Oxidation of 2,6-di-tert-butylphenol by dioxygen catalyzed by tetrasodium phthalocyaninatocobalt(II) tetrasulfonate in aqueous micellar media

Article abstract of DOI:10.1007/BF03245869

The oxidation of 2,6-di-tert-butylphenol by dioxygen has been investigated in aqueous micellar aggregates of cetyltrimethylammonium bromide (CTAB) using tetrasodium phthalocyaninatocobalt(II) tetrasulfonate (CoPcTsNa₄) as catalyst. The CTAB/CoPcTsNa₄ system showed enhanced catalytic activity in the oxidation of 2,6-di-tert-butylphenol compared to that observed in the oxidation reaction in the absence of CTAB. 2,6-Di-tert-butyl-1,4-benzoquinone and 3,5,3',5'-tetra-tert-butyl-4,4-diphenoquinone were identified as reaction products. The initial rate constants of auto-oxidation reaction was found to increase with increasing the pH range from 7.0 to 13.0. The rate constants k_obs of auto-oxidation reaction showed linear dependence on catalyst concentration. The rate of auto-oxidation reaction was found to fit a Michealis-Menten kinetic model for the saturation of catalyst sites with increasing 2,6-di-tert-butylphenol concentration and dioxygen pressure. Tetrasodium phthalocyaninatocobalt(II) tetrasulfonate in aqueous micellar solution of CTAB was found to be mainly monomeric.

Full text of DOI:10.1007/BF03245869

J. Iran. Chem. Soc., Vol. 7, No. 1, March 2010, pp. 128-133.
JOUIRrNaAnLiaOnF THE
Chemical Society
Oxidation of 2,6-Di-tert-butylphenol by Dioxygen Catalyzed by Tetrasodium
Phthalocyaninatocobalt(II) Tetrasulfonate in Aqueous Micellar Media
M. Hassanein*, M. Sakaran and S. Shendy
Chemistry Department, Faculty of Science, Tanta University, Tanta 3152, Egypt
(Received 14 August 2008, Accepted 14 February 2009)
The oxidation of 2,6-di-tert-butylphenol by dioxygen has been investigated in aqueous micellar aggregates of
cetyltrimethylammonium bromide (CTAB) using tetrasodium phthalocyaninatocobalt(II) tetrasulfonate (CoPcTsNa4) as
catalyst. The CTAB/CoPcTsNa4 system showed enhanced catalytic activity in the oxidation of 2,6-di-tert-butylphenol
compared to that observed in the oxidation reaction in the absence of CTAB. 2,6-Di-tert-butyl-1,4-benzoquinone and 3,5,3',5'-
tetra-tert-butyl-4,4-diphenoquinone were identified as reaction products. The initial rate constants of auto-oxidation reaction
was found to increase with increasing the pH range from 7.0 to 13.0. The rate constants kobs of auto-oxidation reaction showed
linear dependence on catalyst concentration. The rate of auto-oxidation reaction was found to fit a Michealis-Menten kinetic
model for the saturation of catalyst sites with increasing 2,6-di-tert-butylphenol concentration and dioxygen pressure.
Tetrasodium phthalocyaninatocobalt(II) tetrasulfonate in aqueous micellar solution of CTAB was found to be mainly
monomeric.
Keywords: Oxidation, Dioxygen, Cobaltphthalocyanine, 2,6-Di-tert-butyl-phenol, Surfactant, Catalysis
INTRODUCTION
Examples are oxidation of alkenes [13,14], aromatic
compounds [15], ascorbic acid [16], thiols [17-24] and
phenols [25-32].
The objective of the present work is to investigate the
oxidation of 2,6-di-tert-butylphenol by dioxygen catalyzed
by tetrasodium phthalocyaninato-cobalt(II) tetrasulfonate (1)
in aqueous micellar media.
The enhancement of the rate of organic reactions by
colloidal assemblies such as aqueous micelles has attracted
considerable interest in recent years. The unique ability of
micelles to solubilize organic substrates in water and to
catalyze chemical reactions, forms the basis for their use as
a medium for investigating organic and bioorganic processes
[1-7].
The oxidation of phenols with transition metal dioxygen
carriers [8-10] is of industrial and biological importance,
because it involves metal dioxygen adduct as an
intermediate in the oxidation process, and serves as an
appro-priate model for the reaction of oxygenized enzymes
[11]. Because of their cheap and facile preparation in a large
scale and their relatively high chemical and thermal
stability, metallophthalocyanines are attractive as potential
catalysts for the oxidation of organic compounds [12-32].
EXPERIMENTAL
Materials and Reagents
2,6-Di-tert-butylphenol (Aldrich) was recrystallized
from n-hexane. Tetrasodium phthalocyaninatocobalt(II)
tetrasulfonate (CoPcTSNa4) was prepared and purified as
reported by Weber and Busch [33]. Hexadecyltrimethyl-
ammonium bromide (Aldrich), sodium dodecylsulfate 98%
(Aldrich) and triton X-100 (Merck) were used as received.
Water used in the reaction was double distilled (deionized
water).
*Corresponding author. E-mail: mthassanein@yahoo.com

Hassanein et al.
SO3Na
NaO3S
Apparatus
The UV-Vis absorption spectra of the catalyst was
N
recorded on Shimadzu 3101 Pc UV-Vis Spectrophotometer.
N
N
N
Autoxidation Reactions
Co
N
N
Oxidations of 2,6-di-tert-butylphenol were performed
as previously [28] by stirring of 100 ml of an aqueous
mixture containing 5 volume% methanol in a 250 ml round
bottomed flask attached to a gas burette. The pH was
adjusted for universal buffer mixture. All reactions were
carried out at 40 ºC at constant oxygen pressure. Lower
partial pressures of dioxygen were obtained by use of
oxygen/nitrogen mixture at 1 atm total pressure. After the
completion of the reaction the mixture was extracted by
diethyl ether. The extract was analyzed quantitatively with
preparative layer chromatography, PLC (PSC-GF254 Silica
gel plates, Merk) plates, 2 mm thickness, 20 cm ꢀ 20 cm
using petroleum ether and ethyl acetate (volume ratio
9.5:0.5) as eluent. The products were identified by
comparison with authentic samples. All kinetic runs were
duplicate and were reproducible within 5%.
N
N
NaO3S
SO3Na
1
O
O
O
2
RESULTS AND DISCUSSION
Oxidation of 2,6-Di-tert-butylphenol
The catalytic activity of tetrasodium phthalocyaninato-
cobalt(II) tetrasulfonate (Scheme 1) was investigated in the
auto-oxidation of 2,6-di-tert-butylphenol in aqueous micellar
aggregates. The oxidation reaction was followed by
measuring dioxygen uptake. Gas burette measurements of
dioxygen consumption, as the reaction proceeded, showed
that after an induction period, the volume of dioxygen
consumed was linear with time, indicating a zero-order
dependence on substrate concentration.
O
3
Scheme 1
tert-butylphenoxide anions by binding them at the surface of
the cationic micelle.
The auto-oxidation of 2,6-di-tert-butylphenol in aqueous
micellar solution of CTAB gave 90% conversion of 2,6-di-
tert-butylphenol within 5 h to 3,5,3',5'-tetra-tert-butyl-4,4-
diphenoquinone 2 and 2,6-di-tert-butyl-1,4-benzoquinone 3
(Table 2). The oxidation reaction in the absence of CTAB
gave only 20% conversion of 2,6-di-tert-butylphenol under
the same reaction conditions. Data illustrated in Table 2
show that micelle could also affect the product distribution
of the auto-oxidation reaction of 2,6-di-tert-butylphenol. The
oxidative coupling product 2 was the major product in the
oxidation reaction catalyzed by aqueous CoPcTSNa4 in the
absence of CTAB. The yield of 2,6-di-tert-butyl-1,4-
benzoquinone 3 increased in the reaction catalyzed by
The data summarized in Table 1 show the enhanced
catalytic activity of CoPcTSNa4 in the presence of surfactant
aggregates in the auto-oxidation of 2,6-di-tert-butyl-phenol
compared to that observed in the absence of surfactant. The
rate acceleration of the oxidation reaction in the presence of
surfactant aggregates could be attributed to the high local
concentration of 2,6-di-tert-butylphenol and the catalyst
CoPcTSNa4 in the pseudo phase in the micelle compared
with the bulk solution [4,7]. The higher rate of auto-
oxidation reaction of 2,6-di-tert-butylphenol catalyzed by
CoPcTSNa4 in the presence of CTAB (Table 1), is also due
to the reduction of the repulsive forces between the active
species phthalocyaninatocobalt(II) tetrasulfonate and 2,6-di-
129

Oxidation of 2,6-Di-tert-butylphenol by Dioxygen
5
Table 1. Auto-oxidation of 2,6-Di-tert-butylphenol
4
3
2
1
0
Catalyzed by Tetrasodium phthalocyanato-
cobalt (II) Tetrasulfonate in Aqueous Micellar
Mediaa
Surfactants
106 kobs
(M min-1)
0.96
0
5
10
15
20
-
104 [CTAB] (M)
CTABb
4.70
1.60
SDSc
Fig. 1. Effect of concentration of CTAB on the auto-
oxidation of 2,6-di-tert-butylphenol catalyzed by
cobalt(II) phthalocyanine complex 1 in aqueous
micellar solution of CTAB. All reactions were
carried out under conditions reported in Table 1.
Triton X-100
1.30
aAll experiments were carried out at 40 °C and oxygen
pressure of 0.9 atm. with magnetic stirring of 1.5 mmol
of 2,6-di-tert-butylphenol in 100 ml distilled water
containing 4 ꢀ 10-5 M of CoPcTSNa4. The pH of the
reaction was adjusted to 11.0 using universal buffer
mixture. bhexadecyltrimethylammonium bromide.
csodium dodecyl sulfate.
8
6
4
2
0
Table 2. Oxidation of 2,6-Di-tert-butylphenol Catalyzed by
Tetrasodium Phthalocyanatocobalt(II) Tetrasulfonate
in Presence of CTABa
6
8
10
12
14
[pH]
Phenol conversion
(%)b
Reaction time
(h)
Selectivity (%)b
Fig. 2. Effect of pH on the auto-oxidation of 2,6-di-tert-
butylphenol catalyzed by cobalt(II) phthalo-
cyanine complex 1 in aqueous micellar solution
of CTAB. All reactions were carried out under
conditions reported in Table 1. The pH of the
reaction mixture was adjusted using universal
buffer mixtures.
DPQc
70
40
BQd
30
60
20e
90
5
5
aAll experiments were carried out under experimental
conditions as reported in Table 1 in presence of 12 ꢀ 10-4
M solution of CTAB. b%Conversion and selectivity are
based on isolated products after completion of the reaction.
cDPQ: 3,5,3',5'-tetra-tert-butyl-4,4'-diphenoquinone. dBQ:
e
2,6-di-tert-butyl-1,4-benzoquinone. Reaction carried out
in absence of CTAB.
of hexadecyltrimethylammonium bromide on the rate
constant kobs of the auto-oxidation of 2,6-di-tert-butylphenol
catalyzed by CoPcTSNa4. The oxidation rate increases as the
CTAB concentration increases and reaches a maximum and
then decreases. Similar results were reported previously
[4,34].
CoPcTSNa4/CTAB system.
Previous results had indicated that CoPcTSNa4/CTAB
system was the best catalyst; it was therefore further
investigated at various reaction conditions.
Effect of pH on the Auto-oxidation of 2,6-Di-tert-
butylphenol
Effect of [CTAB] on the Auto-oxidation of 2,6-Di-
tert-butylphenol
The auto-oxidation of 2,6-di-tert-butylphenol was
studied in the pH range 7.0-13.0, using universal buffer
mixture. The zero-order rate constant kobs was found to
Data illustrated in Fig. 1 show the effect of concentration
130

Hassanein et al.
increase with increasing the pH value of the reaction
medium (Fig. 2). This indicates that 2,6-di-tert-butyl-
phenoxide anion is the active species.
8
6
4
2
0
Effect of Concentration of CoPcTSNa4 on the Auto-
oxidation of 2,6-Di-tert-butylphenol
The dependence of the rate constant kobs of the auto-
oxidation of 2,6-di-tert-butylphenol on the concentration of
CoPcTSNa4 in aqueous micellar solution of hexadecyltri-
methylammonium bromide was investigated in the
concentration range 0.5 ꢀ 10-5 to 5 ꢀ 10-5 M ( Fig. 3). The
rate constants kobs increased linearly with increasing the
concentration of CoPcTSNa4.
0
2
4
6
105 [CoPcTS] (M)
Fig. 3. Dependence of rate constant on catalyst 1
concentration. For reaction conditions see
footnote (a) of Table 1.
6
5
4
3
2
1
0
Effect of Concentration of 2,6-Di-tert-butylphenol
on the Rate of Auto-oxidation
The effect of concentration of 2,6-di-tert-butylphenol on
the rate constants kobs of the auto-oxidation reaction has
been investigated in the concentration range 0.5 ꢀ 10-3 M to
4 ꢀ 10-3 M. The initial rate constants kobs of 2,6-di-tert-
butylphenol consumption depend on the initial concentration
of 2,6-di-tert-butylphenol as shown in Fig. 4. A linear
double reciprocal Lineweaver-Burk (Fig. 5) shows that the
data fit a Michaelis-Menten kinetic model for the saturation
of the catalyst sites with increasing 2,6-di-tert-butylphenol
concentration.
0
1
2
3
4
5
103 [Phenol] ( M)
Fig. 4. Dependence of rate auto-oxidation on 2,6-di-tert-
butylphenol concentration. For reaction conditions
see footnote (a) of Table 1.
0.35
0.3
Effect of Partial Pressure of Dioxygen on the Auto-
oxidation of 2,6-Di-tert-butylphenol
The effect of partial pressure of dioxygen on the auto-
oxidation of 2,6-di-tert-butylphenol in aqueous micelle of
CTAB was investigated by using oxygen/nitrogen mixture
to obtain reduced partial pressure of 1 atm total pressure on
the reaction mixture. The data illustrated in Figs. 6 and 7
show that the observed rate constants kobs depend on the
partial pressure of dioxygen for saturation of catalyst site.
The oxidation of phenols by dioxygen catalyzed by
cobalt(II) phthalocyanine complexes represents a typical
example for a reaction involving the activation of dioxygen
by transition metal cation [26]. Metal-bound oxygen is an
active species in the auto-oxidation of phenols catalyzed by
cobalt(II) phthalocyanine [26].
0.25
0.2
0.15
0.1
0.05
0
0
0.5
1
1.5
2
2.5
10-3/[Phenol] (M)
Fig. 5. Linweaver-Burk plot for the data in Fig. 4.
the high energy absorption band near 620 nm is due to the
dimeric species [30]. The spectroscopic data illustrated in
Fig. 8 show that aqueous CoPcTSNa4 in the presence of
CTAB under reaction conditions is present mainly in the
monomeric form. The surfactant promoted site isolation of
the CoPcTSNa4 prevents the formation of inactive µ-oxo
Metal phthalocyanine complexes are generally known to
exist as dimmers in equilibrium with monomers in aqueous
solutions. Their UV-Vis spectra thus consists of two peaks
in the Q band region. The lower energy absorption band
near 670 nm is associated with the monomeric species while
131

Oxidation of 2,6-Di-tert-butylphenol by Dioxygen
6
5
4
3
2
1
0
dimers. The monomeric CoPcTS-dioxygen adduct (Scheme
2) is therefore the active species in the oxidation reaction of
2,6-di-tert-butyl-phenol in the presence of cationic
surfactant CTAB.
In the literature, there is a general consensus that, the
phenoxy radical is the first intermediate in the mechanism of
the formation of coupled product 2 and the oxygen insertion
product 3 in the oxidation of substituted phenols catalyzed
by dioxygen-cobalt complexes [26-28,35]. The coupling
0
200
400
600
800
[PO2] (mmHg)
product
2 in the oxidation reaction is formed by
dimerization of the phenoxy radical (Scheme 3). The oxygen
insertion product 2,6-di-tert-butyl-1,4-benzo-quinone 3 is
formed through the cobalt(III) complex (Scheme 3).
Fig. 6. Dependence of rate of auto-oxidation of 2,6-di-tert-
butylphenol catalyzed by cobalt(II) phthalocyanine
complex 1 in aqueous micellar aggregates of CTAB
on partial pressure of dioxygen. For reaction
conditions see footnote a of Table 1.
LCoIIIO2
LCoII
DBP
+
O2
(DBP)LCoIIIO2
LCoIIIO2
+
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
ArO.
ArO.
LCoIII
2LCoIII(OH) + O2
(DBP)LCoIIIO2
+
(O2H)
2LCoIII(O2H)
LCoIII(OH)
LCoII + H2O
+
+ DBP
DBP = 2,6-di-tert-butylphenol
0
0.002
0.004
0.006
0.008
ArO.
=
2,6-di-tert-butylphenoxy radical
1/[PO2] (mmHg)
Fig. 7. Linweaver-Burk plot for the data in Fig. 6.
Scheme 2
2.5
2
c
1.5
1
b
a
0.5
0
200
300
400
500
Wavelength
600
700
800
Fig. 8. Electronic absorption spectra of CoPcTSNa4 1: a) electronic absorption spectra of CoPcTSNa4 in water.
b) Spectrum obtained after addition of CTAB. c) Spectrum obtained in presence of surfactant in pH 11.
132

Hassanein et al.
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catalyst
1/2 O2
O.
O
O
+ H2O
2
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2
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H
O.
LCoIIIO2
.
O
+
O
O
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O
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LCoIIIOH
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O
LCo III
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3
Scheme 3
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