Recovery of In Situ-generated Pd Nanoparticles with Linear Polystyrene

doi:10.4236/gsc.2011.12004

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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

E-mail: [email protected]

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.

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.

(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.

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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