Iodine adsorption in tetrathiafulvalene-based covalent organic frameworks
To safeguard the development of nuclear energy, practical techniques for capture and storage of radioiodine are of critical importance but remains a significant challenge. Here we report the synergistic effect of physical and chemical adsorption of iodine in tetrathiafulvalene-based covalent organic frameworks (COFs), which can markedly improve both iodine adsorption capacity and adsorption kinetics due to their strong interaction. These functionalized architectures are designed to have high specific surface areas (up to 2359 m 2 g −1 ) for efficient physisorption of iodine, and abundant tetrathiafulvalene functional groups for strong chemisorption of iodine. We demonstrate that these frameworks achieve excellent iodine adsorption capacity (~ 8.15 g g -1 ) and adsorption kinetics (~ 0.69 g g -1 h -1 ), which are much higher than other materials reported so far, including silver-doped adsorbents, inorganic porous materials, metal−organic frameworks, porous organic frameworks, and other COFs. Furthermore, a combined theoretical and experimental study, including DFT calculations, electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, reveals the strong chemical interaction between iodine and framework of material. Our study thus opens an avenue to construct functional COFs for a critical environment-related application.
<!>Designed synthesis and structural analysis<!>Thermal and chemical stability<!>Analysis of nitrogen gas adsorption<!>Analysis of iodine adsorption<!>Experimental study of the interaction between iodine and framework<!>Theoretical study of the interaction between iodine and framework<!>Outlook<!>References:
<p>Nuclear power is one of the most important energy sources due to its high power density and low carbon emission. 1,2 However, an urgent safety issue related to nuclear power is the administration of waste produced from nuclear fission. 3,4 Radioiodine ( 129 I and 131 I) is a typical pollutant that accompanies nuclear fission, and attracts a particular attention since it can disperse rapidly in air and causes a radiological environment up to ten million years (15.7 million year for half-life of 129 I). 5 For example, radioactive iodine leaked out from Chernobyl nuclear accident has been incorporated into the human metabolic system and resulted in human diseases like thyroid cancer. To safeguard the future development of atomic energy, efficient strategies for adsorption and removal of radioiodine are of necessary but remain elusive.</p><p>The traditional technology to remove radioactive iodine is based on a chemical transformation, that is, using silver-doped adsorbents to produce harmless AgI; however, these materials show a low efficiency. For instance, their theoretical and practical adsorption capacity is 1.18 g g −1 and 0.10-0.31 g g −1 (per gram Ag), respectively, which is far to face the practical needs. 6 In recent years, porous materials including inorganic porous materials, 7,8 metal−organic frameworks (MOFs), 9,10 and porous organic frameworks (POFs), 11,12 have been shown to be a suitable choice for the removal of radioiodine. In particular, covalent organic frameworks (COFs), [13][14][15][16][17] as a unique class of crystalline organic polymers with ordered porous channels, have been studied extensively for the uptake of iodine. These COF materials with specific pore environments and tunable chemistry can be easily functionalized to obtain active sites for more efficient iodine capture by electrostatic forces, Lewis acid-base interactions and coordinative sites. Nevertheless, most studies are still limited to exploitation of physical interaction, i.e., iodine uptake capacity in porous materials is driven by pore volume and uptake kinetics is dominated by pore connectivity and size. For example, although a state-of-the-art COF, TPB-DMTP COF, has achieved remarkable iodine adsorption capacity (6.26 g g -1 ), its adsorption kinetics is quite slow (0.13 g g -1 h -1 ). 18 In principle, the combination of high physisorption and strong chemical interaction in a material will be beneficial to improving both iodine adsorption capacity and adsorption kinetics. However, this avenue is still not properly explored.</p><p>Herein, we report the synergistic effect of physical and chemical iodine adsorption in tetrathiafulvalene (TTF)-based COFs, JUC-560 (JUC = Jilin University China) and JUC-561, leading to ultrahigh iodine adsorption capacity and ultrafast adsorption kinetics. These architectures are designed to have large specific surface areas (1815 m 2 g −1 for JUC-560 and 2359 m 2 g −1 for JUC-561) for high iodine uptakes through the physical process, and plentiful TTF functional groups for powerful iodine chemisorption. As a result, JUC-561 exhibits an unprecedented iodine uptake of 8.15 g g -1 and adsorption kinetics of 0.69 g g -1 h -1 at 333 K under ambient pressure. These values are much higher than other materials reported so far, including silver-doped adsorbents, inorganic porous materials, MOFs, POFs, and COFs. The strong interaction between the COF framework and iodine has been probed by DFT calculations on model molecules, TTF•I 2 , TTF + •I 2 , TTF + •(I -) 2 , TTF + •(I 3 -), and TTF + •(I 5 -), and confirmed by experimental studies including X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy.</p><!><p>To achieve high iodine adsorption, we introduced a TTF-based derivative, tetrathiafulvalenetetrabenzaldehyde (TTF-TBA, Fig. 1b), as a planar 4-connected building unit. TTF or TTF-based derivatives are known to form the radical cations with electron acceptors (e.g., I 2 ). 19 Therefore, by the incorporation of TTF-based units, the resulting COF materials will promote iodine adsorption based on 5 this strong interaction. We further chose two typical monomers, 2',5'-dimethyl-[1,1':4',1''-terphenyl]-4,4''-diamine (DTDA, Fig. 1a) as a linear building unit and 2,4,6-tris(4-aminophenyl)amine (TAPA, Fig. 1c) as a 3-connected building unit. Thus, we developed two novel structures, two-dimensional (2D) JUC-560 with sql topology 20 and three-dimensional (3D) JUC-561 with ffc topology (Fig. 1d-i), 21 constructed from the condensation of TTF-TBA and linear DTDA or non-coplanar 3-connected TAPA (Supplementary Fig. 1), respectively.</p><p>The targeted COFs were synthesized by suspending TFP-TTF and DTDA or TAPT in a mixture of mesitylene/1,4-dioxane or o-dicholobenzene/n-butanol in the presence of acetic acid, followed by heating at 120 °C for 3 days. JUC-560 shows powder X-ray diffraction (PXRD) peaks at 2.69°, 3.48°, 5.38°, 5.60°, and 8.07° for 2θ, which are assigned to the (110), ( 200), ( 220), (310), and (330) facets, respectively. This structure adopts a 2D sql net with an eclipsed AA-stacking mode that yields a PXRD pattern similar to the experimentally observed one (Fig. 2a and Supplementary Figs. 10-12). On the other hand, JUC-561 exhibits PXRD peaks at 1.99°, 3.21°, 4.01°, 5.20°, 6.06°, 6.89°, 7.77°, 8.44°, and 22.00° for 2θ, which correspond to the (020), (001), (040), (041), (060), (061), (042), (110), and (313) facets, respectively. JUC-561 assumes a 3D ffc net that exhibits a PXRD profile that consistent with the experimental one (Fig. 2b and Supplementary Figs. 13-16). PXRD patterns based on Pawley refinements 22 also confirm the assignments of PXRD peaks, as evidenced by their negligible differences (residuals Rp = 1.31%, Rwp = 2.53% for JUC-560, and residuals Rp = 3.07%, Rwp = 4.51% for JUC-561). Based on these results, both TTF-based COFs have mesoporous frameworks with a diameter of about 2.62 nm for JUC-560 and about 2.55 nm for JUC-561 (Fig. 1f and 1g).</p><!><p>Both COFs were observed to be chemically stable in different organic solvents, including acetone, ethanol, cyclohexanone, hexane, tetrahydrofuran, N,N-dimethylformamide and dimethylsulfoxide. After being treated in these solvents for 24 h, JUC-560 and JUC-561 maintained their crystallinity (Supplementary Figs. 17 and 18). Furthermore, both COFs retained their structures in boiling water (100 °C) and even in strong acid (3 M HCl, 25 °C) and strong base (3 M NaOH, 25 °C) for 24 h (Supplementary Figs. 19 and 20). According to the thermogravimetric analysis (TGA), these COFs did not decompose until 400 °C under nitrogen (Supplementary Figs. 21 and 22). Therefore, these TTFbased COFs are chemically and thermally stable, crystalline, porous materials.</p><!><p>The porosities of both COFs were determined by N 2 adsorption and desorption isotherms at 77 K (Fig. 2c and 2d). For each COF, a sharp uptake below P/P 0 = 0.05 and a step, indicating the presence of mesopores, was observed. The inclination of isotherms between P/P 0 = 0.9−1.0 and slight desorption hysteresis is related to the presence of textural mesopores from the agglomeration of COF crystals. 23 The Brunauer−Emmett−Teller (BET) specific surface areas were calculated to be 1815 m 2 g −1 for JUC-560 and 2359 m 2 g −1 for JUC-561, respectively (Supplementary Figs. 23-26). The pore size distribution obtained by the nonlocal density functional theory (NLDFT) showed monomodal mesopores, 2.50 nm for JUC-560 and 2.46 nm for JUC-561 (Fig. 2e and 2f), which are in good agreement with those of the proposed model (2.62 nm for JUC-560 and 2.55 nm for JUC-561). Furthermore, the pore volume was determined to be 1.11 cm 3 g −1 for JUC-560 and 1.92 cm 3 g −1 for JUC-561, matching well with the simulated ones (1.16 cm 3 g −1 for JUC-560 and 2.04 cm 3 g −1 for JUC-561).</p><!><p>We conducted iodine adsorption experiment by exposing COFs to iodine vapor at 60 °C under ambient pressure. JUC-560 exhibited a quick iodine uptake, showing a nearly linear increment in 7 h and then reaching adsorption saturation within 14 h (Fig. 3a). Similarly, JUC-561 showed a rapid adsorption in 8 h and achieved saturation in 18 h (Fig. 3b). JUC-560 obtained iodine uptakes as high as 5.17 g g -1 .</p><p>Remarkably, JUC-561 showed much higher iodine uptakes of 8.15 g g -1 due to its 3D framework with higher specific surface areas and interconnected channels. On the basis of the pore volume together with the iodine density (4.93 g cm -3 ), their theoretical capacities (4.93 g cm -3 × pore volume) upon the full occupation of pores by iodine are 5.47 g g -1 for JUC-560 and 9.47 g g -1 for JUC-561 (Supplementary Table 1). The channels of both COFs are almost entirely occupied by iodine (95% for JUC-560 and 86%</p><p>for JUC-561 based on their theoretical maxima), which is consistent with the high weight lost (Supplementary Figs. 27 and 28) for iodine-loading samples.</p><p>We summarized the iodine uptakes of typical adsorbents in Fig. 4 and Supplementary Table 2. As can be seen, the adsorption capacity of JUC-561 (8.15 g g -1 ) is far superior to those reported previously, such as 29-fold higher than that of the typical silver-doped zeolite mordenite (Ag-MOR, 0.28 g g −1 ), 24 two orders of magnitude higher than those of nonporous materials, and much better to those of representative porous materials, including zeolitic imidazolate framework-8 (ZIF-8, 1.20 g g −1 ), 25 porous aromatic framework 24 (PAF-24, 2.76 g g −1 ), 26 azo-bridged porphyrin-phthalocyanine network (AzoPPN, 2.90 g g −1 ), 27 covalent organic polymers (COP 1 0 , 3.80 g g −1 ), 28 and a state-of-the-art COF (TPB-DMTP COF, 6.26 g g -1 ). 18 Notably, although the specific surface area of JUC-561 (2358 m 2 g −1 ) is lower than that of some of reported porous materials, such as COF-320 (2400 m 2 g −1 ), NiP-conjugated microporous polymer (NiP-CMP, 2630 m 2 g −1 ), and porous carbon (Uassis-PC800, 3053 m 2 g −1 ), its iodine adsorption is higher (COF-320, 4.00 g g −1 ; 29 NiP-CMP, 2.02 g g −1 ; 30 and Uassis-PC800, 2.25 g g −1 ) 31 due to the synergistic effect of physical and chemical adsorption of iodine in JUC-561.</p><p>The adsorption kinetics of iodine in various materials were also summarized in Supplementary Table S3. In fact, the available relevant reports are limited due to low adsorption kinetics in most of previously reported materials. Remarkably, the adsorption kinetics of TTF-based COFs (0.48 g g -1 h -1 for JUC-560 and 0.69 g g -1 h -1 for JUC-561) is superior to those adsorbents reported, such as nitrogen-rich triptycenebased porous polymer (NTP, 0.11 g g -1 h -1 ), 32 TTA-TTB COF (0.15 g g -1 h -1 ), 18 NiP-CMP (0.17 g g -1 h -1 ), 30 AzoPPN (0.18 g g -1 h -1 ), 27 and even more than 34 times higher than that (0.02 g g -1 h -1 ) of Ag-loaded zeolites applied in real environment. 24 These results confirm that the strong physical and chemical interaction between TTF units and iodine is favorable to enhance iodine adsorption ability.</p><p>The iodine-adsorbed COFs can preserve iodine upon exposure to air under ambient conditions (25 °C and 1 bar), and do not display iodine escape from the frameworks (Fig. 3c and 3d). Adsorbed iodine could be removed by immersing these samples in ethanol solution at room temperature. Iodine release was monitored by optical images and UV/Vis spectroscopy, leading to a linear increase based on the absorbance over time (Supplementary Figs. 29-36). After ethanol rinse, these COFs were recyclable and retained similar iodine uptakes (Fig. 3e and 3f). FT-IR spectroscopy verified the chemical integrity of these COFs after cycled application (Supplementary Figs. 37 and 38). These regenerated COFs maintained their crystallinity and porosity, as evidenced by their unchanged PXRD patterns and retained specific surface areas (Supplementary Figs. 39-42). These excellent cycle performances suggest that these materials are robust against oxidative iodine over long-term exposure.</p><!><p>Electron paramagnetic resonance (EPR) studies of the solid samples at 110 K confirmed the generation of TTF radical cations after iodine adsorption (Fig. 5a and 5b). Clearly, the pristine samples showed a very weak EPR signal (black curve) while there is an approximately two orders of magnitude increase in paramagnetic intensity for the iodine-doped samples (red curve). After I 2 doping, sharp peaks were observed at g = 2.0182 for JUC-560 and 2.0189 for JUC-561 respectively, clearly demonstrating the presence of TTF radical cations from oxidation by iodine. 33 Furthermore, we tested the electrochemical behavior of JUC-560 and JUC-561 by cyclic voltammetry (CV). In Supplementary Figs.</p><p>43 and 44, two clear redox processes at 0.72 and 1.15 V vs Ag/AgCl for JUC-560 and 0.69 and 1.08 V vs Ag/AgCl for JUC-561 were observed in the reversible CV profiles, revealing that TTF units retain their redox activities in the frameworks and can be oxidized by iodine which has a reduction potential at 0.78 V. 34 To investigate the existing state of iodine captured in TTF-based COFs, we conducted X-ray photoelectron spectroscopy (XPS). In Fig. 5c and 5d, two conspicuous peaks located at 629.8 eV and 618.3 eV for JUC-560 as well as 630.0 eV and 618.6 eV for JUC-561, belonging to I 3d 3/2 and I 3d 5/2 orbital of iodine molecules respectively, were observed, which indicates that the captured iodine partially exists as a molecule. Furthermore, another two peaks at 630.9 eV and 619.6 eV for JUC-560 as well as 631.4 eV and 619.8 eV for JUC-561 were found, which are attributed to the formation of polyiodine anions (such as I 3and I 5 -). 35 In addition, the species of captured iodine were detected by Raman spectroscopy. As shown in Fig. 5e and 5f, the Raman spectra of TTF-based COFs showed no distinct peaks before iodine adsorption (black curve). However, after iodine capture, the low-frequency spectral region was dominated by three peaks at 107, 136 and 174 cm -1 for JUC-560 and 108, 135 and 181 cm -1 for JUC-561 (red curve). The peaks can be assigned to the symmetric and asymmetric stretching vibrations of I 3as well as the stretching vibrations of I 5 -, respectively. 36 Thus the results show that the adsorbed iodine species in TTF-based COFs exist as both iodine molecule and polyiodine anions, and the process is the combination of physisorption and chemisorption.</p><!><p>To gain insight into how the COF frameworks bind iodine, we performed DFT calculations with the M06-2X-D3 function based on Gaussian 09 package (Revision D.01). 37 Full geometry optimizations of model compounds, TTF•I 2 , TTF •+ •I 2 , TTF •+ •(I -), TTF •+ •(I 3 -), and TTF •+ •(I 5 -), offered related systems with the I 2 molecule, I -, I 3or I 5anion situating on the center of TTF or TTF •+ unit (Supplementary Fig. 45).</p><p>No symmetry constraints were imposed during the optimizations. The standard def2-SVP basis set was used for geometry optimizations, and the vibration analysis and def2-TZVP basis set was applied for calculating single point energy. The binding energy of I 2 molecule to the TTF or TTF •+ unit calculated at the M06-2X-D3 level was low (-8.3 kcal mol -1 and -7.1 kcal mol -1 respectively). However, the association of Ianion with the TTF •+ unit showed a large binding energy as high as -74.6 kcal mol -1 , which is the result of the electrostatic interaction between the anions and the positively charged viologen. 38 Similarly, the I 3or I 5anion also displayed a relatively high binding energy of -70.6 kcal mol -1 or -65.3 kcal mol -1 to TTF •+ unit, respectively. Thus, these results suggest that TTF molecule reducing to its diradical form leads to the change of I 2 to I -, I 3or I 5anion, which at last is tightly bound to the framework of COFs through electrostatic interactions.</p><!><p>By developing TTF-based COF materials, we have successfully obtained high iodine adsorption capacity and quick iodine adsorption kinetics. These results point out that the integration of virtues of physical and chemical adsorption is of paramount importance in the designed synthesis of porous materials for the very challenging capture of radioiodine. The pre-designed COF frameworks exhibiting synergy between physisorption and chemisorption showed a dual impact on the iodine adsorption: substantially improved total iodine capacity, in particular the 3D COF (JUC-561); and accelerated transport rate due to their strong interaction between functionalized units and iodine. Consequently, the TTF-based COFs achieved the highest iodine uptake (~ 8.15 g g -1 ) and adsorption kinetics (~ 0.69 g g -1 h -1 ), which are far superior to other materials reported so far, such as silver-doped adsorbents, inorganic porous materials, MOFs, POFs, and other COFs. The set of experimental results thus demonstrate the potential of functionalized COF materials as a platform for radioiodine capture and storage. Some prospective directions for future work can be exploited to obtain efficient iodine adsorption.</p><p>COF materials feature open pores and excellent specific surface areas, and thus hold great promises in iodine adsorption. Unlike 2D structures with characteristic eclipsed AA stacking, 3D COFs have attracted widespread attentions owing to their interconnected channels and impressive specific surface areas.</p><p>Therefore, the development of 3D COFs with different architectures are interesting for iodine capture due to their unique pore shape and pore environment. In addition, COFs are well-defined in which their organic building units are precisely integrated into extended structures, and thus they can be easily functionalized to obtain strong chemisorption of iodine. A number of functionalized groups as electron donors, such as carbazole and ionic liquid, can be decorated into the frameworks by bottom-up or postsynthetic approaches. Most importantly, by the integration of higher specific surface areas and abundant functional units, we predict that the iodine adsorption in COF materials could be further improved. These functionalized porous frameworks would be applicable in environmental protection for the capture of radioiodine and other hazardous substances.</p><!><p>cm). Then, the mixture of mesitylene (0.7 mL), 1,4-dioxane (0.3 mL) and 0.1 ml of aqueous acetic acid (6.0 mol/L) was added. The Pyrex tube was flash-frozen in a liquid nitrogen bath and evacuated to an internal pressure of ca. 19.0 mbar and flame-sealed, reducing the total length by ca. 10.0 cm. Upon warming to room temperature, the tube was placed in an oven at 120 °C for three days. As a result, a pale brown powder was isolated by centrifugation and washed with acetone (3  5.0 ml) and the yield is about Synthesis of JUC-561. TAPA (0.04 mmol, 13.4 mg) and TFP-TTF (0.03 mmol, 21.7 mg) were weighted into a Pyrex tube (volume: ca. 20.0 ml with a body length of 18.0 cm and neck length of 9.0 cm). Then, the mixture of o-dicholobenzene (0.2 mL), n-butanol (0.8 mL), and aqueous acetic acid (9.0 mol/L, 0.1 mL) was added. The Pyrex tube was flash-frozen in a liquid nitrogen bath and evacuated to an internal pressure of ca. 19.0 mbar and flame-sealed, reducing the total length by ca. 10.0 cm. Upon warming to room temperature, the tube was placed in an oven at 120 °C for five days. As a result, a pale brown powder was isolated by centrifugation and washed with acetone (3  5.0 ml) and the yield is about 81%. General procedure for iodine vapor sorption. An open small vial (2.0 mL) containing the COF sample (20.0 mg) was placed in a large vial (10.0 mL) containing iodine (1.0 g). The large vial was sealed and kept in an oven at 60 °C. After a certain period, the large vial was cooled to room temperature. The small vial containing the COF sample was weighted and placed back into the iodine-containing large vial. The large vial was sealed and put back in the oven at 60 °C to continue the adsorption till the mass of the small vial containing the COF sample did not change.</p><p>General procedure for recycling the COF samples. The iodine-captured COF sample was added to ethanol (20.0 mL) in a vial at 25 °C, and ethanol was refreshed every 2 h until no color of the solution was observed. The COF sample was collected by filtration, washed with ethanol, dried under vacuum at 120 °C overnight and reused for the next cycle.</p><p>General procedure for iodine escape investigation. An open vial (2.0 mL) containing the iodine-loaded COF sample was placed in a large empty vial (10.0 mL). The large vial was sealed and kept in room temperature. After a certain period, the small vial containing the COF sample was weighted and placed back into the large empty vial. The large vial was sealed to continue the desorption. The investigation was carried out for a period of 8 days.</p><p>Cyclic voltammetry (CV) measurements. After grinding each as-synthesized COF (6.0 mg) and carbon black (4.0 mg) in agate mortar and pestle for 10 mins, the grinded powder was subjected to a mixture of water (0.25 mL), ethanol (0.25 mL) and Nafion soluion (50.0 µL) and sonicated for 2 h. Then the electrode was prepared by drop-casting such mixture onto the surface of carbon fiber which was dried before use. The CV measurements were performed using an electrochemical analyzer CHI 760i E and a standard three-electrode setup with a carbon working electrode, platinum wire auxiliary electrode and Ag/AgCl as the reference electrode. The scan rate was 20 mV•s -1 with potential range 0 V to 1.5 V in supporting electrolyte (0.1 M NBu 4 PF 6 in CH 2 Cl 2 ).</p>