Alumina incorporated with mesoporous carbon as a novel support of Pt catalyst for asymmetric hydrogenation

Article abstract of DOI:10.1016/j.catcom.2013.07.044

Mesoporous carbon incorporated with different alumina contents has been prepared by chelate-assisted co-assembly method. These composites were used as supports for Pt particles, and the as-prepared catalysts were reduced at 873 K in hydrogen atmosphere. Our current study by using N₂ sorption, X-ray diffraction and transmission electron microscopy revealed that carbon incorporated with 10-15 wt% alumina was favorable for the high Pt dispersion and retained the mesostructure of carbon. Moreover, 15 wt% alumina-carbon composite supported Pt particles modified by cinchonidine afforded the highest (84.8%) enantiomeric excess and could be reused at least five times for the asymmetric hydrogenation of ethyl 2-oxo-4-phenylbutyrate in acetic acid.

Full text of DOI:10.1016/j.catcom.2013.07.044

Catalysis Communications 42 (2013) 68–72
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Alumina incorporated with mesoporous carbon as a novel support of Pt
catalyst for asymmetric hydrogenation
Qiang Li, Xueqin Zhang ⁎, Meitian Xiao, Yongjun Liu
College of Chemical Engineering, Huaqiao University, Xiamen, 361021, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2013
Received in revised form 31 July 2013
Accepted 31 July 2013
Available online 8 August 2013
Mesoporous carbon incorporated with different alumina contents has been prepared by chelate-assisted co-
assembly method. These composites were used as supports for Pt particles, and the as-prepared catalysts were
reduced at 873 K in hydrogen atmosphere. Our current study by using N sorption, X-ray diffraction and
2
transmission electron microscopy revealed that carbon incorporated with 10–15 wt% alumina was favorable
for the high Pt dispersion and retained the mesostructure of carbon. Moreover, 15 wt% alumina-carbon
composite supported Pt particles modified by cinchonidine afforded the highest (84.8%) enantiomeric excess
and could be reused at least five times for the asymmetric hydrogenation of ethyl 2-oxo-4-phenylbutyrate in
acetic acid.
Keywords:
Pt supported catalyst
Co-assembly
Alumina-carbon composite
Asymmetric hydrogenation
© 2013 Elsevier B.V. All rights reserved.
1
. Introduction
Asymmetric catalysis plays an extremely important role in synthe-
limited the application of the catalyst in industry. Meanwhile, Wang
and co-workers [17] incorporated alumina with well-ordered hexagonal
mesoporous silica structures (SBA-15) by a solvent-free solid-state
grinding method. The Al -SBA-15 composites were proved to be
sizing chiral drugs, agrochemicals, food addictives and fragrances [1].
Enantioselective hydrogenation of activated ketones has featured
considerably in the synthesis of important organic compounds and
has attracted the attention of synthetic chemists [2]. Among these
reactions, the asymmetric hydrogenation of α-ketoesters on the
cinchona-modified supported Pt catalyst has initiated an avalanche of
research in asymmetric heterogeneous catalysis [3–5].
Since the cinchona-platinum/support catalyst was firstly reported
by Orito [3,6], a variety of researches have been done focused on the
supports for platinum nanoparticles. In the beginning, Al
SiO [10,11], zeolite [12], etc., were considered to be suitable supports.
However, these materials supported Pt catalysts showed only mediocre
performance, except cinchonidine(CD)-modified 5 wt% Pt/Al
exhibited excellent enantioselectivity. With the development of
periodic mesoporous materials, MCM-41 [13], FDU-type periodic
mesoporous resols (PMRs) [14] and CMK-8 carbons [15] were
adopted as supports for Pt nanoparticles in the heterogeneous asym-
metric hydrogenation of α-ketoesters, obtaining medium to good
enantiomeric excesses (ees).
2 3
O
remarkable supports for Pt nanoparticles. And the resulting catalyst
modified with CD was found to be effective in the chiral hydrogenation
of ethyl pyruvate. In contrast, the performance of Al
supported catalysts was in medium level and this was attributed to the
weak interaction between Al and FDU-14 PMRs [18]. Interestingly,
2 3
O -FDU-14 PMRs
2 3
O
Böttcher and co-workers observed high enantioselectivity in the
hydrogenation of ethyl pyruvate while using alumina-fused silica
composites as supports for Pt nanoparticles, CD as modifier [19].
Sun and co-workers [20] demonstrated that guest precursors, such
2 3
O [4,7–9],
2
3 3 3 2
as Fe(NO ) and Co(NO ) could be incorporated with phenolic resol
by a chelate-assisted co-assembly method. Followed by calcination,
mesoporous carbon with embedded metal oxide nanoparticles was
formed. Herein, chelate-assisted co-assembly method was used to
prepare mesoporous carbon incorporated with highly dispersed
2 3
O
2 3 2 3
uniform Al O particles. These Al O -carbon composites were used as
supports for Pt particles and the catalytic properties of the resulting
catalysts were studied in the asymmetric hydrogenation of ethyl 2-
oxo-4-phenylbutyrate (EOPB).
Recently, Chen and co-workers [16] reported the usage of carbon
nanotubes (CNTs) as support for Pt nanoparticles. The resulting catalyst
was found to exhibit superior performance in the hydrogenation of α-
2. Experimental section
ketoesters compared to the traditional Pt/Al
O
2 3
system. However, the
2.1. Preparation of Pt catalysts
uncontrollable procedure of the catalyst preparation and costly CNTs
Soluble resol precursors were prepared according to the litera-
ture [20]. The preparation of Al
2 3
O -carbon composites and the Pt/
⁎
Corresponding author. Tel.: +86 592 6162285; fax: +86 592 6162300.
E-mail address: xqzhang2009@hqu.edu.cn (X. Zhang).
2 3
Al O -carbon catalysts were depicted in Fig. 1. Typically, pluronic
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.07.044

Q. Li et al. / Catalysis Communications 42 (2013) 68–72
69
F127 (1.0 g) was dissolved in absolute ethanol (20 mL). The resol
precursor solution (20 wt% in ethanol, 6 mL) was added and stirred
for 10 min. Then a solution of Al(NO ) ·9H O (0.75 g) dissolved in
3 3 2
concentration for the Pt/Al
the asymmetric hydrogenation of EOPB. Herein, we used the same
concentration of CD for the Pt/xAM catalysts. Typically, 0.1 g of catalyst,
2
O
3
[21] and Pt/mesoporous carbon [22] in
ethanol (6 mL) was dropped slowly into the above mixture.
Subsequently, acetyl acetone (acac) (0.61 mL) was added (the
molar ratio of acac/Al is 3/1). The mixture was cast onto Petri dishes
after further stirring for 40 min. After the evaporation of the solvent
2
10 mg of CD, 1 mL of EOPB, 25 mL of solvent, 5 MPa of H and a stirring
speed of 700 rpm were used for the reaction. The reaction was
terminated after 1 h and then the products were analyzed by gas
chromatography (Agilent 6890) equipped with capillary chiral column
(CP-ChiraSil-DEX CB 25×0.25 Agilent). The pure (R)-(+)-EHPB was
used as the marker, and external standard method was used for
identifying the major product. 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.
(
6–10 h) at room temperature, the resulting sticky film was
subjected to thermocuring at 373 K for 24 h. The obtained composite
film was scrapped off and cut into pieces, followed by pyrolysis in
tube furnace at 873 K for 3 h under N
ramp rate was 1 K/min). The resulting composite was ground into
powders. A series of samples with different Al -contents were
synthesized and designated as xAl –MC (MC is the abbreviation
of mesoporous carbon, x refers to Al weight content (wt %) in
the Al –MC), which were denoted hereafter as xAM.
2
atmosphere (the temperature
2 3
O
2 3
O
2 3
O
2
O
3
3. Results and discussion
The 5 wt% Pt/xAM catalysts were prepared by impregnation method:
xAM was impregnated with an aqueous solution of platinum precursor
3.1. XRD
2 6
(H PtCl ) and stirred for 5 h. Then the mixture was evaporated to
remove excess water, followed by drying at 373 K for 12 h. The catalyst
precursors were reduced at 873 K in a hydrogen atmosphere before use.
Fig. 2 exhibited the low-angle and wide-angle XRD patterns of the
xAMs and Pt/xAMs. Low-angle XRD patterns of xAMs (Fig. 2a) showed
a resolved diffraction peak at 2θ = 0.6–1.0°, which could be indexed
to the (110) reflection of a 2-D p6mm hexagonal mesostructure.
2
.2. Characterization
Along with the increase in Al
diffraction intensities decreased obviously, suggesting that excess incor-
poration of Al deteriorated the structure regularity of mesoporous
2 3
O content from 5 to 25 wt%, the
Powder X-ray diffraction (XRD) patterns were obtained in a
Panalytical X'Pert-Pro powder X-ray diffractometer using Cu Kα
radiation (40 kV, 40 mA). N adsorption–desorption isotherms were
2 3
O
2
carbon. That the very weak diffraction intensity at 2θ = 1.0° of MC
attributed to the shrinkage and collapse of the mesopores after the
removal of F127. In the wide-angle XRD patterns of xAMs (Fig. 2b),
two weak and broad diffraction peaks centered at 22.5° and 43.5°
characteristic of amorphous carbon appeared [23]. No distinct diffrac-
tion peaks assigned to the crystalline alumina phases were observed
from all the curves, indicating that the alumina was of low crystallinity
and highly dispersed in the mesoporous carbon. This probably attribut-
recorded on a Micrometitics ASAP 2020M+C analyzer at 77 K. The
BET method was adopted to calculate the special surface areas using
adsorption data in a relative pressure range from 0.05 to 0.20. The
pore size distributions were derived from the adsorption branches of
isotherms using the BJH model. Transmission electronic microscopy
(
TEM) images were taken on a HITACHI H-7650 electronic microscope
with an accelerating voltage of 100 kV. The exact Pt contents of the
catalysts were measured with an Optima 7000 DV inductively coupled
plasma optical emission spectroscopy (ICP-OES).
x
ed to the high melting point of Al(acac) (468 K), which decomposed to
2 3
Al O during pyrolysis at 873 K [24]. Without the addition of acac,
aluminum species in the resol could readily migrate, aggregate and
grow during both thermosetting and pyrolysis treatment because of
the low melting point of aluminum nitrate (346 K). As for the decreased
intensity of (100) diffraction peak after Pt loading and reduction at high
temperature of the catalyst (Fig. 2c), it could be attributed to three
points: First, Pt loading led to pore-filling effects that reduce the scatter-
ing contrast between the pores and the framework [25]; Second, high
temperature catalyst reduction somewhat destroyed the mesoporous
2
.3. Catalytic tests
The catalytic properties of Pt/xAM catalysts for asymmetric hydroge-
nation of EOPB to (R)-(+)-ethyl 2-hydroxy-4-phenylbutyrate ((R)-
+)-EHPB) were evaluated in a 100-mL stainless-steel stirred pressure
(
reactor at room temperature. Based on some precedents, about 10 wt%
cinchonidine (CD, relative to the Pt supported catalyst) was the optimal
Fig. 1. Preparation of alumina-carbon composites and 5 wt % Pt/alumina-carbon catalysts.

70
Q. Li et al. / Catalysis Communications 42 (2013) 68–72
Fig. 2. Low-angle and wide-angle XRD patterns of xAMs (a, b) and Pt/xAMs (c, d).
carbon framework, which could reduce the regularity of mesoporous;
Third, the micropores generated by further carbonization of the
composites after catalyst reduction also made a contribution to reduce
the pores' regularity. The wide-angle XRD patterns of Pt/xAMs
15 wt% to 25 wt%, these pore parameters decreased. These results
implied that the presence of Al could limit the shrinkage of the
mesostructure of carbon during the high temperature reduction, and
pore-blocking occurred upon the incorporation of Al more than 15
wt%. N adsorption–desorption isotherms of 15AM and Pt/15AM
2 3
O
2 3
O
(Fig. 2d) displayed five resolved diffraction peaks, which could be
2
assigned to the (111), (200), (220), (311) and (222) reflections of
face-centered cubic metallic Pt (JCPDS card no. 04-0802). The intensities
(Fig. 3) showed representative type IV curves with pronounced H2
hysteresis loops, which attributed to the presence of well-developed
meso- and microporosity [20]. The pore size distribution curves (Fig. 3
inset) revealed the presence of relatively uniform mesopores in 15AM,
while pore parameters and special surface area became much larger
after Pt loaded and catalyst reduced (Table 1). This probably due to
the generation of much more micropores by further carbonization of
resol and the removal of trace amount of F127 when reduction at 873 K.
of diffraction peaks depended on the Al
amount inclusion of Al facilitated the dispersion of the Pt particles
17], whereas excess amount incorporation of Al resulted in the
aggregation of Pt after high temperature reduction.
2 3
O contents, namely, a certain
2 3
O
[
2 3
O
3
.2. Special surface area and pore size
The textural properties of Pt/xAMs were summarized in Table 1. It
was noted that with a rise of alumina content from 5 wt% to 15 wt%,
the BET surface areas, the total volume and pore size of these catalysts
increased slightly, while with a further increase in alumina from
Table 1
Relevant parameters of 15AM, 5 wt% Pt/xAMs a, Pt/Al
O
and Pt/C.
2
3
2
3
b
Pt (nm) Pt content c (wt %)
Sample
S
BET (m /g)
V
t
(cm /g)
D
p
(nm)
D
Pt/5AM
727
734
704
401
397
526
193
892
0.62
0.68
0.70
0.33
0.32
0.28
0.42
1.07
4.8
5.6
5.6
4.2
5.6
b2
6.8
3.8
7.2
5.2
6.1
8.1
—
6.0
7.7
6.9
6.5
—
Pt/10AM
Pt/15AM
Pt/25AM
15AM
Pt/MC
8.1
3.1
10.0
5.7
3.8
4.0
d
Pt/Al
Pt/C
O
2 3
e
a
The Pt/xAM samples were reduced at 873 K for 2 h in a hydrogen atmosphere.
Average Pt sizes were estimated by the Scherrer equation from XRD patterns.
Exact Pt contents were measured by ICP-OES.
Prepared according to Ref. [24].
Purchased from Alfa Aesar and pretreated in a hydrogen flow at 673 K for 2 h.
b
c
d
e
2
Fig. 3. N sorption isotherms and corresponding pore size distributions (inset) of 15AM
and Pt/15AM.

Q. Li et al. / Catalysis Communications 42 (2013) 68–72
71
Fig. 4. TEM images of 15AM (a) and Pt/15AM (b).
generated Ptδ+ species, and the Ptδ+ species probably responsible for
3
.3. TEM
TEM images of 15AM (Fig. 4a) showed stripe-like and hexagonally
the enantioselection [17]; third, in acetic acid, alumina can form a
+
kind of electrophilic alumina compound, O (Al(OAC)
)
2 3
(oxonium
arranged pore morphology over large domains, indicating an ordered
mesostructure with 2-D hexagonal pore symmetry. After Pt precursor
loading and reduction at high temperature of the catalyst, the 2-D
hexagonal pore morphology was not visible in the TEM images
ions), which may be in favor of the adsorption of CD and plays an
important role in the development of chiral environment [26].
2 3
With a further increase in Al O amount from 15 wt% to 25 wt%,
both the conversion and the ee values decreased (Table 2, Entry 4).
The decrease in the conversion attributed to the obvious decline of the
BET surface area and slightly sintering of Pt particles. The change of
the ee attributed to the decrease in the pore size, which might limit
the diffusion of CD to Pt surface. As for the Pt/MC catalyst only afforded
70.7% conversion and 67.5% ee value (Table 2, Entry 5), it may be due
to the serious destruction of mesostructure after high temperature
reduction resulting in the coverage of some active sites. With regard
(Fig. 4b). These observations were further supported by low-angle
XRD patterns (Fig. 2c). The non-visibility of the 2-D pore morphology
was caused by pore-filled effect and the destruction of the ordered
structure. The particle size distribution (Fig. 4b, inset) showed the
most probable Pt particle size was 6.5 nm, which was consistent with
the analysis of wide-angle XRD patterns (Fig. 2d and Table 1).
3
.4. Asymmetric hydrogenation of EOPB
2 3
to a bit higher ee value on Pt/15AM than on Pt/Al O , it may be caused
by the larger Pt particles of Pt/15AM. These larger particles may
give rise to more flat adsorption of CD on Pt surface and the flat adsorp-
tion of CD was considered to facilitate the reaction with enhanced
enantioselectivity [27].
The results for asymmetric hydrogenation of EOPB on CD-modified
Pt/xAM catalysts are listed in Table 2. It was noted that with an increase
in Al amount from 5 wt% to 15 wt%, the conversion changed slightly,
2 3
O
while the ee value increased obviously, and CD-modified Pt/15AM
afforded the highest ee value of 84.8% (Table 2, Entry 1-3). The
conversion was almost the same which may attribute to the similar
surface area, pore size and Pt size of these catalysts. The alumina played
an important role in determining the ee value could be interpreted as
follows: first, the presence of alumina can stabilize the mesostructure
of carbon and get relatively big pores, which facilitated the diffusion
and adsorption on catalyst surface of chiral modifier; second, the
introduction of alumina embedded into the mesopores of carbon host
The reusability of the CD-modified Pt/15AM in the asymmetric
hydrogenation of EOPB was also investigated. Fig. 5 showed the
conversion and the ee values against the number of runs. It was noted
that Pt/15AM could be reused for more than five times without any
loss of enantioselectivity and activity. The slight increase in ee value
after the first run may be due to the structural reorganization of
mesoporous carbon composite. The superior stability of Pt/15AM mainly
Table 2
a
Reaction results of CD-modified Pt/xAMs for asymmetric hydrogenation of EOPB.
Entry
Catalyst
Conv.%
ee%
1
2
3
4
5
7
8
Pt/5AM
Pt/10AM
Pt/15AM
Pt/25AM
Pt/MC
97.6
97.4
97.0
93.1
70.7
98.9
96.9
74.2
80.7
84.8
80.0
67.5
83.5
69.2
2 3
Pt/Al O
Pt/C
a
Reaction conditions: Pt catalyst (100 mg); CD (10 mg); EOPB (1 mL); acetic acid
(5 MPa); RT; 700 rpm; 1 h and the main configuration of product is R.
Fig. 5. Reusability of Pt/15AM catalyst in the asymmetric hydrogenation of EOPB. The re-
action conditions are same as Table 2, entry 3.
(
25 mL); H
2

72
Q. Li et al. / Catalysis Communications 42 (2013) 68–72
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