J. Phys. Chem. C 2007, 111, 3053-3058
3053
Mesostructured Silica SBA-16 with Tailored Intrawall Porosity Part 1: Synthesis and Characterization Oliver C. Gobin,† Ying Wan,§,| Dongyuan Zhao,| Freddy Kleitz,‡ and Serge Kaliaguine*,† Department of Chemical Engineering, LaVal UniVersity, Quebec, Qc, Canada G1K 7P4, Department of Chemistry, LaVal UniVersity, Ste-Foy, Quebec, Qc, Canada G1K 7P4, Department of Chemistry, Shanghai Normal UniVersity, Shanghai 200237, P R China, and Department of Chemistry, Fudan UniVersity, Shanghai 200433, P R China ReceiVed: June 8, 2006; In Final Form: October 9, 2006
Mesostructured silica materials of SBA-16 type were synthesized under varying conditions affecting the pore size distribution. The Rs-plot method allows us to identify two populations of intrawall pores and measure their respective specific volumes. A semiquantitative treatment of these data yields an estimate for the intrawall pore radius. These results are discussed and related to the behavior of the long polar chains of the F127 surfactant in the synthesis gel.
Introduction Mesoporous silicas have increasingly attracted attention since the development of the M41S family1,2 because of their highly tailorable properties such as pore size, surface area, pore volume and well-ordered structure. Furthermore, recent materials showed a high hydrothermal stability3 and could be synthesized from inexpensive and commercially available reagents.4,5 These special features led to interesting applications in catalysis, separation technology, optoelectronics, and adsorption.6-9 Ordered silicas with large pores are especially suitable for immobilization of biomolecules,10-13 as solid templates for nanostructured materials synthesis,14-17 and for catalysis involving bulky molecules.12,18-22 Furthermore, these materials are available as thin films, fibers, or monoliths with highly tailorable morphology.9,23-25 Cubic materials consisting of a multidirectional pore network14,26-31 are especially interesting for applications where a three-dimensional (3D) diffusion is needed or as membranes with pore openings perpendicular to the film surface. SBA-16 is a highly ordered porous silica with large cage-like mesopores arranged in cubic body-centered Im3hm symmetry.26-27 This material is synthesized under acidic conditions using the triblock copolymer Pluronic F127 (EO106PO70EO106) as a structure-directing agent and thus providing an intrawall complementary porosity comparable to the intrawall porosity of SBA-15.32 The structure of SBA-16 can be described by a triply periodic minimal surface of I-WP (body centered, wrapped package).27 As suggested by electron crystallography studies,27 each mesopore is connected to eight neighboring mesopores. Thereby, the pore entrance size from one mesopore to another is usually significantly smaller than the mesopore size, making this size the limiting factor for applications involving intraparticle mass transfer. Recent studies have shown that the textural properties of large pore cage-like materials could be tuned as a function of synthesis time and temperature. * Corresponding author. Tel: 418-656-2708. Fax: 418-656-3810. Email:
[email protected]. † Department of Chemical Engineering, Laval University. ‡ Department of Chemistry, Laval University. § Department of Chemistry, Shanghai Normal University. | Department of Chemistry, Fudan University.
Particularly, hydrothermal treatment or aging of the synthesis mixture at temperatures above 80 °C usually results in a pronounced increase of the pore volume and enlargement of the pore diameter. Furthermore, the pore entrance size of the mesocages may be tailored from microporous dimensions to about 7 nm.27,29,33-35 The present work is the first part of a series examining the diffusion and sorption properties of SBA-16 materials with varying micro- and mesopore volumes. This paper will focus on the synthesis and characterization of a series of SBA-16 materials with tailored porosity by changing only the hydrothermal synthesis time and temperature. The proper characterization by physisorption, XRD, SAXS, and TEM allows us to identify the Im3hm structure and calculate the relevant parameters for diffusion. In the course of this systematic study, it was established that at temperatures above 80 °C the intrawall micropores start to coalesce, yielding a new population of micropores without a change in total micropore volume. This feature is observed here for the first time. It is specific to SBA-16 because such a phenomenon is not found in SBA-15. It is likely associated with the coalescence of two neighboring PEO chains of the F127 template. A simple model of cylindrical micropores allows us to establish that the pore radius of the initial population varies between 0.6 and 0.7 nm, whereas the radius of the secondary population would be between 0.85 and 1.0 nm. Experimental Section Materials. A series of SBA-16 materials was synthesized exactly as reported recently by Kleitz et al.36,37 using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer in a ternary copolymer-butanol-water system and low-acid concentrations. The aqueous mixture of Pluronic F127 copolymer (EO106PO70EO106) with butanol (1butanol, Aldrich 99%) was used to achieve ordered selfassembly of the silica source tetraethyl orthosilicate (TEOS, Aldrich 98%). In a typical synthesis, 3 g of copolymer F127 was dissolved in a solution of 144 g of distilled water and 5.94 g of concentrated hydrochloric acid (HCl, Fischer 36.5-38.0%). After about 20-30 min, 9 g of the co-surfactant butanol was
10.1021/jp0635765 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/31/2007
3054 J. Phys. Chem. C, Vol. 111, No. 7, 2007 added in order to achieve a 1:3 F127/BuOH mass ratio in the ternary system. After 1 h of stirring, 14.2 g TEOS was added to the solution. At a constant temperature of 318 K, the mixture was further stirred for 24 h. The mixture was then placed under static conditions at varying temperatures and times for hydrothermal treatment. The temperature varied from 318 to 373 K, and the time was varied from 1 to 5 days depending on the specific sample. After the hydrothermal treatment, the precipitated solid was isolated by filtration and dried at 373 K for at least 1 day. The template was removed by a brief extraction in an acidic ethanol solution and further by calcination at 823 K under air for 6 h. The samples are denoted as S-T-t with the time t between 1 and 5 days and the temperature T expressed in degrees Celsius, in the range from 318 to 373 K. For instance, the sample treated for 1 day at low temperature is denoted as S-45-1. Measurements. The powder X-ray diffraction (XRD) patterns of calcined samples were recorded with a STOE STADI P θ-θ powder X-ray diffractometer in reflection geometry (BraggBrentano) using Cu KR(1+2) radiation with a secondary monochromator and a scintillation detector. XRD patterns were recorded in the range of 0.7-2.5° (2θ) with step ) 0.1° (2θ), time/step ) 10 s. The SAXS patterns were acquired on a Nanostar U small-angle X-ray scattering system using Cu KR radiation. Nitrogen physisorption isotherms were measured using an Omnisorp-100 sorptometer at liquid nitrogen temperature (77 K), after outgassing under vacuum at 523 K for at least 6 h. The BET surface area was calculated by applying the BrunauerEmmett-Teller equation to the adsorption isotherms over a relative pressure range from 0.06 to 0.15. The micropore volumes, external surface area, and total pore volume were evaluated using the Rs comparative plot38 with macroporous silica gel LiChrospher Si-1000 as the reference adsorbent.39 Because of the limitations of the Omnisorp-100 instrument, the isotherms were measured at relative partial pressures higher than 0.001. It should be noted that in addition to the primary mesopores SBA-16 materials must contain two intrawall pore populations including micropores and small mesopores. This situation is indeed analogous to SBA-15 materials.32,44 The unit-cell parameter was calculated from the position of the XRD reflections and confirmed by the SAXS measurements. A primary mesopore diameter was obtained from the pore size distribution (PSD) using the modified BJH method (KJS method) calibrated for MCM-41-type cylindrical mesopores and the Harkins and Jura t-plot.40 Furthermore, the model of spherical cavities41 (MSC) was used as a geometric model to determine the wall thickness and the maximal mesopore diameter. The density of the silica walls was considered as constant (2.2 g/mL), however, due to the introduction of the intrawall microporosity; and using the geometry corresponding to SBA-16 materials, the values from the MSC calculations are likely to be less erroneous than from the PSD. For all linear curve fittings, the variance of the fit was calculated and minimized to ensure a good representation of the linear region. Transmission electron microscopy (TEM) images were obtained with a JEOL 2011 microscope operated at 200 kV. For TEM measurements, the samples were prepared by dispersing the powder samples in ethanol, then dispersed and dried on the carbon film on a Cu grid. Results and Discussions For determination of the structure and symmetry of the materials, XRD and SAXS patterns and TEM images were
Gobin et al.
Figure 1. XRD patterns of SBA-16 materials.
Figure 2. SAXS patterns for SBA-16 materials.
analyzed. The reflections of the XRD patterns could be indexed to the cubic body centered space group Im3hm, thus providing an indication of the Im3hm IW-P structure of SBA-16. The positions of the reflections were confirmed by SAXS measurements. In Figures 1 and 2, the XRD and SAXS patterns of representative samples are shown, respectively. The augmentation of the hydrothermal treatment time and temperature caused a pronounced shift toward lower angles, an evidence for the enlargement of the unit cell as a function of temperature and time. The shift toward lower angles was accompanied by changes in the relative intensities in the XRD and SAXS reflections. The (200) reflection could be well resolved by SAXS for all samples, but its relative intensity decreases significantly while the higher reflections tend to increase. This behavior could also be observed by XRD, although the (200) reflection was difficult to resolve for samples with larger unit-cell parameters. This effect can be assigned to the relative change in the pore size to wall ratio and was also observed recently in the case of a mesoporous material with Ia3hd symmetry by Kim et al.5 TEM measurements yielded a confirmation of the Im3h m symmetry. Figures 3 and 4 show images corresponding to [100], [110], and [111] projections of the cubic body-centered (Im3hm) structure for samples S-45-1 and S-100-1 and their correspond-
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J. Phys. Chem. C, Vol. 111, No. 7, 2007 3055
Figure 3. TEM images of sample S-45-1 and their Fourier diffractograms along different projections of the Im3hm structure.
Figure 4. TEM images of sample S-100-1 and their Fourier diffractograms along different projections of the Im3hm structure.
TABLE 1: Structural Properties of SBA-16 Materials
Figure 5. N2 adsorption-desorption isotherms for SBA-16 materials synthesized at different hydrothermal treatment time and temperature. The isotherms (a) S-45-1, (b) S-60-1, (c) S-60-2, (d) S-80-1, (e) S-802, (f) S-80-3, (g) S-80-4, (h) S-80-5, (i) S-100-1, and (j) S-100-2 are shifted by 0, 20, 50, 50, 90, 150, 220, 320, 500, and 600 cm3 STP g-1, respectively.
ing Fourier diffractograms. The Im3hm space group is therefore reliably identified. Large domains of highly ordered materials are visible. The change in structural properties could also be confirmed by TEM. Even if it is not possible to directly access the primary mesopore diameter or the pore wall thickness by TEM, it is obvious from Figures 3 and 4 that for sample S-45-1 the ratio of void to silica wall volume is lower than that in sample S-100-1. The nitrogen physisorption isotherms of the complete series are plotted in Figure 5, showing the shifted isotherms. All of the isotherms are of type IV according to IUPAC classification with a type H2 hysteresis loop typical for materials with ink-
samples
SBET (m2g-1)
Vt (cm3g-1)
a (nm)
DBJH (nm)
me (nm)
Dme (nm)
hw (nm)
S-45-1 S-60-1 S-60-2 S-80-1 S-80-2 S-80-3 S-80-4 S-80-5 S-100-1 S-100-2
370 414 481 621 678 728 800 843 821 886
0.23 0.28 0.32 0.42 0.50 0.55 0.63 0.68 0.66 0.77
12.8 13.0
5.2 5.7 6.1 6.9 7.3 8.3 8.9 9.3 9.4 10.4
0.23 0.28 0.30 0.34 0.40 0.41 0.45 0.46 0.46 0.51
7.72 8.35
8.64 7.24
9.15
5.90
10.52
5.08
11.35 11.33 12.17
4.39 4.43 3.95
13.3 14.4 14.9 14.9 15.5
bottle pores and pore network connectivity like SBA-16.26,42,43 Because the evaporation of the fluid always occurs spontaneously and at the same relative pressure of 0.45 to 0.48 P/P0, as a result of cavitation,41,43 the pore entrance diameter to the main mesoporous cavities must be less than 4 nm.34,43 Varying the hydrothermal treatment time or temperature results in a variation of the total amount of adsorbed nitrogen (estimated at P/P0 ) 0.99) and therefore the total pore volume Vt (Table 1). Sample S-45-1 synthesized at low temperature adsorbs only about 0.23 cm3 liquid nitrogen per gram. In contrast, the sample synthesized at high temperature S-100-1 adsorbs nearly 3 times more. The hysteresis loop increases in height and width with time and temperature, and the capillary adsorption step shifts to higher relative pressures. This is a clear indication of an increasing primary mesopore size and volume. Pore size distributions determined using the BJH methods are not accurate for materials with non-cylindrical and cage-like pore structures. However, in order to compare this series with recent works on cage-like materials pore size distributions (PSDs) were calculated using the modified BJH method on the adsorption isotherm (see Figure 7). The values at maximum of the DBJH are therefore reported in Table 1. For a better evaluation of important geometric properties, we applied the model of spherical cavities43 (MSC) to the materials.
3056 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Gobin et al.
Figure 6. Rs plots of different samples.
Figure 7. Modified BJH pore size distributions.
In this model the diameter of the spherical cavities may be calculated as
Dme ) a
( ) 6 me π ν
1/3
(1)
where a is the cubic cell parameter, ν is the number of cavities per unit cell, which is equal to 2 for the Im3hm of SBA-16, and me is the void fraction associated with the mesopores estimated as
me )
FVVme 1 + FVVt
(2)
Dme 2a3 3 πD2meν
samples
Vt (cm3g-1)
V1 (cm3g-1)
V2 (cm3g-1)
Vt - V 1 - V 2 (cm3g-1)
a VBJH me (cm3g-1)
S-45-1 S-60-1 S-60-2 S-80-1 S-80-2 S-80-3 S-80-4 S-80-5 S-100-1 S-100-2
0.23 0.28 0.32 0.42 0.50 0.55 0.63 0.68 0.66 0.77
0.072 0.071 0.081 0.101 0.086 0.076 0.060 0.045 0.047 0.018
0.001 0.005 0.010 0.021 0.031 0.064 0.087 0.110 0.100 0.132
0.16 0.20 0.23 0.30 0.38 0.41 0.48 0.52 0.51 0.62
0.18 0.23 0.25 0.33 0.39 0.43 0.49 0.53 0.51 0.61
a
where Vme is the specific volume of the cavities evaluated from the Rs plot as Vme ) Vt - V1 - V2 (see below) and Fν is the density of the solid walls (which was assumed to be 2.2 g/cm3 in this initial calculation). The average wall thickness can be calculated as
hw )
TABLE 2: Pore Volumes of SBA-16 Materials
(3)
The values of Dme, hw, and me are also reported in Table 1.
∞ / VBJH me ) ∫2 dV dr dr.
As observed in recent studies on SBA-16 materials,34 the comparative methods like the t plot or Rs plot do not show the usual regular shape below the capillary condensation pressure. As can be seen from Figure 6, two different linear regions were observed below the capillary condensation step in the Rs plot. The low Rs linear region intercept with the ordinate axis yields a volume of micropores (V1) that is unambiguously ascribed to intrawall micropores. The second linear part intercept yields a sum (V1 + V2) where V2 is the volume of a population of larger micropores or small mesopores. The measured values of Vt, V1, and V2 are reported in Table 2. Interestingly, the experimental
Part 1: Synthesis and Characterization
J. Phys. Chem. C, Vol. 111, No. 7, 2007 3057
Figure 9. Confirmation plot of Vx as a function of VBJH me for SBA-16 materials.
Figure 8. Plots of V1 and V2 as functions of the total pore volume for SBA-16 materials.
values of (Vt - V1 - V2) are very close to the mesopore volume estimated using the modified BJH approach. The variations of V1 and V2 as functions of Vt are shown in Figure 8. The exact nature of the pores associated with volume V2 is difficult to assess because these could not uniquely be intrawall mesopores like those reported in materials synthesized with Pluronic-type surfactants.14,32,44-46 There is also a certain possibility that the volumes associated with the pore apertures linking the primary mesopore cavities may contribute to V2. However because the Rs plot is drawn from the adsorption branch of the isotherms this second hypothesis is thought less likely. Indeed in sample S-45-1 where it is believed that there are no intrawall mesopores, the volume of V2 is essentially zero indicating that, at least in this sample, the contribution of the pore apertures is also close to zero. Moreover, the consideration of Figure 8 also suggests that V2 is associated with a population of intrawall micropores. In this Figure, it is seen that both V1 and V2 increase at Vt < 0.42 cm3g-1. This 0.42 value corresponds to S-80-1, which means that below 80 °C the materials have a population of volume V1 representing a large fraction of the intrawall pore volume. Above 80 °C the contribution of these micropores to the overall pore volume decreases drastically, whereas the V2 value increases steadily. In a recent paper, Fajula and Ryoo established that in SBA-15 the intrawall interconnecting mesopores are created by the growth and enlargement of intrawall micropores.44 They specifically pointed out the temperature of 80 °C as the one at which this transition starts occurring. As a consequence, we believe that the data in Figure 8 reflect the same kind of transition with the conversion of intrawall micropores into larger intrawall pores occurring at a significant rate at T g 80 °C. The interconnected nature of the mesocage pore system of SBA16 prepared at 373 K has recently been evidenced independently through the nanocast preparation of CMK-type mesoporous carbon inverse replicas.47 It should be noted from Figure 8 that the slopes of V1(Vt) and V2(Vt) at high Vt are essentially equal, amounting to -0.296
and 0.330 for V1 and V2, respectively. This strongly suggests that the creation of intrawall large pores is not by enlargement of a micropore but rather by the coalescence of micropores because at high V, V1 + V2 is essentially a constant (see Table 2). Assuming that the main process is through coalescence of two micropores, it is possible to use the data in Tables 1 and 2 to estimate micropore size. Indeed, the values of specific surface area associated with the primary mesopores, Sx, may be calculated as
Sx ) SBET - S1 - S2
(4)
Assuming that the micropores are cylindrical, the pores specific areas, S1 and S2, can be estimated as
Si )
2Vi (i ) 1, 2) ri
(5)
If the small mesopores are obtained by mere coalescence of two cylindrical micropores of radius r1, then
r2 ) x2‚r1
(6)
and Sx may be expressed as
Sx ) SBET -
(
)
2 1 V + V r1 1 x2 2
(7)
Using the spherical cavity model (MSC), eq 8 can be developed:41
( )( )
Sme 1 36πν ) Vme a ξme
MSC
(8)
Because ξ ) 0.757 for the Im3hm symmetry, this equation is even simplified as
Sme 6 = Vme Dme
(9)
Table 3 shows the values calculated for S1, S2, Sx, and Vx. In order to fit Vx with the values of VBJH me reported in Table 2, it was necessary to assume that the micropore diameters were not
3058 J. Phys. Chem. C, Vol. 111, No. 7, 2007
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TABLE 3: Fitting Procedure to Estimate the Micropore Radiusa samples S-45-1 S-60-1 S-60-2 S-80-1 S-80-2 S-80-3 S-80-4 S-80-5 S-100-1 S-100-2 a
r1 (nm)
S1 (m2g-1)
S2 (m2g-1)
Sx (m2g-1)
Vx (cm3g-1)
0.6 0.6
240 237 270 404 346 337 300 225 235 102
2.3 12 23 60 89 202 310 393 357 538
125 154 164 219 278 252 277 270 283 301
0.161 0.214
0.65 0.7 0.7 0.7 0.7
0.334 0.442 0.511 0.534 0.611
The error bars in Figure 9 indicate estimates of r ) r1 ( 0.05 nm.
exactly the same in all samples. The values of r1 adopted in these calculations are also reported in Table 3. In Figure 9 the graph of Vx (Table 3) as a function of VBJH me (Table 2) is showing relatively good agreement between these two estimates, which justifies the fitting values adopted for the micropore radius (r1). The error bars in Figure 9 correspond to estimates of the micropore radius r ) r1 ( 0.05 nm. The values of r1 in Table 3 are indeed indicating that this parameter depends slightly on temperature, varying from 0.6 nm at 45 °C to 0.7 nm at 100 °C. The pore population with specific volume V2 was not observed in our previous work on SBA-15.32 This is likely associated with the difference in length of the polar PEO chains of the surfactants used in both syntheses. In SBA-15 this length is of 20 PEO groups, whereas in SBA-16 it is of 106. The longer chains are more prone to being entangled and more likely to contact each other as temperature is raised during synthesis, which leads to micropore coalescence. Conclusions A series of high-quality SBA-16 materials was prepared with different structural properties focusing on the systematic control of intrawall porosity. The mesophase was created using butanol as cosolute. The symmetry and the high quality of the samples was established from XRD, SAXS, and TEM measurements. By varying the hydrothermal temperature and time, materials with a high fraction of intrawall micropores and thick walls (S-45-1) up to mesoporous materials with few micropores, thinner walls, and large primary mesopores (S-100-2) could be obtained. Using the Rs plot, it was possible to distinguish two different types of intrawall pores. Moreover, the variations in intrawall pore volumes allowed us to suggest that the intrawall larger micropores or small mesopores are generated by the coalescence of two micropores, this process happening at a significant rate at temperature exceeding 80 °C. These data also allowed a rough estimate of the micropore radius, which varies from 0.6 nm at 45 °C to 0.7 nm at 100 °C. The model of spherical cavities43 allowed us to calculate estimates of the primary mesopore size, pore wall thickness, and primary mesopore void fraction, which are important structural properties for the correct evaluation of diffusion results to be reported in the next part of this contribution discussing the diffusion and sorption properties of SBA-16 materials. Acknowledgment. S.K. thanks NSERC for funding of the Chair on Industrial Nanomaterials, and F.K. thanks the Canadian government for the Canada Research Chair on Functional nanostructured materials. References and Notes (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.;
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