Enantioselective hydrogenation of α,β-unsaturated carboxylic acid over cinchonidine-modified Pd nanoparticles confined in carbon nanotubes

Article abstract of DOI:10.1016/j.jcat.2013.10.010

We report the enantioselective hydrogenation of α,β-unsaturated acid catalyzed by Pd nanoparticles in carbon nanotubes (CNTs) taking the advantage of the channels as nanoreactors. The Pd nanocatalyst inside the channels of CNTs shows higher activity and e

Full text of DOI:10.1016/j.jcat.2013.10.010

Journal of Catalysis 311 (2014) 1–5
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Journal of Catalysis
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Research Note
Enantioselective hydrogenation of a,b-unsaturated carboxylic acid over
cinchonidine-modified Pd nanoparticles confined in carbon nanotubes
⇑
Zaihong Guan, Shengmei Lu, Can Li
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the enantioselective hydrogenation of a,b-unsaturated acid catalyzed by Pd nanoparticles in
Received 14 June 2013
Revised 10 October 2013
Accepted 12 October 2013
Available online 5 December 2013
carbon nanotubes (CNTs) taking the advantage of the channels as nanoreactors. The Pd nanocatalyst
inside the channels of CNTs shows higher activity and enantioselectivity than that of Pd nanocatalyst out-
side the channels. As high as 92% enantioselectivity is achieved. The enhanced catalytic performance is
attributed to the enrichment of reactant, chiral modifier, and additive in the channels of CNTs. This work
demonstrates the unique feature of CNTs as nanoreactors for asymmetric catalytic reactions.
Ó 2013 Elsevier Inc. All rights reserved.
Keywords:
Carbon nanotubes
Asymmetric hydrogenation
Heterogeneous
Palladium
1. Introduction
pharmaceuticals, fragrances, and flavors [19–21]. The alkaloid-
modified palladium catalyst is the most studied and used hetero-
Due to the unique properties of carbon nanotubes (CNTs), the
improved catalytic performance has been reported when CNTs
were used as supports to disperse the metal catalysts on the outer
surface of the channels [1–3]. The well-defined one-dimensional
channels of CNTs with the diameter ranging from less than
1–100 nm provide the opportunities to use the channels of CNTs
as nanoreactors. The encapsulation of various substrates has been
realized [4–8], making it possible to develop efficient catalytic
systems inside the channels. However, only limited examples of
gas-phase reactions [9–14] and liquid-phase hydrogenation
reactions [15–17] inside the CNTs have been reported, and the
application of CNTs in heterogeneous asymmetric catalysis was
less explored. In 2011, this group reported the first study on
enantioselective hydrogenation in the channels of CNTs as
nanoreactors [18]. The results indicated that Pt nanoparticles
encapsulated in the channels of CNTs showed excellent catalytic
geneous catalyst for this reaction. Different types of support such
as TiO2 [22], Al2O3 [23], and active carbon [24] were used to load
palladium nanoparticles. Inspired by the special properties of the
CNTs, we investigated the enantioselective hydrogenation of
a,b-
unsaturated acid (Scheme 1) on Pd nanoparticles inside CNTs and
studied the essential role of CNTs as nanoreactors played in the
chiral induction and hydrogenation. We found that the catalytic
activity and enantioselectivity were greatly enhanced when the
Pd nanoparticles were located inside the channels of CNTs.
2. Experimental
2.1. Catalyst preparation
Two catalyst samples with Pd nanoparticles confined inside the
channels of CNTs (denoted as Pd/CNTs(in)) and loaded on the outer
surface of CNTs (denoted as Pd/CNTs(out)) were prepared in this
work. Preparation of 5 wt.% Pd/CNTs(in) and 5 wt.% Pd/CNTs(out)
was according to our previous report [18] using an improved wet
chemistry [25,26]. The pristine CNTs (denoted as CNTs(closed))
were first oxidized in concentrated HNO3 (68 wt.%). The as-pre-
pared oxidized CNTs (denoted as CNTs(open)) were used as sup-
port. For the synthesis of 5 wt.% Pd/CNTs(in), the Pd precursor
was introduced into the channels of CNTs by ultrasonic treatment
and prolonged stirring. After a slow drying process, the mixture
was reduced in a sodium formate solution and then washed with
deionized water and was dried to give Pd/CNTs(in). For the
synthesis of 5 wt.% Pd/CNTs(out), the channels of CNTs were filled
performance in the asymmetric hydrogenation of
a-ketoesters,
giving higher activity than the most efficient Pt/Al2O3 catalyst,
which encourages us to further develop other heterogeneous
asymmetric catalysis inside the channels of CNTs and explore the
unique features of CNTs as nanoreactors in catalysis.
Asymmetric hydrogenation of a,b-unsaturated carboxylic acids
is a convenient method for the synthesis of chiral carboxylic acid,
which is an important intermediate for the production of
⇑
Corresponding author. Fax: +86 411 84694447.
E-mail address: canli@dicp.ac.cn (C. Li).
URL: http://www.canli.dicp.ac.cn (C. Li).
0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.10.010

2
Z. Guan et al. / Journal of Catalysis 311 (2014) 1–5
Scheme 1. Asymmetric hydrogenation of
a,b-unsaturated carboxylic acid.
with xylene by ultrasonic treatment. The Pd precursor was added
into the mixture, followed by a controlled stirring and reduction
in a sodium formate solution. Then, the mixture was extracted
with ethanol, washed with deionized water, and dried to give Pd/
CNTs(out). The detailed description of the preparation can be seen
in the text below the transmission electron microscope (TEM) pic-
tures of the catalysts shown in Fig. S1. The preparation of the oxi-
dized multilayer graphene (GO) was similar to that of CNTs(open).
The commercial 5 wt.% STD Pd/AC (AC = activated carbon) used for
comparison is purchased from Alfa Aesar.
solvent were added to the mixture. After adjusting the hydrogen
pressure to the desired H2 pressure, the stirring was started. For
substrates 1–4, the mixture after reaction was first neutralized with
a diluted HCl solution, extracted with ethyl ether and analyzed by
hydrogen nuclear magnetic resonance (1H NMR, Bruker DRX
400 MHz type spectrometer) for conversion and a high perfor-
mance liquid chromatography (HPLC, 6890 Agilent Co.) equipped
with a chiral column (Daicel Chiralcel OJ-H) for enantiomeric excess
(ee). For substrates 5 and 6, the mixture after reaction was first neu-
tralized with a diluted HCl solution. Then, the catalyst and liquid
were separated by centrifugation. After passing the liquid phase
through a short silica column and drying, the conversion and
enantioselectivity of 5 or 6 were determined on a gas chromato-
2.2. Catalyst characterization
graph (Agilent 6890, 30 m ꢀ 0.32 mm ꢀ 0.25
lm HP19091G-B213
Catalysts were usually pre-treated in H2 stream for 0.5 h at
375 °C before characterization. The TEM measurements were ob-
tained on TECNAI G2 electron microscope operating at an acceler-
ating voltage of 110 kV. The samples were outgassed at 120 °C
for 6 h before N2 sorption measurement tested on a Micromeritics
ASAP 2020 volumetric adsorption analyzer and CO chemisorption
measurement performed on a Quantachrome AUTOSORB-1-MS
volumetric adsorption analyzer. X-ray photoelectron spectroscopy
(XPS) analysis of the catalysts was performed on a VG ESCAB mk-2
capillary column).
The activity of the hydrogenation was expressed by the average
turnover frequency (TOF). TOF = ([M0] ꢀ conversion)/([MPd] ꢀ DPd
-
ꢀ (reaction time)). [M0] stands for the initial molar concentration
of the reactant. MPd stands for the Pd molar concentration used
in the reaction. DPd stands for the metal dispersion of Pd obtained
by CO chemisorptions. The conversion used to calculate the TOF
was lower than 30%. The ee (%) of the S isomers was expressed
according to ee (%) = ([S] ꢁ [R]) ꢀ 100/([S] + [R]).
instrument using Al Ka (1486.6 eV, 12.5 kV, 250 W) radiation. The
samples were pressed into a sample holder and evacuated to
0.8–0.4 ꢀ 10ꢁ6 Pa. The Pd loading was measured by inductively
coupled plasma spectroscopy (Shimadzu ICPS-8100). The deter-
mined Pd loadings of Pd/CNTs(in) and Pd/CNTs(out) are 5.02 wt.%
and 4.99 wt.%, respectively.
Adsorption of CD, BA, 1, and 5 by the catalysts was measured in
a similar method. Take CD as an example, the catalysts were pre-
treated in H2 stream at 375 °C for 0.5 h before the adsorption test.
For the adsorption test of CD, the work solution was prepared by
adding 5 mg CD in100 mL of 1,4-dioxane with H2O content of
2.5% (v/v). After stirring, the mixture was centrifuged and 2 mL
of the supernatant was taken out and analyzed on a SHIMADZU
UV-2550 UV/Vis spectrophotometer with a 1,4-dioxane with H2O
content of 2.5% (v/v) as a comparison. The details for adsorption
of BA, 1, and 5 can be found in Figs. S5 and S6.
3. Results and discussion
The TEM characterization shows that the Pd nanoparticles of
Pd/CNTs(in) are uniformly distributed inside the channels of CNT
(Fig. S1(a)) and that the Pd nanoparticles of Pd/CNTs(out) were al-
most exclusively dispersed on the outer surface of CNTs
(Fig. S1(b)). Table 1 gives the catalytic performance of Pd/CNTs
and the commercial STD Pd/AC in the hydrogenation of a-phenyl-
cinnamic acid analogues (1–4) and aliphatic analogues (5 and 6)
using CD as the chiral modifier and BA as additive. The TOF of
Pd/CNTs(in) in racemic hydrogenation of 1–6 is higher than that
of Pd/CNTs(out) and STD Pd/AC. In particular, the highest TOF of
Pd/CNTs(in) for
a-phenylcinnamic acid analogues reaches over
260 hꢁ1 (Table 1, entry 3), about 1.5 times of that Pd/CNTs(out)
and about 2 times of that of STD Pd/AC. For the aliphatic analogues
(5 and 6), the Pd/CNTs(in) also shows the highest TOF value of
1089 hꢁ1 (Table 1, entry 16). It is noteworthy that the addition of
BA decreases the activity of racemic hydrogenation by half (Table
1, entries 4 and 17).
2.3. Catalyst testing
General procedure for hydrogenation: Hydrogenation was car-
ried out in a magnetically stirred autoclave in 1,4-dioxane contain-
ing 2.5% (v/v) of water under an atmospheric pressure of hydrogen
at room temperature. The catalyst (20 mg) was pre-treated in a H2
flow at 375 °C for 30 min. After cooling down and transferred to
the autoclave, the catalyst was pre-treated with modifier CD
(0.02 mmol) in 4 mL solvent for an additional 30 min. Then, certain
amount of substrate and BA, as an effective additive, in 2 mL
Modified by CD, the activity for these three catalysts is de-
creased. This is unlike the ‘‘ligand acceleration’’ for CD-modified
Pt system [27]. The TOF of Pd/CNTs(in) in chiral hydrogenation of
1–6 is also much higher than that of Pd/CNTs(out) and STD Pd/
AC. The addition of benzyl amine as additive promotes not only
the activity but also the enantioselectivity of 1–6. Modified by

Z. Guan et al. / Journal of Catalysis 311 (2014) 1–5
3
Table 1
The catalytic performance of Pd/CNTs(in), Pd/CNTs(out), and STD Pd/AC on hydrogenation of
a
,b-unsaturated carboxylic acid.
Entry
Substrate
Pd/CNTs(in)
Pd/CNTs(out)
STD Pd/AC
TOF (hꢁ1
)
Ee (%)
TOF (hꢁ1
)
Ee (%)
TOF (hꢁ1
)
Ee (%)
1
2
3
4
1a
1b
1c
1d
62
29
261
119
75
65
–
36
24
175
94
61
51
–
42
19
140
77
64
52
–
–
–
–
5
6
7
2a
2b
2c
33
22
101
84
75
–
20
17
90
68
64
–
29
16
84
69
58
–
8
9
10
3a
3b
3c
29
20
84
92
74
–
20
12
56
80
64
–
21
13
52
79
68
–
11
12
13
4a
4b
4c
60
26
237
36
30
–
32
14
118
31
20
–
24
10
96
30
20
–
14
15
16
17
5a
5b
5c
5d
248
176
1089
746
37
30
–
123
140
743
502
31
26
–
106
120
265
174
30
26
–
–
–
–
18
19
20
6a
6b
6c
381
229
1006
42
38
–
204
172
681
37
33
–
176
148
240
35
32
–
Reaction conditions: 20 mg catalyst (pre-treated in H2 stream at 375 °C for 0.5 h before reaction), 6 mg (0.02 mmol) CD, BA/reactant = 0.6 (molar ratio). For substrate 1–4:
0.34 mmol reactant, 6 mL 1,4-dioxane containing 2.5% (v/v) of water, 0.1 MPa H2 pressure. For substrate 5 and 6: 1.5 mmol reactant, 6 mL toluene, 5.0 MPa H2 pressure. The
preparation of substrates 2 and 3 is described in Supplementary material.
a
With BA as additive (BA/substrate molar ratio of 0.6).
Without BA.
Without CD and BA.
With BA and without CD.
b
c
d
CD, Pd/CNTs(in) obtains 65% ee and TOF of 29 hꢁ1 for 1 (Table 1,
entry 2). While in the presence of benzyl amine, Pd/CNTs(in) deliv-
ers 75% ee and TOF of 62 hꢁ1 for 1 (Table 1, entry 1). Compared
with 1, the p-methoxyl substitutions at the benzene rings (2 and
3) further improve the enantioselectivity. Pd/CNTs(in) delivers as
high as 92% ee for 3 (Table 1, entry 8), which is equivalent to the
highest ee obtained on the commercial STD Pd/AC (N.E. Chemcat)
reported by Sugimura [24]. While for the methylcinnamic acid
(4) and aliphatic analogues (5 and 6), Pd/CNTs(in) delivers rela-
tively high activity but only achieves middle or relatively low
enantioselectivity even in the presence of BA. From the catalytic
performance demonstrated in Table 1, it can be concluded that
the catalytic activity and enantioselectivity are greatly enhanced
when the Pd nanoparticles are located inside the channels of CNTs.
The average sizes of Pd nanoparticles in Pd/CNTs(in) and Pd/
CNTs(out) estimated by CO chemisorption are 4.9 nm and 5.1 nm,
respectively (Table S1). The small difference in particle size cannot
explain the enhanced catalytic performance of Pd/CNTs(in). Be-
sides, the XPS analysis shows that no obvious difference in the dis-
tributions of Pd species in Pd/CNTs(in) and Pd/CNTs(out) is
observed (Details in Supplementary material).
In our previous work [18], it is found that the high activity and
enantioselectivity are ascribed mainly to the enrichment of the chi-
ral modifier and reactant in channels of CNTs. Therefore, the
adsorption of the chiral modifier, additive, and reactant by the cat-
alysts is investigated. The chiral modifier, CD, has relatively larger
molecular size than the others. Based on the previous theoretical
calculation results [28], the size of CD is estimated to be around
1 nm2. The inner diameter of channels is about 5–10 nm (Fig. S1).
So the pores are large enough for the substrates related to the
reaction to diffuse into the inner surface of the channels. Since
the adsorbents have the similar morphology and surface areas (Ta-
ble S3), the adsorption molar ratio is used to directly and clearly
demonstrate the adsorption behaviors of the catalysts and the sup-
ports. Fig. 1 shows the adsorption of the chiral modifier (CD) by Pd/
CNTs, CNTs(open), CNTs(closed), and GO. Pd/CNTs(in) exhibits the
highest CD adsorption molar ratio (85%). Pd/CNTs(out) and CNTs(o-
pen) show
a slightly lower adsorption molar ratio, while
CNTs(close) shows a much lower adsorption molar ratio. The
adsorption measurement is unable to discriminate the adsorption
on the outer surface and in the channels. However, due to the cap-
90
85
Pd/CNTs(in)
Pd/CNTs(out)
CNTs(open)
CNTs(closed)
GO
80
75
70
10
0
0
20
40
60
Time (min)
Fig. 1. Comparison of the adsorption molar ratio of CD as a function of adsorption
time for Pd/CNTs(in) (j), Pd/CNTs(out) (d), CNTs(open) (N), CNTs(closed) (.), and
GO (4).

4
Z. Guan et al. / Journal of Catalysis 311 (2014) 1–5
illarity of the channels, the adsorption in the channels may take up
the major part. The adsorption of CD on the GO is test and used to
estimate the adsorption on the outer surface of CNTs. Take CD as an
example, the GO adsorbs about 13% of CD (Fig. 1). So the channels
take up major part of the adsorption (72%). In other words, the
adsorption amount in the channels is 6 times as that on the outer
surface of CNTs. The enrichment of BA and the reactants in the
channels are also estimated: 4 times for BA, 6 times for substrate
1, and 2 times for substrate 5 (Details in Supplementary material).
The influence of CD/BA concentration on the catalytic perfor-
mance of Pd/CNTs(in), Pd/CNTs(out), and STD Pd/AC is investi-
gated. Generally speaking, the TOFs of the three catalysts
decrease with the increase in CD concentration (Fig. 2), and the
enantioselectivity of gradually increases with the increase in CD
concentration (Fig. 2). While it is worth to point out that due to
the enrichment of CD in the channels of CNTs, Pd/CNTs(in) is more
sensitive to the change in CD concentration in the beginning. After
addition of only 1 mg CD, the activity and enantioselectivity of Pd/
CNTs(in) decrease and increase more rapidly than Pd/CNTs(out)
and STD Pd/AC (Fig. 2). As for the influence of BA concentration,
similar to the circumstances of CD, the activity and enantioselec-
tivity of Pd/CNTs(in) increase more rapidly than Pd/CNTs(out)
and STD Pd/AC in the beginning due to the enrichment of BA in
the channels (Fig. S7). These results suggest that the enrichment
of CD/BA obviously influences the reaction inside the channels of
CNTs.
with quinoline ring perpendicular to the Pd surface (denoted as
‘‘N-lone pair bonded CD’’) [30]. It is generally accepted that the
p
-bonded CD plays an important role in the enantiodifferentiation
[27,29]. The carboxyl group of the ,b-unsaturated acid interacts
with the basic quinuclidine N of the -bonded CD [30] and is
a
p
hydrogenated to give the product. Due to the acid–base interaction
between the product and CD, product desorption is found to be the
rate-determining step [31]. Recently, the study by in situ attenu-
ated total reflection infrared spectroscopy shows that the addition
of BA not only improves the desorption of product by competing
with CD through the acid–base interaction, which improves the
activity of the chiral hydrogenation, but also facilitates a gradual
transformation from the N-lone pair bonded CD to the
p-bonded
CD forming a better defined surface for the enantiodifferentiation
[30]. Therefore, as a non-chiral molecule, BA improves both the
activity and the enantioselectivity of the chiral hydrogenation of
a
a
,b-unsaturated acid (Table 1). For the racemic hydrogenation of
,b-unsaturated acid, the high activity of Pd/CNTs(in) is mainly
attributed to the enrichment of reactant in channels of CNTs. For
the chiral hydrogenation, the CD modification decreases the activ-
ity. Modified by CD, we suggest the higher activity of Pd/CNTs(in) is
the outcome of the enrichment of reactant and CD in the channels.
Besides, another benefit of the enrichment effect is that the en-
riched CD and BA within the channels may facilitate the interaction
with each other, thus resulting into a better chiral environment for
enantiodifferentiation on the Pd surface. So Pd/CNTs(in) delivers
relatively higher enantioselectivity than that of Pd/CNTs(out).
Based on the above results, we suggest that in the limited space
of the channels of CNTS, the enrichment of the substrates related
As chiral modifier, CD can adsorb nearly parallel to the Pd sur-
face via the quinoline ring (denoted as ‘‘
p-bonded CD’’) [27,29] or
to the reaction facilitates the chiral hydrogenation of
a,b unsatu-
rated carboxylic acid, resulting into the relatively higher activity
and enantioselectivity of Pd/CNTs(in) than those of Pd/CNTs(out).
a
260
120
100
80
60
40
20
0
4. Conclusions
Pd/CNTs(in)
Pd/CNTs(out)
STD Pd/AC
In summary, we demonstrate CNTs as nanoreactors for the Pd
nanoparticle-catalyzed
enantioselective
hydrogenation
of
a,b-unsaturated acid. Due to the enrichment of reactant, CD and
BA in the channels, the Pd nanocatalyst inside the channels of CNTs
achieves higher activity and enantioselectivity than those of Pd
nanocatalyst outside the channels and the commercial STD Pd/
AC. This work further demonstrates the unique adsorption prop-
erty of CNTs as nanoreactors and the importance of this feature
in a wide range of catalytic reactions.
0
3
6
9
CD (mg)
Acknowledgment
70
60
50
40
30
20
10
0
b
This work was financially supported by the National Natural
Science Foundation of China (NSFC Grant Nos. 20921092 and
20902032).
Appendix A. Supplementary data
Pd/CNTs(in)
Pd/CNTs(out)
STD Pd/AC
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2013.10.010.
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