Reversible Iodine Capture by Nonporous Pillar[6]arene Crystals

Oct 16, 2017 - Errui LiKecheng JieYujuan ZhouRun ZhaoBo ZhangQi WangJiyong LiuFeihe Huang. ACS Applied Materials & Interfaces 2018 10 (27), 23147- ...
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Communication Cite This: J. Am. Chem. Soc. 2017, 139, 15320-15323

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Reversible Iodine Capture by Nonporous Pillar[6]arene Crystals Kecheng Jie, Yujuan Zhou, Errui Li, Zhengtao Li, Run Zhao, and Feihe Huang* State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

Here, we report easily obtained per-ethylated pillar[6]arene17−27 (EtP6) can work as a new adsorbent for iodine capture with high chemical and thermal stability. For comparison, the ability of two similar pillararenes with different cavity sizes, perethylated pillar[5]arene18 (EtP5) and pillar[7]arene19 (EtP7) (Figure 1), to capture iodine were also

ABSTRACT: Here we report that easily obtained perethylated pillar[6]arene (EtP6) is a new adsorbent for iodine capture with high chemical and thermal stability. Nonporous EtP6 solids are shown to capture not only volatile iodine in the air but also iodine dissolved in an organic solvent and aqueous solution. Uptake of iodine leads to a structural transformation of EtP6 in the solid state. In the single crystal structure of iodine-doped EtP6 (I2@EtP6), each adsorbed iodine molecule is located between two adjacent EtP6 molecules to form a linear supramolecular polymer. Iodine is released spontaneously from I2@EtP6 solids when they are immersed in cyclohexane. These EtP6 solids can be reused many times without losing iodine capture capacity.

Figure 1. Chemical structures and cartoon representations of EtP5, EtP6 and EtP7 used in this study.

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investigated. We demonstrated EtP6, other than EtP5 or EtP7, had the ability and the best performance to capture iodine. Nonporous EtP6 crystals were shown to not only capture volatile iodine in the air but also iodine dissolved in an organic solvent and aqueous solution. Each adsorbed iodine molecule was observed to be located between two adjacent EtP6 molecules in the single crystal structure of iodine-doped EtP6 (I2@EtP6), driven by typical charge-transfer interactions between I2 and benzene rings. Uptake of iodine could led to a structural transformation from EtP6β to I2@EtP6, which is different from the cases of most robust MOFs where structural change does not happen upon guest capture.28−30 Interestingly, iodine was released spontaneously from I2@EtP6 crystalline solid when I2@EtP6 solid was immersed in cyclohexane. Iodine was also released in cyclohexane from single crystals of I2@EtP6, accompanied by single-crystal to single-crystal transformation from I2@EtP6 to EtP6-cyclohexane complex (Cy@EtP6). At last, EtP6 was shown to be reused many times without losing iodine capture ability. EtP5, EtP6 and EtP7 were synthesized by a reported method.20 Then desolvated EtP5, EtP6 and EtP7 were prepared as adsorbent materials (see Supporting Information). As characterized by powder X-ray diffraction (PXRD) experiments, they were all crystalline in the solid state (referred to as EtP5α, EtP6β and EtP7α, respectively, Figures S4−S6). However, the N2 sorption experiments showed these pillararene-based crystalline solids were not porous, indicating dense packing of EtP5, EtP6 and EtP7 molecules in the crystal state (Figures S10−S12).

o meet the increasing demand of worldwide energy and reduce greenhouse gas emissions, research on clean, reliable nuclear power has been growing. One challenge is nuclear waste pollution, a safety concern associated with the production of nuclear energy.1,2 Particular attention has been focused on the capture of volatile radionuclide fission products including 129I and 131I, 3H, 14CO2 and 85Kr from inadvertent environmental release or nuclear fuel reprocessing. Of them, radiological iodine species possess exceptional issues: 129I is a hazardous iodine isotope with a long half-life (∼107 years), which means it must be captured and reliably stored. Although 131I is a short-lived (half-life of 8.02 days) radionuclide, it also needs immediate trapping due to its high volatility and the effect on human metabolic processes.3−5 Thus, the search for appropriate adsorbents to capture iodine with long-term storage is crucial. Inorganic composite adsorbents such as silver-based zeolites or aerogels have been used for radioactive iodine capture.6,7 However, these silver containing adsorbents have shown low uptake capacities due to their limited accessible surface areas. Another significant drawback is the high associated cost and adverse environmental impact due to high silver content of these materials. Recently, porous metal−organic frameworks (MOFs) have been intensively explored as adsorbents for iodine capture.8−15 These porous adsorbents have better uptake capacity compared with the silver-based adsorbents, but their relatively low thermal and moisture stability limits their practical applications because nuclear power plants contain high levels of water vapor and the volatile nuclear waste often has relatively high temperature.16 Therefore, the development of new adsorbents with high iodine capture capacity and robust chemical and thermal stability remains a challenge. © 2017 American Chemical Society

Received: September 14, 2017 Published: October 16, 2017 15320

DOI: 10.1021/jacs.7b09850 J. Am. Chem. Soc. 2017, 139, 15320−15323

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Journal of the American Chemical Society

changed for the purple solution (Figure S22a). UV/vis experiment also supported the phenomena (Figure S22b). This is probably due to the higher binding strength between EtP6 and cyclohexane. And this shows iodine capture by EtP6β crystals in alkane solvents has an alkane-shape selectivity. Moreover, EtP6β was capable of adsorbing iodine in aqueous solution, which has rarely been demonstrated by porous materials such as MOFs due to their low water-stability (Figure S23). It took about 10 h to completely adsorb iodine (below 1 ppm) in a saturated aqueous iodine solution, which is much slower than that in n-hexane. The PXRD patterns of EtP6β after complete capture of iodine in solutions are the same and are consistent with that after adsorption of iodine vapor (Figure S24), indicating that EtP6β has the same iodine capture behavior either in air or in solution. As demonstrated above, EtP6β as a kind of nonporous material has a remarkable ability to capture iodine. This prompted us to explore where the adsorbed iodine molecules are located in the nonporous EtP6 crystals. Luckily, by dissolving iodine and EtP6 in a certain solvent (1-chlorobutane), dark red single crystals were grown overnight, which were further revealed by X-ray crystallography to be iodine-EtP6 complexes (I2@EtP6, Figure 3a,b). To our surprise, iodine molecules are not located in

We then investigated their iodine vapor sorption abilities. As can be seen from Figure 2b, EtP5α and EtP7α solids barely

Figure 2. Adsorption of iodine vapor by EtP5α, EtP6β and EtP7α crystalline solids. (a) Photographs showing color change when 5 mg of EtP6β crystals was exposed to iodine vapor. (b) Time-dependent uptake plots of iodine in EtP5α, EtP6β and EtP7α crystals at 358.15 K. (c) Time-dependent powder X-ray diffraction patterns of EtP6β: (I) EtP6β; after adsorption of iodine vapor for (II) 10 min, (III) 30 min, (IV) 60 min and (V) 120 min.

adsorbed iodine vapor and the small quantity adsorbed was ascribed to the crystal surface loading. However, iodine sublimed into nonporous crystals of EtP6 over time, and this was apparent from a color change in the crystals from white to almost black (Figure 2a). Thermogravimetric (TG) measurements taken at various time intervals during the iodine loading indicated the coloration was not merely a result of surface coating. For EtP6β, there was no apparent weight loss below 195 °C (Figure S8). However, the weight loss of the iodine-loaded crystals below 195 °C was significant after 2 h, corresponding to 20.1 wt % of the crystals, which can be calculated as 2.2 iodine atoms per EtP6 molecule or 0.25 g of iodine per gram of EtP6 (Figure 2b and Figure S14). Meanwhile, the iodine release began at ca. 150 °C, indicating the high thermal stability of iodine in EtP6β crystals, which meets the practical demand of iodine storage.12 Furthermore, a large amount of iodine species was detected for these black crystals by energy-dispersive spectroscopy (EDS), confirming the capture of iodine by EtP6β (Figure S19). The results are attractive for a nonporous material and can even be comparable to some porous materials.31 In situ PXRD experiments were carried out to monitor the adsorption of iodine by EtP5α, EtP6β and EtP7α. The time-dependent PXRD patterns of EtP6β changed over time and were unchanged after 1 h, revealing that structural change happened for EtP6β upon capture of iodine (Figure 2c). However, the PXRD patterns did not change for EtP5α and EtP7α, confirming the crystal surface loading of subtle iodine (Figures S16 and S17). FT-IR spectra showed no new peaks appeared for EtP6β after uptake of iodine, indicating physical adsorption rather than chemical adsorption (Figure S18). Besides the iodine vapor uptake ability, we wondered whether it was also possible to load iodine molecules into EtP6β crystals from solutions. Upon addition of EtP6β crystals to an iodine/nhexane solution, the colorless crystals became darker while the purple solution faded over time, and turned completely colorless after 70 min (Figure S21a). A time-dependent UV/vis experiment (Figure S21b) showed iodine concentration decreased over time to nearly zero after 70 min. This meant EtP6β captures iodine from n-hexane with the final concentration of iodine below 1 ppm. However, EtP6β was not able to adsorb iodine from cyclohexane as no coloration was observed for EtP6β in an iodine/cyclohexane solution as well as no color

Figure 3. Single crystal structure of I2@EtP6: (a) side view, (b) top view and (c) packing mode along the a axis. (d) Schematic representation of structural transformation from EtP6β to I2@EtP6 upon capture of iodine by EtP6β crystals.

the cavities of EtP6. Instead, each iodine molecule is located between two distorted EtP6 molecules with the iodine molecule perpendicular to the benzene rings while two opposite repeating units of EtP6 are turned almost perpendicular to their adjacent units and are parallel with each other, giving EtP6 a deformed hexagonal structure (Figure 3a,b). The distance between iodine atom and the benzene ring is 3.405 Å, which is typical for chargetransfer interactions between iodine and benzene (Figure 3a).32 Interestingly, driven by charge-transfer interaction, iodine molecules and EtP6 molecules self-assemble into an AB-type supramolecular polymer along the a axis with an iodine and EtP6 molar ratio of 1:1 in the solid state (Figure 3c). It is rare to obtain a single cocrystal structure between a pillararene and an inorganic species.17 The crystal structure is also in good agreement with iodine vapor adsorption experiments (2.2 iodine atoms per EtP6 molecule; the excess adsorption can be ascribed to crystal surface loading). Moreover, the PXRD patterns of EtP6β after capture of iodine in air, in hexane and in water are in good agreement with the pattern simulated from I2@EtP6 (Figure S24), indicating the structural transformation from EtP6β to I2@EtP6 upon capture of iodine (Figure 3d). This is quite different from most crystalline porous frameworks whose pre-existing pores play a vital role in 15321

DOI: 10.1021/jacs.7b09850 J. Am. Chem. Soc. 2017, 139, 15320−15323

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Journal of the American Chemical Society the capture of iodine molecules. The “pores” in I2@EtP6 crystals are created during the capture of iodine. The release of iodine was triggered rapidly by dissolving the I2@EtP6 crystalline solids in an appropriate solvent such as chloroform (Figure S25), a release route not available in the case of iodine species loaded into insoluble porous frameworks. Iodine loaded in I2@EtP6 crystalline solids was released relatively slowly in poor solvents. When I2@EtP6 crystalline solids were exposed to cyclohexane, a poor solvent for EtP6, the transparent solvent became purple slowly over time, indicating the spontaneous release of iodine (Figure 4a). UV/vis spectros-

Figure 5. (a) Optical microscopy images of (a) I2@EtP6 single crystals, (b) I2@EtP6 single crystals immersed in cyclohexane for 1 day and (c) I2@EtP6 single crystals immersed in cyclohexane for 2 days. The inserted arrows show the colorless small single crystals. (d) Schematic representation of single-crystal to single-crystal (SC−SC) transformation from I2@EtP6 to Cy@EtP6 where cyclohexane molecules were removed due to their high disorder during structure refinement.

crystals became broken over time and were split into numerous small colorless single crystals (Figures 5a−c and S33), which were further refined by X-ray crystallography as Cy@EtP6. This indicates the single-crystal to single-crystal (SC−SC) transformation from I2@EtP6 to Cy@EtP6 upon exposure to cyclohexane, strongly supporting the result that Cy@EtP6 is a more stable complex than I2@EtP6 (Figure 5d). The release of molecular iodine from I2@EtP6 crystals further demonstrated “pores” are generally only stable when occupied by iodine molecules. One disadvantage of most crystalline porous frameworks is that the performance decreases over time, either because of the instability of the porous frameworks or the loss of crystallinity. For example, the iodine uptake performance of a reported hydrogen-bonded cross-linked organic framework (HcOF) decreases to 80% after recycling 4 times.33 To be practically useful, an adsorbent should perform well over multiple cycles without any degradation. After iodine was completely released from I2@EtP6 in cyclohexane, the resultant EtP6 was then desolvated under vacuum. Interestingly, the desolvated EtP6 solids were characterized to be EtP6β by PXRD (Figure S34). We further demonstrated that the newly formed EtP6β still captures iodine without decreasing iodine capture performance after recycling 5 times (Figure 6). Furthermore, we found even

Figure 4. (a) Photographs showing the cyclohexane color change when 3 mg of I2@EtP6 crystals was placed in cyclohexane. (b) Timedependent UV/vis absorption spectra of cyclohexane (1.5 mL) upon addition of I2@EtP6 crystals (3.0 mg). Inset: I2 absorbance at 520 nm at various times. (c) Powder X-ray diffraction patterns: (I) EtP6β after exposure to cyclohexane; (II) I2@EtP6 after exposure to cyclohexane; (III) EtP6β; (IV) EtP6β after adsorption of iodine; (V) simulated from single crystal structure of I2@EtP6.

copy showed that the release of iodine was completely finished within 90 min (Figure 4b). Meanwhile, almost no iodine species were detected for EtP6 after iodine release by energy-dispersive spectroscopy, confirming the complete release of iodine from I2@EtP6 crystals (Figure S26). The PXRD profiles show the pattern of I2@EtP6 after iodine release in cyclohexane is different from that of either I2@EtP6 or EtP6β but is the same as that of EtP6β after exposure to cyclohexane (Figure 4c). We thus deduced EtP6 forms a complex with cyclohexane more stable than I2@EtP6 or EtP6β. To be more confirmative, we obtained colorless rod-like single crystals of EtP6 by cyclohexane diffusion into EtP6 ethylbenzene solution (Figure S28). In this crystal structure, EtP6 molecules with hexagonal pillar structures are assembled in a body-to-body packing mode, forming infinite intrinsic 1D channels (Figures S29a and 5d). But frustratingly, the solvate molecules were not able to be refined by X-ray crystallography due to their high disorder. Nonetheless, we can conclude this crystal structure is solvated with cyclohexane (Cy@EtP6) because it is totally different from reported EB@EtP6,26 EtP6 solvated with ethybenzene (Figure S29b). 1H NMR further confirmed the solvate in these single crystals is cyclohexane rather than ethylbenzene (Figure S30). Thermogravimetric analysis (TGA) showed the weight loss of these single crystals started at about 100 °C, which is about 20 °C higher than the boiling point (80.7) of cyclohexane (Figure S31). This is due to the high binding strength between EtP6 and cyclohexane. Even when the red single crystals of I2@EtP6 grown in 1cholorobutane were placed in cyclohexane, iodine release was also observed (Figure S32). More interestingly, these red single

Figure 6. (a) Schematic representation of EtP6β as the adsorbent for iodine capture and the recycling of EtP6. (b) Iodine capture efficiency in EtP6α after the same material is recycled 5 times. 15322

DOI: 10.1021/jacs.7b09850 J. Am. Chem. Soc. 2017, 139, 15320−15323

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(6) Katsoulidis, A. P.; He, J. Q.; Kanatzidis, M. G. Chem. Mater. 2012, 24, 1937. (7) Chapman, K. W.; Chupas, P. J.; Nenoff, T. M. J. Am. Chem. Soc. 2010, 132, 8897. (8) Zeng, M.-H.; Wang, Q.-X.; Tan, Y.-X.; Hu, S.; Zhao, H.-X.; Long, L.-S.; Kurmoo, M. J. Am. Chem. Soc. 2010, 132, 2561. (9) Sava, D. F.; Rodriguez, M. A.; Chapman, K. W.; Chupas, P. J.; Greathouse, J. A.; Crozier, P. S.; Nenoff, T. M. J. Am. Chem. Soc. 2011, 133, 12398. (10) Falaise, C.; Volkringer, C.; Facqueur, J.; Bousquet, T.; Gasnot, L.; Loiseau, T. Chem. Commun. 2013, 49, 10320. (11) Chapman, K. W.; Sava, D. F.; Halder, G. J.; Chupas, P. J.; Nenoff, T. M. J. Am. Chem. Soc. 2011, 133, 18583. (12) Shi, X.; Yang, J.; Salvador, J. R.; Chi, M. F.; Cho, J. Y.; Wang, H.; Bai, S. Q.; Yang, J. H.; Zhang, W. Q.; Chen, L. D. J. Am. Chem. Soc. 2011, 133, 7837. (13) Liu, Q.-K.; Ma, J.-P.; Dong, Y.-B. Chem. Commun. 2011, 47, 7185. (14) Yin, Z.; Wang, Q.-X.; Zeng, M.-H. J. Am. Chem. Soc. 2012, 134, 4857. (15) Jiang, Z.-Q.; Wang, F.; Zhang, J. Inorg. Chem. 2016, 55, 13035. (16) Luo, Y. H.; Yu, X. Y.; Yang, J. J.; Zhang, H. CrystEngComm 2014, 16, 47. (17) Ogoshi, T.; Yamagishi, T.-a.; Nakamoto, Y. Chem. Rev. 2016, 116, 7937. (18) Cao, D.; Kou, Y.; Liang, J.; Chen, Z.; Wang, L.; Meier, H. Angew. Chem., Int. Ed. 2009, 48, 9721. (19) Han, C.; Zhang, Z.; Chi, X.; Zhang, M.; Yu, G.; Huang, F. Huaxue Xuebao 2012, 70, 1775. (20) Hu, X.-B.; Chen, Z.; Chen, L.; Zhang, L.; Hou, J.-L.; Li, Z.-T. Chem. Commun. 2012, 48, 10999. (21) Duan, Q.; Cao, Y.; Li, Y.; Hu, X.; Xiao, T.; Lin, C.; Pan, Y.; Wang, L. J. Am. Chem. Soc. 2013, 135, 10542. (22) Li, S.-H.; Zhang, H.-Y.; Xu, X.; Liu, Y. Nat. Commun. 2015, 6, 7590. (23) Talapaneni, S. N.; Kim, D.; Barin, G.; Buyukcakir, O.; Je, S. H.; Coskun, A. Chem. Mater. 2016, 28, 4460. (24) Lan, S.; Zhan, S.; Ding, J.; Ma, J.; Ma, D. J. Mater. Chem. A 2017, 5, 2514. (25) Zhao, Q.; Dunlop, J. W. C.; Qiu, X.; Huang, F.; Zhang, Z.; Heyda, J.; Dzubiella, J.; Antonietti, M.; Yuan, J. Nat. Commun. 2014, 5, 4293. (26) Jie, K.; Liu, M.; Zhou, Y.; Little, M.; Bonakala, S.; Chong, S.; Stephenson, A.; Chen, L.; Huang, F.; Cooper, A. I. J. Am. Chem. Soc. 2017, 139, 2908. (27) Tan, L.-L.; Li, H.; Tao, Y.; Zhang, S. X.-A.; Wang, B.; Yang, Y.-W. Adv. Mater. 2014, 26, 7027. (28) Chen, B.; Liang, C.; Yang, J.; Contreras, D. S.; Clancy, Y. L.; Lobkovsky, E. B.; Yaghi, O. M.; Dai, S. Angew. Chem., Int. Ed. 2006, 45, 1390. (29) Li, P.-Z.; Wang, X.-J.; Liu, J.; Lim, J. S.; Zou, R.; Zhao, Y.-L. J. Am. Chem. Soc. 2016, 138, 2142. (30) Luo, F.; Yan, C.; Dang, L.; Krishna, R.; Zhou, W.; Wu, H.; Dong, X.; Han, Y.; Hu, T.-L.; O’Keeffe, M.; Wang, L.; Luo, M.; Lin, R.-B.; Chen, B. J. Am. Chem. Soc. 2016, 138, 5678. (31) Here are two examples: the relative uptakes of iodine in two porous MOFs CdL2 and CAU-1 are 0.18 and 0.31 g per gram of MOF, respectively. For details, see refs 10 and 13. (32) Mulliken, R. S.; Person, W. B. Wiley-Interscience: New York, 1969. (33) Lin, Y.; Jiang, X.; Kim, S. T.; Alahakoon, S. B.; Hou, X.; Zhang, Z.; Thompson, C. M.; Smaldone, R. A.; Ke, C. J. Am. Chem. Soc. 2017, 139, 7172. (34) Hasell, T.; Schmidtmann, M.; Cooper, A. I. J. Am. Chem. Soc. 2011, 133, 14920. (35) Liu, M.; Little, M. A.; Jelfs, K. E.; Jones, J. T. A.; Schmidtmann, M.; Chong, S. Y.; Hasell, T.; Cooper, A. I. J. Am. Chem. Soc. 2014, 136, 7583. (36) Hasell, T.; Culshaw, J. L.; Chong, S. Y.; Schmidtmann, M.; Little, M. A.; Jelfs, K. E.; Pyzer-Knapp, E. O.; Shepherd, H.; Adams, D. J.; Day, G. M.; Cooper, A. I. J. Am. Chem. Soc. 2014, 136, 1438.

amorphous EtP6 was capable of capturing iodine with a phase change from amorphous to I2@EtP6 (Figures S38 and S39). Actually, this iodine capture route is a “self-healing” system: the capture works irrespective of the starting EtP6 structure. This is different from processes involving crystalline porous frameworks where loss of porosity or phase changes are often irreversible and disastrous. In summary, we have demonstrated easily available perethylated pillar[6]arene EtP6 functions as a new adsorbent for iodine capture. From the single crystal structure of I2@EtP6, each adsorbed iodine molecule was observed between two adjacent EtP6 molecules. Compared with previously reported adsorbents such as Ag-based sorbents, MOFs and porous organic polymers, EtP6 with a nonporous character has four advantages. First, its preparation is simple and only one step is necessary from commercially available and cheap 1,4-diethoxybenzene. Second, because EtP6 has more robust moisture and thermal stability than crystalline MOFs, it can capture not only volatile iodine in the air but also iodine dissolved in organic and aqueous solutions. Third, EtP6 as a small organic molecule is soluble in many common organic solvents, which means that the captured iodine can be released instantly upon EtP6 dissolution.34−36 Fourth, its regeneration is simple (only washing and drying is necessary) and it can be reused many times with no decrease in performance (the residual iodine concentration in water is below 1 ppm). Future work will investigate pillararene-based adsorbents in the capture of other hazardous volatile substances such as Freon, BTEXs, etc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09850. Experimental details (PDF) Data for I2@EtP6 (CIF) Data for Cy@EtP6 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Feihe Huang: 0000-0003-3177-6744 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21434005, 91527301) and Open Project of State Key Laboratory of Supramolecular Structure and Materials.

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REFERENCES

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DOI: 10.1021/jacs.7b09850 J. Am. Chem. Soc. 2017, 139, 15320−15323