Enhancing Effect of Residual Capping Agents in Heterogeneous Enantioselective Hydrogenation of α-keto Esters over Polymer-Capped Pt/Al2O3

Article abstract of DOI:10.1021/acscatal.0c04255

Heterogeneous enantioselective catalysis is considered a promising strategy for the large-scale production of enantiopure chemicals. In this work, polymer-capped Pt nanocatalysts having a uniform size were synthesized using poly(vinyl pyrrolidone) (PVP) and poly(vinyl alcohol) and supported on γ-Al2O3. After a facile heat treatment process, their catalytic performance for enantioselective hydrogenation of α-keto esters, a structure-sensitive reaction, was investigated. The presence of residual capping agents on the Pt surface often perturbs the adsorption of reacting species and reduces performance in structure-sensitive reactions. However, the 1 wt % PVP-Pt/Al2O3 catalyst exhibited an enhancement in both activity and enantioselectivity compared to a reference Pt/Al2O3 catalyst prepared by wet impregnation. Under optimized reaction conditions, the cinchonidine-modified PVP-Pt/Al2O3 gave an enantiomeric excess of 95% for the enantioselective hydrogenation of methyl pyruvate despite the low Pt loading. We demonstrate that depending on the type of polymers, the residual capping agents can lead to site-selective blockage of the Pt surface, that is, defects or terraces. Quantitative and qualitative analyses also show that the noticeable improvement in enantioselectivity is attributed to the stable adsorption of chiral modifiers on selectively exposed Pt terrace sites. The findings of this work provide a promising strategy to prepare metal nanoparticles having selectively exposed sites and offer insights into the enhancing effect of residual capping agents on the catalytic properties in structure-sensitive reactions.

Full text of DOI:10.1021/acscatal.0c04255

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Research Article
Enhancing Effect of Residual Capping Agents in Heterogeneous
Enantioselective Hydrogenation of α‑keto Esters over Polymer-
Capped Pt/Al2O3
Iljun Chung,† Byeongju Song,† Jeongmyeong Kim, and Yongju Yun*
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ABSTRACT: Heterogeneous enantioselective catalysis is considered a promising
strategy for the large-scale production of enantiopure chemicals. In this work,
polymer-capped Pt nanocatalysts having a uniform size were synthesized using
poly(vinyl pyrrolidone) (PVP) and poly(vinyl alcohol) and supported on γ-Al2O3.
After a facile heat treatment process, their catalytic performance for
enantioselective hydrogenation of α-keto esters, a structure-sensitive reaction,
was investigated. The presence of residual capping agents on the Pt surface often
perturbs the adsorption of reacting species and reduces performance in structure-
sensitive reactions. However, the 1 wt % PVP-Pt/Al2O3 catalyst exhibited an
enhancement in both activity and enantioselectivity compared to a reference Pt/
Al2O3 catalyst prepared by wet impregnation. Under optimized reaction
conditions, the cinchonidine-modified PVP-Pt/Al2O3 gave an enantiomeric excess
of 95% for the enantioselective hydrogenation of methyl pyruvate despite the low
Pt loading. We demonstrate that depending on the type of polymers, the residual
capping agents can lead to site-selective blockage of the Pt surface, that is, defects or terraces. Quantitative and qualitative analyses
also show that the noticeable improvement in enantioselectivity is attributed to the stable adsorption of chiral modifiers on
selectively exposed Pt terrace sites. The findings of this work provide a promising strategy to prepare metal nanoparticles having
selectively exposed sites and offer insights into the enhancing effect of residual capping agents on the catalytic properties in structure-
sensitive reactions.
KEYWORDS: heterogeneous, enantioselective, hydrogenation, nanocatalyst, capping agent
Cinchona alkaloids, including cinchonidine (CD) and
quinine (QN), are bulky chiral molecules and their quinoline
rings act as anchoring moieties on the Pt surface. The
enantioselective hydrogenation of α-keto esters is structure
sensitive because the adsorption of the chiral modifier
significantly depends on the size and surface structure of Pt
nanoparticles (NPs).11,12 The adsorbed cinchona alkaloids are
stabilized on terrace sites via π−π interactions between the
quinoline ring and the Pt surface, resulting in a flat adsorption
mode.13 It has also been suggested that sufficient Pt sites are
required to accommodate the bulky complex of the chiral
modifier and the substrate.12 Indeed, several studies have
shown that Pt NPs with average sizes of less than 2 nm, which
are expected to expose a large fraction of defect sites, exhibit
1. INTRODUCTION
Chiral molecules are key compounds in the fine chemical
industry, especially for the production of pharmaceuticals,
agrochemicals, and perfumes.1,2 Crucially, the two enantiomers
of a chiral molecule can have distinctly different physiological
effects on the human body because of the homochirality of
living organisms.3,4 Thus, many efforts have been devoted to
the production of enantiomerically pure compounds. Enantio-
selective catalysis over heterogeneous catalysts is considered to
be a promising strategy for the large-scale production of
enantiopure chemicals because of the easy separation of the
products from the catalyst and the high reusability of the
catalyst.5−7 Orito reaction, the enantioselective hydrogenation
of α-keto esters over supported Pt catalysts, is a well-known
example of enantioselective heterogeneous catalysis.8 To create
chirally active sites, the achiral Pt surface is modified by the
adsorption of organic chiral molecules. Cinchona alkaloid-
modified Pt/Al2O3 catalysts are an extensively studied model
system because of their high enantioselectivities in which the
steric hindrance by the chiral modifier leads to enantiodiffer-
entiation of the complex of the modifier with the substrate on
the Pt surface.9,10
Received: September 30, 2020
Revised: November 27, 2020
https://dx.doi.org/10.1021/acscatal.0c04255
© XXXX American Chemical Society
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poor enantioselectivity.14,15 These observations imply that the
coordination of specific surface sites significantly affects the
catalytic performance and that chiral modifiers adsorbed on
terraces are more efficient for enantiodifferentiation than those
adsorbed on defects such as steps and kinks.
selectively exposed sites and shows their application in
structure-sensitive reactions.
2. EXPERIMENTAL SECTION
2.1. Chemicals and Materials. For the preparation of the
Pt catalysts, the following Pt precursors, capping agents, and
support materials were used as received: chloroplatinic acid
hydrate (H2PtCl6, >99.9%, Sigma-Aldrich), potassium tetra-
chloroplatinate (K2PtCl4, >99.9%, Sigma-Aldrich), platinum
acetylacetonate [Pt(acac)2, 98%, Acros], PVP (Mw = 55,000,
Sigma-Aldrich), PVA (Mw = 31,000−50,000, Acros), and γ-
Al2O3 support (PURALOX SCCa 5-150, SBET = 169 m2/g,
Sasol). Ethylene glycol (99.8%, Sigma-Aldrich), ethanol
(99.9%, Samchun Pure Chemical), acetone (99.5%, Samchun
Pure Chemical), and n-hexane (95.0%, Samchun Pure
Chemical) were used as solvents in the synthesis and washing
procedures. For enantioselective hydrogenation, EtPy (98.0%,
Alfa Aesar) and methyl pyruvate (MtPy, >97%, TCI) were
used as reactants. CD (96%, Sigma-Aldrich) and QN (98%,
Sigma-Aldrich) were used as chiral modifiers. The reaction was
conducted in acetic acid (AcOH, 99.7%, Samchun Pure
Chemical).
2.2. Preparation of Polymer-Capped Pt NPs and
Supported Pt Catalysts. PVP-capped Pt NPs having an
average particle size of 3 nm were synthesized by the following
procedure. The Pt precursor, H2PtCl6 (0.1 mmol), and PVP
(0.004 mmol) were added to ethylene glycol (20 mL) in a 100
mL three-neck flask, and the solution was placed under
vacuum for 30 min for degassing. Then, the solution was
heated at 483 K for 10 min under an Ar atmosphere. The
synthesized PVP-Pt NPs were then precipitated in the
presence of excess acetone by centrifugation. Excess PVP on
the Pt NPs was removed by the repeated suspension of Pt NPs
in ethanol and precipitation with hexane. The washed PVP-Pt
NPs were finally dispersed in ethanol. The PVA-capped Pt NPs
having an average particle size of 3 nm were prepared by the
following procedure. PVA (0.006 mmol) was added to
deionized H2O (20 mL), and the mixture was heated at 373
K for 1 h. After the dissolution of K2PtCl4 (0.06 mmol) in the
solution, it was heated at 333 K for 20 min.
Recently, the structure sensitivity of enantioselective hydro-
genation has been studied using metal NPs capped with
polymers such as poly(vinyl pyrrolidone) (PVP) and poly-
(acrylic acid) (PAA).11,16,17 Capping agents control the size
and shape of NPs by preventing aggregation and directing the
growth of particles. In many cases, however, the capping agents
have adverse effects on catalytic performance because they
block the active sites and, thus, hamper the adsorption of
reacting species on the surface.11,17−19 Investigation of the
enantioselective hydrogenation of ethyl pyruvate (EtPy) over
PAA-capped Pt NPs, modified with CD and QN, has revealed
that the presence of PAA on the Pt surface leads to a loss of
catalytic activity and enantioselectivity.11 However, the partial
removal of the capping agent from the Pt surface under
reaction conditions results in an increase in enantioselectivity.
A similar detrimental effect of the capping agent on
enantioselectivity has also been observed for PVP-capped Pd
NPs, modified with S-proline, in the enantioselective hydro-
genation of acetophenone.17 After the partial removal of PVP
from the Pd surface by KBH4 treatment or hot water reflux, the
Pd catalyst showed improved enantioselectivity. These findings
clearly show the importance of understanding the role of
residual species on the metal surfaces in the development of
nanocatalysts for heterogeneous enantioselective catalysis.
In this work, we prepared polymer-capped Pt NPs with a
uniform size by using PVP and poly(vinyl alcohol) (PVA) as
capping agents. After supporting the Pt NPs on γ-Al2O3, the Pt
catalysts were heat-treated to remove the capping agents. The
heat-treated Pt/Al2O3 contained residual capping agents on the
Pt surface. In the presence of chiral modifiers, CD and QN, the
catalytic performance of PVP- and PVA-Pt/Al2O3 catalysts
with residual capping agents was investigated for the
enantioselective hydrogenation of α-keto esters and compared
with that of a reference catalyst with a clean Pt surface
(Scheme 1). Quantitative and qualitative analyses demon-
Supported Pt catalysts were prepared by the following
procedure. A powder of γ-Al2O3 was added to the solution
containing the synthesized polymer-capped Pt NPs. Then, the
solvent containing PVP-Pt or PVA-Pt NPs was evaporated by
stirring the solution overnight at 313 or 328 K, respectively.
The obtained powder was dried at 353 K for 12 h in an oven
and then calcined at 623 K for 10 min to remove the capping
agents. As a reference catalyst, a supported Pt catalyst without
a capping agent (IMP-Pt/Al2O3) was prepared using the wet
impregnation method. Pt(acac)2 (0.064 mmol) was dissolved
in acetone (20 mL). Then, an adequate amount of γ-Al2O3 was
added to the solution, and the mixture was stirred overnight at
313 K. The obtained powder was dried at 383 K for 12 h and
calcined at 623 K for 3 h under Ar flow.
Scheme 1. Enantioselective Hydrogenation of α-keto Esters
over Chirally Modified Pt/Al2O3 Catalysts
2.3. Characterization of Pt NPs and Supported Pt
Catalysts. Transmission electron microscopy (TEM) images
were obtained using a JEOL JEM-2100F FE-TEM at an
acceleration voltage of 200 kV at the National Institute for
Nanomaterials Technology, Pohang, South Korea. Thermog-
ravimetric analysis (TGA) was carried out on a HITACHI
STA 7300 instrument. Samples (30 mg) were charged into the
sample pan and then heated to 1000 K at a heating rate of 5 K/
min in flowing air (200 sccm). The Pt loading was determined
strated that the residual capping agents can selectively block
the Pt sites, leaving terraces or defects exclusively exposed.
Furthermore, the specific exposure of terraces led to a
remarkable enhancement in the catalytic performance in the
enantioselective hydrogenation of α-keto esters by enabling the
stable adsorption of the chiral modifier on the Pt surface. This
work offers a new strategy to prepare nanocatalysts with
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Figure 1. TEM images and size distribution histograms of as-prepared (a,b) PVP-Pt and (c,d) PVA-Pt. Both PVP-Pt and PVA-Pt NPs have uniform
sizes of 2−3 nm.
by inductively coupled plasma-optical emission spectroscopy
(ICP-OES) on a Thermo Scientific model iCAP 6300
spectrometer. The amount of CO chemisorbed on the Pt
catalysts was determined by the CO pulse chemisorption
technique using a MicrotracBEL BELCAT-II. The metal
dispersion and particle sizes were calculated from the amount
of chemisorbed CO on the Pt catalysts using the following
equations: by assuming a CO/Pt adsorption stoichiometry of
1. The metal dispersion (D) and particle size (d) were
calculated using the following equations20,21
measurements were conducted after the samples had been
purged with Ar for 5 min to remove the gaseous CO species.
2.4. Catalytic Performance Tests. The performance of
the Pt catalysts was evaluated under 0.1 MPa H2 pressure using
a 50 mL glass reactor. For reactions at 5.0 MPa H2 pressure, a
stainless-steel autoclave equipped with a 100 mL glass liner was
used. All catalysts were pretreated in a flow of 5% H2/Ar at 673
K for 90 min. The reaction was conducted with a Pt catalyst
(80 mg), a chiral modifier (6 mg), and the substrate (0.1 mL)
in acetic acid (10 mL) at 298 K. Details of the procedures have
been given in previous works.22,23 The compositions of the
reactant and chiral products in the solvent were analyzed by a
gas chromatograph (YL6500) equipped with a Chirasil-Dex
CB column (CP7502, Agilent). Turnover frequencies (TOFs)
[h−1] were measured based on the number of surface Pt atoms
determined by CO chemisorption for IMP-Pt/Al2O3 and based
on the number of surface Pt atoms predicted by the theoretical
Pt NP model with average particle size of 3 nm for PVP- and
PVA-Pt/Al2O3.24−27 The enantiomeric excess (ee) of the chiral
products was calculated as follows: ee [%] = |[R] − [S]|/([R]
+ [S]) × 100, where [R] and [S] are the concentrations of (R)-
and (S)-product, respectively. The standard deviation of
repeatability for conversion and enantioselectivity was less
than 2.3 and 1.4%, respectively. The standard deviation of
reproducibility for conversion and enantioselectivity was less
than 3.4 and 1.7%, respectively.
VCO × SF × MW
WPt × 22414
D[%] =
× 100
(1)
600 × MW
d[nm] =
D × N × ρ × σ
(2)
a
where VCO is the volume of the adsorbed CO [cm3], SF is the
stoichiometry factor, 1 for CO chemisorption on Pt, MW is the
atomic weight of the Pt [g/mol], WPt is the weight of
supported Pt on the sample [g], D is the metal dispersion [%],
Na is the Avogadro constant, ρ is the Pt metal density [g/cm3],
and σ is the cross-sectional area of one Pt atom [nm2]. X-ray
photoelectron spectroscopy (XPS) analysis was performed
with a monochromatic Al Kα X-ray source (hν = 1486.6 eV)
under ultra-high vacuum at the Korea Basic Science Institute,
Busan, South Korea. The binding energies were referenced to
the C 1s peak at 284.6 eV. Fourier transform infrared (FTIR)
transmittance spectra and diffuse reflectance Fourier transform
(DRIFT) spectra were obtained with a Thermo Fisher Nicolet
iS50. Sample measurements for the chemisorption of CO were
carried out in an adsorption cell (PIKE Technologies
DiffuseIR). The FTIR spectra were obtained from 400 to
4000 cm−1 using 32 scans at a spectral resolution of 4 cm−1 at
298 K. For DRIFT analysis of CO on Pt catalysts, the samples
were reduced at 673 K under 5% H2/Ar flow for 90 min and
cooled to 323 K to allow the adsorption of CO. The
3. RESULTS AND DISCUSSION
3.1. Preparation and Characterization of Pt/Al2O3
Catalysts. Enantioselective hydrogenation of α-keto esters
over a Pt surface is structure sensitive because the adsorption
stability and enantiodifferentiation ability of the chiral modifier
are affected by the type of adsorption sites on the Pt
surface.9,28 Thus, both the reaction rate and enantioselectivity
strongly depend on the size of the Pt NPs and the exposed Pt
sites.11,12,29 To exclude size effects on the catalytic perform-
ance, Pt NPs of uniform size were synthesized using two
different capping agents, PVP and PVA. TEM images of the Pt
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NPs capped with PVP (PVP−Pt) and PVA (PVA-Pt) and their
size distributions are presented in Figure 1. The particles have
narrow size distributions and mean diameters of 2−3 nm.
PVP- and PVA-Pt were deposited on γ-Al2O3 supports to
yield the 1 wt % Pt/Al2O3 catalysts, denoted as PVP-Pt/Al2O3
and PVA-Pt/Al2O3, respectively. As a reference sample, 1 wt %
Pt/Al2O3 without a capping agent, IMP-Pt/Al2O3, was
prepared by the conventional wet impregnation method.
Although capping agents are useful for the size control of
NPs, the adsorbed capping agents on the metal surface can
block the active sites, significantly decreasing the catalytic
performance of the nanocatalysts.11,17−19 Thus, remaining
capping agents on the surface are removed by thermal
treatment in air.30,31 Before the heat treatment of PVP- and
PVA-Pt/Al2O3, the optimal temperature was determined by
TGA (Figure 2). The TGA profile of PVP-Pt/Al2O3 contains a
result is consistent with the weight loss of the Pt catalysts on
removal of the capping agents, as shown in Figure 2, in which
the lower thermal stability of PVA than that of PVP was
revealed. The average sizes of the Pt NPs estimated by CO
chemisorption, 7.2 nm for PVP-Pt/Al2O3 and 4.0 nm for PVA-
Pt/Al2O3, are greater than those observed by TEM, 2.8−2.9
nm even after the removal of the capping agents. The
overestimation is attributed to the presence of residual capping
agents on the Pt surfaces, inhibiting CO chemisorption.
The presence of residual capping agents on the Pt surface
was confirmed by comparing the FTIR spectra of PVP- and
PVA-Pt/Al2O3 with those of intact capping agents. The
support material, Al2O3, yielded absorption bands correspond-
ing to the bending vibration of the hydroxyl group (3498
cm−1) and H2O bending vibrations (1632 cm−1), as well as a
broad peak corresponding to Al−O−Al bending (868 cm−1)
(Figure S1). The positions of these absorption bands of Al2O3
are consistent with previous reports.36 The reference catalyst,
IMP-Pt/Al2O3, showed absorption bands identical to those of
the Al2O3 support. These results indicate that IMP-Pt/Al2O3
has clean Pt surfaces without any organic species.
The FTIR spectra of PVP- and PVA-Pt/Al2O3 obtained
before and after heat treatment were compared with those of
intact capping agents (Figure 4). Pure PVP yielded absorption
bands corresponding to the pyrrolidone group (2956 and 2874
cm−1), CO stretching (1660 cm−1), CH2 bending (1461
cm−1), CH bending (1425 cm−1), and C−N bending (1280
cm−1).37−40 Before heat treatment, the spectrum of PVP-Pt/
Al2O3 also contained the characteristic absorption bands of
PVP, indicating that a significant amount of PVP remained
adsorbed on the Pt surface. Compared to pure PVP, the PVP-
Pt/Al2O3 catalyst exhibited a different feature at 1580 cm−1 in
the spectrum. This seems to arise from the formation of strong
CO−Pt bonds.37,38 A red shift of the PVP carbonyl
frequency is attributed to the decrease in electron density in
the CO group by strong charge−transfer interactions with
Pt NPs. After heat treatment, the bands in the spectrum were
attenuated because of the removal of the adsorbed PVP from
the Pt surface. As a result, the bands corresponding to the
pyrrolidone group (2956 and 2874 cm−1) were absent.
However, the heat-treated PVP-Pt/Al2O3 showed a similar
FTIR spectrum to that obtained before heat treatment,
exhibiting absorption bands corresponding to CH2 bending
(1461 cm−1) and CH bending (1425 cm−1) modes. This
indicates the presence of residual PVP on the Pt surface after
heat treatment. The FTIR spectra of PVA-Pt/Al2O3 before and
after heat treatment were also obtained. The characteristic
absorption bands of PVA arising from CH2 asymmetric
stretching (2948 and 2897 cm−1), CH stretching (2850
cm−1), and C−OH stretching (1384 cm−1) were observed for
both catalysts.40−43 These results indicate that despite heat
treatment, residual PVP and PVA remained on the Pt surface.
Moreover, the fact that both heat-treated PVP- and PVA-Pt/
Al2O3 exhibited a lower CO uptake than IMP-Pt/Al2O3 (see
Table 1) suggests that these residual capping agents could
affect the adsorption behavior of reacting species during the
reaction.
Figure 2. TGA profiles of PVP-Pt/Al2O3 and PVA-Pt/Al2O3 under
continuous air flow. This shows that the optimal temperature for the
removal of the capping agents from the Pt/Al2O3 catalysts is
approximately 623 K.
sharp weight loss of 9%, originating from the thermal
decomposition of PVP between 550 and 650 K. In contrast,
the TGA profile of PVA-Pt/Al2O3 shows a weight loss of 21%
over a broad range of 500−700 K. The greater weight loss of
PVA-Pt/Al2O3 is indicative of the lower thermal stability of
PVA than that of PVP, as reported previously.32,33 On the basis
of the TGA results, both PVP- and PVA-Pt/Al2O3 were heat-
treated at 623 K in air.
Heat treatment to remove the capping agent can result in
the aggregation of the Pt NPs under inappropriate conditions.
Thus, after the heat treatment, the size distributions of the Pt
NPs of PVP- and PVA-Pt/Al2O3 were confirmed again and
compared with that of IMP-Pt/Al2O3 (Figure 3). The TEM
images of the heat-treated PVP- and PVA-Pt/Al2O3 catalysts
show that no changes in the size of Pt NPs occurred after
treatment. The high-resolution (HR)TEM images of Pt NPs
clearly show lattice fringes with lattice spacings of 0.23 and
0.20 nm, corresponding to the {111} and {200} facets,
respectively.34,35 These polyhedral Pt NPs have similar mean
diameters of 3 nm for all Pt catalysts.
The physical properties of PVP-, PVA-, and IMP-Pt/Al2O3
are listed in Table 1. Before heat treatment, only a small
amount of CO was adsorbed on the Pt surface of the PVP- and
PVA-Pt/Al2O3 catalysts because of the strongly preadsorbed
capping agents. However, the amount of chemisorbed CO
increased approximately 5−8 times after the heat treatment of
the catalysts. This observation confirms that the capping agents
on the Pt surface had been removed by the treatment at 623 K
in an air atmosphere. The increase in the CO uptake was more
noticeable for PVA-Pt/Al2O3 than for PVP-Pt/Al2O3. This
3.2. Distinct Effects of Residual Capping Agents in
Enantioselective Hydrogenation. The catalytic perform-
ance of 1 wt % Pt/Al2O3 catalysts, the heat-treated PVP- and
PVA-Pt/Al2O3, was investigated in the enantioselective
hydrogenation of α-keto esters, EtPy and MtPy. IMP-Pt/
Al2O3, which has a clean Pt surface, was used as a reference
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Figure 3. TEM images, HRTEM images, and size distribution histograms of Pt NPs for heat-treated (a−c) PVP-Pt/Al2O3, (d−f) PVA-Pt/Al2O3,
and (g−i) IMP-Pt/Al2O3. The Pt NPs of all catalysts have similar average sizes and morphologies.
Table 1. Pt Loading, Total Amount of Adsorbed CO, Pt Dispersion, and Average Size of Pt NPs for PVP-Pt/Al2O3, PVA-Pt/
Al2O3, and IMP-Pt/Al2O3
before heat treatment
after heat treatment
a
b
b
c
d
catalyst
Pt loading [wt %]
CO uptake [μmol/gcat
]
D
[%]
3.3
3.7
CO uptake [μmol/gcat
]
D
[%]
dCO [nm]
dTEM [nm]
PVP-Pt/Al2O3
PVA-Pt/Al2O3
IMP-Pt/Al2O3
1.1
1.0
1.0
1.7
2.2
8.0
17.3
19.7
15.7
28.7
42.6
7.2
4.0
2.7
2.9
2.8
2.5
a
b
c
Pt loading determined by ICP-OES. Pt dispersion determined by CO chemisorption. Average particle size of Pt NPs determined by CO
d
chemisorption. Average particle size of Pt NPs determined by TEM.
catalyst. The TOF and ee’s of the Pt catalysts in the
enantioselective hydrogenation of EtPy at 0.1 MPa H2 pressure
are listed in Table 2. In the absence of the chiral modifier,
PVP- and PVA-Pt/Al2O3 showed lower TOF values than IMP-
Pt/Al2O3. This is consistent with the CO chemisorption results
listed in Table 1, which revealed lower CO uptake for both
catalysts compared to that of IMP-Pt/Al2O3. Indeed, the lower
TOF of PVP- and PVA-Pt/Al2O3 was attributed to the
blockage of Pt sites for the adsorption of reacting species by
residual capping agents.
The significant rate acceleration caused by the presence of the
cinchona alkaloids is an intrinsic phenomenon in enantiose-
lective hydrogenation of α-keto esters over Pt catalysts.5,9,11,44
The rate enhancement induced by the chiral modifier in the
hydrogenation of EtPy decreased in the following order: PVP-
Pt/Al2O3 > IMP-Pt/Al2O3 > PVA-Pt/Al2O3. Compared to use
of the unmodified Pt catalyst, the use of the CD-modified
IMP-Pt/Al2O3 catalyst led to a 1.9 times higher TOF, but that
of the CD-modified PVP-Pt/Al2O3 catalyst led to a 2.5 times
higher TOF. However, the TOF of the CD-modified PVA-Pt/
Al2O3 catalyst was increased by only 1.4 times. A more
noticeable rate enhancement effect in Pt catalysts was observed
Chiral modification of Pt catalysts by CD and QN not only
led to enantioselectivity but also resulted in rate enhancement.
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Figure 4. FTIR spectra of (a) intact PVP, PVP-Pt/Al2O3, and heat-treated PVP-Pt/Al2O3 and (b) intact PVA, PVA-Pt/Al2O3, and heat-treated
PVA-Pt/Al2O3. The presence of the characteristic absorption bands of the capping agents indicates that residual PVP and PVA remain on the Pt
surface after heat treatment.
a
Table 2. TOF [h−1] and ee [%] of Chirally Modified Pt/
enantioselectivity with those obtained from 5 wt % Pt/Al2O3,
Al2O3 Catalysts in the Enantioselective Hydrogenation of
EtPy (Letters in Parentheses Represent the Chirality of the
Major Product)
which is one of the most efficient catalysts, despite its lower
loading.47,48 More importantly, the presence of residual species
of PVP on the Pt catalysts had a positive effect after chiral
modification. To the best of our knowledge, this is the first
demonstration of an enhancing effect of residual species of
capping agents on both activity and selectivity in heteroge-
neous enantioselective catalysis. In contrast, CD- and QN-
modified PVA-Pt/Al2O3 exhibited lower enantioselectivity
than IMP-Pt/Al2O3, yielding 33 and 44% ee, respectively.
These results indicate that the residual PVP on the Pt surface
enhanced the performance for the enantioselective hydro-
genation of EtPy, whereas the residual PVA worsened the
catalytic properties.
b
catalyst
modifier
TOF [h−1
]
ee [%]
c
PVP-Pt/Al2O3
none
CD
QN
none
CD
QN
none
CD
190
480
1480
260
360
460
350
660
c
78 (R)
92 (R)
c
c
PVA-Pt/Al2O3
IMP-Pt/Al2O3
c
33 (R)
44 (R)
c
d
d
46 (R)
64 (R)
d
QN
990
The effect of residual capping agents on the catalytic
performance was also investigated for the enantioselective
hydrogenation of MtPy. The TOFs and ee’s are summarized in
Table 3. Similar trends in the catalytic performance to that of
the enantioselective hydrogenation of EtPy were observed.
Unmodified PVP- and PVA-Pt/Al2O3 exhibited lower TOFs
than IMP-Pt/Al2O3 because the residual capping agents
hindered the adsorption of reacting species on Pt sites.
However, chirally modified PVP-Pt/Al2O3 showed a signifi-
a
b
ee [%] measured at 100% conversion. Reaction conditions: 80 mg
of the catalyst, 4 mg of the chiral modifier, 0.1 mL of EtPy, 10 mL of
c
acetic acid, 0.1 MPa H2 pressure, and 298 K. TOF [h−1] based on
the number of surface Pt atoms predicted by a theoretical Pt NP
d
model with an average particle size of 3 nm (Pt923). TOF [h−1]
based on the number of surface Pt atoms determined by CO
chemisorption.
when QN was used. The QN-modified PVP-Pt/Al2O3 catalyst
showed a TOF of 7.8 times higher than that of the unmodified
catalyst, whereas QN-modified IMP- and PVA-Pt/Al2O3
exhibited only 2.8 and 1.8 times higher TOFs, respectively.
In this case, the differences in the rate increase strongly depend
on the presence and type of residual capping agents.
Considering the critical role of cinchona alkaloids in rate
acceleration, these results suggested that the residual species
preadsorbed on the Pt surface significantly affect the
adsorption behavior of the chiral modifiers.45
For chirally modified Pt catalysts, the ee values at full
conversion are also listed in Table 2. The order of
enantioselectivity of the chiral Pt/Al2O3 catalysts was identical
to the order of rate acceleration: PVP-Pt/Al2O3 > IMP-Pt/
Al2O3 > PVA-Pt/Al2O3. The positive correlation between
enantioselectivity and activity is consistent with the results of
previous reports.11,46 The use of QN-modified Pt catalysts led
to a higher ee than that of the CD-modified catalysts. In the
case of IMP-Pt/Al2O3, CD- and QN-modified catalysts gave 46
and 64% ee, respectively. However, CD- and QN-modified
PVP-Pt/Al2O3 exhibited remarkably higher enantioselectivities
than the modified IMP-Pt/Al2O3, yielding 78 and 92% ee,
respectively. It is noteworthy that the QN-modified PVP-Pt/
Al2O3 with 1 wt % Pt loading showed comparable
a
Table 3. TOF [h−1] and ee [%] of Chirally Modified Pt/
Al2O3 Catalysts in the Enantioselective Hydrogenation of
b
MtPy (Letters in Parentheses Represent the Chirality of
the Major Product)
catalyst
modifier
TOF [h−1
]
ee [%]
c
PVP-Pt/Al2O3
none
CD
QN
none
CD
QN
none
CD
90
820
1810
210
300
630
340
740
c
82 (R)
93 (R)
c
c
PVA-Pt/Al2O3
IMP-Pt/Al2O3
c
38 (R)
67 (R)
c
d
d
54 (R)
72 (R)
d
QN
1190
a
b
ee [%] measured at 100% conversion. Reaction conditions: 80 mg
of the catalyst, 4 mg of the chiral modifier, 0.1 mL of MtPy, 10 mL of
c
acetic acid, 0.1 MPa H2 pressure, and 298 K. TOF [h−1] based on
the number of surface Pt atoms predicted using a theoretical Pt NP
d
model with an average particle size of 3 nm (Pt923). TOF [h−1]
based on the number of surface Pt atoms determined by CO
chemisorption.
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Figure 5. Plots of the conversion and ee of (a) PVP-Pt/Al2O3, (b) PVA-Pt/Al2O3, and (c) IMP-Pt/Al2O3 for the hydrogenation of EtPya as a
function of reaction time. No significant change in ee is observed during the reaction, indicating that residual capping agents, PVP and PVA, are
stable on the Pt surface during the reaction. aReaction conditions: 80 mg of the catalyst, 4 mg of CD, 0.1 mL of EtPy, 10 mL of acetic acid, 0.1 MPa
H2 pressure, and 298 K.
Table 4. Effect of H2 Pressure on TOF [h−1] and ee [%] of CD-Modified Pt/Al2O3 Catalysts in Enantioselective
a
Hydrogenation of EtPy and MtPy
hydrogenation of EtPy
PH = 5.0 MPa
hydrogenation of MtPy
PH = 5.0 MPa
PH = 0.1 MPa
PH = 0.1 MPa
2
2
2
2
catalyst
TOF [h−1
]
ee [%]
TOF [h−1
]
ee [%]
TOF [h−1
]
ee [%]
TOF [h−1
]
ee [%]
b
b
b
c
b
b
b
c
PVP-Pt/Al2O3
PVA-Pt/Al2O3
IMP-Pt/Al2O3
480
78
33
46
2080
1010
1060
91
57
55
820
81
38
54
4500
2630
2130
95
66
63
b
b
360
300
c
c
660
740
a
b
c
Reaction conditions: 80 mg of the catalyst, 4 mg of CD, 0.1 mL of the substrate, 10 mL of acetic acid, 0.1 or 5.0 MPa H2 pressure, and 298 K.
TOF [h−1] based on the number of surface Pt atoms predicted using a theoretical Pt NP model with an average particle size of 3 nm (Pt923).
TOF [h−1] based on the number of surface Pt atoms determined by CO chemisorption.
cantly improved TOF compared to the modified IMP-Pt/
Al2O3. In the case of PVA-Pt/Al2O3, chiral modification led to
the lowest rate enhancement. The enantioselectivity of the
chirally modified Pt catalysts showed a positive correlation
with activity. The CD- and QN-modified IMP-Pt/Al2O3
exhibited 54 and 72% ee, respectively. The chiral PVP-Pt/
Al2O3 led to a significant improvement in enantioselectivity,
yielding 82% ee for CD and 93% ee for QN. However, chiral
PVA-Pt/Al2O3 exhibited lower enantioselectivity than the
corresponding IMP-Pt/Al2O3, showing only 38% ee for CD
and 67% ee for QN. The order of activity and enantiose-
lectivity of the chiral Pt/Al2O3 catalysts in the enantioselective
hydrogenation of EtPy and MtPy was the same: PVP-Pt/Al2O3
> IMP-Pt/Al2O3 > PVA-Pt/Al2O3. These results clearly show
that the enhancement and loss of the catalytic performance can
be attributed to the residual capping agents.
During enantioselective hydrogenation, the residual capping
agents could be desorbed from the Pt surface. Indeed, a Pt/
SiO2 catalyst capped by PAA and PAA-Pt/SiO2 yields a poor
ee during the initial reaction period at 0.1 MPa H2 pressure.11
However, a rapid enhancement in enantioselectivity is
observed once the PAA capping agents have been removed
from the Pt surface under reaction conditions. In our case, the
removal of residual PVP and PVA would lead to a decrease and
increase in the catalytic performance, respectively. The stability
of residual capping agents on the Pt surface was investigated by
monitoring the change in the performance of the PVP-, PVA-,
and IMP-Pt/Al2O3 catalysts as a function of reaction time
during the enantioselective hydrogenation of EtPy (Figure 5).
For both CD-modified PVP- and PVA-Pt/Al2O3 catalysts,
enantioselectivity was determined in the early stages of the
reaction. Subsequently, there was no significant change. A
comparison with the case of IMP-Pt/Al2O3 showed that the
residual PVP and PVA capping agents were stable on the Pt
surface during the reaction at 0.1 MPa H2 pressure.
Additionally, the influence of H2 pressure on the catalytic
performance of the Pt catalysts with residual capping agents
was investigated because CD-modified Pt catalysts, in general,
exhibit enhanced enantioselectivity at high H2 pressures.22,49,50
The TOFs and ee’s of the CD-modified Pt catalysts in the
enantioselective hydrogenation of α-keto esters at 0.1 and 5.0
MPa H2 pressure are compared in Table 4. Similar to the
reference catalyst, IMP-Pt/Al2O3, both PVP- and PVA-Pt/
Al2O3 catalysts showed improved activity and enantioselectiv-
ity. Among the Pt catalysts, PVP-Pt/Al2O3 exhibited the
highest enantioselectivity at 5.0 MPa H2 pressure, yielding 91%
ee in the hydrogenation of EtPy and 95% ee in the
hydrogenation of MtPy, respectively. The increase in the
enantioselectivity of PVP-Pt/Al2O3 at high H2 pressure was
greater than that of IMP-Pt/Al2O3. This indicates that the
residual PVP, which had a positive effect for enantioselective
hydrogenation, was stable on the Pt surface, even at 5.0 MPa
H2 pressure. In the case of PVA-Pt/Al2O3, the lowest TOF and
ee value were observed at 0.1 MPa H2 pressure. At 5.0 MPa H2
pressure, however, it exhibited performance similar to that of
IMP-Pt/Al2O3, showing that the detrimental effect of the
residual PVA was negligible at high H2 pressure. It is also
noteworthy that there have been attempts to use PVP-
stabilized Pt NPs for heterogeneous enantioselective hydro-
genation in which the PVP on Pt NPs of Pt/Al2O3 was rinsed
out before the reaction.51,52 The washed Pt/Al2O3 exhibited
approximately 84% ee in hydrogenation of MtPy at high H2
pressure.52 This suggests that the method for post-treatment of
polymer-capped Pt NPs could significantly affect the catalytic
performance in enantioselective hydrogenation.
Although the nearly constant enantioselectivity during the
reaction at 0.1 MPa H2 pressure (see Figure 5) confirms the
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stability of PVA on the Pt surface, we could not rule out the
possibility that the removal of PVA contributed to the
improvement in enantioselectivity at 5.0 MPa H2 pressure.
Thus, the ee values for enantioselective hydrogenation of EtPy
were measured again at 0.1 MPa H2 pressure after the
treatment of the Pt catalysts at 5.0 MPa H2 pressure for 2 h
(Table 5). There were no significant changes in the TOF and
residual capping agents and was in the metallic state.53−55 For
both PVP- and PVA-Pt/Al2O3, however, there was a peak shift
to lower binding energies by about 1.0−1.5 eV. This is
consistent with previous reports concerning the change in the
electron density of Pt by the residual PVP and PVA
species.56,57 The XPS results show the presence of residual
capping agents on the Pt surface and an increase in the
electron density of Pt by electron transfer from the organic
species. Nonetheless, the fact that PVP- and PVA-Pt/Al2O3
showed opposing catalytic performance indicates that the
increase in the electron density of Pt was not an important
factor determining their enantioselectivity.
a
Table 5. Effect of H2 Treatment of CD-Modified Pt/Al2O3
Catalysts on TOF [h−1] and ee [%] in the Enantioselective
b
Hydrogenation of EtPy
before H2 treatment
after 2 h H2 treatment
Enantioselective hydrogenation of α-keto esters is structure
sensitive, and the enantioselectivity strongly depends on the
exposed sites on the Pt NPs.9,11,12,29 Thus, the exposed Pt sites
of PVP-, PVA-, and IMP-Pt/Al2O3 were examined by DRIFTS
measurements of CO-adsorbed samples. The vibrational
frequencies of CO molecules linearly adsorbed on Pt catalysts
are site-specific and, thus, provide information about the type
of exposed Pt sites, such as well-coordinated (WC; terrace)
sites and under-coordinated (UC; step, edge, and corner)
sites.58 The DRIFT spectra of adsorbed CO on the Pt catalysts
are compared in Figure 7. The spectrum of IMP-Pt/Al2O3
catalyst
TOF [h−1
]
ee [%]
TOF [h−1
]
ee [%]
PVP-Pt/Al2O3
PVA-Pt/Al2O3
IMP-Pt/Al2O3
480
360
660
78
33
46
420
710
550
77
51
45
a
H2 treatment conditions: 80 mg of the catalyst, 4 mg of the chiral
modifier, 10 mL of acetic acid, 5.0 MPa H2 pressure, and 298 K.
Reaction conditions: 80 mg of the catalyst, 4 mg of the chiral
modifier, 0.1 mL of EtPy, 10 mL of acetic acid, 0.1 MPa H2 pressure,
b
and 298 K.
ee of IMP-Pt/Al2O3. As expected, the treated PVP-Pt/Al2O3
yielded a nearly identical performance to that before H2
treatment, showing that the residual PVP was stable on the
Pt surface after H2 treatment at high pressure. However, H2
treatment of PVA-Pt/Al2O3 at 5.0 MPa led to an increase in
activity and enantioselectivity from 33 to 51% ee. A similar
change was observed for the treated Pt catalysts in
enantioselective hydrogenation of MtPy (Table S1). These
results are consistent with the removal of the residual PVA and,
thus, the attenuation of the detrimental effect of PVA on the
catalytic performance under high H2 pressure conditions.
3.3. Selective Exposure of Pt Sites by Residual
Capping Agents. To elucidate the origin of the significant
change in the catalytic performance of PVP- and PVA-Pt/
Al2O3 for enantioselective hydrogenation, the electronic states
of the Pt catalysts were investigated by XPS. The Pt 4d XPS
spectra of PVP-, PVA-, and IMP-Pt/Al2O3 are compared in
Figure 6. The spectrum of the reference catalyst, IMP-Pt/
Al2O3, contains Pt 4d3/2 and Pt 4d5/2 peaks at binding energies
of 315.0 and 332.0 eV, respectively. These binding energies
confirm that IMP-Pt/Al2O3 had a clean Pt surface without
Figure 7. DRIFT spectra of CO adsorbed on PVP-Pt/Al2O3, PVA-Pt/
Al2O3, and IMP-Pt/Al2O3. The peaks at 2084 and 2064 cm−1 can be
assigned to linearly adsorbed CO on WC and UC Pt sites,
respectively. Compared to the spectrum of IMP-Pt/Al2O3, those of
PVP- and PVA-Pt/Al2O3 contain only one peak, indicating the
selective exposure of WC Pt sites for PVP-Pt/Al2O3 and UC Pt sites
for PVA-Pt/Al2O3.
contains two peaks at 2084 and 2064 cm−1. These were
assigned to the vibrational stretches of CO molecules linearly
adsorbed on WC and UC, respectively.58 The noticeable
difference in peak intensity is attributed to the fact that the
extinction coefficient for the stretch arising from CO adsorbed
on UC Pt sites is 2.7 times greater than that arising from CO
adsorbed on WC Pt sites.59 The spectrum of IMP-Pt/Al2O3
confirms that it contains both WC and UC Pt sites for the
adsorption of reacting species during enantioselective hydro-
genation. However, the spectrum of PVP-Pt/Al2O3 exhibited
only one peak at 2086 cm−1, indicating adsorption of CO
molecules on WC Pt sites. The exclusive exposure of WC sites
on the Pt surface showed that PVP on WC Pt sites was
effectively removed during heat treatment, whereas PVP on
UC Pt sites was not. In contrast, the spectrum of PVA-Pt/
Al2O3 exhibited only one peak at 2065 cm−1, indicating
adsorption of CO molecules on the UC Pt sites. This suggests
that residual PVA remained on the WC Pt sites, exposing only
the UC sites after heat treatment. It has been reported that
intact PVP and PVA show preferential adsorption on the
Figure 6. X-ray photoelectron spectra of the Pt 4d region for heat-
treated Pt catalysts. The lower binding energies of the peaks for PVP-
Pt/Al2O3 and PVA-Pt/Al2O3 compared to those of IMP-Pt/Al2O3
indicate the presence of residual capping agents on the Pt surface and
increased electron density of Pt induced by the presence of surface
species.
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Figure 8. DRIFT spectra of CO adsorbed on (a) PVP-Pt/Al2O3, (b) PVA-Pt/Al2O3, and (c) IMP-Pt/Al2O3 before and after H2 treatment at 5.0
MPa H2 pressure. There are no changes in the type of exposed Pt sites for IMP-Pt/Al2O3 and PVP-Pt/Al2O3. In contrast, the appearance of a peak
at 2084 cm−1, arising from adsorbed CO on Pt terrace sites, indicates the removal of the residual PVA under high H2 pressure and the additional
exposure of the WC Pt sites of PVA-Pt/Al2O3.
defect sites of the Pt surface and {111} facets of the Pd surface,
respectively.60−62 Our case is interesting in that heat treatment
of polymer-capped Pt NPs can lead to the exclusive blockage of
defect or terrace sites by residual capping agents. These results
seem to be related to differences in thermal stability rather
than the adsorption energies of the capping agents on surface
sites having different coordination structures.
observation that there was no change in the type of exposed
Pt sites clearly supports the constant enantioselectivity of IMP-
and PVP-Pt/Al2O3 before and after H2 treatment. In the case
of the spectrum of PVA-Pt/Al2O3, the peak at 2065 cm−1 is
still present. However, a new peak, which was not observed
before H2 treatment, appears at 2084 cm−1. Thus, it was
concluded that the removal of the residual PVA by H2
treatment and subsequent exposure of WC Pt sites, that is,
terraces, led to an increase in the enantioselectivity of PVA-Pt/
Al2O3. The DRIFT results demonstrate that site-selective
exposure of the Pt surface by residual capping agents is a key
factor determining the catalytic performance. In addition, these
results show that terrace sites are more efficient than defect
sites in the enantioselective hydrogenation of α-keto esters.
As mentioned above, stable adsorption of a chiral modifier
on the Pt surface is an important prerequisite for achieving
high enantioselectivity. Thus, the amount of CD adsorbed on
the Pt surface was estimated by comparing the CO uptake
before and after CD adsorption in which the CO uptake of a Pt
catalyst is proportional to the number of exposed Pt sites on
the surface. The CO uptake values for IMP-, PVP-, and PVA-
Pt/Al2O3 are summarized in Table 6. Both PVP- and PVA-Pt/
Compared to IMP-Pt/Al2O3, PVP-Pt/Al2O3 exhibited
superior catalytic performance and PVA-Pt/Al2O3 exhibited
inferior performance in the enantioselective hydrogenation of
EtPy and MtPy. The DRIFT spectra of CO adsorbed on Pt
catalysts showed that the residual capping agents directly
affected the catalytic performance by selectively exposing Pt
sites. In enantioselective hydrogenation over cinchona alkaloid-
modified Pt catalysts, large and flat ensembles of Pt atoms are
required to accommodate the bulky CD and QN chiral
modifiers and their complex interacting with the reactant.29
Moreover, cinchona alkaloids are stabilized on terrace sites via
π−π interactions between the quinoline ring and the Pt
surface.13 Therefore, the higher performance of PVP-Pt/Al2O3
having exposed terraces than that of PVA-Pt/Al2O3 having
exposed defects was attributed to the stable adsorption of the
chiral modifiers and the efficient creation of chiral Pt sites. Our
results are consistent with previous observations that larger Pt
NPs with more terrace sites than defect sites show higher
performance.
Table 6. Change in the Amount of CO Uptake Before and
After the Adsorption of CD Modifier on Pt/Al2O3 Catalysts
before CD
after CD
adsorption adsorption
Although PVA-Pt/Al2O3 showed inferior enantioselectivity
at 0.1 MPa H2 pressure, it exhibited nearly the same ee values
as IMP-Pt/Al2O3 at 5.0 MPa H2 pressure. As shown by the
data in Table 5, the improvement in enantioselectivity is most
likely due to the removal of residual PVA from the Pt surface at
high H2 pressures. The DRIFT spectra of CO adsorbed on
PVP-, PVA-, and IMP-Pt/Al2O3 were obtained after H2
treatment at 5.0 MPa to confirm the changes in the exposed
Pt sites (Figure 8). Although there is a change in the intensity
ratio of the peaks, which seems to arise from residual solvent
molecules on the treated catalyst, the spectrum of IMP-Pt/
Al2O3 still contains peaks at 2084 and 2064 cm−1 after
treatment, confirming the presence of WC and UC Pt sites,
respectively. There is no change in the type of the exposed Pt
sites for PVP-Pt/Al2O3, as shown by its spectrum, which only
contains a peak at 2086 cm−1. The appearance of a single peak
arising from the stretch of CO adsorbed on WC Pt sites again
indicates that the residual PVP covering the UC Pt sites was
stable even after H2 treatment at high pressure. The
absolute
relative
CO uptake CO uptake
decrease in
CO uptake
decrease in
a
(B)
(A)
CO uptake
[%]
catalyst
[μmol/gcat
]
[μmol/gcat
]
[μmol/gcat]
PVP-Pt/Al2O3
PVA-Pt/Al2O3
IMP-Pt/Al2O3
8.0
17.3
19.7
1.5
14.8
12.2
6.5
2.5
7.5
81.3
14.5
38.1
a
(B − A)/B × 100 [%].
Al2O3 exhibited less CO uptake than IMP-Pt/Al2O3 because of
the blockage of Pt sites by residual capping agents. After the
adsorption of CD, the strongly adsorbed chiral modifier on the
Pt surface reduced the number of exposed Pt sites for the
adsorption of CO. As a result, the CO uptake decreased
noticeably for all Pt catalysts. Similar changes were observed in
the CO chemisorption results of the Pt catalysts after the
adsorption of QN (Table S2).
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Figure 9. DRIFT spectra of CO adsorbed on (a) PVP-Pt/Al2O3, (b) PVA-Pt/Al2O3, and (c) IMP-Pt/Al2O3 before and after CD adsorption. The
peak at approximately 2085 cm−1 arising from the CO adsorbed on WC Pt sites disappears after CD adsorption. This indicates preferential and
stable adsorption of CD on Pt terrace sites rather than Pt defect sites.
The DRIFT spectra of adsorbed CO in Figure 7 indicate
that PVP-Pt/Al2O3 only exposes WC Pt sites, whereas IMP-
Pt/Al2O3 exposes both WC and UC Pt sites. Therefore, CD
could adsorb on the exposed WC Pt sites for PVP-Pt/Al2O3
and on both WC and UC sites for IMP-Pt/Al2O3.
Interestingly, the absolute decrease in the CO uptake for
PVP-Pt/Al2O3 was similar to that for IMP-Pt/Al2O3 despite
the fact that PVP-Pt/Al2O3 initially has less than half of the
exposed Pt sites of IMP-Pt/Al2O3. However, the absolute
decrease in the number of exposed sites in PVA-Pt/Al2O3 was
noticeably smaller than those of other Pt catalysts. This result
reveals that the selectively exposed defect sites of PVA-Pt/
Al2O3 are inefficient adsorption sites for CD molecules. A
comparison of the relative decrease in the CO uptake between
Pt catalysts clearly shows that terrace sites are more suitable for
the stable adsorption of CD than other defect sites such as
steps, edges, and corners. Furthermore, PVP-Pt/Al2O3 showed
higher enantioselectivity than IMP-Pt/Al2O3 despite the lower
absolute decrease in the CO uptake. The fact that in the case
of PVP-Pt/Al2O3, the chiral modifier was selectively adsorbed
on terraces suggests that CD molecules on terrace sites are
more efficient for enantiodifferentiation than those on defect
sites.
The efficient adsorption of CD molecules on Pt terrace sites
for the creation of chiral sites was also investigated by
comparing the DRIFTS spectra of CO adsorbed on the Pt
catalysts after the adsorption of CD (Figure 9). The spectrum
of surface-clean IMP-Pt/Al2O3 contains two peaks correspond-
ing to linearly adsorbed CO on the WC and UC Pt sites. After
the adsorption of CD, the peak at 2064 cm−1 remains, whereas
the peak at 2084 cm−1 disappears. This change confirms the
strong and selective adsorption of CD molecules on WC Pt
sites rather than on UC sites. Similarly, the peak arising from
CO on the WC Pt sites of PVP-Pt/Al2O3 completely
disappears after CD adsorption. On the other hand, there is
no noticeable change in the CO-DRIFT spectra of PVA-Pt/
Al2O3. The clear appearance of a peak at 2065 cm−1 after CD
adsorption indicates unstable and weak adsorption of CD
molecules on the UC sites, as shown for the case of IMP-Pt/
Al2O3. Moreover, similar changes were observed in the CO-
DRIFT spectra of the Pt catalysts after the adsorption of QN
(Figure S2). These observations again confirm that the
significant improvement in the catalytic performance of PVP-
Pt/Al2O3 in the enantioselective hydrogenation of α-keto
esters originated from the site-selective exposure of WC Pt
sites and the stable adsorption of the chiral modifier on the
terrace sites.
4. CONCLUSIONS
Polymer-capped Pt/Al2O3 containing uniform sized Pt NPs
was used as a heterogeneous catalyst for the enantioselective
hydrogenation of α-keto esters. After the heat treatment of the
Pt catalysts, the residual capping agents led to the site-selective
blockage of Pt sites. As a result, the heat-treated PVP-Pt/Al2O3
exclusively exposed WC terrace sites, whereas the treated PVA-
Pt/Al2O3 selectively exposed UC defect sites. The site-specific
exposure of Pt sites significantly affected both the reaction rate
and enantioselectivity. Compared to IMP-Pt/Al2O3 exposing
both terraces and defects, PVP-Pt/Al2O3 showed remarkably
enhanced catalytic performance, whereas PVA-Pt/Al2O3
exhibited inferior performance. The improvement in the
catalytic performance of PVP-Pt/Al2O3 was attributed to the
stable adsorption of a chiral modifier on terrace sites and their
effective enantiodifferentiation. Our study provides a new
strategy to develop site selectively exposed metal NPs and
demonstrates the enhancing effect of residual capping agents
on the catalytic properties in structure-sensitive reactions.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c04255.
■
sı
Performance for enantioselective hydrogenation of MtPy
after H2 treatment, FTIR spectra of Al2O3 and IMP-Pt/
Al2O3, CO chemisorption results, and CO-DRIFT
spectra of Pt catalysts after QN adsorption (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Yongju Yun − Department of Chemical Engineering, Pohang
University of Science and Technology (POSTECH), Pohang,
Gyeongbuk 37673, Republic of Korea; orcid.org/0000-
0001-6497-4128; Email: yjyun@postech.ac.kr
Authors
Iljun Chung − Department of Chemical Engineering, Pohang
University of Science and Technology (POSTECH), Pohang,
Gyeongbuk 37673, Republic of Korea
Byeongju Song − Department of Chemical Engineering,
Pohang University of Science and Technology (POSTECH),
Pohang, Gyeongbuk 37673, Republic of Korea
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Jeongmyeong Kim − Department of Chemical Engineering,
Pohang University of Science and Technology (POSTECH),
Pohang, Gyeongbuk 37673, Republic of Korea
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c04255
Author Contributions
†I.C. and B.S. equally contributed to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Research
F o u n d a t i o n o f K o r e a ( N R F ) g r a n t ( N R F -
2019R1C1C1002846) funded by the Ministry of Science and
ICT. This work was also supported by LG Chem.
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