Programmed synthesis palladium supported on Fe3O4@C: An efficient and heterogeneous recyclable catalyst for one-pot reductive amination of aldehydes with nitroarenes in aqueous reaction medium

Article abstract of DOI:10.1007/s10562-015-1552-1

A highly efficient Pd/Fe₃O₄@C catalytic system has been developed for direct reductive amination of carbonyl compounds with nitroarenes in aqueous reaction medium. The catalyst was characterized by TEM, XRD, XPS and VSM. 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 at mild temperature. Besides, the catalyst could be recovered in a facile manner from the reaction mixture and recycled six times without obvious loss in activity.

Full text of DOI:10.1007/s10562-015-1552-1

Catal Lett
DOI 10.1007/s10562-015-1552-1
Programmed Synthesis Palladium Supported on Fe3O4@C:
An Efficient and Heterogeneous Recyclable Catalyst for One-Pot
Reductive Amination of Aldehydes with Nitroarenes in Aqueous
Reaction Medium
Xingchun Zhou1 Xinzhe Li1 Lixin Jiao1 Hongfei Huo1 Rong Li1
•
•
•
•
Received: 10 January 2015 / Accepted: 10 May 2015
Ó Springer Science+Business Media New York 2015
Abstract A highly efficient Pd/Fe3O4@C catalytic sys-
tem has been developed for direct reductive amination of
carbonyl compounds with nitroarenes in aqueous reaction
medium. The catalyst was characterized by TEM, XRD,
XPS and VSM. 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 at mild temperature. Besides, the catalyst could
be recovered in a facile manner from the reaction mixture
and recycled six times without obvious loss in activity.
developed for the preparation of amines, such as direct
base-promoted N-alkylation and direct reductive amination
[3, 4]. However, it’s still a challenge to control the
monoalkylation of primary amines [5], the synthesis of a
wide variety of substituted amines relies heavily on the
reductive amination of the carbonyl compounds [6, 7], but
the method is usually carried out with hydride-based
reagents such as NaBH4 [8], pyridine–BH3 [9], Zn–AcOH
[10], Et3SiH–CF3CO2H [11], Bu3SnHDMF [12], InCl3–
Et3SiH [13], and various modified borohydride derivatives
[14]. These reagents have certain limitations such as the
dependence of a stoichiometric amount of reductant, poor
functional group tolerance, side reactions, overalkylation,
toxic waste, thermal instability, handling risks, and rigor-
ous reaction conditions [15]. Therefore, it is desirable to
develop an effective, facile and more economical method
for the synthesis of amides.
Graphical Abstract
It has been found that Palladium nanoparticles, par-
ticularly with dimension less than 10 nm, exhibit unex-
pectedly high catalytic activities toward different types of
reactions, a property not revealed in bulk palladium. In
recent years, many attempts have been made to disperse
palladium nanoparticles on support materials which
showed high catalytic activities toward the reductive
amination reaction [16, 17]. One-pot transformations are
economically, ecologically and highly intriguing for de-
veloping efficient new synthetic processes. Additionally,
one-pot reductive amination using nitroarenes is conceived
to be especially attractive as it does not require prior re-
duction of the nitroarenes [16, 18, 19]. Water is a desirable
solvent comparing with common organic solvents for
chemical reactions considering the cost, safety, and envi-
ronmental concerns [20]. Hence, the development of cat-
alytic protocols employing water as a reaction medium
could be the answer for the future of green chemistry.
Keywords Core–shell Á Pd (0) Á One-pot direct reductive
amination Á Aldehydes Á Nitroarenes
1 Introduction
Amines and their derivatives are important building blocks
possessing wide application in chemical and life science [1,
2]. Due to their importance, several methods have been
& Rong Li
liyirong@lzu.edu.cn
1
The Key Laboratory of Catalytic Engineering of Gansu
Province, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
123

X. Zhou et al.
Based on the above considerations, in this paper, we
report our results about Pd well-dispersed on core–shell
Fe3O4@C (Pd/Fe3O4@C) and it shows excellent catalytic
activity for one-pot reductive amination reaction in aque-
ous medium under a hydrogen atmosphere. The catalyst
can be easily recovered by using a permanent magnet and
recycled six times without obvious loss in activity.
Design vibrating sample magnetometer (VSM) at room
temperature in an applied magnetic field sweeping from
-15 to 15 kOe.
2.3 Typical Procedures for the Catalytic Test
2.3.1 The Catalyst for the Reductive Amination
of Different Aldehydes with Nitroarenes
2 Experimental
0.5 mmol of the different aldehydes, 0.6 mmol of ni-
trobenzene and 20 mg of catalyst (2.08 wt%) were added in
3 mL of water by ball milling under a hydrogen atmo-
sphere. After the reaction, the catalyst was separated by a
small magnet placed at the bottom of the flask, and the
product was extracted with ethyl acetate (5 mL) and the
conversion was estimated by GC (P.E. AutoSystem XL) or
GC–MS (Agilent 6890 N/5973 N). Thereafter, the catalyst
was washed three times with ethanol and dried at room
temperature for the next cycle.
2.1 Catalyst Preparation
The Fe3O4@C was prepared using a published method
[21]. In a typical synthesis, ferrocene (0.30 g) was dis-
solved in acetone (30 mL). After intense sonication for
30 min, 1 mL of hydrogen peroxide was slowly added into
the above mixture solution. Then, it was vigorously stirred
for 30 min with magnetic stirring. After that, the precursor
solution was transferred to the teflon-lined stainless auto-
clave with a total volume of 50.0 mL, and then heated to
and maintained at 210 °C. After 48 h, the precipitates were
washed with acetone three times to remove excess fer-
rocene and dried at room temperature in a vacuum oven.
These Fe3O4@C nanospheres possess functional groups
(–COOH) on the surface [22], the Sn(II) can link to the
nanospheres’ surface through inorganic grafting [23].
200 mg Fe3O4@C was dispersed in 35 mL distilled water
and stirred for 20 min as part A. 200 mg SnCl2 was dis-
solved in 35 mL 0.02 M HCl solution as part B. Parts A
and B were mixed together under stirring for 30 min. The
suspension was separated from the solution by an external
magnet, washed five times with distilled water and dis-
persed in 80 mL distilled water. Then, 50 mL 12 mg PdCl2
was added to the above mixture. 10 min later, 20 mL of
0.15 M sodium formate solution was added, following
stirring for 8 h. The suspension was separated from the
solution by an external magnet, washed with distilled water
and ethanol for several times and dried at 50 °C for 12 h.
2.3.2 The Catalyst for the Reductive Amination
of Aldehydes with Different Nitroarenes
20 mg of catalyst (2.08 wt%), 0.6 mmol nitroarene and
0.5 mmol of aldehyde were added in 3 mL of water. The
resulting suspension was stirred under a hydrogen atmo-
sphere and at 60 °C for the specified period of time. The
reaction process was monitored by thin layer chromatog-
raphy (TLC). After completion of the reaction, the catalyst
was separated from the reaction mixture with the aid of an
external magnet. The conversion was estimated by GC or
GC–MS after separating the catalyst from the reaction
mixture with a magnet. The catalyst was then washed
several times with ethanol and dried at room temperature
for its reuse.
3 Results and Discussions
3.1 Catalyst Preparation and Characterization
2.2 Characterization
The preparation of the Pd/Fe3O4@C catalyst followed the
steps described in Scheme 1. Firstly, Fe3O4@C was pre-
pared. It was proved the existence of carboxyl groups in the
outer carbon layer. Next is to link the Sn(II) to the Fe3-
O4@C precursor surface through inorganic grafting [22].
Thirdly, the linked Sn(II) species acted as a reducing
reagent to reduce Pd(II) in situ. The amount of Pd in the
obtained catalyst was found to be 2.08 wt% based on ICP
analysis.
XRD measurement was performed on a Rigaku D/max-
2400 diffractometer using Cu-Ka radiation as the X-ray
source in the 2h range of 10–90°. The size and morphology
of the magnetic nanoparticles were observed by a Tecnai
G2 F30 transmission electron microscopy and samples
were obtained by placing a drop of a colloidal solution onto
a copper grid and evaporated in air at room temperature.
X-ray photoelectron spectroscopy (XPS) was recorded on a
PHI-5702 instrument and the C1s line at 297.8 eV was used
as the binding energy reference. Magnetic measurement of
Fe3O4@C, Pd/Fe3O4@C were investigated with a Quantum
The morphologies and structures of the products at
different synthetic steps were observed by TEM. Figure 1a
is the TEM image of a single Fe3O4@C nanoparticle.
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Programmed Synthesis Palladium Supported on Fe3O4@C
Scheme 1 Preparation of the
catalysts
Fig. 1 TEM images of a Fe3O4@C, b, c Pd/Fe3O4@C, d size distribution histograms image of the Pd nanoparticles crystal structure in detail on
Pd/Fe3O4@C, e Pd/Fe3O4@C catalyst after six runs, f EDX analysis
123

X. Zhou et al.
Fig. 4 Room temperature magnetization curves of a Fe3O4@C, b Pd/
Fe3O4@C
Fig. 2 XRD patterns of a Fe3O4@C, b Pd/Fe3O4@C
prepared products contain Fe, Pd, Sn, Cu, C and O. Among
these elements, Cu was generally influenced by the copper
network support films, Sn, O, C, Fe and Pd signals resulted
from the Pd/Fe3O4@C particles which formed the
products.
Figure 2 shows the XRD patterns between 10° and 90°
of the samples. Figure 2b shows that apart from the original
peaks, the appearance of the new peaks at 2h = 40.1°,
46.5°, 68.0° and 82.1° corresponding to the reflections of
(111), (200), (220) and (311) crystal planes of palladium,
respectively. The results from XRD implied that the Pd
nanoparticles had been successfully immobilized on the
surface of magnetic nanoparticles.
Figure 3 shows the XPS spectrum of the synthesized Pd/
Fe3O4@C catalyst. Peaks corresponding to oxygen, carbon,
palladium, tin and iron were observed. To ascertain the
oxidation state of the Pd, XPS studies were carried out. The
Pd binding energy of Pd/Fe3O4@C exhibited two strong
peaks centered at 341.5 and 335.5 eV, which were assigned
to Pd 3d5/2 and Pd 3d3/2, respectively.
Fig. 3 XPS wide-scan spectrum of the Pd/Fe3O4@C (inset high-
resolution spectrum of Pd 3d)
Magnetic measurements were carried out by employing
VSM at room temperature. The magnetization curves
measured for Fe3O4@C, Pd/Fe3O4@C are compared in
Fig. 4. There was no hysteresis in the magnetization for the
three tested nanoparticles. It could be seen that the values
of the saturation magnetization were 35.1 emu/g for Fe3-
O4@C, 32.3 emu/g for Pd/Fe3O4@C respectively. The
decrease of the saturation magnetization suggested the
presence of some palladium particles on the surface of
magnetic supports. The catalyst exhibited similar catalytic
performance without visible reduction in the conversion for
the same reaction conditions even after running for more
than 6 cycles (Fig. 5).
Fe3O4@C nanoparticle could be observed from the picture
and the carbon thickness was about 10 nm and the diameter
of the as-synthesized spherical particles was about 160 nm.
Figure 1b, c are the TEM images of Pd/Fe3O4@C catalysts.
From the picture of Fig. 1d, it could be seen that the size
distribution histograms indicated that the most Pd particles
fall in the size range 3–8 nm and the mean particle di-
ameter was about 5.9 nm. In Fig. 1e, we can see that the
catalysts recycled six times without obvious change in
morphology and dispersity. The elemental composition of
the Pd/Fe3O4@C samples was determined by EDX analy-
sis. The result shown in Fig. 1f revealed that the as-
123

Programmed Synthesis Palladium Supported on Fe3O4@C
Table 1 Optimization of
reaction conditions
Entry
Solvent
Temperature (°C)
Time
(h)
Conversion (%)
Yield (%)a
1
Ethanol
DMF
60
60
60
60
60
30
40
50
60
80
60
60
60
8
8
8
8
8
8
8
8
8
8
4
6
8
90
96
99
71
30
79
78
92
99
98
86
92
99
82
81
92
38
28
72
71
87
92
89
44
83
92
2
3
Water
Methanol
Toluene
Water
Water
Water
Water
Water
Water
Water
Water
4
5
6
7
8
9
10
11
12
13
Reaction conditions: benzaldehyde (0.5 mmol), nitrobenzene (0.6 mmol), 20 mg Pd/Fe3O4@C catalyst,
and water (3 mL) at room temperature under a H2 atmosphere
a
Yield was determined by GC analysis and n-tridecane was used as the internal standard
3.1.1 Catalytic Performance
the final optimized reaction parameters for direct reductive
amination were benzaldehyde (0.5 mmol), nitroarene
In the first step of experiments, various reaction parameters
such as effect of solvent, reaction temperature and time
were studied and the results obtained are summarized in
Table 1. Initially, we studied the effect of the solvent on
direct reductive amination. Some polar solvents such as
ethanol (82 %), methanol (38 %), DMF (81 %) and water
(92 %) were good to yield the desired product (entries
1–4). However, a non-polar solvent, namely toluene
(28 %), was screened. Little yield of the desired product
was obtained (entry 5). It was worth mentioning that the
conversion of the desired product was considerably im-
proved by changing solvent from organic solvent to water
in the presence of Pd/Fe3O4@C as the catalyst. These re-
sults clearly showed that the efficiency of the reaction was
affected by different solvents. In order to examine the ef-
fect of temperature on the reaction outcome, reactions were
carried out at different temperatures ranging from
30–80 °C (entries 6–10). It was observed that at 30 °C the
yield of the desired product was low whereas with the
increase in temperature up to 60 °C, 92 % yield of the
desired product was obtained within 8 h. The increase of
reaction temperature up to 80 °C led to low yields (89 %)
due to the generation of by-products. Based on the reaction
yields and environmental consideration, water was proved
to be superior to the other solvent. The reaction yields were
susceptible to temperature changes (entries 6–8). Hence,
(0.6 mmol), catalyst 20 mg, water as solvent (3 mL),
temperature (60 °C), and reaction time (8 h).
Under the optimized reaction conditions, the scope of the
reaction was explored with structurally different aldehydes
and nitroarenes. As shown in Table 2, various aldehydes
were reductively aminated with nitrobenzene in water at
60 °C and under a hydrogen atmosphere. It could be seen
that there was no remarkable electron withdrawing and
steric effects for the aromatic aldehydes during this reaction,
although benzaldehydes are with o-, m-, and p-substituents
(entries 1–7). We also found that salicylaldehyde,
o-chlorobenzaldehyde and 2-chlorobenzaldehyde had low
yields (entries 9, 10, 12), those probably due to the in-
tramolecular hydrogen bond, electron withdrawing and
steric effects for the aromatic aldehydes during this reaction.
Besides, aliphatic aldehydes, regardless of whether they
were linear or R-branched, underwent the reductive amina-
tion rapidly and gave the products in excellent yields nearly
without the formation of any side products (entries 13–15).
Table 3 shows the results obtained for reactions carried
using benzaldehyde with different nitrobenzenes. Unsub-
stituted (Table 3, entry 1) as well as substituted 4-methyl,
4-chlorine, 4-bromine, 4-methoxy nitrobenzenes obtained
the desired product in high yields. It was observed that all
of these reactions have nearly no formation of side
products.
123

X. Zhou et al.
Table 2 Pd/Fe3O4@C catalyst
for the reductive amination of
different aldehydes with
nitrobenzene
Entry
1
Aldehyde
Product
Conversion(%) Yield[%]a
CHO
NH Ph
99
98
92
89
CHO
NH Ph
CH3
NH Ph
2
3
CH3
CHO
99
90
CH3
CH3
CHO
NH Ph
4
5
100
100
94
94
H3C
H3C
CHO
NH Ph
OCH3
NH Ph
OCH3
CHO
6
7
96
90
94
86
OCH3
OCH3
CHO
NH Ph
H3CO
H3CO
H3CO
H3CO
CHO
NH Ph
8
95
80
OCH3
CHO
OCH3
NH Ph
9
75
85
91
65
78
84
OH
OH
CHO
NH Ph
10
11
Cl
Cl
CHO
NH Ph
Cl
Cl
CHO
NH Ph
12
83
75
Cl
Cl
O
13
14
15
87
84
88
86
82
86
NH Ph
H
O
NH Ph
H
O
NH Ph
H
Reaction conditions: benzaldehydes (0.5 mmol), nitrobenzene (0.6 mmol), 20 mg Pd/Fe3O4@C catalyst,
and water (3 mL) at 60 °C under a H2 atmosphere
a
Yield was determined by GC analysis and n-tridecane was used as the internal standard
123

Programmed Synthesis Palladium Supported on Fe3O4@C
Compared with our previous work [24], this work have
several advantages as follows: Firstly, the methods of
synthesis Catalyst are simple and convenient, these ad-
vantages make this methodology attractive for the devel-
opment of large-scale industrial synthesis. Secondly, we
chose water as our reaction solvent, because it was much
cheaper, safer and more environmentally friendly than or-
ganic solvent. Finally, the synthetic methodology can be
extended to the fabrication of other noble metal catalysts
(such as Au- and Pt-based catalysts) with integrated and
enhanced properties for various advanced applications,
multicomponent nanosystems.
Although these catalysts give a good yields in a short
time and mild room temperature. And we chosen water as
our reaction solvent, because it was much cheaper, safer
and environmentally friendly than organic solvent. To
make the synthetic protocol more economical, recyclability
study of the catalyst was examined for direct reductive
amination of aldehydes with nitroarenes (Fig. 4).We ob-
served that the catalyst was highly active under the present
reaction conditions and could be effectively reused for six
consecutive recycles.
Fig. 5 Catalyst recyclability study
The hot-filtration have been performed. After 4 h of the
reaction, we filtered the whole reaction system. We take
part of the filtrate sent for GC–MS and elemental analy-
sis,the conversion is 56 % and the elemental analysis re-
sults indicated that there was no palladium in the filtrate. In
addition, the other part of the filtrate was continued to be
heated under the original reaction condition without cata-
lysts. After 8 h, the solution was sent for GC–MS and
elemental analysis, the conversion is 57 % and there was
no significant change compared with the former.
3.1.2 1H NMR Data of Some Selected Compounds
3.1.2.1 N-Benzylaniline (Table 2, Entry 1) 1H NMR
(CDCl3, 400 MHz) d = 7.38–7.30 (m, 5H), 7.20 (t, 2H),
6.76 (t, 1H), 6.68 (d, 2H), 4.37 (s, 2H), 4.01 (br, 1H);
Table 3 Pd/Fe3O4@C catalyst for the reductive amination of aldehydes with different nitroarenes
Entry
1
Nitroarenes
Product
Conversion(%) Yield[%]a
NH
NO2
98
91
Ph
NO2
NH
2
3
4
97
83
Ph
Cl
Cl
H
N
NO2
NO2
Ph
Ph
94
93
78
86
Br
Br
H
N
H3CO
H3CO
Reaction conditions: benzaldehyde (0.5 mmol), nitroarenes (0.6 mmol), 20 mg Pd/Fe3O4@C catalyst, and water (3 mL) at 60 °C under a H2
atmosphere
a
Yield was determined by GC analysis and n-tridecane was used as the internal standard
123

X. Zhou et al.
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In conclusion, we demonstrated a successful synthesis of
multicomponent Pd/Fe3O4@C magnetic catalysts with well-
defined core–shell nanostructures. Tiny palladium nanopar-
ticles were successfully supported on the surface of core–
shell Fe3O4@C spheres. The obtained multicomponent Pd/
Fe3O4@C magnetic catalysts showed excellent catalytic
performance for one-pot reductive amination under mild
conditions and could be recovered in a facile manner from
the reaction mixture. What’s more, our catalyst had both
convenient separability and excellent reusability. Therefore,
this functional nanostructure held great promise as a novel
Pd-based catalyst system for various catalytic reactions.
Additionally, the design concept for the multifunctional
nanomaterials can be extended to the fabrication of other
multicomponent nanosystems with integrated and enhanced
properties for various advanced applications.
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