Green and Sustainable Chemistry, 2011, 1, 19-25
doi:10.4236/gsc.2011.12004 Published Online May 2011 (http://www.SciRP.org/journal/gsc)
Recovery of in Situ-Generated Pd Nanoparticles
with Linear Polystyrene
Atsushi Ohtaka1*, Ryozo Kuroki1, Takuto Teratani1, Tsutomu Shinagawa2,
Go Hamasaka3, Yasuhiro Uozumi3, Osamu Shimomura1, Ryôki Nomura1,4*
1Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, Osaka, Japan
2Electronic Materials Research Division, Osaka Municipal Technical Research Institute, Osaka, Japan
3Institute for Molecular Science (IMS), Okazaki, Japan
4Nanomaterials and Microdevices Research Center, Osaka Institute of Technology, Osaka, Japan
Received March 17, 2011; revised April 17, 2011; accepted April 24, 2011
Abstract
Palladium nanoparticles generated in situ for Suzuki coupling reactions and aerobic alcohol oxidation in wa-
ter were recovered completely using linear polystyrene. The resultant polystyrene-stabilized palladium
nanoparticles were shown to be reusable without any loss of activity.
Keywords: Polystyrene, Pd Nanoparticles, Water, Suzuki Coupling, Alcohol Oxidation
1. Introduction
In view of the increasing demand for environmentally
benign reaction processes, intensive efforts have been
directed to reducing or eliminating the use of flammable,
hazardous and nonrenewable organic solvents [1,2]. Pal-
ladium-catalyzed reactions such as carbon-carbon cou-
pling and alcohol oxidation, which are some of the most
common transition-metal-catalyzed processes, are more
and more often being performed in aqueous media. Re-
covery of palladium catalysts is also important to enable
reuse of these expensive reagents and to reduce palla-
dium contamination of the isolated products. One reac-
tion system employing a highly active immobilized
catalyst shows good promise in incorporating these con-
siderations [3]. However, the synthesis of heterogeneous
catalysts typically requires several steps, such as prepa-
ration of the polymer support, introduction of ligand
units into the polymer support, and immobilization of the
metal complex. In contrast, palladium nanoparticles
(PdNPs) generated in situ are easy-to-use catalysts and
have high catalytic activities for coupling reactions in
water in the presence of additives such as quaternary
ammonium salts [4,5], cationic and anionic surfactants
[6,7], and poly(ethylene glycol) [8]. Nevertheless, re-
covery of the palladium species can be difficult when the
reaction is complete, as they are dispersed in water.
Recently, we found that PdO nanoparticles (PdONPs)
are readily stabilized on linear polystyrene, and the re-
sultant polystyrene-stabilized PdONPs have high cata-
lytic activities for Suzuki and copper-free Sonogashira
coupling reactions in water [9,10]. Separately, polysty-
rene-stabilized PdNPs have been prepared using a
phenylboronic acid as the reductant. Based on these suc-
cessful results, we proposed that PdNPs generated in situ
for Suzuki coupling reactions might also be stabilized
and recovered by linear polystyrene. Herein we report
that in situ-generated PdNP catalysts can be recovered
completely with linear polystyrene without loss of their
catalytic activity.
2. Results and Discussion
To investigate the recovery of in situ-generated PdNPs,
we examined the Suzuki coupling reaction of bromoben-
zene with 4-methylphenylboronic acid using Pd(OAc)2 as
the catalyst in 1.5 M aqueous KOH solution at 80˚C for
1 h. Under these conditions, the coupling reaction pro-
ceeded efficiently to give 4-methylbiphenyl quantita-
tively, with no palladium species recovered. Simple fil-
tration of the reaction mixture in the presence of linear
polystyrene (Scheme 1, Mn = 6000, styrene/Pd = 9/1),
however, led to recovery of the palladium species. In-
ductively coupled plasma-atomic emission spectroscopy
(ICP-AES) revealed that the quantitative recovery of Pd
was achieved. In contrast, 85% and 86% of Pd were recov-

20 A. OHTAKA ET AL.
Scheme 1. Recovery of in situ-generated PdNPs for Suzuki coupling reaction in water.
Figure 1. (a) XRD patterns of a recovered polystyrene-stabilized PdNPs. (b) JCPDS data (#46-1043) for Pd. (c) TEM image of
a recovered polystyrene-stabilized PdNPs.
ered when the reduced amount of linear polystyrene
(styrene/Pd = 1/1) and cross-linked polystyrene (2%
DVB, styrene/Pd = 9/1) were used as stabilizer. An XRD
pattern of the recovered palladium species is presented in
Figure 1(a). In addition to the broad diffraction with 2θ
ranging from 12° to 30° ascribed to polystyrene, five
other diffraction peaks assigned to the Pd nanoparticles
(JCPDS #46-1043) are clearly observed. A transmission
electron microscopy (TEM) image of recovered polysty-
rene-stabilized PdNPs is shown in Figure 1(c), where a
fairly uniform particle size of 2.8 nm ± 0.4 nm is evident.
These results indicate that PdNPs generated in situ are
stabilized onto the linear polystyrene and recovered
completely, even in the presence of organic compounds.
We next examined the reusability of the recovered
polystyrene-stabilized PdNPs (Table 1). Similar yields
were obtained even after the fifth use. It was confirmed
by ICP-AES analysis that no leaching of the palladium
into either the aqueous or organic solutions occurred.
Furthermore, no change in particle size (2.8 nm ± 0.6 nm)
was observed by TEM (Figure 2). It is noteworthy that a
similar yield was obtained from the reaction of
4-chloroacetophenone with 4-methylphenylboronic acid
at 80˚C for 3 h (1st run, 32%; 2nd run, 30%; 3rd run,
32%), indicating that the catalytic activity of the in
situ-generated PdNPs was retained after stabilization by
polystyrene.
It is known that PdNPs are generally stabilized by
functionalized polymers containing ligand units such as
phosphine [11] and pyridine [12,13]. Encouraged by
these reports, we investigated the recovery of in si-
tugenerated PdNPs by poly(4-diphenylphosphinosty-
rene-co-styrene) (2) and poly(4-vinylpyridine-co-styrene)
(3) (Table 2). In both cases, the catalytic activities of the
recovered PdNPs were decreased. These results can be
explained by the strong binding capability of phosphine
Figure 2. TEM image of a polystyrene-stabilized PdNPs
after the recycling experiment.

A. OHTAKA ET AL.
21
Table 1. Recycling of the linear polystyrene-stabilized PdNPs a.
aReaction conditions: bromobenzene (0.5 mmol), 4-methylphenylboronic acid (0.75 mmol, 1.5 equiv),
Pd(OAc)2 (1.5 mol% of Pd), polystyrene (styrene/Pd = 9/1), 1.5 M KOHaq (1 mL). bIsolated yield.
Table 2. Recovery of Pd with other polymersa.
aReaction conditions: bromobenzene (0.5 mmol), 4-methylphenylboronic acid (0.75 mmol, 1.5 equiv),
Pd(OAc)2 (1.5 mol% of Pd), polymer (monomer/Pd = 9/1), 1.5 M KOHaq (1 mL). Isolated yield.
cDetermined by ICP-AES. dDetermined by TEM.
and pyridine, which hinders the growth of the nanoparti-
cles [14]. Interestingly, the recovery of Pd was decreased
for both of these systems, although similar sizes of Pd
were observed by TEM (Figure 3). The reason for this
differing behavior compared to polystyrene remains un-
clear. Given these results, we determined that polysty-
rene is the best stabilizer for recovery of in situ- gener-
ated PdNPs.
The effect of substituents on the polystyrene was then
evaluated. When the Suzuki coupling reaction of bro-
mobenzene with 4-methylphenylboronic acid was per-
formed in the presence of substituted polystyrene
(monomer/Pd = 1/1), the recovery of Pd increased with
an increase in the electron density on the benzene ring
(Table 3). These results suggest that the PdNPs are im-
mobilized on the polymer through interactions between π
electrons of the benzene rings of the polystyrene and the
vacant orbitals of the metal [15]. Unfortunately, however,
we could not confirm any obvious differences between
polystyrene and polystyrene-stabilized PdNPs in the
FT-IR spectra [16]. In addition, similar sizes of Pd were
observed by TEM in these cases, too (Figure 4).
Lastly, we investigated the recovery and reuse of in
situ-generated PdNPs for aerobic alcohol oxidation in
water (Scheme 2). The moderate catalytic activity was
oberved, probably due to the formation of Pd aggregates s

22 A. OHTAKA ET AL.
(a) (b)
Figure 3. TEM images of recovered PdNPs by polymer 2 (a) and polymer 3 (b).
Table 3. Effect of the substituent on the recovery of Pd.
aReaction conditions: bromobenzene (0.5 mmol), 4-methylphenylboronic acid (0.75 mmol, 1.5 equiv), Pd(OAc)2
(1.5 mol% of Pd), polymer (monomer/Pd = 1/1), 1.5 M KOHaq (1 mL). bIsolated yield. cDetermined by ICP-AES.
dDetermined by TEM.
(Figure 5). The quantitative recovery of Pd was achieved,
and the recovered polystyrene-stabilized PdNPs was re-
used two times without any loss of catalytic activity.
Furthermore, it was confirmed by ICP-AES analysis that
no leaching of the palladium into either the aqueous or
organic solutions occurred.
3. Conclusions
In summary, in situ-generated PdNPs were recovered
completely by linear polystyrene. The scope and appli-
cability of this methodology for aqueous Suzuki coupling
and aerobic oxidation reactions was investigated. The

A. OHTAKA ET AL.
23
(a) (b) (c)
Figure 4. TEM images of recovered PdNPs by polymer 4 (a), polymer 5 (b), and polymer 6 (c).
Scheme 2. Recovery and re use of in situ-generated PdNPs for aerobic alcohol oxidation.
recovered PdNPs were recycled without significant loss
of activity.
4. Experimental
4.1. General Comments
1H-NMR spectra in CDCl3 or DMSO-d6 were recorded
with a 300 MHz NMR spectrometer (UNITY 300,
Varian, Palo Alto, CA) using tetramethylsilane (δ = 0) as
an internal standard. Gel permeation chromatographic
(GPC) analysis in DMF was carried out with a
HPLC-8020 instrument (Tosoh Co., Tokyo, Japan)
(column: Tosoh TSKgel α-3000 and α-5000). The col-
umns were calibrated with polystyrene of narrow mo-
lecular weight distribution standards. Lyophilization was
carried out with a freeze dryer (FDU-830, Tokyo Ri-
kakikai Co., Ltd., Tokyo, Japan). Powder X-ray diffract-
tions were recorded on a Rigaku RINT 2500 diffracto-
meter (Cu Kα radiation) equipped with a monochromator.
Inductively coupled plasma-atomic emission spectros-
copy (ICP-AES) was performed using ICPS-8100 (Shi-
madzu Co., Kyoto, Japan). Pd nanoparticles were invest-
tigated by transmission electron microscopy (TEM) on a
JEM 2100F transmission electron microscope (JEOL
Ltd., Tokyo, Japan). The samples were prepared by
placing a drop of the solution on carbon coated copper
grids and allowed to dry in air. Polystyrene of narrow
molecular weight distribution standards was purchased
from Tosoh Co., Ltd. (Tokyo, Japan). Pd(OAc)2 was
obtained from Sigma-Aldrich Co. (Missouri, USA).
Figure 5. TEM image of a recovered PdNPs for aerobic
alcohol oxidation.

24 A. OHTAKA ET AL.
4.2. Typical Procedures for Suzuki Coupling
Reaction
To a screw-capped vial with a stirring bar were added
bromobenzene (78 mg, 0.5 mmol), 4-methylphenylbo-
ronic acid (102 mg, 0.75 mmol), Pd(OAc)2 (1.7 mg, 7.5
μmol), polystyrene (7.0 mg, 0.068 mmol of styrene unit),
and 1.5 M aqueous KOH solution (1 mL). After stirring
at 80˚C for 1 h, the reaction mixture was cooled to room
temperature. Subsequently, the aqueous phases were
removed, and recovered catalyst was washed with water
(5 × 1.5 mL) and diethyl ether (5 × 1.5 mL), which were
then added to the aqueous phase. The aqueous phase was
extracted five times with diethyl ether. The combined
organic extracts were dried over MgSO4, concentrated
under reduced pressure, and purified by flash column
chromatography on silica gel. The resulting product was
analyzed by 1H-NMR. The recovered catalyst was dried
in vacuo and reused. Furthermore, the amount of Pd
metal in the recovered catalyst determined by ICP-AES
analysis was 15.8 ppm.
4.3. Typical Procedures for Aerobic Alcohol
Oxidation
To a screw-capped vial with a stirring bar were added
benzoin (106 mg, 0.5 mmol), Pd(OAc)2 (1.7 mg, 7.5
μmol), polystyrene (7.0 mg, 0.068 mmol of styrene unit),
and 1.5 M aqueous K2CO3 solution (1 mL). After stirring
at 80˚C for 20 h, the reaction mixture was cooled to
room temperature. Subsequently, the aqueous phases
were removed, and recovered catalyst was washed with
water (5 × 1.5 mL) and diethyl ether (5 × 1.5 mL), which
were then added to the aqueous phase. The aqueous
phase was extracted five times with diethyl ether. The
combined organic extracts were dried over MgSO4, con-
centrated under reduced pressure, and purified by flash
column chromatography on silica gel. The resulting
product was analyzed by 1H-NMR. The recovered cata-
lyst was dried in vacuo and reused. Furthermore, the
amount of Pd metal in the recovered catalyst determined
by ICP-AES analysis was 16.1 ppm.
4.4. Preparation of
Poly(4-Diphenylphosphinostyrene-co-Styrene)
Into a two-necked reaction vessel were added 4- di-
phenylphosphinostyrene (0.29 g, 1.0 × 10–3 mol), styrene
(0.31 g, 3.0 × 10–3 mol), AIBN (16 mg, 1.0 × 10–4 mol),
and THF (5 mL). After stirring at 70˚C for 20 h under N2
atmosphere, the solvent was removed in vacuo to give a
crude product. Reprecipitation was carried out at least
three times in a THF-MeOH system. The last precipitate
was dried under reduced pressure and lyophilized with a
freeze dryer to give 2 (0.17 g, 29% yield) as a white
powder. The number-average molecular weight (Mn) and
the molecular weight distribution (Mw/Mn) determined by
GPC analysis were ca. 6.2 × 103 and 1.5, respectively.
1H-NMR (CDCl3, 300 MHz): δ 7.61 - 7.10 (br, 14 H),
6.98 - 6.25 (br, 10 H), 2.15 - 1.05 (br, 9 H). 31P-NMR
(CDCl3, 121 MHz): δ - 6.22.
4.5. Preparation of
Poly(4-Vinylpyridine-co-Styrene)
Into a two-necked reaction vessel were added 4-vinylpy-
ridine (0.11 g, 1.0 × 10–3 mol), styrene (0.26 g, 2.5 × 10–3
mol), AIBN (16 mg, 1.0 × 10–4 mol), and DMF (5 mL).
After stirring at 70˚C for 20 h under N2 atmosphere, the
solvent was removed in vacuo to give a crude product.
Reprecipitation was carried out at least three times in a
DMF-Et2O system. The last precipitate was dried under
reduced pressure and lyophilized with a freeze dryer to
give 3 (0.11 g, 29% yield) as a white powder. The num-
ber-average molecular weight (Mn) and the molecular
weight distribution (Mw/Mn) determined by GPC analysis
were ca. 5.4 × 103 and 1.6, respectively. 1H-NMR
(DMSO-d6, 300 MHz): δ 8.26 (br, 2 H), 7.11 (br, 11 H),
6.58 (br, 6 H), 2.18 - 0.83 (br, 12 H).
4.6. Preparation of Poly(4-Nitrostyrene)
Into a two-necked reaction vessel were added 4-nitro-
styrene (1.2 g, 8.0 × 10–3 mol), AIBN (33 mg, 2.0 × 10–4
mol), and THF (5 mL). After stirring at 70˚C for 24 h
under N2 atmosphere, the solvent was removed in vacuo
to give a crude product. Reprecipitation was carried out
at least three times in a THF-MeOH system. The last
precipitate was dried under reduced pressure and lyophi-
lized with a freeze dryer to give 4 (0.29 g, 24% yield) as
a white powder. The number-average molecular weight
(Mn) and the molecular weight distribution (Mw/Mn) de-
termined by GPC analysis were ca. 5.0 × 103 and 1.7,
respectively. 1H-NMR (CDCl3, 300 MHz): δ 8.20 - 7.58
(br, 2 H), 7.42 - 6.60 (br, 2 H), 2.15 - 1.05 (br, 3 H).
4.7. Preparation of Poly(4-Methoxystyrene)
Into a two-necked reaction vessel were added 4-me-
thoxystyrene (0.54 g, 4.0 × 10–3 mol), AIBN (16 mg, 1.0
× 10–4 mol), and THF (5 mL). After stirring at 70˚C for
20 h under N2 atmosphere, the solvent was removed in
vacuo to give a crude product. Reprecipitation was car-
ried out at least three times in a THF-MeOH system. The
last precipitate was dried under reduced pressure and
lyophilized with a freeze dryer to give 5 (95 mg, 18%

A. OHTAKA ET AL.
25
yield) as a white powder. The number-average molecular
weight (Mn) and the molecular weight distribution
(Mw/Mn) determined by GPC analysis were ca. 6.4 × 103
and 1.6, respectively. 1H-NMR (CDCl3, 300 MHz): δ
6.80 - 6.25 (br, 4 H), 3.74 (br, 3 H), 1.95 - 1.20 (br, 3 H).
4.8. Preparation of
Poly(4-Dimethylaminostyrene)
Into a two-necked reaction vessel were added 4-dime-
thylaminostyrene (0.88 g, 6.0 × 10–3 mol), AIBN (98 mg,
6.0 × 10–4 mol), and THF (4 mL). After stirring at 70˚C
for 20 h under N2 atmosphere, the solvent was removed
in vacuo to give a crude product. Reprecipitation was
carried out at least three times in a THF-MeOH system.
The last precipitate was dried under reduced pressure and
lyophilized with a freeze dryer to give 6 (0.28 g, 32%
yield) as a white powder. The number-average molecular
weight (Mn) and the molecular weight distribution
(Mw/Mn) determined by GPC analysis were ca. 5.4 × 103
and 1.6, respectively. 1H-NMR (CDCl3, 300 MHz): δ
6.95 - 6.22 (br, 4 H), 2.87 (br, 6 H), 2.15 - 1.05 (br, 3 H).
5. Acknowledgements
This work was grateful to the Nanomaterials and Micro-
devices Research Center (NMRC) of OIT for financial
and instrumental supports, and was supported by the
Joint Studies Program (2010) of the Institute for Mo-
lecular Science.
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