Modified crosslinked polyacrylamide anchored Schiff base-cobalt complex: A novel nano-sized heterogeneous catalyst for selective oxidation of olefins and alkyl halides with hydrogen peroxide in aqueous media

Article abstract of DOI:10.1016/j.apcata.2010.12.001

A new nano-sized polymer-supported Schiff base-cobalt complex catalyst based on crosslinked polyacrylamide was synthesized and characterized. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the catalyst show nanosize fiber-like image of the polymeric catalyst. The catalyst activity was studied in the oxidation of various olefins including non-activated terminal olefin in water with hydrogen peroxide as an oxygen source. The catalyst showed high degree of selectivity. The effects of reaction parameters such as solvent, oxidant, temperature, molar ratio of catalyst and catalyst structure on the oxidation of styrene were investigated. The oxidation of benzyl halides to their corresponding carbonyl compounds in the presence of this heterogeneous catalyst was studied as well. The catalyst can be recycled several times without significant loss in its activity.

Full text of DOI:10.1016/j.apcata.2010.12.001

Applied Catalysis A: General 393 (2011) 242–250
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Modified crosslinked polyacrylamide anchored Schiff base–cobalt complex: A
novel nano-sized heterogeneous catalyst for selective oxidation of olefins and
alkyl halides with hydrogen peroxide in aqueous media
Bahman Tamami∗, Soheila Ghasemi
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2010
Received in revised form
29 November 2010
Accepted 1 December 2010
Available online 8 December 2010
A new nano-sized polymer-supported Schiff base–cobalt complex catalyst based on crosslinked poly-
acrylamide was synthesized and characterized. Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) of the catalyst show nanosize fiber-like image of the polymeric catalyst. The
catalyst activity was studied in the oxidation of various olefins including non-activated terminal olefin in
water with hydrogen peroxide as an oxygen source. The catalyst showed high degree of selectivity. The
effects of reaction parameters such as solvent, oxidant, temperature, molar ratio of catalyst and catalyst
structure on the oxidation of styrene were investigated. The oxidation of benzyl halides to their corre-
sponding carbonyl compounds in the presence of this heterogeneous catalyst was studied as well. The
catalyst can be recycled several times without significant loss in its activity.
Keywords:
Polyacrylamide supported catalyst
Heterogeneous catalysis
Schiff base–cobalt complex
Olefin oxidation
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
aqueous and organic phases. Some examples of these supports are
polyacrylamide, poly(vinylpyrrolidone), and poly(ethylene glycol)
Schiff bases represent one of the most widely utilized classes
of ligands in metal coordination chemistry [1,2]. They are eas-
ily synthesized and form complexes with almost all metal ions.
Over the past few years, there have been many reports in the lit-
erature on their applications in homogenous and heterogeneous
catalysis. These complexes show high catalytic activity in various
reactions such as polymerization reactions, oxidation, decomposi-
tion of hydrogen peroxide, carbonylation reaction, Heck reaction,
Diels–Alder reaction, and Lewis acid assisted organic transforma-
tions [3].
In order to develop environmentally benign methods, there is
currently a rapid growth in the use of supported transition metal
catalysts such as polymer-supported systems [4]. Polymers modify
the reactivity of metal complexes by changing the electronic and
steric properties of the coordinating ligands. In addition, heteroge-
nization of active metal complexes on polymeric supports offers the
practical benefits such as ease in handling and separation, recovery,
recycling, non-toxic characteristics, and amenability for continuous
processing. The incompatibility of conventional polystyrene-
supported species with solvents and substrates has led to the design
of synthetic polymeric supports that are compatible with both
[5–7]. These supported systems have different polarity, solvation,
and reactivity compared to polystyrene-supported species.
Oxidation is among the most important reactions in the
chemical industry which is usually catalyzed by transition metal
complexes [8]. In this field, olefin oxidations that degrade large
compounds or introduce oxygen functionality into molecules are
the most synthetically useful oxidation reactions. Many synthetic
methodologies are widely reported in the literature for oxidizing
double bonds such as ozonolysis [9], oxidation with KMnO4 [10],
OsO4 [11] or RuO4 [12] and combination of oxidant and a transi-
tion metal catalyst [13]. The most well-known methodology is the
combination of oxidant and a transition metal catalyst due to the
transition metal ability in activating oxidants. In this regard, many
different transition metal complexes of Ti (IV), Mo (VI), Mn (III),
Cr (VI), Co (II), Cu (II), Fe (III), etc. have been introduced as cata-
lysts [14–20]. Among these, cobalt complexes are useful catalysts
in oxidizing olefins, as cobalt (II) appears to be a good oxygen trans-
fer agent [21]. Reversible redox cycle between CoII–CoIII oxidation
states causes oxygen atom transfer to the double bonds by cobalt
complexes. A variety of oxygen sources such as NaOCl, PhIO, O2,
ammonium periodate, alkyl hydroperoxides and H2O2 are used as
oxidants in the catalyzed oxidation processes. H2O2 is an attractive
oxidant in catalytic oxidation of organic compounds and widely
used in industry due to its inexpensiveness, cleanliness, easy han-
dling, and availability. In addition, it produces water as its only
by-product in the oxidation reaction [22].
∗
Corresponding author. Tel.: +98 711 2284822; fax: +98 711 2280926.
E-mail addresses: tamami@chem.susc.ac.ir (B. Tamami),
Soheila ghasemi chem@yahoo.com (S. Ghasemi).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.12.001

B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
243
One of the main problems in industry is related to the use
of organic solvents in the processes. The manufacture, transport,
stock, handling and disposal of solvents are aspects that demand
great care and expense. Therefore, water is considered as an alter-
native solvent to expensive, flammable and toxic organic solvents.
Evident show that organic reactions carried out in aqueous media
may offer advantages over those occurring in organic solvents [23].
For example conducting the reactions in aqueous media is usu-
ally more selective, and separating of organic compounds from the
aqueous phase is easily achieved. Therefore, the use of water in
organic reactions is a useful alternative to organic solvents and is a
step in developing green technology.
In continuation of our study on using modified polyacrylamide
support [5], herein, we report the synthesis and characterization
of a new heterogeneous Schiff base–cobalt complex derived from a
modified crosslinked polyacrylamide support, capable of catalyzing
oxidation of olefins and alkyl halides in the presence of H2O2 as
oxidant in aqueous media. This catalytic system allows efficient
oxidation to be carried out with a high degree of selectivity.
ture was refluxed at 100–110 ◦C for 9 h. It was then poured slowly
into a large volume of EtOH (250 mL). The resin was filtered off
and washed with NaCl solution until the filtrate was free from
hydrazine. The gel was washed with distilled water and methanol
and subsequently dried at 60 ◦C under reduced pressure. The
amine content was determined by alkalimetric method. HCl (10 mL,
0.182 M) that was standardized against potassium hydrogen phtha-
late (KHP) (0.1 M) was added to poly(acrylohydrazide) (0.1 g) and
was stirred at room temperature for 12 h. The mixture was then fil-
tered and washed with distilled water (5 mL × 2). The filtrate was
titrated with freshly prepared NaOH (0.196 M). The average amount
of NaOH used in three times titration was 5.3 mL, and the capacity
of the polymer was calculated to be 7.8 mmol of amino group per
gram of the polymer.
2.2.3. Preparation of crosslinked
poly(acrylohydrazide-imino-salicylaldehyde)
Croslinked poly(acrylohydrazide) (1 g, 7.8 mmol NH2) was
added to salicylaldehyde (1.9 g, 15.6 mmol) in ethanol (35 mL).
The reaction mixture was stirred at 60 ◦C for 18 h. The resul-
tant yellow product was filtered, washed with water, ethanol
and acetone and dried under reduced pressure. The imine con-
tent was evaluated from CHN analysis. Poly (acrylohydrazide);
%C = 46.8, %N = 26, %H = 7.2, %O = 20 and poly(acrylohydrazide-
imino-salicylaldehyde); %C = 59.7, %N = 15.4, %H = 6.1, %O = 18.9.
Average imine content was calculated from percent of imine
functionalization using elemental data given. It was found to be
3.1 mmol/g.
2. Experimental
2.1. Materials and techniques
All chemicals were of commercial reagent grade and obtained
from Merck or Fluka companies. Acrylamide (Fluka) was recrys-
tallized from chloroform. Alkenes were passed through a column
containing active alumina to remove peroxidic impurities. Other
reagents and solvents were used without further purification.
Hydrogen peroxide (30%) was stored at 5 ◦C and titrated with potas-
sium permanganate (0.1 N) to check its concentration. All products
were characterized with FT-IR, 1H and 13C NMR spectroscopy; FT-
IR was performed using a Shimadzu FTIR-8300 spectrophotometer
and NMR was performed on a Bruker Avance DPX instrument
(250 MHz). All yields refer to the isolated products. Progress of
reactions was followed by TLC on silica-gel Polygram SIL/UV 254
plates or by GC on a Shimadzu model GC 10-A instrument. The
Co analysis and leaching test were carried out by inductively cou-
pled plasma (ICP) analyzer (Varian, Vista-Pro). CHN analysis was
carried out on thermo finningan FIASHEA 1112 instrument. X-ray
diffraction data obtained with XRD, D8, Advance, Bruker, axs. Scan-
ning electron micrographs were obtained by SEM, XL-30 FEG SEM,
Philips, at 20 kV. All samples are sputter-coated with gold prior to
SEM observation. Transmission electron microscopy (TEM) analy-
ses were performed on a Philips model CM 10 instrument. UV–vis
diffuse reflectance spectroscopy (UV–vis DRS) was performed on
Cary 5 instrument by photometric mode with 300 nm/min scan
rate. The surface area of the catalyst was measured using the
Braunuer–Emmet–Teller (BET) method on Chem BET-3000, Quan-
tachrome instrument and N2 as adsorbent.
2.2.4. Preparation of the polymer supported cobalt complex
Crosslinked poly(acrylohydrazide-imino-salicylaldehyde) (1 g)
was treated with a solution of Co(OAc)2·4H2O (1.4 g) in methanol
(35 mL) at 60 ◦C for 18 h. Upon complexation, the mixture was fil-
tered and washed thoroughly with distilled water, methanol and
acetone. It was then conditioned for a total of 9 h (1× 3 h each
refluxing in acetone, ethanol and acetonitrile). The resultant brown
powder was dried under vacuum overnight. The metal content of
the catalyst found by ICP, was 1.8 mmol/g.
2.3. Catalytic oxidation of olefins with cobalt complex
The catalyst (11 mg, 0.02 mmol) was suspended in water (5 mL),
and olefin (1 mmol), H2O2 30% (0.3 mL, 5 mmol) and dodecane as
internal standard were added to this suspension. The mixture was
heated at 75 ◦C with constant stirring. Small amounts of the reac-
tion mixture were frequently removed from the reaction vessel
and were analyzed by gas chromatography. The conversion was
calculated based on the initial amount of the substrate, and the
selectivity was calculated based on total amounts of products. Upon
reaction completion, the polymeric catalyst was removed by filtra-
tion and the corresponding product was obtained via extraction
with EtOAc and purified on a silica-gel plate. Assignments of prod-
ucts were performed by comparison of their 1H NMR, 13C NMR, and
physical data with those of the authentic samples.
2.2. Preparation of the catalyst
2.2.1. Preparation of 5% NNMBA-crosslinked polyacrylamide
N,Nꢀ-Methylene-bis-acrylamide (NNMBA) (1.14 g, 7.4 mmol)
and acrylamide (10 g, 0.14 mol) were dissolved in ethanol (250 mL).
K2S2O8 (62 mg, 0.23 mmol) was added to this solution and stirred.
The polymer began to precipitate within 30–40 min. The sus-
pension was stirred at 70–75 ◦C for additional 5 h. The polymer
formed was collected by filtration, washed several times with
water, ethanol, and methanol and dried at 60 ◦C under reduced
pressure.
2.4. Catalytic oxidation of benzyl halides with cobalt complex
In a typical experiment, the catalyst (11 mg, 0.02 mmol), ben-
zyl halide (1 mmol) and H2O2 30% (0.3 mL, 5 mmol) were mixed
in H2O (5 mL) and heated at 75 ◦C with constant stirring. The reac-
tion progress was followed by TLC. After completion of the reaction
the above procedure was followed. IR spectroscopy confirmed the
chemical structure of the products.
2.2.2. Preparation of crosslinked poly(acrylohydrazide)
Crosslinked polyacrylamide (1 g) was added in small portions
to an excess of well-stirred hydrazine hydrate (10 mL). The mix-

244
B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
Fig. 3. Powder X-ray diffraction patterns of (a) crosslinked polyacrylamide-
anchored Schiff base and (b) corresponding cobalt complex.
Fig. 4. SEM image of crosslinked polyacrylamide-anchored Schiff base–cobalt com-
plex.
Fig. 1. FT-IR spectra of (a) crosslinked polyacrylamide, (b) crosslinked
Table 1
poly(acrylohydrazide),
(c)
crosslinked
poly(acrylohydrazide-imino-
Electronic transitions of polymeric Schiff-base ligand and its corresponding cobalt
(II) complex.
salicylaldehyde) and (d) polymer supported cobalt complex.
Metal ion Electronic transition Ligand (ꢀmax) (nm) Complex (ꢀmax) (nm)
2.5. General procedure for recycling of the catalyst
Co (II)
␲ → ␲*
n → ␲*
C → T
265
328
–
240
261
380
–
After the oxidation reaction, the suspension was cooled to room
temperature and filtered off. The used polymer was washed with
water, ethanol and acetone and dried under vacuum. It was then
reused in the next reaction cycle with a new portion of reagents.
d → d
–
2.6. Catalyst characterization
IR spectrum of the polymer showed the characteristic absorp-
tion of amide (N–H) group at 3193 and 3360 cm−1, and carbonyl
1
0.8
0.6
a
0.4
b
0.2
0
200
250
300
350
400
450
500
Wavelenght (nm)
Fig. 2. UV–vis diffuse reflectance spectra for the polymeric Schiff-base ligand (a)
Fig. 5. TEM image of crosslinked polyacrylamide-anchored Schiff base–cobalt com-
and its corresponding cobalt catalyst (b).
plex.

B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
245
O
O
H2NNH2 (excess)
100-110 oC
K2S2O8/ EtOH
70 oC
CH2CH
O
N
N
+
H
H
O
NH2
NH2
NNMBA
5% crosslinked
OHC
CH2CH
NHN C
CH2CH CH2CH
CH2CH
O
Co(OAc)2.4H2O
MeOH/ 60 oC
HO
EtOH/ 60 oC
O
HN
N
HN
N
O
NHNH2
O
H
HO
Co
O
O
polymeric Co catalyst
Scheme 1. Synthetic strategy for the preparation of modified polyacrylamide loaded cobalt catalyst.
group (C O) at 1668 cm−1. Poly(acrylohydrazide) was obtained by
transamidation reaction of crosslinked polyacrylamide with excess
of hydrazine hydrate. The resultant polymer had a high content of
amino group per gram of the polymer which was determined by
alkalimetric method and was found to be 7.8 mmol/g of the resin.
IR spectrum of the poly(acrylohydrazide) showed the character-
istic absorption of amino (N–H) group at 3427 cm−1. Crosslinked
poly(acrylohydrazide-imino-salicylaldehyde) was prepared by the
reaction of poly(acrylohydrazide) with salicylaldehyde in ethanol.
IR spectrum of the polymer showed peaks at 1620 cm−1, corre-
sponding to the formation of imine group (CH N) and at 750 and
1488 cm−1 due to the bending of ortho substitution out of phase
(OOP) and C C stretching of aromatic ring, respectively. The imine
content was evaluated from CHN analysis and was found to be
3.1 mmol/g. Treating the solution of Co(OAc)2·4H2O with the Schiff-
base ligand lead to the formation of polyacrylamide supported
cobalt complex. In order to make the process completely hetero-
geneous, the catalyst was conditioned by washing thoroughly with
solvents to remove any loose Co species. The metal content of the
catalyst found by ICP was 1.8 mmol/g of the resin. IR spectra of the
polymer before and after cobalt loading showed a shift in frequency
from 1620 to 1603 cm−1 for ꢁ(C N) and from 1272 to 1283 cm−1 for
ꢁ(C–O). In addition, IR spectrum of the complex showed two new
absorption bands at 450 and 535 cm−1, which were attributed to
the formation of M–N and M–O, respectively. These variations con-
firmed the coordination of cobalt ion with the azomethine nitrogen
and phenolic oxygen of the polymeric Schiff-base ligand. FT-IR
spectra of all polymers are presented in Fig. 1.
Cat.
O
+
O
H2O2/ H2O
CH2CH CH2CH
HN
N
HN
N
Cat. =
O
O
Co
O
O
Scheme 2. Oxidation of olefins using polymeric cobalt catalyst.
Table 2
Effect of different solvents and oxidants on the oxidation of styrene using
polyacrylamide-supported cobalt complex.a
Entry
Solvent
Oxidant
Conversion
(%)b
% Selectivity to
benzaldehydec
1
2
3
4
5
6
7
8
9
CH3CN
EtOH
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
NaIO4
Bu4NIO4
H2O2/urea (UHP)
Tert-BuOOH
–
66
40
55
70
–
d
CH2Cl2
CH3COCH3
EtOAc
H2O
No reaction
30
d
50
62
89
91
90
75
40
–
35
100
100
100
60
H2O
H2O
H2O
H2O
H2O
H2O
10
11
12f
88e
Trace
30
H2O2
70
a
Reaction conditions: styrene (1 mmol), oxidant (5 mmol), cat. (2 mol%), solvent
(5 mL) at 75 ◦C for 6 h.
b
Conversion based on starting material = [1 − (concentration of substrate left after
reaction/initial concentration of substrate) × 100.
c
Table 4
% Selectivity of benzaldehyde = (concentration of benzaldehyde/total concen-
Oxidation of styrene with H2O2 in water catalyzed by different polyacrylamide-
tration of all oxidation products) × 100.
supported metal complex.a
d
At reflux temperature.
e
Crude mixture of oxidation products (benzaldehyde, styrene oxide, phenylac-
Entry Catalyst Metal content
(mequiv./g)
Conversion
(%)b
% Selectivity to
benzaldehyde
TONc
etaldehyde, acetophenone and benzoic acid).
f
Without catalyst for 24 h.
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
G
H
I
1.8
1.29
1.6
1.4
1.1
0.96
1.5
2
100
68
35
62
38
30
15
90
72
35
89
75
82
75
82
70
60
85
72
60
50
34
17.5
31
19
15
7.5
45
36
17.5
Table 3
Effect of molar ratio of catalyst to substrate on the oxidation of styrene.a
Entry
Mol% cat.
Conversionb
% Selectivity to benzaldehyde
TONc
1
2
3
4
5
0.2
0.5
1
45
53
75
100
100
96
225
106
75
50
25
2
4
100
100
89
68
1.8
0.56
10
J
a
a
Reaction condition: styrene (1 mmol), H2O2 (30%) (5 mmol), cat. (0.2–4 mol%) in
H2O (5 mL) at 75 ◦C for 6 h.
Reaction condition: styrene (1 mmol), H2O2 (30%) (5 mmol), cat. (2 mol%) in H2O
(5 mL) at 75 ◦C for 6 h.
b
b
Conversion based on starting material.
mmol of converted substrate/mmol cobalt in the catalyst.
Conversion based on starting material.
mmol of converted substrate/mmol cobalt in the catalyst.
c
c

246
B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
CH2CH CH2CH
CH2CH CH2CH
CH2CH CH2CH
HN
R
HN
R
HN
R
HN
R
O
O
O
O
HN
N
HN
N
O
O
M
N
N
N
N
OAc
Co
Co
O
O
O
O
N
N
OAc
R= 0
R= (CH2)3
R=(CH2)6
Cat. A
Cat. B
Cat. C
M=Co Cat. A M=Fe Cat. I
R= 0
R= (CH2)3
R=(CH2)6
Cat. D
Cat. E
Cat. F
M=Ni
M=Mn
Cat. J
Cat. G
Cat. H
M=Cu
(c)
(a)
(b)
Scheme 3. Different types of Schiff-base metal complexes.
Further evidences for metal ion complexation were obtained
lamide monomer in ethanol using K2S2O8 as an initiator. The polar
nature of the crosslinking agent makes it compatible with the poly-
mer backbone as well as the reaction medium. We found that
polymers with less than 5% NNMBA was sticky and not physically
proper to be used in further reactions.
from electronic spectra of Schiff-base ligand and its corresponding
metal complex (Table 1). The complexation of cobalt (II) ions with
Schiff-base ligand shows a shift in ␲ → ␲* and n → ␲* transitions
from 265 to 240 nm and 328 to 261 nm which provided evidence
for the coordination of cobalt (II) ions through nitrogen atom of
azomethine group. The appearance of a new band at 380 nm for
metal complex is attributed to charge transfer process (L → M) from
ligand to cobalt (II) ions. No d → d transition has been observed nei-
ther for ligand nor for complex. UV–vis diffuse reflectance spectra
for the polymeric Schiff-base ligand and its corresponding cobalt
catalyst is presented in Fig. 2.
XRD pattern of the heterogeneous catalyst is shown in Fig. 3.
The strongest peak of the XRD pattern corresponds to the planes
of amorphous polymer matrix, while other peaks reveal a single
phase of the supported cobalt particles. In addition to XRD, scan-
ning electron microscopy (SEM) was used to study the morphology
of the supported catalyst (Fig. 4). SEM showed a nanometer up to
micrometer fiber-like images for the polymeric catalyst. Transmis-
sion electron microscopy (TEM) image of the catalyst showed that
the fiber lengths mostly range from 50 to 250 nm in size (Fig. 5). The
BET surface area of the polymeric Schiff-base ligand was 4.3 m2/g,
which significantly increased to 25.3 m2/g for its corresponding
cobalt catalyst. It is probably due to the fiber-like images of the
catalyst. Although the BET surface area of the catalyst was rather
small, considerable activity was observed in the oxidation of olefins.
3.2. Catalytic oxidation with polymeric cobalt complex catalyst
The catalytic activity of the polymer-supported cobalt complex
was investigated in the oxidation of different alkenes (Scheme 2).
The oxidation of styrene was initially studied as a model reac-
tion. In the process of oxidation the choice of solvent and oxygen
donor is crucial, so these parameters were optimized and the results
are summarized in Table 2. Among water, acetonitrile, ethanol,
dichloromethane, acetone and ethyl acetate, water was chosen
as the reaction medium. The excellent catalytic activity in water
was attributed to the polarity of solvent and its ability in maxi-
R1 OOH
R2
if R3,R4=H
H2O2
oxidative
cleavage
R2
R4
+
O
CH2O+ H2O
OHR3
R1
iv
peroxy intermediate
LCo(II)
R2 R3
H2O2
+
OH
R1 O R4
R2 R4
R1
HO O
i
through
R3
iii
R2 R4
R1 R3
3. Results and discussion
LCo(III)OOH + H+
LCo(III) + OH + OH
3.1. Preparation of the catalyst
CoIII-peroxo intermediate
ii
The design of the Co catalyst is shown in Scheme 1. Polyacry-
lamide crosslinked with N,Nꢀ-methylene-bis-acrylamide (NNMBA)
(5%) was prepared by free radical solution polymerization of acry-
H2O2
Scheme 4. Proposed oxidation mechanism.
Fig. 7. Effect of successive cycles on catalytic activity of the supported catalyst for
the oxidation of styrene. Reaction condition: styrene (1 mmol), H2O2 (30%) (5 mmol),
2 mol% catalyst in H2O (5 mL) at 75 ◦C for 6 h.
Fig. 6. Effect of reaction time on conversion and selectivity of styrene oxidation over
the polymeric Co catalyst at 75 ◦C using H2O2 as an oxidant.

B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
247
Table 5
Oxidation of olefins with H2O2 in the presence of polyacrylamide-supported cobalt complex in water.a
Entry
Olefin
Product (% selectivity)
Conversion (%)
TONc
50
H2O2 efficiency (%)d
O
CHO
+
1
100
40
89%
11%
CHO
2
3
85
88
42.5
44
36
37
Br
Br
100%
CHO
100%
O
4
100
50
40
100%
O
5b
80
75
40
26
25
+
O
71%
29%
O
O
6b
37.5
100%
7b
65
37
32.5
18.5
22
14
100%
O
H
68%
12%
8b
O
a
The molar ratio for cat.: substrate: H2O2 are 1:50:250. The reactions were performed in H2O (5 mL) at 75 ◦C for 6 h.
The reaction proceed for additional 6 h.
mmol of converted substrate/mmol cobalt in the catalyst.
H2O2 efficiency (%) = products (mol)/consumed H2O2 (mol) × 100.
b
c
d
mum swelling of the polymeric catalyst. Various oxidants such as
decreased. This may be due to the thermal decomposition of hydro-
gen peroxide. About 1.5 equiv. of the H2O2 was decomposed during
this period. On the other hand, higher ratio of H2O2 causes over-
oxidation to benzoic acid. We also studied the effect of temperature
on the oxidation of styrene with H2O2 in water. It was observed
that only small amount of the oxidation product was detected at
room temperature (50% conversion of styrene after 24 h), while by
increasing temperature up to 75 ◦C the conversion was increased
considerably. Finally, the catalyst loading was varied from 0.2 to
4 mol% in the model reaction (Table 3). It is clear that with increas-
ing the amount of the catalyst, the conversion increased, but the
selectivity and TONs (turnover number) decreased. For example, by
increasing the catalyst loading from 0.2 to 4 mol%, the conversion
of styrene reached to 100% but selectivity to benzaldehyde forma-
tion decreased to 68%. Thus 2 mol% of the catalyst was chosen as
an optimum value for acceptable conversion of substrate and good
selectivity.
NaIO4, Bu4NIO4, urea–H2O2 (UHP), t-BuOOH and H2O2 were exam-
ined in the oxidation of styrene in water. H2O2, NaIO4, Bu4NIO4
and t-BuOOH gave excellent conversion of styrene. However, t-
BuOOH gave a crude mixture of oxidation products and NaIO4 and
Bu4NIO4 are expensive and unsafe reagents. As a result, hydrogen
peroxide was chosen as the oxygen source. It is inexpensive, read-
ily available, has a high content of active oxygen and generates
only water as a by-product. Blank experiment under the same con-
ditions showed that without oxidant the reaction does not take
place (Table 2, entry 11) and without the catalyst very poor con-
version was observed (Table 2, entry 12). Percent of selectivity to
benzaldehyde is also shown in Table 2. Higher selectivity to ben-
zaldehyde is observed in water with different oxidants. Even in the
absence of catalyst selectivity toward benzaldehyde is high. Aque-
ous media probably facilitates the ring opening of epoxide leading
to the oxidative cleavage and formation of carbonyl compound. The
optimum H2O2/substrate molar ratio in our system is 5/1. When
lower ratio of H2O2 was used, the amounts of oxidation products
In order to get an insight into the effect of the ligand structure
on the process, a series of catalysts with different Schiff-base lig-

248
B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
Table 6
Oxidation of benzyl halides with H2O2 in the presence of polyacrylamide-supported cobalt complex in water.a
Entry
1
Substrate
Product
Time (h)
6
Isolated yield (%)
87
CH2Cl
CHO
Cl
Cl
Me
CH2Cl
Me
Br
CHO
CHO
CHO
2
3
4
6
4
6
85
90
75
Br
Cl
CH2Br
CH2Cl
Cl
Br
O
5
9
82
a
The molar ratio for cat.: substrate: H2O2 are 1:50:250. The reactions were performed in H2O (5 mL) at 75 ◦C.
ands and different spacer arms were synthesized (Scheme 3, a and
b). The metal contents of the catalysts were determined by ICP as
shown in Table 4. Salicylaldehyde ligands showed higher yields
and efficiency than pyridine carbaldehyde ligands (entries 1–6).
Furthermore, by increasing the length of spacer arms, efficiency
of catalyst in oxidation of styrene decreased (entries 1–3 and 4–6).
Catalysts with six carbon spacer arm (C and F) were nearly inactive.
In order to show the superiority of the cobalt catalyst, Schiff-base
complexes with different metal ions: Fe, Co, Cu, Ni and Mn were
synthesized (Scheme 3, c) and the ability of these metal catalysts
in the oxidation of styrene with H2O2 was investigated. The most
active catalyst, among these catalysts, was cobalt complex due to
its higher yield in the oxidation of styrene with hydrogen peroxide.
This catalyst (A) has the highest TON value which confirmed its
highest catalytic activity between different types of catalyst. The
obtained results are summarized in Table 4. Benzaldeyde is the
main product in almost all cases nevertheless of the structure of the
catalyst and metal ion. It confirmed the role of reaction condition
for oxidative cleavage and formation of carbonyl compounds.
In addition, effect of reaction time on styrene oxidation over the
Co catalyst at 75 ◦C using H2O2 as an oxidant was investigated. The
styrene conversion and selectivity toward benzaldehyde reached
100% and 89%, respectively (Fig. 6). Based on the conversion curve,
it is necessary to have an induction period of about 2 h at the begin-
ning of the reaction. Based on the selectivity curve, a remarkable
increase of the selectivity toward benzaldehyde was obtained in
the last 3 h (Fig. 6).
To determine the general scope of reaction, oxidation of various
olefins including linear, cyclic and phenyl-substituted olefins were
performed under the same reaction condition used for styrene.
The results with respect to conversion and product selectivity
are represented in Table 5. The reactivity of the olefins toward
oxidation depends on the particular structure of the substrate.
In addition, the type of substrate influences the product dis-
tribution. Reports in the literature show that different cobalt
complexes, in the presence of oxidant, catalyze olefin oxidation
to form different products. For example, styrene can produce
benzaldehyde, styrene oxide, phenylacetaldehyde, acetophenone,
1-phenyl-ethane-1,2-diol, and benzoic acid [16,21b,24,25]. Our
catalytic system allows efficient oxidation with high degree of
selectivity. Phenyl-substituted olefins undergo oxidative cleav-
age and efficiently produce aldehydes and ketones selectively
(entries 1–4). Oxidative cleavage occurs, possibly by forming epox-
ide first followed by the nucleophilic attack of hydrogen peroxide
to epoxide and then decomposition of the peroxy intermediate. The
formation of benzaldehyde and styrene epoxide from styrene also
suggests that styrene epoxide may be the intermediate product for
the formation of benzaldehyde. In the case of cyclic olefins, there
is competition between oxidation of the allylic position and oxi-
dation of double bond. This is dependent on the type of substrate.
Table 7
A comparison of the presented catalyst with some previously reported heterogeneous cobalt catalysts for oxidation of styrene or cyclohexene.
No.
Catalyst
Conditions
Yield (% Selectivity)
Ref.
c
c
1a
2a
3b
4b
5b
6b
7b
8a
9b
Polymer supported CoLCl2
Polymer supported CoLCl2
MeCN/rt/4h/Ph-I = O/2 mol% cat.
MeCN/rt/4 h/H2O2/3 mol% cat.
70 (100%d)
73 (100%d)
57 (70%e)
57.3 (100%e)
30 (12%f)
[21a]
[21a]
[21b]
[21c]
[26]
26
[28]
[29]
–
Isomorphously substituted cobalt VSB-5
Co HAP-␥-Fe2O3
Co(cylam)-functionalized SBA-15
Co(cylam)-functionalized SBA-15
Co(Salen)-POM
Acetone/70 ◦C/6 h/H2O2/0.15 g cat.
MeCN/60 ◦C/8 h/H2O2/25 mg cat.
MeCN/40 ◦C/12 h/H2O2/4 mol% cat.
MeCN/40 ◦C/12 h/t-BuOOH/4 mol% cat.
MeCN/60 ◦C/6 h/H2O2/2 mol% cat.
MeCN/r.t/24 h/1 atm. O2/isobutyraldehyde/20 mg cat.
H2O/75 ◦C/6 h/H2O2/2 mol% cat.
40 (60%f)
85 (82%e)
75.7 (65.9d)
85 (89%e)
Colbalt porphyrin immobilized on montmorillonite
Co(Salen)-polyacrylamide
a
Cyclohexene used as substrate.
b
c
d
e
f
Styrene used as substrate.
L = 2-(alkylthio)-3-phenyl-5-(pyridine-2-ylmethylene)-3,5-dihydro-4H-imidazole-4-one.
Selectivity for cyclohexene epoxide.
Selectivity for benzaldehyde.
Selectivity for styrene oxide.

B. Tamami, S. Ghasemi / Applied Catalysis A: General 393 (2011) 242–250
249
Oxidation of cyclohexene was accompanied by allylic oxidation,
and 2-cyclohexene-1-one and cyclohexene-oxide were produced
in the reaction mixture with 29% and 71% selectivity respectively
(entry 5). Cyclooctene was efficiently converted to the correspond-
ing epoxide with no other products (entry 6). Different product
distribution in cyclohexene and cyclooctene is mainly due to the
difference in activity of the double bond which is attributed to the
different ring size of substrates. The allylic hydrogen of cyclohexene
is abstracted because the removal of hydrogen allows formation of
stable intermediate. This intermediate indebted its stability to the
maximum overlap of the n and ␲ molecular orbitals. On contrary,
conformational constraint of the ring system of the cyclooctene
restricts maximum overlap for the allylic radical [26]. Oxidation of
indene gave indone as the only product (entry 7). Although, the
catalyst is able to transform non-activated terminal olefins to the
oxygenated products, but it is not active for oxidation of these lin-
ear alkenes. Oxidation of 1-octene gave 37% conversion with 68%
selectivity to 1-octanal and 12% selectivity to 1-octene oxide (entry
8).
The efficiency of hydrogen peroxide utilization was determined
by KMnO4 titration method at the end of each reaction. As shown
in Table 5, the H2O2 efficiency was obtained from 14% to 40% based
on different substrates. The low conversion of the substrate made
that the H2O2 efficiency was lower. Furthermore, the longer reac-
tion times (entries 5–8) would increase decomposition of hydrogen
peroxide and decrease H2O2 efficiency.
The mechanism for the oxidation of double bonds by H2O2 cat-
alyzed over cobalt catalyst is similar to one reported in the literature
[26,27] and consists of (i) one-electron oxidation of cobalt (II) to
Co (III) by hydrogen peroxide, (ii) activation of the oxidant at the
metal center and formation of CoIII-peroxo intermediate, (iii) the
concerted transfer of oxygen to the C C double bond and epox-
ide formation, and (iv) nucleophilic attack of oxidants following by
decomposition of the peroxy intermediate in the case that oxidative
cleavage occurs. The reversible redox cycle between CoII–CoIII oxi-
dation states which involved the formation of peroxo species and
oxygen atom transfer was the key factor in these cycles (Scheme 4).
Oxidation of benzyl halides to the carbonyl compounds was
also studied. Primary and secondary benzyl chlorides and bromides
yielded their corresponding aldehydes and ketones with H2O2 in
the presence of the supported catalyst. The results are tabulated in
Table 6.
The recyclability of supported catalysts is one of the most impor-
tant benefits and makes them useful for commercial applications.
Thus, the recovery and recyclability of supported catalyst was
investigated using styrene as a model substrate. The catalyst was
recyclable and was used in oxidation of styrene at least six times.
However, there is a progressive loss of activity accompanied by
diminished yield (Fig. 7). The IR spectrum of the recycled polymer
was the same as the original polymer. The amount of cobalt leached
out of the solution was also determined by ICP analysis and it was
about 2.5%.
A comparison of our catalyst with some previous heteroge-
neous cobalt catalysts reported in the literature for oxidation of
olefins is shown in Table 7. It shows that the most common reac-
tion medium for oxidation reactions was organic solvents. Thus, the
present method offers considerable advantages in terms of green
aqueous media, short reaction times, high yields and high degree
of selectivity in addition to inherent advantages of heterogeneous
catalyst.
of the catalyst showed fiber-like image of the polymeric cata-
lyst in nanometer range. The catalyst efficiently oxidized olefins
to the corresponding oxygenated products in the presence of
H2O2 as a sole oxidant in aqueous media. In addition, vari-
ous benzyl halides yielded their corresponding aldehydes and
ketones without further oxidation to carboxylic acids. The cata-
lyst was used for several times without considerable loss in its
efficiency.
Acknowledgment
The authors gratefully acknowledge the partial support of this
study by Shiraz University Research Council.
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