Fabrication of hollow palladium spheres and their successful application to the recyclable heterogeneous catalyst for suzuki coupling reactions

Article abstract of DOI:10.1021/ja026032z

Novel palladium hollow spheres were synthesized using silica spheres as a template, and they were successfully applied as recyclable heterogeneous catalysts for Suzuki cross coupling reactions. Copyright

Full text of DOI:10.1021/ja026032z

Published on Web 06/08/2002
Fabrication of Hollow Palladium Spheres and Their Successful Application to
the Recyclable Heterogeneous Catalyst for Suzuki Coupling Reactions
Sang-Wook Kim, Minsuk Kim, Wha Young Lee, and Taeghwan Hyeon*
National CreatiVe Research Center for Oxide Nanocrystalline Materials and School of Chemical Engineering,
Seoul National UniVersity, Seoul 151-744, Korea
Received February 26, 2002
Scheme 1
The fabrication of uniform hollow spheres with nanometer-to-
micrometer dimensions having tailored structural, optical, and
surface properties has been intensively pursued in a wide range of
applications.1 These hollow spheres are used as photonic crystals,
delivery vehicle systems, fillers, and catalysts. Various hollow
spheres including carbons,2 polymers,3 metals,4 and inorganic
materials5 have been synthesized by using spherical particles such
as polystyrene beads or silica sol as a template.6 Various procedures
have been developed to fabricate these hollow spheres.7
Palladium-catalyzed cross-coupling reactions of aryl halides with
arylboronic acids, often referred as Suzuki coupling reactions, are
versatile methods for synthesizing unsymmetrical biaryls. The
Suzuki coupling reactions have been applied extensively in the
synthesis of natural products, nucleoside analogues, and pharma-
ceuticals.8 Many palladium complexes have been used as homo-
geneous catalysts for these reactions.9 Homogeneous catalytic
systems generally exhibit better activity and selectivity than
heterogeneous systems. For the large-scale applications in liquid-
phase reactions, however, heterogeneous catalysts have many
advantages over homogeneous counterparts, including easy removal
from reaction mixtures and catalyst recyclability. Since the time
of the early development of the Suzuki coupling reactions,
heterogeneous catalysts, including Pd/C, have been used.10 Recently
many transition-metal nanoparticles have been utilized in various
liquid-phase reactions because they have a characteristic high
surface-to-volume ratio, and consequently a large fraction of the
metal atoms are at the surface and available for catalysis.11 Our
research group reported the first heterogeneous Pauson-Khand
Figure 1. Transmission electron micrograph and scanning electron
catalysts using cobalt nanoparticles.12 Nanoparticles of noble metals
micrograph (Inset) of hollow palladium spheres.
such as Pt and Pd have been used as catalysts for olefin
The final step involves the removal of the silica template by treating
hydrogenation,13 and in carbon-carbon coupling reactions including
the Pd-coated silica spheres with 10 M HF to produce hollow Pd
the Heck and Suzuki reactions.14 Here, we report on the fabrication
spheres (Extreme caution!).16 The structure of the hollow Pd spheres
of hollow spheres composed of palladium nanoparticles and their
was investigated by powder X-ray diffraction (XRD) and electron
applications to heterogeneous Suzuki coupling reactions.
microscopy. The XRD pattern of the material revealed a metallic
fcc Pd crystal structure. The scanning electron microscopic (SEM)
image of the Pd spheres, shown by the inset in Figure 1, revealed
Scheme 1 shows the overall procedure used to synthesize the
Pd hollow spheres. The uniform silica spheres were synthesized
using the Sto¨ber method.15 The surfaces of the silica spheres were
that nearly monodisperse 300-nm sized spherical particles were
functionalized with mercaptopropylsilyl (MPS) groups by refluxing
produced. The transmission electron microscopic (TEM) image
the silica spheres and mercaptopropyltrimethoxysilane (HS(CH2)3-
(Figure 1) showed that the individual particles were composed of
Si(OCH3)3) in toluene. The palladium precursor, palladium acetyl-
an empty core with a uniform shell of 15 nm. Shell thickness can
acetonate (Pd(acac)2) was then adsorbed onto the surfaces of these
be controlled by varying the amount of adsorbed Pd precursor. The
MPS-functionalized silica spheres. The resulting Pd2+-adsorbed-
TEM image at high magnification showed that the shell is composed
MPS-functionalized silica spheres were heated at 250 °C for 3 h
of ∼10 nm Pd nanoparticles. The BET surface area of the Pd
spheres was measured to be 64 m2 g-1, which is much larger than
the value of 8.3 m2 g-1 calculated for the surface of dense spheres.
The Suzuki coupling reaction of iodothiophene and phenyl-
boronic acid was used as a test reaction. The cross-coupling
reactions were carried out using 3 mol % Pd catalyst in ethanol
to obtain Pd metal-coated spheres. The CO generated in situ from
the thermal decomposition of acetylacetonate seems to act as the
reductant.
* To whom correspondence should be addressed. E-mail: thyeon@plaza.snu.ac.kr.
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J. AM. CHEM. SOC. 2002, 124, 7642-7643
10.1021/ja026032z CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Suzuki Cross Coupling Reactions Using Hollow Pd
5 times more Pd catalyst was required to achieve a similar yield
(entry 4). The reaction with aryl chloride did not proceed, showing
that the Pd shell is not active enough to catalyze the reaction.
In conclusion, we have synthesized palladium hollow spheres
using silica spheres as a template. The hollow Pd spheres showed
good catalytic activities in Suzuki cross coupling reactions and can
be reused many times without loss of catalytic activity.
Spheres as Catalysta
entry
catalyst
yield (%)b
1
2
3
4
5
6
7
Pd spheres
97
95
95
96
98
95
96c
first recycle
second recycle
third recycle
fourth recycle
fifth recycle
sixth recycle
Acknowledgment. We thank the Korea Research Foundation
(KRF-2001-041-D00172) for financial support.
Supporting Information Available: The detailed synthetic pro-
cedure for hollow Pd spheres (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
a Reaction conditions: 10 mg of hollow Pd spheres (3 mol %), 3 mmol
iodothiophene, 6 mmol phenylboronic acid, 12 mmol K3PO4, and 50 mL
of EtOH under reflux, 3 h, 78 °C. b Yield obtained from GC. c Isolated
yield.
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under reflux conditions. Table 1 shows that the Pd spheres are
highly active for this reaction, in addition, the catalyst can be
recycled and reused seven times without losing its catalytic activity
(entry 2-7). The high surface area of Pd spheres resulting from
the nanoparticular nature of the shell is responsible for the high
catalytic activity. Earlier studies by other research groups reported
that Pd nanoparticles used in Suzuki coupling reactions were
agglomerated after one cycle, resulting in a loss of catalytic
activity.17 Heterogeneous catalysts often suffer extensive leaching
of the active metal species during reactions and eventually lose
their catalytic activity. To our surprise, these catalysts maintained
their catalytic activities even after seven recycles. In addition, simple
filtering can retrieve the catalyst from the reaction pot. Elemental
analysis of the filtrate after the reaction demonstrated no leaching
of Pd, which is very important when Pd catalysts are used for
pharmaceutical production. Recently, the de Vries group demon-
strated the recycling of Pd(OAc)2 catalyst for Heck reactions.18
Table 2 shows that hollow Pd spheres are active catalysts for the
reaction of various aryliodides and phenylboronic acid (entry 1-3).
When arylbromides were used as reactant, instead of aryliodides,
(18) de Vries, J. G. Can. J. Chem. 2001, 79, 1086.
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