Highly efficient and recyclable rhodium nanoparticle catalysts for hydrogenation of quinoline and its derivatives

Article abstract of DOI:10.1039/c5cy00940e

PEG-stabilized rhodium nanoparticles exhibited high activity, selectivity and recyclability for the hydrogenation of quinoline and its derivatives. The selectivity of 1,2,3,4-tetrahydroquinoline was higher than 99%. The catalysts were recycled ten times with a total turnover number of 10592, which is the highest value ever reported for quinoline.

Full text of DOI:10.1039/c5cy00940e

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Highly efficient and recyclable rhodium
nanoparticle catalysts for hydrogenation of
Cite this: DOI: 10.1039/c5cy00940e
quinoline and its derivatives†
Received 30th June 2015,
Accepted 22nd July 2015
Mingming Niu, Yanhua Wang,* Pu Chen, Dongjie Du, Jingyang Jiang and Zilin Jin
DOI: 10.1039/c5cy00940e
www.rsc.org/catalysis
PEG-stabilized rhodium nanoparticles exhibited high activity,
selectivity and recyclability for the hydrogenation of quinoline and
its derivatives. The selectivity of 1,2,3,4-tetrahydroquinoline was
higher than 99%. The catalysts were recycled ten times with a
total turnover number of 10 592, which is the highest value ever
reported for quinoline.
homogeneous when the temperature is increased to a certain
point. Consequently, the reaction proceeds virtually in a
homogeneous system under heating, and on cooling to room
temperature, it separates into a biphasic system composed of
a PEG phase with the transition metal nanoparticle catalysts
and an organic phase containing the products. Such a process
provides both the advantages of homogeneous and heteroge-
neous catalyses. Until now, this thermoregulated PEG
biphasic system has been applied for the hydroformylation of
1
,2,3,4-Tetrahydroquinoline IJ1,2,3,4-THQ) and its derivatives
are very important for the production of fine chemicals,
pharmaceuticals, agrochemicals and petrochemicals.
7a
7b
alkenes,
hydrogenation of cinnamaldehyde
and semi-
Although there are various preparation methods, the catalytic
hydrogenation of quinoline and its derivatives is considered
to be the best one due to its simplicity and high atomic
efficiency. The catalysts most often used for this reaction are
transition metals either in homogeneous or heterogeneous
7c
hydrogenation of alkynes. Driven by our long-standing inter-
est in searching for effective biphasic catalytic systems and
inspired by the aforementioned results, herein, we originally
explore the thermoregulated PEG biphasic system involving
the PEG-stabilized Rh nanoparticle catalysts for the hydroge-
nation of quinoline and its derivatives.
1
2,3g,h
3
2f,4
5
6
form, such as Ru, Rh,
Pd, Ir,
Au, and Ni. Catalytic
systems that are directed towards mild reaction conditions or
Our investigations started with the preparation of PEG-
stabilized Rh nanoparticle catalysts. In a typical experiment,
1
i,2d,3b
green solvents have been reported.
As we have known,
homogeneous catalysts are active and selective, but they face
the problem of separation. On the contrary, heterogeneous
catalysts can be separated easily from the products, but their
activity is low. Therefore, developing a new catalytic system
that combines the merits of homogeneous and heterogeneous
catalyses remains challenging but rewarding. Recently, we
have advanced a thermoregulated polyIJethylene glycol) (PEG)
biphasic system towards transition metal nanoparticle
catalysts, which can realize a totally homogeneous reaction
medium under a certain reaction temperature and achieve
subsequent efficient separation into two phases by cooling
the reaction mixture to room temperature. Namely, the PEG
phase containing PEG-stabilized transition metal nano-
particles is immiscible with a mixture of toluene and
n-heptane at room temperature, but the phases become
RhCl ·3H O (1.35 mg, 0.005 mmol) dissolved in PEG (PEG
4000
3
2
−1
with an average molecular weight of 4000 g mol , 3.0 g, 0.75
mmol) was added into a 75 mL standard stainless steel auto-
clave and stirred under hydrogen (4.0 MPa) at 70 °C for 3 h.
Then, the reactor was cooled to room temperature and
depressurized. The PEG4000-stabilized Rh nanoparticles thus
obtained were characterized by transmission electron micros-
copy (TEM). As depicted in Fig. 1, the TEM image shows that
the average diameter of the Rh nanoparticles is 1.5 ± 0.4 nm.
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
1
16024, PR China. E-mail: yhuawang@dlut.edu.cn; Fax: +86 411 84986033;
Tel: +86 411 84986033
Electronic supplementary information (ESI) available. See DOI: 10.1039/
†
Fig.
1 TEM image and particle size histogram of the
c5cy00940e
PEG4000-stabilized Rh nanoparticles (newly prepared, 200 particles).
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Fig. 2 Schematic illustration of the hydrogenation of quinoline in the
thermoregulated PEG biphasic system with the PEG4000-stabilized Rh
nanoparticles.
Fig.
3
The recycling efficiency of the PEG4000-stabilized Rh
nanoparticle catalysts (reaction time:
recycles).
3 h, the same for the ten
Next, the PEG4000-stabilized Rh nanoparticle-catalyzed
hydrogenation of quinoline in
a thermoregulated PEG
biphasic system was first examined and the schematic illus-
tration of the process is shown in Fig. 2. All the quinoline
hydrogenation reactions were carried out in a 75 mL stan-
dard stainless steel autoclave immersed in a thermostatic oil
bath. The stirring rate was the same for all the experiments
performed. The autoclave was loaded with the Rh nanoparti-
cle catalyst, toluene, n-heptane, n-decane (internal standard),
increase in the temperature to 120 °C, the conversion of
quinoline remained nearly the same (Table 1, entry 3). The
effect of H pressure on the reaction was then studied (Table
2
1, entries 2 and 4–6). In the range from 1.0 to 3.0 MPa, the
conversion of quinoline increased with increasing pressure
and quinoline and was flushed three times with 1.0 MPa H
2
.
and 97% conversion was achieved at 3.0 MPa. When the H
2
The reactor was pressurized with H up to the required pres-
pressure was increased to 4.0 MPa, the 97% conversion
remained unchanged. It was also clear from the data in Table
1 (entries 2 and 7–9) that prolonging the reaction time led to
an increase in the conversion of quinoline. Under the opti-
mized reaction conditions, the conversion of quinoline and
the selectivity of 1,2,3,4-THQ were 97% and more than 99%,
respectively (Table 1, entry 2). The TOF was 320 h and the
TON was 960. In order to gain further insight into the nature
of the catalysis, namely heterogeneous or homogeneous, a
mercury poisoning experiment was carried out. 0.5 g of Hg(0)
(500 equiv. of Rh) was added to the catalytic system at the
beginning of the reaction (Table 1, entry 10) or after 1 h
(Table 1, entry 11). Whenever Hg was introduced, the cata-
lysts were completely deactivated (Table 1, entries 7, 10, and
11). As we know, Hg(0) is a well-known poison for heteroge-
neous catalysts due to its adsorption on the catalyst surface
or amalgamation with the metal catalyst. Therefore, this
2
sure and held at the scheduled temperature with magnetic
stirring for a fixed reaction time. Then, the reactor was
cooled to room temperature and depressurized. The upper
organic phase was carefully removed from the lower PEG
phase by a syringe and immediately analyzed by GC and GC-
MS.
The main results are summarized in Table 1. The major
product is 1,2,3,4-THQ, and the selectivity is more than 99%.
The effect of the reaction temperature was studied in the
−
1
range of 80–120 °C at 3.0 MPa H
Table 1, when the temperature is increased from 80 °C to 100
C, the conversion of quinoline increased evidently (Table 1,
2
pressure. As shown in
°
entry 1 vs. entry 2). This can be explained by the fact that the
catalytic system changed from a biphase to a monophase
(the miscibility temperature of the system is 98 °C) and there-
fore benefited the mass transfer process. With a further
Table 1 Hydrogenation of quinoline catalyzed by the PEG4000-stabilized Rh nanoparticle catalystsa
b
c
−1
d
Entry
Temperature (°C)
Pressure (MPa)
Time (h)
Conversion (%)
Selectivity (%)
TOF (h
)
TON
1
2
3
4
5
6
7
8
80
3.0
3.0
3.0
1.0
2.0
4.0
3.0
3.0
3.0
3.0
3.0
3
3
3
3
3
3
1
2
6
3
3
55
97
98
79
91
97
77
92
98
4
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
182
320
323
261
300
320
762
455
162
13
546
960
969
783
900
960
762
911
972
39
100
120
100
100
100
100
100
100
100
100
9
e
1
1
0
1
f
78
257
771
a
−3
Reaction conditions: PEG4000 3.0 g (containing 5.0 × 10 mmol of Rh), quinoline/Rh = 1000 (molar ratio), toluene 3.0 g, n-heptane 1.0 g,
b
c
n-decane 0.2 g (internal standard). Selectivity of 1,2,3,4-THQ. Turnover frequency (TOF): calculated as the number of moles of 1,2,3,4-THQ
per mole of Rh per hour. Turnover number (TON): moles of product formed per mole of rhodium. For the mercury poisoning experiment,
0
d
e
f
.5 g of Hg(0) (500 equiv. of Rh) was added at the beginning of the reaction. For the mercury poisoning experiment, 0.5 g of Hg(0) (500 equiv.
of Rh) was added after 1 h.
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8
structures. The other reason probably results from the steric
effects of the PEG chain.
The leaching of transition metal nanoparticle catalysts is
frequent, especially for the catalytically active group VIII
9
metals. Therefore, we studied Rh leaching in the recycle
experiments by determination of the Rh content in the upper
organic phase at the end of the reaction. The leaching of Rh
was detected by ICP-AES, and the results indicated that the
average leaching of Rh in the upper organic phase was
Fig.
4 TEM image and particle size histogram of the
PEG4000-stabilized Rh nanoparticles (after ten recycles, 200 particles).
1.0 wt%. Although further optimization is necessary to mini-
mize the Rh leaching, the PEG4000-stabilized Rh nanoparticle
catalysts are not only really easy to handle but also active,
selective and recyclable for the hydrogenation of quinoline.
To expand the application scope of the catalytic system,
various quinoline derivatives including 2-methylquinoline,
result implied the heterogeneous nature of the Rh nanoparti-
cle catalysts.
The lifetime is an important aspect concerning the use of
catalysts; therefore, the reusability of the Rh nanoparticles
was examined. After the reaction, the upper organic phase
was separated from the lower catalyst-containing phase by
simple phase separation in the autoclave under open air con-
ditions. By adding a new batch of substrate and solvent, the
lower catalyst-containing phase can be directly used in the
next reaction run. Under the same reaction conditions as
entry 2 in Table 1, the recovered Rh nanoparticle catalyst
could be reused ten times without evident loss in activity and
selectivity (Fig. 3). The average particle size of the Rh nano-
particles after ten recycles was 1.7 nm, which only increased
slightly compared with the freshly prepared Rh nanoparticles
3
6
-methylquinoline, 6-methylquinoline, 8-methylquinoline and
-methoxyquinoline were screened as substrates. Under the
optimized reaction conditions, they also afforded their corre-
sponding tetrahydroquinolines with high conversion and
>
99% selectivity as shown in Table 2.
Conclusions
In summary, a thermoregulated PEG biphasic system com-
bining the merits of homogeneous and heterogeneous cataly-
ses was explored for the hydrogenation of quinoline and its
derivatives with PEG4000-stabilized Rh nanoparticles as cata-
lysts. These catalysts were not only active and selective but
were also easy to separate from the products by simple phase
separation and were recycled ten times without evident loss
in activity and selectivity. To the best of our knowledge, these
(
Fig. 4). The total TON is 10 592, which is higher than the
2
a
reported value of 940 for a heterogeneous Rh catalyst and
1
2
h
528 for the homogeneous one. The better stability of the
PEG4000-stabilized Rh nanoparticles is perhaps due to the fol-
lowing two aspects. One is the complexation of oxygen in
PEG with Rh through the formation of pseudo-crown ether
Table 2 Hydrogenation of quinoline derivatives catalyzed by the PEG4000-stabilized Rh nanoparticle catalystsa
b
c
−1
d
Entry
1
Substrate
Product
Time (h)
12
Conversion (%)
93
Selectivity (%)
TOF (h
)
TON
>99
77
140
245
921
2
3
7
4
99
99
>99
>99
980
980
4
5
4
6
99
95
>99
>99
245
157
980
941
a
−3
b
Reaction conditions: PEG4000 3.0 g (containing 5.0 × 10 mmol of Rh), PH2 = 3.0 MPa, T = 100 °C, substrate/Rh = 1000 (molar ratio), toluene
c
3
.0 g, n-heptane 1.0 g, n-decane 0.2 g (internal standard). Selectivity of the corresponding products. Turnover frequency (TOF): calculated as
d
the number of moles of product per mole of Rh per hour. Turnover number (TON): moles of product formed per mole of rhodium.
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PEG4000-stabilized Rh nanoparticle catalysts exhibited the
highest TON of 10 592 for the hydrogenation of quinoline.
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Acknowledgements
This work was supported financially by the National Natural
Science Foundation of China (21173031 and 21373039).
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