Biosynthesis of palladium nanoparticles using Poplar leaf extract and its application in Suzuki coupling reaction

Abstract

A green route for the synthesis of palladium (Pd) nanoparticles (Pd NPs) employing Poplar leaf extract as a reducing and capping agent is described. The as‐prepared Pd NPs are spherical with a face centred cubic structure, a particle distribution of 2.2–6.8 nm and an average particle size of 4.2 nm. The application of this catalyst toward homogeneous Suzuki coupling reactions was investigated. The Pd NPs afforded a yield of 98.86% in the Suzuki coupling reaction of 4‐bromotoluene with phenylboronic acid using 0.01 mmol% of the catalyst at 60°C for 30 min under an air atmosphere.

1 Introduction

The Suzuki coupling reaction is a particularly important reaction in organic chemistry. This reaction is the most powerful tool for constructing biaryl structures, which exist in many biologically active compounds and natural products [1, 2]. Palladium (Pd)‐catalysed Suzuki coupling reactions have become the standard methodology for the construction of biaryl units by synthetic organic chemists [3, 4, 5].

Nanostructured catalysts have been extensively investigated because they have much higher activities than that of the corresponding bulk materials [6, 7, 8, 9, 10, 11, 12, 13]. The synthesis of nanoparticles (NPs) using chemical and physical methods requires high pressure, energy and toxic chemicals. In contrast, biomolecules exhibit highly controlled hierarchical assembly, so they are suitable for the development of reliable, ecofriendly processes for metal NP synthesis [14]. Hence, exploration of plant systems as potential bio‐factories has prompted great interest in the biological synthesis of metal NPs [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. For instance, Pd NPs have been synthesised using the leaf extracts of Catharanthus roseus [27], Cinnamomum camphora [28], Solanum trilobatum [29], Euphorbia thymifolia L. [30], Withania coagulans [31], Euphorbia stracheyi Boiss [32], coffee and tea [33]. Plant extracts may act as both reducing and capping agents in the synthesis of metal NPs. The sources of the plant extracts are known to influence the formation of NPs, and different plant‐based extracts contain different combinations of organic reducing and capping agents [34, 35]. Although the present research on the biosynthesis of Pd NPs is extensive, there has been no report on how to confirm the dose of phenolic compounds in the extract.

Poplar is a Salicaceae family plant that is widely distributed in the world (Fig. 1). The main classes of secondary metabolites present in Poplar leaves are phenolic compounds, including alkaloids, terpenes, tannins, phenolic glycosides and flavonoids [36, 37]. These potential antioxidant compounds in this plant can serve as potential sources for the reduction of Pd ions and the green synthesis of Pd NPs.

Fig. 1.

Image of Poplar

In this work, we present a green route to the synthesis of Pd NPs using Poplar leaf extract (PLE) as a reducing and capping agent. Surprisingly, the prepared Pd NPs exhibited superior catalytic activity in Suzuki coupling reactions without toxic solvents, phase transfer agents or inert atmosphere.

2 Materials and methods

2.1 Materials

The Poplar leaves were collected from the area around Harbin, Heilongjiang, China. PdCl₂ and NaCl were purchased from Sinopharm Chemical Reagent Co. Ltd. K₂ CO₃ and galic acid were purchased from Tiajin Kermel Chemical Reagent Co. Ltd. A 0.05 M Na₂ PdCl₄ solution was prepared by adding 0.0887 g of PdCl₂ and 0.0585 g of NaCl in 10 ml of deionised water. Doubly distilled water was used for the experiments.

2.2 Preparation of the extract

The collected leaf sample was washed three times with tap water and twice with distilled water to remove contaminants and dried 1–2 days at environmental temperatures (27–37°C). The sun‐dried Poplar leaves were milled, and ∼2.0 g of the milled powder was dispersed into 100 ml deionised water and maintained in a water bath shaker at 50°C for 30–40 min. The extract obtained was filtered through Whatman no.1 filter paper, and the filtrate was collected in an Erlenmeyer flask and stored at 4°C for further use.

2.3 Synthesis of Pd NPs

To synthesise the Pd NPs, 10 ml Na₂ PdCl₄ (0.05 M) was added to 30 ml of the PLE in conical flasks of 100 ml capacity in a water bath with a constant temperature of 25°C and stirred continuously for 12 h.

2.4 Characterisation

The plant mediated bio‐reduction of Na₂ PdCl₄ solution was monitored by ultraviolet–visible (UV–Vis) spectrometer (Shimadzu spectrophotometer, model UV 2450). The powder X‐ray diffraction (XRD) patterns of the products were recorded with a Bruker D8 advance diffractometer with a Cu Kα (λ  = 0.15418 nm) radiation at 40 kV and 40 mA. Transmission electron microscopy (TEM) images were recorded with a FEI Tecnai G2 S‐TWIN transmission electron microscope operating at 200 kV. The surface chemistry of the synthesised particles was studied with Fourier transform infrared spectroscopy (FTIR) analysis, which gives information about the possible bio‐compound responsible for the formation of Pd NPs in the PLE. FTIR experiments were performed on a Nicolet 6700 spectrophotometer (Thermo Nicolet, USA). X‐ray photoelectron spectrometer (XPS) analyses were conducted on a Kratos AXIS ULTRA DLD spectrometer with monochromatic Al Kα (1486.6 eV) radiation. The C1s peak at 284.6 eV was used for calibration.

2.5 Catalytic Suzuki coupling reaction

The activity of the prepared Pd NPs catalysts was evaluated using a Suzuki coupling reaction. The general procedure for the Suzuki coupling reaction was as follows. A mixture of aryl bromide (2.5 mmol), phenylboronic acid (3.75 mmol), K₂ CO₃ (5 mmol), Pd NPs catalyst (0.01 mmol% relative to the aryl bromide) and EtOH/H₂ O (15 ml/15 ml) was stirred for 30 min at 60°C. The reaction mixture (5 ml) was added to 0.2 mol/l sodium hydroxide solution (5 ml) and extracted with ethyl acetate (10 ml). The combined organic layers were collected and dried in air to obtain the product. High‐performance liquid chromatography (HPLC) analysis was used to evaluate the products of the reactions.

3 Results and discussion

3.1 Determination of the total phenolic content

The amount of total phenolic content in the PLE was determined according to the Graham method using galic acid as a standard [38]. Briefly, 0.5 g galic acid was added to a 100 ml volumetric flask and distilled water was added to bring the volume to 100 ml, affording a 5 mg/ml solution. The galic acid solution was then diluted to 50, 25, 20, 12.5, 8.33, 4.17 and 2.78 µg/ml. The absorbance of the galic acid solution was measured at 267 nm (Fig. 2 a), which is similar to the PLE (inset of Fig. 2 a). The standard curve is shown in Fig. 2 b, where y  = 0.00104 + 0.046x with a correlation coefficient R ²  = 0.99985, which indicates that the standard solution absorbance (and thus concentration) had a linear relationship. Hence, this method has a high sensitivity.

Fig. 2.

UV–Vis spectra and standard curve of galic acid

(a) UV–Vis spectra of galic acid and PLE (inset of a), (b) Standard curve of galic acid

As shown in Table 1, we studied the impact of time and temperature on the extract yield of total phenols in PLE using a uniform design experiment. The per cent of total phenolic content (%TPC) was determined per galic acid equivalent using an equation obtained from the standard galic acid graph: %TPC = (observed concentration/actual concentration) × 100.

Table 1.

Experiment conditions and results of uniform design

No.	Temperature, °C	Time, min	%TPC
1	0	35	6.807
2	10	10	7.952
3	20	45	11.149
4	30	20	16.338
5	40	55	17.209
6	50	30	20.603
7	60	25	19.845
8	70	40	16.875
9	80	15	20.227
10	90	50	18.640
11	100	5	16.723

Reaction conditions: Poplar leaf (200 mg), H₂ O (10 ml).

The analysis yielded a regression equation: $Y_1=8.73725+0.11462X_1+0.06548X_2-0.003703X_1^2$ (Y ₁ is %TPC, X ₁ is temperature and X ₂ is time). From the analysis, the temperature is the main influence factor and time is the secondary influence factor on the %TCP. Therefore, the optimum condition is 50°C for 30 min (Table 1, entry 6).

3.2 UV–Vis spectrum analysis

The bio‐reduction process of the Pd NPs was monitored by UV–Vis spectroscopy at 240–800 nm. Fig. 3 shows the absorption spectra of Pd colloidal solution at 3 h intervals. The colour of the solution gradually turned from brownish yellow into dark brown over 9 h (inset of Fig. 3), which indicated the generation of Pd NPs. The absorption bands appearing in the contrast spectrum of Na₂ PdCl₄ solution were ascribed to the ligand‐to‐metal charge‐transfer transition of the Pd (II) ions. The absence of absorption peaks above 300 nm in all of the samples revealed the complete reduction of the initial Pd (II) ions [39, 40]. A similar observation was made by HO and Nemamcha et al. [41, 42]. Moreover, they noted that the absence of those absorption peaks was consistent with the theoretical study of the surface plasmon resonance absorption of Pd NPs. It was observed from Fig. 3 that the optimal time for the formation of NPs was 12 h.

Fig. 3.

UV–Vis spectrum of Pd NPs prepared with different time and the colour change of the reaction solution (inset of Fig. 3)

3.3 XRD analysis

The XRD profile of the biologically synthesised Pd NPs is shown in Fig. 4. Prominent Bragg reflections at 2θ values of 40.1°, 46.1° and 67.9° were observed, which correspond to the (111), (200) and (220) Bragg reflections of the face centred cubic (fcc) structure of Pd NPs, respectively [43]. The precursor Na₂ PdCl₄ was decomposed into NaCl and PdCl₂ in the synthesis process. The peaks at 28.3°, 31.7°, 56.4° and 75.4° can be ascribed to (111), (200), (420) and (222) of NaCl, respectively. The average particle size, derived using the Scherrer equation, was 4.8 nm, which further confirmed the dimensions of the biologically synthesised Pd NPs.

Fig. 4.

XRD patterns of the as‐prepared Pd NPs

3.4 TEM analysis

The TEM images of the Pd NPs are shown in Fig. 5. The TEM micrograph indicates a fine configuration of crystalline, spherical Pd NPs with sizes ranging from 2.2 to 6.8 nm (Fig. 5 c) and an average size of 4.2 nm, which is similar to the XRD analysis. Selected area electron diffraction (SAED) of the Pd NPs showed Scherrer ring patterns (inset of Fig. 5 b) associated with the (111), (200) and (220) atomic planes and confirmed the formation of the fcc crystal lattice in the Pd NPs. At the same time, the micrographs revealed a uniform distribution of NPs throughout the surface in addition to the variation in size of the synthesised Pd NPs.

Fig. 5.

Low‐ and high‐resolution TEM images, SAED pattern (inset of B) and size histograms of the as‐prepared Pd NPs

(a) Low‐resolution TEM images, (b) High‐resolution TEM images, (c) Size histograms

3.5 XPS analysis

Fig. 6 shows the spectra of the Pd 3d region (Pd 3d5/2 and Pd 3d3/2) for the obtained Pd NPs. The binding energy values are 335.4 eV for Pd 3d5/2 and 340.6 eV for Pd 3/2. The observed binding energy values for Pd 3d coincide with the reported data of Pd (0) within the experimental errors [44], while the binding energy values at 337.8 eV (3d5/2) and 343.3 eV (3d3/2) can be assigned to Pd (II). The quantitative surface composition of the Pd NPs as determined by XPS is C (77.62 atom%), O (14.22 atom%), N (7.61 atom%) and Pd (0.55 atom%). The three elements (C, O and N) originate from the biomass residue of Poplar leaf broth. The proportion of Pd (0) in the Pd NPs is 70.58%. These results demonstrate that most of the Pd ions are reduced by PLE into a metallic state.

Fig. 6.

XPS patterns of the as‐prepared Pd NPs

3.6 FTIR analysis

FTIR measurements were conducted to identify possible functional groups responsible for the bio‐reduction of the metal precursors and the stabilisation of the Pd NPs in plant extract [45, 46]. The FTIR spectrum of the Pd NPs is shown in Fig. 7; the spectrum of the PLE before the reaction is also displayed for comparison. Several absorption peaks located at ∼3400, 2923, 2337, 1728, 1600, 1396, 1265, 1072 and 605 cm⁻¹ are observed (Fig. 7 a) before bio‐reduction. Generally, the peaks at 1072 and 1265 cm⁻¹ might result from the hydroxyl group in carbohydrates and the C–O stretching vibration in cyclic compounds. The peaks at 1396, 2923 and 2337 cm⁻¹ may be ascribed to the bending vibration and stretching vibration of C–H in alkanes. The very strong band at 1600 cm⁻¹ could be assigned to the C–OH vibrations of the protein in the Poplar leaf. The peak at 1396 cm⁻¹ was assigned to the C=C in aromatic compounds; the peak at 3400 cm⁻¹ was assigned to the O–H stretching vibration in alcohols and phenolic compounds. The absorbance band at 1728 cm⁻¹ could be ascribed to the stretch vibration of C=C in aromatic compounds and the C=O stretching vibration in carbonyl compounds. Comparison with PLE (Fig. 7 b) reveals that the intensity of peaks at 2337, 1600, 1072 and 608 cm⁻¹ decreased after the biosynthetic reaction, which suggests that the reaction involved several compounds, such as carbohydrates, cyclic molecules and alkanes. To a great extent, the functional groups such as C=C and C=O might be derived from the water‐soluble heterocyclic compounds in the PLE. From the XPS analysis, one can also deduce that large amounts of C and O elements exist as part of the surface composition of the Pd NPs, and the elements of C and O are very likely to bind to the surface of the Pd NPs via functional groups such as C=C and C=O. Consequently, it is likely that water‐soluble heterocyclic compounds, e.g. flavones and proteins, act as stabilisers for the Pd NPs, and the alkaloids, polyphenols, saccharides or carbonyl compounds are responsible for the reduction of the aqueous metal precursors.

Fig. 7.

FTIR spectrum of

(a) Pd NPs, (b) PLE

3.7 Mechanism of the biosynthesis of Pd NPs

The above research suggests that there are compounds with potential antioxidant activity, such as alkaloids, tannins, phenolic glycosides and flavonoids, in the extract. As these polyphenols have the ability to donate electrons and easily oxidised to its quinones form, we speculate that polyphenols serve as the main reducing agent in the reduction of Pd²⁺ to Pd⁰. According to the previously reported work [47], we propose a possible mechanism of this bio‐reduction (Fig. 8). In this mechanism, we use cianidanol as a typical polyphenols to clarify the reduction mechanism.

Fig. 8.

Biosynthesis mechanism of the Pd NPs

3.8 Catalytic activity evaluation

To explore the catalytic activity, the Suzuki coupling reactions of aryl halides with arylboronic acids were conducted using the present catalyst [48]. We chose 4‐bromotoluene and phenylboronic acid as model reaction substrates (Fig. 9) to study the impact of various reaction conditions, such as base, solvent and temperature, on the reaction.

Fig. 9.

Suzuki coupling reaction of 4‐bromotoluene and phenylboronic acid

The reaction was initially performed using 0.01 mmol% of the Pd NP catalyst at 60°C for 30 min in air atmosphere to study the impact of different bases and solvents. The results are shown in Table 2. Among the various bases used, the bases K₃ PO₄ ·3H₂ O, NaOH, KF, NH₃ ·H₂ O and NaOAc obtained comparatively lower yields, while K₂ CO₃ gave the best result with a 98.86% yield of the coupling product (Table 2, entries 1 versus 2–6).

Table 2.

Effect of base and solvent on the Suzuki coupling reaction

Entry	Base	Solvent	Time, min	Yield, %
1	K2 CO3	EtOH/H2 O	30	98.86
2	K3 PO4 ·3H2 O	EtOH/H2 O	30	96.61
3	NaOH	EtOH/H2 O	30	98.09
4	KF	EtOH/H2 O	30	94.53
5	NH3 ·H2 O	EtOH/H2 O	30	32.45
6	NaOAc	EtOH/H2 O	30	95.07
7	K2 CO3	MeOH/H2 O	30	97.74
8	K2 CO3	Toluene/H2 O	30	34.36
9	K2 CO3	DMAC/H2 O	30	22.53
10	K2 CO3	CH2 Cl2 /H2 O	30	59.27
11	K2 CO3	THF/H2 O	30	20.32
12	K2 CO3	DMF/H2 O	30	32.26
13	K2 CO3	1,4‐dioxane/H2 O	30	96.37

Reaction conditions: 4‐bromotoluene (2.5 mmol), phenylboronic acid (3.75 mmol), Pd NPs (0.01 mmol%), K₂ CO₃ (5.0 mmol), solvent/H₂ O (15 ml/15 ml) at 60°C for 30 min.

Screening with different solvents showed that ethanol was a more effective solvent for this system, while MeOH and 1,4‐dioxane gave slightly lower yields of 97.74 and 96.37%, respectively (Table 2, entries 7 and 13). Poor yields were also obtained for the other solvents (Table 2, entries 8–12). With a consideration toward green chemistry, cheap and non‐toxic ethanol was quite practical and suitable to use as a solvent for the reaction.

As shown in Table 3, the effect of temperature on catalytic activity has been studied. For the reaction system with K₂ CO₃ as the base and ethanol as the solvent at 0.01 mmol% Pd NPs, the reaction temperature does not show a great impact on the catalytic results. When the reaction was performed at 30°C, the yield of the product was 95.16% (Table 3, entry 1). Increasing the reaction temperature to 60°C, increased the yield to 98.86%, similar to the yield at 70°C (Table 3, entries 4 and 5). Based on the yield, it was determined that the reaction at 60°C should be an excellent choice.

Table 3.

Effect of temperature on the Suzuki coupling reaction

Entry	Temperature, °C	Time, min	Yield, %
1	30	30	95.16
2	40	30	97.41
3	50	30	97.85
4	60	30	98.86
5	70	30	98.89

Reaction conditions: 4‐bromotoluene (2.5 mmol), phenylboronic acid (3.75 mmol), Pd NPs (0.01 mmol%), K₂ CO₃ (5.0 mmol), solvent/H₂ O (15 ml/15 ml).

Using these optimised conditions, the Pd NPs were applied to a diverse range of aryl bromides and phenylboronic acids, and the results are shown in Fig. 10. Generally, it was observed that aryl bromides with electron withdrawing groups were more reactive than aryl bromides with electron donating groups in the Suzuki coupling reaction. However, reactions involving the electron‐poor aryl bromide p‐bromobenzaldehyde led to quite a low yield of product in the same reaction time as other aryl bromides. Reactions with groups at the ortho‐position led to a lower yield than the same group at the para‐position, likely due to a steric hindrance effect. This effect was pronounced in the case of p‐bromoanisole and o‐bromoanisole, which showed a yield of 96.79 and 34.38%, respectively (Fig. 10, entries 2 versus 9, entries 6 versus 8). Poor yield was also found with aryl chloride (Fig. 10, entries 11–13).

Fig. 10.

Suzuki coupling reaction of various aryl halides and phenylboronic acid

To show the advantages of the as‐prepared Pd NPs in this work in comparison with other previously reported Pd catalysts, some results for Suzuki coupling reaction of 4‐bromotoluene with phenylboronic acid over different catalysts are summarised in Table 4, which shows that as‐prepared Pd NPs are one of the most efficient catalyst systems, affording the highest yield and requiring the lowest catalyst loading of 0.01mmol%.

Table 4.

Comparison of the present Pd catalyst with other previously reported Pd catalysts in the Suzuki coupling reaction of 4‐bromotoluene and phenylboronic acid

Entry	Catalyst	Catalyst loading	Time, h	Temperature, °C	Isolated Yield
1	Pd‐Schiff base@MWCNTs	0.1 mol%	2	60	98 [49]
2	Pd/Fe3 O4	0.1 mol%	18	60	92 [50]
3	Pd/IL‐NH2 /SiO2 /Fe3 O4	0.5 mol%	6	rt	82 [51]
5	PdCl2	3.0 mol%	5	rt	96 [46]
6	Pd NPs	1.0 mol%	12	100	88 [52]
7	Pd NPs	0.01 mmol%	0.5	60	98.86 (this work)

3.9 Stability of the Pd NPs after reaction

Pd black was observed after the reaction, which indicated that part of the Pd NPs aggregated. After post‐processing, the Pd species stays in the water phase and the products stay in the organic layer after extraction by EtOAc. Then, we added the water phase to the fresh substrate at the same conditions. The product yield decrease to 18%. It indicates that most of Pd NPs aggregated after the reaction, which lead to decrease the catalytic activity. The reaction phenomenon is illustrated in Fig. S1.

4 Conclusions

In summary, this preparation of Pd NPs by the reduction of Poplar leaf aqueous extract is truly a green and cost‐effective approach. The obtained Pd NPs had a small particle size, with average particle size of 4.2 nm, and high metal dispersion. The FTIR analysis indicated that biomolecules such as alkaloids, polyphenols or saccharides compounds were responsible for reducing the Pd (II) to (0) valent ions. The results from the Suzuki coupling reaction showed that as‐prepared Pd NPs were found to possess excellent catalytic performance.