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Mesostructured SBA-16 with excellent hydrothermal, thermal and mechanical stabilities: Modified synthesis and its catalytic application
Journal of Colloid and Interface Science 333 (2009) 317–323
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Mesostructured SBA-16 with excellent hydrothermal, thermal and mechanical stabilities: Modified synthesis and its catalytic application Hui Sun, Qinghu Tang, Yu Du, Xianbin Liu, Yuan Chen, Yanhui Yang ∗ School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 28 August 2008 Accepted 30 January 2009 Available online 6 February 2009
We report a modified method to synthesize SBA-16 mesostructured silica under refluxing condition using block co-polymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (F127) as template, cetyltrimethylammonium bromide (CTAB) as co-template, and tetraethyl orthosilicate (TEOS) as silica source. The physiochemical properties of SBA-16 silica were characterized by X-ray diffraction (XRD), nitrogen physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and 29 Si solid-state nuclear magnetic resonance (NMR). The resulting SBA-16 silica exhibited highly ordered mesoporous structure, mono-dispersed spherical morphology, excellent hydrothermal, thermal and mechanical stabilities. It was worth mentioning that the synthesis time can be significantly reduced from 48 h to 8 h, which opened a feasible way to produce SBA-16 silica in a large scale. Moreover, the “super-cage” pore structure of SBA-16 encapsulated gold nanoparticles in a “ship in a bottle” way. The well-confined gold nanoparticles (mean size of 5 nm) with a narrow particle size distribution were highly active in solvent-free benzyl alcohol selective oxidation with molecular oxygen. © 2009 Elsevier Inc. All rights reserved.
Keywords: SBA-16 CTAB Refluxing Stability Au nanoparticles Benzyl alcohol oxidation
1. Introduction Studies on ordered mesoporous materials have attracted extensive attentions since the discovery of M41S family mesoporous silicas [1,2]. Zhao et al. firstly reported the synthesis of a variety of SBA-type mesoporous silicas using non-ionic block copolymers as template [3,4]. Formation of SBA-type mesoporous silica materials usually occurs in a low-pH solution where the interaction between template and silica precursor proceeds through the S0 H+ X− I+ mechanism [5]. SBA-type mesoporous silicas exhibit large pore size, thick pore wall, and high stability. The important applications of SBA-type silicas include catalyst support [6], template for nanostructure synthesis [7], materials to immobilize or adsorb the biomolecules [8], sensors [9,10]. Among these SBA-type silica materials, SBA-16 is considered to be the most interesting mesostructure; it has a 3D cubic arrangement of mesopores corresponding to the Im3m space group. However, limited SBA-16 synthesis methods were reported because the cage-like SBA-16 mesostructured silica can only be produced in a narrow window of synthesis parameters [4,11–16]. SBA-16 single crystal (particle size ∼1 μm) was obtained under static condition by Yu et al. using block co-polymer F108 as template in the presence of K2 SO4 and HCl [11,12]. Kleitz et al. reported the syn-
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Corresponding author. Fax: +65 6794 7553. E-mail address: [email protected] (Y. Yang).
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thesis of SBA-16 silicas using F127–butanol–H2 O mixture at low HCl concentration [13]. It was reported that the morphology and the particle size distribution are mainly determined by the regularity and the covering degree of the micelles by the silica species, poly-condensing degree (acidity and temperature), and the Brownian movements (temperature) at the moment of the precipitation. The regularity of the structural arrangement and the porosity properties of the final mesoporous silica are likely also related to the same parameters [17]. Recently, SBA-16 has been prepared in the case where CTAB was added as co-template [14–16], CTAB was suggested to help control the morphology and regulate the shape of SBA-16 mesostructure. For instance, Guth et al. [15] proposed that the presence of cationic CTAB can regulate the shape of micelles and their interaction with the silica precursors in the SBA-16 synthesis. An important issue which makes the practical application of SBA-16 material severely hampered is the long synthesis time (usually several days) because it is unfavorable to the large scale production. Enlightening and meaningful update of SBA-16 synthesis with short preparation time is highly desired. Regev et al. [18] observed using Cyro-TEM that the packed elongated micelles of ionic cetyltrimethylammonium chloride (CTAC) were arranged in small and ordered micro-domains after 3 min of mixing at room temperature. Lin et al. [14] reported the synthesis of SBA-16 crystals in a rhombdodecahedronal shape using CTAB–C12 H25 SO3 Na (SDS)–F127 ternary mixture as the template; they suggested that
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Scheme 1. Amine functionalization of SBA-16 and preparation of Au/SBA-16.
ionic surfactant CTAB and SDS acted preferentially as micellization promoters for the F127 block co-polymer. Inspired by these studies, it is speculated that CTAB may facilitate the synthesis of SBA-16 and shorten the synthesis time (accelerate the formation of ordered template of block co-polymer) due to its ionic characteristics. In this study, we report a facile approach to synthesize mesostructured SBA-16 silica material under refluxing condition using F127 as template in the presence of CTAB as co-template. The hydrothermal, thermal and mechanical stabilities of prepared SBA-16 silica are experimentally compared with SBA-16 synthesized by the conventional method. Shortening the synthesis time using this new approach will be discussed in detail. Moreover, the synthesized SBA-16 is used to stabilize gold nanoparticles, and the catalytic application is demonstrated in the solvent-free benzyl alcohol selective oxidation using molecular oxygen. 2. Materials and methods 2.1. Synthesis The synthesis of SBA-16 was carried out under refluxing condition as it was suggested that refluxing can decrease the concentration and temperature gradients in the synthesis mixture. SBA-16 was prepared using triblock co-polymer Pluronic F127 (Sigma-Aldrich) as template, CTAB (Sigma-Aldrich) as co-template, tetraethyl orthosilicate (TEOS, Sigma-Aldrich) as silica precursor. The preparation procedures were as follow: 0.63 g F127 and 0.073 g CTAB were completely dissolved into 71.28 g of 2 M HCl solution, followed by adding 2.083 g TEOS under strong magnetic stirring. The molar ratio was TEOS/F127/CTAB/HCl/H2 O = 1:0.005:0.02:14.3:495. The mixture was transferred into a 250 mL round bottle flask and kept stirring under refluxing condition at 40 ◦ C for 1–6 h, and another 6 h at elevated temperature of 80 ◦ C. The final products were collected by centrifugation and dried at 80 ◦ C overnight. The surfactant was removed by calcination at 550 ◦ C for 6 h. In comparison, conventional SBA-16 (denote as SBA-16-cv) was synthesized under static condition according to the procedure reported elsewhere [13]. The thermal and hydrothermal stabilities of calcined SBA-16 were tested by calcining the sample at 800 ◦ C for 6 h in air flow and boiling for 72 h in distilled water, respectively. Calcined SBA-16 samples were pressed by carver press to test their mechanical stability, the pressure was regulated between 3.15 and 13.5 ton, pressing time was fixed at 10 min. For simplicity, SBA-16 synthesized by this modified method is denoted as SBA-16-xx/yy, e.g., SBA-16-02/06 represents SBA-16 synthesized with self-assembly time of 2 h and aging time of 6 h. For comparison, MCM-41 and SBA-15 were synthesized following the methods reported by Chi et al. [19]. Mesoporous silica supported gold catalysts were prepared by a procedure using surface-functionalized mesoporous silica to adsorb the gold precursor (see Scheme 1). In a typical preparation exemplified by Au/SBA-16-02/06, 1.0 g of SBA-16-02/06 sample was suspended in 30 mL of toluene solution containing 1.0% (3-aminopropyl) triethoxysilane (APS) and refluxed at 110 ◦ C for 5 h. The resulting materials were filtered off, washed with toluene, and then dried at 80 ◦ C to remove the remaining solvent. The amine functionalized SBA-16 sample was denoted as APS/SBA-16-02/06. Im-
mobilization of gold nanoparticles was conducted by adding 1.0 g of APS/SBA-16-02/06 to 100 mL 10−3 M HAuCl4 aqueous solution, followed by stirring at 80 ◦ C for 5 h. The Au/SBA-16-02/06 sample was obtained by filtering and washing with deionized water, followed by drying at 80 ◦ C overnight. The final gold loading was measured by ICP. 2.2. Characterization Powder X-ray diffraction patterns were recorded with a Bruker AXS D86 diffractometer (under ambient conditions) using filtered CuK α radiation. Diffraction data were collected between 0.5◦ and 8◦ (2θ ) with a resolution of 0.02◦ (2θ ). Nitrogen physisorption isotherms were measured at −196 ◦ C with a static volumetric instrument Autosorb-6b (Quanta Chrome). Prior to each measurement, the sample was degassed at 250 ◦ C to a residual pressure below 10−4 Torr. A Baratron pressure transducer (0.001– 10 Torr) was used for low-pressure measurements. Brunauer– Emmett–Teller (BET) method was adopted to estimate the specific surface area. Barrett–Joyner–Halenda (BJH) method was used to calculate the pore size distribution using the desorption branch. The Scanning Electron Microscope (SEM) image was obtained with a JEOL Field Emission Scanning Electron Microscope (JSM-6700FFESEM). Prior to the analysis, the samples were deposited on a sample holder using an adhesive carbon tape and then sputtered with gold. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010, operated at 200 kV. The samples were suspended in ethanol and dried on holey carbon-coated Cu grids. 29 Si solid-state NMR experiments were performed on a Bruker DSX300 spectrometer with a frequency of 59.63 MHz, a recycling delay of 600 s, a radiation frequency intensity of 62.5 kHz, and the reference sample of Si8 O12 . Diffuse reflectance UV–visible (DR UV– vis) spectra were recorded with a Varian–Cary 5000 spectrometer equipped with a diffuse reflectance accessory. The spectra were collected between 200–800 nm at room temperature with BaSO4 as a reference. 2.3. Catalytic reaction The solvent-free oxidation of benzyl alcohol with molecular O2 was carried out using a bath-type reactor operated under atmospheric condition. In a typical reaction run, 200 mg of catalyst was loaded to a glass flask pre-charged with 10 mL (98.5 mmol) benzyl alcohol, the mixture was then heated to the reaction temperature (160 ◦ C) under vigorous stirring. Oxygen flow was bubbled at a flow rate of 20 mL min−1 into the mixture to start the reaction once the reaction temperature was reached. After reaction, the solid catalyst was filtered off and the liquid organic products were analyzed by an Agilent gas chromatograph 6890 equipped with a HP-5 capillary column. Dodecane was the internal standard to calculate benzyl alcohol conversion and benzaldehyde selectivity. 3. Results and discussion Powder XRD patterns of calcined SBA-16 mesoporous silicas prepared by this new method are depicted in Fig. 1, showing
H. Sun et al. / Journal of Colloid and Interface Science 333 (2009) 317–323
Fig. 1. X-ray diffraction patterns of SBA-16, the molar ratio of F127 to TEOS is: (a) 0.001, (b) 0.003, (c) 0.004, (d) 0.005, (e) 0.006, (f) 0.007.
Fig. 2. Powder XRD patterns of calcined SBA-16 with different CTAB/F127 molar ratio: (a) 2, (b) 4, (c) 6, (d) 8.
the effect of F127 content on the mesostructured SBA-16 (selfassembly and aging of 24 h and 24 h, respectively). It is clearly observed that the best synthesis of cubic SBA-16 structure occurs under an optimized F127/TEOS molar ratio of 0.005. No noticeable diffraction peak is observed when the F127/TEOS molar ratio is as low as 0.001. When the F127/TEOS molar ratio increases to 0.003, only one weak diffraction peak centered at 2θ = 0.9◦ appears, indicating a poorly ordered mesostructure. As the ratio increases to 0.005, the sample exhibits one strong diffraction peak and two higher angle peaks in the range of 2θ = 0.8–1.8◦ , assigned to (110), (200) and (211) planes. The FWHM (full width at half maximum) of the (110) peak is about 0.2◦ , indicating the particularly well developed grains of mesostructured silica. When the F127/TEOS molar ratio increases to 0.006, the XRD pattern becomes less resolved; the diffraction peaks are diminished when the ratio is 0.007. As suggested by Kleitz et al. [13], the amount of TEOS added to the system greatly influences the nature of mesophase formed. The Im3m phase is the dominant phase in the presence of optimized amount of TEOS. A lower TEOS to surfactant ratio results in the formation of Fm3m silica mesophase or disordered phase. This fact may be rationalized by pronounced differences in the F127/TEOS ratio in the silica–block co-polymer hybrid mesophase and the repartition of the inorganic and organic domains. Powder XRD patterns of calcined SBA-16 mesoporous silicas with different CTAB/F127 molar ratio are shown in Fig. 2. The content of F127 is fixed at the optimized F127/TEOS molar ratio of
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Fig. 3. X-ray diffraction patterns of SBA-16 of different self-assembly time: (a) SBA16-01/06 (b) SBA-16-1.5/06 (c) SBA-16-02/06 (d) SBA-16-06/06 (e) SBA-16-02/06 without addition of CTAB.
0.005 with various amount of CTAB. XRD pattern of SBA-16 with an optimized CTAB/F127 molar ratio of 4 exhibits well resolved diffraction peaks assignable as (110), (200), and (211) diffractions, implying the well defined long range order cubic mesostructure. When the CTAB/F127 molar ratio is larger, poorly ordered mesostructure is obtained. As the CTAB/F127 molar ratio increases to 8, the sample does not show any discernable XRD diffraction peak in the range of 0.8–8◦ , indicating the amorphous nature of resulting silica material. Synthesis time is an important factor to be concerned in this modified synthesis method. The synthesis of SBA-16 by conventional approach takes approximately 2 days. In this new method, CTAB is added as the co-template, aiming at accelerating the formation of F127 micelle because of CTAB’s ionic characteristics. SBA-16 samples were synthesized under a fixed aging time of 6 h with various self-assembly time; F127/TEOS and CTAB/F127 molar ratios were regulated as 0.005 and 4, respectively. Fig. 3 shows the XRD patterns of SBA-16 mesostructured silicas prepared under different self-assembly time. SBA-16-01/06 sample exhibits only one weak diffraction peak, indicating the mesostructure is poorly constructed within 1 h of self-assembly time. When the self-assembly time increases to 1.5 h, one strong diffraction peak (110) and one weak peak (200) demonstrate the emerging of a good mesoporous structure. Sample SBA-16-02/06 displays three strong and well-resolved (110), (200) and (211) diffraction peaks of Im3m symmetry, suggesting 2 h suffices the self-assembly during the synthesis of a well ordered 3D SBA-16 mesostructured silica. Longer self-assembly time (6 h) gives better mesoporous structure of SBA-16-06/06. The N2 adsorption–desorption isotherms and pore size distributions of SBA-16 prepared with various self-assembly time are shown in Fig. 4. Type IV hysteresis loop of SBA-16-01/06 is poorly formed with the absence of any sharp condensation step. For the sample of SBA-16-1.5/06, N2 isotherms form a type IV hysteresis loop with a sharp capillary condensation step on desorption branch at P / P 0 = 0.45, it implys a good mesoporous structure. SBA-16-02/06 also exhibits a type IV isotherm with sharper capillary condensation step, which are typical features of highly ordered Im3m mesostructure. All the samples exhibit narrow pore size distribution centered at approximately 3.5 nm. The d-spacing, BET surface area, pore size, and pore volume of various SBA-16 samples are summarized in Table 1. The SBA-16 sample synthesized by the conventional method shows a d-spacing of 9.0, BET surface area of 1203 m2 g−1 , and pore volume of 1.35 cm3 g−1 , which is in good agreement with the results reported elsewhere [13]. Although
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Fig. 4. (Left) N2 adsorption/desorption isotherms, (right) pore size distributions calculated from the desorption branch by BJH method: (a) SBA-16-01/06, (b) SBA-16-1.5/06, (c) SBA-16-02/06, (d) SBA-16-06/06.
Table 1 Textural parameters of various SBA-16 samples.a Sample
d-spacing (nm)
S BET (m2 g−1 )
D (nm)
Vp (cm3 g−1 )
a0 (nm)
t (nm)
SBA-16-cv SBA-16-01/06 SBA-16-1.5/06 SBA-16-02/06 SBA-16-06/06
9.01 9.04 8.97 8.95 9.02
1203 691 994 1114 1120
3.42 3.55 3.52 3.41 3.45
1.350 0.701 0.944 1.342 1.356
12.74 12.78 12.69 12.66 12.76
7.61 7.51 7.47 7.55 7.60
SBA-16-cv-hyb SBA-16-02/06-hyb SBA-16-cv-thb SBA-16-02/06-thb
/ 9.02 8.55 8.51
285 527 300 551
3.52 3.51 3.34 3.43
0.482 0.634 0.292 0.386
/ 12.76 12.09 12.03
/ 7.54 7.13 6.99
SBA-16-cv-3.15ton SBA-16-cv-4.17ton SBA-16-cv-10ton SBA-16-cv-13.5ton SBA-16-02/06-3.15ton SBA-16-02/06-4.17ton SBA-16-02/06-10ton SBA-16-02/06-13.5ton
9.52 9.21 9.20 / 8.83 8.82 8.81 8.53
540 446 370 317 1008 769 577 551
3.51 3.53 3.44 3.41 3.37 3.43 3.36 3.25
0.682 0.384 0.362 0.298 0.711 0.554 0.425 0.382
13.46 13.02 13.01 / 12.52 12.47 12.46 12.06
8.15 7.75 7.83 / 7.47 7.37 7.43 7.19
a d-spacing, BET surface area (S BET ), pore diameter (D), pore volume (V p ), unit cell parameter (a0 ) and pore wall thickness (t) of various SBA-16 samples. b “-hy” represents SBA-16 after hydrothermal treatment and “-th” represents SBA16 after thermal treatment. a0 = 21/2 d110 , pore wall thickness t = 31/2 a0 /2 − D.
the d-spacing of SBA-16-01/06 is closed to that of SBA-16-cv, the BET surface area (691 m2 g−1 ) and pore volume (0.70 cm3 g−1 ) are lower than those of SBA-16-cv. With an increase of the selfassembly time, both surface area and pore volume remarkably increase. When the self-assembly time is 2 h, the surface area and pore volume are 1114 m2 g−1 and 1.34 cm3 g−1 , respectively. Thus, the textural properties of SBA-16-02/06 by this new method are comparable to those of SBA-16 by conventional synthesis. It should be noted the current method shortens the synthesis time by a factor of 6 (from 48 h to 8 h). Although SBA-16-06/06 shows a XRD pattern superior to all samples, SBA-16-02/06 is chosen to continue the study because the main focus of this new method is to explore the possibility reducing the synthesis time, SBA-16-02/06 shows comparable properties (as shown in Table 1) compared to SBA-16-06/06 but shorter preparation time.
Fig. 5. SEM image (left) and TEM image (right) of SBA-16-02/06 showing the monodispersed spherical morphology and characteristic planes for a cubic pore structure: (a, b) [111], (c, d) [100].
Scanning electron microscope (SEM) image of as-synthesized SBA-16-02/06 is shown in Fig. 5(left). The particles exhibit monodispersed spherical morphology with diameter ranging from 0.8– 1.2 μm. The mesoporous structure of SBA-16-02/06 was characterized using transmission electron microscopy (TEM). Typical micrographs of the as-synthesized particle are shown in Fig. 5(right). Well-ordered cubic pore structure of ca. 5–6 nm in diameter is clearly observed, which is in a good agreement with the analysis of XRD and N2 physisorption. Results in Fig. 3 suggest that CTAB plays an important role to facilitate the synthesis of SBA-16 within a short time. The role of CTAB was further verified by preparing SBA-16-02/06 in the absence of CTAB (see Fig. 3e). No diffraction peak is observed which indicates amorphous silica characteristic without any long range order structure. It is proposed that cationic surfactant CTAB may enhance the micellization of F127 template by adjusting the conformation of the hydrophilic PEO chains. There can be few hydrogen bonds interacting between F127 micelle and silica precursors without changing the dimension of Im3m mesostructure of SBA-16. In this study, hydrothermal, thermal, and mechanical stability tests were carried out to compare the SBA-16-02/06 by this modified method and SBA-16 by conventional approach. The XRD pat-
H. Sun et al. / Journal of Colloid and Interface Science 333 (2009) 317–323
terns are shown in Fig. 6. Herein, “-hy” represents hydrothermal treatment, “-th” represents thermal treatment. After hydrothermal treatment, SBA-16-cv does not exhibit any diffraction peak, suggesting the mesoporous structure is completely destroyed. It is worth mentioning that the XRD pattern of SBA-16-02/06 remains a strong diffraction peak indexed to (110) plane after severe hydrothermal treatment, indicating the excellent hydrothermal stability of SBA-16-02/06. As shown in Table 1, the surface area of SBA-16-cv after hydrothermal treatment decreases to 285 m2 g−1 , and that of SBA-16-02/06 decreases to 527 m2 g−1 . Only one weak diffraction peak associated with (110) reflection plane is resolved for SBA-16-cv after thermal test. SBA-16-02/06 is able to maintain a better mesostructure as evidenced by the presence of one strong diffraction peak corresponding to (110) reflection plane. Moreover, both surface area and pore volume of SBA-16-02/06 after thermal test are much higher than those of SBA-16-cv (Table 1). Thus, it is clear that SBA-16 from this new approach possesses higher hydrothermal and thermal stabilities than SBA-16 by conventional approach. Although shortening synthesis time has no considerable effect on the order of SBA-16 mesostructure, it is speculated whether the silica local chemical environment is affected. Solid-state nu-
Fig. 6. XRD patterns for SBA-16 after hydrothermal and thermal tests: (a) SBA-16cv-hy, (b) SBA-16-02/06-hy, (c) SBA-16-cv-th, (d) SBA-16-02/06-th.
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clear magnetic resonance (NMR) spectroscopy has become a powerful tool for structural characterization of mesoporous material and other catalytic materials; it detects the inequivalent environments among the silicon sites. Mesoporous silicas are generally characterized by the presence of silicon sites Q 2 , Q 3 , and Q 4 , representing the species of Si(OSi)2 (OH)2 , Si(OSi)3 (OH), and Si(OSi)4 at −90, −102 and −110 ppm chemical shifts, respectively. 29 Si solidstate NMR spectra of SBA-16-02/06 (before and after thermal and hydrothermal treatment) and SBA-16-cv (before and after thermal and hydrothermal treatment) are shown in Fig. 7. Three bands associated with Q 2 , Q 3 and Q 4 Si species for fresh SBA-16-02/06 are similar with those of SBA-16 synthesized by conventional method; there is no significant change in silica local environment when the synthesis time is shortened from 48 h to 8 h by this new synthesis method. After hydrothermal treatment, the peak associated with Q 2 vanishes completely, the peak associated with Q 3 almost can not be discerned, and the intensity of the peak associated with Q 4 remarkably increases, which are analogous with those of SBA-16 synthesized by conventional method after hydrothermal treatment. Only one peak associated with Q 4 Si species can be observed in both SBA-16-02/06 and SBA-16-cv after thermal treatment, which is due to the condensation of Si(OSi)2 (OH)2 and Si(OSi)3 (OH) to Si(OSi)4 during the thermal test. Furthermore, mechanical stability is also concerned and the mechanical pressure test results are shown in Fig. 8. Without applying any press, three well-resolved diffraction peaks indexed to (110), (200) and (211) planes of SBA-16-02/06 characterize a highly ordered Im3m mesoporous structure (Fig. 8(left)). Under low pressure, these three diffraction peaks are well conserved. The SBA-1602/06 sample shows one diffraction peak centered at 2θ = 1.02◦ when the pressure increases to 10.0 ton. The (110) diffraction peak slightly changes as the pressure further increases to 13.5 ton, suggesting ordered mesostructure can be preserved under pressure up to 13.5 ton. As a comparison, conventional SBA-16 was also tested under mechanical pressure (Fig. 8(right)). SBA-16-cv loses its ordered mesoporous structure with the absence of any diffraction peaks under pressure of 13.5 ton. The textural parameters of both samples with an increase of mechanical pressure are also compared in Table 1. Under a pressure of 3.15 ton, the surface area and pore volume of SBA-16-02/06 are 1008 m2 g−1 and 0.71 cm3 g−1 , respectively, whereas, the surface area and pore volume of SBA16-cv under this pressure are 540 m2 g−1 and 0.6 cm3 g−1 , respec-
Fig. 7. (Left) Solid-state MNR spectra of SBA-16-02/06: (a) fresh, (b) after hydrothermal treatment, (c) after thermal treatment. (Right) Solid-state MNR spectra of SBA-16-cv: (a) fresh, (b) after hydrothermal treatment, (c) after thermal treatment.
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Fig. 8. XRD pattern of calcined SBA-16-02/06 (left) and SBA-16-cv (right) under mechanical press at: (a) 0, (b) 3.15 ton, (c) 4.74 ton, (d) 10.0 ton, (e) 13.5 ton. Table 2 Results of benzyl alcohol oxidation over Au-containing mesoporous silica catalysts.a Catalyst
Pretreatment
TOF (h−1 )
Conversion (%)
Benzyldehyde selectivity (%)
SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-cv Au/SBA-15 Au/MCM-41
– Air, Air, H2 , H2 , H2 , H2 , H2 ,
–
<1.0 6 .3 3 .8 13.0 19.0 19.3 16.1 6.5
>99 >99 >99 >99 >99 >99 >99
80 ◦ C 540 ◦ C 200 ◦ C 400 ◦ C 400 ◦ C 400 ◦ C 400 ◦ C
803 484 1655 2419 2457 2048 827
–
a Reaction conditions in this work: the amount of Au in each catalyst 2.5 × 10−3 mmol; benzyl alcohol 10 mL (98.5 mmol), 160 ◦ C, O2 flow rate 20 mL min−1 , reaction time 3 h.
Fig. 10. TEM micrographs of Au-containing mesoporous silica samples as well as the corresponding Au particle size distributions: (a) Au/SBA-16-02/06, (b) Au/SBA-15, (c) Au/MCM-41.
Fig. 9. UV–vis spectra of Au-containing mesoporous silica samples: (a) Au/SBA-1602/06, (b) Au/SBA-15, (c) Au/MCM-41.
tively. With an increase of pressure, surface area and pore volume of both SBA-16-02/06 and SBA-16-cv decrease gradually, but the surface area and pore volume of SBA-16-02/06 under the same pressure are always higher than those of SBA-16-cv. Thus, SBA16-02/06 by this new method exhibits higher mechanical stability than that of the conventional SBA-16, although the reason remains unclear. The catalytic results of solvent-free benzyl alcohol selective oxidation with molecular O2 over mesoporous silica supported gold catalysts pretreated under different conditions are listed in Table 2.
The best catalytic performance is obtained over the SBA-16-cv support pretreated in a H2 flow at 400 ◦ C, benzyl alcohol conversion and TOF are 19.3% and 2457 h−1 , respectively. Almost the same benzyl alcohol conversion and TOF are obtained on Au/SBA-1602/06 catalyst (19.0% and 2419 h−1 , respectively); this further suggests that the properties of SBA-16-02/06 by this new method are comparable to those of SBA-16 by conventional synthesis. The catalytic performances of Au/SBA-15 and Au/MCM-41 catalysts with the same Au content (∼1 wt%) were also investigated for comparison. Although both Au/SBA-15 and Au/MCM-41 can catalyze the benzyl alcohol oxidation at 160 ◦ C, forming benzaldehyde as the main product, both benzyl alcohol conversion and TOF are lower than those of Au/SBA-16. Fig. 9 shows the UV–vis spectra of Au-containing mesoporous silica catalysts pretreated in a H2 flow at 400 ◦ C. All samples show the characteristic surface plasmon resonance absorption peak of Au nanoparticles at around 520 nm. Compared to Au/SBA-16, Au/MCM-41 and Au/SBA-15 exhibit a slightly red-shifted and narrower absorption band, which indicates a larger mean particle size and more particle aggregation [20]. To examine the Au nanoparticle size, the direct TEM microscopic observation was performed. As shown in Fig. 10, we can discern a uniform distribution of Au nanoparticles in the Au/SBA-
H. Sun et al. / Journal of Colloid and Interface Science 333 (2009) 317–323
16 catalyst. The particles are mostly observed in the size range of 2–9 nm, with a maximum of size distribution at 5 nm. Particles larger than ca. 7 nm are hardly seen in this sample. Considering the channel structure of SBA-16, it is reasonable to conclude that most of Au nanoparticles are confined inside the cage of SBA-16 structure. On the other hand, relatively large Au nanoparticles with sizes more than ca. 7 nm can be observed in Au/SBA-15 and Au/MCM-41 catalysts. Gold nanoparticles are not well confined in the pore structure of SBA-15 and MCM-41; many large particles may appear outside of the pores. Moreover, the size distributions of Au particles over SBA-15 and MCM-41 are much broader than that of SBA-16. We note that TEM micrographs of Au-containing mesoporous silica samples as well as the corresponding Au particle size distributions are different from the literature [19]. According to Chi et al. [19], the average sizes of the Au nanoparticles follow the order: MCM-41 (5.1 nm) < SBA-15 (5.8 nm) < MCM48 (6.9 nm). The difference of Au nanoparticle size distribution over SBA-16, MCM-41 and SBA-15 supports in our study may be aroused from the different synthesis method used in this work. The pretreatment of the gold catalyst may facilitate the migration of Au nanoparticles originally encapsulated in the channels, which accounts for the wider distributions of Au nanoparticles in MCM-41 and SBA-15 2D mesoporous silicas. On the other hand, a uniform distribution of Au nanoparticles with a mean size of 5 nm can be obtained in SBA-16. Thus, it is clear that the “supercage” porosity of SBA-16 is superior to MCM-41 and SBA-15 for confining Au nanoparticles. Combined the catalytic performances of Au-containing samples discussed above, it is suggested that the Au nanoparticles with a mean size of 5 nm confined within the SBA-16 cage are more active for benzyl alcohol oxidation with molecular O2 . 4. Summary Highly ordered mesoporous silica SBA-16 was successfully synthesized by a facile method under refluxing condition in the presence of CTAB as co-template. The synthesis time was reduced from 48 h conventionally used to 8 h. It was proposed that CTAB
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plays a crucial role that enhances the micellization of F127 template. SBA-16 by this synthesis method showed high hydrothermal, thermal, and mechanical stabilities. The unique pore structure of SBA-16 can encapsulate Au nanoparticles; the well confined Au nanoparticles with a mean size of 5 nm were highly active in solvent-free benzyl alcohol selective oxidation with molecular O2 . Acknowledgments We are grateful to AcRF tier 1 (RG45/06) for financial support. The authors also thank AcRF tier 2 (ARC 13/07) for funding support. References [1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1999) 10834. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuil, J.S. Beck, Nature 359 (1992) 710. [3] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrckson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [4] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [5] Q. Huo, D.I. Margolese, U. Clesia, P. Feng, T.E. Gler, P. Sleger, R. Leon, P.M. Petroff, F. Schuth, G.D. Stucky, Nature 368 (1994) 317. [6] S.E. Dapurkar, A. Sakthivela, P. Selvam, New J. Chem. 27 (2003) 1184. [7] B. Tian, X. Liu, H. Yang, S. Xie, C. Yu, B. Tu, D. Zhao, Adv. Mater. 15 (2003) 1370. [8] Y.-J. Han, J.T. Watson, G.D. Stucky, A. Butler, J. Mol. Catal. B 17 (2002) 1. [9] G. Wirnsberger, B.J. Scott, G.D. Stucky, Chem. Commun. 3 (2003) 2140. [10] T. Yamada, H.S. Zhou, H. Uchida, M. Tomita, Y. Ueno, I. Honma, K. Asai, T. Katsube, Microporous Mesoporous Mater. 54 (2002) 269. [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
C. Yu, B. Tian, J. Fan, G.D. Stucky, D. Zhao, J. Am. Chem. Soc. 124 (2002) 4556. C. Yu, J. Fan, B. Tian, D. Zhao, Chem. Mater. 16 (2004) 889. F. Kleitz, T.-W. Kim, R. Ryoo, Langmuir 22 (2006) 440. C.-L. Lin, Y.-S. Pang, M.-C. Chao, B.-C. Chen, H.-P. Lin, C.-Y. Tang, C.-Y. Lin, J. Phys. Chem. Solids 69 (2008) 415. M. Mesa, L. Sierra, J. Patarin, J.L. Guth, Solid State Sci. 7 (2005) 990. B.-C. Chen, M.-C. Chao, H.-P. Lin, C.-Y. Mou, Microporous Mesoporous Mater. 81 (2005) 241. M. Mesa, L. Sierra, J.-L. Guth, Microporous Mesoporous Mater. 112 (2008) 338. O. Regev, Langmuir 12 (1996) 4940. Y.-S. Chi, H.P. Lin, C.-Y. Mou, Appl. Catal. A 284 (2005) 199. S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 4212.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Mesostructured SBA-16 with excellent hydrothermal, thermal and mechanical stabilities: Modified synthesis and its catalytic application Hui Sun, Qinghu Tang, Yu Du, Xianbin Liu, Yuan Chen, Yanhui Yang ∗ School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 28 August 2008 Accepted 30 January 2009 Available online 6 February 2009
We report a modified method to synthesize SBA-16 mesostructured silica under refluxing condition using block co-polymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (F127) as template, cetyltrimethylammonium bromide (CTAB) as co-template, and tetraethyl orthosilicate (TEOS) as silica source. The physiochemical properties of SBA-16 silica were characterized by X-ray diffraction (XRD), nitrogen physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and 29 Si solid-state nuclear magnetic resonance (NMR). The resulting SBA-16 silica exhibited highly ordered mesoporous structure, mono-dispersed spherical morphology, excellent hydrothermal, thermal and mechanical stabilities. It was worth mentioning that the synthesis time can be significantly reduced from 48 h to 8 h, which opened a feasible way to produce SBA-16 silica in a large scale. Moreover, the “super-cage” pore structure of SBA-16 encapsulated gold nanoparticles in a “ship in a bottle” way. The well-confined gold nanoparticles (mean size of 5 nm) with a narrow particle size distribution were highly active in solvent-free benzyl alcohol selective oxidation with molecular oxygen. © 2009 Elsevier Inc. All rights reserved.
Keywords: SBA-16 CTAB Refluxing Stability Au nanoparticles Benzyl alcohol oxidation
1. Introduction Studies on ordered mesoporous materials have attracted extensive attentions since the discovery of M41S family mesoporous silicas [1,2]. Zhao et al. firstly reported the synthesis of a variety of SBA-type mesoporous silicas using non-ionic block copolymers as template [3,4]. Formation of SBA-type mesoporous silica materials usually occurs in a low-pH solution where the interaction between template and silica precursor proceeds through the S0 H+ X− I+ mechanism [5]. SBA-type mesoporous silicas exhibit large pore size, thick pore wall, and high stability. The important applications of SBA-type silicas include catalyst support [6], template for nanostructure synthesis [7], materials to immobilize or adsorb the biomolecules [8], sensors [9,10]. Among these SBA-type silica materials, SBA-16 is considered to be the most interesting mesostructure; it has a 3D cubic arrangement of mesopores corresponding to the Im3m space group. However, limited SBA-16 synthesis methods were reported because the cage-like SBA-16 mesostructured silica can only be produced in a narrow window of synthesis parameters [4,11–16]. SBA-16 single crystal (particle size ∼1 μm) was obtained under static condition by Yu et al. using block co-polymer F108 as template in the presence of K2 SO4 and HCl [11,12]. Kleitz et al. reported the syn-
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thesis of SBA-16 silicas using F127–butanol–H2 O mixture at low HCl concentration [13]. It was reported that the morphology and the particle size distribution are mainly determined by the regularity and the covering degree of the micelles by the silica species, poly-condensing degree (acidity and temperature), and the Brownian movements (temperature) at the moment of the precipitation. The regularity of the structural arrangement and the porosity properties of the final mesoporous silica are likely also related to the same parameters [17]. Recently, SBA-16 has been prepared in the case where CTAB was added as co-template [14–16], CTAB was suggested to help control the morphology and regulate the shape of SBA-16 mesostructure. For instance, Guth et al. [15] proposed that the presence of cationic CTAB can regulate the shape of micelles and their interaction with the silica precursors in the SBA-16 synthesis. An important issue which makes the practical application of SBA-16 material severely hampered is the long synthesis time (usually several days) because it is unfavorable to the large scale production. Enlightening and meaningful update of SBA-16 synthesis with short preparation time is highly desired. Regev et al. [18] observed using Cyro-TEM that the packed elongated micelles of ionic cetyltrimethylammonium chloride (CTAC) were arranged in small and ordered micro-domains after 3 min of mixing at room temperature. Lin et al. [14] reported the synthesis of SBA-16 crystals in a rhombdodecahedronal shape using CTAB–C12 H25 SO3 Na (SDS)–F127 ternary mixture as the template; they suggested that
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Scheme 1. Amine functionalization of SBA-16 and preparation of Au/SBA-16.
ionic surfactant CTAB and SDS acted preferentially as micellization promoters for the F127 block co-polymer. Inspired by these studies, it is speculated that CTAB may facilitate the synthesis of SBA-16 and shorten the synthesis time (accelerate the formation of ordered template of block co-polymer) due to its ionic characteristics. In this study, we report a facile approach to synthesize mesostructured SBA-16 silica material under refluxing condition using F127 as template in the presence of CTAB as co-template. The hydrothermal, thermal and mechanical stabilities of prepared SBA-16 silica are experimentally compared with SBA-16 synthesized by the conventional method. Shortening the synthesis time using this new approach will be discussed in detail. Moreover, the synthesized SBA-16 is used to stabilize gold nanoparticles, and the catalytic application is demonstrated in the solvent-free benzyl alcohol selective oxidation using molecular oxygen. 2. Materials and methods 2.1. Synthesis The synthesis of SBA-16 was carried out under refluxing condition as it was suggested that refluxing can decrease the concentration and temperature gradients in the synthesis mixture. SBA-16 was prepared using triblock co-polymer Pluronic F127 (Sigma-Aldrich) as template, CTAB (Sigma-Aldrich) as co-template, tetraethyl orthosilicate (TEOS, Sigma-Aldrich) as silica precursor. The preparation procedures were as follow: 0.63 g F127 and 0.073 g CTAB were completely dissolved into 71.28 g of 2 M HCl solution, followed by adding 2.083 g TEOS under strong magnetic stirring. The molar ratio was TEOS/F127/CTAB/HCl/H2 O = 1:0.005:0.02:14.3:495. The mixture was transferred into a 250 mL round bottle flask and kept stirring under refluxing condition at 40 ◦ C for 1–6 h, and another 6 h at elevated temperature of 80 ◦ C. The final products were collected by centrifugation and dried at 80 ◦ C overnight. The surfactant was removed by calcination at 550 ◦ C for 6 h. In comparison, conventional SBA-16 (denote as SBA-16-cv) was synthesized under static condition according to the procedure reported elsewhere [13]. The thermal and hydrothermal stabilities of calcined SBA-16 were tested by calcining the sample at 800 ◦ C for 6 h in air flow and boiling for 72 h in distilled water, respectively. Calcined SBA-16 samples were pressed by carver press to test their mechanical stability, the pressure was regulated between 3.15 and 13.5 ton, pressing time was fixed at 10 min. For simplicity, SBA-16 synthesized by this modified method is denoted as SBA-16-xx/yy, e.g., SBA-16-02/06 represents SBA-16 synthesized with self-assembly time of 2 h and aging time of 6 h. For comparison, MCM-41 and SBA-15 were synthesized following the methods reported by Chi et al. [19]. Mesoporous silica supported gold catalysts were prepared by a procedure using surface-functionalized mesoporous silica to adsorb the gold precursor (see Scheme 1). In a typical preparation exemplified by Au/SBA-16-02/06, 1.0 g of SBA-16-02/06 sample was suspended in 30 mL of toluene solution containing 1.0% (3-aminopropyl) triethoxysilane (APS) and refluxed at 110 ◦ C for 5 h. The resulting materials were filtered off, washed with toluene, and then dried at 80 ◦ C to remove the remaining solvent. The amine functionalized SBA-16 sample was denoted as APS/SBA-16-02/06. Im-
mobilization of gold nanoparticles was conducted by adding 1.0 g of APS/SBA-16-02/06 to 100 mL 10−3 M HAuCl4 aqueous solution, followed by stirring at 80 ◦ C for 5 h. The Au/SBA-16-02/06 sample was obtained by filtering and washing with deionized water, followed by drying at 80 ◦ C overnight. The final gold loading was measured by ICP. 2.2. Characterization Powder X-ray diffraction patterns were recorded with a Bruker AXS D86 diffractometer (under ambient conditions) using filtered CuK α radiation. Diffraction data were collected between 0.5◦ and 8◦ (2θ ) with a resolution of 0.02◦ (2θ ). Nitrogen physisorption isotherms were measured at −196 ◦ C with a static volumetric instrument Autosorb-6b (Quanta Chrome). Prior to each measurement, the sample was degassed at 250 ◦ C to a residual pressure below 10−4 Torr. A Baratron pressure transducer (0.001– 10 Torr) was used for low-pressure measurements. Brunauer– Emmett–Teller (BET) method was adopted to estimate the specific surface area. Barrett–Joyner–Halenda (BJH) method was used to calculate the pore size distribution using the desorption branch. The Scanning Electron Microscope (SEM) image was obtained with a JEOL Field Emission Scanning Electron Microscope (JSM-6700FFESEM). Prior to the analysis, the samples were deposited on a sample holder using an adhesive carbon tape and then sputtered with gold. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010, operated at 200 kV. The samples were suspended in ethanol and dried on holey carbon-coated Cu grids. 29 Si solid-state NMR experiments were performed on a Bruker DSX300 spectrometer with a frequency of 59.63 MHz, a recycling delay of 600 s, a radiation frequency intensity of 62.5 kHz, and the reference sample of Si8 O12 . Diffuse reflectance UV–visible (DR UV– vis) spectra were recorded with a Varian–Cary 5000 spectrometer equipped with a diffuse reflectance accessory. The spectra were collected between 200–800 nm at room temperature with BaSO4 as a reference. 2.3. Catalytic reaction The solvent-free oxidation of benzyl alcohol with molecular O2 was carried out using a bath-type reactor operated under atmospheric condition. In a typical reaction run, 200 mg of catalyst was loaded to a glass flask pre-charged with 10 mL (98.5 mmol) benzyl alcohol, the mixture was then heated to the reaction temperature (160 ◦ C) under vigorous stirring. Oxygen flow was bubbled at a flow rate of 20 mL min−1 into the mixture to start the reaction once the reaction temperature was reached. After reaction, the solid catalyst was filtered off and the liquid organic products were analyzed by an Agilent gas chromatograph 6890 equipped with a HP-5 capillary column. Dodecane was the internal standard to calculate benzyl alcohol conversion and benzaldehyde selectivity. 3. Results and discussion Powder XRD patterns of calcined SBA-16 mesoporous silicas prepared by this new method are depicted in Fig. 1, showing
H. Sun et al. / Journal of Colloid and Interface Science 333 (2009) 317–323
Fig. 1. X-ray diffraction patterns of SBA-16, the molar ratio of F127 to TEOS is: (a) 0.001, (b) 0.003, (c) 0.004, (d) 0.005, (e) 0.006, (f) 0.007.
Fig. 2. Powder XRD patterns of calcined SBA-16 with different CTAB/F127 molar ratio: (a) 2, (b) 4, (c) 6, (d) 8.
the effect of F127 content on the mesostructured SBA-16 (selfassembly and aging of 24 h and 24 h, respectively). It is clearly observed that the best synthesis of cubic SBA-16 structure occurs under an optimized F127/TEOS molar ratio of 0.005. No noticeable diffraction peak is observed when the F127/TEOS molar ratio is as low as 0.001. When the F127/TEOS molar ratio increases to 0.003, only one weak diffraction peak centered at 2θ = 0.9◦ appears, indicating a poorly ordered mesostructure. As the ratio increases to 0.005, the sample exhibits one strong diffraction peak and two higher angle peaks in the range of 2θ = 0.8–1.8◦ , assigned to (110), (200) and (211) planes. The FWHM (full width at half maximum) of the (110) peak is about 0.2◦ , indicating the particularly well developed grains of mesostructured silica. When the F127/TEOS molar ratio increases to 0.006, the XRD pattern becomes less resolved; the diffraction peaks are diminished when the ratio is 0.007. As suggested by Kleitz et al. [13], the amount of TEOS added to the system greatly influences the nature of mesophase formed. The Im3m phase is the dominant phase in the presence of optimized amount of TEOS. A lower TEOS to surfactant ratio results in the formation of Fm3m silica mesophase or disordered phase. This fact may be rationalized by pronounced differences in the F127/TEOS ratio in the silica–block co-polymer hybrid mesophase and the repartition of the inorganic and organic domains. Powder XRD patterns of calcined SBA-16 mesoporous silicas with different CTAB/F127 molar ratio are shown in Fig. 2. The content of F127 is fixed at the optimized F127/TEOS molar ratio of
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Fig. 3. X-ray diffraction patterns of SBA-16 of different self-assembly time: (a) SBA16-01/06 (b) SBA-16-1.5/06 (c) SBA-16-02/06 (d) SBA-16-06/06 (e) SBA-16-02/06 without addition of CTAB.
0.005 with various amount of CTAB. XRD pattern of SBA-16 with an optimized CTAB/F127 molar ratio of 4 exhibits well resolved diffraction peaks assignable as (110), (200), and (211) diffractions, implying the well defined long range order cubic mesostructure. When the CTAB/F127 molar ratio is larger, poorly ordered mesostructure is obtained. As the CTAB/F127 molar ratio increases to 8, the sample does not show any discernable XRD diffraction peak in the range of 0.8–8◦ , indicating the amorphous nature of resulting silica material. Synthesis time is an important factor to be concerned in this modified synthesis method. The synthesis of SBA-16 by conventional approach takes approximately 2 days. In this new method, CTAB is added as the co-template, aiming at accelerating the formation of F127 micelle because of CTAB’s ionic characteristics. SBA-16 samples were synthesized under a fixed aging time of 6 h with various self-assembly time; F127/TEOS and CTAB/F127 molar ratios were regulated as 0.005 and 4, respectively. Fig. 3 shows the XRD patterns of SBA-16 mesostructured silicas prepared under different self-assembly time. SBA-16-01/06 sample exhibits only one weak diffraction peak, indicating the mesostructure is poorly constructed within 1 h of self-assembly time. When the self-assembly time increases to 1.5 h, one strong diffraction peak (110) and one weak peak (200) demonstrate the emerging of a good mesoporous structure. Sample SBA-16-02/06 displays three strong and well-resolved (110), (200) and (211) diffraction peaks of Im3m symmetry, suggesting 2 h suffices the self-assembly during the synthesis of a well ordered 3D SBA-16 mesostructured silica. Longer self-assembly time (6 h) gives better mesoporous structure of SBA-16-06/06. The N2 adsorption–desorption isotherms and pore size distributions of SBA-16 prepared with various self-assembly time are shown in Fig. 4. Type IV hysteresis loop of SBA-16-01/06 is poorly formed with the absence of any sharp condensation step. For the sample of SBA-16-1.5/06, N2 isotherms form a type IV hysteresis loop with a sharp capillary condensation step on desorption branch at P / P 0 = 0.45, it implys a good mesoporous structure. SBA-16-02/06 also exhibits a type IV isotherm with sharper capillary condensation step, which are typical features of highly ordered Im3m mesostructure. All the samples exhibit narrow pore size distribution centered at approximately 3.5 nm. The d-spacing, BET surface area, pore size, and pore volume of various SBA-16 samples are summarized in Table 1. The SBA-16 sample synthesized by the conventional method shows a d-spacing of 9.0, BET surface area of 1203 m2 g−1 , and pore volume of 1.35 cm3 g−1 , which is in good agreement with the results reported elsewhere [13]. Although
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Fig. 4. (Left) N2 adsorption/desorption isotherms, (right) pore size distributions calculated from the desorption branch by BJH method: (a) SBA-16-01/06, (b) SBA-16-1.5/06, (c) SBA-16-02/06, (d) SBA-16-06/06.
Table 1 Textural parameters of various SBA-16 samples.a Sample
d-spacing (nm)
S BET (m2 g−1 )
D (nm)
Vp (cm3 g−1 )
a0 (nm)
t (nm)
SBA-16-cv SBA-16-01/06 SBA-16-1.5/06 SBA-16-02/06 SBA-16-06/06
9.01 9.04 8.97 8.95 9.02
1203 691 994 1114 1120
3.42 3.55 3.52 3.41 3.45
1.350 0.701 0.944 1.342 1.356
12.74 12.78 12.69 12.66 12.76
7.61 7.51 7.47 7.55 7.60
SBA-16-cv-hyb SBA-16-02/06-hyb SBA-16-cv-thb SBA-16-02/06-thb
/ 9.02 8.55 8.51
285 527 300 551
3.52 3.51 3.34 3.43
0.482 0.634 0.292 0.386
/ 12.76 12.09 12.03
/ 7.54 7.13 6.99
SBA-16-cv-3.15ton SBA-16-cv-4.17ton SBA-16-cv-10ton SBA-16-cv-13.5ton SBA-16-02/06-3.15ton SBA-16-02/06-4.17ton SBA-16-02/06-10ton SBA-16-02/06-13.5ton
9.52 9.21 9.20 / 8.83 8.82 8.81 8.53
540 446 370 317 1008 769 577 551
3.51 3.53 3.44 3.41 3.37 3.43 3.36 3.25
0.682 0.384 0.362 0.298 0.711 0.554 0.425 0.382
13.46 13.02 13.01 / 12.52 12.47 12.46 12.06
8.15 7.75 7.83 / 7.47 7.37 7.43 7.19
a d-spacing, BET surface area (S BET ), pore diameter (D), pore volume (V p ), unit cell parameter (a0 ) and pore wall thickness (t) of various SBA-16 samples. b “-hy” represents SBA-16 after hydrothermal treatment and “-th” represents SBA16 after thermal treatment. a0 = 21/2 d110 , pore wall thickness t = 31/2 a0 /2 − D.
the d-spacing of SBA-16-01/06 is closed to that of SBA-16-cv, the BET surface area (691 m2 g−1 ) and pore volume (0.70 cm3 g−1 ) are lower than those of SBA-16-cv. With an increase of the selfassembly time, both surface area and pore volume remarkably increase. When the self-assembly time is 2 h, the surface area and pore volume are 1114 m2 g−1 and 1.34 cm3 g−1 , respectively. Thus, the textural properties of SBA-16-02/06 by this new method are comparable to those of SBA-16 by conventional synthesis. It should be noted the current method shortens the synthesis time by a factor of 6 (from 48 h to 8 h). Although SBA-16-06/06 shows a XRD pattern superior to all samples, SBA-16-02/06 is chosen to continue the study because the main focus of this new method is to explore the possibility reducing the synthesis time, SBA-16-02/06 shows comparable properties (as shown in Table 1) compared to SBA-16-06/06 but shorter preparation time.
Fig. 5. SEM image (left) and TEM image (right) of SBA-16-02/06 showing the monodispersed spherical morphology and characteristic planes for a cubic pore structure: (a, b) [111], (c, d) [100].
Scanning electron microscope (SEM) image of as-synthesized SBA-16-02/06 is shown in Fig. 5(left). The particles exhibit monodispersed spherical morphology with diameter ranging from 0.8– 1.2 μm. The mesoporous structure of SBA-16-02/06 was characterized using transmission electron microscopy (TEM). Typical micrographs of the as-synthesized particle are shown in Fig. 5(right). Well-ordered cubic pore structure of ca. 5–6 nm in diameter is clearly observed, which is in a good agreement with the analysis of XRD and N2 physisorption. Results in Fig. 3 suggest that CTAB plays an important role to facilitate the synthesis of SBA-16 within a short time. The role of CTAB was further verified by preparing SBA-16-02/06 in the absence of CTAB (see Fig. 3e). No diffraction peak is observed which indicates amorphous silica characteristic without any long range order structure. It is proposed that cationic surfactant CTAB may enhance the micellization of F127 template by adjusting the conformation of the hydrophilic PEO chains. There can be few hydrogen bonds interacting between F127 micelle and silica precursors without changing the dimension of Im3m mesostructure of SBA-16. In this study, hydrothermal, thermal, and mechanical stability tests were carried out to compare the SBA-16-02/06 by this modified method and SBA-16 by conventional approach. The XRD pat-
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terns are shown in Fig. 6. Herein, “-hy” represents hydrothermal treatment, “-th” represents thermal treatment. After hydrothermal treatment, SBA-16-cv does not exhibit any diffraction peak, suggesting the mesoporous structure is completely destroyed. It is worth mentioning that the XRD pattern of SBA-16-02/06 remains a strong diffraction peak indexed to (110) plane after severe hydrothermal treatment, indicating the excellent hydrothermal stability of SBA-16-02/06. As shown in Table 1, the surface area of SBA-16-cv after hydrothermal treatment decreases to 285 m2 g−1 , and that of SBA-16-02/06 decreases to 527 m2 g−1 . Only one weak diffraction peak associated with (110) reflection plane is resolved for SBA-16-cv after thermal test. SBA-16-02/06 is able to maintain a better mesostructure as evidenced by the presence of one strong diffraction peak corresponding to (110) reflection plane. Moreover, both surface area and pore volume of SBA-16-02/06 after thermal test are much higher than those of SBA-16-cv (Table 1). Thus, it is clear that SBA-16 from this new approach possesses higher hydrothermal and thermal stabilities than SBA-16 by conventional approach. Although shortening synthesis time has no considerable effect on the order of SBA-16 mesostructure, it is speculated whether the silica local chemical environment is affected. Solid-state nu-
Fig. 6. XRD patterns for SBA-16 after hydrothermal and thermal tests: (a) SBA-16cv-hy, (b) SBA-16-02/06-hy, (c) SBA-16-cv-th, (d) SBA-16-02/06-th.
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clear magnetic resonance (NMR) spectroscopy has become a powerful tool for structural characterization of mesoporous material and other catalytic materials; it detects the inequivalent environments among the silicon sites. Mesoporous silicas are generally characterized by the presence of silicon sites Q 2 , Q 3 , and Q 4 , representing the species of Si(OSi)2 (OH)2 , Si(OSi)3 (OH), and Si(OSi)4 at −90, −102 and −110 ppm chemical shifts, respectively. 29 Si solidstate NMR spectra of SBA-16-02/06 (before and after thermal and hydrothermal treatment) and SBA-16-cv (before and after thermal and hydrothermal treatment) are shown in Fig. 7. Three bands associated with Q 2 , Q 3 and Q 4 Si species for fresh SBA-16-02/06 are similar with those of SBA-16 synthesized by conventional method; there is no significant change in silica local environment when the synthesis time is shortened from 48 h to 8 h by this new synthesis method. After hydrothermal treatment, the peak associated with Q 2 vanishes completely, the peak associated with Q 3 almost can not be discerned, and the intensity of the peak associated with Q 4 remarkably increases, which are analogous with those of SBA-16 synthesized by conventional method after hydrothermal treatment. Only one peak associated with Q 4 Si species can be observed in both SBA-16-02/06 and SBA-16-cv after thermal treatment, which is due to the condensation of Si(OSi)2 (OH)2 and Si(OSi)3 (OH) to Si(OSi)4 during the thermal test. Furthermore, mechanical stability is also concerned and the mechanical pressure test results are shown in Fig. 8. Without applying any press, three well-resolved diffraction peaks indexed to (110), (200) and (211) planes of SBA-16-02/06 characterize a highly ordered Im3m mesoporous structure (Fig. 8(left)). Under low pressure, these three diffraction peaks are well conserved. The SBA-1602/06 sample shows one diffraction peak centered at 2θ = 1.02◦ when the pressure increases to 10.0 ton. The (110) diffraction peak slightly changes as the pressure further increases to 13.5 ton, suggesting ordered mesostructure can be preserved under pressure up to 13.5 ton. As a comparison, conventional SBA-16 was also tested under mechanical pressure (Fig. 8(right)). SBA-16-cv loses its ordered mesoporous structure with the absence of any diffraction peaks under pressure of 13.5 ton. The textural parameters of both samples with an increase of mechanical pressure are also compared in Table 1. Under a pressure of 3.15 ton, the surface area and pore volume of SBA-16-02/06 are 1008 m2 g−1 and 0.71 cm3 g−1 , respectively, whereas, the surface area and pore volume of SBA16-cv under this pressure are 540 m2 g−1 and 0.6 cm3 g−1 , respec-
Fig. 7. (Left) Solid-state MNR spectra of SBA-16-02/06: (a) fresh, (b) after hydrothermal treatment, (c) after thermal treatment. (Right) Solid-state MNR spectra of SBA-16-cv: (a) fresh, (b) after hydrothermal treatment, (c) after thermal treatment.
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Fig. 8. XRD pattern of calcined SBA-16-02/06 (left) and SBA-16-cv (right) under mechanical press at: (a) 0, (b) 3.15 ton, (c) 4.74 ton, (d) 10.0 ton, (e) 13.5 ton. Table 2 Results of benzyl alcohol oxidation over Au-containing mesoporous silica catalysts.a Catalyst
Pretreatment
TOF (h−1 )
Conversion (%)
Benzyldehyde selectivity (%)
SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-02/06 Au/SBA-16-cv Au/SBA-15 Au/MCM-41
– Air, Air, H2 , H2 , H2 , H2 , H2 ,
–
<1.0 6 .3 3 .8 13.0 19.0 19.3 16.1 6.5
>99 >99 >99 >99 >99 >99 >99
80 ◦ C 540 ◦ C 200 ◦ C 400 ◦ C 400 ◦ C 400 ◦ C 400 ◦ C
803 484 1655 2419 2457 2048 827
–
a Reaction conditions in this work: the amount of Au in each catalyst 2.5 × 10−3 mmol; benzyl alcohol 10 mL (98.5 mmol), 160 ◦ C, O2 flow rate 20 mL min−1 , reaction time 3 h.
Fig. 10. TEM micrographs of Au-containing mesoporous silica samples as well as the corresponding Au particle size distributions: (a) Au/SBA-16-02/06, (b) Au/SBA-15, (c) Au/MCM-41.
Fig. 9. UV–vis spectra of Au-containing mesoporous silica samples: (a) Au/SBA-1602/06, (b) Au/SBA-15, (c) Au/MCM-41.
tively. With an increase of pressure, surface area and pore volume of both SBA-16-02/06 and SBA-16-cv decrease gradually, but the surface area and pore volume of SBA-16-02/06 under the same pressure are always higher than those of SBA-16-cv. Thus, SBA16-02/06 by this new method exhibits higher mechanical stability than that of the conventional SBA-16, although the reason remains unclear. The catalytic results of solvent-free benzyl alcohol selective oxidation with molecular O2 over mesoporous silica supported gold catalysts pretreated under different conditions are listed in Table 2.
The best catalytic performance is obtained over the SBA-16-cv support pretreated in a H2 flow at 400 ◦ C, benzyl alcohol conversion and TOF are 19.3% and 2457 h−1 , respectively. Almost the same benzyl alcohol conversion and TOF are obtained on Au/SBA-1602/06 catalyst (19.0% and 2419 h−1 , respectively); this further suggests that the properties of SBA-16-02/06 by this new method are comparable to those of SBA-16 by conventional synthesis. The catalytic performances of Au/SBA-15 and Au/MCM-41 catalysts with the same Au content (∼1 wt%) were also investigated for comparison. Although both Au/SBA-15 and Au/MCM-41 can catalyze the benzyl alcohol oxidation at 160 ◦ C, forming benzaldehyde as the main product, both benzyl alcohol conversion and TOF are lower than those of Au/SBA-16. Fig. 9 shows the UV–vis spectra of Au-containing mesoporous silica catalysts pretreated in a H2 flow at 400 ◦ C. All samples show the characteristic surface plasmon resonance absorption peak of Au nanoparticles at around 520 nm. Compared to Au/SBA-16, Au/MCM-41 and Au/SBA-15 exhibit a slightly red-shifted and narrower absorption band, which indicates a larger mean particle size and more particle aggregation [20]. To examine the Au nanoparticle size, the direct TEM microscopic observation was performed. As shown in Fig. 10, we can discern a uniform distribution of Au nanoparticles in the Au/SBA-
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16 catalyst. The particles are mostly observed in the size range of 2–9 nm, with a maximum of size distribution at 5 nm. Particles larger than ca. 7 nm are hardly seen in this sample. Considering the channel structure of SBA-16, it is reasonable to conclude that most of Au nanoparticles are confined inside the cage of SBA-16 structure. On the other hand, relatively large Au nanoparticles with sizes more than ca. 7 nm can be observed in Au/SBA-15 and Au/MCM-41 catalysts. Gold nanoparticles are not well confined in the pore structure of SBA-15 and MCM-41; many large particles may appear outside of the pores. Moreover, the size distributions of Au particles over SBA-15 and MCM-41 are much broader than that of SBA-16. We note that TEM micrographs of Au-containing mesoporous silica samples as well as the corresponding Au particle size distributions are different from the literature [19]. According to Chi et al. [19], the average sizes of the Au nanoparticles follow the order: MCM-41 (5.1 nm) < SBA-15 (5.8 nm) < MCM48 (6.9 nm). The difference of Au nanoparticle size distribution over SBA-16, MCM-41 and SBA-15 supports in our study may be aroused from the different synthesis method used in this work. The pretreatment of the gold catalyst may facilitate the migration of Au nanoparticles originally encapsulated in the channels, which accounts for the wider distributions of Au nanoparticles in MCM-41 and SBA-15 2D mesoporous silicas. On the other hand, a uniform distribution of Au nanoparticles with a mean size of 5 nm can be obtained in SBA-16. Thus, it is clear that the “supercage” porosity of SBA-16 is superior to MCM-41 and SBA-15 for confining Au nanoparticles. Combined the catalytic performances of Au-containing samples discussed above, it is suggested that the Au nanoparticles with a mean size of 5 nm confined within the SBA-16 cage are more active for benzyl alcohol oxidation with molecular O2 . 4. Summary Highly ordered mesoporous silica SBA-16 was successfully synthesized by a facile method under refluxing condition in the presence of CTAB as co-template. The synthesis time was reduced from 48 h conventionally used to 8 h. It was proposed that CTAB
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plays a crucial role that enhances the micellization of F127 template. SBA-16 by this synthesis method showed high hydrothermal, thermal, and mechanical stabilities. The unique pore structure of SBA-16 can encapsulate Au nanoparticles; the well confined Au nanoparticles with a mean size of 5 nm were highly active in solvent-free benzyl alcohol selective oxidation with molecular O2 . Acknowledgments We are grateful to AcRF tier 1 (RG45/06) for financial support. The authors also thank AcRF tier 2 (ARC 13/07) for funding support. References [1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1999) 10834. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuil, J.S. Beck, Nature 359 (1992) 710. [3] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrckson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [4] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [5] Q. Huo, D.I. Margolese, U. Clesia, P. Feng, T.E. Gler, P. Sleger, R. Leon, P.M. Petroff, F. Schuth, G.D. Stucky, Nature 368 (1994) 317. [6] S.E. Dapurkar, A. Sakthivela, P. Selvam, New J. Chem. 27 (2003) 1184. [7] B. Tian, X. Liu, H. Yang, S. Xie, C. Yu, B. Tu, D. Zhao, Adv. Mater. 15 (2003) 1370. [8] Y.-J. Han, J.T. Watson, G.D. Stucky, A. Butler, J. Mol. Catal. B 17 (2002) 1. [9] G. Wirnsberger, B.J. Scott, G.D. Stucky, Chem. Commun. 3 (2003) 2140. [10] T. Yamada, H.S. Zhou, H. Uchida, M. Tomita, Y. Ueno, I. Honma, K. Asai, T. Katsube, Microporous Mesoporous Mater. 54 (2002) 269. [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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