Introduction

Volatile radioactive species present in water cooling streams from nuclear fission reactors pose a serious threat to human health and the environment. Radioactive 131I (t1/2 = 8.02 days) and 129I (t1/2 = 15.7 million years) which are common decay products both pose a significant long-term health risk due to β and γ radioactive decay1,2,3. Therefore, there is an urgent need for materials that can capture iodine from aqueous effluents. Recent advances in this field together with synthetic chemistry have led to the development of materials capable of the removal of radioactive iodine. Currently, solid-phase adsorption includes ion exchange4, forming precipitates5, and chemical bonding6, which have notable advantages due to easy handling, avoidance of secondary pollution, and high removal efficiency7. Solid crystalline materials with long-range order are useful models for elucidation of the mechanism of radioactive element capture at the molecular level and may help in the design and synthesis of advanced materials8,9,10. Some crystalline materials, such as zeolites11,12, metal-organic frameworks (MOFs)13,14, and hydrogen-bonded organic frameworks (HCOFs)15, have been applied to water-phase iodide ion capture.

Supramolecular materials and cage compounds have also illustrated the potential for iodine removal as a result of their intramolecular and intermolecular host cavities for guest inclusion16,17,18,19,20. Considering ion exchange, cationic aluminum oxo clusters are more suitable candidates for capturing iodide ions than extensively studied polyoxometalate anions21,22. However, despite their wide application in water treatment (e.g., toxic arsenate adsorption), there are no studies of iodide anion removal from water reported. This may be due to their dense Keggin-type and Brucite-like cage23,24,25 structures, the lack of suitable sites (such as conjugated groups) and appropriate cavities for absorption, and their poor crystallinity (i.e., limiting studies of the binding mechanisms from crystallographic analysis). As noted above, well-studied zeolites are stable with high adsorption and separation properties, while supramolecular cage materials have confined cavities and abundant capture sites. If the characteristics of the two can be combined (microporous zeolite channels and supramolecular macrocyclic sites), it might be possible to form a unique type of porous material for the efficient removal of iodide ions in water.

Based on the above considerations and our previous work on aluminum molecular rings26,27,28,29, we herein report the aggregation of the aluminum molecular rings into cationic core–shell Al24 Archimedean solids and their performance in removing iodide from water. The Al24 Archimedean solids consist of a pure-inorganic truncatedhexahedron (tcu) cage in the core together with six calixarene-like macrocyclic cavities. Notably, this assembly is an unprecedented combination of an inorganic zeolite-like cage with macrocyclic units. The macrocyclic cavities are situated on six faces of the tcu polyhedron, consequently, we refer to this unique class of compounds as “aluminum macrocycle-faced cages” (abbreviated as AlMCs). The AlMCs are well adapted to the guests, exhibiting key features required for iodide extraction—self-assembly, recrystallizability, and reversible ion exchange (Fig. 1). They exhibit high chemical stability, including excellent water stability, and can be prepared in large quantities. The Al24 cation units present not only exhibit a fast adsorption response but also a low capture concentration for iodine ions, indicating their potential as iodide absorbents in water. The iodide capture and aggregation processes have been revealed at the molecular level using crystal-to-crystal diffraction studies.

Fig. 1: The supramolecular assembly of the Al24 cages.
figure 1

a One-step AlMC self-assembly results in the integration of an inorganic tcu cage containing calixarene-like macrocyclic units. b Recrystallization of the AlMC results in a lattice with embedded guest molecules. c Reversible single crystal-to-single crystal ion exchange process of the AlMC.

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Results

Archimedean Al24 cage structure

All of the AlMC host–guest complexes have similar molecular units in which the host is the cation [Al24(BA)12(EtO)24(OH)32]4+ (Al24) (BA = benzoate), constructed from truncated hexahedron (tcu) made up of six octagonal Al8 faces and eight triangular Al3 faces (Fig. 2a, Supplementary Movie 1). Of all the 13 Archimedean polyhedra the tcu is the only one composed of triangles and octagons (Supplementary Figs. 1–4). In previous rarely reported tcu cages, organic ligands are used as edges, such as in [V24O24(C4O4)12(OCH3)32]8– reported by Hartl et al.30, and the Ag24L16 cage reported by Fujita et al.31. Notably, Al24 is a rare example possessing a purely inorganic tcu cage. There are four hydroxyl groups that point towards the center of each octagonal face of the Al24 unit (Supplementary Fig. 5), and one hydroxyl group on every triangular face (Supplementary Fig. 6), forming an internal inorganic cubic cavity (the available volume being ~320 Å3) (Supplementary Fig. 7). The Al24 core is capped by six calixarene-like macrocyclic fragments over each of the octagonal faces of the tcu core. These macrocyclic units are composed of 8 Al3+ cations which are bonded together using a combination of μ2-OH, benzoate, and ethoxide groups. The dimensions of the macrocyclic apertures are very similar to that found in calix[4]arene (the height, lower diameter, and upper diameter are, respectively, 4.83 × 4.97 × 19.18 Å3 vs. 4.51 × 4.48 × 16.16 Å3 for calix[4]arene) (Supplementary Fig. 8). Hence, the whole Al24 arrangement has a molecular diameter of ~ 2.0 nm (Supplementary Figs. 9–11). Compared with traditional metal-organic cages with a single cavity32,33,34, the Al24 cage, therefore, has a ‘two-tier’ cavity arrangement, which combines the characteristics of an inorganic metal cage with that of a calixarene, and possesses inorganic hydrophilic and organic hydrophobic cavities (Supplementary Movie 2).

Fig. 2: The molecular structure of the Al24 Archimedean cage.
figure 2

a Illustration of the assembly and structural model of the inner inorganic tcu core and calixarene-like organic subunits (atom color code: Al: bright green; O: red; C: gray; H: white). The size of the tcu cage is ca. 1.9 nm based on the distance between the BA ligands. b Quintuple structural symmetric anatomy of the Al24 Archimedean solid. The inner shell is an O8 cube, the second shell is an O24 truncatedoctahedral (tro) cage, the third shell is an Al24 truncatedhexahedron (tcu) cage, the fourth shell is an (OR)24 rhombicuboctahedron (rco) cage, and the outermost shell is a (BA)12 cuboctahedron (cuo) cage.

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As shown in Fig. 2b, the molecular polyhedron possesses a highly symmetrical geometry and a five-level nested structure (Supplementary Fig. 12), i.e., from the inner inorganic component to the outer organic components, O8 cube @ tro @ tcu @ rco @ cuo. For the inner inorganic fragment, eight μ3-OH groups form a centered O8 cube (with a diameter of 4.47 Å). The second O24 shell is a truncatedoctahedral (tro) shell made up of 24 μ2-OH groups (with a diameter of 8.16 Å). As far as the organic ligands are concerned, 24 ethoxide groups are situated on the edges of triangular faces constituting an (OR)24 rhombicuboctahedron (rco) subunit (with a diameter of 9.25 Å), while the benzoate ligands bridge adjacent Al3(μ3-OH) segments and assemble into a (BA)12 cuboctahedron (cuo) arrangement (with a diameter of 19.18 Å).

The peripheral macrocyclic units of the Al24 host can trap a large array of guests of different sizes, shapes, and charges, including neutral water molecules, ethanol, n-propyl alcohol, nitrate, and halide anions (Fig. 3a, Supplementary Figs. 13–19). These guest molecules form H-bonding interactions with the four OH groups of the macrocyclic units with O–H···O range 2.823–3.354 Å and O–H···X (X = Cl, Br, I) range 3.131–3.728 Å (Supplementary Figs. 20–25), which are close to those reported in the literature35,36,37,38. The six guests that are accommodated form an octahedron (sizes: 11.19 Å × 7.92 Å–13.76 Å × 9.73 Å, Supplementary Fig. 13). The depth and aperture size of these Al8 macrocyclic subunits vary depending on the guest present (depth: 4.58–5.49 Å, aperture: 16.42–18.76 Å) (Supplementary Table 1).

Fig. 3: Supramolecular lattice assemblies between the cationic Al24 host and a variety of guests.
figure 3

a The Al8 macrocyclic subunits adapt to the accommodated H2O (in AlMC-1), NO3− (in AlMC-1), I− (in AlMC-6), Br− (in AlMC-5), Cl− (in AlMC-4), OnPr− (in AlMC-2) and OEt− (in AlMC-3) guests. The blue dotted lines indicate that there are strong hydrogen bond interactions between two atoms (the details of the hydrogen bond interactions are provided in Supplementary Figs. 20–25). b Packing diagrams of AlMC-1–AlMC-8. Hydrogen-bond interactions between neighboring Al24 units are shown with yellow dotted lines. (NO3– macrocyclic cavities: pink; alcohol/alkoxide macrocyclic cavities: blue; halogen ion macrocyclic cavities: yellow). The molecular formulae of AlMC-1–AlMC-8 are, respectively: Al24·4NO3–·2HOEt·2H2O (AlMC-1), Al24·4NO3−·4HOnPr (AlMC-2), Al24·2NO3–·4HOEt·2OEt– (AlMC-3), Al24·NO3–·3Cl− (AlMC-4), Al24·NO3–·3Br– (AlMC-5), Al24·4I– (AlMC-6), Al24·HNO3·6OEt–·(Al6(BA)6(OEt)6(NO3)2)0.5 (AlMC-7), Al24·NO3–·2Br−·OEt– (AlMC-8) (Al: bright green; O: red; C: gray; H: white; N: blue; Cl: sea green; Br: purple; I: pink).

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The interaction of the Al24 cations with these guests generates a diverse range of supramolecular lattice arrangements, including monoclinic AlMC-1 (space group, C2/c) and AlMC-3 (space group, P21/n), triclinic AlMC-2 and AlMC-4 (space group, P−1), trigonal AlMC-7 (space group, R−3), and cubic AlMC-5 (space group, Ia-3), as well as AlMC-6 (space group, Im-3m) (Fig. 3b). Their supramolecular packing in the solid state correlates with the crystal morphology (Supplementary Fig. 26). Typically, the solid-state packing involves the back-to-back alignment of two Al8 macrocyclic subunits on adjacent Al24 cages which accommodate a pair of guests (Supplementary Figs. 27–32). The shape and size of this coordination pocket and the alignment of the macrocyclic units are highly dependent on the guests (Supplementary Figs. 33–38), being spindle- (Supplementary Fig. 33a), peanut- (Supplementary Fig. 33b), or Z-shaped (Supplementary Fig. 33c), and with a distance between the two macrocyclic units varying in the order H2O cavity (4.794 Å) < Cl− cavity (7.625 Å) < ethanol cavity (7.625–7.706 Å) < n-propanol cavity (7.892 Å) < Br- cavity (9.156 Å) < NO3− cavity (10.483–10.883 Å) (Supplementary Figs. 27–32). The presence of small anions as guests (Cl−, Br− and I−) not only influences the solid-state packing of the cages but these anions are also present within the center of the cage (H-bonded within the eight OH groups of the central cubic O8 unit). AlMC-1 to AlMC-5 contain disordered NO3- anions within their O8 cavities, with the number decreasing from 2 (in AlMC-1 to AlMC-3) to 1 (in AlMC-4 and AlMC-5) in the presence of halide ions (Supplementary Figs. 39–42). In AlMC-6 containing no NO3−, this site is occupied by an I− anion (Supplementary Fig. 42c). AlMC-7 contains a cationic macrocycle in its supramolecular arrangement [(Al6(BA)6(OEt)6(NO3)2)0.5]2+ which links units of Al24 together (Supplementary Figs. 43–49). The Al6-ring unit of the latter contains 6 Al3+ centers held together by 6 benzoate ligands, 6 alkoxides, and two NO3- anions. To the best of our knowledge, such a cationic {Al6} ring has not been reported previously, although similar neutral Al-macrocycles have been observed by us26,28. The Al24 cations are surrounded by six Al6-rings in AlMC-7 (Supplementary Figs. 13 and 46), thus, there are dual-Platonic octahedra present, one is formed by the six capping ethanol guests, and the other is created via six Al6 rings (Supplementary Fig. 46). The isolation of AlMC-7, containing these Al6 macrocyclic cations, provides some potential insight into the mechanism of formation of the Al24 cation itself, which potentially results from the condensation of these smaller rings during the reaction.

Supramolecular assembly can also be achieved by recrystallization from acetonitrile. An interesting stack transformation occurs on crystallization of AlMC-5 from Ia-3 (cube crystals) to P21/n in AlMC-8 (parallelogram crystals) from acetonitrile (Fig. 3, Supplementary Figs. 50–53), with the guests experiencing a slight adjustment in coordination environment. The phase purity for AlMC-1 to AlMC-8 was validated by their powder X-ray diffraction (PXRD) patterns (Supplementary Figs. 54–61). The presence of some of the anion guests can be confirmed by energy-dispersive X-ray dispersive spectroscopies (EDS) (Supplementary Figs. 62–69) and Fourier transform infrared (FT-IR) spectroscopies (Supplementary Figs. 70–77). The apparent band gaps for colorless compounds (except AlMC-6) are in the range of 4.2–4.3 eV (Supplementary Figs. 78–85). Furthermore, thermogravimetric analysis (TGA) of AlMC-1 to AlMC-7 showed that these cages remain stable up to 170–180 °C under N2 atmosphere (Supplementary Figs. 86–92).

Stability characteristics

Stability is a critical issue that needs to be considered in systems with real-world applications. All of the compounds exhibit high air stability (e.g., AlMC-1 is stable for more than half a year, Fig. 4a), thermal stability (e.g., AlMC-1 remains crystalline at 150 °C, Fig. 4a and Supplementary Fig. 93), and a high degree of chemical stability. They are stable in common low-polarity organic solvents (Supplementary Figs. 94–100), and soluble in highly polar aprotic solvents, like acetonitrile (CH3CN), dimethylformamide (DMF) and dimethylsulfoxide (DMSO) (Supplementary Table 2). The stability of the cationic Al24 unit was monitored by ESI-MS spectra (Fig. 4b, c and Supplementary Figs. 101–104, Supplementary Tables 3–8) and 1H NMR spectroscopic analysis (Supplementary Fig. 105) by dissolving single crystals in acetonitrile or DMSO39. For example, ESI-MS analysis of AlMC-5 gave a spectrum with two sets of dominant peaks assigned to [Al24(BA)12(μ3-OH)8(μ2-OH)x(OEt)48−x·NO3]3+ (x = 25–36) and [Al24(BA)12(μ3-OH)8(μ2-OH)x(OEt) 48-x·9CH3CN·NO3·Br]2+ (x = 36–48) due to the loss of ethanol (Fig. 4b)40. The other major consecutive peaks in AlMC-5 and their formulae are provided in Supplementary Table 7. In addition, the presence of the [Al6(BA)6(NO3)2(OH)11·(CH3CN)4]− ion observed in the negative-ion ESI-MS of AlMC-7 strongly supported the crystallographic results (exp: 1363.10; cal: 1363.17) (Fig. 4c and Supplementary Table 8).

Fig. 4: Stability, wettability, and porosity measurements on selected cages.
figure 4

a Structural stability of AlMC-1 under different conditions. b Solution stability of AlMC-5: experimental and simulated mass spectra in MeCN under positive mode. c Solution stability of AlMC-7: experimental and simulated mass spectra in a mixed solvent of MeCN and DMF (volume ratio: 20:1) under negative mode. d The wettabilities of AlMC-1, AlMC-4, AlMC-6, and AlMC-7. e The N2 gas sorption isotherms at 77 K for AlMC-1, AlMC-5, and AlMC-6. Source data are provided as a Source Data file.

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Water stability is an important property with respect to aqueous-phase iodide capture (see later in this paper). Water stability was evaluated by immersing as-prepared crystals of AlMC-1 in water at different pH values for 24 h and was confirmed by the unchanged PXRD pattern (Fig. 4a). In order to evaluate the long-term stability, crystals of AlMC-1 were soaked in water for 7 days at room temperature. Combined PXRD (Fig. 4a), Fourier transform infrared spectroscopy (FTIR) (Supplementary Fig. 106), and the preservation of the morphology of the transparent crystals (Supplementary Fig. 107) indicated its good water stability. In addition, the nearly identical cell parameters encourage us to collect the single-crystal X-ray diffraction data on these water-exposed crystals (Supplementary Fig. 108). We find that in the product (AlMC-1a) the complete Al24 tcu skeleton is preserved (Supplementary Fig. 109). The water stability of AlMC-1 is presumably partly related not only to the presence of hydrophobic ligands which shield the internal Al3+ ions from attack by H2O (Fig. 4d and Supplementary Fig. 110) but also to the presence of robust aluminum–oxygen bonds. Theoretical and experimental studies have proved that the presence of high valent metals, high nuclearity, and the presence of metal-oxygen bonds in metal clusters are key factors influencing water stability41,42,43,44.

The lattice void volumes for AlMC-1 to AlMC-7 are in a range of 16.9–49.0% using PLATON calculations. The N2 sorption isotherms at 77 K were also obtained for AlMC-1 to AlMC-7, and the calculated Brunauer–Emmett–Teller (BET)-specific surface areas are, respectively, 233.36, 132.87, 217.17, 182.39, 557.91, 370.47 and 161.04 m2 g−1 (Fig. 4e and Supplementary Fig. 111). The typical type-I isotherms for them indicate the existence of micropores in the crystals (Supplementary Fig. 112), suggesting they can be employed as potential adsorbents. In addition, they remain stable after the adsorption tests (Supplementary Fig. 113).

Iodine absorption

AlMC-1 was chosen as the ideal candidate for adsorption experiments since it can be prepared on a large scale (Supplementary Figs. 114 and 115) and forms well-defined rectangular crystals (Fig. 5a) of uniform size (~50 μm, passed through a 200-mesh sieve, 20 mg). An aqueous solution of I2/KI was chosen as the reaction medium because it can function as an effective source of I–, I2 and I3– based on the dynamic equilibrium I2 + I– ⇌ I3–13,45. Colorless crystals of AlMC-1 undergo a noticeable color change within 1 minute by eye when immersed in this solution (Fig. 5b, c, Supplementary Movie 3), turning black after 30 min. Single-crystal X-ray diffraction proves that the NO3− and EtOH guests in AlMC-1 have been replaced by iodide ions after 30 min (Supplementary Fig. 116). The characteristic absorptions of I3− and I2 in the UV–vis spectra46 in the aqueous solution of I2/KI decrease in intensity with time until equilibrium is reached after 8 h, with the iodine removal efficiency being up to 99% (Fig. 5c). Using the cationic Al13 cluster ([AlO4Al12(μ2-OH)12(OCH2CH2OH)12]7+) with the same weight for comparison23, AlMC-1 exhibits a faster adsorption rate and a higher removal efficiency (Fig. 5d).

Fig. 5: Iodine removal by AlMC-1 crystals in 400 ppm I2/KI aqueous.
figure 5

a Uniformly sized single-crystal adsorbents (~50 μm) ready for removing iodine from water. b Time-dependent UV–Vis spectra upon the addition of AlMC-1. c The iodine adsorption removal efficiency is based on the absorption peak at 286 nm. Inset: The color change of single crystals during iodine adsorption (1 mL, 400 ppm). d the comparison of iodine adsorption between AlMC-1 and the Al13 cluster reported in the literature23. Source data are provided as a Source Data file.

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To explore the adsorption range further, we evaluated the iodine adsorption behavior of AlMC-1 by using I2/KI solutions with different concentrations (4, 40, 400, 2000 to 100,000 ppm). In all cases, the color of the crystals exhibited a noticeable change after 48 h adsorption, and the color is deeper at this point with higher concentration (Supplementary Figs. 117–123). The iodine-loaded crystalline samples were also characterized by EDS, X-ray photoelectron spectroscopy (XPS), Raman, and PXRD. The EDS and XPS results show increases in the amount of iodine absorbed with increased concentration of the I2/KI solutions used (Supplementary Figs. 124 and 125). As can be seen from the expanded XPS spectra (Fig. 6a), the two peaks for the I3d transition move to the higher binding energy with increased iodine loading, indicating an enhanced degree of aggregation47,48. The intensities of the Raman signals also increase with the concentration of the I2/KI solution used (Fig. 6b). The band of 110 cm−1 can be attributed to the symmetric stretching vibration of I3− 49, the peak of 150 cm−1 belongs to the asymmetric stretching vibration of I3−, and the peak at 220 cm−1 is attributed to the I–I stretching vibration. The latter is different from “free” I2 dissolved in a nonpolar solvent (~211 cm−1)50, indicating that the confined I2 molecules in AlMC-1 have strong interactions with the host. In addition, with increased iodine loading, new diffraction peaks in the PXRD can be observed, indicating that there is a significant host-guest interaction between iodine species and the host lattice (Fig. 6c)15.

Fig. 6: Characterization and mechanism of the iodine capture processes of AlMC-1 in 4–100,000 ppm I2/KI aqueous solutions.
figure 6

Macroscopic characterization of AlMC-1 after immersing in 4–100,000 ppm I2/KI aqueous solutions, including a expanded XPS spectra, b Raman spectra, and c PXRD patterns. d Mechanistic studies at the molecular level (top row: adsorption site; bottom row: corresponding supramolecular interactions between iodine species and Al24 cage hosts; site 1 intermolecular channel; site 2 square window of O24-tro cage; site 3 Al8 macrocyclic cavity; site 4 the O8 cavity of Al24 cages). The strong hydrogen-bond interactions (C–H···I and O–H···I) are represented by yellow dotted bonds, while the I···π interactions are expressed by blue dotted bonds (Al: bright green; O: red; C: gray; H: white; N: blue; Cl: sea green; Br: purple; I: pink). Source data are provided as a Source Data file.

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In order to understand the capture process on a microscopic level, we carried out mechanistic studies using single-crystal X-ray diffraction on crystals obtained from the above iodine absorption experiments at different iodine concentrations. The results from this detailed analysis provide ‘snapshots’ of the entry of iodine species into the host lattice and Al24 unit as the concentration is increased and are shown in Fig. 6d. Porous AlMC-1 provides several sites for iodine incorporation, with the adsorption sites in the crystal lattice being gradually occupied with increased concentration from intermolecular to intramolecular. The iodine guests first enter the hydrophobic intermolecular channels (site 1, 4–400 ppm, C-H···I interactions: 2.317–4.067 Å and I···π interactions: 3.279–4.575 Å) and the intermolecular μ2-OH windows (site 2, 4 ppm, O–H···I: 2.887–3.196 Å) (Supplementary Figs. 126–129), then into the Al8 macrocyclic units of the Al24 units (site 3, 2000 ppm, C-H···I interactions: 2.820–3.387 Å and I···π interactions: 4.799–4.885 Å) (Supplementary Fig. 130), and finally occupying the interior cavity of the Al24 units (site 4, 100,000 ppm, O-H···I interactions: 2.928–3.113 Å) (Supplementary Fig. 131). The exact nature of all of the iodine species present in the lattice at every stage cannot be deduced unambiguously from the X-ray data due to the disordering of the iodine sites. At 4 ppm I2/KI(aq), the intermolecular channels can capture I3− ions (Supplementary Fig. 126b), and the occupancy of these increases with concentration (40 ppm) (Supplementary Fig. 127a). Then iodine atoms continue to accumulate at site 1 until they reach saturation (Supplementary Fig. 128). Subsequently, I2 molecules and disordered iodine species appear at site 3 (2000 ppm) (Supplementary Fig. 130e) and site 4 (100,000 ppm) (Supplementary Fig. 131e). It can be noted that the simulated PXRD patterns are consistent with experimental patterns for the iodine-loaded samples, indicating the evolution of the iodine species proposed analysis from SCXRD is rational (Supplementary Figs. 132–136).

Such a low-concentration iodine/iodide capture ability suggests that AlMC compounds of the type described in this paper may be of value in the capture of environmental iodide/iodine (e.g., waste-water streams). We evaluated the adsorption capacity of AlMC-1 using a 100,000 ppm I2/KI aqueous solution based on the results of single-crystal analysis, which is about 820.3 mg/g. This value is slightly lower than that obtained from gravimetric (1.03 g/g) and titrimetric analysis (890 mg I2/g)13,15,51, which may be due to the disorder of the iodine species in the lattice and difficulty in determining the precise composition by X-ray single-crystal analysis. Even so, the capacity of AlMC-1 is still higher than that of MOFs (Supplementary Table 9)52,53 and noble metal-based adsorbents54,55,56. The iodine-loaded crystals of AlMC-1 can be reused as iodine sponges after the removal of the iodine components by washing them with HOEt (Supplementary Fig. 137). PXRDs of AlMC-1 after 200 kGy β or γ irradiation also indicate no structural degradation (Supplementary Fig. 138a), and the retention of the adsorption capacity compared to non-irradiated samples (Supplementary Fig. 138b). This is important in regard to the potential applications of these adsorbents in the removal of radioactive iodine.

In this paper, we explored the structural landscape of a series of unique solid-state materials based on a cationic Al24 Archimedean host. The highly symmetrical Al24 cage has a purely inorganic tcu kernel and six calixarene-like Al8 shells. The core–shell arrangement is highly adaptive toward a variety of guests (NO3–, OEt–, OnPr–, Cl–, Br–, and I–), generating a broad range of supramolecular lattice arrangements in the solid-state. The unique structural features (simultaneously containing hydrophobic outer channels and a hydrophilic inner cavity) make these materials highly stable in water. Iodine/iodide capture experiments have revealed rapid enrichment, low-concentration capture, high adsorption capacity, recyclability, and radiation-resistant characteristics for AlMC-1, indicating its potential applications in trace iodine extraction in waste-water streams (such as in the radiation industry). These host assemblies provide alternatives to organic hosts such as calixarenes, crown ethers, and pillararenes, for water purification.

Methods

Syntheses of AlMC compounds

AlMC-1 to AlMC-7 were synthesized by mixed Al(OiPr)3 (6 mmol), benzoic acid (3 mmol), HNO3 (60–100 μL) in 8–10 mL alcohols solvent (HOEt or HOnPr). The trace of HNO3 plays an important role in the formation of the Al24 cages. Its use should be controlled between 60 and 100 μL in this reaction. Besides, the addition of extra guests greatly affects the supramolecular assembly of Al24 cages, such as H2O, benzyl alcohol, quaternary ammonium salt and pyrazole. Notably, water helps to increase the yield, for example, it increased from ~11% to ~43% when 60 μL H2O is introduced in the reaction system of AlMC-1. See Supplementary Methods for more details on the synthesis of all of the compounds described in this paper.

Scale-up synthesis of AlMC-1

A mixture of Al(OiPr)3 (6.0 g), benzoic acid (1.8 g), HNO3 (0.5 mL), H2O (300 μL), and ethanol (40 mL) was sealed in an 80 mL vial and heat at 80 °C for 7 days. When cooled to room temperature, the white precipitate and colorless crystals are washed by ethanol repeatedly. After drying, the precipitate and crystals are passed through a 200-mesh sieve, and pure-phase rectangular crystals are obtained. (Yield: ~817 mg, ~16% based on Al(OiPr)3).

Iodine adsorption

Crystals with moderate size were selected for iodine adsorption research. Large crystals tend to fracture during adsorption, while the X-ray diffraction intensity for tiny-size crystals is very weak. Thus, we choose ~15 μm × 50 μm crystals for iodine adsorption measurements. A 20 mg sample was immersed in I2/KI aqueous (10 mL) with various concentrations for 48 h at room temperature. The iodine-loaded samples obtained were filtered and washed prior to characterizations (XPS, EDS, Raman, and PXRD) and SCXRD. To measure the adsorption capacity, AlMC-1 crystalline samples (50 mg) were soaked in 100,000 ppm I2/KI(aq) (300 mg KI and 300 mg I2 in 3 mL H2O) for 48 h. The iodine-saturated samples were collected by filtration, washing with water (2 mL × 30 times) until the filtrate became clear, and dried in air for gravimetric analysis and characterization. The filtrate was collected and 2 mL 2% aqueous starch indicator was added for sodium bisulfite titration analysis.

Regeneration and recycling experiment

Iodine-loaded samples were immersed in HOEt (2 mL) for desorption of the iodine/iodide, during which the solvent was decanted and washing repeatedly several times. The desorption was observed to take place rapidly. Once HOEt is added, the color of the solution turned yellow immediately and then gradually deepened. This process can be investigated using time-dependent UV–Vis spectra.

Irradiation stability measurements

AlMC-1 (100 mg) was irradiated at a dose rate of 20 kGy/h for 10 h using a 60Co β- or γ-irradiation source. β-Irradiation was provided by an electron accelerator located at the CGD Dasheng Electron Accelerator Co., Ltd., in Jiangsu Province, China. While γ-irradiation was conducted by Gansu Tianchen Irradiation Technology Co., Ltd., in Gansu Province, China.

Caution

HNO3 is corrosive and has a pungent odor. Thus, the experiments should be carried out in a fume hood, and gloves and masks should be worn.