Thermoregulated phase-transfer rhodium nanoparticle catalyst for hydrogenation in an aqueous/organic biphasic system

Article abstract of DOI:10.1016/j.catcom.2009.12.015

A simple, effective and reversible aqueous/organic phase-transfer method for rhodium nanoparticles was developed on the basis of the cloud point (Cp) of thermoregulated ligand. The rhodium nanoparticles stabilized by thermoregulated ligand Ph₂P(CH₂CH₂O)_nCH₃ (n = 16) could transfer from aqueous phase to 1-butanol phase and vice versa by means of changing temperature. This thermoregulated phase-transfer rhodium nanoparticle catalyst evaluated in the hydrogenation of olefins in an aqueous/1-butanol biphasic system exhibited high activity and stability.

Full text of DOI:10.1016/j.catcom.2009.12.015

Catalysis Communications 11 (2010) 542–546
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Thermoregulated phase-transfer rhodium nanoparticle catalyst for hydrogenation
in an aqueous/organic biphasic system
*
Kaoxue Li, Yanhua Wang , Jingyang Jiang, Zilin Jin
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 October 2009
Received in revised form 9 December 2009
Accepted 10 December 2009
Available online 14 December 2009
A simple, effective and reversible aqueous/organic phase-transfer method for rhodium nanoparticles was
developed on the basis of the cloud point (Cp) of thermoregulated ligand. The rhodium nanoparticles sta-
bilized by thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16) could transfer from aqueous phase to 1-
butanol phase and vice versa by means of changing temperature. This thermoregulated phase-transfer
rhodium nanoparticle catalyst evaluated in the hydrogenation of olefins in an aqueous/1-butanol bipha-
sic system exhibited high activity and stability.
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
Thermoregulated ligand
Rhodium nanoparticles
Hydrogenation
Aqueous/organic biphasic catalysis
Olefin
1. Introduction
transfer into the organic phase. Thus, the catalyst and the substrate
will be in the same phase and the reaction proceeds in the organic
Soluble transition-metal nanoparticles in catalysis have drawn
much attention due to their high efficiency and unique properties.
However, very similar to traditional homogeneous catalysts, one of
the main disadvantages of soluble nanoparticle catalysts is the
problem of separation the catalyst from the products [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/organic biphasic system [3,4], ther-
moregulated polyethylene glycol (PEG) biphasic system [5] and io-
nic liquid biphasic system [6,7].
Thermoregulated phase-transfer catalysis (TRPTC) in aqueous/
organic biphasic system was suggested by Jin and co-workers
based on the cloud point (Cp) of thermoregulated ligands. Thermo-
regulated ligands exhibit an inverse-temperature-dependent solu-
bility in water, that is, their aqueous solution will undergo a phase
separation on heating to Cp and become water soluble again on
cooling to a temperature lower than the Cp [8–10]. The general
principle of TRPTC is illustrated in Scheme 1. The character of this
catalytic process can be described as follows: the system consists
of aqueous and organic phases. At a temperature lower than the
Cp, the catalyst would remain in the aqueous phase. On heating
to a temperature higher than the Cp, however, the catalyst would
phase. As soon as the reaction is completed and the system is
cooled to a temperature lower than the Cp, the catalyst would re-
turn to the aqueous phase. Therefore, by simple phase separation
the catalyst can be separated from products. TRPTC is not only free
from the shortcomings of classical biphasic catalysis, in which the
scope of application is restrained by the water-solubility of the
substrate, but also a promising approach for recovery and recycling
of noble transition-metal complex catalyst. Till now, several cata-
lytic reactions have been achieved in TRPTC system such as hydro-
formylation [11], hydrogenation [12], CO selective reduction of
nitroarenes [13] and Heck reaction [14]. Although employing ther-
moregulated ligand as stabilizing agent for rhodium has been ex-
plored in the hydroformylation of 1-octene [15], transition-metal
nanoparticle catalyst with the function of TRPTC has not previously
been reported.
In this paper, rhodium nanoparticle catalyst stabilized by ther-
moregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16) was first certified
to be with the function of TRPTC and used as an active, stable and
recyclable catalyst for hydrogenation of olefins.
2. Experimental
2.1. Materials and analyses
* Corresponding author. Tel./fax: +86 411 39893854.
E-mail addresses: likaoxue@yahoo.com.cn (K. Li), yhuawang@dlut.edu.cn
(Y. Wang).
1-Butanol, cyclohexene and styrene were of analytical grade and
were purchased from Kermel. 1-Dodecene and 1,5-cyclooctadiene
1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.12.015

K. Li et al. / Catalysis Communications 11 (2010) 542–546
543
phase turned from light yellow to brownish black, indicating the
formation of rhodium nanoparticle catalyst.
2.3. TEM of the rhodium nanoparticle catalyst
The size of rhodium nanoparticle catalyst was characterized by
transmission electron microscopy (TEM). The solution containing
the rhodium nanoparticle catalyst was diluted with ethanol. Then,
a drop of the aqueous solution (or 1-butanol) was placed onto a
carbon-coated copper grid, which was dried at ambient tempera-
ture. The TEM images were taken with a Philips Tecnai G2 20
TEM at an accelerating voltage of 200 kV.
2.4. Hydrogenation of olefins
All 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 experiments performed. The
autoclave was charged with rhodium nanoparticle catalyst, 1-buta-
nol, water and olefin and flushed three times with 2 MPa H2. The
reactor was pressurized with H2 up to the required pressure and
held at the scheduled temperature with magnetic stirring for a
fixed length of time. Then, the reactor was cooled to room temper-
ature and depressurized. The upper organic phase was separated
by phase separation from the lower aqueous phase and immedi-
ately analyzed by GC and GC–MS.
Scheme 1. General principle of TRPTC.
were supplied from Fluka. 1-Octene was obtained from Acros.
All these chemical agents were purified by distillation from an
appropriate drying agent under inert atmosphere. RhCl3Á3H2O
was received from Beijing Research Institute of Chemical Industry
and used without further purification. Thermoregulated ligand
Ph2P(CH2CH2O)nCH3 (n = 16) was prepared according to the meth-
od reported in the literature [16] (1H NMR (400 MHz, CDCl3):
d = 2.38 (t, 2H), 3.35 (s, 3H), 3.52–3.63 (m, 71H), 7.30–7.41 (m,
10H) ppm; 13C NMR (100 MHz, CDCl3): d = 28.24 (d), 58.42 (s),
67.82 (d), 69.58–71.38 (m), 127.88–137.88 (m) ppm; 31P NMR
(400 MHz, CDCl3): d = À22.01 ppm). Gas chromatography was per-
formed on a Tianmei 7890 GC with an FID detector equipped with
a 50 m OV-101 column. GC–MS measurement was performed on a
HP 6890/5973 MS instrument. ICP-AES analyses of rhodium were
carried out on Optima 2000DV (Thermo Elemental, USA).
3. Results and discussion
3.1. Thermoregulated phase-transfer of thermoregulated ligand
Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized rhodium nanoparticles
A range of methods for the phase-transfer of transition-metal
nanoparticles has been reported [17–19]. However, at present no
reports on using the thermoregulated ligand to realize the phase-
transfer of transition-metal nanoparticle catalyst. Thermoregulated
ligand is generally defined as one kind of nonionic surface-active
phosphine ligands containing polyoxyethylene chains as the
hydrophilic group in the molecular structure, which demonstrates
a special property of inverse-temperature-dependent solubility in
water (Cp). Seeing that the thermoregulated ligand possesses the
property of Cp, meanwhile it can act as a stabilizing agent for tran-
sition-metal nanoparticles, here we developed a simple and effective
phase-transfer approach of noble transition-metal nanoparticles
2.2. Preparation of the rhodium nanoparticle catalyst
In a typical experiment, a mixture of RhCl3Á3H2O (2 mg, 0.0076
mmol), thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16)
(13.9 mg, 0.015 mmol), 1-butanol (4 mL) and water (4 mL) was
added in a 75 mL standard stainless-steel autoclave and stirred un-
der hydrogen (4 MPa) at 70 °C for 2 h. Then, the reactor was cooled
to room temperature and depressurized. The color of the aqueous
Fig. 1. Photographs showing the thermoregulated phase-transfer of rhodium nanoparticle catalyst stabilized by thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16). (a)
Freshly prepared Rh nanoparticle catalyst in water at room temperature. (b) Phase-transfer of Rh nanoparticle catalyst from water phase to 1-butanol phase while heating to
60 °C. (c) Phase-transfer of Rh nanoparticle catalyst from 1-butanol phase to water phase on cooling to room temperature.

544
K. Li et al. / Catalysis Communications 11 (2010) 542–546
Fig. 2. TEM images and particle size histogram of rhodium nanoparticles during the thermoregulated phase-transfer process. (a) Freshly prepared Rh nanoparticles in water
phase (120 particles); (b) Rh nanoparticles after transferring into 1-butanol phase (105 particles); and (c) Rh nanoparticles after going back to water phase (112 particles).
stabilized by the thermoregulated ligand. Among the various meth-
ods available for the generation of nanoparticles we employed
reduction of corresponding transition-metal salt and adopted ther-
moregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16) as stabilizing
agent to prepare rhodium nanoparticle catalyst.
Fig. 1 shows a biphasic water/1-butanol mixture containing
thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized
rhodium nanoparticle catalyst. The upper 1-butanol phase and
the lower water phase were immiscible with each other and there-
fore they separated into two layers with a clear interface when the
stirring ceased at room temperature. The rhodium nanoparticle
catalyst was in the lower water phase (see Fig. 1a). Afterwards,
the water/1-butanol mixture was heated gradually to 60 °C (the
Cp of thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16) in the
presence of 1-butanol) under stirring. Stopping the stirring and
controlling the temperature at 60 °C, we observed that the mixture
separated into two phases and the rhodium nanoparticle catalyst
transferred from the lower water phase into the upper 1-butanol
phase (see Fig. 1b). An evident indication was that the color of both
two phases changed greatly as compared to the respective color
before heating. It should be noted that the transferring of rhodium
nanoparticle catalyst across the water and 1-butanol interface was
reversible. After cooling to room temperature, the rhodium nano-
particle catalyst could go back to the lower water phase from the
upper 1-butanol phase (see Fig. 1c). These thermoregulated
phase-transfer phenomena relating to thermoregulated ligand
Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized rhodium nanoparticle cat-
alyst shown in Fig. 1 are very similar to our previously reported
TRPTC by using transition-metal complex as catalyst [8–10]. Here,
thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16) not only

K. Li et al. / Catalysis Communications 11 (2010) 542–546
545
Table 1
Table 2
Hydrogenation of cyclohexene under various conditions.a
Evaluation of rhodium leaching in the recycling experiments.
Entry
S/Rhb
PH
(MPa)
T (°C)
Time
(min)
Conversion
(%)
TOFc
Recycle number
0
1
2
3
4
5
6
2
(hÀ1
)
Rh leaching (wt.%)
1.10
0.23
0.09
0.07
0.06
0.06
0.06
1
2
3
4
5
6
7
8
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
5000
1
1
1
1
1
1
1
1
1
2
4
40
50
60
70
80
90
60
60
60
60
60
15
15
15
15
15
15
30
45
60
20
60
15
31
59
73
86
100
85
600
1240
2360
2920
3440
4000
1700
1280
1000
3000
5000
also see from Table 1 that by means of increasing temperature,
pressure or prolonging reaction time the conversion of cyclohex-
ene could all reach 100% (entries 6, 9–11). Under the conditions
of hydrogen pressure (4 MPa) at 60 °C with a S/Rh of 5000, the
TOF of the reaction reached up to 5000, suggesting that the ther-
moregulated ligand Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized rho-
dium nanoparticle catalyst was very active for hydrogenation.
The recycling efficiency of the rhodium nanoparticle catalyst
was investigated. After reaction, the upper organic phase was sep-
arated from the lower catalyst-containing aqueous phase by phase
separation. And the aqueous phase was directly used in the next
reaction run. Under the reaction conditions of 1-butanol 4 mL,
water 4 mL containing 7.6 Â 10À3 mmol rhodium, T = 60 °C, hydro-
gen pressure = 1 MPa, cyclohexene/Rh = 1000 (molar ratio), reac-
tion time = 60 min, the catalyst could be recycled for six times
remaining a 100% yield. And no apparent aggregation and size
change of the rhodium nanoparticles have been observed, as
shown by the TEM images in Fig. 3.
96
9
10
11
100
100
100
a
Reaction conditions: 1-butanol 4 mL, water 4 mL containing 7.6 Â 10À3 mmol
rhodium.
b
Molar ratio of substrate to rhodium.
Turnover frequency, calculated as the number of moles of product formed per
c
mol of rhodium per hour.
serves as stabilizing agent, but also endows the rhodium nanopar-
ticle catalyst with the thermoregulated phase-transfer property.
To further know the size change of the rhodium nanoparticle
catalyst before and after the thermoregulated phase-transfer,
TEM images were recorded as shown in Fig. 2. The mean diameter
of the freshly prepared rhodium nanoparticles in water phase was
2.4 nm with a standard deviation of 0.3 (see Fig. 2a). When trans-
ferring into 1-butanol phase, the mean diameter of rhodium nano-
particles was 2.6 nm with a standard deviation of 0.4 (see Fig. 2b).
After going back to water phase, analysis of the rhodium nanopar-
ticles gave average diameter of 2.7 nm with a standard deviation of
0.3 (see Fig. 2c). TEM analysis indicated that no obvious size
increasing of the transferred nanoparticles occurred.
The leaching of transition-metal nanoparticle catalyst is fre-
quent, especially for the catalytically active group VIII metals
[20]. Therefore, we studied the leaching of the aqueous/1-butanol
biphasic system in the recycle experiments by determination of
the rhodium content in the 1-butanol phase at the end of the
Table 3
Hydrogenation of different olefins catalyzed by thermoregulated ligand
Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized rhodium nanoparticle catalyst.a
3.2. Catalytic properties of thermoregulated ligand
Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized rhodium nanoparticles for
hydrogenation
Entry
Substrate
S/Rhb
Time
(h)
Conversion
(%)
TOFc
(hÀ1
)
1
2
3
4
Styrene
1-Octene
1-Dodecene
1,5-Cyclooctadiene
2000
2000
2000
1000
1
1
1
2
100d
95
2000
1900
1740
500
To evaluate the catalytic characteristics of thermoregulated li-
gand Ph2P(CH2CH2O)nCH3 (n = 16)-stabilized rhodium nanoparticle
catalyst, hydrogenation of alkenes was performed. We first studied
the hydrogenation of cyclohexene at various conditions and the re-
sults were listed in Table 1. The conversion of cyclohexene with
cyclohexane as the only product and the TOF of the reaction in-
creased with increasing of temperature (entries 1–6). However,
prolonging the reaction time could increase the conversion but de-
crease the TOF of the reaction (entries 3, 7–9). Meanwhile, we can
87
100e
Reaction conditions: 1-butanol 4 mL, water 4 mL containing 7.6 Â 10À3 mmol
a
rhodium, T = 60 °C, PH = 1 MPa.
2
b
Molar ratio of substrate to rhodium.
Turnover frequency, calculated as the number of moles of product formed per
mol of rhodium per hour.
c
d
Product is ethyl benzene.
Product is cyclooctane.
e
Fig. 3. TEM image and particle size histogram of rhodium nanoparticles after six recycling at 60 °C (160 particles).

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K. Li et al. / Catalysis Communications 11 (2010) 542–546
reaction. The leaching of rhodium was detected by ICP-AES and the
results were presented in Table 2. It can be seen that the leaching
of rhodium decreases with the increase of recycle number. The
reason for this needs further investigation. Nevertheless, compared
with our previous complex catalyst in TRPTC system, the average
leaching of rhodium (0.2 wt.%) is lower.
for New Century Excellent Talents in University (Grant No. NCET-
07-0138) and the Science and Technology Project in Universities
from the Education Department of Liaoning Province (Grant No.
2008T233).
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zene and cyclooctane, respectively, at 60 °C and initial H2 pressure
of 1 MPa (entries 1 and 4).
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