Gas-Phase Total Oxidation of Benzene, Toluene, Ethylbenzene, and

May 17, 2012 - manganese oxide, amorphous manganese oxide (AMO), and ... AMO, and CuO/Mn2O3 as compared to commercial MnO2 were attributed to a ...
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Gas-Phase Total Oxidation of Benzene, Toluene, Ethylbenzene, and Xylenes Using Shape-Selective Manganese Oxide and Copper Manganese Oxide Catalysts Homer C. Genuino, Saminda Dharmarathna, Eric C. Njagi, Michael C. Mei, and Steven L. Suib* Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United States S Supporting Information *

ABSTRACT: Volatile organic compounds (VOCs) continue to be the major source of direct and indirect air pollution. Here, cryptomelane-type octahedral molecular sieve (OMS-2) manganese oxide, amorphous manganese oxide (AMO), and mixed copper manganese oxide (CuO/Mn2O3) nanomaterials were synthesized and, together with commercial MnO2, characterized by various techniques. These catalysts were investigated for gas-phase total oxidation of six VOCs under air atmosphere. Using OMS-2 at 250 °C, the average conversions for toluene, benzene, ethylbenzene, p-xylene, m-xylene, and oxylene were 75%, 61%, 45%, 23%, 13%, and 8%, respectively, whereas using CuO/Mn2O3, 72%, 44%, 37%, 29%, 27%, and 26%, respectively, were obtained. Generally, the conversion of VOCs to CO2 using the synthesized catalysts increased in the order: oxylene ≈ m-xylene < p-xylene < ethylbenzene < benzene < toluene. However, using commercial MnO2, benzene (44% conversion) was more reactive than toluene (37%), and the xylenes showed similar reactivities (13−20%). Differences in reactivity among VOCs were rationalized in terms of degree of substrate adsorption and structural effects. For example, the reactivity of xylenes was dictated by the shape-selectivity of stable OMS-2. The higher oxidative activities exhibited by OMS-2, AMO, and CuO/Mn2O3 as compared to commercial MnO2 were attributed to a combination of factors including structure, morphology, hydrophobicity, and redox properties. The mobility and reactivity of active oxygen species were strongly correlated with catalytic activities. Lattice oxygen was involved in the VOC oxidation, suggesting that the reaction could proceed via the Mars−van Krevelen mechanism. combustion.11,12 Catalytic oxidation has been applied for odor control and treatment of emissions containing evaporated solvents.13 Several recent works have demonstrated that manganese oxide materials are efficient catalysts for the elimination of pollutants.14−18 Manganese oxides have comparable catalytic activities with noble or supported noble metals; hence, they can be a cheaper alternative.4,19−21 Nanoscale potassium-containing manganese oxide octahedral molecular sieve (OMS-2) is a prominent oxidation catalyst under thermal conditions.22,23 OMS-2 has a 2 × 2 tunnel structure and has a chemical composition of KMn8O16 with charge-balancing ions, K+, and H2O residing in the tunnel sites.23,24 Such structure leads to interesting physicochemical properties including porosity and high capacity of adsorption related to catalytic performance.23 The redox activity of OMS-2 is also associated with the presence of Mn3+ and Mn4+ ions, long and open structure, and the formation of OH groups on the surface of the catalyst.25,26 Synthesized and characterized by a variety of methods and techniques, OMS-2 has been used as an effective catalyst for

1. INTRODUCTION Volatile organic compounds (VOCs) are widely used and produced by industrial processes, transportation, and domestic activities.1−3 However, many VOCs are toxic to humans (carcinogenic, mutagenic, or teratogenic) and can cause atmospheric pollution (photochemical smog and destruction of ozone layer).4−7 Collectively known as BTEX, benzene, toluene, ethylbenzene, and xylenes are monocyclic volatile organic compounds mostly found in petroleum derivatives such as gasoline and diesel fuels.8,9 These VOCs are widely used as precursors for manufacturing consumer products and as industrial solvents. However, acute and prolonged exposures to these VOCs can be harmful to skin and can cause neurological, respiratory, and central nervous system damage.1 With the progressive increase in BTEX emissions due to increasing consumption of fuel, there must be a significant effort to control them especially in the gas phase where they are most difficult to contain. Catalytic oxidation is a promising technology for both partial oxidation of petrochemical materials to make products of economical value and complete oxidation of gas-phase VOCs to CO2 and H2O, minimizing formation of undesirable byproduct.10,11 In addition, catalytic oxidation of VOCs occurs at lower temperatures and faster rates as compared to thermal © 2012 American Chemical Society

Received: February 9, 2012 Revised: May 17, 2012 Published: May 17, 2012 12066

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oxidation of benzene,27,28 CO,27,29 benzyl alcohol, 30,31 methane,32 2-thiophenemethanol,33 2-propanol,15 and toluene.27,34 Shape-selectivity of OMS-2 materials has also been illustrated by the liquid-phase oxidation of bulky 1-acenaphthenol versus benzyl alcohol.22 Several studies by other researchers have also found OMS-2 as an excellent catalyst for the oxidation of formaldehyde,35 ethanol,36−38 acetaldehyde,38 and ethyl acetate.37,39 Aside from OMS-2, we have also prepared amorphous manganese oxides (AMO) for gas-phase oxidation of CH3Br40 and dimethyl methylphosphonate.41 We have also used AMO and copper manganese oxides for low temperature CO and ethylene oxidation.17,18,42−44 The high catalytic activities of AMO and copper manganese oxide catalysts for such applications are attributed to the preparation method, high surface area, low crystallinity, morphology, and mixed valency of manganese. It is therefore interesting to further characterize these different nanomaterials and take advantage of their unique properties for the oxidation of other VOCs. This study investigates the catalytic activities of OMS-2, AMO, and CuO/Mn2O3 for gas-phase total oxidation of benzene, toluene, ethylbenzene, o-xylene, m-xylene, and pxylene, including a commercial MnO2 as a reference. These catalysts were characterized by several techniques with the aim of correlating properties with catalytic performance and understanding the possible mechanism for the oxidation reaction. Particular attention was given to the role of lattice oxygen in the catalytic activity.

(FESEM) with a Schottky emitter operating at 2.0 kV with a beam current of 1.0 mA. Transmission electron microscopy (TEM) studies were conducted on a JEOL JEM-2010 FasTEM operating at 200 kV. Each catalyst was suspended in 2propanol. A drop of the suspension was loaded onto a carboncoated copper grid and allowed to dry. Elemental analyses were carried out with an Amray model 1810D operated at 20.0 kV, with the X-ray spectra being acquired and processed with an Amray model PV 9800 EDXS system. Elemental analyses were also performed using a PerkinElmer Model 3100 a Thermo Jarrell Ash model ICAP 61E trace analyzer inductively coupled plasma-atomic emission spectrometer (ICP-AES). Compositions expressed in terms of atomic % and molar % were obtained using the EDXS and ICPAES techniques, respectively. The textural properties of the samples were studied by performing nitrogen sorption measurements using a Micrometrics ASAP 2010 instrument. The adsorption and desorption experiments were conducted at 77 K after initial pretreatment of the samples by degassing at 150 °C for 12 h. Surface area were determined by the Brunauer−Emmett−Teller (BET) method. The pore size distribution was calculated from the desorption branch of the nitrogen adsorption isotherm using the Barrett−Joyner−Halenda (BJH) method. Sorption of water and toluene (representative VOC) by the catalyst was studied at room temperature by static weighing. For sorption experiments, a 200 mg sample of each catalyst was heated at 250 °C for 4 h and then placed in a desiccator containing saturated solutions of NH4NO3 of water or toluene. Uptake data were collected by accurately weighing the catalysts before and after overnight sorption. Hydrophobicity index for each catalyst was determined from the molar ratio of uptake of toluene to uptake of water. Thermogravimetric analyses (TGA) were performed on a TGA Q500 (Q500-1732) Thermogravimetric Analyzer. The samples were loaded into platinum holders and heated from ambient temperature to 30 °C for 10 min and then to 1000 °C in a stream of pure nitrogen (flow rate = 60 mL min−1) at a ramp rate of 20 °C min−1. X-ray photoelectron spectroscopy (XPS) was used for the characterization of surface oxygen and identification of oxidation state of manganese species on the catalysts. XPS data were obtained with a Perkin-Elmer Physical Electronics model 5300 X-ray electron spectrometer equipped with monochromator Al Kα power (250 W) with a pass energy of 50 eV. A hemispherical analyzer was used to study the binding energies (BEs) for oxygen and manganese. BEs were referenced to the C 1s line at 284.6 eV from adventitious carbon. Potentiometric titrations were performed to measure the average oxidation state (AOS) of manganese in the catalysts. The catalysts were first reduced to Mn2+ using HCl and titrated to Mn3+ using a KMnO4 standard solution to obtain the total amount of manganese. The AOS of Mn was determined by reduction of Mn2+ with (NH3)2Fe(SO4)2 and back-titrating the excess Fe2+ with a KMnO4 standard solution. Temperature-programmed desorption (TPD) mass spectrometry experiments were conducted to determine the gaseous species desorbing from the catalysts during the thermal treatment. Typically, a sample catalyst (100 mg) was packed into a quartz tube supported by quartz wool and loaded horizontally inside a Thermolyne 79300 model temperatureprogrammable tube furnace. The catalyst was pretreated by flowing ultra high purity (UHP) Ar at a flow rate of 50 STP

2. EXPERIMENTAL SECTION Catalyst Preparation. OMS-2 was synthesized via the reaction between MnO4− and Mn2+ through a simple refluxing method.36 KMnO4 solution was added into a Mn(CH3COO)2 solution to achieve a MnO4− to Mn2+ molar ratio of 0.72. The mixture was then refluxed for 24 h at a reaction temperature of 100 °C. The resultant precipitate was then filtered, washed with distilled deionized water (DDW), and dried at 110 °C overnight. The sample was calcined at 400 °C for 4 h. AMO was synthesized by the dropwise addition of 1.8 M solution of Mn(CH3COO)2·4H2O to an equal volume of 1.2 M KMnO4 under vigorous stirring.17 The resultant mixture was stirred continuously for 24 h at room temperature, filtered, washed with DDW, vacuum-dried, and ground into powder. Mixed copper manganese oxide was synthesized by a coprecipitation method using 1.0 M NaOH, 0.5 M Cu(NO3)2·2.5H2O, and 0.5 M MnCl2·4H2O solutions. The precipitate was aged for 3 h, filtered, washed with DDW, oven-dried overnight at 100 °C, and calcined in air for 2 h at 200 °C. All chemicals were analytical grade and used without further purification. Commercial MnO2 ( CuO/ Mn2O3 > AMO > commercial MnO2. OMS-2 and CuO/ Mn2O3 switched positions in terms of xylene (most resistant group of VOCs to oxidation) conversion. Using OMS-2, the ease of oxidation decreased in the order: toluene > benzene > ethylbenzene > p-xylene > m-xylene > o-xylene (Figure 10A). Conversion (at least 10%) started at 150 °C for benzene; 200 °C for toluene, ethylbenzene, and p-xylene; and 250 °C for mxylene and o-xylene. At 250 °C, the average % conversions (n = 3) for toluene, benzene, ethylbenzene, p-xylene, m-xylene, and o-xylene were 75%, 61%, 45%, 23%, 13%, and 8%, respectively. As expected, conversion generally increased with reaction temperature, reaching 100% at 300, 350, and 400 °C for toluene, benzene, and ethylbenzene, respectively. Maximum conversions of p-xylene (60%), m-xylene (52%), and o-xylene (47%) were attained at either 350 or 400 °C. AMO showed remarkable activity in the low temperature range (150−250 °C) (Figure 10B). However, lower conversions were generally obtained from 250 to 350 °C as compared to OMS-2, suggesting that in this case, surface area of the catalyst may not be directly correlated to catalytic performance. Using AMO at 350 °C, for example, toluene, ethylbenzene, benzene, p-xylene, m-xylene, and o-xylene achieved conversions of 86%, 77%, 69%, 47%, 37%, and 38%, respectively. Interestingly, the difference in % conversions among xylenes was generally less evident for AMO than for OMS-2. One can also clearly see an enhancement of the activity of AMO when the reaction temperature was increased from 350 to 400 °C. The % increases in conversion were 16%, 30%, 30%, 17%, 38%, and 26% for toluene, ethylbenzene, benzene, pxylene, m-xylene, and o-xylene, respectively, the highest values observed among the catalysts. This is likely due to the transformation of AMO structure to a crystalline phase (cryptomelane) under air atmosphere (Figures 2 and S2) resulting in a change of nature of the active sites. The observed structural change−catalytic activity relationship in AMO cannot be directly connected with those in OMS-2, commercial MnO2, and CuO/Mn2O3 because the physical structures of the latter three catalysts did not change during and after the course of the reaction (air, 400 °C). However, at 400 °C when both AMO and OMS-2 had the same crystal structures but different morphologies, their catalytic activities were comparable. The performance data showed that both the amorphous and the cryptomelane phases of AMO are catalytically active. Among the catalysts used, the commercial MnO2 showed the lowest activity for total oxidation of VOCs (Figure 10C). Benzene was oxidized more easily than toluene using commercial MnO2, as was similarly observed in a recent study.16 Benzene was followed by toluene, ethylbenzene, and then xylene isomers, which had similar but relatively low % conversions. At 250 °C, for example, the % conversions for toluene, benzene, ethylbenzene, p-xylene, o-xylene, and mxylene were 37%, 44%, 31%, 20%, 13%, and 14%, respectively. The plausible reasons for the opposite activity of commercial MnO2 toward benzene and toluene are lower concentration of Mn3+ and lower surface area as compared to the synthesized catalysts. The reactivity trend of toluene, benzene, and ethylbenzene using CuO/Mn2O3 (Figure 10D) was the same as in OMS-2. At 250 °C, the % conversions for toluene, benzene, ethylbenzene, p-xylene, o-xylene, and m-xylene were 72%,

Figure 11. Stability of OMS-2 catalyst toward oxidation of (A) benzene, toluene, and ethylbenzene, and (B) xylenes at 350 °C in air.

°C and oxidizing conditions (21% O2). OMS-2 slightly deactivated (100% to ∼90% conversion) with toluene and benzene after ∼50 h (∼2 d) on-stream (Figure 11A). No significant decay was observed for other VOCs. No CO was detected in all catalytic performance experiments. Intermediates/organic byproduct were also not detected in the liquid phase collected in the cold trap under most experimental conditions. When reactions were run using fresh OMS-2, AMO, and CuO/Mn2O3 catalysts in the absence of any VOC (blank) at each reaction temperature, trace amounts of CO2 (peak area cannot be quantified by the GC-TCD) appeared at ∼350−400 °C, without affecting the peak area of O2. This implies that the small CO2 peak was from the thermal decomposition of unreacted acetates and/or impurities present in the catalysts, and was insignificant as compared to the amount of CO2 produced from oxidation of VOCs under the same experimental conditions. When reactions were started at 350 °C using fresh OMS-2, AMO, and CuO/Mn2O3 catalysts in the presence of toluene but using pure N2 as the feed gas, small CO2 peaks started to appear corresponding to 13% (OMS-2), 5% (AMO), and 12% (CuO/Mn2O3) conversions. No CO2 was formed when the reaction was repeated right after the first run using the same catalysts even at 400 °C. This suggests that the amount of mobile/lattice oxygen species, which can participate in deep oxidation, decreased or consumed resulting in catalyst deactivation. The catalysts were regenerated by supplying gaseous O2 on-stream for ∼30 min (400 °C, air at 20 sccm). Correlating these observations with catalytic performance results (Figure 10) provided further evidence of the dependency of catalysts on gas-phase O2, especially the 12075

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Mn2+ ions.19,51,61,62 The first reduction peak of AMO (Figure 8B) was noticeably smaller as compared to that of OMS-2, which suggests that slightly fewer oxygen species were removed from AMO than OMS-2. Thus, the position and shape of the peaks can be attributed to this compositional difference. Low CO consumption during the TPR experiments could be considered as a low lattice oxygen amount available on the surface of manganese oxide species, indicating the existence of oxygen vacancies.4 Similar to a previous study,62 the oxide with the lowest crystallinity (AMO) was the easiest to reduce. The shifting of the reduction temperature to the lower temperature regimes on incorporation of copper indicates the increased mobility and ease of removal of lattice oxygen species.16 Molecular oxygen can possibly dissociate on the copper surface and spill over from copper to the oxygen vacancies in manganese oxide, which promotes the adsorption/dissociation of VOCs.17 The low temperature signals for the reduction of CuO/Mn2O3 correspond to the reduction of CuO and Mn4+, which leads to an easy release of oxygen species from the lattice and hence a higher oxidation activity. Thus, copper may have a promotional effect on the reduction of manganese ions (Mn3+ and Mn4+).11,44 Meanwhile, the shift of the reduction temperature to a higher temperature means a decrease in the lattice oxygen mobility of the catalyst.37 Oxygen Evolution. The evolution of oxygen results in the formation of framework oxygen vacancies, which may be catalytically active sites for the oxidation reactions.22,29 The higher activity of OMS-2 for VOC oxidation could be due to the increase of these defects resulting from the large distortion with the Mn−O lattice in the cryptomelane structure, promoting oxygen exchange.36,38 The supply of O2 from air is crucial to the activity of AMO. Results of TGA experiments confirmed the higher facility of lattice oxygen dislodged from the structure of the synthesized catalysts. This is important for reactions where oxygen exchange is involved. For example, although the lattice oxygen concentration of AMO was much lower than that of commercial MnO2, AMO had the better performance. This fact suggests that the accessibility may be more important than the concentration of lattice oxygen of the catalyst for oxygenated VOC removal. Moreover, the poor crystallinity of AMO and high defects of both AMO and OMS2 would increase oxygen exchange capacity.67,68 The activity of AMO can be correlated with the loosely bound oxygen with medium BE peak and oxygen vacancies due to its high surface area.25,40,67 These oxygen species play an important role especially for AMO and OMS-2 as such species allow the diffusion of lattice oxygen and OH groups.25,29,69 The involvement of lattice oxygen of catalysts in total oxidation of VOCs and the strong correlation between labile lattice oxygen and catalytic activity suggest that the reaction could proceed via the Mars−van Krevelen mechanism.29,31,37 This is illustrated in the present study and the results of numerous investigations. VOC molecules adsorbed on catalyst surface are oxidized by surface lattice oxygen species. The resultant oxygen vacancies, which would act as active centers in oxidation reactions, are subsequently replenished by O2 in an air stream during catalytic performance experiments. Adsorption properties are usually related to various parameters including porosity, and hydrophobicity of the catalyst and the nature of substrates. For example, toluene and ethylbenzene adsorb on some sites of OMS-2 material even at higher temperatures (∼550 °C) as compared to xylenes. VOCs adsorb on AMO very minimally under deoxygenating conditions. The

AMO, while demonstrating their reusability and ability to utilize lattice oxygen under deoxygenating conditions.

4. DISCUSSION The present work highlights the use of manganese oxides as well as mixed copper manganese oxide as effective catalysts for total oxidation of VOCs. Synthesized via simple redox and coprecipitation methods, the ability of these nanomaterials to selectively oxidize VOCs to CO2 and H2O can be attributed to a combination of factors. Oxidation State of Mn. Mixed valency is an important property in redox catalysis and is common in manganese oxide materials.39,51 Mixed valency of manganese in OMS-2, AMO, and CuO/Mn2O3 is a result of Mn3+, Mn4+, and possibly Mn2+ ions produced from the reduction of Mn7+ during synthesis.14,17,63 A previous study showed that the catalytic performance of manganese oxides for benzene and toluene oxidation decreased in the sequence: Mn3O4 > Mn2O3 > MnO2.16 Here, results of AOS experiments suggest that the relative amount of manganese ions in the synthesized catalysts is essential for catalytic performance. AOS results further suggest that the synthesized catalysts had a relatively higher amount of lower oxidation state manganese (Mn3+) than the less active commercial MnO2 (Mn4+).17,40 The active sites may be derived from the presence of Mn3+ in the near surface of the synthesized catalysts.17,25 The seemingly low AOS values for OMS-2 and AMO obtained in the present study are likely due to a high concentration of oxygen defects.38,64 Hydrophobicity and Morphology. Water vapor can inhibit catalytic reactions by competitive adsorption of water molecules on the active sites.36,65 Hydrophobicity is therefore an important property of a catalyst, particularly in this study where reactions yield significant amounts of water. Our previous study showed that OMS-2 is hydrophobic and capable of selectively absorbing neat benzene in the presence of water vapor.27,29 In the present study, we showed that the hydrophobicity index of OMS-2 is not only higher than AMO, commercial MnO2, and CuO/Mn2O3, but can also be a factor for determining catalytic performance toward oxidation of other VOCs (toluene, ethylbenzene, and xylenes). In other words, the present findings suggest that the effect of moisture on activity could be minimal for OMS-2 as compared to AMO, CuO/Mn2O3, and commercial MnO2. However, the presence of water may be advantageous to inhibit the formation of organic byproduct and increase the selectivity for the less harmful product CO2.38,65 Catalyst poisoning is also more pronounced in oxidation reactions at near ambient temperatures.17,43 Our earlier study showed that a hydrophobic and porous coating such as polydimethylsiloxane was needed to effectively inhibit water deactivation of AMO for lowtemperature CO oxidation under moisture-rich conditions.43 In the present study, AMO had the lowest hydrophobicity index among the catalysts tested. The morphology of CuO/Mn2O3 is also responsible for its high activity. CuO/Mn2O3 consists of aggregates of nearspherical clusters, which resulted from the physical packing of nanometer-sized fibrous particles (Figure 4D).66 These fibrous particles consist of intraparticle disordered mesoporosity, which would furnish a large number of adsorption sites.17,44,66 Reducibility. The activity of manganese-based catalysts for total oxidation of VOCs is associated with their reducibility.45 The peaks in the CO-TPR and H2-TPR profiles can be attributed to the successive reduction of Mn4+ and Mn3+ to 12076

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difference in catalytic activity between OMS-2 and AMO, therefore, may not only be due to the composition of Mn3+ and Mn4+, but also due to the differences in property such as morphology. The catalytic activity can also be correlated with oxygen mobility.39 The rate of reoxidation on the active sites could be slow for commercial MnO2, making this material the least active among the catalysts used. Structural Effects. Structural effects on molecular interactions help explain experimental observations on different reactivities of VOCs. For example, toluene is easier to oxidize than benzene using the synthesized catalysts, presumably due to the −CH3 group in toluene, which is first oxidized. As this happens, oxidizing the aromatic ring becomes easier as compared to dissociating the benzene ring directly. Another reason is that the −CH3 group of toluene could activate the aromatic ring through inductive effect and enhance the reactivity.70 However, as the −CH3 and −CH2CH3 groups have similar inductive effects, the additional C atom in ethylbenzene could make it harder to completely oxidize than toluene. This is expected if one imagines that there will be more interaction between longer chain molecules than shorter ones due to more induced fluctuating charges in the former (van der Waals interaction).70 Furthermore, because the −CH2CH3 group is roughly more hydrophobic than the −CH3 group due to an additional C atom, ethylbenzene is probably adsorbed more strongly on hydrophobic OMS-2, making it more difficult to oxidize than toluene. The last argument is consistent with our CO2-TPD results (Figure 9A). Among xylene isomers, p-xylene is also the easiest to oxidize using OMS-2 catalyst. Because the xylene isomers used in this study are electronically similar, shape-selectivity of OMS-2 could help explain the differences in terms of ease of oxidation. The p-xylene, which presumably can fit in the regular-ordered tunnels of OMS-2, gave higher % conversions than did the oxylene and m-xylene. The active sites of OMS-2 may therefore be derived from Mn3+ present in tunnels of the OMS-2 material.22 The m-xylene may go in the tunnels more easily (through partial anchoring) than the o-xylene, which is somewhat larger in molecular size. This is the first example of shape-selectivity relating to gas-phase total oxidation of xylenes using OMS-2 catalyst.

reducibility by CO. The presence of copper in CuO/Mn2O3 was responsible for the enhanced catalytic activity of manganese oxide. The transformation of an amorphous structure to the cryptomelane phase at 400 °C improved the catalytic activity. The catalytic performance was also associated with the nature of the surface manganese oxide species. The conversion was also found to be influenced by the type of VOC or catalyst (e.g., shape-selectivity of OMS-2). A Mars−van Krevelen mechanism for catalytic oxidation has been proposed on the basis of strong evidence of utilization of lattice oxygen species under conditions devoid of gas-phase O2 and rejuvenation of the catalysts in its presence. This study provides a basis for potential applications of the catalysts in soil-vaporextraction, automobile exhaust, and gasoline-stripping of sites contaminated with gasoline. Particular interest should be given to the removal of BTEX mixtures to realistically model air contaminants and to evaluate the possible inhibitory effects in the oxidation reactions that are presented here.

5. CONCLUSION OMS-2, AMO, and CuO/Mn2O3 have been successfully synthesized using simple refluxing and coprecipitation methods. Along with commercial MnO2, these catalysts have been carefully characterized. The catalytic oxidation of benzene, toluene, ethylbenzene, p-xylene, m-xylene, and o-xylene has been carried out using these catalysts under an air atmosphere. CO2 and H2O were the only products detected under the present conditions. In general, catalytic performance of the catalysts increased in the order: commercial MnO2 < AMO < CuO/Mn2O3 ≈ OMS-2. OMS-2 was the most active catalyst for oxidation of benzene, toluene, and ethylbenzene, whereas CuO/Mn2O3 was the most active for o-xylene, m-xylene, and pxylene. Results of XPS, AOS, and TPD experiments on the synthesized catalysts showed the existence of manganese in a mixed-valent environment. OMS-2 with a hydrophobicity index of 0.91 possessed a strong affinity toward VOCs. OMS-2 was also very stable for VOC oxidation under air atmosphere. TPD and TPR results indicated that the catalysts with high oxygen mobility exhibit high catalytic activity. The oxidation state and the crystallinity or defect concentration determined the

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Supporting Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (860) 486-2797. Fax: (860) 486-2981. E-mail: steven. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for having received research support from the U.S. Department of Energy (DE-FG02-86ER13622.A000), Office of Basic Energy Sciences, Division of Chemical, Biological and Geosciences. We also thank Drs. Lichun Zhang, Heng Zhang, and Hector F. Garces, for collecting additional TEM, XPS, and XRD data, respectively. The insightful comments and suggestions of Drs. Dambar B. Hamal, Naftali N. Opembe, and Hector F. Garces are likewise appreciated. REFERENCES

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dx.doi.org/10.1021/jp301342f | J. Phys. Chem. C 2012, 116, 12066−12078