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Physicochemical and Catalytic Properties of Sol-Gel Aluminas Aged under Hydrothermal Conditions J. Sa´nchez-Valente,*,† X. Bokhimi,‡ and F. Herna´ndez† Instituto Mexicano del Petro´ leo, Eje Central L. Ca´ rdenas 152, A.P. 14-805, 07730 Me´ xico D.F., Mexico, and Institute of Physics, The National University of Mexico (UNAM), A.P. 20-364, 01000 Me´ xico D.F., Mexico Received May 6, 2002. In Final Form: August 22, 2002 Sol-gel boehmite was aged under hydrothermal conditions at 200 °C using water as mineralizer agent. Samples were characterized with X-ray powder diffraction, nitrogen adsorption, and transmission electron microscopy. Calcined solids were also studied by infrared spectroscopy of adsorbed pyridine. Their catalytic properties were determined in the 2-propanol decomposition. The hydrothermal treatment ordered the atoms of the sol-gel boehmite mainly in two dimensions forming thin crystallites, which grew as treatment time increased. Due to the pseudomorphic transformation of boehmite into γ-alumina, the arrangement of crystallites in the corresponding boehmite determined alumina particle morphology and porosity. The pore size distribution was narrow, and the pore size shifted to larger values as the time of hydrothermal treatment increased. The strength and number of acid sites depended also on the treatment time. The catalytic activity correlated well with the acidity and specific surface area.
1. Introduction Aluminas have been extensively used as catalysts or supports in several petrochemical and petroleum refining processes, mainly due to their low cost, good thermal stability, high specific surface area, surface acidity, and the important interaction that they exhibit with deposited transition metals.1-4 Aluminas are generally produced industrially by precipitation, drying, and calcination of aluminum oxy-hydroxides, providing that they are free of impurities such as sodium and iron. The catalytic properties of aluminas largely depend on the crystalline structure and texture. Therefore a great effort has been devoted to master those physicochemical properties.4-12 Differences on surface properties have been reported for aluminas synthesized by different methods, although the final crystalline structure was the same.13 These differences can be partially explained by the fact that the synthesis method produces different hydration of the solid. Of course, a possible effect of impurities incorporated into alumina structure is not discarded. Among the synthesis methods that can be used to prepare aluminas, the sol-gel method provides an at* Corresponding author. E-mail
[email protected]. † Instituto Mexicano del Petro ´ leo. ‡ The National University of Mexico. (1) Kotanigawa, T.; Yamamoto, M.; Utiyama, M.; Hattori, H.; Tanabe, K. Appl. Catal. 1981, 1, 185. (2) Hellgardt, K.; Chadwick, D. Ind. Eng. Chem. Res. 1998, 37, 405. (3) Beguin, B.; Garboswki, E.; Primet, M. J. Catal. 1991, 127, 595. (4) Kno¨zinger, H.; Ratnasamy, P. Cata. Rev.-Sci. Eng. 1978, 17, 31. (5) Lippens, B. C. Thesis, Structure and Texture of Aluminas, Uitgeverij Waltman, Delft, 1961. (6) Ono, T.; Ohguchi, Y.; Togari, O. In Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: New York, 1983; p 631. (7) Trimm, D. L.; Stanislaus, A. Appl. Catal. 1986, 21, 215. (8) Tikhov, S. F.; Salanov, A. N.; Palesskaya, Yu. V. et al. React. Kinet. Catal. Lett. 1998, 64, 2, 301. (9) Music´, S.; Dragcˇevic´, D -; Popovic´, S. Mater. Lett. 1999, 40, 269. (10) Mishra, D.; Anand, S.; Panda, R. K.; Das, R. P. Mater. Lett. 2000, 42, 38. (11) Tsuchida, T. J. Eur. Ceram. Soc. 2000, 20, 1759. (12) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (13) Jiratova, K.; Beranek, L. Appl. Catal. 1982, 2, 125.
tractive and convenient route; this method enables an accurate control on the structural and textural properties: for instance, high specific surface area, and homogeneous distribution of pore size, and high purity. Despite its advantages, the use of the sol-gel method to prepare materials of catalytic interest is not yet as mature as in glasses and ceramics.14-17 About the porosity of sol-gel catalysts, several papers have claimed that the only way to obtain mesoporous solids with a high surface area requires the use supercritical or freeze-drying.17-24 Recently, different alumina aerogels have been prepared using supercritical drying technique. The solids were highly porous and showed a different morphology than solids obtained from xerogels.17-24 The main restriction for using these aerogels, however, comes from the poor knowledge on their physicochemical properties; for example, their thermal stability, phase transformation and morphological changes, together with the surface properties which depend on the structure and morphology of the precursor materials.15-26 To contribute to this knowledge, in the present work, the properties of sol-gel aluminas treated for several days under hydrothermal conditions at 200 °C were studied. Specific surface area and the pore size and shape were measured by nitrogen adsorption-desorption isotherms, because they determine some of the technological properties of the catalyst, such as bed volume, and product retention. X-ray powder diffraction (XRD) and transmis(14) Ward, D. A.; Ko, E. I. IEC Res. 1995, 34, 421. (15) Monaco, S. J.; Ko, E. I. Chem. Mater. 1997, 9, 2404. (16) Livage, J. Catal. Today 1998, 41, 3. (17) Pierre, A. C.; Elaloui, E.; Pajonk, G. M. Langmuir 1998, 14, 66. (18) Armor, J. N.; Carlson, E. J. J. Mater. Sci. 1987, 22, 2549. (19) Fanelli, A. J.; Verma, S.; Engelmann, T.; Burlew, J. V. Ind. Eng. Chem. Res. 1991, 30, 126. (20) Dong, J. S.; Tae-Jin, P. Chem. Mater. 1997, 9, 1903. (21) Pajonk, G. M. Catal. Today 1997, 35, 319. (22) Mizushima, Y.; Hori, M. J. Non-Cryst. Solids 1994, 167, 1. (23) Schneider, M.; Baiker, A. Catal. Rev.-Sci. Eng. 1995, 37, 515. (24) Yoldas, B. E. J. Mater. Sci. 1975, 10, 1856. (25) Wang, J. A.; Bokhimi, X.; Novaro, O.; Lopez, T.; Tzompantzi, F.; Gomez, R.; Navarrete, J.; Llanos, M. E.; Lopez-Salinas, E. J. Mol. Catal. A 1999, 137, 239. (26) Wang, J. A.; Bokhimi, X.; Morales, A.; Novaro, O.; Lopez, T.; Gomez, R. J. Phys. Chem. B 1999, 103, 299.
10.1021/la020423+ CCC: $25.00 © 2003 American Chemical Society Published on Web 04/01/2003
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sion electron microscopy (TEM) techniques were used to characterize the crystal size and morphology, respectively. Acid property plays an important role on the activity and selectivity of catalyst, therefore it was characterized by Fourier transform infrared spectroscopy (FTIR) of pyridine adsorption.25,27 The catalytic properties of aluminas were studied in the 2-propanol decomposition. This test reaction is largely reported,25,28,29 and establishes that dehydration of 2-propanol to propene depends on the quantity and strength of acid sites while dehydrogenation to cetone proceeds on the surface basic sites. The aim of this work was to study the textural, structural and surface properties of aluminas produced from hydrothermal treatment process of boehmite gels obtained by the sol-gel method. 2. Experimental Procedure 2.1. Synthesis. Aluminum tri-sec-butoxide (ATB) (Aldrich 97%) was dissolved and refluxed in absolute anhydrous ethyl alcohol (EtOH) (J. T. Baker) for 1 h. Hydrochloric acid used as a hydrolysis catalyst was added dropwise into the solution while stirring and refluxing for 3 h. Eventually, the system was cooled to room temperature, letting the hydrolysis to complete and form a transparent gel. The molar ratios of reactants were EtOH:ATB ) 60:1, HCl:ATB ) 0.03:1, and H2O:ATB ) 1:1. The gel was mixed with distilled water (1:1 in volume), which expanded it because water incorporated into the gel network. Then it was placed inside an autoclave that was sealed and heated to 200 °C under autogenous pressure for 3, 7 and 15 days. Thereafter the product was dried overnight at 100 °C. The samples were labeled as AlHC3Hy, AlHC7Hy, or AlHC15Hy, according to the days of treatment. Sample labeled AlHC1 corresponded to material without hydrothermal treatment. Dried solids were calcined in air from room temperature up to 400 °C at 2 °C/min, and then from this temperature to 700 °C at 4 °C/min, where it stayed for 4 h. Calcined solids were identified by adding the letter C to the nomenclature used for fresh samples. 2.2. Characterization. 2.2.1. X-ray Powder Diffraction. The X-ray powder diffraction patterns of the samples packed in a glass holder were recorded at room temperature with Cu KR radiation in a Bruker Advance D-8 diffractometer that had θ-θ configuration and a graphite secondary-beam monochromator. Diffraction intensity was measured between 10 and 110°, with a 2θ step of 0.02° for 8 s per point. 2.2.2. Electron Microscopy. Fresh and calcined samples were studied by transmission electron microscopy at a magnification of 400000×, in a JEOL 100CX microscope with a high-resolution polar piece. Samples were milled and dispersed in ethanol before placing them in the copper grill with Formvar support. 2.2.3. Textural Analysis. Specific surface area and pore size distributions were obtained from nitrogen adsorption-desorption isotherms measured in an automatic micromeritics ASAP-2100 analyzer. The adsorption was carried out on fresh and annealed solids, after outgassing at 200 and 400 °C, respectively, under a pressure of 10-5 Torr during 4 h. 2.2.4. Acidity. FTIR-pyridine adsorption technique was used to determine the types of acid sites on calcined samples. Experiments were carried out in a Fourier transform infrared (FTIR) spectrometer Perkin-Elmer model 170-SX. Prior to pyridine adsorption, the sample was put in a vacuum system and then heated to 500 °C at 20 °C/min and cooled to room temperature. After this previous heat treatment, it was exposed for 20 min to pyridine by breaking a capillary tube containing 100 µL of this substance. After adsorption, the infrared spectrum was recorded with the sample temperature fixed at 50, 150, 200, 300, and 400 °C while outgassing. 2.2.5. Catalytic Activity. The catalytic reaction was performed in a down-flow microreactor coupled with a gas chromatograph (HP 3700) equipped with a thermal-conductivity detector and a Porapak-Q packed column. 2-Propanol was mixed with argon as (27) Parry, E. P. J. Catal. 1963, 2, 371. (28) Ai, M.; Suzuki, S. Bull. Chem. Soc. Jpn. 1973, 46, 321. (29) Luy, J. C.; Parera, J. M. Appl. Catal. 1986, 26, 295.
Figure 1. X-ray diffraction patterns of the boehmite samples prepared under hydrothermal conditions for different synthesis times. carrier gas in a double H-type saturator placed in a container cooled at 0 °C with an ice-water mixture; this produced a 2-propanol partial pressure of 8.15 Torr. For the activity measurements, the mixed gas flowed at 1 mL/s over 40 mg of catalyst. Samples were previously activated in air at 400 °C for 1 h. The reaction was carried out at 140, 150, 160, and 170 °C for 2 h, while the product analysis was done every 10 min.
3. Results and Discussion 3.1. X-ray Diffraction. Sample crystallization depended on the hydrothermal treatment time; for a fixed temperature, crystallites were enlarged as treatment time was increased (Figure 1). Boehmite was the phases that was identified. Since crystal dimensions lay in the nanometric range, the corresponding X-ray diffraction peaks were very broad. An extensive study about the crystallography of this boehmite showed that the samples were constituted by crystallite plates with the b axis perpendicular to the plate surface.30 The peak corresponding to (020) plane was absent in the diffraction pattern of the AlHC1 sample, which is characteristic for sol-gel boehmite, which reflected a poor ordering along the b axis.30 From the same study was also concluded that taking two aluminum atoms per projection of the unit cell on any 〈101〉 direction this gives an average of 7.61, 6.76, and 4.11 Al atoms per gram of boehmite for the samples prepared during 3, 7, and 15 days, respectively.30 It was also found that the transformation of boehmite into γ-alumina depended on boemite’s crystallite size, because the transformation was pseudomorphic. The particle size of the γ-alumina also depended on boehmite’s crystallite dimension. These conclusions were derived after modeling the anisotropic morphology of the crystals; more details about these results are found in ref 30. 3.2. Electron Microscopy. To exemplify the effect of the hydrotreatment on aluminas, in parts a-d of Figure 2 are shown the micrographs of the samples treated for 3 and 15 days under hydrothermal conditions, before and after calcining. Boehmite crystallites were plates with dimensions along the plate notoriously larger than perpendicular to it (Figure 2, parts a and c). The micrograph of the fresh sol-gel sample did not show planar objects; instead, the particles look more like folded sheets.30 The main effect of the hydrothermal treatment on the (30) Bokhimi, X.; Sa´nchez-Valente, J.; Pedraza, F. J. Solid State Chem. 2002, 166, 182.
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Table 1. Specific Surface Area, Average Pore Diameter, and Total Pore Volume of Fresh and Calcined Samples sample
BET surface area (m2/g)
average pore diameter (nm)
total pore volume (cm3/g)
AlHC1 AlHC3Hy AlHC7Hy AlHC15Hy AlHC1C AlHC3HyC AlHC7HyC AlHC15HyC
528 104 85 48 345 132 77 66
3.4 19.8 34.0 40.2 6.2 20.0 38.7 48.0
0.70 0.50 0.59 0.40 0.83 0.65 0.58 0.44
sol-gel sample was to order the folded layers formed during gelling, building flat plates with a rhombic morphology.30 When boehmite samples were calcined, the morphology produced by their crystallites did not change (Figure 2, parts b and d); the stacking and orientation of original boehmite layers were preserved. These results disclose that the boehmite morphology is responsible for the final morphology of the γ-alumina. The obtained alumina had a “memory” of the boehmite’s crystallite orientation. Since the transformation from boehmite into γ-alumina is pseudomorphic, the morphology of the particles was kept. The internal composition of the γ-alumina plates, however, was different from that in boehmite: it was made of very small γ-alumina crystallites30 with many pores between them created by the loss of boehmite’s hydroxyls during its transformation. 3.3. Texture Analysis. A decrease of the specific surface area in both boehmite and γ-alumina was observed when the gel was annealed under hydrothermal conditions (Table 1). This treatment also determined the crystallite size,30 and the comparison of both parameters, crystallite size and specific surface area, shows that when the crystal size was increased the surface area decreased. It is wellknown that the range of the specific surface area can vary widely depending upon the particle size and shape and also the porosity. The relationship between surface area Sp and the total volume of pore Vp and the average pore radius rp is given by
Figure 2. TEM micrographs of the samples synthesized for 3 and 15 days, zone axis was parallel to b axis: (a) 3 days fresh sample (boehmite), (b) 3 days calcined sample (γ-alumina), (c) 15 days fresh sample (boehmite), (d) 15 days calcined sample (γ-alumina).
Sp ) f Vp/rp where f depends on the pore’s geometry; for instance, whether they are cylindrical, spherical, or formed by parallel plates.31 In Table 1 we can observe that the size and total volume of pore were determined by the hydrothermal treatment time of boehmite, when the time gets longer, rp increased and Vp decreased, giving rise to the Sp diminution. The sample without hydrothermal annealing, AlHC1, had a high BET surface area (Figure 3 and Table 1). Upon calcination, this area decreased from 528 to 345 m2/g, caused by the high mobility of the layers, because the AlHC1 sample was made of quasi-isolated boehmite layers.30 In fact an ordering was observed in the hysteresis loop of AlHC1C (Figure 3). According to IUPAC classification,32 the hysteresis loop changed from the H2 type for boehmite to a H1 type isotherm for the calcined sample. The H2 hysteresis loop type is typically found on solids with heterogeneous pore size distribution and nonuniform particle size. Conversely, the H1 hysteresis loop corresponds to a homogeneous pore distribution; therefore, the (31) Lecloux, A. In Comptes-rendus de la semaine d’e´ tude de la catalyse; Universite´ de Lie`ge: Lie`ge, 1970; Vol. 64, p 169. (32) Sing, K. S. W.; et al. Pure Appl. Chem. 1985, 57, 603.
Figure 3. Adsorption-desorption isotherms of fresh and annealed alumina gels: AlHC1 [, AlHC1C 9, AlHC7Hy b, AlHC7HyC 2.
pore’s shape and ordering changed considerably during the calcination. In samples prepared under hydrothermal conditions the change in specific surface area was not so large as the one described in the last paragraph for the sample without hydrothermal treatment (Figure 3 and Table 1). The largest change was observed in the sample synthesized for 3 days, AlHC3Hy. In this case the specific surface changed from 104 to 132 m2/g when the sample was calcined, which was produced by the formation of additional pores when a single boehmite crystallite was transformed into many small γ-alumina crystallites.30 The new pores were additional to the initial pores formed by the arrangement of boehmite crystallites, which did not changed when boehmite was transformed into alumina (Figure 2b). The samples synthesized for longer times did not showed any significant changes after calcination (Table 1).
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Figure 4. Effect of thermal treatment on the pore size distribution of boehmites AlHC1 [, AlHC3Hy 9, AlHC7Hy 2, AlHC15Hy b.
Figure 5. Effect of thermal treatment on the pore size distribution of fresh and annealed alumina gels: AlHC1 9, AlHC1C [, AlHC7Hy 2, AlHC7HyC b.
All solids were mesoporous (Figures 4 and 5 and Table 1). In boehmite, crystallite stacking determined the pore formation; therefore, the pore size was strongly related to the boehmite’s crystallite size: large boehmite crystallites produced large pores (Figure 4). This also explains why the average pore size was large when the annealing time was increased (Figure 4 and Table 1), because crystallites grew with synthesis time. Since boehmite crystallites were thin plates, the pore’s form was laminar, characterized by having one of its dimensions smaller than the other two. Since the transformation of boehmite into γ-alumina is pseudomorphic (Figures 2), these laminar pores were also present in the alumina; their dimensions were almost the same, except for the sol-gel sample with no hydrothermal treatment (Figure 5 and Table 1). Although pore dimension increased with the annealing time, the total pore volume decreased (Table 1). This can be related to an effect of the enlargement of crystallites with the synthesis time. They produced larger pores, but they also occupied more space, reducing the volume associated with pores. Boehmite’s crystallite growing occurring during the hydrothermal treatment, and the formation of crystallite plates, can be followed by comparing the adsorptiondesorption isotherms and the hysteresis loops of fresh and calcined solids (Figure 6). The shape of the hysteresis loop changed from H2 to H3 type as the hydrothermal
Sa´ nchez-Valente et al.
Figure 6. Adsorption-desorption isotherms of fresh alumina gels AlHC1 [, AlHC3Hy 2, AlHC7Hy b, AlHC15Hy 9.
treatment time was increased. The H2 hysteresis loop type produced by AlHC1 sample revealed the lack of uniform size and shape of pores,33 which is in agreement with the randomly folded boehmite sheets that were observed for these samples in TEM and XRD.30 The H3 type hysteresis curves are usually found in solids made of aggregates or agglomerates of particles forming slit shape pores:33 this agrees with the observation made by electron microscopy and X-ray diffraction that pores are formed by the arrangement of boehmite crystalline plates. Pores that have a type H3 hysteresis loop can be formed with parallel plates at some distance from each other. Since in a pore, the overlapping potentials of the walls overcome the translational energy of an adsorbate molecule, condensation will occur at a lower pressure in a pore than on an open or a plane surface. Thus, as the relative pressure P/Po is increased, condensation will occur first in pores of smaller radii and will progress onto the larger pores until the relative pressure is equal to one. In Figure 6, we can appreciate an increase on the relative pressure where the hysteresis starts as the crystallite size increased, we do remember that boehmite’s crystallite size determined the pore shape and the pore size distribution. This means that the capillary condensation began at much higher relative pressure than in the solid with no hydrothermal treatment. Besides, hysteresis broadening diminished as the crystallite size increased which gave a decreasing of the total pore volume.34 It is worth noticing that the pore size distribution was narrow (Figure 4), which is very important in catalysis, in addition to the nature of the active sites developed on the pore walls. 3.4. FTIR-Pyridine Adsorption. Pyridine adsorption was carried out on calcined samples. The infrared spectra of adsorbed pyridine showed characteristic absorption bands at 1580, 1490, and 1446 cm-1, which corresponded to pyridine adsorbed on Lewis acid sites.25,27 To illustrate the behavior of the γ-aluminas, the FTIR spectra of them are shown in Figures 7-10. The intensities of the absorption bands at 1445, 1491, and 1579 cm-1, gradually decreased when the desorption temperature increased. The band at 1609 cm-1 corresponded to hydroxyls in the γ-alumina crystalline structure; its intensity diminished when the temperature was increased, due to dehydroxylation.25 (33) Leofanti, G.; Padovan, M.; Tozzola, G.; Venturelli, B. Catal. Today 1998, 41, 207. (34) Lowell, S.; Shields, J. E. Powder Surface Area and Porosity, 3rd ed.; Chapman & Hall: New York, 1991; Chapters 3 and 8.
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Figure 10. Spectra of FTIR-pyridine adsorbed on AlHC15HyC sample, with thermal desorption treatment. Figure 7. Spectra of FTIR-pyridine adsorbed on AlHC1C sample, with thermal desorption treatment.
Table 2. Amounts of Pyridine Adsorbed on γ-Aluminas as a Function of the Desorption Temperature temp (°C)
AlHC1
50 150 200 300 400 500
395 105 20 10 0 0
acidity (µmol/g) AlHC3HyC AlHC7HyC 242 119 55 29 20 0
125 70 40 17 14 0
AlHC15HyC 94 60 42 14 13 0
Table 3. Total Acidity and Intrinsic Acidity on γ-Aluminas sample
total acidity (µmol/g)a
IA (µmol/m2)
AlHC1C AlHC3HyC AlHC7HyC AlHC15HyC
135 223 141 129
0.39 1.69 1.83 1.95
a
Figure 8. Spectra of FTIR-pyridine adsorbed on AlHC3HyC sample, with thermal desorption treatment.
Figure 9. Spectra of FTIR-pyridine adsorbed on AlHC7HyC sample, with thermal desorption treatment.
The strength of the acid sites is related to the maximum of temperature at which the pyridine is retained. When desorption temperature was increased from 50 to 400 °C, the relative intensity of the bands were reduced in a similar way. At 500 °C, absorption bands disappeared entirely.
Total acidity taking into account values above 150 °C.
Table 2 summarizes the results of all samples. The hydrothermal treatment initially produced stronger acid sites and higher total acidity. The solid with no hydrothermal treatment, retained pyridine until 300 °C, while the others aluminas reached the maximum at 400 °C. The quantity of pyridine retained decreased as the treatment time increased (Table 2). Total acidity was calculated considering only the acid sites that retained pyridine above 150 °C. The intrinsic acidity IA was the total acidity referred to the specific surface area of a solid. The largest amount of acid sites was obtained on AlHC3HyC sample (Table 3). If we assume that Lewis sites are associated with aluminum atoms and that the transformation from boehmite to γ-alumina is pseudomorphic, then the amount of acid sites agree with the quantity of aluminum atoms per gram of boehmite measured by XRD.30 However, in terms of IA, the total acidity increased for longer hydrothermal treatment times (See Table 3). The position of the band produced by OH groups changed gradually with the hydrothermal annealing time of the gel: it shifted from 1609 to 1610 and 1613 cm-1 for the samples annealed for 3, 7, and 15 days, respectively. 3.5. Catalytic Activity. The 2-propanol decomposition was carried out between 140 and 170 °C; the reaction showed 100% selectivity to propene. Table 4 reports the intrinsic and specific activities, AlHC1C being the less active catalyst. The samples aged under hydrothermal conditions were more active (Table 4 and Figure 11).
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sample
Vo (mol/g s × 10-6)
Vo (mol/s m2 × 10-8)
Ea (kJ/mol)
Bo (sites/g × 1019)
Kd (1/s × 10-4)
AlHC1C AlHC3HyC AlHC7HyC AlHC15HyC
0.51 3.27 1.79 0.89
0.15 2.48 2.32 1.35
97 108 119
4.63 2.50 1.21
1.71 0.63 0.73 1.52
a
Tr ) 170 °C. Flow: 0.9 mL/s. P2-propanol ) 8.104 Torr.
solid. The tendency observed was proportional to the number of exposed aluminum calculated on boehmites by Rietveld refinement30 and the number of Lewis acid sites measured by pyridine adsorption on calcined solids. The intrinsic reaction rate varied in similar proportion as well. Conclusions
Figure 11. Catalytic activity on 2-propanol dehydration: AlHC1C [, AlHC3HyC 9, AlHC7HyC 2, AlHC15HyC b.
Trends are in agreement with the results obtained by FTIR, where AlHC1C showed the weakest acid centers. It is worth noticing that the catalysts did not show deactivation. The catalytic activities, given in Table 4, decreased with the hydrothermal treatment. This behavior can be understood in function of the total acidity. Thereby the sample annealed for 3 days had the largest activity, which also had the largest total acidity (Table 3) and the largest specific surface area (Table 1). The activation energies Ea calculated from the apparent reaction rate constant are presented in Table 4; they can be directly related to the number and strength of the active sites on a given solid. In our case, for longer treatment time the activation energy increased. A good correlation was obtained using the equation proposed by Wynne-Jones and Eyring,35 which relates the equilibrium rate constant with Ea. According to this theory, the rate constant can be written as
Kr ) kT/hBo exp(-Ea/RT) exp(∆S/R) where k and h are the Boltzmann and Planck constants, Ea and ∆S are the energy and entropy of activation, and Bo the number of sites. This entropy represents the difference of entropy between the adsorbed state and the activated complex, and since the largest term in entropy is due to molecular translation, the assumption that the activation entropy is negligible is quite reasonable. In this sense, we can determine the active sites density Bo on a (35) Wynne-Jones, W. F. K.; Eyring, H. J. Chem. Phys. 1935, 3, 492.
The hydrothermal treatment of sol-gel boehmite produce thins boehmite crystalline layers with dimensions that depended on the treatment time. The calcination of these layers gave rise to γ-alumina particles of the same dimensions along the boehmite plates; but in the direction perpendicular to them, boehmite crystallites collapsed forming thicker layers of alumina than those of boehmite. The morphology of the γ-alumina was mainly determined by the arrangement of boehmite crystallites, which formed laminar pores, that are related to the size and shape of boehmite crystallites. However, some pores were formed within the boehmite platelets upon calcination, as well. The specific surface area and the total pore volume in boehmite and γ-alumina decreased when boehmite crystal size increased. The pore size distribution was narrow and around a pore size that shifted to larger values as the hydrothermal annealing time was increased. Only Lewis acid sites were observed by FTIR of pyridine adsorption on γ-aluminas. The strength and the amount of acid sites depended on the time of hydrothermal treatment: these parameters increased when the treatment was longer. The most important amount of acid sites was obtained for AlHC3HyC sample. Catalytic test showed the interrelation between structural properties and acidity of catalysts, which can be controlled from boehmite precursor using a thermal treatment suited for the intended reaction. The morphology and porosity of the γ-alumina were interdependent with the crystallite dimension and morphology of the corresponding boehmite precursor. The present work is a good example on how the control of boehmite properties provides a direction to control the morphology of the γ-alumina and its surface acidity. Acknowledgment. We thank Mr. A. Morales, Mr. M. Aguilar, and Ms. V. Aranda for technical assistance; this work was financially supported by Project D.01024, IMP-Maya Crude Oil Research Program. LA020423+
