Ce−Al-Pillared Clays: Synthesis, Characterization, and Catalytic

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Ind. Eng. Chem. Res. 2000, 39, 1944-1949

Ce-Al-Pillared Clays: Synthesis, Characterization, and Catalytic Performance G. Fetter* Facultad de Ciencias Quı´micas, Beneme´ rita Universidad Auto´ noma de Puebla, Blvd. 14 Sur y Avenida San Claudio, C.P. 72570, Puebla, Pue., Mexico

P. Salas Instituto de Fı´sica, Universidad Nacional Auto´ noma de Me´ xico, A.P.1, C.P. 1010, Quere´ taro, Qro., Mexico

L. A. Velazquez and P. Bosch Departamento de Quı´mica, Universidad Auto´ noma Metropolitana-Iztapalapa, Michoaca´ n esq. Purı´sima, Iztapalapa, C.P. 09340, Me´ xico, D.F., Mexico

Montmorillonites intercalated with Ce-Al pillars were synthesized. It was found that the precursor of cerium and aluminum determined the presence of Ce-Al mixed oxide pillars. The catalytic performance in the 2-propanol conversion of the various samples is discussed in terms of the catalyst structure. It was found that the surface features of these interlayered materials are different from those of large ceria-alumina particles. Introduction The alumina surface is prone to reacting with other species during a phase change or even during reaction conditions (Figuereido and Ramoa-Ribeiro, 1989). Aluminates are, then, easily formed. Thus, to enhance the thermal stability of γ-alumina, small amounts of divalent ions are incorporated into the network (calcium, magnesium, barium, copper, or nickel) which occupy tetrahedral voids in the spinel and retard the diffusion of Al3+ cations (Alvarez et al., 1995; Alvarez et al., 1997). Nickel or cobalt are known to be used if molybdenum is supported on alumina; tin and rhenium are expected to strongly interact with alumina in Pt-Re or Pt-Sn catalysts. γ-Alumina is a defect spinel which can be described as

{Al2/3+x[ ]2/3-x(Al2-x[ ]x)O4} where x represents the amount of vacancies (0 < x < 1/ ). The introduction of M2+ cations into the structure 3 promotes a new distribution of vacancies which, of course, modifies the surface properties, especially surface acidity. If the doped aluminas (La or Ce) are prepared by the sol-gel technique, the number of Lewis acid sites increases significantly with the high cerium or lanthanum concentrations (5 wt % Ce/Al2O3; i.e., a molar ratio ca. Ce:Al ) 1:52). Low cerium-doping concentrations produce Bronsted acid sites. From theoretical models (Jacobs et al., 1997), the thermostabilizing effect has been described in terms of the hindering of migration of tetrahedral aluminum cations by the presence of highly coordinated lanthanum. Hence, La doping produces a lowering of the coordination numbers of Al3+ cations at the surface of the system La/γ-alumina which tends to increase its porosity as Al and O ions * To whom correspondence should be addressed. E-mail: [email protected].

move upward, whereas the La ions either go down within the structure or remain at the topmost layer. An original focus of the problem is to promote the formation of small alumina aggregates where a high percentage of the constituting atoms are on the surface. To inhibit sintering, these small particles should be in close interaction with compounds which ensure both that the mobility of the particles is avoided and that the size is small and constant. With this purpose, in this work, montmorillonite was chosen. Montmorillonite is, indeed, a clay which presents high swelling properties and an ionic-exchange capacity between 60 and 120 mequiv/100 g (Figueras, 1988). Pillars of alumina can be easily formed between the layers of the clay which limit the size and the location of the alumina particles. Furthermore, we studied, in this case, the effect of cerium as this metal is known (as well as lanthanum) to modify the texture of aluminum-pillared clays. In this context the results obtained by Wenyang et al. (1991) are most relevant. These authors reported on the preparation of Al-Zr-B-pillared clays (Al-Zr-BPILCs) which possess a higher catalytic activity for gas oil cracking than that for the Al-PILC, the Al-ZrPILC, or the Al-B-PILC. It is clear, therefore, that the composition of the pillars and the pillar size determined the catalytic activity. On the other hand, the stabilizing effect of those ions has been confirmed by Figueras et al. (1989) who found that the addition of lanthanide salts to an intercalating solution of zirconium polyhydrocations resulted in PILCs with a higher thermal stability. In a similar approach, Sterte (1991) followed McCauley (1988) and used thermally treated aluminumlanthanum-pillaring solutions to prepare clays with large pore diameters. But this effect depends strongly on the salt added: Trillo et al. (1993) observed a very slight positive effect on the thermal and hydrothermal stability of the pillared clays if cerium was added; however, they did not test their materials catalytically. McCauley (1988) found that hydrothermally stable pillared clays could be

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prepared using pillaring solutions containing mixtures of aluminum chlorohydrate and a cerium salt. Although this work was primarily concerned with pillaring solutions prepared from Ce(NO3)3 and aluminum chlorohydrate, it was concluded that other cerium and aluminum salts could be used to prepare mixed Al-Ce-PILCs. The purpose of the present work is, then, to establish the effect of cerium on the catalytic activity of aluminumpillared clays as well as the effect of cerium on the nature of the intercalate and on the corresponding thermal stability. Experimental Section Initial Clay. The initial clay used for pillaring was a calcium montmorillonite (Bentolite L3, delivered by Southern Clays Products Inc.) whose chemical composition is SiO2 (71.3%), Al2O3 (15.4%), MgO (3.6%), CaO (1.7%), Fe2O3 (1.0%), TiO2 (0.3%), Na2O (0.2%), and K2O (0.1%) and the surface area value is 83 m2/g. The cationexchange capacity is 100 mequiv/100 g of clay. The clay was purified using conventional sedimentation techniques to eliminate impurities as quartz. The less than 2-µm fraction recovered was essentially free from impurities as determined by X-ray diffraction. Pillaring Solutions. The cerium-aluminum-pillaring solutions were prepared by mixing a cerium with an aluminum solution and keeping it under reflux at 367 K for 120, 168, and 312 h. The cerium solution was obtained by hydrolysis of Ce(SO4)2‚6H2O (Merck) or Ce(NO3)3‚6H2O (Aldrich) salts. The 2.5 M aluminum solution was obtained by diluting 40 mL of aluminum chlorohydrate (Chlorhydrol, Reheis Chemical Co., 50 wt % Al2O3; OH/Al ) 2.5) with 60 mL of deionized water or a 1 M aluminum solution obtained by slowly mixing 1 M solutions of NaOH and AlCl3‚6H2O (Merck) to obtain a molar ratio OH/Al ) 2 and aging for 2 days at room temperature, as reported previously (Fetter et al., 1995). In all cases the pillaring solution had a Ce:Al molar ratio of 1:5. The pH of the refluxed solutions ranged from 3.5 to 3.9. Preparation of Alumina and Alumina-CeriaPillared Clays. In a typical preparation, 5 g of Camontmorillonite was dispersed in 2 L of deionized water and stirred for 1 h. The amount of the Ce-Al-pillaring solution required to obtain an Al/montmorillonite ratio of 5 mmol/g was then added dropwise to the clay slurry. The dispersion was kept for 20 h under stirring at room temperature, then washed with deionized water until chloride free (AgNO3 test), and dried in an oven at 318 K. Samples were labeled as S or N if the cerium precursor was sulfate or nitrate, C or CH if the aluminum precursor was chloride or chlorohydrate, and 120, 168, or 312 depending on the refluxing time. For instance, sample NCH312 means that the cerium and the aluminum precursors were nitrate and chlorohydrate, respectively. The refluxing time of the mixed solutions was then 312 h. For comparison purposes commercial γ-alumina (Ketjen) was used and an Al-montmorillonite sample was conventionally synthesized as described in a previous work (Fetter et al., 1996). This sample is labeled as CH. Thermal Treatment. A thin bed of clay was introduced into a horizontal furnace swept by an air flow of 200 mL/min. The heating rate was 50 K/h and the desired temperature was maintained for 5 h. A fresh

sample was used for each calcination. All samples were calcined in air at 673 K. Only samples NCH120, NCH168, and NCH312 were treated at higher temperatures: 773, 873, 973, and 1073 K. Characterization Techniques. Basal spacings were determined conventionally by X-ray powder diffraction from the position of the 001 peak. X-ray diffraction patterns were recorded on a Siemens D-500 diffractometer using Cu KR radiation. Surface areas were determined from nitrogen adsorption isotherms at 77 K using the BET equation on outgassed samples at 573 K for 5 h. The piece of equipment used was a Micromeritics ASAP 2000. The silicon and aluminum content, in some representative samples, was analyzed by atomic absorption using a Perkin-Elmer 500 spectrophotometer. Catalytic Test. The catalytic activity was tested with the conversion of 2-propanol in a continuous flux microreaction system at atmospheric pressure; 100 mg of sample was used for each test with 60 mL/min of helium as the saturation gas, with a molar ratio He/2-propanol ) 5. The kinetic parameters were taken at 90 min of reaction. Results and Discussion Alumina (and Ceria-Doped Alumina) Particles Intercalated into the Clay Structure. The clays intercalated with pillaring solutions prepared by refluxing the Ce(SO4)2‚6H2O solution for 120 h with either AlCl3‚6H2O or chlorohydrol solutions (samples SC120 and SCH120) show an intense (001) diffraction peak (Figure 1). The corresponding interlayer spacing (d001 values) was 1.6 nm (Table 1). But if the pillaring solution was prepared by refluxing the Ce(NO3)3‚6H2O solution with the AlCl3‚6H2O solution (samples NC168 and NC312), then the obtained interlayer spacing was 1.5 nm. This value differs from the value (1.9 nm) obtained if the sample was prepared from Ce(NO3)3‚ 6H2O and aluminum chlorohydrate solutions (samples NCH120, NCH168, and NCH312). Furthermore, a shoulder corresponding to an interlayer distance of 2.6 nm is clearly resolved in the sample NCH168; when the refluxing time was increased to 312 h, the intensity of this secondary peak increased (sample NCH312). Therefore, montmorillonite has been intercalated with complexes whose sizes depend on the cerium as well as on the aluminum precursors. The interlayer spacing of the clay is determined, indeed, by the sizes of the complexes which in turn depend on the solubility of the cerium salt: cerium sulfate is less soluble and reactive than cerium nitrate. The d001 interlayer distance (1.5 nm) is independent of the refluxing time (samples NC168 and NC312) for an aluminum chloride precursor, although if chlorohydrate is the aluminum precursor, for refluxing times longer than 120 h (samples NCH168 and NCH312), a second interlayer distance appears (2.6 nm), showing the influence of refluxing time. It seems, therefore, that aluminum chlorohydrate favors the formation of cerium-aluminum compounds which may intercalate into the clay layers as the reflux time is increased; such is not the case if chloride is used. When the samples were calcined at 673 K (Figure 2), the sharp and well-resolved 001 X-ray diffraction peak previously presented by the intercalated samples SC120, SCH120, NC168, and NC312 turned out to be scarcely observed, indicating that the intercalated structure had collapsed. This did not occur if the samples were

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Figure 2. X-ray diffraction patterns of the 673 K treated catalysts.

Figure 1. X-ray diffraction patterns of the nonthermally treated samples. Table 1. Comparison of Interplanar Distances d001 (nm), before (Intercalated Samples) and after Thermal Treatment at 673 K (Pillared Samples)a sample

d001 (nm) intercalated samples

d001 (nm) pillared samples

SC120 SCH120 NC168 NC312 NCH120 NCH168 NCH312 CH

1.6 1.6 1.5 1.5 1.9 2.6; 1.9 2.6; 1.9 1.9

np np 1.4 (pso) 1.4 (pso) 1.7 2.3; 1.88 2.3; 1.88 1.8

a

np ) no peak; pso ) peak scarcely observed.

prepared from cerium nitrate and aluminum chlorohydrate, NCH120, NCH168, and NCH312; the sample NCH120 presented a more intense 001 line because of an interlayer spacing of 1.7 nm (Table 1); in samples

NCH168 and NCH312 the diffraction peak was broad and asymmetric, showing the presence of two unresolved peaks. Two interlayer distances were present, one because of the more intense line (1.88 nm) and a second minor peak corresponding to a 2.3-nm distance. The cerium precursor determines the formation of pillars into the aluminum chlorohydrate prepared clays for a refluxing time of 120 h, as shown if samples SCH120 and NCH120 are compared. If nitrate is the cerium precursor, only aluminum chlorohydrate favors the formation of pillars after thermal treatment (samples NCH120, NCH168, and NCH312); indeed, calcination at 673 K provides well-structured samples where, depending on the reflux time, the interlayer distances are 1.8 and 2.3 nm. Again, this result indicates that, as previously suggested, species formed from cerium nitrate and aluminum chlorohydrate are maintained; hence, they reach all exchange sites and, therefore, they are anchored homogeneously between the layers. With temperature they form the expected oxides by dehydroxylation. Furthermore, the 1.9-nm interlayer distance observed in the NCH120 sample before thermal treatment is similar to the distance reported in conventional Alintercalated smectites (Tichit et al., 1988; Figueras et al.,1990), and it can be attributed to the intercalation with the Keggin ion (Al13O4(OH)24‚12H2O)7+. Still, the Al-montmorillonite prepared in this work (CH sample) presented also the 1.9-nm interlayer distance. This could mean that no mixed Al-Ce species are formed, although other mechanisms of incorporation of cerium are possible such as ion exchange, replacement of the central Al 3+ by Ce3+, and sorption onto alumina surfaces. The diffraction peak at 2.6 nm increases as the reflux time of the mixed cerium-aluminum solution increases; therefore, this peak represents the intercalation of cerium-aluminum species whose size is close to 1.64 nm as the layer thickness of the montmorillonite is 0.96 nm. The mixed species are probably formed when

Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1947 Table 2. Aluminum and Silicon Ratio Present in the Samples Tested in the 2-Propanol Conversion Reaction samples

wt % Al2O3

wt % SiO2

Al/Si molar ratio

NCH120 NCH168 NCH312 bentolite

17.30 18.54 24.09 15.40

67.40 55.84 55.17 71.30

0.305 0.391 0.513 0.269

Table 3. Comparison of Surface Areas (m2/g) before (Intercalated Samples) and after Thermal Treatment at 673 K (Pillared Samples) sample

surface area (m2/g) intercalated samples

surface area (m2/g) pillared samples

SC120 SCH120 NC168 NC312 NCH120 NCH168 NCH312 CH

97 119 137 136 351 356 355 277

90 63 129 54 341 287 306 234

Ce and Al-Keggin ions interact in solution, although the interaction of rare earths with aluminum polyoxocations is difficult (Caballero et al., 1996; Kloprogge, 1998). Of course, a comparison of SC120, SCH120, and NH120 show that both cerium and aluminum chlorohydrate are required to form the polycation of larger size. Furthermore, reflux increases their concentration (samples NCH120 and NCH312). In this work the stoichiometry of these intercalated species is not clear as a fraction of aluminum is used to build the aluminum oligomers which produce the distance 1.9 nm. The nominal Ce:Al molar ratio is 1:52 which means that one cerium atom corresponds to four Al13. Hence, it seems that the sample is pillared by Al13 entities (d001) 1.8 nm) and by Ce-Al species (d001) 2.6 nm) but cerium oxide is most probably also formed. These speculations may be justified from the elemental analyses obtained from samples tested in the catalytic reaction (Table 2). The NCH312 sample contains twice as much aluminum as the original montmorillonite (Al/Si molar ratio). Surface Areas of the Intercalated and Pillared Montmorillonites. BET surface areas of intercalated samples prepared with aluminum chlorohydrate are systematically higher than those from the corresponding chloride prepared samples (Table 3). Refluxing time does not seem to alter the surface area values except if the cerium precursor is sulfate. From Table 3, it is also clear that cerium nitrate favors larger areas than cerium sulfate (SCH120 compared to NCH120 or SC120 compared to NC168). The surface areas of the intercalated samples present a good correlation with the interlayer spacings; that is, for samples with interlayer distances 1.6-1.5 nm the surface areas are low (67-137 m2/g), but meanwhile the samples NCH120, -168, and -312 showed surface areas close to 355 m2/g. This value is higher than the usual surface area found in a conventionally pillared Almontmorillonite (250-300 m2/g) (Auer and Hofmann, 1993; Pinnavaia et al., 1984); the value obtained in this work for the Al-montmorillonite (CH sample) was 277 m2/g. This indicates again that as shown by the X-ray diffraction (001) peak, in addition to Al13 polycations, larger species are present, most probably Ce-Al mixed ones. After thermal treatment at 673 K, although low surface areas are obtained in PILCs prepared with cerium sulfate and very high areas in preparations from

Figure 3. 001 X-ray diffraction peaks of the samples prepared from cerium nitrate and aluminum chlorohydrol calcined at (a) 673 K, (b) 773 K, (c) 873 K, (d) 973 K and (e) 1073 K.

cerium nitrate and aluminum chlorohydrate, the SCH120 surface area is lower than that of SC120, NC312 is 2.5 times lower than NC168, and NCH120 > NCH312 >NCH168. Nevertheless, the values for NCH120-312 are again higher than those found for the corresponding CH sample (surface area, 234 m2/g). Thermal Stability. Samples NCH120-312 present the highest interlayer spacings and surface areas. They were then calcined from 673 to 1073 K to test their thermal stability. Each sample was studied by X-ray diffraction. In all cases the intensity of the 001 X-ray diffraction peak decreased with increasing temperature (Figure 3). Therefore, the temperature favors disorder in the initially well-pillared structure. However, even at 1073 K, the 001 X-ray diffraction peak is resolved, and the two interlayer distances present in samples NCH168 and NCH312 may be measured. Figure 4 shows how the interlayer distance value remains constant. Hence, at such high temperatures alumina-ceria particles are still located between the montmorillonite layers. Although the total intensity is lower in the hightemperature treated samples, the intensity ratio between the peaks at 1.9 and 2.6 nm is higher in 973 and 1073 K than in 673 K treated sample, indicating that the amount of large pillars (ceria-alumina pillars) is proportionally higher than the amount of alumina pillars in the 973 and 1073 K treated catalysts. Thus, with temperature the small alumina particles seem to sinter and segregate before ceria-alumina agregates. The thermostabilizing role of cerium, even in such small agregates, is confirmed. The variation of surface area as a function of calcination temperature is presented in the same Figure 4. The surface area decreases continuously as the temperature is increased. This behavior indicates that the same cerium-aluminum species is present in the three samples, even if in the NCH120 sample the X-ray diffraction peak was not so clear. The surface area depends on the number of pillars; therefore, the number of these cerium-aluminum agregates, pillaring the montmorillonite layers, is determined by the calcining temperature. Catalytic Performance. These catalysts, previously calcined at 673 K, constituted by Ce-Al mixed oxide particles supported (or interlayered) on the clay, were tested in the selective formation of diisopropyl ether

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Figure 4. Evolution of interlayer spacings and surface areas as a function of calcination temperature. Table 4. Conversion (mol %) of 2-Propanol and Selectivities (mol %) of the Samples Calcined at 673 Ka

sample

2-propanol conversion (mol %)

selectivity to propene (mol %)

selectivity to DIPE (mol %)

montmorillonite γ-alumina CH NCH120 NCH168 NCH312

1 1 43 39 51 35

100 100 100 91 91 89

0 0 0 9 9 11

a Values obtained at 90 min of reaction; reaction temperature ) 423 K.

(DIPE) (Table 4). This reaction depends on the structure as well as on the acidity of the active site. The interlayer spacing (≈1.3 nm) should favor the diffusion of the reactant and product molecules, and hence, there should be free access to the alumina-ceria agglomerates. In these samples the amount of aluminum may vary but the correlation with selectivity has to be established with interlayer spacing: montmorillonite and γ-alumina samples were not active for the conversion of 2-propanol nor selective to DIPE. Still, Al-montmorillonite (CH sample) presents a conversion of 43% and no selectivity to DIPE. The addition of cerium (samples NCH120312) definitely promotes selectivity to 9-11% and conversion is maintained up to 35-51%. As expected, selectivity to propene decreases to 91-89% whereas it was 100% in montmorillonite, γ-alumina, and the CH sample. If the results obtained for samples NCH120 and -168 are compared, a correlation may be established between conversion and the number of Ce-Al pillars: sample NCH168 presents more pillars with the 2.6-nm interlayer distance as well as the highest conversion. Therefore, large and homogeneous Ce-Al particles increase conversion of 2-propanol from 39% to 51%, the increase being 30%.

The value obtained for the CH sample (43%) is, within error range, similar to the value obtained for NCH120; hence, a minimum amount of Ce-Al particles is required to increase conversion and sample NCH120 does not reach this amount. However, this percentage of CeAl particles in NCH120 is enough to differentiate this catalyst from the CH sample as it provides a selectivity to DIPE comparable to the NCH168 sample. Last, if the NCH312 sample is considered (this sample contains the highest amount of alumina and hence a high amount of Ce-Al pillars), it seems that the selectivity is determined by active sites present in the mixed cerium-aluminum pillars and, therefore, to more Ce-Al pillars, higher selectivity (11%). In this catalyst conversion turns out to be the same as that in CH or NCH120 samples and higher than that in γ-alumina. Thus, conversion depends on sites present in small alumina particles or Ce-Al pillars, but selectivity to DIPE is determined by Ce-Al particles. If both parameters have to be improved, a high amount of small alumina particles as well as Ce-Al pillars is required. If the results on La-Al (Jacobs et al., 1997) are extrapolated to Ce-Al mixed oxide particles, the CeAl particles in the clay layers are expected to present a higher amount of low-coordination aluminum cations than pure Al pillars. These sites constituted by the lowcoordination aluminum atoms provide the electronic configuration to obtain a good selectivity to DIPE. Then, the catalyst NCH168 treated at various temperatures (298, 673, 773, 873, and 973 K) was studied in the same reaction. In pillared samples, conversion diminished continuously from 51% (calcining temperature 673 K) to less than 4% in catalysts treated at 973 K. The variation is apparently determined by the amount of Al pillars in the sample, and although the amount of Ce-Al particles is significant in the NCH168 sample treated at 973 K, the conversion is extremely low. Furthermore, the segregated alumina pillars of this

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preparation must be occluded into the collapsed clay as no conversion is observed. The collapse of the clay may be attributed to the well-known dehydroxylation of the montmorillonite with calcining temperature (Figueras, 1988). Conclusion Montmorillonites pillared with Ce-Al mixed oxides were prepared by modifying the aluminum and cerium precursors. It was found that cerium sulfate did not provide the expected Ce-Al pillared samples, whereas cerium nitrate and chlorohydrol promoted mixed species. Still, if instead of chlorohydrol, aluminum chloride was used, again, the Ce-Al pillar formation is inhibited. This effect can be attributed to the highly hygroscopic character of the salt; the amount of water may, then, vary. The Ce-Al prepared samples were composed by two intercalated phases with interlayer spacings of about 1.9 and 2.6 nm. These two phases are stable up to 873 K with interlayer distances of 1.8 and 2.3 nm. For temperatures higher than 873 K the interlayer spacing is reduced to 1.8 nm, showing a single phase. Thus, these PILCs present a fairly high thermal stability. Selectivity to diisopropyl ether is determined by the presence of mixed oxide Ce-Al pillars. Meanwhile, conversion depends on sites present in small alumina particles as well as Ce-Al pillars. It seems that only sites present in Ce-Al pillars provide the low coordination aluminum atoms required by a selective mechanism. This behavior can be interpreted as the confirmation of the theoretically predicted structure (Jacobs et al., 1997) of the doped alumina surface. Acknowledgment G. Fetter gratefully acknowledges financial support of CONACYT. Literature Cited Alvarez L. J.; Leo´n, L. E.; Ferna´ndez, J.; Capita´n, M. J.; Odriozola, J. A. Micropore Formation Mechanisms in γ-Al2O3. Surf. Sci. 1995, 332, 185. Alvarez, L. J.; Ramı´rez-Solı´s, A.; Bosch, P. Mechanisms of Formation of Extra-Framework Al2O3 in Zeolites. Zeolites 1997, 18, 54.

Auer, H.; Hofmann, H. Pillared Clays: Characterization of Acidity and Catalytic Properties and Comparison with Some Zeolites. Appl. Catal. A 1993, 97, 23. Caballero, L.; Dominguez, J. M.; De los Santos, J. L.; Montoya, A.; Navarrete, J. In Synthesis of Microporous Materials: Zeolites, Clays and Nanostructures; Occelli, M., Ed.; Marcel Dekker: New York, 1996; Chapter 30. Fetter, G.; Tichit, D.; De Me´norval, L. C.; Figueras, F. Synthesis and Characterization of Pillared Clays Containing Both Si and Al Pillars. Appl. Catal. A 1995, 126, 165. Fetter, G.; Heredia, G.; Maubert, A. M.; Bosch, P. Synthesis of Al-Pillared Montmorillonites Using Microwave Irradiation. J. Mater. Chem. 1996, 6, 1857. Figueiredo, J. L.; Ramoa-Ribeiro, F. In Catalise Heteroge´ nea; Fundac¸ ao Calouste Gulbenkian: Lisboa, Portugal, 1989. Figueras, F. Pillared Clays as Catalysts. Catal. Rev.-Sci. Eng. 1988, 30, 457. Figueras, F.; Mattrod-Bashi, A.; Fetter, G.; Thrierr, A.; Zanchetta, J. V. Preparation and Thermal Properties of Zr Intercalated Clays. J. Catal. 1989, 119, 91. Figueras, F.; Klapyta, Z.; Massiani, P.; Mountassir, Z.; Tichit, D.; Fajula, F.; Gueguen, C.; Bousquet, J.; Auroux, A. Use of Competitive Ion Exchange for Intercalation of Montmorillonite with Hydroxy-Aluminum Species. Clays Clay Miner. 1990, 38, 257. Jacobs, J.; San Miguel, M. A.; Alvarez, L. J. Studies of LaAlO3 [100] Surfaces by Molecular Dynamics Simulations. J. Mol. Structure: Teochem 1997, 390, 1. Kloprogge, J. T. Synthesis of Smectites and Porous Pillared Clay Catalysts: A Review. J. Porous Mater. 1998, 5, 5. McCauley, J. R. Stable Intercalated Clays and Preparation Method. International Patent Application, PCT/US88/00567, 1988. Pinnavaia, T. J.; Tzou, M. S.; Landau, S. D.; Raythatha, R. H. On the Pillaring and Delamination of Smectite Clay Catalysts by Polyoxo Cations of Aluminum. J. Mol. Catal. 1984, 27, 195. Sterte, J. Preparation and Properties of Large-Pore La-Al-Pillared Montmorillonite. Clays Clay Miner. 1991, 39, 167. Tichit, D.; Fajula, F.; Figueras, F.; Ducourant, B.; Mascherpa, G.; Gueguen, C.; Bousquet, J. Sintering of Montmorillonites Pillared by Hydroxy-Aluminum Species. Clays Clay Miner. 1988, 36, 369. Trillo, J. M.; Alba, M. D.; Castro, M. A.; Poyato, J.; Tobı´as, M. M. Alumina Pillared Montmorillonite: Effect of Thermal and Hydrothermal Treatment on the Accessible Micropore Volume. J. Mater. Sci. 1993, 28, 373. Wenyang, X.; Yizhao, Y.; Xianmei, X.; Shizheng, L.; Taoying, Z. Catalytic Cracking Properties of Al-Zr-B Composite Pillared Clays. Appl. Catal. 1991, 75, 33.

Received for review June 10, 1999 Revised manuscript received December 9, 1999 Accepted December 21, 1999 IE990415X