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Microporous and Mesoporous Materials 262 (2018) 217–226
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Synthesis of hierarchically porous silicate-1 and ZSM-5 by hydrothermal transformation of SiO2 colloid crystal/carbon composites
T
Ping Liu, Li-Na Jin, Chun Jin, Jia-Nan Zhang, Shao-Wei Bian∗ College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchically structure Pore Silicate-1 ZSM-5 Catalysis
Hierarchically porous silicate-1 and ZSM-5 zeolites were successfully prepared by a hydrothermal crystallization process in the presence of SiO2 colloid crystal/carbon composite. The inter-crystalline mesopores and macropores were successfully created inside these zeolite materials. The effect of synthesis conditions including hydrothermal reaction temperature, reaction time, silica source and aluminum source on the morphology and porous structure were studied in detailed. All the samples were characterized by XRD, SEM, N2 adsorptiondesorption analysis and NH3-TPD. The advantage of hierarchically porous structure was evaluated by the catalytic application. The hierarchically porous ZSM-5 as a catalyst displayed higher catalytic activity than the conventional ZSM-5 when using the acetalization reaction of cyclohexanone as a model reaction.
1. Introduction Zeolites are highly ordered microporous materials, which possess excellent properties including open crystalline frameworks, well-defined micropores (typically 0.25–1.5 nm), high surface area, strong acidity, and good hydrothermal stability. They have been widely applied in heterogeneous catalysis [1]. Lots of reported work indicated that the catalytic sites mainly locate in the inner surface of zeolite micropores. Unfortunately, the severe diffusion limitation caused by the small pore size and long micropore length of conventional zeolites restricts not only the catalytic activity and but also the lifetime of the catalysts [2]. Although ordered mesoporous materials with amorphous frameworks facilitate the diffusion of reactant and product molecular, their weak acidity and low hydrothermal stability hinder their catalytic application [3–5]. Two promising strategies for fast mass transfer have been proposed to solve this problem. One is the preparation of nanozeolites, which facilitates to decrease the mass transfer limitation by shortening the intracrystalline diffusion path length. However the nanozeolites suffer from several severe problems, such as low preparation yield, low thermal stability and difficult separation from the reaction mixture [6]. The other is the synthesis of hierarchically porous zeolites, which are gaining more and more attention because of their unique properties including short diffusion length, high external surface area and large pore volume, while promoting the catalyst separation due to their larger secondary particle sizes. The dealumination method is able to create large pores in zeolite ∗
materials through the acid leaching or under the steaming condition [7–12]. For the desilication method, the selective extraction of silicon from the zeolite framework can be realized by alkali treatment [13]. Despite large pores could be created in zeolite materials by these two powerful methods, a decrease in crystallinity and hydrothermal stability were observed, which significantly decrease their catalytic performance in the practical application. A novel and efficient strategy, called template method, has been widely developed using carbon materials (porous carbon, CNTs, active carbon, etc.), organic aerogels, CaCO3 nanoparticles and surfactant molecular as templates to synthesize hierarchically porous zeolites. Viswanadhm et al., prepared hierarchical ZSM-5 by using glucose as a template precursor through a steam-assisted crystallization process [14]. Kim et al., used TPAOH impregnated mesoporous materials containing carbon nanotubes in the pores to synthesize hierarchical ZSM-5 structure [15]. Zhou et al., used three short-chain organosilanes to prepare hierarchically micro-/mesoporous ZSM-5 zeolite by steam-assisted crystallization of dry gels [16]. Despite these reported work successfully created large pores inside ZSM-5 zeolite, the expensive template materials, low preparation yield, pore collapse, tedious preparation process and ununiform morphology limit the further application of the template method [17–23]. Therefore, achieving an ideal hierarchically porous structure within zeolite materials is still a great challenge. Herein we developed a novel strategy to prepare hierarchically porous silicate-1 and ZSM-5 zeolites by in situ assembling of nanozeolites through hydrothermal crystallization process. Using a SiO2 colloid
Corresponding author. E-mail address: [email protected] (S.-W. Bian).
https://doi.org/10.1016/j.micromeso.2017.11.033 Received 16 July 2017; Received in revised form 19 November 2017; Accepted 22 November 2017 Available online 24 November 2017 1387-1811/ © 2017 Elsevier Inc. All rights reserved.
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to remove the template at 550 °C with a heating rate of 5 °C/min in air for 5 h. The conventional ZSM-5 (C-ZSM-5) with a Si/Al ratio of 50 was synthesized according to the previously reported method [25]. 0.065 g of NaAlO2 and 8.35 g of TEOS were added into 12.25 g of H2O. Then 1.644 g of TPAOH (40 wt.%) was added in the above under stirring. After stirred at room temperature for 3 h, the mixture was transferred into an autoclave and heated at 170 °C for 24 h. The resultant Con-ZSM5 was calcined at 550 °C in air for 5 h to remove the template. ZSM-5 samples were treated through ion exchange with 15 ml of 1 mol/L NH4NO3 aqueous solution at 80 °C for 3 h. This process was repeated 3 times. Then the solid product was collected by centrifugation, washed with deionized water and dried at 110 °C overnight in an oven. The final products were converted to H-form by calcination at 550 °C for 5 h [26].
crystal/carbon composite as the silica source resulted in the optimism hierarchically porous structure of silicate-1 and ZSM-5 zeolites. The effect of silica source, aluminum source, hydrothermal reaction temperature and reaction time on the morphology and hierarchically porous structure were investigated in detailed. The catalytic performance of the hierarchically porous ZSM-5 was studied through the acetalization of cyclohexanone with methanol. It showed higher catalytic activity than the conventional ZSM-5. 2. Experimental section 2.1. Materials Tetra-n-propylammonium hydroxide (TPAOH, 40 wt. %) were purchased from Alfar Aesar. Tetraethyl orthosilicate (TEOS), sucrose, sodium hydroxide (NaOH), ammonium hydroxide (NH3·H2O), cyclohexanone, ethanol (EtOH), sodium aluminate, aluminium chloride and aluminum nitrate nonahydrate (Al(NO3)3·9H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. Methanol was obtained from Shanghai Yunli Economic and Trade Co., Ltd.
2.3. Adsorption experiments The adsorption of methylene blue on the ZSM-5 zeolite materials was studied to investigate the effect of hierarchically pores on the diffusion of organic molecular. 30 mg of ZSM-5 zeolite materials were added into 10 ml of methylene blue solution with a concentration of 3.566 × 10−6 mol/L. After that, these suspensions were stirred vigorously at room temperature. The concentration of methylene blue was measured by a Shimadzu UV-1800 spectrophotometer.
2.2. Synthesis 2.2.1. Synthesis of SiO2 colloid crystal The SiO2 colloid crystal was prepared by a reported method with a little modification [24]. The SiO2 spheres were first synthesized by adding 6 mL of TEOS in a solution containing 10 ml of deionized water, 74 ml EtOH and 3.14 ml of NH3·H2O under continuous stirring. After 6 h, the white SiO2 spheres were collected and washed with deionized water 3 times. Then the SiO2 spheres were redispersed in a solution containing 34 ml of EtOH and 34 ml of deionized water by ultrasonic treatment. The SiO2 colloid crystal was obtained by evaporating the solution at room temperature and then calcining at 850 °C for 3 h in a muffle furnace.
2.4. Acetalization of cyclohexanone The acetalization reaction of cyclohexanone was carried out in a three-necked flask connected with a condenser [27]. In a typical reaction, 0.05 g of ZSM-5 catalyst was dispersed in a solution containing 10 mL of methanol and 0.098 g of cyclohexanone. The solution was stirred at 50 °C. The reaction mixture was separated at intervals by centrifugation and then analyzed by a Shimadzu GC-2014 gas chromatography with a Rtx-5 column and FID detector.
2.2.2. Synthesis of SiO2 colloid crystal/carbon composite 1.4 g of SiO2 colloid crystal was soaked with a mixture containing 1.75 g of sucrose, 7 ml of H2O and 0.105 ml of concentrated H2SO4 under stirring. After 1 h, the wet solid sample was heated at 100 °C for 6 h and then 160 °C for 6 h in an oven. This process was repeated three times. The black SiO2 colloid crystal/carbon composite was obtained by carbonizing at 550 °C with a heating rate of 2 °C/min in N2 for 2 h.
2.5. Characterization The powder XRD patterns were recorded on a Rigaku D/Max2550PC X-ray diffractometer with Cu Ka radiation. The material morphologies were characterized by scanning electron microscopy (SEM, Hitachi S-4800). Nitrogen adsorption-desorption analysis was performed at −196 °C using a JWGB-JK122W sorption analyzer. The specific surface area was calculated by the BET (Brunauer-EmmettTeller) method. The average pore size was calculated from the adsorption branch using the BJH (Barrett-Joyner-Halenda) method. The molar ratio of Al/Si was determined by using energy-dispersive X-ray (EDS) spectroscopy. The temperature-programmed desorption of ammonia (NH3-TPD) was performed by using an automated chemisorption analyzer (ChemStar, Quantachrome).
2.2.3. Synthesis of SiO2/carbon composite The SiO2 spheres, instead of SiO2 colloid crystal, were directly used to prepare SiO2/carbon composite. The typical preparation process of SiO2 spheres/carbon composite is similar to that of SiO2 colloid crystal/ carbon composite. The SEM image of this composite is shown in Fig. S1. 2.2.4. Synthesis of hierarchically porous silicate-1 In a typical synthesis process, the SiO2 colloid crystal/carbon composite which contains 0.1 g of SiO2, 0.5083 g of deionized water, 0.0106 g of NaOH and 0.3383 g of TPAOH (40 wt. %) were mixed under stirring for 1 h at room temperature, and then heated in a Teflonlined autoclave at 160 °C for 24 h. After the autoclave naturally cooled to room temperature, the solid product was collected, washed with deionized water 3 times and dried at 110 °C in an oven. Finally, the template inside silicate-1 was completely removed by calcinning it at 550 °C with a heating rate of 5 °C/min in air for 5 h. In a series of control experiments, various SiO2 sources were used to prepare silicate1 to study their effect on the morphology and porous structure.
3. Results and discussion 3.1. Morphology of SiO2 colloid crystal/carbon composite Fig. 1a and b shows the typical SEM images of monodisperse SiO2 spheres with an average diameter of 208 nm. As shown in Fig. 1c and d, SiO2 colloid crystal has a closely packed, ordered three-dimensional structure, which was constructed by numerous SiO2 spheres. The large ordered pores were clearly observed inside the SiO2 colloid crystal. These pores, the interstitial voids between SiO2 spheres in the colloid crystal, were filled with carbon materials after socking a mixture of sucrose and concentrated H2SO4 and then calcining at high temperature in N2. Fig. 1e and f shows that the ordered structure SiO2 colloid crystal remained well.
2.2.5. Synthesis of hierarchically porous ZSM-5 The synthesis procedure of hierarchical porous ZSM-5 (H-ZSM-5) with a Si/Al ratio of 50 was similar to that of silicate-1 except the addition of 0.0124 g of Al(NO3)3·9H2O. The resultant ZSM-5 was calcined 218
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Fig. 1. SEM images of (a and b) monodisperse SiO2 spheres, (c and d) SiO2 colloid crystal and (e and f) SiO2 colloid crystal/carbon composite.
3.2. Morphology, porosity, crystallinity and growth mechanism of hierarchically porous silicate-1 The powder XRD patterns of silicate-1 samples prepared at different temperature are shown in Fig. 2. All silicate-1 samples showed several strong diffraction peaks at 8.0°, 8.9°, 23.2°, 24.0° and 24.5°, confirming the formation of silicate-1 with well-crystallized MFI structure [28]. The line widths of diffraction peaks decreased when the reaction temperature increased from 130 °C to 180 °C, indicating the crystal size of primary silicate-1 particles which construct the hierarchically porous silicate-1 spheres gradually increased. The crystal size of primary silicate-1 was calculated to be 22.0, 27.2 and 41.6 nm using the Scherrer's equation, respectively, when the reaction temperature were 130 °C, 160 °C and 180 °C. The morphology of silicate-1 samples prepared at different hydrothermal reaction temperature was characterized by SEM. Fig. 3a shows the low magnification SEM image of silicate-1 prepared at 130 °C, which exhibited an amorphous appearance without a well-defined morphology after the SiO2 colloid crystal/carbon composite reacting with deionized water, NaOH and TPAOH. High magnification SEM images in Fig. 3b and c clearly show that these irregular particles are composed of many small primary silicate-1 particles. Large silicate-1 spheres with an average diameter of 3.3 μm were observed when increasing the hydrothermal reaction temperature to 160 °C (Fig. 3d). Fig. 3e and f reveal that numerous primary silicate-1 slices with a
Fig. 2. XRD patterns of silicate-1 samples prepared at different temperature: (a) 130 °C, (b) 160 °C and (c) 180 °C.
thickness of around 70 nm stacked together to form these large spheres. The inter-crystalline macropores and mesopores formed inside the silicate-1 spheres due to the self-assembly of primary silicate-1 slices (the inset in Fig. 3e). When further increasing the hydrothermal reaction 219
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Fig. 3. SEM images of hierarchically porous silicate-1 samples prepared at different temperature: (a–c) 130 °C, (d–f) 160 °C and (g–i) 180 °C.
crystalline mesopores in random arrangements. The sharp uptake in N2 adsorption at relative pressure p/p0 < 0.2 and significant adsorption at relative pressure p/p0 0.9–1.0 reveals the presence of micropores and macropores. Fig. 5b shows the BJH pore size distribution of hierarchically porous silicate-1samples prepared at different reaction temperature from 6 to 48 h. These samples showed hierarchical pore size distribution patterns due to the contribution of mesopores and macropores. Table 1 summarizes the pore properties of these silicate-1 samples. The BJH average pore size values of these samples prepared at different hydrothermal reaction time were calculated from the adsorption branch, which follow the order of silicate-1 (48 h) < silicate1 (6 h) < silicate-1 (12 h) < silicate-1 (24 h). These N2 adsorption analysis results are agreement with the results of SEM characterization. The optimum hierarchically porous structure was obtained when the reaction time was 24 h based on the results of N2 adsorption analysis and SEM characterization. In order to investigate the effect of silica source on the morphology and hierarchically porous structure of silicate-1 spheres, a series of control experiments were conducted. Fig. 6a and b shows that using the SiO2/carbon composite prepared by SiO2 spheres only resulted in the formation of large particles without obvious inter-crystalline pores. As shown in Fig. 6c and d, many large irregular solid particles with twinned structure formed when the pure SiO2 colloid crystal was directly used as the silica source. This phenomenon was also observed in the recently reported work [29]. Fig. 6e and f shows that amorphous particles with a porous structure were achieved when the pure SiO2 spheres were directly used as the silica source. These results indicate that the SiO2 colloid crystal/carbon composite greatly affected the morphology and porous structure of silicate-1 due to their unique structure, which facilitate to achieve appropriate SiO2 dissolution rate and crystallization rate and confine effect of SiO2 colloid crystal/carbon composites. The possible mechanism of the present synthetic strategy was investigated by performing some control experiments. Some typical SEM images of SiO2 colloid crystal/carbon composite were collected during the hydrothermal process and characterized by SEM. Fig. 7a and b shows the ordered SiO2 spheres on the outer surface of colloid crystal/
temperature to 180 °C, the morphology of primary silicate-1 slices which constructed the silicate-1 spheres changed to be cuboids. Meanwhile the hierarchically porous structure almost disappeared. It indicates that higher hydrothermal reaction temperature facilitate to form large zeolites due to the dissolution and recrystallization processes. Based on the SEM characterization, the optimum hierarchically porous structure was achieved when the hydrothermal reaction temperature was 160 °C. A series of experiments were performed to explore the effect of reaction time on the morphology and porous structure of silicate-1 samples. Fig. 4a and b shows that a mixture of irregular small silicate-1 particles and amorphous silica nanoparticles appeared when the hydrothermal reaction time was 6 h. Numerous large silicate-1 spheres with an average diameter of 1.5 μm which constructed by many nonuniform primary particles were clearly observed when the hydrothermal reaction time increased to 12 h (Fig. 4c and d). Meanwhile most amorphous nanoparticles disappeared. Fig. 4e shows that a lot of spherical particles with an average diameter of 3.3 μm formed due to the assembly of primary silicate-1 slices when further prolonging the reaction time to 24 h. High magnification SEM image in Fig. 4f displays that the in situ self-assembly of the primary silicate-1 slices during the hydrothermal synthesis process resulted in the formation of intercrystalline mesopores and macropores. The thickness of the primary silicate-1 slices was determined to be around 70 nm. Fig. 4g and h shows that the hierarchically porous structure of the silicate-1 spheres vanished when further increasing the hydrothermal reaction time to 48 h. The large solid silicate-1 particles with an average size of 3.7 μm are composed of coffin-like crystal. Densely stacking of crystals results in no spaces between inter-crystallines. The optimum hydrothermal reaction time was 24 h based on the SEM characterization results. The hierarchically porous silicate-1 prepared at 160 °C for 24 h was mainly used if not specified. The N2 adsorption analysis was used to investigate the pore structure of these silicate-1 samples prepared with different hydrothermal reaction time. As shown in Fig. 5a, all the samples showed the type IV isotherm with type H1 hysteresis loop. The hysteresis loop at p/p0 of 0.2–0.9 indicates the presence of mesoporous structure due to the inter220
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Table 1 Textural properties of silicate-1 samples prepared with different reaction time.
Fig. 4. SEM images of silicate-1 samples prepared at different hydrothermal reaction time: (a and b) 6 h, (c and d) 12 h, (e and f) 24 h and (g and h) 48 h.
Samples
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Pore Size (nm)
Silicate-1 (6 h) Silicate-1 (12 h) Silicate-1 (24 h) Silicate-1 (48 h)
236.5
166.3
108.6
0.313
0.082
0.231
8.5
308.8
251.4
81.5
0.306
0.122
0.184
9.0
365.6
302.0
84.0
0.344
0.147
0.197
9.4
362.9
309.2
85.6
0.313
0.150
0.163
7.6
Fig. 6. SEM images of silicate-1 samples prepared by using different silica sources: (a and b) SiO2/carbon composite, (c and d) SiO2 colloid crystal and (e and f) SiO2 spheres.
Fig. 5. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distributions of silicate-1 samples prepared with different reaction time.
221
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Fig. 7. SEM images of the hierarchically porous silicate-1 with different reaction time before the calcination process: (a and b) 0 h, (c and d) 12 h and (e and f) 24 h.
Fig. 8. Schematic illustration of the possible preparation process of the hierarchically porous silicate-1.
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Fig. 9. SEM images of ZSM-5 prepared with different aluminum sources: (a and d) AlCl3, (b and e) NaAlO2 and (c and f) Al(NO3)3.
Fig. 10. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distribution of the ZSM-5 samples prepared by using AlCl3, NaAlO2 and Al(NO3)3 as the aluminum source and the conventional ZSM-5 sample.
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take place exclusively inside the pores of ordered macroporous carbon, facilitating to form hierarchically porous ZSM-5 [31]. The possible growth mechanism may include three steps (Fig. 8). Firstly, the SiO2 spheres encapsulated in the SiO2 colloid crystal/carbon composite significantly gradually dissolved and shrank when they were immersed in basic solution under the hydrothermal condition at 160 °C [32]. Secondly, the dissolved silica species formed a hydrogel then in situ crystallized into silicate-1 in the presence of TPAOH [30]. The conversion from the SiO2 spheres inside the SiO2 colloid crystal/carbon composite to silicate-1 crystals should be an in situ conversion taking place in/on the solid phase of silica [33]. Finally, the small silicate-1 crystals grow into large ones through the window of ordered macroporous carbon and even through multiple adjacent macropores. The large crystals and in situ assembly of silicate-1 slices could also break up the ordered macroporous carbon [34,35].
Table 2 Textural properties of hierarchically porous ZSM-5 and conventional ZSM-5 zeolites. Samples
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Pore Size (nm)
ZSM-5-AlCl3 ZSM-5NaAlO2 ZSM-5-Al (NO3)3 C-ZSM-5
295.0 263.3
227.8 204.6
97.9 86.7
0.322 0.229
0.111 0.100
0.211 0.129
8.6 5.9
332.4
244.7
122.7
0.356
0.120
0.236
7.7
370.8
261.9
124.3
0.267
0.129
0.138
4.4
3.3. Morphology, porosity and crystallinity of the hierarchically porous ZSM-5 The strategy for hierarchically porous silicate-1 was further applied to prepare hierarchically porous ZSM-5 due to their same MFI structure. The hierarchically porous ZSM-5 was prepared by introducing aluminum source into the hydrothermal synthesis process of hierarchically porous silicate-1. The effect of aluminum source on the ZSM-5 morphology and porous structure was first investigated to achieve the optimum hierarchically porous structure. As shown in Fig. 9a and b, the ZSM-5 sample did not exhibit hierarchically porous structure and uniform spherical morphology when using AlCl3. Only numerous large irregular particles were observed. Fig. 9c and d shows that the spherical morphology appeared when using NaAlO2 as the aluminum source. However, inter-crystalline mesopores and macropores caused by the stacking of primary silicate-1 slices completely vanished. Compared with hierarchically porous silicate-1 spheres, both spherical morphology and inter-crystalline pores well remained when Al(NO3)3 was used as the aluminum source to prepare ZSM-5 (Fig. 9e and f). The presence of 3D hierarchically porous network is beneficial to the diffusion of reactant and product molecules in the zeolite particles due to the inter-crystalline pores and shorter micropore diffusion length of small primary ZSM-5 crystals. Fig. 10a shows the N2 adsorption-desorption isotherm of the ZSM-5 spheres prepared using various aluminum sources including AlCl3, NaAlO2 and Al(NO3)3, which displayed a type IV isotherm with H1 hysteresis loop. The presence of hysteresis loop at relative pressure p/p0 0.2–1.0 reveals that presence of inter-crystalline mesopores and macropores inside ZSM-5 spheres. Fig. 10b shows the BJH pore size distribution of C-ZSM-5 and ZSM-5 spheres. As shown in Table 2, the mesopore volume and average pore size of H-ZSM-5 spheres were determined to be 0.236 cm3/g and 7.7 nm, respectively, which are higher than those of the C-ZSM-5. H-ZSM-5 had lower BET surface area and mesopore surface area than C-ZSM-5. It may due to the mesopores in hierarchical ZSM-5 samples were mainly composed of the intercrystalline mesopores [36]. Fig. 11a shows the XRD pattern of the H-ZSM-5 prepared by using Al (NO3)3 as the aluminum source. It exhibited a series of characteristic diffraction peaks at 7.9, 8.8, 14.8, 23.2, 23.9 and 24.4°, which is well ascribed to a typical MFI zeolite structure (JCPDS no. 49-0657) with high crystallinity [37]. No other impurity was observed in the XRD pattern. The molar ratio of Al/Si was determined to be around 21 by using the EDS spectroscopy.
Fig. 11. XRD pattern of the H-ZSM-5.
Fig. 12. Adsorption performance of H-ZSM-5 and C-ZSM-5. The inset is the photographs of the methylene blue solution containing ZSM-5 zeolite materials at different time.
carbon composite before the hydrothermal process. It indicates that the pores inside the SiO2 colloid crystal were filled with carbon materials due to the evaporation and the carbonation of the glucose aqueous solution. The SiO2 colloid crystal/carbon composite was infused with hydrophilic specific including H2O and TPAOH due to the hydrophilic surface of SiO2 spheres. The reported work indicated that the SiO2 spheres can be gradually etched by NaOH [30]. Fig. 7c–f shows that the hierarchically porous silicate-1 spheres formed above 12 h. Lots of empty ordered macroporous carbon materials were observed inside the inter-crystalline pores. Based on the SEM characterization results (Figs. 6 and 7), the ordered macroporous carbon inside the SiO2 colloid crystal/carbon composite can help the nucleation and allow the growth of the ZSM-5
3.4. Adsorption properties of the hierarchically porous ZSM-5 The adsorption performance of methylene blue on the ZSM-5 zeolite materials was studied to investigate the effect of hierarchically pore structure on the diffusion of organic molecular. Fig. 12 compares the methylene blue adsorption efficiency of H-ZSM-5 and C-ZSM-5. For H224
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Fig. 13. (a) Acetalization of cyclohexanone catalyzed by H-ZSM-5 and C-ZSM-5 at different reaction time. (b) Plots of ln(C0/Ct) of cyclohexanone versus reaction time.
external surface area. Base on the NH3-TPD result, the H-ZSM-5 catalyst is expected to have better catalytic performances than the C-ZSM-5 catalyst. When compared to C-ZSM-5 with large particle size (Fig. S3), HZSM-5 facilitates to realize the fast diffusion of reactant and product molecules in the zeolite particles due to the inter-crystalline pores and shorter micropore diffusion length of small primary ZSM-5 crystals. The acetalization of cyclohexanone catalyzed by H-form ZSM-5 is acid catalysis reaction, which was selected as the model reaction to evaluate the effect of the hierarchically porous structure on the catalytic activity [39].
Fig. 13a show the conversions of cyclohexanone at 50 °C at different reaction time. The conversion values gradually increased when increasing the reaction time from 0 to 6 h. After 6 h, the conversion tended to be stable and reached 90.1%. The C-ZSM-5 catalyst only showed a conversion of 53.7%, which is much less than the H-ZSM-5 catalyst. In the acetalization reaction of cyclohexanone, the concentration of methanol was much higher than that of cyclohexanone, which indicates that these reactions followed the pseudo-first order reaction kinetic model. The reaction kinetic equation can be described as dCt/dt = −Kappt, where Kapp is the apparent rate constant (h−1) and t is the reaction time (h). The liner relationships between ln(Ct/C0) and reaction time (t) were observed for the acetalization of cyclohexanone catalyzed by the H-ZSM-5 and C-ZSM-5 catalysts (Fig. 13b) The apparent reaction rate constants for H-ZSM-5 and C-ZSM-5 catalysts were 1.860 × 10−6 h−1 and 3.405 × 10−7 h−1, respectively. These above results indicate that the excellent catalytic performance of H-ZSM-5 catalyst is due to the low limitation of diffusion caused by the intercrystalline macropores and mesopores, and small crystal size. High-performance catalysts with high catalytic activity at low temperature facilitate to save the cost, realize the energy conservation and reduce the requirements of the equipment. In order to explore the effect of reaction temperature on the conversion of cyclohexanone, these two catalysts (H-ZSM-5 and C-ZSM-5) were tested at 30 °C, 50 °C and 60 °C with a reaction time of 6 h. Fig. 14 displays that the cyclohexanone conversion values for both catalysts gradually increased with reaction temperature, but H-ZSM-5 showed higher catalytic activity in the temperature range due to its hierarchically porous structure.
Fig. 14. The effect of reaction temperature on the acetalization of cyclohexanone catalyzed by the H-ZSM-5 and the C-ZSM-5.
ZSM-5, the adsorption efficiency reached 84% in 10 min and the completely removal of methylene blue could be achieved in 35 min while the time of the C-ZSM-5 was 300 min, was far more slowly. The inset in Fig. 12 shows the photographs of methylene blue solutions containing ZSM-5 zeolite materials at different time. H-ZSM-5 shows higher adsorption efficiency than the C-ZSM-5 due to its unique hierarchically porous structure. The inter-crystalline mesopores and macropores facilitate to shorten the diffusion course of methylene blue molecular. This result suggests that H-ZSM-5 has higher catalytic activity than C-ZSM-5 (see Fig. 13). 3.5. Catalytic application of the hierarchically porous ZSM-5 The nature and distribution of acid sites significantly affect the catalytic performance of ZSM-5 zeolite. Hence, the temperature-programmed desorption of ammonia (NH3-TPD) were performed to investigate the catalytic properties of hierarchically porous ZSM-5 and conventional ZSM-5 catalysts. Fig. S2 shows that the similar curve shapes of both catalysts with two peaks caused by the desorption of NH3 on the weak acidic sites and strong acidic sites, respectively [38]. HZSM-5 has higher and stronger acid sites than C-ZSM-5. The amounts of weak acid and strong acid of H-ZSM-5 are higher than that of C-ZSM-5 due to the presence of more accessible acid sites in H-ZSM-5 with abundant inter-crystalline mesopores and macropores, and larger 225
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4. Conclusions
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A new strategy based on the in situ transformation and crystallization of SiO2 spheres inside the SiO2 colloid crystal/carbon composite was developed. It is a general method and can be applied to prepare hierarchically porous silicate-1 and ZSM-5 zeolites. The structure of SiO2/carbon composite significantly affected the pore structure of zeolites. The acetalization of cyclohexanone was used to investigate the effect of hierarchically architecture on the catalytic activity. The catalytic results indicate that the hierarchical structured catalyst exhibited improved catalytic performance when compared with the conventional ZSM-5. Acknowledgements This work was supported by the National Natural Science Foundation of China (51402048), the Fundamental Research Funds for the Central Universities, DHU Distinguished Young Professor Program and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.micromeso.2017.11.033. References [1] X.Y. Yang, L.H. Chen, Y. Li, J.C. Rooke, C. Sanchez, B.L. Su, Hierarchically porous materials: synthesis strategies and structure design, Chem. Soc. Rev. 46 (2017) 481–558. [2] K.A. Tarach, J. Martinez-Triguero, F. Rey, K. Gora-Marek, Hydrothermal stability and catalytic performance of desilicated highly siliceous zeolites ZSM-5, J. Catal. 339 (2016) 256–269. [3] G.Q. Song, D. Xue, J.W. Xue, F.X. Li, Synthesis and catalytic characterization of ZSM-5 zeolite with uniform mesopores prepared in the presence of a novel organosiloxane, Microporous Mesoporous Mater. 248 (2017) 192–203. [4] S.-W. Bian, Z. Ma, L.-S. Zhang, F. Niu, W.-G. Song, Silica nanotubes with mesoporous walls and various internal morphologies using hard/soft dual templates, Chem. Commun. (2009) 1261–1263. [5] S.-W. Bian, Y.-L. Zhang, H.-L. Li, Y. Yu, Y.-L. Song, W.-G. Song, γ-Alumina with hierarchically ordered mesopore/macropore from dual templates, Microporous Mesoporous Mater. 131 (2010) 289–293. [6] Y.M. Jia, J.W. Wang, K. Zhang, W. Feng, S.B. Liu, C.M. Ding, P. Liu, Nanocrystallite self-assembled hierarchical ZSM-5 zeolite microsphere for methanol to aromatics, Microporous Mesoporous Mater. 247 (2017) 103–115. [7] K. Barbera, F. Bonino, S. Bordiga, T.V.W. Janssens, P. Beato, Structure–deactivation relationship for ZSM-5 catalysts governed by framework defects, J. Catal. 280 (2011) 196–205. [8] M. Guisnet, P. Magnoux, Organic chemistry of coke formation, Appl. Catal. A Gen. 212 (2001) 83–96. [9] U. Olsbye, S. Svelle, M. Bjorgen, P. Beato, T.V. Janssens, F. Joensen, S. Bordiga, K.P. Lillerud, Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity, Angew. Chem. Int. Ed. 51 (2012) 5810–5831. [10] F. Schmidt, C. Hoffmann, F. Giordanino, S. Bordiga, P. Simon, W. Carrillo-Cabrera, S. Kaskel, Coke location in microporous and hierarchical ZSM-5 and the impact on the MTH reaction, J. Catal. 307 (2013) 238–245. [11] Z. Wan, W. Wu, G. Li, C. Wang, H. Yang, D. Zhang, Effect of SiO2/Al2O3 ratio on the performance of nanocrystal ZSM-5 zeolite catalysts in methanol to gasoline conversion, Appl. Catal. A Gen. 523 (2016) 312–320. [12] C. Pagis, A.R. Morgado Prates, D. Farrusseng, N. Bats, A. Tuel, Hollow zeolite structures: an overview of synthesis methods, Chem. Mater. 28 (2016) 5205–5223. [13] W. Schwieger, A.G. Machoke, T. Weissenberger, A. Inayat, T. Selvam, M. Klumpp, A. Inayat, Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity, Chem. Soc. Rev. 45 (2016)
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Synthesis of hierarchically porous silicate-1 and ZSM-5 by hydrothermal transformation of SiO2 colloid crystal/carbon composites
T
Ping Liu, Li-Na Jin, Chun Jin, Jia-Nan Zhang, Shao-Wei Bian∗ College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchically structure Pore Silicate-1 ZSM-5 Catalysis
Hierarchically porous silicate-1 and ZSM-5 zeolites were successfully prepared by a hydrothermal crystallization process in the presence of SiO2 colloid crystal/carbon composite. The inter-crystalline mesopores and macropores were successfully created inside these zeolite materials. The effect of synthesis conditions including hydrothermal reaction temperature, reaction time, silica source and aluminum source on the morphology and porous structure were studied in detailed. All the samples were characterized by XRD, SEM, N2 adsorptiondesorption analysis and NH3-TPD. The advantage of hierarchically porous structure was evaluated by the catalytic application. The hierarchically porous ZSM-5 as a catalyst displayed higher catalytic activity than the conventional ZSM-5 when using the acetalization reaction of cyclohexanone as a model reaction.
1. Introduction Zeolites are highly ordered microporous materials, which possess excellent properties including open crystalline frameworks, well-defined micropores (typically 0.25–1.5 nm), high surface area, strong acidity, and good hydrothermal stability. They have been widely applied in heterogeneous catalysis [1]. Lots of reported work indicated that the catalytic sites mainly locate in the inner surface of zeolite micropores. Unfortunately, the severe diffusion limitation caused by the small pore size and long micropore length of conventional zeolites restricts not only the catalytic activity and but also the lifetime of the catalysts [2]. Although ordered mesoporous materials with amorphous frameworks facilitate the diffusion of reactant and product molecular, their weak acidity and low hydrothermal stability hinder their catalytic application [3–5]. Two promising strategies for fast mass transfer have been proposed to solve this problem. One is the preparation of nanozeolites, which facilitates to decrease the mass transfer limitation by shortening the intracrystalline diffusion path length. However the nanozeolites suffer from several severe problems, such as low preparation yield, low thermal stability and difficult separation from the reaction mixture [6]. The other is the synthesis of hierarchically porous zeolites, which are gaining more and more attention because of their unique properties including short diffusion length, high external surface area and large pore volume, while promoting the catalyst separation due to their larger secondary particle sizes. The dealumination method is able to create large pores in zeolite ∗
materials through the acid leaching or under the steaming condition [7–12]. For the desilication method, the selective extraction of silicon from the zeolite framework can be realized by alkali treatment [13]. Despite large pores could be created in zeolite materials by these two powerful methods, a decrease in crystallinity and hydrothermal stability were observed, which significantly decrease their catalytic performance in the practical application. A novel and efficient strategy, called template method, has been widely developed using carbon materials (porous carbon, CNTs, active carbon, etc.), organic aerogels, CaCO3 nanoparticles and surfactant molecular as templates to synthesize hierarchically porous zeolites. Viswanadhm et al., prepared hierarchical ZSM-5 by using glucose as a template precursor through a steam-assisted crystallization process [14]. Kim et al., used TPAOH impregnated mesoporous materials containing carbon nanotubes in the pores to synthesize hierarchical ZSM-5 structure [15]. Zhou et al., used three short-chain organosilanes to prepare hierarchically micro-/mesoporous ZSM-5 zeolite by steam-assisted crystallization of dry gels [16]. Despite these reported work successfully created large pores inside ZSM-5 zeolite, the expensive template materials, low preparation yield, pore collapse, tedious preparation process and ununiform morphology limit the further application of the template method [17–23]. Therefore, achieving an ideal hierarchically porous structure within zeolite materials is still a great challenge. Herein we developed a novel strategy to prepare hierarchically porous silicate-1 and ZSM-5 zeolites by in situ assembling of nanozeolites through hydrothermal crystallization process. Using a SiO2 colloid
Corresponding author. E-mail address: [email protected] (S.-W. Bian).
https://doi.org/10.1016/j.micromeso.2017.11.033 Received 16 July 2017; Received in revised form 19 November 2017; Accepted 22 November 2017 Available online 24 November 2017 1387-1811/ © 2017 Elsevier Inc. All rights reserved.
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to remove the template at 550 °C with a heating rate of 5 °C/min in air for 5 h. The conventional ZSM-5 (C-ZSM-5) with a Si/Al ratio of 50 was synthesized according to the previously reported method [25]. 0.065 g of NaAlO2 and 8.35 g of TEOS were added into 12.25 g of H2O. Then 1.644 g of TPAOH (40 wt.%) was added in the above under stirring. After stirred at room temperature for 3 h, the mixture was transferred into an autoclave and heated at 170 °C for 24 h. The resultant Con-ZSM5 was calcined at 550 °C in air for 5 h to remove the template. ZSM-5 samples were treated through ion exchange with 15 ml of 1 mol/L NH4NO3 aqueous solution at 80 °C for 3 h. This process was repeated 3 times. Then the solid product was collected by centrifugation, washed with deionized water and dried at 110 °C overnight in an oven. The final products were converted to H-form by calcination at 550 °C for 5 h [26].
crystal/carbon composite as the silica source resulted in the optimism hierarchically porous structure of silicate-1 and ZSM-5 zeolites. The effect of silica source, aluminum source, hydrothermal reaction temperature and reaction time on the morphology and hierarchically porous structure were investigated in detailed. The catalytic performance of the hierarchically porous ZSM-5 was studied through the acetalization of cyclohexanone with methanol. It showed higher catalytic activity than the conventional ZSM-5. 2. Experimental section 2.1. Materials Tetra-n-propylammonium hydroxide (TPAOH, 40 wt. %) were purchased from Alfar Aesar. Tetraethyl orthosilicate (TEOS), sucrose, sodium hydroxide (NaOH), ammonium hydroxide (NH3·H2O), cyclohexanone, ethanol (EtOH), sodium aluminate, aluminium chloride and aluminum nitrate nonahydrate (Al(NO3)3·9H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. Methanol was obtained from Shanghai Yunli Economic and Trade Co., Ltd.
2.3. Adsorption experiments The adsorption of methylene blue on the ZSM-5 zeolite materials was studied to investigate the effect of hierarchically pores on the diffusion of organic molecular. 30 mg of ZSM-5 zeolite materials were added into 10 ml of methylene blue solution with a concentration of 3.566 × 10−6 mol/L. After that, these suspensions were stirred vigorously at room temperature. The concentration of methylene blue was measured by a Shimadzu UV-1800 spectrophotometer.
2.2. Synthesis 2.2.1. Synthesis of SiO2 colloid crystal The SiO2 colloid crystal was prepared by a reported method with a little modification [24]. The SiO2 spheres were first synthesized by adding 6 mL of TEOS in a solution containing 10 ml of deionized water, 74 ml EtOH and 3.14 ml of NH3·H2O under continuous stirring. After 6 h, the white SiO2 spheres were collected and washed with deionized water 3 times. Then the SiO2 spheres were redispersed in a solution containing 34 ml of EtOH and 34 ml of deionized water by ultrasonic treatment. The SiO2 colloid crystal was obtained by evaporating the solution at room temperature and then calcining at 850 °C for 3 h in a muffle furnace.
2.4. Acetalization of cyclohexanone The acetalization reaction of cyclohexanone was carried out in a three-necked flask connected with a condenser [27]. In a typical reaction, 0.05 g of ZSM-5 catalyst was dispersed in a solution containing 10 mL of methanol and 0.098 g of cyclohexanone. The solution was stirred at 50 °C. The reaction mixture was separated at intervals by centrifugation and then analyzed by a Shimadzu GC-2014 gas chromatography with a Rtx-5 column and FID detector.
2.2.2. Synthesis of SiO2 colloid crystal/carbon composite 1.4 g of SiO2 colloid crystal was soaked with a mixture containing 1.75 g of sucrose, 7 ml of H2O and 0.105 ml of concentrated H2SO4 under stirring. After 1 h, the wet solid sample was heated at 100 °C for 6 h and then 160 °C for 6 h in an oven. This process was repeated three times. The black SiO2 colloid crystal/carbon composite was obtained by carbonizing at 550 °C with a heating rate of 2 °C/min in N2 for 2 h.
2.5. Characterization The powder XRD patterns were recorded on a Rigaku D/Max2550PC X-ray diffractometer with Cu Ka radiation. The material morphologies were characterized by scanning electron microscopy (SEM, Hitachi S-4800). Nitrogen adsorption-desorption analysis was performed at −196 °C using a JWGB-JK122W sorption analyzer. The specific surface area was calculated by the BET (Brunauer-EmmettTeller) method. The average pore size was calculated from the adsorption branch using the BJH (Barrett-Joyner-Halenda) method. The molar ratio of Al/Si was determined by using energy-dispersive X-ray (EDS) spectroscopy. The temperature-programmed desorption of ammonia (NH3-TPD) was performed by using an automated chemisorption analyzer (ChemStar, Quantachrome).
2.2.3. Synthesis of SiO2/carbon composite The SiO2 spheres, instead of SiO2 colloid crystal, were directly used to prepare SiO2/carbon composite. The typical preparation process of SiO2 spheres/carbon composite is similar to that of SiO2 colloid crystal/ carbon composite. The SEM image of this composite is shown in Fig. S1. 2.2.4. Synthesis of hierarchically porous silicate-1 In a typical synthesis process, the SiO2 colloid crystal/carbon composite which contains 0.1 g of SiO2, 0.5083 g of deionized water, 0.0106 g of NaOH and 0.3383 g of TPAOH (40 wt. %) were mixed under stirring for 1 h at room temperature, and then heated in a Teflonlined autoclave at 160 °C for 24 h. After the autoclave naturally cooled to room temperature, the solid product was collected, washed with deionized water 3 times and dried at 110 °C in an oven. Finally, the template inside silicate-1 was completely removed by calcinning it at 550 °C with a heating rate of 5 °C/min in air for 5 h. In a series of control experiments, various SiO2 sources were used to prepare silicate1 to study their effect on the morphology and porous structure.
3. Results and discussion 3.1. Morphology of SiO2 colloid crystal/carbon composite Fig. 1a and b shows the typical SEM images of monodisperse SiO2 spheres with an average diameter of 208 nm. As shown in Fig. 1c and d, SiO2 colloid crystal has a closely packed, ordered three-dimensional structure, which was constructed by numerous SiO2 spheres. The large ordered pores were clearly observed inside the SiO2 colloid crystal. These pores, the interstitial voids between SiO2 spheres in the colloid crystal, were filled with carbon materials after socking a mixture of sucrose and concentrated H2SO4 and then calcining at high temperature in N2. Fig. 1e and f shows that the ordered structure SiO2 colloid crystal remained well.
2.2.5. Synthesis of hierarchically porous ZSM-5 The synthesis procedure of hierarchical porous ZSM-5 (H-ZSM-5) with a Si/Al ratio of 50 was similar to that of silicate-1 except the addition of 0.0124 g of Al(NO3)3·9H2O. The resultant ZSM-5 was calcined 218
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Fig. 1. SEM images of (a and b) monodisperse SiO2 spheres, (c and d) SiO2 colloid crystal and (e and f) SiO2 colloid crystal/carbon composite.
3.2. Morphology, porosity, crystallinity and growth mechanism of hierarchically porous silicate-1 The powder XRD patterns of silicate-1 samples prepared at different temperature are shown in Fig. 2. All silicate-1 samples showed several strong diffraction peaks at 8.0°, 8.9°, 23.2°, 24.0° and 24.5°, confirming the formation of silicate-1 with well-crystallized MFI structure [28]. The line widths of diffraction peaks decreased when the reaction temperature increased from 130 °C to 180 °C, indicating the crystal size of primary silicate-1 particles which construct the hierarchically porous silicate-1 spheres gradually increased. The crystal size of primary silicate-1 was calculated to be 22.0, 27.2 and 41.6 nm using the Scherrer's equation, respectively, when the reaction temperature were 130 °C, 160 °C and 180 °C. The morphology of silicate-1 samples prepared at different hydrothermal reaction temperature was characterized by SEM. Fig. 3a shows the low magnification SEM image of silicate-1 prepared at 130 °C, which exhibited an amorphous appearance without a well-defined morphology after the SiO2 colloid crystal/carbon composite reacting with deionized water, NaOH and TPAOH. High magnification SEM images in Fig. 3b and c clearly show that these irregular particles are composed of many small primary silicate-1 particles. Large silicate-1 spheres with an average diameter of 3.3 μm were observed when increasing the hydrothermal reaction temperature to 160 °C (Fig. 3d). Fig. 3e and f reveal that numerous primary silicate-1 slices with a
Fig. 2. XRD patterns of silicate-1 samples prepared at different temperature: (a) 130 °C, (b) 160 °C and (c) 180 °C.
thickness of around 70 nm stacked together to form these large spheres. The inter-crystalline macropores and mesopores formed inside the silicate-1 spheres due to the self-assembly of primary silicate-1 slices (the inset in Fig. 3e). When further increasing the hydrothermal reaction 219
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Fig. 3. SEM images of hierarchically porous silicate-1 samples prepared at different temperature: (a–c) 130 °C, (d–f) 160 °C and (g–i) 180 °C.
crystalline mesopores in random arrangements. The sharp uptake in N2 adsorption at relative pressure p/p0 < 0.2 and significant adsorption at relative pressure p/p0 0.9–1.0 reveals the presence of micropores and macropores. Fig. 5b shows the BJH pore size distribution of hierarchically porous silicate-1samples prepared at different reaction temperature from 6 to 48 h. These samples showed hierarchical pore size distribution patterns due to the contribution of mesopores and macropores. Table 1 summarizes the pore properties of these silicate-1 samples. The BJH average pore size values of these samples prepared at different hydrothermal reaction time were calculated from the adsorption branch, which follow the order of silicate-1 (48 h) < silicate1 (6 h) < silicate-1 (12 h) < silicate-1 (24 h). These N2 adsorption analysis results are agreement with the results of SEM characterization. The optimum hierarchically porous structure was obtained when the reaction time was 24 h based on the results of N2 adsorption analysis and SEM characterization. In order to investigate the effect of silica source on the morphology and hierarchically porous structure of silicate-1 spheres, a series of control experiments were conducted. Fig. 6a and b shows that using the SiO2/carbon composite prepared by SiO2 spheres only resulted in the formation of large particles without obvious inter-crystalline pores. As shown in Fig. 6c and d, many large irregular solid particles with twinned structure formed when the pure SiO2 colloid crystal was directly used as the silica source. This phenomenon was also observed in the recently reported work [29]. Fig. 6e and f shows that amorphous particles with a porous structure were achieved when the pure SiO2 spheres were directly used as the silica source. These results indicate that the SiO2 colloid crystal/carbon composite greatly affected the morphology and porous structure of silicate-1 due to their unique structure, which facilitate to achieve appropriate SiO2 dissolution rate and crystallization rate and confine effect of SiO2 colloid crystal/carbon composites. The possible mechanism of the present synthetic strategy was investigated by performing some control experiments. Some typical SEM images of SiO2 colloid crystal/carbon composite were collected during the hydrothermal process and characterized by SEM. Fig. 7a and b shows the ordered SiO2 spheres on the outer surface of colloid crystal/
temperature to 180 °C, the morphology of primary silicate-1 slices which constructed the silicate-1 spheres changed to be cuboids. Meanwhile the hierarchically porous structure almost disappeared. It indicates that higher hydrothermal reaction temperature facilitate to form large zeolites due to the dissolution and recrystallization processes. Based on the SEM characterization, the optimum hierarchically porous structure was achieved when the hydrothermal reaction temperature was 160 °C. A series of experiments were performed to explore the effect of reaction time on the morphology and porous structure of silicate-1 samples. Fig. 4a and b shows that a mixture of irregular small silicate-1 particles and amorphous silica nanoparticles appeared when the hydrothermal reaction time was 6 h. Numerous large silicate-1 spheres with an average diameter of 1.5 μm which constructed by many nonuniform primary particles were clearly observed when the hydrothermal reaction time increased to 12 h (Fig. 4c and d). Meanwhile most amorphous nanoparticles disappeared. Fig. 4e shows that a lot of spherical particles with an average diameter of 3.3 μm formed due to the assembly of primary silicate-1 slices when further prolonging the reaction time to 24 h. High magnification SEM image in Fig. 4f displays that the in situ self-assembly of the primary silicate-1 slices during the hydrothermal synthesis process resulted in the formation of intercrystalline mesopores and macropores. The thickness of the primary silicate-1 slices was determined to be around 70 nm. Fig. 4g and h shows that the hierarchically porous structure of the silicate-1 spheres vanished when further increasing the hydrothermal reaction time to 48 h. The large solid silicate-1 particles with an average size of 3.7 μm are composed of coffin-like crystal. Densely stacking of crystals results in no spaces between inter-crystallines. The optimum hydrothermal reaction time was 24 h based on the SEM characterization results. The hierarchically porous silicate-1 prepared at 160 °C for 24 h was mainly used if not specified. The N2 adsorption analysis was used to investigate the pore structure of these silicate-1 samples prepared with different hydrothermal reaction time. As shown in Fig. 5a, all the samples showed the type IV isotherm with type H1 hysteresis loop. The hysteresis loop at p/p0 of 0.2–0.9 indicates the presence of mesoporous structure due to the inter220
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Table 1 Textural properties of silicate-1 samples prepared with different reaction time.
Fig. 4. SEM images of silicate-1 samples prepared at different hydrothermal reaction time: (a and b) 6 h, (c and d) 12 h, (e and f) 24 h and (g and h) 48 h.
Samples
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Pore Size (nm)
Silicate-1 (6 h) Silicate-1 (12 h) Silicate-1 (24 h) Silicate-1 (48 h)
236.5
166.3
108.6
0.313
0.082
0.231
8.5
308.8
251.4
81.5
0.306
0.122
0.184
9.0
365.6
302.0
84.0
0.344
0.147
0.197
9.4
362.9
309.2
85.6
0.313
0.150
0.163
7.6
Fig. 6. SEM images of silicate-1 samples prepared by using different silica sources: (a and b) SiO2/carbon composite, (c and d) SiO2 colloid crystal and (e and f) SiO2 spheres.
Fig. 5. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distributions of silicate-1 samples prepared with different reaction time.
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Fig. 7. SEM images of the hierarchically porous silicate-1 with different reaction time before the calcination process: (a and b) 0 h, (c and d) 12 h and (e and f) 24 h.
Fig. 8. Schematic illustration of the possible preparation process of the hierarchically porous silicate-1.
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Fig. 9. SEM images of ZSM-5 prepared with different aluminum sources: (a and d) AlCl3, (b and e) NaAlO2 and (c and f) Al(NO3)3.
Fig. 10. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distribution of the ZSM-5 samples prepared by using AlCl3, NaAlO2 and Al(NO3)3 as the aluminum source and the conventional ZSM-5 sample.
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take place exclusively inside the pores of ordered macroporous carbon, facilitating to form hierarchically porous ZSM-5 [31]. The possible growth mechanism may include three steps (Fig. 8). Firstly, the SiO2 spheres encapsulated in the SiO2 colloid crystal/carbon composite significantly gradually dissolved and shrank when they were immersed in basic solution under the hydrothermal condition at 160 °C [32]. Secondly, the dissolved silica species formed a hydrogel then in situ crystallized into silicate-1 in the presence of TPAOH [30]. The conversion from the SiO2 spheres inside the SiO2 colloid crystal/carbon composite to silicate-1 crystals should be an in situ conversion taking place in/on the solid phase of silica [33]. Finally, the small silicate-1 crystals grow into large ones through the window of ordered macroporous carbon and even through multiple adjacent macropores. The large crystals and in situ assembly of silicate-1 slices could also break up the ordered macroporous carbon [34,35].
Table 2 Textural properties of hierarchically porous ZSM-5 and conventional ZSM-5 zeolites. Samples
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Pore Size (nm)
ZSM-5-AlCl3 ZSM-5NaAlO2 ZSM-5-Al (NO3)3 C-ZSM-5
295.0 263.3
227.8 204.6
97.9 86.7
0.322 0.229
0.111 0.100
0.211 0.129
8.6 5.9
332.4
244.7
122.7
0.356
0.120
0.236
7.7
370.8
261.9
124.3
0.267
0.129
0.138
4.4
3.3. Morphology, porosity and crystallinity of the hierarchically porous ZSM-5 The strategy for hierarchically porous silicate-1 was further applied to prepare hierarchically porous ZSM-5 due to their same MFI structure. The hierarchically porous ZSM-5 was prepared by introducing aluminum source into the hydrothermal synthesis process of hierarchically porous silicate-1. The effect of aluminum source on the ZSM-5 morphology and porous structure was first investigated to achieve the optimum hierarchically porous structure. As shown in Fig. 9a and b, the ZSM-5 sample did not exhibit hierarchically porous structure and uniform spherical morphology when using AlCl3. Only numerous large irregular particles were observed. Fig. 9c and d shows that the spherical morphology appeared when using NaAlO2 as the aluminum source. However, inter-crystalline mesopores and macropores caused by the stacking of primary silicate-1 slices completely vanished. Compared with hierarchically porous silicate-1 spheres, both spherical morphology and inter-crystalline pores well remained when Al(NO3)3 was used as the aluminum source to prepare ZSM-5 (Fig. 9e and f). The presence of 3D hierarchically porous network is beneficial to the diffusion of reactant and product molecules in the zeolite particles due to the inter-crystalline pores and shorter micropore diffusion length of small primary ZSM-5 crystals. Fig. 10a shows the N2 adsorption-desorption isotherm of the ZSM-5 spheres prepared using various aluminum sources including AlCl3, NaAlO2 and Al(NO3)3, which displayed a type IV isotherm with H1 hysteresis loop. The presence of hysteresis loop at relative pressure p/p0 0.2–1.0 reveals that presence of inter-crystalline mesopores and macropores inside ZSM-5 spheres. Fig. 10b shows the BJH pore size distribution of C-ZSM-5 and ZSM-5 spheres. As shown in Table 2, the mesopore volume and average pore size of H-ZSM-5 spheres were determined to be 0.236 cm3/g and 7.7 nm, respectively, which are higher than those of the C-ZSM-5. H-ZSM-5 had lower BET surface area and mesopore surface area than C-ZSM-5. It may due to the mesopores in hierarchical ZSM-5 samples were mainly composed of the intercrystalline mesopores [36]. Fig. 11a shows the XRD pattern of the H-ZSM-5 prepared by using Al (NO3)3 as the aluminum source. It exhibited a series of characteristic diffraction peaks at 7.9, 8.8, 14.8, 23.2, 23.9 and 24.4°, which is well ascribed to a typical MFI zeolite structure (JCPDS no. 49-0657) with high crystallinity [37]. No other impurity was observed in the XRD pattern. The molar ratio of Al/Si was determined to be around 21 by using the EDS spectroscopy.
Fig. 11. XRD pattern of the H-ZSM-5.
Fig. 12. Adsorption performance of H-ZSM-5 and C-ZSM-5. The inset is the photographs of the methylene blue solution containing ZSM-5 zeolite materials at different time.
carbon composite before the hydrothermal process. It indicates that the pores inside the SiO2 colloid crystal were filled with carbon materials due to the evaporation and the carbonation of the glucose aqueous solution. The SiO2 colloid crystal/carbon composite was infused with hydrophilic specific including H2O and TPAOH due to the hydrophilic surface of SiO2 spheres. The reported work indicated that the SiO2 spheres can be gradually etched by NaOH [30]. Fig. 7c–f shows that the hierarchically porous silicate-1 spheres formed above 12 h. Lots of empty ordered macroporous carbon materials were observed inside the inter-crystalline pores. Based on the SEM characterization results (Figs. 6 and 7), the ordered macroporous carbon inside the SiO2 colloid crystal/carbon composite can help the nucleation and allow the growth of the ZSM-5
3.4. Adsorption properties of the hierarchically porous ZSM-5 The adsorption performance of methylene blue on the ZSM-5 zeolite materials was studied to investigate the effect of hierarchically pore structure on the diffusion of organic molecular. Fig. 12 compares the methylene blue adsorption efficiency of H-ZSM-5 and C-ZSM-5. For H224
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Fig. 13. (a) Acetalization of cyclohexanone catalyzed by H-ZSM-5 and C-ZSM-5 at different reaction time. (b) Plots of ln(C0/Ct) of cyclohexanone versus reaction time.
external surface area. Base on the NH3-TPD result, the H-ZSM-5 catalyst is expected to have better catalytic performances than the C-ZSM-5 catalyst. When compared to C-ZSM-5 with large particle size (Fig. S3), HZSM-5 facilitates to realize the fast diffusion of reactant and product molecules in the zeolite particles due to the inter-crystalline pores and shorter micropore diffusion length of small primary ZSM-5 crystals. The acetalization of cyclohexanone catalyzed by H-form ZSM-5 is acid catalysis reaction, which was selected as the model reaction to evaluate the effect of the hierarchically porous structure on the catalytic activity [39].
Fig. 13a show the conversions of cyclohexanone at 50 °C at different reaction time. The conversion values gradually increased when increasing the reaction time from 0 to 6 h. After 6 h, the conversion tended to be stable and reached 90.1%. The C-ZSM-5 catalyst only showed a conversion of 53.7%, which is much less than the H-ZSM-5 catalyst. In the acetalization reaction of cyclohexanone, the concentration of methanol was much higher than that of cyclohexanone, which indicates that these reactions followed the pseudo-first order reaction kinetic model. The reaction kinetic equation can be described as dCt/dt = −Kappt, where Kapp is the apparent rate constant (h−1) and t is the reaction time (h). The liner relationships between ln(Ct/C0) and reaction time (t) were observed for the acetalization of cyclohexanone catalyzed by the H-ZSM-5 and C-ZSM-5 catalysts (Fig. 13b) The apparent reaction rate constants for H-ZSM-5 and C-ZSM-5 catalysts were 1.860 × 10−6 h−1 and 3.405 × 10−7 h−1, respectively. These above results indicate that the excellent catalytic performance of H-ZSM-5 catalyst is due to the low limitation of diffusion caused by the intercrystalline macropores and mesopores, and small crystal size. High-performance catalysts with high catalytic activity at low temperature facilitate to save the cost, realize the energy conservation and reduce the requirements of the equipment. In order to explore the effect of reaction temperature on the conversion of cyclohexanone, these two catalysts (H-ZSM-5 and C-ZSM-5) were tested at 30 °C, 50 °C and 60 °C with a reaction time of 6 h. Fig. 14 displays that the cyclohexanone conversion values for both catalysts gradually increased with reaction temperature, but H-ZSM-5 showed higher catalytic activity in the temperature range due to its hierarchically porous structure.
Fig. 14. The effect of reaction temperature on the acetalization of cyclohexanone catalyzed by the H-ZSM-5 and the C-ZSM-5.
ZSM-5, the adsorption efficiency reached 84% in 10 min and the completely removal of methylene blue could be achieved in 35 min while the time of the C-ZSM-5 was 300 min, was far more slowly. The inset in Fig. 12 shows the photographs of methylene blue solutions containing ZSM-5 zeolite materials at different time. H-ZSM-5 shows higher adsorption efficiency than the C-ZSM-5 due to its unique hierarchically porous structure. The inter-crystalline mesopores and macropores facilitate to shorten the diffusion course of methylene blue molecular. This result suggests that H-ZSM-5 has higher catalytic activity than C-ZSM-5 (see Fig. 13). 3.5. Catalytic application of the hierarchically porous ZSM-5 The nature and distribution of acid sites significantly affect the catalytic performance of ZSM-5 zeolite. Hence, the temperature-programmed desorption of ammonia (NH3-TPD) were performed to investigate the catalytic properties of hierarchically porous ZSM-5 and conventional ZSM-5 catalysts. Fig. S2 shows that the similar curve shapes of both catalysts with two peaks caused by the desorption of NH3 on the weak acidic sites and strong acidic sites, respectively [38]. HZSM-5 has higher and stronger acid sites than C-ZSM-5. The amounts of weak acid and strong acid of H-ZSM-5 are higher than that of C-ZSM-5 due to the presence of more accessible acid sites in H-ZSM-5 with abundant inter-crystalline mesopores and macropores, and larger 225
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4. Conclusions
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A new strategy based on the in situ transformation and crystallization of SiO2 spheres inside the SiO2 colloid crystal/carbon composite was developed. It is a general method and can be applied to prepare hierarchically porous silicate-1 and ZSM-5 zeolites. The structure of SiO2/carbon composite significantly affected the pore structure of zeolites. The acetalization of cyclohexanone was used to investigate the effect of hierarchically architecture on the catalytic activity. The catalytic results indicate that the hierarchical structured catalyst exhibited improved catalytic performance when compared with the conventional ZSM-5. Acknowledgements This work was supported by the National Natural Science Foundation of China (51402048), the Fundamental Research Funds for the Central Universities, DHU Distinguished Young Professor Program and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.micromeso.2017.11.033. References [1] X.Y. Yang, L.H. Chen, Y. Li, J.C. Rooke, C. Sanchez, B.L. Su, Hierarchically porous materials: synthesis strategies and structure design, Chem. Soc. Rev. 46 (2017) 481–558. [2] K.A. Tarach, J. Martinez-Triguero, F. Rey, K. Gora-Marek, Hydrothermal stability and catalytic performance of desilicated highly siliceous zeolites ZSM-5, J. Catal. 339 (2016) 256–269. [3] G.Q. Song, D. Xue, J.W. Xue, F.X. Li, Synthesis and catalytic characterization of ZSM-5 zeolite with uniform mesopores prepared in the presence of a novel organosiloxane, Microporous Mesoporous Mater. 248 (2017) 192–203. [4] S.-W. Bian, Z. Ma, L.-S. Zhang, F. Niu, W.-G. Song, Silica nanotubes with mesoporous walls and various internal morphologies using hard/soft dual templates, Chem. Commun. (2009) 1261–1263. [5] S.-W. Bian, Y.-L. Zhang, H.-L. Li, Y. Yu, Y.-L. Song, W.-G. Song, γ-Alumina with hierarchically ordered mesopore/macropore from dual templates, Microporous Mesoporous Mater. 131 (2010) 289–293. [6] Y.M. Jia, J.W. Wang, K. Zhang, W. Feng, S.B. Liu, C.M. Ding, P. Liu, Nanocrystallite self-assembled hierarchical ZSM-5 zeolite microsphere for methanol to aromatics, Microporous Mesoporous Mater. 247 (2017) 103–115. [7] K. Barbera, F. Bonino, S. Bordiga, T.V.W. Janssens, P. Beato, Structure–deactivation relationship for ZSM-5 catalysts governed by framework defects, J. Catal. 280 (2011) 196–205. [8] M. Guisnet, P. Magnoux, Organic chemistry of coke formation, Appl. Catal. A Gen. 212 (2001) 83–96. [9] U. Olsbye, S. Svelle, M. Bjorgen, P. Beato, T.V. Janssens, F. Joensen, S. Bordiga, K.P. Lillerud, Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity, Angew. Chem. Int. Ed. 51 (2012) 5810–5831. [10] F. Schmidt, C. Hoffmann, F. Giordanino, S. Bordiga, P. Simon, W. Carrillo-Cabrera, S. Kaskel, Coke location in microporous and hierarchical ZSM-5 and the impact on the MTH reaction, J. Catal. 307 (2013) 238–245. [11] Z. Wan, W. Wu, G. Li, C. Wang, H. Yang, D. Zhang, Effect of SiO2/Al2O3 ratio on the performance of nanocrystal ZSM-5 zeolite catalysts in methanol to gasoline conversion, Appl. Catal. A Gen. 523 (2016) 312–320. [12] C. Pagis, A.R. Morgado Prates, D. Farrusseng, N. Bats, A. Tuel, Hollow zeolite structures: an overview of synthesis methods, Chem. Mater. 28 (2016) 5205–5223. [13] W. Schwieger, A.G. Machoke, T. Weissenberger, A. Inayat, T. Selvam, M. Klumpp, A. Inayat, Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity, Chem. Soc. Rev. 45 (2016)
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