Catalytic ozone oxidation of benzene at low temperature over MnOx/Al-SBA-16 catalyst
© Park et al; licensee Springer. 2012
Received: 28 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The low-temperature catalytic ozone oxidation of benzene was investigated. In this study, Al-SBA-16 (Si/Al = 20) that has a three-dimensional cubic Im3m structure and a high specific surface area was used for catalytic ozone oxidation for the first time. Two different Mn precursors, i.e., Mn acetate and Mn nitrate, were used to synthesize Mn-impregnated Al-SBA-16 catalysts. The characteristics of these two catalysts were investigated by instrumental analyses using the Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed reduction. A higher catalytic activity was exhibited when Mn acetate was used as the Mn precursor, which is attributed to high Mn dispersion and a high degree of reduction of Mn oxides formed by Mn acetate than those formed by Mn nitrate.
KeywordsAl-SBA-16 Mn precursors benzene ozone catalytic oxidation
Hazardous air pollutants [HAPs] are airborne species that are known to or are anticipated to cause adverse effects on human health and environment. HAPs are characterized by their toxicity, carcinogenicity, bioaccumulation, persistence, and dispersion. Most HAPs, however, are not regulated/managed, producing secondary pollutants and odor . Benzene, a representative HAP, is a well-known carcinogen. Long-term exposure to benzene can cause blood dyscrasias such as a decrease in erythrocytes, aplastic anemia, and leukemia . Therefore, in recent years, considerable attention has been paid to the removal of benzene and other HAPs.
Ozone has been widely used for pollution treatment in the semiconductor industry, water treatment, and air cleaning [3–5]. In particular, catalytic ozone oxidation has high pollutant-removal efficiency and low energy consumption . In the catalytic ozone oxidation process, ozone is decomposed into activated oxygen species that can oxidize organic compounds. Recently, researches on the catalytic ozone oxidation of volatile organic compounds [VOCs] including HAPs have been performed [7–9]. The HAP removal process involving ozone addition is economically advantageous because it can be performed at a temperature much lower than that required for conventional HAP removal processes. Thus far, Al2O3, SiO2, and zeolite catalysts impregnated with metal have usually been used for catalytic ozone oxidation. In particular, supports with a large specific surface area have good dispersion of metal oxides within the supports, leading to high reaction activity [4, 9].
Recently, mesoporous materials such as MCM-41 and SBA-15 have been widely used as supports for various reactions because of their uniform pores and large specific surface areas. In particular, SBA-16 is expected to exhibit high activity during the catalytic ozone oxidation of benzene because of its super-large cage, large surface area, and high thermal stability. The three-dimensional channel connectivity of SBA-16 makes it even more favorable for mass-transfer kinetics than the other hexagonal mesoporous materials having unidirectional pore structures. To the best of our knowledge, SBA-16 has never been used for the catalytic ozone oxidation of benzene. MnOx is a metal oxide that exhibits high activity during the decomposition of VOCs at a low temperature . Therefore, in this study, Al-SBA-16 was impregnated with Mn by using two different Mn precursors, i.e., Mn(CH3COO)2 (Mn acetate) and Mn(NO3)2 (Mn nitrate), to investigate the effect of Mn precursors on the catalytic ozone oxidation of benzene.
Synthesis of MnOx/Al-SBA-16 catalysts
The detailed procedure for the synthesis of mesoporous silica SBA-16 with cubic Im3m structure is described in the literature . A poly(alkylene oxide)-type triblock copolymer, i.e., F127 (EO106PO70EO106, MW = 12,600, Sigma, St. Louis, MO, USA), was dissolved in an aqueous HCl solution, and tetraethyl orthosilicate [TEOS] (98%) was added at 35°C. The solution was stirred for 15 min by a magnetic stirrer at the same temperature. The molar composition of the mixture was F127:TEOS:HCl:H2O = 0.0040:1.0:4.0:130. This mixture was put in an oven for 24 h at the same temperature. The mixture was then put in an oven at an elevated temperature of 100°C for 24 h. After this hydrothermal aging, the solid product formed was recovered by filtration and was dried at 100°C without washing. The dried sample was washed with ethanol, dried in an oven at 100°C, and calcined at 550°C. Al incorporation in the sample was performed with an ethanolic solution of AlCl3 (Si/Al = 20). After completely evaporating the solvent (ethanol) in a rotary evaporator, the sample was calcined in air at 550°C. The Al-incorporated sample is hereafter referred to as Al-SBA-16.
The amount of Mn impregnated using Mn(NO3)2 (98%, Aldrich, St. Louis, MO, USA) and Mn(CH3COO)2 (> 99%, Aldrich, St. Louis, MO, USA) as the Mn precursors was 15 wt.%. The Mn-impregnated material was calcined at 550°C. Al-SBA-16 catalysts synthesized using Mn nitrate and Mn acetate as the Mn precursors are hereafter referred to as Al-SBA-16-MN15% and Al-SBA-16-MA15%, respectively.
Characterization of MnOx/Al-SBA-16
X-ray diffraction [XRD] patterns of the catalyst were obtained using an X-ray diffractometer (D/MAX-III, Rigaku, Akishima, Japan) with Cu-Kα radiation. The N2 adsorption-desorption isotherms and the Brunauer-Emmett-Teller [BET] surface area of the catalyst were obtained using an ASAP-2010 apparatus (Micromeritics, Norcross, GA, USA). Temperature-programmed reduction [TPR] analysis was performed using a ChemBET 3000 (Quantachrome, Boynton Beach, FL, USA) setup. X-ray photoelectron spectroscopy [XPS] was performed using an AXIS Nova spectrometer (Kratos Inc., NY, USA). A monochromatic Al Kα (1,486.6 eV) of X-ray source and 40 eV of analyzer pass energy were used under ultra-high vacuum conditions (5.2 × 10-9 Torr).
Benzene oxidation with ozone
Catalytic reaction experiments were performed in a fixed-bed flow reactor. Ozone was produced from O2 using a silent-discharge ozone generator. Before each experiment, the sample was heated at 450°C in a Pyrex glass reactor under an O2 flow. The catalyst was then cooled and maintained at 80°C. In each experiment, 0.05 g of the catalyst was used. The ozone flow rate and benzene inlet concentration were set at 120 mL/min and 100 ppm, respectively. The product gas sample was passed through a GC/FID (6000 Series, Young Lin, Anyang, South Korea) with an HP-5 column (Agilent Technologies Inc., Santa Clara, CA, USA) to analyze the benzene conversion, an indoor gas analyzer (ISR-401, WOORI Industrial System Co., Ltd., Chungcheongbuk-do, South Korea) used for the CO and CO2 products, and an ozone analyzer (LAB-S, Ozonetech, Daejeon, South Korea) for the ozone conversion. In this study, the gas-phase reaction of benzene with ozone was shown to be negligible.
Results and discussion
Characterization of Al-SBA-16
Textural properties of the catalysts
Benzene oxidation with ozone
where * represents the catalytic active site. The oxygen species formed during the decomposition of ozone oxidize benzene, producing oxygen-containing by-products. These by-products are further oxidized to COx. The fact that a gas-phase reaction between ozone and benzene did not occur indicates that ozone itself does not function as the oxidizer. Rather, ozone is decomposed into oxygen species by the above-shown mechanisms, and these oxygen species oxidize benzene. As shown in Figure 6, the consumption of ozone had a good correlation with the conversion of benzene: a higher benzene conversion was obtained at a higher consumption of ozone.
Two different Mn precursors were used to synthesize mesoporous catalysts for the catalytic ozone oxidation of benzene by impregnating Al-SBA-16 with Mn. The catalytic activity of Al-SBA-16-MA15% was higher than that of Al-SBA-16-MN15%. It was shown that the type of precursors used for Mn impregnation influenced the dispersion, oxidation state, and oxygen mobility of the impregnated Mn. XRD and TPR analyses showed that Al-SBA-16-MA15% had better Mn dispersion and a higher degree of reduction than Al-SBA-16-MN15%. XPS analysis showed that highly dispersed Mn oxides could form main active sites for Al-SBA-16-MA15%. These catalytic properties appear to have induced the high catalytic activity of Al-SBA-16-MA15%.
X-ray photoelectron spectroscopy
- Raun LH, Marks EM, Ensor KB: Detecting improvement in ambient air toxics: an application to ambient benzene measurements in Houston, Texas. Atmos Environ 2009, 43: 3259–3266. 10.1016/j.atmosenv.2009.03.010View Article
- Nakayama A, Koyoshi S, Morisawa S, Yagi T: Comparison of the mutations induced by p-benzoquinone, a benzene metabolite, in human and mouse cells. Mutat Res 2000, 27: 147–153.View Article
- De Smedt F, De Gendt S, Heyns MM, Vinckier C: The application of ozone in semiconductor cleaning processes: the solubility issue. J Electrochem Soc 2001, 148: G487-G493. 10.1149/1.1385820View Article
- Einaga H, Futamura S: Catalytic oxidation of benzene with ozone over Mn ion-exchanged zeolites. Catal Commun 2007, 8: 557–560. 10.1016/j.catcom.2006.07.024View Article
- Sahledemessie E, Devulapelli V: Vapor phase oxidation of dimethyl sulfide with ozone over V2O5/TiO2catalyst. Appl Catal B Environ 2008, 84: 408–419. 10.1016/j.apcatb.2008.04.025View Article
- Atkinson R: Kinetics and mechanisms of the gas-phase reactions of ozone with organic compounds under atmospheric conditions. Chem Rev 1984, 84: 437–470. 10.1021/cr00063a002View Article
- Naydenov A, Stoyanova R, Mehandjiev D: Ozone decomposition and CO oxidation on CeO2. J Mol Catal A Chem 1995, 98: 9–14. 10.1016/1381-1169(94)00060-3View Article
- Gervasini A, Vezzoli GC, Ragaini V: VOC removal by synergic effect of combustion catalyst and ozone. Catal Today 1996, 29: 449–455. 10.1016/0920-5861(95)00319-3View Article
- Einaga H, Futamura S: Catalytic oxidation of benzene with ozone over alumina-supported manganese oxides. J Catal 2004, 227: 304–312. 10.1016/j.jcat.2004.07.029View Article
- Peña DA, Uphade BS, Smirniotis PG: TiO2-supported metal oxide catalysts for low-temperature selective catalytic reduction of NO with NH3I. Evaluation and characterization of first row transition metals. J Catal 2004, 221: 421–431. 10.1016/j.jcat.2003.09.003View Article
- Kim TW, Ryoo R, Kruk M, Gierszal KP, Jaroniec M, Kamiya S, Terasaki O: Tailoring the pore structure of SBA-16 silica molecular sieve through the use of copolymer blends and control of synthesis temperature and time. J Phys Chem B 2004, 108: 11480–11489. 10.1021/jp048582kView Article
- Kapteijn F, Van Langeveld AD, Moulijn JA, Andreini A, Vuurman MA, Turek AM, Jehng JM, Wachs IE: Alumina-supported manganese oxide catalysts. I. characterization: effect of precursor and loading. J Catal 1994, 150: 94–104. 10.1006/jcat.1994.1325View Article
- Kim SC, Shim WG: Catalytic combustion of VOCs over a series of manganese oxide catalysts. Appl Catal B Environ 2010, 98: 180–185. 10.1016/j.apcatb.2010.05.027View Article
- Radhakrishnan R, Oyama ST, Chen JGG, Asakura K: Electron transfer effects in ozone decomposition on supported manganese oxide. J Phys Chem B 2001, 105: 4245–4253. 10.1021/jp003246zView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.