KINETICS AND MECHANISM OF THE CATALYTIC REACTION OF OZONE
213
С
, mol/l
ArCH3 + Mn(III)
• H2 + O2
ArCH2O•2 + Mn(II) + Н+
2 ArCH2O•2
ArCH2OH + Ас2О + Н+
ArCH2СHО + Ас2О + Н+
→
ArC
• H2 + Mn(II) + Н+, (1)
ArCH2O•2,
0.4
1'
Ar
C
→
(2)
6
1
→
ArCH2O2Н + Mn(III),(3)
0.3
→
ArCH2OH + ArCHO + О2,
(4)
2
0.2
0.1
→
ArCH2OАс + АсОН, (5)
3
→
ArCH(OАс)2.
(6)
4
Benzyl alcohol and benzaldehyde are converted
into benzyl acetate and benzylidene diacetate, which
are relatively stable against oxidation at the moment of
their formation (reactions (5) and (6)).
In contrast to oxygen, ozone oxidizes toluene in
the presence of manganese acetate under the experiꢀ
5
10
20
30
40
50
60
70
, min
τ
Fig. 1. Change in the concentration of the components of
the reaction mixture during toluene oxidation with ozone
in acetic anhydride at 5°С: [ArCH ] = 0.4; [O ] = 4.0 ×
mental conditions at a high rate (at
10–4 mol/l; 10–4 mol/(l s)) and with high
exp = 2
selectivity for the methyl group (Fig. 1; curves ).
5
°
С
and [O3]o = 4
×
w
×
3 о 3 о
–4
ꢀ1
1,
6
10 ; [Mn(ОАс) ] = 0.06; [H SO ] = 0.8 mol l , and
2 4 о
2 о
ꢀ3 ꢀ1
v
= 8.3
×
10 l s ; (
1
) toluene, (1') toluene (after oxidaꢀ
) benzyl acetate, ( ) benzaldeꢀ
) benzylidene diacetate, and ( ) benzoic acid;
6) the total concentration of the products of toluene sideꢀ
The rate and selectivity of toluene oxidation increase
with the increasing ozone concentration (Fig. 2).
Ozone must be introduced into the system continuꢀ
ously. The process slows down as the ozone supply
stops, and passes to the regime of oxidation with dioxꢀ
ygen (Fig. 1, curve 1'); manganese occurs predomiꢀ
nantly in the reduced form under these conditions.
An analysis of the data obtained has shown that this
behavior is observed only in the presence of sulfuric
acid and results from a change in the wо : w1 : w7 ratio.
air
tion with air oxygen), (
2
3
hyde, (
4
5
(
chain oxidation, excluding benzoic acid.
Benzyl acetate (42.0%), benzylidene diacetate
(17.5%), benzaldehyde (30.0%), and traces of benzyl
alcohol were found among the products of toluene
oxidation. Benzoic acid accumulates during the
exhaustive oxidation of toluene in the oxidation mixꢀ
ture (Fig. 1). The form of the rate curves shows (Fig. 2)
that the oxidation develops without an induction
period; the acylated derivatives of benzyl alcohol and
benzaldehyde are formed by parallel pathways, being
intermediate products of oxidation of toluene to benꢀ
zoic acid. The exception is the benzaldehyde case; the
Sꢀshaped form of benzaldehyde buildup rate curves
suggests that this product is formed from benzylidene
diacetate as a result of its hydrolysis in the presence of
water of the reaction (Fig. 2). Notably, the introducꢀ
tion of the hydroxy group into the benzene ring of tolꢀ
uene leads to the preferential formation of the correꢀ
sponding benzyl acetates under the given experimental
conditions [7, 8].
If manganese acetate is introduced into the system
in the reduced form, the products of oxidation at the
methyl group accumulate after a short (10 min) inducꢀ
tion period. The time of attaining the maximum forꢀ
mation rate for the acylated derivatives of alcohol and
aldehyde coincides with the time of transition of
Mn(II) to Mn(III), the point at which the solution
acquires a dark pink color. This implies that the initiꢀ
ating effect of ozone is due to the emergence in the
system of a new fast reaction (7) of formation of an
active catalyst species in the form of Mn(III):
If the oxidation is carried out in the absence of sulfuric
acid, the change in ozone concentration over the
range examined ((0.5–4.0)
not affect the oxidation selectivity, since w7 is always
much higher than w1 in these conditions w7 1 = 200);
×
10–4 mol/l) almost does
(
:
w
as a result, manganese acetate in the solution occurs
predominantly in the oxidized form and toluene is oxiꢀ
dized at the methyl group (at [O3]o = 4
×
10–4;
k
1 = 10–2;
×
[ArCH3]o = 0.4; [Mn(OAc)2]o = 0.06 mol/l,
k
7 = 2
10⎯4mol/(l s); i.e., w7
10⎯4 mol/l, w7
w1 = 62.5).
×
103 l/(mol s),
w
1 = 2
7 = 4.8
×
×
10–2; w1 = 2.4
:
w
102. At [O3]o = 1.25
×
:
The value k7 decreases to 19.2 l/(mol s) in the presꢀ
ence of sulfuric acid (at 5°C), thereby significantly
lowering the Mn(III) formation rate (reaction (7)).
Under these conditions, w7 : w1 = 0.6–1.9; i.e., in
many cases, the rate of manganese reduction (reaction
(1)) exceeds the Mn(II) oxidation rate (reaction (7)).
It is known that in the system, in which both oxidized
and reduced forms of manganese are present, a binuꢀ
clear complex with a considerably decreased redox
potential is formed, whose catalytic activity is lower
than that of Mn(III) [5]. For this reason, under the
above conditions, with the decreasing ozone concenꢀ
tration in the ozone–air mixture, the ratio wо : w1
increases, resulting in a decrease in the oxidation
selectivity at the methyl group.
О3 + 2Mn(II) + 2Н+
→
2Mn(III)+ H2О + О2. (7)
PETROLEUM CHEMISTRY Vol. 51
No. 3
2011