Biosynthesis of supported Pd nanoparticles using poplar leaf as a reducing agent and carrier: A green route to highly efficient and reusable Suzuki coupling reaction catalyst

Article abstract of DOI:10.1080/24701556.2017.1284114

A green process for the synthesis of supported Pd nanoparticles (NPs) using poplar leaf extract (PLE) as a reducing agent and the extracted residue of poplar leaf (RPL) as a carrier is reported. The Pd/RPL nanocomposites were characterized by XRD, TEM, SEM, EDS, XPS, FTIR, and ICP-AES. The spherical Pd NPs had a mean particle size of 3.1 nm and were evenly distributed on the RPL surface. More importantly, the Pd/RPL nanocomposites were used as a heterogeneous catalyst, which presented superior catalytic activity for the Suzuki coupling reaction. In addition, only a slight loss in the catalytic activity was observed after six recycles.

Full text of DOI:10.1080/24701556.2017.1284114

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Biosynthesis of supported Pd nanoparticles using
Poplar leaf as a reducing agent and carrier: A
green route to highly efficient and reusable Suzuki
coupling reaction catalyst
Guanghui Liu, Xuefeng Bai & Hongfei Lv
To cite this article: Guanghui Liu, Xuefeng Bai & Hongfei Lv (2017): Biosynthesis of supported Pd
nanoparticles using Poplar leaf as a reducing agent and carrier: A green route to highly efficient
and reusable Suzuki coupling reaction catalyst, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2017.1284114
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1284114
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Biosynthesis of supported Pd nanoparticles using Poplar leaf as a reducing agent and carrier: A
green route to highly efficient and reusable Suzuki coupling reaction catalyst
Guanghui Liua, Xuefeng Baia,b,*, Hongfei Lvb
aSchool of Chemistry and Material Sciences, Heilongjiang University, Harbin 150080, China
bInstitute of Petrochemistry, Heilongjiang Academy of Sciences, Harbin 150040, China
*Corresponding author: tommybai@126.com
Abstract
A green process for the synthesis of supported Pd nanoparticles (NPs) using Poplar leaf
extract (PLE) as a reducing agent and the extracted residue of Poplar leaf (RPL) as a carrier is
reported. The Pd/RPL nanocomposites were characterized by XRD, TEM, SEM, EDS, XPS,
FTIR and ICP-AES. The spherical PdNPs had a mean particle size of 3.1 nm and were evenly
distributed on the RPL surface. More importantly, the Pd/RPL nanocomposites was used as a
heterogeneous catalyst, which presented superior catalytic activity for the Suzuki coupling
reaction. In addition, only a slight loss in the catalytic activity was observed after six recycles.
Keywords
Pd nanoparticles; Biosynthesis; Heterogeneous catalyst; Poplar leaf; Suzuki coupling reaction
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1. Introduction
The Suzuki coupling reaction is a powerful and popular reaction in the field of organic
synthesis [1], because it is widely used to construct C-C bonds for synthesis biaryl compounds,
which are often considered as partial structures in natural products [2, 3], pharmaceuticals [4] and
advanced functional materials [5].
Traditionally, the coupling reactions are catalyzed by homogeneous Pd catalyst, which is
coordinated with different ligands or other protective agents to produce water-soluble catalysts [6,
7] and can provide high reaction rate or high yield [8-10]. However, the main defect for this
catalytic systems is the product separation and the catalyst reuse in consecutive reactions. In order
to overcome these problems, heterogeneous Pd catalysts have got a considerable concern in
recent years [11-13]. Numerous efforts have been made to immobilize Pd on a variety of solid
supports, such as microporous polymers [14], nano-silica [15],TiO2 [16] and carbon materials [17,
18], which made the catalyst easy to be separated from the reaction mixture by centrifugation.
The synthesis of Pd NPs were employed by chemical or physical methods, which requires
high pressure, energy and toxic chemicals. From the ‘green chemistry’ point of view, the
utilization of biological systems has been became a new method for the synthesis of metal NPs,
because it is suitable to develop a low-cost, reliable and eco-friendly route for preparation nano
materials [19, 20]. An amount of effort has been committed to green synthesis of metal NPs using
bacteria [21-24], fungi [25-27], and plants [28-31]. Among the various biosynthetic methods, the
synthesis of metal NPs using plants, which is a low-cost and renewable resources, has more
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benefits over microbes, which require high-cost to maintain microbial cultures and downstream
processing.
Poplar is the Salicaceae family plant that widely distributed in the world. The main classes
of secondary metabolites presented in this plant are alkaloids, terpenes, tannins, phenolic
glycosides and flavonoid [32, 33]. Due to these potential antioxidant compounds exist in this
plant. So it can serve as a good and green source for the reduction of Pd2+ to synthesize metal Pd
NPs. Although the present study on biological methods for synthesis of Pd NPs was attracted
widespread attention. There is no report on employing the extracted residue of plants as the
carrier to synthesize supported Pd NPs.
In this work, we present a ‘green’ method for the synthesis of supported Pd NPs using
Poplar leaf extract as the reducing agent and the extracted residue of Poplar leaf as the carrier.
Surprisingly, the as-prepared Pd/RPL complexes could serve as a heterogeneous catalyst, which
exhibited excellent catalytic activity for the Suzuki coupling reactions. In addition, only a slight
loss in the catalytic activity was observed after six recycles.
2. Materials and Methods
2.1. Instruments and reagents
The Poplar leaves were collected from in and around Harbin, Heilongjiang, China. All
chemicals were purchased from Tiajin Kermel and Sinopharm Chemical Reagent Co., Ltd. And
all the chemicals were used without further purification. XRD analysis were recorded by a
Bruker D8 advance diffractometer equipped with Cu Kα radiation. TEM was obtained on a
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JEM-2100FX-200kV. ICP-AES was detected by ICP-7510 (Shimadzu). SEM was performed on
a JEOL IT300. FTIR spectra was recorded on Avatar Nicolet 6700 using pressed KBr pellets
with a scanning range from 500 to 4000 cm1. X-ray photoelectron spectrometer (XPS) analysis
was conducted on a Kratos AXIS ULTRA DLD spectrometer with Al X-ray source.
2.2. Preparation of PLE and RPL
The sundried Poplar leaves were milled. Then, 2 g of the milled powder was added into 100
ml deionized water and refluxed at 50°C for 30 min. The PLE obtained was centrifuged at 4000
rpm and filtered through Whatman No.1 filter paper. The filtrate was collected in an
Erlenmeyer flask and stored at 4°C for further use. The filter residue was the RPL, which serves
as the carrier for immobilization Pd NPs.
2.3. Synthesis of Pd/RPL catalysts
To synthesize the Pd/RPL nanocomposites with 5 wt% Pd, which denoted as Pd0.05/RPL,
RPL (0.2 g) was dispersed in the Na2PdCl4 solution (10 ml, 0.01 M) and impregnated for 12 h at
room temperature. Then, the Pd2+ was adsorbed on the RPL surface. Subsequently, 10 ml of the
PLE was added into this precursor mixture with stirring. The color of the mixture gradually
changed from light yellow to deep black in 2 h at 80°C. It indicated that reduction of Pd2+ to Pd0
was completed around 2 h. For comparison, Pd0.01/RPL, Pd0.03/RPL and Pd0.10/RPL
nanocomposites were synthesized under the same experimental conditions.
2.4. Catalytic Suzuki Coupling Reactions
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Aryl halides (2.5 mmol), K2CO3 (5 mmol), EtOH (15 mL), H2O (15 mL) and
Pd0.05/RPL (0.1 mmol%) were uniformly mixed. Then, phenylboronic acid (3.75 mmol)
was added into the mixture with vigorous stirring at 60°C. After the reaction, the product
(5 ml) was added to NaOH (0.2 mol/L, 5 ml) and extracted with EtOAc (10 ml). Then,
the product yields were determined by HPLC analysis.
2.5. Catalyst recyclability
After the reaction, the catalyst was collected by centrifugation step and washed several
times with H2O and EtOAc. In order to investigate the level of reusability and activity of the
present catalyst. The recovered catalyst was reused for the coupling reaction of 4-bromotoluene
with phenylboronic acid.
3. Results and discussion
3.1. Mechanism of synthesis of Pd/RPL
The RPL is mainly composed of hemicellulose and cellulose. These nature compounds
come into being slight negative charge on the RPL surface, because of the hydroxyl ionization
when dispersed in water [34]. Under the electrostatic interaction, the Pd2+ can be absorbed on the
RPL surface. It can occur in mixing of RPL in Na2PdCl4 solution at the beginning of the process.
In addition, potential antioxidant compounds, such as alkaloids, terpenes, tannins, phenolic
glycosides and flavonoid, in this plant extract will be supplied the required reducibility
functional groups (-OH), which can be responsible for the reduction of Pd2+ to synthesize metal
Pd NPs. The synthesis of Pd NPs supported on the RPL is schematically represented in Fig. 1.
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3.2. UV-vis Analysis
The reduction process of the Pd NPs was monitored by UV-visible spectroscopy at 240-800
nm. Fig. 2 shows the absorption spectra of palladium colloidal solution at 0.5 h intervals. The
absorption bands appearing in the contrast spectrum of Na2PdCl4 solution were ascribed to the
ligand-to-metal charge-transfer transition of the Pd (II) ions. The absence of absorption peak
above 420 nm in all of the samples indicates that the initial Pd (II) species was adequately
reduced to Pd (0). It was observed from Fig. 2 that the optimal time for the formation of Pd NPs
was 2 h.
3.3. TEM and ICP-AES Analysis
In Fig. 3, the TEM images are further investigated the size and morphology of the Pd NPs,
which are supported on the RPL surface. With low palladium loads (1 wt% and 3 wt%), the
spherical Pd NPs are quantitatively sparse dispersed on the RPL surface (Fig. 3a and b). At
appropriate palladium loads (5 wt%), the spherical Pd NPs are more uniformly dispersed and
well aligned on the RPL surface (Fig. 3c). According to the particle size distribution histograms
the average particle size of the Pd0.05/RPL is 3.12 nm (Fig. 3g), smaller than Pd0.01/RPL (5.2 nm,
Fig. 3e), Pd0.03/RPL (5.0 nm, Fig. 3f) and commercial 5wt % Pd/C (5.7 nm, Fig. S1). The
high-resolution transmission electron microscopy (HRTEM) pattern further indicated that the Pd
crystal plane is 0.224 nm (Inset Fig. 3C), which is corresponded to the lattice spacing of the
(111) plane of metallic Pd [35]. When the Pd loads increase to 10 wt%, which leads to the
formation of non-spherical Pd NPs, such as triangular, cubic and rod structures (Fig. 3d). It may
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be due to the lack of biomolecules to act as capping agents, so that the most of non-spherical Pd
NPs are formed and the average particle size increase to 10.5 nm (Fig. 3h). Apparently, it was
found that the Pd/RPL complexes have been successfully synthesized with the small-sized Pd
NPs highly distributed on the RPL in an appropriate Pd loads (5 wt%). The exact amounts of Pd
in Pd0.05/RPL are detected by ICP-AES and the content of Pd is 4.56 wt%.
3.4. XRD analysis
The X-ray diffraction was used to confirm the crystalline phase of the Pd/RPL complexes
(Fig. 4). With the low Pd loading of 1wt% (Fig. 4a), the peak of Pd NPs are not obvious due to
the relatively broad reflexes indicating that the quantitatively sparse or small size of Pd NPs are
random dispersion on the RPL surface. The result showed that no the characteristics diffraction
peak of Pd is observed by XRD up to a Pd loading of 3wt%. Only a broad (111) and a weak
(200) peak of metallic Pd are observed (Fig. 4b). Up to a Pd loading of 5 wt% and 10 wt% (Fig.
4c and d), the peaks located at 40.1°, 46.1° and 67.9° are observed, which are indexed to the
lattice planes of (111), (200) and (220), respectively. And this correspond to the Bragg
reflections of face the centered cubic (fcc) structure of Pd (JPDS 87-0643) [36].
3.5. XPS Analysis
The XPS spectrum of the Pd 3d region for the Pd0.05/RPL complexes is shown in Fig. 5. The
peak at 340.6 eV (3d3/2) and 335.2 eV (3d5/2) belong to Pd0, and the other peaks at 342.3 eV
(3d3/2) and 337.1 eV (3d5/2) were assigned to Pd2+ [37]. On the basis of the peak areas analysis,
the percent of zero-valent Pd in the Pd0.05/RPL is 70%. A possible reason for the existence of
Pd2+ is that the reduction of Pd2+ does not been proceed to completion during the fabrication.
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Another reason is that the exposed zero-valent Pd can be further oxidized into divalent Pd at
ambient conditions.
3.6. SEM and EDS Analysis
The SEM image of Pd0.05/RPL catalyst is shown in Fig. 6a. It can be observed that the
sample was consist of platelet-like sheets which consists of crushing plant cell walls. In Fig. 6b,
the EDS spectrum of Pd0.05/RPL shows that the nanocomposite is consisted of C, O and Pd. And
the Pd atomic ratio of the sample was 6.15%, approximately equal to the theoretical value.
3.7. FT-IR Analysis
As show in Fig. 7a, the FTIR absorbance peaks of PLE appear at 3400, 2923, 1728, 1600,
1396, 1265 and 1072, cm1 which are corresponding to O-H stretching vibration (3400 cm1),
carbonyl group (C = O, 1728 and 1600 cm1), C-H bending vibration of alkane (2923 and 1265
cm1), C-OH (1396 cm1) and C-O-C (1072 cm1) stretching vibrations, respectively. Compere
with PLE and Pd NPs revealed that the intensity of peaks at 1600, 1396 and 1072 cm1 decreased
after the reaction. Based on the above analysis, we speculate that the water soluble heterocyclic
compounds such as alkaloids, terpenes, tannins and phenolic glycosides can be acted as a
reducing agent for the reduction Pd2+ to synthesize metal Pd NPs. And the protein compounds,
with the (NH)-C = O groups, can serve as the stabilizing agent for protecting the Pd NPs.
Comparison between the FTIR spectrum of RPL and Pd0.05/RPL (Fig. 7b), the peaks at 1325
and 779 cm1 were distinct weakening after the reaction, which indicates that there is a
covalently coupled interaction between the Pd NPs and the RPL in the Pd/RPL complexes.
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3.8. Catalytic Activity Evaluation
The Suzuki coupling reaction of 4-bromotoluene with phenylboronic acid (Scheme.1) was
selected to compare the catalytic activity of the Pd/RPL complexes (1, 3, 5 or 10 wt%) and
commercial 5 wt% Pd/C (denoted as Pd0.05/C). The reaction was carried out in a rather mild
condition using EtOH/H2O (1:1) as solvent and K2CO3 as base with low amount of catalyst (0.1
mmol%) at 60°C.
Fig. 8 shows the yields of biphenyl at different intervals with various catalysts. In all of the
catalysts, the Pd0.05/RPL exhibits the best and faster catalytic activity, the yield of
4-bromotoluene reached to 98.2% with 0.1 mmol% catalyst at 60°C for 30 min. Lower catalytic
activity is obtained with the high Pd loads (10 wt%) and the low Pd londs (1 wt%), due to the
increase of the particle size and reducing of the degree of dispersion, respectively. This result is
also supported by TEM analysis. Therefore, the amount of dispersion and size uniformity are the
key factors that determine the catalytic activity of catalysts. In addition, the Pd0.05/RPL has a
higher catalytic activity than the commercial 5wt % Pd/C (denoted as Pd0.05/C). The excellent
catalytic activity can be attributed to the close interaction between the Pd NPs and rich hydroxyl
groups on the surface of RPL, which is beneficial to obtain the well-dispersed and small Pd
nanoparticles.
In order to prove catalyst heterogeneity, the possible presence of dissolved palladium was
investigated. After the coupling reaction had completed, the hot-filtered mother liquor was
reacted with fresh substrates. No product was detected even after 30 min (Fig. 9). This result
indicates that Pd0.05/RPL nanocomposite is indeed heterogeneous catalysts in the Suzuki
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coupling reaction. Furthermore, an ICP analysis of the hot-filtered mother liquor and the catalyst
after the reaction indicated that less than 0.1% of the Pd leached from the Pd0.05/RPL during the
reactions (Table S1).
In order to optimize the reaction conditions, we examined the effect of the solvents and base
for the Suzuki coupling reaction, and the results are shown in Tables S2 and S3. According to the
result of optimization and the principles of green chemistry, EtOH/H2O (v/v = 1:1) and K2CO3
was selected as the optimal conditions.
As shown in Table 1, various aryl bromides containing either electron-donating or
electron-withdrawing groups in the para position, the corresponding products were obtained in
good to excellent yields (Table 1, entries 1-5). The m-substituted aryl bromide (Table 1, entry 8),
gave the corresponding products in similar yields. The o-substituted aryl bromides (Table 1,
entries 6 and 7), gave the corresponding products in poor yields due to the steric effects.
Comparing with the commercial Pd0.05/C, the yields of Pd0.05/RPL is higher than the commercial
Pd0.05/C (Table 1, entries 2, 4 and 8).
From the economic point of view, the reusability of catalyst is crucial. As shown in Fig. 10,
the Pd0.05/RPL catalyst could be recycled easily by centrifugation step. In addition, although the
yield gradually reduced, a yield of 81% is still obtained in the sixth cycle. After six recycles, the
amount of Pd in the Pd0.05/RPL detected by ICP-AES is 3.86 wt%.
To determine the reason of the decrease catalytic performance, the TEM image of
Pd0.05/RPL catalyst after six times is shown in Fig. 11. Compering with the TEM of fresh catalyst
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(Fig. 3c), we can find that the Pd NPs have obvious agglomeration on the surface of the carrier
(Fig. 11a). According to the particle size distribution histograms the average particle size of the
Pd0.05/RPL is 16.5 nm (Fig. 11b). As shown in Table S1, less than 0.1% of leaching Pd was
observed in the catalyst recycled once. After recycled six times, the amount of palladium
remained at 3.86 wt%, indicating that the Pd leached from the Pd0.05/RPL during the reactions.
The most important conclusion drawn from these results is that the reduced catalytic activity was
due to the Pd leaching and agglomerating.
Conclusions
In conclusion, a highly efficient and reusable supported Pd NPs catalyst was successfully
prepared by using Poplar leaf extract as the reducing agent and the extracted residue of Poplar
leaf as the carrier. The spherical Pd NPs with small-size and highly-dispersed were successfully
prepared at appropriate Pd loads. And the Pd0.05/RPL catalyst demonstrated excellent catalytic
performance for the Suzuki coupling reaction and could be reused at least six times without
significant loss of activity. This method provides a simple and eco-friendly route for synthesis
supported Pd NPs, which will find potential application in other nano metal materials.
Acknowledgements
This project is financially supported by the National Natural Science Foundation of China
No.21276067, NSFC-RFBR No.214111301884, Program of International S&T cooperation
No.2013DFR40570 and Science Foundation of Heilongjiang Academy of Sciences.
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Table 1 The Suzuki coupling reaction of various aryl halides and phenylboronic acid.a
Yieldb /%
Entry
1
Aryl halide
Time/h
0.5
Pd0.05/RPL
98.2
Pd0.05/C
96.5
2
3
4
5
0.5
0.5
0.5
0.5
95.6
97.5
96.3
97.5
55.0
95.4
69.8
97.3
6
0.5
33.1
26.9
7
8
0.5
0.5
82.4
93.5
70.9
72.6
aReaction conditions: aryl halide (2.5 mmol), phenylboronic acid (3.75 mmol), Pd0.05/RPL (0.1
mmol%), K2CO3 (5 mmol), EtOH/H2O (15 mL/15 mL) at 60°C.
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b Isolated yields.
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Fig. 1. Schematic of biosynthesis of Pd NPs on RPL: Pd nanoparticles supported on (a) cellulose
and (b) hemicellulose.
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3
Na2PdCl4
0.5 h
1 h
1.5 h
2 h
2
1
0
300
400
500
600
700
Wavelength(nm)
Fig. 2. UV-vis spectrum of Pd NPs prepared with different time.
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40
35
30
25
20
15
10
5
e
Max:9.2 nm
Min:3.1 nm
Mean:5.2 nm
0
3
4
5
6
7
8
Diameter (nm)
Max:8.4 nm
Min:2.0 nm
Average:5.0 nm
35
30
25
20
15
10
5
f
0
2
3
4
5
6
7
8
Diameter (nm)
g
Max:5.4 nm
Min:1.6 nm
Mean:3.1 nm
40
30
20
10
0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Diameter (nm)
60
Max:22.07 nm
Min:3.09 nm
Average:10.52 nm
h
50
40
30
20
10
0
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22
Diameter (nm)
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Fig. 3. (a) TEM image and (e) size distribution of Pd0.01/RPL, (b) TEM image and (f) size
distribution of Pd0.03/RPL, (c) TEM image, HRTEM (Inset c) and (g) size distribution of
Pd0.05/RPL, (d) TEM image and (h) size distribution of Pd0.10/RPL.
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111
200
220
d
c
200
b
a
30
40
50
60
70
80
Two Theta (degree)
Fig. 4. XRD patterns of (a) Pd0.01/RPL, (b) Pd0.03/RPL, (c) Pd0.05/RPL and (d) Pd0.10/RPL.
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Pd0(3d5/2
)
Pd0(3d3/2
)
Pd+2(3d5/2
)
Pd+2(3d3/2
)
348
346
344
342
340
338
336
334
332
330
Binding Energy (eV)
Fig. 5. Pd 3d spectrum of the Pd0.05/RPL.
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1400
1200
1000
800
600
400
200
0
C K1
(b)
O  2
Pd L
Pd L
0
1
2
3
4
5
6
Energy/keV
Fig. 6. (a) SEM image and (b) EDS spectrum of Pd0.05/RPL catalyst.
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RPL
(b)
Pd NPs
(a)
Pd
/RPL
0.05
PLE
2337
779
1265
608
1325
1728
2923
1396
1072
1600
3400
4000
3500
3000
2500
2000
1500
1000
500
3900
3510
3120
2730
2340
1950
1560
1170
780
-1
Wavenumber (cm
Wavenumber (cm-1
)
)
Fig. 7. FTIR spectra of (a) PLE and Pd NPs, (b) RPL and Pd0.05/RPL.
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100
90
80
70
60
50
40
30
20
10
0
Pd0.05/RPL
Pd0.05/C
Pd0.10/RPL
Pd0.03/RPL
Pd0.01/RPL
RPL
0
10
20
30
40
50
60
70
80
90
Time/min
Fig. 8. Yields of biphenyl at different intervals with various catalysts.
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1.0
0.8
The hot-filtered mother liquor
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0
10
20
30
40
50
60
Time (min)
Fig. 9. Suzuki coupling reaction in the presence of the hot-filtered mother liquor.
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100
80
60
40
20
0
1
2
3
4
5
6
Cycle
Fig. 10. Reuse of Pd0.05/RPL catalyst for Suzuki coupling reaction.
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20
15
10
5
(b)
Max:23.7 nm
Min:7.6 nm
Average:16.5 nm
0
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21
Diameter (nm)
Fig. 11. (a) TEM image and (b) size distribution of Pd0.05/RPL catalyst after six times.
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