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Supercritical Fluid Extraction of a Nonionic Surfactant Template from SBA-15 Materials and Consequences on the Porous Structure Rafael van Grieken,*,† Guillermo Calleja,† Galen D. Stucky,‡ Juan A. Melero,† Rafael A. Garcı´a,† and Jose Iglesias† Environmental and Chemical Engineering Group, Rey Juan Carlos University, 28933 Mo´ stoles, Madrid, Spain; and Department of Chemistry and Materials, University of California, Santa Barbara, California 93106 Received December 9, 2002. In Final Form: February 4, 2003 SBA-15 mesoporous materials were synthesized using the method reported by Zhao et al. Surfactant was removed from as-made materials by means of different techniques: thermal treatment under air atmosphere, solvent washing at different temperatures, and supercritical CO2 extraction in the presence and absence of cosolvents. The structure of resulting materials was characterized using conventional techniques: nitrogen and argon adsorption measurements, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR), and 29Si MAS NMR. The efficiency of surfactant removal using CO2 under supercritical conditions is similar to that obtained by means of solvent washing under reflux but shows some improvements in the presence of cosolvents. Mesoscopic properties of mildtemperature solvent extracted SBA-15 materials depend on the efficiency of the surfactant removal and the use of supercritical CO2 as solvent. Likewise, the size and volume of the complementary microporosity detected in the treated materials is closely related to the strategy of removal of hydrophilic poly(ethyleneoxide) chains of the triblock copolymer template occluded within the siliceous walls of the SBA-15 mesophase during the synthesis.
Introduction Since poly(alkyleneoxide) triblock copolymers have been used as templates for the synthesis of ordered mesoporous silica,1,2 intensive research has been undertaken on the synthesis and potential application of these new materials.3 Polymer-templated ordered silicas have been synthesized in many forms3-7 as fibers, continuous films, rods, membranes, monoliths, spheres, and so forth. Likewise, the use of triblock copolymer species in nonaqueous media has allowed the preparation of mesoporous transitionmetal oxides and mixed oxides.8,9 Hexagonally ordered SBA-15 silica1,2 has already been applied in different fields of catalysis,10-15 separations,16 and advanced optical * To whom correspondence should be addressed. Phone: 34-914887007. Fax: 34-91-6647490. E-mail:
[email protected]. † Rey Juan Carlos University. ‡ University of California, Santa Barbara. (1) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036. (2) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (3) Yang, P.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033-2036. (4) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1998, 10, 1380-1385. (5) Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 1174-1178. (6) Huo, Q.; Feng, J.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14-17. (7) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275-279. (8) Yang, P.; Zhao, D.; Margolese, D.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152-155. (9) Yang, P.; Zhao, D.; Margolese, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813-2826. (10) Luan, Z.; Maes, E. M.; van der Heide, P. A.; Zhao, D.; Czernuszewicz, R. S.; Kevan, L. Chem. Mater. 1999, 11, 3680-3686. (11) Calleja, G.; van Grieken, R.; Garcia, R.; Melero, J. A.; Iglesias, J. J. Mol. Catal. A: Chem. 2002, 182-183, 215-225. (12) Luan, Z.; Hartmann, M.; Zhao, D.; Zhou, W.; Kevan, L. Chem. Mater. 1999, 11, 1621-1627.
materials17 due to its outstanding thermal stability, pore size adjustment, and tailored particle morphology. In particular, SBA-15 has been functionalized by incorporating different heteroatoms such as Ti10,11 and Al,12,13 as well as by chemical bonding of organosilanes through sylilation and direct synthesis procedures.14-16 These functionalized materials exhibited potential catalytic applications in olefin epoxidation11 and acid-catalyzed processes.15 Removal of templating surfactant molecules occluded within mesoporous materials after the synthesis step is often carried out using thermal treatments, commonly under air atmosphere at temperatures between 773 and 873 K. An alternative method for surfactant removal is through low-temperature solvent extraction under reflux (MCM-4118,19 and SBA-152,14). Solvent extraction of the as-synthesized SBA-15 allows the copolymer surfactant to be removed without decomposition, permitting its recovery and reuse,1,2 but a huge amount of liquid solvent is necessary, which makes eventual solvent disposal a rather challenging step. In contrast, the high-temperature calcination procedure removes completely the surfactant species but does not allow their recovery and leads to a significant structure shrinkage with emission of a sig(13) Yue, Y.; Gedeon, A.; Bonardet, J. L.; Melosh, N.; D’Espinose, J. B.; Fraissard, J. Chem. Commun. 1999, 1967-1968. (14) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448-2459. (15) Melero, J. A.; Stucky, G. D.; van Grieken, R.; Morales, G. J. Mater. Chem. 2002, 12, 1664-1670. (16) Han, Y. J.; Stucky, G. D.; Butler, A. J. Am. Chem. Soc. 1999, 121, 9897-9898. (17) Yang, P.; Wirnsberger, G.; Huang, H. C.; Cordero, S. R.; McGehee, M. D.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Burato, S. K.; Stucky, G. D. Science 2000, 287, 465-467. (18) Chen, C. Y.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (19) Moller, K.; Bein, T.; Fischer, R. X. Chem. Mater. 1999, 11, 665673.
10.1021/la026970c CCC: $25.00 © 2003 American Chemical Society Published on Web 03/11/2003
Nonionic Surfactant Template from SBA-15 Materials
nificant amount of noxious gases. Moreover, these thermal treatments must be substituted for organically modified mesoporous materials to retain the chemical integrity of the organic groups. Oxidative ozone treatment20 and supercritical fluid extraction (SFE) with CO2 in the presence of cosolvents have been successfully used to eliminate organic species from MCM-41 materials synthesized in basic medium21 and from HMS materials synthesized by a neutral assembly pathway.22 Recently, Bozhi et al. have used microwave digestion to remove an organic template from siliceous SBA-15 materials.23 These works prompted us to study the removal of nonionic surfactants from as-made silica-based SBA-15 materials using SFE with CO2. The present contribution describes this novel strategy under a wide range of different experimental conditions: different pressures and temperatures and the presence of additional cosolvents. The procedure is attractive, as it requires relatively low temperatures, is environmentally convenient, and allows an easy surfactant recovery from the reaction medium. Under specific conditions, this technique showed better removal efficiency than the conventional solvent washing under reflux.2,14 Likewise, as will be shown in this work, the extraction methodology has a dramatic influence on the porous structure of the treated materials as well as on the reactivity of the silica surface toward silylation processes. Recently, several works24-27 have demonstrated that the structure of SBA-15 consists not only of large uniform and ordered channels but also of complementary micropores located on the silica wall and providing connectivity between them. Moreover, these studies evidenced that hydrophilic poly(ethylenoxide) chains of triblock copolymers penetrate within the silica walls during the synthesis, inducing the formation of micropores upon calcination. This microporosity has been shown to be controlled via the synthesis conditions (TEOS/surfactant molar ratio, aging temperature, and the presence of salt during the synthesis)28-30 as well as the thermal treatments.25 Several empirical methods such as t-plot,31 Rsplot,32 or β-plot,33 based on suitable nonporous references, have been used to estimate micropore volume within the walls of the mesostructured materials. We have also applied this methodology to calculate the micropore volume within the silica walls of SBA-15 materials after surfactant removal. Likewise, a step forward has been addressed in this work, performing argon isotherms at reliable low-pressure with the purpose of estimating directly the micropore volume. This contribution also includes experimental evidence on the linking between the microporosity extent and the (20) Keene, M. T. J.; Denoyel, R.; Llewellyn, P. L. Chem. Commun. 1998, 2203-2204. (21) Kawi, S.; Lai, M. W. Chem. Commun. 1998, 1407-1408. (22) Kawi, S.; Goh, A. H. Stud. Surf. Sci. Catal. 2000, 129, 131-138. (23) Tian, B.; Liu, X.; Yu, C.; Gao, F.; Luo, Q.; Xie, S.; Tu, B.; Zhao, D. Chem. Commun. 2002, 1186-1187. (24) Kruk, M.; Jaroniec, M. Chem. Mater. 2000, 12, 1961-1968. (25) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuk, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 11465-11471. (26) Impe´ror-Clerc, M.; Davidson, P.; Davidson, A. J. Am. Chem. Soc. 2000, 122, 11925-11933. (27) Go¨ltner, C. G.; Smarsly, B.; Berton, B.; Antonietti, M. Chem. Mater. 2001, 13, 1617-1624. (28) Miyazawa, K.; Inagaki, S. Chem. Commun. 2000, 2121-2122. (29) Galarneau, A.; Cambon, H.; Di Renzo, F.; Fajula, F. Langmuir 2001, 17, 8328-8335. (30) Bharat, L. N.; Komarneni, S. Chem. Mater. 2001, 13, 45734579. (31) Lippens, B. C.; De Boer, J. H. J. Catal. 1965, 4, 319. (32) Sing, K. S. W. Chem. Ind. 1968, 1520. (33) Lukens, W. W.; Schmidt-Winkel, P.; Zhao, D.; Feng, J.; Stucky, G. D. Langmuir 1999, 15, 5403-5409.
Langmuir, Vol. 19, No. 9, 2003 3967 Table 1. Experimental Conditions of Extraction Procedures and Extraction Efficiency
sample
extraction procedure
S-0 S-1 S-2 S-3 S-4 S-5 S-6 S-7
as-made calcination in air ethanol washing ethanol washing SFE with CO2b SFE with CO2b SFE with CO2b SFE with CO2/ethanolb
a
experimental conditions removal T (°C) P (atm) t (h) efficiency 550 RTa reflux 60 90 110 90
1 1 1 125 140 210 130
7 24 24 24 24 24 24
0.0 100.0 68.0 74.0 74.0 79.0 76.0 81.0
Room temperature. b Autogenous pressure.
efficiency of the extraction treatment. Finally, to the best of our knowledge, the work reported here includes for the first time the use of supercritical CO2 for the removal of nonionic block copolymers from mesostructured SBA-15 silica. Experimental Section Materials. As-made SBA-15 materials were synthesized following the procedure reported by Zhao et al.1 using Pluronic 123 triblock copolymer (EO20-PO70-EO20; Aldrich) as a polymeric template. In a typical synthesis: 4 g of Pluronic 123 was dissolved under stirring in 125 g of 1.9 M HCl at room temperature. The solution was heated to 40 °C before adding tetraethyl orthosilicate (TEOS; Aldrich). The molar composition of the mixture for 4 g of copolymer was 0.041TEOS/0.24HCl/6.67H2O. The resultant solution was stirred for 20 h at 40 °C, after which the mixture was aged at 100 °C for 24 h under static conditions. The solid product was recovered by filtration and air-dried at room temperature overnight. Extraction Procedure. The polymeric template was removed from as-made SBA-15 materials through different techniques. Table 1 summarizes the different extraction procedures as well as the experimental conditions. (i) Thermal Treatment. The as-synthesized SBA-15 material was calcined for 7 h in air atmosphere at 550 °C. (ii) Ethanol Washing. Following the method reported by Zhao el al.,1 the block copolymer was removed from as-made materials by washing with ethanol under reflux in a magnetically stirred flask for 24 h (typically 1.5 g of as-synthesized material per 400 mL of ethanol). Additionally, an as-synthesized SBA-15 material was washed with ethanol at room temperature for 24 h. After washing, the resultant materials were filtered off, washed with 100 mL of ethanol to remove the remaining surfactant located on the material surface, and air-dried overnight at room temperature. (iii) Supercritical Fluid Extraction (SFE) with CO2. Different experiments were carried out in a Teflon-lined batch reactor stirred at 500 rpm during 24 h. Typically 1 g of as-made SBA-15 material was treated with 60 g of CO2 under supercritical conditions. The supercritical extractions were performed under autogenous pressure using a range of temperature between 60 and 110 °C. In additional experiments, CO2 was modified with ethanol (60 g of CO2 and 5 g of alcohol per 1 g of as-made material). After supercritical extraction the materials were washed and dried as described above. Functionalization of Mesostructured Materials after Surfactant Removal. Titanium supported on SBA-15 mesoporous silica has been synthesized by chemical grafting over the different extracted mesostructured materials using titanocene dichloride as precursor.11 In a typical experiment, materials, prior to the grafting, were outgassed under vacuum conditions overnight. The sample was then stirred in a solution of (Cp2)TiCl2 (Acros) in 90 g of chloroform under a dry nitrogen atmosphere for 1 h. Thereafter, anhydrous triethylamine was added to the suspension [NEt3/(Cp2)TiCl2 molar ratio of 1] to promote surface silanols activation. The resultant solution was kept under stirring for an additional 2 h. Finally, the solids were recovered by filtration and intensively washed with chloroform.
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The mass ratio of (Cp2)TiCl2 to raw material in the organic solution was of 0.1. Samples Characterization. X-ray powder diffraction (XRD) data were acquired on a PHILIPS diffractometer using Cu KR radiation. Typically, the data were collected from 0.6 to 4° (2θ) with a resolution of 0.02°. Nitrogen adsorption-desorption isotherms at 77 K were determined using an adsorption porosimeter (Micromeritics, TRISTAR 3000). Samples were outgassed for 6 h in the degas unit of the adsorption apparatus at 453 K under nitrogen flow prior to the adsorption test. Argon adsorption isotherms at low relative pressure were determined at 77.5 K using a volumetric adsorption apparatus equipped with a high vacuum turbomolecular pump for the determination of the pore size distribution in the microporous range (Micromeritics, ASAP 2010). Prior to the adsorption test, samples were outgassed for 24 h in the degas port of the adsorption apparatus at 393 K up to a vacuum of 0.0002 mmHg (TGA analysis of outgassed materials did not show evidence of loss of surfactant). TGA measurements were carried out under flowing air on a SETARAM high-resolution thermogravimetric analyzer (detection limit 0.04 µg). Fourier transform infrared (FT-IR) spectra were recorded by means of a MATTSON spectrophotometer using the KBr wafer technique. For each sample, 128 IR spectra were added to achieve acceptable signal-to-noise levels. Solid-state 29Si NMR experiments were performed on a VARIAN spectrometer model VXR300 operating at a frequency of 59.70 MHz with the following conditions: magic-angle spinning at 5 kHz; π/2 pulse, 7 µs; a repetition delay of 600 s; and 200 scans. 29Si MAS NMR spectra were referenced to tetramethylsilane. The titanium content of functionalized materials was determined by ICP-atomic emission spectroscopy. Calculation Methods. The BET surface area was evaluated using adsorption data in a relative pressure range from 0.05 to 0.15. The total pore volume was estimated from the amount adsorbed at a relative pressure of about 0.985 (Vt). The empirical methods described in the next section need a non-microporous sample, with the same chemical composition and surface character as those of the sample tested. In this work we have used as reference a SBA-15 material washed with ethanol at room temperature for 24 h (S-2 sample; 100 mL per g of asmade material). The t-plot method applied to this material using a conventional quartz sample as reference (not shown) demonstrated a complete absence of microporosity. (i) Measurements Using the t-Plot Method. This empirical method31 is based on the comparison of the obtained adsorption data for a specific sample with the adsorption of a nonporous sample. In the t-plot method, adsorption volume for a sample under study is plotted versus the adsorbed layer thickness (t) calculated for the reference material at the same relative pressure. Obviously, the points should lay on a straight line and in the absence of micropores this line should pass through the origin of the coordinates. On the contrary, if micropores are present, the straight line extrapolates to a nonzero intercept of the ordinate, which gives the volume of micropores (Vµp). The t values, at very low partial pressure, for the reference sample (S-2 sample; SBA-15 washed with ethanol at RT) as a function of P/P0 data, have been correlated using the Harkins-Jura equation, resulting in the following empirical equation: t/Å ) [6.1955/(0.1513 - log{P/P0})]0.705. This equation has been used for all the range of partial pressures, simulating a hypothetical nonporous material. The primary mesopore volume (Vmp), primary mesopore area (Smp), and the total micropore volume (Vµp) were evaluated using the t-method in the t range from 3.5 to 6.5 Å for microporosity and from 9.5 to 12.0 Å for structured mesoporosity. Figure 1 illustrates the typical t-plot for the adsorption of nitrogen by a mesoporous sample (S-1 sample; calcined SBA-15). This plot shows four different regions: (1) a low-pressure curved adsorption region due to capillary condensation on micropores; (2) a linear region due to multilayer adsorption on mesopores; (3) a step region due to a capillary adsorbate condensation in primary mesopores; (4) a linear region due to multilayer adsorption in macropores and on the external surface. The linear regressions applied to the aforementioned linear regions combined with the BET surface areas of the reference and tested materials allow the calculation of primary mesopore area (Smp) and volume (Vmp).33
van Grieken et al.
Figure 1. t-plot in the ranges of micro- and mesopores for a SBA-15 material calcined in air at 823 K. The mesopore size distribution was calculated on the basis of adsorption branches of nitrogen isotherms using the BJH34 and BdB35 models, applying the Harkins-Jura multilayer thickness equation fitted at very low partial pressure for a suitable nonmicroporous reference material (S-2 sample; ethanol washed SBA-15 sample). Mesopore size has also been evaluated using geometric calculations taking into account the d100 spacing obtained in XRD analysis for a hexagonal array of uniform pore structure36 and the relationship between pore volume (Vmp) and specific surface (Smp) area for cylindrical pores (Dp ) 4Vmp/Smp). The primary mesopore volume (Vmp) and surface area (Smp) have been calculated by the t-plot method applied to the micropore and mesopore regions of the nitrogen adsorption branch of the isotherm (see Figure 1). (ii) Evaluation of Microporosity Using Very Low P/P0 Isotherm Data. Information about the microporosity of a material can be obtained from changes in the slope of the adsorption curve when the adsorbate amount is plotted versus the relative pressure at very low P/P0 on the logarithmic scale (high-resolution isotherm). Argon at the boiling temperature of nitrogen instead of this one is recommended as adsorbate in order to evaluate accurately the microporosity of the materials.37 Micropore size distribution was calculated from the modified adsorption branches of argon isotherms using the Saito-Foley correction of the HovarthKawazoe (HK) method.38 The micropore volume (VµpAr) was estimated by integration of the pore size distribution calculated by the Hovarth-Kawazoe method.
Results and Discussion X-ray Diffraction. Powder X-ray diffraction patterns were collected for all solids after the different treatments and are illustrated in Figure 2. Table 2 shows the unit cell parameters of the treated SBA-15 materials. Samples treated with conventional extraction procedures (Figure 2(I)) show three distinct Bragg diffraction peaks, which can be indexed to [100], [110], and [200], suggesting the formation of highly ordered 2D hexagonal mesostructures. Removal of nonionic template species using supercritical CO2 yields good hexagonally ordered mesoscopic materials according to the XRD patterns shown in Figure 2(II). All the samples, treated under these particular conditions, exhibited three clear peaks, showing the good quality of the materials after SFE in CO2. (34) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380. (35) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1967, 9, 8. (b) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1967, 9, 15. (c) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1968, 10, 153. (36) Kruk, M.; Jaroniec, M.; Kim, J. M.; Ryoo, R. Langmuir 1999, 15, 5279-5284. (37) Storck, S.; Bretinger, H.; Maier, W. F. Appl. Catal. 1998, 174, 137-146. (38) Hovarth, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470.
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Table 2. Physicochemical and Textural Properties of the Materials upon Removal of the Template 29Si
XRD d100a
NMR
sample
treatment
(Å)
wall thicknessb (Å)
DXRDc (Å)
ratio
S-1 S-2 S-3 S-4 S-5 S-6 S-7
calcined EtOH, RT EtOH, reflux CO2, 60 °C CO2, 90 °C CO2, 110 °C CO2/EtOH, 90 °C
90 100 98
24 30 27
85
0.14 0.38 0.45
96 98 98
23 31 28
93 94 96
93
Q3/Q4
0.48 0.42
N2 adsorption/desorption BdB d
BJH e
Dp (Å)
Dp (Å)
80 86 86 82 88 82 85
74 77 77 76 79 78 79
f
g
SBET Vt (m2/g) (cm3/g) 605 475 508 522 641 600 681
0.922 0.861 0.921 0.885 1.065 1.017 1.105
Dmph (Å) 86 94 88 93 92 95
Ar adsorption i
Vµp (cm3/g)
VµpAr j (cm3/g)
0.069 ref 0.027 0.032 0.060 0.046 0.057
0.077 ref 0.035 0.043 0.055
d (100) spacing. b Calculated by the a0-BdB pore size (a0 ) 2d(100)/x3). c Calculated from the XRD analysis using Vmp.36 d Calculated from the adsorption branch with the BdB method. e Calculated from the adsorption branch with the BJH method. f BET specific surface area. g Total pore volume. h Pore diameter calculated from the primary mesopore volume and surface (4Vmp/Smp). i Micropore volume evaluated using the t-plot method. j Micropore volume evaluated using Ar adsorption at very low relative pressure. a
Figure 2. XRD spectra of SBA-15 materials after extraction treatment: (I) conventional methods; (II) supercritical fluid extraction with CO2.
The XRD spacings of the samples extracted with solvents are higher than those shown by the calcined material (Table 2). The use of solvents at mild temperature avoids the shrinkage of the structure during the removal of the surfactant as described elsewhere.2,21-23 With the purpose of comparing the relative mesoscopic order of the siliceous materials after different extraction procedures, the relative intensities of the d110 and d100 values have been calculated and included in Figure 2. The results reveal that all the materials have similar values, which evidences their good mesoscopic order. Thermogravimetric Analysis. The as-synthesized untreated sample (S-0 sample; Table 1) exhibited an
overall weight loss of 45% at temperatures between 170 °C and 350 °C. This weight loss is attributed largely to the decomposition and desorption of the surfactant and to a smaller extent to a release of water formed due to the condensation of silanol groups in the silica framework. Different treatments have been carried out for the removal of nonionic polymeric species, including conventional washing with ethanol at different temperatures, calcination in air, and SFE in the presence of CO2 and ethanol under different reaction conditions. TG analyses have been used as a measurement of treatment effectiveness for the removal of template species. Table 1 shows the extraction efficiencies under different conditions, taking as reference the weight loss of the as-made sample (S-0 sample; Table 1). The results show that the amount of copolymer species decreases significantly after the different treatments. As is widely known, the treatment of as-made mesoscopic materials under high thermal treatment in the presence of air removes and destroys completely the polymeric species, avoiding their recovery. Conventional washing with ethanol at room temperature eliminates partially the amount of surfactant, but about 30% of the polymer remained in the sample after washing. Similarly, washing with ethanol under reflux2 retained a part of the template although an increase of the extraction efficiency was readily observed. Unlike the case for base-synthesized MCM-41 materials, where an acid wash is required for low-temperature surfactant removal,19 acid leaching did not improve the efficiency of the treatment for SBA-15 materials (result not shown). The silica-surfactant interactions occurring in acid-catalyzed synthesis are weaker than electrostatic interactions prevailing in base-synthesized MCM-41 materials. Washing the as-made material with pure CO2 under supercritical conditions at 60 °C (S-4 sample; Table 1) leads to an extraction efficiency comparable to that obtained with a conventional washing with ethanol under reflux. This relatively high efficiency upon extraction with pure CO2 is somewhat unexpected, since a similar extraction procedure performed on pure silica MCM-41 showed that no surfactant was removed after treatment.21 The solvating strength of pure CO2 is sufficient to extract nonionic surfactants where the silica-template interactions are weak in contrast with stronger interactions predominant in base-synthesized MCM-41 materials. Moreover, SFE efficiency enhances with the increase of temperature up to 90 °C (S-5 sample; Table 1). However, the results in Table 1 demonstrate that there is an optimum temperature for maximum extraction efficiency, since the removal obtained at 110 °C is slightly lower than that obtained at 90 °C. High temperatures lead to lower density of the supercritical fluid; thereby, the solvating capacity and hence the extraction efficiency
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van Grieken et al.
Figure 3. N2 adsorption/desorption isotherms of SBA-15 materials and corresponding pore size distribution in the mesopore range for materials extracted by conventional methods (I and III) and for materials extracted by SFE with CO2 (II and IV).
diminish.21-22 Finally, the addition of ethanol as cosolvent modifies certainly the solvating power of the mixture, enhancing the removal of surfactant species (S-7 sample; Table 1). Nitrogen Adsorption-Desorption. The nitrogen adsorption isotherms of SBA-15 materials have been used to obtain information about their mesoporosity. These isotherms are, for all the materials under study, of type IV according to the IUPAC classification.39 As illustrated in Figure 3(I) the samples extracted with conventional methods showed H1 hysteresis loops with sharp adsorption and desorption branches, characteristic of good-quality SBA-15 material. The sharpness of the adsorption branches, indicative of a narrow pore size distribution, increased with the thermal treatment. Materials treated with supercritical CO2 feature a sharpness of the adsorption branches close to that of the calcined sample. The total pore volume (Vt) present for the ethanol washed at RT material (S-2 sample; Table 1) increases gradually with the efficiency of the removal of the nonionic template species. Pore size distributions of different materials after extraction have been determined using the BJH34 and BdB35 models and by means of the geometric relationship determined by XRD analysis36 and nitrogen adsorption analysis (Dp ) 4Vmp/Smp). With the purpose of comparison, the average pore diameters corresponding to each model are listed in Table 2. The BJH method systematically underestimates pore sizes,33 but the BdB pore size distribution maxima and average pore size calculated by geometric relationships (4Vmp/Smp) are closer to those estimated by XRD.36 It must be pointed out that an exact (39) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscow, L.; Pierotti, R. A.; Rouquerol, J.; Stemieniewska, T. Pure Appl. Chem. 1985, 57, 603-619.
method for calculating pore size is unknown. In this contribution, we are interested in changes occurring in the pore size after extraction treatment, more than in the absolute value; therefore, the comparative use of any of the aforementioned methods is appropriate. Pore size distributions of the different samples using the BdB model are depicted in Figure 3(III and IV). The mean pore diameter of the materials washed with ethanol and CO2 under supercritical conditions correlates with the efficiency of the extraction. A significant decrease of the pore diameter is clearly evidenced for the calcined material due to the shrinkage of the lattice. It is outstanding that the samples treated with CO2 under supercritical conditions show a narrow pore size distribution similar to that exhibited by the calcined sample. This uniformity is not achieved through conventional washing with ethanol under reflux. Finally, it is important to note that the narrowing of the pore size distribution in the mesoporous range seems to be related with the use of supercritical CO2 for surfactant removal. Likewise, it is obvious that the removal of organic templates with supercritical CO2 under temperatures of 90 °C leads to lower structural shrinkage than thermal calcination and better structural quality compared with that for conventional ethanol-washing procedures. The nitrogen isotherms have been further used to evidence the presence of microporosity in the materials after extraction treatments. To obtain good results using the t-plot method, reference isotherms stemming from nonporous materials in the micropore range with identical chemical composition and surface properties to those exhibited by the samples to analyze would be required. In this study the sample washed with ethanol at RT has been the non-microporous reference, as discussed in the Experimental Section (S-2 sample; Table 1).
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Figure 5. Extraction efficiency versus micropore volume for solvent-extracted materials.
Figure 4. t-plot analysis of nitrogen isotherms of SBA-15 materials after extraction. (I): (3) S-1; (/) S-3. (II): (0) S-7; (O) S-5; (4) S-6; (]) S-4.
As shown in Figure 4(I), the t-plot for the calcined sample in the microporous range (S-1 sample; Table 1) gives a straight line between t ) 0.35 and t ) 0.65 nm, and the extrapolation line intercepts the y-axis over the origin, indicating the presence of microporosity in these materials. This microporosity has also been detected in the material after washing with ethanol under reflux, although to a lesser extent. Figure 4(II) exhibits the t-plot analysis for the silica SBA-15 materials after SFE with CO2. All the samples treated under supercritical conditions evidence the presence of microporosity even for the lower treatment at 60 °C. The micropore volume in the mesoporous materials after extraction is closely related to the efficiency of the extraction (see Table 1). The calcined sample with a complete absence of polymeric species showed the highest extent of microporosity (0.069 cm3‚g-1). An increase in the temperature, from 60 to 90 °C in the CO2 SFE, increased the micropore volume from 0.032 to 0.060 cm3‚g-1, in agreement with the enhancement of the solvating power. Under the extraction conditions performed in this work, the micropore volume of the resultant materials assessed using the t-plot method ranged from 0.027 to 0.069 cm3‚g-1. Likewise, a significant enhancement of the BET surface area of the different materials as the microporosity increases is clearly evidenced in Table 2. The data suggest the microporosity seems to be a substantial contributor to the specific surface area. The extraction procedure removes not only hydrophobic poly(propylene) oxide species occluded in the mesopore spaces but also hydrophilic poly(ethylene oxide) chains located within the silica walls.24-28 The later species are more strongly bonded to the silica. Therefore, their removal
is more complex, and more severe treatments need to be accomplished. The change of microporosity with the extraction procedure can be explained in terms of the enhancement of the removal of hydrophilic chains of triblock copolymer occluded within the silica walls during the synthesis. At this point, it must be indicated that the use of CO2 as solvent makes easier its access to the template-filled micropores located within the silica walls in contrast with the case of bulkier ethanol molecules. In conclusion, t-plot analysis of nitrogen adsorption isotherms strongly suggests the formation of micropores within the walls of mesoporous materials after surfactant removal, as reported in other studies.24-28 Likewise, the microporosity extent is significantly related to the removal efficiency of nonionic species occluded within the silica walls, as illustrated in Figure 5. Argon Isotherms at Very Low Relative Pressure. Further evidence for the nature of the micropores featured in extracted SBA-15 materials as well as confirmation of t-plot data was obtained by Ar isotherms at low relative pressure (Figure 6(I)). The argon adsorption data in conjunction with the Horvath-Kawazoe method (HK)39 provide realistic pore-diameter values as well as a realistic micropore size distribution. Argon adsorption tests confirm the existence of micropores occluded within the silica walls and the strong dependence of their volume on the extraction efficiency. The wide pore size distribution in the micropore range illustrated in Figure 6(II) reveals the heterogeneity of these micropores. Likewise, the overall micropore volume calculated from these adsorption tests correlates fairly well with that obtained using t-plot methods, confirming the acceptable choice of the reference material (see Table 2). Silanol Group Concentration on the Silica Surface. FT-IR and 29Si NMR techniques have monitored the evolution of the silanol group concentration on the silica surface with the extraction treatment. Figure 7 depicts the 29Si NMR spectra of the different materials after treatment. All the samples exhibit clearly three peaks associated with Q2 (ca. -90 ppm), Q3 (ca. -100 ppm), and Q4 (ca. -110 ppm) Si conectivities.40 Peaks in the Q2 and Q3 region are due to silanol groups, while those in the Q4 region are attributed to Si connected to four T atoms through O atoms. Therefore, the ratio of Q3/Q4 signals obtained by deconvolution of the overall peak (Figure 7, top left) provides a measurement of the amount of surface (40) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; Wiley: Chichester, U.K., 1987.
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Figure 8. FT-IR spectra of as-made and different extracted SBA-15 materials. Table 3. Functionalization of Treated Materials with Titanium Molecular Species
sample S-1 S-3 S-7
Figure 6. Ar adsorption isotherms of SBA-15 materials (I) and related pore size distribution (II).
Figure 7. 29Si NMR spectra of treated SBA-15 materials. The deconvolution curve analysis for the S-5 sample is attached on the top left.
silanol groups present in the sample. These values are shown in Table 2 for the materials after treatment. It can be seen that the Q3/Q4 ratio decreases very rapidly for the material after calcination due to the silanol decomposition by the thermal treatment. However, this ratio remains constant and similar to that of the as-made material after washing with ethanol or with CO2 under supercritical conditions. Figure 8 shows the FT-IR spectra of SBA-15 samples after different extraction procedures. A broad band around 3400 cm-1 is evidenced for all the samples, which is partially caused by the O-H stretching vibration mode of the adsorbed water, whose bending vibration mode is
titanium content (%) theor mass mass composition composition measured by ICP 2.0 2.0 2.0
1.16 1.32 1.30
incorporation efficiency (%) 58.0 66.0 65.0
responsible for the band recorded at 1630 cm-1. Infrared vibrations around 2850-3000 and 1350-1500 cm-1 are assigned to C-H vibrations of the template. These vibrations are clearly distinguished for the as-synthesized sample whereas in the spectrum of solvent extracted materials these absorption bands are greatly diminished and clearly indiscernible for the calcined sample. Moreover, the relative intensity of Si-OH bending bands centered at 960 cm-1 is nearly the same for as-made (S-0 sample) and supercritical CO2 extracted materials (S-5 and S-7 samples) but much weaker for the calcined sample (S-1 sample). This suggests that SFE with CO2 retains the high level of silanol groups on the pore wall surface, confirming 29Si NMR results. Postsynthetic grafting routes are based on the presence of surface silanol groups as anchoring sites. To achieve a high surface coverage with functional groups or metallic species, it is important to keep a large number of surface silanol groups after surfactant removal. On the basis of avoiding the condensation of many surface groups in a typical template removal by calcination, appropriate mildtemperature extraction methods appeared as an interesting alternative for postsynthetic processes. Supercritical extraction with CO2 yields a high amount of silanol groups as compared to that for the calcined samples and comparable to that for the conventional washing with ethanol, as concluded from FT-IR and 29Si NMR characterization results. This fact is important for the functionalization of these mesoporous materials for prospective catalytic applications, sorption processes, and their use as advanced optical materials.41 Table 3 lists the Ti contents of mesostructured materials prepared using different extracted silica SBA-15 samples as support. In all the cases, the incorporation of titanium is lower than that expected on the basis of the titanium loading of the initial solution. Interestingly, the treatment for surfactant removal has a significant influence on the incorporation degree of Ti species. In this way, ethanolextracted silica based materials (S-3 sample) with a high (41) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12 (19), 1403-1419.
Nonionic Surfactant Template from SBA-15 Materials
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concentration of silanol groups, not thermally reacted by condensation, achieve a more efficient incorporation of titanium species than that found for the calcined material (S-1 sample). Moreover, the material extracted with supercritical CO2 (S-7 sample) evidences a Ti incorporation comparable to that obtained for the ethanol-extracted SBA-15 material. The postsynthetic grafting results are correlated with the concentration of silanol groups measured by 29Si NMR and FT-IR techniques. However, according to the significant differences shown by 29Si NMR and FT-IR results, the increase of Ti efficiency of incorporation should be expected to be higher than that observed in Table 3. At this point, it must be noted that the reactivity of silanol groups located on the silica surface might be dependent on the removal treatment.11
15 materials after SFE retained their uniform pore size distribution and high surface area with any effect of structure shrinkage being negligible. Likewise, a high coverage of surface silanol groups is achieved after supercritical treatment with CO2. The results shown in this work demonstrate again the presence of micropores within the silica walls of SBA-15, which confirms the presence of a bimodal pore system. The presence of both mesopores and micropores is expected to have a significant impact on the potential applications of these materials as catalytic supports and adsorbents. Interestingly, the extent of this microporosity is strongly dependent on the treatment performed for the removal of surfactant species and might be controlled by means of the extraction conditions.
Conclusions
Acknowledgment. This work has been funded by the Comunidad de Madrid through the project “Grupos Estrate´gicos de URJC”. The authors also acknowledge the financial support provided by CICYT (Project QUI 1999-0681-C02-01).
Supercritical fluid extraction (SFE) with pure CO2 exhibits a comparable extraction effectiveness to that obtained with conventional solvent washing for removal of the organic template occluded within the porous structure of as-made SBA-15 materials. This efficiency is even enhanced with the addition of cosolvents. The SBA-
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