Influence of Tungsten Precursors on the Structure and Catalytic

Both W(VI) and W(V) species at W4f7/2 (of 36.0 and 34.7 eV)43,44 for the W4f spin−orbit components have been detected, as shown in Figure 7. Table 3...
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J. Phys. Chem. C 2008, 112, 3819-3826

3819

Influence of Tungsten Precursors on the Structure and Catalytic Properties of WO3/SBA-15 in the Selective Oxidation of Cyclopentene to Glutaraldehyde Xin-Li Yang,†,‡ Ruihua Gao,† Wei-Lin Dai,*,† and Kangnian Fan† Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China, and School of Chemistry & Chemical Engineering, Henan UniVersity of Technology, Henan 450052, People’s Republic of China ReceiVed: October 29, 2007; In Final Form: December 18, 2007

Mesoporous WO3/SBA-15 was synthesized by the conventional incipient wetness impregnation method from various tungsten precursors. The influence of different tungsten precursors on the structure of WO3/SBA-15, as well as their catalytic performance for the selective oxidation of cyclopentene (CPE) to glutaraldehyde (GA), was investigated. The dispersion and nature of the tungsten species were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectra (DRS), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), temperatureprogrammed desorption (NH3-TPD), and temperature-programmed reduction (H2-TPR). Among the different tungsten sources, the tungstenic complex sources produced from the reaction of tungstic acid with oxalic acid showed the highest dispersion of WO3 species, the strongest surface Brønsted and Lewis acid properties, and the strongest interaction with the SBA-15 support. As a consequence, this catalyst gave much higher CPE conversion and excellent GA selectivity. On the other hand, the catalyst prepared from the ammonium paratungstate showed the lowest activity owing to the presence of crystalline WO3 on the surface. Although the activity of the catalyst prepared from the reaction of tungstic acid with oxalic acid decreased slightly after the first recycle due to leaching of small amounts of active tungsten species, it remained nearly the same value after easy regeneration with heat treatment, demonstrating that the WO3/SBA-15 catalyst is a heterogeneous one in the target reaction.

1. Introduction Because of their interesting catalytic properties, tungsten oxide-based catalysts have been widely used in many applications, including metathesis and isomerization of alkenes,1-4 selective oxidation of unsaturated compounds,5-8 catalytically selective reduction of nitric oxide with ammonia,9 dehydrogenation of alcohols,10,11 and hydrodesulfurization and hydrocracking of heavy fractions in the petroleum chemistry.12-14 Previous studies show that the dispersion of tungsten oxide, oxidation state, surface acidity, and structure strongly depend on preparation methods, the tungsten precursors, and the nature of the support. As a consequence, all these factors are likely to affect the catalytic properties intensively. The industrial importance of silica-tungsten mixed oxides has resulted in a large number of studies concerning their properties and catalysis. By using various techniques, Rives and co-workers15 reported the structures, surface acidity, and reducibility of WO3/SiO2 system, which was obtained by impregnation of the silica support with aqueous paratungstate solution. They also reported the formation of a Si-O-W crystalline species where tungsten was present as Keggin-type units similar to those found in dodecatungstosilicates after calcination of the catalyst precursor at 450 °C. Along with the rise in the calcination temperature, these species were decomposed and led to the formation of crystalline WO3, due to the low dispersing * To whom correspondence should be addressed: tel (+86-21)55664678; fax (+86-21)65642978; e-mail [email protected]. † Fudan University. ‡ Henan University of Technology.

ability of silica. Si-O-W species possess strong surface acidity, with both Lewis and Brønsted acid features, and they are more easily reduced than crystalline WO3. Moreover, Wachs and coworkers16 showed that WO3/SiO2 catalysts prepared from (NH4)6H2W12O40 (aqueous method) exhibited very strong Raman features due to the presence of crystalline WO3. Samples prepared from W(η3-C3H5)4 (nonaqueous method) did not show any crystalline WO3, suggesting that the preparation method exerted a big influence on dispersion of the surface tungsten oxide species on SiO2. Somma and Strukul17 described the synthesis of mesoporous tungsten oxide-silica catalysts by a sol-gel method, which allowed the formation of amorphous WO3 entities, well dispersed and tightly held within the silica matrix. This method ensures the formation of materials that are very stable against both leaching and sintering. Pedan and coworkers18 adopted an atomic layer deposition method to graft tungsten oxide species onto SBA-15 surfaces, which could avoid the formation of WOχ oligomers and improve the dispersion and stability of tungsten oxide species. Li and co-workers19 described the synthesis of tungsten-containing MCM-41 with good dispersion at Si/W molar ratios as low as 35 by hydrolysis of TEOS and ammonium tungstate in the presence of cetylpyridinium as template in very strong acidic medium. They found that further decreasing the ratio value would lead to the formation of crystalline WO3. These results were also confirmed by our previous works.20 Bregeaul and co-workers21 and Klepel et al.22 claimed the successful improvement of dispersion of tungsten oxide species by synthesizing tungsten-containing materials in the presence of an excess of H2O2 or under basic conditions.

10.1021/jp710409g CCC: $40.75 © 2008 American Chemical Society Published on Web 02/19/2008

3820 J. Phys. Chem. C, Vol. 112, No. 10, 2008 In this investigation, an attempt has been made to study systematically the effect of various tungsten sources, namely, tungstenic complexes formed through the interaction of tungstic acid with oxalic acid and oxoperoxotungstates formed through the interaction of tungstic acid with hydrogen peroxide, as well as ammonium paratungstate, on the structure and catalytic activity of mesoporous WO3/SBA-15 for the selective oxidation of cyclopentene (CPE) to glutaradehyde (GA), which has been used extensively for disinfection and sterilization in many areas. 2. Experimental Section 2.1. Catalyst Preparation: 2.1.1. Preparation of SBA-15. Mesoporous silica SBA-15 was synthesized according to the reported procedure by use of Pluronic P123 triblock polymer (EO20PO70EO20, Mav ) 5800, Aldrich) as template under acidic conditions.23 Briefly, a solution of EO20PO70EO20/2 M HCl/ TEOS/H2O ) 2:60:4.25:15 (mass ratio) was prepared and stirred for several hours at 40 °C, and then the mixture was hydrothermally treated at 90 °C for 3 days. The solid products were filtered off, and the as-prepared SBA-15 was dried overnight at 100 °C. The occluded surfactant was removed by calcination at 600 °C for 5 h in air, yielding the final mesoprous SBA-15 material. 2.1.2. Preparation of WO3/SBA-15(OA). The WO3-supported catalysts were prepared through the conventional incipient wetness impregnation method from various tungsten sources with 10 wt % WO3 content. The WO3/SBA-15(OA) catalyst was prepared as follows: The required amount of tungstic acid, WO3‚H2O, was dissolved in an aqueous solution of oxalic acid (0.2 M). The molar ratio of WO3:H2C2O4 was about 1:20. After the mixture was stirred at 90 °C for several hours, a transparent tungsten complexcontaining solution was obtained. Pure SBA-15 was dispersed into the stirred solution, and the excess water was evaporated until complete dryness. Then the catalyst was dried overnight in air at 120 °C, followed by calcination at 600 °C for 2 h in air to obtain the WO3/SBA-15(OA) catalyst. 2.1.3. Preparation of WO3/SBA-15(HP). The WO3/SBA-15(HP) catalyst was synthesized according to the aforementioned process but with tungstic acid being dissolved in an aqueous solution of hydrogen peroxide (50%) to obtain the oxoperoxotungstate sources. The molar ratio of WO3:H2O2 was about 1:50. After the same finishing procedure as described above, WO3/ SBA-15(HP) was obtained. 2.1.4. Preparation of WO3/SBA-15(AM). The WO3/SBA-15(AM) catalyst was also synthesized in a manner similar to the process described but with ammonium paratungstate being dissolved in an aqueous solution of ammonia. WO3/SBA-15(AM) was obtained after the same finishing treatments as the above two materials. 2.2. Characterizations. Small-angle X-ray powder diffraction (SAXS) patterns were recorded on a Rigaku D/max-rB diffractometer with Cu KR radiation, operated at 60 mA and 40 kV. Wide-angle X-ray powder diffraction (WAXS) patterns were recorded on a Bruker D8 advance diffractometer with Cu KR radiation, operated at 40 mA and 40 kV. Transmission electron micrographs (TEM) were obtained on a JEOL JEM 2010 scanning-transmission electron microscope. The samples were supported on carbon-coated copper grids for the experiment. UV-visible diffuse reflectance spectra (DRS) were collected on a Shimadzu UV-2540 spectrometer with BaSO4 as a reference. X-ray photoelectron spectra (XPS) were recorded on a PerkinElmer PHI 5000C ESCA system equipped with a dual X-ray

Yang et al. source, of which the Al KR (1486.6 eV) anode and a hemispherical energy analyzer were used. The background pressure during data acquisition was kept below 10-6 Pa. Measurements were performed at a pass energy of 93.90 eV to ensure sufficient sensitivity for the acquisition scan, while a pass energy of 23.50 eV was used for the scanning of the narrow spectra of Si 2p, W 4f, O 1s, and C 1s to ensure sufficient resolution. All the binding energies were calibrated by using contaminant carbon (C1S ) 284.6 eV) as a reference. The surface acidity was monitored from the Fourier transform infrared (FT-IR) spectra recorded after the adsorption of pyridine on a Bruker Vector 22 spectrometer coupled to a conventional high-vacuum system. The sample was compacted to a selfsupporting wafer and was calcined at 400 °C for 1 h in an in situ IR gas cell under vacuum prior to pyridine adsorption.24 Pyridine was adsorbed at room temperature from an argon flow containing 2 vol % pyridine. Then the samples were heated to 100 °C and evacuated to remove physisorbed and weakly chemisorbed pyridine. Temperature-programmed desorption (TPD) of the adsorbed pyridine starting at 100 °C was studied by stepwise heating of the sample under vacuum to characterize the kinds and strength of the acid sites. Difference spectra were obtained by subtracting the background (base spectrum) of the unloaded sample. NH3-TPD experiments were conducted on a home-built flow apparatus. Prior to the TPD experiment, the sample was pretreated with high-purity (99.999%) helium (30 mL min-1) at 500 °C for 2 h. After pretreatment, the sample was saturated with flow of 10% high-purity anhydrous ammonia and balance He mixture (30 mL min-1) at 120 °C for 1 h and subsequently flushed at 120 °C for 2 h to remove physisorbed ammonia. The TPD experiment was carried out from 130 to 530 °C at a heating rate of 10 °C min-1. The amount of desorbed NH3 was calculated by use of CDMC software accompanying the GC workstation. Temperature-programmed reduction (TPR) analysis was carried out on a homemade apparatus loaded with 100 mg of catalyst. The samples were pretreated in flowing air at 600 °C for 2 h in order to ensure complete oxidation. Then the samples were subsequently contacted with 5/95 H2/Ar mixture with a total flow rate of 40 mL min-1 and heated at a ramping rate of 10 K min-1 to a final temperature of 1000 °C. H2 consumption was monitored with a thermal conductivity detector (TCD). The tungsten content was determined by inductively coupled argon plasma spectrometry (ICP; IRIS Intrepid, Thermo Elemental Company) after solubilization of the samples in HF/ HCl solution. 2.3. Activity Test. The activity test was performed at 35 °C for 24 h with magnetic stirring in a closed 100 mL regular glass reactor with aqueous H2O2 as oxygen donor and t-BuOH as the solvent. Quantitative analysis of the reaction products were performed by gas chromatography (GC), and the identification of different products in the reaction mixture was determined by means of gas chromatography-mass spectrometry (GC-MS). Details can be found elsewhere.25,26 3. Results and Discussion 3.1. Characterization of WO3/SBA-15 Catalysts. Small- and wide-angle powder XRD patterns of different WO3/SBA-15 samples are shown in Figure 1. All the samples exhibits three well-resolved peaks indexed to (100), (110), and (200) Bragg reflection, indicating that good mesoscopic order and the characteristic hexagonal features of SBA-15 are maintained (Figure 1A). However, a shift of the diffraction peaks to higher

Structure and Catalytic Properties of WO3/SBA-15

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3821

Figure 1. (A) Small-angle and (B) wide-angle powder XRD patterns of various samples: (a) WO3/SBA-15(OA), (b) WO3/SBA-15(HP), (c) WO3/ SBA-15(AM), and (d) crystalline WO3.

TABLE 1: Physicochemical Parameters of Various Samples

Figure 2. TEM images of various samples: (a) WO3/SBA-15(OA), (b) WO3/SBA-15(HP), and (c) WO3/SBA-15(AM) (inset: selected area electron diffraction pattern).

2θ values is identified for the WO3/SBA-15(AM) sample, probably due to a small shrinkage of the mesostructure as tungsten oxides are loaded.27 Moreover, the intensity of the diffraction peaks decreases drastically, which may be caused by partial pore blocking by crystalline tungsten oxide species.22 This finding could be proved from the WAXS patterns (Figure 1B) and the TEM images shown in context. In the case of the WO3/SBA-15(HP) sample, the intensity of the (100) reflection decreases moderately. This result can be explained by the following two considerations: (i) partial blocking by lowcrystalline tungsten oxide species according to the WAXS patterns (Figure 1B) and (ii) partial destruction of the pore structure confirmed by the TEM image (Figure 2).22 Typical XRD patterns in the wide-angle region for the samples are shown in Figure 1B. The WAXS pattern of pure crystalline WO3 is also presented in Figure 1B for comparison. In the WAXS pattern of the WO3/SBA-15(OA) sample, no peaks corresponding to crystalline WO3 are observed. This indicates the tungsten species obtained from tungstenic complex sources are highly dispersed and have a particle size below 4 nm or are formed by very thin particles with low crystallinity,28 which could not be detected by the XRD technique. However, very weak feature peaks of crystalline WO3 appear on the WO3/SBA-15(HP) sample, suggesting that tungsten species partially congregate and form low-crystalline metal oxide species. For the WO3/ SBA-15(AM) sample, much enhanced intensity of the diffraction peaks from crystalline WO3 can be observed, illustrating that ammonium paratungstate sources would lead to agglomeration and low dispersion of tungsten species on the surface of SBA15 as supported by the TEM image (Figure 2). TEM images of all the above samples show the hexagonal array of uniform channels, except that the channels of the WO3/ SBA-15(HP) sample are partially destroyed. Well-ordered hexagonal arrays of mesopores are observed when the electron beam is parallel to the main axis of these cylinders (not shown here). The two-dimensional hexagonal structure (P6mm) is

catalyst

surface area (m2 g-1)

pore volume (cm3 g-1)

pore diameter (nm)

SBA-15 10% WO3/SBA-15(OA) 10% WO3/SBA-15(HP) 10% WO3/SBA-15(AM)

707.0 590.9 416.9 391.0

1.1 1.0 0.9 0.8

6.4 6.5 7.5 6.2

therefore confirmed. WO3 nanoparticles are highly dispersed as dark objects in the channels of the WO3/SBA-15(OA) sample (Figure 2a). In contrast, obvious blocklike crystalline WO3 species are present on the outer surface of the SBA-15 support in the WO3/SBA-15(AM) sample, which is also confirmed by the selected area electron diffraction pattern (Figure 2c). For the WO3/SBA-15(HP) sample, it is interesting that no metal oxide particles are observed, but there are weak characteristic peaks of crystalline WO3 in the XRD pattern, probably due to the fact that a small quantity of tungsten oxide species could be formed into very thin sheets with low crystallinity. According to these findings, the dispersion of the tungsten species follows the order WO3/SBA-15(OA) > WO3/SBA-15(HP) > WO3/ SBA-15(AM). Ammonium paratungstate sources may favor the sintering of the tungsten species on SBA-15 support. N2 adsorption isotherms of the three different samples were recorded. Irreversible type IV adsorption isotherms with H1 hysteresis loops defined by IUPAC are observed, which is a typical feature of mesoporous materials (not shown here). These results further confirm the maintenance of ordered hexagonal arrangement of the SBA-15 frameworks upon tungsten oxide loading. Pure silica SBA-15 possesses a narrow pore size distribution and a high mesoporous surface area with considerable micropores, as seen from the t-plot.29 Impregnation of tungsten species reduces the surface area and the pore volume (Table 1). For the WO3/SBA-15(OA) sample, the WO3 nanoparticles mainly occupied the micropores, resulting in moderately reduced surface area. However, the bulky tungsten oxide species formed in the WO3/SBA-15(AM) sample can block the pores of SBA-15 and lead consequently to a drastic decrease of the surface area. Although no metal oxide particles are observed on the out surface of the SBA-15 support (Figure 2b), the WO3/SBA-15(HP) sample also has an obviously reduced surface area, which may be caused by the pores partially blocked by low-crystalline WO3 sheet species and by the partially destroyed channels. To obtain information on the chemical nature and coordination states of tungsten species, diffuse reflectance spectra in the UV-vis region of the different WO3/SBA-15 samples were

3822 J. Phys. Chem. C, Vol. 112, No. 10, 2008

Figure 3. UV-Visible diffuse reflectance spectra of various samples after dehydration at 400 °C in air for 2 h: (a) Na2WO4‚2H2O, (b) WO3/ SBA-15(OA), (c) WO3/SBA-15(HP), (d) WO3/SBA-15(AM), and (e) bulk WO3.

recorded and are shown in Figure 3. For comparison, the UVvis DRS spectra of sodium tungstate (Na2WO4‚2H2O) and bulk WO3 are also presented in Figure 3. The spectrum of sodium tungstate, with a spinel structure and isolated [WO4]2- tetrahedra, is characterized by a maximum at 230 nm (curve a).30,31 For pure siliceous SBA-15, there are no evident bands in the spectrum (not shown here); after tungsten oxide is loaded on the SBA-15 support, three types of Raman bands at 230, 290, and 430 nm appear. The broad one at about 430 nm of the WO3/ SBA-15(HP) and WO3/SBA-15(AM) samples (curves c and d) can be attributed to tungsten trioxide in octahedral coordination by comparison with the spectrum of bulk WO3, which is completely in the octahedral symmetry (curve e).22,30,32 The second broad band at about 290 nm represents another kind of tungsten species. The previous work of Weber33 and Iglesia et al.34 shows that the low-energy absorption is shifted toward lower wavelength when the nuclearity of molybdenum or tungsten entities decreases. Therefore, this broad band could be assigned to isolated tungsten species or low-condensed oligomeric tungsten species. The sharp band at 230 nm of the samples (curves b-d) can be attributed to isolated [WO4] tetrahedral species by comparison with the structure of sodium tungstate. The weak broad band at 430 nm and the strong bands at 230 and 290 nm may reflect that the tungsten species are highly dispersed in the WO3/SBA-15(OA) sample, with tungstenic complexes as the precursor formed by the interaction of tungstic acid with oxalic acid (curve b). The band at 430 nm becomes stronger for the WO3/SBA-15(HP) sample, indicating that a small amount of low-crystalline WO3 species are formed, which is in accord with the result of wide-angle XRD (Figure 1B). The intensity of the band at 430 nm of the WO3/SBA15(AM) sample is stronger than that of the other two samples, indicating that a large quantity of crystalline WO3 species are formed and the dispersion of tungsten species is very low in the WO3/SBA-15(AM) sample, which can be further confirmed by the TEM image and XRD pattern. The UV-vis DRS spectra further demonstrate that the tungsten source influences both the dispersion of tungsten species and its coordination in WO3/SBA15 samples. TPR profiles of various WO3-containing samples including bulk WO3 are given in Figure 4. It is found that the tungsten source strongly influences both the position and area of reduction peaks of each catalyst. As shown in Figure 4, the bare siliceous SBA-15 does not show any reduction peak in the temperature range investigated here (Figure 4a), while the TPR profile of

Yang et al.

Figure 4. Temperature-programmed reduction profiles of different samples: (a) SBA-15, (b) WO3/SBA-15(OA), (c) WO3/SBA-15(HP), (d) WO3/SBA-15(AM), and (e) bulk WO3.

Figure 5. NH3-TPD profiles of various samples: (a) SBA-15, (b) bulk WO3, (c) WO3/SBA-15(AM), (d) WO3/SBA-15(HP), and (e) WO3/ SBA-15(OA).

bulk WO3 exhibits three main peaks with maxima at 730, 790, and 910 °C (Figure 4e). These peaks may be assigned to the three-step reduction of WO3 to W(0) [WO3(VI) f WO2.9(V,VI) f WO2(IV) f W(0)] according to the literature.35,36 The two higher temperature peaks at 790 and 910 °C are associated with the reduction of W(VI) species in the tetrahedral coordination,35 while the peak at lower temperature (730 °C) is correlated with the reduction of the supported WO3 crystallites.37 Furthermore, Horsley et al.38 found that the isolated and the low-condensed oligomeric tungsten oxide species (e.g., dipolymer) cannot be easily reduced. The existence of a single H2 consumption peak for the WO3/SBA-15(OA) sample at about 960 °C may indicate high dispersion of the tungsten species, which can be assigned to the reduction of tetrahedrally coordinated species or the low-condensed oligomeric tungsten oxide species according to the literature.35,38 The WO3/SBA15(HP) sample exhibits two weak and broad reduction peaks at around 660 and 920 °C, as well as an intense peak at about 790 °C. Compared with the WO3/SBA-15(OA) sample, an obvious shift of the peak position to lower temperature is observed for the WO3/SBA-15(HP) sample, suggesting that the nuclearity of tungsten species increases. The TPR profile of the

Structure and Catalytic Properties of WO3/SBA-15 TABLE 2: Summary of NH3-TPD Data upon Various Samples samples SBA-15 WO3 10% WO3/SBA-15(OA) 10% WO3/SBA-15(HP) 10% WO3/SBA-15(AM)

Td (°C)

acidic amounts (mmol of NH3/g)

275 275 275, 340, 400

0.172 0.231 0.545 0.460 0.455

WO3/SBA-15(AM) sample is similar to that of bulk WO3. Moreover, the total H2 consumption rises, and the intense peak shifts toward lower temperature, demonstrating the presence of the polymeric tungsten species or crystalline WO3 in the sample corroborated by the XRD pattern. de Lucas et al.39 reported that reducibility of the tungsten-based silica catalysts increases as the strength of interaction of metal oxide species with the support surface decreases. Therefore, it can be concluded that the polymeric tungsten species or crystalline WO3 in the WO3/ SBA-15(AM) catalyst are easier to reduce and have weaker interaction with the SBA-15 support than the low-condensed oligomeric tungsten species in the WO3/SBA-15(OA) catalyst, thus increasing the leaching of tungsten species into the reaction mixture, which could be confirmed by the ICP data to be presented later. NH3-TPD was used to compare the acidic characteristics of the different WO3/SBA-15 samples. It is well-known that tungsten trioxide possesses Lewis acid sites.15 If porous materials contain deposited tungsten oxide species, acid centers will be generated and ammonia adsorption-desorption effects could be observed. NH3-TPD profiles of the different samples are presented in Figure 5. Acidity values and TPD peak temperature positions are given in Table 2. As expected, bulky WO3 and pure siliceous SBA-15 show little ammonia desorption due to the lack of acid centers (0.234 and 0.172 mmol of NH3/g). In contrast, a broad peak of ammonia desorption appears at about 275 °C as the tungsten oxide species are loaded on the SBA-15 support (0.172 f 0.545 mmol of NH3/g). The area under the curve represents the total acid site distribution, while the peaks on the ammonia desorption profile indicates the strength of acid sites. From the desorption profiles of various samples, it can be easily concluded that the WO3/SBA-15(OA) sample possesses comparably higher acidity than the other two samples. For the WO3/SBA-15(AM) sample, in addition to the broad peak

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3823 centered at about 275 °C, another two shoulder peaks are observed at about 340 and 400 °C, suggesting the presence of a small quantity of stronger acid sites on the WO3/SBA-15(AM) sample. For the WO3/SBA-15(HP) and WO3/SBA-15(OA) samples, as the amount of desorbed ammonia gradually increases, the two shoulder peaks become weaker and may be hidden below the tail of the strong band centered at 275 °C, indicating that the strong acid sites of WO3/SBA-15(OA) are less than those of the other two samples. The two hightemperature desorptions are possibly ascribed to Lewis acid centers of bulky WO3 according to the literature.15 The above NH3-TPD results can be correlated with those of XRD and TEM. The appearance of peaks corresponding to crystalline WO3 for the WO3/SBA-15(AM) sample could result in less moderate surface acid sites. Moreover, the CPE conversion during the selective oxidation of CPE to GA is found to decrease over the WO3/SBA-15(AM) catalyst, indicating that the much stronger Lewis acid sites of crystalline WO3 are not beneficial to the target reaction. Pyridine adsorption measured by IR spectroscopy was used to evaluate the strength and types of acid sites of the different samples. Figure 6A shows the FT-IR spectra of the samples recorded after adsorption of pyridine and subsequent evacuation at 150 °C. For bare SBA-15, only two bands at 1595 and 1445 cm-1 can be observed, ascribed to pyridine coordinately bonded to weak surface Lewis acid sites,40 which vanish almost completely after being outgassed at 200 °C (Figure 6B). In addition to the bands at 1595 and 1445 cm-1, the spectra recorded for the WO3/SBA-15 samples show bands at 1488, 1545, and 1635 cm-1, due to protonated pyridine bonded to surface Brønsted acid sites, indicating that the presence of tungsten species leads to the development of surface Brønsted acid sites.15 It can also be seen that the number of Lewis and Brønsted acid sites in the WO3/SBA-15(OA) sample is greater than in the WO3/SBA-15(HP) and WO3/SBA-15(AM) samples. In addition, for the WO3/SBA-15(OA) sample, the intensity of these bands decreases after outgassing at elevated temperature, but they are still recorded even after being outgassed at 300 °C (Figure 6C), illustrating that both types of acid sites are rather strong and are related to tungsten oxide loading. The number of Lewis and Brønsted acid sites decreases in an orderly fashion for the WO3/SBA-15(HP) and WO3/SBA-15(AM) samples. Moreover, all of these bands almost disappear after outgassing

Figure 6. (A) FT-IR spectra of pyridine adsorbed on various samples at 150 °C: (a) SBA-15, (b) WO3/SBA-15(OA), (c) WO3/SBA-15(HP), and (d) WO3/SBA-15(AM). (B, C) FT-IR spectra of pyridine adsorbed on (B) bare SBA-15 and (C) WO3/SBA-15(OA) sample at (a) 150, (b) 200, (c) 300, and (d) 400 °C.

3824 J. Phys. Chem. C, Vol. 112, No. 10, 2008

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Figure 8. Dependence of CPE conversion on reaction time over the various samples: (a) WO3/SBA-15(OA), (b) WO3/SBA-15(HP), and (c) WO3/SBA-15(AM). Figure 7. XPS spectra of the W4f region for different WO3/SBA-15 catalysts.

W(VI) and W(V) species at W4f7/2 (of 36.0 and 34.7 eV)43,44 for the W4f spin-orbit components have been detected, as shown in Figure 7. Table 3 also gives the quantitative results of the molar ratios of W6+/W5+ and Si/W by XPS according to the relative peak intensities of W4f and Si2p after correction with atomic sensitivity factors. For the WO3/SBA-15(AM) sample, the molar ratio of W6+/W5+ is 2.0, which is higher than that of the WO3/SBA-15(OA) and WO3/SBA-15(HP) samples (1.4 and 1.6); that is, the surface W6+ species contents increased, indicating that many more polymeric tungsten oxide or crystalline WO3 species are formed in WO3/SBA-15(AM). On the other hand, the existence of highly polymeric tungsten species in WO3/SBA-15(AM) leads to much more uncovered silica surface, and thus, the obvious increase in the Si/W molar ratio compared with WO3/SBA-15(OA), in which the tungsten species are highly dispersed. The XPS results further demonstrate that the tungsten sources strongly influence the chemical state and dispersion of tungsten species in the WO3/SBA-15 samples. 3.2. Catalytic Performances in the Selective Oxidation of CPE to GA. The catalytic performance of various WO3/SBA15 catalysts is shown in Table 4. For the purpose of comparison, the catalysts used in these experiments contain the same amount of tungsten. As shown in Table 4, unsupported crystalline WO3

at 300 °C, which may be associated with the low dispersion of tungsten species and the appearance of crystalline WO3. From the FT-IR spectra after pyridine adsorption, it can be concluded that both Lewis and Brønsted acid sites evidenced over the WO3/ SBA-15 samples are related to tungsten oxide incorporation and that the strong Lewis and Brønsted acid sites of the highly dispersed tungsten species are essential for selective oxidation of CPE, while the strong Lewis acid sites of crystalline WO3 are unfavorable for selective oxidation of CPE. XPS investigation of binding energies and intensities of the surface elements provides information on the chemical states and relative quantities of the outermost surface compounds. Figure 7 shows the W4f XPS spectra of the different samples. All the samples show a broad XPS peak, but it is possible to distinguish tungsten oxide species in different chemical states from the position of the W4f level by a curve-fitting procedure according to the Doniach and Sunjic theory.41,42 Detailed quantitative results from the peak-fitting results of W4f and Si2p are also listed in Table 3. The measured spectra appear similar for all samples and show identical positions for the W4f peaks, except for the minor charging effect observed and corrected according to the contaminant carbon (C1s ) 284.6 eV). Both

TABLE 3: Peak-Fitting Results of W4f XPS Spectra of WO3/SBA-15 Samples binding energy for W4f (eV) sample

W

c

WO3 WO2.5c 10% WO3/SBA-15(OA) 10% WO3/SBA-15(HP) 10% WO3/SBA-15(AM) c

6+

W4f5/2

W6+ W4f7/2

38.0

36.0

38.2 38.0 38.3

36.0 36.0 36.1

W5+ W4f5/2

W5+ W4f7/2

W6+/W5+ a

Si/Wb

36.8 36.9 36.9 36.9

35.0 34.7 34.7 34.7

1.4 1.6 2.0

77.4 89.0 119.9

a Calculated according to the curve-fitting results of the W XPS spectra of catalysts. b Calculated according to the peak areas of W and Si . 4f 4f 2p See ref 45.

TABLE 4: Catalytic Performance of the Selective Oxidation of CPE over Various Catalystsa selectivity (mol %) catalyst

conversion of H2O2 (mol %)

conversion of CPE (mol %)

GA yield (mol %)

GA

CPDL

CPLE

othersb

TOF c (h-1)

WO3 tungstic acid WO3/SBA-15(OA) WO3/SBA-15(HP) WO3/SBA-15(AM)

0.1 100 98 96 95

2 100 100 99 89

0 65 85 81 70

0 65 85 81 79

0 21 7 7 9

0 6 4 7 6

100 8 4 5 6

0 1.04 1.04 1.03 0.93

d

a Reaction time 24 h, reaction temperature 35 °C, molar ratio of CPE:H O :WO ) 100:210:4, volume ratio of t-BuOH/CPE ) 10. CPLE, 2 2 3 2-(t-butyloxy)-1-cyclopentanol; CPDL, cyclopentane-1,2-diol. b Including cyclopentene oxide, cyclopentenone, and cyclopentanone. c Calculated as TOF ) moles of CPE per mole of WO3 per hour. d Crystalline WO3 obtained by calcination of WO3‚H2O at 600 °C for 2 h.

Structure and Catalytic Properties of WO3/SBA-15

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3825

TABLE 5: ICP Data and Regeneration of the Various Catalystsa catalyst

Si/Wb

Si/Wc

Si/Wd

entry

CH2O2 (%)

CCPE (%)

SGA (%)

YGA (%)

10% WO3/SBA-15(OA)

38.7

39.1

42.8

10% WO3/SBA-15(HP)

38.7

40.3

45.6

10% WO3/SBA-15(AM)

38.7

39.6

57.6

1 2e 1 2e 1 2e

98 98 96 97 95 79

100 98 99 97 89 81

85 81 81 78 79 76

85 80 81 76 70 61

a Same conditions as in Table 4. b Molar ratio in catalyst preparation. c Measured by ICP before the reaction. d Measured by ICP after the second reaction cycle. e Regenerated after calcination at 600 °C.

shows little activity for the selective oxidation of CPE to GA. Furthermore, pure SBA-15 shows no transformation toward the title reaction (not shown here), while those well-dispersed tungsten species on the SBA-15 support show substantial activity and selectivity in the cleavage reaction. This result suggests that the tungsten species loaded on the mesoporous SBA-15 material act as active centers for the selective oxidation of CPE. The TOF values (also shown in Table 4) unambiguously verified the above conclusion. It also can be seen that the WO3/SBA15(OA) and WO3/SBA-15(HP) catalysts show excellent catalytic performance as compared with the WO3/SBA-15(AM) and homogeneous tungstic acid catalysts for the title reaction, not only in terms of the conversion of CPE but also in terms of the selectivity toward GA. To get more information on the activity of these three catalysts, conversion versus time experiments have been carried out and results are shown in Figure 8. It can be seen that WO3/ SBA-15(OA) and WO3/SBA-15(HP) show much higher initial and final catalytic activity as compared with WO3/SBA-15(AM), while WO3/SBA-15(OA) shows the highest initial catalytic activity. The CPE conversions by these three materials (OA, HP, and AM) after 1 h reaction are about 50%, 40%, and 30%, respectively. This finding is easily understood by the above systematic characterizations that tungsten sources influence the nature and dispersion of tungsten species in the WO3/SBA-15 catalysts intensively, thus lead to their different catalytic performance. To investigate tungsten leaching behavior and recycling ability of the three different kinds of WO3/SBA-15 catalysts, the actual tungsten amounts in fresh and regenerated catalysts are determined by the ICP method, and catalytic performances in the selective oxidation of CPE to GA by fresh and regenerated catalysts are tested. From the results shown in Table 5 it can be seen that, after the regeneration, a slight decrease in conversion and selectivity is noticed for the WO3/SBA-15(OA) and WO3/SBA-15(HP) catalysts. The decrease in conversion and selectivity is ascribed to the leaching of small amounts of tungsten species under reaction conditions, which is in accord with the ICP results. The ICP results show that the actual tungsten amounts in the catalysts are very well consistent with the added metal amounts in the as-prepared samples and ca. 7.8% and 11.4% WO3 are leached out after the second cycles of reaction for WO3/SBA-15(OA) and WO3/SBA-15(HP), respectively, while ca. 29.2% WO3 loss is observed for WO3/ SBA-15(AM), leading to an obvious decrease in catalytic performance. Therefore, it can be concluded that the interaction between tungsten species and the mesoporous silica-based matrix for the WO3/SBA-15(OA) and WO3/SBA-15(HP) catalysts is much stronger than for the WO3/SBA-15(AM) catalyst, which is in line with the H2-TPR results. In addition, another experiment has been carried out to test whether the WO3/SBA-15(OA) catalyst is actually heterogeneous. The dependence of CPE conversion on reaction time with the WO3/SBA-15(OA) catalyst and the filtrate after 50%

Figure 9. Dependence of CPE conversion on reaction time over WO3/ SBA (OA) and filtrate after 50% conversion.

conversion of CPE is shown in Figure 9. When the reaction over WO3/SBA-15(OA) catalyst has been carried out for 1 h, the catalyst is removed through simple filtration and the reaction solution is stirred for another 23 h under the same conditions. Only a slight increase of CPE conversion in the further 23 h of reaction is observed, indicating that the small amount of leached W species has little catalytic effect on the reaction. Therefore we can conclude that the W species in the WO3/SBA-15(OA) catalyst show the principal catalytic effect on the reaction, considering that some ultrafine catalyst particles are left in the filtrate. 4. Conclusion In summary, it is clear from this work that the use of different tungsten precursors for the synthesis of WO3/SBA-15 significantly influences the nature of the active tungsten species and therefore the catalytic activity. Under similar synthetic conditions, the tungsten species are highly dispersed in the WO3/ SBA-15(OA) sample by use of tungstenic complex sources formed by the reaction of tungstic acid with oxalic acid, while the use of ammonium paratungstate sources in the WO3/SBA15(AM) sample results in the appearance of both polymeric tungsten species and crystalline WO3 as confirmed by the XRD pattern, TEM image, and UV-vis DRS results. TPR results show a strong interaction between tungsten species and the siliceous SBA-15 matrix in the WO3/SBA-15(OA) sample, which could effectively decrease the leaching of active species into the reaction mixture. The FT-IR-pyridine adsorption experiment confirms the presence of strong Brønsted and Lewis acid sites upon the tungsten oxide species loading for the WO3/ SBA-15(OA) sample, thus enhancing the catalytic performance. The catalytic performance studies show that the GA yield is related to the nature of tungsten species and the WO3/SBA-15(OA) catalyst, with the highly dispersed tungsten species and the strong Lewis and Brønsted acid sites exhibiting 85% GA yield, much higher than for the homogeneous tungstic acid catalyst. Although the activity of the catalyst slightly decreased after the first recycle due to the leaching of small amounts of

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