Synthesis, characterization, and catalytic activity of a polymer-supported copper(II) complex with a thiosemicarbazone ligand

Article abstract of DOI:10.1007/s11243-011-9459-1

A thiosemicarbazone Cu(II) complex anchored to a polystyrene framework has been synthesized and characterized by analytical and spectroscopic techniques. The complex was found to be a highly active catalyst for the oxidation of various organic substrates including alkenes and alcohols using H ₂O₂ as oxidant. The reaction conditions were optimized with respect to temperature, solvent, oxidant, catalyst amount, and substrate to peroxide ratio. The heterogeneous catalyst was reused five times without significant loss of activity. A comparison between the catalytic activities of this polymer-supported Cu(II) complex and its homogeneous analogue was carried out.

Full text of DOI:10.1007/s11243-011-9459-1

Transition Met Chem (2011) 36:223–230
DOI 10.1007/s11243-011-9459-1
Synthesis, characterization, and catalytic activity
of a polymer-supported copper(II) complex
with a thiosemicarbazone ligand
•
Manirul Islam Dildar Hossain Paramita Mondal
•
•
•
Kazi Tuhina Anupam Singha Roy
•
•
Sanchita Mondal Manir Mobarak
Received: 3 November 2010 / Accepted: 31 December 2010 / Published online: 29 January 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A thiosemicarbazone Cu(II) complex anchored
to a polystyrene framework has been synthesized and
characterized by analytical and spectroscopic techniques.
The complex was found to be a highly active catalyst for
the oxidation of various organic substrates including
alkenes and alcohols using H2O2 as oxidant. The reaction
conditions were optimized with respect to temperature,
solvent, oxidant, catalyst amount, and substrate to peroxide
ratio. The heterogeneous catalyst was reused five times
without significant loss of activity. A comparison between
the catalytic activities of this polymer-supported Cu(II)
complex and its homogeneous analogue was carried out.
perfume, pharmaceutical, dyestuff, and agrochemical
industries [4, 5].
In homogeneous catalysis, excellent catalytic efficien-
cies are obtained for the oxidation of alkenes [6–8] under
liquid-phase conditions where the catalyst, substrate, oxi-
dant, and product(s) are in a single phase with the solvent.
Despite the remarkable utility of homogeneous catalysts in
organic synthesis, they suffer from a significant drawback
in that they often remain as contaminants in the organic
products at the end of the reaction. Another problem with
homogeneous catalysts is related to catalyst recovery and/
or reuse, affecting the overall economics of the process [9,
10]. In the recent past, there has been an increasing interest
in the development of greener catalytic processes, which
are also economically viable. Immobilization of the cata-
lyst on an insoluble support is a useful approach because
this enhances the thermal stability, selectivity, recyclabil-
ity, and easy separation of the expensive catalyst from the
reaction products [11, 12]. Many effective and recyclable
heterogeneous catalysts have been studied for liquid-phase
oxidations [13, 14]. Various approaches have been focused
on the incorporation of metal-based catalysts onto or into
inert supports, such as, alumina [15], amorphous silicates
[16], polymers [17], zeolites [18], and MCM-41 [19].
Application of polymer-supported complexes in catalytic
reactions has received much attention in recent years [20–
23]. Chloromethylated polystyrene cross-linked with divi-
nylbenzene is one of the most widely employed macro-
molecular supports for immobilization of homogeneous
catalysts [24]. These polymer-supported complexes have
received considerable attention due to their potential
advantages over homogeneous analogues.
Introduction
The oxidation of alkenes to oxygen-containing value-added
products like alcohols, aldehydes, ketones, acids, epoxides,
etc. is an extremely important reaction in both chemical
and pharmaceutical industries [1–3]. Similarly, the oxida-
tion of primary and secondary alcohols to carbonyl
compounds is one of the simplest and most useful trans-
formations in organic chemistry. The oxidation of benzyl
alcohol to benzaldehyde is an important organic transfor-
mation, since benzaldehyde has applications in the
M. Islam (&) Á D. Hossain Á P. Mondal Á
A. S. Roy Á S. Mondal Á M. Mobarak
Department of Chemistry, University of Kalyani, Kalyani,
Nadia, WB 741235, India
e-mail: manir65@rediffmail.com
In the present study, our initial efforts have focused on
developing a ‘green’ catalytic oxidation system that
employs hydrogen peroxide, an environmentally friendly
K. Tuhina
Department of Chemistry, B.S. College, S-24 P.G.S.,
WB 743329, India
123

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Transition Met Chem (2011) 36:223–230
Synthesis of the catalyst
oxidant whose only oxidation by-product is water [25].
Hydrogen peroxide is a very attractive oxidant due to ready
availability, high oxygen content, and ease of use. Here, we
have developed a polymer-supported [PS-Cu-TSC] cata-
lyst, and catalytic performances are investigated in the
liquid-phase allylic oxidation of various alkenes and alco-
hols using 30% H2O2 as oxidant in acetonitrile medium.
The effects of different experimental parameters were
investigated in order to optimize the reaction conditions.
We have also compared the catalytic activity of the poly-
mer-supported catalyst with its homogeneous analogues.
The Schiff base complex (1) was prepared by condensing
a solution of 5-nitrosalicyl-aldehyde (10 mmol, 1.671 g) in
ethanol (25 mL) with an aqueous ethanolic solution of
thiosemicarbazide (10 mmol, 0.911 g). The reaction mix-
ture was refluxed for 5 h in a water bath. After completion
of the reaction, the resulting mixture was poured onto
crushed ice whereby a precipitate was obtained. The pre-
cipitate of (1) was filtered off, washed several times with
water, recrystallized from ethanol, and finally dried. The
Schiff base complex (1) (1.05 g) was dissolved in DMF
(15 mL) and reduced using Pd/C at room temperature and
one atmosphere pressure of H2 for 24 h. The obtained red
solution (2) was filtered. To the filtrate, chloromethylated
polystyrene (5 mmol, 0.90 g) was added, and the reaction
mixture was stirred and refluxed for 12 h. After completion
of the reaction, the polymer-supported ligand (3) was
separated by filtration and dried in a vacuum oven. Next,
the polymer-supported ligand (3) (1 g) was suspended in
methanol (20 mL). A solution of CuCl2 (0.6 mmol, 0.1 g)
in methanol (5 mL) was added with constant stirring; then,
the suspension was refluxed for 24 h. After cooling to room
temperature, the yellow polymer-supported [PS-Cu-TSC]
catalyst was filtered out, washed thoroughly with methanol,
and dried under vacuum. The outline for the preparation of
the polymer-supported Cu(II) Schiff base complex is given
in Scheme 1.
Experimental
Analytical-grade reagents and freshly distilled solvents
were used throughout. Liquid substrates were predistilled
and dried using the appropriate molecular sieve. Distilla-
tion and purification of the solvents and substrates were
done by standard procedures [26]. Chloromethylated
poly(styrene-divinyl benzene) and aromatic alcohols were
supplied by Sigma–Aldrich. Other reagents were procured
from Merck and used without further purification.
Morphologies of the polymer-anchored ligand and [PS-
Cu-TSC] catalyst were analyzed using a Jeol JSM 6700
scanning electron microscope. The thermal stability of the
polymer-supported ligand and [PS-Cu-TSC] catalyst was
determined using a Mettler Toledo TGA/SDTA 851
instrument. FTIR spectra were recorded on a Perkin-Elmer
FTIR 783 spectrophotometer using KBr pellets. Diffuse
reflectance UV–Vis spectra were taken using a Shimadzu
UV-2401PC double-beam spectrophotometer having an
Results and discussion
1
integrating sphere attachment for solid samples. H NMR
Characterization of the polymer-supported complex
spectra were recorded at 400 MHz using a Bruker DPX-
400 spectrometer. Copper content in the catalyst was
determined using a Varian AA240 atomic absorption
spectrophotometer (AAS).
Due to the insolubility of the polymer-supported [PS-Cu-
TSC] complex in all common organic solvents, character-
ization was limited to its physicochemical properties, SEM,
TGA, IR, and UV–Vis spectral data. The structures of
1
General procedure for catalytic oxidation reactions
ligands were established by H NMR spectroscopy.
1H NMR (DMSO-d6) (Ligand 1): d 11.55 (s, 1H), 9.85
(s, 1H), 8.37 (s, 1H), 8.07 (s, 2H), 7.32 (s, 1H), 6.84 (d,
1H), 6.66 (d, 1H).
1H NMR (DMSO-d6) (Ligand 2): 11.26 (s, 1H), 9.85 (s,
1H), 8.36 (S, 1H), 8.08 (s, 2H), 7.30 (s, 1H), 8.10 (s, 2H),
6.80 (d, 1H), 6.65 (d, 1H).
The oxidation reactions were carried out in a round-bottom
flask fitted with a water condenser and placed in an oil bath
with vigorous stirring. Substrate (5 mmol) was taken in
acetonitrile (10 mL) for different sets of reactions together
with 0.05 g catalyst in which 10 mmol of H2O2 (30% in
aq.) was added. When the reaction was carried out under
O2 atmosphere, O2 gas was purged into the flask continu-
ously. Aliquots of the reaction mixture were withdrawn at
regular intervals and analyzed with a Varian 3400 gas
chromatograph equipped with a 30 m CP-SIL8CB capil-
lary column and a Flame Ionization Detector. Chloroben-
zene was used as an internal standard. All reaction products
were identified by using an Agilent GC–MS.
Chemical analysis suggests 1.69 wt% Cu in the immobi-
lized Cu(II) catalyst.
Field emission scanning electron micrographs for single
beads of the polymer-supported thiosemicarbazone ligand
and its copper complex were recorded to understand the
morphological changes occurring on the polystyrene beads
at various stages of the synthesis. Figure 1 shows the SEM
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Transition Met Chem (2011) 36:223–230
225
Scheme 1 Synthesis of the
polymer-supported Cu(II) Schiff
base complex
S
C
O2N
CHO
OH
O2N
C
S
CH
OH
N
NH
NH2
H2N NH
NH2
DMF
Pd / C
RT
EtOH / Reflux
(1)
C
S
H2N
CH
OH
N
NH
NH2
(2)
NH
N
CH2Cl
P
H2N
C
S
CH
OH
NH2
MeOH
CuCl2
,
Cu
DMF / Reflux
Cl
Cl
(5)
CH2NH
P
C
CH
N
NH
NH2
[Cu-TSC] catalyst
S
OH
MeOH
(3)
CuCl2
NH
CH2NH
P
C
S
CH
OH
N
NH2
Cu
Cl
Cl
(4)
[PS-Cu-TSC] catalyst
Fig. 2 EDX spectra of polymer-supported ligand (a) and polymer-
supported [PS-Cu-TSC] catalyst (b)
light roughening of the top layer of the beads. The image of
the metal complex shows further roughening of the surface
layer, which may be due to interaction of the metal atom
with the ligand to accommodate the fixed geometry of the
complex. The presence of copper along with sulfur and
chlorine was verified by energy dispersive spectroscopy
analysis of X-rays (EDX) (Fig. 2), which is consistent with
the formation of the metal complex with the polymer-
anchored ligand.
Fig. 1 SEM image of polymer-supported ligand (a) and polymer-
supported [PS-Cu-TSC] catalyst (b)
image of the polymer-supported thiosemicarbazone ligand
and the immobilized copper complex on functionalized
polymer. The pure chloromethylated polystyrene bead has
a smooth surface (not shown). Introduction of the ligand
into the polystyrene beads through covalent bonding causes
The coordination mode of the ligand was confirmed by
IR spectral bands in both the mid (4,000–400 cm-1) and
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Transition Met Chem (2011) 36:223–230
the far (600–50 cm-1) IR regions. For ligand 1, the –NH2
and NH groups gave bands at 3,420 and 3,216 cm-1
,
respectively. In addition, a strong band at 1,535 cm-1 was
assigned to m-NO2. The bands due to m(C = S) and
m(C = N) appeared at 1,030 and 1,570 cm-1. In ligand 2,
the reduction of the nitro group to the corresponding amine
is confirmed by the absence of a peak at 1,535 cm-1. On
complexation with copper, the m(C = S) and m(C = N)
bands were shifted toward lower frequency at 1,021 and
1,545 cm-1. This suggests that the ligand chelates through
its N (azomethine) and S atoms. In complex 5, [Cu-TSC],
bands at 412, 320 and 306 cm-1 were assigned to m(Cu–N)
[27], m(Cu–S) [28], and m(Cu-Cl) [29], respectively.
The FTIR spectra of the polymer-supported Schiff base
ligand and polymer-supported complex were also studied.
The intensity of the spectrum of the polymer-supported
complex was weak due to the low concentration of the
complex. The IR spectrum of the supported complex is,
however, very similar to that of the free complex. The IR
spectrum of the polymer-supported thiosemicarbazone
ligand was compared with the polymer-supported [PS-Cu-
TSC] catalyst in order to confirm the coordination of the
metal atom with the thiosemicarbazones ligand. The exis-
tence of a strong band at 1,028 cm-1 due to m(C = S) and
absence of any band in the region 2,500–2,600 cm-1 from
m(C-SH) suggested that the thiosemicarbazone remains in
its thione form. The polymer-anchored thiosemicarbazone
ligand exhibited a strong band at 1,565–1,578 cm-1, which
may be assigned to the m(C = N) stretch of the azomethine
linkage. In the copper complex, this band was shifted to
lower frequency by 32 cm-1 and this was attributed to the
coordination of azomethine nitrogen to the metal with
formation of an M–N band. The m(C = S) band was shifted
to lower frequency by 14 cm-1 for the complex, indicating
the bonding of thionic sulfur to the metal. The spectrum of
the ligand exhibited a broad band in the 3,245–3,360 cm-1
region, which may be due to m(O–H) and m(N–H). These
bands remain at almost the same positions in the [PS-Cu-
TSC] catalyst, suggesting that the terminal amino group
was not involved in coordination [30]. More information
on the coordination of the polymer-supported ligand to the
central metal comes from the far IR data. New bands at
409 cm-1 for m(Cu-Nazomethine), 324 cm-1 for m(Cu–S), and
304 cm-1 for m(Cu–Cl) were present in the spectrum of the
[PS-Cu-TSC] catalyst.
Fig. 3 Electronic spectra of polymer-supported [PS-Cu-TSC] catalyst
[PS-Cu-TSC]
Polymer -supported ligand
100
200
300
400
500
600
Temperature (°C)
Fig. 4 Thermogravimetric weight loss plots for polymer-supported
ligand (a) and polymer-supported [PS-Cu-TSC] catalyst (b)
data, a distorted tetrahedral geometry may be assigned to
the [PS-Cu-TSC] catalyst [31].
Thermogravimetric analysis of the polymer-bound
complex was performed in air at a heating rate of 10 °C
min-1. The TG-data are depicted in Fig. 4. The polymer-
supported ligand decomposed at 380 °C, while the poly-
mer-supported [PS-Cu-TSC] catalyst degraded at the
slightly higher temperature of 400 °C.
The electronic spectrum of the polymer-supported
complex was recorded in diffuse reflectance mode as a
BaSO4 disk, due to the solubility limitations. The UV–Vis
spectra (DRS) are shown in Fig. 3. The UV spectrum for
the catalyst exhibits three bands at ca. 420–440, 260–300,
and 330–360 nm. The first band is assumed to be due to the
2B1g ? 2A1g transition, and the other two are probably
due to charge transfer transitions. From the spectroscopic
Catalytic activity
We explored the catalytic activity of our polymer-anchored
complex toward oxidation reactions, starting with styrene
as a model substrate. The effects of solvent, oxidant,
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Transition Met Chem (2011) 36:223–230
227
temperature, time, catalyst amount, and substrate to oxi-
dant ratio on the oxidation reaction were first surveyed.
Benzaldehyde was identified as the major product of this
reaction, with a small amount of styrene oxide.
A control experiment showed that the reaction did not
occur in the absence of catalyst (Table 1, entry 11). In
order to optimize the catalyst amount, different amounts of
catalyst were used in the oxidation of styrene. The best
results were obtained with 0.05 g Cu catalyst (Table 1,
entry 1).
At first we investigated the effects of different solvents
(both polar and non-polar). The results are summarized in
Table 1. The optimum catalytic activity was observed in
acetonitrile (Table 1, entry 1). In methanol and dichloro-
methane, moderate conversion was obtained. No catalytic
activity was observed when a non-polar solvent, namely
toluene, was employed. The high catalytic activity in
acetonitrile may be due to the polarity as well as high
dielectric constant of the solvent and better solubility of the
substrate and oxidant in the solvent.
The activity of the polymer-anchored catalyst was also
evaluated using three different molar ratios of styrene to
H2O2. The conversion of styrene was 49, 81, and 81% for
styrene to H2O2 ratios of 1:1, 1:2, and 1:3, respectively.
Hence, 1:2 ratio of styrene to H2O2 is sufficient for the
good conversion of styrene.
Thus, the optimal parameters for the oxidation of sty-
rene were follows: cyclohexene (5 mmol), 30% H2O2
(10 mmol), acetonitrile (10 mL), catalyst (0.05 g), and a
reaction temperature of 60 °C. Under these optimum
reaction conditions, the oxidation reactions were carried
with different alkenes and alcohols. The [PS-Cu-TSC]
catalyst efficiently converts a range of alkenes to their
corresponding allylic products with H2O2 as an oxidant.
The results are summarized in Table 2. In the oxidation
of cyclohexene, allylic products 2-cyclohexene-1-one and
2-cyclohexene-1-ol were produced (Table 2, entry 5).
Styrene was converted to benzaldehyde in high yield
(Table 2, entry 1). Substituted styrenes produced selec-
tively the corresponding aldehydes (Table 2, entries 3, 4).
Acetophenone was detected in the oxidation of a-methyl
styrene as major product (Table 2, entry 2). Trans-stilbene
was also oxidized in high yields (Table 2, entry 7), giving
benzaldehyde as major product with small amount of
benzophenone. This catalytic system showed a good
activity in the case of limonene and a-pinene. Limonene
was oxidized to carvone and carvenol with other coprod-
ucts (Table 2, entry 8). In the oxidation of a-pinene, the
major products were verbenone and verbenol (Table 2,
entry 9).
The effect of oxidant on the reaction was also important
because the desired products were not obtained in any
noticeable amounts in the absence of oxidant (Table 1,
entry 6). The ability of different oxidants, such as H2O2,
TBHP, NaOCl, PhIO, and mol. O2, in the oxidation reac-
tion was studied (Table 1, entry 1–5). When NaOCl or O2
was used as oxygen source, low conversion of styrene was
obtained. TBHP and H2O2 were found to be the best
oxygen sources. Considering the green point of view, H2O2
was used as the oxidant for further studies.
The reaction temperature has a very strong influence in
these oxidation reactions. No product formation was
observed at room temperature. As the temperature was
increased from 30 to 60 °C, the conversion of styrene
increased. The reaction performed at 60 °C appeared pro-
vided the highest yield of product.
Table 1 Oxidation of styrene using different oxidants, solvents, and
catalyst amounts catalyzed by [PS-Cu-TSC] complex
Entry
Solvent
Oxidant
Catalyst
amount (g)
Conversion (%)a
Selective oxidation of aromatic alcohols to aldehydes
was investigated with the [PS-Cu-TSC] catalyst using the
same conditions, as for styrene. As shown in Table 3,
benzyl alcohol was oxidized selectively into benzaldehyde
(entry 1). The catalyst also showed good activity toward
the oxidation of other benzyl alcohols containing substit-
uents such as methyl, methoxy, chloro, and nitro groups
(Table 3, entries 2–5). Electron-donating or withdrawing
substituents on the benzyl alcohols did not affect the effi-
ciency of the oxidation. In all the above cases, trace
amount of acids was observed.
1
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3OH
H2O
H2O2
TBHP
NaOCl
Mol. O2
PhIO
–
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
–
81
80
37
22
41
0
2
3
4
5
6
7
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
45
34
19
0
8
9
DCM
10
11
12
13
Toluene
CH3CN
CH3CN
CH3CN
2
0.025
0.10
49
81
Comparison of homogeneous and heterogeneous
catalysts
Reaction conditions: styrene (5 mmol), oxidant (10 mmol), solvent
(10 mL), temperature (60 °C), and time (6 h)
We have also compared the catalytic activity of the present
polymer-anchored [PS-Cu-TSC] with its homogeneous
a
Determined by GC
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Transition Met Chem (2011) 36:223–230
Table 2 Oxidation of alkenes with [PS-Cu-TSC] catalyst
Entry
1
Substrate
Styrene
Time (h)
6
Conversiona (%)
81
Productb (selectivity, %)
TOF (h-1
)
Benzaldehyde (93)
508
Styrene oxide (7)
2
3
a-methylstyrene
8
6
74
78
Acetophenone (100)
4-chloro benzaldehyde (87)
4-Chlorostyrene epoxide (13)
4-nitro benzaldehyde (88)
4-nitro styrene oxide (12)
2-cyclohexene-1-one (78)
2-cyclohexene-1-ol (15)
Cyclohexene epoxide (7)
2-cyclooctene-1-one (74)
2-cyclooctene-1-ol (26)
Benzaldehyde (83)
348
489
4-chlorostyrene
4
5
4-nitrostyrene
Cyclohexene
6
6
80
85
501
533
6
7
Cyclooctene
8
8
69
74
324
348
Trans-stilbene
Benzophenone (12)
trans-Stilbene oxide (5)
Carvenone (75)
8
9
Limonene
8
8
54
57
254
268
Carvenol (18)
Limonene oxide (1,2) (7)
Verbenone (69)
a-pinene
Verbenol (22)
a-pinene oxide (9)
Reaction conditions: alkene (5 mmol), 30% H2O2 (10 mmol), CH3CN (10 mL), and temperature (60 °C)
a
Determined by GC
b
Products were identified by GC and GC–MS analysis
Table 3 Oxidation of aromatic alcohols with [PS-Cu-TSC] catalyst
Entry
1
Substrate
Time (h)
6
Conversiona (%)
Productb (selectivity, %)
TOF (h-1
)
Benzyl alcohol
94
Benzaldehyde (97)
589
Benzoic acid (3)
2
3
4
5
4-methyl benzyl alcohol
4-methoxy benzyl alcohol
4-chloro benzyl alcohol
4-nitro benzyl alcohol
8
8
85
87
78
89
4-methyl benzaldehyde (89)
4-methyl benzoic acid (11)
4-methoxy benzaldehyde (84)
4- methoxy benzoic acid (16)
4-chloro benzaldehyde (79)
4- chloro benzoic acid (21)
4-nitro benzaldehyde (90)
4-nitro benzoic acid (10)
399
409
293
418
10
8
Reaction conditions: alcohol (5 mmol), 30% H2O2 (10 mmol), CH3CN (10 mL), and temperature (60 °C)
a
Determined by GC
b
Products were identified by GC and GC–MS analysis
analogue [Cu-TSC]. The activity of the homogeneous
complex was examined toward oxidation of styrene and
benzyl alcohol using same reaction conditions as for the
heterogeneous catalyst. The results are summarized in
Table 4. The results showed that in the oxidation of styrene
and benzyl alcohol, the turnover frequencies (TOF) for the
homogeneous catalyst were higher than for the heteroge-
neous catalyst. Although styrene conversions in heteroge-
neous systems are comparable with the homogeneous
results, the heterogeneous systems showed higher selec-
tivity to benzaldehyde. Additionally, the supported catalyst
is expected to have several advantages over the
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Transition Met Chem (2011) 36:223–230
229
Table 4 Oxidation of styrene and benzylalcohol with H2O2 catalyzed by [Cu-TSC] and [PS-Cu-TSC]
Substrate
Product (s)
Heterogeneous [PS-Cu-TSC]
Homogeneous [Cu-TSC]
Time
6
Conversion (%)
81
Selectivity (%)
Time
6
Conversion (%)
83
Selectivity (%)
Styrene
Benzaldehyde
Styrene oxide
Benzaldehyde
Benzoic acid
93
7
85
15
81
19
Benzylalcohol
8
94
97
3
8
95
Table 5 Comparison of catalytic activity of styrene oxidation with polymer-anchored [PS-Cu-TSC] complex catalyst and other catalyst reported
in literature
Catalyst
Oxidant
Temp (°C)
Reaction time (h)
Conversion (%)a
Reference
[PS-Cu-TSC]
Vit-B12/MCM-41
PS-[Cu(Ligand)n]
FePcS/NH2-MCM-48
a
H2O2
TBHP
H2O2
TBHP
60
70
80
RT
8
8
81
Present
33
60
6
57
34
24
65.5
35
Conversion obtained from GC analysis
homogenous one. The polymer-based catalysts can with-
stand more drastic conditions. The greatest disadvantage of
homogeneous [Cu-TSC] is the catalyst degradation in the
presence of oxidant, whereas the heterogenized catalyst can
be reused several times without significant loss of activity.
Table 5 provides a comparison of the results obtained
for our present catalytic system with those reported in the
literature [32–34]. The present catalyst exhibited higher
conversions compared to the other reported systems.
oxidation of styrene. After the first run, the catalyst was
separated by filtration, washed thoroughly, dried under
vacuum, and then subjected to another run under the same
reaction conditions. The catalyst was reused up to five
times in this way, without significant loss of activity. Thus,
the yields of the five consecutive runs were 81, 81, 79, 79,
and 78%. The IR spectrum of the recycled catalyst was
quite similar to that of a fresh sample, indicating the het-
erogeneous nature of this complex. The analysis of the
recovered catalyst by atomic absorption spectroscopy
showed no reduction in the amount of copper. The results
indicate that this complex is stable under reuse.
Heterogeneity tests
A hot filtration test was performed in the oxidation of
styrene. During the catalytic reaction, the solid catalyst was
separated from the reaction mixture by filtration after 4 h
and the determined conversion was 52%. The reaction was
then continued out for a further 2 h. The gas chromato-
graphic analysis showed no increase in the conversion.
This confirms that the reaction did not proceed upon the
removal of the solid catalyst. Atomic absorption spectro-
metric analysis of the reaction mixtures collected by fil-
tration confirms that Cu is absent from the reaction
mixture. These results suggest that the Cu is not being
leached out from the catalyst during these catalytic
reactions.
Conclusion
In conclusion, we have demonstrated the effectiveness of a
polymer-supported copper catalyst for liquid-phase oxida-
tions with the environmentally friendly H2O2 as sole
oxidant. The [PS-Cu-TSC] catalyst exhibits good activity
in the oxidation of various alkenes and alcohols to give
selectively the allylic oxidation products. The main
advantages of this catalyst are that the synthesis is very
simple, the catalytic reactions can be conveniently carried
out in air, and the separation of this heterogeneous catalyst
can be achieved easily by filtration. The catalyst can be
recycled five times without much loss in its activity.
Recycling of the catalyst
Acknowledgments We acknowledge Department of Science and
Technology (DST), Council of Scientific and Industrial Research
(CSIR), and University Grant Commission (UGC), New Delhi, India,
for funding. We also thank the DST and UGC, New Delhi, India, for
providing instrumental support under FIST and SAP program.
One of the main reasons of supporting a complex on the
polymer is to enhance the life of the resulting catalyst. The
reusability of the present catalyst was checked in sequential
123

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Transition Met Chem (2011) 36:223–230
17. Lei Z (2000) React Func Polym 43:139–143
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