Thermoregulated phase-separable catalysis for rh nanoparticle catalyzed selective hydrogenation of 1,5-cyclooctadiene

Article abstract of DOI:10.1016/S1872-2067(11)60473-7

Through the study of the critical solution temperature of ionic liquids [CH₃(OCH₂CH₂)_nN⁺Et ₃][CH₃SO₃^-] (IL_PEG, n = 12, 16, 22), IL_PEG-stabilized Rh nanoparticle catalysts have been found to function as thermoregulated phase-separable catalysts and have been shown to be efficient and recyclable for the selective hydrogenation of 1,5-cyclooctadiene (1,5-COD) to cyclooctene (COE). Under optimized conditions, the conversion of 1,5-COD and selectivity for COE were 99% and 90%, respectively. The Rh catalyst could be recovered by simple phase separation and reused for ten times without loss of activity or selectivity.

Full text of DOI:10.1016/S1872-2067(11)60473-7

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 12, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 1871–1876.
Communication
Thermoregulated Phase-Separable Catalysis for Rh
Nanoparticle Catalyzed Selective Hydrogenation of
1,5-Cyclooctadiene
XU Yicheng, WANG Yanhua*, ZENG Yan, SONG Ying, ZHAO Jiaqi, JIANG Jingyang, JIN Zilin
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China
Abstract: Through the study of the critical solution temperature of ionic liquids [CH3(OCH2CH2)nN+Et3][CH3SO3 ] (ILPEG, n = 12, 16, 22),
–
ILPEG-stabilized Rh nanoparticle catalysts have been found to function as thermoregulated phase-separable catalysts and have been shown to
be efficient and recyclable for the selective hydrogenation of 1,5-cyclooctadiene (1,5-COD) to cyclooctene (COE). Under optimized con-
ditions, the conversion of 1,5-COD and selectivity for COE were 99% and 90%, respectively. The Rh catalyst could be recovered by simple
phase separation and reused for ten times without loss of activity or selectivity.
Key words: thermoregulated phase-separable catalysis; ionic liquid; rhodium nanoparticle; selective hydrogenation; 1,5-cyclooctadiene
To overcome the greatest drawback of homogeneous ca-
talysis, namely the separation of the catalyst from the product,
we previously developed a thermoregulated phase-separable
catalysis (TPSC) based on the critical solution temperature
(CST) of phosphine ligands in organic solvents. The catalytic
process of TPSC can be described as follows: before the reac-
tion, at room temperature (T < CST), the system is biphasic and
the catalyst is insoluble in the upper organic solvent. When
heated to T > CST, the catalyst becomes soluble in the solvent
and at the reaction temperature (T > CST) the whole system
remains homogeneous. After the reaction, the reaction mixture
is cooled to room temperature (T < CST) and the catalyst pre-
cipitates out of the organic phase. Thus, by simple phase
separation, the products can be easily separated from the
catalyst. TPSC allows not only for a highly efficient homoge-
neous catalytic reaction, but also for the easy separation and
reuse of catalyst. It has found application in Rh or Ru complex
catalyzed hydroformylation or hydrogenation [1–3].
soluble nanoparticle catalysts come from difficulties in sepa-
rating the catalyst from the products. To overcome these
drawbacks, several methods have been reported, with most
studies focusing on liquid/liquid biphasic systems such as the
use of fluorous/organic biphasic systems [4,5], thermoregu-
lated polyethylene glycol (PEG) biphasic systems [6], ther-
moregulated phase-transfer biphasic systems [7], and ionic
liquid biphasic systems [8,9].
Recently, our group has developed an effective approach for
Rh nanoparticle catalyzed hydrogenation and hydroformyla-
tion of olefins in a thermoregulated ionic liquid and organic
biphasic
system.
A
series
of
ionic
liquids
[CH3(OCH2CH2)nN+Et3][CH3SO3 ] (ILPEG, n = 12, 16, 22 ab-
breviated as ILPEG550, ILPEG750, and ILPEG1000, respectively) has
been developed and ILPEG750 specifically was employed as the
ionic liquid. This ionic liquid exhibits a unique solubility in
organic solvents depending on the temperature. Namely, IL-
–
is immiscible in a mixture of toluene and n-heptane at
PEG750
In recent years, soluble transition-metal nanoparticles in
catalysis have drawn much attention because of their high
efficiency and unique properties. However, as with traditional
homogeneous catalysts, one of the main disadvantages of
room temperature, but becomes homogeneous when the tem-
perature is increased beyond a certain point. Consequently, the
reaction proceeds in a homogeneous system with heating, and
on cooling to room temperature, separates into a biphasic sys-
Received 27 September 2012. Accepted 29 October 2012.
*Corresponding author. Tel/Fax: +86-411-84986033; E-mail: yhuawang@dlut.edu.cn
This work was supported by the National Natural Science Foundation of China (21173031) and the Program for New Century Excellent Talents in Universities of
China (NCET-07-0138).
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60473-7

XU Yicheng et al. / Chinese Journal of Catalysis, 2012, 33: 1871–1876
tem composed of an ionic liquid phase containing the Rh
160
140
120
100
80
catalyst and an organic phase containing the products. Such a
process provides both the advantages of a classic monophasic
and a biphasic system, i.e., highly catalytic efficiency and good
recyclability [10,11].
ILPEG550
ILPEG750
ILPEG1000
Aiming to understand the above-mentioned unique solubil-
ity of ILPEG more clearly and enlightened by TPSC, we initiated
a study on the CST of ionic liquids ILPEG550, ILPEG750, and IL-
60
40
in the mixture of toluene and n-heptane. Herein, IL-
PEG1000
20
PEG-stabilized Rh nanoparticle catalyst has been demonstrated
to function as a TPSC and was shown to be highly active,
selective, and recyclable for the selective hydrogenation of
1,5-COD.
0
0
10
20
30
T/oC
40
50
60
Ionic liquids ILPEG550, ILPEG750, and ILPEG1000 were prepared
according to the reported method [12]. Rh nanoparticles stabi-
lized by ILPEG1000 were prepared as follows. In a typical ex-
periment, a mixture of RhCl3·3H2O (11.30 mg, 0.043 mmol)
and ILPEG1000 (5.0 g, 4.3 mmol) was added to a 75 ml standard
stainless-steel autoclave. The autoclave was flushed five times
with 2.0 MPa H2 and stirred under hydrogen (4.0 MPa) at 70 °C
for 2 h. The reactor was then cooled to room temperature and
depressurized. The ILPEG1000-stabilized Rh nanoparticles thus
obtained were used for the following selective hydrogenation
of 1,5-COD.
Fig. 1. Solubility of ILPEG550, ILPEG750, and ILPEG1000 in the mixture of
toluene and n-heptane as a function of temperature.
tionally, at the same temperature the decrease of solubility with
increasing of n value in ILPEG suggested that ILPEG1000 would be
a better stabilizer for recycling transition-metal nanoparticle
catalysts.
Based on the CST of ILPEG, TPSC using a Rh nanoparticle
catalyst is illustrated in Fig. 2. Before the reaction, at room
temperature (T< CST), the lower ILPEG phase containing IL-
PEG-stabilized Rh nanoparticle catalyst is immiscible with the
upper organic phase containing the substrate. When heated to T
> CST, the ILPEG-stabilized Rh nanoparticle catalyst becomes
miscible with the organic phase. At the reaction temperature (T
> CST), the whole system is homogeneous and the reaction
proceeds smoothly. After the reaction, on cooling to room
temperature (T < CST), the ILPEG-stabilized Rh nanoparticle
catalyst precipitates from the organic phase, thus forming a
biphasic system again, in which the upper organic phase con-
tains products while the lower ILPEG phase contains the Rh
nanoparticle catalyst. By simple phase separation, the lower
ILPEG phase with ILPEG-stabilized Rh nanoparticle catalyst can
be recovered and reused in the next reaction run.
Selective hydrogenation reactions were performed in a 75 ml
stainless-steel autoclave. In a typical experiment, the ILPEG1000
with Rh nanoparticles (0.3 g, containing 2.6 × 103 mmol Rh),
1,5-COD (0.6 g, 5.2 mmol), toluene (3.5 g), and n-heptane (0.9
g) were added to the autoclave and purged five times with 2.0
MPa H2. Subsequently, the autoclave was pressurized with H2
to the desired pressure and held at the desired temperature with
stirring in a thermostatic oil bath for the desired length of time.
After the reaction, the autoclave was cooled to room tem-
perature and depressurized. The upper organic phase was
separated from the lower ionic liquid phase by simple phase
separation and analyzed by GC and GC-MS. Gas chromatog-
raphy analyses was performed on a Tianmei 7890 GC instru-
ment (Shanghai Techcomp Instrument Ltd, Shanghai, China)
equipped with a 50 m OV-101 column (inner diameter 0.25
mm) and an FID detector (N2 as a carrier gas). GC-MS meas-
urements were performed on a HP 6890 GC/5973 MSD in-
strument (with a 30 m HP-5MS column, inner diameter 0.25
mm, and He as a carrier gas). The transmission electron mi-
croscopy (TEM) images were taken with a Tecnai G2 20 Spirit
microscope at an acceleration voltage of 120 kV. ICP-AES
analyses of Rh were carried out on Optima 2000 DV (Perkin
Elmer, USA).
The practicality of this TPSC with a ILPEG1000-stabilized Rh
T (oC)
P : product
C : catalyst
S : substrate
S
P
IL : ionic liquid
Org. : organic solvent
CST : critical solution temperature
C
Org.
S
Org.
P
C
IL
CST
C
IL
The solubility of ILPEG550, ILPEG750, and ILPEG1000 in the mix-
ture of toluene and n-heptane were investigated and are shown
in Fig. 1. These ILs possess a distinctive CST, at which the
solubility of ILPEG550, ILPEG750, and ILPEG1000 in the mixture of
toluene and n-heptane increased dramatically. At 60 °C, IL-
PEG1000 was completely dissolved in the organic solvent. Addi-
Org.
Org.
S
P
C
C
IL
IL
Process
Fig. 2. General principle of TPSC of Rh nanoparticle catalyst.

XU Yicheng et al. / Chinese Journal of Catalysis, 2012, 33: 1871–1876
Selective hydrogenation of 1,5-COD catalyzed by IL-
Table
1
Conversion of 1,5-COD
Selectivity to COE
PEG1000-stabilized Rh nanoparticles
100
80
60
40
20
COE
Temperature Pressure of Time Conversion
Entry
TOF
(h1)
selectivity
(°C)
H2 (MPa) (min)
(%)
(%)
83
82
90
83
47
81
74
81
89
73
1
40
50
60
70
60
60
60
60
60
60
1.0
1.0
1.0
1.0
0.1
0.5
1.5
1.0
1.0
1.0
20
20
20
20
20
20
20
10
15
25
52
58
2590
2854
5346
4980
367
2
3
99
4
100
13
5
6
67
3256
4440
5540
5696
3504
0
1
2
3
4
5
6
7
8
9
10
7
100
57
Recycling number
8
9
80
Fig. 3. Recycling efficiency of the ILPEG1000-stabilized Rh nanoparticle
10
100
catalyst for the selective hydrogenation of 1,5-COD.
Reaction conditions: ILPEG1000 0.3 g (containing 2.6 × 103 mmol Rh),
toluene 3.5 g, n-heptane 0.9 g, 1,5-COD/Rh = 2000 (molar ratio).
TOF: turnover frequency, calculated as the number of moles of product
formed per mol of rhodium per hour.
temperature increased from 50 to 60 °C. This result was as-
cribed to the increase in the solubility of ILPEG1000 over the CST,
causing the biphasic system to transform into a monophasic
system. The effect of H2 pressure on the reaction was also
investigated over the range of 0.1 to 1.5 MPa (Table 1, entries 3
and 5–7). Increasing H2 pressure benefits the conversion of
1,5-COD, but higher pressure decreases the selectivity towards
COE. The best H2 pressure was 1.0 MPa. In addition, the
conversion of 1,5-COD and the selectivity towards COE are
dependent on the reaction time (Table 1, entries 3 and 8–10).
The optimized reaction time was 20 min. The TOF value in the
present study is higher than the reported Pd-catalyzed selective
hydrogenation of 1,5-COD [6].
nanoparticle catalyst was demonstrated by the selective hy-
drogenation of 1,5-COD to COE. The effects of different re-
action parameters have been investigated in detail and the
results are summarized in Table 1. The effect of temperature
was studied over the range of 40–70 °C at 1.0 MPa pressure of
H2. The conversion of 1,5-COD increased with increasing
temperature, but past 60 °C it led to a decrease in selectivity
(Table 1, entries 1–4). At 60 °C, 99% conversion, and 90%
COE selectivity were obtained, respectively, which are higher
than the reported Ir catalyst [13,14]. It was worth noting that
the conversion of 1,5-COD increased remarkably when the
As the lifetime is an important feature in evaluating a cata-
(b)
1.7  0.2 nm
50
(a)
40
30
20
10
0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Diameter (nm)
40
(d)
(c)
2.5  0.4 nm
35
30
25
20
15
10
5
0
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Diameter (nm)
Fig. 4. TEM images and particle size histograms of the ILPEG1000-stabilized Rh nanoparticles newly prepared (a, b) and after ten recycles (c, d) (200 and
100 particles, respectively).

XU Yicheng et al. / Chinese Journal of Catalysis, 2012, 33: 1871–1876
lyst, the reusability of the Rh nanoparticles was examined.
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