PdAu alloy nanoparticles encapsulated by PPI-g-MWCNTs as a novel catalyst for chemoselective hydrogenation of alkenes under mild conditions

Article abstract of DOI:10.1007/s10562-013-1063-x

The synthesis, characterization and catalytic applications of bimetallic PdAu encapsulated on polypropylene imine grafted multi-wall carbon nanotubes hybrid materials have been reported. The results show that the catalyst induces a highly activity and chemoselective hydrogenation of less hindered alkenes to the corresponding alkanes using hydrogen gas in environmentally friendly solvents H₂O/EtOH at 50 °C with high yields. The characterization of catalyst was confirmed by FT-IR, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction.

Full text of DOI:10.1007/s10562-013-1063-x

Catal Lett
DOI 10.1007/s10562-013-1063-x
PdAu Alloy Nanoparticles Encapsulated by PPI-g-MWCNTs
as a Novel Catalyst for Chemoselective Hydrogenation
of Alkenes Under Mild Conditions
•
Ahmad Shaabani Mojtaba Mahyari
Received: 5 May 2013 / Accepted: 1 July 2013
Ó Springer Science+Business Media New York 2013
Abstract The synthesis, characterization and catalytic
applications of bimetallic PdAu encapsulated on polypro-
pylene imine grafted multi-wall carbon nanotubes hybrid
materials have been reported. The results show that the
catalyst induces a highly activity and chemoselective
hydrogenation of less hindered alkenes to the correspond-
ing alkanes using hydrogen gas in environmentally friendly
solvents H2O/EtOH at 50 °C with high yields. The char-
acterization of catalyst was confirmed by FT-IR,
transmission electron microscopy, X-ray photoelectron
spectroscopy, and X-ray powder diffraction.
divided into three main types [6]. Heterostructures, core–
shell structures, and intermetallic or alloyed structures.
Among these types, bimetallic alloy NPs are very important
nanomaterials because of their applications in large variety
of catalytic reactions, including catalytic alcohol oxidation,
reforming reactions, and pollution control [7–11]. The
addition of a second metal is an important approach for
tailoring the geometric and electronic structures of NPs to
promote their catalytic activity and selectivity.
Carbon nanotubes (CNTs) are attractive solid supports
for heterogeneous catalysts due to their unique properties
and surface structures [12], but there are some difficulties
in dispersing metal nanoparticles on the surface of pristine
CNTs.
Keywords Bimetallic Á Polypropylene imine Á
Multi-wall carbon nanotubes Á Chemoselective
hydrogenation
In this work, a well-defined polymer known as dendri-
mer [13, 14] is used as template to control the size, sta-
bility, and solubility of nanoparticles ranging in diameter
from\1 to 5 nm. Dendrimers are suitable for hosting metal
nanoparticles especially for catalytic systems due to the
following reasons: (i) the nanoparticles are stabilized by
encapsulation within the dendrimer, and they do not
agglomerate [15–21]; (ii) the encapsulated nanoparticles
are confined by steric effects, and therefore a substantial
fraction of their surface is unpassivated and available to
take part in catalytic reactions [16, 17, 19–21]; (iii)
moreover, the dendrimer branches can be employed as
selective gates to control access of small substrates to the
encapsulated nanoparticles and therefore inducing selec-
tivity to the reaction.
1 Introduction
The selective hydrogenation is a critical step in the synthesis
of chemical intermediates that have wide applications in
many fields such as foods, pharmaceuticals, cosmetics,
plastics, and lubricants industries [1–3]. Encapsulated noble
metals are industrial and academic important catalytic for
the selective hydrogenation [4, 5]. Bimetallic nanoparticles
(NPs) have appeared as an important class of catalysts.
Bimetallic nanoparticles based on the mixing pattern, can be
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-013-1063-x) contains supplementary
material, which is available to authorized users.
In continuation our research program on the catalytic
transformation of organic compounds [22–25], herein, we
report the synthesis of supported bimetallic PdAu nano-
particles via co-complexation method on first, second, and
third generation of polypropylene imine grafted multi-wall
carbon nanotubes (PPI-g-MWCNTs) for hydrogenation of
A. Shaabani (&) Á M. Mahyari
Department of Chemistry, Shahid Beheshti University,
G. C., P.O. Box 19396-4716, Tehran, Iran
e-mail: a-shaabani@sbu.ac.ir
123

A. Shaabani, M. Mahyari
A CP-Sil-8 fused silica capillary column (25 m–0.32 mm
i.d. and 0.52 mm film thickness) from Chrompack was
employed. X-ray photoelectron spectroscopy (XPS) analy-
sis was performed using a VG multilab 2000 spectrometer
(ThermoVG scientific) in an ultra high vacuum. Catalysis
products were analysed using a Varian 3900 GC (GC con-
version was obtained using n-decane as an internal standard
based on the amount of arylhalide employed relative to
authentic standard product). Ultrasonic bath (EUROSONICÒ
4D ultrasound cleaner with a frequency of 50 kHz and an
output power of 350 W) was used to disperse materials in
solvent. Thermogravimetric analysis (TGA) was carried out
using STA 1500 instrument at a heating rate of 10 °C min-1
in air. X-ray powder diffraction (XRD) data were collected on
an XD-3A diffractometer using Cu Ka radiation.
2.2.1 Preparation of MWCNTs–COCl
Typically 0.6–1.0 g of purified MWCNTs–COOH species
was stirred in a mixture of 160.0–200.0 mL SOCl2 and
20.0 mL DMF at 80 °C for 50 h. The MWCNTs–COCl was
isolated by vacuum filtration through a PTFE membrane
(pore size 0.2 mm) and washed with dry methylene chlo-
ride. Usually MWCNTs–COCl was used almost immedi-
ately after isolation in the next reaction step in order to avoid
the hydrolysis of it [27].
Scheme 1 Selective hydrogenation of alkenes by bimetallic PdAu
nanoparticles encapsulated by PPI-g-MWCNTs
alkenes. The results show that catalyst induces a highly
chemoselective hydrogenation of less hindered alkenes to
the corresponding alkanes under mild conditions with high
yields. The entire concept is summarized in Scheme 1
graphically.
2.2.2 Curtius Rearrangement for the Synthesis
of MWCNTs–NH2
MWCNTs–COCl (1.0 g) and NaN3 (2.0 g, 30 mmol) were
stirred in 800.0 mL DMF at room temperature for 30 h.
Then the temperature was raised up to 100 °C and stirring
continued for an additional 20 h. The reaction product was
isolated by filtration, and after 10 h sonication in conc.
hydrochloric acid yielded MWCNTs–NH2 [13].
2 Experimental
2.1 Materials
MWCNTs were prepared by chemical vapour deposition
procedure in the presence of Co/Mo/MgO as a catalyst at
900 °C [26]. The outer diameter of MWCNTs was between
20 and 40 nm. All solvents and reagents were purchased
from Aldrich or Merck and used without further purification.
2.2.3 Preparation of PPI-Grafted-MWCNTs Hybrid
Materials
First-, second- and third-generation PPI dendrimers were
synthesized on the amino functionalized MWCNTs. The
amino functionalized MWCNTs (0.8 g) was added in
portions at ambient temperature with stirring to acryloni-
trile (3.4 mL, 80 mmol) and methanol (10.0 mL) in a
100 mL round-bottomed flask. The reaction mixture was
stirred at ambient temperature under nitrogen atmosphere
for 5 days. After the completion of reaction, excess reac-
tants and solvent were removed under vacuum. The prod-
uct was washed with MeOH and CH2Cl2. It was dried
under vacuum for 24 h. Then, CoCl2Á6H2O (23.8 g,
100 mmol) in MeOH (300.0 mL) and NaBH4 (19.0 g,
500 mmol) were added to the product of previous step; the
2.2 Instruments and Characterization
1H NMR spectra were recorded with a BRUKER DRX-300
AVANCE spectrometer, and CDCl3 was used as a solvent.
IR spectra were recorded on a Bomem MB-Series FT-IR
spectrophotometer. A transmission electron microscopy
(TEM) analysis was performed by LEO 912AB electron
microscope. Identification and quantification were carried
out on a Varian model 3600 gas chromatograph (Varian
Iberica, Madrid, Spain) equipped with a split/splitless cap-
illary injection port and flame ionization detector (FID).
123

PdAu Alloy Nanoparticles Encapsulated by PPI-g-MWCNTs
mixture was stirred at 20 °C for 24 h. After that, the
mixture was acidified with 3 N HCl (100.0 mL). Next, the
product was filtered under vacuum, washed with ethanol
and water and dried under vacuum at 50 °C for 4 h. Then
the synthesis, titration method [28] was used to calculate
the amount of free primary amines in the periphery of the
dendrimers (Table 1) [13, 29].
Table 1 Amount of –NH2 groups and Pd(0) and Au(0) on each
generation of the PdAu Ns-PPI-g-MWCNTs hybrid materials
Entry
Generation
–NH2
contenta
(mmol g-1
Amount
of Au(0)b
(mmol g-1
Amount
of Pd(0)b
(mmol g-1
)
)
)
1
2
3
4
a
0
1
2
3
0.28
0.41
0.72
1.08
0.11
0.19
0.52
0.90
0.20
0.32
0.80
1.72
2.2.4 Preparation of Bimetallic PdAu Dendrimer-
Encapsulated Catalyst
Determined by titration method
Determined by AAS
b
First, aqueous solution of PdCl2 (0.1 g in 3.0 mL) and PPI-
g-MWCNTs (0.2 g in 10.0 mL) were mixed and placed in
an ultrasonic bath (50 kHz) for 20 min to well disperse
metal ions in the dendritic shell of hybrid materials. Then,
the mixture was stirred for 30 min, aqueous solution of
HAuCl4 (0.2 g in 3.0 mL) was added. The solution was
placed in an ultrasonic bath for 20 min, followed by
addition of 10.5 mL of a 1.0 M NaBH4 solution in 0.3 M
aqueous NaOH [17, 30]. After stirring for several hours, it
was filtered under vacuum, washed well with ethanol and
water and dried under vacuum at 50 °C for 4 h.
that CNT-supported metallic nanoparticles introduce a new
class of recyclable and efficient catalysts for chemical
syntheses specially for catalytic hydrogenation reactions.
The chemical modification of MWCNTs with first, second
and third generation of PPI and subsequent loading of pal-
ladium and gold has been outlined in Schemes 2 and 3. The
direct attachment of amino groups to SWCNTs opened
edges was reported by Gromov and co-workers [27]. First,
Curtius rearrangement for the synthesis of MWCNTs–
CONH2 was carried out. Amino functionalized MWCNT
undergoes Michael addition with acrylonitrile and subse-
quently the cobalt (II)-catalyzed reduction of the nitrile
group to produce first generation of PPI-g-MWCNTs. The
above step has been repeated to synthesis of second and third
generation of PPI-g-MWCNTs. The FT-IR spectra of each
generation PPI-g-MWCNTs were shown in Fig. 1. The
broad band at 3,418 cm-1 is due to the –NH2 stretching, the
absorption at 1,605 cm-1 is attributed to the N–H bending
of the amine groups and the bands at 1,360–1,370 cm-1
could be assigned to the C–N stretching of amine groups.
The bands at 3,000–3,048 cm-1 are assigned to the C–H
stretching. The increase in the relative intensity of the above
absorptions indicates that each generation of dendrimer was
successfully constructed on the surface of MWCNTs.
1H NMR results provide valuable information on the
functional groups in the third generation PPI-g-MWCNTs
samples in CDCl3. The hydrogen signals of the grafted
dendritic units are obviously observable in the corre-
2.3 General Catalytic Hydrogenation
A 10 mL Ace glass pressure tube was loaded with 0.008 g
of third generation PPI-g-MWCNTs (0.2 mol% Pd,
0.2 mol% Au) and 1 mmol of each alkene with 2.0 mL
mixture H2O/EtOH (1:1) and then pressurized to 2 bar with
molecular hydrogen (H2) as a hydrogen source. The pres-
sure tube was placed in an oil bath thermostated at 50 °C to
initiate the reaction. Aliquots of the organic phase were
analyzed by gas chromatography using a CP-Sil-8 fused
silica capillary column.
3 Results and Discussion
3.1 Synthesis and Characterization of Supported
Catalyst
Polyamidoamine (PAMAM) [17, 31] and polypropylenei-
mine (PPI) dendrimers are the most widely used dendrimers
in various applications, but we chose PPI because they are
stable at very high temperatures, whereas PAMAM den-
drimers undergo retro-Michael addition at temperatures
higher than about 100 °C [32]. We used carbon nanotube as
support because they may also act a role in the hydrogena-
tion processes. Kim et al. [33] reported that when hydrogen
gas is in contact with multiwalled carbon nanotubes
(MWCNTs) attached with Ni nanoparticles, the formation
of atomic hydrogen is possible. Although the reasons are not
well understood, experimental evidence supports the fact
1
sponding H NMR spectrum. The most shielded signal at
1.35 ppm is attributed to CH2CH2CH2 and the next signal
at 1.7 ppm could be assigned to CH2N. The peak at
7.6 ppm was assigned to NH2 groups (Fig. S1).
The titration method was used for the quantification of
the growth of the dendrimers on the surface MWCNTs.
The calculated amount of –NH2 groups on each generation
of the PPI-g-MWCNTs hybrid materials was shown in
Table 1.
Finally, bimetallic palladium-gold dendrimer-encapsu-
lated catalyst has been prepared by the co-complexation
123

A. Shaabani, M. Mahyari
Scheme 2 Synthesis route of carbon nanotube-supported poly(propylene imine) dendrimer
method [34]. Atomic absorption spectroscopy (AAS)
determined the amount of Pd and Au in each generation
(Table 1). As the generation of PPI rose, the loading of Pd
and Au nanoparticles was increased. This can be attributed
to an exponential increase of amino groups with the
increase in the generation of dendrimer.
The nitrogen (N 1s) signals are due to PPI produced
during the synthesis of PdAu Ns-PPI-g-MWCNTs hybrid
materials.
As shown in Fig. 2, the XRD pattern of third generation of
PPI-g-MWCNTs PdAu nanoparticles indicated a peak for
the Pd with a 2h value of about 39.24°, 45.46° and 66.19° and
well-defined peaks of the Au nanoparticles with 2h values
of 35.74°, 47.76°, 63.19°, 79.78°. The broad peaks at
2h = 23.27° are relative to the (002) planes of the graphite
like structure of the multi-walled carbon nanotubes [35].
According to literature, the spectrum of monometallic Au
reveled two sharp scattering peaks at 38.2° and 44.4°, which
are assigned to (111) and (200) planes, respectively. The
monometallic Pd revealed one peak centering at about
40.119°, which is identified as the (111) plane palladium.
Our catalyst (PdAu Ns-PPI (G3)-g-MWCNTs hybrid mate-
rials) revealed scattering peaks at 39.24° and 45.46° which
are very close to the peak positions of (111) and (200) planes
of gold. The position of the first peak clearly indicates an
intermediate scattering angle between those of the mono-
metallic nanoparticles. Consequently, it was found that
alloying of the two metal atoms was taking place throughout.
These results are consistent with previous reports of gold–
TEM of third generation PPI-g-MWCNTs contained
PdAu bimetallic nanoparticles and also showed the nano-
particles uniformly dispersed on MWCNTs (Fig. S2A, B).
Figure S2c shows the size distribution of encapsulated
palladium and gold bimetallic nanoparticles in the dendritic
shell of PPI-g-MWCNTs.
XPS data of the third generation of the bimetallic PdAu
nanoparticles PPI-g-MWCNTs showed about a 1:1 ratio of
PdAu content in the catalyst with 18.3 wt% Pd (1.7 mmol
Pd(0)/g catalyst) and 17.8 wt% Au (0.89 mmol Au(0)/g
catalyst) which are in good agreement with AAS results.
XPS spectra of the PdAu catalyst revealed the presence of
Pd(3d5/2) and Pd(3d3/2) peaks at 335.4 and 340.8 eV and
Au(4f7/2) and Au(4f5/2) peaks at 84.0 and 87.8 eV,
respectively (Fig. S3). These binding energy values are in
accordance with those reported for metallic Pd(0) and
Au(0) oxidation states.
123

PdAu Alloy Nanoparticles Encapsulated by PPI-g-MWCNTs
NH
N
2
NH
2
H
2
NH
2
H
N
2
H
N
2
NH
2
N
N
N
H
N
2
N
N
N
N
NH
2
NH
2
N
H
N
2
NH
2
N
H
2
N
N
N
NH
NH
2
H
N
2
N
2
N
N
N
N
N
H
N
2
NH
2
H N
2
N
N
NH
2
N
H
N
2
NH
2
N
N
H
N
2
H
2
4
l
C
NH
N
NH
2
2
NH
2
NH
2
NH
H
N
2
2
2
l
C
NH
u
2
H
N
2
NH
2
A
NH
2
H N
2
H
2
N
d
P
H N
2
H
H
2
N
N
N
N
N
N
N
H
N
2
N
H
N
2
N
N
N
N
NH
2
NH
2
N
N
NH
2
NH
2
N
N
NH
2
NH
2
H
N
N
2
H
N
2
N
NaBH4
N
N
NH
2
NH
2
N
N
N
NH
2
N
NH
H
2
N
H
2
H
2
N
H
N
N
N
N
N
2
N
2
N
N
N
N
N
NH
N
2
NH
2
N
N
NH
2
NH
2
H N
2
H N
2
H
2
N
H
2
N
N
N
N
N
NH
NH
2
2
N
N
H N
2
H N
2
H N
2
H
2
N
H
N
2
H
N
2
=PdAu Alloy Nanoparticles
Scheme 3 Synthesis route of PdAu Ns-PPI-g-MWCNTs hybrid materials
palladium alloy nanoparticles in which the XRD peaks were
found to lie between those of pure gold and pure palladium
nanoparticles [36]. Moreover, according to Scherrer’s
equation, it implies that the crystallite size is 1.9 nm for
PdAu alloy nanoparticles which is in good agreement with
TEM results. Considering the results obtained from XPS,
XRD, and also the synthesis method (co-complexation
method) of the nanoparticles no core–shell structure
observed [34, 36–38].
3.2 The Catalytic Activity of PdAu Ns-PPI-g-
MWCNTs Toward the Hydrogenation Reaction
To evaluate the catalytic properties of the catalyst, hydro-
genation reaction of cyclohexene was performed as a
model reaction. Since Au atoms draw electron density
away from Pd atoms, thereby enhancing the interaction of
Fig. 1 FT-IR spectra for carbon nanotube-supported dendrimer
123

A. Shaabani, M. Mahyari
Pd atoms with olephin, the large efficiency for the bime-
tallic materials is observed. Such synergistic interactions
have also been reported for PdPt and PdRh DECs [39–42].
The reaction was carried out in the presence of various
amounts of catalyst. As the catalyst amount was increased
(0.008 g), the reaction went to completion more rapidly at
50 °C. After screening a variety of solvents, mixed H2O/
EtOH was determined to be the best solvents that was used
due to coincidence to green chemistry principle. The
temperature of the reaction plays an important role,
increasing the temperature to 50 °C on the aqueous solvent
provided higher conversion. The best results were obtained
in the presence of 0.008 g of catalyst in mixed H2O/EtOH
(1:1, 2 mL) at 50 °C (Tables 2, 3).
The first, second, and third generation catalysts was
investigated for hydrogenation reaction of various alkenes
under 2 bar pressure of hydrogen gas at 50 °C. As indi-
cated in Table 2, the third generation catalyst shows the
most activity and selectivity for chemoselective hydroge-
nation of less hindered alkenes. As indicated in Fig. 3, the
sterically hindered alkenes did not undergo hydrogenation;
this was verified by competition reaction between cyclo-
hexene/1-methylcyclohexene with PdAu Ns-PPI (G3)-g-
MWCNTs catalyst.
CNTs are attractive solid supports for heterogeneous
catalysts due to their unique properties and surface struc-
tures [12, 43–45]. Kim et al. [33] reported the possibility
formation of atomic hydrogen on MWCNTs attached with
Ni nanoparticles. Although the reasons are not well
understood, experimental evidences support the fact that
CNT-supported metallic nanoparticles introduce a new
class of recyclable and efficient catalysts for chemical
syntheses especially for catalytic hydrogenation reactions.
We tested activity of MWCNT, PPI (G3)-g-MWCNTs and
PdAu Ns-MWCNTs. As shown in the Table 3 entries 1–3,
the MWCNT and PPI (G3)-g-MWCNTs without using
metals indicate little activity in the hydrogenation reaction,
but PdAu Ns-MWCNTs shows relatively good activity but
due to hydrophobic nature of CNT and its tendency for
agglomeration, we have to use as dendrimer to overcome
these problems [46–49]. Moreover, the existence of den-
drimer induce more selectivity to our system, because as
mentioned before the dendrimer branches can be employed
Fig. 2 X-ray diffraction (XRD) pattern of PdAu Ns-PPI (G3)-g-
MWCNTs hybrid materials
Table 2 Chemoselective hydrogenation of various alkenes with generations of catalyst
Entry
R
Less hindered (yield %)a
More hindered (yield %)a
G1
G2
G3
G1
G2
G3
a
b
c
C4H9
C6H13
C6H5
–
82
88
90
89
90
92
94
94
[99
[95
[98
[99
12
10
15
5
5
3
\2
\5
\4
0
7
d
\3
Reaction conditions: 1.00 mmol of each alkene, catalyst (0.008 g), EtOH/H2O (2 mL), H2 (2 bar) at 50 °C for 4 h
a
Yield determined by GC analysis
123

PdAu Alloy Nanoparticles Encapsulated by PPI-g-MWCNTs
results shows that the presence of gold nanoparticles
enhance the selectivity of catalyst. Palladium nanoparticles
lead to higher yields relative to gold nanoparticles but with
lower selectivity (entries 4 and 5). Therefore, the combi-
nation of gold and palladium in an alloy nanoparticle
configuration is an efficient way to enhance catalytic
activity and selectivity [32, 33] (entries 4 and 5).
Table 3 Comparison of PPI (G3)-g-MWCNTs, PdAu Ns-PPI (G3)-
g-MWCNTs, and monometallic counterparts as catalyst for hydro-
genation reaction
Entry
Catalyst
Yield (%)a
Less
hindered
More
hindered
1
2
3
4
5
6
7
MWCNTs
8
65
6
10
8
The combination of gold and palladium in an alloy
nanoparticle configuration was an efficient way to
enhanced catalytic activity due to the geometric and elec-
tronic effects [32, 33]. In addition, to investigate the
advantages of new catalyst over the AuPd immobilized on
other supports, we used activated carbon, which the results
presented more activity and selectivity for PdAu Ns-PPI
(G3)-g-MWCNTs, because as previously mentioned den-
dritic structures induce narrow size distribution of very
small nanoparticles and also the dendrimer branches can be
employed as selective gates to control access of small
substrates to the encapsulated nanoparticles and therefore
inducing selectivity to the reaction.
PdAu Ns-MWCNTs
PPI (G3)-g-MWCNTs
Au Ns-PPI (G3)-g-MWCNTs
Pd Ns-PPI (G3)-g-MWCNTs
PdAu Ns-C
9
72
\2
20
4
78
70
PdAu Ns-PPI (G3)-g-MWCNTs
[99
0
Reaction conditions: cyclohexene (1.00 mmol), 1-methylcyclohexene
(1.00 mmol), EtOH/H2O (2 mL), H2 (2 bar) at 50 °C for 4 h
a
Yield determined by GC analysis
When heterogeneous catalyst is used, the leaching of
active metal species into solution is an important issue that
should be considered. In a separate experiment, the catalyst
was filtered off after *50 % conversion under the reaction
conditions. The AAS analysis of the filtrate also confirmed
that the Pd and Au content in the solution were below the
detection limit (0.1 ppm). But since testing the filtrate for
Au and Pd may show no metal as they plate out onto the
reactor surface, we analyse the Au and Pd concentrations of
PdAu Ns-PPI (G3)-g-MWCNTs catalyst especially after
the 6 uses which contained 0.77 mmol g-1 Pd(0) and
0.5 mmol g-1 Au(0). So the leaching of Pd and Au after 6
uses was negligible.
Fig. 3 Reaction profile for competitive hydrogenation of cyclohex-
ene with 1-methylcyclohexene with PdAu Ns-PPI (G3)-g-MWCNTs
The recyclability and reusability of the PdAu Ns-PPI-g-
MWCNTs catalyst was tested for the hydrogenation reac-
tion mixed of cyclohexene and 1-methylcyclohexene up to
as selective gates to control access of small substrates to
the encapsulated nanoparticles and therefore inducing
selectivity to the reaction.
To confirm the activity and efficiency of PdAu Ns-PPI
(G3)-g-MWCNTs catalyst relative to corresponding indi-
vidual nanoparticles (Pd or Au) catalysts and also to
investigate the effect of support, hydrogenation reaction of
mixed cyclohexene and 1-methylcyclohexene as a model
reaction was examined. As indicated in Table 3, owing to
strong synergy between the metals, the catalytic hydroge-
nation of alkenes was significantly enhanced in the pres-
ence of bimetallic alloy NPs than their monometallic
counterparts (Table 3, entries 4, 5 and 7). The gold cata-
lysts exhibit much lower activity than the group VIII in
dissociation of H2 [50] and it is strongly dependent on the
amount of low coordination sites available on the gold
nanoparticles and is thus favoured when the size of
the particles decreases [51, 52]. Moreover, experimental
Fig. 4 Effect of recycling on the catalytic efficiency of PdAu Ns-PPI
(G3)-g-MWCNTs
123

A. Shaabani, M. Mahyari
18. Zhao M, Crooks RM (1999) Chem Mater 11:3379–3385
six cycles. The results in Fig. 4 demonstrate that after
every run, the yield and selectivity of reaction does not
change significantly; this finding shows the stability of
catalyst under experimental conditions.
19. Yeung LK, Crooks RM (2001) Nano Lett 1:14–17
20. Chechik V, Zhao M, Crooks RM (1999) J Am Chem Soc
121:4910–4911
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4 Conclusions
25. Shaabani A, Farhangi E (2009) Appl Catal A 371:148–152
26. Yeoh W-M, Lee K-Y, Chai S-P, Lee K-T, Mohamed AR (2009)
New Carbon Mater 24:119–123
In summary, first, second and third generation of PPI-g-
MWCNTs were successfully synthesized and PdAu nano-
particles encapsulated on this surface. Highly dispersed
PdAu Ns-PPI (G3)-g-MWCNTs hybrid materials showed
high chemoselectivity and activity for hydrogenation of
less hindered alkenes in mild and environmentally friendly
conditions. Due to the simplicity and non-hazardous nature
of the catalyst, it could be used in a variety of experiments
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