Characterization of Hexagonal and Lamellar Mesoporous Silicas, Aluminosilicates and Gallosilicates by Small-Angle X-ray Scatteringt

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1 1065 J. Appl. Cryst. (1997). 30, Characterization of Hexagonal and Lamellar Mesoporous Silicas, Aluminosilicates and Gallosilicates by Small-Angle X-ray Scatteringt Guy VAN DEN BOSSCHE, a* ROGER SOBRY, a FRI~DI~RIC FONTAINE, a JEAN-MARC CLACENS b AND ZELIMIR GABELICA b ~Institut de Physique, Universit~ de LiPge, B5, B4000 LiPge, Belgium, and bd~partement de Chimie, Facult~s Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles, B-5000 Namur, Belgium. Guy. VandenBossche@ulg.ac.be (Received 23 July 1996; accepted 28 January 1997) Abstract Various mesoporous silicas and the corresponding aluminosilicates or gallosilicates have been synthesized using a series of literature or 'home-made' recipes. The efficiency of AI or Ga incorporation into the siliceous walls of these materials depends markedly on the trivalent source and the evolution (ageing) of the soformed Si-A/an gel-type phases at different starting ph values and temperatures, alter adding the surfactantstructuring compounds. Crystallization at low temperature (e.g. <373 K) yielded mesoporous compounds with hexagonal topology (MCM-41 type), involving doublelayered Si walls possibly partly substituted by A1 or Ga. Such structures remain stable after calcination in air at 873 K. When the same gels are crystallized at 423 K for 2 d, lamellar frameworks (MCM-50 type) are preferentially stabilized. They readily collapse on heating. The ultra-small-angle X-ray scattering (USAXS) data and the first part of the SAXS data show a power behaviour that indicates a fractal interface before calcination. After calcination, in the case of MCM-50 type materials, the fractal dimension significantly increases, the fractality region being larger than in the precursor. By contrast, in the case of the MCM-41 type materials, the fractal region tends to disappear after calcination. The second part of the SAXS curve reflects the hexagonal or lamellar structure. Some precursors simultaneously exhibit both structures. The hexagonal parameter ranges from 4.6 to 5.8 nm, while the thickness of the wall is estimated to be of the order of 0.7 nm from observations of the satellite peaks in the vicinity of the successive peaks characterizing the hexagonal array. The successive peaks relative to the lamellar structure are consistent with the superposition of two or three layers, the thicknesses of which are of the order of 3.3, 2.85 and 2.5 nm. Predominant hexagonal structures are maintained alter calcination while lamellar structures collapse during calcination. In the case of hexagonal structure, the hexagonal array is slightly contracted. t This paper was presented at the Tenth International Conference on Small-Angle Scattering, Campinas, Brazil, July ~_) 1997 intemational Union of Crystallography Printed in Great Britain - all rights reserved 1. Introduction The recent discovery of a new family of mesoporous materials (Kresge, Leonovicz, Roth, Vartuli & Beck, 1992; Beck et al., 1992) has expanded the range of defined pore sizes (<1.5 nm) currently found in crystalline microporous zeolites into the mesoporous regime (>2 nm). These new materials, belonging to the so-called M41S family and involving pore diameters in the range nm, have many potential applications in catalysis and adsorption science (Kresge et al., 1992; Beck et al., 1992) because of their high thermal and hydrothermal stabilities and their large surface areas, which currently extend over 1000 m 2 g-~. Their quasi-uniform size can be tailored by selecting the appropriate size of the surfactant template molecules as well as an appropriate ratio of the template to other organic additives. MCM-41 is a member of this M41S family which possesses a hexagonal array of uniform monodimensional channels. This material typically has a surface area of 1200 m e g-l (Gabelica, Clacens, Sobry & Van den Bossche, 1995) and a sorption capacity of 0.7 cm 3 g-i or more (Beck et al., 1992; Huo, Margolese & Stucky, 1996). Other members of this family, such as MCM-48 (cubic phase), MCM-50 (lamellar phase) and others with less well defined topologies, have been identified (Vartuli et al., 1994). Their formation and hydrothermal stabilization depends markedly on the synthesis conditions (Gabelica et al., 1995). The framework composition of these materials is based on silica, extending from the pure siliceous end member (Kresge et al., 1992; Beck et al., 1992) to frameworks in which Si is partly substituted by A1 (Luan, Cheng, Zhou & Klinowski, 1995), Ga, A1 + Ga (Gabelica et al., 1995), B (Sayari, Danumah & Mondrakowski, 1995) and a wide series of other transition metals, including Ti, V, Fe, Mn (Huo et al., 1996, and references therein), Co, Cr (Carvalho, Wallau & Schuchardt, 1996), Cu, Zn and their admixtures (Valange, Gabelica, Lopez-Granados, Rojas & Fierro, 1996). A liquid-crystal templating mechanism in which surfactant liquid-crystal structures act as templates has been proposed for the formation of M41S materials (Beck et al., 1992) and the possible variant pathways recently rediscussed (Vartuli et al., 1994). Journal of Applied Crystallography ISSN ,~(~, 1997

2 1066 CHARACTERIZATION OF HEXAGONAL AND LAMELLAR MESOPOROUS SILICAS Table 1. Synthesis characteristics and thermal behaviour of the various mesoporous phases PS = precipitated silica; TMOS = tetramethylorthosilicate; * indicates synthesis performed without an autoclave (at atmospheric pressure). The last column discerns samples that mantain topology after calcination (yes) from the others (no). Sample code Heat treatment Trivalent source Si source ph Synthesis type Calcined K/65 h 0.25 g AI203 Ultrasyl 12 I Yes K/65 h 0.25 g A1203 PS 12 I Yes I I(/65 h 0.42 g sodium aluminate PS >10 II No K/65 h - PS > 10 II No K/65 h 0.64 g sodium gallate PS >10 II No K/65 h 0.25 g Ga2(SO4)3 PS 13 III No K/6 d 0.25 g Ga2(504)3 PS 13 III Yes K/3 d 0.77 g AI2(SO,I)3.18H20 PS 13 III Yes K/4 h 0.77 g AI2(SO4)3.18H20 PS 13 III* Yes K/8 h 0.77 g AlE(SO4)3.18H20 PS 13 III* Yes MSII3 373 K/24 h - Na silicate 10.5 IV Yes MSI K/24 h - TMOS 10.5 V Yes Small-angle X-ray scattering on the MCM-41 materials was first performed by Edler & White (1995) and Gabelica et al. (1995) and later by Gusev, Feng, Bu, Hailer & O'Brien (1996). The present paper is devoted to a systematic analysis of the small-angle X-ray scattering of various mesoporous materials synthesized and crystallized using different methods (Gabelica et al., 1995). The preparations involve the use of various silica sources and trivalent ions (A1 and Ga), and are achieved under different conditions of heating, temperature and/or time. The purpose of the present work is to elucidate the mechanisms of formation of these phases under various synthesis conditions and to describe their structural (porous) characteristics Synthesis 2. Experimental The various synthesis methods used in this paper are referred to as types I, II, III, IV and V. The different synthesis procedures were based on literature methods (Beck et al., 1992; Chen, Li & Davis, 1993; Reddy, Moudrakovski & Sayari, 1994), slightly or thoroughly modified as described below Type I synthesis. Synthesis based on the method proposed by Beck et al. (1992), modified and optimized as follows: 12.5 g of a solution of tetramethylammonium silicate (TMAS) was admixed with 0.25 g of alumina (Catapal B Vista) and 25 g of cetyltrimethylammonium chloride (CTAC) (aq. 25%) under stirring. 2.14g of TMAOH (25%) and 3.13 g of silica were added to this mixture under stirring (Table 1). The final gel was placed in a static autoclave at 423 K for 65 h. Atter cooling at room temperature, the resulting solid was recovered by filtration, washed with water, and dried in air at 373 K Type II synthesis. Synthesis based on the method proposed by Chen et al. (1993), where the trivalent ion (A1, Ga) stems from various origins (Table 1) Type III synthesis. 7 g of silica (precipitated silica) and g of cetyltrimethylamonium bromide (CTAB) were added under vigourous stirring (2 h) to a solution containing 1.6 g of NaOH dissolved in 62 g of water and a variable amount of the trivalent metal source (Table 1). The mixture was placed in a static autoclave heated at different temperatures for variable periods of time, as indicated in Table 1. The treatment of the resulting solid product was the same as described for the type I synthesis Type IV synthesis g of sodium silicate (Merck) was added under vigorous stirring to a solution containing 0.22 g of NaOH, 105 g of water and 3.92 g of CTAB. The ph was fixed at 10.5 by adding dilute HC1. The mixture was placed in a Teflon container at 373 K for 24 h under static conditions. The treatment of the resulting solid product was the same as described for the type I synthesis Type V synthesis g of tetramethylorthosilicate (TMOS) (Janssen) was added under vigorous stirring to a solution containing 2.05 g of NaOH, 115 g of water and 4 g of CTAB. The ph was fixed at 10.5 by adding dilute HC1. The mixture was placed in a Teflon flask at 373 K for 24 h under static conditions. The treatment of the resulting solid product was the same as described for the type I synthesis Calcination The as-synthesized samples were calcined at 773 K in order to eliminate the template molecules completely. The products were t)rpically heated from 293 to 773 K at a rate of 1 K min-' and then maintained at 773 K for 10 h in a dry air flow Characterization All the as-synthesized and calcinated phases were characterized by using a combination of techniques: X-ray powder diffraction (WAXS), atomic absorption

3 G. VAN DEN BOSSCHE et al and/or EDX (amount of trivalent incorporation), 27A1, 71Ga and 295i high-resolution solid-state MAS NMR [coordination and structural quantitative repartition of Ga- and Al-bearing species (Gabelica et al., 1992), as well as the various Qx configurations of the Si atoms], thermogravimetry-differential thermal analysis-differential scanning calorimetry-differential thermogravimetry (TG-DTA-DSC-DTG) combined with in situ n-hexane sorption (thermal stability, template content, pore volume determination, crystallinity), the Brunauer-Emmett- Teller (BET) area (surface area) and ammonia temperature-programmed desorption (NH3-TPD) (acidity). Some of these characteristics have been discussed in more detail in a previous work (Gabelica et al., 1995) Small-angle X-ray scattering SAXS data were collected with a Kratky camera using Cu Kot radiation and a proportional counter by step scanning. Monochromatization was performed by an Ni /3 filter in conjunction with a pulse-height discriminator. The entrance and detector slits were adjusted to 100 and 250 p.m, respectively. However, to approach the origin of the angles as close as possible, the beginning of the curve was also registered with the entrance and detector slits adjusted to 35 and 90 ktm, respectively, and then merged with the main part of the curve. The explored domain was thus < s < 0.98 nm -t, where s = 2 sin 0/~.. All the measurements were made using a step scanning procedure and mostly in the fixed time mode, with a sampling time of 200 s for each step. Nevertheless, on the low-s side, the intensity was high enough to allow the fixed count mode to be used at a rate of counts per step. The number of steps was generally of the order of 380 for each sample, in such a way that the successive A0/0 ratios were roughly similar. Absorption of the X-rays by the samples and scattering of the device without a sample were also taken into account. A constant background was subtracted and the slit-smearing effect was corrected using the method reported by Sobry, Rassel, Fontaine, Ledent & Liegeois (1991) Ultra-small-angle X-ray scattering USAXS data were collected on beamline BW4 of the DORIS ring at HASYLAB (Hamburg). The explored domain was 6 x 10-4 < s < 0.06 nm -t. 3. Results and discussion 3.1. Characteristics of the investigated samples Table 1 lists the samples investigated in this paper. The notations np (precursor phase) and nc (calcined phase), where n is the number of the sample, have been adopted. For n = 19, we distinguish 19P1 and 19P2 before calcination and 19C1 and 19C2 atter calcination. The other notations used in this paper are those used in a previous report (Gabelica et al., 1995), where NMR results are extensively discussed. Samples 3 and 7 only differ in the source of silica. Sample 11 is equivalent to sample 12 except that it contains AI 3+ ions. Samples 44, 56 and 58 also contain A1, but from a different source and heating conditions than sample 11. Samples 14 and 19 were prepared in the presence of Ga, from two different sources. Sample 19/2 was heated at a lower temperature for a longer time than sample 19/1. Samples MSI13 and MSI14 are pure siliceous materials synthesized by using tetramethylorthosilicate along with Na silicate rather than precipitated silica. Syntheses of samples 44, 56, 58, MSI13 and 14 were performed at 373 K; preparation time was considerably reduced for these samples. Note also the relatively low ph used for samples of the MSI type General characterization of the SAXS intensity The SAXS curves of the first 11 samples, members of the MCM-41 family, were recorded before calcination (precursor) and the SAXS curves of all 12 samples were recorded a~er calcination. The first SAXS results allow us to separate the samples into two groups, namely, those that keep their initial morphology on calcination (3, 7, 19/2, 44, 56, 58, MSI13 and 14) and those that do not withstand calcination (11, 12, 14 and 19/1). Fig. 1 shows typical examples of the scattered intensities in both situations. The scattered 1011 I" ~ I01] x50) ~ to +'.~= ~ 107 '.~ C(:I0) ~ 10 3 i s (nm q) (a) io t 14C (xl00) ",,,..l, J ajl,,,l,,,,,hd ) s (nm -I) (b) Fig. 1. Typical log-log SAXS intensity of (a) three hexagonal and (b) four lamellar members after calcination. Intensity has been scaled by an appropriate factor to avoid overlapping of the curves. Labels on the curves identify the samples.

4 1068 CHARACTERIZATION OF HEXAGONAL AND LAMELLAR MESOPOROUS SILICAS intensities of the first group confirm the existence of well defined structures (Fig. l a), while the log-log plots of the scattered intensities of the second group suggest fractal structures (Fig. l b). The first group corresponds to mesoporous materials after calcination while the second group only contains unorganized materials after calcination. With respect to the synthesis methods, it is noted that type II synthesis yields samples that do not withstand calcination, while type III produces samples that withstand calcination at 873 K when the synthesis is performed at a lower temperature (373 K) maintained for a longer time. Confirmation of this result is found for many other synthesized samples, which have not been investigated by SAXS. Figs. 2(a) and 2(b) show the scattered intensities of the various precursors, before calcination. In both cases, a well defined structure is observed at intermediate wavenumbers and a fractal structure is suggested at larger scale. The fractal region extends from to 0.098nm-~; this is too small to conclude a fractal structure. Further on, a peak is often observed at very small angles. It corresponds to a distance d = 1/s ranging from 150 to 200 nm. However, the presence of the peak at the extreme limit of the small-angle domain has to be confirmed. Indeed, the observation of this peak may result from a lack of precision inherent in the measurement of the absorption coefficient or from multiple scattering. By contrast, this peak is not observed for calcinated samples (except 12C) SAXS and USAXS The USAXS intensities have been multiplied by a constant factor in order to compare USAXS and SAXS intensities. Typical results are shown in Fig. 3. For each precursor, the log-log plot from to 0.12 nm -~ yields a straight line, I = ks -u, where a _~ 4. The peak in the SAXS intensity is not observed by USAXS. However, a small dent is detected on the merged curve at nm -~, i.e. at a distance of about 120 nm, in sample 7P. We further investigated by USAXS only the calcined samples that maintain their topology at 873 K. The value of a ranges from 3.3 to 3.4, depending on the sample, and is significantly different from that of the precursor intensity. Nevertheless, it is obvious that this general inclination cannot be related to a fractal structure. Indeed, in all the samples (Fig. 3), a small bump is observed at about nm -~, i.e. at a distance of the order of 320 nm. This bump is also seen in the precursor sample. It could correspond to the channel length. Furthermore, the sample 44C exhibits a second bump at around nm -~. In SAXS experiments, the intensity before calcination can be interpreted by a formula of type I = ks -'~, where 0t ~ 4, except for sample 14P (Fig. 2b) and samples 56P, 58P, MSI13P and MSI14P (Fig. 2a). The 14P intensity plot exhibits a slope ct _~ 3.6, indicating a slightly rough surface in contrast to all the other samples in each of which the surface is smooth. The other four samples do not fit the log-log linear dependence of the intensity li -% 1015 I S P 109,01, zl = lo ~ i 103,,i,I i,,,,,,,i,,,,,,) It s (nm -1) (a) s (nm-l) I (b) Fig. 2. Typical log-log SAXS intensity of (a) five hexagonal and (b) four lamellar precursors. Scattering curves of 58P and MSI14P are not presented because they are very similar to those of 56P and MSI 13P, respectively USAXS 10-3 ~/ISI 13C s \ 1!4c...-~,,, I s ( nm -I) Fig. 3. SAXS and USAXS intensities. The regions investigated by both techniques are clearly indicated. The s -4 line is given for comparison.

5 G. VAN DEN BOSSCHE et al versus s. All of them were prepared at a lower temperature (373 K). This is also the case for samples 19P2 and 44P which, however, show the usual SAXS curve before calcination. 19P2 and 44P were prepared in an autoclave under autogeneous pressure (type III), whereas 56P, 58P, MSI13P and MSI14P were prepared using synthesis procedures III*, IV and V (see Table 1). We conclude that our synthesis methods modify the structure of the precursor at large scale, when compared with the usual methods (types I, II and III). After calcination, the structural behaviour of all the samples is rather disparate. For samples that do not withstand calcination, the fractal zone extends to wide angles with ct = The spatial extension of the fractal regime is between 70 and 7 nm for sample 12C and between 220 and 7 nm for sample 19C 1. For sample 11 C, ot = 3.38 is obtained between 165 and 2 nm. Sample 14C, still marginal before calcination, shows two linear regions, ot = 2.55 between 210 and 30 nm and c~ = 3.48 between 28 and 2.6 nm; the first region would correspond to fractal mass, the second to a fractal surface. It seems that the structure destruction by calcination results in extended fractal behaviour. The generally smooth surface before calcination becomes rough and is characterized by a fractal dimension D (= 6 - a) of the order of For samples that withstand calcination, the fractal zone narrows or disappears with two types of behaviour. Samples 3C and 7C, prepared following the method of Beck et al. (1992), exhibit a linear portion extending from to 0.1 nm -~ (Fig. 3); the mean slope is 3.4, indicating a rough surface. By contrast, all the other samples do not show this linear dependence of log I versus log s. Samples 19C2, 44C, 56C and 58C show the same behaviour, while the scattered intensities of both MSI samples (pure siliceous phases) are not very different from those of samples 3C and 7C (except for the bump at nm-l). Moreover, this bump is better seen for samples 44C and MSI13C than for samples 3C, 7C and MSI14C. The major differences observed at large scale between the samples prepared at 373 K and the others remain after calcination, with MSI14C giving the best agreement with the samples prepared at 423 K, following Beck et al. (1992) The tail end of the SAXS intensity After the fractal zone, all the samples, except those that have not withstood calcination, show a series of small peaks on a mean decay proportional to s -4 (Porod's law), with the exception of sample 14P (Fig. 2b). Let us first consider the case of the precursors of the samples whose structures are destroyed by calcination. All the samples except 14P yield similar SAXS intensities after the fractal region (Fig. 4). Three main peaks are observed at 0.3, 0.6 and 0.9 nm-l; in the vicinity of the first two peaks, satellite peaks are observed at 0.35 and 0.70 nm-'. A third small satellite peak is detected at 0.39nm -1. A similar satellite peak also appears in the vicinity of the second main peak, at 0.80 nm -1, as can be better seen by a plot ofls 4 versus s (Fig. 5, sample 19P1). Ciccariello (1991) has shown that strictly parallel interfaces are responsible for the periodic oscillations observed in a plot of IS 4 versus s. Here we 500 l ~ 105 d lo3 ;2 J / 14P ~ 300 ~ [I 1 Irl"'l' s (nm -1) Fig. 4. Typical SAXS intensities of four lamellar precursors s (nrn -1) Fig. 5. Typical plots of Is 4 versus s for two lamellar precursors.

6 1070 CHARACTERIZATION OF HEXAGONAL AND LAMELLAR MESOPOROUS SILICAS observe a strict periodicity of the three series of peaks. For the main peaks, this periodicity is 0.3 nm -I, exactly corresponding to the position of the first main peak. Following Ciccariello (1991), this indicates that the asymptotic expression of Is 4 is of the form A + B cos 2rr&, where A, B and 8 are three constants. In this case, we have an elliptic contact point which is encountered in the case of spheres or of parallel planes. We attribute these peaks to a lamellar structure characterized by an interlamellar distance 8 such that 3s = 1; thus we have 8 = 3.3 nm. In the same way, the first satellite peaks are also characterized by a periodicity, which is 0.35 nm -1, i.e. the position of the first peak of this satellite series. This also corresponds to a lamellar structure with an interlamellar distance equal to 2.85 nm. The second series of satellite peaks corresponds to a periodicity of 0.39 nm -1, i.e. the position of the first peak of this series. The interlamellar distance is now 2.55 nm. The lamellar structure results from the superposition of three lamellae. The shortest (2.55 nm) could correspond to a distance generated by the particular arrangement of the cetyltrimethylammonium (CTA) surfactant molecules, while the largest (3.3 nm) could correspond to the lamellae of CTA and silica, with a possible replacement of the metal (Ga in 19P1, A1 in 11P). It is easy to estimate that the silica (or AI silicate or Ga silicate) walls would have a thickness of 0.7 nm. As a result, the samples that do not withstand calcination are lamellar and the disappearance of CTA during calcination induces a destruction of the lamellar structure, which was stabilized by these surfactant molecules in the assynthesized phases. In principle, the SAXS intensities of sample 14P show the same features as those of the lamellar samples 19P 1, 12P and lip. Additionally, a peak of relatively large intensity occurs at 0.24 nm -t. The lamellar structure is not so well defined as in the other investigated samples. However, the similarity of the samples is also visible in the plot of Is 4 versus s (Fig. 5), where less-intense oscillations are observed at the same positions as for the other investigated lamellar samples. We will comment later on the interpretation of the additional peak. The SAXS intensities of the five precursor samples that withstand calcination are presented in Fig. 6, while Fig. 7 shows similar results for three typical samples atter calcination. It is obvious that the structure revealed by the SAXS intensities of the precursors (Fig. 6) is essentially maintained in the corresponding calcined samples (Fig. 7). However, details are more obvious in the precursor scattered intensities than in those of the calcined samples. Sample 7P (and 3P) undergoes a more important transformation during calcination than the others. Particularly, we note the disappearance of the peaks that were clearly observed in the corresponding precursors at 0.3 and 0.6 nm -~. These peaks have been assigned to the lamellar structure that collapses during calcination. The form of the peak at 0.3 nm -l in the precursor (3P and, ~ ~(x50)(=msi13c) S 105 ~. ~ \ 3P(xl~ --'~."~ ~'] " ~ ~ 5 6 P ~ 4P, 103 ~ ~ 19C2 ~ (c) ~'~C, 56C, 58C) t ~- 19P2 (=44P) 7C (/1 O) (--3C)~ (f) ~',,~(h) 101 t..,,,,,,...,...,,,.,...,,,,,7.7, s (nm -l) Fig. 6. Typical SAXS intensities of five hexagonal precursors. The successive hexagonal peaks are labelled (a) to (k). MSI14P, 58P and 44P show the same patterns as MSII3P, 56P and 19P2, respectively. l0 t 0. i' o'.' ' o'.7 " 0.9 '" '" s (nm q) Fig. 7. Typical SAXS intensities of three hexagonal members atter calcination. The successive hexagonal peaks are labelled (a) to (k) on the curve of sample 7C.

7 G. VAN DEN BOSSCHE et al also 7P) is the same as that observed in the lamellar samples with the first satellite peak at 0.35 nm -1 (e.g. 19P1). Considering plots of IS 4 versus s for the precursors (Fig. 8) and the calcined phases (Fig. 9), the main lamellar peaks in 7P are observed at 0.33, 0.66 and 0.98 nm -l, indicating a lamellar structure of periodicity 3 nm rather than that of 3.3 nm in the previous samples. These peaks are never observed in the calcined samples (Fig. 9). Similarly, sample 19P2 shows intense narrow peaks in the Is 4 plots at 0.39 and 0.78 nm -1 (Fig. 8), which disappear upon calcination (sample 19C2, Fig. 9). As before, the periodicity is equal to the position of the first peak. These peaks are also probably related to a lamellar structure of periodicity 2.56 nm. Samples 44P, 56P and 58P are also characterized by the presence of two similar peaks which disappear during calcination. By contrast, none of the two MSI precursors shows peaks not observed in the calcined phases. We conclude that most of the precursors are characterized by a doublefaced structure, one lamellar that collapses during the calcination and another that remains after calcination. Comparison of samples 14P (Fig. 4) and 7P or 3P (Fig. 6) shows that the uninterpreted first peak in sample 14P can be assigned to the same origin as the first peak of 7P or 3P. Confirmation of this assignment is found by comparing the is 4 plots of samples 14P (Fig. 5) and 7P (Fig. 8). The fact that sample 7P (or 3P) withstands calcination while sample 14P does not probably results from the relative amount of both structures in the sample. Partial collapse of the structures of samples 7P and 3P provides a possible interpretation of the more 'fractal' behaviour of these samples when compared with the samples that withstand calcination. In some aspects, samples 7C and 3C are related to those samples that do not withstand calcination. Once calcined they are transformed into two types of material: one well organized, corresponding to the remaining structure (hexagonal), and the other resulting from the collapsed lamellar structure. The last part of this study concerns the structures that remain after calcination. Considering Fig. 9, we see that the remaining structure can be characterized by: (i) a strong peak (a) ranging from 0.2 to 0.3 nm -1, depending on the sample; (ii) a satellite peak (b) at the foot of peak (a); (iii) two other relatively strong peaks (c) and (d) (7C and MSI14C), which are merged in 19C2; (iv) a second doublet, (e) and (f), clearly seen in 7C, less resolved in MSI14C and merged in 19C2; (v) some additional weak peaks, (g) to (k), more evident when the first strong peak is closer to the origin. This decomposition of the SAXS intensity, determined from an/s 4 plot, can easily be transposed to the usual I(s) plot (Fig. 7). To determine accurately the peak position, it is better to examine simultaneously the curves I versus s and/s 4 versus s. Indeed, the IS 4 curves correct the effect that the peaks are superimposed to a supposed s -4 decrease of the l(s) curves. Table 2 lists the peak positions for all the investigated samples. For each, we have also calculated the ratio of the successive peak positions to the position of the first one (a) and this sequence is compared with the theoretical sequence Ii, i 'i i i s I Ii i, / * i,i fl l i I I 11 # i,i i, ~I ii L ~", ~,, i l 1 o / i i i i i i i, J 700 /x 600 t:i :'. ii" i i /\,oo t1! i / &: 400 II ' : i I: 300 It: i ~'~d I' "'~ MSI14C It: l//l/, '... "-,--/ 200 I]1 :!: Jill" (e2(l) (~)(,)Vk) r:l; '.',l.ll ~,~'~(g)y~t),l' I s (nm -I) Fig. 8. Typical Is 4 versus s plots of two hexagonal precursors. 0 0-" s (nm -]) Fig. 9. Typical Is 4 versus s plots of three hexagonal members after calcination. The successive hexagonal peaks are labelled (a) to (k) on the curve of sample 7C.

8 1072 CHARACTERIZATION OF HEXAGONAL AND LAMELLAR MESOPOROUS SILICAS Table 2. Peak positions for the investigated samples The s positions (nm-~) of the successive peaks (a)--{k) are reported in the top part of the table. The middle line of the table lists the theoretical ratios of the s position of each peak to that of the first one, in the case of a planar hexagonal array. For each sample, experimental ratios are given in the bottom part of the table. The peak (b) appears as a shoulder on the peak (a) and corresponds to the second hexagonal cell. (a) (b) (c) (at) 3P P P P P P P MSI 13P C C C C C C MSI13C MSI14C s positions (e) (f) (g) (h) (i) (j) (k) Theoretical Experimental ratios 3P P P P P P P MSII3P C C C C C C MSI 13C MS114C expected for a planar hexagonal structure. The experimental sequences agree very well with the theoretical one. It is concluded that the remaining structure after calcination is still hexagonal. In the lamellar structure, we have seen that the satellite peaks are probably related to the thickness of the silica walls. Similar considerations lead us to suggest that they also characterize the thickness of the silica walls in the hexagonal structure. Then, the observed positions (0.26 nm -I in 7C and 0.34 nm -I in MSI14C) compared with the main peak (a) positions (0.21 and 0.29 nm -~, respectively) could indicate that the walls have a thickness of ca 0.9 nm in sample 7C and only 0.4 nm in sample MSI14C. Table 3 summarizes the results. For all the investigated samples, we report the characteristics of the lamellar structure(s), of the hexagonal structure and the thickness of the walls. In every case, the hexagonal cell is slightly smaller after calcination ( nm), as currently observed in other cases (see e.g. Huo et al., 1996). The hexagonal cells of samples 3P and 7P are larger than those of samples 19P2 and 44P, before and after calcination. 4. Conclusions The hexagonal structure is well established for samples that withstand calcination but not for those that collapse after heating at 873 K. It seems reasonable to consider that the presence of the lamellar structure before calcination favours its collapse upon calcination. In each case, the lamellar structure disappears after calcination. By contrast, an initial hexagonal structure better resists calcination even if a lamellar structure is also present, as in samples 3P and 7P. However, the hexagonal structure can also disappear during calcination if it coexists with

9 G. VAN DEN BOSSCHE et al Table 3. Principal structural parameters (nm) of the hexagonal structure and of the lamellar structure P denotes the precursor and C the corresponding calcined sample. The hexagonal structure reveals two distinct hexagonal cells characterized by the axes a and a*. The thickness of the Si walls (WT) results from the difference 3 I/2(a-a*)/2. The contraction of the hexagonal cell after calcination is easily seen. In the case of lamellar structure of the precursor, the characteristic distances L,, L2 and L 3 are given. When a lamellar structure coexists with a well defined hexagonal structure, we only observe the first series of lamellar peaks. Similarly, the second hexagonal cell is not observed in the case of a hexagonal structure coexisting with a well defined lamellar structure (14P). a(v) a(c) / MSII MSI /i Hexagonal structure Lamellar structure a*(p) a*(c) WT(P) WT(C) L1 L2 L an important amount (percentage) of the lamellar structure. This is the case for sample 14P, where the collapse of the predominant lamellar structure induces the disappearance of all short-range structures. When lamellar structure coexists with hexagonal structure and when the latter remains after calcination, the collapse of the lamellar structure is manifested by the more fractal behaviour of the USAXS-SAXS intensities, as in the case of 7C and 3C. At the time of calcination, fractal dimensions decrease strongly as the roughness of the surface increases. When a lamellar structure is absent or minor with respect to the hexagonal one, the fractal dimension does not vary during calcination and the surface remains smooth, as in sample MSI14C. All the above conclusions concern mesoporous silica materials crystallized under various conditions. The most important synthesis parameter defining the material topology and its further thermal stability appears to be the synthesis temperature. Except for the synthesis of type I, a higher temperature appears to favour the formation of a lamellar phase that is not sufficiently stabilized by the organized surfactant molecules and collapses upon calcination. Synthesis of type I involves different ingredients and mixing steps that possibly lead to the partial formation of the hexagonal topology, even at 423 K; however, an appreciable amount of lamellar phase is also found, as ascertained by fractal behaviour. This is possibly caused by heating at a relative high temperature (423 K). The heating time appears to be a less-critical variable provided that the temperature is low enough (samples 44, 56 and 58). Although Ga 3+ and A13+ are found variably coordinated and partitioned in these structures, as suggested by NMR data (Gabelica et al., 1995), the presence of trivalent ions (in minor amounts) does not seem to influence either the initial topology or the stability after calcination. Phases of MSI type (13 and 14) have a very regular hexagonal topology that is quasi integrally maintained upon calcination. This could result from the adequate selection of the synthesis ph, at which the interaction between the positively charged ends of the surfactant molecules and the negative surface of the (partly) ionized silanol groups of the low oligomeric silica particles is optimized. Such preparations also yield pure hexagonal topologies that are remarkably thermostable, even in the presence of odd ionic species added to the initial gel. Indeed, Cu 2+, Zn 2+ or Mo 5+ ions have been found to be incorporated in such frameworks that always keep a stable hexagonal topology (Valange et al., 1996)! GVdB and RS are very much indebted to the FNRS and to the 'Ministrre de l'enseignement suprrieur et de la Recherche de la Communaut6 Fram;aise de Belgique' for financial support. References Beck, S. J., Wartuli, J. C., Roth, W. J., Leonovicz, M. E., Kresge, C. T., Schmitt, K. D., Chu, C. T. W., Olson, D. H., Sheppard, E. W., McCuller, S. B., Higgings, J. B. & Schlenker, J. L. (1992). J. Am. Chem. Soc. 114, Carvalho, W. A., Wallau, M. & Schuchardt, U. (1996). In Proc. 11th Int. Zeolite Conf. Seoul. In the press. Chen, C. Y., Li, H. X. & Davis, M. E. (1993). Microporous Mater. 2, Ciccariello, S. (1991). Phys. Rev. A, 44, Edler, K. J. & White, J. W. (1995). J. Chem. Soc. Chem. Commun. pp Gabelica, Z., Clacens, J. M., Sobry, R. & Van den Bossche, G. (1995). Stud. Surf Sci. Catal. 97, Gabelica, Z., Mayenez, C., Monque, R., Galiasso, R. & Giannetto, G. (1992). In Synthesis of Microporous Materials, Vol. I, Molecular Sieves, edited by M. L. Occelli & H. Robson, pp New York: Van Nostrand Reinhold. Gusev, V. Y., Feng, X., Bu, Z., Haller, G. L. & O'Brien, J. A. (1996). J. Phys. Chem. 100,

10 1074 CHARACTERIZATION OF HEXAGONAL AND LAMELLAR MESOPOROUS SILICAS Huo, Q., Margolese, D. I. & Stucky, G. D. (1996). Chem. Mater. 8, Kresge, C. T., Leonovicz, M. E., Roth, W. J., Vartuli, J. C. & Beck, J. S. (1992). Nature (London), 359, Luan, Z., Cheng, C.-E, Zhou, W. & Klinowski, J. (1995). J Phys. Chem. 99, Reddy, K. M., Moudrakovski, I. L. & Sayari, A. (1994). J. Chem. Soc. Chem. Commun. pp Sayari, A., Danumah, C. & Mondrakowski, I. L. (1995). Chem. Mater. 7, Sobry, R., Rassel, Y., Fontaine, E, Ledent, J. & Liegeois J.-M. (1991). J Appl. Cryst. 24, Valange, S., Gabelica, Z., Lopez-Granados, M., Rojas, S. & Fierro, J. L. G. (1996). In Proc. l lth Int. Zeolite Conf. Seoul. In the press. Vartuli, J. C., Schmitt, K. D., Kresge, C. T., Roth, W. J., Leonowicz, M. E., McCullen, S. B., Hellring, S. D., Beck, J. S., Schlenker, J. L., Olson, D. H. & Sheppard, E. W. (1994). Stud. Surf. Sci. Catal. 84,