J. Phys. Chem. B 2004, 108, 15043-15048
15043
Synthesis of High-Quality MCM-48 Mesoporous Silica Using Gemini Surfactant Dimethylene-1,2-bis(dodecyldimethylammonium bromide) Shuhua Han,*,† Jun Xu,† Wanguo Hou,† Xiaomei Yu,‡ and Youshao Wang§ Key Lab of Colloid and Interface Chemistry (Shandong UniVersity) Ministry of Education, Shandong UniVersity, Jinan, 250100, People’s Republic of China, Chemistry Department, Weifang School, Weifang, 261063, People’s Republic of China, and South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, People’s Republic of China ReceiVed: May 26, 2004; In Final Form: July 12, 2004
Using the Gemini surfactant [C12H25N+(CH3)2-(CH2)2-N+(CH3)2-C12H25]‚2Br- (abbreviated as C12-2-12) with the short spacer group (s ) 2) as structure-directing agent and sodium silicate as precursor, high-quality ordered cubic mesoporous silica (space group Ia3d) was prepared through the S+I- route (S denotes surfactant, I precursor). The samples were characterized by small-angle X-ray diffraction, transmission electron microscopy, and N2 adsorption-desorption techniques. Results showed that the pore structure of the resulting mesoporous silica belonged to the cubic structure (space group Ia3d). The high-quality cubic mesoporous structure was formed at 1:0.33 (molar ratio of sodium silicate to C12-2-12), 2:1 (ethyl acetate to sodium silicate), and at 30 °C. The formation conditions of MCM-48 with C12-2-12 were milder than those with the corresponding monovalent surfactants, such as alkyltrimethylammonium bromide. N2 adsorption-desorption curves revealed type IV isotherms and H1 hysteresis loops; Brunauer-Emmet-Teller (BET) surface areas increased with the decrease of the molar ratio of sodium silicate to C12-2-12 and of ethyl acetate to sodium silicate as well as of the hydrothermal temperatures.
Introduction Gemini surfactants1,2 are a relatively new class of amphiphilic molecules containing two headgroups and two aliphatic chains, linked by a spacer group [CnH2n+1N+(CH3)2-(CH2)s-N+(CH3)2CnH2n+1]‚2Br- (abbreviated as Cn-s-n, in this paper n ) 12 and s ) 2). Compared with the corresponding monovalent surfactants (single chain, single headgroup), Gemini surfactants have the following characteristics: (1) the critical micelle concentration (cmc) of Gemini surfactants was 1 or 2 orders of magnitude lower than that of monovalent surfatants; (2) they are much more efficient for decreasing the surface tension of water; (3) aqueous solutions of Gemini surfactants with a short spacer have a very high viscosity at relatively low surfactant concentration. Thus, Gemini surfactants have a potential application in skin care, antibacterials, construction of porous materials, analytical separations, and solubilization processes. The mesoporous materials of the M41S family were first reported by Mobil researchers in the early 1990s.3,4 Since then, mesoporous silica has attracted considerable attention because of its high surface area, ordered pore structure array, and narrow pore size distribution.5 In the M41S family, the structure of MCM-48 was thought of as two intertwined networks of spherical cages separated by a continuous silicate framework. As an adsorbent, catalyst,6 catalyst support, and template for the synthesis of advanced nanostructures,7-9 MCM-48 may be a more potent candidate than MCM-41 and MCM-50. To obtain a high-quality MCM-48, the following measures have been * To whom correspondence should be addressed. E-mail: shuhhan@ sdu.edu.cn. † Shandong University. ‡ Weifang School. § South China Sea Institute of Oceanology.
tried: using a mixture of cationic alkyltrimethylammonium surfactants (CnTMA) and neutral cosurfactants,10 or a mixture of cetyltrimethylammonium bromide and carboxylate anionic surfactants (CnH2nCOONa, n ) 11, 13, 15, 17),11 by controlling the molar ratio of surfactants to Si source,12 and via temperatureinduced phase transition of hexagonal to cubic.13 Although some measures for MCM-48 synthesis have been taken, the synthesis of MCM-48 was more difficult than that of MCM-41 and MCM50 due to the difficulty in controlling synthesis conditions. Recently, MCM-4814 was synthesized using Gemini surfactants C16-12-16 with the spacer group having 12 carbon atoms (s ) 12) at a base solution. Van Der Voort et al.15-17 have also synthesized high-quality MCM-48 using Cn-s-n (n ) 22, 18, 16; s ) 12, 10). For cationic Gemini surfactants, the surface area occupied by the surfactants at the interface was affected by the length of the spacer group. The surface area first increased and then decreased with the increase of spacer group carbon number, and the maximum value occurred at approximately 1012.18 The increase of surface areas favored formation of MCM48 according to the packing parameter of surfactants (g value);19,20 therefore, MCM-48 was synthesized using Cn-s-n with the spacer group carbon numbers of 10-12 as structuredirecting agent. In this paper, cationic Gemini surfactant C12-2-12 with the short spacer group (s ) 2) was used as a template, and sodium silicate as silica source, to synthesize high-quality MCM-48 through the S+I- route. The pH value of the reaction was controlled by the addition of ethyl acetate.21 The effects of the reaction conditions, for example, the molar ratio of sodium silicate to C12-2-12 and ethyl acetate to sodium silicate as well as hydrothermal temperature, on the pore structure and surface properties of mesoporous silica were studied. The formation mechanism of MCM-48 was also discussed.
10.1021/jp0477093 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/27/2004
15044 J. Phys. Chem. B, Vol. 108, No. 39, 2004
Han et al.
Experimental Section 1. Reagents. C12-2-12 Gemini surfactant was synthesized according to ref 22. The critical micelle concentration of C12-2-12 was 0.84 mmol/L, and the lowest point did not occur in the curve of surface tension versus concentration of the surfactant, indicating that C12-2-12 was pure. Crystalline sodium silicate was obtained from Shanghai Fourth Reagent Plant, which contained 21.05% (w/w) Na2O, and the molar ratio of Na2O to SiO2 was 1.03. Ethyl acetate was analytical grade, obtained from Zhangzhou Second Regent Plant. 2. Synthesis. In a typical procedure for the synthesis of mesoporous silica, an amount of C12-2-12 and 1.71 g of Na2SiO3‚9H2O were dissolved in 29.0 mL of distilled water, respectively. Then the surfactant solution and sodium silicate solution were rapidly mixed with stirring at room temperature. After 20 min, ethyl acetate21 was quickly added with vigorous stirring (the molar composition of the gel was 1 Na2SiO3:0.040.33 C12-2-12:2-8 CH3COOC2H5:537 H2O). After 5 min, a precipitate formed, and the solution was allowed to stand at room temperature for 5 h. The reaction mixture was then kept at different hydrothermal temperatures (30-100 °C) for 3 days in a heating box (the final pH value of the mixture was in the range 8-9). The resulting samples were recovered by filtration, washed with distilled water, and dried at room temperature. The surfactants were removed by calcination at 550 °C for 6 h in air. 3. Characterization. Powder small-angle X-ray diffraction (XRD) data were obtained on a D/max-rB model operating at low angle (2θ from 1° to 10°) with a Cu target at 40 kV and 100 mA, using a speed of 2°/min and a step of 0.01°. The transmission electron microscope (TEM) image was recorded using a JEOL JEM-200CX electron microscope, operating at 200 kV. The sample was crushed in an agate mortar, dispersed in ethanol, and deposited on a microgrid. Nitrogen adsorption-desorption isotherms of the materials were determined at 77 K with a conventional volumetric technique by a Coulter Omnisorp 100CX sorption analyzer. Every sample was degassed at 350 °C for 6 h under a pressure of 10-5 Pa or below. The surface area was calculated using the BET method in a relative pressure (P/P0) range of 0.05-0.25, and the pore size distribution was evaluated using the BarrettJoyner-Halenda (BJH) model. Results and Discussions 1. X-ray Diffraction Patterns of As-Prepared and Calcinated Samples. The powder X-ray diffraction (XRD) patterns (Figure 1) for mesoporous silica showed three peaks at 2θ ) 2.53°, 2.92°, and 4.87° with d values of 3.49, 3.02, and 1.81 nm, respectively. These peaks were indexed as (211), (220), and (332) diffraction peaks, which belonged to a bicontinuous space group (Ia3d) of a cubic system. After calcination at 550 °C for 6 h in air (see Figure 1), these diffraction peaks were slightly shifted to a larger angle due to shrinkage of the silica framework. The degree of shrinkage upon calcination was very small (ca. 2.6%, according to calculation of d211 values in Figure 1). At the same time, the intensities of all diffraction peaks increased after calcinations, showing an increased ordering in the pore channel array. The unit-cell parameters of the cubic system (calculated according to the equation ao ) (h2 + k2 + l2)1/2d211 for as-prepared and calcinated samples) were 8.55 and 8.33 nm, respectively.
Figure 1. XRD patterns for the mesoporous silica prepared from the starting mixture 1 Na2SiO3:0.25C12-2-12:2 CH3COOC2H5:537 H2O under neutral conditions. Upper, calcinated; lower, as-prepared.
Figure 2. TEM of the mesoporous silica prepared from the starting mixture 1 Na2SiO3:0.25C12-2-12:2 CH3COOC2H5:537 H2O.
The TEM image (Figure 2) indicated that the mesostructure silica has a well-ordered cubic mesostructure, which was in agreement with the results of XRD. 2. Effect of the Molar Ratio of Sodium Silicate to C12-2-12 on the Mesoporous Structure of Silica. When the molar ratio of sodium silicate to C12-2-12 (x value denotes a molar ratio of sodium silicate to C12-2-12) was below 1:0.16, three diffraction peaks of (211), (220), and (332) occurred (see Figure 3). When the x value was above 1:0.16, only one diffraction peak of (211) appeared. Also the intensity of the diffraction peak (211) increased with the decrease of x. These results indicated that the ordering of pore structure increased with the decrease of x value. Compared with the corresponding monovalent surfactant, such as CnTMA, MCM-48 was obtained in the molar ratio from 1:1.54 to 1:0.67 (the molar ratio of Si source to CnTMA),12 and the molar ratio of sodium silicate to CnTMA was lower than that of sodium silicate to C12-2-12. These results showed that it was easier for C12-2-12 to self-assemble than for CnTMA. According to the formula ao ) (h2 + k2 + l2)1/2d211, the unit cell ao of cubic system was calculated and listed in Table 1. As we can see from Table 1, the ao value was in the region 8.538.57 nm and not influenced by the x value. At an x value of 1:0.04, a type II isotherm was appeared in the N2 adsorption-desorption curves (see Figure 4). This showed that the pore size of the resulting sample was in the microporous region. But when the x value was below 1:0.08, the isothermal type transformed from type II to type IV, indicating that the pore size of the resulting samples was in the mesoporous region.23 These results indicated that when the x value was below 1:0.08, a mesoporous structure formed, and this conclusion was consistent with that from XRD (Figure 3F).
High-Quality MCM-48 Mesoporous Silica
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Figure 4. N2 adsorption-desorption curves of calcinated sample. From top to bottom the molar ratio of Na2SiO3 to C12-2-12 (x value) is 1:0.04, 1:0.08, 1:0.16, 1:0.33, respectively. Inset: Pore size distribution curve at an x value of 1:0.33 (calculated from the adsorption branch of the isotherms).
Figure 3. Effect of the molar ratio of Na2SiO3 to C12-2-12 (x value) on the pore structure of as-synthesized silica: A (1:0.33); B (1:0.25); C (1:0.16); D (1:0.12); E (1:0.08); F (1:0.04).
TABLE 1: Effect of the Molar Ratio of Na2SiO3 to C12-2-12 (x value) on the Pore Structure and Surface Properties of Mesoporous Silica
x value
d211, nm
1:0.33 1:0.25 1:0.16 1:0.12 1:0.08 1:0.04
3.4892 3.4892 3.5030 3.4883 3.4819 3.5030
H1/2,a ao, nm nm 0.17 0.14 0.20 0.26 0.17 0.17
SBET, most probable H1/2,c m2/g pore size,b nm nm
8.55 1174 8.55 947 8.57 991 8.54 937 8.53 756 8.57 456
2.22 2.14 2.20 2.19 2.19
0.20 0.31 0.30 0.36 0.36
total pore volume, cm3/g 0.84 0.95 0.88 0.78 0.11
a
Half-peak width (H1/2) of the (211) diffraction peak. b Calculated from adsorption data. c Half-peak width (H1/2) of the pore size distribution.
On the other hand, both the BET surface area and the total pore volume increased with the decrease of the x value (Table 1), and the maximum BET surface area was 1174 m2/g at an x value of 1:0.33. Two H1 hysteresis loops appeared in the relative pressure (P/P0) ranges of 0.15-0.30 and 0.8-1, which were attributed to the primary pores of mesoporous silica and to the voids between particles, respectively. The primary pores and voids were consistent with “framework-confined” mesoporosity and “textural” mesoporosity, which were defined and differentiated by Pinnavaia et al.24,25 The framework-confined mesoporosity was the porosity contained within the uniform channels of the templated framework. The textural mesoporosity was the porosity arising from noncrystalline intraaggregate voids and spaces formed by interparticle contacts. The size of the textural mesoporosity was determined mainly by the size, shape, and number of interfacial contacts within the aggregates. The
presence of textural mesoporosity was verified by the appearance of a well-defined hysteresis loop in the P/P0 range from 0.5 to 1.0. In the relative pressure (P/P0) range of 0.15-0.30 the area of the H1 hysteresis loop was very small, and adsorption and desorption curves were almost superposed, indicating that the size of the pore channels of the resulting samples was uniform; that is, “ink bottle” channels was not present. The most probable pore size with the BJH model was in the range 2.14-2.22 nm (see inset in Figure 4 and Table 1), but the half-peak width (H1/2) of the pore size distribution was only about 0.14 nm, and it was hardly affected by the x value. The most probable pore size (BJH model) when C12-2-12 was used as template was smaller than that using dodecyltrimethylammonium bromide (C12TMA) as template (3.07 nm26). Usually, the most probable pore size was determined by the size of the hydrophobic core of the surfactant micelles (not the alkyl chain length of the surfactants). On the basis of the micellar model of the Gemini surfactants, which was proposed by Hirata et al.,27 the shape of the Gemini micelle was prolate and the micelles had a hydrophobic core with a major axis a and minor axis b. The b value was set to be equal to the fully extended length of the portion of the alkyl chain that constituted the hydrophobic core, i.e., the radius of the hydrophobic core of the Gemini micelle. The b and l values (maximum length of the alkyl chain of the surfactant) were calculated by the following equations:
b ) 0.295 + 0.127(nc - nwet)
(1)
l ) 0.295 + 0.127nc
(2)
where nc is the number of carbon atoms in one alkyl chain of the Gemini molecules; nwet is the number of hydrated methylene groups for one alkyl chain. For C12-2-12, nc ) 12 and nwet ) 3.3,27 the b and l values were then calculated to be 1.40 and 1.80 nm, respectively. If
15046 J. Phys. Chem. B, Vol. 108, No. 39, 2004
Figure 5. Effect of the molar ratio of CH3COOC2H5 to Na2SiO3 (y value) on the pore structure of as-synthesized samples (the molar ratio of Na2SiO3 to C12-2-12 is 1:0.25).
framework shrinkage of mesoporous silica caused by calcinations was considered, the pore size (2.80 nm) obtained with b ) 1.40 nm was consistent with that from the N2 adsorptiondesorption curves (see Table 1). The pore size (3.60 nm) obtained with l ) 1.80 nm was, however, greater than that from the N2 adsorption-desorption curves. According to the above method, the pore size using C12TMA as template could be calculated from eq 1 also. For C12TMA, nc ) 12 and nwet ) 2.4,28 the b value was calculated to be 1.51 nm, but the l value did not change. The pore size (3.02 nm) obtained with b ) 1.51 nm was in accord with the reported result (3.07 nm26). Therefore, the difference in the pore size between C12-2-12 and
Han et al. C12TMA was attributed to that of the molecular structures. C12-2-12 and C12TMA had the same alkyl chain length, but C12-2-12 had two quaternary ammonium heads, while C12TMA had only one quaternary ammonium head. The ability of C12-2-12 to bind hydrated water was stronger than that of C12TMA; that is, nwet of C12-2-12 was larger than that of C12TMA. 3. Effect of the Molar Ratio of CH3COOC2H5 to Na2SiO3. The molar ratio of CH3COOC2H5 to Na2SiO3 (y value denotes a molar ratio of CH3COOC2H5 to Na2SiO3) played a key role in the synthesis of high-quality cubic silica (see Figure 5). When the y value was in the range 2:1-3:1, the pattern exhibited three well-resolved diffraction peaks, indexed as (211), (220), and (332). At a y value of 4:1, the pattern showed two diffraction peaks of (211) and (220), while at a y value of 8:1, the pattern showed only one diffraction peak of (211). These indicated that the pore structure of the samples became more ordered with the decrease of the y value. The reason that ethyl acetate affected the pore structure ordering was that ethyl acetate is a good cosolvent for C12-2-12, and excess ethyl acetate could destroy the micellar template and make the pore structure less ordered. So, the ordering of pore structure increased over the y value range of 2:1-3:1. The N2 adsorption-desorption isotherms of a calcinated sample (see Figure 6) showed a typical IV type curve, indicating that the pore size of the resulting cubic structure was in the mesoporous range. There were two H1 hysteresis loops occurring in the relative pressure (P/P0) ranges of 0.15-0.30 and 0.8-1, which were attributed to the primary pores of mesoporous silica and to the voids between particles, respectively. The most probable pore sizes using the BJH model of the calcinated sample (see inset in Figure 6), BET surface areas, and total pore volumes are listed in Table 2. As seen from Table 2, the unit-
Figure 6. N2 adsorption-desorption isotherms at 77 K of the calcinated mesoporous silica at different y values. From upper to lower: 8:1, 4:1, 3:1, 2:1. Inset: Pore size distribution at a y value of 2:1 (calculated from the adsorption branch of the isotherms).
High-Quality MCM-48 Mesoporous Silica
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TABLE 2: Effect of the Molar Ratio of CH3COOC2H5 to Na2SiO3 (y value) on Pore Structure and Surface Properties of Samples y value
d211, nm
2:1 3:1 4:1 8:1
3.4892 3.4839 3.4756 3.8717
H1/2,a a0, SBET, most probable H1/2,c nm nm m2 g-1 pore size,b nm nm 0.14 0.24 0.15 0.18
8.55 8.53 8.52 9.48
991 828 752 689
2.13 2.01 1.97 2.27
total pore volume, cm3 g-1
0.31 0.31 0.33 0.44
0.95 0.89 0.99 1.18
a Half-peak width (H1/2) of the (211) diffraction peak. b Calculated from adsorption data. c Half-peak width (H1/2) of the pore size distribution.
Figure 8. N2 adsorption-desorption isotherm at 77 K of the calcinated mesoporous silica at different hydrothermal temperature. From upper to lower: 100 °C, 80 °C, 50 °C, 30 °C. Inset: Pore size distribution of mesoporous silica at 30 °C.
Figure 7. Effect of the hydrothermal temperature on the pore structure of as-synthesized silica. From upper to lower: 100 °C, 80 °C, 50 °C, 30 °C (the molar ratio of the starting mixture is 1 Na2SiO3:0.25 C12-2-12: 2 CH3COOC2H5:537 H2O).
cell parameter of the cubic structure of silica was in the range 8.52-9.48 nm, the most probable pore size with the BJH model was 2.10 nm or so, and total pore volume was in the range 0.89-1.18 cm3 g-1. The BET surface area increased and halfpeak width (H1/2) of the pore size distribution decreased with the decrease of the y value. The maximum BET surface area was 991 m2 g-1 and the minimum H1/2 (half-peak width of the pore size distribution) was 0.39 nm at a y value of 2:1. Thus, at this y value, high-quality cubic silica was obtained. 4. Effect of the Hydrothermal Temperature. The effect of the hydrothermal temperature on the pore structure of mesoporous silica is shown in Figure 7. When the hydrothermal temperature was lower than 80 °C, three diffraction peaks, (211), (220), and (332), appeared. At the hydrothermal temperature of 100 °C, only the (211) diffraction peak occurred. These results indicated that high-quality cubic mesoporous silica was obtained using lower hydrothermal temperatures. This result is instructive for large-scale production of cubic mesoporous silica. Gemini surfactant C12-2-12 possessed two hydrophobic carbon chains, and it was easier to self-assemble than the corresponding
monovalent surfactants. Thus, using cationic Gemini surfactants as template, the conditions for mesoporous silica synthesis were mild as compared with the corresponding monovalent surfactants as template. The unit cell constant a0 of cubic mesoporous silica was ca. 8.55 nm (Table 3) and is hardly affected by the hydrothermal temperature. Figure 8 shows the nitrogen adsorption-desorption isotherm of the calcinated mesoporous silica at different hydrothermal temperatures. These curves are similar to those in Figures 4 and 6. BET surface area, total pore volume, the most probable pore size with the BJH model, and the half-peak width of pore size distribution are listed in Table 3. With the increase of the hydrothermal temperature, the BET surface area and the total pore volume decreased, the half-peak width of the pore size distribution increased, and the most probable pore size with the BJH model had almost no change. The maximum of the BET surface area was ca. 1200 m2/g at 30 °C. The reasons that high-quality MCM-48 mesoporous silica could be obtained using C12-2-12 with the short spacer were as follows: First, according to the packing parameter of surfactants (g value), g ) V/al, where V is the total volume of the surfactant chains plus any cosolvent (organic molecules) between the chains, a is the effective headgroup area at the micellar surface, and l is the dynamic surfactant tail length. The g values of the hexagonal and cubic phase were in the ranges 1/3-1/2 and 1/22/3, respectively. Compared with the corresponding monomeric
TABLE 3: Effect of the Hydrothermal Temperature on the Pore Structure and Surface Properties of Samples hydrothermal temperature, °C
d211, nm
H1/2,a nm
a0, nm
SBET, m2 g-1
most probable pore size,b nm
H1/2,c nm
total pore volume, cm3 g-1
30 50 80 100
3.4892 3.4892 3.4892 3.5170
0.18 0.20 0.14 0.21
8.55 8.55 8.55 8.61
1238 1100 947 798
2.16 2.19 2.14 2.02
0.21 0.20 0.31 0.34
1.12 0.94 0.95 0.83
a
Half-peak width (H1/2) of the (211) diffraction peak. b Calculated from adsorption data. c Half-peak width (H1/2) of the pore size distribution.
15048 J. Phys. Chem. B, Vol. 108, No. 39, 2004 surfactant C12TMA, the g value of C12-2-12 was larger than that of C12TMA (because the V value of C12-2-12 was double that of C12TMA), but the ratio of the a value of C12-2-12 (0.96 nm2) to C12TMA (0.59 nm2) is less than 2.29 Second, MCM-48 was prepared through the S+I- route, where S+ ) cationic surfactant and I ) precursor. A strong electrostatic interaction existed between cationic surfactants and sodium silicate, and the electrostatic interaction made the a value of C12-2-12 decrease and the g value of C12-2-12 increase. Therefore, the increase of the g value of C12-2-12 favored formation of MCM48. Conclusions 1. Under mild conditions, high-quality MCM-48 mesoporous silica material has been synthesized using Gemini surfactant with the short spacer, s ) 2 (C12-2-12), as structure-directing agent and sodium silicate as precursor. 2. The results showed that the ordering of the pore structure increased with the decrease of the x value (molar ratio of sodium silicate to C12-2-12) and of the y value (ethyl acetate to sodium silicate) as well as the hydrothermal temperature. The highquality cubic mesoporous structure was formed at an x value of 1:0.33, a y value of 2:1, and a hydrothermal temperature of 30 °C. 3. There were type IV adsorption-desorption isotherms and two H1 hysteresis loops in the N2 adsorption-desorption curves. The maximum BET surface area and the total pore volume were about 1200 m2/g and 1.12 cm-3/g, respectively; the most probable pore size with BJH diameter was 2.10 nm or so. Acknowledgment. This research is financially supported by the Key Project Foundation of the Ministry of Education of China. References and Notes (1) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205. (2) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906.
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