- Email: [email protected]
Green synthesis of palladium nanoparticles mediated by black tea leaves (Camellia sinensis) extract: Catalytic activity in the reduction of 4-nitrophenol and Suzuki-Miyaura coupling reaction under ligand-free conditions
Journal of Colloid and Interface Science 485 (2017) 223–231
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Green synthesis of palladium nanoparticles mediated by black tea leaves (Camellia sinensis) extract: Catalytic activity in the reduction of 4-nitrophenol and Suzuki-Miyaura coupling reaction under ligand-free conditions Sadaf Lebaschi a, Malak Hekmati a, Hojat Veisi b,⇑ a b
Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Chemistry, Pharmaceutical Sciences Branch, Islamic Azad University (IAUPS), Tehran, Iran Department of Chemistry, Payame Noor University, Tehran, Iran
g r a p h i c a l a b s t r a c t
PdCl2, H2O NH2 B(OH)2 HO
+ R
X EtOH, H2O K2CO3
Pd NPs
Pd NPs
NaBH4, H2O Poly Phenols
Poly Phenols
NO2
R HO
a r t i c l e
i n f o
Article history: Received 8 July 2016 Revised 11 September 2016 Accepted 13 September 2016 Available online 15 September 2016 Keywords: Green chemistry Black tea leaves (Camellia sinensis) Palladium nanoparticles Suzuki 4-Nitrophenol
a b s t r a c t The present study was conducted to synthesize palladium nanoparticles (Pd NPs) through a facile and green route using non-toxic and renewable natural black tea leaves (Camellia sinensis) extract, as the reducing and stabilizing agent. The as-prepared [email protected] NPs catalyst was characterized by UV–vis spectroscopy, X-ray diffraction (XRD), fourier transformed infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The [email protected] NPs catalyst could be used as an efficient and heterogeneous catalyst for Suzuki coupling reactions between phenylboronic acid and a range of aryl halides (X = I, Br, Cl) and also the reduction of 4-nitrophenol (4-NP) using sodium borohydride in an environmental friendly medium. Excellent yields of products were obtained with a wide range of substrates and the catalyst was recycled 7 times without any significant loss of its catalytic activity. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (H. Veisi). http://dx.doi.org/10.1016/j.jcis.2016.09.027 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
The reaction of aryl halides and aryl boronic acids results in formation of wide variety of unsymmetrical biaryls is Suzuki-Miyaura [1]. Biaryls which are also known as the fourth state of matter are
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significant groups used in liquid crystal materials [2a,2b]. Moreover, there have been many successful examples of total synthesis of biological and medicinal molecules through the approach of constructing new carbon-carbon bonds [2c,2d]. Such molecules generally can be synthesized by CAC bond formation reaction of Suzuki. However, there is still a significant challenge as this reaction suffers from some limitations and drawbacks. One of these is the necessity of inert atmosphere [2e], because the moisture may oxidize the catalyst or its reactive intermediates in the procedure. Furthermore, the ligands might be sensible to air and the existence of air which may diminish the efficiency [3]. Therefore, progress of Suzuki reaction in aqueous media and air condition is of remarkable importance. Biosynthesis of metal nanoparticles using plant materials is currently under exploration. It has received more attention as a suitable alternative to chemical and physical methods [4]. The development of new, efficient, and practical catalysts for organic transformation to synthesize valuable target molecules is an important research area in the pharmaceutical and chemical industries [5,6]. Although various methodologies have been developed for the synthesis of palladium nanoparticles, including chemical and electrochemical reduction [7,8], ion exchange [9], vapor deposition, thermal decomposition [10,11] and polyol method [12], preparation of metal nanoparticles is usually based on the reduction of a metal salt in the presence of a reducing agent such as sodium borohydride, hydrazine, dimethyl formamide and hydrogen, and a stabilizer like polymeric materials, dendrimers, surfactants, organic ligands and polyoxometalates [13,14]. It is important to note that bioinspired, eco-friendly greener methods for the synthesis of metal nanoparticles are among the most attractive aspects of today’s nanoscience and nanotechnology [15,16]. There are few reports available for the synthesis of palladium nanoparticles that effectively utilize Diopyros kaki. leaf [17], Cinnamom zeylanicum bark [18], C. Camphora leaf [19], Curcuma longa tuber [20], banana peel [21], Hippophaerhamnoides Linn [22], Pistacia atlantica kurdica gum [23], Rosa canina fruit [24], pectin [25], Stachys lavandulifolia [26], and Oak gum [27]. The black tea which grows in many parts of world, is used as an herbal tea and Drinking. Camellia sinensis leaves (black tea) have been reported to contain considerable amounts of tannin products [28]. This is motivated us to explore the possible bio-reduction of palladium ions into nanoparticles. Hence, the present work deals with biological (green) synthesis of palladium nanoparticles by black tea extract at ambient conditions. The bioreduction process was monitored by the UV–Visible, XRD, TEM, FESEM, WDX and EDX. Also, the catalytic activity of [email protected] NPs for the Suzuki-Miyaura coupling reaction was studied.
(Hitachi, U-2900) to ensure the formation of nanoparticles. Morphology and particle dispersion was investigated by field emission scanning electron microscopy (FE-SEM) (Cam scan MV2300). The chemical composition of the prepared nanostructures was measured by EDS (Energy Dispersive X-ray Spectroscopy) performed in SEM. TEM images were obtained using a Philips-EM-2085 transmission electron microscope with an accelerating voltage of 100.0 kV. 2.2. Green synthesis of palladium nanoparticles using black tea leaves (Camellia sinensis) Lahijan black tea leaves (Camellia sinensis) were supplied from the Lahijan Tea Research Center, Lahijan, Iran. 10 g of the black tea was added to 100 mL of deionized water and was boiled for 5 min in a water bath. The mixture was then cooled down and was filtered through Whatman filter paper No. 1 to obtain aqueous extract. The filtered extract was stored in refrigerator at 4 °C for further use. The extract was used as reducing as well as stabilizing agent. For preparation of Pd NPs, 10 mL of the prepared plant extract was added drop wise to 100 mL of 1 mM aqueous PdCl2 solution and refluxed at 100 °C for 1 h. The color of the reaction mixture gradually turned over 60 min and indicated the formation of Pd NPs, to which acetone was added to precipitate the catalyst ([email protected] NPs). After addition of acetone (anti-solvent) the precipitated catalyst was then centrifuged at 1000 rpm for 10 min followed by re-suspension of the pellet in Milli-Q water. The Pd loading of the prepared catalyst was measured to be 1.7 mmol/g by ICP and EDX. 2.3. Suzuki-Miyaura coupling reaction In a typical experiment, to a mixture of aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol), and K2CO3 (2.0 mmol) in (4.0 mL, 1:1) water-ethanol, 6.0 mg of the [email protected] NPs catalyst (containing 0.1 mol% of Pd) was added and heated at 60 °C. The progress of the reaction was followed by thin layer chromatography (TLC). After completion of the reaction, the reaction mixture was cooled down to room temperature and (10 mL) EtOAc was added and the catalyst was separated from the reaction mixture by centrifuge and extracted with ethyl acetate (10 mL 3). The combined organic layer was dried over anhydrous sodium sulfate and evaporated in a rotary evaporator under reduced pressure. The product was purified by column chromatography (hexane-ethylacetate, 1:5) to obtain the desired purity. All of the products are known and were identified by comparison of their physical and spectral data with those of authentic samples.
2. Experimental section
2.4. General procedure for the reduction of 4-nitrophenol
2.1. Materials
In a typical experiment, 2.0 mg of the [email protected] NPs catalyst was added to an aqueous solution that contained 4-nitrophenol (2.5 mM, 25 mL), freshly prepared aqueous NaBH4 solution (250 mM, 25 mL) and stirred for 80 s at room temperature. The rate of 4-nitrophenol reduction was evaluated using UV–vis spectroscopy at room temperature. After completion of the reaction, the catalyst was separated from the reaction mixture by centrifugation, washed with doubly distilled water and then dried for the next cycle.
High-purity chemical reagents were purchased from the Merck and Aldrich chemical companies. All materials were of commercial reagent grade. Melting points were determined in open capillaries using a BUCHI 510 melting point apparatus and are uncorrected. 1 H NMR spectra were recorded on a Bruker Avance DRX spectrometer at 400 MHz. FT-IR spectra were recorded on a Bruker Tensor 27 spectrometer (Bruker, Karlsrohe, Germany) using pressed KBr pellets. Lahijan black tea leaves (Camellia sinensis) were supplied from the Lahijan Tea Research Center, Lahijan, Iran. X-ray diffraction (XRD) measurements were carried out using a Philipspowder diffractometer type PW 1373 goniometer (Cu Ka = 1.5406 Å). The scanning rate was in the 2h range of 10–80°. UV–visible spectral analysis was recorded on a double-beam spectrophotometer
3. Results and discussion Biological methods for the synthesis of Pd nanoparticles are gaining importance in the field of nanoparticle synthesis. As a result of the growing success and simple processes for the
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Fig. 1. UV–vis spectra of (a) black tea leaves extract; (b) black tea leaves extract with PdCl2; and (c) [email protected] NPs.
formation of nanoparticles, the biosynthesis of Pd nanoparticles was attempted by reducing Pd2+ in PdCl2 solution using the water extracts of the green tea (Fig. 1, inset). The main cause of reduction of metal to metal nanoparticle is the presence various types of phytochemicals such as flavonoids, phenolics, polysaccharides, and terpenoids in the extracts of different plant and their parts. The preliminary confirmation of the reduction of metallic Pd to Pd nanoparticle was visually confirmed by a change in color of the reaction solution (Fig. 1, inset) to black after 2 h of incubation. Because of the excitation of surface plasmon resonance, the color change in the reaction medium indicated the formation of Pd nanoparticles. In addition to the optical observation, the UV–vis spectra of the reaction mixture was recorded and indicated a significant change with the disappearance of the peak around 400 nm, which showed the conversion of Pd(II) to Pd(0) (Fig. 1). This also proved the large reduction capability of the extract. Moreover, the UV spectrum of the extract shows bonds at kmax 260 nm is for absorbance of ring related to the benzoyl system. They are related to the p ? p transitions and these absorbent bonds demonstrate the presence of polyphenolics in the category of flavonoids that are responsible for the palladium reduction. FT-IR spectroscopy was used to identify the presence of different potential biomolecules in the plant extract which are responsible for reduction of palladium ions and formation of the corresponding Pd NPs, and also capping them by the extract. Fig. 2 shows the compared FT-IR spectrum of black tea leaves extract and green synthesized Pd NPs. The wide peak observed at 3100–3500 cm1, in the spectra of black tea leaves extract and Pd NPs corresponds to free OH in molecule and OH group forming hydrogen bonds of macromolecular association. The band at 3435 cm1 shifted to 3380 cm1 in the presence of Pd NPs, which show the interaction of Pd with the OH group of black tea leaves extract [29–31]. The absorptions shown in Fig. 2a indicate the presence of polyols, carbonyls, and C@C bonds on terpenoids and flavonoids. The IR spectrum of synthesized [email protected] NPs (Fig. 2b) revealed absorption peaks at 3380, 2940, 1708, 1616,1453, 1367, 1240, 1147, and 1087 cm1. Thereby, on the basis of FT-IR data, we could infer that the phenolic hydroxyl groups of flavones, terpenoids, and polysaccharides belonging to the phytomolecules of the black tea played a vital role in reduction of Pd(II) ions and also have strong ability to bind with Pd NPs.
Fig. 2. IR of (a) black tea leaves extract; and (b) [email protected] NPs.
The morphological characteristics of the synthesized [email protected] NPs catalyst were observed under FESEM (Fig. 3). The nanoparticles were formed and the shape was somewhat spherical in nature. Furthermore, the Pd NPs might also have been stabilized due to the interactions such as hydrogen bond and electrostatic interactions between the bioorganic capping molecules that have bound to the Pd NPs. EDX analysis confirmed the elemental composition of the synthesized nanoparticles (Fig. 4). The strong peak at 3 keV indicated the presence of the elemental Pd nanoparticles as evident from previous observations. Apart from Ag, other existing elements revealed by the EDX analysis included carbon, oxygen, and nitrogen which confirmed that the organic metabolites present in the black tea leaves extract were responsible for reducing, capping and stabilization of the Pd nanoparticles. The crystalline nature of green synthesized [email protected] NPs catalyst was also determined by XRD spectroscopy (Fig. 5). In the diffractogram, there are four distinct reflections at 39.7° (1 1 1), 46° (2 0 0), 68° (2 2 0), 82° (3 1 1), and 86° (2 2 2), which according to crystallographic planes the face centered cubic (fcc) crystalline structure was indexed for metallic palladium (Pd0). Particles morphology, size, and crystallinity were studied by TEM. Fig. 6 shows TEM images of typical nanoparticles synthesized
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Fig. 3. SEM images of [email protected] NPs catalyst.
from black tea leaves extract. Particle sizes ranged from 5 to 8 nm (average 7 nm) and most were near spherical. Smaller nanoparticles agglomerated to produce non-spherical, slightly larger particles. It was interesting to note that the dispersed colloidal Pd NPs were surrounded by bio-polymers layer, which appeared to be responsible for reducing and stabilizing the nanoparticles. TGA/DTA experiments were performed at a heating rate of 10 °C min1. The spectrum obtained showed significant weight loss of 74% from Pd NPs due to desorption of biomaterials when heated from 20 °C to 700 °C. This result indicated that bioactive molecules were capped on the Pd NPs and were completely degraded under high temperature (Fig. 7).
3.1. Evaluation the catalytic activity of [email protected] NPs through the Suzuki coupling reaction
Fig. 4. The EDS spectrum of [email protected] NPs catalyst.
Fig. 5. XRD pattern of [email protected] NPs catalyst.
In the next step, we tested the catalytic activity of the [email protected] NPs for the Suzuki-Miyaura coupling reaction (Scheme 1). We initially selected 4-methylbromobenzene and phenylboronic acid as a model reaction to evaluate the effects of solvent (nonpolar, protic and aprotic), base (Et3N, NaOAc and K2CO3) and amount of catalyst at different temperature. Optimization conditions studies are summarized in Table 1. As expected, no target product could be detected in the absence of the catalyst (Table 1, entry 11). However, addition of the catalyst to the mixture has rapidly increased the synthesis of product in high yields. The reactions were conducted using H2O/EtOH (1:1) as the best solvent. Among the bases evaluated, K2CO3 was found to be the most effective. The effect of catalyst loading was investigated employing several quantities of the catalyst ranging from 0.05 mol% to 0.2 mol% (Table 1, entries 6, 7 and 8). The best yield was obtained with 0.006 g (0.1 mol%) of the catalyst (Table 1, entry 8). Several coupling reactions of various aryl halides (I, Br and Cl) with phenyl boronic acid were evaluated using 0.1 mol% of Pd@B. tea NPs catalyst under the optimized conditions and the results presented in Table 2. Phenyl iodides, bromides and chlorides all reacted efficiently with phenylboronic acid (Table 2, entries 1–15). Aryl halides with electron-withdrawing or releasing groups reacted with phenylboronic acid to afford the corresponding products in high yields. As shown in Table 2, steric hindrance of the substituent did not influence the product yield in SuzukiMiyaura reaction of deactivated arylhalides using [email protected] NPs catalyst (Table 2, entries 15). Notably, heteroaryl halides such as 2-bromothiophene and 2-iodothiophene with phenylboronic acid gave the corresponding coupled products in 96% and 90% yields, respectively (Table 2, entries 17 and 18). The coupling reaction of aryl chlorides with phenyl boronic acid required extended reaction time than aryl iodides and bromides, producing the desired products in moderate yield (Table 2, entries 3, 6, 9).
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Fig. 6. TEM images of dispersed colloidal Pd NPs from [email protected] NPs catalyst.
Fig. 7. TGA/DTA analysis of [email protected] NPs catalyst.
After completion of the reaction, catalyst was separated by centrifugation and the recycled catalyst was saved for the next reaction. To investigate the leaching of Pd nanoparticles, the samples of the filtrate plus washings obtained from the above standard reactions were subjected to ICP analysis that showed the concentrations of Pd were less than 3.0 ppm in each measurement. In addition, the standard reaction was conducted with the recovered filtrate, but no product could be isolated. The results suggested that the palladium catalyst remained on the support at elevated temperatures during the reaction and the reaction proceeds on the heterogeneous surface. Based on these evidences, a reaction mechanism for Suzuki coupling using the prepared nanocatalyst was proposed (Scheme 2).
3.2. Reusability the [email protected] NPs catalyst in the Suzuki coupling The reusability of the catalysts is one of their most important advantages, which makes them useful for commercial applications. We studied the reusability of these heterogeneous catalysts in the Suzuki-Miyaura coupling reaction under same conditions as Table 1
(Fig. 8). After completion of the reaction, catalyst was separated by centrifugation from the reaction mixture and washed several times with deionized water and ethanol. Then, it was and dried in an oven at 50 °C and the recycled catalyst was saved for the next reaction. The dried recovered catalyst was successively used for five fresh runs with no significant loss of activity. This reusability demonstrates the high stability and turnover of catalyst under operating conditions.
3.3. Evaluation the catalytic activity of the [email protected] NPs through the reduction of 4-NP In continuation of our works, the catalytic activity of the Pd@B. tea NPs catalyst was examined for reduction of 4-nitrophenol (4NP) in water at room temperature. The progress of the reaction was monitored by recording the absorption spectra as a function of time. Initially, the reaction conditions were optimized for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of different amounts of catalyst (Table 3). The amount of NaBH4, as one of the factors influencing the reduction of 4-NP
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R
[email protected] NPs
+ X
B(OH)2
EtOH, H2O K2CO3
R
Scheme 1. Suzuki coupling reaction in the presence of [email protected] NPs.
Table 1 Optimization of reaction conditions in Suzuki-Miyaura coupling reaction of 4-methylbromobenzene with phenylboronic acid.a
a b c
Entry
Pd (mol%)
Solvent
Base
T (°C)
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11
0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.1 0.2 0.1 0.0
DMF Toluene EtOH H2O EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc
K2CO3 K2CO3 K2CO3 K2CO3 NaOAc Et3N K2CO3 K2CO3 K2CO3 No base K2CO3
80 60 60 90 60 90 60 60 70 90 60
80 65 77 55 65 80 75 98 96 Trace 0.0
Reaction conditions: 4-methylbromobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), catalyst, base (2 mmol) and solvent (4 mL). Isolated yield. (1:1).
Table 2 Suzuki–Miyaura coupling reaction of different aryl halides with phenylboronic acids.a Entry
RC6H4X
R2C6H4B(OH)2
X
Time (h)
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
H H H 4-CH3 4-CH3 4-CH3 4-COCH3 4-COCH3 4-COCH3 4-CH3O 4-CH3O 4-Cl 3-NO2 3-NO2 2-CHO 1-Naphthyl 2-Thienyl 2-Thienyl 4-COCH3 H 4-COCH3 H
H H H H H H H H H H H H H H H H H H 4-NO2 4-NO2 4-CH3 4-CH3
I Br Cl I Br Cl I Br Cl I Br Br I Br Br I I Br Br Br Br Br
0.16 0.66 12 0.20 1 12 0.3 0.8 12 0.3 1.5 1 0.5 3 5 2 1 5 3 3 1.5 1
98 98 70 98 98 75 98 96 65 98 96 96 96 88 70 96 96 90 80 85 92 92
a Reactions were carried out under aerobic conditions in 4 mL of H2O/EtOH (1:1), 1.0 mmol arylhalide,1.1 mmol phenylboronic acid and 2 mmol K2CO3 in the presence of catalyst (0.006 g, 0.1 mol% Pd) at 60 °C. b Isolated yield.
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Scheme 2. Possible mechanism of Suzuki reaction in the presence of [email protected] NPs.
NO2
NO2 NaBH4
NH2 [email protected] NPs
OH
O
OH
Fig. 9. The reduction of 4-NP in the presence of NaBH4 and catalyst.
Fig. 8. Reusability of the catalyst for Suzuki-Miyaura coupling reaction.
into 4-AP, was also investigated. As expected, no target product could be detected in the absence of catalyst. It can be seen that the rate of reduction of 4-NP is increased on increasing the molar ratio of NaBH4 to 4-NP. Nevertheless, the best results were achieved in the presence of 2.0 mg of [email protected] NPs and 100 equivalents of NaBH4 (250 mM). The reduction reaction in the presence of extract as catalyst and in the absence of Pd NPs, was also performed, resulting without product and in 96% conversion respectively. Compared with the Pd NPs, the [email protected] NPs showed apparently higher catalytic activity, which could be attributed to
the effect of increased surface area, smaller size and the well dispersity of the Pd NPs fabricated on the surface of biopolymers. The enhanced catalytic activity observed with our nanocatalyst is attributed to the plenty of active functional groups, such as C@C and AOH that covering the Pd NPs which interact and stabilizes the 4-NP substrate adjacent to the catalytic sites which in turn facilitates the reduction of nitro groups. On the other hand, the covering shell had good adsorption ability to the water-soluble 4-NP, which could accelerate the process of catalytic reduction. It suggested that the extract may play an active part in the catalysis, yielding a synergistic effect. The catalytic mechanism for the conversion of 4-NP into 4-AP relies on the electrons transfer from the BH 4 donor to the acceptor 4-NP through adsorption of the
Table 3 Reduction of 4-NP at different conditions. [4-NP] (mM)
[NaBH4] (mM)
Catalyst (mg)
Time (min)
Conversion (%)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
250 250 250 250 250 250 187.5 125
– Extract (10) Pd NPs (2) [email protected] NPs [email protected] NPs [email protected] NPs [email protected] NPs [email protected] NPs
30:00 30:00 4:00 1:20 10:00 1:20 4:30 7:20
0.0 0.0 96 100 98 100 100 100
catalyst catalyst catalyst catalyst catalyst
(2) (1) (3) (2) (2)
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Fig. 10. UV-visible spectra for catalytic reduction of 4-NP to 4-AP at several interval and the plots of ln(A/A0 ) vs. irradiation time for reduction reaction of 4-NP. Conditions: [4NP] = 2.5 103 M, [NaBH4] = 0.25 M, [email protected] NPs = 2.0 mg.
reactant molecules on to the extract surface. The [email protected] NPs can serve as catalyst to transfer electrons from BH 4 to the 4-NP, which are both absorbed on the catalysts via p-p stacking interactions, leading to the production of amino derivatives. In the present work, when the [email protected] NPs was added to a mixed solution of 4-nitrophenol (oxidant) and NaBH4 (reductant), 4-nitrophenolate ions (Fig. 9) and BH 4 were first adsorbed on the surface of the catalyst via physical adsorption and hydrogen bonding. After electron transfer (ET) to the Pd NPs, the hydrogen atom forms from the hydride, and then attacks 4-nitrophenolate ions to reduce it. This ET-induced hydrogenation of 4-NP occurred spontaneously at the surface of the metal catalyst. Finally, the generated 4-AP was desorbed from the surface of the catalyst. It is observed that 4-NP in aqueous medium has a maximum absorption at 317 nm [29]. However, when freshly prepared NaBH4 solution is added, there was a red shift from 317 nm to 400 nm, and the light yellow color of the solution changes to intense yellow, due to the formation of 4-nitrophenolate ions in alkaline condition. The peak at 400 nm remains unchanged even for a couple of days in the absence of any catalyst. As shown in Fig. 10, after the catalyst was added into the solution containing 4-NP and NaBH4, the intensity of the strong absorption peak at 400 nm gradually decreased and a new peak appeared at about 300 nm which corresponded to the formation of 4-AP. After about 80 s in case of [email protected] NPs, the whole peak at 400 nm almost disappeared and the color became transparent, which indicated that 4-NP was almost turned to 4-AP. In the reduction process, the overall concentration of NaBH4 was 0.25 M and 4-NP was 2.5 103 M. Considering much higher concentration of NaBH4 compared to 4-NP, the pseudo-first-order kinetics has been applied with respect to 4-NP to determine the catalytic activity of [email protected] NPs. The absorbance of 4-NP is proportional to its concentration in solution; the absorbance at time t (A) and time t = 0 (A0 ) are equivalent to the concentration at time t (C) and time t = 0 (C0 ). The rate constant (k) has been determined from the linear plot of ln (A/A0 ) versus reduction time in seconds, and the constant was estimated to be 5.9 104 s1. 3.4. Reusability the [email protected] NPs in the reduction of 4-NP For the application as a practical catalyst, the catalytic stability and the reuse of the catalyst are very important. The recovery and reusability of the [email protected] NPs was investigated in the reduction of 4-NP to 4-AP under the optimized conditions. After completion of the reaction, the catalyst was separated from the reaction mixture by centrifugation, washed three times with 5 mL of acetone
Fig. 11. Reusability of the catalyst for reduction of 4-NP.
and then with doubly distilled water, dried in an oven at 100 °C for 3 h and the recycled catalyst was saved for the next reaction. It is found that catalyst does not lose its catalytic activity during at least nine catalytic cycles in the reduction of 4-NP with NaBH4 (Fig. 11).
4. Conclusions In summary, a novel [email protected] NPs catalyst via the reduction of aqueous Pd2+ ions using bioactive black tea leaves extract was fabricated. The polyols and carbonyl groups present in the aqueous extract act as both reducing and capping/stabilizing agents [12,30–32]. Synthesized [email protected] NPs were characterized by UV– vis, FT-IR, FESEM, EDX, XRD, TGA and TEM analysis. In addition, the synthesized Pd NPs capped by biomolecules showed potent catalytic application for the synthesis of biaryls by Suzuki crosscoupling reaction and also reduction of 4-nitrophenol (4-NP) by NaBH4. This simple synthetic method has the advantages of high yields, elimination of homogeneous catalyst and expensive, unstable and poisonous ligands, simple methodology, easy preparation and handling of the catalyst and also easy work up. It is also observed that the catalyst was recycled several times without any significant loss of catalytic activity. The described strategy for Pd NPs is straightforward, robust, environmentally friendly, and cost-effective, and may find widespread use in palladiumcatalyzed reactions for the preparation of organic compounds. The enhanced catalytic activity observed with our nanocatalyst is attributed to the plenty of active functional groups, such as C@C and AOH that covering the Pd NPs which interact and stabilizes
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the 4-NP substrate adjacent to the catalytic sites which in turn facilitates the reduction of nitro groups. On the other hand, the covering shell had good adsorption ability to the water-soluble 4-NP, which could accelerate the process of catalytic reduction. It suggested that the extract may play an active part in the catalysis, yielding a synergistic effect. The extracted phytochemicals from the black tea leaves showed a colloidal media to interaction with metallic salts to produce the nanostructure. The result showed that the green approach to synthesizing Pd NPs is useful for removing toxic pollutants and dyes such as nitro aromatics from the environment. References [1] (a) N. Miyaura, T. Yanagi, A. Suzuki, Syn. Commun. 11 (1981) 513; (b) N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457; (c) A. Suzuki, in: D. Astruc (Ed.), Modern Arene Chemistry, Wiley-VCH, Weinheim, 2002, p. 53; (d) F. Lu, J. Ruiz, D. Astruc, Tetrahedron Lett. 45 (2004) 9443. [2] (a) Z.G. Zhoua, J.C. Shib, Q.S. Hua, Y.R. Xiea, Z.Y. Dua, S.Y. Zhanga, Appl. Organometal. Chem. 25 (2011) 616; (b) J.P. Wolfe, S.L. Buchwald, Angew. Chem. Int. Ed. 38 (1999) 2413; (c) A.F. Littke, G.C. Fu, Angew. Chem., Int. Ed. 41 (2002) 4176; (d) K.C. Nicolaou, P.G. Bulger, D. Sarlah, Angew. Chem., Int. Ed. 44 (2005) 4442; (e) C. Ornelas, J. Ruiz, L. Salmon, D. Astruc, Adv. Synth. Catal. 350 (2008) 837. [3] (a) E. Negishi, J. Organomet. Chem. 653 (2002) 34; (b) J.J. Li, G.W. Grimble, Palladium in Heterocyclic Chemistry, Pergamon, NY, 2000; (c) I.P. Beletskaya, A.V. Cheprakov, Chem. Rev. 100 (2000) 3009; (d) C. Elsevier, J. Coord. Chem. Rev. 809 (1999) 185. [4] S. Iravani, Green Chem. 13 (2011) 2638. [5] Catalysis from A to Z, in: B. Cornils, W.A. Herrmann, M. Muhler, C.-H. Wong (Eds.), A Concise Encyclopedia, Wiley-VCH, Weinheim, 2007, pp. 1–3. [6] R.A. Sheldon, I.W.C.E. Arends, U. Hanefeld, Green Chemistry and Catalysis, Wiley-VCH, Weinheim, 2007. [7] M. Tristany, J. Courmarcel, P. Dieudonne, M. Moreno-Manas, R. Pleixats, M. Rimola, S. Sodupe, S. Villarroya, Chem. Mater. 18 (2006) 716–722. [8] Y. Xiong, J. Chen, B. Wiley, Y. Xia, Y. Yin, Z.-Y. Li, Nano Lett. 5 (2005) 1237– 1241.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Green synthesis of palladium nanoparticles mediated by black tea leaves (Camellia sinensis) extract: Catalytic activity in the reduction of 4-nitrophenol and Suzuki-Miyaura coupling reaction under ligand-free conditions Sadaf Lebaschi a, Malak Hekmati a, Hojat Veisi b,⇑ a b
Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Chemistry, Pharmaceutical Sciences Branch, Islamic Azad University (IAUPS), Tehran, Iran Department of Chemistry, Payame Noor University, Tehran, Iran
g r a p h i c a l a b s t r a c t
PdCl2, H2O NH2 B(OH)2 HO
+ R
X EtOH, H2O K2CO3
Pd NPs
Pd NPs
NaBH4, H2O Poly Phenols
Poly Phenols
NO2
R HO
a r t i c l e
i n f o
Article history: Received 8 July 2016 Revised 11 September 2016 Accepted 13 September 2016 Available online 15 September 2016 Keywords: Green chemistry Black tea leaves (Camellia sinensis) Palladium nanoparticles Suzuki 4-Nitrophenol
a b s t r a c t The present study was conducted to synthesize palladium nanoparticles (Pd NPs) through a facile and green route using non-toxic and renewable natural black tea leaves (Camellia sinensis) extract, as the reducing and stabilizing agent. The as-prepared [email protected] NPs catalyst was characterized by UV–vis spectroscopy, X-ray diffraction (XRD), fourier transformed infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The [email protected] NPs catalyst could be used as an efficient and heterogeneous catalyst for Suzuki coupling reactions between phenylboronic acid and a range of aryl halides (X = I, Br, Cl) and also the reduction of 4-nitrophenol (4-NP) using sodium borohydride in an environmental friendly medium. Excellent yields of products were obtained with a wide range of substrates and the catalyst was recycled 7 times without any significant loss of its catalytic activity. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding author. E-mail address: [email protected] (H. Veisi). http://dx.doi.org/10.1016/j.jcis.2016.09.027 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
The reaction of aryl halides and aryl boronic acids results in formation of wide variety of unsymmetrical biaryls is Suzuki-Miyaura [1]. Biaryls which are also known as the fourth state of matter are
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significant groups used in liquid crystal materials [2a,2b]. Moreover, there have been many successful examples of total synthesis of biological and medicinal molecules through the approach of constructing new carbon-carbon bonds [2c,2d]. Such molecules generally can be synthesized by CAC bond formation reaction of Suzuki. However, there is still a significant challenge as this reaction suffers from some limitations and drawbacks. One of these is the necessity of inert atmosphere [2e], because the moisture may oxidize the catalyst or its reactive intermediates in the procedure. Furthermore, the ligands might be sensible to air and the existence of air which may diminish the efficiency [3]. Therefore, progress of Suzuki reaction in aqueous media and air condition is of remarkable importance. Biosynthesis of metal nanoparticles using plant materials is currently under exploration. It has received more attention as a suitable alternative to chemical and physical methods [4]. The development of new, efficient, and practical catalysts for organic transformation to synthesize valuable target molecules is an important research area in the pharmaceutical and chemical industries [5,6]. Although various methodologies have been developed for the synthesis of palladium nanoparticles, including chemical and electrochemical reduction [7,8], ion exchange [9], vapor deposition, thermal decomposition [10,11] and polyol method [12], preparation of metal nanoparticles is usually based on the reduction of a metal salt in the presence of a reducing agent such as sodium borohydride, hydrazine, dimethyl formamide and hydrogen, and a stabilizer like polymeric materials, dendrimers, surfactants, organic ligands and polyoxometalates [13,14]. It is important to note that bioinspired, eco-friendly greener methods for the synthesis of metal nanoparticles are among the most attractive aspects of today’s nanoscience and nanotechnology [15,16]. There are few reports available for the synthesis of palladium nanoparticles that effectively utilize Diopyros kaki. leaf [17], Cinnamom zeylanicum bark [18], C. Camphora leaf [19], Curcuma longa tuber [20], banana peel [21], Hippophaerhamnoides Linn [22], Pistacia atlantica kurdica gum [23], Rosa canina fruit [24], pectin [25], Stachys lavandulifolia [26], and Oak gum [27]. The black tea which grows in many parts of world, is used as an herbal tea and Drinking. Camellia sinensis leaves (black tea) have been reported to contain considerable amounts of tannin products [28]. This is motivated us to explore the possible bio-reduction of palladium ions into nanoparticles. Hence, the present work deals with biological (green) synthesis of palladium nanoparticles by black tea extract at ambient conditions. The bioreduction process was monitored by the UV–Visible, XRD, TEM, FESEM, WDX and EDX. Also, the catalytic activity of [email protected] NPs for the Suzuki-Miyaura coupling reaction was studied.
(Hitachi, U-2900) to ensure the formation of nanoparticles. Morphology and particle dispersion was investigated by field emission scanning electron microscopy (FE-SEM) (Cam scan MV2300). The chemical composition of the prepared nanostructures was measured by EDS (Energy Dispersive X-ray Spectroscopy) performed in SEM. TEM images were obtained using a Philips-EM-2085 transmission electron microscope with an accelerating voltage of 100.0 kV. 2.2. Green synthesis of palladium nanoparticles using black tea leaves (Camellia sinensis) Lahijan black tea leaves (Camellia sinensis) were supplied from the Lahijan Tea Research Center, Lahijan, Iran. 10 g of the black tea was added to 100 mL of deionized water and was boiled for 5 min in a water bath. The mixture was then cooled down and was filtered through Whatman filter paper No. 1 to obtain aqueous extract. The filtered extract was stored in refrigerator at 4 °C for further use. The extract was used as reducing as well as stabilizing agent. For preparation of Pd NPs, 10 mL of the prepared plant extract was added drop wise to 100 mL of 1 mM aqueous PdCl2 solution and refluxed at 100 °C for 1 h. The color of the reaction mixture gradually turned over 60 min and indicated the formation of Pd NPs, to which acetone was added to precipitate the catalyst ([email protected] NPs). After addition of acetone (anti-solvent) the precipitated catalyst was then centrifuged at 1000 rpm for 10 min followed by re-suspension of the pellet in Milli-Q water. The Pd loading of the prepared catalyst was measured to be 1.7 mmol/g by ICP and EDX. 2.3. Suzuki-Miyaura coupling reaction In a typical experiment, to a mixture of aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol), and K2CO3 (2.0 mmol) in (4.0 mL, 1:1) water-ethanol, 6.0 mg of the [email protected] NPs catalyst (containing 0.1 mol% of Pd) was added and heated at 60 °C. The progress of the reaction was followed by thin layer chromatography (TLC). After completion of the reaction, the reaction mixture was cooled down to room temperature and (10 mL) EtOAc was added and the catalyst was separated from the reaction mixture by centrifuge and extracted with ethyl acetate (10 mL 3). The combined organic layer was dried over anhydrous sodium sulfate and evaporated in a rotary evaporator under reduced pressure. The product was purified by column chromatography (hexane-ethylacetate, 1:5) to obtain the desired purity. All of the products are known and were identified by comparison of their physical and spectral data with those of authentic samples.
2. Experimental section
2.4. General procedure for the reduction of 4-nitrophenol
2.1. Materials
In a typical experiment, 2.0 mg of the [email protected] NPs catalyst was added to an aqueous solution that contained 4-nitrophenol (2.5 mM, 25 mL), freshly prepared aqueous NaBH4 solution (250 mM, 25 mL) and stirred for 80 s at room temperature. The rate of 4-nitrophenol reduction was evaluated using UV–vis spectroscopy at room temperature. After completion of the reaction, the catalyst was separated from the reaction mixture by centrifugation, washed with doubly distilled water and then dried for the next cycle.
High-purity chemical reagents were purchased from the Merck and Aldrich chemical companies. All materials were of commercial reagent grade. Melting points were determined in open capillaries using a BUCHI 510 melting point apparatus and are uncorrected. 1 H NMR spectra were recorded on a Bruker Avance DRX spectrometer at 400 MHz. FT-IR spectra were recorded on a Bruker Tensor 27 spectrometer (Bruker, Karlsrohe, Germany) using pressed KBr pellets. Lahijan black tea leaves (Camellia sinensis) were supplied from the Lahijan Tea Research Center, Lahijan, Iran. X-ray diffraction (XRD) measurements were carried out using a Philipspowder diffractometer type PW 1373 goniometer (Cu Ka = 1.5406 Å). The scanning rate was in the 2h range of 10–80°. UV–visible spectral analysis was recorded on a double-beam spectrophotometer
3. Results and discussion Biological methods for the synthesis of Pd nanoparticles are gaining importance in the field of nanoparticle synthesis. As a result of the growing success and simple processes for the
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Fig. 1. UV–vis spectra of (a) black tea leaves extract; (b) black tea leaves extract with PdCl2; and (c) [email protected] NPs.
formation of nanoparticles, the biosynthesis of Pd nanoparticles was attempted by reducing Pd2+ in PdCl2 solution using the water extracts of the green tea (Fig. 1, inset). The main cause of reduction of metal to metal nanoparticle is the presence various types of phytochemicals such as flavonoids, phenolics, polysaccharides, and terpenoids in the extracts of different plant and their parts. The preliminary confirmation of the reduction of metallic Pd to Pd nanoparticle was visually confirmed by a change in color of the reaction solution (Fig. 1, inset) to black after 2 h of incubation. Because of the excitation of surface plasmon resonance, the color change in the reaction medium indicated the formation of Pd nanoparticles. In addition to the optical observation, the UV–vis spectra of the reaction mixture was recorded and indicated a significant change with the disappearance of the peak around 400 nm, which showed the conversion of Pd(II) to Pd(0) (Fig. 1). This also proved the large reduction capability of the extract. Moreover, the UV spectrum of the extract shows bonds at kmax 260 nm is for absorbance of ring related to the benzoyl system. They are related to the p ? p transitions and these absorbent bonds demonstrate the presence of polyphenolics in the category of flavonoids that are responsible for the palladium reduction. FT-IR spectroscopy was used to identify the presence of different potential biomolecules in the plant extract which are responsible for reduction of palladium ions and formation of the corresponding Pd NPs, and also capping them by the extract. Fig. 2 shows the compared FT-IR spectrum of black tea leaves extract and green synthesized Pd NPs. The wide peak observed at 3100–3500 cm1, in the spectra of black tea leaves extract and Pd NPs corresponds to free OH in molecule and OH group forming hydrogen bonds of macromolecular association. The band at 3435 cm1 shifted to 3380 cm1 in the presence of Pd NPs, which show the interaction of Pd with the OH group of black tea leaves extract [29–31]. The absorptions shown in Fig. 2a indicate the presence of polyols, carbonyls, and C@C bonds on terpenoids and flavonoids. The IR spectrum of synthesized [email protected] NPs (Fig. 2b) revealed absorption peaks at 3380, 2940, 1708, 1616,1453, 1367, 1240, 1147, and 1087 cm1. Thereby, on the basis of FT-IR data, we could infer that the phenolic hydroxyl groups of flavones, terpenoids, and polysaccharides belonging to the phytomolecules of the black tea played a vital role in reduction of Pd(II) ions and also have strong ability to bind with Pd NPs.
Fig. 2. IR of (a) black tea leaves extract; and (b) [email protected] NPs.
The morphological characteristics of the synthesized [email protected] NPs catalyst were observed under FESEM (Fig. 3). The nanoparticles were formed and the shape was somewhat spherical in nature. Furthermore, the Pd NPs might also have been stabilized due to the interactions such as hydrogen bond and electrostatic interactions between the bioorganic capping molecules that have bound to the Pd NPs. EDX analysis confirmed the elemental composition of the synthesized nanoparticles (Fig. 4). The strong peak at 3 keV indicated the presence of the elemental Pd nanoparticles as evident from previous observations. Apart from Ag, other existing elements revealed by the EDX analysis included carbon, oxygen, and nitrogen which confirmed that the organic metabolites present in the black tea leaves extract were responsible for reducing, capping and stabilization of the Pd nanoparticles. The crystalline nature of green synthesized [email protected] NPs catalyst was also determined by XRD spectroscopy (Fig. 5). In the diffractogram, there are four distinct reflections at 39.7° (1 1 1), 46° (2 0 0), 68° (2 2 0), 82° (3 1 1), and 86° (2 2 2), which according to crystallographic planes the face centered cubic (fcc) crystalline structure was indexed for metallic palladium (Pd0). Particles morphology, size, and crystallinity were studied by TEM. Fig. 6 shows TEM images of typical nanoparticles synthesized
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Fig. 3. SEM images of [email protected] NPs catalyst.
from black tea leaves extract. Particle sizes ranged from 5 to 8 nm (average 7 nm) and most were near spherical. Smaller nanoparticles agglomerated to produce non-spherical, slightly larger particles. It was interesting to note that the dispersed colloidal Pd NPs were surrounded by bio-polymers layer, which appeared to be responsible for reducing and stabilizing the nanoparticles. TGA/DTA experiments were performed at a heating rate of 10 °C min1. The spectrum obtained showed significant weight loss of 74% from Pd NPs due to desorption of biomaterials when heated from 20 °C to 700 °C. This result indicated that bioactive molecules were capped on the Pd NPs and were completely degraded under high temperature (Fig. 7).
3.1. Evaluation the catalytic activity of [email protected] NPs through the Suzuki coupling reaction
Fig. 4. The EDS spectrum of [email protected] NPs catalyst.
Fig. 5. XRD pattern of [email protected] NPs catalyst.
In the next step, we tested the catalytic activity of the [email protected] NPs for the Suzuki-Miyaura coupling reaction (Scheme 1). We initially selected 4-methylbromobenzene and phenylboronic acid as a model reaction to evaluate the effects of solvent (nonpolar, protic and aprotic), base (Et3N, NaOAc and K2CO3) and amount of catalyst at different temperature. Optimization conditions studies are summarized in Table 1. As expected, no target product could be detected in the absence of the catalyst (Table 1, entry 11). However, addition of the catalyst to the mixture has rapidly increased the synthesis of product in high yields. The reactions were conducted using H2O/EtOH (1:1) as the best solvent. Among the bases evaluated, K2CO3 was found to be the most effective. The effect of catalyst loading was investigated employing several quantities of the catalyst ranging from 0.05 mol% to 0.2 mol% (Table 1, entries 6, 7 and 8). The best yield was obtained with 0.006 g (0.1 mol%) of the catalyst (Table 1, entry 8). Several coupling reactions of various aryl halides (I, Br and Cl) with phenyl boronic acid were evaluated using 0.1 mol% of Pd@B. tea NPs catalyst under the optimized conditions and the results presented in Table 2. Phenyl iodides, bromides and chlorides all reacted efficiently with phenylboronic acid (Table 2, entries 1–15). Aryl halides with electron-withdrawing or releasing groups reacted with phenylboronic acid to afford the corresponding products in high yields. As shown in Table 2, steric hindrance of the substituent did not influence the product yield in SuzukiMiyaura reaction of deactivated arylhalides using [email protected] NPs catalyst (Table 2, entries 15). Notably, heteroaryl halides such as 2-bromothiophene and 2-iodothiophene with phenylboronic acid gave the corresponding coupled products in 96% and 90% yields, respectively (Table 2, entries 17 and 18). The coupling reaction of aryl chlorides with phenyl boronic acid required extended reaction time than aryl iodides and bromides, producing the desired products in moderate yield (Table 2, entries 3, 6, 9).
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Fig. 6. TEM images of dispersed colloidal Pd NPs from [email protected] NPs catalyst.
Fig. 7. TGA/DTA analysis of [email protected] NPs catalyst.
After completion of the reaction, catalyst was separated by centrifugation and the recycled catalyst was saved for the next reaction. To investigate the leaching of Pd nanoparticles, the samples of the filtrate plus washings obtained from the above standard reactions were subjected to ICP analysis that showed the concentrations of Pd were less than 3.0 ppm in each measurement. In addition, the standard reaction was conducted with the recovered filtrate, but no product could be isolated. The results suggested that the palladium catalyst remained on the support at elevated temperatures during the reaction and the reaction proceeds on the heterogeneous surface. Based on these evidences, a reaction mechanism for Suzuki coupling using the prepared nanocatalyst was proposed (Scheme 2).
3.2. Reusability the [email protected] NPs catalyst in the Suzuki coupling The reusability of the catalysts is one of their most important advantages, which makes them useful for commercial applications. We studied the reusability of these heterogeneous catalysts in the Suzuki-Miyaura coupling reaction under same conditions as Table 1
(Fig. 8). After completion of the reaction, catalyst was separated by centrifugation from the reaction mixture and washed several times with deionized water and ethanol. Then, it was and dried in an oven at 50 °C and the recycled catalyst was saved for the next reaction. The dried recovered catalyst was successively used for five fresh runs with no significant loss of activity. This reusability demonstrates the high stability and turnover of catalyst under operating conditions.
3.3. Evaluation the catalytic activity of the [email protected] NPs through the reduction of 4-NP In continuation of our works, the catalytic activity of the Pd@B. tea NPs catalyst was examined for reduction of 4-nitrophenol (4NP) in water at room temperature. The progress of the reaction was monitored by recording the absorption spectra as a function of time. Initially, the reaction conditions were optimized for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of different amounts of catalyst (Table 3). The amount of NaBH4, as one of the factors influencing the reduction of 4-NP
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R
[email protected] NPs
+ X
B(OH)2
EtOH, H2O K2CO3
R
Scheme 1. Suzuki coupling reaction in the presence of [email protected] NPs.
Table 1 Optimization of reaction conditions in Suzuki-Miyaura coupling reaction of 4-methylbromobenzene with phenylboronic acid.a
a b c
Entry
Pd (mol%)
Solvent
Base
T (°C)
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11
0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.1 0.2 0.1 0.0
DMF Toluene EtOH H2O EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc EtOH/H2Oc
K2CO3 K2CO3 K2CO3 K2CO3 NaOAc Et3N K2CO3 K2CO3 K2CO3 No base K2CO3
80 60 60 90 60 90 60 60 70 90 60
80 65 77 55 65 80 75 98 96 Trace 0.0
Reaction conditions: 4-methylbromobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), catalyst, base (2 mmol) and solvent (4 mL). Isolated yield. (1:1).
Table 2 Suzuki–Miyaura coupling reaction of different aryl halides with phenylboronic acids.a Entry
RC6H4X
R2C6H4B(OH)2
X
Time (h)
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
H H H 4-CH3 4-CH3 4-CH3 4-COCH3 4-COCH3 4-COCH3 4-CH3O 4-CH3O 4-Cl 3-NO2 3-NO2 2-CHO 1-Naphthyl 2-Thienyl 2-Thienyl 4-COCH3 H 4-COCH3 H
H H H H H H H H H H H H H H H H H H 4-NO2 4-NO2 4-CH3 4-CH3
I Br Cl I Br Cl I Br Cl I Br Br I Br Br I I Br Br Br Br Br
0.16 0.66 12 0.20 1 12 0.3 0.8 12 0.3 1.5 1 0.5 3 5 2 1 5 3 3 1.5 1
98 98 70 98 98 75 98 96 65 98 96 96 96 88 70 96 96 90 80 85 92 92
a Reactions were carried out under aerobic conditions in 4 mL of H2O/EtOH (1:1), 1.0 mmol arylhalide,1.1 mmol phenylboronic acid and 2 mmol K2CO3 in the presence of catalyst (0.006 g, 0.1 mol% Pd) at 60 °C. b Isolated yield.
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Scheme 2. Possible mechanism of Suzuki reaction in the presence of [email protected] NPs.
NO2
NO2 NaBH4
NH2 [email protected] NPs
OH
O
OH
Fig. 9. The reduction of 4-NP in the presence of NaBH4 and catalyst.
Fig. 8. Reusability of the catalyst for Suzuki-Miyaura coupling reaction.
into 4-AP, was also investigated. As expected, no target product could be detected in the absence of catalyst. It can be seen that the rate of reduction of 4-NP is increased on increasing the molar ratio of NaBH4 to 4-NP. Nevertheless, the best results were achieved in the presence of 2.0 mg of [email protected] NPs and 100 equivalents of NaBH4 (250 mM). The reduction reaction in the presence of extract as catalyst and in the absence of Pd NPs, was also performed, resulting without product and in 96% conversion respectively. Compared with the Pd NPs, the [email protected] NPs showed apparently higher catalytic activity, which could be attributed to
the effect of increased surface area, smaller size and the well dispersity of the Pd NPs fabricated on the surface of biopolymers. The enhanced catalytic activity observed with our nanocatalyst is attributed to the plenty of active functional groups, such as C@C and AOH that covering the Pd NPs which interact and stabilizes the 4-NP substrate adjacent to the catalytic sites which in turn facilitates the reduction of nitro groups. On the other hand, the covering shell had good adsorption ability to the water-soluble 4-NP, which could accelerate the process of catalytic reduction. It suggested that the extract may play an active part in the catalysis, yielding a synergistic effect. The catalytic mechanism for the conversion of 4-NP into 4-AP relies on the electrons transfer from the BH 4 donor to the acceptor 4-NP through adsorption of the
Table 3 Reduction of 4-NP at different conditions. [4-NP] (mM)
[NaBH4] (mM)
Catalyst (mg)
Time (min)
Conversion (%)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
250 250 250 250 250 250 187.5 125
– Extract (10) Pd NPs (2) [email protected] NPs [email protected] NPs [email protected] NPs [email protected] NPs [email protected] NPs
30:00 30:00 4:00 1:20 10:00 1:20 4:30 7:20
0.0 0.0 96 100 98 100 100 100
catalyst catalyst catalyst catalyst catalyst
(2) (1) (3) (2) (2)
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Fig. 10. UV-visible spectra for catalytic reduction of 4-NP to 4-AP at several interval and the plots of ln(A/A0 ) vs. irradiation time for reduction reaction of 4-NP. Conditions: [4NP] = 2.5 103 M, [NaBH4] = 0.25 M, [email protected] NPs = 2.0 mg.
reactant molecules on to the extract surface. The [email protected] NPs can serve as catalyst to transfer electrons from BH 4 to the 4-NP, which are both absorbed on the catalysts via p-p stacking interactions, leading to the production of amino derivatives. In the present work, when the [email protected] NPs was added to a mixed solution of 4-nitrophenol (oxidant) and NaBH4 (reductant), 4-nitrophenolate ions (Fig. 9) and BH 4 were first adsorbed on the surface of the catalyst via physical adsorption and hydrogen bonding. After electron transfer (ET) to the Pd NPs, the hydrogen atom forms from the hydride, and then attacks 4-nitrophenolate ions to reduce it. This ET-induced hydrogenation of 4-NP occurred spontaneously at the surface of the metal catalyst. Finally, the generated 4-AP was desorbed from the surface of the catalyst. It is observed that 4-NP in aqueous medium has a maximum absorption at 317 nm [29]. However, when freshly prepared NaBH4 solution is added, there was a red shift from 317 nm to 400 nm, and the light yellow color of the solution changes to intense yellow, due to the formation of 4-nitrophenolate ions in alkaline condition. The peak at 400 nm remains unchanged even for a couple of days in the absence of any catalyst. As shown in Fig. 10, after the catalyst was added into the solution containing 4-NP and NaBH4, the intensity of the strong absorption peak at 400 nm gradually decreased and a new peak appeared at about 300 nm which corresponded to the formation of 4-AP. After about 80 s in case of [email protected] NPs, the whole peak at 400 nm almost disappeared and the color became transparent, which indicated that 4-NP was almost turned to 4-AP. In the reduction process, the overall concentration of NaBH4 was 0.25 M and 4-NP was 2.5 103 M. Considering much higher concentration of NaBH4 compared to 4-NP, the pseudo-first-order kinetics has been applied with respect to 4-NP to determine the catalytic activity of [email protected] NPs. The absorbance of 4-NP is proportional to its concentration in solution; the absorbance at time t (A) and time t = 0 (A0 ) are equivalent to the concentration at time t (C) and time t = 0 (C0 ). The rate constant (k) has been determined from the linear plot of ln (A/A0 ) versus reduction time in seconds, and the constant was estimated to be 5.9 104 s1. 3.4. Reusability the [email protected] NPs in the reduction of 4-NP For the application as a practical catalyst, the catalytic stability and the reuse of the catalyst are very important. The recovery and reusability of the [email protected] NPs was investigated in the reduction of 4-NP to 4-AP under the optimized conditions. After completion of the reaction, the catalyst was separated from the reaction mixture by centrifugation, washed three times with 5 mL of acetone
Fig. 11. Reusability of the catalyst for reduction of 4-NP.
and then with doubly distilled water, dried in an oven at 100 °C for 3 h and the recycled catalyst was saved for the next reaction. It is found that catalyst does not lose its catalytic activity during at least nine catalytic cycles in the reduction of 4-NP with NaBH4 (Fig. 11).
4. Conclusions In summary, a novel [email protected] NPs catalyst via the reduction of aqueous Pd2+ ions using bioactive black tea leaves extract was fabricated. The polyols and carbonyl groups present in the aqueous extract act as both reducing and capping/stabilizing agents [12,30–32]. Synthesized [email protected] NPs were characterized by UV– vis, FT-IR, FESEM, EDX, XRD, TGA and TEM analysis. In addition, the synthesized Pd NPs capped by biomolecules showed potent catalytic application for the synthesis of biaryls by Suzuki crosscoupling reaction and also reduction of 4-nitrophenol (4-NP) by NaBH4. This simple synthetic method has the advantages of high yields, elimination of homogeneous catalyst and expensive, unstable and poisonous ligands, simple methodology, easy preparation and handling of the catalyst and also easy work up. It is also observed that the catalyst was recycled several times without any significant loss of catalytic activity. The described strategy for Pd NPs is straightforward, robust, environmentally friendly, and cost-effective, and may find widespread use in palladiumcatalyzed reactions for the preparation of organic compounds. The enhanced catalytic activity observed with our nanocatalyst is attributed to the plenty of active functional groups, such as C@C and AOH that covering the Pd NPs which interact and stabilizes
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the 4-NP substrate adjacent to the catalytic sites which in turn facilitates the reduction of nitro groups. On the other hand, the covering shell had good adsorption ability to the water-soluble 4-NP, which could accelerate the process of catalytic reduction. It suggested that the extract may play an active part in the catalysis, yielding a synergistic effect. The extracted phytochemicals from the black tea leaves showed a colloidal media to interaction with metallic salts to produce the nanostructure. The result showed that the green approach to synthesizing Pd NPs is useful for removing toxic pollutants and dyes such as nitro aromatics from the environment. References [1] (a) N. Miyaura, T. Yanagi, A. Suzuki, Syn. Commun. 11 (1981) 513; (b) N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457; (c) A. Suzuki, in: D. Astruc (Ed.), Modern Arene Chemistry, Wiley-VCH, Weinheim, 2002, p. 53; (d) F. Lu, J. Ruiz, D. Astruc, Tetrahedron Lett. 45 (2004) 9443. [2] (a) Z.G. Zhoua, J.C. Shib, Q.S. Hua, Y.R. Xiea, Z.Y. Dua, S.Y. Zhanga, Appl. Organometal. Chem. 25 (2011) 616; (b) J.P. Wolfe, S.L. Buchwald, Angew. Chem. Int. Ed. 38 (1999) 2413; (c) A.F. Littke, G.C. Fu, Angew. Chem., Int. Ed. 41 (2002) 4176; (d) K.C. Nicolaou, P.G. Bulger, D. Sarlah, Angew. Chem., Int. Ed. 44 (2005) 4442; (e) C. Ornelas, J. Ruiz, L. Salmon, D. Astruc, Adv. Synth. Catal. 350 (2008) 837. [3] (a) E. Negishi, J. Organomet. Chem. 653 (2002) 34; (b) J.J. Li, G.W. Grimble, Palladium in Heterocyclic Chemistry, Pergamon, NY, 2000; (c) I.P. Beletskaya, A.V. Cheprakov, Chem. Rev. 100 (2000) 3009; (d) C. Elsevier, J. Coord. Chem. Rev. 809 (1999) 185. [4] S. Iravani, Green Chem. 13 (2011) 2638. [5] Catalysis from A to Z, in: B. Cornils, W.A. Herrmann, M. Muhler, C.-H. Wong (Eds.), A Concise Encyclopedia, Wiley-VCH, Weinheim, 2007, pp. 1–3. [6] R.A. Sheldon, I.W.C.E. Arends, U. Hanefeld, Green Chemistry and Catalysis, Wiley-VCH, Weinheim, 2007. [7] M. Tristany, J. Courmarcel, P. Dieudonne, M. Moreno-Manas, R. Pleixats, M. Rimola, S. Sodupe, S. Villarroya, Chem. Mater. 18 (2006) 716–722. [8] Y. Xiong, J. Chen, B. Wiley, Y. Xia, Y. Yin, Z.-Y. Li, Nano Lett. 5 (2005) 1237– 1241.
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