4752
Langmuir 2000, 16, 4752-4755
Mesoporous Gallium Oxide Structurally Stabilized by Yttrium Oxide Mitsunori Yada,* Masahumi Ohya, Masato Machida, and Tsuyoshi Kijima Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan Received December 13, 1999. In Final Form: February 16, 2000
Introduction Since the synthesis of mesoporous silicas such as MCM411 and FSM-162 with large internal surface areas and uniform pore sizes, the surfactant templating method has been used to synthesize mesoporous metal oxides, including titanium,3 aluminum,4,5 niobium,6 and tantalum7 oxides. The mesostructured metal oxides are taken to be useful not only as catalysts and separating or adsorbing agents but also as functional host materials with optically, electrically, or magnetically unique properties, owing to the shape-specific and/or quantum effects of their thin inorganic skeletons. Mesoporous zirconium oxide and phosphate8-11 and hafnium oxide12 are promising as acid catalysts. Layered13 and hexagonal14 mesostructured titanium oxides, for example, were observed to be photocatalytically active. Aluminum and gallium oxides with a mesoporous structure are also expected to serve as a catalytic or other functional material. Though the synthesis of mesoporous alumina has been already demonstrated by several workers,4,5 mesoporous gallium oxide has not yet been reported. We have already reported the synthesis of layered and hexagonal mesostructured aluminum,15-18 gallium,19 and rare earth20,21 oxides templated by alkyl (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) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014. (4) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (5) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (6) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426. (7) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874. (8) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 426. (9) Pacheco, G.; Zhao, E.; Garcia, A.; Sklyarov, A.; Fripiat, J. Chem. Commun. 1997, 491. (10) Wong, M. S.; Ying, J. Y. Chem. Mater. 1998, 10, 2067. (11) Jime´nz-Jime´nz, J.; Marireles-Toorres, P.; Olivera-Pastor, P.; Rodriguez-Castello´n, E.; Jime´nz-Lo´pez, A.; Jones. D. J.; Rozie´re Adv. Mater. 1998, 10, 812. (12) Liu, P.; Liu, J.; Sayari, A. Chem. Commun. 1997, 577. (13) Fujii, H.; Ohtaki, M.; Eguchi, K. J. Am. Chem. Soc. 1998, 120, 6832. (14) Stone, Jr, V. F.; Davis, R. J. Chem. Mater. 1998, 10, 1468. (15) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (16) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Inorg. Chem. 1997, 36, 5565. (17) Yada, M.; Kitamura, H.; Machida, M.; Kijima, T. Langmuir 1997, 13, 5252. (18) Yada, M.; Hiyoshi, H.; Machida, M.; Kijima, T. J. Porous Mater. 1998, 5, 133. (19) Yada, M.; Takenaka, H.; Machida, M.; Kijima, T. J. Chem. Soc., Dalton Trans. 1998, 1547. (20) Yada, M.; Kitamura, H.; Machida, M.; Kijima, T. Inorg. Chem. 1998, 37, 6470. (21) Yada, M.; Kitamura, H.; Ichinose, A.; Machida, M.; Kijima, T. Angew. Chem., Int. Ed. 1999, 38, 3506.
sulfate assemblies by the homogeneous precipitation method using urea. The anion-exchange method reported by Holland et al.22 was found to be applicable to the conversion of the hexagonal mesostructured rare earth oxides into mesoporous materials with specific surface areas of as large as 545 m2 g-1 for the Y system and about 300 m2 g-1 for the lanthanide (Gd-Lu) system.20,21 The Al- and Ga-based hexagonal mesophases structurally collapsed by the same treatment. Our recent study further demonstrated that the Al-based hexagonal mesophase can also be converted into an ordered porous material through the introduction of yttrium into the inorganic framework.23 The synthesis of mesoporous gallium oxide would be interesting because it is expected to be a stronger solid acid than aluminum oxide. If mesoporous gallium oxide were obtainable, it would serve as a strong acid catalyst with molecular shape or size selectivities. In this paper, we report the synthesis and characterization of mesoporous gallium oxide stabilized by yttrium oxide. Experimental Section The mesostructured gallium yttrium mixed oxides were synthesized by the homogeneous precipitation method using urea in the presence of alkyl sulfate. Gallium chloride (GaCl3) was used as the gallium source, yttrium nitrate hexahydrate (Y(NO3)3‚ 6H2O) was used as the yttrium source, and sodium dodecyl sulfate (C12H25OSO3Na) was used as the templating agent. GaCl3, Y(NO3)3, surfactant, urea, and water were mixed at a molar ratio of x:(1 - x):2:30:60 (x ) 0-1.0) and stirred at 40 °C for 1 h. Urea was used to gradually rase the pH of the mixture according to the following reaction at above 60 °C.
(NH2)2CO + 3H2O f 2NH4+ + 2OH- + CO2 After being heated at 80 °C, the mixed solution was then kept at that temperature for 3 and 9 h. As the pH of the reaction mixture increased from 2.6-5.1 at its initial level to 7.1-8.3 after 9 h of reaction time (Table 1), precipitation occurred and developed. 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 repeatedly and then dried at 60 °C in air. The removal of surfactant from the obtained mesophases was done by anion exchange with acetate anions;20-23 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, and resulting solids were washed with ethanol, repeatedly. Powder X-ray diffraction (XRD) measurement was made on a Simadzu XD-D1 diffractometer with Cu KR radiation. Transmission electron microscopy (TEM) was carried out using a Hitachi H-800MU. X-ray microanalysis (XMA) was conducted by a HORIBA EMAX-5770. Prior to the N2 adsorption desorption measurement, samples were heated at 150 °C for 1 h in a vacuum to remove adsorbed water. Specific surface area was calculated by the BET method,24 and pore size distribution was determined by the Clanston-Inkley method,25 a modified procedure of the BJH method.
Results and Discussion Precipitation occurred at pH 3.9 for the yttrium-free (x ) 1.0) system, whereas it was initiated at higher pH values of 4.8-5.7 for the yttrium-gallium mixed ones (22) Holland, B. T.; Isbester, P. K.; Blanford, C. F.; Munson, E. J.; Stein, A. J. Am. Chem. Soc. 1997, 119, 6796. (23) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Chem. Commun. 1998, 1941. (24) Brunauer, S.; Emmett, P. H.; Teller. E. J. Am. Chem. Soc. 1938, 60, 309. (25) Cranston, R. W.; Inkley, F. A. Adv. Catal. 1957, 9, 143.
10.1021/la991628u CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
Notes
Langmuir, Vol. 16, No. 10, 2000 4753
Table 1. Preparation Condition and Characterization of the As-Grown and Acetate-Treated Mesostructured Gallium Oxides Structurally Stabilized by Yttrium Oxide 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
5.1 3.2 3.1 3.0 3.0 2.9 2.9 2.7 2.6
6.9 5.6 5.0 5.7 5.7 5.6 5.5 4.8 3.9
a
acetate-treated after 9 h of reaction 7.3 8.0 8.0 7.1 7.2 7.3 7.4 8.3 7.9
as-grown a/nm Ga/(Ga + Y) 5.5 4.9 4.7 4.4 4.4 4.4 4.2 4.2 4.7
0 0.20 0.42 0.54 0.58 0.62 0.72 0.83 1.00
a/nm
Ga/(Ga + Y)
specific surface area/m2 g-1
5.4 5.2 4.7 4.3 4.4 4.2 4.0 4.7
0 0.30 0.47 0.59 0.57 0.63 0.71 0.83 1.00
447 447 475 558 714 669 642 410 233
x ) Ga/(Ga + Y) in reaction mixture.
Figure 1. (A) XRD patterns of the as-grown mesostructured gallium oxide structurally stabilized by yttrium oxide (a, b, e, f) and those acetate-treated products (c, d, g, h), synthesized at x ) 1.0 (a, c, e, g) and x ) 0.5 (b, d, f, h) in 3 h (a-d) and 9 h (e-h) reaction time. Peak assignment: b, GaO(OH). (B) XRD patterns of the acetate-treated mesostructured gallium oxide structurally stabilized by yttrium oxide synthesized at 9 h of reaction in various nominal molar ratios (x ) 0-1.0). Peak assignment: b, GaO(OH).
with a gallium fraction ()x) of 0.125-0.875 (Table 1). The XRD patterns of the x ) 1.0 and 0.5 mesophases, separated after 3 h of reaction, are shown in spectra a and b of Figure 1A, respectively. Both XRD patterns 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 in each pattern can be assigned to the 100, 110, and 200 reflections for a hexagonal structure
with a unit cell parameter a of 4.2 nm (x ) 1.0) and 4.3 nm (x ) 0.5), respectively. TEM images of the x ) 1.0 and 0.5 products indicated a highly ordered hexagonal structure. Anion-exchange treatment with acetate ions to remove incorporated surfactants from the x ) 1.0 and 0.5 products resulted in the collapse of their hexagonal structure, as confirmed by the XRD patterns indicating no peaks attributable to the 100, 110 and 200 reflections (spectra c and d of Figure 1A). On the other hand, the x ) 1.0 and 0.5 mesophases separated at 9 h of reaction were observed to have a hexagonal structure with their unit cell parameters increased to 4.7 and 4.4 nm, respectively, although the 110 and 200 reflection peaks for both solids were more broadened in comparison with those for the 3 h of reaction mesophases, as shown in spectra e and f of Figure 1A. The TEM images of the x ) 0.5 product indicated a highly ordered hexagonal structure extending for more than several hundred nanometers, as shown in Figure 2, similar to MCM-411 and Al- and Ga-based surfactant mesophases.15,19 Though the x ) 1.0 mesophase coexisted with GaO(OH) formed as a second phase, the x ) 0.5 mesophase was obtained as a single phase, suggesting that yttrium and gallium cations are completely mixed on an atomic scale. The anion exchange treatment of the x ) 1.0 mesophase also led to the structural collapse, as in the case of the 3 h reaction product (spectrum g of Figure 1A). On the other hand, the anion-exchanged x ) 0.5 product yielded a sharp 100 peak and a weak 110 peak for a hexagonal structure with a unit cell parameter of 4.4 nm (spectrum h of Figure 1A). Sulfur to metal (Ga + Y) molar ratio detected by XMA analysis dropped from 0.30 for the as-grown product to 0 for the acetate-treated one, indicating that the incorporated surfactants were completely removed with retention of the hexagonal structure. These results suggest that the substitution of yttrium species into the gallium oxyhydroxide framework is essential for the synthesis of mesoporous materials. A reaction time of as long as 9 h or more is also required for the maintenance of the framework upon the removal of surfactants. The expansion of unit cell parameter and the decrease in crystallinity for the 9 h reaction product in comparison with the 3 h reaction product is most likely due to the more extensive polymerization of the Ga-Y-O framework. Further work was carried out to prepare galliumyttrium based surfactant mesophases by the 9 h reaction method at various molar ratios of x ) 0-1.0 to control Ga to (Ga + Y) molar ratio in products. The x ) 0.25-1.0 mesophases showed XRD patterns characterized by a hexagonal structure, similar to the x ) 0.5 mesophase mentioned above. The unit cell parameter a of the x )
4754
Langmuir, Vol. 16, No. 10, 2000
Notes
Figure 3. N2 adsorption (O) and desorption (4) isotherms for the acetate-treated x ) 0.5 mesostructured gallium oxide structurally stabilized by yttrium oxide separated at 9 h of reaction time.
Figure 2. TEM images of the mesostructured gallium oxide structurally stabilized by yttrium oxide (x ) 0.5) viewed along the channel axis (a) and perpendicular to the channel axis (b) synthesized at 9 h of reaction time.
0.25-0.875 products or gallium-yttrium based mesophases slightly decreased from 4.7 to 4.2 nm with an increase of x (Table 1). In contrast to the x ) 0.25-1.0 mesophases, the x ) 0 and 0.125 mesophases showed a weak and broad 100 reflection, indicating the formation of a hexagonal but poorly crystalline phase. The anion exchange treatment of the x ) 0-0.875 mesophases yielded mesostructured materials giving rise to a sharp 100 reflection at 2θ ) ca. 2.3° in their XRD patterns (Figure 1B). Among them, the x ) 0.25-0.625 products showed a highly ordered hexagonal structure, as judged from their sharp 100 reflections, although the x ) 0.75-1.0 products showed a marked decrease in crystallinity of the hexagonal structure with an increase of x. These results also suggest that the yttrium-rich Ga-Y mixed oxyhydroxide framework is intact upon the removal of surfactant. The Ga/ (Ga + Y) molar ratios for the x ) 0.5-0.875 acetatetreated products are little different from those for the assynthesized ones (Table 1). The difference in Ga/(Ga + Y) between the as-grown and the acetate-treated products in the x ) 0.125, 0.25, and 0.375 systems means that the
three as-grown forms are a little unstable so as to allow a partial release of yttrium species by the acetate treatment. It is therefore likely that the acidity of the Ga-Y mixed oxides would be controllable by the Ga/ (Ga + Y) molar ratio. The N2 adsorption-desorption isotherm for the acetatetreated form of the x ) 0.5 product separated after an elapsed reaction time of 9 h is shown in Figure 3. The rapid adsorption in the range of P/P0 ) 0-0.4 for the sample is due to the monolayer coverage of pores and particle surface and capillary condensation in pores. The desorption isotherm showed no hysteresis relative to the adsorption isotherm. A similar result was observed for MCM-41.1 The specific surface area was as large as 714 m2 g-1. Judging from the Ga0.57Y0.43O1.5 to SiO2 mass ratio of 102.0/60.1, the specific surface area of 714 m2 g-1 corresponds to a specific surface area of 1212 m2 g-1 for a mesoporous silica, nearly equal to 1000-1200 m2 g-1 for MCM-41 or FSM-16. The effective pore size was determined to be 2.0 nm. Taking into consideration a unit cell parameter of 4.0 nm for the sample recovered after the N2 adsorption-desorption isotherm measurement, the thickness of the inorganic wall and acetate layer is thus estimated to be 2.0 nm. The acetate-treated products also showed an increase of specific surface area from 447 m2 g-1 for x ) 0 to 714 m2 g-1 for x ) 0.5 with an increase of x (Table 1), in harmony with an increase in crystallinity. On the other hand, the specific surface area of the x ) 0.625-1.0 products decreased with an increase of x, along with a decrease in crystallinity as shown from their XRD patterns. For example, the x ) 1.0 product showed a specific surface area of as low as 233 m2 g-1 and the disappearance of 100, 110, and 200 XRD peaks. But it is likely that the pore structure of the x ) 1.0 product incompletely collapsed, taking into consideration that the specific surface area of an acetate-treated aluminum-based hexagonal mesophase is as low as 12 m2 g-1. This is because the gallium oxyhydroxide species are more highly polymerized than the aluminum oxyhydroxide species, leading to the appreciable maintenance of the resulting inorganic framework upon the removal of surfactants. We also tried to synthesize other yttrium oxidesupported hexagonal mesostructured metal oxides. When indium chloride, the same group IIIB metal as Al and Ga, was used in place of gallium chloride, layered mesophases and In(OH)3 occurred but no hexagonal mesophase was obtainable for any molar ratios of x. A similar reaction
Notes
using zirconium oxychloride or iron(III) chloride as a second metal component yielded only Y2(CO3)3‚3H2O and amorphous zirconium and iron hydroxides. We also could not synthesize even In-, Zr-, and Fe-based hexagonal mesostructured oxides in the homogeneous precipitation method. On the other hand, the use of tin(IV) chloride led to the formation of a mesostructured tin yttrium oxide, and it was converted into a mesoporous material by the acetate treatment, in contrast to the collapse of a mesostructured tin oxide by the same treatment.26 In the mesostructured yttrium aluminum oxide system, the substitution of yttrium species into the aluminum oxide framework led to a change from the 4-coordinate Al-rich framework to the 6-coordinate Al-rich one to stabilize the hexagonal structure for the removal of surfactants, since the framework composed mainly of edge- and vertexshared YO6 and AlO6 octahedra is much more stable than (26) Yada, M.; Kijima, T. To be described elsewhere.
Langmuir, Vol. 16, No. 10, 2000 4755
that composed of edge-shared AlO4 tetrahedra.27 A similar effect would be exerted in the presence of a mesostructured gallium-yttrium mixed oxide system. It is thus concluded that yttrium species act as a polymerizing agent for inorganic framework to form mesoporous materials. In conclusion, we have synthesized ordered hexagonal mesoporous gallium oxides structurally stabilized by yttrium oxide and demonstrated the capability to control the Ga to (Ga + Y) mole ratio in the resulting solids. The hexagonal mesoporous gallium oxide would be promising as a acid catalyst with size-selective property. This technique, the doping of polymerizing agent into inorganic framework, may be applicable to convert any other unstable hexagonal mesostructured metal oxides for removal of surfactants into stable mesoporous solids. LA991628U (27) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Langmuir 2000, 16, 1535.
