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Mechanical properties of mesoporous silicas and alumina–silicas MCM-41 and SBA-15 studied by N2 adsorption and 129Xe NMR
Microporous and Mesoporous Materials 44±45 (2001) 775±784
www.elsevier.nl/locate/micromeso
Mechanical properties of mesoporous silicas and alumina±silicas MCM-41 and SBA-15 studied by N2 adsorption and 129Xe NMR M.-A. Springuel-Huet a,*, J.-L. Bonardet a, A. Gedeon a, Y. Yue a, V.N. Romannikov b, J. Fraissard a a
Laboratoire de Chimie des Surfaces, CNRS-S.I.E.N., Universit e P. et M. Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France b Boreskov Institute of Catalysis, Pr Akad., Lavrentieva 5, Novossibirsk 630090, Russian Federation Received 24 February 2000; accepted 29 March 2000
Abstract Purely siliceous and Al-containing MCM-41 and SBA-15 materials have been prepared with pore diameters of 2.4± 3.0 nm for MCM-41 and 6.5±7.5 nm for SBA-15. The eect of compression on the mesopore structure has been studied by N2 adsorption±desorption, X-ray diraction and 129 Xe NMR experiments. The pore size distributions obtained from N2 adsorption data show that the mesopore structure is partially destroyed and that small size pores are formed at the expense of the original ones (in the loose powder) when the powder is compressed at high pressure (up to 520 MPa). The 129 Xe NMR spectra are interpreted in terms of an exchange between adsorbed Xe (in the channels) and gaseous Xe atoms (in the interparticle spaces), which depends on the particle size and the compaction of the powder. For both MCM-41 and SBA-15 materials, the Al-containing samples are more fragile than the purely siliceous materials. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous aluminosilicate; Mechanical stability; N2 adsorption;
1. Introduction 129
Xe NMR of adsorbed xenon has proved to be a valuable tool for studying a lot of physicochemical properties of zeolites (pore size, presence of structural defects, localization and distribution of cations, metallic particles, coke or any coadsorbed phase etc.) [1]. The extension of this tech-
*
Corresponding author.
129
Xe NMR
nique to the study of organized mesoporous silicas or alumina±silicas is very tempting. In contrast to zeolites, the ®rst results [2±5] showed that the chemical shift is quasi-independent of the Xe pressure and that a correlation between the chemical shift and the pore size is not evident, noticeable chemical shift dierences are observed for silicas with the same pore size for example. This can be attributed to a fast exchange between adsorbed Xe atoms and gaseous Xe atoms in the interparticle spaces which governs the chemical shift of the coalescence signal. To reduce the Xe
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M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784
exchange between the pores and the interparticle spaces, we compressed the powdered sample or synthesize material with lower external surface (larger particles).
2. Experimental Purely siliceous MCM-41 (denoted S1 and S2) and SBA-15 (denoted SBA), and aluminum-containing (denoted A1, A2 and AlSBA) mesoporous mesophase materials, were prepared according to previous published work [6±8]. Samples were outgassed at 573 K under vacuum (1 Pa) overnight before xenon adsorption and NMR experiments or during a few hours before N2 adsorption at 77 K. The powdered samples were compressed with various pressures ranging from 0 to 520 MPa. The N2 adsorption±desorption isotherms were measured using an ASAP 2010 Micromeretics apparatus. 129 Xe NMR spectra were recorded with a Bruker MSL 400 spectrometer at room temperature, at the resonance frequency of 110.668 MHz using a p=2 r.f. pulse and repetition time of 1±3 s. Chemical shifts are referenced to that of gaseous xenon.
3. Results 3.1. MCM-41 materials 3.1.1. N2 adsorption±desorption The N2 adsorption±desorption isotherms for all samples show a hysteresis loop, very small for S2 but much larger for the Al-containing samples, A1 and A2, in the pressure range 0:45 < P =P0 < 1, which decrease when the pressure of compression increases (Figs. 1A±4A). For all the samples, the BET surface area and the total pore volume decrease when the compaction increases (Table 1) and the pore size distribution, analyzed by the ``density functional theory'' (DFT) method, shows that small pores are formed at the expense of the original ones (Figs. 1B±4B). For the powder samples the average pore size is between 2.4 and 3.0 nm (Table 1). The external surface areas (63,
Fig. 1. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample S1 not compressed (a); compressed at 52 MPa (b); 130 MPa (c); 260 MPa (d); 520 MPa (e).
44, 112 and 237 m2 g 1 for S1, S2, A1 and A2 samples respectively) have been determined from the as -plots according to the method described in Ref. [6].
M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784 Table 1 N2 adsorption±desorption and
129
777
Xe NMR data of the investigated samples
Sample compressed at various pressures in MPa
(BET) Surface areaa (m2 g 1 )
Pore volumeb (cm3 STP g 1 )
Mean pore size (nm)
Chemical shift (ppm)
S1
0 52 130 260 520
1005 975 838 800 555
0.90 0.80 0.72 0.67 0.39
2.9c 2.9c 2.9; 1.7c 2.9; 1.3c 2.9; <1.0c
54 69.5 76.6 86.3; 81.7 91
S2
0 130
1093 879
0.96 0.91
3.0c 3.0c
77; 36 75
A1
0 130 520
1056 904 541
0.82 0.61 0.32
2.4c 2.4; 1.3c 2.4; <1.0c
105; 87; 15 102; 82 92
A2
0 130 520
970 931 411
0.92 0.79 0.26
2.5c 2.5; 1.6c 2.5; 1.2c
72; 60 ± 93
SBA
0 80 260
804 641 621
1.11 0.81 0.69
6.5d 5.5d 5.5; 4.0d
55 ± 90
AlSBA
0 80 260
1004 701 710
1.53 1.07 0.67
7.5d 7.0d No mean pore size
60; 45 ± 93
a
Using 0.162 nm for the N2 molecular cross section. Total pore volume at P =P0 0:97. c Using the ``density functional theory'' method for cylindrical pores assuming Harkins and Jura model. d From the desorption curve, using the BJH method. b
3.1.2. 129 Xe NMR The 129 Xe NMR spectra are practically independent of the Xe pressure for all the samples. Sample S1: The 129 Xe NMR spectrum of the powder sample has one broad signal at about 54 ppm. For compressed samples a ``gas phase line'' (0 ppm) appears and the chemical shift of adsorbed xenon increases with the pressure of compression, from 69.5 to 91 ppm when pressure increases from 52 to 520 MPa (Table 1) while the line width ®rst decreases until pressure of 130 MPa. For 260 MPa the signal splits into two lines (86.3 and 81.7 ppm) as well as the gas phase signal. At 520 MPa there is a single ``adsorbed xenon'' line at 91 ppm (Fig. 1C-e). Sample S2: The 129 Xe NMR spectrum of the powder sample has two signals; one at 77 ppm and the second, very broad at about 36 ppm (Fig. 2C-
b). When the powder is compressed under 130 MPa, only the signal at 77 ppm remains and a gas phase signal (0 ppm) is detectable (Fig. 2C-a). This sample has a lower external surface area (44 m2 s 1 ) than that of S1 (63 m2 s 1 ), determined from N2 adsorption experiment. Sample A1: The 129 Xe NMR spectrum of the powder shows three ``adsorbed Xe'' lines at 105, 87 and 18 ppm and a gas phase signal at 0 ppm (Fig. 3C). When the powder is compressed at 130 MPa, the two high-shifted signals remain, but their relative intensity changes, while the broad line at 18 ppm disappears. For large compression pressure (520 MPa) there is only one asymmetric line around 92 ppm. The X-ray diraction (XRD) pattern of this Al-containing material (loose powder) has a very narrow full-width-at-half-maximum (FWHM), 0.05° (2h°, synchrotron radiation), of the
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Fig. 2. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample S2 not compressed (a); compressed at 130 MPa (b).
[1 0 0] diraction peak (not shown). This suggests that the Al is homogeneously distributed all over the network. Sample A2: The 129 Xe NMR spectra show a single broad ``adsorbed'' line, at about 72 ppm and a gas phase line (0 ppm) (Fig. 4C-a). For a powder
Fig. 3. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample A1 not compressed (a); compressed at 130 MPa (b); 520 MPa (c).
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compression of 520 MPa, the line is narrower and shifted to 93 ppm (Fig. 4C-c). For the loose powder, the FWHM of the [1 0 0] diraction peak is 0.14° (2°) (not shown), broader than that of sample A1, suggesting a heterogeneous Al distribution. 3.2. SBA-15 materials Sample SBA: For non-compressed and compressed samples N2 adsorption±desorption isotherms have the same shape with a hysteresis loop in the range 0:6 < P =P0 < 0:8 (Fig. 5A).
Fig. 4. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample A2 not compressed (a); compressed at 130 MPa (b); 520 MPa (c).
Fig. 5. N2 adsorption±desorption isotherms at 77 K (A) and pore size distribution (B) of sample SBA: not compressed (a); compressed at 80 MPa (b); 160 MPa (c); 260 MPa (d).
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Compression under 80 MPa leads to a decrease in the BET surface area of 160 m2 g 1 ; under higher pressure, (up to 260 MPa) the compression does not aect the surface area but the adsorbed volume at saturation (P =P0 0:97) monotically decreases. The pore size distribution curves (Fig. 5B) obtained with the BJH method [9] show a slight decrease in the average pore diameter (initially 6.5 nm for the powder) which is stabilized around 5.5 nm with the appearance of a secondary porosity centered at 4 nm which increases with the compression pressure. For the powder sample, 129 Xe NMR spectra show a broad line at 55 ppm. After compression this line is strongly down®eld shifted and narrowed. Conner et al. observed similar results for compressed amorphous silica [2]. Nevertheless it can be noticed that the chemical shift slightly depends on the average pore size, increasing when the mean diameter of the synthesized samples decreases (90, 97 and 103 ppm for pore diameter 6.5, 5.5 and 4.5 nm respectively). Sample AlSBA: For samples containing Al atoms, N2 adsorption±desorption isotherms show variations much more important than for pure silica SBA (Fig. 6A). Besides a large decrease (300 m2 g 1 ) in the BET surface area after compression under 80 MPa (Table 1) the shape of the hysteresis loop changes with the pressure to which the sample is submitted indicating an important change in the distribution of the pore size. Analysis of this latter by BJH method (Fig. 6B) shows that powder sample presents homogeneous pores with an average diameter around 7.5 nm as previously described (Fig. 6B-a) [8]. For compressed samples the size and the relative amount of primary pores slightly decrease (as observed with pure SBA) but we can observe the appearance of a secondary porosity with pore diameters lower than 6 nm (Fig. 6B-c). For sample compressed at 260 MPa, a very broad distribution is observed (Fig. 6B-d) indicating the loss of the organized structure of the material. For the powder sample, the 129 Xe NMR spectrum is similar to that of sample S2, showing two signals partially superimposed; a broad one, at about 45 ppm, the other narrower at 60 ppm. After compression spectra present only one line, down®eld shifted at 93 ppm. Nevertheless and in
Fig. 6. N2 adsorption±desorption isotherms at 77 K (A) and pore size distribution (B) of sample AlSBA: not compressed (a); compressed at 80 MPa (b); 160 MPa (c); 260 MPa (d).
contrary to pure SBA samples the signal of xenon gas at 0 ppm is always observed even for loose powder sample. 4. Discussion 4.1. MCM-41 materials The dramatic change of the spectrum when sample S1 and S2 are compressed or not can be
M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784
explained by a rapid exchange between adsorbed and gaseous (in interparticle space) xenon leading to a single broad coalescence line when the sample is not compressed (loose powder). The compression decreases the interparticle space volume and two signals corresponding to the two types of xenon are observable when the powder is compressed. Similar results have been obtained by Ripmeester and Ratclie for Xe adsorbed in Vycor glass (4 nm pore size) for various particle sizes [10]. For high pressure of compression, the adsorbed Xe as well as the gas phase signal is double for sample S1. This can be due to inhomogeneity of compaction. The compression creates pores in which the Xe±Xe interactions can be suciently larger, at the same pressure, than those existing in a non-con®ned gas phase to give a slightly higher chemical shift. It is well known that the chemical shift of Xe gas increases with pressure [11]. For 520 MPa the compaction inhomogeneity may be reduced. It must be noted that, given the short repetition time (1±3 s) used in the pulse program only a small part of the Xe gas magnetization is received due to the very long relaxation time of gaseous Xe, a few thousands seconds in pure gas phase [12], but certainly much less in presence of solid particles. On the other hand, the size of the pellet obtained after the powder compression is dierent from one sample to another, and the amount of Xe gas in the part of the tube in the NMR coil is dierent. Therefore the intensity of the gas phase line is not signi®cant. For sample S2, the coalescence line at 36 ppm disappears when the sample is compressed. This line is due to the small particles while the line at 77 ppm, which does not change with compression, should be due to the presence of large particles for which the exchange with the gas phase is slow compared to the NMR time scale. The external surface is eectively smaller than that of S1. A rapid calculation, assuming spherical particles (D 6 1=q Vme =A, where D is the mean particle size, q, the framework density, Vme the mesoporous volume and A the external surface area) gives a minimum particle size of 0.2 lm for S2 and 0.1 lm for S1. The increase in chemical shift with the degree of compression can be attributed to the proportion of small-diameter pores which are
781
formed during compaction. For crystallized aluminosilicate (zeolites) it has been shown that the chemical shift is roughly inversely proportional to the mean free path of the Xe atoms in the pores, which directly depends on the channel diameter. In the wide pores of these mesoporous solids the diusion of Xe must be at least as rapid as in zeolites and the Xe atoms average all the environments they experienced during the NMR time scale. Therefore the presence of small pores in the pore structure, revealed by N2 adsorption± desorption experiment, increases the observed chemical shift. For sample A1, even in powder form, in addition to the broad coalescence line at 18 ppm, two high-shifted signals (105 and 87 ppm) may be attributed to the presence of large particles for which the exchange of adsorbed and gaseous Xe is slow as for sample S2. A shift of 105 ppm is very high, it corresponds for example to Xe adsorbed in ZSM-5 zeolite channels (tridimensional network) of about 0.54 nm. As it has been stated before [5] the high value of the chemical shift generally observed for Xe adsorbed in mesoporous materials may be associated to the rugosity of the internal surface which has also been detected by other techniques [13,14]. The two distinct signals reveal two dierent types of environment for adsorbed Xe which may result from dierent types of Xe±surface interaction. Thus, while the signal at 87 ppm can be attributed to Xe atoms interacting with the internal surface of the channels having a given rugosity, the signal at 105 ppm could come from Xe atoms interacting with ``normal'' internal surface or microcavities which have been mentioned for explaining the more or less important stability of Alcontaining mesoporous materials [15]. When the powder is compressed at 130 MPa the fraction of Xe atoms, which exchanges between the adsorbed and gaseous phase in the loose powder, remains then inside the channels of the particles and the intensity of the corresponding line increases. On the other hand, since each signal is itself due to the rapid exchange between Xe atoms interacting with the ``normal'' internal surface or microcavities, having therefore high chemical shift (higher for the microcavities than for the ``normal surface''), and Xe atoms inside the pore volume which can be
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considered as gaseous Xe having 0 chemical shift, the increase in this latter population makes the observed chemical shift slightly decreasing. At high compression, the pore network is drastically damaged, a large proportion of small pores are formed with a broad size distribution as it is seen on the pore size distribution (Fig. 3B), the two previous lines broaden, overlap and move a little to higher chemical shift. The hysteresis loop observed for Al-containing samples has the type H4 of the IUPAC classi®cation [16], which can be explained by the presence of aggregates made of platy particles as it has been observed by optical microscopy. The distribution of slit pore width created by the aggregation of these particles is responsible of this particular shape. The more the sample is compressed, the smaller are these pores and the corresponding hysteresis loop. At high compression (520 MPa), especially for sample A2, a hysteresis loop is observable even in the domain of low pressure 0 < P =P0 < 0:45. This may be due to the swelling of the aggregates when N2 condenses in the interparticle spaces during the adsorption. The N2 as well as the X-ray diraction (XRD) data (Fig. 7) show that the pore structure of samples A1 and A2 are more fragile than that of sample S1. The presence of Al in the framework decreases the mechanical properties of these materials as suggested by Gusev et al. [17]. However our mesoporous mesophase materials, at least samples S1 and A1 have better mechanical properties than those of materials studied by these authors. This is certainly due to the better order of our materials as it is revealed by the sharpness of the XRD peaks. The less ordered sample (A2) with the highest FWHM (0.14°) has no XRD pattern when it is compressed at 520 MPa. 4.2. SBA-15 materials The decrease in BET surface area (160 and 300 m2 g 1 for SBA and AlSBA samples respectively) after compression under 80 MPa can be partly attributed to a decrease in the external surface area due to the aggregation of particles. This aggregation is all the more important than the particle size
Fig. 7. XRD patterns (classic RX tube, CuKa radiation) of samples S1 (a); A1 (b); A2 (c); loose powder (A) and powder compressed at 520 MPa (B).
is smaller. The insertion of aluminum atoms in the framework should lead to a particle size lower than with pure SBA material. Nevertheless, the same calculation of particle size, as that described above, gives an average diameter in the range 0.04±0.08 lm which is not consistent regardless to the unit cell parameter (about 0.01 lm) of the solids and the TEM images [8]. Consequently we must admit that the high decrease in the BET surface area after the ®rst step of compression is due not only to aggregation of particles but also because a part of the pores is no longer accessible. In the case of pure SBA samples, the compression under higher pressure (160 and 260 MPa) does not lead to the complete destruction of the organized pore structure. Contrary to M41S materials whose pore size of original channels remains
M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784
constant when the compression increases, the average pore diameter (5.5±6 nm) for SBA decreases after the ®rst step of compression and then remains constant. This shows a certain ¯exibility of the SBA lattice which seems to be not existing for MCM-41 due to thinner walls and smaller pores. The diminution of the total pore volume (Table 1) is the consequence of the reduction of the free volume between the aggregates. The insertion of aluminum atoms in the framework of SBA leads to an increase in the mean pore diameter (7.5 nm) but the sample is more sensitive to the compression, perhaps because of the smaller particle size. All organized structure disappears under 260 MPa as shown by the pore size distribution curve (Fig. 6B). As proved previously, the substitution of a slight part of silicon by aluminum atoms allows to obtain an acidic catalyst with a good catalytic activity [8] but makes the material more fragile regardless the compression which can be prejudicial if the catalyst has to be used in a pellet form. The changes observed in 129 Xe NMR spectra when the SBA and AlSBA samples are in powder form or in compressed pellets can be interpreted in the same way as for S1 and A2 samples described above (fast exchange between Xe gas in the pores and Xe on the surface). The fact that chemical shift slightly increases when the mean pore diameter of SBA materials decreases con®rm this explanation, the smaller the diameter of the pore the higher the probability to ®nd a Xe atom interacting with the surface and the higher the observed chemical shift. For all the samples containing Al atoms (MCM-41 or SBA-15), even in powder form, the presence of a narrow signal at 0 ppm due to xenon in gas phase can be explained by a decrease in relaxation time of xenon, because of quadrupolar interactions between Xe and Al (spin 5=2) atoms on the surface. 5. Conclusion For both MCM-41 and SBA-15 materials, the presence of Al in the framework decreases the mechanical properties, the larger the mean pore diameter the lower the resistance to compression;
783
and also decreases the average particle size of these mesoporous solids. The 129 Xe NMR data are quite dicult to interpret. In contrast to the results obtained with crystallized microporous aluminosilicates, it is not possible to correlate, at room temperature, the chemical shift with the average pore size. In fact, there is an exchange between adsorbed and gaseous Xe atoms and the chemical shift highly depends on the particle size.
Acknowledgements We thank Dr. J.M. Manoli for performing the XRD experiments presented in Fig. 7.
References [1] J.-L. Bonardet, J. Fraissard, A. Gedeon, M.-A. SpringuelHuet, Catal. Rev. Sci. Eng. 41 (2) (1999) 115. [2] W.C. Conner, E.L. Weist, T. Ito, J. Fraissard, J. Phys. Chem. 93 (1989) 4138. [3] V.V. Terskikh, I.L. Moudrakovski, V.M. Mastikhin, J. Chem. Soc., Faraday Trans. 89 (1993) 4239. [4] M.-A. Springuel-Huet, J. Fraissard, R. Schmidt, M. St ocker, W.C. Conner, in: B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodrõguez-Reinoso, K.S.W. Sing, K.K. Unger (Eds.), Characterisation of Porous Solids IV, The Royal Society of Chemistry, Cambridge, 1997, p. 452. [5] M.-A. Springuel-Huet, K. Sun, J. Fraissard, Micropor. Mesopor. Mater. 33 (1999) 89. [6] V.B. Fenelonov, V.N. Romannikov, A.Yu. Derevyankin, Micropor. Mesopor. Mater. 28 (1999) 57. [7] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [8] Y. Yue, A. Gedeon, J.-L. Bonardet, N. Melosh, J.-B. DÕEspinose, J. Fraissard, Chem. Commun. (1999) 1967. [9] E.P. Barrett, L.G. Joyner, P.H. Alenda, J. Am. Chem. Soc. 73 (1951) 373. [10] J.A. Ripmeester, C.I. Ratclie, Analytica Chimica Acta 283 (1993) 1103. [11] A.K. Jameson, C.J. Jameson, H.S. Gutowski, J. Chem. Phys. 53 (1970) 2310. [12] E.R. Hunt, H.Y. Carr, Phys. Rev. 130 (1963) 2302. [13] K.J. Edler, P.A. Reynolds, J.W. White, D. Cookson, J. Chem. Soc., Faraday Trans. 93 (1) (1997) 199. [14] N. Floquet, J.P. Coulomb, S. Giorgio, Y. Grillet, P.L. Llewellyn, in: L. Bonneviot, F. Beland, C. Danumah, S. Giasson, S. Kaliaguine (Eds.), Mesoporous Molecular
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Sieves 1998, Studies in Surface Science and Catalysis, vol. 117, Elsevier, Amsterdam, 1998, p. 583. [15] E.G. Kodenev, A.N. Shmakov, A.Yu. Derevyankin, O.B. Lapina, V.N. Romannikov, J. Mol. Catal. A: Chem. 158 (2000) 349.
[16] K.S.W. Sing, D.H. Everettt, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemenniewska, Pure Appl. Chem. 57 (4) (1985) 603. [17] V.Y. Gusev, X. Feng, Z. Bu, G.L. Haller, J.A. O'Brien, J. Phys. Chem. 100 (1996) 1985.
www.elsevier.nl/locate/micromeso
Mechanical properties of mesoporous silicas and alumina±silicas MCM-41 and SBA-15 studied by N2 adsorption and 129Xe NMR M.-A. Springuel-Huet a,*, J.-L. Bonardet a, A. Gedeon a, Y. Yue a, V.N. Romannikov b, J. Fraissard a a
Laboratoire de Chimie des Surfaces, CNRS-S.I.E.N., Universit e P. et M. Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France b Boreskov Institute of Catalysis, Pr Akad., Lavrentieva 5, Novossibirsk 630090, Russian Federation Received 24 February 2000; accepted 29 March 2000
Abstract Purely siliceous and Al-containing MCM-41 and SBA-15 materials have been prepared with pore diameters of 2.4± 3.0 nm for MCM-41 and 6.5±7.5 nm for SBA-15. The eect of compression on the mesopore structure has been studied by N2 adsorption±desorption, X-ray diraction and 129 Xe NMR experiments. The pore size distributions obtained from N2 adsorption data show that the mesopore structure is partially destroyed and that small size pores are formed at the expense of the original ones (in the loose powder) when the powder is compressed at high pressure (up to 520 MPa). The 129 Xe NMR spectra are interpreted in terms of an exchange between adsorbed Xe (in the channels) and gaseous Xe atoms (in the interparticle spaces), which depends on the particle size and the compaction of the powder. For both MCM-41 and SBA-15 materials, the Al-containing samples are more fragile than the purely siliceous materials. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous aluminosilicate; Mechanical stability; N2 adsorption;
1. Introduction 129
Xe NMR of adsorbed xenon has proved to be a valuable tool for studying a lot of physicochemical properties of zeolites (pore size, presence of structural defects, localization and distribution of cations, metallic particles, coke or any coadsorbed phase etc.) [1]. The extension of this tech-
*
Corresponding author.
129
Xe NMR
nique to the study of organized mesoporous silicas or alumina±silicas is very tempting. In contrast to zeolites, the ®rst results [2±5] showed that the chemical shift is quasi-independent of the Xe pressure and that a correlation between the chemical shift and the pore size is not evident, noticeable chemical shift dierences are observed for silicas with the same pore size for example. This can be attributed to a fast exchange between adsorbed Xe atoms and gaseous Xe atoms in the interparticle spaces which governs the chemical shift of the coalescence signal. To reduce the Xe
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 6 0 - 8
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M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784
exchange between the pores and the interparticle spaces, we compressed the powdered sample or synthesize material with lower external surface (larger particles).
2. Experimental Purely siliceous MCM-41 (denoted S1 and S2) and SBA-15 (denoted SBA), and aluminum-containing (denoted A1, A2 and AlSBA) mesoporous mesophase materials, were prepared according to previous published work [6±8]. Samples were outgassed at 573 K under vacuum (1 Pa) overnight before xenon adsorption and NMR experiments or during a few hours before N2 adsorption at 77 K. The powdered samples were compressed with various pressures ranging from 0 to 520 MPa. The N2 adsorption±desorption isotherms were measured using an ASAP 2010 Micromeretics apparatus. 129 Xe NMR spectra were recorded with a Bruker MSL 400 spectrometer at room temperature, at the resonance frequency of 110.668 MHz using a p=2 r.f. pulse and repetition time of 1±3 s. Chemical shifts are referenced to that of gaseous xenon.
3. Results 3.1. MCM-41 materials 3.1.1. N2 adsorption±desorption The N2 adsorption±desorption isotherms for all samples show a hysteresis loop, very small for S2 but much larger for the Al-containing samples, A1 and A2, in the pressure range 0:45 < P =P0 < 1, which decrease when the pressure of compression increases (Figs. 1A±4A). For all the samples, the BET surface area and the total pore volume decrease when the compaction increases (Table 1) and the pore size distribution, analyzed by the ``density functional theory'' (DFT) method, shows that small pores are formed at the expense of the original ones (Figs. 1B±4B). For the powder samples the average pore size is between 2.4 and 3.0 nm (Table 1). The external surface areas (63,
Fig. 1. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample S1 not compressed (a); compressed at 52 MPa (b); 130 MPa (c); 260 MPa (d); 520 MPa (e).
44, 112 and 237 m2 g 1 for S1, S2, A1 and A2 samples respectively) have been determined from the as -plots according to the method described in Ref. [6].
M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784 Table 1 N2 adsorption±desorption and
129
777
Xe NMR data of the investigated samples
Sample compressed at various pressures in MPa
(BET) Surface areaa (m2 g 1 )
Pore volumeb (cm3 STP g 1 )
Mean pore size (nm)
Chemical shift (ppm)
S1
0 52 130 260 520
1005 975 838 800 555
0.90 0.80 0.72 0.67 0.39
2.9c 2.9c 2.9; 1.7c 2.9; 1.3c 2.9; <1.0c
54 69.5 76.6 86.3; 81.7 91
S2
0 130
1093 879
0.96 0.91
3.0c 3.0c
77; 36 75
A1
0 130 520
1056 904 541
0.82 0.61 0.32
2.4c 2.4; 1.3c 2.4; <1.0c
105; 87; 15 102; 82 92
A2
0 130 520
970 931 411
0.92 0.79 0.26
2.5c 2.5; 1.6c 2.5; 1.2c
72; 60 ± 93
SBA
0 80 260
804 641 621
1.11 0.81 0.69
6.5d 5.5d 5.5; 4.0d
55 ± 90
AlSBA
0 80 260
1004 701 710
1.53 1.07 0.67
7.5d 7.0d No mean pore size
60; 45 ± 93
a
Using 0.162 nm for the N2 molecular cross section. Total pore volume at P =P0 0:97. c Using the ``density functional theory'' method for cylindrical pores assuming Harkins and Jura model. d From the desorption curve, using the BJH method. b
3.1.2. 129 Xe NMR The 129 Xe NMR spectra are practically independent of the Xe pressure for all the samples. Sample S1: The 129 Xe NMR spectrum of the powder sample has one broad signal at about 54 ppm. For compressed samples a ``gas phase line'' (0 ppm) appears and the chemical shift of adsorbed xenon increases with the pressure of compression, from 69.5 to 91 ppm when pressure increases from 52 to 520 MPa (Table 1) while the line width ®rst decreases until pressure of 130 MPa. For 260 MPa the signal splits into two lines (86.3 and 81.7 ppm) as well as the gas phase signal. At 520 MPa there is a single ``adsorbed xenon'' line at 91 ppm (Fig. 1C-e). Sample S2: The 129 Xe NMR spectrum of the powder sample has two signals; one at 77 ppm and the second, very broad at about 36 ppm (Fig. 2C-
b). When the powder is compressed under 130 MPa, only the signal at 77 ppm remains and a gas phase signal (0 ppm) is detectable (Fig. 2C-a). This sample has a lower external surface area (44 m2 s 1 ) than that of S1 (63 m2 s 1 ), determined from N2 adsorption experiment. Sample A1: The 129 Xe NMR spectrum of the powder shows three ``adsorbed Xe'' lines at 105, 87 and 18 ppm and a gas phase signal at 0 ppm (Fig. 3C). When the powder is compressed at 130 MPa, the two high-shifted signals remain, but their relative intensity changes, while the broad line at 18 ppm disappears. For large compression pressure (520 MPa) there is only one asymmetric line around 92 ppm. The X-ray diraction (XRD) pattern of this Al-containing material (loose powder) has a very narrow full-width-at-half-maximum (FWHM), 0.05° (2h°, synchrotron radiation), of the
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Fig. 2. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample S2 not compressed (a); compressed at 130 MPa (b).
[1 0 0] diraction peak (not shown). This suggests that the Al is homogeneously distributed all over the network. Sample A2: The 129 Xe NMR spectra show a single broad ``adsorbed'' line, at about 72 ppm and a gas phase line (0 ppm) (Fig. 4C-a). For a powder
Fig. 3. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample A1 not compressed (a); compressed at 130 MPa (b); 520 MPa (c).
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compression of 520 MPa, the line is narrower and shifted to 93 ppm (Fig. 4C-c). For the loose powder, the FWHM of the [1 0 0] diraction peak is 0.14° (2°) (not shown), broader than that of sample A1, suggesting a heterogeneous Al distribution. 3.2. SBA-15 materials Sample SBA: For non-compressed and compressed samples N2 adsorption±desorption isotherms have the same shape with a hysteresis loop in the range 0:6 < P =P0 < 0:8 (Fig. 5A).
Fig. 4. N2 adsorption±desorption isotherms at 77 K (A), pore size distribution (B) and 129 Xe NMR spectra at 293 K (C) of sample A2 not compressed (a); compressed at 130 MPa (b); 520 MPa (c).
Fig. 5. N2 adsorption±desorption isotherms at 77 K (A) and pore size distribution (B) of sample SBA: not compressed (a); compressed at 80 MPa (b); 160 MPa (c); 260 MPa (d).
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Compression under 80 MPa leads to a decrease in the BET surface area of 160 m2 g 1 ; under higher pressure, (up to 260 MPa) the compression does not aect the surface area but the adsorbed volume at saturation (P =P0 0:97) monotically decreases. The pore size distribution curves (Fig. 5B) obtained with the BJH method [9] show a slight decrease in the average pore diameter (initially 6.5 nm for the powder) which is stabilized around 5.5 nm with the appearance of a secondary porosity centered at 4 nm which increases with the compression pressure. For the powder sample, 129 Xe NMR spectra show a broad line at 55 ppm. After compression this line is strongly down®eld shifted and narrowed. Conner et al. observed similar results for compressed amorphous silica [2]. Nevertheless it can be noticed that the chemical shift slightly depends on the average pore size, increasing when the mean diameter of the synthesized samples decreases (90, 97 and 103 ppm for pore diameter 6.5, 5.5 and 4.5 nm respectively). Sample AlSBA: For samples containing Al atoms, N2 adsorption±desorption isotherms show variations much more important than for pure silica SBA (Fig. 6A). Besides a large decrease (300 m2 g 1 ) in the BET surface area after compression under 80 MPa (Table 1) the shape of the hysteresis loop changes with the pressure to which the sample is submitted indicating an important change in the distribution of the pore size. Analysis of this latter by BJH method (Fig. 6B) shows that powder sample presents homogeneous pores with an average diameter around 7.5 nm as previously described (Fig. 6B-a) [8]. For compressed samples the size and the relative amount of primary pores slightly decrease (as observed with pure SBA) but we can observe the appearance of a secondary porosity with pore diameters lower than 6 nm (Fig. 6B-c). For sample compressed at 260 MPa, a very broad distribution is observed (Fig. 6B-d) indicating the loss of the organized structure of the material. For the powder sample, the 129 Xe NMR spectrum is similar to that of sample S2, showing two signals partially superimposed; a broad one, at about 45 ppm, the other narrower at 60 ppm. After compression spectra present only one line, down®eld shifted at 93 ppm. Nevertheless and in
Fig. 6. N2 adsorption±desorption isotherms at 77 K (A) and pore size distribution (B) of sample AlSBA: not compressed (a); compressed at 80 MPa (b); 160 MPa (c); 260 MPa (d).
contrary to pure SBA samples the signal of xenon gas at 0 ppm is always observed even for loose powder sample. 4. Discussion 4.1. MCM-41 materials The dramatic change of the spectrum when sample S1 and S2 are compressed or not can be
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explained by a rapid exchange between adsorbed and gaseous (in interparticle space) xenon leading to a single broad coalescence line when the sample is not compressed (loose powder). The compression decreases the interparticle space volume and two signals corresponding to the two types of xenon are observable when the powder is compressed. Similar results have been obtained by Ripmeester and Ratclie for Xe adsorbed in Vycor glass (4 nm pore size) for various particle sizes [10]. For high pressure of compression, the adsorbed Xe as well as the gas phase signal is double for sample S1. This can be due to inhomogeneity of compaction. The compression creates pores in which the Xe±Xe interactions can be suciently larger, at the same pressure, than those existing in a non-con®ned gas phase to give a slightly higher chemical shift. It is well known that the chemical shift of Xe gas increases with pressure [11]. For 520 MPa the compaction inhomogeneity may be reduced. It must be noted that, given the short repetition time (1±3 s) used in the pulse program only a small part of the Xe gas magnetization is received due to the very long relaxation time of gaseous Xe, a few thousands seconds in pure gas phase [12], but certainly much less in presence of solid particles. On the other hand, the size of the pellet obtained after the powder compression is dierent from one sample to another, and the amount of Xe gas in the part of the tube in the NMR coil is dierent. Therefore the intensity of the gas phase line is not signi®cant. For sample S2, the coalescence line at 36 ppm disappears when the sample is compressed. This line is due to the small particles while the line at 77 ppm, which does not change with compression, should be due to the presence of large particles for which the exchange with the gas phase is slow compared to the NMR time scale. The external surface is eectively smaller than that of S1. A rapid calculation, assuming spherical particles (D 6 1=q Vme =A, where D is the mean particle size, q, the framework density, Vme the mesoporous volume and A the external surface area) gives a minimum particle size of 0.2 lm for S2 and 0.1 lm for S1. The increase in chemical shift with the degree of compression can be attributed to the proportion of small-diameter pores which are
781
formed during compaction. For crystallized aluminosilicate (zeolites) it has been shown that the chemical shift is roughly inversely proportional to the mean free path of the Xe atoms in the pores, which directly depends on the channel diameter. In the wide pores of these mesoporous solids the diusion of Xe must be at least as rapid as in zeolites and the Xe atoms average all the environments they experienced during the NMR time scale. Therefore the presence of small pores in the pore structure, revealed by N2 adsorption± desorption experiment, increases the observed chemical shift. For sample A1, even in powder form, in addition to the broad coalescence line at 18 ppm, two high-shifted signals (105 and 87 ppm) may be attributed to the presence of large particles for which the exchange of adsorbed and gaseous Xe is slow as for sample S2. A shift of 105 ppm is very high, it corresponds for example to Xe adsorbed in ZSM-5 zeolite channels (tridimensional network) of about 0.54 nm. As it has been stated before [5] the high value of the chemical shift generally observed for Xe adsorbed in mesoporous materials may be associated to the rugosity of the internal surface which has also been detected by other techniques [13,14]. The two distinct signals reveal two dierent types of environment for adsorbed Xe which may result from dierent types of Xe±surface interaction. Thus, while the signal at 87 ppm can be attributed to Xe atoms interacting with the internal surface of the channels having a given rugosity, the signal at 105 ppm could come from Xe atoms interacting with ``normal'' internal surface or microcavities which have been mentioned for explaining the more or less important stability of Alcontaining mesoporous materials [15]. When the powder is compressed at 130 MPa the fraction of Xe atoms, which exchanges between the adsorbed and gaseous phase in the loose powder, remains then inside the channels of the particles and the intensity of the corresponding line increases. On the other hand, since each signal is itself due to the rapid exchange between Xe atoms interacting with the ``normal'' internal surface or microcavities, having therefore high chemical shift (higher for the microcavities than for the ``normal surface''), and Xe atoms inside the pore volume which can be
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considered as gaseous Xe having 0 chemical shift, the increase in this latter population makes the observed chemical shift slightly decreasing. At high compression, the pore network is drastically damaged, a large proportion of small pores are formed with a broad size distribution as it is seen on the pore size distribution (Fig. 3B), the two previous lines broaden, overlap and move a little to higher chemical shift. The hysteresis loop observed for Al-containing samples has the type H4 of the IUPAC classi®cation [16], which can be explained by the presence of aggregates made of platy particles as it has been observed by optical microscopy. The distribution of slit pore width created by the aggregation of these particles is responsible of this particular shape. The more the sample is compressed, the smaller are these pores and the corresponding hysteresis loop. At high compression (520 MPa), especially for sample A2, a hysteresis loop is observable even in the domain of low pressure 0 < P =P0 < 0:45. This may be due to the swelling of the aggregates when N2 condenses in the interparticle spaces during the adsorption. The N2 as well as the X-ray diraction (XRD) data (Fig. 7) show that the pore structure of samples A1 and A2 are more fragile than that of sample S1. The presence of Al in the framework decreases the mechanical properties of these materials as suggested by Gusev et al. [17]. However our mesoporous mesophase materials, at least samples S1 and A1 have better mechanical properties than those of materials studied by these authors. This is certainly due to the better order of our materials as it is revealed by the sharpness of the XRD peaks. The less ordered sample (A2) with the highest FWHM (0.14°) has no XRD pattern when it is compressed at 520 MPa. 4.2. SBA-15 materials The decrease in BET surface area (160 and 300 m2 g 1 for SBA and AlSBA samples respectively) after compression under 80 MPa can be partly attributed to a decrease in the external surface area due to the aggregation of particles. This aggregation is all the more important than the particle size
Fig. 7. XRD patterns (classic RX tube, CuKa radiation) of samples S1 (a); A1 (b); A2 (c); loose powder (A) and powder compressed at 520 MPa (B).
is smaller. The insertion of aluminum atoms in the framework should lead to a particle size lower than with pure SBA material. Nevertheless, the same calculation of particle size, as that described above, gives an average diameter in the range 0.04±0.08 lm which is not consistent regardless to the unit cell parameter (about 0.01 lm) of the solids and the TEM images [8]. Consequently we must admit that the high decrease in the BET surface area after the ®rst step of compression is due not only to aggregation of particles but also because a part of the pores is no longer accessible. In the case of pure SBA samples, the compression under higher pressure (160 and 260 MPa) does not lead to the complete destruction of the organized pore structure. Contrary to M41S materials whose pore size of original channels remains
M.-A. Springuel-Huet et al. / Microporous and Mesoporous Materials 44±45 (2001) 775±784
constant when the compression increases, the average pore diameter (5.5±6 nm) for SBA decreases after the ®rst step of compression and then remains constant. This shows a certain ¯exibility of the SBA lattice which seems to be not existing for MCM-41 due to thinner walls and smaller pores. The diminution of the total pore volume (Table 1) is the consequence of the reduction of the free volume between the aggregates. The insertion of aluminum atoms in the framework of SBA leads to an increase in the mean pore diameter (7.5 nm) but the sample is more sensitive to the compression, perhaps because of the smaller particle size. All organized structure disappears under 260 MPa as shown by the pore size distribution curve (Fig. 6B). As proved previously, the substitution of a slight part of silicon by aluminum atoms allows to obtain an acidic catalyst with a good catalytic activity [8] but makes the material more fragile regardless the compression which can be prejudicial if the catalyst has to be used in a pellet form. The changes observed in 129 Xe NMR spectra when the SBA and AlSBA samples are in powder form or in compressed pellets can be interpreted in the same way as for S1 and A2 samples described above (fast exchange between Xe gas in the pores and Xe on the surface). The fact that chemical shift slightly increases when the mean pore diameter of SBA materials decreases con®rm this explanation, the smaller the diameter of the pore the higher the probability to ®nd a Xe atom interacting with the surface and the higher the observed chemical shift. For all the samples containing Al atoms (MCM-41 or SBA-15), even in powder form, the presence of a narrow signal at 0 ppm due to xenon in gas phase can be explained by a decrease in relaxation time of xenon, because of quadrupolar interactions between Xe and Al (spin 5=2) atoms on the surface. 5. Conclusion For both MCM-41 and SBA-15 materials, the presence of Al in the framework decreases the mechanical properties, the larger the mean pore diameter the lower the resistance to compression;
783
and also decreases the average particle size of these mesoporous solids. The 129 Xe NMR data are quite dicult to interpret. In contrast to the results obtained with crystallized microporous aluminosilicates, it is not possible to correlate, at room temperature, the chemical shift with the average pore size. In fact, there is an exchange between adsorbed and gaseous Xe atoms and the chemical shift highly depends on the particle size.
Acknowledgements We thank Dr. J.M. Manoli for performing the XRD experiments presented in Fig. 7.
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