Formation of the Most Iodine‐Rich Polyiodide [I13]− in Pore‐Partitioned Metal‐Organic Frameworks for Efficient Iodine Capture

Abstract

ABSTRACT

Radioactive iodine produced from nuclear fission in power plants presents substantial environmental risks and requires effective remediation measures. Metal‐organic frameworks (MOFs) containing specifically designed pore geometries with stable skeletons that allow dense packing of guest molecules are sought after for iodine capture. Here, 14 new MOFs were developed through reticular chemistry for a comprehensive study of the iodine capture behavior. Remarkably, one of this family of materials, JOU‐20(FeCo₂), exhibited an exceptional static vapor iodine uptake capacity of 3.08 g/g at 80°C and a high iodine storage density of 4.69 g/cm³. Significantly, single‐crystal X‐ray diffraction revealed the adsorbed iodine in JOU‐20(FeCo₂) forming an unusual aggregation of the giant trigonal antiprismatic polyiodide anion [I₁₃]⁻. To the best of our knowledge, this is the first time that the polyiodide [I₁₃]⁻ was structurally resolved in a crystalline framework, and it represents the most iodine‐rich polyiodide species ever discovered experimentally. Combined spectroscopy and theoretical calculation methods demonstrated that nitrogen/sulfur sites and metal nodes play critical roles in stabilizing [I₁₃]⁻. This work introduces a pore partition strategy to create a confined space with specific pore geometry for the formation of unusual polyiodide [I₁₃]⁻, and multiple binding sites for stabilizing it, which significantly enhances the iodine adsorption performance of MOFs.

Introduction

Nuclear energy is considered a reliable source of power. However, the process of nuclear fission produces a substantial quantity of volatile radioactive substances, including ⁸⁵Kr, ¹²⁷Xe, and ¹²⁹I [1, 2]. Among these, ¹²⁹I is particularly concerning due to its extended half-life (1.57 × 10⁷ years), heightened biological toxicity, and easy diffusion in the environment [3]. Consequently, it is important to capture radioactive iodine before it is discharged into the external environment. Efforts have been directed toward developing adsorbents to effectively capture iodine, including aerogel, microporous polymers, and zeolite-based materials [4–6]. Although some of these materials have exhibited commendable results in the adsorption and capture of iodine, there remains significant interest in the design and synthesis of new types of adsorbents with enhanced iodine capacity and the detailed understanding of structure–property relationships.

Metal-organic frameworks (MOFs) are a new class of porous materials that are periodically arranged by metal ions and organic linkers [7–9]. Their extensive specific surface areas and adaptable structures have led to widespread applications in gas storage and separation, catalysis, and fluorescence [10–12]. The structural advances of MOFs also endow them with the application in iodine adsorption, and many MOFs have been utilized in this field [13–16]. The following strategies, but not limited to, have been utilized to promote the adsorption ability of MOFs. First, functional groups can be introduced into the MOF structure. For instance, the introduction of electron-donating groups (-OH, -NH₂, etc.) can improve the iodine capture capacity of MOFs by forming halogen bonds or charge transfer (CT) interactions [17–20]. Conjugated groups were shown to promote iodine adsorption through the formation of iodine-π interactions [21–23]. Ionic groups also have a promoting effect on the adsorption of iodine [24–26]. Ke et al. created a cationic framework material by protonating melamine groups under acid conditions, resulting in improved performance for iodine adsorption [24]. Second, constructing unsaturated metal sites was employed to enhance the affinity of the framework for iodine [27, 28]. Han et al. designed an azolate-based MOF with unsaturated Cu(I) metal sites, which enable it to have efficient adsorption ability [27]. Third, the pore structure of the MOF can be regulated. Zhang et al. studied the iodine adsorption performance of five MOFs with different pore structures [29]. It was found that DUT-68 with cage-shaped pores showed better constraint effect on adsorbed iodine. In addition, the introduction of metal nanoparticles, such as silver and copper, into the framework is also an effective method to improve the adsorption capacity of MOFs [30, 31]. Although numerous MOFs have been reported for iodine adsorption, there is still a lack of stable MOF platforms with highly adjustable and stable skeletons for systematic study. Moreover, the I₂ adsorbed in MOF pores was usually highly disordered, making it difficult to study the location of iodine and the detailed adsorption mechanism. The position of adsorbed iodine species was rarely resolved at the atomic level [8, 10].

Polyiodide ions of formula [I_2m+n]ⁿ⁻ (where m and n are integers >0, n = 1–4) are a common form of iodine species consisting of the basic building units I⁻, I₂, and I₃⁻ linked by donor-acceptor type I···I interactions [32]. Polyiodides show tunable electrical conductivity and find a wide range of applications in batteries, electronics, solar cells, and optical devices [33, 34]. Up to now, a large number of polyiodides have been structurally characterized, such as I₅⁻, I₇⁻, I₉⁻, and I₂₉³⁻ (with the highest iodine content of I_9.67⁻) [35]. The occurrence of these iodine-rich polyiodides is increasingly rare due to the decreased stability. Previous studies showed that the formation of polyiodides in porous materials can significantly enhance the iodine adsorption capacity [36, 37]. This is because iodine-rich species can increase the packing density of the adsorbed iodine and show multiple interactions with the surrounding pore walls in the confined space. The synthesis of compounds with high iodine content is extremely challenging, and the creation of pores of specific geometry with multiple binding sites from the pore walls in MOFs could facilitate the formation of novel polyiodides.

In this work, pore-partitioned MOFs featuring a pacs network were selected for an iodine adsorption study (Scheme 1a,b). This type of framework exhibits an exceptionally adjustable composition, providing a thorough exploration of the correlation between structure and performance [38–40]. The selected framework is composed of three types of modules, including oxygen core trinuclear cluster ([M₃O(COO)₆]), dicarboxylate, and N-containing ligands (Scheme 1c). The metal ions in [M₃O(COO)₆] may be Mn²⁺, Fe^2/3+, Co²⁺, Ni²⁺, or Zn²⁺. The dicarboxylate ligand is 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H₂DMTDC) or 2,6-naphthalenedicarboxylic acid (2,6-H₂NDC). The N-containing ligand is 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine (TPT), 2,4,6-tris(4-pyridyl)pyridine (TPY), or 1,3,5-tris(4-pyridyl)benzene (TPB). Herein, 14 new MOFs were constructed to investigate the trend for iodine adsorption. Their compositions are [Fe₃O(DMTDC)₃(H₂O)₂Cl] (JOU-10(Fe₃)), [Fe₃O(DMTDC)₃(TPB)]·Cl (JOU-18(Fe₃)), [Fe₃O(DMTDC)₃(TPY)]·Cl (JOU-19(Fe₃)), [(CH₃)₂NH₂]_x[M₃(O/OH)(DMTDC)₃(TPT)]·yCl (JOU-20(M₃); M₃ = Fe₃, x = 0, y = 1; M₃ = Co₃, Ni₃, Mn₃, Fe₂Mn, Fe₂Ni, Fe₂Zn, Fe₂Co, or FeCo₂, x = 0, y = 0; M₃ = Zn₃, x = 1, y = 0), and [Fe₃O(2,6-NDC)₃(TPT)]·Cl (JOU-21(Fe₃)), respectively. It is shown that the iodine adsorption capacities of these MOFs are closely related to the pore structure, N/S atomic content, and metal nodes. Among them, JOU-20(FeCo₂) exhibites impressive iodine adsorption capacities under both static and dynamic conditions. In addition, the host–guest interactions were explored and analyzed via both experimental and theoretical calculation methods. The atomic arrangement of iodine loaded in the MOF was also resolved through single-crystal X-ray diffraction (SCXRD) technique. Significantly, the adsorbed iodine in JOU-20(FeCo₂) represents a remarkable iodine storage density of 4.69 g/cm³ and an unusual aggregation of [I₁₃]⁻ anion, which is the most iodine-rich polyiodide ever discovered experimentally.

Results and Discussion

Characterizations

All MOFs were prepared by solvothermal methods. By appropriately adjusting the amount of hydrofluoric acid, JOU-10(Fe₃), JOU-18(Fe₃), JOU-20(FeCo₂), JOU-20(Co₃), and JOU-20(Mn₃) were isolated with large-sized single crystals. Due to the isoreticular three-dimensional (3D) structure of these MOFs, only the structures of JOU-10(Fe₃) and JOU-20(Fe₃) are discussed in detail. Both JOU-10(Fe₃) and JOU-20(Fe₃) were crystallized in hexagonal P6₃/mmc space group. As shown in Figure S1a, the 3D structure of JOU-10(Fe₃) represents acs network based on [Fe₃O(COO)₆] building blocks and DMTDC²⁻ ligands (Figure S2) with 1D channels extended along the c axis. When tpt ligands were added, the voids of JOU-20(Fe₃) were partitioned into small zones within the pacs network (Figure S1b). The size of open windows of JOU-20(Fe₃) was 8.7 Å × 8.7 Å (Figure S1c). As shown in Figure 1a,b, two types of cage-shaped pores are present in the porous framework of JOU-20(Fe₃), which are trigonal bipyramidal and distorted octahedral pores. The distorted octahedral cage with a diameter of approximately 10 Å is constructed by six [Fe₃(μ₃-O)(COO)₆]⁺ clusters, six DMTDC²⁻ ligands, and two TPT units, whereas the trigonal bipyramidal cage with a diameter of approximately 14 Å is constructed by five [Fe₃(μ₃-O)(COO)₆]⁺ clusters, six DMTDC²⁻ units, and three TPT ligands. All samples were characterized by scanning electron microscopy (SEM) (Figure S3), powder X-ray diffraction (PXRD) (Figure 1c and Figures S4–S8), and Fourier transform infrared spectroscopy (FT-IR) (Figures S9–S11). The measured PXRD patterns match well with the calculated ones, confirming the high purity of all prepared MOFs. The morphology and particle size of Fe-MOFs were analyzed using SEM. As shown in Figure 1d and Figure S3, JOU-18(Fe₃), JOU-20(Fe₃), JOU-20(Fe₂Zn), and JOU-20(Fe₂Ni) have long hexagonal prismatic columnar morphology. JOU-19(Fe₃) and JOU-20(Fe₂Mn) have irregular polyhedral morphology. JOU-20(Fe₂Co) and JOU-20(FeCo₂) have distorted octahedral morphology, while JOU-21(Fe₃) has thick hexagonal plate-shaped morphology. The particle size of these MOF powders was in the range of 1–14 µm. The pore structure of these MOFs was analyzed using N₂ adsorption/desorption technique at 77 K (Figure 1e,f and Figures S12–S15). All MOFs except JOU-10(Fe₃) show typical type-I isotherms. Table 1 presents the specific surface areas calculated by the Brunauer–Emmett–Teller (BET) method and pore size distributions determined by the nonlocal density functional theory (NLDFT). For JOU-10(Fe₃) showed negligible N₂ adsorption with very small specific surface area, which may be due to the collapse of the pores during the degassing process (Figure S15) [38]. This result indicates that partitioning MOF channels into smaller domains can significantly improve their adsorption capacity. It is speculated that these cage-shaped pores with a large number of surface N/S atoms may enhance the confinement effect and promote the interaction between the MOF and iodine atoms, thereby improving the iodine adsorption capacity.

[IMAGE OMITTED. SEE PDF]

TABLE 1 Pore parameters and iodine adsorption capacities of selected MOFs.

MOFs	BET surface area (m2/g)	Pore volume (cm3/g)	Experimental I2 capacity (g/g)
JOU-18(Fe3)	620	0.4285	1.32
JOU-19(Fe3)	619	0.4761	1.94
JOU-20(Fe3)	839	0.6146	2.40
JOU-21(Fe3)	735	0.4532	1.72
JOU-20(Co3)	730	0.4122	/
JOU-20(Ni3)	711	0.3967	/
JOU-20(Zn3)	381	0.3555	/
JOU-20(Fe2Zn)	838	0.4419	1.87
JOU-20(Fe2Ni)	706	0.4396	1.88
JOU-20(Fe2Co)	1107	0.6158	2.87
JOU-20(FeCo2)	1128	0.6567	3.08

Vapor Iodine Adsorption

Iodine adsorption performance of MOF powders was investigated by using the weighting method at atmospheric pressure. As depicted in Figure 2a, the adsorption rates of JOU-18(Fe₃), JOU-19(Fe₃), JOU-20(Fe₃), and JOU-21(Fe₃) exhibited a rapid increase within the initial 2 h, followed by a gradual decline until they attained adsorption saturation. Their equilibrium adsorption capacity is in the order of JOU-20(Fe₃) (2.40 g/g) > JOU-19(Fe₃) (1.94 g/g) > JOU-21(Fe₃) (1.73 g/g) > JOU-18(Fe₃) (1.32 g/g). The larger adsorption capacity of JOU-20(Fe₃) may be due to its larger pore volume and more N/S atoms in the framework [20, 29]. Compared to the latter three MOFs, non-partitioned JOU-10(Fe₃) showed negligible iodine adsorption, suggesting that the MOF pore structure was important for iodine adsorption. Thereafter, we also investigated the effect of MOF metal nodes on their iodine uptakes. Initially, the monometallic variants JOU-20(Co₃), JOU-20(Mn₃), JOU-20(Ni₃), and JOU-20(Zn₃) were prepared. However, these structures were unstable under iodine adsorption conditions (Figure S16). Consequently, we introduced a second metal ion (Co²⁺, Mn²⁺, Ni²⁺, and Zn²⁺) into JOU-20(Fe₃) through a heterometallic cooperative crystallization strategy. These MOFs were labeled as JOU-20(Fe₂Mn), JOU-20(Fe₂Ni), JOU-20(Fe₂Zn), and JOU-20(Fe₂Co), respectively. As shown in Figure 2b, JOU-20(Fe₂Co) exhibits a high iodine adsorption capacity of 2.87 g/g, which is higher than that of JOU-20(Fe₃) (2.40 g/g). On the contrary, JOU-20(Fe₂Mn), JOU-20(Fe₂Ni), and JOU-20(Fe₂Zn) exhibited similar adsorption kinetics curves, reaching adsorption equilibrium after 4 h with relatively low adsorption capacities of 1.78, 1.88, and 1.87 g/g, respectively. When the Fe/Co molar ratio was increased to 1:2, JOU-20(FeCo₂) displayed an exceptional adsorption capacity of 3.08 g/g. These results indicate that the introduction of Co²⁺ ions in JOU-20(Fe₃) can significantly improve iodine adsorption capacity. This may be because the introduction of Co²⁺ significantly enhances the specific surface area and pore volume of JOU-20(Fe₃), thereby improving its iodine adsorption capacity (Table 1). The iodine uptake of JOU-20(FeCo₂) was also confirmed by thermogravimetric analysis (TGA) (Figure S17). The packing density of iodine within the pores was calculated using the literature method [41]. As shown in Table 1, the iodine adsorption capacity and pore volume of JOU-20(FeCo₂) were 3.08 g/g and 0.6567 cm³/g, respectively. Therefore, the iodine storage density in JOU-20(FeCo₂) was calculated to be 4.69 g/cm³, which is close to that of solid iodine (4.93 g/cm³). It is noteworthy that the storage density of iodine within JOU-20(FeCo₂) is comparable to that of the most efficient MOF reported, MFM-174 (Figure 2c, Table S2) [42]. Considering that there may be competitive adsorption of water molecules in the practical conditions, the adsorption of iodine by JOU-20(FeCo₂) was tested under humidity (RH 18%). As shown in Figure 2d, the adsorption capacity of JOU-20(FeCo₂) for iodine is 2.91 g/g in a humid atmosphere, which is almost the same as that in a dry atmosphere. This may be attributed to the hydrophobic skeleton of JOU-20(FeCo₂) (Figure S18), which allows it to still perform effectively in humid conditions in iodine capture applications [43, 44]. The iodine adsorption rate (K_80%) was also investigated [45–47]. K_80% is an important indicator for evaluating the iodine adsorption performance, meaning the average adsorption rate at 80% equilibrium adsorption capacity. As shown in Table S3, the K_80% value of JOU-20(FeCo₂) (0.64 g/(g·h)) surpasses those of other reported MOF adsorbents, suggesting the high efficiency of JOU-20(FeCo₂) toward I₂ adsorption. Pseudo-first-order and pseudo-second-order adsorption kinetics models were used to fit the experimental data of JOU-20(FeCo₂) (Figure S19, Table S4). The correlation coefficient R² generated by the pseudo-first-order model (0.990) is similar to that generated by the pseudo-second-order one (0.985), suggesting that iodine was adsorbed by JOU-20(FeCo₂) through both chemical and physical processes [25]. The fixation effect of MOF adsorbent on adsorbed iodine was studied by exposing I₂@JOU-20(FeCo₂) under ambient conditions. As shown in Figure 2e, unloaded iodine aggregated quickly at room temperature, completing the process within 7 days. Under the same condition, I₂@JOU-20(FeCo₂) showed a much slower iodine release rate. After 20 days, the residue loading amount of iodine within I₂@JOU-20(FeCo₂) was about 2.68 g/g, suggesting the strong fixation effect of iodine by JOU-20(FeCo₂). SEM images proved that the morphology of JOU-20(FeCo₂) remained almost unchanged after loading with iodine (Figure S20), indicating the absence of any bulk I₂ adsorbed on the MOF surface. In the cyclic experiments (Figure S21), the iodine adsorption capacity of powdered JOU-20(FeCo₂) maintained 2.40 g/g even after six adsorption and desorption cycles, suggesting its good reusability. Subsequently, I₂ vapor breakthrough experiments were also performed using a lab-scale fixed-bed reactor at 80°C. As shown in Figure 2f, in a dry atmosphere, JOU-20(FeCo₂) showed a steep breakthrough step at about 11 h, and the I₂ adsorption capacity under dynamic conditions was 0.87 g/g. In a humid atmosphere, JOU-20(FeCo₂) showed a similar breakthrough curve, but with a shorter breakthrough time of 8.5 h and lower adsorption capacity of 0.77 g/g. The I₂ adsorption capacities of JOU-20(FeCo₂) under dynamic conditions are comparable to most reported adsorbents (Table S5). Additionally, the iodine adsorption performance of JOU-20(FeCo₂) at high temperatures was also evaluated (Figure S22). By measuring the mass changes before and after adsorption, its adsorption capacity was determined to be 605 mg/g under conditions of 130°C and 18% RH. In light of the exceptional iodine adsorption capabilities of JOU-20(FeCo₂), we have further explored its practical application prospects. We employed a phase transfer method make a composite of JOU-20(FeCo₂) powder with polyethersulfone (PES) [48], creating beads approximately 3 mm in diameter (Figure 2g,h). Initially, the freshly prepared JOU-20(FeCo₂) powder was thoroughly ground and then mixed with PES and PVA in a DMF solution (for further details, please refer to the Experimental Section). The resulting slurry was then dripped into an ethanol–water mixed solvent using a micro-syringe, soaked overnight to yield MOF beads, which were vacuum-dried for later use. PXRD patterns confirmed that the structure of JOU-20(FeCo₂) remained intact after being loaded in beads (Figure S23). SEM images revealed that JOU-20(FeCo₂) particles were embedded within the beads (Figure S24). Figure 2i shows that the BET surface area of the beads gradually increased with the MOF content. The iodine loading capacity of the beads also increased with the MOF content, with 70% JOU-20@PES achieving an iodine adsorption capacity of 2.3 g/g (Figure 2j). JOU-20(FeCo₂) exhibited excellent iodine adsorption performance after being loaded in beads, demonstrating its significant potential for practical applications.

[IMAGE OMITTED. SEE PDF]

Aqueous Iodine Adsorption

The aqueous iodine adsorption performance of JOU-20(FeCo₂) was also studied. First, the adsorption kinetics in 120 ppm [I₃]⁻ aqueous solution was tested by UV-Vis spectra (Figure 3a). JOU-20(FeCo₂) showed an extremely fast adsorption rate, and the adsorption capacity reached 0.49 g/g in the first 30 s. With time, the equilibrium was reached at 240 s with an adsorption capacity of 0.69 g/g (Figure 3b). The removal efficiency of [I₃]⁻ was as high as 96.3%. Thereafter, the adsorption isotherm was tested to evaluate the saturated iodine adsorption capacity of JOU-20(FeCo₂) in water (Figure 3c). The iodine adsorption capacity of JOU-20(FeCo₂) increased with the increase of the initial concentration of [I₃]⁻ and eventually reached equilibrium. The classical Langmuir and Freundlich isotherm models were used to fit the adsorption isotherm. The R² values obtained by Langmuir and Freundlich fitting were 0.995 and 0.965 (Table S6), respectively, indicating that the aqueous iodine adsorption by JOU-20(FeCo₂) is more in line with the Langmuir isotherm and the adsorption occurs on a single homogeneous surface layer [49]. The maximum aqueous iodine adsorption capacity calculated by the Langmuir model is up to 2.30 g/g, which is comparable to those reported for the state-of-the-art adsorbents (Table S7). The selectivity of JOU-20(FeCo₂) for [I₃]⁻ adsorption in aqueous solution (60 ppm) was investigated by adding classical competitive anions, including SO₄²⁻, CO₃²⁻, Br⁻, and Cl⁻. As shown in Figure 3d, when an equal molar quantity of these competitive anions was introduced, the removal rate of [I₃]⁻ by JOU-20(FeCo₂) barely changed, indicating the high selectivity of JOU-20(FeCo₂) toward iodine adsorption. The cyclic test showed that the removal rate of [I₃]⁻ by JOU-20(FeCo₂) could still reach 95.6% after five adsorption and desorption cycles (Figure 3e). In addition, the dynamic adsorption behavior of JOU-20(FeCo₂) was also investigated. Here, we define the C/C₀ value of 0.05 as the breakthrough point [19], where C₀ and C represent the initial (120 ppm) and interval concentrations of [I₃]⁻, respectively. As shown in Figure 3f, the dynamic adsorption performance of JOU-20(FeCo₂) was much better than that of commercial activated carbon (AC). The penetration volume of JOU-20(FeCo₂) was over 250 mL/g, which is significantly larger than that of AC (<50 mL/g) under similar conditions.

[IMAGE OMITTED. SEE PDF]

Desorption of Iodine

The iodine desorption behavior of I₂@JOU-20(FeCo₂) was investigated by immersing in organic solvents or heating at 120°C. From the desorption kinetic curve in Figure 3g and Figure S25, I₂ was continuously released from I₂@JOU-20(FeCo₂) at a fast rate in the first 40 min, and the ethanol solution transformed from initially colorless to dark yellow. Figure 3h shows the iodine desorption kinetic of I₂@JOU-20(FeCo₂) at 120°C. In the first 10 h, 90% of iodine was released, and almost all iodine (96%) could be released within 20 h. These results indicate that I₂ adsorption and desorption processes by JOU-20(FeCo₂) are reversible, and the adsorbed iodine can be effectively removed by heating at 120°C or immersing in ethanol. The stability of JOU-20(FeCo₂) during iodine adsorption and desorption processes was studied using PXRD. As shown in Figure 3i, the large accumulation of iodine molecules in the pores of I₂@JOU-20(FeCo₂) resulted in diffuse reflection X-ray scattering and significantly weakened diffraction peaks of the MOF [49, 50]. After removing iodine, the diffraction peak of JOU-20(FeCo₂) could be restored. In addition, the BET surface area of JOU-20(FeCo₂) after adsorption of iodine was 1033 m²/g (Figure S26), close to that of the as-synthesized sample (1128 m²/g). These results reveal that JOU-20(FeCo₂) is stable during the iodine adsorption and desorption processes.

The Binding Domains for Adsorbed Iodine

Structural analyses reveal that JOU-20(FeCo₂) contains two types of cage-like pores, the distorted octahedron and the trigonal bipyramid cages, whose voids resemble the shape of a Chinese red drum and a rugby ball, respectively (Figure S27). For the nanoconfined distorted octahedron cage, the interconnected trigonal antiprism-shaped space with multiple binding sites from the pore walls may promote the formation of unusual polyiodide aggregates (Figure 4a). To validate this hypothesis, SCXRD method was used to resolve the binding domains for the adsorbed iodine in JOU-20(FeCo₂). The I₂-loaded crystals used for collecting SCXRD data were prepared by exposing fresh JOU-20(FeCo₂) crystals to I₂ vapor at 80°C for 60 min. Before the test, the I₂@JOU-20(FeCo₂) crystal was embedded in petroleum jelly and maintained at room temperature for several days, which allowed the adsorbed iodine to form a stable configuration. The SCXRD data for the fresh JOU-20(FeCo₂) crystal indicated the largest residual electron density peak of 1.62 (before SQUEEZE procedure) within the pore. By comparison, the I₂-loaded JOU-20(FeCo₂) revealed large residual electron density peaks, demonstrating the adsorption of I₂ within the pore. As shown in Figure 4b, three positions can be observed for the adsorbed iodine. The occupancies of the two iodine atoms located at the center of the triangle window are both 0.35 (the positions are labeled as I¹ and I²). The distance between I¹ and I² is 2.581 Å, suggesting iodine molecules can be located at these positions. The third site is located at the center of the distorted octahedron cage with a total occupancy of 0.35 (the position is labeled as I³). It should be noted that a disordered iodine atom can be found at this position, and the average distance between I¹ and I³ is 3.312 Å, indicating the formation of I⁻ anions. The short distance between I₂ and I⁻ indicates the strong guest–guest interactions between them [50], which reveals the formation of an unusual trigonal antiprismatic polyiodide anion [I₁₃]⁻. The [I₁₃]⁻ anions penetrate through the distorted octahedron cage and are stabilized by the intermolecular interactions between iodine and N atoms from the cage walls (Figure 4b). The average distance of I¹···N₂ is 3.764 Å. In addition, the fourth adsorption position is located at the corner of the trigonal bipyramid cage with an occupancy of 0.15 (I⁴) (Figure S28). Due to the presence of large vacant voids within the trigonal bipyramid cage, the distance between I²···I⁴ measures up to 4.672 Å, suggesting that iodine at the I⁴ position is isolated and encapsulated within the cage (Figure S29). Some strategies like the incorporation of functional groups and the modulation of channel shapes in MOFs have been employed as efficient design methodologies to encapsulate I₂ in unique configurations. The isolation of some polyiodide species has been successfully achieved, including [I₃]⁻, [I₄]²⁻, [I₅]⁻, [I₇]⁻, as well as triple-helical iodine chains [15, 50, 51]. In contrast, I₂-loaded JOU-20(FeCo₂) represents the first example of the crystal structure of [I₁₃]⁻ confined in a porous host. As far as we know, [I₁₃]⁻ anions are the most iodine-rich polyiodides ever discovered experimentally [35].

[IMAGE OMITTED. SEE PDF]

Iodine Adsorption Mechanism

The iodine species in MOF pores were confirmed by Raman spectroscopy (Figure 5a). Compared to the bare JOU-20(FeCo₂), three new peaks appeared in the Raman spectrum of I₂@JOU-20(FeCo₂). The peaks at 107 and 140 cm⁻¹ suggest the presence of [I₃]⁻, whereas the peak at 166 cm⁻¹ indicates the formation of polyiodide [I_n]⁻ (n >3) [52]. This may be because the guest iodine molecules interact with the skeleton of MOF to produce charge-transfer complexes. The binding sites between guest iodine molecules and the MOF skeleton were analyzed through FT-IR spectra (Figure 5b). The characteristic peak of JOU-20(FeCo₂) after iodine adsorption underwent significant changes. For instance, the vibration peaks of C═N on the triazine rings shifted from 1515 and 1373 to 1510 and 1369 cm⁻¹, respectively. The characteristic vibration peak of thiophene group shifted from 1629 to 1575 cm⁻¹ [53, 54]. These indicated effective electrostatic interactions between the adsorbed iodine and the triazine/thiophene groups [55, 56]. In addition, X-ray photoelectron spectroscopy (XPS) was used to determine the valence state of adsorbed iodine in I₂@JOU-20(FeCo₂). During XPS testing, the sample was heated at 120°C for 3 h to remove most captured iodine species. In the high-resolution XPS spectrum of I 3d, the peaks located at 620.93 and 632.38 eV indicate the presence of I₂, while the peaks located at 619.18 and 630.63 eV reveal the presence of reduced iodine species (such as [I₃]⁻) (Figure 5c) [28]. In the high-resolution XPS spectra of S 2p and N 1s, the binding energy of S/N slightly shifted toward higher binding energy after loading iodine, indicating the possible occurrence of charge transfer between S/N sites and iodine (Figure S30a,b) [31, 57]. These observations are consistent with the result of FT-IR spectroscopy. In addition, the high-resolution XPS spectrum of C 1s also shifted toward high binding energy after loading iodine, suggesting the charge transfer between iodine and conjugated ligands (Figure S30c) [22]. As shown in Figure S31, high-resolution XPS spectra of Fe 2p and Co 2p almost have no change after adsorbing iodine, suggesting no charge transfer between iodine and metal ions. Density functional theory (DFT) calculations were also performed to explore the interaction between I₂ and N/S sites of the MOF. The calculated interatomic I–I distances for iodine adsorbed on TPT, TPY, and TPB are all over 2.9 Å (Figure 5d and Figure S32), larger than the bond length in I₂ molecule, revealing that these ligands have significant affinity toward iodine. Besides, the adsorption energies for TPT, TPY, and TPB are about −1.07, −0.63, and −0.36 eV, respectively, indicating that nitrogen sites are important for iodine adsorption [58–60]. Furthermore, the calculated interatomic I–I distance adsorbed on DMTDC²⁻ is about 2.969 Å, suggesting the DMTDC²⁻ ligand with sulfur sites also has significant affinity toward iodine (Figure 4d). The adsorption energy for DMTDC²⁻ is about −0.50 eV, also indicating the strong interaction between DMTDC²⁻ unit and I₂.

[IMAGE OMITTED. SEE PDF]

Based on the above results, a possible adsorption mechanism has been proposed, taking JOU-20(FeCo₂) as an example. First, iodine molecules are adsorbed on the surface of JOU-20(FeCo₂). Then, the adsorbed iodine molecules interact strongly with electron-rich S/N atoms and conjugated ligands, ultimately forming chemical adsorption and producing charge-transfer complexes. In addition, due to the cage-shaped pore structure of JOU-20(FeCo₂), the confinement effect is enhanced, providing more space for accommodating iodine molecules and facilitating the formation of polyiodide [I₁₃]⁻ anions. This work suggests that a combination of suitable pore shape and size and multiple binding sites can provide a unique platform to capture iodine with highly efficient packing. It is feasible to tune the functional groups and pore sizes in the pacs framework based on oxygen core trinuclear clusters. Therefore, it is predictable that more effective pacs Fe-MOFs with designed pore features can be synthesized in the future for iodine adsorption.

Conclusion

In summary, an ultra-tunable MOF platform with pacs network was developed for capturing I₂. Fourteen new MOFs with high specific surface area and cage-shaped pores were successfully constructed for this investigation. It was found that the introduction of conjugated ligands with abundant N/S sites could enhance the host–guest interactions between iodine and the framework, thereby improving packing efficiency and capacity for I₂. The introduction of Fe/Co heterometallic clusters could further enhance the I₂ capture performance. The optimal material JOU-20(FeCo₂) exhibited an exceptionally high iodine adsorption capacity of 3.08 g/g with a high iodine storage density of 4.69 g/cm³. Both experimental and theoretical calculation results revealed that there was a strong interaction between adsorbed I₂ and JOU-20(FeCo₂) skeleton, leading to the formation of charge-transfer complexes and polyiodide anions. Significantly, the binding of the adsorbed I₂ in JOU-20(FeCo₂) was determined using the SCXRD technique, which represented the formation of highly unusual polyiodide [I₁₃]⁻ anions. This is the first time that the giant [I₁₃]⁻ has been structurally resolved in a crystalline host. This is because the interconnected small compartments in the pore-partitioned framework facilitate the formation of the polyhedral [I₁₃]⁻, which has the trigonal antiprismatic symmetry compatible with the confined space accommodating it. In addition, JOU-20(FeCo₂) also exhibited fast I₂ adsorption rate, high water vapor resistance, and good reusability. The above results reveal that JOU-20(FeCo₂), with the advantages of excellent adsorption performance, stability, and recyclability, can serve as a potential adsorbent for trapping I₂. This work not only establishes a platform for designing efficient MOFs for iodine capture but also deepens the understanding of iodine aggregation in confined pores. These insights offer valuable guidance for developing new materials to address challenges in sustainable nuclear energy.

Experimental Section

Details regarding materials and instruments can be found in the Supporting Information. Considering the safety of experimental operations, non-radioactive ¹²⁷I was used to replace ¹²⁹I in the whole experiment. Caution: Hydrofluoric acid is a highly toxic, irritating, and corrosive substance. Due to its hazardous nature, it is imperative to adhere to stringent safety protocols when handling this chemical. Ensure the use of appropriate personal protective equipment (PPE) and conduct all procedures in a well-ventilated fume hood to mitigate exposure risks.

[IMAGE OMITTED. SEE PDF]

Synthesis of JOU-20(Fe₃)

A mixture of FeCl₃·6H₂O (0.5 mmol, 135.15 mg), H₂DMTDC (0.39 mmol, 99.8 mg), TPT (0.19 mmol, 59.59 mg), DMF (2.5 mL), and HF (15 µL, 49 wt%) was transferred into a 10 mL polytetrafluoroethylene-lined reactor, stirred for 2 h, and then heated in an oven at 120°C for 5 days. After cooling to room temperature, the brown microcrystalline powdered product was filtered out, washed using DMF, Soxhlet-extracted using methanol for 1 day, and activated in vacuum at 80°C for 8 h. Yield: 50% (based on H₂DMTDC). FT-IR (KBr, cm⁻¹): v = 3441(m), 2386(w), 2350(m), 2394(w), 1624(w), 1582(w), 1516(w), 1378(s), 1050(w), 874(w), 807(w), 674(m). The preparation methods of other MOFs are provided in the Supporting Information.

Synthesis of JOU-20@PES Beads

First, 100 mg of PES and 50 mg of polyvinyl alcohol (PVA) were added to 1 mL of DMF and stirred until the polymer was completely dissolved. Then, a certain mass of JOU-20(FeCo₂) powder was dispersed in the DMF solution. The resulting slurry was then vertically dripped into an ethanol/water solution (ethanol:water = 1:1) using a 21 G syringe (0.8 mm diameter). The obtained beads were left in the solution for more than 30 min before being filtered out. The beads were dried overnight under vacuum at 80°C and labeled as wt%JOU-20@PES (wt% = 25%, 40%, 55%, or 70%). The content of JOU-20(FeCo₂) (wt%) in the beads was calculated using the formula m_JOU-20/(m_JOU-20 + m_PES), where m_JOU-20 represents the mass of JOU-20(FeCo₂) and m_PES represents the mass of PES.

Vapor Iodine Adsorption

Static adsorption experiments: Adsorption kinetics of iodine vapor was tested through the weighing method; 100 mg of MOF powder was placed into a weighing bottle in a capped glass container containing excessive iodine. The container was heated in an oven (80°C) and weighed at regular intervals. The adsorption capacity of iodine vapor is calculated through the formula (m_t − m₀)/m₀, where m₀ and m_t represent the mass of the sample before and after adsorption, respectively. For the iodine adsorption under humid conditions, saturated calcium chloride solution was used to maintain the relative humidity at RH 18% (80°C).

Dynamic adsorption experiments: The breakthrough experiment was conducted using a lab-scale fixed-bed reactor at 80°C. First, 30 mg of activated JOU-20(FeCo₂) was filled into a quartz column (inner diameter 6.0 mm, length 200 mm) with silane-treated glass wool filling the void space. Then, the adsorbent was purged with N₂ flow (10 mL/min) for 3 h at 80°C. Subsequently, a dry N₂ flow (10 mL/min) containing 400 ppm I₂ was used to pass through the adsorption column (I₂ flow rate: 2.21 mg/h). The exhaust gas from the adsorption column was collected with 3 M NaOH aqueous solution. The iodine concentration in NaOH aqueous solution was detected by UV-Vis spectrophotometer. To achieve the humidity condition, water was injected into the mixed gas using an injection pump (water flow rate: 0.54 µL/h). The iodine adsorption capacities under dynamic conditions were calculated by the weighting method.

Aqueous Iodine Adsorption

Static adsorption experiments: Fresh I₃⁻ aqueous solution, which was prepared by equimolar amounts of I₂ and KI, was used to study the aqueous iodine adsorption properties. The adsorption capacity of JOU-20(FeCo₂) for I₃⁻ was investigated by adsorption isotherms at room temperature. The concentration of I₃⁻ was detected by using a UV-Vis spectrophotometer. Typically, 5 mg of sample powder was added into an aqueous solution containing a certain concentration of I₃⁻ (30 mL) and the mixture stirred gently in the dark for 30 min. The adsorption capacity is calculated by using the formula: q = (C₀ − C_e) V/m, where V (L) represents the total volume of the solution, m (g) denotes the mass of the adsorbent, C₀ and C_e (mg/L) refer to the initial and equilibrium concentrations, respectively, and q (mg/g) signifies the adsorption capacity of the adsorbent for I₃⁻.

Dynamic adsorption experiments: First, 20 mg of MOF adsorbent was filled in the quartz column (inner diameter 6 mm). Then, iodine aqueous solution (I₃⁻: 120 ppm) was flowed through the fixed column (rate: 0.2 mL/min), and the I₃⁻ concentration at the outlet was detected by a UV-Vis spectrophotometer.

Iodine Release

Iodine release in solvents: The adsorbed iodine in MOF samples can be released by Soxhlet extraction with methanol or ethanol for 12 h. The iodine release process was monitored using a UV-Vis spectrophotometer. Iodine release at high temperature: The adsorbed iodine in MOF samples can also be removed by heating at 120°C for 12 h. The complete release of iodine was monitored by the weighting method. For cyclic experiments, the MOF adsorbent was regenerated by heating the I₂-loaded sample at 120°C.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (22271103), Guangdong Provincial Basic and Applied Basic Research Foundation (2024A1515012322), the Postdoctoral Fund of Lianyungang City (LYG20230016), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

CCDC , , , , , and contain the supplementary crystallographic data for JOU-10(Fe₃), JOU-18(Fe₃), JOU-20(Co₃), JOU-20(Mn₃), JOU-20(FeCo₂), and I₂@JOU-20(FeCo₂), respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via

References

L. H. Xie, Z. Y. Zheng, Q. Y. Lin, et al., “Calix[4]pyrrole‐based Crosslinked Polymer Networks for Highly Effective Iodine Adsorption From Water,” Angewandte Chemie International Edition 61 (2022): e202113724.

K. Jin, B. Lee, and J. Park, “Metal‐Organic Frameworks as a Versatile Platform for Radionuclide Management,” Coordination Chemistry Reviews 427 (2021): 213473.

Z. Zhang, X. Dong, J. Yin, et al., “Chemically Stable Guanidinium Covalent Organic Framework for the Efficient Capture of Low‐concentration Iodine at High Temperatures,” Journal of the American Chemical Society 144 (2022): 6821–6829.

M. O'Shaughnessy, J. Glover, R. Hafizi, et al., “Porous Isoreticular Non‐Metal Organic Frameworks,” Nature 630 (2024): 102–108.

Y. G. Zhang, L. W. He, T. T. Pan, et al., “Superior Iodine Uptake Capacity Enabled by an Open Metal‐sulfide Framework Composed of Three Types of Active Sites,” CCS Chemistry 5 (2023): 1540–1548.

K. W. Chapman, P. J. Chupas, and T. M. Nenoff, “Radioactive Iodine Capture in Silver‐Containing Mordenites Through Nanoscale Silver Iodide Formation,” Journal of the American Chemical Society 132 (2010): 8897–8899.

H. Furukawa, K. E. Cordova, M. O'Keeffe, and O. M. Yaghi, “The Chemistry and Applications of Metal‐organic Frameworks,” Science 341 (2013): 1230444.

X. R. Zhang, J. Maddock, T. M. Nenoff, M. A. Denecke, S. H. Yang, and M. Schröder, “Adsorption of Iodine in Metal–Organic Framework Materials,” Chemical Society Reviews 51 (2022): 3243–3262.

J. W. Wang, S. C. Fan, H. P. Li, X. H. Bu, Y. Y. Xue, and Q. G. Zhai, “De‐Linker‐Enabled Exceptional Volumetric Acetylene Storage Capacity and Benchmark C2H2/C2H4 and C2H2/CO2 Separations in Metal–Organic Frameworks,” Angewandte Chemie International Edition 62 (2023): e202217839.

W. Xie, D. Cui, S. R. Zhang, Y. H. Xu, and D. L. Jiang, “Iodine Capture In Porous Organic Polymers And Metal–Organic Frameworks Materials,” Materials Horizons 6 (2019): 1571–1595.

E. A. Dolgopolova, A. M. Rice, C. R. Martin, and N. B. Shustova, “Photochemistry and Photophysics of MOFs: Steps Towards MOF‐Based Sensing Enhancements,” Chemical Society Reviews 47 (2018): 4710–4728.

S. Mallakpour, E. Nikkhoo, and C. M. Hussain, “Application of MOF Materials as Drug Delivery Systems for Cancer Therapy and Dermal Treatment,” Coordination Chemistry Reviews 451 (2022): 214262.

J. T. Hughes, D. F. Sava, T. M. Nenoff, and A. Navrotsky, “Thermochemical Evidence for Strong Iodine Chemisorption by ZIF‐8,” Journal of the American Chemical Society 135 (2013): 16256–16259.

Y. S. Wei, M. Zhang, M. Kitta, Z. Liu, S. Horike, and Q. Xu, “A Single‐Crystal Open‐Capsule Metal–Organic Framework,” Journal of the American Chemical Society 141 (2019): 7906–7916.

Z. Yin, Q. X. Wang, and M. H. Zeng, “Iodine Release and Recovery, Influence of Polyiodide Anions on Electrical Conductivity and Nonlinear Optical Activity in an Interdigitated and Interpenetrated Bipillared‐Bilayer Metal–Organic Framework,” Journal of the American Chemical Society 134 (2012): 4857–4863.

Y. Y. Xue, X. Y. Bai, J. Zhang, et al., “Precise Pore Space Partitions Combined With High‐Density Hydrogen‐Bonding Acceptors Within Metal–Organic Frameworks for Highly Efficient Acetylene Storage and Separation,” Angewandte Chemie International Edition 60 (2021): 10122–10128.

C. Falaise, C. Volkringer, J. Facqueur, T. Bousquet, L. Gasnot, and T. Loiseau, “Capture of Iodine in Highly Stable Metal–Organic Frameworks: A Systematic Study,” Chemical Communications 49 (2013): 10320.

B. Y. Li, X. L. Dong, H. Wang, et al., “Capture of Organic Iodides From Nuclear Waste by Metal‐Organic Framework‐Based Molecular Traps,” Nature Communications 8 (2017): 485.

W. Zhou, A. M. Li, M. Zhou, Y. Y. Xu, Y. Zhang, and Q. He, “Nonporous Amorphous Superadsorbents for Highly Effective and Selective Adsorption of Iodine in Water,” Nature Communications 14 (2023): 5388.

P. H. Andrade, H. Ahouari, C. Volkringer, et al., “Electron‐donor Functional Groups, Band Gap Tailoring, and Efficient Charge Separation: Three Keys to Improve the Gaseous Iodine Uptake in MOF Materials,” ACS Applied Materials & Interfaces 15 (2023): 31032–31048.

A. Gładysiak, T. N. Nguyen, M. Spodaryk, et al., “Incarceration of Iodine in a Pyrene‐Based Metal–Organic Framework,” Chemistry ‐ A European Journal 25 (2019): 501–506.

D. Dai, J. Yang, Y. C. Zou, et al., “Macrocyclic Arenes‐Based Conjugated Macrocycle Polymers for Highly Selective CO2 Capture and Iodine Adsorption,” Angewandte Chemie International Edition 60 (2021): 8967–8975.

S. Y. Yao, W. H. Fang, Y. Y. Sun, S. T. Wang, and J. Zhang, “Mesoporous Assembly of Aluminum Molecular Rings for Iodine Capture,” Journal of the American Chemical Society 143 (2021): 2325–2330.

M. S. Zhang, J. Samanta, B. A. Atterberry, R. Staples, A. J. Rossini, and C. F. Ke, “A Crosslinked Ionic Organic Framework for Efficient Iodine and Iodide Remediation in Water,” Angewandte Chemie International Edition 61 (2022): e202214189.

Y. Z. Tang, H. L. Huang, J. Li, W. J. Xue, and C. L. Zhong, “IL‐induced Formation of Dynamic Complex Iodide Anions in IL@MOF Composites for Efficient Iodine Capture,” Journal of Materials Chemistry A 7 (2019): 18324–18329.

Y. Q. Xie, T. T. Pan, Q. Lei, et al., “Ionic Functionalization of Multivariate Covalent Organic Frameworks to Achieve an Exceptionally High Iodine‐Capture Capacity,” Angewandte Chemie International Edition 60 (2021): 22432–22440.

T. T. Pan, K. J. Yang, X. L. Dong, et al., “Strategies for High‐Temperature Methyl Iodide Capture in Azolate‐Based Metal‐Organic Frameworks,” Nature Communications 15 (2024): 2630.

B. Lee, Y. P. Chen, and J. Park, “Visualization of Iodine Chemisorption Facilitated by Aryl C–H Bond Activation,” ACS Applied Materials & Interfaces 11 (2019): 25817–25823.

L. Y. Wang, T. Li, X. T. Dong, M. B. Pang, S. T. Xiao, and W. Zhang, “Thiophene‐Based MOFs for Iodine Capture: Effect of Pore Structures and Interaction Mechanism,” Chemical Engineering Journal 425 (2021): 130578.

M. B. Kim, J. R. Yu, S. H. R. Shin, H. M. Johnson, R. K. Motkuri, and P. K. Thallapally, “Enhanced Iodine Capture Using a Postsynthetically Modified Thione–Silver Zeolitic Imidazole Framework,” ACS Applied Materials & Interfaces 15 (2023): 54702–54710.

M. L. Li, X. P. Wang, J. Zhang, Y. H. Gao, and W. Zhang, “Cu‐loaded MOF‐303 for Iodine Adsorption: The Roles of Cu Species and Pyrazole Ligands,” Applied Surface Science 619 (2023): 156819.

P. H. Svensson and L. Kloo, “Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide‐Iodine Systems,” Chemical Reviews 103 (2003): 1649–1684.

I. Turkevych, S. Kazaoui, N. A. Belich, et al., “Strategic Advantages of Reactive Polyiodide Melts for Scalable Perovskite Photovoltaics,” Nature Nanotechnology 14 (2019): 57–63.

J. Wu, Z. Lan, J. M. Lin, et al., “Electrolytes in Dye‐Sensitized Solar Cells,” Chemical Reviews 115 (2015): 2136–2173.

K. F. Tebbe and D. R. Buchem, “The Most Iodine‐Rich Polyiodide Yet: Fe3I29,” Angewandte Chemie International Edition 36 (1997): 1345–1346.

J. Hu, Z. Zhang, T. Deng, et al., “Porous Aromatic Frameworks Enabling Polyiodide Confinement Toward High Capacity and Long Lifespan Zinc–Iodine Batteries,” Advanced Materials 36 (2024): 2401091.

Y. Tang, Z. He, W. Xue, H. Huang, and G. Zhang, “Dynamic Polyiodide Anions Formation in Cationic Covalent Organic Framework for Efficient Iodine Capture,” Chemical Engineering Journal 470 (2023): 144211.

Y. S. Wei, M. Zhang, P. Q. Liao, et al., “Coordination Templated [2+2+2] Cyclotrimerization in a Porous Coordination Framework,” Nature Communications 6 (2015): 8348.

S. Y. Li, S. C. Fan, P. Zhang, W. Y. Yuan, and Y. Wang, Q. G. Zhai, “Metal‐Cluster‐Powered Ultramicropore Alliance in Pore‐Space‐Partitioned Metal‐Organic Frameworks For Benchmark One‐Step Ethylene Purification,” Chem 10 (2024): 2761–2775.

Q. G. Zhai, X. H. Bu, C. Y. Mao, et al., “An Ultra‐tunable Platform for Molecular Engineering of High‐performance Crystalline Porous Materials,” Nature Communications 7 (2016): 13645.

L. W. He, L. Chen, X. L. Dong, et al., “A Nitrogen‐rich Covalent Organic Framework for Simultaneous Dynamic Capture of Iodine and Methyl Iodide,” Chem 7 (2021): 699–714.

J. Li, X Zhang, M. Fan, et al., “Direct Observation of Enhanced Iodine Binding Within a Series of Functionalized Metal–Organic Frameworks With Exceptional Irradiation Stability,” Journal of the American Chemical Society 146 (2024): 14048–14057.

D. F. Sava, K. W. Chapman, M. A. Rodriguez, et al., “Competitive I2 Sorption by Cu‐BTC From Humid Gas Streams,” Chemistry of Materials 25 (2013): 2591–2596.

J. Yang, S. J. Hu, L. X. Cai, L. P. Zhou, and Q. F. Sun, “Counteranion‐Mediated Efficient Iodine Capture in a Hexacationic Imidazolium Organic Cage Enabled by Multiple Non‐Covalent Interactions,” Nature Communications 14 (2023): 6082.

H. B. Huo, X. D. Xiao, L. Chang, et al., “Cation‐π and Electrostatic Interactions Co‐Driven Assembly of Two‐Dimensional Heteropore Supramolecular Polymers With Rapid Iodine Capture Capability,” Science China Chemistry 66 (2023): 2070–2082.

X. H. Guo, Y. Li, M. C. Zhang, et al., “Colyliform Crystalline 2D Covalent Organic Frameworks (COFs) With Quasi‐3D Topologies for Rapid I2 Adsorption,” Angewandte Chemie International Edition 59 (2020): 22697–22705.

M. M. Jia, S. Y. Rong, P. C. Su, and W. B. Li, “Designing Functional Terminals and Vacancies Into Crystalline Porous Materials for Iodine Capture,” Chemical Engineering Journal 437 (2022): 135432.

B. Valizadeh, T. N. Nguyen, B. Smit, and K. C. Stylianou, “Porous Metal–Organic Framework@Polymer Beads for Iodine Capture and Recovery Using a Gas‐Sparged Column,” Advanced Functional Materials 28 (2018): 1801596.

Y. Q. Xie, T. T. Pan, Q. Lei, et al., “Efficient and Simultaneous Capture of Iodine and Methyl Iodide Achieved by a Covalent Organic Framework,” Nature Communications 13 (2022): 2878.

X. R. Zhang, I. D. Silva, H. G. Godfrey, et al., “Confinement of Iodine Molecules Into Triple‐Helical Chains Within Robust Metal–Organic Frameworks,” Journal of the American Chemical Society 139 (2017): 16289–16296.

G. Brunet, D. A. Safin, M. Z. Aghaji, et al., “Stepwise Crystallographic Visualization of Dynamic Guest Binding in a Nanoporous Framework,” Chemical Science 8 (2017): 3171–3177.

C. Liu, Y. C. Jin, Z. H. Yu, et al., “Transformation of Porous Organic Cages and Covalent Organic Frameworks With Efficient Iodine Vapor Capture Performance,” Journal of the American Chemical Society 144 (2022): 12390–12399.

G. B. Wang, K. Leus, H. S. Jena, et al., “A Fluorine‐Containing Hydrophobic Covalent Triazine Framework With Excellent Selective CO2 Capture Performance,” Journal of Materials Chemistry A 4 (2016): 51–58.

Y. Wang, J. Tao, S. H. Xiong, et al., “Ferrocene‐Based Porous Organic Polymers for High‐Affinity Iodine Capture,” Chemical Engineering Journal 380 (2020): 122420.

X. H. Guo, Y. Tian, M. Zhang, et al., “Mechanistic Insight Into Hydrogen‐Bond‐Controlled Crystallinity and Adsorption Property of Covalent Organic Frameworks From Flexible Building Blocks,” Chemistry of Materials 30 (2018): 2299–2308.

M. Das, S. K. Sarkar, Y. S. Patra, A. Manna, S. Mukherjee, and S. Das, “Soft Self‐Templating Approach‐Derived Covalent Triazine Framework With Bimodal Nanoporosity for Efficient Radioactive Iodine Capture for Safe Nuclear Energy,” ACS Applied Nano Materials 5 (2022): 8783–8793.

H. L. Li, D. Zhang, K. Cheng, Z. Y. Li, and P. Z. Li, “Effective Iodine Adsorption by Nitrogen‐Rich Nanoporous Covalent Organic Frameworks,” ACS Applied Nano Materials 6 (2023): 1295–1302.

D. Y. Tong, Y. L. Zhao, Z. C. Chen, T. Bo, S. W. Nie, and S. T. Xiao, “A Systematic First‐Principles Study of Volatile Iodine Adsorption Onto Zeolitic Imidazolate Frameworks,” Microporous Mesoporous Materials 317 (2021): 111017.

S. Chibani, M. Chebbi, S. Lebegue, L. Cantrelc, and M. Badawi, “Impact of the Si/Al Ratio on the Selective Capture of Iodine Compounds in Silver‐mordenite: A Periodic DFT Study,” Physical Chemistry Chemical Physics 18 (2016): 25574–25581.

S. Chibani, F. Chiter, L. Cantrel, and J. F. Paul, “Capture of Iodine Species in MIL‐53(Al), MIL‐120(Al), and HKUST‐1(Cu) Periodic DFT and Ab‐Initio Molecular Dynamics Studies,” Journal of Physical Chemistry C 121 (2017): 25283–25291.