XUE Wei et al. / Chinese Journal of Catalysis, 2012, 33: 1913–1918
spectra, there was some elemental boron with a lower BE in the
Table 1 Catalytic Performance of Ru-[bmim]BF4 for benzene selective
catalyst. Elemental boron would donate electrons to elemental
Ru, making boron electron deficient, which also promotes
water adsorption on the catalyst surface, because elec-
tron-deficient boron can readily accept the lone electron pair on
the oxygen atom in a water molecule. Therefore, during selec-
tive hydrogenation of benzene, further hydrogenation of
cyclohexene to cyclohexane is retarded by reducing the read-
sorption of cyclohexene on the hydrophilic catalyst surface.
Ru-[bmim]BF4-II catalyst that had been used once was
separated from the aqueous phase by centrifugation and then
reused for selective hydrogenation of benzene without any
hydrogenation
Benzene
conversion (%)
56.4
Cyclohexene
selectivity (%)
19.0
Entry
Catalyst
1a
2
3b
Ru-[bmim]BF4-I
Ru-[bmim]BF4-II
Ru-[bmim]BF4-II
49.5
34.1
59.6
23.8
Reaction conditions: H2 5.5 MPa, v(H2O):v(benzene) = 2:1, n(C6H6):
n(Ru) = 525, 150 °C, 30 min, c(ZnSO4) = 0.05 mol/L.
aRu-[bmim]BF4-I catalyst was prepared using N2H4·H2O as reducing
agent.
bRu-[bmim]BF4-II was reused for the second time.
further treatment. Entry 3 in Table
1 indicates that
results are listed in Table 1. Ru-[bmim]BF4-II shows slightly
lower activity than that of Ru-[bmim]BF4-I catalyst, which was
prepared using N2H4·H2O as a reductant. However, the selec-
tivity for cyclohexene over the former catalyst is far higher
than that obtained over Ru-[bmim]BF4-I. This difference may
be attributed to the effect of boron.
Ru-[bmim]BF4-II showed low stability during the reaction,
because the selectivity for cyclohexene decreased significantly
when benzene conversion increased to 59.6%.
Figure 1(b) shows an XRD pattern of the Ru-[bmim]BF4-II
catalyst that had been used once. There are obvious differences
between the patterns of the catalysts before and after reaction.
The peaks corresponding to metallic Ru become strong and
sharp, which indicates that Ru particles gradually increase in
size because of the H2 atmosphere in the reaction. It also in-
dicates that the protection from the IL was weakened.
To clarify the influence of boron on catalyst performance,
Ru-[bmim]BF4-II was characterized by XPS (Fig. 3). Because
the Ru 3d3/2 peak overlaps with that of C1s, only the Ru 3d5/2
peak is considered in the following discussion. From Fig. 3,
one can see that almost all of Ru is present in its elemental state
with a binding energy (BE) of 280.7 eV, which is slightly
higher than that of elemental Ru (280.0 eV) reported by Liu et
al. [8]. This positive shift may be assigned to the remaining
[bmim]BF4 on the surface of Ru. The BF4– anion attracts elec-
trons strongly, which can decrease the electron density of Ru
and increase the BE of inner electrons. Moreover, surface
oxidation of Ru by O2 before XPS measurement would also
result in a positive shift of BE [6].
There are also some strong diffraction peaks assigned to zinc
hydroxyfluoride (ZnOHF), which formed during selective
hydrogenation of benzene through a reaction between
[bmim]BF4 and ZnSO4, an additive in the selective hydro-
genation of benzene. Wu et al. [11] synthesized ZnOHF nano-
fibers from Zn5(OH)8(NO3)2·2H2O in the presence of the IL
1,2,3-trimethylimid-azolium tetrafluoroborate.
Fig. 2(b) and (c) show TEM images of the Ru-[bmim]BF4-II
catalyst after one use. There are two main kinds of substance in
the images. Some are agglomerations of small particles, which
are presumed to be metallic Ru particles. Compared with Fig.
2(a), the Ru particles in the used catalyst are clearer because the
IL protection layer is absent, which also allows the Ru particles
to aggregate. The other type of substance in the image is
rod-like particles with widths of up to 200 nm and lengths on
the micro scale. These rod-like particles are ZnOHF.
According to the BE of B 1s (193.5 eV), oxidized boron was
present in the Ru-[bmim]BF4-II catalyst [9]. These oxides may
come from the hydrolysis of NaBH4. Xie et al. [10] found that
the presence of oxidized boron species on the surface of Ru
catalyst could greatly enhance the hydrophilicity of the catalyst
by interacting with water molecules through hydrogen bond-
ing. In addition, according to the asymmetry of the B 1s XPS
284.8
(a)
193.5
(b)
280.7
295
290
285
280
275
175
180
185
190
195
200
205
Binding energy (eV)
Binding energy (eV)
Fig. 3. XPS spectra of Ru 3d (a) and B 1s (b) for Ru-[bmim]BF4-II catalyst.