Use of immobilized transition metal complexes as recyclable catalysts for oxidation reactions with hydrogen peroxide as oxidant

Article abstract of DOI:10.1007/s11243-011-9563-2

A tetradentate Schiff base (teta), obtained from triethylenetetramine and salicylaldehyde, has been covalently bonded to divinylbenzene cross-linked chloromethylated polystyrene. This chelating ligand, abbreviated as PS-teta (PS = polymeric support), reacts with metal chlorides (Cu²⁺, Co ²⁺, and Ni²⁺) in methanol to give polymerbound transition metal complexes, PS-Cu(II)teta/(Cat-1), PS-Ni(II)teta/(Cat-2), and PS-Co(II)teta/(Cat-3), formation of which has been established by various physiochemical methods and spectroscopic techniques. The catalytic potential of these materials has been tested for the oxidation of various alkenes, alkanes, alcohols, and thioethers in the presence of 30% H₂O₂ as an oxidant. At the same time, these catalysts are very stable and could be reused in oxidation reactions for more than five times without noticeable loss of their catalytic activity. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9563-2

Transition Met Chem (2012) 37:97–107
DOI 10.1007/s11243-011-9563-2
Use of immobilized transition metal complexes as recyclable
catalysts for oxidation reactions with hydrogen peroxide
as oxidant
Sk Manirul Islam
Paramita Mondal
•
•
Manir Mobarok
Anupam Singha Roy
•
•
Noor Salam
•
Dildar Hossain
•
Sanchita Mondal
Received: 23 September 2011 / Accepted: 8 November 2011 / Published online: 24 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A tetradentate Schiff base (teta), obtained from
triethylenetetramine and salicylaldehyde, has been cova-
lently bonded to divinylbenzene cross-linked chlorome-
thylated polystyrene. This chelating ligand, abbreviated as
PS-teta (PS = polymeric support), reacts with metal chlo-
transformations in organic chemistry. The oxidation of
benzyl alcohol to benzaldehyde is an important organic
transformation, since benzaldehyde has applications in the
perfume, pharmaceutical, dyestuff, and agrochemical
industries [4, 5].
2
?
2?
2?
rides (Cu , Co , and Ni ) in methanol to give polymer-
bound transition metal complexes, PS-Cu(II)teta/(Cat-1),
PS-Ni(II)teta/(Cat-2), and PS-Co(II)teta/(Cat-3), formation
of which has been established by various physiochemical
methods and spectroscopic techniques. The catalytic
potential of these materials has been tested for the oxida-
tion of various alkenes, alkanes, alcohols, and thioethers in
the presence of 30% H O as an oxidant. At the same time,
Now, from both economic and environmental points of
view, much attention has recently been focused on the
catalytic oxidation of organic compounds using metal
catalysts. Transition metal complexes are widely used in
homogeneous and heterogeneous catalytic oxidations of
different alkenes, alkanes, alcohols, thioethers, etc. [6, 7].
In the recent past, there has been an increasing interest in
developing environmental-friendly greener processes,
which are also economically viable. Homogeneous transi-
tion metal catalyst systems suffer from a major drawback
of the catalyst recovery and/or reuse affecting the overall
economics of the process [8, 9]. Through the past few
decades, there have been significant developments in the
application of heterogeneous catalysts for the industrial
production of organic chemicals. Heterogeneous catalysts,
which are widely used in industry, have good thermal
stability, can be easily separated from the reaction mixture,
and can be often regenerated and reused. Therefore, het-
erogenizing of a homogeneous metal complex by sup-
porting it on an insoluble support has attracted a lot of
interest as a suitable method for solving many practical
problems including the recovery of the catalyst from
reaction mixtures and recycling. Transition metals sup-
ported on materials such as alumina [10], amorphous sili-
cates [11], polymers [12] and zeolites [13], and MCM-41
2
2
these catalysts are very stable and could be reused in oxi-
dation reactions for more than five times without noticeable
loss of their catalytic activity.
Introduction
The oxidation of hydrocarbons to oxygenic compounds is a
pivotal reaction in organic chemistry, for both fundamental
research and industrial manufacturing [1, 2]. Oxidation of
olefins and alkanes to give oxygen containing value-added
products like alcohols, aldehydes, ketones, acids, and
epoxides is an extremely important and useful reaction in
both chemical and pharmaceutical industries [3]. Similarly,
the oxidation of primary and secondary alcohols to
carbonyl compounds is one of the simplest and most useful
[
14] are commonly in use in heterogeneous catalysis.
Polystyrene is one of the most frequently studied het-
S. M. Islam (&) ꢀ M. Mobarok ꢀ P. Mondal ꢀ
A. S. Roy ꢀ N. Salam ꢀ D. Hossain ꢀ S. Mondal
Department of Chemistry, University of Kalyani, Kalyani,
Nadia, WB 741235, India
erogeneous supports due to its environmental stability and
good catalytic activity. Polymer-supported transition metal
catalysts derived from functionalized polystyrene resin
e-mail: manir65@rediffmail.com
123

9
8
Transition Met Chem (2012) 37:97–107
have been employed in various organic reactions [15, 16].
This encourages us to investigate the functionalized poly-
styrene resin-supported transition metal complexes for
oxidation reactions. We have already reported the use of a
polystyrene-anchored transition metal complex as an air-
stable, active, and reusable catalyst in various organic
reactions [17–19]. In the present study, our initial efforts
have focused on developing a ‘green’ catalytic oxidation
system that employs hydrogen peroxide, an eco-friendly
oxidant whose only oxidation by-product is water [20].
Hydrogen peroxide is a very attractive oxidant due to ready
availability, high oxygen content, and ease of use. Here, we
have developed and characterized a polymer-supported
Cu(II) complex of a Schiff base ligand [PS-Cu(II)teta]
catalyst, and catalytic performances are investigated in the
liquid-phase oxidation of various alkenes, alkanes, alco-
hols, and thioethers using 30% H O as oxidant in aceto-
triethylenetetramine, and chloromethylated poly(styrene-
divinyl benzene) were supplied by Sigma-Aldrich chemi-
cals Company, USA. Other reagents were obtained from
Merck Co.
A Perkin-Elmer 2400 C elemental analyzer was used to
collect microanalytical data (C, H, and N). FTIR spectra of
the samples were recorded on a Perkin-Elmer FTIR 783
spectrophotometer using KBr pellets. A Mettler Toledo
TGA/SDTA 851 instrument was used for the thermo-
gravimetric (TGA) analysis. Morphologies of the func-
tionalized polystyrene and complexes were analyzed using
a scanning electron microscope (SEM)(ZEISS EVO40,
England) equipped with EDAX facility. Metal content in
the catalyst was determined using a Varian AA240 atomic
absorption spectrophotometer (AAS).
Preparation of the catalyst
2
2
nitrile medium with excellent conversion and selectivity
for the value-added products like organic epoxides, sulf-
oxides, acetophenone, and benzaldehyde. The effects of
different experimental parameters are investigated in order
to optimize the reaction conditions. We have also com-
pared the catalytic activity of the polymer-supported cat-
alyst with its cobalt and nickel analogs [PS-Co(II)teta, PS-
Ni(II)teta]. The catalytic activities were also tested with
recycled catalyst. The catalytic activity and selectivity of
products did not decrease notably during the recycling
experiments.
The outline for the preparation of polymer-anchored Schiff
base complexes is given in Scheme 1.
Preparation of Schiff base compound (A)
Triethylenetetramine was reacted with salicylaldehyde (in
1:2 molar ratio) in methanol solvent at 70 °C with stirring
for 24 h to prepare the yellow Schiff base compound. The
mixture was cooled to room temperature and then filtered.
The residue was washed with ethanol until the filtrate
became colorless and dried under vacuum.
Experimental
Preparation of polymer-bound ligand (B)
Analytical-grade reagents and freshly distilled solvents
were used throughout the experiment. Liquid substrates
were predistilled and dried by appropriate molecular sieve.
Distillation and purification of the solvents and substrates
were done by standard procedures [21]. Salicylaldehyde,
The polymer-anchored ligand was prepared as follows:
Chloromethylated polystyrene (1.2 g) was added to a
methanolic solution of Schiff base compound (1 g) and
heated to reflux with continuous stirring. After refluxing,
the mixture was cooled to room temperature and then
OH
N=CH
HO
CH Cl
NH2
P
2
N=CH
HO
NH
NH
NH
NH
N
N
CHO
HO
N=CH
MeOH, reflux
MeOH, reflux
HO
N=CH
NH2
A
B
MCl2
EtOH
reflux
N
N
C
NH
O
O
M
M = Cu, Ni, Co
NH
C
Catalyst
Scheme 1 Preparation of polymer-anchored Schiff base complexes
1
23

Transition Met Chem (2012) 37:97–107
99
filtered. The yellow beads were washed with ethanol until
the filtrate became colorless and dried under vacuum.
physicochemical properties, chemical analysis, SEM,
TGA, FTIR, and UV–Vis spectral data. Table 1 provides
the data of elemental analysis. Elemental analysis indicates
that all of the complexes are monomeric, being formed by
the coordination of 1 mol of metal and 1 mol of Schiff base
ligand. The metal content of the polymer-supported cata-
lysts was estimated by atomic absorption spectrometer. The
chemical composition confirmed the purity and stoichi-
ometry of the neat and polymer-supported complexes.
Preparation of polymer-bound Cu(II), Ni(II), and Co(II)
complexes (Cat-1, 2, and 3)
Polymer-anchored Schiff base ligand (1 g) was taken in
ethanol (20 mL). To the reaction mixture, 5 mL of 1% (w/
v) ethanolic solution of metal chlorides (CuCl , NiCl , or
2
2
CoCl ) was added dropwise over a period of nearly 30 min
2
under constant stirring. Then, the reaction mixture was
refluxed for 24 h. The complexes thus formed were filtered
off and washed thoroughly with ethanol and dried at room
temperature under vacuum.
Scanning electron micrographs (SEM) and energy-
dispersive X-ray analyses (EDAX)
Scanning electron micrographs of the polymer-supported
Schiff base ligand and prepared transition metal complexes
were recorded to understand the morphological changes
occurring on the surface of the polymer matrix. In Fig. 1,
the SEM images of the polymer-anchored ligand (Fig. 1a)
and the immobilized Cu(II), Ni(II), and Co(II) complexes
on modified polystyrene (Fig. 1b–d) are shown. SEM pic-
tures showed the morphological difference between the
ligand and the catalysts. Pure polymer beads have a smooth
and flat surface (not shown in Fig. 1). The introduction of
the ligand onto the polystyrene beads causes a roughening
on the top layer. Images of the metal complexes show
further roughening on the top layer, which may be due to
the interaction of the metal atoms with the ligand to
accommodate the fixed geometry of the complexes.
Energy-dispersive X-ray analyses (EDX analysis) of the
complexes show the metal content along with C, N, O, and
Cl, suggesting the formation of metal complex with the
anchored ligand at various sites (Fig. 2). The ratios of
metals to other elements obtained from EDX analysis are
quite similar to the data obtained from elemental analysis.
The presence of the metals can be further proved by
energy-dispersive spectroscopy analysis of X-rays (EDAX)
Procedure for catalytic oxidation
A mixture of 0.1 mmol of catalyst, 10 mL of solvent
(
MeCN), and 5 mmol of substrates was stirred in a 50-mL
two-necked round-bottom flask equipped with a condenser
and placed in an oil bath at 60 °C temperature under vig-
orous stirring, and 10 mmol of H O (30% aqueous solu-
2
2
tion) was added in three intervals. The resulting mixture
was then stirred for 8 h. After filtration, the filtrate was
concentrated and then analyzed by Varian 3400 gas chro-
matograph equipped with a 30-m CP-SIL8CB capillary
column and a flame ionization detector. The products were
determined using cyclohexanone as internal standard.
All reaction products were identified by using an Agilent
GC–MS.
Results and discussion
Characterization of polymer-bound metal Schiff base
complexes
(Fig. 2a–d).
The Schiff base ligand obtained from salicylaldehyde and
triethylenetetramine was well characterized and established
earlier [22]. Due to the insolubilities of the polymer-sup-
ported metal complexes in all common organic solvents,
their structural investigation was limited to their
IR spectral study
The modes of attachment of metals (Cu, Ni, and Co) onto
the support was confirmed by comparison of the FTIR
Table 1 Elemental analysis
C (%)
H (%)
O (%)
N (%)
Cl (%)
Metals (%)
Chloromethylated polystyrene
PS-teta(B)
70.9 (70.82)
76.6 (76.49)
68.2 (68.14)
68.8 (68.73)
68.7 (68.70)
6.0 (5.90)
6.0 (5.97)
4.9 (4.96)
5.1 (5.01)
5.0 (5.00)
–
–
23.3 (23.27)
–
6.4 (6.37)
5.7 (5.67)
5.8 (5.72)
5.8 (5.72)
11.2 (11.15)
10.0 (9.93)
10.1 (10.02)
10.1 (10.01)
–
–
–
–
–
PS-Cu(II)teta
3.1
3.1
2.9
PS-Ni(II)teta
PS-Co(II)teta
Calculated values are given in parentheses
123

1
00
Transition Met Chem (2012) 37:97–107
Fig. 1 FE SEM image of
polymer-anchored Schiff base
ligand (a) and polymer-
anchored complexes Cat-1, Cat-
2
, and Cat-3 (b, c, and d)
-
to 1,298 cm (PS-Cu(II)teta), 1,320 cm (PS-Ni(II)teta),
1
-1
spectra of the polymers before and after loading with
-
metals, both in the mid-IR (4,000–400 cm ) and far-IR
1
-1
and 1,315 cm
(PS-Co(II)teta) confirming the metal
-
600–50 cm ) regions (Table 2). The IR spectrum of pure
1
(
oxygen coordination [25, 26].
TGA-DTA studies
chloromethylated polystyrene has an absorption band at
-
,261 cm due to the C–Cl group, which was absent in the
1
1
ligand and the catalysts. IR spectra show a stretching
-
vibration for CH at 2,918 cm for the polymer-bound
1
Thermal stability of complexes was investigated using
TGA-DTA at a heating rate of 10 °C/min in air over a
temperature range of 30–500 °C. TGA curves of the
polymer-supported metal complexes are shown in Fig. 3.
The polymer-anchored ligand decomposed in the tem-
perature range 190–200 °C (data not shown). The poly-
mer-supported Cu(II) Schiff base complex decomposed at
220 °C, but the polymer-supported Co(II) and Ni(II)
complexes decomposed at 330 and 240 °C. So from the
thermal stability, the polymer-supported metal complexes
degraded at significantly high temperature and the poly-
mer-supported Co(II) complex was more stable than other
two catalysts.
2
ligand and its complexes. The stretching vibrations of C=N
-
1
bonds appeared at 1,639 cm for the Schiff base and
polymer-bound Schiff base ligand which is lowered to
-
,588–1,625 cm in the metal complexes, indicating the
1
1
coordination of azomethine nitrogen to the metal [23]. A
-
1
peak at 3,433 cm for secondary amine (NH) group for
the Schiff base compound is absent for the polymer-bound
Schiff base ligand and metal complexes proving the
attachment of polystyrene and homogeneous Schiff base
through the N-atom of secondary amine group of triethyl-
-
1
enetetramine moiety. Another band at 3,325 cm
is
noticed for O–H stretching, which is not observed for the
metal complexes as the OH group coordinates with metals
through oxygen. The peaks of M–O for the metal com-
Electronic spectral studies
-
1
plexes are observed around 630–680 cm [24]. These are
different since they are influenced by the metal. In the far-
IR data, the bands are due to the M–N stretching vibration
The electronic spectra (Fig. 4) of the polymer-supported
metal complexes were recorded in diffuse reflectance spec-
trum mode as MgCO /BaSO disks due to their solubility
-
1
around 520–580 cm [24], which established the forma-
tion of the metal complexes through M–N bonding.
3
4
limitations in common organic solvents. In the UV spectrum
of the ligand, two absorption bands are observed at 265
-
Another peak at 1,270 cm due to the C–O bond changed
1
1
23

Transition Met Chem (2012) 37:97–107
101
Fig. 2 EDAX data of polymer-
anchored Schiff base ligand
(a) and polymer-anchored
complexes Cat-1, Cat-2, and
Cat-3 (b, c, and d)
123

1
02
Transition Met Chem (2012) 37:97–107
Table 2 IR stretching frequencies of polymer-anchored ligand and complexes
-
1
a
Compound
Color
IR(cm , KBr)
NH (secondary)
M–O
C=N
M–N
O–H
C–O
Schiff base—teta(A)
PS-teta(B)
Yellow
3,433
–
–
1,639
1,639
1,588
1,625
1,606
–
–
3,325
1,270
1,270
1,298
1,320
1,315
Yellow
–
–
–
–
3,325
PS-Cu(II)teta
PS-Ni(II)teta
PS-Co(II)teta
a
Green
652
630
684
548
513
573
–
–
–
Greenish yellow
Pinkish brown
Infrared spectra measured as KBr pellets
c
a
b
PS-Cu-amp-Cl
PS-Co-amp-Cl
PS-Ni-amp-Cl
d
2
50
300
350
400
450
500
550
600
1
00
200
300
400
500
Wavelength (nm)
0
Temperature ( C)
Fig. 4 DRS-UV–visible absorption spectra of polymer-anchored
Schiff base ligand (a) and polymer-anchored complexes Cat-1, Cat-
Fig. 3 Thermal studies of polymer-anchored complexes Cat-1, Cat-2,
and Cat-3
2
, and Cat-3 (b, c, d)
and 348 nm, assigned to the transition n–p* and p–p*,
respectively. The electronic spectrum of the Cat-1 exhibits a
band at 425 nm (sh.) that is assigned to d–d transition [27].
The band at 275 nm is presumably caused by charge transfer.
Cat-3 shows a band at 442 nm (sh.), which indicates d–
d transition [28]. The band at 272 nm is assigned to charge
transfer transition. Cat-2 exhibits three bands at 245, 268, and
parameters, namely amount of oxidant (moles of H O per
2 2
mole of styrene), amount of catalyst, and temperature were
checked to get optimized reaction conditions for the
maximum oxidation of styrene.
To determine the best temperature, the oxidation of
styrene has been investigated with different temperatures
with PS-Cu(II)teta as catalyst. The oxidation reaction was
carried out with various reaction temperatures from 40 to
80 °C using 1:2 mmol ratio of styrene to H O with
4
21 nm (sh.). The first two are due to the charge transfer, and
the last one may be assigned to d–d transition [29].
2
2
acetonitrile as solvent for duration of 8 h, and the results
are shown in Fig. 5. The optimum temperature was found
to be 60 °C. Lower temperatures do not favor the for-
mation of products because the catalyst is less active. At a
higher temperature, the yield and selectivity of product
decreases.
Catalytic activity
Oxidation of styrene
Oxidation of styrene, catalyzed by PS-Cu(II)teta, PS-
Ni(II)teta, and PS-Co(II)teta using H O as an oxidant,
Similarly, for three different amounts viz. 0.05, 0.1, and
0.15 mmol of catalyst, under the above reaction conditions,
using 0.05 mmol catalyst conversion was only 65%, while
using 0.1 and 0.15 mmol catalyst, conversions were max-
imum at 94 and 93%, respectively. However, at the
2
2
gave styrene oxide and benzaldehyde as chief products, as
shown in Scheme 2. Preliminary experiments showed the
good activity of PS-Cu(II)teta among these catalysts, it
was, therefore, taken as a representative, and the different
1
23

Transition Met Chem (2012) 37:97–107
103
Scheme 2 Various oxidation
reaction with PS-M(II)teta
complex
O
CHO
PS-M(II)teta copmlex
+
0
H O , ACN , 60 C, 8hr
2
2
O
OH
PS-M(II)teta copmlex
+
+
O
0
H O , ACN, 60 C, 8hr
2
2
O
CHO
PS-M(II)teta copmlex
+
0
H O , ACN, 60 C, 8hr
2
2
HO
CHO
COOH
PS-M(II)teta copmlex
+
0
H O , ACN , 6 0 C, 8hr
2
2
1
00
Table 3 Styrene epoxidation using different solvents with Cat-1
0
7
6
0 C
0
a
Entry Solvent Conversion (%) Selectivity (%)
a
0 C
0
9
0
0
0
0
0
50 C
0
Styrene oxide Benzaldehyde
4
0 C
8
7
6
5
1
2
3
4
Toluene
MeOH
0
20
60
94
0
18
65
90
0
82
35
10
2
H O
ACN
Reaction conditions: catalyst (0.1 mmol), styrene (5 mmol), solvent
10 mL), H
(10 mmol), time (8 h), 60 °C
Determined by GC
(
2 2
O
a
2
4
6
8
10
12
14
epoxidation selectivity in methanol, and the main product
is benzaldehyde. For the present reaction, the catalysts
exhibit the highest catalytic activity and selectivity for
styrene oxide in acetonitrile media, because it is a polar
solvent, has high dielectric constant, and dissolves a wide
range of chemical compounds.
Time
Fig. 5 Effect of temperature on the oxidation of styrene using Cat-1.
Reaction condition: styrene (5 mmol), ACN (10 mL),
10 mmol), catalyst (0.1 mmol)
2 2
H O
(
expense of H O , 0.1 mmol catalyst can be considered
2
In this study, the effects of different oxidants like TBHP,
H O , PhIO, and molecular oxygen over this catalyst were
2
sufficient to carry out the reaction.
2
2
The oxidation of styrene was examined using different
solvents. The selected solvent should possess certain cri-
teria such as it should be stable and it should dissolve the
substrate and oxidant. Table 3 shows the influence of sol-
vent on the conversion of styrene and selectivity of styrene
oxide using the polymer-supported copper catalyst. The
selectivity of styrene oxide is in the order: acetoni-
examined in the oxidation reaction of styrene. The results
of this study are given in Table 4. The reaction shows
almost no conversion when molecular oxygen was used as
oxidant. Oxidants like molecular O and PhIO are clearly
2
less efficient than TBHP and H O as is evident from the
2
2
percentage of conversion of styrene. TBHP and H O
2
2
proved to be the best among the three oxidants used in
these catalytic oxidation reactions, but in the present study
we chose H O for environmentally benign reaction
trile [ H O [ MeOH [ MePh. The results indicate that
2
the epoxidation reaction does not proceed in toluene sol-
2
2
vent. The reaction, however, shows low activity and
condition.
123

1
04
Transition Met Chem (2012) 37:97–107
Table 4 Oxidation of styrene using different oxidants with Cat-1
temperature (60 °C). PS-Co(II)teta and PS-Ni(II)teta, were
also tested under the above optimized reaction conditions,
and the results are presented in Tables 5 and 6. It is clear
from the data that PS-Cu(II)teta exhibits better catalytic
activity with 94% conversion, followed by PS-Co(II)teta
with 88%. Catalyst PS-Ni(II)teta has shown only 80%
conversion after 8 h of reaction time.
a
Entry Oxidant
Conversion Selectivity (%)
a
(
%)
Styrene oxide Benzaldehyde
1
2
3
4
5
None
PhIO
0
0
75
70
89
90
0
25
30
11
10
15
2
O (1 atm) 32
TBHP
92
94
Oxidation of other substrates with the Cat-1
2 2
H O
Reaction conditions: catalyst (0.1 mmol), styrene (5 mmol), ACN
(
In order to explore further the catalytic activity and
selectivity of the cat-1, oxidation of other olefins, alkanes,
alcohols, and thioethers was investigated under the opti-
mized reaction conditions with some excellent results
10 mL), oxidant (10 mmol), time (8 h), 60 °C
a
Determined by GC
(Table 5). Cyclic olefins such as cyclohexene and cyclo-
octene are oxidized with high yields. The results showed
that the conversion is 54% with 94% epoxide selectivity in
the case of cyclooctene, while the oxidation of cyclohexene
is with 85% conversion and 70% epoxide selectivity with
allylic oxidation products cyclohexene-1-one and cyclo-
hexene-1-ol in small extent.
1
1
1
:3
:2
:1
1
00
8
6
4
2
0
0
0
0
0
It is clear from Table 5 that the selectivity of the dif-
ferent oxidation products formed varies in the order:
epoxide [ aldehyde for the olefins, while ethylbenzene
gave acetophenone with 65% of selectivity and toluene
formed benzaldehyde in 100% selectivity. Benzyl alcohols
gave the corresponding aldehyde with maximum selectiv-
ity also. The thioethers gave the corresponding sulfoxides
in 100% selectivity. Moreover, the conversion % and the
turnover frequency (TOF) are high in most of the reactions.
0
2
4
6
8
10
Time (in hr)
Fig. 6 Effect of substrates to oxidant molar ratio on the oxidation of
styrene as a function of time using. Cat-1. Reaction condition: ACN
Oxidation using Cat-2, Cat-3
(
10 mL), catalyst (0.1 mmol), temperature = 60 °C, and time = 8 h
In search of the activity of other two catalysts (Cat-2 and
3), the oxidation reaction of styrene, cyclohexene, benzyl
alcohol, and ethyl benzene was done under the same
optimized reaction conditions as for cat-1 (Table 6). Cat-2
and 3 are less active than Cat-1. Cat-2 gave styrene oxide
with 84% selectivity in 80% total conversion, while Cat-3
gave epoxide with 92% selectivity in 88% conversion.
For the cyclohexene oxidation, Cat-3 proved superior than
Cat-2 with 80% conversion and 69% selectivity for
epoxide. For the benzyl alcohol and ethylbenzene oxida-
tion, Cat-3 gave better results over Cat-2 with 98 and
62% conversion, respectively, with high selectivity for
benzaldehyde and acetophenone in that order. It is to be
noticed that the order of selectivities of the products using
all Catalysts Cat-1, 2, and 3 are in same array for the
oxidation of styrene, cyclohexene, benzyl alcohol, and
ethylbenzene, etc. with H O . Other catalysts from the
For three different styrene to aqueous 30% H O molar
2
2
ratios, viz. 1:1, 1:2, and 1:3, the amount of styrene
10 mmol) and catalyst (0.1 mmol) was taken in aceto-
(
nitrile (10 mL), and the reaction was carried out at 60 °C.
The formation of products was regularly analyzed at time
intervals. As illustrated in Fig. 6, the conversion of sty-
rene improved from 52 to 94% upon increasing the sty-
rene to aqueous 30% H O molar ratio from 1:1 to 1:2,
2
2
though selectivity in 1:1 condition is high. The oxidation
remained nearly constant upon further increasing this ratio
to 1:3, which suggested that a large amount of oxidant is
not an essential condition to improve the oxidation of
styrene.
Thus, for the maximum oxidation of 10 mmol of sty-
rene, other optimized parameters were catalyst (0.1 mmol),
H O (20 mmol), acetonitrile (10 mL), and reaction
2
2
literature are also compared with our catalysts in Table 6
[15, 30–33].
2
2
1
23

Transition Met Chem (2012) 37:97–107
105
Table 5 Oxidation of different substrates using Cat-1
a
-1
a
Entry
Substrate
Olefins
Conversion (%)
TOF (h
)
Selectivity of the product (%)
Epoxide
Aldehyde/ketone
Alcohol
1
2
3
5
6
Styrene
94
85
54
90
92
83.93
75.89
48.21
80.35
82.16
90
70
94
63
65
10
22
–
Cyclohexene
Cyclooctene
a-Pinene
8
6
12
12
15
13
Limonene
Alkanes
Aldehyde
Ketone
7
8
Ethylbenzene
Toluene
90
35
80.35
31.25
35
65
–
100
Alcohols
Aldehyde
Ketone
Carboxylic acid
9
Benzyl alcohol
100
95
89.30
84.83
80.35
86.62
88
86
80
–
–
12
14
20
–
1
1
1
0
1
2
4-Methyl benzyl alcohol
4-Nitro benzyl alcohol
1-Phenyl ethanol
–
90
–
97
100
Thioethers
Sulfoxide
1
1
a
3
4
Phenyl sulfide
90
87
80.35
77.69
100
100
Methyl phenyl sulfide
Reaction conditions: catalyst (0.1 mmol), substrate (5 mmol), ACN (10 mL, oxidant (10 mmol), 8 h, 60 °C
Determined by GC
b
Heterogeneity test
before reuse in the subsequent run. After the use of catalyst
for five consecutive times, the styrene conversion was 85%.
The nature of the recovered catalyst has been followed by
IR and UV–vis spectra. The results indicated that the cat-
alyst after reusing several times showed no change in its IR
and UV–vis spectra. These observations show that PS-
M(II)teta complexes remain intact after the first oxidation
cycle. The results are presented in Table 7.
To determine whether the catalyst is actually functioning in
a heterogeneous manner, a hot-filtration test was performed
in the oxidation reaction. During the catalytic oxidation of
styrene, the solid catalyst was separated from the reaction
mixture by filtration after 4 h of reaction and the obtained
filtrate was continually stirred under the same reaction
conditions for a further 3 h. The gas chromatographic
analysis showed no increment in the conversion. Atomic
absorption spectrometric analysis of the filtrate confirmed
that no metals were present in the reaction mixture. Metal
content of the recycled catalysts also remained unaltered
indicating no leaching of the metal from the polymer
support. These results confirm that no leaching of the metal
from the catalyst takes place during catalytic reaction.
Conclusion
Polymer-anchored Schiff base complexes of copper(II),
nickel(II), and cobalt(II) have been synthesized and char-
acterized by different spectrophotometric methods. The
catalytic activities of these complexes were tested for the
liquid-phase oxidation of olefins using hydrogen peroxide
as the oxidant. Excellent yield and selectivity of these
catalysts toward the oxidation of olefins were observed.
The complexes are also effective for the oxidation of other
organic substrates like aromatic alcohols, alkanes, and
thioethers. These polymer-anchored catalysts are easy to
build, air-stable, inexpensive, and nonpolluting solids. The
Catalyst recycles
The stability of PS-M(II)teta complexes was studied in
repeated styrene oxidation reactions. The catalysts were
separated from the reaction mixture after each experiment
by filtration, washed with methanol, and dried carefully
123

1
06
Transition Met Chem (2012) 37:97–107
Table 6 A comparison of polymer-anchored complex catalysts Cat-1, 2, and 3
b
b
Substrates
Catalysts
Conversion (%)
Selectivity of the product (%)
Reference
O
CHO
Cat-1
94
90
84
92
10
This work
This work
This work
30
Cat-2
80
26
Cat-3
88
8
PS-[Cu(ligand)
n
]
57
3.6
2.9
63.7
65.7
PS-K[VO(O
2
)
2
(2-pybmz)]
73.2
15
O
OH
O
Cat-1
Cat-2
Cat-3
85
70
22
8
This work
This work
This work
30
76
68
69
20
12
80
21
10
PS-[Cu(ligand)
n
]
51
2.8
12.9
7.3
83.3
13.7
Cu(II)-Cyclam-SBA-16
77.8
1.3
31
O
CHO
Cat-1
Cat-2
Cat-3
90
65
35
This work
This work
This work
32
58
64
36
62
65
35
PS-[Cu(hmbmz)
2
]
60.8
15
66.6
4.5
33.4
70.5
PS-[Cu(ligand)
n
]
30
OH
CHO
COOH
Cat-1
Cat-2
Cat-3
PyC4
100
96
88
12
15
13
This work
This work
This work
33
85
98
87
93.6
90.5
a
Reaction conditions: catalyst (0.1 mmol), substrate (5 mmol), ACN (10 mL), oxidant (10 mmol), 8 h, 60 °C
Determined by GC
b
catalysts were found to exhibit higher catalytic activity
reaction conditions and can be recycled several times with
no appreciable loss of catalytic activities. Easy product
recovery and recycling efficiency along with high selec-
tivity of these catalysts may be useful for the synthesis of
different fine chemicals under eco-friendly conditions.
compared with the corresponding neat complexes. The
remarkable advantages with the use of the catalysts are the
ready accessibility of the catalysts, their reusability, and
storage. The catalysts can withstand more stringent
1
23

Transition Met Chem (2012) 37:97–107
107
9
. Singla M, Mathur P, Gupta M, Hundal MS (2008) Transit Metal
Chem 33:175
Table 7 Recycling efficiency of the catalysts for the oxidation of
styrene
1
1
0. Salavati-Niasari M, Hassani-Kabutarkhani M, Davar F (2006)
Catal Commun 7:955
1. Mureseanu M, Parvulescu V, Ene AR, Cioatera AN, Pasatoiu TD,
Andruh M (2009) J Mater Sci 44:6795
b
Recycle
Conversion (%)
Cat-1
Cat-2
Cat-3
1
1
2. Lei Z (2000) React Funct Polym 43:139
3. Jia A, Lou L, Zhang C, Zhang Y, Liu S (2009) J Mol Catal A
Chem 306:123
1
2
3
4
5
a
94
93
92
90
90
90
89
87
86
85
92
92
92
91
90
14. Thitsartarna W, Gularib E, Wongkasemjita S (2008) Appl
Organometal Chem 22:97
1
5. Maurya MR, Kumar M, Sikarwar S (2006) React Funct Polym
6:808
6
1
1
6. Gupta KC, Sutar AK, Lin CC (2009) Coord Chem Rev 253:1926
7. Islam SM, Tuhina K, Mubarak M, Mondal P (2009) J Mol Catal
A Chem 297:18
8. Islam SM, Mondal P, Singha Roy A, Mondal S, Hossain D (2010)
Tetrahedron Lett 51:2067
9. Islam SM, Mondal P, Mondal S, Mukherjee S, Singha Roy A,
Mubarak M, Paul M (2010) J Inorg Organomet Polym 20:87
0. Strukul G (ed) (1992) Catalytic oxidations with hydrogen per-
oxide as oxidant. Kluwer, Dordrecht
Reaction conditions: catalyst (0.1 mmol), substrate (5 mmol), ACN
10 mL), oxidant (10 mmol), 8 h, 60 °C
(
b
Determined by GC
1
1
2
2
2
2
2
Acknowledgments We thank the Indian Association for the Culti-
vation of Science, Kolkata, for providing the instrumental support. MI
acknowledges Department of Science and Technology (DST),
Council of Scientific and Industrial Research (CSIR), and University
Grant Commission (UGC), New Delhi, India, for funding. PM
acknowledges CSIR for Senior Research Fellowship.
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