Novel polystyrene-anchored zinc complex: Efficient catalyst for phenol oxidation

Article abstract of DOI:10.1016/S1872-2067(14)60113-3

The novel recyclable free -ONNO- tetradentate Schiff base ligand N,N'-bis(2-hydroxy-3-methoxybenzaldehyde)4-methylbenzene-1,2-diamine (3-MOBdMBn) was synthesized. Complexation of this ligand with zinc (3-MOBdMBn-Zn) was performed, and the catalytic activity of the complex was evaluated. The polymer-supported analog of this complex (P-3-MOBdMBn-Zn) was synthesized, and its catalytic activity was studied. These free and polymer-anchored zinc complexes were prepared by the reactions of metal solutions with one molar equivalent of unsupported 3-MOBdMBn or P-3-MOBdMBn in methanol under nitrogen. The catalytic activity of 3-MOBdMBn-Zn and P-3-MOBdMBn-Zn was evaluated in phenol oxidation. The activity of P-3-MOBdMBn-Zn was significantly affected by the polymer support, and the rate of phenol conversion was around 50% for polystyrene-supported 3-MOBdMBn. The experimental results indicated that the reaction rate was affected by the polymer support, and the rate of phenol conversion was 1.64 μmol/(L·s) in the presence of polystyrene-supported 3-MOBdMBn.

Full text of DOI:10.1016/S1872-2067(14)60113-3

Chinese Journal of Catalysis 35 (2014) 1701–1708
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Novel polystyrene‐anchored zinc complex: Efficient catalyst for
phenol oxidation
Alekha Kumar Sutara,*, Yasobanta Dasa,b, Sasmita Pattnaika, Anita Routaraya, Nibedita Natha,
Prasanta Rathb, Tungabidya Maharanac
a Catalysis Research Lab, Department of Chemistry, Ravenshaw University, Cuttack‐3, Odisha, India
b School of Applied Sciences (Chemistry), KIIT University, Bhubaneswar, Odisha, India
c Department of Chemistry, National Institute of Technology, Raipur, India
A R T I C L E I N F O
A B S T R A C T
Article history:
The novel recyclable free –ONNO– tetradentate Schiff base ligand N,N′‐bis(2‐hydroxy‐3‐methox‐
ybenzaldehyde)4‐methylbenzene‐1,2‐diamine (3‐MOBdMBn) was synthesized. Complexation of
this ligand with zinc (3‐MOBdMBn‐Zn) was performed, and the catalytic activity of the complex was
evaluated. The polymer‐supported analog of this complex (P‐3‐MOBdMBn‐Zn) was synthesized, and
its catalytic activity was studied. These free and polymer‐anchored zinc complexes were prepared
by the reactions of metal solutions with one molar equivalent of unsupported 3‐MOBdMBn or
P‐3‐MOBdMBn in methanol under nitrogen. The catalytic activity of 3‐MOBdMBn‐Zn and
P‐3‐MOBdMBn‐Zn was evaluated in phenol oxidation. The activity of P‐3‐MOBdMBn‐Zn was signif‐
icantly affected by the polymer support, and the rate of phenol conversion was around 50% for
polystyrene‐supported 3‐MOBdMBn. The experimental results indicated that the reaction rate was
affected by the polymer support, and the rate of phenol conversion was 1.64 μmol/(L·s) in the
presence of polystyrene‐supported 3‐MOBdMBn.
Received 6 March 2014
Accepted 15 April 2014
Published 20 October 2014
Keywords:
Polystyrene
Schiff base
Polymer support
Catalysis
Zinc
Phenol oxidation
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
homogeneous and unsupported catalysts. Polymer‐supported
palladium complexes have been used in Heck reactions [9–11],
Schiff base complexes have various applications in organic
reactions such as oxidation [1,2], olefin epoxidation [3,4], and
polymerization of ethylenes to give narrow molecular‐mass
distributions [5,6]. However, homogeneous Schiff base catalytic
systems have two major disadvantages: (1) lack of product
control, which causes reactor fouling and (2) limited use in
solution processes. The binding of Schiff base metal catalysts to
polymer supports is a promising solution to these problems. In
general, the catalytic activity of heterogeneous Schiff base sys‐
tems (or supported systems) is lower than that of the homoge‐
neous analogs, but polymer‐supported transition‐metal com‐
plexes have shown high catalytic activity [7,8] compared with
generating the corresponding products in good to excellent
yields. Heterogeneous catalysts are particularly attractive for
phenol oxidation because of their practical advantages and
catalyst reusability. The Schiff base complexes of metal ions
have also been used in phenol oxidation [12,13].
Phenol oxidation is an industrially important reaction be‐
cause its products, i.e., catechol and hydroquinone, have a
range of applications as antioxidants, polymerization inhibi‐
tors, photographic chemicals, flavoring agents, and drug inter‐
mediates. Since 1970, phenol oxidation has been widely inves‐
tigated using various homogeneous and heterogeneous cata‐
lysts [13]. Phenol is an intermediate in the oxidation of many
* Corresponding author. Tel: +91‐671‐2610060; Fax: +91‐671‐2610304; E‐mail: dralekhasutar@gmail.com
DOI: 10.1016/S1872‐2067(14)60113‐3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 10, October 2014

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Alekha Kumar Sutar et al. / Chinese Journal of Catalysis 35 (2014) 1701–1708
aromatic compounds, and it is toxic and resistant to biotreat‐
ment. The treatment of wastewater containing highly concen‐
trated, toxic, or poorly biodegradable compounds by oxidation
of the organic pollutants to non‐toxic products in the presence
of a catalyst is a promising approach [14,15].
(20.00 mmol, 2.725 g) in a round‐bottomed flask at 60 °C for 6
h. All reactions were performed under nitrogen. The metal
complex was recrystallized from dichloromethane and dried in
a vacuum desiccator.
The oxidation of phenols using various chemical reagents
such as H2O2, permanganate, molecular oxygen, and ozone is
widely used [16]. The use of H2O2 has the advantage of pro‐
ducing oxygen, which can be used to augment biological deg‐
radation [17]. The use of H2O2 as an alternative to current in‐
dustrial oxidation processes has environmental advantages
such as (1) replacement of stoichiometric metal oxidants, (2)
replacement of halogens, (3) replacement or reduction of sol‐
vent use, and (4) avoidance of salt by‐products. H2O2 works
either alone or with a catalyst, but the catalytic process gives
better results. Fe is the most common homogeneous catalyst
used with H2O2 [18]. In the present investigation, phenol oxida‐
tion was chosen as the model reaction to study the catalytic
activity of the synthesized unsupported and polymer‐support‐
ed zinc metal complexes; zinc was used because it is a bioactive
metal.
Although the oxidation of phenol in the presence of poly‐
mer‐supported Schiff base transition‐metal complexes has
been reported [8], the catalytic activity of the zinc complex of
the Schiff base N,N′‐bis(2‐hydroxy‐3‐methoxybenzaldehyde)4‐
methylbenzene‐1,2‐diamine (3‐MOBdMBn) has not been re‐
ported in the literature. We therefore attempted to synthesize
and characterize polystyrene‐supported transition‐metal com‐
plexes of 3‐MOBdMBn, evaluate their catalytic activity in phe‐
nol oxidation with H2O2 as the oxidant, and compare their ac‐
tivity with that of the unsupported zinc complex.
2.3. Synthesis of P‐3‐MOBdMBn Schiff base and its zinc complex
Polymer‐anchored zinc complexes were prepared by ni‐
trosation of 3‐MOBdMBn (7.81 g, 20.00 mmol) with sodium
nitrite (20.00 mmol) in 1.0 mol/L hydrochloric acid (100 mL)
in an ice bath. The resultant NO‐3‐MOBdMBn was filtered and
washed with hot and cold water to remove reaction impurities.
Reduction of NO‐3‐MOBdMBn was performed using the ni‐
trosated Schiff base (20.00 mol) in 1.0 mol/L hydrochloric acid
(50 mL) in the presence of metallic iron, which produced N,N′‐
bis(4‐amino‐2‐hydroxy‐3‐methoxybenzaldehyde)‐4‐methylben
zene‐1,2‐diamine (A‐3‐MOBdMBn). Then methanol‐swollen
cross‐linked chloromethylated polystyrene beads (5.0 g) were
refluxed in methanol (50 mL) containing A‐3‐MOBdMBn (20
mmol). After 10 h, the polymer beads with anchored
3‐MOBdMBn were separated and dried in a vacuum desiccator.
Zinc ions were then loaded by keeping P‐3‐MOBdMBn (5.0 g)
for 10 h in 50 mL of an aqueous solution of zinc ions. The metal
ion loadings on the free and polymer‐supported 3‐MOBdMBn
were calculated as a complexation of the metal ions based on
the initial amount of 3‐MOBdMBn and the amount of metal ions
loaded on the polymer beads.
2.4. Characterization of the samples
Infrared (IR) spectra (KBr pellets) were recorded using a
Perkin‐Elmer 1600 Fourier‐transform (FT) IR spectrophotom‐
eter. Electronic spectra were recorded with a Shimadzu 1601
PC ultraviolet‐visible (UV‐Vis) spectrophotometer using sample
mulls in a cuvette. Thermogravimetric (TG) analysis was per‐
formed using a Perkin‐Elmer Pyris Diamond thermal analyzer
under nitrogen at a heating rate of 10 °C/min. The metal ion
loading on the Schiff base was determined by analyzing the
loading solution using a Perkin‐Elmer 3100 atomic absorption
spectrometer at the zinc ion λmax. The compositions of
3‐MOBdMBn and its zinc complex were estimated using a
2. Experimental
2.1. Materials
Divinylbenzene cross‐linked chloromethylated polystyrene
beads were obtained from Ion Exchange India Ltd., Mumbai,
India. Anhydrous zinc chloride was purchased from Thermo
Fisher Scientific India Pvt., Ltd., Mumbai, India and used with‐
out further purifications. Phenol, H2O2 (30.0 wt%), 2‐hydroxy‐
3‐methoxybenzaldehyde (3‐MOBd), and 4‐methylbenzene‐1,2‐
diamine (MBn) were procured from E. Merck, India. Other
chemicals and solvents were of analytical grade (> 99.0 wt%)
and used after drying.
1
Haraeus Carlo Ebra 1108 elemental analyzer. H nuclear mag‐
netic resonance (NMR) spectra were recorded with a Bruker
FT‐NMR 300 MHz spectrometer using DMSO‐d6 as the solvent
and tetramethylsilane as an internal reference. The magnetic
moments (µ) of the metal complexes were measured using a
Vibrating Sample Magnetometer‐155. The molecular mass of
3‐MOBdMBn and its zinc complex was determined using a va‐
por pressure osmometer (Merck VAPRO 5600, Germany).
2.2. Synthesis of 3‐MOBdMBn Schiff base and its zinc complex
The 3‐MOBdMBn Schiff base was synthesized using a modi‐
fied version of the procedure reported in the literature [7]. A
reaction mixture consisting of 3‐MOBd (20.00 mmol, 3.04 g)
and MBn (10.00 mmol, 1.22 g) in methanol was refluxed at 60
°C for 2 h. The reaction mixture was cooled to a low tempera‐
ture, producing light‐ straw‐colored crystals, which were fil‐
tered and recrystallized with methanol. The metal complex of
3‐MOBdMBn was prepared by refluxing a methanolic solution
(100 mL) of Schiff base (20.00 mmol, 7.81 g) and zinc salt
2.5. Catalytic activity of zinc complexes in phenol oxidation
Phenol oxidation was performed using H2O2 as the oxidant,
with a fixed ionic strength (0.10 mol/L) and hydrogen ion con‐
centration (pH = 7.0) in the reaction mixture. A calculated
amount of polymer‐anchored zinc was placed in a two‐necked
round‐bottomed flask containing phenol (4.70 g, 0.05 mol/L);

Alekha Kumar Sutar et al. / Chinese Journal of Catalysis 35 (2014) 1701–1708
1703
100
90
80
70
60
P-3-MOBdMBn-Zn
(1)
P-3-MOBdMBn
(2)
100
200
300
400
500
Temperature (oC)
2500
2000
1500
1000
500
Wavenumber (cm1)
Fig. 1. Thermal stability of P‐3‐MOBdMBn and its zinc complex.
Fig. 2. FTIR spectra of unsupported 3‐MOBdMBn (1) and poly‐
mer‐supported P‐3‐MOBdMBn (2).
H2O2 (5.67 g, 30.0 wt%, 0.05 mol/L) was added to the reaction
mixture with acetonitrile (2 mL) as an internal standard in a
nitrogen atmosphere. The phenol conversion was determined
by removing aliquots of the reaction mixture at different time
intervals and analyzing them using gas chromatography. The
reaction products were identified based on the retention time
of standards, and the peak areas in the chromatograms were
used to measure product selectivity. Phenol, catechol, and hy‐
droquinone were determined using a fused‐silica capillary
column (XE‐60; 30 m × 0.2 mm × 0.3 mm; Perkin‐Elmer Corp.,
Norwalk, CT, USA). The temperatures of the injection port and
column were the same as those in the reactions, and the carrier
gas was supplied at a rate of 20.0 mL/min.
3.2. Characterization of 3‐MOBdMBn Schiff base
The 3‐MOBdMBn Schiff base was produced in substantial
yield (91.6 wt%) by refluxing 3‐MOBd and MBn (Scheme 1).
The IR spectrum (Fig. 2(1)) of 3‐MOBdMBn showed absorption
bands at 1609 (C=N), 1263 (C–O, phenolic), and a broad band
between 3200 and 2910 cm−1, which was assigned to phenolic
OH. Elemental analysis of 3‐MOBdMBn (wt%): C 70.75, N 7.17,
and H 5.68; calcd (%): C 69.82, N 7.03, and H 6.31, correspond‐
ing to C23H22N2O4, the empirical formula of 3‐MOBdMBn [7,12].
The molecular mass of the Schiff base was 390.43 g/mol
(calcd 389.37 g/mol). The electronic spectrum of 3‐MOBdMBn
(Fig. 3) showed absorption bands at 289 and 347 nm, which
were assigned to π → π* and n → π* transitions, respectively.
The 1H‐NMR spectrum of the Schiff base showed signals at δ =
3.71 (6H), 2.79 (3H), 5.13 (2H), 6.60 (2H), 6.98 (2H), 7.35 (3H),
7.41 (2H), and 8.61 (2H), corresponding to the structure of
3‐MOBdMBn.
3. Results and discussion
3.1. TG analysis
The thermal stability of the unsupported and supported cat‐
alysts was determined to assess their applicability in high‐
temperature reactions and to confirm the complexation of met‐
al ions with the polymer‐anchored Schiff base. Thermogravi‐
metric analysis of the polymer‐supported 3‐MOBdMBn
(P‐3‐MOBdMBn) showed a mass loss of 39.1% at 500 °C, but
the zinc(II) ion complex showed a mass loss of 27.0% at the
same temperature, clearly indicating that the zinc ion complex
was more stable [19] than the ligand (Fig. 1). The free and
polymer‐supported metal complexes of 3‐MOBdMBn were also
characterized using IR and UV techniques to confirm complexa‐
tion of metal ions, and the structure and geometry of the metal
complexes were determined on the basis of elemental analysis
and the magnetic properties of the metal complexes.
3.3. Synthesis and characterization of A‐3‐MOBdMBn Schiff
base and its anchoring on polymer beads
The nitrosation of 3‐MOBdMBn was carried out in the
presence of NaNO2/HCl, producing NO‐3‐MOBdMBn in 84.5%
yield (Scheme 2); elemental analysis (%): C 61.60, N 12.49, and
H 4.50; calcd (%): C 62.01, N 12.17, and H 4.95, corresponding
to C23H20N4O6, the formula of the nitrosated Schiff base. The
molecular mass of NO‐3‐MOBdMBn was 448.43 g/mol (calcd
447.02 g/mol). The IR spectrum of NO‐3‐MOBdMBn showed
Scheme 1. Synthesis of N,N′‐bis(2‐hydroxy‐3‐methoxybenzaldehyde)‐4‐methylbenzene‐1,2‐diamine Schiff base (3‐MOBdMBn).

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Alekha Kumar Sutar et al. / Chinese Journal of Catalysis 35 (2014) 1701–1708
(6H), 2.81 (3H), 4.15 (4H), 5.15 (2H), 6.16 (2H), 7.36 (3H), 6.46
(2H), and 8.63 (2H), corresponding to the structure of
A‐3‐MOBdMBn, shown in Scheme 2. A new proton signal from
the amino group appeared at 4.15 (4H).
The synthesized Schiff base was anchored on cross‐linked
chloromethylated polystyrene beads by refluxing A‐3‐MOB‐
dMBn with polymer beads in dimethylformamide at 60 °C for 8
h. The amount of A‐3‐MOBdMBn anchored on the polymer
beads was 86.0 wt% (Scheme 3). Anchoring of A‐3‐MOBdMBn
on the polymer beads was confirmed by comparing the IR
spectrum of 3‐MOBdMBn anchored on polymer beads with that
of pure polymer beads. The IR spectrum of the poly‐
mer‐anchored Schiff base showed absorption bands at 1594
(C=N) and 1246 cm−1 (C–O, phenolic), which were absent in the
IR spectrum of the pure polymer beads but present in the free
Schiff base. The IR spectrum of the pure polymer beads showed
an absorption band at 1262 cm−1, attributed to the chlorome‐
thyl C–Cl bond in the cross‐linked polymer beads [7]. The de‐
crease in the intensity of the absorption band at 1262 cm−1 for
the polymer‐anchored 3‐MOBdMBn compared with that for the
pure polymer beads was evidence of anchoring of 3‐MOBdMBn
on the polymer beads (Fig. 2(2)). The appearance of new ab‐
sorption bands and shifts in the characteristic absorption
bands of 3‐MOBdMBn also confirmed anchoring of 3‐MOB‐
dMBn on the polymer beads.
P-3-MOBdMBn
3-MOBdMBn
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. Electronic spectra of unsupported 3‐MOBdMBn and poly‐
mer‐supported P‐3‐MHBdDBn.
absorption bands at 1604 (C=N), 1260 (C–O, phenolic), and
1500 and 1315 cm−1 for the N–O group. The NMR signals of
NO‐3‐MOBdMBn shifted compared with those of pure
3‐MOBdMBn. The NO‐3‐MOBdMBn had proton signals at δ =
3.75 (6H), 2.82 (3H), 5.15 (2H), 7.15 (2H), 7.36 (3H), 7.66 (2H),
and 8.64(2H), corresponding to the structure of NO‐3‐MOB‐
dMBn. The protons ortho to the nitroso group in 3‐MOBdMBn
were deshielded, therefore their signals appeared at δ = 7.15
and 7.66 instead of δ = 6.60 and 6.98, respectively, for pure
3‐MOBdMBn. The proton signal at δ = 6.91 was missing be‐
cause of substitution of the nitroso group in the benzene ring.
NO‐3‐MOBdMBn was reduced with iron(III) ions in the pres‐
ence of hydrochloric acid, providing A‐3‐MOBdMBn in 87.3
wt% yield; elemental analysis (%): C 65.70, N 13.33, and H
5.75; calcd (%): C 63,92, N 13.21, and H 7.02, corresponding to
C23H24N4O4, the empirical formula of the Schiff base. The mo‐
lecular mass of A‐3‐MOBdMBn was 420.46 g/mol (calcd 418.23
g/mol). The IR spectrum of A‐3‐MOBdMBn had absorption
bands at 1600 (C=N), 1255 (C–O, phenolic), and a band be‐
3.4. Characterization of free and polymer‐anchored zinc
complexes of 3‐MOBdMBn Schiff base
Zinc ions were complexed with free and polymer‐supported
3‐MOBdMBn by refluxing the free Schiff base (Scheme 4) and
polymer‐anchored Schiff base in solutions of a metal salt at 60
°C for 10 h (Scheme 5). The metal complexes 3‐MOBdMBn‐Zn
and P‐3‐MOBdMBn‐Zn were separated and purified, and then
their structures and the amounts of metal ions were deter‐
mined. The amounts of zinc(II) ions complexed with free
3‐MOBdMBn and the polymer‐anchored Schiff base were 74.56
wt% and 78.24 wt%, respectively.
tween 1627 and 1616 cm−1 for the C–N group. The H‐NMR
spectrum of A‐3‐MOBdMBn showed proton signals at δ = 3.72
1
Scheme 2. Synthesis of A‐3‐MOBdMBn.
Scheme 3. Synthesis of polymer‐anchored Schiff base P‐3‐MOBdMBn.

Alekha Kumar Sutar et al. / Chinese Journal of Catalysis 35 (2014) 1701–1708
1705
OCH3
Table 1
O
FTIR frequencies and electronic transitions of 3‐MOBdMBn and its
unsupported and polymer‐supported zinc ion complexes.
OCH3
OCH3
OH
Zn
OCH3
HO
O
N
Zinc salt
solution
Absorption frequency
(cm−1)
Frequency
λmax (nm)
N
N
N
Compound
νC=N νC−O
νOH
νM−O νM−N  n
3‐MOBdMBn
P‐3‐MOBdMBn
3‐MOBdMBn‐Zn 1591 1286
P‐3‐MOBdMBn‐Zn 1575 1278
1609 1263 2910–3200
1594 1246 2900–3340
—
—
—
—
289
282
265
260
347
344
307
301
3-MOBdMBn-Zn (Tetrahedral)
3-MOBdMBn
—
—
560 403
557 401
Scheme 4. Complexation of zinc ions with free 3‐MOBdMBn.
OCH3
OCH3
HO
OH
complexation also led to new bands as a result of formation of
bonds between metal ions and phenolic oxygen (–O–M; Table
1). The complexation of metal ions with the Schiff base was
further confirmed by comparing the electronic spectra of met‐
al‐complexed and pure 3‐MOBdMBn. The complexation of
zinc(II) ions with 3‐MOBdMBn resulted in a hypsochromic shift
of the π → π * transition from 289 to 265 nm, and of the n→ π*
transition from 347 to 307 nm; similar shifts were seen for the
polymer‐supported catalyst (Table 1). These electronic transi‐
tions correspond to the t2g eg configurations of the zinc(II)
ions in these complexes. The magnetic moment (µ) of the Schiff
base complexes with zinc(II) ions was 0.71 BM, indicating a
diamagnetic and tetrahedral structure with sp3 hybridization.
N
N
NH
NH
P
CH2 O P
CH2
P-3-MOBdMBn
Zinc salt solution
O
OCH3
OCH3
O
Zn
6
4
N
N
HN
NH
O P
O P
CH2
CH2
P-3-MOBdMBn-Zn
3.5. Phenol oxidation
Scheme 5. Loading of zinc ions on polymer‐supported Schiff base
(P‐3‐MOBdMBn‐Zn).
The catalytic activity of the free and polymer‐supported
3‐MOBdMBn‐Zn was evaluated by studying the phenol oxida‐
tion in the presence of H2O2. Gas chromatography was used to
determine the product selectivity and estimate the percentage
conversion of phenol. Catechol was the major reaction product
in the oxidation of phenol (Scheme 6). The formation of the
reaction products was attributed to catalytic behavior of the
metal complexes of 3‐MOBdMBn.
These investigations also confirmed that the type of poly‐
mer support plays an important role in controlling the catalytic
activity. In addition, stable active sites on the polymer support
prevent deactivation of the catalyst on repeated use [9]. The
anchoring of active species minimizes poisoning by impurities
and improves the efficiency of active species by reducing their
dimerization and aggregation, as observed under homogeneous
conditions. The activity of the catalyst depends on the proper‐
ties of the support; polymeric supports give better results [20]
because of the flexibility of the polymer backbone [21,22], and
because of their compatibility with reaction media. The an‐
choring of catalysts on polymer supports improves the loading
of metal ions and controls the interactions between catalysts
and substrates; it also enhances the activity and selectivity of
the catalysts [23,24]. A polymer support prevents the aggrega‐
The complexation of metal ions with 3‐MOBdMBn resulted
in significant changes in the IR bands of the C=N and C–O
groups, and new absorption bands appeared as a result of the
formation of M–O and M–N bonds in the metal complexes. The
disappearance of the phenolic absorption band between 2910
and 3200 cm−1 in the IR spectrum of 3‐MOBdMBn after com‐
plexation with metal ions was evidence for complexation of
metal ions with 3‐MOBdMBn. On complexation with zinc(II)
ions, the C=N absorption band of the free Schiff base shifted
from 1609 to 1591 cm−1, and that of the polymer‐supported
Schiff shifted from 1594 to 1575 cm−1 (Fig. 4) [7].
Complexation with zinc(II) ions resulted in a new absorp‐
tion band at 403 cm−1 for the free Schiff base and at 401 cm−1
for the polymer‐anchored Schiff base, as a result of formation of
M–N bonds between zinc(II) ions and the Schiff base. Metal ion
OH
OH
OH
OH
Catalyst
+
OH
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm1)
Phenol
Catechol
Hydroquinone
Scheme 6. Phenol oxidation.
Fig. 4. FTIR spectrum of P‐3‐MOBdDBn‐Zn complex.

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Alekha Kumar Sutar et al. / Chinese Journal of Catalysis 35 (2014) 1701–1708
50
40
30
20
10
0
Table 3
Phenol conversion, selectivity for catechol, and kinetic parameters of
phenol oxidation over 3‐MHBdMBn‐Zn and P‐3‐MOBdMBn‐Zn.
P-3-MOBdMBn-Zn
3-MOBdMBn-Zn
Conversion Selectivity
TON
Rate
Catalyst
(%)
25.9
48.7
(%)
89.4
90.0
(g/(mol·h)) (μmol/(L·s))
5.83
11.14
3‐MOBdMBn‐Zn
P‐3‐MOBdMBn‐Zn
[Phenol] = [Catalyst] = [H2O2] = 0.05 mol/L, 1440 min, 70 °C, CH3CN =
2.0 mL.
0.86
1.64
the presence of the P‐3‐MOBdMBn‐Zn and low (0.86
μmol/(L·s)) in the presence of the unsupported analog (Table
3); a similar trend was observed for the turnover number
(TON) at H2O2:phenol:catalyst molar ratio = 1:1:1.
0
50
100
150
Time (min)
200
750 1500
Fig. 5. Effects of reaction time on phenol oxidation in the presence of
P‐3‐MOBdMBn‐Zn and 3‐MOBdMBn‐Zn. [Phenol] = [Catalyst] = [Oxi‐
dant] = 0.05 mol/L, 70 °C.
3.6. Effect of H2O2, phenol, and catalyst concentrations on
phenol oxidation
tion of active sites and increases the activity of the catalyst.
Supported catalysts are more thermally stable than their un‐
supported analogs [19] as shown in Fig. 1.
The oxidation of phenol was studied using different molar
ratios of H2O2 to phenol, from 0.5 to 2.0, at constant molarities
of the substrate and catalyst (0.05 mol/L), in acetonitrile (2.0
mL) at 70 °C. When the molar ratio of H2O2 was changed from
0.5 to 1.0, phenol oxidation increased in the presence of
P‐3‐MOBdMBn‐Zn (Fig. 6). However, when the molar ratio of
H2O2 was further increased (>1), the phenol oxidation de‐
creased; a similar trend was observed for the unsupported
3‐MOBdMBn‐Zn analog. The decreased phenol conversion was
caused by the decrease in the molar ratios of phenol and the
catalyst with respect to that of H2O2.
The catalytic efficiency of 3‐MOBdMBn‐Zn in phenol oxida‐
tion was evaluated at different molar ratios of phenol in the
reaction mixture, with the molar ratio of H2O2 to the catalyst
being kept constant. The molar ratio of phenol was changed
from 0.5 to 2.0 with respect to the molar ratio of H2O2 to the
catalyst. The concentrations of H2O2 and the catalyst were kept
constant (0.05 mol/L). When the molar ratio of phenol was
increased from 0.5 to 1.0, the conversion of phenol increased
significantly over the P‐3‐MOBdMBn‐Zn (Fig. 6), but when the
molar ratio of phenol was increased further (>1), the conver‐
sion of phenol decreased; this was because of the significant
decrease in the molar ratio of H2O2 to the catalyst in the reac‐
The polymer support facilitated decomposition of interme‐
diates, therefore the percentage conversion of phenol was
higher with the polymer‐supported metal complex than with
the free metal complex of 3‐MOBdMBn (Fig. 5). The conversion
of phenol with the unsupported catalyst was high at 240 min
and then became almost constant because of a substantial de‐
crease in the concentration of oxidant and substrate in the re‐
action mixture. Similar trends in substrate conversion were
observed with the supported catalyst at different time inter‐
vals.
The amount of phenol oxidized by H2O2 was almost equal to
the sum of the amounts of catechol and hydroquinone pro‐
duced, which indicates that almost no other reaction products
such as polymeric phenols were formed. However, the reaction
showed high selectivity for catechol, as determined from the
areas of the gas chromatogram peaks. The supported catalyst
was recycled, and its catalytic activity was evaluated. The effi‐
ciency of the supported catalyst remained almost constant up
to six cycles and then decreased significantly (Table 2), possibly
because of decomposition of the catalyst in the reaction medi‐
um or catalyst extraction into the organic solvent during prod‐
uct isolation [25]. The product selectivity for catechol was un‐
affected by using a recycled catalyst, indicating structural sta‐
bility of the metal complex on the polymer support; this was
confirmed by comparing the IR spectra of the recycled and
freshly prepared catalysts.
50
H2O2
Phenol
45
40
35
The rate of phenol conversion was high (1.64 μmol/(L·s)) in
Catalyst
Table 2
Recycling of P‐3‐MOBdMBn‐Zn in phenol oxidation.
30
25
20
Recycle number
Conversion (%)
Selectivity (%)
90.0
0
2
4
6
8
48.7
47.3
45.2
41.2
24.4
90.3
88.6
84.2
81.7
0.000
0.025
0.050
0.075
0.100
0.125
Concentration (mol/L)
Fig. 6. Effect of H2O2, phenol, and catalyst concentrations on phenol
oxidation in the presence of P‐3‐MOBdMBn‐Zn at 70 °C.
[Phenol] = [Catalyst] = [H2O2] = 0.05 mol/L, 70 °C, CH3CN = 2.0 mL.

Alekha Kumar Sutar et al. / Chinese Journal of Catalysis 35 (2014) 1701–1708
1707
mation of intermediates, Zn‐3‐MOBdMBn‐Ph‐HOO−, through
interactions with phenol in a rapid equilibrium (K). The inter‐
mediate Zn‐3‐MOBdMBn‐Ph‐HOO− facilitated nucleophilic at‐
tack of −OOH species on the ortho and para positions of phenol,
producing hydroxy‐substituted phenols. The reaction products,
catalyst, and hydroxyl ions were formed through decomposi‐
tion of the intermediates; finally, the hydroxyl ions reacted with
hydrogen ions, which were produced in the initial step.
4. Conclusions
Unsupported and polymer‐supported zinc complexes of
3‐MOBdMBn were synthesized, and their structure and cata‐
lytic activity in phenol oxidation were determined. The catalytic
activity of P‐3‐MOBdMBn‐Zn was higher than that of the free
analog. The phenol oxidation showed high selectivity for cate‐
chol. The oxidation rate and TON of the supported catalyst
were higher than those of the unsupported catalyst, which
clearly suggested that the polymer support played a significant
role in increasing the phenol oxidation rate. The effect of H2O2,
phenol, and catalyst concentrations on phenol oxidation was
important for the polymer‐supported zinc complex. Phenol
oxidation conversion was maximum at the phenol:H2O2:cata‐
lyst ratio of 1:1:1.
Scheme 7. Reaction steps in phenol oxidation.
tion mixture compared with the molar ratio of phenol. The
oxidation of phenol was also evaluated at different molar ratios
of P‐3‐MOBdMBn‐Zn at a constant molar ratio (1:1) of the sub‐
strate and oxidant. The molar ratio of the Schiff base complex
with zinc(II) ions was changed from 0.5 to 2.0 at constant mo‐
larities (0.05 mol/L) of phenol and H2O2 in the reaction mix‐
ture. The conversion of phenol showed the same trend as the
changes in substrate concentration.
Acknowledgments
The authors thank DST, CSIR, and UGC, New Delhi, India for
funding. The authors are also grateful to Ravenshaw University,
KIIT University, and the National Institute of Technology, Rai‐
pur for providing research facilities.
3.7. Mechanism of phenol oxidation
Based on the experimental results for the oxidation of phe‐
nol over free and supported 3‐MOBdMBn‐Zn, the following
reaction steps are proposed (Scheme 7). The free and poly‐
mer‐supported 3‐MOBdMBn‐Zn produced active species,
Zn‐3‐MOBdMBn‐HOO−, through fast interactions with H2O2 and
3‐MOBdMBn. The active species were then involved in the for‐
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Graphical Abstract
Chin. J. Catal., 2014, 35: 1701–1708 doi: 10.1016/S1872‐2067(14)60113‐3
Novel polystyrene‐anchored zinc complex: Efficient catalyst for phenol oxidation
Alekha Kumar Sutar*, Yasobanta Das, Sasmita Pattnaik, Anita Routaray, Nibedita Nath, Prasanta Rath, Tungabidya Maharana
Ravenshaw University, India; KIIT University, India; National Institute of Technology, India
The synthesis of a simple, inexpensive, reusable, and highly efficient polymer‐supported –ONNO– tetradentate Schiff base zinc(II) ion
catalyst for the oxidation of phenol with H2O2 to catechol and hydroquinone was described.

1708
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