Periodic Mesoporous Organosilicas with Multiple Bridging Groups and

The PMO samples with different concentrations of organic bridging groups were ..... phenylene−thiophene−ethane PMO particles (Figure 3g and h) exh...
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Langmuir 2007, 23, 11844-11849

Periodic Mesoporous Organosilicas with Multiple Bridging Groups and Spherical Morphology Eun-Bum Cho,† Dukjoon Kim,*,† and Mietek Jaroniec*,‡ Polymer Technology Institute, Department of Chemical Engineering, Sungkyunkwan UniVersity, Suwon, Gyeonggi-do 440-746, Korea and Department of Chemistry, Kent State UniVersity, Kent, Ohio 44240 ReceiVed June 29, 2007. In Final Form: August 23, 2007 Bi- and trifunctional periodic mesoporous organosilicas (PMOs) with phenylene, thiophene, and ethane bridging groups were synthesized using 1,2-bis(triethoxysilyl)ethane (BTEE), 1,4-bis(triethoxysilyl)benzene (BTEB), and 2,5bis(triethoxysilyl)thiophene (BTET) organosilica precursors and a poly(ethylene oxide)-poly(D,L-lactic acid-coglycolic acid)-poly(ethylene oxide) (PEO-PLGA-PEO) triblock copolymer template under low acidic conditions. The PMO samples with different concentrations of organic bridging groups were obtained in the form of spherical particles having average diameters of 2-3 µm and 2D hexagonal (p6m) mesostructure with pore diameters of 7.3-8.4 nm. The particle morphology and chemistry of pore walls were tailored using different mixtures of organosilica precursors. Adsorption and structural properties of the aforementioned PMOs have been studied by nitrogen adsorption and small-angle X-ray scattering, whereas their framework chemistry was quantitatively analyzed by solid-state 13C and 29Si MAS NMR.

1. Introduction A diverse spectrum of periodic mesoporous organosilicas (PMOs) has been synthesized by self-assembly of various bridged silsesquioxane precursors and structure-directing agents.1,2 In comparison to the ordered mesoporous organosilicas obtained by postsynthesis grafting and co-condensation method, PMOs have several advantages; in addition to the structural ordering and narrow pore size distribution they possess a high loading of homogeneously distributed organic bridging groups in the framework, which are used to finely tune their structural and surface properties.2,3 For instance, introduction of phenylene (πinteracting bridging groups) into the framework afforded PMOs with crystalline pore walls.4 Also, it has been reported that introduction of organic bridging groups such as ethane improves the mechanical5 and hydrothermal6 properties of PMOs, making them attractive for versatile applications. Thus far, a variety of PMOs have been reported with different organic bridging groups * To whom correspondence should be addressed. D.K.: phone, 82-31290 7250; e-mail, [email protected]. M.J.: phone, 1-330-672 3790; e-mail, [email protected]. † Sungkyunkwan University. ‡ Kent State University. (1) (a) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Teresaki, O. J. Am. Chem. Soc. 1999, 121, 9611-9614. (b) Melde, B. J.; Holland, B. T.; Blandford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302-3308. (c) Asefa, T.; MacLachlan, M. J.; Coombos, N.; Ozin, G. A. Nature 1999, 402, 867-871. (2) (a) Asefa, T.; Yoshina-Ishii, C.; MacLaclan, M.; Ozin, G. A. J. Mater. Chem. 2000, 10, 1751-1755. (b) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151-3168. (c) Corriu, R.; Mhedi, A.; Rye, C. J. Organomet. Chem. 2004, 689, 4437-4450. (d) Kickkelbick, G. Angew. Chem., Int. Ed. 2004, 43, 31023104. (e) Hatton, B.; Landskron, K.; Whitnall, W.; Perovic, D.; Ozin, G. A. Acc. Chem. Res. 2005, 38, 305-312. (f) Vinu, A.; Hossain, K. Z.; Ariga, K. J. Nanosci. Nanotechnol. 2005, 5, 347-371. (g) Yoshitake, H. New J. Chem. 2005, 29, 11071117. (h) Ford, D. M.; Simanek, E. E.; Shantz, D. F. Nanotechnology 2005, 16, 458-475. (i) Hoffmann, F.; Cornelius, M.; Morell, J.; Fro¨ba, M. J. Nanosci. Nanotechnol. 2006, 6, 265-288. (3) Jaroniec, M. Nature 2006, 442, 638-640. (4) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304307. (5) Burleigh, M. C.; Markowitz, M. A.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Gaber, B. P. J. Phys. Chem. B 2003, 107, 12628-12634. (6) Cho, E.-B.; Char, K. Chem. Mater. 2004, 16, 270-275.

such as methane,7 ethane,1,5,6,8 benzene,4,9 toluene,10 2,5dimethylbenzene,10 thiophene,11,12 p-xylene,13 biphenyl,14 1,3,5benzene,15 interconnected [Si(CH2)]3 rings,16 4-phenylether,17 4-phenylsulfide units,17 isocyanurate rings,18 etc. Recently, multifunctional PMOs containing more than one bridging group have been synthesized using nonionic surfactants, C18H37(EO)10OH (Brij 76)19-21 and EO20PO70EO20 (P123).21,22 Introduction of multifunctional bridging groups allows one to finely tune the surface properties of PMOs to achieve the desired functionality and selectivity. Combining those features with large surface area, (7) (a) Asefa, T.; MacLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A. Angew. Chem., Int. Ed. 2000, 39, 1808-1811. (b) Guan, S.; Inagaki, S.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 5660-5661. (c) Sayari, A.; Hamoudi, S.; Yang, Y.; Moudrakovski, I. L.; Ripmeester, J. R. Chem. Mater. 2000, 12, 3857-3863. (8) (a) Muth, O.; Schellbach, C.; Fro¨ba, M. Chem. Commun. 2001, 20322033. (b) Kapoor, M. P.; Inagaki, S. Chem. Mater. 2002, 14, 3509-3514. (c) Xia, Y.; Yang, Z.; Mokaya, R. Chem. Mater. 2006, 18, 1141-1148. (d) Xia, Y.; Mokaya, R. J. Phys. Chem. B 2006, 110, 3889-3894. (9) (a) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410-2411. (b) Yang, Q.; Kapoor, M. P.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 9694-9695. (c) Kapoor, M. P.; Inagaki, S.; Ikeda, S.; Kakiuchi, K.; Suda, M.; Shimada, T. J. Am. Chem. Soc. 2005, 127, 8174-8178. (d) Sayari, A.; Wang, W. J. Am. Chem. Soc. 2005, 127, 12194-12195. (e) Rebbin, V.; Schmidt, R.; Fro¨ba, M. Angew. Chem., Int. Ed. 2006, 45, 5210-5214. (10) Temtsin, G.; Asefa, T.; Bittner, S.; Ozin, G. A. J. Mater. Chem. 2001, 11, 3202-3206. (11) (a) Yoshina-Ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539-2540. (b) Morell, J.; Woltner, G.; Fro¨ba, M. Chem. Mater. 2005, 17, 804-808. (12) Cho, E.-B.; Kim, D. Chem. Lett. 2007, 36, 118-119. (13) Hunks, W. J.; Ozin, G. A. Chem. Mater. 2004, 16, 5465-5472. (14) Kapoor, M. P.; Yang, Q.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 1517615177. (15) Kuroki, M.; Asefa, T.; Whitnal, W.; Kruk, M.; Yoshina-Ishii, C.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2002, 124, 13886-13895. (16) Landskron, K.; Hatton, B. D.; Perovic, D. D.; Ozin, G. A. Science 2003, 302, 266-269. (17) Hunks, W. J.; Ozin, G. A. Chem. Commun. 2004, 2426-2427. (18) Olkhovyk, O.; Jaroniec, M. J. Am. Chem. Soc. 2005, 127, 60-61. (19) Burleigh, M. C.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Markowitz, M. A.; Gaber, B. P. Chem. Mater. 2004, 16, 3-5. (20) Morell, J.; Gu¨ngerich, M.; Wolter, G.; Jiao, J.; Hunger, M.; Klar, P. J.; Fro¨ba, M. J. Mater. Chem. 2006, 16, 2809-2818. (21) Hoffman, F.; Cornelius, M.; Morell, J.; Fro¨ba, M. Angew. Chem., Int. Ed. 2006, 45, 3216-3251. (22) Grudzien, R. M.; Grabicka, B. E.; Pikus, S.; Jaroniec, M. Chem. Mater. 2006, 18, 1722-1725; Olkhovyk, O.; Jaroniec, M. Ind. Eng. Chem. Res. 2007, 46, 1745-1751.

10.1021/la701948g CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007

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Scheme 1. Schematic Illustration of Bi- and Trifunctional PMOs with Phenylene, Thiophene, and Ethane Bridging Groups

large pore size, and spherical morphology increases an overall attractiveness of these materials for adsorption, catalysis, separations, sensing, etc. While poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers are most often used as soft templates for the synthesis of ordered mesoporous materials, poly(ethylene oxide)-poly(D,L-lactic acid-co-glycolic acid)poly(ethylene oxide) (PEO-PLGA-PEO) triblock copolymers become attractive substitutes because they afford highly ordered PMOs with large pores and large surface area.12 In addition, spherical microsized particles can be formed without any additives.12 The methoxy-terminated PEO-PLGA-PEO triblock copolymers having appropriate composition and sufficiently large molecular weight can be designed and synthesized to achieve the desired structure and solubility in water; for instance, the volume ratio of the PEO block of about 0.38 is used to obtain the highly ordered mesophases of hexagonal symmetry.12 Here we report for the first time the synthesis of spherical bifunctional and trifunctional PMO particles, containing a combination of phenylene, thiophene, and ethane bridging groups, in the presence of a PEO-PLGA-PEO triblock copolymer template under slightly acidic conditions. The uniqueness of this work is a simultaneous control of the structure ordering, surface properties, and morphology of PMOs. As shown in Scheme 1, the surface properties of these particles can be tailored using different molar ratios of the organosilica precursors: 1,2-bis-

(triethoxysilyl)ethane (BTEE), 1,4-bis(triethoxysilyl)benzene (BTEB), and 2,5-bis(triethoxysilyl)thiophene (BTET). Under these conditions the highly ordered 2D hexagonal mesostructures were formed, which was confirmed by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) studies. The spherical morphology of particles was observed by scanning electron microscopy (SEM), while the presence of organic bridging groups was confirmed by solid-state 13C and 29Si MAS NMR measurements.

Table 1. Molar Ratios of Organosilanes in the Reaction Mixturea molar ratios of the organosilanes used sample

BTEB

BTET

BTEE

PE31 PE11 PE13 TE31 TE11 TE31 PT31 PT11 PT13 PTE111 PTE211 PTE121 PTE112

3 1 1 0 0 0 3 1 1 1 2 1 1

0 0 0 3 1 1 1 1 3 1 1 2 1

1 1 3 1 1 3 0 0 0 1 1 1 2

a Notation: BTEB ) 1,4-bis(triethoxysilyl)benzene; BTET ) 2,5bis(triethoxysilyl)thiophene; BTEE ) 1,2-bis(triethoxysilyl)ethane; letters and numbers in the sample codes refer to the bridging groups (P ) phenylene, T ) thiophene, and E ) ethane) and their molar ratios, respectively.

Figure 1. 2D SAXS patterns for PMOs with phenylene, thiophene, and ethane bridging groups: (a) phenylene-ethane-, (b) thiopheneethane-, (c) phenylene-thiophene-, and (d) phenylene-thiopheneethane-bridged organosilicas.

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Table 2. Adsorption and Structural Parameters for the PMO Samples Studieda sample

SBET, m2/g

Vt, cc/g

Vmi, cc/g

wKJS, nm

b, nm

a, nm

PE31 PE11 PE13 TE31 TE11 TE13 PT31 PT11 PT13 PTE111 PTE211 PTE121 PTE112

1026 1022 1031 990 887 984 1108 945 880 1006 963 910 996

1.09 1.13 1.15 1.11 1.06 1.20 1.18 1.01 0.96 1.07 1.02 0.94 1.18

0.14 0.11 0.09 0.10 0.07 0.05 0.17 0.14 0.12 0.14 0.14 0.11 0.11

8.3 8.3 8.2 7.3 7.9 8.0 8.2 8.4 7.9 8.1 8.3 8.0 8.2

2.8 2.7 2.7 3.4 3.0 2.7 2.8 2.5 3.0 2.8 2.6 2.9 2.7

11.1 11.0 10.9 10.7 10.9 10.7 11.0 10.9 10.9 10.9 10.9 10.9 10.9

a Notation: SBET ) BET specific surface area; Vt ) single-point pore volume; Vmi ) micropore volume obtained by Rs-plot analysis; wKJS ) mesopore diameter at the maximum of the PSD curve obtained by the improved KJS method;23 b - pore wall thickness ) a - wKJS; a ) unit cell parameter.

2. Experimental Section 2.1. Synthesis of Triblock Copolymer Template. The methoxyterminated poly(ethylene oxide)-poly(D,L-lactic acid-co-glycolic acid) (mPEO-PLGA) diblock copolymers were synthesized using 50 g of D,L-lactide (Aldrich), 10 g of glycolide (Polyscience), 40 g of monomethoxy poly(ethylene oxide) (Mn ) 750 g mol-1, Aldrich) in the presence of 6.15 mL of stannous 2-ethyl hexanoate (Sigma) as a catalyst, and 710 mL of anhydrous toluene at 393 K under nitrogen atmosphere. Next, EO16(L29G7)EO16 triblock copolymers (denoted as LGE538) were obtained by coupling the mPEO-PLGA diblock copolymers with 4.5 mL of hexamethylene diisocyanate (Sigma). After precipitation of the copolymer in cold diethyl ether, the final product was obtained by successive filtration, evaporation of the solvent, and drying under vacuum for 15 days. The molecular weight and polydispersity index of the triblock copolymer were obtained using a GPC-RI (Waters HPLC) system coupled with a Water 2410 differential refractive index detector. The flow rate of chloroform used as a mobile phase was 0.8 mL min-1. The 1H NMR (500 MHz) spectrum used to determine the chemical structure of the block copolymer was obtained with a Varian Unity Inova 500NB FT-NMR system using dimethyl sulfoxide (DMSO) solvent at 373 K. The average molecular weight of LGE538 was determined to be 5310 Da, and the polydispersity index was 1.28. The volume fraction of the PEO blocks (ΦPEO) estimated by the group contribution method on the basis of NMR measurements was equal to 0.38. 2.2. Synthesis of Periodic Mesoporous Organosilicas. The LGE538 triblock copolymer was used as a soft template, and 1,4bis(triethoxysilyl)benzene (BTEB, Aldrich), 2,5-bis(triethoxysilyl)thiophene (BTET, JSI Silicone), and 1,2-bis(triethoxysilyl)ethane (BTEE, Aldrich) were used as bridged organosilica precursors. The molar ratios of two or three organosilica precursors in the initial reaction mixtures and the corresponding sample codes are listed in Table 1. In a typical synthesis, 0.5 g of the LGE538 triblock copolymer was dissolved in a mixture of 22.35 g of distilled water and 0.5 g of ethanol. After stirring the polymer solution for 1 h, 0.15 g of HCl (37 wt %, Aldrich) was added and the stirring was continued for an additional 2 h. Next, the mixture of organosilica precursors was added and the stirring was continued for about 1 h at 313 K, which resulted in formation of a precipitate. The latter was aged for 24 h at 368 K. The molar ratio of the reaction mixture was LGE538: organosilanes:HCl:ethanol:H2O ) 1.0:19.0-21.9:23.7-30.1:0115.4:12.819-12.830. The block copolymer template was extracted by successive treatment with ethanol (e.g., 60 g of EtOH/0.5 g of organosilica) for 2 h under static conditions and acetone (e.g., 60 g of acetone/0.5 g of the sample) at 329 K under magnetic stirring for 5 h followed by washing with distilled water and acetone using a suction flask and drying at 373 K for 1 day.

Figure 2. TEM images of PMOs containing phenylene, thiophene, and ethane bridging groups: (a) PE11, (b) TE11, (c) PT11, and (d) PTE111 are representative images of the PMOs studied. 2.3. Measurements and Calculations. The SAXS measurements were performed using a Bruker SAXS analyzer having a 2D GADDS diffractometer and Cu KR radiation with λ ) 1.54 Å. The TEM images were obtained with an FE-TEM (JEOL JEM2100F) operated at an accelerating voltage of 200 kV. The samples were sonicated for 30 min in an adequate quantity of ethanol, and the solution was dropped onto a carbon film on a copper grid and then dried. The SEM images were obtained with a field emission SEM (JEOL JSM6700F) operated at an accelerating voltage of 15 kV. Particle size distributions were obtained with the Electrophoretic Light Scattering system (Ostuka ELS-8000) using distilled water as a medium solvent at the Korea Basic Science Institute. Nitrogen adsorption-desorption isotherms were measured with a Micromeritics TriStar analyzer. The samples were degassed at 423 K and 30 µmHg for 6 h. The BET (Brunauer-Emmet-Teller) specific area was calculated from the adsorption data in the relative pressure range from 0.05 to 0.2. The total pore volume was evaluated from the amount adsorbed at a relative pressure of 0.99, and the micropore volume was obtained using the Rs-plot method.23 The pore size distributions were calculated from the adsorption branches of the isotherms by using the improved KJS (Kruk-Jaroniec-Sayari) method, which is applicable for cylindrical pores.24 The pore width was estimated at the maximum of the pore size distribution. The solid-state NMR spectra were obtained with a Bruker DSX400 spectrometer using a 4 mm magic angle (MAS) spinning probe at the Korea Basic Science Institute. All of the samples were spun at rates of 4.5-7 kHz, and the chemical shifts were obtained with respect to the tetramethylsilane reference peak. The 13C CP-MAS NMR spectra were measured under the following experimental conditions: a contact time of 2 ms, a recycle delay of 3 s, and 3000 scans. The 29Si MAS NMR were obtained with a recycle delay of 50 s and 1500 scans. Quantification of the organic components was performed through the decomposition and simulation of the 29Si MAS NMR spectra using the Lorentzian fitting method available in the Microcal Origin 6.0 software.

3. Results and Discussion The 2D SAXS data for bifunctional and trifunctional PMOs synthesized using the LGE538 block copolymer template and the different molar ratios of BTEB, BTET, and BTEE precursors are shown in Figure 1. As can be seen from this figure all samples of the multifunctional PMOs studied are highly ordered 2D (23) (a) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169-3183. (b) Jaroniec, M.; Kaneko, K. Langmuir 1997, 13, 6589-6596. (24) Jaroniec, M.; Solovyov, L. A. Langmuir 2006, 22, 6757-6760.

Periodic Mesoporous Organosilicas

Figure 3. SEM of PMOs containing phenylene, thiophene, and ethane bridging groups. (a, b) PE11, (c, d) TE11, (e, f) PT11, and (g, h) PTE111 are representative images of the PMOs studied.

hexagonal (p6m) mesophases with at least three well-resolved peaks indexed as the (100), (110), and (200) reflections, which are nearly independent of the molar ratios of organosilanes in the reaction mixtures. The most intense (100) peaks are slightly shifted in the d-spacing range from 9.3 to 9.6 nm, which correspond the unit cell parameters (a) varying from 10.7 to 11.1 nm (see Table 2). The PE PMOs, having phenylene and ethane bridging groups, exhibit larger lattice spacings than the remaining bifunctional PMOs. The d values slightly increase from 9.4 to 9.6 nm with increasing amount of phenylene bridging groups. The nitrogen adsorption-desorption isotherms for the PMOs studied are typical type-IV isotherms with a steep increase of adsorption branch at P/P0 ) 0.65-0.75 due to the capillary condensation of nitrogen in the mesopores (see Supporting Information Figure S1). The BET surface areas of the PMO samples are in the range of 880-1108 m2 g-1 (see Table 2); slightly larger values of the surface area are observed for bifunctional phenylene-ethane PMOs. The total pore volumes are in the range of 0.94-1.20 cm3 g-1 with some evidence of intrawall microporosity (about 6-14%), which is a characteristic feature of silica-based materials templated with block copolymers containing PEO segments.25 The PMOs containing a large amount of phenylene bridging groups such as PE31, PT31, and PTE211 show higher microporosity (about 14%) than the other PMO materials prepared in this work. The pore size distributions obtained from the adsorption isotherms by the improved KJS method23 are in the form of narrow peaks having a maximum at the pore width range from 7.3 to 8.4 nm (see Supporting Information Figure S2). The pore wall thickness values calculated (25) Ryoo, R.; Ko, Ch. H.; Kruk, M.; Antochshuk, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 11465-11471.

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Figure 4. Solid-state 13C CP/MAS NMR spectra of PMOs containing phenylene, thiophene, and ethane bridging groups: (a) phenyleneethane-, (b) thiophene-ethane-, (c) phenylene-thiophene-, and (d) phenylene-thiophene-ethane-bridged organosilicas. The resonances with asterisks are due to the spinning sidebands.

by subtracting the pore diameter from the unit cell parameter vary from 2.5 to 3.4 nm, which is in a good agreement with the value estimated from the TEM images. PMOs containing a large amount of phenylene bridging groups such as PE31 and PTE211 showed higher pore widths than those containing thiophene bridging groups such as TE31, TE11, and PTE121. The overall physicochemical properties of the bifunctional and trifunctional PMO materials are summarized in Table 2; all samples studied feature high surface area and large mesopore volume. Formation of well-defined 2D hexagonal (p6m) bifunctional and trifunctional PMOs was also confirmed by TEM analysis as shown in Figure 2. These images are perpendicular to the channels and represent uniform 2D hexagonal mesostructures consistent with the pore sizes estimated by nitrogen adsorption and SAXS analysis. The FE-SEM images shown in Figure 3 represent the particle morphology of the multifunctional PMOs studied. All of those PMOs exhibit a macrostructure consisting of spherical and spherical-like particles with diameters varying from 1 to 10 µm. The average particle sizes estimated by a light scattering method are in the range of 2-3 µm (see Supporting Information Figure S3). The spherical morphology could be beneficial for applications of these materials in separations, catalysis, and microelectronics because spherical particles can be densely and uniformly packed/ organized. The domain boundary of the mesopore channel calculated using the full-width at half-maximum (fwhm) of the Bragg (100) peaks (i.e., domain boundary ≈ 2π/fwhm) is in the range of 0.15-0.17 µm, which suggests that the spherical PMO

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Figure 5. Solid-state 29Si MAS NMR spectra of PMOs containing phenylene, thiophene, and ethane bridging groups: (a) phenyleneethane-, (b) thiophene-ethane-, (c) phenylene-thiophene-, and (d) phenylene-thiophene-ethane-bridged organosilicas.

particles contain 6-67 units of the different axis-oriented mesophasic domains per one particle; this also means that the domain units of anisotropic pore channels are distributed isotropically inside mesoporous particles. The surface roughness as well as the surface curvature of the spherical PMO particles is also noticeable (see Figure 3). The phenylene-thiophene PMO particles (Figure 3e and f) appear to be interconnected, but their surface is smoothest among all types of particles studied. On the other hand, the thiopheneethane PMO particles (Figure 3c and d) show a very rough surface and some of the particles do not appear to have a fully spherical curvature. It is also visible that the trifunctional phenylenethiophene-ethane PMO particles (Figure 3g and h) exhibit better spherical shape than the remaining PMO particles. These results suggest that the similar unit size and chemical reactivity of the phenylene- and thiophene-containing precursors help to form a smooth surface of the PMO particles. Solid-state 13C and 29Si NMR analysis of the polymer-free multifunctional PMOs was performed to verify the composition and structure of the covalently bonded organic bridging groups in the framework. The 13C CP MAS NMR spectra for bi- and trifunctional PMOs are shown in Figure 4a-d, and the resonance peaks at 133, 138, and 5 ppm are assigned to the carbons in the benzene ring, thiophene ring, and ethane chain, respectively. The respective single peaks indicate that the bridging groups are covalently bonded to the Si atoms and that the C-Si bonds are stable in the organosilica framework. The resonances marked by asterisks are due to the spinning sidebands, and the other resonance

Cho et al.

Figure 6. Simulation and decomposition of 29Si MAS NMR spectra for trifunctional PMOs with phenylene, thiophene, and ethane bridging groups: (a) PTE111, (b) PTE211, (c) PTE121, and (d) PTE112.

peaks are negligible to assign them as residual molecules, such as polymer templates, ethoxy groups, ethanol, etc., which indicates that the organosilica precursors are hydrolyzed to a high degree during the co-condensation synthesis and that extraction with acetone without acid addition is quite effective to remove the PEO-PLGA-PEO triblock copolymer templates. The spectra shown in Figure 4 represent: (a) phenylene-ethane, (b) thiophene-ethane, (c) phenylene-thiophene, and (d) phenylenethiophene-ethane organosilicas with the peak ratios corresponding to the molar ratios of the precursors used in the reaction mixtures. The 29Si MAS NMR spectra of the bi- and trifunctional PMOs are shown in Figure 5a-d. The characteristic signals for the phenylene-containing PMOs are assigned to CSi(OSi)3 (T3, δ -78), CSi(OSi)2(OH) (T2, δ -69), and CSi(OSi)(OH)2 (T1, δ -61), respectively, according to the structure of the Si species covalently bonded to the carbon atom. The corresponding Tn signals of the thiophene-containing PMOs appear at δ -83 (T3), δ -74 (T2), and δ -66 (T1), respectively, and the Tn structural units of the ethane-containing PMOs appear at δ -65 (T3), δ -57 (T2), and δ -50 (T1), respectively. The 29Si MAS NMR spectra of the respective bi- and trifunctional PMOs (Figure 5) exhibit the presence of three kinds of organic bridging groups inside the framework and a high degree of condensation of the silanol groups according to the corresponding molar ratios of BTEB, BTET, and BTEE in the initial reaction mixture. The Q peaks such as Si(OSi)4 and Si(OSi)3(OH) between -90 and -120 ppm are negligible to assign, which confirms that cleavage of the C-Si bonds of the BTEB, BTET, and BTEE precursors does not occur during the sol-gel synthesis or any of the postsynthesis

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Table 3. Solid-State 29Si MAS NMR Results Calculated for the PMOs Studied Tn (phenylene, mol %) sample

T1

T2

T3

PE31 PE11 PE13 TE31 TE11 TE13 PT31 PT11 PT13 PTE111 PTE211 PTE121 PTE112

22.4 20.2 14.5

31.8 26.6 21.4

12.7 7.9 10.0

17.9 3.5 1.7 9.7 12.2 7.4 11.4

34.2 7.7 16.4 16.0 19.5 14.3 16.2

14.8 40.7 9.8 11.6 9.9 10.1 5.3

Tn (thiophene, mol %) T1

8.4 12.4 11.2 6.8 2.6 8.8 7.2 12.0 12.0 12.4

T2

40.6 19.5 14.7 21.3 32.5 42.5 22.8 11.7 15.1 8.3

treatment steps. To analyze quantitatively the relative content of bridging groups in the final bi- and trifunctional PMO products, all of the 29Si MAS NMR spectra of the multifunctional PMOs were simulated, deconvoluted, and integrated (Figure 6 and Supporting Information Figure S4). For example, the NMR spectra of the phenylene-thiophene-ethane-bridged PMOs such as PTE111, PTE211, PTE121, and PTE112 were simulated and deconvoluted into seven or eight Tn signals corresponding to the three kinds of organic bridging groups, as shown in Figure 6. The weakest T1 signal of the ethane bridging group was not deconvoluted for several samples, and that of the phenylene bridging group was not used in the analysis of the PTE121 sample. The spectra of the bifunctional PMOs were deconvoluted into five or six signals and integrated to determine the peak areas. The relative molar contents of the respective bridging groups as well as the Tn structural units in each group were calculated by integrating the separated signals, which are listed in Table 3. The calculated results are very similar to the molar ratios of the corresponding bridging groups used in the initial reaction mixtures. The relative ratios can be tuned more precisely by varying the initial reaction conditions such as the molar ratios of reactants and the relative solvent quantity, since the hydrolysis and condensation rates of the three kinds of precursors depend on their composition and experimental conditions. In this study, the appearance of the T1 structural units on the ethane-bridged PMO spectrum is nearly negligible and the T3/T2 ratios for the phenylene- and thiophene-containing PMOs are not very high. The multifunctional PMOs with phenylene, thiophene, and ethane bridging groups also exhibit high thermal stability as confirmed by thermogravimetric analysis. For example, the organosilica framework of PT11 and PTE111 are preserved up to 773 K even in an air flow (see Supporting Information Figure S5).

4. Conclusions Spherical PMO particles with multiple bridging groups and large pores were synthesized for the first time using phenylene-, thiophene-, and ethane-containing organosilica precursors and

Tn (ethane, mol %)

T3

21.2 14.3 13.3 5.0 13.9 20.8 0.4 7.4 9.2 1.9

T1

T2

T3

1.5 3.8 8.6 0.0 3.0 0.0

16.1 26.4 24.4 21.8 26.1 35.9

15.5 15.1 21.1 8.0 24.7 24.9

0.0 5.8 6.6 0.0

26.0 11.6 15.8 20.2

6.3 9.9 9.5 24.2

molar ratio of Σ (Tnphenylene: Tnthiophene: Tnethane) 1:0:0.49 1:0:0.83 1:0:1.18 0:1:0.42 0:1:1.16 0:1:1.55 1:0.49:0 1:0.92:0 1:2.58:0 1:0.81:0.86 1:0.75:0.65 1:1.14:1.00 1:0.69:1.35

a PEO-PLGA-PEO triblock copolymer template. The relative compositions of organic bridging groups in the resulting PMOs were controlled using different molar ratios of the organosilica precursors. In particular, the bifunctional phenylene-ethane PMOs and trifunctional PMOs exhibited better spherical morphology with only a small amount of linkage among the particles. The multifunctional PMO spherical particles with 2D hexagonal mesostructure exhibited large specific surface areas of around 1000 m2 g-1 and large pore diameters of around 8.0 nm with uniform pore size distributions. The relative contents of organic moieties incorporated within the framework were investigated extensively by 29Si MAS NMR measurements, and the results obtained by simulation and deconvolution of the spectra were in a good agreement with the molar ratios of precursors in the initial reaction mixtures. Thus far, no such large pore bi- and trifunctional PMOs with a high surface area of around 1000 m2 g-1 and high thermal stability up to 773 K have been reported using a commercial P123 PEO-PPO-PEO triblock copolymer template. It is believed that the multifunctional PMO spherical particles reported here are promising candidates to serve as highperformance adsorbents, host materials, catalysts, and packing materials. Acknowledgment. This work was supported by the Korean Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (no. R0A-2007-000-10029-0). M.J. acknowledges support by the National Science Foundation under grant CTS-0553014. We thank Prof. J. M. Kim and Ms. G. Ha at the Department of Chemistry in Sungkyunkwan University for nitrogen adsorption measurements. Supporting Information Available: Five figures showing nitrogen adsorption isotherms, pore size distributions, particle size distribution, deconvoluted 29Si MAS NMR spectra, and thermogravimetric profiles for the selected PMO samples. This material is available free of charge via the Internet at http://pubs.acs.org. LA701948G