A novel thermoregulated ionic liquid and organic biphasic system with rh nanoparticles for olefin hydroformylation

Article abstract of DOI:10.1007/s10562-012-0841-1

A thermoregulated ionic liquid and organic biphasic system composed of quaternary ammonium-based ionic liquid N,N-dimethyl-N-(2-(2-methoxyethoxy)ethyl) - N-(2-(2-octyloxyethoxy)ethyl) ammonium methanesulfonate (1) and cyclohexane was first applied for the Rh nanoparticle catalyzed hydroformylation of olefins, which allows for not only a highly efficient homogeneous catalytic reaction, but also an easy separation and reuse of catalyst. Under the optimized conditions, various olefins could be converted completely with C98 % selectivity for aldehydes. After reaction, the Rh nanoparticle catalyst could be recovered by simple phase separation and reused for five times without evident loss of activity. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10562-012-0841-1

Catal Lett (2012) 142:914–919
DOI 10.1007/s10562-012-0841-1
A Novel Thermoregulated Ionic Liquid and Organic Biphasic
System With Rh Nanoparticles for Olefin Hydroformylation
•
Yicheng Xu Yanhua Wang Yan Zeng
•
•
•
Jingyang Jiang Zilin Jin
Received: 19 March 2012 / Accepted: 9 May 2012 / Published online: 23 May 2012
Ó Springer Science+Business Media, LLC 2012
Abstract A thermoregulated ionic liquid and organic
biphasic system composed of quaternary ammonium-based
ionic liquid N,N-dimethyl-N-(2-(2-methoxyethoxy)ethyl)-
N-(2-(2-octyloxyethoxy)ethyl) ammonium methanesulfo-
nate (1) and cyclohexane was first applied for the Rh
nanoparticle catalyzed hydroformylation of olefins, which
allows for not only a highly efficient homogeneous cata-
lytic reaction, but also an easy separation and reuse of
catalyst. Under the optimized conditions, various olefins
could be converted completely with C98 % selectivity for
aldehydes. After reaction, the Rh nanoparticle catalyst
could be recovered by simple phase separation and reused
for five times without evident loss of activity.
organic biphasic system [3, 4], thermoregulated polyeth-
ylene glycol (PEG) biphasic system [5], thermoregulated
phase-transfer biphasic system [6] and ionic liquid biphasic
system [7, 8]. Ionic liquids with negligible vapor pressure
have been explored as stabilizers for transition-metal
nanoparticles. However, in most cases, the reaction sys-
tems involving ionic liquids are either monophase allowing
for rapid transformation or biphase facilitating separation
of product from catalyst [8–13]. Therefore, to search for
new catalyst systems that might combine the advantages of
both homogeneous and heterogeneous catalysis will have a
significant impact.
Very recently, our group has developed an effective
approach for Rh nanoparticle catalyzed hydrogenation and
hydroformylation of olefins in a thermoregulated ionic
Keywords Thermoregulated ionic liquid and organic
biphasic system Á Rhodium Á Nanoparticle Á Olefin Á
Hydroformylation
liquid and organic biphasic system [14, 15], wherein
[CH3(OCH2CH2)16N?Et3][CH3SO3
-
]
(ILPEG750
)
was
employed as ionic liquid. This ionic liquid exhibits very
unique solubility in organic solvent depending on the tem-
perature. Namely, ILPEG750 is immiscible with the mixed
toluene and n-heptane at room temperature, but becomes
homogeneous when the temperature is increased to a certain
point. Consequently, the reaction proceeds virtually in a
homogeneous system under heating, and upon cooling to
room temperature, separates into a biphasic system composed
of ionic liquid phase containing Rh catalyst and organic phase
containing the products. Such a process provides both the
advantages of classic monophasic and biphasic system, i.e.,
highly catalytic efficiency and good recyclability.
1 Introduction
In recent years, considerable attention has been directed
toward the soluble transition-metal nanoparticle catalysts
due to their high efficiency and unique properties. How-
ever, very similar to traditional homogeneous catalysts, one
of the main disadvantages of soluble nanoparticle catalyst
is the problem of separation the catalyst from the product
[1, 2]. To deal with this problem, a range of methods has
been reported, with most studies focusing on the liquid/
liquid biphasic system, for example, the use of fluorous/
With the aim to broaden the scope of this interesting
technology, herein, we report on a novel and more efficient
catalytic system for the biphasic hydroformylation of olefins
by using Rh nanoparticles in a quaternary ammonium-based
ionic liquid N,N-dimethyl-N-(2-(2-methoxyethoxy)ethyl)-
N-(2-(2-octyloxyethoxy)ethyl)ammonium methanesulfonate
Y. Xu Á Y. Wang (&) Á Y. Zeng Á J. Jiang Á Z. Jin
State Key Laboratory of Fine Chemicals, Dalian University
of Technology, Dalian 116024, People’s Republic of China
e-mail: yhuawang@dlut.edu.cn
123

Organic Biphasic System
915
the solution was placed onto a carbon-coated copper grid,
and dried at ambient temperature. The transmission elec-
tron microscopy (TEM) images were taken with a Philips
Tecnai G2 20 TEM at an accelerating voltage of 200 kV.
O
O
O
CH3SO3
N
O
1
2.4 General Procedure for Hydroformylation of Olefins
Scheme 1 The structure of ionic liquid 1
(abbreviated as 1, Scheme 1) as the lower catalyst-contain-
ing phase and cyclohexane as the upper substrate- and
product-containing phase. Higher activity and selectivity for
aldehydes were obtained and recycling of Rh nanoparticle
catalyst was realized.
All hydroformylation reactions were carried out in a 75 mL
stainless-steel autoclave equipped with a magnetic bar. The
autoclave was charged with ionic liquid 1 solution of
rhodium nanoparticles (0.30 g, containing 1.7 9 10-3
mmol Rh), olefin (1.7 mmol), cyclohexane (2.50 g) and
n-heptane (0.05 g, as internal standard). Then, flushed
five times with 2 MPa H2, and pressurized with syngas
(CO/H2 = 1) up to an appointed pressure. The reactor was
held at a scheduled temperature under stirring in a ther-
mostatic oil bath for a fixed length of time. After the
reaction, the reactor was cooled to room temperature and
depressurized. The upper organic phase was separated from
the lower ionic liquid phase and submitted immediately to
GC and GC–MS analysis. The lower ionic liquid phase was
reused to catalyze the hydroformylation of the next run.
2 Experimental
2.1 General Remarks
Cyclohexane and n-heptane were purchased from Kermel
and distilled from sodium under inert atmosphere before
use. RhCl3Á3H2O and Rh(acac)(CO)2 were received from
Beijing Research Institute of Chemical Industry and used
without any further purification. 1-Octene was obtained
from Acros. 1-Decene, 1-dodecene and cyclohexene were
supplied by Fluka. Ionic liquid 1 was prepared according to
our previous reported method [16]. Gas chromatography
analyses were performed on a Tianmei 7890 GC instru-
ment equipped with a 50 m OV-101 column and an FID
detector (N2 as a carrier gas). GC–MS measurement was
performed on a HP 6890 GC/5973 MSD instrument (with a
30 m HP-5MS column, He as a carrier gas). ICP-AES
analyses of rhodium were carried out on Optima 2000 DV
(Perkin Elmer, USA). The IR spectrum was recorded on a
Fourier transform infrared (FTIR) spectrometer (Nicolet
Avatar 360) using the KBr disc technique.
3 Results and Discussion
Rh nanoparticles were synthesized by simple H2 reduction
of RhCl3Á3H2O in ionic liquid 1. The TEM image of the Rh
nanoparticles was shown in Fig. 1. A relatively narrow size
distribution with a diameter of 1.3 0.2 nm was observed.
As shown in Fig. 2, this novel thermoregulated ionic
liquid 1/cyclohexane biphasic system with ionic liquid
1-stabilized Rh nanoparticles also possessed the thermo-
regulated phase-transition property. Namely, at room
temperature, the lower ionic liquid phase containing Rh
nanoparticles was immiscible with the upper cyclohexane
phase (a in Fig. 2). Interestingly, when the temperature was
elevated gradually to 118 °C, the biphasic system merged
into a homogeneous single phase (b in Fig. 2). Then, by
cooling to room temperature, the biphasic system recov-
ered completely to its original form as a (c in Fig. 2). This
phenomenon implies that the thermoregulated ionic liquid
1/cyclohexane biphasic system with ionic liquid 1-stabi-
lized Rh nanoparticles can realize a reaction under homo-
geneous conditions and achieve subsequent efficient
separation of catalyst from products by cooling the reaction
mixture to room temperature. One of the most notable
advantages has been discussed above, the availability and
cost of the ionic liquid 1 are analogous with the most
reported ionic liquids. Till now, Rh nanoparticle catalyzed
hydrogenation has been extensively studied, but less
attention has been paid to hydroformylation [17–21].
Therefore, this novel thermoregulated ionic liquid
2.2 Preparation of Ionic Liquid 1-Stabilized Rh
Nanoparticles
In a typical experiment, a mixture of RhCl3Á3H2O (7.42 mg,
0.028 mmol) and ionic liquid 1 (5.0 g, 11.3 mmol) was
added in a 75 mL standard stainless-steel autoclave. The
autoclave was flushed five times with 2 MPa H2 and stirred
under hydrogen (4 MPa) at 70 °C for 2 h. Then, the reactor
was cooled to room temperature and depressurized. The
nanoparticles thus obtained as ionic liquid 1 solution were
used for the following hydroformylation of olefins.
2.3 TEM Images of Rhodium Nanoparticle Catalyst
The ionic liquid 1 solution containing the rhodium nano-
particle catalyst was diluted with ethanol. Then, a drop of
123

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Y. Xu et al.
Fig. 1 TEM image of ionic
liquid 1-stabilized Rh
nanoparticles (newly prepared,
200 particles)
Fig. 2 Thermoregulated phase-
transition property of ionic
liquid 1/cyclohexane biphasic
system with ionic liquid
1-stabilized Rh nanoparticles
without any noticeable induction period in the second run,
thereby leading to [99 % aldehyde yield within 4 h at
120 °C. The reaction rate is faster than the Rh nanoparticle
catalyst prepared in 1-n-butyl-3-methylimidazolium tetra-
fluoroborate (BMIÁBF4) ionic liquid [17]. The phenomenon
of induction period was very similar to the previously
reported results, which was associated with the dispersion
process or attributed to the transformation of the nano-
particles into catalytically active species [17, 22].
According to the TEM images of ionic liquid 1-stabilized
Rh nanoparticles (Fig. 1 and see below Fig. 5), we spec-
ulate that the latter of the above-mentioned hypotheses is
more possible.
The effects of temperature, pressure of syngas and
reaction time on the hydroformylation of 1-octene cata-
lyzed by ionic liquid 1-stabilized Rh nanoparticles were
summarized in Table 1. The effect of temperature was
studied in the range of 80–120 °C, at a 5 MPa total
syngas pressure (CO/H2 = 1). At 80 °C, 32 % conversion
was observed after 4 h of reaction time. At temperature of
90 °C and higher, conversion was virtually complete. The
selectivity for aldehydes increased with increasing of
temperature (entries 1–4). It is worth noting that the
conversion of 1-octene increased sharply when the tem-
perature increased from 80 to 90 °C. This may be due to
the fact that the reaction system changed from biphase to
Fig. 3 Aldehyde yield as a function of time for hydroformylation of
1-octene
1/cyclohexane biphasic system with ionic liquid 1-stabilized
Rh nanoparticles was first employed for the hydroformylation
of olefins.
The hydroformylation of 1-octene was investigated as a
model reaction. The aldehyde yield as a function of time
was shown in Fig. 3, which revealed a significant induction
period in the first run. However, the reaction started again
123

Organic Biphasic System
917
Table 1 Hydroformylation of 1-octene catalyzed by ionic liquid 1-stabilized Rh nanoparticles
Entry
Temperature (oC)
Pressure (MPa)
Time (h)
Conversion (%)
Yaald (%)
Ybiso (%)
n/b
1
80
5
5
5
5
2
3
4
5
5
5
5
4
4
4
4
4
4
4
1
2
3
4
32
24
84
87
[99
40
58
87
61
86
93
70
8
1.6
1.4
1.4
1.2
0.8
0.7
0.8
1.4
1.0
0.8
1.2
2
90
98
14
13
\1
60
42
13
38
14
7
3
100
120
120
120
120
120
120
120
120
100
100
100
100
100
99
4
5
6
7
8
9
100
100
99
10
11c
29
Reaction conditions: ionic liquid 1 0.30 g (containning 1.7 9 10-3 mmol Rh), 1/Rh = 400 (molar ratio), substrate/Rh = 1,000 (molar ratio),
1-octene 0.19 g (1.7 mmol), cyclohexane 2.50 g, internal standard n-heptane 0.05 g, CO/H2 = 1, n/b molar ratio of normal to branched
aldehyde. The recovered catalyst from the first run was used
a
Yald yield of aldehydes
b
Yiso yield of isomeric octenes
Rh(acac)(CO)2 (1.7 9 10-3 mmol) was used as catalyst
c
Table 2 Hydroformylation of different olefins catalyzed by ionic liquid 1-stabilized Rh nanoparticles
Entry
Olefin
Pressure (MPa)
Time (h)
Conversion (%)
Aldehyde yield (%)
1
2
3
1-Decene
5
5
6
4
4
5
100
100
100
99
98
99
1-Dodecene
Cyclohexene
Reaction conditions: ionic liquid 1 0.30 g (containning 1.7 9 10-3 mmol Rh nanoparticles), 1/Rh = 400 (molar ratio), substrate/Rh = 1,000
(molar ratio), cyclohexane 2.50 g, internal standard n-heptane 0.05 g, T = 120 °C, CO/H2 = 1
monophase (the miscibility temperature of the system was
85 °C in the presence of aldehydes). The effect of syngas
pressure on the reaction was studied in the range from 2
to 5 MPa. The conversion of 1-octene remained 100 %,
and the selectivity for aldehydes increased with increasing
pressure. At 2 MPa two-thirds of the substrate was con-
verted to by-products from isomerization, whereas at
5 MPa by-products account for only less than 1 %
(entries 4–7). In addition, selectivity for aldehydes also
increased with prolonging reaction time in the range
studied (entries 4, 8–10). The molar ratios of normal to
branched aldehydes (n/b = 0.7–1.6) obtained with ionic
liquid 1-stabilized Rh nanoparticles were similar to the
ratios previously reported for reactions catalyzed by Rh
nanoparticles [17, 18]. Furthermore, a catalytic experi-
ment using Rh(acac)(CO)2 as molecular Rh precursor has
been performed in the same conditions as the ionic liquid
1-stabilized Rh nanoparticles. In both cases, virtually
complete conversion was observed, but the yield of
aldehydes was lower with Rh(acac)(CO)2 as catalyst
(entry 4 vs. entry 11).
In addition, this novel thermoregulated ionic liquid 1/
cyclohexane biphasic system with ionic liquid 1-stabilized
Rh nanoparticles was also tested with 1-decene, 1-dode-
cene and less reactive cyclohexene as substrates (see
Table 2). 100 % conversion and C 98 % selectivity for
aldehydes were obtained. The catalytic activity and selec-
tivity for aldehydes of this novel sytem is superior to our
previously reported results [15].
The recycling efficiency of Rh nanoparticle catalyst was
also investigated. After reaction, the upper cyclohexane
phase containing product was separated from the lower
ionic liquid phase containing catalyst by simple phase
separation. And the lower catalyst-containing ionic liquid
phase was directly used in the next reaction run. Under the
identical reaction conditions to entry 4 in Table 1, the
recovered ionic liquid 1-stabilized Rh nanoparticle catalyst
exhibited highly catalytic efficiency without evident loss of
activity after the catalyst was recycled for five times
(Fig. 4). Comparison of the TEM images of Rh nanopar-
ticles between newly prepared and after six runs (Fig. 1 vs.
Fig. 5) showed a slight increase of particle size from
123

918
Y. Xu et al.
original 1.3 nm to 2.4 nm. ICP-AES analysis of the upper
colorless cyclohexane phase separated from the first run
indicated that the leaching of Rh was 0.3 wt%. The IR
spectrum of ionic liquid 1-stabilized Rh nanoparticles with
vibrations at 1,988 and 2,060 cm-1 is an indication of the
presence of Rh-CO band (Fig. 6). However, the nature of
catalysis for Rh nanoparticle catalyzed hydroformylation,
namely homogeneous or heterogeneous, is still open. Some
results point out that the nature of the catalyst is hetero-
geneous [23], whereas other studies show that the active
species are probably Rh complexes [17–19]. Although the
presence of Rh-CO band has been detected, at this stage of
our studies we still cannot certify the existence of molec-
ular Rh species, as it requires different analytical tech-
niques that are difficult to combine for the study of metal
nanoparticles. Nevertheless, in order to make the catalytic
nature of the Rh nanoparticle catalyzed hydroformylation
reaction more clearly, the in situ high-pressure FTIR and
1H-NMR measurements will be helpful.
Fig. 4 Recycling efficiency of ionic liquid 1-stabilized Rh nanopar-
ticle catalyst on the hydroformylation of 1-octene
Fig. 5 TEM image of ionic liquid 1-stabilized Rh nanoparticles (after six runs, 200 particles)
Fig. 6 Part of the IR spectrum of the ionic liquid 1-stabilized Rh nanoparticles
123

Organic Biphasic System
919
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11:542
4 Conclusions
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(2003) Chem Eur J 9:3263
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PJ (2010) Green Chem 12:228
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Synth Catal 350:2077
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In summary, we have developed a novel thermoregulated
ionic liquid 1/cyclohexane biphasic system with ionic
liquid 1-stabilized Rh nanoparticles, which possessed the
thermoregulated phase-transition property. The ionic liquid
1-stabilized Rh nanoparticle catalyst exhibited high activity
and selectivity for aldehydes in the hydroformylation of
olefins. And the Rh nanoparticle catalyst could be sepa-
rated from product by simple phase separation and reused
for five times without evident loss of activity. More
applications of this novel thermoregulated ionic liquid and
organic biphasic system are now under further investiga-
tion and will be reported in due course.
Acknowledgments This work was financially supported by the
National Natural Science Foundation of China (Grant No. 21173031)
and the Program for New Century Excellent Talents in University
(Grant No. NCET -07-0138).
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3:170
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A Chem 252:212
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208:1688
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Chaudret B (2008) Eur J Inorg Chem 22:3460
20. Han DF, Li XH, Zhang HD, Liu ZM, Hu GS, Li C (2008) J Mol
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