Au-Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem synthesis of imines at ambient conditions

Article abstract of DOI:10.1016/j.cattod.2014.04.015

Alloy nanoparticles (NPs) of gold and palladium on ZrO₂ support (Au-Pd@ZrO₂) were found to be highly active in oxidation of benzyl alcohols and can be used for the tandem synthesis of imines from benzyl alcohols and amines via a one-

Full text of DOI:10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
Catalysis Today xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl
alcohol and tandem synthesis of imines at ambient conditions
Wenjing Cuia,b, Qi Xiao , Sarina Sarina , Wulan Ao , Mengxia Xie ,
c
c
a
d
c,∗
a,∗∗
Huaiyong Zhu , Zhaorigetu Bao
a
Inner Mongolia Key Laboratory of Green Catalysis, Inner Mongolia Normal University, Hohhot, Inner Mongolia 010022, China
Department of Chemical Engineering, Inner Mongolia Vocation College of Chemical Engineering, Hohhot, Inner Mongolia 010070, China
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Queensland 4001, Australia
Analytical and Testing Centre of Beijing Normal University, Beijing 100875, China
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Alloy nanoparticles (NPs) of gold and palladium on ZrO2 support (Au–Pd@ZrO ) were found to be highly
2
Received 11 November 2013
Received in revised form 7 April 2014
Accepted 10 April 2014
Available online xxx
active in oxidation of benzyl alcohols and can be used for the tandem synthesis of imines from benzyl
alcohols and amines via a one-pot, two-step process at mild reaction conditions. The first step of the
process is oxidation of benzyl alcohol to benzaldehyde, excellent yields were achieved after 7 h reaction
◦
at 40 C without addition of any base. In the second step, aniline was introduced into the reaction system
to produced N-benzylideneaniline. The benzaldehyde obtained in the first step was completely consumed
within 1 h. A range of benzyl alcohols and amines were investigated for the general applicability of the
Au–Pd alloy catalysts. It is found that the performance of the catalysts depends on the Au–Pd metal
contents and composition. The optimal catalyst is 3.0 wt% Au–Pd@ZrO2 with a Au:Pd molar ratio 1:1. The
alloy NP catalyst exhibited superior catalytic properties to pure AuNP or PdNP because the surface of
alloy NPs has higher charge heterogeneity than that of pure metal NPs according to simulation of density
function theory (DFT).
Keywords:
Tandem synthesis
Gold and palladium alloy nanoparticles
Benzyl alcohol
Amine
Imine
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
Tandem processes involving catalysts can offer unique and pow-
erful strategies for converting simple starting materials into more
complex products in a single reaction vessel. It is recognized as an
atom-economy and green route that avoids intermediate separa-
tion, consumes less energy and decreases waste formation.
Imines, also called as Schiff’s bases, are useful intermediates for
the synthesis of fine chemicals and pharmaceuticals and widely
used as electrophiles in organic reactions [7]. The conventional
method for imine synthesis is through condensation of an amine
and a carbonyl compound in the presence of a Lewis acid [8].
Tandem synthesis of imines from alcohols and amines is a more
advantageous method than the traditional method, because alco-
hols are much more stable and readily available starting materials.
The process involves two reaction steps: selective oxidation of ben-
zyl alcohol to benzaldehyde and the coupling reaction of amines
with benzaldehyde [9]. The rate-determining step was suggested
to be the oxidation of alcohol, due to the use of base [10].
Selective oxidation of alcohols to their corresponding alde-
hydes is of great interest in organic synthesis [1]. Aldehydes are
largely acquired as they are valuable intermediates for the pharma-
ceuticals and agrochemicals industries [2]. Interest in catalytic
oxidation of alcohols by supported gold and gold–palladium alloy
nanoparticles has expanded dramatically [3]. It has been reported
that Au–Pd bimetallic catalysts showed superior performance com-
pared with monometallic catalysts [4]. In most of studies, longer
reaction time, certain O2 pressure and also elevated temperature
are essential for the oxidation of alcohols to aldehydes [5]. Fur-
thermore, selective oxidation of alcohols under mild conditions
typically requires the use of base that often in over-stoichiometric
amounts which results in carboxylate products [3c,6]. Therefore, it
is a challenge to develop selective oxidation of benzyl alcohol to
benzaldehyde at ambient conditions in the absence of base.
The tandem catalytic processes for imine synthesis in homoge-
neous [11] and heterogeneous catalytic systems [9,10e, 12] have
been reported (Table 1). However, the use of base additives, longer
reaction time and higher temperature were required to achieve
high yields of imines. So it would be a significant breakthrough
∗
Corresponding author. Tel.: +61 731381581.
Corresponding author.
E-mail addresses: hy.zhu@qut.edu.au (H. Zhu), zrgt@imnu.edu.cn (Z. Bao).
∗
∗
http://dx.doi.org/10.1016/j.cattod.2014.04.015
920-5861/© 2014 Elsevier B.V. All rights reserved.
0
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
2
Table 1
2
2
.3. Catalytic experiments
Reported reaction conditions of N-benzylideneaniline synthesis from benzyl alcohol
and aniline.
.3.1. Selective oxidation of benzyl alcohol
The selective oxidation reactions of benzyl alcohols were car-
ried out in a 25 mL round-bottomed flask with magnetic stirrer.
0 mg catalyst, benzyl alcohol (3 mmol) and ethanol (10 mL) were
5
added into the flask which was open to ambient air. The suspen-
sion was stirred magnetically at 40 C. During the reactions 0.5 mL
◦
Catalyst
Base
Temp. ( C)
Yield (%)
Time (h)
Ref.
◦
None
KOH
KOH
CH3OK
NaOH
None
None
110
70
20
100
90
110
92
97
7
94
90
100
20
19
24
20
20
24
[12e]
[11a]
[12d]
[12f]
[12b]
[9]
specimens were periodically collected, filtered and analyzed by
GC (Shimadzu GC-2014) with a capillary column of Rtx-5, tem-
Cu(ClO4)2·6H2O
Au/TiO2
◦
◦
None
Pd/AlO(OH)
K-OMS-2
perature of column range from 100 to 220 C (6 C/min), injector
◦
temperature 260 C and flame ionization detector. The products
were identified by comparison with known standard samples.
2
.3.2. Synthesis of imines
Imines were synthesized in a one-pot, two-step method. In the
if we can synthesize imines from alcohols and amines at ambient
conditions in the absence of base.
first step, benzyl alcohol was oxidized to benzaldehyde. During the
reaction, samples were analyzed by GC. The second step reaction
commenced by adding stoichiometric amount of aniline after the
first step reaction was completed (conversion of benzyl alcohol
achieved 100%). The reaction conditions were maintained the same
in the two steps (40 C, air atmosphere). The products were iden-
tified by GC–MS (Thermo DSQ with a DB-5 column). The yield of
imine was calculated based on the converted amine.
In this study, we firstly perform the reaction of selective oxi-
dation of benzyl alcohol to benzaldehyde on 3.0 wt% Au–Pd@ZrO
under ambient conditions (40 C and air atmospheric pressure).
2
◦
Given that high product yield was achieved in oxidation of benzyl
alcohols without base in this work, there might be an opportunity
to provide an approach for base-free synthesis of more complex
compound, imine. Afterwards, the formation of imines by a tan-
dem process was successfully investigated. The present study can
be a facile, economic and green method that using supported
gold–palladium alloy nanoparticles to catalyze tandem reaction
systems with good selectivity under ambient conditions.
◦
2.4. Recycling of catalysts
Catalysts were recycled after the catalytic reactions. The catalyst
was separated by centrifugation, washed with ethanol (twice), and
dried in an oven at 80 C for 12 h.
2
. Experimental
◦
2.1. Catalyst preparation
3
.0 wt% Au–Pd@ZrO2 catalysts (Au:Pd molar ratio 1:1) were
3. Results and discussion
prepared according to the literature [13]. ZrO2 powder 1.0 g was
dispersed into 100 mL aqueous solution of 0.1 mmol HAuCl4 and
3.1. Catalyst synthesis and characterization
0
.1 mmol PdCl . Lysine (0.2 mol/L, 20 mL) was then added to the
2
mixture under vigorous stirring for 30 min. An aqueous solution of
NaBH4 (0.1 mol/L, 10 mL) was added gradually in about 10 min to
the suspension, followed by hydrochloric acid (0.1 mol/L, 10 mL).
The mixture was left to stand for 24 h and then the solid was sepa-
In the present study, alloy NPs of Au and Pd were prepared on
a ZrO2 support (abbreviated as Au–Pd@ZrO ). The particle size and
2
morphology of Au–Pd alloy NPs can be observed from transmission
electron microscopy (TEM) images (Fig. 1). The Au–Pd alloy NPs
disperse evenly on the ZrO2 surface and the mean diameter of the
Au–Pd alloy NPs is 4.6 nm (Fig. 1b). Fig. 1d is a line profile of energy
dispersion X-ray (EDX) spectrum of a typical Au–Pd alloy NP indi-
cated in Fig. 1c, which shows the elemental distribution along the
radial direction of the metal NP. The line profile indicates that the
NP consists of both Au and Pd distributed spherically around a com-
mon centre which means that the two metals exist as binary alloy
NPs in this sample. The TEM images for monometallic Au and Pd
catalysts were shown in Fig. 1e and f, and the average particle sizes
for Au@ZrO2 and Pd@ZrO2 catalyst was measured to be 4.3 nm and
4.7 nm, respectively. Therefore, the sizes of the two monometallic
catalysts are similar to that of the alloy particles.
TEM–EDX spectrum confirms the elemental composition in the
Au–Pd alloy NPs (Fig. 2a). As can be seen from the X-ray diffraction
(XRD) patterns of Au–Pd alloy NP samples (Fig. 2b), no diffrac-
tion peaks corresponding to either metallic Au or metallic Pd are
observed, this is due to the low metal content of the samples, as well
as the small particle size. The X-ray photoelectron spectroscopy
(XPS) spectra of the catalysts are also shown in Fig. 2. The bind-
ing energies of Au 4f7/2 and Au 4f5/2 electrons are 84.3 and 87.9 eV,
respectively (Fig. 2c). In addition, the binding energies of Pd 3d3/2
and Pd 3d5/2 electrons are 340.1 and 334.9 eV, respectively (Fig. 2d).
These results confirmed that the alloy NPs are in the metallic state.
◦
rated, washed with water and ethanol, and dried at 80 C for 12 h.
Preparation of 1.5 wt% Au@ZrO2 or 1.5 wt% Pd@ZrO : ZrO2 pow-
2
der 1.0 g was dispersed in 100 mL aqueous solution of 0.15 mmol
HAuCl4 or 0.28 mmol PdCl . The following procedures were kept
2
identical.
2.2. Catalyst characterization
Transmission electron microscopy (TEM) images were recorded
with a Jeol JEM-1210 transmission electron microscope employ-
ing an accelerating voltage of 200 kV. The samples were suspended
in ethanol and dried on holey carbon-coated Cu grids. The
composition of some samples was determined by using the
energy-dispersive X-ray (EDX) spectroscopy attachment of the
transmission electron microscope. Powder X-ray diffraction (XRD)
patterns were recorded on Philips PW1830 diffractmeter using
filtered Cu-K␣ radiation (ꢀ = 0.15405 nm); the instrument was
operated at 40 kV and 40 mA. Diffraction data were collected
◦
◦
between 10.0 and 80.0 with a resolution of 0.02 (2ꢁ) for high
angles. The X-ray photoelectron spectroscopy (XPS) was measured
with ESCALAB210 of British VG Company. All binding energies were
referenced to the C (1s) hydrocarbon peak at 285.00 eV.
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
3
Fig. 1. (a, c) TEM image of Au–Pd@ZrO2 catalyst, the arrows indicate Au–Pd NPs; (b) particle size distribution of the Au–Pd alloy NPs; (d) EDX spectrum line profile analysis
of a typical Au–Pd NP indicated by the red line in (c), providing the information of the elemental composition and distribution of the NP; (e) TEM image of Au@ZrO2 catalyst;
(
f) TEM image of Pd@ZrO2 catalyst. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
3
.2. Benzyl alcohol oxidation using Au–Pd@ZrO2 under base-free
It is found that the performance of the catalysts depends on
the alloy metal contents. Therefore, the optimization of the Au–Pd
alloy NPs was investigated, and the results are given in Fig. 3. When
the Au–Pd alloy NPs metal content was 3.0 wt%, the conversion of
benzyl alcohol was 100%. Blank experiment shown that negligible
conditions
The results of selective oxidation of benzyl alcohol to benzalde-
hyde using 50 mg 3.0 wt% Au–Pd@ZrO2 with an optimal Au:Pd
molar ratio of 1:1 are presented in Table 2 Entry 1. The con-
trol experiment was carried out at the same conditions using
mechanical mixture of 1.5 wt% Au@ZrO2 and 1.5 wt% Pd@ZrO2
benzaldehyde can be obtained over the support ZrO , this confirms
2
that the catalytic activity is due to the Au–Pd alloy NPs. However,
when the metal content was reduced to 0.8% and 0.5%, the con-
version of benzyl alcohol decreased to 85% and 73%. TON values
were also calculated for these samples, when the metal content
was reduced from 3 to 0.5%, the TON increased from 303 to 1326.
This is because when the total metal content of catalyst is very
low, the amount of converted benzyl alcohol per active metal cata-
lyst site becomes high. We also confirmed the particle size as well
as the distribution of alloy particles kept mainly unchanged with
decreasing of total metal content.
(
Entry 2). Moreover, the result of performance of 50 mg 1.5 wt%
Au@ZrO and 50 mg 1.5 wt% Pd@ZrO are also provided (Entry 3 and
4
2
2
).
As can be seen from Table 2, the Au–Pd alloy NPs exhibited supe-
rior catalytic activity and selectivity to pure monometallic NPs as
well as the mechanical mixture of AuNPs and PdNPs. The significant
enhancement of the catalytic activity can be explained by the elec-
tronic heterogeneity of alloy NPs, which means that interactions
between the alloy NPs and reactant molecules can be enhanced
compared with pure Au and Pd NPs [13,14].
It was also found that the solvent had substantial influence on
the conversion and product selectivity (Fig. 4). Selective oxidation
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
4
Fig. 2. (a) TEM–EDX spectrum of Au–Pd@ZrO2; (b) XRD pattern of ZrO2 (black curve) and Au–Pd@ZrO2 catalyst (red curve); (c) X-ray photoelectron spectrum (XPS) of Au
f in Au–Pd@ZrO2 catalyst; (d) XPS of Pd 3d in Au–Pd@ZrO2 catalyst. (For interpretation of the references to color in text, the reader is referred to the web version of this
4
article.)
Table 2
of benzyl alcohol in organic solvents such as methanol, ethanol,
toluene and DMF are investigated by 3.0 wt% Au–Pd@ZrO2 at 40 C
Oxidation of benzyl alcohol to benzaldehyde on different catalysts.
◦
Entry Catalyst
Conv. (%) Sel. (%) TON TOF (h 1
−
)
for 7 h under base-free condition. The best catalytic performance
was found in ethanol compared with the other solvents: the com-
1
2
3
4
Au–Pd@ZrO2
91
97
88
97
88
551
230
133
209
79
33
19
30
◦
Au@ZrO2 mixed with Pd@ZrO2 38
plete conversions with high selectivity were obtained at 40 C.
Au@ZrO2
Pd@ZrO2
17
49
Meanwhile, high conversion of benzyl alcohol was also achieved in
methanol, but the selectivity is relatively lower. This can be ascribed
to that benzaldehyde and methanol were catalyzed by Au–Pd alloy
to form hemiacetal which was further oxidized to methyl ester. In
contrast, if toluene and DMF were used as solvents, the conversions
were significantly much lower. It is suggested that the reaction
conversion can be improved in more polar solvent. Moreover the
Reaction condition: benzyl alcohol (3 mmol); ethanol (10 mL); catalyst (50 mg);
◦
40
C; 7 h; air atmosphere; TON was calculated as a ratio between the amounts of
converted benzyl alcohol and the total amount of metal. TOF is a ratio of TON value
to the reaction time (measured at t = 1 h).
reactions of alcohol oxidation were studied under solvent-free con-
dition, however, it is a problem to recover and reuse the catalysts
after the reaction [4a,15].
The effect of reaction temperature was shown in Fig. 5. Increas-
ing the reaction temperature can increase the conversion rate of
Fig. 3. Conversion of benzyl alcohol, selectivity to benzaldehyde and TON on cata-
lysts with various Au–Pd alloy NPs metal content. Reaction condition: benzyl alcohol
Fig. 4. Selective oxidation of benzyl alcohol catalyzed by 3.0 wt% Au–Pd@ZrO2 in
different solvents.
◦
(
3 mmol); ethanol (10 mL); catalyst (50 mg); 40 C; 7 h; air atmosphere.
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
5
Fig. 6. The influence of reaction time 1 on the yield of N-benzylideneaniline.
As expected, the yield of N-benzylideneaniline was improved to
7% when Step 1 proceeded for 1 h (aniline was added after 1 h),
2
and Step 2 proceeded for 7 h. Then seven experiments with over-
all reaction time of 8 h were carried out to determine the optimal
reaction time of each step. As shown in Fig. 6, the yield of N-
benzylideneaniline was improved by prolonging the reaction time
in Step 1, and 100% yield was achieved when Step 1 proceeded for
7
h, and Step 2 proceeded for 1 h.
A variety of benzyl alcohols and amines were investigated to
form imines on 3.0 wt% Au–Pd@ZO catalyst under mild conditions
2
(Table 3). The tandem reaction was effective for a wide range of
benzyl alcohols to afford corresponding benzaldehydes in Step 1,
and following addition of amines can obtain various imines in Step
Fig. 5. Conversion of benzyl alcohol (a) and selectivity to benzaldehyde (b) on
3
.0 wt% Au–Pd@ZrO2 at different temperatures.
2
.
benzyl alcohol (Fig. 5a). However, the selectivity to benzaldehyde
decreased, because over-oxidized products appeared [16] (Fig. 5b).
3.4. Mechanism study
The best performance was achieved on Au–Pd@ZrO catalyst at
2
◦
4
0 C at 7 h, as full benzyl alcohol conversion and 94% benzalde-
3.4.1. The charge heterogeneity at the surface of the alloy NPs
Given that the electronegativity of Pd (2.20) is different from
that of Au (2.54) there will be a charge heterogeneity at the alloy
NP surface, with both electron-rich sites and slightly positively
charged sites present [17,18]. The significant enhancement of the
catalytic activity can be explained by the electronic heterogeneity,
which means that interactions between the alloy NPs and reactant
molecules can be enhanced [13,14].
To confirm the charge heterogeneity at the surface of the alloy
NPs, simulations using the Density function theory (DFT) were car-
ried out. Calculation capacity limitations of our DFT simulation
necessitated the examination of an Au32 and Au12Pd20 cluster as a
first step and the charge distributions of the cluster was simulated.
The results of the calculation confirm that charge heterogeneity
exists even in the monometallic Au clusters and the alloy structure
of Au with Pd increases the charge heterogeneity of the nanoparti-
cle surface, as can be seen from the comparison of two lines in Fig. 7.
Considering the electronegativity of the two metals, this charge dis-
tribution is to be expected and in agreement with previous reports
hyde selectivity were obtained. Further increasing the temperature
to 50 C, it did not increase the conversion obviously but decrease
◦
in the selectivity significantly to benzaldehyde (Fig. 5b). We also
calculated the apparent activation energy of the benzyl alcohol
oxidation from the kinetic data of the reaction at different tem-
peratures using the Arrhenius equation, the apparent activation
energy is 78.66 kJ/mol in the present study, which is close to the
value reported in literature (74.5 kJ/mol) [13].
3.3. Tandem synthesis of imines via a one-pot, two-step process
Initially, benzyl alcohol and aniline were used as substrates
to synthesize N-benzylideneaniline in the absence of base on
Au–Pd@ZrO2 catalyst. However, the yield of imine was below 5%
after 8 h at 40 C. Therefore, a two-step in one-pot process was
developed to achieve the better yield of imine. In the first step,
benzyl alcohol was oxidized to benzaldehyde (Step 1), and then in
the second step aniline was added into the reaction system to form
N-benzylideneaniline (Step 2) (Scheme 1).
◦
[
19].
Scheme 1. Synthesis N-benzylideneaniline in one-pot, two-step approach.
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
6
Table 3
Tandem synthesis of imines from various substrates on Au–Pd@ZrO2 catalyst.
Entry
Substrate
Time 1
Time 2
Yield (%)
1
2
3
4
OH
OH
1
2
7
9
1
1
94
92
100
100
NH2
NH2
OH
OH
H C
3
3
4
9
9
1
1
100
80
100
100
NH
2
NH2
OH
OH
H CO
3
5
6
5
7
1
1
90
94
100
100
NH2
NH2
OH
OH
Cl
7
8
9
7
7
1
1
94
94
100
100
NH
2
NH2
OH
OH
7
7
1
1
94
94
100
100
H
N
NH2
1
0
◦
Reaction condition: alcohol (3 mmol); ethanol (10 mL); catalyst (50 mg); 40 C; 7 h; air atmosphere; amine (1:1 stoichiometry for amine/benzyladehyde) was added and the
mixture after time 1.
Fig. 7. The optimized geometry of the Au32 and Au12Pd20 clusters and the natural charge distribution calculated by DFT.
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
7
2
7% at 1 h, if aniline was introduced to the reaction after 1 h and
the benzyl alcohol conversion did not increases in the subsequent
h-reaction course. The catalytic activity for oxidation of benzyl
7
alcohol was practically eliminated by amine addition.
This phenomenon was explained as the result of improvement
of reverse reaction of benzyl alcohol oxidation in the reported lit-
erature [10e]. To clarify whether the reverse reaction of benzyl
alcohol oxidation occurred in our system, benzaldehyde (1 mmol),
aniline (1 mmol) and 3.0 wt% Au–Pd@ZrO catalyst (50 mg) reacted
2
◦
in ethanol (10 mL) at 40 C. No benzyl alcohol was detected from
the reaction, which indicated that the reverse reaction did not
occur.
Another possibility is competitive adsorption between aniline
and benzyl alcohol on the surface sites of the catalyst. We disperse
the 3.0 wt% Au–Pd@ZrO2 catalysts into a solution of 3 mmol ani-
line in 10 mL ethanol and stirred the suspension vigorously for 1 h.
Then the catalyst solid was directly recovered by centrifuging and
used for benzyl alcohol oxidation without washing and drying. No
catalytic activity for the oxidation of benzyl alcohol was observed
in this reaction system. Aniline adsorbed onto the surface of the
catalyst inhibits the benzyl alcohol oxidation reaction. When this
catalyst (with adsorbed aniline) was washed with ethanol twice
Scheme 2. Proposed mechanism of aromatic alcohol oxidation over Au–Pd alloy
NPs.
3.4.2. Mechanism study of the reactions
The first step was oxidation of benzyl alcohols to benzaldehyde
on alloy NPs. The rate-determining step of the alcohol oxidation
on gold catalysts was believed to be hydrogen abstraction from
CH2 of alcohol and formation of Au-H species, while the role of
oxygen in this reaction is to remove the hydrogen from the gold
surface, leading to the catalytic cycle [20]. It was also observed
that the first step of alcohol oxidation with a Pd catalyst was
alcohol dehydrogenation, which resulted in adsorbed hydrogen on
the Pd surface and the formation of aldehyde [15]. We propose
tentative reaction pathways for selective oxidation of aromatic
alcohols to the corresponding aldehydes or ketones, as shown in
Scheme 2.
◦
to remove the adsorbed aniline and dried at 80 C, we found it
regained the catalytic activity for benzyl alcohol oxidation. This
result implies that aniline is weakly adsorbed on the Au–Pd@ZrO2
surface, which is able to impede the adsorption of benzyl alcohol
onto the catalyst. But the adsorbed aniline can be removed readily
and do not lead to permanent poisoning of catalyst.
Step 2 reaction (condensation benzaldehyde and aniline) was
investigated with and without catalyst. In the presence of 3.0 wt%
Au–Pd@ZrO , the reaction rate of Step 2 (100%) is significantly
2
higher than that in the absence of the catalyst (37%). We suggest
that Au–Pd alloy species promoted the condensation step.
The secondary step was coupling of benzaldehyde and amine.
We monitored the conversion of benzyl alcohol before and after
amine addition to get further insight into the mechanism. Given
that benzaldehyde was the product of Step 1, it would be con-
sumed in Step 2. It was expected that amine addition would have
shifted the reaction equilibrium of the benzyl alcohol oxidation
to the side of yielding more benzaldehyde. To our surprise, the
addition of aniline failed to improve the conversion of benzyl
alcohol. As shown in Fig. 8, the conversion of benzyl alcohol was
3
.5. Recover and reuse of the catalyst
One of the important features of a heterogeneous catalyst is its
reusability after the reactions. The 3.0 wt% Au–Pd@ZrO2 catalyst
was tested for the reusability and we found that the catalyst could
be readily recovered and reused without obvious loss of activity
after 3 runs. We did not observe any obvious metal aggregation or
leaching in the recycled catalyst.
4. Conclusions
In summary, the selective oxidation of benzyl alcohols to ben-
zaldehydes and tandem synthesis of imines from benzyl alcohols
and amines on Au–Pd alloy NPs catalyst under mild conditions
are highly effective. The significant enhancement of the catalytic
activity of benzyl alcohol oxidation can be explained by the charge
heterogeneity which is confirmed by DFT simulation results. We
believe that alloy-H species is formed during the oxidation in
the initial step of the reactions. As the competitive adsorption
between aniline and benzyl alcohol on catalyst surface, we can
design a one-pot, two-step process for tandem imine synthesis fol-
lowed by the benzyl alcohol oxidation. Compared with previously
reported systems, this work has several notable advantages: firstly,
benzaldehydes and imines were synthesized on Au–Pd alloy NPs
◦
catalyst under ambient temperature (40 C) and base-free condi-
tions. Additionally, complete yield of imines can be accomplished
in this one-pot reaction system. Finally, it can be highlighted the
potential to develop new heterogeneous catalytic systems for oxi-
dation of benzyl alcohol and tandem synthesis of imines under
ambient conditions.
Fig. 8. Conversion of benzyl alcohol on different time of Step 1, (a) no amine was
added in the reaction, (b) reaction time: 1 h, (c) reaction time: 3 h, (d) reaction time:
5
h.
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015

G Model
CATTOD-9042; No. of Pages8
ARTICLE IN PRESS
8
W. Cui et al. / Catalysis Today xxx (2014) xxx–xxx
Acknowledgements
[7] V. Saggiomo, U. Lüning, Tetrahedron Lett. 50 (2009) 4663–4665.
[
[
8] A.K. Chakraborti, S. Bhagat, S. Rudrawar, Tetrahedron Lett. 45 (2004)
641–7644.
9] S. Sithambaram, R. Kumar, Y. Son, S. Suib, J. Catal. 253 (2008) 269–277.
7
Financial supports from the National Natural Science Foun-
dation of China (No. 20966008), Opening Project of Natural
Science Foundation of Inner Mongolia (No. 2010KF02) and Australia
Research Council (ARC DP110104990) are gratefully acknowl-
edged.
[10] (a) A.S.K. Hashmi, G.J. Hutchings, Angew. Chem. Int. Ed. 45 (2006) 7896–7936;
(
b) N. Zheng, G.D. Stucky, Chem. Commun. 43 (2007) 3862–3864;
(
c) S. Meenakshisundaram, E. Nowicka, P.J. Miedziak, G.L. Brett, R.L. Jenkins, N.
Dimitratos, S.H. Taylor, D.W. Knight, D. Bethell, G.J. Hutchings, Faraday Discuss.
145 (2010) 341;
(
(
d) B.N. Zope, D.D. Hibbitts, M. Neurock, R.J. Davis, Science 330 (2010) 74–78;
e) P. Liu, C. Li, E.J.M. Hensen, Chem. Eur. J. 18 (2012) 12122–12129.
References
[
11] (a) Q. Kang, Y. Zhang, Green Chem. 14 (2012) 1016–1019;
b) C. Xu, L.Y. Goh, S.A. Pullarkat, Organometallics 30 (2011) 6499–6502;
(
[
1] (a) A. Corma, H. Garcia, Chem. Soc. Rev. 37 (2008) 2096–2126;
b) H. Guo, M. Kemell, A. Al-Hunaiti, S. Rautiainen, M. Leskelä, T. Repo, Catal.
Commun. 12 (2011) 1260–1264;
c) V.R. Choudhary, A. Dhar, P. Jana, R. Jha, B.S. Uphade, Green Chem. 7 (2005)
68–770.
(c) J.W. Rigoli, S.A. Moyer, S.D. Pearce, J.M. Schomaker, Org. Biomol. Chem. 10
(2012) 1746–1749.
(
[12] (a) H. Sun, F.-Z. Su, J. Ni, Y. Cao, H.-Y. He, K.-N. Fan, Angew. Chem. Int. Ed. 48
(2009) 4390–4393;
(
7
(b) M.S. Kwon, S. Kim, S. Park, W. Bosco, R.K. Chidrala, J. Park, J. Org. Chem. 74
(2009) 2877–2879;
(c) A. Zanardi, J.A. Mata, E. Peris, Chem. Eur. J. 16 (2010) 10502–10506;
(d) S. Kegnæs, J. Mielby, U.V. Mentzel, C.H. Christensen, A. Riisager, Green Chem.
12 (2010) 1437–1441;
(e) J. Xu, R. Zhuang, L. Bao, G. Tang, Y. Zhao, Green Chem. 14 (2012) 2384;
(f) R.R. Donthiri, R.D. Patil, S. Adimurthy, Eur. J. Org. Chem. 2012 (2012)
4457–4460.
[
[
2] (a) A. Köckritz, M. Sebek, A. Dittmar, J. Radnik, A. Brückner, U. Bentrup, M.M.
Pohl, H. Hugl, W. Mägerlein, J. Mol. Catal. A: Chem. 246 (2006) 85–99;
(
b) M. Sankar, E. Nowicka, R. Tiruvalam, Q. He, S.H. Taylor, C.J. Kiely, D. Bethell,
D.W. Knight, G.J. Hutchings, Chem. Eur. J. 17 (2011) 6524–6532.
3] (a) V.R. Choudhary, D.K. Dumbre, Catal Appl. A 375 (2010) 252–257;
(
(
(
(
b) C.Y. Ma, J. Cheng, H.L. Wang, Q. Hu, H. Tian, C. He, Z.P. Hao, Catal. Today 158
2010) 246–251;
c) M. Zahmakıran, S. Özkar, Mater. Chem. Phys. 121 (2010) 359–363;
d) C.D. Pina, E. Falletta, M. Rossi, Chem. Soc. Rev. 41 (2012) 350–369.
[13] S. Sarina, H. Zhu, E. Jaatinen, Q. Xiao, H. Liu, J. Jia, C. Chen, J. Zhao, J. Am. Chem.
Soc. 135 (2013) 5793–5801.
[14] (a) W. Tang, G. Henkelman, J. Chem. Phys. 130 (2009) 194504;
(b) M. Chen, Science 310 (2005) 291–293.
[15] A. Abad, A. Corma, H. García, Chem. Eur. J. 14 (2008) 212–222.
[16] F. Gao, D.W. Goodman, Chem. Soc. Rev. 41 (2012) 8009.
[17] A. Kotsifa, T.I. Halkides, D.I. Kondarides, X.E. Verykios, Catal. Lett. 79 (2002)
113.
[18] N. Toshima, M. Kanemaru, Y. Shiraishi, Y. Koga, J. Phys. Chem. B 109 (2005)
16326.
[4] (a) Y. Chen, H. Lim, Q. Tang, Y. Gao, T. Sun, Q. Yan, Y. Yang, Appl. Catal. A 380
(
(
(
2010) 55–65;
b) S. Marx, A. Baiker, J. Phys. Chem. C 113 (2009) 6191–6201;
c) N. Dimitratos, A. Villa, D. Wang, F. Porta, D. Su, L. Prati, J. Catal. 244 (2006)
1
13–121.
[
[
5] (a) P.J. Miedziak, Q. He, J.K. Edwards, S.H. Taylor, D.W. Knight, B. Tarbit, C.J. Kiely,
G.J. Hutchings, Catal. Today 163 (2011) 47–54;
(
b) D.I. Enache, Science 311 (2006) 362–365.
6] (a) G.J. Hutchings, Chem. Commun. (2008) 1148;
b) T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037–3058.
[19] J.P. Roth, J.C. Yoder, T.-J. Won, J.M. Mayer, Science 294 (2001) 2524–2526.
[20] D. Liu, J. Gui, F. Lu, Z. Sun, Y.-K. Park, Catal. Lett. 142 (2012) 1330–1335.
(
Please cite this article in press as: W. Cui, et al., Au–Pd alloy nanoparticle catalyzed selective oxidation of benzyl alcohol and tandem
synthesis of imines at ambient conditions, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.015