IR Observation of Selective Oxidation of Cyclohexene with H2O2 over

IR Observation of Selective Oxidation of Cyclohexene with H2O2 over Mesoporous Nb2O5 ... Fax: +81-45-924-5282. E-mail: ... Liquid-phase catalytic oxid...
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J. Phys. Chem. C 2009, 113, 21693–21699

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IR Observation of Selective Oxidation of Cyclohexene with H2O2 over Mesoporous Nb2O5 Hisashi Shima, Manabu Tanaka, Hiroyuki Imai, Toshiyuki Yokoi, Takashi Tatsumi, and Junko N. Kondo* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: September 24, 2009

Liquid-phase catalytic oxidation of cyclohexene by H2O2 (35 wt %) over mesoporous Nb2O5 showed considerable conversion and high selectivity for both cis- and trans-1,2-cyclohexanediols in acetonitrile solvent. The mesoporous structure of the catalyst, in addition to its high surface area, was considered to be advantageous for the reaction due to more frequent interaction of molecules with active sites inside pores. Gradual formation of 1,2-cyclohexanediol from the reaction of adsorbed cyclohexene with diluted (10 wt %) H2O2 was observed at room temperature by an in situ infrared (IR) method. The absence of observation of an IR spectrum of 1,2-epoxycyclohexane confirmed that the hydrolysis of the oxidation product occurred immediately. In addition to the conventional stepwise oxidation and hydrolysis to produce 1,2-cyclohexanediol, a direct hydroxylation of cyclohexene was proposed: the production of cis-1,2-cyclohexanediol with about a half amount of trans1,2-cyclohexanediol suggests the activation of H2O2 on the acidic OH groups of mesoporous Nb2O5 to directly produce both cis- and trans-1,2-cyclohexanediols from cyclohexene without forming 1,2-epoxycyclohexane. The hydrolysis of adsorbed cyclohexene to cyclohexanol was observed when more diluted (2.5 wt %) H2O2 was used. This indicates that H2O can be also activated for hydrolysis of olefin, but that the preferential activation of H2O2 over H2O occurs during the liquid-phase reaction with 35 wt % H2O2 on mesoporous Nb2O5. 1. Introduction Since the pore size of zeolites in the unit of subnanometer restricts the size of reactant molecules, mesoporous silica-based materials are prepared and utilized as catalysts for various reactions.1,2 Whereas specific active sites of zeolites represented by Bro¨nsted acid sites in aluminosilicates3 and Ti oxidation sites in titanosilicates4 are well-identified in rigid crystal structures of zeolites, such active sites cannot function very well in amorphous pore walls of mesoporous silica-related materials.5,6 On the other hand, various mesoporous transition metal oxides have also been prepared recently with the aim of applying them as devices as well as catalysts.7 Mesoporous Nb8 and Ta9 oxides were first prepared by a so-called ligand-assisted templating method, in which amines with long alkyl chains coordinate to metal alkoxides in a 1:1 molecular ratio. The amine-coordinated alkoxides self-assemble with alkyl groups inside to form cylindrical micelles. The resulting materials have small mesopores (about 2.5 nm diameter), thin pore walls (less than 2.0 nm), and a high surface area (above 400 m2 g-1), and their magnetic, electric, and semiconducting properties were applied in various fields.10 More recently, sulfated mesoporous Ta2O5 prepared by this method was found to be a shape-selective acid catalyst.11 One of the disadvantages of the materials is their low thermal stability due to the thin pore walls, which results in structure collapse after thermal treatment. On the other hand, various mesoporous transition metal oxides involving Nb2O512,13 have been prepared by using block copolymers as templates.14 The most advantageous point of the mesoporous materials obtained by this method is their high thermal stability due to thick pore walls, and the mesostructures * To whom correspondence should be addressed. Phone: +81-45-9245239. Fax: +81-45-924-5282. E-mail: [email protected].

can survive after calcinations at 500 °C for the removal of the template. However, such high-temperature treatment of mesoporous Nb2O5 dispossesses the acidic property from hydrated niobic acid (Nb2O5 · nH2O), although the amorphous inorganic structure remains unchanged. This becomes a problem for the application of it as a solid acid catalyst but was solved by improvement of the preparation method: optimization of the aging process and employment of the solvent extraction to the template removal. This method resulted in the formation of supermicroporous (1.5 nm diameter) Nb2O5.13 As a consequence, the supermicroporous Nb2O5 catalyst exhibited much higher activity for dehydration of alcohol and esterification of acetic acid with ethanol than nonporous niobic acid catalyst.15 As an application of mesoporous Nb2O5 for catalytic oxidation reactions, Nb-containing mesoporous silica catalysts16 are reported as well as Nb2O5 materials prepared by using mesoporous silicas as hard templates.17 On the other hand, pure mesoporous Nb2O5, is found to be an active oxidation catalyst for the formation of 1,2-cyclohexanediol from cyclohexene using tert-butyl hydroperoxide (TBHP),18 because mesopores are capable for such large reactant molecules in comparison with zeolite pores. But tert-butyl alcohol is simultaneously produced as a byproduct in the reaction. Thus, a more desirable oxidant, H2O2, which produces only H2O after the reaction, was employed in this study for the oxidation of cyclohexene. In addition, the reaction of cyclohexene adsorbed on mesoporous Nb2O5 with H2O2 aqueous solution was directly observed by infrared (IR) spectroscopy for the discussion on the active sites for oxidation and hydrolysis. 2. Experimental Section Mesoporous Nb2O5 was prepared following the reported method.9 Briefly, a block copolymer template (P-85, PEO26-

10.1021/jp906422z  2009 American Chemical Society Published on Web 12/07/2009

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Figure 1. SEM images of (A) mesoporous and (B) nonporous Nb2O5.

Figure 2. (A) Small- and (B) wide-angle XRD patterns and (C) N2 adsorption-desorption isotherms of (a) mesoporous and (b) nonporous Nb2O5.

PPO39PEO26, 1 g) was dissolved in 1-propanol (10 g), followed by addition of niobium chloride (NbCl5, 7 mmol) and water (70 mmol) with stirring. Then, the solution was aged at 40 °C for 7-10 days until a dry transparent film was obtained. During the aging period, hydrolysis of niobium chloride and successive dehydration among hydroxyl groups occurred. The production of a dry film is a criterion of the completion of these processes. The template was removed by calcination at 400 °C for 10 h. A reference sample, nonporous Nb2O5, was prepared by using the same starting materials and preparation conditions in the absence of template. Characterization was performed by X-ray diffraction (XRD, Rint Ultima with a Cu KR X-ray source, Rigaku), N2 adsorption-desorption measurement (Belsorp mini, Bell Japan), and scanning electron microscopy (SEM, Hitachi S-5200). The Brunauer-Emmett-Taller (BET) surface area and Baret-Joyber-Halenda (BJH) pore size were estimated using adsorption or desorption branches, which resulted in the same values in the present study. The oxidation of cyclohexene was carried out with 0.1 g of catalyst, 2 mmol of cyclohexene, 2 mmol of H2O2 (35 wt % aqueous solution), and 2.5 mL of solvent at 60 °C for 2 h unless otherwise mentioned. The reaction products were analyzed by a gas chromatograph (GC-2014, Shimadzu) with a DB-5 column and a flame ionization detector (FID). A Pt-BDEXcst-TM column, which often is used for the separation of chiral compounds, was used for the analysis of the isomers (cis- and trans-) of 1,2-cyclohexanediol. The amount of consumed H2O2 was estimated by titration of unreacted H2O2 with cerium sulfonate aqueous solution (Ce(SO4)2, 0.1 M). In situ IR observation of the oxidation of adsorbed cyclohexene with aqueous H2O2 solution was performed with an improved IR cell.19 About 40 mg of mesoporous Nb2O5 was pressed into an IR disk (20 mm diameter) and placed in the center of an IR cell with an oblique sample holder. The retention

of the original mesoporous structure after IR experiments was confirmed by XRD. After cyclohexene adsorption from the gas phase, a drop of H2O2 aqueous solution can be supplied by a syringe through an inlet to the inclined catalyst disk with flowing He gas, and a time course of spectral changes was observed at room temperature. Due to the rapid reaction, the concentration of H2O2 was decreased to 10 and 2.5 wt % for the IR experiments. Spectra were recorded on a Jasco FTIR 6100 spectrometer with MCT detector at 4 cm-1 resolution, and 64 scans were averaged for each spectrum. A background spectrum measured before cyclohexene adsorption was subtracted from those measured after cyclohexene adsorption and further reaction with H2O2. 3. Results and Discussion 3.1. Structure and Morphology of Mesoporous Nb2O5. Typical SEM images of mesoporous and nonporous Nb2O5 at the same magnification are compared in Figure 1. Although a nanosized, well-ordered mesoporous structure is clearly observed for mesoporous Nb2O5, a nonporous feature is obvious for the nonporous sample. A sharp diffraction peak at ca. 1.4°, which is attributed to d(100) diffraction in two-dimensional hexagonal symmetry, was observed for mesoporous Nb2O5 (Figure 2A), in agreement with the presence of ordered mesopores in the SEM image. The absence of any peaks in a wide-angle XRD pattern of mesoporous Nb2O5 is attributed to the amorphous pore walls. In the XRD patterns of nonporous Nb2O5, the absence of peaks in small- and wide-angle regions indicates the nonporous and amorphous structure. In N2 adsorptiondesorption isotherms (Figure 2C), a type IV isotherm pattern typical to mesoporous materials was observed for mesoporous Nb2O5, with 222 m2 · g-1 BET surface area, 3.1 nm pore size, and 0.22 mL · g-1 pore volume. The wall thickness estimated from the pore size and the repeat distance obtained from the

Oxidation of Cyclohexene with H2O2 over Nb2O5

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TABLE 1: Comparison of Activity of Oxidation over Amorphous Mesoporous Nb2O5 with H2O2 and TBHP

TABLE 3: Comparison of Activity of Epoxidation over Mesoporous Nb2O5 with H2O2 in Various Solvents

XRD pattern was 4.1 nm. On the other hand, no N2 uptake was observed for nonporous Nb2O5, resulting in a 22 m2 · g-1 BET surface area, no peak in pore size distribution, and a 0.02 mL · g-1 pore volume. Thus, the BET surface area and pore volume of mesoporous Nb2O5 are roughly 10 times those of the nonporous sample. 3.2. Catalytic Oxidation of Cyclohexene. The catalytic performance of mesoporous Nb2O5 for the oxidation of cyclohexene is first compared with using H2O2 and TBHP in Table 1. Among several products, 1,2-epoxycyclohexane is an initial oxidation product, which is rapidly hydrolyzed to 1,2-cyclohexanediol under acidic conditions. It should be noted that both cis- and trans-1,2-cyclohexanediols were formed with both oxidants. This point is discussed below (Section 3.4). The production of 1,2-cyclohexanedione was confirmed to be produced by further oxidation of 1,2-epoxycyclohexane in a separate reaction. In addition to the above-mentioned products, cyclohexene-1-ol and cyclohexene-1-one were formed by radical reactions. A higher conversion was observed for H2O2 oxidant than TBHP in Table 1, and radical reactions are restrained by using H2O2. Thus, H2O2 is found to be the preferable oxidant from the point of view of conversion in addition to the absence of byproduct generation (only H2O production). The structural effect of catalysis of Nb2O5 was studied by using H2O2 oxidant over mesoporous and nonporous Nb2O5 (Table 2). Samples (50 and 500 mg) were used for comparing mesoporous and nonporous Nb2O5 catalysts, considering the different BET surface areas (222 and 22 m2 · g-1 for mesoporous and nonporous samples). Under this condition, almost the same surface area was provided for the reaction; the same number of active sites should be exposed to the reactants. Nevertheless, conversion over nonporous catalyst was less than one-half of that over mesoporous catalyst. Since a similar tendency in selectivity involving the formation of both cis- and trans-1,2cyclohexandiols was observed for two samples in Table 2, the surface properties of them are regarded as almost the same. The

higher efficiency of the reaction over mesoporous catalysts is then supposed to be attributable to the effective contact time of the reactants to the catalyst surface; reactants inside pores have more opportunity to contact with active sites on the catalyst surface than those outside the catalyst particles in an aqueous solution. By using H2O2 oxidant, the solvent effect on the oxidation of cyclohexene was searched (Table 3). While very similar trends for the selectivity were observed for all the solvents, acetonitrile solvent showed the highest conversion, as reported for the case of the titanosilica family.20 It is reported that amorphous Nb2O5 is rich in surface hydroxyl groups21 and that it works as a hydrophilic catalyst. In the present study, OH groups on mesoporous Nb2O5 after evacuation at above 200 °C (Figure S1 of the Supporting Information) were observed as a broad IR band centered at 3707 cm-1, and the acidic property of them was confirmed by the formation of pyridinium ion upon pyridine adsorption (not shown for brevity). So, mesoporous Nb2O5 prepared in the present study is regarded as one of the amorphous Nb2O5 acid catalysts. For oxidation of olefins over hydrophilic catalysts, acetonitrile solvent is found to give a higher conversion than other ones,22 so the mesoporous Nb2O5 catalyst can be analogous to hydrophilic catalysts. It is worth comparing with similar materials, Nb-containing mesoporous silica (MCM-41, Si/Nb ) 32) which was calcined at 500 °C.11 Highest selectivity was observed for 1,2-epoxycyclohexane in the same reaction with alcohol solvents. This indicates that the acidic OH groups were absent on the surface of the catalyst and that the catalyst was rather hydrophobic, most probably due to the different environment of the Nb atoms, in which the Nb atoms were dispersed in silica matrix without forming a niobium oxide network.

TABLE 2: Comparison of Activity for Catalytic Oxidation of Cyclohexene with H2O2 over Nb2O5 Samples

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Figure 3. Time course of the oxidation of cyclohexene over mesoporous Nb2O5: (A) yield of (a) 1,2-epoxycyclohexane, (b) 1,2-cyclohexane-diol, (c) 1,2-cyclohexane-diketone, (d) cyclohexene-1-ol, and (e) cyclohexene-1-one and (B) conversion of H2O2.

Figure 4. Time course of IR spectra of oxidation of adsorbed cyclohexene with 10 wt % H2O2 aqueous solution over mesoporous Nb2O5. Spectra a and f are due to adsorbed cyclohexene and 1,2-cyclohexane-diol on mesoporous Nb2O5, respectively, and spectra b-e were measured after 1, 5, 30, and 120 min, respectively, after addition of H2O2 aqueous solution.

To consider the rate of elementary steps as well as to confirm the scheme of the reaction, the time course of the reaction over mesoporous Nb2O5 catalyst is observed and shown in Figure 3. As clearly observed in Figure 3A, the yield of 1,2-epoxycyclohexane is very low from the beginning, with constant formation of 1,2-cyclohexanediol (with the ratio cis/trans equal to about 1/2), indicating that the hydrolysis of once formed 1,2epoxycyclohexane is rapid in comparison with its formation rate. This is understandable on the basis of the acidic property of amorphous Nb2O5, which has also been reported as a solid acid catalyst.23 By acid-catalyzed hydrolysis of 1,2-epoxycyclohexene, only trans-1,2-cyclohexanediol should be produced. However, cis-1,2-cyclohexanediol was formed with any Nb2O5 catalysts, oxidants, and solvents (Tables 1-3). So the presence of the direct hydroxylation to form cis-1,2-cyclohexanediol is suspected. The consumption of H2O2 oxidant in Figure 3B is very rapid under the optimal condition for titanosilicate catalysts (60 °C, 2 h), so the reaction temperature was decreased to avoid the decomposition of H2O2. However, conversion of cyclohexene

reduced to 3% by decreasing the reaction temperature to 40 °C. Thus, lowering the reaction temperature was not effective for optimizing the reaction condition. 3.3. IR Observation of Cyclohexene Oxidation by H2O2 over Mesoporous Nb2O5. Since the rapid hydrolysis of 1,2epoxycyclohexane and the direct hydroxylation of cyclohexene were supposed to produce 1,2-cyclohexanediol, the oxidation of adsorbed cyclohexene with H2O2 aqueous solution over mesoporous Nb2O5 was observed by IR spectroscopy. The IR spectrum of cyclohexene adsorbed on mesoporous Nb2O5 is shown in Figure 4a. The olefinic CH stretching band is observed at 3017 cm-1, and other typical bands appear at 1435 and 1323 cm-1. The absence of any reactions of the cyclohexene upon adsorption on mesoporous Nb2O5 was confirmed by the reference experiment to obtain the identical IR spectrum of cyclohexene molecularly adsorbed on amorphous SiO2. The negative band at around 3700 cm-1 in Figure 4a is due to the decrease in isolated OH groups in intensity, which was converted to a broadband at around 3300 cm-1 (see the broken vertical line) attributed to hydrogen-bonded interaction with

Oxidation of Cyclohexene with H2O2 over Nb2O5

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Figure 5. IR spectra of 1,2-epoxycyclohexane on mesoporous Nb2O5 (a) under 0.2 kPa of gas phase and (b) after evacuation. Spectrum c was obtained by subtraction of spectrum b from a.

cyclohexene. Spectra b-e were measured in the time course after addition of a drop of 10 wt % H2O2 aqueous solution. All the characteristic bands due to cyclohexene rapidly disappeared, and new bands were observed at around 1350 cm-1 (broad and doublet) and at 1200 cm-1, indicating the occurrence of a rapid reaction. The increase in baseline in spectra b-e is attributed to hydrogen-bonded OH groups of H2O2 and H2O. The spectrum of the reaction product is compared with that of the mixture of cis- and trans-1,2-cyclohexanediol, which were separately adsorbed on mesoporous Nb2O5 (Figure 4f). Because spectra e and f in Figure 4 are very similar except for very broad absorption bands due to H2O2 and H2O, the product of the reaction is identified as 1,2-cyclohexanediol. Thus, stepwise oxidation and hydrolysis were not observed, but only the immediate formation of 1,2-cyclohexanediol was found by IR observation, even with diluted (10 wt %) H2O2. Therefore, the once-formed 1,2-epoxycyclohexane is hydrolyzed most probably on the same site on mesoporous Nb2O5. Otherwise, the direct hydroxylation of cyclohexene may dominate the reaction. The same results were obtained when 10 wt % H2O2 aqueous solution in acetonitrile solvent was added at the same ratio as that used in catalytic reactions. For the confirmation of hydrolysis of 1,2-epoxycyclohexane, adsorption of 1,2-epoxycyclohexane was also attempted. In Figure 5, IR spectra of 1,2epoxycyclohexane are shown. Spectrum a was measured in the presence of gaseous 1,2-epoxycyclohexane at room temperature, and that in Figure 5b was recorded after evacuation of gaseous molecules. Thus, spectrum b is attributed to irreversibly adsorbed 1,2-epoxycyclohexane, and the subtraction of spectrum b from a resulted in 1,2-epoxycyclohexane weakly adsorbed in molecular fashion (Figure 5c).24 Because the spectra of adsorbed species remaining after evacuation and that of adsorbed molecules present only in equilibrium with gaseous molecules are quite different, the immediate reaction of 1,2-epoxycyclohexane with the surface of mesoporous Nb2O5 was suspected. Thus, the step of hydrolysis of 1,2-epoxycyclohexane could not be confirmed in the present IR study. In an attempt to detect 1,2-epoxycyclohexane, more diluted (2.5 wt %) H2O2 aqueous solution was used for the same

reaction as that in Figure 4. Resulting IR spectra are shown in Figure 6, where quick consumption of adsorbed cyclohexene was observed after addition of 2.5 wt % H2O2, similarly to the case in Figure 4. However, the spectrum of the product in Figure 6e is not the same as that in Figure 4e as also compared in spectrum in Figure 6g. The doublet band at around 1350 cm-1 and the 1200-cm-1 band, which are attributed to 1,2- cyclohexanediol, were absent. Instead, a band at 1364 cm-1 with a shoulder peak on the low frequency side and that at 1129 cm-1 were observed. These bands are assigned to absorption bands of cyclohexanol, as indicated in Figure 5f. Therefore, hydrolysis of adsorbed cyclohexene to cyclohexanol was found to proceed instead of oxidation in the case of the reaction with so much diluted (2.5 wt %) H2O2 aqueous solution. Hydrolysis of adsorbed cyclohexene with H2O was confirmed also by the IR method to find that the production of cyclohexanol terminated in 5-10 min. This result supports the ability of mesoporous Nb2O5 for the activation of H2O, probably on acidic OH groups, to hydrolyze olefins. However, predominant activation of H2O2 proceeds under catalytic reaction conditions with 35 wt % H2O2 aqueous solution because cyclohexanol was not detected by GC as a product. 3.4. Active Sites. It is suggested that the activated peroxo species are formed on Ti sites in TS-1, and that the oxidation of the CdC bond proceeds to form 1,2-epoxycyclohexene, as illustrated in Scheme 1A.25 Following a similar mechanism, Nb sites on mesoporous Nb2O5 are regarded as active sites for H2O2 activation to peroxo species. Experimentally, such Nb sites were detected by IR characterization using CO adsorption in the present study to observe a band at 2194 cm-1, corresponding to cationic sites of medium strength.26 Note that the reaction, activation of H2O2, occurred on the surface where cyclohexene was preadsorbed (Figures 4 and 6), indicating that cyclohexene molecules did not strongly interact with Nb cationic sites. Rather, they are considered to adsorb on OH groups (Figure 4a). Consequently, oxidation proceeds when H2O2 is activated on adjacent Nb sites, whereas hydrolysis to cyclohexanol occurs in the case that H2O adsorb on acidic OH groups. From the fact that the former dominated the latter, even in the reaction with 10 wt % H2O2 aqueous solution, the activation of H2O2 is

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Figure 6. Time course of IR spectra of oxidation of adsorbed cyclohexene with 2.5 wt % H2O2 aqueous solution over mesoporous Nb2O5. Spectra a, f, and g are due to adsorbed cyclohexene, cyclohexanol, and 1,2-cyclohexane-diol on mesoporous Nb2O5, respectively, and spectra b-e were measured after 1, 5, 30, and 120 min, respectively, after addition of H2O2 aqueous solution.

SCHEME 1

regarded as more frequent than H2O, and the activated peroxo species are more reactive than H3O+ species. In this mechanism, the absence of detection of 1,2-epoxycyclohexane is attributed to the rapid hydrolysis catalyzed by acidic OH groups. The predominant occurrence of oxidation rather than hydrolysis of cyclohexene with 10 wt % H2O2 provides important information on active sites. Since the rate of hydrolysis of the adsorbed cyclohexene by 2.5 wt % H2O2 (Figure 6) as well as by pure H2O (not shown) was rather high, almost complete exclusion of hydrolysis with 10 wt % H2O2 cannot be interpreted with the notation that Nb sites are the only active sites. In the case of Scheme 1A, only trans-1,2-cyclohexanediol is formed by hydrolysis of 1,2-epoxycyclohexane. On the other hand, cis1,2-cyclohexanediol was simultaneously produced with about a half amount of trans-1,2-cyclohexanediol (Tables 1-3). Thus,

the activation of H2O2 is considered to proceed also on acidic OH groups (Scheme 1B). Assuming this mechanism, cyclohexene molecules on the OH groups may be replaced by H2O2 to move onto other adsorption sites in IR observation. Thus, under the catalytic reaction conditions, cyclohexene molecules weakly adsorb on sites other than OH groups. The protonated H2O2 (H3O2+) species are formed on acidic OH groups, which react with cyclohexene to form 1,2-cyclohexanediol via hydroxylation without production of 1,2-epoxycyclohexane. Scheme 1B explains the formation of cis-1,2-cyclohexanediol. In addition, irreversibly adsorbed 1,2-epoxycyclohexane did not maintain its original structure, but already reacted, which implies the absence of molecularly adsorbed 1,2-epoxycyclohexane. Therefore, Scheme 1B is

Oxidation of Cyclohexene with H2O2 over Nb2O5 considered to be more responsible for the selective production of both cis- and trans-1,2-cyclohexanediols over mesoporous Nb2O5. 4. Conclusions Mesoporous Nb2O5 was found to be an effective catalyst for oxidation of cyclohexene to both cis- and trans-1,2-cyclohexanediols with high selectivity by using H2O2 oxidant. Acetonitrile was the suitable solvent for the reaction under the optimized conditions at 60 °C for 2 h. IR results support that the reaction of 1,2-epoxycyclohexane to 1,2-cyclohexanediol is very rapid and that both H2O2 and H2O can be activated on the catalyst for very rapid oxidation and hydrolysis, respectively. Cyclohexene molecules are considered to adsorb on OH groups, and adjacent Nb sites were regarded as active sites for oxidation, assuming the mechanism proposed for the reaction over TS-1. However, this mechanism does not explain the production of cis-1,2-cyclohexanediol. Thus, acidic OH groups are suggested to activate H2O2 to proceed direct hydroxylation of cyclohexene to form both cis- and trans-1,2-cyclohexanediols without forming 1,2-epoxycyclohexane over mesoporous Nb2O5. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Corma, A. Chem. ReV. 1997, 97, 2373. (2) Taguchi, A.; Schu¨th, F. Microporous Mesoporous Mater. 2005, 77, 1. (3) Corma, A. Appl. Catal. 1989, 47, 125. (4) Notari, B. AdV. Catal. 1996, 41, 253. (5) Araujo, A. S.; Souza, C. D. R.; Souza, M. J. B.; Fernandes, V. J., Jr.; Pontes, L. A. M. Stud. Surf. Sci. Catal. 2002, 141, 467.

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21699 (6) Wu, P.; Tatsumi, T. Catal. SurV. Asia 2004, 8, 137. (7) Alvarez, C. M.; Zilkova, N.; Pariente, J. P.; Cejka, J. Catal. ReV. 2008, 50, 222 and references therein. (8) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426. (9) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874. (10) Ye, B.; Antonelli, D. Angew. Chem., Int. Ed. 2002, 41, 214 and references therein. (11) Kang, J.; Rao, Y.; Trudeau, M.; Antonelli, D. M. Angew. Chem., Int. Ed. 2008, 47, 4896. (12) Lee, B.; Kondo, J. N.; Lu, D.; Domen, K. J. Am. Chem. Soc. 2002, 124, 11256. (13) Lee, B.; Lu, D.; Kondo, J. N.; Domen, K. Chem. Lett. 2002, 31, 1058. (14) Yang, P.; Zhao, D.; Margolese, D. I.; Chemelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (15) Hiyoshi, M.; Lee, B.; Lu, D.; Kondo, J. N.; Domen, K. Catal. Lett. 2004, 98, 181. (16) Nowak, I.; Kilos, B.; Ziolek, M.; Lewandowska, A. Catal. Today 2003, 78, 487. (17) Novak, I.; Jaroniec, M. Top. Catal. 2008, 49, 193. (18) Yamashita, T.; Lu, D.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Lett. 2003, 32, 1034. (19) Shima, H.; Tatsumi, T.; Kondo, J. N. Phys. Chem. Chem. Phys., Submitted. (20) Fan, W.; Wu, P.; Tatsumi, T. J. Catal. 2008, 256, 62. (21) Sun, Q.; Fu, Y.; Yang, H.; Auroux, A.; Shen, J. J. Mol. Catal. A 2007, 275, 183. (22) Carati, A.; Flego, C.; Previde-Massara, E.; Millini, R.; Carluccio, L.; Parker, W. O., Jr.; Bellussi, G. Microporous Mesoporous Mater. 1999, 30, 137. (23) Ushikubo, T.; Iizuka, T.; Hattori, H.; Tanabe, K. Catal. Today 1993, 16, 291. (24) NIST/EPA Gas-Phase Infrared Database; http://webbook.nist.gov/ cgi/cbook.cgi?ID)C286204&Units)SI&Mask)80#IR-Spec. (25) Bonino, F.; Damin, A.; Ricchiardi, G.; Ricci, M.; Spano`, G.; D’Aloisio, R.; Zecchina, A.; Lamberti, C.; Prestipino, C.; Bordiga, S. J. Phys. Chem. B 2004, 108, 3573. (26) Dambournet, D.; Leclerc, H.; Vimont, A.; Lavalley, J.-C. Phys. Chem. Chem. Phys. 2009, 11, 1369.

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