A green approach for the synthesis of palladium nanoparticles supported on pectin: Application as a catalyst for solvent-free Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.molcata.2013.02.023

A green method for the synthesis of palladium nanoparticles supported on pectin has been described. The synthesized nanoparticles were explored in Mizoroki-Heck reaction between different aryl halides and n-butyl acrylate under solvent-free conditions. The catalyst can be recycled for six runs without significant loss in the catalytic activity.

Full text of DOI:10.1016/j.molcata.2013.02.023

Journal of Molecular Catalysis A: Chemical 372 (2013) 160–166
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Journal of Molecular Catalysis A: Chemical
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A green approach for the synthesis of palladium nanoparticles supported on
pectin: Application as a catalyst for solvent-free Mizoroki–Heck reaction
Ardeshir Khazaei∗, Sadegh Rahmati, Zahra Hekmatian, Shahnaz Saeednia
Faculty of Chemistry, Bu-Ali Sina University, P.O. Box 651783868, Hamedan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A green method for the synthesis of palladium nanoparticles supported on pectin has been described.
The synthesized nanoparticles were explored in Mizoroki–Heck reaction between different aryl halides
and n-butyl acrylate under solvent-free conditions. The catalyst can be recycled for six runs without
significant loss in the catalytic activity.
Received 23 January 2013
Received in revised form 21 February 2013
Accepted 25 February 2013
Available online 5 March 2013
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Palladium nanoparticles
Mizoroki–Heck reaction
Solvent-free
Pectin
Catalyst recovery
1. Introduction
freely available hydroxyl groups make it suitable and ideal can-
didate for many practices in different areas of science.
In recent years, the design and development of green catalysts
have attracted a large number of studies in academic and industrial
groups to reduce or eliminate the use of hazardous substances [1,2].
Solid catalysts provide opportunities for recycling and recovering of
the catalysts from reaction environments. Because of having many
advantages such as easy separation, low waste production and the
reduced cost, heterogenization of the catalysts to solid supports by
their immobilization on organic [3] or inorganic [4] polymers is of
great interest [5–7].
In last few years, immobilization of the palladium nanoparticles
on solid supports to prepare active and stable catalytic systems
is an interesting topic, and different supports have been used
to stabilize the nanoparticles [8–15]. Along this line, Pd/gellan
[16], Pd/arabinogalactan [17], Pd/agarose [18], Pd/starch [19] and
Pd/chitosan [20] have been prepared using polysaccharides as the
bed. In order to expand the use of carbohydrate-based materials as
the support for palladium nanoparticles, we decided to introduce
pectin as a suitable and naturally degradable support for stabiliza-
tion of palladium nanoparticles.
Palladium-catalyzed Mizoroki–Heck coupling reaction is one
of the most powerful synthetic methods for the formation of
carbon carbon bonds, which allows the arylation, alkylation or
vinylation of various alkenes through their reaction with aryl, vinyl,
benzyl or allyl halides in the presence of palladium and a suit-
able base [7,22]. Total synthesis of complex organic molecules
has benefited enormously from the Mizoroki–Heck reaction. The
Mizoroki–Heck coupling products find good applications as inter-
mediates in the preparation of materials, natural products, and
bioactive compounds [23]. Herein, we report that palladium
nanoparticles stabilized by pectin can be successfully used in the
Mizoroki–Heck cross-coupling reaction between the activated and
non-activated aryl halides and n-butyl acrylate under solvent-free
conditions.
2. Experimental
2.1. Gram-scale preparation of palladium nanoparticles
supported on pectin
Pectin is a family of complex polysaccharides that is found
extensively in nature [21]. Unique properties of pectin, such as
biodegradability, flexibility, non-toxicity, low price and carrying
Pectin (1 g) was dissolved in water (100 mL) at ambient tem-
perature. To this solution was added a solution of PdCl2 (100 mL,
1 mM) and diluted with water (100 mL). The reaction mixture was
refluxed at 100 ◦C for 4 h. The mixture was cooled down to room
temperature and the solvent was evaporated. The obtained dark
gray composite was dried by the flow of air over night and then
under vacuum for 24 h.
∗
Corresponding author. Fax: +98 811 8257407.
E-mail address: khazaei 1326@yahoo.com (A. Khazaei).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.023

A. Khazaei et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 160–166
161
Fig. 1. Preparation of Pdnp/pectin.
Fig. 2. UV–vis spectra of (a) Pd(II) before reduction and (b) Pd(0) after reduction with pectin.
2.2. General procedure for the Mizoroki–Heck reaction in the
presence of the nanocatalyst
of carboxyl groups and liberation of CO2 gas at the reflux conditions
[22b].
According to the energy dispersive X-ray analysis (EDX) the
presence of palladium is indicated in the Pdnp/pectin catalyst and
its loading amount was measured to be 0.61% (w/w). The EDX spec-
trum also shows signals of carbon and oxygen which are present in
the pectin substrate (Fig. 3).
To a flask, a mixture of pectin supported Pd-nanoparticles
(0.05 g of the composite, contains 0.0028 mmol of palladium),
aryl halide (1 mmol), n-butyl acrylate (1.5 mmol, 0.21 mL) and n-
Pr3N (1.5 mmol, 0.29 mL) were added and heated at 140 ◦C under
solvent-free conditions. After completion of the reaction (moni-
tored by TLC or GC), ethylacetate (10 mL) was added to the flask. The
catalyst was separated by simple filtration. Water (3× 15 mL) was
added to the ethylacetate phase and decanted. The organic layer
was dried over anhydrous Na2SO4. After evaporation of the solvent,
the resulted crude products was purified by column chromatog-
raphy (hexane/ethylacetate) giving the pure products in excellent
yields.
The formation of Pd nanoparticles was confirmed by the XRD
pattern of Pdnp/pectin catalyst. As presented in Fig. 4, the four char-
acteristic peaks at (1 1 1), (2 0 0), (2 2 0) and (3 1 1) can be clearly
observed for Pd particles in various 2Â values. These characteris-
tic peaks shows crystallographic planes of the Pd(0) nanoparticles.
Generally, Solid material can be classified as being either amor-
phous or crystalline. In crystalline materials the ions occupy specific
locations in a regular lattice. The simplest such lattice is a simple
cube with ions on each of the corners of the cube. The most com-
mon simple structures are the face centered cubic (fcc) and the
body centered cubic (bcc). The fcc structure consists of a cube of
ions with six additional ions located at the centers of each of the
six faces of the cube [24]. According to crystallographic planes in the
2.3. The procedure for recycling of the catalyst
After completion of the reaction at the first run, the reaction
mixture was cooled down to room temperature and ethylacetate
(5 mL) was added to the reaction mixture to extract organics. The
ethylacetate phase was sucked from the vial by a syringe and the
catalyst was dried under vacuum. After complete drying, the cata-
lyst was reused for the similar reaction. This process was repeated
for six runs.
3. Results and discussion
Pectin stabilized palladium nanoparticles (Pdnp/pectin) were
prepared by exposure of aqueous solution of PdCl2 (100 mL, 1 mM)
to pectin (1 g dissolved in 100 mL water) without using any extra
reducing agent (Fig. 1). This solution was refluxed for 4 h giving
a gray solution. Evaporation of the solvent followed by drying in
vacuum gave a dark gray solid.
The UV–vis spectrum of the resulted Pd supported nanoparticles
changed significantly after reduction, with the disappearance of the
peak at around 430 nm (Fig. 2). This change showed the conver-
sion of Pd(II) to Pd(0). Since pectin contains free carboxyl groups,
they can form complexes with Pd(II) ions in solution and reduce
them to Pd(0). The process of reduction is performed by oxidation
Fig. 3. The EDX spectrum of Pdnp/pectin.

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A. Khazaei et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 160–166
and in the presence of DMSO and DMF as the solvent after 5 min.
The desired product was obtained in 40, 36 and 94% yields, respec-
tively at 140 ◦C. Comparison of the results clearly shows that the
reaction in solvent-free conditions (Table 1, entry 11) proceeded
much better than in solvents with an excellent yield and shorter
reaction time.
The reaction was also studied at various temperatures under
non-solvent conditions. The results demonstrated that the tem-
perature plays a significant role in the reaction rate. The activity
of the catalyst was increased in high temperatures (Table 1, entries
7–11) with the activity being very high at 140 ◦C. During our opti-
mization studies, various bases were examined and it was found
that the catalyst was very active and selective for the reaction in
the presence of n-Pr3N (Table 1, entry 11) as a liquid organic base,
while the reaction rate was very slow when inorganic bases were
used (Table 1, entries 12–14). Remarkably, the reactions were car-
ried out in air indicating that the Pdnp/pectin catalyst was highly
stable and insensitive to oxygen.
Fig. 4. The XRD spectrum of Pdnp/pectin.
Using the optimized reaction conditions (Table 1, entry 11), a
range of aryl iodides, bromides and chlorides with various sub-
stituent groups were examined in the Mizoroki–Heck reaction
(Table 2).
It is recognized that the relative reactivity of organic halides
in palladium catalyzed reactions is R-Cl < R-Br < R-I. These results
reflect the reactivity toward oxidative addition. Various sub-
stituents on aromatic halides can influence on the Mizoroki–Heck
reaction. Therefore, electron-withdrawing substituents (such as
NO2, CH3CO and F) on aromatic halides increase the reaction rate
and electron-donating substituents (such as Me, OMe and NH2) on
aromatic halides decrease the reaction rate [25].
According to experimental results, high reaction rates and yields
were obtained with both activated and non-activated aryl iodides
(Table 2, entries 1–6). Aryl iodides without any substituent and
activated aryl iodides with electron-withdrawing substituents are
reactive substrates in Heck reactions and the related reactions
went to completion in shorter reaction times. As an example,
the reaction of iodobenzene with n-butyl acrylate went to com-
pletion within 5 min giving the desired product in 94% isolated
yield (Table 2, entry 1). Non-activated aryl iodides with electron-
donating substituents (such as Me and OMe) on the aromatic
diffraction pattern of the spectrum (Fig. 4) the face centered cubic
(fcc) crystalline structure was indicated for Pd(0).
The TEM image of the Pdnp/pectin catalyst revealed that the Pd
nanoparticles with near spherical morphology were formed onto
the surface of pectin with relatively good monodispersity. The TEM
image indicates that the size of the palladium particles to be in the
range 2–6 nm (Fig. 5a and b). The palladium content of the pectin
composite was determined by ICP and EDX to be 0.057 mmol/g of
the pectin support.
After characterization of the composite, the catalytic activities
of Pdnp/pectin were examined in the Mizoroki–Heck reaction. The
coupling reaction of iodobenzene with n-butyl acrylate was used
as a model reaction to investigate the catalytic performance of
Pdnp/pectin and the effect of media upon the reaction. The influ-
ence of bases, solvents and reaction temperature on the yield was
first investigated (Table 1). Employing DMSO and DMF as the sol-
vent in the presence of n-Pr3N at 140 ◦C gave 100% conversion of
the starting material after 15 min (Table 1, entries 1 and 2). The
use of other organic solvents (Table 1, entries 3 and 6) or protic
solvents (Table 1, entries 4 and 5) was not beneficial to the pro-
cess. We have also studied the reaction in solvent-free condition
Table 1
Optimization studies for the reaction of iodobenzene with n-butyl acrylate in the presence of Pdnp/pectin.a
O
I
O
Pdnp/Pectin
O
+
O
Solvent, Base
Temprature
Entry
Base
Solvent
T (◦C)
Time
Yield (%)b,c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
n-Pr3N
NaOH
DMSO
DMF
Toluene
EtOH
H2O
140
140
15 min
15 min
15 h
15 h
15 h
15 h
6 h
6 h
100 min
40 min
5 min
15 h
15 h
15 h
92 (100)
90 (100)
5 (11)
32 (38)
45 (53)
4 (10)
Reflux
Reflux
Reflux
Reflux
80
100
120
130
140
140
140
140
140
THF
None
None
None
None
None
None
None
None
None
8 (14)
71 (76)
91 (100)
92 (100)
94 (100)
68 (73)
78 (85)
86 (91)
93 (100)
KOAc
K2CO3
DABCO
80 min
The bold values (entry 11) show our selected conditions for Mizoroki-Heck reaction.
a
Reactions were carried out using 1 mmol of idobenzene, 1.5 mmol of n-butyl acrylate and 1.5 mmol of base in the presence of Pdnp/pectin (0.05 g; 0.0028 mmol of Pd).
Yields are given for isolated products.
The data presented in the parenthesis refer to the conversion of iodobenzene.
b
c

A. Khazaei et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 160–166
163
Table 2
Reaction of aryl halides 1 with n-butyl acrylate catalyzed by Pdnp/pectin.a
O
X
O
Pdnp/Pectin
O
+
O
n-Pr3N, 140 oC
R
R
1
2
Entry
Aryl halide
Product
Time
Yield (%)b
O
O
I
I
O
O
1
5 min
94
1a
2a
2b
2
30 min
92
Me
Me
1b
O
I
O
3
15 min
93
Me
Me
1c
2c
O
I
O
4
20 min
91
MeO
MeO
O2N
1d
2d
O
I
O
5
6
25 min
35 min
92
90
O2N
Me
Me
1e
2e
2f
I
O
O
1f
O
Br
O
O
7
80 min
91
O2N
O2N
MeO
1g
2g
2d
O
Br
8
18 h
57
MeO
1h

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A. Khazaei et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 160–166
Table 2 (Continued)
Entry
Aryl halide
Product
Time
6 h
Yield (%)b
O
Br
Br
N
N
N
O
9
91
N
1i
1j
2h
O
O
10
18 h
74
58
2a
O
Br
O
11
18 h
Me
Me
1k
2c
2i
O
O
Br
O
12
6 h
61
70
N
1l
N
Cl
O
13
18 h
1m
2a
O
Cl
O
14
18 h
58
MeO
MeO
1n
2d
a
Reactions were carried out using 1 mmol of 1 and 1.5 mmol of 2 in the presence of Pdnp/pectin (0.05 g; 0.0028 mmol of Pd) and 1.5 mmol of n-Pr3N as the base under
solvent free conditions.
b
Yields are given for isolated products.
ring are not reactive substrates in Heck reactions and the related
reactions went to completion in longer reaction times (Table 2,
entries 2–4). The observed longer reaction time for the reaction
of 4-iodoanisole in compare with 4-iodotoluen is due to electron-
donating ability of substituents on aromatic rings (Table 2, entries
3 and 4). The results in Table 2 exhibit that the reaction system
was active for the sterically hindered aryl iodides (Table 2, entries
2, 5 and 6). As shown in Table 2, the coupling reaction of butyl
acrylate with o-substituted in compare with p-substituted aryl
iodides performed in longer reaction times (Table 2, entries 2 and
3).
This catalytic system was also applied for the substituted aryl
bromides and hetero aryl bromide giving the desired products in
moderate to high yields (Table 2, entries 7–12). Literature survey
shows that there are only a few reports that have shown aryl chlo-
rides underwent the Mizoroki–Heck coupling reaction giving the
desired products in the present of supported Pd nanoparticles as
a catalyst [26]. Herein we have reported that Pdnp/pectin catalytic
system can catalyze the reaction of aryl chlorides with n-butyl acry-
late under solvent-free conditions and moderate conversions have
been achieved within 18 h reaction times (Table 2, entries 13 and
14).
There are many reports in the literature applying palladium
nanoparticles for the cross coupling Mizoroki–Heck reaction. To
illustrate the catalytic activity of Pdnp/pectin system, we wish to
compare the previously reported systems in Mizoroki–Heck reac-
tion with this system. Bhaumik et al. reported the application of
palladium nanoparticles tethered into mesoporous polymer MPTA-
1 in the Mizoroki–Heck reaction of iodobenzene with n-butyl
acrylate in the presence of K2CO3 as a base in H2O at reflux. Under
these conditions, the reaction was completed within 5 h [27]. Sim-
ilar reaction under our conditions gave the desired product within
5 min in 94% yield (Table 2, entry 1). The synthesis of poly(N-
vinylimidazole) grafted silica-containing palladium nanoparticles
has been reported. This system has been applied as a catalyst
in Mizoroki–Heck reaction of 4-iodoanisole with n-butyl acrylate
using DMF as solvent in the presence of K2CO3 under phosphine-
free conditions. In the presence of this system the reaction was
completed within 1 h [28]. While for the similar reaction, using
the pectin supported palladium nanoparticles the reaction was
completed after 20 min (Table 2, entry 4). In another try, palla-
dium supported on superparamagnetic nanoparticles was used as
a catalyst for Mizoroki–Heck reaction. The reaction of iodobenzene
with n-butyl acrylate in the presence of this catalyst proceeded to

A. Khazaei et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 160–166
165
Fig. 5. TEM image of the Pdnp/pectin catalyst that shows the morphology of Pd nanoparticles in two different positions on the surface (a and b), TEM image of the Pdnp/pectin
catalyst after the first run recycling of the catalyst (c) and after the 6th run of recycling of the catalyst (d).
completion within 5 h in the presence of K2CO3 in NMP as the sol-
vent at 130 ◦C [29], whereas, the similar reaction, as mentioned
above, went to completion within 5 min. Wang and coworkers
reported the application of Pd nanoparticles supported on func-
tionalized mesoporous silica in the Mizoroki–Heck reaction of
iodobenzene with n-butyl acrylate in the presence of Et3N as a
base in DMF at 120 ◦C. Under these conditions, the reaction was
completed within 1 h [30]. Similar reaction under our conditions
gave the desired product within only 5 min in 94% yield (Table 2,
entry 1).
The notable features of Pdnp/pectin system are in situ generation
of nanoparticles without addition of any external reducing agents,
stability toward air and moisture, easy handling and recycle ability.
Remarkably, some of the reactions proceeded in the presence of
Pdnp/pectin system under solvent-free conditions, are among the
fastest ever reported in the literature.
For practical applications of heterogeneous catalysts, the
reusability is a very significant factor. The reusability of the cat-
alyst was checked upon the reaction of iodobenzene with n-butyl
acrylate as the substrates in the presence of the catalyst and n-
Pr3N at 140 ◦C. After completion of the reaction in the first run,
the organic compounds were separated from the reaction mixture
by a simple extraction and the resulting solid mass was reused for
the next run. At the first run, the reaction was completed within
2 min in 94% yield. The nanocatalyst has been recovered for six runs
with some decrease in the catalytic activity of the catalyst. At the
sixth run, the reaction was completed within 20 min in 93% yield
(see Table 3 in supporting information). The amount of Pd leaching
after the 6th run was determined by ICP analysis to be only 11%.
The TEM picture of the recovered nanocatalysts showed that the
morphology and size of the nanoparticles after first and sixth runs
does not change significantly in comparison with the TEM picture
of the fresh catalyst (Fig. 5a–d). It is interesting that the among the
recycling process the shape and size of the nanocatalyst does not
change notably, for example after the sixth run the catalyst still
remained as a dark gray powder.
In order to examine the leaching of palladium from the solid
catalyst, we have used the hot filtration test. For this purpose,
the reaction of 1-bromo-4-nitrobenzene with butyl acrylate in the
presence of Pdnp/pectin and n-Pr3N in DMF as the solvent at 140 ◦C
was studied. After 10 min (the reaction was completed within
95 min in 92% conversion of the starting material and 88% isolated
yield of product), when 20% conversion of 1-bromo-4-nitrobenzene
was occurred the reaction was stopped and catalyst was removed
by hot filtration from the reaction mixture. Then the mixture with-
out the solid catalyst which contains unreacted substrates was
allowed to continue under the same conditions. After 24 h, only
34% of the product was detected by GC. This result suggested that
the leaching of nanoparticles from the solid support was low.
4. Conclusion
In conclusion, a new class of nano-composite, Pdnp/pectin,
was easily obtained by the in situ reduction of PdCl2 under
green conditions without addition of any external reducing agents.
The Pdnp/pectin system exhibited a high activity toward the
Mizoroki–Heck cross-coupling reaction of various aryl halides (I,
Br and Cl) under aerobic and solvent-free conditions. The catalyst
was very stable and could be easily separated from the products
and reused six times without significant loss in the activity.

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