Room-Temperature Synthesis of Magnetic MetalâOrganic Frameworks

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Room temperature synthesis of magnetic metal-organic frameworks composites in water for efficient removal of methylene blue and As(V) Lijin Huang, Jiayu Cai, Man He, Beibei Chen, and Bin Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05294 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Industrial & Engineering Chemistry Research

Room temperature synthesis of magnetic metal-organic frameworks composites in water for efficient removal of methylene blue and As(V) Lijin Huang, Jiayu Cai, Man He, Beibei Chen, Bin Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, P. R. China

*Corresponding Author Bin Hu Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, P. R. China Tel: 86-27-68752701; Fax: 86-27-68754067; E-mail address: [email protected].

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ABSTRACT: Magnetic metal-organic framework composites (MFCs) were prepared in water under room temperature through a simple, low-cost and green strategy only by employing organic salts as anionic linker sources. Amino-functionalized MFCs (MFC-N) with different amino group content were successfully prepared by varying the ratio of disodium of 2-aminoterephthalic (NH2-Na2BDC)/terephthalic acid (Na2BDC). The prepared nanoscale MFC-N-X (X represents the percentage of NH2-Na2BDC, i.e. X=0, 50, 100) showed high surface area, large pore volume, excellent magnetic response, as well as good water and thermal stability. Among these prepared MFC-N-X, MFC-N-100 exhibited the highest adsorption capacity of 358 mg g-1 for methylene blue (MB) and 71 mg g-1 for As(V) due to its porous structure and abundant amino groups. The prepared MFC-N-100 was successfully applied to remove MB and As(V) from different water samples (tap water, lake water and domestic sewage) with a high removal efficiency and a rapid separation of the sorbent from solution, demonstrating the great application potential of the prepared MFC-N-100 as a fascinating adsorbent for removal of dyes or metal ions from different water samples. KEYWORDS: Metal-organic frameworks; magnetic composites; green strategy; dye removal; As(V) adsorption INTRODUCTION Metal-organic frameworks (MOFs), with permanent porosity and flexible chemical structure, have recently attracted lots of interests for its potential application in catalysis, separation, drug delivery and sensing, etc.1-5 What is particularly intriguing to researchers is the potential of MOFs in adsorbing and pre-concentrating guest molecule species, especially segregating dyes or metal ions from wastewater.6,7 Arsenic, known for its high toxicity, can lead to severe health problems, including lung, liver, kidney, and skin cancers.8 To date, the potential of some MOFs (i.e., ZIF-89, MIL-5310, UiO-6611) as adsorbents for the removal of arsenic has been demonstrated 9-11. However, complicated centrifugation or filtration is required in the synthesis of MOFs and the removal process of MOFs as adsorbents, hindering their 2


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widespread applications. Combination of MOFs with magnetic nanoparticles (MNPs) to construct magnetic metal-organic framework composites (MFCs) is a good approach to solve the above problem. Recently, due to the merits of adjustable magnetic properties, easy-to-prepare, design flexibility and good adsorption performance, MFCs have received tremendous attention recently.12-15 Fe3O4 are considered as one of the most potential magnetic materials due to its easy preparation, low toxicity and inexpensiveness.16 And it has been widely used for MFCs synthesis.17 Currently, several strategies have been applied in the preparation of MFCs, such as embedding, encapsulating, mixing and liquid phase epitaxy (LPE).17 However, some difficulties still exist in synthesis of MFCs with regular structures under environmentally and cost-efficient conditions. Most of the methods are based on solvothermal treatments or LPE. For solvothermal method, high temperature with long reaction time and hazardous organic solvents such as N,N-dimethylformamide (DMF) are usually required.18,19 The process of LPE is time consuming, and the reaction should be conducted in organic ligand and metal precursor solution, separately, which consumes ~45 min for each repetitions.20,21 Moreover, it should be pointed out that most of the MFCs prepared in the lab so far are based on HKUST-1. Nevertheless, HKUST-1 is known to be less robust in water than those aluminium, titanium, or zirconium based frameworks, and copper involved in HKUST-1 is acutely toxic to aquatic biota.22 Therefore, preparation of MFCs in large scale in water under room temperature would be extraordinarily attractive for its low-cost and environmentally friendly applications.

Fig 1. Structure of MIL-53(Al) for its large form (left) and narrow form (right), which 3



can be transformed reversibly with the assistant of guest. MIL-53(Al), containing terephthalic acid (H2BDC) ligands and aluminium (Fig. 1), has been extensively studied due to their easy preparation, structural flexibility, excellent water, chemical and thermal stability, as well as high breathing effect.23,24 Analogous structures of MIL-53 can be synthesized just by using H2BDC derivatives. However, to date, hydrothermal method is the most commonly used for MIL-53(Al) preparation by employing Al(NO3)3·9H2O and H2BDC as precursor at 220 oC for 3 days.23 The large amounts of toxic and corrosive nitric acid generated during the hydrothermal synthesis would be harmful to the environment and operator25, which limit their application greatly. Recently, Manuel et al. pioneered a cost-effective and green strategy for preparation of carboxylate-based MOFs with high-quality via employing organic salts as anionic building blocks.26,27 By using this method, a number of MOFs including MIL-53-Al-X (X=H, NH2, NO2), MOF-74, MOF-5 and MIL-100(Fe) have been obtained in water under room temperature successfully. Therefore, it is feasible to use organic salts instead of organic acid as the anionic building blocks for the preparation of MFCs with water as the solvent, and to grow MOFs on the surface of MNPs with controllable structure and morphology without any surface modification on MNPs beforehand. Herein, we developed a facile and green synthetic method (Scheme 1) and prepared devisable magnetic Al-based MOFs at room temperature by using different ratio of Na2BDC/NH2-Na2BDC as a linker source in water under room temperature (denoted as MFC-N-X, where X represents the percentage of NH2-Na2BDC). This proposed synthetic method for MFCs exhibits some advantages over conventional methods, in which vast organic solvents, high temperature or long reaction time are required. What is more, toxic and harmful corrosive acid (HNO3, HCl, etc., as products of the employed organic linkers) generating in the solvothermal methods can be avoided in the proposed synthesis method, and the corresponding innocuous salts (NaNO3, NaCl, etc.) would form instead. A series of MFC-N-X were prepared and the effect of amino groups in prepared MFC-N-X on the capture of interesting pollutants was investigated. Taking MB and As(V) as the model organic and inorganic pollutants, 4


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respectively, the prepared MFC-N-X were investigated as the adsorbents, and MFC-N-100 exhibited the best adsorption performance for both of MB and As(V) due to its largest amount of amino groups.

Scheme 1. Schematic diagram of the preparation of MFC-N-X composites. EXPERIMENTAL Chemicals H2BDC, Na2HAsO4·7H2O and 2-aminoterephthalic acid (NH2-H2BDC) were supplied by Aladdin (Shanghai, China). Other solid reagents and solvents were commercially available and used without additional treatments. Apparatus The concentration of metal ions was measured by intrepid XSP Radial inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo, Waltham, MA, USA) or 7500a inductively coupled plasma mass spectrometry (ICP-MS) (Agilent Technologies, Japan). Mettler Toledo 320-S pH meter (Mettler Toledo Instruments Co. Ltd., Shanghai, China) was used to control the solution pH. Transmission electron micrograph (TEM, JEM-2010, Tokyo, Japan) and PPMS-9 vibrating sample magnetometer (VSM) (QUANTOM, USA) were employed to character the 5



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microstructure of MFCs and study the magnetic properties of the materials, respectively. Thermo gravimetric analysis (TGA) was carried out to investigate the thermal stability of MFCs by using diamond TG-dta-6300 with a heating rate of 5 oC min-1 under nitrogen. The process for materials dispersion was conducted on KQ 5200DE model ultrasonicator (Shumei Instrument Factory, Kunshan, China). Magnetic separation was realized via Nd-Fe-B magnet (15.0×6.0×1.6 cm). Fourier transform infrared spectrometer (FT-IR) and X-ray diffraction (XRD) were performed on NEXUS 870 (Thermo, Madison, USA) and D/max-IIIC X-ray diffractometer (Shimadzu, 20 Japan), respectively. Scanning electron microscope (SEM) images and compositional analysis of the samples were conducted on X-650 SEM (Hitachi, Tokyo, Japan) equipped with an energy dispersive X-ray (EDX, SUTW-SAPPHIRE) system. The specific surface areas of MFCs were measured on micromeritics ASAP 2020 apparatus (USA). Elemental analysis for organic species was conducted on CARLOERBA-1106

micro-elemental

analyzer.

Spectrophotometer

(Shimadzu

UV-Vis spectrophotometer, UV-1800, Japan) was adopted to determine the concentration of MB at 665 nm. Preparation of MFCs The Fe3O4@SiO2 MNPs (~15 nm) were prepared according to our previous work.11 MFC-N-X (X=0, 50, 100) with different NH2 content were prepared and purified using a modified synthesis protocol based on that reported by Manuel et al..26,27 Briefly, 1.0 g of Fe3O4@SiO2 MNPs were dispersed in 10 mL AlCl3·6H2O (2.5 g, 10.35 mmol) aqueous solution with the help of ultrasonication (Solution 1). Another solution (Solution 2) was prepared by dissolving organic linker H2BDC and NH2-BDC with different molar ratios (0, 50 and 100 % NH2-H2BDC) (10 mmol) into 15 mL of 1.33 mol L-1 NaOH aqueous solution in order to insure the whole and fast deprotonation of the carboxylic groups of the organic linker. Solution 1 was drop-wise added into Solution 2 under stirring and the reaction was maintained at 25 o

C for 24 h. The products were separated and washed with ultrapure water, then dried

at 25 oC under vacuum overnight. The products were heated in DMF at 150 ºC to remove any possible uncoordinated ligands filling the pores of the MOF structure28. 6


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The MFC-N-100 with different amount of MNPs (2.0 g for MFC-N-100-1 and 0.5 g for MFC-N-100-2) were prepared for a comparison. The MIL-53-NH2 was also prepared with the same procedure without the addition of MNPs. Adsorption studies 10 mg of MFCs was mixed with 10 mL of As(V) solution (pH 6.0) and the mixture was shaken vigorously for 12 h under room temperature. After the separation of MFCs, the concentration of As(V) in remaining solution was determined by ICP-OES or ICP-MS. 10 mL of solutions containing MB dyes with different concentration was spiked with 10 mg of MFCs and subjected to the same procedure mentioned above. The concentration of MB was quantified through UV-Vis spectrophotometry. The effect of contact time on the adsorption was carried out to investigate the adsorption kinetics via mixing 50 mg of MFCs with 50 mL of As(V) (20 mg L-1) or MB (50 mg L-1) solution. The remaining concentration of As(V) and MB at predetermined time intervals was measured by ICP-OES/ICP-MS and UV-Vis spectrophotometry, respectively. All adsorption experiments were performed in triplicate.

RESULTS AND DISCUSSION XRD and FT-IR measurements The XRD patterns of MNPs, MFC-N-0, MFC-N-50, MFC-N-100, MIL-53-NH2 and simulated XRD pattern for MIL-53-NH2 are presented in Fig. 2 (a). As can be seen, the corresponding patterns of MFC-N-X materials could be ascribed to crystalline

Fe3O4

(JCPDS

file

19-0629),

phase-pure

of

MIL-53(Al)

and

MIL-53-NH2(Al).29 The XPS spectra of Fe 2p at 711.6 and 724.8 eV confirmed that Fe3O4 is present in the MFC-N-X composites (Fig. S1). The prepared MIL-53-NH2 is in the narrow pore form, and peaks of impure phases were not found, indicating the successful preparation of NH2 functionalized MFCs.

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Fig. 2 XRD patterns (a) and FT-IR spectra (b) of the prepared MFCs. The surface chemistry of MNPs, MFC-N-0, MFC-N-50 and MFC-N-100 obtained by FT-IR is presented in Fig. 2 (b). Compared with the FT-IR spectrum of MNPs, the absorption peak intensity of O-Si-O stretching vibration (at 1090 cm-1) in all MFCs were observed, demonstrating the successful incorporation of MNPs into MOFs. For MFC-N-0, the peak at 3050 cm-1 is ascribed to asymmetric stretching vibration of C-H in H2BDC, and the absorption peaks at 1709 cm-1 and 1645 cm-1 are assigned to the C=O bond of organic linkers. The strong peaks at 1441 cm-1 and 1415 cm-1 are ascribed to the stretching vibration absorption of C=C bonds on the benzene ring. Meanwhile, the broad absorption peaks around 3680 cm-1 is caused by the hydroxyl group of the adsorbed water and MIL-53, i.e. Al-OH-Al.30 Moreover, two sharp absorption peaks at 3393 cm-1 and 3505 cm-1 ascribing to symmetric and asymmetric stretching absorption of NH2 groups are observed for MFC-N-50 and MFC-N-100, respectively31, which is different from the spectra of MFC-N-0. What is more, some minor variations are observed in the spectra of MFC-N-50 and MFC-N-100, and the C-N stretching absorption is found at 1257 cm-1 and 1340 cm-1, while N-H wagging is observed at 764 cm-1. These FT-IR results confirm the presence of NH2 group in the prepared MFC-N-50 and MFC-N-100. It can also be verified by the results of EA (Table 1) and the N2 adsorption-desorption isotherms specified as follows. Table 1 Content (Wt %) of relevant elements in MFC-N-0, MFC-N-50 and MFC-N-100 obtained by EA and ICP-OES Sample

Fe

Al

O

8


C

N

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MFC-N-0

12.9

9.67

31

45

-

MFC-N-50

11.9

8.36

34

45

3.0

MFC-N-100

10.8

7.42

30

40

6.4

Note: the values for Fe and Al were obtained by ICP-OES quantification after acid (HF+HNO3) digestion; the others were obtained by EA detection. N2 adsorption-desorption isotherms The porosity of the MFCs was investigated via N2 adsorption-desorption isotherms (Fig. S2 and Table 2). The surface areas obtained through Brunauer-Emmett-Teller (BET) model calculation were calculated to be 546, 209, 192 and 518 m² g-1 for MFC-N-0, MFC-N-50, MFC-N-100 and MIL-53-NH2(Al), respectively. Compared with that of MNPs,15 all the MFCs showed higher surface area and pore volume (Vt). Compared with MFC-N-0, the surface areas of the MFC-N-50 and MFC-N-100 exhibited a significant decrease after NH2 modification, and the BET surface areas of MFC-N-X are decreased with the increasing amount of NH2, due to the partial blocking of the pores by NH2 groups, which is also noted in previous literature.29 Table 2 Surface areas and pore volumes of MNPs, MFC-N-0, MFC-N-50, MFC-N-100 and MIL-53-NH2

Material

BET surface

Pore volume

Average pore

area (m² g-1)

(mL g-1)

sizes (nm)

MNPs

71

0.27

14.8

MFC-N-0

546

0.48

1.29

MFC-N-50

209

0.38

7.3

MFC-N-100

192

0.19

9.5

MIL-53(Al)-NH2

518

0.71

5.51

Thermal stability

9



Fig. 3 Thermogravimetric (TG) curve of prepared magnetic adsorbents. TGA was carried out to investigate the thermal stability of MFC-N-100 (Fig. 3). For MNPs precursor, little weight loss was observed even when the temperature was increased up to 800 °C. The MFC-N-100 exhibited slight weight loss under 85 °C, which could be attributed to the loss of physically adsorbed water. The second weight loss between 80 oC and 280 °C could be attributed to the loss of water or organic solvents in the MOFs. The main weight loss began at ~450 °C, which is attributed to the decomposition of organic links and conversion to Al2O3.27 The thermal stability of MFC-N-50 and MFC-N-100 exhibited slight decrease than that of MFC-N-0 due to the introduction of -NH2 groups. This behaviour is similar to that reported for MIL-53(Al)-NH2.28, 32 TEM characterization

Fig. 4 TEM images of MNPs (a), MFC-N-100 (b) and the elemental mapping images of MFC-N-100 (c). 10


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TEM analysis was conducted to study the morphologies of the prepared samples (Fig. 4). As can be seen, the MNPs precursor exhibited a typical core-shell structure with SiO2 as shell and Fe3O4 as core (Fig. 4(a)). Remarkably, the TEM image of MFC-N-100 (Fig. 4(b)) revealed a number of MNPs embedded by MIL-53(Al)-NH2 and clear identification of these two components. Elemental mapping images (Fig. 4(c)) confirm the homogeneous distribution of O, C, Al and N in MFC-N-100, while the strong signal of Fe and Si indicated that the MNPs was embedded into the frameworks of MIL-53-NH2 successfully. Magnetic property

Fig. 5 VSM magnetization curves of MNPs and prepared MFCs; the insets show the digital images before and after magnetic separation obtained for MFC-N-100 under an external magnetic field. To investigate the magnetic properties of MFCs, VSM was conducted. Fig. 5 shows the saturation magnetizations (Ms) for precursor, MFC-N-100-1, MFC-N-100 and MFC-N-100-2 are 39 emu g-1, 25 emu g-1, 14 emu g-1 and 10 emu g-1, respectively. Compared with MNPs, the Ms value of MFC-N-100 is lower due to the modification of MOFs. And the Ms value of MFCs increase with the increase of the amount of MNPs during the synthesis. In addition, the magnetic hysteresis curves show that both of remanence and coercivity are negligible for all the composites, demonstrating that they had good superparamagnetism. As shown in the inset of Fig. 4, MFC-N-100 can be separated from solution easily via a commercial magnet in only 10 s. Influence of pH 11



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The influence of solution pH on the adsorption of As(V) and MB onto MFCs was investigated and the experimental results are shown in Fig. 6. As can be seen, the adsorbed amount of As(V) by using MFC-N-100 is increased by increasing pH from 2 to 4, and remains constant in the pH range of 6-12. The adsorbed amount of MB on MFC-N-100 is increasing with the increase of pH from 2 to 5 and levelled off with further increase of pH to 12. Due to its good stability in a wide pH range, MFC-N-100 can be used as good adsorbents when the adsorption/separation have to be performed at higher pH values (6-10).

Fig. 6 Effect of pH on the adsorbed amount of As(V) (100 mg L-1) and MB (500 mg L-1) by MFC-N-100. Adsorption mechanism O -(H +) (+H) -O

N(CH3)2

O

N

Cl (H 3C)2N

O

O-

O

H 3N

O

+

O (H ) O

NH2 O

O

O

O

O

Al NH H

π−π stacking interaction (H 3C)2N Cl-

O

As-O-Al bonds -

O

O

+

As

-

-O

O

(+H) -O

As

Al

Electrostatic attraction O S

O-(H +)

O

As

S

O Al HO H Hydrogen bonding interaction

N(CH3)2

N

Fig. 7 The possible mechanism for the adsorption of As(V) and MB on MFC-N-100. 12


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Based on pKa value of H3AsO4 (2.2, 7.0, 11.5 for pKa1, pKa2, pKa3, respectively), As(V) mainly exists in anionic form under pH>2.2, e.g., H2AsO4− (2.2

Room-Temperature Synthesis of Magnetic MetalâOrganic Frameworks

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