SiO2@Ti-Containing Mesoporous Silica Core

Structural Design of Pd/SiO2@Ti-Containing Mesoporous Silica Core–Shell Catalyst for Efficient One-Pot Oxidation ... Publication Date (Web): June 8,...
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Structural Design of Pd/SiO2@Ti-Containing Mesoporous Silica Core− Shell Catalyst for Efficient One-Pot Oxidation Using in Situ Produced H2O2 Shusuke Okada, Shohei Ikurumi, Takashi Kamegawa, Kohsuke Mori, and Hiromi Yamashita* Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: This study reports the importance of the structural design of heterogeneous catalysts for one-pot reaction. A series of core−shell catalysts were synthesized that consisted of SiO2 core, Pd nanoparticles (NPs), and Ticontaining mesoporous silica (Ti-MS) shell with precise control of the Pd NPs positions, the mesopore diameter, and the thickness of the Ti-MS shell. The catalytic potentials were evaluated for one-pot oxidation reactions involving the direct synthesis of hydrogen peroxide (H2O2) from H2 and O2 gases by the Pd NPs and the subsequent oxidation reaction of sulfide by the isolated TiIV species using in situ produced H2O2 in the mesoporous silica. The precise architecture of the core−shell catalyst significantly enhanced the catalytic performance by a factor of 20, and the catalyst exhibited excellent activity and selectivity toward sulfoxide as compared to the commonly employed Pd-supported/Ti-zeolite (Pd/TS-1) catalyst. This result provides a new perspective of heterogeneous catalysts aimed at developing environmentally benign chemical processes.

1. INTRODUCTION To realize sustainable chemistry, one-pot reactions that involve two or more synthetic steps in the same reaction vessel are

suffers from many disadvantages, such as requiring toxic solvents, large energy, and multiple steps. As an alternative, the direct synthesis of H2O2 from H2 and O2 gases using precious metal catalysts (mainly Pd, Au, or Au−Pd) is a promising method.2,3 However, the key problem is to stabilize the resulting H2O2, because H2O2 simultaneously undergoes hydrogenation and/or decomposition to water in the presence of the same catalysts employed for its formation. Much effort has been directed at preventing the decomposition of H2O2; thus the efficiency has been significantly improved,4−6 but the additional operation for removal of the acid or halide becomes an even more troublesome issue. To avoid this problem, a promising strategy could be the utilization of in situ synthesized H2O2 from H2 and O2 to oxidize organic reactants in a subsequent reaction in the same vessel (one-pot oxidation reaction), which would contribute to savings of both energy and time by avoiding isolation/purification steps, in addition to circumventing risks involved in the transportation of concentrated H2O2. There have been some reports regarding the one-pot oxidation of sulfides,7,8 phenol,9−11 and the one-pot epoxidation of propylene12−16 using in situ produced H2O2. In these reports, the most popular catalysts for the one-pot reaction are Pd nanoparticles (NPs) supported on titanium silicate-1 (Pd/ TS-1) or Pd NPs supported on Ti-containing mesoporous silica

Figure 1. Schematic illustration of the one-pot oxidation reaction using (a) Pd/TS-1 and (b) the Pd/SiO2@Ti-MS core−shell catalyst.

attracting much attention. A one-pot reaction can reduce the number of steps required to obtain the objective product and to isolate/purify the intermediate, establishing a simple synthetic process that saves much energy, reduces the use of organic solvent, and represses the production of byproducts.1 Various one-pot reactions have been reported previously, among which is a one-pot oxidation reaction that combines the direct synthesis of H2O2 from H2 and O2 gases on a Pd catalyst, and an oxidation reaction of an isolated Ti catalyst using in situ produced H2O2 was the focus of this study. H2O2 is known as both a highly selective and environmentally friendly oxidant used in the manufacture of numerous organic and inorganic compounds. However, the current commercial production of H2O2 by the anthraquinone method © 2012 American Chemical Society

Received: March 15, 2012 Revised: June 8, 2012 Published: June 8, 2012 14360

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10−2 M HCl aqueous solution (100 mL) and was stirred for 10 min. A solution of SnCl2·2H2O (500 mg) added to 2 × 10−2 M HCl aqueous solution (100 mL) was poured and stirred for 10 min. The suspension was centrifuged and washed with water several times. The Sn2+ adsorbed SiO2 core was redispersed in water (246 mL) and mixed with 4.08 mL of acidic 11.64 mM PdCl2 solution. After 10 min, 0.15 M sodium formate was added and stirred overnight at room temperature. The suspension was centrifuged and washed several times with water, resulting in Pd/SiO2.32−34 In this step, it was confirmed that no Pd precursor remained in the filtrate. To encapsulate Pd/SiO2 with the Ti-MS shell, Pd/SiO2 was dispersed in a mixture of water (200 mL), ethanol (150 mL), CTAB (0.75 g), and ammonia solution (2.84 mL). A mixture of TEOS (750 μL), TPOT (20.3 μL), and acetylacetone (14.3 μL) was then added at room temperature and stirred overnight.35 The precipitate was isolated by centrifugation, washed with ethanol, and then dried at 373 K in air overnight. The dried composite was calcined at 823 K in air for 6 h to remove the CTAB structure-directing agent. The synthesized sample was treated with H2 (20 mL min−1) at 473 K for 1 h before testing the catalytic performance. To control the pore diameter, the same amounts of C12TMABr, C14TMABr, or C18TMACl were employed instead of C16TMABr. Controlling the thickness of the shell was conducted using the precursors at quantities of 3, 5, and 7 times the original amounts. Synthesis of SiO2@Pd(S)/Ti-MS. The Ti-MS shell was formed on the SiO2 core, followed by loading of the Pd NPs on the Ti-MS surface using the same method as that described for Pd/SiO2@Ti-MS. The precipitate was calcined to remove CTAB and heated in H2 (20 mL min−1) at 473 K for 1 h before testing the catalytic performance. Synthesis of SiO2@Pd(R)/Ti-MS. The Ti-MS shell was formed on SiO2 core and then calcined in air to remove CTAB. Pd NPs were loaded onto the Ti-MS using the same method described. The precipitate was again calcined at 823 K in air for 6 h and treated with H2 (20 mL min−1) at 473 K for 1 h before testing the catalytic performance. Direct Synthesis of H2O2 from H2 and O2. To perform the catalytic reaction using H2/O2 gas mixture with high safety, appropriate precautions should be taken. In particular, the H2/ O2 gas mixture should not be added to dry catalyst directly. Hence, the catalytic reaction was carried out as follows in a protective draft chamber. The catalyst (Pd content adjusted to 3.3 μmol) was placed in a reaction vessel (50 mL), and 1 × 10−2 M HCl aqueous solution (20 mL) was added. The resulting mixture was stirred under bubbling of gaseous H2 and O2 (20 mL min−1, H2:O2 = 1:1) at 303 K for 3 h with magnetic stirring. The amount of H2O2 generated was monitored using a hydrogen peroxide counter (Hiranuma HP-300). One-Pot Oxidation of Methyl Phenyl Sulfide Using in Situ Generated H2O2. The catalyst (Pd content adjusted to 3.3 μmol), methyl phenyl sulfide (0.3 mmol), and acetonitrile (10 mL) were placed in a reaction vessel (50 mL) fitted with a reflux condenser. The mixture was reacted at 303 K in a flow of H2 and O2 (20 mL min−1, H2:O2 = 1:1) with magnetic stirring. The amounts of product and reactant were analyzed by gas chromatography−mass spectrometry (GC−MS) using a spectrometer (Shimadzu CMS-2010 plus) equipped with TC5HT columns. An internal standard technique (biphenyl) was employed.

(Pd/Ti-MS). Unfortunately, the intermediate H2O2 is easily decomposed by the Pd catalyst, thereby competing against its synthesis, because the random location of both active sites accelerate the dispersion and decomposition of H2O2 prior to contact with the Ti site, which results in low utilization efficiency of diluted H2O2 (Figure 1a). To prevent this decomposition, we propose that the efficient dispersion of H2O2 generated on Pd NP sites located adjacent to Ti sites is a key issue. A variety of nanostructured heterogeneous catalysts with unique properties, such as tubular,17 rattle,18 and core−shell catalysts,19 have recently been developed due to the advances in bottom-up technology. Among them, core−shell catalysts are considered to be promising structures for one-pot oxidation reactions. It is expected that by the use of a core−shell catalyst supporting a Pd catalyst in the core region and a Ti catalyst in the shell region (Pd/SiO2@Ti-MS), the in situ produced H2O2 within the inner core region could interact with Ti sites directly without dispersion to the solvent and decomposition, which would ultimately enhance the oxidation activity (Figure 1b). There are many candidates of shell materials,20 such as zeolites,21 mesoporous materials,22−24 polyelectrolytes,25 and porous coordination polymers.26,27 We selected mesoporous silica as a shell material because of its high mechanical and chemical stability, sufficient uniform pores size to accommodate aromatic compounds, and easy incorporation of various transition metal species into the silica network as an isolated species.28 Furthermore, the pore diameter and shell thickness can be controlled by changing the organic surfactant and amount of precursor, respectively, during the preparation of mesoporous silica, which would assist the fabrication of tailormade structures.29,30 Considering this, a series of core−shell catalysts consisting of a SiO2 core, Pd NPs, and a Ti-MS shell were prepared, and their effect on the one-pot oxidation of sulfides was investigated. The position of the deposited Pd NPs was investigated first to clarify the benefit of the core−shell structure. The diameter of the mesoporous channels and the thickness of the Ti-MS shell were further controlled to improve the activity and examine the enhancement mechanism. The core−shell structure was also found to have an effect on not only the catalytic activity, but also on the product selectivity.

2. EXPERIMENTAL SECTION Materials. Tetraethyl orthsilicate (TEOS), tetrapropyl orthotitanate (TPOT), SnCl2·2H2O, PdCl2, ammonia solution (28 wt %), sodium formate, ethanol, acetonitrile, methyl phenyl sulfide, H2O2 aqueous solution (30 wt %), and biphenyl were purchased from Nakalai Tesque. Cetyltrimethylammonium bromide (CTAB) was obtained from Wako Pure Chemical Ind., Ltd. Acetylacetone was purchased from Tokyo Chemical Ind. Co., Ltd. All chemicals were used as-received without further purification. Synthesis of SiO2 Core. TEOS was added to a mixture of ethanol and aqueous ammonia. The TEOS:EtOH:NH3:H2O molar ratio of the solution was 1:70.1:2.1:19.5. The suspension was stirred for 6 h at room temperature. The resultant precipitate was centrifuged and washed with ethanol three times, and dried under vacuum overnight at room temperature. Synthesis of Pd/SiO2@Ti-MS Core−Shell Catalyst.31 To deposit Pd NPs on the SiO2 core, the prepared SiO2 core (500 mg) was dispersed in water (250 mL) under ultrasonication. A solution of SnCl2·2H2O (500 mg) was then added into 2 × 14361

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Figure 2. HR-TEM images and schematic illustrations (insets) of (a,b) Pd/SiO2@Ti-MS, (c,d) SiO2/@Pd(R)/Ti-MS, and (e,f) SiO2/@Pd(S)/TiMS.

Oxidation of Methyl Phenyl Sulfide Using H2O2 Aqueous Solution. The catalyst (Pd content adjusted to 2.0 μmol), methyl phenyl sulfide (0.3 mmol), acetonitrile (5 mL), and H2O2 solution (30 wt %, 0.9 mmol) were placed in a reaction vessel (50 mL) fitted with a reflux condenser. The mixture was reacted at 303 K for 1 h under ambient pressure with magnetic stirring. The amounts of product and reactant were analyzed by GC−MS using an internal standard technique (biphenyl). The efficiency of H2O2 was calculated from the amount of product after the reaction. H 2O2 efficiency =

perpendicular to the core surface, and Pd NPs. The Pd NPs were present at the intended sites: the boundary between the SiO2 core and the mesoporous silica shell, randomly distributed within the mesopores of the silica shell, and on the surface of the mesoporous shell in Pd/SiO2@Ti-MS, SiO2@Pd(R)/TiMS, and SiO2@Pd(S)/Ti-MS, respectively. The average size of the Pd NPs was 3.1−3.2 nm, and no significant difference in the Pd NP size was observed in the three samples (Figure S2). The textural properties, characterized by N2 adsorption− desorption measurements, TEM, and inductively coupled plasma spectroscopy (ICP), are summarized in Figure 3A. The surface area was significantly increased by formation of the Ti-MS shell. The surface area and pore diameter of the SiO2@ Pd(R)/Ti-MS and SiO2@Pd(S)/Ti-MS catalysts were slightly lower than those of Pd/SiO2@Ti-MS, which is probably due to covering with Pd NPs. The shell thickness and the Pd and Ti contents were almost constant for each sample. Ti K-edge Xray absorption fine structure (XAFS) measurements confirmed that the Ti species in Ti-MS were isolated and of tetrahedral coordination geometry, which is suitable for selective oxidation with H2O2 as an oxidizing agent (Figure S3).36 The catalytic activities of the samples were first evaluated for the direct synthesis of H2O2 from atmospheric H2 and O2 gases on the Pd NP sites. The H2O2 concentration in the reaction medium after 1 h was comparable for all three catalysts (Figure 3B); however, that for SiO2@Pd(R)/Ti-MS was slightly lower than those for Pd/SiO2@Ti-MS and SiO2@Pd(S)/Ti-MS due to disturbance of reactant diffusion by the random distribution of Pd NPs within the mesopore channels. The catalytic activities for one-pot oxidation of methyl phenyl sulfide (1) into methyl phenyl sulfoxide (2) were evaluated next. Sulfide oxidation with H2O2 is an important reaction because the sulfoxide produced is used as an intermediate for the preparation of various pharmaceuticals and agrochemicals.37 Sulfone is also obtained by the overoxidation of sulfide; therefore, it is expected that the product selectivity of the catalysts will be influenced by the catalyst structure, which will provide some insight into the most effective core−shell structure.

synthesizedsulfoxide + sulfone(mmol) consumed H 2O2 (mmol)

3. RESULTS AND DISCUSSION 3.1. Effect of the Pd NPs Location in the Core−Shell Catalyst. Pd/SiO2@Ti-MS was synthesized as follows. The PdCl2 precursor was first deposited on the surface of spherical SiO2 NPs (280 nm average diameter (Figure S1)), followed by reduction in the presence of sodium formate. After formation of the Pd NPs, further coating with the Ti-MS shell was performed using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent (SDA), tetraethyl orthosilicate (TEOS) as a silica source, and tetrapropyl orthotitanate (TPOT) as a Ti source. To clarify the effect of the core− shell structure on the catalytic properties, it is not appropriate to compare the core−shell catalyst with a conventional catalyst (e.g., Pd/TS-1), because the environments around the Ti sites are quite different. Therefore, two reference samples, SiO2@ Pd(R)/Ti-MS and SiO2@Pd(S)/Ti-MS, were synthesized as conventional model catalysts. In the case of SiO2@Pd(R)/TiMS, where (R) denotes random, the Pd NPs were deposited after calcination, which allowed random distribution of Pd NPs within the mesoporous shell structure. SiO2@Pd(S)/Ti-MS, where (S) denotes surface, was obtained by first coating the SiO2 core with the Ti-MS shell, followed by loading of the Pd NPs. High resolution-transmission electron microscopy (HRTEM) images of Pd/SiO2@Ti-MS and the two model catalysts are presented in Figure 2. All samples had spherical NPs with nonporous SiO2 cores, Ti-MS shells with channels oriented 14362

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Figure 4. (A) Kinetics of methyl phenyl sulfide oxidation using aqueous H2O2 solution and catalytic efficiencies of H2O2 utilization for the oxidation reaction.

Table 1. Catalytic Activity and Selectivity toward Sulfoxide for the One-Pot Oxidation of Methyl Phenyl Sulfidea

a

Activity = [moles of sulfide converted]/[hour· moles of Pd]. Selectivity = [moles of sulfoxide]/[moles of sulfoxide + sulfone] × 100 (at conversion of sulfide = 50%).

of H2O2 by suitable Pd NP positions, in which the H2O2 synthesized on the inner core Pd NP sites could selectively interact with the Ti-oxide moieties within the mesopores before dispersion to the solvent. In contrast, the H2O2 synthesized on the Pd catalyst in SiO2@Pd(S)/Ti-MS was dispersed to solvent and decomposed by Pd NPs before contact with Ti sites, which resulted in low efficiency similar to the conventional catalysts. Furthermore, the Pd NP position also had an effect on the selectivity. The selectivity of Pd/SiO2@Ti-MS toward the production of 2 was higher than the other two catalysts. In the case of Pd/SiO2@Ti-MS, H2O2 is synthesized in the boundary of the SiO2 core and the Ti-MS shell, in which most of the oxidation reaction is thought to occur on Ti sites within the mesopore channels. In contrast, H2O2 is synthesized on the surface of the Ti-MS shell in the case of SiO2@Pd(S)/Ti-MS; thus, oxidation mainly occurs on the surface shell, and the effect of the mesopores is lessened. The difference in the location of reaction sites had an effect on not only activity, but also selectivity. The same trends were also observed for the one-pot oxidation of diphenyl sulfide (Figure 3D, Figure S4). To confirm the enhancement of catalytic activity, the H2O2 efficiencies of the Pd/SiO2@Ti-MS, SiO2@Pd(R)/Ti-MS, and SiO2@Pd(S)/Ti-MS catalysts for the oxidation reaction were evaluated using commercially available H2O2 aqueous solution (3 equiv to 1) (Figure 4). A clear correlation between the efficiency and oxidation activity was evident, and Pd/SiO2@TiMS exhibited the highest efficiency. In the case of SiO2@ Pd(S)/Ti-MS, the Pd NPs are located on the surface of Ti-

Figure 3. (A) Ti-MS shell parameters and metal contents of samples. (B) Amount of H2O2 synthesized using Pd/SiO2@Ti-MS, SiO2/@ Pd(R)/Ti-MS, and SiO2/@Pd(S)/Ti-MS. (C) (a) Kinetics of methyl phenyl sulfide (1) oxidation using in situ generated H2O2, and (b) dependence of the conversion level on the selectivity. Selectivity = [2/ (2+3)] × 100. (D) Catalytic activity and selectivity for sulfoxide in the one-pot oxidation of methyl phenyl sulfide and diphenyl sulfide.

Time courses for sulfide conversion and selectivity toward sulfoxide based on the conversion of sulfide are shown in Figure 3C for the three catalysts. Both H2 and O2 were necessary reactants to achieve a one-pot oxidation reaction, as sources for the production of H2O2. In addition, no reaction was observed for either SiO2@Ti-MS without Pd NPs or Pd/ SiO2@MS without Ti. The Pd/SiO2@Ti-MS catalyst exhibited the highest activity for the one-pot oxidation of 1. The productivities of H2O2 using the three catalysts were almost the same, and there was no difference in the composition of the TiMS shell of each catalyst. Thus, enhancement of the catalytic activity is ascribed to improvement of the utilization efficiency 14363

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Figure 6. (a,c) Kinetics of methyl phenyl sulfide (1) and diphenyl sulfide (4) oxidation using in situ generated H2O2, and (b,d) dependence of the conversion level on the selectivity using Pd/ SiO2@Ti-MS type core−shell catalysts with controlled pore diameters.

catalyst is an effective structure for one-pot oxidation by enhancement of the H2O2 efficiency. 3.2. Effect of Pore Diameter in the Ti-MS Shell. Uniform pore size is one of the unique characteristics of mesoporous silica materials, and it can be easily controlled by changing the SDA employed.39 It is expected that the mass transport of reactants and products is influenced by the pore size.40 Therefore, mass transport will become easier, and the reaction rate will be higher, as the pore diameter is increased. Moreover, the shorter residence time of sulfoxide in Ti-MS would contribute to the prevention of overoxidation and sulfone production. Thus, the pore diameter was controlled to clarify the role of the Ti-MS shell. The simplest method to control the pore diameter is to change the alkyl length of the surfactant as the SDA. The pore diameter of the Pd/SiO2@Ti-MS type core−shell catalyst was controlled using C12TABr, C14TABr, C16TABr, and C18TACl as the SDAs. The characterization results of these catalysts are summarized in Figure 5. N2 adsorption−desorption analyses confirmed that the pore diameters were systematically expanded according to the alkyl length of the SDA. Uniform core−shell particles with almost the same shell thickness were formed in each sample, as indicated by TEM analysis. The Brunauer−Emmett−Teller (BET) surface area and pore volume were also increased with the pore diameter.

Figure 5. (A) (a) N2 adsorption−desorption isotherms and (b) BJH pore size distributions. (B) TEM images of Pd/SiO2@Ti-MS type core−shell catalysts using (a) C12TABr, (b) C14TABr, (c) C16TABr, and (d) C18TACl as template SDAs. (C) Parameters of the Ti-MS shells.

mesoporous silica shell. The H2O2 must be decomposed by Pd NPs before interaction with the Ti site. As the result, the efficiency and the activity became low. On the other hand, the Pd NPs in Pd/SiO2@Ti-MS existed on the boundary of the SiO2 core and Ti-MS shell. The H2O2 can contact with the Ti site before decomposition by Pd, and Pd/SiO2@ Ti-MS showed high efficiency and activity. A similar phenomenon was observed in the one-pot reaction experiments. Therefore, the highest activity of the Pd/SiO2@Ti-MS catalyst for one-pot oxidation can be attributed to the enhancement of the H2O2 efficiency as compared to that for SiO2@Pd(R)/Ti-MS and SiO2@Pd(S)/Ti-MS. The activities of the conventional Pd/TS-138 catalyst (Figure S5) and a physical mixture of Pd/SiO2 and Ti-MS were evaluated for the same one-pot oxidation reaction to elucidate the effectiveness of the Pd/SiO2@Ti-MS type core−shell structure. These two catalysts were less effective (Table 1), which suggests that the Pd/SiO2@Ti-MS type core−shell 14364

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Figure 9. (a) Kinetics of methyl phenyl sulfide one-pot oxidation and (b) dependence of the conversion level on the selectivity using SiO2@ Pd(S)/Ti-MS (155 nm), SiO2@Pd(S)/Ti-MS (35 nm), and a physical mixture of SiO2@Pd(S)/Ti-MS (35 nm) + Ti-MS.

Figure 7. TEM images of Pd/SiO2@Ti-MS type core−shell catalysts with various shell thicknesses, prepared by changing the amount of precursor: (a) 1×, (b) 3×, (c) 5×, and (d) 7×. (e) Ti-MS shell parameters and metal contents of the catalyst samples.

Figure 10. (a) Kinetics of methyl phenyl sulfide oxidation with H2O2 aqueous solution and (b) dependence of the conversion level on the selectivity with Ti-MS using different H2O2 addition methods. Reaction conditions: Ti-MS (30 mg), methyl phenyl sulfide (0.3 mmol), acetonitrile (5 mL), H2O2 (30% aqueous solution, 0.9 mmol), 30 °C.

other hand, the selectivity toward sulfoxide (at 50% sulfide conversion) was 83% for C18TACl and 61% for C12TABr in the oxidation of 4, while it was 88% for C18TACl and 84% for C12TABr as the SDA in the oxidation of 1. These results confirm the importance of mass transport to achieve high activity and selectivity for a one-pot oxidation reaction. 3.3. Effect of the Ti-MS Shell Thickness. Although it was demonstrated that the core−shell structure influenced the catalytic performance for one-pot oxidation, the role of the TiMS shell remained to be clarified. Therefore, a detailed investigation was performed by varying the shell thickness to elucidate the role of Ti-MS shell for one-pot oxidation. It is anticipated that an extremely thin Ti-MS shell would not allow efficient utilization of the in situ synthesized H2O2 within the mesoporous channels, and that the efficiency would be enhanced by designing an appropriate Ti-MS shell thickness. The thickness of Ti-MS shell can be easily controlled by varying the amount of precursors (TEOS, TPOT) in the presence of C18TACl as the SDA. TEM images of various Pd/SiO2@Ti-MS type core−shell catalysts prepared using 3, 5, and 7 times the amount of precursors than that used for the original experiment are shown in Figure 7. The BET surface area and Barrett− Joyner−Halenda (BJH) pore size determined by N 2 adsorption−desorption analysis, Pd and Ti contents determined by ICP analysis, and the average shell thickness measured by TEM analysis are summarized in Figure 7e. These characteristics can be systematically controlled by varying the amount of precursors, while maintaining mesoporous structures. In the samples of amount of precursor 5× (Figure 7c) and 7× (Figure 7d), there is some different contrast

Figure 8. (a) Kinetics of methyl phenyl sulfide one-pot oxidation and (b) dependence of the conversion level on the selectivity using Pd/ SiO2@Ti-MS or SiO2@Pd(S)/Ti-MS type core−shell catalysts with various shell thicknesses (different amounts of precursors; 1, 3, 5, and 7 times the original amount).

The results of the one-pot oxidation of 1 using these catalysts are shown in Figure 6a,b. The activity and selectivity were improved with an increase in the pore diameter due to enhancement of the diffusion rates of reactants and products within the mesoporous channels. Diffusibility is very important for the catalysis using nanoscale porous materials. The effect of pore diameter in the core−shell catalysts is emphasized by the one-pot oxidation of diphenyl sulfide (4), which is a bulkier reactant than 1 (Figure 6c,d); the catalytic activity was improved in the same way as that for 1. On the 14365

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Table 2. Summary of Catalyst Sample Parameters, Activity, and Selectivity for One-Pot Oxidationa

Activity = [moles of sulfide converted]/[hour·moles of Pd]. Selectivity = [moles of sulfoxide]/[moles of sulfoxide + sulfone] × 100 (at conversion of sulfide = 50%).

a

slightly lower activity as compared to the physical mixture. These results suggest that a thickness of 35 nm is not sufficient to utilize all of the generated H2O2 within the mesopore shell, and the excess amount of leaked H2O2 reacts with the Ti-MS particles after the diffusion into the solvent. Furthermore, to elucidate the advantage of in situ produced H2O2 utilization within the mesopores before dispersion and dilution by the solvent, the oxidation performance of T-MS toward 1 was tested under two different H2O2 concentrations (Figure 10). One reaction employed the addition of 3 equiv of H2O2 to 1 at one time (0.9 mmol × 1). The other reaction was performed by addition of 1 equiv of H2O2 every hour for 2 h (0.3 mmol × 3). Both the activity and the selectivity were higher under higher concentrations of H2O2, which implies that the Ti-MS shell acts not only as a catalytically active site, but also as a nanoreactor to afford concentrated H2O2 to the Ti sites by suppressing dispersion into the solvent. Therefore, a suitable shell thickness of Pd/SiO2@Ti-MS exhibited the best catalytic activity and selectivity.

near the SiO2 core region. We investigated the section of the milled 7× sample by TEM, and it seems that the difference of the contrast is originated from the change of the pore regularity (Figure S6). Ti−K edge XAFS measurements also revealed the formation of isolated and tetrahedrally coordinated Ti species in all samples (Figure S7). Controlling the thickness of the shell was also studied for the SiO2@Pd(S)/Ti-MS type core−shell catalyst. It is thought that the oxidation reaction occurs on the surface of the shell in the SiO2@Pd(S)/Ti-MS catalyst; therefore, the reaction rate would not be changed by varying the shell thickness. TEM images and pore parameters are shown in Figure S8. The catalytic activities for the one-pot oxidation of 1 were evaluated using an equivalent amount of Pd. Thus, the productivities of H2O2 were considered almost the same. The activity and selectivity of the Pd/SiO2@Ti-MS type core−shell catalyst increased with increasing shell thickness and were maximized at a thickness of 155 nm (Figure 8). It is considered that shell thickness of approximately 35 nm is not sufficient to utilize the in situ generated H2O2, and the efficiency of H2O2 within the mesoporous channels was even further enhanced by increasing the shell thickness due to the suppression of reactant dispersion to the solvent. However, further increase in thickness to 185 nm decreased the catalytic activity and selectivity, which is ascribed to the retardation of diffusion with a thicker mesoporous silica shell. A similar tendency was observed in the one-pot oxidation of the bulkier diphenyl sulfide (Figure S9). The catalytic activity of the SiO2@Pd(S)/ Ti-MS type core−shell catalysts was substantially lower than that of Pd/SiO2@Ti-MS (shell: 155 nm), and no significant difference in catalytic activity and selectivity was observed for different shell thicknesses. To confirm the importance of the in situ produced H2O2 utilization within the mesopores before dispersion and dilution by the solvent, the one-pot oxidation reaction was conducted using a physical mixture of Pd/SiO2@Ti-MS (35 nm thick shell) and Ti-MS particles with a Ti content corresponding to that of Pd/SiO2@Ti-MS (155 nm thick shell) (Figure 9). The Pd/SiO2@Ti-MS (155 nm thick shell) exhibited much higher activity, while the Pd/SiO2@Ti-MS (35 nm thick shell) showed

4. CONCLUSION A new type of core−shell structure catalyst was developed that consisted of Pd NP-supported on a SiO2 core covered with a Ti-MS shell. The performance of the catalysts fabricated in this study for one-pot oxidation is summarized in Table 2. The best core−shell catalyst had approximately 20 times higher activity and better selectivity than a conventional catalyst due to control of the Pd NP position, pore diameter, and thickness of the TiMS shell by enhancement of the H2O2 utilization efficiency. Therefore, the core−shell catalyst is considered to be a promising structure for one-pot reactions. It is expected that the present strategy allows flexibility for the selection of metal NPs and catalytically active sites and could be further applied to other one-pot reactions that involve extremely unstable compounds as reaction intermediates.

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ASSOCIATED CONTENT

S Supporting Information *

Size distributions of SiO2 and Pd NPs, Ti K-edge XAFS measurements of the samples, characterizations of Pd/TS-1, 14366

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The Journal of Physical Chemistry C

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kinetics of one-pot reactions of diphenyl sulfide, and TEM images of SiO2@Pd(S)/Ti-MS type core−shell catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +81-6-6879-7457. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate assistance from Dr. Eiji Taguchi at the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, for TEM measurements. This study was supported by priority assistance for the formation of worldwide renowned centers of research, the global COE program (Project: Center of Excellence for Advanced Structural and Functional Materials Design), and JSPS Research Fellowships of Japan Society for the Promotion of Science for Young Scientists. The X-ray absorption experiments were performed at the Photon Factory of KEK-PF(2010G010).

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dx.doi.org/10.1021/jp3025073 | J. Phys. Chem. C 2012, 116, 14360−14367