Catalytic oxidation of benzene using mesoporous α-Mn 2O3

Article abstract of DOI:10.1166/jnn.2013.7910

The catalytic oxidation of benzene was carried out over mesoporous α-Mn₂O₃, MnO_x/KIT-6, and bulk commercial Mn oxides (Mn₂O₃, MnO₂, and MnO). The catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, and temperatureprogrammed reduction analysis. MnO_x/KIT-6, prepared by impregnating MnO_x on KIT-6, exhibited a low activity for the oxidation of benzene, whereas mesoporous α-Mn₂O₃, manufactured using KIT-6 as the template, showed a high activity. The order of the activities of bulk Mn oxides for benzene decomposition was shown to be Mn₂O₃ MnO ₂ MnO. Therefore, the high activity of mesoporous α-Mn ₂O₃ is attributed to the uniform distribution of highly active Mn₂O₃ in the mesoporous structure. Copyright

Full text of DOI:10.1166/jnn.2013.7910

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 7472–7476, 2013
Catalytic Oxidation of Benzene Using
Mesoporous ꢀ-Mn O3
2
1
2
3
3
4
Hyung Bum An , Ji Man Kim , Jongsoo Jurng , Gwi-Nam Bae , Jong-Ki Jeon ,
5
1ꢀ6ꢀ∗
Sung Hoon Park , and Young-Kwon Park
1
Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Korea
Department of Chemistry, BK21 School of Chemical Materials Science and Department of Energy Science,
2
Sungkyunkwan University, Suwon 440-746, Korea
Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology,
3
Seoul 136-791, Korea
4
Department of Chemical Engineering, Kongju National University, Cheonan 330-717, Korea
5
Department of Environmental Engineering, Sunchon National University, Suncheon 540-950, Korea
6
School of Environmental Engineering, University of Seoul, Seoul 130-743, Korea
The catalytic oxidation of benzene was carried out over mesoporous ꢀ-Mn O , MnO /KIT-6, and
2
3
x
bulk commercial Mn oxides (Mn O , MnO , and MnO). The catalysts were characterized using X-ray
2
3
2
diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analysis, and temperature-
programmed reduction analysis. MnO /KIT-6, prepared by impregnating MnO on KIT-6, exhibited a
x
x
low activity for the oxidation of benzene, whereas mesoporous ꢀ-Mn O , manufactured using KIT-6
2
3
as the template, showed a high activity. The order of the activities of bulk Mn oxides for benzene
Delivered by Publishing Technology to: Adelaide Theological Library
decomposition was shown to be Mn O > MnO > MnO. Therefore, the high activity of mesoporous
2
3
2
IP: 117.243.199.182 On: Thu, 21 Jan 2016 01:54:44
ꢀ
-Mn O is attributed to the uniform distribution of highly active Mn O in the mesoporous structure.
2
3
2
3
Copyright: American Scientific Publishers
Keywords: Mesoporous ꢀ-Mn O , Benzene, Oxidation.
2
3
1
. INTRODUCTION
properties as a solvent, although it is a carcinogenic sub-
stance requiring great care when used.
1
Air pollutants are produced as a result of human activi-
ties, directly or indirectly causing discomfort and disease.
Among other air pollutants, volatile organic compounds
There are various techniques for the removal of VOCs,
e.g., direct combustion, catalytic oxidation, and catalytic
ozonation. Although all these techniques are efficient
for the removal of VOCs, direct combustion requires
(VOCs) are defined as compounds of carbon excluding
carbon monoxide, carbon dioxide, carbonic acid, metal
carbonates, and ammonium carbonate. VOCs are emit-
ted from various sources including mobile sources such
as vehicles, petroleum chemical factories, dye works, and
construction materials such as paints, adhesives, and inte-
rior 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
ꢀ
high temperatures (600–900 C), while catalytic oxida-
tion can decompose VOCs at much lower temperatures
ꢀ
300–400 C), thus lowering the maintenance costs.
1–11
(
The catalytic ozonation of VOCs can take place at even
lower temperatures. The products of catalytic ozonation,
however, include hazardous species such as CO, formic
acid, acetic acid, acetaldehyde, oxalic acid, and unreacted
12
ozone, which require post-treatment equipment, while
H O and CO are the main products of catalytic oxidation.
2
2
Noble metals such as Pt and Pd and transition met-
als including Mn and Cu are usually used for the cat-
alytic oxidation of VOCs. In particular, transition metals
are cost-effective compared to noble metals, and are there-
1
become an important issue in air-quality management.
Among VOCs, benzene is the most representative
species. It is widely used because of its outstanding
1–4ꢁ12
fore attracting much attention in this regard.
In this study, the catalytic oxidation of benzene was car-
∗
Author to whom correspondence should be addressed.
ried out over Mn-based catalysts. Mesoporous ꢂ-Mn O
2
3
7
472
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 11
1533-4880/2013/13/7472/005
doi:10.1166/jnn.2013.7910

An et al.
Catalytic Oxidation of Benzene Using Mesoporous ꢂ-Mn O3
2
was applied to the catalytic oxidation of benzene for the
first time. For the investigation of the effect of the addi-
tion of Cu as a promoter, 15 wt% Cu was added to the
mesoporous ꢂ-Mn O (hereafter, 15 wt% Cu/Mesoporous
2.3. Catalytic Oxidation of Benzene
Air containing 100-ppm benzene with a flow rate of
−
1
1
20 mL min was introduced into the reactor through a
2
3
mass flow controller (MFC, Brooks 5850E). The size and
amount of the catalyst used in the experiment were 25–
ꢂ-Mn O ꢀ, and its catalytic activities before and after the
2
3
addition of Cu were compared. Another catalyst, 15 wt%
MnO /KIT-6, was synthesized by impregnating MnO on
35 mesh and 0.1 g, respectively. Before each experiment,
x
x
the catalysts were calcined for 30 min in an electrical fur-
the mesoporous silica (KIT-6), which is the template mate-
ꢀ
nace maintained at 300 C under an oxygen atmosphere.
rial of mesoporous ꢂ-Mn O . The catalytic activity of this
2
3
The reactor was maintained at the initial reaction tem-
material was compared with that of mesoporous ꢂ-Mn O .
2
3
ꢀ
perature of 50 C for 30 min. The temperature was then
The activities of commercial manganese oxides (Mn O ,
2
3
ꢀ
−1
increased for 10 min at a rate of 5 C min , and was
maintained at the elevated temperature for 30 min. This
procedure was repeated until the temperature reached the
MnO , and MnO) for VOC removal were also evaluated.
2
2
. EXPERIMENTAL DETAILS
.1. Synthesis of Catalysts
Mesoporous ꢂ-Mn O was prepared using a nano-
ꢀ
final temperature of 400 C. The benzene content in the
product gas was analyzed using a gas chromatography
flame ionization detector (GC-FID, Young Lin, column
2
HP-5). The CO content was measured using a nondisper-
2
3
3
2
replication method. KIT-6 was used as the template, with
sive infrared (NDIR) sensor (LI-COR, LI-820).
manganese (III) nitrate hexahydrate (Mn(NO ꢀ · 6H O,
3
2
2
Aldrich) as the precursor. The molar ratios of KIT-6,
3
. RESULTS AND DISCUSSION
Mn(NO ꢀ · 6H O, and H O were 0.1136, 0.0158, and
3
2
2
2
0
.1667, respectively. In a typical synthesis, the precursor
3.1. Characterization of Catalysts
was dissolved in distilled water and then impregnated on
the silica template. For the spontaneous infiltration of the
manganese precursor into the pores of the silica template,
Table I shows the BET results for the various cata-
lysts. The specific surface area of MnO /KIT-6 is the
largest, while that of 15 wt% Cu/Mesoporous ꢂ-Mn O
is the smallest. When Cu was added to the mesoporous
x
ꢀ
the sample was dried overnight at 80 C. Subsequently, the
2
3
De
ꢀ
livered by Publishing Technology to: Adelaide Theological Library
sample was heated at 550 C under ambient conditions for
IP: 117.243.199.182 On: Thu, 21 Jan 2016 01:54:44
3
h. The silica template was mostly removed using 2 M
ꢂ-Mn O , the specific surface area and pore volume
Copyright: American Scientific Publishers
2 3
NaOH aqueous solution twice. Eventually, the mesoporous
decreased; this can be attributed to the blocking of pores
by Cu. Figure 1 shows the nitrogen isotherms (a) and
pore-size distributions (b) of mesoporous ꢂ-Mn O and
ꢂ-Mn O material was obtained by washing the sample
2
3
several times with distilled water and acetone, and then
2
3
ꢀ
drying at 80 C.
15 wt% Cu/Mesoporous ꢂ-Mn O . Hysteresis, a charac-
2 3
(
C H O ꢀ Cu · H O (Copper (II) acetate monohydrate,
teristic of mesoporous materials, was observed for both
these materials. The pore size was also observed to be 2.6
and 14.2 nm for mesoporous ꢂ-Mn O and 14.2 nm for
2
3
2
2
2
9
1
8+%) was used as the Cu precursor for the addition of
5 wt% Cu to the mesoporous ꢂ-Mn O using the impreg-
2
3
2
3
ꢀ
nation method. The sample was then dried at 110 C and
calcined at 300 C.
1
Mn acetate on KIT-6. Commercial Mn O , MnO , and
MnO were purchased from Sigma-Aldrich.
15 wt% Cu/Mesoporous ꢂ-Mn O , which are within the
2 3
ꢀ
typical mesopore size range.
5 wt% MnO /KIT-6 was synthesized by impregnating
Figure 2 shows the low-angle (a) and high-angle (b)
XRD patterns of the two mesoporous materials. A char-
acteristic peak of mesoporous ꢂ-Mn O appeared in the
x
1
3
2
3
2
2
3
3
low-angle results (Fig. 2(a)). This peak did not disappear
when Cu was added, although the peak size decreased,
indicating that the impregnation of Cu did not destroy
the mesoporous structure. A characteristic peak of meso-
porous ꢂ-Mn O was also observed in the high-angle
2
.2. Characterization of Catalysts
The powder X-ray diffraction (XRD) patterns were
obtained with an X-ray diffractometer (Rigaku D/MAX-
III) using Cu-Kꢂ radiation. N2 adsorption–desorption
2
3
isotherms were obtained using a Micromeritics ASAP
Table I. Surface areas and total pore volumes of mesoporous material
samples.
ꢀ
2
000 instrument at −196 C (liquid N ꢀ. Brunauer–
2
Emmett–Teller (BET) analysis was used to estimate the
BET surface area. Temperature-programmed reduction
BET
surface area (m g
Pore
volume (cm g ꢀ
2
−1
3
−1
Sample
ꢀ
(
TPR) was carried out with 0.06 g of the calcined cata-
ꢀ
lyst, from room temperature to 700 C, at a heating rate of
1
15 wt% MnO /KIT-6
314
119
55
0.52
0.33
0.22
x
ꢀ
−1
0 C min in a flow of 10 wt% H /He gas. The hydrogen
Mesoporous ꢂ-Mn
5 wt% Cu/mesoporous
ꢂ-Mn O
2 3
O
2
1
consumption was monitored using a thermal conductivity
detector.
2
3
J. Nanosci. Nanotechnol. 13, 7472–7476, 2013
7473

Catalytic Oxidation of Benzene Using Mesoporous ꢂ-Mn O
An et al.
2
3
(
a)
(b)
2
Mesoporous α-Mn O3
Mesoporous α-Mn O3
2
1
5wt% Cu/Mesoporous α-Mn O3
2
1
5wt% Cu/Mesoporous α-Mn O3
2
0.0
0.2
0.4
0.6
0.8
1.0
5
10 15 20 25 30 35 40
Pore size (nm)
P/P0
Fig. 1. (a) Nitrogen isotherms of mesoporous catalysts (b) pore size distributions of mesoporous catalysts.
3
results (Fig. 2(b)). Again, the impregnation of Cu reduced
3.2. Results of Catalytic Oxidation of Benzene
ꢀ
the peak size, while a Cu peak appeared at about 2ꢃ = 35 .
Figure 4 compares the results for the catalytic oxida-
tion of benzene over different catalysts: mesoporous ꢂ-
Mn O , 15 wt% Cu/Mesoporous ꢂ-Mn O , bulk Mn oxides
In the case of 15 wt% MnO /KIT-6, the mesopore struc-
x
ture and even distribution of MnO within the template
structure were demonstrated in a previous study.
Figure 3 shows the TPR analysis results for mesoporous
ꢂ-Mn O and Cu/Mesoporous ꢂI P- M: 1n 1 7O .2.4 T3 h. 1e 9 a9d . d1 i 8t i 2o nO on f: Thu, 21 Jan 2016 01:54:44
Cu resulted in a single peak appearing at 300 C, indicat-
ing that the reduction took place at a lower temperature
than before the addition of Cu. This result implies that
Cu/Mesoporous ꢂ-Mn O has a higher reducing power
i.e., higher oxygen mobility) than mesoporous ꢂ-Mn O .
x
1
3
2
3
2
3
(
Mn O , MnO , and MnO), and 15 wt% MnO /KIT-6.
2 3 2 x
Delivered by Publishing Technology to: Adelaide Theological Library
Generally, carbon balance was achieved between the ben-
2
3
2
3
ꢀ
zene conversion and CO production at 300 C indicating
2
ꢀ
Copyright: American Scientific Publishers
that most of the benzene was converted to CO through
2
the oxidation reaction. At lower temperatures, on the other
hand, carbon balance was not achieved because a consider-
able amount of benzene was not decomposed, but adsorbed
on the surface of the catalysts. When commercial bulk Mn
catalysts were used for the oxidation of benzene, the order
2
3
(
2 3
It was previously reported that MnO /KIT-6 is composed
of well-distributed MnO such as Mn O and MnO .
x
1
3
ꢀ
x
2
3
2
of catalytic activity was Mn O > MnO > MnO at 300 C,
2
3
2
(
a)
(
b)
Mesoporous α-Mn O3
Mesoporous α-Mn O3
2
2
1
5wt% Cu/Mesoporous α-Mn O3
15wt% Cu/Mesoporous α-Mn O3
2
2
CuO (JCPDS 80-1916)
1
2
3
4
5
10
20
30
40
50
60
70
80
2θ
2θ
Fig. 2. XRD patterns of mesoporous catalysts (a) at low angle (b) at high angle.
7
474
J. Nanosci. Nanotechnol. 13, 7472–7476, 2013

An et al.
Catalytic Oxidation of Benzene Using Mesoporous ꢂ-Mn O3
2
100
(
a)
1
5wt% Cu/Mesoporous α-Mn O3
2
Mesoporous
Mn2O3
MnO2
α-Mn O3
2
Mesoporous α-Mn O3
2
8
6
4
2
0
0
0
0
0
15wt% Cu/Mesoporous α-Mn O3
2
MnO
1
5wt% MnO /KIT-6
x
0
50
100
150
200
250
300
Temperature (ºC)
(
b) 600
15wt% Cu/Mesoporous
α
-Mn O
2
3
Mesoporous α
-Mn O3
2
5
4
3
2
00
00
00
00
Mn O3
2
MnO2
MnO
100
200
300
400
500
600
700
1
5wt% MnO /KIT-6
x
Temperature (ºC)
Fig. 3. TPR analysis results of mesoporous catalysts.
that is, the Mn O phase showed the highest activity for
2
3
100
benzene oxidation. The higher activity of mesoporous ꢂ-
Mn O compared with bulk Mn O can be attributed to the
2
3
2
3
0
Delivered by Publishing Technology to: Adelaide Theological Library
0
50
100
150
200
250
300
even distribution of Mn O in the mesoporous structure.
2
3
IP: 117.243.199.182 On: Thu, 21 Jan 2016 01:54:44
Temperature (ºC)
In particular, the benzene conversion of mesoporous ꢂ-
Mn O was more than double that of bulk Mn O at 300 C.
Copyright: American Scientific Publishers
ꢀ
2
3
2
3
Fig. 4. Benzene conversion and CO yield obtained using different
2
This result demonstrates that mesoporous ꢂ-Mn O is an
catalysts.
2
3
excellent catalyst for benzene oxidation. The benzene con-
version and CO yield were slightly increased when 15
2
the manganese oxide is a more important factor than the
specific surface area in determining the catalytic activ-
wt% Cu/Mesoporous ꢂ-Mn O was used, which is con-
2
3
sistent with its higher reducibility compared with meso-
ity. Unlike 15 wt% MnO /KIT-6, in which various man-
x
porous ꢂ-Mn O (Fig. 3). However, further study would be
2
3
ganese oxides with different oxidation states, e.g., Mn O
2
3
required to evaluate the catalytic activity of Cu/Mesoporous
13
and MnO , co-exist, mesoporous ꢂ-Mn O contains only
2
2
3
ꢂ-Mn O more quantitatively.
2
3
well-distributed Mn O , which has the highest activity
2 3
The activity of 15 wt% MnO /KIT-6 was generally
x
among the manganese oxides, resulting in its higher cat-
alytic activity.
low. This result is contrary to that reported in a previ-
ous study for the catalytic ozonation of benzene, in which
MnO /KIT-6 showed a higher activity than mesoporous
x
3ꢁ13
ꢂ-Mn O .
Ozone molecules can be decomposed eas-
4. CONCLUSIONS
2
3
ily over the well-distributed MnO surface of MnO /KIT-6
x
x
1
3
Mesoporous ꢂ-Mn O , which contains well-distributed
2
3
into oxygen radicals via the following reaction:
Mn O , the most active manganese oxide for benzene
2
3
∗
oxidation, showed a very high catalytic activity for the
decomposition of benzene. On the other hand, 15 wt%
O +∗ → O +O
3
2
The oxygen radicals can then oxidize the benzene
adsorbed on the catalyst surface. Because 15 wt%
MnO /KIT-6, which contains various manganese oxides
x
with different oxidation states, exhibited a relatively low
catalytic activity for the decomposition of benzene, in spite
of its large specific surface area.
MnO /KIT-6 has a larger specific surface area than meso-
x
porous ꢂ-Mn O , the amount of benzene adsorbed on the
2
3
catalyst surface is larger for MnO /KIT-6, which leads to
x
a higher reaction activity. In the catalytic oxidation, on
the other hand, it is not ozone but oxygen molecules that
participate in the reaction. Therefore, the characteristic of
Acknowledgments: This research was supported by
the Converging Research Center Program, funded by
the Ministry of Education, Science and Technology
J. Nanosci. Nanotechnol. 13, 7472–7476, 2013
7475

Catalytic Oxidation of Benzene Using Mesoporous ꢂ-Mn O
An et al.
2
3
(
No. 2012K001372). Also, this research was supported
5. B. Izadkhah, S. R. Nabavi, A. Niaei, D. Salari, T. M. Badiki, and
N. Caylak, J. Ind. Eng. Chem. 18, 2083 (2012).
by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by
the Ministry of Education, Science and Technology
6
7
8
9
. C. He, X. Zhang, S. Gao, J. Chen, and Z. Hao, J. Ind. Eng. Chem.
8, 1598 (2012).
. J. B. Kim, J. I. Park, H. S. Kim, and Y. J. Yoo, J. Ind. Eng. Chem.
8, 425 (2012).
. Q. Huang, Z. Y. Zhang, W. J. Ma, Y. W. Chen, S. M. Zhu, and S. B.
Shen, J. Ind. Eng. Chem. 18, 757 (2012).
1
(2012R1A1B3003394).
1
References and Notes
. A. Jodaei, A. Niaei, and D. Salari, Korean J. Chem. Eng. 28, 1665
(
2011).
1
. C. R. Lee, J. Jurng, G. N. Bae, J. K. Jeon, S. C. Kim, J. M. Kim,
M. Jin, and Y. K. Park, J. Nanosci. Nanotechnol. 11, 7303 (2011).
. J. H. Park, J. Jurng, G. N. Bae, S. H. Park, J. K. Jeon, S. C. Kim,
J. M. Kim, and Y. K. Park, J. Nanosci. Nanotechnol. 12, 5942
1
0. M. H. Kim, K. H. Cho, C. H. Shin, S. E. Kang, and S. W. Ham,
Korean J. Chem. Eng. 28, 1139 (2011).
1. L. Laokiat, P. Khemthong, N. Grisdanurak, P. Sreearunothai,
W. Pattanasiriwisawa, and W. Klysubun, Korean J. Chem. Eng. 29,
377 (2012).
12. H. Einaga and S. Futamura, Catal. Commun. 8, 557 (2007).
13. J. H. Park, J. M. Kim, J. Jurng, G. N. Bae, S. H. Park, S. C. Kim,
J. K. Jeon, and Y. K. Park, J. Nanosci. Nanotechnol. 13, 423 (2013).
2
1
(
2012).
3
. M. Jin, J. W. Kim, J. M. Kim, J. Jurng, G. N. Bae, J. K. Jeon, and
Y. K. Park, Powder Technol. 214, 458 (2011).
. J. H. Park, J. M. Kim, M. Jin, J. K. Jeon, S. S. Kim, S. H. Park,
S. C. Kim, and Y. K. Park, Nanoscale Res. Lett. 7, 14 (2012).
4
Received: 15 May 2012. Accepted: 20 December 2102.
Delivered by Publishing Technology to: Adelaide Theological Library
IP: 117.243.199.182 On: Thu, 21 Jan 2016 01:54:44
Copyright: American Scientific Publishers
7
476
J. Nanosci. Nanotechnol. 13, 7472–7476, 2013