How hexagonal mesoporous silica evolves in water on short and long term: Role of pore size and silica wall porosity

Microporous and Mesoporous Materials 183 (2014) 168–176

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How hexagonal mesoporous silica evolves in water on short and long term: Role of pore size and silica wall porosity B. Gouze a, J. Cambedouzou b, S. Parrès-Maynadié b, D. Rébiscoul a,⇑ a b

CEA/DEN/DTCD/LCLT, Centre de Marcoule, F-30207 Bagnols sur Cèze Cedex, France ICSM, UMR 5257 CEA/CNRS/UM2/ENSCM, F-30207 Bagnols sur Cèze Cedex, France

a r t i c l e

i n f o

Article history: Received 24 April 2013 Received in revised form 5 August 2013 Accepted 25 August 2013 Available online 5 September 2013 Keywords: Mesoporous silica Water Silica alteration In situ SAXS

a b s t r a c t In this study, we have determined the evolution of the morphology and the structure of mesoporous silica MCM41 and SBA15 in saturation condition during short and long term alteration by water at 60 °C. These materials were characterized using in situ/ex situ Small Angle X-ray Scattering (SAXS), and ex-situ by nitrogen adsorption–desorption and 29Si Nuclear Magnetic Resonance (NMR). The results have shown that MCM41 lost its hexagonal order of pores. This phenomenon has been attributed to a change of the pore shape probably due to the dissolution of silica wall and to the recondensation of hydrolysed silica on the pore surface leading also to a partial pore clogging at high alteration progress. In the case of SBA15, as soon as the porous silica is in contact with water, an altered silica layer is formed at the pore surface and dissolved at a rate of 120 nm year1 leading to a pore size increase. When an equilibrium between dissolution and recondensation of the silica is reached, the silica dissolution rate strongly decreases (7 nm year1) and the altered layer growth follows a diffusive process with a diffusion coefficient of D = 1.4  1024 m2 s1. The differences of evolution between the two silica are explained by their different pore diameter and the presence of microporosity in the case of SBA15. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The study of silica–water interactions are ubiquitous in a large variety of fields such as oceanography [1], geology [2], nuclear glass disposal [3], membrane technology [4], biology, catalysis [5,6] and microelectronics [7]. The most studied pure silica materials are probably the so-called highly ordered mesoporous silica materials MCM [8] and SBA [9] for their interesting properties at the subnanometer scale and their potential use in all of these fields. Their interactions with water during hydrothermal treatment [10–12], basic or acid treatment [13,14] or water treatment at room temperature [15] were largely investigated. The results obtained in these studies have shown that the silica behaviour with water depends on water treatment, on the morphology (in terms of pore size, pore wall thickness, connectivity) and the structure of the material often imposed by the elaboration method. Generally, these studies were performed on short term, i.e. shorter than a day. As far as we know, the simultaneous analysis of the porosity and the structure of silica was never performed during short term and long term alterations at temperature lower than 100 °C and in saturation condition. Such studies would yet be of great interest to better understand the long term behaviour of

⇑ Corresponding author. E-mail address: [email protected] (D. Rébiscoul). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.08.041

more complex silica-based materials such as minerals, biosiliceous materials and alteration layers of nuclear glasses. In this study, we have tried to determine the evolution of the morphology and the structure of mesoporous silica during short and long term alteration by water. More specifically, we focused on the modification of the pore surface and the silicon local structure when the system is supposed to be in saturation condition regarding silica. To reach this goal, we have used two hexagonal mesoporous silica-based materials: MCM41 and SBA15. These silica-based materials have been chosen for their different pore sizes around 3 and 5 nm respectively and the presence of microporosity in case of SBA15 silica. Both silica materials were altered in water at 60 °C at high silica-surface-area-to-solution-volume ratio (S/V) in order to quickly reach the silica saturation. For the first time, we have determined the evolution of the material porous structure for short term alteration by in situ Small Angle X-ray Scattering (SAXS). Transmission electron microscopy (TEM) images were performed in order to illustrate the structural evolutions in the direct space. We also characterized ex-situ the evolution of the porosity and the local structure of the silicon at longer term by SAXS, nitrogen adsorption–desorption and 29Si Nuclear Magnetic Resonance (NMR).

B. Gouze et al. / Microporous and Mesoporous Materials 183 (2014) 168–176

2. Experimental methods 2.1. Preparation of hexagonal mesoporous silica 2.1.1. Synthesis of MCM41 MCM41 silica was prepared by using the hydrothermal method described by Chen and Wang [16]. The typical process was followed: 1.8 g of octadecyltrimethylammonium bromide (99 wt%, Sigma–Aldrich) and 0.60 g of NaOH were solved in 60 ml of ultrapure water and mixed in a stainless steel autoclave (PARR instruments), and then 11 ml of tetraethoxysilane (TEOS 99 wt%, Sigma–Aldrich) were added to the solution, which was stirred for 60 min at room temperature. Then, the sol was aged during 1 h and placed in an oven at 110 °C during 96 h. After 96 h, the resulting white precipitate was filtered and washed with 1 L of deionized water and 50 mL of ethanol (99.9 wt%, Merck). The resulting powder was dried for 2 h at 80 °C. To remove the octadecyltrimethylammonium bromide template, the resulting powder was mixed with 0.25 mL of HCl (37 wt%, Merck) and 20 mL of ethanol [17]. The mixture was stirred at 60 °C for 90 min, washed with deionized water, dried at 80 °C for 2 h, and then calcined at 540 °C for 2 h (heating rate of 4 °C/min). 2.1.2. Synthesis of SBA15 SBA15 silica was prepared using the method described in [18]. In a typical process, 4 g of P123 pluronic (EO20-PO70-EO20, MW = 5800, Aldrich) were dissolved in 30 mL of ultrapure water and then mixed with 120 mL of a 2 M (37 wt%, Merck) HCl solution at 35 °C. Then, the sol was stirred during 1 h 30 min at the same temperature and 9 mL of tetraethoxysilane (TEOS 99 wt%, Sigma– Aldrich) were slowly added. After 20 h at 35 °C, the sol was placed at 80 °C in an oven during 8 h in order to set the pore size around 5 nm. The resulting white precipitate was filtered and washed using ultrapure water and dried at the laboratory atmosphere. Finally, the powder was calcined at 500 °C for 6 h (heating rate of 1 °C/min). Both resulting materials are presented on Fig. 1. 2.2. Alteration experiments Two silica alteration experiments at high S/V were performed at 60 °C in pure water, namely a short term alteration and a long term alteration experiment. In the short term alteration experiment, the evolution of the silica powder porosity was determined using in situ SAXS characterization. Silica powder and ultrapure water were inserted in a 2 mm diameter capillary at S/V around 35  106 m1. The capillary was sealed and then placed in an oven at 60 ± 2 °C specifically dedicated to SAXS analyses. Scattering

169

measurements were performed over 65 h. In the long term experiment, silica powders were altered in static mode in a PTFE reactor at S/V between 32.5  106 m1 and 37.5  106 m1. Depending on the specific surface area of the powders determined by adsorption– desorption of nitrogen, around 300 mg of silica powders were altered in 4 mL of ultrapure water for SBA15 and 8 mL for MCM41. Then, the reactors were placed in an oven regulated at 60 ± 2 °C. Three reactors, corresponding to three alteration durations, 7, 21 and 28 days were used. At the end of the alteration the powders were characterized by SAXS, adsorption–desorption of nitrogen and 29Si NMR. 2.3. Characterization methods The porosity was determined via nitrogen adsorption–desorption at boiling temperature (77 K) using an ASAP 2020 from Micromeritics. Before the analysis, calcined powders were degassed at 350 °C during 4 h and altered powders were degassed at 150 °C during 24 h in order to prevent the samples from porosity modification. Surface area, pore diameter, and pore volume were obtained using the BrunauerEmmettTeller (BET) method and the BarretJoynerHalenda (BJH) model. The micropore volumes and areas were analyzed using the t-plot method with a statistical thickness t obtained from the Harkins and Jura equation [19]. 29 Si MAS NMR measurements were performed on wet samples using a 400 MHz Bruker Avance III spectrometer, equipped with a 4 mm MAS probe 1H/X, in order to determine the evolution of the Si local structure in unaltered and altered silica. A 90° direct pulse on 29Si nuclei with high power decoupling on protons was used during acquisition (hpdec). The relaxation delay used for hpdec measurements was sufficiently long (200 s) to obtain quantitative results. SAXS experiments were performed in the transmission geometry, using a molybdenum anode delivering a wavelength of 0.71 Å. The monochromation is achieved using a Xenocs Fox2D multishell mirror. Two sets of scatterless slits allow the beam to be collimated and to have a squared shape of side 0.8 mm. SAXS patterns are recorded on a MAR345 2D imaging plate, which enables the simultaneous detection over scattering vectors q ranging from 0.3 to 10 nm1. In-situ temperature SAXS measurements were performed using an oven specially designed for the SAXS set-up, in which glass capillaries can be used as sample holders. TEM experiments were carried out in a JEOL 2200 FS operating at 200 kV. Samples were deposited on 400-mesh carbon-coated copper grids. 3. Results and discussion 3.1. MCM41

4.9 nm dense silica 3.0 nm

5.3 nm

1.7 nm

microporous silica MCM41

SBA15

Fig. 1. Schematic representation of the ordered mesoporous silica materials MCM41 and SBA15 used in the study.

Fig. 2 shows TEM images of the MCM41 sample before and after a 48 h long treatment in water at 60 °C. From these images, it is clear that the hexagonal pore lattice visible before the treatment has disappeared after the water treatment. While a porous structure is still observable, the symmetrical order of pores is no longer visible. More detail can be obtained thanks to in situ SAXS. Fig. 3 shows the SAXS patterns of a MCM41 powder immersed in water at 60 °C in an in situ short term alteration experiment (Fig. 3(a)) and in an ex situ long term alteration experiment (Fig. 3(b)). In the SAXS profile corresponding to 1 h of alteration (short term experiment, Fig. 3(a)), the Bragg peaks relative to the 2D hexagonal lattice of mesopores are clearly visible, and are denoted B1, B2, B3, and B4. They respectively correspond to the 10, 11, 20, and 21 reflections. From the position of these peaks, we determined the mean lattice parameter of the pore lattice a of

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Fig. 2. TEM image of the MCM41 sample before (a) and after (b) immersion in water at 60 °C for 48 h.

dry 1h 12 h 25 h 37 h 50 h 62 h

Intensity (arb. units)

B2

1

B3 B4

0.1 0.01

10

Intensity (arb. units)

B1

10

28 days 21 days dry

1

q-3 0.1

1E-3

(a)

(b)

0.01

1E-4 1

2

3

4

5

q (nm-1)

1

2

3

4

5

q (nm-1)

Fig. 3. Experimental SAXS patterns of the MCM41 sample altered in pure water at 60 °C for the in situ-short (a) and long term (b) experiments. The B1, B2, B3 and B4 peaks respectively stand for the 10, 11, 20 and 21 crystallographic planes of the mesoporous 2D hexagonal lattice.

4.9 nm in our MCM41 sample. When the alteration duration increases, the intensity of the Bragg peaks decreases and after 5 h, Bragg peaks B2, B3, and B4 have almost completely disappeared. Such a disappearance is typical of a progressive loss of the mesoporous order related to a randomization of the position correlation between pores. This phenomenon is still observable on the SAXS patterns of wet MCM41 powders coming from the long term alteration experiment (Fig. 3(b)). Moreover, the overall intensity decreases following a q3 power law, and this power law seems not to depend on the alteration time. Deviation from a Porod regime (q4 power law) has already been observed in similar SAXS measurements [19] on mesoporous silica. It could be related to the roughness of the interface between the pore and the silica wall, and to the contribution of the structure factor coming from the amorphous structure of the silica tetrahedrons inside the walls. The progressive loss of the Bragg peaks related to the hexagonal order of pores can be reproduced through a numerical calculation of the SAXS profiles (Fig. 4). The model used in order to perform these calculations has already been presented in full detail in [20]. In this model, arrays of cylindrical pores of infinite length are disposed on a hexagonal lattice. The pore diameter and the lattice parameter are adjusted in order to optimize the agreement between the experimental and the simulated SAXS pattern. Note that a Gaussian distribution of pore diameters (pd) is considered. The full width at half maximum (FWHM) of this distribution is expressed as a percentage of the lattice parameter. A paracrystalline disorder [21,22] is introduced in the model in order to take into consideration the progressive loss of correlation between the

sitions of the pore centres, as illustrated in Fig. 4(a). A paracrystalline degree (pcd) is therefore introduced and defined as the ratio between the FWHM of the Gaussian distribution describing the dispersion of the position of the first neighbouring pore and the pore lattice parameter. Fig. 4(b) presents calculated SAXS profiles presenting the best agreement with the experimental data, featuring a pore diameter of 3.5 nm with a pd of 20%, and a pcd of 20%. The progressive increase of the pd and pcd parameters results in the progressive smoothing of all Bragg peaks as observed in the experimental data. Of course, the hypothesis of simultaneously increasing pd and pcd is a very coarse approximation of the alteration mechanism that takes place in the sample. A more relevant evolution of the porous structure is proposed and presented in Fig. 5. In the latter picture, the wall of each pore is partly deformed – probably due to the dissolution of silica wall and to the recondensation of hydrolysed silica on the wall, resulting in pore shape change [23]. The centre of the pores is consequently shifted from the original position of the hexagonal lattice, leading to the smoothing of the Bragg peaks. This picture is compatible with the TEM image of altered MCM41 presented in Fig. 2(b). This proposed pore evolution mainly controlled by a hydrolysis–recondensation process is also confirmed by nitrogen adsorption–desorption analyses performed on MCM41. All adsorption– desorption isotherms are shown in Fig. SI1 available as supplementary information. The results are presented on Fig. 6 and in Table 1. After 7 days of alteration in ultrapure water, the pore size distribution shows two mean pore diameters of 2.4 and 3.7 nm, which can respectively come from a decrease and an enlargement of the 3 nm

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(b) 100 Intensity (arb. units)

(a)

10 1 0.1

a b

0.01

c

1E-3

d

1E-4 1

2

3

4

5

q (nm-1) Fig. 4. (a) Schematic representation of the model used to fit the SAXS pattern and representation of the effect of paracrystalline disorder on a 2D hexagonal lattice of pores. The undistorted 2D hexagonal positions are denoted by the diamonds, the extreme positions of the first neighbouring pores are represented by dotted circles. The extreme positions of one pore of the second shell are also depicted by dotted circles. (b) Calculated SAXS patterns of MCM41 structures involving a mesoporous lattice parameter of 4.87 nm and a mesopore radius of 1.75 nm. The FWHM of the pore diameter distribution (pd) and the paracrystalline disorder (pcd) are both equal to (a) 20%, (b) 25%, (c) 30%, and (d) 35%. See text for details on the definition of pd and pcd. The curves have been arbitrarily shifted for clarity.

Fig. 5. Schematic representation of the possible evolution of the porous structure of the MCM41 sample before (a) and during its alteration in pure water at 60 °C (b). Black dots were added to materialize the position of the nodes of the hexagonal lattice of mesopores, which is distorted during the water alteration (red crosses on the figure). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5

dVp/dr (cm3 .g-1 .nm-1)

MCM41

gress, i.e. 21 and 28 days, the amount of pores having a size of 2.4 nm strongly decreases. This pore size evolution during MCM41 alteration goes with the decrease of both the porous volume and the specific surface area probably due to partial pore clogging. The 29Si NMR spectra also show some modifications after 7 days of alteration as presented on Fig. 7. These spectra exhibit well-resolved Qn peaks (where n is the number of bridging oxygen atoms) with n = 1, 2, 3 and 4, allowing the determination of these species [25]. Fig. 8 shows fits of the experimental spectra based on a Gaussian fitting procedure. By measuring the ratio between each Gaussian contribution and the total simulated peaks area, we estimate the Qn species ratio in the sample. The results are presented in Table 2 and highlight two phenomena. First, the ratios of the Qn species are modified after alteration. After 7 days of alteration, an increase of Q2 and Q3 ratios related to the silica pore wall hydrolysis is observable. The Q4 ratio increases after 21 days of alteration traducing the probable repolymerization of the silanols created during the alteration. Second, the positions of the various Qn peaks shift toward higher chemical shifts showing also an increase of the silica polymerization. Such evolution is also observable on alumina–silica gel coming from glass alteration [26,27].

4

unaltered 7 days 21 days 28 days

3 2

3.2. SBA15

1 0 2

3

4

5

6

Pore diameter (nm) Fig. 6. BJH pore diameter distributions of MCM41 before and after 7, 21 and 28 days of alteration at 60 °C in pure water.

pores found in the unaltered MCM41.1 These evolutions would be respectively linked to the silica wall dissolution and to the hydrolyzed silica recondensation on pore surface. At higher alteration pro-

1 The discrepancy between the pore mean diameter obtained using SAXS and the BJH model is a well-known problem [24] originating from the differences in the hypotheses related to each model. As we focus on the evolutions observed with the same technique, this discrepancy does not hamper the discussion.

In order to compare the evolution of MCM41 during its alteration with a silica having larger pore size and presenting microporosity, SBA15 material was studied. Fig. 9 presents TEM images of the SBA15 sample before (a) and after (b) immersion in water at 60 °C during 48 h. From these images, it seems that there is no obvious change in the porous structure of the material. In particular, the pore periodicity and remains roughly the same, and their linear shape resist the alteration process. However, these TEM images are not straightforward to analyse because the contrast between silica and the carbon coating of the grids is low, and because it proved difficult to find particles small enough to avoid superposition effects that blur the images. Fig. 10(a) and (b) present the experimental SAXS profiles obtained as a function of the alteration time at 60 °C for short and long term experiments, respectively. We focused on the range of scattering vectors in which Bragg peaks B2 and B3 are visible, and we denoted their respective intensity I2 and I3. The relative intensities I2 and I3 continuously evolve, showing a progressive increase of I2 and a stagnation of I3 during alteration. The position of B2 and B3 remains the same during the whole experiment, indicating that there is neither contraction nor dilatation of the pore lattice, which stays equal to 10.2 nm. The observed

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Table 1 Porosity characteristics of MCM41 and SBA15 before and after 7, 21 and 28 days of alteration at 60 °C in pure water, obtained from nitrogen adsorption–desorption analysis. D: mean pore diameter, SBET: specific surface area; Vp: porous volume and Vl: microporous volume. Time (days)

MCM41

0 7 21 28

SBA15

D (nm)

SBET (m2 g1)

Vp (cm3 g1)

D (nm)

SBET (m2 g1)

Vp (cm3 g1)

Vl (cm3 g1)

3.0 2.4, 3.7 3.7 3.7

880 950 530 475

0.76 0.96 0.51 0.48

4.9 5.2 5.5 5.6

465 460 500 580

0.42 0.51 0.61 0.77

0.07 0.04 0.04 0.03

change of the I2/I3 ratio can be related to a change of the pore shape, in particular of its mean diameter. As a matter of facts, the cylindrical shape of pores implies that the form factor of the

Q4

MCM41

3

Q Q2 Q1

Q4

ref 7 days 21 days 28 days

Q3

Q2

Q1 -60

-70

-80

-90

-100

-110

-120

-130

29

Si chemical shift (ppm)

29

Fig. 7. Si MAS spectra of MCM41 before and after 7, 21 and 28 days of alteration at 60 °C in pure water.

pores is a cylindrical Bessel function, whose oscillation period is inversely proportional to the pore diameter. Varying the diameter causes a shift of the minima positions. Since the SAXS pattern results from the convolution of the form factor with a structure factor that only depends on the pore lattice parameter, the overall shape of the SAXS pattern (in particular the relative intensities I2 and I3) can be modified by the change of pore diameter. In a first attempt to reproduce the evolution of the I2/I3 ratio in experimental data using our first model, we have tried to vary the pore diameter from 3 to 7.2 nm. Fig. SI2 shows the values obtained for I2, I3, and the ratio I2/I3 versus the pore diameter. Fig. SI3 gives examples of the SAXS profiles calculated using this model. These calculations show that the range of pore diameters for which I2 is lower than I3 is very restricted (6.2 < d < 7 nm). After 23 h of alteration, the I2/I3 ratio becomes greater than 1. This evolution can be interpreted as an increase or a decrease of the pore diameter. However, a closer look at the particular evolution of I2 and I3 in the experimental diagram magnified in Fig. 11(a) indicates that I3 remains constant while I2 increases. Such a tendency is not reproduced by any of the calculations using this model for diameters near from those giving I2 < I3 (Figs. SI2 and SI3). Regarding these results, our first model had to be improved to correctly reproduce the observed trends. That is why we have proposed a second model involving a layer of altered silica in the vicinity of the pores edge,

Q4

Q4

MCM41 7 days MCM41 ref Q3

Q3

Q2

Q2 -80

-90

-100

-110

-120

-130

-80

-90

-100

-110

-120

-130

29

29

Si chemical shift (ppm)

Si chemical shift (ppm) Q4

Q4

MCM41 28 days MCM41 21 days Q3

Q3

Q2

Q2 -80

-90

-100

-110

29

-120

Si chemical shift (ppm)

Fig. 8. Experimental and simulated

29

-130

-80

-90

-100

-110

-120

-130

29

Si chemical shift (ppm)

Si MAS spectra of MCM41 before and after 7, 21 and 28 days of alteration at 60 °C in pure water.

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rffiffiffiffi Dpffiffi t þ r0 R¼2

Table 2 Quantification of the various Qn species in MCM41 and SBA15 before and after 7, 21, and 28 days of alteration at 60 °C in pure water. Alteration time

Ref

7 days

21 days

28 days

MCM41

Q2 proportion (%) Position Q3 proportion (%) Position Q4 proportion (%) Position

1.5 91.6 22.0 101.7 76.5 110.8

4.3 93.6 26.5 103.6 69.2 112.9

2.3 93.8 20.36 102.9 77.3 112.6

1.8 92.6 20.6 102.9 77.7 112.7

SBA15

Q2 proportion (%) Position Q3 proportion (%) Position Q4 proportion (%) Position

3.7 90.5 51.1 101.9 45.3 109.9

6.2 92.2 31.7 102.2 62.1 111.6

4.9 92.4 32.1 102.5 63.0 112.0

4.5 93.4 31.7 103.0 63.8 112.5

ð1Þ

p

where r0 is the value of r at the beginning of the experiment. Such an adjustment is reported in Fig. 12 as dashed line. It gives an apparent water diffusion coefficient D = 1.4  1024 m2s1, in the same order of magnitude than that obtained in glasses [29–33]. This interesting result is to the best of our knowledge, the first determination of an apparent diffusion coefficient of water in the walls of mesoporous silica material using SAXS. From Fig. 12, two silica alteration stages can be identified. During the first stage, – until 25 h, r and R increase at the same rate, and hlw remains constant. At this stage, the complete dissolution rate of the silica pore walls, r = 120 nm year1, is equal to the rate of water diffusion through the pore wall. In the second stage, r stays constant. It highlights a decrease of silica dissolution and the hydration of silica pore wall become the predominant mechanism. We assume that this behaviour could be related to the concentration of Si in the portal solution becoming close or equal to the Si saturation concentration C Si . On the other hand, an increase of the intensity in the low scattering vector area of SAXS patterns is clearly visible after 23 h of alteration. Such a signal could originate from small objects newly formed in the medium, such as recondensed silica nanoparticles. It could also be due to the size reduction of mesoporous silica grains whose size approaches the dimensions detectable with our SAXS set-up. Long term alteration experiments give complementary information. After 7 days of water exposure, nitrogen adsorption– desorption results presented on Fig. 13 and in Table 1 highlight an increase of the pore volume and the pore size, a stagnation of the specific surface area and a decrease of the microporous volume. These results are in good agreement with those of short term SAXS experiments, which highlighted the increase of the mean pore

as represented in Fig. 11(b). In this model, we denote r the pore radius, hlw the width of the altered layer and R the sum of r and hlw. Explanation concerning the choice of the altered layer electron density is presented in supplementary information. Fig. 11(c) and (d) show the results of calculations leading to a good agreement with the experimental data. In particular, the progressive increase of I2 and the simultaneous stabilization of I3 is correctly reproduced. Based on the comparison between the I2/I3 ratio from the calculated patterns and the experimental ones, the evolutions of r, R, and hlw were plotted as a function of alteration time and are presented on Fig. 12. The edge of the altered layer is continuously expanding over the considered time span and the R parameter follows a power law of exponent 1/2, typical of a diffusion process. Assuming that diffusion is the limiting reaction, an apparent water diffusion coefficient D can be calculated from Eq(1) based on the first Fick [28] law:

Fig. 9. TEM images of the SBA15 sample before (a) and after (b) immersion in water at 60 °C for 48 h.

Intensity (arb. units)

10 B2

B3

60 h 40 h 23 h 15 h 8h 1h dry

1

q-3 0.1

(a)

100 10 1

q-3 0.1

(b)

0.01 0.3

28 days 21 days 7 days dry

1000

Intensity (arb. units)

B1

100

1

q (nm ( -1)

5

0.01 1

5 -1

q (nm )

Fig. 10. Experimental SAXS patterns of the SBA sample during alteration in pure water at 60 °C. (a) Short term and (b) long term experiments.

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Intensity (arb. units)

2

60h 40h 23h 15h 8h 1h

(a)

1

1.1

1.2

1.3

1.4

1.5

(b)

1.6

q (nm-1)

(c)

1.3

(d)

1.2 1.1

I2/I3

Intensity (arb. units)

10

r=3.5 hlw=0.8 r=3.5 hlw=0.6 r=3.5 hlw=0.4 r=3.5 hlw=0.3 r=3.5 hlw=0.2 r=3.4 hlw=0.2 r=3.1 hlw=0.2

I2/I3 exp. I2/I3 calc.

1.0 0.9

1

0.8 0.7 1.1

1.2

1.3

1.4

1.5

0

1.6

20

-1

40

60

Time (hour)

q (nm )

Fig. 11. (a) Magnification of experimental SAXS patterns of the SBA15 sample in the [1.1–1.6 nm1] range of scattering vectors. (b) Graphical representation of the model used to calculate the SAXS patterns of SBA15 samples, involving the pore radius r, the altered layer width hlw and the sum of the pore radius and the altered silica layer width R. (c) Calculated SAXS patterns using the model described in the text. (d) Evolution of the I2/I3 ratio in the experimental data (open squares) and in the calculations (filled circles) as a function of alteration time.

4.0

0.7

r

0.4

3.4

0.3

hlw

3.2

0.2

3.0 0

10

20

30

40

50

60

Time (hour) Fig. 12. Evolution of the pore radius (r), the width of the altered silica layer (hlw) and of the sum of r and hlw (R) from the calculated SAXS pattern for the SBA15 sample. The dashed curve is an adjustment of the R value by the squared root of time. The vertical dashed line represents the frontier between the two stages described in the text.

diameter and consequently, of the pore volume. Note that the increase of the pore diameter necessarily implies the disappearance of a part of microporosity located near the pore edge. At longer time, the microporous volume stays constant, while the pore diameter, the porous volume and the specific surface increase. 29Si NRM characterizations of SBA15 reported in Fig. 14 and Fig. 15 shows the deconvolution of NMR lines as Qn contributions. These results summarized in Table 2 show a decrease of the (Q2 + Q3)/Q4 ratio

ref 7 days 21 days 28 days

-1

-1

SBA15

1.2

3

r, R (nm)

0.5

3.6

1.6

dVp/dr (cm .g .nm )

0.6

Hydrated layer width (nm)

R

3.8

2.0

0.8

0.4

0.0 2

3

4

5

6

7

8

9

10

Pore diameter (nm) Fig. 13. BJH pore diameter distribution of SBA15 before and after 7, 21 and 28 days of alteration at 60 °C in ultrapure water.

after 7 days of alteration to reach a constant value for longer alteration durations. The important initial decrease of the (Q2 + Q3)/Q4 ratio after 7 days of alteration is in agreement with the water penetration into silica walls, modifying their structure at the early stage of water exposure. After 7 days of alteration, this proportion does not significantly evolve anymore. However, gas adsorption measurements reveal that pore size still increases. The dissolution rate estimated at r0 = 7 nm year1 is much lower than the value of 120 nm year1 measured before 7 days of alteration. It therefore appears that the dissolution process is probably different from that observed at the early stage of the silica alteration. As for MCM41 silica, the shift of the position of the various Qn peaks in NMR spec-

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Q4

rSiO2 = rH2O

rSiO2’ << rH2O rH2O, DH2O

SBA15

rSiO2

Q3

unaltered 7 days 21 days 28 days

-70

-80

Stage 2

Fig. 16. Schematic representation of the proposed evolution of a pore during the SBA15 alteration at 60 °C by ultrapure water. r H2 O : diffusion rate of water in the silica wall; rSiO2: dissolution rate of silica at the early stage of alteration; r SiO2 :  dissolution rate of the altered silica layer, CSi : concentration of Si into the portal solution close or equal to the Si saturation concentration. DH2 O : Apparent water diffusion coefficient of water in SBA15 silica walls.

-90

-100

-110

-120

-130

29

Si chemical shift (ppm)

29

Fig. 14. Si MAS spectra of SBA15 before and during its alteration at 60 °C in ultrapure water.

tra of SBA15 suggests probable repolymerization mechanisms occurring at the pore surface. Therefore, the slow evolution of silica structure at long term could result from the competition between two antagonist mechanisms of dissolution and recondensation at the pore surface. Fig. 16 summarizes the mechanisms of alteration of SBA15 in the different stages. 3.3. Comparison of MCM41 and SBA15 evolution MCM41 and SBA15 silica showed different evolutions upon water exposure in short and long term experiments. At the beginning of the MCM41 silica alteration, a quick loss of the hexagonal order of pores was revealed by in situ SAXS experiments. This phenomenon has been attributed to a change of the pore shape

probably due to the dissolution of silica wall and to the recondensation of hydrolysed silica on the pore surface leading also to a partial pore clogging at high alteration progress. In the case of SBA15, as soon as the porous silica is in contact with water, an altered silica layer is formed at the pore surface and dissolved at a rate of 120 nm year1 leading to a pore size increase. When an equilibrium between dissolution and recondensation of the silica is reached, the silica dissolution rate strongly decreases (7 nm year1) and the altered layer growth follows a diffusive process with a diffusion coefficient of D = 1.4  1024 m2s1. The differences of evolution between MCM41 and SBA15 silica could be explained by their different pore diameter and the presence of microporosity in the case of SBA15. The smaller size of pores in MCM41 (3 nm) than in SBA15 (4.9 nm) probably exacerbates the distortion of the pore lattice since the position of pore centres is more easily shifted by silica dissolution/recondensation processes. Such a mechanism was already observed in another study focused on the effects of acids on mesoporous silica [23]. In the case of SBA15 silica, the presence of open microporosity al-

Q4 Q4

SBA 7 days

Q3

Q3

SBA ref

Q2

Q2

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-90

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-110

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-80

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SBA 21 days

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29

Si chemical shift (ppm)

Si chemical shift (ppm)

Q4

Q4

SBA 28 days

Q3

Q3

Q2

Q2

-80

rH2O, DH2O

rSiO2 ’

Stage 1 Q2

-60

CSi*

-90

-100

-110

-120

29

Si chemical shift (ppm)

Fig. 15. Experimental and simulated

29

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29

Si chemical shift (ppm)

Si MAS spectra of MCM41 before and after 7, 21, 28 and 56 days of alteration at 60 °C.

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lows water molecules to penetrate silica walls and diffuse away from mesopores edges. Therefore, the kinetics of SBA15 alteration could result from a competition between the water molecules diffusion/reaction trough the microporous silica walls and the hydrolysis–recondensation of the pore surface. Even if MCM41 and SB15 silica materials in water present two types of evolution, these mesoporous silica continue to evolve even in saturation condition. All of these results could be explained by the metastability of these silica materials. Indeed Guthrie et al. [34], have observed that MCM41 was more soluble than amorphous silica and have supposed a precipitation of amorphous silica at the pore surface. This precipitation could delay the MCM41 silica transformation into more stable amorphous silica. 4. Conclusion The investigation of the evolution of highly ordered mesoporous silica in contact with water has shown different alteration behaviours which depend on the porosity and the structure of the silica. For the MCM41 silica having pores size around 3 nm, the alteration behaviour is mainly driven by a dissolution–recondensation process leading to a pore deformation and then, a pore clogging. For SBA15 silica having around 5 nm pores size and presenting microporosity, the alteration is driven by an equilibrium between the reactive diffusion of water and the dissolution of the altered silica wall. In order to confirm these evolutions and to relate such results to the behaviour of other natural materials (minerals, glass alteration layer. . .), supplementary experiments have to be performed incorporating other elements such as Al and Zr for example. Moreover, it would be interesting to determine the water activity in such nanoconfined media, and therefore assess the impact of the confinement on the interaction of water with pore walls. Acknowledgements The authors acknowledge H.P. Brau and X. Le Goff for their help on TEM operation and image processing. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso .2013.08.041. References [1] M. Gallinari, O. Ragueneau, D.J. DeMaster, H. Hartnett, D. Rickert, C. Thomas, Deep Sea Res. Part II 55 (2008) 2451–2464.

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