Green synthesis of palladium nanoparticles using Hippophae rhamnoides Linn leaf extract and their catalytic activity for the Suzuki-Miyaura coupling in water

Article abstract of DOI:10.1016/j.molcata.2014.10.019

During this study, we report the green synthesis of palladium nanoparticles using Hippophae rhamnoides Linn leaf extract and their application as heterogeneous catalysts for the Suzuki-Miyaura coupling in water. The synthesized nanoparticles are characterized by XRD, SEM, TEM and UV-vis techniques. This method has the advantages of high yields, simple methodology, and elimination of ligand, organic solvent and homogeneous catalysts and easy work-up. Furthermore, the catalyst exhibits high catalytic activity, superior cycling stability and excellent substrate applicability.

Full text of DOI:10.1016/j.molcata.2014.10.019

Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Green synthesis of palladium nanoparticles using Hippophae
rhamnoides Linn leaf extract and their catalytic activity for the
Suzuki–Miyaura coupling in water
Mahmoud Nasrollahzadeha,∗, S. Mohammad Sajadib, Mehdi Mahamc
a Department of Chemistry, Faculty of science, University of Qom, P.O. Box 37185-359, Qom, Iran
b Department of Petroleum Geoscience, Faculty of Science, Soran University, P.O. Box 624, Soran, Kurdistan Regional Government, Iraq
c Department of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
During this study, we report the green synthesis of palladium nanoparticles using Hippophae rhamnoides
Linn leaf extract and their application as heterogeneous catalysts for the Suzuki–Miyaura coupling in
water. The synthesized nanoparticles are characterized by XRD, SEM, TEM and UV–vis techniques. This
method has the advantages of high yields, simple methodology, and elimination of ligand, organic solvent
and homogeneous catalysts and easy work-up. Furthermore, the catalyst exhibits high catalytic activity,
superior cycling stability and excellent substrate applicability.
Received 16 September 2014
Received in revised form 15 October 2014
Accepted 16 October 2014
Available online 28 October 2014
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Green synthesis
Hippophae rhamnoides Linn
Palladium nanoparticles
Suzuki–Miyaura
Water
1. Introduction
developed. The recovery and reusability of catalysts is a very impor-
tant factor especially in the case of noble metal (Pd) catalyzed C
C
The Suzuki–Miyaura coupling reaction catalyzed by palladium
(Pd) catalysts in the presence of phosphine ligands is one of the
most powerful tools for selective C C bond formation for the
construction of biaryl skeletons, which are often considered as par-
tial structures in pharmaceuticals, natural products, polymers and
material science [1–3].
However, there are several disadvantages in the literature meth-
ods such as the use of toxic, expensive and sensitive to air or
moisture ligands and toxic organic solvents, low yields, long reac-
tion times, harsh reaction conditions, environmental pollution due
to the side products formed, use of homogeneous catalysts and
tedious work-up [1–3].
Among various palladium catalysts for the Suzuki–Miyaura
coupling reactions, homogeneous catalysts have been widely inves-
tigated, while less expensive heterogeneous catalysts received
scanter attention [1–3]. Homogeneous Pd catalysts are not used
in industrial application due to the difficulty in separating and
recycling, low catalytic efficiency, and the high leaching of metal
species. In order to solve the problem, new strategies should be
coupling reactions like the Suzuki–Miyaura cross coupling reac-
tions, and heterogeneous systems also dispense with the need for
the design and synthesis of often expensive ligands that feature in
homogeneous systems.
The focus today is to make the experimental conditions more
environmentally friendly, e.g. in water and in the presence of
ligand-free catalysts in very small amounts like nanoparticles
(NPs). It is well-known that nano-sized metal particles have
a high surface-to-volume ratio that can dramatically enhance
the interaction between reactants and catalysts [4]. Addition-
ally, nanocatalysis can make the products easily removable from
the reaction mixtures and make the catalysts recyclable. Thus
nanocatalysis is a promising alternative to the homogeneous catal-
ysis to afford the same products with high reaction rates and high
yields.
There are several methods for the synthesis of palladium
nanoparticles using toxic and expensive chemicals. In recent years,
green synthesis of nanoparticles has received much attention due
to the growing need to develop environmentally benign technol-
ogy in nanoparticles synthesis. In addition, the other advantages
of this environmentally benign and safe protocol include a simple
reaction setup, very mild reaction conditions, use of nontoxic sol-
vents such as water, elimination of toxic and dangerous materials
∗
Corresponding author. Tel.: +98 25 32850953; fax: +98 25 32103595.
E-mail address: mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
http://dx.doi.org/10.1016/j.molcata.2014.10.019
1381-1169/© 2014 Elsevier B.V. All rights reserved.

298
M. Nasrollahzadeh et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
a Philips powder diffractometer type PW 1373 goniometer (Cu
◦
˚
K␣ = 1.5406 A). The scanning rate was 2 /min in the 2ꢀ range from
10 to 80◦. Scanning electron microscopy (SEM) was performed on
a Cam scan MV2300. EDS (S3700N) was utilized for chemical anal-
ysis of prepared nanostructures. The shape and size of palladium
nanoparticles crystals were identified by transmission electron
microscope (TEM) using a Philips EM208 microscope operating
at an accelerating voltage of 90 kV. UV–vis spectral analysis was
recorded on a double-beam spectrophotometer (Hitachi, U-2900)
to ensure the formation of nanoparticles.
2.1. Preparation of extract of the leaves of Hippophae rhamnoides
Linn
Leaves of the Hippophae rhamnoides Linn were collected in June
2013 in sarshive region of Kurdistan province in Iran. 100 g of dried
leaves powdered of sea buckthorn were kept into 1000-mL conical
flask containing 500 mL hydroalcoholic solution (30% methanol),
well mixed and then boiled for 30 min. The extract obtained was
centrifuged in 6500 rpm then filtered through a filter paper and
filtrate was kept at refrigerator to use further. Furthermore, HPLC
analysis was used to confirm the presence of antioxidant flavonoids
inside the leaves of the plant.
Fig. 1. Image of Hippophae rhamnoides Linn.
and cost effectiveness as well as compatibility for biomedical and
pharmaceutical applications [5]. Also, in this method there is no
need to use high pressure, energy, temperature and toxic chemi-
cals. Despite the availability of methods for the green synthesis of
Pd NPs by various plants or gums the potential of plants as biolog-
ical materials for the synthesis of nanoparticles is yet to be fully
explored [6,7].
Hippophae rhamnoides Linn (Sea buckthorn) belonging to the
family of Elaeagnaceae is one of the large distributed native plants
of Iran (Fig. 1). The famous of the plant is for its very thorny shrub
which normally grows between 2 and 4 m tall. The fruits, seeds,
berries and leaves of the plant are a rich source of important antiox-
idants such as vitamin C, beta carotene, vitamin E, sterols including
beta sitosterol, stanols, superoxide dismutase (SOD), flavonoids as
aglycones and glycosides, catechins and elagic acid, also significant
values of calcium, magnesium and potassium [8–11].
We recently published green synthesis of Au/Pd bimetallic NPs
and Pd/Fe3O4 NPs using Euphorbia condylocarpa M. bieb root extract
[12,13]. Due to our ongoing interest on the green chemistry and
natural compounds [14-24], we wish to report a new and rapid pro-
tocol for preparation of Pd NPs by using Hippophae rhamnoides Linn
leaf extract (Fig. 1) as a reducing and stabilizing agent. Pd NPs were
characterized by SEM, TEM and UV–vis techniques. Also, the cat-
alytic activity of Pd NPs for the Suzuki–Miyaura coupling in water
was studied (Scheme 1).
2.2. Green synthesis of palladium nanoparticles using Hippophae
rhamnoides Linn leaves extract
In a typical synthesis of Pd NPs, 10 mL extract of the plant leaves
was added dropwise to 50 mL of 0.003 M aqueous solution of PdCl2
with constant stirring at 80 ◦C. Reduction of palladium ions (PdII)
to palladium (Pdo) was completed around 25 min using monitor-
ing by UV–vis and FT-IR spectra of the solution. The color of the
reaction mixtures gradually changed from transparent yellow to
dark brown in 25 min at 80 ◦C indicating the formation of palladium
nanoparticles, then the colored solution of palladium nanopar-
ticles was centrifuged at 7000 rpm for 30 min to completely
dispersing.
2.3. General procedure for Suzuki–Miyaura coupling reaction
To a mixture of aryl iodide (1.0 mmol), phenylboronic acid
(1.2 mmol), and potassium hydroxide (2.0 mmol), 10 mL of the Pd
NPs solution (1.0 mol%) in water was added. The reaction mixture
was then stirred at 100 ◦C for 12 h. The progress of the reaction was
monitored using TLC. After completion of the reaction, the reac-
tion mixture was cooled to room temperature and extracted with
EtOAc three times. The combined extracts were dried over anhy-
drous MgSO4 and the solvents were removed under vacuum. The
crude products were then purified by flash chromatography on sil-
ica gel to give the desired coupling products and characterized by
1H NMR. All the products are known compounds and the spectral
data and melting points were identical to those reported in the
literature [1–3].
2. Experimental
High-purity chemical reagents were purchased from the
Merck and Aldrich chemical companies. All materials were of
commercial reagent grade. Melting points were determined in
open capillaries using a BUCHI 510 melting point apparatus and
are uncorrected. 1H NMR and 13C NMR spectra were recorded
on a Bruker Avance DRX-400 spectrometer at 400 and 100 MHz,
respectively. FT-IR spectra were recorded on a Nicolet 370 FT/IR
spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. The
element analyses (C, H, N) were obtained from a Carlo ERBA Model
EA 1108 analyzer carried out on Perkin-Elmer 240c analyzer.
X-ray diffraction (XRD) measurements were carried out using
3. Results and discussion
In this paper, we develop an ecofriendly, clean, non-toxic, facile
chemically preparative method, for the generation of Pd NPs using
the extract of the leaves of Hippophae rhamnoides Linn, acting as
reducing as well as stabilizing agent. To date, there is no report
on the biosynthesis of Pd NPs by utilizing the leaves extract of
Hippophae rhamnoides Linn.
Scheme 1. Suzuki–Miyaura coupling reaction of different aryl halides in water.

M. Nasrollahzadeh et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
299
nFlO (radical) + nPd0
nFlOX + nPd0 (Nucleation)
nFlOH + Pd+2
nFlO (Radical) + Pd+2
nPd0 + Pd+2
Pdn+2 (Growth)
+2
Pdn+2 + Pdn
+2n
Pd2n
(Pd2n+2n)n + (FlOH)n
Palladium NZV
Scheme 2. Reducing ability of antioxidant phenolics to produce Pd NPs where FlOH
and NZV are flavonoid and nano zero valent, respectively.
Fig. 2. UV–vis spectrum of the hydroalcoholic extracts of the leaves of Hippophae
rhamnoides Linn.
3.1. Characterization of extract of the leaves of Hippophae
rhamnoides Linn
The UV spectrum of the extract of the leaves of Hippophae rham-
noides Linn (Fig. 2) shows bonds at ꢁmax 285 nm (bond I) due to the
transition localized within the ring of cinnamoyl system; whereas
the one centered at 245 nm (bond II) is for absorbance of ring related
to the benzoyl system. They are related to the ␲ → ␲* transitions
and these absorbent bonds demonstrate the presence of polyphe-
nolics in the category of flavonoids.
The FT-IR analysis was carried out to identify the possible
biomolecules responsible for the reduction of Pd nanoparticles and
capping of the bioreduced Pd nanoparticles synthesized by the
leaves extract. The FT-IR spectrum of the crude extract (Fig. 3)
depicted some peaks at 3375 to 3000, 1722 and 1674, 1522, 1396
and 1122 cm−1 which represent free OH in molecule and OH group
Fig. 4. UV–vis spectrum of green synthesized Pd NPs using the extract of the leaves
of Hippophae rhamnoides Linn.
extract of the leaves of Hippophae rhamnoides Linn as a reducing and
stabilizing agent for the synthesis of Pd NPs. As shown in Scheme 2,
metal ions were reduced to nano zero valent (NZV) metallic parti-
cles using flavonoid (FlOH) contents of the plant as green (without
any hazardous impact on the environment) chemicals (reducing
agents) as an alternative to the commonly used hydride reducing
agent, sodium borohydride.
forming hydrogen bonds, carbonyl group (C O), stretching C
C
aromatic ring and C OH stretching vibrations, respectively. These
peaks suggested the presence of flavonoid and other phenolics in
the plant leaves extract.
The progression of the reaction, formation and stability of Pd
nanoparticles were controlled by UV–vis spectroscopy. The UV–vis
spectrum of green synthesized Pd NPs using Hippophae rham-
noides Linn leaves extract (Fig. 4) showed the significant changes
in the absorbance maxima due to surface Plasmon resonance
3.2. Reduction of metal ions
Reduction kinetics plays a key role in controlling the nuclea-
tion and growth of nanoparticles. In this work, we have employed
Fig. 3. FT-IR spectrum of the hydroalcoholic extracts of the leaves of Hippophae rhamnoides Linn.

300
M. Nasrollahzadeh et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
Table 1
demonstrating the formation of Pd NPs. Also, the yellow color of
the PdII solution (ꢁmax 415 nm) immediately changed into dark
brown (ꢁmax 280–320 nm) indicating reduction of PdII to Pdo and
formation of Pd NPs as characterized by UV–vis spectrum. The
synthesized palladium nanoparticles by the this method are quite
stable and no obvious variance in the shape, position and symmetry
of the absorption peak is observed even after 1 month indicating
the stability of Pd NPs.
The presence of flavonoid and other phenolics in the extract
could be responsible for the reduction of metal ions and formation
of the corresponding metal nanoparticles. Furthermore, the FT-IR of
extract after adding PdCl2 while formation of Pd NPs shows demon-
strative differences in the shape and location of signals indicating
the interaction between PdCl2 and involved sites of phytochem-
icals for production of nanoparticles (Fig. 5). The peaks at 3387
to 3000, 1714, 1462, 1381 and 1074 cm−1 represent the OH func-
tional groups, carbonyl group (C O), stretching C C aromatic ring
and C OH stretching vibrations, respectively. Polyphenolics could
be adsorbed on the surface of metal nanoparticles, possibly by
interaction through ␲-electrons interaction in the absence of other
strong ligating agents. In fact the ␲-electrons of carbonyl group
(C O) from C ring of flavonoids in a Red/Ox system can trans-
fer to the free orbital of metal ion and convert that to the free
metal.
Results of HPLC-DAD analysis of the leaves of Hippophae rhamnoides Linn.
HPLC gradient program
T/min
Acetonitrile %
0–2
2–14
15
15–25
25
25–60
60
60–90
90
14–19
19–24
24–28
28–30
30–35
35–40
40–50
90–15
15
3.3. Instrumentation and measurement protocols
The RP-HPLC-DAD instrument consisted of auto injector, sample
cooler, pumps, and column oven and diode array detector. An ODS
5␮ (3) column (250 mm × 4.60 mm, particle size 5 ␮m) was applied.
The effluent consisted of a mixture of water–tetrahydrofuran
(THF)–trifluoroacetic acid (TFA) (98:2:0.1) (solvent A) and acetoni-
trile (solvent B). The gradient profile used in glycoside analysis
is presented in Table 1. Flow rate of the effluent was 1 mL/min,
the detector wavelengths 270 nm and 370 nm and the volume
Fig. 5. FT-IR spectrum of green synthesized Pd NPs using the extract of the leaves of Hippophae rhamnoides Linn.
Fig. 6. HPLC-DAD chromatogram of the leaves extract of Hippophae rhamnoides Linn.

M. Nasrollahzadeh et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
301
45
40
35
30
25
20
15
10
5
Table 2
Optimization of reaction conditions in Suzuki–Miyaura coupling reaction of
iodobenzene with phenylboronic acid.a
Pd NPs
I
B(OH)2
+
Base, H2O
.
Entry
Pd NPs (mol%)
Base
Temperature (◦C)
Yield (%)b
1
2
3
4
5
6
7
8
9
0.0
1.0
1.0
1.0
1.0
0.5
2.0
1.0
1.0
1.0
KOH
KOH
Na2CO3
NaOH
K2CO3
KOH
KOH
KOH
KOH
KOH
100
100
100
100
100
100
100
80
0.0
95
83
92
85
49
95
89
73
35
0
30
40
50
60
70
80
90
Fig. 7. XRD pattern of Pd nanoparticles.
60
25
10
of injection 10 ␮L. Identification was based on co-injections of
authentic compounds and comparisons of absorption spectra. All
solvents were HPLC-grade and chemicals were analytical grade.
The HPLC chromatogram of the hydroalcoholic leaves extract
of the plant showed the qualitative presence of the powerful
antioxidant flavonoids and demonstrated that this plant is a sig-
nificant source to produce the nanoparticles. Glycosides A to E
such as isorhamnetin 3-sophoroside-7-rhamnoside, quercetin 3-O-
rutinoside, quercetin 3-O-glucoside, isorhamnetin 3-O-rutinoside
and isorhamnetin 3-O-glucoside respectively characterized in the
leaves of the plant according to the chromatogram (Fig. 6) are of
great importance for their antioxidant activity.
a
Reaction condition: 1.0 equiv of iodobenzene, 1.2 equiv of phenylboronic acid
and 2.0 equiv of base, 12 h.
b
Yields are after work-up.
3.5. Evaluation of the catalytic activity of Pd NPs through the
Suzuki–Miyaura coupling reaction
In most transition metal catalyzed reactions, ligands play a key
role. Various ligands such as phosphine and nonphosphine used in
palladium catalyzed coupling reactions are described in the litera-
ture. Most of these ligands are air and moisture sensitive, difficult
to prepare, and expensive. Thus, catalysis under ligand-free condi-
tions is an area of high importance. In the next step, we tested the
catalytic activity of the Pd NPs for the Suzuki–Miyaura coupling
reaction in water under ligand-free conditions.
3.4. Characterization of Pd NPs
Pd NPs was characterized using the UV–vis, XRD, SEM and TEM.
The crystalline nature of Pd NPs was confirmed by powder X-ray
diffraction (XRD) study (Fig. 7). XRD studies confirmed the presence
of Pd NPs which was coherent with previous reports [13]. The XRD
analysis of NPs dried powders indicated a crystalline structure with
peaks at 40.13◦, 46.6◦, 50.4◦, 58.01◦ and 66.5◦, values found also by
other authors in Pd NPs synthesized in the presence of a variety of
stabilizing agents [13]. The average particle size was derived using
Scherrer equation, in which the particle size on average is 10 nm.
The size and shape of the products were examined by scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM). Fig. 8 shows a typical SEM image of the as-produced palla-
dium nanosphere. The products are of spherical morphology and
have very narrow diameter distributions. Fig. 9 shows TEM images
of the palladium nanosphere. The particle sizes distributed in the
range of 2.5–14 nm, with an average particle size of 5 2.5 nm syn-
thesized.
We initially selected iodobenzene and phenylboronic acid as a
model reaction. Various temperatures, bases and different amounts
of catalyst were screened in order to establish ideal coupling con-
ditions. As expected, no target product could be detected in the
absence of catalyst (Table 2, entry 1). However, addition of the cat-
alyst to the mixture has rapidly increased the synthesis of product
in high yields. The results indicated that base had a remark effect
on the yield of product. Among the various bases (K2CO3, NaOH,
Na2CO3 and KOH) tested in the presence of Pd NPs in water, KOH led
to significant conversion (Table 2, entry 2). In addition, in order to
optimize the yields, different amounts of Pd NPs were used for the
synthesis of biphenyl from the reaction of iodobenzene (1.0 mmol)
with phenylboronic acid (1.2 mmol) and best result was obtained
with 1.0 mol% of Pd NPs and 2.0 mmol of KOH in water at 100 ◦C,
which gave the product in an excellent yield (Table 2, entry 2). No
surfactant or ligand was required. A decrease in the catalyst loading
from 1.0 to 0.5 mol% afforded the product in lower yield (Table 2,
entry 6). No significant improvement on the yield was observed
using higher amounts of the catalyst (Table 2, entry 7) and 1.0 mol%
of the catalyst was found to be optimum.
Having optimized the conditions, we explored the general appli-
cability of Pd NPs as an effective catalyst for ligand-free coupling of
different phenylboronic acids with aryl halides containing electron-
withdrawing or donating substituents. As indicated in Table 3, it is
evident that our method is reasonably general and can be applied to
several types of aryl halide. In all cases the reaction gave the corre-
sponding products in good to excellent yields. As shown in Table 3,
steric hindrance of the substituent did not influence the product
yield in the ligand-free Suzuki–Miyaura reaction of deactivated aryl
halides using Pd NPs (Table 3, entries 8 and 9).
Compared with literature examples of the Suzuki–Miyaura cou-
pling reaction, the notable features of our method are
•
The reaction system is simple.
Fig. 8. SEM image of Pd nanoparticles.

302
M. Nasrollahzadeh et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
Fig. 9. TEM images of Pd nanoparticles.
•
Elimination of toxic ligands and homogeneous catalysts.
H2O has been employed as a green solvent; no other organic
solvents are needed.
•
•
•
The reactions proceed with higher yields.
The reactions are relatively more environmentally friendly with
easy and efficient recyclability of the catalyst.
Table 3
Suzuki–Miyaura coupling reaction of different aryl halides with phenylboronic
acids.a
Fig. 10. Reusability of Pd NPs for Suzuki–Miyaura coupling reaction.
•
The use of Hippophae rhamnoides L. leaf extract as an eco-
nomic and effective alternative represents an interesting, fast and
clean synthetic route for the large scale synthesis of palladium
nanoparticles.
3.6. Catalyst recyclability
The catalyst recycling is an important step as it reduces the
cost of the process. The reusability of the catalyst was investi-
gated for the Suzuki–Miyaura coupling reaction under the present
reaction conditions (Table 3, entry 1). After completion of the
reaction, catalyst was separated by centrifugation from the reac-
tion mixture and washed with ethanol and dried in an oven and
the recycled catalyst was saved for the next reaction. The dried
recovered catalyst was successively used for four fresh runs with
no significant loss of activity (Fig. 10). This reusability demon-
strates the high stability and turnover of catalyst under operating
conditions.
To check the heterogeneity of catalyst, which is an important
factor, the phenomenon of leaching was studied by inductively
coupled plasma atomic emission spectroscopy (ICP-AES) analysis
of the resulting reaction solution mixture. It was shown that less
than 0.2% of the total amount of the original palladium species was
lost into solution during the course of a reaction.
4. Conclusions
We synthesized palladium nanoparticles with Hippophae rham-
noides Linn leaf extract as a reducing and stabilizing agent. The
use of plant extract as an economic and effective alternative
represents an interesting, fast and clean synthetic route for the
large scale synthesis of palladium nanoparticles. This green pro-
cedure has high yields and easy operations without application of
a
Reaction condition: 1.0 equiv of aryl iodide, 1.2 equiv of phenylboronic acid,
1.0 mol% of catalyst and 2.0 equiv of KOH, 100 ◦C, 12 h.
Isolated yield.
Yield after the 4th cycle.
b
c

M. Nasrollahzadeh et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 297–303
303
toxic reagents or surfactant template. Also, we have established
that the ligand-free heterogeneous catalyst [Pd NPs] is a highly
efficient, stable and recyclable catalyst for the Suzuki–Miyaura
coupling reactions. The present method has the advantages of
readily available starting materials, straightforward and simple
work-up procedures, elimination of ligand, high yields, tolerance
for a wide variety of functionality, and excellent reusability of the
catalyst.
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Acknowledgements
We gratefully acknowledge the Iranian Nano Council and Uni-
versity of Qom for the support to this work.
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