Monodisperse Particles of Bifunctional Periodic Mesoporous

J. Phys. Chem. C 2008, 112, 4897-4902

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Monodisperse Particles of Bifunctional Periodic Mesoporous Organosilica 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: NoVember 10, 2007; In Final Form: January 12, 2008

Monodisperse particles of periodic mesoporous organosilica (PMO) with ethane- and phenylene-bridging groups were prepared using 1,2-bis(triethoxysilyl)ethane (BTEE) and 1,4-bis(triethoxysilyl)benzene (BTEB) organosilica precursors, and a Pluronic P123 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer template under slightly acidic conditions. Highly ordered monodisperse particles having hexagonal mesostructure (p6mm) and coiled shape were obtained only for the BTEE/BTEB molar ratios between 70:30 and 60:40. The average external diameter of these coiled particles was about 2-3 µm, whereas the diameter of mesopores present in these particles was about 7.4 nm.

Introduction Mesoporous silica materials have been developed extensively for versatile applications because of their high surface area, narrow pore size distribution, and adjustable mesopore size.1-7 Because monodisperse particles of mesoporous materials are preferred for many applications, including stationary phases in chromatography and auxiliary components in composites, there is a great interest in the development of recipes for the preparation of these particles, especially spherical ones.8-12 Morphology and texture can alter optical, electronic, mechanical, and catalytic behavior of materials, and all the factors that control morphology are related to surface energy and cooperative organization of the organic and inorganic components used. Thus far, several methods to prepare monodisperse mesostructured particles have been reported via sol-gel synthesis using various quantities of surfactants, co-surfactants, cosolvents, and acids as well as pseudomorphic synthesis.13-25 Surface properties of particles were tuned by attachment of various organic moieties. The development of periodic mesoporous organosilicas (PMOs) opened new possibilities for the synthesis of particles of desired properties.26-37 In contrast to the mesoporous organosilicas obtained by postsynthesis grafting and co-condensation method, PMOs possess a high loading of homogeneously distributed organic bridging groups in the framework in addition to the high surface area, high degree of mesostructural ordering, and narrow pore size distribution. Beside tailoring surface and structural properties of PMOs, organic bridging groups are also used to tune crystallinity, mechanical, and hydrothermal properties of these materials.38-45 Recently, PMOs containing more than one bridging group have been synthesized using nonionic surfactants and block copolymers.46-50 Incorporation of multiple bridging groups allows one to finely tune the surface properties of PMOs to achieve the desired functionality and selectivity. To date, there are several reports on the preparation of spherical PMO particles. For example, PMOs with single bridging groups, such as ethane, ethylene, and phenylene groups, have been fabricated by * Corresponding authors. (D.K.) Phone: 82-31-290 7250. E-mail: [email protected]. (M.J.) Phone: 1-330-672 3790. E-mail: [email protected]. † Sungkyunkwan University. ‡ Kent State University.

adjusting the precursor/surfactant ratio and pH,11,12,51 or using co-surfactants and cosolvents.52-58 However, to our knowledge monodisperse PMO particles with multiple bridging groups have not been reported. Here, we report a facile synthesis of monodisperse PMO particles containing ethane- and phenylene-bridging groups using a Pluronic P123 poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer template under slightly acidic conditions. A unique external morphology of this bifunctional PMO with preserved structural ordering was achieved by adjusting only the molar ratio of the organosilica precursors, 1,2-bis(triethoxysilyl)ethane (BTEE) and 1,4-bis(triethoxysilyl)benzene (BTEB), without any auxiliary additives such as cosolvent and inorganic salts. Namely, monodisperse particles in the form of coiled rods were formed over a certain range of molar compositions. Beyond this range, that is, at higher and lower contents of BTEE, polydisperse spherical and worm-/rodlike particles were formed, respectively. This trial to prepare the monodisperse PMO particle, suggested in this study is a unique method that just uses the cocondensation of two different organosilica precursors, and a coiled form of rods is also a unique one. Experimental Methods Preparation of Periodic Mesoporous Organosilica. Pluronic P123 triblock copolymer (EO20PO70EO20) (BASF) was used as a soft template and 1,2-bis(triethoxysilyl)ethane (BTEE, Aldrich) and 1,4-bis(triethoxysilyl)benzene (BTEB, Aldrich) were used as bridged organosilica precursors. The molar ratios of BTEE and BTEB precursors were 90:10, 80:20, 70:30, 60:40, and 50: 50. In a typical synthesis of the 60:40 PMO sample (EBS64), 0.66 g of the P123 triblock copolymer was dissolved in a mixture of 24 g of distilled water and 0.13 g of HCl (37 wt %, Aldrich) under vigorous stirring. Next, 0.486 g of BTEE and 0.276 g of BTEB organosilica precursors were added and the stirring was continued for about 1.5 h at 313 K, which resulted in the formation of a white precipitate. The solid product was aged for 24 h at 373 K. The block copolymer template in obtained solids was extracted by successive treatment with 60 g of ethanol for 2 h under a static condition and 60 g of acetone

10.1021/jp710772w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/08/2008

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SCHEME 1: Illustration of Morphological Changes during Formation of Ethane-Benzene-PMOs Prepared by Using Two Kinds of Organosilica Precursors

at 329 K under stirring for 5 h, followed by washing with distilled water and acetone using a suction flask and drying at 373 K for 1 day. Measurements and Calculations. The small-angle X-ray scattering (SAXS) experiments were performed using a synchrotron radiation with λ ) 1.541 Å at the 4C1 lines of Pohang Accelerator Laboratory in POSTECH. The transmission electron microscopy (TEM) images were obtained with a field emission TEM (FE-TEM) (JEOL JEM2100F) operated at an accelerating voltage of 200 kV. The samples were sonicated for 60 min in an adequate quantity of ethanol, and the solution was dropped onto a porous carbon film on a copper grid and then dried. The scanning electron microscopy (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 2010 analyzer. The samples were degassed at 423 K to achieve vacuum below 30 µmHg. The BrunauerEmmet-Teller (BET) specific surface 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. The volume of complementary pores was estimated by subtracting the mesopore volume obtained by integration of pore size distribution (PSD) above 3 nm from the single-point pore volume. The PSD curves were calculated from the adsorption branches of the isotherms by using the improved Kruk-Jaroniec-Sayari (KJS) method.60 The pore width was estimated at the maximum of PSD. The solid-state NMR spectra were obtained with a Bruker DSX400 spectrometer using a 4 mm magic-angle spinning (MAS) probe at the Korea Basic Science Institute. Samples for 13C CP-MAS and 29Si MAS NMR were spun at a rate of 6 kHz, and the chemical shifts were obtained with respect to the tetramethylsilane reference peak. The 13C CP-MAS NMR spectra were measured under the experimental conditions of a contact time of 2 ms, a recycle delay of 3 s, and 3,000 scans, and the 29Si MAS NMR were obtained with a recycle delay of 50 s and 2,000 scans. The structural quantification of the carbon-silicon linkages was performed through the simulation and decomposition of the 29Si MAS NMR spectra. Results and Discussion A series of bifunctional PMOs containing ethane- and phenylene-bridging groups has been synthesized (see Experi-

Figure 1. SEM images for ethane-benzene PMOs obtained for the following BTEE/BTEB molar ratios: 90:10 (a,b), 80:20 (c,d), 60:40 (e,f), and 50:50 (g,h).

Figure 2. SEM images for 60:40 ethane-benzene PMO at low (a) and high (b) magnification levels.

mental Methods and Scheme 1) in the presence of Pluronic P123 block copolymer for the following molar ratios of BTEE and BTEB precursors: 90:10, 80:20, 70:30, 60:40, and 50:50. The particle morphology for the aforementioned samples is shown in Figure 1. The left column of panels (a, c, e, g) presents the FE-SEM images at the same magnification levels for the PMO particles obtained using the BTEE/BTEB molar ratios of 90:10, 80:20, 60:40, and 50:50, respectively, whereas the right column presents the enlarged fragments of the images in the left panels to show the shape of particles. For the 60:40 PMO sample, two additional images at low and high magnification levels are shown in Figure 2 to illustrate better the particle uniformity and particle shape. As can be seen from Figure 1 (panels a-d) the PMO samples with high content of BTEE (90: 10 and 80:20 molar ratios) consist mainly of 1-3 µm globular particles accompanied by irregular much smaller particles. At lower content of BTEE (50:50 molar ratio) irregular wormlike

Bifunctional Periodic Mesoporous Organosilica

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Figure 5. TEM images for ethane-benzene (60:40) PMO prepared using P123 triblock copolymer, BTEE, and BTEB.

Figure 3. Particle size distribution for ethane-benzene (60:40) PMO prepared using P123 triblock copolymer, BTEE, and BTEB.

Figure 6. Nitrogen adsorption isotherms and the corresponding pore size distributions for ethane-benzene [(a) [BTEE]/[BTEB] ) 60:40 and (b) ) 70:30] PMOs prepared using P123 triblock copolymer, BTEE, and BTEB.

Figure 4. Synchrotron SAXS pattern for ethane-benzene (60:40) PMO prepared using P123 triblock copolymer, BTEE, and BTEB.

particles are observed (panels g and h). However, at the BTEE/ BTEB molar ratios of 70:30 and 60:40 monodisperse particles of coiled rods were formed (panels e, f and Figure 2). The average size of the latter particles estimated by an electrophoretic light scattering method is 2.22 µm, and their distribution is very narrow in the range of 1-3 µm (see Figure 3). The SEM data suggest that the morphology of organosilica with multiple organic bridging groups can be influenced by the chemical structure and properties of these groups. The synchrotron SAXS data for ethane-benzene (60:40) PMO are shown in Figure 4. As can be seen from this figure, the bifunctional PMO exhibits a highly ordered two-dimensional (2D) hexagonal (p6mm) mesophase with four well-resolved peaks indexed as (100), (110), (200), and (210) reflections. The most intense (100) peak represents the d-spacing value of 9.81 nm, which corresponds the unit cell parameters (a) of 11.3 nm. In the case of 70:30 ethane-benzene PMO, 2D hexagonal mesophase was also obtained with the d-value slightly increased from 9.81 to 9.92 nm. However, the high-order (second and third) peaks of ethane-benzene PMO containing over 80 mol % of BTEE were not observed (Supporting Information Figure S1). The formation of the well-defined 2D hexagonal (p6mm) ethane-benzene PMO was also confirmed by the TEM analysis (see Figure 5). These images are perpendicular and parallel to the channels, respectively, and represent uniform 2D hexagonal

mesostructure consistent with the pore sizes estimated by nitrogen adsorption and SAXS analysis. Nitrogen adsorption-desorption isotherms for the ethanebenzene PMOs with coiled rod morphology are type IV with a steep increase of adsorption branch at P/P0 ) 0.65-0.75 due to the capillary condensation of nitrogen in the mesopores as shown in Figure 6 (panels a-1 and b-1). The BET surface areas of the ethane-benzene 60:40 and 70:30 PMO samples are 975 and 970 m2 g-1, respectively. The total pore volumes are 1.04 and 1.08 cm3 g-1, respectively, and the relative ratios of the volume of complementary pores below 3 nm to the total pore volume are around 16%, which is a characteristic feature of mesoporous silica-based materials templated with block copolymers containing PEO segments.59 The PSD obtained from the adsorption branch of isotherms by the improved KJS method60 are narrow, indicating high uniformity of porous structures as shown in Figure 6 (panels a-2 and b-2). However, a very small quantity of mesopores up to 20 nm also appears in the pore size distribution as shown in Figure 6 (panels a-2, b-2). The pore diameters estimated at the maximum of PSD curves are the same (7.37 nm) for both 60:40 and 70:30 PMO samples; consequently, the wall thickness values are similar too, that is, 4.08 and 3.96 nm for 70:30 and 60:40 PMOs, respectively. It seems that the textural properties of the coiled rods of ethane-benzene PMOs are nearly the same in the range from 60:40 to 70:30 [BTEE]/[BTEB] ratios as evidenced by nitrogen adsorption isotherms and PSDs shown in Figure 6 as well as by the SAXS patterns and TEM images. Overall, physicochemical properties of bifunctional ethane-benzene PMOs with coiled rod morphology are summarized in Table 1. The solid-state 13C CP MAS NMR analysis of the polymerfree bifunctional PMOs studied was performed to verify the composition of the covalently bonded organic bridging groups in the framework. The 13C CP MAS NMR spectra for bifunctional PMOs display the resonance peaks at 5 and 133

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TABLE 1: Physicochemical Properties of Ethane-Benzene PMO Samples Prepared in This Studya sample

[BTEE]/ [BTEB]

SBET (m2 g-1)

Vp (cm3 g-1)

Vcomp (cm3 g-1)

d100 (nm)

DP (nm)

W (nm)

EBS64 EBS73

60:40 70:30

975 970

1.04 1.08

0.17 0.17

9.81 9.92

7.37 7.37

3.96 4.08

a Pore diameters (D ) were determined at the maximum of pore size p distributions (PSDs) calculated by improved KJS method;60 SBET ) BET specific surface area; Vp ) total pore volume; Vcomp ) the volume of complementary pores; W ) pore wall thickness () d100 × 2/x3 Dp).

Figure 9. Thermogravimetric analysis of ethane-benzene PMO (EBS64) prepared using P123 triblock copolymer, BTEE, and BTEB.

TABLE 2: Solid State 29Si MAS NMR Results of Bifunctional PMOs Prepared in This Study Tn (ethane, %)

Figure 7. Solid state 13C CP MAS NMR spectra for ethane-benzene PMOs prepared using P123 triblock copolymer, BTEE, and BTEB. The relative molar ratios of BTEE and BTEB are (a) 60:40 and (b) 70:30, respectively.

Figure 8. Simulation and decomposition of solid state 29Si MAS NMR spectrum for the 60:40 (a) and the 70:30 (b) ethane-benzene PMOs prepared using P123 triblock copolymer, BTEE, and BTEB.

ppm, which are assigned to the carbons in the ethane chain and the benzene ring, respectively (Figure 7). The respective single peaks indicate that the bridging groups are covalently bonded to the Si atoms, which are stable in the organosilica framework and the height of the peaks represent the relative molar ratio of ethane- and benzene-bridging groups approximately. The 29Si MAS NMR spectrum of the bifunctional PMO is shown in Figure 8. The Tn structural units of the ethanecontaining PMO appear at CSi(OSi)3 (T3, δ -65), CSi(OSi)2(OH) (T2, δ -57), and CSi(OSi)(OH)2 (T1, δ -50), respectively, according to the structure of the Si species covalently bonded to the carbon atom. The characteristic signals for the benzenecontaining PMO are assigned to δ -78 (T3), δ -69 (T2), and δ -61 (T1), respectively. The 29Si MAS NMR spectrum for the 60:40 PMO shown in Figure 8 exhibits the presence of two 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 BTEE and BTEB in the initial reaction mixture. The Q-peaks such as Si(OSi)4 and Si(OSi)3(OH) between -90 and -120 ppm are not observed, which

Tn (Phenylene, %)

sample

T1

T2

T3

T1

T2

T3

∑ (TnEthane/ TnPhenylene)

EBS64 EBS73

2.51 5.21

35.34 27.93

20.48 30.60

12.24 1.25

10.59 28.04

18.84 6.97

58.3:41.7 63.7:36.3

confirms that the cleavage of the C-Si bonds of the BTEE and BTEB precursors does not occur during the sol-gel synthesis and any of the postsynthesis treatment steps. To analyze quantitatively the relative content of bridging groups in the bifunctional PMOs, the 29Si MAS NMR spectra for 60:40 and 70:30 PMO samples were simulated, deconvoluted, and integrated as shown in Figure 8. The spectra of the bifunctional PMOs were deconvoluted into six signals and integrated to determine the relative molar content based on the peak areas. The Tn structural units as well as the relative molar contents of the respective bridging groups in each sample were calculated by integrating the separated signals, which are listed in Table 2. The calculated results are very similar to the molar ratios of the corresponding bridging groups used in the initial reaction mixtures. In this study, the T3/T2 ratios for the 60:40 sample (EBS64) are obtained as 0.58 and 1.77 for the ethaneand benzene-bridging groups, respectively. On the contrary, the T3/T2 ratios for the 70:30 sample (EBS73) are 1.09 and 0.24 for the respective ethane- and benzene-bridging groups, which represents the relative condensation efficiency for these bifunctional materials. The T3/T2 ratios were also simulated and calculated for the 80:20 and 50:50 samples; however, there is no general trend observed upon changes in the BTEE/BTEB molar ratio. The EBS64 60:40 bifunctional PMO exhibits thermal stability up to 503 K and delayed degradation temperature up to 673 K in an air flow as confirmed by thermogravimetric analysis (TGA) as shown in Figure 9. TGA profile shows two thermal decomposition events observed between 473 and 773 K and between 773 and 943 K, respectively. It is more likely that ethane-bridging groups undergo the degradation at the first temperature range followed by the degradation of benzenebridging groups. Despite the higher loading of ethane-bridging groups, the TG profile and the corresponding differential (DTG) curve show that the first thermal event is not well separated from the second event (Figure 9), which may suggest that both kinds of bridging groups are randomly distributed in the structure. To verify the sequence of thermal degradation of both bridging groups, the 29Si MAS NMR spectra as well as the 13C CP MAS NMR spectra were recorded for the EBS64 sample treated at 773 K for 10 min after raising the temperature from 303 to 773 K during 47 min in flowing air (Figure 10). A

Bifunctional Periodic Mesoporous Organosilica

J. Phys. Chem. C, Vol. 112, No. 13, 2008 4901 000-10029-0). M.J. acknowledges support by the National Science Foundation under CTS-0553014 grant. We thank Professor W.-C. Zin and Mr. S.-W. Moon at POSTECH for Synchrotron SAXS measurements. Supporting Information Available: Two figures showing small-angle X-ray scattering and SEM images for the selected PMO samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Solid state 13C CP MAS (a) and 29Si MAS NMR (b) spectra for ethane-benzene PMO (EBS64) treated at 773 K for 10 min after heating it in flowing air up to 773 K at the rate of 10 K/min. 29Si MAS NMR spectrum is simulated and deconvoluted to analyze the relative contents of organic bridging groups.

comparison of the solid state 13C CP MAS NMR spectrum in Figure 10a with that in Figure 7a shows that a large quantity of ethane-bridging groups were decomposed during the aforementioned thermal treatment; however, a meaningful fraction of these groups were still retained. Moreover, the 29Si MAS NMR spectrum in Figure 10b shows more clearly the quantity of retained bridging groups after the aforementioned thermal treatment of the sample. The Q-peaks such as Si(OSi)4 (Q4, -111 ppm), Si(OSi)3(OH) (Q3, -101 ppm), and Si(OSi)2(OH)2 (Q2, -92 ppm) are present and the corresponding Q/(T + Q) ratio is 0.64, which confirms the recondensation of silanol groups as well as a gradual cleavage of the C-Si bonds in the BTEE and BTEB units during the observed thermal events. Conclusions Monodisperse particles of bifunctional and highly ordered large-pore PMOs were synthesized for the first time using ethane- and benzene-containing organosilica precursors and a P123 PEO-PPO-PEO triblock copolymer template. This study shows that morphology of particles varies from globular shape through coiled rods to wormlike particles with increasing concentration of BTEB precursor. A unique morphology of coiled rods was obtained for the 60:40 and 70:30 PMO samples; for these molar compositions monodisperse particles with highly ordered 2D hexagonal mesostructure (p6mm), large pores (7.37 nm), an average size of 2.22 µm, and narrow particle size distribution were obtained. Moreover, these ethane-benzene PMOs exhibited large specific surface area of 975 m2 g-1 and large pore volume of 1.04 cm3 g-1. The relative compositions of organic bridging groups in the bifunctional PMOs with the aforementioned unique morphology were restricted to the specific molar ratios between 70:30 and 60:40 of BTEE and BTEB organosilica precursors. The relative contents of organic moieties incorporated within the framework were investigated by 29Si MAS NMR, and the results obtained by simulation and deconvolution of the NMR spectra were in a good agreement with the molar ratios of precursors in the initial reaction mixtures. So far, no such large pore bifunctional PMOs with such unique morphology were reported by using any other soft templates. Acknowledgment. This work was partially supported by the Korea Research Foundation Grant (KRF 2006-005-J04601) and by the Korean Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R0A-2007-

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