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2008, 112, 1316-1320 Published on Web 01/10/2008
Hexagonally Packed Pd Nanoarray Catalyst for Liquid-Phase Enantioselective Hydrogenation of Acetophenone to R-(+)-1-Phenylethanol in the Presence of S-Proline Xueying Chen,† Zhiying Lou,† Minghua Qiao,*,† Kangnian Fan,† Shik Chi Tsang,‡ and Heyong He*,† Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, P. R. China, and Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, U.K. ReceiVed: NoVember 17, 2007; In Final Form: January 2, 2008
Hexagonally packed Pd nanoarray catalyst was prepared via the “two solvents” strategy with mesoporous molecular sieve SBA-15 as a hard template. By using S-proline as the chiral modifier, the catalyst exhibited superior activity and selectivity in liquid-phase hydrogenation of acetophenone to R-(+)-1-phenylethanol with up to 100% chemoselectivity and 28.8% enantiomeric excess. The presence of S-proline and the regular hexagonal mesostructure of the Pd nanoarray catalyst are crucial for its superior catalytic performance.
Introduction Liquid-phase hydrogenation of acetophenone is a model reaction chosen by many researchers for heterogeneous enantioselective catalysis studies. First, it is of extensive industrial relevance because its product (chiral 1-phenylethanol) is used widely in the pharmaceutical and perfume industries.1,2 Second, it is a good example of a multistep reaction. It involves a sequence of competitive parallel and consecutive reactions, and both the unsaturated aromatic ring as well as the carbonyl group in the acetophenone can be reduced.3-5 Third, it is a good probe reaction among the heterogeneous enantioselective catalytic reactions, which have obvious advantages in green chemistry.5 For nonactivated prochiral ketones such as acetophenone, it remains a great challenge to achieve high enantioselectivity to corresponding chiral alcohols by heterogeneous catalysis.6-8 The best result reported so far for acetophenone was 22.5% enantiomeric excess (ee) at 77.9% conversion.8 Thus, it is highly desirable to explore new categories of catalysts with improved low-temperature activity and enantioselectivity for this reaction. As is known, the morphology and structure of the nanoscaled materials have important effects on the performances of materials in practical applications. When the morphologies of the nanoscale materials are transformed from common spherical nanoparticles to one-dimensional nanorods,9 two-dimensional ultrathin films,10 or three-dimensional nanoarrays,11,12 these nanostructured materials usually exhibit unique properties in mesoscopic physics, electronics, semiconductor fabrication, and catalysis. Although enantioselective hydrogenation over metal nanoparticles including supported metal catalysts has been practiced extensively,6-8 the work is lacking over nanostructured metals. In this paper, we report on the enantioselective hydrogenation of acetophenone in the presence of S-proline over an ordered * To whom correspondence should be addressed. Tel: (+86-21) 65643916. Fax: (+86)21-65642978. E-mail:
[email protected] and
[email protected]. † Fudan University. ‡ University of Oxford.
10.1021/jp710962p CCC: $40.75
hexagonally packed mesoporous Pd nanoarray catalyst prepared by a “two solvents” strategy through nanocasting from mesoporous SBA-15. It is expected that the regular mesoporous structure of the Pd nanoarray catalyst is helpful for the enhancement of the enantioselectivity of the asymmetric hydrogenation reaction of acetophenone because the confinement of homogeneous chiral catalyst within micro- or mesopores of solid supports (such as zeolite Y and MCM-41) has been found to improve the enantioselectivity of some asymmetric epoxidation and hydroformylation reactions.13 Experimental Methods The hexagonally packed mesoporous Pd nanoarray catalyst was prepared as follows. One gram of mesoporous molecular sieve SBA-15 synthesized according to literature14 was added into hexane (40 mL) and stirred vigorously for 2 h. Two mL aqueous solution of PdCl2 (1.0 g PdCl2/10 mL) was then added dropwise under vigorous stirring. The excessive solution was decanted, and the resulting brown solids were dried at 393 K for 24 h. After drying, the solids were reduced by adding a 0.3 M aqueous solution of KBH4 dropwise (molar ratio of BH4-/ Pd2+ ) 10). The black solid was centrifuged, and the siliceous template was dissolved by hydrofluoric acid. The product was washed with distilled water until neutrality and then with methanol three times. The catalyst was kept in methanol for characterization and activity test. For comparison, ultrafine Pd black catalyst was prepared by chemical reduction of PdCl2 aqueous solution using KBH4 under the same condition but without involving SBA-15. The BET surface area and pore volume were measured by N2 physical adsorption at 77 K on a Micromeritics TriStar 3000 adsorption apparatus. The active surface area was measured by H2 chemisorption, taking H/Pd(s) ) 1 and a surface area of 7.874 × 10-20 m2 per Pd atom.15 The turnover frequency (TOF) was expressed as the number of H2 molecules consumed per active surface Pd atom per second extrapolated to the beginning of the reaction. The types of active sites on the catalyst were © 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 5, 2008 1317
Figure 1. TEM (a, b, e, and f) and HRTEM (c and d) images of the as-prepared (a-d), the used (e) silica-free hexagonally packed Pd nanoarray catalyst, and the ultrafine Pd black catalyst (f). Parts a, c, and e are viewed along the direction perpendicular to the hexagonal pore axis, and b and d along the hexagonal pore axis. Insets in a and f are the SAED patterns of the Pd nanoarray and ultrafine Pd black catalysts, respectively. Inset in b is the Fourier diffractogram of the Pd nanoarray.
studied by temperature-programmed desorption of H2 (H2-TPD). The mesostructure, selected-area electron diffraction (SAED) pattern, and elemental analysis of the catalyst were acquired on a JEOL 2011 microscope operated at 200 kV fitted with an energy-dispersive X-ray emission analyzer (EDX). XRD patterns were collected on a Bruker AXS D8 Advance X-ray diffrac-
tometer using Cu KR radiation. FTIR spectra were recorded on a Nicolet NEXUS 470 spectrometer. The activity test was carried out in a 75 mL stainless-steel autoclave with a magnetic stirrer. Methanol was used as the solvent. The reaction conditions were as follows: 0.10 mL of acetophenone, 0.50 g of S-proline, 40 mL of methanol, 0.050 g
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of catalyst, H2 pressure of 3.0 MPa, stirring rate of 1000 rpm to exclude diffusion effects, and thermostated at 288 K. Before hydrogenation, the methanol solution of acetophenone and S-proline were boiled at 363 K for 10 min according to Tungler et al.8 Conversion and ee’s were determined by a gas chromatograph equipped with a CP-Chirasil-Dex CB capillary column and a FID detector. The enantioselectivity is expressed as ee (%) ) 100 × (R - S)/(R + S). The assignment of the absolute configuration of the product was based on the retention times of authentic samples (R-(+)-1-phenylethanol and racemic 1-phenylethanol, Alfa Aesar). Reproducibility of ee’s was within (0.5%. Results and Discussion Structural Properties of the Catalysts. The BET surface area and pore volume of the template-free Pd nanoarray catalyst prepared by the “two solvents” strategy are 29.5 m2‚g-1 and 0.216 cm3‚g-1, respectively. The pore size distribution calculated from the desorption branch using the Barrett-Joyner-Halenda (BJH) algorithm indicates that the Pd nanoarray is mesoporous with a pore size of ∼3.2 nm. Figure 1 exhibits the typical TEM images of the Pd nanoarray and ultrafine Pd black catalysts. As shown in Figure 1a, the Pd nanowires in the Pd nanoarray catalyst are in ordered arrangement. EDX reveals that the nanowires are composed of Pd while Si is not detectable, indicating the complete removal of the silica template. The HRTEM image (Figure 1c) reveals that actually the Pd nanowires are constituted by Pd nanocrystallites and are interconnected by self-supported shorter wires due to the occupation of Pd in the channel-interconnecting micropores within the SBA-15 wall.17 The SAED pattern (inset in Figure 1a) displays typical polycrystalline diffraction rings assignable to face-centered cubic (fcc) metallic Pd. Figure 1b clearly shows that the Pd nanoarray consists of hexagonally arranged cylindrical Pd nanowires, which is exactly an inverse replica of SBA15.14 The corresponding Fourier diffractogram additionally supports the hexagonal structure (inset in Figure 1b). The Pd nanowires in the nanoarray are ∼8.6 nm in diameter as determined by HRTEM (Figure 1d), and the center-to-center distance between two adjacent nanowires is ∼10.2 nm. A simple calculation readily shows that the maximum diameter of a cylinder that can be inserted in the void surrounded by hexagonally arranged Pd nanowires is 3.2 nm, in excellent agreement with the nitrogen physisorption result. It is worth noting that, as for the ultrafine Pd black catalyst, Figure 1f shows that the Pd crystallites are aggregated together with the scattered particle size distribution ranging from 7 to 120 nm. The corresponding SAED pattern (inset in Figure 1f) is virtually identical to that of the Pd nanoarray. The XRD patterns of the hexagonally packed Pd nanoarray and ultrafine Pd black catalysts are shown in Figure 2. All of the observed diffraction peaks of the Pd nanoarray and ultrafine Pd black catalysts are similar and can be indexed onto fcc Pd (PDF card no. 046-1043) with the calculated lattice constant a ) 0.3884 nm. No preferential orientation of a specific Pd plane is observed in both samples, indirectly indicating the similar surface structures of the Pd nanoarray and ultrafine Pd black. Enantioselective Hydrogenation of Acetophenone. Figure 3 presents the yield of 1-phenylethanol over the Pd nanoarray and ultrafine Pd black catalysts as a function of reaction time. To gain better insight into the enantioselectivity, the evolutions of ee in terms of R-(+)-1-phenylethanol are also plotted as an inset. For the ultrafine Pd black catalyst, Figure 3a shows that the yield of 1-phenylethanol increased gradually to ∼100% at
Figure 2. XRD patterns of the hexagonally packed Pd nanoarray and ultrafine Pd black catalysts.
Figure 3. Yield of 1-phenylethanol and the evolution of ee as a function of reaction time (inset). (a) Ultrafine Pd black, (b) hexagonally packed Pd nanoarray catalyst. Reaction conditions: 0.10 mL acetophenone, 0.50 g S-proline, 40 mL methanol, T ) 288 K, PH2 ) 3.0 MPa, Wcat. ) 0.050 g, and stirring speed ) 1000 rpm.
a reaction time of 60 min and then dropped at prolonged reaction time. The ee increased with the increase of the reaction time first, reaches its maximum of 16.8% at 45 min, and then dropped gradually. Over the Pd nanoarray catalyst, as depicted in Figure 3b, the increase of the yield of 1-phenylethanol is much steeper than that over ultrafine Pd black catalyst, and only 15 min is needed to achieve 100% yield of 1-phenylethanol. For prolonged reaction time, the yield of 1-phenylethanol remained constant, clearly demonstrating the excellent chemoselectivity of the Pd nanoarray catalyst in hydrogenating acetophenone. Moreover, the maximum ee over the Pd nanoarray catalyst can be as high
Letters
Figure 4. FTIR spectra of (a) the methanolic solution of acetophenone, (b) the methanolic solution of S-proline, (c) the methanolic mixture of S-proline and acetophenone boiled at 363 K for 10 min, and (d) the methanolic mixture of S-proline and acetophenone without boiling.
as 28.8% with the corresponding 1-phenylethanol yield of 94.9% at a reaction time of 10 min, which is much higher than that over the ultrafine Pd black catalyst, signifying the superior enantioselective property of the Pd nanoarray catalyst to the ultrafine Pd black catalyst. The TOF of Pd nanoarray catalyst without considering the possible coverage of S-proline is 0.09 s-1. It is notable that the Pd nanoarray catalyst is stable, as confirmed by its invariable catalytic behavior in 10 successive runs and virtually unchanged morphology of the used catalyst (Figure 1e). It can be attributed to the low reaction temperature related to its high activity and the presence of micropores on the pore wall of SBA-15 through which the hexagonally packed Pd nanowires are self-supported with each other.16,17 Influence of Proline and Pd Nanostructure on the Enhancement of Enantioselectivity. S-proline plays an important role in achieving both the high chemoselectivity and enantioselectivity. Without the addition of S-proline, the major product is ethylbenzene (∼98%) and no enantioselectivity was observed. It is likely that S-proline competes for the adsorption sites of hydrogen, thus suppressing the hydrogenolysis of 1-phenylethanol. Figure 4 exhibits the FTIR spectra of the methanolic solution of acetophenone, S-proline, and the mixture of S-proline and acetophenone before and after being boiled at 363 K for 10 min. As shown in Figure 4a, the band at 1685 cm-1 can be ascribed to the CdO stretching mode of acetophenone.18 S-proline (Figure 4b) presents several typical vibration bands. The vibrations at 2779 and 1379 cm-1 are attributable to the NH2+ stretching and twisting modes, respectively.19,20 The intense bands at 1407, 643, and 453 cm-1 are the symmetric stretching, wagging, and rocking modes of COO-, respectively.19,20 After boiling the methanolic mixture of S-proline and acetophenone, Figure 4c shows that the CdO vibration of acetophenone around 1685 cm-1 disappeared, suggesting the nucleophilic attack of the amino group in S-proline to the carbonyl group in acetophenone and the formation of the carbinol amine intermediate.21 The appearance of the vibration around 1571 cm-1 attributable to iminium species22 due to the dehydration of the carbinol amine intermediate21 is in agreement with this suggestion. The formation of the iminium adduct between S-proline and acetophenone has been proposed by Tungler et al.,8 and a mechanistic study revealed that the chiral 1-phenylethanol was produced in a diastereoselective reaction via hydrogenolysis of the C-N bond of the adduct. Our work directly evidences the presence of the S-proline-acetophenone adduct.
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1319 It is worth noting that without boiling the methanolic mixture of S-proline and acetophenone (Figure 4d) the vibration band of CdO remained in the mixture, while the band assigned to the iminium species did not appear, suggesting that the condensation between S-proline and acetophenone did not occur and no S-proline-acetophenone adduct was formed without boiling. In addition, catalytic results verified that there was no enantiomeric excess over the ultrafine Pd black and Pd nanoarry catalysts without boiling the methanolic mixture of S-proline and acetophenone, implying that enantioselectivity was determined by the S-proline-acetophenone adduct, rather than the effect from separately adsorbed S-proline and acetophenone. The formation of the iminium adduct is only one of the prerequisites that result in a high enantioselectivity. On Pt/C, commercial Raney Ni, Rh/C, and Pd/C catalysts in the presence of S-proline, ee’s of 0.1, 0.2, 0.9, and 22.5%, respectively, were observed for acetophenone hydrogenation,8 signifying that the metal catalyst also involved in the enantio-differentiation step. In this paper, our experimental results further reveal that the Pd nanoarray catalyst exhibits superior enantioselectivity in the hydrogenation of acetophenone to the ultrafine Pd black catalyst prepared under the same reduction condition. Because the Pd nanoarray and ultrafine Pd black catalysts possess the same active component, similar surface structure, but different texture, it is plausible to attribute the improved ee on the Pd nanoarray catalyst to its unique ordered hexagonally packed structure. The ordered mesopores in the Pd nanoarray catalyst, which were generated by the arrangement of Pd nanowires, can form a restricted environment13 that confines the stereo-configuration of the adsorbed S-proline-acetophenone adduct. It seems that such confinement favors the adsorption configuration of the S-proline-acetophenone adduct, which results in the R-isomer of 1-phenylethanol, thus enhancing the enantioselectivity as compared to the ultrafine Pd black catalyst without such steric constraint. This work presents a new possibility for the improvement of the enantioselectivity to chiral alcohols from nonactivated prochiral ketones by tailoring the nanostructure of metal catalysts. Conclusions The hexagonally packed Pd nanoarray catalyst prepared by a facile “two solvents” strategy exhibited superior chemo- and enantioselectivity in liquid-phase acetophenone hydrogenation with the aid of S-proline. 1-Phenylethanol is the only hydrogenation product, and 28.8% ee was obtained at 94.9% yield. It was suggested that S-proline and the ordered hexagonal mesoporous structure of the Pd nanoarray catalyst played important roles in achieving high enantioselectivity on the catalyst. Acknowledgment. This work was supported by the National Basic Research Program of China (2006CB202502), Shanghai Science and Technology Committee (06JC14009, 06DJ14006), the Fok Ying Tong Education Foundation (104022), the Ministry of Education (200602046011), and the NSF of China (20673025, 20528304). References and Notes (1) Mills, P. L.; Ramachandran, P. A.; Chaudhari, R. V. ReV. Chem. Eng. 1992, 8, 1. (2) Kim, I. Chem. Eng. 1993, 100, 50. (3) Bergault, I.; Fouilloux, P.; Joly-Vuillemin, C.; Delmas, H. J. Catal. 1998, 175, 328. (4) Casagrande, M.; Storaro, L.; Talon, A.; Lenarda, M.; Frattini, R.; Rodrı´guez-Castello´n, E.; Maireles-Torres, P. J. Mol. Catal. A 2002, 188, 133.
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