Self-assembly of a new building block of {BMo12O40} with excellent catalytic activity for methylene blue

Self-assembly of a new building block of {BMo

12

O

40

} with excellent

catalytic activity for methylene blue

Mukerrem Findik

_{, Asuman Ucar}

_{, Alper Tolga Colak}

_{, Onur Sahin}

_{, Haluk Bingol}

_{, Ulku Sayin}

_,

Nuriye Kocak

a

Necmettin Erbakan University, A.K. Education Faculty, Department of Chemistry Education, Research Laboratory, Konya, Turkey

b_{Agri Ibrahim Cecen University, Education Faculty, Department of Science Education, Agri, Turkey} c

Dumlupinar University, Faculty of Arts and Science, Department of Chemistry, Kutahya, Turkey

d

Sinop University, Department of Scientific and Technological Research Application and Research Center, Sinop, Turkey

e

Selcuk University, Faculty of Science, Department of Physics, Konya, Turkey

f

Necmettin Erbakan University, A.K. Education Faculty, Department of Science Education, Konya, Turkey

a r t i c l e i n f o

Article history:

Received 16 October 2018 Accepted 22 December 2018 Available online 6 January 2019

Keywords: Polyoxometalate Heteropolymolybdate Catalytic degradation Methylene blue Single crystal structure

a b s t r a c t

A novel organic–inorganic hybrid of 2,20_{-bipyridyl (2,2}0_{-bipy) linked covalently with the first inorganic}

framework based on boron-containing Keggin-type heteropolymolybdate anion [BMo12O40]5has been

hydrothermally synthesized in aqueous solution. The crystal structure was fully characterized by ele-mental analyses, single crystal X-ray diffraction, Fourier-transform infrared spectrum (FT-IR), powder X-ray diffraction (XRD), Ultraviolet–visible spectroscopy (UV–Vis) and electron paramagnetic resonance (EPR) analysis. The catalytic performance of the synthesized catalyst was studied in degradation of methylene blue (MB) at ambient temperature. The catalyst exhibited excellent degradation against MB with a rate constant of 0.506/m, which was much higher than those by other polyoxometalate catalysts. Moreover, it was found to be easily separated from the reaction solution and recycled up to five times without significant loss of degradation activity.

Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Polyoxometalates (POMs), due to their unique physical and chemical properties, have proven to be versatile inorganic building blocks for their potential application such as catalysis [1], mag-netism[2], optics[3], medicine[4], and biology[5]. An important subclass of the Keggin-type POMs, polyoxomolybdates have attrac-tive structures based on their coordinate bonds or hydrogen bonds thanks to their spherical surfaces. Up to now, polyoxomolybdates have been synthesized such as [XMo12O40]n(X = P, Ge, As, Si, Co,

Ni)[6–10]. Our group obtained the first boron-containing Keggin-type heteropolymolybdate anion [BMo12O40]5. In recent years,

inorganic–organic hybrid compounds have attract researchers’ attention owing to their provides a powerful way to acquire versa-tile new compounds. In this regard, functionalization of these clas-sical POMs with organic compounds affords a new method to change the catalytic performance of POMs[11–15].

In the past decades, polyoxomolybdates has attracted consider-able scientific interest in environmental improvement and removal

of organic dye pollutants due to its optical and structural stability, low toxicity and low cost[16–19]. The organic dyes prevent the photosynthetic activity of aquatic organisms by causing sunlight to break. Therefore, it is very important to remove dye pollutants from waste water. In recent years, diverse types of waste water treatment methods have been applied such as chemical, physico-chemical and biological methods for the removal of dye containing wastes [20–23]. However, these techniques are not effective because of the time consumed and are not economical. On the contrary, catalytic reduction is a relatively fast process and more economical[24–28].

In this work, the progress in synthesis of hybrid material based on 2,20_{-bipy organic ligands and Keggin-type heteropolymolybdate}

as inorganic building blocks is referred. We reports a new hybrid material, Na[(Cu(bipy)2)2(BMo12O40)] (1). The compound 1 was

hydrothermally synthesized and fully characterized. Organic dye MB, which is often used in the manufacture of silk, dyeing cotton, ink and so on, is harmful to human and animals. So MB is chosen as a model dye for degradation in aqueous solution. This kind of hybrid crystal material possesses good catalytic activity for the degradation of MB in water solution, and the significant degrada-tion by compound 1 can reach 95% within 5 min.

https://doi.org/10.1016/j.poly.2018.12.043

0277-5387/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. Fax: +90 3323238225.

E-mail addresses:mmukerremm@gmail.com,nkocak@erbakan.edu.tr(M. Findik).

Contents lists available atScienceDirect

Polyhedron

thermal gravimetric analyzer in flowing N2 with a temperature

ramp rate of 1°C min1 _{in the range of 25–1000°C. JEOL}

JesFa-300 X-band EPR spectrometer, located at Selcuk University Advanced Technology Research and Application Center, was used for EPR measurements.

2.2. Preparation of Na[(Cu(bipy)2)2(BMo12O40)] (1)

A mixture of Na2MoO42H2O (0.73 g, 3.00 mmol), H3BO3

(0.093 g, 1.5 mmol), CuCl2.2H2O (0.14 g, 0.825 mmol), 2,20-bipy

(0.117 g, 0.75 mmol) and H2O (10 mL) was stirred for an hour.

The pH was necessarily adjusted to 1.18 with 6 M HCl, then sealed in a 10 mL Teflon-lined reactor and heated at 185°C for 7 days. After the reactor was slowly cooled to room temperature over a period of 10°C/h, turquoise block crystals were filtered, washed with water, and dried at room temperature (yield 87%, based on Mo).

Anal. Calc. for C40H32BCu2Mo12N8NaO40(2576.89): C, 18.63%; H,

1.24%; N, 4.35. Found: C, 18.78; H, 1.46; N, 4.63%. FT-IR (cm1): 1601, 1496, 1444, 1033, 941, 892, 723.

2.3. X-Ray diffraction analysis

Suitable crystal of compound 1 was selected for data collection which was performed on a D8-QUEST diffractometer equipped with a graphite-monochromatic Mo K

a

SHELXL -2013[30]. The H atoms were located from different maps and then treated as riding atoms with C–H distances of 0.93 Å. In the course of structural check of compound 1, we found that some none-H atoms met ADP and NPD problems. In order to avoid them, a restraint comment of EADP was used to refine the non-H atoms, but this cannot be totally avoid considering about the quality of the data. The following procedures were implemented in our anal-ysis: data collection: Bruker APEX2[31]; program used for molec-ular graphics were as follow:MERCURYprograms[32]; software used to prepare material for publication: WinGX [33]. Details of data collection and crystal structure determinations are given in

Table 1.

2.4. Catalytic reduction of MB

The MB was chosen as the degrading pollutions to test the cat-alytic activities of the as-prepared samples. In a typical degrada-tion reacdegrada-tion, the catalyst (6.0 mg) was added to 3.13 102_M

MB aqueous solution (20 mL). Then, 5 102_{M freshly prepared}

NaBH4aqueous solution (2 mL) was added and the resulting

mix-ture was allowed to stir at ambient temperamix-ture. After that, the reduction progress of MB was checked at different intervals of time by a UV–Vis spectrophotometer. For the recycling experiment, the

catalyst was simply separated from the reaction system by brief centrifugation and washed successively with water and ethanol and dried for the next cycle. Effect of NaBH4was investigated by

a blank reaction where degradation studies were performed in absence the catalyst. The concentration of the target dye is calcu-lated by a calibration curve.

3. Results and discussion 3.1. Structural analysis

The X-ray single crystal study shows that compound 1 has 2D coordination polymer. The asymmetric unit of compound 1 con-sists of one Cu(II) ion, one Na(I) ion, a half of [BMo12O40]5anion

and two bipy ligands (Fig. 1). The molecular structure of compound 1 is centrosymmetrical, where the boron atom is situated on the inversion center (0, 1, 0). The anion [BMo12O40]5 has a

Keggin-type structure, which consists of a central BO4[B–O bond lengths

range of 1.56(3)–1.66(3) Å] tetrahedron surrounded by Mo12O36

group (Table 2). For the BO4groups, all oxygen atoms are

disor-dered over two positions. These disordisor-dered situations have been reported in the previous studies[34,35]. The Cu(II) ion is coordi-nated by four nitrogen atoms from bipy ligands and one oxygen atom from [BMo12O40]5anion, thus showing a square-pyramidal

geometry. The value of

s

[Cu–N bond lengths range of 1.965(16)–2.003(18) Å] owing to Jahn–Teller distortion [(i) x 1, y  1/2, z + 1/2]. The [BMo12

-O40]5anions are bridged by Na(I) ions to generate 2D coordination

polymers (Fig. 2). Adjacent [BMo12O40]5anions are stably packed

together constructing the extended 2-D supramolecular structure via C–H  O hydrogen bonds (Table 3).

3.2. FT-IR, TGA, XRD and UV–Vis analyses

The FT-IR spectrum of the crystal share similarly characteristic vibrational features to other typical Keggin structures[36–39]as shown in Fig. 3Aa. Stretching vibration of the different Mo–O bonds are observed as follows: 941 cm1 for terminal Mo–Ot;

892 cm1 for Mo–Ob (Ob interbridges between corner-sharing

octahedra); 723 cm1 for Mo–Oc (Oc intrabridges between

edge-sharing octahedra); peaks at 1033 cm1is assigned to B–O vibra-tion. Bands in the 1601–1444 cm1region are attributed to charac-teristic vibrations of 2,20-bipy groups. Thermal stability for compound 1 was investigated by TGA and the TGA curve is shown

Rint 0.056

S 1.10

Fig. 1. The representation of compound 1.

Table 2

Selected bond distances for compound 1 (Å).

B1–O1 1.56(3) B1–O3 1.60(3) B1–O4 1.60(3)

B1–O2 1.66(3) N1–Cu1 2.003(18) N2–Cu1 1.986(17)

N3–Cu1 1.991(17) N4–Cu1 1.965(16) Cu1–O12i

2.563(2)

N4–Cu1–N2 158.0(7) N4–Cu1–N3 82.6(7) N2–Cu1–N3 101.9(8)

N4–Cu1–N1 103.5(8) N2–Cu1–N1 83.7(9) N3–Cu1–N1 149.5(7)

Symmetry code: (i) x 1, y  1/2, z + 1/2.

ture of the POM. The measured XRD curves (Fig. 3C) are consistent with the corresponding calculated patterns, further verifying the purity of the crystal product. UV–Vis spectrum of compound 1, in the range of 275–400 nm, are presented inFig. 3D. The UV–Vis spectrum of compound 1 displays two shoulder peaks at about 301 and 311 nm assigned to O? Mo charge transfer and n ?

p

transitions of 2,20_{-bipy ligands in crystal.}

3.3. EPR analysis

EPR spectroscopy is unique in its ability to selectively probe paramagnetic molecules and can provide a wealth of information about the electronic structure of paramagnetic molecules, includ-ing those involvinclud-ing metal ions [40]. For this reason EPR spec-troscopy was used to get information on the effective ligand field

of the complex in frozen DMF solution were recorded at 123 K temperature. The spectra given in Figs. 5 and 6 were taken at 500 mT and 200 mT sweep width to understand if there is any other EPR signal coming from paramagnetic impurities and forbid-den transitions. It is understood from the spectra that the sample is highly purified and the Cu(II) complex is well formed.

The EPR spectra of complex have four copper hyperfine lines, by virtue of 3/2 nuclear spin, characteristic of monomeric Cu(II) com-plexes. There is no a super hyperfine interaction between the unpaired electron and nitrogen ligand atom of the complexes. Fur-thermore, there is no signal corresponding to the forbidden mag-netic dipolar transition for the complexes at half-field with g¼ 4 value. This observation displays the absence of any Cu–Cu interac-tion which supporting the mononuclear complexes[42].

The spectral features of the complex with the parallel and per-pendicular components of g value g_k¼ 2:270 and g?¼ 2:056,

Fig. 4. EPR spectra of powder copper complex at 293 K with 200 mT magnetic field sweep width.

Fig. 5. EPR spectra of Cu(II) complex in DMF at 123 K with 500 mT magnetic field sweep width.

The measure of the exchange interaction between the copper centers can be calculated with the geometric parameter (GÞ, using the expression G¼ gk 2:0023

 

= gð ? 2:0023Þ suggested by

Hathaway[48]. If G is greater than four, the exchange interaction is negligible, whereas when G is less than four a considerable inter-action is indicated in the complex[43,49,50]. The obtained G value is 4.99 for the complex which suggests the exchange interaction is negligible. This result is in good agreement with mononuclear complex assumption.

The parallel and perpendicular components of hyperfine split-ting for copper atom are measured as Ak¼ 14:68 mT and

A?¼ 3:29mT, respectively. The isotropic value of hyperfine

split-ting is determined as Aiso¼ 7:09mT by the equation;

Aiso¼ ð2A?þ AkÞ=3. The f parameter given as f ¼ gk=Akcan be used

to predict the geometry adopted by copper complexes. The obtained value of 165 for f parameter suggests the moderate dis-tortion of tetrahedral geometry for the complex[51,52]. In

addi-peaks in the wavelength (k) range of 550–750 nm and typically shows an absorption maxima at kmax= 664 nm, which may be

attributed to the n–

p

Conse-quently, the progress of the reaction can readily be followed by means of UV–Vis absorption spectrophotometry. Typically, the reduction reaction was carried out in ambient conditions, and a complete reduction of MB by NaBH4may be inferred by the

disap-pearance of intense blue color (MB) to colorless leuco-methylene blue (LMB) within 5 min, as shown inFig. 7A. In the absence of cat-alyst, no significant color change was observed within the reaction time (Fig. 7B). Similarly, no significant color change was observed in the presence of catalyst and in the absence of NaBH4(Fig. 7C).

As shown inFig. 7A, the decreasing absorption peak of MB can be clearly seen in the presence the compound 1 with NaBH4, while

only compound 1 or only NaBH4exhibited no significant decrease

of the absorbance (inFig. 7B, C).

Fig. 8a shows the comparative catalytic percentage degradation of MB by compound 1 in the presence of NaBH4, only compound 1

Fig. 7. UV–Vis absorption spectra of MB degradation (a) compound 1/NaBH4(b) only NaBH4(c) only compound 1.

and only NaBH4. The average degradation in terms of the

percent-age of MB in solution was calculated using the following formula: Degredation ð%Þ ¼ ðA0 AÞ=A0 100where, A0 is the initial

absorbance of dye solution and A is the absorbance at different time intervals. The compound 1 catalyzed >95% degradation of MB, which is much higher than that of polyoxometalates as well as other reports[16,19,56].

The catalytic reduction of MB by compound 1/NaBH4system

can be considered to follow pseudo-first order kinetics. Hence, Eq. (1) was used to fit the experimental data:

lnðAt=A0Þ ¼ kt

where, A0and Atare the absorbance of MB at the initial stage and at

time t, respectively. k represents the reaction rate constant.Fig. 8b shows a linear correlation between ln(At/A0) and the reaction time

at ambient temperature. The rate constant (k) is 0.506 min1, the results were compared with which is previous reports[21,25,57]. High catalytic activity of the compound 1 for MB degradation is probably attributed to the presence of monodisperse MB on the sur-face of compound 1, leading to a bigger active contact sursur-face.

The efficiency and applicability of the synthetic catalysts was evaluated by comparison of the results with those of the recently reported methods for the reduction of MB by NaBH4 (Table 4).

The proposed method is the fastest method for the reduction of MB reported so far.

The reaction mechanism invoked for catalytic reduction of MB over the compound 1 was also proposed based on the earlier report

[59]. In the catalytic reaction, MB is electrophilic in nature while BH4ions are nucleophilic, where it can be adsorbed on the catalyst

surface compound 1 accepts electron from BH₄ to form BO3₃ and transmits it to the adsorbed dye molecules quickly. In this way, the catalytic reduction process involves the electron transfer from the BH4 to the adsorbed dye through compound 1 surface [58,60,61]in aqueous media. The possible mechanism of the cat-alytic reduction of MB with compound 1 in the presence of NaBH4

is illustrated inFig. 9.

The stability and reusability of catalysts, is an important factor for its practical applications. The cycling tests were carried out to study the reusability of compound 1. Compound 1 can be easily separated from the reaction mixture by centrifugation due to its

Table 4

Comparison of various catalysts in the reduction of MB dyes with NaBH4.

Sample MB (M) NaBH4 Catalyst (mg) Time (min) Rate constant k (min1) Refs.

AgNPs-Fe3O4@PDA 2.3 102 20 mL 0.1 M 0.5 mL 5 30 1.44 103_{/s [25]} Ni/CPM 3 105 5 mL 1 102_M 2 mL 2 13 0.583 [27] LDHs/POMs 3.13 102 50 mL 0.3 M 0.5 mL (H2O2) 25 180 0.019 [56] Au/CeO2-TiO2 4.8 105 30 mL 0.2 M 2 mL 13 10.5 0.168 [57] RGO/cobalt DND nanocomposite 40 ppm 20 mL 1.5 M 2 mL 6 10 0.463 [58] rGO-SiW 1.1 101 20 mL 5 102_M 5 mL 5 34 0.055 [59] Na[(Cu(bipy)2)2(BMo12O40)] 3.13 102 20 mL 5 102_M 2 mL 6 5 0.506 This work

insolubility in water, washed with water and ethanol several times, then reused in the next catalytic reduction of MB for five times under the same conditions. As can see inFig. 10, there is a partial decrease in the removal efficiency of the compound 1, at the fifth cycle, with removal efficiency of 94% towards MB. Moreover, the FT-IR spectrum of compound 1, after five cycles of use, is shown inFig. 3Ab. The result obtained by the FT-IR confirm the structural integrity of the Keggin structure in compound 1 after five cycles. In addition, XRD before and after catalytic process have been mea-sured. Considerable changes are not seen in any of the distinct XRD patterns inFig. 3C, which definitely indicate that the structure of the compound 1 remains intact after the degradation reaction. 4. Conclusions

In conclusion, we have realized a new organic functionalization of the first example of [BMo12O40]5fragment in aqueous solution.

The new building block of [BMo12O40]5 functionalized by 2,20

-bipyridyl gives a facile route to conduct self-assembly synthesis of POM-based inorganic–organic hybrid materials in aqueous solu-tion, which is a favorable synthesis approach in green chemistry. The MB degradation experimental studies showed that compound 1 in the presence of NaBH4has a fast degradation rate towards MB

and the relatively high value can be reached within the short 5 min. This suggests that compound 1 act as a very good electron transfer system that catalyzes the reactions by reducing the activa-tion energy. The catalytic results confirmed that the compound 1 can degrade MB with high efficiency. It was reused for at least five runs without significant loss of its catalytic activity. This study pre-sents a green, low-cost, simple, procedure for the degradation of dye pollutants in aqueous wastewater solutions.

Acknowledgements

The authors thank the Necmettin Erbakan University Research Foundation (151210002) for the financial support of this study. The authors acknowledge Scientific and Technological Research Application and Research Center, Sinop University, Turkey, for the use of the Bruker D8 QUEST diffractometer.

Appendix A. Supplementary data

CCDC 1529567 contains the supplementary crystallographic data for compound 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road,

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