Kinetics and mechanism of liquid-phase catalytic ozonation of nitrotoluenes

Article abstract of DOI:10.1134/S0965544106050100

The kinetics of catalytic oxidation of nitrotoluenes with ozone was studied. It was shown that the introduction of transition metal salts into the system makes it possible to prevent almost completely the ozonolysis of the aromatic ring and to direct the process towards methyl-group oxidation yielding benzoic acids. A mechanism of the process that explains the experimental findings was proposed. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S0965544106050100

ISSN 0965-5441, Petroleum Chemistry, 2006, Vol. 46, No. 5, pp. 362–366. © Pleiades Publishing, Inc., 2006.
Original Russian Text © A.G. Galstyan, 2006, published in Neftekhimiya, 2006, Vol. 46, No. 5, pp. 391–395.
Kinetics and Mechanism of Liquid-Phase Catalytic Ozonation
of Nitrotoluenes
A. G. Galstyan
East-Ukrainian National University, Rubezhnoe Branch, Rubezhnoe, Ukraine
Received October 13, 2005
Abstract—The kinetics of catalytic oxidation of nitrotoluenes with ozone was studied. It was shown that the
introduction of transition metal salts into the system makes it possible to prevent almost completely the ozonol-
ysis of the aromatic ring and to direct the process towards methyl-group oxidation yielding benzoic acids. A
mechanism of the process that explains the experimental findings was proposed.
DOI: 10.1134/S0965544106050100
Earlier it was shown [1] that toluene ozonation in as absorption in the range 254–290 nm. Oxidation
acetic acid predominantly led to the ozonolysis of the products in the solution were identified and quantified
aromatic ring and that the oxidation selectivity for the by GLC on a LKhM-80 chromatograph with a flame-
methyl group was 16%. The introduction of the elec- ionization detector and a column (2 m in length and
tron-withdrawing nitro group into the toluene molecule 4 mm in inner diameter) packed with Chromaton
increases the stability of the aromatic ring towards N-AW coated with the SE-30 stationary phase, using
ozone electrophilic attack; however, the destructive the following conditions: an evaporator temperature of
oxidation of the compound remains the main route of 250°ë; an oven temperature of 180°ë; and carrier gas
the process as before, and the yield of aromatic prod- (nitrogen), hydrogen, and air flow rates of 1.8, 1.8, and
ucts does not exceed 24% [2]. It is known [3] that the 18 l/h, respectively. Nitrobenzene was used as an inter-
selectivity for side-chain oxidation in ozonation of nal standard. The catalyst concentration, in particular,
methylbenzenes increases with an increase in tempera- that of Co(III) acetate, in the reaction mixture was
ture, as well as in the presence of oxidation catalysts— determined iodometrically against a blank sample lack-
transition metals—in the system. However, the mecha- ing the aromatic products (before sampling, the reac-
nism of catalysis of nitrotoluene oxidation is practically tion mixture was purged with nitrogen to remove dis-
unknown. In this work, the kinetics of this reaction in solved ozone). The effective rate constants of ozone
glacial acetic acid in the presence of transition metal reaction with Co(II) and nitrotoluenes were determined
salts was investigated with the aim of revealing the role spectrophotometrically according to the procedure
of the catalyst in nitrotoluene ozonation.
described in [4].
The rate constant of Co(III) reaction with nitrotolu-
enes was determined graphically, assuming the case of
irreversible second-order reactions [5]. Experiments in
EXPERIMENTAL
Glacial acetic acid (analytical grade) used in the the presence of Mn, Pd, and Cr acetates were carried
experiments was additionally purified by vacuum distil- out in a similar manner.
lation in the presence of potassium permanganate. Liq-
uid 2- and 3-nitrotoluenes were purified on an alumina-
RESULTS AND DISCUSSION
packed column, and crystalline 4-nitrotoluene was
purified by multiple recrystallization from water. Ana-
lytical-grade metal acetates were used without prelimi-
nary purification.
Data on the transition metal-catalyzed oxidation of
nitrotoluene isomers with ozone in acetic acid are given
in Table 1. It is seen that the presence of the catalyst
The experiments were carried out in a glass column prevents to a considerable extent the ozonolysis of the
with a porous membrane at 30–100°ë. The column was aromatic ring and the selective oxidation at the methyl
loaded with 20 ml of glacial acetic acid, 0.5 mol/l of group becomes the main reaction route. The selectivity
nitrotoluene, and a calculated amount of a catalyst; the of oxidation depends on the redox potential of an
column was then thermostated, and an ozone–air mix- åÂn + 1/åÂn+ pair and reaches the maximum value in
ture containing 4 × 10–4 mol/l of ozone was passed the presence of cobalt and manganese salts (Table 2).
through at a flow rate of 30 l/h after establishing steady- Note that the main oxidation products are the corre-
state operation of the ozonizer. The amount of ozone in sponding nitrobenzoic acids regardless of the metal
the gas phase was determined spectrophotometrically nature. Using 4-nitrotoluene oxidation in the presence
362

KINETICS AND MECHANISM OF LIQUID-PHASE
363
Table 1. Oxidation of nitrotoluenes with ozone in the pres-
ence of Co(II) acetate at 100°C, [O2NArCH3]0 = 0.5, and
[Co(OAc)2]0 = 0.15 mol/l
of Co(II) acetate as an example, we observed an
increase in the initial rate of substrate oxidation and in
the yield of 4-nitrobenzoic acid with an increase in the
catalyst concentration (Fig. 1).
Figure 2 presents kinetic data that have been
obtained for 3-nitrotoluene oxidation with ozone in the
presence of cobalt(II) acetate at 100°ë. Intermediate
oxidation products are 3-nitrobenzaldehyde and
3-nitrobenzyl alcohol, and the final product is
3-nitrobenzoic acid. The same kinetics was observed
for the oxidation of 4-nitrotoluene. In the case of 2-
nitrotoluene, the oxidation selectivity for the methyl
group was lower, presumably, owing to the steric hin-
drance of the substituent (Table 1).
Initial oxidation Nitrobenzoic acid
Compound
rate, mol/(l s)
yield, wt %
2-Nitrotoluene
3-Nitrotoluene
4-Nitrotoluene
5.7 × 10–5
7.2 × 10–5
7.4 × 10–5
75.2
94.3
95.6
Table 2. Influence of catalyst nature on the selectivity of 4-ni-
trotoluene oxidation at 100°C; t = 4 h, [O2NArCH3]0 = 0.5;
[M(OAc)2]0 = 0.15 mol/l
In the presence of ozone, a steady-state concentra-
tion of Co(III) ions is established during the first 30–
40 min of oxidation; this time coincides with that of the
achievement of the maximum rate of nitrobenzoic acid
formation. However, when nitrotoluene was introduced
into a system that had been preliminarily ozonized until
the complete conversion of cobalt into the trivalent
state, the formation of nitrobenzoic acid began immedi-
ately at the maximum rate. The characteristic feature of
catalytic oxidation is that the reaction quickly stops if
ozone supply to the oxidation zone is terminated; under
these conditions, the green solution takes a pink color,
thus indicating the rapid reduction of Co(III) to Co(II)
(Fig. 2). The consumption of ozone to yield 1 mol of
4-nitrobenzoic acid is 87% of the theoretically required
amount of oxidant (Table 3). In the case of toluene oxi-
dation, the consumption of ozone does not exceed 33%
[7]. This gives grounds to believe that, upon ozonation
of methylbenzenes that bear electron-withdrawing sub-
stituents on the ring, the oxidation is primarily effected
by the ozone molecule itself, unlike the case of toluene,
in which the oxidant is dioxygen and ozone mainly ini-
tiates the oxidation.
Maximum yield
of 4-nitrobenzoic
acid, wt %
298
eq
E
Mn/Mn + 1
[6]
Catalyst
Co(II)
Mn(II)
Pd(II)
Cr(II)
1.810
1.510
0.987
0.740
95.6
36.4
31.2
27.9
.
O2NÄrCH3 + Co(III)
O2NArCH2 + Co(II) + H+,(7)
.
.
O NAr H + é
O2NÄrCH2O2 ,
(8)
C
2
2
2
O2NÄrCH2O2ç + ëÓ(II)
.
O2NÄrCH2O + ëÓ(III) + çé–
,
(9)
Yeld, %
100
W × 10–5
10
The initial rate of nitrotoluene oxidation in the reac-
tions in question is first-order in substrate, Co(II) ace-
tate, and ozone and is practically independent of diox-
ygen concentration (Fig. 3):
80
60
8
6
d[O2NArCH3]
-----------------------------------
–
dτ
(1)
= k[O2NArCH3]0[Co(OAc2)]0[O3]0.
40
20
4
2
On the basis of the obtained experimental data and
published data on the catalytic oxidation of alkylaro-
matic compounds [8–10], the following nitrotoluene
oxidation scheme may be suggested:
.
.
0.04
0.08
0.12
0.16
O2NÄrCH3 + O3
O2NÄrCH3 + O3
O2NÄrCH3 + O3
O2NArCH2 + HO + O2, (2)
[Cat], mol/l
O2NArCH2OH + O2,
(3)
(4)
ozonolysis products,
Fig. 1. Dependence of the 4-nitrotoluene oxidation rate
.
and the 4-nitrobenzoic acid yield on the concentration of
é3 + ëÓ(II) + ç+
ëÓ(III) + HO + é2,
(5)
Co(II) acetate at 100°C, [ëÓ(éÄÒ) ] = 0.15 mol/l,
2 0
.
[O NArCH ] = 0.5 mol/l, and a space velocity of
.
2
3 0
–1
ëç3ëééç + HO
CH2ëééç + ç2é, (6)
ozone–air mixture of 0.37 s
.
PETROLEUM CHEMISTRY Vol. 46 No. 5 2006

364
C, mol/l
GALSTYAN
–4.0
–3.9
–3.7
–3.5
log[O3]
0.5
0.4
0.3
0.2
0.1
2
3
1
2
1
–4.2
–4.4
–4.6
–4.8
–5.0
5
3
4
–1.6
–0.7
–1.4
–0.5
–1.2
–0.3
–1.0 log[Co(II)]
–0.1 log[ArCH3]
1
2
3
4
τ, h
–0.9
Fig. 2. Kinetics of 3-nitrotoluene oxidation with ozone in
acetic acid in the presence of cobalt(II) acetate at 100°C,
[ëÓ(éÄÒ) ] = 0.15 mol/l, [O NArCH ] = 0.5 mol/l, and
2 0
2
3 0
Fig. 3. Dependence of the 2-nitrotoluene oxidation rate on
the concentration of (1) cobalt(II) acetate, (2) ozone, and (3)
–1
an ozone–air mixture space velocity of 0.37 s : (1) 3-nitro-
tolune, (2) 3-nitrobenzoic acid, (3) 3-nitrobenzaldehyde, (4)
3-nitrobenzyl alcohol, and (5) Co(III) acetate. (The arrow
indicates the time of cessation of ozone supply to the sys-
tem; the dashed lines represent oxidation kinetic curves as
measured 2 h after stopping the ozone supply.)
2-nitrotoluene at 100°C, [ëÓ(éÄÒ) ] = 0.15 mol/l,
2 0
[O NArCH ] = 0.5 mol/l.
2
3 0
methyl group depends on the ratio of the rates of reac-
tions (4), (5), and (7). The data presented in Table 4
show that the rate constant of ozonolysis reaction (4) of
nitrotoluenes exceeds by many times the rate constant
of reaction (7) of their oxidation with Co(III). Hence, in
the presence of cobalt(II) acetate, which is rapidly con-
verted into the oxidized form in reaction (5) with
ozone, the oxidation of nitrotoluene isomers without
destruction of the aromatic system is possible only at
commensurable concentrations of the metal salt and the
substrate, which is in fact the case observed experimen-
tally (Fig. 2).
.
O2NÄrCH2O2 + O2NÄrCH3
.
(10)
O2NArCH2O2H + O2NArCH2,
.
O2NÄrCH2O2 + ëÓ(II) + ç+
(11)
O2NÄrCH2O2ç + ëÓ(III),
.
2O2NÄrCH2O2
products.
(12)
Under noncatalytic oxidation conditions, nitrotolu-
enes are mainly consumed via reactions (2)–(4). In the
presence of Co(II) acetate, the two-stage oxidation by
ozone begins to play a determining role; in this process,
ozone predominantly reacts with the reduced form of
metal (reaction (5)) and the oxidized form of metal is
reduced in reaction (7) with the substrate. An analysis
of the obtained experimental data leads to the conclu-
sion that the selective oxidation of nitrotoluenes at the
In oxidation with an ozone–air mixture, the concen-
tration of dioxygen in the gas mixture is an order of
magnitude above that of ozone; therefore, the benzyl
radical formed via reaction (7) in the system reacts pre-
dominantly with an oxygen molecule (reaction (8)),
yielding arylperoxide radicals which further recombine
according to Eq. (12). The participation of arylperoxide
radicals in propagation reactions (10) and (11) under
the reaction conditions is negligible, as w12 : w11 : w10 ≈
2 × 103 : 20 : 1 (according to published data [11], w12 =
1.5 × 10–2, w11 = 9 × 10–4, and w10 = 8 × 10–6 mol/(l s) for
toluene at 70°C, k10 = 1.6, k11 = 6.1 × 102, k12 = 1.5 ×
Table 3. Ozone consumption in oxidation of nitrotoluenes
with an ozone-air mixture (see Table 2 for conditions)
108 l/(mol s), [O2NÄrCH3]0 = 0.5, [Co(OAc2)]0 = 0.15,
Ozone uptake per mole of nitrotoluene
Compound
.
and [O2NÄrCH2O2 ] ≈ 10–5 mol/l).
mole
% of theoretical amount
These calculations, and the experimental fact that
the selective oxidation is accomplishable only under
commensurable catalyst and substrate concentrations,
as well as the determining role of ozone in the reaction,
2-Nitrotoluene
3-Nitrotoluene
4-Nitrotoluene
4.88
3.33
3.27
130.2
89.3
87.1
PETROLEUM CHEMISTRY Vol. 46 No. 5 2006

KINETICS AND MECHANISM OF LIQUID-PHASE
365
Table 4. Reaction rate constants for the catalytic cycle of oxidation of nitrotoluenes with ozone at 100°ë [ëÓ(éÄÒ)2]0 =
0.15 mol/l; [é2NArCH3]0 = 0.5 mol/l
Reaction no.
Reaction
keff, l/(mol s)
Initial reaction rates, mol/(l s)
4
4
4
5
7
7
7
O3 + 2-nitrotoluene
+ 3-nitrotoluene
+ 4-nitrotoluene
Co2+ + O3
Co3+ + 2-nitrotoluene
+ 3-nitrotoluene
+ 4-nitrotoluene
0.38
0.7 × 10–4
0.9 × 10–4
0.9 × 10–4
5.8 × 10–2
3.0 × 10–4
7.9 × 10–4
9.5 × 10–4
0.45
0.46
9.3 × 102
0.0041
0.0108
0.0130
testify that the process follows the radical ion mecha- acid. The absorption spectrum of the dissolved precipi-
nism. According to this mechanism, arylperoxide radi- tate corresponds to MnO2.
cals recombine, yielding oxidation products and retain-
ing the aromatic structure (reaction (12)).
Thus, the decrease in the selectivity of oxidation of
nitrotoluenes in the presence of manganese(II) acetate
The above assumption that Co(III) regeneration with an increase in temperature is explained by a
mainly occurs via reaction (5) of Co(II) with ozone is decrease in the equilibrium concentration of Mn(IV)
entirely consistent with the calculated kinetic data. ions as a result of the formation of manganese dioxide,
Under experimental conditions at k5 = 9.3 × 102 l/(mol s)
which is insoluble under the reaction conditions.
(Table 4), [Co(OAc2)]0 = 0.15, and [é3]0gas = 4 ×
In agreement with the experimental and published
data [14, 15], it may be assumed that MnO2 formation
10−4 mol/l, w5 is 5.8 × 10–2 mol/(l s) and the value of w11
in the catalyzed reaction of ozone with nitrotoluenes is
calculated above is 9 × 10–4 mol/(l s). It is logical that
due to the accumulation of reaction water according to
the cessation of ozone supply to the system at w5/w11 =
the scheme
Mn(II) + O3 + 2H+
Mn(IV) + Mn(II)
Mn(III) + O3 + 2H+
65 stops the process.
Mn(III) + O2 + H2O, (13)
2Mn(III), (14)
All these results suggest that the reaction of ozone
with nitrotoluenes in the presence of cobalt(II) acetate
follows the (5)–(7)–(8)–(12) route.
Mn(IV) + O2 + H2O, (15)
Note that manganese(II) acetate exhibits the same
catalytic activity at 30°ë as cobalt(II) acetate; oxida-
tion of nitrotoluenes in its presence follows a similar
scheme. However, the activity of manganese gradually
declines with an increase in temperature, so that the
yield of 4-nitrobenzoic acid falls down to 36% at 100°ë
(Table 5), which is almost 3 times lower than in the
presence of cobalt(II) acetate and only 12% higher than
in the case of noncatalytic oxidation.
.
O2NArCH3 + Mn(IV)
+ Mn(III) + H+,
O2NArCH2
(16)
O2NArCH3 + Mn(III)
.
O2NArCH2 + Mn(II) + ç+,
(17)
(18)
(19)
Mn(II) + H2O
MnOH+ + O3
MnOH+ + H+,
MnO2 + O2.
To explain these peculiarities, we studied the behav-
ior of manganese acetate under the conditions of
4-nitrotoluene oxidation with ozone at different tem-
peratures (Fig. 4). Preliminary experiments showed
that Mn(II) in acetic acid is oxidized by ozone to
Mn(IV) (similar results were reported by other
researchers [12, 13]). At 30°ë, Mn(II) transforms into
Mn(IV) within 10 min, and the system reaches an equi-
librium at which manganese occurs predominantly in
the oxidized form Mn(IV) for the entire length of the
experimental run. An increase in temperature decreases
the equilibrium Mn(IV) concentration—the higher the
temperature, the stronger the concentration fall.
In summary, the selectivity of oxidation of nitrotolu-
enes by ozone at the methyl group yielding nitrobenzoic
acids increases in the presence of transition metal com-
Table 5. Temperature dependence of 4-nitrobenzoic acid yield
in the presence of transition metals; [O2NArCH3]0 = 0.5 mol/l;
[M(OAc)2]0 = 0.15 mol/l
Yield of 4-nitrobenzoic acid, wt %
T, °C
Co(II) acetate
Mn(II) acetate
30
50
56.8
62.7
79.5
95.6
53.4
44.9
39.6
36.7
As the equilibrium Mn(IV) concentration decreases
in the system, a dark brown solid precipitates, which is
insoluble in water or glacial acetic acid, is slightly sol-
uble in sulfuric acid, and dissolves well in hydrochloric
70
100
PETROLEUM CHEMISTRY Vol. 46 No. 5 2006

366
C, mol/l
GALSTYAN
2. A. G. Galstyan, G. A. Galstyan, and N. F. Tyupalo,
Neftekhimiya 38, 147 (1998) [Pet. Chem. 38, 136
0.25
(1998)].
3. G. A. Galstyan, T. M. Galstyan, and L. I. Mikulenko,
0.20
0.15
Kinet. Katal. 35, 255 (1994).
4. S. D. Razumovskii and G. E. Zaikov, Ozone and Its
Reactions with Organic Compounds (Nauka, Moscow,
1974) [in Russian].
1
2
0.10
0.05
3
4
5. N. M. Emanuel and D. G. Knorre, Course of Chemical
Kinetics (Vysshaya Shkola, Moscow, 1969) [in Rus-
sian].
6. Yu. Yu. Lur’e, Analytical Chemistry Handbook
(Khimiya, Moscow, 1967) [in Russian].
1
2
3
4
τ, h
7. I. M. Pluzhnik and G.A. Galstyan, Neftekhimiya 39, 120
(1999) [Pet. Chem. 39, 102 (1999)].
Fig. 4. Influence of temperature on the concentration of
Mn(IV) ions in solution upon oxidation of 4-nitrotoluene
with ozone at T = (1) 30, (2) 50, (3) 70, and (4) 100°C;
[O NArCH ] = 0.5 mol/l; [ån(OAc) ] = 0.15 mol/l; and
[O3]gas = 4 × 10 mol/l.
8. N. M. Emanuel, E. T. Denisov, and E. K. Maizus, Liquid-
Phase Oxidation of Hydrocarbons (Nauka, Moscow,
1965; Plenum, New York, 1967).
2
3 0
2 0
–4
0
9. P. S. Bailey, Ozonation in Organic Chemistry (Aca-
demic, New York, 1982), Vol. 2, p. 497.
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lyst consists in the appearance of the metal in the higher
oxidation state after the reaction with ozone, and it this
form that involves the substrate molecule in the selective
oxidation at the methyl group. Exhibiting the same cata-
lytic activity as cobalt(II) acetate at low temperatures,
manganese(II) acetate gradually loses its activity with an
increase in temperature, since the active manganese form
Mn(IV) transforms into an inactive MnO2 precipitate.
10. G. A. Galstyan, N. F. Tyupalo, and S. D. Razumovskii,
Ozone and Its Liquid-Phase Reactions with Organic
Compounds (VUNU, Lugansk, 2004) [in Russian].
11. I. V. Zakharov and Yu. V. Galetii, Neftekhimiya 18, 615
(1978).
12. M. L. Perepletchikov, V. N. Tarunina, B. I. Tarunin, and
Yu. A. Aleksandrov, Zh. Obshch. Khim. 55, 487 (1985).
13. G. R. Hill, J. Am. Chem. Soc. 70, 1306 (1949).
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PETROLEUM CHEMISTRY Vol. 46 No. 5 2006