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Characterization of vesicular mesostructured silica synthesized under alkaline conditions
Recent Progress in Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Published by Elsevier B.V.
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Characterization of vesicular mesostructured silica synthesized under alkaline conditions Cheng Chi, Bo Wang, Wei Shan, Yahong Zhang and Yi Tang* Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433 (P. R. China)
1. Introduction Ordered mesostructured silica has attracted considerable attention in many different areas, such as catalysis, adsorption and separation. Among the various structural types, MCM-41 is one of the most extensively studied mesostructured silica especially in its application in catalysis [1-10]. Many different micronscale morphologies of MCM-41 have been reported by Ozin's group [6-8]. These products were typically prepared by controlling the hydrolysis of tetraethyl orthosilicate (TEOS) under an acidic synthesis condition. A liquid crystal defect mechanism was proposed by Ozin's group to explain these enigmatic curved morphologies [8]. Recently, we reported a series of MCM-41 type vesicular mesostructured silica (VMS) with a rich diversity of micron-scale topologies [10], which was prepared by using the hydrolysis of ester to drive the assembly of silicate and surfactant in an alkaline system [9]. A comparison between VMS and the traditional vesicles suggests that the formation of vesicular structure is a micron-scale self-assembly behavior of MCM-41 mesostructured silica. In this work, the VMS prepared at different reactant concentrations were further systematically characterized by means of thermogravimetric analysis, nitrogen sorption and electron microscopy. 2. Experimental Section The VMS was prepared in a sodium silicate (SS)-cetyltrimethylammionium bromide (CTAB)-ethyl acetate (EA)-water system, as shown in reference [10]. To clearly observe the vesicular structure of VMS, an ammonium treatment was carried out using a diluted ammonium solution (1 wt%) at 80°C overnight. For comparison, a silica gel was also synthesized under the same condition but
212
without adding the surfactant. The thermogravimetric-differential thermal analysis (TG-DTA) was performed on a Rigaku Thermoflex instrument. The samples were heated at a rate of 10 K min"1 from 300 K to 900 K in an air flow. Prior to TG-DTG and DTA experiment the sample was dried at 350 K for 24 h until the mass became constant. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on Philips XL 30 and JEOL JEM-2010, respectively. Nitrogen sorption isotherms were measured using a Micromeritics TriStar 3000 system. Before the sorption measurement, the samples were degassed at 200 °C for about 3 h. 3. Results and Discussion (a)
In order to get a better understanding of the TG-DTA result of the VMS sample, we made a comparison between VMS and the silica gel. Figure 1 shows that the TG-DTA profiles of VMS ranging from 300 K to 900 K are quite different from those of the silica gel. The broad peak within the temperature range 350-450 K in the TG-DTA of silica gel corresponds to the water desorption and the condensation of silanol, which leads to a weight loss of ca 8 wt%. Similarly, the VMS also has a weight loss step beginning at ca 400K, induced by the silanol condensation. However, a larger weight loss peak could be identified above 450 K. Since no obvious exothermic effect was detected from the DTA result, this peak should be ascribed to the surfactant fragmentation/evaporation within the nano-pores of VMS [11, 12]. Another relatively smaller peak at ca 620 K could be observed in the DTG, which has a strong exothermic effect. It should be Temperature/ K induced by the oxidation of surfactant or its Figure 1. TG-DTG curves for fragments at such a high temperature [11,12]. VMS (a), silica gel (b) and DTA When the VMS sample was heated up to curves for both samples (c). 900 K, only the pure silica remains, therefore, the silica content of the VMS could be estimated from its residue weight after calcination at high temperature. It is found by measuring more than 30 samples that the silica content of VMS is ca 45-55 wt%. We further made a comparison between the amount of silica in the VMS product and the silicate sodium we used during the synthesis process. Interestingly, we found that more than 90% of the silicate source transferred into the VMS product within the experimental 300
400
500
600
213
ranges of CTAB and EA concentrations (see figure 2). This result indicates that the surfactant is over-amounted in the synthesis of VMS, as compared to the silicate source. In other words, the over-amounted "soft template" during the self-assembly of vesicular MCM-41 should be an important factor for the formation of ordered 2D hexagonal mesostructure. Insufficient surfactants would lead to the formation of small amorphous silica particles. Our further study shows that amount of the CTAB also plays a role in controlling the dimension of the product. The detailed result will be reported elsewhere. The pore structures of VMS were characterized by nitrogen sorption isotherms. Figure 3 demonstrate a large hysteresis loop at the relative pressure of 0.8-1.0, 0.4 0.6 0.8 Relative pressure (p/p ) corresponding to a large quantity of nonMCM-41 pores in the VMS product. Because Figure 2. Silica yields of VMS of the existence of large pores, such material prepared at different concen-trations was ever considered to be bimodal of CATB and EA (molar ratio, 4.07 SS: x CTAB; y EA: 1,000 H2O). mesoporous silica in the literature [9]. However, we found in this work that these large pores would decrease after a simple hydrothermal treatment in a diluted ammonium solution at 80°C, while the MCM-41 mesopores were well retained. The TEM images of the ammonium-treated VMS exhibit obviously enlarged cavities (Figure 4), suggesting that the non-MCM-41 pores mainly exist in the amorphous phase inside the vesicular structure. (Figure 4f) Figure 3. Nitrogen sorption isotherms Conversely, the MCM-41 pores mainly exist of VMS (molar ratio, 4.07 SS: 1.82 in the shell part of VMS, which could be CTAB; 13.3 EA: 1,000 H2O) before identified in Figure 4d. The assembly process (I) and after (11,111) ammonium leading to the formation of such hybrid pores treatment. Curves I and II were moved in VMS was further discussed as below. up 800 and 400 cm3/g respectively. During the synthesis process various vesicular structures with MCM-41-type mesostructures are formed in the aqueous solution, which is driven by the self-assembly behavior of the silicatesurfactant complexes [10]. However, owing to the closed vesicular structures, some intermediate species (i.e. silicate anion and cationic surfactant) would possibly be encapsulated in the large cavity of VMS. During the recovery process of VMS from the synthesis solution, a fast precipitation might occur to the encapsulated species inside the vesicular structure. It would result in 0
EA (mol /1.000 H;O)
214
unordered silicate-surfactant aggregates with relatively large pores. However, these encapsulated aggregates could be easy to be removed by diluted alkali solution due to the weak interaction between surfactants and silicates. 4. Conclusion Vesicular MCM-41 was further systematically characterized by means of thermogravimetric analysis, nitrogen sorption and electron microscopy. The TG-DTA result of VMS is similar to that of the conventional MCM-41 product [10,11]. The further analysis of the silica yield revealed that more than 90% of the silicate Figure 4. SEM and TEM images of source transferred into the VMS product during VMS (molar ratio, 4.07 SS: x the self-assembly process, and an over-amount CTAB; y EA: 1,000 H2O) before of surfactant is critical to the formation of (a,c,d) and after (b,e,f) ammonium ordered 2D hexagonal mesostructure. Moreover, treatment the combination of the nitrogen sorption and TEM results showed that the nonMCM-41 pores mainly exist in the center of VMS while the MCM-41 pores exist in the shell part. 5. References [1] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 359(1992) 710. [2] A. Firouzi, D. Kumar, L. M. Bull, T. Besier, P. Sieger, Q. Huo, S. A. Walker, J.A. Zasadzinski, C. Glinka, J. Nicol, D. Margolese, G. D. Stucky and B. F. Chmelka, Science, 267(1995)1138. [3] A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke and B. F. Chmelka, Science, 261(1993) 1299. [4] H. P. Lin and C. Y. Mou, Science, 273(1996) 765. [5] H. P. Lin, Y. R. Cheng and C. Y. Mou, Chem. Mater., 10(1998) 3772. [6] H. Yang, N. Coombs and G. A. Ozin, Nature, 386(1997) 692. [7] H. Yang, G. A. Ozin and C. T. Kresge, Adv. Mater., 10(1998) 883. [8] S. M. Yang, H. Yang, N. Coombs, I. Sokolov, C. T. Kresge and G. A. Ozin, Adv. Mater., 11(1999)52. [9] G. Schulz-Ekloff, J. Rathousky and A. Zukal, Inter. J. Inorg. Mater., 1(1999) 97. [10] B. Wang, W. Shan, Y. H. Zhang, J. C. Xia, W. L. Yang, Z. Gao and Y. Tang, Adv. Mater., 17(2005) 578. [11] A. S. Araujo and M. Jaroniec, Thermochimi. Acta 363(2000) 175. [12] J. Goworekl, A. Borowka, R. Zaleski and R. Kusak J. Therm. Anal. Cal., 79(2005) 555.
211 211
Characterization of vesicular mesostructured silica synthesized under alkaline conditions Cheng Chi, Bo Wang, Wei Shan, Yahong Zhang and Yi Tang* Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433 (P. R. China)
1. Introduction Ordered mesostructured silica has attracted considerable attention in many different areas, such as catalysis, adsorption and separation. Among the various structural types, MCM-41 is one of the most extensively studied mesostructured silica especially in its application in catalysis [1-10]. Many different micronscale morphologies of MCM-41 have been reported by Ozin's group [6-8]. These products were typically prepared by controlling the hydrolysis of tetraethyl orthosilicate (TEOS) under an acidic synthesis condition. A liquid crystal defect mechanism was proposed by Ozin's group to explain these enigmatic curved morphologies [8]. Recently, we reported a series of MCM-41 type vesicular mesostructured silica (VMS) with a rich diversity of micron-scale topologies [10], which was prepared by using the hydrolysis of ester to drive the assembly of silicate and surfactant in an alkaline system [9]. A comparison between VMS and the traditional vesicles suggests that the formation of vesicular structure is a micron-scale self-assembly behavior of MCM-41 mesostructured silica. In this work, the VMS prepared at different reactant concentrations were further systematically characterized by means of thermogravimetric analysis, nitrogen sorption and electron microscopy. 2. Experimental Section The VMS was prepared in a sodium silicate (SS)-cetyltrimethylammionium bromide (CTAB)-ethyl acetate (EA)-water system, as shown in reference [10]. To clearly observe the vesicular structure of VMS, an ammonium treatment was carried out using a diluted ammonium solution (1 wt%) at 80°C overnight. For comparison, a silica gel was also synthesized under the same condition but
212
without adding the surfactant. The thermogravimetric-differential thermal analysis (TG-DTA) was performed on a Rigaku Thermoflex instrument. The samples were heated at a rate of 10 K min"1 from 300 K to 900 K in an air flow. Prior to TG-DTG and DTA experiment the sample was dried at 350 K for 24 h until the mass became constant. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on Philips XL 30 and JEOL JEM-2010, respectively. Nitrogen sorption isotherms were measured using a Micromeritics TriStar 3000 system. Before the sorption measurement, the samples were degassed at 200 °C for about 3 h. 3. Results and Discussion (a)
In order to get a better understanding of the TG-DTA result of the VMS sample, we made a comparison between VMS and the silica gel. Figure 1 shows that the TG-DTA profiles of VMS ranging from 300 K to 900 K are quite different from those of the silica gel. The broad peak within the temperature range 350-450 K in the TG-DTA of silica gel corresponds to the water desorption and the condensation of silanol, which leads to a weight loss of ca 8 wt%. Similarly, the VMS also has a weight loss step beginning at ca 400K, induced by the silanol condensation. However, a larger weight loss peak could be identified above 450 K. Since no obvious exothermic effect was detected from the DTA result, this peak should be ascribed to the surfactant fragmentation/evaporation within the nano-pores of VMS [11, 12]. Another relatively smaller peak at ca 620 K could be observed in the DTG, which has a strong exothermic effect. It should be Temperature/ K induced by the oxidation of surfactant or its Figure 1. TG-DTG curves for fragments at such a high temperature [11,12]. VMS (a), silica gel (b) and DTA When the VMS sample was heated up to curves for both samples (c). 900 K, only the pure silica remains, therefore, the silica content of the VMS could be estimated from its residue weight after calcination at high temperature. It is found by measuring more than 30 samples that the silica content of VMS is ca 45-55 wt%. We further made a comparison between the amount of silica in the VMS product and the silicate sodium we used during the synthesis process. Interestingly, we found that more than 90% of the silicate source transferred into the VMS product within the experimental 300
400
500
600
213
ranges of CTAB and EA concentrations (see figure 2). This result indicates that the surfactant is over-amounted in the synthesis of VMS, as compared to the silicate source. In other words, the over-amounted "soft template" during the self-assembly of vesicular MCM-41 should be an important factor for the formation of ordered 2D hexagonal mesostructure. Insufficient surfactants would lead to the formation of small amorphous silica particles. Our further study shows that amount of the CTAB also plays a role in controlling the dimension of the product. The detailed result will be reported elsewhere. The pore structures of VMS were characterized by nitrogen sorption isotherms. Figure 3 demonstrate a large hysteresis loop at the relative pressure of 0.8-1.0, 0.4 0.6 0.8 Relative pressure (p/p ) corresponding to a large quantity of nonMCM-41 pores in the VMS product. Because Figure 2. Silica yields of VMS of the existence of large pores, such material prepared at different concen-trations was ever considered to be bimodal of CATB and EA (molar ratio, 4.07 SS: x CTAB; y EA: 1,000 H2O). mesoporous silica in the literature [9]. However, we found in this work that these large pores would decrease after a simple hydrothermal treatment in a diluted ammonium solution at 80°C, while the MCM-41 mesopores were well retained. The TEM images of the ammonium-treated VMS exhibit obviously enlarged cavities (Figure 4), suggesting that the non-MCM-41 pores mainly exist in the amorphous phase inside the vesicular structure. (Figure 4f) Figure 3. Nitrogen sorption isotherms Conversely, the MCM-41 pores mainly exist of VMS (molar ratio, 4.07 SS: 1.82 in the shell part of VMS, which could be CTAB; 13.3 EA: 1,000 H2O) before identified in Figure 4d. The assembly process (I) and after (11,111) ammonium leading to the formation of such hybrid pores treatment. Curves I and II were moved in VMS was further discussed as below. up 800 and 400 cm3/g respectively. During the synthesis process various vesicular structures with MCM-41-type mesostructures are formed in the aqueous solution, which is driven by the self-assembly behavior of the silicatesurfactant complexes [10]. However, owing to the closed vesicular structures, some intermediate species (i.e. silicate anion and cationic surfactant) would possibly be encapsulated in the large cavity of VMS. During the recovery process of VMS from the synthesis solution, a fast precipitation might occur to the encapsulated species inside the vesicular structure. It would result in 0
EA (mol /1.000 H;O)
214
unordered silicate-surfactant aggregates with relatively large pores. However, these encapsulated aggregates could be easy to be removed by diluted alkali solution due to the weak interaction between surfactants and silicates. 4. Conclusion Vesicular MCM-41 was further systematically characterized by means of thermogravimetric analysis, nitrogen sorption and electron microscopy. The TG-DTA result of VMS is similar to that of the conventional MCM-41 product [10,11]. The further analysis of the silica yield revealed that more than 90% of the silicate Figure 4. SEM and TEM images of source transferred into the VMS product during VMS (molar ratio, 4.07 SS: x the self-assembly process, and an over-amount CTAB; y EA: 1,000 H2O) before of surfactant is critical to the formation of (a,c,d) and after (b,e,f) ammonium ordered 2D hexagonal mesostructure. Moreover, treatment the combination of the nitrogen sorption and TEM results showed that the nonMCM-41 pores mainly exist in the center of VMS while the MCM-41 pores exist in the shell part. 5. References [1] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 359(1992) 710. [2] A. Firouzi, D. Kumar, L. M. Bull, T. Besier, P. Sieger, Q. Huo, S. A. Walker, J.A. Zasadzinski, C. Glinka, J. Nicol, D. Margolese, G. D. Stucky and B. F. Chmelka, Science, 267(1995)1138. [3] A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke and B. F. Chmelka, Science, 261(1993) 1299. [4] H. P. Lin and C. Y. Mou, Science, 273(1996) 765. [5] H. P. Lin, Y. R. Cheng and C. Y. Mou, Chem. Mater., 10(1998) 3772. [6] H. Yang, N. Coombs and G. A. Ozin, Nature, 386(1997) 692. [7] H. Yang, G. A. Ozin and C. T. Kresge, Adv. Mater., 10(1998) 883. [8] S. M. Yang, H. Yang, N. Coombs, I. Sokolov, C. T. Kresge and G. A. Ozin, Adv. Mater., 11(1999)52. [9] G. Schulz-Ekloff, J. Rathousky and A. Zukal, Inter. J. Inorg. Mater., 1(1999) 97. [10] B. Wang, W. Shan, Y. H. Zhang, J. C. Xia, W. L. Yang, Z. Gao and Y. Tang, Adv. Mater., 17(2005) 578. [11] A. S. Araujo and M. Jaroniec, Thermochimi. Acta 363(2000) 175. [12] J. Goworekl, A. Borowka, R. Zaleski and R. Kusak J. Therm. Anal. Cal., 79(2005) 555.