Is There Any Microporosity in Ordered Mesoporous Silicas?

Langmuir 2009, 25, 939-943

939

Is There Any Microporosity in Ordered Mesoporous Silicas? A. Silvestre-Albero,† E. O. Jardim,† E. Bruijn,‡ V. Meynen,‡ P. Cool,‡ A. Sepu´lveda-Escribano,† J. Silvestre-Albero,†,* and F. Rodrı´guez-Reinoso† Laboratorio de Materiales AVanzados, Departamento de Quı´mica Inorga´nica, UniVersidad de Alicante, Ap. 99, E-03080 Alicante, Spain, and Laboratory of Adsorption and Catalysis, UniVersiteit Antwerpen (CDE), UniVersiteitsplein 1, 2610 Wilrijk, Antwerpen, Belgium ReceiVed August 18, 2008. ReVised Manuscript ReceiVed October 10, 2008 The porous structure of nanostructured silicas MCM-41 and SBA-15 has been characterized using N2 adsorption at 77 K, before and after n-nonane preadsorption, together with immersion calorimetry into liquids of different molecular dimensions. Selective blocking of the microporosity with n-nonane proves experimentally that MCM-41 is exclusiVely mesoporous while SBA-15 exhibits both micro- and mesopores. Additionally, N2 adsorption experiments on the preadsorbed samples show that the microporosity on SBA-15 is located in intrawall positions, the micropore volume accounting for only ∼7-8 % of the total pore volume. Calorimetric measurements into n-hexane (0.43 nm), 2-methylpentane (0.49 nm), and 2,2-dimethylbutane (0.56 nm) estimate the size of these micropores to be e0.56 nm.

Introduction The discovery of ordered mesoporous silica materials such as MCM-41 and SBA-15 has attracted much attention in the traditional fields of catalysis and adsorption due to their large pore dimensions when compared to microporous zeolites.1,2 These materials posses a regular structure of mesoporous channels arranged in an ordered hexagonal array. Besides sharing a similar three-dimensional (3D) structure, SBA-15 materials exhibit thicker silica walls than MCM-41, which provide to the material with much higher mechanical stability.3 Interestingly, despite the apparent “exclusiVe” presence of mesoporosity, N2 adsorption isotherms on these materials show a high nitrogen uptake at low relative pressures (p/p0 < 0.1), typically present in microporous materials.4 This finding opened in the last few years an ongoing debate concerning the presence or absence of microporosity. In the case of SBA-15, the application of several characterization techniques, e.g., gas adsorption,5-7 the use of comparative methods (Rs plots and t plots),8-11 positron annihilation spectroscopy,12 temperature-programmed desorption of n-nonane,13 * Corresponding author: [email protected]; tel, +34 96590 9350; fax, +34 96590 3454. † Universidad de Alicante. ‡ Universiteit Antwerpen. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (3) Galarneau, A.; Desplantier-Giscard, D.; Di Renzo, F.; Fajula, F. Catal. Today 2001, 68, 191. (4) Rouquerol, F.; Rouquerol, J.; Sing, K. S. W. Adsorption by Powders and Porous Solids: Principles, Methodology and Applications; Academic Press: London, 1999. (5) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961. (6) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuk, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 11465. (7) Lukens, W. W., Jr.; Schmidt-Winkel, P.; Zhao, D.; Feng, J.; Stucky, G. D. Langmuir 1999, 15, 5403. (8) Galarneau, A.; Cambon, H.; Di Renzo, F.; Ryoo, R.; Choi, M.; Fajula, F. New J. Chem. 2003, 27, 73. (9) Miyazawa, K.; Inagaki, S. Chem. Commun. 2000, 2121. (10) Triantafyllidis, K. S.; Pinnavaia, T. J.; Iosifidis, A.; Pomonis, P. J. J. Mater. Chem. 2007, 17, 3630. (11) Galarneau, A.; Cambon, H.; Di Renzo, F.; Fajula, F. Langmuir 2001, 17, 8328. (12) Ueno, Y.; Tate, A.; Niwa, O.; Zhou, H.-S.; Yamada, T.; Honma, I. Chem. Commun. 2004, 746. (13) Makowski, W.; Kustrowski, P. Microporous Mesoporous Mater. 2007, 102, 283.

formation of carbon and platinum replica,8,14 Hg porosimetry,15 129 Xe NMR,16,17 high-resolution transmission electron microscopy (HRTEM),18 and so on, has provided clear evidence about the presence of complementary porosity (mainly between 0.5 and 3 nm), in addition to the structural mesopores. Additionally, this complementary porosity has been shown to be highly depending on the synthesis conditions.8,9,11 Contrary to SBA15, the situation becomes more controversial in the case of MCM41 materials. In this sense, while the synthesis of a carbon replica using MCM-41 has suggested the absence of micropores, i.e., the carbon replica led to a disordered bunch of single wires,14 the use of gas adsorption (mainly N2 and CO2) together with the application of comparative methods (t plot, Rs plot, etc.) has provided some uncertainty on this particular issue.19-23 Of course, the correct characterization of the textural properties on these materials is of major importance to understand its behavior in a given application. Thus, we propose the combined use of N2 adsorption at 77 K, selective blocking of the microporosity by n-nonane, and immersion calorimetry into liquids of different molecular dimensions as an alternative approach to elucidate the presence or absence of microporosity in these ordered mesoporous silicas.

Experimental Section Synthesis of MCM-41. Hexadecyltrimethylammonium bromide (CTABr; 6.2 g) was dissolved in 40.4 mL of distilled water. The mixture was stirred at room temperature until all the CTABr was completely dissolved. Then, 10 g of 20% tetraethylammonium (14) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712. (15) Galarneau, A.; Lefe`vre, B.; Cambon, H.; Coasne, B.; Valange, S.; Gabelica, Z.; Bellat, J.-P.; Di Renzo, F. J. Phys. Chem. C 2008, 112, 12921. (16) Nossov, A.; Haddad, E.; Guenneau, F.; Galarneau, A.; Di Renzo, F.; Fajula, F.; Ge´de´on, A. J. Phys. Chem. B 2003, 107, 12456. (17) Chen, F.; Zhang, M.; Han, Y.; Xiao, F.; Yue, Y.; Ye, C.; Deng, F. J. Phys. Chem. B 2004, 108, 3728. (18) Liu, J.; Zhang, X.; Han, Y.; Xiao, F.-S. Chem. Mater. 2002, 14, 2536. (19) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267. (20) Long, Y.; Xu, T.; Sun, Y.; Dong, W. Langmuir 1998, 14, 6173. (21) Storck, S.; Bretinger, H.; Maier, W. F. Appl. Catal., A 1998, 174, 137. (22) Berenguer-Murcia, A.; Fletcher, A. J.; Garcı´a-Martı´nez, J.; CazorlaAmoro´s, D.; Linares-Solano, A.; Thomas, K. M. J. Phys. Chem. B 2003, 107, 1012. (23) Berenguer-Murcia, A.; Garcı´a-Martı´nez, J.; Cazorla-Amoro´s, D.; Martı´nezAlonso, A.; Tasco´n, J. M. D.; Linares-Solano, A. Stud. Surf. Sci. Catal. 2002, 144, 83.

10.1021/la802692z CCC: $40.75  2009 American Chemical Society Published on Web 12/22/2008

940 Langmuir, Vol. 25, No. 2, 2009 hydroxide (TEAOH) solution and 4.1 g of fumed silica were added to the solution. The molar ratio of the synthesis gel SiO2:TEAOH: CTABr:H2O was 1:0.19:0.25:39. The mixture was stirred at 343 K for 2 h and aged at room temperature for 24 h. Then it was transferred into a Teflon-lined autoclave and kept at 403 K for 48 h under autogeneous pressure. After the mixture was allowed to cool, the white powder was filtered and washed with plenty of water. Subsequently, the powder was hydrothermally treated by replacing the mother liquor with pure water at 403 K for 72 h. The double hydrothermal treatment allows obtaining an MCM-41 material with higher thermal stability due to the better condensation of the silica, thus preventing the contraction of the porous structure upon calcination.24 Finally, the solid product was filtered, washed, and dried. The resulting powder was calcined under ambient atmosphere at 823 K for 6 h using a heating rate of 1 K/min. Synthesis of SBA-15. For the synthesis of SBA-15, 8 g of pluronic P123 triblock copolymer (EO20PO70EO20) was dissolved in 300 mL of a 2 M HCl aqueous solution. Subsequently, 17 g of tetraethyl orthosilicate (TEOS) was added under vigorous stirring at a temperature of 318 K. The molar ratio of the synthesis solution TEOS:HCl:H2O:P123 was 1:5.87:194:0.017. After 7.5 h, the stirring was stopped and the solution was aged for 15 h at 353 K. Then the white powder was filtered, washed, and dried. Finally, the resulting powder was calcined under ambient atmosphere to 823 K for 6 h with a heating rate of 1 K/min. Characterization Techniques. MCM-41 and SBA-15 were characterized using several techniques. N2 adsorption-desorption isotherms were performed in a Coulter Omnisorb-610 equipment at 77 K. Prior to the adsorption experiment, samples were outgassed under vacuum (10-3 Pa) at 523 K for 4 h. Preadsorption of n-nonane was performed in the same equipment using the following experimental procedure: after an outgassing treatment at 523 K for 4 h, samples were exposed to n-nonane (Aldrich, 99%) for 30 min at 77 K and then left in contact with the liquid for 3 h at room temperature. After the preadsorption, samples were outgassed at 298 K overnight prior to the redetermination of the N2 adsorption isotherm at 77 K. Immersion calorimetry measurements into different liquids (nhexane (Aldrich, >97 %), 2-methylpentane (Fluka, >99.5%), and 2,2-dimethylbutane (Aldrich, 99%)) were performed in a C80D calorimeter at 303 K. Prior to the experiment, the sample was outgassed under vacuum (10-3 Pa) at 523 K for 4 h in a glass bulb. Then the bulb was sealed in vacuum and inserted into the calorimetric chamber containing the immersion liquid. Once thermal equilibration was reached, the brittle end of the glass bulb was broken and the heat released was followed with time. A detailed description of the experimental setup can be found elsewhere.25

Results and Discussion N2 adsorption isotherms of MCM-41 and SBA-15 materials exhibit a combination of type I and type IV isotherms, according to the IUPAC classification (see Figure 1).26 There is in both samples a high N2 uptake at low relative pressures (p/p0 < 0.1), together with the characteristic capillary condensation in the mesopores at p/p0 ∼ 0.32 and p/p0 ∼ 0.68 for MCM-41 and SBA-15, respectively. The higher relative pressure for the capillary condensation on SBA-15 is in accordance with its larger pore diameter (5.5 nm vs 2.6 nm, obtained from the BJHdes). Interestingly, while the capillary condensation and evaporation are completely reversible for MCM-41, SBA-15 exhibits a type H1 hysteresis loop. Although the presence of a type H1 hysteresis loop is characteristic of capillary condensation and evaporation on cylindrical pores open at both ends, the real explanation of (24) Collart, O.; Van Der Voort, P.; Vansant, E. F.; Desplantier-Giscard, D.; Galarneau, A.; Di Renzo, F.; Fajula, F. J. Phys. Chem. B 2001, 105, 12711. (25) Silvestre-Albero, J.; Go´mez de Salazar, C.; Sepu´lveda-Escribano, A.; Rodrı´guez-Reinoso, F. Colloids Surf., A 2001, 187-188, 151. (26) IUPAC Recommendations. Pure Appl. Chem. 1985, 57, 603.

SilVestre-Albero et al.

Figure 1. N2 adsorption-desorption isotherms of MCM-41 and SBA15 materials at 77 K. Table 1. Physical Properties of MCM-41 and SBA-15 Materialsa SBET Smeso Vo Vmeso Vt Vmicro (m2/g) (cm3/g) (cm3/g) (cm3/g) (cm3/g)b (m2/g)b MCM-41 MCM-41 (n-nonane) SBA-15 SBA-15 (n-nonane)

1055 1032

0.36 0.36

0.54 0.53

0.90 0.89

0 0

988 993

595 520

0.22 0.19

0.55 0.55

0.77 0.74

0.04 0

526 507

a Values after n-nonane preadsorption are included. b Obtained from the comparative Rs-plots using a nonporous silica reference.33

the filling and emptying mechanism is still an open question.27-30 In this sense, a detailed analysis of the sorption scanning curves on SBA-15 materials of different quality allowed Esparza et al. to explain these two processes based on the assumption that the pores on SBA-15 are not perfectly cylindrical but they present undulations along the axis of the tubes.27 Thus, the presence of varying cross sectional areas (bulges and necks) along the longitudinal axes favors the appearance of phenomena such as advanced adsorption and single pore blocking, thus explaining the capillary condensation and evaporation on these materials. In the same way, Grosman et al. observed that the hysteresis process could not be described by the classical models (independent domain theory of sorption hysteresis) based on an assembly of independent cylindrical pores.28 However, these authors proposed the presence of cooperative effects, i.e., the pores in SBA-15 do not drain independently of neighboring pore entities, in order to explain the capillary condensation and evaporation (interaction between the pores/network effects). Contrary to these hypotheses, Sonwane et al. explained the presence of hysteresis in this type of material simply based on the pore diameter. According to these authors, all samples with pores above ∼4 nm should always exhibit a hysteresis loop for nitrogen adsorption at 77 K.29 Taking into account that our MCM41 has an estimated pore diameter of 2.6 nm while the SBA-15 has a diameter of 5.5 nm, the absence of hysteresis on the former sample can be successfully explained based on the assumption proposed by Sonwane and co-workers. Table 1 summarizes the textural properties of both mesoporous materials. The micropore volume (V0) on porous solids is usually deduced by application of the Dubinin-Radushkevich (DR) (27) Esparza, J. M.; Ojeda, M. L.; Campero, A.; Domı´nguez, A.; Kornhauser, I.; Rojas, F.; Vidales, A. M.; Lo´pez, R. H.; Zgrablich, G. Colloids Surf., A 2004, 241, 35. (28) Grosman, A.; Ortega, C. Langmuir 2005, 21, 10515. (29) Sonwane, C. G.; Ludovice, P. J. J. Mol. Catal. A: Chem. 2005, 238, 135. (30) Bruschi, L.; Fois, G.; Mistura, G.; Sklarek, K.; Hillebrand, R.; Steinhart, M.; Go¨sele, U. Langmuir 2008, 24, 10936.

Microporosity in Mesoporous Silicas

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Figure 2. Comparative Rs plots for (a) MCM-41 and (b) SBA-15 mesoporous materials. The corresponding Rs plot for SBA-15 after n-nonane preadsorption is included.

Figure 3. N2 adsorption-desorption isotherms at 77 K for (a) MCM-41 and (b) SBA-15 materials before and after n-nonane preadsorption.

equation to the N2 adsorption isotherm. Interestingly, application of the DR equation to the N2 adsorption isotherms in the low relative pressure range for MCM-41 and SBA-15 estimates the micropore volume (V0) in both samples to be ∼30-40 % of the total pore volume (see Table 1). This value is very similar to that elucidated for the intrawall pore volume on SBA-15 materials using both a geometrical model which takes into account the cell parameter from X-ray diffraction, the total pore volume, and the pressure of pore filling11 and nonlocal density functional theory (NLDFT) calculations.32 According to these results, both MCM41 and SBA-15 materials should exhibit an important contribution of microporosity. Is this result meaningful? A common procedure to elucidate the presence or absence of microporosity on porous solids consists of the comparison of the adsorption isotherm of the porous material with that of a nonporous sample with similar surface and chemical characteristics, e.g., Rs method.31 In this sense, Figure 2 shows the comparative Rs plot corresponding to the MCM-41 and SBA-15 materials using as a reference sample the LiChrospher Si-1000 silica reported by Jaroniec and co-workers.33 In the case of MCM41 (Figure 2a), extrapolation of the low Rs range goes to the origin, thus suggesting the absence of microporosity on this sample. On the contrary, the corresponding Rs plot for the SBA15 sample (Figure 2b) provides an extrapolated adsorbed volume for the micropores of Vmicro ∼ 0.04 cm3/g, which corresponds to ∼5 % of the total pore volume. The micropore volume obtained for SBA-15 using the comparative Rs method is quite closed to that reported in the literature on similar materials.5,34 Unfortunately, these values are highly dependent on the Rs range considered and on the reference nonporous sample5,9,23 and, thus,

they cannot provide a conclusive proof about the presence or absence of microporosity on these ordered materials.21 The selective blocking of the microporosity using long hydrocarbons (e.g., n-nonane) was introduced by Gregg and Langford as a relatively direct method to evaluate the microporosity in carbon blacks.35 This method is based on the strong adsorption that these large molecules experience in micropores. In fact, an outgassing treatment at room temperature will only remove the hydrocarbon from large pores, i.e., mesopores and macropores, leaving the micropores blocked. The difference between the isotherms before and after preadsorption will provide the volume of micropores. In this sense, previous studies on activated carbons and mesoporous silicas showed the usefulness of this method for the characterization of the microporosity on porous solids.22,36,37 Figure 3 shows the N2 adsorption-desorption isotherms for the (a) MCM-41 and (b) SBA-15 materials before and after n-nonane preadsorption. Interestingly, n-nonane preadsorption has no effect on the MCM41 sample. N2 adsorption-desorption isotherms, before and after n-nonane preadsorption, perfectly overlap over the whole relative pressure range. This result constitutes an empirical proof that MCM-41 is exclusiVely a mesoporous material. n-Nonane preadsorption on SBA-15 produces important changes when compared with MCM-41. SBA-15 material exhibits a noticeable decrease in the N2 adsorption capacity, this decrease being constant over the whole relative pressure range. Apparently, n-nonane is only blocking the microporosity without affecting the size and volume of the mesopores (the capillary condensation in the adsorption branch perfectly fits before and after preadsorption; see Table 1 and Figure 3b). Interestingly, the decrease in the adsorption capacity is accompanied by a somewhat important change in the hysteresis loop. Although the hysteresis

(31) Marsh, H.; Rodrı´guez-Reinoso, F. ActiVated Carbon; Elsevier: London, 2006. (32) Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. B 2001, 105, 6817. (33) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410. (34) Boskovic, S.; Hill, A. J.; Turney, T. W.; Gee, M. L.; Stevens, G. W.; O’Connor, A. J. Prog. Solid State Chem. 2006, 34, 67.

(35) Gregg, S. J.; Langford, J. F. Trans. Faraday Soc. 1969, 65, 1394. (36) Rodrı´guez-Reinoso, F.; Garrido, J.; Martı´n-Martı´nez, J. M.; Molina-Sabio, M.; Torregrosa, R. Carbon 1989, 27, 23. (37) Go¨ltner, C. G.; Smarsly, B.; Berton, B.; Antonietti, M. Chem. Mater. 2001, 13, 1617.

31

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starts at the same relative pressure, there are now two welldefined slopes, the complete loop closing at a lower relative pressure (p/p0 ∼ 0.45). This finding suggests the presence of partial constrictions in the mesoporosity, this effect being only visible in the desorption branch (BJHdes shows the appearance of a new large peak on SBA-15 centered at 3.8 nm). Most probably, n-nonane blocking the microporosity leaves part of the hydrocarbon chain “hanging” in the mesopores, narrowing them in distinct places and, thus, leading to a two-step desorption branch. A similar widening and tailing of the hysteresis loop, i.e., the presence of a two-step desorption branch, was observed for SBA-15 materials prepared using an excess of silica.38,39 In a similar way, the modified desorption branch compared to the original SBA-15 material was explained based on the presence of plugs and constrictions (amorphous silica deposits) in the mesopores of these modified materials. Interestingly, the presence of clearly observable effects on the desorption branch of the nitrogen isotherm when constrictions and plugs are present within the mesopores rules out the presence of important modulations (bulges and necks) on standard mesoporous SBA-15 and MCM41 materials.27 Quantification of the total volume blocked by n-nonane results in a value of V ∼ 0.06 cm3/g, in close agreement with the Rs method. Finally, application of the Rs method to the SBA-15 with preadsorbed n-nonane gives, at low Rs values, a straight line going through the origin which proves the successful blocking of the micropores by the hydrocarbon (see Figure 2b). In summary, these results prove experimentally that (i) while MCM-41 is exclusiVely mesoporous, SBA-15 exhibits both microand mesopores, (ii) the microporosity in SBA-15 is mainly located in intrawall positions and not on isolated disordered domains, thus modifying the hysteresis loop, and (iii) this microporosity accounts for only ∼7-8 % of the total pore volume, in close agreement with the values obtained using the Rs plots. The formation of microporosity on SBA-15 is commonly attributed to the penetration of the hydrophilic poly(ethylene oxide) chain from the triblock copolymer template into the silica framework.5 This penetration gives rise to a “corona” region around the mesopores of lower density compared to amorphous silica, which is attributed to the presence of the micropores.40 However, the dimensions of these micropores is still an open question. Liu et al. observed, using HRTEM, the presence of spherical cages of ∼0.5 nm randomly distributed on the silica walls of the SBA-15.18 In the same way, the use of positron inhalation spectroscopy together with selective adsorption of aromatic compounds (benzene and toluene) provided a micropore size on SBA-15 of 0.63 nm12 while larger micropores (above 0.8 nm) where suggested from hyperpolarized 129Xe NMR.16 Using chemical modification of the SBA-15 surface with organosilane ligands of different sizes, Ryoo et al. suggested the size of the complementary pores to range from 1 to 3 nm, which corresponds to micropores and small mesopores.6 The large divergence in the values reported in the literature clearly reflects that the size and volume of this complementary porosity highly depend on the synthesis conditions, i.e., synthesis/aging temperature, and on the silica/surfactant ratio.5,9,11 In fact, the presence of small micropores (less than 1 nm) has been suggested for SBA-15 synthesized at low temperature (308-333 K) while small micropores together with large pores (around 2-3 nm), which interconnect the mesopores, has been suggested for samples (38) Van Der Voort, P.; Ravikovitch, P. I.; De Jong, K. P.; Benjelloun, M.; Van Bavel, E.; Janssen, A. H.; Neimark, A. V.; Weckhuysen, B. M.; Vansant, E. F. J. Phys. Chem. B 2002, 106, 5873. (39) Celer, E. B.; Kruk, M.; Zuzek, Y.; Jaroniec, M. J. Mater. Chem. 2006, 16, 2824. (40) Impe´ror-Clerc, M.; Davidson, P.; Davidson, A. J. Am. Chem. Soc. 2000, 122, 11925.

SilVestre-Albero et al. Table 2. Heat of Immersion of MCM-41 and SBA-15 into Liquids with Different Molecular Dimensions (n-Hexane (0.43 nm); 2-Methylpentane (0.49 nm), and 2,2-Dimethylbutane (0.56 nm)) at 303 Ka -∆Himm (J/g)

-∆Himm (mJ/m2)

sample

n-HX

2-MP

2,2-DMB

n-HX

2-MP

2,2-DMB

MCM-41 SBA-15

58 46

63 58

51 34

54.9 77.3

59.7 97.5

48.3 57.1

a Enthalpy of immersion is referred both per unit gram and per unit surface area (obtained using the BET equation).

synthesized at higher temperatures (∼373 K). Furthermore, the development of small mesopores after an increase in the synthesis temperature is accompanied by a simultaneous increase in the density of the silica walls and a decrease in the amount of the small micropores until they disappear for samples synthesized at temperatures nearly 403 K. This behavior has been explained based on the partial dehydration of the PEO chain with temperature which modifies the degree of interaction between the micelles and the silica.8,9,11,15 With this in mind and in order to gain more insight into the size of the micropores present on our SBA-15 material, we have performed immersion calorimetry studies using liquids of different molecular dimensions. It is noteworthy to mention that in the absence of specific interactions, enthalpy of immersion into a certain liquid can be regarded as an indirect measurement of the surface area accessible to this molecule.25 As it can be observed in Table 2, the heat of immersion (J/g) into the different liquids is larger on MCM-41 compared to SBA15. This behavior is attributed to the larger surface area accessible for the different molecules in the former sample and, consequently, it does not permit a direct comparison between them. However, this inconvenience can be overcome using the areal enthalpy of immersion, i.e., the heat of immersion per meter squared, as determined from the BET equation. In this sense, the areal enthalpy of immersion of both MCM-41 and SBA-15 into a linear hydrocarbon, i.e., n-hexane, with a kinetic diameter of 0.43 nm gives values of 55 and 77 mJ/m2, respectively. The larger areal enthalpy of immersion (mJ/m2) on SBA-15 must be attributed to the higher degree of interaction between the hydrocarbon molecule and the silica surface inside the micropores (larger adsorption potential). Interestingly, a larger hydrocarbon molecule, i.e., 2-methylpentane, with a kinetic diameter of 0.49 nm, still provides an enhanced areal enthalpy of immersion on SBA-15, suggesting the absence of steric restrictions for this molecule to access the microporosity on this sample (the increased enthalpy compared with n-hexane should be attributed to a higher packing density). However, the situation becomes different for molecules above 0.56 nm (e.g., 2,2-dimethylbutane). While the areal enthalpy of immersion on MCM-41 decreases only 19 % compared to 2-methylpentane (probably due to the lower uptake on this material for this large molecule), this value falls down 41% for SBA-15. This result confirms that in addition to the normal decrease in hydrocarbon uptake for larger molecules, SBA-15 exhibits molecular sieve effects for molecules above 0.56 nm. Thus, the micropores on our SBA-15 material should be below 0.56 nm. Unfortunately, immersion calorimetric studies are restricted to molecules which are liquid at the temperature of the experiment (303 K) and, consequently, the presence of additional complementary pores in the large micropore-small mesopore range (between 1.5 and 3 nm) cannot be excluded from the calorimetric data.

Conclusions N2 adsorption at 77 K, before and after n-nonane preadsorption, together with immersion calorimetry studies into liquids of

Microporosity in Mesoporous Silicas

different molecular dimensions have been used as an alternative approach to characterize the porous structure of ordered mesoporous silicas MCM-41 and SBA-15. Despite the large N2 uptake observed on both samples at low relative pressures (p/p0 < 0.1), selective blocking of the microporosity using n-nonane preadsorption proves experimentally that MCM-41 is exclusiVely mesoporous while SBA-15 exhibits both micro- and mesopores. Additionally, the appearance of a two-step desorption branch in the N2 adsorption isotherm of the SBA-15 material with preadsorbed n-nonane clearly shows that these micropores are located in intrawall positions, the micropore volume accounting for only ∼7-8 % of the total pore volume. Finally, immersion calorimetry studies into n-hexane (0.43 nm), 2-methylpentane

Langmuir, Vol. 25, No. 2, 2009 943

(0.49 nm), and 2,2-dimethylbutane (0.56 nm) show that, contrary to MCM-41, SBA-15 exhibits molecular sieve effects for molecules above 0.56 nm; i.e., the micropores in SBA-15 should be e0.56 nm. Acknowledgment. Financial support from MEC (MAT200761734) and the Network of Excellence Insidepores (NMP3CT2004-500895) is gratefully acknowledged. J.S.A. acknowledges support from MEC, GV, and UA (RYC2137/06). V.M. gratefully acknowledges the Fund for Scientific research Flanders for her research grant. A.S.A. acknowledges a Ph.D. fellowship from MEC. LA802692Z