Sulfated Zirconia Nanoparticles Synthesized in Reverse Microemulsions

Kawai et al. have used nonionic surfactants for the synthesis of monodisperse ZrO2 particles in reverse micelles but without reporting catalytic prope...
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Sulfated Zirconia Nanoparticles Synthesized in Reverse Microemulsions: Preparation and Catalytic Properties Holger Althues and Stefan Kaskel* Max-Planck-Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D 45470 Mu¨ lheim an der Ruhr, Germany Received March 7, 2002. In Final Form: June 21, 2002 Microemulsion-derived sulfated zirconia was prepared by addition of zirconium butylate to mixtures of aqueous H2SO4, nonionic surfactants, and heptane as the oil phase and subsequent drying and calcination at 873 K. The resulting catalysts have surface areas ranging from 40 up to 175 m2 g-1 depending on the microemulsion composition and show a high activity in n-butane isomerization (4.7 × 10-7 mol s-1 g-1, TOS ) 60 min) comparable to that of other commercially available catalysts. The size of the inverse micelles in the microemulsion can be used to tailor the properties of the catalyst in a wide range. In situ X-ray diffraction studies demonstrate significant differences in the crystallization kinetics of materials derived from low and high water/surfactant ratios (Rw). Transmission electron microscope investigations show that a higher degree of crystallite agglomeration and sintering is responsible for lower surface areas of high Rw derived zirconias. According to thermogravimetric studies coupled with mass spectrometry, the lower activity of low Rw derived materials is due to a lower degree of sulfate incorporation. The parameters in the synthesis procedure which affect the activity of the catalyst are systematically studied, and the advantages and limitations of the microemulsion method are discussed.

Introduction The development of methods which allow effective and controlled preparation of small particles (nanoparticles) is presently one of the most active research areas.1 They have applications in catalysis, electronics, miniaturization, and ceramics; physical properties attributed to the reduction of dimensions, like the quantum-size effect, are of fundamental interest. Of special interest in catalysis is the design of small particles with high and accessible surface area and welldefined surface structure. Sol-gel methods in which alkoxides are hydrolyzed and converted into gels and powders with high surface area have been developed, but nucleation and particle size are difficult to control. Our interest in particle formation is to use inverse micelles filled with water as a space-confined reactant for the condensation of molecular alkoxide precursors to generate small particles. In this space-confined sol-gel process, the amount of water in the micelle determines the size of the primary hydroxide particle. Hydrolysis of alkoxides is a rapid reaction. If the micelles are well separated, the coalescence of the micelles proceeds at a much lower rate than the hydrolysis reaction. In the case of inverse micelles, separation is achieved by dilution with a nonpolar solvent like hexane. For catalytic applications, the introduction of additives in the course of the synthesis procedure is often crucial to achieve a homogeneous distribution of promoters which improve the performance of the catalyst. Since inverse micelles can be viewed as reactors which are only a few nanometers in size, inorganic salts or acids can be dissolved into the core of the micelle. During the addition of the alkoxide precursor, these promoters are incorporated into the hydroxide particles. Due to the separation of the micelles, successive precipitation, a common problem in precipitation reactions with two or more components with * E-mail: [email protected]. Phone: 49-208-3062371. Fax.: 49-208-306-2995. (1) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley VCH: New York, 2001.

differing solubilities, is avoided. The microemulsion technique produces a homogeneous distribution of the promoter (in the following we will use the term w/o microemulsion as a synonym for an inverse micelle system even if a one-phase system in proximity to the phase boundary is meant2). The fundamentals of nanoparticle syntheses in microemulsions have been described by Lade et al.3 and in several monographs.4,5 The microemulsion technique has been used to a wide extent for the preparation of metal catalysts.6-16 Only recently, more and more oxides are reported. Ying has reported the microemulsion-mediated preparation of BHA catalysts for high-temperature applications with surface areas in excess of 100 m2 g-1 even after calcination at 1573 K.17-19 Recent studies have focused on ultrafine particles of aluminum oxidehydroxide,20 Ba(Mg1/3Ta2/3)O3,21 cerium oxide,22-28 TiO2,29-31 (2) Shinoda, K.; Friberg, S. Emulsions and Solubilization; John Wiley & Sons: New York, 1986. (3) Lade, M.; Mays, H.; Schmidt, J.; Willumeit, R.; Schoma¨cker, R. Colloids Surf., A 2000, 163, 3-15. (4) Kumar, P.; Mittal, K. L. Handbook of Microemulsion Science and Technology; Marcel Dekker: New York, 1999. (5) Texter, J. Reactions and Synthesis in Surfactant Systems; Marcel Dekker: New York, 2001. (6) Bradley, J. In Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, 1994; Vol. 1, pp 459-544. (7) Agrell, J.; Hasselbo, K.; Jansson, K.; Ja¨ras, S. G.; Boutonnet, M. Appl. Catal., A 2001, 211, 239-250. (8) Hanaoka, T.; Hayashi, H.; Tago, T.; Kishida, M.; Wakabayashi, K. J. Colloid Interface Sci. 2001, 235, 235-240. (9) Hayashi, H.; Murata, S.; Tago, T.; Kishida, M.; Wakabayashi, K. Chem. Lett. 2001, 34-35. (10) de Jesus, D. M.; Spiro, M. Langmuir 2000, 16, 4896-4900. (11) Ikeda, M.; Takeshima, S.; Tago, T.; Kishida, M.; Wakabayashi, K. Catal. Lett. 1999, 58, 195-197. (12) Kishida, M.; Ichiki, K.; Hanaoka, T.; Nagata, H.; Wakabayashi, K. Catal. Today 1998, 45, 203-208. (13) Kishida, M.; Hanaoka, T.; Hayashi, H.; Tashiro, S.; Wakabayashi, K. Stud. Surf. Sci. Catal. 1998, 118, 265-268. (14) Kishida, M.; Hanaoka, T.; Kim, W. Y.; Nagata, H.; Wakabayashi, K. Appl. Surf. Sci. 1997, 121, 347-350. (15) Kishida, M.; Umakoshi, K.; Ishiyama, J.; Nagata, H.; Wakabayashi, K. Catal. Today 1996, 29, 355-359. (16) Boutonnet, M.; Kizling, J.; Touroude, R.; Maire, G.; Stenius, P. Catal. Lett. 1991, 9, 347-354.

10.1021/la0202327 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/27/2002

Sulfated Zirconia from Reverse Microemulsions

erbium oxide,32 iron oxide,33-36 BaTiO3,37-39 ZnO,40,41 and manganese oxide.42 Sulfated zirconia is a promising catalyst for n-alkane isomerization reactions at low temperatures.43 The microemulsion technique allows introduction of sulfate in the inverse micelles, which may be beneficial as compared to impregnation methods. The reaction of zirconium alkoxides with the H2SO4/H2O swollen inverse micelles is suitable to produce very homogeneous sulfate distributions. In the following, this will be called in situ sulfatation. Kawai et al. have used nonionic surfactants for the synthesis of monodisperse ZrO2 particles in reverse micelles but without reporting catalytic properties.44 Auvray et al. have studied the fundamentals of zirconia nucleation in inverse micelles using small-angle neutron scattering.45 Boutonnet Kizling et al. have studied the performance of microemulsion-derived sulfated zirconia as a support for platinum catalysts in hexane isomerization reactions, but the unpromoted sulfated zirconia did not show catalytic activity at 573 K.46 In the following, we will present unpromoted microemulsion-derived sulfated zirconia with high activity for (17) Zarur, A. J.; Mehenti, N. Z.; Heibel, A. T.; Ying, J. Y. Langmuir 2000, 16, 9168-9176. (18) Zarur, A. J.; Hwu, H. H.; Ying, J. Y. Langmuir 2000, 16, 30423049. (19) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65-67. (20) Berkovich, Y.; Aserin, A.; Wachtel, E.; Garti, N. J. Colloid Interface Sci. 2002, 245, 58-67. (21) Lee, Y. C.; Liang, M. H.; Hu, C. T.; Lin, I. N. J. Eur. Ceram. Soc. 2001, 21, 2755-2758. (22) Zhang, J.; Ju, X.; Wu, Z. Y.; Liu, T.; Hu, T. D.; Xie, Y. N.; Zhang, Z. L. Chem. Mater. 2001, 13, 4192-4197. (23) Masui, T.; Fujiwara, K.; Peng, Y.; Sakata, T.; Machida, K.; Mori, H.; Adachi, G. J. Alloys Compd. 1998, 269, 116-122. (24) Masui, T.; Fujiwara, K.; Peng, Y.; Machida, K.; Adachi, G. Chem. Lett. 1997, 1285-1286. (25) Masui, T.; Fujiwara, K.; Machida, K.; Adachi, G.; Sakata, T.; Mori, H. Chem. Mater. 1997, 9, 2197-2204. (26) Martinez-Arias, A.; Fernandez-Garcia, M.; Hungria, A. B.; Conesa, J. C.; Soria, J. J. Alloys Compd. 2001, 323, 605-609. (27) Martinez-Arias, A.; Fernandez-Garcia, M.; Iglesias-Juez, A.; Hungria, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Appl. Catal., B 2001, 31, 51-60. (28) Fernandez-Garcia, M.; Martinez-Arias, A.; Iglesias-Juez, A.; Belver, C.; Hungria, A. B.; Conesa, J. C.; Soria, J. J. Catal. 2000, 194, 385-392. (29) Kim, E. J.; Oh, S. H.; Hahn, S. H. Chem. Eng. Commun. 2001, 187, 171-184. (30) Kim, E. J.; Hahn, S. H. Mater. Lett. 2001, 49, 244-249. (31) Boutonnet Kizling, M.; Bigey, C.; Touroude, R. Appl. Catal., A 1996, 135, L13-L17. (32) Que, W.; Zhou, Y.; Kam, C. H.; Lam, Y. L.; Chan, Y. C.; Gan, L. H.; Deen, G. R. Mater. Res. Bull. 2001, 36, 889-895. (33) Dresco, P. A.; Zaitsev, V. S.; Gambino, R. J.; Chu, B. Langmuir 1999, 15, 1945-1951. (34) Lopez Perez, J. A.; Lopez Quintela, M. A.; Mira, J.; Rivas, J.; Charles, S. W. J. Phys. Chem. B 1997, 101, 8045-8047. (35) Mira, J.; Lopez-Perez, J. A.; Lopez-Quintela, M. A.; Rivas, J. Mater. Sci. Forum 1997, 235-238, 297-302. (36) Chhabra, V.; Ayyub, P.; Chattopadhyay, S.; Maitra, A. N. Mater. Lett. 1996, 26, 21-26. (37) Beck, C.; Ha¨rtl, W.; Hempelmann, R. J. Mater. Res. 1998, 13, 3174-3180. (38) Herrig, H.; Hempelmann, R. Nanostruct. Mater. 1997, 9, 241244. (39) Herrig, H.; Hempelmann, R. Mater. Lett. 1996, 27, 287-292. (40) Lim, B. P.; Wang, J.; Ng, S. C.; Chew, C. H.; Gan, L. M. Ceram. Int. 1998, 24, 205-209. (41) Hingorani, S.; Pillai, V.; Kumar, P.; Multani, M. S.; Shah, D. O. Mater. Res. Bull. 1993, 28, 1303-1310. (42) Li, J.; Wang, Y. J.; Zou, B. S.; Wu, X. C.; Lin, J. G.; Guo, L.; Li, Q. S. Appl. Phys. Lett. 1997, 70, 3047-3049. (43) Yadav, G. D.; Nair, J. J. Microporous Mesoporous Mater. 1999, 33, 1-48. (44) Kawai, T.; Fujino, A.; Kon-No, K. Colloids Surf., A 1996, 109, 245-253. (45) Auvray, L.; Ayral, A.; Dabadie, T.; Cot, L.; Guizard, C.; Ramsay, J. D. F. Faraday Discuss. 1995, 101, 235-247. (46) Boutonnet Kizling, M.; Regali, F. Stud. Surf. Sci. Catal. 1998, 118, 495-504.

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n-butane isomerization at 573 K comparable to that of commercially available catalysts. Systematic variations in the composition of the microemulsion and preparation conditions reveal the crucial parameters which affect surface area and activity of microemulsion-derived sulfated zirconia catalysts and show the advantages but also limitations of the method. Experimental Section Microemulsion. The phase diagrams were obtained by adding the surfactant (Lutensol AO3: RO(CH2CH2O)xH, x ) 3, R ) C13/ h ) 340, 100%, BASF; Lutensol AO5: RO(CH2C15-oxoalcohol, M h ) 430, 100%, BASF; CH2O)xH, x ) 5, R ) C13/C15-oxoalcohol, M h Marlophen NP5: RO(CH2CH2O)xH, x ) 5, R ) nonylphenyl, M ) 420, 100%, CONDEA; 1-octanol, 99%, Aldrich) to a mixture of n-heptane (>99%, Haltermann) and deionized water under vigorous stirring. The equilibration time was at least 15 min. The reverse microemulsions were set aside to verify that phase separation would not occur after stirring was stopped. If necessary, additional surfactant was added. Syntheses. The microemulsion was prepared as explained above. Stable isotropic solutions were used for particle preparations. The precursor (10 g of zirconium n-butylate, 80% in n-butanol, Aldrich) was added dropwise to the microemulsion under stirring. After 1 h, the white precipitate was filtered out, washed with 100 mL of n-heptane, and dried at 363 K for 20 h. The dried gel was heated at a rate of 5 K min-1 to 873 K and annealed at that temperature for 2 h to give a white, carbon-free powder. Nitrogen Physisorption Measurements. The BrunauerEmmett-Teller (BET) surface area was determined using a micromeritics ASAP2010 instrument. The samples were activated at 473 K for 4 h, and the nitrogen adsorption isotherm was recorded at 77.35 K. Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) Analysis. High-resolution TEM studies were performed on a Hitachi HF2000 electron microscope. The samples were prepared by dipping the copper grid into the powder. XRD powder patterns were recorded on a θ/2θ-STADI-P diffractometer (Stoe) equipped with a position-sensitive detector (7° opening) and a germanium primary beam monochromator. For crystallite size determinations, the single-line size/strain analysis was used (Visual XPOW, Stoe). The monoclinic phase was also taken into account for size/strain analysis if present. For in situ investigations, a θ/θ-STADI-P diffractometer (Stoe) was used equipped with a high-temperature chamber (HTK, Bu¨hler). The dried gel was heated in air in one step to 573 K at a rate of 10 K min-1 and in 50 K steps with the same rate to 1173 K. In each step, the temperature was kept constant for 30 min for the registration of the XRD pattern. Thermal Analyses. Thermogravimetric analyses were carried out using a Netzsch, STA 449 C Jupiter instrument equipped with a Balzer MS Quadrastar 422 mass spectrometer. The samples were heated at a rate of 5 K min-1 to 1273 K in air. Sulfate decomposition was detected by monitoring the SO (m ) 48) and SO2 (m ) 64) signals. Catalytic Tests. The n-butane conversion was measured over 0.5 g of catalyst at 573 K in a vertical flow reactor (10 mm in diameter, 70 cm length) using a 1:1 mixture of n-butane and nitrogen. The total flow rate was 10 mL min-1. The products were analyzed on-line using a Carlo Erba GC 6000 equipped with a DB-1 column and flame ionization detector (FID). The calcined catalyst was pressed into pellets, crushed, and sieved into a fraction of 0.5-1 mm. Activation of the catalyst was carried out at 873 K in air. The conversion was compared to that of a reference MEL (XZO 682) sulfated zirconia catalyst with Sg ) 100 m2 g-1 (predominately tetragonal zirconia) and a reaction rate of 3.1 × 10-7 mol s-1 g-1 after 1 h on stream.

Results and Discussion The micelle-mediated sol-gel process consists of three steps (Figure 1). First, the microemulsion is prepared by adding water to a mixture of the nonionic surfactant and

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Figure 2. Oil-rich region of the phase diagram for the surfactants Lutensol AO3, Lutensol AO5, Lutensol AO5 + cosurfactant, and Marlophen NP5 (I ) isotropic solution, II ) two phase region, x ) volume fraction).

Figure 1. Schematic representation of the synthesis procedure for microemulsion-derived zirconia.

the oil phase n-heptane. The surfactants used in this study are commercial products with a broad molecular weight distribution, and phase diagrams are not available. For this reason, it was necessary to locate the one-phase region in the oil-rich corner of the (pseudo)ternary phase diagram. In the second step, the precursor Zr(n-C4H9O)4 is added slowly. The hydrolysis generates amorphous zirconium oxide hydroxide particles which are agglomerated. Compared to other microemulsion-derived materials, the agglomeration allows the hydroxide to be filtered out and does not require centrifugation. After the solution is aged for 1 h, the precipitate is filtered out and dried. Finally, in the third step the product is heated to 873 K in air in order to convert the hydroxide into crystalline zirconia. Microemulsion. The composition of the microemulsion can be used to tailor the properties of the sol-gel-derived materials since to a first approximation the amount of water in each micelle determines the primary particle size. The inverse micelle size depends on the water/ surfactant ratio:

Rw )

n(H2O) n(surfactant)

The ability of the surfactant/alkane mixture to solubilize water is limited and varies with the structure of the surfactant, the oil phase, and the temperature. At room temperature, a large one-phase region and thus a wide range of Rw values are accessible in the oil-rich side of the phase diagram in mixtures with a phase inversion temperature (PIT) close to room temperature.2 We have not determined PITs but adjusted the microemulsion composition and located the phase boundary of the onephase region for various surfactants (Figure 2). For Lutensol AO5 in combination with n-heptane, the hydrophilic-lipophilic balance (HLB) is too high and the inverse system can solubilize only small amounts of water.2 Octanol may be used as a cosurfactant to realize higher Rw values. For surfactants with lower HLB values such as Marlophen NP5 and Lutensol AO3, the phase boundary is depressed. For example, a 10 wt % mixture of Lutensol

AO5 in n-heptane can solubilize about 1% water whereas AO3 solubilizes 2% and Marlophen NP5 about 6%. The maximum Rw is 10 for AO5, 35 for AO3, and 50 for NP5. For the in situ sulfatation, it is necessary to use sulfuric acid instead of water which also affects the phase diagrams. Our results indicate that for concentrations of 1 mol L-1 and below the phase boundary is not significantly shifted. However, higher concentrations of sulfuric acid and especially salts such as ammonium, iron, or manganese sulfate raise the phase boundary and reduce the solubilization of water.44 Rw values alone are insufficient to characterize microemulsions. In the following, we will use the oil/water ratio

RO )

n(oil) n(H2O)

for this purpose. RO affects the separation (coalescence rate) of the micelles.47 Large amounts of oil are beneficial for the synthesis of ultrafine powders. Materials Characterization. Nitrogen Physisorption Measurements. Effective tailoring of the oxide properties is most clearly demonstrated by comparison of the specific BET surface areas and the Rw values of the microemulsion (Figure 3a). At constant RO, low Rw values give the highest specific surface area as expected. This is quite remarkable since agglomeration of the hydroxide-filled micelles and further crystallite agglomeration and sintering during the calcination could in principle preclude effective prestructuring at the sol stage. Both processes would result in lower specific surface areas due to increasing effective particle sizes. The effective particle size can be estimated from the specific surface area Sg assuming spherical particle morphology:48

d ) 6/FSg This calculated value reflects the effective size of the crystallite agglomerates obtained after the heat treatment to 873 K. From geometrical considerations, a linear correlation of the micelle size and the ratio of the volume fractions of the water and surfactant phase49 and hence to a first approximation a linear increase of the effective particle size with Rw can be expected. Figure 3b shows that this relation is obeyed for Rw > 10 (swollen micelles) (47) Bandyopadhyaya, R.; Kumar, R.; Gandhi, K. S. Langmuir 2000, 16, 7139-7149. (48) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999. (49) Evans, D. F.; Wennestro¨m, H. The Colloidal Domain; WileyVCH: New York, 1999.

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Figure 4. Nitrogen physisorption isotherms at 77.35 K typical for sulfated zirconias synthesized in microemulsions with different Rw and RO.

Figure 3. (a) Correlation of BET surface area (Sg), size broadening of the (111) reflection (tetragonal phase), and Rw for sulfated zirconia catalysts synthesized in w/o microemulsions (RO ) 3.1). (b) Corresponding particle size calculated from the specific surface area.

whereas for Rw ) 3.68 the calculated particle size is lower. Thus for materials synthesized in swollen micelles agglomeration of the hydroxide-filled micelles does not preclude tailoring of the crystallite agglomerate size in the final oxide product. The size of the hydroxide-filled micelle which is controlled by Rw regulates the degree of crystallite agglomeration and sintering and therefore the effective particle size calculated from the specific surface area. The TEM results confirm this view (see below). For very small micelles, the solubilized state of water is different from that in swollen micelles,44 which can cause deviations from the linear relation of the particle size and R w. Higher specific surface areas are produced with increasing RO. At constant Rw ) 3.68, the surface area is as high as 169 m2 g-1 for RO ) 24.7 and thus significantly higher as compared with the 136 m2 g-1 obtained at RO ) 3.1, whereas at a higher Rw of 11.5 the influence of RO is negligible. For very small micelles, it is thus beneficial to achieve effective separation which results in a high BET surface area of the final product. The introduction of sulfate in the microemulsion modifies not only the phase diagrams. Even for materials prepared at the same Rw and RO, the addition of small amounts of sulfuric acid significantly increases the specific surface area of the product. For example, materials calcined at 873 K (Rw ) 36, RO ) 3.1) have surface areas of only 40 m2 g-1 if deionized water is used in the synthesis

but an increase to 100 m2 g-1 is observed even at very low sulfuric acid concentrations of c(H2SO4) ) 0.25 mol L-1. Further increase of the sulfuric acid concentration does not show a significant effect. The role of sulfuric acid in sintering has been described by Chen et al. and is due to sulfate impurities which cause a delay of the crystallization.50 Another parameter affecting the surface area is the water/precursor ratio which affects the zirconium concentration and the pH in the micelles. Thus with increasing concentration of the precursor higher surface areas are obtained. For example, at a ratio of n(H2O)/n(Zr) ) 34 (Rw ) 44), the surface area is only 22 m2 g-1 whereas at n(H2O)/ n(Zr) ) 8.5 it is 106 m2 g-1. This effect is related to variations in the hydrolysis and oxolation rate depending on the precursor concentration, but further studies are necessary to achieve a proper understanding. All microemulsion-derived materials show type II or type IV nitrogen physisorption isotherms (Figure 4). Zirconias prepared at low Rw values consist of small particles and show a distinct type IV hysteresis with the steepest slope of the adsorption branch at P/P0 of about 0.45. Intergranular mesopores are responsible for the hysteresis, and especially samples with low Rw and high RO show predominately pore sizes in the mesopore regime due to the reduced particle size of 5-6 nm. XRD. The powder patterns of all microemulsion-derived samples show that mainly the tetragonal phase of ZrO2 is present after calcination to 873 K. Small amounts of the monoclinic phase were present in some samples, but the amount was not affected by variations in the microemulsion composition. The peak broadening was attributed to the small particle size. A good correlation of the broadening of the (111) reflection and Rw was found, which is in agreement with the decreasing BET surface area with increasing Rw (Figure 3a). Crystallite size determinations using multiple peak analysis come to 5.3 nm (Rw ) 2) and 7.9 nm (Rw ) 40) and are somewhat smaller than the particle sizes determined from the BET measurements (5.7 and 9.2 nm, respectively) assuming spherical crystallite morphology. This is, however, within the error limits of both techniques and the assumptions made. In situ XRD investigations of the crystallization process of microemulsion-derived materials show significant differences of materials obtained in high and low Rw microemulsions (Figure 5). For samples derived at Rw ) 3.5, Bragg reflections of the tetragonal phase can be detected only above 773 K (Figure 5a). Below this (50) Chen, F. R.; Coudurier, G.; Joly, J. F.; Vedrine, J. C. J. Catal. 1993, 143, 616-626.

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Figure 5. (a) In situ XRDs showing the crystallization behavior of microemulsion-derived zirconia (Rw ) 3.5). (b) Evolution of the crystallite size determined using full pattern size/strain analysis for two samples prepared in microemulsions of high and low Rw.

temperature, the materials are X-ray amorphous due to the small particle size (approximately 2 nm). At 873 K, the crystallite size is 6.5 nm and increases slowly up to 31 nm at 1173 K (Figure 5b). For materials synthesized at Rw ) 44, a completely different crystallization behavior is observed (Figure 5b). Crystallization can be detected in the XRD patterns as low as 673 K, and at 873 K the estimated crystal domain size is already about 24 nm and grows to 330 nm at 1173 K (Figure 5b). The variation of the crystallization kinetics with Rw demonstrates effective tailoring of the particle size dependent properties with the microemulsion method. In principle, the surfactant could also prevent particle growth, but according to thermogravimetric analyses coupled with mass spectrometry the surfactant burn-off is completed below 723 K and therefore the surfactant alone cannot explain the growth kinetics above 873 K. Variations in the sulfuric acid concentration in the microemulsion also affect the crystallite sizes as determined from peak broadening in a small range from 8.8 nm (c(H2SO4) ) 0.25 mol L-1) to 6.7 nm (c(H2SO4) ) 2.5 mol L-1, Rw ) 37), which is in agreement with the BET results. TEM. Even though the micelle size controlled by Rw allows one to tailor the surface area and crystallite size as determined from profile analyses of the Bragg reflections, the preparation process is more complicated and the transformation of the zirconium hydroxide particles into ZrO2 crystals has to be taken into account. Thus it was surprising that crystallites seen in TEM pictures are

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of similar size for samples obtained in microemulsions with quite different Rw values (Rw ) 44, Figure 6a,b; Rw ) 1.8, Figure 6c,d). For both samples, the crystallites are 4-8 nm in size according to the TEM pictures. The latter would also contradict the XRD and BET results which show a significant increase of the full width at halfmaximum (fwhm) and Sg with decreasing Rw (Figure 3). Low-resolution TEM pictures (Figure 6a,c) suggest a lower degree of crystallite agglomeration for sulfated zirconias prepared in a microemulsion with Rw ) 1.8 (Figure 6c). High-resolution TEM pictures (Figure 6b,d) show that not only are crystallites synthesized at high Rw (Figure 6b) more agglomerated but several crystallites in contact with each other have the same crystallographic orientation, that is, they show a higher degree of sintering, which is responsible for the bigger particle sizes determined from peak broadening in XRD patterns. The agglomeration and sintering of the crystallites is a process which occurs within the course of the heat treatment and is different from the agglomeration of the hydroxide-filled micelles in the course of the hydroxide precipitation. In our view, the degree of crystallite agglomeration reflects the original size of the hydroxide particle at the gel stage which is controlled by Rw whereas the micelle agglomeration only facilitates the hydroxide separation without completely merging two hydroxide-filled micelles into one. For zirconias prepared in microemulsions without sulfate, the apparent crystallite size in TEM photographs is significantly larger (10-16 nm, Rw ) 2) as compared with sulfated samples (4-8 nm), in agreement with BET and XRD results. Catalytic Activity. The n-butane isomerization was used to compare the activity of materials prepared under different conditions. As a standard reference, sulfated zirconium oxide from MEL (XZO 682) was used with a reaction rate of 3.1 × 10-7 mol s-1 g-1 after 1 h on stream. All tests were carried out under the same conditions at 573 K. As shown above, the particle size of sulfated zirconia can easily be tuned by variations in the microemulsion composition and variations in Rw. With microemulsions of lower Rw, higher surface areas are obtained. However, our results demonstrate that exactly the reverse relation is found in the correlation of catalytic activity and Rw (Figure 7), that is, with higher surface area and low Rw lower activities are obtained, at least for catalysts obtained from in situ sulfatation. The sulfate decomposition was investigated using thermogravimetric analyses coupled with mass spectrometry monitoring the SO2 signal. They show a two-step decomposition. The first step starts at 873 K and is complete at about 973 K. The weight loss in this temperature region is 4.7% for zirconias obtained at Rw ) 46 but only 1.7% for materials obtained at Rw ) 3.7 which were basically inactive in the n-butane isomerization. Even though n(H2O)/n(Zr) was constant (17) and the sulfuric acid concentration was identical for both samples, the sulfate content in the final product differs and is significantly lower for low-Rw materials. The results indicate that large surfactant-to-water ratios hinder the sulfate incorporation. The reason for these variations in activity is probably different solubilized states of water in reverse micelles.44 At Rw < n (n ) approximately the number of EO units), all water molecules are hydrogen bonded to the surfactant, whereas at Rw > n “free” OH species can be detected. This does not mean that samples obtained at low Rw do not contain sulfate. The sorption measurements indicate severe differences in materials prepared with and without sulfuric acid. The surface areas for non-sulfated materials

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Figure 6. TEM pictures of sulfated zirconia crystals synthesized in microemulsions with Rw ) 44 (a,b) and Rw ) 1.83 (c,d).

are significantly lower (57 m2 g-1) as compared with those of sulfated samples (175 m2 g-1) prepared at the same Rw. So even though the sulfated samples show no activity for n-butane isomerization, sulfate doping is evident from the increasing sintering resistance. Thermogravimetric analysis coupled with mass spectrometry clearly indicates a second process of sulfate decomposition at higher temperature between 973 and 1273 K, which can be assigned to bulk sulfate groups. High surfactant concentrations (low Rw) hinder the sulfate incorporation and reduce the number of active sites on the surface of materials calcined at 873 K. However, it is not clear if it is only the overall reduction of sulfate groups in the gel responsible for lower activities, which means the overall concentration in the hydroxide is below a critical threshold needed for active catalysts, or if the surfactant specifically reduces the number of surface sulfate groups and does not affect the bulk doping.

Among the compared surfactants, Marlophen NP5 which can solubilize the largest amounts of sulfuric acid shows the best performance. The latter is suitable to produce active catalysts with reasonable surface area (106 m2 g-1) by in situ sulfatation. Sulfate Concentration. Variations in sulfate concentration in the microemulsion have two effects. First, the phase diagram can be severely changed in a way that the whole system becomes inapplicable. Especially for high sulfate concentrations, larger amounts of surfactant are needed to yield isotropic mixtures which is disadvantageous. When concentrations of 0.5-2.5 mol L-1 sulfuric acid are compared for in situ sulfatation, the differences in activity of the final catalyst are small (Figure 8). Materials prepared with a concentration of 0.25 mol L-1 show no activity, and therefore a threshold concentration is necessary to achieve considerable activity. After a short time

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Althues and Kaskel

Figure 7. n-Butane isomerization rates over microemulsionderived sulfated zirconia after 60 min on stream (surfactant, Marlophen NP5).

Figure 8. Variations of the reaction rate for sulfated zirconia catalysts with increasing sulfuric acid concentration in the microemulsion. The indicated concentration refers to the sulfuric acid concentration of the aqueous phase in the microemulsion. The water/oil/surfactant ratio was constant for all preparations.

on stream (15 min), the differences are a bit more pronounced and an optimum in the concentration is found at 1 mol L-1. Interestingly, not only the sulfate concentration matters but also the water/precursor ratio (Figure 9a). The highest conversion is obtained for a molar water/precursor ratio as low as 4.3, whereas at a ratio of 34.1 the conversion is almost negligible. Since these materials differ considerably in the specific surface area (see above), the comparison normalized to the surface area is more adequate and shows that they produce almost identical conversions after 1 h on stream (Figure 9b). The nature of this effect is still unclear. One aspect is the rate of the oxolation reaction in the precipitation step which is enhanced with lower water/precursor ratios. For very high water/precursor ratios, the agglomeration of the hydroxide-filled micelles is reduced and thus the material cannot be recovered completely by filtration. For example, for n(water)/n(precursor) ) 34.1 the yield was only 50%. However, it is not clear why the activity is affected. Changes in the pH due to a high precursor amount could also produce severe changes in activity. Further investigations are necessary for clarification. Re-Sulfatation. Even though in situ sulfatation produces active catalysts and is beneficial especially to achieve homogeneous materials on the nanometer scale, our

Figure 9. (a) n-Butane isomerization rates over microemulsionderived zirconias prepared with different water/precursor ratios (molar ratios indicated). (b) Conversion normalized to Sg.

results show that it is impossible to produce active catalysts with surface areas as high as 160 m2 g-1 with in situ sulfatation. In situ sulfatation typically produces active catalysts with intermediate surface area (around 100 m2 g-1) or inactive high surface area materials (170 m2 g-1). One way to avoid this compromise is to reimmerse the calcined high surface area materials in sulfuric acid. With this procedure, the inactive materials can be converted to active catalysts but the activity is lower than that of the standard MEL catalyst. Selectivity and Deactivation. The main issue of this work was to compare the activity of sulfated zirconias derived from microemulsions of various compositions and to work out parameters in the synthesis procedure which critically influence the activity of the catalyst. Selectivity and deactivation are not as sensitive to variations in the microemulsion composition. In our study, most of the catalysts with reasonable reaction rates showed a high selectivity to iso-butane. The catalyst with the highest activity showed a selectivity of 80.5% to iso-butane after 15 min on stream at a conversion of 23.9%. For other catalysts, the selectivities were also close to 80%. All microemulsion-derived zirconias showed a rapid deactivation due to coking. Deactivation is the major drawback

Sulfated Zirconia from Reverse Microemulsions

of sulfated zirconia catalysts as compared with other isomerization catalysts. Recent investigations have shown that the acidic sites in sulfated zirconia are not uniform in activity. The more active sites are responsible for a high activity and fast deactivation at TOS < 100 min, whereas at TOS > 100 min the less active sites provide the majority of the catalytic activity.51 According to our studies, the microemulsion method cannot improve the deactivation behavior. Instead, microemulsion-derived sulfated zirconia in general shows higher deactivation rates as compared with the standard catalyst (MEL XZO 682). Obviously, the microemulsion technique preferentially generates the more active sites which are responsible for the higher activity of the microemulsion-derived catalyst as compared to that of standard sulfated zirconia. Conclusion The inverse micelle method has been shown to be an attractive instrument for the preparation of sulfated zirconia catalysts with specific surface areas as high as (51) Kim, S. Y.; Goodwin, J. G.; Galloway, D. Catal. Today 2000, 63, 21-32.

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175 m2 g-1 and well-defined particle size due to prestructuring of the water reactant in the pre-sol stage of the sol-gel process. The smallest particles on average are approximately 5 nm in size after calcination at 873 K. In situ sulfatation produces catalysts active in n-butane isomerization at 573 K. Large amounts of surfactants are necessary to achieve very high surface areas (175 m2 g-1) but reduce the sulfate content and activity of the catalyst probably due to a higher degree of sulfate solvatation in the microemulsion. In situ sulfatation with lower surfactant concentrations produces catalysts which have an activity higher than that of a commercial MEL XZO 682 catalyst and are more active than re-sulfated materials with higher surface area. The results demonstrate the wide applicability of the inverse micelle method and its extendability to oxide materials with high surface area but also shows inherent limitations due to the high surfactant concentrations needed, which may be disadvantageous at least in catalytic applications since the surface structure and composition are affected. Acknowledgment. The generous supply of surfactants by BASF and CONDEA is gratefully acknowledged. LA0202327