On the mechanical stability of mesoporous silica SBA-15

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Microporous and Mesoporous Materials 111 (2008) 134–142 www.elsevier.com/locate/micromeso

On the mechanical stability of mesoporous silica SBA-15 Svatopluk Chytil, Lise Haugland, Edd A. Blekkan

*

Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway Received 10 April 2007; received in revised form 5 July 2007; accepted 10 July 2007 Available online 19 July 2007

Abstract Mesoporous silica SBA-15 was synthesised at 80 C. The calcined solids were exposed to a unilateral external pressure in the range 16–191 MPa in order to monitor the impact of the mechanical pressure on the properties of SBA-15. N2 adsorption–desorption measurements, XRD and UV-Raman spectroscopy was used in order to evaluate the changes occurring in the SBA-15. For the XRD measurement, an internal Si standard was used to correct the position of the SBA-15 patterns. It appeared that the elevated pressure has no influence on the hexagonal cell parameter a. Through the N2 sorption measurements the fraction of the preserved mesoporous structure was estimated to be 60% when the highest pressure has been used. As the remaining part of the material is irreversibly disintegrated into small particles, the pressed sample is considered to be heterogeneous. However, the preserved fraction is slightly modified, showing a smaller pore width and plugs located within the mesopores. The plugs most likely originate from a disintegrated fraction of the SBA15. UV-Raman spectroscopy shows that the relative intensity of the band associated with the siliceous network (x1) has decreased on the pressed samples resulting in a less ordered material possessing an enhanced population of silanols as compared to parent SBA15. We propose that the disorder introduced by pressing is responsible for the observed expansion of the SBA-15 walls, which is detected for the samples treated at higher pressures (112, 191 MPa).  2007 Elsevier Inc. All rights reserved. Keywords: SBA-15; Mechanical stability; XRD; N2 sorption; UV-Raman spectroscopy

1. Introduction Since its discovery in 1998, the SBA-15 material is still receiving considerable attention as a catalyst support [1,2]. This is due to a relatively well defined geometry, involving ordered hexagonal arrays of the siliceous mesostructure with a tuneable pore width and a high surface area. In particular the pore width of SBA-15, which can be adjusted in range 5–30 nm depending on synthesis route, is of interest as it can accommodate bulky reactants and products in the course of a reaction [1,2]. A large variety of functionalization pathways in order to modify the SBA-15 with a desired active component is available, depending on the active species of the final catalyst as well as on the target reaction to be catalysed [3–10].

*

Corresponding author. Tel.: +47 73594157. E-mail address: [email protected] (E.A. Blekkan).

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.07.020

This makes the SBA-15 a very interesting material for academic research. However, to our best knowledge the SBA15 has not yet been applied commercially as a catalyst support, which could be related to some issues discussed below. From a commercial point of view it is important to use a catalyst carrier that can be prepared for a low cost by keeping both the price of the synthesis precursors as well as the investment made on unit operations needed to obtain a demanded product at as low level as possible. This is particularly true when the synthesis of SBA is considered, since the material is obtained at rather mild conditions (temperature, pressure). On the other hand the price of the precursors needed for synthesis of SBA-15 is quite high when compared to other commonly used supports like alumina, zeolites. In this context, it should be mentioned that the cost can be limited through a recovery of the surfactant template and replacement of quite expensive silica source by less costly silica compound [1,2,11,12]. Either strategy

S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142

can be applied still providing a material possessing a high degree of mesoscopic order [1,2,11,12]. Another issue that hinders the commercial use of the SBA-15 is its relatively low hydrothermal stability that is linked to a low degree of condensation of the SBA-15 framework. This can be improved by, e.g. incorporation of titanium ions into the mesoporous framework or alternatively by an increase of the temperature of the SBA-15 synthesis [13]. The latter is, however, limited upwards at temperature of 150 C where the surfactant template tends to decompose [1,13]. In this study, we focus on the mechanical stability of SBA-15. The importance of high pressure on the mesoscopic structure has been pointed out before [14–18]. Here, we describe the impact of external pressure where the mesoporous framework is still partly preserved, which by detailed evaluation of the experimental data (N2 sorption measurement, XRD analysis, and UV-Raman spectroscopy) allows us to draw a simple model of the influence of the external pressure on the mesoporous silica SBA-15. Moreover, we emphasize that such a combination of the experimental data, providing information on the longrange order as well as on the local environment of silicon atoms is presented for the first time. We report data from a starting pressure of 16 MPa, sufficiently high to obtain pellets/discs which can be further used either for spectroscopic measurements (FT-IR), or to prepare SBA-15 particles of various sizes as important for catalytic experiments. However, the range of pressures used here (16–191 MPa) seems to be relevant also in respect to industrial conditions for making pellets as the commonly applied pressure is in the range 10–400 MPa [19].

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pressure using a hand-operated press. The time of exposure was 2 min and the pressures used in this study were 16, 48, 112 and 191 MPa. The pressed materials were carefully ground in a mortar and characterized. 2.2. Instrumentation Nitrogen adsorption–desorption isotherms were measured using a Micromeritics TriStar 3000. Prior to the measurement, the samples were outgassed at 100 C for 12 h. Specific surface area of the solids was evaluated by the BET method (Brunauer–Emmett–Teller) in the range of relative pressures p/p0 = 0.1–0.3. The pore size distributions were obtained from the desorption branch of the isotherm using the Broekhoff–De Boer algorithms. The micropore volumes were assessed by means of a t-plot analysis, where Harkins and Jura’s expression was used to calculate the adsorbed layer thickness [20]. X-ray powder diffraction data was recorded on a Siemens D-5005 diffractometer using Cu Ka-radiation ˚ ). Internal reference Si (NBS standard, Refer(k = 1.542 A ence material No. 640) has been used to reduce the influence of the primary beam on the position of SBA-15 patterns. XRD profiles were collected in the range of 2h from 0.6 to 60, where the standard exhibits 2h reflections of 28.44, 47.30 and 56.12. The content of the internal standard was 20 wt%. UV-Raman spectroscopy measurements were performed on a Horiba Jobin Yvon, LABRAM HR 800 instrument using a laser excitation source of 325 nm. Exposure time of 30 s and accumulation number of 10 was used to obtain the spectra. The spectra were normalized with respect to the D1 band.

2. Experimental 3. Results and discussion All chemicals were used as received without a further purification. The triblock poly(ethylene glycole)20– poly(propylene glycole)70–poly(ethylene glycole)20 copolymer (EG20–PG70–EG20); (Mw 5800 g/mol) as well as HCl were purchased from Sigma–Aldrich. Tetraethoxysilane (TEOS) was purchased from Acro´s Chemicals. 2.1. Synthesis of SBA-15 The SBA-15 material was prepared according to the procedure reported earlier [5,6]. Briefly, a solution of triblock copolymer EG20–PG70–EG20 (4.12 g)/2 M HCl (120 g)/water (32.4 g) was stirred for 5 h at 60 C. 7.35 g of TEOS was added afterwards and the mixture was stirred overnight at 40 C. The molar ratios in the synthesis solution was EG20–PG70–EG20/HCl/H2O/TEOS = 1/330/ 2530/49. The solution was then heated in an oven at 80 C under static conditions for 24 h. The filtered and washed solids were dried at ambient temperature for 24 h, followed by calcination in air at 550 C for 6 h. The mechanical stability of as-calcined solids was examined by exposing 1.5 g of SBA-15 to unilateral external

3.1. N2 adsorption–desorption measurement Nitrogen adsorption–desorption isotherms of the examinated solids are depicted in Fig. 1. For the parent SBA-15 the isotherm shows the characteristic behaviour of the mesoporous materials exhibiting an irreversible type IV of isotherm with a H1 hysteresis loop [1]. The steep capillary condensation step observed as an inflection point at p/p0 of 0.66 indicates a narrow pore size distribution [1]. It can also be noted that such a hysteresis loop obtained from nitrogen sorption measurement performed at 77 K is observable for the solids possessing a pore width above approx. 4 nm [21]. Upon applying an external pressure on the parent SBA15, the changes in its texture are observed as the isotherms gathered on the pressed samples are altered and these alternations are becoming more prominent with increasing pressure applied. The sample pressed at the lowest pressure exhibits an isotherm almost identical to the parent SBA15, while the samples exposed to higher pressures show isotherms which depart from the isotherms of the unpressed

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S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142 500

2000

1800 400

volume adsorbed [cm /g]

1600

1400 SBA-15/16 MPa

3

volume adsorbed [cm /g]

open mesopores (M1)

3

SBA-15

1200

1000 SBA-15/48 MPa

300

partly blocked and blocked mesopores

200 M1 100

800

0

2

4

6

8

10

12

14

16

pore width [nm]

600 SBA-15/112 MPa 0 0.0

400

0.2

0.4

0.6

0.8

1.0

p/p0 SBA-15/191 MPa Fig. 2. Nitrogen adsorption–desorption isotherm of SBA-15 exposed to external pressure of 191 MPa. Corresponding pore size distribution as obtained from the desorption branch shown in the inset.

200

0 0.0

0.2

0.4

0.6

0.8

1.0

p/p0 Fig. 1. Nitrogen adsorption–desorption isotherms of SBA-15 and SBA-15 exposed to a hydraulic pressing (isotherms are shifted from each other by 300 g/cm 3 along the y-axis).

solid. However, all the pressed materials exhibit N2 adsorption–desorption behaviour which is in agreement with mesoporous ordering, both in the terms of the type of the isotherm as well as the type of the hysteresis loop which is still at least partly present. Interestingly, such a deformation of the hysteresis loop as observed for the pressed samples has already been recorded on the so-called plugged hexagonal templated silicas (PHTS) and has been ascribed to materials possessing both open and closed cylindrical pores [22,23]. These materials have been prepared by an identical route as required for synthesis of SBA-15, however, the ratio between the silica source (TEOS) and the structure directing agent (Pluronic P123 triblock copolymer) was here varied, resulting in mesoporous silica containing siliceous nanocapsules within the primary mesopores. This material has been shown to be more resistant towards severe hydrothermal treatments and mechanical pressures [22,23]. By following the adsorption behaviour of the sample pressed at 191 MPa it can be seen (Fig. 2) that the condensation in open and closed pores occurs at the same relative pressure of ca. 0.64, when the metastable adsorption film loses its stability [24].

The desorption branch was used to obtain a pore size distribution for the investigated solids. Here it should be emphasized that the pore width estimation of the SBA-15 is still an object of discussion and a number of different computational procedures has already been proposed to derive the mesopore size distribution from the N2 sorption measurement [25]. Apart from the methods modifying the Kelvin equation by incorporating factors such as shape of the pores, their length or curvature of the meniscus into corresponding expression for the thickness of adsorbed film, the non-local-density-functional-theory (NDLFT) is receiving considerable attention [24]. However, when this method is used to calculate the isotherms, a pronounced layering at pressures below the capillary condensation transition is found. This is an artefact caused by the use of simplified pore wall model [24]. This necessary leads to a detection of artificial mesopores whose size is below the main mesopore width. In this work we use the Broekhoff–De Boer algorithms in order to estimate the pore with [26]. This model is widely accepted for accurate pore width estimation of mesoporous solids [25,27]. As can be seen from Fig. 3, the pore size distribution obtained for the parent SBA-15 shows only the peak corresponding to the pore width of the main channels of the SBA-15. However, the situation is different for the pressed samples as the pore size distribution also displays the second peak corresponding to a lower pore width. The latter can be explained by analyzing the shape of the desorption branch of the isotherms. As already mentioned, the adsorption in both open and closed pores proceeds at the same relative pressure, while the

S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142

dV/dlog (w) [cm3.g-1.nm-1]

30

SBA-15

20 SBA-15/16 MPa

SBA-15/48 MPa

10

SBA-15/112 MPa

SBA-15/191 MPa 0 0

5

10

15

20

25

pore width [nm] Fig. 3. Pore size distribution of SBA-15 and SBA-15 exposed to a hydraulic pressing. For sake of clarity the curves are shifted along the yaxis.

desorption behaviour follows another mechanism. In the open pores, desorption occurs at equilibrium conditions via the receding of the meniscus [22]. This is observed as a high pressure desorption step. As the branch is quite tilted in this section, a rather broad pore size distribution is observed as indicated in Figs. 2 (M1) and 3 [26]. In the closed and partly closed pores the desorption is delayed, which is observed as a pronounced second desorption step [22]. In the limiting case, most likely in the case of blocked pores, the pressure is further delayed until the vapour pressure is reduced below the limit of stability of condensed nitrogen (p/p0  0.45, Fig. 2). These pressures depend weakly on the pore width and geometry [28], and thus the apparent transition from monomodal to bimodal distribution (peak for pore width of 3.6 nm) as detected for the pressed samples has to be interpreted with some caution. However, this inflection, already observed for the sample pressed at 16 MPa (Fig. 3), can be considered as a feature reflecting the blocking or partial blocking of the pores caused by a presence of particles within the pores. Clearly, the inflection follows a trend (Fig. 3), showing an increase with higher pressure, suggesting that the blocking is more prominent for the samples exposed to a higher pressure. It may be noted that in a recent study Konya et al., also detected a bimodal pore size distribution on the SBA-15 that has been exposed to pressures up to 10 MPa [18]. From the N2 sorption isotherms reported therein it is seen

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that the impact of the pressures on the texture of SBA-15 is considerably more detrimental when compared to the data presented here. Furthermore, in the work of Konya et al., the adsorption branch of the N2 sorption isotherm has been used to calculate the pore size distribution [18]. As seen in Fig. 1, the adsorption branch of the N2 isotherms recorded on all the examined samples exhibits only one inflection point corresponding to N2 sorption in the mesopores at relative pressures p/p0 close to 0.6. This indicates that the formation of a true bimodal pore distribution may be excluded. Furthermore, it may be noted that Hartmann and Vinu have employed N2 sorption measurements (using the adsorption branch) as well as mercury porosimetry to evaluate the porosity of SBA-15 upon the pressing [16]. Both experimental techniques showed the same overall trend, a minor decrease in pore width of SBA-15 and a monomodal pore size distribution after its exposure to elevated pressures (52–260 MPa), which is in agreement with data presented here [16]. At relative pressures approaching p/p0 of 1.0, an increase in the N2 adsorption is observed, which is not recorded on the parent SBA-15 (Fig. 1). This might be due to a change in intergranular porosity as suggested by other authors, implying that the interparticle distances are reduced [15,16]. During the exposure of highly porous material such as SBA-15 to a unilateral pressure the pressing force is more elevated at the spots where the primary particles are contacting each other. In this part of the sample a partial disintegration of the material into much smaller particles can be expected, while the remaining part of the sample remains relatively undisturbed. This partial disintegration is most likely an irreversible process and results in a more heterogeneous SBA-15 material formed upon the pressing. The partial breakdown of the SBA-15 structure into the very fine particles will raise the contact area between the particles and might be therefore responsible for the observed increase of the N2 adsorption at relative pressures approaching unity. It should also be pointed out, however, that the preserved part of SBA-15 is modified due to blocking of the mesopores that occurs during the pressing. As discussed above, a part of SBA-15 disintegrates into very fine particles, and it is reasonable to expect that some of the fine particles can enclose the pore mouths of the mesopores or form plugs in the pores of the preserved SBA-15. 3.2. XRD analysis As mentioned, an internal Si standard was used in order to adjust the position of the SBA-15’s XRD patterns. The corrections were performed by shifting the XRD patterns of examinated solids (each containing 20 wt% of Si standard) to 2h reflections of the internal standard. Fig. 4 represents corrected patterns of the materials including the XRD profile of the standard. Patterns for low scattering angles are shown in Fig. 5. The XRD pattern of the parent SBA-15 shows the characteristics of a two-dimensional

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S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142

Si standard

SBA-15 SBA-15/16 MPa

SBA-15/48 MPa

SBA-15/112 MPa SBA-15/191 MPa

0

10

20

30

40

50

60

2-theta Fig. 4. XRD diffraction patterns of SBA-15 and SBA-15 exposed to a hydraulic pressing. Samples were mixed with 20 wt% of internal Si standard. XRD spectrum of the standard is also included.

[100 ]

[11

0]

[20

0]

SBA-15

SBA-15/16 MPa

SBA-15/48 MPa

SBA-15/112 MPa

SBA-15/191 MPa

0

1

2

3

4

5

2 - theta Fig. 5. Low angle XRD patterns of SBA-15 and SBA-15 exposed to a hydraulic pressing.

hexagonal structure (p6mm), where each pore is further surrounded by six pores. The calculated hexagonal cell parameter a is shown in Table 1. The corresponding wall thickness (t) calculated as: a – pore width (dBdB) is 5.1 nm for the parent SBA-15. As shown in Fig. 5 the application of external pressure leads to a decrease of the prominent reflection [1 0 0] as well

as the [1 1 0] and [2 0 0] reflections. This is in agreement with some other studies dealing with the examination of the influence of the external pressures on SBA-15 [15,16,18]. However, the SBA-15 patterns are still detectable even for the material exposed to the highest pressure, confirming that the mesoporous structure is at least partly preserved. The intensity decrease indicates a loss of long range order, which is not surprising as discussed in the previous section. Part of the material is disintegrated and thus cannot contribute to the characteristic pattern. Apart from this, it should be considered that a decrease of the reflections due to the siliceous plugs, particles originating from a partial collapse, can also occur. This effect could be related to a change in electron density within the mesopore interior. Here it should be noted that SBA-15 cannot be regarded as an ideal hexagonal lattice of pores imbedded in a uniform matrix, as the structure of the silica walls is more complex. This can be easily assessed by calculating the dS/V number, where d is pore width, S is specific surface area and V is pore volume. For a material possessing a simple hexagonal geometry this number should be equal to 4 [29]. By using the structural parameters displayed in Table 1. the value of 6.5 is obtained, which is rather far from the theoretical value. This observation agrees well with literature and the reason why the SBA-15 exhibits such behaviour has already been the subject of several studies, see e.g. [29–31]. In this work, we mention this deviation due to other reasons; the dS/V number approaches its theoretical value 4 as the reaction temperature for the synthesis of SBA-15 increases [32]. Additionally, it is believed that the SBA-15 prepared at higher reaction temperature possess more condensed silica walls [13]. However, the mechanical stability of mesoporous silicas has been proposed to be linked to a ratio of wall thickness/ unit cell parameter [15,16]. For other mesoporous silicas, e.g. MCM-41, this ratio is higher as compared to SBA-15 and therefore MCM-41 shows a higher mechanical stability [16]. This ratio is becoming smaller when the reaction temperature for SBA-15 synthesis increases [32] which in turn leads to assumption that SBA-15 prepared at higher temperature would have a lower mechanical stability. 3.3. UV-Raman spectroscopy Representative spectra recorded on calcined SBA-15 as well as on pressed SBA-15 are shown in Fig. 6. The band assignment as shown in Table 2 was done according to a review by Humbert [33] and agrees with the assignment of corresponding bands detected on mesoporous silicas MCM-41 and SBA-15 [14,34]. However, a certain refinement of the D1 band assignment was done according the review by Humbert and references therein [33]. The spectrum of SBA-15 within the range 300– 1050 cm 1 shows the characteristic features of siliceous materials with the dominant Raman lines at 461 and 820 cm 1. These lines are result of inelastic scattering of light by bulk SBA-15. The band x1 at 461 is associated

S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142

139

Table 1 Physico-chemical properties of SBA-15 materials as obtained from N2 adsorption–desorption measurement and low angle XRD analysis Sample

aa (nm)

Surface area (m2/g)

Pore widthb dBdB (nm)

Micropore volumec (cm3/g)

Mesopore volume (cm3/g)

SBA-15 SBA-15/16 MPa SBA-15/48 MPa SBA-15/112 MPa SBA-15/191 MPa

10.6 10.6 10.6 10.6 10.6

920 ± 9 880 ± 11 826 ± 21 758 ± 18 694 ± 7

5.5 5.5/3.6d 5.5/3.6d 5.3/3.6d 5.2/3.5d

0.095 0.096 0.094 0.094 0.092

0.776 0.768 0.684 0.587 0.467

a b c d

Hexagonal cell parameter, a = 2 · 3 1/2 d100 , where d100 is interplanar spacing of primary XRD reflection. From Broekhoff–de Boer algorithm applied on the desorption branch of isotherm. t-plot analysis. Bimodal pore width (see text for explanation). Table 2 UV-Raman bands assignment for SBA-15 Symbol

Band position (cm 1)

Band assignment

x1

461

D1

501

D2

620

x2 x3 SiOH

820 820 933 993

Vibration mode of [SiO4] tetrahedra in the network with four silicon atom neighbours Vibration mode of [SiO4] tetrahedra with one oxygen atom not bonded to another silicon atom Three-membered [SiO3]3 ring located at the surface or in the network Transverse optical (TO) componenta Longitudinal optical (LO) componenta Stretching vibration of silanols Stretching vibration of silanols

a

Fig. 6. UV-Raman spectra of parent SBA-15 and SBA-15 exposed to unilateral external pressure.

with the vibration mode of [SiO4] tetrahedra in network with four silicon atom neighbours, certainly contained in five-, six- or seven-membered rings. In other words, this component can be considered as a building block of the SBA-15 framework. The broad band at around 820 cm 1 is the overlapping x2 and x3 attributed to siloxane linkages (Si–O–Si). The lines arising from vibrations of the siliceous network are rather broad as the Si–O–Si (T–O–T) bond angles in the network are widely distributed [35]. The latter is consistent with 29Si MAS NMR measurements performed on mesoporous silicas, which also indicate a lack of precise

Not resolved.

repeats of Si positions at the second nearest neighbour (T–T) scale [36]. The line D1 is assigned to a vibration mode of [SiO4] tetrahedra, similar to the x1 network mode at 461 cm 1. However, the D1 component contains one non-bridging oxygen as a result of the ring opening [37]. The line D2 detected at 620 is attributed to three membered ring [SiO3]3. This component has probably been formed by a condensation of weakly H-bonded silanols at the surface [37] and is usually observed in higher surface area materials [14]. Finally, the bands assigned to silanols (O–H Raman lines) are detected at 933 and 993 cm 1. Some authors assigned the band at 933 to geminal silanols @Si(OH)2 [38]. Here it should be mentioned that spectra were obtained on pure SBA-15 materials without use of any internal standard, therefore to determine the absolute intensity of each component is not possible. However, the presented spectral profiles show changes in relative intensity between each component or group of components, which can be used to explain observed trends. A further inspection of Fig. 6 reveals that the relative intensity of the band associated with the siliceous network (x1) decrease on the pressed samples. This observation itself illustrates the deformation of SBA-15 framework upon a high pressure treatment, indicating that the framework is becoming less ordered as a result of the external pressure treatment. However, more information can be obtained based on the data presented in Fig. 6. The D1 band is narrower upon

S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142

applying the pressure, which is related to some strain relaxation [37]. The intensity of the D2 band follows two different regimes. The band becomes more intense at low applied pressure (16 MPa). This observation together with a decrease of the x1 for the latter sample indicates that the influence of such pressure on the structure of SBA-15 is similar to that as observed by others for neutron irradiation treatment applied on non-crystalline SiO2 [39]. For a higher applied pressure (>100 MPa), however, a decrease of the D2 is observed. This is in line with others, who have detected a decrease of the D2 as a result of high pressure treatment on SBA-15 [14]. In the spectral region around 990 nm an increase of the silanol population is observed. This is in agreement with observations made by others using either UV-Raman spectroscopy or gravimetrical H2O adsorption [14,40]. Thus, based on the previous observations we propose that the mechanism for SBA-15 disintegration upon external pressure treatment is mostly associated with a transformation of the [SiO4] tetrahedra contained in the ring network to its D1 modification. This probably proceeds through a cleavage of the Si–O–Si bond that is assumed to be distorted by pressing [40]. The cleavage is also necessary for a rupture of the three membered ring [SiO3]3 at the SBA-15 surface. It is likely to expect that the water content or more precisely the activity of H+ and OH ions that increases with pressing [40] is involved in the cleavage of Si–O–Si bond resulting in enhancement of silanol population. It may be noted that the Raman spectroscopy has also been used to monitor the structural changes occurring on amorphous SiO2 that was exposed to very high pressures [41]. It was found that up to 8 GPa the changes in the spectrum are reversible, while pressures above 8 GPa resulted in enhancement in the region of D1 and D2 bands [41]. This indicates that the mechanism of the distortion of the amorphous SiO2 upon the pressing might be partly different from that as observed by UV-Raman spectroscopy for SBA-15. This is plausible taking into account the onset of disintegration (observed by N2 sorption and XRD) at much lower pressures observed for SBA-15. However, a direct comparison of the influence of external pressures on the texture of SBA-15 and amorphous SiO2 calls for a study where the pressures applied as well as the set of characterization techniques would be identical. 3.4. Schematic model of the impact of external pressure on the SBA-15 A schematic picture of SBA-15 prepared at 80 C is depicted in Fig. 8. As already mentioned the material cannot be considered as a material possessing an ideal hexagonal arrangement which is, as proposed by others, shown as a corona surrounding the pores [30,31]. This corona contains a certain fraction of micropores which probably arises from the partial occlusion of the PEG chains in the silica matrix [30,31].

When the SBA-15 is exposed to an external pressure a certain part of its mesostructure collapses, while the remaining part is preserved. Therefore, the pressed material can be considered as heterogeneous. However, some modifications are also observed for the preserved SBA-15. These modifications are basically related to the blocking/ partial blocking of pores, which is proposed to be due to presence of the disintegrated part of SBA-15 within the channels of the remaining SBA-15. Moreover, a slight decrease in pore width (dBdB) is observed for SBA-15 pressed at 112 and 191 MPa as a consequence of a shift of the desorption branch of the N2 isotherm towards a lower p/p0. To assess the amount of the preserved part of SBA-15, we propose the plot depicted in Fig. 7 showing the evolution of the relative mesopore volume and relative surface area as a function of applied pressure. Clearly, the relative mesopore volume decreases more significantly as compared to the surface area. This discrepancy might arise from the possibility that the decrease of mesopore volume is associated with their collapse as well as plugging of the pores. Therefore, we assume that the extent of SBA-15 disintegration upon high pressure treatment is primarily associated with a loss of surface area, while for the decrease in mesopore volume a possibility that certain decrease is caused by the filling of the pores should be considered. Subsequently, the difference between relative surface area and relative pore volume can be used to assess the amount of blocked/partially blocked pores. When doing so, e.g. for SBA-15 pressed at 191 MP, we obtain a value of 25% corresponding to destroyed SBA-15, 15% for SBA-15 containing blocked and partly blocked pores and the remaining 60% of SBA-15 is preserved. These results indicate that the majority of the SBA-15 is preserved. However, this estimation is based only on the N2 sorption measurement and since the evaluation of such a measurement is based on simplified models [42], possibility that the impact of the external pressure on mesoporous silica SBA-15 is in fact relative pore volume and surface area

140

1.0 0.9 SBA-15 destroyed 0.8 SBA-15 preserved SBA-15 with blocked/ partly blocked pores

0.7 0.6

V/V0 S/S0

0.5 0

50

100

150

200

pressure applied [MPa] Fig. 7. Influence of the external pressure on relative surface area and pore volume of SBA-15; blocked pore assessment from N2 sorption. S0, S surface area of unpressed and pressed SBA-15; V0, V pore volume of unpressed and pressed SBA-15, respectively.

S. Chytil et al. / Microporous and Mesoporous Materials 111 (2008) 134–142

141

Fig. 8. Suggested schematic model of the external pressure impact on the structural properties of SBA-15 as derived from the N2 sorption measurement, XRD data and UV-Raman spectroscopy (see text for explanation).

more severe should not be excluded. The latter can be illustrated by findings published by others as it has been reported that the MCM-41 material when functionalized with Pt and subsequently exposed to pressure of 160 MPa shows a decrease in n-hexadecane conversion of 60% as compared to an unpressed sample [15]. The micropore volume is rather constant over the range of experimental conditions. This agrees with observations made by others as it has been shown that through exposing mesoporous silica FSM-16 to an elevated pressure, no additional micropores were formed [40]. This might also suggest that the plugs within the primary SBA-15 channels are not located in the micropores, eventually that the size of the plugs exceeds the micropore range (2 nm). The hexagonal cell parameter a also remains constant over the pressure used in our study. Here, it should be remembered that the conventional XRD diffractometer used in this study is not capable of reliably determining the position of peaks at 2h of about 1 or lower [43]. However, under such circumstances, use of the [2 0 0] reflection has been recommended in order to calculate the cell parameter using an equation a = 4 · 3 1/2 d200, where d200 is interplanar spacing of [2 0 0] reflection [43]. Nevertheless, also these calculations did not show any change of the hexagonal cell parameter upon applying an external pressure on the parent SBA-15 since the position of the [2 0 0] reflection is not altered. As already mentioned, the combination of XRD and N2 sorption data allows the wall thickness (t) of examinated solids to be estimated (t = a dBdB). By using the structural parameters listed in Table 1 the identical value of 5.1 nm for parent SBA-15 as well as for the materials exposed to lower external pressures (16, 48 MPa) is obtained. However, for the SBA-15 pressed at 112 and

191 MPa values of 5.3 and 5.4 nm are obtained respectively. This indicates that the walls of the SBA-15 tend to expand as a result of the high pressure treatment. Therefore, we propose a schematic model for the impact of external pressure when applied on mesoporous silica SBA-15 which is depicted in Fig. 8. This model includes the textural parameters obtained from the XRD and N2 sorption measurement. However, the model gives us a possibility to implement the findings obtained from the UV-Raman measurement, since it was observed that the over-all result of the pressure treatment on SBA-15 is its less ordered structure. Moreover, it was shown that the Raman D2 band associated with the surface of SBA-15 decreases for the samples exposed to 112 and 191 MPa, which appears to be correlated with the corresponding expansion of the wall thickness for those two materials. However, it should be kept on mind that the depicted part of the pressed sample represents only the part of SBA-15 that is preserved. This part was estimated through the N2 sorption measurements to be 60% for SBA-15 exposed to 191 MPa. In this paper, we have examined the effect of pressure on fine powder of SBA-15. If the use of SBA-15 in a technical application is considered, a further issue is important, namely the examination of the mechanical and side-crushing strength of shaped and pressed SBA-15 bodies (including any binder used), as it could bring relevant information concerning the behaviour of SBA-15 close to the real conditions in a reactor. 4. Summary and conclusions It has been shown that the N2 nitrogen adsorption– desorption measurements when coupled with XRD diffrac-

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tion provide a powerful tool with which to characterize structural changes occurring on mesoporous silica. In this particular study, we have used these two fundamental techniques in order to describe the impact of external pressure treatment on the mesoporous silica SBA-15. Based on our results we conclude that in order to obtain complete information from N2 sorption measurement the desorption branch of the isotherm should be used. For the XRD measurement performed using a conventional diffractometer it has been demonstrated that the use of internal standard to evaluate cell parameter a is of great use. Information provided by the UV-Raman spectroscopy brought new insights into the structural changes occurring on the SBA-15 upon its exposure to elevated pressures. The data strongly suggests that the mechanism of SBA-15 damage upon high pressure treatment is associated with an opening of rings containing the [SiO4] units. It is proposed that this proceeds through a cleavage of the Si–O–Si bond originally providing the linkage between the [SiO4] units. This process is accompanied by a rupture of the three membered siloxane rings at the surface of SBA-15. However, the latter is observed only when higher pressures (112, 191 MPa) are applied. Acknowledgments The Norwegian Research Council is gratefully acknowledged for financial support through the Strategic University Programme Scientific Design and Preparation of New Catalysts and Supports, Contract No. 153967/420. We thank Elin Nilsen and Santhosh Kumar Matam for a help with XRD and UV-Raman measurements respectively. Wilhelm R. Glomm is acknowledged for his valuable comments on the manuscript. References [1] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [2] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [3] S. Chytil, W.R. Glomm, E. Vollebekk, H. Bergem, J. Walmsley, J. Sjoeblom, E.A. Blekkan, Micropor Mesopor Mater 86 (2005) 198. [4] A. Taguchi, F. Schueth, Micropor Mesopor Mater 77 (2004) 1. [5] S. Chytil, W.R. Glomm, I. Kvande, Z. Tiejun, E.A. Blekkan, Stud. Surf. Sci. Catal. 162 (2006) 513. [6] S. Chytil, W.R. Glomm, I. Kvande, Z. Tiejun, J.C. Walmsley, E.A. Blekkan, Top. Catal. 45 (2007) 93. [7] H. Song, R.M. Rioux, J.D. Hoefelmeyer, R. Komor, K. Niesz, M. Grass, P. Yang, G.A. Somorjai, J. Am. Chem. Soc. 128 (2006) 3027. [8] D.J. Kim, B.C. Dunn, F. Huggins, G.P. Huffman, M. Kang, J.E. Yie, E.M. Eyring, Energy Fuels 20 (2006) 2608. [9] M. Selvaraj, T.G. Lee, J. Phys. Chem. B 110 (2006) 21793.

[10] Sujandi, S.-C. Han, D.-S. Han, M.-J. Jin, S.-E. Park, J. Catal. 243 (2006) 410. [11] R. Van Grieken, G. Calleja, G.D. Stucky, J.A. Melero, R.A. Garcia, J. Iglesias, Langmuir 19 (2003) 3966. [12] P.F. Fulvio, S. Pikus, M. Jaroniec, J. Coll. Interf. Sci. 287 (2005) 717. [13] F.-S. Xiao, Top. Catal. 35 (2005) 9. [14] Y. Borodko, J.W.I. Ager, G.E. Marti, H. Song, K. Niesz, G.A. Somorjai, J. Phys. Chem.B 109 (2005) 17386. [15] A. Galarneau, D. Desplantier-Giscard, F. Di Renzo, F. Fajula, Catal. Today 68 (2001) 191. [16] M. Hartmann, A. Vinu, Langmuir 18 (2002) 8010. [17] K. Cassiers, T. Linssen, M. Mathieu, M. Benjelloun, K. Schrijnemakers, P. Van Der Voort, P. Cool, E.F. Vansant, Chem. Mater. 14 (2002) 2317. [18] Z. Konya, E. Molnar, G. Tasi, K. Niesz, G.A. Somorjai, I. Kiricsi, Catal. Lett. 113 (2007) 19. [19] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, New Jersey, 2005. [20] G. Jura, W.D. Harkins, J. Chem. Phys. 11 (1943) 430. [21] C.G. Sonwane, P.J. Ludovice, J. Mol. Catal. [A] Chem. 238 (2005) 135. [22] P. Van Der Voort, P.I. Ravikovitch, K.P. De Jong, M. Benjelloun, E. Van Bavel, A.H. Janssen, A.V. Neimark, B.M. Weckhuysen, E.F. Vansant, J. Phys. Chem. B 106 (2002) 5873. [23] E. Van Bavel, P. Cool, K. Aerts, E.F. Vansant, J. Phys. Chem. B 108 (2004) 5263. [24] P.I. Ravikovitch, A.V. Neimark, J. Phys. Chem. B 105 (2001) 6817. [25] K. Morishige, M. Tateishi, Langmuir 22 (2006) 4165. [26] J.C.P. Broekhoff, J.H. De Boer, J. Catal. 10 (1968) 377. [27] A. Galarneau, H. Cambon, F. Di Renzo, F. Fajula, Langmuir 17 (2001) 8328. [28] P.I. Ravikovitch, A.V. Neimark, Langmuir 18 (2002) 1550. [29] R. Ryoo, C.H. Ko, M. Kruk, V. Antochshuk, M. Jaroniec, J. Phys. Chem. B 104 (2000) 11465. [30] M. Imperor-Clerc, P. Davidson, A. Davidson, J. Am. Chem. Soc. 122 (2000) 11925. [31] A. Galarneau, H. Cambon, F. Di Renzo, R. Ryoo, M. Choi, F. Fajula, New J. Chem. 27 (2003) 73. [32] A. Sayari, B.-H. Han, Y. Yang, J. Am. Chem. Soc. 126 (2004) 14348. [33] B. Humbert, J. Non-Crystal. Solids 191 (1995) 29. [34] Z. Luan, P.A. Meloni, R.S. Czernuszewicz, L. Kevan, J. Phys. Chem. B 101 (1997) 9046. [35] A.E. Geissberger, F.L. Galeener, Phys. Rev. B Condens. Matter. 28 (1983) 3266. [36] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, et al., J. Am. Chem. Soc. 114 (1992) 10834. [37] B. Humbert, A. Burneau, J.P. Gallas, J.C. Lavalley, J. Non-Crystal. Solids 143 (1992) 75. [38] D.M. Krol, J.G. Van Lierop, J. Non-Crystal. Solids 68 (1984) 163. [39] J.B. Bates, R.W. Hendricks, L.B. Shaffer, J. Chem. Phys. 61 (1974) 4163. [40] T. Ishikawa, M. Matsuda, A. Yasukawa, K. Kandori, S. Inagaki, T. Fukushima, S. Kondo, J. Chem. Soc. Faraday Trans. 92 (1996) 1985. [41] K. Awazu, H. Kawazoe, J. Appl. Phys. 94 (2003) 6243. [42] K.S.W. Sing, J. Rouquerol, Characterization of Solid Catalysts, in: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, VCH, Weinheim, 1997, pp. 427–439. [43] M. Kruk, M. Jaroniec, C.H. Ko, R. Ryoo, Chem. Mater. 12 (2000) 1961.