Electrochemical synthesis and characterization of palladium nanoparticles on nafion-graphene support and its application for Suzuki coupling reaction

Article abstract of DOI:10.1016/j.tetlet.2013.04.092

The electrochemically deposited palladium nanoparticles on nafion-graphene support showed an excellent catalytic activity for Suzuki coupling reactions. The as obtained catalyst was characterized by SEM, TEM, EDAX, XRD, and TGA. The particle size of palladium nanoparticles (Pd NPs) determined from TEM was in the range of 4-12 nm. The mean diameter of Pd NPs was found to be 8.1 ± 1.9 nm. The recyclability of the catalyst was examined and it did not show any significant loss of catalytic activity for five consecutive cycles.

Full text of DOI:10.1016/j.tetlet.2013.04.092

Tetrahedron Letters 54 (2013) 3457–3461
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Electrochemical synthesis and characterization of palladium nanoparticles on
nafion–graphene support and its application for Suzuki coupling reaction
⇑
Suresh S. Shendage, Umakant B. Patil, Jayashree M. Nagarkar
Department of Chemistry, Institute of Chemical Technology (Deemed University), Nathalal Parekh Marg, Matunga (E), Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemically deposited palladium nanoparticles on nafion–graphene support showed an excel-
lent catalytic activity for Suzuki coupling reactions. The as obtained catalyst was characterized by SEM,
TEM, EDAX, XRD, and TGA. The particle size of palladium nanoparticles (Pd NPs) determined from TEM
was in the range of 4–12 nm. The mean diameter of Pd NPs was found to be 8.1 1.9 nm. The recyclability
of the catalyst was examined and it did not show any significant loss of catalytic activity for five consec-
utive cycles.
Received 4 April 2013
Revised 18 April 2013
Accepted 20 April 2013
Available online 27 April 2013
Keywords:
Electrochemical
Ó 2013 Elsevier Ltd. All rights reserved.
Palladium nanoparticles
Nafion–graphene
Heterogeneous catalyst
Palladium-catalyzed Suzuki cross-coupling reaction of aryl ha-
lides with aryl boronic derivatives is a very suitable and exten-
sively studied approach to C–C bond formation. This reaction is
important because biaryl moiety is present in natural products,
pharmaceuticals, and herbicides.1–4 Metallic palladium and palla-
dium complexes are widely used as catalysts for coupling reactions
because of high activity and selectivity.5–8
However, separation of the catalyst from the product is always a
challenging task. A heterogeneous catalyst in such reactions finds
an attractive solution to these problems as the Pd catalyst sup-
ported on microporous polymers,9 carbon materials,10 amorphous
or mesoporous silica,11 inorganic material,12 molecular sieve-
s,13and zinc ferrite14 can be easily separated from the product
and successfully reused. It is well known that support material
plays a very important role in catalytic activity. Recently graphene
has attracted much attention because of its unique properties such
as, large specific surface area,15 and thermal and electrical conduc-
tivity.16 Graphene, has exhibited huge application potential in dif-
ferent fields, such as electronic devices17 nanocomposite
materials,18 and fuel cells.19
because they are not aggressive to the environment. They demon-
strate high reproducibility, simple methodology and can be used at
ambient temperature. Besides these advantages, electrochemical
processes are of heterogeneous nature which allows easy recovery
of products. The other advantages are short reaction time, simple
operation, no side reaction, and purity of product. The catalytic
activity of metal nanoparticles (NPs) very much depends on their
sizes, shapes, and method of preparation.21 Most of the previous
Letters demonstrate the electrochemical preparation of Pd NPs
and their application as catalysts for Suzuki coupling reaction.22,23
Moreover, in spite of the progress offered by the electrochemi-
cal synthesis of NPs, the use of electrochemically prepared Pd NPs
as catalysts in coupling reactions is scanty.24 Similarly, little
However, growing world population with increasing industrial
demands has led to the situation where the protection of the envi-
ronment has become a major concern and crucial factor for the fu-
ture growth of industrial processes. Electrochemical methods are
suitable for the preparation of metal nanoparticles because size
and morphology of nanoparticles can be controlled by changing
electrochemical conditions.20 These methods are generally chosen
⇑
Corresponding author. Tel.: +91 22 3361 1111/2222; fax: +91 22 3361 1020.
E-mail address: jm.nagarkar@ictmumbai.edu.in (J.M. Nagarkar).
Figure 1a. SEM image of Pd/Nf–G composite on platinum plate.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.092

3458
S. S. Shendage et al. / Tetrahedron Letters 54 (2013) 3457–3461
solution at constant potential and their application as heteroge-
neous catalysts for Suzuki coupling reaction. Nafion is thermally
and chemically stable in the operating temperature of the reaction.
Here nafion is used to disperse graphene and stabilize it on the
electrode surface.
To the best of our knowledge electrochemically deposited palla-
dium NPs on Nf–G support has not been reported earlier for Suzuki
coupling reaction. The as-prepared catalyst shows excellent activ-
ity and good recyclability for Suzuki coupling reaction.
Graphene was prepared by chemical reduction (hydrazine hy-
drate as a reducing agent) of graphene oxide (GO). GO was pre-
pared by modified Hummers method (Supplementary data). The
catalyst Pd/Nf–G composite was prepared by electrochemical
Figure 1b. TEM image of Pd/Nf–G composite.
attention has been given toward electrochemical deposition of Pd
NPs on different supporting material and its applications for Suzuki
coupling reaction. We herein, report the electrochemical deposi-
tion of Pd NPs on Nafion–graphene (Pd/Nf–G) from aqueous
Figure 2c. TGA curve of Pd/Nf–G composite.
Figure 2a. XRD pattern of Pd/Nf–G composite.
Figure 2b. EDAX of Pd/Nf–G composite.

S. S. Shendage et al. / Tetrahedron Letters 54 (2013) 3457–3461
3459
X
B(OH)2
Pd/Nf-G, K2CO3
R1
R2
+
R1
Ethanol/H2O, 80oC
R2
R1=CH3, OCH3, NO2, COCH3, OH, H
R2=CH3, OCH3, Cl, H
X=I, Br
Scheme 1. Suzuki coupling reaction of aryl halides with phenylboronic acid.
urated calomel electrode, platinum rod, and Nf–G modified
platinum plate were the reference, counter, and working elec-
trodes respectively (Supplementary data).
The morphology and formation of Pd NPs on Nf–G support was
confirmed by analytical techniques such as scanning electron
microscopy-energy dispersive X-ray spectroscopy (SEM-EDAX,
Quanta-200 at 20 kv as operating voltage), XRD (Rigaku Miniflex
Table 1
Optimization of reaction parameters for Suzuki reaction catalyzed by Pd/Nf–Ga
Entry Solvent
Base
Ethanol/H2O K2CO3
Ethanol/H2O Na2CO3 0.3
Pd loading (mol %) Time (h) Yieldb (%)
1
2
3
4
0.3
1
1
94
93
80
55
37
90
52
96
94
94
80
Ethanol/H2O K3PO4
Ethanol/H2O Cs2CO3
0.3
0.3
0.3
0.3
0.3
0.5
0.4
0.3
0.2
1
1
1
1
24
1
1
1
1
5
Water
Ethanol
K2CO3
K2CO3
model by using CuKa = 1.54 Å with scanning range 0–50°), trans-
6
mission electron microscopy (TEM, PHILIPS Model: CM200, Oper-
ating voltages: 20–200 kv Resolution: 2.4 Å), thermogravimetric
analysis (TGA).
Figure 1a displays the SEM image of Pd/Nf–G composite depos-
ited on platinum plate. Figure 1a shows well dispersed spherical Pd
NPs. The TEM image shows the presence of uniform well-dispersed
Pd nanoparticles on Nf–G support. The particle size distribution
histogram of Pd NPs is shown in Fig. S4 Supplementary data. Par-
ticle size of Pd NPs ranges from 4 nm to 12 nm (Fig. 1b). The aver-
7c
9
Ethanol/H2O K2CO3
Ethanol/H2O K2CO3
Ethanol/H2O K2CO3
Ethanol/H2O K2CO3
Ethanol/H2O K2CO3
10
11
12
a
Reaction condition: iodobenzene (1 mmol), phenyl boronic acid (1.5 mmol),
base (3 mmol), solvent (5 mL), catalyst Pd/Nf–G. Temp 80 °C.
Isolated yield based on column chromatography.
b
deposition of Pd NPs on Nf–G film from aqueous solution contain-
ing palladium salt at room temperature at À0.2 V constant poten-
tial. The conventional three electrode cyclic voltammetry
PGSTAT302N (potentiostat/galvanostat) equipped with GPES soft-
ware was employed for electrochemical deposition of Pd NPs. Sat-
age particle size was found to be 8.1
1.9 nm. EDAX analysis
(Fig. 2b) evidently shows the presence of Pd in the sample and that
the sample consists mostly of carbon with oxygen most likely due
to the presence of a few unreduced oxygen functional groups. It
also shows the presence of fluorine and sulfur indicating the exis-
Table 2
Pd/Nf–G catalyzed Suzuki reaction of various aryl halides with phenylboronic acida
Entry
1
Aryl halide
Boronic acid
Product(s)
Time (h)
1
Yieldb (%)
94
I
B(OH)2
Br
I
B(OH)2
B(OH)2
2
3
3
1
88
89
I
I
B(OH)2
Cl
4
1
92
93
Cl
B(OH)2
H3CO
5
6
1.5
1.5
OCH3
I
B(OH)2
93
(continued on next page)

3460
S. S. Shendage et al. / Tetrahedron Letters 54 (2013) 3457–3461
Table 2 (continued)
Entry
Aryl halide
Boronic acid
Product(s)
Time (h)
1.5
Yieldb (%)
I
B(OH)2
H3CO
7
8
9
94
OCH3
I
B(OH)2
B(OH)2
HO
1.5
1.5
93
95
OH
I
O2N
NO 2
I
B(OH)2
H3COC
10
1.5
92
COCH3
I
B(OH)2
B(OH)2
11
12
1
3
92
89
OCH3
Br
Br
B(OH)2
B(OH)2
O2N
13
3
4
90
90
NO2
B(OH)2
14
a
Reaction condition: aryl halide (1 mmol), phenyl boronic acid (1.5 mmol), base (3 mmol), solvent (5 mL), catalyst Pd/Nf–G (1 mg). Temp 80 °C.
Isolated yield based on column chromatography.
b
tence of nafion in Pd/Nf–G composite. Figure 2a displays the XRD
pared Pd/Nf–G composite as heterogeneous catalyst (Scheme 1).
The reaction was optimized for parameters such as, solvent, cata-
lyst loading, and base. Solvent: ethanol/water (1:1), catalyst load-
ing (1 mg), and base: K2CO3 at 80 °C temperature were the
optimized reaction conditions26 (See Table 1).
The applicability of the prepared Pd/Nf–G catalyst for Suzuki
coupling reaction of the different aryl halides and boronic acids un-
der optimized conditions (Table 2, entries 1–14) was studied. The
electron donating and electron withdrawing aryl iodides and phe-
nyl boronic acids gave excellent yields of corresponding products
(Table 2, entries 6–10). However, longer reaction time was re-
quired for substituted aryl bromide but afforded excellent yield
of the products (Table 2, entries 2, 12–14).
The reusability of the Pd/Nf–G catalyst was tested for the reac-
tion of iodobenzene and phenylboronic acid. The catalyst was sep-
arated from the reaction mixture by decantation, washed with
ethyl acetate and dried at room temperature. It was then re-used
in a subsequent run. It was found that the Pd/Nf–G catalyst could
be recycled for five times without losing the catalytic activity
(Table 3). The activity of the catalyst only dropped slightly in 5th
pattern of Pd/Nf–G composite which show characteristic peak of
nafion at 16.5°. A broad peak between 24° and 28° corresponds
to graphene.25 The other two peaks at 40.1° and 46.8° are the char-
acteristic peaks of palladium. We have also examined the thermal
stability of the prepared Pd/Nf–G composite under nitrogen atmo-
sphere using thermal gravimetric analysis (TGA). As shown in Fig-
ure 2c, the Pd/Nf–G exhibits initial weight loss below 200 °C which
may be due to moisture loss. Next peak from 300 to 600 °C is
attributed to the decomposition of the –SO3H groups and degrada-
tion of the polymer main chain. Pd–graphene shows much higher
thermal stability with much less mass loss up to 800 °C. The mass
loss of Pd graphene is attributed to the existence of a few oxygen
functional groups since it is well known that chemical reduction
of GO results in thermal degradation pattern similar to that of
GO. The amount of Pd in the catalyst (Pd/Nf–G) measured by
inductively coupled plasma atomic emission spectrometry (ICP-
AES) was 32 wt.%.
Initially we have carried out the Suzuki reaction of iodobenzene
with phenylboronic acid in the presence of electrochemically pre-

S. S. Shendage et al. / Tetrahedron Letters 54 (2013) 3457–3461
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94
2nd
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3rd
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4th
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5th
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Yieldb (%)
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a
Reaction condition: iodobenzene (1 mmol), phenyl boronic acid (1.5 mmol),
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Isolated yield based on column chromatography.
b
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run. The amount of Pd lost after 5th cycle was found to be 0.4 wt.%
as determined by ICP-AES. This clearly indicates the high catalytic
activity and stability of P/Nf–G.
In conclusion we have fabricated a Pd/Nf–G catalyst by electro-
chemical deposition of palladium on Nf–G support from aqueous
solution containing palladium ions. The catalyst was characterized
by various techniques. This was further used as an excellent cata-
lyst for the Suzuki–Miyaura coupling reactions. The high catalytic
activity was attributed to the small size and uniform dispersion
of Pd NPs in ethanol/water (1:1 v/v) solvent. In addition it can be
recycled for five cycles without much loss in catalytic activity.
21. Guo, D. J.; Li, H. L. Electrochem. Commun. 2004, 6, 999.
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Acknowledgement
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26. General procedure for Suzuki reactions:
The authors are thankful to UGC New Delhi, India for the award
of Faculty improvement programme (FIP).
Supplementary data
Chemicals:
Aryl bromide and chloride, potassium carbonate, aryl substituted boronic acid,
and olefins were all purchased from Aldrich and used as received. A mixture of
ethanol/deionized water was used for the Suzuki coupling reactions.
The weighed amount of catalyst (Pd/Nf–G) is placed in a 25 mL round bottom
flask and 5 mL of mixture of H2O:EtOH (1:1) as a solvent was added and
ultrasonicated for 120 s in order to disperse the catalyst. Then aryl halide
(1 mmol,), aryl boronic acid (1.5 mmol,), and potassium carbonate (3 mmol),
were added to the above solution. Then this mixture was heated at a certain
temperature and time which is indicated in Table 2. Upon the completion of
the reaction period, the reaction mixture was cooled and allowed to stand for
20 min. The catalyst was separated by decantation and washed with ethyl
acetate and dried in air, which was used for next run. The organic layers were
combined, dried over anhydrous Na2SO4, and filtered. The solvent in the filtrate
was then removed in vacuum to give a solid. The pure products were obtained
by column chromatography using hexane/ethyl acetate (70:30) as the eluent or
by washing the solid products. The products were characterized by GC–MS and
NMR spectroscopy (Supplementary data).
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
04.092.
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