Green synthesis of palladium nanoparticles using ...

Materials Letters 205 (2017) 138–141

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Green synthesis of palladium nanoparticles using fenugreek tea and their catalytic applications in organic reactions Koduru Mallikarjuna a, Chinna Bathula b,⇑, Kezia Buruga c, Nabeen K. Shrestha b, Yong-Young Noh b, Haekyoung Kim a,⇑ a b c

School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712 749, Republic of Korea Department of Energy and Materials Engineering, Dongguk University, Seoul 100-715, Republic of Korea Department of Chemical Engineering, National Institute of Technology, Suratkal 575025, Karnataka, India

a r t i c l e

i n f o

Article history: Received 10 May 2017 Received in revised form 13 June 2017 Accepted 16 June 2017 Available online 17 June 2017 Keywords: Nanoparticles Green catalyst Nitrophenol Suzuki-Miyaura reaction FTIR

a b s t r a c t In this communication, we present the fenugreek tea aided green synthesis of Pd nanoparticles (PdNPs@FT). The synthesized PdNPs were characterized by UV–Visible spectroscopy, scanning electron microscopy (SEM), small area electron diffraction (SAED), fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). FT-IR manifested that polyol and amide groups present in fenugreek may have participated in the synthesis of palladium nanoparticles. PdNPs@FT exhibited high catalytic activity towards hydrogenation of 4-nitrophenol to 4-aminophenol. PdNPs@FT catalyzed Suzuki– Miyaura coupling reaction between bromobenzene and phenyl boronic acid gave desired biphenyl in excellent yield. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Admirable research has been progressed on the utilization of phytochemicals in the field of green nanoscience and nanotechnology [1–3]. Tea extracted from natural sources are basically enriched with proteins, phenols, glycosides, terpenoids, flavonoids, polysaccharides and vitamins. These compounds serve as reducing/stabilizing agents, and therefore, aid in fabrication of nanoparticles [4–7]. Noble and transition metal nanoparticles such as Pt, Pd, Ru, Au, Ag, Fe, Ni, Co and Cu are widely employed as active catalysts in various synthetic and industrial processes. Some of the conventional routes for synthesis of metal nanoparticles include chemical reduction in an aqueous solution or non-aqueous solution, micro-emulsion method, template method, and microwaveassisted synthesis using hazardous reducing agents [8,9]. In harmonization with environment, green synthesis, consisting of environmentally friendly protocols, for metal nanoparticles has emerged out over the last decade [10]. For example, synthesis of palladium nanoparticles (PdNPs) employing biological sources such as leaves, fruits, roots, honey, coffee, tea, and peel extracts has been reported [11–13]. These natural products are expected to play efficient role in reduction of metal ions, and formation of ⇑ Corresponding authors. E-mail addresses: [email protected] (C. Bathula), [email protected] (H. Kim). http://dx.doi.org/10.1016/j.matlet.2017.06.081 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.

corresponding nanoparticles [14–17]. Fenugreek is an economically significant medicinal seed, as it contains very high concentrations of the proteins, vitamins and dietary fibers [18]. However, it has not been employed yet for reduction of Pd ions. Herein, we report the synthesis of palladium nanoparticles (PdNPs@FT) by using tea of fenugreek seeds as green reducing agent. As an example of the functional properties, the obtained PdNPs@FT were investigated as catalyst for hydrogenation of 4nitrophenol, which manifested highly catalytic activity. In addition, PdNPs@FT are utilized as a catalyst in Suzuki-Miyaura coupling reaction as an alternative to traditionally used Pd complexes to obtain biphenyl in excellent yield. 2. Experimental procedure PdNPs were prepared by stirring 30 ml of 0.01 M PdCl2 with 3 ml of aqueous extract at room temperature. The reaction mixture gradually turned into a black indicating the formation of PdNPs@FT. The progress of the reaction was monitored via UV– Vis absorption spectroscopy. For the study of catalytic performance of PdNPs@FT on 4-nitrophenol reduction, aqueous solution of 5 ml of 10 mM NaBH4 and 50 ml of 10 mM 4-nitrophenol was added in a quartz cuvette, followed by addition of a 20 ml aliquot of PdNPs (1 mg/mL in DI water). The time-dependent UV/Vis absorption spectra were measured at room temperature. Further, to demon-

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3. Result and discussion 3.1. Formation of Pd nanoparticles The formation of PdNPs@FT was studied by UV–Vis absorption spectroscopy Fig. 1. The reaction media showed a transition in color from dark yellow to black, which indicates the bioreduction of Pd ions and formation of PdNPs@FT. The completion of reaction is confirmed by the disappearance of d-d transition band of Pd2+ (i.e., PdCl2) exhibited by the reaction mixture at about 480 nm, which suggests the reduction of Pd2+ (d8 electron configuration) to Pd0 (d10 electron configuration).

3.2. Morphology and crystallinity of nanoparticles

Fig. 1. (a) UV–Vis absorption spectroscopy of during synthesis of PdNPs@FT. The spectrum of PdNPs@FT (PdCl2 + Fenugreek seed extract) was recorded after 3 h of reaction.

strate the catalytic application of PdNPs@FT in Suzuki–Miyaura coupling reaction, bromobenzene, benzeneboronic acid, PEG-400 and K2CO3 in water were mixed with PdNPs@FT catalyst in a round bottomed flask, and the mixture was refluxed for 6 h. After completion of reaction, the filtrate containing the product was evaporated to obtain desired biphenyl in excellent yield (96%). The 1H NMR data of the recrystallized product were indistinguishable to those reported in the literature [19].

FE-SEM images of the synthesized biogenic PdNPs@FT are presented in Fig. 2(a) and (b). These images illustrate spherical nanoparticles with diameter ranging 20 to 50 nm. We also examined particles carefully at the edges by lowering the accelerator voltage. In all cases, the brighter edges of the PdNPs@FT was observed. The observed phenomenon of color contrast between edge and central part of the PdNPs@FT indicates the capping of PdNPs by phyto-chemical moieties. The capping was further confirmed using FT-IR analysis, which will be discussed in later part. The formation of PdNPs by bioreduction of Pd ions and their crystal structure was confirmed using powder X-ray diffraction (XRD) technique. The XRD pattern of the PdNPs@FT is displayed in Fig. 2(c), showing the peaks at 2-Theta of 40.2°, 46.6°, 68.6°, 81.8° and 86.8°, which correspond to the diffraction from (1 1 1), (2 0 0), (2 2 0) (3 1 1) and (2 2 2) planes, respectively. Based on the XRD patterns (JCPDS 05-0681 PDF file), PdNPs can be indexed to

Fig. 2. (a) and (b) Different magnification of SEM images of PdNPs@FT, (c) XRD patterns of PdNPs@FT, and (d) SAED pattern of PdNPs@FT.

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Fig. 3. (a) FTIR spectrum of PdNPs@FT, and (b) FTIR spectrum of Fenugreek seed extract.

(b) OH Br

+

B

PEG/ PdNPs@F T

OH Bromobenzen e

P henylboron ic acid

1,1' -b ip henyl

Fig. 4. (a) UV–Vis spectra of catalytic kinetics of 4-nitrophenol to 4-aminophenol in presence of NaBH4 by PdNPs@FT (Inset is the scheme for reduction reaction). (b) Schematic presentation of Suzuki cross coupling reaction between bromobenzene and phenyl boronic acid catalyzed by PdNPs@FT.

tions of amides and polyphenol groups. Peaks at 2922 and 2848 cm 1 represents asymmetric and symmetric stretching C–H vibrations. The band at 2348 cm 1 is due to the coordination of C-N groups of proteins with the metal atoms at the surface of the PdNPs@FT. The peaks between 1738–1170 cm 1 arises from the N–H bending, C@O and C–OH stretching vibrations of amides, carboxylic groups of protein/polysaccharide in the fenugreek tea [22]. This FT-IR spectrum looks more or less similar to that of the tea extract (Fig. 3b), suggesting the encapsulation of PdNPs by phytochemical moieties of the tea extract. This encapsulation probably provided excellent stabilizing against agglomeration of PdNPs. Catalytic behavior of the PdNPs@FT was investigated through reduction of 4- nitrophenol to 4-aminophenol in the presence of NaBH4. The kinetics of reaction was monitored using UV–Vis absorption spectroscopy. When NaBH4 was added to 4nitrophenol, the solution turned yellow and produced an absorption peak at 400 nm in the UV–Vis spectra (Fig. 4a), suggesting the formation of 4-nitrophenolate anion. The addition of nanoparticles demonstrated a noticeable decrease in absorption peak at 400 nm. Meanwhile, instantaneous appearance of a reduction product (4-aminophenol) was monitored by the emergence of two new peaks at 300 nm and 230 nm [22,23]. The results revealed that the PdNPs@FT successfully catalyzed the reduction reaction completely within 5 min. A continuous decrease in the peak intensity at 400 nm illustrates the utilization of 4-nitrophenol, and the progress of the reaction. However, no proportional increase in the aminophenol peak intensity is observed, which is mainly due to the difference in molar extinction coefficients of 4-nitrophenol and 4-aminophenol. The catalysis by the PdNPs@FT is probably as a result of hydrogen transport shuttle between the NaBH4 and 4-nitrophenol. In this shuttling, PdNPs@FT adsorbs hydrogen from the NaBH4, and release it efficiently during the reduction reaction. On the other hand, Suzuki–Miyaura coupling reaction is most widely used protocol in most challenging C–C bond formation reaction, and has significance in organic synthesis. Traditionally, Pd(PPh3)4 is used as catalyst in this coupling reaction. However, owing to the high cost and complex procedure in catalysis, PdNPs obtained from various techniques have been explored as an alternative (see Table S1 in SI section). In this work, the coupling reaction between bromobenzene and phenyl boronic acid was catalyzed by the PdNPs@FT (Fig. 4b), which yielded desired biphenyl in excellent yield. The catalytic performance is highly competitive to the previously reported PdNPs, and found even better in some cases (Table S1). 4. Conclusion

the face centered cubic (fcc). The comparatively highly intense diffraction peak from (1 1 1) planes suggests that the biogenic PdNPs@FT inclined to develop into particles with planes bound by the lowest energy (1 1 1) lattice plane. This growth indicates that crystals grown on the (1 1 1) planes were bottled-up by the selective adsorption of bioorganic moieties onto the (1 1 1) facet [20,21]. Further, SAED pattern (Fig. 2(d)) shows the characteristic diffraction ring patterns, which are in good agreement with the XRD patterns.

In summary, we have successfully demonstrated the environmental friendly synthetic route of PdNPs@FT as a green catalyst. Phyto-chemical moieties within fenugreek tea not only contributes to effective reduction of Pd(II) to PdNPs, but also aids in capping in order to provide excellent stabilizing against agglomeration. The PdNPs@FT grew with their surfaces inclined by the lowest energy (111) facets. PdNPs@FT displayed high catalytic activity in nitrophenol reduction, and in Suzuki-Miyaura coupling reactions. The green synthetic route presented here to synthesize greener catalysts can be beneficial in food or pharmaceutical industries. Further exploration of PdNPs@FT in organic synthesis is currently under progress.

3.3. Surface properties and catalytic performance

Appendix A. Supplementary data

FT-IR spectroscopy is used to elucidate the role of fenugreek tea in the surface adsorbed biomolecules on PdNPs@FT (Fig. 3a). Peak at 3418 cm 1 is attributed to N–H stretching, H bonded O–H vibra-

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.06. 081.


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