Catalytic oxidation of benzene with ozone over alumina-supported manganese oxides

Article abstract of DOI:10.1016/j.jcat.2004.07.029

Catalytic oxidation of benzene with ozone over alumina-supported manganese oxides was carried out at room temperature (295 K) to investigate the behavior of benzene oxidation and CO_x formation. The ratio of the decomposition rate for ozone to that for benzene was found to be 6, independent of ozone concentration, reaction times, and the amount of Mn loading. A linear correlation was observed for the amount of ozone decomposed and that of CO_x formed, whereas deviation from linearity was observed between the amount of ozone decomposed and that of benzene reacted. Carbon balance was in the range of 26-37% due to the formation of two types of intermediates, weakly bound compounds including formic acid and strongly bound surface formate and carboxylates. Catalyst was significantly deactivated due to the buildup of the intermediates on the catalyst surface during the course of benzene oxidation. The weakly bound compounds were removed by heat treatment at 573 K in the O₂ flow. The strongly bound surface formate and carboxylates were oxidized to CO_x by further heating up to 723 K. The deactivated catalyst was regenerated by the heat treatment.

Full text of DOI:10.1016/j.jcat.2004.07.029

Journal of Catalysis 227 (2004) 304–312
www.elsevier.com/locate/jcat
Catalytic oxidation of benzene with ozone over alumina-supported
manganese oxides
∗
Hisahiro Einaga , Shigeru Futamura
National Institute of Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan
Received 21 May 2004; revised 25 July 2004; accepted 28 July 2004
Available online 26 August 2004
Abstract
Catalytic oxidation of benzene with ozone over alumina-supported manganese oxides was carried out at room temperature (295 K) to
investigate the behavior of benzene oxidation and COx formation. The ratio of the decomposition rate for ozone to that for benzene was
found to be 6, independent of ozone concentration, reaction times, and the amount of Mn loading. A linear correlation was observed for
the amount of ozone decomposed and that of COx formed, whereas deviation from linearity was observed between the amount of ozone
decomposed and that of benzene reacted. Carbon balance was in the range of 26–37% due to the formation of two types of intermediates,
weakly bound compounds including formic acid and strongly bound surface formate and carboxylates. Catalyst was significantly deactivated
due to the buildup of the intermediates on the catalyst surface during the course of benzene oxidation. The weakly bound compounds were
removed by heat treatment at 573 K in the O flow. The strongly bound surface formate and carboxylates were oxidized to COx by further
2
heating up to 723 K. The deactivated catalyst was regenerated by the heat treatment.

2004 Elsevier Inc. All rights reserved.
Keywords: Benzene oxidation; Catalytic ozonation; Manganese oxide; Catalyst deactivation; Catalyst regeneration
1
. Introduction
Mehandjiev have used unsupported manganese oxides as the
catalysts to investigate the effect of the reaction tempera-
ture on the rate of benzene oxidation [12]. The use of ozone
can lower the reaction temperature for benzene oxidation
over manganese oxides. They have also reported that appar-
ent activation energy estimated in the presence of ozone is
three times lower than that in the absence of ozone. The
mixed oxide catalysts, such as alumina-supported Cu–Cr
and Co–Cr [13], NiMnO3-ilmenite, and NiMn2O4-spinel
Ozone is widely used for various industrial and envi-
ronmental processes, and its reactivity toward organic com-
pounds has been extensively studied both in the liquid phase
[
1,2] and in the gaseous phase [3]. Catalytic oxidation reac-
tions using ozone are promising technologies for the treat-
ment of air polluted with hazardous compounds [4–6]. Hith-
erto, the reactions have been reported for the oxidation of
CO [7,8], methane [9], alkanes [10], aromatic compounds
[
14] have also been reported to be effective for benzene oxi-
dation with ozone.
[
11–15], acrylonitrile [15], alcohols [15–17], and chlorohy-
Oyama and his co-workers have mainly studied the ozone
decomposition on supported-manganese oxides in the ab-
sence of organic substrates [4–6,19–24]. The reaction mech-
anism was clarified by in situ Raman spectroscopy, ab initio
molecular orbital calculations, and kinetic studies [19,20].
They have also used temperature-programmed oxygen des-
orption techniques and EXAFS studies to determine the
structure of the manganese active centers on the supports
[21,22]. On the other hand, we have investigated the oxida-
drocarbons [15,18].
Benzene is one of the hazardous compounds removed
from flue gases due to its carcinogenicity. Generally, the el-
ements of the first transition series have been used for the
catalytic oxidation of benzene with ozone. Naydenov and
*
Corresponding author.
E-mail address: h-einaga@aist.go.jp (H. Einaga).
0021-9517/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2004.07.029

H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
305
tion of benzene and cyclohexane with ozone over alumina-
supported metal oxides, and have reported that manganese
oxide showed higher activity than the oxides of Fe, Co, Ni,
Cu, and Ag [25].
We herein report the catalytic oxidation of benzene with
ozone over supported manganese oxide catalysts at room
temperature. Our attention is concentrated on the behavior
of benzene oxidation, ozone decomposition, and product for-
mation on the manganese oxide catalysts. Alumina is used as
a catalyst support based on the high reactivity of manganese
oxides on the support for ozone decomposition [21]. In addi-
tion, manganese oxide can be highly dispersed on γ -alumina
at low loading levels [26–29].
in N2, N2 (>99.9995%, total hydrocarbon <1 ppm), and
O2 (>99.9995%, total hydrocarbon <1 ppm) in cylinders
by using the sets of thermal mass-flow controllers. Ozone
was synthesized from O2 by a silent discharge ozone gen-
erator. Prior to the catalytic reaction, the sample in a Pyrex
glass reactor was heated at 723 K in O2 flow. Then, the cata-
lyst was cooled and thermostated at 295 K with a water bath.
After the adsorption–desorption equilibrium was achieved
for benzene between the catalyst surface and the gas phase,
ozone was continuously fed into the reactor. Unless other-
wise stated, ozone concentration was 1000 ± 30 ppm. Cat-
alyst weight was generally 20–75 mg and flow rate was
−1
250–1000 ml min . Here, weight hourly specific velocity
−1
−1
(
WHSV) was 200–3000 L h
g . Analysis of the gas sam-
ple was performed with a Fourier-transform infrared spec-
trophotometer (Perkin-Elmer Spectrum One) equipped with
a 2.4 m optical length gas cell (volume 100 ml). The dead
volume, namely, the volume of the reactor and apparatus in
the reaction system, was smaller than 20 ml and the resi-
dence time under the reaction condition was 1.2–5 s. Both
benzene oxidation and ozone decomposition did not proceed
in the gas phase with an empty reactor. Thus, homogeneous
gaseous reaction of benzene with ozone can be neglected.
Reaction rates were evaluated under the conditions where
the conversions were linear to W/F.
2
. Experimental
2
.1. Catalyst materials
Alumina-supported manganese oxides were prepared by
the impregnation of γ -Al2O3 (Catalysis Society of Japan,
2
−1
JRC ALO-4, SBET 170 m g ) with the aqueous solution
containing an appropriate amount of Mn(CH3COO)2 ·4H2O
(
Wako Pure Chemical, >99.9%). Catalyst samples were
dried at 383 K and then calcined at 773 K for 3 h in air. For
simplicity, alumina-supported manganese oxide is denoted
by MnO2/Al2O3, although the oxidation states of Mn on
alumina supports are mixtures of II–IV [26]. For the prepa-
ration of MnO2/SiO2 and MnO2/TiO2, SiO2 (JRC SiO-1,
2.3. Other measurements
FTIR spectra were recorded with a Jasco FT-IR480 Plus
spectrometer equipped with a TGS detector. Catalyst sam-
ples were pressed into thin self-supporting wafers of 10 mm
in diameter (ca. 7 mg) and set in an IR cell with KBr win-
dows, which was connected to the flow-type reaction system
described above. Prior to the measurements, the sample was
preheated at 723 K in O2 and then cooled to room temper-
ature. The reaction gas containing 100 ppm benzene was
fed to the IR cell. After the adsorption–desorption equilib-
rium was achieved for benzene between the catalyst surface
and gas phase, the feed of 1000 ppm ozone was started. The
2
−1
2
−1
)
SBET = 110 m g ) and TiO2 (P25, SBET = 43 m g
were used as the catalyst supports. MnO2/ZrO2 was pre-
2
−1
pared from ZrO2 (SBET = 58 m g ), which had been ob-
tained by the calcination of Zr(OH)4 at 823 K for 3 h in air.
2
.2. Catalytic reactions
Catalytic reactions were carried out with a fixed-bed flow
reactor. Fig. 1 shows the schematic of the reaction system
used in this study. Reaction gases were prepared by benzene
Fig. 1. Schematic of the reaction system for catalytic oxidation by using ozone.

306
H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
spectra were collected with the resolution of 4 cm 1 at room
temperature.
−
reaction. After 120 min, the feed of ozone and benzene was
stopped, and the reactor was purged out until benzene and
Temperature-programmed oxidation was conducted with
the flow reactor described above. After the benzene oxi-
dation with ozone was carried out, the reaction gas was
switched to N2–O2 (9-to-1 in volume) with the flow rate of
CO were not observed. When ozone was continuously fed
to the reactor without benzene feed (Stage II in Fig. 2a), CO2
x
and CO were evolved from the catalyst surface. Their evolu-
tion shows that intermediates were deposited on the catalyst
surface in the benzene oxidation. The formation rate of CO2
for the first 10 min in Stage II was higher than that after
120 min in Stage I, indicating that the presence of benzene
−1
5
00 ml min . Then, the used catalyst was heated at a rate
−1
of 10 K min to 773 K. The effluent gas from the reac-
tor was analyzed by the FTIR spectrometer (Perkin-Elmer
Spectrum One) with the 2.4 m optical length gas cell. Prod-
uct compounds on catalyst surface were determined by a
gas chromatograph (Hewlett-Packard 6890) equipped with
a mass selective detector (Hewlett-Packard 5973).
inhibits the oxidation of the intermediates to CO forma-
2
tion. The mole fraction of CO in this period was higher
2
than that in Stage I. Thus, the fraction in the oxidation of the
intermediates is higher than that in benzene oxidation. Car-
bon balance based on the total amount of COx formed and
benzene reacted through Stages I and II was 50%. Fig. 2b
shows the time course for ozone decomposition in these two
stages. The conversion decreased with time on stream in the
presence of benzene (Stage I in Fig. 2b). However, no such
decrease was observed for ozone conversion after the ben-
zene feed was stopped (Stage II in Fig. 2b).
3
. Results
3
.1. Decomposition behavior of benzene on MnO2/Al2O3
Fig. 2 shows the time courses for benzene oxidation and
products formation by using ozone with 5 wt% MnO2/Al2O3
catalyst. The conversions of benzene and ozone gradually
decreased with time on stream (Stage I in Fig. 2a). CO2
and CO were observed as the products, and their con-
centrations also decreased with time. No other products
were detected in the gas phase. The mole fraction of CO2
Table 1 summarizes the results for benzene oxidation
with ozone over supported-manganese oxides. Here, the re-
action rates were measured 2 h after the reactions were
started. When γ -Al2O3 without Mn was used for the reac-
tion, the rates for benzene oxidation and ozone decomposi-
tion were lower than the limit of experimental error, although
the catalyst was slightly browned. No significant difference
was observed for the rates by changing the Mn loading of
[
= CO2/(CO2 + CO)] and the carbon balance were 80 and
3
0%, respectively, which were almost unchanged during the
Fig. 2. Time profiles for benzene oxidation, ozone decomposition, and products formation with 5 wt% MnO /Al O . Catalyst 0.05 g, ozone 1000 ppm,
2
2 3
−1
−1
g ). Benzene: 100 ppm (Stage I); 0 ppm (Stage II).
oxygen 10%, flow rate 250 ml/min (WHSV 300 L h

H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
307
Table 1
Catalytic activities of supported manganese oxides for benzene oxidation with ozone
a
b
−1 −1
c
−1 −2
d
e
Catalyst
Mn loading
Rate (mol min
g
)
Normalized rate (mol min
m
)
Surface area
Ratio
(%)
2
−1
Benzene
Ozone
Benzene
Ozone
(m g
)
γ -Al O
–
5
10
20
5
–
–
–
–
170
161
151
146
106
43
–
2
3
−5
−5
−5
−5
−6
−6
−5
−5
−5
−5
−5
−5
−8
−8
−8
−8
−8
−8
−7
MnO /γ -Al O
1.38 × 10
1.40 × 10
1.13 × 10
1.02 × 10
3.09 × 10
3.10 × 10
8.29 × 10
8.46 × 10
6.50 × 10
4.59 × 10
2.07 × 10
2.34 × 10
8.55 × 10
9.29 × 10
7.73 × 10
9.66 × 10
7.18 × 10
5.41 × 10
5.14 × 10
5.62 × 10
4.45 × 10
4.33 × 10
4.82 × 10
4.07 × 10
6.0
6.0
5.8
4.5
6.7
7.5
2
2
3
3
3
−7
−7
−7
−7
−7
MnO /γ -Al O
2
2
MnO /γ -Al O
2
2
MnO /SiO
2
2
MnO /TiO
5
5
2
2
MnO /ZrO
2
57
2
a
Conditions: benzene 100 ppm, ozone 1000 ppm, oxygen 10%.
b
c
d
e
−1
Rate = C X(W/F) ; C , initial concentration of benzene or ozone; X, conversion.
0
0
Reaction rate normalized by surface area.
−6
Surface area was determined from BET plots. The samples were evacuated (< 2 × 10 Torr) at 573 K for 3 h prior to N adsorption measurements.
2
Ratio of rate for ozone decomposition to that for benzene oxidation. The values were obtained from the slope of the plots between the rates for ozone
decomposition and those for benzene oxidation under the conditions where the conversions were linear to W/F .
SiO2-, TiO2-, and ZrO2-supported catalysts are also listed
in Table 1. The catalyst supports without manganese oxides
were also inactive for the reaction. The rates for benzene ox-
idation normalized by surface area with SiO2-, TiO2-, and
ZrO2-supported catalysts were comparable or slightly lower
than those with the Al2O3-supported catalysts. The ratio of
the rate for ozone decomposition to that for benzene oxida-
tion slightly depended on the support.
Fig. 3a shows the effect of ozone concentration on the rate
of benzene oxidation at various reaction times. The oxida-
tion rate at 30 min monotonically increased with the ozone
concentration. As the reaction proceeded, the increment of
the rate became smaller. At 120 min, the rate was almost sat-
urated at higher concentration levels than 1000 ppm. Fig. 3b
shows the relationship between the decomposition rates of
benzene and ozone through the reactions at various reaction
times. The rate for benzene oxidation increased linearly with
that for ozone decomposition. The ratio of the rate for ozone
decomposition to that for benzene oxidation was estimated
to be 6.1 from the slope, independent of the ozone concen-
tration and the reaction times.
3
.2. Formation behavior of COx
Fig. 4 shows the dependence of the mole fractions of CO2
and CO on the benzene conversion. Here, the conversion was
−1
−1
varied by changing the WHSV from 200 to 1500 L h
g .
The values at 1 and 2 h are plotted in this figure. Both the
factions of CO2 and CO were unchanged in the low con-
version region (<50%). In the higher conversion region,
however, the mole fraction of CO2 increased and that of CO
decreased with increasing the conversion. The carbon bal-
ance was 26% in the low conversion region and increased
slightly in the high conversion region. Fig. 5 shows the re-
lationship between the amount of benzene oxidized and that
Fig. 3. (a) Effect of ozone concentration of the rate of benzene oxidation at
various reaction times. (b) Relationship between the rate of benzene oxida-
tion and that of ozone decomposition. Benzene 100 ppm, ozone 1000 ppm,
−6
−1
−1
,
oxygen 10%. Experimental error: benzene ± 0.3 × 10 mol g min
−
5
−1
−1
.
ozone ± 0.3 × 10 mol g min
MnO2/Al2O3 catalysts from 5 to 20 wt%. In this study, here-
after, the catalyst with 5 wt% was used for the catalytic tests
and spectroscopic studies. The ratio of the rate for ozone de-
composition to that for benzene oxidation was about 6 for the
Al2O3-supported catalysts. For comparison, the activities of
of CO formed and that of ozone reacted. The values at 1
x
and 2 h are also plotted in this figure. A good linear correla-
tion was observed between the amount of CO formed and
x
that of ozone decomposed. According to the slope, the ratio

308
H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
Fig. 4. Dependence of the formation of COx and carbon balance on benzene
conversion at various WHSV. The values were evaluated at 1 h (!) and
2
h ("). Benzene 100 ppm, ozone 1000 ppm, oxygen 10%.
Fig. 6. FTIR spectra of MnO /Al O catalyst. (a) During the course of
2
2 3
benzene oxidation with ozone: benzene 100 ppm, ozone 1000 ppm, oxy-
gen 10%. Spectra were taken after 0, 2, 4, 8, 12, 16, 20, and 30 min.
(b) During the ozone feed without benzene: ozone 1000 ppm, oxygen 10%.
Spectra were taken after 0, 30, and 120 min.
Fig. 5. Relationship between the benzene consumption and COx formation
with change of ozone consumption. The values were evaluated at 1 h (!)
and 2 h ("). Benzene 100 ppm, ozone 1000 ppm, oxygen 10%.
bands of adsorbed water and hydroxyl groups on the cat-
−1
alyst surface (1620 and 2500–3700 cm ), even after the
catalyst was equilibrated with dilute benzene (100 ppm).
No bands assigned to benzene were observed probably due
to its low concentration and low absorption coefficiency.
When the ozone feed was started, new bands appeared in
of the amount of ozone decomposed to that of COx formed
was estimated to be 4.2. On the other hand, deviation from
the linearity was observed between the amounts of benzene
reacted and that of ozone decomposed in the higher conver-
sion region. The ratio of the amount of benzene oxidized to
that of ozone decomposed was estimated to be 6 in the low
conversion region and increased to 9 in the high conversion
region.
−
1
the range of 1000–1800 cm with the maximum intensities
−
1
at 1412, 1636, and 1754 cm . Their intensities monotoni-
cally increased with time on stream, indicating that organic
by-products were accumulated on the catalyst surface. The
−1
band at 1754 cm is assigned to the C=O stretching vibra-
3
.3. FTIR spectroscopic studies
tion of formic acid on the catalyst surface (see below). The
−1
broader band at around 1636 cm and the bands at around
−1
Catalytic oxidation reactions were carried out in an FTIR
1412 cm may be the overlapping of several bands includ-
ing OH bending of adsorbed water and the OH stretching of
alcohols and carboxylic acids. The intensity of a broad band
in the range of 2500–3700 cm assignable to OH stretching
of water and/or organic intermediates also increased during
the reaction. These bands were not observed in the spectrum
cell at room temperature. Fig. 6a shows the spectra of the
catalyst during the benzene oxidation with ozone. The spec-
tra were collected without ozone at first, and then at the
intervals of 2–10 min from the start of ozone feed. The spec-
trum of the catalyst before the reaction exhibited only the
−
1

H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
309
Fig. 8. TPO profiles of the used MnO /Al O catalyst. Temperature ramp:
2
2 3
−1
10 K min
.
−
1
region of 1620–1780 cm and that in the range of 1000–
−1
1
500 cm . Instead, the bands at 1320, 1380, 1400, 1440,
−1
and 1610 cm appeared. These new bands are assigned
to the antisymmetric and symmetric COO– stretching with
the CH deformation of the surface formate and carboxylate
species [30–32]. Further heating up to 623 K greatly re-
duced the intensities of the bands of the residual species. On
−1
the other hand, the band in the region of 2500–3700 cm
monotonically decreased in its intensity by increasing the
heating temperature. The residual species on the catalyst sur-
face were almost completely removed after the catalyst was
heated up to 723 K.
When the intermediate compounds on the used catalyst
were extracted with methanol and analyzed on GC-MS,
formic acid was detected as a major by-product. As the
minor by-products, 2,5-furandione, phenol, acetic acid, and
oxalic acid were also detected. However, these compounds
were not detected after the used catalyst was heated at 573 K
Fig. 7. FTIR spectral changes in MnO /Al O surface by heating the cat-
2
2 3
alyst in O flow at room temperature (a), 373 K (b), 423 K (c), 473 K (d),
2
523 K (e), 573 K (f), 623 K (g), 673 K (h), and 723 K (i).
when ozone was contacted with the MnO2/Al2O3 catalyst
without benzene, although the data are not shown.
in the O flow. Thus, the intermediate compounds such as
carboxylic acids and phenols on the catalyst were all decom-
2
After the benzene oxidation with ozone was conducted,
gaseous benzene was purged out and ozone was contin-
uously introduced into the reactor again without benzene
feed. As shown in Fig. 6b, the intensity of the newly ob-
served bands gradually decreased with time on stream in the
posed by the heating up to 573 K, and the residual formate
and carboxylates could not be extracted with the organic sol-
vents.
3.4. Temperature-programmed oxidation
−1
presence of ozone. Especially, the band at 1754 cm was
greatly diminished after 120 min, while the bands at 1636
Fig. 8 shows the TPO profiles obtained when the used
catalyst was heated from room temperature to 773 K. The
catalyst had been used for the benzene oxidation with ozone
−
1
and 1412 cm and the broad band in the range of 2500–
3
−
1
700 cm remained.
Fig. 7 shows the FTIR spectra obtained after the heating
for 4 h (benzene, 100 ppm; ozone, 1000 ppm; O , 10%;
2
−1
−1
of the catalyst in the flow of O2, which had been used for
the benzene oxidation with ozone for 120 min. The catalyst
sample was heated for a few minutes at the desired tempera-
ture. Spectra were taken after the sample was cooled to room
temperature. When the sample was heated to 473 K, the band
WHSV, 1200 L h
g
). Formic acid was mainly observed
at the temperature region of 300–500 K and was not ob-
served at temperatures higher than 623 K. On the other hand,
two peaks were observed for CO at 360 and 620 K. Es-
2
pecially, larger amounts of CO were observed at the tem-
2
−
1
−1
at 1750 cm and that in the range of 1050–1300 cm dis-
perature range of 573–673 K. The amount of CO was com-
−1
appeared, while the band at around 1630 cm and that at
around 1400 cm were almost unchanged. The heating up
to 573 K decreased the intensity of the band especially in the
parable with that of CO in the lower temperature region.
2
−1
However, its amount was much smaller than that of CO2 in
the higher temperature region. According to the TPO pro-

310
H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
Fig. 10. Effect of water vapor contact on the reaction rate of ozone decom-
position with MnO /Al O catalyst. Benzene 100 ppm, ozone 1000 ppm,
2
2 3
−1
−1
g ).
oxygen 10% (WHSV 2000 L h
the first 10 min. After the gas saturated with water vapor
was fed to the reactor for 20 min, the ozone decomposi-
tion was carried out again (Stage II). By this treatment, to-
−
4
tally 6.9 × 10 mol of water vapor was contacted with the
Fig. 9. Time courses for benzene oxidation with ozone over MnO /Al O
2 3
2
catalyst. Benzene 100 ppm, ozone 1000 ppm, oxygen 10%. Exper-
catalyst. The ozone conversion was dropped to the rate of
−6
−1
−1
−4
−1 −1
g
imental error: benzene ± 0.3 × 10 mol g min , ozone ± 0.3 ×
4
1
0
.3 × 10 mol min
. However, it increased to 5.1 ×
−5
−1
−1
10
mol g min .
−4
−1 −1
0
mol min
g
after 300 min, which corresponded to
.73 times the rate for the first 10 min in Stage I.
files, the amounts of CO2, CO, and formic acid were 0.1462,
−1
0
.0548, and 0.0465 mmol-C (g-catalyst) , respectively.
4
. Discussion
3
.5. Effect of heat treatment on the time course for benzene
decomposition with ozone
In the present study, we demonstrate that benzene oxida-
tion catalytically proceeds with MnO2/Al2O3 to give CO2
and CO with the mole fractions of around 80 and 20%,
respectively, which slightly depend on reaction conditions.
The low carbon balance (26–37%) is ascribed to the forma-
tion of intermediates on the catalyst surface in the benzene
oxidation. Although Mn is crucial for obtaining catalytic ac-
tivity, the rate of benzene oxidation is not so much sensitive
to the amount of Mn loaded. Our investigation on the ef-
fect of the supports reveals that Al2O3 is an effective support
for manganese oxides in the benzene oxidation with ozone.
However, the reaction rate normalized by surface area for
SiO2-supported catalysts is comparable to that for Al2O3. In
addition, the rates with TiO2- and ZrO2-supported catalyst
are 20 and 40% lower than those with Al2O3-supported cata-
lysts, respectively. Therefore, the surface area of the support
is one of the important factors for catalytic activities.
Fig. 9 shows the time courses for benzene oxidation with
ozone when the catalyst was repeatedly used for the reaction.
Both the rates of benzene oxidation and ozone decomposi-
tion decreased with time on stream (first run). Steady-state
activity was not obtained. After the benzene oxidation was
carried out for 4 h, the catalyst was heated in O2 flow up
to 723 K for 1 h. Then, the benzene decomposition was car-
ried out again under the same conditions as those of first run.
Almost the same time course plots were obtained for these
two reaction profiles. After the 4-h reaction, the total amount
of benzene reacted was estimated to be 3.8 ± 0.7 mmol per
1
g of catalyst. By using this value, the turnover number per
a Mn site was estimated to be 4.3 ± 0.8, indicating that the
reaction catalytically proceeded.
Fig. 10 shows the ozone decomposition with the MnO2/
Al2O3 catalyst without benzene feed. No deactivation was
observed until 80 min, after which the rate of ozone decom-
position slightly decreased with time on stream (Stage I).
Oyama and his co-workers have investigated the mecha-
nism for O3 decomposition on manganese oxide supported
on Al2O3 without organic substrates, and have reported that
the reaction proceeds through two irreversible steps, the ad-
−
4
The decomposition rate at 240 min was 6.3 × 10 mol
−1
−1
g , which corresponded to 0.9 times the rate of
min

H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
311
sorption of ozone on the catalyst surface and the desorption
of molecular oxygen,
on the catalyst surface. Although the rate for ozone de-
composition significantly decreased in the benzene oxida-
tion, no such deactivation was observed without benzene
feed (Figs. 2 and 10). The accumulation of the organic in-
termediates is also evidenced by the FTIR spectra, which
shows the appearance of new bands in the wavenumber
range of 1000–800 and 2500–3700 cm . The detection of
carboxylic acids, 2,5-furandione and phenol, urged us to as-
sign these bands to the overlapping of the stretching and
∗
∗
O3 + → O2 + O ,
(1)
(2)
(3)
∗
∗
O + O3 → O2 + O2 ,
∗
∗
O2 → O2 + ,
−1
∗
where denotes the surface site on the catalyst [19,20].
As the intermediate species, atomic oxygen and peroxides
have been observed [19]. Imamura et al. [7] and Naydenov
et al. [8] have suggested on the basis of ESR studies that
the bending of oxygen-containing groups, –C=O, –OH, and
−1
–
C–O–. The bands at 1754 and 1100 cm are assigned to
−
the oxygen anions O are formed on Ag2O and CeO2 in the
formic acid on the catalyst surface [32]. The disappearance
of these bands during the ozone feed without benzene is also
consistent with the finding that the organic intermediates are
decomposed in the reaction with ozone.
presence of ozone and these species oxidize CO and ben-
zene. Such active oxygen species are responsible for benzene
oxidation on manganese oxide catalysts.
The ratio of decomposition rate of ozone to benzene is es-
timated to be 6, and the value does not depend on the ozone
concentration, reaction times, or the amount of Mn loading.
The value is almost the same as that obtained in the ben-
zene oxidation over other types of metal oxides (Fe, Co, Ni,
Cu, and Ag) supported on Al2O3 [25]. Thus, the activities
for benzene oxidation with ozone strongly depend on those
for ozone decomposition. This finding is consistent with the
consideration that the active oxygen species formed in ozone
decomposition are responsible for benzene oxidation. Based
on these findings, the reaction stoichiometry is estimated. If
only one atom of O3 was reacted to oxidize benzene, the re-
action equation can be written as
According to the stoichiometry described in Eq. (4), three
molecules of water are formed when one molecules of ben-
zene is completely oxidized. Our results show that the ad-
sorbed water on the catalyst surface also inhibits the benzene
−4
oxidation on active sites. After 6.9 × 10 mol of water va-
por (corresponding to 53 times the amount of Mn sites on
the catalyst surface) was contacted with catalyst surface, the
rate of ozone decomposition decreased to around 0.5 times
the initial reaction rate (Fig. 10). However, the rate of ozone
decomposition was recovered to some extent after the feed
of water vapor was stopped, very probably due to the des-
orption of adsorbed water. Thus, adsorbed water is not the
dominant factor for the severe deactivation of MnO2/Al2O3
catalyst in the benzene oxidation.
C6H6 + 15O3 = 6CO2 + 3H2O + 15O2
(4)
FTIR studies combined with TPO measurements pro-
vide further information on the intermediates on the MnO2/
Al2O3 catalyst. By heating the catalyst up to 423 K, formic
acid on the catalyst surface was desorbed from the catalyst
according to the reaction mechanism shown in Eqs. (1)–(3).
The value of 6 for the ratio of the decomposition rates is
lower than that estimated from the above equation (O3/
C6H6 = 15). This indicates that not only ozone but also
molecular oxygen participates as an important role in the
benzene decomposition: O2 may be involved in the autox-
idation processes, in which the radical intermediates formed
in benzene oxidation are oxidized by O2
−1
−1
surface and the band at 1754 cm and around 1100 cm
disappeared, which confirms the assignment of these two
bands to formic acid. The weakly bound intermediates in-
cluding formic acid, carboxylic acids, 2,5-furanedione, and
phenol were completely decomposed by the subsequent
heating up to 573 K. The new bands due to strongly bound
surface formate and carboxylates remained on the catalyst
surface, which give the bands of 1320, 1380, 1400, 1440,
R·+ O2 → RO2· → CO2, CO.
(5)
A linear relationship between the amount of COx formed
and that of ozone decomposed (Fig. 5) implies that the for-
mation behavior of COx is dominated by the decomposition
behavior of ozone. On the other hand, deviation from linear
plots between the amount of benzene oxidized and that of
ozone decomposed can be explained in terms of the follow-
ing mechanism: ozone is mainly consumed in the benzene
oxidation at low conversion levels, while ozone is much
more consumed in the oxidation of the intermediates at high
conversion levels, which leads to the increase in carbon bal-
ance and the mole fraction of CO2 (Fig. 4). This considera-
tion is supported by the findings that the presence of benzene
inhibits the oxidation of intermediates to COx, and that the
mole fraction of CO2 in the oxidation of the intermediates is
higher than that in benzene oxidation (Fig. 2).
−1
and 1610 cm . These surface formate and carboxylates
may be derived from formic acid and the corresponding car-
boxylic acids during the course of benzene oxidation with
ozone, because surface formate was observed when gaseous
formic acid was adsorbed on the catalyst in a separate exper-
iment. Thus, benzene is converted to two types of intermedi-
ates, weakly bound compounds and strongly bound surface
formate and carboxylates on the catalyst in the oxidation
reaction. These species needs higher temperatures for the
complete decomposition: heating the catalyst from 573 K to
673 K leads to the decomposition of the residual formate and
carboxylates with the evolution of large amounts of COx.
Further heating up to 723 K leads to the complete decom-
position of the intermediates on the catalyst surface. The
formation ratio of CO to CO2 decreases with increasing the
MnO2/Al2O3 catalyst is gradually deactivated during the
benzene oxidation due to the buildup of the intermediates

312
H. Einaga, S. Futamura / Journal of Catalysis 227 (2004) 304–312
heating temperature. This may reflect the different distribu-
tion of adsorbed species and/or the oxidation of CO to CO2
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