Understanding the Stability in Water of ... - ACS Publications

8268

J. Phys. Chem. C 2007, 111, 8268-8277

Understanding the Stability in Water of Mesoporous SBA-15 and MCM-41 Anne Galarneau,*,† Mirella Nader,‡ Flavien Guenneau,‡ Francesco Di Renzo,† and Antoine Gedeon*,‡ Institut Charles Gerhardt Montpellier, UMR 5253 CNRS/UM2/ENSCM/UM1, Mate´ riaux AVance´ s pour la Catalyse et la Sante´ , ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France, and Laboratoire des Syste` mes Interfaciaux a` l’Echelle Nanome´ trique, UMR 7142, UniVersite´ Pierre et Marie Curie-Paris 6, 4 place Jussieu, 75252 Paris Cedex 5, France ReceiVed: December 12, 2006; In Final Form: March 13, 2007

Surprisingly, SBA-15 mesoporous silicas are not as stable as expected in water, even at room temperature, despite their thick walls. The microporosity of SBA-15, synthesized at a temperature below 110 °C, is lost during water treatment, leading to a strong decrease in specific surface area and an increase in mesopore size. Only SBA-15s without microporosity, such as the ones synthesized at 130 °C, are stable under water treatment. Investigations by nitrogen adsorption isotherms and hyperpolarized 129Xe NMR spectroscopy have been performed in an effort to understand the silica dissolution/redeposition processes occurring during water treatment at room temperature and at the boiling point for three SBA-15s synthesized at different temperature levels: 60, 100, and 130 °C. The differences between the local curvatures of silica in the different structures explain the difference of behavior in water with respect to silica dissolution/redeposition. Similar experiments on MCM-41 lead to a totally different dissolution/redeposition process because of its thinner walls: decrease of pore size, surface area and pore volume.

Introduction The family of highly ordered mesoporous siliceous materials, prepared in the presence of long-chain quaternary ammonium (designated as M41S: MCM-41 and MCM-48)1 or triblock copolymers (SBAs:2,3 SBA-15, SBA-16, MCF4,5) as surfactants, has attracted a great attention as new potential supports. In the last 10 years, many studies6,7 have been dedicated to the immobilization of different species solubilized in water solutions adsorbed in mesoporous micelle-templated silicas (MTS) as supports. Species such as proteins, enzymes, and whole cells have been immobilized in MTS by adsorption (more than 150 publications, see for example the review by Hartmann et al.)6,7 and especially since the discovery of SBA-15 materials exhibiting larger pore diameter (10 nm) than the previous M41S family (4 nm). More than 45 publications used these large-pore SBA15s as supports for enzyme or protein immobilization. These materials present high surface areas, high pore volumes, and well-ordered uniform mesopores. Their high capacities and adjustable pore sizes offer new opportunities to develop inorganic hosts for protein adsorption and other biological applications, such as biocatalysis, biosensors, and biofuel cells. SBA-15 materials are claimed to be very stable2 because of their wall thickness, around 4 nm, compared to MCM-41 materials with only 1 nm wall thickness. Most of the studies concerning the stability of SBA-15 concern mechanical strength, thermal or hydrothermal stability in boiling water or in steam,8-11 and address their potential use in catalysis such as oil hydrotreatment or in the extreme conditions of catalysts regeneration. No study of water stability at room temperature * Corresponding authors. E-mail: [email protected]; [email protected]. † Institut Charles Gerhardt Montpellier. ‡ Laboratoire des Syste ` mes Interfaciaux a` l’Echelle Nanome´trique.

of SBA-15 materials has been performed to our knowledge, despite the widespread use of these materials as supports to immobilize species from water solutions at room temperature.6,7 During the past decade, 129Xe NMR has been used widely for studying mesoporous materials,12-16 in particular since the introduction of hyperpolarized xenon by optical-pumping,17-19 which leads to an increase in the sensitivity of detection. Adsorption of atomic xenon (129Xe), which is highly polarizable, gives precious information about structural properties of nanoporous networks. In previous papers,20-23 it was shown that, as for zeolites, there is a relationship between the chemical shift, δ, and the pore diameter, D. However, the pore size is not the only parameter affecting the xenon NMR spectra. Textural properties such as the size and packing of the particles, as well as the chemical composition and the structure of the surface, influence the observed chemical shift leading to a non obvious correlation between δ and D. Indeed, because of the fast exchange process on the NMR time scale, xenon atoms can probe a lot of distinct adsorption sites with different chemical shift values, δi. Hence, the observed chemical shift of adsorbed xenon-129 cannot be directly related to a specific environment and can be expressed as δobs ) Σ δi Ni where Ni is the relative xenon population on site i. These populations are affected by the variation of the xenon partial pressure, and the effect on δobs brings out valuable information such as the presence of a microporosity within the mesoporous walls.24 By combining hyperpolarized xenon NMR and nitrogen adsorption measurements, SBA-15 materials treated with water at room temperature and under reflux have been characterized in order to identify the process of dissolution/redeposition of the silica inside the SBA-15 structures. A comparison with MCM-41 mesoporous materials was performed.

10.1021/jp068526e CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007

Stability in Water of Mesoporous SBA-15 and MCM-41

Figure 1. Silica solubility as a function of nanoparticle diameter (plain line) as described in ref 27 and expected MCM-41 solubility as a function of wall thickness (dashed line). r is the corresponding radius of curvature. The nanoparticles have been obtained from polymerization at pH 8 and 25 °C of pure silicic acid at various times and have been subsequently stabilized at pH 2. Changing the synthesis mode will lead to different solubilities due to various degrees of hydration or to the presence of impurities.27

Experimental Section A. Materials. SBA-15 samples have been prepared by using a (EO)20(PO)70(EO)20 triblock copolymer and tetraethylorthosilicate according to the method described in literature3 but in more concentrated media. Syntheses were performed with 1 g Pluronic P123 [(EO)20(PO)70(EO)20, Aldrich], 5 g H2O, 10 g HCl 2 M, 2.1 g tetraethylorthosilicate (TEOS, Aldrich), corresponding to a molar ratio of 1 SiO2/0.017 P123/2 HCl/80 H2O. The mixtures were maintained at 35 °C for 24 h and then for 2 days at a given temperature 60, 100, and 130 °C under static

J. Phys. Chem. C, Vol. 111, No. 23, 2007 8269 conditions in a Teflon-lined autoclave. In this paper, the three samples are identified by their temperature of synthesis in degrees centigrade. MCM-41 material was synthesized according to the ref 25 in a stainless steel autoclave at 115 °C by using cetyltrimethylammonium bromide (CTAB, Aldrich), pyrogenic silica (Aerosil 200V Degussa), sodium hydroxide (Prolabo), and deionized water in the molar ratio 1 SiO2 /0.26 NaOH/0.1 CTAB/20 H2O. All materials were then filtered, washed with water, and dried at 80 °C for 24 h. All samples have been calcined at 550 °C in air flow and characterized by X-ray diffraction (XRD), N2 adsorption-desorption at 77 K, and hyperpolarized 129Xe NMR. B. Water Treatments. In 100 mL of deionized water, 1 g of calcined sample (10 g silica L-1) was stirred under magnetic stirring for 4 h at room temperature or at reflux at 100 °C. Samples were then filtered and dried before characterization by XRD, N2 adsorption-desorption at 77 K, and hyperpolarized 129Xe NMR. C. Characterization. Powder X-ray diffraction (XRD) data were obtained on a Bruker AXS D8 diffractometer by using Cu KR radiation and a Ni filter. The adsorption/desorption isotherms of nitrogen at 77 K were measured using a Micromeritics ASAP 2010 instrument. Each sample was outgassed at 250 °C until a stable static vacuum of 3 × 10-3 Torr was reached. The BET surface area, SBET, was calculated in the domain of validity of the BET equation, that is, between 0.1 < p/p0 < 0.2. Pore diameters, D, were evaluated from the nitrogen desorption branch according to the Broekhoff and de Boer method,26 validated as one of the most accurate methods to evaluate mesopore size.25 Prior to the NMR measurements, samples were compressed at 40 MPa, placed in an NMR tube with two J. Young valves, and treated at 400 °C overnight under dynamic vacuum. 129Xe NMR spectra were collected on a Bruker DSX 300 spectrometer operating at 83.03 MHz. Hyperpolarized (HP) xenon was produced in an optical pumping cell placed in the fringe field of the spectrometer cryomagnet. The Xe-He mixtures containing 8-1000 Torr of Xe polarized to ca. 1%, were delivered at 70 cm3 min-1 flow rate to the samples via plastic tubings. FIDs (64-256) were accumulated with 10 µs (π/2) pulses and 1s delays.

Figure 2. Transmission electron microscopy of MCM-41 in side view showing infinite plane of wall thickness, t, and ended by a positive radius of curvature, r. The solubilization in water will start at the border of the wall, and silica will redeposit on the plane inside the pore as shown in the cartoon.

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Figure 3. (A) Nitrogen adsorption/desorption isotherms at 77 K. (B) Hyperpolarized 129Xe NMR spectrum under different Xe pressure. (C) Evolution of Xe chemical shift versus Xe pressure of (a) calcined MCM-41, (b) treated with water at 25 °C, and (c) under reflux.

Results and Discussion Silica Solubility. The solubility of amorphous silica in water at a neutral pH has been evaluated from 70 to more than 250 ppm (mg kg-1) at 25 °C.27 Such variations have been attributed to electrolyte concentration,28 differences in particle size, state of internal hydration, and the presence of impurities either in the silica itself or adsorbed on its surface during the treatment. Currently, accepted values are 120 ppm at 25 °C29 and 400 ppm at 100 °C under autogenous pressure.30 Silica dissolution equilibrium is established very slowly, unless the amorphous silica is finely divided or microporous so as to furnish an area of hundreds or thousands of square meters per liter of water. A high dispersion of the solid not only accelerates the kinetics of dissolution but also modifies the corresponding thermodynamics. The solubility of any solid phase is higher when the surface is convex and lower when it is concave.31 In fact, the solubility

depends on the radius of curvature of the surface: the smaller the absolute value of the radius, the greater the effect on solubility. One can distinguish the case of particles with r > 0 (convex surface) from that of pores with r < 0 (concave surface). For particles with diameters below 2 nm, the silica solubility, S, increases very rapidly with decreasing the particle size. This is illustrated in Figure 1 where the solid line (a) represents the experimental silica solubility in water at 25 °C as a function of silica particle diameter.27 S follows a reverse exponential relationship with the radius of curvature r, and can be expressed by the Ostwald-Freundlich equation

S ) Sinf exp [(2EV)/(RTr)] where Sinf is the solubility of a flat surface (infinite radius), E is the interfacial surface energy (5 × 10-2 J m-2 for amorphous


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Figure 4. Schematic representation of the side view of MCM-41 showing the solubilization of silica, which occurs at the end of the infinite plane. Further redeposition on the plane takes place mainly at the entrance of the channels, creating some constrictions. Small differences in wall thicknesses lead to great effects on solubility, the thinnest walls being the most attacked. D0, the initial pore diameter, is unaffected by this process, but the resulting channel lengths are strongly thickness-dependent.

silica at 25 °C), V is the molar volume for amorphous silica (27.2 cm3 mol-1), R is the gas constant, T is the temperature, and r is the radius of curvature (cm). Alternatively, in a pore or crevice the radius of curvature is negative and the equilibrium concentration is systematically lower than that for a positive radius, the smallest negative radii with the smallest absolute value presenting solubilities close to zero (Figure 1b). MCM-41 Solubility. On the basis of Figure 1, micelletemplated silicas with particle sizes of several micrometers should be stable in water. However, it is known that the structural properties of these materials can be drastically affected by exposure to water or even to air humidity during long-time storage. Concerning pure silica MCM-41, the dissolution of silica may only come from the curvature occurring at the end of each silica wall in contact with the solution as shown in Figure 2. Indeed, the silica wall at the border of the particles of MCM-41 shows a positive radius of curvature, r, which is half of its thickness, t. Along the pore direction, the silica walls are planar with an infinite radius of curvature, giving a very low solubility. To compare the solubilities of MCM-41 and nanoparticles, one needs to calculate the whole curvature, C, for both structures:

For a spherical nanoparticle C ) 1/r1 + 1/r2 ) 2/r with r1 ) r2 ) r For the edge of a silica wall C ) 1/r1 + 1/r2 ) 1/r with r1 ) r and r2 ) ∞ The solubility depends mainly on the curvature present in the silica, and from this calculation it appears that the border of the silica walls of MCM-41 present a solubility equivalent to a silica particle two times larger. The dashed line (c) in Figure 1 depicts the expected MCM-41 solubility as a function of its wall thickness, obtained by adjusting the nanoparticles data (taking r’ ) 2r). In MCM-41 materials, the solubility increases rapidly when wall thicknesses decreases below 1.5 nm. Because most MCM-41s present wall thicknesses around 1 nm, their solubility in water is relatively high. However, no weight loss of silica was observed after water treatment of MCM-41. The dissolved Si(OH)4 species were redeposited inside the pores, which are the most stable part of MCM-41 particles (because r < 0) and contributed to increase the wall thickness (Figure 2). Water treatment at higher temperature increases the concentration of silica in solution and accelerates the deposition process because of rapid polymeri-

TABLE 1: Properties of SBA-15 Synthesized at 60, 100, and 130 °C, Calcined at 550 °C, after 4 h in Water at 25 °C, and after 4 h in Water at Refluxa d a D V SBET Vmes Vs t SBA-15 (nm) (nm) (nm) (mL/g) (m2/g) (mL/g) (mL/g) (nm) 60 °C water reflux 100 °C water reflux 130 °C water reflux

7.9 7.2 8.1 9.4 9.6 9.3 9.4 9.6 9.5

9.1 8.3 9.3 10.9 11.1 10.8 10.9 11.1 11.0

4.5 4.9 6.9 7.3 8.4 8.7 9.7 9.8 9.9

0.75 0.64 0.79 1.32 1.09 1.14 1.19 1.13 1.18

937 556 484 1042 525 509 501 457 464

0.27 0.34 0.61 0.72 0.80 0.94 1.18 1.12 1.21

0.48 0.30 0.17 0.60 0.29 0.20 0.01 0.01 -0.03

4.8 3.7 2.8 4.0 3.2 2.5 1.7 1.8 1.6

F 1.07 1.33 1.59 0.95 1.34 1.53 2.15 2.17 2.20

a d-Spacing (d), cell parameter (a), pore diameter (D), total pore volume (V), specific surface area (SBET), mesoporous volume (Vmes), secondary porosity volume (Vs), wall thickness (t), wall density (F). V ) Vmes + Vs. Vs, secondary porosity volume (microporous and connections volume).

zation of monomers during the contact with the surface. This is why the dissolution/redeposition process is going further at 100 °C compared to 25 °C, as shown by nitrogen sorption measurements (Figure 3A). Nitrogen adsorption-desorption isotherms of MCM-41 clearly show the decrease of the pore size (from 3.5 to 2.8 nm) and the decrease of the BET surface area (from 918 to 609 m2 g-1), both caused by the increase of the wall thickness. This is, of course, accompanied by the decrease of the pore volume (from 0.68 to 0.57 mL g-1). The hyperpolarized (HP) 129Xe NMR spectra of xenon adsorbed in native MCM-41 consist of two peaks (Figure 3B). All of the 129Xe NMR chemical shifts are referenced with respect to the signal of xenon gas at zero pressure. Therefore, the NMR peak appearing around 0 ppm in every spectrum corresponds to free xenon gas between the material particles. The downfield peak is ascribed to xenon adsorbed in MCM-41 channels. Its chemical shift is almost pressure-independent, revealing uniform pores of a defined diameter, without defects on their surface.16 Once MCM-41 has been in contact with water at room temperature, in addition to the signal at 0 ppm, two overlapped peaks are observed (Figure 3B), suggesting two different pore structures. This is not consistent with nitrogen adsorption isotherms, which give only one average pore diameter. For both peaks, the chemical shift decreases with xenon pressure, suggesting the presence of strong adsorption sites. The latter were assigned to the presence of constrictions created by a preferential redeposition of silica near the entrance of the pores as explained in the schematic representation in Figure 4. As a consequence, at low partial pressure, Xe preferentially occupies


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Figure 5. Schematic representation of the three SBA-15s used in this study and synthesized at (A) 60 °C, (B) 100 °C, and (C) 130 °C, revealing different pore diameters, wall thicknesses, microporosities, and interconnections between main channels.

these constrictions and presents a relatively high chemical shift value. As the pressure is raised, Xe starts to populate the rest of the channels. Therefore, at high pressure the chemical shift tends toward values that reflect the “true” pore diameter. This explains the decrease of the chemical shift with Xe pressure observed for both peaks. For peak b1, one can see that the highpressure chemical shift is close to the one of native MCM-41. This shows that the redeposition process is definitely a local process and the pore diameter was not modified by the water treatment in most parts of the channels. In contrast, peak b2 has a noticeably smaller shift at high Xe pressure than the initial signal. The average diameter of the pores (measured by nitrogen adsorption) diminishes with water treatment; this is possible only if the exchange with the gas phase is favored in the latter case. Therefore, the general behavior of peak b2 is consistent with the presence of considerably shorter channels due to the

preferential dissolution of the thinner walls, a small difference in radius having dramatic effects on solubility. In the N2 adsorption data, these structural modifications only manifest themselves as a larger distribution of the pore sizes visible in Figure 3A. The increase of the chemical shift at low partial pressure is even more pronounced after the treatment in water at reflux (peaks c1 and c2), reflecting the extension of the constrictions. However, because the high-pressure limit of the xenon chemical shift stays approximately the same for both peaks, the shortening of the channels has probably stopped. At some point, the redeposition of silica inside the pore has created a radius of curvature large enough to hinder the dissolution process. SBA-15 Solubility. To examine SBA-15 water stability, three samples were prepared at different synthesis temperatures (Figure 5).23,24,32,33 SBA-15 prepared at 60 °C features a 5 nm



Figure 6. SBA-15 synthesized at 60 °C: (A) nitrogen adsorption/desorption isotherms at 77 K; (B) XRD pattern; (C) hyperpolarized 129Xe NMR spectrum under different Xe pressure; (D) evolution of Xe chemical shift versus Xe pressure of (a) calcined SBA-15, (b) treated with water at 25 °C, and (c) under reflux.

pore diameter, a wall thickness of 5 nm, and no connections between main mesopores. Previous experiments also pointed out the presence of ultramicropores, identified by an increase of the chemical shift with the increase of the Xe pressure, and HP 129Xe NMR results suggested that (at least) one of their dimensions was larger than 1 nm.24 At 130 °C, SBA-15 features 10 nm pore diameter, 2 nm wall thickness, and connections (1.5-5 nm) between main mesopores. No ultramicropores were detected at this synthesis temperature, but some preferential adsorption sites described as shallow micropores at the borderline with surface roughness gave rise to a decrease of the chemical shift with the Xe pressure. SBA-15s synthesized at 100 °C are described as being in between the two previous samples, with a 7 nm pore diameter, a wall thickness of 4 nm, and connections (1.5-4 nm) between main mesopores. The microporosity is intermediary between ultramicropores and rugosity. All of the characteristics of the native and posttreated

SBA-15 samples reported in Table 1 were obtained from nitrogen adsorption curves (total pore volume, V, obtained at the end of the pore filling) and XRD results (cell parameter a). Further calculations33 allowed us to evaluate the mesopore volume, Vmes (due only to the main channels), the secondary pore volume, Vs (microporosity and inter-mesopore connections volume), as well as the wall thickness, t, and the wall density, F, thanks to the following equations

Vmes ) (D/1.05a)2 (V + 1/FSi) Vs ) V - Vmes 1/F ) Vs + 1/FSi t ) a - 0.95D where FSi is the density of amorphous silica (2.2).


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Figure 7. Schematic representation of the side view of SBA-15 synthesized at 60 °C, showing important microporosity and the solubilization/ deposition process occurring during water treatment. D0 is the initial pore diameter, and DW is that after water treatment.

Figure 8. SBA-15 synthesized at 130 °C: (A) nitrogen adsorption/desorption isotherms at 77 K; (B) XRD pattern; (C) hyperpolarized 129Xe NMR spectrum under different Xe pressure; (D) evolution of Xe chemical shift versus Xe pressure of (a) calcined SBA-15, (b) treated with water at 25 °C, and (c) under reflux.

For SBA-15s synthesized at 60 °C, the water treatment, even at room temperature, causes a strong decrease in nitrogen adsorption in the region below p/p0 ) 0.4 (Figure 6), revealing the loss of microporosity (Table 1). The secondary pore volume,

equal to the microporous volume for SBA-15 (60 °C), is decreasing from 0.48 to 0.30 mL g-1 (water treatment at 25 °C), and even to 0.17 mL g-1 (water treatment under reflux). The loss of microporosity is also confirmed by Xe NMR at

Stability in Water of Mesoporous SBA-15 and MCM-41 variable Xe partial pressure. Although in our previous study a single peak was observed,24 the spectrum of adsorbed xenon for native SBA-15 (60 °C) consists of two overlapping peaks, a1 and a2. The chemical shift of both signals increases with the Xe partial pressure, following a trend characteristic of zeolitic microporous materials. This behavior is unexpected for mesoporous systems and was attributed to the presence of the previously cited ultramicropores in the walls of the mesopore channels. The occurrence of two distinct peaks reveals the existence of two different environments in the mesopores with slow Xe exchange between them on the NMR time scale. This cannot be explained by the existence of particles with different sizes because all samples were compressed prior to the experiment. The nitrogen adsorption results do not show evidence of a double mesoporosity, so the two peaks can only come from two types of ultramicropores present in mesopores. Furthermore, the chemical shift increase with Xe pressure implies that in both cases at least one dimension is more than twice the Xe diameter, that is, 0.8 nm. From the known properties of pluronic micelles, one cannot imagine micropores deeper than ∼1 nm32 and the only remaining explanation for the presence of two Xe signals involves different micropore orientations. For instance, micropores can be oriented perpendicular to the surface (∼1 nm in depth) or parallel to it (∼1 nm in width). In our Xe NMR experiments, the chemical shift extrapolated to zero pressure, δs, reflects the ease of exchange of Xe atoms between micropores and the gas phase. Because δs(a1) is higher than δs(a2), Xe diffusion must be easier in the micropores attributed to the a2 peak. Thus, the a1 peak must correspond to mesopores mainly associated with deep micropores, restricting Xe exchange with the gas, whereas a2 relates to mesopores associated with a substantial fraction of wide micropores, enhancing Xe diffusion. When SBA-15 (60 °C) is been treated with water, the slope of the chemical shift variation with Xe pressure becomes negative for both peaks (b1 and c1), revealing the loss of microporosity. The very negative slope at the beginning of the curves shows that preferential adsorption of Xe exists in some sites. In the meantime, the pore diameter of the main channel increases from 4.5 to 5 nm for the treatment in water at 25 °C and to 7 nm for the treatment at reflux. The increase in pore diameter is also confirmed by Xe NMR because the chemical shift decreases after water treatment. Meanwhile, the cell parameter remains almost the same and therefore the wall thickness decreases from 5 to 3 nm and the wall density increases from 1.1 to 1.6 (Table 1). This behavior of SBA-15 (60 °C) under water treatment is explained by the dissolution of the silica with a positive radius of curvature at the micropores’ entrance and the redeposition of the solubilized silica into the more stable crevices of the micropores as illustrated in Figure 7. The end of the silica walls in contact with the solution is not solubilized as for MCM-41 because of its larger radius of curvature due to large wall thickness. This process explains why the global wall thickness decreases and the pore diameter increases. The silica solubilization/deposition phenomenon leads to a rearrangement toward a smoother surface, still presenting a certain surface roughness as suggested by Xe NMR. The two peaks are still present in the NMR spectra, but the one with a higher chemical shift has a much lower intensity, showing that the solubilization process tends to homogenize the surface by widening the micropores. After the treatment at reflux, the second peak (c1) is barely distinguishable and the material can be considered as totally homogeneous. As for the c2 peak, its chemical shift variation is similar to the one of b2 so the nature


Figure 9. Schematic representation of the side and front views of SBA15 synthesized at 130 °C, showing the silica solubilization/deposition mechanism leading to the slight increase of the interconnection diameter occurring during water treatment.

of the microporosity is unchanged. The overall decrease of the chemical shift reflects the increase of the pore diameter. For SBA-15 synthesized at 130 °C, the material is very stable under water treatments at room temperature and under reflux. No changes were observed in the XRD pattern, pore volume, or pore diameter (Figure 8). Only a small decrease in nitrogen adsorption is noticed at p/p0 below 0.4, leading to an 8% decrease in specific surface area from 500 to 460 m2 g-1. Nevertheless, some changes were revealed by Xe NMR, which is a more local probe for characterizing porous solids. BET analysis demonstrated that the mesopore size is not affected by water treatments; thus, the significant decrease of the Xe chemical shift cannot be attributed to an increase in the pore diameter. In terms of silica dissolution, the end of the silica walls at the border of the particles of SBA-15 (130 °C) cannot be dissolved as for MCM-41 because of its larger thickness (>1.5 nm), and no dissolution due to micropores is expected as in SBA-15 (60 °C). However, SBA-15 (130 °C) features some connections between main mesopores exhibiting a low positive radius of curvature, as illustrated in Figure 9, triggering silica dissolution. A local silica dissolution/redeposition process occurring at this position in the mesopore channel is responsible for a slight increase of the aperture size of the connections between main channels facilitating the diffusion of Xe between two adjacent mesopores. This increase of the Xe mean free path leads to the observed decrease of the chemical shift after water treatment. Besides, this behavior has already been observed in the case of zeolite dehydration followed by Xe NMR.34,35 Nitrogen will no longer condense into these small connections, and therefore this very small part of adsorption in secondary mesopores will disappear from the nitrogen adsorption isotherm. A higher-temperature water treatment enhances this phenomenon. SBA-15 synthesized at 100 °C, exhibiting at the same time micropores and connections between main mesopores, reveals


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Figure 10. SBA-15 synthesized at 100 °C: (A) nitrogen adsorption/desorption isotherms at 77 K; (B) XRD pattern; (C) hyperpolarized 129Xe NMR spectrum under different Xe pressure; (D) evolution of Xe chemical shift with Xe pressure of (a) calcined SBA-15, (b) treated with water at 25 °C, and (c) under reflux.

a combined behavior of the two previous cases. Indeed, in nitrogen adsorption isotherms (Figure 10) the loss of microporosity is clearly evidenced by the loss of volume for pressure below p/p0 ) 0.6. This is confirmed by calculations (Table 1), with a decrease in secondary pore volume from 0.60 to 0.25 mL g-1 after water treatments. The pore diameter of the main channels increases from 7.3 to 8.5 nm, without any variation of cell parameters, which is due to a decrease of wall thickness. The increase in pore diameter is accompanied by a strong decrease in Xe chemical shift, from 85 to 66 ppm. For the untreated SBA-15 (100 °C), δXe remains almost constant with the Xe pressure. This reflects a compromise between the effect of micropores, which give a positive slope, and that of some preferential adsorption sites, which cause a negative slope.24 After water treatment, the slope becomes slightly negative, in accordance with the disappearance of the micropores as for SBA-15 (60 °C). Surprisingly, there is no difference

between the δXe ) f(PXe) curves for water treatment at room temperature and at reflux. It probably means that, contrary to SBA-15 (130 °C), there is no increase in Xe diffusion between main channels. In conclusion, no silica dissolution occurs in the connections of SBA-15 (100 °C) owing to its thicker walls. Conclusions Once more, this paper demonstrates the potential of hyperpolarized Xe NMR for studying mesoporous materials. Because the nature of the structural modifications underwent by the studied samples is significantly different, we prefer not to use alone the absolute value of the chemical shift for comparison purposes. We rather chose to rely on the pressure dependence of δ and to combine it with N2 adsorption results. Those two methods complement each other nicely to unveil the presence of defects in the nanostructures such as microporosity and to


J. Phys. Chem. C, Vol. 111, No. 23, 2007 8277 References and Notes

Figure 11. Nitrogen adsorption/desorption isotherms at 77 K of (a) calcined Al-MCM-41 (Si/Al ) 15), (b) treated with water at 25 °C, and (c) under reflux, in the same conditions as those for SBA-15.

finely monitor local changes such as the occurrence of constrictions inside mesoporous channels. In general, to get water-stable micelle-templated silica materials, one is supposed to use thick pore walls materials or to apply very low amounts of water for the impregnation of different species with a silica concentration threshold of 100 g L-1, as for MCM-41.36 Another way consists of adding certain impurities such as iron or aluminum incorporated in the silica or grafted on its surface,8,27 which reduces the rate of silica dissolution and also diminishes the solubility of silica at equilibrium. For MCM-41, a large amount of alumina was necessary (molar ratio Si/Al ) 15) to stabilize it in the conditions examined in the present study (Figure 11). For SBA-15, we have demonstrated that it is not necessary to add alumina if SBA-15 synthesis is performed at 130 °C instead of the 100 °C used widely in literature. Classical SBA-15 materials (synthesized at temperatures close to 100 °C) are not as stable as claimed in literature mainly because of the presence of micropores, exhibiting local positive curvature on the pore surface, which promotes silica solubilization even at room temperature. It has been shown by xenon NMR that strong restructuration occurs in SBA-15 materials in water at room temperature, featuring the loss of microporosity, and therefore the loss of the half of the specific surface area for materials synthesized at low temperature (60 and 100 °C). The absence of micropores in SBA-15 (130 °C) avoids any major restructuration of the material in the presence of water. Only a small increase of the size of the connections between mesopores has been evidenced by 129Xe NMR. These materials are better candidates as supports for immobilizing species in aqueous solution.

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