Excellent Selectivity with High Conversion in the Semihydrogenation of Alkynes using Palladium-Based Bimetallic Catalysts

Article abstract of DOI:10.1002/cctc.201700800

A series of active-carbon-supported PdPb and PdCu bimetal catalysts were prepared for the selective semihydrogenation of alkynes in the liquid phase. The Pd_0.33Pb_0.67/C catalyst showed the best performance for various alkynes under mild reaction conditions (room temperature and ambient H₂ pressure) and achieved 100 % conversion with 98 % selectivity to alkenes. In particular, over-hydrogenation was avoided at complete alkyne conversion.

Full text of DOI:10.1002/cctc.201700800

Accepted Article
Title: Excellent selectivity with high conversion in semi-hydrogenation
of alkynes using Pd-based bimetallic catalysts
Authors: Jian Liu, Yanan Zhu, Chang Liu, Xiaoshi Wang, Changyan
Cao, and Wei-Guo Song
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10.1002/cctc.201700800
ChemCatChem
COMMUNICATION
Excellent selectivity with high conversion in semi-hydrogenation
of alkynes using Pd-based bimetallic catalysts
Jian Liu,[a, b] Yanan Zhu,[a, b] Chang Liu,[a, b] Xiaoshi Wang,[a, b] Changyan Cao*[a, b] and
Weiguo Song*[a, b]
Abstract: A series of active carbon supported PdPb and PdCu
bimetal catalysts were prepared for selective semi-hydrogenation of
alkynes in liquid-phase. Pd0.33Pb0.67/C catalyst showed the best
performance for various alkynes under mild reaction conditions (room
temperature and ambient H2 pressure), showing 100% conversion
and 98% selectivity to alkenes. Particularly, over hydrogenation was
avoided under complete alkyne conversion.
leading to desired selectivity. During the past few years, PdCu,[11]
PdSn,[12] PdAg,[13] PdGa,[14] PdPb,[15] etc. alloys have been
developed. One of the most attracting properties of these
bimetallic alloy nanoparticles is their highly tunable composition,
resulting in different catalytic properties.[16] For example, Niu et al.
produced PdPb alloy nanocrystals with various shape and
composition.[15] They found that the PdPb alloy octahedral
nanocrystals showed the best performance for liquid-phase
hydrogenation of phenylacetylene. Although up to 99% selectivity
to alkenes was obtained under optimal conditions, the alkene
selectivity decreased rapidly with the prolonged reaction time on
most of the above Pd-based alloys. Thus, semi-hydrogenation of
alkynes to the corresponding alkenes with excellent selectivity
under high conversion is quite difficult and challenging.
Herein, we report active carbon supported PdPb and PdCu
bimetal catalysts, which exhibit high selectivity and high stability
towards the liquid-phase selective hydrogenation reaction of
alkynes. Pd0.33Pb0.67/C catalyst shows the best performance for
various alkynes under mild reaction conditions (room temperature
and ambient H2 pressure), showing 100% conversion and 98%
selectivity to alkenes. Particularly, over hydrogenation was
avoided under complete alkyne conversion. The catalyst can also
be easily separated and reused for at least 5 times without loss of
activity and selectivity.
Fine control of the desired product selectivity is an essential issue
in catalysis reaction to reduce waste emission, save cost and
improve performance. Selective semi-hydrogenation of alkynes to
alkenes is one of the most important and fundamental reaction in
fine chemical industries.[1] Lindlar catalysts (Pd/CaCO3 treated by
Pb salts or Pd/BaSO4 with excessive addition of quinoline) are
frequently used for this kind of reactions.[2] However, Lindlar
catalysts suffer from unsatisfactory selectivity and stability due to
over hydrogenation at high alkyne conversion[1d]. Several kinds of
other noble metals, such as Au,[3] Ag,[4] Ru,[5] etc., have been
developed to substitute Pd. For example, Mitsudome et al.
prepared new core-Au/shell-CeO2 nanoparticles for highly
selective semi-hydrogenation of various alkynes at ambient
temperature under additive-free conditions.[3a] However, the
activity of these substituted noble metals is much lower than that
of Pd.
Active carbon supported PdPb and PdCu bimetal catalysts
(denoted as PdxMy/C. M refers to Cu or Pb, C refers to XC-72R,
and x/y refers to the ratio of Pd/M) were prepared by reduction of
the corresponding metal hydroxide in enviromental friendly EG
solution followed by a rapid thermal treatment.[17] Typical TEM
images show that PdCu (Figure 1) and PdPb (Figure 2)
nanoparticles with different ratios were uniformly dispersed on the
activated carbon support. The mean diameters of nanoparticles
were 5.6 ± 0.9 nm. XRD peaks of PdCu/C shifted slightly to higher
2θ values with increasing Cu contents (Figure S1 in the
Supporting Information), suggesting formation of the PdCu alloys.
Similarly, XRD peaks of PdPb/C shifted to lower 2θ values with
increasing Pb contents, also confirming formation of PdPb alloys
(Figure 2e). Furthermore, STEM-EDS line scanning of one
nanoparticle in Pd0.67Pb0.33/C (insert of Figure 2c) showed that Pd
and Pb elements were uniformly dispersed, further confirming the
alloy structure.
It needs to be mentioned that the fast thermal treatment is very
important to remove the organic layer of ethylene glycol on the
surface of catalysts. In fact, the hydrogenation reaction cannot be
catalyzed without thermal treatment. FTIR analysis showed that
the organic layer had been removed totally by the fast thermal
treatment (Figure S2). In addition, the fast thermal treatment didn’t
change the states of Pd species (Figure S3).
On the other hand, considering the high price and limited
availability of these noble metals, it is highly attractive to develop
non-noble metal catalysts for this kind of reactions.[6] Some
catalysts based on transition metals (such as Cu,[7] Ni,[8] Co,[9] and
Fe[10]) are also developed for semi-hydrogenation of alkynes.
Nevertheless, hydrogenation reactions catalyzed by these non-
noble metals are carried out under harsh conditions.[9] It still
remains a challenge to develop a kind of cost-effective catalysts
to realize highly selective semi-hydrogenation of alkynes under
mild conditions.
The most effective strategy is to partially substitute Pd with a
second metal to form Pd-based alloy, in which the presence of
other metal atoms can modify the electron density on Pd atoms,
[a]
J. Liu, Y. Zhu, C. Liu, X. Wang, Dr. C. Cao, Prof. W. Song
Beijing National Laboratory for Molecular Sciences
Laboratory of Molecular Nanostructures and Nanotechnology
CAS Research/Education Center for Excellence in Molecular
Sciences
Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190 (China)
E-mail: cycao@iccas.ac.cn,wsong@iccas.ac.cn
J. Liu, Y. Zhu, C. Liu, X. Wang, Dr. C. Cao, Prof. W. Song
University of Chinese Academy of Sciences, Beijing100049 (China)
[b]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201700800
ChemCatChem
COMMUNICATION
reaction of phenylacetylene under the same conditions. As
expected, the activity was improved extensively while the high
selectivity was retained (Figure S5 in the Supporting Information).
As Pd/Pb ratio decreased, the activity of the catalyst decreased.
But different from PdCu/C catalysts, the activity did not decrease
sharply. Pd0.33Pb0.67/C showed the best performance with 97%
conversion and 98% selectivity in 2 h (Table 1, entry 5).
Particularly, the selectivity can be maintained 97% after 12 h
(Table 1, entry 6), suggesting superior selectivity of this catalyst
for semi-hydrogenation of phenylacetylene. For comparison,
commercial Pd/C (denoted as C-Pd/C) showed very high activity
with 99% conversion, but only 50% selectivity in 1 h (Table 1,
entries 1-2). Lindlar catalysts were also tested under the same
reaction conditions, although high selectivity could achieve at
lower conversion, the alkene selectivity decreased rapidly with the
prolonged reaction time when the alkyne conversion was high
(Table 1, entries 3-4).
Figure 1. TEM images of the PdxCuy/C catalysts: (a) Pd/C; (b) Pd0.67Cu0.33/C;
(c) Pd0.5Cu0.5/C; (d) Pd0.33Cu0.67/C; (e) Pd0.25Cu0.75/C; (f) Cu/C. Scale bars: 50
nm.
Table 1 Hydrogenation of phenylacetylene with different catalystsa
Sel/%lb
TOF/h-1
Time
/ h
Entry
Catalyst
Con%
1
2
3
4
5
6
7
C-Pd/C
C-Pd/C
0.5
1.0
1.0
4.0
1.0
2.0
12.0
62
99
64
99
53
97
99
95
50
99
42
98
98
97
1128
-
Lindlar
582
Lindlar
-
Pd0.33Pb0.67/C
Pd0.33Pb0.67/C
Pd0.33Pb0.67/C
482
-
-
Figure 2. TEM images of the PdxPby/C catalysts: (a) Pd0.67Pb0.33/C; (b)
Pd0.5Pb0.5/C; (c) Pd0.33Pb0.67/C; STEM-EDS line scanning image (insert); (d)
Pd0.25Pb0.75/C. Scale bar: 50 nm. (e) XRD patterns of the PdxPby/C catalysts.
[a] Reaction conditions: Pd loading: 0.10 mol%, phenylacetylene (100 μL,
0.91 mmol) in 2.0 mL of acetonitrile using 1,3,5-trimethylbenzene as the
internal standard, 1 atm H2, 30 °C. [b] Alkene selectivity.
The catalytic performance of PdCu/C and PdPb/C with different
Pd/M ratios for the selective hydrogenation of phenylacetylene
were first tested. As illustrated in Figure S4 in the Supporting
Information, with a decrease of Pd/Cu ratio, the activity dropped
sharply. For Pd/C catalyst, when the conversion was more than
85%, the selectivity dropped immediately (Figure S4a in the
Supporting Information). As expected, for Cu/C, no hydrogenation
reaction occurred even after 30 h under the same conditions,
showing that Pd was the essential one to active the hydrogen. For
Pd0.67Cu0.33/C and Pd0.5Cu0.5/C, it took 4 h and 30 h to achieve
99 % conversion, respectively. More importantly, the selectivity
can be maintained for at least two hours after the conversion
reached 98%, which was meaningful in real industrial process.
We believe that the high selectivity would maintained for a longer
time after the conversion reached 98% for Pd0.25Cu0.75. Despite
the excellent selectivity, the activity is too low for PdCu/C catalysts
with higher Cu ratios. It would increase the process cost for real
production. The goal for a good catalyst requires both high
selectivity and high activity.
Table 2 Hydrogenation of alkynes using the Pd0.33Pb0.67/C catalysta
Conver
Time
/ h
Selectivityb
/ %
Entry
Substrate
Product
sion
/ %
1
2
2.0
1.5
98
98
99
98
3
4
1.5
1.5
98
98
99
99
5
1.0
97
99
Inspired by Lindlar catalyst, the catalytic performance of
PdPb/C was then examined for liquid-phase hydrogenation
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10.1002/cctc.201700800
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COMMUNICATION
In summary, a series of active carbon supported PdPb and
PdCu bimetal catalysts were prepared by reduction of the
corresponding metal hydroxide in environmental friendly EG
solution followed by a rapid thermal treatment. They exhibited
unique selectivity and high stability towards the liquid-phase
selective hydrogenation reaction of alkynes. Among all the
catalysts, Pd0.33Pb0.67/C catalyst shows the best performance for
various alkynes under room temperature and ambient H2
pressure, showing 100% conversion and 98% selectivity to
styrene. Particularly, over hydrogenation was avoided under
complete alkyne conversion.
6
7
8
1.5
1.5
1.0
96
98
98
99
99
99
99
0
99
--
9c
2.0
[a] Reaction conditions: Pd loading: 0.10 mol%, 0.9 mmol substrates in 2.0
mL of acetonitrile using 1,3,5-trimethylbenzene as the internal standard. 1
atm H2, 30 oC. [b] Alkene selectivity. [c] 0.45 mmol phenylacetylene and 0.45
mmol nitrobenzene was used.
Experimental Section
The size distributions of PdM nanoparticles showed no obvious
difference for these samples (Figure S6-S7). Compared with the
effect of size on the catalytic property, Pd/M ratio showed obvious
influence on the activity/selectivity. Higher Pd/M ratios, higher
catalytic activity while lower selectivity. Addition of Cu or Pb could
poison the active Pd sites through lower the d-band energy of Pd
and reduce the adsorption energy of alkenes, and/or to suppress
the formation of the β-palladium hydride phase.[15, 18] This is the
key of selectivity.
The excellent catalytic performance of Pd0.33Pb0.67/C can be
further tested using a wide range of alkynes under the same mild
conditions. Various alkynes with functional groups such as
halogen, carbonyl and nitro were successful converted to the
corresponding alkenes with > 99% selectivity, while the reduction
of the reducible functional groups were avoided and no
corresponding alkanes from excessive hydrogenation of alkynes
were detected (Table 2). These results indicate that the bimetallic
Pd0.33Pb0.67/C catalyst works significantly well for a wide range of
alkynes to produce desired alkenes. Furthermore, Pd0.33Pb0.67/C
catalyst showed excellent stability during recycles. There was no
significant activity and selectivity loss at least for five runs (Figure
3). Compared with other catalysts in the literatures (Table S1) and
the commercial Pd/C and Lindlar catalysts, Pd0.33Pb0.67/C shows
high activity and obvious better selectivity and stability for liquid-
phase hydrogenation reaction of phenylacetylene.
Synthesis of Pd-M/C Pd-base bimetallic catalyst was prepared according
to a previous report with modification[17a]. In a typical procedure, 5.7 mL of
3 mM Cu(CH3COO)2·H 2O ethylene glycol (EG) solution was mixed with
335 μL of 56 mM of H2PdCl4 ethylene glycol and further diluted to 40 mL
with EG, followed by adjusting the pH to 10 by adding appropriate 0.25 M
NaOH EG solution. The mixture was dropped into the suspension of XC-
72R (98 mg) which had been dispersed in EG (80 mL). The mixture was
then heated to 198 ºC and maintained for 4 h under Ar atmosphere. After
cooling to room temperature, the mixture was filtered and the solid product
was washed thoroughly by water and ethanol for several times. Then the
product was dried at room temperature overnight in a vacuum oven for 12
h. Finally, the samples were treated under rapid thermal conditions
followed literature procedures[17b]. A square furnace was preheated to
500 °C, and hold for 15 minutes. Then, the sample was introduced by a
pair of long, heat resistant tongs (CAUTION: the temperature in the
furnace is very high, so that it can cause burns and dangers. The
introduction and withdraw of the catalyst must be performed carefully). The
treatment was performed in static air. The sample was quickly withdrawn
from the furnace one minute later and left cooling down to the ambient
temperature. Different atom ratio PdCu and PdPb catalysts were prepared
by the same method except for the kind and amount of the EG solution of
the metal salt. The resultant materials are named as PdxMy/C (M refers to
Cu or Pb, C refers to XC-72R, and x/y refers to the ratio of Pd/M). For all
catalysts, the loading content of Pd was fixed at 2 wt%. The contents of Pd
and Cu/Pb were determined by ICP (Table S2).
Partial hydrogenation of acetylene. In a typical procedure, a 10 mL
round bottle was charged with phenylacetylene (100 μL, 0.91 mmol), 1,3,5-
trimethylbenzene (internal standard, 100 μL, 0.72 mmol), catalyst (5 mg,
Pd 2 wt%) and 5 mL acetonitrile. The mixture was dispersed by sonication
for a few seconds. The air in the reactor was replaced by H2 for three times.
Then, the reactor was placed in a 25 ºC water base with magnetic stirring.
The samples were taken by a needle at different reaction time following by
centrifugal separation. The products were analyzed by
a gas
chromatograph (Shimadzu GC-2010) equipped with a flame ionization
detector (FID) and a Rtx-5 capillary column (0.25 mm in diameter, 30 m in
length). The identity was ascertained by GC-MS (Shimadzu GCMS-
QP2010S).
Recycling test. After the reaction, the catalyst was recovered by
centrifugation, rinsed with acetonitrile for three times, and dried at room
temperature overnight in a vacuum oven for next test. The next run was
performed as the typical procedure, and the products was analyzed by GC.
Acknowledgements
Figure 3. Recycle property of Pd0.33Pb0.67/C in the phenylacetylene
hydrogenation.
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Chinese Academy of Sciences-Peking University Pioneer
Cooperation Team and the Youth Innovation Pro-motion
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10.1002/cctc.201700800
ChemCatChem
COMMUNICATION
COMMUNICATION
Jian Liu, Yanan Zhu, Chang Liu,
Xiaoshi Wang, Changyan Cao* and
Weiguo Song*
Active carbon supported PdPb and
PdCu bimetal catalysts showed
excellent selectivity under complete
alkyne conversion in semi-
hydrogenation of alkynes in the
liquid-phase.
Page No. – Page No.
Excellent selectivity with high
conversion in semi-hydrogenation
of alkynes using Pd-based
bimetallic catalysts
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