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SBA-15 nanostructures and their catalytic properties for epoxidation of styrene
Microporous and Mesoporous Materials 215 (2015) 199e205
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Temperature difference effect induced self-assembly method for Ag/SBA-15 nanostructures and their catalytic properties for epoxidation of styrene Yinhai Tang, Mu Yang, Wenjun Dong, Li Tan, Xiaowei Zhang, Peng Zhao, Chaohao Peng, Ge Wang* Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 19 May 2015 Accepted 22 May 2015 Available online 5 June 2015
In this paper the first dynamics-controlled self-assembly method for Ag nanostructures in mesoporous channels is developed, and Ag/SBA-15 nanostructures are synthesized by a temperature difference effect induced self-assembly method with AgNO3 as Ag precursor and SBA-15 as support. Different morphologies of Ag nanomaterials such as nanoparticles, nanorods, and nanowires can be controlled by changing the weight ratio of AgNO3/SBA-15. In the synthesis procedure, the temperature difference effect induces air volume shrinkage in the channel, and the drop in pressure pushes the molten AgNO3 assembled into the channels step by step. Finally, Ag/SBA-15 nanostructures are obtained by calcination at 350 C for 2 h. Furthermore, the catalytic activities of Ag/SBA-15 nanostructures toward the epoxidation of styrene are investigated, and Ag nanoparticles/SBA-15-130 shows 77.7% of conversion and 73.7% selectivity for the epoxidation of styrene. © 2015 Elsevier Inc. All rights reserved.
Keywords: Ag/SBA-15 Temperature difference effect Epoxidation of styrene
1. Introduction Noble metals with quantum confinement properties represent a breakthrough in creating possibilities for catalytic, electronic, photonic, and biotechnical applications due to their structure and configuration efficiency [1e5]. Silver as a noble metal for the epoxidation of olefins has attracted considerable attention, and silver nanostructures with controllable morphologies that improve their catalytic properties have been extensively investigated in recent years [6e10]. One effective method has been developed to combine silver materials within mesoporous material channels due to their robust structures, tunable pore sizes and mesoporous structures [11]. Therefore, several strategies, such as conventional wetness impregnation [12e14], in-situ “pH-adjusting” method [15e17], double solvent technique [18], and solvent-free grinding method [19], have been developed to incorporate Ag into mesoporous channels. For example, Han et al. synthesized Agnanoparticle-loaded mesoporous silica SBA-15 through a one-step
* Corresponding author. Tel.: þ86 10 62333765. E-mail address: [email protected] (G. Wang). http://dx.doi.org/10.1016/j.micromeso.2015.05.040 1387-1811/© 2015 Elsevier Inc. All rights reserved.
method, in which Ag nanoparticles were spontaneously embedded in channels and even implanted in frameworks of mesoporous silica SBA-15 [20]. Huang et al. prepared different morphologies of Ag/SBA-15 via a double solvent technique, and both Ag nanowires and nanorods were successfully incorporated into SBA-15 with different surface properties [21]. Liam et al. utilized the phenomenon of salt occlusion to incorporate silver within mesoporous materials. Silver nanowires both included in and extruded from the siliceous mesoporous support have been produced [22]. Conventionally, the static driven force to incorporate Ag into the meso-channels, such as chemical adsorption between silver ions and functionalized chemical groups or physical adsorption driven by capillary force, is untunable [23e25]. As such a dynamicscontrolled self-assembly method to synthesize Ag nanostructures within meso-channels still remains as a challenge. In this paper, a facile and efficient temperature difference effect induced self-assembly method is developed to fabricate Ag/SBA-15 nanostructures. The dynamic assembly driving force generated by temperature difference effect induces the AgNO3 precursor selfassembly and pushes it into the mesoporous channels in a controllable manner. During the synthesis process, the homogeneous AgNO3/SBA-15 powder is first placed in an oven at a set
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temperature, and then the homogeneous powder is ground quickly as the temperature is decreased. The temperature difference induces air volume shrinkage in the channels of SBA-15, and then the molten AgNO3 is sucked into the channels as a result of the drop in pressure step by step. Finally, the Ag/SBA-15 nanostructures are obtained by calcination at 350 C for 2 h. Ag nanoparticles, nanorods and nanowires/SBA-15 can be controlled by changing the weight ratio of AgNO3/SBA-15. Due to the mesoporous structure, high specific surface area and good thermal stability, the Ag/SBA-15 nanostructures exhibit enhanced catalytic properties. The optimized catalytic conversion of styrene is 77.7% and the selectivity of styrene oxide is 73.7%. 2. Experiment 2.1. Materials Silver nitrate (AgNO3), tetraethyl-orthosilicate [(OC2H5)4Si, TEOS], tert-butyl hydroperoxide (TBHP) and HCl (36e38 wt.%) were purchased from Beijing Chemical Reagent Company. Pluronics P123 (Mw ¼ 5800) and styrene (99.5%) were supplied by SigmaeAldrich and Alfa Aesar, respectively. All reagents were used as received without further purification.
The products were filtered off, rinsed with de-ionized water, and subjected to thermal treatment at 350 C in air for 2 h to decompose the impregnated AgNO3. The final product was obtained and named as 0.5 Ag/SBA-15-w and 0.25 Ag/SBA-15-w. 2.4. Characterization High-resolution transmission electron microscopy (HRTEM) was conducted on a JEM-2100F equipped with an energydispersive X-ray (EDX) operated at the accelerating voltages of 200 kV. Nitrogen adsorptionedesorption isotherms at 77 K were obtained using an AUTOSORB-1C analyzer (USA Quantachrome Instruments). The specific surface area was calculated using the BrunauereEmmetteTeller (BET) method. Pore size distributions were calculated using the BarretteJoynereHalenda (BJH) method from the adsorption branches of the isotherms. The X-ray diffraction (XRD) patterns were taken on a M21X diffractometer (MAC Science Co. Ltd., Japan) using a Cu Ka radiation (l ¼ 1.541 Å) source at 40 kV and 200 mA. Small-angle X-ray diffraction (SAXRD) patterns were recorded on a D/MAX-2550 HB/PC (Rigaku Co., Tokyo, Japan) at 40 kV and 150 mA. Finally, the catalytic properties were analyzed by Agilent 7890/5975C-GC/MSD. 2.5. Catalytic properties
2.2. Preparation of mesoporous silica SBA-15 Mesoporous silica SBA-15 was synthesized according to a previous report [26]. 4.0 g of P123 as the structure-directing agent was first dissolved in 30 mL H2O, and 120 mL (2.0 mol/L) HCl aqueous solution was added under stirring, and then the mixture was stirred until the solution became transparent, following which 8.5 g of tetraethyl-orthosilicate (TEOS) was added as the silica source. The resultant mixture was stirred at 35 C for 24 h, and then the solution was transferred into a Teflon-sealed autoclave and aged at 110 C for 24 h. After cooling to room temperature, the precipitates were filtrated and washed with de-ionized water several times. The powder was dried and calcined at 550 C for 4 h under air flow with a heating rate of 2 C/min to remove the organic template.
Typically, the epoxidation reactions were carried out in a 50 mL round-bottom flask equipped with a reflux condenser. 5.0 mmol of styrene, 50 mg of solid catalyst, 5.0 mL acetonitrile and 0.02 mL of nitrobenzene (as the internal standard for GC-MSG analysis) were mixed in the flask. After the temperature was raised to 80 C with a reflux system, 2.5 mL tert-butyl hydroperoxide (TBHP) was added drop-wise to the flask under vigorous stirring. After allowing a certain reaction time (9 h), the flask was cooled to room temperature. Then the catalyst was filtered out, and the liquid organic products were analyzed by gas chro-matography-mass spectrometry (GC-MSD, HP5-MS column, Ar carrier gas, 200 C) using an internal standard. 3. Result and discussion
2.3. Preparation of Ag/SBA-15 nanostructures 3.1. Synthesis of Ag/SBA-15 nanostructures Ag/SBA-15 nanostructures were synthesized by a temperature difference effect induced self-assembly method. In the synthesis procedure, the SBA-15 was first dried at 120 C for 4 h at a vacuum condition in order to remove the absorbed water, and then, a certain amount of AgNO3 was ground with 0.2 g of SBA-15 in an agate mortar at room temperature. Then, the mixture was placed in an oven at 130 C for 30 min. After that, the agate mortar was taken outside and ground quickly for one minute, and the temperature was decreased to 95 C. The agate mortar was then put back into the oven again for 5 min. The process was repeated for 3 times, and the mixture was calcined at 350 C for 2 h with a heating rate of 2 C/min. The final product was obtained and named as Ag/SBA-15130. To investigate the effect of the weight ratio of AgNO3/SBA-15 and the setting temperature, Ag/SBA-15 nanostructures obtained by different weight ratios of AgNO3/SBA-15 or different setting temperatures were investigated in detail, and these samples were denoted as X Ag/SBA-15-Y, with X representing the weight ratio of AgNO3/SBA-15, and Y representing the setting temperature. Moreover, to compare the catalytic performances of Ag/SBA-15, samples prepared by conventional wetness impregnation were obtained as follow [13]: 20 mg of SBA-15 powder was soaked in 10 mL of 1 g/L (and 0.5 g/L) AgNO3 EtOH-H2O (1 þ 1 v/v) solutions, and the suspension was stirred overnight at room temperature.
Ag/SBA-15 nanostructures were synthesized by a temperature difference effect induced self-assembly method. Homogeneous AgNO3/SBA-15 powder, obtained by mixing certain amount of AgNO3 and 0.2 g of SBA-15, was placed in an oven at 130 C for 30 min, then the homogeneous powder was ground quickly with a temperature decrease to 95 C. The above process was repeated three times, and the Ag/SBA-15 nanostructures were obtained by calcination at 350 C for 2 h. Interestingly, different morphologies of Ag/SBA-15 nanostructures could be prepared via temperature difference effect induced self-assembly method by changing the weight ratio of AgNO3/SBA-15. When the weight ratio was 0.5 (0.5 Ag/SBA-15-130, 0.5 represented the weight ratio of AgNO3/SBA-15, and 130 represented the setting temperature), Ag nanoparticle structures with a diameter about 6.0 nm were observed in the SBA15 channels (Fig. 1a). When the weight ratio was 0.25 (0.25 Ag/SBA15-130), Ag nanorods/SBA-15 were prepared accompanied by small amounts of nanoparticles in the meso-channels (Fig. 1b). The Ag nanorods along the SBA-15 meso-channels were between 20 and 120 nm in length, 4.0 nm in diameter. When the weight ratio was 0.15 (0.15 Ag/SBA-15-130), long nanowires with nanoparticles were obtained in the channels (Fig. 1c). The nanowires were between 300 and 800 nm in length. It appeared that the temperature difference effect induced self-assembly method had successfully
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Fig. 1. HRTEM images of (a) 0.5 Ag/SBA-15-130, (b) 0.25 Ag/SBA-15-130, (c) 0.15 Ag/SBA-15-130.
driven AgNO3 into the mesoporous channels. These products were obtained by repeating the dynamic self-assembly process three times, and the precursor was pushed into the mesoporous channels step by step. The small amounts of AgNO3 which were pushed into the channels easily formed nanorods and nanowires, and large amounts of AgNO3 coalesced together to form nanoparticles in the channels, and the formation of Ag nanoparticles led to jamming of the pores [27]. XRD patterns of Ag/SBA-15-130 nanostructures show a broad band at 23 , which corresponds to the amorphous structure of silica (Fig. 2). Moreover, four distinct peaks at about 38.0 , 44.3 , 64.5 and 77.3 are observed in all three samples, which can be indexed as the (111), (200), (220), and (311) planes of face-centered cubic Ag
Fig. 2. Power X-ray diffraction patterns of Ag/SBA-15-130 nanostructures prepared by different weight ratio of AgNO3/SBA-15.
(JCPDS card no. 4-783) [28], respectively, indicating the presence of silver after the subsequent calcinations. At the relative-low weight ratio of AgNO3/SBA-15, the obtained Ag nanorods and nanowires/ SBA-15 with high crystallinity exhibit enhanced Ag characteristic peaks. When the weight ratio was increased to 0.5, the formation of nanoparticles with poor crystallinity, leading to the lowest intensities of XRD characteristic peaks [29]. Moreover, the broad peak in the sample of 0.5 Ag/SBA-15-130 at about 33 is indexed to Ag2O, which may be due to the incomplete decomposition of AgNO3 [21], leading to the decrease of the intensities of four characteristic peaks of Ag. Ag/SBA-15-130 nanostructures with different morphologies were further characterized by N2 adsorptionedesorption isotherms, and the pore size distributions are calculated by the BarretteJoynereHalenda (BJH) method (Fig. 3). The isotherm for all these samples can be classified as type IV for mesoporous materials, and the hysteresis loop of type H1 proves the reservation of ordered mesopores (Fig. 3a), which is in conformity with the HRTEM result [30]. The physicochemical properties of the samples are summarized in Table 1. Attributable to the silver successfully incorporated into the mesopores of SBA-15 (0.15 Ag/SBA-15-130), the BET surface area decreased from 836.4 to 486.8 m2 g1, and the pore volume dropped from 0.89 to 0.70 cm3 g1 [31]. When the weight ratio of AgNO3/SBA-15 was increased from 0.15 to 0.5, large amounts of AgNO3 precursor caused a significant decrease in the surface area and pore volume to 273.5 m2 g1 and 0.39 cm3 g1, respectively. The pore size decreases from 6.50 nm for 0.15 Ag/SBA-15-130 to that 4.91 nm for 0.25 Ag/SBA-15-130, and then increases to 5.67 nm for 0.15 Ag/SBA-15-130. It is suggested that, with the increase of the weight ratio of AgNO3/SBA-15 to 0.5, the obtained Ag nanoparticles in the channels with 6.0 nm in diameter may block the channels.
Fig. 3. N2 adsorptionedesorption isotherms of pure SBA-15 and Ag/SBA-15-130 nanostructures prepared by different weight ratios of AgNO3/SBA-15.
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Table 1 Physicochemical properties of SBA-15 and Ag/SBA-15-130 nanostructures. Sample
BET (m2 g1)
Vp (cm3 g1)
Dp (nm)
SBA-15 0.15 Ag/SBA-15-130 0.25 Ag/SBA-15-130 0.5 Ag/SBA-15-130
836.4 486.8 422.4 273.5
0.89 0.70 0.51 0.39
6.62 6.50 4.91 5.67
Therefore, both open and blocked mesopores will be formed among the nanoparticles in the channel, leading to a bimodal porosity distribution (Fig. 3b) [32], in which one pore size is 5.91 nm, and the other one is 3.80 nm. 3.2. The effect of the setting temperatures on the formation of Ag nanostructures/SBA-15 To investigate the effect of the setting temperature, 0.5 Ag/SBA15 nanostructures obtained using different setting temperatures (90 C, 130 and 170 C) were investigated in detail. The results are shown in Fig. 4. When the setting temperature was 90 C, bulk particles with sizes of tens to hundreds of nanometers were deposited on the exterior surface of the SBA-15 (Fig. 4a). When the setting temperature was increased to 130 C, a great number of Ag nanoparticles in the meso-channels was observed (Fig. 1a), suggesting that the moving interface of AgNO3/SBA-15 relaxes the elastic energy under thermal grinding, which increases the driving force for melting and reduces the melting temperature of AgNO3. So melting process can occur at the small size solid AgNO3 particle surface at 130 C in an extremely short time [33]. When the temperature reached 170 C, the mesostructure of SBA-15 was severely damaged during the thermal grinding process (Fig. 4b), which might be due to the conversion between solid and melton of AgNO3 in the repeating thermal grinding process, leading to destroy the mesostructure of SBA-15. Fig. 5 displays small-angle XRD patterns of SBA-15 and 0.5 Ag/ SBA-15 nanostructures. The pure SBA-15 exhibits three diffraction peaks, which can be indexed as (100), (110), and (200) reflections of the ordered 2D-hexagonal (p6mm) symmetry structures [34]. In the 0.5 Ag/SBA-15 nanostructures, 0.5 Ag/SBA-15-90 shows a shifted (100) peak and illegible diffraction peaks of (110) and (200), suggesting that the ordered 2D-hexagonal symmetry structure has been distorted, which may be caused by the formation of large number of bulk particles deposited on the exterior surface of the SBA-15. The 0.5 Ag/SBA-15-130 exhibits a lower diffraction peak than the parent SBA-15, which may be ascribed to the loading of Ag. It also indicates that the ordered mesostructures were well preserved under the thermal grinding and subsequent calcinations processes [35]. When the setting temperature reaches 170 C, the
Fig. 4. HRTEM images of Ag/SBA-15 obtained with different grinding temperature: (a) 0.5 Ag/SBA-15-90, (b) 0.5 Ag/SBA-15-170.
Fig. 5. Small-angle XRD patterns of SBA-15 and 0.5 Ag/SBA-15 nanostructures obtained with different grinding temperatures.
three diffraction peaks of 0.5 Ag/SBA-15-170 are severely distorted, meaning that the deteriorating mesoscale order of the SBA-15. These results are in conformity with the HRTEM results (Figs. 1 and 4). 3.3. The mechanism of the temperature difference effect induced self-assembly method Experimental results confirmed that AgNO3 precursor was embedded into the channels effectively through temperature difference effect induced self-assembly method during the thermal grinding process. In order to investigate the formation mechanism of Ag/SBA-15 nanostructures, some controlled experiments were designed. A weight ratio of AgNO3/SBA-15 at 0.25 and a grinding temperature at 130 C were adopted to investigate the mechanism. When the sample was obtained without a temperature decrease (0.25 Ag/SBA-15-130-a), small amounts of Ag nanoparticles and nanorods were obtained in the mesopores accompanied by some Ag particles coalescing together on the exterior surface (Fig. 6a). When the thermal grinding process took place with a temperature difference of about 20 C (0.25 Ag/SBA-15-130-b), a large number of nanorods formed in the pore, and some large particles formed on the surfaces of the SBA-15 (Fig. 6b). When the grinding process was carried out with a temperature difference of about 35 C, Ag nanorods were the primary morphology accompanied by small amounts of nanoparticles in the meso-channels (Fig. 1b). These results suggest that the temperature difference induces air volume shrinkage in the channel, leading to the drop in pressure in the channels. Therefore, the melting AgNO3 obtained at 130 C, which is lower than the melting temperature of AgNO3 [36], could be
Fig. 6. HRTEM images of (a) 0.25 Ag/SBA-15-130-a prepared without temperature difference, (b) 0.25 Ag/SBA-15-130-b produced with a temperature difference of about 20 C.
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Fig. 7. N2 adsorptionedesorption isotherms of SBA-15, 0.25 Ag/SBA-15-130-a and b which were obtained by different temperature difference effect.
sucked into the channels step by step under the influence of the drop in pressure. N2 adsorptionedesorption isotherms of SBA-15 and 0.25 Ag/ SBA-15-130-a and b nanostructures, which were obtained using different temperatures, are shown in Fig. 7. The isotherms of the samples are of classical type IV with a hysteresis loop of H1, indicating the existence of ordered mesopores after grinding at 130 C and the subsequent calcinations, which is well in agreement with the HRTEM result. The physicochemical properties of the samples are summarized in Table 2. With the increase of the temperature difference to 20 C (0.25 Ag/SBA-15-130-b), the BET surface area decreases from 548.2 to 509.5 m2 g1, and the pore volume drops from 0.85 to 0.79 cm3 g1. When the temperature difference is about 35 C (0.25 Ag/SBA-15-130), the sample displays the lowest surface area of about 422.4 m2 g1 and pore volume of about 0.51 cm3 g1 (Table 1). The loading of Ag is considered to be the main reason for the decrease in the surface area and pore volume [37]. Fig. 8 shows the TG-DSC curves of 0.25 AgNO3/SBA-15-m (a mixture of AgNO3 and SBA-15) and 0.25 AgNO3/SBA-15-130. The DSC curve of 0.25 AgNO3/SBA-15-m shows four exothermic peaks at 197 C, 207 C, 297 and 450 C (Fig. 8a). The exothermic peaks at 197 and 207 C correspond to the formation of Ag2O and the melting point of AgNO3, respectively, indicating that AgNO3 mixed together with SBA-15 can reduce the melting point of AgNO3 [38,39]. The exothermic peak at ~297 C corresponds to the decomposition of Ag2O to metallic Ag, and the broad exothermic peak at ~450 C represents the decomposition of AgNO3. The AgNO3 loaded in SBA-15 channels without calcination (0.25 AgNO3/SBA15-130) exhibits similar exothermic peaks to the mixture of AgNO3/ SBA-15 (Fig. 8b). As shown in the TGA curves, 0.25 AgNO3/SBA-15130 exhibits a main weight loss of around 250 C, and 0.25 AgNO3/ SBA-15-m shows an obvious weight loss of around 450 C, which suggests that the temperature difference effect induced selfassembly method drove the precursor into the SBA-15 channels, and Ag nanostructures were formed during the calcinations process under a lower temperature [18]. The weight loss of 0.25 Ag/SBA-15-130 between 197 C to 450 C assigned to the
Table 2 Physicochemical properties of SBA-15, 0.25 Ag/SBA-15-130-a and b. Sample
BET (m2 g1)
Vp (cm3 g1)
Dp (nm)
SBA-15 0.25 Ag/SBA-15-130-a 0.25 Ag/SBA-15-130-b
836.4 548.2 509.5
0.89 0.85 0.79
6.62 6.54 6.52
Fig. 8. TG-DSC curves of (a) 0.25 AgNO3/SBA-15-m (mixture of AgNO3 and SBA-15) and (b) 0.25 AgNO3/SBA-15-130.
decomposition of AgNO3 shows that the Ag content is about 12.4%, which is close to the theoretical value of 13.7%. This result suggests that temperature difference effect induced self-assembly is an extremely efficient method for the preparation of Ag nanostructures in mesoporous channels. The mechanism of the temperature difference induced selfassembly method is illustrated in Scheme 1. During the synthesis procedure, a certain amount of AgNO3 powder is ground with 0.2 g of SBA-15, and the mixture is placed in an oven at 130 C for 30 min. Then the homogeneous powder is ground quickly for one minute outside the oven with a temperature decrease to 95 C. According to PV ¼ nRT, the temperature difference effect (DT ¼ 35 C) induces air volume shrinkage in the channel, and the drop in pressure induced by air volume shrinkage pushes the precursor assembled into the channels with a length of DL ¼ L1 L2 (L represents the long axis of mesochannels, which does not incorporate with AgNO3). The relation between the temperature decrease and the drop in volume in the channels can be described as:
DL/L1 ¼ DT/T1 DL is the moving distance of AgNO3 into the channel of the SBA15 each time, DT is the temperature difference. After the thermal grinding process, the homogeneous powder is put into the oven again. After repeating the above process 3 times, the products are obtained by calcination at 350 C for 2 h. During the synthesis process, the small amount of AgNO3 is pushed into the channels easily. With the effect of the drop in pressure, the precursor is pushed into the channel step by step, which leads to form Ag nanorods and nanowires, and large amount of AgNO3 coalesces together in the channels, which leads to form nanoparticles, and the formation of Ag nanoparticles leads to the jamming of the pores.
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Scheme 1. Illustration of the mechanism of the temperature difference induced self-assembly method.
3.4. Catalytic studies The Ag/SBA-15 nanostructures were used to catalyze the epoxidation of styrene (Table 3). Styrene epoxidation has a strong preference for the formation of styrene oxide on Ag (111) [40e42]. The 0.5 Ag/SBA-15-130 sample exhibits the optimized catalytic conversion of styrene at 77.7% and selectivity of styrene oxide at 73.7%, which can be attributed to the supported silver nanoparticles primarily expose the (111) facet accompanied with minor amounts of less-stable, leading to higher styrene epoxidation selectivity [43]. When the setting temperature is 90 C (0.5 Ag/SBA15-90), as a consequence of the coalescing of Ag nanoparticles on the exterior surfaces of the SBA-15, the conversion and selectivity reduce to 31.6% and 63.3%, respectively. 0.5 Ag/SBA-15-170 exhibits lower catalytic activity than that of 0.5 Ag/SBA-15-130, due to the deterioration of the mesoscale order of the SBA-15 which causes a serious decline of the specific surface area. The catalytic efficiency of Ag nanorods and nanowires/SBA-15 was compared. Ag nanorods/SBA-15 (0.25 Ag/SBA-15-130) with enhanced diffraction peak intensity of Ag (111) shows a lower conversion of styrene at 46.4% and selectivity of styrene oxide at 54.2%, which may be due to the accompanied with amounts of lessstable planes. The less-stable planes causes breaking of the C]C bond to form benzaldehyde or converts the styrene oxide into benzaldehyde via further oxidation, leading to the decrease of the selectivity of styrene oxide. The Ag nanowires/SBA-15 (0.15 Ag/ SBA-15-130) with the lowest weight ratio of AgNO3/SBA-15 has a conversion at 63.7% and the highest styrene oxide selectivity at 79.6%, which may be attributed to the existence of Ag nanoparticles with more Ag (111) facets exposed in the channel.
Table 3 Catalytic activities of Ag/SBA-15 nanostructures for the oxidation of styrene.
Samples prepared by conventional wetness impregnation were carried out to compare the catalytic performances of Ag/SBA-15 prepared by temperature difference effect. The HRTEM images of 0.5 Ag/SBA-15-w and 0.25 Ag/SBA-15-w indicates that no nucleation of Ag is observed because of the well-dispersion of the Ag in the channels (Fig. S1). As shown in Table 3, both 0.5 Ag/SBA-15-w and 0.25 Ag/SBA-15-w catalysts show higher conversion (79.3% and 64.5%, respectively) than that of samples prepared by temperature difference effect method with the same weight ratio. However, the selectivity of styrene oxide decreases to 52.7% and 48.3%, respectively. Generally speaking, Samples prepared by temperature difference effect method display better catalytic proprieties than that prepared by conventional wetness impregnation. 4. Conclusions In this paper, a facile and efficient temperature difference effect induced self-assembly method has been developed to fabricate Ag/ SBA-15 nanostructures. In the synthesis procedure, the dynamic assembly driving force generated by temperature difference effect induces the AgNO3 precursor self-assembly and pushes it into the mesoporous channels in a controllable manner. The morphologies of Ag nanoparticles, nanorods and nanowires/SBA-15 can be easily controlled by changing the weight ratio of AgNO3/SBA-15. Ag/SBA15 nanostructures show high activity in the epoxidation of styrene to styrene oxide. In particular, the Ag nanoparticle/SBA-15 shows 77.7% of conversion and 73.7% selectivity for the epoxidation of styrene. Acknowledgment We are grateful to the 863 Program (grant no. 2013AA031702) and the Co-building Special Project of Beijing Municipal Education for financial support of this work. Appendix A. Supplementary data
Catalyst
Cs (%)
Sa (%)
Sb(%)
0.5 Ag/SBA-15-90 0.5 Ag/SBA-15-130 0.5 Ag/SBA-15-170 0.25 Ag/SBA-15-130 0.15 Ag/SBA-15-130 0.5 Ag/SBA-15-w 0.25 Ag/SBA-15-w
31.6 77.7 46.5 46.4 63.7 79.3 64.5
36.7 26.3 25.6 45.8 20.4 47.3 51.7
63.3 73.7 74.4 54.2 79.6 52.7 48.3
Reaction condition: catalyst 50 mg, styrene 5 mmol, TBHP 2.5 mL, acetonitrile 5 mL, nitrobenzene 0.02 mL, temperature 80 C, time 9 h. Cs ¼ conversion of styrene, Sa ¼ selectivity of benzaldehyde, Sb ¼ selectivity of styrene oxide.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.05.040. References [1] J.X. Gong, F. Zhou, Z.Y. Li, Z.Y. Tang, Langmuir 28 (2012) 8959e8964. [2] T.K. Sau, A.L. Rogach, F. Jackel, T.A. Klar, J. Feldmann, Adv. Mater. 22 (2010) 1805e1825. [3] X.Y. Zhang, F. Li, Q. Wei, B. Du, D. Wu, H. Li, Sens. Actuators B 194 (2014) 64e70. [4] X.Y. Deng, C.M. Friend, J. Am. Chem. Soc. 127 (2005) 17178e17179. [5] X.Y. Liu, A.Q. Wang, X.F. Yang, T. Zhang, C.Y. Mou, D.S. Su, J. Li, Chem. Mater. 21 (2009) 410e418.
Y. Tang et al. / Microporous and Mesoporous Materials 215 (2015) 199e205 [6] Y. Chi, Q. Yuan, Y.J. Li, J.C. Tu, L. Zhao, N. Li, X.T. Li, J. Colloid Interface Sci. 383 (2012) 96e102. [7] R.J. Kalbasi, N. Mosaddegh, J. Porous Mater. 19 (2011) 557e565. [8] D.B. Zhang, L.M. Qi, J.M. Ma, H.M. Cheng, Chem. Mater. 13 (2011) 2753e2755. [9] F.J. Williams, D.P.C. Bird, A. Palermo, A.K. Santra, R.M. Lambert, J. Am. Chem. Soc. 126 (2004) 8509e8514. [10] F.J. Williams, D.P.C. Bird, E.C.H. Sykes, A.K. Santra, R.M. Lambert, J. Phys. Chem. B 107 (2003) 3824e3828. [11] V. Ambrogi, A. Donnadio, D. Pietrella, L. Latterini, F.A. Proietti, F. Marmottini, G. Padeletti, S. Kaciulis, S. Giovagnoli, M. Ricci, J. Mater. Chem. B 2 (2014) 6054e6063. [12] Z.J. Wang, Y.B. Xie, C.J. Liu, J. Phys. Chem. C 112 (2008) 19818e19824. [13] M.H. Huang, A. Choudrey, P.D. Yang, Chem. Commun. (2000) 1063e1064. [14] Y. Zhu, K. Morisato, W.Y. Li, K. Kanamori, K. Nakanishi, ACS Appl. Mater. Interfaces 5 (2013) 2118e2125. [15] X.D. Zhang, Z.P. Qu, X.Y. Li, Q.D. Zhao, X. Zhang, X. Quan, Mater. Lett. 65 (2011) 1892e1895. [16] X.D. Zhang, Z.P. Qu, X.Y. Li, Q.D. Zhao, Y. Wang, X. Quan, Catal. Commun. 16 (2011) 11e14. [17] G.P. Yong, D. Tian, H.W. Tong, S.M. Liu, J. Mol. Catal. A: Chem. 323 (2010) 40e44. [18] X.B. Huang, M. Yang, G. Wang, X.X. Zhang, Microporous Mesoporous Mater. 144 (2011) 171e175. [19] A.L. Garay, A. Pichon, S.L. James, Chem. Soc. Rev. 36 (2007) 846e855. [20] J. Han, P. Fang, W.J. Jiang, L.Y. Li, R. Guo, Langmuir 28 (2012) 4768e4775. [21] X.B. Huang, W.J. Dong, G. Wang, M. Yang, L. Tan, Y.H. Feng, X.X. Zhang, J. Colloid Interface Sci. 359 (2011) 40e46. [22] L.M. Worboys, P.P. Edwards, P.A. Anderson, Chem. Commun. (2002) 2894e2895. [23] L. Tian, Q.Y. Yang, Z. Jiang, Y. Zhu, Y. Pei, M.H. Qiao, K.N. Fan, Chem. Commun. (2011) 6168e6170. n, Chem. Mater. 19 (2007) 5023e5034. [24] J.M. Rosenholm, M. Linde [25] Y.Y. Chen, C. Wang, H.Y. Liu, J.S. Qiu, X.H. Bao, Chem. Commun. (2005) 5298e5300.
205
[26] X.W. Zhang, N. Huang, G. Wang, W.J. Dong, M. Yang, Y. Luan, Z. Shi, Microporous Mesoporous Mater. 177 (2013) 47e53. [27] B. Naik, S. Hazra, V.S. Prasad, N.N. Ghosh, Catal. Commun. 12 (2011) 1104e1108. [28] W.P. Zhu, Y.C. Han, L.J. An, Microporous Mesoporous Mater. 80 (2005) 221e226. ~o, I. Kirm, F. Medina, X. Rodríguez, Y. Cesteros, P. Salagre, [29] R.J. Chimenta J.E. Sueiras, J.L.G. Fierro, Appl. Surf. Sci. 252 (2005) 793e800. [30] L.F. Chen, P.J. Guo, L.J. Zhu, M.H. Qiao, W. Shen, H.L. Xu, K.N. Fan, Appl. Catal. A 356 (2009) 129e136. [31] A. Ungureanu, B. Dragoi, A. Chirieac, C. Ciotonea, S. Royer, D. Duprez, A.S. Mamede, E. Dumitriu, ACS Appl. Mater. Interfaces 5 (2013) 3010e3025. [32] J.C. Groen, L.A.A. Peffer, J.P. Ramírez, Microporous Mesoporous Mater. 60 (2003) 1e17. [33] V.I. Levitas, B.F. Henson, L.B. Smilowitz, B.W. Asay, J. Phys. Chem. B 110 (2006) 10105e10119. [34] A. Carrero, J.A. Calles, A.J. Vizcaíno, Appl. Catal. A 327 (2007) 82e94. [35] Y.M. Wang, Z.Y. Wu, H.J. Wang, J.H. Zhu, Adv. Funct. Mater. 16 (2006) 2374e2386. [36] Q. Jiang, Z.Y. Wu, Y.M. Wang, Y. Cao, C.F. Zhou, J.H. Zhu, J. Mater. Chem. 16 (2006) 1536. [37] X. Wei, X.F. Yang, A.Q. Wang, L. Li, X.Y. Liu, T. Zhang, C.Y. Mou, J. Li, J. Phys. Chem. C 116 (2012) 6222e6232. [38] R.V. Kumar, G. Raza, Ionics 15 (2009) 579e587. [39] R. Georgekutty, M.K. Seery, S.C. Pillai, J. Phys. Chem. C 112 (2008) 13563e13570. [40] L. Zhou, C.F. Gorin, R.J. Madlx, J. Am, Chem. Soc. 132 (2010) 434e435. [41] R.J. Madix, L. Zhou, J. Phys. Chem. C 112 (2008) 4725e4734. [42] M. Enever, S. Linic, K. Uffalussy, J.M. Vohs, M.A. Barteau, J. Phys. Chem. B 109 (2005) 2227e2233. [43] R. Xu, D.S. Wang, J.T. Zhang, Y.D. Li, Chem. Asian J. 1 (2006) 888e893.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Temperature difference effect induced self-assembly method for Ag/SBA-15 nanostructures and their catalytic properties for epoxidation of styrene Yinhai Tang, Mu Yang, Wenjun Dong, Li Tan, Xiaowei Zhang, Peng Zhao, Chaohao Peng, Ge Wang* Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 19 May 2015 Accepted 22 May 2015 Available online 5 June 2015
In this paper the first dynamics-controlled self-assembly method for Ag nanostructures in mesoporous channels is developed, and Ag/SBA-15 nanostructures are synthesized by a temperature difference effect induced self-assembly method with AgNO3 as Ag precursor and SBA-15 as support. Different morphologies of Ag nanomaterials such as nanoparticles, nanorods, and nanowires can be controlled by changing the weight ratio of AgNO3/SBA-15. In the synthesis procedure, the temperature difference effect induces air volume shrinkage in the channel, and the drop in pressure pushes the molten AgNO3 assembled into the channels step by step. Finally, Ag/SBA-15 nanostructures are obtained by calcination at 350 C for 2 h. Furthermore, the catalytic activities of Ag/SBA-15 nanostructures toward the epoxidation of styrene are investigated, and Ag nanoparticles/SBA-15-130 shows 77.7% of conversion and 73.7% selectivity for the epoxidation of styrene. © 2015 Elsevier Inc. All rights reserved.
Keywords: Ag/SBA-15 Temperature difference effect Epoxidation of styrene
1. Introduction Noble metals with quantum confinement properties represent a breakthrough in creating possibilities for catalytic, electronic, photonic, and biotechnical applications due to their structure and configuration efficiency [1e5]. Silver as a noble metal for the epoxidation of olefins has attracted considerable attention, and silver nanostructures with controllable morphologies that improve their catalytic properties have been extensively investigated in recent years [6e10]. One effective method has been developed to combine silver materials within mesoporous material channels due to their robust structures, tunable pore sizes and mesoporous structures [11]. Therefore, several strategies, such as conventional wetness impregnation [12e14], in-situ “pH-adjusting” method [15e17], double solvent technique [18], and solvent-free grinding method [19], have been developed to incorporate Ag into mesoporous channels. For example, Han et al. synthesized Agnanoparticle-loaded mesoporous silica SBA-15 through a one-step
* Corresponding author. Tel.: þ86 10 62333765. E-mail address: [email protected] (G. Wang). http://dx.doi.org/10.1016/j.micromeso.2015.05.040 1387-1811/© 2015 Elsevier Inc. All rights reserved.
method, in which Ag nanoparticles were spontaneously embedded in channels and even implanted in frameworks of mesoporous silica SBA-15 [20]. Huang et al. prepared different morphologies of Ag/SBA-15 via a double solvent technique, and both Ag nanowires and nanorods were successfully incorporated into SBA-15 with different surface properties [21]. Liam et al. utilized the phenomenon of salt occlusion to incorporate silver within mesoporous materials. Silver nanowires both included in and extruded from the siliceous mesoporous support have been produced [22]. Conventionally, the static driven force to incorporate Ag into the meso-channels, such as chemical adsorption between silver ions and functionalized chemical groups or physical adsorption driven by capillary force, is untunable [23e25]. As such a dynamicscontrolled self-assembly method to synthesize Ag nanostructures within meso-channels still remains as a challenge. In this paper, a facile and efficient temperature difference effect induced self-assembly method is developed to fabricate Ag/SBA-15 nanostructures. The dynamic assembly driving force generated by temperature difference effect induces the AgNO3 precursor selfassembly and pushes it into the mesoporous channels in a controllable manner. During the synthesis process, the homogeneous AgNO3/SBA-15 powder is first placed in an oven at a set
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temperature, and then the homogeneous powder is ground quickly as the temperature is decreased. The temperature difference induces air volume shrinkage in the channels of SBA-15, and then the molten AgNO3 is sucked into the channels as a result of the drop in pressure step by step. Finally, the Ag/SBA-15 nanostructures are obtained by calcination at 350 C for 2 h. Ag nanoparticles, nanorods and nanowires/SBA-15 can be controlled by changing the weight ratio of AgNO3/SBA-15. Due to the mesoporous structure, high specific surface area and good thermal stability, the Ag/SBA-15 nanostructures exhibit enhanced catalytic properties. The optimized catalytic conversion of styrene is 77.7% and the selectivity of styrene oxide is 73.7%. 2. Experiment 2.1. Materials Silver nitrate (AgNO3), tetraethyl-orthosilicate [(OC2H5)4Si, TEOS], tert-butyl hydroperoxide (TBHP) and HCl (36e38 wt.%) were purchased from Beijing Chemical Reagent Company. Pluronics P123 (Mw ¼ 5800) and styrene (99.5%) were supplied by SigmaeAldrich and Alfa Aesar, respectively. All reagents were used as received without further purification.
The products were filtered off, rinsed with de-ionized water, and subjected to thermal treatment at 350 C in air for 2 h to decompose the impregnated AgNO3. The final product was obtained and named as 0.5 Ag/SBA-15-w and 0.25 Ag/SBA-15-w. 2.4. Characterization High-resolution transmission electron microscopy (HRTEM) was conducted on a JEM-2100F equipped with an energydispersive X-ray (EDX) operated at the accelerating voltages of 200 kV. Nitrogen adsorptionedesorption isotherms at 77 K were obtained using an AUTOSORB-1C analyzer (USA Quantachrome Instruments). The specific surface area was calculated using the BrunauereEmmetteTeller (BET) method. Pore size distributions were calculated using the BarretteJoynereHalenda (BJH) method from the adsorption branches of the isotherms. The X-ray diffraction (XRD) patterns were taken on a M21X diffractometer (MAC Science Co. Ltd., Japan) using a Cu Ka radiation (l ¼ 1.541 Å) source at 40 kV and 200 mA. Small-angle X-ray diffraction (SAXRD) patterns were recorded on a D/MAX-2550 HB/PC (Rigaku Co., Tokyo, Japan) at 40 kV and 150 mA. Finally, the catalytic properties were analyzed by Agilent 7890/5975C-GC/MSD. 2.5. Catalytic properties
2.2. Preparation of mesoporous silica SBA-15 Mesoporous silica SBA-15 was synthesized according to a previous report [26]. 4.0 g of P123 as the structure-directing agent was first dissolved in 30 mL H2O, and 120 mL (2.0 mol/L) HCl aqueous solution was added under stirring, and then the mixture was stirred until the solution became transparent, following which 8.5 g of tetraethyl-orthosilicate (TEOS) was added as the silica source. The resultant mixture was stirred at 35 C for 24 h, and then the solution was transferred into a Teflon-sealed autoclave and aged at 110 C for 24 h. After cooling to room temperature, the precipitates were filtrated and washed with de-ionized water several times. The powder was dried and calcined at 550 C for 4 h under air flow with a heating rate of 2 C/min to remove the organic template.
Typically, the epoxidation reactions were carried out in a 50 mL round-bottom flask equipped with a reflux condenser. 5.0 mmol of styrene, 50 mg of solid catalyst, 5.0 mL acetonitrile and 0.02 mL of nitrobenzene (as the internal standard for GC-MSG analysis) were mixed in the flask. After the temperature was raised to 80 C with a reflux system, 2.5 mL tert-butyl hydroperoxide (TBHP) was added drop-wise to the flask under vigorous stirring. After allowing a certain reaction time (9 h), the flask was cooled to room temperature. Then the catalyst was filtered out, and the liquid organic products were analyzed by gas chro-matography-mass spectrometry (GC-MSD, HP5-MS column, Ar carrier gas, 200 C) using an internal standard. 3. Result and discussion
2.3. Preparation of Ag/SBA-15 nanostructures 3.1. Synthesis of Ag/SBA-15 nanostructures Ag/SBA-15 nanostructures were synthesized by a temperature difference effect induced self-assembly method. In the synthesis procedure, the SBA-15 was first dried at 120 C for 4 h at a vacuum condition in order to remove the absorbed water, and then, a certain amount of AgNO3 was ground with 0.2 g of SBA-15 in an agate mortar at room temperature. Then, the mixture was placed in an oven at 130 C for 30 min. After that, the agate mortar was taken outside and ground quickly for one minute, and the temperature was decreased to 95 C. The agate mortar was then put back into the oven again for 5 min. The process was repeated for 3 times, and the mixture was calcined at 350 C for 2 h with a heating rate of 2 C/min. The final product was obtained and named as Ag/SBA-15130. To investigate the effect of the weight ratio of AgNO3/SBA-15 and the setting temperature, Ag/SBA-15 nanostructures obtained by different weight ratios of AgNO3/SBA-15 or different setting temperatures were investigated in detail, and these samples were denoted as X Ag/SBA-15-Y, with X representing the weight ratio of AgNO3/SBA-15, and Y representing the setting temperature. Moreover, to compare the catalytic performances of Ag/SBA-15, samples prepared by conventional wetness impregnation were obtained as follow [13]: 20 mg of SBA-15 powder was soaked in 10 mL of 1 g/L (and 0.5 g/L) AgNO3 EtOH-H2O (1 þ 1 v/v) solutions, and the suspension was stirred overnight at room temperature.
Ag/SBA-15 nanostructures were synthesized by a temperature difference effect induced self-assembly method. Homogeneous AgNO3/SBA-15 powder, obtained by mixing certain amount of AgNO3 and 0.2 g of SBA-15, was placed in an oven at 130 C for 30 min, then the homogeneous powder was ground quickly with a temperature decrease to 95 C. The above process was repeated three times, and the Ag/SBA-15 nanostructures were obtained by calcination at 350 C for 2 h. Interestingly, different morphologies of Ag/SBA-15 nanostructures could be prepared via temperature difference effect induced self-assembly method by changing the weight ratio of AgNO3/SBA-15. When the weight ratio was 0.5 (0.5 Ag/SBA-15-130, 0.5 represented the weight ratio of AgNO3/SBA-15, and 130 represented the setting temperature), Ag nanoparticle structures with a diameter about 6.0 nm were observed in the SBA15 channels (Fig. 1a). When the weight ratio was 0.25 (0.25 Ag/SBA15-130), Ag nanorods/SBA-15 were prepared accompanied by small amounts of nanoparticles in the meso-channels (Fig. 1b). The Ag nanorods along the SBA-15 meso-channels were between 20 and 120 nm in length, 4.0 nm in diameter. When the weight ratio was 0.15 (0.15 Ag/SBA-15-130), long nanowires with nanoparticles were obtained in the channels (Fig. 1c). The nanowires were between 300 and 800 nm in length. It appeared that the temperature difference effect induced self-assembly method had successfully
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Fig. 1. HRTEM images of (a) 0.5 Ag/SBA-15-130, (b) 0.25 Ag/SBA-15-130, (c) 0.15 Ag/SBA-15-130.
driven AgNO3 into the mesoporous channels. These products were obtained by repeating the dynamic self-assembly process three times, and the precursor was pushed into the mesoporous channels step by step. The small amounts of AgNO3 which were pushed into the channels easily formed nanorods and nanowires, and large amounts of AgNO3 coalesced together to form nanoparticles in the channels, and the formation of Ag nanoparticles led to jamming of the pores [27]. XRD patterns of Ag/SBA-15-130 nanostructures show a broad band at 23 , which corresponds to the amorphous structure of silica (Fig. 2). Moreover, four distinct peaks at about 38.0 , 44.3 , 64.5 and 77.3 are observed in all three samples, which can be indexed as the (111), (200), (220), and (311) planes of face-centered cubic Ag
Fig. 2. Power X-ray diffraction patterns of Ag/SBA-15-130 nanostructures prepared by different weight ratio of AgNO3/SBA-15.
(JCPDS card no. 4-783) [28], respectively, indicating the presence of silver after the subsequent calcinations. At the relative-low weight ratio of AgNO3/SBA-15, the obtained Ag nanorods and nanowires/ SBA-15 with high crystallinity exhibit enhanced Ag characteristic peaks. When the weight ratio was increased to 0.5, the formation of nanoparticles with poor crystallinity, leading to the lowest intensities of XRD characteristic peaks [29]. Moreover, the broad peak in the sample of 0.5 Ag/SBA-15-130 at about 33 is indexed to Ag2O, which may be due to the incomplete decomposition of AgNO3 [21], leading to the decrease of the intensities of four characteristic peaks of Ag. Ag/SBA-15-130 nanostructures with different morphologies were further characterized by N2 adsorptionedesorption isotherms, and the pore size distributions are calculated by the BarretteJoynereHalenda (BJH) method (Fig. 3). The isotherm for all these samples can be classified as type IV for mesoporous materials, and the hysteresis loop of type H1 proves the reservation of ordered mesopores (Fig. 3a), which is in conformity with the HRTEM result [30]. The physicochemical properties of the samples are summarized in Table 1. Attributable to the silver successfully incorporated into the mesopores of SBA-15 (0.15 Ag/SBA-15-130), the BET surface area decreased from 836.4 to 486.8 m2 g1, and the pore volume dropped from 0.89 to 0.70 cm3 g1 [31]. When the weight ratio of AgNO3/SBA-15 was increased from 0.15 to 0.5, large amounts of AgNO3 precursor caused a significant decrease in the surface area and pore volume to 273.5 m2 g1 and 0.39 cm3 g1, respectively. The pore size decreases from 6.50 nm for 0.15 Ag/SBA-15-130 to that 4.91 nm for 0.25 Ag/SBA-15-130, and then increases to 5.67 nm for 0.15 Ag/SBA-15-130. It is suggested that, with the increase of the weight ratio of AgNO3/SBA-15 to 0.5, the obtained Ag nanoparticles in the channels with 6.0 nm in diameter may block the channels.
Fig. 3. N2 adsorptionedesorption isotherms of pure SBA-15 and Ag/SBA-15-130 nanostructures prepared by different weight ratios of AgNO3/SBA-15.
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Table 1 Physicochemical properties of SBA-15 and Ag/SBA-15-130 nanostructures. Sample
BET (m2 g1)
Vp (cm3 g1)
Dp (nm)
SBA-15 0.15 Ag/SBA-15-130 0.25 Ag/SBA-15-130 0.5 Ag/SBA-15-130
836.4 486.8 422.4 273.5
0.89 0.70 0.51 0.39
6.62 6.50 4.91 5.67
Therefore, both open and blocked mesopores will be formed among the nanoparticles in the channel, leading to a bimodal porosity distribution (Fig. 3b) [32], in which one pore size is 5.91 nm, and the other one is 3.80 nm. 3.2. The effect of the setting temperatures on the formation of Ag nanostructures/SBA-15 To investigate the effect of the setting temperature, 0.5 Ag/SBA15 nanostructures obtained using different setting temperatures (90 C, 130 and 170 C) were investigated in detail. The results are shown in Fig. 4. When the setting temperature was 90 C, bulk particles with sizes of tens to hundreds of nanometers were deposited on the exterior surface of the SBA-15 (Fig. 4a). When the setting temperature was increased to 130 C, a great number of Ag nanoparticles in the meso-channels was observed (Fig. 1a), suggesting that the moving interface of AgNO3/SBA-15 relaxes the elastic energy under thermal grinding, which increases the driving force for melting and reduces the melting temperature of AgNO3. So melting process can occur at the small size solid AgNO3 particle surface at 130 C in an extremely short time [33]. When the temperature reached 170 C, the mesostructure of SBA-15 was severely damaged during the thermal grinding process (Fig. 4b), which might be due to the conversion between solid and melton of AgNO3 in the repeating thermal grinding process, leading to destroy the mesostructure of SBA-15. Fig. 5 displays small-angle XRD patterns of SBA-15 and 0.5 Ag/ SBA-15 nanostructures. The pure SBA-15 exhibits three diffraction peaks, which can be indexed as (100), (110), and (200) reflections of the ordered 2D-hexagonal (p6mm) symmetry structures [34]. In the 0.5 Ag/SBA-15 nanostructures, 0.5 Ag/SBA-15-90 shows a shifted (100) peak and illegible diffraction peaks of (110) and (200), suggesting that the ordered 2D-hexagonal symmetry structure has been distorted, which may be caused by the formation of large number of bulk particles deposited on the exterior surface of the SBA-15. The 0.5 Ag/SBA-15-130 exhibits a lower diffraction peak than the parent SBA-15, which may be ascribed to the loading of Ag. It also indicates that the ordered mesostructures were well preserved under the thermal grinding and subsequent calcinations processes [35]. When the setting temperature reaches 170 C, the
Fig. 4. HRTEM images of Ag/SBA-15 obtained with different grinding temperature: (a) 0.5 Ag/SBA-15-90, (b) 0.5 Ag/SBA-15-170.
Fig. 5. Small-angle XRD patterns of SBA-15 and 0.5 Ag/SBA-15 nanostructures obtained with different grinding temperatures.
three diffraction peaks of 0.5 Ag/SBA-15-170 are severely distorted, meaning that the deteriorating mesoscale order of the SBA-15. These results are in conformity with the HRTEM results (Figs. 1 and 4). 3.3. The mechanism of the temperature difference effect induced self-assembly method Experimental results confirmed that AgNO3 precursor was embedded into the channels effectively through temperature difference effect induced self-assembly method during the thermal grinding process. In order to investigate the formation mechanism of Ag/SBA-15 nanostructures, some controlled experiments were designed. A weight ratio of AgNO3/SBA-15 at 0.25 and a grinding temperature at 130 C were adopted to investigate the mechanism. When the sample was obtained without a temperature decrease (0.25 Ag/SBA-15-130-a), small amounts of Ag nanoparticles and nanorods were obtained in the mesopores accompanied by some Ag particles coalescing together on the exterior surface (Fig. 6a). When the thermal grinding process took place with a temperature difference of about 20 C (0.25 Ag/SBA-15-130-b), a large number of nanorods formed in the pore, and some large particles formed on the surfaces of the SBA-15 (Fig. 6b). When the grinding process was carried out with a temperature difference of about 35 C, Ag nanorods were the primary morphology accompanied by small amounts of nanoparticles in the meso-channels (Fig. 1b). These results suggest that the temperature difference induces air volume shrinkage in the channel, leading to the drop in pressure in the channels. Therefore, the melting AgNO3 obtained at 130 C, which is lower than the melting temperature of AgNO3 [36], could be
Fig. 6. HRTEM images of (a) 0.25 Ag/SBA-15-130-a prepared without temperature difference, (b) 0.25 Ag/SBA-15-130-b produced with a temperature difference of about 20 C.
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Fig. 7. N2 adsorptionedesorption isotherms of SBA-15, 0.25 Ag/SBA-15-130-a and b which were obtained by different temperature difference effect.
sucked into the channels step by step under the influence of the drop in pressure. N2 adsorptionedesorption isotherms of SBA-15 and 0.25 Ag/ SBA-15-130-a and b nanostructures, which were obtained using different temperatures, are shown in Fig. 7. The isotherms of the samples are of classical type IV with a hysteresis loop of H1, indicating the existence of ordered mesopores after grinding at 130 C and the subsequent calcinations, which is well in agreement with the HRTEM result. The physicochemical properties of the samples are summarized in Table 2. With the increase of the temperature difference to 20 C (0.25 Ag/SBA-15-130-b), the BET surface area decreases from 548.2 to 509.5 m2 g1, and the pore volume drops from 0.85 to 0.79 cm3 g1. When the temperature difference is about 35 C (0.25 Ag/SBA-15-130), the sample displays the lowest surface area of about 422.4 m2 g1 and pore volume of about 0.51 cm3 g1 (Table 1). The loading of Ag is considered to be the main reason for the decrease in the surface area and pore volume [37]. Fig. 8 shows the TG-DSC curves of 0.25 AgNO3/SBA-15-m (a mixture of AgNO3 and SBA-15) and 0.25 AgNO3/SBA-15-130. The DSC curve of 0.25 AgNO3/SBA-15-m shows four exothermic peaks at 197 C, 207 C, 297 and 450 C (Fig. 8a). The exothermic peaks at 197 and 207 C correspond to the formation of Ag2O and the melting point of AgNO3, respectively, indicating that AgNO3 mixed together with SBA-15 can reduce the melting point of AgNO3 [38,39]. The exothermic peak at ~297 C corresponds to the decomposition of Ag2O to metallic Ag, and the broad exothermic peak at ~450 C represents the decomposition of AgNO3. The AgNO3 loaded in SBA-15 channels without calcination (0.25 AgNO3/SBA15-130) exhibits similar exothermic peaks to the mixture of AgNO3/ SBA-15 (Fig. 8b). As shown in the TGA curves, 0.25 AgNO3/SBA-15130 exhibits a main weight loss of around 250 C, and 0.25 AgNO3/ SBA-15-m shows an obvious weight loss of around 450 C, which suggests that the temperature difference effect induced selfassembly method drove the precursor into the SBA-15 channels, and Ag nanostructures were formed during the calcinations process under a lower temperature [18]. The weight loss of 0.25 Ag/SBA-15-130 between 197 C to 450 C assigned to the
Table 2 Physicochemical properties of SBA-15, 0.25 Ag/SBA-15-130-a and b. Sample
BET (m2 g1)
Vp (cm3 g1)
Dp (nm)
SBA-15 0.25 Ag/SBA-15-130-a 0.25 Ag/SBA-15-130-b
836.4 548.2 509.5
0.89 0.85 0.79
6.62 6.54 6.52
Fig. 8. TG-DSC curves of (a) 0.25 AgNO3/SBA-15-m (mixture of AgNO3 and SBA-15) and (b) 0.25 AgNO3/SBA-15-130.
decomposition of AgNO3 shows that the Ag content is about 12.4%, which is close to the theoretical value of 13.7%. This result suggests that temperature difference effect induced self-assembly is an extremely efficient method for the preparation of Ag nanostructures in mesoporous channels. The mechanism of the temperature difference induced selfassembly method is illustrated in Scheme 1. During the synthesis procedure, a certain amount of AgNO3 powder is ground with 0.2 g of SBA-15, and the mixture is placed in an oven at 130 C for 30 min. Then the homogeneous powder is ground quickly for one minute outside the oven with a temperature decrease to 95 C. According to PV ¼ nRT, the temperature difference effect (DT ¼ 35 C) induces air volume shrinkage in the channel, and the drop in pressure induced by air volume shrinkage pushes the precursor assembled into the channels with a length of DL ¼ L1 L2 (L represents the long axis of mesochannels, which does not incorporate with AgNO3). The relation between the temperature decrease and the drop in volume in the channels can be described as:
DL/L1 ¼ DT/T1 DL is the moving distance of AgNO3 into the channel of the SBA15 each time, DT is the temperature difference. After the thermal grinding process, the homogeneous powder is put into the oven again. After repeating the above process 3 times, the products are obtained by calcination at 350 C for 2 h. During the synthesis process, the small amount of AgNO3 is pushed into the channels easily. With the effect of the drop in pressure, the precursor is pushed into the channel step by step, which leads to form Ag nanorods and nanowires, and large amount of AgNO3 coalesces together in the channels, which leads to form nanoparticles, and the formation of Ag nanoparticles leads to the jamming of the pores.
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Scheme 1. Illustration of the mechanism of the temperature difference induced self-assembly method.
3.4. Catalytic studies The Ag/SBA-15 nanostructures were used to catalyze the epoxidation of styrene (Table 3). Styrene epoxidation has a strong preference for the formation of styrene oxide on Ag (111) [40e42]. The 0.5 Ag/SBA-15-130 sample exhibits the optimized catalytic conversion of styrene at 77.7% and selectivity of styrene oxide at 73.7%, which can be attributed to the supported silver nanoparticles primarily expose the (111) facet accompanied with minor amounts of less-stable, leading to higher styrene epoxidation selectivity [43]. When the setting temperature is 90 C (0.5 Ag/SBA15-90), as a consequence of the coalescing of Ag nanoparticles on the exterior surfaces of the SBA-15, the conversion and selectivity reduce to 31.6% and 63.3%, respectively. 0.5 Ag/SBA-15-170 exhibits lower catalytic activity than that of 0.5 Ag/SBA-15-130, due to the deterioration of the mesoscale order of the SBA-15 which causes a serious decline of the specific surface area. The catalytic efficiency of Ag nanorods and nanowires/SBA-15 was compared. Ag nanorods/SBA-15 (0.25 Ag/SBA-15-130) with enhanced diffraction peak intensity of Ag (111) shows a lower conversion of styrene at 46.4% and selectivity of styrene oxide at 54.2%, which may be due to the accompanied with amounts of lessstable planes. The less-stable planes causes breaking of the C]C bond to form benzaldehyde or converts the styrene oxide into benzaldehyde via further oxidation, leading to the decrease of the selectivity of styrene oxide. The Ag nanowires/SBA-15 (0.15 Ag/ SBA-15-130) with the lowest weight ratio of AgNO3/SBA-15 has a conversion at 63.7% and the highest styrene oxide selectivity at 79.6%, which may be attributed to the existence of Ag nanoparticles with more Ag (111) facets exposed in the channel.
Table 3 Catalytic activities of Ag/SBA-15 nanostructures for the oxidation of styrene.
Samples prepared by conventional wetness impregnation were carried out to compare the catalytic performances of Ag/SBA-15 prepared by temperature difference effect. The HRTEM images of 0.5 Ag/SBA-15-w and 0.25 Ag/SBA-15-w indicates that no nucleation of Ag is observed because of the well-dispersion of the Ag in the channels (Fig. S1). As shown in Table 3, both 0.5 Ag/SBA-15-w and 0.25 Ag/SBA-15-w catalysts show higher conversion (79.3% and 64.5%, respectively) than that of samples prepared by temperature difference effect method with the same weight ratio. However, the selectivity of styrene oxide decreases to 52.7% and 48.3%, respectively. Generally speaking, Samples prepared by temperature difference effect method display better catalytic proprieties than that prepared by conventional wetness impregnation. 4. Conclusions In this paper, a facile and efficient temperature difference effect induced self-assembly method has been developed to fabricate Ag/ SBA-15 nanostructures. In the synthesis procedure, the dynamic assembly driving force generated by temperature difference effect induces the AgNO3 precursor self-assembly and pushes it into the mesoporous channels in a controllable manner. The morphologies of Ag nanoparticles, nanorods and nanowires/SBA-15 can be easily controlled by changing the weight ratio of AgNO3/SBA-15. Ag/SBA15 nanostructures show high activity in the epoxidation of styrene to styrene oxide. In particular, the Ag nanoparticle/SBA-15 shows 77.7% of conversion and 73.7% selectivity for the epoxidation of styrene. Acknowledgment We are grateful to the 863 Program (grant no. 2013AA031702) and the Co-building Special Project of Beijing Municipal Education for financial support of this work. Appendix A. Supplementary data
Catalyst
Cs (%)
Sa (%)
Sb(%)
0.5 Ag/SBA-15-90 0.5 Ag/SBA-15-130 0.5 Ag/SBA-15-170 0.25 Ag/SBA-15-130 0.15 Ag/SBA-15-130 0.5 Ag/SBA-15-w 0.25 Ag/SBA-15-w
31.6 77.7 46.5 46.4 63.7 79.3 64.5
36.7 26.3 25.6 45.8 20.4 47.3 51.7
63.3 73.7 74.4 54.2 79.6 52.7 48.3
Reaction condition: catalyst 50 mg, styrene 5 mmol, TBHP 2.5 mL, acetonitrile 5 mL, nitrobenzene 0.02 mL, temperature 80 C, time 9 h. Cs ¼ conversion of styrene, Sa ¼ selectivity of benzaldehyde, Sb ¼ selectivity of styrene oxide.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.05.040. References [1] J.X. Gong, F. Zhou, Z.Y. Li, Z.Y. Tang, Langmuir 28 (2012) 8959e8964. [2] T.K. Sau, A.L. Rogach, F. Jackel, T.A. Klar, J. Feldmann, Adv. Mater. 22 (2010) 1805e1825. [3] X.Y. Zhang, F. Li, Q. Wei, B. Du, D. Wu, H. Li, Sens. Actuators B 194 (2014) 64e70. [4] X.Y. Deng, C.M. Friend, J. Am. Chem. Soc. 127 (2005) 17178e17179. [5] X.Y. Liu, A.Q. Wang, X.F. Yang, T. Zhang, C.Y. Mou, D.S. Su, J. Li, Chem. Mater. 21 (2009) 410e418.
Y. Tang et al. / Microporous and Mesoporous Materials 215 (2015) 199e205 [6] Y. Chi, Q. Yuan, Y.J. Li, J.C. Tu, L. Zhao, N. Li, X.T. Li, J. Colloid Interface Sci. 383 (2012) 96e102. [7] R.J. Kalbasi, N. Mosaddegh, J. Porous Mater. 19 (2011) 557e565. [8] D.B. Zhang, L.M. Qi, J.M. Ma, H.M. Cheng, Chem. Mater. 13 (2011) 2753e2755. [9] F.J. Williams, D.P.C. Bird, A. Palermo, A.K. Santra, R.M. Lambert, J. Am. Chem. Soc. 126 (2004) 8509e8514. [10] F.J. Williams, D.P.C. Bird, E.C.H. Sykes, A.K. Santra, R.M. Lambert, J. Phys. Chem. B 107 (2003) 3824e3828. [11] V. Ambrogi, A. Donnadio, D. Pietrella, L. Latterini, F.A. Proietti, F. Marmottini, G. Padeletti, S. Kaciulis, S. Giovagnoli, M. Ricci, J. Mater. Chem. B 2 (2014) 6054e6063. [12] Z.J. Wang, Y.B. Xie, C.J. Liu, J. Phys. Chem. C 112 (2008) 19818e19824. [13] M.H. Huang, A. Choudrey, P.D. Yang, Chem. Commun. (2000) 1063e1064. [14] Y. Zhu, K. Morisato, W.Y. Li, K. Kanamori, K. Nakanishi, ACS Appl. Mater. Interfaces 5 (2013) 2118e2125. [15] X.D. Zhang, Z.P. Qu, X.Y. Li, Q.D. Zhao, X. Zhang, X. Quan, Mater. Lett. 65 (2011) 1892e1895. [16] X.D. Zhang, Z.P. Qu, X.Y. Li, Q.D. Zhao, Y. Wang, X. Quan, Catal. Commun. 16 (2011) 11e14. [17] G.P. Yong, D. Tian, H.W. Tong, S.M. Liu, J. Mol. Catal. A: Chem. 323 (2010) 40e44. [18] X.B. Huang, M. Yang, G. Wang, X.X. Zhang, Microporous Mesoporous Mater. 144 (2011) 171e175. [19] A.L. Garay, A. Pichon, S.L. James, Chem. Soc. Rev. 36 (2007) 846e855. [20] J. Han, P. Fang, W.J. Jiang, L.Y. Li, R. Guo, Langmuir 28 (2012) 4768e4775. [21] X.B. Huang, W.J. Dong, G. Wang, M. Yang, L. Tan, Y.H. Feng, X.X. Zhang, J. Colloid Interface Sci. 359 (2011) 40e46. [22] L.M. Worboys, P.P. Edwards, P.A. Anderson, Chem. Commun. (2002) 2894e2895. [23] L. Tian, Q.Y. Yang, Z. Jiang, Y. Zhu, Y. Pei, M.H. Qiao, K.N. Fan, Chem. Commun. (2011) 6168e6170. n, Chem. Mater. 19 (2007) 5023e5034. [24] J.M. Rosenholm, M. Linde [25] Y.Y. Chen, C. Wang, H.Y. Liu, J.S. Qiu, X.H. Bao, Chem. Commun. (2005) 5298e5300.
205
[26] X.W. Zhang, N. Huang, G. Wang, W.J. Dong, M. Yang, Y. Luan, Z. Shi, Microporous Mesoporous Mater. 177 (2013) 47e53. [27] B. Naik, S. Hazra, V.S. Prasad, N.N. Ghosh, Catal. Commun. 12 (2011) 1104e1108. [28] W.P. Zhu, Y.C. Han, L.J. An, Microporous Mesoporous Mater. 80 (2005) 221e226. ~o, I. Kirm, F. Medina, X. Rodríguez, Y. Cesteros, P. Salagre, [29] R.J. Chimenta J.E. Sueiras, J.L.G. Fierro, Appl. Surf. Sci. 252 (2005) 793e800. [30] L.F. Chen, P.J. Guo, L.J. Zhu, M.H. Qiao, W. Shen, H.L. Xu, K.N. Fan, Appl. Catal. A 356 (2009) 129e136. [31] A. Ungureanu, B. Dragoi, A. Chirieac, C. Ciotonea, S. Royer, D. Duprez, A.S. Mamede, E. Dumitriu, ACS Appl. Mater. Interfaces 5 (2013) 3010e3025. [32] J.C. Groen, L.A.A. Peffer, J.P. Ramírez, Microporous Mesoporous Mater. 60 (2003) 1e17. [33] V.I. Levitas, B.F. Henson, L.B. Smilowitz, B.W. Asay, J. Phys. Chem. B 110 (2006) 10105e10119. [34] A. Carrero, J.A. Calles, A.J. Vizcaíno, Appl. Catal. A 327 (2007) 82e94. [35] Y.M. Wang, Z.Y. Wu, H.J. Wang, J.H. Zhu, Adv. Funct. Mater. 16 (2006) 2374e2386. [36] Q. Jiang, Z.Y. Wu, Y.M. Wang, Y. Cao, C.F. Zhou, J.H. Zhu, J. Mater. Chem. 16 (2006) 1536. [37] X. Wei, X.F. Yang, A.Q. Wang, L. Li, X.Y. Liu, T. Zhang, C.Y. Mou, J. Li, J. Phys. Chem. C 116 (2012) 6222e6232. [38] R.V. Kumar, G. Raza, Ionics 15 (2009) 579e587. [39] R. Georgekutty, M.K. Seery, S.C. Pillai, J. Phys. Chem. C 112 (2008) 13563e13570. [40] L. Zhou, C.F. Gorin, R.J. Madlx, J. Am, Chem. Soc. 132 (2010) 434e435. [41] R.J. Madix, L. Zhou, J. Phys. Chem. C 112 (2008) 4725e4734. [42] M. Enever, S. Linic, K. Uffalussy, J.M. Vohs, M.A. Barteau, J. Phys. Chem. B 109 (2005) 2227e2233. [43] R. Xu, D.S. Wang, J.T. Zhang, Y.D. Li, Chem. Asian J. 1 (2006) 888e893.