Mechanistic Insight into Hydrogen-Bond-Controlled Crystallinity and

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Mechanistic Insight into Hydrogen-Bond-Controlled Crystallinity and Adsorption Property of Covalent Organic Frameworks from Flexible Building Blocks Xinghua Guo, Yin Tian, Meicheng Zhang, Yang Li, Rui Wen, Xing Li, Xiaofeng Li, Ying Xue, Lijian Ma, Chuanqin Xia, and Shoujian Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05121 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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Chemistry of Materials

Xinghua Guo,a,§ Yin Tian,b,§ Meicheng Zhang,a Yang Li,a Rui Wen,a Xing Li,a Xiaofeng Li,a Ying Xue,a Lijian Ma,*a Chuanqin Xia,*a and Shoujian Lia College of Chemistry, Sichuan University, Key Laboratory of Radiation Physics & Technology, Ministry of Education, No. 29 Wangjiang Road, Chengdu, 610064, P. R. China. b Southwestern Institute of Physics, Chengdu 610041, P. R. China. a

ABSTRACT: The effective control of crystallinity of covalent organic frameworks (COFs) and the optimization of their performances related to the crystallinity have been considered as big challenges. COFs bearing flexible building blocks (FBBs) generally own larger lattice sizes and broader monomer sources, which may endow them with unprecedented application values. Herein, we report the oriented synthesis of a series of two-dimensional (2D) COFs from FBBs with different content of intralayer hydrogen bonds. Studies of H-bonding effects on the crystallinity and adsorption properties indicate that partial structure of the COFs is “locked” by the H-bonding interaction, which consequently improves their microscopic order degree and crystallinity. Thus, the regulation of crystallinity can be effectively realized by controlling the content of hydrogen bonds in COFs. Impressively, the as-prepared COFs show excellent and reversible adsorption performance for volatile iodine with capacities up to 543 wt%, much higher than all previously reported adsorbents, although the variation tendency of adsorption capacities is opposite to their crystallinity. This study provides a general guidance for the design and construction of highly/appropriately crystalline COFs and ultrahigh-capacity iodine adsorbents.

For the practical application of materials, high crystallinity and ordered periodic structure will greatly increase the performance of materials in optoelectronics, energy storage and other fields.12,15 However, good crystallinity does not necessarily imply a good application performance in many ways, such as the application in the fields of adsorption, separation and catalysis.16,17 Therefore, how to effectively control the crystallinity of COFs and thus regulate their related application performance is a very practical research subject, especially to COFs based on flexible building block, who have usually large lattice sizes and various monomer sources and are of great importance for type and performance expansion over conventional COFs. Researches on this issue may lead to unprecedented properties and application prospects for COFs. Since the extremely long radioactive half-life (1.57 × 107 years), volatile and biocompatible, 129I is the most important radioactive pollutant in airborne radioactive nuclear waste and greatly harmful to the ecological environment and human health.18,19 Hence, design and preparation of suitable materials for efficient capture and storage of iodine are essential to public and nuclear safety. Natural or synthetic molecular sieves and other inorganic composites are generally used as iodine adsorbents.20 But the unsatisfactory adsorption performance of these materials is an obstacle to their practical applications. Burgeoning porous materials, including porous inorganic materials,21 metal-

INTRODUCTION Covalent organic frameworks (COFs),1-3 a kind of porous crystalline materials are prepared by molecular organic building units connected via covalent bonds. COFs have recently emerged as promising materials for practical applications in gas storage,4 separation,5 catalysis,6 detection,7 optoelectronics8,9 and energy storage materials.10 Despite the progress achieved over the past decade, simple and effective preparation of COFs with well-defined structure and high crystallinity is still an enormous challenge. In order to obtain ordered structure and favorable crystallinity, rigid building blocks and special bonding mode, such as boron oxygen bond, carbon-nitrogen and carbon-carbon double bond, acetylenic bond etc.,11,12 are generally chosen to construct COFs, whose molecular chains are integrally linear or near-linear after the bonding. This strategy significantly limits the species and structure-type of COFs that can be built. Choosing a variety of FBBs to prepare COFs provides the possibility to enrich the structural diversity and complexity of COFs.13,14 Due to the high rotational degree of freedom of flexible units, COFs based on FBBs exhibit greater potential extensibility and more diversity of packing pattern than that based on rigid units. However, the high rotational degree of freedom of flexible units also greatly increases the difficulty to obtain highly ordered structure and good crystallinity. Therefore, it is still a great challenge to construct highly crystallized COFs with FBBs.

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Scheme 1. Schematic of the synthetic route of TPT-BD COF, TPT-DHBD COF and TPT-DHBDX COFs (X= 25, 50, 75) with adjustable content of H-bonding structures.

organic frameworks (MOFs),22 porous organic polymers (POPs)23 and COFs,24 etc., are considered to be of great potential for iodine enrichment due to their superior surface area, large porosity and average pore diameter structure. Among them, COFs possess simultaneously the most regular and ordered structures and excellent physicochemical stabilities which may endow them with more unprecedented practical performance. As reported before, nitrogen-rich structure and electronrich π-conjugated system (such as aromatic ring) can provide effective binding sites for iodine adsorption. In particular, the presence of nitrogen atoms can greatly improve the adsorption capacity of the materials to iodine.25,26 Hence, a nitrogen-rich flexible monomer containing triazine ring and ether bond was chose as a building block for preparation of COFs in this paper. The as-prepared COFs have fairly high nitrogen content, good crystal quality and large lattice size, which is extremely rare in COFs based on flexible units. The effects of intralayer hydrogen bonds on the crystallinity and the adsorption properties was discussed for the first time through a combined experimental and theoretical study and the results indicated that the regulation of crystallinity can be smoothly realized by controlling the content of hydrogen bonds in COFs. Surprisingly, the obtained COFs exhibited unreported ultrahigh capacities (up to 543 wt%) for iodine enrichment, which gives the materials great possibility to be applied in the enrichment of radioactive iodine in nuclear waste disposal. EXPERIMENTAL SECTION Materials and Equipment. All the reagents and solvents are commercial available and used without further purification. All the reactions were performed under ambient atmosphere using oven-dried glassware unless otherwise mentioned. The Fourier transform infrared (FT-IR) spectra were recorded on Nicolet Nexus 670FT-IR. Solid-

state nuclear magnetic resonance (SSNMR) experiments were carried out on a Bruker Avance III 400 MHz. X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos ASAM800 spectrometer. Powder X-ray diffraction data (PXRD) were collected on a Shimadzu XRD6100 diffractometer and using Cu Kα radiation. Scanning electron microscopy (SEM) images were performed on a JEOL JSM5900LV instrument. Transmission electron microscopy (TEM) images were performed on a FEI Tecnai G2 F20 STWIN instrument. The surface areas and pore properties were investigated by nitrogen adsorption and desorption at 77.3 K using ASAP 2020 V4.00. The pore-sizedistribution curves were obtained from the adsorption branches using non-local density functional theory (NLDFT) method. Thermogravimetric analyses (TGA) were carried out under N2 atmosphere with a heating rate of 10 °C/min on a Shimadzu DTG-60 (H) analyzer. Raman spectra were recorded on a LabRAM HR spectrometer. Element analysis (EA) were performed on a CARLO ERBA 1106. DFT calculations. The electronic structure calculations for all species were performed with Kohn-Sham density functional theory (DFT) in Gaussian 03 program. The hybrid exchange-correlation functional B3LYP was employed in this study. Moreover, the triple split valence basis set 6311+G(d) was applied to describe hydrogen, carbon nitrogen and oxygen atoms. The geometry optimizations and energy calculations were carried out in the gas phase (298.15 K, 0.1 MPa). The harmonic frequencies were calculated to confirm the stationary point as true minimum with no imaginary frequency. The Mayer bond orders (MBO) were analyzed at the same level of theory. Synthetic Procedure. A typical procedure exemplified by TPT-DHBD COF: 2,4,6-tris-(4-formylphenoxy)-1,3,5triazine (TPT-CHO) (88.4 mg, 0.2 mmol) and 3,3'dihydroxybenzidine (DHBD) (64.8 mg, 0.3 mmol) were placed respectively in 10 mL glass vials, then mesity-

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lene/dioxane (1:1 v/v, 1 mL) was added and the two mixtures were sonicated for 5 min to afford homogeneous dispersions. Afterwards, the two dispersions were mixed together, and the mixture was briefly sonicated for about 15 s. Subsequently, acetic acid (6 M, 0.2 mL) was slowly added and the vial was then sealed and left undisturbed for 4 days at 80 °C. The formed solid was collected by filtration and washed with anhydrous DMF, anhydrous acetone and anhydrous THF separately until the filtrate was colorless. The resultant powder was dried at 50 °C under vacuum overnight to afford a yellowish crystalline solid in ~62.4% isolated yield and a molecular formula of (C42H27N6O6)n (% calc/found: C 70.87/65.49, H 3.83/4.05, N 11.81/9.87, O 13.49/20.63). Synthesis of TPT-DHBD COF on a gram scale. TPT-CHO (1.4 g, 30 mmol) and DHBD (1.0 g, 45 mmol) were placed in a 100 mL glass vial, then mesitylene/dioxane (1:1 v/v, 20 mL) was added and the mixture was sonicated for 30 min to afford a homogeneous dispersion. Subsequently, acetic acid (6 M, 2.5 mL) was slowly added and the vial was then sealed and left undisturbed for 4 days at 80 °C. The formed solid was collected by filtration and washed with anhydrous DMF, anhydrous acetone and anhydrous THF separately until the filtrate was colorless. The resultant powder was dried at 50 °C under vacuum overnight to afford a yellowish crystalline solid in ~70.3% isolated yield. RESULTS AND DISCUSSION Synthesis and Characterization. The newly synthesized COFs, namely, TPT-BD COF and TPT-DHBD COF, were formed from a large flexible monomer 2,4,6-tris-(4formylphenoxy)-1,3,5-triazine (TPT-CHO) as vertices and benzidine (BD) or 3,3'-dihydroxybenzidine (DHBD) as edges respectively, which were linked via an aldehydeamine polycondensation reaction (Scheme 1; see the Supporting Information (SI) for details). Compared their experiment data, TPT-BD COF and TPT-DHBD COF exhibited similar isolation yields, indicating the similar reactivities of BD and DHBD under the reaction conditions.

ing bands at ~3300 cm-1 disappeared and the typical C=O stretching bands at 1702 cm-1 practically vanished, suggesting the consumption of aldehyde and amino group of monomers. Meanwhile, the characteristic C=N stretching band (1622 cm-1 and 1621 cm-1, respectively) appeared, confirming the occurrence of aldehyde-amine condensation reaction in the polymers (Figure S2). The solid-state 13C spectra of the COFs showed that the sharp characteristic resonance peak of imine carbons appeared approximately at 154 ppm, which demonstrated the presence of the imine linkages in materials again (Figures 1a,b). The N1s high resolution spectra measured by X-ray photoelectron spectroscopy (XPS) are presented in Figures 1c,d. The N1s core level peak showed that the peaks with binding energies of 399.4 eV and 398.7 eV for TPT-BD COF, and 399.4 eV and 399.0 eV for TPT-DHBD COF were ascribed to the nitrogen atoms within the triazine rings and the imine linkages, respectively.27,28 There are only two peaks in the spectra and the area are basically identical, illustrating only triazine and imine nitrogen atoms exist in the materials and the ratio is 1:1, which matches well with the predicted structures. In summary, the highly polymerized COFs have been prepared successfully with condensation reactions. Theoretical simulations and powder X-ray diffraction (PXRD) experiments were conducted to determine the exact structures of the as-obtained materials. The PXRD pattern of TPT-DHBD COF had multiple distinct diffraction peaks, as shown in Figure 2a. The main diffraction peak caused by the (100) facet, located at 2.27°, and others clearly visible peaks at 4.07°, 4.69°, 6.22° and 8.13° were attributed to (110), (200), (210) and (220) facets, respectively. Meanwhile, the PXRD pattern with background deduction of TPT-DHBD COF was shown in Figure S5a, whose sharp diffraction peaks mean excellent crystallinity of the materials. The crystal structure was then simulated using the Materials Studio software and the results showed that this experimental pattern accorded well with the simulated pattern based on the eclipsed stacking (AA) structure. Pawley refinement produced unit cell parameters of a = b = 43.26 Å, c = 3.44 Å, α = β = 90°, and γ = 120°, with factors of RP = 2.80% and RWP = 3.91%, which matched with the observed pattern quite well (SI, Section 9). The highresolution transmission electron microscope (HR-TEM) images in Figures 3h,i proved that TPT-DHBD COF is a twodimensional layered material with the interlayer distance of d = 3.50 Å, which is fairly close to the simulated value (d = 3.44 Å). The PXRD pattern of TPT-BD COF had obvious diffraction peaks in 2.27, 4.03, 4.53, 6.16, 8.12°(Figure 2b), which arise from (100), (110), (200), (210), (220) facets. This PXRD pattern agreed with the simulated pattern based on the AA stacking structure. Pawley refinement produced unit cell parameters of a = b = 43.46 Å, c = 3.50 Å, α = β = 90°, and γ = 120°, with factors of RP = 4.48% and RWP = 6.11%. It was found that the diffraction peak intensity of TPT-DHBD COF was very strong and the experimental pattern was less affected by baseline according to the PXRD results. However, the intensity of (100) diffraction peak of TPT-BD COF was weak and the PXRD spectrum was greatly disturbed by the baseline. Moreover, an obvi-

Figure 1. 13C solid state NMR spectra of TPT-BD COF (a) and TPT-DHBD COF (b). High resolution XPS spectra of N1s for TPT-BD COF (c) and TPT-DHBD COF (d).

Fourier transform infrared spectroscopy (FT-IR) was used to characterize the as-prepared COFs. In the spectra of TPT-BD COF and TPT-DHBD COF, the N-H stretch3



ous amorphous peak appeared at ~21.5° in the spectrum of TPT-BD COF. The results distinctly illustrate that TPTDHBD COF has better crystallinity than TPT-BD COF, which could be due to the formation of hydrogen bonds between C=N and -OH groups in TPT-DHBD COF.

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caused by H-bonding interaction existing in the structure of TPT-MA (Figure S2). 1H nuclear magnetic resonance spectra were employed for further study of the H-bonding interaction in TPT-MA and the results revealed that the intramolecular H-bonding interaction between C=N and – OH groups can’t form due to the strong solvation effect of DMSO when using DMSO-d6 as solvent.29,30 At this time, the chemical shift of –OH group locates in 9.00 ppm (Figure 4a), which is quite close to the hydroxyl peak of phenol (at 9.33 ppm) in the same solvent (Figure 4b). However, when CDCl3-d1 was used as solvent, the chemical shift of –OH group in TPT-MA is 7.29 ppm (Figure 4c), much greater than the hydroxyl peak of phenol at 4.88 ppm (Figure 4d). Moreover, the comparison of 1H NMR spectra of TPT-MA and TPT-MB in CDCl3-d1 also reveals that the chemical shift of hydrogen in CH=N group shifted sharply from 8.43 ppm in TPT-MB to 8.67 ppm in TPT-MA (Figure S3f). The results indicate the formation of hydrogen bond between –OH group and the N atom of imine linkage in TPT-MA which tends to construct a stable five-membered ring structure, somewhat similar to benzoxazole, and leads to the increase of the chemical shift of phenolic hydroxyl group, and the Hbonding interaction has been identified by single crystal structure analysis in a similar structure with TPT-MA (Figure S3g).31 In addition, the binding energy of imine linkage of TPT-DHBD COF from the N1s high resolution spectra is 0.3 eV higher than that of TPT-BD COF because the intralayer H-bonding interaction reduced the electron density of the nitrogen atom (Figures 1c,d).

Figure 2. Experimental (black), Pawley refined (red), and predicted (blue) PXRD patterns of TPT-DHBD COF (a) and TPTBD COF (b), and the differences between the experimental and refined PXRD patterns in dark cyan (insert: views of spacefilling models along the c-axis with the layer distances).

H-bonding Interaction. TPT-DHBD COF has additional phenolic hydroxyl groups in the adjacent positions of the benzene rings with imine linkages at edges compared with TPT-BD COF. Judged from the characteristics of spatial structure and functional groups, it is highly possible that the phenolic hydroxyl group can form hydrogen bond with the imine-nitrogen atom through a five-membered bridge ring way, thus making the structure more stable. In order to demonstrate the existence and the formation mechanism of hydrogen bonds in TPT-DHBD COF, methods of simulation and simplification of materials structure have been adopted in this paper. Two model compounds, TPTMA and TPT-MB, were prepared to describe the local structural characteristics of the materials (SI, Section 2). Comparing FT-IR spectra of the two model compounds, it was found that the position of C=N stretching vibration peak varied from 1625 cm-1 to 1628 cm-1, which may be

Figure 3. SEM images of TPT-BD COF (a), TPT-DHBDX COFs (X=25 (b), 50 (c), 75 (d)) and TPT-DHBD COF (e). TEM images of TPT-BD COF (f) and TPT-DHBD COF (g) (insert: SAED patterns). HR-TEM images of TPT-DHBD COF (h, i).

To further demonstrate the likelihood of hydrogen bonding in the materials’ structure, density functional theory (DFT) method was utilized to explore the influence of hydrogen bonds on the COFs.32,33 As described in Figure 4e, two optimized structure including structure-A with hydrogen bond and structure-B without hydrogen bond, were obtained by DFT calculations. The relative energy of the structure-A is -6.1 kcal mol-1, much lower than the struc-

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Figure 4. 1H NMR (DMSO) of TPT-MA (a) and phenol (b). 1H NMR (CDCl3) of TPT-MA (c) and phenol (d). DFT calculation of the relative energies of hydrogen bond (e).

Scheme 2. Schematic of the effects of hydrogen bonds on crystallinity. The intralayer H-bonding interaction suppress the rotation of the flexible building block and “lock” COFs structure in a planar conformation.

ture-B (0 kcal mol-1), which suggests that structure-A is the low energy and high fraction stable structure. Furthermore, for the hydrogen bond in structure-A, the bond length is 2.094 Å and the Mayer bond orders (MBO) is 0.087.34,35 The MBO between two atoms is a measure of the bond strength between these two atoms, which corresponds to the more stable structure-A. The hydrogen bond in structure-A also make the structure in a planar conformation (SI, Section 4 for details). Both the theoretical calculation and the experiment data indicated that intramolecular hydrogen bonds between C=N and –OH groups would form positively in TPT-DHBD COF. The conclusion also can be extended to other materials with similar regional structures containing phenolic hydroxyl group and ortho-position carbon-nitrogen double bond. H-bonding Effects on Crystallinity. According to the previous characterization data, TPT-DHBD COF has better crystallinity than TPT-BD COF. It is speculated that the key reason is due to the H-bonding interaction in the intralayer of TPT-DHBD COF, which causes the formation of a stable pentacyclic structure, somewhat similar to benzoxazole in a planar conformation. Therefore, the structure of TPT-

DHBD COF is more stable and its laminar layer and spatial arrangement are more ordered than TPT-BD COF, which ultimately leads to the obvious optimization of its crystalline form. In other words, the COF structure is “locked” and optimized by the intralayer H-bonding interaction (Scheme 2). Then, is it possible to realize the regulation of crystallinity and other related properties of the product COFs by setting and controlling the formation and the number of hydrogen bonds? To confirm the possibility, combinatorial copolymerization strategy using TPT-CHO as COF vertices and BD as well as DHBD as conjunct edges was employed in this study. A series of products denoted as TPT-DHBDX COFs (X = 25, 50, 75) whose proportions of H-bonding sites are adjustable through changing the molar percentage of the monomer DHBD versus BD (X = [DHBD] / ([BD] + [DHBD]) × 100) were synthesized under the same condition (Scheme 1). According to the results of elemental analysis, the increasing content of oxygen is in good agreement with the increasing X value, which is directly related to the proportion of DHBD (containing –OH groups) in the structure of TPT-DHBDX COFs (Table S7). Observed from the images of scanning electron microscope (SEM), the microscopic morphology of TPT-DHBDX COFs are all

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uniform, suggesting that the mixed three reaction monomers are likely to form homogeneous copolymerized crystalline materials (Figures 3a-e).

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with the increasing X. Based on all the phenomena, it can be concluded that the COF structure tends to be more ordered and its crystallization tends to be more exquisite and complete with the increasing content of hydrogen bonds inside the materials. In other words, the crystallinity of COFs based on FBBs can be conveniently controlled by adjusting the number of H-bonding interaction sites in the COFs. The micromorphology of the products observed by SEM showed that TPT-BD COF was a broad and thick nanosheet and TPT-DHBD50 COF was a curled irregular rod. However, TPT-DHBD COF with the maximum H-bonding sites appeared as a regular nanowire with a length up to micron scale (Figures 3a-e). It was found that the morphology of the TPT-DHBDX COFs had an obvious tendency toward small and exquisite along with the proportion increase of H-bonding sites. The change in morphology could be attributed to the increasing degree of microscopic order that was also mentioned in previous reports.36 In order to further detect and characterize the structural characteristics of TPT-DHBDX COFs (X= 0, 50, 100), transmission electron microscopy (TEM) was employed after the treatment of ultrasonic stripping. As shown in Figures 3f,g, TPT-BD COF and TPT-DHBD50 COF exhibited sheet-like morphology, but TPT-DHBD COF was shown as a nanowire, which was in accordance with the SEM observation (Figure S7). Selected area electron diffraction (SAED) patterns revealed that TPT-DHBDX COFs (X= 0, 50, 100) had obvious electrondiffraction spots, which confirmed the PXRD analysis (Figure 3). Furthermore, comparing the pore size distributions of TPT-DHBDX COFs (X= 0, 25, 50, 75, 100), it was found that TPT-BD COF had partial distributions in the range of 5-100 nm beside the main distribution in 3.43 nm. As the number of H-bonding sites increased, the pore size distributions of TPT-DHBDX COFs (X= 0, 25, 50, 75, 100) became centralized gradually in about 3.43 nm and the pore sizes were close to the theoretical values, meaning the regular pore diameter and ordered internal configuration (Figure S8). In particular, TPT-DHBDX COFs possess the largest crystal lattice size among COFs from FBBs reported before. In summary, the formation of intralayer hydrogen bonds in 2D COFs based on FBBs has been demonstrated by theoretical calculation and experimental study and the hydrogen bonds make partial structure of the COF system “locked”, which decreases the rotational freedom of the FBBs and increases the flatness degree of the 2D layer and reduces the total energy of the system. As a result, the degree of microscopic order of the materials is increased, which exhibits that the crystallization of the COFs tends to be more complete and the crystallinity increases significantly. Another prominent advantage of H-bonding in this system is that the positive effect on the crystallinity also simplifies the synthetic process, making the synthetic conditions more moderate and convenient. Compared with the conventional solvothermal method for COF synthesis, the preparation of COFs in this paper is achieved without sealing tube. The reaction vessel is only ordinary glass bottle containing a screw cap and the reaction temperature is 80 °C, relatively lower than the usual 120 °C. More importantly, this reaction does not require a cumbersome degassing operation and rigid protection by inert atmos-

Figure 5. PXRD patterns of the TPT-DHBDX COFs (a) (insert: normalized amorphous peaks) and TPT-DHBDX COFs at 2θ in the 3-12°region (b).

Powder X-ray diffraction (PXRD) was used to evaluate the crystallinity of TPT-DHBDX COFs (X=25, 50, 75) and the results showed obvious diffraction peaks at ~2.27°, 4.07°, 4.70°, 6.22° and 8.20°, which were assigned to the (100), (110), (200), (210) and (220) facets, respectively (Figure S5). As shown in Figure 5a, there is a significant characteristic that the intensities of PXRD diffraction peaks increased little by little with increasing molar ratio of DHBD under the same detection conditions. For example, without H-bonding sites inside the molecular, the PXRD diffraction peak intensity of TPT-BD COF, i, e. TPT-DHBD0 COF, on the (100) facet is 2211 cps. When hydrogen bonds were introduced and the proportion of DHBD was increased to 25%, the diffraction intensity of the (100) facet significantly increased to 10067 cps. In particular, the intensity of the PXRD diffraction peak of the (100) facet of TPT-DHBD COF, i, e. TPT-DHBD100 COF, reached up to 42000 cps, much higher than other materials in the paper, when the proportion of DHBD was increased to 100%. Moreover, along with the increase of H-bonding sites in TPT-DHBDX COFs, the intensities of the amorphous peaks (broad peak at 22°) relative to the (100) facet decreased gradually with the ratios about 8.9%, 3.0%, 1.4%, 0% and 0%, respectively. It was also revealed in Figure 5 that the full-width halfmaximum values of each diffraction peaks, such as (100), (110) and (210) facets, had a regular decreasing tendency

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phere.37 Therefore, a gram-scale preparation of TPT-DHBD COF was realized easily in similar conditions via increasing the volume of reaction vessel and the quantity of raw materials (see SI for details). PXRD profile of TPT-DHBD COF on a gram scale has the same clear diffraction peaks caused by corresponding facets, indicating excellent crystal quality and high crystallinity of the product (Figure S5b). This strategy provides the possibility for preparing well-defined high-crystalline COFs on a large scale, which, to the best of our knowledge, has rarely been reported until now.

the materials under different adsorption time was detected by gravimetric measurement and the materials after iodine adsorption are denoted as TPT-DHBDX COFs@I2 (X = 0, 25, 50, 75, 100) (Figure 6a). The results showed that the adsorption capacities increased almost linearly within the initial 6 h of adsorption time, reaching over 70% of the total adsorption capacities (Table S8). The adsorption was almost close to equilibrium at 12 h and no longer changed after 32 h, indicating the adsorption equilibrium had been reached. In the process of adsorption, the color of the materials kept deepening continuously from light yellow to the final dark black (Figure S10). The kinetics simulation data shows that iodine adsorption conforms to a pseudosecond-order kinetic model within 600 min, which means that chemical adsorption is the major process (Figure S11, Tables S3-6).39 Under the adsorption conditions, the saturated adsorption capacities of TPT-DHBDX COFs are about 5.43, 4.65, 4.30, 4.12, 3.88 g/g when X = 0, 25, 50, 75, 100, respectively. The adsorption capacities are fairly high for all the products, especially TPT-BD COF, whose iodine uptake is definitely higher than all the previously reported solid adsorbents, including MOFs, amorphous POPs and other known COFs (Table S11). In additions, the iodine uptake of TPT-DHBD COF prepared on a gram scale can reach up to 4.03 g/g, suggesting the potential for largescale production and practice application of the materials (Figure S12). More importantly, the FT-IR spectra of the materials are fairly similar before and after the irradiation of 105 Gy γ-ray, which indicates that the as-prepared COFs possess good irradiation stabilities (Figure S2). All the results indicate that TPT-DHBDX COFs are excellent iodine adsorbents and have great potential value for the enrichment and removal of radioactive iodine in specific environment, such as nuclear accident site. The desorption process of TPT-DHBDX COFs@I2 were achieved with a faster iodine release efficiency and more than 80% of the adsorbed iodine could be released during 40 minutes at 125 °C (Figure 6b). The desorption was tolerably balanced after 6h and almost all iodine was released at this moment (Table S9), suggesting that TPT-DHBDX COFs are easy to recycle and reuse. The iodine uptake of TPT-DHBDX COFs (X = 0, 50, 100) were calculated again by thermogravimetric analysis (TGA), which were 4.77 g/g, 4.24 g/g, 3.77 g/g, respectively. The results are basically consistent with the measured values of the gravimetric method, and the subtle differences may be caused by some incomplete release of iodine at the calculated temperature. Recycling experiment shows that TPT-BD COF can maintain the iodine uptake capacity of 515 wt% (up to 96% recycling percentage) upon completion of the first cycle. And it still retained remarkable capacity of 472 wt% (up to 87.9% recycling percentage) after the third cycle (Figure S13). Obviously, the iodine adsorption process of TPTDHBDX COFs is efficient and reversible, which can be operated expediently in the enrichment and storage of volatile iodine. Furthermore, TPT-DHBDX COFs can also carry out the iodine enrichment and release with high efficiency in solution (Figures S14 and S15). The mechanism of the iodine enrichment was preliminary studied by FT-IR spectra, Raman spectra and PXRD patterns. It was found from FT-IR spectral analysis that the characteristic peaks position of the materials changed sig-

Figure 6. Gravimetric iodine uptake of TPT-DHBDX COFs (a) as a function of time at 75 °C and ambient pressure. Controlled release of iodine upon heating the TPT-DHBDX COFs@I2 (b) at 125 °C.

Iodine Adsorption. According to the designed structures, TPT-DHBDX COFs (X= 0, 25, 50, 75, 100) are rich in nitrogen and contain abundant π-conjugated system, including aromatic rings and C=N bonds. These characteristics provide the COFs many possibilities for their interaction with iodine.26,38 TPT-DHBDX COFs also have good thermal stabilities (Figure S9). In particular, the weight loss of TPT-BD COF is only 7% when the temperature ranges from 25 to 400 °C. Therefore, the materials are very suitable for the application in adsorption and desorption experiments of iodine vapor. To evaluate the enrichment capability for iodine vapor, TPT-DHBDX COFs powder was exposed to excess iodine vapor in a closed system at 75 °C and ambient pressure, which is close to the typical nuclear fuel reprocessing conditions. The adsorption capacities of

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Scheme 3. Schematic illustration of the iodine uptake mechanism of TPT-DHBDX COFs (N, blue; C, gray; O, red; H, white; R=H or OH, pink).

show any characteristic peaks belonging to molecular iodine under the condition of saturated adsorption, suggesting the transformation of iodine as well (Figures 7b and S5c). Based on the results, the possible mechanism of volatile iodine uptake on TPT-DHBDX COFs is put forward and shown in Scheme 3. Table 1. The characteristic structural units in TPTDHBDX COFs (X=0, 25, 50, 75, 100) and the corresponding iodine adsorption capacities.

Figure 7. FT-IR spectra of TPT-BD COF and TPT-BD COF@I2 (a). PXRD patterns of the pristine iodine and iodine loadedCOFs (b). Raman spectra of TPT-BD COF (c) and TPT-DHBD COF (d) before and after volatile iodine adsorption.

nificantly before and after the adsorption. For example, the C=C and C―H bands of phenyl ring in TPT-BD COF shifted from 1499 and 815 cm-1 to 1492 and 805 cm-1, and the C=N bands of triazine ring at 1566 and 1363 cm-1 shifted to 1562 and 1359 cm-1. Moreover, the imine linkages changed markedly from 1622 to 1631 cm-1. TPT-DHBDX COFs (X = 0, 25, 50, 75) also had a similar pattern, which indicated that the iodine adsorption could occur simultaneously at imine linkage, triazine ring and phenyl ring in the materials (Figures 7a and S2).26,38 The species of iodine in TPT-BD COF@I2 and TPT-DHBD COF@I2 were detected by Raman spectroscopy (Figures 7c,d). The spectra showed that TPTBD COF@I2 and TPT-DHBD COF@I2 have the strongest peak at ~167 cm-1 which has been confirmed to be the signature peak of I5- (consistent with I5- in the V or L configuration).40,41 Combined with the literature reports, some charge-transfer complex was formed through charge transfer interaction between iodine guest molecules and the electron-rich TPT-DHBDX COFs and led to the generation of I5-, which is consistent with the analysis of FT-IR spectra.42,43 In addition, diffraction peaks of TPT-BD COF@I2 and TPT-DHBD COF@I2 in PXRD patterns did not

COFs

Phenyl ring

Triazine ring

imine linkage

Hbonds

AdsI2 (g/g)

X=0

6

1

3

0

5.43

X=25

6

1

3

0.75

4.65

X=50

6

1

3

1.5

4.30

X=75

6

1

3

2.25

4.12

X=100

6

1

3

3

3.88

Interestingly, it was found that the adsorption capacities of TPT-DHBDX COFs for iodine decreased gradually along with the increasing proportion of the intralayer H-bonding interaction sites. Seen from Table S10, the iodine uptake has no apparent correlation to the surface area and porosity of the materials, indicating they are not the decisive factor for iodine adsorption. Usually, the iodine adsorption process is related not only to the surface properties of the materials, but also to the type and the quantity of functional groups in the materials directly.44 Analysis of the functional groups in TPT-DHBDX COFs was found that the structure units (C42H27N6OY)n (Y=3~6) and the number of functional components except H-bonding interaction sites were identical for all the COFs in the paper (Table 1). While, the iodine adsorption capacities decreased gradually with the increasing proportion of H-bonding sites, from 5.43g/g without hydrogen bonds to 3.88 g/g containing the most hydrogen bonds. The declination of iodine adsorption capacities, on the one hand, should be attributed to the H-bonding interaction between C=N and –OH groups which reduced the electron density of nitrogen

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atoms in the imine linkages, consequently impairing the chance of the charge transfer interaction with iodine. Namely, hydrogen bonds occupied some active sites for iodide adsorption and thus led to the adsorption capacities decline. On the other hand, H-bonding interaction resulted in the formation of a stable five-element ring structure inside the materials, which also gave rise to a certain space baffle effect hindering the approach and the adsorption of iodine to the imine linkage active sites. The discovery of this mechanism is of important guiding significance for rational and directional design and preparation of ultrahigh iodine capture materials. CONCLUSIONS In summary, a series of 2D COFs based on the flexible modules are constructed through combinatorial copolymerization. The effects of the existence and the proportion of Hydrogen bonds on the structure and properties of the as-prepared COFs have been studied for the first time, and the results exhibit that H-bonding interaction makes partial structure of the COFs “locked”, which reduces the degree of rotational freedom of the flexible unit and increase the flatness degree of the 2D layers, consequently leading to the significant improvement of microscopic order degree as well as crystallinity of the COFs. Therefore, COFs with high crystallinity and the largest lattice dimension among all reported congeneric materials can be obtained under moderate and convenient conditions, which also can be expanded to a rarely reported large-scale preparation of high quality COFs. Due to the high nitrogen content and abundant electron-rich π-conjugated system, TPT-DHBDX COFs show efficient, reversible and ultrahigh adsorption capacities for volatile iodine and the maximum capacity can reach up to an unreported 543 wt%. Moreover, the asprepared COFs are proved to possess good irradiation stabilities and can withstand 105 Gy γ-ray irradiation, suggesting their potentials to capture radioactive iodine, such as 129I or 131I in specific circumstances. Studies of mechanism also reveal that H-bonding interaction occupies some adsorption active sites and hinders the approach of iodine to the imine linkages by space baffle effect, which will give helpful guidance for the design and preparation of iodine capture materials. In addition, the ordered spatial arrangement, large porosity and multiple-triazine-ring structure of TPT-DHBDX COFs are able to afford the materials a promising application in the field of CO2 selective capture and metal-supported catalysis.

§ These

authors contributed equally.

The authors declare no competing financial interest.

The financial support from the Science Challenge Project TZ2016004, the National Natural Science Foundation of China (Grants 21771128, 21671140, 11575122 and 11475120) and the international cooperation of Sichuan province 2017HH0056 are gratefully acknowledged.

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Detailed experimental procedures; FT-IR and NMR spectra; XPS data; XRD data; TEM images; BET data; TGA data and iodine uptake data. This material is available online from the http://pubs.acs.org.

* E-mail: [email protected] * E-mail: [email protected] Lijian Ma: 0000-0002-6317-6287

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Table of Content


11

Mechanistic Insight into Hydrogen-Bond-Controlled Crystallinity and

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