406
Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
method under low alkaline conditions. The incorporation of Zn
100
80
60
40
20
0
species led to a decrease in the activity of the Ru catalyst alt‐
hough the selectivity was improved significantly. The optimal
Zn content in the Ru‐Zn catalysts was determined to be 16.7
wt% (i.e., Ru‐Zn‐3). The use of a stirring rate in the range of
1100 to 1300 r/min had very little impact on the catalytic reac‐
tion. The addition of small amount of ZnSO4 to the reaction
solution led to a decrease in the benzene conversion and a sig‐
nificant improvement in the selectivity, with a ZnSO4 concen‐
tration of >0.3 mol/L leading to an increase in the benzene
conversion. The optimal concentration of ZnSO4 was found to
be 0.42–0.45 mol/L. The selectivity for cyclohexene reached
80% (yield > 45%) when the conversion of benzene was 57%
in ZnSO4·7H2O solution of 0.45 mol/L under the optimal reac‐
tion conditions (i.e., hastelloy reactor, 150 °C, 5 MPa of H2
pressure, 1200 r/min). The presence of ZnO crystals in the
Ru‐based catalysts was very important to obtain a high selec‐
tivity for cyclohexene (>80%). Taken together, these results
demonstrate that Ru‐Zn catalysts prepared under low alkaline
conditions could be used for industrial application.
Conversion
Selectivity
Yield
1
2
3
4
5
Recycle times
Fig. 7. Recyclability of the Ru‐Zn‐3 catalyst for the selective hydrogena‐
tion of benzene to cyclohexene. Reaction conditions: Ru‐Zn‐3 catalyst
0.12 g, ZrO2 0.6 g, ZnSO4·7H2O 8.4 g, C6H6 35 mL, H2O 70 mL, 150 °C, 5
MPa of H2 pressure, stirring rate 1200 r/min, 45 min.
down the hydrogenation of cyclohexene to cyclohexane be‐
cause it would suppress the direct hydrogenation of the ad‐
sorbed cyclohexene. The presence of a water layer would also
slow down the rate of cyclohexene re‐adsorption because of the
poor solubility of cyclohexene in water. Liu’s group [22] re‐
cently proposed that the synergistic effect of ZnO and ZnSO4
enhanced the selectivity for cyclohexene. Namely, the
(Zn(OH)2)3(ZnSO4)(H2O)5 salt formed by ZnO on the surface of
the catalyst would react with ZnSO4 and play a key role in im‐
proving the selectivity of the catalyst for cyclohexene. However,
no explanation has been provided to date in the literature to
account for the observed increase in the benzene conversion
with increasing ZnSO4 concentration. We propose that the pH
value of the reaction solution decreases with increasing ZnSO4
concentration, which would result in the dissolution of the ZnO
on the surface of the catalyst, leading to an increase in the ben‐
zene conversion. However, further research would be required
to investigate this hypothesis in detail.
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3.2.4. Stability of the catalyst
The stability of the Ru‐Zn‐3 catalyst was investigated. The
catalyst was recycled five times without the inclusion of any
additives, and the results are shown in Fig. 7. The results show
that the benzene conversion was stable above 48%, and that
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and 40% in the first 4 recycles, respectively, which indicated
that the catalyst was stable. The activity of the catalyst was
slightly decreased in 5th recycle because of the inevitable loss
of catalyst during the recycling process and the occurrence of 4
recycles without regeneration, however, the selectivity to cy‐
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Unsupported Ru‐Zn catalysts with different Zn contents
have been successfully prepared using the coprecipitation