Catalytic oxidation of toluene by ozone in the acetic acid-sulfuric acid system

Article abstract of DOI:10.1134/S1070427211060140

Oxidation of toluene by ozone was studied in the system constituted by acetic and sulfuric acids in the presence of manganese(II) acetate and sodium bromide. The effect of sulfuric acid and the catalyst on the yield of benzoic acid and on the oxidation ra

Full text of DOI:10.1134/S1070427211060140

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 6, pp. 984–987. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © E.V. Potapenko, P.Yu. Andreev, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 6, pp. 960–963.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Catalytic Oxidation of Toluene by Ozone
in the Acetic Acid–Sulfuric Acid System
E. V. Potapenko and P. Yu. Andreev
Institute of Chemical Technologies, Dal’ Eastern-Ukrainian National University, Rubezhnoe,
Lugansk oblast, Ukraine
Received August 13, 2010
Abstract—Oxidation of toluene by ozone was studied in the system constituted by acetic and sulfuric acids in
the presence of manganese(II) acetate and sodium bromide. The effect of sulfuric acid and the catalyst on the
yield of benzoic acid and on the oxidation rate was considered. The optimal ozonization conditions were
determined. A scheme of redox catalysis that accounts for experimental data was suggested.
DOI: 10.1134/S1070427211060140
Toluene is oxidized by ozone in acetic acid
predominantly with destruction of the aromatic ring
[1]. The selectivity of oxidation at the methyl group
can be raised by performing ozonization in the
presence of variable-valence metals at 100°C [2]. The
maximum yield of benzoic acid (71%) is achieved with
Co(II) acetate. A less effective catalyst is Mn(II)
acetate, in whose presence the yield of benzoic acid
does not exceed 32%. The oxidation selectivity in the
presence of Mn(II) acetate is low because the activity
of Mn(IV) formed in the course of ozonization is low
and it cannot compete under catalytic conditions with
the reaction of disintegration of the aromatic system.
At the same time, it is known that, in catalytic
ozonization of nitrotoluenes in acetic anhydride in the
presence of sulfuric acid additive, only Mn(III) is
formed in the reaction mixture and oxidation occurs
with high selectivity even at a temperature of 20°C
[3, 4]. Of certain interest in this context is a study of
the oxidation of toluene by ozone in acetic acid in the
presence of Mn(II) acetate and sulfuric acid.
The amount of ozone consumed in catalytic
oxidation conditions for formation of the carboxy
group is 93% of that theoretically required. Cutting off
the supply of ozone to the reaction mixture results in
that Mn(III) is reduced to Mn(II) and the oxidation
process is terminated. Thus, the role of ozone consists
in permanent generation of the oxidized form of the
metal that can selectively interact with the methyl
group.
The experimental data obtained in the study and
published evidence suggest the following scheme of
toluene oxidation [2–4]:
Products of oxidation of the methyl group
ArCH3 + O3
(1)
Ozonolysis products
H+
Mn3+ + НО. + О2,
ArСH2. + Mn2+ + H+,
ArСH2ОО.,
(2)
(3)
Mn2+ + O3
ArСH3 + Mn 3+
With sulfuric acid used in ozonization f toluene in
acetic acid in the presence of Mn(II) acetate, benzoic
acid could be obtained in 82.5% yield (Fig. 1). Benzyl
alcohol and benzaldehyde are formed as intermediates
in the reaction mass. As in the case of oxidation in
acetic anhydride, 90% of manganese is in the trivalent
state under the experimental conditions of the study
(Fig. 1, curve 5).
ArСH2. + O2
(4)
(5)
ArСH2ОО. + O3
ArСH2О. + 2O2,
H+
ArСH2О. + Mn2+
ArСH2ОО.
ArCH2ОН + Mn3+,
(6)
(7)
Molecular products.
984

CATALYTIC OXIDATION OF TOLUENE BY OZONE
985
c, M
In the absence of a catalyst, the main reaction
pathway is the destruction of the aromatic ring [reac-
tion (1)] (Table 1, run no. 1). Introduction of Mn(II)
acetate into the reaction zone makes it possible, owing
to the faster reaction (2), to preclude direct interaction
of ozone and toluene, with the methyl group oxidized
by a reaction with Mn(III) (Table 2).
Because the rate constant of the interaction of
ozone with toluene substantially exceeds that of its
interaction with the oxidized form of the catalyst, the
maximum yield of benzoic acid is possible at
comparable concentrations of toluene and catalyst
(Table 1, run no. 6).
The study demonstrated that sulfuric acid not only
favors formation of Mn(III) in the system, but also
affects the yield of the reaction product (Table 1, run
nos. 6, 10–13). The yield of benzoic acid increases
with the H2SO4 concentration primarily because the
activity of the oxidized form of the metal in the
reaction with toluene grows (Fig. 2) [the rates of
interaction of ozone with Mn(II) acetate and ozone
with toluene are independent of the sulfuric acid
concentration in the range studied]. At sulfuric acid
concentrations higher than 1.2 M, the consumption rate
of toluene decreases (Fig. 3) and the yield of benzoic
acid continues to grow (Table 1, run nos. 12 and 13).
τ, min
Fig. 1. Kinetics of (1) toluene consumption and (2–5)
accumulation of (2) benzoic acid, (3) benzyl alcohol, (4)
benzaldehyde, and (5) Mn(III) in ozonization in acetic acid
in the presence of Mn(II) acetate and sulfuric acid at 60°C.
[ArCH3] = 0.35 M, [O3] = 4 × 10–4 M, [H2SO4] = 1.2 M,
[Mn(OAc)2] = 0.14 M. (c) Concentration and (τ) reaction
duration; the same for Figs. 2–4.
This occurs because the activity of Mn(III) increases at
high concentrations of strong acids to such an extent
that the reduction of trivalent manganese in the
reaction with the solvent becomes noticeable (Fig. 2,
Table 1. Oxidation of toluene by ozone–air mixture. [ArCH3] = 0.35 M, [O3] = 4 × 10–4 M, gas flow rate 6.0 × 10–3 l s–1
[Mn(ІІ)]
[Н2SO4]
Yield of benzoic
acid, %
Run no.
Т, °С
τ, min
M
1
2
–
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
–
60
60
60
60
60
60
20
40
60
60
60
60
60
140
120
110
100
90
11.2
41.8
64.2
74.1
78.2
82.5
70.6
75.4
24.6
68.3
76.1
84.7
86.8
0.02
0.05
0.08
0.1
3
4
5
6
0.14
0.14
0.14
0.14
0.14
0.14
0.14
0.14
75
7
90
8
80
9
130
100
85
10
11
12
13
0.4
0.8
1.6
2.0
95
125
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 6 2011

986
POTAPENKO, ANDREEV
c, M
c, M
τ, min
τ, min
Fig. 3. Kinetics of Mn(III) consumption in acetic acid at
various sulfuric acid concentrations. [ArCH3] = 0.35 M,
temperature 20°C. [H2SO4] (M): (1) 0 (2) 0.4, (3) 0.8,
(4) 1.2, (5) 2.0, and (6) 2.0 (in the absence of toluene).
Fig. 2. Kinetics of toluene consumption at various sulfuric
acid concentrations at 60°C. [ArCH3] = 0.35 M, [O3] = 4 ×
10–4 M, [Mn(OAc)2] = 0.14 M. [H2SO4] (M): (1) 1.2,
(2) 1.6, and (3) 2.0.
curve 6), together with the process of toluene oxida-
tion.
substitution of AcO– situated in the inner coordination
sphere with a substrate molecule.
It is known that the catalytic activity of variable-
valence metal ions increases in the presence of alkali
metal bromides [5]. As can be seen from the ex-
perimental data (Fig. 4), use of sodium bromide made
it possible to accelerate the consumption of toluene
and raise the yield of benzoic acid to 93.4%.
Benzaldehyde and trace amounts of benzyl bromide
were also found in the reaction mixture.
Consequently, with the ratio [Br–]/[Mn2+] increase-
ing to above 1.1, manganese forms a complex with a
higher content of bromine so that the rate at which
toluene is involved in oxidation by reaction (11) de-
creases, and the fraction of methylbenzene under-
going destruction grows.
The fact that an excess amount of Br– in the starting
mixture is necessary is accounted for by the carryover
The increase in the catalytic activity of manganese
acetate in the presence of NaBr is due to the formation
of a manganese bromide complex Mn2+Br– [5], which
can involve a toluene molecule into oxidation at a
higher rate, compared with Mn3+ (Table 2):
c, M
Mn2+ + Br– → Mn2+Br–,
(8)
Н+
Mn2+Br– + O3
Mn2+Br· + НО· + О2,
(9)
→
Mn3+ + Br– → Mn2+Br·,
(10)
The yield of benzoic acid is the highest at [Br–]/
[Mn2+] = 1.1 (Table 3). The observed decrease in the
yield at higher contents of Br– is in good agreement
with the data of [6], according to which a number of
complexes are formed, depending on the molar ratio
between the metal and bromine, with the catalytic
activity being the highest for metal monobromide. In
the opinion of the authors of [6], this is due to the
increased mobility of this complex, which facilitates
τ, min
Fig. 4. Kinetics of (1) toluene consumption and (2, 3)
accumulation of (2) benzoic acid and (3) benzaldehyde in
ozonization in acetic acid in the presence of Mn(II) acetate,
NaBr, and sulfuric acid at 60°C. [ArCH3] = 0.35 M, [O3] =
4 × 10–4 M, [H2SO4] = 1.2 M, [Mn(OAc)2] = 0.11 M,
[NaBr] = 0.1 M.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 6 2011

CATALYTIC OXIDATION OF TOLUENE BY OZONE
987
Table 3. Oxidation of toluene by ozone–air mixture. [ArCH3] =
Table 2. Kinetic parameters of elementary reactions in
catalytic oxidation of toluene by ozone in acetic acid in the
presence of sulfuric acid. [H2SO4] = 1.2 M
0.35 M, [O3] = 4 × 10–4 M, gas flow rate 6.0 × 10–3 l s–1
Yield of
benzoic acid,
%
[Mn(ІІ)]
[NaBr]
Oxidation
duration, min
k, M–1, at indicated T, °C
M
Е,
Reaction
kJ mol–1
0.1
0.1
0.1
0.1
0.14
0.7
0.03
0.07
0.11
0.15
0.11
0.11
60
50
40
55
55
65
75.4
86.1
94.3
85.6
87.5
73.2
20
0.78
60
5.2
1
2
3
38.5
24.2
47.8
1.3×102
7.2×10–3
4.3×102
7.6×10–2
0.23
10
1.9×10–2
50.6
of molecular bromine formed in the course of
oxidation and by the loss of bromine for formation of
benzyl bromide.
CONCLUSIONS
(1) The main reaction pathway in oxidation of
toluene in the system constituted by acetic and sulfuric
acids is the destruction of the aromatic ring, with the
yield of benzoic acid not exceeding 11.2%.
EXPERIMENTAL
Toluene was oxidized in a hermetically sealed
thermostated glass reactor equipped with a fast turbine
stirrer and a bubbler 3 mm in diameter. With the stirrer
working, the reactor was charged with 0.05 l of glacial
acetic acid and calculated amounts of toluene, catalyst,
and a strong acid, and an ozone–air mixture containing
4 × 10–4 M of ozone was delivered into the reactor. At
a stirrer speed of 29.2 s–1 and the ozone–air mixture
delivered at a rate of 6.0 × 10–3 l s–1, the oxidation
occurred under a kinetic control. The ozone concentra-
tion in the gas phase was determined spectrophoto-
metrically from the absorption in the spectral range
254–259 nm, and the Mn(II) concentration in the
reaction mixture, by the iodometric method. The
variation of the concentrations of toluene and its oxida-
tion products was monitored by gas-liquid chromato-
graphy with a flame-ionization detector on 3 m × 2 mm
column packed with Inerton AW-DMCS support on
which an SE-30 stationary phase is immobilized in an
amount of 5% relative to the support mass under the
following conditions: thermostat temperature increase-
ing by a program in the range 100–200°C in 10 min;
flow rates of carrier gas (nitrogen), hydrogen, and air
equal to 1.8, 1.8, and 18 l h–1, respectively. As an
internal standard served 4-nitrochlorobenzene.
(2) Use of manganese(II) acetate as a catalyst
makes it possible to raise the yield of the aromatic acid
to 82.5%.
(3) Introduction of sodium bromide into the
catalytic system raises both the oxidation rate of
toluene and the yield of benzoic acid.
REFERENCES
1. Galstyan, G.A. and Pluzhnik, I.M, Vіsn. Skhіdnouk-
raїns’k, Derzhavn. Unіver., 1999, vol. 20, no. 4, pp. 57–
60.
2. Pluzhnik, I.M. and Galstyan, G.A., Neftekhimiya, 1999,
vol. 39, no. 2, pp. 120–123.
3. Potapenko, E.V., Galstyan, G.A., Galstyan, A.G., and
Kudyukov, Yu.P., Kinet. Katal., 2001, vol. 42, no. 6,
pp. 872–875.
4. Potapenko, E.V. and Galstyan, G.A., Zh. Prikl. Khim.,
2000, vol. 73, no. 7, pp. 1218–1220.
5. Galstyan, G.A., Reactions of Alkylbenzenes and Their
Oxygen Derivatives with Ozone, Doctoral Dissertation,
Lviv, 1992.
6. Gaevskii, V.F., Evmenenko, N.P., and Budarin, L.N.,
Kinet. Katal., 1976, vol. 17, no. 2, pp. 353–356.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 6 2011