Highly Improved Water Resistance and Congo Red Uptake Capacity

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Highly Improved Water Resistance and Congo Red Uptake Capacity with a Zn/Cu-BTC@MC Composite Adsorbent Qing Liu,† Yuan Gao,† Yitian Zhou,† Ning Tian,† Gangfeng Liang,‡ Na Ma,§ and Wei Dai*,† †

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Science and §College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, People’s Republic of China ‡ Xingzhi College, Zhejiang Normal University, Lanxi 321100, People’s Republic of China

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S Supporting Information *

ABSTRACT: A novel composite material (Zn/Cu-BTC@ MC) composed of bimetallic Zn/Cu-BTC and mesoporous carbon (MC) has been successfully synthesized by one-pot hydrothermal reaction. We have approved that MC was successfully wrapped around the surface of Zn/Cu-BTC and the existence of the crystal state by various characterization methods. The Zn/Cu-BTC of this composite material is microporous while the MC is mesoporous, which is beneficial for Congo red (CR) to diffuse in the MC and adsorb in the Zn/Cu-BTC. Because of the excellent surface area and bimetallic synergistic effect, the Zn/Cu-BTC@MC exhibits a high uptake capacity and selectivity toward CR; this is even more outstanding than other adsorbents reported previously. In addition, the novel composite adsorbent possessed a good water resistance for CR capture from aqueous solution. Saturated adsorbent Zn/Cu-BTC@MC can be regenerated by solvent elution and nitrogen purging. Over 91% of the CR absorption capacity could be recovered with water molecules after regeneration. The distinctive structure of Zn/Cu-BTC@MC might be a hopeful adsorbent for anionic dye purification such as CR.

1. INTRODUCTION Congo red (CR, Scheme S1), one of typical anionic dyes, is widely utilized in textiles, cosmetics, plastics, pharmaceuticals, paper and food processing, and so forth.1,2 However, the surge of application of CR in all walks of the life has been creating environmental pollution because of its non-biodegradability and high toxicity features.3,4 Thus, CR capture in the water treatment process is necessary. Adsorption with porous materials is being considered because it is cost-efficient, highly selective, chemically stable, efficiently adsorptive, and easily regenerated and so forth, which make it superior over the other pollutant removal techniques.5,6 Importantly, an adsorbent will be evaluated as excellent if it possesses several outstanding adsorption performances such as high uptake capacity, good selectivity, desirable kinetics, and recyclability. In this aspect, much attention has been paid to the development of new porous materials, especially metal− organic frameworks (MOFs).7,8 The excellent chemical properties and flexibility, accessible coordinative unsaturated sites, exceptionally high surface area, and large pores make MOFs a very promising candidate for CR capture.9 For instance, we have reported the potential application of a Cubased MOF to be used as absorbent to remove CR.10 Because CR is an anionic dye, cationic MOFs with copper-unsaturated sites show rapid CR uptake ability and increased adsorption capacity. However, to date, the most widely investigated © XXXX American Chemical Society

porous MOFs are usually constructed from a single metal with an organic ligand. Therefore, it is relatively “simple” in terms of composition and structure. On the contrary, bimetallic MOFs have two different metals in the same structural role. A synergistic interaction between the introduced bimetallic ions might obviously increase the uptake capacity and selectivity of CR adsorbate over MOFs. In addition, compared with some common adsorbents such as porous carbon, zeolite, and porous resin, the MOFs have lower density which cannot provide strong enough dispersion to bind the pollutant molecules during the adsorption process. If its atomic density is further improved, the CR capture characteristics of MOF can be further improved.11 Carbon materials are relatively suitable for realizing the improvement of the dispersion of MOFs. Thus, the synthesis of MOF carbon composites could be a promising trial, and the formation of MOF can be improved by the increased dispersion force, inhibiting its aggregation and controlling the physical and chemical properties of MOF. Petit and Bandosz12,13 have reported that the MOF-5@GO composite has increased the uptake capacities of N2, NH3, H2S, and NO2 more than virgin MOF-5 because of the presence of GO in the process of MOF formation. Received: February 16, 2019 Accepted: June 20, 2019

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DOI: 10.1021/acs.jced.9b00159 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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2.4. Batch Adsorption Procedure. The equilibrium isotherms of CR adsorption were obtained by performing batch tests in 100 mL Erlenmeyer flasks where 50 mL of CR solutions with different initial concentrations (100−1000 mg/ L) was placed. The as-prepared materials (0.02 g) were added to every flask, and those flasks were kept in a shaker with 150 rpm for 4 h to reach equilibrium. The adsorbent was filtered, and the residual CR concentration in the filtrate was analyzed using a UV−visible spectrophotometer (Thermo Fisher Scientific EVO300) at the maximum wavelength of 497 nm. The CR calibration curve is highly reproducible and linear over the concentration range (R2 > 0.99). The CR uptake capacity at equilibrium, qe (mg/g), was calculated by the following equation

In fact, there are two serious problems for adsorption application in aqueous solution with MOF materials. First, MOFs have weak water resistance ability. Water molecules are stronger nucleophiles than organic ligands and could attack the metal−ligand bonds of MOFs, exposing the metal ions and consequently coordinating with them. This fatal action would lead to the partial or even total collapse of the MOF structure. The collapse of the MOF structure in the presence of humidity limits its applications. Second, for adsorption kinetics, the microporous feature of MOFs could reduce the adsorbate diffusion coefficient during the adsorption process in the pore structure. Zhu et al.11 have reported adsorption performances of the thiophenic rings with a composite material MOF-5@ AC. The larger pore size of activated carbon makes the adsorbate molecules easily diffuse in the pores and enter the unsaturated active sites of the MOF-5. Thus, the uptake capacities of thiophenic rings can be obviously improved because of the introduction of activated carbon in MOF-5. Similar results were also addressed in the literature.14−16 To tackle the abovementioned weakness, a new composite material, Zn/Cu-BTC@MC, was facilely designed and prepared by the one-pot hydrothermal synthesis method. Existence of Zn(II) and Cu(II) in the MOF structure can improve the CR adsorption performance. A larger pore channel in the mesoporous carbon (MC) makes the CR molecules easily diffuse inside the pore structure because of the threshold effect of MC. In order to find out the possibility of using these MOF-carbon composite materials as an effective adsorbent for CR capture and better understand the adsorption mechanism of the material, the adsorption rate and the adsorption controlling steps are investigated. Kinetic and equilibrium parameters are also carried out to support our results.

(Co − Ce) ·V (1) W where Co and Ce (mg/L) are the initial and equilibrium concentrations of CR, respectively; V (L) is the volume of solution; and W (g) is the weight of the adsorbents added. qe =

3. RESULTS AND DISCUSSION 3.1. Textural Properties of the Material. N2 adsorption−desorption isotherms and summarized textural information of Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/CuBTC@MC samples at −196 °C are exhibited in Figure 1 and

2. EXPERIMENTAL SECTION 2.1. Chemicals. Benzene tricarboxylic acid (H3BTC, 98%), cupric nitrate hydrate (Cu(NO3)2·3H2O, ≥99.5%), zinc nitrate (Zn(NO 3 ) 2 ·3H 2 O, ≥99.5%), ethanol (>99.5%), CR (≥99.5%), and N,N-dimethylformamide (≥99.5%) were supplied by Sigma-Aldrich Chemical Company. MCs (200 meshes) were purchased from Fujian Yuanli Active Carbon Co., Ltd. None of the chemicals selected in this work are purified further before use. 2.2. Zn/Cu-BTC and Zn/Cu-BTC@MC Synthesis. The Zn/Cu-BTC crystal was similarly prepared according to our previous works10,14 (Scheme S2). The (1.0)Zn/Cu-BTC@ MC, (3.0)Zn/Cu-BTC@MC, and (4.0)Zn/Cu-BTC@MC adsorbents were produced by the Zn/Cu-BTC preparation process while doping MC using the starting MOF/MC weight ratios of 0.1:1, 0.3:1, and 0.4:1, respectively. 2.3. Sample Characterization. N2 physical adsorption was used to determine the structural properties of the sample using an ASAP 2020 physical adsorption instrument at −196 °C. The scanning electron microscopy (SEM) images were characterized using a Hitachi S4800 field emission scanning electron microscope. The samples’ crystal phase was recorded on an X-ray powder diffractometer from Nakaguchi Corporation of the Netherlands using Cu Kα. Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer at room temperature using KBr disks. The obtained FT-IR spectrograms have a good repeatability with ±0.01%. The water contact angle was measured by an instrument of Data Physics OCT-20.

Figure 1. N2 adsorption/desorption isotherms for Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC samples at −196 °C, respectively.

Table S1, correspondingly. Based on the IUPAC (International Union of Pure and Applied Chemistry) regulations,5,14 the MC and Zn/Cu-BTC@MC adsorbents belong to a typical IV-type curve with a small hysteresis loop characteristic, indicating the presence of a few slit mesopores. A relative high fraction (>40%) of micropores in these composites can be observed from the steep ascending at the low pressure, as shown in Table S1. The average pore size of Zn/Cu-BTC@MC increased significantly with more MC doping. The pore sizes of Zn/Cu-BTC (microporous structure) and MC (mesoporous structure) are 1.87 and 4.17 nm, respectively. Especially, the average pore size of our Zn/Cu-BTC@MC appeared at 3.86−4.08 nm. This may be due to the enlargement of the pore volume during the crystallization of Zn/Cu-BTC, or the B



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fracture pores accumulated by MC, during the MC selfassembly coating onto the surface of Zn/Cu-BTC. 3.2. Powder X-ray Diffraction. The Powder X-ray diffraction (PXRD) patterns of the MC, Zn/Cu-BTC, and Zn/Cu-BTC@MC samples are compared and shown in Figure 2. The patterns of MC have a “steamed buns” wide peak in the

Figure 2. XRD analysis for Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC samples, respectively.

range of 16−25° before loading, which is the characteristic peak of carbon materials. The amorphous structure of MC is used to determine the baseline of wide peak and inhomogeneity. The three main feature peaks of Zn/CuBTC appeared at 2θ of 9.1°, 10.2°, and 12.5° correspond, respectively, to crystal facets in (222), (400), and (422) planes, which are consistent with relevant literature reports.10,14 The peaks of Zn/Cu-BTC@MC were mainly superimposed on Zn/ Cu-BTC and MC. The obvious feature peaks of MC (2θ = 16−25°) and Zn/Cu-BTC (2θ = 9.1°, 10.2°, and 12.5°) can still be observed from the Zn/Cu-BTC sample. The baseline of Zn/Cu-BTC@MC is significantly rougher than that of Zn/CuBTC, but better than MC. Compared with the PXRD diffraction spectra, we realized that the Zn/Cu-BTC and MC recombination occurred, and MC was distributed onto the Zn/ Cu-BTC surface. 3.3. SEM Images. The SEM images of MC, Zn/Cu-BTC, and Zn/Cu-BTC@MC are presented in Figure 3. The crystal morphology of the Zn/Cu-BTC material is a consistent octahedron with particle size of 10−20 μm. The irregularly shaped MC can be observed on the surface of the Zn/CuBTC@MC sample, which indicated that the shaped small granular MC was attached to the MOF surfaces to form the composite structure. In addition, the particle shape and uniformity of Zn/Cu-BTC@MC are relatively intact and no regular Zn/Cu-BTC crystals were found. 3.4. FT-IR Analysis. The information of surface functional groups on the MC, Zn/Cu-BTC, and Zn/Cu-BTC@MC is exhibited by FT-IR curves in Figure 4. The typical Cu−O and Zn−O stretching vibrations appeared at 729 cm−1. The distinguished suggestions of these feature peaks are the stretching VCO, VC−O and bending O−H vibrational frequencies at 1645, 1447, 3440, and 1360 cm−1, correspondingly, indicating the existence of a carboxylic acid group.5,14 For MC and Zn/Cu-BTC@MC samples, the peaks at 1610,

Figure 3. SEM images of MC, Zn/Cu-BTC, and (3.0)Zn/Cu-BTC@ MC samples. (A) MC; (B) (3.0)Zn/Cu-BTC; (C) (3.0)Zn/CuBTC@MC, respectively.

Figure 4. FT-IR curves for Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC samples, respectively.

879, 1100, and 2270 cm−1 could be a result of the vibrational and feature spectrum of H−O−H, C−H, and CC, respectively. This is another indirect evidence to support the existence of MC in the Zn/Cu-BTC structure. C



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3.5. Thermogravimetric Analysis. Based on the thermogravimetric (TG) results (Figure 5), at 50−200 °C,

Figure 6. Adsorption isotherms of Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC samples, respectively. Figure 5. TG analysis of MC, Zn/Cu-BTC, and (3.0)Zn/Cu-BTC@ MC samples, respectively.

the CR sorption force over the adsorbent could get weakly accompanied by the change of adsorption heat. Therefore, the adsorption of CR onto Zn/Cu-BTC is obviously affected by the temperature. On the other side, the maximum CR uptake capacity is significantly higher on (3.0)Zn/Cu-BTC@MC (1078 mg/g) than the other adsorbents such as MC (934 mg/ g) and Zn/Cu-BTC (892 mg/g). As proved from our previous work,10 the CR uptake capacity of Zn/Cu-BTC significantly increased in comparison with the Cu-BTC. Such phenomena may be due to the synergistic effect of Cu(II) and Zn(II) on the anionic CR molecules, which enhanced the ability of adsorption for CR onto MOFs. While more Zn(II) were introduced into the support, it may block the pore and lead to the increase of the diffusion resistance during the mass transfer reactions, which can actually decrease the amount of accessible active sites, and thus, the (4.0)Zn/Cu-BTC@MC shows a smaller CR uptake capacity than that with lower Zn(II) doping [(3.0)Zn/Cu-BTC@MC]. Therefore, among all the samples, (3.0)Zn/Cu-BTC@MC shows the highest CR capacity. The CR uptake capacities of MC, Zn/Cu-BTC, and Zn/Cu-BTC@ MC in this work are summarized in Table S4, together with those of the control adsorbents. It is clear that the adsorption capacity of (3.0)Zn/Cu-BTC@MC is superior to the other reported adsorbents (Table S4). Both Zn/Cu-BTC and MC have important contributions to the CR uptake capacity over Zn/Cu-BTC@MC. As known from Table S4, the uptake capacities of CR followed the order Zn/Cu-BTC@MC > MC > Zn/Cu-BTC. The existence of Zn(II) and Cu(II) in the Zn/ Cu-BTC structure might improve the CR uptake capacity because of the synergistic effect. For MC, much larger pore channel and surface area (Table S1) could promote the CR molecule diffuse rate and adsorption capacity. The effect of adsorption time on CR uptake capacity over MC, Zn/Cu-BTC, and Zn/Cu-BTC@MC is presented in Figure 7. This figure revealed that the CR adsorbed amount increases with the increase of adsorption time, and the adsorption reaches equilibrium within about 5 h. It is also shown that rapid increase in capacity for CR is achieved during the first 0.3 h. The fast adsorption at the initial stage may be due to the availability of the uncovered surface area and the remaining active sites of the adsorbents. The CR kinetic parameter was calculated according to three kinetic models:

there is a slight loss of weight about 3.3%, which may be due to the loss of moisture. A significant weight loss began at 500 °C, for which the weight loss was about 10%. Therefore, MC has a decomposition temperature of 500 °C. For Zn/Cu-BTC, in the 30−120 °C temperature range, the first step of weight loss can be attributed to the loss of the water and solvent molecules. The second step is due to the fact that organic linkers are broken down. No significant mass loss was observed from 200 to 300 °C, indicating that the prepared MOF should be heated at a flow of nitrogen below 300 °C, the temperature at which the structure collapsed badly. When the temperature rises to 350 °C, the weight of Zn/Cu-BTC decreased about 60%. For (3.0)Zn/Cu-BTC@MC, in the 50−120 °C range, the sample underwent a weight loss (about 10%), which could be due to the residual moisture. This weight loss is slightly larger than that of MC, but far less than Zn/Cu-BTC. At 500 °C (total weight loss of 20%), significant decomposition of (3.0)Zn/CuBTC@MC began to occur. In general, the thermal stabilities of (3.0)Zn/Cu-BTC@MC and MC are basically the same, but better than Zn/Cu-BTC. 3.6. Evaluation Performance of CR Capture. The adsorption isotherms of MC, Zn/Cu-BTC, and Zn/Cu-BTC@ MC at pH 7 at 25 °C and a solid/liquid ratio of 0.4 g/L are shown in Figure 6. The Langmuir, Freundlich, Temkin, and D−R isothermal models were used to fit the experimental equilibrium adsorption data (Table S2).5,17,18 The calculated constants R2 values are presented in Table S3. For MC and Zn/Cu-BTC@MC, the good fittings (R2 > 0.99) to the Langmuir model (Figure S1) suggest that the CR adsorption is limited with monolayer coverage and the surface is relatively homogeneous. However, the Zn/Cu-BTC conforms to the Freundlich adsorption model (R2 = 0.95), indicating that the adsorption occurs on a nonuniform surface, and the 1/n < 1 indicates it is a favorable adsorption process.22 The fitting data are in accordance with the Temkin adsorption model (Figure S2), which again verifies the heterogeneity of the adsorption surface, and the adsorption process is directly related to the surface activity. First, the CR adsorbates might be strongly adsorbed on the highest active sites of Zn/Cu-BTC. Second, D



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of the reaction increases with increasing temperature. The positive values of S (Table S7) exhibited that entropy increases as a result of adsorption because of the redistribution of energy between the adsorbate and the adsorbent. Before adsorption occurs, the CR near the adsorbent surface will be more ordered than in the subsequent adsorbed state and the ratio of free CR molecules interacting with the adsorbent will be higher than in the adsorbed state. As a result, the distribution of rotational and translational energy among a small number of molecules will increase with increasing adsorption by producing a positive value of S and randomness will increase at the solid−solution interface during the process of adsorption. Adsorption is thus likely to occur spontaneously at normal and high temperatures because H > 0 and S > 0. There are four main stages in the process of CR adsorption over as-prepared adsorbents: (i) CR solute transfer from the bulk solution to the boundary film that surrounds the adsorbent’s surface, (ii) CR solute transport from the boundary film to the adsorbent’s surface, (iii) CR solute transfer from the adsorbent’s surface to active intraparticular sites, and (iv) interactions between the CR solute molecules and the available adsorption sites on the internal surfaces of the adsorbent. Therefore, diffusion and liquid film resistances also directly affect the CR adsorption process. After a certain time of diffusion, CR in the pore gradually becomes very weak, the diffusion coefficient decreases, and the adsorption rate tends to zero, reaching the dynamic equilibrium of adsorption. 3.7. Recyclability of Zn/Cu-BTC@MC in Water. CR multiple capture by the Zn/Cu-BTC@MC in water is important for practical application. Literature studies10,14,19,20 reported that the water molecules are stronger nucleophiles than H3BTC ligands and could coordinate with Zn(II)/Cu(II) ions, which can lead to total collapse of the Zn/Cu-BTC structure in presence of a water environment. Based on the solvent elution and heating process of the literature, saturated (3.0)Zn/Cu-BTC@MC was washed with excess deionized water and ethanol solution, which retained CR and was obtained after the adsorption experiment. Then, the sample was put in a tube furnace with nitrogen sweeping at 250 °C for 2 h. After regeneration, the color of Zn/Cu-BTC@MC turned from slight red to its original black color. The CR adsorption experiment of regenerated Zn/Cu-BTC@MC were then tested by multiple batch tests for five times (Figure S6), and the results showed that the CR uptake capacities of the regenerated adsorbent are as high as 91% of the initial values after five recycles. As known from Table S3, the reusability of Zn/Cu-BTC@MC is comparatively higher than some other reported adsorbents. In addition, as known from the water contact angle tests (Figure S7), the Zn/Cu-BTC showed complete water wetting with water contact angles to 20.4°. In comparison, the (3.0)Zn/Cu-BTC@MC sample exhibits water contact angles of 39.7°, increasing its water resistance. From the results of XRD patterns and N2 adsorption isotherms (Figure S8 and S9), the Zn/Cu-BTC@MC after recycling five times was in good agreement with virgin Zn/Cu-BTC@MC, and the surface area and pore volume were similar, indicating that the stability of Zn/Cu-BTC@MC in the CR capture process was satisfactory. 3.8. Mechanism Discussion. The formation mechanism of Zn/Cu-BTC@MC might occur via the coordination of carboxylates groups (and thus oxygen groups) and metallic centers. Band assignments for the spectrum (Figure 4) of MC

Figure 7. Effect of contact time on the CR uptake capacity of Zn/CuBTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC samples, respectively.

pseudo-first order, pseudo-second order, and intraparticle diffusion models5,14 (Table S5). Based on the correlation coefficient R2 of the three kinetic equations (Table S6) and Figure S3, the pseudo-second order adsorption kinetics (R2 > 0.99) can better describe the adsorption process. Additionally, a plot of the amount of CR adsorbed (qt) versus the square root of time (t1/2) are not linear over the total time range (Figure S4), which indicates that the intraparticle transport is not the rate-limiting step. Thermodynamic behavior of CR adsorbed onto adsorbent was evaluated by the thermodynamic parameters including the changes in free energy (G), enthalpy (H), and entropy (S). These parameters are calculated with the following equations5,14 ln(Kd) =

ΔS ΔH − R RT

ΔG = −RT ln(Kd) Kd =

(2) (3)

qe(W /V ) Ce

(4)

where R is the universal gas constant 8.314 (J/mol K), T is the temperature (K), and Kd is the distribution coefficient for the adsorption. According to eqs 2−4, the ΔH and ΔS parameters (Tables S7) for CR adsorption can be calculated from the slope and intercepts of the plot of ln(Kd) versus 1/T (Figure S5). Usually, the adsorption enthalpy ranging from 2.1 to 20.9 kJ/ mol corresponds to a physical adsorption.5 The ΔH parameter is 30.13 kJ/mol, which shows the CR adsorption process over (3.0)Zn/Cu-BTC@MC might mainly depend on chemisorption. In contrast, the CR adsorption on MC and Zn/Cu-BTC is mainly physical adsorption based on the ΔH parameters (20.34 and 20.47 kJ/mol), which may be a monomolecular or multimolecular layer; it occurs rapidly. The positive ΔH is an indicator of the endothermic nature of the adsorption and its magnitude also gives information on the type of adsorption, which can be either physical or chemical. The negative value for the Gibbs free energy shows that the CR adsorption over Zn/Cu-BTC, MC, and (3.0)Zn/CuBTC@MC is spontaneous and that the degree of spontaneity E



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indicate that the MC contains a lot of atomic groups such as hydroxyl and carboxyl groups. Usually, these oxygencontaining functional groups are of negative charge in water. Additionally, Zn/Cu-BTC are crystalline porous solids composed of a three-dimensional network of Zn(II) and Cu(II) metal ions held in place by the H3BTC organic linker (BTC is benzoic acid radical). According to the reports,21,22 there are still a few open-metal sites in the Zn/Cu-BTC structure. Therefore, Zn/Cu-BTC@MC could be obtained by the coordination bond between the open metal ions with the oxygen-containing functional groups. Similar results can be found in the literature.21,22 In addition, the mechanism of CR adsorption is controlled by various factors like physical and/or chemical properties of adsorbents, mass transfer process, and so forth.5,14 First, as one of the typical anionic dyes (Scheme S1), CR (1-napthalenesulfonic acid, 3,3′-(4,4′-biphenylene bis(azo))bis(4-amino)disodium salt) usually exist in negative forms in water at pH = 7.2,4,10 Therefore, there would be an electrostatic interaction with active sites of metal centers [Cu(II) and Zn(II)] on the surfaces of Zn/Cu-BTC@MC, attracted to the anionic CR molecules. Second, literature studies23−25 have reported that there are two different metals with the same structural role in the bimetal-organic frameworks. The possibilities of introducing advanced complexity into bimetal−organic framework materials hold enormous potential because of the ability to adjust or even control the ratios of the organic and inorganic components in the structure, providing novel control over pore metrics and compositions and, hence, tailoring the physicochemical properties in a particular way beyond the options available to the single-metal parent MOFs. For example, increasing the capacity of selective adsorption and improving the selective adsorption of adsorbate and the catalytic activity have been achieved. It is well known that Zn and Cu are adjacent position in periodic table of elements, and they have the similar features such as atomic quantity (Cu: 63.55, Zn: 65.39), atomic radius (Cu: 135 (145) pm, Zn: 135 (142) pm), and electron configuration (Cu: [Ar]3d104S1, Zn: [Ar]3d104S2).2 The paddle wheel complex built from the axial Cu2+ and Zn2+ ions and 1,3,5-benzenetricarboxylic acid (H3BTC), is very interesting because of its easy preparation, flexibility, and open metal site. The choice of Zn and Cu is based on the expected higher affinity for the CR molecule, according to their adsorption behavior in Zn/Cu-BTC.2,10,14 Third, the CR are ideally planar molecules and therefore can easily adsorb on MC by π−π stacking interaction between the aromatic backbone of the CR (Scheme S1) and hexagonal skeleton of MC. Notably, we have also calculated the coefficients of kinetic diffusion (Diq) according to the Dunwald−Wagnen equation26,27 below ÄÅ É 2Ñ ÅÅ i q zy ÑÑÑÑ ÅÅ j π2 lnÅÅÅ1 − jjjj t zzzz ÑÑÑ = Diq ·t ÅÅ j q z ÑÑÑ R p2 ÅÅÇ k e { ÑÖ (5)

Figure 8. Linear fitting of CR adsorption diffusion coefficients of Zn/ Cu-BTC, MC, and (3.0)Zn/Cu-BTC@MC samples, respectively.C

Because the mesoporous structure could provide a relatively larger space for CR adsorption diffusion during the adsorption, the Diq values of CR adsorption onto MC and (3.0)Zn/CuBTC@MC (Table S8) are 0.0021 and 0.0018, which are almost 10 times than Zn/Cu-BTC. Therefore, there are more CR molecules adsorbed onto the MC surface than Zn/CuBTC before 200 min. For Zn/Cu-BTC, it has been proved that the electrostatic force is also involved in adsorption of negatively charged CR on Cu(II) and Zn(II) and positively charged open metal sites of Zn/Cu-BTC. Thus, the final CR uptake capacity of Zn/Cu-BTC is larger than MC at 300 min later (Figure 7). Therefore, the anionic CR could be enhanced adsorption by Zn/Cu-BTC@MC with open metal sites. The improved adsorption capacity is attributed to the synergetic effect of Zn(II) and Cu(II) as well as to the mesoporous features of the as-prepared composite.

4. CONCLUSIONS CR capture performances of Zn/Cu-BTC@MC composite adsorbents are assessed through batch tests. Experimental results confirm that high CR uptake amount and selectivity of Zn/Cu-BTC@MC can be attributed to the micro−mesoporous structure and presence of Cu(II)/Zn(II) in the material’s framework. Compared with some other adsorbents in the literature, the (3.0)Zn/Cu-BTC@MC sample has a significantly higher CR absorption capacity. Diffusion coefficient of CR adsorption onto (3.0)Zn/Cu-BTC@MC are almost 10 times than Zn/Cu-BTC. Adsorption kinetics of the CR capture process suit well with the pseudo-second-order model. Adsorption equilibrium data are more consistent with Langmuir isotherm model equations. Besides, the high CR selectivity and capacity, Zn/Cu-BTC@MC sorbents can be easily regenerated by solvent elution and nitrogen sweeping method. More than 91% of the CR uptake capacity was recovered after regeneration. In general, Zn/Cu-BTC@MC, not confined by the vulnerability toward moisture like other MOFs, has been confirmed to be potential value in the application for anionic dyes purification in industry.

where qt (mg/g) is the CR uptake amount per unit mass adsorbent at t min, mg/g; qe is the saturation capacity, mg/g; Rp is the average pore size of adsorbent; Diq is the CR diffusion coefficient. Then the Dunwald−Wagnen model is used to fit the experimental equilibrium adsorption data obtained (before 50 minutes) by eq 5, and the results are shown in Figure 8.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00159. F



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Chemical structures of CR; prepared procedures of the Zn/Cu-BTC crystal; textural properties of Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC; adsorption isotherm models used in this study and their linear forms; constants and correlation coefficients of different adsorption models; comparison of the CR uptake capacity and reusability on different adsorbents; kinetic calculation equations; kinetic parameters for CR adsorption on Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC; thermodynamic adsorption parameters for CR on Zn/Cu-BTC, MC, and (n = 1.0, 3.0, and 4.0)Zn/Cu-BTC@MC; kinetic diffusion coefficients for CR on Zn/Cu-BTC, MC, and (3.0)Zn/Cu-BTC@ MC; Langmuir model fitting of the adsorption process; Temkin model fitting of the adsorption process; fitting curves of pseudo-second-order equation; Weber−Morris intraparticle diffusion plots for the adsorption of CR on MC, Zn/Cu-BTC, and Zn/Cu-BTC@MC, respectively; plot of ln(Kd) versus 1/T for CR adsorption on MC, Zn/Cu-BTC, and Zn/Cu-BTC@MC, respectively; effect of recycle times of (3.0)Zn/Cu-BTC@MC on the regeneration; contact angle for Zn/Cu-BTC, MC, and (3.0)Zn/Cu-BTC@MC samples, respectively; XRD patterns of (3.0)Zn/Cu-BTC@MC before and after adsorption/desorption experiment; and N2 adsorption− desorption isotherms of (3.0)Zn/Cu-BTC@MC before and after adsorption/desorption experiment (PDF)

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Corresponding Author

*E-mail: [email protected]. Phone: +86-579-82282269. Fax: +86-579-82282325. ORCID

Wei Dai: 0000-0001-9377-1151 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank to the Zhejiang Provincial Natural Science Foundation of China under grant no. LY19B060014 and national undergraduate innovation and entrepreneurship training project (201910345029), China.

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