Palladium nanoparticles stabilized by polyethylene glycol: Efficient, recyclable catalyst for hydrogenation of styrene and nitrobenzene

Article abstract of DOI:10.1016/j.jcat.2011.11.001

We developed an efficient, simple chemical reduction method to produce highly active palladium (Pd) nanoparticles in polyethylene glycol (PEG) with no other stabilizer. The as-prepared Pd/PEG catalyst demonstrated a remarkable catalytic activity toward hydrogenation of both styrene and nitrobenzene under mild conditions. The Pd-PEG catalyst could be easily removed from the reaction mixture and its recyclability with no loss of activity was possible for seven times in case of styrene and three cycles for nitrobenzene. The catalytic performance was found to depend essentially on the catalyst and target concentrations and the reaction time. The catalyst was fully characterized by a variety of techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and N₂ adsorption-desorption isotherm. The reactivity of the Pd/PEG catalyst toward hydrogenation reactions is attributed to the high degree of dispersion of Pd(0) nanoparticles in PEG with small average particle size distribution of 5 nm. Results of the synthesis and characterization of Pd/PEG catalyst and its catalytic performance for hydrogenation reactions are presented and thoroughly discussed.

Full text of DOI:10.1016/j.jcat.2011.11.001

Journal of Catalysis 286 (2012) 184–192
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Palladium nanoparticles stabilized by polyethylene glycol: Efficient, recyclable
catalyst for hydrogenation of styrene and nitrobenzene
a
b
a
F.A. Harraz a, , S.E. El-Hout , H.M. Killa , I.A. Ibrahim
⇑
a Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute (CMRDI), P.O. 87 Helwan, Cairo 11421, Egypt
b 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:
Received 10 June 2011
Revised 27 September 2011
Accepted 1 November 2011
Available online 9 December 2011
We developed an efficient, simple chemical reduction method to produce highly active palladium (Pd)
nanoparticles in polyethylene glycol (PEG) with no other stabilizer. The as-prepared Pd/PEG catalyst
demonstrated a remarkable catalytic activity toward hydrogenation of both styrene and nitrobenzene
under mild conditions. The Pd-PEG catalyst could be easily removed from the reaction mixture and its
recyclability with no loss of activity was possible for seven times in case of styrene and three cycles
for nitrobenzene. The catalytic performance was found to depend essentially on the catalyst and target
concentrations and the reaction time. The catalyst was fully characterized by a variety of techniques
including X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis spectroscopy, Fourier
transform infrared (FT-IR) spectroscopy, and N2 adsorption–desorption isotherm. The reactivity of the
Pd/PEG catalyst toward hydrogenation reactions is attributed to the high degree of dispersion of Pd(0)
nanoparticles in PEG with small average particle size distribution of 5 nm. Results of the synthesis and
characterization of Pd/PEG catalyst and its catalytic performance for hydrogenation reactions are
presented and thoroughly discussed.
Keywords:
Palladium nanoparticles
Polyethylene glycol
Heterogeneous catalysis
Hydrogenation reaction
Styrene
Nitrobenzene
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
particles [22,23]. It should be noted, however, that the choice of
a suitable support is crucial because the interaction with the active
Transition-metal nanoparticles science is a strategic research
area in material development due to their particular physical and
chemical properties and its diverse application fields including
catalysis, photochemistry, electronics, optics, and magnetism
[1–9]. Particularly for catalysis applications, palladium (Pd) nano-
particles is a unique class of heterogeneous catalysts that is widely
used and effective for a variety of organic reactions due to its high
surface-to-volume ratio. However, active surface atoms often led
to aggregation of Pd(0) nanoparticles (Pd black), and hence the cat-
alytic activity and selectivity would decrease. Therefore, it is an
essential task to stabilize the nanoparticles for effective utilization.
Various supports have been developed to stabilize Pd nanoparti-
cles, including mesoporous silica support [10,11], surfactants
[12,13], dendrimers [14,15], ionic liquids [16,17], block copolymer
[18], and polyvinylpyridine [19]. In a recent report, Siamaki et al.
reported a microwave-assisted method to generate active Pd nano-
particles supported on graphene for carbon–carbon cross-coupling
reactions [20]. In another work, Wu et al. used polyphenol-grafted
collagen fibers to stabilize the Pd nanoparticles [21]. Different inor-
ganic materials, such as hydrotalcite, MgO, zeolite Beta, and
phase plays a critical role during the catalytic reactions [24]. The
main recorded disadvantages of the above supports are either its
complexity or high cost, some are not environmentally accepted
media, and poor selectivity and recyclability are sometimes
observed. Therefore, it is desirable to develop Pd nanoparticles
based catalytic systems supported/dispersed on a suitable mobile
phase that are active, selective, stable, nonhazardous, and
recyclable.
Metal-polymer based catalysts, on the other hand, are efficient
category of catalysts as they have advantages over traditional cat-
alysts including: large surface areas of the metal particles, effective
stabilization of the nanoparticles, possible control of particle shape
and size by varying the nature of the polymer, and the presence of
polymer layers on the metal particles can improve the catalytic
selectivity [25]. Polyethylene glycol (PEG) is a promising candidate
to serve as soluble polymeric support. This particular choice of PEG
is justified by its capability to act as both reducing agent and sta-
bilizer to prevent the agglomeration and precipitation of particles
in the solution [26]. In addition, PEG is inexpensive, nontoxic,
and its properties can be tuned by simply changing molecular
weight [27]. PEG is benign that is already approved for use in food
industry. These features make it ideal stabilizer for synthesis of Pd
nanoparticles, and therefore PEG can combine the advantages of
both homogeneous and heterogeneous catalysis.
c-Al2O3 have also been utilized as effective supports for Pd nano-
⇑
Corresponding author. Fax: +202 25010 639.
E-mail address: fharraz@cmrdi.sci.eg (F.A. Harraz).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.11.001

F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
185
Generally, there are four main synthetic approaches for prepa-
ration of catalysts [28]: (i) transition metal salt reduction, (ii) ther-
mal decomposition and photochemical methods, (iii) ligand
reduction and displacement from organometallics, and (iv) metal
vapor synthesis, in addition to the electrochemical synthesis ap-
proach [29]. Among the aforementioned methods, the chemical
reduction method that involves the reduction of corresponding
metal salts with suitable reducing agents is merely one of the most
cost-effective, since it requires simple equipment to produce the
metallic nanoparticles [30].
nation reactions could be achieved in this study, because the reac-
tants are miscible in PEG, while the products are immiscible.
Therefore, pure final products could be separated by simple decan-
tation. The catalytic performance of both systems is evaluated and
will be thoroughly addressed.
2. Experimental
2.1. Preparation of Pd nanoparticles
Pd nanopartciles based catalytic systems are increasingly inves-
tigated by several research groups and being used in various organ-
ic transformations. For example, Corma et al. used both PEG and
imidazolium ionic liquids as solvents for developing a homoge-
neous and reusable Pd catalytic system for the Suzuki, Sonogashira,
and Heck reactions [31,32]. A ligand-free Heck reaction has been
also catalyzed by in situ-generated Pd nanopartciles in PEG-400
as reported by Jin and co-workers [33]. As recent works of de Souza
et al. [34] and Wang and co-workers [35], Suzuki–Miyaura cross-
coupling reactions have been catalyzed efficiently using Pd/PVP
and Pd/PEG as catalysts under various conditions. Li et al. used
functionalized PEG supported Pd nanoparticles as a catalyst for
aerobic oxidation of alcohols [36]. A series of novel benzazepines
has been synthesized using a phosphine-free Pd-catalyzed Heck
reaction in PEG polymeric support [37], in addition organostannox-
ane-supported Pd nanopartciles have been recently developed for
Heck-coupling reactions [38].
The procedure was adapted to that reported by other authors
[26]. Pd(II) acetate [(CH3COO)2Pd] Aldrich with 99.9% purity was
added into PEG with different molecular weights (400, 800, 1000,
2000, 4000, and 6000) in a 50-ml round-bottomed flask. Different
wt.% of Pd acetate/ PEG, mainly 1.2%, 2.5%, 3.75% and 4.8%, were
essentially used. The mixture was heated in a water bath at 75 °C
under vigorous, continuous stirring for 2 h. During this stage, the
light yellow color of the solution turned brown and finally to gray
dark, indicating the formation of Pd nanoparticles. The mixture of
Pd nanoparticles in PEG solidified upon cooling at room
temperature.
2.2. Characterization of Pd/PEG catalyst
Phase identification and crystallite size measurements of Pd
nanoparticles were performed by X-ray diffraction (XRD, Bruker
AXS D8 Advance, Germany) with Cuk
a radiation (k = 1.5406 Å).
Of particular importance of Pd/PEG catalytic systems, there are
some precedents reporting the synthesis of Pd nanoparticles in PEG
[26,39,40]. Pillai et al. carried out selective hydrogenation of olefins
using phenanthroline stabilized Pd nanoparticles in PEG [39]. Luo
et al. used PEG with different chain lengths to generate Pd nano-
particles catalyst for Heck reaction [26]. Here, the most prominent
synthetic characteristic of the catalyst is the use of PEG alone with-
out using any additional ligands. Following the procedure of Luo et
al., Pd nanoparticles catalyst has been recently synthesized in PEG
and used for the hydrogenation of different types of olefins [40].
However, a definite understanding of the limits of Pd/PEG catalyst
formation and its morphology control is still lacking. To better
understand the conditions necessary for generating such a catalytic
system using a similar method as that proposed by Luo et al., a
detailed investigation was conducted in which the catalyst phase
and morphology were correlated to systematic variation in operat-
ing parameters. The effects of controlling reaction parameters such
as Pd loading, effect of using different molecular weights of PEG
(400–6000), the operating temperature, and the reaction time have
been investigated in detail in this work.
Measurements were taken with a tube power of 40 kV and
40 mA, from 10° to 80° 2h, with a 0.02° 2h step size and 0.4 s count
time. The specific surface area (SBET), pore volume, and pore size
distribution of the samples were determined by surface area and
pore size analyzer (QUANTACHROME-NOVA 2000 Series, UK). For
detailed morphological and structural analysis, an JEOL JEM-1230
transmission electron microscope (TEM) operating at 200 kV was
used. Selected-area electron diffraction (SAED) patterns for some
prepared catalysts were also recorded. Before TEM observation,
the samples were first prepared by dispersing a trace amount of
the catalyst in ethanol followed by ultrasonic vibration for
20 min. A carbon film coated copper grid was then quickly im-
mersed into the dispersion and left in air for drying. UV–vis spec-
troscopy measurements (V570 spectrophotometer Jasco Co.) were
performed for Pd(II) ions before and after reduction with PEG. FT-
IR spectra were measured for as-prepared catalyst by means of
3600 JASCO spectrophotometer. The spectra were collected over
the frequency range of 4000–400 cmꢀ1
.
In heterogeneous catalysis, the progressive deactivation of cat-
alysts is a major economic concern and mastering their stability
has become as essential as controlling their activity and selectivity.
For that reason, there is a strong motivation to develop new cheap-
er and cleaner approaches for synthesizing catalysts that are stable
and recyclable and to understand the mechanisms leading to any
loss in activity and selectivity. Accordingly, the second main objec-
tive of the present work is to examine/describe in details the cata-
lytic performance of as-synthesized Pd/PEG catalysts for the
hydrogenation of styrene and nitrobenzene as two reaction models
under mild conditions. Hydrogenation of C@C double bonds is of
significant importance due to great demands in chemical, petro-
chemical, pharmaceutical, and food industries [40]. The catalytic
hydrogenation of nitrobenzene, on the other hand, is an industri-
ally important reaction for the production of aniline, which is an
important intermediate for polyurethanes, dyes, pharmaceuticals,
explosives, and agricultural products [41].
2.3. Hydrogenation reactions
Hydrogenation of styrene and nitrobenzene was conducted in
liquid phase in a 50-ml round-bottomed flask equipped with a
reflux condenser and a magnetic stirrer. In a typical reaction proce-
dure, known concentrations of the target compound, 1 mmol of
biphenyl as standard and a certain weight of the Pd/PEG catalyst
were mixed in 20 ml of absolute ethanol as a solvent, stirring
and heating the mixture for 4 h at room temperature in hydrogen
atmosphere. After the reaction, the mixture was extracted with
diethyl ether and analyzed by Agilent GC 7890A model: G3440A
Gas Chromatography using 19091J-413 capillary column (30 m
ꢁ 320
l
m ꢁ 0.25
lm). Chromatographic conditions applied for
hydrogenation of styrene and nitrobenzene are summarized in
Table 1. In the catalyst recycling experiments, the reaction mixture
was cooled to room temperature, and 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. The
In this work, we show that the ideal green catalytic process of
‘‘one-phase catalysis and two-phase separation’’ for both hydroge-

186
F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
Table 1
Chromatographic conditions applied for hydrogenation of styrene and nitrobenzene.
Pd acetate
Pd/PEG catalyst
Hydrogenation of styrene
Reduction of nitrobenzene
Oven
60 °C for 3 min ? 200 °C
50 °C for 5 min ? 200 °C for
1 min
60 ? 200 °C 10 °C/min
50 ? 200 °C 10 °C/min
Detector Type: FID
Heater: 300 °C
Type: FID
Heater: 300 °C
H2 flow: 35 ml/min
Air flow: 400 ml/min
Makeup flow: 20 ml/min
H2 flow: 30 ml/min
Air flow: 400 ml/min
Makeup flow: 25 ml/min
Injection Heater: 250 °C
Heater: 250 °C
Pressure:10 psi
Pressure:13.762 psi
Total flow: 93.099 ml/min
Septum purge flow: 3 ml/
min
Total flow: 41.384 ml/min
Septum purge flow: 3 ml/min
300
400
500
600
700
800
Wavelength / nm
Mode: split with ratio (40:1) Mode: split with ratio (10:1)
Column
30 m ꢁ 320 m ꢁ 0.25 30 m ꢁ 320 m ꢁ 0.25
l
l
m
l
lm
Fig. 2. UV–vis spectra of palladium acetate and Pd/PEG4000 catalyst.
1.97, respectively). No significant change of the diffraction pattern
was observed upon changing the Pd(II) precursor, except the rela-
tively more intense peak obtained in case of using Pd(II) acetate,
which probably related to the different degree of chemical reduc-
tion of both Pd salts in PEG. The reflection peak shapes are broader
than that of bulk Pd, indicating that Pd(0) is highly dispersed into
PEG with small crystallite size. The crystallite size was calculated
using Scherer’s equation, and the value corresponding to the refec-
tion plane (111) was found to be 3 nm.
extracted Pd content was analyzed by an inductively coupled plas-
ma-optical emission spectrometer (ICP-OES) model Perkin/Elmer
Optima 2000. Photographs of recycling procedure are provided in
Supporting information (Fig. S1).
3. Results and discussion
3.1. Characterization of Pd/PEG catalyst
To ensure the complete reduction of Pd(II) ions, UV–vis spectra
of Pd(II) acetate before and after reduction with PEG4000 was mea-
sured, and the recorded spectra are shown in Fig. 2. As can be seen,
the UV–vis spectrum of Pd(II) acetate shows absorption maximum
at around 400 nm, which is characteristic of Pd(II) ions [26,43].
After the reaction of Pd(II) ions in PEG4000, the peak observed at
400 nm disappeared, indicating the complete conversion of Pd(II)
to Pd(0) nanoparticles.
The morphology of as-prepared catalyst was further observed
with TEM. The recorded image is shown in Fig. 3 with the SAED
pattern and particle size distribution depicted in the inset. The
TEM image visualized the presence of narrow size distributed
spherical Pd nanoparticles. It can be seen that the average dimen-
sion of Pd nanoparticles fabricated at 75 °C using PEG4000 is
around 4 nm, in good agreement with the crystallite size calculated
from XRD data. The SAED pattern shown in the inset exhibits the
Upon addition of Pd(II) acetate into PEG4000 with a 3.75 wt.%
(Pd(II) acetate 0.15 g (0.67 mmol)/ PEG 4 g (1 mmol) under vigor-
ous stirring at 75 °C for 2 h, the chemical reduction of Pd(II) ions
with PEG resulted in the in situ formation of Pd(0) nanoparticles
that were still stabilized by PEG matrix. Herein, PEG serves as both
a reducing agent and a stabilizer for the Pd(0) nanoparticles. The
crystalline structure of as-synthesized catalyst was obtained using
powder XRD. The typical patterns of catalysts prepared using Pd(II)
acetate and Pd(II) chloride in PEG4000 are presented in Fig. 1. The
inset shows two peaks at about 19.2° and 23.4° indicating the pres-
ence of pure PEG polymer [42]. As shown in the XRD patterns,
broad peak near 2h = 40° is detected, which can be indexed to
the characteristic reflection (111) plane for face-centered-cubic
Pd(0) with another weak peak around 2h = 43°, consistent with
the (200) crystalline plane (JCPDS card No. 87–641, d = 2.28,
Pd0
PEG
111
(200)
200
Pd chloride
(111)
SAED
5
10
15
20
25
30
Pd acetate
60
50
40
30
20
10
0
40
45
50
55
60
65
70
1
2
3
4
5
6
7
8
9 10 11
Particle size / nm
2 Theta / degree
Fig. 1. XRD for Pd nanoparticles prepared by different Pd sources. The inset is
Fig. 3. TEM image of Pd nanoparticles in PEG4000 with 3.75 wt.%. Particle size
related to the pattern of PEG.
distribution for Pd/PEG4000 (3.75%) and the SAED pattern are shown in the inset.

F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
187
Table 2
Textural properties from N2 adsorption–desorption isotherm measurements of Pd/PEG4000 catalysts prepared using different wt.% of Pd(II)/PEG.
a
b
c
d
e
f
Sample
BET (m2/g)
MicroPA (m2/g)
MicroPV (cc/g)
Total PV (cc/g)
Average PD (nm)
MicroPW (nm)
Pd/PEG (1.2%)
Pd/PEG (2.5%)
Pd/PEG (3.75%)
Pd/PEG (4.8%)
11.38
70
112.75
10.97
21.72
0.00766
0.0471
0.0759
0.00739
0.00294
0.0181
0.0291
0.00283
1.072
6.6
10.62
1.033
35.71
133.62
215.22
20.94
219.64
353.77
34.42
a
b
c
d
e
f
BET surface area calculated from the linear part of the BET plot.
Micropore area.
Micropore volume.
Total pore volume, taken from the volume of N2 adsorbed at about p/p° = 1.
Average pore diameter.
Micropore width.
(111) and (200) directions confirming again the cubic structures
and the crystalline nature of the catalyst, in consistent with XRD
result of Fig. 1. The TEM analysis implies that the long-chain
structure of PEG could provide good stability and dispersing effects
to the Pd nanoparticles and prevent its agglomeration.
The formation of Pd(0) nanoparticles by a thermal heating of Pd
precursor in the presence of PEG without adding any further reduc-
ing agent may proceed through the oxidation of the hydroxyl
group (OAH) in PEG chains. The FTIR analysis of PEG after the reac-
tion with Pd(II) ions revealed a new absorption peak appeared at
around 1739 cmꢀ1, which could be attributed to the formation of
aldehyde group (ACHO) during the reduction of Pd(II) ions (see
Fig. S2 of Supporting information). This is actually in agreement
with a similar analysis done by 1H NMR and confirmed the forma-
tion of aldehyde group for a similar reaction system [26].
Table 3
Crystallite size for Pd/PEG prepared at different temperatures and different times.
Temperature (°C)/time (h)
Crystallite size (nm)
BET (m2/g)
50/2
–
–
75/2
3
112.75
82.7
70
–
–
100/2
125/2
150/2
75/1
75/3
75/4
7.7
7.7
–
9.4
6.3
6.9
66.2
–
further conducted in which the phase formation, particle size,
and morphology were correlated to systematic variation in operat-
ing parameters.
Different Pd loading/ wt.% of Pd acetate/PEG (1.2%, 2.5%, 3.75%,
and 4.8%) were used to investigate the evolution of Pd nanoparti-
cles morphology and surface area. All samples were synthesized
at 75 °C for 2 h. The BET surface area (SBET) and pore parameters
of Pd/PEG4000 synthesized using different wt.% are summarized
in Table 2. The results reveal that the surface area of as-formed cat-
alyst remarkably increased from 11.4 to 112.8 m2/g by increasing
3.2. Influence of Pd loading, temperature, reaction time, and PEG
molecular weight on as-formed Pd nanoparticles
To better understand the conditions necessary for the formation
of Pd nanoparticles in PEG matrix, series of experiments were
Fig. 4. TEM images of Pd nanoparticles in PEG4000 prepared at different temperatures: 75 °C (a); 100 °C (b); 125 °C (c).

188
F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
Pd(II) acetate/PEG wt.% from 1.2% to 3.75%, whereas the surface
area dropped again with further increase in wt.%. The maximum
SBET and total pore volume were achieved at 3.75 wt.%. This result
is likely due to that the Pd/PEG wt.% affected the rate of chemical
reduction of Pd(II) ions by PEG as well as the coalescence degree
of as-formed Pd particles, which in turn controlled the particle size
and surface area of as-formed catalyst.
Reaction temperature and duration are also key operating
parameters during the thermal heating process of the reduction
of Pd(II) ions with PEG. Experiments using 3.75 wt.% and operating
at different temperatures—50, 75, 100, 125, and 150 °C—and at dif-
ferent times—1, 2, 3 and 4 h—were conducted. Table 3 shows the
dependence of crystallite size and surface area of as-synthesized
Pd/PEG4000 catalyst on temperature and time. It is worthy to note
that appropriate reaction time is necessary for obtaining an effi-
cient reduction of Pd(II) ions in PEG, which finally led to the de-
sired particle size and surface area of as-formed catalyst. Under
the present conditions, complete reduction of Pd(II) is likely
achieved after 2 h reaction duration. The finding indicates also that
at 50 °C, the chemical reduction of Pd(II) ions was not initiated or
the reaction itself is a very slow process at such low temperature.
To speed up the reduction of Pd(II) by PEG, the reaction tempera-
ture was increased to 75 °C. Pd(0) nanoparticles with good disper-
sion, small crystallite size and high SBET were accordingly formed.
However, by increasing the temperature above 75 °C, the crystal-
lite sizes increased with a consequence decrease in surface area,
which may be related to some degree of particle sintering. It has
been reported that the reduction in the specific surface area is
related to the reduction of energy associated with the interface be-
tween particles and environment as a thermally activated natural
phenomenon [44]. Another possibility is due to a decrease in pore
volume of the samples with a temperature rising, and hence the
surface area is expected to decrease. Examples of these catalyst
products are presented in TEM images of Fig. 4. One can see that
spherical Pd nanoparticles with average particle size 5 nm are
obviously visible in the polymeric matrix in the sample prepared
at 75 °C, Fig. 4a, compared to other samples prepared at 100 °C
or 125 °C, Fig. 4b and c, respectively. In addition, the separation
of Pd nanoparticles prepared at 75 °C was easier than those pre-
pared at higher temperature. This may be a consequence of the
good stability effect of PEG to the Pd nanoparticles at lower tem-
peratures [26].
Fig. 5. TEM images of Pd nanoparticles prepared in different PEG molecular weights
at 75 °C with 3.75 wt.% of Pd/PEG for 2 h: PEG2000 (a) and PEG6000 (b).
3.3. Catalytic performance
3.3.1. Hydrogenation of styrene
We also examined the effect of different PEG molecular weights
(400, 800, 1000, 2000, 4000, and 6000) on the formation of Pd/PEG
catalyst. It is an experimental fact in this study that the reactivity
or reducing power of PEG was found to be a function of its average
molecular weight. No reaction was observed, under the current
experimental conditions, using PEG with low molecular weights
of 400, 800, or 1000. Pd nanoparticles were partially developed
in PEG2000. A complete conversion of Pd(II) ions to Pd(0) was
achieved in PEG4000 and 6000. Fig. 5 displays the TEM images
taken for as-formed Pd nanopartciles prepared in PEG2000 and
6000 using the same Pd/PEG wt.% at 75 °C for 2 h. Small particle
size with high particle density and good dispersion of Pd(0) nano-
particles can be observed in case of using larger chain length of
PEG6000, Fig. 5 image b. This is indicative of rapid, easier reduction
of Pd(II) to Pd nanoparticles in higher PEG molecular weight. The
exact reason behind such behavior of different PEG molecular
weights is not very clear at the moment. However, it might be
related to the formation of metal complexes. It has been reported
that metal complexation would stabilize the metal ions against
reduction [45]. Based on this assumption, the coordination of Pd(II)
ions is probably more effective in lower PEG molecular weight,
which in turn may inhibit the electron transfer necessary for the
reduction process. Thus, a larger PEG chain length is preferred for
maximum rate of Pd(II) reduction.
The catalytic activity of as-synthesized Pd nanoparticles in
PEG4000 was firstly investigated using styrene hydrogenation as
a model catalytic reaction in a 1 mmol biphenyl as a standard
and 20 ml ethanol as a solvent at room temperature in a hydrogen
atmosphere (Scheme 1). Fig. 6 illustrates the% conversion of
reaction in Scheme 1 for different concentrations of styrene,
mainly 0.5, 1, 1.5, 2, and 2.5 mmol at room temperature using
the same weight of 25 mg catalyst for 90 min duration. The cata-
lyst demonstrated superior activity at concentration 61.5 mmol
styrene; complete conversion (100%) of styrene to ethylbenzene
product was achieved after 90 min at room temperature. At higher
concentration of styrene, the% conversion decreased to 80% and
Pd/PEG catalyst - H2
Room Temp
Ethylbenzene
Styrene
Scheme 1. Hydrogenation of styrene with Pd/PEG catalyst.

F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
189
120
100
80
100
80
60
40
20
0
60
40
20
0.5
1.0
1.5
2.0
2.5
0
Concentration of styrene / mmol
0
20
40
60
80
100
Conversion / %
Fig. 6. Effect of styrene concentration on the hydrogenation of styrene using Pd/
PEG4000 catalyst.
Fig. 7. Effect of reaction time on styrene hydrogenation using Pd/PEG4000 catalyst.
product per moles of Pd) of 990 and TOF of 660 hꢀ1 (Entry 4 of
Table 4). This TON is actually twice higher than a previously
reported value of (TON = 448) achieved for hydrogenation of sty-
rene using 1,10-phynanthroline-stabilized Pd nanoparticles in
PEG [39]. The reported particle size of phynanthroline-stabilized
Pd nanoparticles dispersed in PEG matrix was 2–6 nm size, and
thus cannot explain the different catalytic performance of their
catalyst and the Pd/PEG catalyst of the present work. A possible
explanation for this is likely attributed to the presence of phy-
nanthroline that could induce some surface deactivation for Pd
nanoparticles by sterically hindering the diffusion of reactants
toward the active sites of catalyst and eventually led to a lower
catalytic activity in comparison with the present work.
We carried out a few control experiments to gain a better
understanding of the hydrogenation reaction of styrene. Control
experiments in absence of Pd failed to give any activity. Pd nano-
particles were then extracted from the Pd/PEG catalyst by dissolv-
ing the catalyst in pure water, followed by filtration to separate the
Pd nanoparticles. The hydrogenation reaction of styrene was con-
ducted in presence of as-extracted Pd nanoparticles with no PEG
matrix. A conversion of 14% of styrene to ethylbenzene was ob-
tained compared to 100% conversion in case of using Pd/PEG4000
under the same experimental conditions (Entry 4 of Table 4). This
result suggests that the presence of PEG matrix is essential, which
could act as a mobile supporting phase thereby giving higher
stability and consequently enhancing the catalytic activity of Pd
nanoparticles.
39% using 2 and 2.5 mmol concentrations, respectively. Under the
current experimental conditions, the complete conversion to ethyl-
benzene was achieved after 90 min, while a 78% conversion was
obtained after 75 min. Further decrease in reaction time led to a
gradual decrease in the% conversion as shown in Fig. 7. At shorter
reaction time of 15 min, only 13% conversion was detected.
Determination of the minimum concentration of catalyst neces-
sary for full conversion of styrene is indispensable step in optimiz-
ing the hydrogenation reaction. Different weights of Pd/PEG
catalyst, mainly 150, 100, 50, 25, 19, and 13 mg in 20 ml of ethanol
solvent, 1.5 mmol styrene under hydrogen atmosphere at room
temperature were accordingly utilized. The results of styrene
conversion%, ethylbenzene product selectivity% and the values of
turnover frequency (TOF), calculated as the number of moles of
product per mol of Pd per h are summarized in Table 4. At smaller
weight of catalyst, 13 mg, only 10% conversion of styrene was
obtained after 90 min, in comparison with 60% for using 19 mg cat-
alyst for the same time. With a moderate catalyst weight of 25 mg,
the hydrogenation reaction was completed after 90 min at room
temperature, affording 100% styrene conversion to ethylbenzene
with 100% selectivity. Interestingly, the selectivity to the desired
product, ethylbenzene, was always 100% over the whole conver-
sion range of 10–100%. The selectivity of the present catalyst sys-
tem is mainly ascribed to the nanoparticles size of Pd and the
nature of Pd metal itself. This result demonstrates a remarkable
catalytic activity of Pd nanoparticles stabilized in PEG4000 toward
styrene hydrogenation with a turnover number (TON: moles of
Table 4
Pd nanoparicles in PEG for the hydrogenation of styrenea and nitrobenzeneb.
Entry
Substrate
Product
Catalyst weight (mg)
Time (min)
Substrate conversion (%)
Product selectivity (%)
TOF (hꢀ1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Aniline
Aniline
Aniline
Aniline
Aniline
150
100
50
25
19
13
100
50
25
13
90
90
90
90
90
100
100
100
100
60
10
100
84
41
16
93
42
100
100
100
100
100
100
100
91
83
71
93
86
126
178
322
660
385
189
83
126
90
73
90
180
180
180
180
120
90
100
100
100
71
32
18
Aniline
Aniline
60
22
87
TOF: turnover frequency calculated as the number of moles of product per mol of Pd per h.
a
Reaction conditions: 1.5 mmol styrene, stirring, H2 balloon, room temperature.
Reaction conditions: 1 mmol nitrobenzene, stirring, H2 balloon, room temperature.
b

190
F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
3.3.2. Reduction of nitrobenzene
sion% with time since the beginning of the catalytic reaction clearly
indicates that the Pd nanoparticles are well stabilized by PEG
matrix.
To extend the application of the Pd/PEG catalyst to the hydroge-
nation of aromatic nitro compound, we examined its catalytic per-
formance for the reduction of nitrobenzene to convert it to aniline.
The reduction reaction was evaluated using Pd/PEG4000 catalyst in
a liquid phase composed of 1 mmol biphenyl and 20 ml of ethanol
at room temperature in a hydrogen atmosphere (Scheme 2). For
those experiments, we chose first to apply the optimum conditions
achieved during the hydrogenation of styrene (1.5 mmol target,
25 mg (3.75 wt.%) Pd/PEG4000 catalyst, 1.5 h, room temperature)
for the reduction reaction of nitrobenzene. However, this experi-
ment showed that Pd/PEG has almost no activity: extremely
low% conversion of nitrobenzene (3%) to aniline was obtained. This
means that under identical experimental conditions, the hydroge-
nation of styrene over Pd/PEG catalyst is much easier and faster
compared to the reduction of nitrobenzene. We then changed the
nitrobenzene concentration, catalyst weight, and reaction time to
define the optimum levels necessary to achieve efficient catalytic
performance of Pd/PEG catalyst for the reaction described in
Scheme 2.
The% conversion of nitrobenzene to aniline was investigated
using different concentrations of nitrobenzene, mainly 1, 1.5, and
2 mmol at room temperature using 100 mg catalyst for 180 min.
With 1 mmol nitrobenzene, the catalyst showed 100% yield of ani-
line. While at higher nitrobenzene concentrations of 1.5 and
2 mmol, sharp decrease in% conversion was detected: the catalyst
yields the product with only 57% and 40% conversion, respectively.
The dependence of conversion%, selectivity% of aniline, and val-
ues of calculated TOF on different reaction times and catalyst
weights is presented in Table 4. As can be seen, under the present
conditions, the as-prepared Pd/PEG catalyst is effective to com-
pletely convert 1 mmol nitrobenzene to aniline within 180 min
at room temperature with 100% selectivity. By decreasing the reac-
tion time, a conversion of 93% was obtained after 120 min. With
further decrease in reaction time, a significant decrease in the%
conversion was observed: 42% conversion after 90 min and only
22% after 60 min. The observed increase in nitrobenzene conver-
The effect of using different Pd/PEG concentrations on the cata-
lytic hydrogenation of nitrobenzene is also listed in Table 4. As a
general trend, the% conversion was found to increase with an in-
crease in catalyst weight. Only 16% conversion of nitrobenzene
was achieved using 13 mg catalyst, in comparison with 41% ob-
tained at a doubled weight of 25 mg catalyst. With a catalyst
weight of 50 mg, the hydrogenation reaction reached 84%, whereas
a full conversion of nitrobenzene to pure aniline with 100% selec-
tivity was observed at 100 mg catalyst (Entry 7 of Table 4). It
should be noted that controls using analogous samples without
Pd showed no catalytic activity toward the conversion to aniline.
The above results indicate that the Pd nanoparticles stabilized in
PEG could exhibit a remarkable catalytic performance for the
reduction of nitrobenzene producing aniline with 100% yield with
a good TOF value of 83 hꢀ1. In addition, the product extraction
from the current catalytic system is extremely facile, which offers
the possibility for catalyst recycling as addressed in the following
section.
The reaction mechanism for the reduction of nitrobenzene to
aniline has been reported by a group of authors [46–49]. They indi-
cated that the hydrogenation reaction may proceed via successive
steps, which can be simply written as follows:
C6H5ANO2 ! C6H5ANO ! C6H5ANHOH ! C6H5ANH2
In another possible mechanism proposed by Gelder et al. [50], the
hydrogenation reaction may take place through the formation of
nitrosobenzene as an intermediate according to the following form:
C6H5ANO2 ! ½C6H5ANOHðadsÞꢂ ! C6H5ANHOH ! C6H5ANH2
where [C6H5ANOH(ads)] is the adsorbed species. As shown, phenyl-
hydroxylamine/ C6H5ANHOH, is an intermediate in both mecha-
nisms. It has been reported that the accumulation of this hazardous
intermediate is undesirable as it may lead to rapid exothermic
decomposition and/or formation of condensation products during
the hydrogenation of nitrobenzene [48,51]. In view of the consider-
ation mentioned above, a proposed mechanism of the reduction of
nitrobenzene using the current Pd/PEG catalyst involving phenyl-
hydroxylamine intermediate can be schematically depicted in
Scheme 3.
O
O-
N+
NH2
Pd/PEG catalyst - H2
Room Temp
3.3.3. Recycling of the Pd/PEG catalyst
In heterogeneous catalysis, it is essential, indispensable practi-
cal step to release and recover the catalyst from the reaction med-
ium and reuse it for subsequent reaction cycles until the catalyst
showed inactivity. Accordingly, to test the catalyst lifetime, the
ability to recycle the Pd/PEG catalyst was studied for the hydroge-
Nitrobenzene
Aniline
Scheme 2. Reduction of nitrobenzene with Pd/PEG catalyst.
+
N
N
N
N
+
N
-
HO
H
HO
H
H
H
O
O
-
O
O
-
H2O
H +
H
-
H+
H2O
H
Pd
Pd
Pd
Pd
Pd
H2
H2
H2
Scheme 3. Proposed mechanism for the Pd/PEG-catalyzed hydrogenation of nitrobenzene.

F.A. Harraz et al. / Journal of Catalysis 286 (2012) 184–192
191
observe any further increase in reactivity (conversion efficiency)
after the removal of the supported Pd nanoparticles from the reac-
tion medium. This means that the leached Pd, even if occurred, has
nothing to do with the catalytic hydrogenation reactions, indicat-
ing again that the hydrogenation reactions most probably proceed
heterogeneously. In addition, this observation suggests that Pd
leaching is not the direct reason behind the reduction in catalytic
activity during the catalyst recycling.
Another important piece of information concerning the practi-
cal use of heterogeneous catalyst is its storage stability. Generally,
the storage of supported Pd nanoparticles leads to a decrease in
catalytic activity, due to the oxygenation of Pd nanoparticles in
air [21,55]. Interestingly, the Pd/PEG catalyst prepared in this work
exhibits excellent storage stability for 3 months even upon expo-
sure to air, and thus no special care is required for its storage. This
actually indicates that Pd nanoparticles in PEG are stable enough to
retain high catalytic activity in recycling as well as during storage.
Encouraged by these promising results, further studies to utilize
this Pd/PEG catalyst system in other heterogeneous catalytic reac-
tions, including the hydrogenation of crotonaldehyde, the oxida-
tion of benzene, and the oxidative desulfurization of fuel are in
progress in our laboratories.
Styrene
120
100
80
60
40
20
0
Nitrobenzene
1
2
3
4
5
6
7
8
9
10
Number of cycles
Fig. 8. Recycling of Pd/PEG4000 catalyst used for hydrogenation of styrene and
nitrobenzene.
nation reactions of styrene and nitrobenzene. After each reaction
run, the organic products were extracted into diethyl ether fol-
lowed by simple decantation. The ether-immiscible PEG layer con-
taining Pd nanoparticles catalyst was recovered and reused with
fresh reaction mixture in a subsequent run; photographs of Sup-
porting information (Fig. S1) show the recycling procedure. The re-
sults of recycling uses of Pd/PEG catalyst for hydrogenation of
styrene and nitrobenzene are presented in Fig. 8. The Pd/PEG cata-
lyst could be successfully recycled for seven times for styrene
hydrogenation without any loss in activity, giving 100% conversion
to ethylbenzene. The catalyst activity slightly dropped in runs 8
and 9, yielding 97% and 93% conversions, respectively. In the
10th run, the catalyst showed 74% conversion. The deactivation
of the Pd/PEG catalyst is likely related to the formation of aggre-
gated Pd nanoparticles, which led to a decrease in the surface area
of the catalyst.
Similarly, the recyclability of Pd/PEG catalyst was also exam-
ined for the reduction of nitrobenzene to aniline. In this case, com-
plete conversions were achieved in the runs 1–3 as illustrated in
Fig. 8. The catalytic activity was then slightly dropped in the 4th
run yielding 98% conversion. The activity was further decreased
to 90% in run 7. A gradual decrease in activity was observed again
in runs 8–10 where conversions of 88%, 80%, and 67% were
obtained, respectively. As explained above, the agglomeration of
Pd nanoparticles is likely to take place which in turn led to the
deactivation of Pd/PEG catalyst during the recycling procedure.
Based on the above observations, the catalyst recyclability was
more efficient in case of styrene hydrogenation compared to the
reduction of nitrobenzene. The main reason may be related to
the different degrees of Pd aggregation in both reactions. In addi-
tion, the different leaching behavior of Pd nanoparticles from cat-
alyst into the reaction medium in both cases may be another
reason. The leaching of active species of catalyst is often a serious
problem for supported metal catalysts, which leads to catalyst
depletion and causes difficulty in catalyst separation and recycling.
Previous reports on Heck reaction catalyzed by supported Pd nano-
particles indicated that the Pd leaching process is essentially
dependent on different factors including the solvent, base, sub-
strate and product, and the surface nature of support [52–54].
We examined the leaching behavior of Pd/PEG catalyst for both
reactions. The Pd content was determined in the extracted solu-
tions using ICP-OES. Undetectable levels for Pd leaching (our ICP-
OES detection limit for Pd = 0.04 ppm) were obtained for both reac-
tion solutions, indicating extremely low degree of Pd leaching un-
der the current experimental conditions. In addition, we did not
4. Conclusion
A heterogeneous Pd(0) nanoparticles catalyst was successively
synthesized by simple, mild, and efficient chemical approach using
PEG as stabilizer with no additional reducing agent. The XRD and
TEM analysis confirmed the presence of highly dispersed Pd(0)
nanoparticles in PEG with small average particle size distribution
of 5 nm. The as-synthesized Pd/PEG catalyst exhibited an
outstanding catalytic activity and selectivity for the hydrogenation
of styrene and nitrobenzene under mild conditions in an environ-
mentally friendly solvent system. The catalytic performance is
markedly dependent on catalyst and target concentrations and
reaction time. Importantly, the catalysts demonstrated excellent
stability for easily removal from the reaction mixture and could
be reused for seven times for styrene and three times in case of
nitrobenzene with no loss in activity. No Pd leaching was detected
on examined samples during or after their use over ten reaction cy-
cles for both styrene and nitrobenzene, indicating the good stabi-
lizing effect of PEG for Pd nanoparticles. Extending the scope of
applications, this catalyst is expected to demonstrate a broad range
of utility for other heterogeneous catalytic reactions, which is
being underway.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2011.11.001.
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