Effect of Amino Functionality on the Uptake of Cationic Dye by

Article pubs.acs.org/jced

Effect of Amino Functionality on the Uptake of Cationic Dye by Titanium-Based Metal Organic Frameworks Rehana Bibi,† Lingfei Wei,† Quanhao Shen,† Wei Tian,† Olayinka Oderinde,† Naixu Li,*,† and Jiancheng Zhou*,†,‡,§ †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, P. R. China Department of Chemical and Pharmaceutical Engineering, Southeast University, Chengxian College, Nanjing, 210088, P. R. China § Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Southeast University, Nanjing 211189, P. R. China ‡

S Supporting Information *

ABSTRACT: Titanium-based metal−organic frameworks (MOFs), named MIL-125 and NH2-MIL-125, have been successfully synthesized by the hydrothermal method. They were specially designed for the application of methylene blue (MB) removal from aqueous solution. The maximum adsorption capacity of MB was found to be 321.39 mg/g and 405.61 mg/g. The amount of dye that has been adsorbed 99.9% was 300 ppm within 20 and 120 intervals of time. The results show that NH2MIL-125 is more effective in terms of both selectivity and capacity for the adsorption of MB cationic dye compared with MIL-125. The high adsorption selectivity was due to the unique electrostatic interaction between the amino groups of the dye molecules and NH2-MIL-125 but, on the other hand, owing to the more negative zeta potential (−32.4 mV), resulted from the charge balance for the protonation of −NH2. Moreover, reaction parameters including exposure time, adsorbent dose (0.02−0.05 mg), initial dye concentration (100−500 mg/L), and temperature were studied in detail. The adsorption processes in the two MOFs were determined to follow a pseudo-second-order pathway and obey a Langmuir isotherm model. Furthermore, the reaction was found to be spontaneous in nature yet thermodynamically an endothermic process. Characterization and structural analysis of the samples were evaluated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectrometry, N2 adsorption/desorption (BET), X-ray photoelectron spectroscopy (XPS), and zeta potential. oxidation, and adsorption.7 As a classic technique that is promising to use, adsorption could operate in a more simple, efficient, and environmental benign way. Numerous materials have been investigated on the adsorptive removal of dye from wastewater such as activated carbon or zeolite,8 banana peel,9 polymeric resins,10 graphene,11 and clay.12 However, these adsorbents often face some problems like a low adsorption capacity, complex preparation process, and difficulty in regenerating up to some extent.13 The development of an effective adsorbent that possesses good adsorption ability and selectivity toward dye removal is thus highly desired. Recently, metal−organic frameworks (MOFs) have received considerable research interest. They comprise a new class of coordination network structures consisting of metal or cluster linked by organic ligands.14,15 As a result, MOFs are usually characterized by a high specific surface area and uniform and tunable pores. This unique textural property enables them with a wide variety

1. INTRODUCTION The last few decades have witnessed a tremendous growth of industries. However, this rapid development has also caused many crises in the world. One of the most critical issues, for example, is water pollution.1 The industrial wastes containing organic dyes are considered as the major reason for water pollution. These dyes could destroy the symbiotic process rising from their intrinsic carcinogenic and mutagenic effects and thus are highly toxic to aquatic life and also threaten human life.2,3 Generally, effluents from industries like paints, rubber, cosmetics, dyeing, textile, food coloring, printing, paper making, and so forth are the major contributors of organic dyes in water.4 So far, all around the world, more than 100 000 kinds of dyes are produced with a yield rate of more than 7 × 105 tons/ year. During the manufacturing process, 2% of dyes are directly discharged, and another 10% of them may leak during the textile coloring process.5,6 Various methods have been investigated for the removal of dyes. Notable approaches include electrochemical destruction, flocculation, precipitation, coagulation, ozonation, biological treatment, photocatalytic © 2017 American Chemical Society

Received: December 7, 2016 Accepted: April 13, 2017 Published: April 20, 2017 1615

DOI: 10.1021/acs.jced.6b01012 J. Chem. Eng. Data 2017, 62, 1615−1622

Journal of Chemical & Engineering Data

Article

dicarboxylic acid [H2BDC; C8H6O4 (99%)], N,N-dimethylformamide [DMF; (CH3)2NCHO], and methanol (CH3OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2.2. Synthesis of MIL-125 and NH2-MIL-125. MIL-125 and NH2-MIL-125 were synthesized by a slight modification of the method provided by Zlotea et al.4,28 Specifically for the preparation of MIL-125, 15 mmol of 1,4-benzene dicarboxylic acid (H2BDC, 99%) were added to a 60 mL solution containing N,N-dimetylformamide (DMF) and methanol with a (9.1 v/v) ratio. Then, 9 mmol of tetra-isopropyl ortho titanate (98%) under the flow of nitrogen gas was added to the above solution. The mixture was stirred at room temperature for 30 min, then transferred to 100 mL Teflon-lined stainless steel autoclave and heated at 413 k for 24 h in an oven. After completing the reaction, a product was obtained by centrifugation and washed repeatedly with DMF and methanol, respectively. Finally, the obtained white solid was activated under vacuum at 423 K for 12 h. NH2-MIL-125 was prepared by dissolving 3 mmol of tetra-isopropyl orthotitanate and 6 mmol of 2-amino-1,4benzene dicarboxylic acid (H2BDC-NH2, 99%) in a 60 mL DMF/methanol mixture (1:1, v/v) under the flow of nitrogen gas and heated in a 100 mL Teflon-lined stainless steel autoclaves at 413 K. The same procedure was applied for product recovery and vacuum activation as shown above. 2.3. Instrumentation. X-ray powder diffraction (XRD) patterns of the MIL-125 and NH2-MIL-125 were recorded with a D8-Discover (Bruker, Germany) operating with Cu Ka radiation at 30 kV and 10 mA with a scanning angle (2θ) range from 5 to 60 at the scanning rate of 10 deg/min. The morphological observation was observed through scanning electron microscope (SEM) model (EPMA-8705QH2, Shimadzu Co, Japan). Nitrogen adsorption−desorption isotherms were measured by a Micromeritics ASAP 2020 M system at liquid nitrogen temperature (77 K). The specific surface area (SBET) was calculated using the Brunauer, Emmett, and Teller (BET) method. The pore size distributions and the average pore diameter were measured using the DFT method. X-ray photoelectron spectroscopy (XPS) was conducted on a 2000 XPS system with a monochromatic Al-Kα source and a multichannel detector. Before each test, the calcined sample was reduced in H2 at 250 °C for 2 h. The obtained binding energies were calibrated using the C 1s peak as a reference at 286.4 eV. The experimental error was given within ±0.1 eV. The zeta potential was measured by ZETASIZER nano-NS through Malvern instruments. Fourier transform infrared spectroscopy (FT-IR) was carried out with a Germany Bruker Company EQUI-NOX55 by means of KBr pellet technique at a scanning wavelength range of 4000−400 cm−1. 2.4. Adsorption Experiments. In a typical adsorption test, 40 mg of MIL-125 or NH2-MIL-125 was added into 50 mL solution containing 300 mg/L dye with continuous stirring. Samples were taken at respective predetermined adsorption time and were filtered through syringe filter (PTFE, hydrophobic, 0.45 μm). At the end, the amount of remaining dye in the supernatant liquid was determined over a UV−vis spectrophotometer (PerkinElmer, Lambda 35) by measuring its maximum wavelength 664 nm. All of the experiments were repeated for three times in order to check the reproducibility, and the average results were employed for further data analysis. The percentage of removal dye in the adsorbate phase as well as the residuals in the solution at equilibrium (qe, mg/g) or at time

of applications ranging from gas storage, to drug delivery, catalysis, and adsorption of chemicals.16,17 Thanks to the efforts of many researchers, there are many reports available regarding the gas adsorption properties of MOFs like O2, N2O, CH4, CO2, CO, H2, and N2.18,19 They also generate research interest in dye removal from aqueous solution.20 In addition to their structural superiority, MOF adsorbents have also found indispensable value in finely divided states known as surface physicochemical interactions between the dye molecules and MOF external surfaces, for example, in hydrogen bonding, electrostatic interactions, π−π interaction, aromatic cycles, and so forth. It is worth pointing out that MOFs are microporous in nature, whereas most of the dye molecules are too big to get inside of the pores. Therefore, they mostly adsorb on the external surface of the MOFs, different from the gas molecule adsorption process that could occur inside. Therefore, the MOF surface properties play a vital role in the adsorption of dyes.21 Among the various surface interactions, electrostatic interaction22−24 could be easier to be modulated by altering the surface zeta potential. For example, through replacing the organic ligands like 1,4-benzenedicarboxylic acid to 2-amino1,4-benzenedicarboxylic acid, the electrostatic interaction can be varied because of protonation of −NH2.21 The adsorption selectivity and adsorption capacity could thus be increased for specific dyes. Among the various reported MOFs, MIL-125 (MIL = Material of Institute Lavoisier) is an interesting candidate, first synthesized by Dan-Hardiet al. The basic unit of it is Ti8(OH)4−(O2C−C6H5−CO2)6, a three-dimensional quasi-cubic tetragonal structure built up from a cyclic octamer. More specifically, the unit is constructed from corner sharing of titanium octahedra and are further connected to 12 other cyclic octamers through BDC linkers.25 The isoreticuler NH2-MIL125 of this MIL-125 can be prepared by replacing H2BDC by NH2−H2BDC in the substrate. MIL-125 and NH2-MIL-125 were then evaluated as adsorbents for the removal of methylene blue (MB, see Scheme 1 for the structure), a typical cationic Scheme 1. Molecular Structure of Methylene Blue Dye

dye from wastewater. While both of them showed a good adsorption capacity, NH2-MIL-125 exhibited a better performance. The possible mechanisms for this improvement as well as the reaction kinetics and thermodynamics were investigated and discussed in detail. While to the best of our knowledge MOFs have been successfully investigated for the removal of hazardous organics,26,27 there is no report available for the removal of MB dye by using MIL-125 and NH2-MIL-125 through adsorption.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents and solvents of analytical grade were obtained and used as received without further purification. 2-Amino-1,4-benzenedicarboxylic acid (H 2 BDC-NH 2 , C8H7NO4 (99%) was obtained from Alfa Aesar, China Co., Ltd. Tetra isopropyl-ortho-titanate (C12 H28O4Ti), 1,4-benzene 1616



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“t” (qt, mg/g) were calculated by using eqs 1, 2, and 3, respectively. % removal =

C i − Cf × 100 Ci

be assigned to the vibration of the benzene ring, while those around 1583 and 1418 cm−1 could be attributed to the carboxylate linker. Generally, NH2-MIL-125 was found with a similar FTIR spectrum. A careful investigation reveals some differences. Specifically, a series of bands in the region of 1400− 1700 cm−1 are obtained due to the carboxylic acid functional group. The bands at 3415.62 and 3547.88 cm−1 provide a direct evidence of the existence of bridged NH2 and OH groups. Moreover, the asymmetric stretching vibration of carbonyl group should be responsible for the absorption bands around 1600 and 1500 cm−1, whereas the symmetric stretching vibrations of carbonyl groups occur at 1440 and 1400 cm−1.31 Other vibrations, at 1385 and 1257.13 cm−1, for instance, are due to the binding terephthalate group to titanium and C−N stretching of an aromatic amine group, respectively.32,33 The (Ti−O−Ti-O) vibration can be observed in the region of 400− 800 cm−1.34 Figure 3 shows the SEM images of prepared two kinds of MOFs. It is found that both of them possess a disc shape with a particle size 1.37 and 1.25 μm, respectively.

(1)

qe =

(C i − Cf )V W

(2)

qt =

(C i − Cf )V W

(3)

where Ci is the initial concentration of MB dye in aqueous solution and Ce and Ct (mg/L) are the dye concentrations at equilibrium and at a certain adsorption time “t”, respectively. V (L) is the volume of solution, and W (g) is the mass of adsorbent.

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbents. Figure 1 represents the XRD pattern of prepared samples. Clearly, MIL-125 and

Figure 3. SEM image of (a) NH2-MIL-125 and (b) MIL-125.

Figure 1. XRD pattern of (a) NH2-MIL-125 and (b) MIL-125 (Ti).

The permanent porosity and architectural stability of samples were confirmed by the N2 adsorption/desorption isotherm and DFT pore size distribution (Figure 4). The specific surface area

NH2-MIL-125 are found with similar diffraction profiles, which are also consistent with reported ones.28,29 The presence of the −NH2 group in the catalyst does not affect the crystalline structure of the MIL-125. Furthermore, there are no characteristic peaks regarding bulk titanium dioxide phase, e.g., anatase and rutile, observed in both samples. This result indicates that small titanium cluster should present in the framework of NH2MIL-125 and excludes the possibility of huge aggregates of titanium oxide species.. We further investigated the FTIR spectra of the prepared samples, as shown in Figure 2. For MIL-125, the broad bands at 3430 cm−1 are due to the free solvent molecules entrapped within the pores.30 The bands appear at 1310 and 748 cm−1 can

Figure 4. (a) N2 adsorption−desorption isotherm of NH2-MIL-125 and MIL-125. (b) DFT pore size distribution MIL-125 and NH2-MIL125.

for NH2-MIL-125 BET was determined to be 1028 m2/g, while it was 470 m2/g for MIL-125. The total pore volume of NH2MIL-125 was 0.486 cm−3/g, and the pore diameter is 1.820 nm, which was smaller than that of MIL-125 (3.620 nm). The smaller pore volume (Table 1) may be because of the aggregation of amine molecule in the empty space of organic linker. Both samples represent a type IV isotherm at 77 K with a hysteresis loop at p/p0 = 0.4−0.9, indicating the presence of mesoporous along with microspores. The electronic structure and chemical composition of the prepared samples were further characterized by XPS as shown in Figure 5. The XPS survey

Figure 2. FTIR spectra of prepared sample (a) NH2-MIL-125 and (b) MIL-125. 1617



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Table 1. Textural Properties samples

SBET (m2 g−1)

average pore volume

references

MIL-125 NH2-MIL-125 MIL-125(Ti) NH2-MIL-125(Ti) NH2-MIL-125(Ti) NH2-MIL-125(Ti) NH2-MIL-125 NH2-MIL-125

470 1028 714 1660 1360 1469 1300 1298

0.43 0.56 0.16 0.57 0.50 0.60 0.56 0.55

this work this work 34 28 35 36 37

Figure 6. Effect of contact time on the adsorption of MB dye on (a) NH2-MIL-125 and (b) MIL-125 at 30 °C, Ci, 300 mg/g.

be easily understood because adsorbent surface were largely available at the beginning and later it becomes saturated with dye molecules. 3.3. Adsorption Kinetics for MB Dye on NH2-MIL-125. To uncover the reaction mechanism between the catalyst surface and dye molecules, the kinetic data were checked by pseudo-first order38 and pseudo-second order models.39 These two models were used to fit the experimental data, represented by eqs 4 and 5. ln(qe) = ln(qe) − k1t

(4)

t 1 t = + qt k 2q2e qe

(5)

where k1 and k2 are the rate constants of first-order and secondorder models, respectively. So far the values of qe, k1, and coefficient R2 for the first model can be calculated from the linear plot of ln(qe − qt) vs t; similarly, for pseudo-secondorder, the values of qe, k2, and R2 can be calculated from the linear plot of t/qt vs t (Figure S3), where the coefficients of pseudo-second-order and pseudo-first-order models are given in Table 2. It is clearly seen from the given values that the correlation coefficient R2 of pseudo-second-order are 0.999 and 0.994, while that of pseudo-first-order are not satisfactory. Furthermore, calculated qe values (qe,cal) given by pseudosecond-order (372 and 333.3 mg/g for MIL-125 and NH2MIL-125) are closer to the experimental results (qe,exp, 384 and 299.04 mg/g for MIL-125 and NH2-MIL-125) in comparison to the pseudo-first-order model. The much more efficient adsorption of MB taken by NH2-MIL-125 (equilibrium time: 20 and 120 min for MIL-125 and NH2-MIL-125) indicates the indispensable role of the −NH2 group. The effect of the −NH2 group was further validated by a designed experiment, in which methyl orange (MO, intrinsic absorption at 464 nm), an anionic dye, instead of MB, as the model pollutant. As shown in Table S1, even the adsorption process was prolonged for 120 min, no significant decrease of the absorption spectrum could be observed, in consistent with the little changed solution color (Supporting Information, S5). These results demonstrate that, while both of the adsorbents possessed a good removal efficiency for cationic dye, the NH2-MIL-125 showed a much better behavior as induced by the amino group present. 3.4. Adsorption Isotherm for MB Dye on NH2-MIL-125. The adsorption isotherms of MB dye at 30 °C on MIL-125 and NH2-MIL-125 are given in Figure 7. The maximum adsorption capacity of NH2-MIL-125 reaches 405.61 mg/g within 20 min, which is comparatively larger than MIL-125 after 120 min adsorption. The performance again demonstrated the significant role of the −NH2 group for dye adsorption. The adsorption isotherm was determined to be the Langmuir40 adsorption isotherm and based upon the following eq 6:

Figure 5. Survey XPS spectra of (a) MIL-125, (b) NH2-MIL-125, and (c) NH2-MIL-125 after adsorption of MB dye.

spectrum shows four notable peaks 284.6, 458.8, 399.6, and 531.3 eV, which could correspond to C 1s, Ti 2p, N 1s, and O 1s, respectively. No apparent peak for N 1s was observed in MIL-125, indicating the negligible amount of N. It also gives proof that the NH2 group has been functionalized in MIL-125 MOF. The two peaks in the N 1s spectrum at 399.33 and 402.6 eV can be assigned to the N of amino functionality stretching out or protruding into the surface cavities and the positive nitrogen (−N+, −NH2+), respectively. The symmetric peaks at 458.8 and 464.6 eV in the Ti 2p spectrum for Ti 2p3/2 and Ti 2p1/2 imply that the Ti are in the IV oxidation state in the titanium oxo-cluster. Considering the similar XRD patterns of the NH2-MIL-125 and MIL-125 MOFs, it is reasonable to speculate that the NH2 group does not affect the electronic structure of the MIL-125. The NH2 group may not directly coordinate with the metal ion but protrude into the empty space of mesoporous. 3.2. Effect of Operation Conditions on the Adsorption of Methylene Blue. 3.2.1. Effect of Contact Time and Initial Dye Concentration. We then investigated the absorption ability of the MOFs toward MB removal from aqueous solution. Figure 6 shows the effect of contact time (30 and 120 min) on the adsorption capacity of MB dye on MIL-125 and NH2-MIL-125 at different initial concentrations (100−500 mg/g−1, Figure S2. The equilibrium contact time is not the same because of the presence of NH2 group in MIL-125 MOF which plays a vital role in MB removal. It can be clearly observed that the adsorption capacity increases at the initial stage by extending the reaction time and then slowly remained constant until the equilibrium reached. The phenomenon can 1618



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Table 2. Parameters of Pseudo-First-Order and Pseudo-Second-Order Models pseudo-first-order

pseudo-second-order

adsorbent

qe(exp) (mg/g)

qe (mg/g)

K1 (min−2)

R2

qe (mg/g)

K2 (mg/g min)

R2

MIL-125 NH2-MIL-125

372.42 299.04

108.84 449.84

4.6 × 10−2 2.6 × 10−2

0.972 0.988

372.4 333.3

1.009 × 10−3 1.009 × 10−3

0.999 0.999

3.5. Adsorption Thermodynamics of MB Dye on NH2MIL-125. Figure S4 represents the adsorption of MB dye at different temperatures ranging from 15 to 35 °C. The adsorption time was set to be 20 and 120 min over NH2MIL-125 and MIL-125, respectively. As expected, the MB adsorption capacity increases by elevating the temperature and finally reaches 99.57 mg/g. The result suggests that MB adsorption favors a relatively high temperature. The reason may be ascribed to the improved mobility of MB molecules as well as the increased active sites of the MOF. Thermodynamic parameters, Gibbs free energy (ΔGo), standard enthalpy (ΔHo), and standard entropy (ΔSo) were calculated. According to the thermodynamic theory the relationships between ΔGo, ΔSo, and ΔHo are given by the following equation.

Figure 7. Adsorption isotherm for MB dye: (a) Langmuir adsorption isotherm for MIL-125 and NH2-MIL-125) and (b) Freundlich adsorption isotherm for MIL-125 and NH2-MIL-125.

Ce C 1 = + e qe kLqmax qmax

ln

(6)

where KL is the Langmuir constant related to the binding energy of adsorption, qmax (mg/g) was the maximum adsorption capacity, and generally, larger KL is more favorable for the adsorption.41 The parameters of Langmuir model obtained from the linear plot (Figure 7a) are given in Table 3.

adsorbents MIL-125 NH2-MIL125

KF (L/mg) 1.766 1.442

n 0.460 0.189

R2 0.865 0.927

321.39 405.61

KL (L/mg) −2

1.9 × 10 4.5 × 10−2

Ce n

(8) (9)

Table 4. Thermodynamic Parameters for the Adsorption of MB Dye onto NH2-MIL-125 R2

ΔG0 (kJ mol−1)

0.999 0.994

ΔH0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

15 °C

25 °C

30 °C

26.101

91.26

- 0.3

−6.6

−18.36

As shown in the table, a ΔHo value was found positive which indicated that the reaction was an endothermic reaction and mainly associated with the electrostatic attraction.42 In this work, the adsorption of MB on MIL-125 and NH2-MIL125 was endothermic in nature which might be due to the adsorbed water molecules on the catalyst surface. They must be desorbed to restore the active sites for cationic dye adsorption. The desorption process was endothermic, while the adsorption was an exothermic reaction. Thus, the heat absorbed by water molecules for desorption is supposed to be greater than that released from dye adsorption, resulting in a net-endothermic reaction. The value of ΔGo for physical adsorption is usually in the range of −20 to 0 kJ mol−1, while for chemical adsorption it is from −400 to −80 kJ mol−1.43 Furthermore, the negative value of ΔGo as shown in Table 4 indicates that the reaction was spontaneous and endothermic in nature. The value of ΔGo slowly increases with the increment of temperature, in accordance with above results of temperature effect. The obtained positive ΔSo value indicates that the adsorption of MB dye on the catalyst surface was related to entropy and enthalpy changes. In fact, the desorbed water molecules were more than compared adsorbed dye molecules, which leads to an increasing entropy of the system.44

All of the R2 values of Langmuir isotherm model were thus calculated to be 0.99. It could be also concluded that the values of KL and qmax increase at higher temperature. The results demonstrates that the adsorption process obeys the Langmuir isotherm model. The increment of temperature is beneficial for the adsorption of MB on both MOFs. In addition, the adsorption isothermal capacity was also checked by the Freundlich isotherm model according to eq 7 for comparison: ln qe = ln KF + ln

ΔS 0 ΔH 0 − R RT

where T is the absolute temperature in Kelvin (K), R is the gas constant (8.314 J mol−1 K−1). Thus, ΔSo and ΔHo were calculated from slope and intercept of the linear plot of ln KD (ln qe/Ce) vs T. The thermodynamic parameters are given in Table 4.

Langmuir isotherm qmax (mg/g)

Ce

=

ΔG° = ΔH ° − T

Table 3. Parameters of Langmuir and Freundlich Adsorption Isotherms Freundlich isotherm

Qe

(7)

where KF is the Freundlich constant, 1/n is concerned with the heterogeneous surface of the adsorbent, and n represents the driving force strength of adsorption, where a larger n values represent that it would be easy or convenient to the adsorption. The Freundlich isotherm was based on the assumption that there are multilayer as well as monolayer adsorption in an adsorption system. Therefore, when this model was used, physiochemical adsorption should be considered in the system. The experimental data were fitted to a Freundlich isotherm model as shown in Figure 7b, and the parameters are given in Table 3. 1619



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3.6. Adsorption Mechanism. According to above analyses on adsorption kinetics and thermodynamics, it is clear that NH2-MIL-125 has a good adsorption capacity for MB molecules. Many researchers investigated that the surface area has a major role in adsorption phenomena.45 On one hand, the bigger surface area of NH2-MIL-125 than MIL-125 may be one of the reasons for the high adsorption of MB dye, but it should not be the conclusive one. As validated by the data shown in Table S1, NH2-MIL-125 was found to be less active than MIL125 for the adsorption of anionic dye. We therefore suppose that the zeta potential should be more crucial for their adsorption capacity. The zeta potential of as-prepared samples and MB adsorbed samples were thus checked. The results showed that NH2-MIL-125 and MIL-125 have zeta potential values of −32.4 and −7.6 eV, respectively. This difference was definitely attributed to the introduction of -NH2 group. After the adsorption, it can be seen from Figure 8b(xii) that the zeta

Figure 9. Effect of the recycle time of NH2-MIL-125 on methylene blue.

4. CONCLUSION In summary, MIL-125(Ti) and NH2-MIL-125 were successfully synthesized by a hydrothermal process. Both of them showed good capabilities as absorbents for the removal of cationic dye. One possible reason was because of the electrostatic attraction between the adsorbent and the adsorbate surface as revealed by XPS and zeta potential analyses before and after adsorption. Our FT-IR study suggests that some π−π stacking interaction between the phthalic group from NH2-MIL-125 and benzene ring from MB dye was also existed. However, this interaction was not very effective in comparison to electrostatic attraction. The adsorption processes in the two MOFs were determined to follow a pseudo-second-order pathway and obey a Langmuir isotherm model. More interestingly, the reaction was found to be spontaneous in nature yet thermodynamically an endothermic process.

Figure 8. (a) N 1s XPS spectra after the adsorption of MB dye and (b) zeta potential of (xi) NH2-MIL-125 before dye adsorption, (xii) NH2MIL-125 after adsorption of MB dye, and (xiii) MIL-125.

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ASSOCIATED CONTENT

S Supporting Information *

potential of NH2-MIL-125 decreased to −18.06 eV thus capable of adsorbing more cationic dye then anionic one through electrostatic attraction. Generally, the zeta potential shall become more positive with the introduction of -NH2 group, but in this work we find it more negative. We consider that, during dye adsorption on the external surface, protons can get in the insides of the micropore and captured by amidogen to form NH3+.24 This notion can also explained by the analysis of XPS. XPS analysis over the adsorbed NH2-MIL-125 was also performed as shown in Figure 8a. The high-resolution XPS spectrum of N 1s from NH2-MIL-125 indicates a slight shift of the binding energy to 401.59 and 399.23 eV in comparison to that before adsorption (see Figure 5). This result suggests that NH2 can capture protons and make NH3+.45 3.7. Recycling of the Catalyst. According to economical demands, a catalyst should not only possess a high adsorption capacity but also possess high desorbing capability in order to reduce the overall cost of the adsorbent. In this study, the used adsorbent was recycled by washing with anhydrous methanol and water. After drying in a reduced pressure, the used adsorbent was used for the next cycle of dye removal. As shown in Figure 9, the dye removal efficiency was 99% and can be well-preserved for the first four cycles, after which it started to sharply decrease to 54.88% (the fifth cycle). This phenomenon demonstrates that NH2-MIL-125 can effectively remove MB dye even after used, yet further research needs to be addressed for recovering the activity when the MOF has been seriously used.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b01012. Adsorbent dose effect on MB dye adsorption, initial dye concentation, adsorption kinetics for MB dye on NH2MIL-125 and MIL-125(Ti), FTIR spectra after dye adsorption, effects of temperature, van’t Hoff for thermodynamic parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: (+86) 025-52090621; fax: (+86) 025-52090620. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiancheng Zhou: 0000-0002-5551-782X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities of China (nos. 3207045403 and 3207045409), National Natural Science Foundation of China (nos. 21576050 and 51602052), Jiangsu Provincial Natural Science Foundation of China (BK20150604), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

(1) Borah, L.; Goswami, M.; Phukan, P. Adsorption of methylene blue and eosin yellow using porous carbon prepared from tea waste:

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