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Microemulsion-directed synthesis of zeolite A nano-crystals
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
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Microemulsion-directed synthesis of zeolite A nano-crystals Jianan Zhang, Wenfu Yan, Hong Ding, Yang Liu, Kangjian Tang, Jihong Yu* and Ruren Xu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012. Email: [email protected] ABSTRACT This contribution reports the synthesis of zeolite A (LTA) nanocrystals with narrow particle size distribution and sphere-shaped morphology in the presence of cationic microemulsion. The results presented here show that the microemulsion environment can accelerate the crystallization of LTA nanocrystals. The composition of the microemulsion and the reaction time can significantly affect the crystallinity and the particle size distribution of the resulting LTA nanocrystals. X-ray diffraction and scanning electron microscopy (SEM) were used to confirm zeolite crystallinity, phase purity, and morphology. 1. INTRODUCTION In recent years, the synthesis of nanocrystalline zeolites has received much attention. The reduction of particle size of zeolites from micrometer to nanometer scale leads to substantial changes of their properties. Previous studies revealed that the particle size and morphology of the zeolite crystals play an important role in their applications in the areas of catalysis and separation [1]. Thus, the development of new synthesis strategy in the preparation of nanosized crystals of zeolites is highly desired. So far, a couple of methods have been developed to control the crystal size of low-silica zeolites X (FAU) and A (LTA), which were called clear solutions or gels methods [2-5] or confined-space methods [6,7]. One promising approach that could be used to control both the size and morphology of zeolite crystals is the utilization of microemulsion system. Generally, microemulsions comprise surfactant, co-surfactant, oil phase, and water phase [8]. Small water droplets are formed in the oil phase, which supply an excellent confined space for the crystallization of zeolite crystals with controllable morphology and particle size distribution. For example, Dutta and coworkers firstly synthesized zeolite A from a microemulsion system consisting of 1,2-bis(2-ethylhexyloxycarbonyl)-1-ethane sulfonate (AOT), isooctane, and starting mixture for the crystallization of zeolite phase. The resulting zeolite A crystals had a uniform size of 1-2ȝm [9]. Recently, Shantz and colleagues studied the rapid growth of LTA in non-ionic microemulsions [10]. This contribution presents the synthesis of nanocrystals of zeolite A in the cetyltrimethylammonium bromide (CTAB)-n-butanol- heptane microemulsion system, and further investigates how the synthetic parameters including the composition of the zeolite synthesis mixture, the reaction time affect the particle sizes of the resulting LTA crystals.
476 2. EXPERIMENTAL 2.1. Preparation of starting mixture for the crystallization of LTA The molar composition of the starting mixture for the crystallization of LTA is 0.22 Na2O : 5.0 SiO2 : 1.0 Al2O3 : 8.0 (TMA)2O : 400 H2O. Typically, 11.2 mL of tetramethylammonium hydroxide (25 wt%) was added to an aqueous solution of sodium hydroxide (0.22 mM, 3.75 mL) and aluminum isopropoxide (0.8g) with stirring, followed by the addition of 1.7 mL of aqueous colloidal silica sol (Ludox HS-30, 30 wt%, Aldrich). The mixture was sealed and further aged for 24h without stirring for completely dissolving aluminum isopropoxide. 2.2. Preparation of reverse microemulsion and LTA nanocrystals Microemulsion was prepared by dissolving cetyltrimethylammonium bromide (CTAB, Aldrich, 1.5-1.7g) in a mixture of n-butanol (99.8%, Aldrich, 0.9-1.06 mL) and 8.77mL of heptane (99.5%, Aldrich). CTAB, n-butanol, and heptane were used as the surfactant, co-surfactant, and the oil phase, respectively. The mass ratio of CTAB/ 1-butanol was 2:1. The weight fractions of surfactant were 0.233-0.257. Subsequently, the pre-weighed starting mixture for the crystallization of LTA (1.3-1.6g) was added under stirring. The final mixture was optically transparent, which indicated the formation of a microemulsion system. The resulting mixture was transferred into a Taflon-lined stainless steel autoclave and heated at 100°C for 6.3 h. The product was then separated from the mother liquid by centrifugation, washed with distilled water and ethanol by centrifuging for several times to completely remove the surfactants, and dried at 60°C in air for 24h. 2.3. Characterization The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2500/PC X-ray diffractometer with graphite-filtered Cu KĮ radiation, at 40 kV and 200 mA, and collected at 2ș angles of 5-40° with a scan rate of 3°/min. The morphologies of samples were investigated by scanning electron microscopy (SEM) on a JSM-6700F electron microscope. 3. RESULTS AND DISCUSSION 3.1. Phase diagram of the microemulsion The microemulsion is treated as a pseudoternary system with oil, “surfactant”, and “aqueous” components. Its phase diagram can be used as a guide for the selection of different microemulsion compositions for hydrothermal synthesis. In this study, the phase diagram, as shown in Fig. 1, was plotted with the surfactant (CTAB)/butanol ratio (weight) of 2:1. The optically transparent single phase microemulsion can be formed in the region enclosed by dashed line on the phase diagram. The composition of the microemulsion in this region will affect the crystallinity, particle size and its distribution of the LTA crystals. All experiments conducted in this investigation were performed in the
Fig.1. Phase diagram of microemulsion made from a surfactant phase consisting of CTAB and butanol in a mass ratio of 2:1, heptane, and aqueous phase consisting of synthetic mixture of LTA.
477 microemulsion with the composition in this region. 3.2. Influence of microemulsion on the crystallization of LTA The X-ray diffraction patterns of the LTA crystals crystallized from microemusion system (0.6 heptane: 0.233 surfactant: 0.167 aqueous phase) and conventional hydrothermal system under identical crystallization conditions are shown in Fig. 2. The stronger intensity of the diffraction peaks of the LTA crystals crystallized from the microemusion system indicates that the microemulsion system accelerates the crystallization process, which is consistent with previous report [10]. The typical SEM images of the corresponding samples are shown in Fig. 2. The particle size of the LTA crystals obtained from microemusion system is between 100 and 120 nm (Fig. 2A), while that of the LTA crystals crystallized from conventional hydrothermal system varies from 150 to 700 nm (Fig. 2B). Obviously, the microemulsion system plays an important role in the formation of the uniform LTA nanocrystals. It can be speculated that the uniform confined space of the water droplets in the oil phase and the presence of surfactant and co-surfactant supply an excellent “hard” template in the formation of LTA nanocrystals.
Fig. 2. The XRD patterns and SEM images of the LTA crystals formed from (A) microemulsion system and (B) conventional hydrothermal system.
3.3. Influence of the composition of microemulsion on the crystallization of LTA The influence of the composition of microemulsion on the crystallization of LTA crystals was investigated. Highly crystallized LTA crystals could be obtained if the weight percent of heptane was fixed at 0.6 or 0.7 and the mass ratio of the surfactant to LTA starting mixture was varied from 1.4 to 2.0 for each heptane concentration. The detailed experimental conditions are summarized in Table 1. The XRD patterns and the SEM images of the resulting LTA crystals for batches 1, 3, 5, 6, 7, and 8 are shown in Fig. 3. XRD analyses indicate that batch 5 gives the highest crystallinity of the resulting LTA crystals in these experiments, which suggest that the amount of the surfactant in the microemulsion system is also critical for the crystallization of LTA phase. The increase of the content of the surfactant and co-surfactant results in the decrease of the crystallinity of the LTA crystals.
478 Table 1 The composition of microemulsion Batch Number 1 2 3 4 5 6 7 8
Composition of the microemulsion (wt %) Heptane Aqueous phase CTAB+Butanol 0.7 0.125 0.175 0.7 0.115 0.185 0.7 0.107 0.193 0.7 0.100 0.200 0.6 0.167 0.233 0.6 0.154 0.246 0.6 0.143 0.257 0.6 0.133 0.267
Surfactant/ Aqueous phase 1.4 1.6 1.8 2.0 1.4 1.6 1.8 2.0
Fig. 3. The XRD patterns and the SEM images of the resulting LTA crystals crystallized from batch 1 (A), 3 (B), 5 (C), 6 (D), 7 (E), and 8 (F). The particle sizes estimated from the SEM images: 50~300nm (A), 80~300nm (B), 100~120nm (C), 110nm~140nm (D), 120nm~200nm (E), and 150~ 200nm (F).
The SEM images in Fig. 3 show that the LTA crystals crystallized from batch 5 posses the narrowest particle size distribution (100~120 nm). The decrease or increase of the weight percent of the surfactant and co-surfactant broadens the particle size distribution. 3.4. Influence of the reaction time on the crystallinity of LTA crystals Fig. 4 shows the XRD patterns and the SEM images of the products obtained from the experiments conducted in the microemulsion of batch 5. The reaction time for five different autoclaves was set 2h, 4h, 6.3h, 24h, and 48h, respectively. The data in Fig. 4 indicate that no XRD detectable crystalline LTA crystals were formed after a 2 h reaction and very weak diffraction peaks of the LTA phase appeared only after a 4 h reaction. After a 6.3 h reaction, the LTA crystals reach the highest crystallinity and the narrowest particle distribution. Prolongation of the reaction time to 24 or 48 h significantly decreases the crystallinity of the resulting LTA crystals and broadens their particle size distribution (SEM images in Fig. 4). These results imply that a better crystallinity will result in a narrower particle size distribution for the LTA crystals crystallized from the microemusion system.
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Fig. 4. The XRD patterns of the products obtained from the experiments conducted in the microemusion of batch 5. Reaction time: 2h (A), 4h (B), 6.3h (C), 24h (D), and 48h (E). The SEM images of a-d correspond to the products of B-D in XRD patterns, respectively.
4. CONCLUSIONS The synthesis of LTA nanocrystals with high crystallinity and narrow particle size distribution (100-120 nm) and sphere-shaped morphology is achieved in the presence of cationic microemulsions. This investigation reveals that the microemulsion system can accelerate the crystallization process of the LTA. In the formation region of the stable microemulsion, the concentration (weight percent) of the surfactant and co-surfactant can significantly affect the crystallinity and particle size distribution of the resulting LTA crystals. It is found that the reaction time plays an important role in the formation of highly crystallized and uniform sphere-shaped LTA nanocrystals. The control of zeolite nanocrystals with small and uniform size as well as the sphere-morphology from microemusion system might facilitate new possibilities in their use as seeds for zeolite thin film formation via secondary growth. ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China and the State Basic Research Project of China. REFERENCES [1] [2] [3] [4] [5] [6] [7]
L. Tosheva, V. P. Valtchev, Chem. Mater., 17 (2005) 2495. G. Zhu, S. Qiu , Y. Sakamoto, F. Xiao, R. Xu, O. Terasaki, Chem. Mater., 10 (1998) 1483. R. A. Rakoczy, Y. Traa, Microporous Mesoporous Mater., 60 (2003) 69. S. Mintova, N. H. Olson, V. Valtchev, T. Bein, Science, 283 (1999) 958. S. Mintova, V. Valtchev, Stud. Surf. Sci. Catal., 125 (1999) 141. I. Schmidt, C. Madsen, C. J. H. Jacobsen, Inorg. Chem., 39 (2000) 2279. C. J. H. Jacobsen, C. Madsen, T. V. W. Janssens, H. J. Jakobsen, J. Skibsted, Microporous Mesoporous Mater., 39 (2000) 393. [8] P. Kumar, K. L. Mittal, Handbook of Microemulsion Science and Technology. Marcel Dekker: New York, 1999. [9] P. K. Dutta, D. Robins, Langmuir 7 (1991) 1048. [10]C. S. Carr, D. F. Shantz, Microporous Mesoporous Mater. 85 (2005) 284.
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Microemulsion-directed synthesis of zeolite A nano-crystals Jianan Zhang, Wenfu Yan, Hong Ding, Yang Liu, Kangjian Tang, Jihong Yu* and Ruren Xu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012. Email: [email protected] ABSTRACT This contribution reports the synthesis of zeolite A (LTA) nanocrystals with narrow particle size distribution and sphere-shaped morphology in the presence of cationic microemulsion. The results presented here show that the microemulsion environment can accelerate the crystallization of LTA nanocrystals. The composition of the microemulsion and the reaction time can significantly affect the crystallinity and the particle size distribution of the resulting LTA nanocrystals. X-ray diffraction and scanning electron microscopy (SEM) were used to confirm zeolite crystallinity, phase purity, and morphology. 1. INTRODUCTION In recent years, the synthesis of nanocrystalline zeolites has received much attention. The reduction of particle size of zeolites from micrometer to nanometer scale leads to substantial changes of their properties. Previous studies revealed that the particle size and morphology of the zeolite crystals play an important role in their applications in the areas of catalysis and separation [1]. Thus, the development of new synthesis strategy in the preparation of nanosized crystals of zeolites is highly desired. So far, a couple of methods have been developed to control the crystal size of low-silica zeolites X (FAU) and A (LTA), which were called clear solutions or gels methods [2-5] or confined-space methods [6,7]. One promising approach that could be used to control both the size and morphology of zeolite crystals is the utilization of microemulsion system. Generally, microemulsions comprise surfactant, co-surfactant, oil phase, and water phase [8]. Small water droplets are formed in the oil phase, which supply an excellent confined space for the crystallization of zeolite crystals with controllable morphology and particle size distribution. For example, Dutta and coworkers firstly synthesized zeolite A from a microemulsion system consisting of 1,2-bis(2-ethylhexyloxycarbonyl)-1-ethane sulfonate (AOT), isooctane, and starting mixture for the crystallization of zeolite phase. The resulting zeolite A crystals had a uniform size of 1-2ȝm [9]. Recently, Shantz and colleagues studied the rapid growth of LTA in non-ionic microemulsions [10]. This contribution presents the synthesis of nanocrystals of zeolite A in the cetyltrimethylammonium bromide (CTAB)-n-butanol- heptane microemulsion system, and further investigates how the synthetic parameters including the composition of the zeolite synthesis mixture, the reaction time affect the particle sizes of the resulting LTA crystals.
476 2. EXPERIMENTAL 2.1. Preparation of starting mixture for the crystallization of LTA The molar composition of the starting mixture for the crystallization of LTA is 0.22 Na2O : 5.0 SiO2 : 1.0 Al2O3 : 8.0 (TMA)2O : 400 H2O. Typically, 11.2 mL of tetramethylammonium hydroxide (25 wt%) was added to an aqueous solution of sodium hydroxide (0.22 mM, 3.75 mL) and aluminum isopropoxide (0.8g) with stirring, followed by the addition of 1.7 mL of aqueous colloidal silica sol (Ludox HS-30, 30 wt%, Aldrich). The mixture was sealed and further aged for 24h without stirring for completely dissolving aluminum isopropoxide. 2.2. Preparation of reverse microemulsion and LTA nanocrystals Microemulsion was prepared by dissolving cetyltrimethylammonium bromide (CTAB, Aldrich, 1.5-1.7g) in a mixture of n-butanol (99.8%, Aldrich, 0.9-1.06 mL) and 8.77mL of heptane (99.5%, Aldrich). CTAB, n-butanol, and heptane were used as the surfactant, co-surfactant, and the oil phase, respectively. The mass ratio of CTAB/ 1-butanol was 2:1. The weight fractions of surfactant were 0.233-0.257. Subsequently, the pre-weighed starting mixture for the crystallization of LTA (1.3-1.6g) was added under stirring. The final mixture was optically transparent, which indicated the formation of a microemulsion system. The resulting mixture was transferred into a Taflon-lined stainless steel autoclave and heated at 100°C for 6.3 h. The product was then separated from the mother liquid by centrifugation, washed with distilled water and ethanol by centrifuging for several times to completely remove the surfactants, and dried at 60°C in air for 24h. 2.3. Characterization The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2500/PC X-ray diffractometer with graphite-filtered Cu KĮ radiation, at 40 kV and 200 mA, and collected at 2ș angles of 5-40° with a scan rate of 3°/min. The morphologies of samples were investigated by scanning electron microscopy (SEM) on a JSM-6700F electron microscope. 3. RESULTS AND DISCUSSION 3.1. Phase diagram of the microemulsion The microemulsion is treated as a pseudoternary system with oil, “surfactant”, and “aqueous” components. Its phase diagram can be used as a guide for the selection of different microemulsion compositions for hydrothermal synthesis. In this study, the phase diagram, as shown in Fig. 1, was plotted with the surfactant (CTAB)/butanol ratio (weight) of 2:1. The optically transparent single phase microemulsion can be formed in the region enclosed by dashed line on the phase diagram. The composition of the microemulsion in this region will affect the crystallinity, particle size and its distribution of the LTA crystals. All experiments conducted in this investigation were performed in the
Fig.1. Phase diagram of microemulsion made from a surfactant phase consisting of CTAB and butanol in a mass ratio of 2:1, heptane, and aqueous phase consisting of synthetic mixture of LTA.
477 microemulsion with the composition in this region. 3.2. Influence of microemulsion on the crystallization of LTA The X-ray diffraction patterns of the LTA crystals crystallized from microemusion system (0.6 heptane: 0.233 surfactant: 0.167 aqueous phase) and conventional hydrothermal system under identical crystallization conditions are shown in Fig. 2. The stronger intensity of the diffraction peaks of the LTA crystals crystallized from the microemusion system indicates that the microemulsion system accelerates the crystallization process, which is consistent with previous report [10]. The typical SEM images of the corresponding samples are shown in Fig. 2. The particle size of the LTA crystals obtained from microemusion system is between 100 and 120 nm (Fig. 2A), while that of the LTA crystals crystallized from conventional hydrothermal system varies from 150 to 700 nm (Fig. 2B). Obviously, the microemulsion system plays an important role in the formation of the uniform LTA nanocrystals. It can be speculated that the uniform confined space of the water droplets in the oil phase and the presence of surfactant and co-surfactant supply an excellent “hard” template in the formation of LTA nanocrystals.
Fig. 2. The XRD patterns and SEM images of the LTA crystals formed from (A) microemulsion system and (B) conventional hydrothermal system.
3.3. Influence of the composition of microemulsion on the crystallization of LTA The influence of the composition of microemulsion on the crystallization of LTA crystals was investigated. Highly crystallized LTA crystals could be obtained if the weight percent of heptane was fixed at 0.6 or 0.7 and the mass ratio of the surfactant to LTA starting mixture was varied from 1.4 to 2.0 for each heptane concentration. The detailed experimental conditions are summarized in Table 1. The XRD patterns and the SEM images of the resulting LTA crystals for batches 1, 3, 5, 6, 7, and 8 are shown in Fig. 3. XRD analyses indicate that batch 5 gives the highest crystallinity of the resulting LTA crystals in these experiments, which suggest that the amount of the surfactant in the microemulsion system is also critical for the crystallization of LTA phase. The increase of the content of the surfactant and co-surfactant results in the decrease of the crystallinity of the LTA crystals.
478 Table 1 The composition of microemulsion Batch Number 1 2 3 4 5 6 7 8
Composition of the microemulsion (wt %) Heptane Aqueous phase CTAB+Butanol 0.7 0.125 0.175 0.7 0.115 0.185 0.7 0.107 0.193 0.7 0.100 0.200 0.6 0.167 0.233 0.6 0.154 0.246 0.6 0.143 0.257 0.6 0.133 0.267
Surfactant/ Aqueous phase 1.4 1.6 1.8 2.0 1.4 1.6 1.8 2.0
Fig. 3. The XRD patterns and the SEM images of the resulting LTA crystals crystallized from batch 1 (A), 3 (B), 5 (C), 6 (D), 7 (E), and 8 (F). The particle sizes estimated from the SEM images: 50~300nm (A), 80~300nm (B), 100~120nm (C), 110nm~140nm (D), 120nm~200nm (E), and 150~ 200nm (F).
The SEM images in Fig. 3 show that the LTA crystals crystallized from batch 5 posses the narrowest particle size distribution (100~120 nm). The decrease or increase of the weight percent of the surfactant and co-surfactant broadens the particle size distribution. 3.4. Influence of the reaction time on the crystallinity of LTA crystals Fig. 4 shows the XRD patterns and the SEM images of the products obtained from the experiments conducted in the microemulsion of batch 5. The reaction time for five different autoclaves was set 2h, 4h, 6.3h, 24h, and 48h, respectively. The data in Fig. 4 indicate that no XRD detectable crystalline LTA crystals were formed after a 2 h reaction and very weak diffraction peaks of the LTA phase appeared only after a 4 h reaction. After a 6.3 h reaction, the LTA crystals reach the highest crystallinity and the narrowest particle distribution. Prolongation of the reaction time to 24 or 48 h significantly decreases the crystallinity of the resulting LTA crystals and broadens their particle size distribution (SEM images in Fig. 4). These results imply that a better crystallinity will result in a narrower particle size distribution for the LTA crystals crystallized from the microemusion system.
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Fig. 4. The XRD patterns of the products obtained from the experiments conducted in the microemusion of batch 5. Reaction time: 2h (A), 4h (B), 6.3h (C), 24h (D), and 48h (E). The SEM images of a-d correspond to the products of B-D in XRD patterns, respectively.
4. CONCLUSIONS The synthesis of LTA nanocrystals with high crystallinity and narrow particle size distribution (100-120 nm) and sphere-shaped morphology is achieved in the presence of cationic microemulsions. This investigation reveals that the microemulsion system can accelerate the crystallization process of the LTA. In the formation region of the stable microemulsion, the concentration (weight percent) of the surfactant and co-surfactant can significantly affect the crystallinity and particle size distribution of the resulting LTA crystals. It is found that the reaction time plays an important role in the formation of highly crystallized and uniform sphere-shaped LTA nanocrystals. The control of zeolite nanocrystals with small and uniform size as well as the sphere-morphology from microemusion system might facilitate new possibilities in their use as seeds for zeolite thin film formation via secondary growth. ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China and the State Basic Research Project of China. REFERENCES [1] [2] [3] [4] [5] [6] [7]
L. Tosheva, V. P. Valtchev, Chem. Mater., 17 (2005) 2495. G. Zhu, S. Qiu , Y. Sakamoto, F. Xiao, R. Xu, O. Terasaki, Chem. Mater., 10 (1998) 1483. R. A. Rakoczy, Y. Traa, Microporous Mesoporous Mater., 60 (2003) 69. S. Mintova, N. H. Olson, V. Valtchev, T. Bein, Science, 283 (1999) 958. S. Mintova, V. Valtchev, Stud. Surf. Sci. Catal., 125 (1999) 141. I. Schmidt, C. Madsen, C. J. H. Jacobsen, Inorg. Chem., 39 (2000) 2279. C. J. H. Jacobsen, C. Madsen, T. V. W. Janssens, H. J. Jakobsen, J. Skibsted, Microporous Mesoporous Mater., 39 (2000) 393. [8] P. Kumar, K. L. Mittal, Handbook of Microemulsion Science and Technology. Marcel Dekker: New York, 1999. [9] P. K. Dutta, D. Robins, Langmuir 7 (1991) 1048. [10]C. S. Carr, D. F. Shantz, Microporous Mesoporous Mater. 85 (2005) 284.