Chem. Mater. 2005, 17, 3019-3024
3019
Synthesis and Characterization of Phosphonic Acid Functionalized Organosilicas with Bimodal Nanostructure Qihua Yang,* Jie Yang, Jian Liu, Ying Li, and Can Li State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ReceiVed January 28, 2005. ReVised Manuscript ReceiVed March 24, 2005
We report on the synthesis of phosphonic acid functionalized mesoporous organosilicas (MES-PA) with bimodal nanostructure by a postsynthesis treatment method. Phosphonate ester functionalized mesoporous organosilicas (MES-P) were first synthesized by one-step cocondensation of 1,2-bis(trimethoxysilyl)ethane (BTME) and diethoxyphosphorylethyltriethoxysilane (PETES) using triblock copolymer Pluronic P123 as the structure-directing agent in acidic medium. MES-PA were prepared by refluxing the as-synthesized MES-P in concentrated HCl. Further condensation of silicon species during the postsynthesis treatment results in material with more ordered mesostructure compared with the untreated one. Bimodal nanostructure is formed by refluxing MES-P (synthesized with PETES/(PETES + BTME) ) 30 mol %) in concentrated HCl. The existence of the bimodal mesopore was confirmed by N2 sorption isotherms and TEM images. The primary pore structure with pore diameter of 6.5 nm is long-range ordered. The secondary pore structure formed by further condensation of the nanostructured aggregates is disordered with pore diameter centered at 65 nm. The generation of phosphonic acid functionalities, the removal of surfactant, and the formation of secondary pore were accomplished at a single step.
1. Introduction The periodic mesoporous organosilicas (PMOs) synthesized using [(R′O)3SiRSi(OR′)3] as a precursor were one of the novel breakthroughs in the fields of mesoporous materials.1 This novel route offers a new approach for the synthesis of organic group modified mesoporous materials. The bridged organic groups in the wall should endow the PMOs different physical and mechanical properties from those of mesoporous silicas, such as the improved hydrothermal and mechanical stability.2 Many kinds of organic functionalities (methylene, ethylene, phenylene, biphenylene, thiophene, ferrocene, and so on) have been successfully incorporated in the wall of mesoporous materials.1,3 However, most PMOs reported have MCM-411,3a-g or SBA-154,5 type mesostructure consisting of a uniform one-dimensional channel, which will hamper the mass transfer during their applications in catalysis * To whom correspondence should be addressed. Tel: 86-411-84379552; Fax: 86-411-84694447; email:
[email protected] (1) (a) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (b) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (c) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. (2) (a) Lu, Y.; Fan, H.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258. (b) Burleigh, M. C.; Markowitz, M. A.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Gaber, B. P. J. Phys. Chem. B 2003, 107, 12628. (3) (a) Yoshina-ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539. (b) Landskron, K.; Hatlon, B. D.; Perovic, D. D.; Ozin, G. A. Science 2003, 302, 266. (c) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. (d) Kapoor, M. P.; Yang, Q.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 15176. (e) Hunks, W. J.; Ozin, G. A. Chem. Commun. 2004, 2426. (f) Hamoudi, S.; Kaliaguine, S. Chem. Commun. 2002, 2118. (g) Sayari, A.; Yang, Y. Chem. Commun. 2002, 2582. (h) Hunks, W. J.; Ozin, G. A. Chem. Mater. 2004, 16, 5465. (i) Olkhovyk, O.; Jaroniec, M. J. Am. Chem. Soc. 2005, 127, 60.
and separation. The structural and morphological control of mesoporous organosilicas remains to be a challenge. Many efforts have been devoted to control the structure and morphology of PMOs, for example, 3-D ordered mesoporous ethanesilicas with cross-linked channels are synthesized using either copolymer surfactants (F127, F108, B50-660, and PEO-PLGA-PEO) under acidic conditions or cationic surfactants [C18TMACl and CH3(CH2)15N(CH3)2N(CH3)3]2Br (C16-3-1)] under basic conditions.6,7 Recently, the mesoporous ethanesilicas with bimodal pore structure were synthesized using cetyltrimethylammonium bromide and a complexing polyalcohol (2,2′,2′-nitrilotriethanol) as an additive.8 The existence of bimodal pore structure may result in an improved diffusion rate of guest molecules in the mesoporous network.9 The bifunctionalized periodic mesoporous organosilicas (BPMOs) synthesized by cocondensation of [(R′O)3SiRSi(4) (a) Muth, O.; Schellbach, C.; Fro¨ba, M. Chem. Commun. 2001, 2032. (b) Zhu, H.; Jones, D. J.; Zajac, J.; Rozie`re, J.; Dutartre, R. Chem. Commun. 2001, 2568. (c) Burleigh, M. C.; Makowitz, M. A.; Wong, E. M.; Lin, J.-S.; Gaber, B. P. Chem. Mater. 2001, 13, 4411. (d) Guo, W.; Park, J. Y.; Oh, M.-O.; Jeong, H.-W.; Cho, W.-J.; Kim, I.; Ha, C.-S. Chem. Mater. 2003, 15, 2295. (e) Bao, X. Y.; Zhao, X. S.; Li, X.; Chia, P. A.; Li, J. J. Phys. Chem. B 2004, 108, 4684. (5) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410. (6) (a) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Asefa, T.; Coombs, N.; Ozin, G. A.; Kamiyama, T.; Terasaki, O. Chem. Mater. 2002, 14, 1903. (b) Zhao, L.; Zhu, G.; Zhang, D.; Di, Y.; Chen, Y.; Terasaki, O.; Qiu, S. J. Phys. Chem. B 2005, 109, 764. (c) Cho, E.B.; Kwon, K.-W.; Char, K. Chem. Mater. 2001, 13, 3837. (d) Guo, W.; Kim, I.; Ha, C.-S. Chem. Commun. 2003, 2692. (7) (a) Guan, S.; Inagaki, S.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 5660. (b) Liang, Y.; Hanzlik, M.; Anwander, R. Chem. Commun. 2005, 525. (8) Haskouri, J. E.; Za´rate D. O.; Guillem, C.; Beltra´n-Porter, A.; Calde´s, M.; Marcos, M. D.; Beltra´n-Porter, D.; LaTorre, J.; Amoro´s P. Chem. Mater. 2002, 14, 4502.
10.1021/cm050198s CCC: $30.25 © 2005 American Chemical Society Published on Web 04/23/2005
3020 Chem. Mater., Vol. 17, No. 11, 2005
(OR′)3] with (R′O)3SiR will not only enrich the fields of PMOs but also enable the easy synthesis of mesoporous materials with multifunctional groups.10-12 With organic groups both in the wall and in the pore, BPMOs will have a much wider application potentials than PMOs. Phosphonic acid functionalized microporous silicas are efficient solid catalysts for a number of reactions such as pinacolpinacolone rearrangement and dehydration of cyclohexanol.13 The phosphonic acid groups will allow the incorporation of numerous transition metals, which should make them potential candidates as catalysts. However, there have been no reports on the synthesis of phosphonic acid functionalized mesoporous organosilicas (MES-PA). Here, we report on the synthesis and structural control of MES-PA. The bifunctionalized mesoporous organosilica with bimodal nanostructure was synthesized by refluxing the as-synthesized phosphonate ester containing mesoporous organosilicas in concentrated HCl. The postsynthesis treatment provides an opportunity for structural and morphological control of the mesoporous materials. 2. Experiment 2.1. Chemicals and Reagents. 1,2-Bis(trimethoxysilyl)ethane (BTME) and poly(alkylene oxide) block copolymer (Pluronic 123) were purchased from Sigma-Aldrich Company, Ltd. (USA). Diethoxyphosphorylethyltriethoxysilane (PETES) was from Gelest, Inc. (Germany). Other reagents were purchased from ShangHai Chemical Reagent. Inc., of Chinese Medicine Group. All materials were of analytical grade and used as purchased without further purification. 2.2.1. Synthesis of Phosphonate Ester Functionalized Mesoporous Organosilicas (MES-P). In a typical synthesis process, P123 (0.55 g) and KCl (3.49 g) were dissolved in HCl solution (16.5 g, 2 M) and H2O (3.75 g) at 45 °C under vigorous stirring. A mixture of BTME and PETES was added to the above solution, and the resulting mixture was stirred at 45 °C for 24 h and aged at 100 °C under static conditions for additional 24 h. The solid product was recovered by filtration and air dried at room temperature overnight. The molar ratio of the original gel was 1.000 Si/6.598 KCl/158.5 H2O/4.64 HCl/0.0133 P123. The template was extracted by refluxing 0.5 g of as-synthesized materials in 150 mL of ethanol for 24 h. The phosphonate ester functionalized mesoporous organosilicas were denoted as MES-Pn, where n (n ) 10, 20, 30) is the mol % of PETES/(PETES + BTME) in the initial gel mixture. 2.2.2. Synthesis of MES-PA. As-synthesized MES-Pn (0.5 g) was refluxed in concentrated HCl (50 mL, 36.5 wt%) at 100 °C for 6 h. The powder product was filtered and washed with capacious (9) (a) Sun, J.; Shan, Z.; Maschmeyer, T.; Coppens, M.-O. Langmuir 2003, 19, 8395. (b) Sun, J.; Shan, Z.; Maschmeyer, T.; Moulijn, J. A.; Coppens, M.-O. Chem. Commun. 2001, 2670. (c) Coppens, M.-O.; Sun, J.; Maschmeyer, T. Catal. Today 2001, 69, 331. (10) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2001, 123, 8520. (11) (a) Yang, Q.; Kapoor, M. P.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 9694. (b) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. J. Phys. Chem. B 2001, 105, 9935. (c) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Langmuir 2001, 17, 7923. (d) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Chem. Mater. 2001, 13, 4760. (12) (a) Yang, Q.; Liu, J.; Yang, J.; Kapoor, M. P.; Inagaki, S.; Li, C. J. Catal. 2004, 228, 265. (b) Yang, Q. Liu, J.; Yang, J.; Zhang, L.; Feng, Z.; Zhang, J.; Li, C. Microporous Mesoporous Mater. 2005, 77, 257. (c) Hamoudi, S. R.; Kaliaguine, S. Microporous Mesoporous Mater. 2004, 71, 17. (13) Aliev, A.; Ou, D. L.; Ormsby, B.; Sullivan, A. J. Mater. Chem. 2000, 10, 2758.
Yang et al. amounts of deionized water and dried at 100 °C. The sample was denoted as MES-PAn, where n (n ) 10, 20, 30) is the mol % of PETES/(PETES + BTME) in the initial gel mixture. 2.3. Characterization. X-ray powder diffraction (XRD) patterns were recorded on a Rigaku RINT D/Max-2500 powder diffraction system using Cu KR radiation. The nitrogen sorption experiments were performed at 77 K on an ASAP 2000 system. The samples were outgassed at 100 °C for 4 h before the measurement. The Brunauer-Emmett-Teller (BET) surface area was evaluated from data in the relative pressure range from 0.1 to 0.2. The total pore volume was estimated from the amount adsorbed at a relative pressure of ∼0.99. Pore diameters were determined from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2010 at an acceleration voltage of 100 kV. 13C (100.5 MHz) cross-polarization magic-angle spinning (CP-MAS), 31P (161.8 MHz), and 29Si (79.4 MHz) MAS solid-state NMR experiments were recorded on a Bruker DRX-400 spectrometer equipped with a magic-angle spin probe in a 4-mm ZrO2 rotor. 13C and 29Si signals were referenced to tetramethylsilane, and the 31P NMR signal was referenced to H3PO4 (85 wt %). Experimental parameters for 13C CP-MAS NMR experiments, 8-kHz spin rate, 3-s pulse delay, 4-min contact time, 1500-3000 scans; for 29Si MAS NMR experiments, 8-kHz spin rate, 3-s pulse delay, 10-min contact time, 2000-3000 scans; for 31P MAS NMR experiments, 8-kHz spin rate, 2-s pulse delay, 100 scans.
3. Results and Discussion 3.1. Synthesis of MES-P and MES-PA. MES-P were prepared by cocondensation of BTME and PETES using triblock copolymer Pluronic P123 as a structure-directing agent and inorganic salt (KCl) as an additive in acidic medium. We have synthesized thiol and sulfonic acid functionalized mesoporous ethansilicas using the same cocondensation method in the presence of P123 as a structuraldirecting agent and KCl as an additive.12a,b Phosphonic acid functionalities can be formed by dealkylation of phosphonate ester groups. The dealkylation of phosphonate ester is always performed by refluxing the sample in concentrated HCl. Postsynthesis treatment has been used to modify the textural properties of the mesoporous materials, for example, to enlarge the pore diameter of mesopore.14 Therefore, we tried to combine the dealkylation together with the surfactantremoving process by refluxing the as-synthesized MES-P in concentrated HCl. By use of this method, we may not only avoid the tedious surfactant-removing process but may also modify the textural properties of the materials. The detailed synthetic process was shown in Scheme 1. Scheme 1. Schematic Description for the Synthesis of MES-P and MES-PA
(14) Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Garces, J.; Olken, M. M.; Coombs, N. AdV. Mater. 1995, 7, 842.
Phosphonic Acid Functionalized Organosilicas
Figure 1.
13C
CP-MAS and
29Si
Figure 2.
31P
MAS NMR spectra of MES-P20 and MES-PA20.
Chem. Mater., Vol. 17, No. 11, 2005 3021
MAS NMR spectra of MES-P20 and MES-PA20.
3.2. Characterization of MES-P and MES-PA by SolidState NMR. 13C CP-MAS NMR spectrum of MES-P20 displays three resonances at 5.6, 17.1, and 63.0 ppm (Figure 1). The signal at 5.6 ppm can be assigned to the mixture of carbon species of Si-CH2CH2-Si1b and Si-CH2CH2-P.13 The resonances at 17.1 and 63.0 ppm represent the phosphonate ester group. The resonance at 17.1 ppm also masks the signal of carbon species of Si-CH2CH2-P.13 31P MAS NMR spectrum of MES-P20 exhibits one signal at 33.3 ppm due to P(O)(OCH2CH3)2 (Figure 2).13 29 Si MAS NMR spectrum of MES-P20 exhibits a broad peak centered at -62.3 ppm. This broad peak can be assigned to the mixture of silicon species bonded with -CH2CH2P(O)(OCH2CH3)2 and silicon species bridged by -CH2CH2- (Figure 1).1b,13 The NMR results indicate that the phosphonic acid diethyl ester group is covalently incorporated into the framework of mesoporous ethanesilicas. Phosphonate ester groups can be transformed readily to phosphonic acid groups by acid-catalyzed hydrolytic dealkylation in concentrated HCl. We tried to generate phosphonic acid groups during the template removing process; therefore, the as-synthesized MES-Pn was treated with concentrated HCl. Two signals were observed in 13C CP-MAS NMR spectrum of MES-PA20 (Figure 1). The signals at 5.2 and 20.0 ppm can be assigned to the mixture of carbon species of Si-CH2CH2-Si and Si-CH2CH2P and carbon species of Si-CH2CH2P, respectively. The disappearance of the signal at 63.0 ppm strongly confirms the dealkylation of the ester group. No other signals were observed, indicating the complete removal of template during the dealkylation process. The chemical shift of PO(OH)2 is very close to that of PO(OEt)2; therefore the signal at 32.3 ppm in 31P MAS NMR spectrum of MES-PA20 could be assigned to phos-
Figure 3. XRD patterns of MES-Pn (a, MES-P10; b, MES-P20; c, MESP30) and MES-PAn (a, MES-PA10; b, MES-PA20; c, MES-PA30).
Figure 4. TEM images of MES-P10 (a) and MES-P20 (b).
phonic acid. The additional signal at 23.2 ppm is due to P species formed between phosphonic acid and Si-OH (Figure 2).13,15 29 Si NMR spectrum of MES-PA20 exhibits a broad peak centered at -68.4 ppm (Figure 1). Compared with the chemical shift of silicon species of MES-P20 (-62.3 ppm), this chemical shift was altered to upfield. The result of 29Si NMR suggests that silicon species undergo further condensation during the postsynthesis treatment process. The Si-C bond is stable enough to survive the dealkylation process as evidenced by the fact that no signals in the range of -90 to -110 ppm (SiOn(OH)4-n) were observed in 29Si NMR spectrum of MES-PA20. (15) Corriu, R. J. P.; Datas, L.; Guari, Y.; Mehdi, A.; Reye´, C.; Thieuleux, C. Chem. Commun. 2001, 763.
3022 Chem. Mater., Vol. 17, No. 11, 2005
Yang et al.
Figure 5. N2 sorption isotherms of MES-Pn and MES-PAn (n ) 10, 20, 30).
3.3. Structural Characterization of MES-P and MESPA. XRD patterns of MES-P10 and MES-P20 display three well-resolved diffraction peaks at low-angle regions, corresponding to d(100), d(110), and d(200) reflections, respectively, indicating MES-P10 and MES-P20 have well-ordered 2-D hexagonal structure (Figure 3). TEM images clearly show the hexagonal symmetry mesopores throughout the sample, in good agreement with XRD results (Figure 4). Only one diffraction peak with decreased intensity is observed in XRD pattern of MES-P30. These results show that the mesostructure of the material is getting disordered with molar ratio of PETES/(PETES + BTME) in the initial gel mixture increasing. N2 sorption isotherms of MES-P10 are of typical type IV with H1 hysteresis loop, characteristic of SBA-15-type mesoporous materials (Figure 5).16 The isotherms of MESP20 are also of typical type IV with uniform pore size distributions, as evidenced by the sharp capillary condensation step (Figure 5). H2 hysteresis loop and a two-step desorption branch with the substep at P/P0 of 0.6, were observed, indicating the pore mouth of MES-P20 is blocked.17 Compared with MES-P10 and MES-P20, the pore size distributions of MES-P30 are broad determined from the lesspronounced capillary condensation step (Figure 6). The hysteresis loop at P/P0 ) 0.65-1.0 can be attributed to the textural mesoporosity, which is generated from the packing of nanoparticles.18 XRD patterns of MES-PAn (n ) 10, 20, 30) are almost the same as those of MES-Pn (n ) 10, 20, 30) (Figure 3). (16) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (17) Voort, P. V. D.; Ravikovitch, P. I.; Jong, K. P. D.; Benjelloun, M.; Bavel, E. V.; Janssen, A. H.; Neimark, A. V.; Weckhuysen, B. M.; Vansant, E. F. J. Phys. Chem. B 2002, 106, 5873. (18) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area and porosity; Academic Press: London, 1982.
Figure 6. BJH pore size distributions of MES-P30 and MES-PA30 calculated from the adsorption branch of N2 sorption isotherms. Table 1. Physicochemical Properties of MES-P and MES-PA
sample MES-P10 MES-PA10 MES-P20 MES-PA20 MES-P30 MES-PA30
total pore volume d(100) SBET (nm) (m2 g-1) (cm3 g-1) 11.12 10.53 11.07 10.77 10.51 11.06
692 662 478 686 382 443
0.825 0.806 0.782 0.802 0.775 0.966
pore diameter (nm) 7.54 7.38 7.49 6.44 6.79 6.50, 65.00
P wall thickness (mmol g-1)b (nm)a 5.30 4.78 5.29 6.00 5.35
0.48 0.68 1.00
Wall thickness was calculated by a0 - pore diameter, where a0 ) 2d(100)/x3. b Determined by elemental analysis. a
The decrease of d(100) spacing was observed for MES-PA10 and MES-PA20, indicating the contraction of a unit cell after refluxing in HCl (Table 1). On the contrary, the d(100) spacing of MES-PA30 is increased after HCl treatment, implying the lattice expansion of the material. MES-PA10 has almost the same N2 sorption isotherms as that of MES-P10 with only a slight decrease of surface area, total pore volume, and pore diameter. This result shows that the mesostructure of MES-P10 is very stable and cannot be altered by HCl treatment (Figure 5 and Table 1). N2 sorption isotherms of MES-PA20 are of typical type IV with H1
Phosphonic Acid Functionalized Organosilicas
Chem. Mater., Vol. 17, No. 11, 2005 3023
Figure 7. TEM images of MES-PA30 in the direction parallel to (a, b) and in the direction perpendicular to(c) the pore axis.
hysteresis loop. The two-step desorption branch disappears from the N2 isotherms of MES-PA20, implying that the pore mouth is freed from the clogging. The BET surface area of MES-P20 is increased from 478 to 686 m2/g with pore diameter decreasing after HCl treatment (Table 1). Connected with XRD results, it can be proposed that further condensation between silicon species during the treatment makes the mesostructure of MES-P20 more ordered. The isotherms of MES-PA30 exhibit two consecutive capillary condensation steps. The first one occurs at P/P0 ) 0.6-0.8. The second one is much steeper and occurs at P/P0 ) 0.8-0.99, proposing the presence of a significant amount of secondary pores.9 From the corresponding pore size distributions, the existence of two types of pores was clearly observed (Figure 6). This means that MES-PA30 has bimodal pore structure. The primary pore diameter of MES-PA30 is 6.50 nm, which is almost the same as that of MES-P30. The pore size distributions of secondary pore are broad with pore diameter centered at 65 nm. The bimodal nanostructure of MES-PA30 is more evident from TEM images (Figure 7). TEM images show that the primary pore structure is longrange ordered and that the secondary pores are scattered irregularly in the direction or in the direction perpendicular to the pore axis of MES-PA30. The primary mesopores are effectively cross linked through the secondary pores, which may increase the diffusion rate of guest molecules through the channels of MES-PA30. The shape and size of the secondary pore are not uniform. From the above results, we can suggest that the material synthesized with higher molar ratio of PETES/(PETES + BTME) in the initial gel mixture goes through greater structural alteration after HCl treatment. Generally, the PMOs synthesized in acidic conditions have much more silicon species of [SiC(OH)(OSi)2] than that of [SiC(OSi)3] due to the incomplete condensation of Si-OSi bonds in acidic medium.5 Therefore, further condensation between silicon species with enriched surface hydroxyl groups may occur when the sample is treated under acidic/ basic conditions or at high temperature. After HCl treatment, further condensation of silicon species in MES-P was verified by 29Si MAS NMR (chemical shift of silicon of MES-PA20 shift to upfield compared with that of MES-P20). Also the decrease of d(100) spacing of MES-PA10 and MES-PA20 further confirms the result of 29Si MAS NMR. For the synthesis of MES-P, the self-assembly of silane precursor
around the micelle is disturbed because of the different hydrolysis and condensation rates of the bridged and the terminal-bonded silane precursors.10 The material synthesized with higher molar ratio of PETES/(PETES + BTME) tends to have more disordered mesostructure with a lower crosslinking degree of the silicon species; therefore the aggregates of silicon species is getting softer. This may be the main reason for the greater alteration of the structural properties from MES-P10 to MES-P20. The mesostructure of MESP30 is disordered, and there are lots of interparticle voids derived from the packing of mesostructured aggregates, as evidenced by XRD and N2 sorption isotherms. The mesostructured aggregates are not so robust and can further condense to siloxane bridges around some of micelle assemblies in concentrated HCl (formed by dissolution of P123 from the as-synthesized material).9 The expansion of the unit cell of MES-PA30 proved the occurrence of condensation between the mesostructured aggregates (Table 1). However, because of the low concentration of P123, the ordered assembly is difficult to achieve, which results in the formation of irregularly distributed secondary pore with broad pore size distributions. The formation of bimodal structure cannot be ascribed to a specific functional group such as the phosphonate ester group because MES-PA10 and MES-PA20 do not have bimodal structure. However, the formation of the bimodal nanostrucure may involve the “soft” aggregates of silicon species with enriched surface hydroxyl groups. 4. Conclusions This paper reports on the synthesis and characterization of MES-P and MES-PA. The MES-PAs with bimodal structure were prepared by treatment of the as-synthesized MES-Ps in concentrated HCl. The structural properties of the primary nanoparticles influence greatly the mesostructure of the materials after HCl treatment. The aggregates with disordered mesostructure and “soft” nature (not fully cross linked and with nanoparticle packing) can further condense to siloxane bridges around micelle assemblies in concentrated HCl, which leads to the formation of bimodal structure without affecting the primary mesopore. The primary mesopores are efficiently cross-linked by the secondary pores, which could enhance the transportation ability of molecules
3024 Chem. Mater., Vol. 17, No. 11, 2005
in the bifunctionlized mesoporous organosilicas for their applications as catalysts and adsorbents. Acknowledgment. Financial support of this work was provided by the National Natural Science Foundation of China
Yang et al.
(20303020), National Basic Research Program of China (2003CB615803), and Talent Science Program of the Chinese Academy of Sciences. CM050198S
