Optimizing Silica Synthesis for the Preparation of Mesoporous Ti-SBA

Jan 26, 2010 - Furthermore, the results showed that higher conversions of cyclohexene were obtained with catalysts prepared using SBA-15 calcined at 5...
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Ind. Eng. Chem. Res. 2010, 49, 6977–6985

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Optimizing Silica Synthesis for the Preparation of Mesoporous Ti-SBA-15 Epoxidation Catalysts Franc¸ois Be´rube´,† Abdelkarim Khadhraoui,‡ Michael T. Janicke,§ Freddy Kleitz,*,‡ and Serge Kaliaguine*,† Departments of Chemical Engineering and Chemistry, UniVersite´ LaVal, Quebec, QC, Canada G1 V 0A6, and Chemistry DiVision, Los Alamos National Laboratory, Mail Stop J514, Los Alamos, New Mexico 87545

The influence of the synthesis conditions of SBA-15 silica support in the development of Ti-SBA-15 epoxidation catalysts has been studied in detail. In this study, efficient and stable Ti-SBA-15 epoxidation catalysts were prepared using a recently developed postgrafting method based on the insertion of a chelated titanium alkoxide precursor inside the SBA-15 silica mesophase. First, the nature of the SBA-15 supports was analyzed as a function of different synthesis parameters, such as hydrothermal aging temperature and calcination temperature, by solid-state NMR spectroscopy, thermal analysis, and nitrogen physisorption at -196 °C. Subsequently, titanium-substituted SBA-15 materials were characterized by DR UV-vis spectroscopy, elemental analysis, and nitrogen physisorption. As a catalytic test reaction, the activity, selectivity, and catalyst regenerability were studied in the epoxidation of cyclohexene. Our findings show that the density of silanol groups on the support greatly influences the retention and coordination number of the grafted titanium species. This characteristic of the mesoporous silica supports also has an influence on the catalytic activity of the resulting titanosilicate materials. The conversion of cyclohexene obtained with Ti-SBA-15 catalysts synthesized using noncalcined mesoporous silica was found to increase with the hydrothermal aging temperature of the support. Furthermore, the results showed that higher conversions of cyclohexene were obtained with catalysts prepared using SBA-15 calcined at 550 °C prior to the Ti grafting step, although lower dispersion of the titanium species was usually observed for these materials. A higher accessibility of the substrates to the active sites on the surface of these materials can explain these results. However, a dramatic deactivation of the catalysts was observed upon recycling of these active materials. In contrast, the grafted catalysts prepared using uncalcined mesoporous silica supports showed a substantially lower catalytic deactivation upon different reaction cycles as compared to those grafted after calcination. 1. Introduction The production of epoxides is of growing interest in the chemical and petrochemical industries because these compounds can serve as organic intermediates in pharmaceutical synthesis or as monomers in the production of various functional polymers. Porous titanosilicate materials have been found to be highly effective catalysts in various oxidation reactions using hydrogen peroxide under mild conditions.1-3 In this field of research, the recent development of ordered mesoporous silica materials exhibiting a variety of pore structures and pore sizes has opened new perspectives.4-7 In particular, mesostructured titanosilicate materials have attracted great interest for their use as catalysts owing to better activities as compared to microporous zeolites such as TS-1 for oxidation reactions involving bulky molecules.8 Several attempts have been made to incorporate titanium into the mesoporous silica framework of MCM-41,8-10 mesocellar foams (MCFs),11 MCM-48,12,13 MSU,14,15 and SBA-1516-22 by simultaneous co-condensation of silicon and titanium precursors. However, the diversity of Ti precursors that can be used for this co-condensation pathway is greatly limited because the syntheses are restricted to conditions suitable for mesostructure formation. On the other hand, the postsynthesis grafting pathway * To whom correspondence should be addressed. Tel. +1 418 656 2708 (S.K.), +1 418 656 7812 (F.K.). Fax: +1 418 656 3810 (S.K.), +1 418 656 7916 (F.K.). E-mail: [email protected] (S.K.), [email protected] (F.K.). † Department of Chemical Engineering, Universite´ Laval. ‡ Department of Chemistry, Universite´ Laval. § Los Alamos National Laboratory.

represents a viable alternative and allows for the use of a larger variety of titanium precursors such as titanium chloride,23,24 titanocene organometallic precursors,25-27 and titanosilicate molecular precursors.28 Nevertheless, most of these precursors are sensitive to moisture, and their high cost is a drawback. We recently proposed a new method based on the use of a titanium alkoxide specifically modified with acetylacetone (acac) acting as a chelating agent for the postgrafting of SBA-15 supports. The use of such a chelated Ti precursor enables the preparation of mesoporous titanosilicates with improved catalytic activity as compared to those synthesized by co-condensation.29 Furthermore, postgrafting of these titanium precursors can be performed in aqueous media over a wide range of temperatures and pH values. The interaction between the acac-chelated titanium precursor and the silanol groups present on the pore surface of the material leads to highly dispersed titanium species on the surface of the silica without the formation of external Ti species outside on the particle surface. Although the catalytic activity is not increased for Ti-SBA-15 materials with Ti/Si molar ratios above 6%, it was found that the high-titaniumcontent materials exhibited remarkable stability in terms of catalytic activity even after several reaction cycles. Because the acac-chelated titanium alkoxide reacts with silanol groups leading to the formation of Ti-O-Si bonds, the density of silanols on the material surface is thus believed to influence the retention and the dispersion of the titanium species in the mesoporous material. However, the correlation between the silanol density and pore structure of SBA-15 and the resulting properties of the Ti-SBA-15 catalysts remains to be substantiated as a function of the SBA-15 synthesis conditions.

10.1021/ie901659k  2010 American Chemical Society Published on Web 01/26/2010

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Therefore, in the present study, mesoporous SBA-15 materials were synthesized with varying hydrothermal conditions and calcination temperatures in order to explore silica supports with different physicochemical properties. The Ti retention and the chemical environment of the titanium species grafted on the surfaces of these different SBA-15 silica materials were then determined. The effects of the properties of a given SBA-15 silica support on the catalytic activity and regenerability of the obtained titanosilicate materials could thus be elucidated. 2. Experimental Section Materials. SBA-15 materials were synthesized using Pluronic P123 (BASF, Mw ) 5800 g/mol) as a structure-directing agent and tetraethylorthosilicate (TEOS, 98%, Aldrich) as a silicon source following the procedure reported by Choi et al..30 The synthesis was carried out with the following initial molar gel composition: 0.99 TEOS/0.54 HCl/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 was 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 (35, 100, or 140 °C) to ensure further framework condensation. The solution containing the solid products was then cooled to room temperature prior to the postgrafting procedure without further treatment. In the case of the postgrafting on calcined SBA-15, the pure silica SBA-15 samples were recovered by filtration and dried in air at 100 °C for 24 h. Then, the materials were calcined either at 250 °C for 2 h or at 550 °C for 3 h to remove the P123 copolymer template. These calcined materials were then dispersed in hydrochloric acid solutions of the same concentration (0.3 M) as used in the initial synthesis medium. Titanium postgrafting with the acac-modified titanium alkoxide was performed with tetrapropylorthotitanate (TPOT, 97%, Aldrich) as the titanium alkoxide and acetylacetone (acac, 98%, Aldrich) as a chelating ligand.29 The temperature of the postgrafting and the chosen acac/Ti molar ratio were 5 °C and 3.15, respectively. After grafting of the titanium precursor, the titanosilicate materials were calcined under air at 550 °C for 3 h. The recovered titanosilicate samples are designated as TiSBA-15-(HT)-(CT)-(z), where HT and CT stand for the temperatures (°C) of the hydrothermal treatment and calcination, respectively, of the SBA-15 used as silica support and z stands for the Ti/Si atomic ratio in the recovered products. When assynthesized materials are used as silica supports, these samples are then designated as (AS) instead of (CT). Characterization. Solid-state 29Si MAS NMR experiments were performed on a Bruker Avance 400 MHz spectrometer. The rotor was 4 mm and was spun at 10000 Hz. The data were collected with a π/2 pulse of 6.0 µs and high-power 1H decoupling. A delay time of 60 s between each acquisition for T1 relaxation was allowed, and the number of acquisitions was at least 3000 scans. The deconvolution of the 29Si MAS NMR spectra was performed with the TopSpin software package from Bruker-Biospin using pure Gaussian functions. Thermogravimetric analysis (TGA) was carried out under flowing air with a Netzsch STA 449C Jupiter thermo-microbalance using a heating rate of 10 °C min-1. Nitrogen adsorption-desorption isotherms were measured at -196 °C using an Autosorb-1C sorption analyzer. Prior to analysis, samples were outgassed for 12 h at 120 and 250 °C for materials calcined at 250 and 550 °C, respectively. The pore size distributions and the cumulative surface area curves were determined using the nonlocal density

Figure 1. Solid-state 29Si MAS NMR spectra of SBA-15-(HT)-(CT) synthesized with different hydrothermal aging temperatures (HT) and calcination temperatures (CT).

functional theory (NLDFT) method31 supplied by the Autosorb-1 1.55 software from Quantachrome Instruments. The kernel selected was N2 sorption on silica at -196 °C assuming a cylindrical pore geometry and implementing the model of (metastable) NLDFT adsorption isotherms (adsorption branch).31 Elemental analysis was performed by atomic absorption using an M1100B Perkin-Elmer atomic absorption spectrophotometer. Diffuse-reflectance UV-visible (DR UV-vis) spectra were recorded using a Varian Cary 500 spectrophotometer equipped with a praying mantis. A Spectralon reflectance standard was used as the reference. The epoxidation of cyclohexene was chosen as a test reaction, with experimental conditions following a previously published procedure.32 Specifically, 20 mmol of cyclohexene, 4 mmol of TBHP (70% in H2O), 10 mL of acetonitrile, and 0.1 g of catalyst were mixed in a 50 mL roundbottom flask and were heated to 70 °C under stirring for 3 h. The products were analyzed by gas chromatography (Varian CP-3800 gas chromatograph equipped with a Varian CP-SIL capillary column and coupled with a Varian Saturn 2200 mass spectrometer). For the recycling of the catalysts, the samples were calcined at 500 °C for 2 h between each reaction cycle. This step is essential for the recovery of the catalysts because water and/or organic compounds present in the reaction mixture could adsorb on the active titanium sites, increasing their coordination number and thus deactivating the catalysts. 3. Results and Discussion Synthesis and Physicochemical Properties of the SBA-15 Silica Supports. High-quality mesoporous SBA-15 silica materials were synthesized under mild acidic aqueous conditions ([HCl] ) 0.3 M) following the procedure reported by Choi et al.32 Solid-state 29Si MAS NMR spectra of the SBA-15 supports that were hydrothermally treated and calcined at different temperatures are presented in Figure 1. Silicon species with four neighboring silicon atoms (Q4) appear as a signal centered at -110 ppm. The substitution of neighboring silicon atoms by silanol groups contributes in increasing the 29Si NMR chemical shift. The chemical shifts of silicon species with one (Q3) and two (Q2) silanol groups are -102 and -91 ppm, respectively. Here, as-synthesized materials aged at different temperatures showed appreciable differences in their Q3/Q4 ratio, indicating that the condensation degree of the SiO2 framework is greatly influenced by the hydrothermal aging temperature. The uncal-

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Figure 2. TGA and DTA curves of as-synthesized and calcined SBA-15(HT)-(CT) materials synthesized at different hydrothermal aging temperatures (HT).

cined material aged at 35 °C showed more abundant Q3 silicon species as compared to Q4, whereas for the material aged at 140 °C, the amount of Q4 silicon species was larger than the amount of Q3 species. The material aged at 100 °C exhibited an intermediate Q3/Q4 ratio that did not vary significantly after the subsequent thermal treatment at 250 °C. All of the materials calcined at 550 °C under air showed rather similar 29Si MAS NMR spectra with a significant decrease in the Q3 silicon species. These results suggest that substantial framework condensation took place between 250 and 550 °C and that only a low amount of residual silanol groups remained in the materials treated at high temperature.33 One can therefore conclude that the extent of framework condensation upon calcination was larger for materials that were synthesized at low hydrothermal treatment temperatures (e.g., 35 °C). TGA and differential thermal analysis (DTA) curves obtained for as-synthesized and calcined SBA-15 materials are shown in Figure 2. The TGA curves of all of the as-synthesized SBA15 materials indicate a major weight loss between 180 and 300 °C that is attributed to the combustion of the triblock copolymer. As evidenced by the figure, the hydrothermal aging temperature strongly influences the fraction of residual template in the materials. An increase of the aging temperature leads to a decrease in organic content in the P123/SBA-15 composite. At higher hydrothermal aging temperatures, dehydration of the hydrophilic chains of the block copolymer is believed to lead to segregation of the polyalkene oxide groups away from the silica framework, thus making them more easily removed during the filtration step. Moreover, several studies of SBA-15 materials have confirmed that the hydrophilic poly(ethylene oxide) (PEO) chains of the copolymer template interact with the silica framework during mesostructure formation.34-38 It was suggested that these interactions are mainly hydrogen-bondingmediated, possibly also involving Cl- counterions and Si-OH2+ groups depending on the pH conditions and extent of synthesis.39 A lower density of Si-OH2+ or Si-OH groups in favor of Si-O-Si siloxane bonds in the framework of materials synthesized at high hydrothermal treatment temperatures (Figure 1) could also result in decreased interactions between silica and the block copolymer. The TGA curves of the SBA-15 materials

Figure 3. NLDFT pore size distributions of SBA-15-(HT)-(CT) materials synthesized at different hydrothermal aging temperatures (HT) and calcination temperatures (CT) (kernel of NLDFT metastable adsorption isotherms).

aged at 100 °C, before and after calcination under air at 250 °C, showed weight losses above 100 °C of 20% and 6%, respectively. This indicates that a substantial amount of the organic copolymer template was removed by thermal treatment at 250 °C. According to previous studies on the calcination process of SBA-15, the weight loss estimated above 250 °C is attributed to the oxidation of PEO chain moieties located in the intrawall micropores-small mesopores and loss of water due to framework condensation.40,41 The NLDFT pore size distributions of SBA-15 materials hydrothermally treated at the different temperatures and then calcined at 250 and 550 °C are presented in Figure 3. The mesopore distributions of the SBA-15 materials calcined at 250 °C reveal that most of the block copolymer could be extracted from the cylindrical mesopores at this temperature, as also shown by the TGA measurements (Figure 2). As expected, the mesopore diameter increased with higher hydrothermal treatment temperature. Several studies have established that higher aging temperatures lead to an increase in the size of the cylindrical mesopores of SBA-15-type materials.36,42,43 In addition, it was observed that the calcination temperature also affected the size of the mesopores of SBA-15. The materials calcined at lower temperatures exhibited larger mesopore diameters. Moreover, contraction of the cylindrical channels upon calcination between 250 and 550 °C was reduced as a function of the hydrothermal treatment temperature. Interestingly, the pore size distribution revealed the presence of a significant micropore volume (Dp e 2 nm) for materials aged at temperatures lower than 140 °C (see Figure 3B). Several previous studies have demonstrated that SBA-15 materials contain intrawall pores that can interconnect the ordered primary mesopores.36,44-46 As discussed above, this porosity in the framework walls is generated because the hydrophilic part of the triblock copolymer (EO) is occluded within the silica walls. Furthermore, Galarneau et al. showed that the size of these intrawall pores in the SBA-15 materials is

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Figure 4. Cumulative surface area curves for SBA-15-(HT)-(CT) materials synthesized with different hydrothermal aging temperatures (HT) and calcination temperatures (CT).

a function of the hydrothermal treatment temperature.36 Large interconnecting mesopores were observed when the SBA-15 materials were aged at 140 °C. On the other hand, the framework pores of materials aged at 35 °C were mainly constituted by small intrawall mesopores (2 < Dp < 3 nm) and micropores (Dp e 2 nm). The pore size distributions (PSDs) of the framework pores also showed that a significant decrease of the intrawall pore volume occurred upon calcination between 250 and 550 °C. Furthermore, as for the primary mesopores, the decrease of this volume associated with intrawall pores during calcination was more significant for SBA-15 aged at lower temperatures. Studies on the influence of the calcination temperature on the physicochemical properties of SBA-15 have shown that substantial lattice shrinkage occurs upon high-temperature treatment.40,41 It was shown there by XRD and N2 sorption at -196 °C that the framework contraction takes place mainly at temperatures higher than 250 °C. One can thus conclude that this framework contraction arises from the condensation of the silanol groups, as confirmed by 29Si MAS NMR spectroscopy (Figure 1). A lower framework contraction is expected for the materials hydrothermally treated at higher temperatures because the degree of condensation of the silica framework increases with this synthesis parameter. The NLDFT cumulative surface area curves of SBA-15 materials aged at different temperatures and calcined at 250 and 550 °C are shown in Figure 4. These graphs allow the specific surface area associated with the framework pores to be distinguished from the specific surface area related to the mesopores. Note that the materials under study presented very low external surface areas, and, therefore, the specific surface area corresponding to pores with diameters lower than 12 nm is a good approximation of the total specific surface area of these silica materials. For materials aged at 35 °C, the specific surface area associated with the intrawall micropores (Dp e 2 nm) corresponded to about 40% of the entire surface area, whereas the specific surface area of the material aged at 140 °C mostly corresponded to pores wider than 6 nm. Moreover, as viewed from the PSD plot (Figure 3), the materials underwent substantial lattice shrinkage between 250 and 550 °C, as expected, reducing the specific surface areas. This observed decrease of the specific surface areas upon calcination corresponded to 45%, 26%, and 1% for SBA-15 samples aged at 35, 100, and 140 °C, respectively, confirming smaller lattice contractions for materials aged at the higher temperatures. Postgrafted Ti Species in SBA-15 Materials and Catalytic Properties. The effects of the hydrothermal treatment and calcination temperatures of the SBA-15 support on the resulting

Figure 5. Effects of temperatures of hydrothermal aging (HT) and calcination (CT) on the Ti/Si atomic ratio (z) of Ti-SBA-15-(HT)-(CT) materials synthesized by postgrafting using different Ti/Si molar ratios in the initial mixture.

Ti/Si molar ratio in the final titanosilicates (z) after the grafting of Ti(acac)2(OiPr)2 are presented in Figure 5. The highest Ti/Si atomic ratio that could be obtained in the case of the assynthesized material aged at 35 °C was 4 times higher than the Ti/Si ratio obtained for the as-synthesized material aged at 140 °C. For the efficient postgrafting of Ti(acac)2(OiPr)2, we showed that reaction between the titanium precursor and the surface silanol groups is needed, yielding Ti-O-Si bonds.29 A high density of silanol groups on the surface of the SBA-15 material is therefore expected to enhance the retention of the titanium precursor in the recovered materials. These differences in silanol group density as a function of hydrothermal treatment were indeed observed by 29Si MAS NMR spectroscopy (see Figure 1). Furthermore, the higher titanium retention observed for materials aged at lower temperatures can also be explained by an increased micropore volume, as the silanol density was possibly higher in micropores. For the titanosilicate materials synthesized using SBA-15 precalcined at 550 °C, the highest Ti/Si atomic ratio that could be grafted on the surface of these materials was found to be proportional to the specific surface area (see Figure 4). The density of the silanol groups on the surface of the materials is expected to be appreciably the same for all of the materials calcined at 550 °C, as observed by 29Si NMR spectroscopy. For low Ti/Si atomic ratios in the initial synthesis mixture, all of the curves showed steep increases in the Ti/Si atomic ratio in the recovered materials (z). This retention at low values of initial Ti/Si ratio used for grafting was nevertheless lower

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Figure 7. Conversion of cyclohexene as a function of Ti/Si atomic ratio (z) obtained for calcined Ti-SBA-15-(HT)-(CT) materials synthesized by postgrafting using SBA-15 samples prepared with different hydrothermal aging (HT) and calcination (CT) temperatures.

Figure 6. DR UV-vis spectra obtained for calcined Ti-SBA-15-(HT)-(CT)(z) materials synthesized by postgrafting on SBA-15 prepared with different hydrothermal temperatures (HT) and calcination temperatures (CT) and with different Ti/Si atomic ratios (z).

for as-synthesized SBA-15 materials aged at 100 °C as compared to the same materials after calcination at 250 or 550 °C. In the latter case, the higher retention occurring at low Ti/ Si ratios used in the initial mixture could be attributed to the presence of free micropores in the calcined materials in which the density of silanol groups could be higher. For as-synthesized materials, residual PEO chains of the block copolymer occluded in the micropores of the silica material might partially block the access to these framework micropores. For higher titanium precursor concentrations in the postgrafting solution, the resulting Ti/Si atomic ratio of the recovered materials seemed to reach a plateau for all Ti-SBA-15-(HT)-(550 °C) samples. We attribute this observation to the lower amount of accessible silanol groups on the surface of the materials, as most silanols were already saturated with titanium precursor. Interestingly, the extent of the retention of the Ti species in the as-synthesized materials and in those that were calcined at 250 °C continued to rise with higher initial Ti/Si. An overall higher density of Si-OH groups on the surface of these materials could explain this result (see Figure 1). The DR UV-vis spectra of the calcined Ti-SBA-15 materials synthesized using the different SBA-15 samples described above are presented in Figure 6. It is important to note that the Ti/Si atomic ratio used initially for the grafting (initial mixture) was carefully selected to recover materials with similar Ti/Si atomic ratios (z ≈ 0.03). No extraframework anatase TiO2 was observed in these spectra, as no absorption signal was detected between 350 and 400 nm.47-49 The presence of acac in the coordination

sphere of the titanium species is known to inhibit the formation of anatase TiO2 clusters on the surface of the silica materials.29 Moreover, the dispersion of the titanium species on the surface of as-synthesized silica materials was greatly enhanced by a lower hydrothermal treatment temperature. From Figure 6, the Ti-SBA-15 material synthesized using uncalcined SBA-15 silica material aged at 35 °C indeed exhibits a narrow absorption signal centered at 220 nm. This feature is believed to originate from ligand-to-metal charge transfer associated with titanium in tetrahedral coordination.47 Increasing the hydrothermal treatment temperature led to a broadening of the absorption signal toward higher wavelengths. According to previous studies, bands between 300 and 350 nm can be ascribed to titanium species in intermediate coordination states.47,50,51 Although major differences were seen between the DR UV-vis spectra of the different Ti-SBA-15 samples prepared using as-synthesized SBA-15 materials as supports, no substantial changes were found in those of the titanosilicate materials prepared with mesoporous silica precalcined at 550 °C. Also, similar DR UV-vis spectra were obtained for Ti-SBA-15-(100 °C)-(AS)-(0.030) and Ti-SBA15-(100 °C)-(250 °C)-(0.031) samples. These results strongly suggest that the chemical environment of the titanium species in the solids is directly related to the Q3/Q4 ratio in the silica materials (Figure 1). Increased density of accessible silanol groups on the surface of the materials both aged and calcined at lower temperatures could lead to more Ti-O-Si bonds, thus yielding higher dispersion of titanium sites in tetrahedral coordination, that is, narrower UV-vis absorption signals. The epoxidation of cyclohexene by tert-butyl hydroperoxide (TBHP) was performed in order to measure the catalytic activity of these Ti-SBA-15 materials. The conversions of cyclohexene for the Ti-SBA-15 materials synthesized using different silica supports obtained as a function of aging and calcination temperatures are presented in Figure 7. When as-synthesized

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Figure 8. NLDFT pore size distributions of SBA-15-(35 °C)-(250 °C) and Ti-SBA-15-(35 °C)-(AS)-(0.13) materials (kernel of NLDFT metastable adsorption isotherms).

SBA-15 materials were used as silica supports, the conversion of cyclohexene increased with titanium content and reached a maximum value, after which it tended to remain constant with further increase in the titanium content. Reaching of this threshold value is related to the gradual increase of the coordination number of titanium as observed from the DR UV-vis spectra.29 Interestingly, the maximum conversion of cyclohexene obtained for the catalysts synthesized using uncalcined supports was higher with increased hydrothermal aging temperature of the silica support. This result is in contradiction with the DR UV-vis data (Figure 6), which showed a higher dispersion of titanium species when the postgrafting was performed on as-synthesized materials hydrothermally aged at lower temperatures. Indeed, isolated tetrahedral titanium species are thought to be the active species for the catalytic epoxidation of cyclohexene. The conversion of cyclohexene as a function of the Ti/Si atomic ratio in the recovered Ti-SBA-15 materials (z) prepared using mesoporous silica calcined at 550 °C showed quite a different behavior. For low Ti/Si atomic ratios in the recovered materials, a steeper increase in the conversion of cyclohexene was observed as a function of increasing titanium content. The conversion of cyclohexene also reached a maximum value for a rather low Ti/Si atomic ratio, the value of which was substantially higher than for the catalysts synthesized using uncalcined SBA-15. However, these maximum conversion values were found to be almost independent of the hydrothermal aging temperature. Moreover, because these materials showed pronounced differences in terms of pore diameter (from 5 to 10 nm), these results suggest that pore size within this range is

not a variable affecting the conversion of cyclohexene under the present conditions, namely, a reaction time of 3 h. For TiSBA-15-(HT)-(550 °C) samples, a further increase in the titanium content led to lower cyclohexene conversions. Fewer accessible surface silanols in materials calcined at 550 °C as compared to those in as-synthesized samples could lead to the formation of titanium species exhibiting progressively higher coordination number as the Ti/Si ratio increased. Finally, the presence of organic template in the silica supports used for the postgrafting did not substantially influence the resulting catalytic activity of the titanium species in the calcined titanosilicate materials. Indeed, only a slightly lower conversion was obtained for the Ti-SBA-15-(100 °C)-(250 °C)-(z) samples compared to the Ti-SBA-15-(100 °C)-(AS)-(z) catalysts. All of the above results suggest that the catalytic activity of the different grafted Ti-SBA-15 materials was mainly influenced by the accessibility of the reactants to the active sites. To confirm the location of the grafted titanium species in the materials, a comparison between the PSDs of SBA-15-(35 °C)-(250 °C) and Ti-SBA-15-(35 °C)-(AS)-(0.13) after calcination at 550 °C is presented in Figure 8. No substantial differences can be observed in the diameter of the primary mesopores and the framework mesoporosity between these two materials. However, one can see a significant decrease of the micropore volume for the titanosilicate material compared to the silica material calcined at 250 °C. This result suggests that the titanium species are mainly placed in these small framework micropores, where the density of silanol groups might be higher.52 In this case, the condensation of the framework silanols during calcination is susceptible to isolate the active sites in these small pores, making them fairly inaccessible for the reactant (see Scheme 1A). Although the titanium species present in these materials exhibit the proper coordination number, these tetrahedral titanium sites become less accessible for the substrate species, thus leading to a decreased conversion. For materials aged and/or calcined at higher temperatures (e.g., 140 °C for aging and 550 °C for calcination), only a slight structural shrinkage occurred upon the last calcination step performed to remove acac from the titanium coordination sphere. Even if these materials exhibit significant differences in their porosity (see Figures 3 and 4), a higher amount of grafted titanium species present on their surface might remain accessible compared to noncalcined materials prepared at 35 °C (see Scheme 1B and Scheme 2). Differences in the selectivity toward cyclohexene oxide are presented in Figure 9. Previous investigations showed that byproducts such as 2-cyclohexen-1-one and 2-cyclohexen-1-ol could also be formed through the allylic oxidation of cyclo-

Scheme 1. Schematic Representation of the Synthesis of Ti-SBA-15 Materials Using As-Synthesized SBA-15 Silica Supports

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Scheme 2. Schematic Representation of the Synthesis of Ti-SBA-15 Materials Using SBA-15 Aged at 35 °C and Subsequently Calcined at 550 °C before Insertion of Chelated Ti Precursors

hexene due to the decomposition of the peroxide and resulting formation of radicals.32,53 Interestingly, the values for the epoxide selectivity as a function of Ti/Si ratio followed a tendency similar to that of the cyclohexene conversion. Casuscelli et al. made similar observations and postulated that the yields of these byproducts could be less affected by the number of active sites of the titanosilicate catalysts than the yield of the epoxide.54

Figure 9. Selectivity for cyclohexene oxide as a function of Ti/Si atomic ratio (z) obtained for calcined Ti-SBA-15-(HT)-(CT) materials synthesized by postgrafting on SBA-15 treated with different hydrothermal aging temperatures (HT) and calcination temperatures (CT).

Figure 10. Recycling of calcined Ti-SBA-15-(HT)-(CT) materials synthesized by postgrafting using SBA-15 samples treated with different hydrothermal temperatures (HT) and calcination temperatures (CT) and with different Ti/Si atomic ratios (z).

Finally, catalyst recycling was also studied, as presented in Figure 10. In this system, catalyst deactivation through succesive reaction cycles is known to depend on the titanium content of the materials.29 Therefore, the influence of the physicochemical properties of the SBA-15 supports on the regenerability of the postgrafted Ti-SBA-15 catalysts was evaluated in the case of Ti-SBA-15 samples having similar Ti/Si atomic ratios (z ≈ 0.03). Dramatic decreases in the conversion of cyclohexene and therefore in the selectivity toward the epoxide were observed between the first and third reaction cycles for mesoporous titanosilicate materials synthesized using SBA-15 precalcined at 550 °C. Interestingly, the reduced conversion of cyclohexene upon three reaction cycles showed a different behavior when an uncalcined SBA-15 sample was used as a support for the grafting of the titanium centers. Although the initial conversions appeared quite different between the three samples prepared with varying hydrothermal aging temperatures for reasons discussed above, lower losses in conversion were observed for these catalysts as compared to titanosilicate materials synthesized using SBA-15 support precalcined at 550 °C. For the Ti-SBA15-(140 °C)-(AS)-(0.033) sample, the conversion of cyclohexene in the third reaction corresponded to 70% of the original value, which was substantially higher than that of the other catalysts. An increased resistance to leaching for materials with more substituted titanium species remaining accessible could explain the reduced deactivation of the catalysts. Even though the TiSBA-15-(140 °C)-(AS)-(0.033) was synthesized using a silica support with a substantially lower silanol density compared to that used for Ti-SBA-15-(35 °C)-(AS)-(0.031), the catalytic deactivation was less significant for the former sample. In the latter, the repeated calcination step between each reaction cycle could contribute to isolating more active titanium species in the micropores for the same reasons as discussed above.

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4. Conclusions In conclusion, the effects of the physicochemical properties of mesoporous SBA-15 supports used for the postgrafting of a chemically modified titanium alkoxide have been substantiated. It was shown that the hydrothermal treatment and calcination temperatures of the silica supports greatly influence the catalytic properties of the resulting mesoporous titanosilicate materials. It was found that the retention and dispersion of the titanium species in the silica materials decreased with higher degrees of condensation of the silica framework. The higher density of silanols present on the surface of SBA-15 silica prepared at lower temperatures is a possible explanation for the higher substitution of the titanium species. Although the titanium species of Ti-SBA-15 synthesized using SBA-15 precalcined at 550 °C seemed to be in higher coordination (e.g., > 4), substantially higher conversion of cyclohexene was obtained for these catalysts as compared to materials obtained without a precalcination step. An increased accessibility of the titanium active sites for the reactants is suggested to explain this result. However, these highly active materials showed a rather poor regenerability upon several reaction cycles compared to titanosilicates synthesized using uncalcined SBA-15. As a conclusion, as-synthesized SBA-15 materials aged at elevated temperatures (e.g., 140 °C) appeared to be the best silica supports for synthesizing Ti-SBA-15 epodixation catalysts, affording a viable compromise between high catalytic activity and stability. Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and le Fonds Que´be´cois de la Recherche sur la Nature et les Technologies (FQRNT) is gratefully acknowledged. We thank Professor M. Leclerc from the Department of Chemistry of Universite´ Laval for access to UV-vis spectrometer. F.K. thanks the Canadian Government for the Canada Research Chair on Functional Nanostructured Materials. Literature Cited (1) Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. ReV. 1997, 97, 2373. (2) Corma, A. Preparation and catalytic properties of new mesoporous materials. Top. Catal. 1997, 4, 249. (3) Gallot, J. E.; Kaliaguine, S. Oxidation of hydrocarbons by hydrogen peroxide over Ti catalysts: Kinetics and mechanistic studies. Can. J. Chem. Eng. 1998, 76, 833. (4) Huo, Q. S.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Organization of organic molecules with inorganic molecular species into nanocomposite biphase arrays. Chem. Mater. 1994, 6, 1176. (5) Wan, Y.; Shi, Y. F.; Zhao, D. Designed synthesis of mesoporous solids Via nonionic-surfactant-templating approach. Chem. Commun. 2007, 897. (6) Wan, Y.; Zhao, D. On the controllable soft-templating approach to mesoporous silicates. Chem. ReV. 2007, 107, 2821. (7) Soler-Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chemical strategies to design textured materials: From microporous and mesoporous oxides to nanonetworks and hierarchical structures. Chem. ReV. 2002, 102, 4093. (8) Blasco, T.; Corma, A.; Navarro, M. T.; Pe´rez-Pariente, J. Synthesis, characterization, and catalytic activity of Ti-MCM-41 structures. J. Catal. 1995, 156, 65. (9) Corma, A.; Camblor, M. A.; Esteve, P.; Martinez, A.; Perez-Pariente, J. Activity of Ti-Beta catalyst for the selective oxidation of alkenes and alkanes. J. Catal. 1994, 145, 151. (10) Zhang, W.; Fro¨ba, M.; Wang, J.; Tanev, P. T.; Wong, J.; Pinnavaia, T. J. Mesoporous titanosilicate molecular sieves prepared at ambient temperature by electrostatic (S+I-, S+X-I+) and neutral (S°I°) assembly

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ReceiVed for reView October 25, 2009 ReVised manuscript receiVed December 23, 2009 Accepted January 5, 2010 IE901659K