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Highly ordered mesoporous silica structures templated by poly(butylene oxide) segment di- and tri-block copolymers
Microporous and Mesoporous Materials 44±45 (2001) 65±72
www.elsevier.nl/locate/micromeso
Highly ordered mesoporous silica structures templated by poly(butylene oxide) segment di- and tri-block copolymers Chengzhong Yu a,b, Yonghao Yu a, Lei Miao a, Dongyuan Zhao a,* a
Department of Chemistry, Fudan University, Shanghai, 200433, People's Republic of China b School of Medicine, Fudan University, Shanghai, 200433, People's Republic of China Received 3 February 2000; accepted 29 August 2000
Abstract Highly ordered mesoporous silica structures including body-centered cubic (Im 3 m), two-dimensional hexagonal (p6mm) and lamellar La symmetries have been synthesized by using hydrophobic poly(butylene oxide) moiety diblock and triblock copolymers as structure-directing agents. Under acidic condition, highly ordered, caged cubic mesoporous can be formed in the presence of triblock silica structures (FDU-1, Im 3 m) with a large cell parameter a 220 A poly(ethylene oxide)±poly(butylene oxide)±poly(ethylene oxide) (PEO±PBO±PEO) copolymer such as EO39 BO47 EO39 . among all known cubic silica mesostructures, a pore volume of 0.77 Calcined FDU-1 has a largest pore size of 120 A cm3 /g, and a BET surface area of 740 m2 /g. Diblock poly(butylene oxide)±poly (ethylene oxide) (PBO±PEO) copolymer such as BO10 EO16 favors to yield highly ordered two-dimensional hexagonal (p6mm) silica mesostructures with a well and a large BET surface area of 902 m2 /g. Lamellar La mesostructures can also be uniformed pore size of 60 A obtained when high concentration diblock copolymer is used as the structure-directing agent. The calcined cubic mesostructured silica FDU-1 is hydrothermally stable in boiling water for at least nine days. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous materials; Block copolymer; Synthesis
1. Introduction Since the discovery of M41S family of mesoporous silicates and aluminosilicates in 1992 [1,2], numerous mesoporous or nanoporous materials have been with pore sizes between 20 and 300 A synthesized because of their potential applications on catalysis, separation of large molecules, medical implants, semiconductor, magnetoelectric devices,
*
Corresponding author. E-mail address: [email protected] (D. Zhao).
etc. [3±14]. In most cases, ionic and neutral surfactants have been employed as templates to direct the mesophase formation based on the electrostatic and hydrogen-bonding interactions, respectively. Using triblock poly(ethylene oxide)±poly (propylene oxide)±poly(ethylene oxide) (PEO± PPO±PEO) copolymers as the structure-directing agents, a series of mesoporous silica structures including hexagonal (H1 ), lamellar La and cubic (I1 ) phases have been synthesized [3,4]. However, the attempt to synthesize mesoporous silica materials by using PEO±PPO diblock copolymers as the templates was unsuccessful. Three-dimensional (3D)
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cubic mesostructures, such as MCM-48 (Ia 3 d) [1], SBA-1 (Pm 3 n) [12,13] and SBA-16 (Im 3 m) [4], have more advantages compared to hexagonal mesoporous structures with one-dimension channels. However, such cubic structures could be synthesized only under strict conditions. Calcined SBA-16 synthesized with PEO±PPO±PEO triblock copolymers has the largest pore size of 54 A among these cubic silica structures, and can only be synthesized with large PEO segment amphiphilic PEO±PPO±PEO block copolymers, such as EO106 PO70 EO106 in a narrow range of the reaction composition at room temperature [4]. The minor dierence of hydrophilicity/hydrophobicity between PEO and PPO chains in room temperature and the low ratio of PO/EO might be the reasons that result in hard formation of ordered mesostructures and such small pore size. Here we report the preparation of mesostructured silicas by using diblock and triblock copolymers with more hydrophobic poly(butylene oxide) (PBO) moiety as the structure-directing agents. The resulted mesoporous silica products with highly ordered mesostructures have been characterized by the means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption/desorption measurements, 29 Si magic angle spinning nuclear resonance (MAS NMR) spectroscopy and thermogravimetric analysis and dierential thermal analyses (TGA/DTA). 2. Experimental section 2.1. Chemicals All chemicals were used as received without further puri®cation. Block copolymers including diblock PEO±PBO copolymer BO10 EO16 (BL501500, Mn 1500) and triblock PEO±PBO±PEO copolymer EO39 BO47 EO39 (B50-6600, Mn 6800) were obtained as gifts from Dow Chemical Co., Midland, MI. 2.2. Syntheses Mesoporous silica structures were synthesized at room temperature following the similar proce-
dure as previously reported [3,4]. In a typical preparation, 0.50 g of B50-6600 was dissolved in 30 g of 2 mol/l HCl. To this homogeneous solution, 2.08 g (0.01 mol) of tetraethyloxysilane (TEOS) was added with vigorous stirring for 24 h (TEOS:B50-6600:HCl:H2 O 1:0.0074:6:166, molar ratio). The resulting solid was aged in 100°C for another 24 h. The solid product was ®ltered, washed, and dried in vacuum at room temperature. The calcination was carried out in an oven by slowly increasing temperature from room temperature to 550°C in 4 h and heated at 550°C for 6 h in air. 2.3. Analyses XRD patterns were recorded on a Rigaku D/ Max-IIA diractometer using CuKa radiation. The nitrogen adsorption/desorption isotherms were performed at 196°C using a Micromeritics ASAP 2010 analyzer utilizing Barrett±Emmett± Teller (BET) calculations of surface area and Barrett±Joyner±Halenda (BJH) and/or Broekho and de Boer (BdB) model [15] calculations of pore volume and pore size distributions for the adsorption branch of the isotherm. TEM photographs were obtained with a JEM-1200EX microscope operated at 80 kV. For TEM measurements, the samples were ground, embeded in epoxy resin, and ultramicrotomed. SEM images were recorded with a Hitachi S-520. 29 Si MAS NMR spectra were recorded on a Bruker MSL-300 Spectrometer, operating at a 29 Si resonance frequency of 59.63 MHz. Tetramethylsilane, Si(CH3 )4 , was used as a reference for 29 Si MAS spectra measurements. TGA/DTA were performed on a Rigaku PTC-10A analyzer with temperature rate of 10°C/min in air. 3. Results and discussion 3.1. Triblock copolymer The powder XRD patterns of as-synthesized and calcined mesoporous silica FDU-1 prepared in the presence of EO39 BO47 EO39 triblock copolymer are shown in Fig. 1a. As-synthesized FDU-1 shows a well-resolved XRD pattern and the ®rst
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Fig. 1. (a) XRD patterns of as-synthesized and calcined cubic mesoporous silica FDU-1 prepared by using EO39 BO47 EO39 triblock copolymer as a structure-directing agent at room temperature; (b) Plot of the reciprocal d spacing of the re¯ections for calcined FDU-1 versus m h2 k 2 l2 1=2 .
diraction peak appears at a low angle 2h 0:68, after calcination at 550°C in air the ®rst diraction peak is shifted to a little higher angle 2h 0:73 since the shrinkage. Calcined FDU-1 shows well-resolved nine Bragg peaks. Combin-
ing with TEM analysis (see below), we can exactly index the diraction peaks to (1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (2 2 2), (4 0 0), (4 1 1) and (4 2 0) re¯ections for space group Im 3 m (Q229 ), respectively [16]. The relative intensity of these
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observed peaks follows the trend reported in previous literatures for surfactant±water systems [17,18]. The straight line passing through the ori1=2 gin of the 1=dh k l versus m h2 k 2 l2 plot (Fig. 1b) indicates the good ®t of the data to Im3 m space group. The values of the cubic cell lat and tice parameter, a, are calculated to be 183 A 171 A for as-synthesized and calcined FDU-1, respectively. The cubic Im 3 m mesophase has been observed in a ganglioside surfactant±water binary system [19±21] and PBO±PEO diblock copolymer± water systems [18]. The formation of surfactant/ water ``gels'' (of presumably micellar cubic morphology) has also been reported for PBO±PEO and PEO±PBO±PEO block copolymers in water, but no detail information regarding their structure has been reported [22,23]. SEM image (Fig. 2a) reveals that calcined FDU-1 sample has a wheat-like morphology with the particle size of 20 lm. As-synthesized samples of FDU-1 shows almost the same morphology as that for calcined samples, indicating that the macroscopic structures are thermally stable. TEM images of dierent orientations for calcined FDU1 are shown in Fig. 3a±c. Although both simple (P) and centered (I) cubic structures show a square lattice along the [1 0 0] direction and a hexagon lattice along [1 1 1] direction, the value of d1 0 0 versus d1 1 1 is equal to 0.866 (I) and 1.224 (P) respectively from geometrical calculations. From high-dark contrast in the TEM images (Fig. 3a and b) for calcined FDU-1, the value of d1 0 0 =d1 1 1
Fig. 2. SEM images of (a) calcined cubic silica mesostructure FDU-1 templated by EO39 BO47 EO39 triblock copolymer and (b) calcined hexagonal silica mesostructure templated by BO10 EO16 diblock copolymer.
Fig. 3. TEM images of (a), (b), and (c), calcined cubic mesoporous silica FDU-1 synthesized by using EO39 BO47 EO39 triblock copolymer without TMB at room temperature. Along the direction (a), [1 0 0]; (b), [1 1 1] and (c), [1 1 0]; (d) calcined cubic FDU-1 synthesized with TMB as the swelling agent. TEM images of (e) and (f) calcined hexagonal mesoporous silica structure templated by BO10 EO16 diblock copolymer along dierent zone axes, (e) [1 0 0], (f) [1 1 0].
is estimated to be 0.88, giving further evidence that the mesostructure is centered cubic with Im 3 m space group. The cell parameter, a, is estimated to from TEM images, in good agreement be 170 A with that determined from the XRD data. Moreover, TEM measurements also reveal that all areas for calcined FDU-1 have well-ordered mesoscopical arrays, and 3D caged structure can be observed along the edge of the sample (Fig. 3b), indicating that calcined FDU-1 has high quality caged cubic mesostructure.
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Fig. 4. Nitrogen adsorption±desorption isotherm plots and pore size distribution curve of calcined mesoporous silica FDU-1.
Representative N2 adsorption±desorption isotherms and the corresponding pore size distributions (analyzed by using BJH model) are shown in Fig. 4. Calcined cubic mesoporous silica FDU-1 synthesized with B50-6600 triblock copolymer yields a type IV isotherm with a large type-H2 hysteresis loop. The large hysteresis loop gives another evidence that FDU-1 has a large caged mesostructure [15]. A narrow pore-size distribu with a mean value of 120 tion FWHM 10 A A is also obtained from adsorption branch, indicating that FDU-1 has well-de®ned uniform pore dimensions. Calcined cubic mesoporous silica has a pore volume of 0.77 cm3 /g and a BET surface area of 740 m2 /g. Although the cell parameter for the FDU-1 is similar to that of SBA-16 (176 A), pore size and pore volume for FDU-1 are greatly 0.45 cm3 /g, respectively) larger than those (54 A, for SBA-16 [4], suggesting that the pore size is much dependent on the hydrophobic domain of the block copolymer. This might be caused from the curvature of copolymer micelles. The ratio of EO/BO (1.7) for EO39 BO47 EO39 is lower than that of EO/PO (3.0) for EO106 PO70 EO106 , resulting in higher curvature energy between the copolymer and silica species, therefore yields larger pore size. To our best knowledge, FDU-1 has
the largest pore size in all reported cubic silica structures. 29 Si MAS NMR spectroscopy of as-synthesized FDU-1 shows three broad peaks at 95, 105 and 115 ppm. These peaks correspond to Q2 , Q3 and Q4 silica species, respectively, which are associated with progressively increased silica cross-linking. From the relative peak areas, the ratios of these species are estimated to be Q2 :Q3 :Q4 0:1:1.0:0.8. However, the 29 Si MAS NMR spectroscopy of calcined FDU-1 shows mainly two broad peaks at 105 and 115 ppm, which correspond to Q3 and Q4 silica species, respectively. The ratio of these two species is estimated to be Q3 :Q4 1:0:0.9, indicating that the silica species are a little condensed after calcination [3,4]. The Q3 :Q4 ratio is much higher than that for conventional calcined MCM41, it may relate with its bilayer mesostrucutre of Im 3 m. TGA/DTA of as-synthesized FDU-1 show total losses of 36.7 wt.%. At 56°C TGA registers a weight loss of 5.5 wt.% because of desorption of water [12±14]. This is followed by a weight loss of 31.2 wt.%, which coincides with two exothermic DTA peaks at 192°C and 245°C, and associates with desorption and decomposition of the copolymer.
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Cubic mesoporous silica can be synthesized over a relatively wide range of reaction mixture compositions (TEOS:B50-6600:HCl:H2 O 1:0.0029± 0.012:0.3±12:142±167, molar ratio) at the temperature of 0±60°C. At room temperature, the cubic structure can be formed in a range of PEO±PBO± PEO concentrations (0.6±2.4 wt.%). The cell pa with the rameter (a) decreases from 200 to 178 A increasing of copolymer concentration [19]. At HCl concentrations from 4 to 0.1 mol/l, cubic mesoporous FDU-1 can be formed with high quality, while the cubic structures remains almost the same cell parameter. At pH values from 2 to 7, only amorphous silica or silica gel is obtained. Moreover, a low temperature such as 0°C yields a but it can large cell parameter (up to a 220 A), cause slight disorder for FDU-1 based on the results from XRD and TEM. At high temperature such as 60°C, the cell parameter and crystalline quality of FDU-1 remain almost unchanged. Adding swelling agents, such as trimethylbenzene (TMB), can slightly enlarge the cell parameter up with highly ordered to the certain value (220 A) mesostructure. Fig. 3d shows TEM image of calcined FDU-1 obtained with TMB. The cell in agreement parameter is estimated to be 220 A, with that determined from XRD pattern. However, the weight ratio of TMB to triblock copolymer B50-6600 cannot be higher than 25% to preserve the high order of the cubic structures. In spite of such large lattice dimensions, calcined cubic FDU-1 is hydrothermally stable. After calcined FDU-1 is heated in boiling water (100°C) for more than nine days, at least ®ve diraction peaks can be observed in the XRD pattern and the intensity for (1 1 0) re¯ection is even increased, indicating that large pore mesoporous FDU-1 is hydrothermally stable. 3.2. Diblock copolymer The Powder XRD pattern (Fig. 5a) of as-made mesoporous silica prepared by using diblock copolymer BO10 EO16 as a template in aqueous solution shows three re¯ection peaks at 2h values between 0.5° and 2.5°, which can be indexed to (1 0 0), (1 1 0), (2 0 0) diractions of hexagonal (p6mm) symmetry with a cell parameter of 101 A.
Fig. 5. Powder XRD patterns of (a) as-made and calcined hexagonal mesoporous silica structure and (b) as-made and calcined silica lamellar mesostructure prepared by using diblock copolymer BO10 EO16 at room temperature.
After calcination at 500°C for 6 h in air, these peaks are retained and a better-resolved XRD pattern is observed with a little shrinking of cell as expected [24]. The results parameter a 93 A indicate that the mesoporous materials prepared with diblock copolymer have even better longrange ordered 2D hexagonal mesostructure. TEM measurements further con®rm the conclusion. As shown in Fig. 3e and f, a large area of homogeneous ordered hexagonal mesostructure and highly ordered 1D channels can be observed. SEM images show that the calcined hexagonal mesoporous silica has a spherical morphology with a relative uniform diameter of 3 lm (Fig. 2b).
C. Yu et al. / Microporous and Mesoporous Materials 44±45 (2001) 65±72
Fig. 6. Nitrogen adsorption±desorption isotherm plots and pore size distribution curves of calcined hexagonal mesoporous silica prepared with BO10 EO16 diblock copolymer. The pore size distribution curves were calculated based on BdB model from both the adsorption and desorption branch of the isotherm.
N2 adsorption±desorption isotherm (Fig. 6) of calcined hexagonal mesoporous silica is a clear type IV curve with a type-H1 hysteresis loop. The pore size distribution curves calculated from BdB model [10] (Fig. 6) are quite narrow at the mean indicating that the material has a value of 60 A, quite uniform pore size. The pore diameter calcu which is lated from the adsorption branch is 59 A, quite consistent with that (61 A) calculated from the desorption branch by using a cylinder model, suggesting that the mesoporous silica synthesized with BO10 EO16 diblock copolymer has a high quality cylinder mesosturcture. The calcined sample has a BET surface area of 902 m2 /g, and a pore volume of 1.03 cm3 /g. TGA and DTA of as-synthesized hexagonal silica mesostructures show three loss steps with a total weight loss of 45 wt.%. The exothermic weight loss at 185°C and 245°C (totally 43.2 wt.%) can be assigned to the desorption and decomposition of the diblock copolymer, suggesting that after calcined at 245°C, most of the copolymer species can be removed from the channels. Hexagonal mesoporous silica structures can be synthesized under acidic conditions over a relatively wild range of reaction mixture compositions (TEOS:BO10 EO16 :HCl:H2 O 1:0.0067±0.0053:1.5
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±12:114±142, molar ratio) at the room temperature to 60°C. At room temperature, the hexagonal mesostructure can be prepared in a range of PBO± PEO concentrations (0.3±3.0 wt.%), with the cell The parameters (a) in the range from 108 to 95 A. cell parameter remains almost the same when the concentration of HCl changes, but it increases with the decreasing of copolymer concentrations [18]. When the concentration of the diblock copolymer is higher than 20 wt.%, lamellar silica mesostructure can be formed. XRD pattern (Fig. 5b) of as-synthesized products shows three well-resolved diraction peaks with d spacing of 88.2, 44.1, and respectively, which can be indexed as the 30.1 A, (1 0 0), (2 0 0), (3 0 0) re¯ections of a lamellar mesostructure. While after calcination at 500°C, all the peaks disappear as expected, because of the collapse of the lamellar structure when the diblock copolymer species are removed [1]. Higher concentration of the block copolymer can result in the lamellar mesostructures with larger d spacing, in accordance with the previously works [4]. BO10 EO16 diblock copolymer can form six lyotropic liquid crystalline and two solution phases in the presence of water and p-xylene [18]. The attempt to synthesize cubic mesoporous silica materials by using BO10 EO16 diblock copolymer as the template in water solution was unsuccessful. However, our previous results have shown that reverse 2D hexagonal (H2 ) silica mesostructure can be formed in toluene solution. Thus this is the ®rst example to show that in acidic media three mesostructures can be synthesized with one surfactant as the template. 4. Conclusions Highly ordered mesoporous silica structures including body-centered cubic (Im 3 m), 2D hexagonal (p6mm) and lamellar La symmetries have been synthesized by using hydrophobic poly(butylene oxide) moiety diblock and triblock copolymers as structure-directing agents. Caged cubic mesoporous silica structures (FDU-1, Im 3 m) with the largest cell parameter a 220 A can be easily formed by using triblock copolymer
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EO39 BO47 EO39 as the template. Calcined FDU-1 a pore volume of has a large pore size of 120 A, 3 0.77 cm /g, and a BET surface area of 740 m2 /g. Diblock copolymer BO10 EO16 favors to yield highly ordered, uniform sphere morphology (3 lm), hexagonal (p6mm) silica mesostructures with and a large BET well-uniformed pore size of 60 A 2 surface area of 902 m /g. Lamellar La mesostructures can also be obtained with the same diblock copolymer at high concentration. The calcined cubic mesostructured silica FDU-1 is hydrothermally stable in boiling water for at least nine days. The new mesoporous materials with large cage structure, uniform sphere morphology, and highly hydrothermal stability are expected to be of great value in electrochemistry, catalysis and separation for large molecules. Acknowledgements This work was supported by the National Science Foundations of China (Grant no. 29925309 and 29873012) and National Education Ministry. We thank Dr. Weiming Hua, Yinghong Le and Ms. Haiying Chen for BET and TGA measurements. We also thank Dow company for providing block copolymers.
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www.elsevier.nl/locate/micromeso
Highly ordered mesoporous silica structures templated by poly(butylene oxide) segment di- and tri-block copolymers Chengzhong Yu a,b, Yonghao Yu a, Lei Miao a, Dongyuan Zhao a,* a
Department of Chemistry, Fudan University, Shanghai, 200433, People's Republic of China b School of Medicine, Fudan University, Shanghai, 200433, People's Republic of China Received 3 February 2000; accepted 29 August 2000
Abstract Highly ordered mesoporous silica structures including body-centered cubic (Im 3 m), two-dimensional hexagonal (p6mm) and lamellar La symmetries have been synthesized by using hydrophobic poly(butylene oxide) moiety diblock and triblock copolymers as structure-directing agents. Under acidic condition, highly ordered, caged cubic mesoporous can be formed in the presence of triblock silica structures (FDU-1, Im 3 m) with a large cell parameter a 220 A poly(ethylene oxide)±poly(butylene oxide)±poly(ethylene oxide) (PEO±PBO±PEO) copolymer such as EO39 BO47 EO39 . among all known cubic silica mesostructures, a pore volume of 0.77 Calcined FDU-1 has a largest pore size of 120 A cm3 /g, and a BET surface area of 740 m2 /g. Diblock poly(butylene oxide)±poly (ethylene oxide) (PBO±PEO) copolymer such as BO10 EO16 favors to yield highly ordered two-dimensional hexagonal (p6mm) silica mesostructures with a well and a large BET surface area of 902 m2 /g. Lamellar La mesostructures can also be uniformed pore size of 60 A obtained when high concentration diblock copolymer is used as the structure-directing agent. The calcined cubic mesostructured silica FDU-1 is hydrothermally stable in boiling water for at least nine days. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous materials; Block copolymer; Synthesis
1. Introduction Since the discovery of M41S family of mesoporous silicates and aluminosilicates in 1992 [1,2], numerous mesoporous or nanoporous materials have been with pore sizes between 20 and 300 A synthesized because of their potential applications on catalysis, separation of large molecules, medical implants, semiconductor, magnetoelectric devices,
*
Corresponding author. E-mail address: [email protected] (D. Zhao).
etc. [3±14]. In most cases, ionic and neutral surfactants have been employed as templates to direct the mesophase formation based on the electrostatic and hydrogen-bonding interactions, respectively. Using triblock poly(ethylene oxide)±poly (propylene oxide)±poly(ethylene oxide) (PEO± PPO±PEO) copolymers as the structure-directing agents, a series of mesoporous silica structures including hexagonal (H1 ), lamellar La and cubic (I1 ) phases have been synthesized [3,4]. However, the attempt to synthesize mesoporous silica materials by using PEO±PPO diblock copolymers as the templates was unsuccessful. Three-dimensional (3D)
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 1 6 9 - X
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cubic mesostructures, such as MCM-48 (Ia 3 d) [1], SBA-1 (Pm 3 n) [12,13] and SBA-16 (Im 3 m) [4], have more advantages compared to hexagonal mesoporous structures with one-dimension channels. However, such cubic structures could be synthesized only under strict conditions. Calcined SBA-16 synthesized with PEO±PPO±PEO triblock copolymers has the largest pore size of 54 A among these cubic silica structures, and can only be synthesized with large PEO segment amphiphilic PEO±PPO±PEO block copolymers, such as EO106 PO70 EO106 in a narrow range of the reaction composition at room temperature [4]. The minor dierence of hydrophilicity/hydrophobicity between PEO and PPO chains in room temperature and the low ratio of PO/EO might be the reasons that result in hard formation of ordered mesostructures and such small pore size. Here we report the preparation of mesostructured silicas by using diblock and triblock copolymers with more hydrophobic poly(butylene oxide) (PBO) moiety as the structure-directing agents. The resulted mesoporous silica products with highly ordered mesostructures have been characterized by the means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption/desorption measurements, 29 Si magic angle spinning nuclear resonance (MAS NMR) spectroscopy and thermogravimetric analysis and dierential thermal analyses (TGA/DTA). 2. Experimental section 2.1. Chemicals All chemicals were used as received without further puri®cation. Block copolymers including diblock PEO±PBO copolymer BO10 EO16 (BL501500, Mn 1500) and triblock PEO±PBO±PEO copolymer EO39 BO47 EO39 (B50-6600, Mn 6800) were obtained as gifts from Dow Chemical Co., Midland, MI. 2.2. Syntheses Mesoporous silica structures were synthesized at room temperature following the similar proce-
dure as previously reported [3,4]. In a typical preparation, 0.50 g of B50-6600 was dissolved in 30 g of 2 mol/l HCl. To this homogeneous solution, 2.08 g (0.01 mol) of tetraethyloxysilane (TEOS) was added with vigorous stirring for 24 h (TEOS:B50-6600:HCl:H2 O 1:0.0074:6:166, molar ratio). The resulting solid was aged in 100°C for another 24 h. The solid product was ®ltered, washed, and dried in vacuum at room temperature. The calcination was carried out in an oven by slowly increasing temperature from room temperature to 550°C in 4 h and heated at 550°C for 6 h in air. 2.3. Analyses XRD patterns were recorded on a Rigaku D/ Max-IIA diractometer using CuKa radiation. The nitrogen adsorption/desorption isotherms were performed at 196°C using a Micromeritics ASAP 2010 analyzer utilizing Barrett±Emmett± Teller (BET) calculations of surface area and Barrett±Joyner±Halenda (BJH) and/or Broekho and de Boer (BdB) model [15] calculations of pore volume and pore size distributions for the adsorption branch of the isotherm. TEM photographs were obtained with a JEM-1200EX microscope operated at 80 kV. For TEM measurements, the samples were ground, embeded in epoxy resin, and ultramicrotomed. SEM images were recorded with a Hitachi S-520. 29 Si MAS NMR spectra were recorded on a Bruker MSL-300 Spectrometer, operating at a 29 Si resonance frequency of 59.63 MHz. Tetramethylsilane, Si(CH3 )4 , was used as a reference for 29 Si MAS spectra measurements. TGA/DTA were performed on a Rigaku PTC-10A analyzer with temperature rate of 10°C/min in air. 3. Results and discussion 3.1. Triblock copolymer The powder XRD patterns of as-synthesized and calcined mesoporous silica FDU-1 prepared in the presence of EO39 BO47 EO39 triblock copolymer are shown in Fig. 1a. As-synthesized FDU-1 shows a well-resolved XRD pattern and the ®rst
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Fig. 1. (a) XRD patterns of as-synthesized and calcined cubic mesoporous silica FDU-1 prepared by using EO39 BO47 EO39 triblock copolymer as a structure-directing agent at room temperature; (b) Plot of the reciprocal d spacing of the re¯ections for calcined FDU-1 versus m h2 k 2 l2 1=2 .
diraction peak appears at a low angle 2h 0:68, after calcination at 550°C in air the ®rst diraction peak is shifted to a little higher angle 2h 0:73 since the shrinkage. Calcined FDU-1 shows well-resolved nine Bragg peaks. Combin-
ing with TEM analysis (see below), we can exactly index the diraction peaks to (1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (2 2 2), (4 0 0), (4 1 1) and (4 2 0) re¯ections for space group Im 3 m (Q229 ), respectively [16]. The relative intensity of these
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observed peaks follows the trend reported in previous literatures for surfactant±water systems [17,18]. The straight line passing through the ori1=2 gin of the 1=dh k l versus m h2 k 2 l2 plot (Fig. 1b) indicates the good ®t of the data to Im3 m space group. The values of the cubic cell lat and tice parameter, a, are calculated to be 183 A 171 A for as-synthesized and calcined FDU-1, respectively. The cubic Im 3 m mesophase has been observed in a ganglioside surfactant±water binary system [19±21] and PBO±PEO diblock copolymer± water systems [18]. The formation of surfactant/ water ``gels'' (of presumably micellar cubic morphology) has also been reported for PBO±PEO and PEO±PBO±PEO block copolymers in water, but no detail information regarding their structure has been reported [22,23]. SEM image (Fig. 2a) reveals that calcined FDU-1 sample has a wheat-like morphology with the particle size of 20 lm. As-synthesized samples of FDU-1 shows almost the same morphology as that for calcined samples, indicating that the macroscopic structures are thermally stable. TEM images of dierent orientations for calcined FDU1 are shown in Fig. 3a±c. Although both simple (P) and centered (I) cubic structures show a square lattice along the [1 0 0] direction and a hexagon lattice along [1 1 1] direction, the value of d1 0 0 versus d1 1 1 is equal to 0.866 (I) and 1.224 (P) respectively from geometrical calculations. From high-dark contrast in the TEM images (Fig. 3a and b) for calcined FDU-1, the value of d1 0 0 =d1 1 1
Fig. 2. SEM images of (a) calcined cubic silica mesostructure FDU-1 templated by EO39 BO47 EO39 triblock copolymer and (b) calcined hexagonal silica mesostructure templated by BO10 EO16 diblock copolymer.
Fig. 3. TEM images of (a), (b), and (c), calcined cubic mesoporous silica FDU-1 synthesized by using EO39 BO47 EO39 triblock copolymer without TMB at room temperature. Along the direction (a), [1 0 0]; (b), [1 1 1] and (c), [1 1 0]; (d) calcined cubic FDU-1 synthesized with TMB as the swelling agent. TEM images of (e) and (f) calcined hexagonal mesoporous silica structure templated by BO10 EO16 diblock copolymer along dierent zone axes, (e) [1 0 0], (f) [1 1 0].
is estimated to be 0.88, giving further evidence that the mesostructure is centered cubic with Im 3 m space group. The cell parameter, a, is estimated to from TEM images, in good agreement be 170 A with that determined from the XRD data. Moreover, TEM measurements also reveal that all areas for calcined FDU-1 have well-ordered mesoscopical arrays, and 3D caged structure can be observed along the edge of the sample (Fig. 3b), indicating that calcined FDU-1 has high quality caged cubic mesostructure.
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Fig. 4. Nitrogen adsorption±desorption isotherm plots and pore size distribution curve of calcined mesoporous silica FDU-1.
Representative N2 adsorption±desorption isotherms and the corresponding pore size distributions (analyzed by using BJH model) are shown in Fig. 4. Calcined cubic mesoporous silica FDU-1 synthesized with B50-6600 triblock copolymer yields a type IV isotherm with a large type-H2 hysteresis loop. The large hysteresis loop gives another evidence that FDU-1 has a large caged mesostructure [15]. A narrow pore-size distribu with a mean value of 120 tion FWHM 10 A A is also obtained from adsorption branch, indicating that FDU-1 has well-de®ned uniform pore dimensions. Calcined cubic mesoporous silica has a pore volume of 0.77 cm3 /g and a BET surface area of 740 m2 /g. Although the cell parameter for the FDU-1 is similar to that of SBA-16 (176 A), pore size and pore volume for FDU-1 are greatly 0.45 cm3 /g, respectively) larger than those (54 A, for SBA-16 [4], suggesting that the pore size is much dependent on the hydrophobic domain of the block copolymer. This might be caused from the curvature of copolymer micelles. The ratio of EO/BO (1.7) for EO39 BO47 EO39 is lower than that of EO/PO (3.0) for EO106 PO70 EO106 , resulting in higher curvature energy between the copolymer and silica species, therefore yields larger pore size. To our best knowledge, FDU-1 has
the largest pore size in all reported cubic silica structures. 29 Si MAS NMR spectroscopy of as-synthesized FDU-1 shows three broad peaks at 95, 105 and 115 ppm. These peaks correspond to Q2 , Q3 and Q4 silica species, respectively, which are associated with progressively increased silica cross-linking. From the relative peak areas, the ratios of these species are estimated to be Q2 :Q3 :Q4 0:1:1.0:0.8. However, the 29 Si MAS NMR spectroscopy of calcined FDU-1 shows mainly two broad peaks at 105 and 115 ppm, which correspond to Q3 and Q4 silica species, respectively. The ratio of these two species is estimated to be Q3 :Q4 1:0:0.9, indicating that the silica species are a little condensed after calcination [3,4]. The Q3 :Q4 ratio is much higher than that for conventional calcined MCM41, it may relate with its bilayer mesostrucutre of Im 3 m. TGA/DTA of as-synthesized FDU-1 show total losses of 36.7 wt.%. At 56°C TGA registers a weight loss of 5.5 wt.% because of desorption of water [12±14]. This is followed by a weight loss of 31.2 wt.%, which coincides with two exothermic DTA peaks at 192°C and 245°C, and associates with desorption and decomposition of the copolymer.
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Cubic mesoporous silica can be synthesized over a relatively wide range of reaction mixture compositions (TEOS:B50-6600:HCl:H2 O 1:0.0029± 0.012:0.3±12:142±167, molar ratio) at the temperature of 0±60°C. At room temperature, the cubic structure can be formed in a range of PEO±PBO± PEO concentrations (0.6±2.4 wt.%). The cell pa with the rameter (a) decreases from 200 to 178 A increasing of copolymer concentration [19]. At HCl concentrations from 4 to 0.1 mol/l, cubic mesoporous FDU-1 can be formed with high quality, while the cubic structures remains almost the same cell parameter. At pH values from 2 to 7, only amorphous silica or silica gel is obtained. Moreover, a low temperature such as 0°C yields a but it can large cell parameter (up to a 220 A), cause slight disorder for FDU-1 based on the results from XRD and TEM. At high temperature such as 60°C, the cell parameter and crystalline quality of FDU-1 remain almost unchanged. Adding swelling agents, such as trimethylbenzene (TMB), can slightly enlarge the cell parameter up with highly ordered to the certain value (220 A) mesostructure. Fig. 3d shows TEM image of calcined FDU-1 obtained with TMB. The cell in agreement parameter is estimated to be 220 A, with that determined from XRD pattern. However, the weight ratio of TMB to triblock copolymer B50-6600 cannot be higher than 25% to preserve the high order of the cubic structures. In spite of such large lattice dimensions, calcined cubic FDU-1 is hydrothermally stable. After calcined FDU-1 is heated in boiling water (100°C) for more than nine days, at least ®ve diraction peaks can be observed in the XRD pattern and the intensity for (1 1 0) re¯ection is even increased, indicating that large pore mesoporous FDU-1 is hydrothermally stable. 3.2. Diblock copolymer The Powder XRD pattern (Fig. 5a) of as-made mesoporous silica prepared by using diblock copolymer BO10 EO16 as a template in aqueous solution shows three re¯ection peaks at 2h values between 0.5° and 2.5°, which can be indexed to (1 0 0), (1 1 0), (2 0 0) diractions of hexagonal (p6mm) symmetry with a cell parameter of 101 A.
Fig. 5. Powder XRD patterns of (a) as-made and calcined hexagonal mesoporous silica structure and (b) as-made and calcined silica lamellar mesostructure prepared by using diblock copolymer BO10 EO16 at room temperature.
After calcination at 500°C for 6 h in air, these peaks are retained and a better-resolved XRD pattern is observed with a little shrinking of cell as expected [24]. The results parameter a 93 A indicate that the mesoporous materials prepared with diblock copolymer have even better longrange ordered 2D hexagonal mesostructure. TEM measurements further con®rm the conclusion. As shown in Fig. 3e and f, a large area of homogeneous ordered hexagonal mesostructure and highly ordered 1D channels can be observed. SEM images show that the calcined hexagonal mesoporous silica has a spherical morphology with a relative uniform diameter of 3 lm (Fig. 2b).
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Fig. 6. Nitrogen adsorption±desorption isotherm plots and pore size distribution curves of calcined hexagonal mesoporous silica prepared with BO10 EO16 diblock copolymer. The pore size distribution curves were calculated based on BdB model from both the adsorption and desorption branch of the isotherm.
N2 adsorption±desorption isotherm (Fig. 6) of calcined hexagonal mesoporous silica is a clear type IV curve with a type-H1 hysteresis loop. The pore size distribution curves calculated from BdB model [10] (Fig. 6) are quite narrow at the mean indicating that the material has a value of 60 A, quite uniform pore size. The pore diameter calcu which is lated from the adsorption branch is 59 A, quite consistent with that (61 A) calculated from the desorption branch by using a cylinder model, suggesting that the mesoporous silica synthesized with BO10 EO16 diblock copolymer has a high quality cylinder mesosturcture. The calcined sample has a BET surface area of 902 m2 /g, and a pore volume of 1.03 cm3 /g. TGA and DTA of as-synthesized hexagonal silica mesostructures show three loss steps with a total weight loss of 45 wt.%. The exothermic weight loss at 185°C and 245°C (totally 43.2 wt.%) can be assigned to the desorption and decomposition of the diblock copolymer, suggesting that after calcined at 245°C, most of the copolymer species can be removed from the channels. Hexagonal mesoporous silica structures can be synthesized under acidic conditions over a relatively wild range of reaction mixture compositions (TEOS:BO10 EO16 :HCl:H2 O 1:0.0067±0.0053:1.5
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±12:114±142, molar ratio) at the room temperature to 60°C. At room temperature, the hexagonal mesostructure can be prepared in a range of PBO± PEO concentrations (0.3±3.0 wt.%), with the cell The parameters (a) in the range from 108 to 95 A. cell parameter remains almost the same when the concentration of HCl changes, but it increases with the decreasing of copolymer concentrations [18]. When the concentration of the diblock copolymer is higher than 20 wt.%, lamellar silica mesostructure can be formed. XRD pattern (Fig. 5b) of as-synthesized products shows three well-resolved diraction peaks with d spacing of 88.2, 44.1, and respectively, which can be indexed as the 30.1 A, (1 0 0), (2 0 0), (3 0 0) re¯ections of a lamellar mesostructure. While after calcination at 500°C, all the peaks disappear as expected, because of the collapse of the lamellar structure when the diblock copolymer species are removed [1]. Higher concentration of the block copolymer can result in the lamellar mesostructures with larger d spacing, in accordance with the previously works [4]. BO10 EO16 diblock copolymer can form six lyotropic liquid crystalline and two solution phases in the presence of water and p-xylene [18]. The attempt to synthesize cubic mesoporous silica materials by using BO10 EO16 diblock copolymer as the template in water solution was unsuccessful. However, our previous results have shown that reverse 2D hexagonal (H2 ) silica mesostructure can be formed in toluene solution. Thus this is the ®rst example to show that in acidic media three mesostructures can be synthesized with one surfactant as the template. 4. Conclusions Highly ordered mesoporous silica structures including body-centered cubic (Im 3 m), 2D hexagonal (p6mm) and lamellar La symmetries have been synthesized by using hydrophobic poly(butylene oxide) moiety diblock and triblock copolymers as structure-directing agents. Caged cubic mesoporous silica structures (FDU-1, Im 3 m) with the largest cell parameter a 220 A can be easily formed by using triblock copolymer
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EO39 BO47 EO39 as the template. Calcined FDU-1 a pore volume of has a large pore size of 120 A, 3 0.77 cm /g, and a BET surface area of 740 m2 /g. Diblock copolymer BO10 EO16 favors to yield highly ordered, uniform sphere morphology (3 lm), hexagonal (p6mm) silica mesostructures with and a large BET well-uniformed pore size of 60 A 2 surface area of 902 m /g. Lamellar La mesostructures can also be obtained with the same diblock copolymer at high concentration. The calcined cubic mesostructured silica FDU-1 is hydrothermally stable in boiling water for at least nine days. The new mesoporous materials with large cage structure, uniform sphere morphology, and highly hydrothermal stability are expected to be of great value in electrochemistry, catalysis and separation for large molecules. Acknowledgements This work was supported by the National Science Foundations of China (Grant no. 29925309 and 29873012) and National Education Ministry. We thank Dr. Weiming Hua, Yinghong Le and Ms. Haiying Chen for BET and TGA measurements. We also thank Dow company for providing block copolymers.
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