82
G.-Y. Fan, J. Wu / Catalysis Communications 31 (2013) 81–85
reaction mixture turned to a black suspension after 3 h, excess water
10 mL) was added. The reaction mixture was further stirred for
0 min at 100 °C and cooled to room temperature. The black solid
was filtered, washed with acetone, and dried in air at 100 °C for 1 h
to obtain a bluish-gray powder.
were detected as the intermediates in the hydrogenation process.
For the reaction carried out at 100 °C and an H pressure of
(
3
2
0.8 MPa, the Rh/AlO(OH) catalyst achieved a 99.4% conversion of
quinoline within 0.5 h with a selectivity of 88.2 % toward 1THQ
(entry 1). Furthermore, the selectivity to DHQ increased from 7.9%
to 87.4% when the reaction time was increased to 5.0 h (entries
2–4). The increase in the amount of DHQ was equal to the decrease
in the amount of 1THQ, which indicates that the hydrogenation of
quinoline first occurred in the N-heterocycle of quinoline to yield
2
.2. Characterization of the catalyst
The content of the metal was determined by inductively coupled
1
plasma (ICP) (IRIS Intrepid). The morphology of the catalysts was
determined with a transmission electron microscope (JEM-2010) at
an accelerating voltage of 200 kV. X-ray diffraction patterns (XRD)
were recorded on a PHILIPHS X'Pert MPD. X-ray photoelectron
spectroscopy (XPS) spectra were obtained with Kratos XSAM800.
The Fourier transform-infrared spectra (FT-IR) of the sample were
obtained on a Nicolet 510SX FTIR spectrometer using the KBr wafer
technique.
THQ, and then further hydrogenated to DHQ.
The effect of the catalyst amount on quinoline hydrogenation
shows that the yield of DHQ increased from 50.4% to 99.6% when
the catalyst amount was increased from 30 mg to 50 mg (entries 3,
5–6). Moreover, the results of the tests illustrate that hydrogen
pressure has a great effect on the yield of DHQ. When the hydrogen
pressure was increased from 0.4 MPa to 1.4 MPa at a reaction
temperature of 100 °C, the selectivity towards DHQ increased from
2
0.9% to 87.5% (Entry 3, 7–8) because of the rapid hydrogenation
1
2
.3. Activity tests
of the partial hydrogenated product THQ to form DHQ. In addi-
tion, the yield of DHQ increased from 15.9% to 99.3% when the
reaction temperature elevated from 75 °C to 125 °C (entry 3,
9–10). Consequently, the optimum reaction conditions were a
reaction temperature of 125 °C, a hydrogen pressure of 0.8 MPa,
a substrate/catalyst mole ratio of 120, and a reaction time of
3.5 h. on the basis of our experimental data, this is the optimum
conditions compared with those reported in the Ru and Rh catalyst
systems. For example, Sánchez-Delgado [10] only reported a 1THQ
yield of a 92% over a Ru/P4VPy catalyst with a quinoline/Rh ratio of
The hydrogenation of quinoline was carried out in a 60 mL steel
autoclave equipped with a glass liner and a magnetic stirrer. The
desired amounts of the catalyst, quinoline, and the solvent were
introduced to the reactor. The autoclave was flushed with pure
hydrogen for five times. When the designated reaction temperature
was reached, hydrogen was fed to the desired pressure, the stirring
rate was adjusted to 1500 rpm, and the reaction time was measured.
Parallel experimental data were obtained by repeating the reaction
two or three times, and they had good repeatability. All liquid
samples were analyzed by gas chromatography (GC-960) with an
FID detector and OV-17 Supelco column (30 m×0.25 mm, 0.25 μm
film).
84 at 200 °C and an H
obtained a maximum DHQ yield of 41.9% over Rh/PLC at a quino-
line/Rh ratio of 560 at 200 °C and an H pressure of 2.0 MPa for
2
pressure of 4.0 MPa for 1 h, Campanati only
2
2.5 h. On the other hand, Fache obtained a 100% yield of DHQ with
a quinoline/Rh ratio of 40 in expensive hexafluoroisopropanol for
19 h.
3
. Results and discussion
3
.1. Catalyst characterization
3.3. Effect of calcination temperature
The ICP results indicate that the rhodium loading was 5.0 wt.%.
The transmission electron microscopy (TEM) images of Rh/AlO(OH)
Fig. 1) show nanofiber morphologies that are characteristic of the
The effects of calcination temperature of Rh/AlO(OH) on the
hydrogenation of quinoline were also investigated, and the results
are listed in Table 2. From Table 2, the calcination temperature has a
significant influence on the catalytic properties of the Rh/AlO(OH)
catalyst for the hydrogenation of quinoline. When the calcination
temperature was increased from 100 °C to 350 °C, a sharp decrease
in the conversion of quinoline from 100% to 23.6% was observed,
and the selectivity to DHQ decreased from 50.4% to 0%. A contrary
trend was found for the selectivity to 1THQ, which increased from
47.3% to 99.3%. Fig. 2 shows the XRD pattern of Rh/AlO(OH) calcined
at different temperatures. The corresponding mean Rh particle sizes
are listed in Table 2. When the reduction temperature was increased
from 100 °C to 350 °C, the mean Rh particle sizes increased from
2.9 nm to 7.5 nm (entries 1–3). TEM results of the fresh and calcined
catalysts indicate that the Rh particle size was consistent with the
XRD results. The aggregation of Rh particles of the Rh/AlO(OH) cata-
lyst during the heat treatment may account for the decrease in the
conversion of quinoline.
(
aluminum oxyhydroxide matrix [14–16]. The mean particle size of
Rh was estimated to be 3.2 nm based on a high-resolution transmis-
sion electron microscopy (HRTEM). XRD pattern of the fresh Rh/
AlO(OH) (Fig. 2a) shows an existence of metallic Rh. The Rh particle
size was calculated to be 2.9 nm from the half-width of the diffraction
line (111), which is consistent with the HRTEM results. The binding
energy values at 308.1 and 312.4 eV in XPS assigned to the Rh3d5/2
and Rh3d3/2 peaks, indicating that Rh (III) was reduced to a
low-oxidation state (Fig. 3). The infrared spectrum of Rh/AlO(OH)
−
1
(
Fig. 4) indicates that the O―H stretching band at 3439 cm
the O―H bending mode at 1630 cm
and
−
1
[7] are rather strong, which
implies the presence of structure water and surface hydroxyl groups
on the surface of the catalyst.
3
.2. Optimization of reaction conditions
It should be noted that this remarkable decrease in reaction activ-
ity and selectivity of the Rh/AlO(OH) catalyst could be attributed to
not only the aggregation of Rh particles but also other reasons. The
results listed in Table 2 indicate a drastic loss of water when the
calcination temperature of the Rh/AlO(OH) catalyst was increased
from 100 °C to 350 °C. As the calcination temperature increased
from 100 °C to 250 °C, the loss of water, which is expressed as weight
percent of the fresh catalyst, was 23.3% (entry 2 in Table 2). This
result is attributed to the reversible elimination of physisorbed
water [17]. Furthermore, a water loss of about 6.7 % was observed
when the calcination temperature was increased to 350 °C because
The catalytic properties of the Rh/AlO(OH) catalysts were investi-
gated using the hydrogenation of quinoline as a model reaction. The
results are presented in Table 1. The conversion of quinoline was neg-
ligible without catalyst or in the presence of Rh-free AlO(OH) at the
same conditions, which shows that the presence of Rh is indis-
pensable for high catalytic activity. The Rh/AlO(OH) catalyst was
extremely catalytically active for quinoline hydrogenation at relative-
ly mild conditions. Special attention was given to distinguish the
intermediate products of the reaction with gas chromatography–
mass spectrometry (GC–MS). 1THQ and a small amount of 5THQ