Oxidation of ethylbenzene using some recyclable cobalt nanocatalysts: The role of linker and electrochemical study

Article abstract of DOI:10.1016/j.molcata.2011.01.027

In this paper some Co(II)-Schiff base complexes were immobilized on SiO₂-Al₂O₃ mixed-oxide using two linkers with different flexibilities (3-aminopropyl and 2-aminoethyl-3-aminopropyl). The synthesized materials were characterized by FT-IR spectroscopy, UV-Vis, CHN elemental analysis, ICP-MS, EPR, SEM, TEM, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The catalytic activity of the heterogenized Co(II)-Schiff base complexes in the oxidation of ethylbenzene was studied without the need of any solvent, at 353 K, using tert-butyl hydroperoxide as oxygen source. The best catalyst has a higher catalytic activity (86%) and selectivity to acetophenone (99%) at TBHP/ethylbenzene molar ratio (1:1), and could be reused at least 4 times without significant loss in acetophenone yield, suggesting that no complex leaching took place under the reaction conditions. The electrochemical data about oxidation of the immobilized Co(II)-Schiff base complexes at the surface of multi-walled carbon nanotubes was also studied. The results indicate that the Co-complexes anchored on the modified SiO₂-Al₂O₃ mixed-oxide have an easily oxidizable environment, which led to higher catalytic activity and selectivity.

Full text of DOI:10.1016/j.molcata.2011.01.027

Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Oxidation of ethylbenzene using some recyclable cobalt nanocatalysts: The role
of linker and electrochemical study
M. Arshadia, M. Ghiacia,∗, A.A. Ensafia, H. Karimi-Maleha, Steven L. Suibb
a Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
b Department of Chemistry, University of Connecticut, Storrs, CN 06269-3060, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 October 2010
Received in revised form 25 January 2011
Accepted 26 January 2011
Available online 5 March 2011
In this paper some Co(II)–Schiff base complexes were immobilized on SiO2–Al2O3 mixed-oxide using two
linkers with different flexibilities (3-aminopropyl and 2-aminoethyl-3-aminopropyl). The synthesized
materials were characterized by FT-IR spectroscopy, UV–Vis, CHN elemental analysis, ICP-MS, EPR, SEM,
TEM, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The catalytic activity
of the heterogenized Co(II)–Schiff base complexes in the oxidation of ethylbenzene was studied without
the need of any solvent, at 353 K, using tert-butyl hydroperoxide as oxygen source. The best catalyst has
a higher catalytic activity (86%) and selectivity to acetophenone (99%) at TBHP/ethylbenzene molar ratio
(1:1), and could be reused at least 4 times without significant loss in acetophenone yield, suggesting that
no complex leaching took place under the reaction conditions. The electrochemical data about oxidation
of the immobilized Co(II)–Schiff base complexes at the surface of multi-walled carbon nanotubes was also
studied. The results indicate that the Co-complexes anchored on the modified SiO2–Al2O3 mixed-oxide
have an easily oxidizable environment, which led to higher catalytic activity and selectivity.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Cobalt
Ethylbenzene
Oxidation
Acetophenone
Carbon nanotubes
SiO2–Al2O3
1. Introduction
mentally unfriendly nature. One of the major drawbacks of using
organometallic compounds in homogenous solutions is the forma-
Indeed, catalytic oxidation is the single most important technol-
ogy for the conversion of hydrocarbons to industrially important
oxygenated derivatives [1]. Selective oxidation of the C–H bonds
in ethylbenzene is an attractive field as well as unsolved problem
in chemistry. In this oxidation, the intermediate such as acetophe-
none is more susceptible to secondary oxidation reaction to the
corresponding carboxylic acid which lowers the selectivity. Thus,
the formation of ketone with high selectivity is more difficult. The
production of acetophenone imparts high value addition, because
it is used as intermediate for the manufacture of pharmaceuticals,
resins, alcohols, esters, aldehydes and tear gas and is also used as a
component in perfumery, and as a solvent for cellulose ethers.
Cobalt-based catalysts, due to their plausible potential as com-
mercial catalysts, are the ones among the most investigated. In this
regard Co(II) complexes are an important class of oxidants for this
type of reactions and quite a number of studies have been con-
ducted [2,3]. The current industrial production of acetophenone
is via the oxidation of ethylbenzene with molecular oxygen using
cobalt cycloalkanecarboxylate or cobalt acetate as catalyst in acetic
acid solvent [4]. This method suffers from its corrosive and environ-
tion of ␮-oxo dimers and other polymeric species which lead to
irreversible catalyst deactivation. The problem has been avoided
in principle by isolating the metal complexes from each other by
immobilization through covalent bond on the solid supports. In
recent years, encapsulation of metal complexes on the solid sup-
ports, typically zeolites and mixed oxides, have been extensively
studied in an attempt to prepare isolated catalytically active centers
that do not undergo rapid degradation as the homologous analo-
gous [5,6]. Homogeneous and heterogeneous catalysts offer their
own distinct advantages. Heterogeneous catalysts have an advan-
tage that at the end of reaction the catalyst can be removed by
simple filtration. In principle the product is un-contaminated with
a transition metal or ligand and allows the catalyst to be recycled
into the next reaction.
Recently there has been an increased interest in the develop-
ment of clean and economical processes for the selective oxidation
of ethylbenzene to higher-value added product acetophenone over
both homogenous and especially heterogeneous cobalt-based cat-
alysts [7–11]. As part of our continuing efforts directed toward
the elucidation of the correlation between catalytic activity and
electrochemical behavior of the synthesized heterogeneous com-
plexes, we have further explored the behavior of the immobilized
Co(II)–Schiff base complexes in a model reaction, i.e., the oxidation
of ethylbenzene [12].
∗
Corresponding author. Tel.: +98 311 3913254; fax: +98 311 3912350.
E-mail address: mghiaci@cc.iut.ac.ir (M. Ghiaci).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.027

72
M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
Transition metal Schiff base complexes have played for some
Electrochemical impedance measurements were carried out in
a conventional three-electrode cell, powered by an electrochemical
system comprising the Autolab (AUT83593) at a frequency range
of 0.1–10,000 Hz. The AC voltage amplitude was 5 mV.
time an important role in the oxidation of hydrocarbon feedstocks
(olefins, alkanes and aromatics) to their respective oxygenated
products. Herein, we report the immobilization of three cobalt
Schiff base complexes onto SiO2–Al2O3 mixed oxide, covalently
bound through two typical linkers with different flexibility and
spacer (3-aminopropyl and 2-aminoethyl-3-aminopropyl), and
their remarkable catalytic activities and excellent selectivity in the
catalytic oxidation of ethylbenzene in the liquid phase using tert-
butyl hydroperoxide as the oxidant without the need of any solvent
and reducing reagent.
The reason for choosing this particular mixed oxide was the
properties such as low thermal expansion and conductivity, low
dielectric constant, excellent creep resistance, robust chemical and
thermal stability, good high temperature strength and oxidation
resistance [13].
The synthesized materials were characterized by FT-IR spec-
troscopy, UV–Vis, CHN elemental analysis, ICP-MS, SEM, TEM, EPR,
cyclic voltammetry (CV) and electrochemical impedance spec-
troscopy (EIS). Also, the influence of electrochemical potential and
study of electrochemical behavior of the complex-immobilized
materials on the catalytic activity of them was studied by
electrochemical methods such as cyclic voltammetry (CV) and elec-
trochemical impedance spectroscopy (EIS).
2.3. Reaction conditions
A typical procedure for the liquid-phase oxidation of ethylben-
zene to acetophenone was as follows: 50 mg of the catalyst, 1.14 mL
of ethylbenzene (2.0 mmol), 1.34 mL of 80% TBHP (2.0 mmol) and
n-decane as internal standard were in turn introduced into a 50 mL
one-neck flask equipped with a condenser. The flask was immersed
in an oil bath in order to make the working temperature constant
at 353 K for a predetermined time (24 h) with continuous stirring.
The small aliquots of samples were withdrawn from the reaction
flask at different time intervals with a HPLC syringe. The con-
tents were centrifuged to separate the catalyst and the liquid layer
was analyzed. Products were monitored by GC (Agilent technolo-
gies 6890N), HP-50 capillary column (30.0 m × 250 ␮m × 0.25 ␮m),
FID detector, and high purity helium as carrier gas, and the prod-
ucts were identified by GC–MS (Shimadzu QP 5000; DB 1 column;
30 m × 0.25 mm): column temperature, 353 K; injection and detec-
tion port temperature, 523 K; and using a temperature program
(the oven temperature held at 353 K for 1 min then raised from
353 K to 473 K at 15 K/min and held at 473 K for 3 min). Identifica-
tion 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 Co content using
atomic absorption spectrophotometry (AAS).
2. Experimental
2.1. Materials
All reagents (A.R.) were purchased from Merck or Fluka and
were used without further purification, except that solvents were
treated according to standard methods. Graphite powder (parti-
cle diameter = 0.1 mm) and carbon nanotubes [>90% MWCNT basis,
d × l = (110–70 nm) × (5–9 ␮m) (Fluka)] were used as the substrates
for the preparation of the carbon paste electrode as a working elec-
trode (WE).
2.4. Preparation of the organometallic functionalized SiO2–Al2O3
mixed-oxide
SiO2–Al2O3 mixed-oxide was prepared according to the proce-
dure reported before [15]. The support is denoted as SiO2–Al2O3
(1:1). The modified support (Scheme 1) was prepared by reflux-
ing 5.2 g SiO2–Al2O3 (activated at 823 K for 6 h under air) with
3.5 mL of 3-aminopropyl-trimethoxysilane (0.0195 mol) in dry
dichloromethane (100 mL) for 24 h. The solid was filtered and
washed off with methanol, dichloromethane and dried at 373 K
under vacuum for 6 h. The same reaction condition was used
for the functionalization of SiO2–Al2O3 (1:1) with 2-aminoethyl-
3-aminopropyl-trimethoxysilane. The functionalized SiO2–Al2O3
(1:1) mixed oxides are identified hereafter by Si/Al-pr-NH2 and
Si/Al-pr-NH-et-NH2. Then, aldehyde or ketone (salicylaldehyde,
methyl-2-pyridylketone or pyridine-2-carbaldehyde) was added to
a suspended solution of Si/Al-pr-NH2 or Si/Al-pr-NH-et-NH2, in dry
methanol. The mixture was refluxed for 24 h to prepare a Schiff base
(vide Scheme 1) on the surface of the mixed-oxide (a bi-dentate
ligand).
2.2. Characterization
Diffuse reflectance spectra were recorded on a JASCOV-550
UV–Vis spectrophotometer. Fourier transform IR spectra were
measured using a JASCO FT/IR (680 plus) spectrometer. The spectra
of solids were obtained using KBr pellets. The vibrational transi-
tion frequencies are reported in wave numbers (cm−1). Elemental
analysis was performed by a CHNO-Rapid Heraeus elemental ana-
lyzer (Wellesley MA). Chemical analyses were carried out by
inductively coupled plasma optical emission spectroscopy (ICP-
OES) using a Shimadzu ARL 34000 instrument (spectroflamed;
typically, 30 mg sample was dissolved in 500 ␮L 40% HF solu-
tion, 4 mL 1:4 H2SO4:H2O solution and 45 mL H2O). Nitrogen
(99.999%) adsorption experiments have been performed at 76 K
using a volumetric apparatus (Quantachrome NOVA automated
gas sorption analyzer). Before the adsorption experiments, the
sample was outgassed at 393 K for 16 h. The specific surface
areas are calculated from the BET method [14]. Transmission
electron microscopy (TEM) was carried out on the powder
samples with a Tecnai F30TEMoperating at an accelerating volt-
age of 300 kV. In addition, energy dispersive X-ray analysis
was conducted on each sample. Electrochemical measurements
were carried out with Micro-Autolab, ptentiostat/galvanostat
instrument (␮3AUT70751), connected to a three-electrode cell.
Conventional three electrodes cells were used for all experiments.
The modified multi-walled carbon nanotubes paste electrode used
as a working electrode, platinum wire as an auxiliary electrode, and
an Ag/AgCl/KCl electrode as a reference electrode were used.
Catalysts containing Co(III)–Schiff base complexes were
obtained by stirring 0.5 g of the hybrid material, Si/Al mixed-oxide-
Schiff base ligand, with Co (OCOCH3)2 4H2O (5.4 mmol), and LiCl
(8.5 mmol) in 30 mL of ethanol at reflux for 24 h. Then, the result-
ing catalyst (brown powder) was filtered off, washed with copious
of ethanol and methanol and dried in a vacuum at 60 ◦C.
2.5. Preparation of the electrode
10 mg of the immobilized Co catalyst onto the SiO2–Al2O3 hand
mixed with 79 mg of graphite powder and 10 mg of multi-wall
carbon nanotubes in a mortar and pestle. Using a syringe, 0.88 g
paraffin was added to the mixture and mixed well for 40 min until
a uniformly wetted paste was obtained. The paste was then packed

M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
73
Scheme 1. Immobilization procedure of the immobilized Co(II)-Schiff base complexes onto the SiO2–Al2O3 mixed-oxide.
into a glass tube. Electrical contact was made by pushing a copper
2.6. Recommended procedure for electrochemical analysis
wire down the glass tube into the back of the mixture. When neces-
sary, a new surface was obtained by pushing an excess of the paste
out of the tube and polishing it on a weighing paper.
The blend of immobilized Co catalyst and multiwall carbon nan-
otubes paste electrode was polished with a white and clean filter

74
M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
paper. To record of cyclic voltammograms for the immobilized Co
catalyst onto the SiO2–Al2O3, 10.0 mL of 0.1 M phosphate buffer
at (pH 10.0), was transferred into an electrochemical cell. The ini-
tial and final potentials were adjusted to +0.00 and +1.00 V versus
Ag/AgCl, respectively. The cyclic voltammograms were recorded
with scan rate 100 mV s−1 to give the oxidation and reduction sig-
nals.
3. Results and discussion
The structures of the obtained organometallic-modified Si/Al
mixed oxides were confirmed by elemental analysis, BET (N2
adsorption–desorption technique), FTIR (Infrared spectroscopy
serves as a main investigation tool), DR UV–Vis, ICP-MS, TEM,
EPR, cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS). Elemental analysis showed that the addi-
tion of salicylaldehyde, methyl-2-pyridylketone and pyridine-2-
carbaldehyde to the surface amines was approximately stoichio-
metric.
In fact, during the preparation of the organometallic-modified
Si/Al mixed oxides, the same amounts of alkoxysilane agents
were deliberately introduced in the anchoring solution (moles
of alkoxysilane agent/grams Si/Al mixed oxide = 3.7 mmol/g) to
obtain the similar amounts of linkers on the surface of Si/Al mixed
oxide. In addition, during the condensation and immobilization, the
dosage of aldehyde (salicylaldehyde or pyridine-2-carbaldehyde)
or ketone (methyl-2-pyridylketone) were excessive to minimize
the amounts of the undesired residual linkers on the support. The
immobilized Schiff base ligands readily reacts with [Co(OCOCH3)2]
in EtOH to give the heterogeneous complexes. A change in color of
the resulted powders can also be visualized during the reactions
(Scheme 1). Loading of the immobilized catalyst was estimated
from the cobalt content determined by ICP-AES and the Schiff
base ligand content determined by elemental analysis (Table 1).
Moreover, the densities of cobalt complexes per surface area in all
immobilized complexes were not similar, showing the almost non-
identical condensation and coordination degrees of active moieties
present in the complex-immobilized materials. On the other hand,
the difference in catalyst loading could be due to the difference in
ligands structure and linker too. The result is summarized in Table 1.
The textural properties of the samples, evaluated from the low
temperature nitrogen adsorption revealed a decrease in the spe-
cific surface area (SBET). The surface area decreased further on the
modified support with 3-aminopropyl with the exception of sal-
iclaldehyde. Pore diameters of all organometallic-modified Si/Al
˚
mixed oxides (Table 1) are similar in the range of 16–20 A.
3.1. IR spectroscopy
Infrared wave numbers (cm−1) of significant valence vibra-
tions helpful for identification of the immobilized ligands and
of complexes are collated in Table 2 and Fig. 1. The band at
around 1050 cm−1 is due to the asymmetric stretching vibration
of (Si/Al)O4 units of Si/Al mixed oxide. The bands at 2943–2921
and 2877–2845 cm−1 are assigned to the stretching mode of –CH2
groups. From the presence of these bands, it can be inferred that the
Si/Al mixed oxide is modified by amine spacer groups successfully.
The N–H deformation peak at 1540–1560 cm−1 confirms the suc-
cessful functionalization of 3-APTES and 2-AE-3-APTMS on the Si/Al
mixed oxide. In agreement with previously reported data [16,17],
C
N (Schiff base) absorptions are in the 1639–1631-cm−1 range,
the skeletal vibration of benzene in the 1600–1590 cm−1 range and
the vibration of C N in pyridine groups are in the 1571–1569 cm−1
range [18] (Table 2).

M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
75
Table 2
Infrared attributions of significant vibrations for complexes.
System
FT-IR data (cm−1
)
ꢀ(C–H) aliphatic
ꢀ(C–H) aromatic
ꢀ(N–H)
ꢀ(C N)
ꢀ(C C)
Ring
ꢀ(C–O)
Si/Al-pr-NH2
Si/Al-pr-NH-et-NH2
Si/Al-pr-N=salicylaldehyde
2933, 2853
2927, 2854
2925, 2854
2925, 2854
2927, 2852
2940, 2877
2925, 2852
2931, 2856
2930, 2855
2924, 2855
2929, 2853
2928, 2856
2927, 2855
2960, 2923,2851
–
–
1560–1545 –
1560–1545 –
1560–1545 1633,
1560–1545 1639,
1560–1545 1635,1571
1560–1540 1635
1560–1550 1633, 1569
1560–1540 1631 1569
1560–1545 1644,
–
–
1540
1540
1590,1434
1591,1435
1540,1591, 1437
1590, 1435
1535,
–
–
–
–
3068, 3056
3068, 3056
3066
3069
3064, 3052
3054
3057, 3013
3065
3065
3061, 3013
3058
3058
1457, 760
1457, 760
1458
1455, 751
1457, 750
1469, 752
1440
1453,
1473
1457, 1473
1470
1465
1278
1278
Si/Al-pr-NH-et-N = salicylaldehyde
Si/Al-pr-N=methyl-2-pyridylketone
Si/Al-pr-NH-et-N=methyl-2-pyridylketone
Si/Al-pr-N=pyridine-2-carbaldehyde
Si/Al-pr-NH-et-N=pyridine-2-carbaldehyde
Si/Al-pr-N=salicylaldehyde-Co
Si/Al-pr-NH-et-N = salicylaldehyde-Co
Si/Al-pr-N=methyl-2-pyridylketone-Co
Si/Al-pr-NH-et-N=methyl-2-pyridylketone-Co
Si/Al-pr-N=pyridine-2-carbaldehyde-Co
Si/Al-pr-NH-et-N=pyridine-2-carbaldehyde-Co
–
–
–
–
1280
1310
–
1560–1545 1642,
1538
1560–1545 1640, 1568
1560–1545 1640, 1568
1560–1545 1638, 1571
1555–1540 1639, 1569
1542, 1535
1540, 1535,1436
1542, 1438
1544, 1535,1439
–
–
–
100
80
60
40
20
4000
3000
2000
1000
400
Wavenumber [cm-1]
Fig. 1. FT-IR spectra of organo-functionalized SiO2–Al2O3 mixed-oxide and
immobilized cobalt nanoparticles. Expanded FT-IR spectra of: (above insert)
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co (bottom insert) Si/Al-pr-NH-et-
N = methyl-2-pyridylketone.
Fig. 2. UV–Vis diffuse reflectance spectra of the immobilized Co(II)-Schiff base
complexes onto the SiO2-Al2O3 mixed-oxide modified with 3-aminopropyl-
trimethoxysilane.
The peaks in the 3070–3052 cm−1 range are attributed to the
C–H stretching vibrations of phenyl group. The FT-IR spectra of all
Schiff base systems clearly show the C–H vibration of Ph and Py
group at 3070–3052 cm−1, which further confirming the existence
of phenyl and pyridine groups on the Si/Al mixed oxide after the
immobilization of the ligands. The modes of metal coordination
with the two N and O donor ligands through the phenolic oxygen
and azomethine nitrogen of Schiff bases are evidenced from the
blue shift (ca. 32 cm−1) of ꢀ(C–O) and the red shift (ca. 3–11 cm−1
)
of the ꢀ(C N) vibrations with respect to the free ligand (Table 2).
The FT-IR results demonstrate the formation of the Co(II) complexes
immobilized on the Si/Al mixed oxide through 3-APTES and 2-AE-
3-APTMS linkers.
3.2. DR UV–Vis spectroscopy
To gain insight into the effect of Si/Al mixed oxide modifying
with Co(II)–Schiff base-complexes, the UV–Vis diffuse reflectance
spectra of the synthesized heterogeneous materials were recorded
at room temperature in the range of 230–800 nm. The UV–Vis
spectra of the Si/Al mixed oxides and different immobilized Co(II)
complexes are shown in Figs. 2 and 3. The UV–Vis spectrum
of the Si/Al mixed oxide only had a side-band adsorption near
244 nm, while the spectra of the immobilized Co(II) complexes
were dominated by strong absorptions in the 230–370 nm due to
Fig. 3. UV–Vis diffuse reflectance spectra of the immobilized Co(II)-Schiff base
complexes onto the SiO2–Al2O3 mixed-oxide modified with 2-aminoethyl-3-
aminopropyl-trimethoxysilane.
the ␲ → ␲* and n → ␲* transitions of the phenyl, pyridine and C
N
groups. Furthermore, the Co(II)-complexes exhibited the observed

76
M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
Fig. 4. SEM micrographs of SiO2–Al2O3 mixed-oxide (left) and Si/Al-pr-NH-et-N = methyl-2 pyridylketone-Co (right).
shoulder around 390 and 490 nm, probably attributable to the
ligand-to-metal charge transfer of the d → ␲* transitions, similar to
metallosalen compounds [19,20]. Several low intensity asymmetric
shoulder broad bands appeared over 500 nm in the visible region
at ꢁ = 500–650 nm are attributed to the d → d transitions expected
(Table 2), elemental analyses (Table 1) and thermal analyses (data
not published) clearly confirm that the Schiff base ligand and C
groups are not affected or destroyed through immobilization on the
functionalized Si/Al mixed oxide.
N
for the cobalt complexes with a square pyramidal geometry, (dxz
→
3.3. SEM and TEM images
dx 2 ), (dyz, dxy → d
2 ) and (d → dx 2 ) (decreasing energy),
2
2
2
2
−y
x
−y
z
−y
which were similar to related metal (salen) compounds described
in the literatures [21–23]. The UV–Vis spectra of the heterogenized
catalysts revealing that the organocobalt complexes were immo-
bilized by chemical bonds over the Si/Al mixed oxide structure
and also confirmed the coordination of the ligands to the central
metallic ion.
SEM images of the parent SiO2–Al2O3 mixed-oxide and the
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co sample are shown
in Fig. 4. The surface modification has changed the morphology
of the surface. The images show that the organic modifier has
increased the porosity of the surface.
The TEM micrographs and size distributions (100 and 50 nm)
of the nanoparticles are shown in Fig. 5, respectively. TEM images
indicated that the most of the prepared nanoparticles are spherical
shaped and have size less than 70 nm.
Consequently, DRUV–Vis and FT-IR spectra expose that the
immobilized Co–Schiff base ligands are synthesized upon coor-
dination to the organo-functional groups. The charge transfer
bands (Figs. 2 and 3), the stretching frequency at 1630–1645 cm−1
Fig. 5. Co nanoprticles: TEM micrographs of Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co.

M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
77
OH
O
CH3
Si/Al-pr-N= salicylaldehyde-Co
Si/Al-pr-NH-et-N=salicylaldehyde-Co
Si/Al-pr-N=pyridine-2-Carbaldehyde-Co
Si/Al-pr-N=methyl-2-pyridylketone-Co
Si/Al-pr-NH-et-N=pyridine-2-Carbaldehyde-Co
Si/Al-pr-NH-et-N=methyl-2-pyridylketone-Co
O
H
O
CH2CH3
Catalyst
60000
50000
40000
30000
20000
10000
0
80%TBHP,
without solvent
3
1
2
Scheme 2. Formation of the products over Co nanocatalysts.
80% (TBHP), because aqueous hydrogen peroxide (H2O2) did not
show any reactivity in this reaction. The results of the % selec-
tivity to acetophenone and % conversion of ethylbenzene with
the reaction time are presented in Table 3. The conversion and
selectivity were monitored by GC analysis. Products were iden-
tified by considering their GC–MS and by comparison with the
GC of the authentic samples. The oxidation takes place on the
␣-carbon of the alkylbenzene. Formation of the products acetophe-
none, 1, benzaldehyde, 2, and benzoic acid, 3, are illustrated in
Scheme 2. It is remarkable to mention that no oxidation was
observed in the aromatic ring of the ethylbenzene. The catalytic
activity of the solid catalysts was higher in solvent free condi-
tion than in the presence solvent such as CH3CN (data not shown).
The decrease in conversion under solvent conditions is attributed
to the blocking of active sites by solvent molecules or may be
explained by the diffusion competition of the solvent and the sub-
strate [27]. Generally, acetophenone was found to be the main
product, along with minor quantities of benzaldehyde and benzoic
acid. In fact, acetophenone selectivity was initially increased grad-
ually (on Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co, Si/Al-
pr-N = salicylaldehyde-Co, Si/Al-pr-N = methyl-2-pyridylketone-Co
and Si/Al-pr-NH-et-N = salicylaldehyde-Co) and on some catalysts
sharply (Si/Al-pr-NH-et-N = pyridine-2-carbaldehyde-Co and Si/Al-
pr-N = pyridine-2-carbaldehyde-Co) from 2 to 12 h. This indicates
that the side reactions become less plausible with the progress of
the reaction leading to increases in the selectivity to acetophenone
and decreases in the selectivity to benzaldehyde and benzoic acid.
After 12 h, the acetophenone selectivity was more or less constant
up to 24 h and probably inhibits the side reactions and therefore
the selectivity of acetophenone did not change. The formation of
other products viz. benzaldehyde may arise from the cleavage of
the C–C bond while the presence of benzoic acid may result from
the oxidation of benzaldehyde.
-10000
2200
2700
3200
3700
4200
Field (G)
Fig. 6. EPR spectra of of the immobilized Co(II)-Schiff base complexes onto the
SiO2–Al2O3 mixed-oxide.
3.4. EPR spectroscopy
EPR spectroscopy has been employed to confirm on the oxida-
tion states and the chemical environment of Co species present in
our samples. The X-band EPR spectra of the immobilized Co–Schiff
base-complexes measured at room temperature are depicted in
Fig. 6.
The appearance of these EPR signals, characteristic of Co(II), indi-
cates that, part of the Co ions are presented in from the +2 oxidation
state upon immobilization on the functionalized SiO2/Al2O3 mixed
oxide. The ESR spectra all appear to have a very broad line and the
most prominent is a broad feature centered around g = 1.95, with
the wide wings. In fact, the peak broadening and wings are due to
dipole–dipole interactions [24], and they are predominant in the
spectra of higher loaded samples (samples containing more than
1.5 wt% Co)[25].
Microcrystalline nanoparticles often yield characteristic EPR
spectra with features distinctly different from those observed for
magnetically isolated ions [26]. The detailed features of the reso-
nances depend strongly on shape and size distributions and on the
magnetic properties of the particles. Consequently, with regard to
the EPR spectra and TEM photo, the broad features around gav = 2.05
are probably characteristic of superparamagnetic relaxation as it
concerned with the magnetic moment of the whole particles and
either may be due to the highly dispersed nanoparticle of Co species
(Fig. 6).
As presented in Table 3, the selectivity over different immo-
bilized Co-complexes after 24 h changed in the following order:
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co
NH-et-N = pyridine-2-carbaldehyde-Co (85%) > Si/Al-pr-NH-et-
N = salicylaldehyde-Co (78%) > Si/Al-pr-N = salicylaldehyde-Co
(99%) > Si/Al-pr-
(75%) > Si/Al-pr-N = pyridine-2-carbaldehyde-Co (67%) > Si/Al-pr-
N = methyl-2-pyridylketone-Co (63%). Probably, the electronic
effects of the ligands may change the selectivity to acetophenone.
As a result, high selectivity to acetophenone was achieved using
catalysts with pyridine groups instead of phenyl groups in the
ligand structure. The most effective system in terms of selectivity
was Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co, where the
backbone of the catalyst was modified with 2-AE-3-APTMS.
Ethylbenzene conversion and catalytic activity (turnover) were
monitored after 2 h of the reaction and was increased gradually
up to 24 h (Table 3). The reaction was almost complete after about
24 h, when the TBHP efficiency reaches a maximum. Majority
of the prepared catalysts were highly reactive and selective
for the oxidation of ethylbenzene. As shown in Table 3 and
Fig. 7, the catalyst, Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co,
showed the most desired catalytic performances (86%), that could
be due to the cobalt content of the catalyst (0.57 mmol g−1).
The EB conversion after 24 h over different catalysts follows
3.5. Catalytic performance
The performance of the Co nanocatalysts was tested in ethyl-
benzene oxidation without the need of any solvent and reducing
reagent at 353 K. The best oxidant was t-butyl hydroperoxide

78
M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
Table 3
Oxidation of ethylbenzene: variation with reaction time over different cobalt nanocatalysts.a
Catalyst
Time (h)
Conversion (mol%)
TONb
Selectivity (mol%) (acetophenone)
Si/Al-pr-N = salicylaldehyde-Co
2
6
12
18
24
20.0
32.8
37.8
40.0
43.8
19.0
–
–
–
41.6
60.0
65.1
68.0
74.0
75.2
Si/Al-pr-NH-et-N = salicylaldehyde-Co
Si/Al-pr-N = pyridine-2-carbaldehyde-Co
Si/Al-pr-NH-et-N = pyridine-2-carbaldehyde-Co
Si/Al-pr-N = methyl-2-pyridylketone-Co
2
6
12
18
24
25.3
37.6
46.4
57.3
59.6
18.6
–
–
–
43.7
60.1
66.8
73.5
76.9
78.2
2
6
12
18
24
16.8
24.1
40.2
47.4
48.5
15.2
–
–
–
43.8
37.9
51.1
59.7
65.9
67.7
2
6
12
18
24
24.0
41.8
57.6
68.0
77.3
19.8
–
–
–
63.8
53.0
66.8
73.5
80.0
85.8
2
6
12
18
24
21.4
31.0
47.0
52.0
55.7
17.0
–
–
–
44.1
47.1
50.3
58.5
61.0
63.1
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co
2
6
12
18
24
24.6
43.1
58.4
76.9
86.8
17.4
–
–
–
61.4
78.7
81.3
87.1
95.0
99.9
a
Reaction condition: substrate (ethyl benzene) = 2.0 mmol, TBHP = 2.0 mmol, weight of the catalyst = 50 mg, T = 353 K.
TON, turn over number, moles of substrate converted per mole of metal.
b
the order of: Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co
With regard to the obtained results, ethylbenzene oxidation
activity of different immobilized Co-complexes over modified Si/Al
mixed-oxide prepared with three different Schiff base ligands by
the condensation between methyl-2-pyridylketone, pyridine-2-
carbaldehyde and salicylaldehyde with two different amine linkers,
3-APTES and 2-AE-3-APTMS, varied significantly depending on
their structures.
(86%) > Si/Al-pr-NH-et-N = pyridine-2-carbaldehyde-Co (77%) > Si/
Al-pr-NH-et-N = salicylaldehyde-Co (59%) > Si/Al-pr-N = methyl-2-
pyridylketone-Co (55%) > Si/Al-pr-N = pyridine-2-carbaldehyde-Co
(48%) > Si/Al-pr-N = salicylaldehyde-Co
(43%).
The
Si/Al-pr-
N = salicylaldehyde-Co catalyst showed the lowest conversion
among the studied systems (43%). The poor reactivity of the last
catalyst was probably related to the low cobalt content of this
sample (0.42 mmol g−1 from ICP results).
When comparison was made between the two catalysts pre-
pared on 3-APTES and 2-AE-3-APTMS, significant differences are
detected: the selectivity toward acetophenone as well as the
activities was not the same for both systems. Among these, the cat-
alyst prepared with 2-AE-3-APTMS showed the best performance
compared with those synthesized with 3-APTES. Thus, as a com-
petent statement, the conversion and selectivity of Co(II)–Schiff
base-complexes on the modified Si/Al mixed oxide through 2-
AE-3-APTMS were higher than those on 3-APTES. This difference
apparently results from the different surface properties of the mod-
ified Si/Al mixed oxide with two spacers. When 3-APTES is the
spacer, the steric hindrance and accessibility issues that caused by
the surrounding alkyl chains at the catalytic site, leading to a signifi-
cant decrease in the selectivity and reactivity (this point of view will
be more discussed in Section 3.6). On the other hand, the increase
in conversion and selectivity may be attributed to the improved
surface hydrophobicity of modified Si/Al mixed oxide with 2-AE-
3-APTMS and the reduced steric interaction of the immobilized
Co-complexes with host support (mimicking homogeneous con-
ditions) and also has increased the mobility of them (Keeping in
mind that 2-AE-3-APTMS has a longer carbon chain than 3-APTES,
and certainly enlarges accessibility issues of the catalytic sites to
the substrate).
Fig. 7. Productdistribitionand catalytic activity versus timein theoxidation of ethyl-
benzene with TBHP catalysed by Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co at
353 K.
Besides coordination to a Schiff base ligand, the cobalt may also
interact with the surface Si–OH and Al–OH groups of the Si/Al

M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
79
Table 4
Effect of temperature on the oxidation of ethylbenzene with TBHP over Si/Al-pr-NH-
et-N = methyl-2-pyridylketone-Co.
Time (h)
Conversion %
323 K
Selectivity %
323 K
353 K
353 K
2
6
12
18
24
19
32
47
64
76
24
43
58
76
86
67
75
85
91
95
79
81
87
95
99
mixed oxide structure. Thus, the materials of blank Si/Al mixed
oxide that adsorbed cobalt were also used for oxidation experi-
ment. It turned out that the conversion of ethylbenzene was almost
0%. Therefore, in the oxidation of ethylbenzene with TBHP, the
possibility of absorbed cobalt onto the Si–OH and Al–OH groups
bestow no contribution. The above observations suggest that the
oxidations occur due to the catalytic nature of the chemically
immobilized Schiff base complexes. Clearly, the Si/Al-pr-NH-et-
N = methyl-2-pyridylketone-Co catalyst was vastly more selective
to acetophenone (99%) and high active catalyst for oxidation of
ethylbenzene to acetophenone (86%), therefore the rest of research
was devoted on this catalyst.
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co, the promising
catalyst from the screening test, was further evaluated for catalytic
activity for the oxidation of ethylbenzene with TBHP at 323 and
353 K, and the results are given in Table 4. It has been shown that the
conversion and selectivity enlarged significantly with increasing
temperature from 50 to 80 ◦C.
It must be emphasized that a successful complex immobiliza-
tion must involve not only the efficiency of the immobilization
strategy, but also the stability of the resulting material (support
and complex) under the experimental catalytic reaction condi-
tions. Consequently, to confirm the true heterogeneous nature of
immobilized Co-complexes in ethylbenzene oxidation reactions
with TBHP, the Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co cat-
alyst was reused in the ethylbenzene oxidation several times. This
catalyst was reused after the solid was separated from the liquid
reaction mixture by centrifugation and followed by several washes.
The results are presented in Fig. 8. Comparing the result in Fig. 8,
shows that the selectivity of the reused catalyst remained almost
unchanged after fourth cycles, but the catalytic reactivity (Fig. 8)
fell gradually (in the fourth cycle, the conversion was only 61%,
and about 25% reduction of conversion took place after 24 h). This
could be due to the loss of the catalyst during filtration and wash-
ing. The AAS analysis indicates that the activity reduction was not
mainly attributed to the leaching of cobalt during the catalytic runs
because the AAS analysis of the reaction solutions after 24 h showed
no leaching of cobalt, confirming that immobilized Co-complexes
on the modified SiO2–Al2O3 mixed-oxide have high stability dur-
ing the catalytic process. Based on a reduced activity of the catalyst
Fig. 8. Influence of the number of uses on the catalytic activity (above figure) and
selectivity (lower figure) of Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co for oxi-
dation of ethylbenzene to acetophenone under solventless condition at 353 K.
after last run, the authors conclude, that probably most of the cat-
alytic activity has to be attributed to heterogeneous catalysis, and
the leached metal should deposited on the surface of the support
without playing any active role in the oxidation reaction. To further
confirm that the reaction was catalysed by the heterogeneous cat-
alyst, we added extra ethylbenzene to the filtrate after the removal
of the catalyst and continued the reaction under the same condi-
tions; the GC analysis showed that no more oxygenated products
were produced.
Table 5
Comparison of figure of merit of the present work with other studies in the literature.
Catalyst (amount)a
Oxidantb
Reaction time
(h)
T (K)/solvent
Ethylbenzene
conversion (%)
Selectivity (%)c
AP
98
90.2
85.0
100
79
99
BZ
–
0.7 9.1
9.1 5.9
–
14
–
Other products
2
Ref.
CoTPP–P(4VP-co-St)/SiO (0.09 g)
Cobalt(III) complexe (2 mmol)
Co/MCM-41(0.05 g)
Co(OAc)2–SiO2 (0.005 g)
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co (0.05 g)
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co (0.05 g)
O2
O2
TBHP
TBHP
TBHP
TBHP
0.2
4
24
24
2
368/–
423/–
353/–
363/isooctane
353/–
24
[18]
[8]
[10]
[28]
This work
This work
70.4
26.8
65
24
86
–
7
1
24
353/–
a
CoTPP–P(4VP-co-St)/SiO: Co tetraphenylporphyrins on P(4VP-co-St)/SiO2.
TBHP, tert-butyl hydroperoxide.
AP, acetophenone; BZ, benzaldehyde.
b
c

80
M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
Fig. 9. Cyclic voltammograms of the different immobilized Co(II)-Schiff base complexes onto the SiO2/Al2O3 mixed oxide through two linker at surface of multi walled carbon
nanotube paste electrode.
Table 5 compares the catalysts used in the present study
and the other catalysts used for oxidation of ethylbenzene,
under different environmental conditions reported in the per-
taining literature. When the ethylbenzene conversion (86%) and
selectivity to acetophenone (99%) obtained in this study are
compared with those published in the literature it can be
observed that the catalyst employed here behave in an outstanding
way.
3.6. Electrochemical study
As a follow up of this study, herein we have reported the study
on electrochemical behavior of the immobilized Co(II)–Schiff base
complexes onto the SiO2–Al2O3 mixed-oxide. In this section, we
describe application of electrochemical methods such as cyclic
voltammetry CV and EIS to study the electrochemical behavior of
the synthesized catalysts and comparing the dependence of the cat-

M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
81
Fig. 10. Nyquist diagrams of the imaginary impedance (Zim) versus the real impedance (Zre) of the EIS obtained at the multi walled carbon nanotube electrode recorded for
the different immobilized Co(II)-Schiff base complexes onto the SiO2/Al2O3 mixed oxide through two linker.
alytic effect of the immobilized Co(II)–Schiff base complexes with
their electrochemical behaviors. To our knowledge, there may be
a few reports [13] that have used the electrochemical study of this
type to demonstrate the reactivity of a catalyst by EIS technique
with carbon paste and especially multi walled carbon nanotube
paste electrodes.
Fig. 9 shows cyclic voltammograms obtained from different
Co(II)-complexes in 0.1 M phosphate buffer solution (PBS) at
(pH 10.0). The cyclic voltammograms exhibits an anodic peaks
(Epa) and corresponding cathodic peaks (Epc) (with low inten-
sity) versus Ag|AgCl|KClsat (as a reference electrode) related to
the Co3+/Co2+ redox couple with quasi reversible behavior. The
shift in the oxidation potential over different immobilized Co-
complexes from low potential to high potential increased in the
following order: Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co
(Epa = 0.673) < Si/Al-pr-NH-et-N = pyridine-2-carbaldehyde-
took place at lower potentials when the Co-complexes anchored
on the modified SiO2–Al2O3 mixed-oxide by 2-AE-3-APTMS than
3-APTES. As further discussed, the voltammograms of the modified
SiO2–Al2O3 mixed-oxide by 2-AE-3-APTMS indicate an easily
oxidizable environment, in close agreement with the catalytic test
(selectivity to acetophenone and conversion of ethylbenzene). The
results show that anodic peak oxidation for the Si/Al-pr-NH-et-
N = methyl-2-pyridylketone-Co has lower potential than the rest
of complexes, this again being in agreement with the selectivity
and conversion data of the catalysts (lower oxidation potential
leads to the higher activity).
EIS was also employed to investigate the oxidation ability of
the complexes at a surface of multi wall carbon nanotube paste
electrode (MWCNTPE) as a powerful electrochemical technique
in characterization and determination of kinetic parameters such
as charge transfer resistance (Rct) of the immobilized complexes
and solution resistance (Rs). Fig. 10 presents Nyquist diagrams of
the imaginary impedance (Zim) versus the real impedance (Zre) of
the EIS obtained at the MWCNTPE recorded at 0.62 V dc-offset in
the presence of immobilized Co(II)–Schiff base complexes onto
the SiO2–Al2O3 mixed-oxide in 0.1 mol L−1 PBS (pH 10.0). In the
presence of the Co(II)-complexes, the Nyquist diagram comprises
a depressed semicircle at high frequencies which may be related to
the combination of charge transfer resistance for electro oxidation
of these compounds and the double-layer capacitance (Cd), is
generally a function of potential. The equivalent circuit compatible
with the Nyquist diagram recorded is depicted in Scheme 3. In this
Co
(Epa = 0.703) < Si/Al-pr-NH-et-N = salicylaldehyde-Co
(Epa = 0.800) < Si/Al-pr-N = salicylaldehyde-Co (Epa = 0.950). As it is
observed from Fig. 9, the Si/Al-pr-N = pyridine-2-carbaldehyde-Co
and Si/Al-pr-N = methyl-2-pyridylketone-Co complexes did not
show any oxidation peak at a surface of multi walled carbon
nanotube paste electrode, probably, because the oxidation of
those complexes occur after the solvent oxidation wall, that is, the
oxidation peak was not observed. The electrochemical data about
the oxidation correlate well with the selectivity and conversion
data of the catalysts (vide Fig. 7). Moreover, the electrochemical
measurements indicate that the oxidation of cobalt was easier and

82
M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
(vide Table 1S). Noticeably, with regard to the cyclic
voltammograms data we could not obtained any Rct for
Si/Al-pr-N = pyridine-2-carbaldehyde-Co and Si/Al-pr-N = methyl-
2-pyridylketone-Co complexes (may be due to the oxidation of the
considered complexes after the solvent oxidation wall). Therefore,
this is certainly obvious that these complexes have a high Rct rather
than the other complexes. In view of the above observations, that
is, the diminishing in the charge transfer resistance and influence
of the charge transfer resistance on the selectivity and conversion
of the catalysts, we consider that the immobilized Co-complexes
on the modified Si/Al mixed oxide through 2-AE-3-APTMS indicate
a lower Rct than 3-APTES, probably, due to raised the mobility of
them (2-AE-3-APTMS has a longer carbon chain than 3-APTES and
might make the catalytic sites more accessible to the substrate)
and decreased their interaction with the Si/Al mixed oxide support.
Thus, it leads to the decreasing of Rct and increasing the ability
of the catalysts for presence and partnership in electrooxidation
reaction.
Scheme 3. The equivalent circuit compatible with the Nyquist diagram recorded.
circuit, Rs, CPE, and Rct represent solution resistance, a constant
phase element corresponding to the double-layer capacitance, and
the charge transfer resistance associated with the oxidation of low-
valence Co(II)-complexes species, respectively. The result shows
that in the presence of different Co(II)-complexes the value of Rct
was altered. The value of charge transfer resistance is equal to the
ability of a compound for participation in electrooxidation reaction
[29]. Furthermore, in the mentioned circuits, the charge-transfer
resistance of the electrode reaction is the only circuit element
that has a simple physical meaning describing how fast the rate
of charge transfer during electro-oxidation of the Co(II)–Schiff
base-complexes changes with the electrode potential. Conse-
quently, each compound that has high charge transfer resistance
is inferior due to the inefficiency of the compound in oxidation
of reactant to the desired product. Therefore, we determined the
value of this parameter (Rct) for Co(II) complexes used in this study,
and comprehending the activity of these complexes in oxidation
reaction. The charge transfer resistance of different catalysts
follows the order Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co
(Rct = 0.549 Mohm) < Si/Al-pr-NH-et-N = pyridine-2-carbaldehyde-
This was very interesting that the results of selective oxida-
tion efficiency of immobilized Co(II)–Schiff base complexes on
SiO2–AI2O3 mixed-oxide in the oxidation of ethylbenzene to ace-
tophenone with TBHP, have close compatibility and correlation
with observations of the Epa and charge transfer resistance (Rct
)
that obtained by cyclic voltammetry and EIS methods, respectively.
As further discussed, the oxidation potential of the Si/Al-pr-
NH-et-N = methyl-2-pyridylketone-Co was lower than the other
catalysts (Fig. 9). Also, in EIS, the Si/Al-pr-NH-et-N = methyl-2-
pyridylketone-Co has lower charge transfer resistance than the
other complexes (Fig. 10). Therefore, with regard to the above con-
sideration, the catalyst could better associate in oxidation reaction
Co (Rct = 1.990 Mꢂ) < Si/Al-pr-NH-et-N = salicylaldehyde-Co (Rct
=
2.179MKꢂ) < Si/Al-pr-N = salicylaldehyde-Co (Rct = 4.853 Mꢂ)
Scheme 4. Proposed mechanism for the oxidation of ethylbenzene with tert-butyl hydroperoxide over the Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co catalyst.

M. Arshadi et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 71–83
83
due to the low Epa and charge transfer resistance that would be
caused for increasing the selectivity of ethylbenzene oxidation with
TBHP rather than the other complexes, which is roughly consistent
with the catalytic activity and selectivity of the Co(II)–Schiff base
complexes.
of Excellence in the Chemistry Department of Isfahan University of
Technology for supporting of this work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.01.027.
3.7. Proposed mechanism for the oxidation of ethylbenzene with
tert-butyl hydroperoxide over the
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Co catalyst
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The mechanism for the oxidation of ethylbenzene with
tert-butyl hydroperoxide over the Si/Al-pr-NH-et-N = methyl-2-
pyridylketone-Co catalyst proceeded in several steps as shown in
Scheme 4. TBHP is activated by coordinating with the immobilized
Co(II)–Schiff base complexes, that is, the activated distant oxygen
of co-coordinated TBHP reacted with ethylbenzene to yield the
products. In the reaction mixture, no products (2-phenyl ethanol,
2-phenyl acetaldehyde and 2-phenyl acetic acid) were detected by
GC–MS (unfavored path in Scheme 4). The oxidation of the ethyl-
benzene with TBHP is supposed to occur by free radical mechanism,
yielding primarily ethylbenzene hydroperoxide [30]. As shown in
Scheme 4, the intermediate ethylbenzene hydroperoxide can react
in two different routes to form two different products, acetophe-
none and benzaldehyde. However, the possibility of benzoic acid
formation from the over oxidation of benzaldehyde on the immo-
bilized Co(II)–Schiff base complexes cannot be excluded.
4. Conclusion
We have described a new highly recoverable and efficient
cobalt-nanocatalyst for the oxidation of ethylbenzene with TBHP
without the need of any solvent at 80 ◦C. We have also demon-
strated that the combination of an organic ligands and SiO2–Al2O3
mixed-oxide resulted in an interesting synergistic effect that led
to enhanced activity and selectivity of the heterogenized organo-
cobalt nanocatalyst, the obstruction of the agglomeration of the
Co(II) nanoparticles, and the generation of a durable catalyst.
Among the catalysts investigated, cobalt nanoparticles immobi-
lized over Si/Al-pr-NH-et-N = methyl-2-pyridylketone was found to
be the ideal heterogeneous catalyst system for the selective oxida-
tion of ethylbenzene. A maximum conversion (86%) and excellent
selectivity (99%) were observed under milder reaction conditions
with a substrate-to-oxidant ratio one time. The mutual relation of
the efficiency of the prepared catalysts for ethylbenzene oxidation
with the redox potential and charge transfer resistance of them
were demonstrated by cyclic voltammetry (CV) and electrochemi-
cal impedance spectroscopy (EIS), respectively.
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Acknowledgements
Thanks are due to the Iranian Nanotechnology Initiative and the
Research Council of Isfahan University of Technology and Center