Electrochemical synthesis of Pd nano particles on pencil- graphite and application for suzuki coupling reactions

Article abstract of DOI:10.1007/s10562-015-1571-y

Synthesis of Pd nano particles by electrochemical deposition on pencil graphite has been carried out. Catalytic activity of these nanoparticles was tested for Suzuki coupling reactions. The heterogeneous catalyst was synthesized in 10 s and without use of hazardous reducing reagent. The catalyst showed excellent catalytic performance towards Suzuki cross coupling reaction in aqueous medium. The catalyst was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray analysis, thermo gravimetric analysis, inductively coupled plasma and EDX mapping techniques. Pd nano particles were found to have monodispersed nature with average particle size of 9-10 nm in diameter. The developed catalyst can be recycled up to five cycles without significant decrease in the product yield.

Full text of DOI:10.1007/s10562-015-1571-y

Catal Lett
DOI 10.1007/s10562-015-1571-y
Electrochemical Synthesis of Pd Nano Particles on Pencil-
Graphite and Application for Suzuki Coupling Reactions
1
1
1
Kishor E. Balsane
•
Radheshyam S. Shelkar
•
Jayashree M. Nagarkar
Received: 20 February 2015 / Accepted: 27 June 2015
Ó Springer Science+Business Media New York 2015
Abstract Synthesis of Pd nano particles by electrochem-
ical deposition on pencil graphite has been carried out.
Catalytic activity of these nanoparticles was tested for
Suzuki coupling reactions. The heterogeneous catalyst was
synthesized in 10 s and without use of hazardous reducing
reagent. The catalyst showed excellent catalytic perfor-
mance towards Suzuki cross coupling reaction in aqueous
medium. The catalyst was characterized by scanning
electron microscopy, transmission electron microscopy,
X-ray diffraction, energy dispersive X-ray analysis, thermo
gravimetric analysis, inductively coupled plasma and EDX
mapping techniques. Pd nano particles were found to have
monodispersed nature with average particle size of 9–10 nm
in diameter. The developed catalyst can be recycled up to
five cycles without significant decrease in the product yield.
Graphical Abstract
Keywords Electrochemical Á Palladium nanoparticles Á
Pencil graphite Á Heterogeneous recyclable catalyst Á
Greener media
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1571-y) contains supplementary
material, which is available to authorized users.
&
Jayashree M. Nagarkar
jm.nagarkar@ictmumbai.edu.in;
jayashreenagarkar@yahoo.co.in
1 Introduction
Suzuki cross-coupling reaction is one of the most important
reactions for the synthesis of biaryl moiety present in
pharmaceutical and agrochemical compounds [1–4].
1
Department of Chemistry, Institute of Chemical Technology,
Matunga, Mumbai 400019, India
123

K. E. Balsane et al.
Nowadays, there is an increasing demand for the use of
water as a solvent for heterogeneous, metal catalyzed reac-
tions for environmental benefits and cost and safety purposes
2 Experimental Section
2.1 Synthesis of PdNP–PG Catalyst
[5]. Various palladium based homogeneous catalysts such as
palladium diaminocarbene complexes [6], oxime based
palladacycle [7], chalcogenated Schiff base [8], some
nitrogen based ligands [9], N–H heterocyclic carbene com-
Conventional three electrode cyclic voltammetry poten-
tiostat/galvanostat (PGSTAT302N) equipped with GPES
software was used for electrodeposition on PG for the
synthesis of PdNP–PG. Saturated calomel electrode, plat-
inum rod, and pencil graphite rod were used as the refer-
ence, counter, and working electrodes, respectively. A
5 cm long pencil graphite bar having 3 mm diameter was
cleaned with nitric acid and subsequently with acetone for
removing the surface impurities. Electrodeposition of
PdNPs was carried out by using 5 mmol aqueous solution
plex [10] and morpholine-Pd(OAc) [11] were employed in
2
the Suzuki cross coupling reactions. However, these repor-
ted catalysts could not be recovered and hence, researchers
moved towards heterogeneous supports like alumina based
catalyst [12], MCM-41 [13], MIL-101 [14], Al-MOF [15],
Montmorillonite clay [16], sepiolite clay [17], SBA-15/
CCMet/Pd(II) [18], Pd acetate in imine functionalized silica
gel [19], and nanoporus polymeric ionic liquid loaded with
palladium [20]. Additionally, some carbon allotropes are
also used as support such as carbon nanotube [21], graphene
of PdCl having pH *1. The deposition was carried out at
2
constant potential of -0.2 V at 25 °C for 10 s. PG rod was
removed and air dried. The deposited Pd nano particles
were scraped along with graphite support with the help of
stainless steel blade, dried at 60 °C for 20 min and directly
used as catalyst.
oxide-NH [22], GO supported NHC [23], graphene [24],
2
and Pd-Au bimetallic on graphite support [25]. Recently
some magnetite supported catalysts such as Pd on zinc fer-
rite [26], Fe O @SiO @mSiO –Pd(II) catalyst [27] and Pd-
3
4
2
2
Fe3O4@Alg [28] were also used for the Suzuki reaction.
However, most of the above reported catalysts required
harsh reducing reagents in their synthesis, which eventually
cause agglomeration of nano particles. Our research group is
working on electrochemical synthesis of metal nano parti-
cles like Pd on nafion-graphene as a support [29] and Cu on
nafion graphene ribbon [30]. However, multistep synthesis
of supporting material and use of costly nafion polymer as
an electrolyte are the drawbacks of the above reported
methods. Herein we report a single step synthesis of
monodispersed Pd nano particles (PdNPs) on pencil graphite
2.2 Material Characterization
The chemicals were purchased from Sigma-Aldrich and
S.d.Fine chemicals. The prepared catalyst was character-
ized by various analytical techniques such as by scanning
electron microscopy (SEM), transmission electron micro-
scopy (TEM), X-ray diffraction (XRD), energy dispersive
X-ray analysis (EDAX), thermo gravimetric analysis
(TGA), and inductively coupled plasma (ICP). The mor-
phological information of the catalyst PdNP–PG was
obtained by using scanning electron microscopy (SEM-
EDAX, Quanta 200 at 20 kV as operating voltage) and
transmission electron microscopy (TEM, PHILIPS Mod-
el:CM200, Operating voltages: 20–200 kv Resolution:
(
PG) by using electrochemical process. Low cost, high
surface area and good electrical conductivity are the
advantages of graphite. It plays a dual role; as a working
electrode as well as support. Initially Seyed Habi Tabian and
co-workers worked on mechanism of development of PdNPs
on pencil graphite [31]. Abdul Aziz and et al. showed
application of Pd-Pencil graphite electrode in hydrogen
peroxide sensor [32]. Herein we present simple, efficient
electrochemical method for the synthesis of monodispersed
palladium nano particles on pencil graphite support (PdNP–
PG) in aqueous medium. The PdNP–PG were characterized
by various analytical techniques and applied to Suzuki cross
coupling reactions in water. To our knowledge, this is the
first report on the application of electrochemically synthe-
sized PdNP on PG support for suzuki coupling reaction in
aqueous medium.
˚
2.4 A). The crystallinity of the catalyst was determined by
X-ray diffraction (XRD, Rigaku Miniflex model by using
˚
Cu Ka = 1.54 A with scanning range 0–80) and energy
dispersive X-ray analysis (EDAX). Thermo gravimetric
(TGA) analysis was carried by using Mettler Toledo
instrument under N atmosphere.
2
2.3 General Procedure for Suzuki Coupling
Reaction
Mixture of iodobenzene (0.5 mmol), boronic acid
(0.75 mmol), K CO (1.25 mmol), PdNP–PG catalyst
2 3
We have carried out the synthesis of biaryl compounds
by using aryl halides and various substituted boronic acids
in water as the solvent and K CO as the base at 90 °C
(0.1 mol %), and water as a solvent (2 mL) was stirred at
90 °C for appropriate time as indicated in Table 1 entry 5.
The progress of the reaction was monitored by TLC. The
reaction mass was cooled after completion of the reaction
and the catalyst was separated by filtration. The product
2
3
temperature. The reaction time varies from 10 to 120 min
depending on the nature of the reactant.
1
23

Electrochemical Synthesis of Nano Pd Particles on Pencil-Graphite and Application for Suzuki…
Table 1 Optimization reaction
condition for Suzuki coupling
reaction by using PdNP–PG
Entry
Solvent
Base
Temp (°C)
Catalyst loading (mol%)
Time (min)
Yield
a
(
%)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Water
Water
Water
Water
Water
Water
Water
Water
Water
KOH
90
90
90
90
90
50
25
90
90
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.15
150
120
100
240
10
30
55
60
40
98
78
35
56
98
NaOH
CsCO
3
Na CO
2
3
2
K CO
2
K CO
2
K CO
2
K CO
2
K CO
3
3
3
3
3
60
20 h
120
07
2 3
Reaction condition: Iodobenzene (0.5 mmol), phenyl boronic acid (0.75 mmol), K CO (1.25), water
(
2 mL), catalyst PdNP–PG (0.1 mol%)
a
Isolated yield based on column chromatography
was extracted from aqueous layer by using 10 mL ethyl
acetate. The combined organic layer was dried on anhy-
drous sodium sulphate, concentrated under reduced pres-
sure and was purified by column chromatography on silica
gel (Hexane/EtOAc: 98:2) to get the product.
3
Results and Discussion
3
.1 Catalyst Characterization of PdNP–PG
The typical SEM image of PdNP–PG (Fig. 1) shows the
distribution of Pd nano particles on the surface of graphite
flake. Figure 2 is the TEM image of scraped PG particles
without palladium deposition which shows flake like
structures. Figure 3 indicates the electrodeposited palla-
dium nano particles on pencil graphite. The image shows
highly dispersed Pd-NPs on PG support without any
agglomeration. It also indicates the monodispersed well
separated and spherical Pd-NPs on scraped graphite flake.
The PdNPs size was found in the range of 9–12 nm by
TEM analysis. Similarly the average size of PdNPs was
found to be 9–10 nm with the help of particle size distri-
bution histogram depicted in Fig. 4. The XRD (Fig. 5)
peaks at 26.93°, 44.5°, and 55° are due to graphite. They
matched with hexagonal crystal structure with space group
p63mmc of standard graphite structure. The clay impurity
in pencil graphite having 2h values 20°, 35° are due to
Fig. 1 SEM image of PdNP–PG
SiO . The peaks in the region of 40.2° (111), 46.5° (200)
2
and 68.2° (220) are attributed to the PdNPs. These char-
acteristic reflections indicate the face centered cubic (fcc)
crystalline planes of the of PdNPs. EDAX analysis (Sup-
porting Information Fig. 4) clearly shows the presence of
Pd in the catalyst along with that of O, Al, Mg, and Ca due
to blended clay containing pencil graphite [33, 34]. The
palladium content is determined quantitatively in the
Fig. 2 TEM image of pencil graphite (PG)
123

K. E. Balsane et al.
Fig. 3 TEM image of PdNP–PG
Fig. 6 EDX mapping image of PdNP–PG
PdNP–PG catalyst with the help of inductively coupled
plasma equipped with atomic emission spectrometry (ICP-
AES) and they are amounted to be 0.1 mol%. The thermal
stability of the catalyst was studied by TGA under nitrogen
atmosphere with ramp rate 30°per minute up to 890 °C
(
Supporting Information Fig. 5). TGA curve shows loss of
.65 % of weight in the sample and remaining 95.35 %
was left undecomposed up to 890 °C. The curve shows
.6 % weight loss in the range of 40–140 °C which is due
4
Fig. 4 Particle size histogram
0
to the loss of absorbed water on the surface of the catalyst.
In the temperature range 140–700 °C, the observed weight
loss is 2.5 % which is due to dehydroxylation of the clay in
the pencil graphite material [35]. In the temperature range
7
00–890 °C, the graphite in the material is slowly oxidized
to carbon dioxide due to which the observed weight loss
.55 %. The energy dispersive X-ray elemental mapping
1
for palladium nanoparticles (Fig. 6, red color), they shows
PdNPs are homogeneously distributed on the PG support.
3.2 Catalytic Study
Our first aim was to optimize the reaction conditions for the
model reaction using iodobenzene and phenyl boronic acid
(Table 1 entry 5) and the results are given in Table 1.
Initially we tried the variety of bases, out of which K CO
2
3
was found to be the most suitable base for the model
reaction (Table 1 entries 1–5). This can be explained by
considering the effect of size and solvation of cations on
Fig. 5 XRD of PdNP–PG
1
23

Electrochemical Synthesis of Nano Pd Particles on Pencil-Graphite and Application for Suzuki…
Table 2 Suzuki coupling reactions of aryl halide and aryl boronic acid by using PdNP–PG
a
Entry
Arylhalides
Boronic acid
Products
Time (min)
10
Yield (%)
1
.
X
B(OH)2
98
96
38
X=I
X=Br
X=Cl
2
1
0
20
X
B(OH)2
X=I
X=Br
2
.
H3
C
20
95
92
4
0
CH3
X
B(OH)2
B(OH)2
X=I
X=Br
H3CO
3
.
.
20
45
96
91
OCH3
X
X=I
X=Br
4
H3COC
15
95
85
5
0
COCH3
X
B(OH)2
B(OH)2
X=I
X=Br
5
.
.
H
40
70
91
84
O
OH
X
X=I
X=Br
O2N
6
10
98
96
2
0
NO2
X
B(OH)2
B(OH)2
X=I
7
8
9
1
1
1
.
F
20
20
40
30
40
94
93
90
91
89
F
X
X=I
.
CN
CN
X
B(OH)2
B(OH)2
OH
OH
X=I
.
X
CH3
CH3
X=I
0.
1.
2.
X
F
X=I
F
X
B(OH)2
X=I
25
30
90
94
B(OH)2
X
1
3.
4.
X=I
B(OH)2
X=I
1
X
S
S
20
20
95
96
X
B(OH)2
B(OH)2
1
1
1
1
5.
6.
7.
8.
N
X=I
N
X
X=I
X=Br
Cl
Cl
30
93
89
4
5
Cl
B(OH)2
X
X=I
X=Br
40
91
86
9
0
Cl
X
B(OH)2
X=I
H3CO
Cl
50
92
OCH3
Cl
123

K. E. Balsane et al.
Table 2 Suzuki coupling reactions of aryl halide and aryl boronic acid by using PdNP–PG
a
Entry
Arylhalides
Boronic acid
Products
Time (min)
45
Yield (%)
X
B(OH)2
X=I
1
9.
0.
H3COC
Cl
94
COCH3
X
Cl
B(OH)2
X=I
X=Br
2
O2N
Cl
25
95
90
5
0
NO2
X
Cl
B(OH)2
Cl
Cl
X=I
2
2
1.
2.
45
20
87
96
B(OH)2
X
X=I
COCH3
OCH3
COCH3
B(OH)2
X
X=I
X=Br
2
3.
15
98
94
3
0
OCH3
2 3
Reaction conditions: aryl halide (0.5 mmol), phenyl boronic acid (0.75 mmol), K CO (1.25 mmol), water (2 mL), catalyst catalyst PdNP–PG
(
a
0.1 mol%), temperature 90 °C
Isolated yield based on column chromatography
Table 3 Recyclability study of catalyst for Suzuki cross coupling
reaction using PdNP–PG
a
Entry
Run
Time (min)
Yield (%)
1
2
3
4
5
1st
10
10
10
10
10
98
98
98
97
97
2nd
3rd
4th
5th
Reaction condition: iodobenzene (0.5 mmol), phenyl boronic acid
0.75 mmol), K CO (1.25 mmol), water (2 mL), catalyst catalyst
(
2
3
PdNP–PG (0.1 mol%), temperature 90 °C
a
Isolated yield based on column chromatography
the rate of reaction. The rate of reaction increases with
?
increase in size and solvation of cations. K ion has large
?
size and greater solvation than Na ion. The base effect
Fig. 7 TEM image of PdNP–PG after 5th cycle
PdNP-PG, K CO
also depends on the affinity of counter ion for halide ions.
The reaction is fast for counter ion with high stability
2
3
X
(HO) B
2
Water, 90 °C
?
constant for halide ions. K ion has high stability constant
R1
X= I, Br, Cl
R2
R1
R2
?
and affinity for halide ions than Na ions. Therefore
K CO gave maximum yields when used in Suzuki reac-
2
3
tion. Then we increased the temperature from 25 to 90 and
0 °C was found to be the best temperature for this
transformation. It also gave maximum yield and selectivity
Table 1 entries 5–7). The optimum concentration for
Scheme 1 Suzuki coupling reaction catalyzed by PdNP–PG
9
0.15 mol% Pd content, slightly less reaction time with no
change in the yield was observed (Table 1 entry 9). Hence
we concluded that 0.1 mol% is the optimum concentration
of catalyst loading for this transformation (Table 1 entry
5). Under the optimized reaction conditions, we carried out
substrate study of various aryl halides and different phenyl
(
catalyst loading was found to be 0.1 %. When we
decreased the Pd concentration up to 0.05 mol% the
reaction required more time and gave less yields (Table 1
entry 8). However, when the reaction was carried out with
1
23

Electrochemical Synthesis of Nano Pd Particles on Pencil-Graphite and Application for Suzuki…
Table 4 Comparison of various
Entry
Catalyst
Reaction parameters
Yield (%)
Reference
catalysts and PdNP–PG with
respect to solvent, time and
yield on the formation of diaryl
product
1
2
3
4
5
6
7
8
Nano Pd-PG
H
2
O, 90 °C, 10 min
EtOH–H O, RT, 1 h
O, 60 °C, 1 h
DMF,100 °C, 1 h
98
95
90
80
98
87
96
94
Current work
[14]
Pd@MIL-101
2
Pd-Montmorillonite clay
Pd-sepiolite
H
2
[16]
[17]
SBA-15/CCMet/Pd(II)
2
EtOH–H
EtOH–H
EtOH–H
EtOH–H
2
O, 50 °C, 1 h
2
O, 60 °C, 0.5 h
2
O 80 °C,1 h
2
O 80 °C,1 h
[18]
GO-NH -Pd
2
GO supported NHC-Pd ?
2
[22]
[23]
Pd@Nafion-Graphene
[29]
boronic acids as starting materials. Iodo and bromo sub-
stituted aromatic compounds gave excellent yields where
as chloro substituted aromatic compound gave only 38 %
coupling product (Table 2 entry 1). We tried the reaction of
substituted aryl halides with phenylboronic acid. Aryl
halides with electron withdrawing group such as –NO2
carried out for model reaction (Table 1, entry 5). The
reaction mass was filtered after 5 min in hot condition
and cooled to room temperature. The filtrate was analyzed
using GC which showed 65 % conversion. This filtrate
was heated at 90 °C for another 15 min and again ana-
lyzed by GC. The GC results did not show any increase
in the conversion. This clearly indicates that the catalyst
is very stable and did not leach out during the course of
reaction. The efficiency of PdNP–PG compared with
some previously reported catalyst is demonstrated in
Table 4. The observed result found that PdNP–PG cat-
alytic system for Suzuki reaction with higher yield and
low reaction time.
(
Table 2 entry 6) gave more yield as compared to electron
donating groups like –CH and –OCH (Table 2 entries 2,
3
3
3
). Excellent yields of respective products are also obtained
when the reaction was carried out with ortho and meta
substituted aryl halides (Table 2 entries 9–12). Hetero-
cyclic iodide derivatives also showed good selectivity and
yield with less reaction time (Table 2 entries 14–15).
Substituted boronic acids having electron donating as well
as electron withdrawing groups gave good to excellent
yields of coupling products with aryl halides (Table 2
entries 16–20). We also carried out reaction between
4 Conclusion
2
-chloroboronic acid and iodobenzene which gave com-
We have developed heterogeneous, highly efficient palla-
dium nanoparticles on pencil graphite catlyst. The PdNP–
PG catalyst which is used for the Suzuki cross-coupling
reaction in greener medium. The catalyst can be easily
prepared and reused without any significant loss in the
activity up to 5th cycle. The reaction was done without any
external ligands. The catalyst tolerates withdrawing as well
as donating substituents on aryl halides and different
arylboronic acids.
paratively less yield which may be due to sterically hin-
dered boronic acid (Table 2 entry 21). Boronic acid having
electron donating groups also reacted efficiently with aryl
halides affording excellent product yields (Table 2 entries
2
2–23).
Recyclability of catalyst is an important parameter for
industrial as well as research applications (Table 3). We
examined the recyclability of catalyst for the model reac-
tion (Table 1, entry 5) up to fifth cycle and catalyst showed
excellent efficiency for this coupling reaction. The reaction
mass was filtered after completion of reaction and sepa-
rated catalyst was washed with ethyl acetate and dried at
Acknowledgments The authors are thankful to UGC-SAP, India for
the award of fellowship.
6
0 °C for 30 min. This recovered catalyst was again reused
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