Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 371–377

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Novelmetal–organic photocatalysts: Synthesis, characterization and decomposition of organic dyes N.B. Gopal Reddy a,b, P. Murali Krishna a,⇑, Nagaraju Kottam a,⇑ a b

Department of Chemistry, M. S. Ramaiah Institute of Technology, Bangalore 560 054, India Visvesvaraya Technological University, Belgaum 590 018, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The chemical and structural

Tentative mechanism of the photodegradation of dye in the presence of metal complex is.

environments of the metal complexes were specified.  The interaction of the hydrazone with MCl2nH2O afforded square planar complexes.  Transition metal complexes are used as photocatalysts for the degradation of methylene blue dye under UVlight.  Ni(II) complex shows higher photodegradation of MB than Co(II) complex at alkaline pH.  Tentative mechanism of the photodegradation of dye in the presence of metal complex is described.

a r t i c l e

i n f o

Article history: Received 23 April 2014 Received in revised form 30 June 2014 Accepted 24 August 2014 Available online 30 August 2014 Keywords: Metal–organic catalysts Methylene blue Photocatalytic activity

a b s t r a c t An efficient method for the photocatalytic degradation of methylene blue in an aqueous medium was developed using metal–organic complexes. Two novel complexes were synthesized using, Schiff base ligand, N0 -[(E)-(4-ethylphenyl)methylidene]-4-hydroxybenzohydrazide (HL) and Ni(II) (Complex 1)/ Co(II) (Complex 2) chloride respectively. These complexes were characterized using microanalysis, various spectral techniques. Spectral studies reveal that the complexes exhibit square planar geometry with ligand coordination through azomethine nitrogen and enolic oxygen. The effects of catalyst dosage, irradiation time and aqueous pH on the photocatalytic activity were studied systematically. The photocatalytic activity was found to be more efficient in the presence of Ni(II) complexes than the Co(II) complex. Possible mechanistic aspects were discussed. Ó 2014 Published by Elsevier B.V.

Introduction Dyes are applied in textile manufacturing, leather tanning, and paper production and food technology industries as dyeing agents ⇑ Corresponding authors. Tel.: +91 80 2360 0822x318; fax: +91 80 2360 3124. E-mail addresses: [email protected] (P. Murali Krishna), [email protected] com (N. Kottam). http://dx.doi.org/10.1016/j.saa.2014.08.045 1386-1425/Ó 2014 Published by Elsevier B.V.

[1]. The release of those colored waste waters in the ecosystem is a dramatic source of esthetical pollution, of eutrophication and of perturbations in the aquatic life. Hence effluents from industries contain a high level of environmentally hazard dyeing agents. Therefore, the effluent water has to be treated before disposal. There are many physicochemical techniques that can remove dyes from its aqueous solution which includes biological techniquesaerobic and anaerobic process, physical techniques-membrane

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O

O N H

O 5% Acetic acid, Ethanol

NH2

N H

Reflux, 2 hs

H N C

HO

HO

Ethanol Stirring 2-3 hrs

MCl2

Metal Complex Scheme 1. Synthesis of ligand and its complexes.

filtration, coagulation/flocculation, precipitation, flotation, adsorption, and chemical techniques chemical oxidation process of chlorination, bleaching, ozonation, Fenton oxidation and photocatalytic oxidation [2]. However, low removal efficiency or high cost of operation often limits their application [3]. In this concern, extensive work has been done recently on the photocatalytic degradation of environmentally hazard pollutants using metal oxides due to their superior photocatalytic performance, non-toxicity, low production cost and high persistence to photocorrosion [4–7]. The process is based on the generation of the hydroxyl radicals (OH) that can oxidize a broad range of organic contaminants non-selectively in a short period of time. Interestingly, ‘‘advanced oxidation processes’’ (AOPs) also offer different routes to OH production, allowing easier tailoring of the process for specific treatment requirements [8–12]. Since, structurally, dyes are double bonded such as [email protected], [email protected] and heterocyclic compounds, therefore transition metal ions are able to coordinate with most of the organic substances containing this type of bonds. This is because of their lenience in change of the oxidation state and presence of unpaired electrons the metal ions react readily with molecular oxygen, thereby mediating oxygenation of other compounds easily [13]. These properties have made use of transition metal complexes for the chemical, biological, photochemical and photo-biological degradation of the organic ligands. So far a number of metal complexes involving degradation of dyes and other pollutants has been reported from time to time [14]. From the previous reports it is evident that in all the cases hydrogen peroxide plays an important role in generating reactive species such as hydroxyl free radical and is used in the destruction of dispersed dyes. However, higher concentration of H2O2 is hazardous and it is harmful to eyes, lungs and skin. Recently reported degradation of MB in the absence of H2O2 using Co(II), Ni(II) and Cu(II) complexes of (2E)-2-[(2E)-3-phenylprop-2-en-1ylidene]hydrazinecarbothioamide [15]. In the continuation of our research on MB degradation using metal complexes, herein reporting with Ni(II) and Co(II) complexes of N0 -[(E)-(4-ethylphenyl)methylidene]-4-hydroxybenzohydrazide as a catalysts and also investigated the influence of various parameters such as

Fig. 1. Experimental setup for the photocatalytic degradation.

catalyst dosage, irradiation time and aqueous pH on the photocatalytic activity. Experimental Materials Methylene Blue (C16H18ClN3SCl) was used in this work which was purchased from s.d fine-chem limited, Bombay [99.0% pure]. 4-Hydroxybenzhydrazide [98% pure] and 4-Ethylbenzaldehyde [98% pure] were purchased from Sigma–Aldrich, Bangalore. Nickel chloride hexahydrate and Cobalt chloride hexahydrate [99% pure] were purchased from HiMedia laboratory Pvt. Ltd. Bombay and were used as received to prepare the precursor for the synthesis of photocatalytic complexes. Double-distilled water was used throughout the experiments. Instruments used C, H and N estimated on a Perkin Elmer CHN 2400 analyzer. Melting points of the compounds were determined by using capillaries in Sigma melting point apparatus, Sigma instruments, Chennai, India. Magnetic susceptibility measurements were carried at room temperature on a magnetic susceptibility balance (Sherwood Scientific, Cambridge, England) using CuSO45H2O as standard. Electronic spectra were recorded on Elico-150 in DMSO solvent.

Table 1 The analytical and physical data of the ligand and its complexes. Compound

Mol. Wt (g/mol)

Color

Melting point (°C)

leff BM

Elemental analysis data (%calculated/%observed) C

H

N

O

HL

268.37

Color less

200–202

71.62 (71.56)

6.01 (5.89)

10.44 (10.39)

11.92 (11.78)



Complex I

593.30

Light blue

>300

64.78 (64.73)

5.10 (4.98)

9.44 (9.23)

10.79 (10.59)

0.77

Complex II

593.54

Rose

>300

64.75 (64.71)

5.09 (5.01)

9.44 (9.31)

10.78 (10.61)

0.99

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Scheme 2. Tentative fragmentation scheme for complex 1.

Infrared spectra were recorded in the 4000–400 cm1 region (KBr disc) on a Nicolet portage 460 FT-IR spectrophotometer. 1H- and 13 C{1H} NMR spectra were obtained in d6-DMSO using tetra methyl silane (TMS) as an internal reference on Advanced 200.12 and 50.32 MHz NMR spectrometer. The LC-MS studies of the complexes were recorded in aqueous methanol on a Shimadzu LCMS 2010A spectrometer.

CH3

HO H

Synthesis Preparation of N0 -[(E)-(4-ethylphenyl)methylidene]-4hydroxybenzohydrazide (HL) Ligand HL, was prepared according to the procedure given in literature [16]. To an ethanolic solution of 4-hydroxybenzhydrazide (1.11 g, 7.29 mmol) equimolar ethanolic solution of 4-ethylbenzaldehyde (0.979 g, 7.29 mmol) was mixed and the mixture was refluxed for 2–3 h that resulted in a clear solution. The clear solution was cooled to room temperature overnight to give a solid product, then, was filtered, recrystallized in methanol and dried under vacuum (Scheme 1).

N

O

N

M N

N

O

H OH

CH3

Where M= Ni(II) and Co(II) Preparation of the Ni(II)/Co(II) complexes Nickel(II) and Cobalt(II) complexes of HL was prepared by following the general procedure. A warm ethanolic solution (20 mL) containing HL ligand (1.0 g, 3.72 mmol) was added to an ethanolic solution (10 mL) of MCl26H2O (0.442 g for Ni and 0.443 g for Co, 3.72 mmol). Then, the reaction mixture was stirred for 2 h at room temperature and the solid formed was filtered off, washed with cold ethanol, and dried under vacuum.

Fig. 2. Tentative structures of the complexes.

Photocatalytic activity The experimental setup for the degradation of MB is shown in Fig. 1. The photocatalytic activities of the metal complexes were evaluated by the degradation of MB for both before and after

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Fig. 3. Photocatalytic efficiency of MB under ultraviolet irradiation at 10 mg/L initial MB concentration; (A) Catalyst dosage, (B) time variation, (C) pH variation.

irradiation of UV light (320 nm). A reaction suspension was prepared by adding metal complexes into 100 mL of aqueous MB solution (10 mg/L), and was allowed to react at ambient condition under stirring. After irradiation, the catalyst was separated by centrifuging and the absorbance of MB was determined by the UV–visible spectrophotometer at the absorption wavelength of 664 nm [17]. The photocatalytic efficiency (%) was calculated using the formula

To study the effect of important parameters like pH (acidic, neutral and alkaline), contact time (30, 60, 90, and 120 min) and photocatalyst dose (0–40 mg) on the MB dye, batch experiments were conducted for each experiment.

Degradation ð%Þ ¼ ½ðA0  At Þ=A0   100

The prepared hydrazone, N0 -[(E)-(4-ethylphenyl)methylidene]4-hydroxybenzohydrazide is air stable, readily soluble in common organic solvents. The results of elemental analysis (Table 1) are in good agreement with the proposed formula. IR spectrum (ES Fig. 1) of the ligand spectrum exhibits characteristic absorption bands at

ð1Þ

where A0 is the absorbance of MB solution before the photocatalytic reaction, and At is the absorbance of MB solution after catalyzed by the metal complexes for known intervals of time.

Results and discussion Characterization of the ligand

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MB Catalyst I Catalyst II

90

% degradation

1770, 1562 and 1108 cm1 due to the m([email protected]), m([email protected]) and m(NAN) vibrations, respectively [18]. Due to strong intramolecular hydrogen bonding in ligand, the m(OAH) groups exhibited at 3715 cm1. On the other hand, the electronic absorption spectrum (ES Fig. 2) of the hydrazone in DMSO exhibit two intense bands at 310 and 350 nm characteristic for transitions of intra-ligand p–p* transition and the ligand-to-metal charge transfer (LMCT) transitions. The mass spectrum of the ligand showed the molecular ion at m/z = 269 confirming its formula weight (FW = 268). Also, the 1 H and 13C{1H} NMR spectral data of the ligand in d6-DMSO relative to TMS (ES Figs. 3 and 4) lend a further support of its structure.

60

30

0

Characterization of the metal complexes The interaction of the hydrazone with Ni(II)/Co(II) salts affords the corresponding mononuclear metal complexes. The structural elucidation of the isolated complexes was achieved via elemental, magnetic susceptibility measurements as well as spectral studies viz. electronic, vibration, and mass. The analytical data (Table 1) of the complexes are compatible with 1:2 metal-to-ligand stoichiometry. Room temperature magnetic susceptibility data of the complexes suggesting the mixed stereochemistry of square-planar and tetrahedral structure. The magnetic moment values observed in the present study are much less than the expected values for spin free square-planar or tetrahedral complex. Mixed stereochemistry in solid state may be due to molecular association [19]. IR spectra of the complexes The IR spectra (ES Fig. 1) of the complexes are compared with those of the free ligand in order to study the binding mode(s) of the hydrazone ligand to the metal. The ligand spectrum exhibits characteristic absorption bands at 3715, 1770, 1562 and 1108 cm1 due to the m(OAH), m([email protected]), m([email protected]) and m(NAN) vibrations, respectively [18]. The IR spectra of the complexes reveal significant differences from those of the free ligand. On coordination of azomethine nitrogen, m([email protected]) shifts to lower wavenumbers by 30–40 cm1, as the band shifts from 1770 cm1 in the free hydrazone spectrum to lower wavenumber in the spectrum of the complexes [20]. The decrease in the stretching frequency of m([email protected]) band from 1562 cm1 in the hydrazone ligand to lower wave number upon complexation indicates coordination through oxygen of the carbonyl group [21]. Electronic spectra of the complexes The electronic spectra of the complexes were recorded in DMSO solvent (ES Fig. 2). The bands appeared in 300–309 nm region in the spectra of the complexes assigned to the intra-ligand p–p* transition due to the formation of conjugated double bond after complexation to the metal ions [22]. The bands observed at approximately 350–400 nm are attributable to the ligand-to-metal charge transfer (LMCT) transitions [23]. The spin allowed transitions 3T1 ? 3T1(P) and 3T1 ? 3A2 are assigned to corresponding square planar geometry around Ni(II) and Co(II) ion, respectively [24]. The nature and position of the electronic spectral absorptions of the complexes indicate square planar geometry around metal ions in these complexes. Mass spectra of the complexes The mass spectrum of complexes 1 (ES Fig. 5) and 2 (ES Fig. 6), showed the molecular ion at m/z 592.97 and 594.27 Conforming their formula weights (FW = 593.3 (Ni) and 595.5 (Co)). Tentative fragmentation scheme for complex 1 is shown in Scheme 2. On the basis of analytical, magnetic and spectral studies the tentative structure of the isolated complexes is shown in Fig. 2.

3

6

9

12

pH Fig. 4. Effect of solution pH on the photocatalytic degradation efficiency of MB at 10 mg/L of MB concentration, optimum catalyst load 20 mg and 10 mg and optimum time irradiation 60 min and 90 min for catalysts I and II respectively under UV light irradiation.

Degradation of methylene blue Degradation of methylene blue was studied with and without the catalyst under UV light irradiation. There was no appreciable degradation with catalyst in dark. So that UV light irradiation was necessary for efficient degradation. Photocatalytic decolorization efficiency of the catalyst was monitored by performing UV–visible spectral studies at different intervals of time, pH and catalyst dose. To optimize the amount of catalyst dose, the experiments were conducted by taking catalyst amounts from 5 to 40 mg (Fig. 3A). It was observed that the 76.50% degradation efficiency was achieved with 20 mg of complex I and 64.40% with 10 mg of complex II. In general, increase in the amount of catalyst increases the degradation because it increases the number of active sites on the catalysts surface. Furthermore increase may cause the aggregation of free catalyst and also increased opacity and a decrease in UV light penetration, as a result of increased scattering effect and therefore percentage degradation starts decreasing [25,26]. Then, the effect of irradiation time on the photocatalytic degradation of MB was investigated from 0 to 120 min. with MB concentration of 10 mg/L, for optimum loading of catalyst I and II respectively at neutral pH. Fig. 3B shows the absorption spectra of aqueous solutions of MB tested at different time intervals in the presence of the catalyst I and II. The degradation efficiency observed in catalysts I (76.5%) in 60 min and catalyst II (64.6%) in 90 min of irradiation. Further increase in irradiation time, the degradation efficiency is decreasing. In general, if an increase in irradiation time, the recombination of charge carriers and also the desorption process of adsorbed reactant species, resulting a decrease of photocatalytic activity [27]. In photocatalytic degradation the solution pH is also has a special significance. Most of the processes are favoured by alkaline medium that is connected to the formation of hydroxo complexes and generation of the hydroxyl radicals, which are the main oxidizers in photoinduced degradation of pollutants [28]. Hence to study the effect of pH on the decolorization efficiency, the experiments were carried out at pH values 3, 7 and 12 with catalysts (20 mg; 60 min for catalyst I and 10 mg; 90 min for catalyst II). Fig. 3C shows color removal efficiency of catalysts I and II at different pH values. The photocatalytic efficiency of the catalysts at difference pH values is in the order of 12 (91.0% for catalyst I and 88.5% for catalyst II), 7 (76.5% for catalyst I and 64.6% for catalyst II) and 3 (8.0% for I, 3.86% for II) (Fig. 4).

M3þ þ H2 O ! M2þ þ Hþ þ  OH photons   ML2 Cl2 þ 2H2 O ƒƒƒƒ! ML2 ðH2 0Þ2 þ 2Cl

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Table 2 Degradation (%) efficiency of catalyst I and II at different condition. Catalyst

MB I II

Irradiation time (min)

Dosage amount (mg)

pH

30

60

90

120

10

20

30

3.0

7.0

12.0

– 17.9 37.7

– 76.5 64.4

– 23.10 87.70

– 25.9 68.6

– 44.7 64.4

– 76.5 40.1

– 43.4 25.53

5.69 8.00 3.86

29.93 76.50 64.40

47.73 91.00 88.50

Bold values indicate the maximum percentage degradation of MB in the presence of catalysts I and II at optimization of catalytic dose, time interval and pH variation.

Scheme 3. Mechanism for the photodegradation of dye in the presence of metal complex. 



Cl þ NaOH ! NaCl þ OH

Appendix A. Supplementary material

Photocatalytic degradation efficiency of these materials was calculated and shown in Table 2. On the basis of the experimental observations and corroborate the existing literature, a tentative mechanism has been proposed for the degradation of MB in presence of metal complexes and light can be explained as represented in Scheme 3. When metal complex is illuminated under UV light, absorb photons and then charge separation occurs at the interface and promoting the photocatalytic activity of photocatalyst. The photogenerated holes, reducing their recombination with the electrons, and the surface OH groups allow the adsorption of O2 from water. Then, the photo formed electrons reduce O2 to O 2 species, which in turn can interact with water to form further oxygenated radicals (mainly hydroxyl radicals OH). Consequently, the presence of hydroxyl radicals MB dye decolourises effectively [29].

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.08.045.

Conclusions Ni(II) and Co(II) complexes of N0 -[(E)-(4-ethylphenyl) methylidene]-4-hydroxybenzohydrazide were prepared and characterized using various spectroscopy techniques. The photocatalytic effect of prepared complexes for MB was studied systematically under UV light irradiation. The photocatalytic activity was found to be more efficient in the presence of Ni(II) complexes than the Co(II) complex. Studies revealed that these metal complexes with higher photocatalytic efficiency will be a potential candidate for environmental purification under UV light at ambient conditions. The prepared metal complexes in this work may find other applications like tackling a variety of substrates including azo and anthraquinone dye pollutants, catalysis etc. Acknowledgements Authors acknowledge financial support from the Visvesvaraya Technological University, Belgaum (VTU/Aca./2010-11/A-9/ 11341) for financial support and the management of M.S. Ramaiah Institute of Technology, Bangalore.

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Novel metal-organic photocatalysts: synthesis, characterization and decomposition of organic dyes.

An efficient method for the photocatalytic degradation of methylene blue in an aqueous medium was developed using metal-organic complexes. Two novel c...
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