Pd immobilized on amine-functionalized magnetite nanoparticles: A novel and highly active catalyst for hydrogenation and Heck reactions

Article abstract of DOI:10.1039/c0gc00854k

A palladium-based catalyst supported on amine-functionalized magnetite nanoparticles was successfully prepared by a facile one-pot template-free method combined with a metal adsorption-reduction procedure. The catalyst was characterized by TEM, XRD, XPS, FT-IR and VSM. The catalyst afforded fast conversions for various aromatic nitro and unsaturated compounds, and with a turn-over frequency (TOF) of 83.33 h^-1 under a H₂ atmosphere in ethanol, even at room temperature. Furthermore, it was found that the catalyst showed a high activity for the Heck reaction, affording over a 93% yield in all the cases investigated. Interestingly, the novel catalyst could be recovered in a facile manner from the reaction mixture and recycled eight times without any significant loss in activity.

Full text of DOI:10.1039/c0gc00854k

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PAPER
Pd immobilized on amine-functionalized magnetite nanoparticles: a novel
and highly active catalyst for hydrogenation and Heck reactions
Fengwei Zhang, Jun Jin, Xing Zhong, Shuwen Li, Jianrui Niu, Rong Li* and Jiantai Ma*
Received 26th November 2010, Accepted 1st March 2011
DOI: 10.1039/c0gc00854k
A palladium-based catalyst supported on amine-functionalized magnetite nanoparticles was
successfully prepared by a facile one-pot template-free method combined with a metal
adsorption–reduction procedure. The catalyst was characterized by TEM, XRD, XPS, FT-IR and
VSM. The catalyst afforded fast conversions for various aromatic nitro and unsaturated
compounds, and with a turn-over frequency (TOF) of 83.33 h-1 under a H2 atmosphere in ethanol,
even at room temperature. Furthermore, it was found that the catalyst showed a high activity for
the Heck reaction, affording over a 93% yield in all the cases investigated. Interestingly, the novel
catalyst could be recovered in a facile manner from the reaction mixture and recycled eight times
without any significant loss in activity.
enhancement of catalytic activity was found and attributed
Introduction
to the mass transfer restriction. To improve the rate of the
It is well known that Pd-catalyzed hydrogenation and Heck
reaction, much attention has been paid to non-porous and small
reactions (the Pd with nanometre-scale dimensions with a high
supports because they can effectively decrease the pore diffusion
surface area per unit volume) are of significant importance in
resistance. Very recently, magnetic nanoparticles have emerged
modern chemical transformations.1 Recently, the hydrogenation
as viable alternatives to conventional materials as robust, readily
of nitro compounds to amines has become one of the most im-
available, high surface area heterogeneous catalyst supports.12 In
portant chemical reactions because organic amines are essential
particular, much attention has been focused on amine function-
materials for the production of agrochemicals, dyes, pharmaceu-
alization, since amines are well known to stabilise nanoparticles
ticals, polymers and rubbers.2 Additionally, much recent work
against aggregation without disturbing their desirable properties
has been directed toward the Heck reaction because it is the most
and are also recognised to increase their catalytic activity.13
powerful and widely used method to couple alkenes with organic
For example, amine-functionalized magnetic nanoparticles have
moieties bearing suitable leaving groups, such as halide, triflate
been employed in a range of organic transformations, showing
or diazonium.3 The resulting products have found extensive
excellent catalytic activities in oxidation,14 hydrogenation 15 and
applications as intermediates in the preparation of materials,4
C–C coupling reactions.16 However, a drawback is that the
natural products5 and bioactive compounds.6 Unfortunately, the
amine-functionalized magnetic catalyst needs further chemical
efficient separation and subsequent recycling of homogeneous
functionalization, and the process is relatively complicated.
Pd catalysts remains a scientific challenge. Therefore, developing
Therefore, developing a simple and facile approach to obtain
a facile and expedient approach to separate and recycle homo-
amine-functionalized magnetic nanoparticles with a good dis-
geneous catalysts is highly desirable. One approach to such an
persibility is highly desirable.
environmentally benign process is based on the development
Herein, we report a novel and one-pot template-free synthesis
of heterogeneous catalysts because these heterogeneous systems
are easy to handle, recover and are “green” processes.7
In the last few years, numerous methods have been developed
of a nano-magnetite-supported, magnetically recyclable and
highly active Pd catalyst, and its application in hydrogena-
tion and Heck reactions. To the best of our knowledge, no
to immobilize palladium on a large variety of solid supports,
examples of the one-pot template-free synthesis of an amine-
such as microporous polymers,8 mesoporous silica,9 amorphous
functionalized magnetite nanoparticle-supported Pd catalyst
silica10 and carbon nanofibers,11 making it really easy and simple
to separate the catalyst from the mixture. Nevertheless, a limited
have been reported.
Experimental
Materials
College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou, 730000, China.
Ferric chloride hexahydrate (FeCl3·6H2O, >99%) as a single
E-mail: liyirong@lzu.edu.cn;majiantai@lzu.edu.cn; Fax: +86 931
8912311; Tel: +86 931 8912311
iron source, anhydrous sodium acetate, ethylene glycol, 1,
1238 | Green Chem., 2011, 13, 1238–1243
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6-hexanediamine and ethanol. All chemicals were of analytical
grade and used as received without further purification. De-
ionized water was used throughout the experiments.
twice (2 ¥ 15 mL) with ethyl acetate. The organic fraction was
dried over MgSO4, the solvents evaporated under vacuum and
the residue redissolved in 5 mL of dichloromethane. An aliquot
was taken with a syringe and subjected to GC or GC-MS
analysis. Yields were calculated against the consumption of the
aryl halides. After the first cycle of the reaction, the catalyst was
recovered with the help of a magnet, successively rinsed with
NMP, distilled water (to remove excess of base) and ethanol,
and dried at room temperature ready for the next cycle.
Synthesis of amine-functionalized magnetite nanoparticles
(Fe3O4-NH2)
Amine-functionalized magnetite nanoparticles were synthesized
via the versatile solvothermal reaction reported by Li.17 Typi-
cally, a solution of 6.5 g 1,6-hexanediamine, 2.0 g anhydrous
sodium acetate and 1.0 g FeCl3·6H2O as a ferric source in
30 mL ethylene glycol was stirred vigorously at 50 ◦C to give
a transparent solution. The solution was then transferred into
a Teflon-lined autoclave and maintained at 200 ◦C for 6 h. The
magnetite nanoparticles were then thoroughly rinsed with de-
ionized water and ethanol to effectively remove the solvent a◦nd
unbound 1,6-hexanediamine, and then vacuum dried at 50 C
to obtain a black powder for further use. During each rinsing
step, the nanoparticles were separated from the supernatant by
using a magnetic force.
Characterization
FT-IR spectra were recorded on a Nicolet NEXUS 670 FTIR
spectrometer with a DTGS detector, and samples were measured
with KBr pellets. XRD measurements were performed on a
Rigaku D/max-2400 diffractometer using Cu-Ka radiation as
the X-ray source in the 2q range of 10–80◦. 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 evaporating the solvent in air at room temper-
ature. X-Ray photoelectron spectroscopy (XPS) was recorded
on a PHI-5702 instrument and the C1S line at 292.1 eV was
used as the binding energy reference. Magnetic measurements
of Fe3O4-NH2 and Fe3O4-NH2–Pd were investigated with a
Quantum Design vibrating sample magnetometer (VSM) at
room temperature in an applied magnetic field sweeping from
-15 to 15 kOe.
Loading of Pd on amine-functionalized magnetic nanoparticles
(Fe3O4-NH2–Pd)
500 mg of as-synthesised Fe3O4 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 mM 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 de-
ionized water and then dried in a vacuum at room temperature
overnight. The weight percentage of Pd in the Fe3O4-NH2–Pd, as
determined by atomic absorption spectroscopic (AAS) analysis,
was 8.43 wt%.
Results and discussion
The process for the preparation of the catalyst Fe3O4-NH2–
Pd is schematically described in Scheme 1. First, amine-
functionalized magnetite nanoparticles were synthesized by a
facile one-pot solvothermal synthetic strategy. Second, Pd(0)
nanoparticles were immobilized on amine-functionalized mag-
netic nanoparticles through the reduction of PdCl2 by NaBH4
with the assistance of ultrasound radiation. Briefly, magnetite
particles were first prepared by a one-pot method and then
immobilized Pd(0) on the surface of the magnetite nanoparticles
(amine groups on the surface of nanoparticles act as a robust
anchor and avoid Pd leaching).
Fe3O4-NH2–Pd catalyst for hydrogenation reaction
In a typical experiment, 1 mmol of the reagent was dissolved in
5 mL ethanol with 20 mg of catalyst under a H2 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 reaction vessel, and the
conversion was estimated by GC (P.E. AutoSystem XL) or
GC-MS (Agilent 6,890N/5,973N). Thereafter, the catalyst was
washed three times with ethanol and dried at room temperature
for the next cycle. The catalytic reactions were repeated, and
even after 5 cycles, there was no deterioration in the catalytic
efficiency.
Scheme 1 Preparation of palladium nanoparticles supported on amine-
functionalized magnetite nanoparticles.
Fe3O4-NH2–Pd catalyst for the Heck reaction
For Heck coupling reactions, 0.5 mmol of the aryl halides,
0.6 mmol of styrene or n-butyl acrylate, and 0.6 mmol of K2CO3
were taken into 5 mL of N-methyl-2-pyrrolidone (NMP). The
amount of catalyst used in each reaction was 32 mg, and the
Fig. 1a shows the typical TEM image of Fe3O4-NH2 nanopar-
ticles prepared by the versatile solvothermal reaction. As can be
seen from the image, the average diameter of the as-synthesized
spherical particles was about 35 nm and nearly monodisperse.
The TEM image in Fig. 1b shows that the Fe3O4-NH2–Pd
catalyst didn’t change considerably after attachment of the
palladium onto the surface of the magnetic nanoparticles. From
Fig. 1b, it can also be concluded that the palladium particle size
distribution was centered at 8.9 nm. Meanwhile, the figure also
◦
reaction mixture was refluxed at 130 C. The reaction process
was monitored by thin layer chromatography (TLC). After
completion of the reaction, the mixture was cooled to room
temperature, separated by magnetic decantation, the resultant
residual mixture diluted with 20 mL H2O, followed by extraction
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Fig. 1 TEM images of (a) Fe3O4-NH2 and (b) Fe3O4-NH2–Pd (the
upper right image in Fig. 1b is the HRTEM image).
shows the photograph of the catalyst being pulled magnetically
(shown in the inset of Fig. 1b).
Unambiguous evidence of palladium particles on Fe3O4-
NH2 is provided via X-ray powder diffraction analysis (XRD).
The XRD pattern of Fe3O4-NH2 shows characteristic peaks of
magnetite nanoparticles, and the sharp and strong peaks confirm
the products are well crystallized. The XRD results reveal their
high crystallinity, which is coincident with the results reported
by Li.17 Fig. 2b shows that apart from the original peaks, the
appearance of the new peaks at 2q = 40.1, 46.5 and 68.0 are
attributed to the Pd species. The results from XRD imply that
the Pd nanoparticles have been successfully immobilized onto
the surface of the magnetic nanoparticles. The average crystallite
size, calculated using Scherrer’s equation, is about 12.8 nm and
the crystallite size nearly matches with the average particle size
seen in the HRTEM analysis.
Fig. 3 XPS spectrum of the elemental survey scan of Fe3O4-NH2–Pd.
Fig. 4 XPS spectrum of the Fe3O4-NH2–Pd showing Pd 3d5/2 and Pd
3d3/2 binding energies.
binding energy of 335.3 eV18 and the second corresponding to
smaller Pd particles with a binding energy of 336.8 eV.19
We also measured the Fourier transform infrared (FTIR)
spectra of Fe3O4-NH2 and Fe3O4-NH2–Pd. Fig. 5a shows the
IR spectrum of Fe3O4-NH2, indicating that plenty of 1,6-
hexanediamine molecules are immobilized on the surface of the
nanoparticles.20 The peaks at 1043, 1618, 3420 and around 2845–
2929 cm-1 correspond to C–N stretching, N–H deformation and
C–H stretching models of the alkyl chain, respectively.21 The
major peaks for the palladium-based catalyst in Fig. 5b can
be assigned as follows: 3420 cm-1 (N–H stretching vibration),
1557 cm-1 (N–H deformation vibration) and 1402 cm-1 (C–
H symmetric blending vibration). The IR spectrum of the
palladium-based catalyst demonstrates that almost no change
occurs after immobilization of palladium on the magnetite
nanoparticle surface.
Fig. 2 XRD patterns of (a) Fe3O4-NH2 and (b) Fe3O4-NH2–Pd.
Fig. 3 presents XPS elemental survey scans of the surface
of the Fe3O4-NH2–Pd catalyst. Peaks corresponding to oxygen,
carbon, nitrogen, palladium and iron are clearly observed. To
ascertain the oxidation state of the Pd, X-ray photoelectron
spectroscopy (XPS) studies were carried out. In Fig. 4, the
Pd binding energy of Fe3O4-NH2–Pd exhibits two strong peaks
centered at 340.6 and 335.3 eV, which are assigned to Pd 3d3/2 and
Pd 3d5/2, respectively. However, only after close inspection can
one find another weak peak, appearing at 336.8 eV (Pd 3d5/2).
The results from XPS reveal that the Pd(0) exists as two types of
species, the first being larger Pd nanoparticles >10 nm having a
Magnetization curves revealed the superparamagnetic behav-
ior of the magnetic nanoparticles. The hysteresis loops of Fe3O4-
NH2 and Fe3O4-NH2–Pd are shown in Fig. 6. It can be seen that
the magnetic saturation values of these are 71.0 and 60.7 emu
g-1, respectively. The decrease of the saturation magnetization
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after the catalyst had been reused ten times. The recyclability of
the catalyst can be attributed to the efficient stabilisation of the
active Pd species by the amine groups on the magnetite surface.
In contrast, commercial 10% Pd/C was used to catalytically
hydrogenate nitrobenzene; it demanded at least 2 h to complete
the reaction, whereas the Fe3O4-NH2–Pd completed the reaction
in no more than 1 h with a relatively low Pd loading under similar
experimental conditions.
After completion of the reaction, the catalyst was recovered in
a facile manner from the reaction mixture by using a permanent
magnet. The magnetic property of the nanoparticles facilitated
the separation of the catalyst while ensuring redispersion of the
particles in the solvent for further use. Moreover, the Pd content
(8.29 wt%) of the recovered Fe3O4-NH2–Pd catalyst remained
just about the same as the fresh catalyst, indicating that Pd
leaching was negligible.
Fig. 5 FT-IR spectra of (a) Fe3O4-NH2 and (b) Fe3O4-NH2–Pd.
Catalyst testing for the Heck reaction
Having demonstrated that the Fe3O4-NH2–Pd catalyst was
highly effective for the hydrogenation reaction, its activity was
also investigated for the Heck reaction. Table 2 displays the
results of the Heck reaction of aryl halides with styrene or
n-butyl acrylate. A range of activities were observed from 93
to >99% yields, depending on the nature of the aryl bromo
and iodo derivatives. For both styrene and n-butyl acrylate,
shorter reaction times were observed for 4-bromonitrobenzene
and 4-bromoacetophenone (Table 2, entries 3, 4, 8 and 9),
however, longer reaction times were observed for bromobenzene
(Table 2, entries 1 and 6). When using iodobenzene and
styrene as substrates, the reaction afforded almost a 100% yield
(Table 2, entry 5). The results show that electron-withdrawing
substituents enhance the coupling product formation, while
electron-donating groups have a negative influence on the
reaction process.
The recyclability of Fe3O4-NH2–Pd was further investigated
because the recyclability of the heterogeneous catalyst is one
of the most important issues for practical applications. We
therefore turned attention to the reusability of our Pd catalyst.
As shown in Table 3, the catalyst was recycled in the Heck
coupling of 4-bromonitrobenzene with styrene. It is worth
noting that the catalyst gave a complete conversion of 4-
bromonitrobenzene, an over 95% yield was achieved within
10 h under each cycle and without a significant loss of activity.
Furthermore, the catalyst could be easily separated magnetically
from the reaction mixture, washed three times with ethanol and
finally dried for the next run. The results further confirmed the
high recyclability of Fe3O4-NH2–Pd.
Fig. 6 Room temperature magnetization curves of (a) Fe3O4-NH2 and
(b) Fe3O4-NH2–Pd.
suggests the presence of some palladium particles on the surface
of the magnetic supports. Even with this reduction in the
saturation magnetization, the catalyst can still be efficiently
separated easily from the solution with the help of an external
magnetic force.
Catalyst testing for hydrogenation reactions
Initially, the catalytic activity of Fe3O4-NH2–Pd was tested for
the hydrogenation of a variety of aromatic nitro and unsaturated
compounds to their corresponding products. The reactions were
carried out in ethanol at room temperature and under a 1 atm
pressure of H2. Detailed observations of all the reactions are
given in Table 1. We observed that the catalyst was very active
for the hydrogenation reaction under such mild conditions, since
the amine groups are known to enhance the catalytic activity of
Pd. As shown in Table 1, the hydrogenation reaction could be
completed within 45 min, except for 1-nitronaphthalene and
trans-stilbene, which required for 75 min. This is probably due
to the relatively large steric hindrance. Furthermore, the Fe3O4-
NH2–Pd catalyst, with a Pd content of 1.6 mol%, afforded a fast
conversion of the aromatic nitro and unsaturated compounds to
the corresponding products, and the turn-over frequency (TOF)
for the hydrogenation reaction was up to 83.33 h-1 under mild
conditions. A satisfactory yield (>99%) was still obtained even
Conclusions
In the present study, a nanosized Fe3O4-NH2–Pd catalyst with a
high magnetic responsivity and excellent dispersibility was pre-
pared through a facile one-pot template-free strategy combined
with a metal adsorption–reduction procedure. In this system,
the novel magnetite nanomaterials play two important roles:
one is in increasing the stability of the nanoparticles, because of
their surface amino groups, and the other in immobilizing Pd
by means of coordination. On the basis of the above-mentioned
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Table 1 Hydrogenation of various substrates using Fe3O4-NH2–Pd as the catalysta
Entry
1
Substrate
Product
t/min
Yield (%)
45
45
120
120
45
>99
>99b
>99c
97d
2
3
4
5
6
7
>99
45
45
45
45
45
92
96
>99
98
92
8
9
45
75
99
98
10
75
97
a Reaction conditions: 20 mg catalyst; 1 atm H2; 1 mmol substrate; 5 mL solvent. b Yield after 5 runs. c Hydrogenation with 20 mg of 10% Pd/C.
d Hydrogenation with 1.6 mol% Pd/C.
Table 2 Heck reactions in the presence of Fe3O4-NH2–Pda
Table 3 Recycling of the Fe3O4-NH2–Pd catalyst in a Heck reactiona
Yieldb (%)
Run
Yield (%)
Run
Yield (%)
Entry
R
X
Z
t/h
1
2
3
4
>99
>99
99
5
6
7
8
97
98
95
96
1
2
3
4
5
6
7
8
H
Br
Br
Br
Br
I
Br
Br
Br
Br
I
Ph
Ph
Ph
Ph
24
24
10
10
10
24
24
10
10
10
96
93
>99
>99
>99
98
4-Me
4-NO2
4-COMe
H
98
Ph
a Reaction conditions: 4-bromonitrobenzene (0.5 mmol), styrene (0.6
mmol), recycled Pd catalyst, K2CO3 (0.6 mmol), NMP (5 mL) under an
N2 atmosphere.
H
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
4-Me
4-NO2
4-COMe
H
96
>99
>99
>99
9
10
a The reaction was carried out with 0.50 mmol of aryl halides, 0.6 mmol
of styrene (or n-butyl acrylate), 0.6 mmol of K2CO3, 5 mol% Pd with
respect to the aryl halides and 5 mL of NMP under an N2 atmosphere.
b Determined by GC or GC-MS.
properties, the loss of Pd was greatly suppressed during catalytic
cycles. The novel catalyst was successfully applied for the first
time to hydrogenation and Heck reactions with the fascinating
nature of both a high activity and durability.
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8 S. Ogasawara and S. Kato, J. Am. Chem. Soc., 2010, 132, 4608.
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for financial support.
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