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hydrogenation of benzene to cyclohexene. The corresponding
plots of the selectivity for cyclohexene versus the conversion
of benzene are shown in Figure 6 (reaction temperature:
1
508C; H pressure: 5.0 MPa). The benzene content decreased
2
gradually and the completely saturated product, cyclohexane,
increased linearly with reaction time. Regarding the cyclohex-
ene content, there was a maximum at a certain reaction time,
dependent on the catalyst, exhibiting the well-known behavior
of consecutive reactions. Among the four catalysts, the Ru/
MgAl-LDH catalyst shows the highest activity of benzene hy-
drogenation, but the lowest selectivity to cyclohexene. With
the enhancement of incorporated Cu, the activity of benzene
hydrogenation over the resulting Ru Cu /MgAl-LDH decreased
x
y
gradually, whereas the selectivity to cyclohexene increased and
reached a maximum value at a Ru/Cu molar ratio of 1:0.5. As
a result, a maximum cyclohexene yield of 44.0% at a benzene
Figure 5. In situ FTIR spectra of CO adsorption on a) Cu/MgAl-LDH-2, b) Ru/
MgAl-LDH, c) Ru1.0Cu0.2/MgAl-LDH, d) Ru1.0Cu0.5/MgAl-LDH, and e) Ru1.0Cu1.0
MgAl-LDH.
/
conversion of 71.8% was obtained over the Ru Cu /MgAl-
1.0
0.5
LDH catalyst. Although the obtained catalytic yield is not as
high as some reported values, the Cu-decorated Ru catalyst in
this work does not involve the use of any additive (e.g.,
served (Figure 5a), in accordance with the CO-TPD result (Fig-
0
ure 4e), further confirming that Cu hardly adsorbs CO species
ZnSO ), which has been commonly applied previously (see
4
[
38]
at room temperature.
In the case of the Ru/MgAl-LDH
Table S1). Therefore, this additive-free catalyst can serve as
a green and eco-friendly candidate for the selective hydroge-
nation of benzene.
sample, three bands can be identified (Figure 5b). The bands
ꢀ
1
at 1993 and 2062 cm can be attributed to the bridge-
bonded and linearly adsorbed CO on Ru atoms, respectively;
It can be seen from Figure 6 that all three Ru Cu /MgAl-LDH
x
y
ꢀ
1
whereas the band at 2120 cm can be assigned to the stretch-
ing mode of multicarbonyl species, which often adsorb on
low-coordinated sites (such as corners, steps, or defective sites)
catalysts display inhibited activity of benzene hydrogenation
with the addition of Cu, in comparison with the Ru/MgAl-LDH
catalyst. As summarized in Table 2, the weight-specific activity
[
23,45]
of finely dispersed noble metals.
Generally, low-coordinat-
(R ) of these catalysts also decreased with an increase of Cu
0
ed metal atoms possess relatively low electron density and
lead to the occurrence of the stretching vibration of multicar-
bonyls at a high wavenumber, owing to the weak electron
feedback to the adsorbed CO. Of the four Ru-based samples,
Ru/MgAl-LDH exhibits the strongest multi-CO adsorption (Fig-
ure 5b), indicating a high exposure of unsaturated Ru. With
the introduction of Cu, the adsorption intensity of multi-CO
decreases gradually and reaches a negligible value at a Ru/Cu
molar ratio of 1:0.5 and 1:1 (Figure 5d and e), implying almost
complete coverage of Cu on the low-coordinated Ru. The cov-
content. This inhibiting effect, to some extent, can be attribut-
ed to the coverage of Cu on the surface of Ru, resulting in a de-
creased exposure of Ru active sites. To give a deep insight into
the inhibition effect, the turnover frequency (TOF) of benzene
over these catalysts was further calculated on the basis of Ru
dispersion and R data. A plot of the TOF value as a function of
0
the coverage degree of Cu (q ) is shown in Figure 7a. It is ob-
Cu
served that initially the TOF value exhibits a linear decrease
with an increase of q , and is balanced at a q value of
Cu
Cu
29.1%. It has been proven that metal catalysts with high unsa-
turation/low coordination number generally show excellent hy-
erage degree of Cu (q ) for these two samples is ꢁ29.1 and
Cu
[47]
46.6%, respectively, according to the CO-TPD result (Table 1).
drogenation activity. For benzene hydrogenation, a high effi-
ciency of hydrogenation is also dependent on the exposure of
The results indicate a preferential coverage of Cu on the low-
coordinated Ru, in good agreement with previous investiga-
[20]
low-coordinated Ru atoms.
Compared with the Ru/MgAl-
[
46]
tions.
For the bridge-bonded CO adsorption peak at
LDH catalyst, the low-coordinated Ru atoms in the Ru Cu /
x
y
ꢀ
1
1
993 cm , a continuous decrease is observed with the en-
MgAl-LDH catalysts are continuously covered by Cu and under-
go a complete coverage at qCu value of 29.1%, according to
the CO-TPD result, which accounts for the gradually decrease
in the TOF of benzene.
hancement of Cu content to the Cu/Ru molar ratio of 1:1 (Fig-
ure 5e), implying a further coverage of Cu on the high-coordi-
nated Ru. Therefore, a tunable exposure of the active sites of
Ru NPs can be successfully achieved by changing the surface
coverage of Cu originating from a LDH support. This finding
imposes great influence on the hydrogenation selectivity,
which is discussed below.
Although the reaction rate of benzene hydrogenation over
the Cu-decorated Ru catalysts was inhibited to some extent,
the hydrogenation selectivity to cyclohexene was largely en-
hanced. In terms of the effect of Cu on the cyclohexene selec-
tivity for these catalysts, the value of S50,CHE was also obtained
(
defined as the cyclohexene selectivity at 50% benzene con-
Catalytic evaluation for selective hydrogenation of benzene
version). Unlike the negative correlation between the TOF
value and the coverage of Cu on the Ru surface, a positive re-
lationship was established for S50,CHE (Figure 7b), indicative of
The catalytic performances of the as-synthesized Ru/MgAl-LDH
and Ru Cu /MgAl-LDH samples were evaluated by the selective
x
y
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