2234
BOIKOV et al.
of the reactor. In this work, we only considered the 1∆g
Yields of the main toluene oxidation products (%) and
1
amounts of generated ∆gO2 (c, molecules/g) on massive
singlet oxygen form because the 1Σg form was formed
in negligibly small amounts under our experimental
conditions [10].
oxides at tpc = 400°C (η is the conversion of toluene)
t, °C c × 10–15 η, %
V2O5 · MoO3
BAl
BAc
MA
COx
RESULTS AND DISCUSSION
300
400
500
0
17.3
2.7
0.0
0.0
1.1
12.5
9.4
0.9
28.1
23.2
12.7
59.2
67.1
33.9
56.1
100
100
The reaction on the catalysts studied occurred with
the formation of benzaldehyde (BAl), benzoic acid
(BAc), maleic anhydride (MA), and carbon oxides
(COx) as the major products. In addition, benzo-
quinone, anthraquinone, etc. were detected as impuri-
ties. Because of their insignificant amounts (yield less
than 1%), their contribution to the composition of the
product was ignored. The oxidation of toluene reached
stationary conditions immediately after the beginning
of the process. The activity and selectivity of the cata-
lysts remained unchanged for at least 8 h if the experi-
mental conditions did not change during the reaction.
V2O5
300
400
500
0
29.3
99.6
100
3.1
0.8
5.2
9.6
0.0
38.9
29.0
21.0
50.3
60.5
19.5
34.4
0.0
10.0
MoO3
0.2
300
400
500
0
1.2
8.9
1.0
2.3
8.0
0.0
0.0
0.0
0.0
2.8
8.7
0.45
0.55
3.8
47
30.0
According to the table, the ability to generate singlet
oxygen decreased in the series V2O5 · MoO3 > V2O5 >
MoO3. These results corresponded to the catalytic
activity of the catalysts in the oxidation of toluene; that
is, the catalytic activity of the samples correlated with
their ability to generate 1∆gO2. At 300°C, the generation
traps for liquid and gaseous products were changed.
Carbon balance in flows before and after the reactor
proved the completeness of trapping transformation
products. Under constant experimental conditions, the
activity and selectivity of catalysts did not decrease. No
catalyst regeneration was therefore performed.
1
of ∆gO2 was not observed, and conversion did not
exceed 30% on the most active vanadium–molybde-
num catalyst.
The liquid reaction products were analyzed by gas-
liquid chromatography on a Cambridge GC95 chro-
matograph (flame ionization detector) with a capillary
column 50 m long and a FFAP deposited phase under
programmed heating conditions (35°C, 10 min, and
35–150°C, 278 K/min). Gases were analyzed on a Kri-
stall-2000 gas chromatograph using a packed column
(activated carbon) 5 m long under programmed heating
conditions (35–160°C, 281 K/min). The content of
maleic anhydride in an aqueous solution of reaction
products was determined by titration with NaOH.
The amount of 1∆gO2 was determined by the chemi-
luminescent method [9]. A catalyst sample was placed
into a quartz reactor with low thermal inertia. At the
exit of the reactor, air was cooled to room temperature
and directed into a cell, where active oxygen selectively
reacted with a chemiluminescent dye. The intensity of
chemiluminescence was measured by a FEU-79 photo-
electron multiplier.
The fraction of deep toluene oxidation products
increased as the temperature of the catalytic process
grew. Already at 400°C, the products of oxidation on
the most active V2O5 · MoO3 sample did not contain
benzaldehyde, but the yields of benzoic acid, maleic
anhydride, and COx increased substantially. Simulta-
1
neously, the generation of ∆gO2 increased to a maxi-
mum (compared with the other samples).
The yield of benzoic acid at the same temperature
was somewhat lower in the presence of V2O5, and the
yield of maleic anhydride was noticeably higher than
on V2O5 · MoO3. The products obtained on vanadium-
containing catalysts at 500°C did not contain benzalde-
hyde; conversely, the degree of toluene conversion into
COx was high. At the same time, the yield of benzoic
acid decreased insignificantly on the vanadium–molyb-
denum catalyst and even increased on vanadium oxide.
On low-activity MoO3, the products of the oxidative
destruction of the toluene benzene ring were absent
over the whole temperature range (300–500°C), but the
yield of benzaldehyde increased, which is consistent
with the lower ability of this catalyst to generate the sin-
glet molecular oxygen form.
The amount of stabilized singlet oxygen was esti-
mated from its desorption into a flow passing above a
sample layer. The sample was preliminarily calcined at
500°C for 1 h in a flow of air at a residual pressure of
1 kPa, rapidly cooled to minus 60°C, and again heated
to 200–300°C. The amount of singlet oxygen desorbed
from the sample into the air flow was measured at the
exit of the reactor. The generation of singlet oxygen
Additional evidence in favor of correlation between
1
catalytic activity and the ability to generate ∆gO2 is
was studied by heating a catalyst sample in a flow of air provided by the experimental data on the influence of
to the required temperature at a rate of 293 K/min, and the temperature of preliminary calcining on these pro-
the amount of singlet oxygen was determined at the exit cesses (Fig. 1). At a 400°C temperature of preliminary
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 13 2008