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2006, 110, 2972-2975 Published on Web 01/31/2006
Periodic Mesoporous Organosilicas with Im3m Symmetry and Large Isocyanurate Bridging Groups Rafal M. Grudzien,† Stanisław Pikus,‡ and Mietek Jaroniec*,† Department of Chemistry, Kent State UniVersity, Kent, Ohio 44240, and Department of Crystallography, Maria Curie-Skłodowska UniVersity, 20-031 Lublin, Poland ReceiVed: December 22, 2005; In Final Form: January 13, 2006
Adsorption and structural properties of periodic mesoporous organosilica with Im3m symmetry (SBA-16) and large heterocyclic bridging groups in cage-like mesopores are studied. The core of this bridging group is an isocyanurate ring integrated with three trimethoxysilyls through flexible propyl chains.
The area of ordered mesoporous materials (OMMs),1-3 which are of great interest to nanoscience and nanotechnology, has been expended enormously. This is due to their interesting structural, adsorption, catalytic, conductive, and magnetic properties. Among many achievements in the field of OMMs, the discovery of periodic mesoporous organosilicas (PMOs)4 has attracted a lot of attention. A PMO is a special type of hybrid organic-inorganic material, in which organic moieties are integrated into the silica framework. PMOs can be synthesized by using various structure-directing agents such as ionic surfactants,4-6 oligomeric surfactants,7 and nonionic block copolymers.8-10 Because of uniform distribution of organic bridging groups inside mesopore walls, PMOs become very promising materials for potential applications ranging from highly selective adsorbents and catalysts to sensing devices and hosts for biomolecules.11 So far, the research on PMOs was intensively concentrated on the development of channel-like PMO structures such as MCM-414 and SBA-15,8 whereas a little progress has been made in the direction of cage-like PMO structures such as FDU-19 with Fm3m symmetry (face-centered cubic) and SBA-1610 with Im3m symmetry (body-centered cubic). In particular, PMOSBA-1610 was found to be very interesting because of its 3D structure with uniformly distributed cage-like mesopores, in which each cage is connected with eight neighboring cages via small apertures that form a multidirectional system of mesopore network. This type of structure can be applicable for separation of molecules with certain dimensions as well as a host for various species. Initially, research on PMOs was focused on the incorporation of small aliphatic and aromatic bridging groups8-10,12 such as methylene, ethane, ethylene, phenylene, and thiophene mostly into the channel-like structures, i.e., MCM-414 and SBA-158 using both ionic surfactants4 and nonionic block copolymers.8 There are only a few reports on PMOs with FDU-1-9 and SBA16-type structures.10 Recently, several efforts have been made to synthesize PMOs with large bridging groups,13 such as * Corresponding author: Mietek Jaroniec (phone 330-672 3790; e-mail
[email protected]). † Kent State University. ‡ Maria Curie-Skłodowska University.
10.1021/jp0574652 CCC: $33.50
SCHEME 1: Schematic Illustration of the SBA-16-type PMO with Large Heterocyclic Isocyanurate Bridging Groupsa
a Triangles represent tris[3-(trimethoxysilyl)propyl]isocyanurate bridging groups incorporated into the silica framework.
bipyridine, biphenylene, tetraazacyclotetraadecane, and benzene ring linked with three silicon atoms, but those refer to the channel-like structures only. Later, others14 were stimulated to synthesize the channel-like PMOs with large bridging group such as tris[3-(trimethoxysilyl)propyl]isocyanurate (ICS) by cocondensation with tetraethyl orthosilicate (TEOS) in the presence of triblock copolymer Pluronic P123. The synthesized PMOs exhibited high BET specific surface area, good structural ordering, and high loading of incorporated bridging groups. Here, we report the synthesis of PMO with Im3m symmetry (SBA-16) and large heterocyclic bridging groups in cage-like mesopores (see Scheme 1). The core of this bridging group is an isocyanurate ring integrated with three trimethoxysilyls through flexible propyl chains. This PMO was synthesized by co-condensation of tetraethyl orthosilicate and tris[3-(trimethoxysilyl)propyl]isocyanurate in the presence of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127, EO106PO70 EO106) and sodium chloride under low acidic conditions. The recipe is analogous to that reported by Qiu et al.10 for ethane-bridged SBA-16. In addition, an efficient removal of the polymeric template was achieved by heating of the extracted sample in nitrogen at 315 °C for 4 h, which improves the quality of the resulting material. To the best of our knowledge, there is no report on the cage© 2006 American Chemical Society
Letters
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TABLE 1: Adsorption, Structural Parameters, Synthesis Gel Compositions, Elemental Analysis Data, and Thermogravimetric Weight Loss for the PMOs Studieda
sample
X ICS
nt mmol
a nm
SBET m2/g
Vt cc/g
Vm cc/g
wKJS nm
wd nm
TG wt. loss (%)
S1-e S1-ec S2-e S2-ec S3-e S3-ec S4-e S4-ec S5-e S5-ec S6-e S6-ec
0.017 0.017 0.036 0.036 0.056 0.056 0.077 0.077 0.100 0.100 0.125 0.125
39 39 37.6 37.6 36.3 36.3 34.9 34.9 33.6 33.6 32.2 32.2
16.7 16.0 16.5 15.4 16.6 16.6 17.2 16.2 17.8 17.2 16.8 15.7
712 777 748 761 870 886 881 891 840 776 736 785
0.49 0.49 0.49 0.48 0.59 0.57 0.56 0.53 0.53 0.49 0.44 0.45
0.27 0.29 0.29 0.29 0.33 0.33 0.33 0.34 0.32 0.29 0.29 0.29
7.44 7.04 7.23 7.05 7.24 6.89 7.20 7.06 7.41 7.04 6.58 6.37
10.1 9.40 9.70 8.92 10.3 10.1 10.3 9.20 10.5 10.1 9.14 8.66
20.1 14.6 21.7 15.9 21.9 18.0 24.1 18.8 26.5 22.4 28.8 24.0
synthesis gel mixture %C %N
elemental analysis %C %N
3.76 3.76 7.10 7.10 10.78 10.78 12.76 12.76 15.18 15.18 17.38 17.38
10.35 8.83 11.05 9.44 12.30 10.87 12.88 11.14 15.53 13.32 16.48 15.53
1.10 1.10 2.07 2.07 2.94 2.94 3.72 3.72 4.42 4.42 5.07 5.07
1.18 1.05 1.87 1.60 2.25 2.04 3.20 2.83 3.91 3.60 4.53 4.09
a Notation: X, mole fraction of ICS in the synthesis gel; nt, total number of mmoles of TEOS and ICS in the synthesis gel; a, unit cell parameter calculated from the observed characteristic Bragg’s reflections; SBET, BET specific surface area; Vt, single-point pore volume; Vm, volume of micropores and interconnecting pores of the diameter below 4 nm; wKJS, mesopore cage diameter; wd, mesopore cage diameter calculated on the basis of the unit cell parameter and pore volumes according to the relation derived for the cubic Im3m symmetry (see ref 15); TG, weight loss recorded in flowing nitrogen in the range between 100 and 800 °C.
like PMOs containing high loadings of incorporated large bridging groups and showing long-range structural ordering as well as good adsorption characteristics as evidenced by smallangle X-ray scattering and nitrogen adsorption data. The molar composition of the synthesis gel mixture was (13x) TEOS/x ICS/0.0037 F127/0.86 HCl/2.77 NaCl/100.2 H2O, where x stands for the number of moles of ICS (see Table 1 for the values of x). The amount of the TEOS used was (1-3x) to maintain the Si/polymer ratio at the same level. To make the PMO mesostructre accessible for adsorption, the polymeric template was removed from the as-synthesized materials by extraction with sulfuric acid/ethanol solution followed by temperature-controlled calcination (see below). All reagents were used as received. The template-free SBA-16 PMO samples are denoted either as Sy-e or Sy-ec, where y, e, and c stand for the sample number, extracted SBA-16 PMO, and calcined SBA16 PMO, respectively (see Table 1). Synthesis of the SBA-16 mesoporous organosilica with ICS bridging groups was carried out similarly to the Qiu procedure: 10 2 g of F127 and 7.05 g of sodium chloride were dissolved in 20 mL of 2 M HCl and 60 mL of deionized water (DW) at 40 °C. After 2-3 h stirring, a specified volume of TEOS was pipetted dropwise to this solution under vigorous mixing, and then after 15 min, ICS was added to achieve a desired molar composition of both silanes. After further stirring for 20 h at 40 °C, the resulting white precipitate was transferred into a polypropylene bottle and subsequently heated at 100 °C for 24 h. The product was filtered, washed with DW, and dried in the oven at 80 °C. The as-synthesized nanocomposites were extracted twice with 20 mL of H2SO4 and 100 mL of 95% EtOH at 70 °C, followed by calcination at 315 °C for 4 h in flowing nitrogen with a ramp of 3 °C per minute, to remove the polymeric template from mesopores. The symmetry group of the extracted PMOs was determined by small-angle X-ray scattering data (SAXS), which is shown in Figure 1, whereas the unit cell parameters are listed in Table 1. For instance, the S6-e sample synthesized with the largest amount of ICS exhibits a strong peak at 2θ ) ∼0.74 attributed to the (110) reflection and three minor well-resolved peaks at 2θ ) ∼1.05, 1.29, and 1.96 attributed to the (200), (211), and (310) reflections, respectively. This assignment is consistent with the cubic Im3m symmetry. As can be seen from Figure 1, an increase in the percentage of ICS in PMO led to a pronounced
Figure 1. Small-angle X-ray scattering patterns for the extracted PMO samples.
change in the SAXS profiles, indicating that the structural order in the PMOs studied is dependent to a certain degree on the amount of introduced ICS. The minor peak intensities attributed to the second and third reflections increased upon increasing the molar ratio of the bridging groups, which may suggest some improvement of the mesophase order. This finding indicates that upon addition of a small amount of large bridging groups (low concentration of ICS) some disruption in the formation of ordered mesostructure may occur, because the aforementioned amount of these groups may not be sufficient to ensure their uniform distribution in the entire framework. Also, at high concentrations of ICS, a structure deterioration may be observed because of geometrical constrictions to accommodate a high
2974 J. Phys. Chem. B, Vol. 110, No. 7, 2006
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Figure 2. (A) Nitrogen adsorption-desorption isotherms measured at -196 °C for the extracted PMO samples. The isotherms for S2-e, S3-e, S4-e, S5-e/S5-ec, and S6-e were offset vertically by 120, 205, 309, 471, and 600 cm3 STP g-1, respectively. (B) Pore size distributions (PSDs) calculated according to the KJS method from adsorption branches of nitrogen adsorption isotherms for the PMOs shown in Figure 2A. (C) A comparison of PSDs for the S5e and S5-ec PMOs.
Figure 3. (A) Thermogravimetric weight change (TG) curves measured in flowing nitrogen for S5 PMOs: as-synthesized (S5-F127), extracted (S5-e), and extracted-calcined (S5-ec); and (B) the corresponding differential thermogravimetric (DTG) curves. (C) A comparison of the TG curves for the extracted-calcined S3-ec and S6-ec samples and (D) the corresponding DTG curves.
amount of ICS groups in the framework. Thus, the SAXS data suggest that there is an optimal concentration range of large bridging groups to form highly ordered mesostructures. For the SBA-16-ICS system and experimental conditions studied, the best samples were obtained for the mole fractions of ICS between ∼0.07 and ∼0.12. Shown in Figure 2A is a comparison of nitrogen adsorptiondesorption isotherms measured at -196 °C for a series of the PMOs studied. All isotherms are type IV with sharp capillary condensation/evaporation steps and a pronounced hysteresis loop starting at a relative pressure of about 0.7 and abruptly ending at ∼0.45. This is typical for good-quality cage-like structures with narrow pore size distributions (see Figure 2B) and uniform pore openings.15 As can be seen from Figure 2A, incorporation of a large amount of ICS causes a shift of the capillary condensation step toward lower relative pressures, which reflects a decrease in the mesopore diameter [see in Table 1 the pore diameter values evaluated according to the KJS (Kruk, Jaroniec, Sayari) method16 applicable for cylindrical mesopores as well as those estimated on the basis of SAXS and pore volume data]. It should be mentioned that the pore diameters estimated on the basis of SAXS and pore volume data for the cubic Im3m symmetry15 are greater by about 2-3 nm than those obtained with the KJS method.16 For the sample with the highest concentration of ICS (S6-e), the isotherm features a smaller uptake volume of adsorbate that indicates a decrease in the volume of primary mesopores. This can be expected because of the geometrical constrictions associated with accommodation of a high amount of large isocyanurate bridging groups into the mesopore walls. An example of PMO prepared by extraction followed by calcination at 315 °C in flowing nitrogen is shown by the dotted line in Figure 2A. There is only a slight difference between adsorption isotherms for S5-e and S5-ec because of the expected shrinkage of the structure during calcination, which caused a decrease in the total pore volume, and mesopore cage diameter (see PSDs in Figure 2C and data in Table 1).
High-resolution thermogravimetry (TG) was used to monitor the incorporation of large isocyanurate bridging groups into the silica framework. The TG patterns and the corresponding differential TG (DTG) curves for the S5-F127, S5-e, and S5-ec (where S5-F127 denotes PMO with polymeric template) recorded in flowing nitrogen are shown in Figure 3. The TG weight loss data for the extracted as well as extracted-calcined samples are listed in Table 1. As can be seen from Figure 3B (see curve for S5-127), the DTG profile exhibits two peaks at the temperature ranges 300-400 °C and 400-650 °C, respectively, related to the thermodesorption and degradation of triblock copolymer template and cross-linked heterocyclic rings. Because of the specific 3D structure and narrow apertures of the cage-like materials, a small residue of the polymer template remained after extraction, and its further removal was performed by controlled-temperature calcination in flowing nitrogen. This was possible because of a good separation of thermogravimetric events reflecting the removal of polymeric template and decomposition of bridging organic groups. As shown in Figure 3B for S5-e and S5-ec, an additional heating of the extracted ICS-SBA-16 samples in flowing nitrogen at 315 °C did not remove the ICS group but eliminated the residual amount of the polymeric template. Further evidence of successful incorporation of ICS into the mesopore walls is shown in Figure 3C,D for the extracted-calcined PMOs with different concentrations of ICS. As can be seen from that figure, a gradual increase in the concentration of ICS caused an increase in the TG weight loss that is reflected by increased height of the DTG peak. Elemental analysis was performed to estimate the concentration of incorporated isocyanurate rings into the silica framework (see %N and %C values in Table 1). Nitrogen and carbon percentages obtained from elemental analysis are similar to those estimated on the basis of the initial gel composition, which indicates a good incorporation of isocyanurate bridging group into PMO.
Letters In conclusion, this work demonstrates a successful incorporation of bulky isocyanurate bridging groups into the cage-like silica framework of Im3m symmetry. A more complete removal of the Pluronic F127 template from extracted samples was achieved by their additional calcination at 315 °C in nitrogen. The resulting PMOs possess relatively high loadings of large heterocyclic isocyanuarte rings, high total pore volume, and high surface area. Acknowledgment. The NSF support (grant CHE-0093707) is acknowledged. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710. (2) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (3) Zhao, D.; Feng, J.; Huo, Q,; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Teresaki, O. J. Am. Chem. Soc. 1999, 121, 9611. Melde, B. J.; Holland, B. T.; Blandford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. Asefa, T.; MacLachlan, M. J.; Coombos, N.; Ozin, G. A. Nature (London) 1999, 402, 867. YoshinaIshii, C.; Asefa, T.; Coombos, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539. (5) Rebbin, V.; Jakubowski, M.; Potz, S.; Froba, M. Microporous Mesoporous Mater. 2004, 72, 99. (6) Liang, Y.; Hanzlik, M.; Anwander, R. J. Mater. Chem. 2005, 15, 3919. Liang, Y.; Hanzlik, M.; Anwander, R. Chem. Commun. 2005, 525. (7) 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. McInall, M. D.;
J. Phys. Chem. B, Vol. 110, No. 7, 2006 2975 Scott, J.; Mercier, L.; Kooyman, P. J. Chem. Commun. 2001, 2282. Fan, H.; Reed, S.; Bear, T.; Schunk, R.; Lopez, G. P.; Brinker, C. J. Microporous Mesoporous Mater. 2001, 44, 625. Dag, O.; Ozin, G. A. AdV. Mater. 2001, 13, 1182. Dag, O.; Yoshina-Ishii, C.; Asefa, T.; MacLachlan, M. J.; Grondley, H.; Coombos, N.; Ozin, G. A. AdV. Funct. Mater. 2001, 11, 213. (8) Muth, O.; Schellbach, C.; Froba, M. Chem. Commun. 2001, 2032. Cho, E.-B.; Kwon, K.-W.; Char, K. Chem. Mater. 2001, 13, 3837. Burleigh, M. C.; Markowitz, M. A.; Wong, E. M.; Lin, J. S.; Gaber, B. P. Chem. Mater. 2001, 13, 4411. Zhu, H.; Jones, D. J.; Zajac, J.; Dutartre, R. Chem. Commun. 2001, 2568. Bao, X. Y.; Zhao, X. S.; Li, X.; Chia, P. A.; Li, J. J. Phys. Chem. B 2004, 108, 4684. Bao, X. Y.; Zhao, X. S. J. Phys. Chem. B 2005, 109, 10727. (9) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Asefa, T.; Coombos, N.; Ozin, G. A.; Teresaki, O. Chem. Mater. 2002, 14, 1903. (10) Zhao, L.; Zhu, G.; Zhang, D.; Di, Y.; Teresaki, O.; Qiu, S. J. Phys. Chem. B 2004, 109, 764. (11) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. Hatton, B.; Landskron, K.; Whitnall, W.; Perovic, D.; Ozin, G. A. Acc. Chem. Res. 2005, 38, 305. Stein, A. AdV. Mater. 2003, 15, 763. (12) Asefa, T.; Yoshina-Ishii, C.; MacLaclan, M.; Ozin, G. A. J. Mater. Chem. 2000, 10, 1751. Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. Kickkelbick, G. Angew. Chem., Int. Ed. 2004, 43, 3102. Vinu, A.; Hossain, K. Z.; Ariga, K. J. Nanosci. Nanotechnol. 2005, 5, 347. Ford, D. M.; Simanek, E. E.; Shantz, D. F. Nanotechnology 2005, 16, 458. (13) Kapoor, M. P.; Yang, Q.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 15176. Corriu, R. J. P.; Mehedi, A.; Reye, C.; Thieuleux, C. Chem. Commun. 2002, 1382. Kuroki, M.; Asefa, T.; Whitnal, W.; Kruk, M.; Yoshina-Ishii, C.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2002, 124, 13886. Llandskrn, K.; Hhatton, B. D.; Pervic, D. D.; Ozin, G. A. Science 2003, 302, 266. (14) Olkhovyk, O.; Jaroniec, M. J. Am. Chem. Soc. 2005, 127, 60. (15) Matos, J. R. Mercuri, L. P.; Kruk, M.; Jaroniec, M. Langmuir 2002, 18, 844. (16) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267.
