Reversible dehydrogenation-hydrogenation of tetrahydroquinoline-quinoline using a supported copper nanoparticle catalyst

Article abstract of DOI:10.3987/COM-10-S(E)90

Copper nanoparticles synthesized on a titania surface (Cu/TiO₂) act as an efficient heterogeneous catalyst for the reversible dehydrogenation and hydrogenation of tetrahydroquinoline-quinoline under an atmospheric pressure of H₂. The Japan Institute of Heterocyclic Chemistry.

Full text of DOI:10.3987/COM-10-S(E)90

HETEROCYCLES, Vol. 82, No. 2, 2011
1371
HETEROCYCLES, Vol. 82, No. 2, 2011, pp. 1371 - 1377. © The Japan Institute of Heterocyclic Chemistry
Received, 1st July, 2010, Accepted, 13th August, 2010, Published online, 16th August, 2010
DOI: 10.3987/COM-10-S(E)90
REVERSIBLE
DEHYDROGENATION-HYDROGENATION
OF
TETRAHYDROQUINOLINE-QUINOLINE USING
COPPER NANOPARTICLE CATALYST
A
SUPPORTED
Yusuke Mikami,a Kaori Ebata,a Takato Mitsudome,a Tomoo Mizugaki,a
Koichiro Jitsukawa,a and Kiyotomi Kanedaa,b,*
a
Department of Materials Engineering Science, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan. E-mail: kaneda@cheng.es.osaka-u.ac.jp
Fax: +81 6-6850-6260; Tel: +81 6-6850-6260
b
Research Center for Solar Energy Chemistry, Osaka University, 1-3
Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Abstract – Copper nanoparticles synthesized on a titania surface (Cu/TiO2) act as
an efficient heterogeneous catalyst for the reversible dehydrogenation and
hydrogenation of tetrahydroquinoline-quinoline under an atmospheric pressure of
H2.
INTRODUCTION
Hydrogen has been proposed as a clean energy source and an alternative to fossil fuels for
next-generation automobile and fuel cells, because only water is formed as a by-product after the
consumption of hydrogen.1 For a hydrogen-based energy infrastructure, efficient and safe storage,
transportation, and supply of hydrogen are crucial issues.2 Among the several approaches to accomplish
this, the organic hydride strategy has attracted much attention as a promising method, wherein H2 is
stored or released from organic molecules through reversible hydrogenation and dehydrogenation using
catalysts.3 The organic hydride method has significant advantages such as the safe use of recyclable
organic hydrides, good handling, and no emission of CO, CO2 or other by-products. Although reversible
hydrogenation and dehydrogenation using cyclohexane/benzene and decalin/naphthalene pair systems
have been reported, severe conditions of high temperature (> 200oC) and high H2 pressure (30-80 atm)
are required in these systems.3a-e
This paper is dedicated to Prof. Albert Eschenmoser on the occasion of his 85th birthday

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HETEROCYCLES, Vol. 82, No. 2, 2011
Recently, Pez and Crabtree discovered that the incorporation of nitrogen atoms into the aromatic rings
facilitated
the
dehydrogenation
process.4
Heteroaromatics
such
as
carbazole
and
o
1,2,3,4-tetrahydroquinoline were easily dehydrogenated under a milder temperature (120 C) using Pd/C
and Rh/C catalysts. These catalyst systems, however, still required a high H2 pressure (50 atm) in the
hydrogenation process. While successful dehydrogenation systems of nitrogen heterocycles have been
reported,4,5 few catalytic systems for reversible dehydrogenations are known and these systems still suffer
from the need for severe reaction conditions and the use of expensive metals such as Pt, Pd or Ir.6 Thus,
the development of reusable and less expensive base-metal catalysts for efficient reversible
dehydrogenation and hydrogenation systems is highly desirable from the ecological and practical points
of view.
Herein, we report that copper nanoparticles synthesized on a titania surface act as an efficient
heterogeneous
catalyst
for
the
reversible
dehydrogenation
and
hydrogenation
of
tetrahydroquinoline/quinoline. This hydrogen storage system can be operated under mild conditions
(Scheme 1). This is the first example of a reversible organic hydride system using a heterogeneous
catalyst with an atmospheric pressure of H2.
-2H2
Cu/TiO2
150 oC, Ar
N
H
N
Cu/TiO2
1,2,3,4-tetrahydroquinoline
150 oC, H2 (1 atm)
quinoline
Scheme 1. Reversible dehydrogenation-hydrogenation of tetrahydroquinoline-quinoline using Cu/TiO2
RESULTS AND DISCUSSIONS
TiO2-supported copper nanoparticles (Cu/TiO2) were prepared as follows. TiO2 (JRC-TIO-4) (1.0 g) was
added to an aqueous solution of copper (II) trifluoromethanesulfonate (16 M), and then the mixture was
stirred at 25 oC for 2 min. Next, a 25 wt% aqueous NH3 solution was slowly dropped into the mixture to
adjust the pH to 8.0 while stirring. After stirring for 1 h, the slurry was filtered, washed with deionized
water, and dried at room temperature in vacuo, providing divalent copper species on TiO2 (CuII/TiO2) as a
light grey powder. The resulting CuII/TiO2 was reduced with 1 atm of H2 to afford Cu/TiO2 as a dark
powder (Cu: 4.91 wt%). UV spectra of Cu/TiO2 showed an absorption around 572-575 nm, which is
characteristic of the plasmon band of a zero valent copper species.7
o
Initially, Cu/TiO2 was examined in the dehydrogenation of 1,2,3,4-tetrahydroquinoline (1) at 150 C
under Ar flow for 16 h. 1 was dehydrogenated to give quinoline (2) in 99% yield and H2 was generated in

HETEROCYCLES, Vol. 82, No. 2, 2011
1373
two equimolar amounts to 2. Other inorganic materials-supported copper nanoparticles of Cu/Al2O3,
Cu/SiO2, Cu/MgO, and Cu/Hydroxyapatite (Cu/HAP) were prepared in similar ways and tested in the
dehydrogenation of 1. As summarized in Table 1, Cu/Al2O3 was also found to be an effective catalyst,
whereas Cu/SiO2, Cu/HAP, and Cu/MgO showed lower activities.8 The present dehydrogenation system
using Cu/TiO2 was applicable to scale-up conditions (20-mmol scale of 1) and an identical yield of 2
could be obtained (entry 2). In heterogeneous catalysts in the liquid-phase reaction, leaching of the active
metal species from the support into the reaction solvent is a significant issue. In the course of the
dehydrogenation of 1 using Cu/TiO2, the solid Cu/TiO2 was filtered off at a 50% conversion of 1. Further
stirring of the filtrate under similar reaction conditions did not afford any products, showing that the
dehydrogenation took place on the surface of Cu/TiO2. In addition, no copper ions in the filtrate were
confirmed by ICP analysis (detection limit: 0.054 ppm ).
Table 1. Dehydrogenation of 1,2,3,4-tetrahydroquinoline catalyzed by various Cu catalystsa
Cu catalyst (Cu: 0.1 mmol)
+
2H2
mesitylene, 150 oC
Ar flow, 16 h
N
H
N
2
1 (0.5 mmol)
Entry
Cu catalyst
Conv. (%)b
Yield (%)b
1
Cu/TiO2
Cu/TiO2
Cu/Al2O3
Cu/SiO2
Cu/HAP
Cu/MgO
>99
98
92
69
64
29
>99
97
92
68
64
29
2c
3
4
5
6
a Reaction conditions: Cu catalyst (Cu: 0.1 mmol), mesitylene (5.0 mL), 16 h.
c 1
b Determined by GC using an internal standard technique.
(5.1 g), mesitylene (50 mL), 12 h.
(20 mmol), Cu/TiO2
Next, the hydrogenation of 2 into 1 was carried out using Cu/TiO2. We found that 2 could be successfully
hydrogenated with Cu/TiO2 at 150 oC under 5 atm of H2 in mesitylene for 3 h, affording 1 in 92% yield. It
is noted that the hydrogenation quantitatively proceeded even under an atmospheric pressure of H2
(Scheme 2). The previously reported systems using copper catalysts required high pressure (> 30 atm)
and/or temperature to achieve a high yield of 1.9 From the above results, Cu/TiO2 was found to be an
effective catalyst for both the dehydrogenation of 1 and hydrogenation of 2.

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HETEROCYCLES, Vol. 82, No. 2, 2011
Cu/TiO2 (Cu: 0.1 mmol)
150 oC, H2 (1 atm), 6 h
conv. = 95%
yield = 95%
N
N
H
2
(0.5 mmol)
1
Scheme 2. Hydrogenation of quinoline catalyzed by Cu/TiO2 under 1 atm of H2
To further investigate the reversibility of the present tetrahydroquinoline/quinoline system, successive
dehydrogenation-hydrogenation of 1/2 using Cu/TiO2 was repeated (Table 2). After the quantitative
dehydrogenation of 1 into 2 (entry 1), the Ar atmosphere was replaced with 1 atm of H2 and the reaction
mixture was heated again and stirred at 150 oC. The above treatment gave the product 1 in 95% yield for
6 h, which is shorter reaction time than that of the dehydrogenation (entry 2). Molecular hydrogen could
be released and stored using the present tetrahydroquinoline/quinoline catalyst system. These reversible
dehydrogenation and hydrogenation reactions could be repeated with retention of high efficiency (entries
3 and 4).
Table 2. Reversible dehydrogenation and hydrogenation between 1 and 2 using Cu/TiO2 catalysta
-2H2
Cu/TiO2 (Cu: 0.1 mmol)
mesitylene, 150 oC, Ar flow
N
N
H
Cu/TiO2 (Cu: 0.1mmol)
mesitylene, 150 oC, H2 (1 atm)
2
1 (0.5 mmol)
Entry
Time (h)
Conv. (%)b
1
2
3
4
dehydrogenation (1st)
hydrogenation (1st)
dehydrogenation (2nd)
hydrogenation (2nd)
16
6
16
6
99
95
98
94
a Reaction conditions: Cu/TiO2 (0.13 g, Cu: 0.1 mmol), mesitylene (5 mL).
b Determined by GC.
CONCLUSION
In conclusion, we prepared TiO2-supported copper nanoparticles that acted as a heterogeneous catalyst for
the reversible dehydrogenation and hydrogenation of tetrahydroquinoline/quinoline. The
Cu/TiO2-catalyzed organic hydride system could be conducted under mild reaction conditions and
repeated. This is the first report of a reversible organic hydride system using a heterogeneous catalyst
with 1 atm of H2. This strategy will give a clue to the development of a highly efficient, practical organic
hydride system.

HETEROCYCLES, Vol. 82, No. 2, 2011
1375
EXPERIMENTAL
1) General
All organic reagents were purified before use. Copper (II) trifluoromethanesulfonate was purchased from
Tokyo Kasei Kogyo Co., Ltd. TiO2 (JRC-TIO-4), Al2O3 (JRC-ALO-3), SiO2 (JRC-SIO-4), MgO
(JRC-MGO-1) were obtained from the Catalysis Society of Japan as reference catalysts. Hydroxyapatite
(HAP) was purchased from Wako Pure Chemical Industries, Ltd. Inductively coupled plasma
1
13
measurement was performed by SII NanoTechnology SPS7800. H and C nuclear magnetic resonance
(NMR) spectra were recorded on JEOL JNM-AL400 and JNM-GSX270 spectrometers, respectively. Gas
chromatography (GC-FID) was carried out a Shimadzu GC-2014 equipped with a KOCL-3000T column
(2 m).
2) Characterization of Cu/TiO2
Elemental analysis showed that the copper loading was 4.91 wt%. UV-vis spectra were recorded on a
JASCO V-550 spectrometer equipped with integration spheres at room temperature. BaSO4 was used as a
standard reflection sample.
0.8
(b)
0.7
0.6
0.5
(a)
0.4
0.3
400
500
600
700
800
Wavelength (nm)
Figure 1S. UV-vis absorption spectra of (a) CuII/TiO2 (dashed line) and (b) Cu/TiO2 (solid line).
3) Typical reaction procedure
3-1) Dehydrogenation of 1,2,3,4-Tetrahydroquinoline (1)
A typical procedure for the dehydrogenation of 1 by Cu/TiO2: CuII/TiO2 (0.13 g, 0.01 mmol of Cu) was
placed in a Schlenk tube equipped with a reflux condenser. The vessel was evacuated and filled with 1
atm of molecular hydrogen (H2) and the solid catalyst was stirred at 180 oC for 0.5 h. After replacing the
H2 with Ar, a mineral oil bubbler was attached to the top of the condenser, mesitylene (5 mL) and 1 (0.5

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HETEROCYCLES, Vol. 82, No. 2, 2011
mmol) were added, and the reaction mixture was vigorously stirred at 150 oC under a stream of Ar for 16
h. The solid catalyst was removed by centrifugation and the yield was determined by GC analysis with
naphthalene as an internal standard [Shimadzu GC-8A; carrier gases: N2 (50 kPa), H2 (50 kPa), and air
o
o
(50 kPa); column: 5% KOCL-3000T (5.0 m); oven temperature program: from 100 C, 10 C/min up to
220 oC, hold for 10 min; injection and detection temperature: 270 oC; retention time: 1 (10.9 min), 2 (9.3
min), naphthalene (7.16 min)].
3-2) Hydrogenation of quinoline (2)
A typical procedure for the hydrogenation of 2 by Cu/TiO2: CuII/TiO2 (0.13 g, 0.01 mmol of Cu) was
placed in a Schlenk tube equipped with a reflux condenser. The vessel was evacuated and filled with 1
o
atm of molecular hydrogen (H2) and the solid catalyst was stirred at 180 C for 0.5 h. A balloon filled
with hydrogen was connected to the top of the condenser. Mesitylene (5 mL) and 2 (0.5 mmol) were
added, and the reaction mixture was vigorously stirred at 150 oC for 6 h. The solid catalyst was removed
by centrifugation and the yield was determined by GC analysis with naphthalene as an internal standard.
3-3) Reversible dehydrogenation and hydrogenation between 1 and 2
A typical procedure for the dehydrogenation of 1 by Cu/TiO2: CuII/TiO2 (0.13 g, 0.01 mmol of Cu) was
placed in a Schlenk tube equipped with a reflux condenser. The vessel was evacuated and filled with 1
atm of molecular hydrogen (H2) and the solid catalyst was stirred at 180 oC for 0.5 h. After replacing the
H2 with Ar, a mineral oil bubbler was attached to the top of the condenser, mesitylene (5 mL) and 1 (0.5
mmol) were added, and the reaction mixture was vigorously stirred at 150 oC under a stream of Ar for 16
h. The yield of 2 was determined by GC analysis. Next, the atmosphere of the flask was replaced with 1
atm of H2. The mixture was magnetically stirred for 6 h at 150 oC. The yield of 1 was determined by GC
analysis. The above procedures were repeated.
3-4) Measurement of H2
The dehydrogenation was carried out in a closed vessel under standard reaction conditions. After the
reaction, the gas phase was subjected to GC-TCD for qualitative and quantitative analysis of H2. GC
conditions were as follows: GC using thermal conductivity detector (Shimadzu GC-8A); column:
o
o
molecular sieves 13X (4.0 m); oven temperature: 40 C; injection and detection temperature: 70 C;
carrier gas: Ar (100 kPa); current: 60 mA; retention time: H2 (3.0 min). Evolution of H2 was also
confirmed volumetrically using a gas buret attached to the reactors.
ACKNOWLEDGEMENTS
This investigation was supported by a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan. This work was also supported by a
Grant-in-Aid for Scientific Research on Priority Areas (No.18065016, "Chemistry of Concerto

HETEROCYCLES, Vol. 82, No. 2, 2011
1377
Catalysis") from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. One of the
authors Y.M. thanks the JSPS Research Fellowships for Young Scientists. Y. M. expresses his special
thanks for The Global COE (center of excellence) Program “Global Education and Research Center for
Bio-Environmental Chemistry” of Osaka University.
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