Highly selective aerobic oxidation of alkylarenes catalyzed by cobalt-based nanocatalyst in aqueous solution

Article abstract of DOI:10.1002/aoc.4193

The catalytic aerobic oxidation of alkylarenes catalyzed by cobalt supported on a highly crystalline γ-Al₂O₃ support (Co/Al₂O₃ nanocatalyst) is reported. The catalyst was prepared by a co-precipitation method and characterized using scanning and high-resolution transmission electron microscopies, energy-dispersive X-ray spectroscopy, X-ray diffraction and surface area measurements. A wide range of alkylarenes were converted to corresponding ketones. The catalyst can be recovered by simple filtration is recyclable for up to six consecutive runs.

Full text of DOI:10.1002/aoc.4193

Received: 29 August 2017
DOI: 10.1002/aoc.4193
Revised: 31 October 2017
Accepted: 1 November 2017
F U L L P A P E R
Highly selective aerobic oxidation of alkylarenes catalyzed
by cobalt‐based nanocatalyst in aqueous solution
1
2
3
4
Jalal Albadi
| Amir Alihosseinzadeh | Mehdi Jalali | Azam Mansournezhad
1
Department of Chemistry, Faculty of
Science, Shahrekord University,
Shahrekord, Iran
The catalytic aerobic oxidation of alkylarenes catalyzed by cobalt supported
on a highly crystalline γ‐Al O support (Co/Al O nanocatalyst) is reported.
2
3
2 3
2
School of Chemical Engineering,
The catalyst was prepared by a co‐precipitation method and characterized
using scanning and high‐resolution transmission electron microscopies,
energy‐dispersive X‐ray spectroscopy, X‐ray diffraction and surface area
measurements. A wide range of alkylarenes were converted to corresponding
ketones. The catalyst can be recovered by simple filtration is recyclable for up
to six consecutive runs.
University of Tehran, Tehran, Iran
3
National Petrochemical Company,
Petrochemical Research and Technology
Company, Tehran, Iran
4
Department of Chemistry, Payame Noor
University, Tehran, Iran
Correspondence
KEYWORDS
Jalal Albadi, Department of Chemistry,
Faculty of Science, Shahrekord University,
Shahrekord, Iran.
2 3
aerobic oxidation, alkylarenes, co/Al O nanocatalyst, ketones
Email: jalal.albadi@gmail.com;
albadi@sci.sku.ac.ir
1
| INTRODUCTION
Until now various kinds of catalysts have been
prepared for the aerobic oxidation of alkylarenes, and
the catalytic activity, reaction mechanism, synthesis
procedure and chemical character of such catalysts are
different from each other. Based on the literature, the
The oxidation reaction of alcohols is one of the most
important transformations in synthetic chemistry that
represents a significant methodology for the introduction
and modification of functional groups. In this way, the
oxidation of alkylarenes to their corresponding carbonyl
compounds is of significant importance in organic
chemistry, not only in academic research but also in the
[5–9]
performance of metal‐based catalysts such as Co,
[
10,11]
[12]
[13]
Cu,
Mn
and Au
nanocatalysts in the aerobic
oxidation of alkylarenes has been are widely reported.
Distinctive surface properties of metal oxide nanopar-
ticles of definite shape and size have motivated the explo-
ration of new synthesis strategies for nanostructured
materials under controlled synthesis conditions. There-
fore a convenient synthesis method for designing a
nanomaterial having large surface area is highly attrac-
tive. Among the metal oxides studied in this context,
Al O is one of the most intensively investigated due to
[1–3]
chemicals industry.
In previous research, various catalytic procedures
have been developed for the oxidation of alkylarenes
using various oxidants, solvents and reaction conditions.
In this view, environmentally benign procedures such as
using green oxidants and heterogeneous recyclable
catalysts are especially attractive for the oxidation of
alkylarenes.
Due to the availability of oxygen in the air, and the
fact that a by‐product of oxidation in an atmosphere of
oxygen is water, aerobic oxidation of alkylarenes using
oxygen as the oxidant has drawn special attention in
2
3
its wide‐scale potential in catalysis, optics, electronics,
[
14,15]
adsorption and biomedical applications.
Cobalt catalysts supported on alumina are important
material systems in the field of heterogeneous catalysis.
It has been reported that metal–support interactions
may appreciably affect surface properties, and hence
[4]
organic chemical synthesis.
Appl Organometal Chem. 2017;e4193.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4193

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ALBADI ET AL.
[
16,17]
catalytic activities.
Especially, Co nanoparticles sup-
100 °C and then calcined in air at 450 °C for 4 h. Also, a
batch of Al O support without Co content was prepared
ported on Al O substrates exhibit an attractive perfor-
2
3
2 3
mance in oxidation reactions because of the favourable
using the same method for supplementary analysis.
[18–26]
mechanical properties of the alumina support.
It
has been reported that metal–support interactions can
significantly improve the physical properties of the sur-
face of a catalyst and hence the catalytic activity.
2
.3 | Catalyst characterization
The Brunauer–Emmett–Teller (BET) surface area of the
catalyst was measured using the nitrogen adsorption–
desorption method. The analysis was carried out using
an automated gas adsorption analyser (Tristar 3020,
Micromeritics). The sample was purged with nitrogen
gas for 3 h at 300 °C using a VacPrep 061 degas system
(Micromeritics).
Supported metallic nanoparticles play an important
role in nanoscience and nanotechnology. Moreover, the
immobilization of nanoparticles on high‐surface‐area
supports allows a higher stability and dispersity of the
particles as well as a further exploitation of the special
[27]
activity and recycling properties of catalysts.
To the best of our knowledge, the performance of
cobalt nanocatalysts supported on γ‐alumina has not been
investigated for organic synthesis reactions, especially for
the oxidation of alkylarenes. Therefore, in continuation
of our studies of the catalytic activity of various
X‐ray diffraction (XRD) analysis was performed using
an X‐ray diffractometer, Cu Kα monochromatized radia-
tion source and a nickel filter (PANalytical X'Pert‐Pro),
in order to investigate the structure and crystallinity of
the catalyst. The Scherrer equation was used to determine
the average crystallite size of the sample. A flame atomic
absorption spectrophotometer (GBC 906AA) was applied
to determine the cobalt content of the catalyst.
[28–34]
nanocatalysts in aerobic oxidation reactions,
herein
the performance of a Co nanocatalyst supported on a
highly crystalline γ‐Al O3 support is reported for the
2
aerobic oxidation of alkylarenes to their corresponding
ketones (Scheme 1).
The morphology of the prepared catalyst was analysed
using scanning electron microscopy (SEM) with a
JEOL JSM‐6500F instrument, equipped with an energy‐
dispersive X‐ray spectroscopy (EDS) analytical system to
study the presence of various components of the catalyst.
Transmission electron microscopy (TEM) analysis was
also performed using a JEOL JEM‐2100 (200 kV) micro-
scope equipped with an EDS analytical system. The sizes
of about 200 cobalt nanoparticles were measured in order
to determine the size distribution and average particle size
of the cobalt nanoparticles.
2
2
| EXPERIMENTAL
.1 | General
Chemicals were purchased from Merck. All products were
characterized by comparison of their spectroscopic data
(NMR, IR) and physical properties with those reported
in the literature. Yields refer to isolated pure products.
2.2 | Catalyst preparation
2.4 | General procedure
The nanocatalyst of cobalt oxide supported on Al O3
A heterogeneous mixture of alkylarene (1 mmol) and
2
(
Co/Al O3 nanocatalyst) was synthesized using a co‐
Co/Al O3 nanoparticles (0.07 g) in water was stirred
2
2
precipitation method. An aqueous solution of 0.5 M
NaOH was added to a mixture of 0.5 M cobalt nitrate
and aluminium nitrate under vigorous stirring at 50 °C.
The obtained solution was aged at pH = 9 for 1 h, then
filtered and washed with deionized water to remove the
interfering ions. The sample was dried overnight at
under oxygen atmosphere at reflux condition. The
progress of the reaction was followed using TLC. After
reaction completion, the catalyst was recovered by filtra-
tion, then washed with hot ethanol (2 × 5 ml), and dried
for subsequent reaction runs. The filtrate was extracted
with n‐hexane. Evaporation of the solvent followed
by column chromatography on silica gel afforded the
pure products.
3
| RESULTS AND DISCUSSION
.1 | Catalyst characterization
3
The XRD patterns of the cobalt nanocatalyst and the
Al O3 support are shown in Figure 1. The Co/Al O
2 3
SCHEME 1 Aerobic oxidation of alkylarenes catalyzed by Co/
2
Al
2
O
3
nanocatalyst
nanocatalyst shows diffraction peaks at 2θ = 37.60°,

ALBADI ET AL.
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2 3
FIGURE 1 XRD patterns of (a) Al O support and (b) cobalt
nanocatalyst
2 3
FIGURE 2 Nitrogen adsorption–desorption profiles of Co/Al O
nanocatalyst
Table 1 presents structural properties of the samples.
The amount of Co in the catalyst analysed using atomic
absorption spectroscopy is 9.6 wt%. The alumina support
4
5.79°, 66.76° and 85.02°, which are attributed to [110],
[
111], [211] and [300] planes of γ‐Al O3 (code no.
2
2
−1
0
4
1‐1303). The diffraction peaks at 19.04°, 31.35°, 36.94°,
4.92°, 59.51°, 65.41° and 77.56° respectively correspond
has a high BET surface area of about 270 m g ; however,
after incorporation of Co content the BET surface area is
2
−1
to [111], [220], [311], [400], [511], [440] and [533] planes
of cubic Co O (code no. 01‐074‐1657). In addition, the
XRD pattern of the cobalt nanocatalyst shows a distinct
peak at 94.38° which is attributed to [731] plane of
CoAl O (code no. 03‐0896). This peak indicates the incor-
decreased to about 252 m g . Also, the decrease in the
average pore size of the support is consistent with the pore
size distributions of the samples in Figure 2.
3
4
Figure 3 displays SEM micrographs, TEM images and
2
4
EDS results for the Co/Al O3 catalyst. SEM analysis
2
poration of Co species into the structure of the Al O
support at calcination temperatures as high as 450 °C.
(Figure 3b,c) shows a uniform aggregate of nanoparticles
with a diameter of about 21 nm. This result is in
accordance with the type IV H2 hysteresis isotherm of
the nitrogen adsorption–desorption experiment.
2
3
The average crystallite size of Co oxide in the Co/Al O3
2
nanocatalyst, calculated using the Debye–Scherrer equa-
tion, is about 11.1 nm.
As shown in Figure 3(e), cobalt nanoparticles are
dispersed on the alumina support with a particle size
distribution of between 3 and 13 nm, and an average size
of about 6.5 nm. The high‐resolution TEM image of the
catalyst (Figure 3g) exhibits reflections with d‐spacing
values of about 0.24 nm that can be attributed to Co O
Figure 2 shows the nitrogen adsorption–desorption
profiles of the catalyst. The results show that the alumina
support and the Co nanocatalyst exhibit adsorption–
desorption isotherm profiles with hysteresis loops
0
between p/p = 0.45 and 0.9. Based upon the IUPAC
3
4
classification, this type of isotherm is categorized as
type IV, and ascribed to mesoporous structures. While
the alumina support adsorption profile shows an H3‐type
hysteresis loop, which can demonstrate aggregates of
nonuniform narrow slit‐like pores of plate‐like and/or
cubic nanoparticles, the Co/Al O sample shows an H2
(311) lattice plane.
TABLE 1 Structural properties of Al
nanocatalyst
2
O
3
support and Co/Al
2
O
3
Co
BET
Average
2
3
Co
crystallite surface pore
area size
(m g ) (nm)
Pore
hysteresis loop which is common for mesoporous carrier
and catalysts. This type of hysteresis loop is attributed to
aggregates and agglomerates of spheroidal particles with
content size
volume
a
b
2
−1
3
−1
Sample (wt%)
(nm)
(cm g
)
[35,36]
Al
2
O
3
0.0
9.6
—
269
252
2.8
4.0
0.21
0.24
nonuniform size and shape pores.
As exhibited in
Co/Al
2
O
3
11.1
Figure 2, the alumina support and the cobalt nanocatalyst
display narrow pore size distributions with a maxima at
a
Measured by atomic absorption method.
b
3
and 4 nm, respectively.
Average Co crystallite size calculated by XRD.

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ALBADI ET AL.
2 3
FIGURE 3 (a–c) SEM micrographs, (e–g) TEM images and (d, h) EDS results of Co/Al O nanocatalyst
The EDS analysis shows a cobalt content of about
5.4 wt% on the surface of the catalyst. The higher
of ethylbenzene in various solvents such as water,
ethanol, methanol, acetonitrile or DMSO, under oxygen
atmosphere and reflux conditions. The obtained results
demonstrated a high yield of 90% after 4 h of reaction in
water solution. However, ethanol, as the second most
active solvent, resulted in a yield of only 50% for a
reaction time of 12 h. Therefore, water was adjudged as
the most efficient solvent for the subsequent catalytic
investigations.
1
Co loading in comparison to the nominal Co loading
10 wt%) could be due to the procedure of the synthesis
of the catalyst. By considering the higher K number of
(
sp
−
16
Co(OH) (3 × 10 ), in comparison to Ksp of Al(OH)3
3 × 10 ), it can be suggested that with a gradual
2
−
34
(
increase of the pH of solution during the co‐precipitation
procedure, the Co clusters tend to precipitate at higher pH
values; this results in an increase in the amount of Co on
the surface of the Co/Al O catalyst.
Moreover, optimization of temperature and amount of
catalyst was conducted. The best results were achieved in
the presence of 0.07 g of catalyst in water under reflux
condition (Table 2). Therefore, the reactions were
performed in the presence of 0.07 g of Co/Al O in water
2
3
2
3
3.2 | Catalytic activity
under reflux condition.
As presented in Table 2, the catalytic performance of the
Co/Al O nanocatalyst was studied for aerobic oxidation
The scope of our procedure was investigated for the
aerobic oxidation of various alkylarenes under oxygen
balloon under optimized conditions (Table 3). As is
evident from Table 3, alkylarenes were converted into
the corresponding ketones in high yields under mild
conditions. A wide range of ethylbenzene derivatives
including those containing electron‐donating and elec-
tron‐withdrawing groups could be oxidized efficiently
to the corresponding ketones with good conversions
2
3
a
TABLE 2 Optimization of reaction conditions
Catalyst Time
Yield
(%)
b
Entry Solvent
Condition (g)
(h)
1
2
3
4
5
6
CH
DMSO
CH CH
Toluene
n‐hexane
3
CN
Reflux
Reflux
0.07
0.07
0.07
0.07
0.07
0.07
12
25
12
12
12
12
4
20
(
Table 3, entries 1–10). All the reactions occurred with
3
2
OH Reflux
Reflux
50
absolute selectivity for ketones and no other products
were detected in the reaction mixture. Moreover, the
oxidation of other cyclic arenes such as diphenylmethane
and fluorene under the same reaction conditions afforded
the corresponding ketones in high yields (Table 3,
entries 12–14).
10
Reflux
Reflux
Trace
90
2
H O
a
Reaction conditions: ethylbenzene (1 mmol), O
2
, reflux.
b
Isolated yield.

ALBADI ET AL.
5 of 7
TABLE 3 Aerobic oxidation of alkylarenes catalyzed by Co/Al
nanocatalyst
2
O
3
TABLE 3 (Continued)
a
b
Entry Substrate
Product
Time (h) Yield (%)
87
b
Entry Substrate
Product
Time (h) Yield (%)
14
7
1
4
90
a
2 3
Reaction conditions: alkylarenes (1 mmol), Co/Al O nanocatalyst (0.07 g),
2
3
4
87
in water at reflux conditions.
b
Isolated pure products.
5
87
TABLE 4 Recyclability of Co/Al
2
O
3
nanocatalyst
Run
1
2
3
4
5
6
4
5
4
4
88
90
Time (h)
4
4
5
5.5
5.5
7
a
Yield (%)
90
90
89
88
85
80
a
Isolated pure products.
Further studies were carried out to investigate the
recyclability of the catalyst. As evident from Table 4, the
aerobic oxidation of ethylbenzene was carried out six
times in consecutive runs with only a slight decrease
in the yield of products. After reaction completion, the
catalyst was washed with hot ethanol, dried and stored
for the subsequent reaction run.
6
7
8
9
4.5
4
88
90
91
89
A possible mechanism of the reaction is shown in
Scheme 2. It possibly proceeds through the removal of
a hydrogen atom from the benzylic position of the
4
[
5–7]
alkylarene by the Co/Al O nanocatalyst.
2
3
In order to show the merit of our method, Table 5
compares the results for the aerobic oxidation of
4.5
alkylarenes catalysed by Co/Al O3 nanocatalyst with
2
those for some other reported catalysts. Compared with
other reported catalysts, the Co/Al O3 nanocatalyst
2
promotes the reaction very effectively and gives the
desired products in high yields in water. The catalyst
can be recovered simply by filtration and can be reused
in subsequent runs without significant decrease in its
1
1
1
0
1
2
4
90
88
87
6.5
8
1
3
7
88
(
Continues)
SCHEME 2 Plausible mechanism of aerobic oxidation catalyzed
2 3
by Co/Al O nanocatalyst

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ALBADI ET AL.
2 3
TABLE 5 Comparison between Co/Al O nanocatalyst and some other catalysts employed for aerobic oxidation of ethylbenzene
a
Entry
Condition
Catalyst
Time (h)
Yield (%)
Ref.
[
[
[
5]
6]
7]
1
[bmim]Br/100 °C
Co‐pc
7
77
2
3
4
6
o‐xylene/reflux
p‐xylene/reflux
Acetonitrile/80 °C
CoPc@cell
10
6
89
75
39
90
Co‐NHAp
4
−
[8]
[CoTSPc] /SiO
2
24
4
H
2
O/reflux
Co/Al nanocatalyst
2
O
3
This work
a
Isolated yield.
activity. Reactions in the presence of some of other
catalysts were performed in organic solvents. Moreover,
other reported methods required longer reaction times
and also excess amounts of base need to be used for
reaction completion.
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8396.
4
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1
4
| CONCLUSIONS
5
We have discovered an efficient catalytic procedure for
the aerobic oxidation of alkylarenes catalysed by Co sup-
ported on a highly crystalline γ‐Al O support (Co/Al O
5
2
3
2 3
nanocatalyst) in water. The Co/Al O nanocatalyst dem-
2
3
onstrated a good performance for the aerobic oxidation
[14] A. K. Patra, A. Dutta, A. Bhaumik, J. Hazard. Mater. 2012,
201, 170.
of various alkylarenes with O as the oxidant. Moreover,
2
the introduced nanocatalyst was easily separated from
the reaction mixture with simple filtration and showed
good recyclability and stability.
[
15] M. Trueba, S. P. Trasatti, Eur. J. Inorg. Chem. 2005, 17,
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2
We are grateful to the Research Council of Shahrekord
University for the support of this research.
[
[
6
ORCID
6
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21] C. Zhang, G. P. Ling, J. H. He, Mater. Lett. 2003, 58, 200.
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