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Pore Size Engineering of Mesoporous Silicas Using Decane as Expander J. L. Blin, C. Otjacques, G. Herrier, and Bao-Lian Su* Laboratoire de Chimie des Mate´ riaux Inorganiques, ISIS, Universite´ de Namur, 61 rue de Bruxelles, 5000 Namur, Belgium Received November 8, 1999. In Final Form: January 19, 2000 Decane has been used as swelling agent to enlarge the pore size of pure silica MCM-41 materials. The synthesis conditions such as the swelling agent/surfactant molar ratio, the adding sequence of swelling agent, etc., have been studied. The role of decane and the effect of crystallization time and temperature on the synthesis have also been discussed. Final compounds were intensively characterized by several techniques (XRD diffraction, SEM, TEM, and nitrogen adsorption-desorption analysis). The present work shows that decane is an effective expander to enlarge the pore size of mesoporous materials. Two possible mechanisms have been proposed to describe the swelling effect of decane molecules. The synthesis of mesoporous materials can be explained by different steps, which have been observed in the synthesis course of zeolites.

Introduction Since the discovery of MCM-41 in 1992 by Mobil scientists,1,2 numerous studies concerning the conditions and the mechanism of synthesis, the characterization, and the use of these materials as catalystss and catalyst supports in various reactions3-6 have been reported. Important effort has been devoted to enlarge the pore size of MCM-41 as the large pore siliceous supports with high surface area and high thermal stability are of particular interest in the preparation of catalysts for some technologically important catalytic treatments of organic molecules such as the ammoxidation of hydroxyacetophenones,7 the selective oxidation of aromatics,8 the hydroxylation of phenols,9 and vinyl acetate production.10 The synthesis of MCM-41 materials consists of the preparation of micelles in aqueous solution, the polycondensation of the inorganic source, and the removal of surfactants from the pores of materials. It is known that the size of formed micelles determines the pore size of final mesoporous materials.11 Postsynthesis treatments,12,13 surfactants of different chain lengths,14 and polymers such as * Corresponding author. Phone: 32-81-72-45-31. Fax: 32-81-7254-14. E-mail: [email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCulle, S. B.; Higgins, J. B.; Schlender, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Llewellyn, P. L.; Ciesla, Y.; Decher, H.; Stadler, R.; Schu¨th, F.; Unger, K. K. Stud. Surf. Sci. Catal. 1994, 84, 2013. (4) Aguado, J.; Serrano, D. P.; Romero, M. D.; Escola, J. M. J. Chem. Soc., Chem. Commun. 1996, 765. (5) Corma, A.; Iglesias, M.; Sanchez, F. Catal. Lett. 1996, 39, 153. (6) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (7) Clerici, M. G. Appl. Catal. 1991, 68, 249. (8) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (9) Clerici, M. G.; Ingallina, P. J. Catal. 1993, 140, 71. (10) Nakamura, S.; Yasui, T. J. Catal. 1970, 17, 336. (11) Corma, A. Chem. Rev. 1997, 97, 2373. (12) Huo, Q.; Margolez, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (13) Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, 2499. (14) Sayari, A.; Karra, V. R.; Sudhakar Reddy, J. Presented at the Symposium on Synthesis of Zeolites, Layered compounds and other Microporous Solids, 209th National Meeting of the American Chemical Society, Anaheim, CA, 1995.

triblock copolymers15 used as templates or incorporation of swelling agent in the formed micelles can lead to the formation of large pore mesoporous materials. In the literature, 1,3,5-trimethylbenzene (TMB),16-18 triisopropylbenzene,19 amines,20 and tetraalkylammonium cations (TAA+)21 have been used as expanders, and materials with pore size superior to 80 Å16-18 have been obtained. Recently, Kunieda et al.22 studied the effect of oil on the structure of liquid crystals in polyoxyethylene dodecyl ether-water systems and found that the penetration tendency was very large for alcohol and aromatic hydrocarbons such as m-xylene; in this case, no change in the micelle size was observed. Whereas the swelling tendency is favored for saturated hydrocarbons such as decane and squalane, which lead to large size micelles in aqueous solution. In the present study, decane has been used as a swelling agent in order to obtain large pore silica mesoporous materials. Experimental Section Synthesis. Cetyltrimethylammonium bromide (CTMABr) was first dissolved in water with stirring at 40 °C to obtain a clear micellar solution. Sodium silicate and decane were then separately added drop by drop to the solution, and the pH value of the gel obtained was adjusted with sulfuric acid to around 10. After being stirred for several minutes at room temperature, the homogeneous gel with the molar composition of 1 CTMABr:x C10H22:0.63 SiO2:102 H2O (0 e x e 4) was sealed in Teflon autoclaves. The effect of the molar ratio of decane/surfactant, the sequence of introduction of alkane, and the effect of crystallization time and temperature on the final phases were (15) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1995, 279, 548. (16) Beck, J. S. U.S. Patent, 5,057,57296, 1991. (17) Branton, P. J.; Dougherty, J.; Lockhart, G.; White, J. W. Charact. Porous Solids IV 1997, 668. (18) Desplantier-Giscard, D.; Galarneau, A.; Di Renzo, F.; Fajula, F.; 15e`me reúnion du Groupe Franç ais des Zeólithes, Carry Le Rouet, Abstracts of Posters, 15th Annual Meeting of French Zeolite Association, IFP, Carry Le Rouet, 1999. (19) Kimura, T.; Sugahara, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1998, 559. (20) Sayari, A.; Kruk, M.; Jaroniec, M.; Moudrakovski, I. L. Adv. Mater. 1998, 10, 1376. (21) Corma, A.; Kan, K.; Navarro, M. T.; Pe´rez-Pariente, J.; Rey, F. Chem. Mater. 1997, 9, 2123. (22) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831.

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Table 1. Specific Surface Area, Pore Volume and Pore Diameter of Products Obtained from Different Synthesis Conditions (Decane/Surfactant Molar Ratio Equal to 1.0 and Crystallization Time of 8 days at 100 °C Used) sample sample A: without using decane sample B: decane incorporated during the micellar solution preparation at 40 °C sample C: decane incorporated at room temp after preparation of micellar solution but before adding silica sample D: decane incorporated at room temp after adding silica

SBET (m2/g)a

V (cm3/g)a

ø (nm)a

770 661

0.465 0.908

2.6 4.9

511

0.627

4.5

640

0.661

4.1

a These results were obtained on the samples only after ethanol extraction; the values obtained are probably very underestimated.

studied. The final products were obtained after ethanol extraction with a Soxhlet apparatus during 30 h and calcination in a flow of dry nitrogen and then air atmosphere at 500 °C for 18 h. It should be noted that the samples obtained from the study on the effect of the introduction sequence of decane had undergone only the ethanol extraction. Instrumentation. X-ray powder diffraction patterns of obtained materials were recorded with a Philips PW 170 diffractometer, using Cu KR (1.541 78 Å) radiation, equipped with a thermostatization unit (TTK-ANTON-PAAR, HUBER HS60). The scanning micrographs of obtained phases were made from a Philips XL-20 scanning electron microscope (SEM) using conventional sample preparation and imaging techniques. For TEM observations, the samples were prepared using two different methods. First, sample powders were embedded in epoxy resin and then sectioned on an ultramicrotome. The thin films were supported on copper grids previously coated by carbon to improve the stability and reduce the accumulation of the charges. Other samples were prepared by dispersing the powder products in ethanol. The slurry was then dried on a holey carbon film placed on a Cu grid. The transmission electron micrographs were taken using a Philips EM 301 microscope equipped with a tungsten gun at an accelerating voltage of 100 kV. Nitrogen adsorptiondesorption isotherms were obtained from a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were first degassed for several hours at 320 °C. The measurements of adsorption-desorption were then carried out at -196 °C over a wide relative pressure range from 0.01 to 0.995. The average pore diameter and the pore size distribution were determined by the BJH method using the adsorption isotherm branch.

Results and Discussion Effect of the Introduction Sequence of the Swelling Agent. The introduction sequence of the swelling agent can strongly affect the formation of the micelles and consequently the formation of final phase and the pore size of the materials. The effect of the introduction sequence of the swelling agent was studied to find the best way to incorporate the swelling agent into the micelles formed based on CTMABr in an aqueous solution. For this goal, decane was added before or after the introduction of sodium silicate and at different temperatures. Table 1 gives the synthesis conditions as well as the characterization results of obtained products by nitrogen adsorption. Figure 1 depicts the pore size distribution of four samples prepared under different conditions. It is evident that the addition of decane during the synthesis has a beneficial effect on the pore diameter. The pore size is expanded from 2.6 to more than 4.0 nm. However the largest pore size is obtained if the swelling agent is added during the preparation of micellar solution. In fact, if the decane is added during the preparation of micellar solution, decane molecules can be easily incorporated into the core of the

Figure 1. Pore size distribution of samples obtained: without decane (A), decane incorporated during the micellar solution preparation at 40 °C (B), and decane incorporated at room temperature before (C) and after (D) the addition of the silica source.

micelles to form aggregates and the swelling effect occurs as indicated by Kunieda et al.22 The large micelles and finally the large pore materials are obtained. When decane is introduced after the formation of micelles, especially after the addition of silica source, the micelles are already formed, only part of decane added can penetrate into the center of the micelles, the swelling effect is disfavored, and the enlargement of the pore size is moderated. It should be noted that, after autogenous crystallization in the autoclaves, the gel products were obtained and the surfactant and swelling agent incorporated were then extracted by ethanol with a Soxhlet apparatus during 30 h. After being dried in a vacuum at 100 °C for 24 h, the obtained fine powders were subjected to a further evacuation treatment at 320 °C before nitrogen adsorption measurement. It is possible that the treatments used cannot eliminate completely the surfactant and swelling molecules incorporated in the pores. The specific surface area, the pore volume, and the pore diameter obtained from nitrogen adsorption are certainly underestimated.

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Figure 3. X-ray diffraction patterns of products synthesized with various molar ratios of decane/surfactant: a, 0.5; b, 1; c, 2; d, 2.5; e, 3.

Figure 2. X-ray diffraction patterns of products synthesized: a, sample A; b, sample D. 23

Our recent study shows that even though an extraction of 30 h by solvent is carried out to remove the surfactant and swelling agent, around 5-10% of surfactants and swelling agent will still remain in the pores of final products. An additional calcination is necessary to exclude all organic compounds to liberate the pores of the materials. Figure 2 reports the XRD patterns of the samples A and D. Those of samples B and C (not shown here) give the similar XRD patterns to sample D. Few amorphous phases are detected in the final products. In the explored 2θ range (1° e 2θ e 10°), three peaks located at 2.09°, 3.6°, and 4.0° for sample A and at 1.5°, 2.6°, and 3.0° for sample D are detected. These peaks are characteristic of the 100, 110, and 200 reflections of the hexagonal MCM41 phase. This part of the study indicates that the introduction of decane during the synthesis can enlarge the pore diameter of final products, but the best way to incorporate decane is during the micellar solution preparation. The final product has a high specific surface area and a high pore volume despite their underestimation due to the remainder of surfactants and swelling agents in the pores. The XRD results show clearly that these materials have a hexagonal MCM-41 structure. In all the following studies, the swelling agents will be introduced during the micellar solution preparation and the final samples will be subjected to a supplementary calcination in dried nitrogen and then in air at 500 °C after extraction by ethanol. Effect of the Decane/Surfactant Ratio. According to Kunieda et al.,22 the decane is incorporated in the core of the micelles and the size of micelles is therefore increased. The questions can arise as to how much decane can be introduced without destroying the hexagonal phase and how much can the pore size be expanded? The effect (23) Blin, J. L.; Herrier, G.; Otjacques, C.; Su, B. L. Submitted for publication in Int. J. Inorg. Mater.

Table 2. Values of Pore Diameter Calculated from the BJH Method, Unit Cell Parameter, and Wall Thickness decane/ surfactant molar ratio

pore diameter (BJH) (nm)

value of unit cell param (nm)

estimated wall thickness (nm)

0 0.5 1 1.5 2 2.5

2.50 3.60 4.90 4.40 4.80 4.80

4.90 6.90 7.30 7.60 7.90 8.00

2.40 3.30 2.40 3.20 3.10 3.20

a Samples were prepared at 100 °C with a crystallization time of 8 days.

of the molar ratio of decane/surfactant in a range of 0.54.0 on the pore size of final materials was therefore studied. Figure 3 depicts the XRD patterns of samples prepared using different decane/surfactant molar ratios of 0.5 (a), 1.0 (b), 2.0 (c), 2.5 (d), and 4.0 (e). All these samples were synthesized at 100 °C with a crystallization time of 8 days. Three distinct peaks at 2θ ) 1.50°, 2.56°, and 2.97° are clearly identified by XRD on the sample obtained using a decane/surfactant molar ratio of 0.5 (Figure 3a). These peaks correspond respectively to the 100, 110, and 200 reflections of hexagonal MCM-41 materials, indicating the regular arrangement of channels in the compound. At higher decane/surfactant molar ratio up to 2.5, these three reflections are also observed but become less well resolved due to the broadening of the peaks and their positions shifted toward smaller angle region. The increase in d100 value observed as the decane/surfactant molar ratio is raised indicates the enlargement effect of decane on the pore size. Using d100 values, according to a0 ) 2d100/(3)1/2, the unit cell parameter of these products can be calculated. The a0 values are listed in Table 2. A linear relationship (Figure 4) is observed between the value of the unit cell parameter and the decane/surfactant molar ratio in the range of 0.5-2.5. However, when the molar ratio of decane/ surfactant is higher than 3.0 (Figure 3e), no XRD peak is observed. There are two possibilities. First, the pore size of the materials is so large and the positions of the peaks will be situated at so small angle region that no peaks can be detected by our conventional XRD diffractometer. On the other hand, no MCM-41 phase is present since the

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Figure 4. Change in cell parameter a0 (nm) of samples prepared at 100 °C with a crystallization time of 8 days as a function of the decane/surfactant molar ratio.

Figure 6. Variation of the pore diameter (a) and the specific surface area (b) with the decane/surfactant molar ratio.

Figure 5. SEM (a) and TEM (b) micrographs of sample synthesized with a decane/surfactant molar ratio of 0.5.

amount of decane introduced is so high that the formation of the MCM-41 structure is disfavored. Our nitrogen adsorption results which will be presented in the next paragraph suggest the second possibility. Figure 5 represents micrographs of SEM (a) and TEM (b) of solid products obtained using a decane/surfactant molar ratio of 0.5. MCM-41 crystals with variable size and form are observed (Figure 5a). The surface of these crystals is rather porous. The transmission electron micrographs depict a well-ordered hexagonal array of the channels (Figure 5b), which confirm the MCM-41 structure of this compound. The pore diameter of this sample determined by the BJH method from the nitrogen adsorption branch of the isotherm is around 3.6 nm. Figure 6 shows the variation of the pore diameter (Figure 6a) and the specific surface area (Figure 6b) as a function of decane/surfactant molar ratio. From the molar decane/surfactant ratio of 0 to 1, the value of the pore diameter increases from 2.6 to 4.8 nm, and then it remains constant around 5.0 nm until a decane/surfactant molar ratio of 3.5; the incorporation of more decane has no further effect on the value of the pore diameter. It is observed that when 3.5 mol of decane is added, the pore diameter decreases and no homogeneous pore size distribution is observed. The introduction of more swelling agent leads very likely to a destruction of the formed micelles. Looking at the variation of the specific surface area (Figure 6b),

Figure 7. Nitrogen adsorption isotherm (a) and pore size distribution (b) of the sample obtained with a decane/surfactant molar ratio of 2.0.

the value increases from 435 m2/g for a molar decane/ surfactant molar ratio of 0.5 to 802 m2/g for a decane/ surfactant molar ratio of 2.0. Then the specific surface area decreases with the quantity of swelling agent introduced during the preparation of the micellar solution. The formed micelles are probably partially destroyed. So we can conclude that the optimal value for the molar decane/surfactant ratio is between 1.0 and 2.0. Figure 7 depicts the nitrogen adsorption-desorption isotherm (Figure 7a) and the pore size distribution (Figure 7b) of the material obtained with a decane/surfactant molar ratio of 2.0. An IV type nitrogen isotherm (Figure 7a), characteristic of mesoporous compounds, according to the BDDT classification,24 is obtained. The sharp increase in the adsorbed volume of nitrogen due to capillary condensation occurs at relative pressure p/p0 ) 0.60. This part of the curve is almost vertical which indicates the (24) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.


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Figure 8. Different steps observed during zeolites and mesoporous materials synthesis. Scheme 1. Swelling Mechanism Proposed by (A) Ulagappan et al.25 (a, from Pentane to Octane; b, for Higher Alkane) and (B) Kunieda et al.22 for Decane

Figure 9. Variation of the specific surface area at three different temperatures (a, 80 °C; b, 100 °C, c, 120 °C) as a function of crystallization time at decane/surfactant molar ratio of 0.5 (A) and 1.0 (B).

homogeneity in pore size distribution of the final product since the p/p0 position of the inflection point is related to the pore diameter. The pore diameter distribution determined by using the BJH method is quite narrow and centered at around 5.0 nm (Figure 7b). For a hexagonal structure, the dimension of the unit cell is the sum of the pore diameter and the thickness of the walls which separate two adjacent pores. The wall thickness can be therefore deduced by subtracting the pore size determined by BJH method from the dimension of unit cell, obtained by XRD method (Table 2). It is found that the incorporation of decane not only can enlarge the pore size but also can increase the thickness of the walls. This implies that the presence of decane enhances the condensation of silicium source around the micelles. Two mechanisms can be used to explain the swelling effect. The first one is proposed by Ulagappan et al.25 In their study, dealing with incorporation and the organization of alkane and surfactant molecules in the process of (25) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759.

Figure 10. Morphology of some samples obtained at different synthesis conditions: a, 4 days at 80 °C; b, 1, c, 4, d, 6, and e, 11 days, at 100 °C; f, 11 days at 120 °C (decane/surfactant molar ratio ) 0.5).

forming mesoporous silica, the authors indicated that, from pentane to octane, alkanes and the surfactant molecules can be described as molecular dispersions of the solubilizing agent between the tails of surfactant

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Figure 11. Nitrogen adsorption-desorption isotherms of some samples obtained at different synthesis conditions: at 80 °C (A), a, 4, b, 11 days; at 100 °C (B), a, 1, b, 4, c, 6, d, 8, e, 11 days; at 120 °C (C), a, 1, b, 4 days (decane/surfactant molar ratio ) 0.5).

molecules (Scheme 1A, a). Whereas for higher alkanes such as decane, the alkane molecules form a core which is then surrounded by a layer of the cationic surfactant molecules; thus, this swelling mechanism for decane as expander involves one molecule of surfactant for one molecule of expander (Scheme 1A, b). The authors concluded that the optimal alkane/surfactant molar ratio is one. However, our results are quite different from those of Ulagappan. In the present work the optimum decane/ surfactant molar ratio is found between 1.0 and 2.0. It is possible that, in our situation, the decane molecules forming the core of the micelles are surrounded by surfactant, but the molar ratio of decane/surfactant is higher than one as described by Ulagappan et al. for alkanes from pentane to octane. The mechanism proposed by Kunieda et al.22 (Scheme 1B), who justify the expansion of micelles size of the polyoxyethylene dodecyl ether-water system by decane and squalane, can also be used here to explain the swelling effect of decane. During the micellar solution preparation, the decane molecules are incorporated in the core of the micelles to form aggregates. The volume of the micelle is increased whereas the effective cross-sectional area of one surfactant molecule remains constant, contrary to the case of penetration effect. The decane molecules are not aligned with the alkyl groups of surfactants. Complementary investigations are needed if any conclusion concerning the swelling mechanism could be made.

Blin et al.

Figure 12. TEM micrographs of some samples: a, 1, b, 6 days, at 100 °C; c, 1 day at 120 °C (decane/surfactant molar ratio ) 0.5).

Kinetic Study of Large Pore Mesoporous Materials Synthesis. When sodium silicate is added into the prepared micellar solution and pH adjusted, the condensation and polymerization of the silica take place around the micelles of surfactant. The mesoporous materials are therefore formed. Moreover, the physical properties of obtained compounds are found to be also dependent on the crystallization time and temperature. So we have investigated the effect of those factors on the formation of large pore mesoporous materials with two different decane/surfactant molar ratios. The scheme, which is represented in Figure 8, describes the typical change in the crystallinity of materials as a function of crystallization time during zeolite synthesis. Four different steps can be observed and correspond respectively to the nucleation (I), the crystallization (II), the growth of crystals (III), and the amorphization (IV). The same variations, i.e., different steps, have been also found in the synthesis course of mesoporous materials.26 If the specific surface area is used to express the crystallinity of materials only, in the formation of mesoporous materials, the first step (I) is related to the hydrolysis of inorganic silicium source in aqueous solution and no mesostructured solid phase can be obtained. This step is rarely observed due to the high rate of hydrolysis and condensation of silicium source around the micelles. The resulting solid phase, at this step, should be amor(26) Blin, J. L.; Herrier, G.; Otjacques, C.; Su, B. L. Stud. Surf. Sci. Catal., in press.


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Figure 13. Variation of the pore diameter with crystallization time and temperature at two decane/surfactant molar ratios of 0.5 (A) and 1.0 (B).

phous, and the specific surface area should be quite low. The progressive polycondensation of the source of silicium around highly organized micelles occurs at step II, and the mesostructured phase is progressively formed. After removal of the surfactants, the highly ordered solid with surface area higher and higher can be obtained. In the third step, the surface area and pore volume stop growing. However, the thermal stability of the obtained solid phase increases. This indicates that, during this step, the polycondensation of silicium source continues and the thickness of the wall separating two adjacent pores increases with heating time as we have previously observed.26 The last step (IV) corresponds to the amorphization. The high temperature and long crystallization time lead often to the formation of the amorphous phase both in the case of synthesis of zeolites27 and mesoporous materials.26 This analysis can help us to adjust the heating temperature and time for the mesoporous materials synthesis. The kinetic study was performed at two different decane/ surfactant molar ratios of 0.5 and 1.0. The crystallization time and temperature vary respectively from 1 to 11 days and 80 to 120 °C. Figures 9-14 depict respectively the variation of the specific surface area (Figure 9), the morphology (Figure 10), the nitrogen adsorption-desorption isotherms (Figure 11), TEM micrographs (Figure 12), pore diameter (Figure 13), and pore size distribution of materials (Figure 14) obtained at three different crystallization temperatures as a function of crystallization time using two decane/surfactant molar ratios. At Low Decane/Surfactant Molar Ratio. Step II is detected only at 80 °C (curve a of Figure 9A); the value of the specific surface area increases from 300 m2/g for 4 days to 688 m2/g for 8 days. Spherical grains of silica are observed by SEM (Figure 10a), and the sharp increase (27) Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1974.

Figure 14. Pore size distribution of some samples: a, 4 days at 80 °C; b, 1, c, 8 days, at 100 °C; d, 4 days at 120 °C (decane/ surfactant molar ratio ) 0.5).

due to the capillary condensation is not clearly observed in isotherm curve (Figure 11Aa). This indicates the presence of an important quantity of amorphous phase. Only a small part of the gel is transformed into MCM-41. Then the value of the specific surface area reaches a plateau of around 700 m2/g, indicating the period of step III. At 100 °C, the maximum value of surface area is reached from 1 day and maintained to 4 days of crystallization. During this period, the capillary condensation is quite evident (Figure 11Ba) and the morphology characteristic of MCM-41 described in the literature28,29 is detected (Figure 10b,c) by scanning electron microscopy. The crystals have variable sizes and forms, and their surface is very porous. After 4 days, the value of the specific surface area decreases progressively. Even if TEM micrographs still show domains with a well-ordered channel array (Figure 12), the capillary condensation of nitrogen within the mesopores begins to disappear (Figure 11Bb-d) and in the same way, crystals with a “sandy rose like” structure are detected (Figure 10d). According to our previous work,26 we attribute these crystals to the lamellar MCM50 phase. Part of the sample has the hexagonal MCM-41 structure with a well-ordered channel array, and the other (28) Tanev, P. T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 2068. (29) Elder, K. J.; Dougherty, J.; Durand, R.; Iton, L.; Lockhart, G.; Wang, Z.; Whiters, R.; White, J. W. Colloids Surf., A 1995, 102, 213.

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belongs to the lamellar MCM-50 one. The amorphization step (step IV) has begun. At 120 °C, the important drop of the specific surface area occurs only after 1 day of crystallization. The lamellar MCM-50 and spherical grains of silica can be seen by SEM (Figure 10f) and are main products. Neither crystals of MCM-41 from SEM nor the sharp increase in nitrogen adsorption volume due to the capillary condensation from nitrogen adsorption isotherm is detected. The volume of nitrogen adsorbed is rather low (Figure 11Cb). The amorphization step is already completely reached. At High Decane/Surfactant Molar Ratio. At 80 °C, the surface area with a value of around 500 m2/g is found and remains constant throughout the crystallization from 1 to 11 days (curve a of Figure 9B). At 100 and 120 °C, the similar variation is observed as in the case of low decane/ surfactant molar ratio. Only the decline of the value of the specific surface area is retarded. The pore diameter calculated on the basis of the BJH method from the adsorption branch of the isotherm (Figure 13) is found to be constant for a given temperature. Nevertheless, a slight increase up to 5 nm can be noted for a crystallization time of 8 days at 100 °C with the molar ratio of decane/surfactant of 1.0. So crystallization time and temperature do not affect significantly the swelling effect. For the samples obtained during step III,

Blin et al.

the pore size distribution is quite narrow and centered at around the mean value of the pore diameter (Figure 14b). Then it becomes larger, and finally, no homogeneous pore size distribution can be noted when the amorphization is reached (Figure 14d). Conclusion The present study reveals that decane can be used as swelling agent to enlarge up to two times the pore size of mesoporous materials. The best way to incorporate decane is during the micellar solution preparation. The maximum molar decane/surfactant ratio that can be used without destruction of the formed micelles is found to be between 1.0 and 2.0. The kinetic study reveals that crystallization time and temperature affect only slightly the swelling effect but strongly the quality (structural and textural properties) of the samples. Two swelling mechanisms proposed by Ulagappan et al.25 and Kunieda et al.22 can be used to explain the expander effect of decane. Acknowledgment. This work has been performed within the framework of PAI/IUAP 4-10. G.H. thanks the FNRS (Fond National de la Recherche Scientifique of Belgium) for a FRIA scholarship. LA9914615

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