Engineering the Structures of Nanoporous Clays with Micelles of Alkyl

Jan 6, 2001 - Composite clay nanostructures (CCNs) were observed in intercalating Laponite clay with alumina in the presence of alkyl polyether surfac...
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Langmuir 2001, 17, 588-594

Engineering the Structures of Nanoporous Clays with Micelles of Alkyl Polyether Surfactants H. Y. Zhu and G. Q. Lu* Nanomaterials Centre and Department of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia Received May 17, 2000. In Final Form: October 23, 2000 Composite clay nanostructures (CCNs) were observed in intercalating Laponite clay with alumina in the presence of alkyl polyether surfactants which contain hydrophobic alkyl chains and ether groups. Such nanostructured clays are highly porous solids consisting of randomly orientated clay platelets intercalated with alumina nanoparticles. The pores in the product solids are larger than the dimension of the surfactant molecules, ranging from 2 to 10 nm. This suggests that the micelles of the surfactant molecules, rather than the molecules, act as templates in the synthesis. Interestingly, it is found that the size of the framework pores was directly proportional to the amount of the surfactants in terms of moles, but shows no evident dependence on the size of the surfactant molecules. Broad pore size distributions were observed for the product CCNs. This study demonstrates that introducing surfactants in the pillaring process of clays is a powerful strategy for tailoring the pore structures of nanoporous clays. With this new technique, it is possible to design and engineer such composite clay nanostructures with desired pore and surface properties by the proper choice of surfactant amounts and preparation conditions.

Introduction In searching for materials with larger pore sizes than zeolites, a class of thermally stable porous materials, pillared layered clays (PILCs) were developed from swellable layered clays, such as smectite, in the late 1970s.1-6 When dispersed in water, the layered clays swell because of hydration of the interlamellae cations which act as counterions to balance the negative charges of clay layers. Therefore, inorganic polycations, the so-called pillar precursors, in aqueous solutions can be intercalated into the interlayer gallery by cation exchange. During the subsequent heating above 400 °C, the intercalated polycations are converted to oxide pillars which prop the clay layers apart. A permanent micropore system is thus formed. Naturally, the pore structure of thus obtained PILCs strongly depends on the type of pillars.5,6 For instance, a solution containing polycations [Al13O4(OH)24]7+ (Keggin ions, abbreviated as Al13 ions) is widely used as a pillaring agent for the synthesis of aluminapillared clays (Al-PILCs). The pillar precursors, Al13 ions, are about 1 nm in size.1-3,5,6 During calcination at elevated temperatures, the Al13 ions are dehydrated and converted into alumina pillars of a reduced size. The predominant slit-shaped pores in a calcined Al-PILC have a width of about 0.7-0.8 nm. The micropore volume of Al-PILC is about 0.1 cm3/g, and the BET surface area is about 300 m2/g.1-6 Various oxides can be intercalated as pillars. Pillaring has also become a well-established technique for the synthesis of porous materials. However, this approach is limited to preparing microporous solids of a moderate porosity (pore volume of 0.15-0.40 cm3/g and * To whom correspondence should be addressed. E-mail: maxlu@ cheque.uq.edu.au. (1) Brindley, G. W.; Semples, R. E. Clay Miner. 1977, 12, 229. (2) Lahav, N.; Shani, U.; Shabtai, J. Clays Clay Miner. 1978, 26, 107. (3) Vaughan, D. E. W.; Lussier, R. J.; Magee, J. S. U.S. Patent 4,176,090, 1979. (4) Pinnavaia, T. J. Science 1983, 220, 365-371. (5) Burch, R., Ed. Pillared Clays, Catalysis Today; Elsevier: New York, 1988; Vol. 2. (6) Mitchell, I. V., Ed. Pillared layered structures, current trends and applications; Elsevier Applied Science: London, 1990.

BET surface area of 150-450 m2/g).5,6 It is extremely difficult to obtain large pillar precursors that are identical in size, and the pillaring technique appears incapable of creating very high porosity. In recent years, templated synthesis of mesoporous molecular sieves of silica or aluminosilicate, first reported in 1992 by scientists at Mobil,7,8 has stimulated a great research interest. In this novel approach, surfactant (quaternary ammonium salts) in an aqueous medium forms micelles that orient into a well-defined structure. The added silica source is condensed around the micelles, forming a silica matrix embedded with organic templates. The templates are removed normally by calcination to create a continuous network of pores that mimic the size and shape of the template. The advantages of this approach are that the pore volume is controlled by the volume fraction of the template constituents, and the pore size is controlled by the size of the surfactant micelles. Solids with uniform mesoporous structure and large pore volumes can be obtained by the templating approach. Recently, efforts have also been made to create a mesoporous system in the galleries between the clay layers with the templating approach. Galarneau et al reported a successful synthesis of mesoporous solids termed as porous clay heterostructures (PCHs) from layered clays using quaternary ammonium surfactants as template agents.9 Layered clays were first intercalated with surfactants. Tetraethoxide orthosilicate (TEOS) was then allowed to hydrolyze and condense, surrounding the surfactants in the galleries. An open framework of silica formed in the galleries after removal of the surfactants by heating. It is noted, however, that the PCH solids and most of the M41S solids have a silica surface. Weak acidity and poor ion exchange capacity are usually observed on (7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (8) Beck, J. S.; Vartuli, C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (9) Galarneau, A.; Barodawalla, A.; Pinnavaia, T. J. Nature 1995, 374, 529.

10.1021/la0006871 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001

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these surfaces, due to the lack of active surface groups which are very useful for loading active metal components for catalysis and adsorbing polar molecules. Partial substitution of alumina and some transition metal oxides into MCM41 has been extensively investigated recently.10 The latter can bring catalytic function to the solids while alumina substitution results in changes in surface properties: enhancing acidity and creating new surface groups. However, the fraction of the substituted oxide is limited, being small relative to that of silica. Otherwise products of poor porosity are obtained. If various metal oxides can be incorporated into highly porous clays, similar to what has been done in pillared clays, while their porosity is retained, the resultant solids will have not only high porosity but also superior surface properties. It has been found that the introduction of poly(vinyl alcohol) (PVA) or alkyl polyether surfactants in the synthesis of Al-PILCs brings about a significant change in the pore structure of the products.11-13 It has been argued that PVA acts as a template. The interlayer distance of the clay is significantly influenced by the amount of PVA introduced, indicating that the polymers exist in the interlayer gallery.12 They enlarge the gallery space but do not disturb the ion exchange of the clays. The Keggin ions not only can disperse into the gallery by cation exchange but also can polymerize to larger species due to hydrolysis. However, no significant increase in specific surface area has been observed for the products thus obtained. In a previous study,14 we found that the AlPILC prepared in the presence of PVA has poor longrange order, but a relatively large pore volume, which mainly arises from mesopores. In contrast, the long-range order in the alumina-pillared clay prepared in the presence of alkyl polyether was substantially improved.13 Binding of the surfactant molecules to the pillar precursors was observed.15 This provides insight into the mechanism of the pore structure formation in the presence of the surfactants. However, there was no significant increase in porosity of the calcined products reported. In the present study, we report the preparation and formation mechanism of composite clay nanostructures using an alumina pillaring agent and Laponite (Al-LPC) in the presence of poly(ethylene oxide) (PEO) surfactants. Importantly distinct features that are different from those of the known templated synthesis process were observed for this synthesis. For instance, the porosity of the products depends on the quantity, rather than the size, of the surfactant molecules in the system. In this paper, we term this new heterogeneous nanostructure as a “composite clay nanostructure (CCN)”. The results of this study show that introduction of these surfactants into the synthesis could be an effective means for engineering the pore structures of pillared clays, thus providing new opportunities for developing catalysts or adsorbents of tailorable pore structure and desired active sites from clays. Experimental Section Materials. The clay used for this study was Laponite RD, supplied by Fernz Specialty Chemicals, Australia. The clay powder has a BET specific surface area of 370 m2/g and a cation (10) Kosslick, H.; Landmesser, H.; Frickle, R. J. Chem. Soc., Faraday Trans. 1997, 93 (9), 1849. (11) Suzuki, K.; Mori, T. Appl. Catal. 1990, 63, 181. (12) Suzuki, K.; Horio, M.; Masuda, H.; Mori, T. J. Chem. Soc., Chem. Commun. 1991, 873. (13) Michot, L. J.; Pinnavaia T. J. Chem. Mater. 1992, 4 (6), 1433. (14) Zhu, H. Y.; Vansant, E. F.; Lu, G. Q. J. Colloid Interface Sci. 1999, 210, 352. (15) Michot, L. J.; Barre`s, O.; Hegg, E. L.; Pinnavaia, T. J. Langmuir 1993, 9, 1794.

Langmuir, Vol. 17, No. 3, 2001 589 exchange capacity (CEC) of 55 mequiv/100 g of clay. A commercial solution of aluminum hydroxychloride (Locron L from Hoechst, Germany) was used as the alumina source. It contains polyoxycations of aluminum hydrate with an Al2O3 content of 23.5 + 0.5 wt %, a ratio of OH/Al of 2.5, and a pH of about 3.5-3.7. Four nonionic PEO surfactants, Tergitol 15S-n (n ) 5, 7, 9, and 12) from Aldrich, were used in this study. The PEO surfactants have a general chemical formula of C12-14H25-29O(CH2CH2O)nH and an average molecular weight of 420 for Tergitol 15S-5 (n ) 5) and of 730 for Tergitol 15S-12 (n ) 12). Preparation of Al-LPC Samples. A 4.0 g sample of Laponite was dispersed in 200 mL of water. The suspension was stirred until it became clear. A certain amount of an alkyl polyether surfactant was added to the Laponite suspension, which then became opaque. Stirring was prolonged for 2 h to allow sufficient mixing. To this mixture was added 20 mL of the Locron L solution dropwise with continuous stirring. The suspension was transferred to an autoclave after being stirred for 2 h and maintained at 373 K for 2 days. A white precipitate was recovered from the mixture by centrifuging and washed with deionized water until it was free of Cl- ions. The wet cake was dried in air and calcined at 773 K for 20 h. The temperature was raised at a rate of 2 K/min. The samples are labeled Al-LPC-m, where m denotes the amount of the surfactant used (in grams). For instance, Al-LPC20 is the sample prepared with 20 g of the surfactant. A series of samples was prepared using 0, 2, 4, 6, 8, 10, 12, and 20 g of the surfactant. To examine the effect of the molecular size of the surfactants on the structure of Al-LPC, another series of samples was obtained by using 8.0 g of different surfactants, Tergitol 15-TS-5, -TS-7, and -TS-12. They are named Al-LPC-Tn. Characterization. N2 adsorption/desorption isotherms were measured at liquid nitrogen temperature, using a gas sorption analyzer (Quantachrome, NOVA 1200). The samples were degassed at 523 K under a vacuum below 10-3 Torr for 16 h prior to the measurement. The surface area was calculated by the BET equation, and the external surface area and framework pore volume were determined through the t-plot method of Lippens and de Boer.16 The difference between the BET surface area and external surface area is attributed to the surface area from the framework pores of the sample. The adsorption isotherms of carbon dioxide were measured at 273 K using the same instrument and degassing conditions. Thermogravimetric analysis (TGA) of the samples as prepared was performed on a Shimazu TGA-50. About 10 mg of solid before calcination was loaded onto a platinum pan and heated from room temperature to 1173 K at a heating rate of 5 K/min in an air flow of 80 mL/min. X-ray diffraction (XRD) patterns of the sample powder were recorded on a Philips PW 1840 powder diffractometer with cobalt KR radiation and a nickel filter. Fourier transform infrared (FTIR) spectra of the samples were recorded by a Perkin-Elmer 2000 FTIR spectrometer. Specimens for the measurements were prepared by mixing 2 mg of the sample powder with 100 mg of KBr and pressing the mixture into pellets. Spectra were acquired in a wavenumber range between 580 and 4000 cm-1 at 2 cm-1 resolution and averaged over 100 scans. Transmission electron microscopy images were taken with a JEOL 2010 microscope on powder samples deposited onto a copper microgrid.

Results XRD and TEM Results. Generally, the intercalated structure of alumina-pillared clay can be examined by the d001 peak in the XRD patterns.1-3,5,6 However, it is hard to observe the peak in the XRD patterns of the starting clay, Laponite, because the clay platelets are small, about 20-30 nm in diameter, aggregating in a poor long-range order. In this work, we did not observe a clear peak for the starting clay, Laponite, and the resultant CCN samples. This is also attributed to the lack of longrange order in the structure of these solids. Figure 1 shows a TEM image of Al-LPC-8, which is typical for the sample prepared with surfactants. The TEM (16) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319.

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Figure 1. TEM image of Al-LPC-8. The thin hairy lines with a thickness of about 1 nm are the edges of the Laponite layers.

micrograph clearly shows the layered sheets of Laponite are somewhat distorted and randomly oriented. These layers (of uniform thickness of about 1 nm) do not stack together closely, having a separation space of several nanometers, suggesting intercalation with nanosized alumina particles. Essentially, we have developed a highly porous clay nanostructure consisting of randomly orientated clay platelets that compart alumina nanoparticles, resulting in a highly heterogeneous porous structure. For convenience in description and to avoid possible confusion with the PCH definition as defined in ref 9, we term this structure as a CCN. This term is used throughout this paper. The results of in situ elemental analysis also support this composite nanostructure. The chemical composition at several spots of a sample particle, randomly chosen, is found to be consistent with that of the particle. The size of a spot is larger than 10 nm, which ensures that the consistency of composition is due to the intercalated structure. On the other hand, the poor order in the layer stacking observed in Figure 1 suggests that the alumina pillars are not uniform in size. The separation between silicate layers is substantially larger than their thickness (Figure 1). This means the size of the pillars is larger than that of Al13 ions, which are about 1 nm. We know that aging at 373 K can cause polymerization of the Al13 ions,17 and the large polyoxyl ions of aluminum are not uniform in size. The preponderance of mesopores in this sample is anticipated according to Figure 1. More detailed information of the pore structure of these solids can be derived from the data of N2 adsorption. N2 Adsorption Data. N2 adsorption and desorption isotherms of Al-LPCH-6 dried at 373 K and calcined at 773 K are presented in Figure 2. The adsorption by the dried sample is very low. The calcination at 773 K brings

about a pronounced increase in the porosity of the sample, and thus in adsorption. During heating, the polycations of aluminum hydrate were converted into alumina particles, forming rigid intercalated nanostructures. Meanwhile removal of the surfactant from the sample was observed. The removal of surfactant creates a large number of mesopores in the sample. This argument is also supported by other experimental results that will be discussed later. The isotherms of Al-LPC-6 have the shape of a type IV isotherm, which is characteristic of mesoporous solids.18 A steep increase in adsorption and a hysteresis loop are observed in the P/P0 range between 0.4 and 0.8. The N2 isotherms of other samples prepared with surfactants have a quite similar shape to that of calcined Al-LPC-6.

(17) Zhu, H. Y.; Xia, J. A.; Vansant, E. F.; Lu, G. Q. J. Porous Mater. 1997, 4, 17.

(18) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982.

Figure 2. N2 adsorption and desorption isotherms of Al-LPC-6 dried at 373 K and calcined at 773 K.

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Table 1. Surface Area, Pore Volume, and Mean Diameter of the Framework Pores in the Calcined Al-LPC-m Samples m 20 12 8 6 4 2 0 10 (alumina prepared without Laponite)

SBET (m2/g) Sf (m2/g) Vf(cm3/g) df (nm)a 531 495 542 499 437 417 278 239

503 477 524 482 430 379 267

0.925 0.652 0.709 0.641 0.530 0.405 0.233 -0.631

7.4 5.5 5.4 5.3 4.9 4.3 3.5 10.6

a d , the mean diameter of the framework pores, is the hydraulic f diameter, derived from the ratio of the pore volume to the pore surface area.

Figure 4. Thermogravimetric analysis (weight losses) for uncalcined Al-LPC-m samples. Table 2. Surface Area, Pore Volume, and Mean Diameter of the Framework Pores in the Calcined Al-LPC-Tn Samples

Figure 3. Pore size distributions (PSDs) of some samples, calculated from the data of N2 isotherms: (A, top) Al-LPC-m samples, (B, bottom) Al-LPC-Tn samples.

An evident trend is that adsorption (and thus the porosity) increases with m, the amount of the surfactant (Tergitol TS-15-9) introduced in the synthesis gel (Table 1). To clearly illustrate the influence of the surfactant properties on the structure of the calcined products, pore size distributions (PSDs) of some samples were calculated from the N2 isotherms and are given in Figure 3. The data of specific surface area, pore volume, and mean pore size were calculated from the isotherms and t-plots (not shown), as listed in Tables 1 and 2. Figure 3A shows that both pore size (indicated by the position of the peak) and pore volume (indicated by the area under the PSD curve) increase with m. The data in Table 1 show generally that the BET surface area, pore volume, and mean pore size in Al-LPC-m samples increase with m, the amount of the surfactant Tergitol TS-15-9 used. Besides, a mesostructured alumina sample obtained

n

SBET (m2/g)

Sf (m2/g)

Vf (cm3/g)

df (nm)

5 7 9 12

557 545 542 514

505 527 524 500

1.162 0.752 0.709 0.559

9.2 5.7 5.4 4.5

under equivalent conditions in the absence of Laponite exhibits a substantially smaller surface area and pore volume (the last row in Table 1). Surprisingly Figure 3B and the data in Table 2 demonstrate clearly that the pore size, pore volume, and BET surface area of Al-LPC-Tn samples decrease with n. In the preparation of these samples, the same amount, 8.0 g, of surfactants with a molecular weight ranging from 420 to 730 was used. According to the general formula for this class of surfactants, the molecular weight and the dimension of a surfactant molecule increase with n, i.e., the number of ethylene oxide groups in the surfactant. In the templating mechanism, the pore size of the product is closely related to the molecular size of the surfactant. It is known that the micelles of quaternary ammonium surfactants are uniform cylindrical rods and the diameter of the rods is proportional to the molecular size of the surfactants.7,8 But for the alkyl polyether surfactants used in this study, the trend appears quite different. For a given amount, 8.0 g, the number of molecules for a surfactant with a smaller n is larger than that for a surfactant with a larger n. Therefore, the results in Figure 3B and Table 2 also suggest that the more surfactant molecules added in the synthesis, the larger the pore size and pore volume in the product. In effect, this is similar to the trend demonstrated by Figure 3A and Table 1. The molecular size of the surfactant is not a sole determinant of the pore size of the product solids. Thermogravimetric Analysis. The weight losses during the course of calcination in air up to 1173 K are shown in Figures 4 and 5 for Al-LPC-m and Al-LPC-Tn samples, respectively. The sample prepared without surfactant, Al-LPC-0, loses about 15% of its weight below 423 K mainly due to dehydration (Figure 4). In contrast, the weight loss below 423 K for samples Al-LPC-m and Al-LPC-Tn is quite small (Figures 4 and 5). For instance, the weight loss for Al-LPC-8 is below 3%. This suggests that the samples prepared with surfactant have much lower water content. However, for this sample a sharp weight loss of about 40% is observed in the temperature range between 423 and 523 K, whereas a loss of 9% was recorded for Al-LPC-0 in the same temperature range. It

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Figure 5. Thermogravimetric analysis (weight losses) for uncalcined Al-LPC-Tn samples.

Figure 6. Influence of the calcination temperature on the surface areas and pore volumes of Al-LPC-10 (squares), which is the representative of samples prepared with surfactants, and Al-LPC-0 (circles).

is also noted that, as n increases, the sharp weight loss commences at higher temperatures (Figure 5). The boiling point of the surfactant increases with the number of ethylene oxide groups in the molecule, n. Therefore, the weight loss between 423 and 523 K is mainly attributed to the evaporation of the surfactants. It was observed that a large amount of surfactants was released at around 473 K during calcination. This is a potential advantage for this synthesis method because the surfactants used can be collected simply by a cooling trap and reused. Thermal Stability. Heating at high temperatures always results in a loss of surface area and collapse of small pores. The influence of heating on the surface areas and pore volumes of Al-LPC-10, as a representative of samples prepared with surfactants, and Al-LPC-0 is illustrated in Figure 6. It is known that clay layers are generally unstable above 973 K.19 The damage to the clay layers inevitably results in collapse of the pore structure in the intercalated derivatives of the clay. Heating at 1073 K causes serious damage to the clay layers and a complete loss in crystallinity of the layers.17 For instance, a normal alumina-pillared montmorillonite calcined at 773 K has a specific surface area of about 300 m2/g and a pore volume of about 0.15 cm3/g. They became 30 m2/g and 0.05 cm3/g, respectively, after heating at 1073 K, losing most of the porosity. The collapse in the pore structure is also observed in Figure 6, which indicates that heat treatment at 1073 K results in serious losses in surface area and pore volume. However, the pore structures of the sample prepared with (19) Occelli, M. L. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 553.

Figure 7. FTIR spectra of Tergitol 15-TS-9, a mixture of this surfactant and Laponite, which is the specimen taken prior to introducing the solution of aluminum hydroxychloride, and AlLPC-8 before calcination: (A, top) in the range between 1500 and 700 cm-1, (B, bottom) in the range between 3200 and 2700 cm-1.

surfactants exhibit much better resistance to heating at high temperatures. Calcination at 1073 K causes a decrease of about 45% in surface area and of about 1/3 in pore volume for Al-LPC-10, while the losses are about 2/3 and 9/10, respectively, for the reference sample, Al-LPC0. After heat treatment at 1073 K, all Al-LPC-m samples still retain substantial porosity, having a surface area of about 180-200 m2/g. FTIR Spectra. In Figure 7, FTIR spectra of Tergitol 15-TS-9, a mixture of this surfactant and Laponite, which is the specimen taken prior to introducing the solution of aluminum hydroxychloride, and Al-LPC-8 before calcination are compared. Figure 7A shows the spectra in the range between 1500 and 700 cm-1, and the characteristic bands of the ethylene oxide chain15 can be clearly seen at 1350, 1250, 1105, 950, and 845 cm-1 for the pristine surfactant and the mixture of the surfactant and Laponite, while only the most intense characteristic bands at 1350 and 1250 cm-1 can be found for Al-LPC-8. Strong bands at 1070 and 1005 cm-1 are observed for the mixture and Al-LPC-8, due to the vibration modes of Si-O-Si in the clay layers.20 The bands at 2873 and 2928 cm-1 (Figure 7B), as well as at 1466 cm-1 (Figure 7A), are due to the -CH2- groups of both the alkyl and ethylene oxide chains.15 Consistent with the literature, Figure 7B also shows that the intensity ratio of the asymmetric band (2928 cm-1) to the symmetric band (2873 cm-1) is greater for Al-LPC-8 (1.65) than that for the pristine surfactant (20) Farmer, V. C.; Russell, J. D. Clays & Clay Minerals, Proceedings of the 15th Conference; Pergamon Press: Oxford, 1968; p 121.

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Figure 8. Water adsorption by calcined Al-LPC-0, Al-LPC8A, and a commercial γ-alumina.

(1.32). According to Michot et al.,15 this is due to the surfactant binding to the aluminum hydroxide, the precursor of alumina pillars. The FTIR spectra confirm that there are strong interactions among the surfactant molecules, pillar precursors, and clay layers. Water Adsorption. The behavior of water adsorption by a solid is associated with the surface polarity and pore structure of the solid. Hydrophilic surfaces are of a strong polarity, which exhibit strong adsorption of the polar molecules of water at low vapor pressures. This is the case of water adsorption by a commercial γ-alumina illustrated in Figure 8. Water adsorption by calcined AlLPC-0 and Al-LPC-8 (as a representative of the intercalated clays prepared with surfactants) is also shown in the figure. The adsorption at low vapor pressures (P/P0 < 0.1) by Al-LPC-6 and Al-LPC-8 is weak, compared with that by activated alumina. This means that the surfaces of the CCN samples have a poor hydrophilicity, similar to the surface of alumina-pillared montmorillonite, which has been reported to be weakly hydrophilic.21,22 The adsorption by Al-LPC-8 is evidently larger than that by Al-LPC-0. However, the specific surface area of Al-LPC-0 is about half that of Al-LPC-8, so the surface hydrophilicity of the two samples may be comparable. CO2 Adsorption. The ability of a solid to adsorb CO2 also provides information about an important surface property of the solid, the surface basicity. CO2 adsorption by three Al-LPC samples prepared with surfactants is compared with that of a commercial γ-alumina in Figure 9. The CO2 adsorption capacity of Al-LPC-m samples is larger than that of the alumina sample. This could be due to two reasons: First, the surface of Laponite in the AlLPC samples is still available for CO2 adsorption. It is known that the Laponite surface is basic, adsorbing CO2 strongly. Second, the surface areas from the alumina pillars in Al-LPC samples are relatively large so the number of active sites for CO2 adsorption could still be large even if the surface does not have a high density of the sites. Discussion Formation of the Mesopores. As shown above, introducing surfactants during the synthesis leads to pronounced increases in the pore volume and pore size of the calcined products. In Figure 10, the weight loss below (21) Yamanaka, S.; Malla, P. B.; Komarneni, S. J. Colloid Interface Sci. 1990, 134, 51. (22) Zhu, H. Y.; Gao, W. H.; Vansant, E. F. J. Colloid Interface Sci. 1995, 171, 377.

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Figure 9. CO2 adsorption by three CCN samples and a commercial γ-alumina.

Figure 10. Relation between the weight loss below 773 K and the volume of the framework pores in the calcined products.

773 K, obtained from TGA analysis, is plotted against the volume of the framework pores in the calcined products. The pore volume increases with the weight loss, and a linear relation between them is evident. Mesopores in the samples clearly arise from removal of water and the surfactants. The surfactants acted as templates. A sample was also prepared by changing the sequence of mixing reactants in the synthesis. The pillaring solution was first added to the clay suspension, followed by the surfactant addition. The porosity of the sample thus obtained is lower than that of the sample prepared in the sequence described in the Experimental Section but with the same amounts of the starting materials. However, the pore volume and pore size are significantly greater, compared to those of the sample prepared without the surfactant. Therefore, the primary reason for the large porosity is the presence of the surfactants. Function of the Surfactants. According to the data shown in Figures 2, 4, and 5, the surfactant templates exist in the interlayer region, (the galleries) of the intercalated clay. During heat treatment, first the pillars and thus framework of the intercalated clay, which is propped by the templates, stiffen, and then the surfactants evaporate (as indicated by the TGA results), leaving large voids as mesopores in the structure. However, the template is unlikely a single surfactant molecule, considering that the mean pore size of the calcined solids is about 4-6 nm, being much larger than the molecular sizes of the surfactants. On the other hand, PEO surfactants of similar molecular formulas were found to form rodlike or wormlike

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micelles with a diameter of about 6 nm.23 Thus, the templates in our synthesis should be the micelles of the surfactants. Besides, during the removal of water in the CCN systems (drying and heat treatment), the strong interfacial force of water usually causes the collapse of pores. Smaller pores have higher interfacial tension and aare easier to collapse.24 According to the TGA results, the content of water in the intercalated mixtures is reduced due to the existence of the surfactants. It appears that most of the water is squeezed out by the surfactant micelles. This is supported by the fact that the micelles exhibit a limited miscibility with water and have a strong affinity for the surface of clay layers and aluminum hydroxide pillars (as FTIR results indicate). As a consequence, the interfacial tension is reduced significantly, and the pore collapse due to the interface tension is mitigated. The presence of PEO surfactants in the pores has two functions: reducing the interfacial tensions and propping up the structure of aluminum hydroxide and Laponite during the drying and dehydration process. The dehydration of the alumina precursors leads to a consolidated nanostructure of composite clay layers and aluminum hydroxide. The subsequent heating at higher temperatures removes the surfactant, leaving a nanoporous framework of Laponite and aluminum oxide nanoparticles (CCN). Mechanism of the Formation of CCN. The most plausible mechanism for the synthesis appears to be that the surfactant molecules form micelles in the interlayer region because of low solubility in water and strong affinity for the clay surface. The Keggin ions replace the interlayer sodium ions and polymerize further to form larger pillars which are separated, however, by the micelles. The micelles act as templates, preventing the intercalated framework from collapsing during the dehydration process. Meanwhile the framework hardens, and removal of the surfactant results in a highly heterogeneous nanostructure of pillared Laponite and alumina nanoparticles. The surfactants are removed due to evaporation at temperatures between 423 and 473 K rather than burnoff as in many other templated syntheses. It is possible to collect the surfactants with a cooling trap for cyclical use, and this feature brings about economical and environmental advantages to the synthesis. Influence of the Surfactant Properties on Porosity. All surfactants we used have the same alkyl chain; the only difference between them is the number of ether groups, (CH2CH2O)n. Michot et al. have found that the surfactant molecules interact with the precursors of alumina pillars through the ether groups, (CH2CH2O)n.15 A surfactant with large n is expected to have a much stronger interaction with the precursor surface, compared to a surfactant with small n. The strong interaction must influence the formation and configuration of the micelles. The observation that the mean diameter of the framework pores decreases with n could be attributed to the fact that the stronger interaction results in micelles of a smaller diameter. In addition, the miscibility of the surfactants with water increases with n. Alkyl polyether surfactants with larger n are easier to disperse in water, and thus easier to be lost during the washing with water. Therefore, (23) Lin, Z.; Scriven, L. E.; Davis, H. T. Langmuir 1992, 8, 2200. (24) Occelli, M. L.; Peaden, P. A.; Ritz, G. P.; Iyer, P. S.; Yokoyama, M. Microporous Mater. 1993, 1, 99.

Zhu and Lu

the amount of the surfactants with larger n remaining in the intercalated solid before calcination is less than that of the surfactants with smaller n. In Figure 5, the weight loss between 423 and 623 K decreases with n, supporting our argument. The pore volume of the final product greatly depends on the amount of surfactants left prior to the drying and heating treatments because the surfactants act as templates. Surface Properties. The water adsorption data (Figure 9) indicated that the surfaces of the clay samples are weakly hydrophilic. Besides, the CCN samples exhibit a stronger CO2 adsorption than commercial γ-alumina. Such adsorption behavior should be attributed to the surface of Laponite clay rather than to the surface of alumina. The clay layers are believed to be important constituents of the walls of the framework pores in Al-LPC samples. Since there are a large number of basic sites on these samples, it is anticipated that these solids are good supports for catalysts used in reactions involving acidic reactants, such as CO2. We have found that the catalytic performance of catalysts supported on these clay samples is good for the methane re-forming with CO2. The details of these studies on application will be reported elsewhere. Summary Highly porous CCNs of alumina and laponite can be synthesized with polyether surfactants. The surfactant molecules form micelles in the interlayer region, and the precursors of alumina pillars polymerized beside the micelles because of the affinity of the surfactant molecules for the surface of pillar precursors. The micelles act as templates, preventing the intercalated framework from collapsing during the dehydration process, during which the framework hardens. The surfactants can be removed at temperatures between 423 and 523 K due to evaporation, leaving a highly nanoporous framework of aluminaintercalated Laponite. In contrast to the well-known template synthesis, the mean diameter and volume of the framework pores in the products decrease as the molecular size of the surfactants increases. This is because the increase in the molecular size comes from the ether groups, (CH2CH2O)n, rather than from the alkyl chain. These groups have a strong interaction with the precursors of alumina pillars, and the configuration and formation of the micelles are affected. We can tailor the pore sizes and their distribution by the choice of the surfactants and adjusting the amount of the surfactants added. Another significant advantage of this synthesis technique is that the surfactants can be recovered with a cooling trap for reuse. Besides, there are a large number of basic sites on the surface of the calcined clay samples, so the solids are ideal for use as supports of the catalyst for the reaction involving acidic reactants such as CO2. Acknowledgment. Most experimental and data collection work was conducted by our skillful research assistant, Mr. Chaoqing Lu, who deserves a grateful acknowledgment. Financial support from the Australian Research Council (ARC) and the University of Queensland is also gratefully acknowledged. H.Y.Z. is indebted to the ARC for the QE II fellowship. Thanks are also due to Dr. John Barry for TEM measurements and some useful discussion on TEM. LA0006871