Direct Synthesis and Structural Characteristics of Ordered SBA-15

J. Phys. Chem. C 2007, 111, 15173-15184

15173

Direct Synthesis and Structural Characteristics of Ordered SBA-15 Mesoporous Silica Containing Tungsten Oxides and Tungsten Carbides Linhua Hu,† Shengfu Ji,*,† Zheng Jiang,‡ Huanling Song,§ Pingyi Wu,† and Qianqian Liu† State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, 15 Beisanhuan Dong Road, P.O. Box 35, Beijing, 100029, People’s Republic of China, State Key Laboratory of Multi-Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, People’s Republic of China, and State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 342 Tianshui Road, Lanzhou 730000, People’s Republic of China ReceiVed: June 22, 2007; In Final Form: August 11, 2007

A series of WO3-SBA-15 materials with different Si/W ratios have been hydrothermally synthesized using tetraethyl orthosilicate (TEOS) as silica precursor, ammonium paratungstate as tungsten precursor, and EO20PO70EO20 (P123) as structure-directing reagent. After temperature-programmed carburization (TPC) in flowing CH4/H2 (20/80 v/v mixture), the materials were converted to the corresponding WxC-SBA-15 materials. The structure of the oxide and carbide materials has been characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen adsorption-desorption measurements, 29Si magic-angle spinning (MAS) NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and thermogravimetric and differential scanning calorimetric analysis (TG-DSC) measurements. The results show that after hydrothermal synthesis using different amounts of tungsten and subsequent carburization, the materials retain the mesopore structure of SBA-15. When Si/W ) 30-15, the majority of the tungsten is dispersed in the channels of SBA-15 with the remainder being incorporated into the framework of SBA-15 with the formation of Si-O-W bonds. The tungsten carbide exists as a single W2C phase after carburization. At higher tungsten content (Si/W ) 7.5), the amount of tungsten in the framework of SBA-15 increases with the formation of both Si-O-W bonds and W-O-W bonds. The tungsten carbide formed after carburization exists as a mixture of W2C and WC phases. A model for the distribution of tungsten in SBA-15 is proposed involving three different tungsten species: R-W inside SBA-15 channels, β-W embedded in the internal surfaces of the SBA-15 channels, and γ-W inside the framework of SBA-15. After temperature-programmed carburization, R-W sites are transformed into W2C, whereas β-W sites afford WC; in contrast, γ-W sites show little change after carburization.

1. Introduction (OMS)1-4

Ordered mesoporous silicas such as SBA-15 and MCM-41 have attracted considerable attention in the fields of heterogeneous catalysis and nanoscale materials because of their large surface areas and uniform pore structures.5-8 In recent years, many highly active catalysts have been prepared by means of the introduction of noble metals and transition metals and their oxides into the channels of preformed OMS as well as by the direct incorporation of transition-metal ions into the framework of OMS during the synthesis process. Wang et al.9 found that TiO2 and ZrO2 can be inserted into SBA-15 and that after platinization the resulting Pt/TiO2 (ZrO2)/SBA-15 composite materials displayed high activity in ethyl acetate combustion. Raja et al.10 prepared a bimetallic catalyst with a uniform distribution of Pd-Ru nanoparticles encapsulated within the pores of MCM-41 which was a highly effective catalyst for olefin hydrogenation. Inumaru et al.11 synthesized a highly active photocatalyst for decomposition of 4-nonylphenol by insertion of crystalline TiO2 particles into the pores of MCM-41. * To whom correspondence should be addressed. Tel.: +86 10 64412054; fax: +86 10 64419619; e-mail: [email protected]. † Beijing University of Chemical Technology. ‡ Institute of Process Engineering, Chinese Academy of Sciences. § Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.

M-containing mesoporous silicas by direct synthesis have attracted considerable attention such as Ti,12 Al,13,14 Zr,15,16 Au,17 Pt,18 and Ce19 because of their potential applications in catalysis. Tungsten-containing mesoporous silica is an important catalytic material for metathesis and selective oxidation reaction. Zhang et al.20 prepared W-MCM-41 mesoporous silica in strongly acidic medium and found that tungsten was incorporated in the tetrahedrally coordinated positions of the MCM-41 framework when its amount in the mesophase did not exceed 5.6 wt % (Si/W > 50). Klepel et al.21 prepared tungsten-containing ordered MCM-41 by the incorporation of tungsten into the framework of MCM-41 (Si/W ) 17) without extraframework oxide species under basic conditions. Tungsten-containing ordered MCM-48 has been synthesized under hydrothermal conditions via pH adjustment by Yang et al.,22 and the as-synthesized W-MCM-48 material is very active as a heterogeneous catalyst for the selective oxidation of cyclopentene to glutaraldehyde. Hu et al.23 prepared tungsten-substituted SBA-15 in a one-step co-condensation sol-gel method using sodium tungstate as tungsten precursor under acidic conditions and found that the tungsten-substituted mesoporous SBA-15 catalysts show excellent catalytic performance in the metathesis of 1-butene to high-value olefins with highly dispersed tungsten oxide species in the silica-based framework structure.

10.1021/jp074879h CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

15174 J. Phys. Chem. C, Vol. 111, No. 42, 2007 Transition-metal carbides are promising catalysts which are comparable in performance to noble metals in many cases,24-29 and some cases, such as deep hydrodesulfurization (HDS) of oil and gas,30-32 hydrodenitrogenation (HDN),33 hydrocarbon isomerization,34 and methane reforming to synthesis gas,35-38 have an even better performance. Transition-metal carbides have a serious disadvantage, however, in that they are readily agglomerated under the high-temperature reaction conditions involved. Usually, the highly dispersed transition-metal carbides in OMS can be prepared by the impregnation of the host mesoporous material with aqueous solutions of different metal salts and subsequently temperature-programmed carburization (TPC). Piquemal et al.39 prepared highly dispersed molybdenum carbides in MCM-41 mesoporous silica and found that the material had a good catalytic activity for propene transformation (hydrogenation and metathesis). In our previous work,40 the W2C/SBA-15 material was prepared by impregnation of aqueous ammonium paratungstate solutions into SBA-15 mesoporous silica host material; subsequently, carburization and the structure was investigated. It was found that a tungsten carbide layer was well confined in the channels of SBA-15. In fact, the highly dispersed transition-metal carbides in OMS can also be prepared by TPC of W-containing mesoporous silica with direct synthesis. To the best of our knowledge, the specific structural analysis and tungsten distribution in ordered silicas of the SBA-15 type containing tungsten oxides and tungsten carbides have not been reported in the literature so far. In this paper, the direct synthesis of a series of WO3-SBA-15 and the corresponding WxC-SBA-15 is described and the resulting materials are characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen adsorption-desorption measurements, Fourier transform infrared (FTIR) spectroscopy, 29Si magic-angle spinning (MAS) NMR spectroscopy, transmission electron microscopy (TEM), and thermogravimetric and differential scanning calorimetric analysis (TG-DSC) measurements. A model of tungsten distribution in WO3-SBA-15 is proposed, and the mechanism of carburization is analyzed in detail. 2. Experimental Section 2.1. Preparation of Samples. SBA-15 was synthesized as described in ref 3. Samples of WO3-SBA-15 with different tungsten contents (molar ratio Si/W from 30 to 7.5) were synthesized using a triblock copolymer (P123, EO20PO70EO20, M ) 5800, Aldrich) as structure-directing reagent, tetraethyl orthosilicate (TEOS) as silica precursor, and ammonium paratungstate ((NH4)10·[H2W12O42]‚4H2O) as tungsten precursor. The triblock copolymer (4 g) was dissolved in a mixture of deionized water (60 mL) and 4 M hydrochloric acid (60 mL) and was stirred at 40 °C for 2 h; tetraethyl orthosilicate (TEOS) (8.5 g) and aqueous solutions of ammonium paratungstate with different concentrations (30 mL) were then slowly added to the mixture which was subsequently stirred at 40 °C for 24 h. The gel mixture was then transferred to a Teflon bottle and was aged at 100 °C for 24 h under static conditions. The resulting solid product was filtered and washed with deionized water until the washings had neutral pH, was dried at room temperature, and finally was calcined at 550 °C for 6 h in air to remove the triblock copolymer organic template. The samples are denoted as WO3-SBA-15 (Si/W ) n), where n is the molar ratio of silicon to tungsten. Samples of WxC-SBA-15 were prepared by temperatureprogrammed carburization (TPC) of WO3-SBA-15 in a flow of CH4/H2 (20/80 v/v) mixed gases. The temperature was raised

Hu et al. linearly from room temperature to 300 °C at the rate of 10 °C min-1 and then from 300 °C to 700 °C at the rate of 1 °C min-1. The temperature was kept at 700 °C for 2 h, and then the CH4/ H2 mixed gas switched to hydrogen and the sample cooled to room temperature. After cooling, the samples of WxC-SBA-15 were passivated by 1% O2/Ar (v/v) prior to characterization. The samples are denoted as WxC-SBA-15 (Si/W ) n), where n is the molar ratio of silicon to tungsten. The measured value of the molar ratio of silicon to tungsten in WxC-SBA-15 is approximately equal to that in WO3-SBA-15. 2.2. Characterization of Samples. X-ray powder diffraction (XRD) patterns of the samples were recorded using a Rigaku D/Max 2500 VB2+/PC diffractometer with Cu KR radiation at 40 kV and 50 mA or 200 mA. Chemical composition of samples was analyzed semiquantitatively using X-ray fluorescence (XRF) spectrometry (PANalytical Magix PW2403) with Rh radiation at 24 kV. The samples were finely ground and were mixed with methylcellulose. The mixture was placed in a standard fluorescent aluminum cup mold and was compressed into a wafer with a diameter of 40 mm and a thickness of 35 mm at 20 tons with a dwell time of 30 s. N2 adsorptiondesorption experiments were performed with a Quantachrome Autosorb-1 automatic surface area and pore size analyzer. The samples were pretreated at 300 °C for 4 h, and the specific surface area of samples was determined using the BrunauerEmmett-Teller (BET) method. The pore volume and pore size distribution were derived from the desorption profiles of the isotherms using the Barrett-Joyner-Halanda (BJH) method. High-resolution 29Si magic-angle spinning (MAS) NMR spectra were recorded with 4 mm zirconia rotors spinning at 5 kHz on a Bruker Avance 300M solid-state spectrometer operating at 59.62 MHz. Quantitative 29Si single-pulse excitation MAS experiments were recorded using a recycle delay of 15 s and a pulse length of 3.6 µs; 2000-3000 scans were collected. Chemical shifts are referenced to tetramethylsilane (TMS). Specimens for high-resolution transmission electron microscope (HRTEM) were prepared by dispersing a few milligrams of the sample in chloroform for 15 min in an ultrasonic bath. The suspension was transferred dropwise onto to a lacey carbon grid (Agar Scientific, 300 mesh) and was allowed to dry in air. A Jeol JEM-3000F HRTEM was subsequently used to examine the prepared specimens. Additionally, focal series of selected WxC-SBA-15 samples were acquired on the Jeol JEM-3000F HRTEM equipped with a Gatan 794 (1 k × 1 k pixel) CCD camera. IR spectra were recorded on a Bruker Tensor 27 FTIR spectrometer at 2 cm-1 resolution using a KBr pellet technique. Before measurement, all samples and KBr were dried at loft drier at 373 K overnight. The sample diluted in KBr (2 wt %) was pressed into a wafer (40.5 mg cm-2 thickness). The wafer was evacuated at 373 K in a quartz cell sealed with KBr windows for 30 min. The spectra were collected in absorbance mode after the cell was cooled to room temperature. Thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis was conducted on Netzsch STA 449C thermal analyzer from room temperature up to 1000 °C with a heating speed of 5 °C min-1 in an argon atmosphere. 3. Results 3.1. XRD and XRF. The low-angle and large-angle XRD patterns of SBA-15 and WO3-SBA-15 (Si/W ) 30-7.5) are shown in Figure 1 and Figure 2. The XRD patterns of WO3SBA-15 (Si/W ) 30-15) samples (Figure 1B-E) display three well-resolved peaks at low angle which can be indexed to (100), (110), and (200) reflections in the hexagonal space group p6mm,

Synthesis and Structure of Ordered SBA-15 Mesoporous Silica

Figure 1. Low-angle XRD patterns of WO3-SBA-15 samples: (A) SBA-15, (B) WO3-SBA-15 (Si/W ) 30), (C) WO3-SBA-15 (Si/W ) 25), (D) WO3-SBA-15 (Si/W ) 20), (E) WO3-SBA-15 (Si/W ) 15), and (F) WO3-SBA-15 (Si/W ) 7.5).

Figure 2. Large-angle XRD patterns of WO3-SBA-15 samples: (A) SBA-15, (B) WO3-SBA-15 (Si/W ) 30), (C) WO3-SBA-15 (Si/W ) 25), (D) WO3-SBA-15 (Si/W ) 20), (E) WO3-SBA-15 (Si/W ) 15), and (F) WO3-SBA-15 (Si/W ) 7.5).

which are very similar to those of SBA-15 itself (Figure 1A). The intensity of the (100) peak is attenuated with increasing tungsten content and, in particular, is not observed when the Si/W ratio reaches 7.5 (Figure 1F). This is a clear indication that although the ordered mesoporous structures are retained at lower tungsten content, higher tungsten content leads to some disruption of the structure. As shown in Figure 2, WO3-SBA-15 materials show peaks at 23.08°, 23.58°, 24.26°, 26.54°, 28.75°, 33.28°, 41.66°, 49.74°, and 55.71° (2 theta) (Figure 2B-F), which are not present in the pattern of the SBA-15 itself (Figure 2A) and which are characteristic of WO3 (monoclinic (P21/n), JCPDS#72-0677). The crystallite size of WO3 particles can be estimated by Scherrer’s method. The full width at half-maximum (fwhm) of the peak at 41.66° is independent of tungsten content, showing that the crystallite size of WO3 particles does not increase with increasing tungsten content. This suggests that the WO3 crystallites are uniformly dispersed in the SBA-15 host and that the confinement effect of the SBA-15 channels inhibits agglomeration and growth of the particles.

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15175

Figure 3. Large-angle XRD patterns of WxC-SBA-15 samples: (A) SBA-15, (B) WxC-SBA-15 (Si/W ) 30), (C) WxC-SBA-15 (Si/W ) 25), (D) WxC-SBA-15 (Si/W ) 20), (E) WxC-SBA-15 (Si/W ) 15), and (F) WxC-SBA-15 (Si/W ) 7.5).

The low-angle XRD patterns of WxC-SBA-15 (Si/W ) 307.5) are similar to that of WO3-SBA-15 (see Figure S1 in Supporting Information). The characteristic (100) peak of the SBA-15 hexagonal structure is obvious in the patterns of WxCSBA-15, but its intensity shows a dramatic decrease when the Si/W ratio reaches 7.5. The large-angle XRD patterns of WxCSBA-15 (Si/W ) 30-7.5) are shown in Figure 3. The peaks in the diffraction patterns of WxC-SBA-15 at 34.34°, 37.84°, 39.35°, 61.59°, 69.55°, and 74.77° (2 theta) (Figure 3B-E) correspond to the characteristic reflections reported for W2C (JCPDS #72-0677). The diffraction pattern of WxC-SBA-15 (Si/W ) 7.5) shows additional peaks at 31.32°, 35.62°, 48.20°, 64.79°, and 73.18° (2 theta) (Figure 3F) which can be attributed to WC (JCPDS #51-0939). Thus, we can conclude that tungsten carbide exists as a single phase, W2C, at lower tungsten content but as a mixture of W2C and WC phases at higher tungsten content. Similar to WO3, the fwhm of the peak at 39.35° does not vary with tungsten content, suggesting that the W2C crystallites are uniformly dispersed in the SBA-15 mesoporous silica and do not agglomerate even at higher tungsten content. The chemical composition of WxC-SBA-15 samples is listed in Table 1. When Si/W is 30, the nominal value of W% is 9.22 wt %, whereas the measured value of W% is 6.13 wt %. The difference for tungsten between nominal and measured value is about 3.09 wt %, whereas the measured Si/W is 48.3. The tungsten loss can be attributed to washing with deionized water in hydrothermal synthesis. When Si/W is 15 and 7.5, the nominal value of W% is 16.84 wt % and 28.61 wt %, whereas the measured value of W% is 11.80 wt % and 22.85 wt %. The difference for tungsten between nominal and measured value is about 5.04 wt % and 5.76 wt %, whereas the measured Si/W is 22.7 and 10.2. The lost tungsten gradually increases with increasing tungsten content. 3.2. N2 Adsorption-Desorption Measurements. 3.2.1. WO3-SBA-15. The N2 adsorption-desorption isotherms and the calculated pore distributions of calcined SBA-15 mesoporous silica and WO3-SBA-15 with different tungsten contents are shown in Figure 4. The pore structure parameters of different samples are listed in Table 2, including BET specific surface area (SBET), total pore volume (VP), pore diameter (DBJH) from N2 adsorption-desorption measurements, and the d(100) spacing and cell parameter obtained from XRD data. Other parameters such as the wall thickness of SBA-15 and the thickness of the

15176 J. Phys. Chem. C, Vol. 111, No. 42, 2007

Hu et al.

TABLE 1: Chemical Composition of WxC-SBA-15 Samples nominal value samples

Si/W

Si/%c

WxC-SBA-15 (Si/W ) 30)a WxC-SBA-15 (Si/W ) 15)a WxC-SBA-15 (Si/W ) 7.5)b

30 15 7.5

42.29 38.61 32.79

measured value

O/%

W/%

C%

Si/W

Si/%

O/%

W/%

C%

difference value of W

48.19 44.00 37.36

9.22 16.84 28.61

0.30 0.55 1.24

46.8 22.7 10.2

43.79 41.05 35.60

49.89 46.77 40.56

6.13 11.80 22.85

0.19 0.38 0.99

3.09 5.04 5.76

a The phase of W C is W C when Si/W ) 30 and 15. b The phases of W C are W C and WC when Si/W ) 7.5. c % is defined as the weight x 2 x 2 percent.

Figure 4. Nitrogen sorption isotherms and pore size distribution of WO3-SBA-15 samples: (A) SBA-15, (B) WO3-SBA-15 (Si/W ) 30), (C) WO3-SBA-15 (Si/W ) 15), and (D) WO3-SBA-15 (Si/W ) 7.5) The isotherms of A, B, and C are offset by 300, 200, and 100 cm3 g-1, respectively.

TABLE 2: Pore Structure Parameters of Samples Measured Using the Desorption Branch of N2 Adsorption-Desorption Isotherms and XRD samples

BET area (m2 g-1)

DBJH (nm)

Vp (cm3 g-1)

XRD d(100) spacing (nm)

cell parametera (nm)

SBA-15 (Si/W ) ∞) WO3-SBA-15 (Si/W ) 30) WO3-SBA-15 (Si/W ) 15) WO3-SBA-15 (Si/W ) 7.5) WxC-SBA-15 (Si/W ) 30) WxC-SBA-15 (Si/W ) 15) WxC-SBA-15 (Si/W ) 7.5)

736 624 491 384 604 476 409

7.90 6.95 6.59 5.57 6.64 6.35 5.82

1.10 0.99 0.87 0.74 0.96 0.85 0.80

10.40 10.43 10.62 12.20 10.51 10.83 12.96

12.00 12.04 12.26 14.08 12.13 12.50 14.96

Dn(WO3-SBA-15)b (nm)

Dn(WxC-SBA-15)c (nm)

4.13 4.32 5.90

Dn(WO3)d (nm)

Dn(WxC)e (nm)

0.48 0.68 1.30 4.21 4.53 6.66

0.64 0.81 1.24

a Cell parameter: a ) 2d (100)/x3. b D (WO -SBA-15): the wall thickness of WO -SBA-15, calculated from eqs 1 and 2. c D (WO ): the n n 3 3 n 3 WO3 layer thickness of WO3-SBA-15, calculated from eq 3. d Dn (WxC-SBA-15): the wall thickness of WxC-SBA-15, calculated from eqs 1 and 4. e Dn (WxC): the WxC layer thickness in WxC-SBA-15, calculated from eq 5.

WO3 and WxC layers are also listed in Table 2 and are discussed in section 4.1. As observed in Figure 4, the isotherms of SBA-15 and WO3SBA-15 (Si/W ) 30-15) show typical type IV features and have an H1 hysteresis loop in which the two branches are almost vertical and nearly parallel over an appreciable range of relative pressure (P/P0).41-42 The volume of adsorbate shows a sharp increase at a P/P0 of approximately 0.70 arising from capillary condensation of nitrogen within the uniform mesoporous structures. In contrast, the isotherms of WO3-SBA-15 (Si/W ) 7.5) show typical IV features and have an H3 hysteresis loop in which the two branches are acclivitous and unparallel. Moreover, the H3 hysteresis does not exhibit any limiting

adsorption at high P/P0.41-42 The volume of adsorbate gradually increases with relative pressure (P/P0) up to 1.0 indicating that the mesoporous structures in which capillary condensation of nitrogen occurs are not uniform. The values of SBET and VP both show a monotonic decrease with increasing concentration of tungsten (see Table 2) consistent with the formation of WO3 within the mesoporous structure of SBA-15. Even for the highest tungsten content, WO3-SBA-15 (Si/W ) 7.5), the values of SBET and VP are more than 50% of the corresponding values for the host itself. SBA-15 has a primary mesopore diameter (DBJH) of 7.90 nm, while samples of WO3-SBA-15 have smaller pore diameters (see Table 2). The value of DBJH for WO3-SBA-15 (Si/W )

Synthesis and Structure of Ordered SBA-15 Mesoporous Silica 30) is 0.95 nm lower than that of SBA-15, and the values of SBET and VP are also significantly reduced. This is consistent with the presence of a layer of WO3 on the surface walls of SBA-15. The value of DBJH for WO3-SBA-15 (Si/W ) 15) shows a further decrease of 0.36 nm, suggesting an increased thickness of the layer of WO3 on the walls. The value of DBJH for WO3-SBA-15 (Si/W ) 7.5) shows a much more dramatic decrease, being 1.04 nm lower than that of WO3-SBA-15 (Si/W ) 15). This can be attributed to formation of WO3 particles in the channels of SBA-15 in addition to the layers of WO3 on the intrachannel surfaces, which is consistent with the observed H3 hysteresis loop and abnormal Gaussian pore distribution. As shown in Table 2, the cell parameter increases with tungsten content from 12.00 nm in SBA-15 itself to 14.08 nm for SBA15 (Si/W ) 7.5). The increase in the cell parameter is consistent with the incorporation of tungsten atoms into the framework of SBA-15. These textural and structural properties all clearly demonstrate that the distribution of tungsten within SBA-15 is highly dependent on the tungsten content. For lower tungsten content, the WO3 is uniformly distributed within the SBA-15 channels, and there is partial tungsten atom incorporation into the framework of SBA-15. For higher tungsten content, the distribution of WO3 within SBA-15 is not uniform, occurring in the channels and on intrachannel surfaces, and there is a higher proportion of framework tungsten atoms. 3.2.2. WxC-SBA-15. The nitrogen adsorption-desorption isotherms and pore distributions of WxC-SBA-15 are qualitatively similar to those of WO3-SBA-15 (see Figure S2 in Supporting Information); however, the values of SBET, VP , and DBJH for WxC-SBA-15 are significantly different from those of WO3-SBA-15 with the same molar ratio of silicon to tungsten (see Table 2). Compared with WO3-SBA-15 (Si/W ) 30), the values of SBET, DBJH, and VP of WxC-SBA-15 (Si/W ) 30) show decreases of 20 m2 g-1, 0.31 nm, and 0.03 cm3 g-1, respectively, while compared with WO3-SBA-15 (Si/W ) 15), the values of SBET, DBJH, and VP of WxC-SBA-15 (Si/W ) 15) show similar decreases of 15 m2 g-1, 0.24 nm, and 0.02 cm3 g-1, respectively. Similar to the case of WO3-SBA-15 discussed above, the data indicate that the layer of W2C in the carburized species is thicker in WxC-SBA-15 (Si/W ) 15) than in WxC-SBA-15 (Si/W ) 30) and suggest that the mesoporous structure of SBA-15 is not substantially influenced by carburization for materials with Si/W ) 30-15. The value of DBJH decreases more sharply than SBET and VP with increasing tungsten content which can be attributed to the influence of the tungsten atoms incorporated into the framework of SBA-15 since they cannot be carburized because of the presence of strong Si-O-W bonds. In contrast, the values of SBET, DBJH, and VP of WxC-SBA-15 (Si/W ) 7.5) show increases of 25 m2 g-1, 0.25 nm, and 0.06 cm3 g-1 compared with the respective values for WO3-SBA-15 (Si/W ) 15). This indicates that the mesoporous structure of SBA-15 is strongly influenced by carburization at higher tungsten content, which is consistent with the significant decrease in the intensity of the low-angle reflections and the formation of the mixture of W2C and WC phases indicated by the large-angle reflections in the XRD pattern of WxC-SBA-15 (Si/W ) 7.5) discussed above. 3.3. NMR Measurements. High-resolution solid-state magicangle spinning (MAS) NMR spectroscopy has become a powerful tool for structural characterization of zeolites and other catalytic materials. The high resolution now available allows small structural differences such as those induced by temperature variations or by the presence of crystallographically inequivalent

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15177 environments among the silicon sites to be detected; in contrast, the XRD technique provides information about the overall longrange structure.43-45 Mesoporous silicas are generally characterized by the presence of three silicon sites Q2, Q3, and Q4 representing the species Si(OSi)2(OH)2, Si(OSi)3(OH), and Si(OSi)4, respectively. Comparing the relative amounts of the three silicon sites in the SBA15 precursor, after tungsten-doped hydrothermal synthesis and after carburization, can provide valuable information about the influence of different tungsten moieties, such as surface WO3, WxC, or framework tungsten atoms, on the mesoporous structure of SBA-15. The chemical shifts of the Q2, Q3, and Q4 species are directly obtained from 29Si MAS NMR spectra, and a Gaussian/Lorentzian model is used to find the best set of parameters and shapes to minimize the difference between simulated and experimental spectra. Using this method, Zotov and Keppler have previously succeeded in analyzing the influence of water on the structure of hydrous sodium tetrasilicate glasses by fitting their 29Si MAS NMR spectra.46 The possibility of overlap of peaks for Q2, Q3, and Q4 species in the 29Si MAS NMR spectra cannot be ignored, however, especially for the case of Q2 and Q3 species in calcined samples. The peaks due to Q2, Q3, and Q4 species in as-synthesized SBA15 are clearly resolved, however, and therefore the chemical shifts of these peaks are used as the basis for the subsequent fitting of the spectra of other samples. 3.3.1. SBA-15. The 29Si MAS NMR spectra of SBA-15 (assynthesized) is different from that of SBA-15 (calcined) (see Figure S3 in Supporting Information). The spectrum of the assynthesized species shows three well-resolved peaks with chemical shifts of -91.2, -101.2, and -110.7 ppm which can be assigned to the Q2, Q3, and Q4 species, respectively, whereas the calcined species shows a broad asymmetric peak centered at about -110 ppm. It appears that during the calcination process some of the hydroxyl groups in Q2 and Q3 species undergo condensation so that the intensity of these peaks decreases relative to that of the Q4 species. The experimental 29Si MAS NMR spectrum of SBA-15 (as-synthesized) can be closely fitted by a superposition of three Gaussian/Lorentzian components because of Q2, Q3, and Q4 sites (see Figure S4A in Supporting Information). The Q3/Q4 ratio, on the basis of the ratio of the areas of the corresponding peaks in the fitted spectrum, provides important information regarding the structure and mechanism of formation of the silica walls since the lower the ratio Q3/Q4 the higher the degree of condensation of the silica structure.45 The broad peak in the experimental 29Si MAS NMR spectrum of SBA-15 (calcined) can also be closely fitted by superposition of three Gaussian/Lorentzian components because of Q2, Q3, and Q4 sites (see Figure S4B in Supporting Information). The results are reported in Table 3. The ratio of Q3/Q4 in SBA-15 (calcined) is lower than that in SBA-15 (as-synthesized) since some of the hydroxyl groups in Q3 sites undergo condensation during calcination as suggested above. 3.3.2. WO3-SBA-15. The fitted 29Si MAS NMR spectra of WO3-SBA-15 (Si/W ) 30, 15, 7.5) are shown in Figure 5. The Q3/Q4 ratios for different tungsten contents are listed in Table 3. The Q3/Q4 ratio decreases gradually with increasing tungsten content. As discussed above, XRD and N2 adsorption data indicate that there are two different types of tungsten sites within SBA-15. One is on the intrachannel walls of SBA-15 involving formation of Si-O-W bonds by the combination of tungsten and hydroxyl groups of Q3 sites during calcination. This results in a decrease in the number of Q3 sites since they are converted into Si(OSi)3(OW), which is denoted below as Si (3 Si, 1 W).

15178 J. Phys. Chem. C, Vol. 111, No. 42, 2007 TABLE 3:

29Si

Hu et al.

MAS NMR Chemical Shifts and Calculated Q2/Q4 and Q3/Q4 Ratios for Different Samples chemical shift/ppma

samples

Q2

Q3

Q4

Q2/Q4 (ratio of peak area)

Q3/Q4 (ratio of peak area)

SBA-15 (as-synthesized) SBA-15 (calcined) WO3-SBA-15 (Si/W ) 30) WO3-SBA-15 (Si/W ) 15) WO3-SBA-15 (Si/W ) 7.5) WxC-SBA-15 (Si/W ) 30) WxC-SBA-15 (Si/W ) 15) WxC-SBA-15 (Si/W ) 7.5)

-91.2 -91.2 -90.9 -92.2 -90.4 -90.2 -91.5 -89.8

-101.2 -101.2 -100.5 -100.4 -100.0 -100.2 -99.9 -98.8

-110.7 -109.5 -109.6 -109.3 -109.1 -108.9 -108.6 -108.1

0.062 0.035 0.020 0.022 0.054 0.020 0.023 0.050

0.510 0.440 0.415 0.361 0.314 0.435 0.405 0.390

a

ppm, the unit of chemical shift, is abbreviation of part per million.

Figure 5. Fitting of 29Si NMR MAS spectra of WO3-SBA-15 samples: (A) WO3-SBA-15 (Si/W ) 30), (B) WO3-SBA-15 (Si/W ) 15), and (C) WO3-SBA-15 (Si/W ) 7.5).

The other is in the framework of SBA-15, which involves the formation of Si-O-W bonds in the framework of SBA-15 during hydrothermal synthesis of tungsten-containing mixtures and results in a decrease in the number of Q4 sites that are also

converted into Si (3 Si, 1 W). The Q3/Q4 ratios of WO3-SBA15 are less than that of SBA-15, indicating that the amount of tungsten on the surface is larger than that in the framework of SBA-15. For each different tungsten content, the Q2/Q4 ratio in WO3SBA-15 is much smaller than the Q3/Q4 ratio. As shown in Table 3, SBA-15 itself also shows an excess of Q3 sites over Q2. The Q2/Q4 ratio shows a significant increase for Si/W ) 7.5 compared with that of Si/W ) 30 and Si/W ) 15, however, which suggests that the mesoporous structure of SBA-15 is strongly influenced at higher tungsten content. 3.3.3. WxC-SBA-15. The fitted 29Si MAS NMR spectra of WxC-SBA-15 (Si/W ) 30, 15, 7.5) are different from that of WO3-SBA-15 (Si/W ) 30, 15, 7.5; see Figure S6 in Supporting Information). The values of the Q3/Q4 ratio increase gradually with increasing tungsten content as shown in Table 3. The Q3/ Q4 ratios of WxC-SBA-15 are higher than those of WO3-SBA15 with the same tungsten content. This is as expected; while carburization leads to the transformation of W-O-Si units on the intrachannel surfaces of SBA-15 into H-O-Si units, giving an increase in the number of Q3 sites, the tungsten sites in the framework of SBA-15 do not undergo reaction and the number of Q4 sites is not affected by carburization. 3.4. FTIR Spectroscopy. The FTIR spectra of SBA-15 (calcined), WO3-SBA-15, and WxC-SBA-15 samples in the wavenumber range 4000-2600 cm-1 and 1050-600 cm-1 are shown in Figure 6 and Figure 7, respectively. The band areas between the wavenumbers from 3800 cm-1 to 2800 cm-1 are listed in Table 4. As shown in Figure 6, the bands at 3745 cm-1 can be assigned to the stretching vibration mode of isolated terminal silanol (Si-OH) groups,47 whereas the bands at 3430 cm-1 can be assigned to the hydrogen-bonded Si-OH groups because of geminal Si-OH of Q2 and adjacent Q3. It is consistent with the FTIR result of MCM-41 by Zhao et al.,48 who deemed the band at 3222 cm-1 to hydrogen-bonded SiOH groups perturbed by physically adsorbed water. The band area of 3430 cm-1 for SBA-15 is 115.8, whereas for WO3SBA-15 samples the band area displays gradual decrease with increasing tungsten content. This indicates that the amount of the O-H bond in Si-O-H decreases after tungsten-doped hydrothermal synthesis. It is consistent with the formation of Si-O-W bonds by the combination of tungsten and hydroxyl groups of Q3 sites during calcination on the intrachannel surfaces of SBA-15 in the NMR result discussed above. The band areas at 3430 cm-1 for WxC-SBA-15 samples are larger than those of WO3-SBA-15 samples with the same molar ratio of silicon to tungsten, implying that Si-O-W bonds on the intrachannel walls of SBA-15 have been partially transformed to Si-O-H bonds after carburization. As shown in Figure 7, the band at 966 cm-1 and 810 cm-1 corresponds to characteristic stretching vibration of nonbridging oxygen atoms (Si-Oδ-)49 of Si-O-H bonds and symmetric


Figure 6. FTIR spectra of different samples between 4000 and 2600 cm-1: (1) SBA-15, (2) WO3-SBA-15 (Si/W ) 30), (3) WO3SBA-15 (Si/W ) 15), (4) WO3-SBA-15 (Si/W ) 7.5), (5) WxC-SBA15 (Si/W ) 30), (6) WxC-SBA-15 (Si/W ) 15), and (7) WxC-SBA-15 (Si/W ) 7.5).

Figure 7. FTIR spectra of different samples between 1050 and 600 cm-1: (1) SBA-15, (2) WO3-SBA-15 (Si/W ) 30), (3) WO3-SBA15 (Si/W ) 15), (4) WO3-SBA-15 (Si/W ) 7.5), (5) WxC-SBA-15 (Si/W ) 30), (6) WxC-SBA-15 (Si/W ) 15), and (7) WxC-SBA-15 (Si/W ) 7.5).

TABLE 4: FTIR Band Areas of Different Samples between 3800 and 2800 cm-1 samples

band area (au)

SBA-15 (calcined) WO3-SBA-15 (Si/W ) 30) WO3-SBA-15 (Si/W ) 15) WO3-SBA-15 (Si/W ) 7.5) WxC-SBA-15 (Si/W ) 30) WxC-SBA-15 (Si/W ) 15) WxC-SBA-15 (Si/W ) 7.5)

115.8 87.2 76.8 65.9 99.5 96.7 80.6

stretching vibration (Si-O-Si)sym of tetrahedral SiO44- for SBA-15.49 For WO3-SBA-15, the band at 966 cm-1 gradually shifts to lower wavenumbers with increasing tungsten content. When Si/W ) 7.5, the band at 966 cm-1 shifts to as low as 945 cm-1. This indicates that Si-O-W bonds form both on the intrachannel walls and in the framework of SBA-15. The formation of Si-O-W is similar to that of Si-O-M in other


Figure 8. HRTEM of WxC-SBA-15 (Si/W ) 15) viewed along pore axis.

mesoporous sieves such as HMS and MCM-41.50-52 After carburization of WO3-SBA-15, the bands at 945 cm-1 for higher tungsten content (Si/W ) 7.5) shift to 954 cm-1, whereas for lower tungsten content (Si/W ) 30 and 15) peak shifts to higher wavenumbers are also observed. This indicates that Si-O-W units on the surface of SBA-15 may be converted to tungsten carbides and Si-O-H units. As far as (Si-O-Si)sym is concerned, in WO3-SBA-15, the band at 810 cm-1 gradually shifts to higher wavenumbers with increasing tungsten content. When Si/W ) 7.5, the band shifts to 816 cm-1. After carburization, the band at 816 cm-1 shifts again to 810 cm-1. It is a clear indication that after carburization the partial transformation of Si-O-W bonds to Si-O-H bonds is confirmed. Especially, in WO3-SBA-15, there is an additional band at 775 cm-1, which is attributed to W-O-W bond formation.53 The intensity of the band increases with increasing tungsten content. Furthermore, remarkable intensity is observed when Si/W ) 7.5. This confirms that more W-O-W bonds form on the intrachannel walls of SBA-15 at higher tungsten content. After carburization, the band at 775 cm-1 disappears, suggesting that W-O-W units have been converted to tungsten carbides. 3.5. TEM Measurements. HRTEM images of WxC-SBA15 (Si/W ) 15) are shown in Figure 8. The hexagonal channels of WxC-SBA-15 (Si/W ) 15) are well defined when viewed along the pore axis or normal to the pore axis (see Figure S7 in Supporting Information). There is a (gray) layer along the (black) SBA-15 wall which can be attributed to the W2C layer consistent with the XRD pattern (Figure 3) and pore size distribution data (Figure S2). The channels of WxC-SBA-15 (Si/W ) 7.5) are distorted in some respects when viewed normal to the pore axis (see Figure S8 in Supporting Information). Thus, it is apparent that the mesoporous structure of SBA-15 is significantly affected by high tungsten content, which is consistent with the XRD pattern (Figure S1) and pore distribution data (Figure S2). 3.6. TG-DSC Measurements. The TG and DSC traces of WO3-SBA-15 with different tungsten content are shown in Figure 9. The weight loss on heating WO3-SBA-15 (Si/W ) 30) from room temperature to 400 °C is ca. 2.5 wt %, whereas


Hu et al.

Figure 9. TG-DSC traces of WO3-SBA-15 samples.

from 400 °C to 1000 °C it is ca. 0.9 wt %. The temperature corresponding to the maximum peak for WO3-SBA-15 (Si/W ) 30) in the DSC trace is 790 °C. The corresponding weight losses for WO3-SBA-15 (Si/W ) 15) are ca. 2.5 wt % and ca. 2.3 wt %. The temperature corresponding to the maximum peak for WO3-SBA-15 (Si/W ) 15) in the DSC trace is 757 °C. In the case of WO3-SBA-15 (Si/W ) 7.5), the weight loss on heating from room temperature to 400 °C is ca. 6.4 wt %, from 400 °C to 730 °C it is ca. 2.3 wt %, and from 730 °C to 1000 °C it is ca. 1.3 wt %. The temperatures corresponding to the maximum peaks for WO3-SBA-15 (Si/W ) 7.5) in the DSC trace are 83, 254, 691, and 788 °C. The former two peaks can be attributed to the loss of H2O at low temperature, and the latter two can be attributed to the decomposition of the two different types of tungsten centers within SBA-15. Thus, the data indicate that WO3-SBA-15 having low tungsten content displays better thermal stability than WO3-SBA-15 with high tungsten content. The TG and DSC traces of WxC-SBA-15 are different from that of WO3-SBA-15 (see Figure S9 in Supporting Information). The weight loss on heating WxC-SBA-15 (Si/W ) 30) from room temperature to 270 °C is ca. 2.1 wt %, between 270 °C to 750 °C there is no significant weight loss, and from 750 °C to 1000 °C a further weight loss of ca. 1.2 wt % is observed. The temperature corresponding to the maximum peak for WxCSBA-15 (Si/W ) 30) in the DSC trace is 767 °C. The total weight loss on heating WxC-SBA-15 (Si/W ) 15) from room temperature to 1000 °C is ca. 4.6 wt %. The temperature corresponding to the maximum peak for WxC-SBA-15 (Si/W ) 15) in the DSC pattern is 747 °C. The total weight loss on heating WxC-SBA-15 (Si/W ) 7.5) from room temperature to 1000 °C is ca. 6.8 wt %. The weight loss on heating WxCSBA-15 (Si/W ) 7.5) from room temperature to 115 °C is ca. 2.2 wt %, from 115 °C to 650 °C it is ca. 3.5 wt %, and from 650 °C to 1000 °C it is ca. 1.1 wt %. The temperature corresponding to the maximum peak for WxC-SBA-15 (Si/W ) 7.5) in the DSC trace is 687 °C. For high tungsten content, the total weight loss on heating is significantly smaller for WxCSBA-15 than that for WO3-SBA-15, whereas is it slightly smaller for low tungsten content. In addition, the temperature corresponding to the maximum peak in the DSC trace is slightly smaller for WxC-SBA-15 than that of WO3-SBA-15 with lower tungsten content. This indicates that the carburization influence

on WO3-SBA-15 with higher tungsten content is significant larger than that with lower tungsten content. 4. Discussions 4.1. Model of Tungsten Distribution in WO3-SBA-15. The results of XRD, N2 adsorption-desorption, FTIR, and NMR measurements all demonstrate that there are three distinct tungsten sites in WO3-SBA-15. The first, inside SBA-15 channels, is denoted as R-W; the second, embedded in the intrachannel surfaces of the SBA-15, is denoted as β-W; the third, in the framework of SBA-15, is denoted as γ-W. The mechanism of formation and distribution of the different tungsten sites are discussed in detail in this section. SBA-15 is synthesized by using neutral nonionic surfactants as templates acting through hydrogen bonding and other electrostatic interactions. In acid media, cationic silica species will be present as precursors, and the assembly process can be expected to proceed through an intermediate of the form S0H+X-I+, where S0 is the neutral nonionic surfactant, X- is a halide anion, and I+ is a protonated Si-OH moiety (SiO+).4 The micellar rods are encased in a layer of SiO+, and subsequent auto-polymerization is expected to form the hexagonal silica structure. When tungsten is present in the acidic medium, WO3SBA-15 is expected to form via an intermediate of the form S0H+X-I+W+, where W+ is a protonated W-OH moiety (WO+). The micellar rods are encased in a layer of SiO+ and WO+, and subsequent auto-polymerization is expected to form the hexagonal silica structure doped with tungsten. A schematic representation of the complete process from micellar rods to ordered WO3-SBA-15 materials with different tungsten contents is shown in Figure 10. The orderly mesoporous structures are still retained at lower tungsten content where a small moiety of tungsten is incorporated into the framework of SBA-15 and a large moiety of tungsten is confined into the channels of SBA-15. In other words, the number of R-W sites are larger than that of β-W sites and γ-W sites. However, the number of β-W sites and γ-W sites at higher tungsten content are significantly increased than at lower tungsten content. The specific sites of R-W, β-W, and γ-W are shown in Figure 11. Incorporation of R-W, β-W, and γ-W has different effects on the structure of SBA-15 because of their different chemical environments. As shown in Figure 4 and Table 2, the meso-



Figure 10. Schematic representation of tungsten in the surface and wall of WO3-SBA-15: (A) low tungsten content and (B) high tungsten content.

porous structures of WO3-SBA-15 vary with composition in some respects, such as the gradual decrease in pore diameter and increase in cell parameter as the tungsten content increases. Formation of WO3 involves R-W sites, and the observed decrease in pore diameter can be attributed to the bonding of R-W with Si sites on the internal surfaces of SBA-15. The formation of framework tungsten sites (γ-W) is responsible for the observed increase in cell parameter since a W-O bond is longer (1.90 Å) than Si-O (1.60 Å). This is also consistent with the observed decrease in the Q3/Q4 ratio with increasing tungsten content in that there are more R-W sites than γ-W sites. As shown in Table 3, at high tungsten content (Si/W ) 7.5), a slight increase in Q2/Q4 ratio is observed. This can be attributed to the presence of β-W sites, which favors the formation of shorter micellar rods and results in a larger number of shorter hexagonal channels; since the Q2 sites are usually found at the entrances to the channels, their number increases with increasing number of β-W sites. The extent of ordering in the mesoporous structure is significantly reduced at higher tungsten content, where a small amount of tungsten is embedded in the intrachannel surfaces of SBA-15 in addition to the majority in the framework and the channels of SBA-15. As shown in Figure 7, incorporation of tungsten into the framework of SBA-15 is demonstrated by the presence of the IR band at 945 cm-1 associated with SiO-W units and an additional band at 775 cm-1 at higher

tungsten content resulting from the presence of W-O-W units; this latter band suggests that some R-W sites are located adjacent to β-W sites giving rise to R-W-O-β-W moieties. The pore structure of WO3-SBA-15 is shown schematically in Figure S10 (see Supporting Information), where the wall is shaded yellow, the WO3 layer is shaded green, and the pores are white. The influence of R-W and γ-W sites on the structure of SBA-15 can be determined by calculation of specific pore parameters, which are defined in eqs 1-3.

Lwall )

2 d∞(100) - D∞(BJH) x3

(1)

Dn(WO3-SBA-15) - Lwall ) dn(100) - d∞(100) Dn(WO3-SBA-15) + 2Dn(WO3) + Dn(BJH) )

(2)

2 dn(100) x3 (3)

In eqs 1-3, Lwall is the wall thickness of parent SBA-15, d∞(100) is the lattice spacing from the (100) peak in parent SBA15, D∞(BJH) is the pore diameter of parent SBA-15 as given by the BJH method, dn(100) is the lattice spacing from the (100) peak in WO3-SBA-15, Dn(WO3-SBA-15) is the wall thickness of WO3-SBA-15, Dn(WO3) is the thickness of the WO3 thin layer on the wall of WO3-SBA-15, Dn(BJH) is the pore diameter


Hu et al.

Figure 11. Schematic representation of R-W, β-W, and γ-W in WO3-SBA-15: (A) low tungsten content and (B) high tungsten content.

of WO3-SBA-15, and 2/x3 dn(100) is the cell parameter of WO3-SBA-15. The change in Dn(WO3-SBA-15) is estimated from the change in dn(100) in eq 2. The thickness of the WO3 thin layer increases gradually with tungsten content while the wall thickness of WO3-SBA-15 increases slightly at lower tungsten content and increases more significantly at higher tungsten content (see Table 2). In WO3-SBA-15 (Si/W ) 7.5), a combination of thicker WO3 layers resulting from incorporation of R-W sites and thicker walls resulting from incorporation of γ-W sites results in a major change in the mesoporous structure, which is consistent with the observation of the H3 hysteresis loop and abnormal Gaussian pore distribution (see Figure 4). 4.2. Mechanism of Formation of Tungsten Carbides in WxC-SBA-15. As shown in Figure 3, the identity of the tungsten carbide phase formed during carburization of WO3-SBA-15 varies with tungsten content, with a single W2C phase being observed at lower tungsten content and a mixture of W2C and WC phases being observed at higher tungsten content. The results of carburization of WO3-SBA-15 are significantly different from those previously reported for powdered WO3 in flowing CH4/H2 (20/80 v/v mixture).54 The carburization temperature decreases significantly from WO3-SBA-15 to WxC-SBA-15. This can be attributed to the high dispersion of tungsten and the combination of tungsten and Si-O-H within SBA-15. The ordered mesoporous structures of SBA-15 are retained in the materials after carburization for which the pore parameters are defined in eqs 1, 4, and 5.

Dn(WxC-SBA-15) - Lwall ) dn(100) - d∞(100) Dn(WxC-SBA-15) + 2Dn(WxC) + Dn(BJH) )

(4)

2 dn(100) x3 (5)

In eqs 4 and 5, dn(100) is the spacing of the (100) peak in WxCSBA-15, Dn(WxC-SBA-15) is the wall thickness of WxC-SBA15, Dn(WxC) is the thickness of the WxC thin layer on the wall of WxC-SBA-15, Dn(BJH) is the pore diameter of WxC-SBA15, and 2/x3 dn(100) is the cell parameter of WxC-SBA-15. The change in Dn(WxC-SBA-15) is estimated from the change in dn(100) in eq 4. Compared with WO3-SBA-15, the cell parameter of WxC-SBA-15 shows a slight increase with the same molar ratio of silicon to tungsten. The thickness of the WxC layer is slightly larger than that of the WO3 layer at lower tungsten content but smaller than that of the WO3 layer at higher tungsten content (see Table 2). This is presumably related to the formation of WC at higher tungsten content and the concomitant significant changes in the mesoporous structure. As shown in Table 2, the thickness of the W2C layer in WxCSBA-15 (Si/W ) 15) is ca. 0.81 nm, and the increase in the cell parameter resulting from incorporation of γ-W into the framework is ca. 0.50 nm. The thickness of the WxC layer in WxC-SBA-15 (Si/W ) 7.5) is ca. 1.24 nm, and the increase in the cell parameter associated with incorporation of γ-W into the framework is ca. 2.96 nm. A mechanism of carburization can be postulated by taking into account the formation of different tungsten sites as follows. The first carburization pathway, shown in Figure 12A, occurs



Figure 12. Schematic representation of carburization pathway from WO3-SBA-15 to WxC-SBA-15: (A) low tungsten content and (B) high tungsten content.

at lower tungsten content. Species A involves R-W sites in the channels of SBA-15. After carburization, R-W is converted into W2C and the R-W sites are replaced by H atoms to form species B. Since there are very few β-W sites in A, no WC is formed (any γ-W sites are not susceptible to carburization). The carburization temperature of WO3-SBA-15 is lower than that of powdered WO3, which is attributed to the confinement effect of SBA-15 and the resulting high dispersion of R-W in the channels of SBA-15. The other carburization pathways, shown in Figure 12B, occur at higher tungsten content. Species C involves β-W sites embedded into the intrachannel surfaces of SBA-15. After carburization, these β-W sites are converted into WC and the β-W is replaced by H atoms to form species D. Species E involves both R-W in the channels and β-W in the intrachannel surfaces of SBA-15 as shown by the presence of the band at 775 cm-1 in the FTIR spectrum (see Figure 7). Carburization involves two steps, where the first step is the transformation of R-W to W2C and the formation of F, and the second step is the transformation of β-W to WC leading to formation of G and H2O. As shown in Table 3, Figure 5, and Figure S6, the NMR results indicate that for these processes the number of Q2 sites is essentially unchanged after carburization; this is because the increased number of Q2 sites associated with the formation of Si-OH is compensated by the decrease associated with the dehydration step. The above conclusions regarding the presence of different tungsten sites in WO3-SBA-15 and the different carburization mechanisms are supported by the TG-DSC data. As shown in Figure 9, the maximum peak in the DSC of WO3-SBA-15 (Si/W

) 30) and WO3-SBA-15 (Si/W ) 15) occurs at 790 °C and 757 °C, respectively. The weight losses for WO3-SBA-15 (Si/W ) 30) show a slight increase as compared with WO3-SBA-15 (Si/W ) 15), which can be attributed to the influence of R-W in that γ-W is believed to be present in the framework of SBA15 and is unchanged during the high temperature. However, the weight losses of WO3-SBA-15 (Si/W ) 30 and 15) are significantly smaller than that of WO3-SBA-15 (Si/W ) 7.5). It can be attributed to the influence of significant increasing β-W sites at higher tungsten content in which a larger number of shorter hexagonal channels form giving rise to the decrease of thermal stability. The presence of two maxima at 788 °C and 691 °C in the DSC of WO3-SBA-15 (Si/W ) 7.5) indicates that there are two different tungsten sites within SBA-15, namely, R-W and β-W. The presence of a single peak at 687 °C in the DSC of WxCSBA-15 (Si/W ) 7.5) suggests that R-W and β-W have been converted into WxC, and the transformation of Si-O-W bonds to Si-OH bonds is therefore also expected. 5. Conclusions Ordered SBA-15 silica containing tungsten oxides and tungsten carbides have been directly synthesized, and their structures have been investigated in detail. All the WO3-SBA-15 and WxCSBA-15 samples have an ordered mesoporous structure in which tungsten is dispersed within SBA-15 in three different types of sites, R-W inside SBA-15 channels, β-W embedded in the intrachannel surfaces of SBA-15, and γ-W inside the framework

15184 J. Phys. Chem. C, Vol. 111, No. 42, 2007 of SBA-15. The different chemical environments of R-W, β-W, and γ-W lead to their having different influences on the mesoporous structure of SBA-15. After carburization, R-WO-Si moieties can be transformed into H-O-Si and R-W is converted into thin layer of W2C whose thickness gradually increases with increasing tungsten content. The number of γ-W sites in the framework increases slightly with tungsten content at lower values and increases more significantly at higher tungsten content. The β-W sites are only present in significant quantities at higher tungsten content, since a WC phase is present in WxC-SBA-15 (Si/W ) 7.5) but is absent in WxC-SBA-15 (Si/W ) 30 or 15). The high degree of dispersion of tungsten within SBA-15 and the confinement effect of the SBA-15 channels as well as the interaction between tungsten and hydroxyl groups result in a decrease in carburization temperature compared with that of powdered WO3. Acknowledgment. Financial support by the National Natural Science Foundation of China (Grant No. 20473009) and the National Basic Research Program of China (973 Program) (Grant No. 2006CB202503) is gratefully acknowledged. Supporting Information Available: Low-angle XRD patterns of WxC-SBA-15 samples, nitrogen sorption isotherms and pore size distribution of WxC-SBA-15 samples, 29Si NMR MAS spectra of SBA-15 (as-synthesized) and SBA-15 (calcined), fitting of 29Si NMR MAS spectra of SBA-15 (A) as-synthesized (B) (calcined), fitting of 29Si NMR MAS spectra of WO3-SBA15 samples (as-synthesized), fitting of 29Si NMR MAS spectra of WxC-SBA-15 samples, HRTEM of WxC-SBA-15 (Si/W ) 15) viewed normal to pore axis, HRTEM of WxC-SBA-15 (Si/W ) 7.5) viewed normal to pore axis, TG-DSC traces of WxCSBA-15 samples, and schematic representation of the pore of WO3-SBA-15. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. T.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (5) Schuth, F.; Schmidt, W. AdV. Eng. Mater. 2002, 4, 269. (6) Wight, A. P.; Davis, M. E. Chem. ReV. 2002, 102, 3589. (7) Stein, A. AdV. Mater. 2003, 15, 763. (8) Rolison, D. R. Science 2003, 299, 1698. (9) Wang, X.; Landau, M. V.; Rotter, H.; Vradman, L.; Wolfson, A.; Erenburg, A. J. Catal. 2004, 222, 565. (10) Raja, R.; Sankar G.; Hermans, S.; Shephard, D. S.; Bromley, S.; Thomas, J. M.; Johnson, B. F. G. Chem. Commun. 1999, 1571. (11) Inumaru, K., Kasahara, T.; Yasui, M.; Yamanaka, S. Chem. Commun. 2005, 2131. (12) Zhang, W. H.; Lu, J.; Han, B.; Li, M.; Xiu, J.; Ying, P.; Li, C. Chem. Mater. 2002, 14, 3413. (13) Yue, Y. H.; Ge´de´on, A.; Bonardet J. L.; Melosh, N.; Espinose, J. B.; Fraissard, J. Chem. Commun. 1999, 1967. (14) Vinu, A.; Murugesan, V.; Bohlmann, W.; Hartmann, M. J. Phys. Chem. B 2004, 108, 11496.

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Direct Synthesis and Structural Characteristics of Ordered SBA-15

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