Sugar-Assisted Synthesis of Ordered and Stable Cubic Mesoporous

Oct 26, 2010 - Chun-Chiang Ting,† Hao-Yiang Wu,‡ Yu-Chi Pan,† Shanmugam Vetrivel,† George T. K. Fey,§ and Hsien-Ming Kao*,†. Department of ...
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Sugar-Assisted Synthesis of Ordered and Stable Cubic Mesoporous Silica SBA-1 Chun-Chiang Ting,† Hao-Yiang Wu,‡ Yu-Chi Pan,† Shanmugam Vetrivel,† George T. K. Fey,§ and Hsien-Ming Kao*,† Department of Chemistry, National Central UniVersity, Chung-Li, Taiwan 32054, R.O.C., Department of Neurological Surgery, Tri-SerVice General Hospital, National Defense Medical Center, 325, Sec. 2, Cheng-Kung Rd, Nei-Hu Dist, Taipei 11490, Taiwan, R.O.C., Department of Chemical and Materials Engineering, National Central UniVersity, Chung-Li, Taiwan 32054, R.O.C. ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: September 29, 2010

Highly ordered and hydrothermally stable cubic mesoporous silicas SBA-1 (Pm3jn mesophase) have been synthesized over a broad temperature range using tetraethoxysilane (TEOS) and cetyltriethylammonium bromide (CTEABr) as silicon source and template agent, respectively, under highly acidic conditions via addition of sugar molecules such as D-fructose or sucrose as an auxiliary agent. Various synthesis parameters such as D-fructose content, acid concentration and reaction time have been optimized under different synthesis conditions in order to obtain highly ordered mesoporous silica SBA-1 materials with good hydrothermal stability. In contrast to the conventional synthesis methods requiring low synthesis temperature, this strategy allows the preservation of the SBA-1 mesophase under high temperature synthesis conditions and leads to a highly cross-linked silica framework. In particular, the framework locations of the derivatives of D-fructose molecules and their interactions with the surfactant molecules were investigated by two-dimensional 1H-1H exchange NMR. The present NMR results confirmed that the derivatives of D-fructose are in close spatial proximity to the surfactant molecules to form stable spherical micelles with adequate curvatures for the formation of cubic mesophase in a wide range of synthesis conditions. Introduction The favorable characteristics of mesoporous silica materials, such as high surface area, large pore volume, and controllable pore sizes, have stimulated extensive research for their potential use as prospective catalysts, separation, adsorbents, and templates for the synthesis of nanostructures.1-8 Soon after the discovery of ordered M41S mesoporous materials,9,10 Huo et al.11-13 reported another variety of mesoporous silica materials, among which the cubic SBA-1 (Pm3jn mesophase) is of particular interest because of its cage-type structure. The conventional synthesis route for SBA-1 is based on the S+X-I+ pathway, where S, X, and I correspond to surfactant, halide, and inorganic species, respectively, under highly acidic conditions by using surfactants with large headgroups, for example, cetyltriethylammonium bromide (CTEABr), as the templating agent. However, it was soon recognized that poor stability of SBA-1, compared to other mesoporous silica materials such as MCM-41 and SBA-15, is the major limitation for its widespread application.14,15 This can be attributed to the fact that its synthesis requires more strict conditions involving the use of a large headgroup surfactant at a preferred low synthesis temperature of 0 °C. Kim and Ryoo14 have found that the low temperature condition, e.g., at 0 °C, favors the formation of the cubic SBA-1 mesostructure, which undergoes a phase transformation to the hexagonal SBA-3 mesophase when the synthesis temperature is above 40 °C. As a result, SBA-1 often exhibits an imperfectly condensed mesoporous wall with a very low cross-linking * Corresponding author. Fax: +886-3-4227664. E-mail: hmkao@ cc.ncu.edu.tw. † Department of Chemistry, National Central University. ‡ National Defense Medical Center. § Department of Chemical and Materials Engineering, National Central University.

degree, as revealed by the previous 29Si MAS (magic angle spinning) NMR studies.14 The drying procedure was also found to play a key role in phase transformation.15 It has been reported that drying at higher temperatures promotes the phase transformation not only from hexagonal p6mm to cubic Ia3d but also from cubic Ia3d to a lamellar mesophase.16 The anions present in the interfacial space between the pore directing CTEA+ micelle and the silica wall of mesostructured silica can also influence the phase transformation and morphology change.17 It has been known that the as-synthesized SBA-1 material is not stable and loses its cubic structure upon washing with water.14,18 Although there are some efforts to incorporate metal elements into SBA-1 for specific catalytic functions,19-25 the synthesis of SBA-1 with a good long-range structural ordering and high stability is still a challenging objective. As with silica-based materials, a critical factor in increasing stability is to have more silica condensation on the pore walls. However, the synthesis of mesoporous silica materials with a higher degree of silica condensation has proven to be particularly difficult with the S+X-I+ templating pathway under strongly acidic conditions.13 The situation is even worse for the synthesis of SBA-1. In the original recipe of SBA-1 synthesis, the preferred low synthesis temperature (e.g., 0 °C) precludes the undesirable phase transformation, but results in imperfectly condensed mesoporous walls with large amounts of silanol groups that make the mesostructure unstable.14 Several researchers have observed phase transformation from hexagonal (SBA3, p6mm) to cubic (SBA-1, Pm3jn) at early stage of crystallization.26,27 Both the surfactant packing parameter (g) and the concept of charge density matching theory were employed to comprehensively explain the phase transformation observed.28,29 To further stabilize the cubic SBA-1 structure, Kim and Ryoo14 suggested a subsequent drying process after synthesis under N2

10.1021/jp104844b  2010 American Chemical Society Published on Web 10/26/2010

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19323 atmosphere at 160 °C. The material after such treatment was stable toward washing with ethanol-HCl mixture for the template removal. Another method is to prolong the hydrothermal treatment at 100 °C. Unfortunately, there was a noticeable structure change from Pm3jn to an undetermined space group with lower symmetry after excessively long hydrothermal treatment.18 To enhance the stability of SBA-1 and thus its widespread application, the development of a simple approach to enhance the cross-linking of SBA-1 silica framework by optimizing the synthesis conditions is essential. A straightforward strategy is to increase the synthesis temperatures and durations. However, attention should be paid to avoid any uncontrollable phase transformation. In our earlier papers, we have demonstrated a successful synthesis of highly ordered SBA-1 with enhanced stability toward washing with water, under strongly acidic conditions using CTEABr as the template and TEOS as silicon source, simply using sugars like D-fructose and D-sorbitol as auxiliary agents, over a broad temperature range without any phase transformation.29-31 Similar to the silicate condensation reaction, D-fructose and its derivatives can undergo polymerization with hydroxyl carboxylic acid derivatives, formed by oxidation, to form saccharoidal-like resins under acidic conditions. Since D-fructose and saccharoidal-like resins contain large numbers of hydroxyl groups, they should be dominantly located at the hydrophilic part of the CTEABr molecules to form spherical micelles through hydrogen bonding under the synthesis condition. We extend our studies herein to report the optimized synthesis condition for the preparation of highly ordered SBA-1 structures with excellent hydrothermal stability. In particular, attempts have been devoted to implement multinuclear solidstate NMR (1H, 13C, and 29Si) and two-dimensional (2D) 1H-1H exchange NMR spectroscopy to investigate the locations of the derivatives of D-fructose and their possible interactions with the surfactant molecules in the micelle for better understanding the role of sugar played in the sugar-assisted synthesis route. Experimental Section Synthesis of SBA-1 in the Presence of Sugar. The synthesis procedures of cubic SBA-1 are as follows. Sugar (D-fructose or sucrose), HCl, and TEOS were mixed in water at room temperature to obtain a homogeneous solution. The surfactant CTEABr was then added into the solution. The surfactant CTEABr was synthesized by the reaction of 1-bromohexadecane with an equimolar amount of triethylamine in ethanol under reflux conditions for several days. The resulting CTEABr was purified three times by recrystallization from a chloroform/ethyl acetate mixture. Generally, the synthesis conditions of SBA-1 include two heating steps: one is performed at a self-assembly temperature and then followed by hydrothermal treatment at 100 °C. After addition of CTEABr into the parent silicate solution, the reaction was performed at a desired self-assembly reaction temperature in the range of 0 to 90 °C (T) under vigorous stirring for different periods ranging from 4-72 h (H). Afterward, the reaction mixture was then hydrothermally treated at 100 °C for only 1 h, since prolonged hydrothermal treatment may do harm to the preservation of the cubic mesostructure.18 The composition of the reaction mixture was varied in the range of 1 CTEABr: 5 TEOS: 48-460 HCl: 0.04-40.8 sugar: 3500 H2O. The resultant precipitate was filtered (without washing) and dried at 70 °C overnight. The as-synthesized samples were then calcined at 550 °C for 6 h in order to remove templates. The resulting samples prepared in the presence of D-fructose and sucrose were respectively denoted as F(A)/T(H) and S(A)/

T(H), where F and S represent the samples synthesized in the presence of D-fructose and sucrose, respectively, A is the HCl/ CTEABr molar ratio, and T and H denote the temperature and time interval for the self-assembly reaction, respectively. For comparison purposes, similar procedure was used to prepare the pure SBA-1 samples in the absence of sugar and the resulting samples were denoted as P(A)/T(H), where P represents pure silicas. Tests for Hydrothermal Stability. In order to explore the hydrothermal stability of SBA-1, the calcined samples were added in water (1 L/g) at 100 °C under reflux conditions. After treatment, the samples were filtered, without washing, and dried at 70 °C overnight. Characterization Methods. Powder X-ray diffraction (XRD) patterns were collected on Wiggler-17A1 beamline (λ ) 0.133295 nm) at the National Synchrotron Radiation Research Center in Taiwan. N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. The sample was degassed at 180 °C for several hours before measurements. Specific surface areas were calculated by using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of P/P0 ) 0.05-0.3. The pore size distribution was obtained from the analysis of the adsorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method. Pore volumes were calculated from the volumes of N2 adsorbed at P/P0 ) 0.95 or in the vicinity. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer TGA7 thermogravimetric analyzer with a heating rate of 10 °C/min under air in a flow of 50 mL/min. Scanning electron microscopic (SEM) images were taken on a Hitachi S-3500N electron microscope. Multinuclear (1H, 13C, and 29Si) solid-state NMR spectra were recorded on a Varian Infinityplus-500 NMR spectrometer, equipped with Chemagnetics probes. The Larmor frequencies for 1H, 13C, and 29Si nuclei are 498.54, 125.37, and 99.04 MHz, respectively. 13C CPMAS (cross-polarization magic angle spinning) NMR spectra were recorded by using a contact time of 3 ms and a recycle delay of 2 s at a spinning speed of 9 or 10 kHz. Single pulse experiments with a π/6 pulse of 2 µs and a recycle delay of 200 s were used to acquire the quantitative 29 Si MAS NMR spectra. The 1H, 13C, and 29Si chemical shifts were externally referenced to tetramethylsilane (TMS) at 0.0 ppm. The 2D 1H-1H exchange NMR experiments were performed with a NOESY-type sequence with three π/2 pulses. After the initial excitation, an additional π/2 pulse is incorporated after the evolution time (t1) to store the 1H magnetization along the z axis, followed by a mixing time (tmix), during which proton spin diffusion can occur. Magnetization is exchanged only between homonuclear dipolar-coupled proton species and their separation can be probed by varying the mixing time. The 1 H-1H exchange sequence produces homonuclear correlated spectra giving rise to off-diagonal intensities at positions where nuclei undergo chemical exchange or spin diffusion during the mixing time tmix. Detailed experimental conditions for the individual NMR spectra are presented in the figure captions. For 1H-1H exchange NMR experiments, the samples were dehydrated in vacuum at 100 °C for at least 12 h before NMR measurements. The sample was tightly packed into the rotor in a glovebox under a dry nitrogen atmosphere to avoid sample rehydration. Results and Discussion Effect of Reaction Temperature. We have previously demonstrated that ordered SBA-1 can be synthesized over a

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Figure 1. Powder XRD patterns of as-synthesized S(230)/T(4) samples, where T ) (a) 0, (b) 20, (c) 30, (d) 50, (e) 70, and (f) 90.

wide synthesis temperature with the aid of D-fructose and D-sorbitol.29,30 Sucrose was also chosen as an auxiliary agent in the present study for comparison purposes because it can decompose into fructose and glucose in the solution. Figure 1 shows the powder XRD patterns of the as-synthesized S(230)/ T(4) samples, which were synthesized at various reaction temperatures for 4 h in the presence of sucrose. When the S(230)/T(4) samples were synthesized at lower temperature (70 °C), the thermal disorder due to the motion of surfactant tails dominates and thus causes an increase in the effective volume of the surfactant, leading to the surfactant parameter g > 1/3. As a result, a phase transformation from cubic to hexagonal mesophase is observed. Thermal Behavior. The TGA-DTA combinational curves of the as-synthesized F(230)/T(4) samples are illustrated in Figure 2. The weight-loss curves show three relatively large weight-loss steps. The first weight-loss from 50 to 150 °C is approximately 2 wt % due to the desorption of physically adsorbed water. The second weight-loss of 30-50 wt.% is decomposition and combustion of the organic template and D-fructose in the temperature ranging from 150 to 320 °C. Finally, the weight-loss for the decomposition of the residual organic template and D-fructose and the condensation of silicate walls is around 10-15 wt % in the temperature range between 330 and 500 °C. There was an additional weight-loss of about 15 wt.% in the temperature range between 500 and 800 °C for F(230)/90(4) due to further condensation of silicate walls and decomposition of some carbonaceous species. The TGA results suggested the existence of carbonaceous species resulting from the derivatives of D-fructose in the F(230)/90(4) sample (Figure 2c). Morphology Studies. Both the SEM micrographs of F(230)/ 0(4) and P(230)/0(4) show crystallites with a relatively uniform and decaoctahedral shape (Figure 3a and b). It should be noted that the SBA-1 particle morphology is very sensitive to

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Figure 2. TGA and DTA curves of as-synthesized F(230)/T(4) samples, where T ) (a) 0, (b) 50, and (c) 90.

Figure 4. 13C CPMAS NMR spectra of as-synthesized (a) F(230)/ 0(4) after washing with water, and F(230)/70(72) before (b) and after (c) washing with water. The samples in a, b, and c were prepared with a D-fructose/CTEABr ratio of 10.2, 20.4, and 20.4, respectively. Asterisks denote spinning sidebands.

Figure 3. SEM images of the SBA-1 samples synthesized with various conditions: (a) F(230)/0(4), (b) P(230)/0(4), (c) F(230)/70(4), (d) F(230)/ 70(36), (e) F(230)/70(48), and (f) F(460)/70(48).

temperature. The morphology of the F(230)/T(4) samples varied from a decaoctahedral shape to an almost entirely spherical shape while increasing the reaction temperature from 0 to 70 °C. Although the particle size remained about 2 µm (Figure 3a,c), it is expected that higher temperature conditions would make crystallization more kinetically controlled, leading to the

formation of particle with a spherical shape. However, the particle size remained about 2 µm regardless of temperature changes from 0 to 70 °C. With increasing the reaction period from 4 to 48 h at a reaction temperature of 70 °C, the F(230)/ 70(48) samples showed a decaoctahedral shape and the particle size was increased from 2 to 3 µm (Figure 3c-e). From the viewpoint of crystallization kinetics,32 the crystal morphology is determined by the relative rates of growth of different crystal facets, with the slow-growing surfaces dominating the final form. The growth of the F(230)/T(H) mesoporous silica crystal would favor along the optimum orientation as the crystallization time is prolonged. The effect of acid concentration on the morphology was also studied. The morphology significantly varied from decaoctahedral to spherical shape with increasing the HCl/ CTEABr ratio from 230 to 460 in the presence of D-fructose at 70 °C (Figure 3f). These results are in agreement with the observation of Chao et al.33 and Tanglumlert et al.,34 who suggested that the growth of SBA-1 facets could be regarded as a reaction process where the surfactant aggregates and silicate species take on specific orientations at a particular pH. At lower acidity, the crystallization is slow and proceeds under more thermodynamically controlled, near equilibrium conditions, leading to the formation of decaoctahedral particles (Figure 3e). Higher acid concentrations, on the other hand, would render crystallization more kinetically controlled, leading to the formation of spherical particles (Figure 3f). 13 C CPMAS and 29Si MAS NMR. In order to explore the role of sugar in the synthesis, the as-synthesized F(230)/T(H) were characterized by 13C CPMAS NMR measurements as a function of reaction temperature and time (Figure 4). No 13C

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signal associated with D-fructose or its derivatives was observed in the samples synthesized at reaction temperatures lower than 70 °C, suggesting the absence of sugar moiety within the SBA-1 sample. The peaks in the range of 10-55 ppm are due to the signals from the surfactant CTEABr. The 13C CPMAS NMR measurements were carefully performed on the pure D-fructose and the SBA-1 samples synthesized in the presence of D-fructose at different status (i.e., before and after washing with water). There are two possible reasons why the 13C NMR signals of D-fructose in the SBA-1 samples were not detectable. First, the signal-to-noise (S/N) ratio of 13C CPMAS NMR spectrum for the pure D-fructose was about 10 times worse than that of the SBA-1 sample under the identical NMR experimental conditions. This could be due to the poor cross-polarization (CP) efficiency in D-fructose since D-fructose contains C(OH) groups instead of the CH2 groups as in the case of CTEABr. The fast exchange property of the protons in C(OH) groups could make the CP transfer from the protons to the carbon atoms very difficult. Second, the amount of D-fructose remained in the assynthesized SBA-1 sample is low when the synthesis temperature is lower than 70 °C. Although the SBA-1 sample with a high D-fructose/CTEABr ratio of 10.2 was prepared, the 13C NMR signals due to D-fructose could not be detected. As shown in Figure S3 (Supporting Information), a D-fructose/CTEABr molar ratio of 0.18 is sufficient to preserve the SBA-1 mesophase at high synthesis temperature. The color of the F(230)/T(4) sample also changed from white to slightly brown as the synthesis temperature is raised from 0 to 70 °C. This suggests that some carbonization has occurred. However, the amount of the carbonaceous species is low as compared to the surfactant CTEABr. This situation became more favorable when the sample was prepared with a longer synthesis time up to 72 h. As shown in Figure 4b and c, four broad peaks at around 180 and 60 ppm were observed for the samples synthesized at 70 °C for 72 h. These peaks are tentatively ascribed to the carbons of the furfuran ring of 5-hydroxymethylfurfural: the two protonated carbons of the furan ring (δ ) 110 ppm), the two unprotonated carbons of the furan ring (δ ) 151 ppm), and the aliphatic carbon of the aldehyde group (δ ) 174 ppm).35 This species remained within the sample and cannot be washed out. Due to the poor baseline and S/N ratio of the spectrum, the peak at δ ) 60 ppm expected for the aliphatic carbon of the methylene group can not be clearly observed. This clearly demonstrates that under such reaction conditions more derivatives of D-fructose, instead of pure D-fructose molecules, are observed by 13C CPMAS NMR. Therefore, to explore the role of D-fructose in the synthesis of SBA-1 at high temperature, a high D-fructose/CTEABr ratio of 10.2 or 20.4 is used and the 1 H spins, instead of the 13C spins, are chosen to be a more sensitive probe. 29 Si MAS NMR spectroscopy can provide direct information about the extent of silica condensation raised by variation of synthesis conditions. We have shown that the cross-linking degree of the as-synthesized F(230)/T(H) framework increases with increasing the reaction temperature from 0 to 90 °C.29 The effect of the reaction time on the condensation degree of silica framework was further investigated in this study. Three signals at -90, -100, and -110 ppm, corresponding to Q2 (Si(OSi)2(OH)2), Q3 (Si(OSi)3(OH)), and Q4 (Si(OSi)4) species, were observed in the 29Si MAS NMR spectra of the as-synthesized the F(230)/70(H) samples. When the reaction period was prolonged from 12 to 48 h (Figure 5), the fraction of Q4 sites at the expense of both Q2 and Q3 framework sites was progressively increased. This implied that promotion of reaction

Ting et al.

Figure 5. 29Si MAS NMR spectra of as-synthesized F(230)/70(H) samples, where H ) (a) 12, (b) 24, (c) 36, and (d) 48. The dashed lines are the components used for spectral deconvolution.

period at 70 °C from 12 to 48 h, which are only possible via the addition of D-fructose, assists the formation of a higher crosslinked silica framework. Effect of Acid Concentration and D-Fructose Content. As the stability of mesoporous silica materials strongly depends on the cross-linking degree of the silica framework, the use of high temperature conditions opens new possibility to improve the poor stability problems associated with SBA-1. Therefore, the synthesis parameters such as synthesis time and temperature and acid concentration are optimized in order to obtain SBA-1 with excellent hydrothermal stability, which is not obtainable without the aid of D-fructose. To investigate the influence of the acid concentration on the formation mesopore silica structure, the proper molar ratios of HCl to CTEABr were also investigated for the as-synthesized SBA-1 samples in the presence and absence of D-fructose (Figure S4, Supporting Information). Consistent with the previous studies,18 the F(A)/50(4) samples prepared with the ratio of HCl/CTEABr ranging from 48 to 460 were found to exhibit good structural ordering. These results demonstrated that, in the presence of D-fructose, highly ordered SBA-1 materials could be obtained in a wide HCl concentration range without any undesirable phase transformation. The influence of the ratio of D-fructose to surfactant on the synthesis of SBA-1 was also investigated. The powder XRD patterns (Figure S3, Supporting Information) of the assynthesized F(230)/50(4) samples as a function of D-fructose content exhibited well-ordered cubic structures as long as the D-fructose/surfactant ratio is over 0.18. This implies that the structural ordering is not very sensitive to the amount of D-fructose used. Hydrothermal Stability. To investigate the hydrothermal stability, each calcined sample was directly tested in boiling water at 100 °C for different periods. Figure 6 shows the XRD patterns of the SBA-1 samples prepared in the absence and presence of D-fructose with different acid concentrations, i.e., HCl/CTEABr ) 230 and 460, after treatment with boiling water for various periods of time. As seen in Figure 6, all of the

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19327 TABLE 1: Textural Properties of the P(230)/0(4), P(230)/ 0(48), and P(460)/0(4) Samples after Treatment with Boiling Water

sample P(230)/0(4)

P(230)/0(48)

P(460)/0(4)

time interval (h) for Vt DP boiling water a0 ABET treatment (nm) (m2/g)a (cm3/g)b (nm)c 0 1 4 12 24 0 1 4 12 24 96 120 0 1 8 12 24 96

7.6 8.6 8.5 8.5 8.6 7.4 7.0 6.8 7.0 7.0 7.0 7.7 7.7 7.7 7.5 7.5

1252 832 804 465 330 1198 1244 1170 1197 1032 739 140 988 791 686 634 533 140

0.68 0.55 0.55 0.42 0.32 0.64 0.64 0.59 0.60 0.86 0.71 0.36 0.57 0.48 0.55 0.47 0.42 0.36

2.4 2.2 2.4 3.0 3.8 2.3 2.3 2.3 2.3 3.5 4.1 9.5 2.1 2.1 2.6 2.6 5.1 9.5

a ABET is the surface area. b Vt is the total pore volume. c Dp is the pore size.

TABLE 2: Textural Properties of F(230)/70(48) and F(460)/70(48) Samples after Treatment with Boiling Water Figure 6. Powder XRD patterns of boiling water treated (A) P(230)/ 0(4), (B) P(460)/0(4), (C) F(230)/70(48), and (D) F(460)/70(48) samples. The duration of the boiling water treatment at 100 °C is indicated in the figure.

samples exhibited the cubic mesostructures even after treatment with boiling water for 8 h. With a short reaction time, the structural ordering of the conventional P(230)/0(4) and P(460)/ 0(4) samples prepared in the absence of D-fructose degraded significantly after treatment with boiling water for 12 h, and the structure was collapsed after 24 h. For comparison purposes, the P(230)/0(48) sample was also prepared to investigate the effects of the reaction time. As shown in Figure S5 (Supporting Information), the structural ordering of P(230)/0(48) was significantly degraded after treatment of boiling water for 24 h and completely collapsed for 120 h. On the other hand, the F(230)/70(48) sample prepared in the presence of D-fructose still preserved quite well with their structural ordering of the cubic mesostructures after treatment with boiling water up to 120 h. Remarkably, three XRD diffraction peaks are still clearly resolved for F(460)/70(48) after treatment with boiling water up to 192 h. Clearly, the samples prepared with a higher HCl/ CTEABr ratio using D-fructose as an additive exhibited much better hydrothermal stability to keep the integrity of well-ordered mesostructures. The N2 adsorption-desorption measurements were performed on P(A)/0(4), P(230)/0(48), and F(A)/70(48) after treatment with boiling water for different durations. The textural properties of these boiling water treated samples are summarized in Tables 1 and 2. The N2 adsorption-desorption isotherms of boiling water treated F(460)/70(48) samples are shown in Figure 7, along with those of P(460)/0(4) shown in Figure S6 (Supporting Information) for comparison. In general, the surface area and pore volume are progressively reduced as the duration of boiling water treatment was increased. For P(230)/0(4), the BET surface area and pore volume were significantly reduced when the

sample F(230)/70(48)

F(460)/70(48)

time interval (h) for boiling water a0 Vt DP ABET treatment (nm) (m2/g)a (cm3/g)b (nm)c 0 1 12 24 48 120 192 0 1 24 192

8.1 7.8 7.8 7.8 7.8 7.8 8.0 8.0 8.0 8.0

1195 954 951 890 726 660 36 1028 984 816 778

0.62 0.56 0.61 0.59 0.63 0.63 0.16 0.84 0.84 0.75 0.72

2.3 2.3 2.4 2.5 3.2 3.7 2.3 2.6 3.0 3.2

a ABET is the surface area. b Vt is the total pore volume. c Dp is the pore size.

duration of boiling water treatment increased from 0 to 24 h. However, the stability can be significantly enhanced when the synthesis time was increased from 4 to 48 h. As seen in Table 1, the BET surface area of P(230)/0(48) decreased about 38% after treatment of boiling water for 96 h. After 120 h, the structure of P(230)/0(48) was completely collapsed and its surface area was only 140 m2/g. As seen in Table 2, on the other hand, the F(230)/70(48) sample exhibited much better textural properties after the treatment of boiling water for 120 h. This is also in line with the fact that the Q4/(Q3 + Q2) ratio is 1.38 and 0.70 for F(230)/70(48) and P(230)/0(48), respectively, as revealed by 29Si MAS NMR studies. With a higher HCl concentration, the boiling water treated F(460)/70(48) sample still exhibited a cubic mesostructure with the decrease in the BET surface area about 30% after treatment with boiling water for 192 h (Table 2). Based on the above results, it can be concluded that the SBA-1 samples synthesized at 70 °C for 48 h, followed by short hydrothermal treatment at 100 °C for 1 h in the presence of the D-fructose, is an efficient way to produce the hydrothermally stable cubic SBA-1 mesostructure. 1 H MAS NMR. It is often difficult to attribute 1H resonance in solid-state NMR spectra to particular chemical species in

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Figure 7. Nitrogen adsorption-desorption isotherms of F(460)/70(48) after treatment with boiling water for (a) 0, (b) 1, (c) 24, and (d) 196 h.

complex systems, because the narrow proton chemical shift range and huge homonuclear 1H-1H dipolar coupling cause 1H NMR peaks to be often overlapping and broad, resulting in poor resolution in the spectrum. To observe the contribution from D-fructose and its derivatives more clearly, a D-fructose/CTEABr ratio of 10.2 or 20.4 is used for 1H related NMR studies. If less D-fructose is used, the 1H peaks from the CTEABr surfactant are dominant and the tiny peaks from D-fructose and its derivatives are difficult to be observed. Figure 8 shows the 1H MAS NMR spectra of the as-synthesized P(230)/0(4) and F(A)/ T(H) samples with different treatments. The 1H MAS NMR spectra showed three main peaks at 1.0 and 1.4 ppm, which are attributed to the alkyl chain CH3 protons or the protons in the polar head β-position of N-CH2-CH3 and the alkyl chain CH2 protons (exception made for N-CH2 in the R-position), respectively, and at 3.6 ppm mainly due to the protons in the polar head R-position of N-CH2-CH3.36 Although the 3.6 ppm might also be due to the CH2 protons of D-fructose,37 this possibility can be excluded by the fact that P(230)/0(4) and F(230)/0(4) exhibit nearly the same 1H MAS NMR spectra. As the self-assembly temperature was increased to 70 °C for 72 h, a new broad peak around 6.6 ppm appeared, which can be attributed to the protons of the furfuran ring of 5-hydroxymethylfurfural (inset, Figure 8) created by the dehydration of D-fructose in an acidic medium.35 The downfield shift of the protons of the furfuran ring of 5-hydroxymethylfurfural in the present study can be related to the electron-withdrawing effect from the nearby polar N+(C2H5)3 headgroup of the CTEABr surfactant. This could serve as a good indicator for the spatial proximity between the surfactant and the derivatives of Dfructose, and was further investigated by using 2D 1H-1H homonuclear exchange experiments. 2D 1H-1H Exchange NMR. In order to probe the spatial proximity between the derivatives of D-fructose and the surfactant CTEABr, 1H-1H 2D homonuclear exchange experiments were utilized in this study to trace the 1H magnetization transfer between different proton species in the materials. The 1H-1H magnetization transfer is governed by spin diffusion of the

Figure 8. 1H MAS NMR of as-synthesized (a) P(230)/0(4), (b) F(230)/ 0(4),(c) F(460)/70(72) (both b and c were without washing with water), and (d) F(460)/70(72) after washing with water. The samples in b, c, and d were prepared with a D-fructose/CTEABr ratio of 10.2, 20.4, and 20.4, respectively. All of the spectra were acquired at a spinning speed of 10 kHz.

proton nuclei involved. The efficiency of spin diffusion depends on the dipole-dipole interactions among different proton species and thus the molecular distances between the protons. Therefore, the diffusion of the proton magnetization allows the dipolarcoupled species to interact during the variable mixing period, and thus can be regulated to probe the increasing internuclear distances. Figure 9 shows the 2D 1H-1H homonuclear exchange experiments of the as-synthesized F(460)/70(72) sample as a function of mixing times, during which magnetization transfer may occurs between the various protons. These experiments were recorded for the sample dried overnight at 100 °C under vacuum to minimize the presence of adsorbed water in the samples. Cross peaks were not visible for a short mixing time of 1 ms (Figure 9a), indicating that the derivatives of D-fructose and the surfactant molecules can be considered to be isolated at this time scale. Upon increasing the mixing time to 5 ms (Figure 9b), the cross peaks labeled as “A” were observed. The presence of such cross peaks indicates that there is a correlation between the R-position protons of -NCH2CH3 (headgroup) of the surfactant and the protons of the furfuran ring of 5-hydroxymethylfurfural, which was derived from D-fructose. By further increasing the mixing time, the magnetization was transferred toward the protons of the headgroup of the surfactant, and then progressively transferred to the protons of the aliphatic chains (the cross peak “B” associated with the 1H peak at 1.4 ppm and the cross peaks for the 1H peak at 1.0 ppm as well), i.e., the interior of the surfactant micelle. The present 1H-1H exchange NMR results imply that the derivatives of D-fructose

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Figure 9. 2D 1H-1H exchange NMR spectra of as-synthesized F(460)/ 70(72) samples as a function of mixing time, tmix ) (a) 1, (b) 5, (c) 20, and (d) 100 ms, acquire with a spinning speed of 20 kHz.

are preferably located at the hydrophilic headgroup of the surfactant molecules. Thus, the simple 2D 1H-1H exchange

NMR technique provides a useful mean to establish the spatial proximity of the sugar molecules to the surfactant in the present complicated system. Proposed Formation Mechanism for Self-Assembly between CTEABr and D-Fructose. In contrast to the conventional synthesis condition, the transition of the mesophases from cubic to hexagonal is not observed in the present study even at reaction temperature above 40 °C. Although the chemistry of D-fructose in aqueous solution under strong acidic conditions could be very complicated, it must play a critical role to direct the formation of micellar structure for the cubic mesostructure. It has been known that sugar containing a large number of hydroxyl groups can undergo hydrolysis and oxidize to form a mixture of hydroxyl carboxylic acid such as gluconic acid, saccharic acid, glycolic acid and tartaric acid, under strong acidic conditions.38 Furthermore, similar to the silicate condensation reaction, D-fructose and its derivatives containing hydroxyl carboxylic acid undergo polymerization to form the saccharoidal-like resins. A possible explanation can be based on the classical micelle theory. The shape of the surfactant assemblies is dependent on the packing parameter of the surfactant molecules, g ) V/a0l, where g is the surfactant packing parameter, V is the total volume of the surfactant chain, a0 is the effective headgroup area at the micelle surface, and l is the surfactant tail length. With increasing the self-assembly reaction temperature, the thermal disorder due to the motion of surfactant tails causes an increase in the effective volume of the surfactant, leading to the surfactant parameter g > 1/3. As a result, phase transformation proceeds from cubic to hexagonal mesophase (1/3 < g < 1/2) is observed, as schematically illustrated in Scheme 1a,b. Without the aid of sugar, this is often observed for the synthesis of SBA-1 at high reaction temperatures. In contrast, such phase transformation can be avoided by adding simple sugar moiety in the gel in the present work. The sugar molecules act as a stabilizer for the cubic micelle during the synthesis, especially at high reaction temperatures. Sugar molecules such as D-

SCHEME 1: Proposed Schematic Representation for the Change of Surfactant Packing Parameter in the (a and b) Absence and (c) Presence of D-Fructose

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fructose, glucose, and maltose have been used as nonsurfactant templates in the sol-gel reactions of TEOS under basic or near neutral conditions to synthesize mesoporous materials.39,40 As revealed by 1H-1H exchange NMR, the derivatives of D-fructose containing a large number of hydroxyl groups are anticipated to be dominantly located at the hydrophilic headgroup of the surfactant molecules. It is possible that CTEABr and the derivatives of D-fructose are entangled with each other to form a mixed micelle, which is very stable with adequate curvature, irrelevant of temperature variation, to cause an increase in the effective headgroup area (a0) for the S+X-I+ micelle. Under such conditions, the g value can still be preserved to be smaller than 1/3 to form the necessary spherical micelles for the Pm3jn mesophase, as illustrated in Scheme 1c. The present 1H-1H 2D exchange NMR results also provide direct evidence that the derivatives of D-fructose are spatially in close proximity to the headgroups of the surfactant molecules. Conclusions We have presented a synthesis route for the preparation of well-ordered and stable SBA-1 at high temperatures simply by using low-cost and environmentally friendly D-fructose as an auxiliary agent in the synthesis mixture. In contrast to earlier methods requiring the low synthesis temperature (e.g., at 0 °C), this synthesis strategy would allow for the cubic SBA-1 framework structure to be assembled in a wide range of reaction times and temperatures. The role of D-fructose and its derivatives played in the synthesis, along with their spatial proximity to the surfactant headgroups, is unambiguously established through the use of 2D 1H-1H exchange solid-state NMR techniques. Based on these NMR results, a comprehensive explanation for the mesophase formation is proposed to account for the results observed at different synthesis temperatures. Acknowledgment. The financial support of this work by the National Science Council of Taiwan is gratefully acknowledged. Supporting Information Available: Powder XRD patterns of SBA-1 synthesized in the presence of glucose and inorganic salts (Figures S1 and S2) and with different D-fructose contents and HCl concentrations (Figures S3 and S4). Powder XRD patterns of P(230)/0(48) after treatment of boiling water for various periods (Figure S5) and N2 adsorption-desorption isotherms of boiling water treated P(460)/0(4) (Figure S6). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367. (2) Macquarrie, D. J. Chem. Commun. 1996, 1961. (3) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (4) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (5) Lim, M. H.; Blanford, C. F.; Stein, A. J. Am. Chem. Soc. 1997, 119, 4090. (6) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285.

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