Functionalized Mesostructured Silicas As Supports for Palladium

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J. Phys. Chem. C 2010, 114, 57–64

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Functionalized Mesostructured Silicas As Supports for Palladium Catalysts: Effect of Pore Structure and Collapse on Catalytic Activity in the Suzuki-Miyaura Reaction Stephanie MacQuarrie,† Bendaoud Nohair,‡ J. Hugh Horton,† Serge Kaliaguine,‡ and Cathleen M. Crudden*,† Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada K7L 3N6 and Department of Chemical Engineering, LaVal UniVersity, Quebec (Quebec), Canada GIV 0A6 ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: NoVember 10, 2009

Two different Pd complexes are supported on the mercaptopropyltrimethoxy silyl-functionalized mesoporous material KIT-6 and their activity in the Suzuki-Miyaura cross coupling reaction is examined. Remarkably, these materials show a much greater recyclability than the corresponding SBA-15-based materials, which lose activity quickly in the presence of hot aqueous base. The high recyclability of the KIT-6 materials is proposed to result from a greater redistribution of Pd to the external surface of the material, resulting in Pd nanoparticles that are not constrained by the size of the pores. These Pd nanoparticles are then available for catalysis after the material has collapsed, whereas in the case of SBA-15-based materials, the Pd is captured inside the pores such that catalytic activity ceases when the material collapses. Eventually, Ostwald ripening leads to larger particles and a decrease in catalytic activity. 1. Introduction The Suzuki-Miyaura cross-coupling reaction of organoboron compounds and organic halides or pseudohalides is one of the most efficient methods for the construction of carbon-carbon bonds.1-5 Unlike many other coupling reactions, the organometallic coupling partners (boronic acids and their derivatives) in the Suzuki-Miyaura reaction are functional group compatible, have low toxicity, and are stable to air and water.6 Furthermore, the boron-containing byproduct of the SuzukiMiyaura cross-coupling can readily be separated from the desired compound. These factors, combined with the variety of partners that can be employed, even including secondary sp3hybridized boronic esters7 and secondary alkyl halides,8 makes this a very powerful reaction. Although other metals such as iron9,10 and nickel11,12 have been employed in coupling reactions, palladium remains the metal of choice for this transformation, especially in the Suzuki-Miyaura coupling. Although high activity can be obtained with low catalyst loadings, the high cost of Pd makes its recovery desirable, and the application of this reaction in the synthesis of pharmaceuticals makes removal at the end of reaction a necessity. Entire reviews are devoted to issues with removal of Pd from organic products at the end of the reaction, since FDA regulations limit the amount of all heavy metals that can be present in any product for human consumption.13 Considering these facts, the application of heterogeneous catalysts would seem logical to permit facile recovery and removal of the Pd after reaction. Although this is effective, the majority of heterogeneous catalysts require harsher conditions than well-tuned homogeneous catalysts, which often results in leaching of Pd from the support.14 However many supports appear to function by a “release and catch” mechanism, in which the actual catalysis takes place in solution, but the Pd is * To whom correspondence should be addressed. Tel. (+1 613) 533 6755; Fax: (+1 613) 533 6669; E-mail: [email protected]. † Department of Chemistry, Queen’s University. ‡ Department of Chemical Engineering, Laval University.

recaptured on support at the end of the reaction with varying degrees of efficiency.15,16 It is then the extent of recapture that determines the “effective heterogeneity” of the catalyst.17 Within the family of metal oxides, modified silica is by far the most utilized support for immobilizing palladium complexes in the Suzuki reaction. Following up in the work of Ying et al.18 with unfunctionalized mesoporous silica, the groups of Shimizu,19 Davis,20 and Crudden21,22 have shown that mesoporous silica with high surface areas and long-range ordered structures such as FSM-16 and SBA-15 can be modified with commercially available thiol ligands resulting in highly leachresistant recoverable catalysts. Mechanistic studies showed that the catalysis likely occurs in solution via leached Pd.21 One of the hallmarks of this process is the presence of Pd nanoparticles, which result from the redistribution of Pd as it dissolves and is recaptured on the surface. Remarkably, in the case of SBA-15, the Pd nanoparticles are primarily observed within the pores of the catalyst, such that their size is limited by the size of the pores.21 While examining the use of other types of supports, including cubic materials such as SBA-16 and KIT-623-25 it was found that KIT-6 showed a significantly improved catalyst lifetime, as evidenced by its ability to be recycled up to eight times.26 This is in comparison with regular SBA-15-based catalysts, which have decreased lifetimes, shown by Crudden et al. to be the result primarily of degradation of the silica walls by the action of the aqueous base required in the Suzuki-Miyaura reaction.21 This latter study showed conclusively that the catalysis either takes place within the pores, or that the bulk of the Pd that is catalytically active is held in a reservoir within the pores, since when these pores collapse, complete loss of catalytic activity was observed. Pore collapse could be remedied by several means, the most effective of which was doping with aluminum,27 however the key point for this work, is that without surface protection or doping with aluminum, catalytic activity is lost after 1-3 runs, and is clearly attributed to a loss of structural integrity.

10.1021/jp908260j  2010 American Chemical Society Published on Web 12/16/2009

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In this work, we examine the catalytic activity of various heterogeneous Pd catalysts prepared using KIT-623-25 with different pore sizes (from 6 to 10 nm) and with different Pd sources. The materials were prepared by standard methods, introducing the thiol ligand by co-condensation. Pd was introduced by adsorbing a controlled amount of soluble Pd on the surface of the functionalized support. Two Pd sources were employed, tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), and palladium acetate (Pd(OAc)2). The catalytic performance of the resulting materials in the Suzuki-Miyaura cross-coupling reaction of aryl halides with aryl boronic acids and the efficiency of recycling the resulting catalysts are reported herein. 2. Experimental Section 2.1. Catalyst Preparation. 2.1.1. KIT-6 Material. A 2.0 g portion of block copolymer Pluronic 123 (P123, Aldrich) was dissolved with stirring in 72.4 g of water and 3.7 g of HCl (Fischer 36.5-38.0%) at room temperature. The solution was heated to 35 °C before adding 2.0 g of butanol. After about 1 h, 5.2 g of TEOS (tetraethyl orthosilicate, Si(OEt)4) was added to the solution. The molar composition of the reaction mixture TEOS/P123/BuOH/HCl/H2O is 1.0/0.017/1.31/1.83/195.28,29 The mixture is maintained at 35 °C for 24 h with vigorous stirring, followed by heating at 60-130 °C for 24 h. The solid products were filtered off and then washed at least twice with 100 mL deionized water each time. After being dried at 60 °C overnight, the surfactant was removed by Soxhlet extraction with ethanol for 48 h. 2.1.2. KIT-6-SH Materials. The preparation of thiol-functionalized mesoporous silica follows a route similar to that described above for pure KIT-6, with the exception that 3-mercaptopropyl trimethoxysilane (Aldrich, 95%) (MPTMS) was added during the synthesis of the material, after the prehydrolysis of TEOS. MPTMS was added at a molar ratio of 2% to TEOS. The resulting mixture was stirred at the desired temperature of 35 °C for 24 h and then transferred to Teflon bottles to allow hydrothermal treatment under static conditions at different temperatures 60, 80, 100, and 130 °C for 24 h. The product was filtered and air-dried, followed by surfactant removal by Soxhlet extraction with ethanol for 48 h. The final material was dried at 60 °C overnight. Sample compositions were established by elemental analysis, particularly to determine the amount of sulfur on the surface, which is used to determine the amount of Pd employed. The resulting materials are referred to as KIT-6(X)-SH where X represents the temperature of hydrothermal treatment expressed in degrees Celsius. 2.1.3. Pd Complex Adsorption. Catalysts were prepared by adsorption of the desired Pd complex on the surface of a dried, characterized, thiolated material.26 For example, 1 g of KIT6(X)-SH was stirred in a Schlenk flask under argon with either Pd(PPh3)4 or Pd(OAc)2 at a 1:2 molar ratio (of S:Pd, where the amount of S on the surface is determined by elemental analysis of KIT-6(X)-SH) in benzene or acetone respectively at room temperature for 12 h. After this time the color of the solution, which represents the Pd complex that it itself colored, will have transferred entirely to the solid, indicating complete adsorption. This has been confirmed in previous work by ICPMS analysis of the solution.20-22 The resulting catalysts were filtered in air and washed by refluxing in respectively ethanol, toluene, and then acetonitrile to remove any physisorbed Pd. A final filtration was followed by drying under vacuum at room temperature (ca. 22 °C). The catalysts prepared are listed in Table 1. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using a Bruker AXS Discover D8 X-ray diffractometer with Cu KR radiation. Transmission electron

MacQuarrie et al. TABLE 1: Characteristics of the Prepared Catalysts entry

sample

Pd adsorbed mmol/ga

sulfur content mmol/gb

S:Pdc

1 2 3 4 5 6

KIT-6(60)-SH•TPdd KIT-6(80)-SH•TPd KIT-6(100)-SH•TPd KIT-6(130)-SH•TPd KIT-6(60)-SH•PdAcd KIT-6(130)-SH•PdAc

0.10 0.11 0.12 0.14 0.16 0.11

0.19 0.25 0.32 0.29 0.29 0.20

1.9 2.2 2.6 2.0 1.8 1.8

a Determined by ICPMS analysis. b Determined by elemental analysis. c Molar ratio. d TPd: Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), PdAc: Palladium(II) acetate Pd(OAc)2.

micrographs (TEM) were obtained using a JEOL JEM 1230 microscope operated at 80 kV. 13C MAS NMR spectra were obtained at room temperature on a Bruker Avance 300 MHz at a frequency of 76 MHz and at 6 kHz spinning rate. Adamantane was used as external reference for analyses. Weight loss curves (TGA) were recorded on a TA Instruments TGA model Q500 from ambient temperature to 700 °C at a heating rate of 5 °C/ min under nitrogen. The samples were dehydrated at 110 °C for 2 h before TGA analysis. Adsorption measurements were performed at -196 °C using a Quantachrome Autosorb1 volumetric adsorption analyzer. Before any measurements were performed, the samples were outgassed at 120 °C for 5 h under vacuum, until a residual pressure of 10-6 mbar was reached. The Brunauer-EmmettTeller (BET) equation was used to calculate the specific surface area SBET from adsorption data obtained at P/P0 between 0.1 and 0.2. The total pore volume was estimated from the volume of N2 adsorbed at a relative pressure of P/P0 ) 0.99. Nonlocal density functional theory (NLDFT) analyses were also performed to evaluate surface areas, pore volumes, and pore sizes.30,31 For these analyses, the kernel of NLDFT equilibrium capillary condensation isotherms of N2 at -196 °C on silica was selected for the model isotherms (using desorption model, and assuming cylinder pores). The t-plot method was used for the estimation of the micropore volume. X-ray photoelectron spectra of the catalyst samples were acquired at room temperature using an Axis-Ultra (Kratos-UK)

Figure 1. HRXRD patterns of KIT-6(X)-SH•TPd (Successive samples are offset for clarity).

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J. Phys. Chem. C, Vol. 114, No. 1, 2010 59 mmol, 0.106 g as internal standard for GC analysis), and the palladium catalyst (1 mol % Pd) were mixed in a sealed tube and flushed with Ar. Solvent (dry, degassed dimethylformamide (DMF)/distilled H2O mixture (20:1), total volume 5 mL) was added to this reaction mixture via syringe. The reaction mixture was stirred at 80 °C under Ar for 12 h. After completion of the reaction (as determined by GC), the solution was filtered via a syringe filter and diluted to 25 mL with EtOAc before ICPMS measurement. For recycling experiments, after reaction, the recovered solid was washed with EtOAc and H2O to remove residual salts and organics before being used in the next recycling step.

Figure 2. (A) N2 adsorption-desorption, (B) NLDFT pore size distributions calculated from the adsorption branches of the isotherms of KIT-6(X)-SH•TPd materials before reaction.

photoelectron spectrometer and Al-monochromated (hν ) 1486.6 eV) X-ray radiation. The source, operated at 300 W, sensed ∼800 × 400 µm on the wafers. The high-resolution spectra of C 1s and Pd 3d were acquired at a pass energy of 40 eV and a step size of 0.1 eV, respectively. Correspondingly, nominal resolutions were 0.6 eV. The binding energies were compared with literature data.32 2.3. Catalytic Runs: Suzuki-Miyaura Coupling Reaction. p-Bromoacetophenone (1.0 mmol, 0.199 g), the pinacol ester of phenyl boronic acid, (PhBpin), (1.5 mmol, 0.306 g), potassium carbonate (2 mmol, 0.076 g), dimethoxybenzene (1

3. Results and Discussion 3.1. Catalyst Characterization. Mesoporous silica supports (KIT-6) containing thiol groups as ligands for palladium were synthesized by the co-condensation route. For all catalysts, the S:Pd ratio was kept close to 2:1 based on previous reports from the Crudden group which showed that this ratio gave the optimum reactivity and minimum leaching.20-22 At higher ratios, lower levels of leaching are observed, but activity is eventually quenched, and at lower ratios, Pd leaching becomes problematic. Effective functionalization of the surface by the thiol is determined by elemental analysis to quantify loading, and by CPMAS 13C NMR. X-ray diffraction patterns were collected for the KIT-6 materials after functionalization with the palladium complexes Pd(PPh3)4 and Pd(OAc)2. All of the samples have a single intense reflection at 2θ angle around 0.88° (Figure 1) as is typical for KIT-628,29 suggesting that the cubic Ia3d symmetry was retained during the introduction of the mercaptopropyl groups and the Pd complex. The sample aged at 80 °C shows more intense 110 and 200 diffraction lines which suggests a more regular mesostructure. The N2 adsorption-desorption isotherm of KIT-6(X)-SH•Pd (Figure 2) also confirms this observation, showing a more narrow DFT pore size distribution for sample KIT-6(80)-SH•TPd. All samples show the expected type IV isotherm (Figure 2A) with a sharp capillary condensation step at high relative pressures, typical of highly organized mesostructures. The N2adsorption-desorption results for these samples are presented in Table 2. In accordance with previous reports,28,29,33,34 a significant increase in pore volume and pore size is observed with increasing aging temperature (Figure 2B). Furthermore,

TABLE 2: Physical Characteristics of Catalysts before and after Reactiona SBET

SDFT

materials

2

(m /g)

2

(m /g)

KIT-6(60)-SH KIT-6(60)-SH•TPd (unused) KIT-6(60)-SH•TPd (after 1st run) KIT-6(80)-SH KIT-6(80)-SH•TPd(unused) KIT-6(60)-SH•TPd (after 1st run) KIT-6(100)-SH KIT-6(100)-SH•TPd(unused) KIT-6(100)-SH•TPd (after 1st run) KIT-6(130)-SH KIT-6(130)-SH•TPd(unused) KIT-6(130)-SH•TPd (after 1st run) KIT-6(60)-SH•PdAc (unused) KIT-6(130)-SH•PdAc (unused)

377 357 196 758 512 205 773 467 122 482 506 239 309 457

312 291 156 600 411 165 611 392 110 410 465 209 251 406

Vmic 3

(cm /g) 0.019 0.023 0.008

0.012

Vt 3

VDFT 3

WDFT

(cm /g)

(cm /g)

(nm)

0.47 0.40 0.22 0.98 0.67 0.21 1.15 0.73 0.23 1.03 1.05 0.39 0.35 0.85

0.45 0.38 0.20 0.95 0.65 0.19 1.11 0.70 0.21 1.00 1.01 0.37 0.33 0.82

5.9 5.0 4.8 7.5 6.5 5.7 8.6 7.3 6.0 9.2 8.5 6.0 5.0 9.4

a SBET is the nitrogen BET specific surface area calculated from the isotherm analysis in the relative pressure range of 0.10-0.20; Vt is the total pore volume at relative pressure 0.99. SDFT is the specific surface area; VDFT is the total pore volume, Vmic is the micropore volume and WDFT is the mesopores diameter, calculated by the DFT method using the kernel of NLDFT equilibrium capillary condensation isotherms of N2 at 77 K on silica.

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Figure 3. KIT-6 catalysts before use. (A) KIT-6(60)-SH•TPd, scale bar ) 100 nm; (B) KIT-6(80)-SH•TPd, scale bar ) 50 nm; (C) KIT-6(100)SH•TPd, scale bar ) 50 nm; (D) KIT-6(130)-SH•TPd, scale bar ) 100 nm.

TABLE 3: Pd 3d3/2,5/2 for Various Pd-Loaded KIT-6 Catalysts Pd 3d3/2,5/2 binding energy (eV)

area ratio

support

Pd source

catalyst treatment

state 1

state 2

St 2:St 1

KIT-6(80)-SH KIT-6(80)-SH KIT-6(130)-SH KIT-6(130)-SH KIT-6(130)-SH KIT-6(130)-SH

Pd(PPh3)4 Pd(PPh3)4 Pd(PPh3)4 Pd(PPh3)4 Pd(OAc)2 Pd(OAc)2

none after use none after use none after use

336.9/342.2 337.5/342.8 337.4/342.7 337.6/342.8 337.5/342.7 337.3/342.6

335.7/340.9

0.150

335.4/340.7

0.322

335.4/340.6

0.548

when comparing the various parent materials (not shown) with those loaded with MPTMS and doped with Pd complexes, decreases in surface area, pore volume and pore diameter were observed. This can be explained taking into account the fact that the mercaptopropyl groups and the Pd complexes are grafted on the interior of the material as well as the exterior, which causes a gradual decrease in average pore diameter. Figure 3A-D shows the TEM images of Pd(PPh3)4 complexloaded KIT-6-X functionalized samples before reaction. As expected based on the XRD results, TEM analysis indicated that the functionalization and loading steps had no negative impact on the order of the material. Similar results were obtained for materials prepared using palladium acetate in place of palladium tetrakistriphenylphosphine (see Supporting Information, Figure 4S). TGA analysis was also employed with the various catalysts, the results of which are shown in the Supporting Information (Figure 1S). To establish the chemical state of the palladium particles deposited on KIT-6-SH, X-ray photoelectron spectroscopy (XPS) measurements were carried out for various catalysts before and after the coupling reaction. Pd 3d3/2,5/2 binding energies for these catalysts are listed in Table 3. Typical Pd 3d XP spectra for the catalyst KIT-6(130)-SH loaded with Pd(PPh3)4 before and after the coupling reaction are shown in Figure 4A,B. In Figure 4B, it is evident that two Pd species are present, state 1 being at a lower binding energy than state 2. The data in Table 3 demonstrate that in the unreacted KIT6(80)-SH catalyst, a single chemical state is present with 3d5/2 and 3d3/2 peaks at binding energies of 336.9 and 342.2 eV, respectively. The 3d5/2 binding energy is, within experimental error, identical to the value of 337.0 eV previously observed35 for Pd(PPh3)4, suggesting that before reaction the Pd(PPh3)4 remains intact within the silicate support. Upon reaction, a new state is observed in which the binding energy for the Pd 3d5/2 peak is 335.7 eV. This value is considerably lower than that previously reported for a wide range of SBA-15-supported Pd catalysts, which had an average binding energy of 337.5 eV,20 a value more typical of a Pd(II) species. In that case, a

combination of TEM and Auger parameter data demonstrated that the Pd had indeed been reduced; however, the small size of the Pd nanoparticles formed (between 5 and 7 nm)20 led to an apparent increase in binding energy due to the relatively low polarizability of the few surrounding Pd atoms. Pd catalysts prepared in powdered form, or as Pd supported on carbon or BaSO4 supports showed binding energies of 335.7 eV, much closer to published values for bulk Pd, variously reported as 334.6-335.1 eV.36 Such particles were significantly larger (10-20 nm)37 than those observed in the case of catalysts prepared on SBA-15. The observation here of binding energies at 335.7 eV is consistent with Pd nanoparticles of a significantly larger size being formed on KIT-6 compared with those on SBA15. Further evidence of this comes from the Pd XPS data from the KIT-6(130)-SH loaded with Pd(PPh3)4 given in Table 3. Here, the binding energy data are similar to that observed with

Figure 4. XPS spectra show the Pd 3d regions for KIT-6(130)-SH•TPd, (A) before and (B) after first reuse of coupling reaction.

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J. Phys. Chem. C, Vol. 114, No. 1, 2010 61

Figure 5. Plot of yield versus time for Suzuki-Miyaura coupling with fresh KIT-6(X)-SH•Pd catalysts (A) Pd(PPh3)4; (B) Pd(PPh3)4 and Pd(OAc)2 (X ) 60, 130 °C). Using, 4-bromoacetophenone (1 mmol), PhBpin (1.5 mmol), base (2 mmol), Pd (1% mol relative to aryl halide), and DMF/H2O (20:1-5 mL), at 80 °C under Ar. The yield was determined by GLC using hexamethylbenzene as internal standard.

Pd supported on KIT-6(80)-SH, but the ratio of reduced (State 2) to nonreduced (State 1) Pd species following reaction is considerably larger, indicating a more significant formation of Pd nanoparticles, presumably due to the larger pore size in the latter material. Since our previous work with SBA-15-supported Pd catalysts used Pd(OAc)2 as the Pd source, similar KIT-6-based catalysts were prepared using Pd(OAc)2. The Pd 3d XPS spectra obtained before and after the coupling reaction are given in Table 3. Before reaction, the Pd 3d55/2 binding energy was found to be 337.5 eV. This is slightly lower than the 338.8 eV previously observed35 for Pd(OAc)2, but similar to the 337.7-338.0 eV observed for Pd(OAc)2 adsorbed on either SBA-15-SH or amorphous silica before any reaction. Two chemical states can again be resolved in the Pd 3d spectra for the reacted catalyst, with Pd 3d5/2 binding energies of 337.3 and 335.4 eV. This suggests that, unlike SBA-15-SH-Pd catalysts formed using Pd(OAc)2, in the case of KIT-6-based catalysts, some of the Pd(OAc)2 remains unreacted, while the state at lower binding energy is again consistent with the formation of relatively large Pd nanoparticles. 3.2. Catalytic Efficiency of KIT-6(X)-SH•Pd. In order to test the catalytic activity of the supported Pd complexes, the Suzuki-Miyaura reaction between the pinacol ester of phenylboronic acid and p-bromoacetophenone was studied. The results are shown in eq 1 and Figure 5A. As expected from our previous

Figure 6. Reusability study of palladium complex Pd(PPh3)4 supported on KIT-6(X)-SH for the Suzuki-Miyaura coupling of 4-bromoacetophenone with PhBpin. Reaction conditions: 4-bromoacetophenone (1 mmol), PhBpin (1.5 mmol), base (2 mmol), Pd (1% mol relative to aryl halide), and DMF/H2O (20:1-5 mL), Ar for 12 h at 80 °C.

Figure 7. TEM micrographs of KIT-6(X)-SH•TPd after one use or multiple uses; (A) X ) 60, scale bar after first use ) 100 nm, after 7th use ) 50 nm; (B) X ) 80, scale bars in both cases ) 50 nm; (C) X ) 100, scale bars in both cases ) 50 nm; and (D) X ) 130 °C, scale bars in both cases ) 100 nm.

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Figure 8. N2 adsorption-desorption, NLDFT pore size distributions calculated from the adsorption branches of the isotherms of (A) KIT-6(130)SH•TPd before and after first use and (B) KIT-6(60)-SH•TPd before and after first use.

results, all catalysts gave excellent yields (90 to 100%) after 4-5 h at 80 °C. Examination of the various KIT-6 supports showed that the activity of the catalyst was not influenced by mesopore size. The slight difference in catalytic activity observed between KIT-6-X-SH•Pd (X ) 80 and 100 °C) and (X ) 60 and 130 °C) is likely related to small changes in the S:Pd molar ratio for these materials20 (Table 1).

In order to determine whether the form of Pd has an impact on the catalytic activity, the catalysts prepared with Pd(PPh3)4 were compared with those using Pd(OAc)2 at the same S:Pd (∼2:1) ratio (Table 1: entry 5, 6). Interestingly, even though the Pd(OAc)2 catalysts presumably require prereduction to Pd(0), no significant difference in catalytic activity was observed (Figure 5B). 3.3. Catalyst Reusability. To evaluate the reusability of KIT6(X)-SH•TPd, we performed a series of consecutive runs by simply reisolating the catalyst after reaction by filtration. The results, given in Figure 6, show that all catalysts KIT-6(X)SH•TPd (regardless of X) could be reused 6-8× in the previously described reaction conditions, with high catalytic activity maintained after multiple reactions. In all cases, a gradual decrease in catalytic activity occurred somewhere between the sixth and eighth reuse, which necessitated prolonging the reaction for 22 or 24 h to reach the yield of 70-90%. These results are remarkable considering the behavior of SBA15-based catalysts, which, when untreated, show considerable deactivation after 1-2 runs. Careful studies in the Crudden lab demonstrated conclusively that this deactivation could be

attributed to the collapse of the porosity in the thin-walled material.20,27 This collapse could be prevented by essentially buffering the surface with boric acid or alumina,27 but untreated materials showed significant base-induced degradation after 1-2 runs.38 Thus, the high reusability of the KIT-6-based materials is intriguing. Analysis of XRD and N2 adsorption data after one run indicated that the material was beginning to collapse at this time, as expected based on our results with SBA-15-based catalysts.27 After multiple uses, TEM analysis indicated that a few small areas of order remained, but that large fractions of the material were rendered amorphous (compare Figures 3 and 7). Physisorption analysis of catalysts before use and after one use shows a drastic diminution of mesopore volume (and pore size) in the case of KIT-6(130)-SH•Pd which could result from partial pore blockage by organic residues. It is however clear that collapse of the pore structure also contributes to decreasing mesopore volume. The latter explanation is indeed confirmed by TEM pictures after the first use (Figure 8). It is likely, therefore, that after multiple uses, the mesostructure is degraded even further. Remarkably, however, unlike SBA-15-based catalysts, catalytic activity does not seem to correlate with retention of order. Further explanation of these results was also found in the TEM images of spent catalysts. As shown in Figure 9, Pd nanoparticles are observed on the surface of the material, even after a single use. Both catalysts prepared from Pd(OAc)2 and from Pd(PPh3)4 displayed nanoparticles on their surfaces. Pd nanoparticles ranging in size from 5 to 12 nm were observed, consistent with the lower Pd 3d5/2 binding energy observed as described above. Unlike our previous results with SBA-15, where the size of the nanoparticles does not exceed the size of the pores (Figure 10),20 with KIT-6-based materials, the nanoparticles appear to form preferentially on the exterior surfaces after exposure to coupling reaction conditions, and are not

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J. Phys. Chem. C, Vol. 114, No. 1, 2010 63 TABLE 4: Pd Leaching in the Case of the First and the Last Reuse of KIT-6(X)-SH•Pd for the Suzuki-Miyaura Coupling of 4-Bromoacetophenone with PhBpin Suzuki-Miyaura reaction entry

catalysts

1 2

KIT-6(60)-SH•TPd KIT-6(80)-SH•TPd

3

KIT-6(100)-SH•TPd

4

KIT-6(130)-SH•TPd

cycle Pd leaching number time (h) yield (%) (ppm) 1 1 6 1 8 1

12 12 24 12 22 12

98 93 79 93 69 100

1.78 1.5 0.49 1.17 0.90 0.50

4. Conclusions

Figure 9. Bright field and dark field TEM micrographs of KIT-6(X)SH•TPd after one use (A) and KIT-6(X)-SH•PdAc after one use (B). All scale bars ) 100 nm.

Figure 10. TEM micrograph of SBA-15-SH•PdAc (prepared by grafting of the thiol onto a preformed support) after one use.20

constrained in size by the pores of the material. In fact, Pd nanoparticles ranging in size from 5 to 12 nm were observed on the exterior surface of the used KIT-6 based materials. The presence of these nanoparticles on the exterior enables access of reagents to the catalytically active Pd on the surface of the material, even after the pore structure has collapsed. However, eventually the formation of large Pd nanoparticles is likely responsible for the decrease in catalyst activity. 3.4. Pd Leaching. Palladium leaching was measured by removing the solid catalyst by filtration after the reaction, and analyzing the resulting solutions by ICPMS. In two cases, catalysts were examined after the first and the last reuse (Table 4). Although the Pd levels were low, around 1-2 ppm, these levels were higher than we observed with the SBA-15-SH catalysts, which are generally on the order of 0.03 ppm after the first run, eventually increasing to 0.6 ppm after 5 runs for cases where catalyst activity was retained for that number of runs.20,39 On the contrary, with those catalysts examined herein, the Pd leaching was higher in the first run than the last run.

In summary, we have developed a new reusable heterogeneous catalyst, KIT-6-SH•Pd, which has one of the highest levels of recyclability in the Suzuki-Miyaura reaction among catalysts supported on mesoporous silica materials. Interestingly, even though the material appears to degrade early on in the process, due to the action of aqueous base, catalytic activity remains. XPS and transmission electron microscopy of the used catalyst showed the presence of large Pd nanoparticles, indicating that with KIT-6 materials, unlike SBA-15, the size of the nanoparticles was not constrained by pores. This is likely associated with the large difference in mesopore connectivity. In the 2Dhexagonal p6 mm structure of SBA-15, the long parallel cylindrical pores likely restrict diffusion of Pd species out of the pore lattice. On the contrary, in the cubic Ia3d of KIT-6 (similar to MCM-48), the pores have not only a 3-D arrangement, but also a very high connectivity still enhanced by the wall porosity at aging temperatures higher than 80-100 °C.29,40 Under these conditions, Pd clusters may diffuse more readily and reach the outer surface of the KIT-6 particles. On the exterior surface, the size of the Pd nanoparticles is thus not controlled and particles larger than the pore size are observed. As a result, the Pd nanoparticles present on the external surface can continue to catalyze the reaction even after collapse of the materials, which, in the case of SBA-15, traps Pd on the interior rendering the catalyst inactive once the material has degraded. Although this phenomenon may lead to slightly higher Pd leaching into solution, the resulting catalysts can be reused many more times compared to similarly prepared catalysts on SBA15-type materials. Acknowledgment. Dr. David Dube´ (Silicycle) is thanked for CNS analyses and Mr. Richard Janvier for TEM analyses. CMC thanks the Natural Science and Engineering Research Council of Canada (NSERC) and Johnson & Johnson for support of this work. Supporting Information Available: N2 adsorption-desorption, NLDFT pore size distributions of KIT-6(130)-SH•TPd and KIT-6(60)-SH•TPd before and after first reuse, N2 adsorptiondesorption; HRXRD patterns of KIT-6-SH and KIT-6-SHX•PdAc (X ) 60, 130 °C); and TEM micrograph of KIT-6SH-X•PdAc; (A) X ) 60 °C, (B) X ) 130 °C. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (2) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513. (3) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979, 3437. (4) Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J. , Eds.; Wiley-VCH: New York, 1998.

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