Preparation of Pd (0) and Pd (II) nanotubes and nanoparticles on modified bentonite and their catalytic activity in oxidation of ethyl benzene to acetophenone

Article abstract of DOI:10.1016/j.apcata.2010.03.062

The synthesis and application of palladium nanotubes and nanoparticles on modified bentonite was studied. In the first step, an organo-bentonite was prepared by the exchange of the exchangeable Na⁺ cations of a homoionic Na-bentonite by cetyl p

Full text of DOI:10.1016/j.apcata.2010.03.062

Applied Catalysis A: General 381 (2010) 121–131
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Preparation of Pd (0) and Pd (II) nanotubes and nanoparticles on modified
bentonite and their catalytic activity in oxidation of ethyl benzene to
acetophenone
M. Ghiacia,∗, Z. Sadeghib, M.E. Sedaghata, H. Karimi-Maleha, J. Safaei-Ghomib, A. Gilc
a Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
b Department of Chemistry, Faculty of Sciences, University of Kashan, Kashan, Iran
c Department of Applied Chemistry, Building Los Acebos, Public University of Navarra, Campus of Arrosadia, E-31006 Pamplona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and application of palladium nanotubes and nanoparticles on modified bentonite was
studied. In the first step, an organo-bentonite was prepared by the exchange of the exchangeable Na+
cations of a homoionic Na–bentonite by cetyl pyridinium cations (CP–bentonite), especially in the range of
low coverage ratios where surfactant ions are adsorbed through cation exchange with the counter ions of
bentonite. At this stage, there will be a disordered liquid-like monolayer arrangement of alkyl chain within
the gallery. This modified bentonite was loaded with the first generation of amidoamine hyperbranch
cascade, 3,3ꢀ-(dodecylazanediyl) bis(N-(2-(2-aminoethylamino)ethyl)propanamide) (DAEP), which has
a long aliphatic tail (C12) and a hydrophilic head. The solid/liquid interfacial layer of this architecturally
designed bentonite (DAEP–bentonite) was utilized as a nanoreactor for the synthesis of nanoparticles
of Pd2+ and Pd0. The structure, specific surface area, and porosity of bentonite are significantly altered
by the incorporation of nanoparticles. These alterations were monitored by several techniques such as
N2 adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and electrochemical
impedance spectroscopy (EIS). The size of the palladium nanoparticles prepared in this work was in the
range of 5–15 nm. Solvent free oxidation of ethyl benzene using tert-butyl hydroperoxide as an oxidant
showed that the palladium nanocatalysts prepared in this work, were highly active and selective.
© 2010 Elsevier B.V. All rights reserved.
Received 25 February 2010
Received in revised form 26 March 2010
Accepted 29 March 2010
Available online 4 April 2010
Keywords:
Palladium
Modified bentonite
Ethylbenzene
Oxidation
1. Introduction
ticles have been successfully applied to catalyze various chemical
reactions. Supported noble metals catalysts are widely used in
In the last decade, new directions of modern research, broadly
defined as nanoscale science and technology, have emerged [1,2].
These new trends involve the ability to fabricate, characterize, and
manipulate artificial structures, whose features are controlled at
the nanometer level. They embrace areas of research as diverse
as engineering, physics, chemistry, materials science, and molec-
ular biology [3]. Research in this direction has been triggered by
the recent availability of revolutionary instruments and approaches
that allow the investigation of material properties with a resolution
close to the atomic level. Strongly connected to such technolog-
ical advances are the pioneering studies that have revealed new
physical properties of matter at a level intermediate between
atomic/molecular and bulk [4].
industrial processes such as chemical synthesis, oil refining and
exhaust gas treatment [5].
Nanosized noble metal particles (e.g. Pt, Pd, Rh and Ru) dis-
persed on high surface area supports (e.g. Al2O3, SiO2 and clay) have
demonstrated high oxidation activity, good thermal stability and
selectivity [6–9]. In 1993, Balogh and Laszlo applied clays in het-
erogeneous systems as catalysts or catalyst supports [10]. In 1999,
Dekany and his co-workers developed preparation of palladium
nanoparticles on the surface of modified clays [11].
We have reported the synthesis of nanocrystalline semiconduc-
tors (CdS, ZnS) on the surface of hydrophobized Na–bentonite [12].
In the present work, and in continuation of our research interest
on modification of solid supports [13–15], the adsorption layer of
modified bentonite at the solid/liquid interface was utilized as a
nanophase reactor for preparation of nanocatalysts of Pd and for
their stabilization. The main point of the procedure, as described
deeply by Dekany et al. [11], was to modify the surface in such a
way that it could adsorb the active precursor, and especially the
noble metals in the adsorption layer of the surface of solid particles
dispersed in a liquid medium. In this work we have made a com-
Nanoparticles possess unique physical and chemical properties
different from bulk materials due to drastic reduction of particle
size. Among those nanoparticles, palladium and its alloy nanopar-
∗
Corresponding author. Tel.: +98311 391 3254; fax: +98311 391 2350.
E-mail address: mghiaci@cc.iut.ac.ir (M. Ghiaci).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.03.062

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M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
parison between a modified bentonite with hydrophobic nature,
and a bentonite which was modified in two steps: in the first step,
with a cationic surfactant (cetyl pyridinium bromide, CP) which
form a disordered liquid-like monolayer arrangement of surfac-
tant in the gallery, and in the second step with the first generation
of an amidoamine cascade, named 3,3ꢀ-(dodecylazanediyl) bis(N-
(2-(2-aminoethylamino)ethyl)propanamide) (DAEP) that adsorbs
as a nonionic surfactant on the hydrophobized bentonite, through
Van der Waals interactions. We used n-hexane as the dispersion
medium because the precursor ions are practically insoluble in it.
When solubility in the liquid phase is poor, it is expected that the
precursor ions are preferentially adsorbed on the solid surface and
their concentration within the bulk phase will be close to zero. With
this strategy in our mind, the objective of our study was to exper-
imentally investigate the effect of the modified bentonites on the
size of the nanoparticles generated on the surface, and the catalytic
activity of the resulting immobilized palladium nanoparticles in
oxidation of ethyl benzene.
The aromatic ketones are important starting materials for many
organic reactions, some of which find also industrial applications.
The usual way for their production is Friedel–Crafts acylation by
acid chlorides or anhydrides in the presence of acid catalysts of
Brønsted or Lewis type. The present demand for cleaner chemical
processes requires the use of environmentally friendly catalysts
avoiding the evolution of aggressive gases or corrosive solutions.
The palladium nanoparticles present a promising future for the
Friedel–Crafts reactions. We herein report the use of Pd nanoparti-
cles immobilized on modified bentonite as catalyst in the oxidation
of ethyl benzene using t-butyl hydroperoxide as oxidant for produc-
tion of acetophenone.
FTIR spectra of the catalyst were recorded on a JASCO FTIR 680
plus spectrometer with the KBr pellet method.
2.3. General procedures
2.3.1. Synthesis of 3,3ꢀ-(dodecylazanediyl)bis(N-(2-(2-
aminoethylamino)ethyl)propanamide)
(DAEP)
Methyl acrylate (3 mmol) was added slowly to a stirred solution
of dodecylamine (1 mmol) in dry CH3OH (20 mL). After stirring for
24 h at reflux temperature under nitrogen atmosphere, the solution
was evaporated giving 3,3ꢀ-(dodecylazanediyl)dipropanoate, (I),
which was used in the subsequent step without need for purifica-
tion. In the next step, diethylenetriamine (DETA) (5.25 mmol) was
slowly added to 1.75 mmol of 3,3ꢀ-(dodecylazanediyl)dipropanoate
(I) in dry CH3OH (20 mL), and the mixture was left stir-
ring under N2 at reflux temperature for 4 days. The solution
was poured into water, the solid residue dissolved in chlo-
roform and the organic extract was dried (MgSO4) and
evaporated
affording
3,3ꢀ-(dodecylazanediyl)bis(N-(2-(2-
aminoethylamino)ethyl)propanamide) (DAEP) (II) (Scheme 1).
The product was characterized using FTIR, 1H NMR, CHN and mass
spectrometry. Yield 78%, m.p. 38 ◦C. IR (KBr): ꢁmax/cm−1, 3553
(m), 3413(w), 1657(m), 1589(s). 1H NMR (CDCl3): ı (ppm), 0.60
(CH3, 3H), 0.98 (CH2, 2OH), 1.20 (NH2, NH, 6H), 1.53 (CH2, 12H),
2.05 (CH2, 2H), 2.42 (CH2, 4H), 2.44 (CH2, 2H), 2.53 (CH2, 4H),
2.57 (CH2, 2H), 4.65 (NH, 2H). Elemental analysis % calculated for
C26H57N7O2, C = 62.48, H = 11.50, N = 19.62 and observed % was
C = 62.43, H = 11. 57, N = 19.70. Mass (m/z): 499.
2.3.2. Purification and modification of bentonite
Since the XRD analysis revealed the presence of significant
amount of quartz and feldspar, the raw material was purified using
the following procedure. Bentonite (5% by mass), was dispersed
in water under continuous stirring. After separation of quartz
and feldspar, 1 M NaCl solution was added. The suspension was
stirred overnight followed by decantation. The solid was washed
until a negative test for chloride in solution was obtained. The
cation-exchange capacity (CEC) of the Na–bentonite was measured
(0.70 mequiv. g−1), by the method of Ming and Dixon [16].
In order to modify the bentonite, on the basis of the critical
micelle concentration of the 3,3ꢀ-(dodecylazanediyl)bis(N-(2-(2-
aminoethylamino) ethyl)propanamide) (DAEP), this surfactant was
loaded on the bentonite to obtain surfactant modified bentonite
(DAEP–bentonite). 1 g of Na–bentonite was dispersed in 100 mL
of DAEP solution (20%, w/w, ethanol–water) (Scheme 2). The dis-
persion was stirred at 200 rpm for 24 h and centrifuged. The solid
was washed with water to remove excess surfactant and surfactant
loosely attached to the bentonite particles.
2. Experimental
2.1. Materials
The parent bentonite had the following chemical composition
(in wt%): SiO2 (65.04), Fe2O3 (1.67), MgO (1.87), Al2O3 (13.61), CaO
(2.01), TiO2 (0.19), Na2O (2.26), K2O (0.75). It was obtained from
Salafchegan mine (Salafchegan, Iran). Cetyl pyridinium bromide
(CP), methyl acrylate, dodecylamine, and diethylene triamine were
purchased from Merck and used as received. The metal precursor
PdCl2 (purity 99%) was obtained from Aldrich. All other chemicals
used in this study were of analytical grade.
2.2. Characterization techniques
X-ray powder diffraction was performed on a Phillips diffrac-
tometer with CuK␣ radiation (40 kV, 30 mA) over a 2ꢀ range
between 3 and 80◦.
Scanning electron micrographs were obtained using a Cam-
bridge Oxford 7060 Scanning Electron Microscope (SEM) connected
to a four-quadrant backscattered electron detector with resolution
of 1.38 eV. The samples were dusted on a double sided carbon tape
placed on a metal stub and coated with a layer of gold to minimize
charging effects.
Transmission electron microscopy (TEM) was carried out on the
powder samples with a Tecnai F30 TEM operating at an accelerating
voltage of 300 kV. In addition, energy dispersive X-ray analysis was
conducted on each sample.
2.3.3. Preparation of immobilized palladium (II) and Pd (0) on
modified bentonites
In a typical experiment to prepare Pd (II)–bentonite composite,
500 mg of the monolayer surfactant coverage modified bentonite
(DAEP–bentonite), was dispersed in 100 mL of n-hexane, to which
100 ␮L of 10−2 M aqueous solutions of PdCl2 were added in 10
steps (each step 10 ␮L) to the suspension during 72 h, under vigor-
ous stirring in a Morton flask. The resulting product was separated
via centrifugation, and washed few times with distilled water. The
product was dried under vacuum at room temperature before char-
acterization and catalytic activity measurements.
Chemical reduction of Pd (II) loaded on modified bentonites
with excess NaBH4 results in formation of intralayer Pd clus-
ters or interdendrite Pd clusters (Scheme 3) on CP–bentonite and
DAEP–bentonite, respectively. Evidence for this reduction comes
from the immediate change in solid color.
The solid-state UV–vis spectra were recorded using
PerkinElmer model Lambda 650 spectrophotometer.
a
BET surface area and pore size distribution were measured on a
Micromeritics Digisorb 2600 system at −196 ◦C using N2 as adsor-
bate. Before measurements, the samples were degassed at 450 ◦C
for 3 h under vacuum (0.1333 Pa).

M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
123
Scheme 1. Synthesis of 3,3ꢀ-(dodecylazanediyl)bis(N-(2-(2-aminoethylamino)ethyl) propanamide).
2.3.4. General procedure for oxidation of ethyl benzene
madzu QP 5000; DB 1 column). Identification of products was done
by comparing the GC retention times of expected products with
those of standard samples. Leaching experiments were carried out
in order to prove the heterogeneous character of the reactions. In
representative tests, the catalyst was filtered out and the filtrate
was analyzed for Pd content using inductively coupled plasma (ICP).
A 25 mL round bottom flask was charged with 1 mmol of
ethyl benzene, 0.03 g (0.021 wt% Pd) of catalyst, and the amount
of oxidant (0.42 mL of 80% tert-butyl hydroperoxide in 1,2-
dichloroethane), and placed in a thermostatic oil bath and fitted
with water cooled condenser. The reaction mixture was stirred
at reflux temperature (80 ◦C) for 24 h. The progress of the reac-
tion was monitored by gas chromatography (Agilent technologies
6890N, HP-50 capillary column). Finally, the catalyst was filtered
off and washed with ethyl acetate. The filtrate and washings were
collected. Finally, the products were analyzed using GC–MS (Shi-
2.3.5. General procedure for the electrochemical measurements
2.3.5.1. Apparatus. Cyclic voltammetry (CV) and electrochemical
impedance spectroscopy (EIS), were performed in an analytical
system, Autolab with PGSTAT 12 (Eco Chemie B.V., Utrecht, The
Scheme 2. Model for the sorption of surfactant on the clay surface.

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M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
Scheme 3. A model of modification of clay surface by surfactant and preparation of Pd nanoparticles.
Table 1
Characterization of clay modified monolayer with CP and then modified bilayer with DAEP.
c
d
Entry
Support
Composition
N (%)
Amount of CP (mmol m−2
)
Amount of CP + DAEP (mmol m−2
)
C (%)
H (%)
C/N
1
2
CP–bentonitea
0.27
1.69
1.75
3.55
1.44
1.68
6.48
2.10
3.04 × 10−5e
3.17 × 10−5f
DAEP–bentoniteb
2.93 × 10−5f
a
Bentonite with monolayer CP coverage.
Bentonite modified with CP and DAEP.
Determined by elemental analysis.
Determined by back titration with NaOH [17].
Amount of CP on the bentonite for preparation of monolayer modified bentonite (CP–bentonite).
Amount of DAEP on the DAEP–bentonite.
b
c
d
e
f
Netherlands). For impedance measurements, a frequency range of
100–0.10 Hz was employed. The AC voltage amplitude used was
5 mV, and the equilibrium time was 10 min. A conventional three-
electrode cell assembly consisting of a platinum wire as an auxiliary
electrode and an Ag/AgCl (KClsat) electrode as a reference electrode
was used. The working electrode was either a carbon nanotube
paste electrode modified with Pd (0) and Pd (II). A pH-meter (Corn-
ing, Model 140) with a double junction glass electrode was used to
check the pH of the solutions.
isotherm of DAEP on modified bentonite with monolayer cover-
age of CP–bentonite (Fig. 1). From the estimation of amino groups
and CHN analysis (Table 1), it was found that the final modified
bentonite contained 3.35 mmol of DAEP per gram of the final solid.
Thus, there are 6.7 mmol of primary amine groups, 6.7 mmol of sec-
ondary amino groups, 3.35 mmol of tertiary amine, and 6.7 mmol
of amide groups per gram of the final solid. This final modified
bentonite was designated as DAEP–bentonite as mentioned before.
Complexation behaviour of Pd (II) ions and Pd (0) with
the immobilized dendrite was studied. In this regard, 0.5 g of
the DAEP–bentonite was dispersed in 100 mL of dry n-hexane
overnight. The reason for choosing n-hexane as the medium was
that DAEP–bentonite, swells very easily in a non-polar solvent. In
the next step, 100 ␮L of a PdCl2 solution (10−2 M) was added during
10 days in 10 steps (10 ␮L × 10 ␮L). This is because the dendrimeric
head of the DAEP was more solvated in polar solvent, in this case
2.3.5.2. Preparation of the electrode. Supported nanoparticles
(0.1 g) were dispersed in diethyl ether and hand mixed with 70-
times their weight of graphite powder and 20-times their weight
of carbon nanotube in a mortar and pestle. The solvent was evapo-
rated by stirring. Using a syringe, paraffin was added to the mixture
and mixed well for 20 min until a uniformly wetted paste was
obtained. The paste was then packed into a glass tube. Electrical
contact was made by pushing a copper wire down the glass tube
into the back of the mixture. When necessary, a new surface was
obtained by pushing an excess of the paste out of the tube and
polishing it on a weighing paper.
3. Results and discussion
3.1. Synthesis of bentonite-supported dendrimer–palladium
complex
DAEP-dendrite was immobilized on
a modified bentonite
starting from bentonite with monolayer cetyl pyridinium cation
coverage (CP–bentonite). The scheme of the process is shown in
Scheme 2. Immobilization of DAEP proceeded smoothly on the
modified bentonite. This can be observed from the adsorption
Fig. 1. Isotherm of adsorption against concentration of DAEP on modified bentonite
with monolayer coverage of CP at room temperature.

M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
125
Table 2
Specific surface area and pore volume for bentonite, modified bentonite and ben-
tonite with palladium nanoparticle.
Entry Catalyst
Metal content
(wt%)
SBET (m2 g−1
)
Vp (cm3 g−1
)
1
2
3
4
5
6
7
Na–bentonite
124
26
43
34
34
51
75
0.051
0.102
0.074
0.105
0.094
0.079
0.060
CP–bentonitea
DAEP–bentoniteb
CP–bentonite–Pd (II)
CP–bentonite–Pd (0)
DAEP–bentonite–Pd (II) 0.021
DAEP–bentonite–Pd (0) 0.021
0.021
0.021
a
Bentonite with monolayer CP coverage.
Bentonite modified with CP and DAEP.
b
water, and so the ligand and metal ions were in a more interacting
state, which facilitated the metal intake. To prepare Pd (0) com-
plexes 100 fold excess of NaBH4 aqueous solution was applied [18].
This kind of behaviour by dendrimers under homogenous condi-
tions was previously reported [19]. The amount of palladium ions
attached on the immobilized dendrimeric ligand under various
conditions is given in Table 2.
Fig. 2 represents the FTIR spectra of several samples starting
from Na–bentonite to immobilized dendrite and after complexa-
tion with Pd (II) and Pd (0). The peaks corresponding to the primary
amino groups of the dendrite appear at 3390 cm−1 and 3344 cm−1
Fig. 3. UV–vis spectra of (A) bentonite modified with a monolayer coverage of CP; (B)
DAEP–bentonite; (C) CP–bentonite support with Pd (II); (D) CP–bentonite support
with Pd (0); (E) DAEP–bentonite with Pd (II); (F) DAEP–bentonite with Pd (0).
which shifted to 3358 cm−1 and 3300 cm−1 after complexation with
the metal, respectively. This shift is due to participation of the pri-
mary amino groups in forming coordination bonds with the metal.
There is no change in the band corresponding to the amide groups;
this means that the amide groups do not take part in complexa-
tion. The electronic spectrum of the complex showed a broad band
at 450 nm, which may be due to the charge transfer transition, and
a band at 650 cm−1 due to first excited state transition. Complexes
of the types [PdL4]2+ and [PdL2Cl2] can be assigned to the clusters
of the encapsulated Pd (II) ions, and probably complexes of the type
[Pd0L2Cl]− [20] could be produced after reduction (Fig. 3). Clearly,
complex formation between the metal and amino groups of various
branches of the same dendrite is possible.
Transmission electron microscopy (TEM) images (Fig. 4) clearly
show that Pd encapsulated particles are nearly monodisperse and
that their shape is spherical. The metal particle diameters are less
than 10 nm.
Table 2 gives the specific surface area and the pore volume data
of the immobilized palladium nanoparticles on DAEP–bentonite.
Modification of bentonite with cetyl pyridinium bromide as a
monolayer decreased the specific surface area. Interestingly, by
adding DAEP surfactant to the monolayer modified bentonite
to obtain bilayers modified bentonite, the specific surface area
increases which might be reasonable, because the hydrophilic
head of the DAEP molecules have a water solvation core and
one expect that it swells the bentonite to some extent. By
loading these modified bentonites with Pd (II) and Pd (0),
Fig. 2. FTIR spectra of (A) Na–bentonite; (B) bentonite modified with a mono-
layer coverage of CP; (C) DAEP–bentonite; (D) DAEP–bentonite with Pd (II); (E)
DAEP–bentonite with Pd (0).

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M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
Fig. 4. TEM image of (A) immobilized DAEP on modified bentonite with a monolayer coverage of CP; (B) DAEP–bentonite with Pd (II); (C) DAEP–bentonite with Pd (0).
it is plausible to observe an increase in the specific surface
area.
Palladium nanoparticles seem to change the porosity of the aggre-
gated particles.
The XRD patterns (not shown) of the air-dried surfactant
complexes with clay (air humidity, 59%) shows that at the
first step of modification with CP, a peak corresponding to
d(0 0 1) of 1.32 nm is found, and after loading the bentonite with
monolayer surfactant coverage with DAEP, the interlamellar dis-
tances do not change. This suggests that DAEP cannot penetrate
into interlamellar spaces, and just occupy the edges and the
pores.
Scanning electron microscope images of the modified ben-
tonites with or without palladium nanoparticles are shown in Fig. 5.
Palladium nanoparticles had a strong influence on the particle
structure. The modified bentonite before loading with palladium
showed a pack type aggregation of the particles completely differ-
ent from the SEM images of palladium loaded modified bentonites.
3.2. Synthesis of palladium nanoparticles and nanotubes on
surfactant modified bentonite
The sorption of cationic surfactant on a negatively charged
surface involves both cation exchange and hydrophobic bond-
ing. At low loading levels, surfactant monomers are retained by
ion exchange and eventually form a monolayer. At this stage, by
suspending the modified bentonite in an organic solvent such as
n-hexane, and then adding water to this mixture, water molecules
will try to find a site on the surface of the bentonite, especially
in places where electrostatic charges exist. One would expect that
these electrostatic charges develop where the negative charges of
the surface have interaction with the polar head of the surfactant.

M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
127
Fig. 5. SEM image of (A) Na–bentonite; (B) bentonite modified with a monolayer coverage of CP; (C) CP–bentonite support with Pd (II); (D) CP–bentonite support with Pd
(0) (E) DAEP–bentonite; (F) DAEP–bentonite with Pd (II); (G) DAEP–bentonite with Pd (0).
–CH2 units, and the absorption band appeared between 1000 cm−1
and 1260 cm−1 are assigned to the Si–O–Si stretching vibrations.
The electronic spectrum of the Pd (0) hybrid material is clearly sim-
ilar to the electronic spectrum of monolayer modified bentonite,
although the spectrum of Pd (II) shows some charge transfer tran-
sitions (Fig. 3).
The specific surface area and the pore volume data for the immo-
bilized palladium nanoparticles and nanotubes on CP–bentonite
are reported in Table 2. The BET surface areas of the new materi-
als along with reference Na–bentonite were measured both before
and after the addition of palladium. The specific surface area of
CP–bentonite decreased when it compared to Na–bentonite, but by
Therefore, by adding palladium solution to the suspended ben-
tonite in n-hexane, and depending on the size of the added aqueous
solution (in each step, 10 ␮L), separate aqueous pools are formed on
the surface. The size of the created water pools exclusively deter-
mines the size of the palladium chloride clusters.
Fig. 6 represents the FTIR spectra of various samples starting
from Na–bentonite to CP–bentonite and after loading with Pd (II)
and Pd (0). All characteristic peaks exist but give the vibrations of
organic moieties in the spectra of CP–bentonite, and modified ben-
tonite with palladium nanoparticles compared with Na–bentonite.
The vibrations between 2850 cm−1 and 2960 cm−1 are assigned to
the asymmetric and symmetric stretching vibrations of –CH3 and

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M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
immobilizing palladium the specific surface area of CP–bentonite
has increased that might be due to the water pools entered between
the layers of bentonite. The presence of Pd (II) nanoparticles or Pd
(0) nanotubes in these water pools does not have any significant
effect on the surface area.
Transmission electron microscopy (TEM) images (Fig. 7) show
that the immobilized palladium nanotubes are empty tubes with a
diameter of about 50 nm. Although the preparation of Pd nanopar-
ticles supported on various supports has been reported elsewhere
[21], to best of our knowledge there have been no reports on the
preparation of palladium nanotubes in the absence of a template.
3.3. Oxidation of ethyl benzene catalyzed by DAEP–bentonite–Pd
complexes
In order to study the catalytic properties of the modified
bentonite-supported palladium complexes, we carried out the oxi-
dation of ethyl benzene as a model reaction. Initially, the ratio of
substrate to oxidant was changed in the presence of various cata-
lysts (Table 3). The ratio of 1:5 was selected as the best molar ratio
of substrate to oxidant, and the DAEP–bentonite–Pd (II) was chosen
as the best catalyst. Then, the reaction was carried out at various
temperatures (Table 4). The conversion was increased to about 92%
at reflux conditions. By optimization of the amount of catalyst and
the reaction temperature, the time (Table 5) was also optimized.
After completion of the reaction, the catalyst was recovered easily,
and after washing with water it can be reused without any diffi-
culty. A positive dendrite effect was observed, when we compared
the catalytic efficiency of the DAEP–bentonite–Pd complexes with
the palladium catalyst immobilized on bentonite modified with
monolayer coverage of CPB surfactant (Table 6). Better results were
obtained by DAEP–bentonite–Pd (II) complex. This can be explained
if we consider that, with DAEP on the surface of modified bentonite,
it prevents the metal leaching and thereby increases the stability
Fig. 6. FTIR spectra of (A) Na–bentonite; (B) bentonite modified with a monolayer
coverage of CP; (C) CP–bentonite support with Pd (II); (D) CP–bentonite support
with Pd (0).
Table 3
Effect of the amount of catalyst, DAEP–bentonite with Pd (II), in oxidation of ethyl benzene.
Entry
Amount of catalyst (g)
Conversion (%)
Selectivity (%)
1
2
3
4
5
0.00
0.01
0.03
0.05
0.07
0.0
86.9
92.3
86.8
85.3
4.6
0.6
0.1
1.1
15.2
5.4
7.4
66.5
93.5
90.7
76.9
13.7
0.5
0.9
0.4
21.6
Reaction conditions: 80% TBHP; EB:TBHP (mol ratio) = 1–5; time 24 h, temperature = reflux, without solvent.
Table 4
Effect of temperature on the catalytic activity of DAEP–bentonite with Pd (II) in oxidation of ethyl benzene.
Entry
Temperature (◦C)
Conversion (%)
Selectivity (%)
1
2
3
4
5
25
40
50
60
3.6
5.5
8.8
21.4
92.3
42.6
33.1
22.0
4.3
7.1
8.5
10.9
15.6
5.4
27.1
42.6
58.9
78.7
93.5
23.2
15.8
8.2
1.4
0.5
Reflux
0.6
Reaction conditions: catalyst weight = 0.03 g; 80% TBHP; EB:TBHP (molar ratio) = 1:5; time 24 h, without solvent.

M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
129
Fig. 7. TEM image of (A) Na–bentonite; (B) bentonite modified with a monolayer coverage of CP; (C) CP–bentonite support with Pd (II); (D) CP–bentonite support with Pd
(0).
of the catalyst. The catalytic sites of the DAEP–bentonite may work
in a cooperative manner due to the peculiar structure of the den-
drimeric head of the DAEP, which makes the DAEP–bentonite–Pd
complex more efficient catalyst. In addition, the hydrophilic tail
of the DAEP acts as a spacer between the support and the cat-
alytic site, which reduces the steric factors offered by the support
to some extent. This effect cannot be seen by the CP–bentonite–Pd
complex and so it acted as a less efficient catalyst compared to
DAEP–bentonite–Pd complex.
was active after the fifth cycle, but the drop in activity and metal
leaching may be due to the partial decomposition of the dendritic
ligand due to prolonged exposure to an oxidizing atmosphere. FTIR
analysis of the catalyst before and after the reaction showed that
the intensity of the band at 1660 cm−1 due to amide group started
decreasing; this indicates a possible decomposition of the DAEP
backbone during the course of the reaction. After the fifth cycle,
the catalyst lost about one third of its activity.
The results of reusability of the best catalyst are summarized in
Table 7. The catalyst obtained after the first reaction was used in the
subsequent runs after washing with ethyl acetate and drying. The
catalyst remained active for the next few cycles. There was some
drop in the activity of the catalyst, and the metal leaching may be
the reason for the drop in activity. Although practically the catalyst
3.4. Electrochemical study using carbon nanotube paste electrode
We used cyclic voltammetry (CV) and electrochemical
impedance spectroscopy (EIS) to study the electrochemical
behaviour of the prepared nanoparticles at the surface of carbon
paste electrode. Fig. 8 shows cyclic voltammogram of Pd (0) and
Table 5
Effect of time on the catalytic activity data in the oxidation of ethylbenzene with DAEP–bentonite–Pd (II).
Entry
Time (h)
Conversion (%)
Selectivity (%)
1
2
3
4
5
1
4
6
12
24
24.7
36.3
47.3
59.4
92.3
33.2
29.3
25.1
29.8
0.6
9.4
12.2
14.6
11.8
5.4
41.3
46.1
47.0
30.3
93.5
16.1
12.4
13.3
28.1
0.5
Reaction conditions: catalyst weight = 0.03 g; 80% TBHP; EB:TBHP (molar ratio) = 1:5; reflux, without solvent.

130
M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
Table 6
Effect of the EB:TBHP molar ratio on the catalytic activity of palladium catalysts in oxidation of ethyl benzene.
Entry
Catalyst
EB:TBHP (mol)
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
CP–bentonite–Pd (II)
CP–bentonite–Pd (II)
CP–bentonite–Pd (II)
CP–bentonite–Pd (0)
DAEP–bentonite–Pd (II)
DAEP–bentonite–Pd (II)
DAEP–bentonite–Pd (II)
DAEP–bentonite–Pd (II)
DAEP–bentonite–Pd (II)
DAEP–bentonite–Pd (0)
1:2
1:3
1:4
1:3
1:1
1:2
1:3
1:4
1:5
1:3
29.5
21.3
23.5
20.6
36.9
40.3
51.6
76.3
92.3
22.4
49.2
37.0
38.3
18.5
4.5
10.8
4.3
1.8
12.3
11.0
6.4
23.8
2.8
24.6
27.4
10.9
5.4
37.0
45.9
38.2
53.2
92.6
59.9
65.1
86.3
93.5
73.5
1.5
6.1
17.1
4.5
0.1
4.6
3.2
1.0
0.5
3.7
0.6
16.2
10
6.6
Reaction conditions: catalyst weight = 0.03 g; 80% TBHP; time 24 h, reflux, without solvent.
Table 7
Activity of recycled catalyst, DAEP–bentonite with Pd (II), in oxidation of ethyl benzene.
Entry
Cycle
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
1
2
3
4
5
6
92.3
64.1
68.7
61.2
59.0
57.0
0.6
8.9
9.1
5.6
8.5
6.8
5.4
9.3
6.9
4.2
8.6
9.7
93.5
80.2
82.7
88.7
81.5
81.9
0.5
1.6
1.2
1.5
1.3
1.6
Reaction conditions: catalyst weight = 0.03 g; 80% TBHP; EB:TBHP (molar ratio) = 1:5; time 24 h, reflux, without solvent.
Pd (II) complexes at pH 11.0 in an universal buffer solution. Results
show an oxidation peak potential for Pd (II) at 620 mV vs. Ag/AgCl
as a reference electrode. We did not observe any oxidation peak
for Pd0 in this potential. Therefore, oxidation of Pd (II) is much
faster than of Pd (0) in these conditions, because it oxidizes at a
lower potential. One might judge that electron transfer from Pd (II)
complex is much easier than that of Pd (0). On the basis of these
data one would expect that the Pd (II) complex should be a better
catalyst for the oxidation reaction, as experimental data supports.
Electrochemical impedance spectroscopy is a powerful and very
informative technique for probing charge transfer properties at the
electrode–solution interface [22]. We have used electrochemical
impedance spectroscopy to study Pd (0) and Pd (II) complexes to
evaluate their charge transfer coefficients. The values of the charge
transfer coefficients have direct relation with electron transfer in
electro active materials. The compounds with lower charge transfer
coefficient are more efficient in redox reactions.
Fig. 9 shows the Nyquist diagrams and bode plots of the imag-
inary impedance (Zim) vs. the real impedance (Zre) of the EIS
obtained at the carbon nanotube paste electrode which includes
Pd (0) (curve a) or Pd (II) (curve b) nanoparticles. They have been
recorded at 0.40 V dc-offset in 0.10 mol L−1 universal buffer (pH
11.0), respectively. Result shows that the charge transfer coeffi-
Fig. 9. Impedance for palladium (0) and (II) nanoparticles at pH 11.0 with Ep of
+0.40 V vs. Ag/AgCl reference electrode.
Fig. 8. Cyclic voltammogram of Pd (0) and Pd (II) complexes at pH 11.0.

M. Ghiaci et al. / Applied Catalysis A: General 381 (2010) 121–131
131
cient in the presence of Pd (0) complex is bigger than that of
the charge transfer coefficient of the Pd (II) complex. Therefore,
the Pd (II) complex can contribute easily in oxidation reaction. It
is very interesting that these results are in a good accord with
data obtained with cyclic voltammetry data, and oxidation of ethyl
benzene.
would like to thank Professor A.A. Ensafi for his helpful comments
in the electrochemical measurements.
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In conclusion, palladium complexes of modified bentonite
with 3,3ꢀ-(dodecylazanediyl)bis(N-(2-(2-aminoethylamino)ethyl)
propanamide) (DAEP) were prepared and characterized. These
complexes were found to be efficient and reusable catalysts for
the oxidation of ethyl benzene to acetophenone under desirable
conditions. The catalysts remained stable under several reaction
conditions. The catalytic activity was higher for the Pd (II) immo-
bilized on modified bentonite with cetyl pyridinium bromide and
DAEP in comparison with Pd (0) on the same support. Moreover, the
catalytic activity of the Pd (II) and Pd (0) on bentonite with mono-
layer surfactant coverage was lower than that of the other catalysts.
Oxidation was efficient in the presence of tert-butyl hydroperox-
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catalysts based on dendrimers.
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Thanks are due to the Research Council of Isfahan University of
Technology and Center of Excellence in the Chemistry Department
of Isfahan University of Technology for supporting of this work. We