin, P. Nerisson, L. Cantrel, C. Volkringer, T. Loiseau, *Chem. – A Eur. J.* **2022**, *28*, e202104437.

[10] P. H. M. Andrade, N. Henry, C. Volkringer, T. Loiseau, H. Vezin, M. Hureau, A. Moissette, *ACS Appl. Mater. Interfaces* **2022**, *14*, 29916.

[11] Y. Lei, G. Zhang, Q. Zhang, L. Yu, H. Li, H. Yu, Y. He, *Nat. Commun.* **2021**, *12*, 4483.

[12] X. Zhang, J. Maddock, T. M. Nenoff, M. A. Denecke, S. Yang, M. Schröder, *Chem. Soc. Rev.* **2022**, *51*, 3243.

[13] M. Leloire, J. Dhainaut, P. Devaux, O. Leroy, H. Desjonqueres, S. Poirier, P. Nerisson, L. Cantrel, S. Royer, T. Loiseau, C. Volkringer, *J. Hazard. Mater.* **2021**, *416*, 125890.

[14] Z. J. Li, Y. Ju, H. Lu, X. Wu, X. Yu, Y. Li, X. Wu, Z. H. Zhang, J. Lin, Y. Qian, M. Y. He, J. Q. Wang, *Chem. – A Eur. J.* **2021**, *27*, 1286.

[15] T. Assaad, B. Assfour, *J. Nucl. Mater.* **2017**, *493*, 6.

[16] N. A. Khan, Z. Hasan, S. H. Jhung, *J. Hazard. Mater.* **2013**, *244*, 444.

[17] C. Falaise, C. Volkringer, J. Facqueur, T. Bousquet, L. Gasnot, T. Loiseau, *Chem. Commun.* **2013**, *49*, 10320.

[18] P. H. M. Andrade, M. Moreau, N. Henry, M. T. Bakouche, S. Duval, C. Volkringer, T. Loiseau, M. Hureau, A. Moissette, *J. Phys. Chem. C* **2022**, *2*, 4618.

[19] P. H. M. Andrade, H. Ahouari, C. Volkringer, T. Loiseau, H. Vezin, M. Hureau, A. Moissette, *ACS Appl. Mater. Interfaces* **2023**, *15*, 31032.

[20] J. C. Hye, P. S. Myunghyun, *J. Am. Chem. Soc.* **2004**, *126*, 15844.

[21] X. Zhang, I. Da Silva, R. Fazzi, A. M. Sheveleva, X. Han, B. F. Spencer, S. A. Sapchenko, F. Tuna, E. J. L. McInnes, M. Li, S. Yang, M. Schröder, *Inorg. Chem.* **2019**, *58*, 14145.

[22] G. Brunet, D. A. Safin, M. Z. Aghaji, K. Robeyns, I. Korobkov, T. K. Woo, M. Murugesu, *Chem. Sci.* **2017**, *8*, 3171.

[23] X. Yang, X. Liu, Y. Liu, X.-F. Wang, Z. Chen, X. Wang, *Front Chem Sci Eng* **2022**, *17*, 395.

[24] S. Kolay, Jagannath, *J. Solid State Chem.* **2022**, *312*, 123182.

[25] Z. Meng, K. Mirica, *Nano Res.* **2021**, *14*, 369.

[26] B. Lee, Y. P. Chen, J. Park, J. Park, *ACS Appl. Mater. Interfaces* **2019**, *11*, 25817.

[27] J. Tang, S. Zhou, M. Huang, Z. Liang, S. Su, Y. Wen, Q. L. Zhu, X. Wu, *Inorg. Chem. Front.* **2022**, *9*, 3436.

[28] M. Dan-Hardi, C. Serre, T. Frot, L. Rozes, G. Maurin, C. Sanchez, G. Férey, *J. Am. Chem. Soc.* **2009**, *131*, 10857.

[29] M. A. Nasalevich, C. H. Hendon, J. G. Santaclara, K. Svane, B. Van Der Linden, S. L. Veber, M. V. Fedin, A. J. Houtepen, M. A. Van Der Veen, F. Kapteijn, A. Walsh, J. Gascon, *Sci. Rep.* **2016**, *6*, 23676.

[30] Y. Yang, J. Zhu, P. Wang, H. Liu, W. Zeng, L. Chen, Z. Chen, S. Mu, *Wuli Huaxue Xuebao/Acta Phys. – Chim. Sin.* **2022**, *38*, 1.

[31] Y. Yang, J. Zhu, P. Wang, H. Liu, H. Tang, J. Xiao, L. Chen, S. Mu, *J. Alloys Compd.* **2022**, *906*, 164321.

[32] P. George, N. R. Dhabarde, P. Chowdhury, *Mater. Lett.* **2017**, *186*, 151.

[33] N. Kolobov, M. G. Goesten, J. Gascon, *Angew. Chemie – Int. Ed.* **2021**, *60*, 26038.

[34] Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo, M. Matsuoka, *J. Phys. Chem. C* **2012**, *116*, 20848.

[35] S. N. Kim, J. Kim, H. Y. Kim, H. Y. Cho, W. S. Ahn, *Catal. Today* **2013**, *204*, 85.

[36] P. Yang, Y. Huang, Z. W. Zhang, N. Li, Y. Fan, *Dalt. Trans.* **2020**, *49*, 10052.

[37] M. El-Shahat, A. E. Abdelhamid, R. M. Abdelhameed, *Carbohydr. Polym.* **2020**, *231*, 115742.

[38] A. S. Yasin, J. Li, N. Wu, T. Musho, *Phys. Chem. Chem. Phys.* **2016**, *18*, 12748.

[39] M. De Miguel, F. Ragon, T. Devic, C. Serre, P. Horcajada, H. García, *ChemPhysChem* **2012**, *13*, 3651.

[40] J. G. Santaclara, M. A. Nasalevich, S. Castellanos, W. H. Evers, F. C. M. Spoor, K. Rock, L. D. A. Siebbeles, F. Kapteijn, F. Grozema, A. Houtepen, J. Gascon, J. Hunger, M. A. Van Der Veen, *ChemSusChem* **2016**, *9*, 388.

[41] Y. Li, Y. Fu, B. Ni, K. Ding, W. Chen, K. Wu, X. Huang, Y. Zhang, *AIP Adv.* **2018**, *8*, 035012.

[42] C. Zlotea, D. Phanon, M. Mazaj, D. Heurtaux, V. Guillerm, C. Serre, P. Horcajada, T. Devic, E. Magnier, F. Cuevas, G. Férey, P. L. Llewellyn, M. Latroche, *Dalt. Trans.* **2011**, *40*, 4879.

[43] P. H. M. Andrade, A. L. M. Gomes, H. G. Palhares, C. Volkringer, *J. Mater. Sci.* **2022**, *57*, 4481.

[44] S. Sircar, J. R. Hufton, *Adsorption* **2000**, *6*, 137.

[45] T. Ahnfeldt, N. Guillou, D. Gunzelmann, I. Margiolaki, T. Loiseau, G. Férey, J. Senker, N. Stock, *Angew. Chemie – Int. Ed.* **2009**, *48*, 5163.

[46] J. M. Gardner, M. Abrahamsson, B. H. Farnum, G. J. Meyer, *J. Am. Chem. Soc.* **2009**, *131*, 16206.

[47] L. Yang, S. S. Saavedra, N. R. Armstrong, *Anal. Chem.* **1996**, *68*, 1834.

[48] B. Bhowmik, G. L. Jendrasiak, B. Rosenberg, *Nat. Publ. Gr.* **1967**, *216*, 615.

[49] L. A. Walker, S. Pullen, B. Donovan, R. J. Sension, *Chem. Phys. Lett.* **1995**, *242*, 177.

[50] H. A. Benesi, J. H. Hildebrand, *J Am Chem Soc* **1949**, *71*, 2703.

[51] P. Klaeboe, *J. Am. Chem. Soc.* **1967**, *89*, 3667.

[52] W. Kiefer, H. J. Bernstein, *J. Raman Spectrosc.* **1973**, *1*, 417.

[53] W. Guo, D. Wang, J. Hu, Z. K. Tang, S. Du, *Appl. Phys. Lett.* **2011**, *98*, 2011.

[54] P. Deplano, F. A. Devillanova, J. R. Ferraro, F. Isaia, V. Lippolis, M. L. Mercuri, *Appl. Spectrosc.* **1992**, *46*, 1625.

[55] S. S. Lobanov, J. A. Daly, A. F. Goncharov, X. Chan, S. K. Ghose, H. Zhong, L. Ehm, T. Kim, J. B. Parise, *J. Phys. Chem. A* **2018**, *122*, 6109.

[56] A. J. Blake, F. A. Devillanova, R. O. Gould, W. S. Li, V. Lippolis, S. Parsons, C. Radek, M. Schröder, *Chem. Soc. Rev.* **1998**, *27*, 195.

[57] C. Wang, Y. Wang, R. Ge, X. Song, X. Xing, Q. Jiang, H. Lu, C. Hao, X. Guo, Y. Gao, D. Jiang, *Chem. – A Eur. J.* **2018**, *24*, 585.

[58] W. Zhang, Y. Mu, X. He, P. Chen, S. Zhao, C. Huang, Y. Wang, J. Chen, *Chem. Eng. J.* **2020**, *379*, 122365.

[59] R. P. Cooney, P. J. Hendra, M. Fleischmann, *J. Raman Spectrosc.* **1977**, *6*, 264.

[60] P. Deplano, J. R. Ferraro, M. L. Mercuri, E. F. Trogu, *Coord. Chem. Rev.* **1999**, *188*, 71.
[1] A. P. Møller, T. A. Mousseau, *Biol. Conserv.* **2011**, *144*, 2787.

[2] W. Xie, D. Cui, S. R. Zhang, Y. H. Xu, D. L. Jiang, *Mater. Horiz.* **2019**, *6*, 1571.

[3] L. Pesce, P. Kopp, *Int J Pediatr Endocrinol* **2014**, *14*, 1.

[4] B. J. Riley, J. D. Vienna, D. M. Strachan, J. S. McCloy, J. L. Jerden, *J. Nucl. Mater.* **2016**, *470*, 307.

[5] S. U. Nandanwar, K. Coldsnow, V. Utgikar, P. Sabharwall, D. E Aston, *Chem. Eng. J.* **2016**, *306*, 369.

[6] B. J. Riley, J. Chun, J. V Ryan, J. Matyáš, X. S. Li, D. W. Matson, S. K. Sundaram, D. M. Strachan, J. D. Vienna, *RSC Adv.* **2011**, *1*, 1704.

[7] P. Wang, Q. Xu, Z. Li, W. Jiang, Q. Jiang, D. Jiang, *Adv. Mater.* **2018**, *30*, 1801991.

[8] T. Geng, S. Ye, Z. Zhu, W. Zhang, *J. Mater. Chem. A* **2018**, *6*, 2808.

[9] M. Leloire, C. Walshe, P. Devaux, R. Giovine, S. Duval, T. Bousquet, S. Chibani, J. Paul, A. Moissette, H. Vezin, P. Nerisson, L. Cantrel, C. Volkringer, T. Loiseau, *Chem. – A Eur. J.* **2022**, *28*, e202104437.

[10] P. H. M. Andrade, N. Henry, C. Volkringer, T. Loiseau, H. Vezin, M. Hureau, A. Moissette, *ACS Appl. Mater. Interfaces* **2022**, *14*, 29916.

[11] Y. Lei, G. Zhang, Q. Zhang, L. Yu, H. Li, H. Yu, Y. He, *Nat. Commun.* **2021**, *12*, 4483.

[12] X. Zhang, J. Maddock, T. M. Nenoff, M. A. Denecke, S. Yang, M. Schröder, *Chem. Soc. Rev.* **2022**, *51*, 3243.

[13] M. Leloire, J. Dhainaut, P. Devaux, O. Leroy, H. Desjonqueres, S. Poirier, P. Nerisson, L. Cantrel, S. Royer, T. Loiseau, C. Volkringer, *J. Hazard. Mater.* **2021**, *416*, 125890.

[14] Z. J. Li, Y. Ju, H. Lu, X. Wu, X. Yu, Y. Li, X. Wu, Z. H. Zhang, J. Lin, Y. Qian, M. Y. He, J. Q. Wang, *Chem. – A Eur. J.* **2021**, *27*, 1286.

[15] T. Assaad, B. Assfour, *J. Nucl. Mater.* **2017**, *493*, 6.

[16] N. A. Khan, Z. Hasan, S. H. Jhung, *J. Hazard. Mater.* **2013**, *244*, 444.

[17] C. Falaise, C. Volkringer, J. Facqueur, T. Bousquet, L. Gasnot, T. Loiseau, *Chem. Commun.* **2013**, *49*, 10320.

[18] P. H. M. Andrade, M. Moreau, N. Henry, M. T. Bakouche, S. Duval, C. Volkringer, T. Loiseau, M. Hureau, A. Moissette, *J. Phys. Chem. C* **2022**, *2*, 4618.

[19] P. H. M. Andrade, H. Ahouari, C. Volkringer, T. Loiseau, H. Vezin, M. Hureau, A. Moissette, *ACS Appl. Mater. Interfaces* **2023**, *15*, 31032.

[20] J. C. Hye, P. S. Myunghyun, *J. Am. Chem. Soc.* **2004**, *126*, 15844.

[21] X. Zhang, I. Da Silva, R. Fazzi, A. M. Sheveleva, X. Han, B. F. Spencer, S. A. Sapchenko, F. Tuna, E. J. L. McInnes, M. Li, S. Yang, M. Schröder, *Inorg. Chem.* **2019**, *58*, 14145.

[22] G. Brunet, D. A. Safin, M. Z. Aghaji, K. Robeyns, I. Korobkov, T. K. Woo, M. Murugesu, *Chem. Sci.* **2017**, *8*, 3171.

[23] X. Yang, X. Liu, Y. Liu, X.-F. Wang, Z. Chen, X. Wang, *Front Chem Sci Eng* **2022**, *17*, 395.

[24] S. Kolay, Jagannath, *J. Solid State Chem.* **2022**, *312*, 123182.

[25] Z. Meng, K. Mirica, *Nano Res.* **2021**, *14*, 369.

[26] B. Lee, Y. P. Chen, J. Park, J. Park, *ACS Appl. Mater. Interfaces* **2019**, *11*, 25817.

[27] J. Tang, S. Zhou, M. Huang, Z. Liang, S. Su, Y. Wen, Q. L. Zhu, X. Wu, *Inorg. Chem. Front.* **2022**, *9*, 3436.

[28] M. Dan-Hardi, C. Serre, T. Frot, L. Rozes, G. Maurin, C. Sanchez, G. Férey, *J. Am. Chem. Soc.* **2009**, *131*, 10857.

[29] M. A. Nasalevich, C. H. Hendon, J. G. Santaclara, K. Svane, B. Van Der Linden, S. L. Veber, M. V. Fedin, A. J. Houtepen, M. A. Van Der Veen, F. Kapteijn, A. Walsh, J. Gascon, *Sci. Rep.* **2016**, *6*, 23676.

[30] Y. Yang, J. Zhu, P. Wang, H. Liu, W. Zeng, L. Chen, Z. Chen, S. Mu, *Wuli Huaxue Xuebao/Acta Phys. – Chim. Sin.* **2022**, *38*, 1.

[31] Y. Yang, J. Zhu, P. Wang, H. Liu, H. Tang, J. Xiao, L. Chen, S. Mu, *J. Alloys Compd.* **2022**, *906*, 164321.

[32] P. George, N. R. Dhabarde, P. Chowdhury, *Mater. Lett.* **2017**, *186*, 151.

[33] N. Kolobov, M. G. Goesten, J. Gascon, *Angew. Chemie – Int. Ed.* **2021**, *60*, 26038.

[34] Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo, M. Matsuoka, *J. Phys. Chem. C* **2012**, *116*, 20848.

[35] S. N. Kim, J. Kim, H. Y. Kim, H. Y. Cho, W. S. Ahn, *Catal. Today* **2013**, *204*, 85.

[36] P. Yang, Y. Huang, Z. W. Zhang, N. Li, Y. Fan, *Dalt. Trans.* **2020**, *49*, 10052.

[37] M. El-Shahat, A. E. Abdelhamid, R. M. Abdelhameed, *Carbohydr. Polym.* **2020**, *231*, 115742.

[38] A. S. Yasin, J. Li, N. Wu, T. Musho, *Phys. Chem. Chem. Phys.* **2016**, *18*, 12748.

[39] M. De Miguel, F. Ragon, T. Devic, C. Serre, P. Horcajada, H. García, *ChemPhysChem* **2012**, *13*, 3651.

[40] J. G. Santaclara, M. A. Nasalevich, S. Castellanos, W. H. Evers, F. C. M. Spoor, K. Rock, L. D. A. Siebbeles, F. Kapteijn, F. Grozema, A. Houtepen, J. Gascon, J. Hunger, M. A. Van Der Veen, *ChemSusChem* **2016**, *9*, 388.

[41] Y. Li, Y. Fu, B. Ni, K. Ding, W. Chen, K. Wu, X. Huang, Y. Zhang, *AIP Adv.* **2018**, *8*, 035012.

[42] C. Zlotea, D. Phanon, M. Mazaj, D. Heurtaux, V. Guillerm, C. Serre, P. Horcajada, T. Devic, E. Magnier, F. Cuevas, G. Férey, P. L. Llewellyn, M. Latroche, *Dalt. Trans.* **2011**, *40*, 4879.

[43] P. H. M. Andrade, A. L. M. Gomes, H. G. Palhares, C. Volkringer, *J. Mater. Sci.* **2022**, *57*, 4481.

[44] S. Sircar, J. R. Hufton, *Adsorption* **2000**, *6*, 137.

[45] T. Ahnfeldt, N. Guillou, D. Gunzelmann, I. Margiolaki, T. Loiseau, G. Férey, J. Senker, N. Stock, *Angew. Chemie – Int. Ed.* **2009**, *48*, 5163.

[46] J. M. Gardner, M. Abrahamsson, B. H. Farnum, G. J. Meyer, *J. Am. Chem. Soc.* **2009**, *131*, 16206.

[47] L. Yang, S. S. Saavedra, N. R. Armstrong, *Anal. Chem.* **1996**, *68*, 1834.

[48] B. Bhowmik, G. L. Jendrasiak, B. Rosenberg, *Nat. Publ. Gr.* **1967**, *216*, 615.

[49] L. A. Walker, S. Pullen, B. Donovan, R. J. Sension, *Chem. Phys. Lett.* **1995**, *242*, 177.

[50] H. A. Benesi, J. H. Hildebrand, *J Am Chem Soc* **1949**, *71*, 2703.

[51] P. Klaeboe, *J. Am. Chem. Soc.* **1967**, *89*, 3667.

[52] W. Kiefer, H. J. Bernstein, *J. Raman Spectrosc.* **1973**, *1*, 417.

[53] W. Guo, D. Wang, J. Hu, Z. K. Tang, S. Du, *Appl. Phys. Lett.* **2011**, *98*, 2011.

[54] P. Deplano, F. A. Devillanova, J. R. Ferraro, F. Isaia, V. Lippolis, M. L. Mercuri, *Appl. Spectrosc.* **1992**, *46*, 1625.

[55] S. S. Lobanov, J. A. Daly, A. F. Goncharov, X. Chan, S. K. Ghose, H. Zhong, L. Ehm, T. Kim, J. B. Parise, *J. Phys. Chem. A* **2018**, *122*, 6109.

[56] A. J. Blake, F. A. Devillanova, R. O. Gould, W. S. Li, V. Lippolis, S. Parsons, C. Radek, M. Schröder, *Chem. Soc. Rev.* **1998**, *27*, 195.

[57] C. Wang, Y. Wang, R. Ge, X. Song, X. Xing, Q. Jiang, H. Lu, C. Hao, X. Guo, Y. Gao, D. Jiang, *Chem. – A Eur. J.* **2018**, *24*, 585.

[58] W. Zhang, Y. Mu, X. He, P. Chen, S. Zhao, C. Huang, Y. Wang, J. Chen, *Chem. Eng. J.* **2020**, *379*, 122365.

[59] R. P. Cooney, P. J. Hendra, M. Fleischmann, *J. Raman Spectrosc.* **1977**, *6*, 264.

[60] P. Deplano, J. R. Ferraro, M. L. Mercuri, E. F. Trogu, *Coord. Chem. Rev.* **1999**, *188*, 71.
[61] P. H. Svensson, L. Kloo, *Chem. Rev.* **2003**, *103*, 1649.

[62] S. L. Hsu, A. J. Signorelli, G. P. Pez, R. H. Baughman, *J. Chem. Phys.* **1978**, *69*, 106.

[63] M. Leloire, *Utilisation de Matériaux Poreux de Type Metal–Organic Framework (MOF) Pour l'adsorption de Molécules Gazeuses* (12, RuO4) Dans Le Contexte d'un Accident de Réacteur Nucléaire, Université de Lille, France **2021**, https://pepite-depot.univlille.fr/LIBRE/EDSMRE/2021/2021LILUR009.pdf (accessed: January 2021).

[64] J. T. Hughes, D. F. Sava, T. M. Nenoff, A. Navrotsky, *J. Am. Chem. Soc.* **2013**, *135*, 16256.

[65] R. Pénélope, L. Campayo, M. Fournier, A. Gossard, A. Grandjean, *J. Nucl. Mater.* **2022**, *563*, 153635.

[66] D. F. Sava, K. W. Chapman, M. A. Rodriguez, J. A. Greathouse, P. S. Crozier, H. Zhao, P. J. Chupas, T. M. Nenoff, *Chem. Mater.* **2013**, *25*, 2591.

[67] D. Banerjee, X. Chen, S. S. Lobanov, A. M. Plonka, X. Chan, J. A. Daly, T. Kim, P. K. Thallapally, J. B. Parise, *ACS Appl. Mater. Interfaces* **2018**, *10*, 10622.

[68] G. Massasso, J. Long, J. Haines, S. Devautour-Vinot, G. Maurin, A. Grandjean, B. Onida, B. Donnadieu, J. Larionova, C. Guérin, Y. Guari, *Inorg. Chem.* **2014**, *53*, 4269.

[69] W. J. Albery, P. N. Bartlett, C. P. Wilde, J. R. Darwent, *J. Am. Chem. Soc.* **1985**, *107*, 1854.

[70] M. Hureau, A. Moissette, A. Legrand, F. Luchez, M. Sliwa, C. Bremard, *J. Phys. Chem. C* **2012**, *116*, 9092.
[61] P. H. Svensson, L. Kloo, *Chem. Rev.* **2003**, *103*, 1649.

[62] S. L. Hsu, A. J. Signorelli, G. P. Pez, R. H. Baughman, *J. Chem. Phys.* **1978**, *69*, 106.

[63] M. Leloire, *Utilisation de Matériaux Poreux de Type Metal–Organic Framework (MOF) Pour l'adsorption de Molécules Gazeuses* (12, RuO4) Dans Le Contexte d'un Accident de Réacteur Nucléaire, Université de Lille, France **2021**, https://pepite-depot.univlille.fr/LIBRE/EDSMRE/2021/2021LILUR009.pdf (accessed: January 2021).

[64] J. T. Hughes, D. F. Sava, T. M. Nenoff, A. Navrotsky, *J. Am. Chem. Soc.* **2013**, *135*, 16256.

[65] R. Pénélope, L. Campayo, M. Fournier, A. Gossard, A. Grandjean, *J. Nucl. Mater.* **2022**, *563*, 153635.

[66] D. F. Sava, K. W. Chapman, M. A. Rodriguez, J. A. Greathouse, P. S. Crozier, H. Zhao, P. J. Chupas, T. M. Nenoff, *Chem. Mater.* **2013**, *25*, 2591.

[67] D. Banerjee, X. Chen, S. S. Lobanov, A. M. Plonka, X. Chan, J. A. Daly, T. Kim, P. K. Thallapally, J. B. Parise, *ACS Appl. Mater. Interfaces* **2018**, *10*, 10622.

[68] G. Massasso, J. Long, J. Haines, S. Devautour-Vinot, G. Maurin, A. Grandjean, B. Onida, B. Donnadieu, J. Larionova, C. Guérin, Y. Guari, *Inorg. Chem.* **2014**, *53*, 4269.

[69] W. J. Albery, P. N. Bartlett, C. P. Wilde, J. R. Darwent, *J. Am. Chem. Soc.* **1985**, *107*, 1854.

[70] M. Hureau, A. Moissette, A. Legrand, F. Luchez, M. Sliwa, C. Bremard, *J. Phys. Chem. C* **2012**, *116*, 9092.