Palladium supported on hollow magnetic mesoporous spheres as recoverable catalyst for one-pot reductive amination of aldehydes with nitroarenes under mild conditions

Article abstract of DOI:10.1016/j.molcata.2013.10.018

We described here a method to stabilize Pd(0) on the surface of hollow magnetic mesoporous spheres (HMMS), with Fe₃O₄ nanoparticles embedded in the mesoporous shell. The catalyst was characterized by TEM, XRD 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 in ethanol, even at room temperature. Besides, the catalyst could be recovered in a facile manner from the reaction mixture and recycled six times without loss in activity.

Full text of DOI:10.1016/j.molcata.2013.10.018

Accepted Manuscript
Title: Palladium supported on hollow magnetic mesoporous
spheres as recoverable catalyst for one-pot reductive
amination of aldehydes with nitroarenes under mild conditions
Author: Jinghui Zhou Zhengping Dong Peng Wang Zhiqiang
Shi Xingchun Zhou Rong Li
PII:
S1381-1169(13)00391-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.10.018
MOLCAA 8911
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
2-8-2013
10-10-2013
20-10-2013
Please cite this article as: J. Zhou, Z. Dong, P. Wang, Z. Shi, X. Zhou,
R. Li, Palladium supported on hollow magnetic mesoporous spheres as
recoverable catalyst for one-pot reductive amination of aldehydes with nitroarenes
under mild conditions, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.10.018
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1Palladium supported on hollow magnetic mesoporous spheres as
recoverable catalyst for one-pot reductive amination of aldehydes
with nitroarenes under mild conditions
Jinghui Zhou, Zhengping Dong, Peng Wang,Zhiqiang Shi,
Xingchun Zhou , Rong Li *
College of Chemistry and Chemical Engineering Lanzhou University, The Key
Laboratory of Catalytic engineering of Gansu Province,
Lanzhou 730000, China
Abstract
We described here a method to stabilize Pd (0) on the surface of hollow magnetic
mesoporous spheres (HMMS), with Fe3O4 nanoparticles embedded in the mesoporous
shell. The catalyst was characterized by TEM, XRD 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 in ethanol, even at room
temperature. Besides, the catalyst could be recovered in a facile manner from the
reaction mixture and recycled six times without loss in activity.
Keywords: hollow magnetic mesoporous spheres, Pd (0), one-pot direct reductive
amination, aldehydes, nitroarenes
1
∗ Corresponding author. Tel.: +86 0931 891 3597; fax: +86 0931 891 2582.
E-mail address: liyirong@lzu.edu.cn (Rong Li ).
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1. Introduction
The amide is one class of most common intermediates in synthetic organic chemistry
and biological chemistry. Amidation reaction is one of the most fundamental synthetic
tools in modern organic synthesis. [1] However, the application scope of this method
is still limited by some flaws, such as the demand on activated carboxylic acids, harsh
conditions, and tedious workup, etc. [2] The direct reductive amination of carbonyl
compounds is still the most practical way to get target amines. [3-4] Numerous
methods have been developed to achieve this transformation including biocatalytic [5],
metal or metal oxide [6-8], dirhodium tetracarboxylate-catalyzed amidation reaction
[9], catalytic hydrogenation [10], or use a reducing agent [11-12], PEG/oxidant
system [13], etc. Unfortunately, there have been one or more drawbacks in these
methods, such as poor stability, toxic byproducts and in acidic conditions. Therefore,
development of facile, effective and more economical methods for the synthesis of
amide has been attracting a great deal of 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 [14-17]. In recent years,
many attempts have been made to immobilize palladium nanoparticles on different
solid supports for reductive amination of carbonyl compounds, such as polymers
[18-20], metal oxide [21-22], and inorganic materials [23]. Hollow mesoporous silica
spheres (HMS) with well-defined structures have received great interest, owing to
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their unique properties such as low toxicity, good compatibilities, low density, and
very high specific surface area with abundant Si–OH bonds on the pore surface, which
can provide high catalyst loading and homogeneous spherical morphologies [24-27].
Although these supported catalysts possess extremely high catalytic activity, the
separation step becomes a more troublesome issue. To overcome the problem,
magnetic nanoparticles (Fe3O4 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 due to the paramagnetic character of the
support [28-34]. In addition, in the point of environmental and economic, 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.
Based on above considerations, in this paper, we report our results about Pd immobile
on mesoporous Fe3O4 (Pd/Fe3O4) catalyzed one-pot reductive amination reaction
under a hydrogen atmosphere, even at room temperature (Scheme 1).
2. Experimental
2.1 Synthesis of the hydrophobic magnetite nanoparticles (Fe3O4 NPs)
The Fe3O4 NPs were prepared using a published method with a slight modification.
[35] First, 4.8 g of FeCl3•6H2O, 2.0 g FeCl2•4H2O and 1.0 mL oleic acid were added
to 40 mL of deionized water under vigorous stirring. Second, the mixture solution was
purged with nitrogen gas for 30 min under nitrogen atmosphere. Third, the mixture
solution was heated to 90 ºC. Finally, 10 mL of ammonium hydroxide (28 wt%) was
added rapidly to the solution, and it immediately turned black. The reaction was kept
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at 90 ºC for 2.5 h. The black precipitate was obtained with the help of a magnet and
dried at 323 K overnight.
2.2 Preparation of polystyrene latex with attached Fe3O4 nanoparticles
Briefly, carboxylic polystyrene (PS) latex was prepared by soap-free emulsion
polymerization of St with AA. [36] 1.5 g Fe3O4 nanoparticles and 2.5 g negatively
charged PS latex were dispersed in 84 mL and 60 mL hydrochloric acid solution
(pH=2.3), respectively. Then the latter suspension was added dropwise into the former
under vigorous stirring at room temperature. After 6 h, the heteroaggregates, i.e.
Fe3O4 nanoparticles attached on the surface of PS latex particles, were separated from
the solution by an external magnet and washed several times with water until the pH
value of the solution became close to 7.
2.3 Preparation of hollow magnetic mesoporous silica spheres (HMMS)
First, 2.0 g heteroaggregate of the PS latex and Fe3O4 nanoparticles were dispersed in
a solution composed of 75 mL of ethanol, 5 mL of H2O, and 3.8 mL of NH3.H2O (28
wt %). After stirring for 10 min, 0.4 mL of TEOS and C16TMS mixture with a molar
ratio of 4.7:1 were added dropwise under vigorous stirring. Then, the reaction
proceeded for 8 h at room temperature under strong stirring. The resultant particles
were separated by centrifugation, purified by three cycles of magnetic
separation/washing in ethanol, and dried at room temperature for 12 h. Finally, the
as-prepared products were dried at 313 K overnight and calcined in air at 823 K for 7
h.
Hollow magnetic mesoporous silica spherical magnetite nanoparticleswere
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synthesized via the versatile solvothermal reaction reported by Liao. [37]
2.4 –NH2 modify the surface of hollow magnetic mesoporous silica spheres
(HMMS-NH2)
Briefly, 1.0 g HMMS samples were dispersed in 80 mL ethanol solution, then, 2.5 g
aminopropyltriethoxysilane (APTES) was droped into the system. The mixture was
refluxed for 24 h. After filtration, washing and drying, the functionalized HMMS was
obtained and denoted as HMMS-NH2.
2.5 Loading of Pd on aminopropyl-modified silica coated HMMS
(HMMS-NH2-Pd)
500 mg of HMMS-NH2 samples were first dispersed in a 50 mL ethanol solution
under ultrasonication for 0.5 h. The formed black suspension was ultrasonically
mixed with 3.0 mmol of a PdCl2 solution for 1 h, then an excess 0.01 M NaBH4
solution was slowly dropped into the above mixture with vigorous stirring. After 2 h
of reduction, the products were obtained with the help of a magnet, washed
thoroughly with deionized water and then dried in a vacuum at room temperature
overnight.
2.6 Typical procedure for Catalytic reductive amination reactions
2.6.1 The catalyst for the reductive amination of different aldehydes with
nitroarenes
1.0 mmol of the different aldehydes, 1.2 mmol of nitrobenzene and 20 mg of catalyst
were added in 5 mL of ethanol at room temperature by ball milling under a hydrogen
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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 (P.E. AutoSystem XL) or
GC–MS (Agilent 6,890 N/5,973 N).
2.6.2 The catalyst for the reductive amination of aldehydes with different
nitroarenes
20 mg of catalyst and 1.2 mmol nitroarene were added in 5 mL of 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
chromatography (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.
Characterization
XRD measurement was performed on a Rigaku D/max-2400 diffractometer using
Cu-Kα radiation as the X-ray source in the 2θ range of 10–70°. 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.
Magnetic measurement of HMMS, HMMS-NH2 and HMMS-NH2-Pd was
investigated with a Quantum Design vibrating sample magnetometer (VSM) at room
temperature in an applied magnetic field sweeping from -15 to 15 kOe.
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Results and discussions
Catalyst preparation and characterization
Calcined at 550℃
C16TMS
PH=2-3
TEOS
PS sphere
Sphere absorbs Fe3O4 particles
NH2
H2N
NH2
PdCl2
APTES
H2N
H2N
NH2
NaBH4
NH2
NH2
Scheme 1 Preparation of the catalysts
The preparation of the HMMS-NH2-Pd catalyst follows the steps described in Scheme
1. Firstly, HMMS were prepared. Secondly, the functionalization of the silica coated
HMMS with 3-(2-aminoethylamino) propyltrimethoxysilane. Thirdly, Palladium
chloride was supported on the surface of HMMS, and the palladium chloride reduced
to palladium particles with sodium borohydride. The amount of Pd in the obtained
catalyst was found to be 4.04 wt % based on ICP analysis.
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Fig. 1 TEM images of (a, b) HMMS, (c, d) HMMS-NH2-Pd
Fig. 1a shows the typical TEM image of HMMS nanoparticles. As can be seen from
the image, the average diameter of the as-synthesized spherical particles was about
350 nm and they were nearly monodisperse. Fig. 1b is HRTEM image of a single
HMMS. HMMS can be observed from the picture and the wall thickness is about 30
nm. The TEM image in Fig. 1c shows that the HMMS-NH2-Pd catalyst didn’t
change considerably after attachment of the palladium onto the surface of the hollow
magnetic silica sphere. It can also be concluded that the palladium particle size
distribution was centered at 7-8 nm in Fig. 1d.
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Fig. 2 XRD patterns of (a) HMMS, (b) HMMS-NH2 and (c) HMMS-NH2-Pd
Unambiguous evidence of palladium particles on HMMS-NH2 is provided via X-ray
powder diffraction analysis (XRD). The XRD pattern of the HMMS shows
characteristic peaks of magnetite nanoparticles and the sharp, strong peaks confirm
the products are well crystallized. The XRD results revealed their high crystallinity,
which is coincident with the results reported by Frank Caruso [39]. Fig. 2c shows that
apart from the original peaks, the appearance of the new peaks at 2θ=40.1 and 46.5 is
attributed to the Pd species. The results from XRD imply that the Pd nanoparticles
have been successfully immobilized on the surface of magnetic nanoparticles. The
average crysallite size, calculated using Scherrer’s equation, is about 9 nm and the
crystallite size nearly matches with the average particle size seen in the TEM analysis.
The elemental composition of the HMM-NH2-Pd samples was determined by EDX
analysis. The result shown in Fig. 3 reveals that the as-prepared products contain Fe,
Si, Pd, Cu, C and O. Among these elements, Cu C and O are generally influenced by
the copper network support films and their degree of oxidation, Si, O, Fe and Pd
signals result from the HMMS and Pd particles which form the products.
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Fig. 3 EDX spectrum of HMMS-NH2-Pd
Fig. 4 shows the XPS spectrum of the synthesized HMMS-NH2-Pd catalyst. Peaks
corresponding to oxygen, carbon, nitrogen, palladium, silicon, and iron are observed.
To ascertain the oxidation state of the Pd, X-ray photoelectron spectroscopy (XPS)
studies were carried out.
Fig. 4 XPS spectrum of the elemental survey scan of HMMS-NH2-Pd
In Fig. 5 the Pd binding energy of HMMS-NH2-Pd exhibits two strong peaks centered
at 339.7 and 335.5 eV, which are assigned to Pd 3d3/2 and Pd 3d5/2, respectively.
Interestingly, there is no the oxidation state of Pd species in the figure. In the
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experiment we used the amount of reducing agent is ten times for palladium. Pd(Ⅱ)
would be reduced into Pd(0) completely, which is in favourable to amination reaction.
So the oxidation state of Pd species is hard to find in the figure. On the basis of above
analysis, it can be concluded that the XPS data further identified the hollow
mesoporous structure of the synthesized HMMS-NH2-Pd.
Fig. 5 XPS spectrum of the HMMS-NH2-Pd showing Pd 3d5/2 and Pd 3d3/2 binding
energies.
Magnetic measurements were carried out by employing vibrating sample
magnetometer (VSM) at room temperature. The magnetization curves measured for
HMMS, HMMS-NH2 and HMMS-NH2-Pd are compared in Fig. 6. There was no
hysteresis in the magnetization for the three tested nanoparticles. It can be seen that
the values of the saturation magnetization were 63.64 emu/g for HMMS, 50.84 emu/g
for HMMS-NH2 and 37.65 emu/g for HMMS-NH2-Pd respectively. The decrease of
the saturation magnetization suggests the presence of some palladium particles on the
surface of the surface of the magnetic supports. Even with this reduction in the
saturation magnetization, the catalyst still can be efficiently separated easily from the
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solution with the help of an external magnetic force. Meanwhile, the figure also shows
the photograph of the catalyst being pulled magnetically.
Fig. 6 Room temperature magnetization curves of (a) HMMS, (b) HMMS-NH2 and (c)
HMMS-NH2-Pd
Catalytic activity for one-pot reductive amination of aldehydes with nitroarenes
In the first step of experiments, the reductive amination of benzaldehyde with
nitrobenzenewas compared with various reaction conditions. Based on the reaction
yields and environmental consideration, ethanol was proved to be superior to the other
alcohols [38]. As can be seen in Table 1, increasing the reaction time from 6 to 10 h
resulted in a little decrease of reaction yield (entries 3–5). The main by-products are
Benzenamine,
N-(phenylmethylene)-
(C13H11N),
Benzenemethanamine,
N-ethyl-N-phenyl- (C15H17N), and Methanone, diphenyl-, oxime (C13H11NO).
Benzenamine, N-(phenylmethylene)- is more than others, which is about 2%.
Therefore, all of the following experiments were performed using the optimized
reaction conditions.
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Table 1 Optimization of reaction conditions. a
NO2
CHO
N
H
catalyst
+
Entry Aldehyde
Product
Time/ Conversion/
Selectivity/%
b
h
%
CHO
NH Ph
NH Ph
NH Ph
NH Ph
NH Ph
2
4
30
70
100
98
1
2
3
4
5
CHO
CHO
CHO
CHO
6
100
100
100
97
8
94.2
90
10
a Reaction conditions: benzaldehyde (1.2 mmol), nitrobenzene (1 mmol), catalyst
(0.76 mol%), and solvent (5 mL) at room temperature under a H2 atmosphere.
b Yield was determined by GC analysis.
Under the optimized reaction conditions, the scope of the reaction was explored with
structurally different aldehydes and nitroarenes. As shown in Table 2, various
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aldehydes were reductively aminated with nitrobenzene in ethanol at room
temperature 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 1–8).
We also found that 2-methoxybenzaldehyde and mutil-substituent benzaldehyde had
low yields (entries 9, 12, 13), this was probably due to the relatively large steric
hindrance. Besides, 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 14–16).
On the contrary, commercial 10% Pd/C was used to this reaction, the reaction result
showed that the yield was only 17% (Entry 1), at the same time, the catalyst can be
reused 6 times without loss in activity. As we all know, the HMMS-NH2-Pd catalyst is
hollow mesoporous spheres, which has a higher specific surface area than Pd/C
catalyst. The Pd NPs may disperse into the catalyst. Meanwhile the Pd/C catalyst in
the reaction easily form hydroxylamine compound, then the content main product
would decreased.
Table 2 HMMS-NH2-Pd catalyst for the reductive amination of different
aldehydes with nitrobenzene.a
NO2
+
CHO
N
catalyst
H
R
R
Entry
Aldehyde
Product
Time/h
Yield/%b
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CHO
CHO
NH Ph
6
6
6
6
97
17c
95d
90
1
NH Ph
NH Ph
2
3
Cl
Cl
CHO
6
97
Cl
Cl
CHO
NH Ph
6
6
6
99
92.3
92
4
5
6
Cl
F
Cl
F
CHO
NH Ph
NH Ph
CH3
CHO
CH3
NH Ph
CHO
6
94
7
CH3
CH3
NH Ph
CHO
6
6
6
99
80
95
8
9
H3C
H3C
NH Ph
OCH3
CHO
OCH3
CHO
NH Ph
10
OCH3
OCH3
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NH Ph
NH Ph
CHO
6
6
98
60
11
12
H3CO
H3CO
H3CO
CHO H3CO
OCH3
CHO
OCH3
NH Ph
OH
NH Ph
6
6
6
86
96
93
13
14
15
OH
O
H
O
NH Ph
H
H
O
6
95
16
NH Ph
a Reaction conditions: aldehyde (1 mmol), nitrobenzene (1.2 mmol), Pd/Fe3O4
catalyst (0.76 mol%), and ethanol (5 mL) at room temperature under a H2
atmosphere.
b Yield was determined by GC analysis.
c catalyst with of 10% Pd/C.
d Yield after 6 runs.
Table 3 shows the results obtained for reactions carried using benzaldehyde with
different nitrobenzenes. Unsubstituted (Table 2, entry 1) as well as substituted
4-methyl, 2-methyl, 4-chlorine, 4-Bromine, 4-methoxy nitrobenzenes obtained the
desired product in high yields, except for nitromethane (Table 3 Entry 6), which only
achieved 45%. This is maybe due to the –CH3 does not have π-πconjugated. It was
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observed that all of these reactions have nearly no formation of side products.
Table 3 HMMS-NH2-Pd catalyst for the reductive amination of aldehydes with
different nitroarenes.a
NO2
CHO
N
catalyst
+
H
R
R
Entry
1
nitroarenes
Product
Time/h
Yield/%b
NH
NO2
6
95
Ph
NH
NO2
6
6
6
90
85
80
2
3
4
Ph
NH
NO2
Ph
Cl
Br
Cl
Br
NO2
H
N
Ph
H
N
NO2
6
6
92
45
5
6
Ph
H3CO
H3CO
Ph
CH3NO2
CH3NH
a Reaction conditions: benzaldehyde (1 mmol), nitroarenes (1.2 mmol), Pd/Fe3O4
catalyst (0.76 mol%), and ethanol (5 mL) at room temperature under a H2
atmosphere.
b Yield was determined by GC analysis.
Conclusions
In conclusion, we describe here a method to stabilized Pd (0) on the surface of hollow
magnetic mesoporous spheres (HMMS), with Fe3O4 nanoparticles embedded in the
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mesoporous shell. The simple catalyst exhibits high activity for one-pot reductive
amination undermild conditions. The simple catalyst exhibits high activity even after
six cycles. These advantages make this methodology attractive for the development of
large-scale industrial synthesis.
Acknowledge
The authors are grateful to Projects in Gansu Province Science and Technology Pillar
Program (1204GKCA047).
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[39] F. Caruso, Adv. Mater., 13 (2001) 11-22.
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Calcined at 550℃
C16TMS
TEOS
PH=2-3
PS sphere
Sphere absorbs Fe3O4 particles
NH2
H2N
NH2
NH2
PdCl2
APTES
H2N
H2N
NaBH4
NH2
NH2
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Highlights
The synthesized HMMS‐NH2‐Pd shows excellent catalytic activity in amination reaction with over
97% yield in 6h.
The catalyst can be recycled 6 times without loss activity.
The catalysts can be easily recovered by using an external magnet.
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