Highly selective oxidation of allylic alcohols catalysed by monodispersed 8-shell Pd nanoclusters in the presence of molecular oxygen

Article abstract of DOI:10.1039/b207098g

Treatment of Pd₄phen₂(CO)₂(OAc)₄ with metal nitrates such as Cu(NO₃)₂ produced monodispersed Pd nanoclusters with a mean diameter and standard deviation (d±σ) of 38±2.1 A (σ/d = 6%). The Pd nanoclusters act as heterogeneous catalysts for the selective oxidation of primary aromatic allylic alcohols using molecular oxygen as an oxidant. This unique catalysis can be ascribed to multiple interactions between the alcohol and specific ensemble sites consisting of Pd⁰, Pd⁺, and Pd²⁺ on the cluster surface.

Full text of DOI:10.1039/b207098g

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Highly selective oxidation of allylic alcohols catalysed by
monodispersed 8-shell Pd nanoclusters in the presence of
molecular oxygen
a
Kwang-Min Choi, Tomoki Akita, Tomoo Mizugaki, Kohki Ebitani and Kiyotomi Kaneda*
b
a
a
a
a
Department of Chemical Science and Engineering, Graduate School of Engineering Science,
Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
E-mail: kaneda@cheng.es.osaka-u.ac.jp; Fax: +81-6-6850-6260; Tel: +81-6-6850-6260
Special Division for Green Life Technology, National Institute of Advanced Industrial Science
and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
b
Received (in Montpellier, France) 18th July 2002, Accepted 27th September 2002
First published as an Advance Article on the web 13th December 2002
4 2 2 4 3 2
Treatment of Pd phen (CO) (OAc) with metal nitrates such as Cu(NO ) produced monodispersed Pd
˚
nanoclusters with a mean diameter and standard deviation (d ꢀ s) of 38 ꢀ 2.1 A (s/d ¼ 6%). The Pd
nanoclusters act as heterogeneous catalysts for the selective oxidation of primary aromatic allylic alcohols using
molecular oxygen as an oxidant. This unique catalysis can be ascribed to multiple interactions between the
0
+
2+
alcohol and specific ensemble sites consisting of Pd , Pd , and Pd on the cluster surface.
Transition metals, especially Pd, are among the most impor-
tant catalysts in organic synthesis. Adjusting the steric and
strategy to design highly selective Pd catalysts for many
organic syntheses.
1
electronic properties of metal centers using organic ligands
generally affords high-performance Pd complex catalysts with
1
b
respect to the target organic transformations. On the other
hand, the heterogeneous catalysis properties of Pd particles
depend upon their size, because of alterations in the geometric
and electronic properties of the surface Pd atoms. Recently,
synthesis of giant Pd clusters containing more than 100 Pd
Experimental
Materials and characterization
Pd(OAc)
used without further purification. 1,10-Phenanthroline (phen)
and Cu(NO O were obtained from Nacalai Tesque Co.
2
was purchased from N. E. Chemcat Co. Ltd. and
2
–6
atoms has opened up an exciting field, offering the possibi-
lity of unprecedented catalytic reactions based on specific sur-
face ensemble Pd sites within a regular arrangement of
3
)
2
ꢁ3H
2
Ltd. Acetic acid and all commercially obtained alcohols were
purified by standard procedures. Noncommercial alcohols, 4-
methoxycinnamyl alcohol, 4-chlorocinnamyl alcohol, 4-phenyl-
6
multiple Pd species.
Previously, we reported the synthesis of anion ligand-
preserved giant Pd clusters by a novel method using treatment
of Pd (CO) (OAc) ꢁ2AcOH (PCA) with metal nitrates [e.g.
3-buten-2-ol and 4-methyl-3-penten-2-ol, were synthesized
according to literature procedures. TiO was supplied by the
1
2
2
4
4
4
Catalysis Society of Japan as a reference catalyst of JRC-TIO-
2. X-Ray diffraction (XRD) was performed on an X’pert
3 2 3 3
Cu(NO ) and Fe(NO ) ] in the presence of 1,10-phenanthro-
7
7
line (phen) under an atmospheric O . This method yielded
2
0
+
mixed-valence states on the surface with Pd , Pd , and Pd
2+
diffractometer (Phillips Co., Ltd.). The procedure for deter-
mining the number of surface Pd O species using CO con-
2
species to form specific ensemble Pd sites. The giant Pd clusters
7
sumption has been described in a previous paper. The
+
with the highest percentage of Pd species showed high cataly-
tic activity for the oxidative acetoxylation of toluene to benzyl
amount of O absorption was measured volumetrically using
2
a gas buret directly connected to the rotary vacuum pump.
The O2 uptake during preparation of the Pd nanoclusters
acetates under an atmospheric pressure of O
conventional Pd catalysts such as Pd/carbon, Pd/Al
Pd(OAc) . The unique catalytic ability of these Pd nanoclus-
2
, compared with
2 3
O
, and
was determined by trapping the evolved CO
ꢃ
2
in a cold trap
2
at ꢂ120 C. Field emission scanning electron microscopy
ters was attributed to the ensemble Pd sites on the cluster
surface.
(
(
FE-SEM) was done on a Hitachi S-5000 L microscope
18.0 kV). The high-resolution transmission electron micro-
Attention has focused on Pd-catalysed oxidation of alcohols
into the corresponding carbonyl compounds using mole-
scopy (HR-TEM) measurement of the single Pd nanocluster
was performed on a JEOL JEM-3000F (300 kV).
8
–11
cular oxygen as an environmentally friendly oxidant.
In
this report, we explore the catalytic potential of Pd nanoclus-
ters for the aerobic oxidation of alcohols. The Pd nanoclusters
show high catalytic activity for the oxidation of primary allylic
alcohols, which may be due to the unique surface ensemble Pd
sites of the monodispersed Pd nanoclusters. We also demon-
Synthesis of monodispersed Pd nanoclusters. A solution of
Pd(OAc) (0.20 g; 0.89 mmol) in AcOH (20 mL) was stirred
2
ꢃ
at 50 C for 2 h under continuous CO flow, yielding 0.12 g of
1
3
Pd (CO) (OAc) ꢁ2AcOH (PCA) as a yellow precipitate. The
4
4
4
0
+
2+
strate that each surface Pd species (i.e. Pd , Pd , and Pd
)
obtained PCA (0.155 mmol; Pd 0.62 mmol) and phen
(0.31 mmol) were stirred in AcOH (2.5 mL) at room tempera-
ture for 30 min in air to give a dark brown solution of
does not act individually as a catalytic site, but together
work cooperatively as a ‘‘trio’’ on the same cluster surface
to provide unique selectivity. The Pd cluster provides a new
1
4
Pd phen (CO) (OAc) . Anal. calcd for Pd C H N O : C,
10
4
2
2
4
4
34 28
4
3
24
New J. Chem., 2003, 27, 324–328
DOI: 10.1039/b207098g
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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3
0
7.9; H, 2.6; N, 5.2; found: C, 34.1; H, 2.7; N, 5.2%. Then,
.0155 mmol of Cu(NO ) ꢁ3H O was added to the mixture
resulted in a longer reaction time to obtain a high yield of
the aldehyde (entry 7). It should be noted that these Pd
nanoclusters show lower reactivity for primary aliphatic allylic
alcohols [e.g. 3-methyl-2-buten-1-ol and 2-hexen-1-ol (entries 8
and 10, respectively)] than for aromatic alcohols. Furthermore,
oxidation of secondary aromatic allylic alcohols proceeded
slowly; prolonged reaction times afforded a high yield of the
corresponding ketone (entry 14). Interestingly, benzylic alco-
hols were oxidized more slowly than allylic alcohols (entries
18 and 21). This high reactivity for allylic alcohols was also
exemplified in a competitive reaction of an equimolar mixture
of cinnamyl alcohol and benzyl alcohol, in which cinnamalde-
hyde was selectively obtained in 92% yield with only 1% of
benzaldehyde after 2 h. Oxidation of saturated alcohols
such as 2-octanol and 1-octanol hardly occurred using Pd
nanoclusters.
3
2
2
ꢃ
2
and heated at 90 C under an atmosphere of O . After 15
min, a black solid precipitated with the consumption of
.067 mmol O (O
0
2
2
/Pd ¼ 0.108 mol/mol). The precipitate
was washed with AcOH several times and dried in vacuo to
yield the Pd nanoclusters (0.032 g, ca. 30% yield based on
Pd). Anal. calcd for Pd2060(NO )360(OAc)360O80 : Pd, 83.02;
3
C, 3.28; H, 0.41; N, 1.91; found: Pd, 86.50; C, 3.21; H, 0.23;
N, 1.34%. No appreciable copper signals were detected by ele-
mental analysis and XPS measurement of the precipitate. The
ꢃ
XRD pattern of the precipitate exhibited three peaks, at 40 ,
ꢃ
ꢃ
46 , and 68 , which correspond to the {111}, {200}, and
{220} planes of a face-centerd cubic (fcc) lattice, respectively.
7
The following eqn. (1) represents the formation of the Pd
clusters:
Generally, divalent Pd catalysts are prone to facilely oxidize
benzyl alcohols, but are not suitable for the oxidation of allylic
alcohols because of irreversible coordination of allylic alcohols
9
0½Pd4phen2ðCOÞ ðOAcÞ ꢄ þ 9½CuðNO3Þ ꢁ 3H2Oꢄ þ 172AcOH
2
4
2
þ 137O2 ! 0:05½Pdꢅ2060ðNO3Þ
ðOAcÞ
Oꢅ80ꢄ
ꢅ360
ꢅ360
1
0
2
to the Pd species. Indeed, the Pd(OAc) –pyridine catalyst
þ 180½PdphenðOAcÞ ꢄ þ 77½PdðOAcÞ ꢄ þ 9½CuðOAcÞ ꢄ
2
2
2
system does not efficiently oxidize cinnamyl alcohol (entry 3).
The difference between the giant Pd clusters described here
and conventional Pd complexes lies in the high reactivity
þ 113H2O þ 180CO2
ð1Þ
2
+
During preparation of the Pd clusters, molecular oxygen was
consumed to form CO , H O, and the oxygen species (Pd O)
on the cluster surface.
A typical procedure for the immobilization of the Pd clusters
of allylic alcohols over benzylic alcohols. As compared with
2
2
2
+
the Pd561phen (OAc) cluster, which has Pd cation species
60
180
6
over all its surface, the present Pd nanoclusters prefer aro-
matic allylic alcohols to aliphatic ones. Cinnamyl alcohol
was completely oxidized after only 1 h, while oxidation of 3-
methyl-2-buten-1-ol required 5 h (entry 1 vs. 8). In contrast,
Pd561phen60(OAc)180 catalysed the oxidation of both allylic
alcohols at similar rates (entry 2 vs. 9).
7
2
on TiO was described in a previous paper.
Alcohol oxidation
A typical procedure for the alcohol oxidation is as follows.
Acetic acid (4 mL) and cinnamyl alcohol (2 mmol) were added
to a reaction vessel containing the Pd nanoclusters (0.05 mmol
of Pd atoms). The heterogeneous mixture was stirred at 60 C
2
for 2 h under an atmospheric pressure of O . The yield of pro-
In order to consider the origin of the unique Pd nanocluster
catalysis of aerobic alcohol oxidation, the particle size distribu-
tion and ordering of Pd atoms of the Pd clusters were esti-
mated by FE-SEM and HR-TEM. The Pd clusters were
ꢃ
2
immobilized on a TiO surface because this does not change
ducts was determined by GC analysis using an internal stan-
dard technique. Filtration of the heterogeneous mixture gave
a solution devoid of any oxidizing ability. The amount of Pd
species in the filtrate was less than 0.01% by ICP analysis.
their original cluster size or the local ordering of the Pd atoms
on the cluster surface. As shown in Fig. 1, the FE-SEM image
6
of the Pd clusters on the TiO
nanoclusters were well dispersed with a quite narrow size dis-
tribution over the TiO surface. The mean diameter and stan-
dard deviation (d ꢀ s) of the Pd clusters were 38 ꢀ 2.1 A (s/
instead of Cu(NO also
2
surface revealed that the Pd
2
˚
Results and discussion
d ¼ 6%). The use of Fe(NO
3
)
3
3 2
)
Treatment of Pd phen (CO) (OAc) with Cu(NO ) ꢁ3H O at
afforded Pd nanoclusters having a narrow size distribution
with a mean diameter and standard deviation (d ꢀ s) of
4
2
2
4
3 2
2
ꢃ
9
0 C under an O
2
atmosphere produces Pd nanoclusters.
˚
The Pd nanoclusters act as efficient heterogeneous catalysts
for the selective oxidation of cinnamyl alcohol to cinnamalde-
hyde in the presence of molecular oxygen. In monitoring O2
uptake during the oxidation of cinnamyl alcohol, the ratio of
38 ꢀ 4.4 A (s/d ¼ 11%).
Concerning the size distribution of Pd clusters, Teranishi and
Miyake also reported a narrow size distribution for giant Pd
2 4
clusters prepared by reduction of aqueous H PdCl in the pre-
O consumed to cinnamaldehyde was 1:2, indicating molecular
2
oxygen was quantitatively used for the dehydrogenation as an
oxidant.
sence of poly(N-vinyl-2-pyrrolidone), which had a standard
˚
deviation of the mean diameter of ꢅ14% (s ¼ 3.6 A,
˚
15a
d ¼ 24.4 A). To the best of our knowledge, the Pd nanoclus-
ters prepared by our method using metal nitrates have the nar-
rowest size distribution of Pd nanoparticles obtained by
1
5–19
chemical and electrochemical methods.
One reason for
the narrow size distribution of these Pd nanoclusters could be
the strong ability of the metal nitrate for the disproportionation
of monovalent Pd ions. This leads to the formation of a rela-
0
Results of the aerobic oxidation of various alcohols cata-
lysed by the Pd nanoclusters are displayed in Table 1 together
tively small Pd assembly through fast nucleation of Pd spe-
7
0
cies. Furthermore, partial oxidation of surface Pd species by
the metal nitrate retards further growth of the nanoclusters.
Fig. 2 represents the HR-TEM image of the Pd nanocluster
6
cluster and Pd-
catalytic systems. Interestingly, the giant
with those using the Pd phen (OAc)
5
61
60
180
1
0e
(
OAc)
2
–pyridine
Pd clusters efficiently catalysed the oxidation of primary aro-
matic allylic alcohols using molecular oxygen. For example,
cinnamyl alcohol and its para-substituted derivatives having
electron-donating groups were smoothly transformed into
the corresponding a,b-unsaturated aldehydes in high yields
2
immobilized on the TiO surface. A regular arrangement of Pd
atoms on the Pd nanocluster surface can be clearly observed,
with 17 Pd atoms along the {111} planes. A possible arrange-
ment of the Pd atoms of the eight-shell Pd cluster is illustrated
0
+
in Fig. 3, where the Pd and Pd species are situated on the
face of the cluster. These observations indicate that the pre-
7
(
entries 1 and 4), while electron-withdrawing groups such as
chloro and nitro markedly retarded the reaction rates (entries
and 6). Introduction of a methyl group onto the olefinic bond
sent giant Pd clusters have an fcc cuboctahedral shape with
an eight-shell structure composed of 2060 Pd atoms as the
5
New J. Chem., 2003, 27, 324–328
325

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a
Table 1 Oxidation of alcohols catalysed by Pd nanoclusters under an oxygen atmosphere
b
Entry
Substrate
Product
Catalyst
Time/h
% Conv
% Yield
1
2
3
Pd2060(NO
3
)
360(OAc)360
O
80
2
1
2
93
100
46
91
94
35
Pd561phen60(OAc)180
c
2
Pd(OAc) –pyridine
4
Pd2060(NO
Pd2060(NO
Pd2060(NO
Pd2060(NO
Pd2060(NO
3
3
3
3
3
)
)
)
)
)
360(OAc)360
360(OAc)360
360(OAc)360
360(OAc)360
360(OAc)360
O
80
O
80
O
80
O
80
O
80
2
10
10
4
100
43
91
35
37
87
d
5
d
6
46
7
91
8
9
5
1
90
100
83
89
Pd561phen60(OAc)180
e
1
0
1
Pd2060(NO
3
)
360(OAc)360
O
80
5
1
66
100
49
1
Pd561phen60(OAc)180
79
f
1
2
3
Pd2060(NO
3
)
360(OAc)360
O
80
80
5
2
77
100
50
100
1
Pd561phen60(OAc)180
1
4
5
Pd2060(NO
3
)
360(OAc)360
O
14
4
94
47
80
36
1
Pd561phen60(OAc)180
1
6
7
Pd2060(NO
3
)
360(OAc)360
O
80
80
24
24
< 1
8
Trace
1
1
Pd561phen60(OAc)180
18
Pd2060(NO
3
)
360(OAc)360
O
14
4
95
45
92
44
1
2
9
0
Pd561phen60(OAc)180
24
2
78
100
24
100
c
Pd(OAC)
2
–pyridine
2
1
Pd2060(NO
3
)
360(OAc)360
O
80
14
4
95
53
97
91
50
95
c
22
Pd(OAC)
2
–pyridine
2
a
ꢃ
b
Reaction conditions: substrate (2 mmol), Pd (0.05 mmol), AcOH (4 mL), 60 C, O
2
atmosphere. Yields of aldehydes and ketones were deter-
(0.05 mmol), pyridine (0.2 mmol), MS3A (0.5 g),
c
mined by GC analysis using an internal standard technique. Substrate (2 mmol), Pd(OAc)
2
ꢃ
d
atmosphere. Substrate (1.5 mmol). 15% of 2-hexenyl acetate was formed. 22% of 2,4-hexadienyl acetate was formed.
e
f
toluene (4 mL), 80 C, O
2
+
magic number. Pd (Pd
7
2
O) species comprised 23% of the sur-
2
species. Then, the oxygen from Pd O reacts with the hydride
2
+
0
face Pd atoms; the percentages of Pd and Pd species were
0
to give Pd . In a similar fashion, the highly selective oxidation
of aromatic alcohols by the Pd nanoclusters can be explained
by a p-bond interaction between the aromatic allylic alcohols
7
5
6% and 20%, respectively. This agrees with the composition
of the Pd nanocluters, Pd2060(NO 80 ; the surface
divalent Pd cations were preserved at corner/edge sites by
3
)360(OAc)360O
2
+
and the cationic Pd species.
ꢂ
ꢂ
NO
to the two monovalent Pd ions on the face of the cluster.
Selection of the Cu(NO /PCA molar ratio can control the
3
and OAc anions, where oxygen species were bound
The initial step is a p-bond interaction between the phenyl
group of the primary aromatic allylic alcohol and the
Pd species, followed by oxidative addition of the O–H bond
2
+
3
)
2
+
7
0
2+
fraction of surface Pd species in the Pd nanocusters. To
determine the effect of surface Pd fraction on alcohol oxida-
of a coordinated alcohol to a neighboring Pd species. A Pd –
alcoholate species subsequently undergoes b-hydride elimina-
tion to produce an a,b-unsaturated aldehyde and a Pd–H
+
tion, the catalytic activity of the Pd clusters was examined in
the aerobic oxidation of cinnamyl alcohol as a function of
8e,20
species.
a neighboring Pd
the Pd O species is restored by the dissociative adsorption of
Then, the hydride readily reacts with oxygen from
0
Cu(NO
3
)
2
/PCA molar ratio. As depicted in Fig. 4, the highest
2 2
O to give H O, regenerating the Pd . Finally,
catalytic activity was obtained for the Pd2060 cluster prepared
with a 0.10 Cu(NO ) /PCA molar ratio. The most active
2
0
molecular oxygen on the surface of Pd . As shown in Fig. 4,
the Pd nanocluster having the maximum fraction of Pd spe-
cies effectively catalysed the oxidation. This behavior is respon-
sible for the high concentration of the ensemble sites consisting
of three components (i.e. Pd , Pd O, and Pd ) on the cluster
3
2
+
+
Pd2060 cluster had the highest fraction (23%) of Pd among
the surface Pd atoms.
The ensemble Pd sites of the Pd nanocluster catalyst
described here play a pivotal role in the oxidative acetoxyla-
tion of toluene using molecular oxygen, where the reaction
is initiated by a p-bond interaction between toluene and
0
2+
2
+
2
surface (Fig. 3), where an interconversion between Pd (Pd O)
0
7
and Pd facilely occurs in the presence of molecular oxygen.
The above mechanism involving electron donation from an
aromatic ring to the Pd cation to form the p-arene Pd complex
accounts for the para-substitution effect of cinnamyl alcohols;
2
+
the cationic Pd species, followed by rupture of the methyl
C–H bond of adsorbed toluene by a neighboring Pd to form
0
a p-benzyl Pd adduct together with formation of a Pd–H
3
26
New J. Chem., 2003, 27, 324–328

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Fig. 3 Proposed structural model of the 8-shell Pd cluster,
Pd2060(NO )360(OAc)360O80 , with surface ensemble Pd sites. The anio-
3
nic ligands in the outer sphere are omitted.
Fig. 1 FE-SEM photograph of the Pd nanoclusters dispersed on
TiO and their particle size distribution.
2
Fig. 4 Aerobic oxidation of cinnamyl alcohol as a function of
/PCA molar ratio. Reaction conditions: Pd (0.05 mmol),
Cu(NO
3
)
2
ꢃ
cinnamyl alcohol (2 mmol), AcOH (4 mL), 60 C, 2 h, O
2
atmosphere.
of the oxidation of alcohols using molecular oxygen as the oxi-
dant. Research continues into the synthesis of monodispersed
polynuclear metal clusters (e.g. 5, 6, and 7-shell nanoclusters)
as heterogeneous catalysts for the selective oxidation of
2
2
organic compounds in relation to the preparation chemistry
of nanocluster materials.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan. We are grateful to the
Department of Chemical Science and Engineering, Graduate
School of Engineering Science, Osaka University for scientific
support by the ‘‘Gas-Hydrate Analyzing System (GHAS)’’.
Fig. 2 HR-TEM image of the single Pd nanocluster prepared with a
.10 Cu(NO /PCA molar ratio.
0
3 2
)
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˚
7
average distance between the Pd species was 2.75 A. The distance
1
between a terminal b-carbon of the C=C bond and oxygen atom
of the OH function of cinnamyl alcohol was ca. 3 A , calculated by
˚
the PM3 semiempirical method as implemented in MOPAC.
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1
1
2
1
3
28
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