A Comprehensive Study of Titanium-Substituted SBA-15 Mesoporous

J. Phys. Chem. C 2008, 112, 14403–14411

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A Comprehensive Study of Titanium-Substituted SBA-15 Mesoporous Materials Prepared by Direct Synthesis Franc¸ois Be´rube´,† Freddy Kleitz,*,‡ and Serge Kaliaguine*,† Chemical Engineering Department and Chemistry Department, LaVal UniVersity, Quebec, Canada G1V 0A6 ReceiVed: May 1, 2008; ReVised Manuscript ReceiVed: July 2, 2008

The influence of different synthesis parameters on the physicochemical properties of titanium-substituted SBA-15 silicas prepared by direct one-step synthesis was systematically studied by a combination of N2 physisorption at -196 °C, X-ray diffraction (XRD), diffuse reflectance UV-vis, and elemental analysis. The results showed that for low titanium precursor concentrations used in the initial synthesis gel, incorporation of titanium in the final material is not complete. Under these synthesis conditions, Ti atoms are well-dispersed in the silica framework and mainly exist in tetrahedral coordination. When the isolated titanium species present in the gel reach a critical concentration, an increase in incorporation ratio is observed due to the formation of anatase TiO2 clusters on the materials surface. It was also shown that this critical titanium loading is mainly influenced by synthesis temperature, hydrothermal treatment time, and silicon precursor concentration and does not depend on acid concentration. Moreover, it was shown that when the formation of anatase TiO2 takes place, the amorphous titanium species dispersed into the silica materials are resolubilized in the acidic solution increasing the fraction of bulk anatase into the mesoporous silica. 1. Introduction Titanium-substituted silicalites have attracted considerable attention due to their excellent properties as mildly acidic catalysts for selective oxidation reactions. In particular, titanium silicalites with MFI (TS-1) or MEL (TS-2) zeolite structure are remarkable redox catalysts for numerous catalytic reactions involving hydrogen peroxide as the oxidant.1,2 However, these zeolite-type materials are not active for oxidation reactions involving bulky molecules as they are limited to substrates having kinetic diameters lower than ∼0.55 nm. Significant advances in liquid-phase heterogeneous catalysis have been made with the discovery of M41S mesoporous molecular sieves.3 MCM-41 and related materials show a uniform arrangement of large pores which makes them suitable for processes involving diffusion of bulky molecules to internal active sites. Using the corresponding surfactant templating pathway, many efforts have been deployed to synthesize new materials with a variety of mesostructures and pore sizes.4-7 One of the most promising developments in this research field was the discovery of SBA-15-type materials synthesized with triblock-copolymer P123 as the structure-directing agent.8,9 The growing interest for this new family of molecular sieves is mainly due to their large pore size (6-12 nm), thick pore wall (3-6 nm), and consequently enhanced hydrothermal stability. However, as these mesoporous material frameworks consist of amorphous silica with no acid sites and very low ion exchange capacity, their applications in catalysis remain mainly limited to their use as catalytic supports.10,11 Many efforts were made to incorporate heteroelements such as aluminum,12,13 titanium,13-28 and vanadium29,30 into ordered mesoporous silica materials. Ti-substituted MCM-4114-17 and Ti-mesoporous mesocellular foam (Ti-MCF)18 have been syn† Chemical Engineering Department. Phone: +1 418 656 2708. Fax: +1 418 656 3810. E-mail: [email protected]. ‡ Chemistry Department. Phone: +1 418 656 7812. Fax: +1 418 656 7916. E-mail: [email protected].

thesized and showed potential for oxidation of bulky molecules. Later on, other silica mesostructures were substituted with titanium such as Ti-MSU19,20 and Ti-MCM-48.21,22 These new mesoporous catalysts seem to offer remarkable possibilities for partial oxidation reactions involving products selectivity.31-34 The first synthesis of Ti-SBA-15 was carried out by a nonaqueous postgrafting procedure.23 Since then several works have dealt with the incorporation of titanium into the SBA-15 framework by one-pot direct synthesis. Several optimization methods have been tried such as fluoride addition,24 microwave hydrothermal treatment,25 and pH adjustment of the synthesis solution.26 However, these synthetic procedures involve strongly acidic media and, in most cases, only a small fraction of the heteroelement added to the initial gel mixture was successfully incorporated into the final silica framework. Also, under these conditions, materials with only small loadings of isolated Ti4+ into the silica framework could be achieved and extensive formation of extra-framework anatase TiO2 is observed. In the first synthetic procedure of SBA-15 proposed by Zhao et al. in 1998,8 it was reported that this mesoporous material can be synthesized over a wide range of gel composition and reaction conditions. However, the original procedure proposed was based on a high concentration of hydrochloric acid (1.6 M) and reaction solution with relatively low block copolymer concentration (2.6 wt %). In 2003, Choi et al. proposed a simple procedure that implies a substantially lower HCl concentration (0.3 M) with higher block copolymer concentration in solution (5.3 wt %).35 These synthesis conditions allowed a highly reproducible preparation of SBA-15 materials with high yields. Moreover, under these conditions, it is possible to adjust the SiO2/P123 molar ratio over a wider range to tailor pore network connectivity. Because the addition of heteroelements in SBA-15 materials performed by direct synthesis may be influenced by these synthetic parameters, studies were undertaken to understand their effect on titanium substitution. Vinu et al. recently showed that a decrease in HCl concentration enhances the amount of titanium

10.1021/jp803853m CCC: $40.75  2008 American Chemical Society Published on Web 08/23/2008

14404 J. Phys. Chem. C, Vol. 112, No. 37, 2008 incorporated into the silica framework.27 On the other hand, other studies showed that increasing pH could also lead to extraframework TiO2 formation.28 Although studies were performed to clarify the influence of hydrochloric acid concentration on titanium-substituted SBA-15 prepared by direct synthesis,27,28 little is known about the effect of temperature on titanium incorporation. However, it is known that titanium species in aqueous solution are very sensitive to the temperature of hydrothermal treatment, a high value of which can induce bulk anatase TiO2 formation.36 We report a comprehensive study of titanium-substituted SBA-15 prepared by direct synthesis (co-condensation). In particular, the influences of hydrochloric acid concentration, silicon precursor concentration, and hydrothermal treatment temperature and time on the Ti incorporation ratio and its chemical environment were investigated. Our approach consists of systematically studying these different parameters over a wide range of titanium content in the initial gel mixture. 2. Experimental Section 2.1. Materials. Ti-SBA-15 materials with different molar ratios Ti/Si were synthesized by using Pluronic P123 (Aldrich, MW ) 5800) as a structure-directing agent, tetraethylorthosilicate (TEOS 98%, Aldrich) as a silicon source, and tetrapropylorthotitanate (TPOT 97%, Aldrich) as a titanium precursor. The synthesis was carried out with the following initial molar gel composition: 0.24-0.98 TEOS/0.0061-0.29 TPOT/0.18-2.88 HCl/0.0041-0.016 P123/100 H2O. In a typical synthesis, 6.0 g of Pluronic P123 was dissolved in 114 g of deionized water and 3.5 g of hydrochloric acid (37%) at 35 °C under magnetic stirring. Then, 13.0 g of TEOS and the selected amount of TPOT were premixed and rapidly added to the initial homogeneous solution. The resulting mixture was stirred for 24 h at 35 °C and subsequently hydrothermally treated for an additional 24 h at a given temperature to ensure further framework condensation. The solid products were recovered by filtration and dried in air at 100 °C for 24 h. Finally, the products were calcined at 550 °C for 3 h to remove the template. Calcined titanosilicate samples are designated as Ti-SBA-15(V)-(w)-(x)-(y)-(z), where V stands for silicon precursor concentration (in g/L), w stands for the HCl concentration (in mol/L), x stands for hydrothermal treatment temperature (in °C), y stands for hydrothermal treatment time (in h), and z stands for the titanium precursor concentration (in mol/L) used in the initial gel. In what follows, the titanium incorporation ratio is also defined as the ratio between the Ti/Si molar ratio of the recovered solid products divided by the Ti/Si molar ratio in the initial gel (%incorporation ) Ti/Sirecovered products/Ti/Siinitial gel × 100%). The titanium concentration of the final solution ([Ti]final solution) is calculated by subtracting the titanium recovered in the solid product from the titanium used in the initial gel composition ([Ti]final solution ) [Ti]starting solution - {Ti/Sirecovered products × [Si]}). The silica yield is assumed to be 100%. Different series of experiments were performed to evaluate the effect of these synthesis parameters on the titanium incorporation ratio and coordination number. In a first set of experiments, the effect of HCl concentration (w) on the titanium incorporation ratio was analyzed. For these samples, other parameters were kept as followed: Ti-SBA-15(114)-(w)-(100)(24)-(0.010). A second set of samples was synthesized to evaluate the effect of the HCl concentration and hydrothermal treatment conditions on the titanosilicate materials. In this case, nine series of titanosilicate materials with different titanium loadings (z ) 0.005-0.15 mol/L) were prepared at three

Be´rube´ et al. different HCl concentrations (w ) 0.1, 0.3, and 0.6 M) and aging temperatures (x ) 35, 60, and 100 °C). A third set of samples was synthesized by using three different P123 concentrations in the solution mixture (1.3, 2.6, and 5.3 wt %) while keeping a constant silica/P123 molar ratio (r ) 60). For each silicon precursor concentration used (V ) 28, 56, and 114 g/L), different titanium precursor concentrations (z ) 0.005 to 0.050 mol/L) were used in the initial gel. Finally, a set of samples were synthesized by using four different hydrothermal treatment times at 60 °C (y ) 6, 12, 24, and 48 h). For each hydrothermal treatment duration, different titanium precursor concentrations in the initial gel (z ) 0.005 to 0.010 mol/L) were used. To evaluate the accessible amorphous titanium contents (Ti/ Si) of the Ti-SBA15 synthesized with different hydrothermal treatment times (Ti-SBA-15(114)-(0.3)-(60)-(y)-(0.050)), a posttreatment in acidic solution was done. In this case, the different solid products recovered by filtration were added to hydrochloric acid solutions of the same concentration (0.3M) as used in the initial synthesis medium and the solutions were stirred for 24 h at 35 °C. The solid products were recovered by filtration, dried in air at 100 °C for 24 h, and calcined at 550 °C for 3 h. 2.2. Characterization. Low-angle X-ray diffraction (XRD) patterns were recorded on a Bruker D4 X-ray diffractometer with Ni-filtered Cu KR radiation (40 kV, 40 mA). Wide-angle XRD patterns were recorded on a SIEMENS D5000 diffratometer and Cu KR radiation (λ) 1.5496 Å). The reference pattern of crystalline anatase was obtained from the Powder Diffraction File 2 (PDF-2) database licensed by the International Center for Diffraction Data (ICDD). Elemental analysis was performed by atomic absorption with a M1100B Perkin-Elmer atomic absorption spectrophotometer. Nitrogen adsorption and desorption isotherms were determined at -196 °C, using a Quantachrome Autosorb-1 sorption analyzer. Prior to analysis, samples were outgassed at 250 °C for 12 h under vacuum. The pore size distributions were obtained by the nonlocal density functional theory (NLDFT) method and calculated by using the Autosorb-1 1.52 software supplied by Quantachrome Instruments. The kernel selected was N2 on silica at -196 °C assuming cylindrical pore geometry and the equilibrium model based on the desorption branch.37 UV-vis diffuse reflectance spectra were recorded with a Varian Cary 500 spectrophotometer equipped with a praying mantis. A Spectralon reflectance standard was used as reference. 3. Results To verify the influence of HCl concentration, we synthesized Ti-SBA-15 samples using three different HCl concentrations (w ) 0.1, 0.3, and 0.6 M). It is important to note that these acid concentrations are much lower than in the original SBA15 synthesis at 1.6 M.8 Figure 1 shows the XRD patterns for calcined Ti-SBA-15(114)-(0.1)-(100)-(24)-(z) synthesized using different titanium precursor concentrations. All these patterns clearly show three well-resolved diffraction peaks associated with the (100), (110), and (200) reflections consistent with the two-dimensional (2-D) hexagonal p6mm symmetry. These diffractograms show that Ti-SBA-15 exhibiting a high structural quality can be obtained even at these substantially decreased acid concentrations. Since the synthesis of SBA-15 with TEOS as silica precursor at acid concentrations lower than 0.2 M seems to be difficult, the presence of TPOT appears beneficial for the mesophase formation. Figure 2 shows the evolution of the titanium incorporation ratio for Ti-SBA-15 synthesized with different hydrochloric acid concentrations. The other synthesis parameters were fixed as

Titanium-Substituted SBA-15 Mesoporous Materials

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Figure 1. XRD patterns of calcined Ti-SBA-15(114)-(0.1)-(100)-(24)(z) synthesized by using different titanium precursor concentrations in the initial gel composition: (A) z ) 0.005, (B) z ) 0.01, (C) z ) 0.015, and (D) z ) 0.02.

Figure 2. Titanium incorporation ratios of Ti-SBA-15(114)-(w)-(100)(24)-(0.01) synthesized at different hydrochloric acid concentrations.

Ti-SBA-15(114)-(w)-(100)-(24)-(0.010). The results show that under these conditions, the incorporation ratio decreases exponentially with an increase in the HCl concentration. Moreover, at low hydrochloric acid concentration, the Ti/Si atomic ratios of the materials are close to those in the initial gel. These results are in agreement with the report of Vinu et al.27 and confirm that titanium incorporation is favored at low acid concentrations. Values of incorporation ratio higher than 100% are obtained only in conditions where some fraction of the Si in the initial gel may remain in solution. For different titanium precursor concentrations in the initial gel, a combination of the effects of hydrothermal treatment temperature and hydrochloric concentration on the titanium incorporation ratios, the Ti/Si atomic ratios of solid products, and the calculated titanium concentrations of final solutions were systematically studied. Results corresponding to hydrothermal treatment temperatures of 35, 60, and 100 °C are presented in Figures 3-5, respectively. At low initial titanium precursor concentrations, titanium incorporation ratios in the recovered solid products are essentially the same for a given HCl concentration, irrespective of the temperature (plateau 1), and correspond to the values presented in Figure 2. When the titanium precursor concentration in the initial gel is higher than a critical value, a pronounced increase in the titanium incorporation ratio is observed. Then, at high titanium precursor concentrations in the starting solution, the Ti/Si atomic ratios of the solid products are close to those of the corresponding

Figure 3. Titanium incorporation ratio, Ti/Si atomic ratio, and [Ti] in final solution for Ti-SBA-15(114)-(w)-(35)-(24)-(z) synthesized at various HCl concentrations and with different titanium precursor concentrations in the initial gel. The hydrochloric acid concentrations are 0.1 (squares), 0.3 (triangles), and 0.6 M (diamonds).

gel composition (plateau 2) indicating that most of the Ti has precipitated under these conditions. This second plateau is not observable for samples aged at 35 °C with a hydrochloric acid concentration of 0.6 M indicating that at this temperature the plateau 2 might only take place for titanium precursor concentration higher than 0.15 mol/L in the starting solution. In the second part of Figures 3-5, the previous experimental results are represented in a different way. Here, Ti/Si atomic ratios are given instead of Ti incorporation ratios. These curves make it possible to compare the titanium loadings in the solid products corresponding to each of the plateaux described before. From these results it has become clear that the Ti/Si atomic ratio in the solid products corresponding to the end of plateau 1 is independent of the HCl concentration. On the other hand, this critical titanium concentration in final products is strongly influenced by the hydrothermal treatment temperature. Indeed, critical Ti/Si atomic ratios of titanosilicate materials are reduced from 5.3% to 1% when the hydrothermal treatment temperature increases from 35 to 100 °C (Figures 3-5). To visualize the solubility of titanium in the reaction mixture, the residual concentrations of titanium species that are not retained in the mesoporous silica materials are also calculated and presented in the bottom part of Figures 3-5. As previously shown, it is clear that the solubility of titanium increases according to the hydrochloric acid concentration. Moreover, for materials corresponding to plateau 1, the concentration of solubilized titanium is directly proportional to the titanium precursor concentration and is not affected by the hydrothermal

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treatment temperature. For materials synthesized at 35 °C for 24 h, the concentration of titanium species that are not substituted to mesoporous silica and that remain in solution increases until saturation. For higher titanium precursor concentrations in the initial gel, the concentrations of solubilized titanium do not change resulting in an increase of titanium incorporation ratios (see the upper graph). For higher hydrothermal treatment temperatures (Figures 4 and 5), the concentration of titanium species that are not incorporated into the mesoporous silica decreased drastically when the titanium loading of solid products (Ti/Si) is above its critical value. For materials aged at 60 °C, this pronounced decrease in the solubilized titanium concentration was followed by a slight increase and a stabilization of this value. Similar experiments were carried out to establish the influence of the silicon precursor concentration on titanium incorporation. Titanium incorporation ratios, Ti/Si atomic ratios, and titanium concentrations of final solutions for Ti-SBA-15 synthesized with three different silicium precursor concentrations and at different titanium precursor concentrations in the initial gel are shown in Figure 6. Other parameters such as the SiO2/P123 ratio, hydrochloric acid concentration, and hydrothermal treatment conditions were kept constant. At low titanium precursor concentrations in the initial gel the results showed that the titanium incorporation ratios increased according to the silicon precursor concentration for a given titanium precursor concentration in the initial gel. Furthermore, an increase in the Ti/Si atomic ratio and a decrease in the solubility of titanium were obtained with the increase of silicon precursor concentration. For lower silicon precursor concentrations in the initial gel (e.g.,

28 and 56 mol/L), a sudden increase in the titanium incorporation ratio and decrease in the concentration of solubilized titanium occurred when a critical titanium loading was reached. Interestingly, the titanium content in the solid products (Ti/Si) corresponding to the end of plateau 1 increased according to the silica precursor concentration. The last synthesis parameter under study was the duration of hydrothermal treatment. In Figure 7, the same titanium incorporation parameters were calculated for Ti-SBA-15(114)-(0.3)(60)-(y)-(z) synthesized with three different hydrothermal treatment times and with different titanium precursor concentrations in the initial gel. Only slight differences in the titanium incorporation were perceptible between the materials synthesized with a hydrothermal treatment at 60 °C for 6 h and those synthesized at 35 °C for 24 h (Figure 3). As the hydrothermal treatment time increased, the pronounced step in the titanium incorporation ratio between both plateaux became clear and was also steeper. Moreover, the critical titanium precursor concentration corresponding to the upper value at the end of plateau 1 shifted to lower values according to the hydrothermal treatment time. Indeed, critical titanium contents (Ti/Si) in the solid products are reduced from 4.4% to 2.5% when the duration of hydrothermal treatment increases from 6 to 24 h. It was also shown above (Figures 3-5) that the pronounced increase in the titanium incorporation ratio was accompanied by a decrease of titanium solubility. Moreover, the concentration of solubilized titanium species corresponding to plateau 2 decreased depending on the hydrothermal treatment time. Wide-angle X-ray diffraction (XRD) patterns of as-recovered and calcined Ti-SBA-15 prepared under different synthesis


Figure 6. Titanium incorporation ratio, Ti/Si atomic ratio, and [Ti] in final solution for Ti-SBA-15(V)-(0.3)-(60)-(24)-(z) synthesized with various TEOS concentrations and with different titanium precursor concentrations in the initial gel. TEOS concentrations are 28 (squares), 56 (triangles), and 114 g/L (diamonds).

conditions are presented in Figure 8. The data points corresponding to these materials are encircled in Figures 3 and 4 for the sake of clarity. No crystalline phase was detected by XRD for as made materials synthesized at 35 °C (Figure 8, 2.A and 2.B) and those synthesized at 60 °C with a titanium precursor concentration lower than the critical value (Figure 8, 2.C). When the titanium precursor concentrations in the initial gel were higher than the critical value (Figure 8, B and D), XRD patterns of as made and calcined materials synthesized at 60 °C (Figure 8, 1.D and 2.D) and XRD patterns of calcined material synthesized at 35 °C (Figure 8, 1.B) showed the reflexions characteristic of the anatase TiO2 structure. It is thus clear that the increase in the titanium incorporation ratio is related to the bulk anatase formation. The absence of crystalline phase in the as made material synthesized at 35 °C with the higher titanium precursor concentration (Figure 8, 2.B) could be associated with the increase of crystal size occurring during calcination considering the limit of detection of the XRD that is around 4 nm. To characterize the titanium environment, UV-vis diffuse reflectance spectroscopy (UV-vis DRS) measurements were also performed on the different titanosilicate materials. Previous studies have shown that a band between 200 and 240 nm should be attributed to a ligand-to-metal charge-transfer transition in isolated TiO4 units.38 This feature is generally believed to be a direct proof of Ti4+ in tetrahedral coordination incorporated into the silica framework. Another band centered at 330 nm is attributed to a ligand-to-metal charge transfer of titanium in bulk anatase TiO2.38-40 Absorption signals between these bands may


Figure 7. Titanium incorporation ratio, Ti/Si atomic ratio, and [Ti] in final solution for Ti-SBA-15(114)-(0.3)-(60)-(y)-(z) synthesized with different titanium precursor concentrations in the initial gel and recovered after different aging times. The aging times are 6 (squares), 12 (triangles), and 24 h (diamonds).

Figure 8. Wide angle XRD patterns of calcined (1) and as-recovered (2) Ti-SBA-15(114)-(0.3)-(x)-(24)-(z) synthesized at different hydrothermal treatment temperature and with different titanium precursor concentrations in the initial gel composition: (A) x ) 35 °C and z ) 0.05, (B) x ) 35 °C and z ) 0.1, (C) x ) 60 °C and z ) 0.02, and (D) x ) 60 °C and z ) 0.04. The reference pattern of anatase TiO2 is given in E.

be assigned to titanium in intermediate coordination and to small TiO2 clusters through quantum size effect.41-43 Figure 9 displays UV-vis DR spectra of calcined Ti-SBA15 samples synthesized at various hydrothermal treatment

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Figure 9. UV-vis diffuse reflectance spectra of calcined Ti-SBA-15 (114)-(0.3)-(x)-(24)-(z) synthesized at various hydrothermal temperatures and with different titanium precursor concentrations in the initial gel composition.

temperatures and with different titanium-to-silica ratios in the initial gel. Other parameters such as hydrochloric acid and silica precursor concentrations were constant, respectively w ) 0.3 M and V ) 114 g/L. For each hydrothermal treatment temperature, UV-vis DR spectra of calcined Ti-SBA-15 materials with a Ti/Si ratio in the solid product below the critical titanium loading show a predominant signal between 200 and 240 nm indicating that under these conditions titanium is essentially well-dispersed in the silica framework. Moreover, these spectra also show a broad band between 250 and 340 nm that can be associated to TiO2 nanoclusters. For each hydrothermal treatment temperature used for synthesis, this absorption signal at higher wavelength increases with titanium loading. For higher hydrothermal treatment temperature, when the titanium amount used in synthesis is above the critical titanium content, the UV-vis DR spectra reveal a dramatic change in titanium coordination. Further increase in the titanium-to-silicon ratio is accompanied by the appearance of a band between 300 and 375 nm, which corresponds to bulk anatase TiO2. To clarify the effect of HCl concentration and silicon precursor concentration on the titanium dispersion into the SBA15 silica framework, Ti-SBA-15 samples were synthesized at 60 °C with different hydrochloric acid concentrations and different silicon precursor concentrations. These samples were also synthesized with an appropriate amount of titanium precursor to obtain materials with similar Ti/Si molar ratio, but slightly below the critical value (plateau 1). UV-vis DR spectra of these samples (not shown) reveal no significant differences according to their synthesis conditions. To quantify the accessible amorphous TiO2, a leaching test was performed for materials synthesized with different hydro-

Be´rube´ et al. thermal treatment times. This experiment consists of determining the titanium contents of as recovered materials and those of the same samples following a leaching post-treatment in an aqueous HCl solution at the same concentration as the initial synthesis medium. Since the crystalline TiO2 species are not soluble in slightly acidic solution, it was thus possible to determine the accessible amorphous titanium contents substituted to the mesoporous silica. Furthermore, by considering that no solubilized titanium species has precipitated after the first synthesis step at 35 °C, the further increase in the titanium incorporation ratio can be associated to the anatase TiO2 produced from the titanium present in the solution during the hydrothermal treatment at 60 °C. The different Ti/Si molar ratios calculated for Ti-SBA-15(114)-(0.3)-(60)-(0 to 96)-(0.05) samples are presented in Table 1 and Figure 10. It was shown that the titanium content leached from the solid product recovered after the first synthesis step (aging time ) 0 h) is equal to 1.8% Ti/ Si. At the beginning of the hydrothermal treatment (t e 6 h), these results revealed a decrease in the titanium species that were leached during the post-treatment. During this period, only a slight increase in the titanium incorporation ratio was observed suggesting changes in the chemical environment of the substituted titanium. However, UV-vis DR experiments (not shown) showed that within this period, no changes were detected in the coordination state of the substituted titanium species. Between 9 and 24 h, as discussed above, the solubilized titanium species were no longer stable in solution and precipitated leading to the formation of bulk anatase. The leaching test also revealed that the increase in the titanium incorporation ratio depicted in Figure 7 was accompanied by a decrease in the accessible amorphous titanium content. N2 adsorption-desorption isotherms at -196 °C of TiSBA15(114)-(0.1)-(100)-(0.005 to 0.020)-(24) are shown in Figure 11. Ti-SBA-15 materials synthesized with a titanium precursor concentration of 0.005 and 0.010 show a type-IV isotherm with steep capillary condensation/evaporation steps revealing high-quality samples. Titanosilicate materials synthesized with an initial titanium precursor concentration higher than 0.010 show a hysteresis loop with a slightly less pronounced capillary condensation/evaporation step. This may be due to surface roughness and/or structural defects. For a better visualization of hysteresis loop changes upon increase of titanium loading, the NLDFT pore size distributions (PSD) of these samples were calculated from the desorption branch by using NLDFT calculations,37 as shown in Figure 12. PSD of Ti-SBA-15 samples with lower titanium content (z ) 0.005 and 0.010) exhibit a narrow peak at 8.5 nm associated with primary cylindrical mesopores. Under these conditions, an increase of z above 0.010 leads to a decrease of the primary mesopore diameter, but also to the appearance of a small shoulder around 7.5 nm. Since this shoulder is not present in the pore size distribution calculated from the adsorption branch (results not shown), one can conclude that this porosity is due to plug formation, which delays capillary evaporation toward lower relative pressures (the pore blocking effect).44,45 Moreover, this shoulder becomes more pronounced at higher titanium loading suggesting that these plugs could be related to extra-framework titanium oxide. 4. Discussion Our results showed that the titanium incorporation and dispersion into the SBA-15 framework is highly influenced by synthesis conditions. A schematic representation of the effects of synthesis parameters on the incorporation and dispersion of titanium during direct synthesis of titanium-substituted meso-



TABLE 1: Titanium Contents of Ti-SBA-15(114)-(0.3)-(60)-(y)-(0.05) Synthesized with Different Hydrothermal Treatment Time Determined by Using a Leaching Test Ti/Si hydrothermal treatment time (h) 0 3 6 9 12 24 48 96 a

% incorporation

before leaching testa (%)

after leaching testa (%)

leached titaniumb (%)

anatase TiO2 formed during hydrothermal treatmentc (%)

54.0 55.0 56.1 59.8 81.3 91.8 94.0 94.4

5.4 5.5 5.6 6.0 8.1 9.2 9.4 9.4

3.5 3.8 4.2 4.7 7.8 9.0 9.2 9.3

1.8 1.6 1.4 1.3 0.3 0.2 0.1 0.1

0.0 0.1 0.2 0.6 2.7 3.8 4.0 4.0

Determined by elemental analysis. b Ti/Si before leaching test - Ti/Si after leaching test. c Ti/Si before leaching test - 5.4%.

Figure 10. Titanium contents of Ti-SBA-15(114)-(0.3)-(60)-(y)-(0.05) synthesized with different hydrothermal treatment time and determined with a leaching test. The different calculated titanium over silicon ratios are the leached titanium (black) and the increases in the titanium loading during hydrothermal treatment at 60 °C (gray).

Figure 12. NLDFT pore size distributions calculated from the desorption branch of calcined Ti-SBA-15(114)-(0.1)-(100)-(24)-(z) synthesized with different titanium precursor concentrations in the initial gel.

SCHEME 1: Schematic Representation on the Modification of Incorporation and Chemical Environment of Titanium during the Direct Synthesis of Titanium-Substituted Mesoporous Silica

Figure 11. N2 adsorption-desorption isotherms at -196 °C of calcined Ti-SBA-15(114)-(0.1)-(100)-(24)-(z) synthesized with different Ti/Si atomic ratios in the initial gel composition. For the sake of clarity, the data are given with an offset of 200 (z ) 0.01), 400 (z ) 0.015), and 600 cm3/g (z ) 0.02).

porous silica is presented in Scheme 1. At low titanium precursor concentration in the synthesis gel, the titanium incorporation ratio is found to be independent of the amount of titanium introduced into the reaction mixture. Under these conditions, the titanium incorporation ratio is strongly influenced by hydrochloric acid concentration (Figures 2-5) and silicon precursor concentration (Figure 6). These results suggest that equilibrium has been reached between the titanium species

present in the solution and those incorporated into the silica framework (Scheme 1A). The decrease of titanium incorporation with the increase of hydrochloric acid concentration can be explained by the higher solubility of titanium and/or the fragility of Si-O-Ti bound in strongly acidic solution. Furthermore,

14410 J. Phys. Chem. C, Vol. 112, No. 37, 2008 when the silica precursor concentration in the reaction mixture is increased, a higher substitution of titanium to the mesoporous silica is observed. This can be attributed to the faster growth of silica oligomers due to the condensation reaction rate of silanol groups that increases according to the concentration of TEOS. Consequently, the hydrolyzed titanium species are more susceptible to condense with silanol groups before the precipitation of the silica mesophase. Furthermore, when the silica precursor concentration in the reaction mixture is increased, condensation reactions between silicon and titanium precursors are favored. It was also found that hydrothermal treatment conditions do not influence this equilibrium, probably because it is reached predominantly during the first low temperature synthesis step at 35 °C. Wide-angle XRD patterns of low titanium content Ti-SBA-15 samples showed that no anatase TiO2 was formed during the synthesis of materials corresponding to plateau 1 (see Figure 8). However, although UV-vis DR spectra showed that titanium is mainly well-dispersed into the silica framework in tetrahedral coordination (see Figure 9), there is some indication that small TiO2 clusters and/or penta-coordinated titanium species could also be formed during synthesis even at low hydrothermal treatment temperature (e.g., 35 °C). UV-vis experiments also confirm that under these synthesis conditions, hydrochloric acid concentration and silica precursor concentration have no significant influence on the titanium coordination (not shown) for materials corresponding to plateau 1. Sanchez et al. showed that more acidic conditions tend to inhibit the Ti-O-Ti formation.46 However, the H+/Ti ratios used by these authors (close to 1) were much higher than the one in the present study. A particularity of transition metal alkoxides is their high reactivity upon hydrolysis due to the coordination expansion of the metal.47 Hydrolysis rates are expected to be much higher than that for silicon alkoxide where the 4-fold coordination of silicon is already satisfied. Furthermore, their tendency to fulfill their coordination sphere makes them very unstable in solution and liable to form precipitates. Condensation reactions such as alcoxolation and oxolation occur upon the hydrolysis process of transition metal alkoxides and lead to the irreversible formation of M-O-M bonds unless the solution is sufficiently acidic to solubilize the metal.47 In our case, the formation of oxo titanates upon the hydrolysis of the titanium alkoxides could explain the presence in the solid phases of titanium atoms with a coordination number higher than four. When the titanium precursor concentration used in the initial gel is above a critical value, the titanium incorporation ratio is no longer only driven by the above-described equilibrium and the measured increase in titanium incorporation ratio likely corresponds to TiO2 precipitation over the solid. XRD patterns and UV-vis spectra showed that this increased titanium incorporation ratio and decreased concentration of solubilized titanium are due to the formation of bulk anatase TiO2 with titanium atoms in octahedral coordination (see Figures 8 and 9). However, this crystallization was not detected by XRD for as made materials synthesized at 35 °C for 24 h suggesting low hydrothermal treatment temperatures do not allow the production of bulk anatase exhibiting crystal sizes detectable by XRD. As described above, hydrochloric acid concentration strongly influences the titanium incorporation ratio at low titanium contents. However, this parameter does not affect the maximum titanium content above which extra-network TiO2 formation is observed (see Figures 3-5). This result suggests that the critical titanium precursor concentration in the initial gel is associated with a critical titanium content in the solid product and is not

Be´rube´ et al. related to a critical concentration of solubilized titanium species. Moreover, it was shown that the hydrothermal treatment temperature and the silicon precursor concentration are predominant factors for this maximum substitution of titanium into SBA-15 framework (see Scheme 1B). It was thus suggested that when mesoporous materials reach a critical titanium loading for a given combination of hydrothermal temperature and silicon precursor concentration, the solubilized titanium species are no longer stable in solution and start to precipitate. This critical titanium loading diminution with an increase of the hydrothermal treatment temperature can be associated with the endothermic nature of the anatase TiO2 crystallization reaction.36 Furthermore, these results are in agreement with other studies and showed that only small loading (Ti/Si < 1%) of tetrahedrally coordinated titanium dispersed into SBA-15 framework can be achieved at relatively high hydrothermal treatment temperature (g100 °C).24 The influence of silicon precursor concentration may be explained by the production of ethanol during the hydrolysis process, which can slow down the anatase formation for a given titanium precursor concentration. Even if the solubilized titanium participates in the formation of anatase TiO2, it was shown that the incorporated titanium species are first involved in this process. At the beginning of the hydrothermal treatment at 60 °C, the fraction of amorphous titanium that can be leached decreases depending on the hydrothermal treatment time whereas the titanium incorporation ratio increases significantly (see Figure 10). Even if no anatase TiO2 was detected by UV-vis DR (not shown), one can consider the formation of irreversible and insoluble Ti-O-Ti bonds with the densification of the silica walls. All the above results suggest that when the titanium content is higher than a critical value, the proximity of titanium species on the material surface that increases with the hydrothermal treatment temperature and time is sufficient to allow the precipitation of solubilized titanium species into bulk anatase TiO2. N2 physisorption at -196 °C and pore size distribution of calcined TiSBA-15 confirmed that these extra-network anatase TiO2 species are formed onto the material surface, blocking mesopores and delaying capillary evaporation to lower relative pressures (Figures 11 and 12). When the incorporation ratio starts to increase, the fraction of the accessible amorphous titanium species also decreases rapidly indicating that these species are resolubilized in the acidic solution following the equilibrium discussed above and/or that the crystal formation on the material surface prevents the leaching of these titanium species. The fluctuation of solubilized titanium concentration at the beginning of plateau 2 supports the first hypothesis since lower concentrations of solubilized titanium can be attributed to the previous equilibrium that is not reached at the end of the hydrothermal treatment (see Scheme 1C). The above results also showed that this bulk anatase formation is a slow process that can spread out over several days depending on the synthesis conditions (see Figure 7). The comparison between the concentrations of the solubilized titanium species corresponding to plateau 2 indicates that the rate of the anatase TiO2 formation increases according to the hydrothermal treatment temperature and decreases depending on the hydrochloric acid concentration and silicon precursor concentration (see Figures 3–6) for the same reasons discussed above. 5. Conclusion The influence of synthesis parameters on incorporation and dispersion of titanium was elucidated in the case of titaniumsubstituted SBA-15 materials prepared by direct synthesis. At

Titanium-Substituted SBA-15 Mesoporous Materials low titanium ratio in synthesis gel (below critical titanium loading), pH and silica precursor concentration both influence the titanium incorporation ratio, which is believed to be driven by an equilibrium between titanium in the silica framework and titanium species dissolved into the reaction medium. It was shown that during this step, a low titanium precursor concentration used in the initial gel leads to a material with incorporated titanium species that are mainly tetrahedrally coordinated into the silica framework. However, even at low temperature (35 °C), the increase of titanium precursor concentration can induce the formation of small clusters and/or titanium species with intermediate coordination. When the titanium content is above a critical value, which depends on the synthesis conditions, the incorporation ratio increases due to TiO2 anatase formation onto the material surface. It was also found that the maximal titanium loading that can be well-dispersed into the silica framework is influenced by hydrothermal treatment temperature and time and also by silica precursor concentration but is not affected by hydrochloric acid concentration. Furthermore, it was shown that when the formation of anatase TiO2 takes place, the amorphous titanium species dispersed into the silica materials are resolubilized in the acidic solution. This dissolution of these titanium species leads to a decrease of accessible TiO2 tetrahedral coordination dispersed into the mesoporous silica framework. To avoid the anatase TiO2 formation, it is necessary to synthesize materials at low hydrothermal treatment temperature and time. In this case, Ti remains amorphous and mainly as well-dispersed titanium into the silica framework. However, under these conditions, SBA-15 pore diameters are limited to relatively small values (

A Comprehensive Study of Titanium-Substituted SBA-15 Mesoporous

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