Synthesis of Ultralarge-Pore FDU-12 Silica with Face-Centered Cubic

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Synthesis of Ultralarge-Pore FDU-12 Silica with Face-Centered Cubic Structure Liang Huang,†,‡ Xuewu Yan,†,§ and Michal Kruk*,†,‡ †

Department of Chemistry, College of Staten Island, City University of New York, 2800 Victory Boulevard, Staten Island, New York 10314, and ‡Graduate Center, City University of New York, 365 Fifth Avenue, New York, New York 10016. § Present address: School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Received June 2, 2010. Revised Manuscript Received August 4, 2010

Ultralarge-pore FDU-12 (ULP-FDU-12) silicas with face-centered cubic structures (Fm3m symmetry) of spherical mesopores were synthesized at low initial temperature (∼14 °C) using commercially available PEO-PPO-PEO triblock copolymer Pluronic F127 as a micellar template and xylene as a micelle expander. Xylene was selected on the basis of its predicted higher swelling ability for the Pluronic surfactant micelles in comparison to 1,3,5-trimethylbenzene that was used previously to obtain large-pore FDU-12. The optimization of the synthesis conditions afforded as-synthesized ULP-FDU-12 materials with unit-cell parameters up to 56 nm, which is comparable to the highest reported values for Fm3m structures templated by custom-made surfactants. Calcined silicas were obtained with unit-cell parameters up to 53 nm and pore diameters up to ∼36 nm (for N2 adsorption at 77 K, the capillary condensation relative pressure was up to 0.938). The preferred silica source was tetraethylorthosilicate, but tetramethylorthosilicate was also found suitable. The pore diameter was dependent on the unit-cell size of the as-synthesized material, but was further tuned by adjusting the time and temperature of the treatment in the HCl solution. If the synthesis was performed at low temperature only, highly ordered closed-pore silicas were obtained at calcination temperatures as low as 450 °C. On the other hand, the hydrothermal treatments, including the acid treatment at 130 °C, afforded silicas with large pore entrance sizes. The present synthesis constitutes a major advancement in the synthesis of ordered silicas with very large open and closed spherical mesopores.

Introduction Ordered silicas with spherical mesopores (OSSMs) constitute a vast family of novel porous materials. They offer a diversity of structural symmetries, pore diameters, and pore entrance sizes, which surpasses their widely known counterparts with approximately cylindrical mesopores (such as MCM-41,1,2 FSM-16,3 SBA-15,4,5 MCM-48,1,2,6 and KIT-6,7-10 the latter two having pores with three-way intersections). First OSSMs, SBA-1 silica11,12 of Pm3n symmetry and SBA-2 silica13 of P63/mmc symmetry were *To whom correspondence should be addressed. E-mail: Michal.Kruk@ csi.cuny.edu. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (3) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680–682. (4) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Frederickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552. (5) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024–6036. (6) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299–1303. (7) Chan, Y.-T.; Lin, H.-P.; Mou, C.-Y.; Liu, S.-T. Chem. Commun. 2002, 2878– 2879. (8) Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136–2137. (9) Kim, T.-W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 7601–7610. (10) Che, S.; Garcia-Bennett, A. E.; Liu, X.; Hodgkins, R. P.; Wright, P. A.; Zhao, D.; Terasaki, O.; Tatsumi, T. Angew. Chem., Int. Ed. 2003, 42, 3930–3934. (11) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schueth, F.; Stucky, G. D. Nature 1994, 368, 317–321. (12) Sakamoto, Y.; Kaneda, M.; Terasaki, O.; Zhao, D. Y.; Kim, J. M.; Stucky, G.; Shin, H. J.; Ryoo, R. Nature 2000, 408, 449–453. (13) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324– 1327.

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templated by alkylammonium surfactants and exhibited pore diameters ∼4 nm. The introduction of oligomer and blockcopolymer surfactant templates, including poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers (commercially available as Pluronics),4,5,14,15 brought about several new OSSM structures, including SBA-16 silica of Im3m symmetry,5,12,16 as well as face-centered cubic (Fm3m symmetry) and related structures known as SBA-12,17 FDU1,18,19 FDU-12,20 and KIT-5.21 The block-copolymer-templated OSSMs share a number of remarkable features, such as a tailorable pore cage diameter and pore entrance size (from below 1 nm to the diameter close to the pore cage size), which are adjusted simply through the selection of synthesis conditions.19,20,22 OSSMs also exhibit superior hydrothermal stability.19 In addition, OSSMs can be synthesized in the form of powders,5 thin films,23 fibers,24 and monoliths,25 and are available as (14) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242–1244. (15) Templin, M.; Franck, A.; Du Chesne, A.; Leist, H.; Zhang, Y.; Ulrich, R.; Schadler, V.; Wiesner, U. Science 1997, 278, 1795–1798. (16) Tattershall, C. E.; Aslam, S. J.; Budd, P. M. J. Mater. Chem. 2002, 12, 2286– 2291. (17) Sakamoto, Y.; Diaz, I.; Terasaki, O.; Zhao, D.; Perez-Pariente, J.; Kim, J. M.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 3118–3123. (18) Yu, C.; Yu, Y.; Zhao, D. Chem. Commun. 2000, 575–576. (19) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Zhao, L.; Kamiyama, T.; Terasaki, O.; Pinnavaia, T. J.; Liu, Y. J. Am. Chem. Soc. 2003, 125, 821–829. (20) Fan, J.; Yu, C.; Gao, F.; Lei, J.; Tian, B.; Wang, L.; Luo, Q.; Tu, B.; Zhou, W.; Zhao, D. Angew. Chem., Int. Ed. 2003, 42, 3146–3150. (21) Kleitz, F.; Liu, D.; Anilkumar, G. M.; Park, I.-S.; Solovyov, L. A.; Shmakov, A. N.; Ryoo, R. J. Phys. Chem. B 2003, 107, 14296–14300. (22) Kim, T. W.; Ryoo, R.; Kruk, M.; Gierszal, K. P.; Jaroniec, M.; Kamiya, S.; Terasaki, O. J. Phys. Chem. B 2004, 108, 11480–11489. (23) Zhao, D. Y.; Yang, P. D.; Melosh, N.; Feng, Y. L.; Chmelka, B. F.; Stucky, G. Adv. Mater. 1998, 10, 1380–1385. (24) Yang, P.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033–2036.

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open-pore5 or closed-pore26,27 materials. Due to their remarkable structural features, OSSMs attracted interest in a variety of areas. Heterogeneous catalysis,28 including enzymatic catalysis,29 benefits from the unique pore connectivity and ability to tailor the pore entrance size in OSSMs. Appropriately modified OSSMs were used as sensors.30 OSSMs were also found suitable as templates for isolated nanoparticles (including nanospheres)31 and for ordered arrays of nanospheres of a variety of compositions.20,32-34 OSSMs were used as media for immobilization of biomolecules35-37 and as supports for high-surface-area polymer brushes.38 Moreover, OSSMs and surface-modified OSSMs were found promising as closed-pore low-dielectric-constant materials.39-41 One of the crucial aspects of the development of OSSMs is the ability to tailor their pore diameter and pore entrance diameter and to maximize their mesopore volume. In most cases, blockcopolymer-templated OSSMs are synthesized using block copolymer surfactants with relatively large hydrophilic block(s), such as Pluronic F127 (EO106PO70EO106),5,42 which have a strong tendency to aggregate into spherical micelles and thus to template spherical mesopores. However, such a selection of the template usually results in a relatively low mesopore volume,19 because the mesopore voids in the final material primarily correspond to the space occupied by the hydrophobic domains of the surfactant micelles, which in such cases have moderate or even small volume fraction. On the other hand, the hydrophilic blocks of the surfactant are occluded in the silica framework and thus contribute to the intrawall porosity (typically microporosity)19 rather than to the generation of the mesopore volume. As a result, the micropore volume is significant, the mesopore volume is moderate or small, the mesopore diameter is moderate, and the pore walls are very thick. The simplest way to increase the mesopore diameter and volume is to use appropriate swelling agents, such as 1,3,5-trimethylbenzene (TMB)20,25,33,43 or linear hydrocarbons,25 which resulted in (25) El-Safty, S. A.; Hanaoka, T.; Mizukami, F. Chem. Mater. 2005, 17, 3137– 3145. (26) Yu, K.; Hurd, A. J.; Eisenberg, A.; Brinker, C. J. Langmuir 2001, 17, 7961– 7965. (27) Kruk, M.; Hui, C. M. J. Am. Chem. Soc. 2008, 130, 1528–1529. (28) Yang, H.; Li, J.; Yang, J.; Liu, Z.; Yang, Q.; Li, C. Chem. Commun. 2007, 1086–1088. (29) Shui, W.; Fan, J.; Yang, P.; Liu, C.; Zhai, J.; Lei, J.; Yan, Y.; Zhao, D.; Chen, X. Anal. Chem. 2006, 78, 4811–4819. (30) Balaji, T.; El-Safty, S. A.; Matsunaga, H.; Hanaoka, T.; Mizukami, F. Angew. Chem., Int. Ed. 2006, 45, 7202–7208. (31) Yiu, H. H. P.; Niu, H.-j.; Biermans, E.; Tendeloo, G. v.; Rosseinsky, M. J. Adv. Funct. Mater. 2010, 20, 1599–1609. (32) Tian, B.; Liu, X.; Yang, H.; Xie, S.; Yu, C.; Tu, B.; Zhao, D. Adv. Mater. 2003, 15, 1370–1374. (33) Fan, J.; Yu, C.; Lei, J.; Zhang, Q.; Li, T.; Tu, B.; Zhou, W.; Zhao, D. J. Am. Chem. Soc. 2005, 127, 10794–10795. (34) Kruk, M.; Dufour, B.; Celer, E. B.; Kowalewski, T.; Jaroniec, M.; Matyjaszewski, K. J. Phys. Chem. B 2005, 109, 9216–9225. (35) Fan, J.; Lei, J.; Wang, L.; Yu, C.; Tu, B.; Zhao, D. Chem. Commun. 2003, 2140–2141. (36) Hudson, S.; Cooney, J.; Hodnett, B. K.; Magner, E. Chem. Mater. 2007, 19, 2049–2055. (37) Urrego, S.; Serra, E.; Alfredsson, V.; Blanco, R. M.; Dı´ az, I. Microporous Mesoporous Mater. 2010, 129, 173–178. (38) Kruk, M.; Dufour, B.; Celer, E. B.; Kowalewski, T.; Jaroniec, M.; Matyjaszewski, K. Macromolecules 2008, 41, 8584–8591. (39) Smarsly, B.; Xomeritakis, G.; Yu, K.; Liu, N.; Fan, H.; Assink, R. A.; Drewien, C. A.; Ruland, W.; Brinker, C. J. Langmuir 2003, 19, 7295–7301. (40) Yu, K.; Smarsly, B.; Brinker, C. J. Adv. Funct. Mater. 2003, 13, 47–52. (41) Yu, K.; Wu, X.; Brinker, C. J.; Ripmeester, J. Langmuir 2003, 19, 7282– 7288. (42) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y.-U.; Terasaki, O.; Park, S.-E.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552–2558. (43) Kruk, M.; Hui, C. M. Microporous Mesoporous Mater. 2008, 114, 64–73. (44) Han, Y.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 643–649. (45) El-Safty, S. J. Porous Mater. 2008, 15, 369–387. (46) Xu, Y.; Wu, Z.; Zhang, L.; Lu, H.; Yang, P.; Webley, P. A.; Zhao, D. Anal. Chem. 2009, 81, 503–508.

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a wide array of OSSMs suitable for a range of applications.20,29,44-46 An optimal selection of swelling agents for the surfactanttemplated synthesis of ordered mesoporous silicas (OMSs) has been a topic of significant research activity nearly since inception of the OMS research field.47-53 It was recently proposed54 that this selection can be based on two criteria. First, swelling agents that are known to afford well-ordered OMSs33,51 appear to solubilize in the surfactant micelles to a small or moderate extent,43,55 whereas the substances that are strongly solubilized are likely to afford disordered and/or heterogeneous structures of the templated materials.50,56 Therefore, it was postulated54 that in order to achieve an appreciable pore size increase with the retention of the ordered nanostructure, the swelling agent needs to exhibit moderate extent of solubilization in surfactant micelles. Second, the extent of the solubilization of substances57,58 (swelling agent candidates) in micelles of a particular surfactant in solution can be used to predict the extent of solubilization relevant to the micelle-templated synthesis.54 If the data for a given surfactant/ swelling agent combination are not available, but trends in the behavior of similar compounds are clear, one may extrapolate the data over families of compounds.54 The above criteria were used to identify swelling agents suitable for the synthesis of ultralargepore SBA-15 silica54,59 with 2-dimensional hexagonal structure, and related periodic mesoporous organosilicas (PMOs).60 These materials were template by Pluronic P123 and the commonly used swelling agent TMB was inferred54 to produce excessively strong swelling for SBA-15 synthesis.56,61 The use of a 1,3,5-trialkylbenzene swelling agent with larger alkyl substituents was proposed to reduce the extent of solubilization, which paved the avenue to ultralarge-pore SBA-15 silicas with (100) interplanar spacings up to 26 nm and pore diameters up to ∼26 nm.54 On the other hand, the synthesis of large-pore SBA-15 with hexane as a swelling agent, which affords highly ordered products with (100) interplanar spacing up to ∼14 nm,51,55 was inferred55 to involve a very modest uptake of the swelling agent. It was proposed54 to replace hexane with cyclohexane, which is known to solubilize more strongly in Pluronics.57 This led to a major breakthrough in the synthesis of periodic mesoporous silicas (PMOs) with 2-D hexagonal structures, as their (100) interplanar spacing reached ∼23 nm and the pore diameter reached ∼20 nm,60 which is nearly two times larger than the previously reported values.62-66 (47) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759–2760. (48) Namba, S.; Mochizuki, A. Res. Chem. Intermed. 1998, 24, 561–570. (49) Jana, S. K.; Nishida, R.; Shindo, K.; Kugita, T.; Namba, S. Microporous Mesoporous Mater. 2004, 68, 133–142. (50) Sayari, A.; Kruk, M.; Jaroniec, M.; Moudrakovski, I. L. Adv. Mater. 1998, 10, 1376–1379. (51) Sun, J.; Zhang, H.; Ma, D.; Chen, Y.; Bao, X.; Klein-Hoffmann, A.; Pfaender, N.; Su, D. S. Chem. Commun. 2005, 5343–5345. (52) Blin, J. L.; Su, B. L. Langmuir 2002, 18, 5303–5308. (53) Luechinger, M.; Pirngruber, G. D.; Lindlar, B.; Laggner, P.; Prins, R. Microporous Mesoporous Mater. 2005, 79, 41–52. (54) Cao, L.; Man, T.; Kruk, M. Chem. Mater. 2009, 21, 1144–1153. (55) Kruk, M.; Cao, L. Langmuir 2007, 23, 7247–7254. (56) Schmidt-Winkel, P.; Lukens, W. W., Jr.; Yang, P.; Margolese, D. I.; Lettow, J. S.; Ying, J. Y.; Stucky, G. D. Chem. Mater. 2000, 12, 686–696. (57) Nagarajan, R. Colloids Surf., B 1999, 16, 55–72. (58) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2, 210–215. (59) Cao, L.; Kruk, M. Colloids Surf., A 2010, 357, 91–96. (60) Mandal, M.; Kruk, M. J. Mater. Chem. 2010, DOI:10.1039/C0JM01170C, published on Web 8/2/2010. (61) Lettow, J. S.; Han, Y. J.; Schmidt-Winkel, P.; Yang, P.; Zhao, D.; Stucky, G. D.; Ying, J. Y. Langmuir 2000, 16, 8291–8295. (62) Cho, E.-B.; Kim, D.; Gorka, J.; Jaroniec, M. J. Phys. Chem. C 2009, 113, 5111–5119. (63) Bao, X. Y.; Li, X.; Zhao, X. S. J. Phys. Chem. B 2006, 110, 2656–2661. (64) Bao, X. Y.; Zhao, X. S.; Li, X.; Chia, P. A.; Li, J. J. Phys. Chem. B 2004, 108, 4684–4689.

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It was demonstrated earlier that TMB is very effective as a swelling agent in the synthesis of ordered silicas with spherical mesopores.20,33,43,67-74 First, it was demonstrated that when Pluronic F127 (EO106PO70EO106) was used in combination with TMB at above room temperature, OSSM (dubbed FDU-12) with face-centered cubic structure and pore diameters of ca. 10-12 nm was obtained with the pore size and pore entrance size controllable by the temperature of the second step of the synthesis.20 Later, it was reported that when the initial synthesis temperature was lowered to 14-15 °C, the unit-cell parameter and pore diameter increased to as much as 44 and 22-27 nm, respectively.33 The resulting silicas are known as large-pore FDU-12 (LP-FDU-12). A further slight increase in the unit-cell parameter (up to 50.5 nm) and pore diameter was achieved when the same synthesis strategy was optimized74 or used to synthesize silicas with organic groups on the surface through the co-condensation approach.68 These unit-cell sizes are the largest among the reported OMSs, except for the OSSM similar to LP-FDU-12 synthesized using poly(ethylene oxide)-polystyrene (PEO-PS) surfactant, which is not commercially available.75 In the latter case, the unit-cell parameter was 56.7 nm for an uncalcined acidtreated material and 54.4 nm for a calcined sample. Herein, the identification of xylene as a superior swelling agent for Pluronic F127 surfactant is outlined, and the application of xylene to synthesize a new generation of face-centered cubic silicas, which are referred to herein as ultralarge-pore FDU-12 (ULP-FDU-12), is presented. These novel silicas exhibit unit-cell parameters approaching those achieved earlier by using PEO-PS surfactants,75 which are not commercially available, and have exceptionally large pore diameters. In addition, it is shown that highly ordered closed-pore silicas can be obtained using one-step low-temperature synthesis and remarkably low calcination temperature of 450 °C.

Materials and Methods Synthesis. In a typical synthesis procedure, 1.00 g of Pluronic F127 (EO106PO70EO106) and 2.5 g of KCl were dissolved in 60 mL of 2 M HCl in a glass container with magnetic stirring, and then a certain amount (see text) of xylenes (mixed isomers, which will be referred to as xylene throughout the manuscript) was added and the mixture was stirred at 350 rpm at 14 °C for one day in a covered container. Next, 4.5 g of tetraethyl orthosilicate (TEOS) was added. In some cases, tetramethyl orthosilicate (TMOS) was used instead of TEOS. The reaction mixture was stirred at 14 °C for one day, and then it was transferred to a polypropylene bottle and kept at 100 °C for one day. The product was filtered and dried in a vacuum oven at 60 °C. The resulting as-synthesized material was calcined at 550 °C under air for 5 h to remove the surfactant template. (65) Zhang, W.-H.; Zhang, X.; Hua, Z.; Harish, P.; Schroeder, F.; Hermes, S.; Cadenbach, T.; Shi, J.; Fischer, R. A. Chem. Mater. 2007, 19, 2663–2670. (66) Landskron, K.; Ozin, G. A. Science 2004, 306, 1529–1532. (67) Yu, T.; Zhang, H.; Yan, X.; Chen, Z.; Zou, X.; Oleynikov, P.; Zhao, D. J. Phys. Chem. B 2006, 110, 21467–21472. (68) Budi Hartono, S.; Qiao, S. Z.; Jack, K.; Ladewig, B. P.; Hao, Z.; Lu, G. Q. M. Langmuir 2009, 25, 6413–6424. (69) Budi Hartono, S.; Qiao, S. Z.; Liu, J.; Jack, K.; Ladewig, B. P.; Hao, Z.; Lu, G. Q. M. J. Phys. Chem. C 2010, 114, 8353–8362. (70) Zhou, X.; Qiao, S.; Hao, N.; Wang, X.; Yu, C.; Wang, L.; Zhao, D.; Lu, G. Q. Chem. Mater. 2007, 19, 1870–1876. (71) Ersen, O.; Parmentier, J.; Solovyov, L. A.; Drillon, M.; Pham-Huu, C.; Werckmann, J.; Schultz, P. J. Am. Chem. Soc. 2008, 130, 16800–16806. (72) Gao, F.; Botella, P.; Corma, A.; Blesa, J.; Dong, L. J. Phys. Chem. B 2009, 113, 1796–1804. (73) Tang, J.; Liu, J.; Wang, P.; Zhong, H.; Yang, Q. Microporous Mesoporous Mater. 2010, 127, 119–125. (74) Ma, G.; Yan, X.; Li, Y.; Xiao, L.; Huang, Z.; Lu, Y.; Fan, J. J. Am. Chem. Soc. 2010, 132, 9596–9597. (75) Deng, Y.; Yu, T.; Wan, Y.; Shi, S.; Meng, Y.; Gu, D.; Zhang, L.; Huang, Y.; Liu, C.; Wu, X.; Zhao, D. J. Am. Chem. Soc. 2007, 129, 1690–1697.

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The resulting samples are denoted EHx or MHx, where E stands for TEOS, M stands for TMOS, H denotes the hydrothermal treatment at 100 °C, and x is the sample number. In some cases, a part of the dried as-synthesized material was subjected to an acid treatment, in which 0.5 g of the as-synthesized sample was placed in 30 mL of 2 M HCl solution and heated at 100 °C (in a polypropylene bottle) or 130 °C (in a Teflon-lined autoclave) for 1-4 days. The resulting materials were filtered and dried. The acid-treated samples are denoted using the EHx (or MHx) with -ATt ending, where A denotes the acid treatment, T indicates the treatment temperature in °C, and t shows the treatment time in days. For the low-temperature synthesis of closed-pore silicas, 4.5 g of xylenes was used and the sample was filtered out after one day of stirring at 14 °C. After drying at at 60 °C in a vacuum oven, the assynthesized material was calcined under air for 5 h at different temperatures in the range 300-450 °C. The calcined samples were denoted E40 -T, where T represents the calcination temperature. Measurements. Small-angle X-ray scattering (SAXS) measurements were performed using a Bruker Nanostar U SAXS/ wide-angle X-ray scattering instrument with a rotating anode X-ray source and Vantec-2000 two-dimensional detector. The instrument was in high flux mode. In some cases, SAXS measurements were performed at station D1 of the Cornell High Energy Synchrotron Source (CHESS). Nitrogen adsorption measurements were performed at -196 °C on a Micromeritics ASAP 2020 volumetric adsorption analyzer. Before the analysis, samples were outgassed under vacuum at 200 °C for 2 h in the port of the adsorption analyzer. Transmission electron microscopy (TEM) images were acquired on a FEI Tecnai G2 Twin microscope operated at 120 kV. Before the imaging, the samples were dispersed in ethanol using sonication and subsequently deposited on a carbon-coated copper grid. Calculations. The specific surface area was calculated using the Brunauer Emmett Teller (BET) method in the relative pressure range from 0.04 to 0.2.76 The total pore volume, Vt, was estimated from the amount adsorbed at a relative pressure of 0.99.76 The micropore volume, Vmi, was evaluated using the Rs plot method in the standard reduced adsorption range from 1.0 to 1.3.76,77 For many OMSs,43,78 the sum of the mesopore volume, Vp, and the micropore volume (Vp þ Vmi) can be estimated from the Rs plot method from data corresponding to pressures above the capillary condensation pressure. However, the pore diameter for the present materials was so large that the capillary condensation took place very close to the saturation vapor pressure, making such calculations inaccurate or even impossible. Therefore, Vp þ Vmi was estimated as 0.957  Vt, the proportionality factor being estimated from the data for LP-FDU-12, most of which were reported elsewhere.43,79 The pore size distribution (PSD) was calculated using the KJS method for cylindrical mesopores80 with the statistical film thickness curve for a macroporous silica gel.77 This implementation of the PSD calculation is known to underestimate the pore diameter for large spherical mesopores by at least 2 nm,19,43 but it reflects well the width of the mesopore size distribution and allows one to reliably compare the pore size among different samples with the same pore geometry. To obtain a more accurate assessment of the pore diameter, an equation relating the unit-cell parameter, a, the mesopore volume, and the micropore volume was used:81 6 Vp F wd ¼ a πv 1 þ Vp F þ Vmi F

!1=3 ð1Þ

(76) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619. (77) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410–5413. (78) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961– 1968. (79) Kruk, M.; Hui, C. M. In Hybrid Nanomaterials: Synthesis, Characterization, and Applications; Chauhan, B. P. S., Ed.; Wiley: New York, in press. (80) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267–6273. (81) Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 1550–1560.

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Figure 1. SAXS patterns for as-synthesized and calcined EH4 sample before (0 d) and after acid treatment for different periods of time. where a is determined from the position of (111) peak on SAXS pattern, F is the density of silica framework (assumed to be 2.2 g cm-3), and v = 4 is the number of spherical pores in the facecentered cubic unit cell.

Results and Discussion Identification of Xylene As Swelling Agent for Pluronic F127. Our work on LP-FDU-12 with face-centered cubic (Fm3m) structure of spherical mesopores synthesized in the presence of TMB indicated that the uptake of TMB by the Pluronic F127 micelles was small.43 Following the criteria of the selection of the micelle expanders54 outlined above, it was hypothesized that the use of a swelling agent that has a higher extent of solubilization in Pluronics than TMB would allow one to attain larger unit-cell parameters and pore diameters while maintaining the Fm3m structure. As the decrease in the number and size of alkyl substituents on the benzene ring is known to increase the solubilization of benzene derivatives in Pluronics,57 xylene was identified and evaluated as a swelling agent candidate. While xylene was used as a swelling agent for Pluronic P123 (EO20PO70EO20),82,83 silica mesocellular foams (MCFs) were obtained.82 As discussed elsewhere,54 TMB appears to solubilize excessively strongly in micelles of Pluronic P123, because it induces the formation of MCFs. Therefore, xylene that is expected to solubilize even to a greater extent is expected to cause poorly controlled swelling of Pluronic P123. Although xylene is not expected to be an appropriate swelling agent for P123 with ∼70 wt % of the hydrophobic domain, the template for FDU-12 synthesis (Pluronic F127) has only ∼30 wt % of the hydrophobic block, and thus an uptake of a hydrophobic swelling agent, such as xylene, is more limited,57 thus making xylene, a potent swelling agent, potentially suitable for improving the synthesis of LP-FDU-12. Indeed, as shown herein, the use of xylene allowed us to synthesize a new generation of face-centered cubic mesoporous silicas, which are referred to herein as ULP-FDU-12. (82) Choi, D.-G.; Yang, S.-M. J. Colloid Interface Sci. 2003, 261, 127–132. (83) Kirmayer, S.; Neyshtadt, S.; Keller, A.; Okopnik, D.; Frey, G. L. Chem. Mater. 2009, 21, 4387–4396.

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Synthesis of Fm3m Structure with Ultralarge Unit-Cell Size. The replacement of TMB by xylene in the synthesis analogous to that of LP-FDU-12 afforded a highly ordered face-centered cubic structure with large unit-cell parameter of 50 nm for as-synthesized and 45.6 nm for calcined material (see synchrotron SAXS data in Supporting Information Figure S1 and the comparison between LP-FDU-12 synthesized in the presence of xylene and TMB in Supporting Information Figure S2). Twelve well-resolved reflections characteristic of Fm3m structure were accurately identified for the as-synthesized sample, while the assignment of the last four peaks (indexes 551 and higher) for the calcined sample was somewhat less clear, although it can be done convincingly by referring to the data for the as-synthesized sample. It was found that the initial step of the synthesis should be performed in a semiclosed container (glass container with glass cover) and magnetic stirring, as the use of an open container with mechanical stirring tended to afford materials with lower unit-cell size or even those devoid of the structural ordering. This may be due to volatility of xylene. In the case of the semiclosed setup described above, the stirring speed needed to be optimized, and 350 rpm was found to be suitable to obtain materials with the largest unit-cell parameters (see below). The stirring was maintained throughout the low-temperature step of the synthesis. A series of samples was synthesized at a stirring speed of 350 rpm with different amounts of xylenes and fixed molar ratio of the other components: TEOS/F127/KCl/HCl/ H2O = 1.1:0.004:1.68:6:157. The low-temperature step of the synthesis was followed by heating of the synthesis mixture for 1 day at 100 °C. SAXS patterns and N2 adsorption isotherm for a selected sample (EH4) are shown in Figures 1 and 2, while the data for the entire series of samples are shown in Supporting Information Figures S3 and S4, and the structural parameters derived from these data are summarized in Table 1. The SAXS patterns resembled those shown in Supporting Information Figure S1, although they were less well resolved, which can be attributed, at least in part, to the use of a laboratory SAXS setup in high flux mode, which limits the resolution of peaks. With the increase in the amount of xylenes, the unit cell parameter of the face-centered cubic structure increased until the mass of xylenes was 6 g per 1 g of Pluronic F127. It should be noted that in all Langmuir 2010, 26(18), 14871–14878

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Figure 2. N2 adsorption isotherms and pore size distributions for calcined sample EH4 before (0 d) and after acid treatment for different periods of time. The adsorption isotherms for samples acid-treated at 130 °C for 2 and 4 days were shifted vertically by 400 and 900 cm3 STP g-1. Table 1. Structural Parameters of FDU-12 Silicasa unit cell parameter sample

mass of xylenes (g) per 1 g F127

aas (nm)

ac (nm)

ac/aas

wKJS (nm)

SBET (m2/g)

Vt (cm3/g)

EH1 3.7 52.2 43.1 0.83 23.1 367 EH2 3.9 52.6 44.3 0.84 24.1 307 EH3 4.1 54.5 44.6 0.82 23.5 80 EH4 4.5 55.2 45.3 0.82 23.5 130 EH5 5.2 55.8 45.9 0.82 23.5 249 EH6 6.0 55.8 45.9 0.82 23.6 157 EH7 6.9 51.3 41.9 0.82 22.0 271 a Composition of the synthesis mixture: 4.5 g (21.6 mmol) TEOS, 1 g F127, and 2.5 g KCl in 60 mL of 2 M HCl solution.

cases, the available amount of xylene had to be much higher than the actual uptake of xylene by the micelles of F127, because otherwise the pore volume generated in the final material would be much higher. The largest unit cell parameter (∼55.8 nm) obtained for as-synthesized samples EH5 or EH6 was larger than the unit-cell size reported for LP-FDU-12 synthesized in the presence of TMB33,43,67,74 and comparable to the unit-cell size reported for the face-centered cubic silica templated by PEO-PS diblock copolymer.75 After calcination at 550 °C, the shrinkage was large (the unit-cell parameter decreased by ∼18%), which is similar to the case of LP-FDU-12 silicas prepared in the presence of TMB.43 Even after this significant shrinkage, the unit-cell size was 45-46 nm for some of the samples, being higher than the unit-cell parameter for LP-FDU-12 silica33 with the template removed through the microwave digestion that is known to greatly reduce the structural shrinkage (unlike the calcination that was used for our samples considered here). The ordered structure was also confirmed using TEM (Figure 3). The specific surface areas and total pore volumes of the FDU-12 silicas were relatively low, especially for sample EH3, which indicates that some mesopores may have closed during the calcination at 550 °C due to the structural shrinkage that reduced the pore entrance diameter to such an extent that nitrogen adsorbate molecules were unable to access the pore interiors, which is known for similarly hydrothermally treated LP-FDU-12.27 Our previous study of LP-FDU-12 has shown43 that the acid treatment, which was introduced by Fan et al.33 and Kleitz et al.,84 (84) Yang, C.-M.; Schmidt, W.; Kleitz, F. J. Mater. Chem. 2005, 15, 5112–5114.

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0.51 0.37 0.16 0.24 0.40 0.29 0.41

Figure 3. TEM images of ULP-FDU-12: (a) calcined EH4 sample ([110] incidence); (b and c) calcined EH6-A130C4d sample ([100] and [110] incidence).

greatly reduces the shrinkage upon calcination, allowing one to significantly increase the total pore volume and pore diameter. Therefore, as-synthesized ULP-FDU-12 samples were treated in 2 M HCl solution at 100 or 130 °C for periods of time from 1 to 4 days. SAXS patterns for a selected series of as-synthesized and calcined samples, as well as nitrogen adsorption isotherms and PSDs are shown in Figures 1 and 2. The corresponding structural parameters for these and some additional acid-treated ULPFDU-12 silicas (see Supporting Information Figures S5 and S6) are listed in Table 2. After the acid treatment at 100 °C for 2 days, the shrinkage upon calcination at 550 °C decreased to 8%, while DOI: 10.1021/la102228u

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Huang et al. Table 2. Structural Parameters for FDU-12 Samples after the Acid Treatment unit cell parameter

sample

aas (nm)

ac (nm)

ac /aas

wKJS (nm)

SBET (m2/g)

Vt (cm3/g)

Vmi (cm3/g)

wd (nm)

EH4-A100C2d EH4-A130C2d EH4-A130C4d EH5-A100C2d EH5-A130C2d EH5-A130C4d

54.4 54.4 54.4 54.4 54.4 54.4

50.2 52.6 53.3 49.7 53.1 53.5

0.92 0.97 0.99 0.91 0.98 0.98

27.0 29.5 31.6 26.2 31.4 31.7

575 380 246 558 363 236

0.81 0.97 0.97 0.77 1.04 1.00

0.14 0.04 0.02 0.14 0.04 0.00

31.5 35.5 36.3 30.9 36.2 36.7

Figure 4. N2 adsorption isotherms (left) and pore size distributions (right) of E40 ULP-FDU-12 silicas calcined at different temperatures. The adsorption isotherms for the samples calcined at 300, and 350 °C were shifted vertically by 400 and 200 cm3 STP g-1.

the pore size and the total pore volume increased significantly. The N2 adsorption isotherm retained the same shape as before the acid treatment, which indicates the cage-like mesopores with narrow entrances (diameter below 5 nm in the narrowest point).85 For the samples treated at 130 °C, the shrinkage upon calcination at 550 °C decreased to 1-4%. Consequently, the pore size and the total pore volume of these samples were larger than those of the samples treated at 100 °C. The KJS pore diameter approached 32 nm. It should be noted that the original BdB work86 for spherical mesopores indicates a pore diameter of 43-46 nm for the capillary condensation at relative pressures of 0.9325-0.9375, which correspond to the largest capillary condensation relative pressures observed for our samples (0.9350.938). The use of eq 1 allows one to assess the pore diameters of 31-37 nm for our acid-treated samples (see Table 2). Equation 1 is expected to closely reflect the actual pore diameter, and therefore the BdB “spherical” pore diameters appear to overestimate and the KJS “cylindrical” pore diameters appear to underestimate the actual diameter of very large spherical mesopores. The hysteresis loops on nitrogen adsorption isotherms for the considered samples became narrower when the acid treatment was performed at 130 °C, and the narrowing became even more pronounced as the acid treatment was prolonged. This provides evidence of a significant pore entrance widening. In fact, the hysteresis loops observed for the samples subjected to the acid treatment at 130 °C for 4 days were so narrow that the pore entrance diameter was likely to approach the pore cage diameter. The specific surface area decreased as the temperature and time of acid treatment were increased, as already reported for LP-FDU-12.43 (85) Kruk, M.; Jaroniec, M. Chem. Mater. 2003, 15, 2942–2949. (86) Broekhoff, J. C. P.; De Boer, J. H. J. Catal. 1968, 10, 153–165.

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The acid-treated samples showed several strong peaks on their SAXS patterns, indicating that the ordered Fm3m structure was well preserved, although some reflections (for instance, (220) and (311)) were not well-separated in some cases and the relative intensity of peaks changed (which was also observed earlier).43,87 The latter can be attributed to changes in the relation between the mesopore diameter and the unit-cell parameter. TEM (Figure 3) also confirmed that the acid-treated materials had a highly ordered structure. ULP-FDU-12 prepared with TMOS. Tetramethyl orthosilicate (TMOS) is a faster hydrolyzing silica source when compared to TEOS. The same procedure as that introduced above for TEOS was used to prepare ULP-FDU-12 from TMOS. Structural parameters determined by SAXS and N2 adsorption are listed in Supporting Information Table S1. For comparison, samples MH1 and EH4 were prepared with the same molar composition of the synthesis mixture (except for the different silica source) and the same stirring speed. The sample prepared using TMOS exhibited a slightly smaller unit-cell parameter. The acid treatment effectively reduced the shrinkage caused by calcination and increased the specific surface area and total pore volume. SAXS patterns for these samples (Supporting Information Figure S7) revealed ordered cubic Fm3m structures (additionally confirmed by TEM), and N2 adsorption isotherms exhibited narrow pore size distributions (Supporting Information Figure S8). Effect of Salt and Use of Toluene. The synthesis procedures described above involved the use of an inorganic salt, KCl. It is known that the use of such additives facilitates the synthesis of copolymer-templated OMSs.88 However, it was found that (87) Schmidt, W. Microporous Mesoporous Mater. 2009, 117, 372–379. (88) Yu, C.; Tian, B.; Fan, J.; Stucky, G. D.; Zhao, D. Chem. Commun. 2001, 2726–2727.

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Article Table 3. Structural Parameters for E40 ULP-FDU-12 Silica Calcined at Different Temperatures unit cell parameter

sample

aas (nm)

ac (nm)

ac/aas

wKJS (nm)

SBET (m2/g)

Vt (cm3/g)

Vmi (cm3/g)

E40 -300C 49.7 41.7 0.84 19.8 458 0.48 0.14 49.7 40.3 0.81 19.4 378 0.41 0.12 E40 -350C 49.7 39.5 0.79 18.9 248 0.32 0.06 E40 -400C a 0 49.7 38.5 0.77 NA NA 0.27 NA E4 -450C a The pore volume was calculated through extrapolation. NA;not available from gas adsorption data due to the closed-pore nature of the material.

volume, and the pore diameter decreased (Table 3). The isotherm of the sample calcined at 450 °C indicated no accessible mesoporosity. On the other hand, the SAXS patterns (Figure 5) revealed that all the considered silicas had highly ordered facecentered cubic structures, whose unit-cell parameter decreased as the calcination temperature increased. The unit-cell parameter of the as-synthesized E40 ULP-FDU-12 was 49.7 nm, whereas the samples calcined at 300 to 450 °C exhibited unit-cell sizes from 41.7 to 38.5 nm. An approximately linear relationship is shown in Supporting Information Figure S9 between the total pore volume and the unit cell volume of E40 ULP-FDU-12 silica calcined at different temperatures. The total pore volume of the closed-pore sample, E0 -450C, estimated by extrapolation79 was 0.27 cm3/g. It was thus demonstrated that closed-pore OSSM with an appreciable volume of ordered mesopores can be prepared using a one-step procedure without a hydrothermal treatment and the transition to closed-pore materials takes place at as low as 450 °C. In an earlier study, the pore closing of LP-FDU-12 prepared with hydrothermal treatment was observed at 550-640 °C, which is at least 100 °C higher.27 While LP-FDU-12 synthesized without hydrothermal treatment was a closed-pore material after calcination at 550 °C,27 the exact pore closing temperature was not investigated.

Conclusions

Figure 5. Small angle X-ray scattering patterns for E40 ULPFDU-12 silica calcined at different temperatures.

large-pore Fm3m structures form in the absence of a salt when F127 and xylene are used. Based on the considerations outlined above, toluene is expected to be suitable as a swelling agent for the synthesis of ultralarge-pore FDU-12. While the use of toluene instead of xylene afforded silicas with large unit-cell sizes, the latter were smaller than those attained with xylene. Reasons for this behavior are not clear at present, and may include the excessive volatility of toluene in our semiclosed (but not airtight) setup. Synthesis of Closed-Pore Silicas. Figure 4 shows N2 adsorption isotherms for ULP-FDU-12 silica prepared using xylene in the same manner as the sample EH4, but at 14 °C only (without further heating of the synthesis mixture). It is noteworthy that the as-synthesized sample E40 prepared without the hydrothermal treatment had somewhat lower unit-cell size than the as-synthesized sample EH4 prepared with the hydrothermal treatment (compare Tables 1 and 3). A similar behavior was already observed for LP-FDU-12 and related materials.43,75 With the increase in the calcination temperature in the 300-400 °C range, the specific surface area, the total pore volume, the micropore Langmuir 2010, 26(18), 14871–14878

The selection of swelling agents for the surfactant-templated synthesis of ordered mesoporous materials can be based on the data for the extent of solubilization of compounds in surfactant micelles and their extrapolation within families of compounds. The solution data (or predictions based on them) can be considered along with results of surfactant-templated syntheses involving known micelle expanders to identify suitable swelling agent candidates by assessing whether a more strongly solubilized (“stronger”) or a more weakly solubilized (“weaker”) micelle expander is needed to achieve the desired pore size expansion without the loss of structural ordering. The judicious selection of xylene as a “stronger” swelling agent (in comparison to commonly used TMB) for the low-temperature synthesis templated by Pluronic F127 micelles allowed us to extend the range of the unitcell parameters achievable in ordered mesoporous silicas templated by commercially available surfactants. The resulting ultralarge-pore silicas exhibit significant shrinkage during calcination, which can be greatly reduced by the acid treatment. The selection of acid treatment conditions allows one to generate silicas with a range of pore diameters from a single synthesis mixture. The pore entrance sizes can be varied in a wide range. Highly ordered closed-pore ordered mesoporous silica with appreciable pore volume (as evaluated by extrapolation) can be synthesized in a one-step low-temperature synthesis followed by calcination at as low as 450 °C. Acknowledgment. NSF is gratefully acknowledged for partial support of this research (award DMR-0907487) and for funding the acquisition of the SAXS/WAXS system through award CHE0723028. Acknowledgment is made to the Donors of the American DOI: 10.1021/la102228u

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Chemical Society Petroleum Research Fund for partial support of this research (Award PRF No. 49093-DNI5). Some SAXS measurements were performed at the Cornell High Energy Synchrotron Source (CHESS, Cornell University), which is supported by the National Science Foundation under award DMR-0225180. Dr. Detlef M. Smilgies (CHESS, Cornell University) is gratefully acknowledged for assistance in the SAXS measurements at CHESS. The Imaging Facility at CSI is acknowledged for providing access to TEM. Mr. Letian Lin (University of North Carolina at Chapel Hill) is acknowledged for preliminary TEM imaging of

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several samples. BASF is acknowledged for the donation of the F127 block copolymer. Supporting Information Available: Figures with experimental SAXS patterns, nitrogen adsorption isotherms, and pore size distributions derived from them, as well as the relation between the pore volume and the unit-cell volume; table with structural parameters derived from SAXS and nitrogen adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org.

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