102
S. Liu et al. / Applied Catalysis A: General 400 (2011) 99–103
NH2
NH2
+H2
NH2
H
N
H
N
-NH3
+H2
NH2
NH
-H2
+H2
H
N
NH2
H
N
-NH3
+H2
NH2
Fig. 7. The pathway of side reaction over Ni/␥-Al2O3.
3.2. Reaction parameters
Finally, the reaction was conducted at a 1:1 benzaldehyde/aniline
molar ratio with a yield of 96.5%.
To optimize the reaction conditions, the reduction of ben-
zaldehyde in presence of aniline was performed at different
temperatures and different H2 pressures. As shown in Figs. 3 and 4,
no matter how the temperature and H2 pressure changed, the con-
version of benzaldehyde was 100%, but the yield of toluene changed
with the variation in the conditions. From the GC–MS data, we
found benzaldehyde transformed to N-compounds with aniline at
any conditions examined. It support that the hydrogenation and
hydrogenolysis of N-compounds are the key step of the reduction
of benzaldehyde. Furthermore, those conditions have a big influ-
ence on the selectivity of the benzaldehyde. As shown in Fig. 3, with
the increase of temperature, the highest selectivity was obtained
at 200 ◦C. When the temperature was above 210 ◦C, the selectiv-
ity decreased obviously, because toluene was further reduced to
methyl cyclohexane, and when the temperature was below 190 ◦C,
the N-compounds cannot be hydrogenolysis. As shown in Fig. 4,
the best selectivity was obtained with a H2 pressure of 2 MPa. At
1 MPa, the hydrogenolysis of N-compounds was incomplete, while
at 3 MPa, methyl cyclohexane could be detected.
From the GC–MS data of the reaction mixture of hydrogenation
of benzaldehyde with aniline, the mechanism proposed in Fig. 1
was further determined. All the intermediates and products had
been detected. In addition, cyclohexylamine, dicyclohexylamine
and N-phenylhexylamine that were detected in the reaction mix-
ture were the byproducts from the hydrogenation of aniline over
The acidity of ␥-Al2O3 facilitated the adsorption of substrate
on the surface of catalyst due to the basicity of substrate.
After the introduction of aniline to the reaction, the basicity of
efficiency of benzaldehyde increased, which lead to a better con-
version afterwards. Meanwhile, aniline was also adsorbed on the
catalyst. The adsorption of aniline caused the side reaction, and the
corresponding pathway was shown in Fig. 7. The adsorption of ani-
line might decrease the activity of Ni30/␥-Al2O3, and it would stop
the further hydrogenation of toluene. Therefore, methyl cyclohex-
ane could not be detected under such conditions.
In the present study, 15 g of Ni30/Al2O3 was fixed in the reac-
tion zone, which was the maximum amount catalyst to occupy the
heat area in the tube reactor. 15 g of catalyst is about 25 mL, and
the inner diameter of the tube reactor is 15 mm, so the length of
the catalytic area is 141 mm. Therefore, the reaction time of the
solution can be reflected by the flow rate of the solution (10 wt.%
of benzaldehyde in the dioxane, molar ratio of benzaldehyde and
aniline was 1:1), the faster the flow rate, the lesser is the time the
solution goes through the reaction zone and lesser is the reaction
time. To optimize the flow rate, the reaction was performed with
0.3, 0.5, 0.8, 1.0 mL/min. As shown in Fig. 5, the best flow rate of the
solution in the experiments was 0.3 mL/min. With the increase of
the flow rate of the solution, the selectivity dropped sharply. When
the flow rate was faster than 0.5 mL/min, the amount of the stock
was beyond 15 g catalyst’s capacity.
After the reduction, the dioxane and most of aniline (>90%) can
be recovered by distillation. Since the whole process was continu-
ous, the lifetime of the catalyst was also examined. The reaction was
performed under optimum reaction conditions over 120 h. Dur-
ing this period, the catalyst showed good stability and the yield
of toluene remained around 95%.
4. Conclusion
toluene over Ni/␥-Al2O3 in the presence of aniline and H2 was
established in this work. Toluene was obtained in the yield of 96.5%.
The added aniline played an important role in the reduction of
benzaldehyde by a mechanism shown in Fig. 1. The catalyst was
characterized by XRD. Ni0 was believed to be the active site for the
hydrogenation and hydrogenolysis.
3.3. Effects of aniline in the reduction of benzaldehyde
Acknowledgement
line might be enough for the reaction. To study the influence of the
amount of aniline to the reduction of benzaldehyde, an experiment
was performed at a benzaldehyde/aniline molar ratio of 1:0, 1:0.1,
1:0.3, 1:0.5, 1:0.7 and 1:1. As shown in Fig. 6, the results indicated
that the conversion was 100% after the addition of any amount
of aniline and the selectivity rises along with the increase in the
amount of aniline. When the amount of aniline is more than 0.7
times of the amount of benzaldehyde, the selectivity is above 92%.
Financial support was provided by the National Natural Science
Foundation of China (Grant No. 20706043).
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