Green synthesis of [email protected] nanocomposites for catalytic reduction of methylene blue

Accepted Manuscript Green synthesis of MOF@Ag nanocomposites for catalytic reduction of methylene blue

Aseman Lajevardi, Mohammad Tavakkoli Yaraki, Ali Masjedi, Afsaneh Nouri, Moayad Hossaini Sadr PII: DOI: Reference:

S0167-7322(18)35532-6 https://doi.org/10.1016/j.molliq.2018.12.002 MOLLIQ 10078

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

25 October 2018 19 November 2018 1 December 2018

Please cite this article as: Aseman Lajevardi, Mohammad Tavakkoli Yaraki, Ali Masjedi, Afsaneh Nouri, Moayad Hossaini Sadr , Green synthesis of MOF@Ag nanocomposites for catalytic reduction of methylene blue. Molliq (2018), https://doi.org/10.1016/ j.molliq.2018.12.002

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ACCEPTED MANUSCRIPT Green synthesis of MOF@Ag nanocomposites for catalytic reduction of methylene blue Aseman Lajevardia,, Mohammad Tavakkoli Yarakib, c,,*, Ali Masjedia, Afsaneh

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Nourid, Moayad Hossaini Sadre,*

Department of Chemistry, Islamic Azad University, Science and Research Branch, Tehran, Iran.

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Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive,

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Institute of Materials Research and Engineering, The Agency for Science, Technology and Research (A*STAR),

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c

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Singapore 117585, Singapore

2 Fusionopolis Way, #08-03 , Innovis 138634 , Singapore

Department of Chemical Engineering, University of Mohaghegh Ardabili, Ardabil, Iran

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Department of Chemistry, Faculty of Science, Azarbaijan Shahid Madani University, Tabriz, Iran

A.L and M.T.Y contributed equally to this work.

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*Corresponding Authors: M. Tavakkoli Yaraki, E-mail: [email protected]; [email protected] Prof. M. Hossini Sadr, E-mail: [email protected], [email protected]

Abstract 1

ACCEPTED MANUSCRIPT In this study, a novel catalyst (ZBD@Ag) based on silver nanoparticles (AgNPs) grown on the surface of Zn2(bdc)2(dabco) metal-organic framework (ZBD), was synthesized using a green in-situ approach. The ZBD@Ag nanocomposite was characterized by various techniques, and its catalytic performance was investigated in the reduction of methylene blue (MB) in the presence of NaBH4. It was found that the catalyst possesses outstanding

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performance even at deficient concentrations and that acidic pH is more suitable for the

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catalytic reduction. The catalytic reduction of MB completed in 6 min, and the reaction

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followed the first-order kinetics (K=0.37 min-1 ≈ 30.5 s-1g-1). The developed catalyst could well suppress the other similar catalysts for the same reaction, showing its potential to be used

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for the removal of hazardous materials like organic dyes.

Keywords: Metal-Organic Framework; Dye Degradation; Silver Nanoparticles; Catalytic

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Reduction; Wastewater treatment

1. Introduction 2

ACCEPTED MANUSCRIPT Dyes, a class of synthetic organic compounds, are widely used in the textile industry. They are produced in a vast quantity around the world each year. Dye pollutants are harmful to our ecosystems due to the formation of toxic complexes with some heavy metal ions in effluents. Hence, it is essential to discover a practical method for eliminating the color effluents from

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textile wastewaters [1-5]. Various physical and chemical treatment techniques including biological degradation,

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adsorption, chemical precipitation, reverse osmosis, and coagulation have been developed to

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eliminate the dyes from wastewater [6, 7]. These treatment methods have some disadvantages. Indeed, some of them cannot entirely decompose the dyes but only transfer the pollutants from

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one phase to another [8-10]. Hence, developing a method like chemical reduction to degrade a

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wide range of chemical pollutants in a straightforward step is of interest. However, this method usually suffers from a low rate of degradation at low concentrations of harsh reducing agents like sodium borohydride. Therefore, in order to decrease the amount of reducing agent as well

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as increasing the reaction rate, developing an efficient catalyst for such a chemical reduction process is of importance. Nanoremediation, based on nanostructures for the removal of chemicals either through the catalytic process or chemical reduction[11, 12], is highly effective

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and has been explored to treat polluted environment [13].

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Gold, silver and palladium nanoparticles are attractive as catalysts in the degradation of both anionic and cationic dyes [14-17]. Silver nanoparticles (Ag NPs), the class of nanomaterials that show remarkable optical properties and high catalytic activities, are mainly used in reduction reactions [18-20] by reducing the activation energy for the degradation reactions to be thermodynamically and kinetically favorable [21-23]. However, the catalytic properties of Ag NPs depend on the shape and size [24]. In addition, Ag NPs are limited in practical application due to their cost and agglomeration [25, 26]. Hence, immobilization of the metallic

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ACCEPTED MANUSCRIPT nanoparticles onto the appropriate support is of interest for decreased consumption of Ag NPs and their agglomeration [27]. Several materials such as graphene[28], metal oxides[29], and organic-inorganic[30] nanocomposite materials have been employed as supports in different catalytic reactions.

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Metal-organic frameworks (MOFs) are porous nanostructures based on the interaction between multifunctional organic ligands and metallic ions. Recently, they have attracted much attention

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to be used for catalysis due to their porosity and stability at harsh conditions [31-34]. The

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porous structure of MOFs can efficiently reduce the agglomeration of metal nanoparticles, and

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therefore is capable support for metal nanoparticles [35-37].

This study reports in-situ synthesis of Ag NPs on the surface of a Zn2(bdc)(dabco)2 MOF,

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(ZBD@Ag) and its superior catalytic activity in chemical reduction of a model dye, methylene blue (MB). The effective parameters were well studied and the performance of the new catalyst

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2. Materials and Methods

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was evaluated by comparing the normalized kinetic reaction rate with the reported literature.

2.1. Materials

acid

(C76H52O46), sodium

borohydride

(NaBH4),

zinc

acetate

dehydrate

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tannic

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Silver nitrate (AgNO3, 99.99%), potassium carbonate (K2CO3), methylene blue (C16H18ClN3S),

(Zn(OAC)2.2H2O), 1,4-benzene dicarboxylic acid (BDC), 1,4-diazabicyclo[2.2.2]octane (DABCO), dimethyl formamide (DMF) and acetic acid glacial (CH3COOH) were purchased from Sigma-Aldrich, Germany. All the reagents were analytical grade and used without further purification. 2.2. Instrumentations

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ACCEPTED MANUSCRIPT The transmission electron micrograph (TEM) image of the prepared materials was captured on a Philips CM30 electron microscope (Netherland). Field emission Scanning Electron Microscopy (FESEM) (TESCAN mira3, Czech Republic) was used for EDX and elemental mapping analysis. A Bruker Fourier transform infrared spectrometer (FT-IR) (Thane, Maharashtra, India) was used for the surface functionality characterization of samples. The

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crystalline structure was identified by Rigaku Ultima IV (Japan) using Cu Kα radiation (40 kV,

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35mA) with a scan rate of 4° min-1 and step size of 0.02°. A micromeritics ASAP 2020

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analyzer was used for N2 adsorption-desorption isotherms of the samples and specific surface area and pore size distribution were evaluated using Brunauer-Emmett-Teller (BET) method

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and Barrett–Joyner–Halenda (BJH) method, respectively. The thermal behavior of the samples was determined by thermogravimetric analysis (TGA) on a Mettler-Toledo TGA 851e

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apparatus with a heating rate of 10 °C min-1 under N2 atmosphere. 2.3. Synthesis of MOF

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ZBD was prepared using direct mixing of zinc salt solutions and organic ligands under solvothermal conditions in the presence of acetic acid and DMF as the solvent and modulator, respectively. Briefly, Zn(OAC)2.2H2O (0.132 g) was added to 10 mL of acid acetic solution in

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DMF (1 M) (solution A). Then, DABCO (0.035 g) and BDC (0.1 g) were dissolved in 25 mL

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of DMF (solution B). The mixture of solutions A and B was sealed and stirred under reflux at 100 ◦C for 24 h. A white product was washed several times and dried in a vacuum oven at 150 ◦

C for 15 h [38].

2.4. In situ synthesis of ZBD@Ag 2.4.1. Modification with Tannic acid

ZBD (60 mg) was dispersed in 38 mL of H2O and then 2 mL of tannic acid (40 mg/mL) was added into the mixture and stirred for 1 h. The product was centrifuged and washed with 5

ACCEPTED MANUSCRIPT deionized (DI) water for three times to remove excess tannic acid (TA). The supernatant was collected to estimate the amount of adsorbed TA by ZBD. 2.4.2. Growth of Ag NPs on ZBD surface

The TA-functionalized ZBD was suspended in 38 mL DI water. K2CO3 (0.05 M) was used for

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adjusting the pH of the suspension to 8.5. Finally, 4 mL of AgNO3 (0.2 M) was added slowly under vigorous stirring for 2 h. The dark product, ZBD@Ag, was isolated by a centrifuge and

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washed with DI water several times. The product was dispersed in water for catalytic

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application. The final concentration of the product could be considered as 12 mg/mL.

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2.5. Catalytic study

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The catalytic activity of ZBD@Ag was studied in reduction of a thiazine dye, methylene blue (MB), to leuco form (LMB) by a reducing agent, sodium borohydride. The catalytic study was performed in aqueous media and in a standard quartz cell. The solution in a cuvette was made

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up to 3 mL with deionized water. The experimental procedure was as follows: First, 1.26 mL of NaBH4 in different concentrations (1.66×10-2, 2.14×10-2 and 3.57×10-2 M)

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was mixed with 1 mL of MB (5×10-5 M). Then, 0.5 mL of ZBD@Ag solution at a certain concentration was added. The degradation of the MB was monitored using a UV–visible

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absorption spectrophotometer (Perkin Elmer, Lambda 25, USA). The decrease in the absorbance of MB was recorded periodically in the wavelength range of 200−800 nm.

3. Results and discussions 3.1. Synthesis of ZBD@Ag UV-visible absorbance spectra of TA solution and the supernatant after functionalization of the ZBD with TA were measured to confirm the adsorption of TA on the surface of ZBD. The

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ACCEPTED MANUSCRIPT reduction of 26% in the absorbance peak of tannic acid at 275 nm indicates that the adsorption of tannic acid on ZBD structure occurred. (Fig. 1a).

2.5 Supernatant after mixing with ZBD Initial TA solution ZBD

Absorbance

2.0

1.5

1.0

0.5

300

400

500

600

700

b

ZBD ZBD@AgNPs

3.0

2.5

2.0

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Absorbance

800

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Wavelenght(nm)

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Wavelength(nm)

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Fig.1 a) UV-Vis spectra of a) TA solution; b) ZBD@Ag dispersed in water

To confirm the formation of AgNPs on the surface of ZBD, UV-vis analysis was carried out

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for the synthesized ZBD@Ag. As can be seen in Fig. 1b, the AgNPs/MOF spectrum indicates a broad absorption in a range of 351-800 nm due to the conduction electrons on the metal

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surface. The broad SPR absorption peak centered at around 420 nm could be attributed to the formation of Ag nanoparticles on the surface of ZBD. ZBD@Ag spectrum exhibits two additional peaks before 420 nm, which are related to tannic acid molecules on the ZBD. In addition, the absorbance peak of ZBD could not be observed on ZBD@Ag due to high extinction coefficient of Ag NPs that suppresses the absorbance peak of ZBD. In the synthesis of ZBD@Ag, the successful adsorption of tannic acid on the surface of ZBD and pH of the TA-functionalized ZBD solution before addition of AgNO3 are key parameters. 7

ACCEPTED MANUSCRIPT Tannic acid is a polyphenolic compound, which could be oxidized to quinones at the alkaline condition and consequently, leads to the reduction of Ag+ ions in the solution and formation of AgNPs. As tannic acid-functionalized ZBD has been washed after adsorption of tannic acid, free tannic acid molecules do not exist in the solution. Hence, AgNPs only form on the surface of TA-functionalized MOF, where tannic acid molecule exists. Tannic acid also could be

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bonded to the surface of grown AgNPs and plays as a capping agent[39].

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3.2. Characterization of ZBD@Ag

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3.2.1 Fourier-transform infrared spectroscopy

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FT-IR characterization (Fig.2a) shows the high-intensity bands at 1390 and 1620 cm-1 assigning to the carboxylic groups and C=O stretch. These bands are the characteristic bands

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of ZBD. The bands at 1020 and 1200 cm-1 in tannic acid spectra (* symbol) are corresponding to the stretching vibrations of C-O-C and C-O bending, which were observed in the spectra of

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TA-modified ZBD, and reveal the successful TA bonded with Zn2 units on the surface of ZBD.

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The minor shift of 1450 to 1440 cm-1, which is assigned to the in-plane bending mode of hydroxyl groups in TA, shows the complexation of Zn2 units with OH groups in the TA.

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However, the band at 1440 cm-1 in ZBD@Ag disappeared due to the oxidation of hydroxyl groups into quinone, and the reduction of Ag ions to AgNPs [40, 41]. Table. 1 shows the main

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bands of the FT-IR spectra.

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ACCEPTED MANUSCRIPT Table 1. The leading bands of the FT-IR spectrum Description

1390

Carboxylic groups stretch of ZBD

1620

C=O stretch of ZBD

1450

In-plane bending mode of the hydroxyl group in C-O-H of TA

1440

The shifted band of C-O-H bending mode of the hydroxyl group of TA

1200

Stretching vibrations of C-O bending in TA spectrum

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Stretching vibrations of C-O-C bending in TA spectrum

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Band (cm-1)

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3.2.2 X-ray Diffractions

Fig.2b shows X-ray diffraction patterns of ZBD and ZBD@Ag. The diffraction peaks of ZBD

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are in a good agreement with a previously reported pattern of ZBD [38]. However, the main diffraction peaks of ZBD were not detected in ZBD@Ag due to the existence of Ag NPs on

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the surface of ZBD. Indeed, the main peak of ZBD was concealed by Ag NPs peaks suggesting

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that ZBD has lost its crystalline structure after the modification with tannic acid and growth of Ag NPs on its surface. The main characteristic peaks at about 38◦ and 44.2◦ in the XRD pattern

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of ZBD@Ag corresponded to the (111) and (200) lattice planes of AgNPs, respectively [42].

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ACCEPTED MANUSCRIPT a

b ZBD@Ag NPs ZBD ZBD@Ag NPs

111

* 200

*

* *

TA

*

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*

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1440

Intensity

1450

C=O

ZBD 3500

C=O

3000

2500

2000

1500

1000

5

500

10

15

20

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4000

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Transmittance

TA-ZBD

Wavenumber (cm-1)

25

30

35

40

45

50

55

60

2 (degree)

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Fig. 2. a) FTIR spectra and b) Powder X-ray diffraction patterns for different samples.

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3.2.3 FESEM/TEM measurements

FESEM images of ZBD and ZBD@Ag are shown in Fig. 3a and Fig. 3b, respectively. Fig.

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3a indicates a rod-like crystal of ZBD with a length of more than 10 μm. After AgNPs formation on the surface of ZBD, the morphology of ZBD@Ag has changed. The result

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indicates a change in the morphology of the ZBD [43, 44], which is in consistent with the XRD results. The change in the morphology of ZBD after growth of AgNPs could be due to either slight degradation of ZBD under acidic conditions during the first step, which was treatment with tannic acid[45] or galvanic replacement of some zinc ions in the ZBD structure by Ag ions during growth of AgNPs on ZBD[46]. However, The TGA results (Fig. S1) shows very similar degradation rate for both ZBD and TA-functionalized ZBD[45], which indicates that overall composition of ZBD has remained the same after functionalization with tannic acid.

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ACCEPTED MANUSCRIPT However, the overall thermal stability of the ZBD was improved slightly after growth of

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AgNPs.

Fig.3. a) The SEM images of ZBD; b) The SEM image of ZBD@Ag; c) Ag; g) Zn; e) C; f) O; g) N; h) combination of all elements.

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ACCEPTED MANUSCRIPT The elemental mapping (Fig. 3c) also confirms the formation of Ag NPs on ZBD surface where this nanostructure contains Ag element. Also, the developed nanostructure contains 19% Ag, 16% Zn, 23% C, 33% O and 8% N atoms (Fig. 3d-g). TEM image of ZBD@Ag (Fig. 4) reveals that the particle size of AgNPs is less than 20 nm

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and they have a uniform size distribution in the range of 10 nm to 20 nm. The formation of small AgNPs is of interest as the catalytic performance of AgNPs is size-dependent, which

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enhances dramatically with the decrease in the size of Ag NPs. The TEM image is another

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evidence for embedding of AgNPs on the surface of ZBD and ZBD could support the AgNPs

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during the catalytic process.

Fig.4. TEM images of ZBD@Ag

3.2.4 Porosity and Surface area Nitrogen adsorption and pore size distribution of ZBD and ZBD@Ag were evaluated, and the results are presented in Fig. S2. As can be seen, the amount of nitrogen uptake ZBD@Ag dropped about 60%, which could be attributed to lower BET surface area of the ZBD@Ag

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ACCEPTED MANUSCRIPT compared with free ZBD. A similar trend has been observed for loading of Pd on Zn2(azoBDC)2(dabco) [35]. The BET surface area was 353 𝑚2 𝑔−1 and 211.8 𝑚2 𝑔−1 for ZBD and ZBD@Ag samples, respectively. Furthermore, the pore size distributions for these two samples indicate that some pores might be blocked by Ag NPs; however, the pore size

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distribution has not changed a lot after the growth of Ag NPs. This result suggests that the final

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nanocomposite is a porous material and could be a good candidate for catalytic reactions.

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3.3. Catalytic study 3.3.1. Effect of catalyst

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The reduction of MB molecules by NaBH4 was studied in the absence and presence of ZBD@Ag. Fig. 5a shows the variation of the spectra of dye molecules by the addition of

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NaBH4 (3.57*10-2 M). As can be seen, the spectra of MB undergoes a slow change in the intensity of the profile, and MB can be degraded by 18% in 45 min. Upon addition of ZBD@Ag

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(0.2 mg/mL), the time of methylene blue degradation is obviously decreased and 99% of the dye molecules degraded in only 6 min (Fig. 5b). As it can be seen in Fig. 5b, the MB band at 664 nm gradually vanishes, and a new band related to LMB develops at 250 nm [47]. This

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phenomenon can be explained by the oxidation-reduction reaction and electron transfer on the

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surface of ZBD@Ag. Briefly, 𝐵𝐻4− ions donate electrons to the ZBD@Ag, whereas the methylene blue molecules capture the electrons from the surface of ZBD@Ag. The mechanism is presented in Scheme 1. To find out the role of AgNPs in the catalytic activity of the ZBD@Ag, the catalytic activity of bare ZBD (0.2 mg/mL) was investigated in the presence of NaBH4 (3.57*10-2 M). The bare ZBD has no catalytic activity (Fig. 5c). To further confirm this observation, the activity of ZBD at a higher concentration (12 mg/mL) was also studied, but no catalytic activity was observed (Fig. S3). The difference between the poor catalytic activity of ZBD and promising catalytic 13

ACCEPTED MANUSCRIPT activity of ZBD@Ag could be attributed to the presence of AgNPs as well as synergic effect between the ZBD and AgNPs in the facilitating the electron transfer process. Fig. 5d shows the pseudo-first-order kinetic model of MB reduction in the absence and presence of ZBD@Ag (0.2 mg/mL). The results illustrated that the rate constant of the MB

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reduction in the presence of ZBD@Ag (k=0.37min-1) is about 74 times higher than that in the absence of the catalyst(k=0.005 min-1).

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To evaluate the effect of ZBD@Ag concentration on the chemical dye degradation, the

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experiments were performed with different amounts of the catalyst (0.1, 0.2, 0.3, 0.4, 0.5, 0.6,

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0.8, 1, 1.5, 2, 4, 6, 8, 10 and 12 mg/mL) at the constant concentration of NaBH4 (3.57*10-2 M). At the concentrations higher than 0.5 mg/mL, the MB degradation occurred immediately, and

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it is not possible to monitor the change by UV-visible spectrometry. Some videos were recorded for mentioned concentrations and are presented as supplementary information. The

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results of MB degradation at ZBD@Ag concentrations of 0.1, 0.3, 0.4 and 0.5 mg/mL are

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shown in Fig. S4(a-d).

Furthermore, as the catalytic activity of metallic NPs is size dependent [24, 48], the ZBD@Ag

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was synthesized at lower pH (pH=6.5), to obtain bigger Ag NPs on the surface of ZBD. As can be seen in Fig. S5a, the size of grown Ag NPs are about 50 nm. The catalytic activity of this

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catalyst was compared with ZBD@Ag synthesized at alkaline condition (Fig. S5b). It is obvious that the ZBD@Ag with smaller Ag NPs has a better catalytic activity compared with ZBD@Ag with bigger NPs. This trend is in agreement with findings of Shi et al.[48], where smaller Ag NPs showed a better catalytic activity in the reduction of 4-nitrophenol.

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ACCEPTED MANUSCRIPT − Methylene Blue

Reduction

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Leucomethylene Blue

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ZBD@AgNPs Catalyst

b

1.5

1.5

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Scheme 1. The mechanism of reduction of MB on the surface of ZBD@Ag

0.5

0.0 200

300

400

500

600

700

0 min

Absorbance (a.u)

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46 min

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1.0

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6 min 0.0 200

800

300

d

700

800

In absence of ZBD@AgNPs In presence of AZBD@AgNPs 2.0

45 min

1.5

0.5

0.0 200

600

2.5

0 min

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Absorbance (a.u)

1.0

500

Wavelength (nm)

-Ln(A/A0)

c

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Wavelength (nm)

400

1.0

0.5

0.0 300

400

500

600

700

0

800

10

20

30

40

50

Time (min)

Wavelength (nm)

Fig.5. (a): Variation of full spectrum of MB at different time in the absence of ZBD@Ag, (b): Variation of full spectrum of MB at different time in the presence of ZBD@Ag, (c): Full in presence of bare ZBD (0.2 mg/mL) (d): The first-order kinetic model of MB reduction in the absence and presence of ZBD@Ag.

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ACCEPTED MANUSCRIPT 3.3.2. Effect of NaBH4 concentration

In order to study the effect of NaBH4 concentration, different concentrations of NaBH4 at a constant concentration of ZBD@Ag (0.2 mg/mL) were used for the reduction of dye molecules and the NaBH4 concentration was varied in 0.0357, 0.0214 and 0.0166 M. It was found that as

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the concentration of NaBH4 increases, the time for the dye reduction decreases. The faster reduction rate was observed at the concentration of 0.0357 M NaBH4. This observation

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recommends that a higher concentration of NaBH4 increased local electron density on the

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surface of Ag NPs that could lead to an enhanced reaction rate [49]. The obtained data are fitted

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to the first order kinetic model and are shown in Fig. 6 and Table 2.

2.5

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2.0

-Ln(A/A0)

1.5

1.0

NaBH4 (M)

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2.1410

-2

3.5710

-2

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Time (min)

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Fig.6. The first order kinetic model for the different amount of NaBH4, (Yellow): 3.57*10-2M, (Blue):2.14*10-2 M and (Red): 1.66*10-2 M.

NaBH4.

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Table 2. Parameters and regression coefficients for the first kinetic model of different concentration of

Kinetic model

NaBH4 (mM)

Parameters

First-Order

16.6

𝐾 = 0.03, 𝑅2 = 0.98

− ln (𝐴⁄𝐴 ) = 𝐾𝑡

21.4

𝐾 = 0.12, 𝑅2 = 0.95

35.7

𝐾 = 0.37, 𝑅2 = 0.98

0

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ACCEPTED MANUSCRIPT 3.3.3. Influence of dye solution pH

The pH value of dye solution is one of the effective factors influencing the rate of degradation. Hence, the catalytic experiments were carried out at pHs of 5, 7, 9 and 10 with a constant amount of ZBD@Ag (0.2 mg/mL) to explore the effect of pH on MB reduction. The methylene

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blue solution has a pH of 5. The catalytic activity at 0.2 mg/mL of ZBD@Ag and dye solution of pH=5 showed the reduction of dye molecules completed in only 6 min. Increasing the pH to

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higher values of 5 resulted in decreased reduction rate. The surface of ZBD@Ag is presumably

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positively charged in pH<7 and negatively charged in pH>7 (Fig. 7a). Methylene blue has a pKa of 3.14. It is positively charged at pH below 3.14 and negatively charged at pH above 3.14

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[49]. Hence, the acidic solution favors the dye adsorption onto the catalyst surface and the degradation increases. The amount of degradation of MB after 6 min of reaction at different

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pH is presented in Fig.7b. In addition, the degradation amount at pHs 5, 7, 9 and 10 in the presence of 0.1 mg/mL of the catalyst was studied, and the results are shown in Fig. S6 where

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the same trend could be observed for 0.1 mg/mL of the catalyst. It should be noted that more acidic conditions (pH<5) were not considered to be examined due to the very fast

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decomposition of sodium borohydride at acidic conditions.

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ACCEPTED MANUSCRIPT a

15 10

Zeta Potential (mV)

5 0 2

4

6

8

10

pH

-5 -10 -15 -20

b

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40

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pH=7

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Degradation of MB at t=6 min

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pH=10

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Fig.7. a) Zeta potential of the ZBD@Ag catalyst at different pH, b) The amount of MB degradation (after 6 min) in the presence of 0.2 mg/mL catalyst at different pH.

In order to compare the catalytic performance of ZBD@Ag with other catalysts reported in the

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literature, the reaction rate constant, 𝐾𝑎𝑝𝑝 can be compared. In addition, a normalized rate

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constant, 𝐾𝑛𝑜𝑟 , was also considered. 𝐾𝑛𝑜𝑟 was obtained as follows [50]:

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𝐾𝑛𝑜𝑟 = 𝐾𝑎𝑝𝑝 /𝑚

Where m (g) is the mass of the catalyst used. As it can be seen in Table. 3, the developed

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ZBD@Ag well suppresses the other catalysts for the chemical reduction of MB dye. The high normalised first-order reaction rate constant of ZBD@Ag could be attributed to: 1) high porosity of metal-organic framework as a host for Ag NPs , where it provides sufficient place for growing Ag NPs on its the surface and consequently, providing a good substrate with many sites for reactant molecules to be adsorbed on the surface, reacting and desorbed quickly with controlled diffuision[51]; 2) tannic acid at alkaline conditions has led to growth of small size Ag NPs[39], which have more catalytic activity than Ag NPs with bigger size [52-54]. In

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ACCEPTED MANUSCRIPT addition, tannic acid serves as both reducing agent and capping agent for NPs, leading to better interaction between the MB and catalyst at pH=5; 3) ZBD with its porous structure has adsorption affinity to MB molecules at optimum pH (pH=5); hence, this leads to increase in local concentration of MB around the Ag NPs and catalyze the reaction.

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Degradation of MB at t=6 min

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Cycle Number

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Fig. 8 Reusability of the ZBD@Ag catalyst for different cycle

Figure 8 shows the reusability of the ZBD@Ag catalyst. As can be seen, the MB reduction

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performance remained similar even up to five cycles of experiments. This high activity is due to the strong attachment of the Ag NPs on the surface of the ZBD that results in less leaching of the nanoparticles from the catalyst, which could be supported by TEM image after the

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reaction (Fig. S7). It should be noted that the ZBD@Ag could be easily separated from the

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reaction solution using simple filtration or low-speed centrifugation for reuse.

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ACCEPTED MANUSCRIPT Table 3. Comparison of the performance of developed catalyst with the other catalysts in the literature for catalytic reduction of MB Catalyst Preparation Mass of Ref 𝑲𝒂𝒑𝒑 𝑲𝒏𝒐𝒓 −𝟏 −𝟏 −𝟏 Catalyst (g) (𝒔 𝒈 ) (𝒔 ) 0.0061

30.5

Present work

0.02

0.00003

0.001

[55]

Reaction of Ag ions with asprepared P(St-Nass)

0.01

0.002

0.2

[56]

Reaction of Ag ions with Fe3O4@PDA core-shell

0.01

0.007

0.71

[57]

Sulfurization and calcination of MOF-Derived

0.04

0.0003

0.0092

[58]

0.00038

0.0019

[59]

0.004

0.08

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[60]

0.027

0.015

0.55

[61]

0.01

0.0014

0.14

[62]

Adding Fe3O4@PDA to silver ammonia solution

0.005

0.029

5.93

[63]

3D-graphene/Ag

Reaction of Ag ions with GO solution

0.005

0.0041

0.83

[64]

Au@TA-GH

Reaction of TA-GH with HAuCl4 solution

0.002

0.0051

2.5

[65]

Fe3O4@PDA-Ag-10 MOF-Derived MxSy@Composites(ZnS@C and CdS@C)

MHNTS-PDA-Au P(TA)-Cu ILs Cu/CuO-Ag

0.2

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Fe3O4@PDA-Ag hollow microsphere

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Fe-GAC

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Reaction of Fe3+ ions with GAC(Granular Activated Carbon) Reaction of Au ions with Polydopamine functionalized MHNTs Reaction of P(TA) particle with CuCl2 solution Reaction of Ag ions with Cu/CuO suspension

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Monodisperse raspberrylike multihollow polymer/Ag

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Anionic AgNPs Colloids

Reaction of TA/ZBD with Ag ions Reaction of Ag ions with oleic acid and n-butylamine

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0.0002

ZBD@Ag

4. Conclusions

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We have synthesized a superior catalyst via in-situ growth of Ag NPs on the surface of tannic

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acid-modified Zn2(bdc)(dabco)2 using a facile, efficient and green approach. The ZBD metalorganic framework provides a large surface area for the efficient in-situ synthesis of silver nanoparticles. The ZBD@Ag catalyst could accelerate the reaction at a very low concentration, suppressing other noble metal-based catalysts. The catalytic reduction of MB clearly depends on the initial NaBH4 concentration, pH of the dye solution and the amount of catalyst. The results suggest that other MOF-metal nanocomposite (based on Au, Pd, Pt, and Cu, etc.) catalysts could be developed in the future for different environmental purposes.

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ACCEPTED MANUSCRIPT Contributions M.T.Y developed the idea and designed the experiments. A.L and A.M. performed all the experiments. A.L., M.T.Y, and A.N. analyzed the data. A.L wrote the first draft of the manuscript. All authors confirmed the final draft of the manuscript. M.T.Y and M.H.S lead the

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project.

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Conflict of interest

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The authors declare no conflict of interest.

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Acknowledgment

A.L. would like to thank Islamic Azad University, Science and Research Branch (Tehran, Iran) to

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provide the facilities for this project.

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Graphical abstract

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Research Highlights

Ag NPs decorated ZBD metal-organic framework (ZBD@AgNPs) was prepared using

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in-situ approach.

ZBD@AgNPs showed high catalytic activity for reduction of MB.



The MB molecules could be degraded in a few seconds.

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ZBD@AgNPs can be easily recovered and reused for five cycles.

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