Liquid-phase catalytic oxidation of organic substrates by a recyclable polymer-supported copper(II) complex

Article abstract of DOI:10.1007/s11243-013-9740-6

Chloromethylated polystyrene beads cross-linked with 6.5 % divinylbenzene were functionalized with 2-(2′-pyridyl) benzimidazole (PBIMH) and on subsequent treatment with Cu(OAc)₂ in methanol gave a polymer-supported diacetatobis(2-pyridylbenzimidazole)copper(II) complex [PS-(PBIM)₂Cu(II)], which was characterized by physicochemical techniques. The supported complex showed excellent catalytic activity toward the oxidation of industrially important organic compounds such as phenol, benzyl alcohol, cyclohexanol, styrene, and ethylbenzene. An effective catalytic protocol was developed by varying reaction parameters such as the catalyst and substrate concentrations, reaction time, temperature, and substrate-to-oxidant ratio to obtain maximum selectivity with high yields of products. Possible reaction mechanisms were worked out. The catalyst could be recycled five times without any metal leaching or much loss in activity. This catalyst is truly heterogeneous and allows for easy work up, as well as recyclability and excellent product yields under mild conditions.

Full text of DOI:10.1007/s11243-013-9740-6

Transition Met Chem (2013) 38:705–713
DOI 10.1007/s11243-013-9740-6
Liquid-phase catalytic oxidation of organic substrates
by a recyclable polymer-supported copper(II) complex
M. L. Shilpa V. Gayathri
•
Received: 5 April 2013 / Accepted: 28 May 2013 / Published online: 8 June 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Chloromethylated polystyrene beads cross-
linked with 6.5 % divinylbenzene were functionalized with
drawback of homogeneous catalysts is the difficulty of
separation of the catalyst from the reaction mixture.
Immobilization of metal complexes onto a solid support
has proved to be one of the most effective methods to
overcome this problem. The search for environmentally
benign synthesis has stimulated interest in developing
polymer-supported catalysts that combine high activity and
selectivity with easy workup and reusability.
0
2
-(2 -pyridyl) benzimidazole (PBIMH) and on subsequent
treatment with Cu(OAc) in methanol gave a polymer-
2
supported diacetatobis(2-pyridylbenzimidazole)copper(II)
complex [PS-(PBIM) Cu(II)], which was characterized by
2
physicochemical techniques. The supported complex
showed excellent catalytic activity toward the oxidation of
industrially important organic compounds such as phenol,
benzyl alcohol, cyclohexanol, styrene, and ethylbenzene.
An effective catalytic protocol was developed by varying
reaction parameters such as the catalyst and substrate
concentrations, reaction time, temperature, and substrate-
to-oxidant ratio to obtain maximum selectivity with high
yields of products. Possible reaction mechanisms were
worked out. The catalyst could be recycled five times
without any metal leaching or much loss in activity. This
catalyst is truly heterogeneous and allows for easy work up,
as well as recyclability and excellent product yields under
mild conditions.
Transition metal complexes based on nitrogen hetero-
cycles with benzimidazole rings have been studied exten-
sively [1–5]. Some of the Cu(II) complexes show good
catalytic activity, particularly toward the synthetically
important oxidation of substituted phenols [6]. The prod-
ucts of such reactions include high value-added chemicals
of use in the pharmaceutical, agriculture, and perfumery
industries [7].
The applicability of polymer-supported Cu(II) com-
plexes as catalysts in the degradation of phenol is of current
interest [8]. Styrene oxide has important applications as an
intermediate in many organic syntheses. Several oxidizing
reagents are used for such oxidation reactions. The use of
TBHP (tert-butyl hydroperoxide) as oxidant in such reac-
tions is convenient [9], and this reagent can selectively
oxidize olefins, alcohols, ethers, and other organic com-
pounds when transition metal catalysts are used [10–12].
Introduction
Application of transition metal complexes as homogeneous
catalysts has been the focus of much research. The major
0
Vanadium, molybdenum, and tungsten complexes of 2-(2 -
pyridyl)benzimidazole anchored onto a polymer support
[
13] were found to be active for the oxidation of both
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9740-6) contains supplementary
material, which is available to authorized users.
phenol and styrene. The conversion for phenol under
optimized conditions was, however, only 34 % for the
vanadium catalyst.
M. L. Shilpa ꢀ V. Gayathri (&)
As a part of our interest in polymer-supported com-
plexes [14, 15], our aim is to synthesize simple, selective,
inexpensive, and environmental-friendly polymer-sup-
ported catalysts and develop their catalytic activity under
Department of Studies in Chemistry, Bangalore University,
Central College Campus, Ambedkar Veedi,
Bangalore 560 001, Karnataka, India
e-mail: gayathritvr@yahoo.co.in
123

7
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Transition Met Chem (2013) 38:705–713
0
mild conditions. Herein, we have synthesized a polymer-
supported diacetatobis(2-pyridylbenzimidazole)copper(II)
2-(2 -pyridyl)benzimidazole (0.22 g, 1 mmol) in meth-
anol (10 mL) was added to a solution of copper(II)
acetate (0.10 g, 0.5 mmol) in methanol (10 mL), and the
mixture was refluxed for about 7 h during which green
precipitate separated out. This was filtered off, washed
with water and then with ethanol, and dried in vacuum
over P O (yield 0.14 g, 52 %).
complex [PS-(PBIM) Cu(II)] and investigated its catalytic
2
activity toward oxidation of cyclohexanol, phenol, benzyl
alcohol, ethylbenzene, and styrene using 70 % TBHP as
oxidant in acetonitrile medium. The reaction conditions
have been optimized for high activity and selectivity.
2
5
Functionalization of polymer support
Experimental
Beads of chloromethylated polystyrene (5.0 g) cross-linked
-
1
Materials and methods
with 6.5 % divinyl benzene (5.12 mmol Cl g ) were
washed with THF:water (4:1) and dried. They were then
allowed to swell in DMF (50 mL) for 2 h. A DMF solution
(100 mL) of PBIMH (6.5 g, 6.73 mmol) was added to the
suspension, followed by the addition a solution of trieth-
ylamine (9.0 g, 20 mL) in ethyl acetate (150 mL), and the
mixture was heated at 60 °C for 45 h. After cooling, the
functionalized beads were filtered off, washed with etha-
nol, Soxhlet extracted with ethanol to remove any unre-
acted PBIMH, dried at 100 °C for 24 h, and then finally
vacuum dried.
Chloromethylated polystyrene divinyl benzene copolymer
with 6.5 % cross-linking (PS-DVB) obtained as a gift from
THERMAX Ltd, Pune, India, was used as a support for
anchoring the metal complex. Analytical-grade copper
acetate and TBHP were purchased from Merck and used as
such. Laboratory-grade solvents were purified according to
the literature methods before use [16].
Elemental analyses were obtained with an Elementar
Vario micro cube CHNS analyzer. Surface area measure-
ments by the BET (Brunauer, Emmett and Teller) method
were carried out with a Micromeritics surface area analyzer
model ASAP 2,020. Copper content was determined using a
Perkin–Elmer atomic absorption spectrometer, after diges-
tion of the polymer-supported catalyst with concentrated
sulfuric acid and subsequent decomposition with H O .
Preparation of the polymer bound complex
A sample of the functionalized polymer with PBIM (2.0 g,
4.3 % N) was soaked in toluene/acetonitrile mixture
(100 mL, 1 : 1) for 1 h. A solution of copper(II) acetate
(0.16 g, 0.8 mmol) in methanol (5 mL) was added, and the
mixture was heated for 48 h at 60 °C, whereupon the beads
turned green. The mixture was filtered, and the beads washed
with hot ethanol were then Soxhlet extracted with ethanol to
remove excess copper(II) acetate. The sample was dried in an
oven for 24 h and vacuum dried over P O (Scheme 1).
2
2
2
-Methyl-1-phenyl-2-propyl hydroperoxide (MPPH) was
prepared according to a literature method [17]. Thermo-
gravimetric analysis of the polymer support and the
anchored complex were carried out using a TA Instrument,
SDT analyzer model Q 600, under nitrogen atmosphere with
-
1
heating rate of 10 °C min . Infrared spectra in the range
2
5
-
00–4,000 cm were recorded in KBr disks on a Shimadzu
1
4
8
,400 s FTIR spectrometer. Diffuse reflectance spectra were
Typical procedure for the catalytic oxidation reaction
recorded as BaSO disks on a Shimadzu UV–Vis–NIR
4
model UV-3101P spectrophotometer having an integrating
sphere attachment for the solid samples. Magnetic moments
of the supported and unsupported complexes were deter-
mined at room temperature using Guoy’s method. ESR
spectra were recorded using a Bruker EMX X-band ESR
spectrometer at liquid nitrogen temperature. ESI–MS was
recorded using a Thermo LCQ Deca XP Max instrument at
the spectroscopy/analytical test facility, IISc., Bangalore.
All the reaction products were analyzed using a Shimadzu
In a typical reaction, a mixture of catalyst (0.02 mmol with
respect to copper) and 70 % aqueous TBHP (5.0 mmol) in
acetonitrile (5 mL) was heated with continuous stirring at
60 °C. Substrate (5.0 mmol) was added, and the progress of
the reaction was monitored by a Shimadzu 14B gas chro-
matograph with FID detector using BP-5 capillary column.
When there was no further conversion of the reactants, the
catalyst was filtered off and the products were analyzed.
1
4B gas chromatograph fitted with flame ionization detector
connected to a BP-5 capillary column.
Results and discussion
Preparation of [Cu(OAc) (PBIMH) ]
2
Characterization of the materials
2
0
2
-(2 -Pyridyl)benzimidazole (PBIMH) was prepared
according to the literature method [18]. A solution of
Polymer beads were functionalized with PBIMH, followed
by the reaction with Cu(OAc) . Elemental analysis showed
2
1
23

Transition Met Chem (2013) 38:705–713
707
2
Scheme 1 Synthesis of PS-(PBIM) Cu(II) complex
Table 1 Physical properties and analytical data of polymer beads, functionalized beads, PS-(PBIM)
complex, and Cu(OAc)
2
Cu(II) complex, recycled supported
2 2
(PBIMH)
Parameters
Color
Analytical data
Apparent bulk
density (g cm
Surface
area
(
Pore
volume
-
3
)
C %
H %
N %
Cu %
2
m g
-1
3
(cm g
-1
)
)
PS-DVB
White
73.9
66.9
6.2
8.9
8.6
–
–
0.3
0.4
0.5
20.8
17.8
16.4
0.2
0.2
0.1
Functionalized beads with PBIM Yellow
4.3
3.8
–
Functionalized PBIM beads
anchored with Cu(OAc)
Recycled supported complex
Cu(OAc) (PBIMH)
Olive green 72.7
2.3
2
Olive green 71.4
8.1
3.9
2.3
0.5
–
16.2
–
0.1
–
2
2
*
Green
57.7 (58.0) 4.2 (4.2) 14.9 (14.5) 10.8 (10.4)
*
Calculated values are in parentheses
that the functionalized beads had 4.3 % nitrogen content,
which indicated that the PBIM had bonded on to the
polymer support. Metal estimation by AAS indicated 2.3 %
Cu in the immobilized catalyst (Scheme 1). The equivalent
complex Cu(OAc) (PBIMH) was also synthesized by
up to 330 °C, but on anchoring the metal complex, its
stability decreased to 210 °C.
The IR spectrum of the chloromethylated polystyrene
-
1
beads exhibited bands at 1,263 and 829 cm due to m
CH2ꢁCl
and tC–Cl (Fig. 1). On functionalization, these bands
decreased in intensity, indicating the bonding of PBIM onto
the polymer support. The free PBIMH exhibited a band
2
2
reacting copper acetate with PBIMH in methanol. It was
characterized by IR, electronic, ESR, ESI mass spectral
methods, and magnetic susceptibility measurements (sup-
plementary data). Surface area and pore volume were
determined by the BET method, and the values are tabu-
lated along with apparent bulk density in Table 1. Swelling
studies are an important method to study the mechanical
properties of the swollen beads, effects of cross-linking,
and their interaction with the solvents, which in turn
influences the rate of the catalytic reaction. The swelling of
the polymer-supported complex was in the order: acetoni-
trile [ methanol [ THF [ ethanol [ ethylacetate [ ben-
zene. Maximum swelling occurred in acetonitrile; hence, it
was chosen as the solvent for the oxidation reactions.
Thermogravimetric analysis of the polymer support and
the anchored complex was carried out in a nitrogen
-
1
around 3,400 cm due to tN–H, which was absent in the
functionalized polymer, as the bonding of the ligand to the
polymer beads is accomplished through the nitrogen.
-
1
The functionalized beads exhibited a band at 1,616 cm
-
1
due to tC=N, which was shifted to 1,609 cm in the anchored
beads, indicating that the nitrogen atoms of the pyridine and
benzimidazole moieties of the ligand are coordinated to
copper. The anchored beads displayed bands at 1,520 and
-
1
1,484 cm due to tC=O and tC–O, respectively, character-
istic of monodentate coordination of the acetate group to the
metal [19]. The IR spectra of the anchored complex before
and after recycling were similar. The unsupported complex
-
1
(E) exhibited a band due to tN–H at 3,253 cm , which was
absent in the polymer-supported metal complex, confirming
-
1
0
atmosphere with heating rate of 10 °C min up to 600 °C.
The results indicated that the polymer support was stable
that the 2-(2 -pyridyl)benzimidazole is bonded through
nitrogen to the polymer support.
123

7
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Transition Met Chem (2013) 38:705–713
Fig. 2 Effect of the amount of oxidant on the oxidation of styrene
Fig. 1 Infrared spectra of A PS-DVB, B functionalized polymer,
C polymer-supported Cu(II) complex, D recycled polymer-supported
complex and E Cu(OAc) (PBIMH)
2 2
Oxidation of styrene
The oxidation of styrene catalyzed by PS-(PBIM) Cu(II)
2
using TBHP as oxidant gave styrene oxide as the major
product and benzaldehyde as minor product. The activity
The diffuse reflectance spectra of the unsupported and
polymer-supported metal complexes were similar. PBIMH
exhibited an absorption band at 314 nm due to n ? p* and
p ? p* transitions. The electronic spectra of both sup-
ported and unsupported complexes showed bands around
of PS-(PBIM) Cu(II) in the oxidation of styrene was
2
evaluated using four different molar ratios of styrene :
TBHP, with fixed amounts of styrene (5 mmol) and
PS-(PBIM) Cu(II) (0.02 mmol) at 60 °C in acetonitrile.
2
2
2
4
14, 623 and 823 nm, which are assigned to B ? E ,
Using a 1:0.5 styrene:TBHP molar ratio, only 61 % con-
version of styrene (Fig. 2) occurred, with 47 % selectivity
to styrene oxide. At 1:1 styrene:TBHP ratio, nearly 95 %
conversion of styrene with 93 % of styrene oxide and 2 %
of benzaldehyde was obtained. Further increases in sty-
rene:TBHP ratio (1:2 and 1:3) gave only 2 % increase in
conversion, with decrease in selectivity for styrene oxide
(93–88 %) and increase of benzaldehyde (2–6 %).
1
g
g
2
2
2
2
B1g ? B and B ? A transitions [20] of Cu(II)
2g
1g
1g
9
with d configuration in a distorted octahedral geometry.
The magnetic moments of the polymer-supported and
unsupported complexes at room temperature were found to
be 1.72 and 2.14 BM, respectively. The ESR spectra of the
Cu(OAc) (PBIMH) and PS-(PBIM) Cu(II) complexes at
2
2
2
liquid nitrogen temperature exhibited one peak, with cal-
culated g values of 2.12 and 2.14, respectively. Hence, both
the magnetic moment and ESR spectral studies indi-
cated that the copper is paramagnetic with one unpaired
electron.
The concentration of catalyst was also varied from 0.01
to 0.05 mmol, keeping all other parameters constant. At
0.01 mmol of catalyst, 64 % conversion of styrene was
observed (Fig. 3), and at 0.02 mmol of catalyst, there was
92 % conversion of styrene. At higher catalyst concentra-
Catalytic activity
tions (0.03–0.05 mmol), the conversion of styrene remained
almost constant. Hence, 0.02 mmol of the catalyst was
chosen for the oxidation of styrene, which gave 88 %
conversion to styrene oxide and 3 % benzaldehyde.
The catalytic activity of the polymer-supported complex
was initially tested for the oxidation of cyclohexanol,
phenol, and styrene at 60 °C using both H O and TBHP as
2
2
The oxidation of styrene was also carried out by varying
the substrate concentration from 2.5 to 10 mmol. At
2.5 mmol, there was 97 % conversion of styrene with 95 %
selectivity to styrene oxide. With further increase in sty-
rene concentration up to 10 mmol, the conversion and the
selectivity to styrene oxide gradually decreased. At
10 mmol of styrene concentration, there was only 87 %
conversion of styrene. Hence, 5 mmol of styrene was
oxidants. The catalyst decomposed when H O was used.
2
2
Hence, TBHP was chosen for further catalytic studies. In
search of suitable reaction conditions to achieve maximum
conversion, variations of the catalyst and substrate con-
centration, reaction temperature, and substrate-to-TBHP
ratio were investigated. Use of the unsupported complex as
a catalyst under similar conditions gave low yields (30 %).
1
23

Transition Met Chem (2013) 38:705–713
709
Fig. 3 Effect of catalyst concentration on the oxidation of styrene
Fig. 4 Effect of catalyst concentration on the oxidation of
ethylbenzene
optimal for the effective oxidation of styrene. The effect of
temperature on the performance of the catalyst was studied
in the range from 30 to 70 °C. At 30 °C, low conversion
(
79 %) of styrene was observed. The optimum temperature
was found to be 45 °C, giving maximum conversion of
styrene (95 %) with 93 % selectivity to styrene oxide. At
higher temperatures, the selectivity decreased. The opti-
mized operating conditions for the oxidation of styrene
were therefore 0.02 mmol of catalyst plus 5 mmol of sty-
rene and TBHP at 45 °C in acetonitrile.
Oxidation of ethylbenzene
Oxidation of ethylbenzene is difficult to achieve under
normal conditions. Generally, oxidation of ethylbenzene
yields three products, namely acetophenone, benzaldehyde,
and styrene. PS-(PBIM) Cu(II) catalyzed the oxidation of
2
ethylbenzene to acetophenone, with 90 % selectivity under
mild conditions. The influence of TBHP concentration
on the oxidation of ethylbenzene was studied by using
ethylbenzene-to-TBHP ratios of 1:0.5, 1:1, 1:2, and 1:3
with 5 mmol of ethylbenzene plus 0.02 mmol of PS-
Fig. 5 Effect of substrate concentration on the oxidation of
ethylbenzene
ethylbenzene at 60 °C with 1:2 ethylbenzene:TBHP mole
ratio in acetonitrile.
(
PBIM) Cu(II) at 60 °C in 10 ml of acetonitrile. As the 1:2
2
molar ratio gave higher conversion of ethylbenzene, this
ratio was chosen for further reactions. Catalyst concentra-
tion was varied from 0.01 to 0.05 mmol, showing that
Oxidation of alcohols
PS-(PBIM) Cu(II) also proved to be capable of catalyzing
2
0
.04 mmol gave highest selectivity to acetophenone (78 %,
the oxidation of cyclohexanol, benzyl alcohol, and phenol.
Oxidation of phenol gave catechol as the major product
with hydroquinone as a minor product. The optimal con-
ditions for phenol oxidation (Table 2) were 0.03 mmol of
the catalyst with 5.0 mmol of phenol at 60 °C with 1:2
molar ratio of substrate:TBHP, resulting in complete
conversion with 95 % catechol and 3 % hydroquinone
(Entry 15).
Fig. 4). Substrate variation studies at 60 °C (Fig. 5) indi-
cated that 5 mmol of ethylbenzene was the suitable con-
centration with 90 % conversion to acetophenone plus 7 %
of benzaldehyde. The reaction was also carried out at 45,
60, and 70 °C; maximum conversion occurred at 60 °C.
Hence, the optimum conditions for ethylbenzene oxidation
were 0.04 mmol of PS-(PBIM) Cu(II) plus 5 mmol of
2
123

7
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Transition Met Chem (2013) 38:705–713
Oxidation of cyclohexanol gave cyclohexanone, with
bond, then the reaction with MPPH and the catalyst should
give 2-methyl-1-phenyl-2-propanol. On the other hand,
homolytic cleavage of the hydroperoxide leads to a radical
mechanism yielding acetone, benzyl alcohol, and benzal-
dehyde as some of the b-scission fragmentation products.
The reaction was carried out using MPPH (1.7 g) with
0.01 mmol of catalyst in acetonitrile (5 mL) for 5 h, and
the reaction products analyzed by GC revealed the pres-
ence of 2-methyl-1-phenyl-2-propanol. Under the same
reaction conditions, when styrene was used as the substrate,
the product analysis showed the presence of styrene oxide,
benzaldehyde, unreacted styrene, and 2-methyl-1-phenyl-
2-propanol. Thus, in this case, products arising from the
homolytic cleavage of MPPH were not detected [23],
indicating that the oxidation of styrene occurred by het-
erolytic cleavage of the O–O bond of hydroperoxide and
not by homolytic cleavage.
the best conditions identified as 0.02 mmol of the catalyst
plus 20 mmol of cyclohexanol at 60 °C with 1:1 sub-
strate:cyclohexanol molar ratio. Benzylalcohol (25 mmol)
on oxidation with PS-(PBIM) Cu(II) catalyst (0.02 mmol)
2
gave benzaldehyde with 98 % selectivity, using a 1:2
substrate to TBHP ratio at 60 °C. Further details of the
optimization experiments are given in the Supplementary
Data.
Reaction mechanism
Catalytic oxidation reactions using TBHP can proceed by
either homolytic or heterolytic decomposition of the alkyl
hydroperoxide [21]. The oxidation reaction of styrene was
carried out in the presence of 50 equivalents of 2,6-di-t-
butyl-4-methyl phenol (BHT) with 0.03 mmol of the
PS-(PBIM) Cu(II) catalyst plus 5.0 mmol of styrene in
2
Based on these results, tentative mechanisms can be
proposed for the oxidation of styrene (Scheme 2) and
ethylbenzene (Scheme 3). The reaction mechanism for the
phenol had been discussed earlier [24].
5
mL of acetonitrile in the presence of TBHP (Sub-
strate:TBHP = 1:1) at 60 °C. BHT acts as a scavenger for
free peroxy radicals. The results showed that conversion of
styrene (96 %) and formation of the reaction products were
not altered in presence of BHT, indicating that the reaction
does not involve free radicals.
Heterogeneity and recycling tests
The oxidation of styrene under the optimized reaction
conditions was also carried out using methyl-1-phenyl-
A heterogeneity test was performed by taking 0.02 mmol
of the catalyst plus 5.0 mmol of cyclohexanol with 1:1
substrate:TBHP ratio at 60 °C in acetonitrile solvent. The
reaction was carried out for 30 min, giving a conversion of
37 %. The catalyst was then filtered off from the reaction
mixture, and the filtrate was stirred for a further 1 h at
2
-propyl hydroperoxide (MPPH) as a mechanistic probe, to
check for homolytic versus heterolytic scission of the
peroxide O–O bond. According to Sorokin et al. [22], if the
reaction proceeded via heterolytic cleavage of the O–O
Table 2 Effect of catalyst and
phenol concentration,
phenol:TBHP ratio,
temperature, reaction time on
the oxidation of phenol
Entry Catalyst
conc.
Temp Phenol
(°C)
Substrate Reaction
: TBHP
ratio
%
Selectivity (%)
conc.
(mmol)
time (h)
Conversion
of phenol
Catechol Hydroquinone
(
mmol)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
0.01
0.02
0.03
0.04
0.05
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
60
60
60
60
60
30
45
70
60
60
60
60
60
60
60
60
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
2.5
7.5
10
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:0.5
1:2
1:3
5
5
65
84
96
93
92
68
95
98
99
53
50
57
66
75
100
100
62
80
95
87
84
65
89
94
98
39
42
47
53
87
95
96
35
16
1
5
5
7
5
8
5
32
5
5
5
2
5
1
0
1
2
3
4
5
6
5
47
46
43
34
5
5
10
10
24
5
10
5.0
5.0
5.0
5
3
5
3
1
23

Transition Met Chem (2013) 38:705–713
711
Scheme 2 Plausible reaction
mechanism for the oxidation of
styrene
Scheme 3 Plausible reaction
mechanism for the oxidation of
ethylbenzene
6
0 °C. The GC analysis indicated there was no further
Table 3 Recycling ability of the PS-(PBIM) Cu(II) catalyst for the
2
oxidation of styrene, ethylbenzene, and phenol
conversion of cyclohexanol. The filtrate was tested for
metal content by AAS. The absence of copper in the filtrate
confirmed that there was no leaching of the metal from the
polymer support during the catalytic reaction, and the
polymer-supported complex was truly heterogeneous.
The recyclability of the catalyst was tested for oxida-
tions of cyclohexanol, phenol, and styrene under the opti-
mized conditions. The results are shown in Table 3. After
the reaction, the catalyst was separated by simple filtration,
washed with ethanol and dried before use, and recycled five
times. The catalytic activity of the supported complex
toward oxidation of substrates remained almost constant.
Analytical and spectral studies of the recycled catalyst
indicated that the metal did not leach out from the support,
Conversion* (%) Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5
Styrene
92
87
84
90
87
84
90
87
83
89
85
82
89
85
82
Ethylbenzene
Phenol
* Reaction condition: 0.02 mmol catalyst, 5 mmol of the substrates at
0 °C with 1:1 substrate:TBHP ratio in acetonitrile
6
and the structure of the polymer-supported complex
remained unchanged, even after recycling it for five times.
We conclude that the catalyst is purely heterogeneous in
nature. Comparison of the catalytic activity of PS-
(PBIM) Cu(II) with other reported systems (Table 4)
2
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Transition Met Chem (2013) 38:705–713
2
Table 4 Comparison of the catalytic activity of PS-(PBIM) Cu(II) catalyst with other reported systems
Catalyst
Oxidant Temperature
°C)
Time
(h)
%
Selectivity (%)
Ref
(
Conversion
of Styrene
Styrene
oxide
Benzaldehyde
Styrene
PS-(PBIM)
2
Cu(II) catalyst
TBHP
TBHP
TBHP
45
50
60
4
7
8
95
53
79
93
52
79
2
48
21
This
work
Polymer-anchored Cu(II) Schiff base
complex
25
26
Polymer-anchored Schiff base copper
complex
a
PS-[Cu(hpbmz)
2
]
H
2
O
2
80
70
60
7
6
6
59
87
74
2
78
12
58
16
57
27
28
29
b
PS-[Cu(Hfsal-aepy)Cl]
Polymer-anchored Cu(II) azo complex
TBHP
TBHP
0
Catalyst
Oxidant Temperature ( C) Time (h) % Conversion of phenol Selectivity (%)
Catechol Hydroquinone
Ref
Phenol
PS-(PBIM)
2
Cu(II) catalyst
TBHP
TBHP
60
70
70
70
5
5
100
99
95
89
3
This work
c
PS-(QBIM)
2
Cu(II) catalyst
d
4
24
30
31
(
P-HPHZ-Cu)
e
H
2
O
O
2
2
24
24
43
89
89
–
–
(
P-HPPn-Cu)
H
2
53
Catalyst
Oxidant Temperature (°C) Time (h) % Conversion of Selectivity (%)
Ethylbenzene
Ref
Acetophenone Benzaldehyde
Ethylbenzene
PS-(PBIM)
2
Cu(II) catalyst TBHP
60
70
90
6
8
99
15
11
90
5
7
71
–
This work
f
]
PS-[Cu(tmbmz)
g
2
H
2
2
O
2
32
33
(
PS-PAR-Cu)
O
10
68
a
b
c
d
e
f
0
2
] = polymer-supported 2-(2 -hydroxyphenyl)benzimidazole copper complex
PS[Cu(hpbmz)
PS-[Cu(Hfsal-aepy)Cl] = polymer-supported[(3-formyl salicylic acid) with 2-(2-aminoethyl)pyridine] copper complex
PS-(QBIM)
2
Cu(II) = polymer-supported diacetatobis(2-quinolylbenzimidazole)copper(II) complex
0
(
P-HPHZ-Cu) = polymer-supported [N,N -bis(o-hydroxy acetophenone)hydrazine Schiff base]copper complex
0
(
P-HPPn-Cu) = polymer-supported [N,N -bis(o-hydroxy acetophenone) propylene diamine Schiff base]copper complex
PS-[Cu(tmbmz) ] = polymer-supported [2-thiomethylbenzimidazole]copper complex
PS-PAR-Cu) = polymer-supported 4-(2-pyridylazo)rosorcinol-copper complex
2
g
(
reveals that the present catalyst exhibits higher conversion
and selectivity compared to other systems [24–33].
Acknowledgments Authors wish to thank UGC, New Delhi, India,
for Major Research Project [F no. 39-741/2010(SR)]; Department of
Chemistry, Bangalore University, Bangalore, for instrumentation
facilities; Thermax Ltd., is gratefully acknowledged for providing PS-
DVB .
Conclusion
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