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Synthesis and characterization of mesostructured silica sphere particles with core space
Recent Progress in Mesostructured Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
607 607
Synthesis and characterization of mesostructured silica sphere particles with core space Jung-Sik Choi3, Kyung-Ku Kangb and Wha-Seung Ann3* "Department of Chemical Engineering, Inha University, Incheon, 402-751, Korea Research Institute of Chemicals and Electronic Materials, Cheil Industries, INC. Gyeonggi-do, 437-711, Korea
Spherical mesostructured silica particles with inner core space were prepared under acidic condition using TEOS and a block copolymer template in an aqueous/butanol emulsion system. Mesoporous silica particles obtained were highly uniform in 125 um - 1 mm size range with core space in 100 - 170 um depending on the synthesis condition. The sphere diameter was directly proportional to the concentrations of TEOS and butanol. Butanol plays an important role of making the silica particles hollow and spherical as well as being involved in mesopore formation; an increase in butanol concentration produced mesoporous silica with larger pore diameter. 1. Introduction The preparation of mesoporous silica materials has evolved to a new stage in which the hierarchical structures having at least two length scales of micro- and nanometer can be achieved [1]. The controlling of structures in both length scales has important impacts on design of nanomaterials and their potential applications such as catalyst/supports, drug release, micro reactor, separation, and chromatography packing materials. Making spherical silica particles with mesopore structure by chemical and hydrodynamic approach has advantages on cost and performance. In this work, spherical mesoporous silica particles with core space were prepared by promoting condensation of silica/surfactant assembly around an organic phase in an aqueous/butanol emulsion system. Synthesis process is very simple and uses cheaper reagents than the previous report by Stucky's group on a similar material [2].
608
2. Expermental Section TEOS was used as silica source and 1-butanol with Pluronic 123 dissolved in water was used to form an O/W emulsion. In a typical synthesis, surfactant solution was prepared by adding P123 to 2 M HCl solution and stirred at room temperature. Independently, TEOS was added in 1-butanol with stirring and this solution was added to the surfactant solution. The substrate mixture was then stirred for 24 h at 35°C. Particles obtained were recovered by filtration, washed, and dried. Finally, the product was carefully calcined in air at 550°C. The morphology of the samples was examined by SEM (Hitachi S-4300) and TEM (Philips, CM 200). The specific surface area and average pore diameters were determined by N2 physisorption using a Micromeretics ASAP 2000 automatic analyzer. 3. Results and Dicussion In this work, spherical mesoporous silica particles were synthesized by solgel method under acidic condition with controlled morphology, using butanol as the organic phase in O/W emulsion. Scheme 1 represents the formation process of the sphere particles as suggested by Schacht et al. [1]. In our synthesis, TEOS mixed with butanol was added to acidic surfactant solution with stirring.
Direction of silica growth
r c?s> v
Surfactant as emulsion
stabilizer
Organic phase (Emulsion)
>{
Surfactant as rjoi^ej) template with random orientation
Aqueous phase Scheme 1. Proposed growth mechanism of spherical mesoporous silica [1].
Surfactant plays an important role both as emulsion stabilizer and as micelle template. TEOS also contributes to the stabilization of this emulsion phase after partial hydrolysis. TEOS is eventually fully hydrolyzed under acidic conditions at the organic/water interface and forms the mesostructure under the influence of the surfactant.
609
1^ (a)
y^
Macro pore
(b)
V
100 µm 100
1 mm
µmy
Core Core space space Fig. 1. SEM micrographs of the spherical mesoporous silica and the particle cross section.
Amount of adsorption (cc/g, STP)
SEM image (Fig. la) shows uniform spherical particles with ca. 500 urn in diameter. It was also possible to prepare particles up to 1 mm by controlling the synthesis condition. Fig. lb is a cross section image of a spherical mesoporous silica particle with ca. 200 um in diameter, which are made of 3 different types of pores: core space with 100-170 urn, macro pores with 1-5 um and mesopores as measured by nitrogen adsorption-desorption isotherm. Spherical particle size and core space could be controlled by changes in stirring rate and concentrations of TEOS and butanol. Increasingly large particles were obtained as the stirring speed decreased from 1200 to 300 rpm. At a fixed amount of butanol, increases of TEOS amount led to increases in particle size and also increased the particle thickness. TEOS concentration did not sensitively govern the particle size as butanol concentration. At a fixed amount of TEOS, increases of butanol/TEOS molar ratio led to particle size increase from 125 urn to 1 mm. Increase of butanol concentration was 800 related not to only particle size but also to mesostructure formation. Pore 600 diameter was increased from 3.2 to 4.9 nm with increasing concentration of butanol. 400 Pore diameter As shown in Fig. 2, nitrogen adsorption-desorption isotherm of 200 type IV for mesopore structure and hysteresis loop of type H2 was 0 0.0 0.2 0.4 0.6 0.8 1.0 observed, which is known due to the Relative Pressure (P/Po) presence of pores with narrow mouths (cage-like pores). The isotherm Fig. 2. Nitrogen adsorption-desorption isotherm inflection point for the spherical plot and BJH desorption pore size distribution mesoporous silica particles was (inset) of spherical mesopore silica. 10
100
1000
610
located at relatively high P/Po between 0.6 and 0.7, which indicates larger pore diameter than other mesoporous materials such as MCM-41 and HMS. BET surface area was ca. 890 m /g, single point total pore volume at relative pressure of 0.98 was 0.96 cm3/g, and the BJH desorption pore diameter was 4.9 nm. Fig. 3 shows the TEM micrograph of the spherical silica material prepared. Disordered pore structure but with uniform diameters similar to the disordered mesoporous silica materials obtained using nonionic surfactant or in the presence of organic salt [3,4] was observed. Ordered pore structures are seldom found in the case of spherical particles for which an oil phase is employed to control the morphology. It seems that its mesostructure was disordered because of the presence of butanol in the synthesis mixture. Increasing amount of butanol added to the synthesis batch for SBA-15 in acidic condition led to transition from 20 nm 2D hexagonal to the cubic Ia3d mesophase and finally to the disordered phase through the smaller Fig. 3. TEM micrograph of the spherical domains of intermediate mixed phase mesoporous silica. [5]. 4. Conclusion Spherical mesostructured silica particles with core space were synthesized under acidic condition in an aqueous/butanol emulsion system. Concentration of TEOS or butanol was directly proportional to the sphere diameter in such a manner that high concentration of both TEOS and butanol resulted in particle size increases. Butanol, however, was the more critical variable controlling the particle size and also found to affect the pore diameter of the silica obtained. 5. References [1] S. Schacht, Q. Huo, I. G. Voigt-Martin, G. D. Stucky and F. Schuth, Science, 273 (1996) 768. [2] Q. Huo, J. Feng, F. Schuth and G. D. Stucky, Chem. Mater., 9 (1997) 14. [3] P. Tanev and T. Pinnavia, Science, 269 (1995) 1242. [4] R. Ryoo, J. Kim, C. Ko and C. Shin, J. Phys. Chem., 100 (1996) 17718. [5] T. Kim, F. Kleitz, B. Paul and R. Ryoo, J. Am. Chem. Soc, 127 (2005) 7601.
607 607
Synthesis and characterization of mesostructured silica sphere particles with core space Jung-Sik Choi3, Kyung-Ku Kangb and Wha-Seung Ann3* "Department of Chemical Engineering, Inha University, Incheon, 402-751, Korea Research Institute of Chemicals and Electronic Materials, Cheil Industries, INC. Gyeonggi-do, 437-711, Korea
Spherical mesostructured silica particles with inner core space were prepared under acidic condition using TEOS and a block copolymer template in an aqueous/butanol emulsion system. Mesoporous silica particles obtained were highly uniform in 125 um - 1 mm size range with core space in 100 - 170 um depending on the synthesis condition. The sphere diameter was directly proportional to the concentrations of TEOS and butanol. Butanol plays an important role of making the silica particles hollow and spherical as well as being involved in mesopore formation; an increase in butanol concentration produced mesoporous silica with larger pore diameter. 1. Introduction The preparation of mesoporous silica materials has evolved to a new stage in which the hierarchical structures having at least two length scales of micro- and nanometer can be achieved [1]. The controlling of structures in both length scales has important impacts on design of nanomaterials and their potential applications such as catalyst/supports, drug release, micro reactor, separation, and chromatography packing materials. Making spherical silica particles with mesopore structure by chemical and hydrodynamic approach has advantages on cost and performance. In this work, spherical mesoporous silica particles with core space were prepared by promoting condensation of silica/surfactant assembly around an organic phase in an aqueous/butanol emulsion system. Synthesis process is very simple and uses cheaper reagents than the previous report by Stucky's group on a similar material [2].
608
2. Expermental Section TEOS was used as silica source and 1-butanol with Pluronic 123 dissolved in water was used to form an O/W emulsion. In a typical synthesis, surfactant solution was prepared by adding P123 to 2 M HCl solution and stirred at room temperature. Independently, TEOS was added in 1-butanol with stirring and this solution was added to the surfactant solution. The substrate mixture was then stirred for 24 h at 35°C. Particles obtained were recovered by filtration, washed, and dried. Finally, the product was carefully calcined in air at 550°C. The morphology of the samples was examined by SEM (Hitachi S-4300) and TEM (Philips, CM 200). The specific surface area and average pore diameters were determined by N2 physisorption using a Micromeretics ASAP 2000 automatic analyzer. 3. Results and Dicussion In this work, spherical mesoporous silica particles were synthesized by solgel method under acidic condition with controlled morphology, using butanol as the organic phase in O/W emulsion. Scheme 1 represents the formation process of the sphere particles as suggested by Schacht et al. [1]. In our synthesis, TEOS mixed with butanol was added to acidic surfactant solution with stirring.
Direction of silica growth
r c?s> v
Surfactant as emulsion
stabilizer
Organic phase (Emulsion)
>{
Surfactant as rjoi^ej) template with random orientation
Aqueous phase Scheme 1. Proposed growth mechanism of spherical mesoporous silica [1].
Surfactant plays an important role both as emulsion stabilizer and as micelle template. TEOS also contributes to the stabilization of this emulsion phase after partial hydrolysis. TEOS is eventually fully hydrolyzed under acidic conditions at the organic/water interface and forms the mesostructure under the influence of the surfactant.
609
1^ (a)
y^
Macro pore
(b)
V
100 µm 100
1 mm
µmy
Core Core space space Fig. 1. SEM micrographs of the spherical mesoporous silica and the particle cross section.
Amount of adsorption (cc/g, STP)
SEM image (Fig. la) shows uniform spherical particles with ca. 500 urn in diameter. It was also possible to prepare particles up to 1 mm by controlling the synthesis condition. Fig. lb is a cross section image of a spherical mesoporous silica particle with ca. 200 um in diameter, which are made of 3 different types of pores: core space with 100-170 urn, macro pores with 1-5 um and mesopores as measured by nitrogen adsorption-desorption isotherm. Spherical particle size and core space could be controlled by changes in stirring rate and concentrations of TEOS and butanol. Increasingly large particles were obtained as the stirring speed decreased from 1200 to 300 rpm. At a fixed amount of butanol, increases of TEOS amount led to increases in particle size and also increased the particle thickness. TEOS concentration did not sensitively govern the particle size as butanol concentration. At a fixed amount of TEOS, increases of butanol/TEOS molar ratio led to particle size increase from 125 urn to 1 mm. Increase of butanol concentration was 800 related not to only particle size but also to mesostructure formation. Pore 600 diameter was increased from 3.2 to 4.9 nm with increasing concentration of butanol. 400 Pore diameter As shown in Fig. 2, nitrogen adsorption-desorption isotherm of 200 type IV for mesopore structure and hysteresis loop of type H2 was 0 0.0 0.2 0.4 0.6 0.8 1.0 observed, which is known due to the Relative Pressure (P/Po) presence of pores with narrow mouths (cage-like pores). The isotherm Fig. 2. Nitrogen adsorption-desorption isotherm inflection point for the spherical plot and BJH desorption pore size distribution mesoporous silica particles was (inset) of spherical mesopore silica. 10
100
1000
610
located at relatively high P/Po between 0.6 and 0.7, which indicates larger pore diameter than other mesoporous materials such as MCM-41 and HMS. BET surface area was ca. 890 m /g, single point total pore volume at relative pressure of 0.98 was 0.96 cm3/g, and the BJH desorption pore diameter was 4.9 nm. Fig. 3 shows the TEM micrograph of the spherical silica material prepared. Disordered pore structure but with uniform diameters similar to the disordered mesoporous silica materials obtained using nonionic surfactant or in the presence of organic salt [3,4] was observed. Ordered pore structures are seldom found in the case of spherical particles for which an oil phase is employed to control the morphology. It seems that its mesostructure was disordered because of the presence of butanol in the synthesis mixture. Increasing amount of butanol added to the synthesis batch for SBA-15 in acidic condition led to transition from 20 nm 2D hexagonal to the cubic Ia3d mesophase and finally to the disordered phase through the smaller Fig. 3. TEM micrograph of the spherical domains of intermediate mixed phase mesoporous silica. [5]. 4. Conclusion Spherical mesostructured silica particles with core space were synthesized under acidic condition in an aqueous/butanol emulsion system. Concentration of TEOS or butanol was directly proportional to the sphere diameter in such a manner that high concentration of both TEOS and butanol resulted in particle size increases. Butanol, however, was the more critical variable controlling the particle size and also found to affect the pore diameter of the silica obtained. 5. References [1] S. Schacht, Q. Huo, I. G. Voigt-Martin, G. D. Stucky and F. Schuth, Science, 273 (1996) 768. [2] Q. Huo, J. Feng, F. Schuth and G. D. Stucky, Chem. Mater., 9 (1997) 14. [3] P. Tanev and T. Pinnavia, Science, 269 (1995) 1242. [4] R. Ryoo, J. Kim, C. Ko and C. Shin, J. Phys. Chem., 100 (1996) 17718. [5] T. Kim, F. Kleitz, B. Paul and R. Ryoo, J. Am. Chem. Soc, 127 (2005) 7601.