Efficient clay supported Pd nanoparticles as heterogeneous catalyst for arylation of alkenes

Article abstract of DOI:10.1016/j.catcom.2012.02.001

A new pincer-type ligand containing Pd(II) was immobilized on a modified bentonite. The material was evaluated as catalyst in the arylation of iodobenzene and several alkenes under room conditions. The catalyst combines high activity and selectivity, and can be reused three times with no observed deactivation.

Full text of DOI:10.1016/j.catcom.2012.02.001

Catalysis Communications 21 (2012) 82–85
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Catalysis Communications
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Short Communication
Efficient clay supported Pd nanoparticles as heterogeneous catalyst for arylation
of alkenes
a
b
c
Mehran Ghiaci a, , Fatemeh Ansari , Zahra Sadeghi , A. Gil
⁎
a
Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
b
Payam-Nor University, Isfahan, Iran
c
Department of Applied Chemistry, Los Acebos Building, Public University of Navarra, Campus of Arrosadia, 31006-Pamplona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2012
Received in revised form 31 January 2012
Accepted 1 February 2012
Available online 13 February 2012
A new pincer-type ligand containing Pd(II) was immobilized on a modified bentonite. The material was eval-
uated as catalyst in the arylation of iodobenzene and several alkenes under room conditions. The catalyst
combines high activity and selectivity, and can be reused three times with no observed deactivation.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Amine modified bentonite
Palladium catalyst
Alkenes arylation
1. Introduction
2. Experimental section
The arylation of alkenes under a palladium catalyst is referred as
the Heck reaction. This reaction has become an important tool in or-
ganic synthesis [1–3]. The homogeneous palladium-based catalysts
suffer from low stability and high costs, which prevent their applica-
tion in industrial processes. As a metal, palladium is also highly unde-
sirable as a contaminant of pharmaceutical products. In efforts to
develop a heterogeneous catalyst for the Heck reaction, palladium
salts or its complexes have supported on materials such as active
carbon [4,5], mesoporous silica [6], inorganic oxide [7,8], molecular
sieves [9], polymers [10], zeolites [11], hydrotalcites [12,13], and clays
[14,15].
Our interest is the use of a modified clay material as a catalyst for
the arylation of olefins. For that, we report the immobilization of a
nonionic surfactant with multiple N groups on a bentonite modified
with a cationic surfactant and the use of this material as support for
the immobilization of palladium acetate. The amino groups in the sur-
factant were designed to coordinate with Pd(ΙΙ) and additional amino
groups could be varied in order to improve the performance of the
complexes in catalytic reactions and prevention of palladium leach-
ing. The new catalyst reported in this work does not require addition
of phosphins, commonly introduced in to palladium catalyzed reac-
tions, and exhibit high activity toward the Heck reaction of iodoben-
zene with olefins with small amount of palladium.
2.1. Preparation of immobilized palladium (II) on a modified bentonite
Synthesis, characterization, and immobilization of 3,3′-
(dodecylazanediyl)bis(N-(2-(2-aminoethylamino)ethyl)propanamide)
(DAEP) on a modified bentonite have been described in a previous work
[16]. The modified bentonite at this stage has a layer of cationic surfactant
(cetyl pyridinium) and a layer of DAEP. With the multiple amine groups
on the surface of the bentonite, a suitable pincer-type ligand for immobi-
lization of palladium nanoparticles have been designed (see Scheme 1).
In a typical experiment to prepare Pd(II)-bentonite composite,
0.5 g of the modified bentonite (DAEP-bentonite) was dispersed in
100 cm3 of n-hexane to which 10−4 dm3 of 10−2 mol/dm3 of a aque-
ous solution of palladium acetate were added in ten steps to the sus-
pension during 72 h, under vigorous stirring in a Morton flask. The
resulting product was separated via centrifugation, and washed few
times with distilled water. The solid was dried under vacuum at
room temperature, before characterization and catalytic activity mea-
surements. The amount of palladium in the catalyst was determined
by ICP-AES method. The maximum palladium load achieved was
164 μmol/gcatalyst
.
2.2. Characterization techniques
X-ray diffraction (XRD) patterns were recorded by using a Siemens
D-500 diffractometer, at 40 kV and 30 mA, and employing filtered Cu
Kα radiation over a 2θ range between 5 and 80°.
Scanning electron micrographs were obtained using a Cambridge
Oxford 7060 Scanning Electron Microscope (SEM) connected to a
⁎
Corresponding author.
E-mail address: mghiaci@cc.iut.ac.ir (M. Ghiaci).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.02.001

M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
83
Scheme 1. Modification of the clay surface with a surfactant and impregnation of Pd particles.
four-quadrant backscattered electron detector with resolution of
1.38 eV. The samples were dusted on a double sided carbon tape
placed on a metal stub and coated with a layer of gold to minimize
charging effects.
the support with palladium, the interlayer space increased to
13.21 Å, which probably means that some of the palladium ions
with their water shells penetrated into the bentonite layers.
FTIR spectra of the catalyst were recorded on a JASCO FTIR 680
plus spectrometer with the KBr pellet method.
3.3. SEM
2.3. Arylation of alkenes with iodobenzene
The SEM images of the modified bentonites with or without palla-
dium nanoparticles are shown in Fig. 1. Palladium nanoparticles had a
strong influence on the morphology of the support. The Na-bentonite
after modification with cetyl pyridinium bromide has found a pack
type aggregation of the particles [18]. This packed structure
completely changed when it was modified with DAEP in the next
step (Fig. 1B). The hydrophilic nature of the head of DAEP seems to
The reactions were carried at 130 °C in a sealed reactor immersed
in a thermo-stated bath. A typical reaction run was as follows: 0.05 g
of catalyst, 1 mmol of iodobenzene, 2 mmol of alkene, 2 mmol of base
(NaOAc or triethylamine) and 3 cm3 of DMF. The reaction mixture
was heated to the desired temperature and stirred for 20–24 h.
When the reaction was completed, the reaction mixture was centri-
fuged and the reaction products were analyzed by gas chromatogra-
phy (Agilent, model 6890, equipped with a wide bone OV-17
capillary column and a FID detector). The products were identified
by GC–MS (Fisons Instruments, model 8060).
3. Results and discussion
3.1. Synthesis of the catalyst
DAEP-dendrite was immobilized on a modified bentonite starting
from bentonite with monolayer cetyl pyridinium cation coverage.
This process was monitored by the adsorption isotherm of DAEP on
modified bentonite. The FTIR spectrum of the catalyst contains
peaks corresponding to the primary amino groups of the dendrite
which appear at 3390 cm−1 and 3344 cm−1
,
and shifted to
3358 cm−1 and 3300 cm−1 after complexation with metal [16]. The
electronic spectrum of the complex showed broad band at
a
450 nm, which may be due to charge transfer transition, and a band
at 650 nm due to first excited transition [16].
3.2. X-ray diffraction analysis
The interlayer spacing of the original bentonite (Na-bentonite) is
12.3 Å [17], which has changed to 12.95 Å after adsorption of cationic
surfactant, cetyl pyridinium bromide. However, by adsorption of the
pincer-type surfactant (DAEP) onto the monolayer modified benton-
ite, the interlamellar distances did not change to a meaningful value
(13.0 Å). Therefore, it was presumed that DAEP molecules adsorbed
on the external surface of the monolayer modified bentonite and
edges of the layers, and could not penetrate into the interlamellar
spaces of the bentonite. Also it should be mentioned that by loading
Fig. 1. SEM images of (A) Na-bentonite; (B) DAEP-bentonite with Pd(II).

84
M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
Table 2
change the porosity of the support by allowing it to adsorb more
water. These changes were continued by loading the modified ben-
tonite with palladium nanoparticles.
Comparison of the data obtained in the present work with other studies in the
literature.
Entry Haloarene
Alkene
Conversion Pd (mol)
(%)
Reference
3.4. The arylation of alkenes
1
Iodobenzene
(0.5 mmol)
Bromobenzene Styrene
(1 mmol)
Iodobenzene
(1 mmol)
Iodobenzene
(1 mmol),
Iodobenzene
(1 mmol)
Iodobenzene
(0.5 mmol)
Iodobenzene
(2.23 mmol)
Iodobenzene
(1 mmol)
Methyl acrylate
(0.75 mmol)
99
89
99
95
99
94.5
87
95
95
0.5
0.1
1
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
In order to test the activity of the catalyst, the Heck reaction of
iodobenzene with several alkenes, active and deactivated alkenes
were considered. The results are shown in Table 1. An important
point about these reactions is the small amount of Pd loaded on the
catalyst. This scale of Pd was enough to perform the reaction between
iodobenzene and alkenes with high yield and excellent selectivity.
The reaction time was long but the preference of the amount of Pd,
made it suitable catalyst for using in sensitive systems toward metals,
especially for pharmaceutical systems. As shown in Table 1, in most
cases (entries 1–4, and 7), the selectivity is 100% and only trans
alkenes were found.
The reaction of methyl acrylate and iodobenzene is also carried
out with 0.05 g catalyst (8.2 μmol Pd) at several reaction temperature
and several times. Maximum conversion for this reaction was
achieved at 130 °C after 20 h . In all cases, the stability of the catalyst
allowed us to work with it under non inert atmosphere and high tem-
perature (130 °C). The catalytic activity was also studied in water as
solvent. For the reaction of iodobenzene with methylacrylate in H2O
only 60% of conversion was achieved at 130 °C after 20 h.
2
(1.4 mmol)
Butyl acrylate
(1.5 mmol)
Methyl acrylate
(3 mmol)
Butyl acrylate
(1.5 mmol)
Acrylic acid
(6 mmol)
Butyl acrylate
(3.14 mmol)
Methyl acrylate
(2 mmol)
methyl acrylate
(1.5 mmol)
Methyl acrylate 100
(1.5 mmol)
3
4
0.5
0.5
0.1
1
5
6
7
8
1
9
Iodobenzene
(1 mmol)
Iodobenzene
(1 mmol)
Iodobenzene
(1 mmol)
4.36×10−7 [9]
9.33×10−5 [10]
10
11
Methyl acrylate 100
(2 mmol)
8.2×10−6
Current
work
For comparison of the effect of several bases, the reaction of iodo-
benzene with alkenes using NaOAc or Et3N were also examined
(Table 1). Et3N exhibited fairly more selectivity toward the main
product, in the Heck reaction between methyl vinyl ketone and iodo-
benzene (Table 1, entries 5 and 6). In this reaction, the second prod-
uct was 4-phenyl-2-butanone which is produced by reduction of the
double bond. For other alkenes, activity and selectivity were the
same for both bases.
The catalyst was also tested to catalyze the Heck reaction of
bromobenzene with alkenes. Clearly, aryl bromides are much less
reactive than aryl iodides. The reaction between methyl acrylate
and bromobenzene with even 0.1 g catalyst (16.4 μmol Pd) just had
11% of conversion, so this catalyst is not suitable for less reactive
halobenzenes.
of palladium used in this study is compared with those published in
the literature, it can be observed that the catalyst employed here be-
haves in an outstanding way.
After each experiment, the Pd in solution after removal of the solid
catalyst by centrifugation has been examined. The Pd content in solu-
tion was not detectable even after five runs. It's possible to consider
that the decrease in activity could be related to leaching of Pd from
the active sites of the catalyst and reprecipitation of palladium on
the surface of bentonite.
For recycling study, reaction of methyl acrylate and iodobenzene
in the presence of NaOAc in DMF with 0.05 g of catalyst (8.2 μmol
Pd) was performed. After each run of reaction, the catalyst was recov-
ered by simple centrifugation and subsequently washed with water
followed with chloroform, then dried in the oven at 100 °C and
reused. The catalyst retained the same activity after three reuse
runs and after five run only lost 12% of its activity (Table 3).
The catalyst used in the present study, and the other catalysts used
for the Heck reaction under various environmental conditions
reported in the literature are compared in Table 2. When the amount
Table 1
The Heck reaction between iodobenzene and several alkenes.a.
Entry
1
Substrate
Base
Pd (μmol)
Conversion (%)
100
Selectivity (%)
100
Products
O
O
NaOAc
8.2
O
O
O
O
O
2
3
4
5
6
7
Et3N
8.2
100
100
100
100
100
98
100
100
100
67b
80b
100
85c
O
O
O
NaOAc
Et3N
8.2
O
O
O
O
O
8.2
O
O
NaOAc
Et3N
8.2
O
O
O
8.2
O
OH
NaOAc
NaOAc
0.164
0.164
OH
O
8
93
a
Reactions condition: 1 mmol of iodobenzene, 2 mmol of alkene, 2 mmol of base, 3 cm3 of DMF, 130 °C, 20 h.
Second product was 4-phenyl-2-butanone.
Second product was cis stilbene.
b
c

M. Ghiaci et al. / Catalysis Communications 21 (2012) 82–85
85
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a
Recycling and reuse of the catalyst
.
Run
1
2
3
4
5
Conversion/selectivity (%)
100
100
100
97
88
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a
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The commercial success of Heck reaction depends on two main
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mides and aryl chlorides as the aryl halide source; and (ii) the use
of active and recyclable palladium catalyst that can compensate for
the cost of transition metal [19,20]. In this work, small amount of
palladium are required to perform the reaction of iodobenzene with
several alkenes with high yield and selectivity.
Acknowlegdments
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Thermodynamics 36 (2004) 707–713.
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York, 2002, pp. 1205–1208.
Thanks are due to the Iranian Nanotechnology Initiative and the
Research Council of Isfahan University of Technology and Center of
Excellence in the Chemistry Department of Isfahan University of
Technology for supporting of this work.
[20] A. Zapf, M. Beller, Topics in Catalysis 19 (2002) 101–109.
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