Direct synthesis of in-situ chirally modified palladium nanocrystals without capping agents and their application in heterogeneous enantioselective hydrogenations

Article abstract of DOI:10.1021/acscatal.9b00762

Shape-controlled metal nanocrystals possess the advantages of specific atomic arrangement on the surface and uniform size, which can act as ideal model catalysts for heterogeneous enantioselective catalytic studies. Nevertheless, capping agents are commonly retained on the surface of the as-synthesized metal nanocrystals, which may hinder the necessary interaction between the substrate and the chiral modifiers coadsorbed on the metal surface for chiral recognition. In this paper, we directly synthesized dendritic or cubic palladium nanocrystals in-situ modified by chiral modifiers such as cinchonidine (CD) or S-proline through a one-pot strategy by replacing the conventional capping agents with the chiral modifiers. The as-prepared CD-modified Pd nanodendrite catalyst already exhibits enantioselectivity in the asymmetric hydrogenation of (E)-α-phenylcinnamic acid without subsequent chiral modification processes. The in-situ S-proline-modified Pd nanocube catalyst shows higher enantioselectivity than nanocubes synthesized with polyvinylpyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB) as traditional capping agents in asymmetric hydrogenation of acetophenone. Chiral modifiers have dual functions (both shape-control agents and catalytic functional molecules), which can not only eliminate the adverse effect of traditional capping agents (such as PVP) on heterogeneous enantioselective hydrogenations but also simplify the follow-up chiral modification step of metal catalysts, providing a simple and efficient synthetic protocol to directly prepare in-situ chirally modified metal nanocrystal catalysts.

Full text of DOI:10.1021/acscatal.9b00762

Subscriber access provided by Stockholm University Library
Article
Direct Synthesis of In-situ Chirally Modified Palladium
Nanocrystals without Capping Agents and Their Application
in Heterogeneous Enantioselective Hydrogenations
Xiuyun Gao, Xueying Chen, Zhen Hua Li, and He-Yong He
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00762 • Publication Date (Web): 28 May 2019
Downloaded from http://pubs.acs.org on May 29, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
Direct Synthesis of In-situ Chirally Modified
Palladium Nanocrystals without Capping Agents and
Their Application in Heterogeneous Enantioselective
Hydrogenations
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0

Xiuyun Gao, Xueying Chen , Zhen Hua Li, and Heyong He
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Fudan University, Shanghai 200433, P. R. China
ABSTRACT: Shape-controlled metal nanocrystals possess the advantages of specific atomic
arrangement on the surface and the uniform size, which can act as ideal model catalysts for
heterogeneous enantioselective catalytic studies. Nevertheless, capping agents are commonly
retained on the surface of the as-synthesized metal nanocrystals, which may hinder the necessary
interaction between the substrate and the chiral modifiers co-adsorbed on the metal surface for
chiral recognition. In this paper, we directly synthesized dendritic or cubic palladium
nanocrystals in-situ modified by chiral modifiers such as cinchonidine (CD) or S-proline through
a one-pot strategy by replacing the conventional capping agents with the chiral modifiers. The
as-prepared CD-modified Pd nanodendrite catalyst is already to exhibit enantioselectivity in the
asymmetric hydrogenation of (E)-α-phenylcinnamic acid without subsequent chiral modification
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
processes. The in-situ S-proline-modified Pd nanocube catalyst shows higher enantioselectivity
than nanocubes synthesized with polyvinyl pyrrolidone (PVP) or cetyl trimethyl ammonium
bromide (CTAB) as traditional capping agents in asymmetric hydrogenation of acetophenone.
Chiral modifiers have dual functions (both shape-control agents and catalytic functional
molecules), which can not only eliminate the adverse effect of traditional capping agents (such as
PVP) on heterogeneous enantioselective hydrogenations, but also simplify the follow-up chiral
modification step of metal catalysts, providing a simple and efficient synthetic protocol to
directly prepare in-situ chirally modified metal nanocrystal catalysts.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Key words: enantioselective hydrogenation, chiral modification, palladium nanocrystals, (E)-α-
phenylcinnamic acid, acetophenone
1
. INTRODUCTION
The increasing demand of optically pure compounds in pharmaceutical, agrochemical and
perfume industries has provided tremendous impetus for developing efficacious synthetic
1
methods for the preparation of single-enantiomer chiral products. Heterogeneous asymmetric
hydrogenation of prochiral chemicals with C=C or C=O bonds is an ideal way to synthesize
important chiral products due to its significant economic and ecological advantages.2-8 Up till
now, chirally modified supported metal catalysts are most extensively studied for heterogeneous
enantioselective hydrogenations.3-8 However, the difference between the energy of S and R
transition states in heterogeneous asymmetric catalysis is the same as the range of various weak
interactions existed on the surface of the support (e.g. hydrogen bond, physical adsorption, and
9
van der Waals force). Baiker and coworkers found that the chemoselectivity and the
enantioselectivity are both greatly affected by adjusting the basicity and acidity of the support.10
ACS Paragon Plus Environment
2

Page 3 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
Thus, the existence of support increases the complexity, which is disadvantageous for the in-
depth study of the chiral recognition nature of heterogeneous asymmetric hydrogenation on
metal surface. Therefore, it is extraordinary desirable to prepare new and effective chirally
modified metal catalysts without support.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
As is known, the catalytic performance of metal catalysts intensively depends on the
morphology and atomic arrangement at different facets.11-15 Thus, the preparation of metal
nanocrystals exposing specific catalytically active facets has aroused great concern for
decades.13-18 However, up to the present, the application of shape-controlled metal nanocrystals
is mainly focused on electrochemical tests rather than heterogeneous catalysis, especially in
1
5
heterogeneous asymmetric catalysis . The main reason is that the surface cleanliness of the
metal nanocrystals cannot meet the requirements of heterogeneous asymmetric catalysis. As we
all know, for the synthesis of shape-controllable metal nanocrystals, capping agents such as cetyl
trimethyl ammonium bromide (CTAB) and polyvinyl pyrrolidone (PVP) are commonly
introduced to accurately manipulate the exposed facets of the nanocrystals and inhibit
aggregation.16-19 As the capping agents have a strong adsorption on the surface of metals, they
usually tend to be retained on the surface of the as-prepared metal nanocrystals and are difficult
to be completely removed without changing the nanocrystal morphology, which have a major
effect on the catalytic behavior of catalysts.19-25 Our previous study demonstrated that in the
enantioselective hydrogenation of acetophenone, PVP molecules residued on Pd concave-
2
5
tetrahedron nanocrystals caused the enantioselectivity loss. The remained PVP can interact
with S-proline and hinder the necessary interaction between acetophenone and S-proline co-
adsorbed on the Pd surface, inhibiting enantio-differentiation.25 Therefore, how to prepare
chirally modified metal nanocrystals without capping agents is of great significance.
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Chiral modifiers, such as cinchona alkaloids and chiral amino acids, also contain specific
functional groups (e.g. amino or carboxyl groups), which could bind on metal surface like
conventional capping agents.1-3, 26-27 Bönnemann and Braun applied dihydrocinchonidine to
stabilize colloidal Pt nanoparticles and the nanoparticle size of Pt can be varied from 1.5 to 4 nm
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
6
by changing the ratio between platinum and dihydrocinchonidine during the synthesis. This
system showed remarkable enantioselectivity in C=O bond hydrogenation of ethyl pyruvate.26-27
Therefore, the chiral modifiers are possible to be used as a new category of shape-control agents
in the preparation of metal nanocrystals, which can not only stabilize the metal particles but also
control the morphology and exposed facets of metal nanocrystals. As both shape-control agents
and chiral modification molecules, the chiral modifiers could probably not only eliminate the
adverse effects of traditional capping agents on heterogeneous asymmetric hydrogenation, but
also simplify the follow-up chiral modification steps.
In the present work, by replacing traditional capping agents with cinchonidine and S-proline,
the two most commonly used chiral modifiers in the heterogeneous enantioselective
hydrogenations, we successfully synthesized in-situ chirally modified palladium nanocrystals
with different morphologies. The catalytic behaviors of the in-situ chirally modified palladium
nanocrystals in the heterogeneous asymmetric hydrogenation of C=C or C=O bonds were studied.
For comparison, Pd nanocrystals with similar morphology and particle size were synthesized by
using conventional capping agents (PVP or CTAB) and their catalytic behaviors were also tested
for evaluating the effect of chiral modifier instead of residual capping agents on the
enantioselectivity. The work provides a new tactic to synthesize in-situ chirally modified metal
nanocrystal catalysts with controllable shapes for heterogeneous enantioselective catalysis by
eliminating the disadvantageous effect of conventional capping agents.
ACS Paragon Plus Environment
4

Page 5 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
2
. EXPERIMENTAL SECTION
2
.1 Chemical and materials
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cinchonidine (CD), cinchonine (CN), S-proline, polyvinyl pyrrolidone (PVP, Mw = 55000),
sodium tetrachloropalladate (Na PdCl ) and (E)--phenylcinnamic acid (PA) were bought from
2
4
Aldrich. Palladium chloride (PdCl ) was obtained from J&K Scientific Ltd. Cetyl trimethyl
2
ammonium bromide (CTAB), benzylamine (BA), ascorbic acid, hydrochloric acid, potassium
bromide (KBr), 1,4-dioxane, and acetophenone were bought from Sinopharm Chemical Reagent
Co., Ltd.
2.2 Catalysts synthesis
The reducing agent we used in all the synthesis is ascorbic acid. As reported in the literature28,
the reduction power of ascorbic acid is strongly related to the pH values. With the increase of pH,
ascorbic acid is progressively deprotonated and the higher the pH values, the faster the reduction
2
8
rate. Thus, to accurately manipulate the morphology and size of Pd nanocrystals, choosing
suitable Pd(II) precursors with appropriate pH values is necessary to obtain ideal reduction rates.
Otherwise, Pd nanocrystals with mixed morphologies will be produced. Under acid conditions,
the self-prepared H PdCl solution with different pH values prepared by dissolving PdCl in
2
4
2
hydrochloric acid solution was used. Under neutral conditions, the solubility of PdCl is low and
2
thus commercial Na PdCl with higher solubility is used.
2
4
Cinchonidine-modified Pd nanodendrite (Pd-dendrite-CD): 29 mg of CD was dissolved in
distilled water (100 mL) at 50 C with stirring and nitrogen protection. Then 5 mL of the
aqueous solution of H PdCl (10 mM, pH = 0.9) was added. After 3 min, ascorbic acid aqueous
2
4
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
solution (0.1 M, 0.8 mL, freshly prepared) was supplemented. The synthesis proceeded for 20
min at 50 C in nitrogen atmosphere with stirring. The resulting colloids were centrifuged,
washed by water once and methanol twice, then kept in 1,4-dioxane for activity test.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cinchonine-modified Pd nanodendrite (Pd-dendrite-CN): The synthetic process is identical
to the procedure described in the preparation of Pd-dendrite-CD, except using CN instead of CD.
S-Proline-modified Pd nanocube (Pd-cube-proline): 1.2 g of S-proline was added into 300 mL
of deionized water under stirring and nitrogen protection at 95 C. Then, ascorbic acid aqueous
solution (0.1 M, 2.4 mL, freshly prepared) was added and stirred for 5 min, and subsequently 15
mL of H PdCl aqueous solution (10 mM, pH = 0.9) was added. The synthesis proceeded for 20
2
4
min in nitrogen atmosphere with stirring at 95 C. The resulting products were centrifuged,
washed by water once and methanol twice, subsequently kept in methanol for activity test.
CTAB-capped Pd nanocube (Pd-cube-CTAB): The synthesis of CTAB-capped Pd nanocube
29
was based on the previous report. At 95 °C under nitrogen protection and stirring, 15 mL of
H PdCl aqueous solution (10 mM, pH = 1.7) was mixed with aqueous solution of CTAB (12.5
2
4
mM, 300 mL). 5 min later, aqueous solution of ascorbic acid (0.1 M, 2.4 mL, freshly prepared)
was pipetted and the synthesis proceeded for additional 20 min. The resulting colloids were
centrifuged, washed by water once and methanol twice, subsequently kept in methanol for
activity test.
PVP-capped Pd nanocube (Pd-cube-PVP): The synthesis of PVP-capped Pd nanocube was
3
0
based on the previous report. Ascorbic acid (60 mg), KBr (600 mg), and PVP (105 mg) were
dissolved in water (8.0 mL) and stirred in air at 80 °C for 10 min. Then, Na PdCl aqueous
2
4
solution (65 mM, 3.0 mL) was pipetted. The synthesis proceeded for 3 h at 80 °C. The resulting
ACS Paragon Plus Environment
6

Page 7 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
products were centrifuged and washed by water once and methanol twice, subsequently stored
with methanol for activity test.
2.3 Catalysts characterization
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
A JEOL 2011 transmission electron microscope (200 kV) was utilized to obtain the
2
transmission electron microscopic (TEM) images of the samples. A FEI Tecnai G F20 S-Twin
field-emission transmission electron microscope (200 kV) was utilized to obtain the scanning
transmission electron microscopic (STEM) images, the high-resolution TEM (HRTEM) images,
the selected-area electron diffraction (SAED) patterns, and the EDX-mapping results. A Fourier
transformation infrared spectrometer (FT-IR, Thermo Fisher Nicolet iS10) was used to record
-1
the spectra. The recording range is from 400 to 4000 cm with transmission mode. The spectral
-1
resolution is 32 scans and 16 cm . A Bruker D8 Advanced X-ray diffractometer (40 kV, 40 mA,
λ = 0.15418 nm, Cu-Kα radiation) was used to record the powder X-ray diffraction (XRD)
patterns.
The adsorption geometries and energies of S-proline on Pd(100) and Pd(111) surfaces were
optimized by density functional theory (DFT) methods. The SIESTA package was used for
calculation by utilizing the Troullier-Martins norm-conserving pseudopotentials and numerical
atomic orbital basis sets31-32. A four-atomic-layer periodic slab with a (33) surface unit cell was
utilized to simulate the two surfaces and the top half of atoms were allowed to relax. A vacuum
layer more than 15 Å vertical to the surface was utilized to separate the slabs from their periodic
images. The Monkhorst-Pack k-point sampling we utilized was (3×3×1). For the exchange-
correlation functional, we applied a double-polarization basis set and the generalized gradient
3
3
approximation PBE method . An energy shift of 0.01 eV was applied to determine the orbital-
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
confining cutoff radii. 150 Rydberg was the energy cutoff set for representing the density of the
real space grid. To accelerate calculations, an iterative parallel diagonalization method was
applied to solve the Kohn-Sham equations and the ScaLAPACK subroutine pdsygvx was utilized
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
34
employing 2D block cyclically distributed matrix . We used the Broyden method to relax the
-1
geometry until all the relaxation atom forces were lower than 0.05 eV Å . In all the
3
5
optimizations, we consider spin polarization and fix the lattice constant of Pd to 3.95 Å . The
following is the formula for calculating the adsorption energies:
Eads = Eadsorbed - E - E
Pd
proline
where Eadsorbed: optimized energy of S-proline molecule adsorbed on Pd(111) or Pd(100) surface,
EPd: energy of isolated Pd surface, Eproline: energy of vacuum S-proline molecule.
2
.4 Activity test
Enantioselective hydrogenation of (E)--phenylcinnamic acid (PA)
The reaction was conducted in a three-necked flask in 1,4-dioxane (2.5 vol% H O) under
2
-1
atmospheric pressure at ambient temperature and the H flow rate was kept at 60 mL min . 0.022
2
mL of BA, 4 mg of Pd-dendrite catalyst, and 76 mg of reactant PA were dispersed in 6 mL of
1
,4-dioxane (2.5 vol% H O). BA was used as an additive in the enantioselective hydrogenation
2
of PA, which not only enhanced the reaction rate by restructuring of the surface acid−base
adducts from substrate−chiral modifier adduct to product−amine adduct, but also increased the
enantioselectivity by changing the adsorption configuration of the chiral modifier on the Pd
surface from N-lone pair bonded CD to -bonded CD.36-37 A stirring rate of 1000 rpm was kept
for excluding diffusion effects. The catalytic products were identified by a high performance
liquid chromatography (Agilent-1260) equipped with a chiral column (Chiralpak IG-3, 4.6 mm ×
ACS Paragon Plus Environment
8

Page 9 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
2
50 mm). The retention time of the products were identified with standard chemicals. The
enantioselectivity was calculated by e.e. % = |(S − R)|/(S + R) ×100.
Enantioselective hydrogenation of acetophenone
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
The reaction was conducted in methanol under atmospheric H pressure (60 mL min ) at 273 K
2
in a three-necked flask. Pd-cube catalyst (4 mg), S-proline (0.40 g) and acetophenone (20 L)
were dispersed in methanol (17 mL). A stirring rate of 1000 rpm was kept for excluding
diffusion effects. The catalytic products were analyzed by a gas chromatography (Agilent
7
2
820A) equipped with a chiral capillary column (CP-CHIRASIL-Dex CB, 0.25 μm × 0.32 mm ×
5 m) and a flame ionization detector (FID). The products were identified with standard samples.
3. RESULTS AND DISCUSSION
3
3
.1 Cinchonidine-modified Pd nanodendrite catalyst
.1.1 Catalyst characterization
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment
1
0

Page 11 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
Figure 1. TEM (a), HRTEM (b), and elemental mapping results (c) of the Pd-dendrite-CD
catalyst. The inset in (a) is a HAADF-STEM image of the nanodendrites. Bottom-left and top-
right insets in (b) are the enlarged HRTEM image of the squared area and the corresponding
SAED pattern. TEM images of (d) Pd nanocrystals prepared using 58 mg of CD, (e) Pd
nanocrystals synthesized in the absence of CD, and (f) Pd-dendrite-CN catalyst.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 1 presents the typical TEM, STEM, HRTEM, EDX-mapping images and SAED pattern
of the Pd-dendrite-CD catalyst. As shown in Figure 1a, nanodendrites with worm-like
nanostructures were produced and the average size was ~35 nm. The dendritic nature of the
nanoparticles was further verified by a high-angle annular dark-field STEM image (HAADF-
STEM, the inset in Figure 1a). Figure 1b shows the typical HRTEM image of a single
nanodendrite. The nanodendrite was aggregated by tiny grains with the size of ~6.0 nm. SAED
pattern (top-right inset in Figure 1b) reveals the polycrystalline nature of the nanodendrite. A d-
spacing of 0.23 nm is observed in Figure 1b, ascribable to (111) planes of face-centered cubic
3
8
(fcc) Pd . Elemental analysis of the Pd-dendrite-CD sample was performed to determine the
amount of CD adsorbed on the catalyst. Figure 1c presents the typical elemental mapping images
of the Pd-dendrite-CD sample. The atomic ratio of N/Pd is 0.036 and the amount of CD adsorbed
on Pd-dendrite-CD sample is calculated to be ca. 0.20 mg, which is ca. 5.0 wt % of the Pd
catalyst. To see if it is possible to increase the amount of CD adsorbed on the catalyst, we
tentatively doubled the amount of CD added in the synthetic system. Experimental results show
that the resulted Pd nanocrystals no longer retain nanodendrite morphology (Figure 1d), although
the N/Pd atomic ratio in this sample is slightly increased from 0.036 to 0.047.
We also synthesized palladium nanocrystals under the same synthetic conditions except for not
adding CD in the synthetic system. Without the addition of CD, large Pd nanoparticles with
ACS Paragon Plus Environment
1
1

ACS Catalysis
Page 12 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
irregular shapes were formed and the size varied in the range of 40~100 nm (Figure 1e). This
indicates that the introduction of CD is a key element for producing dendritic Pd nanocrystals.
With CD, the reduction of the Pd(II) precursor leads to the production of a great number of small
Pd grains and they tend to agglomerate to lower the total surface energy and form
nanodendrites.39-41
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. XRD pattern of Pd-dendrite-CD catalyst. Vertical lines represent fcc Pd (JCPDS 89-
4
897).
The XRD pattern of Pd-dendrite-CD catalyst (Figure 2) exhibits characteristic reflection of the
Pd black (JCPDS No. 89-4897) and the lattice parameter is calculated to be 0.389 nm. The ratio
of the (200) to (111) peaks in the nanodendrite sample is similar to that of Pd black sample,
indicating no observed preferred orientation of the Pd-dendrite-CD catalyst.
ACS Paragon Plus Environment
1
2

Page 13 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. FT-IR spectra of Pd-dendrite-CD (a) and CD (b). Insets are enlarged spectra of the
rectangular areas.
The FT-IR spectra of Pd-dendrite-CD catalyst and pure CD are shown in Figure 3. As for CD,
the typical vibrational modes of vinyl ν(C=C) and the quinuclidine moieties are observed at 1636
-1
42-44
and 1454 cm , respectively.
Three bands corresponding to the vibrational bands of quinoline
ring are observed at 1590, 1567 and 1510 cm-1.42-44 In addition, two other bands ascribable to C-
-1 44
H stretch modes of quinuclidine moiety from CD are also observed at 2934 and 2866 cm . As
for the Pd-dendrite-CD catalyst, these vibrational bands can still be observed although there are
some shifts, which is probably due to the interaction between palladium and CD.42-44 FT-IR
spectra clearly indicate that there is CD adsorbed on the as-prepared Pd nanodendrite catalyst.
3.1.2 Enantioselective hydrogenation of (E)--phenylcinnamic acid (PA)
The CD-modified Pd catalyst is the most widely studied heterogeneous catalyst for
7
enantioselective hydrogenation of ,-unsaturated carboxylic acids. Therefore, we tested the
ACS Paragon Plus Environment
1
3

ACS Catalysis
Page 14 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
catalytic property of the as-prepared Pd-dendrite-CD catalyst by using the enantioselective
hydrogenation of PA as a probe.
The yield of 2,3-diphenylpropionic acid and the enantioselectivity evolution (inset) with time
on the as-synthesized Pd-dendrite-CD catalyst are plotted in Figure 4. With reaction time
increasing, the yield of 2,3-diphenylpropionic acid exhibits a gradual increase. Interestingly, an
average enantioselectivity of ~15% with the dominant S-enantiomer is obtained over the as-
prepared Pd-dendrite-CD catalyst without the introduction of additional CD chiral modifier in
the reaction mixture. This result suggests that the CD remained on the as-prepared dendritic Pd
nanocrystals is already sufficient to induce chiral recognition. When we introduce another 6 mg
of CD in the reaction mixture, the enantioselectivity can be further increased to 33%, although
the yield of 2,3-diphenylpropionic acid decreased from 49.8% to 17.6% (Table 1, entry 2). The
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
7
phenomenon of reduction in activity is similar to the literature report , which is probably due to
4
5
that the Pd active sites are occupied by the adsorbed modifier CD . After three successive runs,
there was no decrease in the enantioselectivity, demonstrating the good enantioselective ability
of the Pd-dendrite-CD catalyst (Table 1, entry 2-4). The decrease in the yield of 2,3-
diphenylpropionic acid over the recycled Pd-dendrite-CD catalyst probably concerns with the
occupation of Pd active sites by the accumulated CD adsorbed on the recycled catalysts.7,45
As for the heterogeneous asymmetric hydrogenation of PA, literature studies focused on
supported Pd catalysts and it was found that the enantioselectivity varied significantly associated
with support materials, reaction conditions and preparation methods of Pd catalysts.36, 46-47 The
highest ee reported so far is ~90% over a CD-modified Pd-TiO (40 wt %) catalyst at 15 C
2
4
7
under optimized conditions. The difficulty in achieving high ee is considered to arise from the
fact that the reaction rate on unmodified Pd sites is ca. ten times faster than that on CD-modified
ACS Paragon Plus Environment
1
4

Page 15 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
36
Pd sites. Thus, the much lower ee on the Pd-dendrite-CD catalyst is probably related to the fact
that only small amount of CD is adsorbed on the catalyst, as verified by the EDX-mapping
analysis. The low surface coverage of adsorbed CD on the Pd-dendrite-CD catalyst is
unfavorable to obtain high enantio-differentiation.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. The yield of 2,3-diphenylpropionic acid and the enantioselectivity evolution (inset)
with time on the as-synthesized Pd-dendrite-CD catalyst. Reaction conditions: ambient
temperature, 76 mg reactant, 0.022 mL BA, 4 mg catalyst, 6 mL 1,4-dioxane (2.5 vol% H O), H
2
2
-1
(
60 mL min ).
Table 1. The catalytic property of as-prepared Pd-dendrite-CD and Pd-dendrite-CN samples in
enantioselective hydrogenation of PA.
Entry
Catalyst
Amount of
additional CD/CN
added
Yield
(%)
ee
(%)
Dominant
Enantiomer
(mg)
1
2
3
4
5
Pd-dendrite-CD
Pd-dendrite-CD
0
6
6
6
49.8
17.6
15.4
12.8
48.4
17
33
35
34
9
S
S
S
S
R
nd
Pd-dendrite-CD-2 run
rd
Pd-dendrite-CD-3 run
Pd-dendrite-CN
0
ACS Paragon Plus Environment
1
5

ACS Catalysis
Page 16 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Reaction conditions: ambient temperature, 76 mg reactant, 0.022 mL BA, 4 mg catalyst, 6 mL
-1
1
,4-dioxane (2.5 vol% H O), H (60 mL min ), reaction time 2 h.
2 2
To further verify that the chirality is induced by the chiral modifier CD adsorbed on the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
dendritic Pd nanocrystals, we tentatively replaced CD with CN (epimer of CD) in the shape-
controlled synthesis and obtained similar dendritic Pd nanocrystals (Figure 1f). The as-
synthesized Pd-dendrite-CN catalyst also exhibited enantioselectivity in the enantioselective
hydrogenation of PA, except that R-enantiomer was dominant (Table 1, entry 5) in contrast to
the S-enantiomer dominant for the Pd-dendrite-CD catalyst. The results clearly demonstrate that
CD has the dual functions of both the shape-control agent and the chiral modification molecule
in our shape-controlled synthetic process.
3
3
.2 S-Proline-modified Pd nanocube catalyst
.2.1 Catalyst characterization
ACS Paragon Plus Environment
1
6

Page 17 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment
1
7

ACS Catalysis
Page 18 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 5. (a) TEM, (b) histograms of particle size distribution, (c) STEM, (d) HRTEM and (e)
elemental mapping results of the Pd-cube-proline catalyst. Inset in (d) shows the Fourier
diffractogram. (f) TEM image of Pd nanocrystals synthesized without adding S-proline.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5 shows the TEM, HRTEM and STEM results of the Pd-cube-proline catalyst. As shown
in Figure 5a-5c, the dominant products are cubic nanocrystals and the average size is ~24 nm.
The HRTEM image of a nanocube lying flat on the grid is shown in Figure 5d. Two
dimensionally perpendicular lattice fringes are clearly observed. The d-spacing (0.20 nm)
corresponds to {200} planes of fcc Pd. The Fourier diffractogram (inset in Figure 5d) exhibits
regular square diffraction spot array, further confirming the enclosing of the Pd nanocubes with
{100} facets and the zone axis is [001]. EDX-mapping results (Figure 5e) reveal that the atomic
ratio of N/Pd is ca. 0.015 in the Pd-cube-proline sample. The amount of S-proline adsorbed on
Pd-cube-proline sample is calculated to be ca. 0.06 mg, which is ca. 1.5 wt % of the Pd catalyst.
Only a very small proportion (~0.005%) of S-proline added in the synthetic procedure can
adsorb on the Pd-cube-proline catalyst. This fact indicates that it is unlikely to significantly
increase the amount of S-proline adsorbed on the catalyst by simply increasing the quantity of S-
proline added in the synthesis. How to modify the preparation procedure to dramatically increase
the amount of S-proline adsorbed on the Pd nanocrystals still requires further study.
A comparison experiment was also performed under similar synthetic conditions without
adding S-proline in the synthetic system. As Figure 5f shows, without S-proline, Pd nanocrystals
with mixed morphologies and variable sizes were formed and tended to aggregate together. This
demonstrates that the formation of {100} facets in the Pd-cube-proline catalyst is correlated with
S-proline.
ACS Paragon Plus Environment
1
8

Page 19 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. Adsorption of S-proline on (a, b) Pd(111) and (c, d) Pd(100) surface. (a, c) is the
hydrogen-adsorption geometry and (b, d) is the oxygen-adsorption geometry.
For better understanding why S-proline can stabilize the {100} facets of fcc Pd, the adsorption
of S-proline on the Pd(100) and Pd(111) surfaces was simulated using the DFT methods. S-
proline is an amino acid molecule and either carboxylic group or pyrrolidine ring in S-proline
has the possibility to bind to the surface Pd atoms. XPS study revealed that S-proline adsorbed
4
8
on Pd(111) as a zwitterion . Thus, the adsorption through the nitrogen atom from pyrrolidine is
excluded. Then, only two most likely adsorption structures of S-proline were optimized. The first
starting geometry is the hydrogen-adsorption from pyrrolidine ring. The second one is the
oxygen-adsorption from carboxylate group. Figure 6 shows the optimized geometries. The
-1
adsorption energies (E ) for the hydrogen-adsorption geometry are -10.4 and -13.1 kcal mol
ads
on the Pd(111) and Pd(100) surfaces, respectively. As for the oxygen-adsorption geometry, the
-1
adsorption energies are -14.1 and -16.7 kcal mol on the Pd(111) and Pd(100) surfaces,
ACS Paragon Plus Environment
1
9

ACS Catalysis
Page 20 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
respectively. Obviously, the adsorption energy of S-proline on Pd(100) surface is much lower
than that on Pd(111) surface for both hydrogen- and oxygen-adsorption geometry, indicating that
S-proline adsorbs stronger on Pd(100) surface. Thus, S-proline probably prefers to stabilize {100}
facets than {111} facets in our synthetic process, leading to the production of Pd nanocubes.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7. FT-IR spectra of Pd-cube-proline (a) and S-proline (b).
FT-IR spectra of Pd-cube-proline and pure S-proline are shown in Figure 7. The vibration
-1
-
bands at 1622 and 1562 cm in the Pd-cube-proline catalyst correspond to COO asymmetric
+
49
stretch vibrational mode and scissor bending vibration of NH in S-proline, respectively. The
2
similar FT-IR spectra between Pd-cube-proline catalyst and pure S-proline indicate that there is
S-proline binded to the surface of Pd-cube-proline catalyst.
ACS Paragon Plus Environment
2
0

Page 21 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 8. TEM images (a, c) and particle size distribution (b, d) of Pd-cube-CTAB (a, b) and Pd-
cube-PVP (c, d).
For comparison, we also synthesized Pd nanocubes with similar particle size by using CTAB or
PVP as the capping agents. Figure 8 shows the typical TEM images and histograms of particle
size distribution. All the nanocube samples have similar average particle size of ~23 nm.
Figure 9. XRD patterns of Pd-cube-proline (a), Pd-cube-CTAB (b), Pd-cube-PVP (c) catalysts.
Vertical lines represent fcc Pd (JCPDS 89-4897).
ACS Paragon Plus Environment
2
1

ACS Catalysis
Page 22 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The XRD patterns of the Pd-cube-proline, Pd-cube-CTAB, and Pd-cube-PVP samples are
shown in Figure 9. In all Pd nanocube samples, there are five diffraction peaks which are
ascribable to reflections of fcc Pd. The Pd-cube-proline, Pd-cube-CTAB, and Pd-cube-PVP
samples exhibit lattice parameter identical to that of fcc Pd (0.389 nm). The ratio of the (200) to
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(111) reflections in all Pd nanocube samples is similar and ~1.4 times that of JCPDS (0.60 versus
0
.44). As the Pd nanocubes are prone to lying flat on the XRD sample holder, its (200) preferred
orientation indicates that the nanocubes are bounded by {100} facets, consistent with the TEM
results.
Figure 10. FT-IR spectra of CTAB (a), Pd-cube-CTAB (b), PVP (c) and Pd-cube-PVP (d).
The infrared spectrum of CTAB (Figure 10a) shows three strong vibrational bands at 1478,
-1
2850 and 2920 cm , corresponding to C-H scissoring vibrational mode of -N-CH moiety in the
3
50-51
CTAB, symmetric and asymmetric -CH stretch vibrational mode of the CTAB, respectively.
2
As for the Pd-cube-CTAB sample (Figure 10b), we can still observe the two characteristic
-1
vibrations at 2920 and 2850 cm , suggesting that CTAB is retained on the Pd nanocubes. As for
pure PVP (Figure 10c), it gives the characteristic C=O stretching vibrational band at 1670 cm-
ACS Paragon Plus Environment
2
2

Page 23 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
1 52-54
.
Other characteristic bands, including the C-N stretching vibrational band of N-C=O (1494
-1
-1
-1
cm ), C-H scissoring vibrational mode (1424 cm ), C-H bending vibrational band (1373 cm ),
-1
52-54
and C-N stretching vibrational mode of N-CH (1288 cm ), are also observed.
In the Pd-
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
cube-PVP sample (Figure 10d), although the relative intensity of the vibrational modes ranging
-1
-1
from 1500 to 1300 cm decreases dramatically, the stretching vibration of C=O (1655 cm ) can
still be clearly observed. The red shift and weakening of C=O stretching vibrational mode is
5
4
probably related to the chemisorption of PVP on Pd surface. FT-IR result suggests that PVP is
retained on the surface of Pd cubic nanocrystals. Washing and centrifugation cannot totally clear
the PVP bonded on the nanocubes.
3.2.2 Enantioselective hydrogenation of acetophenone
5
As for the S-proline modified Pd catalyst, the literature results verify that it can work as an
enantioselective catalyst for heterogeneous asymmetric hydrogenation of acetophenone. Thus,
we tested the properties of Pd-cube-proline, Pd-cube-CTAB, and Pd-cube-PVP catalysts utilizing
the acetophenone hydrogenation reaction as a probe.
ACS Paragon Plus Environment
2
3

ACS Catalysis
Page 24 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 11. The enantioselective behaviors of (a) Pd-cube-proline, (b) Pd-cube-CTAB, and (c)
Pd-cube-PVP catalysts. Reaction conditions: 17 mL methanol, 273 K, 20 L acetophenone, 4 mg
-1
catalyst, 0.40 g S-proline, H (60 mL min ).
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Without the introduction of additional S-proline in the reaction mixture, no enantioselectivity
was observed over all three nanocube catalysts. The reason of no enantioselectivity over the as-
prepared Pd-cube-proline catalyst is probably due to that the amount of S-proline adsorbed on
the catalyst is relatively small and not sufficient to induce chirality in the asymmetric
hydrogenation of acetophenone.
Table 2. The catalytic performance of Pd-cube-proline catalyst in asymmetric hydrogenation of
acetophenone after three successive runs.
Run
Yield
%)
30.9
28.3
26.2
ee
(%)
15
15
15
Dominant
Enantiomer
(
1
2
3
R
R
R
Reaction conditions: 17 mL methanol, 273 K, 20 L acetophenone, 4 mg catalyst, 0.40 g S-
-1
proline, H (60 mL min ), reaction time 1 h.
2
With the introduction of additional S-proline in the hydrogenation reaction mixture, as shown in
Figure 11, the activity sequence of the three nanocube catalysts is Pd-cube-CTAB > Pd-cube-
PVP > Pd-cube-proline. The Pd-cube-proline catalyst exhibits an average enantioselectivity of
15% with the dominant R-enantiomer, which is 1.5 times that of Pd-cube-CTAB catalyst (10%)
and 2.5 times that of Pd-cube-PVP catalyst (6%). The ee sequence reveals that the adverse effect
of residual CTAB on the enantioselectivity is smaller than that of PVP in acetophenone
hydrogenation. Table 2 shows the catalytic performance of the Pd-cube-proline catalyst after 1
ACS Paragon Plus Environment
2
4

Page 25 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
h’s reaction for three successive runs. The invariable ee for three consecutive runs demonstrates
that the Pd-cube-proline catalyst is highly stable, although a decrease in activity is observed. The
decrease of activity after recycling is probably related to the fact that Pd nanocubes tend to
aggregate in the recycled Pd-cube-proline catalyst, which is verified by TEM images of the
recycled catalysts. The accumulation of S-proline adsorbed on the recycled catalyst during
recovery may also lead to the decrease of activity. Despite the relative low enantioselectivity on
the in-situ synthesized Pd-cube-proline catalyst, our work demonstrates that in-situ chirally
modified Pd catalyst is more favorable to achieve higher enantioselectivity than CTAB or PVP-
capped Pd catalysts.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
With respect to acetophenone asymmetric hydrogenation, the enantioselectivities reported so
far over supported Pd and Pt catalysts are generally low.5-6, 55-56 The best result was reported by
Tungler et al. over S-proline modified 10% Pd/C (Dutral), which gave 22.5% ee at 77.9%
5
conversion. However, they also found that the origin of the commercial Pd/C catalysts strongly
5
affected the ee, which was generally below 16.7% and could be as low as 7.9%. Previously, we
found that by altering the confinement effect, the enantioselectivity in acetophenone
hydrogenation on free-standing mesoporous Pd catalysts could be effectively tuned from 22% to
4
2% and the mesoporous Pd catalyst with higher (111) preferred orientation exhibited increased
5
7
enantioselectivity. Thus, the low enantioselectivity of the present Pd-cube-proline catalyst may
derive from the fact that the Pd-cube-proline catalyst has large Pd particle size (small surface
area) and exposes {100} facets. If someone could prepare in-situ chirally modified Pd
nanocrystal catalyst with smaller particle size and with other specific facets, higher
enantioselectivity is expected.
ACS Paragon Plus Environment
2
5

ACS Catalysis
Page 26 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
4
. CONCLUSION
In summary, in-situ chirally modified palladium nanocrystals without capping agents were
successfully prepared via a simple one-pot strategy by replacing the conventional capping agents
with chiral modifiers such as CD and S-proline. Their applications in the asymmetric
hydrogenation of C=O or C=C bond were studied. In the enantioselective hydrogenation of (E)-
α-phenylcinnamic acid, the as-prepared CD-modified Pd nanodendrite catalyst does not require
follow-up chiral modification process and shows enantioselectivity, indicating that the CD
remained on the as-prepared dendritic Pd nanocrystals is already sufficient to induce chiral
recognition. In the asymmetric hydrogenation of acetophenone, the as-prepared S-proline-
modified Pd nanocube catalyst shows higher enantioselectivity than nanocubes synthesized with
PVP or CTAB as capping agents. The direct synthesis of in-situ chirally modified palladium
nanocrystals with controllable shapes by using chiral modifiers as shape-control agents offers a
simple and effective method to prepare enantioselective metal nanocrystal catalysts, which may
be promising in heterogeneous asymmetric catalytic reactions.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
AUTHOR INFORMATION
Corresponding Author
*E-mail: xueyingchen@fudan.edu.cn
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
ACKNOWLEDGMENT
ACS Paragon Plus Environment
2
6

Page 27 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
This work was supported by the National Natural Science Foundation of China (21573045,
2
1773034).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
REFERENCES
1) Blaser, H. U.; Pugin, B. In Chiral Reactions in Heterogeneous Catalysis; Jannes, G., Dubois,
V., Eds.; Plenum Press: New York, 1995; pp 5-20.
2) Klabunovskii, E.; Smith, G. V.; Zsigmond, Á. Heterogeneous Enantioselective
(
(
Hydrogenations-Theory and Practice; Springer: Dordrecht, 2006; Vol. 31, pp 161-170.
(3) Baiker, A. Crucial Aspects in the Design of Chirally Modified Noble Metal Catalysts for
Asymmetric Hydrogenation of Activated Ketones. Chem. Soc. Rev. 2015, 44, 7449-7464.
(
4) Meemken, F.; Baiker, A. Recent Progress in Heterogeneous Asymmetric Hydrogenation of
C=O and C=C Bonds on Supported Noble Metal Catalysts. Chem. Rev. 2017, 117, 11522-11569.
5) Tungler, A.; Tarnai, T.; Mathe, T.; Petro, J. Enantioselective Hydrogenation of
(
Acetophenone in the Presence of S-proline. J. Mol. Catal. 1991, 67, 277-282.
(6) Hess, R.; Vargas, A.; Mallat, T.; Bürgi, T.; Baiker, A. Inversion of Enantioselectivity in the
Platinum-catalyzed Hydrogenation of Substituted Acetophenones. J. Catal. 2004, 222, 117-128.
(7) Guan, Z. H.; Lu, S. M.; Li, C. Enantioselective Hydrogenation of α,β-unsaturated
Carboxylic Acid over Cinchonidine-modified Pd Nanoparticles Confined in Carbon Nanotubes.
J. Catal. 2014, 311, 1-5.
(8) Bui Trung, T. S.; Kim, Y.; Kang, S.; Lee, H.; Kim, S. The Enantioselective Hydrogenation
of (E)-α-phenylcinnamic Acid: Role of TiO Coated on Al O as a Novel Support for
2
2
3
Cinchonidine-modified Pd Catalysts. Catal. Commun. 2015, 66, 21-24.
(9) Li, C. Chiral Synthesis on Catalysts Immobilized in Microporous and Mesoporous Materials.
Catal. Rev. - Sci. Eng. 2004, 46, 419-492.
(10) Hoxha, F.; Schimmoeller, B.; Cakl, Z.; Urakawa, A.; Mallat, T.; Pratsinis, S. E.; Baiker, A.
Influence of Support Acid–base Properties on the Platinum-catalyzed Enantioselective
Hydrogenation of Activated Ketones. J. Catal. 2010, 271, 115-124.
(
11) Davis, M. E.; Davis, R. J. Fundamentals of Chemical Reaction Engineering; McGraw-Hill
Higher Education: New York, 2003; pp 149-153.
12) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chemistry and Properties of
Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, 1025-1102.
13) Zhang, Y.; Wang, M. S.; Zhu, E. B.; Zheng, Y. B.; Huang, Y.; Huang, X. Q. Seedless
(
(
Growth of Palladium Nanocrystals with Tunable Structures: From Tetrahedra to Nanosheets.
Nano Lett. 2015, 15, 7519-7525.
(14) Zhao, X. J.; Zhao, Y.; Fu, G.; Zheng, N. F. Origin of the Facet Dependence in the
Hydrogenation Catalysis of Olefins: Experiment and Theory. Chem. Commun. 2015, 51, 12016-
1
2019.
15) Schmidt, E.; Vargas, A.; Mallat, T.; Baiker, A. Shape-selective Enantioselective
Hydrogenation on Pt Nanoparticles. J. Am. Chem. Soc. 2009, 131, 12358-12367.
16) Zhang, H.; Jin, M. S.; Xiong, Y. J.; Lim, B.; Xia, Y. N. Shape-controlled Synthesis of Pd
(
(
Nanocrystals and Their Catalytic Applications. Acc. Chem. Res. 2013, 46, 1783-1794.
(17) Ruditskiy, A.; Peng, H.-C.; Xia, Y. N. Shape-controlled Metal Nanocrystals for
Heterogeneous Catalysis. Annu. Rev. Chem. Biomol. Eng. 2016, 7, 327-348.
ACS Paragon Plus Environment
2
7

ACS Catalysis
Page 28 of 31
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
18) Wu, B. H.; Zheng, N. F. Surface and Interface Control of Noble Metal Nanocrystals for
Catalytic and Electrocatalytic Applications. Nano Today 2013, 8, 168-197.
19) Koczkur, K. M.; Mourdikoudis, S.; Polavarapu, L.; Skrabalak, S. E. Polyvinylpyrrolidone
(
(
PVP) in Nanoparticle Synthesis. Dalton. Trans. 2015, 44, 17883-17905.
(20) Lopez-Sanchez, J. A.; Dimitratos, N.; Hammond, C.; Brett, G. L.; Kesavan, L.; White, S.;
Miedziak, P.; Tiruvalam, R.; Jenkins, R. L.; Carley, A. F.; Knight, D.; Kiely, C. J.; Hutchings, G.
J. Facile Removal of Stabilizer-ligands from Supported Gold Nanoparticles. Nat. Chem. 2011, 3,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
51-556.
21) Crespo-Quesada, M.; Andanson, J. M.; Yarulin, A.; Lim, B.; Xia, Y. N.; Kiwi-Minsker, L.
(
UV-ozone Cleaning of Supported Poly(vinylpyrrolidone)-stabilized Palladium Nanocubes:
Effect of Stabilizer Removal on Morphology and Catalytic Behavior. Langmuir 2011, 27, 7909-
7916.
(22) Li, D. G.; Wang, C.; Tripkovic, D.; Sun, S. H.; Markovic, N. M.; Stamenkovic, V. R.
Surfactant Removal for Colloidal Nanoparticles from Solution Synthesis: The Effect on Catalytic
Performance. ACS Catal. 2012, 2, 1358-1362.
(23) Niu, Z. Q.; Li, Y. D. Removal and Utilization of Capping Agents in Nanocatalysis. Chem.
Mater. 2014, 26, 72-83.
(24) Luo, M. H.; Hong, Y. C.; Yao, W. F.; Huang, C. P.; Xu, Q. J.; Wu, Q. Facile Removal of
Polyvinylpyrrolidone (PVP) Adsorbates from Pt Alloy Nanoparticles. J. Mater. Chem. A 2015, 3,
2
770-2775.
25) Su, N.; Gao, X. Y.; Chen, X. Y.; Yue, B.; He, H. Y. The Enantioselective Hydrogenation
(
of Acetophenone over Pd Concave Tetrahedron Nanocrystals Affected by the Residual Adsorbed
Capping Agent Polyvinylpyrrolidone (PVP). J. Catal. 2018, 367, 244-251.
(26) Bönnemann, H.; Braun, G. A. Enantioselective Hydrogenations on Platinum Colloids.
Angew. Chem. Int. Ed. 1996, 35, 1992-1995.
(27) Bönnemann, H.; Braun, G. A. Enantioselectivity Control with Metal Colloids as Catalysts.
Chem. Eur. J. 1997, 3, 1200-1202.
(28) Rodrigues, T. S.; Zhao, M.; Yang, T.-H.; Gilroy, K. D.; da Silva, A. G. M.; Camargo, P. H.
C.; Xia, Y. Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the
Reductants. Chem. Eur. J. 2018, 24, 16944-16963.
(
29) Niu, W. X.; Zhang, L.; Xu, G. B. Shape-controlled Synthesis of Single-crystalline
Palladium Nanocrystals. ACS Nano 2010, 4, 1987-1996.
30) Jin, M. S.; Liu, H. Y.; Zhang, H.; Xie, Z. X.; Liu, J. Y.; Xia, Y. N. Synthesis of Pd
(
Nanocrystals Enclosed by {100} Facets and with Sizes <10 nm for Application in CO Oxidation.
Nano Res. 2010, 4, 83-91.
(31) Junquera, J.; Paz, O.; Sanchez-Portal, D.; Artacho, E. Numerical Atomic Orbitals for
Linear-scaling Calculations. Phys. Rev. B 2001, 64, 235111.
(32) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for Plane-wave Calcualations. Phys.
Rev. B 1991, 43, 1993-2006.
(
33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made
Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.
34) ScaLAPACK Home Page. http://www.netlib.org/scalapack/scalapack_home.html
accessed Aug 17, 2018).
35) Eichler, A.; Mittendorfer, F.; Hafner, J. Precursor-mediated Adsorption of Oxygen on the
(
(
(
(111) Surfaces of Platinum-group Metals. Phys. Rev. B 2000, 62, 4744-4755.
(36) Nitta, Y. Importance of Product Desorption in Enantioselective Hydrogenation of (E)-α-
ACS Paragon Plus Environment
2
8

Page 29 of 31
ACS Catalysis
1
2
3
4
5
6
7
8
9
phenylcinnamic Acid with a Cinchonidine-modified Pd/TiO Catalyst: Effect of Additives. Chem.
Lett. 1999, 635-636.
2
(37) Meemken, F.; Maeda, N.; Hungerbühler, K.; Baiker, A. Heterogeneous Asymmetric
Hydrogenation of Prochiral Alkenoic Acid: Origin of Rate and Enantioselectivity Enhancement
by Amine Addition. ACS Catal. 2012, 2, 464-467.
(38) Huang, X. Q.; Tang, S. H.; Mu, X. L.; Dai, Y.; Chen, G. X.; Zhou, Z. Y.; Ruan, F. X.;
Yang, Z. L.; Zheng, N. F. Freestanding Palladium Nanosheets with Plasmonic and Catalytic
Properties. Nat. Nanotechnol. 2011, 6, 28-32.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(39) Lim, B.; Jiang, M. J.; Yu, T.; Camargo, P. H. C.; Xia, Y. N. Nucleation and Growth
Mechanisms for Pd-Pt Bimetallic Nanodendrites and Their Electrocatalytic Properties. Nano Res.
2
010, 3, 69-80.
(40) Lim, B.; Xia, Y. N. Metal Nanocrystals with Highly Branched Morphologies. Angew.
Chem. Int. Ed. 2011, 50, 76-85.
41) Watt, J.; Cheong, S.; Tilley, R. D. How to Control the Shape of Metal Nanostructures in
Organic Solution Phase Synthesis for Plasmonics and Catalysis. Nano Today 2013, 8, 198-215.
42) Ferri, D.; Burgi, T. An in situ Attenuated Total Reflection Infrared Study of a Chiral
(
(
Catalytic Solid-liquid Interface: Cinchonidine Adsorption on Pt. J. Am. Chem. Soc. 2001, 123,
12074-12084.
(43) Ferri, D.; Burgi, T.; Baiker, A. In situ ATR-IR Study of the Adsorption of Cinchonidine on
Pd/Al O : Differences and Similarities with Adsorption on Pt/Al O . J. Catal. 2002, 210, 160-
2
3
2
3
1
70.
44) Behzadi, B.; Vargas, A.; Ferri, D.; Ernst, K.-H.; Baiker, A. Cinchonidine Adsorption on
(
Gold and Gold-containing Bimetallic Platinum Metal Surfaces: An Attenuated Total Reflection
Infrared and Density Functional Theory Study. J. Phys. Chem. B 2006, 110, 17082-17089.
(45) Reschetilowski, W.; Böhmer, U.; Wiehl, J. Pt-containing Zeolites for Enantioselective
Catalytic Hydrogenation of Ethylpyruvate – Effects of Zeolite Structures, Solvent and Modifier
Concentration. Stud. Surf. Sci. Catal. 1994, 84, 2021-2028.
(46) Nitta, Y.; Kobiro, K.; Okamoto, Y. Enantioselective Hydrogenation of (E)-α-
phenylcinnamic Acid on Cinchonidine-modified Palladium Catalysts: Influence of Support. Stud.
Surf. Sci. Catal. 1997, 108, 191-198.
(47) Kubota, T.; Kubota, H.; Kubota, T.; Moriyasu, E.; Uchida, T.; Nitta, Y.; Sugimura, T.;
Okamoto, Y. Enantioselective Hydrogenation of (E)-α-phenylcinnamic Acid over Cinchonidine-
modified Pd Catalysts Supported on TiO and CeO . Catal. Lett. 2009, 129, 387-393.
2
2
(
48) Gao, F.; Wang, Y. L.; Burkholder, L.; Tysoe, W. T. Chemistry of L-proline on Pd(111):
Temperature-programmed Desorption and X-ray Photoelectron Spectroscopic Study. Surf. Sci.
007, 601, 3579-3588.
49) Herlinger, A. W.; Long, T. V. Laser-Raman and Infrared Spectra of Amino Acids and
2
(
Their Metal Complexes. III. Proline and Bisprolinato Complexes. J. Am. Chem. Soc. 1970, 92,
6481-6486.
(50) Wang, Y.; Chen, X. Y.; Yue, B.; He, H. Y. Self-assembly of Mesoporous Ni–P
Nanosphere Catalyst with Uniform Size and Enhanced Catalytic Activity in Nitrobenzene
Hydrogenation. Top. Catal. 2012, 55, 1022-1031.
(51) Mehta, S. K.; Kumar, S.; Chaudhary, S.; Bhasin, K. K.; Gradzielski, M. Evolution of ZnS
Nanoparticles via Facile CTAB Aqueous Micellar Solution Route: A Study on Controlling
Parameters. Nanoscale Res. Lett. 2009, 4, 17-28.
(52) Borodko, Y.; Habas, S. E.; Koebel, M.; Yang, P.; Frei, H.; Somorjai, G. A. Probing the
ACS Paragon Plus Environment
2
9