Catalytic hydrogenation of crotonaldehyde and oxidation of benzene over active and recyclable palladium nanoparticles stabilized by polyethylene glycol

Article abstract of DOI:10.1016/j.molcata.2013.01.011

Polyethylene glycol (PEG)-stabilized Palladium (Pd) nanoparticles have been successfully synthesized by a simple hydrothermal approach. The as-prepared catalyst was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption-desorption measurements. Pd nanoparticles show a high degree of dispersion in PEG with a small average particle size distribution in the range of 3-6 nm. The Pd/PEG catalyst exhibited a remarkable catalytic activity toward selective hydrogenation of crotonaldehyde and oxidation of benzene in liquid phase under mild conditions. The catalyst could be easily removed from the reaction mixture and its recyclability was possible for ten times for crotonaldehyde and two times for benzene. The catalytic performance was found to depend essentially on the catalyst and target concentrations and reaction time.

Full text of DOI:10.1016/j.molcata.2013.01.011

Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic hydrogenation of crotonaldehyde and oxidation of benzene over active
and recyclable palladium nanoparticles stabilized by polyethylene glycol
F.A. Harraza,b,∗, S.E. El-Houta, H.M. Killac, I.A. Ibrahima
a Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), P.O. 87 Helwan, Cairo 11421, Egypt
b Advanced Materials and NanoResearch Centre, Najran University, P.O. Box: 1988, Najran 11001, Saudi Arabia
c Faculty of Science, Zagazig University, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Polyethylene glycol (PEG)-stabilized Palladium (Pd) nanoparticles have been successfully synthesized by
a simple hydrothermal approach. The as-prepared catalyst was characterized by X-ray diffraction (XRD),
transmission electron microscopy (TEM) and N2 adsorption-desorption measurements. Pd nanoparticles
show a high degree of dispersion in PEG with a small average particle size distribution in the range of
3–6 nm. The Pd/PEG catalyst exhibited a remarkable catalytic activity toward selective hydrogenation
of crotonaldehyde and oxidation of benzene in liquid phase under mild conditions. The catalyst could
be easily removed from the reaction mixture and its recyclability was possible for ten times for croton-
aldehyde and two times for benzene. The catalytic performance was found to depend essentially on the
catalyst and target concentrations and reaction time.
Received 5 December 2012
Received in revised form 15 January 2013
Accepted 16 January 2013
Available online 24 January 2013
Keywords:
Palladium nanoparticles
Polyethylene glycol
Hydrothermal
Crotonaldehyde
Benzene
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
an excellent activity, but its recycling was hampered due to the
collapse of mesopores. Reetz and Westermann described the for-
Palladium (Pd) nanoparticles play an important role in many
industrial applications. For example, Pd nanoparticles have shown
high catalytic activity in the reduction of automobile pollutants [1]
and facilitation of organic reactions such as Suzuki reactions [2] and
Heck reaction [3]. They have also been used as gas sensors owing
to their specific sensitivity toward hydrogen [4].
In the field of catalysis, Pd nanoparticles have a wide range
of applications in catalysis, due to high surface-to-volume ratio.
However, active surface atoms often led to aggregation of Pd(0)
nanoparticles (Pd black), and hence the catalytic activity and selec-
tivity would decrease. Therefore, it is an important task to stabilize
the nanoparticles by immobilizing Pd nanoparticles onto inorganic
porous materials [5] or stabilizing them by surface modification
using surfactants [6] or functionalized polymers [7]. Stabiliza-
tion could be accomplished in two ways: electrostatic (charge
or inorganic) stabilization and steric (organic) stabilization. The
general stabilization mechanisms of colloidal materials have been
described in Derjaguin–Landau–Verway–Overbeck (DLVO) theory
[8,9].
mation of Pd nanoparticles stabilized by surfactant [12]. Polymers
provide metal nanoparticles stabilization not only because of the
steric bulk of their framework, but also by weak binding to the
nanoparticles surface by the heteroatom, playing the role of ligands.
Metal-polymer based catalysts were also reported as efficient
catalysts having effective stabilization of the nanoparticles and the
catalytic selectivity would be improved due to the presence of
polymer matrix [13,14]. Polyethylene glycol (PEG) was successfully
utilized as soluble polymeric support acting as both reductant and
stabilizer during the catalyst synthesis [3]. Due to unique charac-
teristics of Pd/PEG based catalysts, there is an increasing interest
in synthesis and utilization of this system in different hydrogena-
tion reactions [15,16]. In our recent work [17] we have described an
ideal green catalytic process of “one-phase catalysis and two-phase
separation” for the hydrogenation of styrene and nitrobenzene over
Pd/PEG catalyst. The catalyst has been synthesized via a simple
thermal heating of Pd precursor with PEG without any further
additives. In the literatures, less attention has been given to the
preparation of Pd/PEG system using the hydrothermal method. In
this work, we have synthesized Pd nanoparticles containing PEG
matrix by a hydrothermal method and employed the as-prepared
catalyst for hydrogenation and oxidation reactions. This proce-
dure is based on the fact that the PEG would act as stabilizer of
Pd nanoparticles while it is miscible with the mixture of toluene
and n-octane at a higher temperature followed by a phase split
In previous studies, Ying et al. reported on highly dispersed Pd
in the mesoporous silica [10,11]. Although this solid catalyst has
∗
Corresponding author. Tel.: +2 02 25010 640; fax: +2 02 25010 639.
E-mail addresses: fharraz@cmrdi.sci.eg, fharraz68@yahoo.com (F.A. Harraz).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.01.011

F.A. Harraz et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
183
Table 1
at a lower temperature, which could combine the advantages of
Chromatographic conditions applied for hydrogenation and oxidation reactions.
homogeneous and heterogeneous catalysis.
Selective hydrogenation of ␣,␤ unsaturated aldehydes, ketones
and esters, in general, is useful in perfumery, hardening of fats,
drugs and in synthesis of organic chemical intermediates [18].
The reaction becomes challenging in olefinic compounds having
functional groups, since the hydrogenation of C C is thermo-
dynamically more favorable than C O or CHO. The catalytic
performance of the present catalyst in selective hydrogenation of
␣,␤ unsaturated aldehydes, mainly crotonaldehyde has been exam-
ined.
Hydrogenation of crotonaldehyde
Oxidation of benzene
Oven
35 ◦C for 1 min → 210 ◦C for 1 min
40 ◦C for 5 min → 260 ◦C for
1 min
35 → 210 ◦C 5 ◦C/min
40 → 260 ◦C 10 ◦C/min
Type: FID
Detector Type: FID
Heater: 300 ◦
C
Heater: 300 ◦
C
H2 flow: 30 ml/min
H2 flow: 30 ml/min
Air flow: 400 ml/min
Makeup flow: 30 ml/min
Air flow: 400 ml/min
Makeup flow: 30 ml/min
Heater: 250 ◦
C
Heater: 250 ◦
C
Injection
Column
Pressure: 1 psi
Pressure: 1 psi
Total flow: 22.027 ml/min
Septum purge flow: 3 ml/min
Mode: split with ratio (100:1)
30 m × 320 ␮m × 0.25 ␮m
On the other hand, direct oxidation of benzene to phenol is of
great interest not only for its industrial importance, but also from
a purely scientific viewpoint. Although a direct oxidation of ben-
zene to phenol would be the most economical route, until now
only the indirect manufacturing process has been operated [19].
The reason is that the activation of the C H bonds in benzene is
difficult due to the resonance stability. Benzene is a hazardous
volatile organic compound and its catalytic oxidation is consid-
ered as the most promising method for its effective removal. In
addition, phenol, one of the important chemical intermediates for
the synthesis of various industrial products such as agrochemicals,
petrochemicals and plastics has been mainly manufactured using
the conventional “cumene-process” which has several disadvan-
tages [20]. Even though, the selectivity for the phenol is high, this
process consists of three steps and produces acetone as a byproduct,
which results in many difficulties due to economical and environ-
mental concerns [21]. The direct oxygenation of the energetically
stable benzene to produce phenol has been one of the most difficult
oxidation reactions. Therefore, a one-step production of phenol by
direct insertion of oxygen into the C H bond of benzene ring is
an attractive and challenging task. Oxidation of benzene to phenol
was examined using the present Pd/PEG catalyst. Evaluation of cat-
alytic performance and catalyst recycling for both hydrogenation
and oxidation reactions are thoroughly addressed and discussed.
Total flow: 22.555 ml/min
Septum purge flow: 3 ml/min
Mode: split with ratio (100:1)
30 m × 320 ␮m × 0.25 ␮m
prepared by dispersing a trace amount of catalyst in ethanol fol-
lowed by ultrasonic treatment and a carbon film coated copper
grid was then immersed into the dispersion and left in air for dry-
ing. Selected-area electron diffraction (SAED) patterns for some
prepared catalysts were also recorded.
2.3. Hydrogenation of crotonaldehyde
As-synthesized catalyst was tested in the selective hydrogena-
tion of crotonaldehyde. In a typical reaction procedure, known
concentrations of the target compound, 1 mmol of biphenyl as
standard and a certain weight of Pd/PEG catalyst were mixed in
20 ml of absolute ethanol as a solvent, stirring the mixture for
4 h at room temperature in hydrogen atmosphere. After the reac-
tion, the mixture was extracted with diethyl ether and analyzed
by Agilent GC 7890A model: G3440A Gas Chromatography using
19091 J-413 capillary column (30 m × 320 ␮m × 0.25 ␮m). Chro-
matographic conditions applied for hydrogenation crotonaldehyde
are summarized in Table 1. In the catalyst recycling experiments,
the organic products were extracted into diethyl ether and finally
removed by a simple decantation. The Pd/PEG catalyst is immiscible
in ether and could be easily recovered and used for the next reaction
cycle.
2. Expermintal
2.1. Preparation of Pd nanoparticles
PEG-stabilized palladium nanoparticles were prepared by the
following procedure: Pd(II) acetate [(CH3COO)2Pd] Aldrich (99.9%
purity) and PEG with different molecular weights (2000, 4000 and
6000) were dissolved in a mixture of (3:1) toluene and n-octane
in a 50-ml round-bottomed flask. The mixture was heated to 70 ◦C
under vigorous, continuous stirring for half an hour. The solution
was then transferred into a 75 ml standard stainless-steel auto-
clave. The autoclave was held at 100 ◦C for 24 h. After finishing the
time of the procedure and the autoclave reached the room temper-
ature, the content is heated to 70 ◦C and transferred to flask. The
mixture solidified upon cooling at room temperature.
2.4. Oxidation of benzene
The catalyst was also used for the selective oxidation of ben-
zene to phenol with hydrogen peroxide (H2O2) as an oxidant.
In this reaction, a certain weight of catalyst (Pd/PEG) and 1 ml
(11.28 mmol) of benzene were added to 7.5 ml of acetic acid glacial
in 50 ml flask placed in a water bath, then 2 ml (19.4 mmol) of 50%
(w/v) H2O2 was added drop wise during 30 min. The reaction mix-
ture was consequently stirred for a desired time. Then the resulting
solution was analyzed by gas chromatography. Table 1 shows the
chromatographic conditions applied for oxidation of benzene.
2.2. Characterization of Pd/PEG catalyst
3. Results and discussion
Phase identification and crystallite size measurements of Pd
nanoparticles were performed by X-ray diffraction (XRD, Bruker
axs D8 Advance, Germany) with Cu-k␣ radiation (␭ = 1.5406 A) in
3.1. Characterization of Pd/PEG catalyst
˚
From previous works on metal nanoparticles, it becomes
evident that the catalytic activity of Pd based catalysts depends on
a large extent of particle size, dispersion of the nanoparticles and
on the way the nanoparticles are stabilized against agglomeration
[22–24]. Such properties are determined by the preparation
procedure and controlled in some extent by the support. The
preparation procedure is crucial to obtain particles in nanoscale
size with adequate stability and dispersion. In the present work,
we have prepared PEG-stabilized Pd nanoparticles by dissolving
the range 2ꢀ from 10 to 80◦. Crystallite size is automatically calcu-
lated from XRD data. The specific surface area (SBET) of the catalyst
was obtained by nitrogen adsorption with a QUANTACHROME-
NOVA 2000 Series, UK. Morphology and pore size distribution of the
samples are investigated using transmission electron microscopy
(TEM, JEOL instrument – Japan-model JSM-2010). The electron
microscope was operated at 200 KV, with a high resolution of
0.194 nm and magnification of 1.5 million. TEM samples were

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F.A. Harraz et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
Table 2
Crystallite size and BET surface area for Pd/PEG4000 prepared at different weight
percents of solvents.
Sample
Crystallite size (nm)
BET (m2/g)
Toluene:n-octane (3:1)
Toluene:n-octane (2:1)
Toluene:n-octane (1:1)
3
14.6
16.8
148.4
67.2
49.6
4–6 nm, whereas the frequency of particles smaller than 2 nm and
larger than 8 nm was considerably low <5%. In addition, the number
of Pd particles of 3 nm represented 15%, whereas the frequency of
larger particles of 7–8 nm accounted to be 10%. It is therefore estab-
lished from TEM measurement that the majority (75%) of particle
size of Pd fabricated using PEG4000 located in the range 3–6 nm.
TEM analysis implies also that the PEG matrix could provide good
stability and dispersing effects to Pd nanoparticles and prevent its
agglomeration. The SAED pattern shown in the inset exhibits the
(1 1 1) and (2 0 0) directions confirming again the cubic structure
and the crystalline nature of the catalyst, in consistent with XRD
result of Fig. 1.
The effect of changing the wt.% of the solvents mixture dur-
ing the hydrothermal reaction was further studied. Different wt.%
of (Toluene: n-octane) was used under the conditions: Pd/PEG
wt.% (0.5%), PEG molecular weight (4000), preparation temperature
(100 ◦C) and time (24 h). Table 2 shows the crystallite size and the
SBET of as-synthesized Pd/PEG catalysts. The results show that the
crystallite size of sample prepared in Toluene: n-octane wt.% (3:1)
is the smallest compared to other samples and increasing (toluene:
n-octane wt.%) leads to an increase in surface area. This may be
related to the fact that toluene has a higher polarity and dielectric
constant compared to octane. This means that toluene has a higher
ability for interaction between PEG and Pd nanoparticles and hence
good dispersion of Pd particles in PEG matrix which finally led to a
higher surface area. The dielectric constant measures the solvent’s
ability to reduce the field strength of the electric field surrounding
a charged particle immersed in it. The Hansen solubility parameter
values of toluene and octane are given in Table 3. Different solu-
bility parameters led to different abilities for salvation with PEG,
which would essentially affect the separation of Pd nanoparticles
and finally the surface area [25].
Fig. 1. XRD patterns of Pd nanoparticles synthesized by a hydrothermal method
using different PEG molecular weights; PEG2000, PEG4000 and PEG6000.
Pd(II) acetate and PEG in a mixture of solvents (3 g toluene and 1 g
n-octane) then heated at 100 ◦C for 24 h; subsequently the Pd(0)
nanoparticles formed by the reduction of Pd(II) ions are stabilized
by PEG matrix. XRD characteristics of Pd nanoparticles prepared
in different molecular weights of PEG are given in Fig. 1. As shown
in XRD patterns, broad peak near 2ꢀ = 40◦ is detected which can
be indexed to the characteristic reflection (1 1 1) plane for face-
centered-cubic Pd(0) with another weak peak around 2ꢀ = 43◦,
consistent with the (2 0 0) crystalline plane (JCPDS card no. 87-641,
d = 2.28, 1.97, respectively). No significant change of the diffraction
patterns was observed upon changing the molecular weight of
polymer, except the relatively more intense peak obtained in case
of PEG4000, which related to the high reducing reactivity of high
molecular weight PEG. Therefore, because of partial reduction in
case of using PEG2000 and high viscosity of PEG6000, PEG4000
was accordingly selected and used. The crystallite size was cal-
culated using Scherrer’s equation, and the value corresponding
to the reflection plane (1 1 1) was found to be 3 nm, whereas the
crystallite size estimated using (2 0 0) plane was around 6 nm.
The morphology of as-prepared catalyst was further observed
with TEM. The recorded image is shown in Fig. 2, together with the
particle size distribution and SAED pattern depicted as insets. Nar-
row size distributed spherical Pd nanoparticles could be observed.
For Pd particle size distribution, 60% of the particles fell in the range
3.2. Catalytic performance
3.2.1. Hydrogenation of crotonaldehyde
Crotonaldehyde is a C4-straight-chain compound containing
a pair of conjugated C C and C O bonds. This reaction aims to
examine the catalytic performance of Pd/PEG catalyst in selective
hydrogenation of crotonaldehyde. Partial hydrogenation usually
produces saturated aldehyde (butyraldehyde) or unsaturated alco-
hol (crotyl alcohol), while complete hydrogenation produces
butanol according to Scheme 1.
Fig. 3 illustrates the conversion % of hydrogenation reaction of
Scheme 1 for different concentrations of crotonaldehyde, mainly
0.5, 1, 1.5, 2 and 2.5 mmol at room temperature using 25 mg catalyst
for 90 min duration. The catalyst demonstrated remarkable activity
at concentration ≤1.5 mmol crotonaldehyde; complete conversion
(100%) of crotonaldehyde to butyraldehyde product was achieved
after 90 min at room temperature. At higher concentration of
Table 3
The Hansen solubility parameter of toluene and octane.
Solvent
␦D dispersion
␦P polar
␦H hydrogen
bonding
Dielectric
constant
Octane
Toluene
14.9
18
0.0
1.4
0.0
2.0
1.96
2.38
Fig. 2. TEM image of Pd nanoparticles prepared in PEG4000. Particle size distribu-
tion and the SAED pattern are shown in the insets.

F.A. Harraz et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
185
O
butyraldehyde
Pd/PEG4000 - H2
O
( major product)
crotonaldehyde
room temperature - 1.5 hour
HO
HO
crotyl alcohol
(minor product)
butanol
Scheme 1. Hydrogenation of crotonaldehyde using Pd/PEG4000 catalyst.
catalyst weight, whereas decreasing the catalyst weight to 13 mg,
the conversion dropped to 62.3%. Interestingly, the selectivity to
butyraldehyde was always 100% over the whole conversion % range
during studying the effect of different concentrations of croton-
aldehyde, different times or catalyst loadings. The selectivity of the
present catalytic system is mainly ascribed to the nanoparticles size
of Pd and the nature of Pd metal itself.
Two mechanisms have been proposed for the hydrogenation of
the C C bond in ␣,␤-unsaturated aldehydes: The classical Horiuti-
polanyi mechanism which involves a 1,2-addition process on a
di-␴ adsorbed species and a mechanism involving a 1,4- addition
process which would be preferred on a ␩ geometry. The latter
4
one leads to the formation of an enol that isomerizes to the same
saturated aldehydes as the former mechanism of 1,2-addition pro-
cess. Therefore the two processes finally lead to the hydrogenation
of C C bond. The possible adsorption modes of ␣,␤-unsaturated
aldehydes on mono crystalline metal surfaces were described by
Delbecq and Sautet as shown in Scheme 2 [26,27]. The adsorption
mode of crotonaldehyde is the determining factor, which orientates
the selectivity toward the formation of the monohydrogenated
products [28].
Fig. 3. Effect of crotonaldehyde concentration on the hydrogenation of crotonalde-
hyde using Pd/PEG4000 catalyst.
Three types of products as in Eq. (1) could be obtained from the
hydrogenation of crotonaldehyde: the C C bond is hydrogenated
giving a saturated aldehydes or the C O bond is involved yielding
an unsaturated alcohol; finally, a total hydrogenation would occur
and a saturated alcohol is essentially obtained. For the catalytic
hydrogenation, it is generally assumed that the reactive bond is
the one involved in chemisorptions on the surface. Therefore the
problem of selective hydrogenation can be reduced in a first rough
approximation to determine the electronic factors which control
the adsorption mode of ␣,␤-unsaturated aldehydes on the surface
[26].
crotonaldehyde, the % conversion decreased to 58% and 39% using
2 and 2.5 mmol concentrations, respectively. Under the current
experimental conditions, the complete conversion to butyralde-
hyde was achieved after 90 min. A 73.5% conversion was obtained
after 60 min. as shown in Fig. 4. Different weights of Pd/PEG catalyst,
mainly 50, 25 and 13 mg were also used to determine the min-
imum catalyst weight necessary for complete the hydrogenation
reaction. The hydrogenation reaction was completed using 25 mg
RR* CH-CH2-CH=O
RR* C=CH-CH=O
RR* CH-CH2-CH2-OH
RR* C=CH-CH2-OH
(1)
A d-band width of Pd is low which means that the radial expan-
sion of Pd d orbitals is small. As a consequence, the overlaps of
these orbitals with the orbitals of the adsorbate molecule (croton-
aldehyde) are reduced and hence a weakening of the interactions
between the molecule and the surface is expected. This is why the
␩
2
modes are more strongly bound on Pd (1 1 1), particularly the
␲ geometries [29–32]. On Pd (1 1 1), the ␲ and ␲ adsorption
co
cc
geometries become competitive with the di-␴ ones. As the bind-
ing energy of the di-␴ mode does not decrease much when one
cc
Fig. 4. Effect of reaction time on the crotonaldehyde hydrogenation using
methyl group attached on C C bond, the geometry is the best one
Pd/PEG4000 catalyst.

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F.A. Harraz et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
Scheme 2. Adsorption modes of the molecule of crotonaldehyde on metal surface [27].
Table 4
Oxidation of benzene using hydrogen peroxide in presence of Pd/PEG4000 catalysta
Entry
Substrate
Product
Catalyst weight (mg)
Amount of H2O2 (ml)
Time (h)
Substrate conversion (%)
Product selectivity (%)
1
2
3
4
5
Benzene
Benzene
Benzene
Benzene
Benzene
Phenol
Phenol
Phenol
Phenol
Phenol
50
100
100
100
100
2
2
3
3
3
2.5
2.5
2.5
3.5
5
35
63
81
95
100
100
100
100
100
100
a
Reaction conditions: stirring, H2 balloon,1 ml benzene,7.5 ml acetic acid, room temp.
for crotonaldehyde where adsorption of Pd occurs through both
double bonds in di-␲ geometry.
to 3 ml in presence of 100 mg catalyst, the conversion % increased
from 63 to 81%. It is worthy to mention that with further increase in
amount of H2O2, the % conversion decreased. This may be due to the
possibility of blocking the active sites of the catalyst by the water
molecules from 30% H2O2 solution. Upon increasing the reaction
time from 2.5 to 3.5 h, the conversion % increased from 81.3 to 95%.
Further increase in time to 5 h led to a complete conversion with
100% selectivity to phenol.
In similar reaction conditions, Zhang et al. [33] reported a yield
of 26% and a selectivity of 91% for a direct catalytic oxidation of
benzene to phenol at room temperature in a liquid-phase using
vanadium-substituted heteropolyacids as a catalyst. A comparison
of the catalytic performance of various catalysts for the oxidation
of benzene to phenol with H2O2 as oxidant is given in Table 5 (data
collected from Refs. [19,33–36]).
The reaction mechanism for oxidation of benzene to phenol has
been reported by a group of authors [19,37,38]. They indicated
that the oxidation of benzene proceeds via a free radical mecha-
nism in which it involves the interaction of Pd/PEG catalyst with
H2O2 which yields HO· and HO2· species via a redox mechanism.
Then the OH· radical attacks the benzene ring and yields phenol as
3.2.2. Oxidation of benzene
To extend the application of the Pd/PEG catalyst, we examined
its catalytic performance in the oxidation of benzene to convert it
to phenol under mild reaction conditions using hydrogen perox-
ide (H2O2) as an oxidant according to Scheme 3. Here, we changed
the catalyst weight, reaction time and amount of H2O2 to reach the
optimum levels necessary to achieve the best performance of cat-
alyst during the oxidation reaction. Results of conversion % and
selectivity of phenol are shown in Table 4. The conversion % of
benzene to phenol was investigated using 50 and 100 mg Pd/PEG
catalyst at room temperature for 150 min in presence of 2 ml H2O2
to oxidize 11.28 mmol benzene. As a general trend, the % conversion
was found to increase with an increase in catalyst weight. Using
50 mg catalyst, 35% conversion of benzene was obtained, in com-
parison with 63% obtained at a double weight of 100 mg catalyst.
These conversion values are quite higher than previously reported
values of 21% and 24% under similar experimental conditions that
achieved for the oxidation of benzene using CuO-impregnated
mesoporous silica [19]. By increasing the amount of H2O2 from 2
Table 5
Catalytic performance of various catalysts during oxidation of benzene.
OH
Catalyst
Reaction
Conversion
(%)
Reference
temperature (◦C)
H2O2 - acetic acid - Room Temperature
4Cu/MCM-41
H4PMo11VO40
1% CuAPO-5
5Fe/NACH-600N
Ti-MCM-41 (Si/Ti = 25)
Pd/PEG4000
30
25
60
65
65
25
21
26
28
50
98
[19]
[33]
[34]
[35]
[36]
Present work
Pd / PEG4000
Benzene
Scheme 3. Oxidation of benzene by H2O2 in presence of Pd/PEG4000 catalyst.
Phenol
100

F.A. Harraz et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
187
- H2O2 + Pd 2+
- H2O2 + Pd 1+
PEG
PEG
HO2 + H+ + Pd 1+
PEG
OH + OH + Pd 2+ PEG
OH
-
-
+ OH
OH
OH
O
OH
OH
+ OH
+
H2O
O
benzoquinone
hydroquinone
- Pd 1+ + H2O2
- Pd 2+ + HO2
Pd 2+ + OH + H2O
Pd 1+ + H+
+ O2
Scheme 4. Mechanism for oxidation of benzene by H2O2 in presence of Pd/PEG catalyst.
the major product with hydroquinone and benzoquinone as the
side products. Oxygen and water are formed as the side prod-
ucts by the decomposition of the hydroperoxy radical and H2O2.
Based on above explanation, a proposed mechanism for benzene
oxidation using the current Pd/PEG catalyst can be schematically
depicted in Scheme 4.To gain a better understanding of the cat-
alytic hydrogenation of crotonaldehyde and oxidation of benzene,
a few control experiments was carried out. Experiments in the
absence of Pd failed to give any activity. Pd nanoparticles were
then extracted from the Pd/PEG catalyst by dissolving the catalyst
in pure water, followed by filtration to separate the Pd nanopar-
ticles. The hydrogenation and oxidation reactions were conducted
in presence of as-extracted Pd nanopartilces with no PEG matrix.
Conversions of 18% and 4% were obtained, respectively for hydro-
genation and oxidation reactions compared to 100% conversions in
case of using Pd/PEG4000 under the same experimental conditions.
This result suggests that the presence of PEG matrix is indispensi-
ble which could act as a mobile supporting phase thereby giving
higher stability and consequently enhancing the catalytic activity
of Pd nanoparticles.
with fresh reaction mixture in a subsequent run. The results of
recycling of Pd/PEG for hydrogenation of crotonaldehyde and oxi-
dation of benzene are presented in Fig. 5. The Pd/PEG catalyst
could be successfully recycled for ten times for crotonaldehyde
hydrogenation without any loss in activity, giving 100% conver-
sion. The catalyst activity dropped in runs 11 and 12, yielding
72% and 61% conversions, respectively. The deactivation of Pd/PEG
catalyst during the recycling procedure is most likely related to
the agglomeration of Pd nanoparticles which led to a decrease in
3.2.3. Recycling of the Pd/PEG catalyst
It is essential to recover the catalyst from the reaction medium
and reuse it for subsequent reaction cycles without loss of activ-
ity. Here, we studied the recyclability of the Pd/PEG catalyst
after the hydrogenation reaction of crotonaldehyde and oxida-
tion of benzene. After each reaction run, the organic products
were extracted into diethyl ether followed by simple decantation
[17], while Pd/PEG precipitated at the bottom of flask and reused
Fig. 5. Recycling of Pd/PEG catalyst after hydrogenation of crotonaldehyde and oxi-
dation of benzene.

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F.A. Harraz et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 182–188
the surface area of the catalyst. Fig. S1 (supplementary informa-
tion) shows a TEM image taken for the Pd/PEG catalyst after 12th
run. One could observe clearly the dissolution of the polymeric
matrix simultaneously with some degree of agglomeration of Pd
nanoparticles. Similarly, the recyclability of Pd/PEG catalyst was
also performed for the oxidation of benzene where complete con-
versions were achieved in the runs 1 and 2 as shown in Fig. 5.
The catalytic activity was then decreased in the 3rd run yield-
ing 71% conversion, then rapidly decreased down to 4% in the
6th run. With further reusing the catalyst after 6th run, no cat-
alytic activity was detected. As explained above, the agglomeration
of Pd nanoparticles takes place during the recycling procedure,
leading to the deactivation of the catalyst. The dissolution of PEG
matrix was found to be much faster during the oxidation reac-
tion of benzene. Based on above observation, the Pd/PEG catalyst
is much more recyclable in case of crotonaldehyde hydrogenation
compared to benzene oxidation. This is probably due to that the
hydrogenation on Pd surface takes place more easily than the oxi-
dation. Finally it should be mentioned that, the Pd/PEG catalyst
exhibits excellent storage stability for three months even upon
exposure to air and thus no special care is required for its stor-
age.
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