J. Liang et al. / Journal of Molecular Catalysis A: Chemical 411 (2016) 95–102
99
Fig. 6. The conversion of octadecanol and the yield of products as a function of reac-
tion times over -Mo2C/CNTs. Experimental conditions: octadecanol (0.5 g), decane
(50 mL), catalyst (0.25 g), 4 MPa H2, 180 ◦C.
Fig. 5. The conversion of stearic acid and the yield of products as a function of reac-
tion times over -Mo2C/CNTs. Experimental conditions: stearic acid (0.5 g), decane
(50 mL), catalyst (0.25 g), 4 MPa H2, 180 ◦C.
n-octadecane decreases to 86.28% because of the enhancement of
heptadecane yield via decarbonylation.
stearic acid, because of the fast hydrogenation of the double bond
to n-octadecane over -Mo2C/CNTs catalyst. No other products
are found in the deoxygenation process, such as esters or ketones,
which indicates the good hydrodeoxygenation performance of -
Mo2C/CNTs. Gas products obtained over the -Mo2C/CNTs were
also analyzed, and the main oxygen-containing product is water.
Trace amounts of CO2 and CH4 were also detected in the reaction
system.
Large amounts of octadecanol can be achieved at a lower tem-
perature (<180 ◦C) during the process of catalytic hydrogenation,
which indicates that the dehydration reaction of octadecanol to n-
octodecane requires a higher temperature in the overall reaction
when -Mo2C/CNTs used as the catalyst. With the increase of reac-
tion temperatures, the yield of n-heptadecane gradually increases,
and the highest n-heptadecane yield of 13.72% is achieved at 200 ◦C.
This result suggests that higher temperatures favor the decar-
bonylation reaction of octadecanal to produce n-hepadecane over
-Mo2C/CNTs. By tracing the conversion course of stearic acid,
intermediate octadecanal was confirmed. Because only traces of
octadecanal were detected in the overall reaction, the octadecanal
reduction rate to the octadecanol was proved to be faster than its
production rate over -Mo2C/CNTs. In addition, a blank test with-
out catalyst was carried out to evaluate the contributions of the
used catalyst at a reaction temperature of 200 ◦C, and a very lower
stearic acid conversion can be obtained. That is to say the presence
of the -Mo2C/CNTs catalyst plays a crucial role in the hydrogena-
tion reaction of stearic acid. On the basic of the results mentioned
above, 180 ◦C is selected as the favorable reaction temperature in
this paper.
To understand the deoxygenation step of stearic acid to alka-
nes better, the representative intermediate, octadecanol, was
employed in
a catalytic hydrogenation experiment over -
Mo2C/CNTs with the same reaction conditions mentioned in
conversion of stearic acid. As shown in Fig. 6, the yield of n-
octadecane and the conversion of octadecanol increase obviously
with reaction times, and the highest n-octadecane yield of 88.26%
is obtained with the octadecanol conversion of 100% at 2.5 h, which
also supports the view that hydrogenation–dehydration of octade-
canol is the major reaction path of deoxygenation of stearic acid. In
addition, the equilibrium between the octadecanol and octadecanal
exists in the fact that yields of individually produced n-heptadecane
and n-octadecene increase linearly with reaction times.
3.2.3. The effect of hydrogen pressures
tion pressures were investigated over -Mo2C/CNTs catalyst. The
results indicate that initial hydrogen pressures have a significant
effect on the conversion of stearic acid, and the conversion of
stearic acid is proportional to the initial hydrogen pressure in
the range of 0–1 MPa. As shown in Fig. 7, a complete conver-
sion of stearic acid is obtained when the initial hydrogen pressure
increases to 1 MPa. With further increase of the hydrogen pres-
sure to 4 MPa, the yield of n-octadecane enhances obviously to
91.24%, whereas the n-heptadecane yield reduces from 36.77 to
8.76%. This is related to the fact that the higher hydrogen pres-
sure shifts the equilibrium from octadecanal to octadecanol, and
benefits the hydrogenation–dehydration route of octadecanol to
n-octadecane, and thus, the yield of n-octadecane without car-
bon atom loss is enhanced, whereas the yield of n-heptadecane
obtained from decarbonylation of octadecanal is suppressed [14].
This suggested that higher hydrogen pressures heavily favor the
hydrogenation–dehydration pathway over -Mo2C/CNTs catalyst.
In order to investigate the influence of reaction times on deoxy-
genation of stearic acid, the experiments of catalytic hydrogenation
as the increase of reaction times were carried out, and the results
are shown in Fig. 5. The conversion of stearic acid and the yields
of liquid hydrocarbons almost linearly increase with the reaction
times. The major liquid product is n-octadecane with the high-
est yield of 91.24% and the stearic acid conversion of 100% after
3 h, whereas the yield of n-heptadecane enhances slightly from
5.12 to 8.76%, which indicates that the hydrogenation–dehydration
of octadecanol is the major reaction route during the deoxygena-
tion process of stearic acid. The content of octadecanol is relatively
stable in the overall reaction, and the yield declines sharply with
almost complete conversion of stearic acid after 2.5 h, which can
been explained that the initial deoxygenation reaction proceeds
firstly by a fast hydrogenation of the carboxylic group to octade-
canol, and then the obtained octadecanol undergoes dehydration
to produce the major product of n-octadecane over -Mo2C/CNTs.
Only traces of n-octadecene are detected during transformation of