Synthesis of Pd/SiO2 nanobeads for use in suzuki coupling reactions by reverse micelle sol-gel process

Article abstract of DOI:10.1007/s10562-012-0798-0

Pd/SiO₂ nanobeads containing tiny Pd clusters with a diameter of about 2 nm were prepared via a sol-gel process for SiO₂ by using a water-in-oil microemulsion with Pd complexes and subsequent hydrogen reduction by heat treatment. The Pd/SiO₂ nanostructures were employed in Suzuki coupling reactions with various substrates, and they served as good catalysts in these reactions.

Full text of DOI:10.1007/s10562-012-0798-0

Catal Lett (2012) 142:588–593
DOI 10.1007/s10562-012-0798-0
Synthesis of Pd/SiO Nanobeads for Use in Suzuki Coupling
2
Reactions by Reverse Micelle Sol–gel Process
Mijong Kim
Ji Chan Park
•
Eunjung Heo
•
•
Aram Kim
Kang Hyun Park
•
Hyunjoon Song
•
Received: 4 January 2012 / Accepted: 6 March 2012 / Published online: 27 March 2012
Springer Science+Business Media, LLC 2012
ꢀ
Abstract Pd/SiO nanobeads containing tiny Pd clusters
2
even though these catalysts have high activity [2, 3].
Recently, heterogeneous catalysis in which Pd nanoparti-
cles are supported on solid surfaces such as those of carbon
and metal oxides has been studied with the aim of pre-
venting the Pd leaching problem and increasing the activ-
ities of catalysts in carbon coupling reactions [4–7].
Metal–silica hybrid structures have been considered to
be excellent heterogeneous catalytic systems because silica
is stable, has pore structures that can be modified, facili-
tates easy surface modification, and is chemically inert
with a diameter of about 2 nm were prepared via a sol–gel
process for SiO by using a water-in-oil microemulsion
2
with Pd complexes and subsequent hydrogen reduction by
heat treatment. The Pd/SiO nanostructures were employed
2
in Suzuki coupling reactions with various substrates, and
they served as good catalysts in these reactions.
Keywords Palladium nanoparticles Á Silica coating Á
Suzuki coupling Á Bifunctional catalyst Á Microemulsion
[
8–11]. Initially, metal nanoparticles were supported on
mesoporous silica materials such as mesostructured cellu-
lar foam (MCF) and SBA-15 by various techniques
including impregnation, inclusion, and deposition–precip-
itation [12–14]. However, the sizes of the supports are on
the order of micrometers, and this has a negative impact on
the reactant’s diffusion. More recently, we reported well-
designed metal hybrid nanocatalysts, which have silica
supports whose sizes are in the nanometer range, such as a
yolk-shell nanoreactor framework [15, 16]. It has been
observed that metal–silica nanocomposites have high
thermal stability and that they completely prevent particle
aggregation/agglomeration. Even though the well-designed
nanostructures have high stability and activity, they were
not given preference over conventional catalysts, especially
in industrial catalytic applications, because of the compli-
cated fabrication process involving the separated particle
synthesis. In the present study, we report the fabrication
of Pd/SiO2 nanobeads consisting of Pd nanoparticles
1
Introduction
Pd-catalyzed Suzuki coupling reactions have been carried
out many types of organic syntheses and a significant
number of synthetic transformations for obtaining phar-
maceutical and natural products [1]. However, it is difficult
to carry out these reactions in industrial applications by
using homogeneous catalysts such as soluble Pd complexes
with phosphine ligands because of the recycling problem,
M. Kim Á H. Song
Department of Chemistry, Korea Advanced Institute of Science
and Technology, Daejeon, Korea
E. Heo Á A. Kim Á K. H. Park (&)
(diameter: *2 nm) in porous silica beads (diameter:
Department of Chemistry and Chemistry Institute for Functional
Materials, Pusan National University, Busan 609-735, Korea
e-mail: chemistry@pusan.ac.kr
*20 nm) by the simple modification of the conventional
impregnation and reduction methods. The developed
Pd/SiO nanobeads with the resulting morphology were
2
J. C. Park
Clean Fuel Department, Korea Institute of Energy Research,
Daejeon, Korea
successfully used in Suzuki coupling reactions with various
substrates.
1
23

Synthesis of Pd/SiO
2
Nanobeads
589
2
Experimental Procedure
.1 General Remarks
of methanol and centrifuging at 10,000 rpm for 30 min.
The precipitates were washed by carrying out a repetitive
dispersion/precipitation cycle in ethanol; then, the precip-
itates were dried with acetone. Finally, the Pd(II) com-
2
Reagents were purchased from Aldrich Chemical Co. and
Strem Chemical Co. and used as received. Reaction prod-
plexes/SiO were obtained as a yellowish powder.
2
1
1
ucts were analyzed by H NMR. H NMR with spectra
obtained on Varian Mercury Plus spectrometer
300 MHz). Chemical shift values were recorded as parts
2.4 Synthesis of Pd/SiO Nanobeads
2
a
(
The yellowish Pd(II) complexes/SiO powders were placed
2
per million relative to tetramethylsilane as an internal
standard, unless otherwise indicated, and coupling
constants in Hertz. Reaction products were assigned by
comparison with the literature value of known compounds.
The nanoparticles were characterized by high-resolution
transmission electron microscopy (HRTEM; Philips F20
Tecnai operated at 200 kV, KAIST) and high-angle annu-
lar dark field transmission electron microscopy (HAADF–
TEM; Hitachi HD-2300A operated at 200 kV; National
Nanofabrication Center, NNFC, at KAIST). Samples were
prepared by placing a few drops of the corresponding
colloidal solutions on carbon-coated copper grids (Ted
Pellar, Inc). X-ray powder diffraction (XRD) patterns were
recorded on a Rigaku D/MAX-RB (12 kW) diffractometer.
The Pd loading amount was measured by inductively
coupled plasma-atomic emission spectrometry (ICP-AES;
POLY SCAN 60 E). Nitrogen sorption isotherms were
measured at 77 K using a BELSORP mini-II (BEL Japan
Inc.). Before measurements, the samples were degassed in
a vacuum at 423 K for at least 6 h.
in a ceramic boat in a glass tube oven, and they were slowly
heated at a ramping rate of 4 K/min up to 773 K under a
hydrogen flow of 200 cc/min. In order to carry out complete
reduction of the Pd(II) complexes to Pd(0) clusters and to
remove organic moieties from the sample, the samples were
allowed to stand at 773 K for 4 h under the continuous
hydrogen flow. After the reduction, the resulting dark-
brown Pd/SiO powder was cooled to room temperature.
2
2.5 Suzuki Coupling Reactions by the Pd/SiO2
Nanocatalysts
In order to create the optimal reaction conditions, the Pd/
SiO2 catalyst (2 mg, 1.5 lmol), aryl halide (1.57 mL,
15.0 mmol), phenylboronic acid (2.38 g, 19.5 mmol),
DMF (10.0 mL), water (0.5 mL), and K CO (4.15 g,
2
3
10.06 mmol) were added to a 25 mL stainless steel reactor.
The mixture was stirred for 3 h at 200 ꢁC. After the reac-
tion, the Pd/SiO nanobeads were separated from the clean
2
solution by centrifugation. The reaction products were
1
analyzed by H NMR using Varian Mercury Plus
2
.2 Chemicals
(300 MHz). The chemical shifts were recorded in units of
parts per million relative to tetramethylsilane as an internal
standard, unless otherwise indicated, and the coupling
constants were determined in units of Hertz.
Disodium tetrachloropalladate (Na PdCl , [98 %) was
2
4
purchased from TCI. Tetramethyl orthosilicate (TMOS,
8 %), Igepal CO-630 (polyoxyethylene (9) nonylphenyle-
ther (C H O) •C H O), and octadecyl trimethoxysilane
9
2
4
n
15 24
(
C TMS, 90 %) were purchased from Aldrich. Ammonium
3 Results and Discussion
18
hydroxide (NH OH, 28 % aqueous solution) and cyclohex-
4
ane (99.5 %) were purchased from Junsei. All chemicals
were used as received without further purification.
Pd/SiO nanobeads were synthesized in two simple steps,
2
as shown in Scheme 1: (i) loading of the Pd(II) complexes
into the silica bead using microemulsions and (ii) hydrogen
reduction of Pd(II) complexes to Pd(0) clusters and the
simultaneous generation of silica pores by heat treatment.
2
.3 Encapsulation of Pd(II) Complexes in Silica Matrix
In a 125 mL polypropylene (PP) bottle containing 50 mL
of cyclohexane and 1.0 mL of NH OH solution (28 %
4
3.1 Catalyst Preparation and Characterization
aqueous solution), Na PdCl (0.2 g, 0.68 mol) was added
2
4
2
?
and completely mixed by sonication for 20 min. After
sufficient stirring for 20 min and the subsequent addition of
The Pd complexes were deposited inside the silica spheres
by employing a modified version of the reverse micro-
emulsion method [17, 18]. A water-in-oil microemulsion
was prepared by mixing Igepal CO-630 as a nonionic sur-
factant, aqueous ammonia solution, and cyclohexane.
Disodium tetrachloropalladate powder (Na PdCl ) as a Pd
8
.0 mL of Igepal CO-630 to the mixture solution, a pale
yellow sediment was formed at the bottom of the mixture
solution. Then, 1.0 mL of TMOS and 1.0 mL of C TMS
1
8
were simultaneously added and stirred at 200 rpm for 1 h.
After 1 h, the products were precipitated by adding 15 mL
2
4
precursor was added to the mixture solution containing the
123

5
90
M. Kim et al.
Scheme 1 The proposed
procedures for the formation of
2
encapsulated Pd/SiO nanobead
Fig. 1 a TEM and b HRTEM
images of Pd(II) complexes/
SiO
Pd(II) complexes/SiO
bars represent 20 nm (a) and
nm (b), respectively
2
beads. c XRD spectrum of
2
. The
5
microemulsion, and it was completely dissolved by sonica-
tion. After sonication for 20 min, a pale yellow sediment was
observed at the bottom of the mixture; this indicates the
formation of the tetraammine Pd(II) chloride complex. The
formation of silica spheres and encapsulation of Pd(II)
complexes were simultaneously initiated by adding TMOS
as a silica precursor and adding C TMS as a porogen in
electrons can reduce Pd(II) complexes to Pd clusters. The
XRD spectrum of Pd(II) complexes/SiO beads in Fig. 1c
2
corresponds to that of tetragonal Pd ammine chloride hydrate
(JCPDS No. 46-0875).
The Pd(II) complexes/SiO2 beads were calcined at
773 K for 4 h under hydrogen atmosphere. By heat treat-
ment under a hydrogen flow, the Pd ammine chloride
hydrate complex in the silica matrix was reduced [19], and
this led to the formation of tiny Pd clusters. The TEM
image in Fig. 2a clearly shows the dark spots that indicate
the presence of Pd clusters, and the inset presents the size
distribution of Pd NPs verifying that an average diameter
of Pd NPs measured from TEM images is 1.7 ± 0.3 nm. In
the HAADF–TEM image (Fig. 2b), the ‘‘tiny’’ Pd particles
are observed as white spots. The HRTEM image in Fig. 2c
shows the single-crystalline nature of Pd nanoclusters. The
distance between adjacent lattice fringe images was mea-
sured to be 0.224 nm, which matches that in the case of
(111) planes in face-centered cubic (fcc) Pd. The XRD
1
8
order to introduce irregular pores after calcination. Igepal
CO-630and ammonia were used as the reactionmedia for the
formation of the microemulsion and as a catalyst for the
decomposition of TMOS, respectively. After the mixture
solution was stirred at room temperature for 1 h, the yellow
product comprising Pd(II) complexes/SiO2 beads was
obtained by centrifugation. The TEM image in Fig. 1a shows
spherical Pd(II) complexes/SiO nanobeads with an average
2
diameter of 19 ± 1 nm. In the case of direct irradiation with
the electron beam during the TEM measurement, the Pd(II)
complex loaded in silica beads appeared as a few dark dots on
the image, as shown in Fig. 1b, because the high-energy
1
23

Synthesis of Pd/SiO
2
Nanobeads
591
Fig. 2 a TEM, b HAADF–
TEM, and c HRTEM images of
Pd/SiO
spectrum of Pd/SiO
2
nanobeads. d XRD
beads. The
inset images of a and c show a
2
histogram of Pd NP size
distribution and single Pd/SiO
nanobead, respectively. The
2
bars represent 20 nm ((a), (b)),
nm (inset of (c)), and 2 nm
c), respectively
5
(
Table 1 Optimization of reaction condition for Suzuki reaction of
bromobenzene with phenylboronic acid
Brunauer–Emmett–Teller (BET) surface area and total pore
2
volume were calculated to be 203 m /g and 0.56 cm /g,
3
respectively. The load of Pd in Pd/SiO nanobeads was
2
Br
+
B(OH)2
measured to be 8.27 wt % by ICP-AES.
3.2 Catalytic activity
Entry
Cat.
(mol%)
Temp.
( ꢁC)
Time
(h)
Solvent
Conv.
a
(%)
1
2
3
4
5
a
0.01
0.01
0.5
200
200
200
200
200
3
DMF
22
38
83
The Pd/SiO catalyst was employed in the Suzuki coupling
2
5
DMF
reaction of bromobenzene with phenylboronic acid. The
systematically optimized reaction conditions are presented
in Table 1. The reaction time and catalyst amounts affected
the conversion rates. By utilizing a small amount of water
in dimethylformamide (DMF), the reactivity of Pd/SiO2
catalysts was increased further (Table 1, entry 3). The
optimized reaction was carried out using K CO as a base
1.5
3
DMF:H
DMF:H
DMF:H
2
2
2
O(10:0.5)
0.01
0.005
O(10:0.5) [99
O(10:0.5) 65
3
1
Determined by H NMR spectra. K
base
2 3
CO (2.0 equiv.) was used as a
2
3
spectrum of Pd/SiO beads in Fig. 2d is similar to that of
2
at 200 ꢁC. When the catalyst was used at a concentration of
0.01 mol % with respect to the substrate amount, bromo-
benzene was completely converted to biphenyl within 3 h
(Table 1, entry 4). Even when the concentration of the
catalyst was 0.005 mol %, a satisfactory conversion of
65 % was achieved.
bulk Pd (JCPDS No. 46-1043; space group, Fm-3 m). The
broad peak at 2h = 40.1ꢁ reflects entirely regular and small
Pd single-crystalline domain sizes; by using the value of
the full-width half-maximum of the (111) peak and the
Debye–Scherrer equation, the domain sizes were calculated
as 2.8 nm. The small irregular pores in the silica matrix
were simultaneously generated via heat treatment by
removing the long alkyl carbon chains of C TMS [20]. To
Recently, direct fabrications of Au–Pd core–shell het-
erostructures and catalysts for Suzuki coupling reaction
under mild reaction conditions (reflux at 80 ꢁC, 0.5 h) have
been reported [21]. This can be a good example of the
Suzuki reaction using various types of Pd metals. This will
1
8
evaluate the porosity of silica nanobeads, a N sorption
2
experiment was conducted at 77 K. The resulting
123

5
92
M. Kim et al.
Table 2 Pd/SiO
2
catalyzed
a
Entry
1
Substrate
Product
Conv. (%)
Suzuki coupling reaction of
various substrates with phenyl
boronic acid
82
O N
Br
O N
2
2
O
O
2
3
4
5
6
82
Br
Br
O
O
66
EtO
EtO
F3C
Br
Br
F C
59
3
100
53
NC
NC
MeO
Br
Br
MeO
7
100
a
1
Determined by H-NMR
spectra. K CO (2.0 equiv.)
was used as a base
2
3
O
O
help us to review the reaction to the mild reaction condi-
tions and understand the behaviors of catalysts. The cou-
pling reaction between iodobenzene and phenylboronic
acid to form biphenyl was performed. In the case of using
including electron-rich and electron-deficient substrates
proceeded readily, and various functional groups were
tolerated in the reaction. High reactivity with excellent
yield was also observed in the production of biaryls via the
coupling of activated o-bromobenzaldehyde with phenyl-
boronic acid (Table 2, entries 7). Furthermore, the reaction
with electron-rich 4-methoxyphenylboronic acid proceeded
readily to afford the corresponding 4-methoxybiphenyl in a
high yield (100 %).
our Pd/SiO nanobeads, 73 % yield was found under the
2
same conditions. The lower activity of Pd/SiO nanobeads
2
than Au–Pd core–shell heterostructures (91 % yield) can
explained by their effective high-index surface facets.
The substrate scope of the Pd/SiO -catalyzed Suzuki
2
coupling reaction of aryl bromides is summarized in
Table 2. From the table, it is clear that this reaction can be
extended to a wide variety of substituted aryl bromides
4 Conclusions
(
Table 2, entries 1–7). In our catalytic system, both
bromobenzenes and chlorobenzenes, which include either
Pd/SiO nanobeads consisting of tiny Pd clusters with a
2
electron-withdrawing substituents such as NO , COMe,
2
diameter of about 2 nm were prepared via a sol–gel process
for silica by using a water-in-oil microemulsion with
Pd complexes and subsequent hydrogen reduction along
COOEt, CF , and CN or electron-donating ones such as
3
OMe, were readily coupled with arylboronic acids in
moderate yields. Reactions of various aryl bromides
with heat treatment. The Pd/SiO catalyst was used as a
2
1
23

Synthesis of Pd/SiO
2
Nanobeads
593
high-temperature catalyst in Suzuki coupling reactions of
various aryl bromides with phenylboronic acids. One
important factor affecting the yield of the reaction is the
superior chemical reactivity of the nanoparticles. The
present result indicates that transition-metal nanoparticles
can be used as catalysts for a wide variety of organic
transformations.
7. Chen Y-H, Hung H-H, Huang MH (2009) J Am Chem Soc
1
31:9114
. Bedford RB, Singh UG, Walton RI, Williams RT, Davis SA
2008) Chem Mater 17:701
8
(
9. Lambert S, Tran KY, Arrachart G, Noville F, Henrist C, Bied C,
Moreau JJE, Man MWC, Heinrichs B (2008) Microporous
Mesoporous Mat 115:609
0. Budroni G, Corma A (2006) Angew Chem Int Ed 45:3328
1. Park J-N, Forman AJ, Tang W, Cheng J, Hu Y-S, Lin H,
McFarland EW (2008) Small 4:1694
1
1
Acknowledgments This research was supported by Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology (2009-0070926, 2010-0002834) and through the Active
Polymer Center for Pattern Integration (No.R11-2007-050-00000-0).
K. H. P thank to the TJ Park Junior Faculty Fellowship.
12. Song H, Rioux RM, Hoefelmeyer JD, Komor R, Niesz K,
Grass M, Yang P, Somorjai GA (2006) J Am Chem Soc 128:3027
13. Burattin P, Che M, Louis C (1999) J Phys Chem B 103:6171
14. Ko CH, Park JG, Park JC, Song H, Han S-S, Kim J-N (2007)
Appl Surf Sci 253:5864
15. Park JC, Lee HJ, Bang JU, Park KH, Song H (2009) Chem
Commun 47:7345
1
6. Park JC, Lee HJ, Kim JY, Park KH, Song H (2010) J Phys Chem
C 114:6381
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