A silica supported cobalt (II) Salen complex as efficient and reusable catalyst for the selective aerobic oxidation of ethyl benzene derivatives

Article abstract of DOI:10.1016/j.catcom.2010.11.024

A silica supported cobalt (II) Salen complex catalyst has been successfully employed in the aerobic oxidation of alkyl aromatics at atmospheric pressure in the presence of N-hydroxyphthalimide (NHPI). The reaction is particularly selective for the oxidation of the benzylic CH₂ group and the major product obtained was ketone. The immobilized catalyst can be easily recovered and reused for at least four reaction cycles without any significant loss of its catalytic activity.

Full text of DOI:10.1016/j.catcom.2010.11.024

Catalysis Communications 12 (2011) 510–513
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A silica supported cobalt (II) Salen complex as efficient and reusable catalyst for the
selective aerobic oxidation of ethyl benzene derivatives
b
c
d
c
Fatemeh Rajabi a, , Rafael Luque , James H. Clark , Babak Karimi , Duncan J. Macquarrie
⁎
a
Department of Science, Payame Noor University, PO Box 878 Qazvin, Iran
Departamento de Quimica Organica, Universidad de Cordoba, Ctra Nnal Iva, Km 396, E-14014, Córdoba, Spain
Green Chemistry, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P O Box 45195-159, Gava Zang, Zanjan, Iran
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 October 2010
Received in revised form 24 November 2010
Accepted 27 November 2010
Available online 4 December 2010
A silica supported cobalt (II) Salen complex catalyst has been successfully employed in the aerobic oxidation
of alkyl aromatics at atmospheric pressure in the presence of N-hydroxyphthalimide (NHPI). The reaction is
particularly selective for the oxidation of the benzylic CH2 group and the major product obtained was ketone.
The immobilized catalyst can be easily recovered and reused for at least four reaction cycles without any
significant loss of its catalytic activity.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Aerobic oxidation
Supported catalyst
Ethyl benzene
NHPI
1. Introduction
mental impact of the process, the heterogenization of homogeneous
catalysts onto inorganic solids [20–25] via covalent anchoring is an
The oxidation of alkyl aromatic compounds is an important
transformation in the chemical industry, and is generally performed
with corrosive or toxic reagents [1]. Green chemistry teaches us
that there is a need for non-toxic and clean methods for carrying out
organic reactions [2–4]. Therefore, much research has been carried
out to develop environmentally friendly methods for the oxidation of
these compounds [5–9]. One approach is to design cleaner catalytic
systems for oxidation reactions using molecular oxygen as oxidant.
This is becoming increasingly attractive both from the economical and
environmental points of view [10–13]. The efficient aerobic oxidation
of various types of organic compounds has been carried out using
N-hydroxyphthalimide (NHPI) as a radical generator [14–19]. It is
believed that the phthalimide N-oxyl (PINO) radical generated in-situ
from the reaction of O2 and NHPI, abstracts the hydrogen atom from a
sp3 (saturated) carbon, forming the corresponding alkyl radical. Under
aerobic conditions, these radicals subsequently react with O2, to give
various types of oxygen-containing compounds such as alcohols and
ketones. In order to facilitate this pathway, transition metal catalysts
(specifically Co(II)) have been utilized. However, these catalysts are
generally homogeneous and are not easily recoverable after oxidation.
Also, the work up procedure is not convenient and generates a lot of
waste. In order to avoid these problems and to lower the environ-
attractive pathway [26–28]. Among the various types of support
materials, a silica matrix has been previously selected for the
immobilization of catalytic complexes such as Schiff base derivatives
[29,30].
Inspired by these previous findings, we recently reported an
efficient method for the aerobic oxidation of a variety of alcohols to
the corresponding carbonyl compounds with molecular oxygen using
NHPI combined with a silica supported Schiff base cobalt (II) complex
catalyst 1 [31]. The catalyst exhibited a good catalytic activity in this
reaction. Following our previous investigations in this area, herein, we
wish to disclose our results on the aerobic oxidation of alkyl aromatics
using the silica supported Co(II) Schiff base catalyst 1 in combination
with homogeneous NHPI under oxygen at atmospheric pressure.
2. Experimental
All chemicals were obtained from Aldrich. DRIFT spectra were
performed in a Bruker Equinox 55 FTIR instrument. Nitrogen
adsorption/desorption experiments were carried out using a Coulter
SA3100 surface area analyser. Prior to analysis, samples were
degassed at 130 °C for 5 h. Surface areas were determined using the
BET equation. Thermogravimetric analysis was performed using a
Polymer Laboratories STA 625, using a heating rate of 10 K min−1
under flowing nitrogen. Atomic absorption spectroscopy was per-
formed using a Philips PU 9200 instrument. 1H NMR spectra were
recorded on a 400 MHz JEOL NMR spectrometer in CDCl3 solution.
⁎
Corresponding author. Tel.: +98 281 3336366; fax: +98 281 3344081.
E-mail address: faprajabi@yahoo.com (F. Rajabi).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.11.024

F. Rajabi et al. / Catalysis Communications 12 (2011) 510–513
511
Table 1
2.1. Cobalt (II) catalyst preparation
Effect of temperature and amount of catalyst on the yield and selectivity of the products
from the catalytic oxidation of ethyl benzene.
Salicylaldehyde (2 mmol, 0.244 g) was added to an excess of
absolute MeOH, to which 3-aminopropyl (trimethoxy) silane
(2 mmol, 0.352 g) was subsequently added. The solution instantly
became yellow due to imine formation. After 3 h, cobalt (II) acetate,
Co(OAc)2·2H2O (1 mmol, 0.248 g) was added to the solution, and
the mixture was further stirred for 3 h to allow the new ligands
to complex the cobalt. A colour change from pink to olive green is
observed. Mesoporous silica (average pore diameter 60 Å, 3 g) was
activated by refluxing in concentrated hydrochloric acid (6 M) and
then washed thoroughly with the deionized water and dried before
undergoing chemical surface modification. This activation treatment
readily hydrolyses the siloxane Si–O–Si bonds to Si–OH species
which will be key to anchor the Cobalt complex. The complex
and the activated silica were then mixed and the mixture was
stirred overnight. The solvent is removed using a rotary evaporator,
and the resulting olive green solid dried at 80 °C overnight. The
final product was washed with MeOH and water (to remove all
physisorbed metal species) until the washings were colourless.
Further drying of the solid product was carried out in an oven at
80 °C for 8 h (Scheme 1).
Entry
Catalyst 1
(g)
Temp
(°C)
Selectivity
alcohol
Selectivity
acetophenone
Yield
(mol%)a
1
2
3
6
7
8
9
–(Blank)
0.063
0.125
0.125
0.125
0.125
0.125
40
40
25
40
60
59
34
49
38
30
21
10
41
66
51
62
70
79
90
6
21
49
57
64
69
76
80
100
a
Yield corresponds to the total oxidation products, based on 1H NMR.
the surface area as 197 m2/g (Kieselgel 60 has a surface area of
458 m2/g) and the catalyst has a single point total pore volume of
0.461 cm3/g, compared to 0.69 cm3/g for the support. This indicates
that supporting the complex brings about a substantial reduction in
surface area, possibly due to pore blockage from the bulky silane
limiting access to some very narrow-entrance pore in the material.
The infrared spectrum of the catalyst shows the expected bands [6]
including a distinctive band at 1561 cm−1 due to C = N stretching
lowered in frequency on complexation to cobalt (II).
2.2. General procedure for oxidation of alkyl aromatics catalysed by
silica supported
3.2. Oxidation of alkyl aromatics
In a typical reaction, a suspension of supported cobalt catalyst
(0.025 mmol, 0.125 g), N-hydroxyphthalimide (0.5 mmol), and alkyl
aromatic (5 mmol) in acetic acid (15 mL), under an oxygen atmo-
sphere (1 atm) were rapidly mixed together using a high shear reactor.
Upon reaction completion, the catalyst was removed by filtration, and
the solvent was evaporated in vacuo. Dichloromethane was added
and the organic phase was washed with 5% NaOH, water, dried and
evaporated to dryness.
Table 1 shows representative results for the oxidation of ethyl
benzene with molecular oxygen using NHPI and immobilized cobalt
(II) catalyst 1 under various conditions (Scheme 2). Several solvents
were examined in the reaction. Among them, acetic acid was found to
be the optimum solvent. It can be observed from Table 1 that the
benzylic position of ethyl benzene can be oxidized very selectively
and efficiently. Remarkably, the conversion and selectivity towards
acetophenone increased with an increase in temperature. The
optimized conditions were found to be as follows: ethyl benzene
(5 mmol), heterogeneous cobalt (II) catalyst 1 (0.025 mmol), NHPI
(0.5 mmol), and acetic acid (5 mmol) under oxygen at atmospheric
pressure at 100 °C. The oxidation of ethyl benzene in the absence of
supported cobalt (II) catalyst 1 produced a trace amount of oxidation
products under these conditions (Table 1, entry 1).
3. Results and discussion
3.1. Catalyst characterization
The loading of cobalt on the modified silica is about 0.3 mmol g−1
(determined from thermogravimetric analyses and atomic absorption
analysis of the HNO3-digested material). The surface analysis shows
To test the heterogeneity of the catalytic system, the catalyst
was filtered after different times and temperatures during the
CHO
N
Si(OMe)3
MeOH
(MeO)3Si
NH2
+
OH
OH
Co(OAc)2
N
N
(MeO)3Si
(MeO)3Si
O
SiO2
O
Co
N
Co
N
O
O
Scheme 1. Preparation of silica supported Cobalt (II) catalyst 1.

512
F. Rajabi et al. / Catalysis Communications 12 (2011) 510–513
O
OH
Catalyst 1 (0.025eq)
+
NHPI (0.5eq)
R
R
R
5eq
AcOH, O2 (1atm)
12h
Scheme 2. Catalytic aerial oxidation of ethyl benzene derivatives.
reaction and the filtrate was allowed to react further. We found that
after filtration of the supported cobalt catalyst 1, the filtration liquor
reacted significantly more slowly and at a similar rate to the reaction
performed in the absence of the catalyst (Table 2). The catalyst was
filtered off after the end of the first reaction and AAS confirmed that
no cobalt could be detected in the solution phase (detection limit
0.5 ppm Co).
In order to determine the scope of the oxidation, we investigated
the aerobic oxidation of a range of alkyl aromatics using NHPI/O2/
supported Co (II) catalyst 1 as catalytic system. Results are presented
in Table 3. Reactions take place at a good rate with maximum
conversions reached within 12 h. A mixture of the alcohol and the
ketone is always obtained with higher ketone selectivities (Table 3).
As can be seen in Table 3, an electron donating substituent (e.g.
methoxy group) at the para position of the substrate has a positive
effect on the oxidation yield. It is surprising that electron withdrawing
substituents including the NO2 or Cl group in the para position of the
substrate suppresses the oxidation. Para-nitroethyl benzene is not
oxidized at all under these conditions under equivalent homogeneous
reaction conditions using NHPI and soluble Co (II) species. While the
reasons for this observation are not clear, it is an indication that the
reaction is catalysed by the supported complex, which is the active
catalyst, and that catalysis takes place on the surface. Due to steric
effects, all substituents in the ortho position have a negative effect on
the oxidation reaction.
Table 2
Studies on the heterogeneity of the catalytic oxidation system showing conversions
before (“Co-cat”) and after (“Filtrate”) removal of the catalyst.
Entry
Co-cat
Time
(h)
Temp
(°C)
Selectivity
alcohol
Selectivity
acetophenone
Yield
(mol%)
1
1
2
2
3
3
4
4
Co-cat
Filtrate
Co-cat
Filtrate
Co-cat
Filtrate
Co-cat
Filtrate
3
8
2
10
2
10
3
27
27
40
40
60
60
100
100
30
28
27
25
24
21
22
18
70
72
73
75
76
79
78
82
22
23
24
28
30
34
52
57
8
We have also found that the heterogeneous cobalt catalyst can be
reused at least four times with only a small reduction in the rate of
reaction under similar experimental conditions (Table 4). No
preconditioning step was used, although the recovered catalyst was
washed with ca. 5 mL AcOH to remove traces of the previous reaction
mixture.
Table 3
Catalytic oxidation of ethyl benzene derivatives using the supported Co (II) catalyst
under the investigated reaction conditions.
Isolated
Substrate
Selectivity
alcohol
Selectivity
ketone
Yield (%)a
4. Conclusion
1
8
92
80
We have demonstrated that a silica supported cobalt (II) complex
is an efficient and recyclable catalyst for the oxidation of ethyl
benzene derivatives in the presence of NHPI under an atmospheric
oxygen pressure. The cobalt catalyst is highly active and easily
recyclable by a simple filtration. This reaction provides a new envi-
ronmentally friendly and economical route for the oxidation of alkyl
aromatics.
MeO
O2N
2
3
11
10
89
90
60
72
CI
Br
Acknowledgement
4
5
9
89
90
75
78
B. K. acknowledges IASBS Research Councils and Iran National
Science Foundation (INSF) for the partial support of this work. RL
gratefully acknowledges Ministerio Ciencia e Innovacion, Gobierno de
España for the concession of a Ramon y Cajal contract (RYC-2009-
04199). F. R. gratefully acknowledges PNU for its partial support of
this work.
10
HO
Br
12
12
78
78
58
55
6
Table 4
Reused silica supported co (II) catalyst 1 for oxidation of ethyl benzene.
Entry
Number of
uses
Temp
(°C)
Selectivity
alcohol
Selectivity
acetophenone
Yield
(mol%)
NO2
1
2
3
4
First
100
100
100
100
9
10
10
10
91
90
90
90
78
78
77
70
7
Second
Third
Fourth
a
Isolated yield.

F. Rajabi et al. / Catalysis Communications 12 (2011) 510–513
[13] T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037.
513
References
[14] Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 343 (2001) 393.
[15] H. Ma, J. Xu, Q. Zhang, H. Miao, W. Wu, Catal. Commun. 8 (2007) 27.
[16] Y. Ishii, S. Sakaguchi, Catal. Today 117 (2006) 105.
[17] B. Karimi, J. Rajabi, Org. Lett. 6 (2004) 2841.
[18] I. Hermans, J.V. Deun, K. Houthoofd, J. Peeters, P.A. Jacobs, J. Catal. 251 (2007) 204.
[19] F. Recupero, C. Punta, Chem. Rev. 107 (2007) 3800.
[20] P.T. Tanev, T.J. Pinnavaia, Science 269 (1995) 865.
[21] T. Matsushita, K. Ebitani, K. Kaneda, Chem. Commun. (1999) 265.
[22] F. Schuth, W. Schmidt, Adv. Mater. 14 (2002) 629.
[23] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicsz, C.T. Kresge, K.D. Schmitt, C.T.W.
Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am.
Chem. Soc. 114 (1992) 10834.
[24] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403.
[25] J. Chen, Q. Zhang, Y. Wang, H. Wan, Adv. Synth. Catal. 350 (2008) 453.
[26] J. Rafelt, J.H. Clark, Catal. Today 57 (2000) 33.
[1] M. Hudlicky, Oxidations in Organic Chemistry, American Chemical Society,
Washington, 1990.
[2] J.H. Clark, D.J. Macquarrie (Eds.), Handbook of Green Chemistry and Technology,
Blackwell, Oxford, 2002.
[3] M. Ghaffarzadeh, M. Bolourtchian, M. Gholamhosseni, F. Mohsenzadeh, Appl.
Catal. A 333 (2007) 131.
[4] S.I. Murahashi, N. Komiya, Y. Oda, T. Kuwabara, T. Naota, J. Org. Chem. 65 (2000)
9186.
[5] F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti, C. Punta, R. Paganelli, G.F.
Pedulli, F. Fontana, Org. Process Res. Dev. 8 (2004) 163.
[6] I.C. Chisem, J. Rafelt, M.T. Shieh, J. Chisem, J.H. Clark, R. Jachuck, D.J. Macquarrie, C.
Ramshaw, K. Scott, Chem. Commun. (1998) 1949.
[7] V. Parvulescu, C. Anastasescu, B.L. Su, J. Mol. Catal. A. 211 (2004) 143.
[8] S.C. Murahashi, N. Komiya, Y. Hayashi, T. Kumano, Pure Appl. Chem. 73 (2001)
311.
[27] T. Pruss, D.J. Macquarrie, J.H. Clark, J. Mol. Catal. A 211 (2004) 209.
[28] Z. Wang, T.J. Pinnavaia, J. Mater. Chem. 13 (2003) 2127.
[29] P. Sutra, D. Brunel, Chem. Commun. (1996) 2485.
[9] J. Warnatz, Pure Appl. Chem. 72 (2000) 2101.
[10] S. Vetrivel, A. Pandurangan, J. Mol. Catal.A 217 (2004) 165.
[11] R.A. Sheldon, in: L.L. Simandi (Ed.), Dioxygen Activation and Homogeneous
Catalytic Oxidation, Elsevier, Amsterdam, 1991.
[30] R.J.P. Corriu, E. Lancelle-Beltran, A. Mehdi, C. Reye, S. Brandes, R. Guilard, Chem.
Mater. 15 (2003) 3152.
[31] F. Rajabi, B. Karimi, J. Mol. Catal. A 232 (2005) 95.
[12] B.R. James, in: L.L. Simandi (Ed.), Dioxygen Activation Catalytic Oxidation,
Elsevier, Amsterdam, 1991.