Reversible hydrogenation-oxidative dehydrogenation of quinolines over a highly active pt nanowire catalyst under mild conditions

Full text of DOI:10.1002/cctc.201300136

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DOI: 10.1002/cctc.201300136
Reversible Hydrogenation–Oxidative Dehydrogenation of
Quinolines over a Highly Active Pt Nanowire Catalyst
under Mild Conditions
Danhua Ge, Lei Hu, Jiaqing Wang, Xingming Li, Fenqiang Qi, Jianmei Lu, Xueqin Cao,* and
Hongwei Gu*[a]
Catalytic hydrogenation (reduction) and dehydrogenation (oxi-
dation) reactions are of fundamental importance in the synthe-
sis of organic molecules.[1] The hydrogenation of aldehydes/ke-
tones and imines yields the corresponding alcohols and
amines. In the reverse direction, dehydrogenation of alcohols
and amines forms the corresponding aldehydes/ketones and
imines. The catalytic hydrogenations and dehydrogenations of
N-heterocycles are particularly important transformations, as
both saturated and unsaturated heterocycles are important
structural units in natural products and pharmaceuticals.[2]
Previous methods for the hydrogenation of N-heterocycles
have been catalyzed with homogeneous precious-metal cata-
lysts, such as Ir,[3] Ru,[4] Rh,[5] and Mo.[6] Those catalytic systems
all exhibit high activity and selectivity, but separation of the
catalyst from the reaction mixture is quite difficult, and amines
in particular suffer from contamination by the heavy-metal
ions. Heterogeneous catalysis has also been used for the hy-
drogenation of N-heterocycles. Liao and Shi used Pd nanoparti-
cles supported on tannin-grafted collagen fibers as a recyclable
catalyst for the hydrogenation of quinoline.[7] This catalyst
showed excellent catalytic activity at an initial hydrogen pres-
sure of 2 MPa. Bianchini found that silica-supported Pd nano-
particles could catalyze the hydrogenation of 1,10-phenanthro-
line at 30 bar (1 bar=100 kPa) of hydrogen.[8] Raney-Ni can
also be used as a catalyst for the hydrogenation of phenan-
throlines with good to excellent yields under 27 bar hydrogen
pressure.[9] As encouraging as these results are, the process is
not suitable for industrial production because of the high
working pressure. It is also energy intensive and suffers from
some safety issues. Therefore, the development of a heteroge-
neous metal catalyst with superior catalytic activity and reus-
ability is a very attractive research target. Kobayashi reported
that sub-nanometer-sized Pd clusters stabilized by random co-
polymers act as catalysts for the hydrogenation of quinoline to
1,2,3,4-terahydroquinoline under mild reaction conditions.[10]
Recently, Au/TiO2,[11] Rh/AlO(OH),[12] and heterogeneous multi-
metallic nanoparticle catalysts (72 catalysts based on Rh, Pt, Ir,
and Ru) supported on metal oxides[13] were also screened.
They also need high hydrogen pressure (more than 8 bar).
[14]
Pd@OmpG-C3N4 (OmpG=outer membrane protein G) is the
best catalyst that has been reported for the hydrogenation of
quinolones, and it shows high activity and selectivity under
mild reaction conditions (1 bar hydrogen and 40–1008C). Plati-
num oxide (PtO2) can also be used in the hydrogenation of
N-heterocycles in good yields.[15] However, the utilization of
Pt-based catalysts has been less investigated, even though Pt
is regarded as one of the best metal catalysts for hydrogen ac-
tivation. Kobayashi successfully applied polymer-incarcerated
Pt nanoparticles (<3 nm) to the catalytic hydrogenation of un-
saturated compounds including heterocycles in good yields
under mild reaction conditions [5 atm of hydrogen (1 atm=
101.3 kPa)].[16] Somorjai and Yang demonstrated that the struc-
ture of the nanomaterials can affect the selectivity of the reac-
tions: Pt nanocubes were used as a catalyst for the hydrogena-
tion of pyrrole, and they showed selectivity towards n-butyl-
amine that was higher than that shown by nanopolyhedra.[17]
These reported Pt-based catalysts are highly active and selec-
tive, which encouraged us to explore Pt-based heterogeneous
catalysts in the hydrogenation of N-heterocycles.
The direct oxidation of saturated N-heterocycles is an equal-
ly important organic transformation, as the structure unit can
be found in many biologically active natural products and
pharmacologically active molecules.[2] Pd/C as a commercial
catalyst proved to be effective in the oxidation of 1,2,3,4-tetra-
hydroquinolines to quinolines under mild reaction condi-
tions.[18] Ru-based catalysts can also be used in the oxidation
of N-heterocycles to provide the products in good to excellent
yields.[19] Pt has proved to be one of the best catalysts for oxi-
dation[20] and has also been used as a heterogeneous catalyst
in the oxidation of saturated N-heterocycles. Kaneda et al.[21]
found that supported Cu nanoparticles can be used in the de-
hydrogenation of 1,2,3,4-tetrahydroquinoline at 1508C with
the evolution of hydrogen gas. This is good for hydrogen
transformations with organic molecules. This catalyst also
shows excellent catalytic activity and selectivity for the hydro-
genation of quinoline, and it is the first catalytic system that
can be used in the hydrogenation of quinoline and in the de-
hydrogenation of hydrogenated quinoline. However, oxidative
dehydrogenation is also an important transformation of organ-
ic molecules, and to the best of our knowledge, no catalytic
system has been reported that can be used as an effective cat-
[a] D. Ge, Dr. L. Hu, J. Wang, Prof. X. Li, F. Qi, Prof. J. Lu, Prof. X. Cao,
Prof. H. Gu
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering and Materials Science
Soochow University, Suzhou 215123 (China)
Fax: (+86)512-65880905
E-mail: xqcao@suda.edu.cn
hongwei@suda.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300136.
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alyst for the reversible hydrogenation and oxidative dehydro-
genation of N-heterocycles.
entry 1). Alcohols (methanol or ethanol) could also be used as
the solvent in this reaction (Table 1, entries 2 and 3). Of all the
solvents we used, water was the most suitable for this reac-
tion, and it is green and effectively free. Without a catalyst or
hydrogen, no hydrogenated products were observed. We also
used Pt nanoparticle (NP), Pt nanorod (NR), and Pd NW cata-
lysts (Figure S3, Supporting Information) to compare their cata-
lytic activities with that of our Pt NW. The Pt NW showed the
highest catalytic activity and selectivity of these catalysts
(Table 1, entries 6–8).
In our previous work, we demonstrated that Pt nanowire
(NW) shows high catalytic activity and selectivity towards the
hydrogenation of unsaturated compounds under mild reaction
conditions.[22] The NW can be used multiple times with no de-
tected Pt ion leakage. In this paper, we report a heterogeneous
catalytic system that is equally effective in the hydrogenation
and oxidative dehydrogenation of quinolines by using an ultra-
thin Pt NW catalyst. In addition, the remarkable stability of the
Pt NW catalyst without a supporter and the exceptionally high
catalytic activity fulfill all the requirements for catalysis under
mild reaction conditions, which is in sharp contrast to most
Vierhapper reported that in neutral or weakly acidic media,
the reaction is selective towards 2, and in strong acid 3 is pre-
ferred.[24] Five additives were examined in this catalytic system
to detect the reaction activity of the Pt NW catalyst (Table 2).
All reactions continued to give high selectivity for 2. If HCl or
supported nanoparticle
metal catalysts.
catalysts and
homogeneous
To test the feasibility of our concept, the ultrathin Pt NW cat-
alyst was synthesized by acidic etching of FePt NW in air fol-
lowed by multiple washings in methanol.[23] The NWs are sever-
al hundreds of nanometers in length and have an average di-
ameter of about 1.5 nm with a very narrow size distribution
(Figure S1, Supporting Information). The predominant plane is
(111) from the XRD pattern (Figure S2, Supporting Information),
and all Fe was removed, as confirmed by inductively coupled
plasma mass spectrometry (ICPMS).
Table 2. Hydrogenation of quinoline with different additives.[a]
Entry
Additive
Conversion[b] [%]
Selectivity for 2[b] [%]
1
2
3
4
5
CH3COOH
HCl
H2SO4
H3PO4
NH3·H2O
97.9
59.9
74.4
97.8
80.3
92.8
98.0
97.9
94.2
96.8
[a] Quinoline (1 mmol), additive (1 mmol), water (3 mL), and H2 (1 bar)
with the catalyst (0.005 mmol) at 808C for 24 h. [b] Determined by GC
and GC–MS.
To investigate the catalytic activity of this Pt NW catalyst for
the partial hydrogenation of quinolines, quinoline was selected
as the model reaction substrate to optimize the reaction condi-
tions. The partial hydrogenation of quinoline (1) generated
two main products: 1,2,3,4-tetrahydroquinoline (2) and 5,6,7,8-
tetrahydroquinoline (3, Scheme 1). Table 1 shows the partial
hydrogenation results of quinoline under different reaction
conditions. Water was first used as the solvent under an initial
hydrogen pressure of 1 bar. Compound 2 was obtained in
92.1% yield after 24 h with a selectivity of 94.1% (Table 1,
H2SO4 (1 equiv.) was added, the conversion decreased to less
than 75%, which shows that strong acids decrease the hydro-
genation of quinoline under these conditions. If acetic acid or
H3PO4 was added, the conversion remained around 97%. If am-
monia was added, the reaction activity also decreased. None
of these additives increased the catalytic activity of this reac-
tion, but the selectivity remained constant. These results con-
firm that Pt NW is an excellent catalyst for the catalytic partial
hydrogenation of 1 to 2.
The Pt NW catalyst can be conveniently recycled and reused.
Upon completion of the reaction, the Pt NW can be separated
by centrifugation after stirring is stopped, and the catalyst can
be reused after washing with methanol and hydrochloric acid
(0.1 m). The conversion and selectivity were constant over
10 cycles (Figure 1) and no leached Pt was detected by atomic
absorption spectroscopy (AAS) and ICPMS.
Scheme 1. Partial hydrogenation of quinoline (1) to 1,2,3,4-tetrahydroquino-
line (2) or 5,6,7,8-tetrahydroquinoline (3).
Table 1. Optimization of the reaction conditions for the hydrogenation
of quinoline.[a]
This catalyst can also be used in the selective oxidation of
amines, which is an equally important organic transformation.
When the hydrogenation was finished, 2 could be dehydro-
genated back to quinoline with the Pt NW catalyst if the at-
mosphere was changed to oxygen or air (Scheme 2). The yield
was above 99% for the dehydrogenation reaction under mild
reaction conditions (408C, 1 bar). These results demonstrate
that Pt NW can be used as an effective catalyst for the hydro-
genation of quinoline and for the oxidative dehydrogenation
of 1,2,3,4-tetrahydroquinoline.
Entry
Catalyst
Solvent
Conversion[b] [%]
Selectivity for 2[b] [%]
1
2
3
4[c]
5[c]
6
Pt NW
Pt NW
Pt NW
Pt NW
Pt NW
Pt NP
H2O
97.9
91.3
95.5
60.4
75.1
58.8
97.7
18.8
94.1
97.1
93.1
82.2
98.0
51.8
75.4
94.1
CH3OH
C2H5OH
CHCl3
THF
H2O
H2O
7
8
Pt NR
Pd NW
H2O
[a] Quinoline (1 mmol), solvent (3 mL), and H2 (1 bar) with the catalyst
(0.005 mmol) at 808C for 24 h. [b] Determined by GC and GC–MS. [c] Re-
action performed at 408C.
The general applicability of this catalytic hydrogenation and
oxidative dehydrogenation was tested with
a series of
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Table 3. Hydrogenation of N-heterocycles and oxidative dehydrogena-
tion of the products.[a]
Entry
Compound 1
Compound 2
Yield[b] [%]
1 to 2
2 to 1
1
2
70.0 (65)
79.3 (77)
53.9
68.2
75.1
3[c]
80.7 (76)
Figure 1. Catalytic stability of the Pt NW catalyst in the hydrogenation of
quinoline.
4
5
6
7
89.1 (85)
92.9 (90)
97.8 (96)
84.4 (80)
97.5
100
N-heterocyclic compounds. For the hydrogenations, the reac-
tion was remarkably selective towards the pyridine ring with-
out affecting the fused benzene rings. Table 3 shows the re-
sults of the hydrogenation and oxidative dehydrogenation of
N-heterocycles. The Pt NW showed high catalytic activity and
selectivity in all the hydrogenations of substituted quinolines
and the corresponding oxidative dehydrogenations of the
products (Table 3, entries 1–9).
99.8
91.9
Direct hydrogenation of acridine was also achieved. Al-
though the yield of the hydrogenation process was low,
9,10-dihydroacridine could be oxidized to acridine easily under
the oxidative conditions (Table 3, entry 10). Under the standard
reaction conditions, 1,10-phenanthroline was hydrogenated to
1,2,3,4-tetrahydrophenanthroline in 80.4% yield, and the yield
of the oxidative dehydrogenation was 100% (Table 3, entry 11).
If pyridine or a substituted pyridine was used as the sub-
strate, only trace amounts of the products were obtained
under the mild reaction conditions. Increasing the initial hydro-
gen pressure to 10 bar still resulted in low yields (<50%;
Table S1, Supporting Information). This structure-sensitive ac-
tivity can be explained by the molecular adsorption of the cat-
alyst onto the surface. Quinoline, one of the more basic N-het-
erocycles, can adsorb onto the surface of a heterogeneous cat-
alyst though two different modes: end-on through the nitro-
gen atom and side-on through the aromatic ring.[25] Lambert
has shown that quinoline preferentially adsorbs in the side-on
configuration through the aromatic p system onto the Pt (111)
surface plane.[25b] This adsorption configuration is good for the
hydrogenation of quinoline and its derivatives by using active
hydrogen on the surface of the Pt NW. However, the adsorp-
tion of pyridine is both coverage and temperature dependent.
At the reaction temperature (808C), pyridine adopts an upright
8
97.8 (97)
76.9
9
46.0 (40)
35.1 (30)
80.4 (75)
91.7
100
10
11
100
[a] Hydrogenation (1 to 2): compound 1 (1 mmol), CH3OH (3 mL), and H2
(1 bar) with the Pt NW catalyst (0.005 mmol) at 408C; oxidative dehydro-
genation (2 to 1): compound 2 (1 mmol), CH3OH (3 mL), and O2 (1 bar)
with the Pt NW catalyst (0.005 mmol) at 408C. [b] Determined by GC and
GC–MS. The value in parentheses is the yield of the isolated product.
[c] Initial pressure of hydrogen=10 bar.
configuration and covers the Pt (111) surface plane,[26] which is
not suitable for hydrogenation.
In summary, we have developed a simple and highly effi-
cient method for the reversible hydrogenation–oxidative dehy-
drogenation of N-heterocycles by using Pt NW as the catalyst
under mild reaction conditions. Pt NW shows high selectivity
towards the hydrogenation of N-heterocycles, and the hydro-
genation products can be easily oxidized under the
same conditions with oxygen or air. This method
avoids high temperatures and pressures, and the cat-
alyst can be recycled easily. We are convinced that
this procedure is an appealing option in organic syn-
thesis.
Scheme 2. Reversible hydrogenation and oxidative dehydrogenation of quinoline and
1,2,3,4-tetrahydroquinoline by using Pt NW as the catalyst.
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Experimental Section
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General procedure for the catalytic hydrogenation of
N-heterocycles
Pt NWs in methanol (0.005 mmol), quinoline or its derivatives
(1 mmol), and solvent (2 mL) were added to a Schlenk tube, and
the tube was then sealed. The reaction tube was evacuated
(3 times) and flushed with hydrogen, which took place at a certain
temperature under a hydrogen atmosphere. Upon completion of
the reaction, the hydrogen atmosphere was removed, and the re-
sultant product mixtures were analyzed by GC and GC–MS.
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Keywords: hydrogenation
·
nanocatalyst
·
oxidative
dehydrogenation · platinum · reversible reactions
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Received: February 21, 2013
Revised: March 15, 2013
Published online on && &&, 0000
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Over the wire and back again: The re-
versible hydrogenation–oxidative dehy-
drogenation of quinolines is reported
by using Pt nanowire (NW) as the cata-
lyst under mild reaction conditions. Pt
NW shows high activity and selectivity
in the hydrogenation of quinolines
under H2 pressure (1 bar=100 kPa), and
these hydrogenation products can be
easily oxidized under the same condi-
tions in an atmosphere of oxygen
(1 bar) or in air.
D. Ge, L. Hu, J. Wang, X. Li, F. Qi, J. Lu,
X. Cao,* H. Gu*
&& – &&
Reversible Hydrogenation–Oxidative
Dehydrogenation of Quinolines over
a Highly Active Pt Nanowire Catalyst
under Mild Conditions
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