Palladium nanoparticles supported on reduced graphene oxide as an efficient catalyst for the reduction of benzyl alcohol compounds

Article abstract of DOI:10.1016/j.catcom.2015.05.023

Palladium nanoparticles were prepared on reduced graphene oxide (Pd NPs/rGO) by using a sonochemical procedure. Pd NPs with a mean diameter of 37 ± 22 nm were deposited on reduced graphene oxide sheets by the reaction between PdCl₄^{2 -} and graphene oxide (GO) under sonochemical conditions. The catalyst was characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The Pd NPs/rGO nanocomposite was successfully applied as a reusable catalyst for the reduction of benzyl alcohol derivatives into the corresponding methylene compounds in the presence of triethylsilane. The reductive dehydroxylation of benzyl alcohols takes place under mild conditions affording high yields of the corresponding methylene compounds in short reaction times.

Full text of DOI:10.1016/j.catcom.2015.05.023

Catalysis Communications 69 (2015) 97–103
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Catalysis Communications
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Short communication
Palladium nanoparticles supported on reduced graphene oxide as an
efficient catalyst for the reduction of benzyl alcohol compounds
a
b
Maryam Mirza-Aghayan a, , Mahdieh Molaee Tavana , Rabah Boukherroub
⁎
a
Chemistry and Chemical Engineering Research Center of Iran (CCERCI), P.O. Box 14335-186, Tehran, Iran
Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN, UMR 8520), Cité Scientifique, Avenue Poincaré - CS 60069, 59652 Villeneuve d'Ascq, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium nanoparticles were prepared on reduced graphene oxide (Pd NPs/rGO) by using a sonochemical pro-
cedure. Pd NPs with a mean diameter of 37 22 nm were deposited on reduced graphene oxide sheets by the
reaction between PdCl42− and graphene oxide (GO) under sonochemical conditions. The catalyst was character-
ized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), infrared
spectroscopy (FT-IR), transmission electron microscopy (TEM) and inductively coupled plasma optical emission
spectroscopy (ICP-OES). The Pd NPs/rGO nanocomposite was successfully applied as a reusable catalyst for the
reduction of benzyl alcohol derivatives into the corresponding methylene compounds in the presence of
triethylsilane. The reductive dehydroxylation of benzyl alcohols takes place under mild conditions affording
high yields of the corresponding methylene compounds in short reaction times.
Received 9 February 2015
Received in revised form 9 May 2015
Accepted 28 May 2015
Available online 1 June 2015
Keywords:
Palladium nanoparticles
Reduced graphene oxide
Sonochemical
Benzyl alcohols
Hydrogenolysis
Triethylsilane
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Organosilanes are used to reduce alcohols to alkanes in the presence
of strong Lewis and Brønsted acids [9]. Egi et al. have reported on the di-
The transformation of a hydroxyl functional group into the corre-
sponding alkyl group is a delicate process. Alcohol deoxygenation consti-
tutes a powerful synthetic tool especially used in complex natural
product synthesis. The Barton–McCombie methodology is the most com-
monly used, mainly for secondary alcohols, due to its versatility and com-
patibility with different functional groups [1,2]. The main disadvantage of
this reaction is the use of tributylstannane which is toxic, expensive and
difficult to remove from the reaction mixture. Since hydroxyl group is a
poor leaving group, its transformation into a good leaving group is gener-
ally employed [3]. Several reagents and catalysts have been reported for
the reduction of benzyl alcohols [4–6]. For example, Pd/C and H2 in com-
pressed CO2/water [4], iridium catalyst with hydrazine at 160 °C [5],
palladium chloride (PdCl2) and polymethylhydrosiloxane (PMHS) as hy-
dride source [6] have been investigated for this chemical transformation.
However, it should be noted that high pressure autoclave or high temper-
atures and long reaction times were required for this process. Different
benzylic alcohols were successfully reduced into the corresponding deox-
ygenated compounds by using 2.0 equiv. of Ti(III) and 1.5 equiv. of reduc-
ing agent such as Mn dust in tetrahydrofuran as solvent [7]. Recently,
direct electrolysis of primary alcohols in excess of methyl toluate was re-
ported for the synthesis of the corresponding deoxygenated products in
high yield [8].
rect catalytic deoxygenation of propargyl alcohols with the combination
of H3[PW12O40]·nH2O (1 mol%) and Et3SiH in Cl(CH2)2Cl or CF3CH2OH
as solvent [10]. Chan et al. demonstrated the efficiency of FeCl3 as cata-
lyst for the selective dehydroxylation of secondary benzylic alcohols
using PMHS as hydride source [11].
We have investigated the efficiency of Et3SiH/PdCl2 system for several
transformations [12–14] such as reduction of benzyl alcohols to the cor-
responding methylene compounds under homogeneous conditions [15].
Graphene oxide and graphite oxide have been successfully applied
as effective heterogeneous catalysts for several organic transformations
[16–21].
Noble metal nanoparticles have widely been used as catalysts to pro-
mote various chemical reactions [22,23]. Graphite oxide, graphene
oxide (GO), reduced graphene oxide (rGO), and graphene in combina-
tion with metal nanoparticles have been utilized as catalyst supports
[24,25].
In continuation of our efforts on the use of PdCl2/Et3SiH system
[12–15] and graphite oxide [18–21], we propose in the present study
a simple procedure for the preparation of palladium nanoparticles sup-
ported reduced graphene oxide (Pd NPs/rGO) under ultrasound condi-
tions (Scheme 1a). The nanocomposite material was characterized
using XRD, TGA, SEM, FT-IR, TEM and ICP-OES. The catalytic activity of
the Pd NPs/rGO was further investigated for the reduction of benzyl al-
cohol derivatives into the corresponding methylene compounds in the
presence of triethylsilane (Scheme 1b).
⁎
Corresponding author.
E-mail address: m.mirzaaghayan@ccerci.ac.ir (M. Mirza-Aghayan).
http://dx.doi.org/10.1016/j.catcom.2015.05.023
1566-7367/© 2015 Elsevier B.V. All rights reserved.

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M. Mirza-Aghayan et al. / Catalysis Communications 69 (2015) 97–103
Scheme 1. (a) Synthesis of Pd NPs/rGO using ultrasonication; (b) reduction of benzyl alcohol compounds using Et3SiH and Pd NPs/rGO nanocomposite catalyst.
2. Experimental
determined using ICP-OES Varian 735 ES configuration torch redial
instrument after each catalyst sample was completely dissolved in the
mixture of HNO3/HCl (1/3 ratio).
2.1. General information
Elmasonic P ultrasonic cleaning unit (bath ultrasonic) and ultrasonic
homogenizer Bandelin Sonoplus HD 3100 (probe ultrasonic) were used
to prepare the Pd NPs/rGO nanocomposite samples. X-ray diffraction
(XRD) data were collected using a Bruker D8 Advance Theta–2theta dif-
fractometer. Thermogravimetric analysis (TGA) was performed on a
NETZSCH TG 209 F1 analyzer. Scanning electron microscopy (SEM)
and energy dispersive X-ray (EDX) data were acquired using a VEGA3
LMU TESCAN SEM. IR spectra were recorded from KBr disks with a
Bruker Vector 22 FT-IR spectrometer. Transmission electron microscopy
(TEM) images were obtained using a Zeiss em900 transmission electron
microscope. Palladium content of the Pd NPs/rGO catalyst was
2.2. Synthesis of graphite oxide
Graphite oxide utilized in this work was synthesized according to a
previously reported procedure [20].
2.3. Synthesis of palladium nanoparticles/reduced graphene oxide
(Pd NPs/rGO)
Typically, 200 mg of graphite oxide powder in 800 mL of deionized
water was mixed in a reaction container using bath ultrasonic with a
frequency of 37 kHz for 2 h. Then 2 mL of the graphite oxide dispersion
Fig. 1. XRD patterns of (a) graphite, (b) graphite oxide, and (c) Pd NPs/rGO.

M. Mirza-Aghayan et al. / Catalysis Communications 69 (2015) 97–103
99
Fig. 2. TGA analysis of (a) graphite, (b) graphite oxide, and (c) Pd NPs/rGO.

100
M. Mirza-Aghayan et al. / Catalysis Communications 69 (2015) 97–103
with 8 mL of deionized water were mixed for 20 min using bath ultra-
sonic with a frequency of 80 kHz. Next 0.1 mL of aqueous solution of
H2PdCl4 (50 mM), prepared by completely dissolving 177.3 mg of
PdCl2 in 100 mL of 20 mM HCl solution, was added into the container
and mixed for 40 min using probe ultrasonic.
typical XRD patterns of the starting graphite, graphite oxide and Pd
NPs/rGO nanocomposite. Graphite displays a single and sharp peak at
2θ = 26.4° (Fig. 1a). The disappearance of the graphite peak at 2θ =
26.4° and appearance of a new peak at 2θ = 11.8° confirmed the forma-
tion of graphite oxide upon graphite oxidation (Fig. 1b) [20]. The in-
crease of the interlayer spacing value from 3.36 to 4.70 Å for graphite
and graphite oxide, respectively, indicates the inclusion of various oxy-
gen containing functional groups and trapped water molecules between
the layers during the oxidation process. As shown in Fig. 1c, the XRD
pattern of Pd NPs/rGO exhibits characteristic peaks at 2θ = 39.5°,
45.9°, 67.4° which can be assigned to the (111), (200), and (220) crys-
talline planes of the face centered cubic structure of palladium, respec-
tively [26]. A broad peak of very low intensity at about 2θ = 26°
indicates the presence of few-layered reduced graphene oxide (rGO)
[27]. This result suggests graphite oxide reduction to rGO by partial re-
moval of oxygenated functional groups. The absence of characteristic
diffraction peaks of PdO at 2θ = 33.9°, 41.9° and 54.8° is in accordance
with the reduction of Pd(II) to Pd(0) in this process [28]. The results
are in agreement with recent reports on the direct and simultaneous re-
duction of metal ions and GO using a sonochemical method for the
preparation of Au NPs/rGO [29] and Ag NPs/rGO [30] nanocomposites.
TGA confirms the simultaneous conversion of graphite oxide to rGO
nanosheets under ultrasonication with the deposition of Pd NPs. Fig. 2
2.4. General procedure for benzyl alcohol hydrogenolysis
To a solution of benzyl alcohol (1 mmol) and triethylsilane (amount
indicated in Table 2) in ethanol (5 mL) was added 10 mg of Pd NPs/rGO
under an argon atmosphere. The resulting mixture was stirred for the
time indicated in Table 2 prior to GC–MS analysis. The resulting mixture
was filtered and washed by ethyl acetate for catalyst separation. The
pure product in entry 1 was isolated by distillation; for entries 2–10
the products were isolated by column chromatography using hexane/
ethyl acetate (9/1) as eluent.
3. Results and discussion
3.1. Preparation and characterization of Pd NPs/rGO nanocomposite
The morphology and chemical composition of the Pd NPs/rGO were
characterized by XRD, TGA, SEM, TEM and ICP-OES. Fig. 1 displays
Fig. 3. SEM images of (a) graphite, (b) graphite oxide, (c) Pd NPs/rGO and their respective EDX spectra.

M. Mirza-Aghayan et al. / Catalysis Communications 69 (2015) 97–103
101
displays the TGA curves of graphite, graphite oxide, and Pd NPs/rGO
nanocomposite samples. Graphite material shows a weight loss of near-
ly 1% in the experimental temperature range (Fig. 2a) [31]. In contrast,
the weight loss of graphite oxide occurs in three successive steps. The
first one is a steady weight loss (7%), attributed to the vaporization of
adsorbed water molecules and occurs at around 100 °C. It is followed
by a rapid loss (25%) due to the decomposition of the labile oxygen-
containing functional groups (hydroxyl, epoxy, carbonyl, and carboxyl
groups) in the temperature range of 100–208 °C. Finally, a weight loss
(19%) due to the combustion of the carbon skeleton is observed in the
temperature range of 208–900 °C (Fig. 2b) [32]. Pd NPs/rGO exhibited
overall less than 10% weight loss in the same temperature range
(Fig. 2c). This result indicates the effective reduction of graphite oxide
to rGO by removing most of the carboxyl, epoxy, carbonyl, and hydroxyl
groups.
Fig. 3 depicts the SEM images and the corresponding EDX spectra of
graphite flakes, graphite oxide and Pd NPs/rGO nanocomposite.
As clearly seen in the SEM images, the structure of graphite (Fig. 3a)
was completely changed to flower-like structure with appearance of
graphitic sheets after the oxidation process (Fig. 3b). EDX analysis
shows that the initial graphite comprises mainly carbon (97.29 at.%)
with a small amount of oxygen (Fig. 3d). After extensive oxidation of
graphite to graphite oxide, EDX spectrum reveals the formation of a
new structure with higher oxygen content of 17.91 at.% with C/O ratio
of 3.24 (Fig. 3e). It should be noted that the presence of sulfur
(4.64 at.%) is most likely due to impurities from H2SO4 used in this pro-
cedure. The SEM image of the Pd NPs/rGO nanocomposite exhibits a
high density of nanoparticles in form of aggregates (Fig. 3c). EDX anal-
ysis of Pd NPs/rGO nanocomposite indicates that the palladium loading
on the rGO is about 4.29 at.% (Fig. 3f). The FTIR spectrum of graphite
oxide exhibits an intensive broad peak of O\\H stretching vibrations
at about 3393 cm−1, and several bands at 1719, 1574, 1215 and
1044 cm−1 assigned to C_O, C_C, (C\\O) epoxy and (C\\O) alkoxy
groups, respectively (Fig. 4a) [33]. The comparison of FT-IR spectra of
graphite oxide and Pd NPs/rGO clearly shows that chemical changes oc-
curred during Pd NPs deposition using ultrasonication. The strong peaks
at 1719 and 1044 cm−1 disappear and a weak peak at 1217 cm−1 still
remains in the FTIR spectrum of Pd NPs/rGO. The FTIR spectrum is dom-
inated by a broad band at ~1636 cm−1 due to C_C stretching modes,
suggesting partial restoration of the aromatic network during the chem-
ical process (Fig. 4b) [33].
Fig. 5. TEM image of Pd-NP/rGO.
average diameter (Fig. 5). The TEM image of the Pd NPs/rGO also shows
nanoparticles in form of aggregates.
3.2. Catalytic performances of Pd NPs/rGO
In connection with our interest in heterogeneous catalysis, we have
investigated rGO as a supporting matrix for Pd NPs and Et3SiH as a hy-
dride source in the chemoselective reductive dehydroxylation of ben-
zylic alcohols. In a control experiment, we have studied the efficiency
of graphite oxide (50 mg) for the hydrogenolysis of benzyl alcohol
(1 mmol) in ethanol (5 mL). The resulting mixture was stirred for the
time indicated in Table 1 prior to GC–MS analysis. Only the starting ma-
terial was recovered after 5 h (entry 1, Table 1), suggesting that graphite
oxide alone is not active for this chemical transformation. In another
control experiment, we have tested the efficacy of triethylsilane
(2 mmol) for the reductive dehydroxylation of benzyl alcohol in anhy-
drous ethanol in an inert atmosphere. Again Et3SiH alone was not effec-
tive for the reduction of benzyl alcohol and toluene was obtained only in
8% yield after 5 h (entry 2, Table 1). Finally, we have investigated graph-
ite oxide and Et3SiH system for this chemical process. After 5 h (entry 3,
Table 1), only the starting material was recovered. From this set of con-
trol experiments, it is clear that graphite oxide, Et3SiH or graphite oxide/
Et3SiH is not effective for the reductive dehydroxylation of benzyl
alcohol.
TEM analysis of the synthesized Pd NPs/rGO material clearly reveals
highly exfoliated rGO sheets covered by Pd nanoparticles of 37 22 nm
In contrast when the reaction of benzyl alcohol (1 mmol) and Et3SiH
(2 mmol) in dry ethanol was performed in the presence of Pd NPs/rGO
(50 mg), an exothermic reaction occurred during the first 5 min. GC/MS
analysis of the crude product indicated that toluene was obtained in 99%
yield after 5 min (entry 4, Table 1). It should be noted that similar results
were obtained using only 10 mg of Pd NPs/rGO under otherwise identi-
cal experimental conditions (entry 5, Table 1). Decreasing the catalyst
amount to 1 mg led to a decrease of the reaction yield to 65% even
Table 1
Different conditions for the hydrogenolysis of benzyl alcohol.
Entry
Catalyst
(mg)
50
Et3SiH (mmol)
Time (min)
Yielda (%)
1
2
3
4
5
6
Graphite oxide
–
Graphite oxide
Pd NPs/rGO
Pd NPs/rGO
Pd NPs/rGO
–
2
2
2
2
2
300
150
300
5
5
5
–
8
–
99
99
65
50
50
10
1
a
Fig. 4. FT-IR spectra of (a) graphite oxide and (b) Pd NPs/rGO.
Determined by GC–MS.

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M. Mirza-Aghayan et al. / Catalysis Communications 69 (2015) 97–103
Table 2
Reduction of benzyl alcohol derivatives using Pd NPs/rGO catalyst.
Entry
1
Substrate
Product
Et3SiH (mmol)
2
Time (min)
5
Yielda (%)
99
2
2
2
15
10
94
92
3
4
2
2
10
10
90
96
5
6
7
2
4
2
15
15
20
47
90
93
8
9
2
5
98
2
4
30
30
42
82
10
11
2
4
2
40
60
240
240
240
35
80
–
–
–
–
6
6b
a
Determined by GC–MS.
The reaction was conducted at reflux.
b
after 1 h. We concluded that 10 mg of Pd NPs/rGO are required for the
hydrogenation reaction (entry 6, Table 1).
conditions as compared to the methods described in the literature
[4–6]. For example, benzyl alcohol was reduced in 54% yield after 3 h
using Pd/C catalyst in compressed CO2/water and 1 MPa H2 [4]. Inde-
pendently, Huang et al. [5] obtained toluene in 98% yield through the re-
duction of benzyl alcohol using iridium catalyst with hydrazine at
160 °C after 3 h. Wang et al. [6] reported benzyl alcohol hydrogenolysis
in 99% yield using PdCl2 and PMHS after 12 h at 40 °C.
To evaluate the reusability of the Pd NPs/rGO after completion of the
reaction, acetonitrile was added and the mixture was filtered through a
sintered funnel to recover the catalyst. The reaction of benzyl alcohol
and Et3SiH in dry ethanol in the presence of 10 mg of the recovered
Pd NPs/rGO was performed for seven consecutive cycles. The recycled
Pd NPs/rGO was efficient for the hydrogenolysis of benzyl alcohol into
the corresponding toluene even after seven consecutive times with an
activity loss of about 20% (Table 3). Moreover, to ascertain the leaching
of Pd metal, we analyzed the recovered catalyst after seven runs by ICP-
OES. The value of Pd in the recovered catalyst was determined to be
0.1 wt.%, which is much lower than 4.38 wt.% determined in the fresh
Pd NPs/rGO catalyst. This result indicates that the activity loss of the cat-
alyst after seven consecutive runs was due to Pd leaching.
Under the best reaction conditions, various benzyl alcohols (1 mmol)
reacted smoothly with Pd NPs/rGO (10 mg), Et3SiH (2 mmol) in dry
ethanol to give the corresponding methylene compounds at room tem-
perature (Table 2). The reaction of 2-hydroxybenzyl alcohol, 4-
hydroxy-3-methoxybenzyl alcohol, 4-tert-butyl-benzyl alcohol and 1-
phenyl-1-propanol gave the corresponding products in 90–98% yields
(entries 1–5, Table 2). We also observed the reduction of conjugated al-
cohols such as cinnamyl alcohol; propylbenzene was obtained using
2 mmol of Et3SiH in 47% yield after 15 min at room temperature (entry
6, Table 2). The yield increased to 90% by increasing the amount of
Et3SiH to 4 mmol under the same conditions (entry 6, Table 2). The
reaction of benzhydrol and 1-indanol led to the formation of
diphenylmethane and indane in 93% and 98% yield, after 20 and 5 min,
respectively (entries 7–8, Table 2).
Hydrogenation of 2-furanmethanol and 3-pyridinylmethanol
afforded the corresponding 2-methylfuran and 3-methylpyridine in 42
and 35% yield after 30 and 40 min, respectively (entries 9–10, Table 2).
Using an excess of triethylsilane (4 mmol) gave 2-methylfuran and 3-
methylpyridine in 82 and 80% yield after 30 and 60 min, respectively
(entries 9–10, Table 2). Finally, we investigated the reductive dehydrox-
ylation of aliphatic alcohols such as 1-octanol using this procedure. The
reduction failed to proceed under these conditions even in the presence
of a large excess of Et3SiH at reflux of solvent (entry 11, Table 2).
It should be noted that this procedure allowed the reduction of ben-
zyl alcohol derivatives in relatively short reaction times and mild
4. Conclusion
A simple technique was used to synthesize Pd NPs/rGO from graph-
ite oxide and Pd salt in aqueous media using ultrasonic activation. This
method does not require any reducing agent. A catalytic amount of Pd
NPs/rGO was successfully applied for the hydrogenation of benzyl
alcohols to the corresponding methylene derivatives using Et3SiH in
dry ethanol. This new and simple protocol is advantageous as the
hydrogenolysis takes place at room temperature, in short reaction
times and in high yields, reusable catalyst with a very simple work-up
procedure.
Table 3
Reusability of Pd NPs/rGO catalyst.
Run
1st
99
2nd
99
3rd
93
4th
92
5th
80
6th
80
7th
79
Yield (%)

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