Functionalized poly(ethylene glycol)-stabilized water-soluble palladium nanoparticles: Property/activity relationship for the aerobic alcohol oxidation in water

Article abstract of DOI:10.1021/la9027755

The preparation, characterization, and catalytic properties of water-soluble palladium nanoparticles stabilized by the functionalized- poly(ethylene glycol) as a protective ligand were demonstrated for aerobic oxidation of alcohols in aqueous phase. UV/vis spectra and X-ray photoelectron spectroscopy (XPS) proved that there was an electronic interaction between the bidentate nitrogen ligand and palladium atoms. Transmission electron microscopy and XPS analysis showed that the particle size and surface properties of the generated palladium nanoparticles can be controlled by varying the amount of protective ligand and the kinds of reducing agents. It was found that both the size and surface properties of palladium nanoparticles played very important roles in affecting catalytic performance. The stabilized metallic palladium nanoparticles were proven to be the active centers for benzyl alcohol oxidation in the present system, and the water-soluble Pd nanocatalysts can also be extended to the selective oxidation of various alcohols.

Full text of DOI:10.1021/la9027755

pubs.acs.org/Langmuir
© 2009 American Chemical Society
Functionalized Poly(ethylene glycol)-Stabilized Water-Soluble Palladium
Nanoparticles: Property/Activity Relationship for the Aerobic Alcohol
Oxidation in Water
Bo Feng,† Zhenshan Hou,*,† Hanmin Yang,‡ Xiangrui Wang,† Yu Hu,† Huan Li,† Yunxiang Qiao,†
Xiuge Zhao,† and Qingfa Huang†
†Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of
Science and Technology, Shanghai, 200237, China and ‡Key Laboratory of Catalysis and Materials Science of
the State Ethnic Affairs Commission and Ministry of Education, Hubei Province, South-Central University for
Nationalities, Wuhan, 430074, China
Received July 28, 2009. Revised Manuscript Received November 9, 2009
The preparation, characterization, and catalytic properties of water-soluble palladium nanoparticles stabilized by the
functionalized-poly(ethylene glycol) as a protective ligand were demonstrated for aerobic oxidation of alcohols in
aqueous phase. UV/vis spectra and X-ray photoelectron spectroscopy (XPS) proved that there was an electronic
interaction between the bidentate nitrogen ligand and palladium atoms. Transmission electron microscopy and XPS
analysis showed that the particle size and surface properties of the generated palladium nanoparticles can be controlled
by varying the amount of protective ligand and the kinds of reducing agents. It was found that both the size and surface
properties of palladium nanoparticles played very important roles in affecting catalytic performance. The stabilized
metallic palladium nanoparticles were proven to be the active centers for benzyl alcohol oxidation in the present system,
and the water-soluble Pd nanocatalysts can also be extended to the selective oxidation of various alcohols.
1. Introduction
to reactants, and unique nature which is freely rotational and
three-dimensional.5 However, by referring to the previous studies
on the alcohol oxidation by soluble-metal nanoparticle catalysts
in water, only several examples have been demonstrated to date,
that is, Pd, Pt, and Au nanoparticles stabilized by microgels or
polymers (PVP, P123, or vinyl ether star polymer), respectively.6
The water-soluble polymer poly(ethylene glycol) (PEG) is
known to be inexpensive, thermally stable, nontoxic, nonvolatile,
and recoverable media for catalysis.7 In particular, it was reported
that adding PEG in water as a cosolvent would lead to an
apparent decrease in the polarity of the aqueous solution, then
the consequent increase in the solubility of organic molecules.8
Moreover, the use of PEG as the support has attracted particular
attention because of the easy recovery of the catalyst/ligand and
attractive catalytic performance of the resultant catalysts. Re-
cently, PEG-supported Pd catalysts have been successfully used in
many reactions, including oxidation reactions,9 but Pd catalysts
always sufferfrom deactivation due to the formationof palladium
black. Therefore, the bidentate nitrogen ligands havebeenutilized
to stabilize palladium(0) active species.10 However, there is no
The selective oxidation of alcohols to the corresponding
carbonyl compounds has been extensively studied, because of
its ubiquitous importance in production of fine chemicals and
intermediates.1 As a consequence of the ever-growing concerns
over green chemistry and chemical processes, numerous efforts
have been made to develop new catalytic protocols for the
oxidation of alcohols particularly with O2 as oxidant in place of
the stoichiometric metal oxidants.2 Among these studies, the
progress for the alcohol oxidation in green solvents has received
much attention in recent years.3 Especially, the aerobic oxidation
of alcohols in water is considerably notable. A number of
homogeneous or heterogeneous catalysts including palladium
complexes or palladium nanoparticles have been investigated
for the aerobic oxidation of alcohols in water.4 In contrast to
the traditional supported metal nanoparticles that are restricted
by support surfaces, soluble metal nanoparticles have many
superiorities, such as controllable size, more active sites accessible
*Corresponding author.
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Compounds; Plenum Press: New York, 1986. (b) Miyamura, H.; Matsubara, R.;
Miyazaki, Y.; Kobayashi, S. Angew. Chem., Int. Ed. 2007, 46, 4151.
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Adv. Synth. Catal. 2006, 348, 275. (c) Seki, T.; Baiker, A. Chem. Rev. 2009, 109, 2409.
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Leitner, W. Green Chem. 2007, 9, 127.
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N.; Park, C. M.; Lee, J. S.; Kang, K. Y.; Park, J. Org. Lett. 2005, 7, 1077. (d) Yamada, Y.
M. A.; Arakawa, T.; Hocke, H.; Uozumi, Y. Angew. Chem., Int. Ed. 2007, 46, 704. (e)
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Langmuir 2010, 26(4), 2505–2513
Published on Web 12/29/2009
DOI: 10.1021/la9027755 2505

Article
Feng et al.
previous report of PEG anchoring bidentate dipyridyl ligand as a
ligand to stabilize palladium nanoparticles employed for aerobic
oxidation in water.
Scheme 1. The Chemical Structure of the Functionalized-PEG2000
(L)
Although many papers have been contributed to the Pd
nanoparticle-catalyzed oxidation of alcohols and moderate to
excellent turnover numbers or selectivities can be obtained,
studies on the relationships between nanoparticle properties
and catalytic performance are unexpectedly scarce.11 For better
understanding the structure/activity relationship in the Pd nano-
particle-catalyzed aerobic oxidation of alcohols, further investi-
gations are still highly required. Needless to say, an accurate
control of the nanoparticle sizes is very crucial to the further
application. It has been reported that the size control of the
nanoparticles can be achieved by various methods.12 But the
preparation of Pd nanoparticles with controllable sizes is still a
challenging task.
2.2. Preparation of Various Pd Nanoparticle Catalysts.
The bidentate nitrogen ligand functionalized PEG (L) (Scheme 1)
was prepared as previously reported 14 First, Pd(OAc)2 was added
to a solution of L in water, and then the mixture was stirred at
room temperature overnight to give a clear pale yellow solution
(Pd(II)-L complex). The different reducing agents such as
NaBH4, benzyl alcohol, H2, and ethanol were used to prepare
various Pd nanoparticles, and then the corresponding nanopar-
ticleswere designatedasPd-NaBH4, Pd-benzyl alcohol, Pd-H2
and Pd-ethanol, respectively.
For the synthesis of Pd-NaBH4 catalysts, four samples were
prepared by varying the molar ratio of L to Pd. For example, 1.0
mL of a fresh 0.1 M NaBH4 solution was rapidly added to the
mixture of 9.6 ꢀ 10-3 mmol Pd(OAc)2 and 9.6 ꢀ 10-3 mmol L in
5.0 mL water under vigorous stirring for 30 min. The color of the
reaction mixture immediately turned to dark brown, indicating
the formation of small Pd nanoparticles (Pd-NaBH4-1, where 1
designated the molar ratio of L/Pd). Afterward, the solution was
dialyzed overnight (through a cellulose ester dialysis membrane
with a cutoff molecular-weigh of 1000) using deionized water to
completely remove the inorganic impurities. Then the solution of
the Pd nanoparticles after dialysis was concentrated to 6.0 mL
under vacuum at room temperature. The pH value of the solution
changed from 8.8 to 7 after dialysis. The other Pd-NaBH4
catalysts (Pd-NaBH4-0.5, Pd-NaBH4-1.5, and Pd-NaBH4-3)
were obtained in the same way by only varying the amount of L
(4.8 ꢀ 10-3, 1.44ꢀ 10-2, and 2.88ꢀ 10-2 mmol) correspondingly.
For the synthesis of Pd-benzyl alcohol, the mixture of 9.6 ꢀ
10-3 mmol L and 9.6 ꢀ 10-3 mmol Pd(OAc)2 in 6.0 mL of water
was stirred with benzyl alcohol (0.52 g, 4.8 mmol) at 90 °C for 12
h. After reduction, the brown mixture was extracted with ethyl
ether three times to remove excess of benzyl alcohol, and then
evaporated ethyl ether under reduced pressure at room tempera-
ture to give a brown aqueous solution. The solution was subse-
quently used for catalysis without further purification.
Recently, Hou et al.13 have studied the structure/activity
relationship in the aerobic alcohol oxidation using Pd nanopar-
ticles supported on hybrid mesoporous silica under supercritical
carbon dioxide (scCO2) conditions, and found that the size and
agglomeration of the nanoparticles have a prominent influence on
the catalytic performance of the materials. In the previous work,
we have demonstrated that the functionalized PEG-stabilized Pd
nanoparticles can catalyze aerobic oxidation of alcohols effi-
ciently in PEG or scCO2/PEG biphasic media.14 These results
indicated that the bidentate nitrogen ligand functionalized-PEG
played an important role in stabilizing and immobilizing cataly-
tically active palladium(0) species. Herein, we presented the
synthesis and characterization of a water-soluble palladium
nanocatalyst stabilized by the functionalized-PEG, and also
employed the nanocatalyst for aerobic oxidation of alcohols in
aqueous phase. We attempted to establish the methods to control
the size of ligand-protected Pd nanoparticles by changing the
synthetic conditions systematically, and then explored the rela-
tionships between nanoparticle properties and catalytic perfor-
mance in detail. Although the size effect of water-soluble metal
nanoparticles on the catalytic oxidation of alcohols has been
observed in the previous publication,15 to the best of our knowl-
edge, no more detailed research about the effect of the surface
properties of water-soluble metal nanoparticles on the catalytic
activity for alcohols oxidation is reported.
For the preparation of Pd-H2, the mixture of 9.6 ꢀ 10-3 mmol
L and 9.6 ꢀ 10-3 mmol Pd(OAc)2 in 6.0 mL water was placed into
a 50 mL autoclave and reduced by H2 (3.0 MPa) at 25 °C for 1 h.
Afterward, the autoclave was cooled down. A black solution was
obtained and then used directly for catalysis.
Pd-ethanol was preparedaccording to the method reportedby
Miyake et al.16 with slight modifications. To the mixture of 9.6 ꢀ
10-3 mmol L and 9.6 ꢀ 10-3 mmol Pd(OAc)2 in 6.0 mL of water
was added 12.0 mL of ethanol, and the mixture was further
refluxed for 3 h. After reaction, the color of the solution was
brown. Then the aqueous solution was evaporated under reduced
pressure to remove ethanol and water. Afterward, the brown
residue was redispersed in 6.0 mL of water. The brown aqueous
solution was subsequently used for catalysis without further
purification.
2. Experimental Section
2.1. Materials. PEG-2000 and potassium carbonate (K2CO3)
were obtained from SCRC (Sinopharm Chemical Reagent Co, Ltd.,
Shanghai). 2,20-Dipyridylamine and palladium acetate were pur-
chased from Aldrich. All organic solvents used in this work were
dried by standard procedures.
(11) (a) Chen, J.; Zhang, Q. H.; Wang, Y.; Wan, H. L. Adv. Synth. Catal.
2008, 350, 453. (b) Li, F.; Zhang, Q. H.; Wang, Y. Appl. Catal. A: Gen. 2008,
334, 217.
€
(12) (a) Schimpf, S.; Lucas, M.; Mohr, C.; Rodemerck, U.; Bruckner, A.;
€
Radnik, J.; Hofmeister, H.; Claus, P. Catal. Today 2002, 72, 63. (b) Bonnemann,
2.3. Characterization of Pd Nanoparticle Catalysts. The
Pd nanoparticle catalysts were characterized by UV/vis spectra,
powder X-ray diffraction (XRD), transmission electron micros-
copy (TEM), and X-ray photoelectron spectroscopy (XPS). UV/
vis spectra were obtained in aqueous solution using a Varian Cary
500 spectrophotometer. The XRD analysis was performed in D/
MAX 2550 VB/PC using a graphite crystal as monochromator.
The palladium nanoparticles were isolated by centrifuging and
washing before XRD measurements. The TEM micrographs were
H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 10, 2455. (c) Bianchi, C. L.; Biella, S.;
Gervasini, A.; Prati, L.; Rossi, M. Catal. Lett. 2003, 85, 91. (d) Konya, Z.; Puntes, V. F.;
ꢀ
Kiricsi, I.; Zhu, J.; Ager, J. W., III; Ko, M. K.; Frei, H.; Alivisatos, P.; Somorjai, G. A.
Chem. Mater. 2003, 15, 1242. (e) Corain, B.; Jerabek, K.; Centomo, P.; Canton, P.
Angew. Chem., Int. Ed. 2004, 43, 959.
€
(13) Hou, Z. S.; Theyssen, N.; Brinkmann, A.; Klementiev, K. V.; Grunert, W.;
€
Buhl, M.; Schmidt, W.; Spliethoff, B.; Tesche, B.; Weidenthaler, C.; Leitner, W. J.
Catal. 2008, 258, 315.
(14) (a) Feng, B.; Hua, L.; Hou, Z. S.; Yang, H. M.; Hu, Y.; Li, H.; Zhao, X. G.
Catal. Commun. 2009, 10, 1542. (b) Wang, X. R.; Yang, H. M.; Feng, B.; Hou, Z. S.;
Hu, Y.; Qiao, Y. X. Li, H.; Zhao, X. G. Catal. Lett. 2009, 132, 34.
(15) (a) Biffis, A.; Minati, L. J. Catal. 2005, 236, 405. (b) Tsunoyama, H.; Sakurai,
H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. (c) Wang, T.; Shou, H.;
Kou, Y.; Liu, H. C. Green Chem. 2009, 11, 562.
(16) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594.
2506 DOI: 10.1021/la9027755
Langmuir 2010, 26(4), 2505–2513

Feng et al.
Article
Figure 1. UV/vis spectrum of (a) the functionalized PEG (L), (b)
Pd(II)-L complex, (c) Pd-NaBH4-1, (d) Pd(II)-L complex in
aqueous solution after oxidation of benzyl alcohol at 80 °C for 8 h.
Figure 2. X-ray diffraction pattern of Pd-NaBH4-1 nanoparti-
cles. The peaks are labeled with the hkl of the planes for the
corresponding Bragg angles.
recorded on a JEOL JEM 2010 transmission electron microscope
at 200 kV. Samples were prepared by directly dropping the Pd
nanoparticles aqueous solutions onto carbon-coated Cu grids.
More than 200 particles for each sample were randomly counted
to determine the particle size distributions. XPS measurements
were performed on a Thermo ESCALAB 250 spectrometer.
Nonmonochro Al KR radiation was used as a primary excitation.
The binding energies were calibrated with the C1s level of
adventitious carbon (284.8 eV) as the internal standard reference.
The samples for XPS analysis were prepared by evaporating the
catalyst aqueous solution to dryness at 50 °C for 24 h under high
vacuum. The molar ratio of Pd(0)/Pd(II) was approximately
obtained as the ratio of peak area of the Pd(0) species of Pd 3d5/2
to that of the Pd(II) species of Pd 3d5/2. Generally, the errors for
the molar ratios of Pd(0)/Pd(II) are in the range of 10-20%.17
2.4. Typical Procedures for Oxidation of Alcohols. The
typical procedure for the alcohol oxidation was described briefly.
Into a reaction vessel equipped with a reflux condenser and
balloon were placed the catalyst solution (0.0096 mmol Pd in
6.0 mL water), K2CO3 (0.2 g, 1.45 mmol), benzyl alcohol (0.10 g,
0.96mmol). Thenthe mixture was heatedat 80°C for 8 h under an
atmospheric pressure of O2 with vigorous stirring. After reaction,
the solution was extracted with ethyl ether three times. Then, the
catalyst solution was reused after removal of diethyl ether from
the aqueous phase at room temperature under vacuum. For each
reaction, a parallel experiment was carried out to analyze the
aqueous phase according to the following steps: after reaction, the
aqueous mixture was acidified with 7.5 mL of 2 M HCl and then
extracted with diethyl ether three times.
Figure 3. XPS analysis for (a) N 1s spectra of L and Pd-NaBH4-
1, (b) Pd 3d spectra of the surface Pd centers in Pd-NaBH4-1.
solution. Consecutively, various Pd nanoparticle catalysts were
obtained by reducing the Pd(II)-L complex with different redu-
cing agents.
The products in diethyl ether phase were analyzed by GC and
GC-MS equipped with a HP-5MS column (30 m long, 0.25 mm
i.d., 0.25 μm film thickness). The conversion and selectivity were
determined using n-heptane as an internal standard in all reac-
tions. The selectivity was defined as the moles of the designated
product divided by the total moles of all the products. In the
oxidation of secondary alcohols, no other byproducts were
detected. But for the oxidation of primary alcohols, the very small
amounts of the corresponding acids were produced as byproducts.
Turnover frequency (TOF) was based on substrate turnover per
mole of Pd per hour.
The preparation processes were monitored and confirmed with
UV/vis spectrum, XRD, XPS, and TEM. As shown in Figure 1,
the UV/vis absorption spectrum of L appeared in the range of
200-350 nm. The peaks at 231.6 and 271.4 nm were ascribed to
the intraligand π-π* transition of pyridyl rings, and the band at
312.3 nm may be due to the n-π* transition.18,19 After L was
coordinated with Pd(OAc)2 in aqueous phase, the blue shifts of
the absorption bands of L were observed (Figure 1a,b), which
implied the charge transfer between Pd(II) species and L, in other
words, the formation of the Pd(II)-L complex. After reduction,
the absorption spectrum of the solution was totally different from
that of the Pd(II)-L complex, which indicated the formation of the
Pd nanoparticles (Figure 1b,c). And some changes were also
3. Results and Discussion
3.1. Synthesis and Characterization of Various Pd Na-
noparticle Catalysts. The catalyst solution was prepared by
stirring Pd(OAc)2 and L overnight in water to give a clear yellow
(18) (a) Frolov, A. N. Russ. J. Gen. Chem. 2001, 71, 222. (b) Ho, K. Y.; Yu, W. Y.;
Cheung, K. K.; Che, C. M. Chem. Commun. 1998, 2101. (c) Anbalagan, V.; Srivastava,
T. S. J. Photochem. Photobiol. A: Chem. 1994, 77, 141.
(19) (a) Lee, J. R.; Liou, Y. R.; Huang, W. L. Inorg. Chim. Acta 2001, 319, 83. (b)
Burgos, M.; Crespo, O.; Gimeno, M. C.; Jones, P. G.; Laguna, A. Eur. J. Inorg. Chem.
2003, 2170.
(17) Nefedov, V. I. X-ray Photoelectron Spectroscopy of Solid Surfaces; VSP BV:
Utrect, The Netherlands, 1988.
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Feng et al.
Figure 4. TEM micrographs and size distributions of Pd nanoparticles with different reducing agents (scale bar = 50 nm): (a) Pd-benzyl
alcohol; (b)Pd-H2; (c) Pd-ethanol.
observed between the absorption spectra of Pd-NaBH4-1 and L
(Figure 1a,c), but the changes were rather small probably because
of a decrease in both an electron-withdrawing force and the
number of Pd atoms coordinated to N atoms for the reduced
Pd(0) species compared with Pd(II)-L complex.
XRD was also used to examine the crystal structure of the Pd
nanoparticles (Figure 2). XRD pattern of the Pd-NaBH4-1
nanoparticles clearly showed the diffraction peaks corresponding
to (111), (200), (220), and (311) lattice planes, indicating a face-
centered-cube (fcc) phase.
To gain insight into the surface characteristic of the modified
Pd nanoparticles, the XPS spectra of L and Pd-NaBH4-1 were
measured. Figure 3 showed the spectra of Pd 3d and N 1s regions.
As shown in Figure 3a, compared to the N 1s binding energy of L
(397.6 eV), that of Pd-NaBH4-1 was shifted toward higher
binding energy (þ1.1 eV), indicating that the N atoms of L was in
an electron-poor state due to the coordination of these atoms to Pd
atoms.20 Furthermore, the Pd 3d spectrum of Pd-NaBH4-1 showed
two distinct signals, which indicated the presence of two surface-
bound species (Figure 3b). As presented in Figure 3a, because the N
atoms of L was in an electron-poor state, the surface Pd atoms
coordinating to N atoms should be in an electron-rich state, so the
binding energy of the surface palladium atoms would shift to a lower
value. Nevertheless, the main peak, Pd 3d5/2, was virtually the same
as the specimens of the giant clusters of Pd(0) or the bulk metal Pd
(334.8 eV), corresponding to palladium in a zero oxidation state.21
This may be ascribed to the size effect that the values of binding
energy have a tendency to become larger for the smaller nanoparti-
cles.20a,22 The competition of the opposite factors resulted in a little
(21) NIST X-ray Photoelectron Spectroscopy Database. NIST Standard Re-
ference Database 20, version 3.5. http://srdata.nist.gov/xps/, 1979.
(22) (a) Jiang, L. C.; Gu, H. Z.; Xu, X. Z.; Yan, X. H. J. Mol. Catal. A: Chem.
2009, 310, 144. (b) Oberli, L.; Monat, R.; Matieu, H. J.; Landolt, D.; Buttet, J. Surf. Sci.
1981, 106, 301.
(20) (a) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B
1999, 103, 3818. (b) Evangelisti, C.; Panziera, N.; Pertici, P.; Vitulli, G.; Salvadori, P.;
Battocchio, C.; Polzonetti, G. J. Catal. 2009, 262, 287.
2508 DOI: 10.1021/la9027755
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Feng et al.
Article
Figure 5. TEM micrographs and size distributions of different Pd nanoparticle catalysts at various molar ratio of L/Pd (scale bar = 50 nm):
(a) Pd-NaBH4-0.5; (b) Pd-NaBH4-1; (c) Pd-NaBH4-1.5; (d) Pd-NaBH4-3.
change of the binding energy. Besides, a very small shoulder
with the binding energy Eb = 336.8 eV in the spectra might be
assigned to the interaction between Pd(II) species and 2,20-dipy-
ridylamine ligand,13 which indicated that the Pd(II) species were in
an electron-rich state, compared with Pd(OAc)2 (Eb = 338.6 eV).21
As a result, the XPS results indicated that the Pd centers were reduced
to a great extent by NaBH4 treatment, resulting in the 9.9 ( 1.98
molar ratio of Pd(0)/Pd(II) at the surface of Pd nanoparticles.
Langmuir 2010, 26(4), 2505–2513
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Feng et al.
Table 1. Comparison of the Catalytic Performance of Different Pd
Speciesa
Table 3. Comparison of the Catalytic Performance of Various Pd
Nanoparticles with Different L/Pd (Molar Ratio)a
entry
catalyst
Pd(II)-L
t (h)
conversion (%)b
entry
catalyst
particle size (nm)
TOF (h-1
)
1c
2
1
8
1
8
0.2
1.9
28.0
38.9
1
2
3
4
Pd-NaBH4-0.5
Pd-NaBH4-1
Pd-NaBH4-1.5
Pd-NaBH4-3
2.8 ( 0.4
2.6 ( 0.4
2.4 ( 0.2
2.2 ( 0.2
41.2
56.0
32.4
30.0
3c
4
Pd-NaBH4-1
a Reaction conditions: 0.96 mmol benzyl alcohol, 6 mL of catalyst
solution, without K2CO3, 80 °C, 1 atm O2. b Based on GC, benzaldehyde
was detected as the only product. c A total of 1.92 mmol benzyl alcohol.
a Reaction conditions: 1.92 mmol benzyl alcohol, 6 mL of catalyst
solution, without K2CO3, 80 °C, 1 atm O2, 1 h. Benzaldehyde was
detected as the only product.
Table 2. Comparison of the Catalytic Performance of Various Pd
Nanoparticles Using Different Reducing Agentsa
Table 4. Aerobic Oxidation of Benzyl Alcohol in Water Using
Pd-NaBH4-1a
oxidation state of
surface Pd (XPS)
(Pd(0)/Pd(II))
entry
T (°C)
t (h)
run
conversion (%)b
selectivity (%)b
particle
size (nm)
1c
2d
3
4
5
6
7
8
80
80
50
80
80
8
8
8
4
8
1
1
1
1
1
2
3
4
53.6(38.9)e
83.0
60.2
99.8(100)e
99.5
99.8
entry
catalyst
TOF (h-1
)
1
2
3
4
Pd-NaBH4-1
Pd-ethanol
Pd-H2
Pd-benzyl
alcohol
2.6 ( 0.4
2.4 ( 0.3
3.5 ( 0.5
3.8 ( 0.5
9.9 ( 1.98
0.3 ( 0.06
3.7 ( 0.74
0.7 ( 0.14
56.0
28.4
49.8
32.0
83.5
99.0
96.4(98.1)e
96.1(96.5)e
95.2(94.0)e
93.0
98.8(99.0)e
98.2(98.0)e
98.5(98.8)e
97.1
a Reaction conditions: 1.92 mmol benzyl alcohol, 6 mL of catalyst
solution, without K2CO3, 80 °C, 1 atm O2, 1 h. Benzaldehyde was
detected as the only product.
a Reaction conditions: 0.96 mmol benzyl alcohol, 0.2 g of K2CO3, 6
mL of Pd-NaBH4-1 catalyst solution without dialysis/purification, 1
atm O2. b Based on GC, benzoic acid was detected as a byproduct.
c Without K2CO3. d A total of 0.1 g of K2CO3. e The conversion and
selectivity in parentheses were obtained with the Pd-NaBH4-1 catalyst
purified by dialysis.
It has been reported that the reduction method used in the
preparation of nanoparticles has a decisive influence on the
particle size and surface properties, which in turn defines
the catalytic performance of the materials.23 For comparison,
three other water-soluble Pd nanoparticle catalysts were prepared
using different reducing agents instead of NaBH4, including
benzyl alcohol (Pd-benzyl alcohol), H2 (Pd-H2), and ethanol
(Pd-ethanol). As shown in Figure 4, these Pd nanoparticles were
well dispersed in aqueous phase and the TEM images showed
mean diameters of 3.8, 3.5, and 2.4 nm, respectively. XPS analysis
was also performed to assess the surface characteristic of these Pd
nanoparticles, and the results were summarized in Table 2.
Compared with Pd-NaBH4-1, the other three Pd nanocatalysts
contained significant amounts of unreduced Pd sites, correspond-
ing to 0.7 ( 0.14, 3.7 ( 0.74, and 0.3 ( 0.06 molar ratio of Pd(0)/
Pd(II) at the surface, respectively.
UV/vis spectrum showed that the functionalized-PEG would
coordinate with the surface Pd atoms of the nanoparticles.
Besides, the analysis of XPS confirmed the ligands (L) were
strongly adsorbed on the surface Pd atoms. These results ex-
plained perfectly the reasons why the Pd nanoparticles can be
stabilized effectively by the functionalized-PEG (L). As a result,
the amount of L added to the solution was expected to affect the
growth process of the Pd nanoparticles. Therefore, the influence
of the molar ratio of L to Pd on the size of the Pd nanoparticles
was investigated. Figure 5 showed the TEM micrographs and size
distributions of different Pd nanoparticle catalysts at various
molar ratios of L/Pd. As shown in Figure 5, the particles were
roughly spherical and possessed mean diameters of 2.8, 2.6, 2.4,
and 2.2 nm, respectively, with narrow size distributions. Notwith-
standing limited dependence of Pd nanoparticle size on the
amount of L under the present conditions, a small decrease in
size with increasing L was still observed. As a consequence, the
size control of the Pd nanoparticles could be achieved by varying
the kind of reducing agents or the amount of L. Moreover, the
surface composition of the nanoparticles can also be tuned by
adopting appropriate reducing agents in the preparation of
nanoparticles.
3.2. Active Pd Species for Aerobic Oxidation of Alco-
hols. First, in an effort to investigate the potential application
of different Pd species, oxidation of benzyl alcohol was carried
out under atmospheric O2 pressure using Pd catalysts in its
oxidized and reduced state. The results were summarized in
Table 1. It was important to note that Pd(II)-L complex
exhibited hardly any catalytic activity for benzyl alcohol
oxidation under the present conditions (entries 1 and 2,
Table 1), where metallic Pd nanoparticles were much more
active (entries 3 and 4, Table 1). Experimentally, no concomi-
tant color change was observed and then no noticeable O2
uptake was detected by using water-soluble Pd(II)-L complex
as a catalyst. Furthermore, the UV/vis spectra of recovered
Pd(II) catalyst was identical to that of fresh Pd(II) species,
revealing that electronic configuration of the Pd species did not
change (Figure 1b,d). TEM examination also proved that there
was no formation of Pd particles after the alcohol oxidation.
From the above results, it could be concluded that the cataly-
tically active species were not the Pd(II) species but the Pd
nanoparticles in the present catalytic system. Baiker et al.24
investigated the oxidation of benzyl alcohol over Pd/Al2O3
catalysts by using FT-IR spectroscopy and in situ X-ray
absorption spectroscopy combined with online catalytic mea-
surements, and they found that the catalysts after in situ
reduction by hydrogen-saturated cyclohexane were much more
active (over 50 times) than before reduction. The studies on
active species in Pd-catalyzed aerobic alcohol oxidation by the
other groups also came to the similar conclusion.25
(24) Grunwaldt, J.-D.; Caravati, M.; Baiker, A. J. Phys. Chem. B 2006, 110,
25586.
(25) (a) Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem.
Soc. 2004, 126, 10657. (b) Murata, M.; Hara, T.; Mori, K.; Ooe, M.; Mizugaki, T.;
Ebitani, K.; Kaneda, K. Tetrahedron Lett. 2003, 44, 4981. (c) Wu, H. L.; Zhang, Q. H.;
Wang, Y. Adv. Synth. Catal. 2005, 347, 1356. (d) Karimi, B.; Zamani, A.; Abedi, S.;
Clark, J. H. Green Chem. 2009, 11, 109.
(23) (a) Toshima, N.; Shiraishi, Y.; Teranishi, T.; Miyake, M.; Tominaga, T.;
€
Watanabe, H.; Brijoux, W.; Bonnemann, H.; Schmid, G. Appl. Organomet. Chem.
2001, 15, 178. (b) Bell, A. T. Science 2003, 299, 1688. (c) Rolison, D. R. Science 2003,
299, 1698. (d) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257.
2510 DOI: 10.1021/la9027755
Langmuir 2010, 26(4), 2505–2513

Feng et al.
Article
Figure 6. TEM micrographs and size distributions of Pd-NaBH4-1 after four successive runs in the oxidation of benzyl alcohol (scale bar =
50 nm).
that the catalytic performance was not consistent with the size
effect that the smaller the size, the higher the activity. Especially,
two representative Pd nanoparticles (Pd-NaBH4-1 and
Pd-ethanol) with the similar size (2.6 and 2.4 nm) showed
remarkable different catalytic activities. Similar phenomenon
was also observed for Pd-H2 (3.5 nm) and Pd-benzyl alcohol
(3.8 nm) catalysts. The differences in the activities of four Pd
nanoparticle catalysts indicated that some other factors may also
contribute to the catalytic performance of the Pd nanoparticles.
These factors can be ascribed to the different metal oxidation
states of the surface of Pd nanoparticles. From the XPS analysis,
the oxidation states of the various Pd nanoparticles were greatly
different, and the oxidation state of Pd in the nanocatalyst had a
significant influence on catalytic activity for benzyl alcohol
oxidation under the present conditions. Upon consideration of
the palladium(0) species as catalytically active species, the cata-
Figure 7. (a) Pd nanoparticles in Pd-NaBH4-1 were dispersed
lytic performance of the four Pd nanoparticle catalysts could
homogeneously in aqueous phase; (b) the bulk Pd particles were
correlate well with their states (entry 1 vs 2, 3 vs 4, Table 2). As a
result, Pd-NaBH4-1 with smaller particle size and more active
metallic palladium showed the highest catalytic activity (entry 1,
completely precipitated from aqueous solution when L was re-
placed by PEG2000.
3.3. The Relationships between Pd Nanoparticle Proper-
ties and Catalytic Performance in the Oxidation of Benzyl
Alcohol. Catalytic activities of metal nanoparticles have been
Table 2).
Then, Pd-NaBH4 catalysts at different molar ratios of L/Pd
were examined in the oxidation of benzyl alcohol under the
same reaction conditions and the results were listed in Table 3.
The TOF exhibited a notable dependence on the mole ratio of
L/Pd that maximized at a L/Pd ratio of 1:1 (entry 2, Table 3). At
the ratio of L/Pd below 1:1, the nanocatalyst was accompanied
by Pd aggregation and then the activity was lower (entry 1,
shown to depend strongly on the size of nanoparticles, the metal
oxidation states, etc.26 Generally, the alcohol oxidation is con-
sidered “structure-sensitive”, and smaller metal particles contain-
ing more coordinately unsaturated metal sites are believed to be
more active.27 To further understand the nature of the Pd
nanocatalysts, we examined the effects of the size of Pd nano-
Table 3). Though the Pd nanoparticles were more stable in
particles and their surface properties on the catalytic activity. The
catalytic activities of various Pd nanoparticle catalysts were
aqueous phase and also their sizes became smaller at increased
ratio of L/Pd, the reaction rate was strongly inhibited possibly
evaluated for aerobic oxidation of benzyl alcohol at the initial
because of the poison of the excess ligands (entries 3 and 4,
Table 3). The catalytic composition of L/Pd = 1:1 appeared to
reflect a balance between optimal turnover rate and catalyst
reducing agents, and the results were summarized in Table 2. The
stability. Accordingly, we anticipated that these properties of
reaction stage in water (conversion < 30%). We first examined
the catalytic activities of various Pd nanoparticles with different
TOF for oxidation of benzyl alcohol decreased as follows:
the metal particles can be controlled by finely tuning the
interactions between the ligand with the metal nanocluster
surfaces, thus leading to improvement in the activities and
Pd-NaBH4-1 (56.0 h-1) > Pd-H2 (49.8 h-1) > Pd-benzyl
alcohol (32.0 h-1) > Pd-ethanol (28.4 h-1). It was worth noting
stability of soluble Pd nanoparticles. These results indicated
(26) (a) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921. (b) Bars,
J.-L.; Specht, U.; Bradley, J. S.; Blackmond, D. G. Langmuir 1999, 15, 7621. (c) Haider,
P.; Kimmerle, B.; Krumeich, F.; Kleist, W.; Grunwaldt, J.-D.; Baiker, A. Catal. Lett.
2008, 125, 169.
(27) (a) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (b)
Yin, H. M.; Zhou, C. Q; Xu, C. X; Liu, P. P.; Xu, X. H; Ding, Y. J. Phys. Chem. C 2008,
112, 9673. (c) Tsunoyama, H.; Sakurai, H.; Tsukuda, T. Chem. Phys. Lett. 2006, 429,
528.
that the chosen reduction method of the Pd(II) precursor
exerted a controlling effect on the particle size and oxidation
state distribution of the obtained palladium species, resulting
in very significant differences in the catalytic activity.
3.4. Catalytic Performance of Pd-NaBH4-1 in the Aero-
bic Oxidation of Benzyl Alcohol. In detail, then we examined
Langmuir 2010, 26(4), 2505–2513
DOI: 10.1021/la9027755 2511

Article
Feng et al.
Table 5. Aerobic Oxidation of Various Alcohols Using Pd-NaBH4-1a
a Reaction conditions: 0.96 mmol alcohol, 6 mL of Pd-NaBH4-1 catalyst solution without dialysis/purification, 1 atm O2, 0.2 g K2CO3. b Based on
GC. c The corresponding acid was detected as a byproduct. d A total of 0.48 mmol alcohol. e There was 3.0 g of PEG-2000 in 3.0 mL of H2O as cosolvent.
f The conversion and selectivity in parentheses were obtained with the Pd-NaBH4-1 catalyst purified by dialysis.
the catalytic activity of Pd-NaBH4-1 toward aerobic oxida-
tion of benzyl alcohol in water, and the results were summar-
ized in Table 4. It was found that the activity and the
selectivity to benzyladehyde were little affected by dialysis
for purification of Pd nanoparticles in the presence of a
base (entries 5, 6 and 7, Table 4). As shown in Table 4,
the presence of K2CO3 can significantly promote selective
oxidation of benzyl alcohol, and the amount of K2CO3 had
an influence on the activity (entries 1, 2, and 5, Table 4).
This may be explained by the previously reported mechanism
of alcohol oxidation over Pd nanoparticles,25a,28 which
involved β-H elimination of a dissociated alcohol on
the Pd surface, followed by reaction of oxygen with Pd-H
species, while the base can promote the deprotonation of
the alcohol toward dehydrogenation. With the optimized
conditions (entries 3, 4, and 5, Table 4), benzyl alcohol
could be oxidized to form benzaldehyde with high conver-
sion and selectivity. Compared with other recently reported
Pd nanoparticle catalyst for alcohols oxidation with oxygen
in water, the productivity of present catalysts appeared to
be higher than that of palladium nanoparticles supported
on hydroxyapatite (95% yield after 7 h at 100 °C).29 More-
over, the present catalyst can be used under more mild
conditions (80 °C).
(Figure 5b vs Figure 6). Such a stability was superior to that of
the above-mentioned soluble metal nanoparticle catalysts in the
alcohols oxidation in water.6a,15a
It should be worth noting that when L was replaced by
PEG2000 as the stabilizer to prepare a new catalyst under the
same conditions, the bulk Pd particles were formed rapidly and
deposited completely from the solution after reduction with
NaBH4 (Figure 7b), and the aqueous phase became colorless
and clear, which was totally different from the case of Pd-
NaBH4-1 (Figure 7a). Therefore, we believed that almost all the
Pd nanoparticles in Pd-NaBH4-1 should be coordinated with the
functionalized-PEG (L) that was effective for preventing the
aggregation of Pd nanoparticles, resulting in the stabilization
and recycling of Pd-NaBH4-1.
3.5. The Scope of Pd-NaBH4-1 Catalyst. Then, Pd-
NaBH4-1 was examined for the aerobic oxidation of various
alcohols (Table 5). Allyl alcohols such as 3-methyl-2-buten-1-ol,
geraniol, and cinnamyl alcohol were oxidized selectively into
the corresponding aldehydes in moderate to high conversions,
that is, 41.4, 95.4, and 99.3%, respectively (entries 1-3, Table 5).
The high reactivity toward the oxidation of allyl alcohols could
be ascribed to the high coordination ability of the CdC double
bond of the alcohols to the Pd species as an anchor, which could
also enhance the interaction of the hydroxyl group with Pd
species. Unfortunately, some other nonactivated and poorly
water-soluble aliphatic secondary alcohols exhibited much lower
catalytic productivity than the benzylic ones; 2-octanol and
cyclohexanol reached conversions of only 5.7% and 8.0%,
respectively (entries 4 and 6, Table 5). This was not entirely
surprising, however, because aliphatic alcohols were known to be
much more demanding substrates for this kind of reaction
whenever Pd metal catalysts were used, as can be inferred from
the literatures.15a,25a However, these nonactivated alcohols could
be converted to corresponding ketones with moderate conver-
sions (45.7% and 31.8%) in the presence of PEG (entries 5 and 7,
Table 5). The improvement of the conversion may attribute to the
increase in solubility of the substrates in water, due to an apparent
The recyclability of the Pd nanoparticle catalyst was also
examined in the oxidation of benzyl alcohol. This catalyst
can be readily recycled after simple extraction by ethyl ether.
As shown in Table 4, the conversion and selectivity to benzalde-
hyde remained essentially constant for the four successive cycles
(entries 5-8, Table 4); no Pd deposits were observed, reflect-
ing high stability and reusability of the catalyst. Indeed, after
four catalytic runs, the Pd nanoparticles were still well dispersed
in water with a slight increase in the particle size (3.0 nm)
(28) (a) Muzart, J. Tetrahedron 2003, 59, 5789. (b) Mallat, T.; Baiker, A. Chem.
Rev. 2004, 104, 3037. (c) Grunwaldt, J.-D.; Caravati, M.; Baiker, A. J. Phys. Chem. B
2006, 110, 9916.
(29) Jamwal, N.; Gupta, M.; Paul, S. Green Chem. 2008, 10, 999.
2512 DOI: 10.1021/la9027755
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Feng et al.
Article
decrease of the aqueous solution polarity after adding PEG as a
cosolvent.8
performance depended strongly on the properties (size and
oxidation state) of Pd nanoparticles. The oxidation of various
alcohols was carried out smoothly by using the water-soluble Pd
nanoparticles. We have also illustrated that the nanocatalyst can
be recycled at least four times without any loss of catalytic
activity. Further efforts to extend the application of this system
in other reactions are in progress in our laboratory.
4. Conclusions
In the present work, we have synthesized stable, size-con-
trolled, water-soluble Pd nanoparticle catalysts using bidentate
nitrogen ligand functionalized PEG as a stabilizing reagent, and
then investigated their properties as well as their catalytic perfor-
mance for the aerobic oxidation of alcohols in water. UV/vis
spectra and XPS analysis showed that there was an electronic
interaction between the bidentate nitrogen ligand and palladium
atoms. It was found that the particle size decreased with increas-
ing the amount of the functionalized PEG. Also, the reduc-
tion methods used in the preparation of nanoparticles had a
decisive influence on the particle size and surface properties. By
relating activity data to the properties of Pd nanoparticles, we
found that the metallic palladium was the active species for benzyl
alcohol oxidation in the present system, moreover, the catalytic
Acknowledgment. We are grateful for the support from the
National Natural Science Foundation of China (No. 20773037),
ECUST (No. YJ0142136), the Commission of Science and
Technology of Shanghai Municipality (No.07PJ14023), China,
and Key Laboratory of Catalysis and Materials Science of the
State Ethnic Affairs Commission and Ministry of Education,
Hubei Province, South-Central University for Nationalities (No.
CHCL08001), China. The authors would also like to express their
thanks to Prof. Walter Leitner’s helpful suggestions and Dr. Nils
Theyssen’s assistance.
Langmuir 2010, 26(4), 2505–2513
DOI: 10.1021/la9027755 2513