Facile Synthesis of High-Quality Large-Pore Periodic Mesoporous

Chapter 34

Facile Synthesis of High-Quality Large-Pore Periodic Mesoporous Organosilicas Templated by Triblock Copolymers *

Downloaded by CORNELL UNIV on October 6, 2016 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch034

Wanping Guo, Jin-Woo Park, and Chang-Sik Ha Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea

One exciting development in the surfactant-templated synthesis strategies is the discovery of a novel class of hybrid materials called periodic mesoporous organosilicas (PMOs) through surfactant-templated condensation of organosilanes with two organically bridged trialkoxysilyl groups. We report a facile synthesis of high-quality PMOs with large pores using commercially available poly(ethylene oxide)-b-poly(propylene oxide)-b-poly (ethylene oxide) (PEO-PPO-PEO) triblock copolymers as the structure directing agents under strongly acidic media in the presence of inorganic salts. These P M O materials exhibit not only highly ordered hexagonal (p6mm) or cubic (Im3m) pore structures, but also characteristic external morphologies.

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© 2006 American Chemical Society

Schubert et al.; Metal-Containing and Metallosupramolecular Polymers and Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Background In recent years, surfactant-templated synthesis strategies have been successfully applied to the preparation of a variety of mesoporous materials. One of the most exciting new developments was the discovery of a novel class of organic-inorganic hybrid materials called periodic mesoporous organosilicas (PMOs) through surfactant-templated condensation of organosilanes with two organically bridged trialkoxysilyl groups. " To date, P M O materials have been prepared using various bridged organic groups including methane, ethane, * ethylene, benzene, thiophene, biphenylene, etc. In addition, successful syntheses of PMOs have been achieved under a wide range of pHs from highly basic to strongly acidic conditions using cationic, " anionic, neutral, and nonionic oligomeric surfactants. Furthermore, P M O materials were already found to have such potential applications as novel catalysts, selective adsorbents, and hosts for nanocluster synthesis. A t present, particular interest is focused on large-pore P M O materials for the immobilization and encapsulation of large molecules. There are, however, only a few reports " on the synthesis of PMOs with large pores. Two communications first described the use of triblock copolymer Ρ123 (EO20PO70EO20) in strongly acidic media to prepare large-pore PMOs with poorly ordered mesostructures. Later, an attempt to synthesize large-pore PMOs using triblock copolymer B50-6600 (EO39BO47EO39) under low-acidic conditions led to the formation of PMOs with large cagelike pores in limited long-range order. 1

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A very recent report demonstrated the preparation of well-ordered phenylene-bridged PMOs using Ρ123 triblock copolymer. The other report on the synthesis of highly ordered large-pore PMOs involved direct crystal templating of Ρ123 triblock copolymer. In spite of a little success mentioned above, a general and facile route to the synthesis of highly ordered large-pore P M O materials using commercial triblock copolymer surfactants and bridged organosilanes has not been realized yet. 17

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Objective In this paper, we report a facile method for the synthesis of high-quality PMOs with large pores using commercially available poly(ethylene oxide)-èpoly(propylene oxide)-A-po!y(ethylene oxide) (PEO-PPO-PEO) triblock copolymers as the structure directing agents under strongly acidic media in the presence of inorganic salts, based on our preliminary works published previously. Pluronic Ρ123 (EO20PO70EO20) and Pluronic F127 (EO106PO70EO106) were employed to prepare hexagonal P M O material 19


488 (designated PMO-SBA-15) and cubic P M O material (designated P M O - S B A 16), respectively, as did in the preparation of silica-based SBA-15 and SBA-16 mesoporous materials. [l,2-bis(trimethoxysilyl)ethane] ( B T M E ) was chosen as the organically bridged silica source only because of its commercial availability. 20


Experimental Synthesis l9a

The following procedure was the typical preparation of P M O - S B A - 1 5 : 1.2 g of P123 (Aldrich) and 3.3 g of NaCl (Aldrich) were dissolved in 10 g of water and 30 g of 2.0 M HC1 solution (Wako, Japan) with stirring at 40 °C. To this homogeneous solution was added 1.6 g of B T M E (Aldrich) and then the mixture was stirred for 24 h at the same temperature. Subsequently, the resulting mixture was transferred into a Teflon-lined autoclave and heated at 80 °C for an additional 24 h under static conditions. The final reactant molar composition was B T M E / P123 / HC1 / NaCl / H 0 = 0.5 : 0.017 : 5.07 : 5.07 : 178. The solid products were obtained by filtration, washed thoroughly with water, and airdried at room temperature. The surfactant was removed by stirring 1.0 g of assynthesized sample in 150 mL of ethanol with 3.8 g of 36% HCI aqueous solution at 50 °C for 6 h. The resulting solid was recovered by filtration, washed with ethanol, and dried in air. This extraction process was repeated in order to remove the surfactant thoroughly. The complete extraction was confirmed by the disappearance of the characteristic IR adsorptions of the surfactant and by the absence of surfactant carbon signals in the C C P M A S N M R spectrum. According to the same procedure, PMO-SBA-16 was synthesized using F127 triblock copolymer as the template under strongly acidic media in the presence of K S 0 , following a molar ratio of 0.5 B T M E / 0.004 F127 / 4.51 HCI / 2.61 K S 0 / 116 H 0 . Two control samples (P123 blank and F127 blank) corresponding to PMO-SBA-15 and PMO-SBA-16, respectively, were prepared according to the protocols described above, without the addition of inorganic salts. 2

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489 Characterization S A X S measurements were carried out using 4C2 beam lines with Co Κα synchrotron radiation (λ = 0.1608 nm) at 2.5 GeV and 140 mA in the Pohang Accelerator Laboratory, P O S T E C H , Korea. Nitrogen adsorption-desorption isotherms were obtained using a Quantachrome Autosorb-1 apparatus at liquid nitrogen temperature and the pore size distributions were calculated by the BdB method. T E M images were acquired on a JEOL JEM-2010 microscope operating at 200 k V . S E M images were collected with a Hitachi S-4200 field emission scanning microscope. S i CP M A S N M R and C CP M A S N M R spectra were recorded on a Bruker DSX400 spectrometer at a S i resonance frequency of 59.63 M H z and a C resonance frequency of 75.47 M H z with tetramethylsilane as the reference. 29

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Results & Discussion Small angle X-ray scattering The framework structures of obtained samples were investigated by smallangle X-ray scattering (SAXS) experiments. The S A X S pattern of the solventextracted PMO-SBA-15 (Figure lc) shows three well-resolved peaks with interplanar d spacings of 10.5, 6.06, and 5.25 nm, which can be assigned to (100), (110), and (200) reflections of the two-dimensional hexagonal space group (p6mm), similar to that reported for the silica-based mesoporous counterpart S B A - 1 5 . The unit cell parameter of the hexagonal lattice is 12.1 nm. Analogously, three well-resolved peaks at very small scattering angles, with interplanar d spacings of 12.8, 9.03, and 7.38 nm, are observed in the S A X S pattern of the solvent-extracted PMO-SBA-16 (Figure Id). These three peaks can be indexable as (110), (200), and (211) reflections corresponding to the body-centered cubic space group (Im3m) with the unit cell parameter as large as 18.1 nm. 20a

20b

The well-resolved S A X S data demonstrate that highly ordered large-pore hexagonal and cubic P M O materials have been obtained from strongly acidic media in the presence of inorganic salts. By contrast, the S A X S patterns of two solvent-extracted control samples Ρ123 blank (Figure lb) and F127 blank (Figure la), which were prepared in the absence of inorganic salts, exhibit the


490 formation of either disordered mesoporous powder with a single broad peak or amorphous gel.

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Nitrogen adsorption Nitrogen adsorption technique was used to characterize the pore structures of P M O materials. Figure 2 shows nitrogen adsorption-desorption isotherms and the corresponding pore size distributions calculated by the B d B (Broekhoff and de Boer) method for solvent-extracted PMO-SBA-15 and PMO-SBA-16 samples. The isotherm of PMO-SBA-15 (Figure 2a) is of type IV with a clear H t y p e hysteresis loop at high relative pressure characteristic of large-pore mesoporous materials with one-dimensional cylindrical channels. The pore sizes determined from the adsorption and desorption branches of the isotherm (see Figure 2a inset) using the cylindrical model give the nearly same values (6.4 nm and 6.6 nm, respectively), which attests the pore structures of PMO-SBA-15 are certainly cylindrical. * The solvent-extracted PMO-SBA-15 has a B E T surface area of 737 m /g and a pore volume of 0.88 cm /g. The pore wall thick ness of PMO-SBA-15 evaluated from the unit cell parameter and the pore size data is around 5.5-5.7 nm. In comparison, the solvent-extracted P M O - S B A 21

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492 16 yields a type IV isotherm with a large H i hysteresis loop (Figure 2b), indicating bottle-shaped pore structures. This P M O material has a B E T surface area of 989 m /g and a pore volume of 0.63 cm /g, with a cell size of 9.8 nm determined from the adsorption branch of the isotherm and a window size of 5.5 nm determined from the desorption branch of the isotherm (see Figure 2b inset). 23

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The combination of S A X S and nitrogen adsorption data provides strong evidence of the high quality of PMO-SBA-15 and PMO-SBA-16 materials. The structural order of these two P M O materials is analogous to that observed in silica-based SBA-15 and SBA-16 counterparts. Furthermore, the high-quality large-pore P M O materials exhibit high hydrothermal stability. For example, i f PMO-SBA-15 with a pore size of 6.4 nm, a B E T surface area of 737 m /g, and a pore volume of 0.88 cm /g was treated in boiling water for 6 days, it then showed a pore size of 6.7 nm, a B E T surface area of 772 m /g, and a pore volume of 0.92 cm /g. The S A X S pattern of the P M O material was essentially unchanged after hydrothermal treatment. The high hydrothermal stability of P M O materials is believed to result from the thick pore walls and hydrophobicity imparted by the organic components in the P M O framework. 20


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Electron microscopy Transmission electron microscopy (TEM) images shown in Figure 3 provide direct visualization of the P M O pore structures. The T E M images of the solvent-extracted PMO-SBA-15 (Figure 3a,b) further corroborate well-ordered hexagonal p6mm arrays of one-dimensional mesoporous channels. A detailed analysis results in an estimated pore size of 6.5 nm and a pore wall thickness of about 5.5 nm. These values are in good agreement with those determined from the nitrogen adsorption measurement. The T E M images of the solvent-extracted PMO-SBA-16 recorded along the [100] and [110] directions (Figure 3c,d) clearly show well-ordered domains of three-dimensional cubic mesostructrues. Thus, the assignment of Im3m space group to PMO-SBA-16 in S A X S experiments is strongly supported by the T E M images. It should be noted that the P M O materials synthesized under strongly acidic media in the presence of inorganic salts have characteristic external morphologies. As seen in Figure 4, The NaCl-assisted PMO-SBA-15 exhibits a rod-like morphology with the diameter of around 5 μηι, whereas the PMO-SBA-16 synthesized with K S 0 shows a cauliflower-type morphology. 24

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Figure 3. TEM images of solvent-extracted PMO-SBA-15 recorded along the a) [100], b) [110] directions, and solvent-extracted PMO-SBA-16 recorded along the c) [100], d) [110] directions.

N M R spectroscopy The basic structural unit ethane-silica (Si-CH -CH -Si) in the P M O materials was confirmed by solid-state N M R spectroscopy. The C crosspolarization (CP) M A S N M R spectrum (not shown) of the solvent-extracted PMO-SBA-15 shows a strong resonance at 6.8 ppm that is attributed to ethane 2

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Figure 4. SEM images of as-synthesized a) PMO-SBA-15; b) PMO-SBA-16.

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carbon atoms in the P M O framework. The S i C P M A S N M R spectrum of the solvent-extracted PMO-SBA-15 (Figure 5b) exhibits two signals at -56.8 and 64.2 ppm corresponding to T [RSi(OSi) OH] and T [RSi(OSi) ] resonances, respectively. Alike, one signal at -57.6 ppm assigned to T resonance and the other signal at -64.6 ppm corresponding to T resonance are observed in the S i CP M A S N M R spectrum of the solvent-extracted PMO-SBA-16 (Figure 5a). In addition, the control sample synthesized using Ρ123 triblock copolymer as the template without NaCl also presents similar two signals T at -57.8 ppm and T at -64.9 ppm in the S i C P M A S N M R spectrum (Figure 5c). The absence of signals due to Q [Si(OSi) (OH) . ] species between -90 and -120 ppm indicates 2

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495 that all silicon atoms are covalently connected to carbon atoms in the P M O materials, and that no carbon-silicon bond cleavage occurred under the saltassisted strongly acidic synthesis conditions used herein.


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Figure 5. Si CP MAS NMR spectra of solvent-extracted a) PMO-SBA-16, b) PMO-SBA-15, andc) Ρ123 blank.

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Inorganic salts were used to improve the hydrothermal stability, control the morphology, extend the synthesis domain, and to tailor the framework porosity during the formation of mesoporous materials, which could be attributed to the specific effect of inorganic salts on the self-assembly interaction between surfactant headgroups and inorganic species. Our study confirms a similar effect of inorganic salts on the formation of high-quality large-pore P M O materials. In addition, the well-ordered PMO-SBA-15 can also be prepared in the presence of other neutral inorganic salts such as K G , N a S 0 and K S 0 . Instead of K S 0 , N a S 0 can also be used to synthesize the well-ordered P M O SBA-16 materials. However, the use of NaCl or K C l resulted in the deterioration 26

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496 of the long-range order of PMO-SBA-16, suggesting that highly charged salts favor the formation of the well-ordered PMO-SBA-16 materials. Therefore, we believe that the addition of inorganic salts, especially highly charged salts, can increase the self-assembly interaction between the headgroups of triblock copolymer surfactant and the organosilane species. The following consideration gives further elucidation. PEO-PPO-PEO triblock copolymers form micelles in water with the core of the PPO block surrounded by the shell of hydrated PEO end blocks. The addition of inorganic salts causes dehydration of ethylene oxide units from the hydrated P E O shell remaining adjacent to the PPO cores, leading to an increase of hydrophobicity in the PPO moieties and a reduction of hydrophilicity in the PEO moieties. By the counterion-mediated (S°H )(XT) pathway for the formation of the P M O materials under strongly acidic conditions, the low-hydrophilic P E O headgroups in the positively charged triblock surfactant are expected to have increased interaction with the positively charged organosilane species with low hydrophilicity due to the organic components. This enhanced self-assembly interaction can result in long-range ordered domain of organosilica-surfactant mesostructures. 29

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On the other hand, the more condensed pore walls could make contribution to the long-range structural order of the P M O materials, which is demonstrated by the S i C P M A S N M R spectra shown in Figure 5. After deconvolution, the T / T ratio of the solvent-extracted PMO-SBA-15 sample synthesized with NaCl is calculated to be 0.69, whereas the T / T ratio of the solvent-extracted corresponding control sample synthesized without NaCl (see Figure 5c) is found to be 0.48. The higher T / T ratio indicates that there is higher degree of organosilane cross-linking during the formation of the P M O materials, which is comparable to that reported for the synthesis of silica-based S B A - 1 5 . Moreover, the T / T ratio of the solvent-extracted PMO-SBA-16 is as high as 0.82, indicating the presence of much more condensed pore walls. In short, the increased self-assembly interaction between the headgroups of triblock copolymer surfactant and the organosilane species, and the much high degree of organosilane cross-linking, which are two kinds of effects initiated by the addition of inorganic salts, result in the highly ordered large-pore P M O materials. 29

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Conclusions In conclusion, we have provided a facile route to the first synthesis of highly ordered large-pore hexagonal (p6mm) and cubic (lm3m) P M O materials under strongly acidic media in the presence of inorganic salts using Ρ123 triblock copolymer and F127 triblock copolymer as the templates, respectively. These P M O materials with well-ordered large pores show not only characteristic


497 external morphologies, but also much high hydrothermal stability. Although only two important two-dimensional hexagonal and three-dimensional cubic P M O materials were researched in this paper, we believe that the salt-assisted synthesis strategy could be extended to the preparation of other types of highquality P M O materials with large pores using other kinds of triblock copolymers as the templates. Moreover, the resulting highly ordered large-pore P M O materials are expected to have promising applications involved in large molecules.


Acknowledgments This work was supported by the National Research Laboratory Program, the Center for Integrated Molecular Systems, and the Brain Korea 21 Project. We thank Prof. R. Ryoo (KAIST, Korea) for his help in nitrogen adsorption measurements, and Prof. D . Zhao (Fudan University, China) for providing BdB program. The Pohang Accelerator Laboratory, P O S T E C H , Korea is also acknowledged for S A X S measurements. Dr. W. Guo is now working as a research fellow at Department of Chemical and Biomolecular Engineering, National University of Singapore.

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Facile Synthesis of High-Quality Large-Pore Periodic Mesoporous

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