Hexagonally Packed Pd Nanoarray Catalyst for Liquid-Phase Enantioselective Hydrogenation of Acetophenone to R-(+)-1-Phenylethanol in the Presence of S-Proline

Abstract

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. IntroductionLiquid-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

Preparation of free-standing mesoporous metal catalysts and their applications in heterogeneous enantioselective hydrogenations

“…2a-b) via the "two solvents" strategy with the mesoporous molecular sieve SBA-15 as the hard template. 45 The Pd nanoarray consists of hexagonally arranged cylindrical Pd nanowires, which is exactly an inverse replica of SBA-15, and the corresponding Fourier diffractogram additionally supports the hexagonal structure. The BET specific surface area and the pore size of the hexagonally packed Pd nanoarray are 29.5 m 2 g −1 and 3.2 nm, respectively.…”

“…2a-b) via the "two solvents" strategy with the mesoporous molecular sieve SBA-15 as the hard template. 45 The Pd nanoarray consists of hexagonally arranged cylindrical Pd nanowires, which is exactly an inverse replica of SBA-15, and the corresponding Fourier diffractogram additionally supports the hexagonal Wang et al prepared mesoporous gyroid Pd network catalysts with different topologies and pore diameters via the "two solvents" strategy with the mesoporous molecular sieve KIT-6 as the hard template hydrothermally treated at different temperatures. 47 Generally, the BET specific surface areas are from 45 to 80 m 2 g −1 for the gyroid Pd samples, which are significantly larger than the ultrafine Pd black sample (23 m 2 g −1 ).…”

Recent developments in the control of selectivity in hydrogenation reactions by confined metal functionalities

“…This confinement seemed to favor the adsorption of the adduct which results in the R-isomer, thereby enhancing the ee. 159 This reaction was also performed on freestanding Pd gyroidal structures prepared with various KIT-6 mesoporous silicates as templates. The use of KIT-6 templates prepared at temperatures below 375 K resulted in palladium with a gyroidal morphology (Fig.…”

Chemical Design of Palladium‐Based Nanoarchitectures for Catalytic Applications

“…TEM images of b) silica‐free 3DOM Pd networks recorded along zone axis (100) with an enlarged image (inset in part (b)) (reproduced with permission . Copyright 2011, The Royal Society of Chemistry) and c) as‐prepared silica‐free hexagonally packed Pd nanoarray with the SAED pattern (inset in part (c)) (reproduced with permission . Copyright 2008, American Chemical Society).…”

“…Pd nanoarrays with hexagonally closed mesoporous structures were previously fabricated using SBA‐15 as a template (Figure c) . These nanoarrays exhibited an average pore size of ≈3.2 nm and a moderate surface area of 29.5 m 2 g −1 .…”