Ind. Eng. Chem. Res. 2004, 43, 2485-2492
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Study on the Synthesis of High-Surface-Area Mesoporous TiO2 in the Presence of Nonionic Surfactants Guillermo Calleja,* David P. Serrano, Rau ´ l Sanz, Patricia Pizarro, and Ahitana Garcı´a Group of Environmental and Chemical Engineering, School of Experimental Sciences and Technology (ESCET), Rey Juan Carlos University, c/ Tulipa´ n s/n, 28933 Mo´ stoles, Spain
Mesoporous titanium dioxide has been prepared using a nonionic block copolymer (Pluronic P-123) as the structure-directing agent. A detailed characterization of the TiO2 materials so prepared indicates that a locally ordered mesostructure with high surface area and narrow pore size distribution is obtained under controlled synthesis conditions. The roles of the main factors affecting the synthesis have been studied, including the hydrolysis ratio (H2O/alkoxide molar ratio), the type of solvent, the surfactant concentration, and the titanium source. These variables influence the final properties of the inorganic oxide by directing the type of interactions between the species involved in the process. Thus, a variety of mesoporous TiO2 materials with different properties can be effectively obtained by adjusting the synthesis conditions. Moreover, the pore walls of the materials prepared are formed by incipient nanocrystallites of anatase, which is a significant achievement regarding the possible photocatalytic applications of these mesoporous TiO2 solids. Introduction The ability of surfactant materials to assemble and organize inorganic frameworks into ordered structures was first demonstrated with the development of the mesoporous silica material called MCM-41.1 The synthesis procedure is based on a liquid-crystal templating mechanism in which the silica source hydrolyzes and condenses into inorganic walls around ordered micelles of a cationic surfactant. This material exhibits a high surface area and a narrow hexagonal pore size distribution with dimensions that can be tailored through the controlled modification of the synthesis conditions. The subsequent application of this synthesis procedure to other inorganic oxides, such as Al2O3, Nb2O3, and ZrO2, has been carried out, leading to materials with similar textural properties.2 Titanium dioxide is one of the most studied semiconductors for use in photocatalytic reactions and as a catalyst support.3-6 Its low cost, ease of handling, resistance to photoinduced corrosion, and harmlessness are the main properties that make this material so interesting. Commercial crystalline TiO2, however, has a very low surface area and a minute particle size, making its recovery quite difficult. For the types of applications intended for titania, an increase of the surface area is desirable, but full structural order is not crucial as shape-selectivity effects are not involved in most of its applications. In recent years, the synthesis of mesoporous TiO2 under many different conditions has been extensively studied.7-15 In the case of surfactantassisted routes, a variety of compounds can be used that provide different interactions between the surfactant and the titania precursor such as electrostatic, hydrogenbonding, covalent-bonding, and van der Waals interactions. When these interactions are well balanced, the self-assembly mechanism is achieved, giving materials with a wide range of structural orders and textural properties. * To whom correspondence should be addressed. Tel.: 34 91 488 70 06. Fax: 34 91 664 74 90. E-mail: g.calleja@escet. urjc.es.
Ionic surfactants were used initially in these synthesis approaches because their amphiphilic nature provides well-organized micelles around which the titania framework can be assembled by electrostatic interactions. Antonelli et al.7 synthesized TiO2 with a system of hexagonally arranged mesopores by a modified sol-gel method using an alkyl phosphate surfactant. Mesostructured titania was later prepared by the same author8 using dodecylamine as the template. The TiO2 samples so obtained exhibited high surface areas after surfactant removal by solvent extraction with p-toluene sulfonic acid in ethanol. Trong On9 used cetyltrimethylammonium chloride as the cationic surfactant and soluble peroxytitanate as the Ti precursor. This route led to either hexagonal or lamellar mesophases of titania that retained their structure up to 300 °C. More recently, Yoshitake et al.10 reported the synthesis of titanium dioxide with the largest BET surface area registered until now and pore sizes in the range of mesopores. Dodecylamine was used as the surfactant, and the stability of the material was slightly improved by a chemical vapor deposition posttreatment. The use of ionic surfactants presents limited potential applications because of their strong interactions with titania walls, which result in the impossibility of completely removing the surfactant by extraction procedures and in the collapse of the inorganic structure when postsynthesis thermal treatment is employed for surfactant elimination. Thus, nonionic surfactants appeared to be a potential alternative given that, in this case, hydrogen bonding mediates the formation of the metal oxide-surfactant composites involved in the inorganic framework organization. Yang et al.11,12 prepared mesoporous TiO2 by a neutral route using a block copolymer as the structuredirecting agent and TiCl4 as the Ti precursor. The resulting materials exhibited a high pore ordering but a lower surface area than those obtained by other methods based on a ligand-assisted templating mechanism. Moreover, the extreme reactivity of the titania precursor in the presence of water makes this procedure
10.1021/ie030646a CCC: $27.50 © 2004 American Chemical Society Published on Web 04/14/2004
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poorly amenable to proper and safe development. SolerIllia et al.13,14 studied the influence of different synthesis conditions on the interactions that take place during the formation of mesostructured TiO2 when a nonionic block copolymer is used as the surfactant. They found that the water and acid contents play an important role in the synthesis, yielding vermicular mesophases at low water concentrations, but ordered mesostructures at higher concentrations. Previously, we reported the synthesis of mesoporous TiO2 with a high surface area using the block copolymer PEOx-PPOy-PEOx [PEO ) poly(ethylene oxide), PPO ) poly(propylene oxide)] and titanium alkoxide as the initial reaction agents.15,16 The weak surfactant-precursor interactions involved in the synthesis reactions made possible the effective elimination of the block copolymer by solvent extraction. Moreover, the use of acid synthesis conditions provided titania pore walls composed of nanocrystalline anatase. In this work, we report the preparation and characterization of mesoporous TiO2 (m-TiO2) through the solgel route developed in our previous work,15,16 as well as the influence of the main synthesis variables. The procedure developed here provides appropriate conditions for the organization of TiO2 into a mesostructure having both a high surface area and pore walls formed by incipient anatase nanocrystals. These materials are very promising for use in the preparation of catalytic materials, such as photocatalysts. Experimental Section Chemicals. Reagents used for the synthesis included triblock poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (PEO20PPO70PEO20, Pluronic P-123, Aldrich) and different titanium alkoxides purchased from Alfa Aesar and Acros. All chemicals were used as received. Synthesis. In a typical synthesis, Pluronic P-123 (5 g) was first dissolved in a mixture of 2-propanol and 0.8 M HCl diluted in water. The resulting solution was stirred for 4 h, to favor organization of the surfactant into micelles. The clear solution formed was added to a solution of titanium isopropoxide (0.05 mol) and 2-propanol under vigorous stirring at 40 °C, and the sol obtained was kept under stirring until a gel appeared. The gel was aged at 40 °C so that the total synthesis time was 20 h, and then it was dried under ambient conditions. The solid product (m-TiO2) was washed twice with ethanol, and removal of the surfactant was completed by extraction with boiling ethanol under reflux for 24 h. The effects of the following synthesis variables were studied: hydrolysis molar ratio (H ) H2O/Ti), type of organic solvent, type of titanium alkoxide used and surfactant/Ti ratio (S). Characterization. TG/DTA measurements were carried out in flowing air with a simultaneous DSC-TGA apparatus (SDT 2960, TA Instruments). Samples were heated to 600 °C at 5 °C min-1 with flowing air at 100 mL min-1. Spectroscopic analyses of TiO2 were performed using an FT-IR spectrometer (Mattson Infinity Series) and a UV-visible spectrophotometer (Varian Cary 500). The UV spectra were analyzed with the Kubelka-Munk function F(R) to calculate the band gaps of TiO2 samples. Nitrogen adsorption-desorption isotherms at 77 K were obtained using a Micromeritics Tristar 3000 apparatus. The Brunauer-Emmet-Teller (BET) equation
was used to calculate the specific surface area. Pore size distributions were obtained using the Barret-JoynerHalenda (BJH) model in the range of mesopores and the Horvath-Kawazoe approach for micropore determinations. Low-angle and wide-angle X-ray powder diffraction (XRD) patterns were obtained on a Philips X’PERT MPD diffractometer using Cu KR radiation. Transmission electron microscopy (TEM) images were recorded on a Philips Technai 20 instrument working at 20 kV. Results and Discussion 1. Properties of m-TiO2. Prior to analysis of the effects of the main synthesis variables on the properties of titanium dioxide, an m-TiO2 sample synthesized with a hydrolysis ratio of 6 was extensively characterized after surfactant extraction. Figure 1 summarizes the results obtained in the characterization of the extracted m-TiO2. The material presents a type IV isotherm, according to the IUPAC classification, with a small hysteresis loop indicating the existence of some pore necking (Figure 1A). The high BET surface area of 381 m2 g-1 and the relatively narrow pore size distribution centered at 2.8 nm, with a pore volume of 0.249 cm3 g-1 (Figure 1B), indicate that Pluronic acted effectively as a structure-directing agent during the synthesis reactions. Adsorption in the microporous range also occurs, which is attributed to the inclusion of the hydrophilic groups of the surfactant into the walls of the titania framework and their subsequent removal by solvent extraction. This effect has been previously observed in the synthesis of the pure silica material SBA-15 employing the same type of surfactant.17 Thermogravimetric analysis of the extracted sample (not shown here) indicates a maximum organic matter content of 4%, verifying the effective elimination of surfactant from the inorganic structure by the treatment of the as-synthesized m-TiO2 with boiling ethanol under reflux. The low-angle X-ray diffraction pattern (Figure 1C) exhibits a broad peak, indicating the presence of some ordering of the m-TiO2 porous structure. On the other hand, wide-angle XRD analysis (Figure 1D) shows the amorphous nature of the pore walls of the material. However, in a previous work,16 we reported that the addition of HCl into the synthesis medium not only favors hydrolysis of the titania precursor and development of the mesostructure, but also induces the nucleation of the anatase phase within the pore walls. By transmission electron microscopy (Figure 2) the low ordering degree of the m-TiO2 porous structure is revealed. When these images are magnified (Figure 2B), several crystal planes that are randomly oriented across the particle can be detected. The distance between planes agrees well with that of anatase, corroborating the nucleation assumption mentioned above. The presence of this crystalline phase is not observed in the wideangle X-ray pattern because of the low degree of crystallization of the material and the small size of the nuclei. Therefore, it can be concluded that the synthesis procedure presented here provides mesostructured titanium dioxide with a high specific surface area and a relatively uniform structure whose pore walls are incipiently crystallized into the allotropic anatase phase. At present, we are conducting further investigations to
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Figure 1. Characterization of a typical m-TiO2 sample: (A) N2 adsorption-desorption isotherm; (B) pore size distribution; (C) XRD spectra, low-angle and wide-angle diffraction patterns; and (D) UV-vis spectrum. Table 1. Properties of m-TiO2 Prepared at Different Hydrolysis Ratiosa Compared to Commercial Samples sample anatase rutile m-TiO2-H(3) m-TiO2-H(6) m-TiO2-H(10) m-TiO2-H(20) m-TiO2-H(50) m-TiO2-H(100) a
Figure 2. TEM images of a typical m-TiO2 sample at different magnifications.
complete this crystallization process so that nanocrystallized mesostructured TiO2 is obtained.16 Thus, a novel TiO2 material combining remarkable textural and crystalline properties has been developed that might result in an increase in photocatalytic applications of titania.
SBET VPore DPore (m2 g-1) (nm) (cm3 g-1)
phase
band gap (eV)
2.4 2.8 4.2 2.6 2.6 2.0
anatase rutile amorphous amorphous anatase anatase anatase-rutile anatase-rutile
3.44 3.14 3.45 3.48 3.42 3.40 3.30 3.28
7.0 4.2 582 381 245 223 180 137
0.342 0.249 0.252 0.127 0.143 0.083
H(x), where x ) moles of H2O per mole of Ti.
2. Effect of the H2O/Ti Molar Ratio. The hydrolysis molar ratio (H) or the water content during the synthesis reaction is one of the most important variables affecting the textural and structural properties of TiO2. It is well-known that the incorporation of water has three main functions:13,14,18 (a) it facilitates the hydrolysis and condensation reactions of the titanium dioxide precursor; (b) it can influence the type of interactions between the surfactant and the metallic centers; and (c) it increases the polarity of the reaction medium so that the surfactant self-arranges into more ordered micelles, around which the titania framework will condense. The effect of the hydrolysis ratio on the overall properties of m-TiO2 was investigated. For this purpose, the H2O/Ti molar ratio was varied between 3 and 100, while the rest of the synthesis variables were kept constant. The resulting samples were labeled as m-TiO2H(x), where x corresponds to the value of the hydrolysis ratio. Table 1 summarizes the results obtained from the characterization of the prepared samples. Nitrogen
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Figure 3. Nitrogen adsorption-desorption data for m-TiO2 prepared at different hydrolysis ratios: (A) isotherms and (B) BJH pore size distributions.
adsorption-desorption isotherms (Figure 3A) reveal a decrease in the specific surface area and pore volume as the water content is increased. This effect is more dramatic for H e 10, indicating the crucial role of this variable in the final properties of m-TiO2. Hydrolysis molar ratios higher than 10 seem to lead to uncontrolled hydrolysis and condensation reactions, almost instantly giving TiO2 precipitates, which hardly interact with the
surfactant micelles. Consequently, materials with a significant lack of uniformity, broader pore size distributions, and isotherms closer to type I are obtained. In conclusion, values of H below 10 are adequate for controlling the synthesis process, considerably improving the textural properties of the resulting titania. From wide-angle XRD patterns, it can be deduced that the addition of acidic water solution to the reaction
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2489 Table 2. Properties of m-TiO2 Prepared from Different Solvent Compositions solvent
SBET (m2 g-1)
DPore (nm)
VPore (cm3 g-1)
phase
band gap (eV)
2-propanol ethanol methanol
381 547 426
2.8 2.0 2.9
0.249 0.278 0.269
amorphous amorphous amorphous
3.48 3.57 3.50
Table 3. Properties of m-TiO2 Prepared with Different Surfactant/Ti Ratios (S)a
Figure 4. UV-vis spectra of m-TiO2 samples prepared at different hydrolysis ratios.
medium causes crystallization to the anatase form for H ) 10 and a partial phase transition to rutile for larger water contents. The optical band gaps of these materials were calculated from the linear absorption branch of the UV-vis data (Figure 4), assuming that a proportionality exists between the Kubelka-Munk function, F(R), and the absorption coefficient.19,20 The resulting parameter, reported in Table 1, clearly decreases as the crystallization process proceeds, approaching that of either pure anatase or rutile as these crystalline phases appear. 3. Effect of the Type of Solvent. According to the literature, the progress of hydrolysis and condensation reactions is strongly determined by the nature of the solvent, as the solvent can provide hydroxyl groups or water molecules when polar or aqueous solvents are used.6,21-23 In addition, the micelle arrangement and, consequently, the structure of the final metal oxide will depend on the polarity of the solvent. In this study, several solvents were used (2-propanol, ethanol, and methanol) to determine their effects on the structure and properties of TiO2. Two hydrolysis ratios were employed, H ) 6 and 10, according to the discussion above. However, no significant differences in the final results were observed for the two ratios, so only the former is considered here for the discussion. Both the appearance of the gel and the time required for its formation are good indicators of the type of material being obtained.21 In this way, clear gels denote the absence of TiO2 precipitates, whereas turbidity reflects larger amounts of precipitates. When 2-propanol is used as the solvent, a slightly turbid gel is obtained, whereas the use of ethanol gives completely transparent gels. Under these synthesis conditions, transalcoholysis reactions between the isopropoxide groups of the titanium source and the solvent are favored.13,14 In the case of ethanol, Ti species of lower reactivity are created, decreasing the hydrolysis rate and, consequently, avoiding the prompt precipitation of titania. On the other hand, white precipitates are immediately generated when titanium isopropoxide is added to methanol. This result can be explained in terms of the greater polarity and reactivity of the methanol molecules, so that more accelerated hydrolysis and condensation reactions occur. The different gelation behaviors described above have a strong influence on the textural and optical properties of titania samples obtained, as shown in Table 2. Substitution of 2-propanol by either methanol or ethanol lead to similar isotherm shapes, although the pore size
sample
SBET (m2 g-1)
DPore (nm)
VPore (cm3 g-1)
phase
band gap (eV)
m-TiO2-S(0.6) m-TiO2-S(0.8) m-TiO2-S(1.0) m-TiO2-S(1.2) m-TiO2-S(1.4)
166 300 381 377 337
6.3 4.5 2.8 2.7 2.4
0.247 0.259 0.249 0.259 0.204
amorphous amorphous amorphous amorphous amorphous
3.50 3.51 3.48 3.51 3.52
a
S(x), where x ) grams of P-123 per 0.01 mol of Ti.
and BET surface areas are notably modified, especially for the ethanol solvent. In this case, the specific surface area was increased significantly to approximately 550 m2 g-1, with the average pore diameter being reduced to 2.0 nm. 4. Effect of the Pluronic/Ti Ratio. Surfactant organization into micelles takes place when the critical micelle concentration (cmc) is reached. This concentration depends on the type of surfactant as well as the chemical nature and impurities of the solvent employed.24-26 It is important, therefore, to work with appropriate amounts of surfactant to ensure effective TiO2 condensation around the ordered surfactant structures, providing final materials with mesostructured frameworks. In this section, the influence of the concentration of surfactant in the synthesis solution is examined. For this purpose, the other synthesis variables were selected as follows: titanium isopropoxide as the Ti source, 2-propanol as the solvent, and hydrolysis ratio H ) 6. TiO2 materials so obtained were labeled as m-TiO2S(x), where x refers to the fraction grams of P-123 per 0.01 mol of Ti. Variation of the Pluronic P-123 amount leads to noticeable changes in m-TiO2 properties, as is evidenced in Figure 5. Increasing surfactant concentrations to S ) 1 results in higher specific surface areas and smaller pore sizes (Table 3). Because a type IV isotherm and an ordered mesostructure are yielded for the lowest surfactant/Ti ratio (S ) 0.6), it is clear that the critical micelle concentration of Pluronic is reached under those synthesis conditions. When the amount of surfactant added to the reaction medium is increased, equilibrium of the concentration between the micellar and dispersed species is established. Thus, the larger the quantity of P-123 incorporated, the higher the concentrations of both phases, which justifies both the higher BET areas (from 166 to 381 m2 g-1) and the smaller pore sizes (from 6.3 to 2.8 nm). On the other hand, for S > 1, a slow but progressive decrease in the specific surface area is produced, whereas the pore sizes remain essentially unchanged. Moreover, the hysteresis-cycle shape suggests a structural modification affecting the pore geometry. Therefore, both dispersing effects and micelle organization changes might account for these latter results, clearly indicating that the surfactant concentration is also a critical process variable. 5. Effect of the TiO2 Source. The influence of the titanium dioxide precursor is directly related to its reactivity and, consequently, to the rates of the hydroly-
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Figure 5. Nitrogen adsorption-desorption data for m-TiO2 prepared at different P-123/Ti ratios: (A) isotherms and (B) BJH pore size distributions.
sis and condensation reactions through which the inorganic oxide framework is created. Inorganic sources, such as TiCl4, have been extensively studied,11,12 but their high reactivity and difficult handling have forced the need for other materials that are easier to control. Among them, titanium alkoxides have become the most promising precursors because of their higher chemical stability and safer handling. Moreover, alkoxyl groups act as reaction controllers by steric inhibitions during the synthesis reactions, giving time for interactions between TiO2 and surfactant to be established. In this work, different titania precursors were tested as shown in Table 4: Ti isopropoxide [Ti(OPri)4], titanium ethoxide [Ti(OEt)4], titanium n-butoxide [Ti(OBun)4], titanium tert-butoxide [Ti(OBut)4], and titanium isopropoxide chemically modified with 2,4-pentanodione [Ti(OPri)4-AcAc]. In this case, 2-propanol was
Table 4. Properties of m-TiO2 Prepared from Different Titania Sources Ti source
DPore SBET VPore (m2 g-1) (nm) (cm3 g-1)
Ti(OPri)4 Ti(OEt)4 Ti(OBun)4 Ti(OBut)4
381 333 280 383
2.8 3.0 4.9 2.8
0.249 0.256 0.229 0.311
Ti(OPri)4-AcAc
398
2.0
0.218
phase amorphous amorphous amorphous amorphousanatase amorphous
band gap (eV) 3.48 3.46 3.46 3.55 3.34
used as the solvent, the hydrolysis ratio was fixed to H ) 6 and the surfactant/Ti ratio to S ) 1. The textural properties of the mesoporous materials obtained with these Ti sources, derived from nitrogen adsorption-desorption isotherms, are quite similar, except when either titanium n-butoxide or chemically modified titanium isopropoxide are employed. This
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modification consisted of treating the titanium isopropoxide with 2,4-pentanodione (0.1 mol of AcAc/mol of Ti) so that metallo-acetylacetonato complexes are formed, which are kinetically more stable to the hydrolysis reactions. The lower reactivity of these compounds increases the contact time between the surfactant and the inorganic phase. However, the bonding strength of the acetylacetonate groups is too high for them to be removed either under the synthesis conditions or by solvent extraction, so they remain linked to the TiO2 structure and partially block the pores, explaining the pore size reduction observed. On the other hand, upon use of titanium n-butoxide, a significant reduction of the BET area (280 m2g-1) and an enlargement of the pore size (4.9 nm) are observed, the shape of the isotherm being closer to that of a typical siliceous SBA-15 material. The different reactivity and spatial configuration of the n-butoxide group, derived from its linearity and larger dimensions, are assumed to be responsible for this particular behavior. Finally, for the m-TiO2 material prepared from titanium tertbutoxide, a pore volume increase occurs, due again to the influence of the alkoxyl group on the geometric and structural configuration of the pores. Conclusions A sol-gel method has been successfully applied to the preparation of mesostructured titanium dioxide (mTiO2) using titanium alkoxides as the inorganic precursors, and a nonionic copolymer surfactant (Pluronic) as the structure-directing agent. The weak metallo-surfactant interactions, based on hydrogen bonds, make surfactant removal by solvent extraction of the assynthesized materials feasible. Hydrolysis molar ratios of less than 10 provide homogeneous solids characterized by high specific surface areas (>300 m2 g-1); narrow pore size distributions, in the range of mesopores; and walls composed of incipient nanocrystallites of anatase. By varying different synthesis conditions, such as the type of solvent and titanium alkoxide or the surfactant concentration, the preparation of different types of mesoporous TiO2 materials can be tailored. The chemical nature of the solvent employed determines the type of interactions between the species involved in the synthesis reactions, producing, for instance, accelerated hydrolysis of the titania precursor or even partial incoporation of the solvent into the inorganic framework. On the other hand, as the surfactant content is increased beyond the critical micellar concentration, dispersion effects and micellar arrangement changes occur, giving rise to materials of higher specific surface areas and smaller pore sizes. Finally, the reactivity and spatial configuration of the alkoxides have to be taken into account, as the pore diameter depends on the size and the linear nature of the alkoxyl group, increasing with these two structural parameters. These results show that the properties of mesoporous TiO2 can be effectively tailored to improve its potential capabilities as a catalyst support or photocatalyst, among other applications. Acknowledgment The authors acknowledge Consejerı´a de Educacio´n, Comunidad de Madrid, for the financial support of this
research through Project Grupos Estrate´gicos and Ministerio de Ciencia y Tecnologı´a for support through Project PPQ2000-1287. Literature Cited (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartulli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism. Nature 1992, 359, 710. (2) Sayari, A.; Liu, P. Non-Silica Periodic Mesostructured Materials: Recent Progress (Review). Microporous Mater. 1997, 12, 149. (3) Anpo, M. Applications of Titanium Oxide Photocatalysts and Unique Second-Generation TiO2 Photocatalysts Able to Operate under Visible Light Irradiation for Reduction of Environmental Toxins on a Global Scale. Stud. Surf. Sci. Catal. 2000, 130, 157. (4) Hermann, J.-M. Heterogeneous Photocatalysis: Fundamentals and Applications to the Removal of Various Types of Aqueous Pollutants. Catal. Today 1999, 53, 115. (5) Ollis, D. F. Heterogeneous Photocatalysis. Cattech 1998, 2, 149. (6) Schneider, M.; Baiker, A. Titania-based Aerogels. Catal. Today 1997, 35, 339. (7) Antonelli, D. M.; Ying, J. Y. Synthesis of Hexagonally Packed Mesoporous TiO2 by a Modified Sol-Gel Method. Angew. Chem., Int. Ed. Engl. 1995, 34 (18), 2014. (8) Antonelli, D. M. Synthesis of Phosphorus-Free Mesoporous Titania Via Templating with Amine Surfactants. Microporous Mesoporous Mater. 1999, 30, 315. (9) Trong On, D. A Simple Route for the Synthesis of Mesostructured Lamellar and Hexagonal Phosphorus-Free Titania (TiO2). Langmuir 1999, 15, 5, 8561. (10) Yoshitake, H.; Sugihara, T.; Tatsumi, T. Preparation of Wormhole-like Mesoporous TiO2 with an Extremely Large Surface Area and Stabilization of its Surface by Chemical Vapor Deposition. Chem. Mater. 2002, 14, 1023. (11) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Generalized Syntheses of Large-Pore Mesoporous Metal Oxides with Semicrystalline Frameworks. Nature 1998, 396, 152. (12) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Block Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Framework. Chem. Mater. 1999, 11, 2813. (13) Soler-Illia, G. J. de A. A.; Sanchez, C. Interactions Between Poly(ethylene oxide)-based Surfactants and Transition Metal Alkoxides: Their Role in the Templated Construction of Mesostructured Hybrid Organic-inorganic Composites. New J. Chem. 2000, 24, 493. (14) Soler-Illia, G. J. de A. A.; Scolan, E.; Louis, A.; Albouy, P.-A.; Sanchez, C. Design of Meso-Structured Titanium Oxo Based Hybrid Organic-Inorganic Networks. New J. Chem. 2001, 25, 156. (15) Serrano, D. P.; Calleja, G.; Sanz, R.; Pizarro, P. Synthesis of Micelle Templated TiO2 Mesophases by a Sol-Gel Approach: Effect of the Surfactant Removal. Stud. Surf. Sci. Catal. 2001, 135, 251. (16) Pizarro, P.; Serrano, D. P.; Calleja, G.; Sanz, R. Photocatalytic Degradation of Trichloroethylene in Water by High Surface Area Mesoporous Titania. Chem. Ind. Environ. IV 2003, 1,113. (17) Bennadja, Y.; Beaunier, P.; Margolese D.; Davidson, A. Fine Tuning of the Interaction Between Pluronic Surfactants and Silica Walls in SBA-15 Nanostructured Materials. Microporous Mesoporous Mater. 2001, 44-45,147. (18) Soloviev, A.; Tufeu, R.; Sanchez, C.; Kanaev, A. V. Nucleation Stage in the Ti(OPri)4 Sol-Gel Process. J. Phys. Chem. B 2001, 105, 4175. (19) Klaas, J.; Schulz-Ekloff, G.; Jaeger, N. I. UV-Visible Diffuse Reflectance Spectroscopy of Zeolite-Hosted Mononuclear Titanium Oxide Species. J. Phys. Chem. B 1997, 101, 1305. (20) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. PbS in Polymers. From Molecules to Bulk Solids. J. Chem. Phys. 1987, 87 (12), 7315.
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Received for review August 4, 2003 Revised manuscript received December 8, 2003 Accepted December 13, 2003 IE030646A
