CHEN Zhijun et al. / Chinese Journal of Catalysis, 2011, 32: 1133–1137
standard stainless steel autoclave immersed in a thermostatic
oil bath. The stirring rate was the same for all the experiments.
The autoclave was charged with the Rh nanoparticle catalyst,
1-pentanol, water, olefins, and Àushed three times with 2.0
MPa H2. The reactor was pressurized with H2 up to the required
pressure and held at the scheduled temperature with stirring
magnetically for a fixed length of time. The reactor was then
cooled to room temperature and depressurized. The upper
organic phase was separated by phase separation from the
lower aqueous phase and immediately analyzed by GC and
GC-MS.
heating
cooling
(b)
(c)
(a)
Fig. 1. Scheme of the thermoregulated phase-transfer of the rhodium
nanoparticle catalyst stabilized by the thermoregulated ligand
Ph2P(CH2CH2O)22CH3 in the aqueous/1-pentanol biphasic system. (a)
Freshly prepared Rh nanoparticles in the water phase at room temperature
(b) Phase-transfer of Rh nanoparticles from water phase to 1-pentanol
phase while heating to 60 oC; (c) Rh nanoparticles after returning to the
water phase while cooling to room temperature.
Gas chromatography was performed on a Tianmei 7890 GC
equipped with a 50 m OV-101 column and a FID detector.
GC-MS was performed on a HP 6890/5973 MS instrument.
The TEM images were taken with a Philips Tecnai G2 20 TEM
at an accelerating voltage of 200 kV.
The choice of the thermoregulated ligand Ph2P-
(CH2CH2O)22CH3 with the longer polyether chain as a stabi-
lizer of Rh nanoparticles and the use of the aqueous/1-pentanol
biphasic system for the TRPTC was for the following reasons.
First is that the thermoregulated ligand Ph2P(CH2CH2O)22CH3
with a longer polyether chain will result in the catalyst re-
turning to the aqueous phase more easily after the reaction. The
other reason is that the solubility of water in 1-pentanol (7.2%,
30 oC) is lower than our previously reported aqueous/1-butanol
biphasic system (20.6%, 30 oC) and this will result in a lower
amount of Rh leaching into the organic phase.
transfer of the rhodium nanoparticle catalyst across the water
and 1-pentanol interface was reversible. After cooling to room
temperature, the rhodium nanoparticle catalyst returned to the
lower water phase from the upper 1-pentanol phase (Fig. 1(c)).
The mean diameter of the rhodium nanoparticles was 2.2 nm
with a standard deviation of 0.2 nm, and this remained un-
changed before and after the transfer (Fig. 2).
To evaluate the catalytic characteristics of the thermoregu-
lated ligand Ph2P(CH2CH2O)22CH3-stabilized rhodium
nanoparticle catalyst, the hydrogenation of olefins was per-
formed. We first studied the hydrogenation of cyclohexene
under various conditions and the results are listed in Table 1.
Table 1 indicates that upon increasing the temperature, the
pressure or by prolonging the reaction time the conversion of
cyclohexene increases. At a temperature of 60 °C, a hydrogen
pressure of 1 MPa, a substrate/Rh molar ratio of 1000, and a
time of 60 min the conversion of cyclohexene and the yield of
cyclohexane were 99%. When the temperature was increased
from 50 to 60 °C the conversion of cyclohexene increased
sharply. This may be a result of the catalyst having transferred
into the organic phase and the reaction proceeding homoge-
neously in the organic phase [9–11]. After the reaction, the
upper organic phase separated from the lower cata-
lyst-containing aqueous phase by phase separation and the
Figure 1 shows the aqueous/1-pentanol biphasic system
containing the thermoregulated ligand Ph2P(CH2CH2O)22CH3-
stabilized rhodium nanoparticle catalyst. The upper 1-pentanol
phase and the lower water phase were immiscible and, there-
fore, they separated into two layers with a clear interface at
room temperature. The rhodium nanoparticle catalyst was in
the lower water phase (Fig. 1(a)). When the water/1-pentanol
biphasic system was heated gradually to 60 °C (the Cp of the
thermoregulated ligand Ph2P(CH2CH2O)22CH3 in the presence
of 1-pentanol), we observed that the rhodium nanoparticle
catalyst transferred from the lower water phase into the upper
1-pentanol phase (Fig. 1(b)). An indication of this was that the
color of both phases changed markedly compared with their
respective colors before heating. It should be noted that the
Fig. 2. TEM images of the thermoregulated ligand Ph2P(CH2CH2O)22CH3-stabilized rhodium nanoparticles. (a) Freshly prepared Rh nanoparticles in the
water phase; (b) Rh nanoparticles after transfer to the 1-pentanol phase; (c) Rh nanoparticles after returning to the water phase.