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A Simple Route for the Synthesis of Mesostructured Lamellar and Hexagonal Phosphorus-Free Titania (TiO2) Do Trong On† Department of Chemical Engineering, Laval University, Sainte Foy, Quebec G1K 7P4, Canada Received May 4, 1999. In Final Form: October 6, 1999 A new synthetic approach to generate mesostructured phosphorus-free titania is reported. Mesostructured hexagonal and lamellar titania were prepared by using the cationic surfactant C16TMA+ and soluble peroxytitanates in the presence of Na+ and TMA+, respectively. Moreover, upon hydrothermal postsynthesis, the lamellar mesophase can be converted to the hexagonal mesophase in the presence of Na+ in the liquid phase. In contrast, the lamellar structure is stable and improves under similar conditions when Na+ is replaced by TMA+.
Introduction The discovery of a new family of mesoporous materials designated as M41S by researchers at Mobil Oil Corporation1,2 has opened up a new perspective in the hydrothermal synthesis of porous materials. A variety of investigations have been carried out on this family, for example, preparative techniques, modification of oxide framework, and applications of such materials as catalysts.3-8,10-15 Further recent efforts have suggested that it is possible to synthesize mesoporous materials based on transition metal oxides.5-8 Semiconducting transition metal oxides participate in a variety of photocatalytic reactions. Titanium dioxide is one of the most studied semiconductors for these photocatalytic reactions.9 Mesoporous titania has been synthesized by using alkyl phosphate surfactants and titanium isopropoxide bisacetylacetonate;8-10-12 however, phosphorus from the template was bound so strongly to the molecular sieve that it could not be removed by either calcination or solvent † Tel.: 1(418) 656 2131, ext. 4356. Fax: 1(418) 656 5993. Email:
[email protected].
(1) Kresge, C. T.; Leonowick, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowick, 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, 10834-10843. (3) Trong On, D.; Joshi, P. N.; Kaliaguine, S. J. Phys. Chem. 1996, 100, 6743-6748. (4) Trong On, D.; Zaidi, S. M. J.; Kaliaguine S. Microporous Mesoporous Mater. 1998, 22, 211-224. (5) Huo, Q.; Margolese, D. I.; Cielsa, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317-321. (6) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D. I.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P. M.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299-1303. (7) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426-430. (8) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014-2017. (9) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (10) Putnam, R. L.; Nakagawa, N.; McGrath, K. M.; Yao, N.; Aksay, I. A.; Gruner, S. M.; Navrotsky, A. Chem. Mater. 1997, 9, 2690-2693. (11) Stone, V. F., Jr.; Davis, R. J. Chem. Mater. 1998, 10, 14681474. (12) Fujii, H.; Ohtaki, M.; Eguchi, K. J. Am. Chem. Soc. 1998, 120, 6832-6833. (13) Antonelli, D. M. Microporous Mesoporous Mater. 1999, 30, 315319. (14) Yang, P.; Zhao, D.; Margolese, D. I.; Chelka, B. F.; Stucky, G. D. Nature 1998, 396, 152-155. (15) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147-1160.
extraction. This will limit its possible use as a catalyst or catalyst support; for example, one of the reasons for the relative low photocatalytic activity of these materials is attributed to the poisoning of catalytic surface sites by the residual phosphorus.11,12 However, the method above for the synthesis of mesostructured titania was not successful with cationic surfactants and other anionic surfactants such as those with carboxylate or sulfate headgroups. Recently, phosphorus-free mesoporous titania was synthesized by using amine surfactants in combination with a dry aging technique; however, these materials are not thermally stable.13 Yang et al.14 also reported the synthesis of thermally stable and large-pore mesoporous metal oxides, including TiO2, ZrO2, etc., by using block copolymers as structure-directing agents. In this study, I report a relatively simple and effective route for synthesizing mesostructured lamellar and hexagonal titania by using a modified sol-gel method in conjunction with a cationic surfactant, cetyltrimethylammonium chloride (C16TMA+Cl-), and soluble peroxytitanates. Moreover, the lamellar titania mesophase transforms into the hexagonal mesophase upon hydrothermal postsynthesis. This strategy thus obviates the presence of phosphorus in mesoporous titania8,10-12 as well as the use of acetylacetone and other chelating agents to control the condensation reaction. Experimental Section Two series of Na+-free and Na+ mesostructured titania were hydrothermally prepared from cetyltrimethylammonium chloride and soluble peroxytitanates, [TiO2(OH)(H2O)]OH. The molar composition of the Na+-free gel was, TiO2-10H2O2-C16TMACl1.95TMAOH-300H2O. In a typical synthesis procedure, solution 1 was prepared by adding 5.84 g (0.0256 mol) of tetraethylorthotitanate (TEOT) to doubly distilled deionized water (50 g) with constant stirring, whereupon a white precipitate (hydrous titanium oxide) was formed. An aqueous H2O2 solution (30 g, 30 wt % H2O2) was added to the stirred slurry at room temperature and stirring continued for 2 h. The hydrous titanium oxide dissolved on reaction with H2O2 to form soluble peroxytitanates.16 A second solution was prepared by mixing 32.8 g of 25% cetyltrimethylammonium chloride (C16TMACl), 20 g of 25% tetramethylammonium hydroxide (TMAOH), and 25 g of water. Solution 1 was added dropwise to the well-stirred solution 2, and upon complete addition, the mixture was blended for an additional 1 h. The pH was adjusted with dilute HCl solution to 11.5 and (16) Trong On, D.; Kaliaguine, S.; Bonneviot, L. J. Catal. 1995, 157, 235-243.
10.1021/la9905463 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/07/1999
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Figure 1. Powder X-ray diffraction patterns of the as-made lamellar Na+-free titania followed by hydrothermal treatment in TMAOH solution, pH ) 11.5 (50 mL of TMAOH solution/g of sample) at 100 °C for (a) 0 day (as-made sample), (b) 7 days and (c) 14 days. the homogeneous mixture was then transferred into a Teflonlined autoclave and heated to 80 °C for 48 h. The synthesis conditions chosen were based on 18 syntheses performed by the variation of aging time (1-5 days) and hydrothermal temperature (40-120 °C); in all cases, the pH was held constant (pH ) 11.5). Another series of mesoporous titania with various Na+ concentrations in the gel mixtures (Na+/Ti ) 1-2.5) was also synthesized by using the same above procedure except that NaOH was used instead of TMAOH. The solid products were filtered, washed, and dried in air at room temperature. The hydrothermal posttreatment was carried out in an autoclave using the Na+free as-made sample: the sample was mixed with ∼50 times its weight (i) in TMAOH solution and (ii) in NaOH solution both at pH ) 11.5 and the slurry was heated at 100 °C for a given time.15 The treated samples were also washed with doubly deionized water in order to eliminate TMA+ and Na+ ions and then dried at room temperature.
Results and Discussion The as-made Na+-free sample exhibited a well-defined X-ray diffraction (XRD) pattern typical of lamellar mesophase with a d spacing of 31.5, 15.5, and 10.2 Å due to the 001, 002, and 003 reflections, respectively (Figure 1a), in agreement with those described by Beck et al.1,2,10,12 This strongly suggests that the as-made Na+-free sample is a lamellar mesophase. Two approaches were used for obtaining higher quality lamellar phase and transformed hexagonal phase. The first involved the as-made lamellar Na+-free sample followed by hydrothermal posttreatment in TMAOH solution, pH ) 11.5 at 100 °C for various times. Figure 1 shows the XRD patterns of the as-made sample and the samples treated for 7 and 14 days. The XRD peaks were invariant in comparison with the as-made sample, even after 14 days of hydrothermal treatment. However, the treated samples exhibited narrower and clearer XRD peaks. Figure 3A also shows a TEM image of the sample after 14 days of posttreatment, clearly indicating a welldefined lamellar structure with large domains. It suggests that the presence of TMA+ in the liquid-phase improves the lamellar mesophase during the hydrothermal posttreatment (Figure 1). The second approach is the use of NaOH instead of TMAOH in the liquid phase. A solid-
Figure 2. (A) Powder X-ray diffraction patterns of as-made lamellar Na+-free titania followed by hydrothermal treatment in NaOH solution, pH ) 11.5 (50 mL of NaOH solution/g of sample) at 100 °C for (a) 0 day (as-made sample), (b) 7 days and (c) 14 days. (B) Nitrogen adsorption-desorption isotherm plots of the mesoporous titania (sample c in Figure 2A) calcined at 300 °C for 4 h. The inset is the pore size distribution curve obtained from nitrogen desorption branch.
phase transition was observed during this posttreatment. Figure 2A presents the XRD patterns of the as-made lamellar sample and the treated sample in the presence of Na+ at 100 °C for various times. After 7 days, the sample contained mixed lamellar/ hexagonal phases, as seen in Figure 2A. Furthermore, after 14 days, the lamellar phase disappeared and only the hexagonal phase (d100 ) 40.5 Å) was observed. The XRD pattern of this sample shows a major peak along with a shoulder, which is characteristic of a hexagonal phase with a pore system lacking longrange order (Figure 2A, curve c). This is also reminiscent of HMS17 and MSU18 mesoporous materials, which are wormhole frameworks. Figure 2B also presents the N2 adsorption/desorption isotherm and the pore size distribution of this sample after calcination at 300 °C for 4 h (heating rate 1 °C/min). Its surface area was ∼310 m2/g. And the pore size diameter calculated from the desorption isotherm branch using the Horvath-Kawazoe model19 was (17) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (18) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (19) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470.
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Figure 3. TEM images of (A) lamellar and (B) hexagonal titania obtained after hydrothermal posttreatment in TMAOH and NaOH solution, respectively, at 100 °C for 14 days.
∼36 Å. Elemental analysis of this calcined sample shows 0.2% N and 1.2% C; only a trace of P was detected. Calcination of this sample at higher temperature (>300 °C) led to loss of surface area, and the sample totally collapsed at 700 °C with the surface area of 40 m2/g. Note that this pore size diameter (∼36 Å) is greater than that reported (d ∼ 30 Å) for mesoporous aluminosilicate MCM41 using the same surfactant (C16TMACl).4 This is consistent with that obtained from XRD (d100 ) 40.5 Å). These results indicate that a larger pore size can be obtained upon hydrothermal postsynthesis.15 Figure 3B also shows a TEM image of this sample, clearly displaying the hexagonal array of mesopores with the 120° hexagonal symmetry. Moreover, the XRD pattern of the calcined sample was slighly broader and less intense than those of the as-synthesized sample; the TEM images show the calcined sample still containing a disordered hexagonal phase. Hence, the presence of Na+ in the liquid phase favors the change from the lamellar phase to the hexagonal phase. The existence of the biphasic product (Figure 2A, curve b) is evidence that the transition occurred in the solid phase and not through dissolution to the liquid phase.15 The direct synthesis of hexagonal phosphorus-free titania using soluble peroxytitanate and cationic surfactant, C16TMA+, was also studied. The hexagonal titania phase can be obtained under similar conditions as for the synthesis of the lamellar phase by using NaOH in place of TMAOH in the gel mixtures. However, its crystallinity depends strongly on the sodium concentration. Figure 4 presents the XRD patterns of the mesophase samples obtained as a function of Na+ concentration in the gels. The XRD pattern of the sample (with Na+:Ti ) 0.5:1) consisted of one broad peak for the (100) reflection. However, its intensity increased and then went through a maximum at a gel Na:Ti ratio of 1.5:1, and no lamellar phase was observed in these samples. Thus, it is interesting to note that the most distinct hexagonal phase pattern
Figure 4. Powder X-ray diffraction patterns of as-made mesophase samples obtained by the direct synthesis using NaOH in place of TMAOH in the gels with various Na+ concentrations. Na:Ti ratio of (a) 0.5:1, (b) 1:1, (c) 1.5:1, and (d) 2.5:1.
was associated with the sample prepared with 1.5:1 Na/ Ti ratio and had a d spacing for the (100) reflection at 38 Å. The low intensity (110), (200), and (210) reflections are not clearly distinguished. This phenomenon is attributed to disordered packing of individual tubes in the mesostructure with a long range ordering of 20-40 nm, according to the transmission electron microscopy (TEM) images and wide-angle diffaction pattern results of the as-synthesized and calcined samples.14 Calcination of this sample (at a gel Na:Ti ratio of 1.5:1) at 300 °C yielded a mesoporous phase with 275 m2/g surface area and a pore
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size of ∼30 Å, as determined by nitrogen adsorption. It is clear that the factor governing the structural features of the hexagonal phase is the Na+ concentration in the gel mixture. As demonstrated above, under hydrothermal posttreatment, the nature of cations in the liquid-phase affects the formation of the product structure. In the presence of Na+, the titania framework continues to condense and undergo reconstruction under hydrothermal posttreatment. During condensation, the framework negative charge density decreases; therefore, the packing of the surfactant must change in order to maintain charge matching with the framework. The solid-phase transitions can be explained in terms of reorganization of the surfactant and condensation of the solid phases. In this study, ionic additives, such as TMA+ and Na+, penetrate into the hydrophobic and palisade regions of surfactant arrays and thereby
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induce a structural rearrangement of the surfactant phase to reoptimize the interface charge density matching and the surfactant packing.6 However, the behavior depends on the nature of the cation. Thus, under hydrothermal postsynthesis, the presence of Na+ in the liquid phase favors the lamellar to hexagonal phase transformation. In contrast, the lamellar structure is stable and improves under similar conditions when Na+ is replaced by TMA+. The designed synthesis in this study should be extendable to a wide range of mesoporous materials based on transition metal oxides. Work is in progress to gain further insight into the mechanistic aspects of this process using different types of surfactants as well as the study of photocatalytic activities of these materials. LA9905463
