Rh nanoparticle catalyzed hydrogenation of olefins in thermoregulated ionic liquid and organic biphase system

Article abstract of DOI:10.1016/j.catcom.2011.11.037

Rh nanoparticle catalyzed hydrogenation of olefins in a biphase system composed of [CH₃(OCH₂CH₂)₁₆N ⁺Et₃][CH₃SO₃^-] (IL _PEG750) ionic liquid and organic solvent allows for not only a highly efficient homogeneous catalytic reaction, but also an easy separation and reuse of catalyst attributed to the thermoregulated phase-transition property of the IL_PEG750 and organic biphase system. Under the optimized conditions, various alkenes could be hydrogenated to the corresponding alkanes as the sole product with 100% conversion. The Rh nanoparticles can be recovered by simple phase separation and reused for nine times without evident loss of activity.

Full text of DOI:10.1016/j.catcom.2011.11.037

Catalysis Communications 19 (2012) 70–73
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Rh nanoparticle catalyzed hydrogenation of olefins in thermoregulated ionic liquid
and organic biphase system
Yan Zeng, Yanhua Wang ⁎, Jingyang Jiang, Zilin Jin
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Rh nanoparticle catalyzed hydrogenation of olefins in a biphase system composed of [CH (OCH CH ) N+Et ]
Article history:
Received 25 May 2011
Received in revised form 5 September 2011
Accepted 27 November 2011
Available online 29 December 2011
3
2
2
16
3
−
3 3
[CH SO ] (ILPEG750) ionic liquid and organic solvent allows for not only a highly efficient homogeneous cata-
lytic reaction, but also an easy separation and reuse of catalyst attributed to the thermoregulated phase-
transition property of the ILPEG750 and organic biphase system. Under the optimized conditions, various
alkenes could be hydrogenated to the corresponding alkanes as the sole product with 100% conversion. The
Rh nanoparticles can be recovered by simple phase separation and reused for nine times without evident loss
of activity.
Keywords:
Thermoregulated ionic liquid and organic
biphase system
© 2011 Elsevier B.V. All rights reserved.
Rhodium catalyst
Nanoparticles
Hydrogenation
Olefin
1
. Introduction
Transition-metal nanoparticles in homogeneous catalysis have
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 biphase system com-
posed of ionic liquid phase containing Rh catalyst and organic phase
containing the products. Such a novel process provides both the advan-
tages of classic monophase and biphase system, i.e., highly catalytic
efficiency and good recyclability.
To best use this interesting technology as well as to make the cat-
alytic process more effective, we initiated a study of Rh nanoparticle
catalyzed reaction in this thermoregulated biphase system because,
in addition to the thermoregulated phase-transition property, the
PEG-based ionic liquid is also anticipated to be acting to stabilize
the Rh nanoparticles via the interaction of PEG linear chain and Rh,
and ultimately, improving the recyclability of the nanoparticles. Here-
in, we disclose that the ILPEG750-stabilized Rh nanoparticle is a highly
efficient, stable, and reusable catalyst for the hydrogenation of olefins.
attracted growing interest owing to the highly catalytic efficiency and
unique properties. However, the recycling and stabilization of the nano-
particles remain a considerable challenging [1,2]. To overcome these
drawbacks, several methods have been reported, with most studies
focusing on the liquid/liquid biphase system, for example, the use
of fluorous/organic biphase system [3,4], thermoregulated polyethylene
glycol (PEG) biphase system [5], thermoregulated phase-transfer biphase
system [6] and ionic liquid biphase system [7,8]. Ionic liquids with negli-
gible vapor pressure have been explored as stabilizers for transition-
metal nanoparticles. However, in most cases, the reaction systems
involving ionic liquids are either monophase allowing for rapid
transformation or biphase facilitating separation of product from
catalyst [9–14]. Therefore, to search for new catalyst systems that
might combine the advantages of both homogeneous and heteroge-
neous catalysis will have a significant impact.
2. Experimental
Recently, we have developed an effective approach for Rh-
catalyzed hydroformylation of olefins in a thermoregulated ionic
2.1. Materials and analyses
+
liquid/organic biphase system [15], wherein [CH
3
(OCH
2
CH
2
)
16N Et
3
]
−
[CH SO
3 3
] (ILPEG750) was employed as ionic liquid. This ionic liquid ex-
Toluene and n-heptane were purchased from Kermel and purified
by distillation according to the standard method. Cyclohexene and
styrene were analytical grade and were purchased from Kermel. 1-
Dodecene was supplied by Fluka. 1-Octene was obtained from Acros.
hibits 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
3 2
RhCl ·3H O was received from Beijing Research Institute of Chemical
Industry and used without further purification. ILPEG750 was prepared
according to the reported method [15]. Gas chromatographic analyses
were performed on a Tianmei 7890 GC instrument equipped with a
⁎
Corresponding author. Tel.: +86 411 84986033; fax: +86 411 84986033.
E-mail address: yhuawang@dlut.edu.cn (Y. Wang).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.11.037

Y. Zeng et al. / Catalysis Communications 19 (2012) 70–73
71
Fig. 1. TEM images and particle size histogram of Rh nanoparticles stabilized by ILPEG750 (200 particles).
5
0 m OV-101 column (inner diameter 0.25 mm) and an FID detector
as a carrier gas; injector and detector temperature 260 °C; oven
temperature 70 °C). GC–MS measurement was performed on a HP
890 GC/5973 MSD instrument (with a 30 m HP-5MS column, inner
was flushed three times with 1.0 MPa H
2
. The reactor was then pres-
(
N
2
surized with H up to an appointed pressure and heated at a sched-
2
uled temperature under stirring. 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 analyses. The lower ionic liquid phase
was reused to catalyze the hydrogenation of the next run.
6
diameter 0.25 mm; He as a carrier gas; injector temperature 300 °C;
oven temperature 60 °C). ICP-AES analyses of rhodium were carried
out on Optima 2000 DV (Perkin Elmer, USA).
2
.2. Preparation of the ILPEG750-stabilized rhodium nanoparticles
3. Results and discussion
A mixture of RhCl
.6 mmol) was added in a 75 mL standard stainless-steel autoclave
and stirred under hydrogen (4 MPa) at 70 °C for 2 h. Then, the reactor
was cooled to room temperature and depressurized. The nanoparti-
cles thus obtained as ILPEG750 solution were used for the following hy-
drogenation of olefins.
3
·3H
2
O (20 mg, 0.076 mmol) and ILPEG750 (7.0 g,
3.1. TEM image and thermoregulated phase-transition property of
ILPEG750 and organic biphase system with ILPEG750-stabilized Rh
nanoparticles
7
Rh nanoparticles were prepared by the reduction of RhCl
3 2
·3H O
with H in the ILPEG750 media. As depicted in Fig. 1, TEM image
2
showed that the Rh nanoparticles thus obtained display a homoge-
neous distribution in ILPEG750 with an average particle size of 2.1±
2
.3. TEM image of the rhodium nanoparticle catalyst
0
.3 nm in diameter. This observation suggested that reductive hydro-
The ILPEG750 solution containing the rhodium nanoparticle catalyst
genation is a reliable and simple method for the synthesis of Rh
nanoparticles.
was diluted with ethanol. Then, a drop of the solution was placed
onto a carbon-coated copper grid, and dried at ambient temperature.
The TEM images were taken with a Philips Tecnai G2 20 TEM at an
accelerating voltage of 200 kV.
The thermoregulated phase-transition property of the ILPEG750
and organic biphase system with ILPEG750-stabilized Rh nanoparticles
is shown in Fig. 2. At room temperature, the lower ionic liquid phase
containing Rh nanoparticles was immiscible with the upper organic
phase consisting of toluene and n-heptane (a in Fig. 2). Interestingly,
when the temperature was elevated gradually to 60 °C, the biphase
system merged into a clear single phase (b in Fig. 2). Finally, by de-
creasing the temperature to room temperature, the biphase system
recovered completely to its original form as a (c in Fig. 2). The
2
.4. Hydrogenation of olefins
A 75 mL stainless-steel autoclave charged with ILPEG750 solution of
−
3
rhodium nanoparticles (0.5 g, containing 5.5×10
mmol Rh), olefin
(
5.5 mmol), toluene (3.5 g), n-heptane (0.7 g), and a magnetic bar
Fig. 2. Thermoregulated phase-transition property of the ILPEG750 and organic biphase system with ILPEG750-stabilized Rh nanoparticles.

72
Y. Zeng et al. / Catalysis Communications 19 (2012) 70–73
Fig. 3. Schematic illustration of thermoregulated phase-transition process for Rh nanoparticle catalyzed reaction.
observation of a colorless organic phase in c implies that after a heat-
ing and cooling cycle, no or only negligible Rh nanoparticles remain
in organic phase.
should be noted that when using n-heptane instead of the mixed
toluene and n-heptane as organic phase, a much lowered conver-
sion and TOF were obtained (entry 7). These results further con-
firmed that a proper choice of organic phase that enables the
reaction to be carried out in a homogeneous system under heating
is crucial because we observed that ionic liquid ILPEG750 and n-
heptane cannot be merged into a homogeneous phase under the re-
action temperature.
With the optimized reaction conditions established, we exam-
ined the scope of this thermoregulated phase-transition process by
varying structures of olefins. 100% conversion was also occurred for
styrene, 1-octene, and 1-dodecene to afford the corresponding hy-
drogenated products.
The observed thermoregulated phase-transition process would
provide a simple, atom-efficient, and environmentally benign way
for various Rh-catalyzed reactions when the reaction procedure is
appropriately designed. As illustrated in Fig. 3, for a reaction whose
product is soluble in organic phase, one can make the reaction pro-
ceed in a homogeneous phase under heating (middle panel in
Fig. 3). When the reaction is completed, the product and the catalyst
are collected easily from the organic and ionic liquid phase, respec-
tively, just by cooling the reaction system to room temperature
(
right panel in Fig. 3). The recovered catalyst can be reused without
further purification or activation (right to left panel in Fig. 3).
Finally, the reusability of the Rh nanoparticles was examined.
Under the identical reaction conditions to entry 2 in Table 1, the re-
covered Rh nanoparticles exhibited highly catalytic efficiency with-
out evident loss in activity after the catalyst was reused for nine
times (Fig. 4). Detection of the Rh content in the upper organic
phase after each run by using ICP-AES revealed that the Rh leaching
was under the minimum detectable amount (0.005 μg/mL). TEM
image showed that the morphology of the nanoparticles remained
unchanged after nine recycles (Fig. 5).
The good reusability and high stability of the Rh nanoparticles
presented herein are in good agreement with our original assump-
tion (vide supra). Namely, the presence of PEG linear chain in the
ILPEG750 may be acting to stabilize the Rh nanoparticles via the com-
plexation of oxygen with Rh. To compare with the classic ionic liq-
uids which stabilize the Rh nanoparticles via the electrostatic
interaction and/or steric effects [19], it is assumed that the introduc-
tion of complexation effect as an extra factor should play an impor-
tant role in stabilizing the nanoparticles, although the detailed
reasons deserve a further clarification.
3
.2. Catalytic properties of ILPEG750-stabilized Rh nanoparticles for
hydrogenation
The practicality of the newly developed approach was demon-
strated by the hydrogenation of olefins. Here, the hydrogenation of
cyclohexene was first employed as a model reaction. The results are
listed in Table 1. A general observation is that under various condi-
tions, cyclohexane was provided as a single product. However, both
the conversion and turnover frequency are influenced by the reaction
parameters such as temperature (entry 1 vs. entry 2), reaction time
(
entries 2–5), and pressure (entry 2 vs. entry 6). The optimized reac-
tion conditions are heating the reaction mixture at 60 °C for 1 h under
.0 MPa of H pressure (entry 2). Under these conditions, 100% con-
1
2
version as well as 1000 TOF was observed. The TOF was comparable
to that of the Rh nanoparticle catalysts in the ionic liquid n-butyl-
+
−
methyl-imidazolium tetrafluoroborate([BMIm ][BF
4
]) [16–18]. It
Table 1
Hydrogenation of cyclohexene catalyzed by ILPEG750-stabilized Rh nanoparticle catalyst.a
Entry
T (°C)
Time (min)
Pressure (MPa)
Conversion (%)
TOF (h−1)b
1
2
3
4
5
6
50
60
60
60
60
60
60
60
60
30
40
50
60
50
1.0
1.0
1.0
1.0
1.0
0.5
1.0
83
100
58
74
89
830
1000
1160
1110
1068
750
75
15
c
7
180
a
Reaction conditions: ILPEG750 0.5 g (containing 5.5×10−3 mmol rhodium), substrate/
Rh=1000 (molar ratio), toluene 3.5 g, n-heptane 0.7 g, miscibility temperature of the
system is 60 °C.
b
Turnover frequency (TOF), calculated as the number of moles of product formed
per mol of rhodium per hour.
c
n-Heptane as the only organic solvent.
Fig. 4. The recycling efficiency of ILPEG750-stabilized Rh nanoparticle catalyst.

Y. Zeng et al. / Catalysis Communications 19 (2012) 70–73
73
Fig. 5. TEM images and particle size histogram of Rh nanoparticles stabilized by ILPEG750 after nine recycles at 60 °C (200 particles).
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