Effective one-step reduction of Pt/alumina-carbon catalysts for asymmetric hydrogenation of α-ketoesters

Article abstract of DOI:10.1016/j.apcata.2014.04.031

Platinum (Pt) particles supported on 15% alumina-carbon composites(Pt/15AM) were reduced by liquid phase reduction methods and one-step high temperature methods. The catalyst was reduced by one-step method with hydrogen at 600°C (Pt/15AM-600) showed superior structure and surface properties such as high special surface area, large pore size, high surface Pt/Al atomic ratio and no residual chlorine on the surface. Moreover, CD-modified Pt/15AM-600 catalyst afforded the highest enantioselectivity of 87.5% and 84.8% in the asymmetric hydrogenation of ethyl pyruvate and EOPB, respectively. Of particular note was the reusability of Pt/15AM-600 catalyst, which could be reused for 23 times without distinct loss in catalytic activity in the asymmetric hydrogenation of ethylpyruvate. The excellent reusability of these alumina-carbon composites supported Pt catalysts indicated the possibility of these novel catalysts in industrial application.

Full text of DOI:10.1016/j.apcata.2014.04.031

Applied Catalysis A: General 480 (2014) 50–57
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Effective one-step reduction of Pt/alumina–carbon catalysts for
asymmetric hydrogenation of ␣-ketoesters
Xueqin Zhang∗, Qiang Li, Meitian Xiao, Yongjun Liu
College of Chemical Engineering, Huaqiao University, Xiamen, Fujian, 361021, PR China
a r t i c l e i n f o
Article history:
Received 9 February 2014
Received in revised form 10 April 2014
Accepted 16 April 2014
Available online 24 April 2014
1. Introduction
property of Pt/Al2O3-FDU-14 was in moderate level and this may
due to the weak interaction between alumina and FDU-14 [32].
Interestingly, Böttcher and co-workers [33] observed high enan-
tioselectivity (ee up to 94.1%) in the enantioselective hydrogenation
of ethyl pyruvate when aluminized fumed silica were used as sup-
ports for Pt nanoparticles, CD as modifier. However, all the work
about these alumina–X composites supported Pt catalysts was not
involved in reusability of relative catalysts.
Asymmetric hydrogenation of activated ketones, using hetero-
geneous chiral catalysts, plays a key role in the production of fine
chemicals [1]. Since the original discovery of Pt-Cinchona system
by Orito and co-workers [2,3], the asymmetric hydrogenation of ␣-
ketoesters over cinchonidine(CD)-modified Pt supported catalysts
have been studied extensively [4–14]. Particular attention is given
to the supports of Pt nanoparticles. Al2O3 [4–8], zeolite [15,16],
SiO2 [17], SWNTs [18], MCM-41 [19], CMK-type mesoporous carbon
[20,21], FDU-type mesopolymer [22], ionic liquid [23] and carbon
nanotubes (CNTs) [24] are considered as suitable supports, fur-
nishing medium to good enantioselectivity. And Al2O3 supported
Pt catalysts are proved to be the best catalyst for the asymmetric
hydrogenation of ␣-ketoesters. However, the hydrogenation of the
aromatic part of CD [25,26] and the intrinsic drawback of Al2O3
with inferior stability in acetic acid [27,28] limits the reusability of
CD-modified Pt/Al2O3 catalyst system, while acetic acid is proved to
be one of the best solvents in the asymmetric hydrogenation of ␣-
functionalized ketones on the CD-modified Pt supported catalysts
[6,28,29]. In comparison with Pt/Al2O3, FDU-type mesopolymer
supported Pt catalysts show moderate enantioselectivity but long-
term stability by adding modifier after each reaction cycle [22,30].
For the drawbacks of the single-component materials supported
catalysts, alumina–X (X represents a kind of material) composites
have attracted attention of catalytic chemists gradually. Wang et al.
[31] incorporated SBA-15 with alumina (Al2O3-SBA-15) by solvent-
free solid-state grinding method and the obtained CD-modified
Pt/Al2O3-SBA-15 catalyst furnished 93.4% ee value in the asym-
metric hydrogenation of ethyl pyruvate. Nevertheless, the catalytic
Pt supported catalysts are commonly prepared by impregna-
tion of supports with chlorine containing platinum precursor and
the presence of chlorines limits the aggregation of Pt to obtain
high dispersed nanoparticles [5,8]. However, the residual chlo-
rine originating from the chlorine containing precursor hinders the
adsorption of reactant and poisons the active metal sites [34,35].
Generally, the residual chlorine cannot be totally removed from the
surface by direct reduction in hydrogen [35,36]. The most common
reduction method of Cl-containing platinum catalyst precursors is
liquid phase reduction, which includes the follow three steps: (a)
calcination in air or vacuum; (b) reduction by the aqueous solu-
tion of NaCOOH and washing by ionized water; (c) pretreatment
before use [5,20–22,24,29,31,37]. The procedures of calcination and
aqueous reduction can remove the surface residual chlorines. The
pretreatment of the catalyst can be carried out by high tempera-
ture [8,38] or sonication [39–41]. Both two pretreatment methods
affect the Pt particle size and the surface adsorption of cinchoni-
dine, which can induce asymmetric sites on the catalyst surface
and plays an important role in achieving high ee values [13,42,43].
Nevertheless, this traditional multi-steps liquid reduction method
is complicated and the hydro-thermal treatment of the catalyst
during the aqueous reduction results in Pt loss and structure
destruction of the catalyst.
Lately, our group incorporated alumina with mesoporous
carbon by chelate-assisted co-assembly method [44]. The
Al2O3–mesoporous carbon (Al2O3–MC) composites were proved
∗
Corresponding author. Tel.: +86 0592 6162285; fax: +86 0592 6162300.
E-mail address: xqzhang2009@hqu.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.apcata.2014.04.031
0926-860X/© 2014 Elsevier B.V. All rights reserved.

X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
51
to be favorable for high Pt dispersion and the resulting Pt supported
catalyst modified with CD showed excellent catalytic performance
in the asymmetric hydrogenation of ethyl 2-oxo-4-phenylbutyrate
(EOPB). In the further exploration, we found that reduction
method played a vital role in the catalytic performance of these
Pt/Al2O3–MC catalysts and the residual chlorine on the catalyst
surface could be removed by one-step reduction in hydrogen.
Herein, we mainly focus on uncovering the superiorities of the
one-step reduction by using various characterization methods.
Additionally, the reusability of Pt/Al2O3–MC in the asymmetric
hydrogenation of ethyl pyruvate is also studied.
2. Experimental
2.1. Catalyst preparation
The mesoporous carbon incorporated with 15 wt% alumina
was prepared according to Ref. [44]. The obtained 15 wt%
Al2O3–MC composites was denoted as 15AM. The 5 wt% Pt/15AM
was prepared by impregnation method. 15AM composites were
impregnated with an aqueous solution of platinum precursor
(H2PtCl6) and stirred for 5 h. Then the mixture was evaporated
to remove the excess water, followed by drying at 373 K for 12 h.
One portion of the catalyst precursor was reduced in an aqueous
solution of sodium formate at 368 K for 1 h, then the mixture was
washed by deionized water for three times and dried at 373 K for
12 h. Before use, the catalyst was pretreated at different tempera-
ture in a hydrogen flow. The other portion was directly reduced in
a hydrogen flow. The obtained catalysts were denoted as Pt/15AM-
SF-T, where SF is the abbreviation of sodium formate, and T stands
for the temperature at which the catalyst precursor was reduced.
The description without SF stands for the catalyst directly reduced
in a hydrogen flow. As a comparison, Al2O3 supported Pt cata-
lyst was prepared. A portion of Pt/Al2O3 precursor was reduced
directly at 600 ◦C in hydrogen and denoted as Pt/Al2O3-600. The
other portion Pt/Al2O3 precursor was reduced according to Ref. [29]
and the obtained catalyst was denoted as Pt/Al2O3-SF-400.
2.2. Characterization
Powder X-ray diffraction (XRD) patterns were collected on a
Panalytical X’Pert-Pro powder X-ray diffractometer using Cu K␣
radiation (40 kV, 40 mA). The low and wide scan ranges were 0.5–5◦
and 20–90◦, respectively. N2 adsorption–desorption isotherms
were recorded on a Micrometitics ASAP 2020M+C analyzer at
−196 ◦C. The BET method was adopted to calculate the special
surface areas using adsorption data in a relative pressure range
from 0.05 to 0.25. The pore size distributions were derived from
the adsorption branches of isotherms using the BJH model. Trans-
mission electronic microscopy (TEM) images were taken on a
HITACHI H-7650 electronic microscope with an accelerating volt-
age of 100 kV.
Fig. 1. Low-angle (a) and wide-angle (b) XRD patterns of Pt/15AM-T and Pt/15AM-
SF-T catalysts.
stainless-steel stirred pressure reactor at room temperature.
Pt/15AM catalyst (100 mg), CD (10 mg), ethyl pyruvate (2 mL) or
EOPB (1 mL), solvent (25 mL) and H2 (5 MPa) were used for the
reaction. The reaction was terminated after a while and then
the products were analyzed by gas chromatography (Agilent
6890) equipped with capillary chiral column (CP-ChiraSil-DEX
CB 25 m × 0.25 mm × 0.25 ␮m, Agilent). The optical yield was
expressed as ee value: ee (%) = ([R] − [S])/([R] + [S]) × 100.
After the reaction, the catalyst was separated and washed with
fresh acetic acid. Fresh reactant, CD and acetic acid were added to
the reactor together with the recovered catalyst to carry out the
next cycle reaction.
The XPS spectra were recorded on a Physical Electronics
PHI Quantum-2000 spectrometer with a monochromatized AlK␣
source (hꢀ = 1486.6 eV). The measurements were performed at
room temperature in a high vacuum of 2 × 10−7 Pa.
The exact Pt contents of the catalysts were measured with
an Optima 7000 DV inductively coupled plasma optical emission
spectroscopy (ICP-OES). Before ICP-OES measurement, the samples
were calcined at 873 K for 1 h in air and then dissolved in aqua regia.
3. Results and discussion
2.3. Catalytic testing
3.1. XRD
The catalytic performance of Pt/15AM-SF-T for asymmetric
hydrogenation
phenylbutyrate ((R)-(+)-EHPB) were evaluated in
of
EOPB
to
(R)-(+)-ethyl
2-hydroxy-4-
100 mL
Fig. 1a shows the low-angle XRD patterns of Pt/15AM-T and
a
Pt/15AM-SF-T catalysts. The diffraction peaks at 2Â = 0.6−1.0◦

52
X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
Table 1
Relevant parameters of Pt/15AM and Pt/Al2O3 reduced by different methods.
Entry
Sample
SBET (m2/g)
Vt (cm3/g)
Dp (nm)
Dpt (nm)a
Pt content (wt %)b
1
2
3
4
5
6
7
8
Pt/15AM-400
Pt/15AM-SF-400
Pt/15AM-500
Pt/15AM-SF-500
Pt/15AM-600
Pt/15AM-SF-600
Pt/Al2O3-600
421
527
650
751
704
866
170
193
0.29
0.33
0.47
0.54
0.70
0.82
0.40
0.42
5.6
4.8
5.6
4.8
5.6
4.8
6.1
6.8
3.8
4.8
4.3
6.4
6.1
8.1
3.0
3.1
4.9
4.3
5.5
4.9
6.9
6.0
4.8
3.8
Pt/Al2O3-SF-400
a
Average Pt particle sizes were measured by TEM.
Exact Pt contents were measured by ICP-OES.
b
can be indexed to the (1 0 0) reflection of 2-D P6mm hexagonal
mesostructure [45]. Along with reduction temperature from 400 to
600 ◦C, the diffraction intensities decreased obviously, suggesting
that high temperature deteriorated the regularity of mesoporous
carbon. In comparison with Pt/15AM-T catalysts, obvious decreases
in (1 0 0) diffraction intensities were observed for Pt/15AM-SF-
T catalysts. The hydro-thermal treatment in aqueous solution of
sodium formate during the reduction might cause the collapse of
mesopores and changed the structural rigidity of carbon structure.
The wide-angle XRD patterns of the Pt/15AM-T and Pt/15AM-
SF-T catalysts (Fig. 1b) displayed four resolved diffraction peaks at
2Â = 39.8◦, 46.3◦, 67.4◦ and 81.3◦, which can be indexed to the (1 1 1),
(2 0 0), (2 2 0), (3 1 1) reflections of face-centered cubic metallic
Pt (JCPDS card No. 04-0802). The intensities of diffraction peaks
depended on the reduction temperature suggested that high tem-
perature were harmful to the dispersion of Pt particles. Compared
to Pt/15AM-T catalysts, a bit stronger diffraction intensities of
Pt/15AM-SF-T catalysts were observed. This could be attributed
to the removal of chlorine during the aqueous reduction, while
the chlorine was considered to inhibit the aggregation of the Pt
particles [5].
reduction process. The further carbonization of the resol resulted
in the decrease amount of support, and thus Pt content was
increased relatively. Pt/15AM-T catalysts had higher Pt contents
than Pt/15AM-SF-T catalysts suggested that directly high tempera-
ture reduction can inhibit the Pt loss, which always occurred during
the aqueous reduction.
3.2. N2 adsorption-desorption
The relevant physico-chemical parameters of Pt/15AM and
Pt/Al2O3 by nitrogen adsorption-desorption are summarized in
Table 1. It is noted that with an increase in reduction temperature,
the special surface area and total pore volume of these catalysts
increased obviously. This implied that high temperature made the
further carbonization of resol composites and the removal of the
trace of template. And the extent of carbonization depended on the
reduction temperature.
The nitrogen adsorption-desorption isotherms and the Barrett-
Joyner-Halenda (BJH) pore size distributions of Pt/15AM-T catalysts
and Pt/15AM-SF-T catalysts are shown in Fig. 2. These samples
exhibited representative type IV curves with H2 hysteresis loops,
which attributed to the presence of well-developed mesopores and
micropores [46]. The BJH pore size distribution curves revealed that
the mean pore sizes of Pt/15AM-T catalysts were 5.6 nm (Fig. 2a,
insert), the same with 15AM [44], while Pt/15AM-SF-T catalysts
only obtained mean pore sizes of 4.8 nm (Fig. 2b, insert). More-
over, compared to Pt/15AM-T catalysts, much broader pore size
distributions were observed for the Pt/15AM-SF-T catalysts. All
these results implied that hydro-thermal treatment during aque-
ous reduction weakened the structure rigidity of the support, and
these results were consistent with the analysis of low-angle XRD
patterns (Fig. 1a).
The exact Pt contents of these catalysts measured by ICP-OES
are listed in Table 1. With an increase in reduction temperature,
the Pt content increase obviously (Table 1, entries 1, 3, 5 and 2, 4,
6, respectively). This also suggested that the mesoporous carbon
support was in a dynamic change state during high temperature
Fig. 2. N2 adsorption-desorption isotherms: (a) Pt/15AM-T catalysts; (b) Pt/15AM-
SF-T catalysts. The insets are the corresponding pore size distribution curves based
on the adsorption branch of isotherm.

X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
53
Fig. 3. TEM images of catalysts reduced by different methods. (a) Pt/15AM-400; (b) Pt/15AM-SF-400; (c) Pt/15AM-500; (d) Pt/15AM-SF-500; (e) Pt/15AM-600; (f) Pt/15AM-
SF-600. Insets in (a–f) are the corresponding particle size distributions.
3.3. TEM
Fig. 3 displays TEM images of Pt/15AM-T and Pt/15AM-SF-T
catalysts. TEM images of Pt/15AM-400 (Fig. 3a) catalyst showed
stripe-like and hexagonally arranged pore morphology, confirming
mesostructure with 2-D hexagonal pore symmetry [46], while the
other catalysts displayed non-visibility hexagonal pore structure.
This observation indicated that the loading of Pt precursor and
reduction at 400 ◦C did not destroy the mesostucture of 15AM
support, while hydro-thermal treatment or higher temperature
may cause the collapse of mesopores. All these results were in
good agreement with the low-angle XRD analysis. The particle

54
X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
Table 2
XPS analysis of various catalysts.
Entry
Catalyst
Cl/Pt
Pt/Al
a
1
2
3
4
5
Pt/15AM-600
Pt/15AM-SF-600
Pt/15AM-400
Pt/15AM-SF-400
Pt/Al2O3-600
0.13
0.01
0.09
0.03
–
0.44
0.56
0.61
0.88
a
Below detection limit.
size distribution of Pt/15AM-400 (Fig. 3a, inset) displayed a mean
diameter centered at 3.8 nm. When the catalyst reduced at a higher
temperature or reduced by aqueous solution of sodium formate,
much larger Pt particles could be obtained. This observation was
consistent with the wide-angle XRD analysis (Fig. 1b).
3.4. X-ray photoelectron spectra analysis
The chemical species on the catalysts surfaces and their propor-
tions were evaluated by XPS. The C1s peak at 284.6 eV was used
for calibration. The Cl/Pt and Pt/Al atomic ratios of the catalysts
are summarized in Table 2. It is interesting to observe that the
Pt/15AM-600 catalyst had no surface chlorine residues, whereas
the Cl/Pt ratios of Pt/15AM-400 and Pt/Al2O3-600 were 0.56 and
0.88, respectively. The considerable amount of chorine on the sur-
face of Pt/Al2O3-600 also verified the residual chlorine could not
totally removed by directly reduction of Pt/Al2O3 in hydrogen
[35,36,47,48]. The non-chlorine residues on the surface of Pt/15AM-
600 could be attributed to the water, which was generated from
the further carbonization of alumina–carbon composites. And the
water may take away the chlorine, which was originated from the
decomposition of Pt precursor during the high temperature reduc-
tion. For the Pt/15AM-SF-400 and Pt/15AM-SF-600 catalysts, there
were still amount of chlorine left on the surface after reduction
by sodium formate solution and washing by deionized water. This
must be caused by the hydrophobicity and porous structure of the
catalyst support, and the special structural properties barred the
water from rinsing the chlorine residuals. The resulting free chlo-
rine attached on the surface of the support was hard to remove
by post-treatment in hydrogen. As shown in Fig. 4, the binding
energy of Cl2p3/2 in Pt/Al2O3-600 and Pt/15AM catalysts were about
198.0 eV and 198.8 eV, respectively. The shift binding energy of
Fig. 5. Pt4f and Al2p photoelectron spectra of Pt/15AM-T and Pt/15AM-SF-T catalysts.
Pt/Al2O3 and Pt/15AM catalysts indicated that residual chlorines
were more strongly absorbed on the surface of alumina support
than on alumina–carbon composites. The residual chlorines on the
Pt/Al2O3 surface might present as a stable Al–Cl complex [36] or
PtClxOy [35], while the chlorines on Pt/15AM catalysts surface prob-
ably had a weak interaction with mesoporous carbon, and this weak
interaction made the binding energy shifted to a higher value.
Fig. 5 shows the spectra region comprising the Al2p and Pt4f
signals of Pt/15AM catalysts. The peak at 74.3 eV (Al2p) could be
assigned to Al2O3 [49] and the peak at 71.1–71.5 eV (Pt4f7/2) was
assigned to Pt0 [50]. The Pt/Al molar ratios of these catalysts were
calculated according to fitted spectra and Pt/15AM-600 furnished
the highest Pt/Al ratio of 0.13 (Table 2). Compared to Pt/15AM-T
catalysts, Pt/15AM-SF-T catalysts obtained much lower Pt/Al ratios.
This observation could be attributed to three points: firstly, the
hydro-thermal treatment during aqueous reduction destroyed the
mesostructure of Pt/15AM-SF-T catalysts (Table 1, Fig. 2b), thus
much more alumina exposed to the surface and more platinum par-
ticles were masked (Fig. 5); secondly, the aqueous reduction may
result in somewhat platinum loss, which was proved by the ICP-
OES test (Table 1); thirdly, the aqueous reduced catalysts may have
a change in the platinum particles, from plat particles to spherical
or hemispherical, resulting in a low Pt/Al ratio [36].
In summary, for our alumina–carbon supported Pt catalysts,
one-step 600 ◦C reduction method avoided the Pt loss and struc-
ture deterioration during traditional aqueous reduction. Moreover,
one-step reduction procedure saved the extra time without calcina-
tion and drying. This simple, effective reduction method can replace
the fussy traditional reduction method and also provides an alter-
native for the preparation of other carbon composites supported
metal catalysts.
3.5. Catalytic performance of Pt catalysts with different reduction
methods
The conversions and ee values for asymmetric hydrogenation
of ethyl pyruvate on CD-modified Pt/15AM-T and Pt/15AM-SF-T
are listed in Table 3. It is note that all the catalysts afforded over
96% conversions with the exception of the Pt/15AM-400 catalyst
(Table 3, entry 1). With an increase in reduction temperature, the
ee value increased obviously (Table 3, entries 1, 3, 5 and entries
Fig. 4. Cl2p photoelectron spectra of several catalysts.

X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
55
Table 3
Effects of catalyst reduction methods on the enantioselective hydrogenation of ethyl pyruvate on CD-modified Pt/15AM and CD-modified Pt/Al2O3.a
H
O
O
Pt/Alumina-Carbon
Cinchonidine, Solvent, H2
H
N
O
O
C OH
H
O
OH
ethyl pyruvate
(R)-(+)-ethyl lactate
N
Cinchonidinne
Entry
Sample
Conv.%
ee%
1
2
3
4
5
6
7
8
Pt/15AM-400
Pt/15AM-SF-400
Pt/15AM-500
Pt/15AM-SF-500
Pt/15AM-600
Pt/15AM-SF-600
Pt/Al2O3-600
83.9
96.7
69.7
73.4
79.0
80.5
87.5
82.2
67.3
90.4
100
99.6
100
100
86.8
100
Pt/Al2O3-SF-400
a
Reaction conditions: Pt catalyst (100 mg); CD (10 mg); ethyl pyruvate (2 mL); acetic acid (25 mL); H2 pressure (5 MPa); RT; 1000 rpm; 0.5 h and the main
configuration is R.
2, 4, 6, respectively) and the Pt/15AM-600 afforded the highest
ee value of 87.5% (Table 3, entry 5). These observations could
be attributed to the changes of structure properties and surface
chemical species by different reduction methods. Firstly, the higher
temperature resulted in the further carbonization of the support,
which facilitated an increase in exact Pt content relatively (Table 1)
and provided more active sites on the surface. Secondly, much more
chlorines were taken away by the higher temperature reduction
(Table 2) while the chlorines were considered to be poison to active
Pt [35]. Thirdly, the further carbonization of carbon composites
resulted in an increase relative amount of alumina, which could
form a kind of electrophilic alumina compound in acetic acid and
provide acidity sites. The alumina compound might be in favor of
the adsorption of CD and played an important role in the develop-
ment of chiral environment [51], and acidity sites linked with the
enantioselectivity directly [52]. Pt/15AM-SF-600 catalyst only fur-
nished 82.2% ee value, which was much lower than Pt/15AM-600
catalyst. The poor performance of Pt/15AM-SF-600 catalyst could
be attributed to its smaller average pore size (Fig. 2), the slightly
aggregation of Pt particles (Fig. 3), the lower surface Pt/Al atomic
ratio and the residual chlorine on the surface (Table 2). The former
limited the diffusion of CD to Pt surface, the later three gave rise to
a decrease in Pt active sites.
Table 4 shows the effects of catalyst reduction methods on the
asymmetric hydrogenation of EOPB. It is note that Pt/15AM-600
obtained the highest ee value of 84.8%. And the same tendency
with the hydrogenation of ethyl pyruvate is also found: the ee value
increased with the reduction temperature of corresponding cata-
lyst. Of particular note is the difference between the asymmetric
hydrogenation of ethyl pyruvate and EOPB. In the asymmet-
ric hydrogenation of ethyl pyruvate, Pt/Al2O3-SF-400 afforded
90.4% ee value, higher than Pt/15AM-600. However, Pt/15AM-600
obtained a bit higher ee value than Pt/Al2O3-SF-400 in the enantio-
selective hydrogenation of EOPB. This could be explained partly by
the large molecular size of EOPB. The Pt/15AM-600 had much larger
Pt size than Pt/Al2O3-SF-400 and the larger Pt size may beneficial to
Table 4
Effects of catalyst reduction methods on the enantioselective hydrogenation of EOPB on CD-modified Pt/15AM and CD-modified Pt/Al2O3.a
OH
O
OEt
OEt
Pt/Alumina-Carbon
O
O
Cinchonidine, Solvent, H2
Ethyl 2-oxo-4-phenylbutyrate
(EOPB)
(R)-(+)-Ethyl 2-hydroxy-4-
phenylbutyrate
((R)-(+)-EHPB)
Entry
Sample
Conv.%
ee%
1
2
3
4
5
6
Pt/15AM-400
Pt/15AM-SF-400
Pt/15AM-500
Pt/15AM-SF-500
Pt/15AM-600
Pt/15AM-SF-600
Pt/15AM-600
Pt/15AM-600
Pt/Al2O3-600
85.9
98.0
98.2
99.0
97.0
98.4
97.4
95.4
98.7
98.9
57.3
57.4
61.1
65.5
84.8
75.4
84.0
84.9
65.3
83.5
7b
8c
9
10
Pt/Al2O3-SF-400
a
Reaction conditions: Pt catalyst (100 mg); CD (10 mg); EOPB (1 mL); acetic acid (25 mL); H2 pressure (5 MPa); RT; 700 rpm; 1 h and the main configuration is R.
The amount of Pt catalyst was 50 mg.
EOPB (10 mL), CD (20 mg), acetic acid (50 mL), 2 h.
b
c

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X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
the co-adsorption of CD and EOPB simultaneously [20]. Ethyl pyru-
vate had a smaller molecular size, thus the steric hindrance of the
diffusion may be not very important, while the surface states of the
catalysts may play an important role in determining the catalytic
enantioselectivity [29].
From the foregoing, many factors influenced the enantioselec-
tivity for the asymmetric hydrogenation of ethyl pyruvate and EOPB
on CD-modified Pt/15AM catalysts. On the surface of the catalyst,
Pt content may be the main factor. Pt/15AM-600 catalyst with the
highest exact Pt content (6.9 wt%) may result in the highest ee value.
However, Li and co-workers [21] had reported that there was no
obvious increase in both activity and enantioselectivity when the
Pt loading increased from 5 wt% to 10 wt%. And Azmat [53] also
had the similar report. Accordingly, we deduced that the Pt content
may be a main factor, which influenced the catalytic performance
not because of Pt absolute content, but by changing the Pt parti-
cle size, morphology and the interaction between Pt particle and
the support. 50 mg Pt/15AM-600 catalyst was applied to the asym-
metric hydrogenation of EOPB for proving the assumption. It is
note that 84.0% ee value (Table 4, entry 7) was obtained on 50 mg
catalyst in this reaction system, only slightly lower than 100 mg
catalyst. Meanwhile, the reaction system of 10 mL EOPB, 100 mg
Pt/15AM-600, 20 mg CD and 50 mL acetic acid also furnished 84.9%
ee value (Table 4, entry 8). These observations further confirmed
our assumption and indicated that the adsorption of CD on the
catalyst surface was irreversible as well [54].
In order to investigate the influence of residual chlorine on enan-
tioselectivity, Pt/Al2O3 catalysts were applied for comparisons.
Compared to Pt/Al2O3-SF-400, Pt/Al2O3-600 furnished much lower
enantioselectivity, only 67.3% and 65.3% ee values were obtained
for the enantioselective hydrogenation of ethyl pyruvate (Table 3,
entry 7) and EOPB (Table 4, entry 9), respectively. The physical
parameters of Pt/Al2O3-600 and Pt/Al2O3-SF-400 were very sim-
ilar (Table 1) except surface chemical species. Pt/Al2O3-600 had a
high surface chlorine concentration (Table 2), while Pt/Al2O3-SF-
400 was proved to be no surface chlorine residues [48]. Thus, the
low enantioselectivity on Pt/Al2O3-600 could be mainly attributed
to the surface chlorine residues. Similarly, Pt/15AM-600 had no
surface residual chlorine but the other Pt/15AM catalysts all had
chlorine residues on the surface. Therefore, we drew the follow-
ing speculations: One-step reduction method changed pore size,
Pt particle size, Pt exact content, surface atomic ratio and surface
chlorine residues; all of these factors partly influenced the enan-
tioselectivity of the catalyst in the hydrogenation reaction, but the
surface chlorine residue was the critical one.
Fig. 6. (A and B) Reusability of Pt/15AM-600 catalyst for the asymmetric hydrogena-
tion of ethyl pyruvate in acetic acid. Reaction conditions: Pt/15AM-600 (100 mg);
CD (10 mg); ethyl pyruvate (2 mL); acetic acid (25 mL); H2 pressure (5 MPa); RT;
1000 rpm; 0.5 h and the main configuration of product is R.
Besides the catalytic performance, the reusability of heteroge-
neous catalyst is also a very important matter to consider. Fig. 6
shows the conversions and ee values against the number of runs in
the asymmetric hydrogenation of ethyl pyruvate. It was note that
Pt/15AM-600 catalyst could be reused for 23 times without distinct
loss of activity (Fig. 6a). Since the fresh CD was added to each reac-
tion cycle, the enantioselectivity could maintained over 85% upon
23 times re-use (Fig. 6b). Compared to Pt/15AM-600 catalyst, tradi-
tional Pt/Al2O3 showed much more inferior reusability, only could
be re-used at best six times, which mainly caused by the Pt leaching
and the peptization of alumina in acetic acid [20,27,28]. The excel-
lent reusability of Pt/15AM-600 was mainly ascribed to the good
stability of the mesoporous carbon structure in acid conditions [55].
Furthermore, Pt particles could be stability by the ꢁ-donating from
the benzene rings of carbon composites to Pt particles [20]. In addi-
tion, one-step 600 ◦C reduction may result in more facet Pt particles,
which had much stronger interaction with the support [56]. These
three factors all reduced the Pt loss during each cycle in acetic acid.
The excellent reusability of these alumina–carbon composites sup-
ported Pt catalysts indicated the possibility of these novel catalysts
in industrial application.
4. Conclusion
Pt particles supported on 15% alumina–carbon composites
(Pt/15AM) were reduced by liquid phase reduction methods and
one-step high temperature methods. The catalyst was reduced by
one-step method with hydrogen at 600 ◦C (Pt/15AM-600) showed
superior structure and surface properties: high special surface area,
large pore size, high surface Pt/Al atomic ratio and no residual
chlorine on the surface. Moreover, CD-modified Pt/15AM-600 cat-
alyst afforded the highest enantioselectivity of 87.5% and 84.8%
in the asymmetric hydrogenation of ethyl pyruvate and EOPB,
respectively. Of particular note was the reusability of Pt/15AM-
600 catalyst, which could be reused for 23 times without distinct
loss in catalytic activity in the asymmetric hydrogenation of ethyl
pyruvate. This remarkable reusable heterogenous catalyst and the
simple, effective one-step reduction in the process of the cat-
alyst preparation may give catalyst workers inspiration on the
researches of carbon composites supported catalysts, and provide
a potential application in industry.

X. Zhang et al. / Applied Catalysis A: General 480 (2014) 50–57
57
Acknowledgement
[28] X. Li, Ph D thesis, Chinese Academy of Sciences, 2004.
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This research was supported by the Natural Science Foundation
of Fujian Province, China (Grant No. 2012J05026).
[33] S Böttcher, C. Hoffmann, W. Reschetilowksi, DE102011003540A1, Technische
Universitaet Dresden, Germany, 2012.
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