Hydrogenation of Methyl Propionate to Propanol
all alcohol solvents, the best selectivity rate was 81.7%
and the highest conversion rate was 57.6%. However,
an excellent catalytic performance of Ru/ZrO •xH O
2 2
ture. When the dry temperature increased from room
temperature to 300 ℃, the conversion rate decreased
from 96.0% to 84.0% and the selectivity rate also de-
creased from 98.6% to 90.8%. To heat a hydrous cata-
lyst can not only remove the hydroxyl groups or struc-
tural water, but also cause the change of the structure
characterizations of a catalyst. In order to find the real
reason which led to the decrease of the catalytic activity
and selectivity with the increase of the drying tempera-
ture, the specific surface area, pore distribution, and
pore volume of the catalysts prepared at various drying
temperature were identified. The results in Table 2 show
that the catalyst dried at room temperature has the
smallest specific surface area and no pore volume as it
is a colloid substance. It is well known that this kind of
surface structure is not the characters of a good catalyst.
In spite of this, the catalyst showed the highest activity
and selectivity. When the drying temperature elevated to
100 ℃, the specific surface area greatly increased due
to the loss of structural water and formation of some
small pore. If the drying temperature was further ele-
vated to 200 ℃, the specific surface area, pore distribu-
tion, and pore volume of catalysts achieved a maximum
value. However, the activity and selectivity of
was exhibited in water. Both the conversion of methyl
propionate and the selectivity to propyl alcohol ex-
ceeded 99% in the similar reaction conditions. In our
2
0
previous work, it was found that AlO(OH) supported
Ru-Pt catalyst Ru-Pt/AlO(OH) is easy to cause the
transesterification between substrate and alcohol as a
solvent or product. And other catalyst systems for the
hydrogenation of esters also cause a serious transesteri-
1
1,12,19
fication.
In this system, the transesterification was
completely inhibited both in alcohol and water. Al-
though TOF (ratio of converted substrate molecules to
-
1
supported metal atoms) of 7.3 h was lower than 8.6
over Ru-Pt/AlO(OH), the conversion of 96% over
Ru/ZrO
2
•xH
2
O
was higher than 89% over
Ru-Pt/AlO(OH) and the hydrogenation temperature of
1
50 ℃ over Ru/ZrO
2
•xH
2
O was lower than 180 ℃
2
0
over Ru-Pt/AlO(OH). In addition, this result was also
higher than 4.0 h reported on the gas phase hydro-
-
1
6
genation of ethyl acetate at 300 ℃ over Rh-Sn/SiO
2
.
2 2
Above result demonstrates that Ru/ZrO •xH O in water
is an excelllent catalyst for the ester hydrogenation and
water plays an important promotion role in the hydro-
genation of methyl propionate. To the best of our
2 2
Ru/ZrO •xH O gradually decreased with the improve-
ment of its structural characters. The TEM images of
ruthenium particles did not exhibit a visible change with
increasing the drying temperature from room tempera-
ture to 200 ℃ (Figure 1a and 1b). The distribution of
particle size showed a little change and the average di-
ameter increased from 6.5 nm at room temperature to
7.5 nm at 200 ℃. When the drying temperature was up
to 300 ℃, the average diameter of ruthenium particles
increased to about 8.5 nm and the small particles (<6.0
nm) obviously reduced (Figure 1c). Furthermore, XRD
patterns of the catalysts dried at different temperatures
were consistent with HRTEM data. When the dry tem-
perature of the catalyst was lower than 200 ℃ (Figure
2 2
knowledge, ZrO •xH O is not only the first time to be
used as a catalyst carrier of ester hydrogenation, but a
monometallic catalyst is also the first time to show so
high efficiency for the hydrogenation of esters.
Table 1 Effect of solvents on hydrogenation of methyl propi-
onate
Selectivity/%
-
1
Solvent
Conversion/%
TOF/h
Propanol Propionic acid
Methanol
Ethanol
30.2
40.0
57.6
11.8
35.9
96.0
99.5
62.8
81.2
81.7
59.3
30.9
98.6
99.8
37.1
18.8
18.3
40.6
69.1
1.4
0.49
0.58
3.60
0.54
0.85
7.3
2), the XRD patterns did not show the diffraction peak
i-Propanol
Hexane
of ruthenium. But a weakened ruthenium peak appeared
in XRD pattern after the catalyst was dried at 300 ℃.
However, the aggregation of ruthenium nanoparticles
seemed not to be the major reason to cause the decrease
of the catalyst activity and selectivity. According to the
Ethylene glycol
H
H
2
2
O
O
a
0.2
4.8
Reaction condition: Catalyst, 125 mg; temperature, 150 ℃; hy-
2 2
TG curve of Ru/ZrO •xH O dried under vacuum at
drogen pressure, 5.0 MPa; time, 10 h; solvent, 2.0 mL; substrate,
room temperature (Figure 4), the weight of the catalyst
was gradually reduced with rising temperature. When
the temperature rose from room temperature to 200 ℃,
the catalyst lost the weight of about 20% and the con-
version of methyl propionate decreased from 96.0% to
85.2%. If the drying temperature was up to 300 ℃, the
weight loss of the catalyst was about 5% and the size of
Ru particles increased from 7.5 to 8.5 nm with the ob-
vious decrease of the number of small metal particles
(8.6 nm calculated by Scherrer formula), but the con-
version of methyl propionate only decreased from
85.2% to 84.0%. Obviously, the activity and selectivity
are not sensitive to the size of ruthenium particles,
a
0.4 mL; catalyst. Time: 16 h.
Effect of surface hydroxyl group of carrier on the
hydrogenation
In order to understand what role the hydroxyl group
on the catalyst surface plays in the hydrogenation of
methyl propionate, the catalyst was dried in a tempera-
ture range of r.t. to ca. 300 ℃. The results are shown in
Table 2. The catalyst, dried under vacuum at room tem-
perature, showed the highest conversion and selectivity
among all catalysts, and the activity and selectivity of
the catalyst decreased with the increase of dry tempera-
Chin. J. Chem. 2011, 29, 229— 236
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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