Singlet oxygen in the heterogeneous catalytic oxidation of benzene

Article abstract of DOI:10.1134/S0036024407060052

The partial oxidation of benzene on vanadium and molybdenum oxides and the participation of singlet oxygen in this reaction were studied. A correlation between the activity of the catalysts in the partial oxidation of benzene and their ability to generate singlet oxygen was observed. The character of the initiation of oxidative benzene transformations under the action of molecular singlet oxygen was analyzed. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036024407060052

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2007, Vol. 81, No. 6, pp. 861–865. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © E.V. Boikov, M.V. Vishnetskaya, A.N. Emel’yanov, Yu.N. Rufov, N.V. Shcherbakov, I.S. Tomskii, 2007, published in Zhurnal Fizicheskoi Khimii, 2007,
Vol. 81, No. 6, pp. 993–997.
CHEMICAL KINETICS
AND CATALYSIS
Singlet Oxygen in the Heterogeneous Catalytic Oxidation of Benzene
a
a
a
b
a
E. V. Boikov , M. V. Vishnetskaya , A. N. Emel’yanov , Yu. N. Rufov , N. V. Shcherbakov ,
a
and I. S. Tomskii
a
Gubkin State University of Oil and Gas, Leninskii pr. 65, Moscow, 117917 Russia
b
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 117977 Russia
e-mail: ev-boykoff@yandex.ru
Received April 7, 2006
Abstract—The partial oxidation of benzene on vanadium and molybdenum oxides and the participation of sin-
glet oxygen in this reaction were studied. A correlation between the activity of the catalysts in the partial oxi-
dation of benzene and their ability to generate singlet oxygen was observed. The character of the initiation of
oxidative benzene transformations under the action of molecular singlet oxygen was analyzed.
DOI: 10.1134/S0036024407060052
INTRODUCTION
For this reason, the authors of [8], who performed
the reduction of the vanadium–molybdenum catalyst
The partial oxidation of hydrocarbons is the main with benzene in the absence of oxygen by the pulsed
method for the preparation of valuable oxygen-contain- method, came to the conclusion that maleic anhydride
ing products, such as organic acids and their anhy- was formed on more oxidized catalyst active centers
drides, olefin oxides, etc. The most popular industrial and it was lattice oxygen that directly participated in the
catalysts for these reactions are systems based on V and oxidation of benzene, because physically adsorbed é2
Mo oxides; they are used in the oxidation of alkanes, was certainly removed in an inert carrier gas flow. It
olefins, and aromatic hydrocarbons. In recent years, was, in addition, shown in [9] that the oxidation of ben-
ever increasing attention has been given to seeking zene ceased not immediately after the interruption of
methods for the direct oxidation of hydrocarbons by oxygen supply to the initial mixture. Even in seven
molecular oxygen. The question of the nature of active hours, traces of benzene oxidation products were
particles that participate in oxidative transformations observed at the exit of the reactor.
on catalysts of this type is central to solving this prob-
All these results are evidence that systems of this
lem. In particular, the hypothesis of the participation of
type can possess unique ability to generate an active
electronically excited oxygen molecules in these reac-
tions was advanced in several works. Experimental
studies in this direction were, however, largely per-
formed under the conditions of low-temperature homo-
geneous catalysis.
oxygen form responsible for the oxidation of benzene.
As early as the mid-1960s, the suggestion was made
1
that the singlet form of molecular oxygen ( ∆ O ) might
g
2
participate in the oxidation of aromatic hydrocarbons on
heterogeneous oxide catalysts. A comparison of the acti-
At the same time, a large number of works were vation energies necessary for the oxidative transforma-
concerned with the partial oxidation of aromatic hydro- tions of naphthalene, toluene, and benzene (all of them
were virtually equal and close to 29 kcal/mol) led the
authors of [4] to conclude that this energy is consumed
in the triplet–singlet transition (~23 kcal/mol) of
adsorbed oxygen molecules. The methods that allowed
singlet oxygen to be detected qualitatively and quanti-
tatively on heterogeneous systems, however, appeared
somewhat later.
carbons, benzene in particular, on heterogeneous cata-
lysts. For instance, the kinetics of benzene oxidation on
vanadium–molybdenum catalysts was studied in [1–6].
The conclusion was drawn that the major fraction of
benzene was oxidized by mobile weakly chemisorbed
oxygen molecules on the surface of vanadium pentox-
ide, because the rate of benzene oxidation by adsorbed
oxygen was higher than the rate of catalyst lattice oxi-
The major contribution to studies of the generation
dation by oxygen. It was also shown [7] that vanadium of singlet oxygen on transition metal oxides was made
oxide deposited on a carrier in an amount close to a by researchers of the Karpov Research Institute of
monolayer was not selective in the oxidation of ben- Physical Chemistry. The main objects of study were
zene. This was evidence that just the presence of the mechanically activated quartz [9, 10], vanadium,
oxide crystal lattice was required either for the forma- molybdenum, and tungsten oxides [11, 12], and photo-
tion of oxygen adsorption centers or for the accumula- stimulated emission of singlet oxygen from vanadium
tion of lattice oxygen.
and molybdenum oxides deposited on silica gel [13,
8
61

8
62
BOIKOV et al.
1
4]. The partial oxidation of naphthalene into phthalic was placed into a quartz reactor with low thermal iner-
anhydride was found to occur with the direct participa- tia. At the exit of the reactor, air was cooled to room
1
tion of ∆ O oxygen formed on the surface of catalysts temperature and introduced into a cell, where active
as a result of structure-chemical transformations [15].
The participation of singlet molecular oxygen in the
one-electron oxidation of benzene on ZSM-5 zeolites
was studied in [15]. The adsorption of oxygen was
g
2
oxygen selectively reacted with a chemiluminescent
dye. The intensity of chemiluminescence was recorded
by a FEU-79 photoelectron multiplier.
The amount of stabilized singlet oxygen was esti-
found to activate the catalysts and result in the forma- mated from its desorption into a flow that passed above
tion of benzene radical cations as intermediates of sub- a sample layer. The sample was preliminarily calcined
sequent transformations of hydrocarbon substrates. The at 500°ë for 1 h in a flow of air at a 1 kPa residual pres-
catalyst with adsorbed oxygen also acquired the ability sure, rapidly cooled to –60°ë, and again heated to 200–
to transform benzene molecules adsorbed on the sur- 300°ë. The amount of singlet oxygen desorbed from
face into polycondensation products.
the sample into air was measured at the exit of the reac-
tor. To study singlet oxygen generation, a sample was
heated in a flow of air to the required temperature at a
It is therefore clear that the singlet molecular oxy-
gen form can interact with aromatic hydrocarbons. In
agreement with the chemical properties of singlet oxy-
gen [16], such transformations most probably occur
with the intermediate formation of endoperoxides. The
2
0 K/min rate, and the amount of singlet oxygen was
measured at the exit.
Catalytic experiments were performed on a labora-
3
present work is concerned with the elucidation of the tory unit with a flow reactor (working volume 3 cm )
role played by singlet oxygen in the partial catalytic over the temperature range 300–500°ë at atmospheric
–
1
oxidation of aromatic hydrocarbons (benzene) on vana- pressure and volume benzene flow rate 0.02–0.45 h .
dium–molybdenum catalysts.
Mixtures of massive metal oxides (0.25–0.5 g) with
ground quartz (1.5–3 g) were placed into the reactor.
Deposited oxides (0.5–2 g) were studied without diluting
them with quartz. Benzene was introduced as a mixture
with air (benzene concentration 0.3–7.0 mol %), the flow
rate was 120–140 ml/min. Reaction products were ana-
lyzed by gas–liquid chromatography. Liquid products
were determined on a capillary column 50 m long with
an FFAP deposited phase, and gasses were analyzed on
a column 5 m long packed with activated carbon. The
content of maleic anhydride was determined by titrat-
ing an aqueous solution of reaction products with a
EXPERIMENTAL
The oxidation of benzene was performed on mas-
sive V é and Moé oxides, mixed V é · Moé oxide
2
5
3
2
5
3
(
molar ratio V O : MoO = 3 : 1), and V O and MoO
2 5 3 2 5
oxides deposited on silica gel (silochrom, s
3
=
sp
2
1
20 m /g). The massive V O and MoO samples were
2
5
3
prepared by the thermal decomposition of ammonium
metavanadate and metamolybdate at 400–500°ë for
5
h. TheV–Mo mixed oxide was prepared by the copre-
0
.1 M solution of NaOH.
cipitation of metal hydroxides from a saturated aqueous
solution of ammonium metavanadate and metamolyb-
date taken in the required ratio followed by calcining.
The deposited catalysts were obtained by impreg-
nating silica gel with solutions of pure metal nitrates
and ammonium metavanadate and metamolybdate
The EPR spectra were recorded at room temperature
on an X-band EPR-V spectrometer. The references
used were CuSO · 5ç é and MnCl –MgO.
4
2
2
RESULTS AND DISCUSSION
[
17]. The amount of salts for preparing the solutions
was calculated to satisfy the requirement that the con-
The catalytic oxidation of benzene with air oxygen
centration of metal oxides with respect to silica gel on transition metal oxides resulted in the formation of
should be 3.8–7.5 wt % V O and 12 wt % MoO . The such products as benzoquinone, maleic anhydride
2
5
3
necessary amount of silica gel was placed into a solu- (MA), CO, ëé , and ç é. Only traces of benzo-
2
2
tion of the corresponding salt. Water was then removed quinone were detected, and its contribution to the prod-
by evaporation at 110°ë for 12 h, while the solution ucts of partial benzene oxidation was therefore ignored.
was periodically stirred with a magnetic stirrer. The cat- The activity of the catalysts in oxidative benzene trans-
alysts were then calcined at 400°ë for 5 h, pressed into formations was therefore estimated from the yields of
pellets, and ground. A 0.25–0.5 mm fraction was used. MA and ëé . Note that the activity and selectivity of
x
The specific surface area of the samples was deter- all the samples studied in this work reached constant
mined by the thermal desorption of nitrogen. All the values immediately after the beginning of catalytic
massive metal oxide samples had similar specific sur- experiments and did not change for at least 8 h.
2
face areas of 5–6 m /g; the specific surface areas of
The activity of massive oxides in the oxidation of
2
deposited catalysts were 150–200 m /g.
benzene decreased in the series V O · MoO > V O >
2
5
3
2
5
The amount of singlet oxygen generated and stabi- MoO (Fig. 1). The data presented in Fig. 2 show that
3
lized on the surface of the samples was determined by benzene transformations on massive oxides begin at
the jet method [18] at a ~1 kPa residual air pressure and 300–350°ë. The conversion of benzene increases as the
a 260 cm/s flow rate. A weighed amount of a catalyst temperature grows, whereas the selectivity of the cata-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 6 2007

SINGLET OXYGEN IN THE HETEROGENEOUS CATALYTIC OXIDATION OF BENZENE
863
%
%
4
3
2
1
0
0
0
0
0
60
1
1
'
2
4
2
0
0
0
2
'
1
1
2
2
1
2
I
II
III
3
00
400
500
t, °C
Fig. 1. Characteristics of benzene oxidation ((1) benzene
conversion and (2) selectivity with respect to MA) on mas-
sive catalysts; preliminary calcining and reaction were
Fig. 2. Partial oxidation of benzene at various temperatures:
(1, 1') conversion and (2, 2') selectivity on V O and V–Mo,
performed at 400°ë: I, V O ; II, V O · MoO ; and III,
2
5
2
5
3
2 5
MoO3.
respectively.
lysts in partial oxidation decreases. The contribution of selectivity values during at least 8 h of measurements.
homogeneous noncatalytic benzene oxidation reactions The use of deposited oxides as catalysts increases the
is insignificant in this temperature range. This was sub- conversion of the substrate and simultaneously
stantiated by no-load experiments, which showed that decreases selectivity compared with massive samples.
benzene–air mixtures were stable in the absence of cat- This effect is well known; it is explained by an increase
alysts up to 500°ë.
in the effective time of contact between substrates and
catalysts because of steric hindrances to the escape of
substances from carrier pores. The V O /SiO samples
At the same time, chemiluminescent analyses
showed that these catalysts were capable of generating
singlet molecular oxygen on their surfaces. The amount
of singlet oxygen generated decreased in order of cata-
lyst activity lowering (Table 1). In addition, according
to Table 1 and Fig. 2, the temperature at which the gen-
eration of singlet molecular oxygen began was very
close to the lowest temperature of the beginning of the
partial oxidation of benzene.
2
5
2
exhibited the highest activity, whereas MoO /SiO
3
2
was virtually inactive in the oxidation of benzene.
The amount of singlet molecular oxygen generated
on the samples (Table 3) changed accordingly,
V O /SiO and MoO /SiO had the highest and lowest
2
5
2
3
2
activity in this reaction, respectively.
It follows that, as with massive oxides, the oxidative
transformations of benzene are favored by high singlet
molecular oxygen concentrations on the surface of the
The temperature of preliminary thermal treatment of
oxides strongly influences the activity of catalysts in
both partial benzene oxidation and singlet molecular
samples. In addition, the EPR data on deposited cata-
–
•
–
oxygen generation. Table 2 shows how the main pro- lysts show that the O and é radical oxygen particles
2
cess characteristics change depending on the tempera-
ture of the preliminary calcining of the catalysts. The
activity of vanadium oxide and vanadium–molybde-
num catalysts in the partial oxidation of benzene
decreased as the temperature of the preliminary thermal
treatment of the catalysts increased from 400 to 500°ë.
do not make a significant contribution to these pro-
cesses. This follows from the absence of the corre-
sponding signals in the EPR spectra of the samples
immediately after their participation in the oxidation of
–
•
–
benzene. The O and é particles in amounts sufficient
2
Chemiluminescent analyses showed that the activity of for detecting them were only observed after the adsorp-
these samples in the generation of active molecular tion of é at room temperature on the samples sub-
oxygen also decreased as the temperature of calcining jected to special reducing treatment, that is, enriched in
increased. Note that the specific surface area of the cat-
alysts did not change depending on thermal treatment
conditions.
2
4
+
V ions. The EPR spectra recorded for these samples
Table 1. Amount of singlet molecular oxygen (×10–15 mol-
ecules/(g h)) on V and Mo oxides at various temperatures
It follows that the oxidation of benzene on oxide cat-
alysts is favored by an increase in the concentration of
singlet molecular oxygen on the surface.
Oxide
V O
300°C
400°C
500°C
Deposited metal oxide samples on silica gel were
studied in the partial oxidation of benzene under the
same conditions as massive metal oxides. All the
deposited samples gave stable benzene conversion and
<0.1
<0.1
<0.1
19.5
33.9
0.45
34.5
56.5
0.55
2
5
V O · MoO
3
2
5
MoO3
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 6 2007

8
64
BOIKOV et al.
Table 2. Characteristics of the catalytic oxidation of benzene and amount of singlet molecular oxygen (×10–15 molecules/(g h))
on metal oxides calcined at various temperatures tcalc
V O
V O · MoO
MoO3
2
5
2
5
3
treac, °C
tcalc = 400°C
tcalc = 500°C
tcalc = 400°C
tcalc = 500°C
tcalc = 400°C
tcalc = 500°C
Y
X
Y
X
Y
X
Y
X
Y
X
Y
X
3
3
4
5
00
50
00
00
2.6
8.1
1.3
4.2
0.9
2.5
5.1
0.5
1.3
5.7
18.4
33.7
56.6
4.0
12.8
22.2
33.9
2.1
7.9
1.6
5.8
0
0
0
0
–
–
–
–
16.5
32.8
8.0
2.6
19.1
56.6
13.0
33.9
0.7
1.2
0.5
0.8
0.7
1.3
0.5
0.8
14.0
32.8
14.0
[1
∆ O ]
19.5
6.9
34
12.1
0.5
0.4
g
2
1
Note: treac is the temperature of the reaction, Y is the conversion of benzene, and X is the yield of MA; the [ ∆ O ] values were measured
g
2
at 400°C.
Table 3. Activity of samples in the partial oxidation of benzene and amounts of singlet molecular oxygen generated (×10−15
molecules/(g h))
,
1
Sample
[ ∆ O ]
Y
X
Y
X
Y
X
g
2
3
00°C
400°C
500°C
7
3
.5% V O /SiO
16.1
13.6
2.4
5.1
2.1
1.64
–
40.3
24.8
5.2
14.5
46.3
34.2
8.5
3.8
4.0
5.9
2
5
2
.8% V O /SiO
2.05
6.25
4.5
2
5
2
1
2.0% MoO /SiO
<0.2
3
2
Note: Y is the conversion of benzene, %, and X is the yield of MA, %.
coincided with the spectra of oxygen radicals reported
ACKNOWLEDGMENTS
–
•
2
–
This work was financially supported by the Russian
Foundation for Basic Research, project no. 05-03-
in [19–22]. In addition, the O and é particles disap-
peared as the samples were heated to as low as 200°ë.
3
2221.
2
+
On the other hand, VO vanadyl ions, whose pres-
ence on the surface of partially reduced deposited cata-
lysts was proved by the EPR method, can be centers of
singlet molecular oxygen generation. Note that EPR
spectra were obtained for deposited catalysts only,
because massive samples were characterized by signal
broadening and the disappearance of hyperfine struc-
ture caused by dipole–dipole interaction at a small dis-
tance between paramagnetic centers on their surface.
This effect considerably complicates more detailed
qualitative and quantitative studies of such systems.
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