Preparation of high-magnetization Fe3O4-NH 2-Pd (0) catalyst for heck reaction

Article abstract of DOI:10.1016/j.catcom.2011.10.015

A magnetically separable Fe₃O₄-NH₂-Pd (0) catalyst was easily synthesized by immobilizing Pd nanoparticles on the surface of magnetic Fe₃O₄-NH₂ microspheres. It was found that the combination of Fe₃O₄ and triethylene tetramine (TETA) could give rise to structurally stable catalytic sites. Furthermore, the high-magnetization Fe₃O₄-NH ₂-Pd(0) catalyst can be recovered by magnet and reused for six runs for Heck reaction without significant loss in catalytic activity.

Full text of DOI:10.1016/j.catcom.2011.10.015

Catalysis Communications 17 (2012) 168–172
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of high-magnetization Fe3O4–NH2–Pd (0) catalyst for Heck reaction
⁎
Mingliang Ma, Qiuyu Zhang , Dezhong Yin, Jinbo Dou, Hepeng Zhang, Hailong Xu
Key Laboratory of Applied Physics and Chemistry in Space, Ministry of Education, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University,
Xi'an 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2011
Received in revised form 12 October 2011
Accepted 14 October 2011
Available online 30 October 2011
A magnetically separable Fe3O4–NH2–Pd (0) catalyst was easily synthesized by immobilizing Pd nanoparticles on
the surface of magnetic Fe3O4–NH2 microspheres. It was found that the combination of Fe3O4 and triethylene
tetramine (TETA) could give rise to structurally stable catalytic sites. Furthermore, the high-
magnetization Fe3O4–NH2–Pd(0) catalyst can be recovered by magnet and reused for six runs for Heck
reaction without significant loss in catalytic activity.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Fe3O4–NH2–Pd (0)
Triethylene tetramine (TETA)
Heterogeneous catalyst
Heck reaction
1. Introduction
nanocomposite of Pd nanoclusters supported on silica-coated Fe2O3
nanoparticles. The Pd/H2N–SiO2/Fe2O3 catalyst provides excellent
In modern chemical research, catalysts and catalytic reactions are
always attracting considerable attention in fundamental research and
industrial applications [1–5]. Conventionally, heterogeneous catalysis
is favored over homogeneous catalysis due to its ease of handling and
regenerability. But the application of heterogeneous catalysts is limited
because of less active site than their homogeneous counterparts.
Recently, nanomaterials have emerged as sustainable alternatives to
conventional heterogeneous catalysts and catalyst supports [6–9].
The nanomaterials present high specific surface area of the active
component, thereby enhance the contact between reactants and
catalyst support [10]. Although a higher surface area affords nano-
materials with more active sites, they are easy to agglomerate and
hard to be separated. Therefore, it is essential to design an easily
recovery and well-dispersed catalysts.
In this regard, magnetic nanoparticles (MNPs) as catalyst supports
are very promising due to their large specific surface area and magnetic
property [11–19]. It can be easily collected by a magnet for reuse to
prevent loss of the catalyst. Recently, much attention has been focused
on the surface modification with appropriate capping agents onto the
MNP surface to anchor the catalytically active complexes [20–26]. As
we know, amines have a strong affinity for general catalysts and can
keep the MNPs from aggregating without disturbing their desired
properties [27]. Thus, more attention has been focused on amine
functionalization procedures to anchor the catalytically active com-
plexes onto the MNP surface. Yi et al. reported on the synthesis of a
reactivity and reusability in the hydrogenation of nitrobenzene
[27]. Guin et al. reported the Pd(0) immobilized on the surface of
amine-terminated Fe3O4 and NiFe2O4 nanoparticles. The catalysts can
be completely recovered with an external magnetic field, and their cat-
alytic efficiency remains unaltered after 10 repeated cycles [20]. In our
previous work [28], a magnetically separable palladium catalyst has
been synthesized by immobilizing Pd on the surface of superparamag-
netic composite microspheres. It can be conveniently recovered by
magnet and reused at least six times. However, an additional amine
process is needed after the preparation of MNPs. Therefore, it would
be intriguing to synthesize Fe3O4–NH2 nanoparticles in one step.
Described in this communication is a straightforward preparation
of Fe3O4–NH2–Pd(0) nanocatalysts with high magnetic sensitivity
and the application in Heck reactions. The Fe3O4–NH2 nanoparticles
were synthesized through a one-pot solvothermal reaction using
FeCl3·6H2O as a single iron source and triethylene tetramine(TETA)
as a ligand. This reaction simplified the procedure of surface amine
modification. The TETA on the surface of MNP makes them convenient
for coordination with Pd(0). Finally, the catalytic behavior of Fe3O4–
NH2–Pd(0) nanoparticles was measured by the Heck reaction.
2. Experimental section
2.1. Materials
Ferric chloride hexahydrate (FeCl3·6H2O), sodium acetate
(NaAc), ethylene glycol (EG), triethylene tetramine(TETA), Pd(OAc)2,
acrylic acid(AA), aryl halides, N,N-dicyclohexylmethylamine,
⁎
Corresponding author. Fax: +86 2988431653.
E-mail address: qyzhang@nwpu.edu.cn (Q. Zhang).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.10.015

M. Ma et al. / Catalysis Communications 17 (2012) 168–172
169
dimethylformamide(DMF) and potassium borohydride (KBH4) were
purchased from Sigma-Aldrich and used as received.
a
2.2. Preparation of amine-functionalized magnetic nanoparticles
The Fe3O4–NH2 nanoparticles were prepared according to Li's
method with some modification [29]. A solution of TETA (4.5 g), anhy-
drous NaAc (2.0 g) and FeCl3·6H2O (2.5 g) in EG solution (30 mL) was
stirred vigorously at 30 °C to give a transparent solution. This solution
was then transferred into a teflon autoclave and then kept at 200 °C
for 6 h. The magnetic nanoparticles were then rinsed with water and
ethanol for several times to effectively remove the solvent and unbound
TETA, and then dried under vacuum at ambient temperature. During
each rinsing step, the nanoparticles were separated from the superna-
tant by magnet.
2.3. Preparation of Fe3O4–NH2–Pd (0) magnetic catalyst
b
0.18 g of as-synthesized Fe3O4–NH2 powder was added to 30 mL
of 0.01 g Pd(OAc)2 aqueous solution and stirred for 24 h at 35 °C
(S1). And then the powder was reduced by 30 mL of 0.1 M KBH4
aqueous solution to get a dark black powder (Fe3O4–NH2–Pd (0))
(Scheme 1). The solid catalyst was separated by magnet and was
washed several times with ethanol. The Pd content in Fe3O4–NH2–Pd
(0) catalyst was determined to be 4.26 wt.% by AAS.
2.4. General procedures for heck reactions and recovery of the supported
magnetic catalyst
The novel magnetic supported catalyst (0.5 mol%), acrylic acid
(7.5 mmol), aryl halides (5 mmol), N,N-dicyclohexylmethylamine
(10 mmol) and DMF(5 mL) were added into a round bottomed flask
and stirred at 95–140 °C in atmosphere for 3–14 h. After the reaction
mixture was cooled to room temperature, the catalyst was separated
by magnet.
c
3. Results and discussion
3.1. Characterization of Fe3O4–NH2–Pd (0) catalyst
Fe3O4–NH2–Pd (0) catalyst was prepared by ligand-mediated
immobilization of Pd (0) nanoparticle on functionalized surfaces of
Fe3O4–NH2, as shown in Scheme 1. Fig. 1a shows the TEM micrograph
of the Fe3O4–NH2 nanoparticles prepared by solvothermal reduction
method. It is observed that the synthesized particles are spherical
and nearly monodisperse and with an average diameter of 100 nm.
Fig. 1b and c shows the TEM micrographs of Fe3O4–NH2–Pd catalysts
before and after reaction, respectively. Molecular simulation (S2) and
FTIR (S3) also suggest that the surface of magnetic nanoparticles has
been successfully functionalized with amino groups in the synthetic
process. As can be seen, the morphology and size of these particles
did not change obviously even after several reactions.
Fig. 1. TEM images of the samples (a) Fe3O4–NH2 (b) Fe3O4–NH2–Pd (II) (c) Fe3O4–
NH2–Pd (0).
Scheme 1. Procedure of the preparation of Fe3O4–NH2–Pd (0) catalyst.

170
M. Ma et al. / Catalysis Communications 17 (2012) 168–172
The crystalline structures of nanoparticles were determined by
XRD. As presented in Fig. 2a, six characteristic diffraction peaks
(2θ=30.1°, 35.3°, 43.0°, 53.4°, 56.9°and 62.5°) corresponded well to
the standard for Fe3O4 reflections. These peaks are sharp and intense,
indicating their well crystallized structure. Each particle is composed
of a lot of small Fe3O4 nanoparticles with a size around 11.5 nm calcu-
lated from the Debye–Scherrer equation, which is comparable with
the result in Fig. 2c (around 10 nm). The agglomeration of plenty of
small single-crystal particles leads to the formation of bulky hollow
nanosphere [30]. As seen from Fig. 2b, the diffraction peaks at
2θ=39.3°, 45.9° and 67.1° are the characteristic signals of the face
centered cubic crystalline phase of Pd. It suggests that Pd nanoparticles
successfully deposited onto the Fe3O4–NH2 supports and Pd remained
highly dispersed on the surface of the Fe3O4–NH2–Pd (0) catalyst
after reduction. Other diffraction peaks in Fig. 2b corresponded to the
reflections of Fe3O4, indicating that the crystalline structure of Fe3O4
did not change after the immobilization of organometallic species.
Magnetic properties of the Fe3O4–NH2–Pd (0) complex catalyst
were studied by a vibrating sample magnetometer (VSM) at 300 K.
The magnetization curves show that the magnetization saturated up
to 75 emu/g at an external filed of 15 KOe and the coercivity is almost
negligible, indicating this sample the superparamagnetic property
(Fig. 3a). These magnetic properties allow a fast separation of the cat-
alyst from the reaction media (Fig. 3b) and the redispersion of the
catalysts in solution without severe assembly and/or aggregation in
successive reactions (Fig. 3c).
Fig. 3. The magnetization curves of Fe3O4–NH2–Pd (0) measured at 300 K(a). The
nanocrystals are drawn from the solution to the sidewall of the vial by an assistant
magnet field (b) and also can be easily dispersed in DMF(c).
3.2. Heck reaction catalyzed by Fe3O4–NH2–Pd (0) catalyst
XPS measurements were carried out to confirm that Pd nanoparti-
cle are adsorbed by amino groups of Fe3O4–NH2. In Fig. 4, the XPS of a
Fe3O4–NH2–Pd (0) sample shows Pd 3d5/2 and Pd 3d3/2 binding ener-
gies (BE) at 336.4 and 341.5 ev, respectively. According to the litera-
ture [31], the binding energy of Pd 3d5/2 for Pd (0) is 335.1 ev.
However, we found that the Pd 3d5/2 peak shifted to higher values
as a result of the presence of the surrounding positively charged am-
monium groups of the Fe3O4–NH2 nanoparticles. It indicates that
there is a good interaction to make Pd more stable on the surface of
Fe3O4–NH2 nanoparticles. It is also well recognized that amino groups
can suppress the agglomeration of Pd nanoclusters, and enhance the
catalytic efficiency of Pd nanoparticles [27].
(311)
In general, the most important aspect for catalyst was its efficiency
and stability. As we know amino groups can enhance the catalytic ac-
tivity of Pd [31], the TETA on the surface of Fe3O4–NH2 provided the
ideal environment for better conversions. In order to make Pd more
stable on the surface of Fe3O4–NH2 nanoparticles, we used the exces-
sive amount of TETA to increase the content of amino group in this
experiment and chose the amount of TETA was 4.5 g after a series of
optimizations in our experiment. To investigate the catalytic activity
of the Fe3O4–NH2–Pd(0)(40 m2/g), Heck reaction of cross-coupling
of aryl halide with acrylic acid was examined (Scheme 2). For the
(440)
(220)
(511)
(400)
(422)
a
Pd(111)
Pd(200)
Pd(220)
b
30
40
50
60
70
2
θ / deg
c
336.4
341.5
346
344
342
340
338
336
334
332
330
Binding energy (ev)
Fig. 2. XRD pattern of Fe3O4–NH2 (a) and Fe3O4–NH2–Pd (0) (b) and the HRTEM of
Fe3O4–NH2 microspheres (c).
Fig. 4. XPS spectrum of Fe3O4–NH2–Pd (0) catalyst.

M. Ma et al. / Catalysis Communications 17 (2012) 168–172
171
Scheme 2. Heck reaction of aryl halide with acrylic acid catalyzed by Fe3O4–NH2–Pd (0) catalyst b.
Heck reaction shown in Table 1, the catalyst showed good catalytic ef-
ficiency. The reaction temperature between iodobenzene and vinyl
substrate for Heck reaction was low (95 °C) and the yield can reach
97.8%. For bromobenzene, the yield of corresponding product was
12.4% (entry 5) under the same reaction condition. However, when
the reaction time was extended to 14 h and the reaction temperature
was increased to 140 °C (entry 6), a moderate yield (50.2%) was
obtained. It was demonstrated that the reactivity of aryl bromides
with electron-withdrawing substituents was higher than that of aryl
bromides with electron-donating substituents. Meanwhile, to prove
the complete separation of catalyst from the reaction solution, the
verification has been done [33] (S4).
Table 1
Heck reactions of various aryl halides and acrylic acid catalyzed by the magnetic
catalysta.
Entry
R1
X
Time (h)
Temperature(°C)
Yield (%)b
TON [32]
Further experiments were performed to verify the stability and re-
cycle of the Fe3O4–NH2–Pd(0) in Heck reactions. We can see from
Fig. 5 that the Pd particles dispersed well on the surface of the
Fe3O4–NH2 microspheres before the Heck reaction (Fig. 5a). However,
the Pd particles agglomerated after the six runs of Heck reaction
(Fig. 5b), leading to the yield gradually decreased to about 90% of
the initial run after six runs of Heck reaction (Fig. 6). Meanwhile,
the AAS result showed that only 0.6 wt.% Pd leached away from the
surface of catalyst after six runs of Heck reaction, which indicated
the coordination link of Pd (0) with \NH2 ligand is strong. All these
results showed that the as-synthesized magnetic catalyst is an excel-
lent potential candidate for reusable and recoverable catalyst.
1
2
3
4
5
6
7
8
H
I
I
I
I
Br
Br
Br
Br
3
3
3
3
3
14
14
14
95
95
95
95
95
140
140
140
97.8
92.1
97.4
93.2
12.4
50.2
89.1
39.4
978
921
974
932
124
502
891
394
CH3
NO2
OCH3
H
H
NO2
CH3
a
Reaction conditions: aryl halide(5 mmol),acrylic acid (7.5 mmol), N,N-dicyclohex-
ylmethylamine (10 mmol), DMF (5 mL). The supported magnetic catalyst (0.5 mol%).
b
HPLC yield based aryl halide.
a
4. Conclusions
In summary, a high-magnetization (75 emu/g) Fe3O4–NH2–Pd(0)
catalyst was prepared by immobilizing Pd(0) nanoparticles on the
surface of Fe3O4–NH2 microspheres. The catalyst is recoverable mag-
netically and could be reused for six runs without significant loss in
catalytic activity and selectivity for Heck reaction. Further studies on
the effects of Fe3O4–NH2–Pd(0) catalyst on the catalytic activity for
other catalyst systems are now in progress.
Pd
Acknowledgments
Financial support from the National Natural Science Foundation of
China (No. 50773063), National Basic Research Program of China
(2010CB635111) and Basic Research Foundation of Northwestern
Polytechnical University (G9KY1020) are highly appreciated.
b
100
80
60
40
20
0
Pd
1
2
3
4
5
6
reused times
Fig. 5. HRTEM images of the morphology of Pd particles before (a) and after (b) Heck
Fig. 6. Reused times of the Fe3O4–NH2–Pd (0) catalyst.
reaction.

172
M. Ma et al. / Catalysis Communications 17 (2012) 168–172
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