Azo-bridged calix[4]resorcinarene-based porous organic frameworks

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Azo-bridged calix[4]resorcinarene-based porous organic frameworks with highly efficient enrichment of volatile iodine Kongzhao Su, Wenjing Wang, Bei-Bei Li, and Daqiang Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05203 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Sustainable Chemistry & Engineering

Azo-bridged

calix[4]resorcinarene-based

porous

organic frameworks with highly efficient enrichment of volatile iodine Kongzhao Su,† Wenjing Wang† Beibei Li†‡ and Daqiang Yuan*† † State

Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155

Yangqiao Road West, Fuzhou, Fujian 350002, P. R. China; E-mail: [email protected]; Fax: +86-591-83794946; Tel: +86-591-83792460. ‡

University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing 100049, P. R. China.

KEYWORDS: Calix[4]resorcinarene; Porous organic framework; Diazocoupling reaction, Iodine vapor uptake; Chemisorption

ABSTRACT: The effective capture and storage of volatile radionuclide iodine from the nuclear waste stream is of paramount importance for environment remediation. In this work, we report the first examples of azo-bridged calix[4]resorcinarene-based porous organic frameworks (CalPOFs), synthesized by diazocoupling reaction of 4,4'-biphenyldiamine and C-alkylcalix[4]resorcinarenes (RsCns; n stands for the associated alkyl chain length) under mild conditions. The resulting CalPOFs are permanently porous, and their porous properties could be adjusted by varying the alkyl chain lengths of RsCns. With the alkyl chain length increasing from methyl, ethyl to propyl, the

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Brunauer−Emmett−Teller surface areas are decreasing from 303, 154 to 91 m2g-1 for CalPOF-1, CalPOF-2 and CalPOF-3, respectively. The presence of a great many of effective sorption sites including azo (-N=N-) groups, macrocyclic π-rich cavities and phenolic units in the skeleton as well as permanent porous structures provides these materials with ultrahigh iodine vapor uptake up to 477 wt%. Further, thorough studies revealed that the capacities for removing iodine vapor are in the order of CalPOF-1 (477 wt%) > CalPOF-2 (406 wt%) > CalPOF-3 (353 wt%), which are dependent on their surface areas, and also the densities of the azo and RsCn units. In addition, detailed analyses of iodine-loaded CalPOF-1 suggested that chemisorption is the major process in this adsorbent, illustrating the big chance to explore versatile CalPOFs to capture volatile toxic vapors.

Introduction The quest for clean and safe nuclear energy is becoming more and more important for purpose of meeting the energy needs in the 21st century for a sustainable world. Of particular major concern together with nuclear energy is capture of radioiodine (129I) generated from the exhaust fumes of nuclear power plants regularly, not only because its long radioactive half-life (15.7 million years), but also due to it can easy access into the human body and can hence cause mutations and even death.1 In order to effectively capture and reliably store the radioactive iodine, up to now, inorganic porous materials2,3 and metal-organic frameworks (MOFs)4-6 have been explored and tested for I2 capture. More recently, porous organic framework (POF), a new type of adsorbent, has also been gradually applied in this field and also gas adsorption and separation, heterogeneous catalysts and energy storage.712

A variety types of POFs such as covalent organic frameworks (COFs),13,14 conjugated microporous polymers (CMPs),15-18 porous polymer networks (PPNs),19,20 crystalline triazine-based frameworks (CTFs),21,22 hyper-cross-linked polymers (HCPs),23-25 polymers of intrinsic microporosity (PIMs),26-28 and porous aromatic frameworks (PAFs),29-32 by using a variety of building blocks in different coupling reactions have been prepared. With the good physical and chemical properties such as structural diversity, low mass


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density, good stability, and high porosity and easy functionality, several very promising POF candidates for I2 capture have been reported.33-43 For example, charged PAFs with a good uptake of 276 wt% prepared Zhu,30 novel thiophene-beared CMPs with an excellent uptake up to 345 wt% by Li,44 a nitrogen-rich fluorescent CMP achieving exceptional capacity of 443 wt% prepared by Geng,45 and novel 2D COFs constructed from the flexible modules with the highest value of 543 wt % obtained by Ma up to now in POFs have been reported.46 The recent increasing interest is focused on the synthesis of POFs by fixing supramolecular molecules, especially which contain hosts and complementary binding groups, and have the abilities to enclathration and confinement of various guest molecules into their backbones, because it can often create material with unexpected properties.47 For example, William R. Dichtel et al. prepared a mesoporous POF, crosslinked by β-cyclodextrin and tetrafluoroterephthalonitrile, showed much higher affinity (15 to 200 times) for sequestering a range of organic micropollutants compared to activated carbons in water purification.48 Ali Coskun et al. reported a pillar[5]arene-based POF could be used in the separation of propane and methane based on host−guest interaction.49 Ali Trabolsi et al. prepared an alkyne-rich calix[4]arene-based POF exhibited significantly higher sorption capacity for organic solvents, oils, and toxic dyes from aqueous mixtures than most of the reported adsorbents.50 Yu Liu et al. obtained two POFs constructed from (β-cyclodextrin and sulfonatocalix[4]arene) and tetraphenylethlene could absorb different fluorophores through the efficient host-guest complexations, and exhibit photo-luminescence tunable properties via the fluorescence resonance energy transfer (FRET) process from the tetraphenylethlene to encapsulated fluorophores.51 In this light, calix[4]resorcinarenes, typically bowl-shaped molecules with eight phenolic groups in their upper rims, have been decided to be excellent building components in preparation of supramolecular host compounds. They possess a hydrophobic πrich cavity with eight upper-rim phenolic hydroxyl groups. On account of these structural characteristics, they have been determined to have the abilities to encapsulate a variety of guest molecules.52-54 Inspired by their favorable recognition properties, we plan to incorporate the calix[4]resorcinarene macrocycles into the backbones of polymeric networks that may create excellent


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absorbent materials. To our best of knowledge, there are only two kinds of polymers based on calix[4]resorcinarene so far. One is prepared using Bakelite type chemistry through condensation calix[4]resorcinarenes with formaldehyde,55,56 while the other one is synthesized utilizing dioxane-forming polymerization reaction through the combination of calix[4]resorcinarene phenolic groups and fluorinated aromatic monomers.57 However, none of the sorption behavior of these polymers has been detailed studied, and no calix[4]resorcinarene-based POF in iodine enrichment and removal has been reported. In this work, we present the synthesis and characterization of a series of novel calix[4]resorcinarene-based porous organic framework (CalPOFs) by diazocoupling reaction coupling under mild conditions. With the features of calix[4]resorcinarene units and azo-nitrogen atoms doping in the porous framework, the CalPOFs exhibits excellent efficiency for removing iodine vapor. Experimental Section Chemicals and Materials Staring materials, C-methylcalix[4]resorcinarene (RSC1), C-ethylcalix[4]resorcinarene (RSC2) and C-propylcalix[4]resorcinarene (RSC3) were prepared according to the literature method.58 4,4'-Diaminobiphenyl was purchased from J&K Scientific Co. Ltd, while other reagents and solvents were of reagent grade and purchased from National Medicines Corporation Ltd. of China and used without further purification. Preparation of CalPOFs CalPOF-1: The diazo-coupling reaction was performed in two steps. First, a 250 mL flat-bottomed flask was charged with 4,4'biphenyldiamine (1.5 mmol, 276.4 mg), deionized water (50 mL) and concentrated HCl (0.7 mL). The mixture was stirred for 20 minutes, then 10 mL of aqueous solution of sodium nitrite (3.1 mmol, 217 mg) was added dropwise at such a rate to maintain the temperature at 0~5 °C, and further stirred for half an hour to make sure the diazonium process was complete. Second, the resulting mixture was neutralized with dilute aqueous solution of Na2CO3, and then slowly added to a solution of RSC1 (0.75 mmol, 408 mg) and NaOAc•3H2O (15 mmol, 2.04 g) in N,N-dimethylformamide (DMF, 30 mL) at 0~5 °C. The reaction mixture was stirred overnight, a red solid precipitate formed. The precipitates were collected by filtration, washed by the solvents in the order: dilute HCl, DMF, water, methanol,


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tetrahydrofuran (THF), methanol, water, Soxhlet extracted exhaustively with methanol for 24 h. Treated by freeze drying method, the sample was obtained in a high yield of 96%. Elemental analysis (%) for CalPOF-1. Expected C, 70.28; H, 4.63; N 11.71. Found C, 65.16; H, 4.93; N 10.65. CalPOF-2: Synthesis as for CalPOF-1, except RSC2 (0.75 mmol, 450 mg) was used in place of RSC1. Yield, 95%. Elemental analysis (%) for CalPOF-2. Expected C, 71.13; H, 5.17; N 11.06. Found C, 72.09; H, 5.33; N 10.75. CalPOF-3: Synthesis as for CalPOF-1, except RSC3 (0.75 mmol, 492 mg) was used in place of RSC1. Yield, 93%. Elemental analysis (%) for CalPOF-3. Expected C, 71.89; H, 5.66; N 10.48. Found C, 73.05; H, 5.89; N 10.25. Material Characterization Elemental compositions (C, H, N) was determined using a German Elementary Varil EL III service. Powder X-ray diffraction (PXRD) pattern was recorded in the range of 2θ = 4-60° on a desktop X-ray diffractometer (Rigaku Mini 600) with CuKα radiation (λ = 0.154 Å). Thermal gravimetric analysis (TGA) was carried out under a dynamic N2 flow on a NETZSCH STA 449C thermal analyzer by heating samples from 25 to 900 °C. The solid-state 13C cross polarization/magic angle spinning nuclear magnetic resonance (13C CP/MAS) NMR spectrum was carried out on a Bruker SB Avance III 500 MHz spectrometer. Field-emission scanning electron microscopy (SEM) was performed on a JEOL JSM-7500F microscope under an accelerating scanning voltage of 3.0 kV. Before testing, gold granules was labeled with the samples. X-ray photoelectron spectroscopy (XPS) was measured on a Thermo ESCALAB 250 spectrometer. Electron paramagnetic resonance (EPR) spectrum was carried out at ambient temperture on a Bruker ER-420 spectrometer with a 100 kHz magnetic field in the X band. Fourier transform infrared (FT-IR) spectrum was collected on KBr disks in transmission mode in the region 400-4000 cm-1 using a VERTEX70 FT-IR spectrometer. UV-vis adsorption spectrum was collected on a PerkinElmer Lambda 35 spectrophotometer at room temperature. Moreover, N2 adsorption-desorption isotherm of the CalPOF was measured at 77 K with a Micromeritics ASAP 2020 surface area and porosity analyzer. Before adsorption characterizations, the CalPOF sample was degassed at 120 °C in the analysis chamber under


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vacuum of 10-5 bar for 12 h. Brunauer−Emmett−Teller (BET) method was applied to determine the cumulative apparent surface area, and the pore size distribution (PSD) was evaluated from the aforementioned isotherm utilizing the non-local density functional theory (NLDFT) equation. Results and Discussion CalPOF Materials Characterization As depicted in Scheme 1, CalPOF-1, CalPOF-2 and CalPOF-3 were synthesized from three RSCns with different alkyl chain length (RSC1, RSC2 and RSC3) and 4,4'-diaminobiphenyl by diazo-coupling reaction in mild conditions, which has been recently proved to be a good approach to prepare azo-bridged POFs.59 It should be noted that utilizing RSCns with different alkyl chains is mainly for the following two reasons: (1) Whether it would be a general method to prepare CalPOFs by the diazocoupling reaction coupling method; (2) How do the alkyl chain lengths influence the surface areas, and also the iodine sorption capacities of these materials. The CalPOFs were insoluble in water and common organic solvents including acetonitrile, chloroform, THF, dioxane, acetone and DMF, suggesting their highly stable, covalently cross-linked and reticular structure. The solid-state CP/MAS 13C NMR spectra of the CalPOF materials (Fig. 1d) showed chemical shift values in the range from d = 102 to 178 ppm, corresponding to the aromatic carbons that constitute the polymeric framework. The broad peaks near 27 and 16 ppm are associated with the bridged methylene carbons (-CH2-) and -CH3 of the RSCns (n = 13), respectively. Meanwhile, the aliphatic methylene moieties (-CH2-) of RSC2 and RSC3 in the POFs have also been observed in about 35 ppm. The FT-IR absorption spectra exhibited the typical bands of phenolic groups of the RSCns (n = 1-3) units around 3430 cm-1, and 1415 cm-1 due to the asymmetric vibration of the -N=N- bonds (Figure 1e). TG analyses revealed that the CalPOFs were thermally stable up to 280 °C under nitrogen atmosphere (Supporting Information, Figure S1). SEM image showed that these materials were made up of rough sphere shape (Figures. 1a-1c). PXRD patterns of the CalPOFs showed characteristic broad peaks and suggest their amorphous nature (Supporting Information, Figure S2).


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Scheme 1. Typical design method based on diazo-coupling reaction between diazonium salts and RsCns for the synthesis of CalPOF materials. The associated alkyl chains are omitted for clarity in the molecule structures of RsCns.


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Figure 1. SEM images of CalPOF-1 (a), CalPOF-2 (b), and CalPOF-3 (c). Solid 13C CP/MAS NMR spectra (d) and FT-IR spectra of the CalPOFs (e). Porosity Measurement The pore features of the CalPOFs were investigated by nitrogen adsorption−desorption experiments on the activated samples at 77 K (Figure 2). The sorption isotherms reveal that these materials exhibit IV isotherms with type hysteresis loops corresponding to desorption, suggesting the presence of mesopores and swelling in these networks. Calculation from the N2 adsorption data gives BET surface areas of 303, 154 and 91 m2 g-1 for CalPOF-1, CalPOF-2 and CalPOF-3, respectively, with decreased surface area related to the length of associated alkyl chains. Their PSDs were calculated using NLDFT method, and confirmed the presence of primary mesopores (Supporting Information, Figures S3), which were similar to those of o-hydroxyazobenzene polymers.59 Further analysis using the t-plots method


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revealed that ~ 95 % and ~ 98 % surface areas of CalPOF-1 and CalPOF-2 were from the mesopores, while CalPOF-3 was almost mesoporous. Such results revealed that the ratios of the micropores decreased with the associated alkyl chain length increasing.

Figure 2. N2 gas sorption isotherm curves of the CalPOFs. The hysteresis loop suggests swelling. Iodine Capture and Release The presence of good porous properties and swelling phenomena combined with plenty of functional groups as effective iodine sorption sites in the CalPOFs prompt us to explore their iodine enrichment behaviors. For each experiment, 20 mg of powder samples were placed in a pre-weighed glass vial, and excess solid iodine in a sealed polypropylene container at 75 °C and atmosphere pressure. The iodine-loaded samples were weighed directly at various time intervals (Figure 3). The amount of loaded iodine for the CalPOF materials increased quickly at first 6 h, and then gradually from 6 to 20 h, and almost close to equilibrium at 24 h. As the iodine-loaded


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samples show no longer changed after 32 h, suggesting that the adsorption was basically saturated. The maximum uptake capacities of CalPOF-1, CalPOF-2 and CalPOF-3 are about 477 wt%, 406 wt% and 353 wt%, respectively, which are not only referred to the specific surface areas, but also the densities of the functional azo and calix[4]resorcinarene units (Supporting Information, Table S1).36 The iodine adsorption capacities of these three materials are fairly high for all these materials, and their capacities are considerably higher than the reported azo-bridged POFs including Azo-Trip60 and AzoPPN,61 whose capacities are 290 wt% and 238%, and also the calix[4]arene-based POFs with iodine uptake capacities in the range from 88.4 wt% to 312 wt%. Notably, the iodine uptake value of CalPOF-1 is somewhat lower than the reported COFs with ordered internal structures, but much higher than most of the reported solid porous materials including zeolites, MOFs, and amorphous POFs in capturing iodine vapor (Supporting Information, Table S5). Further analyses of these iodine adsorption data within 600 minutes by three different kinetic models showed the pseudo-second-order kinetic model support the assumption of the strong interaction, indicating that the chemical adsorption maybe the main process in these adsorbents (Supporting Information, Figures S4-S6, and Table S2-S4).62

Figure 3. Gravimetric iodine uptake of CalPOFs at 348 K.


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The mechanism of the iodine vapor adsorption in CalPOF-1 was taken as an example and detailed studied by PXRD, FT-IR spectrum, XPS and EPR measurements. PXRD measurements revealed that the saturated iodine-loaded CalPOF-1 was amorphous, and not showed any characteristic peaks of I2 crystalline diffraction peaks (Figure 4a). In fact, the FT-IR spectrum (Figure 4b) of the iodine-loaded material also not showed an additional peak at about 731 nm, belonging to the crystalline iodine crystals. Moreover, it was found that the characteristic peaks of the CalPOF-1 showed many differences compared to the iodine-loaded sample in FT-IR spectra. Specifically, the iodine-loaded sample showed considerably enhanced –OH stretching vibrations. The C=C and C-H bands on phenyl ring of CalPOF-1 at 1480 and 823 cm−1 shifted to 1475 and 818 cm−1, respectively, meanwhile the N=N band at1415 shifted to 1400 cm−1 after loading iodine. These phenomena suggested that the iodine vapor adsorption might occur at –OH groups, phenyl ring and N=N linkage in this adsorbent. The XPS measurement was measured to further determine the existence of chemisorption between iodine and CalPOF-1 (Figure 4c). It is obvious that the peak corresponding to neural I2 is much weaker than that of I3-, revealing the chemisorption plays the major role in the iodine adsorption process, and such chemisorption is also mainly responsible for the high iodine uptake capacity. It should be noted that such I3- anions and neural I2 could also form the polyiodides [I3-·I2], because the chemical adsorption is the major process based on the aforementioned results together with the kinetic simulation data. The polyiodide anions are more likely due to the formation of a charge transfer interaction between iodine guest molecules and the electron-rich CalPOF-1, which has also been authenticated by EPR analysis (Figure 4d). This analysis suggested the existence of radicals, which might arise from the hydroxyl groups and π-rich cavities of RsC1 units as well as N=N linkers. According to the aforementioned analyses, the possible mechanism for CalPOF-1 with high iodine enrichment behavior is most likely due to the electrons that arise from this adsorbent transfer to the adsorbed iodine molecules, and make most of them transform into polyiodide anions (Scheme 2).


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Figure 4. (a) PXRD patterns of I2 and I2@CalPOF-1. (b) FT-IR spectra of CalPOF-1 and the iodine-loaded sample. (c) XPS spectrum of CalPOF-1 after iodine capture. (d) EPR spectrum of iodine-loaded CalPOF-1.

Scheme 2. The mechanism of the iodine adsorption process in CalPOF-1. C, gray; O, red; N, blue; H, white.


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Further study revealed that CalPOF-1 could also trap iodine in solution. As can be seen from Figure 5a, after 30 mg active CalPOF-1 sample was added into the iodine hexane solution (0.01 mol L-1, 3 mL) at ambient temperature, the purple color of the solution faded gradually to pale red and finally to nearly colorless after 48 h, suggesting that this adsorbent can capture iodine in organic solvents efficiently. When compared to other adsorbents (including classical activated carbon and zeolite), CalPOF-1 shows a remarkable better iodine inclusion rate and capability as shown in Figure S7. This result indicates that accessible interaction between the affinity sites and iodine, in this case, macrocyclic π-rich cavities, phenolic units and azo groups, may lead to the result of much higher iodine capture capability than those adsorbents for comparison.


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Figure 5. (a) Photographs showing the iodine-absorbed process and (b) iodine-released process of CalPOF-1. (c) UV-vis spectra for the iodine release from I2@CalPOF-1 at different times. Notably, we found that the iodine sorption of CalPOF-1 is reversible mostly in organic solvents. Once the iodine-loaded sample of CalPOF-1 was immersed in ethanol at 25 °C, the color of the ethanol gradually varied from colorless to dark brown (Figure 5b), indicating that the iodine released gradually from the polymer skeleton. To further study such releasing process, about 3 mg iodine-loaded sample of CalPOF-1 was immersed in 7 mL ethanol, and the delivery of iodine was measured by UV−vis spectroscopy (Figure 5c). The signals


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corresponding to iodine released at 204, 288, and 360 nm normally increased within the first hour and then reached a dynamic equilibrium. This finding revealed that CalPOF-1 could be an attractive recyclable iodine uptake adsorbent in practical application. To prove this, the recyclability of CalPOF-1 has further been studied, and we noted that CalPOF-1 still retains 458 wt% of its initial capacity after five cycles (Supporting Information, Figure S8). Conclusion In conclusion, we have presented three new calix[4]resorcinarene-based POFs by diazocoupling reaction. The azo and calix[4]resorcinarene units in these POFs could act as effective iodine sorption sites, which significantly increase the capture of iodine vapor by chemisorption. Notably, CalPOF-1 displays excellent iodine adsorption capacity of 477 wt%, which is among the highest value reported to date in solid porous materials. We anticipate that this study will increase the awareness of using calix[4]resorcinarenes in the area of capturing harmful volatile pollutants. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. TGA, PXRD and PSD patterns, kinetic simulation figures and data of the CalPOF materials; summary of iodine uptake properties of porous materials; recycle test of CalPOF-1 in iodine vapor adsorption; other additional tables and figures (PDF) Acknowledgements This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SLH019), the National Nature Science Foundation of China (51603206, 21707143 and 21771177), and the Nature Science Foundation of Fujian Province (2016J05056). References (1)

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TOC Picture

Porous azo-bridged calix[4]resorcinarene-based porous organic frameworks (CalPOFs) prepared via mild diazocoupling reaction exhibited ultrahigh iodine vapor uptake up to 477 wt%.


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Azo-bridged calix[4]resorcinarene-based porous organic frameworks

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