Microwave-assisted synthesis of Pd nanoparticles and their catalysis application for Suzuki cross-coupling reactions

Article abstract of DOI:10.1080/15533174.2016.1212228

Pd nanoparticles (PdNPs) were prepared via microwave-assisted chemical reduction method (PdNPs-MW) and a conventional chemical reduction method (PdNPs-C) using ethanol as the reducing agent in the presence of polyvinylpyrrolidone (PVP), respectively. The as-prepared PdNPs were characterized by UV–vis spectroscopy, TEM, XRD, and X-ray photoelectron spectroscopy analysis. The results showed that PdNPs- MW had a uniform structure and narrower size distribution with diameters of 3.6–5.4 nm. An obvious increase in size of the PdNPs was apparent for PdNPs-C. The PdNPs-MW demonstrated 68.7% yield toward the Suzuki cross-coupling reaction of 4-bromobenzonitrile and phenylboronic acid for 10 min with a higher turnover frequency of 41220 h^-1 compared with the value of PdNPs-C (20580 h^-1).

Full text of DOI:10.1080/15533174.2016.1212228

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Microwave-assisted synthesis of Pd nanoparticles
and their catalysis application for Suzuki cross-
coupling reactions
Jian Zhang & Xuefeng Bai
To cite this article: Jian Zhang & Xuefeng Bai (2016): Microwave-assisted synthesis of
Pd nanoparticles and their catalysis application for Suzuki cross-coupling reactions,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI:
10.1080/15533174.2016.1212228
To link to this article: http://dx.doi.org/10.1080/15533174.2016.1212228
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Published online: 12 Aug 2016.
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Microwave-assisted synthesis of Pd nanoparticles and their catalysis application for Suzuki
cross-coupling reactions
Jian Zhanga，Xuefeng Baia,b*
a School of Chemistry and Material Sciences, Heilongjiang University, Harbin 150080, China
b Institute of Petrochemistry, Heilongjiang Academy of Sciences, Harbin 150040, China
*Corresponding author. E-mail: tommybai@126.com
Abstract
Pd nanoparticles (PdNPs) were prepared via microwave-assisted chemical reduction
(PdNPs-MW) and a conventional chemical reduction method (PdNPs-C) using ethanol as the
reducing agent in the presence of polyvinylpyrrolidone (PVP), respectively. The as-prepared
PdNPs were characterized by UV-Vis spectroscopy, TEM, XRD and XPS analysis. The results
showed that PdNPs-MW had a uniform structure and narrower size distribution with diameters
of 3.6-5.4 nm. An obvious increase in size of the Pd nanoparticles was apparent for PdNPs-C.
The PdNPs-MW demonstrated 68.7% yield towards the Suzuki cross-coupling reaction of 4-
bromobenzonitrile and phenylboronic acid for 10 min with a higher turnover frequency (TOF) of
41220 h-1 compared with the value of PdNPs-C (20580 h-1).
Keywords: microwave-assisted chemical reduction; Pd nanoparticles; Suzuki cross-coupling
reactions
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1. Introduction
The Suzuki cross-coupling reactions of arylboronic acids and aryl halides provide an
effective and attractive synthetic approach to biaryls [1-3], which play an important role as
precursors in the synthesis of natural product, fine chemicals, pharmaceuticals, and advanced
materials [4-5]. The transition metal palladium has become one of the most available catalysts in
organic syntheses, which is mainly applied in cross-coupling reactions for the formation of C–C
bonds.
The Pd catalysts used in Suzuki cross-coupling reactions has drawn a great deal of attention
in both homogenous catalysis in the presence of different ligands [6-8] and heterogeneous
catalysis in which the nanoparticles are supported on solid surfaces (e.g., polymer [9], silica [10],
zeolite [11], graphene [12] or activated carbon [13]). The homogeneous catalysis suffers from a
number of drawbacks which lie in the preparation of the ligands and separation of the catalyst,
despite its remarkable catalytic activity. Alternatively, heterogeneous catalysis allows efficient
separation and avoids the preparation of ligands, while the applications of heterogeneous
catalysis are also limited due to a poor mass transfer result in a low reaction rates. By contrast,
Pd nanoparticles highly dispersed in reaction system result in an excellent mass transfer exhibits
high activity. Moreover, a reduction for preparation cost in the absence of ligands is satisfied. Pd
nanoparticles used as a catalyst for Suzuki cross-coupling reactions have attracted a growing
interest in recent years.
The general trend of increased catalytic activity with the decrease in the particle size is
frequently observed [14]. However, there remains challenging for the aggregation and
precipitation of nano-sized particles out of solution during the preparation process in the absence
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of stabilizers. In order to get large quantities of uniform, stable, and monodisperse Pd
nanoparticles in nano-scale range. The selection of a suitable stabilizer is an available way to
avoid the aggregation of nanoparticles in the preparation process. A common method to prepare
metal nanoparticles involves the reduction of metal ions in the presence of stabilizing ligands
such as polymers [15], surfactants [16], dendrimers [17], phosphine [18] and thioether [19].
Polyvinylpyrrolidone (PVP) is one of most used polymers for stabilization due to its water-
solubility, non-toxic and cheap price [20-21].
The particle size distribution and surface structure of the nanoparticles play an important
role in the catalytic activity. Therefore, numerous efforts have been conducted to explore
effective and facile synthetic method to get well-defined and uniform-sized metal NPs [22]. In
addition to the conventional chemical reduction, microwaves-assisted chemical reduction
provides a simple and fast way to prepare nanoparticles. The main advantage of microwave-
assisted chemical reduction over other conventional water bath heating is that the reaction
system achieves a uniform and rapid heating. Therefore, a more homogeneous nucleation as well
as a shorter crystallization time can be obtained, generating metal particles in nanometer size
with narrow size distribution and uniform structure.
In this paper, the Pd nanoparticles were prepared by microwave-assisted chemical
reduction. The catalytic performances of obtained Pd nanoparticles for the Suzuki cross-coupling
reaction were examined and compared with the Pd nanoparticles prepared by a conventional
chemical reduction.
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2. Materials and methods
2.1 Materials
All commercially available reagents were used without further purification. The PdCl2,
NaCl, ethanol and polyvinylpyrrolidone (PVP) were purchased from Sinopharm Chemical
Reagent Co. Ltd. K2CO3 was purchased from Tiajin Kermel Chemical Reagent Co. Ltd.
2.2 Characterization
The 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. TEM images were obtained using
a FEI Tecnai G2 S-TWIN Transmission electron microscope operated at 200 kV. Samples for
TEM were prepared by placing a droplet of a colloid suspension in ethanol on a 300-mesh
copper grid and allowed to evaporate in air at room temperature. The X-ray photoelectron
spectroscopy (XPS) analysis was performed on a Thermo Fisher Scientific ESCALAB 250 using
a monochromatic Al KR X-ray. Absorption measurements were carried out on a UV 2450
Shimadzu spectrophotometer. The spectra were recorded at room temperature using a quartz
cuvette as sample-holder. Samples for UV-Vis spectroscopy were prepared by diluting the as
prepared suspension of Pd nanoparticles with deionized water in order to load the same metal
concentration for every sample into the cuvette.
2.3 Preparation of PdNPs with microwave-assisted chemical reduction
In a typical synthesis procedure, a 0.05M Na2PdCl4 solution was first prepared by adding
0.0887 g of PdCl2 and 0.0585 g NaCl into 10 mL of deionized water. Then 0.555 g of
polyvinylpyrrolidone (PVP) was added into the prepared Na2PdCl4 solution under stirring. After
0.5 h, 6 mL of ethanol was injected into solution with further stirring for another 0.5 h. Then the
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mixture was transferred to the microwave reaction vessel (XinYi MDS-6) equipped with a
temperature sensor. The mixture was then subjected to microwave irritation for 2 h at 80°C.
After the reaction, a black stable suspension of Pd nanoparticles was obtained. The as-prepared
PdNPs was denoted as PdNPs-MW.
2.4 Preparation of PdNPs with conventional chemical reduction
The experiments were performed following the same procedure mentioned above except for
the ethanol was slowly added as the reducing agent and the mixture was stirred for an additional
4h heating at 80°C in water bath. The resultant PdNPs was denoted as PdNPs-C.
2.5 General procedure for Suzuki reactions
Aryl halides (4 mmol, 1 eq.) was dissolved in a mixture of 48 mL H2O: EtOH (1:1) and
placed in a three-necked bottle (100 mL). To this were added the phenylboronic acid (6 mmol,
1.5 eq.) and potassium carbonate (8 mmol, 2 eq.). PdNPs catalyst (0.4 mmol‰ Pd) was then
added and the mixture was vigorously stirred at 60°C under air atmosphere for 10 min. 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 organic layers were combined, dried in air to give a solid
product which was detected in high performance liquid chromatograph (HPLC).
3. Results and discussion
The progression of the reaction and forming of the PdNPs-MW and PdNPs-C are recorded
by the UV-vis spectrum, respectively. As shown in Fig. 1, a strong characteristic absorption peak
at around 420 nm which is attributed to the Pd (II) ions in the absence of reducing agent. With
the increase of reduction time, the intensity of the absorption peak turns weak and the yellow
color of Pd (II) solution gradually changed into black indicating the forming of Pd (0). For the
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PdNPs-MW, the peak at 420 nm totally disappears with a reduction time for 2 h indicating the
reduction of Pd (II) to Pd (0) is completed. However, a longer reduction time is needed for the
PdNPs-C. The result demonstrates that microwave irritation is an effective and fast method to
generate Pd nanoparticles.
The particles size distribution of the Pd nanoparticles displays an important influence on
their catalytic activities. The TEM images and particles size distribution of as-prepared PdNPs-
MW and PdNPs-C are presented in Fig. 2. From the image, the clear crystal lattice fringes with a
spacing of 0.226 nm corresponding to the (111) facets of palladium are shown in Fig. 2 (b).
Besides, the PdNPs-MW with near spherical shapes uniformly distributed in the system; these
particles have a narrow size distribution with diameters ranging from 3.6 to 5.4 nm according to
the particle size distribution histogram, and the average particle sizes is about 4.71 nm. However,
the PdNPs-C of different morphology such as tetrahedron, cube and polyhedron with an obvious
increase in particle size are observed. The particle size of PdNPs-C is between 1.3 and 26.8 nm
(average diameter of 9.54). A possible reason for this is the ability to heat the catalytic system
uniformly in microwave irritation reduction progress forming the Pd nanoparticles with uniform
structure and narrow size distribution. Otherwise, the conventional chemical reduction method
possess a longer reduction time than microwave result in a further growth of Pd nanoparticles.
The XRD patterns of PdNPs-MW and PdNPs-C dried powders are displayed in Fig. 3.
Three characteristic peaks observed at 2θ= 40.1°, 46.6°, 68.1° which are corresponding to Pd
(111), Pd (200), Pd (220) lattice phases, respectively. The position of all diffraction peaks
matched well with the standard face centered cubic structure according to the JCPDS card no.
46-1043. However, the diffraction peaks of PdNPs-MW are weaker and broader than those of
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PdNPs-C, indicating that the PdNPs-MW has a smaller Pd nanoparticles size than PdNPs-C. In
addition, the peaks with 2θ of 28.3°, 31.7°, 56.4° which are ascribed to (111), (200) and (222) of
NaCl are also observed in all samples.
XPS is employed to obtain the information on the electronic state of the surface region of
the Pd nanoparticles. As shown in Fig. 4, the observed binding energies of Pd3d5/2 and Pd3d3/2
at 335.0 eV and 340.4 eV indicate the presence of Pd (0). The binding energy is coincident
with the value reported in literatures which is 335.1 eV and 340.4 eV, respectively. However, the
binding energy values at 337.2 eV (3d5/2) and 342.4 eV (3d3/2) are assigned to Pd2+. Similarly,
four peaks of PdNPs-C are ascribed to Pd (0) and Pd2+. Besides, the reduction degree of 66%
obtained under microwave reduction method is slightly higher than the value of 65% obtained
under conventional reduction method.
To evaluate the catalytic activity of PdNPs-MW and PdNPs-C, Suzuki cross-coupling
reaction was carried out as the model reactions. The influence of using EtOH/H2O, EtOH and
H2O as the solvent in the Suzuki reaction was studied. As shown in Table 1, the reaction in pure
water was sluggish and the low yields of 14.4% for PdNPs-MW and 10.8% for PdNPs-C were
obtained in neat ethanol (Table 1, entries 2, 3). The satisfactory result in terms of yield is
obtained by using EtOH/H2O (1:1) co-solvent (Table 1, entry 1). The merit of the co-solvent is
attributed to the good solubility of the organic reactants and the inorganic base.Thus, we adopt
EtOH/H2O (1:1) as the solvent for this catalytic reaction.
To survey the generality of the catalytic protocol, we investigated the reaction using Suzuki
cross-coupling reactions of aryl halides with phenylboronic acid in the presence of K2CO3 using
EtOH/H2O (1:1) as solvent. Unsurprisingly, aryl bromides are more reactive than aryl chlorides.
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Although extending the reaction time and increasing the reaction temperature, the poor yields
were obtained for aryl chlorides (Table 1, entries 8-10). On the other hand, the PdNPs-MW
obtains a higher catalytic activity with higher turnover frequency (TOF) than PdNPs-C by
comparing the yield for same substrates under the same reaction conditions. For instance, the
reaction between 4-bromobenzonitrile and phenylboronic acid, PdNPs-MW provides a 68.7%
yield of the coupling product. A lower yield of 34.3% is obtained by using PdNPs-C as catalyst.
These results display a higher catalytic activity towards Suzuki cross-coupling reaction for
PdNPs-MW with a higher TOF of 41220 h-1 (Table 1, entry 1). The excellent catalytic activity of
PdNPs-MW may be attributed to the uniform structure and small particle size of Pd nanoparticles
(3.6-5.4 nm). We then further extended the scope of reaction substrates to heteroaryl bromides,
and 22.8%-32.2% yields were observed (Table 1, entries 11-13).
4. Conclusions
In summary, a simple, versatile and fast synthesis method to generate highly active Pd
nanoparticles was prepared by microwave-assisted chemical reduction method. The obtained
PdNPs of near spherical shapes have a narrow size distribution with particle size of 3.6-5.4 nm.
Moreover, the PdNPs-WM were of smaller particle size and more dispersed than PdNPs-C
result in a higher catalytic activity for Suzuki cross-coupling reactions.
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Table 1 Representative Suzuki cross-coupling reaction performed using the PdNPs catalyst.a
Entr
y
ArX
Time
Solvent (v/v)
Yield b/%
Yield c/%
Temp/℃
1d
10min
10min
10min
10min
10min
60
60
60
60
60
EtOH/H2O (1:1)
Neat EtOH
68.7
14.4
Trace
99.0
95.0
34.3
10.8
Trace
91.0
82.0
2
3
4
5
Pure water
EtOH/H2O (1:1)
EtOH/H2O (1:1)
6
7
10min
10min
60
60
EtOH/H2O (1:1)
EtOH/H2O (1:1)
85.1
39.1
58.3
28.8
8e
6h
80
EtOH/H2O (1:1)
26.5
20.8
9e
6h
6h
6h
80
80
80
EtOH/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
18.2
13.4
32.2
14.6
11.5
27.5
10e
11e
12e
13e
6h
6h
80
80
EtOH/H2O (1:1)
EtOH/H2O (1:1)
25.5
30.7
22.8
28.6
a Reaction conditions: aryl bromides (4 mmol), phenylboronic acid (6 mmol), PdNPs (0.4
mmol‰), K2CO3 (8 mmol), ethanol/H2O (24 mL/24 mL) .
b catalyst: PdNPs-MW
c catalyst: PdNPs-C
d TOF values in (mol aryl bromides)(molPd)-1h-1 of the PdNPs was determined on the basis of the
yield of biphenyl.
e 4mmol‰ of Pd was used.
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100
(a)
80
60
40
20
0
0min
30min
60min
120min
240
320
400
480
560
640
720
800
Wavelength/nm
100
(b)
80
60
40
20
0
0min
60min
120min
180min
240min
240
320
400
480
560
640
720
800
Wavelength/nm
Fig. 1. UV–vis spectrum of PdNPs: (a) PdNPs-MW; (b) PdNPs-C.
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Fig. 2. TEM images of PdNPs: (a) PdNPs-MW; (b) a close vision of PdNPs-MW; and (c)
PdNPs-C.
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(111)
(a)
(b)
(200)
(220)
20
30
40
50
60
70
80
2θ /degree
Fig. 3. XRD patterns of PdNPs: (a) PdNPs-MW; (b) PdNPs-C.
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Pd0(3d5/2
)
(a)
Pd0(3d3/2
)
Pd+2(3d5/2
)
Pd+2(3d3/2
)
344
342
340
338
336
334
332
330
Binding Energy/eV
Pd0(3d5/2
Pd+2(3d5/2
)
(b)
)
Pd0(3d3/2
Pd+2(3d3/2
)
)
344
342
340
338
336
334
332
330
Binding Energy/eV
Fig. 4. XPS spectra of PdNPs: (a) PdNPs-MW; (b) PdNPs-C.
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