A Mild Simple Method for Liquid-Phase Selective Catalytic Oxidation

Nov 24, 2008 - The total conversion and yield to the selective products exhibited by STC were superior to those catalysts exhibited in the presence of...
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Ind. Eng. Chem. Res. 2009, 48, 440–445

A Mild Simple Method for Liquid-Phase Selective Catalytic Oxidation of Toluene with Ozone over CeO2 Promoted Sulfated TiO2 Bo Wang,* Wei Mao, and Hongzhu Ma Institute of Energy Chemistry, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an, 710062, People’s Republic of China

A new clean catalytic process for liquid phase selective oxidation of toluene has been carried out on CeO2 promoted sulfated TiO2 (SO42-/TiO2-CeO2, abbreviated as STC) catalyst with ozone-air at ambient temperature and atmospheric pressure. Neither solvent nor promoters are needed in the reaction system. STC were found to exhibit good catalytic reactivity for the selective oxidation of toluene. For comparison, a series of single and complex oxides were also investigated including pure TiO2, SO42-/TiO2 (abbreviated as ST). The total conversion and yield to the selective products exhibited by STC were superior to those catalysts exhibited in the presence of ozone. The highest conversion of toluene was found to be 9.7% with 77% selectivity to benzaldehyde and benzyl alcohol on the STC. The anatase phase was found to be dominating in the STC by means of X-ray diffraction. By utilization of the N2 adsorption method, it was detected that the Brunauer-Emmett-Teller surface area of STC was higher than other samples. Other techniques such as scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and energy dispersive X-ray spectroscopy were also employed to characterize the property of catalysts. 1. Introduction Selective oxidation of toluene to benzaldehyde is an important organic reaction,1,2 which is more valuable than benzoic acid as it is an important raw material for the synthesis of many other valuable chemicals.3 However, the selective oxidation of toluene could not produce benzaldehyde with air because it is difficult to effectively prevent the overoxidation of benzaldehyde into benzoic acid during the oxidation of toluene. Up to the present, benzaldehyde is mainly synthesized by the chlorination of toluene followed by hydrolysis in industry, and it is a severe pollution process.4 In recent years, some researchers have tried the selective aerobic oxidation of toluene to benzaldehyde catalyzed by the transition metal oxides and metalloporphyrins. V-Zr-O, V-Ag-Ni-O, cobalt tetraphenylporphyrin, and manganese tetraphenylporphyrin have been studied for the selective oxidation of toluene to benzaldehyde in the gas phase or liquid phase.3-6 Unfortunately, there is no commercial plant available currently for the production of benzaldehyde from the oxidation of toluene by molecular oxygen due to the low activity and/or selectivity. Moreover, the general conditions of above synthetic methods are quite rigorous such as high temperature and pressure, which not only spend many special and costly materials but also consume a large amount of energy. Thus, there is an exigent need for “green” technologies in the synthesis of benzaldehyde. However, not much attention has been paid on the selective catalytic oxidation of toluene in liquid phase with ozone over solid acid catalysts. In view of environmental and economical reasons, there is an ongoing effort to replace the conventional liquid catalysts with new solid acids. It is well-known that the solid acid SO42-/ MxOy have been used in isomerization, alkylation, polymerization, esterification, oxidation, etc. for their high strength of acidity, nontoxicity, and high activity at low temperatures.7 Sulfated titania is one of the strongest solid superacids, which possesses high activity and stability.8 Furthermore, it is generally * To whom correspondence should be addressed. Phone: +86 29 85308442. Fax: +86 29 85307774. E-mail: [email protected].

accepted that anatase titania is the best support catalyst found for oxidation of toluene by now.9,10 On the other hand, the reason for the successful use of ceria-based oxides in catalysis is just connected to the ability to transport oxygen in combination with the ability to cycle easily between reduced and oxidized states (i.e., Ce3+/Ce4+).11 Here, liquid-phase selective oxidation of toluene to benzaldehyde has been carried out over CeO2 promoted sulfated TiO2 catalyst (SO42-/TiO2-CeO2, abbreviated as STC) with ozone-air at ambient temperature and atmospheric pressure. For comparison, a series of single and complex oxides, pure TiO2, SO42-/ TiO2 (ST) were also prepared. Various techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), N2 adsorption method, and energydispersive X-ray spectroscopy were employed to characterize the phases and structures of the catalysts. 2. Experimental Section 2.1. Catalyst Preparation. All chemicals used in the experiment were analytical grade and used without any further purification, which were obtained from Xi’an Reagent Co. of China. The STC catalysts were prepared by adopting a two-step route.12 The detailed method was described as below. In the first stage, Ce-Ti hydroxide gel were obtained by precipitating a mixture solution of cerium nitrate Ce(NO3)3 · 6H2O and titanium tetrachloride TiCl4 with dilute aqueous ammonia solution (25 wt %) under vigorous stirring continuously until the pH was 8. Then the obtained precipitate was filtered, washed until free from chloride ions, and then dried at 373 K for 12 h. In the second stage, the hydroxide gel was sulfated by H2SO4 (0.5 M) solution under ambient temperature, deposited overnight, dried at 373 K for 12 h, and calcined at 773 K for 3 h in air. Finally, Ce-doped SO42-/TiO2 catalysts were obtained (the theoretical molar ratio of S, Ce and Ti is 1:15:175). For comparison, TiO2 catalyst and SO42-/ TiO2 catalyst were also prepared using the same procedure.

10.1021/ie800725h CCC: $40.75  2009 American Chemical Society Published on Web 11/24/2008

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2.2. Characterization Techniques. The morphology of variant catalysts was investigated by SEM (Philips at 30 kV) measurements. To determine the component and atomic ratios in the catalyst along the surface cross-section, the signals of surface elements were detected by the EDXS (Oxford Instruments Microanalysis 1350) mapping method. The surface area of the catalyst, fresh and used was measured using nitrogen adsorption at 77 K and the Brunauer-Emmett-Teller (BET) method using a Micrometerics ASAP2020 system. Prior to analyses, 0.2 g of sample catalysts were outgassed at 120 °C for 60 min in a quartz reactor. XRD analyses were performed to characterize the bulk properties of the catalysts. XRD patterns of the as-prepared samples were collected with a Rigaku D/max-γA rotation anode X-ray diffractometer (Cu KR, λ ) 0.15418 nm). The pattern for the structure refinement was taken in a 2θ range of 20-80° with a step width of 0.02° (2θ). The infrared absorptions were studied adopting FT-IR spectrometer (Perkin-Elmer 2000, USA) for judging the formation of surface acid sites of catalysts. The IR spectra of catalysts samples were recorded at room temperature. Before disks were made, all the samples were degassed at 383 K for 1 h in vacuum. The information on surface elemental compositions of catalyst was analyzed by XPS using a XSAM800 (Kratos) equipment operated in FAT mode with nonmonochromatic Mg X radiation (hν ) 1253.6 eV). The base pressure in the chamber was in the range of 1 × 10-8 Pa. Charging of catalyst sample was corrected by setting the binding energy of adventitious carbon (C 1s) at 284.6 eV. The samples were outgassed in a vacuum oven overnight before XPS measurements. 2.3. Catalytic Activity Studies. The finished catalyst was activated at 423 K for 3 h in vacuum before catalytic runs. Toluene was analyzed before the reaction by gas chromatography to ensure the absence of auto-oxidation products before use. Ozone was generated by corona discharge using an ozone generator (Harbin Jiujiu Electric Chemistry Engineering and Technique Co., Ltd., DHX-SS-03C, China) with O3 at a rate of 2 g h-1. Toluene oxidation with ozone-air mixtures in this paper was carried out under the following typical procedures: toluene (120 mL) and catalyst (10 g) were charged into a simple slurry reactor system. A gas of ozone-air mixtures (contained approximate 5% ozone) was passed through the catalyst with a constant flow rate of 0.25 L min-1 at ambient temperature (the temperature range 301-305 K). The total reaction time was about 3 h. When the reaction finished, the products were collected and analyzed by gas chromatography-mass spectrometry (GC-MS, QP2010, Japan). The conditions of chromatography (RTX-5MS 30 m/0.25 mm/250 mm polyethyleneglycol column): injection temperature, 260 °C; initial temperature, 70 °C for 2 min; ramp 1, 10.0 °C/min to 260 °C, held for 2 min. 3. Results and Discussion 3.1. Characterization of Catalysts. XRD analyses of pure TiO2, ST, STC samples (calcined at 773 K) before reaction are shown in Figure 1. The diffractogram of TiO2 samples show that the samples used in this study were mixtures of anatase and rutile phases. The most important features of the diffractogram were observed between 20 and 40° (2θ), with two sharp and intense peaks (25.4 and 27.4°) dominantly indexed to the anatase phase (PDF-ICDD 89-4921) and rutile phase (PDFICDD 89-4920), respectively. The typical temperature needed to transform the pure TiO2 phase into rutile is ca. 823 K.13 From diffractogram, it can be also concluded that, at the calcination

Figure 1. XRD patterns of TiO2 (A), ST (B), and STC (C) calcined at 773 K (A) 25.4, 27.4, 36.2, 38.1, 41.1, 48.2, 54.4, 56.8, 62.9, 69.2, 75.3°. (B) 25.5, 38, 48.2, 54.8, 62.9, 70.1, 75.4°. (C) 25.5, 28.7, 33.2, 37.9, 47.6, 54.5, 56.6, 59.2, 69.5, 76.8, 79.3°. Table 1. Surface Parameters of the Prepared Systems

samples

BET surface area (cm2/g)a

pore volume (cm3/g)b

DBJH (nm)c

TiO2 ST STC

67 100 118

0.079 0.166 0.197

5.02 4.73 4.11

crystallite size (nm)d 14.7 6.68 5.84 (anatase) 39.03 (cubic)

a BET surface area calculated from the linear part of the BET plot (P/P0 ) 0.2). b Total pore volume, taken from the volume of N2 adsorbed at P/P0 ) 0.995. c Average pore diameter, estimated using the desorption branch of the isotherm and the Barrett-Joyner-Halenda (BJH) formula. d Average crystal size of anatase-TiO2 or cubic-CeO2, estimated using the Scherrer equation.

temperature of 773 K, anatase phase was a bit more abundant than rutile phase in pure titania, whereas the completely different situation was encountered at sulfated titania samples. It can be definitely observed that only typical anatase phase was displayed on diffraction profiles of ST, which indicated the existence of SO42- can effectively restrain the metastable TiO2 anatase phase to the TiO2 rutile phase.14 However, no diffraction peak due to titanium sulfate compounds emerged, maybe due to the fact that the sulfate group decomposed or the amount of these compounds is insufficient to be detected by XRD. Otherwise, in comparison with pure TiO2 sample, the diffraction peaks due to the TiO2 anatase in ST mixed oxides became broader and weaker, suggested that the presence of sulfate ions retarded the crystallization process of anatase TiO2 phase and the crystallite size of the anatase TiO2 became smaller.14,15 Under identical calcination conditions, sulfated samples displayed a smaller crystallite size and increased amounts of amorphous phase, in contrast to the corresponding nonsulfated samples. Up to CeO2 adding into the ST, the titania only displayed a single further broad and weak diffraction peak at 25.5°, corresponding the (101) plane. This indicates that along with the doped CeO2 into ST samples, the crystallinity of TiO2 anatase phase deteriorated more seriously and the crystallite size of the TiO2 anatase became further smaller than the ST. This result was in agreement with previous reports that the existence of CeO2 can effectively stabilize to some extent of the TiO2 anatase phase and retarded the transformation from amorphous to anatase phase.16,17 Furthermore, a typical diffraction pattern of CeO2 with a cubic fluorite structure (PDF-ICDD 81-0792)

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Figure 2. SEM micrographs of ST, STC (×5000): (a) ST, (b) STC; EDX spectrum of STC, ST.

can be observed in the diffractogram of STC samples, and no diffraction peaks due to titanium sulfate or cerium sulfate compounds appeared. Similar findings were reported by Gao et al.18 BET surface areas of the samples and the average crystallite sizes of TiO2 anatase and cubic CeO2 in the mixed oxides used in this study are displayed in Table 1. A substantial increase in the surface area and decrease in the particle size could be observed when the sulfate ion was introduced into samples, which was in line with the XRD results. This indicates that the sulfate ion impregnated samples enhanced the effective dispersion and stabilization of TiO2 and STC particles over its surface.

This was in agreement with previous literature reports.19 The increase in the surface area of sulfated samples may be owing to the sulfates ions, which retard the samples from amorphous to the crystalline and protect oxides from sintering.15,20 The SEM image also sustained the inference mentioned above. The surface morphological features of the samples are shown in Figure 2. As can be seen, for the catalyst of ST and STC, the cubical shape transformed to a rough and loose shape, and the average size of particles decreased. Moreover, it was observed that a microsized and pored structure with bigger surface had formed during STC samples. Otherwise, the transformation tendency of morphology mentioned above

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weak shoulder peaks, the former absorption was more intense than the latter, which indicated the organic sulfate structure was primary on surface. More specifically, the sulfate species modified the electronic environment around the Ti4+ ion by anchoring SO42- in either bridging bidentate or chelating bidentate complexes shown as follow:25

Figure 3. FT-IR spectra of catalysts calcined at 773 K. (a) neat TiO2; (b) SO42-/TiO2; (c) SO42-/TiO2-CeO2; (d) SO42-/TiO2-CeO2 (after reaction).

became further clear when Ce was added into ST that was consistent with the results of XRD and BET. In addition, it can be detected clearly that the expected elements such as Ti, Ce, S, O were involved on the surface of catalysts (STC) confirmed by EDX survey, and the atom rate of Ti, Ce, S, O on the surface is 15:1:5:79 from the EDX results, which is much different from the theoretical atom rate of those elements in the catalysts, indicating that S element mainly existed on the surface of the catalysts compared with that Ce mainly existed in the bulk of catalysts. Moreover, the atom rate of 22:5:73 for Ti, S, O is also obtained on the surface of ST. The FT-IR absorption spectra of different catalysts calcined at the same temperature (773 K) are shown in Figure 3. In the curve of neat TiO2, the broad peaks (intense) at 3419 cm-1 were the stretching vibrating absorption peaks of O-H bond in the surfaced hydroxyl and in the planar water.8 A wide sharp band appeared at 1630 cm-1 corresponding to the flexion vibration band of the H2O molecule, which was ascribed to the water of physisorption on the catalyst.8,20 In general, for the metal oxides modified with sulfate ions followed by evacuation above 400 °C, a strong band assigned to S-O asymmetric stretching frequency is observed at 1360-1410 cm-1, which demonstrates the superacidic property of solid acid.21-23 As shown in Figure 3, a sharp band at 1402 cm-1 was observed in the curve of ST and STC samples but absent on the pure TiO2, which belonged to the stretching frequency of the free sulfate groups.24 Moreover, The spectrum of ST contained three shoulders but split bands at around 1230, 1123, and 1041 cm-1, and the spectrum of STC contained four shoulders but split bands at 1240, 1123, 1049, and 975 cm-1 in curves, respectively, which were the characteristic frequencies of a bidentate SO42coordinated to metals such as Ti4+.27,28 It indicates the presence of two kinds of sulfated species on the surface of catalysts. In addition, in comparison of the IR spectrum of STC before and after reaction, no significant change was found except the reduction of absorption intensity near the 1400 cm-1, which can be ascribed to loss of surface sulfate ion and/or the adsorbed reagents on surface acid sites. It should be pointed out that the bands of 900-1250 and 1360-1410 cm-1, respectively, belonged to the inorganic chelate, in which the S-O was mainly in the shape of an ionic bond with partial covalent bond, and organic sulfate structure, in which it was a covalent double bond.21-23 Although it can be observed in Figure 3 that there were both inorganic chelate and organic chelate in sulfated metal oxide, the peaks near 1400 cm-1 were sharp peaks, and the peaks near 1230 cm-1 were

It was noted that such bridge bidentate or chelating bidentate structure could strongly withdraw electrons from the neighboring Ti cations, resulting in a number of electron-deficient metal centers on the Ti cations that act as strong Lewis acid sites. Such results are similar to those of other workers26 and are believed to be a driving force in the generation of a large amount of surface acidic sites on solid acids of sulfated metal oxides.26 By use of a color change method (utilizing the Hammett indicator dissolved in benzene),27 the Hammett acidity function of STC is g -13.16 and ST is g -12.4. These results means that STC and ST act as super solid acid catalysts and indicates that the strengthening in acid strength of the modified catalyst is attributed to the inductive effect of S-O.8,24 Above results was also confirmed by followed XPS analysis. The XPS spectra of STC after reaction are presented in Figure 4. The desired composition of catalysts was shown in Figure 4a, and no other element was detected; the atom rate of various elements (Ti, Ce, S, O) on the surface is 16:1:4:79, calculated based on XPS spectra, which is according with EDX results. Figure 4b shows the XPS spectrum in the S2p binding energy (BE) region for STC, and the BE of S2p3/2 was observed at 168.6 eV corresponding to the typical S6+ of sulfate reported in the literature,8 and it also confirmed the IR results, which indicates that sulfur compound might exist in a form of bidentate sulfate on the surface of TiO2. Furthermore, comparing various the proportions of the S element in the bulk of and on surface of catalysts, it may be believed that sulfur species mainly exist on the surface of catalysts. The spin-orbit (2p3/2) signals at 459.2 eV indicated that Ti element mainly existed as the chemical state of Ti(IV). Normally, the Ti 2p3/2 line in the case of pure TiO2 samples can be observed at 458.5 eV.8 An increase in the binding energy of the same in the present study may be due to the formation of bidentate complexes between titanium cation and sulfate anion, which could strongly withdraw electrons from the neighboring Ti cations resulting in an up-shifting of BE of the Ti cations. It was observed in Figure 4d that the Ce 3d5/2 binding energy of CeO2 in STC catalysts was 881.8 eV, implying that Ce4+ dominates in STC. Besides, comparing with XRD results and various atom rate in the bulk of or on the surface of catalysts, it may be reasonable to infer that the Ce element mainly existed in the bulk of catalyst so that the signal of cerium element was not strong in the XPS spectrum, and it also indicated that the titanium element was dominating metal element on the surface of catalysts. 3.2. Liquid-Phase Selective Oxidation of Toluene on Various Catalysts. It was interesting to find that the blank reaction of only ozone-air mixtures in existence and all the catalysts calcined at 773 K exhibited catalytic performance in synthesis of benzaldehyde at ambient temperature and atmospheric pressure. Reported in Figure 5 is the catalytic behavior

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Figure 5. Conversion of toluene over a series of catalysts as a function of reaction time at ambient temperature and atmospheric pressure. Table 2. Selectivity of Products on the Oxidation of Toluene at 301-303 K and Atmospheric Pressure selectivity (%)

Figure 4. XPS survey spectrum of STC: (a) ESCA peak of STC; (b) S 2p XPS spectrum; (c) Ti 2p XPS spectrum, and (d) Ce 3d XPS spectrum.

of these samples reactivity tests as a function of time (all in a constant flow of ozone-air mixtures). Conversion and selectivity to products are expressed as mol % on a C atom basis. As can be seen, while the conversion of toluene is very low on blank systems (only ozone-air mixtures) and TiO2 catalyst, it is clear that the ST and STC catalysts favor the conversion of toluene in oxidation reaction, which is obvious higher compared with two mentioned previously. The poor effect of using ozone alone and pure titania may be due to the fact that the residence time was not enough for ozone to react with the toluene. In other words, these data maybe indicate that the chemical adsorption of ozone on the catalysts after H2SO4 treatment is enhanced notably so that the conversion is increased in oxidation reaction. ST and STC samples have larger BET surface and stronger

catalyst

blank (air-ozone)

pure titania

ST

STC

benzaldehyde benzyl alcohol CP

88 12 0

81 6 13

62 16 22

58 19 23

acidity than other catalysts. Thus, it is reasonable to infer that the surface acidity of metal oxides may play an important role in selective oxidation of aromatic molecules, and large surface area is also facilitating to the conduct of reaction. Moreover, it can also suggest that the acidity and the surface area of sulfated titania were improved by Ce addition, so that the highest conversion is obtained when STC was applied as catalyst. On the other hand, benzaldehyde, benzyl alcohol, and several coupling products (CP) are the main reaction products observed on either catalyst. Here, the CP consists of 1-benzyl-4methylbenzene, 1-benzyl-2-methylbenzene, 1-benzyl-3-methylbenzene, and benzoin. No benzoic acid formation was detected in the reaction. The selectivity of products distribution in toluene oxidation over various catalysts are shown in Table 2. It is interesting to mention that the selectivity to desired products: benzaldehyde and benzyl alcohol were 88 and 12%, respectively, over the blank reaction system; this result indicates that the selectivity in the toluene oxidation with ozone alone is excellent in the partial oxidation of toluene. However, it is very regrettable that the conversion of blank experiment is so low (less than 3.3%); this process will not have favorable industrial prospects. Although the STC catalysts are less selective compared with those catalysts, the total selectivity for benzaldehyde and benzyl alcohol is still 77%. Therefore, taking into account the high conversion of the STC system, the average yield of benzaldehyde and benzyl alcohol is 7.1% in STC that is far more than other catalytic systems. Actually, the traditional industrial process usually exhibits a 15% conversion of toluene and 85% selectivity to benzoic acid with little amount of benzaldehyde.3 Therefore, it is quite promising to have an 9.7% conversion of toluene with 77% selectivity to benzaldehyde and benzyl alcohol. Besides, to elucidate the necessity of ozone in the oxidation of toluene, the reaction was also conducted with only air as the oxidant agent, and all the other reaction conditions maintained constantly. Finally, it was very regrettable that no desired

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oxidized products were detected in aforementioned reaction systems, which may be ascribed to the stability of molecular oxygen under normal pressure and temperature that can hardly be activated by the catalysts in our study. On the contrary, ozone is a powerful oxidant (E0 ) 2.07 V), so it can easily react with many compounds under mild conditions. Thus, on the basis of the above experiments, ozone is an effective oxidant agent in the partial oxidation of toluene under normal pressure and temperature. Studies have shown that the decomposition of ozone molecules is based on the existence of Lewis acid sites where significant amount of active oxygen atoms were generated in acid catalyst.28 On the other hand, in the presence of catalysts, the toluene molecule may be activated on the Lewis acid sites, which will benefit the reaction between the oxygen atom and toluene molecule.29 The atomic oxygen would attack the C-H bonds of toluene and oxidize toluene into benzaldehyde and benzyl alcohol. According to the above results and the literature, we suggested potential reaction mechanism by a catalytic reaction as follows:

4. Conclusions It was interesting to find that the blank reaction of only ozone-air mixtures in existence exhibited activity in partial oxidation of toluene at room temperature and air pressure, but the low conversion of this method (less than 3.3%) will restrain tempting industrial prospects. Fortunately, ceria supported on sulfated TiO2 were found to be active and selective catalysts for the liquid-phase selective oxidation of toluene in the presence of ozone-air. XRD, FT-IR, and XPS confirm that ceria supported on sulfated TiO2 have larger surface area and stronger acidity which can influence the partial oxidation of toluene remarkably at room temperature and air pressure. The conversion of toluene was found to be 9.7% with 77% selectivity to benzaldehyde and benzyl alcohol on STC. In these systems, although the addition of catalysts into reaction restricts the selective oxidation of toluene to desired products slightly, the conversion increased evidently so that the yield of desired products (the average yield 7.1%) was much more than that of other system. Thus, it is quite promising catalytic route for partial oxidation of toluene environmentally friendly. Although the conversion of toluene was relatively low (the highest 9.7%), the total selectivity to benzaldehyde and benzyl alcohol was relatively high (77%) at ambient temperature and atmospheric pressure. Literature Cited (1) Chang, C. J.; Deng, Y.; Shi, C.; Chang, C. K.; Anson, F. C.; Nocera, D. G. Electrocatalytic four-electron reduction of oxygen to water by a highly flexible cofacial cobalt bisporphyrin. Chem. Commun. 2000, 5, 1355. (2) Chang, C. C.; Weng, H. S. Deep Oxidation of Toluene on Perovskite Catalyst. Ind. Eng. Chem. Res. 1993, 32, 2930. (3) Xue, M. W.; Ge, J. Z.; Zhang, H. L.; Shen, J. Y. Surface acidic and redox properties of V-Ag-Ni-O catalysts for the selective oxidation of toluene to benzaldehyde. Appl. Catal., A 2007, 330, 117. (4) Guo, C. C.; Liu, Qiang.; Wang, X. T.; Hu, H. Y. Selective liquid phase oxidation of toluene with air. Appl. Catal., A 2005, 282, 55. (5) Huang, G.; Jin, L.; Cao, C. D.; Yong, A. G.; Shu, K. Z.; Hong, Z.; Shan, W. Catalytic oxidation of toluene with molecular oxygen over manganese tetraphenylporphyrin supported on chitosan. Appl. Catal., A 2008, 338, 83.

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ReceiVed for reView May 05, 2008 ReVised manuscript receiVed August 18, 2008 Accepted October 19, 2008 IE800725H