Novel One-Step Synthesis Route to Ordered Mesoporous Silica

Ind. Eng. Chem. Res. 2010, 49, 583–591

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Novel One-Step Synthesis Route to Ordered Mesoporous Silica-Pillared Clay Using Cationic-Anionic Mixed-Gallery Templates Huihui Mao,†,‡ Baoshan Li,*,† Xiao Li,† Liwen Yue,† Zhenxing Liu,† and Wei Ma† State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P.R China, and Department of Chemical Engineering, Jiangsu Polytechnic UniVersity, Changzhou, Jiangsu ProVince 213164, P.R. China

A novel one-step synthesis route to ordered mesoporous silica-pillared clay (SPC) with cantonic-anionic mixed-gallery templates has been successfully established. This approach involves the type and formation of surfactant mixture for SPC through intragallery ammonia-catalyzed tetraethoxysilane (TEOS). The formation of a precursor with a micromesoporous framework is a result of TEOS hydrolysis and the presence of surfactant micelles in the gallery regions under ammonia-catalyzed conditions. The type of structure was confirmed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and low-temperature N2 physisorption. Addition of the anionic surfactant resulted in changes of the layered structure and ratio of micropores to mesopores. The formation mechanism of SPC derivatives is proposed. 1. Introduction The development and utilization of clay minerals has long history.1-4 Today, clay and its derivatives have become indispensable materials in many fields such as high-technology areas and industrial and agricultural production. Because clay minerals are natural catalyst materials, however, many defects such as poor thermal stability and wide distributions of pore sizes limit the application of these materials. To bridge the gap between the properties of clay and the need of catalysts in industry, modifications based on the clay minerals is necessary. Montmorillonites are a family of clay minerals with a layered structure. Their special properties, such as cation-exchange capacity, swelling, and resource richness, make them attractive for catalytic applications. Since the introduction of the M41S family of ordered mesoporous silicates in 1992, impressive progress in the development of many new mesoporous solids based on the same mechanism of templating has been reported. In 1995, clay-based mesoporous solids were synthesized by the formation of an MCM-like porous silica structure between clay layers.5 Such materials are designated porous clay heterostructures (PCHs), bridging microporous zeolites and pillared clays on one hand and mesoporous silica or alumina on the other. PCHs are interesting materials because of their large surface areas and unique combined micro- and mesoporosity. The preswelling method can be used in the synthesis of PCHs with ordered pore sizes.6-10 However, it requires the handling of large amounts of surfactants and long-chain amines, and therefore, it is not suitable for large-scale industrial application. In recent research, we synthesized mesoporous silica-pillared montmorillonite materials with ordered mesopores between the layers by intragallery ammonia-catalyzed hydrolysis of tetraethoxysilane (TEOS).11-14 The reaction involved the hydrolysis and condensation of tetraethoxysilane in the presence of intragallery surfactant templates. Our results indicate that surfactants play a decisive role in pore formation, because they * To whom correspondence should be addressed. Address: Baoshan Li, No. 15, Beisanhuan East Road, Chaoyang District, Beijing 100029, P.R. China. Tel.: +86-010-64445611. E-mail: [email protected]. † Beijing University of Chemical Technology. ‡ Jiangsu Polytechnic University.

act as micelle-like templates during the hydrolysis of TEOS. Upon addition of a gel mixture of the surfactants and a Si source, TEOS and the surfactants form micellar templates, in interaction with polymerized silicate species at the outer surface of the micelles. Based on the polymerization of the silicate species, a dense Si network is formed in situ on the clay host. The removal of the templates creates ordered mesopores in the gallery regions of the layered clay. The excellent acid catalytic activity, together with a well-organized and stable porous structure, opens up new opportunities for these materials for applications in catalysis such as in heavy oil cracking, selective oxidation, and hydrogenation. In 1996, research on the synthesis of MCM-41 using mixed cationic surfactants as templates was reported.15 The use of mixed micellar templates allows the possibility of gaining a better understanding of how to control the pore structure. From 1997 to 1999, use of a mixture of cetyltrimethylammonium bromide and carboxylate anionic surfactant as templates to prepare siliceous MCM-48 with a low molar ratio of mixed surfactants to silica and a low concentration of mixed surfactants was reported.16,17 Much information has been supported by the new novel method. Montmorillonite is characterized by an excellent ion-exchange capacity. A great variety of organic compounds can be adsorbed by montmorillonite. Research about organic modifications of montmorillonite has been widely reported, with most modification agents being cationic surfactants. In 2000, Zhu and other scientists reported montmorillonites modified by cationic-anionic surfactants.18 Additionally, these materials exhibit excellent adsorption properties. In this study, we report a new synthetic route for the preparation of silica-pillared clays (SPCs) using cationic-anionic mixed surfactants. This strategy involves the type and formation of surfactant mixtures for SPCs through intragallery ammoniacatalyzed tetraethoxysilane. The use of mixed micellar templates allows for the possibility of gaining a better understanding of how to control this interaction. We have researched the synthesis and characterization of silica-pillared clays prepared by cationic-anionic mixed surfactants as the gallery templates and wish to use this new method to control the micropores and mesopores formed in the gallery. The synthesis of SPC materials with ordered mesopores is of significant importance in industrial

10.1021/ie9011563  2010 American Chemical Society Published on Web 11/24/2009

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Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010

applications as a consequence of low cost, benignity to the environment, and synthetic convenience of these materials. 2. Experimental Details 2.1. Procedure. The raw clay had a basal spacing of d001 ) 14.7 Å and an anhydrous structural (layer) formula of [Si7.86Al0.14][Al2.84Fe0.30Mg0.86]O20(OH)4. The natural montmorillonite clay (MMT) was obtained from Inner Mongolia and was used without any further purification or cationic-exchange process. Acetone, benzyl chloride, cetyltrimethylammoniumethyl bromide (C16TMAEB), bromoethane, and N,N-dimethyl octadecylamine (A.R.) were purchased from Beijing Yijing Fine Chemicals Company, Beijing, China. Tetraethoxysilane (TEOS) (A.R.), ammonia (25%), acetone (A.R.), and ethanol (95%) were purchased from Beijing Chemical Reagents Company, Beijing, China. Sodium dodecyl sulfate (SDS) and dodecyl dimethyl benzyl ammonium chloride (C12DMBACl) (A.R.) were purchased from Tianjin Surfactants Company, Tianjin, China. In the preparation of octadecyl dimethyl ethyl ammonium bromide (C18DMEAB), N,N-dimethyl octadecylamine was combined with bromoethane at molar ratio of 1:1.2 at 40 °C for 10 h. The product was filtered and washed by acetone. The molecular structure of C18DMEAB was characterized by solidstate 29Si (59.6 MHz) MAS NMR spectroscopy on a Bruker AV 300 spectrometer. The following experimental parameters were employed for 29Si MAS NMR experiments: 5.0-kHz spinning frequency, 90° pulse width of 10 µs, 40-s recycle delay, 2000 scans. The preparation of octadecyl dimethyl benzyl ammonium chloride (C18DMBACl) was similar to the method for C18DMEAB, except the bromoethane was replaced by benzyl chloride. Natural montmorillonite clay (MMT) was first suspended in 30 mL of water. Cationic surfactant and SDS were dissolved in ethanol, to which tetraethoxysilane (TEOS) was added, and the mixture was stirred for 0.5 h until a clear solution was obtained. This solution was slowly dropped into the clay suspension. The gel mixture with a clay/cationic surfactant/SDS/ TEOS/ethanol/water molar ratio of 1:2:0.4:30:1.2:250 was stirred for 0.5 h. Then, the pH of the gel mixture was adjusted by ammonia solution to 10, and the gel mixture was stirred for 2 h at room temperature. The product was recovered by filtration, dried in an oven at 90 °C, and subsequently calcined at 600 °C for 3 h using a programmed furnace in air (with temperature increasing at a rate of 2 °C/min). The prepared SPC derivatives samples are denoted based on the cationic surfactants used: C12DMBACl, MSPC-12-7; C16TMAB, MSPC-16-1; C18DMEAB, MSPC-18-2; and C18DMBACl, MSPC-18-7. To compare the layered and porous structures of the SPC derivatives, comparable samples were prepared. The synthetic process was the same as for the SPC derivatives using cationic-anionic mixed surfactants except that no SDS was added. These samples are correspondingly denoted as SPC-127, SPC-16-1, SPC-18-2, and SPC-18-7. 2.2. Characterization. The obtained solid products were characterized by X-ray diffraction on a Rigaku D/Max 2500 VBZ+/PC diffractometer using Cu KR radiation (40 kV, 50 mA) in the range between 0.5° and 10° 2θ (small-angle range) and Cu KR radiation (40 kV, 200 mA) in the range between 3° and 90° 2θ (wide-angle range). N2 adsorption isotherms were obtained using a Micromeritics ASAP 2000 instrument. The samples were degassed at 115 °C for 8 h before measurements. The specific surface area (SBET) was estimated by the Brunauer-Emmett-Teller (BET) equation, and the mesopore size distribution and mesopore analysis

Figure 1. Small-angle XRD patterns of (A) SPC-12-7, SPC-16-1, SPC18-2, and SPC-18-7 and (B) MSPC-12-7, MSPC-16-1, MSPC-18-2, and MSPC-18-7.

were obtained from the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. The micropore size distributions were obtained using the Horvath-Kawazoe (HK) method. Scanning electron microscopy (SEM) micrographs were obtained on a Hitachi S-4700 microscope operated at 30 kV. Thermal analyses of the products were performed on a PerkinElmer differential-scanning-calorimeter-type thermal analysis system (DSC-2). The measurements were carried out in static air with a heating rate of 10 °C/min. 3. Results and Discussion 3.1. X-ray Diffraction Analysis. Figure 1A shows X-ray diffraction patterns of the four calcined SPC samples. The order of the (001) X-ray reflection, indicating layered structures, was observed for all of the products.19 All of the products have (001) reflections corresponding to basal spacings of approximately 3.2-4.5 nm. It is very important to note that the molecular length of the surfactants used is an important factor in the enlargement of the basal spacing of the products. Increasing the molecular length of the surfactants increases the basal spacing of the products. However, SPC-18-7 has a relatively smaller basal spacing than SPC-16-1 and SPC-18-2. This is because the surfactant C18DMBACl has weaker polarity than C16TMAB and C18DMEAB. The arrangement of C18DMBACl in the gallery regions is more compact because of the van der


Waals forces between the molecules and the relatively weaker electrostatic repulsion between the hydrophilic heads of the surfactant. Moreover, the basal spacings are near the values expected based on the long chain lengths of the surfactants. Because the clay layer sheet thickness is 0.96 nm, the corresponding gallery heights are nearly double the molecular length of the surfactants. This implies that the surfactant molecules in the gallery are arranged as molecular lamellar bilayers and that TEOS produces firm enough silica pillars to prop up the expanded gallery after the removal of the surfactants.20 Figure 1B shows X-ray diffraction patterns of MSPC-12-7, MSPC-16-1, MSPC-18-2, and MSPC-18-7. MSPC-12-7 exhibits a 001 reflection that is similar to that of SPC-12-7 in Figure 1A. This indicates that MSPC-12-7, which was prepared using a cation-anionic mixed template keeps the same lamellar structure and high basal spacing (3.2 nm). However, for MSPC16-1 and MSPC-18-2, the (001) reflections are much different from those of SPC-16-1 and SPC-18-2. The MSPC-16 sample exhibits the (001) reflection at 2θ ) 4.5°, and the corresponding basal spacing is much smaller than that of SPC-16-1. For MSPC18-2, the peak cannot even be observed, which indicates that the disordered collapse of interlayer spaces occurred.19 The XRD pattern for MSPC-18-7 exhibits a wide, rather than sharp, (001) peak. This indicates that MSPC-18-7 keeps a lamellar structure but that the degree of regularity is limited. As for the synthetic process of the SPC samples, the cationic surfactants intercalated into the clay interlayers by ion exchange, and the ionic polarity of the surfactants is an important factor for successful ion exchange. Both C16TMAB and C18DMEAB have the relatively strong polarities and exposed hydrophilic heads. With the addition of anionic SDS, an association reaction between the cationic and anionic surfactants probably occurred. The association reaction affects the ion-exchange process, so the quantity of surfactant that intercalates into the interlayers is limited. As a result, the organic Si source cannot intercalate into the gallery regions enough by solvent. Most of the Si source stays in the outer gallery areas, and firm enough pillars cannot form in the gallery. Accordingly, in this situation, the removal of the surfactant by calcination can cause the disordered collapse of the interlayer spaces, which results in severe peak broadening or peak disappearance, as observed for MSPC-18. On the other hand, the disordered collapse of interlayer spaces might not occur, but a limited quantity of interlayer surfactants and TEOS might not be able to form high basal spacing, as observed for MSPC-16. This implies that the pillar size and strength depend on the quantities of gallery surfactants and TEOS. C12DMBACl has weak polarity and an unexposed hydrophilic head; nevertheless, with added SDS, this “pillared” process still occurred successfully. Moreover, the gallery height was found to decrease slightly (as shown in Figure 2). This phenomenon can be rationalized by a combination of factors, the most important being the force between the surfactants. For cationic surfactants, there are two types of forces: (1) van der Waals force between hydrocarbon chains and (2) electrostatic repulsion between the hydrophilic heads of the surfactant. The added anionic surfactant increases the van der Waals force between hydrocarbon chains and decreases the cationic electrostatic repulsion. As a result, the surfactants in the gallery regions arrange more closely and reduce the basal spacing of the clay. For C18DMBACl, the weak polarity and unexposed hydrophilic head interfere with the association reaction between the cationic and anionic surfactants. However, because its polarity is too weak, C18DMBACl cannot form an ordered arrangement

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Figure 2. Comparable small-angle XRD patterns of SPC-12-7 and MSPC12-7.

Figure 3. Wide-angle XRD patterns of (A) SPC-12-7, (B) SPC-16-1, (C) SPC-18-2, (D) SPC-18-7, (E) MSPC-12-7, (F) MSPC-16-1, and (G) MSPC18-7. (See the Supporting Information for the wide-angle XRD pattern of MSPC-18-2.)

with added SDS in the gallery regions. In the XRD pattern, MSPC-18-7 exhibits a wide peak. Wide-angle XRD patterns for the samples are shown in Figure 3. Typical peaks for all samples of the trioctahedral subgroup of 2:1 phyllosilicates are observed, which are ascribed to (110), (020), (004), (130), (200), (330), and (060) diffractions. The results indicate that the crystalline structure of the clay sheets has not been destroyed.21 3.2. SEM Image Analysis. The typical morphologies of the samples are shown in Figure 4. MMT particles are composed of plates, and all of the SPC derivatives synthesized using cationic surfactants alone exhibited similar morphologies, except that the particles seemed to be swelled. Most of the platelets in the samples were unaffected by hydrolysis, although they swelled slightly more than the natural clay. Both samples MSPC-12-7 and MSPC-18-7 exhibited SEM morphologies similar to those of the SPC derivatives. In the image of MSPC16-1, some small particles can be observed around platelets (arrow). The small particles are probably broken platelets and amorphous SiO2 caused by the hydrolysis of surface TEOS. However, this platelet destruction did not reflect MSPC-16 gallery structure destruction, probably because the gallery

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Figure 4. SEM images of (A) MMT, (B) SPC-12-7, (C) SPC-16-1, (D) MSPC-18-7, (E) MSPC-12-7, (F) MSPC-16-1, and (G) MSPC-18-2.

structure within broken platelet particles is preserved.19,20 In the image of MSPC-18-2, the surface of the clay is different from those of the other samples. The layered structure of clay can hardly be observed because almost all of the surface was covered by the amorphous SiO2. Most of the TEOS hydrolysis occurred in the surface of the sheets and the extragallery regions

of the layered clay. Moreover, the layered structure was probably destroyed during the removal of the organic compounds. The SEM images of the samples are in great agreement with the small-angle X-ray diffraction data. 3.3. Analysis of the Porous Structure. Typical N2 adsorption-desorption isotherms of the samples are illustrated in


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Figure 5. N2 adsorption-desorption curves for MMT, SPC-12-7, SPC-16-1, SPC-18-2, SPC-18-7, MSPC-12-7, MSPC-16-1, MSPC-18-2, and MSPC-18-7.

Figure 5 for MMT, SPC-12-7, SPC-16-1, SPC-18-2, SPC-187, MSPC-12-7, MSPC-16-1, MSPC-18-2, and MSPC-18-7. The data on the surface areas, pore volumes, and pore sizes are shown in Table 1. From Figure 5 and Table 1, it can be seen that the N2 adsorption and surface area of MMT are very low. The nitrogen adsorption isotherm of MMT is of type IV, indicating that the pore size is in the mesopore range. The

mesopores are from the intragallery regions between the sheets. The SPCs have a pronounced increase in porosity and surface area, and thus in adsorption, which is due to the fact that TEOS, as the pillaring precursors are converted to firm enough silica pillars, forms a rigid intercalated porous structure. Aside from MSPC-18-2, all of the SPC samples can be characterized as hybrid type IV according to the BDDT (Brunauer-Deming-

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Table 1. Basal Spacing (d001), Gallery Height, Surface Area (SBET), Pore Volume (Vp), Micropore Volume (Vmp), Mesopore Volume (Vmesp), and HK and BJH Pore Sizes of the Samples sample

d001a,b (nm)

SBET (m2/g)

Vp (cm3/g)

Vmp (cm3/g)

Vmesp (cm3/g)

HK pore size (nm)

BJH pore size (nm)

SPC-12-7 SPC-16-1 SPC-18-2 SPC-18-7 MSPC-12-7 MSPC-16-1 MSPC-18-2 MSPC-18-7 MMT

3.32 (2.36) 3.91 (2.95) 4.85 (3.89) 4.23 (3.27) 3.24 (2.28) 2.10 (1.14) 4.92 (3.96) 1.47 (0.51)

1156.8 622.7 542.8 901.7 1008.1 292.4 173.2 532.4 10

0.72 0.81 0.83 0.85 0.88 0.55 0.27 0.92 0.05

0.56 0.42 0.40 0.45 0.39 0.21 0.17 0.48 0.05

0.16 0.39 0.43 0.40 0.49 0.34 0.10 0.44 -

0.82 0.81 0.85 0.82 0.80 0.83 0.83 0.84 0.21

2.1 2.5 3.5 3.1 2.0 7.1 7.2

a

Gallery height (nm) in parentheses. b Gallery height ) basal spacing -0.96 nm (thickness of MMT layer).

Deming-Teller) classification with a hysteresis loop, whose features correspond to type B in Boer’s five types representing the presence of open slit-shaped or cylindrical pores formed in gallery regions.22-24 A gradual increase in N2 adsorption at low to medium partial pressures (P/P0 ) 0.05-0.2) suggests that these materials contain supermicropores and small mesopores.5 The adsorption jumps in the above isotherms appear at partial pressures P/P0 of 0.2-0.4 as a result of capillary condensation in the mesopores.25 In addition, the hysteresis loop represents a slit-shaped pore structure, indicating that the layered structure is preserved. Pillaring results in an increase in Vmes, which might be caused by the fact that intercalated silica increases the basal spacing of the clay, during which well-organized mesopores are generated by the surfactants. Furthermore, the hysteresis loop indicates that the pores formed between parallel layers are quite open.26 However, there are some differences in N2 adsorption-desorption isotherms among the SPC samples, especially for MSPC18-2, whose N2 adsorption-desorption isotherm is similar to that of MMT. Moreover, the adsorption and surface area of MSPC-18-2 are much lower than those of the other SPC samples. This suggests that firm pillars and a porous structure did not form in the gallery regions or that the structure was destroyed. For MSPC-16-1 and MSPC-18-7, the adsorption jump at P/P0 ) 0.2-0.4 is not so clear. This is probably because the Si pillars in the gallery regions do not form mesopores or the formed mesopores are not ordered. The BJH analysis of the N2 desorption data (as shown in Figure 6) yielded pore sizes of 2.1-3.5 nm. Furthermore, the narrow pore size distributions (0.8-nm peak width at peak halfmaximum) of SPC-12-7, SPC-16-1, SPC-18-2, SPC-18-7, and MSPC-12-7 are similar to that of the related MCM-41 (0.5 nm peak width), which is produced using a surfactant template. This indicates that the surfactants used play a decisive role in pore formation in SPC derivatives, and that they form molecular assemblies.14 In these porous materials formed by this gallerytemplated reaction, the specific surface areas are composed of the surfaces of micropores, where pore walls act as pillars and mesopores. Figure 7 presents a schematic representation of mesoporous silica-pillared layered MMT. The pore size distribution curves for MSPC-16-1, MSPC18-2, and MSPC-18-7 do not indicate narrow sharp peaks or even any peak. It can be seen that the pore size distributions of MSPC-18-7 and MSPC-16-1 are mainly below 2 nm with combined micropores and mesopores. This suggests that an ordered mesoporous structure did not form in the gallery, in accord with the above XRD and SEM observations. A schematic representation of MSPC-16-1 is shown in Figure 8. According to the XRD pattern, N2 adsorption-desorption isotherm, and SEM image of MSPC-18-7, the layered structure is preserved, and the Si source is pillared in the space between the sheets of

clay. However, the pore size distribution curve for MSPC-18-7 does not exhibit the same peak as SPC-18-7. This is because the pillar and gallery mesoporous structure is formed but the mesopores are not regular. A schematic representation of MSPC18-7 is shown in Figure 9. From the BJH pore size distributions, it can be observed that MSPC-18-2 exhibits a broad pore size distribution in the range of 2-12 nm, whereas the pore size distribution of MSPC-18-2 is similar to that of natural MMT. Figure 10 shows comparable N2 adsorption-desorption isotherms and pore size distribution curves for SPC-12-7 and MSPC-12-7. MSPC-12-7 has a surface area of ca. 1000 m2/g, whereas SPC-12-7 has a higher surface area of ca. 1100 m2/g. The two samples have comparable pore volumes, however. The pore volume of MSPC-12-7 is higher than that of SPC-12-7. Moreover, for SPC-12-7, most of the pore volume is from the micropores, but nearly half of the pore volume of MSPC-12-7 is from mesopores. A key consideration is the cooperative effect of the mixed surfactants. During the hydrolysis of TEOS, the organic TEOS hydrolyzes around the micelles formed by gallery surfactants. The added anionic surfactant weakens the electrostatic repulsion of the cationic surfactant, resulting in more ordered and stable mixed micelles. Ultimately, this process translates into an enhanced degree of order of mesopores in the gallery. In the N2 adsorption-desorption isotherms, from medium to high pressures (P/P0 ) 0.4-1.0), the adsorption of MSPC-12-7 is much more than that of SPC-12-7, suggesting that Vmes of MSPC-12-7 is higher than that of SPC-12-7. The pore size distribution curve obtained from the BJH method for MSPC-12-7 is relatively narrower than that of SPC-12-7. The BJH analysis also confirms that the mixed cation-anionic surfactants contribute to the more ordered gallery mesopores. 3.4. Thermal Analysis. The differential scanning calorimetry (DSC) results for the samples are shown in Figure 11. All of the uncalcined samples synthesized using cationic surfactant alone exhibit an endothermic process caused by dehydration in gallery below 110 °C. This suggests that there is some water in the galleries of the uncalcined samples. Moreover, the long endothermic process (150-300 °C) suggests the dehydration of crystalline water. These samples have exothermic peaks in the range of 300-550 °C because of the decomposition of surfactant and other organic materials. The exothermic processes of the samples when the temperature is above 760 °C are induced by Si frame collapse and layer collapse. The uncalcined samples of MSPC-12-7 and MSPC-18-7 have exhibit same process as the SPC derivatives. However, for the MSPC-16-1 and MSPC-18-2 samples, the process is different. First, the exothermic process of evaporation of surfactant and other organic materials is the range of 230-480 °C. This is due to the fact that extragallery surfactants are easier to evaporate than those in the gallery regions. Second, the exothermic process of layer collapse is below 530 °C, because


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Figure 6. Pore size distribution curves for SPC-12-7, SPC-16-1, SPC-18-2, SPC-18-7, MSPC-12-7, MSPC-16-1, MSPC-18-2, and MSPC-18-7. (See the Supporting Information for the micropore size distribution pattern for SPC-18-7.)

firm-enough silica pillars did not form in the gallery. The DSC results are in great agreement with the XRD data. 3.5. Proposed Mechanism of Mesoporous Silica-Pillared Clay Formation Using Surfactants Mixture As Gallery Templates. From the above analysis, it can be concluded that ordered mesoporous SPC derivatives can be obtained through two-step reaction processes, including use of an intragallery Si source and the formation of pillars.

In the first step, TEOS and a cationic surfactant are intercalated into the interlayer regions simultaneously. Moreover, the cationic surfactant molecules intercalate into clay interlayers by ion exchange, and TEOS also intercalates with cationic surfactant by solvation. The cationic surfactant molecules in the gallery are arranged as molecular lamellar bilayers, and the intragallery TEOS expands the gallery (as shown in Figure 7a).

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Figure 7. Schematic for the preparation of silica-pillared clay with ordered gallery mesopores such as SPC-12-7, SPC-16-1, SPC-18-2, SPC-18-7, and MSPC-12-7.

Figure 10. Comparable N2 adsorption-desorption pore size distribution curves for SPC-12-7 and MSPC-12-7. Figure 8. Schematic for the preparation of MSPC-16-1.

Figure 11. DSC thermal analyses of (A) SPC-16-1, (B) MSPC-12-7, (C) MSPC-18-7, (D) MSPC-16-1, and (E) MSPC-18-2.

Figure 9. Schematic for the preparation of MSPC-18-7.

In the second step, with the added ammonia solution, subsequent hydrolysis of the gallery TEOS affords hydrous silica templated around rod-like micellar surfactant assemblies (as shown in Figure 7b). In this process, TEOS does not flow out from the interlayer during hydrolysis because of its water insolubility and rapid hydrolysis in the presence of ammonia.

Therefore, this process can minimize the extragallery silica, easily allowing siloxane-pillared clay. The calcination process produces the final SPC derivative products (as shown in Figure 7c). Among additional anionic surfactants, cationic surfactants with weak polarity and unexposed hydrophilic heads such as C12DMBACl and C18DMBACl are quite hard to associate with anionic surfactant. The pillaring process can be completed successfully. However, the formed gallery mesoporous structure


is different. For C12DMBACl, MSPC-12-7 shows a relatively narrower width of the pore size distribution than SPC-12-7. A key consideration is the cooperative effect of the mixed surfactants. During the hydrolysis of TEOS, the organic TEOS hydrolyzes around the micelles formed by gallery surfactants. The added anionic surfactant weakens the electrostatic repulsion of the cationic surfactant, resulting in more ordered and stable mixed micelles. On the other hand, the polarity of C18DMBACl is too weak, so that when SDS is added into the synthetic gel mixture, the regular arrangement of surfactant in the gallery has been destroyed and ordered gallery mesopores cannot form. Cationic surfactants with strong polarity and exposed large hydrophilic heads would associate with SDS easily. The added SDS affects the pillaring process, as some of the surfactants cannot intercalate into the gallery regions of the clay and TEOS cannot intercalate sufficiently into the gallery regions by solvent. Surfactant removal by calcination destroys the layered structure, resulting in the collapse of the sheets and a decrease of the surface areas. 4. Conclusion A suitable mixed-gallery templating synthetic route has been established for ordered mesoporous silica-pillared clay using cationic-anionic surfactants as gallery templates. The molecular length of the surfactants affects the gallery height and the gallery pore size of the formed SPC derivatives. The SPC derivatives synthesized by cationic surfactants alone have high surface areas and ordered gallery mesoporous structures. Cationic surfactants with suitable polarity and unexposed hydrophilic heads cooperating with anionic surfactants effects a decrease of the electrostatic repulsion between the surfactants. This process results in more ordered and stable gallery mixed micelles. Moreover, the mesopores formed in the gallery are more regular. Additionally, the mixed surfactants can be used to control the pore volume and the ratio of micropores and mesopores in the gallery regions. Cationic surfactants with unsuitable polarity and unexposed hydrophilic heads would cause random gallery porous structures and even the failure of the Si source intragallery process. In our research, the best results were obtained using the mixed surfactants of C12DMBACl and sodium dodecyl sulfate. This study provides an effective, environmentally benign approach for the synthesis of silica-pillared clays with controllable gallery porous structures. Acknowledgment We thank senior engineer Xinmei Pang, Mr. Zhiyuan Zhou, and Ms. Xiaohui Gao of Sino petrochemical research institute (Beijing) for characterization of instrument support. Supporting Information Available: Wide-angle XRD pattern of MSPC-18-2 and micropore size distribution pattern for SPC-18-7. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Han, Y. S.; Yamanaka, S.; Choy, J. H. Acidic and Hydrophobic Microporous Clays Pillared with Mixed Metal Oxide Nano-Sols. J. Solid. State. Chem 1999, 144, 45. (2) Ding, Z.; Zhu, H. Y.; Greenfield, P. F. Photocatalytic Properties of Titania Pillared Clays by Different Drying Methods. J. Colloid Interface Sci. 1999, 209, 193. (3) Zhu, H. Y.; Ding, Z.; Barry, J. C. Porous solids from layered clays by combined pillaring and templating approaches. J. Phys. Chem. B 2002, 106, 11420.

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ReceiVed for reView July 19, 2009 ReVised manuscript receiVed September 20, 2009 Accepted November 8, 2009 IE9011563

Novel One-Step Synthesis Route to Ordered Mesoporous Silica

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