Environmental remediation by chitosan-carbon nanotube supported palladium nanoparticles: Conversion of toxic nitroarenes into aromatic amines, degradation of dye pollutants and green synthesis of biaryls

Separation and Purification Technology 247 (2020) 116987

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Environmental remediation by chitosan-carbon nanotube supported palladium nanoparticles: Conversion of toxic nitroarenes into aromatic amines, degradation of dye pollutants and green synthesis of biaryls

T

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Idris Sargina, , Talat Baranb, Gulsin Arslana a b

Department of Biochemistry, Faculty of Science, Selcuk University, 42075 Konya, Turkey Department of Chemistry, Faculty of Science and Letters, Aksaray University, 68200 Aksaray, Turkey

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon nanotube Pd(0) nanoparticle Suzuki-Miyaura coupling reactions Heterogeneous catalyst Chitosan beads Nitrobenzene reduction

Nitroarenes, organic dyes and solvents are recalcitrant pollutants commonly found in industrial effluents. These organic compounds are toxic to aquatic organisms, hence their removal from aquatic systems is of significance. Remediation systems based on metallic nanoparticles are superior to the conventional systems with respect to their efficiency. Biopolymers like chitosan are considered to be suitable support material because biopolymers are capable of forming gel and exhibit affinity for metal cations. Catalysis systems with palladium nanoparticles are efficient in reducing nitroarenes to aromatic amines through hydrogenation, degradation of dye molecules and forming C-C bonds in organic coupling reactions. This study reports the preparation of chitosan-carbon nanotube supported palladium catalyst and the results of characterization studies by FTIR, XRD, TGA and SEMEDX. The catalyst system was effective in catalytic reduction of 4-nitrophenol, 2-nitroaniline, 4-nitro-o-phenylenediamine and 2,4-dinitrophenol, and in degradation of dyes i.e., Congo red, Methyl orange, Methylene blue and Methyl red. The heterogeneous catalyst system also showed an excellent activity in green and solvent-free synthesis of 18 different biaryl compounds simply by six-minute microwave irradiation. The study demonstrated that chitosan-carbon nanotube supported-Pd(0) nanoparticles can be used for environmental remediation and protection, and for green synthesis of biaryls through Suzuki-Miyaura coupling reactions.

1. Introduction Nanotechnology, an interdisciplinary field of research, has become the most progressive field in recent years due to the widespread and successful use of nanomaterials in various applications including environmental remediation and protection [1,2]. Owing to the growing interest in design of metallic nanoparticles, significant progress has been observed in use of nanotechnological materials for water remediation [3] and green synthesis of organic molecules [4,5]. Researchers have synthesized and successfully used different metallic nanostructures such as Ag, Cu, Au, Ni, Pt and Pd nanoparticles as nanotechnological solution to environmental contamination [6]. Among metallic nanostructures, palladium particles stand out as effective catalyst due to its high activity and stability [7]. One of the problems widely encountered in synthesis of metallic nanoparticles is the aggregation of nanoparticles, which adversely affects the characteristics of the particles and their application [8]. It has been shown that the aggregation problem can be overcome by choosing

⁎

suitable support materials exhibiting stable and strong interactions with metals [9]. It is important that the catalyst should be low-cost and nontoxic and the production of the support should be easy and environmentally friendly as well. Chitosan, natural polysaccharide derivative, is a non-toxic, abundant, low-cost biopolymer. This carbohydrate derivative dissolves in water by forming hydrogel, and this makes it possible to chemically and physically modify chitosan and to mould into a range of forms such beads, films and nanostructures [10,11]. Besides, due to its gel forming feature in aqueous solutions, chitosan is capable of forming composites with particulate materials easily. Many chitosan composites have been synthesized by using chitosan as matrix and these composites have been applied in various catalysis systems [12], food packaging [13], drug delivery [14] and remediation technologies for organic and inorganic pollutants [15]. Chitosan is widely used to immobilize nanoparticles due to the hydrophilic hydroxyl and amino groups on its polymeric chains [16]. Previous studies demonstrated that blending chitosan with multi-walled carbon nanotubes (MWCNT) is capable of improving the mechanical properties of

Corresponding author. E-mail address: [email protected] (I. Sargin).

https://doi.org/10.1016/j.seppur.2020.116987 Received 6 December 2019; Received in revised form 24 March 2020; Accepted 2 April 2020 Available online 28 April 2020 1383-5866/ © 2020 Elsevier B.V. All rights reserved.

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Scheme 1. Drawing showing the preparation of the catalyst; Pd NPs@chitosan-MWCNT (CS: Chitsan, CNT: multiwalled carbon nanotubes).

Fig. 2. X-ray diffractograms of (a) chitosan-MWCNT and (b) Pd NPs@chitosanMWCNT catalyst.

wastewater treatment applications, yet novel CNT-based systems should be developed and tested in water treatment technologies [23]. Aromatic nitroarenes and dyes are organic pollutants commonly found in water bodies. These organic pollutants pose risks to the environment and aquatic living organisms. For example, 4-Nitrophenol is a nitroaromatic organic compound that is toxic and bio-refractory. The treatment of wastewater effluents contaminated with 4-Nitrophenol is very difficult owing to recalcitrant and stable nature of 4-Nitrophenol. The reduction product, 4-Aminophenol, is an intermediate chemical widely used in many industries and is removed more easily compared to 4-Nitrophenol [24]. Therefore, 4-Nitrophenol removal or reduction to nontoxic derivative is of significance [25]. Similarly, organic dye molecules have damaging effects on aquatic life. Congo red, for example, is an azo dye and known as highly carcinogenic. Therefore, removal of azo dyes from the environment is of great importance [26,27]. Many physical, chemical and biological techniques have been developed and frequently used in remediation of contaminants. In a recent study a sulphur-doped carbon quantum dots-based photocatalyst was efficiently used for degradation of antibiotic tetracycline and for 99.99% destruction of bacteria (Escherichia coli) in a contaminated wastewater under visible-light irradiation [28]. Recent advances in environmental remediation and protection demonstrated that catalysis systems involving metallic nanoparticles are desirable due to their simple but effective nature in cleaning up hazardous materials [29,30]. Particularly, metallic nanoparticles with catalytic activity are superior to their conventional equivalents. Metallic nanoparticles are effective in removal of organic contaminants and green and facile synthesis of

Fig. 1. FT-IR spectra of (a) chitosan-MWCNT and (b) Pd NPs@chitosanMWCNT.

chitosan films [17]. Another study reported that the presence of carbon nanotubes on the surface of chitosan enabled to produce Pd nanoparticles with unique properties [18]. Numerous studies have shown that metallic nanoparticles can be used effectively as nanoadsorbents [19] or catalysts [20–22] in water treatment technologies. Organic contaminants interact with pristine and/or functionalized carbon nanotubes (CNT) through hydrophobic interaction, π-π binding and formation of chemical bonds with surface functionalities. An earlier study by Sarkar et al. (2014) revealed that CNT-based remediation technologies are promising in remediation of water bodies contaminated with organic contaminants [23]. However, as stated by the authors of the paper, although CNTs have a potential in 2

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Fig. 3. Thermograms of the chitosan-MWCNT and Pd NPs@chitosan-MWCNT catalyst.

microwave oven.

organic compounds in solvent-free media [31]. Catalytic hydrogenation of nitroaromatics and dyes are considered as an effective way of environmental remediation [32,33]. Transition metal catalyzed Kumada, Stille, Sonogashira and Negishi coupling reactions are important organic synthesis methods to form C-C bonds [34–37]. Among these coupling reactions, Suzuki-Miyaura coupling reactions are widely used because of their wide functional group tolerance, cheap and easy availability of the reagents, their high selectivity. Suzuki-Miyaura coupling reactions can also be carried out in water or organic solvents [38–40]. Furthermore, biaryl compounds synthesized by Suzuki-Miyaura coupling reactions have a widespread use in pharmacology, medicine and cosmetics due to their chemical and biological properties, and this makes Suzuki-Miyaura coupling reactions even more important [41,42]. Palladium is one of the most important transition metals used in Suzuki-Miyaura coupling reactions [43,44]. Suzuki-Miyaura C-C coupling reactions give high reaction yields in the presence of homogeneous catalysts. However, difficulties experienced in separating the biaryl products and recovering the used catalyst from the reaction medium are major drawbacks in such systems with homogeneous catalysts [45,46]. This adversely affects the catalytic performance and repeatability of the catalyst. These problems can be avoided by synthesizing transition metal catalysts on a solid support. Therefore, there is a need for design of ideal support material for Suzuki-Miyaura coupling reactions and heterogeneous catalysts with high catalytic performance. In the study, we hypothesized that chitosan in gel form could form an excellent composite with MWCNT. After moulding chitosan gelMWCNT into solid beads, the chitosan-MWCNT surface could be used for synthesis of Pd nanoparticles (Pd NPs) without using any reducing agent. The catalytic performance of the Pd NPs@chitosan-MWCNT catalyst system was tested in reduction of nitroaromatic compounds, which are considered as pollutants, into “environmentally safer” form i.e., amino compounds and in degradation of dye pollutants. Also, the catalytic system was tested in green synthesis of biaryls, which are desirable in medicine and cosmetics products, through forming C-C bonds. The heterogeneous catalyst showed an excellent activity in the catalytic reduction of 4-nitrophenol, 2-nitroaniline, 4-nitro-o-phenylenediamine and 2,4-dinitrophenol and in the degradation of Congo red, Methyl orange, Methylene blue and Methyl red. Using this catalysis system, green and solvent-free synthesis of 18 different biaryl compounds was also achieved by simply irradiating the reactants in a

2. Materials and methods 2.1. Materials The materials and chemicals used in each section of the study are listed below. In preparation of chitosan beads as the catalyst support: medium molecular weight chitosan (deacetylation degree, 75–85%) (Sigma-Aldrich), multi-walled carbon nanotubes (Sigma-Aldrich), acetic acid solution (2% by volume), sodium hydroxide (Sigma-Aldrich) and methanol (Sigma-Aldrich). In cross-linking of chitosan-MWCNT gel beads: methanol, glutaraldehyde solution (25%, Sigma-Aldrich) and ethanol (Sigma-Aldrich). In conversion of Pd(II) to Pd(0) nanoparticles on the surface of chitosan-MWCNT gel beads: PdCl2 and ethanol. In reduction of nitroaromatic compounds to aromatic amines: 4nitrophenol (Sigma-Aldrich), 2-nitroaniline (Sigma-Aldrich), 2,4-dinitrophenol (Merck), 4-nitro-o-phenylenediamine (Acros Organics) and the hydrogen source NaBH4 (Merck). In degradation of dyes: Congo red (Sigma-Aldrich), Methyl orange (Sigma-Aldrich), Methylene blue (Sigma-Aldrich), Methyl red (SigmaAldrich) and sodium borohydride. In “green” synthesis of biaryls: (i) in the model reaction, in which 4‐methoxybiphenyl was synthesized from the coupling reaction of phenylboronic acid with 4-iodoanisole. Phenylboronic, 4-iodoanisole and inorganic bases (NaOH, KOH, Cs2CO3 and K2CO3). (ii) In synthesis of biaryl: phenylboronic acid, aryl halides (I, Br, Cl) and the base system K2CO3. (iii) In the extraction procedure: water, toluene and MgSO4. 2.2. Instruments Physicochemical and surface features of the catalyst support (glutaraldehyde cross-linked chitosan-MWCNT beads) and the catalyst itself (Pd(0)-loaded cross-linked chitosan-MWCNT beads) (Pd NPs@chitosanMWCNT) were examined using FT-IR spectrophotometry (Perkin Elmer Spectrum 100 FT‐IR spectrophotometer), X-ray diffraction (XRD) (Bruker D8-Advance; 2θ scan angle of 0–80°), Thermal gravimetric analysis (EXSTAR S11 7300, under nitrogen atmosphere and in the heating range of 30–700 °C) and Scanning Electron Microscopy (SEM ZEISS EVO LS 10). Reduction of nitroaromatic compounds and 3

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Once neutral pH was attained, the wet beads were treated with crosslinking medium (90 mL methanol and 0.9 mL glutaraldehyde solution) at 70 °C for 6 h in a reflux system. Glutaraldehyde cross-linked chitosanMWCNT beads were recovered by filtration and washed with water and ethanol to get rid of unreacted glutaraldehyde molecules. The beads were finally dried under room conditions and used in the synthesis of carbon nanotube-chitosan supported palladium catalyst.

a

2.4. Synthesis of Pd NPs@chitosan-MWCNT catalyst Glutaraldehyde cross-linked chitosan-carbon nanotube beads (1.0 g) (chitosan-MWCNT beads) and PdCl2 (0.25 g) were boiled in ethanol (30 mL) for 5 h in a reflux system. During the heating treatment, the solution, which had been clear, was turned into dark, which was an indication of reduction of Pd(II) to Pd(0). Finally, the Pd(0)-loaded beads (Pd NPs@chitosan-MWCNT) were collected by filtration (Scheme 1).

b

2.5. Conversion of nitroaromatic compounds into aromatic amines Freshly prepared sodium borohydride solution (0.1 mL, 0.05 M) was added into the solution of nitroaromatic compound (1.0 mL, 1.0 × 10−4 M 4-nitrophenol, 2-nitroaniline, 4-nitro-o-phenylenediamine or 2,4-dinitrophenol) and the reaction was initiated with addition of the catalyst (7 beads weighing about 4 mg). The progress of the conversion reaction was followed on the UV–vis spectrophotometer at certain absorbance peak for each compound. A complete disappearance of the maximum absorbance peak of nitroaromatic compound solution, which had been yellowish, and emergence of a distinct absorbance peak indicated that the reduction reaction went completion by giving a colourless solution of the corresponding aromatic amine compound. The reduction experiments were conducted at room temperature.

c

2.6. Degradation of dye molecules The catalyst Pd NPs@chitosan-MWCNT (4 mg) was added into the dye solution (1.0 mL, 1.0 × 10−4 M Congo red, Methyl orange, Methylene blue or Methyl red) and the degradation reaction was initiated by putting sodium borohydride (3 mg) into the solution. The reaction was monitored on the UV–vis spectrophotometer. Addition of sodium borohydride yielded a colourless solution with disappearance of the maximum absorption peak of the intact dye solution. The dye degradation experiments were conducted at room temperature.

Fig. 4. SEM micrographs of (a) chitosan-MWCNT beads and (b, c) Pd NPs@ chitosan-MWCNT catalyst.

2.7. Synthesis of biaryls through Suzuki-Miyaura coupling reaction degradation of dyes experiments were monitored using a UV–vis spectrophotometer (Shimadzu 1800). A domestic microwave oven was used in Pd NPs@chitosan-MWCNT catalyzed synthesis of biaryls. Identification of the biaryls synthesized throughout the study was done by GC–MS analysis (Agilent GC-7890A-MS 5975).

Optimum parameters for the microwave assisted-synthesis of biaryls through Suzuki-Miyaura coupling reaction were determined on a model reaction in which 4-methoxybiphenyl was synthesized from the coupling reaction of phenylboronic acid with 4-iodoanisole. On this model reaction, optimum operational conditions i.e., base system (NaOH, KOH, Cs2CO3 and K2CO3), catalyst loading and duration of microwave irradiation (at 400 W) were studied. Synthesis of biaryls was done under the optimized conditions; base system (K2CO3), catalyst loading (2.5 × 10−3 mol %) and duration of microwave irradiation (6 min). The following general procedure was followed in the coupling reactions; 1.12 mmol aryl halide, 1.87 mmol phenylboronic acid, catalyst (2.5 × 10−3 mol%) and K2CO3 (3.75 mmol) were irradiated in the microwave oven for 6 min. Then, the product was extracted out in water:toluene mixture (1:2) three times. After the extraction, residual water was removed from the organic phase by addition of the drying agent MgSO4 and the product was collected in a vacuum evaporator. The reusability tests of Pd NPs@chitosan-MWCNT were conducted in the model reaction (the coupling reaction of phenylboronic acid with 4iodoanisole) under the specified optimum conditions.

2.3. Preparation of chitosan-MWCNT beads Chitosan gel solution was prepared by dissolving chitosan flakes (3.0 g) in acetic acid solution (a mixture of 148 mL water and 2 mL acetic acid) and incubating the solution overnight. MWCNTs (1.5 g) were then blended with the chitosan solution and the resulting blend was stirred for 3 h to disperse the CNTs homogeneously. After transferring into a burette, the chitosan-carbon nanotube blend was dropped into alkaline water-methanol solution (200 mL water, 300 mL methanol and 60.0 g sodium hydroxide). The gel beads formed in the coagulation medium was rested in the same solution overnight to ensure a complete gelation of chitosan. The beads were collected by filtration and rinsed with distilled water to remove excess amount of sodium hydroxide. 4

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2

3

4

30

25

20

Ag Pd Cl Fe C O

Pd C l Ag

Si

Fe

15

10

5

0

2

4

6

keV

8

10

12

14

Fig. 5. SEM-EDX spectrum of the Pd NPs@chitosan-MWCNT catalyst.

3. Results and discussion

suitability of the chitosan-MWCNT beads as a support material for various transition metal catalysts. In case of Pd NPs@chitosan-MWCNT catalyst, a decrease in thermal stability was observed and Tmax value of Pd NPs@chitosan-MWCNT catalyst was recorded as 217 °C. This reduction in the thermal stability of the catalyst can be explained by the catalytic activity of palladium ions on the decomposition of the polymeric backbone of Pd NPs@chitosan-MWCNT catalyst beads [50].

3.1. Characterisation of Pd NPs@chitosan-MWCNT catalyst 3.1.1. Analysis of the FT-IR spectra In the spectrum of chitosan-MWCNT beads the vibration band at 1633 cm−1 was the indication of imine bond formation (–C=N–), confirming the condensation reaction of chitosan amino groups with aldehyde groups of the cross-linking agent glutaraldehyde (Fig. 1 a). Also, the bands that are characteristics of chitosan were recorded; 3269 cm−1 (stretching of OH groups), 2923–2852 cm−1 (stretching of C–H groups), 1557 cm−1 (N–H bending) and 1015 cm−1 (stretching of C–O–C). On the other hand, in the spectrum of the Pd NPs@chitosanMWCNT catalyst the bands appeared at relatively lower wavenumbers (Fig. 1 b). The shifting of the bands to lower wavenumbers could be attributed to the interaction of palladium with functional groups of chitosan polymer.

3.1.4. Surface features The surface properties of the synthesized chitosan-MWCNT beads and Pd NPs@chitosan-MWCNT catalyst were examined by SEM analysis and the surface images are depicted in Fig. 4. The images of both chitosan-MWCNT beads and Pd NPs@chitosan-MWCNT catalyst clearly show that they are spherical in shape and confirm that the cross-linked procedure was achieved successfully. Furthermore, the surface features of Pd NPs@chitosan-MWCNT catalyst at high magnifications confirmed the formation of Pd NPs on the surface of chitosan-MWCNT beads (Figs. 4 and 5). Fig. 5 display EDX spectrum of Pd NPs@chitosanMWCNT catalyst. Pd peak is the indication of deposition of palladium particles on the chitosan gel beads (Fig. 5). The EDX analysis results for metallic content were recorded as follows; Pd: 8.98%, Fe: 0.22%, Ag: 0.38% and Si: 0.13%. The metallic contents except for palladium most probably resulted from the impurities of MWCNT or chitosan used to prepare the catalyst support.

3.1.2. XRD patterns In the X-ray diffractogram of chitosan-MWCNT beads (Fig. 2a), the characteristic peaks of chitosan (at 20.07°) and CNT (at 26.08 and 43.22°) were observed [47,48]. In the X-ray diffractogram of Pd NPs@ chitosan-MWCNT catalyst (Fig. 2b) additional peaks were observed at 40.48, 46.90 and 68.13°. These emerged peaks could be corresponded to (1 1 1), (2 0 0) and (2 2 0) of face centred cubic (fcc) structure of Pd, respectively [49]. This result confirms that palladium nanoparticles were successfully formed on chitosan-MWCNT beads.

3.2. Conversion of nitroaromatic compounds into aromatic amines The catalytic performance of Pd NPs@chitosan-MWCNT catalyst was investigated in the catalytic reduction of 4-nitrophenol, 2-nitroaniline, 4-nitro-o-phenylenediamine and 2,4-dinitrophenol at room temperature at the presence of NaBH4 in water. 4-nitrophenol is a toxic compound. Aqueous solution of 4-nitrophenol has a typical absorption band at about 300 nm and upon addition of NaBH4 the absorption band shifts to 400 nm due to the formation of the 4-nitrophenolate ion. In the

3.1.3. Thermal stability Thermal properties of chitosan-MWCNT beads and Pd NPs@chitosan-MWCNT catalyst were examined by TGA analysis (Fig. 3). The maximum temperature (Tmax), at which maximum degradation occurred, was recorded as 285 °C for chitosan-MWCNT beads, demonstrating the thermal stability of the chitosan-MWCNT beads and the 5

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4-nitrophenol 4-nitrophenolate 4-aminophenol

2-nitroaniline o-phenylenediamine

Absorbance

Absorbance

2

1

0

1

0 300

400

500

600

300

Wavelength (nm)

400

500

2

2

2,4-dinitrophenol 2,4-dinitrophenolate 2,4-diaminophenol

Absorbance

4-nitro-o-phenylenediamine 4-amino-o-phenylenediamine

Absorbance

600

Wavelength (nm)

1

0

1

0 300

400

500

600

300

Wavelength (nm)

400

500

600

Wavelength (nm)

Fig. 6. UV–vis spectra of (a) 4-nitrophenol, (b) 2-nitroaniline, (c) 4-nitro-o-phenylenediamine and (d) 2,4-dinitrophenol and the reduction products.

Table 1 Comparison of Pd NPs@chitosan-MWCNT-catalyzed reduction of nitroarenes into aromatic amines to some of reported studies. Nitroaromatic

Catalyst

Time

Reference

4-nitrophenol

Silver/iron oxide nanoparticles Ag(0)@chitosan gel beads Ag NPs on fibrous nano-silica Pd NPs@Sch-boehmite Pd NPs@chitosan-MWCNT Pd NPs@Sch-boehmite Ag(0)@chitosan gel beads Ag NPs on fibrous nano-silica V-doped Bi2(O,S)3 Pd NPs@chitosan-MWCNT Ag(0)@chitosan gel beads Pd NPs@chitosan-MWCNT Ag(0)@chitosan gel beads Pd NPs@chitosan-MWCNT

30 min. 2 min. 8.5 min. 4 min. 12 min. 3 min. < 1 min. 9 min. 2.5 min. 5 min. 1 min. 1.5 min. 1 min. 8 min.

[51] [52] [53] [54] This work [54] [52] [53] [55] This work [53] This work [53] This work

2-nitroaniline

4-nitro-o-phenylenediamine 2,4-dinitrophenol

was achieved. The catalytic activity of Pd NPs@chitosan-MWCNT catalyst was further tested for catalytic reduction of 2-nitroaniline, 4-nitro-o-phenylenediamine and 2,4-dinitrophenol. 2-nitroaniline, 4-nitro-o-phenylenediamine and 2,4-dinitrophenol give a typical band of λmax at 410, 404 and 360 nm respectively. Addition of Pd NPs@chitosan-MWCNT catalyst to the reaction medium led to a decrease in these adsorption bands and the bands disappeared completely within 5, 1.5 and 8 min, respectively (Fig. 6 b, c and d). It was also observed that during all

study, 4-nitrophenolate formation was observed once NaBH4 was added into 4-nitrophenol. With the addition of Pd NPs@chitosan-MWCNT catalyst to the reaction medium, the band at 400 nm gradually decreased and completely disappeared within 12 min. (Fig. 6 a). Colour of the solution, which had been pale yellow, became colourless following the catalytic reduction. In the reduced form, a new band appeared at about 300 nm due to the formation of 4-amino phenol. The results showed that the catalytic reduction of toxic pollutant 4-nitrophenol to 4-aminophenol in the presence of Pd NPs@chitosan-MWCNT catalyst 6

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Table 2 Comparison of Pd NPs@chitosan-MWCNT-catalyzed degradation of dyes to some of reported studies. Nitroaromatic

Catalyst

Time

Reference

Congo red

Bentonite/Cu NPs CeO2/Nylon NCTF photocatalyst DLP-AuNPs SWNT-Ru nanoparticle Keggin-type polyoxometalate catalyst Pd NPs@chitosan-MWCNT CeO2/Nylon NCTF photocatalyst DLP-AuNPs CDs/Znln2S4 microspheres Keggin-type polyoxometalate catalyst Pd NPs@chitosan-MWCNT Bentonite/Cu NPs V-doped Bi2(O,S)3 CoO/NaHSO3 system Pd NPs@chitosan-MWCNT Keggin-type polyoxometalate catalyst NiS-SiO2 and Cr2S3-TiO2 nano-catalyst SiO2 NPs Pd NPs@chitosan-MWCNT

5 min. 2h 10 min. 40 s 30 min. 2 min. 1h 8 min. 40 min. 51 min. 1 min. 40 s 2.5 min. 6 min. Instantly 30 min. 50 min. 120 min. Instantly

[56] [57] [58] [59] [60] This work [57] [58] [61] [60] This work [56] [55] [62] This work [60] [63] [64] This work

Methyl orange

Methylene blue

Methyl red

catalytic reductions of 4-nitrophenol, 2-nitroaniline, 4-nitro-o-phenylenediamine and 2,4-dinitrophenol. A comparison of Pd NPs@chitosanMWCNT-catalyzed reduction of nitroarenes to some of reported studies is presented in Table 1. Considering the conversion time, the performance of Pd NPs@chitosan-MWCNT in reduction of nitroarenes is moderate. 3.3. Degradation of dye molecules To demonstrate the applicability of the designed Pd NPs@chitosanMWCNT catalyst in different catalytic reactions, the catalytic performance of the catalyst was also tested in degradation of different organic dyes i.e., Congo red, Methyl orange, Methylene blue and Methyl red. The degradation reactions were carried out at room temperature and monitored on the UV–vis spectrophotometer. Aqueous solutions of Congo red, Methyl orange, Methylene blue and Methyl red gave maximum absorption at λmax 493, 465, 663 and 500 nm band, respectively. The changes in these absorption bands were recorded after the addition of Pd NPs@chitosan-MWCNT catalyst (Fig. 7). The catalyst was able to degrade Congo red molecules in 2 min, whereas Methyl orange molecules were degraded in only 1 min. The catalytic performance of Pd NPs@chitosan-MWCNT catalyst was excellent in case of Methylene blue and Methyl red. Their degradation occurred instantaneously upon addition of the catalyst. A comparison of Pd NPs@chitosan-MWCNT-catalyzed degradation of dyes to some of reported studies is presented in Table 2. In the presence of Pd NPs@chitosan-MWCNT the degradation of dyes except from Congo red occurred in shorter time, demonstrating the potential use of Pd NPs@chitosan-MWCNT in environmental remediation. An earlier study reported the catalytic degradation of azo dyes including Congo red and Methyl orange in the presence of sodium borohydride [32]. In that study, the analysis of the degradation products of the azo dyes by liquid chromatography-mass spectrometry demonstrated that decolourization of the dye solutions could be attributed to the hydrogenation of azo groups and then to the cleavage of the azo bonds. Considering the earlier reports, the degradation of Congo red, Methyl orange, Methylene blue and Methyl red could be related to the catalytic cleavage of –N=N– groups in the structure of the dyes.

Fig. 7. UV–vis spectra of (a) Congo red, (b) Methyl orange, (c) Methylene blue and (d) Methyl red and the degradation products.

3.4. Green synthesis of biaryls by microwave irradiation catalytic reduction experiments the coloured solutions became clear once catalytic reductions went to completion. All these observations demonstrated that Pd NPs@chitosan-MWCNT catalyst was effective in

The catalytic activity of Pd NPs@chitosan-MWCNT catalyst was also evaluated in C–C bond formation reactions by following “green” synthesis route. Before the catalytic tests, the coupling reaction 7

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Table 3 Activity of Pd NPs@chitosan-MWCNT catalyst in Suzuki–Miyaura coupling reactions of phenylboronic acid with various substituted aryl iodides, bromides and chlorides.

Entry

X

R

Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

I I I I I I Br Br Br Br Br Br Br Br Br Cl Cl Cl

4-OCH3 3-NO2 4-NH2 2-CH3 3-CH3 4-CH3 2-OCH3 3-OCH3 4-OCH3 4-CN 3-NO2 4-NO2 4-NH2 3-CH3 4-CH3 3-NO2 4-CN 4-CH3

99 95 91 72 77 80 82 87 92 98 92 97 84 69 77 80 82 61

Reaction conditions: 1.8 mmol aryl halides, 1.2 mmol phenyl boronic acid, 3.5 mmol K2CO3 and 0.0025 mol% Pd NPs@chitosan-MWCNT catalyst, 6 min, 400 W.

catalyst exhibiting very good activity in Suzuki-Miyaura coupling reactions. A comparison of the performance Pd NPs@chitosan-MWCNT catalyst in the model Suzuki-Miyaura coupling reaction to the Pd catalysts reported in earlier studies is presented in Table 4. Catalysts with high reusability provide a significant advantage for both academic and industrial applications. Thus, the reusability of synthesized Pd NPs@chitosan-MWCNT catalyst was investigated on the model Suzuki-Miyaura coupling reaction. After the typical SuzukiMiyaura coupling reaction, Pd NPs@chitosan-MWCNT were easily recovered by simple filtration and then regenerated by rinsing with distilled water. The washing procedure was repeated at the end of each cycle. Reusability tests showed that Pd NPs@chitosan-MWCNT was catalyst with high stability, still producing 85% yield even after 6 cycles.

Table 4 Comparison of the performance Pd NPs@chitosan-MWCNT catalyst in the model Suzuki-Miyaura coupling reaction to some Pd catalysts reported in the literature. Catalyst

Yield (%)

Time

Reference

Pd@chitosan/starch nanocomposite N-heterocyclic carbene Pd magnetic nanocatalyst PdCl2 and phosphorous containing imidazolium salt PdNRs (5)/SBA-15 Fe3O4/o-PDA–Pd nanoscale system Pd NPs@chitosan-MWCNT

99 > 99

6 min. 0.5 h

[65] [66]

> 99

24 h

[67]

> 99 98 99

90 min. 10 min. 6 min.

[68] [69] This work

between 4 and iodoanisole and phenylboronic acid PhB(OH)2 was studied as a model through which the optimum reaction conditions were determined under microwave heating and organic solvent-free media. In the preliminary studies, the base system K2CO3, reaction time of 6 min and 0.0025 mol% catalyst amount gave the highest reaction yield (99%). Under the specified optimized conditions, the activity of Pd NPs@chitosan-MWCNT catalyst in C–C bond formation was tested in coupling reaction of PhB(OH)2 with various substituted aryl iodides, bromides and chlorides. The results are summarized in Table 3. Pd NPs@chitosan-MWCNT catalyst gave higher reaction yields in reactions of aryl iodides and bromides bearing substituents such as –OCH3, –CH3, –NO2, –CN and –NH2, demonstrating its high functional group tolerance. For example, in the presence of the catalyst the coupling reaction with 4-bromobenzonitrile occurred in 98% yield. In addition, Pd NPs@ chitosan-MWCNT catalyst also showed high activity against para, ortho- and meta-substituted aryl halides. For example, p-bromoanisole, m-bromoanisole and o-bromoanisole produced the desired biaryl compounds by producing 92, 87 and 82% yield, respectively. On the other hand, Pd NPs@chitosan-MWCNT catalyst were able to bind aryl chlorides by giving high reaction yields. This observation is of significance because aryl chlorides usually exhibit low activity in C–C coupling reactions. The coupling reaction between 4-chlorobenzonitrile and PhB(OH)2 was performed by giving 82% yield. The catalytic activity tests demonstrated that the synthesized Pd NPs@chitosan-MWCNT is an ideal

4. Conclusions Palladium-catalysed reduction of toxic nitroaromatic through hydrogenation in the presence of sodium borohydride was achieved using a chitosan-carbon nanotube composite supported Pd(0) particles. The catalyst system also exhibited excellent catalytic activity in degradation of four organic dye pollutants. Apart from the environmental remediation studies, the catalyst showed high activity in “green” synthesis of 18 biaryl compounds through microwave-assisted Suzuki–Miyaura coupling reactions. The designed catalyst was proved to be reusable in the coupling reactions. The Pd NPs@chitosan-MWCNT catalyst can be used in applications including environmental remediation and protection and green synthesis of biaryls through SuzukiMiyaura coupling reactions. However, further studies should test the performance of the catalyst in various applications. Acknowledgment This study was funded by Selcuk University Research Foundation, Konya, Turkey (project number: BAP-19401112). Declaration of Competing Interest None. 8

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