Catalytic Oxidation of Benzene with Ozone Over Nanoporous Mn/MCM-48 Catalyst

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1 Copyright 12 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, , 12 Catalytic Oxidation of Benzene with Ozone Over Nanoporous Mn/MCM-48 Catalyst Jong-Hwa Park 1, Jongsoo Jurng 2, Gwi-Nam Bae 2, Sung Hoon Park 3, Jong-Ki Jeon 4, Sang Chai Kim 5, Ji Man Kim 6 1 7, and Young-Kwon Park 1 Graduate School of Energy and Environmental System Engineering, University of Seoul, Seoul , Korea 2 Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology, Seoul , Korea 3 Department of Environmental Engineering, Sunchon National University, Suncheon , Korea 4 Department of Chemical Engineering, Kongju National University, Gongju , Korea 5 Department of Environmental Education, Mokpo National University, Muan , Korea 6 Department of Chemistry and BK21 School of Materials Science and Department of Energy Science, Sungkyunkwan University, Suwon , Korea IP : Mon, 22 Oct 12 17:3:37 7 School of Environmental Engineering, University of Seoul, Seoul , Korea The catalytic oxidation of a representative volatile organic compound, benzene, with ozone at a low temperature was investigated. A nanoporous MCM-48 material with a high specific surface area was used as the support for the catalytic oxidation for the first time. Mn, which has high activity at a low temperature, was used as the metal catalyst. To examine the effect of the Mn precursor, MCM-48 was impregnated with two different Mn precursors: Mn acetate and Mn nitrate. The characteristics of the synthesized catalysts were analyzed by Brunauer Emmett Teller surface area, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed reduction. MCM-48 impregnated with Mn acetate showed higher catalytic activity than MCM-48 impregnated with Mn nitrate. This result was attributed to the better dispersion within nanoporous MCM-48 and higher oxygen mobility of Mn oxides produced by Mn acetate. The catalytic activity was also shown to depend closely on the ozone concentration. Keywords: MCM-48, Benzene, Ozone, MnOx, Mn Precursor, Catalytic Oxidation. 1. INTRODUCTION Volatile organic compounds (VOCs) are well-known air pollutants that cause photochemical smog and ozone production. In addition, some VOC species, such as benzene have adverse health effects, causing heart disease and cancer. Accordingly, VOC emissions are becoming more stringent all over the world. Considerable efforts have been made to develop efficient and economical methods for reducing VOC emissions. Among these, catalytic oxidation is cheaper than direct combustion while producing little secondary pollutants. In particular, catalytic oxidation with ozone, which makes use of the strong oxidizing power of ozone, can decompose VOCs at temperatures < C, whereas conventional catalytic oxidation requires a much higher temperature. 1 Various types of catalysts have been studied for conventional catalytic oxidation but there have been few studies Author to whom correspondence should be addressed. on catalytic oxidation with ozone. Thus far, metal/al 2 O 3, 2 metal/zeolite, 3 and noble metals 4 have been used for catalytic oxidation with ozone, showing an increase in catalytic activity with the increasing surface area of the support. Also, various nanoporous materials with large surface areas have been successfully applied to many catalytic reactions Therefore, it is expected that a good catalytic oxidation performance of VOCs with ozone will be achieved if metal catalysts can be impregnated on nanoporous material supports with large surface areas. In this study, MCM-48, which has a three-dimensional pore structure and mutually connected nanopores as well as good adsorption ability for VOCs due to its large specific surface area, was used for the catalytic oxidation of VOCs with ozone for the first time. Benzene was used as a model species for VOCs. Mn was adopted for the metal catalyst because it has high activity at a low temperature. Different Mn precursors were used to examine their effects on the activity of the generated catalyst J. Nanosci. Nanotechnol. 12, Vol. 12, No /12/12/5942/5 doi:1.1166/jnn

2 Park et al. Catalytic Oxidation of Benzene with Ozone Over Nanoporous Mn/MCM-48 Catalyst 2. EXPERIMENTAL DETAILS 2.1. Preparation of MnOx/MCM MCM-48 was prepared using the following procedure. First, pure MCM-48 was prepared by mixing 1. g of cetyltrimethylammonium bromide, 1.5 g of Brij-3, and 19.5 g of distilled water. After the mixture became transparent, g of a sodium silicate solution with a Na/Si ratio of.5 was added dropwise during stirring. The prepared solution underwent a reaction in a C oven for 48 h and was then allowed to cool. The ph of the solution was adjusted to 1 using 5 wt% acetic acid, and the solution was allowed to react for another 48 h. After repeating the ph-adjusting/reaction process three times, the solution was washed with distilled water, filtered and dried in the oven for 24 h. The dried sample was again washed with ethanol, filtered, dried for 24 h, and baked at 55 C for 4h. As the Mn precursors, Mn(NO 3 ) 2 (Aldrich, 98%) and Mn(CH 3 COO) 2 (Aldrich, 99%+) were used. The amount of impregnated Mn was 1 wt%. The Mn-impregnated materials were calcined at 55 C. The MCM-48 catalysts that were impregnated using manganese nitrate and manganese acetate as the Mn precursors are referred to as MCM-48-MN 1% and MCM-48-MA 1%, respectively Characterization of MnOx/MCM-48 X-ray diffraction (XRD, Rigaku D/MAX-III) was performed for phase analysis. The analysis was conducted using a Cu K X-ray source, within the scan range 9, in a step size of.2. The N 2 adsorption desorption isotherms and the Brunauer Emmett Teller (BET) surface areas were obtained using an ASAP 1 apparatus (Micromeritics). After.3 g of calcined sample was outgassed for 5 h at 25 C in a vacuum, nitrogen was introduced as an adsorption gas at the liquid nitrogen temperature. Temperature programmed reduction (TPR) was performed to compare the reduction temperatures of the samples with different manganese types using a ChemBET 3 (Quantachrome) setup. The H 2 consumption was measured with an analysis gas of 5% H 2 /N 2 and temperature rise rate of 1 C/min, from 5 to 7 C. X-ray photoelectron spectroscopy (XPS) measurements were made using an AXIS-NOVA (Kratos. Inc). A monochromatic Al K ( ev) X-ray source and 4 ev of analyzer pass energy were used under ultra-high vacuum conditions ( Torr). IP : Mon, 22 Oct 12 17:3:37 under an O 2 flow and then cooled for maintenance at C. The catalyst mass of.5 g, the ozone flow rate of 1 ml/min, and the benzene concentration of ppm were used for each experiment. GC analysis was carried out for the inlet and outlet gas samples to measure the level of benzene conversion. The CO and CO 2 concentrations and ozone concentration were measured using an indoor gas analyzer and an ozone analyzer, respectively. A homogeneous gas-phase reaction of benzene with ozone can be neglected in this system. 3. RESULTS AND DISCUSSION 3.1. Characterization of MCM-48 Table I lists the BET surface area and pore size measured for the catalysts generated in this study. The surface area and pore size of MCM-48 MA1% and MCM-48 MN1% were similar. Figure 1 shows the XRD patterns of the Mnimpregnated MCM-48. MCM-48 MA1% did not exhibit Mn oxide peaks at high angles, indicating that the Mn oxides had been dispersed well within MCM-48. On the other hand, MCM-48 MN1% exhibited distinct Mn oxide peaks indicating that the Mn oxides existed as large-sized particles due to the poor dispersion within MCM-48. Figure 2 shows the XPS spectra of MnOx/MCM-48. MCM-48 MA1% showed two maximum peaks at and ev indicating Mn 2 O 3 and MnO 2 respectively by deconvolution The main oxidation state of MCM-48 MA1% was Mn 2 O 3. In the case of MCM-48 MN1%, three peaks were identified through peak deconvolution representing Mn 2 O 3 (641.2 ev), MnO 2 (642.2 ev) and Mn-nitrate (644.2 ev), respectively, indicating that various Mn oxides were produced when Mn nitrate was used as a precursor. The order of the catalytic activity for VOC oxidation of the different Mn catalysts was reported to be Mn 3 O 4 > Mn 2 O 3 > MnO Therefore, high catalytic activity was expected when Mn acetate was used as the precursor based on the large content of well-dispersed MnO x, especially Mn 2 O 3. Figure 3 shows the H 2 -TPR analysis results for the two different catalysts synthesized using different Mn precursors. As shown in this figure, MCM-48 MA1% exhibited a single broad peak at approximately 41 C. On the other hand, MCM-48 MN1% showed two distinct peaks at approximately 41 and 56 C. This suggests that MCM- 48 MA1% has a higher reducing ability than MCM-48 MN1%, indicating that the lattice oxygen mobility was 1 18 higher for MCM-48 MA1% Benzene Oxidation with Ozone The catalytic reactions were conducted in a fixed bed flow reactor. Ozone was produced from O 2 using a silent discharge ozone generator. Prior to the catalytic reaction, the catalyst was heated at 45 C in a Pyrex glass reactor Table I. Physical properties of the catalysts. S BET V total Average pore (m 2 /g) (cm 3 /g) size (nm) MCM-48 MA1% MCM-48 MN1% J. Nanosci. Nanotechnol. 12, ,

3 Catalytic Oxidation of Benzene with Ozone Over Nanoporous Mn/MCM-48 Catalyst Park et al. Fig. 1. XRD patterns of the MnOx/MCM-48 catalysts Benzene Oxidation with Ozone Figure 4 shows the conversions of benzene and ozone obtained using the different Mn precursors. The level of benzene conversion of MCM-48 MA1% was more than 1% larger than that of MCM-48 MN1% at a reaction time of min. In terms of ozone conversion, MCM-48 MA1% showed an approximately 3% higher value than MCM-48 MN1%. As some benzene molecules might not participate in the reaction and might only be adsorbed onto MCM-48 with a high specific surface area at C, part of the benzene conversion of MCM-48 MN1% might be attributed to adsorption, which resulted in a smaller difference in benzene conversion than in ozone conversion between the MCM-48 catalysts impregnated using the two different Mn precursors. In addition, the level of conversion rapidly decreased with time in the case of MCM-48 MN1% compared to MCM-48 MA1%. IP : Mon, 22 Oct 12 17:3:37 Figure 5 shows the benzene conversion and yield of COx (CO 2 + CO) obtained with MCM-48 MA1% and MCM-48 MN1%. In the case of MCM-48 MA1%, most of the decomposed benzene had been converted to COx. In contrast, the COx yield of MCM-48 MN1% was 3% lower than that of MCM-48 MA1%. This can be attributed to the difference in catalyst characteristics, as explained below. As shown in the TPR result, MCM-48 MA1% has higher reduction ability than MCM-48 MN1%. This means that MCM-48 MA1% has higher lattice oxygen mobility, leading to higher activity for the oxidation. In addition, as mentioned earlier, the order of catalytic activity for VOC oxidation of Mn oxides was reported to be Mn 3 O 4 > Mn 2 O 3 > MnO In MCM-48 MA1%, the highly active Mn 2 O 3 was dispersed well, whereas MCM-48 MN1% contained large-sized Mn 2 O 3 particles resulting in low activity. Moreover, MCM-48 MN1% also contained MnO 2 and Mn nitrate with low activity, which MCM-48 MA1% MCM-48 MN1% MCM-48 MA1% MCM-48 MN1% Intensity (a.u.) Intensity (a.u.) Binding Energy (ev) Temperature ( C) Fig. 2. XPS of the MnOx/MCM-48 catalysts. Fig. 3. TPR of the MnOx/MCM-48 catalysts J. Nanosci. Nanotechnol. 12, , 12

4 Park et al. Catalytic Oxidation of Benzene with Ozone Over Nanoporous Mn/MCM-48 Catalyst 4 4 O 3 Conversion (%) MCM-48 MA1% Benzene Conversion MCM-48 MN1% Benzene Conversion MCM-48 MA1% O 3 Conversion MCM-48 MN1% O 3 Conversion 4 Delivered Time by (min) Ingenta to: IP : Mon, 22 Oct 12 17:3:37 Fig. 4. Benzene and ozone conversion over the MnOx/MCM-48 catalysts at C. is believed to be another reason for the low activity of MCM-48 MN1%. Figure 6 shows the level of benzene conversion and COx yield as a function of ozone consumption obtained for min when MCM-48 MA1% was used. Both the level of benzene conversion and the COx yield increased with increasing ozone consumption. The mechanism by which ozone is decomposed through interaction with the Mn oxides forming active catalytic sites is as follows: O 3 O 2 + O (1) O + O 3 O 2 + O 2 (2) O 2 O 2+ (3) where represents the active catalytic site. The oxygen species produced by the decomposition of ozone oxidize benzene into oxygen-containing species. These species are oxidized further to COx. The fact that ozone did not react directly with benzene in the gas phase indicates that ozone itself is not the active species. Ozone is believed to decompose into O 2 and oxygen radicals over Mn oxide catalysts through the above-shown mechanism, followed by the oxidation of benzene by these oxygen products. Benzene Conversion CO x Yield 4 4 CO x Yield (Carbon wt%) MCM-48 MA1% MCM-48 MN1% Fig. 5. COx yields of the MnOx/MCM-48 catalysts on the time stream at min and C. J. Nanosci. Nanotechnol. 12, ,

5 Catalytic Oxidation of Benzene with Ozone Over Nanoporous Mn/MCM-48 Catalyst Park et al CO x Yield Benzene Conversion COx Yield (Carbon wt%) Delivered O 3 Consumption by Ingenta (ppm) to: IP : Mon, 22 Oct 12 17:3:37 Fig. 6. Effect of ozone on the conversion of benzene and on the COx yield over MCM-48 MA1% on the time stream at min and C. 4. CONCLUSIONS The activity of the nanoporous material, MCM-48, for the catalytic oxidation of benzene with ozone was examined by impregnating it with Mn using different precursors. The catalytic activity of MCM-48 MA1% was higher than that of MCM-48 MN1%. The precursors used for Mnimpregnation affected the degree of dispersion, the oxidation state, and the oxygen mobility of Mn. Also, the catalytic activity was affected by the ozone consumption. Acknowledgments: This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (No. 11K752). References and Notes 1. M. Jin, J. W. Kim, J. M. Kim, J. Jurng, G. N. Bae, J. K. Jeon, and Y. K. Park, Powder Technol. 214, 458 (11). 2. H. Einaga and S. Futamura, J. Catal. 227, 34 (4). 3. H. Einaga, Y. Teraoka, and A. Ogat, Catal. Today 164, 571 (11). 4. C. L. Chang and L. Tser-Sheng, React. Kinet. Catal. Lett. 86, 91 (5). 5. K. H. Park, H. J. Park, J. Kim, R. Ryu, J. K. Jeon, J. Park, and Y. K. Park, J. Nanosci. Nanotechnol. 1, 355 (1). 6. S. S. Kim, H. S. Heo, S. G. Kim, R. Ryoo, J. Kim, J. K. Jeon, S. H. Park, and Y. K. Park, J. Nanosci. Nanotechnol. 11, 6167 (11). 7. J. H. Yim, D. I. Kim, J. A. Bae, Y. K. Park, J. H. Park, J. K. Jeon, S. H. Park, J. Song, and S. S. Kim, J. Nanosci. Nanotechnol. 11, 1714 (11). 8. D. I. Kim, J. H. Park, S. D. Kim, J. Y. Lee, J. H. Yim, J. K. Jeon, S. H. Park, and Y. K. Park, J. Ind. Eng. Chem. 17, 1 (11). 9. H. W. Lee, H. J. Cho, J. H. Yim, J. M. Kim, J. K. Jeon, J. M. Sohn, K. S. Yoo, S. S. Kim, and Y. K. Park, J. Ind. Eng. Chem. 17, 54 (11). 1. S. J. Choi, Y. K. Park, K. E. Jeong, T. W. Kim, H. J. Chae, S. H. Park, J. K. Jeon, and S. S. Kim, Korean J. Chem. Eng. 27, 1446 (1). 11. H. I. Lee, J. M. Kim, J. Y. Lee, Y. K. Park, J. K. Jeon, J. H. Yim, S. H. Park, K. J. Lee, S. S. Kim, and K. E. Jeong, J. Nanosci. Nanotechnol. 1, 3639 (1). 12. 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, 736 (11). 13. 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 (12). 14. S. H. Lee, H. S. Heo, K. E. Jeong, J. H. Yim, J. K. Jeon, K. Y. Jung, Y. S. Ko, S. S. Kim, and Y. K. Park, J. Nanosci. Nanotechnol. 11, 759 (11). 15. H. W. Lee, J. K. Jeon, S. H. Park, K. E. Jeong, H. J. Chae, and Y. K. Park, Nanoscale Res. Lett. 6, 5 (11). 16. J. Li, J. Chen, R. Ke, C. Luo, and J. Hao, Catal. Commun. 8, 1896 (7). 17. D. A. Peña, B. S. Uphade, and P. G. Smirniotis, J. Catal. 221, 421 (4). 18. S. C. Kim, and W. G. Shim, Appl. Catal. B: Environ. 98, 1 (1). 19. C. Reed, Y. K. Lee, and S. T. Oyama, J. Phys. Chem. 11, 47 (6). Received: 2 August 11. Accepted: 31 January J. Nanosci. Nanotechnol. 12, , 12