Nuclear energy, a clean and low carbon-emitting power resource, is an efficient and reliable alternative source to fossil energy if the challenge of nuclear waste pollution can be tackled efficaciously1,2. Appropriate nuclear waste management has received considerable attention especially after the explosion of the Fukushima nuclear power plant in 2011. Volatile and radioactive iodine species (e.g., 129I and 131I) possess exceptional issues, mainly because 129I has a very long half-life (~107 years) and 131I has direct effects on human metabolic processes despite its short half-life (8.02 days). Therefore, efficient capture and reliable storage of radioactive I2 is of great significance. To realize the effective I2 capture, great efforts have been devoted to developing various physico- and chemo- absorbents/adsorbents. Metal–organic frameworks3,4,5, microporous polymer6,7,8, charged porous aromatic frameworks9, hydrogen-bonded cross-linked organic framework10, nonporous pillar[6]arene crystal11, carbon-based materials12,13, deep eutectic solvents14, and ionic liquids (ILs)15,16,17, have been reported for the efficient capture of I2 via physical interaction. However, these materials usually suffer from low I2 uptake and unstable I2 storage. Ag2O@NFC aerogels18, silver-containing mordenites19, alkene/alkyne perovskites20, alkali-TCNQ salts21, and functionalized Mg-Al layered double hydroxide22 have been reported as chemical adsorbents, whose capture efficiency are considerably dependent on their activity to react with I2, generally with shortcomings such as slow reaction rate and low capture capacity. Consequently, although such progress has been made in I2 capture, the absorbents/adsorbents capable of fast capturing and reliably storing I2 is still highly desirable.

ILs, composed entirely of organic cations and inorganic/organic anions, can be designed with specific and tunable chemical reactivity via careful selection of component ions, and have been widely applied in gas capture and chemical wastes management, showing promising potentials23,24,25. For example, 1-butyl-3-methylimidazolium tetrafluoroborate/water system has been demonstrated to be a tunable media for sustainable waste management26. Heterocyclic anions-based ILs, including azolate and pyridinolate ILs, have been reported as effient sorbents for CO2, SO2, or NO due to the formation of complexes between the ILs and the gases27,28,29,30,31,32. Especially, the azolate and pyridinolate anions of the ILs have been found to be capable of chemically capturing CO2 efficiently27,28,29. ILs also have been applied in I2 capture and 1-butyl-3-methylimidazolium bromide was reported to show an I2 capture capacity of 1.9 mol I2 per molar IL15.

Here we report a series of imidazolate ILs (Im-ILs) for fast capture and reliable storage of I2. These Im-ILs are capable of chemically capturing I2 rapidly via the reaction of the Im anions with I2, forming I-substituted imidazoles together with new ILs with [I]- anion, and the in situ-generated ILs can absorb I2 to form polyiodide species, resulting in very high I2 uptake capacity. For example, choline  imidazolate ([Ch][Im]) as the initial absorbent shows increased I2 capture capacities with temperature in the range of 30–100 °C, giving the capacities of 8.7 and 17.5 g of I2 per gram IL at 30 and 100 °C, respectively, which is an improvement over typical state-of-the-art capacities of 0.5–4.3 g/g. More importantly, the captured I2 can be stably stored in the absorbent system even at 100 °C. It is suggested that the Im-ILs reported here are highly efficient not only for fast capture of I2 with high capacity, but also for the safe storage of this volatile compound.

Results

Synthesis and characterization of Im-ILs

The Im-ILs shown in Fig. 1a were synthesized via the neutralization of bases (e.g, choline) with weak proton donors including imidazole (Im), methyl-substituted Im, as detailed in the Methods. The 1H and 13C nuclear magnetic resonance (NMR) spectra confirmed the formation of these Im-ILs (Supplementary Fig. 1), and  thermogravimetric analysis (TGA) showed that they are thermally stable from 131 °C to 223 °C (Supplementary Fig. 2). These Im-ILs are viscous liquids at room temperature (Supplementary Table 1). Choline iodide ([Ch][I]) and imidazolium iodide ([ImH][I]) were synthesized via the reactions of HI with choline and imidazole in aqueous solutions, and they have the melting points of 270 °C and 204 °C, respectively.

Fig. 1
figure 1

Structure and function of Im-ILs. Chemical structures of Im-ILs (a) and possible pathway of I2 capture by R+[Im]− (b). R+[Im]− could capture I2 chemically, forming I-containing compounds together with new ILs with [I]− anion, meanwhile the new ILs could absorb I2 to form polyiodide species

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Extraction of I2 from cyclohexane solution by Im-ILs

The resultant Im-ILs were first used to extract I2 from cyclohexane solutions at 30 °C. It was found that all Im-ILs could rapidly and efficiently remove I2 from the cyclohexane solutions. As illustrated in Fig. 2, 51 mg of [Ch][Im] (0.3 mmol) almost removed I2 completely from the cyclohexane solution (0.01 M, 50 mL, 126.9 mg of I2) within 15 min, reaching extraction equilibrium with an I2 uptake of 3.1 mol I2 per molar IL corresponding to an I2 uptake of 4.6 g I2 per g IL (Fig. 2c). Moreover, it was found that [Ch][Im] could also rapidly and efficiently remove I2 from other solvents, for instance, n-hexane and n-heptane, affording lower I2 uptakes at extraction equilibrium of 4.2 and 2.7 g/g, respectively.

Fig. 2
figure 2

Extraction of I2 from cyclohexane solution by [Ch][Im]. a Time-dependent UV-visible spectra of the I2 cyclohexane solutions (50 ml, 0.01 M,  126.9 mg of I2) extracted by [Ch][Im] (0.3 mmol, 51 mg). b Time-dependent removal efficiency of I2. c Time-dependent I2 uptake from cyclohexane solution (100 ml, 0.01 M) by [Ch][Im] (0.3 mmol)

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To reveal the interaction mechanism of extraction I2 by [Ch][Im], the absorbent solution obtained at the extraction equilibrium was examined by 1H and 13C NMR spectroscopy. Compared with those of [Ch][Im], the 1H and 13C NMR spectra of the absorbent solution changed significantly (Fig. 3a, b and Supplementary Fig. 3). In contrast, the signals at 141.67 and 124.22 ppm. assigned to the carbons in the [Im]− anion of [Ch][Im] disappeared and two new peaks appeared at 134.21 and 119.18 ppm., which were attributed to the carbons in [ImH]+ cation according to the 1H and 13C NMR spectra of [ImH][I] (Supplementary Fig. 4). As shown in Fig. 3b, a new signal appeared at 89.68 ppm, which was ascribed to the carbon of C–I bond from 2,4,5-triiodoimidazole based on the 13C NMR spectrum of 2,4,5-triiodoimidazole (Supplementary Fig. 5). This is also confirmed by the heteronuclear singular quantum correlation (HSQC) spectrum (Supplementary Fig. 6). The heavy atom effect was responsible for the signal of I-connected carbons shifting upfield. Diffusion ordered spectroscopy (DOSY) analysis (Supplementary Fig. 7) was performed to further confirm the assignment of the carbon peaks. The diffusion sequences of the signals at 134.21 and 119.18 ppm. were similar, whereas that at 89.68 ppm. was significantly different (Supplementary Fig. 7). This indicates that the signals at 134.21 and 119.18 ppm. belonged to the same compound and the signal at 89.68 ppm. was attributed to another one, which agreed well with the above analysis. Notably, it is accepted that the carbon chemical shifts of C2 and C4/5 in 2,4,5-triiodoimidazole should be different. This is true if the NH proton is not changing position between two N atoms in 2,4,5-triiodoimidazole, whereas an exchange usually occurs in solution and on the NMR time scale, thus resulting in one peak. In addition, the signals of the carbons in [Ch]+ cation only shifted slightly. The above NMR results indicate that the [Im]− anion was so active that it chemically captured I2 to form new species including [ImH]+ and 2,4,5-triiodoimidazole, whereas the [Ch]+ cation kept unchanged. From the 1H NMR spectra of [Ch][Im] before and after capturing I2 to reach equilibrium (Supplementary Fig. 3), it was found that the amount of H atoms in the Im ring of the absorbent solution declined to half value compared to that of [Ch][Im], implying that the [Im]− anions of [Ch][Im] were converted equally into [ImH]+ cations and 2,4,5-triiodoimidazole molecules. From the above analysis, it can be deduced that during the I2 extraction process the C–H bonds in [Im]− were first iodinated to form C–I bonds accompanied with production of HI and the resultant HI subsequently induced a series of reactions to form 2,4,5-triiodoimidazole, [Ch][I], and [ImH][I]. That is, I2 was chemically captured and stored in these I-containing compounds.

Fig. 3
figure 3

13C NMR spectra of the absorbent systems. a–d 13C NMR spectra of [Ch][Im]-based absorbent systems before and after extracting I2 from cyclohexane solution at different time (a = 0 min, b = 30 min, c = 5 min, d = 1 min), [D6][DMSO], 0.6 mL, 298 K

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To further understand the chemical reactions in the I2 extraction process, the absorbent solutions obtained at different extracting time were examined by one- and two-dimensional NMR analysis. In the spectrum of the absorbent solution obtained at 1 min, the signals at 141.67 and 124.22 ppm. assigned to C2 and C4@C5 in the [Im]− anion disappeared, accompanied with the appearance of some new signals, whereas the signals assigned to the carbons 6, 7, and 8 in [Ch]+ cation shifted slightly. These results confirm that the [Im]− anion could rapidly and efficiently capture I2 via chemical reactions, resulting in formation of new species. According to the HSQC, 1H detected heteronuclear multiple bond correlation (HMBC) and 13C NMR analysis (Supplementary Figs. 8, 9), the new signals at 79.34, 125.33, and 140.05 ppm. were assigned to the carbons in [4-iodo-Im]−, whereas the signals at 121.69 and 135.19 ppm. were ascribed to the carbons in Im molecule as remarked in Fig. 3d. These findings indicate that the [Im]− anion was iodinated to form [4-iodo-Im]− preferientially together with HI and HI was further trapped rapidly by the unreacted [Im]− anions to form Im molecule and [Ch][I], as described in Fig. 4a, b. The disappearance of signals ascribing to the [Im]− anions also indicate that both reactions of [Im]− with I2 to form [4-iodo-Im]− and with HI to form Im occurred rapidly, and half of [Im]− anions involved in the reaction with I2, followed by the reaction of the other half with HI, identical to the results obtained from the 1H NMR spectrum of the absorbent solution at extraction equilibrium (Supplementary Figs. 3, 10). In the 13C NMR spectrum of this absorbent solution, the signals at 88.43 and 148.42 ppm. were ascribed to C4/C5 binding with I and C2 in [4,5-diiodo-Im]−, respectively, whereas the signals at 96.64 and 90.26 ppm. belonged to the carbons in [2,4,5-triiodo-Im]− anion. These results suggest that the C–H bonds in [4-iodo-Im]− and [4,5-diiodo-Im]− anions further reacted with I2 quickly to form [2,4,5-triiodo-Im]−, as shown in Fig. 4c, d. In the spectrum of the absorbent solution obtained at 5 min, the signals to the carbons of [4-iodo-Im]− and [4,5-diiodo-Im]− disappeared, whereas the signals to C2 and C4/5 in the iodinated imidazolium ring enhanced and became closer. This further indicates that not only the reactions shown in Fig. 4c, d took place but also that in Fig. 4e occured, based on the 13C NMR spectra of the mixture of 2,4,5-triiodoimidazole and [Ch][2,4,5-triiodo-Im] with different molar ratio (Supplementary Fig. 11). In the 13C NMR spectrum of the absorbent solution at extraction equilibrium (Fig. 3b), the peaks at 134.21 and 119.18 ppm. were ascribed to the carbons in [ImH]+, suggesting the formation of [ImH][I] from the reaction of Im with HI (Fig. 4f).

Fig. 4
figure 4

Possible reactions in the I2 extraction process by [Ch][Im]. a Generation of [Ch][4-iodo-Im] and HI from [Ch][Im] and I2. b Reaction between [Ch][Im] and the generated HI. c, d Iodination reactions of [Ch][4-iodo-Im] and [Ch][4,5-diiodo-Im], respectively. e Formation of 2,4,5-triiodo-Im from [Ch][2,4,5-triiodo-Im] and HI. f Trap of HI by Im to form [ImH][I]

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To further verify the reactions illustrated in Fig. 4, the control reaction of HI (2 mmol) with [Ch][Im] (1 mmol) was conducted in aqueous solution at room temperature. Indeed, the reaction occurred rapidly and [Ch][I] was obtained together with [ImH][I], as confirmed by the NMR analysis (Supplementary Figs. 4 and 12). These results indicate that during the I2 extraction process from cyclohexane solution, the generated HI could be trapped by [Ch][Im] to form [Ch][I] and [ImH][I]. Notably, 2,4,5-triiodoimidazole could be isolated from the absorbent solution, verified by the NMR analysis (Supplementary Figs. 5, 13), which confirmed the reaction shown in Fig. 4e.

Based on the mass balance and the equations shown in Fig. 4, the molar ratio of [Ch][I]:[Im][I]:2,4,5-triiodoimidazole should be 1:0.5:0.5 at the reaction equilibrium, which means that 1.5 mol of I2 can be theoretically captured by per molar [Ch][Im]. In fact, the I2 uptake by [Ch][Im] reached 3.1 mol/mol (Table 1). This suggests that the mixture of [Ch][I] + [Im][I] + 2,4,5-triiodoimidazole contributed to the extraction of I2 from the cyclohexane solution as well.

Table 1 I2 uptakes of Im-ILs at different temperatures
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Encouraged by the above results, other Im-ILs shown in Fig. 1a were also used to extract I2 from cyclohexane solutions at 30 °C and the results are included in Table 1. Obviously, these Im-ILs could efficiently extract I2 with high efficiency and the I2 uptake of each Im-IL was almost the same. These results imply that the anions of these Im-ILs might behave similarly in extracting I2. NMR analyses (Supplementary Fig. 14) indicate that besides all the Csp2-H bonds in the Im ring, one Csp3-H bond in each –CH3 of [2-MIm]−, [4-MIm]−, and [2,4-DMIm]− anions was iodinated to form –CH2I, suggesting that the –CH3 group in these anions was active to attract I2. That is, similar to [Im]−, each anion, [2-MIm]−, [4-MIm]−, and [2,4-DMIm]−, has three C–H bonds that can react with I2, resulting in the chemical capture of I2. Similarly, the generated HI was also found to induce a series of reactions to form corresponding I-substituted imidazoles and ILs with [I]− anions, which were illustrated in Supplementary Fig. 15.

DFT calculation

To understand the reactivity of the anions of these Im-ILs, we calculated the Hirshfeld charges of carbons in the Im ring of imidazole, [Im]− and [ImH]+, and the results are shown in Supplementary Fig. 16. In general, the carbon site possessing more negative Hirshfeld charge has stronger ability to attract electrophiles and is thus more possible to be the reactive site33,34. The calculation results indicate that the Hirshfeld charges of the carbon atoms in [Im]− become more negative compared with those in imidazole, whereas those in [ImH]+ are more positive. Similarly, the Hirshfeld charges of the carbons, especially the methyl carbons, in the [methyl-substituted-Im]− anion are also negative (Supplementary Fig. 16). These results imply that both Csp2-H and Csp3-H in the anions have the capability to react with I2. Therefore, it can be deduced that Im anion-directed electron redistribution is favorable to promoting C–H bond activation, thus providing multiple-sites to capture I2 chemically with different reactivity.

I2 vapor capture by [Im]-ILs

As I2 capture is generally performed in gas atmosphere, the resultant Im-ILs were applied in capturing I2 vapor and the results are shown in Table 1. Obviously, each Im-IL could capture I2 with a very high capacity, much higher than those of various absorbents or adsorbents reported in literature (Supplementary Table 2)6,9,14,15. For example, using [Ch][Im] as the absorbent, its I2 capture capacity increased with temperature in the range of 30–100 °C, reaching the highest value of 11.8 mol/mol (i.e., 17.5 g/g) at 100 °C. Further increasing temperature caused decline in I2 capture capacity, decreasing to 9.4 mol/mol at 110 °C (Fig. 5a).

Fig. 5
figure 5

Influences of temperature and vapor pressure on I2 uptakes. Dependence of I2 uptakes of [Ch][Im] on temperature (a) and on the vapor pressures of I2 (b)

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As described above, the I2 capture capacity of [Ch][Im] includes two parts: the chemical capture capacity of 1.5 mol I2 per molar IL and the I2 uptake by the mixture of [Ch][I] + [ImH][I] + 2,4,5-triiodoimidazole. The latter is actually the I2 solubility in the mixture. In general, the solubility of a solute in a solvent is related to its vapor pressure. Calculating the vapor pressures of I2 at different temperatures in the range of 30–100 °C (Supplementary Table 3 and Supplementary Fig. 17), the dependence of the I2 uptakes of [Ch][Im] on its vapor pressures is plotted in Fig. 5b. Obviously, the I2 uptakes of [Ch][Im] increased with the I2 vapor pressures, suggesting that higher vapor pressure, i.e., higher temperature, is favorable to the I2 absorption by the mixture of [Ch][I], [ImH][I], and 2,4,5-triiodoimidazole. However, further increasing temperature to 110 °C, the I2 uptake of [Ch][Im] reduced, which may be ascribed to the weaker interactions among I2 and the absorbent molecules at higher temperature.

I2 storage reliability

Besides rapid and efficient I2 absorption, it is also very important for volatile I2 to be stably stored in the absorbent system for a long time. To examine the I2 storage reliability in the [Ch][Im]-based absorbent system, TGA analysis with N2 sweeping was performed on sample A and sample B as shown in Fig. 6. Clearly, the mass losses of these samples were hardly observed at 30 °C under a N2 sweeping flow rate of 40 mL/min for 10 h, whereas pure I2 powder showed ~30 wt% mass loss under the same condition. At 100 °C, only < 5% of mass losses for sample A and sample B were observed after N2 sweeping with a flow rate of 5 ml/min for 10 h, whereas the same amount of powdered I2 almost completely evaporated within 50 min. These results indicate that even at high temperature (e.g., 100 °C), the absorbent system still has relatively reliable capability to store I2. Compared with the reported I2 absorbents in literature14,15, this IL absorbent system exhibits much better performance for I2 storage.

Fig. 6
figure 6

TGA curves of [Ch][Im] after absorbing I2. Sample A presents the IL absorbing 3.1 mol/mol of I2 from cyclohexane solution and sample B presents the IL absorbing 5.9 mol/mol of I2 vapor; N2 sweeping rates: 40 mL/min at 30 °C, 5 mL/min at 100 °C

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Melting points of [Ch][Im]-based absorbent systems

[Ch][I], [ImH][I], and 2,4,5-triiodoimidazole are solids at room temperature with melting points at 263, 204, and 189 °C, respectively. In the process of I2 absorption by [Ch][Im], the mixture of [Ch][I], [ImH][I], and 2,4,5-triiodoimidazole was formed, but they displayed liquid state after capturing I2 at room temperature. This indicates that the absorbed I2 considerably decreased the melting point of the [Ch][I] + [ImH][I] + 2,4,5-triiodoimidazole mixture. To explore the influence of I2 on the melting points of [Ch][I] and [ImH][I], we determined the phase diagrams of [ImH][I]-I2 and [Ch][I]-I2 binaries. It was found that [Ch][I] and [ImH][I] could form low-temperature eutectic salts with I2, significantly decreasing the melting points of [Ch][I]-I2 and [ImH][I]-I2 binaries (Supplementary Figs. 18, 19). Moreover, the mixture of [Ch][I], [ImH][I], and 2,4,5-triiodoimidazole with a molar ratio of 1:0.5:0.5 was found to have a melting point of 78 °C, much lower than that of each compound, and the melting point further decreased after capturing I2. Therefore, the above findings suggest that although [Ch][I] and [ImH][I] are solids with high melting points, they are highly efficient absorbents for I2, because they can form low-temperature eutectic salts with I2.

Analysis on iodide species

In contrast, in the case of extracting I2 from cyclohexane solution the I2 capture capacity of [Ch][Im] reached 3.1 mol/mol at the extraction equilibrium at 30 °C and it could not increase further even if enough amount of I2 was present in the cyclohexane solution, whereas it reached 5.9 mol/mol in the case of capturing I2 vapor. These results suggest that iodide species may exist in different forms in the IL solutions in these two cases. To reveal the forms of the iodide species in the IL solutions, we conducted the electrospray ionization mass spectrometry (ESI-MS) in negetive mode on the IL solutions (Supplementary Figs. 20, 21). It was observed that the iodide species mainly exsited as 2,4,5-triiodoimidazole, I− and I3− in the extraction solution, whereas besides these species polyiodide species (including I5−, I7−) combined with IL cations were present in the IL solution absorbing I2 vapor, identical to that reported in literature35. The ESI-MS results enclose why the absorption capacity of the IL capturing I2 vapor was much higher than that extracting I2 from cyclohexane solution.

Discussion

A series of Im-ILs were designed, which were found to be capable of rapidly and efficiently capturing I2 via the reactions of the Im anions with I2 and the formation of polyiodide species (Fig. 1b), showing high I2 capture capacity. Moreover, the Im-ILs systems were safe materials for the storage of I2, and they could be tolerant to high temperature (e.g., 100 °C). This work presents green absorbent systems to capture I2 rapidly and efficiently, which have promising potential applications in capturing radioactive I2 from the nuclear waste with stable storage.

Methods

Materials

Iodine, choline chloride, imidazole, 2-methylimidazole, 4-methylimidazole, 2,4-dimethylimidazole, tetrabutylammonium bromide, and ion exchange resin (Ambersep∣r 900(OH)) were obtained from Beijing Innochem Science & Technology Co., Ltd. and J&K Scientific Ltd., respectively. All chemicals were used without further purification.

General procedure for the synthesis of Im-ILs

The ILs as depicted in Fig. 1a, including [Ch][Im], [Ch][2-MIm], [Ch][4-MIm], and [Ch][2,4-DMIm]] were prepared by neutralizing [Ch][OH] with corresponding weak proton donors. Typically, for the synthesis of [Ch][Im], an ethanol solution of [Ch][OH] was first obtained from [Ch][Cl] via anion-exchange resin. To the ethanol solution of [Ch][OH] equimolar imidazole was added and the mixture was then stirred at room temperature for 24 h. Subsequently, ethanol and generated water were distilled off at 70 °C under reduced pressure. The obtained [Ch][Im] was dried in vacuum for 24 h at 70 °C to remove trace amount of water. The water content of these ILs was determined with a Karl Fisher titration and found to be < 0.1 wt%.

Typical procedure for I2 extraction from cyclohexane solution

In a typical experiment, [Ch][Im] (51 mg, 0.3 mmol) was added into a cyclohexane solution containing I2 (0.01 M, 100 ml) in 250 mL flask equipped with a magnetic stirrer at r.p.m. of 400 r/min. The I2 quantitative analysis at different extraction time was conducted by UV/Vis spectroscopy analysis. The intensity of absorption peaks at λ = 523 nm is proportional to the quantity of I2.

I2 vapor capture

Typically, [Ch][Im] (26 mg, 0.15 mmol) loaded in a pre-weighed culture dish (5 cm in diameter) was exposed to an I2 vapor environment in a desiccator at a given temperature, in which I2 crystals (2 g) were placed in lieu of the desiccant. The amount of I2 absorbed was determined by an analytical balance within an accuracy of ± 0.0001 g.

Characterization

1H and 13C NMR analyses were performed on a Bruker Avance NMR spectrometer (400 MHz) (Germany) with [D6][DMSO] as a solvent. The DOSY, HSQC, and HMBC spectra were collected on a Bruker Avance NMR (600 MHz) (Germany) with [D6][DMSO] as a solvent. The thermogravimetric analysis was conducted at a Perkinelmer TGA 4000. The UV-vis analysis was performed at TU-1901 spectrophotometer.

DFT calculation

Stable structures of Im in different environment were optimized at the B3LYP/6-311 + g(d,p) level using Gaussian 09 package. Solvation (cyclohexane) corrections were calculated by a self-consistent reaction field using the CPCM model. Hirshfeld atomic charge was calculated based on the wave function of the optimum structures using the Multiwfn code.

The calculation of the vapor pressures of I2 at different temperatures

The temperature for the indicated pressure of I2 solid (Supplementary Table 3) was obtained from the CRC handbook of chemistry and physics (90th edition). The temperature–vapor pressures (Supplementary Fig. 17) curve fits with the Clausius–Clapeyron equation (\(\ln P = \frac{{ - \Delta H}}{{RT}} + C\)), which was then used for calculating the vapor pressures of I2 at different temperatures in the range of 30–100 °C