Highly dispersed manganese oxide catalysts grafted on SBA-15: Synthesis, characterization and catalytic application in trans-stilbene epoxidation

Microporous and Mesoporous Materials 132 (2010) 501–509

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Highly dispersed manganese oxide catalysts grafted on SBA-15: Synthesis, characterization and catalytic application in trans-stilbene epoxidation Qinghu Tang a,b, Shuangquan Hu a, Yuanting Chen a, Zhen Guo a, Yu Hu b, Yuan Chen a, Yanhui Yang a,* a b

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore Department of Chemistry, Henan Normal University, Xinxiang 453007, China

a r t i c l e

i n f o

Article history: Received 10 October 2009 Received in revised form 14 February 2010 Accepted 28 March 2010 Available online 1 April 2010 Keywords: Manganese Grafting method SBA-15 Epoxidation trans-Stilbene

a b s t r a c t Manganese oxide catalysts supported on mesoporous silica SBA-15 (Mn-SBA-15) were synthesized following a controlled post-synthesis grafting process through the atomic layer deposition. N2-physisorption, X-ray diffraction, transmission electron microscopy, Raman spectroscopy, H2 temperature-programmed reduction, diffuse reflectance UV–Vis, and X-ray absorption near-edge structure were employed to characterize the physicochemical properties. This grafting method can efficiently upload a large amount of Mn species, and the Mn-SBA-15 samples exhibited high surface area, large pore volume, and uniform pore size. The spectroscopic results revealed that the manganese oxides with the coexistence of Mn2+ and Mn3+ either highly dispersed on the mesoporous silica surface or formed small size MnOx clusters in the SBA-15 channels depending on the manganese oxide contents. These manganese catalysts with ordered pore structure exhibited superior activity and selectivity towards desired product in the liquid-phase epoxidation of trans-stilbene using tert-butyl hydroperoxide as the oxidant. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Mesostructured SBA-15 silica material synthesized using a triblock co-polymer as template under strong acidic conditions has attracted rapidly growing attention due to its large uniform pore diameter (5–30 nm), high specific surface area (600–1000 m2/g), thick pore wall (3.1–6.4 nm), excellent thermal stability, and a vast number of potential applications in heterogeneous catalysis [1,2]. In most cases, purely siliceous SBA-15 materials do not have sufficient intrinsic activities as catalysts, and thus numerous efforts have been made on introducing catalytically active sites such as metals, metal oxides and metal complexes into the mesoporous SBA-15 silica [3–15]. The most popular method is the direct hydrothermal (DHT) method, which involves the direct addition of metal precursors to the synthesis gel before hydrothermal treatment. Several studies reported the incorporation of heteroatoms such as Co [16], Al [17], Fe [18], V [19], Ti [17,20], Ga [21], and Mn [9– 11] into SBA-15 framework by the DHT synthesis. Our group investigated the direct synthesis of Co-SBA-15 following a ‘‘pH-adjusting” method. Serious drawback of this ‘‘pH-adjusting” procedure lies in the way that the amount of incorporated metal is extremely low [16] because metals exist only in the cationic form under

* Corresponding author. Address: 62 Nanyang Drive, N1.2-B1-18, Singapore 637459, Singapore. Tel.: +65 6316 8940; fax: +65 6794 7553. E-mail address: [email protected] (Y. Yang). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.03.033

strongly acidic synthesis condition rather than their corresponding oxo species. Moreover, a large part of the metal sites are inactive because they are buried inside the pore walls [6]. The conventional impregnation method was applied to deposit active components onto SBA-15. Nevertheless, the impregnation method cannot ensure the incorporation of active species into SBA-15 mesopores, and the contraction of channels may also occur during wet impregnation [22]. The unsatisfactory dispersion of catalytically active centers and the inherent leaching problem also cause the confusion on the heterogeneity of the impregnated catalysts [6]. Several groups reported the synthesis of mesoporous silica supported catalysts using grafting methods [23–26], e.g., grafting of organometallic complexes or metal ions onto the surface of mesoporous silica by using surface silanol groups as anchoring sites. Oldroyd et al. [26] found that Ti supported on mesoporous silica prepared by a surface grafting method was more active than that prepared by DHT method for the epoxidation of cyclohexene using tert-butyl hydroperoxide (TBHP). A novel controlled post-synthesis grafting process through atomic layer deposition (ALD) was successfully developed to prepare tungsten and vanadium oxide catalyst supported on SBA-15 mesoporous silica [27,28]. It was suggested that the isolated vanadium sites resulted in the high selectivity in the partial oxidation of methanol to formaldehyde. More recently, Ni-grafted MCM-41 and SBA-15 catalysts have also been successfully synthesized with nickel acetylacetonate as metal precursor by a modified ALD method [29]. Highly dispersed Ni nanoparticles anchored by silica matrix resulted in the excellent

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catalytic activity and long-term stability for CO2 reforming of CH4 under atmospheric pressure. To our knowledge, the synthesis, characterization, and catalytic performance of Mn supported on SBA-15 with a large amount of Mn dispersed on silica pore walls by ALD method have yet been reported. In this report, grafting manganese oxide species onto the surface of mesoporous silica SBA-15 (Mn-SBA-15) was attempted by the modified ALD method using Mn(II) acetylacetone as the Mn precursor. This grafting approach was performed under strictly anhydrous conditions in the presence of an organic solvent to avoid the unnecessary hydrolysis of Mn precursors and the aggregation of manganese species to form large clusters. To gain further insight into the nature of manganese species on the SBA-15 support, the physicochemical properties of Mn-SBA-15 samples were extensively characterized using N2-physisorption, X-ray diffraction (XRD), transmission electron microscope (TEM), Raman, H2 temperature-programmed reduction (H2-TPR), UV–Vis, and X-ray absorption near-edge structure (XANES). The catalytic performances were tested and benchmarked against impregnated and incorporated Mn catalysts in the epoxidation of trans-stilbene with TBHP as the oxidant.

2. Experimental 2.1. Catalyst synthesis Siliceous SBA-15 mesoporous material was prepared according to a well-established procedure reported by Stucky et al. [2] using tetraethylorthosilicate (TEOS, Sigma) as the silica source and triblock co-polymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) EO20PO70EO20 (Mav = 5800, Aldrich) as the structure-directing agent under acidic conditions. Typically, a solution of EO20PO70EO20:HCl:TEOS:H2O = 2:60:4.25:12 (mass ratio) was prepared by dissolving 4 g of EO20PO70EO20 in 80 g of 2 M HCl and 20 g of H2O under stirring before adding 8.8 g of TEOS dropwise. The synthesis solution was stirred at 40 °C for 20 h followed by aging at 100 °C for 48 h. After cooling to room temperature, the resulting solid was recovered by filtration, washed with deionized water, and dried under ambient condition overnight. The pre-dried powder was heated at a constant ramping rate from room temperature to 540 °C over 17 h under He and held for 1 h under the same conditions, followed by calcination at 540 °C for 6 h to remove the residual organic template materials. The manganese-grafted SBA-15 catalysts were synthesized as follow: the calcined SBA-15 sample was suspended in anhydrous toluene (99%, Aldrich) and refluxed under an inert nitrogen atmosphere for 5 h to remove the adsorbed moisture. Subsequently, an appropriate amount of manganese (II) acetylacetonate (Sigma–Aldrich) dissolved in anhydrous toluene was added dropwise to above suspension. The grafting process was performed under an inert nitrogen atmosphere by refluxing at 110 °C for 8 h, involving the reaction of manganese (II) acetylacetonate with surface silanol groups of the calcined SBA-15. The resulting mixture was cooled, filtered, washed with anhydrous toluene, and dried at room temperature overnight. The grafted sample was calcined following the same procedures as mentioned above to obtain the final sample denoted as wt.% Mn-SBA-15, where wt.% represents the final Mn content in the sample tested by ICP analysis. For comparison, Mn/SBA-15-IMP was prepared by a conventional wet impregnation method. The calcined SBA-15 powder was immersed in an aqueous solution of Mn(NO3)2, stirred for 3 h, and allowed to rest for 20 h. The impregnated sample was obtained by heating at 70 °C to evaporate water, followed by drying in vacuum at 50 °C and calcination at 540 °C for 6 h.

2.2. Characterization N2-physisorption isotherms were measured at 196 °C with a static volumetric Autosorb 6B (Quanta Chrome). Prior to the measurement, the sample was degassed at 250 °C to a residual pressure below 10 4 Torr. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. Pore size and pore size distribution were calculated by the Barrett–Joyner–Halenda (BJH) method [30] using the desorption branch. Powder XRD patterns were recorded on a Bruker Advance 8 X-ray diffractometer equipped with a rotating anode using Cu Ka radiation (k = 0.154 nm, 40 kV, 40 mA). TEM measurement was carried out on a JEOL JEM-2100F operated at an acceleration voltage of 200 kV. Samples were suspended in ethanol and ultrasonically dispersed. Drops of the suspensions were applied to a copper grid coated with carbon. The Mn content was analyzed with a Dualview Optima 5300 DV ICP-OES system after the sample was dissolved in a HF (40%) solution. Raman spectra were collected on a Renishaw in Via Raman Microscope system with a 514.5 nm laser as the excitation source. A laser output of 30 mW was employed and the maximum incident power at the sample was approximately 6 mW. Diffuse reflectance UV–Vis spectra were recorded with a Varian Cary-5000 spectrometer. The spectra were collected in the range of 200–800 nm at room temperature with BaSO4 as the reference. X-Ray absorption measurements at Mn K-edge were performed at the X-ray Demonstration and Development beam line of the Singapore Synchrotron Light Source where a Si (1 1 1) channel-cut monochrometer was equipped [31]. The samples were ground into fine powders, pressed into self-supporting wafers, placed in a stainless steel cell, and measured in transmission mode at room temperature. The electron energy in the storage ring was about 700 MeV with a current of about 200 mA. Energy was calibrated using Mn foil (6539.0 eV). The spectra collected were analyzed using the WinXAS 2.3 code. The reducibility of Mn species supported on SBA-15 was investigated by a H2-TPR technique using Autosorb-1C (Quanta Chrome) equipped with a thermal conductivity detector (TCD). Prior to each H2-TPR run, 100 mg of sample was purged by ultra zero grade air at 500 °C for 1 h, and cooled down to room temperature. This procedure produced a clean surface before running the H2-TPR. The gas flow was switched to 5 vol.% hydrogen in argon balance, and the base line was monitored until stable. After baseline stabilization, the sample cell was heated at 8 °C min 1 rate to 800 °C. 2.3. Catalytic reaction trans-Stilbene (>98%, Sigma) and TBHP (70% aqueous solution, Aldrich) were employed as received without further purification. The reaction was conducted using a bath-type reactor according to the following procedure. Typically, 1 mmol of trans-Stilbene, 10 ml solvent (9:1 solvent ratio (v/v) of MeCN/DMF) and 0.2 g of catalyst were introduced into a round-bottom flask. After adding 5 mmol TBHP, the reaction was started by immersing the flask into water bath kept at 65 °C. The reaction was carried out under vigorous magnetic stirring. After the reaction, the solid catalyst was filtered off and the liquid samples were analyzed by an Agilent gas chromatograph (GC) 6890 equipped with a flame ionization detector and a HP-5 capillary column (30 m long and 0.32 mm in diameter, packed with silica-based supel cosil). trans-Stilbene oxide (>99%), benzaldehyde (>99%) and benzoic acid (>98%) (Aldrich) were used as the references for liquid product analysis. To further confirm, gas chromatograph–mass spectrometer (GC–MS) analysis using Agilent 6890 N GC–MS was also carried out with the GC oven temperature, helium as the carrier, and MS in the EI mode with a 69.9 eV ion source.

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uniform pore diameter. The broad hysteresis loop is caused by the presence of a uniform array of mesopores connected by micropores, thus hampering the filling and emptying of the accessible volume [33]. The physical properties of these manganese-grafted SBA15 samples are summarized in Table 1. Slightly decreased total pore volume (from 1.15 to 1.04 cc g 1) and BET surface area (from 946 to 808 m2 g 1) are observed when the manganese loading is increased to 7.2 wt.%. The monotonic decrease in average pore diameter from 6.3 to 5.9 nm might reflect a uniform coating of MnOx on the inner walls of SBA-15, which is consistent with the deceased pore volume [28]. It is noted that both BET surface area and total pore volume of 7.2Mn/SBA-15-IMP are substantially lower than those of 7.2Mn-SBA-15 which may be due to the contraction of channels during wet impregnation [22]. A dramatic change in the hysteresis loop of nitrogen physisorption isotherm is observed for 18.8Mn-SBA-15. 18.8Mn-SBA-15 also shows a decreased BET surface area, total volume and pore size, probably implying the aggregation of manganese oxide nanocrystals in the SBA-15 channels. The similar feature was also reported by Han et al. for nanosized Mn3O4/SBA-15 catalyst [13]. The XRD patterns in Fig. 2a show that SBA-15 exhibits three well-resolved reflections at 2h of 0.5–3o, including one strong (1 0 0) peak and 2 weak peaks (1 1 0) and (2 0 0). These results complement the N2-physisorption measurements that the SBA15 synthesized in this study possesses a highly ordered hexagonal mesoporous silica framework. Grafting Mn species onto SBA-15 decreases the intensity of these diffraction peaks to some extent. The decrease in intensity becomes noticeable when the Mn content is 18.8 wt.%, although the hexagonal regularity of SBA-15 support is still attained. The decrease in peak intensity suggests the irregular organization at long-range order of the mesoporous structure, arising from either the incorporation of Mn species into the channels or the collapse of SBA-15 mesopores. TEM observations helped

3. Results and discussion 3.1. Physicochemical properties of fresh Mn-SBA-15 As expected, the results showed that the successive steps, i.e., drying and calcination, do not affect the total manganese loading which is only determined by the grafting step. The amount of available manganese precursors and the number of surface silanol groups on SBA-15 pore walls as well as the efficiency of grafting affect the final manganese content. With a certain amount of SBA-15 and a fixed volume of solution for grafting, the manganese content can be regulated by changing the concentration of manganese (II) acetylacetonate solution. As shown in Table 1, at a low concentration of manganese (II) acetylacetonate, e.g., Mn content is 8.0 wt.% or less in the synthesis solution, more than 90% of the Mn can be grafted onto SBA-15. Further increasing the concentration of manganese (II) acetylacetonate or repeating the grafting procedures results in a high manganese content, but the efficiency of manganese grafting remarkably decreases. In situ DRIFTS studies have revealed that Mn(II) acetylacetone can be anchored onto SBA-15 support via silanol groups, resulting in a decreased number of silanol groups on the surface of SBA-15 [32]. We propose the following mechanism to explain how the grafting of Mn onto SBA-15 silica pore walls occurs. As shown in Scheme 1, Mn(II) acetylacetone is anchored onto SBA-15 support via silanol groups and subsequently the adsorbed Mn(II) acetylacetone decomposes to form MnOx species by calcination at 540 °C. The N2 adsorption/desorption isotherms and the corresponding pore size distributions of these manganese-grafted SBA-15 samples are shown in Fig. 1. All samples exhibit type IV isotherms with a broad H1-type hysteresis loop and a sharp step increase in nitrogen uptake at relative pressures of 0.5–0.8, arising from the capillary condensation of nitrogen inside the primary mesopores with a

Table 1 Physical properties of synthesized Mn-SBA-15 catalysts.

a b c d

Sample

Mn contenta (wt.%)

Proportion of Mn incorporated (%)

Mn contentb (wt.%)

BET (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

SBA-15 0.9Mn-SBA-15 1.8Mn-SBA-15 2.7Mn-SBA-15 4.6Mn-SBA-15 7.2Mn-SBA-15 7.2Mn-SBA-15c 7.2Mn/SBA-15-IMPd 12.1Mn-SBA-15 18.8Mn-SBA-15

– 1.0 2.0 3.0 5.0 8.0 8.0 – 15.0 24.0

– 93 90 91 93 90 90 – 81 78

–

1028 946 915 892 820 808 798 651 723 570

1.25 1.15 1.14 1.12 1.05 1.04 1.02 0.86 0.92 0.77

6.3 6.2 6.1 6.1 6.0 5.9 5.8 6.0 5.4 4.0

0.9 1.8 2.7 4.6 7.2 7.2 7.2 12.1 18.8

Mn content used in the synthesis gel. Mn content measured by ICP analysis. The sample after reaction. Prepared by the impregnation method.

Scheme 1. Plausible mechanism of Mn(II) acac grafted on SBA-15.

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1.2

2000

(b)

(a)

SBA-15 1.8Mn-SBA-15 4.6Mn-SBA-15 7.2Mn-SBA-15 18.8Mn-SBA-15

1.0 SBA-15

0.8

1200

1.8Mn-SBA-15

Dv(d)

Adsorbed volume / cm g

3 -1

1600

4.6Mn-SBA-15

0.6

800

0.4 7.2Mn-SBA-15

400

0.2

18.8Mn-SBA-15

0 0.0

0.0 0.2

0.4

0.6

0.8

2

1.0

4

6

8

10

12

Pore size / nm

P/P0

Fig. 1. Nitrogen physisorption of Mn-SBA-15 samples with different manganese loadings.

(a)

(b) * *

**

(e)

Intensity /a.u.

Intensity /a.u.

*

(a) (b)

(f)

(d) (c)

(c)

(b)

(d) (e)

(a)

(f) 1

2

3

4

2θ / degree

10

20

30

40

50

60

70

2θ /degree

Fig. 2. XRD patterns of Mn-SBA-15 samples at (a) low angles and (b) high angles: (a) SBA-15, (b) 1.8Mn-SBA-15, (c) 4.6Mn-SBA-15, (d) 7.2Mn-SBA-15, (e) 7.2Mn/SBA-15-IMP, (f)18.8Mn-SBA-15.

to rule out the possibility of mesopore collapse during the grafting procedures. Fig. 3 clearly shows that the hexagonal array of mesoporous channels of SBA-15 is well sustained for the Mn-SBA-15 samples with manganese loadings are 18.8 wt.% or less. These results suggest that the procedures for grafting manganese onto SBA-15 preserve the long-range order of the mesoporous structure probably due to the anhydrous condition and hydrophobicity of SBA-15 pore wall surface. In spite of the low-intensity of diffraction peaks, 7.2Mn/SBA-15-IMP still shows three distinct reflections at 2h of 0.5–3o, indicating the hexagonal array of mesoporous channels of SBA-15 is still attained after the impregnation. The XRD patterns at high diffraction angles (see Fig. 2b) show that no any Mn crystalline phase appears when the Mn content is 7.2 wt.% or less. As shown in Scheme 1, Mn grafting via this modified ALD method relies on silanol groups. Therefore, it is reasonable to suggest that Mn species are incorporated into the SBA-15 channels due to the preferential reaction between Mn precursors and silanol groups on the pore wall surfaces. We suggest that with 7.2 wt.% or less manganese contents, the Mn species are highly dispersed on the pore wall surface or formed small clusters with diameters below the detection limit of XRD. This is consistent with

the TEM observations (Fig. 3), which does not show any large MnOx clusters for Mn-SBA-15 sample with manganese content lower than 7.2%. Several weak diffraction peaks, which cannot be attributed to one type of manganese oxide, become noticeable for the sample with manganese loading of 18.8 wt.%. Large manganese oxide clusters located in the mesoporous channels of SBA-15 can also be observed from TEM micro-images. These results indicate that significantly high loading of Mn may result in the aggregation of manganese oxide microcrystalline. The manganese oxide microcrystalline is the coexistence of different manganese oxides of low crystallinity, which do not form a distinct crystalline structure. We also note the XRD pattern of 7.2Mn/SBA-15-IMP shows a weak diffraction peak at about 36o, implying the presence of aggregated manganese oxide species in the impregnated sample. The Raman spectra of Mn-SBA-15 samples along with reference Mn3O4 are shown in Fig 4. Mn3O4 spectrum exhibits a strong Raman peak at 658 cm 1 and two low-intensity peaks at 318 and 374 cm 1, which are in a good agreement with the literature reported elsewhere [34–36]. The vibrations attributed to Si–O–Si and Si–O–Mn of Mn-SBA-15 are not detected when a 514.5 nm laser was employed as the excitation source due to the fluorescence.

Q. Tang et al. / Microporous and Mesoporous Materials 132 (2010) 501–509

505

Fig. 3. The TEM images of (a) 1.8Mn-SBA-15 (viewed along the pore direction), (b) 1.8Mn-SBA-15 (parallel fringes, side on view), (c) 7.2Mn-SBA-15 (viewed along the pore direction), (d) 7.2Mn-SBA-15 (parallel fringes, side on view), (e) 18.8Mn-SBA-15 (viewed along the pore direction), (f) 18.8Mn-SBA-15 (parallel fringes, side on view).

For 18.8Mn-SBA-15, only one weak Raman band at 658 cm 1 attributed to the Mn–O–Mn stretching vibrations of crystalline MnOx can be observed [37,38]. This Raman peak broadens with decrease in manganese loading, implying the smaller size of MnOx cluster. For the low Mn loading samples, i.e., 1.8Mn-SBA-15 and 4.6Mn-SBA-15, this Mn–O–Mn stretching vibration band at 658 cm 1 cannot be discerned, suggesting that manganese oxides supported on SBA-15 are highly dispersed. H2-TPR profiles in Fig. 5 show that H2 uptake for bulk Mn2O3 starts from 280 °C. A main reduction peak centered at 455 °C along with a shoulder peak around 370 °C is pronounced, representing the two-step reduction of Mn2O3 to MnO via Mn3O4 [39]. Bulk

Mn3O4 starts reducing from about 340 °C, a single peak representing the reduction of Mn3O4 to MnO is observed at 478 °C. The H2TPR profile of 18.8Mn-SBA-15 sample shows that the reduction onset is 180 °C, the main reduction peak appears at 342 °C, followed by two small reduction peaks at 440 and 473 °C. The significant shift of main reduction is possibly due to the easy reduction of dispersed manganese oxides or smaller clusters compared to the bulk manganese oxides [13]. In addition, the small reduction peaks at high temperatures, which are close to the reduction of bulk Mn2O3 and Mn3O4, may be attributed to the reduction of aggregated manganese oxide microcrystalline supported on SBA-15. With the decrease of Mn loading, the intensity of the reduction

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270

658

514 nm

(f) (e) (d) (c)

Absorbance /a.u.

374

318

Intensity / a.u.

(e)

500

(d) (c)

(b) (a) 400

600

800

1000

(b) (a)

1200

-1

Raman shift / nm

300

Fig. 4. Raman spectra of Mn-SBA-15 samples: (a) SBA-15, (b) 1.8Mn-SBA-15, (c) 4.6Mn-SBA-15, (d) 7.2Mn-SBA-15, (e) 18.8Mn-SBA-15, (f) Mn3O4.

400

500

600

700

800

Wavelength /nm Fig. 6. Diffuse reflectance UV–Vis spectra of Mn-SBA-15 samples: (a) 1.8Mn-SBA15, (b) 4.6Mn-SBA-15, (c) 7.2Mn-SBA-15, (d) 18.8Mn-SBA-15, (e) 7.2Mn-SBA-15IMP.

320

H2 consumpation /a.u.

(g) 455 370 478

(f) (e)

342

180

(d) (c) (b) (a)

200

400

600

800

o

Temperature / C Fig. 5. H2-TPR profiles of Mn-SBA-15 samples and the reference compounds: (a) 1.8Mn-SBA-15, (b) 4.6Mn-SBA-15, (c) 7.2Mn-SBA-15, (d) 18.8Mn-SBA-15, (e) Mn3O4, (f) Mn2O3, (g) 7.2Mn/SBA-15-IMP.

peaks at 440 and 473 °C remarkably decreases. When the Mn loading is less than 1.8 wt.%, only one broad reduction at 352 °C is discerned, which can be ascribed to the presence of manganese species with different reducibility resulted from the different strength of Mn–O interaction. A shift of maximum reduction peak towards high temperatures suggests the presence of MnOx species strongly bounded to silica walls. The H2-TPR of 7.2Mn/SBA-15-IMP indicates the presence of MnO2 species. The first peak (Fig. 5g) is the reduction of MnO2 to Mn3O4, and the second is the reduction of Mn3O4 to MnO. Fig. 6 shows the diffuse reflectance UV–Vis spectra of Mn-SBA15 samples with various Mn loadings. Two broad bands at 270 and 500 nm are observable for all Mn-SBA-15 samples. The O2 ? Mn3+ charge transition in Mn3O4 in which Mn is octahedrally coordinated with oxygen exhibits a typical band at 320 nm [40,41]. The band at 270 nm can be attributed to the charge transfer transition of O2 ? Mn3+ in tetrahedral oxygen coordination [5,41,42]. However, the existence of the octahedral oxygen coordinated Mn3+ cannot be ruled out as for the broad band at 270 nm may be formed by overlapping the octahedral oxygen coordinated Mn3+ with the tetrahedral oxygen coordinated Mn3+. The band at 500 nm is due to the 6A1 g ? 4T2 g crystal field transition of

Mn2+ as observed in Mn3O4 and MnO [41]. This latter band was also assigned to Mn2+ species on the surface of Mn-MCM-41 [5]. Therefore, the presence of the two broad bands at around 270 and 500 nm in the UV–Vis spectra of Mn-SBA-15 indicates the coexistence of Mn2+ and Mn3+ in these samples. The UV–Vis spectra, however, do not allow a quantitative interpretation with respect to the fractions of Mn2+ and Mn3+ species. The UV–Vis spectrum of 7.2Mn/SBA-15-IMP is different from that of Mn-SBA15 prepared by the grafting method. The absorption in 350–420 and 450–550 nm regions are relatively pronounced. The former absorption is also shown in pyrolusite, where the Mn is presented in the 4+ valence associated to Mn4+ [43]. The latter one, which is Mn3+ charge transfer presented in Mn2O3, is attributed to O2 [44]. Fig. 7 shows the normalized Mn K-edge XANES spectra along with the characteristic E0 absorption energy for Mn-SBA-15 samples and the reference manganese oxides. The absorption energies of Mn-SBA-15 samples increase with elevated Mn loading (6.5471– 6.5476 keV). These absorption energies are in between the energies obtained for Mn2O3 (6.5481 keV) and Mn3O4 (6.5467 keV), evidencing the oxidation state of manganese oxide

18.8Mn-SBA-15

Normalized absorption

400

E0 =

7.2Mn-SBA-15

6.5475

4.6Mn-SBA-15

6.5474

1.8Mn-SBA-15

6.5474 MnO 6.5472 Mn3O4 6.5440

Mn2O3

6.5467

MnO2

6.5485

7.2Mn/SBA-15-IMP

6.5519 6.5498

6.50

6.55

6.60

6.65

6.70

Photon energy / keV Fig. 7. Normalized Mn K-edge XANES spectra of Mn-SBA-15 samples along with reference compounds.

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phase in the grafted Mn-SBA-15 sample is in between Mn2O3 (+3) and Mn3O4 (+2.67). The XANES results also suggest that at a low Mn loading, the oxidation state is similar to that of Mn3O4 while the oxidation state is close to that of Mn2O3 at a high Mn loading. The absorption energy for 7.2Mn/SBA-15-IMP is in between Mn2O3 (6.5481 keV) and MnO2 (6.5519 keV), suggesting the coexistence of Mn3+ and Mn4+ in this particular sample, which further confirms the H2-TPR and UV–Vis results.

Table 2 Epoxidation of trans-stilbene over highly dispersed manganese oxides supported on SBA-15.a

– SBA-15 0.9Mn-SBA-15 1.8Mn-SBA-15 2.7Mn-SBA-15 4.6Mn-SBA-15 7.2Mn-SBA-15 7.2Mn/SBA-15-IMPb 12.1Mn-SBA-15 18.8Mn-SBA-15 MnSBA-15c

8.0 9.4 45.4 61.7 65.1 72.0 86.5 68.8 74.5 71.8 64.3

Selectivity (%) TSO

Benzydehyde

Benzonic acid

74.1 73.6 80.3 86.1 88.9 88.1 91.4 89.1 88.1 89.1 76.7

25.9 26.4 17.7 12.1 8.3 7.7 7.2 7.2 7.3 7.1 15.6

0 0 2.0 1.8 2.8 4.2 2.8 3.5 4.6 3.8 7.8

Catalyst, 0.2 g; t = 24 h; T = 65 °C; MeCN + DMF(9:1), 10 ml. Prepared by the impregnation method. Ref. [9] (by the DHT method).

100

90

100

80

80

70

70 Conv. (%) Select. (%)

60

95

Conversion /%

90

Epoxide selectivity / %

Conversion / %

90

80 90 70 85 Conv. (%) Select. (%)

60

50

60

50 5

100

(b)

(a)

10

15

80 55

20

60

90

20

Conv. (%) Select. (%) Efficiency (%)

0 6

80 80 70 70

Conv. (%) Select. (%)

10

20 4

90

Conversion /%

60

TBHP efficiency / %

Conversion and epoxide selectivity / %

30

100

(d)

40 80

2

75

100

50

0

70 o

(c)

40

65

Temperature / C

Solvent amount / ml 100

Epoxide selectivity /%

c

TS Conv. (%)

The epoxidation of trans-stilbene (TS) with TBHP was carried out at 65 °C for 24 h over Mn-SBA-15 catalysts, and the results are shown in Table 2. tran-Stilbene oxide (TSO) was the main products along with benzaldehyde and benzoic acid as by-products. In the absence of any catalyst, TS conversion is lower than 8.0%. Siliceous SBA-15 gives a TS conversion around 9.4%. Adding Mn remarkably increases the TS conversion, and the TS conversion increases with the Mn loading, suggesting that the Mn in SBA-15 acts as the active sites for the epoxidation of TS with TBHP. The 7.2MnSBA-15 catalyst exhibits the best performance with a TS conversion of 86.5% and a TSO selectivity of 91.4%. Further increasing the Mnloading, nevertheless, lowers the TS conversion. Only 71.9% TS conversion is achieved over 18.8Mn-SBA-15. As evidenced by XRD, N2physisorption, Raman, and H2-TPR, Mn species are located inside the mesopores of SBA15 forming small clusters or highly dispersing on the pore wall surface if the loading amount is low than 7.2%. High loading amount leads to the appearance of aggregated manganese oxide microcrystalline. Thus, it can be concluded that the superior catalytic activity of 7.2Mn-SBA-15 catalyst should be attributed to the highly dispersed manganese oxide species mainly exist as a mixture of Mn2+ and Mn3+ on SBA-15. The TSO selectivity slightly increases with Mn loading up to 7.2 wt.%. Further increasing the Mn loading does not increase the selectivity towards TSO significantly. Ramanathan et al. [42] also found the TSO selectivity was dependent on the Mn content of MnTUD-1 catalysts to some extent. The reason of such Mn content-dependent TSO selectivity

8

Molar ratio (TBHP/ST)

10

Epoxide selectivity /%

a b

Catalyst

3.2. Epoxidation of trans-stilbene

60

60

50 0.1

0.2

0.3

Catalyst amount /g

Fig. 8. Effect of various reaction parameters on epoxidation of trans-stilbene over 7.2Mn-SBA-15: (a) solvent amount, (b) temperature, (c) molar ratio of TBHP/EB, (d) catalyst amount.

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remains unclear up to now. One possible explanation is that the TSO selectivities are also related to the number of available silanol groups on the surface of mesoporous materials. Compare to a MnSBA-15 catalyst prepared by the DHT method under the same reaction condition, the grafted Mn-SBA-15 exhibits remarkably higher TS conversion (86.5% vs. 64.3%) and TSO selectivity (91.4% vs. 76.7%). Zhang et al. [5] reported that the Mn sites on the surface of mesoporous channels are more active in the trans-stilbene epoxidation than those incorporated inside the framework. As described above, almost all the manganese sites are highly dispersed on the surface of SBA-15 in the grafted samples with 7.2 wt.% or less Mn loadings, thus a higher TS conversion than that of the DHT catalyst can be expected. Although the Mn/ SBA-15-IMP catalyst by a wet impregnation method also shows a high TS conversion (68.8%), about 15% of the manganese in fresh catalyst leached out during the reaction, presumably leading to a partly homogeneously catalyzed reaction. Based on XRD and H2TPR analysis, there are aggregated manganese oxide species with large particle size in the Mn/SBA-15-IMP catalyst. These large MnOx particles may easily leach out during the reaction due to the weak interactions between the particles and SBA-15 support. In addition, Mn4+ in Mn/SBA-15-IMP may be less active than Mn2+ or Mn3+ in 7.2Mn-SBA-15 by grafted method.

100

100

70 70

Conv. (%) Select. (%)

60

60

50 1

2

3

4

Reaction recylce Fig. 9. Recycling of the 7.2Mn-SBA-15 catalyst for the epoxidation of trans-stilbene with TBHP. Catalyst amount = 0.2 g, T = 65 °C, time = 24 h, trans-stilbene = 1 mmol, TBHP = 5 mmol, MeCN + DMF(9:1) = 10 ml.

Various reaction parameters such as solvent amount, molar ratio of TBHP/TS, temperature and catalyst amount were investigated to optimize the reaction conditions. The effect of solvents on the epoxidation of trans-stilbene with TBHP over manganese-containing catalysts has been investigated previously and the highest trans-stilbene conversion was achieved using MeCN as the solvent [5]. Yonemitsu et al. reported that the selectivity to epoxide can be improved when a small amount of DMF was added to MeCN during the epoxidation of trans-stilbene over a Mn-MCM-41 catalyst [45]. A mixture of MeCN and DMF (9:1) was employed as the solvent in this study. The effect of solvent amount on the reaction is shown in Fig. 8a. trans-Stilbene conversion increases with the amount of solvent up to 10 ml, while the epoxidation selectivity remains constant. Nevertheless, an adverse effect is observed by further increasing the solvent amount. Yonemitsu et al. proposed that the epoxidation using TBHP over Mncontaining catalyst proceeded through a radical intermediate [45]. A suitable amount of solvent may maintain the concentration of radical intermediate which is beneficial for the high conversion of trans-stilbene. Fig. 8b shows the effect of reaction temperature. As the temperature increases from 55 to 65 °C, the trans-stilbene conversion and selectivity towards epoxide increase from 78.6% to 86.5% and 86.7% to 91.4%, respectively. Further increasing the temperature decreases the trans-stilbene conversion and epoxide selectivity, which may be due to the decomposition of TBHP at a high temperature. The decrease in the epoxide selectivity can be explained by the further oxidation of epoxide at a high temperature. The effect of TBHP amount on the epoxidation of trans-stilbene was also studied and the results are shown in Fig. 8c. With the variation of nTBHP/nTS molar ratio from 1:1 to 10:1, the conversion of trans-stilbene increases from 32% to 99%, while the epoxide selectivity remains constant. Zhang et al. suggested the efficiency of TBHP is also a vital standard for evaluating the reaction condition [5]. As show in Fig. 8c, the efficiency of TBHP decreases remarkably with increasing the nTBHP/nTS molar ratio. High TBHP efficiency is obtained only at a low nTBHP/nTS molar ratio. Catalyst amount plays an important role in controlling the catalytic performance, and it was also examined in this study, as shown in Fig. 8d. When the catalyst amount is increased from 0.05 to 0.2 g, trans-stilbene conversion increases from 64% to 86.5%. Further adding more catalyst leads to a lower trans-stilbene conversion. Parida et al. reported that high metal concentration might inhibit the oxidation process [46], which explains why a decreased trend in the trans-stilbene conversion can be seen by

(a)

Intensity /a.u.

Intensity /a.u.

(b) (b)

(a)

10

20

30

40

50

60

70

2 θ /degree

(b)

H2-consumption /a.u.

Conversion /%

80 80

Epoxide selectivity /%

90 90

3.3. Optimization of reaction conditions

(b)

(a)

(a) 1

2

3

2θ /degree

4

5

200

400

600 o

Temperature / C

Fig. 10. XRD patterns and H2-TPR profiles of 7.2Mn-SBA-15 catalyst: (a) before and (b) after reaction.

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adding a large amount of catalyst. There is no significant effect on the epoxide selectivity with the variation of catalyst amount. 3.4. Recyclability of the catalyst It has been verified that no observable leaching of Mn from MnSBA-15 catalyst occurs during the reaction. The manganese content in the sample after several reaction cycles remained the same as that of the fresh catalyst. Moreover, manganese cannot be detected by ICP analysis in the filtrate after reaction. Recycling tests of 7.2Mn-SBA-15 catalyst was carried out at 65 °C. Fig. 9 shows that the trans-stilbene conversion and epoxide selectivity are essentially constant in the repeated runs, suggesting that the reaction proceeds heterogeneously over the catalyst. 3.5. Physicochemical properties of spent Mn-SBA-15 Fig. 10a shows the XRD patterns of Mn-SBA-15 catalysts before and after reaction. The hexagonal regularity of SBA-15 support is still attained and there is no obvious decrease in the peak intensity. The physical properties of reacted Mn-SBA-15 are almost the same as that of the fresh catalyst (as shown in Table 1). Fig. 10b shows the H2-TPR profiles of Mn-SBA-15 catalysts before and after reaction. There are no significant changes in the profiles of spent catalyst compared to that of the fresh catalyst, implying that the nature of MnOx species remains unchanged after reaction. XRD results (Fig. 10a insert) further prove the MnOx species in Mn-SBA-15 are almost the same before and after reaction. 4. Conclusion Manganese-grafted SBA-15 catalysts with various manganese loadings (Mn content: 0.9–18.8 wt.%) by an atomic layer deposition method have been successfully synthesized. These Mn-SBA15 samples exhibited high surface area, large pore volume, and uniform pore size. With the Mn loadings of 7.2 wt.% or less, the manganese oxide species were highly dispersed on the surface of SBA-15 or forming smaller size of MnOx clusters, whereas high manganese loadings resulted in microcrystalline phase manganese oxides. As evidenced from UV–Vis and XANES, the Mn atoms in Mn-SBA-15 exist in the oxidation states of +2 and +3. These manganese oxide species highly dispersed on the surface of SBA-15, coexistence of Mn2+ and Mn3+, exhibited superior catalytic activity in the epoxidation of trans-stilbene with TBHP. Acknowledgments This work was mainly supported by AcRF grant tier 2 (ARC 13/ 07). We also thank to the Creative Research Foundation for the Scientists working in the Universities of Henan Province of China (2010HASTIT028) for partial financial support. Work partially performed at SSLS under NUS Core Support C-380-003-003-001, A*STAR/MOE RP 3979908M and A*STAR 12 105 0038 grants. References [1] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmeleka, G.D. Stucky, Science 279 (1998) 548–552. [2] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024–6036.

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