Palladium supported on magnetic nanoparticles as recoverable catalyst for one-pot reductive amination of aldehydes with nitroarenes under ambient conditions

Article abstract of DOI:10.1016/j.catcom.2012.10.024

A high-performance palladium-based catalyst has been prepared by the co-precipitation method. The catalyst was characterized by TEM, XRD, VSM and ICP. It was found that the catalyst showed a high activity for the one-pot direct reductive amination of aldehydes with nitroarenes in the presence of molecular hydrogen in ethanol, even at room temperature. Furthermore, the catalyst was easily recovered by applying an external magnet and reused for eight cycles without significant loss of activity.

Full text of DOI:10.1016/j.catcom.2012.10.024

Catalysis Communications 30 (2013) 40–44
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Palladium supported on magnetic nanoparticles as recoverable catalyst for one-pot
reductive amination of aldehydes with nitroarenes under ambient conditions
Shuoyun Wei, Zhengping Dong, Zongyan Ma, Jian Sun, Jiantai Ma ⁎
The Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A high-performance palladium-based catalyst has been prepared by the co-precipitation method. The catalyst
was characterized by TEM, XRD, VSM and ICP. It was found that the catalyst showed a high activity for the
one-pot direct reductive amination of aldehydes with nitroarenes in the presence of molecular hydrogen in
ethanol, even at room temperature. Furthermore, the catalyst was easily recovered by applying an external
magnet and reused for eight cycles without significant loss of activity.
Received 11 September 2012
Received in revised form 29 October 2012
Accepted 30 October 2012
Available online 6 November 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Magnetic nanoparticles
One-pot reductive amination
Aldehydes
Nitroarenes
1
. Introduction
[23], and metal oxides [24–26]. Although these supported catalysts
possess extremely high catalytic activity, the separation step becomes
a more troublesome issue. To overcome the problem, magnetic
Amidation reaction is one of the most fundamental synthetic tools
in modern organic synthesis [1]. Amines and their derivatives exist in
many biologically active molecules, and are relevant intermediates in
the synthesis of pharmaceutically active substances, dyes and fine
chemicals [2]. Although several new methodologies have been
reported for imine synthesis in recent years [3–7], the direct reductive
amination of carbonyl compounds is still the most practical way to get
target amines [8–11]. A variety of methods have been developed to
achieve this transformation including catalytic hydrogenation [12],
or using a reducing agent such as sodium borohydride [13], sodium
cyanoborohydride [14], sodium triacetoxyborohydride [15], pyridine-
3 4
nanoparticles, such as Fe O nanoparticles, are used as the support
materials because they have unique physical properties and can be
easily separated from reaction mixture by using an external magnet
[27–29]. In addition, from the environmental and economic view-
points, the one-pot reductive amination of aldehydes with
nitroarenes have gained more and more attention, since it avoids
the isolation of the intermediary imine or iminium.
3 4
Here, we report our results about Pd/Fe O catalyzed one-pot
reductive amination reaction under a hydrogen atmosphere, even at
room temperature (Scheme 1). To the best of our knowledge, it has
not been reported for examples of the one-pot reductive amination
3
BH [16], and N-methyl piperidine zinc borohydride [17]. Unfortunately,
there have been one or more drawbacks in these methods, such as poor
stability, toxic byproducts and in acidic conditions. Therefore, it is quite
important to study an economically viable and environmentally benign
method under mild reaction conditions for reductive amination of
carbonyl compounds.
of aldehydes with nitroarenes using Pd/Fe
as solvent.
3 4
O as catalyst and ethanol
2. Experimental
In this context, the development of efficient and easily recyclable
heterogeneous catalysts has attracted much attention. It has been
found that palladium nanoparticles (particularly with dimension
less than 10 nm) exhibit unexpectedly high catalytic activities toward
different types of reactions but do not display the property in bulk
palladium [18–20]. Recently, attempts have been made to immobilize
palladium nanoparticles on diverse solid supports for reductive
amination of carbonyl compounds, such as polymers [21,22], carbon
2.1. Characterization
XRD measurements were performed on a Rigaku D/max-2400
diffractometer using Cu–Ka radiation as the X-ray source in the 2θ
range of 10–80°. The size and morphology of the magnetic
2
nanoparticles were observed by a Tecnai G F30 transmission electron
microscopy and samples were obtained by placing a drop of a colloidal
solution onto a copper grid and evaporating the solvent in air at room
temperature. Pd content of the catalyst was measured by inductively
coupled plasma (ICP) on IRIS Advantage analyzer. Magnetic measure-
⁎
Corresponding author. Tel./fax: +86 931 891 2311.
E-mail address: majiantai@lzu.edu.cn (J. Ma).
3 4
ments of Pd/Fe O were investigated with a Quantum Design vibrating
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.10.024

S. Wei et al. / Catalysis Communications 30 (2013) 40–44
41
3 4
Scheme 1. One-pot reductive amination of aldehydes with nitroarenes by Pd/Fe O .
sample magnetometer (VSM) at room temperature in an applied
separated from the reaction mixture with the aid of an external
magnet. The conversion was estimated by GC or GC-MS.
magnetic field sweeping from −15 to 15 kOe. BET surface area was
2
acquired by physisorption of N on a Micromeritics TriStar 3020
apparatus.
3
. Results and discussion
2
.2. Catalyst preparation
3.1. Catalyst preparation and characterization
The Pd/Fe O
3 4
catalyst had been prepared by the co-precipitation
·9H O was dissolved
M) at room temperature. Then,
3
solution was slowly dropped into the mixed solution, and the
method [30,31]. Under stirring, 4.1 g of Fe(NO
3
)
3
2
3 4
The Pd/Fe O catalyst had been prepared by the co-precipitation
−
2
in 14 mL of PdCl
Na CO
2
solution (3.2×10
method. The preparation process involved combing of the precursor
of active components and support, drying and calcining at elevated
temperatures, and then further reducing. Compared to the traditional
impregnation method, the co-precipitation method usually led to a
better interaction between metal and solid support.
2
pH of the final solution was controlled to ca. 8.5. After stirring for 6 h,
the resulting precipitate was filtered, washed with deionized water
for several times, air-dried at 323 K overnight, and calcined in air
at 573 K for 3 h. The resulted brown powder was reduced in
3 4
Fig. 1 shows TEM images of the Pd/Fe O catalyst. In Fig. 1a, the
2
0 vol.%H
as Pd/Fe
be 5.18 wt.% based on ICP analysis.
2
/Ar at 473 K for 3 h. The as-reduced product was denoted
O . The amount of Pd in the obtained catalyst was found to
3 4
particle diameter of the catalyst was about 20 nm and the particle
was nearly uniform in size. The free Pd nanoparticle was found highly
dispersed on Fe
3
O
4
nanoparticles (Fig. 1b). The HRTEM image of
component with an
Fig. 1c shows the (220) planes of the Fe
3 4
O
2
2
.3. Catalytic reductive amination reactions
.3.1. Pd/Fe catalyst for the reductive amination of different aldehydes
interplanar spacing of 0.296 nm, which corresponded well to the
plane of magnetite. From Fig. 1c, it can also be seen that the interplanar
spacing of 0.22 nm corresponded to the (111) plane of fcc Pd (JCPDS
46–1043). It is observed that the diameter of most Pd particles on
3 4
O
with nitroarenes
1
.0 mmol of the different aldehydes, 1.2 mmol of nitrobenzene
Fe
the diffraction peaks of metallic Pd on the Pd/Fe
Fig. 2 presents the XRD pattern of the Pd/Fe
3 4
O crystal was less than 3 nm, which accounted for the absence of
and 27 mg of catalyst were added in 5 mL of ethanol at room temper-
ature by ball milling under a hydrogen atmosphere. The reaction
process was monitored by thin layer chromatography (TLC). After
the reaction, the catalyst was separated by a small magnet placed at
the bottom of the flask, and the conversion was estimated by GC
O
3 4
catalyst [30,31].
3
O
4
catalyst. It showed
that all the characteristic diffraction peaks match well with standard
Fe reflections (JCPDS card No. 86–1354). These diffraction peaks
3 4
O
were sharp and intense, indicating that products were well crystal-
lized, and the average crystallite size was about 11 nm by using
Scherrer equation. There was no diffraction peaks of metallic Pd,
indicating that Pd remained highly dispersed in the catalyst which
corresponded to the HRTEM analysis.
(
P.E. AutoSystem XL) or GC–MS (Agilent 6,890 N/5,973 N).
2
.3.2. Pd/Fe
different nitroarenes
7 mg of catalyst and 1.2 mmol nitroarene were added in 5 mL of
3 4
O catalyst for the reductive amination of aldehydes with
2
3 4
The magnetic properties of the Pd/Fe O catalyst were character-
ethanol. The resulting suspension was stirred under a hydrogen
atmosphere and at room temperature for the specified period of
time. After amine formation was completed, 1.0 mmol of aldehyde
was added. The reaction process was monitored by thin layer chro-
matography (TLC). After completion of the reaction, the catalyst was
ized using a vibrating sample magnetometer. It can be seen that the
saturation magnetization value and the coercivity value of the
Pd/Fe
was
3
O
a
4
was 83.3 emu/g and 161 Oe (Fig. 3) respectively. There
little remanence and coercivity, suggesting that such
nanoparticles were soft ferromagnetic [32], and the figure (Fig. 3)
3 4
Fig. 1. TEM images (a), (b) and HRTEM image (c) of the Pd/Fe O catalyst.

42
S. Wei et al. / Catalysis Communications 30 (2013) 40–44
3 4
Fig. 2. XRD pattern of the Pd/Fe O catalyst.
2 3 4
Fig. 4. N adsorption–desorption isotherm of the Pd/Fe O catalyst.
also showed that the catalyst can be efficiently separated from the
solution by an external magnetic field.
2
Fig. 4 shows the N adsorption–desorption isotherm of the
Pd/Fe catalyst. The catalyst exhibited a type-IV isotherm and a
type H2 hysteresis loop at the relative pressure range of 0.5–0.8,
3 4
O
Table 1
Comparative study of different catalysts on reductive amination of benzaldehyde with
nitrobenzene.
which was characteristic of the mesoporous structure [33]. The BET
a
2
−1
surface area of the Pd/Fe
3
O
4
catalyst was 192 m g
.
Entry
Catalyst
Time (h)
Yield (%)b
1
2
3
4
5
6
7
Pd/C (5%)
Pd/C (10%)
Ru/C (5%)
6
6
10
24
6
16
18
0
3
.2. Catalytic activity for one-pot reductive amination of aldehydes with
nitroarenes
Ni/SiO
2
(10%)
0
Pd/Fe
Pd/Fe
Pd/Fe
3
O
3
O
3
O
4
4
4
92
88
82
In the first screening of experiments, the reductive amination of
8
10
benzaldehyde with nitrobenzene was compared with various catalysts.
As can be seen from Table 1, the reaction could be carried out in 16%
yield in the presence of 5% Pd/C (entry 1). While the amount of Pd/C
to 10% increases, the reaction result showed that the yield did slow
improvement (entry 2). And no product was formed when the reaction
was run in the absence of the catalyst, or in the presence of the Ru/C
a
Reaction conditions: benzaldehyde (1 mmol), nitrobenzene (1.2 mmol), catalyst
(1.3 mol%), and solvent (5 mL) at room temperature under a H atmosphere.
2
b
Yield was determined by GC analysis.
(
5%) and the Ni/SiO
Pd/Fe catalyst showed the highest catalytic activity and produced
amine in 92% yield (entry 5). Increasing the reaction time from 6 to
0 h resulted in a little decrease of reaction yield (entries 6–7). There-
fore, all of the following experiments were performed using Pd/Fe
as catalyst.
In the following studies, the above reactions were explored in the
presence of Pd/Fe under conditions of various solvents and
2
(10%) catalyst (entries 3–4). In contrast, the
different temperatures. These results clearly showed that the efficiency
of the reaction was affected by different solvents (Table 2). Based on
the reaction yields and environmental consideration, ethanol was
proved to be superior to the other alcohols. The reaction yields were
susceptible to temperature changes (entries 6–8). The increase of reac-
tion temperature up to 60 °C led to low yields (38%).
Under the optimized reaction conditions, the scope of the reaction
was explored with structurally different aldehydes and nitroarenes.
As shown in Table 3, various aldehydes were reductively aminated
with nitrobenzene in ethanol at room temperature and under a
hydrogen atmosphere. It was found that there was no remarkable
electron and position effects for the aromatic aldehydes during this
reaction, although benzaldehydes are with o-, m-, and p-substituents
3 4
O
1
3 4
O
3 4
O
Table 2
a
Optimization of reaction conditions.
Entry
Solvent
Temp. (°C)
Yield (%)b
1
2
3
4
5
6
7
8
MeOH
i-PrOH
n-BuOH
i-BuOH
s-BuOH
EtOH
rt
rt
rt
rt
rt
rt
40
60
93
81
87
55
40
92
82
38
EtOH
EtOH
a
Reaction conditions: benzaldehyde (1 mmol), nitrobenzene (1.2 mmol), catalyst
(
1.3 mol%), and solvent (5 mL) at room temperature under a H
Yield was determined by GC analysis.
2
atmosphere.
b
Fig. 3. Room temperature magnetization curve of the Pd/Fe
3
O
4
catalyst.

S. Wei et al. / Catalysis Communications 30 (2013) 40–44
43
Table 3
Pd/Fe
Table 4
3
3 4
Pd/Fe O
catalyst for the reductive amination of aldehydes with different nitroarenes.a
O
4
catalyst for the reductive amination of different aldehydes with nitroarenes.a
Entry
1
Aldehyde
Product
Time(h)
6
Yield(%)b
92
Entry
1
Aldehyde
Product
Time (h)
6
Yield (%)b
84
2
3
6
6
99
96
2
3
6
6
90
86
4
5
6
6
91
93
4
5
6
7
87
89
6
7
7
8
99
95
a
3 4
Reaction conditions: aldehyde (1 mmol), nitrobenzene (1.2 mmol), Pd/Fe O cata-
lyst (1.3 mol%), and ethanol (5 mL) at room temperature under a H atmosphere.
2
b
Yield was determined by GC analysis.
8
9
8
91
It was observed that all of these reactions have nearly no formation of
any side products.
3 4
The recyclability of Pd/Fe O catalyst was further investigated for
8
8
82
81
commercial application. After completion of the reaction, the catalyst
was recovered from the reaction mixture by using an external
magnet. The recovered catalyst was washed with ethanol for several
times, and dried at room temperature. The recovered catalyst was
added to reaction mixture under same conditions for eight cycles
without a significant loss of yield and catalytic activity (Table 5).
Metal leaching was studied by ICP analysis of the catalyst after the
1
0
1
1
1
2
8
8
85
78
3 4
reaction. The Pd content (5.06 wt.%) of the recovered Pd/Fe O cata-
lyst remained just about the same as the fresh catalyst, indicating
that Pd leaching was negligible. These results further confirmed the
high recyclability of the catalyst.
1
1
1
3
4
5
8
7
6
96
98
90
4
. Conclusions
In conclusion, we have developed a simple and efficient and
eco-friendly method for one-pot reductive amination using easily
recoverable catalyst that immobilized Pd on magnetic support
under mild conditions. This protocol can be used to generate a diverse
range of amines in good to excellent yields. The simple catalyst
exhibits high activity even after eight cycles. These advantages make
this methodology attractive for the development of large-scale indus-
trial synthesis.
1
6
7
94
a
Reaction conditions: aldehyde (1 mmol), nitrobenzene (1.2 mmol), Pd/Fe
3
O
4
cata-
lyst (1.3 mol%), and ethanol (5 mL) at room temperature under a H
2
atmosphere.
b
Yield was determined by GC analysis.
Acknowledgments
This work was supported financially by the Key Laboratory of Non-
ferrous Metals Chemistry and Resources Utilization, Gansu Province.
(
entries 1–8). We also observed that 2-methoxybenzaldehyde and
mutil-substituent benzaldehyde had low yields (entries 9–12), this
was probably due to the relatively large steric hindrance. Furthermore,
aliphatic aldehydes, regardless of whether they were linear or
R-branched, underwent the reductive amination rapidly and gave the
products in excellent yields nearly without the formation of any side
products (entries 13–16).
Table 5
a
Reductive amination of aldehydes with nitrobenzene over eight cycles.
Entry
Aldehyde
–CHO
1
2
3
4
5
6
7
8
Yield(%)b
C
6
H
6
92
89
93
90
92
88
90
91
Table 4 shows the results obtained for reactions carried using
benzaldehyde with different nitrobenzenes. Unsubstituted (Table 3,
entry 1) as well as substituted 4-chlorine, 4-methyl, 2-methyl, 4-ethyl,
a
Reaction conditions: aldehyde (1 mmol), nitrobenzene (1.2 mmol), Pd/Fe
3
O
4
cata-
lyst (1.3 mol%), and ethanol (5 mL) at room temperature for 6 h under a H
sphere.
2
atmo-
b
4
-methoxy nitrobenzenes obtained the desired product in high yields.
Yield was determined by GC analysis.

44
S. Wei et al. / Catalysis Communications 30 (2013) 40–44
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