MgCl2 Hybrid Products as Dye

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Preparation of Carbonized MOF/MgCl2 Hybrid Products as Dye Adsorbent and Supercapacitor: Morphology Evolution and Mg Salt Effect Ting Li, Shuai Ma, Hu Yang,* and Zhen-liang Xu State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

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S Supporting Information *

ABSTRACT: Metal organic frameworks (MOFs) containing salt impurity provide a new idea for carbon material design. Presently, Mg−MOF/MgCl2 hybrid polycrystallines with different morphologies are solvothermally synthesized using 1,3,5-benzenetricarboxylic acid (BTC) or 1,4-benzenedicarboxylic acid (BDC) as the ligand, with or without polyethylene terephthalate (PET) inducer, respectively. After calcination, the products look like a flower, bud, cube, or nanosheet. When tested as adsorbent, the maximum methyl orange adsorption capacity reaches 3250 mg/g, the highest reported to date. The reason is attributed to the carboncovered Mg salt inside the carbonized MOF. When tested as supercapacitor, carbonized MOFs based on BTC ligand show a high specific capacitance (127F/g) but a low rate capability, whereas a lower specific capacitance (121F/g) but a better rate capability (80% retention at 10A/g) are found for carbonized MOFs based on BDC. The reason is due to their different pore structures.

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INTRODUCTION Carbon-based structures are the most versatile materials used in various industry applications.1,2 Novel carbon materials with a high surface area and different shapes, morphology, and pore size distributions are always expected, while the formation of pore structure, pore tortuosity, or microstructural defects of the carbon materials is hard to be predicted during the preparation process. On the other hand, those features have an important influence on the performance of porous carbon materials in various applications.3,4 Thus, in spite of many methods used to prepare porous carbon, the synthesis of carbon materials with designed structure could be appreciated. Metal organic frameworks (MOFs) are new types of materials constructed by metal ion connectors and organic bridging ligands.5,6 To date, thousands of different MOFs have been synthesized. Their regular morphology and controllable pore size make them attractive in applications from gas storage, separation, to energy field.7−9 Moreover, MOF can be further modified for different applications, such as doping with different metal ions.10,11 The calcination of MOF precursor is another popular approach to produce many useful functional products, such as metal oxide,12,13 carbon materials,14 hybrid materials,15,16 etc. Among them, the preparation of nanoporous carbon from MOF precursors is a recent research highlight.17,18 The uniform pore structure of MOF ensures the regular pore size of its carbonized derivatives, which makes it possible to design porous carbon materials at the molecular level.19,20 Moreover, © XXXX American Chemical Society

the carbonized MOF can be further adjusted to have different appearances, such as nanorods,16 hollow,21 and 1D structure,22 which are especially attractive for energy storage materials. The pure MOF crystals are very important in scientific research, but MOF products containing structure defect or impurity are very popular. Such phenomena could be due to different reasons, such as the side reaction, inappropriate reactant ratio, or imperfect crystallization, etc. In most cases, it is still a challenge to separate the desired MOF from crystalline and amorphous contaminants cogenerated or purify the mixed products after synthesis.22 Those primary MOF products containing defect or impurity may be referred to “low grade” if judged by their crystallinity. However, they could greatly outnumber those perfect MOFs in quantity. Thus, if those lowgrade MOF products could be utilized in practice it will notably add their value. Besides, those low-grade MOFs may have the different properties or morphologies in terms of structure. Thus, it would be an interesting topic to explore the potential of those mixed products. During synthesis of the MOF, the ligand plays an important role in determining its morphology and pore structure.23 Among numerous ligands, terephthalic acid and trimesic acid are two cheap and popular linkers used for the synthesis of many famous Received: December 28, 2018 Accepted: January 8, 2019

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DOI: 10.1021/acs.iecr.8b06437 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Mg−MOF and Carbonized Products Prepared at Different Conditions MOF precursor

carbonized product

Mg−BDC Mg3−BTC2 Mg−BDC/PET Mg3−BTC2/PET

C-IIa C-III CP-II CP-III

carbonized product

washing condition 1 g of MOF washed in 100 mL of 1% HCl for 2 h, repeated.

C-II/2 C-III/2

washing condition further washed in 100 mL of 5%HCl

a

II, products using BDC as ligand; III, products using BTC as ligand.

Nicolet iS10 FTIR spectrometer. Scanning electron microscopy (SEM) images were carried out on a NOVA Nano SEM450 (Thermo Scientific, America). Nitrogen adsorption−desorption experiments was carried out by JW-BK122F (Beijing JWGB Sci.&Tech.). The sample was activated at 150 °C for 2 h. The crystal phase was investigated by a BRUKER AXS D8-Advance X-ray diffraction (XRD) system. Thermal stability was measured by TGA (NETZSCH STA 449 F3) under N2 atmosphere. Transmission electron microscope (TEM) and selected area electron diffraction (SAED) patterns were taken by a JEM-2010 (TEM; Hitachi, Japan). X-ray photoelectron spectrometry (XPS) analysis was performed with an Escalab 250Xi. The zeta potential was measured by a Zeta potentiometer (Zetasizer, Nano ZS, Malvern Uk). Dye Adsorption. The adsorption of methyl orange (MO) and methyl blue (MB) onto carbonized MOF was performed by batch sorption experiments at room temperature. The adsorbent was separated from the dye solution by centrifutration. A 0.02 g amount sample was mixed with 100 mL of dye solution of different concentrations (from 100 to 800 ppm) in a beaker. The dye concentration of the solution was analyzed at a wavelength of 464 nm for MO and 628 nm for MB by UV−vis spectroscopy (UV-1800 Vis spectrophotometer, Suzhou Daojin Ltd.). The amount of dye adsorbed at equilibrium was calculated from the following equation

MOFs, like HKUST-1, UiO66, etc. Moreover, MOFs formed by light main group metals can reduce the density of the MOF, i.e., Mg/DOBDC shows the highest CO2 adsorption capacity at low pressure.24 Although Mg−MOFs based on those two linkers were synthesized a long time ago, no application has been reported until now.25,26 Considering the abundant and cheap source of both ligands and Mg salts, it would be a great pity to leave it unused. Moreover, we would like to prove our opinion by turning those useless Mg−MOFs into useful carbon materials. Herein, four Mg−MOFs based on terephthalic acid or trimesic acid linker were synthesized. PET has been added as an inducer for comparison since it may improve the crystallinity of MOF products and reduce the MgCl2 impurity in the products.27 Afterward, the prepared Mg−MOFs were calcinated to prepare porous carbon materials. The obtained carbon materials were tested as dye adsorbent and supercapacitor. It was found that Mg salt can strongly influence the morphology and structure of both MOF and its carbonized products, which provided a new approach to adjust the new carbon materials for different applications. In all, this study provides new knowledge to design useful materials from those primary MOF products.

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EXPERIMENTAL SECTION Materials. Magnesium chloride (MgCl2·6H2O), dimethylacetamide (DMAc), HCl, and KOH were purchased from Sinopharm Chemical Reagent Co. 1,3,5-Benzenetricarboxylic acid, 95% (BTC), and 1,4-benzenedicarboxylic acid (BDC) were from Aladdin Co. Methyl blue (C37H27N3Na2O9S3) and methyl orange (C14H14N3SO3Na) were purchased from Linfeng Chemical Reagent, China. Chitosan was purchased from Macklin Co. Super P was purchased from Tianjin Union Chem, China. All chemicals in the experiment were analytical grade and used without further purification. Synthesis of Mg−MOFs and Its Carbonized Derivatives. The synthesis of Mg−MOFs was carried out by a solvothermal method. Mg salt and ligand of an equal stoichiometric ratio were weighted and mixed in DMAc; then the solution was put into a PFTE vessel to react at 120 °C for 12 h. Afterward, the vessel was cooled to room temperature. The solid products were collected by filtration, continuously rinsed with methanol, and then dried at ambient condition. For comparison, a parallel experiment was carried out by adding polyethylene terephthalate (PET) nonwoven fabric in the solution as an inducer to produce purer MOFs. Carbonization Process. The obtained MOF crystals were calcinated under nitrogen gas at 800 °C in a furnace for 5 h to obtain carbon material. The product was then washed thoroughly with plenty of water and excessive HCl solution for 4 h and afterward dried under vacuum for 12 h. Table 1 lists the conditions adopted to prepare Mg−MOFs and their carbonized products. Characterization of Samples. Fourier transform infrared spectroscopy (FTIR) results were recorded on a Thermo

qe =

(C0 − Ce)V m

(1)

where qe is the amount of dye taken up by the adsorbent (mg/g), C0 is the initial dye concentration (mg/L), Ce is the dye concentration after the adsorption process (mg/L), m is the adsorbent mass (g), and V is the volume of the dye solution (L) . The equilibrium adsorption characteristics were analyzed by using the Langmuir and Freundlich isotherm models, as shown in eqs 2 and 3 Ce C 1 = e + qe qm KLqm

(2)

qe = KFCe1/ n

(3)

where qm is the maximum adsorption capacity (mg/g), qe is the adsorption uptake at equilibrium (mg/g), Ce is the concentration (mg/g), and KL and KF are adsorption constants (L/g). The adsorption kinetics was evaluated by pseudo-first-order and pseudo-second-order equations, as shown in eqs 4 and 5, respectively log(qe − qt ) = log qe − t 1 1 = + t qt k 2qe qe 2

B

k1 t 2.303

(4)

(5) DOI: 10.1021/acs.iecr.8b06437 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. SEM images of MOF products prepared from MgCl2 and BDC or BTC with or without PET inducer at 120 °C.

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RESULTS AND DISCUSSION Preparation of Mg−MOF Product. The structures of the Mg−MOF products were investigated. Virtually, Mg(OH)2 was also tried to synthesize a MOF, but it is unable to obtain the product. This phenomenon is different compared with other metals, such as Cu and Fe, as those metal ions can easily react with those ligands at the room temperature.28,29 The reason should be due to the low reaction activity between Mg and BDC or BTC. The SEM images of Mg−MOF products are shown in Figure 1. Although the MOF products were synthesized at similar conditions, they show a great difference. Mg−BDC shows an assembled nanosheet morphology, while Mg−BDC/PET appears as many smaller oval crystals. Similarly, a big difference in appearance also exists between Mg3−BTC2 and Mg3−BTC2/ PET; the former looks like a big nut with a porous structure inside. The latter shows an aggregated structure of less regular solids or particles, indicating a low crystallinity degree. Here, PET was added in the solution since its structure unit is similar to BDC. The nucleation of MOF crystal could start from the PET surface, not from the solution. Hence, the obtained crystal could be more pure. As for the morphology difference between MOF products using different ligands, BDC tends to form a MOF with a lamellar structure but BTC tends to form a network structure. The reason is related to the different numbers of functional groups of the ligand. According to SEM images, the products, except Mg-BDC/ PET, showed polymorph structures, indicating a mixed component. This means that MgCl2 salt has changed the crystallization process of MOF, so a crystal with different morphology was formed. On the basis of the low reaction activity between Mg2+ and ligand, this could easily happen.

where k1 is the rate constant of pseudo-first-order adsorption, k2 is the rate constant of pseudo-second-order adsorption, t is the adsorption time, and qt and qe are the adsorption capacity at time t and at equilibrium, respectively. For repeated adsorption experiment, the product adsorbed with dye was centrifugated and then washed with 5% NaOH to release absorbed dye for 12 h. Electrode Preparation and Electrochemical Measurement. The working electrode was prepared as follows: a mixture of the as-prepared sample, Super P, and Chitosan at a weight ratio of 75:15:10 was prepared and coated on nickel foams to form the working electrode. Chitosan was first dissolved in water with a few drops of acetic acid; then Super P and sample was added with strong stirring to get a mixture for electrode preparation. The obtained electrode was dried in an oven at 100 °C for 12 h. The typical mass loading of the active material is approximately 1−2 mg/cm.2 The electrochemical performance of the carbonized MOFs was investigated using a three-electrode system with Hg/HgO as the reference electrode, Pt plate as the counter electrode, and 6 M KOH solution as the aqueous electrolyte. Cyclic voltammograms (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were recorded on a CHI 760e electrochemical workstation within a potential window from −0.8 to 0 V. The specific capacitances (C) of the electrodes were calculated using the following equation based on the discharge curves, C = IDt/mDV. In this equation, m (g) is the mass of the active material, DV (V) is the operating potential window, I (A) is the galvanostatic discharge current, and Dt (s) is the discharge time. C


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Figure 2. XRD curves of Mg−MOF products prepared without (a) and with inducer (b); (c) schematic illustration of the possible MOFs structure units with the existence of Mg salts.

Figure 3. SEM images of carbonized MOFs from different Mg−MOF precursors.

some character peaks of Mg−MOF (BDC) can be observed.30,31 A slight shift of XRD peaks compared with those reported in the literature could be due to their different composition. Besides, two character peaks of MgCl2 can also be found in XRD curves,32 indicating the existence of Mg salt in MOF crystals.

To further test the hybrid structure of the Mg−MOF products, XRD patterns were measured. The results are shown in Figure 2. In Figure 2, the complex XRD patterns are revealed, indicating the polycrystalline structure of MOF and MgCl2. For Mg−BDC, D


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Figure 4. TEM of carbonized Mg−MOF products.

Figure 5. XRD pattern (a,b) and Ramam spectra (c) of carbonized MOF products.

is in consistent with their XRD data. Besides, the low BET surface area means a strong interaction between the guest solvent molecule and the MOF, which can cause collapse of the pore structure during solvent removal.33 This can be proved by their TGA curves, as shown in Figure S2. Carbonized Mg−MOF Products. Four Mg−MOF products were further calcinated to prepare porous carbon materials. The samples were maintained at 800 °C for 5 h to ensure the full carbonization of MOFs. The SEM images of carbonized MOFs are shown in Figure 3. The morphology evolution from MOF to porous carbon derivative can be clearly observed. In Figure 3, C-II (carbonized MOFs based on BDC) shows a flower-like morphology assembled by lamellas (enlarged images are shown in Figure S3), while CP-II (carbonized MOFs using PET inducer) appears as a popcorn structure. Similarly, a big difference exists between C-III and CP-III, a bud-like structure compared with a nanosheet structure. Such an expansion structure is due to the decomposition effect. Virtually, both ligand and MgCl2 can produce gas during the thermal decomposition process. If the crystal structure is less stable, an expansion phenomenon can be observed. Usually the expansion effect is not so significant compared with their MOF precursors.

Mg−BDC/PET shows a higher crystallinty, which proved the nucleation effect of PET. For Mg3−BTC2, the characteristic peak of Mg salt can be observed, while the weak XRD peak intensity of Mg3−BTC2/PET indicated a lower crystallinity. It means that PET has no effect to improve the crystallinity of Mg3−BTC2. Without the help of MgCl2,, the formation of crystal structure becomes difficult due to the low reaction activity between Mg2+ and BTC. Moreover, since the products are soluble in water, so those MgCl2 impurities cannot be removed from the product. According to the results of XRD and SEM, we assume that the possible structure of Mg−MOF could be that MgCl2 may be mixed between the MOF crystal units, as shown in Figure 2c. The reason could be due to the low activity between Mg and ligand, so the existence of MgCl 2 interferes with the crystallization process of Mg−MOF. Thus, the morphology and the shape of the products can be changed with the participation of MgCl2 during the crystal formation process of Mg−MOF. Finally, the N2 adsorption/desorption was measured, as shown in Figure S1. As expected, Mg−MOFs showed a lower BET surface area than those prepared with PET inducer, which E


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Figure 6. (a,b) XPS spectra of carbonized Mg−MOFs, (c) high-resolution C 1s XPS spectra, (d) high-resolution O 1s XPS spectra, and (e) highresolution Mg 1s XPS spectra.

Figure 7. (a) N2 adsorption/desorption isotherms and (b) pore-size distributions calculated from the desorption isotherms by the DFT method, and (c) cumulative prove volume via pore size of different samples

Here, such a big crack effect further proved that MgCl2 molecules were located between MOF crystals. In Figure 4, TEM images showed that C-II has a laminated packing and CP-II is randomly assembled. C-III has a network structure with smaller pores, and CP-III shows a trace of monosheets. The difference mainly comes from the different morphology of their MOF precursors. If C-II (or CP-II) and CIII (or Cp-III) were further compared, the difference should be attributed to their different MOF crystal units. Mg−BDC tends to form a lamellar structure. Mg3−BTC2 tends to form a 3D network structure. XRD patterns are shown in Figure 5a. It is surprising to find that C-II and C-III show some extra strong peaks. The peaks at 2θ of 37° and 62.3° correspond to the (111) and (220) planes of cubic MgO.34 The peaks at 2θ of 18.4° and 38.0° matched with the (001) and (101) of the hexagonal Mg(OH)2 phase,35 which means that some Mg salts remain inside the carbonized MOFs. This can be also proved by the area electron diffraction pattern (SAED) (shown in Figure S4). Since C-II and C-III were washed by excessive 1% HCl solution for a few hours, those remaining salts should be covered by carbon. Those carbons come from the vapor released by the decomposition of ligands, which further deposited onto the surface of Mg salt and

transformed into carbon. Such a phenomenon is very popular during the calcination process.36 Thus, the second washing was adopted; the new XRD pattern exhibits two peaks at around 24° and 43°, corresponding to the (002) and (100) diffraction peaks of carbon, respectively. However, the second washing caused damage of the microstructure of C-II and C-III (shown in Figure S5). However, CPII and CP-III show the XRD pattern of pure carbon materials, indicating that they can be easily cleaned by an acid solution. The reason is due to the low salt content of their MOF precursors. Raman spectra were measured and are shown in Figure 5c. All samples display both D (1350 cm−1) and G (1590 cm−1) bands. Generally, the D band corresponds to a disordered carbon structure.37 The relative ratios of the G band to the D band (ID/ IG) reveal the crystallization degree of graphitic carbon.38 An increased ID/IG ratio means an enhanced structural distortion. Here, the intensity ratios (R = ID/IG) of all samples were calculated. Samples based on II showed higher R data than the counterpart based on III. Moreover, a weak peak at 2700 cm−1 presents in the Raman curve of samples based on II, which correspondes to the 2D or G′ band of graphene. This result further proves that BDC ligand tends to form a laminar F


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Industrial & Engineering Chemistry Research structure, which can be further transformed into a graphitized structure after carbonization. XPS spectra were measured to determine the chemical composition of the carbonized MOFs. The results are shown in Figure 6. A weak Cl 1s peak and an obvious Mg 1s peak can be observed in the XPS spectra of C-II and C-III. Usually 1304.0 eV is attributed to MgO and 1305 eV to Mg(OH)2.39,40 In Figure 6e, the deconvoluted peak is close to 1305 eV, assigned to Mg(OH)2. Here, Mg (OH)2 was formed by the reaction between MgO and water, and MgO is the decomposed product of MgCl2. Thus, this result is consistent with the XRD result. The other samples show the characteristic XPS spectra of carbon materials. The peaks of C 1s spectra of all samples were located between 284.4 and 284.6 eV, corresponding to the standard sp2 graphitic carbon (C−C bonding). The O 1s spectra of C-II and C-III can be deconvoluted into two peaks: a big one at 532.4 eV (MgO) and a small one at 531.4 eV (CO). The O 1s spectra of C-II/2, C-III/2, CP-II, and C-III only contained one peak at 532.7 eV (C−O). The elemental compositions of all samples are given in Table S1. C-II contained 6.7% Mg atom, and C-III contained 3.7% Mg. For CP-II and CP-III, they do not contain a Mg atom. The higher Mg ratio of C-II should be attributed to the higher MgCl2 content in its precursor. Besides, the other samples showed a high carbon content, indicating a pure carbon material. N2 adsorption/desorption isotherm analysis was carried out. The results are shown in Figure 7a. The isotherms of all samples show I/IV-type adsorption/desorption with an obvious H1 hysteresis loop at a relative pressure P/P0 of 0.4−1.0, indicating a hierarchical morphology containing mesopores. C-II and C-III showed lower BET surface areas than CP-II and CP-III due to the residual Mg salt. The second washing caused an increase in both the BET surface area and the total pore volume. Their pore size distributions are shown in Figure 7b and 7c. All samples show a typical bimodal pore size distribution,41 i.e, C-II and CIII contained mainly two types of pores (3.8 and 7.5 nm). Also, a weak shoulder peak around 11 nm can be observed. A second washing enlarged the pore size. While CP-II contained pores of 3.8 and 10 nm, CP-III contained pores of 3.8 and 7 nm. The pore structure difference between samples based on II and III originated from their MOF precursors. While the laminated nanosheet formed a larger pore window, the 3D network formed a smaller pore window. The cumulative pore volume is shown in Figure 7b and 7c. All samples consist of a higher volume of the larger pores and lower volume of the small pores. Dye Adsorption of the Carbonized Product. The environmental remedy is one important application of the porous carbon materials.42 The adsorption performances of MB and MO on carbonized MOFs were first measured. Figure 8a and 8b shows the adsorption capacity of carbonized MOFs at different dye concentrations. It is found that the MO adsorption capacity of C-II or C-III is much higher than that of CP-II or CP-III. The adsorption capacity of C-III reached above 3000 mg/g. To the best of our knowledge, this is the highest report to date. To give a reasonable explanation a literature survey was made. It is found that MgO and Mg(OH)2 are good dye adsorbents.43−45 Thus, the high adsorption capacity of C-II and C-III should be attributed to the carbon-covered Mg salt inside it. The higher adsorption capacity of C-III than C-II should be related to their different pore structure. C-II shows a nanosheet structure, so dye molecules can easily diffuse out of the channel, whereas since C-

Figure 8. (a,b) Dye adsorption of carbonized products at different concentrations, (c) Freundlich isotherm fitting curve (MO), and (d) Langmuir isotherm fitting curve (MB) (points, experimental data; lines, fitting curves).

III has a network structure with a nanocage, it can easily entrap dye molecules. To further explain the mechanism of the dye adsorption on carbonized product, FTIR spectra before and after dye adsorption were measured and are shown in Figure 9a.

Figure 9. FTIR spectra (a) of carbonized MOF before and after adsorption of MO, and zeta potentials (b) of carbonized MOF at pH = 7.0.

In Figure 9a, the band at 1242 cm−1 corresponds to C−O−C. The broad band at 3200−3600 cm−1 is related to OH. The sharp peak at 3725 cm−1 can be related to the O−H stretching vibration of H2O molecules bound to MgO; 446 cm−1 is assigned to the Mg−O stretching vibration.35 After adsorption, the sharp peaks at 3725 and 446 cm−1 become very weak. While MO shows a weak acidic property, carbon-covered MgO or Mg(OH)2 acts as a weak base.44 Thus, the vibrating state of Mg−O is changed after the adsorption of MO on Mg−O through its sulfonate groups, and the surface water bound on Mg−O is replaced with dye simultaneously. Zeta potentials are shown in Figure 9b. According to the literature, MgO shows a zeta potential about 15 mV;29 Mg(OH)2 is about 30 mV.42 The zeta potentials of C-II and C-III are close to those data. After the second washing, the zeta potential of the samples increased. This result reveals the change of the surface electrostatic property caused by their surface G


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Figure 10. (a,b) Effect of contact time on the dye adsorption onto carbonized products: initial dye concentration 200 mg/L, sample dosage 0.2 g/L, pH 7. (c,d) Pseudo-second-order kinetic model fitting curves (points, experimental data; lines, fitting curves).

composition, i.e, the higher zeta potential of sample C-III/2 could be due to its hydrophobic property and low acidic functional groups,46 since its O atom content is quite low (2.4%). Besides, C-III (the isoelectric point at about pH = 10.3) shows a positive net charge at pH = 7.0, whereas the pKa of MO molecules is 3.8, indicating negative surface charge formed by −SO3− groups. The result proves that carbon-covered MgO or Mg(OH)2 plays an important role in improving the adsorption capacity of the carbonized product. Moreover, the adsorption capacity of MB becomes much smaller in Figure 8b. To explore the reason, the selective experiment was further carried out by putting the carbonized products in the mixed solution of two dyes with the same concentration. No selective adsorption behavior is observed (Figure S6). This means the low adsorption capacity of MB is mainly attributed to their size difference. MB has a much larger molecule size than that of MO (Figure S7). Compared with MO, the spaces occupied by MB molecules are much larger. Thus, the smaller voids inside the carbonized products become unreachable for MB; the adsorption capacity of MB decreases. As for CP-II and CP-III, their dye adsorption capacity is close to the reports about the carbon materials, such as GO and active carbon.47 Langmuir and Freundlich isotherm models were further used to analyze the adsorption equilibrium data. The fitting curves and parameters are shown in Figure 8c and 8d. The Langmuir isotherm assumes a monolayer adsorption of adsorbate molecules without any interaction with each other on the adsorbent surface with identical binding sites. The Freundlich isotherm is an empirical equation derived by assuming a heterogeneous adsorbent surface with its adsorption sites being at different energy levels.48,49 Obviously, the Freundlich isotherm is more suitable to describe the equilibrium data of MO adsorbed on C-II and C-III. The reason could be that the uneven distribution of Mg salts inside the carbonized

MOF leads to a heterogeneous surface. During the adsorption of MB onto C-II and C-III or dye adsorption onto CP-II and CPIII, Langmuir isotherms give a better fitting result. Afterward, the adsorption kinetics were investigated. Figure 10a and 10b shows the effect of contact time on the adsorption process of MB and MO. All samples reached the adsorption equilibrium in 1−2 h. The fitting curves of pseudo-second-order kinetic models and parameters are presented in Figure 10 c and 10d. It can be concluded that the experimental data fit very well with the pseudo-second-order kinetic model, which is popular for the adsorption of porous carbon materials. Fitting with the pseudo-first-order model showed a very low correlation coefficient (R2); thus, the results were not presented. The detailed data on the thermodynamic and kinetic fitting parameters are given in Tables S2 and S3 respectively. Moreover, the repeated use of C-II and C-III was also measured. The second adsorption capacity recovery reached about 90% for C-II and 80% for C-III after washing with 5% NaOH solution. Thus, it means that our products can be used in dye removal. Comparison with Literature. The MO adsorption capacities of different adsorbents were collected from the literature for comparison.50−60 The data are shown in Figure 11. The detailed information is listed in Table S4. The typical adsorbents from different materials, such as the polymer,50 carbon materials,51,52,56,57 metal oxide,53,55 and layered hydroxides,58−60 are included. Obviously, our samples show a higher dye adsorption capacity than those reports. The reason has been given in the above discussion. Electrochemical Performance. The carbonized MOFs were further tested as supercapacitors since application in the energy field is a very important research topic of carbon materials. H


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The cyclic voltammetry (CV) curves of carbonized MOF electrodes at a scan rate of 100 mV/s are shown in Figure 12a (CV curves of the individual electrode at scan rates from 5 to 100 mV/s are shown in Figure S8a−c). All of the CV curves exhibit a nearly rectangular shape, indicating a well-defined EDLC behavior without redox reaction. It can be further found that CP-III displays the largest area in the CV curve, indicating the highest capacitance. Galvanostatic charge/discharge (GCD) curves of carbonized MOF electrodes at a current density of 1 A/g are shown in Figure 12b. The symmetrical charge−discharge curves indicate good reversibility and conductivity. Figure 12c shows the calculated values of Csp as a function of scan rate. The highest Csp = 127 F/g is achieved at a scan rate of 1 A/g for the CP-III electrode. Furthermore, the specific capacitances of electrode at different current densities are shown in Figure 12d. The specific capacitance of CP-III decreases quickly at high current density, indicating a low rate capability. However, the specific capacitance of CP-II was quite stable even at a current density

Figure 11. Comparison with the different adsorbents reported in the literature.

Figure 12. (a) CV curves of different electrodes within a potential range from −0.8 to 0 V (vs Hg/HgO) in 6 mol/L KOH aqueous solution at a scan rate of 100 mV/s, (b) galvanostatic charge/discharge curves of the different electrodes at a current density of 1 A/g, (c) galvanostatic charge/discharge curves of CP-III electrode at different current densities (1−10 A/g), (d) specific capacitance of the carbonized electrodes at different scan rates, (e) EIS curves of the different electrodes, and (f) cycling performance of CP-III electrode at a current density of 1A/g (insert curves, GCD of the last 10 cycles at a current density of 1 A/g). I


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Industrial & Engineering Chemistry Research Table 2. Structure Properties and Specific Capacitance (obtained by a three-electrode system) of Carbonized MOFs sample

SBET (m2/g)

pore volume (cm3/g) (≤4 nm/≤10/full)

main pore size (nm)

IG/ID

morphology

specific capacitance (1 A/g)

C-II C-III CP-II CP-III

447 370 768 732

0.05/0.28/1.29 0.05/0.28/0.94 0.17/0.5/1.71 0.2/0.6/1.14

3.8/7.5 3.8/7.5 3.8/10 3.8/7

1.41 2.01 1.87 1.93

flower bud cube nanosheet

74 83 121 127

the literature was found. When used as a supercapacitor, carbonized MOFs has a specific capacitance about 120 F/g. Besides, their pore structures can significantly change their capacitive behavior. In all, this study provides new idea to design new carbon materials for different applications.

of 10A/g, about 80% of its initial capacitance at 1 A/g. This could be related to their different pore structure. Electrochemical impedance spectra (EIS) reflected the internal resistance and interfacial contact resistance on the electrode. The results are shown in Figure 12e. At a low frequency, a typical 45° segment was observed, which was due to the resistance of the ions during diffusion into the electrode particles.61 The high-frequency intercept on the real axis corresponds to the total ohmic resistance. The equivalent series resistances are approximately less than 1Ω, and no distinct semicircle can be observed for all of the electrodes, indicating a fast ion diffusion toward the surface of the electrode. The reason could be that our carbonized MOFs contain a very small amount of microporous structure. The cycling performance of CP-III electrode at a current density of 1 A/g is shown in Figure 12f. The specific capacitance was reduced by only 3% after 1200 cycles. The corresponding GCD curves of the last 10 cycles are shown in Figure 12f. It appears to be similar without obvious changes, further indicating a stable cycling performance. Finally, the structure parameters and specific capacitance of all samples are summarized in Table 2. Usually the specific capacitance of normal carbon materials falls into the range between 100 and 200 F/g.62,63 However, if compared with the reports about the porous carbon material with the similar morphology in the literature, our data is lower.64 The reason could be due to the different porous structure or composition.65 According to many reports, mesopores (2−50 nm) and micropores (0.5−2 nm) contribute the most to the capacitance in an electrical double-layer capacitor;62,66 but our samples showed a low microporous structure. Besides, our samples have a high carbon content close to 98% without other heteroatoms except O according to XPS data in Table S1. Moreover, the higher specific capacitance of CP-II and CP-III than that of C-II and C-III was due to the difference in their surface area. The different electrochemical behavior between CP-II and CP-III is related to their different pore structure since they have a similar surface. According to the literature, the different porous structure can influence the capacitance of the sample.67 CP-II showed a laminated structure. At a high current density, the laminated structure can provide a quick response for ion diffusion. That is why CP-II shows a better rate capability (80% retention at 10 A/g).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b06437. N2 adsorption/desorption and TGA curves of Mg−MOF; enlarged SEM images and SAED of carbonized Mg− MOF; TEM image of carbonized Mg−MOF after second washing; XPS data; thermodynamic and kinetic fitting parameters of adsorption MO; MB on carbonized Mg− MOF; selective adsorption of MO and MB on carbonized Mg−MOF; molecular structure of MO and MB; CV curves of carbonized Mg−MOF at various scan rates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 021- 64252989. E-mail: [email protected]. ORCID

Hu Yang: 0000-0001-9411-2553 Zhen-liang Xu: 0000-0002-1436-4927 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 (No.21276075). REFERENCES

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CONCLUSION Mg−MOFs were synthesized by the solvothermal method with or without PET inducer. The obtained products showed polymorph structures with different morphologies, indicating a hybrid product of MOF/salt. After carbonization, the products showed an expansive structure similar to a flower, bud, cube, or nanosheet depending on their precursors. The reason can be explained by the thermal decomposition effect of MgCl2 and ligand. When the carbonized MOFs were used as dye adsorbent, it was found that the residual carbon-covered Mg salt inside carbonized MOFs could greatly increase the MO adsorption capacity. A higher MO adsorption capacity than that reported in J


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