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Catalysis Today 204 (2013) 108– 113

Contents lists available at SciVerse ScienceDirect

Catalysis Today

j ourna l ho me p ag e: www.elsev ier .com/ lo cate /ca t tod

enzene oxidation with ozone over MnOx/SBA-15 catalysts

ingshi Jina, Jung Hwan Kimb, Ji Man Kima, Jong-Ki Jeonc, Jongsoo Jurngd,wi-Nam Baed, Young-Kwon Parkb,e,∗

Department of Chemistry, BK21 School of Chemical Materials Science and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of KoreaGraduate School of Energy and Environmental System Engineering, University of Seoul, Seoul 130-743, Republic of KoreaDepartment of Chemical Engineering, Kongju National University, Cheonan 330-717, Republic of KoreaCenter for Environment, Health and Welfare Research, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of KoreaSchool of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea

r t i c l e i n f o

rticle history:eceived 15 May 2012eceived in revised form1 September 2012ccepted 20 September 2012vailable online 12 December 2012

a b s t r a c t

Catalytic oxidation of benzene with ozone has been studied using manganese oxides with two differ-ent manganese precursors, Mn(NO3)2 and Mn(CH3COO)2, supported on SBA-15 (MnOx/SBA-15). Thecatalysts were characterized by X-ray diffraction, N2 adsorption–desorption, Raman spectroscopy, andH2-temperature programmed reduction. The manganese nitrate (MN) precursor primarily resulted inlarge particles on the silica support, while the manganese acetate (MA) precursor mainly resulted in ahighly dispersed manganese oxide on the silica support. The catalytic activity was dependent upon ozone

eywords:nOx/SBA-15

atalytic oxidationenzenezone

concentration, reaction times, and the amount of Mn loading. Higher benzene conversion, O3 conversion,and COx yield were observed for MnOx-MA/SBA-15 catalyst over MnOx-MN/SBA-15, due to the highly dis-persed manganese oxides on the supports, and the higher oxygen mobility. The 15 wt% MnOx-MA/SBA-15catalyst shows the highest catalytic activity of all the catalysts considered in this study.

© 2012 Elsevier B.V. All rights reserved.

n acetaten nitrate

. Introduction

Volatile organic compounds (VOCs), whether directly or indi-ectly, are known to be major contributors to air pollution.herefore, control of VOCs has been one of the most importantssues within the research area of environmental catalysis [1]. Thereave been numerous reported methods for the removal of VOCs,

ncluding catalytic oxidation, thermal oxidation, and adsorptionrocesses. Among these methods, catalytic oxidation is a promisingethod to control the emission of VOCs. On the other hand, if oxida-

ion is performed at 80 ◦C rather than at 200 ◦C, the required energyost of the oxidation process would be much lower. Therefore, itight be better if oxidation is performed at a lower temperature.

o do this, the use of ozone as an alternative oxidant for the catalyticxidation of VOCs has been reported in the literature [2,3].

Supported, and unsupported, manganese oxides such as Mn3O4,

n2O3, and MnO2 are known to exhibit high activity for hydrocar-

on and VOC catalytic combustion, producing CO2 upon completeeactant conversion [4]. In particular, they have shown higher

∗ Corresponding author at: Graduate School of Energy and Environmental Systemngineering, University of Seoul, Seoul 130-743, Republic of Korea.el.: +82 2 2210 5623; fax: +82 2 2244 2245.

E-mail address: catalica@uos.ac.kr (Y.-K. Park).

920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2012.09.026

activity for the complete oxidation of benzene and cyclohexanecompared to the oxides of Fe, Ni, Co, Cu, and Ag [5].

The catalytic properties of MnOx-based catalysts are attributedto the ability of manganese to form oxides with different oxi-dation states, and to their high oxygen storage capacity. Theircatalytic application is primarily due to their high efficiency in thereduction/oxidation reaction cycles. Redox abilities are stronglyenhanced when combined with other elements [6]. In general,manganese oxide catalysts are prepared with a manganese nitrateprecursor, probably because of its high solubility and the easyremoval of the nitrate anion during calcination. However, the use ofmanganese acetate as a precursor has been found to give superiorperformance over manganese nitrate [7,8].

Meanwhile, Oyama and co-workers have studied ozone decom-position on supported manganese oxides in the absence of organicsubstrates [9–11]. Einaga et al. reported that an alumina-supportedmanganese oxide catalyst exhibited high reactivity for the oxida-tion of benzene with ozone [12,13]. In addition, SiO2-, TiO2-, andZSM-5-supported manganese oxide materials were used for thecatalytic oxidation of benzene with ozone, with it noted that thesurface area of the catalysts is one of the most important factors for

the reaction [5].

Recently, mesoporous materials have been considered aspromising catalyst supports with well-defined pore size, large sur-face area, and higher thermal stability. In addition, the application

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M. Jin et al. / Catalysis

f mesoporous materials as catalysts and catalyst supports has beeneviewed [14,15]. In these reviews, a range of catalytic processessing mesoporous materials were described. Therefore, the char-cteristics of mesoporous materials might induce high catalyticctivity for the oxidation of benzene with ozone. Siliceous SBA-5 is considered a representative mesoporous material [14–17]. Tohe best of our knowledge, SBA-15 was used for the first time in theatalytic conversion of benzene with ozone.

In the present study, benzene was used as a model species forOCs. Also, manganese oxides supported on highly ordered meso-orous SBA-15 (MnOx/SBA-15) were prepared using two kinds ofanganese precursor, namely manganese nitrate and manganese

cetate, in order to investigate the effect of manganese precursorsn the catalytic oxidation of benzene with ozone for the first time.

. Experimental

.1. Preparation of SBA-15

Mesoporous silica SBA-15 was obtained following the pro-edures described elsewhere [18]. Typically, a triblock polymer,luronic P123 (EO20PO70EO20, Mav = 5800, Aldrich), was used ashe structure-directing agent and tetraethylorthosilicate (TEOS,AMCHUN) was used as the silica source for the SBA-15 mate-ial. Typically, 30.0 g of P123 was dissolved in a mixture of 721 gf double-distilled water and 182 g of HCl (35%, SAMCHUN). Sub-equently, 64 g of TEOS was added to the polymer solution underigorous stirring at 40 ◦C for 24 h, and heated at 100 ◦C for 24 h. Theroduct was filtered, washed with double-distilled water, and dried

n an oven at 80 ◦C. The white powder thus obtained was washedith EtOH, dried at 80 ◦C for 12 h, and finally calcined at 550 ◦C in

tatic air for 3 h in order to remove the template.

.2. Synthesis of MnOx/SBA-15 catalysts

MnOx/SBA-15 was synthesized by incipient wetness impreg-ation. Mn(NO3)2 (Aldrich, 98%) and Mn(CH3COO)2 (Aldrich, 99%) were used as the Mn precursors. The catalysts were dried at10 ◦C overnight, and then calcined at 550 ◦C. The impregnated Mn

oadings were 5 wt% and 15 wt%. For the synthesis of 5 wt% MA/SBA-5 and 5 wt% MN/SBA-15 catalysts, 2.87 mmol of Mn(NO3)2·4H2Or 2.87 mmol Mn(CH3COO)2·4H2O, respectively, was dissolved in

cm3 of distilled water. The obtained solution was added slowlyo 3 g of SBA-15. The Mn loading was determined by ICP. Fromere on, the SBA-15 catalysts prepared from manganese nitratend manganese acetate are denoted as MN/SBA-15 and MA/SBA-5, respectively. In addition, 15 wt% Mn was impregnated on SiO2hich was purchased from Grace Davison (XPO-2412) using Mn

cetate.

.3. Benzene oxidation

Catalytic reactions were carried out with a fixed-bed flow reac-or. Ozone was synthesized from O2 by a silent discharge ozoneenerator. Prior to the catalytic reaction, the sample, under O2 flow,as heated at 450 ◦C in a Pyrex glass reactor. Then, the catalyst was

ooled with its temperature maintained at 80 ◦C. 0.05 g of catalystas used. 60 ml min−1 of 200 ppm benzene in N2 was mixed with

he 60 ml min−1 of O2 flow. Analysis of the gas sample was per-ormed by gas chromatography for benzene conversion, an indooras analyzer for CO and CO2 products, and an ozone analyzer forzone conversion. In this system, the homogeneous gaseous reac-ion of benzene with ozone can be neglected.

204 (2013) 108– 113 109

2.4. Characterization

X-ray powder diffraction (XRD) patterns were collectedwith a Cu K� X-ray source using a Rigaku D/MAX-II instru-ment. N2 adsorption–desorption isotherms were obtainedusing a Micromeritics ASAP 2000 at −196 ◦C (liquid N2).Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)methods were used to estimate the BET surface area and poresize distributions, respectively. Transmission electron microscopy(TEM, JEOL JEM 3010) was performed at an accelerating voltage of200 kV. Raman spectra were recorded under ambient conditions atroom temperature with an Ar ion laser (Renishaw Inc., U.K.). H2-temperature-programmed reduction (H2-TPR) was performed in aquartz microreactor. 0.06 g samples were first pretreated under anairflow of 30 ml min−1 at 200 ◦C for 1 h, followed by purging with aHe flow of 30 ml min−1 at the same temperature for 30 min beforethey were cooled to room temperature. Subsequently, 10 vol% H2in a He flow of 40 ml min−1 was applied, at room temperature, andmaintained for 30 min. Finally, the sample was ramped to 700 ◦Cat 10 ◦C min−1.

3. Results and discussion

Fig. 1 shows the XRD patterns of MnOx/SBA-15 catalysts withtwo manganese precursors and Mn loadings. As shown in the low-angle XRD patterns, all of the samples exhibit an intense peak andtwo weaker peaks, corresponding to peaks at 1 0 0, 1 1 0, and 2 0 0,that are characteristic of a 2-dimensional hexagonal mesostruc-ture (plane group, P6 mm) [19]. The high-angle XRD patterns of5% MA/SBA-15 and 15% MA/SBA-15 show no diffraction intensity,except for the peak that corresponds to amorphous silica, therebyimplying that manganese oxides that use manganese acetate arehighly dispersed on the support materials. Conversely, manganeseoxides using manganese nitrate precursor gave crystalline diffrac-tion peaks for both MnO2 and Mn2O3, and as the Mn load increasedto 15%, the peak became sharper with increased intensity, thus indi-cating formation of large-sized particles. This is consistent withthe TEM observations (Fig. 2.), which does not show any largemanganese oxide clusters for MA/SBA-15 and highly dispersedmanganese oxide clusters located in the mesoporous channels ofSBA-15. Some large manganese oxides clusters, however, wereobserved on the external surface of MN/SBA-15.

N2 adsorption–desorption isotherms of the SBA-15, 5% MA/SBA-15, 15% MA/SBA-15, 5% MN/SBA-15, and 15% MN/SBA-15 are shownin Fig. 3(a). All of these materials exhibit a Type IV isotherm, which,according to the IUPAC nomenclature, is characteristic of a meso-porous material [20]. Furthermore, these catalysts possess uniformmesopores, as can be seen in the corresponding pore-size distri-bution curves in Fig. 3(b). Textural parameters of all the catalystsare summarized in Table 1. After introducing the manganese pre-cursor, an obvious decrease in the BET surface area, pore volume,and pore size, as well as an increase in the wall thickness wereobserved. This may be due to partial pore blockage by the intro-duction of manganese oxide. In particular, the surface area of theMA/SBA-15 catalysts was significantly lower than MN/SBA-15 cata-lysts. As illustrated by XRD and TEM, MnOx from manganese acetateprecursor might form a highly dispersed small particle. Therefore,the particles can be located mainly in the mesopores and are welldistributed over the internal surface. This might result in a signif-icant decrease in the surface area of MA/SBA-15. MnOx from themanganese nitrate precursor can form large particles, which are

difficult to enter the mesopores and become distributed mainly onthe external surface. Therefore, pore blockage by the introductionof MnOx can be prevented somewhat. Furthermore, these materialsexhibit a unit cell parameter (a0) within the range of 10.6–10.9 nm.

110 M. Jin et al. / Catalysis Today 204 (2013) 108– 113

Fig. 1. XRD pattern of MA/SBA-15 and MN/SBA-15 catalysts.

Fig. 2. HR-TEM images of (a) 15% MA/SBA-15, (b) 15% MN/SBA-15.

Fig. 3. N2-isothem and pore size distribution of MA/SBA-15 and MN/SBA-15 catalysts.

M. Jin et al. / Catalysis Today 204 (2013) 108– 113 111

Table 1Physicochemical properties of MA/SBA-15 and MN/SBA-15 catalysts.

Samples SBET (m2/g)a Vtotal (cm3/g)b DBJH (nm)c Unit cell (nm)d Wall thickness (nm)e

SBA-15 597 0.84 7.4 10.8 3.45% MA/SBA-15 463 0.81 7.4 10.9 3.515% MA/SBA-15 371 0.62 6.3 10.6 4.35% MN/SBA-15 574 0.79 7.4 10.9 3.515% MN/SBA-15 532 0.74 6.9 10.9 4.0

a BET surface areas calculated in the range of p/p0 = 0.05–0.20.b Total pore volumes measured at p/p0 = 0.99.

ze.

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z

COx yields, the most important parameter for benzene oxidation

c BJH pore sizes obtained from the adsorption branches.d The unit cell parameter a0 = 2d100/

√3.

e Wall thickness was obtained from wall thickness = unit cell parameter − pore si

or SBA-15 and SBA-15 impregnated with manganese oxide, therere few changes observed for the lattice parameters of the largenit cell (d100). Table S1 lists the surface area and pore volume ofiO2 and MA/SiO2 (supporting information).

Fig. 4 shows Raman spectra of the catalysts considered in thisork. The 5% MN/SBA-15 and 15% MN/SBA-15 catalysts showed

wo bands at 528 cm−1 and 649 cm−1; and 308 cm−1 and 644 cm−1,espectively. The MnO2 and Mn2O3 gave the most intense Ramaneak at 654 cm−1 [7]. However, the peak for the MN/SBA-15 cat-lyst shows a weak Raman shift, due to the large particle size ofn2O3 [21]. The MA/SBA-15 catalyst gave only a broad, down-

hifted peak at approximately 638 cm−1, assignable to the highlyispersed smaller manganese oxide particles on the support [22].nalogous results, reported previously, have been attributed to theffect of phonon confinement [23,24].

H2-TPR experiments were then carried out (Fig. 5). From thecquired data, the MA/SBA-15 showed higher reduction ability,ndicating that the lattice oxygen mobility was higher than in

N/SBA-15. The broad peak, and easy reduction peak, may beue to the small, more highly dispersed, Mn particles on the sup-ort [22]. The MN/SBA-15 catalysts exhibited one main reductioneak and a small shoulder peak because of the formation of differ-nt particle sizes. The reduction peak that appeared in the loweremperature region could be attributed to the reduction of smallarticles, while the reduction in the high-temperature region coulde attributed to the large-sized particles on the support [7,22].herefore, we expected that the catalytic activity of MA/SBA-15

ould be high due to its high dispersion and high oxygen mobility.

Fig. 6 shows typical time progressions for the oxidation of ben-ene with ozone over manganese oxides, supported on SBA-15.

Fig. 4. Raman spectra of MA/SBA-15 and MN/SBA-15 catalysts.

Remarkably, higher benzene conversion and O3 conversion wereobserved over 5% MA/SBA-15 and 15% MA/SBA-15 catalysts thanwere observed for 5% MN/SBA-15 and 15% MN/SBA-15. It is note-worthy that, with the exception of the 5% MN/SBA-15 catalyst, aclose to constant rate of benzene conversion by the catalysts wasmaintained, even after 150 min of reaction. Although the degreeof aggregated Mn oxides increased with Mn content, the catalyticactivity of 15% MN/SBA-15 was higher than that of 5% MN/SBA-15.These dependencies revealed that not only were the highly dis-persed Mn oxides, but also the aggregated Mn oxides were theactive sites for benzene oxidation with ozone.

The COx (CO + CO2) yields from the 5% MA/SBA-15 and 15%MA/SBA-15 catalysts are higher than those of 5% MN/SBA-15 and15% MN/SBA-15 (Fig. 7). The yields from the 5% MA/SBA-15 and 15%MA/SBA-15 catalysts were 80 and 94%, respectively. On the otherhand, the 5% MN/SBA-15 and 15% MN/SBA-15 catalysts gave lessthan 70%. From the results shown Figs. 5 and 6, it can be concludedthat the 15% MA/SBA-15 catalyst shows the best catalytic activityfor the reaction. This may be ascribed to the fact that an acetateprecursor mainly results in a highly dispersed manganese oxidesurface phase, homogeneously distributed throughout the SBA-15,even though there is a higher Mn content.

The catalytic activity of MA/SBA-15 was also compared with thatof MA/SiO2 to address the efficiency of the mesoporous structure(Fig. 7). The benzene conversion of 15% MA/SBA-15 (98.2%) wasslightly higher than that of 15% MA/SiO2 (95.6%). However, the

with ozone, over 15% MA/SBA-15 and 15% MA/SiO2 were 93.9% and84.1%, respectively. Because adsorption of benzene or conversion

Fig. 5. TPR profiles of MA/SBA-15 and MN/SBA-15 catalysts.

112 M. Jin et al. / Catalysis Today 204 (2013) 108– 113

Time(min)

0 20 40 60 80 100 120 140 160

O3 C

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0

20

40

60

80

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5% MA/SBA-1515% MA/SBA-155% MN/SBA-1515% MN/SBA-15

Time(min)

0 20 40 60 80 100 120 140 160

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)

0

20

40

60

80

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5% MA/SBA-15

15% MA/SBA-15

5% MN/SBA-15

15% MN/SBA-15

Fig. 6. (a) Benzene conversion, (b) O3 conversion of MA/SBA-15 and MN/SBA-15 catalysts at ozone concentration of 1000 ppm.

5% MA/SBA-15

15% MA/SBA-15

5% MA/SBA-15(H2O)

5% MN/SBA-15

15% MN/SBA-15

15% MN/SBA-15(H2O)

15% MA/SiO2

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and MN/SBA-15 catalysts at ozone concentration of 1000 ppm.

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15% MA/SBA-15(H2O)

15% MN/SBA-15

15% MN/SBA-15(H2O)

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0

20

40

60

80

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Fig. 7. Benzene conversion and COx yield of MA/SBA-15

nto intermediates may be included in benzene conversion, it cane regarded that the catalytic activity of 15% MA/SBA-15 which hasigher COx yield may be higher than that of 15% MA/SiO2. As shown

n Fig. S1 (Supporting Information), Mn peaks were observed in theRD pattern of 15% MA/SiO2. In addition, TEM showed that some

arge manganese oxides were observed on MA/SiO2 (Fig. S2). Thisuggests that larger MnOx is formed on the SiO2. From these results,BA-15 appears to have the potential to be a better support for Mnxide for benzene oxidation with ozone than SiO2.

Fig. 8 shows the distributions of CO and CO2 in the total COx

ver 15% Mn/SBA-15. For the MA/SBA-15, a high selectivity to CO2as observed and no gaseous byproducts were detected. In addi-

ion, as shown in Fig. 7, the carbon balance of 15% MA/SBA-15eached 96%. The CO2 selectivity was lower on the 15% MN/SBA-5 catalyst than on the 15% MA/SBA-15 catalyst, and the carbonalance of 15% MN/SBA-15 was much lower than 100%. Theoor carbon balance of 15% MN/SBA-15 might be due to theuildup of intermediates on the catalyst surface because no gaseousyproducts were detected when the 15% MN/SBA-15 catalyst wassed. To identify the intermediates on the spent catalyst sur-ace, the temperature programmed desorption (TPD) of spent 15%

N/SBA-15 was performed under a He atmosphere. The prod-

cts desorbed were analyzed directly by gas chromatography/masspectrometry. For 15% MN/SBA-15, high amounts of intermediates,uch as formic acid, acetic acid, acetaldehyde, 2(5H)-furanone, etc.ere detected (data not shown). Among them, formic acid was

Fig. 8. Selectivity of CO and CO2 in COx .

detected as a major by-product. On the other hand, very small

amount of formic acid was detected when the 15% MA/SBA-15 cat-alyst was used, which might explain the high carbon balance of 15%MA/SBA-15.

M. Jin et al. / Catalysis Today

O3 Concentration (ppm)

0 20 0 40 0 60 0 80 0 100 0

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ig. 9. Effect of ozone concentation on benzene conversion over 15% MA/SBA-15atalyst.

In addition, the effect of water vapor (0.7 vol%) on benzene oxi-ation with ozone was investigated (Figs. 7 and 8). As water wasdded, the CO2 selectivity and carbon balance increased for the 15%N/SBA-15 catalyst. When the TPD of MN/SBA-15 with water vaporas carried out, the amount of intermediates of the by-productsecreased significantly (data not shown). This is consistent withhe result reported by Einaga and Ogata [25], who performed ben-ene oxidation with ozone on Mn/SiO2. They suggested that waterapor added might promote the decomposition of intermediateompounds on the catalyst surface. Therefore, for 15% MN/SBA-15,ater vapor might facilitate the oxidation of intermediate com-ounds. For MA/SBA-15, the selectivity to CO2 and carbon balanceas similar irrespective of the addition of water. Furthermore the

mount of byproducts on spent MA/SBA-15 with water vapor wasimilar to that on spent MA/SBA-15 (data not shown).

The effect of ozone concentration on benzene conversion andOx yield by the 15% MA/SBA-15 catalyst is illustrated in Fig. 9.enzene conversion and COx yield increased with an increase inzone concentration. When the concentration of ozone reached000 ppm, the highest conversion of benzene and COx yieldere exhibited. This indicates that the behavior of benzene oxi-ation and COx yield strongly depend on the concentration ofzone.

. Conclusions

For the purpose of catalytic benzene oxidation, manganesexide, using a manganese acetate precursor, was highly dispersedn a SBA-15 support, whereas manganese oxide with manganese

itrate precursor formed large particles on SBA-15. MA/SBA-15atalysts showed higher catalytic activity and stability than thosef MN/SBA-15 catalysts, due to the highly dispersed manganesexide and high oxygen mobility. The COx yield was also observed

[[

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204 (2013) 108– 113 113

to be higher with the MA/SBA-15 catalyst, compared to MN/SBA-15catalysts. Among the catalysts considered herein, 15% MA/SBA-15 showed the best catalytic activity, due to highly dispersedmanganese oxides on the SBA-15 supports, and the high Mncontent.

Acknowledgements

This research was supported by the Converging Research CenterProgram, funded by the Ministry of Education, Science and Tech-nology (no. 2012K001372). Young-Kwon Park acknowledges Prof.Ryong Ryoo’s valuable discussion and comments.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.cattod.2012.09.026.

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