Selective hydrogenation of quinolines over a CoCu bimetallic catalyst at low temperature

Article abstract of DOI:10.1016/j.mcat.2019.04.005

Quinoline derivatives are widely exist in the environment, and mainly separated from the coal tar pitch fraction. Hydrogenation of these compounds to 1,2,3,4-tetrahydroquinolines, an important class of natural products and medicinal agents, is a significant transformation of waste to valuable chemicals. In the present work, we developed a cheap and highly efficient Co₃Cu₁O_x bimetallic catalyst and used it for the hydrogenation of quinolines at a temperature down to 60 °C. The introduction of Cu into Co catalyst changed the physical and chemical features of Co catalyst, which was characterized by Raman spectra, N₂-adsorption/desorption isotherms, H₂-TPR and H₂-TPD tests. The recycling experiments indicated the catalyst was stable and possessed good reusability. Importantly, the gram-scale experiment provided a high yield (92%) to the target product, demonstrating that the catalytic system has a potential practical application.

Full text of DOI:10.1016/j.mcat.2019.04.005

MolecularCatalysis470(2019)120–126
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Selective hydrogenation of quinolines over a CoCu bimetallic catalyst at low
temperature
⁎
⁎
Zhen-Hong Hea,c, , Na Lia,c, Kuan Wanga,c, Wei-Tao Wanga,c, Zhao-Tie Liua,b,
a College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi’an, 710021, China
b Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an,
710119, China
c National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science & Technology, Xi’an, 710021, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Bimetallic catalysis
CoCu catalyst
Quinoline derivatives
Selective hydrogenation
Tetrahydrogenquinoline
Quinoline derivatives are widely exist in the environment, and mainly separated from the coal tar pitch fraction.
Hydrogenation of these compounds to 1,2,3,4-tetrahydroquinolines, an important class of natural products and
medicinal agents, is a significant transformation of waste to valuable chemicals. In the present work, we de-
veloped a cheap and highly efficient Co3Cu1Ox bimetallic catalyst and used it for the hydrogenation of quinolines
at a temperature down to 60 °C. The introduction of Cu into Co catalyst changed the physical and chemical
features of Co catalyst, which was characterized by Raman spectra, N2-adsorption/desorption isotherms, H2-TPR
and H2-TPD tests. The recycling experiments indicated the catalyst was stable and possessed good reusability.
Importantly, the gram-scale experiment provided a high yield (92%) to the target product, demonstrating that
the catalytic system has a potential practical application.
1. Introduction
ZrO2·xH2O [14], RuCu nanocages and Cu@Ru nanocrystals [15], Ru
NPs immobilized on PVP [16], multimetallic nanoparticle catalysts [4],
N-containing heterocycle compounds are common intermediates in
pharmaceutical and biological active molecules [1,2]. For example,
1,2,3,4-terthydrogenquinoline derivatives (THQs) are very important
chemicals in organic synthesis, pharmaceuticals, biologically active
natural products etc [3]. The hydrogenation of quinolines which are the
by-products of petroleum refining processes, is considered to be a
promising strategy to generate THQs in view of the high atom efficiency
[3,4–6]. However, the reaction is difficult for the highly unsaturated
feedstocks and the potential poisoning on the metal catalysts [2,5].
Despite great progresses have been achieved in the last decades, the
efficient hydrogenation of quinolines to THQs still remains challenge-
able.
A variety of homogeneous and heterogeneous catalysts have been
developed for the titled reaction. The homogeneous catalysts such as Ir
[7], η6-arene/TsDpen-Ru(II) [8] and metal-free catalyst [9] etc. were
active for the titled reaction. However, the difficulties in separation of
the catalyst from reaction mixtures and the high cost inhibited their
further applications. The heterogeneous catalysts always involved
precious metals, for example, Ru/ordered mesoporous N-doped carbon
[10,11], Au/HSA-TiO2 (HSA denoted as high surface area) [3], nano-
porous Au catalyst [12], Ru/HAP (HAP is hydroxyapatite) [13], Ru/
Pt/TiO2 [17], Ru-SiO2@mSiO2 [18], Pd NPs [19–21] and PEG-stabi-
lized Rh NPs [22] etc. The precious catalysts are unfavorable for in-
dustrial applications, while the base metal catalysts, especially Co-
based catalysts, are potential substitutions for these precious catalysts.
Recently, Co-based catalysts were developed and utilized to catalyze
hydrogenation of quinolines, however, the reaction operated at ele-
vated temperature (> 120 °C) [23–25].
The bimetallic catalysts have caught much attention due to some
individual properties originated from the regulable electronic, structure
and chemical effects between two metals [26,27]. Consequently, de-
veloping a cheap bimetallic catalyst with precious metal-free and high
efficiency is extremely desirable. In recent years, lots of bimetallic
catalysts were developed in hydrogenation [28], hydrogen generation
[29], CeC and C–X coupling reactions [26], furan transformation [30],
energy and environmental applications [31], CO hydrogenation [32]
etc. However, to the best of our knowledge, it is rare for hydrogenation
of quinolines. Herein, we developed
a CoCu bimetallic catalyst
(Co3Cu1Ox) and applied in the titled reaction under relatively low
temperature conditions. The stable Co3Cu1Ox catalyst, which could be
magnetically separated from the reaction mixtures, exhibited a high
efficiency for the selective hydrogenation of unsaturated aromatic
⁎ Corresponding authors at: College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi’an, 710021, China.
E-mail addresses: hezhenhong@sust.edu.cn (Z.-H. He), ztliu@snnu.edu.cn (Z.-T. Liu).
https://doi.org/10.1016/j.mcat.2019.04.005
Received 8 December 2018; Received in revised form 29 March 2019; Accepted 3 April 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

Z.-H. He, et al.
MolecularCatalysis470(2019)120–126
amines. The catalytic performances and characterization of catalysts
suggested that a synergistic effect existed between Co and Cu species.
min was performed to remove residual H2 from the surface of catalysts.
H2 adsorption test was conducted from 50 to 600 °C at a heating rate of
10 °C/min and maintained at 600 °C for 30 min with 50 mL/min of He.
The desorption amount of H2 was tested by using a TCD detector.
2. Experimental section
2.1. Materials and methods
2.4. Hydrogenation of quinolines
Co(NO3)2·6H2O (99.0%), Cu(NO3)2·3H2O (99.0%) were provided by
Adamas-Beta. Ni(NO3)2·6H2O (≥98.0%) was obtained from Tianjin
Beilian Fine Chemicals Development Co., Ltd. Fe(NO3)3·6H2O
(≥98.5%) and tetrahydrofuran (99%) were afforded by Guangdong
Guanghua Sci-Tech Co., Ltd. Na2CO3 was purchased from Tianjin
Sheng’ao Chemicals Development Co., Ltd. Quinoline (96.0%) was
gained by Aladdin Industrial Corporation. N2 (99.999%), CO2
(99.995%) and H2 (99.99%) were purchased from Xi’an Teda Cryogenic
Equipment Co., Ltd. All other chemicals were obtained from commer-
cial companies and used without further purification.
The hydrogenation reactions of quinolines were carried out in
16 mL stainless steel autoclaves with a Teflon inner container in a batch
mode of operation [33,34]. In a typical reaction, quinoline (0.129 g,
1 mmol), catalyst 20 mg and THF (1 mL) were added into the inner
container, which was transferred into the autoclave. The reactor was
sealed and charged with 1 MPa of H2 to remove the air for three times.
After that, it was charged with 4 MPa of H2 and heated to 60 °C and
stirred for 15 h. Upon completion of reaction, the solid catalyst was
separated using an extra magnet, and the liquid was analysed on a gas
chromatograph (GC9720, Zhejiang Fuli Analytical Instruments Co.,
Ltd., China) equipped with a flame ionization detector and a HP-5 ca-
pillary column (30 m × 0.32 mm × 0.25 μm) with toluene as an in-
ternal standard and GC–MS (Agilent 6890N-5975) with a HP-5 capillary
column (30 m × 0.32 mm × 0.25 μm).
2.2. Catalyst preparation
The catalysts were prepared by a simple co-precipitation method.
The Co3Cu1Ox catalyst was manufactured as follows: Co(NO3)2·6H2O
(0.873 g, 3.0 mmol), Cu(NO3)2·3H2O (0.242 g, 1.0 mmol) were dis-
solved in 20 mL of deionized water and the mixture was stirred for 1 h
until Co2+ and Cu2+ were well dispersed. Then the solution was
dropped slowly to 0.5 M of Na2CO3. Meanwhile, the precipitate was
formed and the mixture was stirred for another 16 h at room tem-
perature. The solid was filtrated and washed with 500 mL of deionized
water to remove Na+ and CO32−. After that, the solid was calcined in a
muffle furnace in air from room temperature to 500 °C with a heating
rate of 5 °C/min and kept at 500 °C for 3 h. The catalyst was further
reduced in a tube furnace under pure H2 from room temperature to
500 °C with the same rate and kept at 500 °C for 2 h, after reduction, the
catalyst was passivated by 1% O2/N2 at room temperature for 30 min to
avoid the over-oxidation.
2.5. The reusability of the catalyst and the hot filtration tests
After the reaction, the solid catalyst was separated by an extra
magnet and washed with THF for five times, and then it was used in the
next run. Other operations were the same as mentioned. The hot fil-
tration test was carried out as follows: after the reaction proceeded for
9 h, the catalyst was filtrated and the solution was kept under the cat-
alytic conditions for another 6 h. After the reaction completion, the
solution was analyzed by GC.
3. Results and discussion
Initially, we chose the abundant and cheap quinoline as the sub-
strate to screen the catalyst, and the results are summarized in Table 1.
The reaction did not occur spontaneously in the absence of the catalyst
(entry 1). The pure Co catalyst gave THQ with the yield of only 2%. The
pure Cu showed a low activity even prolonged the time to 20 h, which
gave 2% yield. The results revealed that Co and Cu were both inactive
Other Co-Cu catalysts with different compositions were synthesized
by the same procedures as the above, and the catalysts of the pure Co
and Cu, Co3Zn1Ox, Co3Mn1Ox, Co3Fe1Ox, Co3Ni1Ox and Co3Ce1Ox were
also prepared by the above co-precipitation method with corresponding
nitrate salts as precursors.
2.3. Catalyst characterization
Table 1
Hydrogenation of quinoline over divers catalysts.a.
The as prepared catalysts were characterized by TEM, XRD, XPS,
Raman, H2-TPR and H2-TPD, and N2-adsorption/desorption. The TEM
images of the catalysts were measured using a JEM-2100 electron mi-
croscope operated at 200 kV. The XRD was carried out on Rigaku D/
max 2500 with nickel filtered Cu-Kα (λ = 0.154 nm) operated at 40 kV
and 20 mA. The XPS spectra were performed using an ESCALab 220I-XL
electron spectrometer from VG Scientific using 300 W AlKα radiation
with a hemispherical energy analyzer. The Raman spectra were tested
on a Laser microscopic Raman imaging spectrometer (Thermo Fisher,
USA) equipped with a 532 nm Ne laser and a high-grade Leica micro-
scope (long working distance objective 50). The single crystal silicon
was used for the position correction.
The H2-TPR tests were performed on a chemisorption analyzer
(Autochem II 2920, Micromeritics). Before the test, the catalyst was
pretreated at 300 °C for 30 min in He with a flow rate of 50 mL/min,
and then was cooled down to 50 °C. Then a mixture gas of 10%H2-
90%Ar was passed through the catalysts with a flow of 50 mL/min. The
temperature was linearly raised from 50 °C to 700 °C at a heating rate of
10 °C/min. The effluent gas was analysed using a thermal conductivity
detector (TCD). The H2-TPD analysis was carried out as follows: the
catalyst was heated to 400 °C in He for 30 min, and then was cooled to
50 °C. After that, the catalyst was saturated with the 10%H2-90%Ar
mixture gas for 1 h. Then the purging in He with a flow rate of 50 mL/
Entry
Catalyst
Time (h)
Conv.b (%)
1
2
3
4
5
6
7
8
–
15
15
20
15
15
15
15
15
15
15
15
15
15
15
15
15
0
2
2
97
7
23
27
15
40
8
5
0
0
< 1
< 1
2
pure Co
pure Cu
Co3Cu1Ox
Co1Cu6Ox
Co1Cu3Ox
Co1Cu1Ox
Co2Cu1Ox
Co4Cu1Ox
Co5Cu1Ox
Co6Cu1Ox
Co3Zn1Ox
Co3Mn1Ox
Co3Fe1Ox
Co3Ni1Ox
Co3Ce1Ox
9
10
11
12
13
14
15
16
a
Reaction conditions: catalyst 20 mg, quinoline 1 mmol, THF 1 mL, H2
4 MPa, 15 h, 60 °C.
b
Conversion was detected by GC with toluene as an internal standard, and
the selectivity of THQ was > 99% in all cases.
121

Z.-H. He, et al.
MolecularCatalysis470(2019)120–126
for the reaction (entries 2 and 3). Interestingly, the CoCu bimetallic
catalyst with the molar ratio of 3/1, denoted as Co3Cu1Ox exhibited a
97% conversion of quinoline and > 99% selectivity to THQ, which
were much higher than that of pure Co and Cu (entry 4). Importantly,
the high catalytic activity was obtained at a low temperature (60 °C),
which is within an acceptable temperature range in industry. The cat-
alyst was synthesized by a simple co-precipitation method, and the
details are given in the Supporting Information. Interestingly, the other
CoCu catalysts with different molar ratios such as Co1Cu6Ox, Co1Cu3Ox,
Co1Cu1Ox, Co2Cu1Ox, Co4Cu1Ox, Co5Cu1Ox, Co6Cu1Ox etc. showed
lower yields to THQ (entries 4 vs 5–11), which was probably due to the
complex properties of structure and electronic of bimetallic catalyst.
The Co/Cu molar ratios were confirmed by ICP-OES (Table S1). For
comparison, the Co catalysts combined with other metals were also
evaluated. All of them exhibited low activities, for example, the
Co3Zn1Ox and Co3Mn1Ox catalysts could not catalyse the reaction at all
(entries 12 and 13), while Co3Fe1Ox, Co3Ni1Ox and Co3Ce1Ox bimetallic
catalysts yielded trace amounts of desirable product (entries 14–16).
Based on these results, the outstanding performances were probably
due to the existence of a synergistic effect between Co and Cu metals in
Co3Cu1Ox catalyst.
It is well known that the catalytic performances of the catalyst are
related closely to the structures. The N2 adsorption/desorption iso-
therms indicated the BET surface area of the fresh Co3Cu1Ox catalyst
(41 m2/g) was smaller than that of the pure Co catalyst (77 m2/g). The
pore sizes of Co3Cu1Ox and Co catalysts calculated from the distribution
curves were approximately 50 and 46 nm, respectively, which were
mainly formed through particle accumulation (Fig. S1 and Table S2 in
supporting information). This was also confirmed by SEM image, which
showed the nanoparticles were aggregated (Fig. S2a). The EDS spec-
trum of the fresh catalyst was also tested, and the result revealed the
Co/Cu/O surface atom ratio was 71.7/20.5/7.8 (Fig. S2b and Table S3).
TEM images showed that the fresh Co3Cu1Ox catalyst was mostly ag-
gregated by nanoparticles with the size of approximately 60–100 nm
(Fig. 1a). The HR-TEM images of fresh catalyst showed the lattice
Fig. 2. XRD patterns of the as-synthesized catalysts (a, pure Co, ♥), (b, pure Cu,
♦), (c, the fresh Co3Cu1Ox), (d, the unreduced Co3Cu1Ox), (e, the used
Co3Cu1Ox), (f, Co standard JCPDS:15-0806), (g, Cu, JCPDS:04-0836), (h, CuO,
♠, JCPDS:48–1548) and (i, Co3O4, JCPDS:42–1467).
fringes of the planes with a D-spacing of 1.8 Å, 2.4 Å and 2.0 Å, corre-
sponding to Cu (200), CoO (111) and Co (002) (Fig. 1c-d), respectively.
The HR-TEM tests indicated the fresh catalyst was composed of Co°, Cu°
Fig. 1. TEM images of the fresh Co3Cu1Ox catalyst (a), the used catalyst (b) and HR-TEM images of fresh catalyst (c–e).
122

Z.-H. He, et al.
MolecularCatalysis470(2019)120–126
Fig. 3. XPS tests of the fresh catalyst (a, Co), (b, Cu), the used catalyst Co3Cu1Ox, (c, Co) (d, Cu), and the unreduced catalyst (e, Co), (f, Cu).
and CoO species. The Co° and Cu° species were also confirmed by the
XRD tests.
of the unreduced catalyst were in good agreement with that of Co3O4
(JCPDS file No. 42-1467) (Fig. 2d vs Fig. 2i) which were indexed as
(111), (220), (311), (222), (400), (511) and (400) diffraction lines,
respectively (2d vs 2i) [36]. The weak peaks at 35.5° and 38.8° are
assigned to (002) and (111) of CuO (JCPDS file No. 48-1548) (2d vs 2 h)
[5,35].
To investigate the surface atomic composition, the catalysts were
examined by XPS (Fig. 3). The peaks of Co species in the unreduced
catalyst were attributed to Co3O4, which was verified by the weak
shake-up satellite structure of from the minor Co2+ component (Fig. 3e)
[37]. The surface Cu atom of the unreduced catalyst mainly was Cu2+
The bulk phase of these nanoparticles was mainly composed of Co°
and Cu° species, which were confirmed by the XRD patterns (Fig. 2)
[35]. XRD patterns of different catalysts are shown in Fig. 2. Compared
with the unreduced catalysts, fewer peaks were found in the fresh
catalyst. The peaks at 44.3°, 51.7°, and 75.8° were ascribed to the (111),
(200), and (220) of Co° (JCPDS file No. 15-0806), respectively (2c vs 2a
and 2f) [35]. The peaks around 43.3° and 50.6° were attributed to (111)
and (200) of Cu° (JCPDS file No. 04-0836), respectively (2c vs 2b and
2 g) [35]. The peaks at 19.0°, 31.3°, 36.9°, 38.6°, 44.9°, 59.4° and 65.3°
123

Z.-H. He, et al.
MolecularCatalysis470(2019)120–126
were characterized by H2-TPR and H2-TPD tests (Fig. 5). In H2-TPR
tests, two peaks emerged around 282 and 375 °C in the pure Co catalyst,
which could be attributed to the reduction of Co3O4 to CoO and CoO to
Co°, respectively [40,41]. These corresponding temperatures were re-
duced to 180 and 221 °C in Co3Cu1Ox catalyst. The TPR curve of CuO
showed a wide peak at 208 °C and two shoulder peaks at 139 and
262 °C, which were assgined to the reduction of isolated CuO to Cu2O,
the reduction of surface CuO clusters and the reduction of Cu2O to Cu°,
respectively (Fig. 5a). The results demonstrated the addition of Cu
could reduce the reduction temperature of Co catalyst. In H2-TPD tests,
the Co3Cu1Ox catalyst exhibited two peaks presented at approximately
120 and 600 °C, respectively. The H2 adsorption quantities were cal-
−1
culated from the curves, which were 0.25 and 0.44 mmolH2·gcat
,
respectively. However, for the pure Co and Cu catalysts, both of them
were much less than that of Co3Cu1Ox, which were only 0.14 and
0.10 mmolH2·gcat−1, respectively. The results indicated the Cu and Co
gave the strong synergistic effect for adsorption of H2, which probably
contributed to the outstanding catalytic performances in hydrogenation
of quinoline.
Fig. 4. Raman spectra of the pure Co catalyst (pink), Co3Cu1Ox catalyst (green)
and the pure Cu (red) (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article).
Effects of reaction conditions such as the reaction temperature,
pressure of H2 and reaction time on the catalytic performances were
investigated and the results are given in Fig. 6. The temperature af-
fected the catalytic efficiency remarkably (Fig. 6a). The reaction did not
occur at 40 °C, while it proceeded smoothly at 50 °C. Interestingly, the
conversion increased to 97% at 60 °C. The pressure of H2 also affected
the reaction obviously (Fig. 6b). The conversion of quinoline increased
with increasing the pressure of H2. The dependence of reaction time on
the conversion and selectivity was given in Fig. 6c, which showed the
conversion increased with prolonging the time to 18 h. For example, the
conversion of quinoline at 13 h was 20%, which increased remarkably
to 97% at 15 h. The reason for this rapid increase was speculated to
exist of an activation period between the catalyst and substrate. During
the period, the surface of oxidized catalyst was in-situ reduced to Co°
species under H2 in THF. The dependence of solvent on the catalytic
performances was listed in Table S4. Among the solvents tested, THF
was considered to be the best solvent for the reaction (Table 1, entry 4).
Cyclohexane and toluene gave THQ with a moderate yield of 77% and
67%, respectively. Water, DMF, DMI (1,3-dimethyl-2- imidazolidinone)
and DCM (dichloromethane) exhibited low activities for the reaction.
The reasons for these divers activities were probably due to the low
solubility of substrate in water, strong coordination between solvent,
Co3Cu1Ox catalyst and substrate.
species (Fig. 3f). In the fresh catalyst, the binding energy of the Co 2p
peaks observed at 782.0 eV and 796.5 eV could be assigned to Co3+ 2p
3/2 and Co3+ 2p 1/2, respectively. The peaks at 781.3 eV and 797.4 eV
could be attributed to Co2+ 2p 3/2 and Co2+ 2p 1/2, and the peaks at
778.0 eV and 795.3 eV could be designated to Co° 2p 3/2 and Co° 2p 1/
2, respectively. Small amount of Co° species was found because it was
easily oxidized to CoO in air at room temperature (Fig. 3a), which was
further confirmed by the strong shake-up peaks [37]. The peaks at
932.9 eV and 934.8 eV could be assigned to Cu0/+ and Cu2+ species,
respectively (Fig. 3b). It is difficult to distinguish between Cu° and Cu+
species based on their Cu 2p binding energies, which are almost in-
distinguishable [38]. The XPS spectra of used catalyst were nearly the
same as the fresh one, which indicated the surface of catalyst stayed
stable after using.
Raman scattering is very sensitive to the microstructure of nano-
crystalline materials [39], and the pure Co, Cu and the fresh Co3Cu1Ox
catalysts were examined (Fig. 4). The peaks at 190, 470, 514, 607 and
676 cm−1 in the pure Co catalyst were attributed to F2g, Eg, F2g, F2g and
A
1g mode of Co3O4, respectively [35]. Notably, Co3O4 was formed from
partial oxidization of CoO by local laser heating [39]. The pure Cu
catalyst showed no obvious peak in the range of 100-1000 cm-1. Com-
pared with the pure Co catalyst, the intensities of Co3Cu1Ox decreased
remarkably, meanwhile, the peak of Co3O4 was broadened for the
structural disorder. The Raman spectra indicated the lattice defects
were created in Co3O4 via the penetration of Cu atoms [35].
The results of Raman spectra indicated that the Co3Cu1Ox catalyst
probably possessed a strong interaction between Co and Cu species. To
further substantiate this, the fresh catalyst, the pure Co and Cu catalysts
The reusability of Co3Cu1Ox catalyst in hydrogenation of quinoline
was also studied (Fig. 7a). The results showed that it could be used at
least 5 times without significant loss in activity, indicating that it pos-
sessed a good stability in the recycling tests, which was confirmed by
the TEM, XRD, XPS and ICP-OES results. TEM image of the used catalyst
displayed that it was mainly nano-particles with the size in the range of
60–100 nm, which was the same as the fresh catalyst (Fig. 1a vs 1b).
Fig. 5. (a) H2-TPR tests of Co3O4 (pink), bimetallic Co3Cu1Ox catalyst (green) and CuO (red); (b) H2-TPD studies of the Co3Cu1Ox (green), the Co (pink) and the Cu
(blue) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
124

Z.-H. He, et al.
MolecularCatalysis470(2019)120–126
Fig. 6. Effects of temperature (a), the pressure of H2 (b) and the reaction time (c) on the catalytic performances (catalyst 20 mg, quinoline 1 mmol, THF 1 mL).
The XRD patterns of the used catalyst were the same as the fresh one
(Fig. 2e vs 2c), and the XPS results showed the used catalyst were the
same as the fresh catalyst (Fig. 3c vs 3a, and 3d vs 3b). The ICP-OES test
of the used catalyst showed the same molar ratio of Co/Cu (3.16) as the
fresh one (Table S1). These characterizations indicated the catalyst
possessed good stability during the reaction. The Co3Cu1Ox catalyst
could be easily separated from the reaction mixture by an extra magnet
(Fig. S3). The hot-filtration experiments indicated that the yield of
1,2,3,4-tetrahydroquinoline did not increase after the catalyst was fil-
trated, indicating that the catalyst was performed in a true hetero-
geneous way (Fig. 7b).
Subsequently, we evaluated the substrate scope of the reaction over
Co3Cu1Ox bimetallic catalyst and the results are given in Table 2. The
substrates bearing with an electron-donating group, such as CH3- and
CH3O- groups on the 6-position of quinoline, worked well under the
given conditions (entries 1 and 2). CH3- group at the 8-position of
quinoline, afforded the target product in excellent yield (up to 99%)
under 120 °C (entry 3). Electron-withdrawing groups such as chloride,
bromide at the 6-position of the quinoline structure showed 77% and
23% yield (entries 4 and 5). No corresponding product was detected
when nitro- group at 7-position of quinoline (entry 6). Isoquinoline was
tolerated in the present protocol, which produced 1,2,3,4-tetra-
hydrogenisoquinoline with a 99% yield (entry 7). Interestingly, the
gram-scale experiment was also conducted, and a satisfying yield of
92% for THQ was obtained (entry 9), indicating that the catalyst has
great potential application in industry.
Table 2
Hydrogenation of various substrates over Co3Cu1Ox catalyst.a.
Entry
R
Time (h)
Temp. (oC)
Yieldb
1
2
3
4
6-CH3
6-OCH3
8-CH3
6-Cl
6-Br
7-NO2
H
15
15
15
15
15
15
24
15
80
80
> 99
> 99
> 99
77
23
0
120
80
130
130
130
80
5
6
7c
8d
> 99
92
H
a
Reaction conditions: catalyst 20 mg, solvent 1 mL, substrate 1 mmol, H2
4 MPa.
b
c
GC yield.
isoquinoline was used as the substrate.
gram-scale experiment, quinoline 10 mmol, solvent 10 mL, catalyst
d
200 mg.
Co3Cu1Ox catalyst for hydrogenation of quinolines to 1,2,3,4-tetra-
hydroquinolines under low temperature. The catalyst could be simply
fabricated by a co-precipitate method, reused for several times and
worked in a heterogeneous way. The high efficiency was partly proved
to be originated from the strong adsorption of H2 over Co3Cu1Ox cat-
alyst. The gram-scale experiment indicated the catalyst had a great
potential application in industry. The developed approach demon-
strates the technical feasibility of replacing precious metal catalysts by
non-precious bimetallic catalysts, and we expect this protocol will
4. Conclusions
In summary, we have developed a highly efficient bimetallic
Fig. 7. The reusability and the hot filtration tests for Co3Cu1Ox catalyst. Reaction conditions were similar to that of entry 4, Table 1.
125

Z.-H. He, et al.
MolecularCatalysis470(2019)120–126
extend the utilization in hydrogenation of analogous core structure.
(2008) 965–970.
[15] Y. Chen, Z. Yu, Z. Chen, R. Shen, Y. Wang, X. Cao, Q. Peng, Y. Li, Nano Res. 9 (2016)
2632–2640.
Acknowledgements
[16] R.A. Sánchez-Delgado, N. Machalaba, N. Ng-a-qui, Catal. Commun. 8 (2007)
2115–2118.
[17] S. Li, Y. Yang, Y. Wang, H. Liu, J. Tai, J. Zhang, B. Han, Catal. Sci. Technol. 8 (2018)
The authors gratefully acknowledge the financial supports from
National Natural Science Foundation of China (21776170,
21811530292 and 21706152).
4314–4317.
[18] L. Zhang, X. Wang, Y. Xue, X. Zeng, H. Chen, R. Li, S. Wang, Catal. Sci. Technol. 4
(2014) 1939–1948.
[19] M. Guo, C. Li, Q. Yang, Catal. Sci. Technol. 7 (2017) 2221–2227.
[20] M.M. Dell`Anna, V.F. Capodiferro, M. Mali, D. Manno, P. Cotugno, A. Monopoli,
P. Mastrorilli, Appl. Catal. A-Gen. 481 (2014) 89–95.
Appendix A. Supplementary data
[21] M.M. Dell’Anna, G. Romanazzi, S. Intini, A. Rizzuti, C. Leonelli, A.F. Piccinni,
P. Mastrorilli, J. Mol. Catal. A-Chem. 402 (2015) 83–91.
[22] M. Niu, Y. Wang, P. Chen, D. Du, J. Jiang, Z. Jin, Catal. Sci. Technol. 5 (2015)
4746–4749.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.04.005.
[23] Z. Wei, Y. Chen, J. Wang, D. Su, M. Tang, S. Mao, Y. Wang, ACS Catal. 6 (2016)
5816–5822.
[24] F. Chen, B. Sahoo, C. Kreyenschulte, H. Lund, M. Zhang, L. He, K. Junge, M. Beller,
Chem. Sci. 8 (2017) 6239–6246.
References
[1] W. Zhou, Q. Tao, F. Sun, J. Qian, J. Xu, M. He, Q. Chen, J. Xiao, J. Catal. 361 (2018)
1–11.
[2] Z. He, H. Liu, Q. Qian, L. Lu, W. Guo, L. Zhang, B. Han, Sci. Chin. Chem. 60 (2017)
[25] I. Sorribes, L. Liu, A. Doménech-Carbó, A. Corma, ACS Catal. 8 (2018) 4545–4557.
[26] D.R. Pye, N.P. Mankad, Chem. Sci. 8 (2017) 1705–1718.
[27] W. Yu, M.D. Porosoff, J.G. Chen, Chem. Rev. 112 (2012) 5780–5817.
[28] X. Jiang, N. Koizumi, X. Guo, C. Song, Appl. Catal. B-Environ. 170-171 (2015)
173–185.
[29] Z. Wei, J. Sun, Y. Li, A.K. Datye, Y. Wang, Chem. Soc. Rev. 41 (2012) 7994–8008.
[30] S.K. Singh, Asian J. Org. Chem. 7 (2018) 1901–1923.
[31] S. De, J. Zhang, R. Luque, N. Yan, Energy Environ. Sci. 6 (2016) 3314–3347.
[32] Y. Xiang, R. Barbosa, N. Kruse, ACS Catal. 4 (2014) 2792–2800.
[33] W. Wang, N. Li, H. Tang, Y. Ma, X. Yang, Mol. Catal. 453 (2018) 113–120.
[34] W. Wang, N. Li, L. Shi, Y. Ma, X. Yang, Appl. Catal. A-Gen. 553 (2018) 117–125.
[35] J. Su, Z. Zhang, D. Fu, D. Liu, X. Xu, B. Shi, X. Wang, R. Si, Z. Jiang, J. Xu, Y. Han, J.
Catal. 336 (2016) 94–106.
[36] J. Wang, R. Nie, L. Xu, X. Lyu, X. Lu, Green Chem. 21 (2019) 314–320.
[37] M.M. Natile, A. Glisenti, Chem. Mater. 14 (2002) 3090–3099.
[38] S. Natersakhawat, J.W. Lekse, J.P. Baltrus, P.R. Ohodnichki Jr., B.H. Howard,
X. Deng, C. Matranga, ACS Catal. 2 (2012) 1667–1676.
[39] S. Farhadi, M. Javanmard, G. Nadri, Acta Chim. Slov. 63 (2016) 335–343.
[40] Z. He, Q. Qian, J. Ma, Q. Meng, H. Zhou, J. Song, Z. Liu, B. Han, Angew. Chem. Int.
Ed. 55 (2016) 737–741.
927–933.
[3] D. Ren, L. He, L. Yu, R. Ding, Y. Cao, H. He, K. Fan, J. Am. Chem. Soc. 134 (2012)
17592–17598.
[4] N.A. Beckers, S. Huynh, X. Zhang, E.J. Luber, J.M. Buriak, ACS Catal. 2 (2012)
1524–1534.
[5] Z. He, H. Liu, H. Liu, Q. Qian, Q. Meng, Q. Mei, B. Han, ChemCatChem 9 (2017)
1947–1952.
[6] G.H.C. Prado, Y. Rao, Ad. Klerk, Energy Fuels 31 (2017) 14–36.
[7] W. Tang, S. Zhu, L. Xu, Q. Zhou, Q. Fan, H. Zhou, K. Lam, A.S.C. Chan, Chem.
Commun. 0 (2007) 613–615.
[8] T. Wang, L. Zhou, Z. Li, F. Chen, Z. Ding, Y. He, Q. Fan, J. Xiang, Z. Yu, A.S.C. Chan,
J. Am. Chem. Soc. 133 (2011) 9878–9891.
[9] Z. Zhang, H. Du, Org. Lett. 17 (2015) 6266–6269.
[10] G. Li, H. Yang, H. Zhang, Z. Qi, M. Chen, W. Hu, L. Tian, R. Nie, W. Huang, ACS
Catal. 8 (2018) 8396–8405.
[11] X. Yu, R. Nie, H. Zhang, X. Lu, D. Zhou, Q. Xia, Microporous Mesoporous Mater. 256
(2018) 10–17.
[12] M. Yan, T. Jin, Q. Chen, H.E. Ho, T. Fujita, L. Chen, M. Bao, M. Chen, N. Asao,
Y. Yamamoto, Org. Lett. 15 (2013) 1484–1487.
[41] Z. He, Q. Qian, Z. Zhang, Q. Meng, H. Zhou, J. Jiang, B. Han, Phil. Trans. R.Soc. A
373 (2015) 20150006.
[13] Y. Sun, H. Fu, D. Zhang, R. Li, H. Chen, X. Li, Catal. Commun. 12 (2010) 188–192.
[14] R. Zhang, G. Fan, C. Li, Y. Wang, R. Li, H. Chen, X. Li, Acta Phys. -Chim. Sin. 24
126