An Ultrastable Metal–Organic Framework with Open Coordinated

Article pubs.acs.org/crystal

An Ultrastable Metal−Organic Framework with Open Coordinated Sites Realizing Selective Separation toward Cationic Dyes in Aqueous Solution Meng-Ke Wu,† Fei-Yan Yi,*,†,§ Yuan Fang,† Xun-Wen Xiao,‡ Shi-Cheng Wang,† Lu-Qing Pan,† Shuai-Ru Zhu,† Kai Tao,† and Lei Han*,† †

State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo 315211, China ‡ Department of Chemical Engineering, Ningbo University of Technology, Ningbo 315016, China § Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou, Guangdong 515063, P. R. China S Supporting Information *

ABSTRACT: A novel Zn-based metal−organic framework (MOF) was synthesized from the mixed ligands, 3,3′,5,5′-azoxybenzenetetracarboxylic acid (H4AOBTC) and 1,4-bis(1H-benzo[d]imidazol-1-yl)benzene (phenDIB) ligand. The crystal structure of the complex exhibits an interpenetrated three-dimensional framework that can be simplified as a (4,4)-connected 2-nodal bbf net. This MOF displays extraordinary thermostability in boiling water for 12 h and chemical stability in a wide pH range of 2−13. The most intriguing feature is that it can successfully separate cationic dyes from mixed dye molecules in aqueous solution with high effectivity and selectivity, even for rhodamine B molecule with a large size. Furthermore, this material is reusable, and the adsorbed dye molecules can be released and recovered completely. It is very important for the practical application from a view of environmental protection and resource recycling.

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metal centers aid the prediction of structure;11,12 however, the successful predictions of functional channels in target frameworks rely heavily on the organic linkers.13−16 So it is extremely important to select an appropriate organic linker to construct porous MOFs. Herein, an novel azo-carboxylate ligand 3,3′,5,5′-azoxybenzenetetracarboxylic acid (H4AOBTC) with an open negative charged O site was adopted since the strategy of open sites has been proven effectively to realize the selectivity.17−19 Another N-donor ligand, 1,4-bis(1H-benzo[d]imidazol-1-yl)benzene (phenDIB), was also used based on a dual-ligand strategy because two complementary ligands can provide an additional level of control in the framework structure and charge density distribution. On the basis of the aforementioned consideration, a Zn-based 2-fold interpenetrated three-dimensional (3D) MOF, namely, [Zn(phenDIB)(AOBTC)0.5] (1), was synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric analyses (TGA), power X-ray diffraction (PXRD), and elemental analyses. Interestingly, 1 is ultrastable in various organic solvents, the aqueous solution in a wide pH

INTRODUCTION Over the past decade, millions of tons of dyes were used in various industries every year, such as paper, plastics, printing, nylon, wool, silk, etc.1 As a result, the treatment of a large number of dye wastewater has attracted great attention, since most dye molecules with high stability are difficult to degrade by simple oxidation and light.2 In particular, most of them are highly toxic, which not only causes serious environmental pollution, but also poses a great threat to human health. Hence, it becomes an urgent project to remove these toxic dye molecules with high efficiency and low cost. At present, it is a popular method to degrade dyes by chemical catalysts;3 however, it has high costs and consumes too much energy. Adsorption technology is regarded to be most promising from the point of application because it is ecofriendly and economical, since the adsorbed dyes can be released and recycled.4,5 Metal−organic frameworks (MOFs) as a class of new porous crystalline materials are considered to be promising adsorption materials due to their tunable pore sizes and functional active sites, such as Lewis basic/acidic sites and open metal/ligand sites, which ensure the adsorbed performance and realize the selective separation.6−10 Essentially, MOFs are constructed by metal centers or clusters and organic ligands. The adopting © 2017 American Chemical Society

Received: July 15, 2017 Revised: August 20, 2017 Published: August 22, 2017 5458

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range of 2−13 as well as in boiling water for 12 h. Furthermore, it can separate selectively cationic dyes from aqueous solution containing mixed dye molecules, even for a rhodamine B (RhB) molecule with a large size.

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Table 1. Crystal Data for 1 CCDC empirical formula formula weight temperature λ, Å crystal size, mm3 crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z calculated density, mg/mm3 F(000) adsorption coefficient, mm−1 reflections collected/unique completeness, % goodness-of-fit on F2 final R indexes [I > 2σ(I)] final R indexes (all data)

EXPERIMENTAL SECTION

Materials and Physical Measurements. The 3,3′,5,5′-azoxybenzenetetracarboxylic acid was synthesized according to previous literature (see Supporting Information, Figure S1).20 1H NMR data were collected on a Bruker 500 MHz spectrometer. Other chemicals and reagents for the syntheses were commercially obtained and used without further purification. Powder X-ray diffraction patterns (PXRD) were performed on a D8Focus (Bruker) equipped with Cu Kα radiation field-emission (λ = 1.5405 Å, continuous, 40 kV, 40 mA, increment = 0.02°). Thermogravimetric and differential thermal analysis (TG-DTA) patterns were carried out on a SII TG/ DTA7300 apparatus at a ramp rate of 10 °C/min under an air atmosphere in the temperature range of 30−800 °C. IR spectra were obtained on a Shimadzu FTIR-8900 spectrometer on a KBr disk in the wavelength range of 4000−500 cm−1. UV−vis spectra studies were performed at room temperature on a PerkinElmer Lambda 900 UV/ vis/NIR spectrophotometer equipped with an integrating sphere in the wavelength range of 200−1200 nm. The contents of C, H, and N in the sample were measured by using an Elementar Vario EL III analyzer. Synthesis of [Zn(phenDIB)(AOBTC)0.5] (1). Zn(NO3)2·6H2O (0.2 mmol, 59.4 mg), H4AOBTC (0.05 mmol, 18.7 mg), and phenDIB (0.05 mmol, 15 mg) in mixed solvent of dimethylacetamide (DMA) (3 mL) and fluoroboric acid (1.2 mL) were sealed in a Teflon-lined stainless steel vessel (20 mL) and heated to 120 °C in 300 min; maintained at this temperature for 4 days; and then cooled to room temperature at a rate of 0.1 °C/min. The resulting yellow brick crystals were obtained, then washed with DMA three times. The yield is ca. 66% based on H4AOBTC. Anal. Calcd (%) for 1 C28H17N5O5Zn (Mr = 568.84): C, 59.10; H, 2.99; N, 12.30. Found: C, 59.23; H, 2.73; N, 12.21. FT-IR data (cm−1, KBr): 3436 (m), 3106 (m), 1718 (w), 1643 (vs), 1620 (s), 1573 (w), 1524 (vs), 1464 (m), 1384 (m), 1326 (s), 1238 (s), 1157 (w), 1101 (w), 995 (m), 913 (m), 851 (m), 754 (s). X-ray Crystallography. Suitable single crystal with dimensions of 0.35 × 0.32 × 0.16 mm3 of 1 was selected for single-crystal X-ray diffraction analyses. Crystallographic data were collected at 293 K on a Bruker Apex II CCD diffractometer with graphite monochromated Mo−Kα radiation (λ = 0.71073 Å). Data reduction was performed with SAINT.21 The absorption corrections was applied by using the multiscan program SADABS.22 The structure was solved by direct methods and refined on F2 by full-matrix least-squares using SHELXTL-97.23 All non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. The hydrogen atoms associated with the organic molecules were placed in calculated positions based on geometrical considerations and assigned isotropic thermal parameters. The formula for 1 was determined by combining single-crystal structure, elemental microanalysis, and TGA. The detailed crystallographic data for the structure are listed in Table 1. Selected bond distances and angles are given in Table S1.

1529834 C28H17N5O5Zn 568.84 298(2) 0.71073 0.35 × 0.32 × 0.16 monoclinic P21/c 10.7444(4) 14.0078(6) 17.0348(6) 90 113.500(2) 90 2351.18(16) 4 1.607 1160 1.098 29996/5250 [Rint = 0.0479] 97.1 1.053 R1 = 0.0595, wR2 = 0.1643 R1 = 0.1025, wR2 = 0.2046

Figure 1. ORTEP representation of the asymmetric unit of 1. Thermal ellipsoids are drawn at the 30% probability level.

ligand lies on the 2-fold axis, so only a half one is crystallographically unique, in which each carboxylate group is unidentate with a noncoordinated oxygen and connects one Zn center adopting a μ1-η1η0 coordination mode. Each AOBTC ligand is quadridentate and bridges four adjacent Zn(II) centers via four carboxylate groups into a two-dimensional (2D) wavetype layer extending along the [100] direction, as shown in Figure 2a and Figure S2a. The separation of adjacent layers is ca. 10.74 Å (a axis). Such layers are pillared by phenDIB ligands to form a three-dimensional (3D) framework with large channels (15.91 × 14.01 Å2 along the c-axis, and 10.74 × 8.52 Å2 along the b-axis, including van der Waals radii) (Figure 2b and Figure S2b,c). Each phenDIB ligand as a 2-connected node links two Zn(II) centers by two N atoms (N2 and N4) into a one-dimensional (1D) zigzag chain (Figure 2c). Because of the existence of such large channels in each single net, a 2fold interpenetrating net for 1 was fabricated in order to stabilize the whole structure (Figure 2c). The independent net in 1 is related by a single vector of [1,0,0] (10.74 Å), which can be assigned to the class Ia interpenetration. Apparently, each Zn center is connected by two AOBTC ligands and two phenDIB

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RESULTS AND DISCUSSION Description of the Crystal Structure. Single-crystal X-ray diffraction analyses revealed that the compound 1 crystallized in the monoclinic P21/c space group. As shown in Figure 1, the asymmetric unit of 1 contains one Zn center, one phenDIB ligand, and a half AOBTC ligand. Each Zn center is tetrahedral coordination with two carboxylate oxygen atoms (O1 and O4A) from two AOBTC ligands and two imidazole N atoms (N2 and N4B) from two phenDIB ligands. The Zn−O bond distances are between 1.917(3) and 1.978(4) Å, and the Zn−N distances range from 2.019(3) to 2.035(3) Å, which are similar to those in other reports.24−26 The deprotonated AOBTC 5459

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Such high thermostability, good water stability under hydrothermal conditions, and wonderful chemical stability with high acid and base resistance in a wide pH range, as well as recyclability are very important to realize its practical use, especially for industry. Combining its structural features in 1 with noncoordinated carboxylate oxygen atoms and open negative charged O site of azoxybenzene, these characteristics lay a firm foundation for the further practical study in selective separation. Dyes Adsorption and Separation Properties. Motivated by the above properties of 1, it is employed as separated material for dyes. First, to systematically investigate the adsorption ability of 1 toward organic dyes from contaminated water, three organic dye molecules, cationic methylene blue (MB), anionic methyl orange (MO), and neutral solvent yellow 2 (SY2) with different charges are selected as the typical pollutant targets for experiments. The samples of 1 with different amounts (1 mg, 3 mg, 5 mg, 8 mg, and 10 mg) were immersed in aqueous solution containing MB, MO, and ethanol solution of SY2 (20 mg/L, 20 mL) for 24 h, respectively. Then, these samples were centrifuged to remove the suspended particles, and the upper clear solutions were tested using a UV−vis spectrophotometer. The test results obviously show that MB dye molecules can be completely adsorbed and removed (Figures 4a and S8), but the removals of MO and SY2 dyes are almost negligible (Figures S9 and S10). In order to calculate the saturated adsorption capacity of 1 toward MB molecules, the standard adsorption curve of pure MB solution with different concentrations was obtained, in which all the absorbance plots as a function of concentration (C0, mg/L) had the best linear fit (Figure S11). The uptake capacity of 1 is 139.4 mg/g at room temperature, which is higher than that of commercial activated carbon and some other materials reported up to now (Table S2).4,5,28−31 Furthermore, the absorption efficiency for 1 toward MB molecules was tested. After each freshly prepared 1 (3 mg) was dispersed into aqueous solution of MB (20 mg/L, 10 mL) for a given time interval, the UV−vis spectra of each solution after centrifugation were obtained, as shown in Figure 4b and Figure S12. The results are excellent and show that 97% MB molecules can be rapidly adsorbed by compound 1. The above-mentioned experimental results demonstrate that 1 can efficiently remove MB dye molecules from aqueous solution. And the preliminary investigations also suggest 1 has the potential application of removal of cationic dye molecules for effluents, because it

Figure 2. (a) A Zn-OABTC 2D sheet; (b) the 3D single net; (c) a 2Dfold interpenetrating net; (d) the simplified 2-fold net.

ligands in opposite orientation, which is considered as a 4connected node. Each OABTC ligand acts as 4-connected node in a sheet. All phenDIB ligands as 2-connected linkers are transformed into the connective pillars between Zn centers. On the basis of this simplification, the whole interpenetrated 3D framework can be simplified as a (4,4)-connected 2-nodal bbf network with a point symbol of {64.82}{66}2 defined by TOPOS software (Figure 2d).27 Purity and Stability. The experimental PXRD pattern for 1 is in good agreement with its simulated pattern from the singlecrystal X-ray diffraction, demonstrating the phase purity for 1 (Figure S3). The TGA result shows that 1 is very stable up to 400 °C (Figure S4). The more important one is that it still remains intact after being boiled for 12 h in water, soaked in various water solutions with different pH values of 1−13 for 2 days, and immersed in common organic solvents such as methanol, ethanol, acetone, isopropanol, n-propanol, n-butanol, N,N′-dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile, dichloromethane (CH2Cl2), chloroform (CHCl3), and cyclohexane for 4 days. All PXRD patterns, FTIR spectra, and N2 sorption isotherm of treated samples strongly verify that the crystalline integrity of 1 is retained (Figure 3, Figures S5− S7). In addition, 1 is insoluble in water and common organic solvents, and can easily be recovered by centrifugal operation.

Figure 3. PXRD patterns of 1 after immersion in water solution of different pH values for 2 days (a) as well as various common organic solvents for 4 days (b). 5460

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Figure 4. UV−vis spectra and the related photographs of the adsorption rate for 1 toward pure MB (20 mg/L) aqueous solution, respectively, with different quantity (mg) of 1 (a) and the given time (b).

Figure 5. UV−vis spectra for selective adsorption of 1 toward the mixed dyes: (a) MO and MB, (b) MO and RhB, as well as the related photographs of color change at a given interval.

exhibits clearly superior adsorption properties for cationic MB compared with anionic MO and neutral SY2 dye molecules. As expected, the structural synergistic effect in 1 causes preferentially adsorbent toward cationic MB molecules rather than anionic MO molecules,32,33 containing a large number of noncoordinated carboxylate oxygen atoms and open negative charged O sites of azoxybenzenes with high electron densities on the material surface of 1. Such rapid adsorption should be due to the surface adsorption, since no residual solvent accessible void in 1 can be found based on the calculation performed using the PLATON program.34 In order to investigate the surface charge of as-prepared samples, zeta potential measurements were carried out as shown in Figure S13. A highly negative zeta potential (−19.2 mV) can be observed for 1 in aqueous solution. Thus, the strong electrostatic attractions between cationic dye molecules and highly negative charged surface of 1 play a vital role during such separation procedure. On the other hand, the impact of particle sizes on the adsorption property was also investigated. First, the crystals were ground for different times (0, 5, 15, 30, and 60 min) to obtain different particle sizes (Figure S14). Then the samples with different sizes were immersed in MB aqueous solution for adsorption experiments. The results show that the faster

adsorption rate of compound 1 toward MB dye can be observed as the smaller particle size in the same condition (Figure S15). So particle size has a certain degree of impact on the adsorption property for surface adsorption. Of course, some other parameters also affect the adsorption performance, such as temperature, pH, contact time, etc.35 In this work, all experiments are based on the sample of 1 after being ground for 30 min. But in general, the impact is very limited, and all samples show very good adsorption properties. The functionality of materials is directly related to their textures. Therefore, the designed structure still plays the key role in selective removal toward dye molecules with different charges. To confirm this assumption, another cationic dye, RhB, with a much larger molecular size was selected for further study. As shown in Figure S16a,b, the different quality of 1 was used to adsorb RhB (20 mg/L) in aqueous solution, and the results clearly exhibit that RhB molecules can be almost completely adsorbed and removed. In addition, the UV−vis adsorption spectra of RhB dye solution (20 mg/L) with 3 mg sample of 1 at given time intervals display that RhB dye molecules can be adsorbed rapidly with a removal efficiency of 94% (Figure S16c,d). By similar calculation by a linear equation from pure RhB with different concentrations (Figure S17), the adsorption capacity of 1 at room temperature is 127.5 mg/g for RhB. 5461

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Figure 6. UV−vis spectra of three adsorption and desorption cycles for 1 toward MB (a) and RhB (b) dye molecules. The first cycle: Ad-first and De1st; the second cycle: Ad-second and De2nd; the third cycle: Ad-third and De3rd, as well as the comparative photographs after three cycles.

azoxybenzene successfully enhances the selectivity and removal efficiency of material. Furthermore, dye-release experiments were performed for 1 loaded with MB and RhB in order to test the stability and recyclability, which are two crucial factors for practical application. Take MB@1 as an example: the sample after the adsorption process was immersed in various eluting solutions containing the pure aqueous solution of NaCl, pure ethanol solution, and the mixed solution of NaCl with ethanol and water of different volume ratios (2:1, 1:1, and 1:2) at room temperature. Since 1 is insoluble in water and solvents, after 1 h, the solution of MB and RhB released from MB- or RhBloaded MOF after centrifugation was monitored by UV−vis spectra. The results (Figure S20) show that the intensity of released MB from MB@1 in the solution of NaCl with ethanol and water (1:1) remain basically consistent with the initial pure one, confirming that they have been almost completely released over other eluting solutions. Such fast release of dye molecules is owing to weak electrostatic interactions between dye molecules and the framework. Then the cycling tests for adsorption and desorption were observed using a solution of NaCl with ethanol and water (1:1) as an eluting solution. The sample of 1 after desorption was simply heated at 80 °C for 3 h, and then continuously being used for the next cycle test for adsorption and desorption. The solution of each adsorption− desorption (the removal and regeneration of MB and RhB) was evaluated through UV−vis spectroscopy. The spectroscopic investigations of the supernatants and digital images show that this material still displays high effective removal capability toward MB and RhB molecules after several cycles (Figure 6). In addition, we have also demonstrated that this separation material is very stable and reusable by PXRD patterns and FTIR spectra of samples after several cycles. The results of PXRD patterns and FTIR spectra are consistent with the assynthesized products, suggesting that the framework integrity of 1 can be well retained (Figures S21−S23). All adsorbed and removal performances of 1 toward dye molecules are very significant for practical applications.

Although its uptake capacity is a little lower than MB molecules, to our knowledge, the MOFs that can absorb such large dye molecules are quite rare.30,36 These results further demonstrate that particle size has a certain degree of impact on the adsorption property for surface adsorption; however the crystal structure plays the key role in its functionality, especially for selective adsorption. The other types of nonporous compounds also successfully realize the removal of dye molecules through electrostatic attractions between materials and dye molecules.37,38 It will be very attractive and challenging if 1 can realize selective adsorption and separation of cationic dyes by virtue of opposite charges not but a size-exclusion effect.39 To validate that this material possesses the ability to separate different dyes with opposite charges, two mixed dye solutions were prepared in a centrifuge tube, MB and MO (10 mg/L, 20 mL), RhB and MO (10 mg/L, 20 mL) like the chromatographic column, where a yellow solution of MO is mixed with a blue solution of MB into olive solution, while a pink RhB solution becomes orange solutions (Figure S18). After 10 mg of freshly samples of 1 were exposed to above the binary mixture, respectively, a clear color change could be observed after a given time (Figure 5), from olive to yellow for MB and MO, as well as from orange to yellow for RhB and MO, indicating that the material 1 could selectively capture cationic MB and RhB molecules from the mixed dye solution, respectively. The UV− vis absorption spectra also confirm this phenomenon, where the absorption peaks of MB and RhB all disappeared, just leaving the characteristic absorption peaks of MO (Figure 5). Their separation efficiency is very high. Then, the mixed dye solutions with higher concentrations were used for these tests. A similar selective adsorption can be still clearly observed (Figure S19). The above inquiry proves the selective adsorption ability of 1 toward cationic dyes of opposite charges, even dyes with a similar size, such as MO and MB, as well as dyes with a very large size, such as RhB. To our knowledge, this is a quite rare realization of a MOF separation for such large dye molecules from mixed solutions. Our experimental results unambiguously demonstrate that the removal process of dyes is based on strong electrostatic attractions between cationic dye molecules and highly negative charged surfaces as designed. The strategy of an open negative charged O site of

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CONCLUSION The open charged site approach has been established to enhance the selective adsorption of MOF material. A Zn-based 5462

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MOF was successfully constructed, which possesses high thermostability, strong acid−base tolerance in the aqueous solution of pH 2−13, wonderful water stability in boiling water for 12 h, as well as chemical stability in various organic solvents. Giving full play of its structural and performance advantages, this material realizes selective adsorption and separation toward cationic dyes (MB and RhB) from dye wastewater solution. Such a separation method toward dyes is quite different from the previously reported the ones based on a size-exclusion effect.40−42 Furthermore, this material is reusable, since its sample after adsorption−desorption treatment does not affect the next removal of dyes. Therefore, this material with open charged sites offers great technological promise in the field of dye wastewater treatment, cationic dye recovery, and material recycling. In particular, the material is highly stable in harsh environments, which makes it a candidate for applications in industry.

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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/acs.cgd.7b00984. Images of crystal structure, PXRD, TGA, and FTIR, and UV−vis results (PDF) Accession Codes

CCDC 1529834 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(L.H.) E-mail: [email protected]. *(F.-Y.Y.) E-mail: [email protected]. ORCID

Lei Han: 0000-0002-2433-9290 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 (91122012, 21471086), the Social Development Foundation of Ningbo (No. 2014C50013), the Xinmiao Talent of Zhejiang Province, and the K.C. Wong MagnaFund at Ningbo University.

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REFERENCES

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Crystal Growth & Design

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DOI: 10.1021/acs.cgd.7b00984 Cryst. Growth Des. 2017, 17, 5458−5464