Microwave-assisted facile synthesis of palladium nanoparticles in HEPES solution and their size-dependent catalytic activities to Suzuki reaction

Article abstract of DOI:10.1166/jnn.2011.4722

Palladium nanoparticles (NPs) were successfully synthesized via a rapid and facile microwave route in HEPES (2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid) buffer solution. The shape- and size-controlled Pd nanoparticles could be obtained by one-step method without dependence of seed-mediated growth. The capping agent plays a key role in the formation of Pd NPs with different shape and size, which could be tuned by varying capping agents such as polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), sodium citrate (Na₃(cit)) and potassium bromide (KBr). The size-dependent catalytic activities of the obtained Pd NPs for Suzuki coupling reaction were also investigated. It demonstrated that the catalytic activity of Pd NPs was enhanced regularly with the decrease of particle size. Pd NPs less than 10 nm exhibited better catalytic activities for Suzuki reaction than the commercial Pd/C catalyst. Pd/MWCNTs and Pd/SBA-15 nanocomposites were also prepared by a facile method and afforded good catalytic activity and reusability. This "green" synthetic protocol could be used as a general method for the rapid synthesis of transition metal nanoparticles. Copyright

Full text of DOI:10.1166/jnn.2011.4722

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 7794–7801, 2011
Microwave-Assisted Facile Synthesis of
Palladium Nanoparticles in HEPES
Solution and Their Size-Dependent
Catalytic Activities to Suzuki Reaction
Wei Zhang1, Qiao Wang1, Fan Qin1, Huamin Zhou2, Zhong Lu1, and Rong Chen1ꢀ∗
1Key Laboratory for Green Chemical Process of Ministry of Education and School of Chemical Engineering and Pharmacy,
Wuhan Institute of Technology, Wuhan, 430073, P. R. China
2State Key Laboratory of Materials Processing and Die and Mould Technology,
Huazhong University of Science and Technology, Wuhan, 430074, P. R. China
Palladium nanoparticles (NPs) were successfully synthesized via a rapid and facile microwave route
in HEPES (2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid) buffer solution. The shape- and
size-controlled Pd nanoparticles could be obtained by one-step method without dependence of
seed-mediated growth. The capping agent plays a key role in the formation of Pd NPs with different
shape and size, which could be tuned by varying capping agents such as polyvinylpyrrolidone
(PVP), cetyltrimethylammonium bromide (CTAB), sodium citrate (Na3(cit)) and potassium bromide
(KBr). The size-dependent catalytic activities of the obtained Pd NPs for Suzuki coupling reaction
Delivered by Ingenta to: Nanyang Technological University
were also investigated. It demonstrated that the catalytic activity of Pd NPs was enhanced regularly
IP: 46.161.63.19 On: Wed, 08 Jun 2016 02:02:09
with the decrease of particle size. Pd NPs less than 10 nm exhibited better catalytic activities for
Copyright: American Scientific Publishers
Suzuki reaction than the commercial Pd/C catalyst. Pd/MWCNTs and Pd/SBA-15 nanocomposites
were also prepared by a facile method and afforded good catalytic activity and reusability. This
“green” synthetic protocol could be used as a general method for the rapid synthesis of transition
metal nanoparticles.
Keywords: Palladium Nanoparticles, Microwave, Size-Dependent, Suzuki Reaction, Catalytic
Activity.
1. INTRODUCTION
For example, palladium-catalyzed Suzuki coupling reac-
tion of aryl halides with arylboronic acids is recognized as
the most promising and powerful method for construction
of noble biaryls, which has been extensively used in the
synthesis of drugs and fine chemicals.20–25 Although pal-
ladium complexes and palladium salts were widely used
and exhibited good catalytic activities in Suzuki reactions,
there were clear drawbacks such as environmental prob-
lems, poor recyclability and violent reaction conditions.
Furthermore, the use of aqueous phase reaction medium
including neat water and water/organic co-solvent in
Suzuki reaction has received much more attention because
of its environmental friendliness.26–28 From the standpoint
of green chemistry, palladium nanoparticles catalysis in
water is beneficial for carrying out efficient reactions
under environmentally benign conditions and also facili-
tates separation of products from palladium nanoparticles
Metal nanoparticles have broad applications in electronic,
magnetic, optical and biological materials due to their
remarkable physical, chemical and biological properties,
which is quite different from bulk materials.1–4 However,
research into the catalytic activities of metal nanoparticles
in organic reactions is at an early stage of development
and still remains a great challenge. Transition metal
nanoparticles have been successfully applied as catalysts
in many organic reactions over the past decades.5–11
Among them, palladium nanoparticles have been found
to exhibit remarkable catalytic activities in carbon-
carbon
cross-coupling,12–15
hydrodechlorination,16ꢀ17
methoxycarbonylation18 and hydrogenation19 reactions.
∗Author to whom correspondence should be addressed.
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Zhang et al.
Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution
dispersed in water. Hence, many efforts have been devoted
to the development of novel Pd nanoparticles-based
catalyst with high catalytic activity, good recyclability,
simple product purification and little pollution to the
environment. On the other hand, it is well known that the
sizes of metal nanoparticles are determinants of catalytic
activity. Narayanan and El-Sayed addressed the size and
shape dependence of the catalytic properties of platinum
nanoparticles.29–31 However, few detailed investigations
into the catalytic activity of palladium nanoparticles with
different size were reported.18
To date, numerous methods about the synthesis of
palladium nanoparticles have been reported,32–34 such
as polyol reduction in the presence of capping agents,
and the decomposition of organometallic precursors.
However, many methods suffered from the drawbacks
of using hazardous organic solvents and chemicals or
performing reaction under rigorous conditions, which
were undesirable in the context of green chemistry
and restrictive for their subsequent application. Hence,
it is highly expected to develop a new method for
the synthesis of palladium nanoparticles in aqueous
medium and without the use of toxic reagents. Our
recent research work showed that 2-[4-(2-hydroxyethyl)-
1-piperazinyl]ethanesulfonic acid (HEPES) can serve as
efficient reductant and stabilizer in the preparation of tran-
best of our knowledge, there are few relevant literatures
about the investigation of catalytic activities of palladium
nanoparticles with different sizes.
2. EXPERIMENTAL DETAILS
2.1. Chemicals
All chemicals were purchased as reagent grade from
commercial suppliers and used without further purifi-
cation. 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic
acid (HEPES), polyvinylpyrrolidone (PVP, with aver-
age MW of 55,000) and palladium chloride (PdCl2ꢁ
were purchased from Sigma-Aldrich. Cetyltrimethylam-
monium bromide (CTAB) was purchased from Lancaster.
Ethanol (EtOH), potassium bromide (KBr), sodium cit-
rate (Na3(cit)), tetrabutyl ammonium bromide (TBAB)
and all organic substrates were purchased from Aladdin
(Shanghai, China). Multi-walled carbon nanotubes (MWC-
NTs) were provided by Shenzhen Nanotech Port Co., Ltd.
(China), and palladium (10%) on carbon catalyst (Pd/C)
were purchased from Sinopharm Chemical Reagent Co.,
Ltd. (Shanghai, China). SBA-15 was synthesized accord-
ing to literature.44
2.2. Preparation of Palladium Nanoparticles by
sition metal nanoparticles, including Au, Ag, Ru, Os, Rh,
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Microwave Irradiation Method
35–40 IP: 46.161.63.19 On: Wed, 08 Jun 2016 02:02:09
Ir, Pd and Pt nanoparticles.
In this synthesis, there
Copyright: American Scientific Publishers
are three aspects engaged in green chemistry, includ-
ing the choice of water as solvent, the reducing agent
employed, and the capping agent applied. However, these
palladium nanoparticles were obtained under hydrother-
mal conditions in HEPES solution. The problems of rigor-
ous experimental conditions and long reaction time limited
its further applications. Microwave-assisted technique is a
new promising method for one-pot synthesis of metallic
nanostructure in solution,41–43 which produces a uniform
heating of the solution, a more homogeneous nucleation
as well as a shorter crystallization time. It is therefore
very useful for the formation of monodisperse metal col-
loids. Microwave irradiation also has short thermal induc-
tion period, no convection processes, easy control and low
cost.
4 mL of 25 mM PdCl2 solution was added into a round-
bottom flask which contained 20 mL of HEPES buffer
solution (500 mM, pH = 7ꢂ4) under vigorous stirring.
ꢀ
Then the mixture was heated up to 100 C by an 800 W
microwave irradiation for 30 min with vigorous stirring.
After cooling down to room temperature, a dark-black
homogeneous palladium colloidal solution was obtained.
Palladium nanoparticles were collected by centrifugation
and washed with deionized water to remove impurity
and surfactants (Sample S1). Other palladium nanoparti-
cles (Samples S2–S5) were also prepared using the same
method in the presence of different capping agents. The
detailed procedure is same as described above and all the
experiment parameters are listed in Table I.
In this work, we develop a facile rapid microwave
irradiation route to prepare palladium nanoparticles in
HEPES buffer solution. By properly monitoring the exper-
imental condition, Pd nanoparticles with different shapes
and sizes could be obtained. We have also demonstrated
that the obtained palladium nanoparticles exhibited size-
dependent catalytic activities for Suzuki coupling reaction
in water/ethanol solution. Moreover, carbon nanotube-
supported and SBA-15-supported palladium nanocatalysts
have successfully been synthesized and used in Suzuki
reaction, which was found to proceed with the advantages
of both homogeneous and heterogeneous catalysts. To the
2.3. Preparation of Palladium Nanoparticles by
Refluxing Method
4 mL of 25 mM PdCl2 solution was added into a round-
bottom flask which contained 20 mL of HEPES buffer
solution (500 mM, pH = 7ꢂ4) and 0.066 g PVP under
vigorous stirring. Then the mixed solution was heated up
to 100 ꢀC and refluxed for 3 h with vigorous stirring.
After cooling down to room temperature, a dark-black
homogeneous palladium colloidal solution was obtained.
Palladium nanoparticles were collected by centrifugation
and washed with deionized water to remove impurity and
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Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution
Zhang et al.
Table I. Experimental parameters for the preparation of palladium col-
loids, and obtained nanoparticle size.
and energy-dispersive X-ray microanalysis (EDX). X-ray
diffraction (XRD) was carried on Bruker axs D8 Dis-
cover (Cu Kꢃ = 1ꢂ5406 Å). TEM, HRTEM images and
ED pattern were recorded on a Philips Tecnai 20 electron
microscope, using an accelerating voltage of 200 kV. A
small drop of the suspension in ethanol was deposited onto
a carbon-coated copper grid and dried at room tempera-
ture. Organic reactions were monitored by thin layer chro-
Synthesis
method
Capping
agenta
Average
size (nm)
Sample
Salt
Time
S1
S2
S3
S4
S5
S6
S7
S8
microwave
microwave
microwave
microwave
microwave
refluxing
—
CTABb
PVP
PVP
PVP
PVP
PVP
PVP
—
—
—
30 min
30 min
30 min
30 min
30 min
3 h
∼100
20∼50
9∼10
15∼20
5∼7
Na3(cit)c
KBrd
—
1
matography (TLC). All H NMR (400 MHz) spectra were
8∼12
13∼20
5∼6
refluxing
refluxing
Na3(cit)
KBr
3 h
3 h
recorded on a Bruker-Avance II 400 MHz spectrometer
using CDCl3 as the solvent. Flash column chromatography
was performed with silica gel (200–300 mesh).
aThe amount of capping agent PVP (Mw = 55000) is 0.066 g, bthe amount of
capping agent CTAB is 0.109 g, cthe amount of Na3(cit) is 0.088 g, dthe amount
of KBr is 0.149 g. All the other parameters were kept the same as sample S1. Note
that the concentration of HEPES buffer solution is 500 mM and 4 mL of 25 mM
PdCl2 solution was used in the reaction.
2.7. Suzuki Coupling Reactions
Catalytic activity of the synthesized palladium colloids
was tested through Suzuki reaction of aryl halides with
phenylboronic acid. A mixture of aryl halide (1 mmol),
phenylboric acid (1.5 mmol), K2CO3 (2 mmol), tetra-
butylammonium bromide (1 mmol), and 0.01 mmol of
the Pd nanoparticles were added into 9 mL ethanol/H2O
(v/v = 1/2) solution. The reaction mixture was then
surfactants (Sample S6). Samples S7 and S8 were also pre-
pared using the same method in the presence of different
capping agents under identical conditions (Table I).
2.4. Synthesis of Pd/MWCNTs Nanocomposites
Raw MWCNTs (diameter 40–60 nm) were firstly acidified
by ultrasonicating in 65 wt% nitric acid solution for 4 h
ꢀ
refluxed under the temperature of 86 C for 25–90 min.
ꢀ
and refluxed at 83 C for 4 h. The resultant MWWNTs
After cooling down to the room temperature, the product
was extracted with diethyl ether twice. Then the organic
phase was dried with anhydrous Na2SO4 and evaporated
were then filtered and washed with deionized water until
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the pH value of the filtrate was 6∼7. Solid product was
IP: 46.161.63.19 On: Wed, 08 Jun 2016 02:02:09
ꢀ
then collected and dried in a vacuum at 80 C for 12 h.
in vacuum. Product was obtained and purified by col-
umn chromatography using either petroleum ether or ace-
tone (5–12%)/petroleum ether (for yields see Table II). All
products are known in the literature and were identified
by comparing their 1H NMR spectra with the standard
spectrum.
Copyright: American Scientific Publishers
0.10 g of treated MWCNTs were added into the 24 mL
mixed solution of PdCl2 and HEPES, which contained
0.149 g KBr. Then the mixture was sonicated for 1 min
and treated under 800 W microwave irradiation for 30 min.
After cooling down to room temperature, the solid prod-
uct was collected by filtration and washed with deionized
ꢀ
water twice, then dried under vacuum at 40 C.
Table II. Products and yields for Suzuki coupling reactions of phenyl-
boronic acid and various aryl halides catalyzed by S5a.
2.5. Synthesis of Pd/SBA-15 Nanocomposites
Mesoporous silica SBA-15 supports were synthesized
according to literature.44 Then, 0.13 g of prepared SBA-15
were added into the mixed solution of PdCl2 and HEPES,
which contained 0.149 g KBr. Then the mixture was soni-
cated for 1 min and treated under 800 W microwave irradi-
ation for 30 min. After cooling down to room temperature,
the solid product was collected by filtration and washed
with deionized water twice, then dried under vacuum at
Entry
Substrate
Products
Time (min) Yieldb (%)
1
2
3
4
5
60
90
30
45
25
98
98
97
99
96
ꢀ
40 C.
2.6. Characterization
The Pd nanoparticles, Pd/MWCNTs and Pd/SBA-15
nanocomposites were characterized by the powder X-ray
diffraction (XRD), transmission electron microscopy
(TEM), high-resolution transmission electron microscopy
(HRTEM), selected area electron diffraction (SAED)
Notes: aAryl bromide (1 mmol), phenylboronic acid (1.5 mmol), K2CO3 (2 mmol),
TBAB (1 mmol), Pd nanoparticles (0.01 mmol) in 9 mL ethanol/H2O (v/v = 1/2)
solution. All reactions were performed at 86 ꢀC. bIsolated yield after purification
by column chromatography (eluent: petroleum ether or acetone/petroleum ether).
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Zhang et al.
Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution
3. RESULTS AND DISCUSSION
further investigate the function of the capping agent in
the growth of palladium nanoparticles, Na3 (cit) and KBr
3.1. Characterization of Palladium Nanoparticles
was introduced into the PVP stabilized reaction system.
It was observed by TEM image that irregular polyhedron-
like palladium nanoparticles were obtained when Na3 (cit)
was used as a capping agent in the synthesis. The size of
most of Pd nanoparticles varied from 15 to 20 nm, which
had a narrow size distribution (Fig. 1(D)). However, when
KBr instead of Na3(cit), well-separated uniform palladium
nanoparticles with an average diameter of 6 nm were fab-
ricated, which was demonstrated by its TEM observation
(Figs. 1(E and F)). The diameters of Pd nanoparticles had
a narrow size distribution which varied from 5 to 7 nm.
The enlarged TEM image of the Pd nanoparticles further
confirmed the spherical-like morphology and the unifor-
mity of the obtained palladium nanoparticles (Fig. 1(F)).
To verify the quick reduction of palladium salt in
HEPES buffer solution under microwave irradiation, pal-
ladium nanoparticles were also prepared by using sim-
ple refluxing method. Figure 2 showed the TEM images
of the as-prepared palladium nanoparticles (S6∼S8) by
simple refluxing for 3 h. Figure 2(A) showed a typical
image of the palladium nanoparticles prepared by reflux-
ing in the presence of PVP. It indicated that highly dis-
persed and well crystallized palladium nanoparticles were
obtained, whose particle size varied from 8∼12 nm. The
Palladium nanoparticles were successfully prepared by a
facile microwave-assisted heating method within 30 min.
As shown in Figure 1(A), anisotropic polygonal palladium
nanoparticles with an average size ∼100 nm were obtained
in the absence of any capping agent. However, the parti-
cle size varied greatly by adding different capping agent
under identical microwave heating condition. In the pres-
ence of capping agent CTAB, the sizes of obtained pal-
ladium nanoparticles with anisotropic structures were in
the range of 20∼50 nm and most of them were about
50 nm, which was shown in Figure 1(B). Among them,
some short palladium nanobars were observed. Figure 1(C)
showed the TEM image of the palladium nanoparticles
prepared in the presence of PVP, which indicated that uni-
form spherical-like particles with a diameter of 10 nm
were fabricated under such condition. It has a narrow size
distribution from 7 to 13 nm (inset of Fig. 1(C)). To
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corresponding HRTEM image of a selected Pd nanoparti-
IP: 46.161.63.19 On: Wed, 08 Jun 2016 02:02:09
cle revealed clear lattice fringes with a measured d-spacing
Copyright: American Scientific Publishers
of 0.229 nm as shown in Figure 2(B). When Na3 (cit) were
introduced into the reaction system, palladium nuclei also
Fig. 1. TEM images of as-obtained palladium nanoparticles prepared by
microwave irradiation method (A) Sample S1; (B) Sample S2; (C) Sam-
ple S3; (D) Sample S4; (E) and (F) Sample S5. Insets of (C) and (E) are
the corresponding size distribution graphs.
Fig. 2. TEM and HRTEM images of as-obtained palladium nanopar-
ticles prepared by refluxing method (A) and (B) Sample S6; (C) Sam-
ple S7; (D) Sample S8. Inset of (D) is the corresponding size distribution
graph.
J. Nanosci. Nanotechnol. 11, 7794–7801, 2011
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Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution
Zhang et al.
showed a tendency to form irregular polyhedron-like palla-
dium nanoparticles. It has a narrow size distribution from
13 to 20 nm as showed in the TEM image (Fig. 2(C)).
Figure 2(D) showed the TEM image of palladium nanopar-
ticles prepared by refluxing in the presence of KBr. Highly
dispersed uniform spherical-like palladium nanoparticles
were obtained, which had a very narrow particle size distri-
bution of 5∼6 nm (inset of Fig. 2(D)). There is no obvious
difference in morphology and size compared with the sam-
ple prepared by microwave heating under same condition.
The experiments established clearly demonstrated that
shape- and size-controlled palladium nanoparticles could
be synthesized by microwave irradiation method and
refluxing method via introducing different capping agents
in HEPES buffer solution. Compared with refluxing
method and reported hydrothermal method, the pro-
posed water-based microwave heating method provided a
more rapid and facile route for the synthesis of palla-
dium nanoparticles. It was found that capping agent has
remarkable influence on the size and shape of palladium
nanoparticles in this synthesis. As we known, palladium is
a face-centered cubic (fcc) metal, which can only be forced
to grow into anisotropic nanostructures through the kinetic
control. An extremely slow reduction rate could induce
the formation of stacking faults and thus break the cubic
symmetry.45 In this synthesis, HEPES served as a mild
reducing agent for the generation of palladium nanoparti-
cles with anisotropic nanostructures. The shape of product
could be effectively altered by manipulating the reduction
rate. The reduction rate increased when PVP was added,
which was also a mild reducing agent for the generation
of metal nanoparticles.29ꢀ46 As expected, Pd atoms nucle-
ate and grow into sphere-like nanoparticles in the pres-
ence of PVP. This point was also supported by the result
of a synthesis where DEG instead of EG was used as
the reductant.47ꢀ48 Moreover, capping agents can alter the
surface free energies via adsorption and thus induce new
shapes in a solution-phase process. In the present synthe-
sis, potassium bromide played a critical role in the for-
mation of palladium nanoparticles with a nearly spherical
shape. The bromide is able to chemisorb onto the surface
of Pd seeds and alter the order of surface free energies
for different facets, leading to the formation of sphere-
like shape when atomic addition is fast enough. How-
ever, when CTAB was introduced into the reaction system,
anisotropic palladium nanostructures were formed, such
as one-dimensional nanobar. It might be due to the driv-
ing force of the anisotropic growth generated by the long
surfactant tails, which was similar with the function of
CTAB in the formation of Au nanorods.49ꢀ50 The addition
of sodium citrate also could efficiently induce morphology
transformation, because that the formation of some special
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Copyright: American Scientific Publishers
Fig. 3. TEM images of the 9.6 wt% Pd/MWCNTs nanocomposites (A) and the 7.8 wt% Pd/SBA-15 nanocomposites (B); XRD patterns of the
Pd/SBA-15 and Pd/MWCNTs nanocomposites (C); EDX spectra of the Pd/SBA-15 and Pd/MWCNTs nanocomposites (D).
7798
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Zhang et al.
Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution
facets was promoted, resulting in the fabrication of irreg-
ular polyhedron-like palladium nanostructures.
Although zero valence palladium exhibited good cat-
alytic in C-C bond formation, there is an obvious problem
for the recycle of palladium in the further large-scale appli-
cation. Hence, it is highly desired to develop Pd-supported
heterogeneous catalysts, which were easily separated
from reaction system for recycled usage. In this study,
Pd/MWCNTs and Pd/SBA-15 nanocomposites were pre-
pared via a facile method. The detailed morphology and
structures of the 9.6 wt% Pd/MWCNTs and 7.8 wt%
Pd/SBA-15 nanocomposites was studied by transmission
electron microscopy and X-ray diffraction. As shown in
Figure 3(A), abundant small and highly dispersed palla-
dium nanoparticles were strongly adhered on the sidewalls
of the MWCNTs. The particle size was estimated to be
5∼6 nm. Figure 3(B) showed the magnified TEM image
of the Pd/SBA-15 nanocomposites. It revealed that pal-
ladium nanoparticles with a diameter about 5 nm were
confined in the hexagonal channels of SBA-15. No free
palladium nanoparticles were observed in the whole sam-
ple, indicating the thorough removal of unsupported pal-
ladium nanoparticles in the catalysts. Figure 3(C) showed
the XRD pattern of the as-synthesized Pd/MWCNTs and
Pd/SBA-15 nanocomposites. The typical palladium faced-
centered cubic crystalline structure could be found. The
Fig. 4. Yields for Suzuki coupling reactions between 4-iodophenyl
methyl ether and phenylboronic acid catalyzed by palladium catalysts
(1 mol% of palladium). (1) S8; (2) S5; (3) S3; (4) S2; (5) S1; (6) com-
mercial Pd/C; (7) Pd/MWCNTs; (8) Pd/SBA-15.
96% for Pd/MWCNTs and 99% for Pd/SBA-15 nanocom-
posites, which was comparable with S3, S5, S8 and better
than commercial Pd/C catalyst. It clearly demonstrated that
the size was a key factor affecting the catalytic activity of
palladium catalyst.
S5 was also used to catalyze the Suzuki coupling reac-
tions of a wide range of aryl halides with phenylboronic
acid and the results were summarized in Table II. As
EDX analysis also confirmed that the species supported on
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shown in Table II, aryl bromides (entries 1–3) and aryl
IP: 46.161.63.19 On: Wed, 08 Jun 2016 02:02:09
the MWCNTs and SBA-15 was Pd (Fig. 3(D)).
iodides (entries 4–5) efficiently reacted with phenylboronic
Copyright: American Scientific Publishers
acid to yield coupling products in good to excellent yields
(96%∼98%), indicating of high catalytic activity of S5 for
Suzuki reaction.
3.2. Catalytic Activity for Suzuki Reaction
To investigate the catalytic activity of the as-prepared
Pd nanoparticles with different sizes (S1, S2, S3, S5
and S8) and nanocomposites, the Suzuki coupling reac-
tion of 4-iodophenyl methyl ether with phenylboronic
acid was conducted and examined under identical con-
ditions, as shown in Figure 4. 1 mol% of the Pd cata-
lyst was employed in each reaction. It showed that the
yields of Suzuki coupling reaction generally decreased as
the increase of the palladium particle size: 98% for S8,
99% for S5, 98% for S3, while 73% for S2 and 71% for
S1. It revealed that the catalytic activity of Pd nanopar-
ticles varied obviously with its particle size. As depicted
in Figure 4, the catalytic activity was enhanced regularly
with the decrease of particle size. Pd nanoparticles with
small average size of 10 nm or less exhibited remarkable
catalytic activity for this coupling reaction, which afforded
better catalytic activities than the commercial Pd/C cata-
lyst (88%). Importantly, when the particle size was more
than 50 nm, Pd nanoparticles showed poor catalytic activ-
ity, which were worse than that of commercial Pd/C cat-
alyst (S1 and S2). The coupling reaction of 4-iodophenyl
methyl ether and phenylboronic acid was also carried out
under identical conditions using Pd/MWCNTs or Pd/SBA-
15 as a catalyst, respectively. It afforded good yield of
For practical application in the Suzuki reaction, the life-
time of the heterogeneous catalysts and their reusability
are very important factors. The recycle of Pd/SBA-15
nanocomposites was investigated. We have carried out
recycling uses of the 7.8 wt% Pd/SBA-15 catalyst for the
Suzuki reaction between 4-iodophenyl methyl ether and
phenylboronic acid, the coupling product was obtained
with a yield of 99% in the first cycle. The solid catalyst can
Fig. 5. Recycling of the 7.8 wt% Pd/SBA-15 nanocomposites in Suzuki
coupling reaction between 4-iodophenyl methyl ether and phenylboronic
acid.
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Microwave-Assisted Facile Synthesis of Palladium Nanoparticles in HEPES Solution
Zhang et al.
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ꢀ
As showed in Figure 5, the recycled catalyst was then used
in the model reaction under the same reaction conditions
giving 99%, 95%, 96%, and 93% isolated yield for the sec-
ond, third, forth and fifth recycles, respectively. After five
recycle, the Pd/SBA-15 catalyst almost maintains the orig-
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tering contrast between walls and pores. It indicated that
it was stable and could be readily recovered and reused
several times without significant loss of catalytic activity.
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4. CONCLUSION
In summary, we developed a rapid microwave synthe-
sis method of palladium nanoparticles in HEPES buffer
solution. The proposed water-based microwave route pro-
vided a more facile and environment-friendly method for
the synthesis of palladium nanoparticles. The shape and
size of palladium nanoparticles could be tuned by adding
different capping agents in HEPES buffer solution. We
also demonstrated that the obtained palladium nanoparti-
cles showed size-dependent catalytic activities for Suzuki
coupling reaction. The palladium nanoparticles less than
10 nm exhibited remarkable catalytic activities, which was
better than commercial Pd/C catalyst. The Pd/MWCNTs
23. S. Lightowler and M. Hird, Chem. Mater. 17, 5538 (2005).
Delivered by Ingenta to: Nanyang Technological University
and Pd/SBA-15 composites were also prepared by a facile
method and displayed high catalytic activity for Suzuki
IP: 46.161.63.19 On: Wed,2048. XJ.unZh2an0,1S6. 0W2an:0g,2:Y0. 9Liu, X. Wu, and D. Zhu, Chem. Mater.
15, 1963 (2003).
Copyright: American Scientific Publishers
25. S. D. Walker, T. E. Barder, J. R. Martinelli, and S. L. Buchwald,
reaction. The resulting Pd/SBA-15 nanocomposites exhib-
ited good recyclable quality and could be reused for the
Suzuki coupling reaction. This “green” synthetic protocol
also provided a general method for the rapid synthesis of
transition metal nanoparticles.
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Acknowledgments: This work was supported by Pro-
gram for New Century Excellent Talents in University
(NCET-09-0136), Outstanding Youth Scientific Foundation
of Hubei Province (2009CDA075), Open Research Fund
of State Key Laboratory of Materials Processing and Die
and Mould Technology, Huazhong University of Science
and Technology (2010-P06) and the Program for Excel-
lent Talents of Hubei Provincial Department of Education
(Q20081504).
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Received: 17 January 2011. Accepted: 22 February 2011.
Delivered by Ingenta to: Nanyang Technological University
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 11, 7794–7801, 2011
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