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High-temperature synthesis of stable ordered mesoporous silica materials using mesoporous carbon as a hard template
Microporous and Mesoporous Materials 86 (2005) 81–88 www.elsevier.com/locate/micromeso
High-temperature synthesis of stable ordered mesoporous silica materials using mesoporous carbon as a hard template Lifeng Wang, Kaifeng Lin, Yan Di, Daliang Zhang, Caijin Li, Qing Yang, Chengyang Yin, Zhenhua Sun, Dazhen Jiang, Feng-Shou Xiao * State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University, Changchun 130012, China Received 18 March 2005; received in revised form 17 July 2005; accepted 19 July 2005 Available online 26 August 2005
Abstract Replicated mesoporous silica materials from CMK-3 (denoted as RSC-3-X, X stands for synthesis temperature) have been successfully synthesized in acidic media at high temperatures (160–240 C) and subsequently characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen sorption isotherms, and 29Si nuclear magnetic resonance (NMR) spectroscopy. XRD patterns and nitrogen sorption isotherms exhibit the highly hydrothermal stable mesostructure of RSC-3 samples. Transmission electron microscopy images of RSC-3-220 show ordered hexagonal arrays of mesopores with 1D channels and further confirm the 2D hexagonal (p6mm) mesostructure of RSC-3-220. 29Si MAS NMR spectra indicate that as-synthesized RSC-3 samples are primarily made up of fully condensed Q4 silica units (d = 112 ppm) with a small contribution from incompletely cross-linked Q3 (d = 102 ppm) as deduced from the high Q4/Q3 ratio of 5.5–9.5, implying the fully condensed walls of RSC-3. Such unique structural features should be attributed directly to the high-temperature synthesis, which could be responsible for the observed highly hydrothermal stability of RSC-3. 2005 Elsevier Inc. All rights reserved. Keywords: High-temperature synthesis; Mesoporous silica; Mesoporous carbon; Hard template; Hydrothermal stability
1. Introduction Mesoporous materials have attracted much attention because of their potential use as versatile catalysts and catalyst supports for conversion of large molecules. However, as compared with conventional zeolites, mesostructured materials have relatively low hydrothermal stability, which severely hinders their practical applications in catalytic reactions for the petroleum industry [1,2]. To improve the hydrothermal stability of mesostructured materials, couples of synthetic routes have been reported, such as mesoporous MCM-41 with more
*
Corresponding author. Tel.: +86 431 5168590; fax: +86 431 5168624. E-mail address: [email protected] (F.-S. Xiao). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.07.019
highly condensed pore walls synthesized by increasing crystallization time [3], ordered hexagonal SBA-15 with thicker walls prepared in strongly acidic media (pH < 0) [4], vesicle-like MSU-G materials with a high SiO4 crosslinking templated by neutral gemini surfactants [5], disordered KIT-1 obtained by using inorganic salts as additives [6], and stable mesoporous aluminosilicates synthesized through a grafting route [7,8], or from zeolite seeds and preformed zeolite nanoclusters [9–13]. Among above routes, a critical factor in improving hydrothermal stability is the highly condensed silica walls [5,7,8]. It can be expected that the level of silica condensation will be enhanced by increasing the crystallization temperature [3]. However, mesostructured materials prepared at relatively low temperatures (80–150 C) generally exhibit imperfectly condensed walls with large amounts of terminal hydroxyl groups, resulting in the
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unstable mesostructure in hydrothermal conditions [14,15]. Recently, there have been the syntheses of stable mesoporous silica-based materials at the temperatures of 160–220 C using high-temperature stable surfactants of fluorocarbon–hydrocarbon surfactant mixtures and cationic modified ionic liquids as templates [16–18]. However, it is no way to prepare ordered mesoporous materials at much higher temperatures yet (>220 C). It is well known that carbon materials are generally high-temperature stable, even up to 300 C. Interestingly, the discovery of ordered mesostructured carbon materials [19–26] with large surface area and tunable pore sizes offers a novel synthetic route to ordered mesoporous materials [27–32]. For examples, ordered hexagonal mesoporous silica was successfully synthesized using Na2SiO3 as a silica precursor and CMK-3 as a template [27]; ordered hexagonal mesoporous silica (NCS-n) was effectively nanocast by TEOS into CMK3 [28–30]; ordered cubic mesoporous silica (HUM-1) was particularly templated from mesoporous carbon of CM48T-C [31]; ordered mesoporous aluminosilicates (RMM-n) were completely prepared from ordered mesoporous carbons and preformed aluminosilicate precursors [32]. Notably, the temperatures for all the hydrothermal syntheses are less than 100 C. We demonstrate here that, when an ordered mesoporous carbon (CMK-3) is used as the template, ordered mesoporous silica materials with unusual hydrothermal stability are successfully synthesized in strongly acidic media at high temperatures (160–240 C).
2. Experimental section 2.1. Materials Triblock copolymer P123 (EO20PO70EO20) was supplied from BASF Co., and TEOS, NaOH, HF, HCl, sulfuric acid, sucrose and ethanol were purchased from Beijing Chemical Co. (China).
dried in air and calcined at 600 C for 6 h to remove the template of CMK-3. The final product is denoted as RSC-3-X (X stands for synthesis temperature). After characterization of these samples by XRD and N2 isotherms, it was found that the sample with ratio of SiO2/C at 1.7 showed the most ordered mesostructure in all samples. It is mentionable that the high temperatures in hydrothermal synthesis are not limited to 240 C, and much higher temperatures can be carried out, because ordered mesoporous carbon materials are generally stable at higher temperatures than 240 C. However, due to the use of polytetrafluoroethylene (PTFE) in autoclaves, we cannot heat the autoclaves over 240 C. If we choose suitable autoclaves, the hydrothermal synthesis of ordered stable mesoporous silica from mesoporous carbon at the temperatures higher than 240 C could be carried out. SBA-15 and CMK-3 were synthesized according to the literatures [4,22]. 2.3. Characterization X-ray diffraction patterns (XRD) were obtained with a Siemens D5005 diffractometer using CuKa radiation. Transmission electron microscopy 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 using a Micromeritics ASAP 2020 M system. The samples were outgassed for 10 h at 300 C before the measurements. MAS NMR measurements were performed on a Varian Infinity plus 400 spectrometer at a rotation frequency of 4 kHz. The corresponding Larmor frequency was 79.4 MHz for 29Si nuclear. Free induction decays (FID) were recorded with 45 flip angle preparation pulses and 5.0 s recycle delay. The deconvolution method to resolve the Q3 and Q4 signals was supported by software in Varian Infinity plus 400 spectrometer. Chemical analysis for carbon element was performed on a Perkin–Elmer 2400 element analyzer.
2.2. Synthesis 3. Results and discussion As a typical run, replicated mesoporous silica from mesoporous carbon of CMK-3 was synthesized as follows: (1) 5 mL of tetraethyl orthosilicate (TEOS) were dissolved in 10 mL of ethanol, followed by adding 1 g of CMK-3. The mixture was subjected to aging at 40 C for 48 h to form silica/CMK composite. (2) The procedure was repeated until the desired amount (SiO2/C mass ratios of 1.3, 1.5, 1.7, 1.9, and 2.0 using 0, 0.75, 1.5, 2.25, and 2.63 mL of TEOS, respectively) of silica was reached. The resultant composite then crystallized in the presence of HCl vapor (1 M) at various temperatures (160–240 C) for 48 h. The product was
3.1. X-ray diffraction (XRD) Fig. 1 shows XRD patterns of as-synthesized, calcined and hydrothermally treated samples of replicated mesoporous silica from CMK-3 at synthetic temperature of 180 C (RSC-3-180), replicated mesoporous silica from CMK-3 at synthetic temperature of 220 C (RSC-3-220), and SBA-15. Compared with CMK-3 (Supporting Fig. 1), RSC-3-180 and RSC-3-220 samples exhibit a relatively weak peak (Fig. 1a-A and b-A), indicating that the silica sources have been successfully
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Fig. 1. XRD patterns of (A) as-synthesized samples, (B) calcined samples, and (C) treated samples in boiling water for 80 h. Samples include (a) RSC-3-180, (b) RSC-3-220, and (c) SBA-15.
nanocast into the mesopores of CMK-3. After calcination at 600 C for 6 h to remove the CMK-3 template, both samples (Fig. 1a-B and b-B) show clearly three peaks that can be indexed as the (1 0 0), (1 1 0) and (2 0 0) diffractions associated with the p6mm hexagonal symmetry. Obviously, peak intensities of calcined samples increase significantly, compared with those of as-synthesized samples. These results confirm that the mesostructure of CMK-3 has been successfully replicated into RSC-3 samples. Moreover, no significant changes in the basal spacing (d1 0 0) between as-synthesized and calcined RSC-3 samples are observed, suggesting their good thermal stabilities. Whereas, the unit cell ˚ during the same calcination of SBA-15 contracts 8 A (Fig. 1c-B). Furthermore, the wide-angle XRD patterns in the region of 6–50 for RSC-3 samples show a very broad peak as that of SBA-15, indicating the amorphous state of the silica walls (Supporting Fig. 2). Moreover, RSC-3 samples were investigated by a Perkin–Elmer 2400 element analyzer, and obtained results showed that the carbon was almost absent in all of calcined samples (<0.1 wt.%), indicating the complete removal of the carbon template. Very interestingly, after treatment in boiling water for 80 h, the XRD patterns of RSC-3 samples (Fig. 1a-C and b-C) still retain those peaks associated with the p6mm hexagonal symmetry. In contrast, after the same hydrothermal treatment, SBA-15 (Fig. 1c-C) loses most of its mesostructure. These results demonstrate that RSC-3 samples are much more hydrothermally stable than SBA-15. 3.2. Transmission electron microscopy (TEM) As a typical example, Fig. 2 shows images of calcined RSC-3-220, which exhibit ordered hexagonal arrays of mesopores (Fig. 2A and B) with one-dimensional chan-
nels (Fig. 2C), further confirming the ordered hexagonal mesostructure of RSC-3-220. These results demonstrate that CMK-3 can serve as a hard template for fabrication of ordered mesoporous silica at high temperatures (160– 240 C), in agreement with the XRD results. Additionally, partially discontinuous mesochannels and minute externally deposited silica can be observed in RSC-3220 (Fig. 2B and C). The discontinuity may be generated from the strong condensation of silica walls during the high-temperature synthesis. Furthermore, from highdark contrast in the TEM images of the sample, the cen˚ apart, ters of adjacent mesopores are estimated to be 92 A in good agreement with the value determined from XRD. 3.3. NMR spectroscopy Fig. 3 shows 29Si NMR spectra of as-synthesized RSC-3-180, RSC-3-220, and SBA-15 samples, providing direct evidence of the extent of silica condensation. In general, ordered mesoporous silica materials exhibit three bands centered at chemical shifts of 92, 102, and 112 ppm, which can be attributed to Si(OSi)x(OH)4 x framework units where x = 2 (Q2), x = 3 (Q3), and x = 4 (Q4), respectively. Notably, RSC-3-180 (Fig. 3a) and RSC-3-220 (Fig. 3b) are primarily made up of fully condensed Q4 silica units (d = 112 ppm) with a small contribution from incompletely cross-linked Q3 (d = 102 ppm), while no Q2 units are observed. The Q4/Q3 ratios for RSC-3-180 and RSC-3-220 are 5.5 and 9.5, respectively, indicating the fully condensed walls of RSC-3 samples. In contrast, as-synthesized SBA-15 exhibits the ratio of Q4/Q3 + Q2 at 1.6, indicating the presence of a large amount of terminal hydroxyl groups in the walls. The differences imply that high temperature is favor of the fabrication of mesoporous materials with more completely
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Fig. 2. TEM images of calcined RSC-3-220 (A) and (B) taken in the [1 0 0] direction, and (C) taken in the [1 1 0] direction. The partially discontinuous channels and minute externally deposited silica are marked by white circles and rectangle, respectively.
cross-linked frameworks, which are responsible for the higher thermal and hydrothermal stabilities [3,16–18].
Fig. 3. 29Si MAS NMR spectra of as-synthesized (a) RSC-3-180, (b) RSC-3-220, and (c) SBA-15.
3.4. N2 isotherms N2 isotherms of calcined RSC-3-180, RSC-3-220, and SBA-15 are shown in Fig. 4a, and the textural parame-
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Fig. 4. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of calcined samples before and after treatment in boiling water for 80 h: (A) RSC-3-180 (before); (B) RSC-3-180 (after); (C) RSC-3-220 (before); (D) RSC-3-220 (after); (E) SBA-15 (before); (F) SBA-15 (after). Isotherms A and C have been offset by 100 cm3 g 1 along the vertical axis for clarity.
ters are presented in Table 1. Notably, both RSC-3-180 and RSC-3-220 basically exhibit type-IV isotherms with a broad hysteresis loop at relative pressure of 0.55–0.95 (Fig. 4a-A and a-C), which is due to capillary condensation in mesopores. In contrast, SBA-15 exhibits isotherms (Fig. 4a-E) with a well-developed step in relative pressure ranged 0.65–0.8, which is characteristic of capillary condensation in uniform mesopores. Possibly, broader hysteresis loops in RSC-3-180 and RSC3-220 samples than that in SBA-15 may be related to partially discontinuous mesopores and minute externally deposited silica in the samples, as observed in TEM images (Fig. 2B and C). Generally, the discontinuous mesopores in mesostructured silica materials replicated from mesoporous carbon (CMK-3) are strongly influenced by the loading amount of silica in CMK-3 (SiO2/C, mass ratio) [30]. When the ratio of SiO2/C is less than 1.3, the mesopores of CMK-3 are not completely filled with silica. When the ratio reaches about 1.3–1.4, the filling of silica in mesopores of CMK-3 is almost complete, and obtained mesoporous silica exhibits
standard type-IV isotherms [30,32]. In this work, the ratio of SiO2/C changes among 1.3–2.0 for preparation of RSC-3-180 and RSC-3-220 samples. After characterization of these samples by XRD and N2 isotherms, it is found that the sample with ratio of SiO2/C at 1.7 shows the most ordered mesostructure in all samples. Depending on the pore volume of CMK-3, the ratio of SiO2/C at 1.7 is consistent with previous reports [28,30,32]. However, partially discontinuous mesopores still exist. Moreover, it is observed that the amount of discontinuous mesopores in RSC-3-220 is much more than that in RSC-3-180 (Fig. 4b-A and b-C). Possibly, high-temperature synthesis (160–240 C) results in silica condensation with relatively high degree, forming partially discontinuous mesopores in the samples [3,16–18]. The N2 adsorption isotherms (Fig. 4, Table 1) also confirm the highly hydrothermal stability of RSC-3 samples. After hydrothermal treatment in boiling water for 80 h, RSC-3-180 and RSC-3-220 are still type-IV isotherms (Fig. 4a-B and a-D) and the pore size distribution curves (Fig. 4b-B and b-D) are as sharp as the
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Table 1 Physical properties of the samples before and after treatmenta ˚ ) Pore size (A ˚ ) Wall thickness (A ˚ ) Pore volume (cm3 g 1)b Sample d(1 0 0) (A SBA-15 Calcined Treatedc
BET surface area (m2 g 1)
106 98
73
40
1.09 0.8
794 220
RSC-3-180 Calcined Treatedc
84 84 84
52 52
45 45
0.93 0.77
469 326
RSC-3-220 Calcined Treatedc
82 82 82
53 53
42 42
0.81 0.73
251 190
CMK-3
88
42
60
1.25
1100
Q4/Q3 + Q2
wS/V
1.6 5.3 5.5 2.6 9.5 1.7
a
Pore sizes and pore volumes determined from N2 adsorption isotherms at 77 K and the wall thickness was calculated as: thickness = a pore size (a = 2 · d(1 0 0)/31/2); in the last column, w, S, and V denote pore size, surface area, and pore volume, respectively. b ˚ diameter). The pore volume here is the primary mesopore volume (BJH adsorption cumulative pore volume of pores between 20 and 250 A c Treated in boiling water for 80 h.
untreated samples (Fig. 4b-A and b-C). In contrast, the treated SBA-15 shows a poor isotherm (Fig. 4a-F) with
an undiscernible pore size distribution (Fig. 4b-F). Correspondingly, the BET surface area and primary meso-
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pore volume of RSC-3-180 reduce by 30% from 469 m2 g 1 to 326 m2 g 1 and by 17% from 0.93 cm3 g 1 to 0.77 cm3 g 1, respectively. Those of RSC-3-220 reduce by 24% from 251 m2 g 1 to 190 m2 g 1 and by 10% from 0.81 cm3 g 1 to 0.73 cm3 g 1, respectively (Table 1). These results imply that the mesostructures of RSC-3-180 and RSC-3-220 samples are basically retained, and RSC-3-220 has a higher hydrothermal stability than RSC-3-180. While for the treated SBA-15, there are obvious decreases in BET surface area of 73% from 794 m2 g 1 to 220 m2 g 1 and in primary mesopore volume of 27% from 1.09 cm3 g 1 to 0.8 cm3 g 1 (Table 1). These results also confirm that RSC-3-180 and RSC-3-220 have much higher hydrothermal stability than SBA-15. Furthermore, it is interesting to note that pore sizes of RSC-3-180 and RSC-3-220 (Fig. 4b-A and b-C) are ˚ and 53 A ˚ (BJH model), respectively, centered at 52 A in good agreement with the wall thickness of CMK-3 (Table 1). These results indicate the successful structure transformation from CMK-3 to RSC-3-180 and RSC-3220 [27–30]. From XRD and N2 adsorption data, the wall thickness of RSC-3-180 and RSC-3-220 are calculated to be ˚ and 42 A ˚ , respectively (Table 1). Obviously, walls 45 A of RSC-3-180 and RSC-3-220 are thicker than those of SBA-15. It is proposed that the thicker walls and larger wall thickness/pore size ratio of RSC-3-180 and RSC-3220 are favorable for their stabilities [8], which also result from the advantage of carbon template. Generally, the fundamental relation between the structural parameters for materials with uniform pores of simple cylindrical geometry is as follows: wS/V 4 (w, S, and V denote pore size, surface area, and pore volume, respectively) [33–35]. However, RSC-3-180 and RSC-3-220 give the values of wS/V at only 2.6 and 1.7, due to the presence of partially discontinuous mesopores and externally deposited silica in the samples. In contrast, the value of wS/V for SBA-15 is 5.3, which is assigned to the presence of a large number of micropores in the walls of SBA-15 [22,33]. It is worthy noting that RSC-3-180 and RSC-3-220 have the relatively low surface areas (469 and 251 m2 g 1) when compared with SBA-15. Fig. 5 shows t-plots for N2 adsorption isotherms over RSC-3-180 and RSC-3-220 samples. Notably, the sample t-plots pass zero of axis, meaning no micropores in RSC-3-180 and RSC-3-220 [33–35]. The elimination of micropores in RSC-3-180 and RSC-3-220 may be related to the strong condensation of silica synthesized at high temperatures, and similar phenomena have been reported previously [16,17]. Considering the free-microporosity, partially discontinuous mesopores, minute externally deposited silica, and thicker mesoporous walls, the relatively low surface areas in RSC-3-180 and RSC-3-220 are reasonable.
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Thickness-Harkins & Jura / Å Fig. 5. N2 adsorption isotherm t-plots of calcined (a) RSC-3-180 and (b) RSC-3-220.
4. Conclusions and perspective In summary, we have shown a possibility to use ordered mesoporous carbon such as CMK-3 as a hard template for high-temperature (160–240 C) synthesis of ordered mesoporous silica. Consequently, these mesoporous silica materials show much higher hydrothermal stability than conventional silica materials synthesized at relatively low temperatures (<150 C). More importantly, high-temperature synthesis is not limited to replication of silica materials from CMK-3, and many inorganic materials and carbons with various mesostructures can be used if the inorganic materials could effectively interact with mesoporous carbons. This method may offer an approach for fabrication of a series of hydrothermally stable ordered mesoporous materials with various mesostructures, such as hexagonal p6mm, cubic Ia3d, cubic Pm3n, and cubic Im3m. Additionally, the high temperatures are not limited to 240 C, and much higher temperatures can be carried out, because ordered mesoporous carbon materials are generally stable at higher temperature than 240 C.
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Acknowledgements We thank Prof. D. Zhao and Dr. F. Zhang in Fudan University for the helpful discussion. This work is supported by NSFC, CNPC, BASF, the National High Technology Research and Development Program of China (863 Program) and State Basic Research Project (973 Program).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.micromeso.2005.07.019.
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High-temperature synthesis of stable ordered mesoporous silica materials using mesoporous carbon as a hard template Lifeng Wang, Kaifeng Lin, Yan Di, Daliang Zhang, Caijin Li, Qing Yang, Chengyang Yin, Zhenhua Sun, Dazhen Jiang, Feng-Shou Xiao * State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University, Changchun 130012, China Received 18 March 2005; received in revised form 17 July 2005; accepted 19 July 2005 Available online 26 August 2005
Abstract Replicated mesoporous silica materials from CMK-3 (denoted as RSC-3-X, X stands for synthesis temperature) have been successfully synthesized in acidic media at high temperatures (160–240 C) and subsequently characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen sorption isotherms, and 29Si nuclear magnetic resonance (NMR) spectroscopy. XRD patterns and nitrogen sorption isotherms exhibit the highly hydrothermal stable mesostructure of RSC-3 samples. Transmission electron microscopy images of RSC-3-220 show ordered hexagonal arrays of mesopores with 1D channels and further confirm the 2D hexagonal (p6mm) mesostructure of RSC-3-220. 29Si MAS NMR spectra indicate that as-synthesized RSC-3 samples are primarily made up of fully condensed Q4 silica units (d = 112 ppm) with a small contribution from incompletely cross-linked Q3 (d = 102 ppm) as deduced from the high Q4/Q3 ratio of 5.5–9.5, implying the fully condensed walls of RSC-3. Such unique structural features should be attributed directly to the high-temperature synthesis, which could be responsible for the observed highly hydrothermal stability of RSC-3. 2005 Elsevier Inc. All rights reserved. Keywords: High-temperature synthesis; Mesoporous silica; Mesoporous carbon; Hard template; Hydrothermal stability
1. Introduction Mesoporous materials have attracted much attention because of their potential use as versatile catalysts and catalyst supports for conversion of large molecules. However, as compared with conventional zeolites, mesostructured materials have relatively low hydrothermal stability, which severely hinders their practical applications in catalytic reactions for the petroleum industry [1,2]. To improve the hydrothermal stability of mesostructured materials, couples of synthetic routes have been reported, such as mesoporous MCM-41 with more
*
Corresponding author. Tel.: +86 431 5168590; fax: +86 431 5168624. E-mail address: [email protected] (F.-S. Xiao). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.07.019
highly condensed pore walls synthesized by increasing crystallization time [3], ordered hexagonal SBA-15 with thicker walls prepared in strongly acidic media (pH < 0) [4], vesicle-like MSU-G materials with a high SiO4 crosslinking templated by neutral gemini surfactants [5], disordered KIT-1 obtained by using inorganic salts as additives [6], and stable mesoporous aluminosilicates synthesized through a grafting route [7,8], or from zeolite seeds and preformed zeolite nanoclusters [9–13]. Among above routes, a critical factor in improving hydrothermal stability is the highly condensed silica walls [5,7,8]. It can be expected that the level of silica condensation will be enhanced by increasing the crystallization temperature [3]. However, mesostructured materials prepared at relatively low temperatures (80–150 C) generally exhibit imperfectly condensed walls with large amounts of terminal hydroxyl groups, resulting in the
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unstable mesostructure in hydrothermal conditions [14,15]. Recently, there have been the syntheses of stable mesoporous silica-based materials at the temperatures of 160–220 C using high-temperature stable surfactants of fluorocarbon–hydrocarbon surfactant mixtures and cationic modified ionic liquids as templates [16–18]. However, it is no way to prepare ordered mesoporous materials at much higher temperatures yet (>220 C). It is well known that carbon materials are generally high-temperature stable, even up to 300 C. Interestingly, the discovery of ordered mesostructured carbon materials [19–26] with large surface area and tunable pore sizes offers a novel synthetic route to ordered mesoporous materials [27–32]. For examples, ordered hexagonal mesoporous silica was successfully synthesized using Na2SiO3 as a silica precursor and CMK-3 as a template [27]; ordered hexagonal mesoporous silica (NCS-n) was effectively nanocast by TEOS into CMK3 [28–30]; ordered cubic mesoporous silica (HUM-1) was particularly templated from mesoporous carbon of CM48T-C [31]; ordered mesoporous aluminosilicates (RMM-n) were completely prepared from ordered mesoporous carbons and preformed aluminosilicate precursors [32]. Notably, the temperatures for all the hydrothermal syntheses are less than 100 C. We demonstrate here that, when an ordered mesoporous carbon (CMK-3) is used as the template, ordered mesoporous silica materials with unusual hydrothermal stability are successfully synthesized in strongly acidic media at high temperatures (160–240 C).
2. Experimental section 2.1. Materials Triblock copolymer P123 (EO20PO70EO20) was supplied from BASF Co., and TEOS, NaOH, HF, HCl, sulfuric acid, sucrose and ethanol were purchased from Beijing Chemical Co. (China).
dried in air and calcined at 600 C for 6 h to remove the template of CMK-3. The final product is denoted as RSC-3-X (X stands for synthesis temperature). After characterization of these samples by XRD and N2 isotherms, it was found that the sample with ratio of SiO2/C at 1.7 showed the most ordered mesostructure in all samples. It is mentionable that the high temperatures in hydrothermal synthesis are not limited to 240 C, and much higher temperatures can be carried out, because ordered mesoporous carbon materials are generally stable at higher temperatures than 240 C. However, due to the use of polytetrafluoroethylene (PTFE) in autoclaves, we cannot heat the autoclaves over 240 C. If we choose suitable autoclaves, the hydrothermal synthesis of ordered stable mesoporous silica from mesoporous carbon at the temperatures higher than 240 C could be carried out. SBA-15 and CMK-3 were synthesized according to the literatures [4,22]. 2.3. Characterization X-ray diffraction patterns (XRD) were obtained with a Siemens D5005 diffractometer using CuKa radiation. Transmission electron microscopy 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 using a Micromeritics ASAP 2020 M system. The samples were outgassed for 10 h at 300 C before the measurements. MAS NMR measurements were performed on a Varian Infinity plus 400 spectrometer at a rotation frequency of 4 kHz. The corresponding Larmor frequency was 79.4 MHz for 29Si nuclear. Free induction decays (FID) were recorded with 45 flip angle preparation pulses and 5.0 s recycle delay. The deconvolution method to resolve the Q3 and Q4 signals was supported by software in Varian Infinity plus 400 spectrometer. Chemical analysis for carbon element was performed on a Perkin–Elmer 2400 element analyzer.
2.2. Synthesis 3. Results and discussion As a typical run, replicated mesoporous silica from mesoporous carbon of CMK-3 was synthesized as follows: (1) 5 mL of tetraethyl orthosilicate (TEOS) were dissolved in 10 mL of ethanol, followed by adding 1 g of CMK-3. The mixture was subjected to aging at 40 C for 48 h to form silica/CMK composite. (2) The procedure was repeated until the desired amount (SiO2/C mass ratios of 1.3, 1.5, 1.7, 1.9, and 2.0 using 0, 0.75, 1.5, 2.25, and 2.63 mL of TEOS, respectively) of silica was reached. The resultant composite then crystallized in the presence of HCl vapor (1 M) at various temperatures (160–240 C) for 48 h. The product was
3.1. X-ray diffraction (XRD) Fig. 1 shows XRD patterns of as-synthesized, calcined and hydrothermally treated samples of replicated mesoporous silica from CMK-3 at synthetic temperature of 180 C (RSC-3-180), replicated mesoporous silica from CMK-3 at synthetic temperature of 220 C (RSC-3-220), and SBA-15. Compared with CMK-3 (Supporting Fig. 1), RSC-3-180 and RSC-3-220 samples exhibit a relatively weak peak (Fig. 1a-A and b-A), indicating that the silica sources have been successfully
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Fig. 1. XRD patterns of (A) as-synthesized samples, (B) calcined samples, and (C) treated samples in boiling water for 80 h. Samples include (a) RSC-3-180, (b) RSC-3-220, and (c) SBA-15.
nanocast into the mesopores of CMK-3. After calcination at 600 C for 6 h to remove the CMK-3 template, both samples (Fig. 1a-B and b-B) show clearly three peaks that can be indexed as the (1 0 0), (1 1 0) and (2 0 0) diffractions associated with the p6mm hexagonal symmetry. Obviously, peak intensities of calcined samples increase significantly, compared with those of as-synthesized samples. These results confirm that the mesostructure of CMK-3 has been successfully replicated into RSC-3 samples. Moreover, no significant changes in the basal spacing (d1 0 0) between as-synthesized and calcined RSC-3 samples are observed, suggesting their good thermal stabilities. Whereas, the unit cell ˚ during the same calcination of SBA-15 contracts 8 A (Fig. 1c-B). Furthermore, the wide-angle XRD patterns in the region of 6–50 for RSC-3 samples show a very broad peak as that of SBA-15, indicating the amorphous state of the silica walls (Supporting Fig. 2). Moreover, RSC-3 samples were investigated by a Perkin–Elmer 2400 element analyzer, and obtained results showed that the carbon was almost absent in all of calcined samples (<0.1 wt.%), indicating the complete removal of the carbon template. Very interestingly, after treatment in boiling water for 80 h, the XRD patterns of RSC-3 samples (Fig. 1a-C and b-C) still retain those peaks associated with the p6mm hexagonal symmetry. In contrast, after the same hydrothermal treatment, SBA-15 (Fig. 1c-C) loses most of its mesostructure. These results demonstrate that RSC-3 samples are much more hydrothermally stable than SBA-15. 3.2. Transmission electron microscopy (TEM) As a typical example, Fig. 2 shows images of calcined RSC-3-220, which exhibit ordered hexagonal arrays of mesopores (Fig. 2A and B) with one-dimensional chan-
nels (Fig. 2C), further confirming the ordered hexagonal mesostructure of RSC-3-220. These results demonstrate that CMK-3 can serve as a hard template for fabrication of ordered mesoporous silica at high temperatures (160– 240 C), in agreement with the XRD results. Additionally, partially discontinuous mesochannels and minute externally deposited silica can be observed in RSC-3220 (Fig. 2B and C). The discontinuity may be generated from the strong condensation of silica walls during the high-temperature synthesis. Furthermore, from highdark contrast in the TEM images of the sample, the cen˚ apart, ters of adjacent mesopores are estimated to be 92 A in good agreement with the value determined from XRD. 3.3. NMR spectroscopy Fig. 3 shows 29Si NMR spectra of as-synthesized RSC-3-180, RSC-3-220, and SBA-15 samples, providing direct evidence of the extent of silica condensation. In general, ordered mesoporous silica materials exhibit three bands centered at chemical shifts of 92, 102, and 112 ppm, which can be attributed to Si(OSi)x(OH)4 x framework units where x = 2 (Q2), x = 3 (Q3), and x = 4 (Q4), respectively. Notably, RSC-3-180 (Fig. 3a) and RSC-3-220 (Fig. 3b) are primarily made up of fully condensed Q4 silica units (d = 112 ppm) with a small contribution from incompletely cross-linked Q3 (d = 102 ppm), while no Q2 units are observed. The Q4/Q3 ratios for RSC-3-180 and RSC-3-220 are 5.5 and 9.5, respectively, indicating the fully condensed walls of RSC-3 samples. In contrast, as-synthesized SBA-15 exhibits the ratio of Q4/Q3 + Q2 at 1.6, indicating the presence of a large amount of terminal hydroxyl groups in the walls. The differences imply that high temperature is favor of the fabrication of mesoporous materials with more completely
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Fig. 2. TEM images of calcined RSC-3-220 (A) and (B) taken in the [1 0 0] direction, and (C) taken in the [1 1 0] direction. The partially discontinuous channels and minute externally deposited silica are marked by white circles and rectangle, respectively.
cross-linked frameworks, which are responsible for the higher thermal and hydrothermal stabilities [3,16–18].
Fig. 3. 29Si MAS NMR spectra of as-synthesized (a) RSC-3-180, (b) RSC-3-220, and (c) SBA-15.
3.4. N2 isotherms N2 isotherms of calcined RSC-3-180, RSC-3-220, and SBA-15 are shown in Fig. 4a, and the textural parame-
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Fig. 4. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of calcined samples before and after treatment in boiling water for 80 h: (A) RSC-3-180 (before); (B) RSC-3-180 (after); (C) RSC-3-220 (before); (D) RSC-3-220 (after); (E) SBA-15 (before); (F) SBA-15 (after). Isotherms A and C have been offset by 100 cm3 g 1 along the vertical axis for clarity.
ters are presented in Table 1. Notably, both RSC-3-180 and RSC-3-220 basically exhibit type-IV isotherms with a broad hysteresis loop at relative pressure of 0.55–0.95 (Fig. 4a-A and a-C), which is due to capillary condensation in mesopores. In contrast, SBA-15 exhibits isotherms (Fig. 4a-E) with a well-developed step in relative pressure ranged 0.65–0.8, which is characteristic of capillary condensation in uniform mesopores. Possibly, broader hysteresis loops in RSC-3-180 and RSC3-220 samples than that in SBA-15 may be related to partially discontinuous mesopores and minute externally deposited silica in the samples, as observed in TEM images (Fig. 2B and C). Generally, the discontinuous mesopores in mesostructured silica materials replicated from mesoporous carbon (CMK-3) are strongly influenced by the loading amount of silica in CMK-3 (SiO2/C, mass ratio) [30]. When the ratio of SiO2/C is less than 1.3, the mesopores of CMK-3 are not completely filled with silica. When the ratio reaches about 1.3–1.4, the filling of silica in mesopores of CMK-3 is almost complete, and obtained mesoporous silica exhibits
standard type-IV isotherms [30,32]. In this work, the ratio of SiO2/C changes among 1.3–2.0 for preparation of RSC-3-180 and RSC-3-220 samples. After characterization of these samples by XRD and N2 isotherms, it is found that the sample with ratio of SiO2/C at 1.7 shows the most ordered mesostructure in all samples. Depending on the pore volume of CMK-3, the ratio of SiO2/C at 1.7 is consistent with previous reports [28,30,32]. However, partially discontinuous mesopores still exist. Moreover, it is observed that the amount of discontinuous mesopores in RSC-3-220 is much more than that in RSC-3-180 (Fig. 4b-A and b-C). Possibly, high-temperature synthesis (160–240 C) results in silica condensation with relatively high degree, forming partially discontinuous mesopores in the samples [3,16–18]. The N2 adsorption isotherms (Fig. 4, Table 1) also confirm the highly hydrothermal stability of RSC-3 samples. After hydrothermal treatment in boiling water for 80 h, RSC-3-180 and RSC-3-220 are still type-IV isotherms (Fig. 4a-B and a-D) and the pore size distribution curves (Fig. 4b-B and b-D) are as sharp as the
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B
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Table 1 Physical properties of the samples before and after treatmenta ˚ ) Pore size (A ˚ ) Wall thickness (A ˚ ) Pore volume (cm3 g 1)b Sample d(1 0 0) (A SBA-15 Calcined Treatedc
BET surface area (m2 g 1)
106 98
73
40
1.09 0.8
794 220
RSC-3-180 Calcined Treatedc
84 84 84
52 52
45 45
0.93 0.77
469 326
RSC-3-220 Calcined Treatedc
82 82 82
53 53
42 42
0.81 0.73
251 190
CMK-3
88
42
60
1.25
1100
Q4/Q3 + Q2
wS/V
1.6 5.3 5.5 2.6 9.5 1.7
a
Pore sizes and pore volumes determined from N2 adsorption isotherms at 77 K and the wall thickness was calculated as: thickness = a pore size (a = 2 · d(1 0 0)/31/2); in the last column, w, S, and V denote pore size, surface area, and pore volume, respectively. b ˚ diameter). The pore volume here is the primary mesopore volume (BJH adsorption cumulative pore volume of pores between 20 and 250 A c Treated in boiling water for 80 h.
untreated samples (Fig. 4b-A and b-C). In contrast, the treated SBA-15 shows a poor isotherm (Fig. 4a-F) with
an undiscernible pore size distribution (Fig. 4b-F). Correspondingly, the BET surface area and primary meso-
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pore volume of RSC-3-180 reduce by 30% from 469 m2 g 1 to 326 m2 g 1 and by 17% from 0.93 cm3 g 1 to 0.77 cm3 g 1, respectively. Those of RSC-3-220 reduce by 24% from 251 m2 g 1 to 190 m2 g 1 and by 10% from 0.81 cm3 g 1 to 0.73 cm3 g 1, respectively (Table 1). These results imply that the mesostructures of RSC-3-180 and RSC-3-220 samples are basically retained, and RSC-3-220 has a higher hydrothermal stability than RSC-3-180. While for the treated SBA-15, there are obvious decreases in BET surface area of 73% from 794 m2 g 1 to 220 m2 g 1 and in primary mesopore volume of 27% from 1.09 cm3 g 1 to 0.8 cm3 g 1 (Table 1). These results also confirm that RSC-3-180 and RSC-3-220 have much higher hydrothermal stability than SBA-15. Furthermore, it is interesting to note that pore sizes of RSC-3-180 and RSC-3-220 (Fig. 4b-A and b-C) are ˚ and 53 A ˚ (BJH model), respectively, centered at 52 A in good agreement with the wall thickness of CMK-3 (Table 1). These results indicate the successful structure transformation from CMK-3 to RSC-3-180 and RSC-3220 [27–30]. From XRD and N2 adsorption data, the wall thickness of RSC-3-180 and RSC-3-220 are calculated to be ˚ and 42 A ˚ , respectively (Table 1). Obviously, walls 45 A of RSC-3-180 and RSC-3-220 are thicker than those of SBA-15. It is proposed that the thicker walls and larger wall thickness/pore size ratio of RSC-3-180 and RSC-3220 are favorable for their stabilities [8], which also result from the advantage of carbon template. Generally, the fundamental relation between the structural parameters for materials with uniform pores of simple cylindrical geometry is as follows: wS/V 4 (w, S, and V denote pore size, surface area, and pore volume, respectively) [33–35]. However, RSC-3-180 and RSC-3-220 give the values of wS/V at only 2.6 and 1.7, due to the presence of partially discontinuous mesopores and externally deposited silica in the samples. In contrast, the value of wS/V for SBA-15 is 5.3, which is assigned to the presence of a large number of micropores in the walls of SBA-15 [22,33]. It is worthy noting that RSC-3-180 and RSC-3-220 have the relatively low surface areas (469 and 251 m2 g 1) when compared with SBA-15. Fig. 5 shows t-plots for N2 adsorption isotherms over RSC-3-180 and RSC-3-220 samples. Notably, the sample t-plots pass zero of axis, meaning no micropores in RSC-3-180 and RSC-3-220 [33–35]. The elimination of micropores in RSC-3-180 and RSC-3-220 may be related to the strong condensation of silica synthesized at high temperatures, and similar phenomena have been reported previously [16,17]. Considering the free-microporosity, partially discontinuous mesopores, minute externally deposited silica, and thicker mesoporous walls, the relatively low surface areas in RSC-3-180 and RSC-3-220 are reasonable.
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500 400 300 200 100 0 0
2
4
6
8
10
12
14
Thickness-Harkins & Jura / Å Fig. 5. N2 adsorption isotherm t-plots of calcined (a) RSC-3-180 and (b) RSC-3-220.
4. Conclusions and perspective In summary, we have shown a possibility to use ordered mesoporous carbon such as CMK-3 as a hard template for high-temperature (160–240 C) synthesis of ordered mesoporous silica. Consequently, these mesoporous silica materials show much higher hydrothermal stability than conventional silica materials synthesized at relatively low temperatures (<150 C). More importantly, high-temperature synthesis is not limited to replication of silica materials from CMK-3, and many inorganic materials and carbons with various mesostructures can be used if the inorganic materials could effectively interact with mesoporous carbons. This method may offer an approach for fabrication of a series of hydrothermally stable ordered mesoporous materials with various mesostructures, such as hexagonal p6mm, cubic Ia3d, cubic Pm3n, and cubic Im3m. Additionally, the high temperatures are not limited to 240 C, and much higher temperatures can be carried out, because ordered mesoporous carbon materials are generally stable at higher temperature than 240 C.
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Acknowledgements We thank Prof. D. Zhao and Dr. F. Zhang in Fudan University for the helpful discussion. This work is supported by NSFC, CNPC, BASF, the National High Technology Research and Development Program of China (863 Program) and State Basic Research Project (973 Program).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.micromeso.2005.07.019.
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