A synergistic effect of a cobalt Schiff base complex and TiO2 nanoparticles on aerobic olefin epoxidation

Article abstract of DOI:10.1039/c6ra16167g

In this study, a cobalt Schiff base complex and TiO₂ nanoparticles exhibited a synergistic effect on the visible-light photocatalytic activity in the aerobic oxidation of various olefins in the absence of a reducing agent. The catalyst structure was found to be well preserved after the oxidation reaction and the catalyst could be reused at least five times.

Full text of DOI:10.1039/c6ra16167g

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A synergistic effect of a cobalt Schiff base complex
and TiO2 nanoparticles on aerobic olefin
epoxidation†
Cite this: RSC Adv., 2016, 6, 79085
Received 22nd June 2016
Accepted 12th August 2016
*
*
Maasoumeh Jafarpour, Hossein Kargar and Abdolreza Rezaeifard
DOI: 10.1039/c6ra16167g
www.rsc.org/advances
In this study, a cobalt Schiff base complex and TiO2 nanoparticles reaction mixtures, making the processes more cost effective.
exhibited a synergistic effect on the visible-light photocatalytic activity Nevertheless, in many of these aerobic epoxidation reactions
in the aerobic oxidation of various olefins in the absence of a reducing under the heterogeneous or homogeneous catalysis conditions,
agent. The catalyst structure was found to be well preserved after the a sacricial co-reductant was needed. Moreover, limited activity
oxidation reaction and the catalyst could be reused at least five times. towards different olens was observed.7–18 Thus, there is a great
demand for designing a novel heterogeneous cobalt(II)-based
catalyst for aerobic epoxidation of structurally and electroni-
cally different olens in the absence of any reducing agent.
1. Introduction
Research on functional nanohybrid materials has been
Transition metal-catalyzed aerobic oxidation reactions to
widely recognized as one of the most promising and rapidly
produce valuable oxygen containing chemicals are an impres-
emerging research areas in material chemistry. The unique
sive example of green processes. Not only is molecular oxygen
properties of inorganic/organic nanohybrid materials can be
(or air) an inexpensive and renewable and the simplest available
oxidant, but also the only by-product of the reaction, water, is
also a non-toxic and safe chemical.1–4
Co-containing material catalyzed epoxidation methods have
been widely developed.5–9 The heterogeneous catalyst based-
Co(II) can be easily separated, recovered, and reused from the
advantageous to many elds such as optical and electronic
materials, biomaterials, sensing, coating, energy storage and
catalysis. To the best of our knowledge, a few reports have
approached the catalytic activity of transition-metal-
coordinated organic–inorganic hybrid materials for the
aerobic epoxidation of olens.19–22
In continuation of our ongoing research on the development
of novel catalytic methods for aerobic oxidation of organic
compounds,21–24 just recently, we have discovered the catalytic
efficiency of the CoL2@TiO2 nanocomposite in the selective
aerobic benzylic C–H oxidation of alcohols and hydrocarbons in
acetonitrile.22 Herein, we wish to describe its catalytic activity
and selectivity in the aerobic epoxidation of olens under
heterogeneous conditions, that takes place in the absence of
reducing agent (Scheme 1). The catalyst was reused in the
procedure for many times without any loss of activity.
2. Results and discussion
Scheme
1 Aerobic olefin oxygenation catalyzed by CoL2@TiO2
The activity of title catalyst was screened in the oxidation of
norbornene (1 mmol) under continues stream of oxygen (1 atm)
in acetonitrile (2 ml). No activity was observed in the absence of
the catalyst under any conditions. Then, the catalytic system
was performed and the inuence of different factors such as
solvent nature and amount, temperature, catalyst and NHPI
amounts and oxidant nature on the oxidation of norbornene
nanohybrid.
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414 Iran. E-mail: mjafarpour@birjand.ac.ir;
rrezaeifard@birjand.ac.ir
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra16167g
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Fig. 1 The screening of the (i) solvent nature (ii) temperature (iii) solvent amount (iv) catalyst amount (v) NHPI amount and (vi) oxidant nature on
the oxidation of norbornene (1 mmol) catalyzed by CoL2@TiO2 after 5 h.
was investigated (Fig. 1). According to our nding the catalytic
system showed the best performance using a molar ratio of
100 : 5 : 0.75 for olen–NHPI–catalyst in 2 ml acetonitrile at 65
Fig. 2 The comparison of catalytic activity of CoL2@TiO2 nano-
composite with other nanocomposites and nanooxometals on the
Fig. 3 The screening of photocatalytic activity of CoL2@TiO2 nano-
composite in the oxidation of norbornene using NHPI (5 mol%) under
continues stream of O2 (5–7 ml minÀ1) at 65 ꢀC.
oxidation of norbornene (1 mmol) in CH3CN (2 ml) using NHPI (5 mol%)
under continues stream of O2 (5–7 ml minÀ1) and visible light at 65 ꢀC.
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ꢀC. Under the optimized conditions, norbornene converted presence of 2,6-di-tert-butyl-4-methylphenol as
completely within 5 h and 96% of exo-norbornene oxide was scavenger.
a radical
secured as the sole product.
It is worth to mention that, replacing the catalyst (CoL2@-
The efficiency of oxidation was affected noticeably by NHPI TiO2 nanocomposite) by Co-complex (CoL2) and TiO2 nano-
amount demonstrating a radical processes prevail to a high particles alone decreased the yield of norbornene oxide to 32
extent in the present system.25 More evidence for this claim was and 42% respectively in the presence of NHPI under the same
obtained by retardation of the norbornene oxidation in the conditions (Fig. 2). Also, another nanosized oxometals such as
Table 1 Aerobic oxidation of olefins catalyzed by CoL2@TiO2 nanohybrid in CH3CNa
Conversion
(yield%)
Selectivity
(%)
Entry
1
Alkene
Productb
Time (h)
5.5
100 (95)
96 (88)
100
100
2
3
5
30 (22)
100
10
4
5
89 (82)
100
84c
8.5
7
100 (84)
6
7
94 (87)
65 (57)
100
100
6.5
9
8
9
43
40
100
100
10
5
10
93
88
82d
76d
10
11
9.5
a
The molar ratio of substrate/NHPI/catalyst was 1000 : 50 : 7.5. The reactions were run under continues stream of O2 (5–7 ml minÀ1) in CH3CN (2
ml) at 65 ꢀC. b The products were identied by 1H NMR or by comparison with authentic samples retention times of GC analysis. c The remainder is
the related a,b-unsaturated ketone. d The remainder is the related epoxide.
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MoO3,26 m-ZrO2,27 g-Fe2O3 and TiO2, as well as and their
nanocomposites such as CoL2@MoO3, CoL2@SMNP and SMNP
the CoL2@TiO2 nanocomposite, prolonged the oxidation of
norbornene under the same conditions (Fig. 2).
So, we inferred that, the catalytic activity of CoL2@TiO2
nanohybrid may be attributed to oxidation activity of Co(II)
centers5–9 combined with photocatalytic activity of TiO2 core
which also acts as support.22
To conrm of photocatalytic property of TiO2 at the core of
title nanohybrid, the aerobic oxidation of norbornene in the
presence of NHPI (5 mol%) at 65 ꢀC under UV and visible light
as well as in dark were investigated aer 3 and 5 h. Accelerated
reactions under light radiation particularly UV light proved the
photocatalytic effect of TiO2 core on the oxidation efficiency of
CoL2@TiO2 nanocomposite (Fig. 3). The smaller band-gap of
CoL2@TiO2 nanohybrid (2.9 eV) than that of TiO2 nanoparticles
(3.2 eV)22 may be a good reason for such an activity.
Fig. 4 Recycling of the catalytic system for oxidation of norbornene
under conditions mentioned in this study.
To establish the general applicability of the method, various
olens were subjected to the oxidation protocol under the
optimized conditions (Table 1). Table 1 showed the several
useful features of this catalytic method. The present oxidation
method selectively produced the corresponding epoxides of
norbornene, a-methylstyrene, 1-phenylcyclohexene and indene
(Table 1, entries 1–4). The forming of norbornene oxide exclu-
sively as an exo-isomer declared stereoselectivity of the method
(entry 2). The oxidation of 1-phenylcyclohexene (entry 3) pro-
ceeded slowly demonstrating the steric effect around the active
site of the catalyst is dominant.
When cyclooctene was subjected to the oxidation procedure,
cyclooctene oxide was formed in 84% yield along with 16% of
the related a,b-unsaturated ketone resulting from allylic
oxidation (entry 5). Allylic oxidation pathway was dominant in
the oxidation of cylohexene, and 2-cylohexen-1-one was
appeared as sole product (entry 6). Accordingly, cyclic allylic
alcohol, 2-cyclohexene-1-ol oxidized easily to the related a,b-
unsaturated ketone (entry 7).
Fig. 5 The FT-IR spectra of fresh (a) and reused (b) CoL2@TiO2
nanocatalyst.
The FT-IR spectrum of a fresh and a reused sample of the
catalyst did not show any signicant differences, suggesting
that the CoL2@TiO2 nanohybrid remained in its initial state
(Fig. 5).
To demonstrate the practical utility of this method, a larger
scale oxidation of norbornene (20.0 mmol) in the presence of
CoL2@TiO2 was performed, giving norbornene oxide in 92%
yield within 4 h.
Contrary to other olens, a terminal C]C bond conjugated
with phenyl rings mainly gave the related carbonyl compounds
(entries 10, 11). It seems that benzaldehyde and acetophenone
are favorably formed at high temperature in the oxidation of
styrene and a-methylstyrene, respectively, because high
temperature will supply enough energy to break the C]C
bond.28–30
The chemoselectivity of the procedure was notable. The
linear allylic alcohols (entries 8, 9) converted selectively to the
corresponding epoxides in moderate yield and no product
resulting from oxidation of hydroxyl group was detected.
3. Conclusions
It should be noted that no activity was observed for 1-octene In this work, we improved TiO2 visible light photocatalytic
as a less reactive linear terminal olen. properties by immobilizing of cobalt Schiff base complex on
To assess the stability of catalyst during the epoxidation nanocrystalline TiO2 for aerobic oxidation of various olens. A
process, the catalyst was recovered efficiently over at least ve synergistic effect of cobalt Schiff base complex and TiO2 nano-
times in aerobic epoxidation of norbornene under condition particles on the visible-light photocatalytic activity was
used (Fig. 4). The leaching of the catalyst was screened by explored. These favorable characteristics, plus the facile and
centrifuging and decantation of the reaction mixture. No cata- efficient reusability of the solid catalyst, are the strengths of the
lytic activity was observed in the ltrate solution and based on presented work providing the scalability of the methods. Thus,
ICP-AES results, amount of Co in the catalyst aer recycle are our method possesses high generality which makes it suitable
comparable with the fresh catalyst.
for industrial goals.
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14 N. Maksimchuk, M. Melgunov, Y. A. Chesalov, J. Mrowiec-
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