Hydrolytically Stable Nanotubular Cationic Metal–Organic Framework

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Hydrolytically Stable Nanotubular Cationic Metal−Organic Framework for Rapid and Efficient Removal of Toxic Oxo-Anions and Dyes from Water Shu-Qi Deng, Xiao-Jing Mo, Sheng-Run Zheng,* Xia Jin, Yong Gao, Song-Liang Cai, Jun Fan, and Wei-Guang Zhang* School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, PR China Downloaded via IOWA STATE UNIV on February 5, 2019 at 20:58:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Cationic framework materials capable of removing anionic pollutants from wastewater are highly desirable but relatively rarely reported. Herein, a cationic MOF (SCNU-Z1-Cl) possessing tubular channels with diameter of 1.5 nm based on Ni(II) and a nitrogen-containing ligand has been synthesized and applied to capture hazardous anionic contaminants from water. The SCNU-Z1-Cl exhibits high BET surface area of 1636 m2/g, and shows high hydrolytically stability in pH range from 4 to 10. Owing to the large tubular channels and the uncoordinated anions in the framework, the aqueous-phase anion-exchange applications of SCNU-Z1-Cl were explored with environmentally toxic oxo-anions including CrO42−, Cr2O72−, MnO4−, and ReO4−, and organic dyes. The adsorption of oxoanions exhibits high uptake kinetics and the adsorption capacities of CrO42−, Cr2O72−, MnO4−, and ReO4− are 126, 241, 292, and 318 mg/g, respectively, which were some of the highest values in the field of MOF/ COF. In additional, the selectively is high when other concurrent anions are exist. The anionic dyes with different sizes including methyl orange, acid orange A, congo red, as well as methyl blue can be adsorbed by SCNU-Z1-Cl in few minutes to about 1 h. The adsorption capacities for them are 285, 180, 585, and 262 mg/g, respectively. In contrast, the adsorption kinetics for catinionic dyes with different sizes is obviously lower and exhibit a size-selectively adsorption that only cationic dye with suitable size (rhodamine B) can be adsorbed by SCNU-Z1-Cl. Consequently, SCNU-Z1-Cl can sepearate organic dyes in three different modes: size-dependent, charge-dependent, and kinetics-dependent selective adsorption. The excellent adsorption and separation properties of SCNU-Z1-Cl is attribute to the cationic framework, large tubular channel, as well as the high positive Zeta potential.

1. INTRODUCTION Water pollution has become a concern of the world and the removal of toxic contaminants from wastewater prior to their discharge has attracted worldwide research attention. These organic or inorganic pollutants mainly contain food, textiles, paper, plastics, cosmetics and pharmaceuticals industrial waste, the use of pesticides and fertilizers, as well as radioactive pollutants generated from nuclear power plants, and so on. Several inorganic pollutants that exist as oxo-anion in water, are priority pollutants according to the U.S. Environmental Protection Agency (EPA).1 In particular, hexavalent chromium Cr(VI) has serve harm to human health, thus it is classified as a Group “A” human carcinogen by the EPA.2,3 Pertechnetate (TcO4−) ion is one of a problematic radionuclides in the nuclear fuel cycle because its high mobility in the environment, thus the studies on removal of TcO4− or its nonradioactive surrogates, ReO4− and MnO4−, become more and more important.4−6 Besides, organic dye is extensively used in a wide variety of manufacturing industries and produces large © XXXX American Chemical Society

amounts of wastewater, which can be a great hazard to both the environmental safety and the human health.7−10 Many technologies including chemical oxidation/reduction,11,12 electrocatalytic/photocatalytic degradation,13,14 biological treatments,15,16 membrane filtration,17,18 adsorption,19,20 and so on, have been developed to eliminate toxic contaminants from wastewater. Among them, adsorption is considered to be one of the better methods because of its high feasibility and efficiency, relatively low cost, fewer byproducts, and simple operation.21,22 Therefore, many materials including zeolites, activated carbon synthesized from different precursors, clay and modified clay, cellulosic derivative materials, and graphene-based composites, have been applied in the adsorption of toxic and radioactive contaminants.23−26 Great progress has been made in this field; however, these materials still have some inherent disadvantages such as relative low Received: January 11, 2019

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DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry adsorption capacity, slow adsorption rate, poor selectivity, and complicated preparation processes. Thus, developing new adsorbent materials for more effectively and efficient adsorption of contaminants from wastewater is desirable. Metal−organic frameworks (MOFs), constructed from metal ions/clusters and organic ligands, comprise an important class of porous framework materials with diverse topology structures, large surface area, and adjustable pore size and shape.27−32 Recently, MOFs have been used for water remediation, mainly acting as an absorbent or catalyst to remove pollutants from wastewater.33−37 For removing anionic pollutants, cationic framework materials seem a good choice; however, such materials are scare compared to the neutral and anionic framework materials.38−42 As a subclass of MOFs family, cationic MOFs composed of cationic framework and uncoordinated and exchangeable anions represent a cationic framework material that has great potential application on adsorb anionic pollutants via anion-exchange.43−47 However, only a few cationic MOFs have been used for the adsorption of CrO42−/C2O72− and TcO4−/ReO4−/MnO4−.41−45 Furthermore, dye adsorption and separation on cationic MOFs are also very limited compared with those on anionic MOFs.48−51 In order to be used in water treatment, it is necessary for the cationic MOFs to possess not only enough pores for exchanging and accommodating guest anions but also high stability in water. The use of high-valence metal ions (such as ZrIV) and carboxylic acid ligands is an effective strategy to synthesize MOFs with high water stability.52,53 However, using carboxylic acid ligands is generally easier to form neutral MOFs rather than cationic MOFs. One method of assembling cationic MOFs is to utilize metal ions and nitrogen-containing ligands, but many of the MOFs synthesized by this method have unsatisfactory water stability. Therefore, cationic MOFs based on nitrogen-containing ligands with high water stability have rarely been reported.54,55 In addition, for many MOFs, although the pore size is large, the adsorption resistance is increased due to the smaller window, thereby reducing the adsorption kinetics. It is reasonable to expect that the adsorption kinetics would be increased if the pores of MOFs are tubular. For a tubular pore, the diameter of windows and pores are the same, and the conjugate planes of ligands are basically parallel to the direction of pores. In this case, the guest molecules more easily enter the pore, and it is easier to form various interaction with the wall of the pore. However, fewer cationic MOFs with tubular holes have been constructed.54,55 Fortunately, herein, a cationic MOFs with tubular channels by the reaction of bifunctional tripodal ligands with Ni (II) ions, namely, {[Ni2(DITP)3]·Cl· xsolvent}n (SCNU-Z1-Cl; HDITP = 3,5-di(1H-imidazol-1yl)phenyl)-2H-tetrazole) were constructed in the present work. SCNU-Z1-Cl exhibits high hydrolytically stability over a wide pH range. It can be used for the adsorption of oxo-anion pollutants (CrO42−, Cr2O72−, MnO4−, and ReO4−) and anionic dyes from water rapidly, as well as for the size-selectively separation of cationic dyes, as depicted in Figure 1.

Figure 1. Schematic representation of the adsorption of oxo-anions and dyes by SCNU-Z1-Cl in water. temperature in the wavelength range from 4000 to 400 cm−1. Powder X-ray diffraction (PXRD) measurements were recorded on an Ultima IV X-ray powder diffractometer (Kurary, Tokyo, Japan) at 40 kV, 40 mA equipped with Cu Kα radiation (k = 1.5406 Å). Thermogravimetric analyses (TGA) were carried out with a PerkinElmer TGA8000 under N2 atmosphere from room temperature to 800 °C with a heating rate of 10 °C/min. Zeta potential was measured by the Zetasizer Nano ZS90 from Malvern Instruments with the suspension concentration of 0.75 mg/mL. UV−vis spectra were performed on a Shimadzu UV-2550 spectrophotometer equipped with a Labsphere diffuse reflectance accessory. EDX images were performed on a MERLIN Compact scanning electron microscope at 15.0 kV. 1H NMR spectra were recorded at room temperature on Varian 500 (500 MHz) using TMS as an internal standard. 2.2. Syntheses of SCNU-Z1-Cl. A mixture of NiCl2·6H2O (0.05 mmol), HDIPT (0.1 mmol), NaOH (0.1M, 1 mL), and DMF (9 mL) was put in a 20 mL Teflon-lined stainless-steel autoclave. Then, the mixture was heated at 80 °C for 72 h and cooled at 5 °C h−1 to room temperature under autogenous pressure. Very pale-purple crystalline powder of SCNU-Z1-Cl was obtained. A few crystals large enough for single X-ray data collection were picked up from the crystalline powder. Yield 85% (based on the Ni). IR (KBr, ν/cm−1): 3392(s), 3129(m), 1619(s), 1500(s), 1449(m), 1364(w), 1314(w), 1245(s), 1110(m), 1068(s), 1016(m), 838(m), 753(s), 661(s). 2.3. Surface Area Measurements. The Brunauer−Emmett− Teller (BET) method was applied in order to calculate the specific surface areas of SCNU-Z1-Cl. Its pore structure was evaluated by measuring the N2 adsorption−desorption isotherms at 77 K via an adsorption instrument (TriStarII, Micromeritics Company, USA). Before testing, the SCNU-Z1-Cl crystals were immersed in acetone solvent for 1 day, and then the solvent was further replaced with fresh acetone every day for another 3 days to remove the guest solvent molecules. Then, the SCNU-Z1-Cl crystalline powder was collected through centrifugation and activated at 100 °C under vacuum for 24 h. 2.4. X-ray Data Collection and Structure Refinement. Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) for SCNU-Z1-Cl. Multiscan absorption correction was applied by using SADABS.56 Structural solutions and refinements based on F2 were performed with the SHELXS-201657 and SHELXL201657 program packages, respectively. SQUEEZE55 was applied to exclude the contributions of disordered solvent in the pores. All nonH atoms were refined with anisotropic thermal parameters. All the hydrogen atoms were placed in idealized positions. The crystal

2. EXPERIMENTAL SECTION 2.1. Materials and Measurements. Ligand 3,5-di(1H-imidazol1-yl)phenyl)-2H-tetrazole (HDIPT) was bought from Jinan Camolai Trading Company. All other reagents were of analytical grade and were purchased from Guangzhou Chemical Reagent Factory without further purification prior to use. IR spectra were performed on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets at room B

DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) Coordination environment of Ni(II) in SCNU-Z1-Cl. Symmetric codes: (i) 1 − x, 1 − y, −z; (ii) y, −x + y, −z; (iii) 1 + x − y, x, −z; (iv) 1 − y, x − y, z; (v) 1 −- x + y, 1 − x, z. (b) Binuclear SBU in SCNU-Z1-Cl. (c) M2L3 cage in SCNU-Z1-Cl. (d) Side view (left) and top view (right) of the 1D SBU in SCNU-Z1-Cl. (e) 3D framework in SCNU-Z1-Cl. (f) Nine- and three-connected nodes in SCNU-Z1-Cl. (g) 3D network topology in SCNU-Z1-Cl. parameters, data collections, and refinements are listed in Table S1. Selected bond lengths and angles are shown in Table S2. The CCDC reference number is 1556071. 2.5. Oxo-Anion Uptake Experiments. 2.5.2. General Procedure. Oxo-anions uptake by SCNU-Z1-Cl was studied by immersing 10 mg of SCNU-Z1-Cl in 30 mL of a 10−4 M aqueous solution of CrO42−, Cr2O72−, MnO4−, and ReO4−, respectively. The concentration of CrO42−, Cr2O72−, MnO4−, and ReO4− at given time were determined by UV−vis spectrophotometer. The adsorption isotherms of oxo-anions were performed by adding 5 mg of SCNU-Z1-Cl into 25 mL of solution with concentration ranging from 10 to 200 ppm. After the adsorption equilibriums were reached, the concentrations of oxo-anions were determined by UV− vis spectrophotometer. The uptake of oxo-anions in the presence of competing anions including Cl−, N3−, NO3−, ClO4−, and SO42− was studied using mixed equimolar (1 mM, 1:1) anion solutions of targeted oxo-anions and the competing anion. First, 5 mg of SCNU-Z1-Cl was added to 25 mL of mixed solution and stirred for 3 h; then, the concentrations of targeted oxo-anions were analyzed by UV−vis spectrophotometer. The desorption experiments were carried out by immersing 50 mg of SCNU-Z1-oxo-anion into 20 mL of 1 M NaCl solution and keeping them for 3 h. 2.6. Dye Adsorption and Separation. Eight dyes, namely, positively charged Methylene Blue (MLB), Crystal Violet (CV), Rhodamine B (RhB), and alcian blue 8GX (AB) and negatively

charged methyl orange (MO), acid orange A (AO), congo red (CR), and methyl blue (MB), were selected to study the dye adsorption and separation abilities of the as-synthesized SCNU-Z1-Cl powder. The adsorption kinetics experiments were carried out by immersing 10 mg of activated SCNU-Z1-Cl into 30 mL of 20 ppm aqueous solutions containing different organic dyes at room temperature. Next, 3 mL of the upper clear solution was taken at given time for UV−vis measurement and the solution then poured back into the original suspension. A similar method was also used to investigate the performance of dye separation; 10 mg of activated SCNU-Z1-Cl was dispersed into 30 mL of mixed MLB and RB, MO and MLB, and MO and RhB dye solution. To determine the adsorption isotherms, 5 mg of activated SCNU-Z1-Cl was added into 25 mL of aqueous solutions of different initial concentrations ranging from 10 to 200 ppm. After soaking and stirring for 1 day, the solution was taken out, filtered with a syringe, and the equilibrium concentration measured. 2.7. Recyclability Test of SCNU-Z1-Cl. SCNU-Z1@oxo-anion (25 mg) was regenerated by immersing the materials in a 1 M NaCl solution (20 mL) for 4 h at room temperature. Recyclability of the regenerated SCNU-Z1-Cl was examined by using 20 mL of the 250 ppm oxo-anion (CrO42−, Cr2O72−, and ReO4−) solutions. For each oxo-anion, these recyclability studies were performed repeatedly for four cycles. 2.8. Calculation of Capacity. The adsorption capacity of oxoanions and dyes on the SCNU-Z1-Cl at certain time and equilibrium are expressed as eqs 1 and 2, respectively: C

DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Q t = V (C0 − Ct )/m

(1)

Q e = V (C0 − Ce)/m

(2)

where V is the solution volume, C0 is the initial solution concentration, Ct is the solution concentration at certain time, Ce is the dye solution concentration at the equilibrium, and m is the SCNU-Z1-Cl adsorbent mass.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure Depiction. The X-ray diffraction analysis reveals that SCNU-Z1-Cl crystallizes in P2(1)/c space group. There are one Ni(II) ion with occupancy of 1/3, half of a DIPT− ligand, one Cl− with occupancy of 1/3, as well as some uncertain distorted solvent molecules in the asymmetric unit. As shown in Figure 2a, the Ni(II) ion is six-coordinated by three tetrazolyl and three imidazolyl nitrogen atoms from six different DIPT− ligands. A C2 axis bisects the benzene and tetrazole rings; thus, only half of DIPT- ligand is observed in the asymmetric unit. The DIPT− ligand adopts a μ4coordiantion mode that bridge two Ni(II) ions via 2- and 3position of nitrogen atoms on tetrazolyl group and another two Ni(II) ions via two monodentate imidazolyl groups. Two Ni(II) ions are bridged by three tetrazolyl groups, generating a binuclear secondary building block with Ni···Ni separation of 3.796 Å, as shown in Figure 2b. In contrast, three DIPT− ligands using their two imidazolyl groups coordinate to two Ni(II) ions simultaneously, leading to a triangular prism cage, as shown in Figure 2c. The distance between two Ni(II) ions in the cage is 10.0393(44) Å. One C−H bond on benzene and two C−H bonds on two imidazole rings in the unique DIPT− ligand point into the cage; thus, there are in total nine C−H bonds pointing into the center of the cage, forming nine C− H···Cl hydrogen bonds to a Cl− anion that locates inside the cage with C···Cl distance varying from 3.4082 to 3.4093 Å. The alternating interconnection of the binuclear SBUs and the triangular prism cages produces a 1D chain extending along the c direction, as shown in Figure 2d. Such types of 1D chain are connected by DIPT− ligands to form a 3D porous framework with 1D hexagonal tubular channels of 15 × 15 Å2 extending along the c direction (Figure 2e). The wall of this tubular channel is composed of DIPT− ligands, in which the benzene and tetrazole rings are parallel to the channel. The void volume of SCNU-Z1-Cl was calculated to be 60.4% using PLATON.58 In order to better understand the structure of compound SCNU-Z1-Cl, topological analysis is also applied to compound SCNU-Z1-Cl. As shown in Figure 2f, the binuclear SBU connects to nine DIPT− ligands can be considered as nineconnected node with point symbol (412·615·89), whereas the DIPT− ligand link to three binuclear SBUs can be considered as three-connected node with point symbol (43). Because the ratio of 9- to 3-connected nodes is 1:3, the overall topology of this highly connected framework is a (3,9)-connected binodal gfy network topology59 with point symbol of (412·615·89)(43)3 (Figure 2g). 3.2. Measurement of BET and Stability. PXRD data of SCNU-Z1-Cl and activated SCNU-Z1-Cl suggest that the sample is of high purity and the activated sample retained its crystallinity (Figure S1). Nitrogen adsorption−desorption isotherms measured at 77 K on an activated SCNU-Z1-Cl. As can be seen from Figure 3, the nitrogen adsorption is very high at low relative pressure and display a type I sorption isotherm, indicating a typical microporous structure of SCNUZ1-Cl framework. The BET surface area and total pore volume

Figure 3. N2 adsorption isotherm for SCNU-Z1-Cl at 77 K. Inserted Figure is the corresponding pore size distributions.

are 1636 m2/g and 0.353241 cm3/g, respectively. It should be noted that the specific surface area of most cationic MOFs has not been reported.44 This may be due to the small pores and the additional blocking effect caused by the anions that occupying the channels or the instability of many cationic MOFs under negative pressure.44,60−62 The BET surface area is larger than the most recent reported mesoporous cationic thorium−organic framework (1360 m2/g), which is identified as the MOFs display the highest BET surface area among cationic MOFs.44 TGA revealed the guest molecules can be removed from room temperature to 150 °C, and the framework is stable up to 350 °C (Figure S2). The TGA curve of activated sample showed obvious less weight loss before 150 °C, which confirms the removal of the guest molecules under vacuum at 100 °C for 24 h. The hydrolytic stability of SCNU-Z1-Cl was checked by immersing the crystals into aqueous solutions with different pH values ranging from 4 to 10 for 30 h. After immersion, the PXRD data of the crystal indicate that the structure remains unexchanged (Figure 4). The stability in common organic

Figure 4. PXRD of SCNU-Z1-Cl crystals immersed in aquatic solution at different pH values after 30 h.

solvents is also checked, and the structure is also maintained as indicated by PXRD (Figure S3). These results indicate that SCNU-Z1-Cl has good stability in both water and organic solvents, probably due to the formation of stable cagelike building blocks, and the metal ion has saturated coordination that does not easily interact with water or organic solvent molecules, resulting in high chemical stability. 3.3. Oxo-Anions Removal Studies. Four oxo-anions, CrO4−, Cr2O72−, MnO4−, and ReO4−, were selected to investigate the adsorption capacities of SCNU-Z1-Cl on oxoD

DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. UV−vis spectroscopy of the water solution of (a) CrO42− ions, (b) Cr2O72− ions, and (c) MnO4− and (d) ReO4− anions after adding SCNU-Z1-Cl at different time intervals; (e) adsorption isotherms of four oxo-anions on SCNU-Z1-Cl; (f) bar diagrams show the removal efficiency of SCNU-Z1-Cl in the existing of competing anions.

SCNU-Z1-Cl for CrO4−, Cr2O72−, MnO4−, and ReO4− based on Langmuir mode are 180, 285, 585, and 262 mg/g, respectively, whereas the corresponding maximum sorption capacity under the maximum selected experimental concentrations are 149.5, 305.8, 313.5, and 303.0 mg/g, respectively, indicating the removal of MnO4− is the largest deviation from the Langmuir equation, which may due to chemical reaction between MnO4 − and SCNU-Z1-Cl (see below). The adsorption capacities of these three oxo-anions are high compared with other reported materials.38,54,65−72 Notably, the adsorption capacity of SCNU-Z1-Cl for Cr(IV) oxo-anions is one of the highest values in MOF and COF materials, as far as we known (Tables S5 and S6). The adsorption capacity of SCNU-Z1-Cl for ReO4− is moderate (Figure S7), but the kinetics is extremely high. The anion exchange selectivity toward oxo-anions was investigated by the uptake of oxo-anions with the coexisting of 1 equiv of competitive anion, including Cl−, N3−, NO3−, ClO4−, and SO42−. In most cases, the removal percentage is still over 90%. Only the mixtures of CrO4− and N3−, CrO4− and ClO4−, and Cr2O72− and NO3−, show a relatively lower removal percentage (from 70 to 80%), as shown in Figure 5f. This binary mixture study implies the good selectively of SCNU-Z1-Cl. The adsorption of oxo-anions by SCNU-Z1-Cl crystals was analyzed by IR and EDX analyses. As shown in Figures S9− S11, the peaks at 930 and 918 cm−1 in the FT-IR spectra are attribute to the CrO4−/Cr2O72− and ReO4− anions, respectively, which confirm the adsorption of CrO4−, Cr2O72− and ReO4− anions. In addition, the presence of Cr and Re and the significantly decrease or absence of Cl after adsorption were revealed by EDX, which indicated that the adsorption of CrO4−, Cr2O72−, and ReO4− is an anion-exchanged process (Figures S12−S15). The PXRD of SCNU-Z1-Cl crystals after adsorption of them were performed, indicating the framework

anions. Crystals of SCNU-Z1-Cl were immersed in an aqueous solution of 30 mL of 0.1 mM oxo-anions, and the exchange process was detected by UV/vis absorption spectroscopy according to the intensity variation of the maximum adsorption peak of CrO4−, Cr2O72−, MnO4−, and ReO4− in solution (372, 352, 525, and 203 nm, respectively). As shown in Figure 5a−d, the concentration of all oxo-anions decreased significantly only within 1 min, with the removal rates of 70.0, 51.2, 84.0, and 83.6% for CrO4−, Cr2O72−, MnO4−, and ReO4−, respectively. The adsorption equilibrium reached after about 30 min for CrO4− and Cr2O72−, while it only takes about 12 min and 30 s for MnO4− and ReO4− anions, respectively, which indicates the removal kinetics is very high compared with many recent reports.43−47 Such high kinetics may be attributed to the tubular channels in SCNU-Z1-Cl, where the anions are able to move in smoothly. The final removal rate for CrO4−, Cr2O72−, MnO4−, and ReO4− are 88.9, 67.7, 100, and 84.9%, respectively. Sorption kinetics study of all the oxo-anions were performed by using pseudo-first-order and pseudosecond-order equations. The results reveal that the data fit better with the pseudo-second-order model. All of them had R2 values higher than 0.99 (Figures S4 and S5 and Table S3); thus, the adsorption of oxo-anions may be govered by a chemical process.63,64 Moreover, the adsorption isotherms of SCNU-Z1-Cl toward the oxo-anions were tested (Figure 5e). The Langmuir, Freundlich, as well as Themkin equations were used to fit the data, as shown in Figures S6−S8 and Table S4. For CrO4−, Cr2O72−, and MnO4−, although the data best fit to the Freundlich equation, they also fit well to the Langmuir equation; therefore, the adsorption of them may have both chemical and physical mechanisms. For ReO4−, the data are best fit to the Langmuir equation (with R2 of 0.9974), so the adsorption of ReO4− may be a typical monomolecular-layer adsorption. The calculated maximum sorption capacity of E

DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. UV−vis spectra of anionic MO (a), AO (b), CR (c), MB (d), and cationic RhB (e) in aqueous solutions at given times during the adsorption process with SCNU-Z1-Cl as the host. (f) Corresponding color changes of solution and solid before and after the dyes adsorption. (g) Dye removal rate to anionic dyes (MO, CR, MB, and AO) as a function of time by SCNU-Z1-Cl. (h) Dye removal rate to cationic dyes (MLB, CV, RhB, and AB) as a function of time by SCNU-Z1-Cl. (i) Adsorption isotherms of AO, CR, MO, MB, and RhB on SCNU-Z1-Cl (t = 24 h, pH 7, T = 25 °C).

four cycles are similar to that of the original sample (Figure S19), which indicated that the framework of SCNU-Z1-Cl is stale during the recyclability test. For the adsorption of MnO4−, PXRD showed that the intensity of the peaks of SCNU-Z1-Cl was significantly decreased after immersed in the MnO4− solution. The crystallinity of SCNU-Z1-MnO4 loss gradually as time increased (Figure S20). In addition, by measuring the UV of the solution, it was found that the peak attributed to MnO4− was decreased and that no new peak was formed within 1 h (Figure S21), indicating that the MnO4− anion was adsorbed. Moreover, the EDS analysis revealed that no Cl was observed, indicating the anion-exchanged process (Figure S22). However, when the time was prolonged, the peak of the MnO4− was further decreased and a new peak observed (Figure S21). In addition, no obvious peaks attributed to MnO4− but a peak at 550 cm−1 attributed to MnO2 were observed in IR spectra (Figure S23). The SCNU-Z1-Cl and SCNU-Z1-MnO4 were digested by NaOH solution (in D2O), and the resulting solution were measured by 1H NMR. As shown in Figure S24, the peaks corresponding to organic ligands only display some shifts, which means that the structures of organic ligands in the

is stable during these anion-exchanged process, as shown in Figure S16. Then, the resulting anion-exchanged samples, SCNU-Z1-CrO4, SCNU-Z1-Cr2O7, and SCNU-Z1-ReO4, were immersed in 1 M NaCl solution to desorb the oxo-anions. The peaks corresponding to CrO4− (Cr2O72− converts to CrO4− when desorbing in neutral solution) and ReO4− were observed in UV of the solution (Figure S17), which is clearly showed that these oxo-anions are removed only by adsorption. If all the Cl− anions can be exchanged, then the adsorption capacities of CrO4−, Cr2O72−, and ReO4− can be calculated as 51, 96, and 222 mg/g, respectively, which are all obviously lower than those observed in experiments. Thus, the high adsorption capacities may be due to that the adsorption of oxo-anions by SCNU-Z1-Cl is not only via anion-exchanged with Cl− ions but also through anion-π interaction73,74 and hydrogen bonding between the tubular wall and the oxo-anions that further adsorb more oxo-anions. The reusability of SCNU-Z1-Cl on the adsorption of CrO4−, Cr2O72−, and ReO4− was explored with a 1 M NaCl solution. As shown in Figure S18, the relative adsorption capacity is over 90% after 3 cycles, and it is still higher than 80% after 4 cycles. PXRD of the samples shows that after anion-exchanged for F

DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) UV−vis spectra of the mixed MO and MLB dyes solution and the scheme of charge-dependent selective separation mode of dyes. (b) UV−vis spectra of the mixed RhB and MLB dyes solution and the scheme of size/shape-dependent selective separation mode of dyes. (c) UV−vis spectra of mixed MO and RhB dyes solution and the scheme of kinetics-dependent selective separation mode of dyes.

(75%) than did the other selected cationic dyes. The size of RhB is bigger than the MLB and CV, but is smaller than the AB; thus, the adsorption of cationic dyes by SCNU-Z1-Cl crystals demonstrates a size-selectively behavior. Although the removal rate is close, the adsorption kinetic is significantly lower than those for the anionic dye adsorption. The adsorption equilibrium for RhB can take up to 4 h to reach. The color changes of solution and SCNU-Z1-Cl crystals were demonstrated in Figure 6f, which further confirm the adsorption of dyes. The time-resolved adsorption capacities for them were explored to further study the adsorption mechanism. The pseudo-first-order and pseudo-second-order equations were applied to study the adsorption data were, and the results show that good linear fits were obtained by the latter equations (Figures S27 and S28). The equilibrium capacities values, the correlation coefficients (R2), as well as the pseudo-secondorder rate constants are listed in Table S9. In addition, the adsorption isotherms of selected dyes with SCNU-Z1-Cl materials reveal that the highest adsorption capacities of organic dyes MO, AO, CR, MB, and RhB for SCNU-Z1-Cl are 285, 180, 585, 262, and 130 mg/g, respectively, as shown in Figure 6i. In addition, the above three adsorption equations were also selected to fit the data, and the results showed that the Langmuir model is the best-fitting mode (Figures S29− S33). Therefore, the adsorptions of MO, AO, CR, MB, and RhB were regarded as typical monomolecular-layer adsorp-

SCNU-Z1-Cl and SCNU-Z1-MnO4 may similar. In addition, a peak at 254 cm−1 attributed to 3,5-di(1H-imidazol-1-yl)benzoic acid (DIBA) was observed in MS. Thus, we tentatively supposed that the chemical reaction between MnO4− and ligand in SCNU-Z1-Cl happened, which cause the MnO4− reduces to Mn2+ or MnO2 and the HDIPT oxidized as DIBA, leading to the decrease in the crystallinity of SCNU-Z1-Cl. Therefore, the removal of MnO4− is not only by anionexchange process, but also by redox reaction. Thus, the adsorption capacity of SCNU-Z1-Cl for MnO4− measured after 1 h indicates that it displays some of the highest values in MOFs or COFs (Table S8). 3.4. Dye-Adsorption and Separation Studies. Considering the large 1D tubular channels in SCNU-Z1-Cl, organic dye adsorption was performed to examine its porosity. Eight organic dyes with various charges and sizes, including positively charged MLB, CV, RhB, and AB and negatively charged MO, AO, CR, and MB were selected (Figure S25). As expected, the SCNU-Z1-Cl adsorb anionic dyes rapidly and effectively. The adsorption of MO, AO, CR, and MB can be completed in 26, 60, 30, and 120 min, respectively, with the removal rates of 99, 98, 97, and 87%, respectively (Figures 6a− d,g). On the contrary, as shown in Figures 6e,h and S26, most of the selected cationic dyes including MLB, CV and AB only display a slight adsorption with removal rate of 10, 23, and 3%, respectively, by the SCNU-Z1-Cl crystals. Interestingly, the cationic RhB exhibits a significant higher adsorption rate G

DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry tion.75 The adsorption isotherm parameters calculated from the three equations are shown in Table S10. The desorption of dyes on SCNU-Z1-Cl were explored by selecting the anionic dye MO and cationic dye RhB as representative. As shown in Figure S34a,b, unlike for oxoanions, the adsorbed MO or RhB cannot be released when immersing in water or NaCl solution, which may due to the strong interactions between the host SCNU-Z1-Cl framework and the dye guests. The adsorbed MO can only partly release by using DMF or NaNO3 solution (in DMF), whereas the adsorbed RhB retained almost unrelease under the same conditions. Again, the framework retained stable under these desorption experiments, as indicated by PXRD (Figure S34c,d). Considering that the desorption of dyes by organic solvent is not beneficial for real water treatment applications, the reusability of SCNU-Z1-Cl on the adsorption of dyes were not further performed. On the basis of the above investigation, the SCNU-Z1-Cl can achieve dye separation by three modes. First, the mixture of anionic and cationic dyes can be selective separation based on charge-dependent selective separation mode. To confirm this, the adsorption properties of SCNU-Z1-Cl were further investigated in a mixed MO and MLB solution. As shown in Figure 7a, the MO solution was almost completely adsorbed (removal rate of 93.7%) after 20 min, while the MLB was only adsorbed by a small amount (removal rate of 22.7%). At the same time, the color of the solution turned from green to blue, as shown in the inserted figure of Figure 7a, confirming the selective adsorption of MO. Second, the cationic dyes with different sizes can be separated via a size-dependent selective separation mode. In the current system, the separation of MLB and RhB is selected for examination. As shown in Figure 7b, the RhB is adsorbed in 60 min, while the MLB is hardly absorbed. The color of the solution turned from dark blue to relative pale blue, confirming the selective adsorption. Third, for some dyes with large different adsorption kinetics, the mixed dyes can be separated via kinetics-dependent selective separation mode (Figure 7c). The separation of RhB and MLB can be carried out by this method. Although they are all adsorbed by the SCNU-Z1-Cl crystals, the adsorption kinetics is significantly different. Therefore, after adding SCNU-Z1-Cl for a short time (5 min), the MO is almost completely adsorbed, while only a small amount of RhB is adsorbed. Therefore, they can be separated in a short time. If the time is further increased, then RhB will also be adsorbed. The color changes from orange to pink in 5 min, indicating the selective adsorption of MO. After 120 min, both dyes are absorbed, and the color changes to pale pink, indicating the further adsorption of RhB. As far as we known, this kinetics-dependent dye separation mode has not been reported except by us.76 FTIR spectra are measured before and after dye adsorption. From Figures S36−S39, it is shown that the selected dye molecules are adsorbed on the SCNU-Z1-Cl. In addition, PXRD of SCNU-Z1-Cl after dye adsorption clearly indicates that crystal structure was maintained during dye adsorption (Figure S40). The adsorption properties of SCNU-Z1-Cl to the dye molecules are related to its structure. Since SCNU-Z1Cl is a cationic framework and the anions in the channels are easily exchanged, the anionic dyes can easily enter the channels via anion-exchange process. In addition, since the channel is tubular, the aromatic ring on the tube wall also provides a supramolecular interaction site for the adsorption of dye

molecules, which further makes the accommodation of dye molecules effective. For cationic dyes, because of the inability to perform anion-exchange, the adsorption of SCNU-Z1-Cl on them is mainly attributed to the interaction between the channels and the dye molecules. RhB molecule with a suitable size that best matches the pore size of SCNU-Z1-Cl is more likely to form supramolecular interaction to the wall of the channel, so it is the most easily adsorbed cationic dye. Other cationic dyes are too small or too large to form an interaction or to enter the channel, resulting in lower adsorption capacities. In addition to the pore structure, the surface charge of the material also influences the adsorption kinetic and capacity of dye adsorption,77,78 thus the zeta potential of SCNU-Z1-Cl was tested. The results show that it exhibits a positive zeta potential with a value of 37.1 (Figure S41). Therefore, it is more likely to adsorb anionic dyes rather than cationic dyes, which is one of the main reasons for the significant difference in the adsorption kinetics toward anionic dyes and cationic dyes adsorption.

4. CONCLUSIONS To summarize, a cationic MOF with a tubular channel with high specific surface area and high hydrolytically stability was synthesized. The uncoordinated anions in this MOF are easily exchanged and thus can be used for rapid adsorption of oxoanions and anionic dyes from water, and exhibit high adsorption kinetics and capacity. In addition, the tubular channels facilitate the interaction of the MOF with the guest molecules, thereby also exhibiting size-selective adsorption of the cationic dye. The results show that this porous cationic MOF framework does show strong potential application in wastewater treatment.

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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.inorgchem.9b00104. TGA curves, PXRD, kinetic model plots, equilibrium capacities and rate constants, fitting of experimental data to Freundlich, Themkin, Langmuir, isotherm modes, adsorption parameters, comparison with literature data, IR spectra, EDS analysis, UV−vis spectra, recyclability test results, structures, dye desorption data, zeta potential of SCNU-Z1-Cl (PDF) Accession Codes

CCDC 1556071 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 [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

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sheng-Run Zheng: 0000-0002-9071-0748 Song-Liang Cai: 0000-0002-5399-9036 H

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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant nos. 21473062, 21603076, and 21571070).

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DOI: 10.1021/acs.inorgchem.9b00104 Inorg. Chem. XXXX, XXX, XXX−XXX