True Microporosity and Surface Area of Mesoporous SBA-15 Silicas

Role of Liquid vs Vapor Water in the Hydrothermal Degradation of SBA-15 ...... Calleja, Galen D. Stucky, Juan A. Melero, Rafael A. García, and Jose I...
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Langmuir 2001, 17, 8328-8335

True Microporosity and Surface Area of Mesoporous SBA-15 Silicas as a Function of Synthesis Temperature Anne Galarneau,* He´le`ne Cambon, Francesco Di Renzo, and Franc¸ ois Fajula Laboratoire de Mate´ riaux Catalytiques et Catalyze en Chimie Organique, UMR 5618 ENSCM/CNRS, Ecole Nationale Supe´ rieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France Received April 14, 2001. In Final Form: October 2, 2001 A model based on hexagonal mesopores connected by micropores allows evaluation of the porosity of SBA-15 silicas and elucidation of the literature problems on the subject. The presence of micropores in the walls between the mesopores renders unreliable the surface area determination by the BrunauerEmmett-Teller equation and the microporosity assessment by the t-plot method. A geometrical model allows calculation of the true surface area, as well as the real mesoporous and microporous pore volumes, the wall thickness and the wall density from the total pore volume, the pressure of pore-filling, and the cell parameter. The increase of synthesis temperature leads to a continuous increase of mesopore size and a continuous decrease of microporous volume until the disappearance of micropores for materials synthesized at nearly 130 °C. The results are discussed on the basis of the collective properties of nonionic surfactants.

Introduction Since 1991, great interest has been focused on the new class of mesoporous materials templated by micellar systems foreran by MCM-411 for their potential applications in catalysis, adsorption, support for electronic devices, and luminescence applications; see for instance the review of Corma.2 Ordered mesoporous material prepared by cooperative assembly of silica and micelles have high surface area (>700 m2/g), high pore volume (>0.7 mL/g), and tunable pore size (20-150 Å). In 1998, a new synthesis of ordered hexagonal mesoporous silica was proposed by Stucky and co-workers.3,4 They prepared a material named SBA-15 by using triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers (EOn-POm-EOn) as the structure-directing agents in an acidic medium. SBA-15 materials show a remarkable hydrothermal stability, being claimed stable for at least 48 h in boiling water.4 SBA-15 materials prepared with EO20-PO70-EO20 exhibit BrunauerEmmett-Teller (BET) surface areas of 690-920 m2/g, pore volumes between 0.80 and 1.23 mL/g, pore sizes between 47 and 89 Å, and unusually thick walls between 31 and 53 Å. The pore size increases and the wall thickness decreases as the temperature of synthesis increases from 35 to 100 °C. Different particle morphologies can be tailored.5 Because of their large pores, high hydrothermal stability, and easy preparation, SBA-15 materials have been considered very promising materials and have been tested for several applications: as catalysts (Al-SBA156,7 or Ti-SBA-158,9), supports for grafted catalysts,10,11 * Corresponding author. Fax: 33 4 67 14 43 49. Tel: 33 4 14 43 29. E-mail: [email protected]. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmidt, 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. (2) Corma, A. Chem. Rev. 1997, 97, 2373. (3) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (5) Zhao, D. Y.; Sun, J. Y.; Li, Q. Z.; Stucky, G. D. Chem. Mater. 2000, 12, 275. (6) Luan, Z. H.; Hartmann, M.; Zhao, D. Y.; Zhou, W. Z.; Kevan, L. Chem. Mater. 1999, 11, 1621.

sorbents for heavy metal,12 advanced optical materials,13 templates for metal nanowires (Pt,14,15 Ag16), or selective sorbents for proteins.17 Nevertheless, some surprising results have been obtained in the characterization of these materials by nitrogen sorption. In the case of large-pore MCM-41, the more accurate methods to analyze pore size of cylindrical pores from nitrogen sorption are the Broekhoff and de Boer (BdB) method18 or the recent nonlinear functional density theory from Ravikovitch and Neimark.19 The results of these two methods are close to the results obtained by other methods such as the commonly used Gurvitch (4V/S) method. The pore size measurement differs by no more than 20%, and these differences can be easily accounted for, if the physical assumptions on which each method is based are taken into account.18 In the case of SBA-15, the results of different methods of pore diameter evaluation widely differ. As an example, SBA-15 synthesized at 100 °C, with a cell parameter of 120 Å, a pore volume of 1.17 mL/g, and a BET surface area of 850 m2/g, features pores with a diameter of 89 Å3,4 as evaluated by (7) Yue, Y. H.; Gedeon, A.; Bonardet, J. L.; Melosh, N.; DEspinose, J. B.; Fraissard, J. Chem. Commun. 1999, 1967. (8) Luan, Z. H.; Maes, E. M.; vanderHeide, P. A. W.; Zhao, D. Y.; Czernuszewicz, R. S.; Kevan, L. Chem. Mater. 1999, 11, 3680. (9) Morey, M. S.; OBrien, S.; Schwarz, S.; Stucky, G. D. Chem. Mater. 2000, 12, 898. (10) Bae, S. J.; Kim, S. W.; Hyeon, T.; Kim, B. M. Chem. Commun. 2000, 31. (11) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448. (12) Liu, A. M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145. (13) Yang, P. D.; Wirnsberger, G.; Huang, H. C.; Cordero, S. R.; McGehee, M. D.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Buratto, S. K.; Stucky, G. D. Science 2000, 287, 465. (14) Han, Y. J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. (15) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuck, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 11465. (16) Huang, M. H.; Choudrey, A.; Yang, P. D. Chem. Commun. 2000, 1063. (17) Han, Y.-J.; Stucky, G. D.; Butler, A. J. Am. Chem. Soc. 1999, 121, 9897. (18) Galarneau, A.; Desplantier, D.; Dutartre, R.; DiRenzo, F. Microporous Mesoporous Mater. 1999, 27, 297. (19) Ravikovitch, P. I.; Neimark, A. V. Stud. Surf. Sci. Catal. 2000, 129, 597.

10.1021/la0105477 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/20/2001

Microporosity and Surface Area of Silicas

the pressure of the pore-filling step, a diameter of 55 Å as evaluated by the equivalent diameter formula 4V/SBET and a diameter of 107 Å as evaluated by the geometrical (Vmes + XRD) method.18 In the latter method, the diameter D is a function of the mesopore fraction mes and equal to D ) 1.05ames1/2 ) 1.05a[Vmes/(Vmes + 1/FSi)]1/2 where FSi is equal to the density of amorphous silica for MCM-4120 (FSi ) 2.2 g cm-3), a is the cell parameter, and Vmes is the mesoporous volume. These results suggested that some assumptions at the basis of the measurement methods were not valid in the case of SBA-15. Especially sensitive assumptions were the absence of micropores, on which the BET equation and the wall density are based, and the constant section of the pores, on which the equivalent diameter formula is founded. To explain these discrepancies, several researchers have analyzed the nitrogen adsorption data of SBA-15 to detect the presence of micropores. Fenelov et al.21 found no micropores by t-plot analysis but claimed a surface roughness, implying BET surface areas ∼1.5-2.0 times higher than those of a smooth surface. Jaroniec and coworkers22 found by Rs-plot analysis that SBA-15 exhibits a micropore volume increasing from 0.08 mL/g at 35 °C to 0.12 mL/g at 80 °C and then decreasing to 0.06 mL/g for a synthesis at 100 °C. By grafting different alkylsilanes on the SBA-15 surface, the micropore size was estimated to be larger than 10 Å and smaller than 30 Å.15 Impe´rorClerc et al.23 investigated the pore structure of SBA-15 at 35 and 100 °C. They observed micropores by β-plot analysis for SBA-15 synthesized at 35 °C, but no micropores could be detected by β-plot analysis for SBA-15 synthesized at 100 °C. Moreover, by exploiting the X-ray diffraction (XRD) reflection intensities, the authors showed for both synthesis temperatures that the wall density was lower than the density of amorphous silica due to the presence of micropores arising from the partial occlusion of PEO chains into the silica walls. Other authors suggested that micropores formed some connections between mesopores. Indeed Ryoo and co-workers prepared a platinum replica15 and a carbon replica24,25 of SBA-15, the latter being called CMK-3, and showed the carbon network to have a hexagonal symmetry. They attributed the stiffness of the carbon replica to microporous interconnections between the mesopores of SBA-15. Indeed, the carbon replica technique applied to MCM-41 led not to a hexagonal structure but to a disordered bunch of single wires, in agreement with the unconnected mesopores of MCM-41 materials. To untangle the common thread of these partially contradictory results, in this paper the nitrogen sorption data of SBA-15 synthesized in a wide temperature field have been analyzed. Experimental Section Materials. The SBA-15 materials used in this paper have been synthesized according to the methods described in the literature4 from typical synthesis batches with the composition of 1 g of Pluronic P123 [(EO)20(PO)70(EO)20, Aldrich], 15 g of H2O, (20) DiRenzo, F.; Testa, F.; Chen, J. D.; Cambon, H.; Galarneau, A.; Plee, D.; Fajula, F. Microporous Mesoporous Mater. 1999, 28, 437. (21) Fenelov, V. B.; Derevyankin, A. Y.; Kirik, S. D.; Solovyov, L. A.; Shmakov, A. N.; Bonardet, J.-L.; Gedeon, A.; Romannikov, V. N. Microporous Mesoporous Mater. 2001, 44-45, 33. (22) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961. (23) Impe´ror-Clerc, M.; Davidson, P.; Davidson, A. J. Am. Chem. Soc. 2000, 122, 11925. (24) Ryoo, R.; Joo, S. H.; Jun, S.; Tsubakiyama, T.; Tearasaki, O. Stud. Surf. Sci. Catal. 2001, 135, 07-O-01. (25) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712.

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Figure 1. (a) Nitrogen isotherm at 77 K of SBA-15 synthesized at 50 °C together with the nitrogen isotherm of a 900 m2/g MCM-41 of 50 Å pore diameter. (b) Nitrogen isotherm at 77 K of SBA-15 synthesized at 130 °C together with the nitrogen isotherm of a 900 m2/g MCM-41 of 115 Å pore diameter. 30 g of HCl 2 M, and 2.1 g of tetraethyl orthosilicate (TEOS, Aldrich). The mixture has been maintained at 35 °C for 24 h and then for 2 days at a given temperature between 35 and 130 °C under static conditions in a Teflon-lined autoclave. For synthesis temperatures higher than 130 °C, the surfactant starts to decompose. The aging step at 35 °C is very important. In the absence of room-temperature ripening, a disordered material is obtained. Reference MCM-41 materials were synthesized at 115 °C by using cetyltrimethylammonium bromide (CTAB, Aldrich), 1,3,5-trimethylbenzene (TMB, Aldrich), pyrogenic silica (Aerosil 200V Degussa), sodium hydroxide (Prolabo), and deionized water in molar ratios 1 SiO2/0.26 NaOH/0.035 NaAlO2/0.1 CTAB/20 H2O/x TMB.1,26 All materials were then filtered, washed with water, and dried at 80 °C for 24 h. The solids were then calcined in air at 550 °C for 8 h. Measurements. Powder XRD data were obtained on a CGR Theˆta-60 diffractometer with Inel drive, using monochromated Cu KR radiation. The adsorption/desorption isotherms of nitrogen at 77 K were measured using a Micromeritics ASAP 2000 instrument. Each sample was outgassed at 250 °C until a stable static vacuum of 3 × 10-3 Torr was reached. Pore diameter was measured by the Broekhoff and de Boer method,27 which has been demonstrated as one of the best for MCM-41 materials.18 Aerosil silica was used as a nonporous reference material for the t-plot analyses. BET surface area and the CBET parameter were calculated using adsorption data in the relative pressure range from 0.15 to 0.26 included in the validity domain of the BET equation.28

Results Nitrogen Sorption Isotherms at 77 K. The nitrogen isotherms of SBA-15 and MCM-41 materials with different pore sizes are reported in Figure 1. In all cases, the isotherms are of type IV and exhibit hysteresis loops of H1 type according to the IUPAC classification, typical of (26) Schmidt, R.; Sto¨cker, M.; Hansen, E.; Akporiake, D.; Ellestad, O. H. Microporous Mater. 1995, 3, 443. (27) Broekhoff, J. C. P.; Boer, J. H. d. J. Catal. 1968, 10, 377. (28) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London: 1982; p 44.

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materials with pores of constant cross section (for instance, cylindrical or hexagonal). The pore-filling step in adsorption and desorption curves is sharp, corresponding to a narrow pore size distribution. SBA-15 and MCM-41 materials share these overall features but differ when their isotherms are quantitatively examined. The slope of the isotherm after the low-pressure adsorption step is much lower in the case of SBA-15, corresponding to a lower area of the mesopore surface, and the pore volume as measured at the top of the mesopore-filling step is also much lower for SBA-15 than for MCM-41. Both features strongly suggest that SBA-15 pores are separated by silica walls thicker than the walls of MCM-41. Pore-size analysis is traditionally based upon the application of Kelvin’s relation between vapor pressure of a capillary condensed phase and pore size. But Kelvin’s relation was developed for large capillaries and basically ignores physical adsorption of the adsorbate on the pore walls. This phenomenon is of importance for pores in the mesosize range and necessitates therefore additional correction terms to Kelvin’s relation. This fact was well considered by Broekhoff and de Boer in 196827 who developed a complete thermodynamical analysis of physical adsorption and capillary condensation phenomenon, named the BdB method. Using MCM-41 as reference materials, it has been shown that the BdB method was one of the most reliable methods to evaluate the mesopore diameter, while the widely used BJH method underestimates pore sizes by more than 20%.18 Recently, a new approach, named the nonlinear functional of density (NLFD),19 provides an accurate analysis of MCM-41 isotherms, and the results obtained for the determination of pore size distribution are very close to the pore diameters obtained by the BdB method. In this paper, the BdB method will be used for the pore size determination for the mesoporous MCM-41 and SBA-15 materials. In the case of MCM-41, larger pores are formed by the addition of a growing amount of swelling agent to the alkyltrimethylammonium micelles, as can be observed from the isotherms of Figure 1. The molar ratio TMB/ surfactant was 2.7 for the MCM-41 of Figure 1a, with a pore diameter of 50 Å, and 13 for the MCM-41 of Figure 1b, with a pore diameter of 115 Å. In the case of SBA-15, larger pores are formed at higher temperature of synthesis,3,4 as witnessed by the shift to higher relative pressure of the pore-filling step of the isotherms of Figure 1. The pore diameter, reported in Figure 2a as a function of the synthesis temperature, increases continuously with the temperature of synthesis from 55 Å (for syntheses between 35 and 60 °C) to 92 Å for a synthesis at 130 °C. The observed increase of pore diameter corresponds to a well-known property of nonionic surfactants: the increase of micelle size with temperature. A rise of temperature brings about a partial dehydration of the PEO units and decreases the volume of the hydrophilic corona (and so decreases the surface of the hydrophilic part of the micelle). The corresponding decrease of the surface/volume ratio of the micelle is the driving force for an increase of the aggregation number and the volume of each micelle, leading to an increase of pore size. XRD Results. In the case of MCM-41 materials, the increase of pore size corresponds to the increase of cell parameter. In the case of SBA-15, the XRD patterns are very well-defined and characteristic of ordered hexagonal materials. The cell parameters are reported in Figure 2b as a function of the temperature of synthesis. The cell parameters for all as-synthesized SBA-15 samples are about 110 Å, notwithstanding their very different pore sizes. After calcination, a 20% contraction is observed for

Galarneau et al.

Figure 2. Evolution as a function of SBA-15 synthesis temperature of (a) pore diameters determined by the Broekhoff and de Boer method (DBdB), (b) the cell parameter of assynthesized (aas-synthesized) and calcined (acalcined) SBA-15, and (c) the pore volume of calcined SBA-15 (Vp).

samples synthesized between 35 and 60 °C, giving cell parameters around 90 Å. For samples synthesized at temperatures higher than 80 °C, the cell parameters of calcined samples are nearly 105 Å, revealing a slighter 5% contraction, similar to what is observed for MCM-41 materials. In the case of the samples synthesized at lower temperature, the difference between cell parameter and pore size is higher than 35 Å, suggesting extreme thickness of the pore walls. Walls seem to become thinner at higher temperatures, as the difference between cell parameter and pore diameter decreases. Also, the pore volume (Vp) increases with the synthesis temperature, as reported in Figure 2c. For a pure mesoporous solid, an increase in pore volume together with a constant cell parameter would correspond to materials with thinner pore walls. For an ideal hexagonal honeycomb structure, a decrease of wall thickness is associated with an increase of surface area,29 which is not clearly observed for SBA-15 materials. SBET and CBET. The specific surface area as determined by the BET equation, SBET, is reported in Figure 3a. Surface areas of about 700 m2/g are measured for SBA-15 synthesized at a temperature between 35 and 60 °C. An increase to 850 m2/g is indeed observed for a synthesis temperature of 80 °C, but for a higher synthesis temperature, the surface area remains constant and finally decreases to 550-600 m2/g for samples synthesized at 120-130 °C. Moreover, when the isotherms of SBA-15 and MCM-41 of equivalent pore size and similar SBET are compared (Figure 1a), it is obvious that these two materials feature very different profiles in the adsorption region below p/p0 ) 0.3, where the nitrogen multilayer adsorption takes place (domain of validity of the BET equation). All this unexpected trend of the surface area requires a careful examination of the measurement technique. (29) DiRenzo, F.; Desplantier, D.; Galarneau, A.; Fajula, F. Catal. Today 2001, 66, 73.

Microporosity and Surface Area of Silicas

Figure 3. Evolution as a function of SBA-15 synthesis temperature of (a) the BET surface area and (b) the CBET parameter calculated from the nitrogen adsorption at 77 K of calcined SBA-15, in the relative pressure range 0.15 < p/p0 < 0.26.

The CBET parameters from the BET equation are reported in Figure 3b. Negative values are observed for samples synthesized at low temperature. The value of CBET strongly decreases for a synthesis temperature of 90 °C (CBET ) -480), sharply shifts to an unusual high positive value for a synthesis temperature of 100 °C (CBET ) 350), and slowly approaches the usual values for calcined silica or MCM-41 for samples synthesized at higher temperature (CBET ∼ 80-110). With the oxides most often used as mesoporous or macroporous adsorbents (e.g., silica and alumina), CBET values for nitrogen adsorption at 77 K are in the range 80-150.30 CBET parameters are characteristic of the adsorbent-adsorbate interactions. A high CBET value evidences strong interaction of nitrogen molecules with the hydroxylated silica surface. CBET is expressed as follows:28

CBET ) K exp[(E1 - Eliq)/RT] and

K ) a1n2/a2n1 where E1 is the enthalpy of nitrogen adsorption in the first adsorbed layer on the solid surface and Eliq is the enthalpy of adsorption in the second layer, assumed to be equal to the enthalpy of nitrogen liquefaction. The coefficent K is composed of ai, a condensation coefficient (fraction of incident molecules which actually condense), and ni, the frequency of oscillation of adsorbed molecules in the direction normal to the surface, indexed i ) 1 or 2 for the first and second adsorbed layer, respectively. Neither the exponential nor the pre-exponential terms K of the equation can assume negative values; hence, a (30) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by powders & porous solids: principles, methodology and applications; Academic Press: San Diego, CA, 1999.

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negative CBET value has no physical meaning and indicates that the system is outside the field of application of the BET equation. The assumptions of the BET model limit its scope to a monolayer-multilayer adsorption and prevent it from being meaningfully applied to microporous solids, like zeolites for instance, where the pore sizes are of the same order of magnitude as the size of nitrogen molecules and prevent the formation of a monolayer-multilayer process. To calculate the surface area of a porous material, the pore size should be at least 6 times the size of the adsorbate molecules. The presence of micropores in SBA-15 synthesized at low temperature15,22-25 could account for the negative values measured for the CBET parameter. The CBET parameter is measured from the linear fit, in the range 0.15 < p/p0 < 0.26, of the BET curve (p/p0)/(V[1 (p/p0)]) versus (p/p0). CBET ) (S/I) + 1, where S is the slope of the BET linear fit and I is the intercept with the y-axis. When micropores are present, an important adsorption takes place at very low pressure. As a consequence, the adsorbed amount V at the pressure of measurement of the BET equation is higher than the amount which would correspond to the monolayer-multilayer adsorption. A higher V value shifts the BET Y terms toward lower values, and I, the intercept to the y-axis, can assume negative values. At increasing synthesis temperature, a decrease of the microporous volume is observed23 which will shift upward the BET linear fit toward more normal values, and negative I will increase (its absolute value will decrease). When negative I will tend to zero, CBET, proportional to I-1, will pass through a discontinuity from infinite negative value (for negative I) to infinite positive value (for positive I). When I is positive, a further increase allows CBET to decrease toward the values typical of a nonmicroporous calcined silica (CBET ∼ 100), accounting for the trend observed in Figure 3b. Nitrogen t-Plot Analysis. Microporosity can be assessed by tracing comparison plots with reference isotherms, usually called t-plots. The nonporous solids chosen for reference should have a similar nature of surface as the studied material. For MCM-41, a good nonporous reference is the pyrogenic silica Aerosil 200. The thickness of the adsorbed nitrogen layer, t, is equal to t/Å ) 3.54(Va/Vm) where Va is the volume adsorbed of the nonporous surface at p/p0 and Vm is the volume of the monolayer. The t-plot analysis consists of plotting the volume of the porous materials, V, as a function of the previously calculated thickness of the monolayer t (for the same p/p0). As long as the multilayer of adsorbate is formed unhindered on the solid surface, V is a straight line passing through the origin. The total surface area is then equal to the slope of the t-plot (divided by 646, the ratio between the liquid and gas densities of nitrogen). At higher relative pressures (higher t values), deviations from the straight line may occur. An upward deviation indicates the presence of a capillary condensation in mesopores. A downward deviation is observed when micropores or slitshaped pores are present in the solid. For MCM-41 materials, an upward deviation is observed for MCM-41 with a pore size higher than 24 Å and a downward deviation is observed for MCM-41 of 19 Å pore size (Figure 4). The t-plot analysis allows one to point out the limit between mesoporous and microporous materials. t-Plot curves of SBA-15 materials are very different depending on the synthesis temperature, as reported in Figure 5. At low temperatures, between 35 and 90 °C, t-curves do not pass through the origin showing distinct microporosity (Figure 5a), with a micropore volume classically deter-

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Figure 4. Nitrogen t-plot curves of MCM-41 with 19 and 24 Å pore diameter showing the presence of a downward deviation for MCM-41 (19 Å) characteristic of the presence of micropores and an upward deviation for MCM-41 (24 Å) characteristic of the presence of mesopores.

Figure 6. Schematic representation of a hexagonal cell showing the cell parameter (a), the wall thickness (t), and the cylindrical pore of DBdB diameter which possesses the same section as the corresponding hexagonal pore.

adsorbed at each pressure level is unlikely to be the same on such a heterogeneous surface and the homogeneous silica surface used as reference, decreasing the significance of the t-plot analysis. Discussion

Figure 5. (a) Nitrogen t-plot curve of SBA-15 synthesized at 60 °C showing the presence of micropores. The volumes determined by the t-plot are the total pore volume (Vp), the microporous volume (Vµ-t), and the volume corresponding to the downward deviation of the adsorption (Vµbreak). (b) Nitrogen t-plot of SBA-15 at 100 °C.

mined at the intercept with the y axis, Vµ-t. The t-plot shows a downward deviation at t ) 4.2 Å which corresponds to a volume Vµbreak. The following upward deviation is characteristic of the mesopore filling. For synthesis temperatures of 100 °C and beyond, the t-plot passes through the origin (Figure 5b) and no clear downward deviation is observed. This means that the t-plot does not allow observation of any micropores for samples synthesized at 100 or 110 °C, for which CBET values strongly suggest some micropores still may be present. This discrepancy can be probably accounted for by a basic assumption of the t-plot analysis, which is only meaningful when the surface of the porous material analyzed has the same nature as the surface of the nonporous material used to determine the reference isotherm. In the present case, the surface of the mesopores of SBA-15 is composed of silica and micropore mouths. The amount of nitrogen

A quantitative assessment of the porosity of SBA-15 can be attempted on the basis of the only available parameters which seem to be independent of any assumption on the nature of the surface: the cell parameter (a), the total pore volume (Vp), and the mesopore diameter evaluated by the pressure of the mesopore-filling step of the isotherm (DBdB). It can be assumed that the presence of microporosity does not modify the pressure of the mesopore-filling step; hence, the mesopore diameter evaluated by the BdB method has been assumed to be valid also in the case of SBA-15. In the case of MCM-41, the geometrical model of a honeycomb structure formed by hexagonal pores accounts for all the porosity parameters.18 The very well defined X-ray diffraction pattern of SBA-15 clearly indicates an ordered hexagonal array of mesopores; hence, it seems reasonable to apply the MCM-41 model to these solids, once the modifications needed to take into account the presence of a microporosity inside the walls are provided. From the geometrical model schematically depicted in Figure 6, the wall thickness (t) can be evaluated as

t ) a - 0.95DBdB

(1)

The present model has been developed for MCM-41 materials,18 in which the section of the pores has been shown to be hexagonal. If the pore were cylindrical, the wall thickness would not be constant and the value determined here would be an average value. The pore diameter calculation resulting from the model developed for MCM-41 is expressed as a function of the mesopore fraction mes and has been shown to be equal to the pore diameter determined by the BdB method;18 the pore diameter for any silica honeycomb structure (MCM-41 or

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SBA-15) then followed the relationship

DBdB ) 1.05ames1/2 The mesopore fraction is mes ) Vmes/VTotal ) Vmes/(Vp + 1/FSi), where Vp, determined by nitrogen adsorption at the end of the mesopore-filling step, is the total pore volume. For MCM-41, the total pore volume is directly the mesoporous pore volume: Vp ) Vmes. For SBA-15, the total pore volume Vp is the sum of the microporous and mesoporous pore volumes: Vp ) Vmes + Vµ. The density of the silica walls FSi is equal to the density of amorphous silica (2.2 g cm-3), shown to be a valid density value for the walls of MCM-4120 and assumed here as a proper value for the solid volume between micropores in the SBA-15 walls. Hence, the mesopore fraction can be expressed as mes ) (DBdB/1.05a)2, and the mesoporous volume Vmes and the microporous volume Vµ for SBA-15 can be calculated by the following equations:

Vmes ) (DBdB/1.05a)2(Vp + 1/FSi)

(2)

Vµ ) Vp - Vmes

(3)

and

In this model of honeycomb structure for SBA-15, the average density of the walls can be evaluated as well as the true surface area of the mesopores of SBA-15. The average density Fw of the walls between mesopores of SBA-15 is the result of the contributions of micropore volume and silica volume:

1/Fw ) Vµ + 1/FSi

(4)

The surface area of the mesopores can be determined29 as

Smes ) 4 × 104/Fwt[(1 - t/a)/(2 - t/a)]

(5)

with t and a expressed in Å. In Figure 7a, the surface area of the mesopores (Smes) calculated by eq 5 is compared to the experimental surface area SBET*. SBET* is the BET surface area calculated by assuming that the N2 molecule covers an average surface of 13.5 Å2, a value shown to be more correct than the usual value 16.2 Å2, in the case of adsorption on the surface of calcined silica.18,31 SBET* is 1.2 times lower than SBET. The true mesoporous surface area Smes is much lower than the BET surface area SBET* (300 instead of 600 m2/g) for SBA-15 synthesized between 35 and 110 °C, and for SBA-15 synthesized at 120 and 130 °C the calculated mesopore surface area is in good agreement with the BET surface area (∼500 m2/g). This confirms that the porosity of SBA-15 synthesized at high temperature corresponds to an array of constant-diameter mesopores, with no contribution from microporosity. In Figure 7b, the mesopore volume Vmes calculated by eq 2 is compared to the measured pore volume Vp. The two values evolve in a similar way for synthesis temperatures lower than 100 °C and converge at the highest synthesis temperature. The difference between Vp and Vmes is the calculated micropore volume Vµ (eq 3) reported in Figure 7c. Vµ-t is the intercept with the y-axis of the t-plot as shown in Figure 5a. The use of Vµ-t as an indicator of micropore volume is based on the assumption that the filling of the mesopores does not modify to a significant extent the outer surface of the solid. Vµ-t usually provides (31) Jelinek, L.; Kova`ts, E. s. Langmuir 1994, 10, 4225.

Figure 7. Evolution as a function of SBA-15 synthesis temperature of (a) SBET* (BET surface area determined using a value of 13.5 Å, for the molecular area of adsorbed nitrogen) and Smes determined by eq 5; (b) total pore volume (Vp) and mesoporous pore volume (Vmes) determined by eq 2; (c) microporous volumes (Vµ) calculated by eq 3, (Vµ-t) evaluated by t-plot at the intercept with the y-axis, and (Vµbreak) evaluated by t-plot at the break of the slope (downward deviation) in the low pressure domain.

a good evaluation of the micropore volume when an important nonmicroporous area is present in the sample, as in the case of mixtures of microporous zeolites and mesoporous matrixes. The other evaluation of microporous volume in Figure 7c is Vµbreak, corresponding to the adsorbed amount at the end of the portion of the isotherm with a microporous contribution, measured at the corresponding break of the slope in the t-plot (downward deviation) as shown in Figure 5a. Vµbreak can be considered as an indicator of microporous volume if the adsorption on the mesoporous surfaces can be neglected at low pressure at which the micropores are filled. A surface with a continuous distribution of supermicropores-small mesopores (pore diameter between 10 and 30 Å as supposed for SBA-1515) is a new kind of surface, and it could present adsorption properties significantly different from the outer surface of ultramicroporous solids (pores smaller than 10 Å) like zeolites. For SBA-15 synthesized at temperatures lower than 90 °C, the value Vµ-t is nearly 0.12 mL/g, whereas the value of Vµbreak is slightly higher than 0.3 mL/g. The latter value is very similar to the calculated micropore volume Vµ and could indicate that most of the mesopore surface is occupied by micropore mouths and is not available for monolayer adsorption at low pressure. Nevertheless, the validity of these t-plot results is questioned by another observation: both Vµ-t and Vµbreak suddenly fall to zero for materials synthesized at 100 °C or higher temperatures. This behavior is different from the proportional decrease of the calculated micropore volume Vµ with temperature and suggests that the adsorption of N2 on pyrogenic silica Aerosil 200 (nonporous reference material used for t-plot analysis) does not represent the proper reference for the

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Figure 8. Evolution as a function of SBA-15 synthesis temperature of (a) wall thickness (t) determined by eq 1 and pore diameters determined by the BdB method of calcined SBA15; (b) wall density determined by eq 4.

adsorption on the mesopore surface of SBA-15. The adsorption on a heterogeneous surface largely occupied by the mouths of nitrogen-filled micropores is probably less energetic than the adsorption on a homogeneous silica surface. In this case, the use of Aerosil as the reference isotherm in t-plot analysis brings about an overevaluation of the thickness of the nitrogen adsorbed layer at each pressure level. The thickness of the walls between mesopores, as calculated by eq 1, is reported in Figure 8a. For the solids prepared at low temperature, the wall thickness is nearly 40 Å. It steadily decreases with temperature beyond 80 °C. This trend parallels the disappearing of microporosity (Vµ in Figure 7c). For the solid synthesized at 130 °C, which presents almost no microporosity, the wall thickness is nearly 20 Å. Such wall thicknesses have been observed for MCM-41 materials prepared in alkaline32 or neutral conditions,20 but they cannot be obtained in the presence of alkylammonium surfactants for solids prepared in acid conditions such as SBA-3.33 The average density of the walls, calculated by eq 4, is reported in Figure 8b and is effectively lower than the one of amorphous silica as suggested previously34 due to the presence of micropores. The density of the SBA-15 walls increases with the temperature of synthesis from 1.3 g cm-3 toward the density of amorphous silica (2.2 g cm-3) as the microporosity disappears. The presence of micropores probably accounts for the corona of lower density around mesopores found by Impe´ror-Clerc et al.23 The microporosity seems to be an important property of SBA-15 silicas. As evidenced by Ryoo and co-work(32) Coustel, N.; Renzo, F. D.; Fajula, F. J. Chem. Soc., Chem. Commun. 1994, 967. (33) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (34) Lukens, W. W.; Schmidt-Winkel, P.; Zhao, D. Y.; Feng, J. L.; Stucky, G. D. Langmuir 1999, 15, 5403.

Galarneau et al.

Figure 9. (Left) Schematic representation of SBA-15 structure where mesopores are connected through micropores. The amount of micropores decreases with the increase of synthesis temperature, to end up with no micropores for synthesis close to 130 °C. (Right) The micropores are formed due to interactions of PEO chains of different micelles. When the synthesis temperature increases, the PEO chains are dehydrated, the pore sizes increase, and progressively PEO chains no longer interact with the other PEO chains of the other micelles.

ers,15,24,25 the micropores represent connections between mesopores, as schematically represented in Figure 9. The microporous network is disordered: no XRD reflections at an appropriate angle are detected in the well-defined diffraction pattern of SBA-15. The microporous connections might be explained by the occurrence of some interactions between micelles of Pluronic surfactant through ethylene oxide headgroups. Interactions between micelles of nonionic surfactants are a common occurrence, having been postulated already in 1958 to explain the dependence on shear rate of the viscosity of poly(ethylene oxide) solutions.35 Probably, the most widely studied interaction between nonionic micelles is the formation of the dense micellar solution which originates the phenomenon of cloud point. In this phenomenon, the partial dehydration of PEO chains at higher temperature makes a sharing of the hydration spheres of different micelles easier, with an increased contact between micelles also if no ordered mesophase is formed.36 In the case of the formation of silica-copolymer mesophases, the use of nonionic surfactants with hydrophilic chains with different lengths leads to different structures: a silica lamellar phase with EO5PO70EO5, a silica hexagonal phase with EO20PO70EO20, and a silica cubic phase with EO106PO70EO106.4 The Pluronic P123, EO20PO70EO20, used in SBA-15 synthesis, has hydrophilic PEO chains much shorter than its hydrophobic portion. Short PEO chains decrease the repulsion between micelles of PEO-PPO-PEO and render more difficult the formation of distinct objects such as globular micelles formed by surfactants with longer EO moieties, like EO106PO70EO106 used in the synthesis of SBA-16.4 This effect of the length of the PEO chains on the interaction between micelles in the formation of different silica structures closely parallels (35) Bailey, F. E.; Powell, G. M.; Smith, K. L. Ind. Eng. Chem. 1958, 50, 8. (36) Magid, L. J. Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 677.

Microporosity and Surface Area of Silicas

the observations made for the cloud point of the surfactant-water systems. For all nonionic surfactants, shorter hydrophilic chains correspond to a lower cloud point, an indicator of easier interactions between micelles.37 The formation of a network between micelles through interactions between EO headgroups is affected by the chain length of the surfactant and is responsible for the formation of the different silica structures. The microporosity in SBA-15 results from interactions between PEO chains of micelles sharing their hydration sphere. When the synthesis temperature is increased, the PEO is partially dehydrated, with several effects on the mesophase properties. On one hand, the volume of the outer hydrophilic layer decreases and the micelle diameter increases. On the other hand, the partial dehydration of the PEO moiety decreases the interactions between micelles through PEO chains (PEO chains then get closer to the PPO part of the same micelle) (Figure 9), with a consequent decrease of microporosity. The interactions between micelles are directly satisfied by the sharing of the hydration sphere of the micelles (as for cloud point). A preorganization step at low temperature is needed to form ordered mesophases; disordered systems are obtained when the synthesis begins at a temperature higher than the cloud point (85 °C, in the case of a 6% solution of Pluronic P123, the concentration used in our syntheses), in conditions in which micelles are aggregated also in the absence of silica. Conclusion Differences and correspondences between SBA-15 and MCM-41 have been hot topics of debate in recent literature. Is SBA-15 the same material as MCM-41, differing only in the method of synthesis? In most cases, the answer to this question seems to be definitely negative: the properties of nonionic surfactants used in the synthesis of SBA15 are reflected in several properties of the final solid. Intermicellar interactions between hydrophilic heads of nonionic surfactants are at the origin of micropores which connect the mesopores. The presence of supermicropores (37) Clint, J. H. Surfactant Aggregation; Blackie: Glasgow, 1992; p 155.

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affects the adsorption properties of the mesopore surface in such a way that the adsorption isotherm on pyrogenic silica, a proper reference isotherm for MCM-41, is not a correct reference for SBA-15 synthesized at most temperature levels. The properties of the solids formed are extremely dependent on the synthesis temperature, in good agreement with the sensitivity of nonionic micelles to their hydration state. All the solids formed at a temperature lower than 60 °C are alike. The large microporosity observed in these solids is due to the interactions between PEO chains of micelles sharing their hydration spheres. For higher synthesis temperature, partial dehydration of PEO heads decreases the hydrophilic volume (and hence the surface) of the micelles and so their surface/volume ratio. The size of the micelles increases, with a corresponding increase of pore size. The partial dehydration of PEO chains reduces the interactions between the outer layers of different micelles (PEO chains become closer to the PPO fraction), reducing at the same time the resulting microporosity of the solids. The simultaneous presence of interconnected micropores and mesopores renders most usual methods of texture evaluation unreliable. For the solids prepared at temperatures lower than 90 °C, a large volume of micropores visible by t-plot analysis renders the surface area measurement by BET equations unreliable. The true surface areas are much lower than the literature claims (around 300 instead of 750-850 m2/g) due to the presence of micropores. SBA-15 synthesized between 100 and 110 °C possesses micropores which are not revealed by nitrogen t-plot analysis, although their micropore volume represents 20-30% of the total pore volume, due to a lack of an appropriate reference isotherm. SBA-15 without micropores could be synthesized for temperatures as high as 130 °C. The pore size of SBA-15 synthesized at these temperatures is then ∼90 Å, and their wall thickness is near 20 Å. Such wall thickness is higher than the one obtained in the synthesis of large-pore MCM-41 by swelled trimethylammonium micelles and accounts for their higher stability. LA0105477