Efficient preparation and application of monodisperse palladium loaded graphene oxide as a reusable and effective heterogeneous catalyst for suzuki cross-coupling reaction

Article abstract of DOI:10.1016/j.molliq.2019.111967

A homogeneously dispersed graphene oxide supported palladium nanomaterial (Pd?GO) has been successfully synthesized and used as a catalyst in cross-coupling reactions at room temperature. Various analytical techniques such as X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) were used to characterize the monodisperse Pd?GO. Monodisperse Pd?GO nanomaterials were used for the cross-coupling reactions which brought together organic molecules with functional significance. This catalyst showed superior catalytic activity and stability for these coupling reactions. High product yields, short reaction times and mild reaction conditions, obtained by the using of developed catalysts. Importantly, the catalyst can be used at least five experiments successfully without losing its efficiency.

Full text of DOI:10.1016/j.molliq.2019.111967

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Efficient preparation and application of monodisperse palladium loaded graphene
oxide as a reusable and effective heterogeneous catalyst for suzuki cross-coupling
reaction
Fatma Diler, Hakan Burhan, Hayriye Genc, Esra Kuyuldar, Mustafa Zengin, Kemal
Cellat, Fatih Sen
PII:
S0167-7322(19)35388-7
DOI:
https://doi.org/10.1016/j.molliq.2019.111967
MOLLIQ 111967
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To appear in: Journal of Molecular Liquids
Received Date: 29 September 2019
Revised Date: 8 October 2019
Accepted Date: 17 October 2019
Please cite this article as: F. Diler, H. Burhan, H. Genc, E. Kuyuldar, M. Zengin, K. Cellat, F. Sen,
Efficient preparation and application of monodisperse palladium loaded graphene oxide as a reusable
and effective heterogeneous catalyst for suzuki cross-coupling reaction, Journal of Molecular Liquids
(2019), doi: https://doi.org/10.1016/j.molliq.2019.111967.
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GRAPHICAL ABSTRACT

Efficient preparation and application of monodisperse palladium loaded
graphene oxide as a reusable and effective heterogeneous catalyst for
Suzuki cross-coupling reaction
Fatma Dilera, Hakan Burhanb, Hayriye Genca, Esra Kuyuldarb, Mustafa Zengina*,
Kemal Cellatb, Fatih Senb*
aSakarya University, Faculty of Arts and Sciences, Department of Chemistry, Esentepe
Campus, 54187 Serdivan, Sakarya, Turkey
bSen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar
University, Evliya Çelebi Campus, 43100 Kütahya, Turkey
Email: mzengin@sakarya.edu.tr, fatih.sen@dpu.edu.tr
ABSTRACT
A homogeneously dispersed graphene oxide supported palladium nanomaterial (Pd@GO) has
been successfully synthesized and used as a catalyst in cross-coupling reactions at room
temperature. Various analytical techniques such as X-ray Diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HR-TEM) were used to characterize the monodisperse
Pd@GO. Monodisperse Pd@GO nanomaterials were used for the cross-coupling reactions
which brought together organic molecules with functional significance. This catalyst showed
superior catalytic activity and stability for these coupling reactions. High product yields, short
reaction times and mild reaction conditions, obtained by the using of developed catalysts.
Importantly, the catalyst can be used at least five experiments successfully without losing its
efficiency.
Keyword: Nanocatalyst; Organic chemistry; Palladium; Suzuki–Miyaura Cross-coupling
reactions

1. Introduction
Recently, researchers have been utilizing the nanoparticles (NPs) in various reactions as a
catalyst [1–3], and Palladium (Pd) is one of the most popular metal-nanocatalysts. Pd is
widely used to catalyze chemical reactions such as alcohol oxidation [4], carbon-carbon
coupling [5], electrochemical formic acid oxidation [6] and olefin hydrogenation [7].
Specifically, Pd NPs have been tested extensively as robust catalysts for the optimization of
reaction kinetics due to their high surface-to-volume ratio [8–13]. Moreover, hydrogenation
with the help of heterogeneous palladium is one of the oldest, most important and widely used
applications in organic synthesis. Herein, the synthesis and application of monodisperse
Pd@GO NPs for the Suzuki-Miyaura cross-coupling reaction were reported. The Suzuki–
Miyaura cross-coupling reaction of aryl halides with aryl boronic acids has emerged as one of
the most powerful, attractive, and convenient synthetic routes for the construction of biaryls
that are essential compounds in medicinal chemistry [14–16]. Pd, especially a homogeneous
Pd complex, is an inevitable catalyst option for these coupling reactions due to the short
reaction time, high synthetic yield, and selectivity [17]. However, the necessity of using
flammable hydrogen gas at high pressure and high temperatures are serious disadvantages
[18]. In addition, the recovery of homogeneous Pd complexes from the reaction medium is
difficult and cannot be recycled for the next round of reaction. For this reason, the
development of hydrogenation catalysts that are effective and reusable, as well as providing
mild conditions has attracted scientists’ attention [19–21]. This study suggests a solution for
the reusability of Pd NPs by using a graphene oxide support [22]. Heterogeneous or
homogeneous palladium catalysts present wide utilization areas in organic chemistry and also
industrial chemistry [23,24]. Since hydrogenation reactions provide the most fundamental
transformations in chemical processes, their applications in hydrogenation reactions are
remarkable [25,26]. On the other hand, Pd catalysts are most preferred for the cross-coupling
reactions [27,28].
Hydrogenation is one of the most important and widely used synthetic transformations in the
chemical industry and organic synthesis. Hydrogenation of alkenes [29], vegetable oils [30],
and nitroreduction in different catalytic systems [31,32] have been reported in the literature.
In this study, the catalytic activity of monodisperse Pd@GO nanoparticles as a catalyst on
hydrogenation was investigated using the Suzuki-Miyaura reaction. In addition to the
determination of the activation energy of Suzuki-Miyaura reaction with Pd@GO catalyst, the
effects of the electron-rich ligand structures and clustering of Pd were also investigated in the
support structure. Results indicated that the Pd@GO catalyst exhibited a relatively high

catalytic activity. Furthermore, the decrease in activation energy resulted in an increase in the
reaction rate of the catalyst.
2. Experimental
2.1. Chemicals
PdCl2, DMAB, graphite, NN–Dimethylformamide (DMF), acids (nitric, hydrochloric,
sulfuric) were purchased from Sigma-Aldrich. Methanol (≥99.5%), HClO4 (60%)
tetrahydrofuran (THF; 99.5%), and 2-propanol were obtained from Merck. Millipore water
distillation system (18 Mꢀ) was used to obtain pure water. All glassware and other
apparatuses used in experiments were cleaned with distilled water.
2.2. The general procedure of hydrogenation of multiple bonds
The synthesis of graphene oxide was performed according to the Hummers method, given in
supporting information in detail. In order to the preparation of Pd@GO catalyst, 10 mL of
ethanol, 10 mg of graphene oxide, and 10 mg of palladium chloride were stirred in an
ultrasonic bath at 300W for 30 minutes. The mixture was then mixed for 3 hours to provide
palladium cation exchange with acidic functional groups in the graphene oxide. Finally, 148
o
mg of DMAB was added slowly into the mixture. The final mixture was refluxed at 100 C
for 12 hours. The resulting mixture was centrifuged and washed with ethanol and water. The
o
resultant dried under the vacuum oven for 12 hours at 50 C. The synthesized catalyst was
characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), High-
resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy
(XPS), and Raman spectroscopy techniques.
2.3. General procedure for Suzuki-Miyaura reaction with the Pd@GO
Dimethylformamide (10 mL) was mixed with 4-iodotoluene (0.218 g, 1 mmol) (4-
acetylphenyl), boronic acid (0.164 g, 1 mmol), K2CO3 (0.179 g, 1.4 mmol), 10 mg Pd@GO.
The mixture was stirred at 110 °C for 6 hours under nitrogen gas. The mixture was allowed to
cool down to room temperature and then centrifuged. The resultant washed using 1M HCl (2
x 10 mL) and water (2 x 10 mL). 10 mL of ethyl acetate was added to dissolve the resulting
supernatant. The organic layer was dried using MgSO4 and then taken under reduced pressure
to concentrate.
2.4. General procedure for Heck reaction with the Pd@GO

4-Iodotoluene (0.218 g, 1 mmol), butyl acrylate (0.128 g, 1 mmol), K2CO3 (0.179 g, 1.4
mmol), and 10 mg of Pd@GO were added into dimethylformamide (10 mL), under 80 °C was
mixed. The mixture was cooled to room temperature and centrifuged. After centrifugation, 10
mL of ethyl acetate was added to dissolve the resulting supernatant. After washing with 1M
hydrochloric acid (2 x 10 mL), washing was carried out with water (2 x 10 mL). Using
MgSO4, the organic layer was dried and then taken under reduced pressure to concentrate.
The product was filtered through a small silica gel pad and eluted with (30:70) ethyl acetate
and hexane mixture.
3. Result and Discussion
3.1. Characterization of Prepared Catalyst
TEM analysis was used to determine the particle size and morphology of Pd@GO catalyst.
TEM and HRTEM images of Pd@GO catalyst was shown in Figure 1 (a). The Pd
nanoparticles are well dispersed on GO support and spherical shape. No-agglomeration is
observed. Atomic lattice fringe was measured as 0.22 nm as shown in HR-TEM image. The
average particle size of the Pd@GO catalyst was found to be 3.94 ± 0.72, as shown in the
particle size histogram in Figure 1 (b). Furthermore, the existence of palladium metal in
prepared Pd@GO catalyst was confirmed by EELS (Transmission electron microscopy-
Elemental energy loss spectroscopy) line profile analysis as shown in Figure 1c.

Figure. 1. (a) TEM and HRTEM images, (b) particle size histogram and (c) TEM-EELS line
profile.
X-ray diffractogram of Pd@GO NPs is shown in Figure 2. Palladium peaks located on 2θ=
40.6°, 46.5°, 68.5°, and 82.5° are related to the Pd (111), (200), (220) and (311) planes,
respectively. In addition, the mean crystallite particle size of the monodisperse Pd@GO NPs
was calculated as approximately 3.88 nm using Scherrer equation [18]. These results are in
good agreement with the results obtained in TEM analysis. Besides, C (002) plane was shown
apparently in 12o which indicates that graphene oxide has remained in unreduced form after
Pd immobilization.

Figure 2. XRD patterns of Pd@GO NPs
The chemical oxidation state and the surface compositions of the elements were determined
by the XPS analysis. The peak couple observed at energy binding of 335.7 eV and 337.4 eV
are corresponded to Pd0. While the peaks located at 341.1 eV and 343.1 eV related to Pd2+
species. The comparison of peak areas revealed that Pd0 is the dominantly exist in the
prepared catalyst. The main reason for the formation of a small amount of Pd2+ species may
be due to the oxidation of the surface or unreduced species throughout the preparation of the
catalyst [30].

Figure 3. XPS spectra Pd 3d region and
Raman spectra show two distinct signals at 1350 and 1600 cm-1 related to sp3 hybridized
carbon (D band) and crystallized graphite sp2 carbon (G band), respectively. The ratio of D-
band to G-band (ID/IG) was increased from 1.01 to 1.15 after doping of palladium to the
graphene oxide surface as shown in Figure 4 [10,31,32]. This is due to the functionalization of
GO by Pd NPs.

Figure 4. Raman spectra of GO and Pd@GO catalyst
3.2. Hydrogenation of Unsaturated Compounds
In order to determine the optimal reaction conditions of the Pd@GO catalyst, a series of
experiments were carried out. Trans-stilbene was taken as the base substrate, and
tetrahydrofuran, methanol, ethanol, and acetone for 1 mmol of trans-stilbene were used for the
determination of the ideal solvent. The shortest conversion time was achieved with methanol
and acetone in 60 min, while ethanol and tetrahydrofuran provided the complete conversion in
75 min and 90 min, respectively. Since acetone and methanol exhibited similar performance,
methanol was preferred for further reactions due to the toxicity of acetone (Table 1). The
optimal amount of the catalyst was also studied. The complete conversion was achieved with
7 mg of catalyst in 1 hour, 15 mg of catalyst in 40 min, and 20 mg of catalyst in 30 min.
Although the conversion rate was increased with the catalyst amount, 7 mg of catalyst was
used in further experiments for cost-effectiveness. As a result, the optimum condition was
determined for 1 mmol of the substrate as in methanol with 7 mg of catalyst in 1 h. The
optimum amount of solvent was identified by the using of different amounts of methanol. The
conversion was completed within 90 and 60 min when 10 mL and 40 of methanol were used,
respectively, as shown in Table 2. This can be as a result of that higher concentration of

reactants provides high yields by increasing the contact of the reactant at the catalytic surface
and H2 gas.
Table 1. Evaluation of monodisperse Pd@GO in the hydrogenation of trans-stilbenea
H
atm, Pd@GO
rt
2
Run
Solvent (10ml)
Time (min)
Conversion (%)
THF
THF
THF
15
30
60
90
15
30
60
75
15
30
60
15
30
60
11
54
78
>99
13
27
95
>99
31
94
>99
47
1
2
3
4
5
6
7
8
THF
EtOH
EtOH
EtOH
EtOH
MeOH
MeOH
MeOH
Acetone
Acetone
Acetone
9
10
11
12
13
14
92
>99
a Solvent (10 mL), rt, trans-stilbene (1.00 mmol) and 1 atm H2 reaction conditions
Table 2. Effect of the catalyst amount and concentration in hydrogenation reaction of trans-
stilbenea
H2 atm, Pd@GO
rt
Run
Pd@GO (mg) MeOH (ml)
Time (min)
Converted (%)
None
7
7
15
20
10
10
40
10
10
60
60
90
40
30
0
1
2
3
4
5
>99
>99
>99
>99
a Solvent (10 mL), rt, trans-stilbene (1.00 mmol) and 1 atm H2 reaction conditions
The scope of this catalytic method was examined with the availability of this catalyst in
hydrogenation reactions of various unsaturated compounds. It was tested with 11 different
compounds such as chain or cyclic, isolated or conjugated alkene, alkyne, nitrile, or carbonyl
groups as shown in Table 3.

Table 3. Catalytic hydrogenation in the presence of new Pd@GOa,b using various alkenes
Timec
Entr
y
1
Reactant
Product
Yield
(%)
>99
60 min
90 min
90 min
>99
>99
2
3
60 min
>99
4
90 min
120 min
120 min
>99
>99
>99
5
6
7
120 min
180 min
>99
>99
8
9
180 min
240 min
>99
>99
10
11
a % Conversion ratio determined by 1H NMR spectroscopy.
bReaction conditions: 1.00 mmol of the substrate, 20 ml of methanol, 7mg of Pd@GO at room temperature under
1 atm of H2.
c The reaction time was adjusted using TLC analysis.
a 20ml EtOH/H2O and 4mmol N2H4.H2O and 1mmol nitroarene reaction conditions
b Isolated yields
c Using 1H NMR and 13C NMR spectra, products were characterized and compared with other authentic samples.
3.3. Cross-Coupling Reactions
In the experiment as mentioned above, it was seen that the new Pd@GO nanomaterial might
be used in many reactions where Pd@C is used. From this point of view, Heck [33,34] and
Suzuki-Miyaura [35–37] cross-coupling reactions, which is known to be able to perform with

Pd@C as a catalyst, have been successfully tested in the presence of the new Pd@GO NPs
with superior catalytic activity as shown in Scheme 2.
O
Pd@GO
Ar atm
O
O
K CO
2
3
+
H C
I
3
O
o
DMF, 80 C H C
3
4 h
O
Pd@GO
N2 atm
CH3
HO
HO
O
K2CO3
+
H3C
I
B
DMF,110 oC
6 h
CH3
H3C
Scheme 2. Heck and Suzuki-Miyaura cross-coupling reactions with new Pd@GO
3.4. The reuse of the catalyst
In some cases of carbonous–noble metal composites; nano-sized metal nanoparticles can be
detached from the carbon support, and tend to agglomeration. This is a serious problem that
also reduces the catalytic activity and lifetime of the catalyst [38]. The stability of the catalyst
was investigated by repetitive using of catalyst. For this aim, at the end of the reactions, the
catalyst was removed by centrifugation. The catalyst was washed with acetone, ethanol, and
water. Then, dried and used in successive reactions for testing the reusability and stability. It
has been proved that the catalyst can be reused at least five times without losing its efficiency
as shown in Scheme 3. As a result, the synthesized Pd@GO NPs exhibited a superior catalytic
activity compared to the previous catalysts for coupling reactions [39]. This unique catalytic
activity can be ascribed to the large surface area of the catalysts, the synergic effects of Pd
with GO and ultrafine structure. Moreover, optimizing the electronic interactions between the
metal and the support material may also increase the catalytic activity. It has been observed
that the interaction between metal and the graphene oxide layer contributed to the electron
transfer.

Scheme 3. Reusability performance of the catalyst after tandem reactions
4. Conclusions
Pd@GO catalyst was synthesized using a facile and simple method for the utilization as a
catalyst in Suzuki–Miyaura Cross-coupling reactions. The prepared catalyst showed superior
catalytic activity and durability, without losing its efficiency after five times of using in
reactions. This unique catalytic activity can be explained by the large surface area of the
catalysts and the synergic effects between Pd and GO. In this approach, the conductive
property of the GO has supplied an increased electron transfer rate. Results support that the
synthesized Pd@GO catalyst cost-effective and environmentally friendly alternative to
classical Pd-based catalysts. According to the obtained results, we strongly believe that the
performance and the synthetic applicability of Suzuki–Miyaura Cross-coupling reactions will
be employed strategically in future actions by the use of Pd@GO nanocatalysis. Moreover,
with a suitable design, these technologies can be generated cheaper and profitable
technologies.
Acknowledgements
K.C. would like to acknowledge support from the Scientific and Technical Research Council
(TUBITAK) BIDEB - 2218 Grant.
The author(s) declare no competing interests.

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Author Contributions
M.Z. and F.S. organized all experiments and wrote the manuscript. F.D., H.B., H.G., E.K., and
K.C. performed all experiments and characterizations. They have also drawn the figures.

Highlights
•
Pd@GO as nanocatalyst on hydrogenation was investigated by Suzuki-Miyaura
reaction.
•
•
Pd@GO nanocatalyst exhibited superior catalytic activity and durability.
The effects of the electron-rich ligand structures and clustering of Pd on support were
also investigated.
•
The reaction rate was increased by the decreasing of activation energy.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: