Temperature effect on the rate of formation of free radicals in CTAB-catalyzed decomposition of hydroperoxides

Article abstract of DOI:10.1023/A:1025624621871

The temperature effect on the rate of the decomposition of hydroperoxides and the rate of the formation of free radicals in the oxidation of ethylbenzene with molecular oxygen in the presence of α-phenylethyl hydroperoxide-cetyltrimethylammonium bromide (CTAB) as a catalytic system for free radical generation was studied by kinetic methods (from the oxygen consumption and hydroperoxide decomposition rates) and the inhibition method involving different acceptors of free radicals.

Full text of DOI:10.1023/A:1025624621871

1500
Russian Chemical Bulletin, International Edition, Vol. 52, No. 7, pp. 1500—1503, July, 2003
Temperature effect on the rate of formation
of free radicals in CTABꢀcatalyzed decomposition of hydroperoxides
ꢀ
L. M. Pisarenko, T. V. Maksimova, Z. S. Kartasheva, and O. T. Kasaikina
N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences,
4 ul. Kosygina, 119991 Moscow, Russian Federation.
Fax: +7 (095) 938 2156. Eꢀmail: kasaikina@chph.ras.ru
The temperature effect on the rate of the decomposition of hydroperoxides and the rate of
the formation of free radicals in the oxidation of ethylbenzene with molecular oxygen in the
presence of αꢀphenylethyl hydroperoxide—cetyltrimethylammonium bromide (CTAB) as a
catalytic system for free radical generation was studied by kinetic methods (from the oxygen
consumption and hydroperoxide decomposition rates) and the inhibition method involving
different acceptors of free radicals.
Key words: cetyltrimethylammonium bromide, 2,2´ꢀbis[2ꢀ(pꢀdimethylaminophenyl)indaneꢀ
1,3ꢀdione], 4ꢀmethylꢀ2ꢀspirocyclohexylꢀ[3,4:3´,2´]ꢀtetrahydrofuranoꢀ1,2,3,4ꢀtetrahydroꢀ
quinolinꢀ1ꢀoxyl, oxidation of ethylbenzene, αꢀphenylethyl hydroperoxide, catalysis of hydroꢀ
peroxide decomposition, energy of activation.
The catalytic effect of cationic amphiphilic surfacꢀ
Experimental
tants, particularly cetyltrimethylammonium bromide
(CTAB), in freeꢀradical oxidation is due to the accelerꢀ
ated decomposition of hydroperoxide ROOH into free
radicals in mixed microaggregates of the inverted micelle
type (ROOH—CTAB).1—5 Earlier,5 the kinetics of free
radical generation in ethylbenzene oxidation with moꢀ
lecular oxygen at 60 °C in the presence of αꢀphenylethyl
hydroperoxide (ROOH)—CTAB as a catalytic system
was studied by the inhibition method using dimeric
2,2´ꢀbis[2ꢀ(pꢀdimethylaminophenyl)indaneꢀ1,3ꢀdione] as
a free radical acceptor. It was found6,7 that under experiꢀ
mental conditions, CTAB affects only the radical initiaꢀ
tion rate (Wi). The oxygen consumption rate during
Reagents and reaction mixtures were prepared and experiꢀ
ments and measurements were carried out as described earꢀ
lier.2,5 Ethylbenzene oxidation was carried out in a temperaꢀ
tureꢀcontrolled reaction vessel with a magnetic stirrer and a gas
volumeter for monitoring the oxygen consumption rate. In exꢀ
periments of hydroperoxide decomposition, a flow reactor was
used. Free radical acceptors were the earlier successfully used
dimeric 2,2´ꢀbis[2ꢀ(pꢀdimethylaminophenyl)indaneꢀ1,3ꢀdione]
(1)5 and a stable nitroxyl radical, namely, 4ꢀ(spirotetrahydroꢀ
furanꢀ2´ꢀyl)ꢀ2ꢀspirocyclohexylꢀ1,2,3,4ꢀtetrahydroquinolinꢀ1ꢀ
oxyl (2) synthesized according to a known procedure.8 This
nitroxyl radical is a specific trap for peroxyl radicals since their
spin adducts, quinone nitrones, show a characteristic absorption
band in the visible spectrum.9 Electronic absorption spectra of
the nitroxyl radical and quinone nitrone were recorded on an
Ultrospec 1100 pro spectrophotometer.
ethylbenzene oxidation in this system (WO ) is deꢀ
2
scribed by the known equation for the liquidꢀphase oxidaꢀ
tion rate:
WO = kp/(2kt)0.5[RH](Wi)0.5
,
(1)
2
Results and Discussion
where kp and kt are the rate constants of chain propagaꢀ
tion and termination.
In the present work, the temperature effects on the
oxygen consumption and radical initiation rates were studꢀ
ied for oxidation of ethylbenzene in the presence of hydꢀ
roperoxide and CTAB. In addition, we verified the vaꢀ
lidity of equation (1) at different temperatures and comꢀ
pared data for the temperature effects on the radical iniꢀ
tiation and hydroperoxide decomposition rates in microꢀ
aggregates formed by CTAB.
Typical kinetic curves for oxygen consumption durꢀ
ing ethylbenzene oxidation initiated by the catalytic
ROOH—CTAB system are shown in Fig. 1 for different
temperatures. It can be seen that at equal initial concenꢀ
trations of the reagents, the oxygen consumption rate
increases with temperature. Addition of inhibitor 1 interꢀ
rupts the oxygen consumption, but then its rate returns to
the original level. In all experiments (Table 1),5 the
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1419—1422, July, 2003.
1066ꢀ5285/03/5207ꢀ1500 $25.00 © 2003 Plenum Publishing Corporation

CTABꢀcatalyzed decomposition of hydroperoxides
[O2]•10–3/mol L–1
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
1501
of ROOH and CTAB. The experimental WO and Wi
2
values and the Wi values calculated by the formula
Wi = {WO /([RH]kp/(2kt)0.5)}2
(3)
1
2
3
2
1
0
2
from experimental WO values and the literature data
2
3
for the parameter kp/(2kt)0.5 are given in Table 1. Accordꢀ
ing to known data,6,7 (kp/(2kt)0.5)•104 is 3.67, 5.5, 8.04,
and 11.5 L0.5 mol–0.5 s–0.5 at 50, 60, 70, and 80 °C, reꢀ
spectively.
4
The apparent rate constant of radical initiation in the
process ROOH—CTAB → RO2• is given by the formula
20
40
60
80
100 t/min
Fig. 1. Kinetic curves for oxygen consumption in the oxidation
kapp = Wi/([CTAB] [ROOH]).
(4)
of ethylbenzene containing ROOH (1.26•10–2 mol L–1) and
CTAB (1.2•10–3 mol L–1) at (1) 80 °C ([1] = 3.0•10–4 mol L–1
,
The kapp values are rather widely scattered (see
Table 1), which indicates that the system is complex and
that the reaction rate is affected by many factors.
τ
ind = 23 min), (2) 70 °C ([1] = 1.7•10–4 mol L–1, τind = 41 min),
(3) 60 °C ([1] = 1.1•10–4 mol L–1, τind = 52 min), and (4) 50 °C
([1] = 0.34•10–4 mol L–1, τind = 35 min). Addition of inhibitor 1
is marked with arrows. (The curves of oxygen consumption are
shifted in time for clarity.)
The radical initiation rates in this system during
ethylbenzene oxidation were measured in a flow reactor
in a temperature range from 50 to 70 °C with the use of
stable nitroxyl radical 2 as an acceptor of radicals. Twenty
minutes after the beginning of oxidation, a solution of
nitroxyl radical 2 in chlorobenzene was added to ethylꢀ
benzene containing CTAB (1.2 mmol L–1) and a hydroꢀ
peroxide (20 mmol L–1) so that the concentration of radiꢀ
cal 2 in the system was 0.13 mmol L–1. The reaction
solution was sampled every five minutes and the absorpꢀ
tion spectra were recorded. The initiation rate is equal to
the rate of formation of quinone nitrone (λmax = 371 nm;
ε = 1.98•104 L mol cm–1).9
amount of consumed oxygen (∆Ο2) is virtually equal to
the amount of the hydroperoxide formed.
[ROOH]t = [ROOH]0 + ∆O2,
(2)
where [ROOH]t and [ROOH]0 are the current and initial
concentrations of hydroperoxide, respectively.
Taking this into account, inhibitor 1 was added at
such an instant t1 that [ROOH]t1 was (14 1) mmol L–1
;
the induction period (τ) and therefore the initiation rate
(Wi = (2[1]0/τ) were determined at equal concentrations
The plot for kapp vs. temperature in Arrhenius coꢀ
ordinates is shown in Fig. 2 (curve 1). The average
Table 1. Kinetic characteristics of ethylbenzene oxidation initiated by a ROOH—CTAB system (1.2 mmol L–1) at different temperaꢀ
tures
T/°C
WO *•106
Wi•107
([ROOH]0 + ∆О2)•102
[ROOH]t**•102
kapp•102
Inhibition
2
2
Inhibition From WO
method (1)
mol L–1
From WO
2
method
mol L–1 s–1
1
2
50
60
60
70
70
70
80
80
80
80
0.41
0.91
0.91
2.81
3.00
2.50
6.60
7.10
7.20
6.90
0.32
0.67
0.72
1.70
1.80
1.40
3.80
4.19
4.30
4.30
0.18
0.44
0.44
1.80
2.10
1.40
4.90
5.70
5.90
5.30
1.28
1.61
1.62
1.70
1.60
1.60
1.80
1.85
1.72
1.78
1.27
1.59
1.63
1.67
1.62
1.62
1.87
1.83
1.70
1.75
0.13
0.26
0.26
1.00
1.30
0.80
2.20
2.60
2.80
2.50
0.19 0.14
0.39 0.32
0.42
—
0.98 0.84
1.00
0.83
2.10
2.20
2.40
2.40
—
—
—
—
—
—
* Experimental values.
** At the end of the experiment.

1502
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
Pisarenko et al.
7 + lnkapp(kd
)
1 + ln([ROOH]t/[ROOH]0)
1.0
app
4.0
0.9
0.8
0.7
0.6
0.5
0.4
2
1
2
1
3.0
2.0
1.0
0
3
20
40
60
80
100
t/min
2.5
2.7
2.9
103 T/K–1
Fig. 3. Kinetic curves for the decomposition of αꢀphenylethyl
hydroperoxide in ethylbenzene in the presence of CTAB in a
nitrogen atmosphere at (1) 90 °C (11•10–2 mol L–1 ROOH +
+ 1•10–2 mol L–1 CTAB), (2) 100 °C (2.7•10–2 mol L–1
Fig. 2. Plots for (1) the apparent rate constant kapp and (2) kd
vs. temperature in Arrhenius coordinates.
app
energy of activation is 91.0 3.3 kJ mol–1, kapp
=
ROOH
+
2•10–3 mol L–1 CTAB), and (3) 120 °C
7.2•1011exp(–91000/(RT )) L mol–1 s–1
.
((2.7—4.5)•10–2 mol L–1 ROOH + 2•10–3 mol L–1 CTAB).
Earlier,2 the energy of activation of this process was
estimated at 50.2 kJ mol–1 by comparing the rates of the
CTABꢀcatalyzed decomposition of αꢀphenylethyl hydroꢀ
peroxide at 120 and 90 °C. Because of such discrepancies
in temperature coefficients, the kinetics of accumulation
and decomposition of ROOH in the presence of CTAB
was studied in more detail.
The kinetic curves for ROOH decomposition in an
inert atmosphere in the presence of CTAB can be deꢀ
scribed by the firstꢀorder equation (Fig. 3)
kJ mol–1) is higher than the E value found previously,2
but is nearly 1.5 times lower than the energy of activation
of radical formation (ER) in this system in the presence of
oxygen. As can be seen in Table 1, at 50—80 °C in the
presence of oxygen, αꢀphenylethyl hydroperoxide is virꢀ
tually the sole product of the initial oxidation of ethylꢀ
benzene in the concentration range studied. This was conꢀ
firmed by oxidation of ethylbenzene in a flow reactor with
periodical sampling for analysis of the ROOH content.
The kinetic curve for ROOH accumulation at equal iniꢀ
tial concentrations of the reagents ([ROOH]0 = 9•10–3
mol L–1, [CTAB] = 1.2•10–3 mol L–1, 70 °C) virtually
coincides with the kinetic curve for oxygen consumption
(see Table 1).
d[ROOH]/dt = k´app[ROOH],
(5)
or
ln[ROOH]t/[ROOH]0 = k´appt,
(6)
At high current concentrations of hydroperoxide and
elevated temperatures with oxygen as an oxidant, the
ROOH concentration rapidly reaches a maximum value.
The higher the CTAB concentration in ethylbenzene oxiꢀ
where the rate constant k´app depends on the concentraꢀ
tion of CTAB. The initial segments of the kinetic curves
deviate from a linear anamorphosis in semilogarithmic
coordinates (see Fig. 3); the deviation the larger, the higher
the CTAB concentration and the lower the temperature.
Earlier,5 it was noted that some time is required for
the formation of quasistable surfactant—ROOH microꢀ
aggregates in a microheterogeneous system obtained by
mixing a surfactant with an ROOHꢀcontaining hydrocarꢀ
bon medium. Taking this into account, the effect of CTAB
on the rate of ROOH decomposition in an inert atmoꢀ
sphere at different temperatures was quantitatively charꢀ
Table 2. Kinetic characteristics of the CTABꢀcatalyzed decomꢀ
position of αꢀphenylethyl hydroperoxide
T/°C
[CTAB] [ROOH]
mmol L–1
k´app•104
/s–1
kd •102
app
/L mol–1 s–1
190
190
100
120
120
120
120
120
1
10
2
10
10
3
100
110
27
100
115
60
0.13
1.03
0.44
5.40
6.20
1.80
1.10
1.14
1.3
1.0
2.2
5.4
6.2
6.0
5.5
5.7
acterized by k´app and kd
= k´app/[CTAB] (Table 2).
app
The temperature dependence of kd is satisfactorily deꢀ
app
scribed by the Arrhenius equation (Fig. 2, curve 2)
kdapp = (1.67•107)exp(–63430/(RT)) L mol–1 s–1
.
2
2
27
45
The obtained apparent energy of activation of ROOH
decomposition in an inert atmosphere (ED = 63.43 3.16

CTABꢀcatalyzed decomposition of hydroperoxides
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 7, July, 2003
1503
[ROOH]/mol L–1
0.8
tions suggest that oxygen is involved in the catalytic deꢀ
composition of ROOH in CTAB microaggregates.
1
2
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos. 01ꢀ03ꢀ32222
and 00ꢀ15ꢀ97404).
0.6
0.4
References
3
0.2
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4
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0
2
4
6
8
10
12
14 16 t/h
Fig. 4. Kinetic curves for accumulation of hydroperoxides in the
oxidation of ethylbenzene in the presence of CTAB at 120 °C.
The concentration of CTAB is (1) 0, (2) 1, (3) 10, and
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be reached (Fig. 4). Earlier,2 it was noted that the oxidaꢀ
tion of ethylbenzene in the presence of CTAB and its
autooxidation yield the same products.
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[Dokl. Chem., 1979, 245 (Engl. Transl.)].
Comparison of the kapp and kd
values (see Fig. 2)
app
showed that the rate of the catalytic decomposition of
ROOH into radicals in the presence of oxygen is higher
than the overall rate of ROOH consumption in an inert
atmosphere (at least, at temperatures >60 °C). At elevated
temperatures, inhibitors and the radicals formed from
them react with hydroperoxides, a hydrocarbon, and oxyꢀ
gen, which hinders direct determination of the radical
initiation rate by the inhibition method. However, a large
discrepancy between ED and ER values and the higher
rates of ROOH decomposition under oxidation condiꢀ
Received December 11, 2002