Benzene oxidation with ozone over MnOx/MSU-H and MnOx/Mesoporous-SAPO-34 Catalysts

Article abstract of DOI:10.1166/jnn.2015.8361

Two mesoporous silica materials, MSU-H and mesoporous SAPO-34 (Meso-SAPO-34), were applied to the catalytic oxidation with ozone of benzene. The catalysts were characterized by X-ray diffraction, N₂ adsorption-desorption, Brunauer-Emmett-Teller

Full text of DOI:10.1166/jnn.2015.8361

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Benzene Oxidation with Ozone Over MnOx/MSU-H and
MnOx/Mesoporous-SAPO-34 Catalysts
Hyung Bum An1, Sung Hoon Park2, Sung Hwa Jhurng3, Jong Soo Jurng4,
Gwi-Nam Bae4, Jong-Ki Jeon5, and Young-Kwon Park1ꢀ6ꢀ∗
1Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Korea
2Department of Environmental Engineering, Sunchon National University, Suncheon 540-950, Korea
3Department of Chemistry, Kyungpook National University, Daegu 702-701, Korea
4Center for Environment, Health and Welfare Research, KIST, Seoul 136-791, Korea
5Department of Chemical Engineering, Kongju National University, Cheonan 330-717, Korea
6School of Environmental Engineering, University of Seoul, Seoul 130-743, Korea
Two mesoporous silica materials, MSU-H and mesoporous SAPO-34 (Meso-SAPO-34), were
applied to the catalytic oxidation with ozone of benzene. The catalysts were characterized by
X-ray diffraction, N2 adsorption–desorption, Brunauer–Emmett–Teller specific surface area, and
H2-temperature programmed reduction. When MnOx /MSU-H was used at three different temper-
ature, 50 ꢀC, 80 ꢀC, and 100 ꢀC, the ozone conversion and COx yield increased with increasing
temperature. MnO /MSU-H exhibited much higher catalytic activity than that of MnO /Meso-SAPO-
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x
x
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34. The high catalytic activity of MnOx /MSU-H seems to be due to the high oxygen ability and
Copyright: American Scientific Publishers
structural stability of MSU-H. The CO2 yield was somewhat enhanced by the addition of steam
because steam facilitated desorption of the reaction intermediates adsorbed on the catalyst surface,
enabling them to be oxidized further.
Keywords: Benzene, VOC, Oxidation, Ozone, MSU-H, Mesoporous SAPO-34.
1. INTRODUCTION
been used for removing VOCs, catalytic oxidation and cat-
alytic ozonation are the most efficient ones. In particular,
catalytic ozonation can oxidize VOCs at low temperatures
Air pollutants are produced as a result of human activ-
ities, directly or indirectly causing discomfort and dis-
ease. Among them, volatile organic compounds (VOCs)
are emitted from various sources including mobile sources
such as vehicles, petroleum chemical factories, dye works,
and construction materials such as paints, adhesives, and
interior materials. VOCs emitted from factories and mobile
sources react photochemically with nitrogen oxides (NOxꢀ
in the air to produce ground-level ozone and smog,
whereas those emitted from construction materials cause
sick building syndrome and respiratory disorders. There-
fore, the control and restriction of VOC emissions has
become an important issue in air-quality management.1
Among various methods, including condensation,
absorption, direct oxidation, catalytic oxidation, and cat-
alytic ozonation (catalytic oxidation with ozone) that have
ꢀ
below 100 C, while catalytic oxidation is usually effi-
cient above 200 ꢀC. Reduction of process temperature
can reduce the operation cost for VOC removal consider-
ably and provide the possibility of removing indoor VOCs
efficiently.
In this study, benzene, one of the representative haz-
ardous air pollutants attracting increasing attention due
to their relevance to public health, was chosen as the
subject VOC. Benzene is widely used for solvent of
organic compounds, building material, paint, and adhe-
sives. Evaporated benzene causes respiratory irritation,
skin allergy, dizziness, and headache. Benzene is also a
carcinogen. When waste benzene needs to be disposed,
it should be separated from other wastes due to its high
inflammability.2
Mn-based catalysts have widely been used for low-
∗Author to whom correspondence should be addressed.
temperature catalytic oxidation of VOCs.3–8 Support
454
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 1
1533-4880/2015/15/454/005
doi:10.1166/jnn.2015.8361

An et al.
Benzene Oxidation with Ozone Over MnOx/MSU-H and MnOx/Mesoporous-SAPO-34 Catalysts
materials with large surface area are advantageous for dis-
persion of Mn. Among other support materials, meso-
porous materials have large specific surface area and large
pores (2 nm–50 nm), enabling large VOC molecules
like benzene to diffuse easily to active metal sites present
in the pores. MCM-41, MCM-48, SBA-15, KIT-6, and
MSU-H are representative mesoporous materials that have
been applied effectively to a number of catalytic reactions.
Mesoporous materials converted from microporous mate-
rials are also widely used.
In this study, benzene, MSU-H, and mesoporous SAPO-
34 (Meso-SAPO-34) were selected as the model VOC,
mesoporous material, and mesoporous material from
microporous material, respectively. MSU-H and Meso-
SAPO-34 have never been applied before for catalytic
oxidation or catalytic ozonation of VOCs. These two
mesoporous materials were impregnated by 15 wt% Mn
and applied to catalytic oxidation with ozone of benzene
to examine their potential for the removal of benzene.
60 ml/min controlled by an MFC. The total flow rate was
120 ml/min, while the concentrations of ozone and ben-
zene were 1000 ppm and 100 ppm, respectively. The size
and mass of the catalyst used for each experiment were
25∼35 mesh and 0.05 g, respectively. Before experiments,
ꢀ
catalysts were calcined for 30 min at 300 C under oxy-
gen condition. Three different reaction temperatures were
used: 50 ꢀC, 80 ꢀC, and 100 ꢀC. Each reaction was allowed
to continue for 150 min. Product gas was analyzed by gas
chromatography equipped with a flame ionization detector
(Young Lin, column HP-5) to determine the concentration
of benzene. An indoor gas analyzer was used to measure
the concentrations of CO and CO2 produced.
3. RESULTS AND DISCUSSION
3.1. Characteristics of Catalysts
Table I compares the specific surface areas and pore vol-
umes of the catalysts. The specific surface area of MSU-H
was 820 m2/g, while it decreased to 370 m2/g after the
impregnation of Mn. The pore volume was also reduced
from 0.98 cm3/g to 0.65 cm3/g by the impregnation of Mn.
This result is attributed to partial blockage of pores by
the impregnated Mn. Similarly, the specific surface area
and pore volume of Meso-SAPO-34 were also reduced
by the impregnation of Mn from 499 m2/g to 225 m2/g
and from 0.58 m3/g to 0.19 m3/g, respectively. The nitro-
2. EXPERIMENTAL DETAILS
2.1. Synthesis of Catalysts
Meso-SAPO-34 was prepared in the presence of carbon
black as a hard template.9 MSU-H was purchased from
Sigma-Aldrich. MSU-H and Meso-SAPO-34 were impreg-
nated by 15 wt% Mn using Mn(CH COO) (Aldrich,
99%+) following the incipient wetness method. The
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gen sorption isotherms shown in Figure 1(a) demonstrate
that both MSU-H and Meso-SAPO-34 exhibit hysteresis,
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15 wt% Mn-impregnated mesoporous catalysts will be
referred to, hereafter, as MnOx/MSU-H and MnOx/Meso-
SAPO-ꢀ34, respectively. These catalystsꢀwere dried for 12 h
at 110 C and calcined for 3 h at 550 C.
Copyright: American Scientific Publishers
a characteristic of mesoporous materials. The slopes of
the isotherms of MSU-H and Meso-SAPO-34 increased
rapidly at P/P0 = 0ꢂ7 and P/P0 = 0ꢂ9, respectively.
When they were impregnated by Mn, MSU-H retained
hysteresis, while the hysteresis of Meso-SAPO-34 was
weakened substantially. As shown in Figure 1(b), the pore
size of MSU-H was ca. 10 nm, while that of Meso-SAPO-
34 was ca 4 nm and 15 nm. After the impregnation of Mn,
similar pore size was observed for MSU-H, whereas most
mesopores of Meso-SAPO-34 disappeared.
Figure 2 compares the XRD patterns of MSU-H and
Meso-SAPO-34 observed before and after the impregna-
tion of Mn. For both MSU-H and Meso-SAPO-34, no Mn
peak was observed after the impregnation of Mn, indicat-
ing MnOx was dispersed well.3ꢃ4 The peak intensity at low
angle shown in Figure 2(a) indicates that the structure of
MSU-H was preserved well. On the other hand, the peak
intensity of Meso-SAPO-34 was dramatically reduced at
2.2. Characterization of Catalysts
The specific surface areas of the catalysts were estimated
using the Brunauer–Emmett–Teller (BET) equation. Pore
size distribution was calculated by the Barrett–Joyner–
Halenda method. The powder X-ray diffraction (XRD)
was determined by X-ray diffractometer (Rigaku D/MAX-
III) using Cu-Kꢁ radiation. N2 adsorption–desorption
isotherms were obtained using a Micromeritics ASAP
2000 at −196 ꢀC (liquid N2ꢀ. H2-Temperature pro-
grammed reduction (TPR) analysis was performed with
0.06 g of calcined catalyst, increasing temperature from
ꢀ
ꢀ
room temperature to 700 C at a rate of 10 C min−1 in
10 wt% H2/He gas flow. The hydrogen consumption was
monitored via a thermal conductivity detector.
Table I. BET surface area, pore volume of the catalysts.
2.3. Benzene Oxidation
BET surface
Pore
A fixed-bed flow reactor was used for the catalytic oxi-
dation with ozone of benzene. An ozone generator was
used to produce ozone with a concentration of 2000
50 ppm. The ozone flow rate was controlled by a mass flow
controller (MFC, Brooks 5850E) at 60 ml/min. 200 ppm
benzene (N2 balance) was introduced at a flow rate of
Sample
area (m2 g−1
ꢀ
volume (cm3 g−1
ꢀ
MSU-H
820
370
499
225
0.98
0.65
0.58
0.19
MnOx/MSU-H
Meso-SAPO-34
MnOx/Meso-SAPO-34
J. Nanosci. Nanotechnol. 15, 454–458, 2015
455

Benzene Oxidation with Ozone Over MnOx/MSU-H and MnOx/Mesoporous-SAPO-34 Catalysts
An et al.
(a)
(a)
MSU-H
MnOx/MSU-H
MSU-H
MnOx/MSU-H
Meso-SAPO-34
MnOx/Meso-SAPO-34
0.0
0.2
0.4
0.6
0.8
1.0
1
2
3
4
5
P/P0
2θ (degree)
(b)
(b)
MSU-H
MSU-H
MnOx/MSU-H
MnOx/MSU-H
Meso-SAPO-34
Meso-SAPO-34
MnOx/Meso-SAPO-34
MnOx/Meso-SAPO-34
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15
20
25
30
Copyright: American Scien5tific Publi1s0hers
0
10
20
30
40
50
60
70
2θ (degree)
Pore Size (nm)
Figure 2. XRD patterns of mesoporous materials (a) at low angle (b)
at high angle.
Figure 1. (a) N2-isotherms and (b) pore size distributions.
high angle (Fig. 2(b)) by the impregnation of Mn, indicat-
ing that the destruction of Meso-SAPO-34 structure was
more considerable. This result implies that MSU-H has
better structural stability than Meso-SAPO-34 against the
impregnation of Mn via Mn acetate.
However, the COx yield increased with increasing tem-
perature. At 50 ꢀC, the COx yield was only 57.2% although
the benzene conversion was 97%, breaking the carbon
balance.
Figure 3 shows the TPR results of the two Mn-
impregnated catalysts. MnOx/MSU-H and MnOx/Meso-
SAPO-34 exhibited the maximum reduction peak at
420 ꢀC and at 550 ꢀC, respectively, indicating that
the lattic oxygen mobility of MnOx/MSU-H is supe-
rior to that of MnOx/Meso-SAPO-34, promoting the oxi-
dation reaction more effectively.3ꢃ4ꢃ7ꢃ8 The peak area,
which indicates the amount of hydrogen consumed, was
much larger for MnOx/MSU-H, implying that the reduc-
ing ability of MnOx/MSU-H is greater than that of
MnOx/Meso-SAPO-34.
MnOx/MSU-H
MnOx/Meso-SAPO-34
3.2. Catalytic Oxidation of Benzene
Figure 4 shows the results of the catalytic oxidation with
ozone of benzene over MnOx/Meso-SAPO-34 and over
MnOx/MSU-H. The benzene conversion obtained with
MnOx/MSU-H was shown to be larger than 95% at all
temperature.
100
200
300
400
500
600
700
Temperature (ºC)
Figure 3. TPR curvess of MnOx/MSU-H and MnOx/Meso-SAPO-34.
J. Nanosci. Nanotechnol. 15, 454–458, 2015
456

An et al.
Benzene Oxidation with Ozone Over MnOx/MSU-H and MnOx/Mesoporous-SAPO-34 Catalysts
Ozone conversion (%)
Benzene conversion (%)
To examine the effect of steam on the catalytic activity,
0.7% steam was addedꢀin the experiment performed with
CO yield (%)
100
80
60
40
20
0
x
100
80
60
40
20
0
MnOx/MSU-H at 100 C. The COx yield was increased
a little by the addition of steam (Fig. 5). This is because
steam is adsorbed on the catalyst surface, preventing the
reaction intermidiates from accumulating on the catalyst
surface. Jin et al.10 also reported that the addition of steam
enhanced the COx yield. In particular, the increase in the
CO2 yield was more profound than that of CO.
The ozone conversion and COx yield obtained with
MnOx/MSU-H increased with increasing temperature. This
is because ozone was consumed to convert benzene to
COx.3ꢃ4 The following mechanism3ꢃ4ꢃ11ꢃ12 has been sug-
gested for the oxidation with ozone of benzene, which
is believed to be valid also in the experiments of this
study.
Figure 4. Ozone conversion, benzene conversion and COx yield of
MnOx/MSU-H and MnOx/Meso-SAPO-34 catalysts at ozone concentra-
tion of 1000 ppm.
O3 → O2 +O∗
O∗ +O3 → O2 +O∗2
O∗2 → O2 +∗
(1)
(2)
(3)
This result implies that benzene or reaction intermedi-
ates were adsorbed on the catalyst surface, inhibiting the
completion of the oxidation reaction.
∗
where represents the active sites on the catalyst.
This phenomenon disappeared as temperature increased.
Meanwhile, the benzene conversion and COx yield
obtained at 100 ꢀC with MnOx/Meso-SAPO-34 were
shown to be 41.5% and 32.7%, being much smaller than
4. CONCLUSIONS
When MnOx/MSU-H was applied to the catalytic oxi-
dation with ozone of benzene at 50 ꢀC, 80 ꢀC, and
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those obtained with MnOx/MSU-H. The lower activity
of MnOx/Meso-SAPO-34 can be attributed to its lower
lattice oxygen mobility and structural stability, as was
deduced from the TPR and XRD analyses. Previous stud-
ies have also reported that the reducing ability of Mn
(lattice oxygen ability) is an important factor determining
the activity of mesoporous catalysts.3ꢃ4ꢃ7ꢃ8
ꢀ
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100 C, the ozone consumption and COx production
increased with increasing temperature. At 50 ꢀC, con-
siderable amount of benzene was removed by adsorp-
tion. As temperature increased, the effect of adsorption
decreased. The catalytic activity of MnOx/MSU-H was
much higher than that of MnOx/Meso-SAPO-34. This is
attributed to the higher structural stability and reducing
ability (high oxygen mobility) of MSU-H. Addition of
0.7% steam increased the COx yield. In particular, the
selectivity toward CO2 increased. This result indicates that
the reaction intermidiates adsorbed on the catalyst surface
were desorbed by steam, to be oxidized further.
Copyright: American Scientific Publishers
100
CO
CO2
80
60
40
20
0
Acknowledgments: This research was supported by the
Converging Research Center Program, through the Min-
istry of Science, ICT and Future Planning, Korea (No.
2013K000394). Also, this research was supported by Basic
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education (2012R1A1B3003394).
References and Notes
O
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(2012).
/MSU-H 50 ºC/MSU-H 80 ºC
/MSU-H 100 ºC
x
x
x
MSU-H 100 ºC H
MnO
MnO
/
MnO
x
/Meso-SAPO-34 100 ºC
x
MnO
MnO
Figure 5. Selectivity of CO and CO2 in COx yield.
J. Nanosci. Nanotechnol. 15, 454–458, 2015
457

Benzene Oxidation with Ozone Over MnOx/MSU-H and MnOx/Mesoporous-SAPO-34 Catalysts
An et al.
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Received: 20 February 2013. Accepted: 25 March 2013.
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