Porous Yttrium Aluminum Oxide Templated by Alkyl Sulfate

Langmuir 2000, 16, 1535-1541

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Porous Yttrium Aluminum Oxide Templated by Alkyl Sulfate Assemblies Mitsunori Yada,* Masahumi Ohya, Kaoru Ohe, Masato Machida, and Tsuyoshi Kijima Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan Received April 23, 1999. In Final Form: October 4, 1999 Mesostructured hexagonal yttrium aluminum oxides templated by alkyl sulfate assemblies were synthesized by the homogeneous precipitation method using urea. The unit cell parameter a of the hexagonal mesophases is controllable by the alkyl chain length of the incorporated surfactant molecules as in the aluminum-based end member system, although the cell dimension of the yttrium-based system is little dependent on the alkyl chain length. The hexagonal yttrium aluminum based dodecyl sulfate mesophases with Al/(Al+Y) molar ratios of 0.54 or above were obtained as a single phase, whereas those with lower Al/(Al+Y) ratios coexisted with the yttrium-based mesophase. The Y-Al based hexagonal mesophases with 0.54 e Al/(Al+Y) e 0.74 were converted into an ordered porous material with a specific surface area of 662-797 m2g-1 by the anion exchange of alkyl sulfate surfactants with acetate ions, in contrast to the collapse of the hexagonal mesophases with Al/(Al+Y) > 0.74 by the same treatment. The much higher stability of the former hexagonal phase upon acetate treatment was attributed to the increased ratio of 6-coodinate to 4-coordinate Al polyhedra.

Introduction Since the discovery of mesoporous silicas such as MCM411 and FSM-162 templated by surfactant assemblies, surfactant templating approaches have been applied to the synthesis of various mesoporous metal oxides, because of their great applicabilities not only as catalysts and separating or adsorbing agents but also as host materials based on their large internal surface areas and uniform pore sizes. Such layered and hexagonal mesostructured metal oxides are also expected to serve as functional materials with optically, electrically or magnetically unique properties, due to the shape-specific and/or quantum effects of their thin inorganic skeletons. The templating approach using cationic surfactants such as alkyltrimethylammonium halogenides yielded a variety of mesostructured metal oxides, including zirconium oxide3 and hafnium oxide.4 The ligand-assisted liquid crystal templating method using primary amines was also developed to prepare various microporous and mesoporous transition metal oxides such as niobium and tantalum oxides.5,6 We found that the homogeneous precipitation method using urea in the presence of alkyl sulfate is effective for the synthesis of layered and hexagonal mesostructured materials such as aluminum,7-9 gallium,10 and yttrium-based11 surfactant mesophases. Holland et * E-mail: [email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (3) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Angew. Chem. Int. Ed. Engl. 1996, 35, 426. (4) Liu, P.; Liu, J.; Sayari, A. Chem. Commun. 1997, 577. (5) Antonelli, D. M.; Ying, J. Y. Angew. Chem. Int. Ed. Engl. 1996, 35, 426. (6) Sun, T.; Ying, J. Y. Nature 1997, 389, 704. (7) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (8) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Inorg. Chem. 1997, 36, 5565. (9) Yada, M.; Kitamura, H.; Machida, M.; Kijima, T. Langmuir 1997, 13, 5252. (10) Yada, M.; Takenaka, H.; Machida, M.; Kijima, T. J. Chem. Soc., Dalton Trans. 1998, 1547.

al.12 reported another anionic surfactant templating approach to prepare aluminophosphate and galloaluminophosphate and convert them into mesoporous composites by anion exchange with acetate anions. This anion exchange method was also applicable to the conversion of the yttrium-based mesophase into a mesoporous material with a specific surface area of as large as 545 m2g-1, in contrast to the Al and Ga-based hexagonal mesophases leading to collapse by the same acetate treatment.11 Porous aluminas with large specific surface areas are promising especially as catalysts or catalytic supports. The synthesis of mesoporous alumina was reported by Bagshaw et al.13 and Vaudry et al.14 These aluminas, however, are clearly distinguished from MCM-41 characterized by its ordered hexagonal pore structure, because the mesopores in the formers are disordered in arrangement and/or show no definite dependence of their diameter upon the chain length of incorporated surfactants. In contrast, we first reported the synthesis of an Al-based surfactant mesophase with an ordered hexagonal structure similar to MCM-41. It was converted by calcination at 600 °C into a less ordered microporous form with a specific surface area of as small as 365 m2g-1, although a 100 peak was slightly observed in its XRD pattern.7,8 The removal of surfactants in the Al-based surfactant mesophase by anion exchange with acetate ions also led to structural collapse. Attempts to dope Al into the silica framework of MCM-41 have been performed to improve the catalytic activities of the mesoporous material as solid acids. But they resulted in Al/Si molar ratios of as low as 6.67 × 10-2-1.67 × 10-1,15-17because of the structural collapse induced by excess doping of Al. Recently, on the other hand, we synthesized porous yttrium aluminum (11) Yada, M.; Kitamura, H.; Machida, M.; Kijima, T. Inorg. Chem. 1998, 37, 6470. (12) Holland, B. T.; Isbester, P. K.; Blanford, C. F.; Munson, E. J.; Stein, A. J. Am. Chem. Soc. 1997, 119, 6796. (13) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem. Int. Ed. Engl. 1996, 35, 1102. (14) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451.

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oxide templated by dodecyl sulfate assemblies and having an ordered hexagonal structure with an Al/Y ratio of as large as 2.78.18 In contrast to the Y-free mesophase, the Y-Al combined mesophase was also observed to be so stable as to keep its hexagonal mesostructure even after the removal of surfactant with acetate anions. Thus, the Y-Al combined templating approach would be largely effective to suppress the structural destabilization of aluminum oxyhydroxide frameworks by the removal of surfactants. Mesoporous metal oxides composed of two or more metal components would also be useful as new functional materials because of their electronically and/or structurally combined effects. Almost all of the mesoporous materials so far reported, however, are limited to single metal oxides. This would be because the mixed use of two or more metal components does not generally lead to the formation of any stable hexagonal framework, due to the difference in their ionic radii and/or bonding characters. Thus, the porous yttrium aluminum mixed oxide system in our report would also contribute to an understanding of the mechanism of formation of the mesoporous mixed metal oxides. In this paper, we report the synthesis of mesostructured hexagonal yttrium aluminum oxides templated by various alkyl sulfate assemblies and their detailed characterization including their stability field, specific surface areas, and short-range structural properties. Experimental Section The mesostructured yttrium aluminum oxides templated by alkyl sulfate assemblies with a hexagonal structure were synthesized by the homogeneous precipitation method using urea. Yttrium nitrate hexahydrate (Y(NO3)3‚6H2O) was used as a yttrium source and aluminum enanhydrate (Al(NO3)3‚9H2O) was used as an aluminum source and alkyl sulfate sodium salt (CnH2n+1OSO3Na) was used as a templating agent. Yttrium nitrate, aluminum nitrate, sodium dodecyl sulfate, urea, and water were mixed at a molar ratio of (1 - x):x:2:30:60 (x ) 0-1.0) and stirred at 40 °C for 1 h to obtain a transparent mixed solution. Urea was used to gradually raise the pH of the reaction mixture because on heating to above 60 °C it is hydrolyzed to release ammonia according to the following reaction.

(NH2)2CO + 3H2O f 2NH4+ + 2OH- + CO2 The mixed solution was heated at 80 °C and then kept at that temperature for 20 h. As the pH of the reaction mixture increased from 3.4 to 5.2 at its initial level to 7.4-8.3, due to the enhanced hydrolysis of urea, precipitation occurred and developed, as shown in Figure 1 and Table 1. The resulting mixture was immediately cooled to room temperature to prevent further hydrolysis of urea. After centrifugation, the resulting solid, white in color, was washed with water a few times and then dried at 60 °C in air. Powder X-ray diffraction (XRD) measurement was made on a Simadzu XD-D1 diffractometer with Cu KR radiation. X-ray microanalysis (XMA) was conducted by a HORIBA EMAX-5770. SEM observations were made on a Hitachi H-4100. NMR spectra were measured on a Bruker AC-250 with a MAS frequency of 3 kHz.

Result and Discussion Synthesis of Dodecyl Sulfate Mesophases. Precipitation using sodium dodecyl sulfate occurred at nearly (15) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (16) Luan, Z.; Cheng, C.-F.; Zhou, W.; Klinowski, J. J. Phys. Chem. 1996, 99, 1018. (17) Tuel, A.; Gontier, S. Chem. Mater. 1996, 8, 114. (18) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Chem. Commun. 1998, 1941.

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Figure 1. Plots of pH at three stages against x (x ) Al/(Al+Y) in each starting reaction mixture): (O) starting, (4) immediately after precipitation occurred, (0) after 20 h reaction time.

the same pH of 5.7-6.1 for the reaction mixtures with the Al fraction () x) of 0.125-1.00, whereas the x ) 0 precipitate appeared at pH 7.3, as shown in Figure 1 and Table 1. After a 20 h’s reaction, the pH increased up to 7.4-7.8 and 8.3 for the reaction mixtures of x ) 0-0.875 and x ) 1.00, respectively. The growth of mesostructured yttrium aluminum oxides was initiated by the formation of a layered phase to undergo its transition into a hexagonal structure,18 as observed in the aluminum7-9 and yttrium-based11 systems. The layer to hexagonal transition was completed in a reaction time of 20 h. The XRD patterns of the solids separated after a 20 h reaction are shown in Figure 2A. The x ) 0 product corresponds to a mesostructured yttrium oxide previously reported.11 The major peak located at 2 ) ca. 1.7° is assignable to the 100 reflection based on a hexagonal unit cell with a ) 6.2 nm. On the other hand, the x ) 0.375-1.00 products are characterized by a major peak located at 2θ ) ca. 2.3° and two weak peaks at 2θ ) 3-5°, along with a halo band at 2θ ) ca. 20°. The former three peaks can be assigned to the 100, 110, and 200 reflections for a single phase with a hexagonal structure. The x ) 0.125 and 0.250 products are assumed to be a mixture of the mesostructured yttrium oxide and the yttrium aluminum oxide, according to their broad and unsymmetric 100 peaks. The TEM image of the x ) 0.625 product viewed along the channel axis indicates a highly ordered hexagonal structure (Figure 3(a)) and the TEM image viewed perpendicular to the channel axis (Figure 3(b)) shows that the hexagonal channels extend for more than several hundred nanometers. The TEM images of the x ) 1.00 products also showed a similar highly ordered hexagonal structure.7 On the other hand, the x ) 0 product, yttrium-based mesophase, showed a less ordered hexagonal structure missing any long range order.11 The IR spectrum of the x ) 0.625 product indicated a broad band in the range of 1300-1700 cm-1 attributable to the CO32- group, along with some peaks attributable to dodecyl sulfate groups, similar to the x ) 0 product11 but different from the x ) 1.00 product,7,8 indicating that the mesostructured yttrium aluminum oxides are composed of yttrium aluminum oxyhydroxide, dodecyl sulfate species, and the carbonate species incorporated or taken up along with yttrium species. The unit cell parameter a of the x ) 0.375-1.00 products slightly decreased from 4.6 to 4.2 nm with an increase of x, in contrast to the x ) 0-0.125 products indicative of larger unit cell parameters of 5.9-6.2 nm, as shown in Figure 4 and Table 1. This cell contraction of the inorganic framework as well as the above TEM observations suggest that the microstructure of the inorganic frameworks is largely influenced by the com-

Porous YAlO Templated by Alkyl Sulfate Assemblies

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Table 1. Preparation Condition and Characterization of the As-Grown and Acetate-Treated Mesostructured Yttrium Aluminum Oxides Templated by Dodecyl Sulfate Assemblies preparation condition

product

pH entry

xa

starting

initial precipitation

1 2 3 4 5 6 7 8 9

0 0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000

5.2 4.3 3.9 3.8 3.4 3.6 3.6 3.4 3.6

7.3 5.9 6.1 5.8 6.0 6.1 6.1 5.7 6.1

a

as-grown after a 20 h reaction 7.4 7.6 7.6 7.5 7.5 7.5 7.7 7.8 8.3

acetate-treated

a/nm

S/ (Al+Y)

Al/ (Al+Y)

a/nm

Al/ (Al+Y)

specific surface area m2g-1

6.2 5.9 4.6 4.6 4.4 4.3 4.3 4.2 4.2

0.28 0.25 0.32 0.29 0.32 0.31 0.35 0.33 0.33

0 0.17 0.39 0.54 0.68 0.74 0.78 0.87 1.00

6.1 6.1 5.4 4.4 4.0 3.8 s s s

0 0.22 0.44 0.55 0.67 0.74 0.78 0.87 1.00

545 592 480 797 732 662 426 268 12

x ) Al/(Al+Y) in a reaction mixture.

Figure 2. XRD patterns of (A) mesostructured yttrium aluminum oxides templated by dodecyl sulfate assemblies and (B) their acetate treated products: (a) x ) 0, (b) x ) 0.125, (c) x ) 0.250, (d) x ) 0.375, (e) x ) 0.500, (f) x ) 0.625, (g) x ) 0.750, (h) x ) 0.875, (i) x ) 1.00. (C) XRD patterns of the mesostructured yttrium aluminum oxides templated by assemblies of various alkyl sulfate ions (CnH2n+1OSO3-; n ) 8-16); (a) n ) 8, (b) n ) 10, (c) n ) 12, (d) n ) 14, (e) n ) 16. The initial mixing ratio x of Al to (Al+Y) is 0.625.

position of the solids. The surfactant-to-metal and Al-to(Al+Y) molar ratios for the mesostructured yttrium aluminum oxides were determined by XMA (Table 1), both being plotted against the Al fraction x in Figure 5. The S-to-metal molar ratio for these products was 0.25-0.35 or little dependent on x, indicating that nearly the same molar amount of surfactant relative to metal is incorporated in these mesophases. The Al to (Al+Y) molar ratio for the resulting solids increased with an increase of x and the ratios were larger than those for the starting reaction mixture up to x ) 0.750. These observations would be attributable to the more preferred precipitation of the aluminum species, due to the enhanced polymerization of aluminum oxyhydroxide species7,8 at a low pH compared with yttrium-based analogues11 (Table 1). Acetate Exchanged Mesophases. An attempt was made to remove the surfactant species from the above

mesophases by the anion exchange with acetate anions; the mesophase sample (0.5 g) was mixed with 0.05 M ethanol solution of sodium acetate (40 mL), and then stirred at 40 °C for 1 h, as described in a previous report.11,12,18 XMA analysis revealed that no sulfur species are detectable for the acetate-treated samples, suggesting the complete removal of the incorporated dodecyl sulfate species. This was also confirmed by the IR spectrum of the acetate-treated solid which shows no absorption peaks attributable to dodecyl sulfate. The XRD patterns of the acetate-treated mesophases are summarized in Figure 2B. The x ) 0 product gave a sharp 100 peak and broad 110 and 200 bands, indicating the formation of hexagonal mesoporous material with a unit cell parameter of 6.1 nm.11 The x ) 0.375-0.625 products also gave a sharp 100 peak for a hexagonal structure, but with an accompanying disappearance of the 110 and 200 peaks,

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Figure 5. S-to-metal (O) and Al-to-(Al+Y) (4,0) molar ratios against x for the mesostructured yttrium aluminum oxides templated by dodecyl sulfate assemblies (O,4) as-grown products and (0) their acetate-treated forms.

Figure 3. TEM images of mesostructured yttrium aluminum oxide templated by dodecyl sulfate assemblies synthesized with x ) 0.625 viewed along the channel axis (a) and perpendicular to the channel axis (b).

Figure 4. Plots of unit cell parameter a against x for the mesostructured yttrium aluminum oxides templated by dodecyl sulfate assemblies: (O) as-grown products and (4) their acetatetreated forms.

indicating a decrease of their crystallinity due to the acetate treatment. The Y-Al based hexagonal mesophases with 0.54 e Al/(Al+Y) e 0.74 were converted into an ordered porous material, as shown in Figures 2B and 5 and Table 1. The unit cell parameters a of the x ) 0.3750.625 products decreased significantly from 4.4 to 3.8 nm with an increase of x, in contrast to the as-grown mesophases indicative of a slight decrease from 4.6 to 4.3 nm (Figure 4 and Table 1). The marked cell contraction for the exchanged forms would be attributable to the

condensation of the aluminum species, taking into consideration that the Al/(Al+Y) molar ratios for the acetatetreated products were little different from those for the as-synthesized ones (Figure 5 and Table 1) and that the Al-free or x ) 0 product showed no cell contraction. The acetate treatment of the x ) 0.750-1.00 products resulted in structural collapse without compositional change, as shown in Figure 2B(g-i) and Table 1. This is probably due to the destabilization of the Al-based framework. In the x ) 0.125 and 0.250 systems, on the other hand, the value of Al/(Al+Y) for the acetate treated products significantly increased relative to that for the as-grown ones, indicating partial release of yttrium species from the yttrium aluminum mesophases by the acetate treatment. The XRD data for the x ) 0.250 product also showed that the yttrium aluminum mesophase identified by the peak attributable to a unit cell parameter of a ) 4.6 nm considerably decreased in amount upon the acetate treatment to leave the a ) 5.4 nm phase as the main phase (Figure 2Ac and 2Bc). The yttrium aluminum mesophase in the x ) 0.125 product was also mostly lost upon acetate treatment to leave the a ) 6.1 nm phase as the major phase (Figure 2Ab and 2Bb). The phase change with x observed above could be explained as follows. The large difference in size of the Y and Al-based clusters would make the stability field of the Y-rich solid solutions so narrow that the solid formed at low x would be separated into Y-rich and Y-Al-based solid solutions. On the other hand, the solids formed at high x would undergo no phase separation because the structural distortion of the yttrium aluminum mixed framework would be fully relaxed by the increase of AlO4 content. Porous Properties. The specific surface areas of the acetate treated products with varying Al/(Al+Y) ratios were determined by the BET method,19 as shown in Figure 6 and Table 1. Prior to the N2 adsorption measurement, samples were heated at 150 °C for 1 h in a vacuum to remove adsorbed water. The specific surface area of the x ) 0.375-0.625 products as large as 662-797 m2g-1, in comparison to 480-592 m2g-1 for the x ) 0-0.250 products. Furthermore, the pore structures of the yttrium single oxide and Al-rich Y-Al mixed oxides would be clearly distinguished from each other, as noted from the TEM images of their precursor as-grown forms as described above. For the x g 0.375 products, the specific surface area decreased with an increase of x due to the partial collapse of the hexagonal structure, as judged from their XRD patterns (Figure 2B(d-i)). Especially, the x ) 1.00 product showed an extremely low specific surface area of 12 m2g-1, consistent with the complete collapse of structure suggested from its X-ray diffraction patterns (19) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.


Figure 6. Specific surface area as a function of x for the acetatetreated mesostructured yttrium aluminum oxides templated by dodecyl sulfate assemblies.

A

B

Figure 7. N2 adsorption isotherms (A) and pore size distribution (B) for the acetate treated form of the x ) 0.375 mesostructured yttrium aluminum oxide.

(Figure 2B(i)). The N2 adsorption isotherm for the acetate treated x ) 0.375 product yielded the largest specific surface area of 797 m2g-1 (Figure 7A). The rapid adsorption in the range of P/P0 ) 0-0.3 for this sample is due to the monolayer coverage of pores and particle surface and capillary condensation in pores. The pore size of 1.8 nm was obtained by the Clanston-Inkley method,20 a modified procedure of the BJH method, as shown in Figure 7B. The pore sizes of the x ) 0.500 and 0.625 single phase products are 1.6 nm, in close agreement with that for the x ) 0.375 product, although the unit cell parameters of the former two products are quite different from each other, as shown in Figure 4 and Table 1. This is because the preheating of these products at 150 °C for dehydration caused a reduction in their crystallinity, as suggested from a decrease in intensity of the 100 reflection and its slight (20) Cranston, R. W.; Inkley, F. A. Adv. Catalysis 1957, 9, 143.

Langmuir, Vol. 16, No. 4, 2000 1539

shift to a higher angle.18 The mesoporous materials thus obtained were observed to be thermally unstable: upon calcination at 300 °C for 5 h the acetate treated mesoporous form of the x ) 0.625 phase became highly disordered to show no 100 XRD peak and a marked decrease of specific surface area from 662 to 295 m2g-1. The specific surface area of the same acetate treated product decreased to 193 and 90 m2g-1 on 5 h calcination at 500 and 700 °C, respectively. The attempt to improve the thermal stability of the porous materials is underway. The hexagonal mesoporous yttrium aluminum oxides may be thus applicable to catalytic bodies with solid acidity and an adsorbent or separating agent as well as a host matrix for optical or other functional species, although they are not suitable for high-temperature processes or applications. Microstructural and Morphological Properties. Taking into consideration the pore size distribution and unit cell parameter of the solids after the N2 adsorption experiment, the thickness of the wall including the acetate moiety for the acetate treated x ) 0.625 product was determined to be 1.9 nm. This value is nearly equal to 2.2 nm for the x ) 0.500 product, but is much less than 3.0 nm for the x ) 0 or mesoporous yttrium oxide.11,18 The XRD patterns for all of the present solids also showed a weak halo near 2θ ) 20° (Figure 2A and 2C), indicating that the wall structure of the porous yttrium aluminum oxides is amorphous independent of their Al to Y ratio. Since the structural stability of the Y-Al based surfactant removed mesophases depends on the Al content x, the 27Al MAS NMR spectra of the as-grown and acetate treated products were measured, as shown in Figure 8. With an increase of x, the 6-coordinate Al peak near 4 ppm characteristic of the yttrium-poor mesophase decreased gradually in intensity, whereas the 4-coordinate Al peak near 60 ppm increased remarkably. These data suggest that the aluminum species coexisting with yttrium ones are solidified primarily as 6-coordinate oxide groups or clusters linked to yttrium hydroxide groups, though those in the yttrium-free or low yttrium content system are stabilized as 4-coordicate Al oxide groups. The 27Al MAS NMR spectrum of the acetate-treated x ) 1.00 product showed a remarkable increase of 6-coordinate Al peak in intensity compared with that for the as-grown x ) 1.00 product, indicating a marked conversion of 4-coordinate Al into 6-coordinate Al induced by the acetate treatment, as shown in Figure 8(f). This is in contrast to both the as-grown and acetate-treated x ) 0.625 products giving rise to nearly the same NMR patterns (Figure 8(c,e)). These XRD and NMR observations indicate that the surfactant templated aluminum or aluminum rich framework would be destabilized with an accompanying appreciable change in the coordination state of Al on dodecyl sulfate/acetate ion exchange. Thus, the structural stability of the as-grown x ) 0.375-0.625 mesophases against the acetate treatment would be due to their frameworks composed mainly of edge- and vertex-shared AlO6 and YO6 octahedra, in contrast to the as-grown x ) 1.00 mesophase with a flexible but unstable framework of vertex-shared AlO4 tetrahedra. Especially, the edgeshared AlO6 would contribute to the structural stability, since aluminum and oxygen ions are more closely packed in the structure. These results are also in agreement with mesoporous aluminas reported by Bagshaw13 and Vaudry14 which are mainly composed of 6-coordinate Al. The yttrium aluminum based mesophases with a hexagonal structure were also morphologically characterized. The yttrium-based end member product with x ) 0 was of platelike shaped particles, and the aluminum-based one with x ) 1.00 was of winding-rod shaped particles, as

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Figure 8. 27Al MAS NMR spectra of the as-grown (a-d) and acetate-treated (e,f) mesostructured yttrium aluminum oxides templated by dodecyl sulfate assemblies, synthesized with x ) 0.125 (a), x ) 0.375 (b), x ) 0.625 (c,e), and x ) 1.00 (d,f).

previously reported.8,11 The morphologies of the mesostructured yttrium aluminum oxides varied depending on x. The aluminum rich x ) 0.500-0.875 products were of winding-rod shaped particles similar to the hexagonal mesostructured aluminum oxide, whereas the yttrium rich x ) 0.125-0.375 products occurred in undefinite and platelike morphologies, as shown in Figure 9. These SEM observations suggest that the morphological properties of the mixed oxide mesophases are significantly influenced by the composition of their inorganic frameworks. These morphological differences would be due to the flexibility of inorganic framework. The 4-coordinate Al rich yttrium aluminum oxide framework would be structurally more flexible than the 6-coordinte Al rich one. This is because AlO4 tetrahedral units sharing only their vertexes are more advantageous in linking to rodlike surfactant molecules than AlO6 octahedral units sharing their edges or vertexes, due to the smaller strain energy. Thus the 4-coordinate Al rich x ) 0.500-1.00 products are crystallized in a winding rod shape reflecting the shape of rodlike surfactant micelle. Control in Mesostructural Size. Similar experiments using sodium alkyl sulfates (CnH2n+1OSO3Na, n ) 8-16) in place of SDS were conducted with a reaction time of 20 h and x ) 0.625. The XRD patterns of the resulting solids are shown in Figure 2C. The solids for n ) 12-14 indicated three sharp peaks attributable to the 100, 110, and 200 reflections for a hexagonal structure, whereas those for shorter chain surfactants significantly decreased in crystallinity giving rise to only a broad 100 peak (n ) 10)

Yada et al.

Figure 9. SEM images of the mesostructured yttrium aluminum oxides synthesized with (a) x ) 0.375, (b) x ) 0.500.

or a shoulder peak (n ) 8) below 2θ ) 5°. Furthermore, the S-to-metal mole ratios for the n ) 8 and 10 products were as small as 0.15 and 0.23, respectively, in comparison with 0.31 and 0.42 for the n)12 and 14 products. The XRD pattern of the n ) 16 product also indicated three diffraction peaks below 2θ ) 5° in which the 200 reflection peaks are much stronger in intensity than those observed for the n ) 12 and 14 products. The SEM image also showed that the n ) 16 product is of lamellar particles. The XRD and SEM observations suggest that the n ) 16 product is a mixture of layered and hexagonal mesophases. The layer to hexagonal transition observed in the mesostructured yttrium aluminum oxide system could be explained as follows. The increase in pH due to decomposition of urea would cause interlayer condensation and crosslinking in any adjacent yttrium aluminum oxyhydroxide clustered layers, leading to the rearrangement of the bilayered surfactant molecule into a more stable rodlike assembly as a result of their partial release by ion exchange with the hydroxyl anion. In the n ) 12 and 14 systems such a molecular rearrangement would induce the layer to hexagonal transition of the framework structure. However, the bilayer assembly of the much longer (n ) 16) surfactant molecules would be stabilized through the strong van der Waals attraction between their alkyl chains to prevent a structural transition through their partial release. The unit cell parameter a for the mesostructured yttrium aluminum oxides increased from 4.2 nm for n ) 10 to 4.9 nm for n ) 14, via 4.3 nm for n ) 12. For both


end members, on the other hand, the mesostructured aluminum oxide based system was obtainable as a hexagonal form only for n ) 8, 10, and 12, whereas the yttrium oxide based system yielded a hexagonal form only for n ) 10 and 12.8,11 The cell parameter a for the mesoporous silica1 and our reported aluminum-based surfactant mesophases7,8 shows a tendency to increase with an increase of the carbon number of the surfactant used, because the inorganic cluster units are small. The unit cell parameter a for the mesostructured aluminum oxide phases showed a tendency to linearly increase from 3.7 nm for n ) 8 to 4.3 nm for n ) 12 with an increase of the carbon number of the surfactant used. On the other hand, the unit cell parameter a of the yttrium-based hexagonal mesophase11 is little dependent on the length of surfactant molecule or a ) ca. 6 nm and essentially determined by the dimension of the framework of large yttrium-oxyhydroxide-carbonate clusters linked to sur-

Langmuir, Vol. 16, No. 4, 2000 1541

factant assemblies. Thus, the unit cell parameter a for the mesostructured yttrium aluminum oxide is moderately variable, and the nonlinear variation of a with the length of surfactant is due to the inorganic framework being constructed from both the aluminum and yttrium-based clusters with largely different dimensions. Conclusion In conclusion, we have synthesized ordered hexagonal porous yttrium aluminum oxides templated by alkyl sulfate assemblies and found the capability to control the Al to (Al+Y) mole ratio in the resulting solids. The present approach for the synthesis of mesoporous mixed metal oxides would further contribute to the development of functional mesostructured materials. LA990493P

Porous Yttrium Aluminum Oxide Templated by Alkyl Sulfate

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