Selective semihydrogenation of alkynes on shape-controlled palladium nanocrystals

Article abstract of DOI:10.1002/asia.201201166

A systematic study on the selective semihydrogenation of alkynes to alkenes on shape-controlled palladium (Pd) nanocrystals was performed. Pd nanocrystals with a cubic shape and thus exposed {100} facets were synthesized in an aqueous solution through the reduction of Na₂PdCl₄ with L-ascorbic acid in the presence of bromide ions. The Pd nanocubes were tested as catalysts for the semihydrogenation of various alkynes such as 5-decyne, 2-butyne-1,4-diol, and phenylacetylene. For all substrates, the Pd nanocubes exhibited higher alkene selectivity (>90 %) than a commercial Pd/C catalyst (75-90 %), which was attributed to a large adsorption energy of the carbon-carbon triple bond on the {100} facets of the Pd nanocubes. Our approach based on the shape control of Pd nanocrystals offers a simple and effective route to the development of a highly selective catalyst for alkyne semihydrogenation. Catalysis³: The semihydrogenation of various alkynes by Pd nanocubes was investigated. The nanocubes exhibited high alkene selectivity and complete cis-selectivity, thus surpassing the Lindlar catalyst. The shape control of Pd nanocrystals provides a simple and efficient way for generating highly selective catalysts for the semihydrogenation of alkynes.

Full text of DOI:10.1002/asia.201201166

FULL PAPER
DOI: 10.1002/asia.201201166
Selective Semihydrogenation of Alkynes on Shape-Controlled Palladium
Nanocrystals
[
a]
[b]
[a]
[c]
[d]
Jooyoung Chung, Chanhoi Kim, Hansaem Jeong, Taekyung Yu, Do Huy Binh,
[
b]
[d]
[a]
[d]
Jyongsik Jang,* Jaichan Lee,* B. Moon Kim,* and Byungkwon Lim*
Abstract: A systematic study on the se-
lective semihydrogenation of alkynes
to alkenes on shape-controlled palladi-
um (Pd) nanocrystals was performed.
Pd nanocrystals with a cubic shape and
thus exposed {100} facets were synthe-
sized in an aqueous solution through
the reduction of Na PdCl with l-ascor-
catalysts for the semihydrogenation of
various alkynes such as 5-decyne, 2-
butyne-1,4-diol, and phenylacetylene.
For all substrates, the Pd nanocubes ex-
hibited higher alkene selectivity (>
90%) than a commercial Pd/C catalyst
(75–90%), which was attributed to
a
large adsorption energy of the
carbon–carbon triple bond on the {100}
facets of the Pd nanocubes. Our ap-
proach based on the shape control of
Pd nanocrystals offers a simple and ef-
fective route to the development of
a highly selective catalyst for alkyne
semihydrogenation.
Keywords: chemoselectivity
hydrogenation nanocrystals
nanocubes · palladium
·
·
2
4
·
bic acid in the presence of bromide
ions. The Pd nanocubes were tested as
Introduction
been the method of choice for semihydrogenation of alkynes
owing to its high alkene selectivity, although Pd/C catalysts
[3–5]
Selective semihydrogenation of alkynes to alkenes is of
great importance in the production of olefins in relation to
the synthesis of natural products and fine chemicals. Palla-
dium is one of the most widely used catalysts for hydrogena-
tion reactions, but it usually exhibits low alkene selectivity
in alkyne hydrogenation owing to its high efficiency in pro-
moting the addition of hydrogen to carbon–carbon double
bonds, which leads to further reduction into alkanes. Tradi-
tionally, the use of the Lindlar catalyst, which consists of de-
have also been employed.
The Lindlar catalysts are usu-
ally poisoned by a lead complex, with quinoline as an addi-
tional poison, to enhance the alkene selectivity. This makes
the Lindlar catalysts often suffer from the production of
[
1]
[
2]
[5]
harmful wastes due to the incorporation of toxic additives.
The catalytic properties of metal nanocrystals can be dras-
tically enhanced by the control of their shapes, which deter-
mine surface atomic arrangements and thus affect activity
[6]
and/or selectivity toward a specific catalytic reaction. Here
we demonstrate that for alkyne hydrogenation, high alkene
selectivity can be achieved simply by shaping Pd nanocrys-
tals to expose facets that favor the adsorption of alkyne. In
this work, we synthesized Pd nanocrystals with a cubic
shape and thus exposed {100} facets and then applied them
to selective semihydrogenation of various alkynes including
activated palladium supported on CaCO3 or BaCO , has
3
+
[
a] J. Chung, H. Jeong, Prof. B. M. Kim
Department of Chemistry
College of Natural Sciences
Seoul National University, Seoul 151-747 (Korea)
Fax : (+82)2-872-7505
E-mail: kimbm@snu.ac.kr
5
-decyne, 2-butyne-1,4-diol, and phenylacetylene. For all
substrates, the Pd nanocubes exhibited higher alkene selec-
tivity (>90%) as compared to commercial Pd/C catalyst
+
[
b] C. Kim, Prof. J. Jang
World Class University (WCU) program of Chemical Convergence
for Energy & environment (C2E2)
(
75–90%). Our results show great promise for the develop-
School of Chemical and Biological Engineering
Seoul National University, Seoul 151-747 (Korea)
E-mail: jsjang@plaza.snu.ac.kr
ment of highly efficient and reliable catalysts for selective
semihydrogenation of alkynes without employing toxic addi-
tives.
[
c] Prof. T. Yu
Department of Chemical Engineering
College of Engineering
Kyung Hee University
Youngin 446-701 (Korea)
Results and Discussion
[
d] D. H. Binh, Prof. J. Lee, Prof. B. Lim
School of Advanced Materials Science and Engineering
Sungkyunkwan University
Palladium nanocubes were synthesized using a previously
reported method. A typical transmission electron micros-
copy (TEM) image of the as-prepared Pd nanocrystals
showed the formation of nanocubes as major product in ad-
dition to a small portion of nanobars that were slightly elon-
gated along one direction (Figure 1a). The nanocubes exhib-
[7]
Suwon 440-746 (Korea)
E-mail: jclee@skku.edu
blim@skku.edu
+
[
] These authors contributed equally to this work.
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B. Moon Kim et al.
commercial Pd/C catalyst, how-
ever, semihydrogenation was
accompanied by significant
overhydrogenation, even at low
conversion of 5-decyne (Fig-
ure 3b), resulting in the forma-
tion of a 75:20 mixture of 5-
decene/decane at about 95%
conversion
of
5-decyne
(
Table 1). This observation
clearly demonstrates the higher
alkene selectivity of the Pd
nanocubes for the semihydroge-
nation of 5-decyne as compared
to the commercial Pd/C cata-
lyst.
We also carried out hydroge-
nation of 2-butyne-1,4-diol and
phenylacetylene
(Scheme 1,b
and c). At full conversion, the
Pd nanocubes exhibited high
alkene selectivities of 98 and
9
1% for the semihydrogenation
of 2-butyne-1,4-diol and phenyl-
acetylene, respectively (Table 1
Figure 1. Characterization of Pd nanocubes. a) TEM image, b) particle size distribution, and c) powder XRD
pattern. d) HRTEM image of a single Pd nanocube shown in (a) and the corresponding FT pattern (inset). and Figure 3,c and e). On the
contrary, the commercial Pd/C
catalyst resulted in the forma-
tion of an 88:11 mixture of 2-
it an average edge length of approximately 11 nm (Fig-
ure 1b). The powder X-ray diffraction (XRD) pattern of the
Pd nanocubes (Figure 1c) shows that they have a face-cen-
tered cubic (fcc) structure (Fm3m, a=3.958 ꢁ, JCPDS Card
No. 87-0641). The six faces of a Pd nanocube are bounded
by {100} facets, as revealed by high-resolution TEM
butene-1,4-diol/butane-1,4-diol at a full conversion of 2-
butyne-1,4-diol and an 80:19 mixture of styrene/ethylben-
zene at 99% conversion of phenylacetylene (Table 1 and
Figure 3,d and f). These results suggest that the Pd nano-
cubes could be applied as a highly selective catalyst for the
semihydrogenation of various alkynes.
(
HRTEM) imaging and the array of spots with square sym-
metry in the corresponding Fourier-transform (FT) pattern
(
Figure 1d).
We first investigated the catalytic properties of the Pd
nanocubes toward the hydrogenation of 5-decyne
Scheme 1a) and compared them with those of a commercial
(
Pd/C catalyst (10 wt.% of ca. 3 nm Pd nanoparticles on acti-
vated carbon support, Figure 2). The hydrogenation reac-
tions of 5-decyne were conducted at room temperature
under 1 atm hydrogen pressure with initial sample concen-
trations of about 0.20m in ethanol and 1.0 mol% of the Pd
nanocubes or commercial Pd/C catalyst. The reactant and
product distribution was evaluated by gas chromatography
(
GC) analysis. When the reaction was catalyzed by the Pd
nanocubes, high selectivity (95%) toward cis-5-decene was
observed at about 98% conversion of 5-decyne (Table 1).
After full conversion of 5-decyne, 5-decene was readily over-
hydrogenated to decane (Figure 3a), which indicates that
the {100} facets on the Pd nanocubes also catalyzed the hy-
drogenation of 5-decene effectively. It is worth noting that
the semihydrogenation and the overhydrogenation stages
are well separated, as shown in Figure 3a. In the case of the
Scheme 1. Reaction pathways for the hydrogenation of a) 5-decyne, b) 2-
butyne-1,4-diol, and c) phenylacetylene.
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B. Moon Kim et al.
that the nanocrystal shape was indeed responsible for the
observed catalytic behavior. Inductively coupled plasma
atomic emission spectroscopy (ICP–AES) analysis revealed
that Pd apparently did not leach from the Pd nanocubes
into the solution during the reaction. The as-synthesized Pd
nanocubes may contain residual bromide ions adsorbed on
their surface. The X-ray photoelectron spectroscopy (XPS)
analysis of the Pd nanocubes revealed the presence of Br
signals even after extensive washing of the nanocubes with
ethanol and water (Figure 6), which implies that it is very
difficult to completely remove bromide ions from the nano-
cube surface. We therefore examined whether bromide ions
adsorbed on the surface of Pd nanoparticles affect their cat-
alytic behavior. To this end, the hydrogenation of 5-decyne
was carried out with a Pd/C catalyst that had been pre-treat-
ed with KBr. We did not observe any noticeable change in
the catalytic behavior of the Pd/C catalyst before and after
the treatment with bromide ions (Figure 7). Our results sug-
gest that bromide ions adsorbed on the surface of Pd nano-
particles do not have a significant effect on the behavior of
the catalyst in alkyne hydrogenation.
We propose that the higher alkene selectivity of the Pd
nanocubes may arise from a large difference in the adsorp-
tion energies of alkynes and alkenes on their surface. In hy-
drogenation, the adsorption strengths of substrates on
a given metal surface are known to increase in the order of
Figure 2. a) TEM image of the commercial Pd/C catalyst used.
b) HRTEM image of a single Pd nanocrystal shown in (a). The inset
shows the corresponding FT pattern.
[8]
alkane<alkene<alkyne. To investigate the surface struc-
ture-sensitive adsorption of alkynes and alkenes, we per-
formed density functional theory (DFT) calculations taking
acetylene and ethylene as model molecules. Figure 8 illus-
trates the lowest energy states for the adsorption of acety-
[
a]
Table 1. Product distribution in the hydrogenation of alkynes.
[
b]
Substrate
Catalyst
Product distribution [%]
[c]
cis-Alkene
Alkane
lene and ethylene on Pd
ACTHUNGTNERNUNG( 100) and Pd ACHTNUGTRENNUNG
Pd nanocubes
Pd/C
Pd nanocubes
Pd/C
Pd nanocubes
Pd/C
95
75
98
88
91
80
3
20
2
11
9
19
5
2
-decyne
DFT calculations revealed that, on Pd
ACHTUNGTRENNUNG
energy of acetylene (ꢀ2.61 eV) is much larger than that of
ethylene (ꢀ0.96 eV; see Table 2). Furthermore, the differ-
-butyne-1,4-diol
phenylacetylene
Table 2. Adsorption energies of acetylene and ethylene on Pd
Pd(111) surfaces.
ACHTUNGTRENNUNG( 100) and
[
a] Reaction conditions: Substrate (0.20m) in ethanol and 1.0 mol% of
AHCTUNGTRENNUNG
catalyst at room temperature and 1 bar hydrogen pressure. [b] All the
products were identified by comparison with authentic samples using GC
analysis. [c] Formation of trans-alkene was not detected by GC analysis.
Substrate
Acetylene
Ethylene
Pd surface
(100)
(111)
(100)
(111)
Adsorption energy [eV]
A
H
U
G
R
N
U
G
ꢀ2.61
ꢀ2.01
ꢀ0.96
ꢀ0.88
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
We then investigated the effect of the catalyst concentra-
tion on the product distribution in the hydrogenation of 5-
decyne. Higher alkene selectivities were still observed in the
reactions in which Pd nanocubes were used at concentra-
tions ranging from 0.5 to 2.0 mol%; by contrast, reactions
with the Pd/C catalyst showed lower alkene selectivity in
the same range of concentrations (Figure 4). The result illus-
trate that higher alkene selectivity of the Pd nanocubes in
the semihydrogenation of alkynes originated from their
shape rather than catalyst concentration; thus, the chemose-
lective reduction of alkynes by shape-controlled Pd nano-
crystals is a feasible process.
ACHTUNGTRENNUNG
ence in binding energies between acetylene and ethylene
was larger on Pd(100) (DE=1.65 eV) than on Pd(111)
(DE=1.13 eV), thus suggesting that Pd(100) is better suited
for the selective binding of acetylene than Pd(111). The Pd
A
H
U
G
E
N
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
nanocubes would favor the adsorption of alkynes owing to
the predominant exposure of {100} facets on their surfaces.
During the semihydrogenation, unreacted alkynes could
readily substitute freshly formed alkenes on the {100} facets
of the Pd nanocubes and thus impede the overhydrogena-
tion into alkanes, which apparently contributes to the high
alkene selectivity. Once alkynes have been consumed, al-
kenes could be adsorbed on the {100} facets of the Pd nano-
Representative TEM images of the Pd nanocubes ob-
tained after the hydrogenation of 5-decyne confirmed their
morphological stability (Figure 5), thus strongly suggesting
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B. Moon Kim et al.
overhydrogenation to alkane
could be suppressed almost
completely by lowering the re-
action temperature to 08C
(
Table 3, entry 6). For compari-
son, reduction of 5-decyne to 5-
decene was carried out employ-
ing 1.0 mol% Lindlar catalyst.
After a reaction time of 18 h,
a mixture of products was ob-
tained consisting of 93.2% cis-
decene and 4.9% trans-decene
(
Table 3, entry 7). The Lindlar
catalyst is known to be very ef-
ficient for the semihydrogena-
tion of alkynes to cis-alkenes,
but reactions with this catalysts
often suffer from Z/E isomeri-
zation, low reactivity, and poor
[11]
reproducibility.
On the con-
trary, the reactions with Pd
nanocubes consistently showed
higher reactivity and much
more enhanced cis-alkene se-
lectivity than those with the
Lindlar catalyst. Furthermore,
Pd nanocubes could be easily
recovered by filtration, which
offers a very convenient way to
recycle the catalyst.
Conclusions
In summary, we have studied
the hydrogenation of alkynes
Figure 3. Hydrogenation of various alkynes on a, c, e) Pd nanocubes and b, d, f) commercial Pd/C catalyst. on shape-controlled Pd nano-
Substrates: a, b) 5-decyne; c, d) 2-butyne-1,4-diol; and e, f) phenyl acetylene.
crystals enclosed by {100}
facets. The Pd nanocubes exhib-
ited higher alkene selectivity in
cubes and then be hydrogenated to alkanes, as demonstrated
in Figure 3. Meanwhile, small Pd nanoparticles on the Pd/C
catalyst have a truncated octahedral shape, whose surface is
bound by a mix of {100} and {111} facets (Figure 2b). This
shape also contains a large fraction of edge atoms, which
serve as active sites not only for the semihydrogenation but
also for the overhydrogenation due to increased adsorption
the semihydrogenation of various alkynes including 5-
decyne, 2-butyne-1,4-diol, and phenylacetylene than a com-
mercial Pd/C catalyst owing to the predominant exposure of
[
a]
Table 3. Selective hydrogenation of alkynes using Pd nanocubes.
Entry Substrate Reaction time Product distribution [%]
cis-Alkene trans-Alkene Alkane
[9]
strength of alkenes and thus lower the overall alkene se-
lectivity.
1
2
3
4
5
6
7
1-octyne
80 min
85.7
81.9
84.4
85.2
89.3
93.6
93.2
0.0
0.0
0.0
0.0
0.0
0.0
4.9
8.6
12.7
7.3
6.6
7.8
1-decyne 90 min
2-decyne 90 min
3-decyne 90 min
4-decyne 80 min
5-decyne 10 h
[
[
[
a]
a]
a]
We have also carried out hydrogenation of several inter-
nal and terminal alkynes using the Pd nanocubes as cata-
lysts. As summarized in Table 3, hydrogenation reactions
using the Pd nanocubes exhibited extremely high cis-alkene
selectivities regardless of the location of the triple bond in
alkynes. In all the cases examined in the reduction of inter-
nal alkynes with the Pd nanocubes, no trace of trans-alkene
as product was detected (Table 3, entries 3–6). Moreover,
[
b]
0.2
0.0
[c]
5-decyne 18 h
[
a] Reaction conditions: Substrate (0.20m) and 1.0 mol% of catalyst in
ethanol were stirred at room temperature under 1 atm hydrogen pres-
sure. [b] The reaction was carried out at 08C. [c] Lindlar catalyst
(1 mol%, Acros) was used instead of Pd nanocubes.
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B. Moon Kim et al.
from Aldrich (Catalog no. 330108).
All other commercial materials were
purchased from Aldrich Chemical Co.
or Tokyo Chemical Industry Co. and
were used without further purification.
Synthesis of Pd Nanocubes
In a typical procedure, an aqueous so-
lution containing poly(vinylpyrroli-
done) (PVP, MW=55000, 11 mL,
1
05 mg), l-ascorbic acid (60 mg), KBr
(
200 mg), and Na PdCl (57 mg) was
2
4
heated at 808C in air under magnetic
stirring for 3 h and then cooled to
room temperature.
Alkyne Hydrogenation
Pd catalyst and degassed ethanol
(
2.0 mL) were added to a 5 mL round-
bottom flask, and the mixture was so-
nicated in an ultrasonic bath for
1
(
0 min. After sonication, substrate
0.40 mmol) was added to the flask
and the mixture was stirred at room
temperature under hydrogen bal-
a
loon. The course of the reaction was
periodically followed by analyzing ali-
quots (10 mL) by gas chromatography
(
GC) on a Hewlett Packard 5890 in-
strument.
Characterization
TEM studies were done with
a LIBRA 120 (Carl Zeiss) microscope
operated at 120 kV by drop casting
a
dispersion of nanoparticles on
a carbon-coated copper grid. HRTEM
analyses were performed using
a JEOL 2100F instrument operated at
2
00 kV accelerating voltage. Powder
XRD measurements were conducted
using an M18XHF-SRA (Mac Sci-
ence) diffractometer equipped with
a
CuKa radiation source (l=
Figure 4. Hydrogenation of 5-decyne on a) 0.5 mol% Pd nanocubes, b) 1.5 mol% Pd nanocubes, c) 2.0 mol%
Pd nanocubes, d) 0.5 mol% Pd/C, e) 1.5 mol% Pd/C, and f) 2.0 mol% Pd/C.
1.5406 ꢁ) at 40 kV and 300 mA
(12 kW). GC analyses were performed
with
using a HP-5 capillary column (30 mꢂ
.25 mm; coating thickness 0.25 mm).
a HP-6980 gas chromatograph
0
{100} facets on their surface. The Pd nanocubes also had
a cis-selectivity superior to that of the traditional Lindlar
catalyst in the reduction of internal alkynes. Our results
demonstrate that shape control of Pd nanocrystals provides
a simple and efficient way for generating highly selective
catalysts for alkyne semihydrogenation. The shape-con-
trolled Pd nanocrystals reported herein could serve as at-
tractive candidates for the fundamental study of shape ef-
fects of nanocrystals on various Pd-based catalytic reactions.
DFT calculations
First-principles calculations were performed to estimate the adsorption
energies of acetylene and ethylene on the Pd surface using the Vienna
Ab Initio Simulation Package (VASP). In the calculations, the mole-
cules are adsorbed on a Pd slab, which has a periodic 2ꢂ2 unit cell with
[
12]
4
and 5 layers for (100) and (111) surfaces, respectively. A vacuum layer
of 12 A was used to separate the metal slabs. Then, the adsorption
energy of the molecules was calculated by the following equation;
DEads ¼ Eadsorbed systemꢀðEclean metal slab þ Egas
Þ
Experimental Section
[
13]
Ultra-soft pseudopotentials were employed within a cut-off energy of
00 eV. Exchange correlation potential used in this calculation was
5
Materials
[
14]
Perdew Wang (PW-91), and the electron density was converged within
a tolerance of 1ꢂ10 eV. The Monkhorst–Pack scheme was used to gen-
erate a k-point grid in the Brillouin zone, and the 7ꢂ7ꢂ1 k-point grid
ꢀ
5
Lindlar catalyst (5 wt% palladium on calcium carbonate; poisoned with
lead) was purchased from Acros Organics (CAS no. 7440-05-3) and Pd/C
(
Palladium, 10 wt% on activated carbon, Degussa type) was purchased
was adapted for both Pd AHCTUNTGRENNUNG( 111) and Pd ACHTUNGERTNNUNG( 100) slabs.
Chem. Asian J. 2013, 8, 919 – 925
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B. Moon Kim et al.
Figure 7. Hydrogenation of 5-decyne on 2.0 mol% Pd/C catalyst pre-
treated with bromide ions. The pre-treatment was conducted by heating
1
1 mL of an aqueous mixture containing the commercial Pd/C catalyst
(
200 mg) and KBr (200 mg) at 808C for 3 h.
Figure 5. TEM images of Pd nanocubes obtained after the hydrogenation
of 5-decyne.
Figure 8. Illustrations of the lowest energy adsorption modes of acetylene
and ethylene on Pd(100) and Pd(111): a) acetylene on a 4-fold hollow
site of Pd(100); b) ethylene on a 2-fold aligned bridge site of Pd(100); c)
acetylene on a 3-fold hcp site of Pd(111); and d) ethylene on a 2-fold
aligned bridge site of Pd(111).
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Figure 6. Br 3d XPS core level spectrum taken of the Pd nanocubes after
extensive washing with ethanol and water. In the spectrum, the Br 3d5/2
and Br 3d3/2 binding energies were 67.6 and 68.5 eV, respectively, which
[
ꢀ
[9]
are close to the literature values for Br .
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Acknowledgements
B.M.K. thanks Mid-career Researcher Program through National Re-
search Foundation (NRF) grant funded by the Ministry of Education,
Science, and Technology (MEST) (No. 2011-0016187). J.J. was supported
by the World Class University (WCU) program through the NRF of
Korea funded by the MEST (R31-10013). J.L. was supported by the NRF
of Korea through Basic Science Research Program (2009-0092809). B.L.
was supported by Basic Science Research Program through NRF of
Korea funded by the MEST (2011-0009852 and 2009-0083540). The com-
putations were supported by National Institute of Supercomputing and
Networking/KISTI with supercomputing resources (KSC-2012-C2-33).
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Received: December 7, 2012
Revised: January 30, 2013
Published online: March 6, 2013
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