Stable Ordered Mesoporous Silica Materials Templated by High

Ordered hexagonal mesoporous silica material (JLU-30) has been successfully synthesized in alkaline media at high temperature (>160 °C), using cation...
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4696

J. Phys. Chem. B 2004, 108, 4696-4700

Stable Ordered Mesoporous Silica Materials Templated by High-Temperature Stable Surfactant Micelle in Alkaline Media Xiaoyu Yang,† Suobo Zhang,‡ Zhiming Qiu,‡ Ge Tian,† Yefei Feng,† and Feng-Shou Xiao*,† College of Chemistry & State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, Jilin UniVersity, Changchun 130023, China, and State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy Sciences, Changchun 130022, China ReceiVed: December 27, 2003

Ordered hexagonal mesoporous silica material (JLU-30) has been successfully synthesized in alkaline media at high temperature (>160 °C), using cationic (1,3-dimethyl-2-imidazolidin-2-ylidene)hexadecylmethylammonium bromide (DIHAB) as a template, and characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption isotherms, differential thermal analysis (DTA), and thermogravimetric analysis (TG), as well as 27Al and 29Si nuclear magnetic resonance (NMR) spectroscopy. Mesoporous JLU-30 shows much higher hydrothermal stability than MCM-41. 29Si NMR spectra indicate that the pore walls of JLU-30 samples synthesized at high temperature (160 °C) are fully condensed, giving a Q4/Q3 ratio as high as 6.2. In contrast, MCM-41 synthesized at relatively low temperature (100 °C) shows the Q4/Q3 + Q2 ratio at 1.1. Such unique structural feature might be responsible for the observed highly hydrothermal stability of the mesoporous silica materials (JLU-30). Furthermore, heteroatoms such as Al and Ti species have been easily introduced into the mesoporous walls of JLU-30 by using a simple mixture of TEOS with aluminum or titanium source in the initial reaction gel.

Introduction Mesostructured materials with well-ordered and uniform mesopores have been given much attention due to their potential applications in adsorption and catalysis, but inadequate hydrothermal stability of mesostructured materials such as MCM-41 greatly limits their wide uses.1,2 Recently, there have been a number of successful examples of the preparation of mesoporous materials that have reasonably good hydrothermal stability.1-14 For example, mesoporous SBA-15 with thicker pore walls has been prepared by using triblock copolymers as templates.4,5 Vesicle-like MSU-G materials with a high SiO4 cross-linking have been achieved by using neutral gemini surfactants as the templates.6 The preparation of disordered KIT-1 involves the use of inorganic and organic salts as additives.7 Assembly of preformed zeolite nanoclusters or seeds solution with surfactant micelle results in formation of ordered mesoporous aluminosilicates with zeolite primary and secondary building units.8-11 Through the grafting route, some stable mesoporous aluminosilicates have been obtained.12,13. More recently, high-temperature (150-220 °C) synthesis of ordered mesoporous silica-based materials, using fluorocarbonhydrocarbon surfactant mixtures, has been reported. The hightemperature synthesis of mesoporous silica-based materials led to more silica condensation on the mesoporous walls than relatively low-temperature synthesis (150 °C) stable surfactant micelles, such as cationic phase transfer catalysts and cationic modified ionic liquids,15-18 has been reported. Similar to cationic surfactants such as cetyltrimethylammonium bromide (CTAB), they have hydrophilic heads and hydrophobic chains. We demonstrate here that when a cationic phase transfer catalyst, namely (1,3-dimethyl-2-imidazolidin-2-ylidene)hexadecylmethylammonium bromide18 (DIHAB, Figure 1), is used as a template, an ordered mesoporous silica material with unusual hydrothermal stability, designated JLU-30, is successfully synthesized in alkaline media at relatively high temperatures (160-180 °C). DIHAB has exceptional stability at high temperature because the positive charge in DIHAB is delocalized over one carbon and three nitrogen atoms, which renders them high degree of thermal stability compared with that of tetraalkylammonium salts.17,18 Therefore, DIHAB would seem to be a good candidate for a high-temperature stable template. Experimental Section In a typical synthesis of JLU-30, DIHAB and tetra-methylammonium (TMAOH) were added to water, followed by addition of tetraethyl orthosilicate (TEOS). After being stirred for 8 h, the gel mixture with a molar ratio of SiO2/DIHAB/ TMAOH/H2O at 1/0.35/0.2/50 was aged at room temperature for 24 h. Then, the gel was transferred to a Teflon-lined stainless

10.1021/jp0380226 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/19/2004

Synthesis of Hexagonal Mesoporous Silica Material

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Figure 1. The structure of (1,3-dimethyl-2-imidazolidin-2-ylidene)hexadecylmethylammomium bromide (DIHAB).

Figure 2. XRD patterns of as-synthesized JLU-30 (A), calcined JLU-30 (B), and JLU-30 treated in boiling water for 60 (C) and 80 h (D); XRD patterns of calcined MCM-41 (E), MCM-41 treated in boiling water for 40 h (F), calcined JLU-30(100) (G), and JLU-30(100) treated in boiling water for 40 h (H).

steel autoclave for crystallization at 100 °C for 24 h. After further crystallization at 160 °C for 48 h, the solid product was filtered, washed with water, and dried in air for 12 h. Calcination of the product was carried out at 550 °C for 8 h in flowing oxygen to remove organic templates such as DIHAB. For a comparison, ordered mesoporous silica denoted as JLU-30(100) was prepared at 100 °C with the same procedures as for JLU30, except for a higher crystallization temperature (160 °C). Preparation of MCM-41 was synthesized according to a literature procedure.1 In the preparation of DIHAB,18,19 cyclic pentaalkylguanidine was prepared by the reaction of a corresponding urea with phosphorus oxychloride to yield a chloroformamidinium salt, frequently referred to as a “Vismeier salt”, followed by reaction of said salt with MeNH2. The reaction of cyclic pentaalkylguanidine with C16H33Br in acetonitrile gave a high yield (95%) of the final product (DIHAB). Ti- or Al-substituted mesoporous silica (Ti-JLU-30 or AlJLU-30) was prepared from the molar ratio of 5Al2O3(or TiO2)/ 30SiO2/16[TMA]2O/45DIHAB/5000H2O. In a standard run, sodium aluminate or Ti(OC4H9)4 was dissolved in tetraethylammonium hydroxide (TMAOH) solution. The resulting solution was slowly added to TEOS together with the DIHAB and excess deionized water. After being stirred for 8 h, the gel mixture was aged at room temperature for 24 h. Then, the gel was transferred to a Teflon-lined stainless steel autoclave for crystallization at 100 °C for 24 h. After further crystallization at 160 °C for 48 h, the solid product was filtered, washed with water, and dried in air for 12 h. Finally, calcination of the product was carried out at 550 °C for 8 h in flowing oxygen to remove the organic templates. X-ray diffraction patterns (XRD) were obtained with a Siemens D5005 diffractometer using Cu KR radiation. Transmission electron microscopy (TEM) experiments were performed on a JEM-3010 electron microscope (JEOL, Japan) with

an acceleration voltage of 300 kV. The nitrogen adsorption and desorption isotherms at the temperature of liquid nitrogen were measured with a Micromeritics ASAP 2010M system. The samples were outgassed for 10 h at 300 °C before the measurements. The pore-size distribution for mesopores was calculated by using the Barrett-Joyner-Halenda (BJH) model. 29Si and 27Al NMR spectra were recorded on a Bruker MSL400WB spectrometer, fitting the samples in a 7 mm ZrO2 rotor, spinning at 8 kHz. A Perkin-Elmer TGA 7 unit was used to carry out the thermogravimetric analysis (TGA) in air at a heating rate of 20 °C/min. Results and Discussion Figure 2 shows XRD patterns for MCM-41, JLU-30(100), and JLU-30 materials before and after hydrothermal treatments. Notably, as-synthesized JLU-30 (Figure 2A) exhibits clearly well-resolved peaks that can be indexed as the (100), (110), (200), and (210) diffractions associated with the p6mm hexagonal symmetry with a lattice constant a ) 71.6 Å. After calcination in air at 550 °C for 8 h, the sample XRD pattern (Figure 2B) shows that the four diffraction peaks are still present, confirming that hexagonal JLU-30 is thermally stable. Interestingly, after treatment of the calcined sample in boiling water for 60 (Figure 2C) and 80 h (Figure 2D), the sample XRD patterns also show those peaks assigned to the hexagonal symmetry. In contrast, upon hydrothermal treatment in boiling water for 60 h, both JLU-30(100) and MCM-41 lose most of their mesostructures (Figure 2F,H and Table 1). These results indicate that JLU-30 sample synthesized at high temperature has much higher hydrothermal stability than that of conventional mesoporous silica synthesized at relatively low temperature. This may be attributed to the fact that the pore walls of JLU-30 are fully condensed at relatively high temperature. The TEM image of calcined JLU-30 (Figure 3) exhibits ordered hexagonal arrays of mesopores with one-dimensional

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Figure 3. TEM images of calcined JLU-30 taken in the (100) and (110) directions and the corresponding Fourier diffractogram (inset).

Figure 4. N2 isotherms and the corresponding pore size distribution (inset) of calcined JLU-30 before (A) and after treatment in boiling water for 80 h (B). N2 isotherms and pore size distribution of calcined JLU-30(100) before (C) and after treatment in boiling water for 60 h (D). The isotherms B and D are offset from 250 cm3/g and 700 cm3/g at the beginning for clarity, respectively. The corresponding pore size distribution curve (insert) is obtained from the adsorption branch of the isotherm, using the BJH method.

TABLE 1: Physical Parameters of Mesoporous Silica Materials Including JLU-30, MCM-41, and JLU-30(100)

sample JLU-30 calcined treatedc treatedd MCM-41 calcined treatedc JLU-30(100) calcined treatedc

surface wall d(100) pore size pore vol area a0a thicknessb (Å) (Å) (cm3/g) (m2/g) (Å) (Å) 62 61.8 61.2 61 44 43.5 44.3 44.1 62

53 54 55

0.68 0.61 0.60

510 472 459

39.2

0.88 0.35

891 220

40.1

0.82 0.41

810 310

71.6 71.2 70.7 70.4 50.8 50.2

11

51.2 50.9

10.8

18.2 16.7 15.4

a a (lattice parameter) ) 2d 1/2 b 0 100/3 . Wall thickness ) a0 - pore size. c Boiling water for 60 h. d Boiling water for 80 h.

channels and further confirms that JLU-30 has a 2-D hexagonal (P6mm) mesostructure. The N2 adsorption isotherms of calcined JLU-30, JLU30(100), and MCM-41 (Figure 4, Figure 2 in the Supporting Information, and Table 1) exhibit a typical adsorption curve of type IV. The steps can be identified in the adsorption curve at a relative pressure 0.5 < P/P0 < 0.7 and 0.3 < P/P0 < 0.4 for JLU-30 and MCM-41, which is due to the characteristic of

mesopores. After hydrothermal treatment in boiling water for 60 h, there is only a 7% decrease in BET surface area (from 510 m2/g to 472 m2/g) for JLU-30 whereas the decrease is 75% (from 891 m2/g to 220 m2/g) for MCM-41. Moreover, the isotherm of the treated JLU-30 sample is still a typical IV isotherm (Figure 4A), implying the preservation of the uniform mesopores. Its pore size distribution curve is as sharp as that of the untreated sample (Figure 4A, inset). In contrast, the treated MCM-41 shows a poor isotherm (Figure 2B in the Supporting Information) with an undiscernible pore size distribution (Figure 2B, inset, in the Supporting Information). Correspondingly, the primary mesopore volume of JLU-30 is reduced by only 10% from 0.68 cm3/g to 0.61 cm3/g while that of MCM-41 is reduced by 60% from 0.88 cm3/g to 0.35 m2/g (Table 1). All of these results confirm that JLU-30 exhibits much higher hydrothermal stability than MCM-41. From XRD and N2 adsorption data, the wall thickness and mesopore size of JLU-30 are calculated to be 18 Å and 53 Å, and those of MCM-41 are 11 Å and 39 Å, respectively (Table 1). Obviously, the walls of JLU-30 are much thicker than those of MCM-41, in good agreement with the results obtained from high-dark contrast in the sample TEM image (Figure 3). This is reasonably assigned to the high temperature of the synthesis,

Synthesis of Hexagonal Mesoporous Silica Material because JLU-30(100) synthesized at 100 °C shows similar pore size and wall thickness to those of MCM-41. It is interesting to note that calcined JLU-30 has a relatively low surface area (510 m2/g) and pore volume (0.68 cm3/g) when compared with MCM-41. However, JLU-30 strictly fulfills the fundamental relation between the structural parameters for materials with uniform pores of simple cylindrical geometry: wS/V ≈ 4 (w, S, and V denote pore size, surface area, and pore volume, respectively).20 This suggests that like MCM-41, JLU30 is also free of micropores in the mesoporous walls, which is confirmed by N2 adsorption isotherm t-plots (Supporting Information, Figure 3). Possibly the relatively low surface area of JLU-30 is assigned to thicker mesoporous walls, compared with that of MCM-41. Furthermore, the 29Si MAS NMR spectrum of as-synthesized JLU-30 (Supporting Information, Figure 4) provides direct evidence of the extent of silica condensation. JLU-30 is primarily made up of fully condensed Q4 silica units (-112 ppm) with a small contribution from incompletely cross-linked Q3 (-102 ppm) as deduced from the very high Q4/Q3 ratio of 6.2 while no Q2 units were observed. In contrast, MCM-41 has typical peaks correspond to Q2, Q3, and Q4 silica species respectively, and the ratio of Q4/Q3 + Q2 is 1.1 (Table 1), indicating the presence of large amounts of terminal hydroxyl group in the mesoporous walls, in good agreement with published results.13 Obviously, full condensation of the walls would increase hydrothermal stability of mesoporous materials greatly.13 The complete silica condensation and high stability of JLU30 should be attributed directly to the high-temperature synthesis rather than other reasons such as DIHAB surfactant. If JLU-30 is prepared at 100 °C instead of at 160 °C, it shows no difference in both structural properties (surface area, pore volume, and Q4/ Q3 ratio) and hydrothermal stability with conventional MCM41 although DIHAB is used. Interestingly, the use of DIHAB surfactant allows full condensation via a high-temperature synthesis. This method is not limited to the DIHAB surfactant, and many cationic phase transfer catalysts and cationic ionic liquids can be used if they effectively form a regular micelle in solution with suitable interactions between the chosen organic template and inorganic species. Moreover, this method also can be extended to prepare ordered mesoporous materials with various mesostructures such as cubic Ia3d, cubic Pm3n, and three-dimension hexagonal P63/mmc in alkaline media at high temperature if the suitable stable micelle is chosen. These studies are under investigation. Additionally, under alkaline media (pH 11), heterogeneous atoms of Al and Ti substituted materials, with the goal of introducing catalytically active sites, have been easily introduced into the mesoporous walls of JLU-30 by using a simple mixture of TEOS with aluminum or titanium source in the initial reaction gel. For example, the 27Al MAS NMR spectrum of the as-synthesized Al-JLU-30 shows a strong signal centered near 53 ppm (Supporting Information, Figure 5), indicating that Al species have been successfully incorporated into the mesoporous walls with 4-coordinated number, which suggests that AlJLU-30 may be used as an acidic catalyst and an ion exchanger. Particularly, after calcination at 550 °C for 5 h, in addition to a very small peak at 0 ppm associated with octahedral Al, the peak assigned to tetrahedral Al is almost unchanged, indicating high thermal stability of tetrahedral Al sites in JLU-30. Furthermore, we observed that the mesostructured order of heteroatom-substituted JLU-30 is reduced significantly, as compared with pure silica JLU-30. This is a sign that the

J. Phys. Chem. B, Vol. 108, No. 15, 2004 4699 mesostructures become less uniform upon the introdction of heteroatoms into the walls, in very good agreement with a previous report.21 For example, Ti-JLU-30 exhibits only two peaks associated with hexagonal mesostructure indexed as (100) and (110) diffractions (Supporting Information, Figure 6). In contrast, JLU-30 exhibits four well-resolved peaks associated with hexagonal mesostructure indexed as (100), (110), (200), and (210) diffractions (Figure 2A). Interestingly, after introduction of heteroatoms into pure silica materials, both Al-JLU-30 and Ti-JLU-30 exhibit much higher hydrothermal stabilities than pure silica JLU-30. This is due to combining the advantages of high-temperature synthesis14 and incorporation of heteroatoms.22 Conclusions Ordered hexagonal mesoporous silica material (JLU-30) has been successfully synthesized in alkaline media at high temperature (>160 °C) with use of cationic (1,3-dimethyl-2imidazolidin-2-ylidene)hexadecylmethylammonium bromide (DIHAB) as a template. Mesoporous JLU-30 shows much higher hydrothermal stability than that of MCM-41. 29Si NMR spectra indicate that the pore walls of JLU-30 samples synthesized at high temperature (>160 °C) are fully condensed, giving a Q4/ Q3 ratio as high as 6.2, which would be responsible for the observed highly hydrothermal stability of the mesoporous silica materials (JLU-30). Furthermore, heteroatoms such as Al and Ti species have been easily introduced into the mesoporous walls of JLU-30 by using a simple mixture of TEOS with aluminum or titanium source in the initial reaction gel. Acknowledgment. This work is supported by NSFC, CNPC, the National High Technology Research and Development Program of China (863 Program), and State Basic Research Project (973 Program). Supporting Information Available: Figures showing the synthetic steps for DIHAB; N2 isotherms and pore size distribution of calcined MCM-41; t-plots of calcined JLU-30 for the N2 adsorption isotherm; 29Si NMR spectrum of assynthesized JLU-30; 27Al NMR spectrum of as-synthesized and calcined Al-JLU-30; and the XRD pattern of calcined TiJLU-30. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Corma, A. Chem. ReV. 1997, 97, 2373. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 352, 710. (3) Beck J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, C. T.; 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. (4) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science. 1998, 279, 548. (5) Janssen, A. H.; Van Der Voort, P.; Abraham, J.; Kosterc, A. J.; de Jong, K. P. Chem. Commun. 2002, 1632. (6) Kim, S. S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 1032. (7) Ryoo, R.; Kim, J. M.; Shin, C. H. J. Phys. Chem. 1996, 100, 17718. (8) (a) Liu, Y.; Zhang, W.; Pinnavaia, T. J. J. Am. Chem. Soc. 2000, 122, 8791. (b) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 2001, 40, 1255. (9) Zhang, Z.; Han, Y.; Zhu, L.; Wang, R.; Yu, Y.; Qiu, S.; Zhao, D.; Xiao, F.-S. Angew. Chem., Int. Ed. 2001, 40, 1258. (10) Han, Y.; Wu, S.; Sun, Y.; Li, D.; Xiao, F.-S. Chem. Mater. 2002, 14, 1144. (11) Xiao, F.-S.; Han, Y.; Meng, X.-J.; Yu, Y.; Yang, M.; Wu, S. J. Am. Chem. Soc. 2002, 124, 888. (12) (a) Mokaya, R. Chem. Commun. 1998, 1839. (b) Mokaya, R. Angew. Chem., Int. Ed. Engl. 1999, 38, 2930.

4700 J. Phys. Chem. B, Vol. 108, No. 15, 2004 (13) (a) Mokaya, R. J. Phys. Chem. B 1999, 103, 10204. (b) Mokaya, R. Chem. Commun. 2001, 633. (14) Han, Y.; Li, D. F.; Zhao, L.; Song, J. W.; Yang, X. Y.; Li, N.; Li, C. J.; Wu, S.; Xu, X. Z.; Meng, X. J.; Lin, K. F.; Xiao, F.-S. Angew. Chem., Int. Ed. 2003, 42, 3633. (15) Wasserscheid, P.; Keim, W. Angnew. Chem., Int. Ed. 2000, 39, 3772. (16) Ionic Liquids in Synthesis; Wasserscheid, P., Weltion, T., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (17) Brunelle, D. J.; Haiko, D. A.; Barren, J. P.; Singh, S. U.S. Patent 5,132,423. (18) Duan, H. F.; Lin, Y. J.; Zhang, S. B.; Qiu, Z. M.; Wang, Z. M. Gaodeng XuexiaoHuaxue Xuebao 2003, 24, 2024. (19) The details for preparation and characterization of DIHAB are as follows (Supporting Information, Figure 1): (1) POCl3 (50.5 g, 0.33 mol) was added to a solution of 1,3-dimethy-2-imidazolidinone (33.6 g, 0.29 mol) in 30 mL of toluene. After the mixture was stirred at 60 °C for 24 h, excess methylamine was passed through the solution at 0 °C, then the

Yang et al. solution was refluxed for 18 h. The solution was treated with 120 mL of aqueous 37% sodium hydroxide and extracted with methylene chloride (4 × 40 mL). The organic phase was dried, filtered, evaporated, and distilled under redued pressure to yield 1 (34.1 g, 91.4%). 1H NMR (CDCl3) δ 2.82 (6H, s, 2CH3), 3.14 (3H, s, CH3), 3.17 (4H, s, 2CH2). (2) A mixture of 1 (12.7 g, 0.10 mol) and hexadecyl bromide (36.6 g, 0.12 mol) was refluxed in 100 mL of acetonitrile for 6 h. Then the solution was evaporated; the crude product was recrystallized by acetonituile-ethyl acetate to give 2 (41.7 g, 96.5%). 1H NMR (D2O) δ 0.65 (3H, t, J ) 6 Hz, CH2-CH3), 1.00-1.23 (28H, m, CH2), 2.82 (9H, s, N-CH3), 3.13 (2H, t, J ) 6 Hz, N-CH2), 3.55 (4H, s, N-CH2-CH2-N); 13C NMR (D2O) δ 14.08 (CH2CH3), 27.76-32.22 (14CH2), 36.43 (2N-CH3), 37.52 (N-CH3), 49.73 (NCH2-CH2-N), 52.49 (N-CH2). (20) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuk, M.; Jaroniec, M. J. Phys. Chem. B. 2000, 104, 11465. (21) Shen, S.-C.; Kawi, S. J. Phys. Chem. B 1999, 103, 8870. (22) Han, Y.; Xiao, F.-S.; Sun, Y. Y.; Meng, X. J.; Li, D. S.; Lin, S.; Deng, F.; Ai, X. J. J. Phys. Chem. B 2001, 105, 7963.