G.-Y. Fan et al. / Catalysis Communications 11 (2010) 451–455
453
Table 1
ZrOÁxH
2
O was blocked in the presence of water and it did not need
The effect of solvents on the hydrogenation.
any special additive. Wang et al., reported that the electropositive
nitrogen atom of nitro group was easily absorbed on the surface of
the electron-rich metal atom [29], so it leads to the activation of
N@O bond in CNB. The activated N@O bond becomes to be highly
susceptible to hydrogen attack and can be quickly hydrogenated.
According to XPS results, the binding energy 60.4 eV and 63.3 eV
Entry
Solvent
Conv. (%)
Sel. (%)
p-CAN
p-CNSB
Aniline
1
2
3
4
5
6
7
8
9
EtOH + H
EtOH + H
2
Oa
O
>99
>99.9
96.7
95.0
89.3
92.1
96.2
89.9
93.0
86.1
0
0
0
b
2
73.2
59.4
51.1
13.9
54.3
27.7
15.4
35.3
3.3
2.2
10.4
6.3
2.7
8.1
4.2
13.9
a
Toluene
2.8
0.3
1.5
1.2
1.9
2.8
0
a
EtOH
of Ir 4f in Ir/ZrO
Ir 4f in Ir/ZrO
ÁxH
was more electron-rich than that on Ir/ZrO
tron-rich Ir supported on commercial ZrO showed a lower activity
than Ir/ZrO O. According to the previous works [15,30–32], the
ÁxH
(omitted) are lower than 60.8 eV and 63.9 eV of
O. It indicated that Ir nanoparticles on ZrO
ÁxH O, but the elec-
2
THFa
2
2
2
a
Cyclohexane
a
2
2
Isopropanol
EtOHc
2
EtOH + H
2
Oc
2
2
modification role of metal cation in the hydrogenation of haloaro-
matic nitro compound results from chemical interaction between
oxygen atom in nitro group of substrate and metal cation adsorbed
on the surface of catalyst [30–32] or between oxygen atom in nitro
group of substrate and metal cation in the solution [15]. The inter-
Reaction conditions: solvent, 7 ml; p-CNB, 1.0 mmol; catalyst, 25 mg; temperature,
5 °C; hydrogen pressure, 1 atm; reaction time, 2.5 h, p-CNSB: p-chloronitrosobenzene.
2
a
Ir/ZrO
2
ÁxH
Reaction time, 1.5 h.
Ir/ZrO
2
O.
b
c
2
.
action is favorable for the activation of polar N@O bond in ÀNO
2
group. Subsequently, the attack of the dissociated H atoms to the
activated N@O bond results in the rapid reduction of the nitro
group in substrate molecule. The coordination of the nitro group
in substrate molecule with metal cation enhances the electron-
withdrawing ability of nitro group and induces the lone pair
electron on chlorine atom transferring to phenyl ring, so that the
polarity of C–Cl is weakened and the hydrogenolysis of C–Cl is sup-
pressed [15]. The Mervein–Ponndorf–Verley reduction of corbonyl
compounds has been proved that the formation of hydrogen bond
between surface hydroxyl group and substrate is a key step to this
reaction [33]. Similarly, the formation of the hydrogen bond be-
tween substrate and modifier adsorbed on the catalyst surface
could improve the hydrogenation rate and enantioselectivity to
product in the asymmetric hydrogenation of carbonyl group over
cinchonidine modified Pt catalyst [34,35]. Being the similar with
the actions of metal cation additives and modifiers [15,31–35], In
this case, it is suggested that the hydroxyl groups on the surface
of the catalyst form the hydrogen bond with N@O bond of CNB
and cause the polarization of N@O bond, so the activated hydrogen
on the surface of Ir particles is more easily to attack the polarized
N@O bond. However, if the interaction only occurs between sub-
strate and hydroxyl group on the surface of support, it seems not
to explain the great improvement of this catalyst activity because
in this way the promotional role can be only caused in the fringes
of the metal particles. According to the promotional role of water
as a solvent in Table 1, we suggest further that besides the hydro-
gen bond between substrate and hydroxyl group on the surface of
support, the hydrogen bond between the substrate and water as a
solvent also plays an important role to the activation of substrate
molecule. At the same time, the formation of hydrogen bond be-
tween the hydrogenation product and water as a solvent promotes
the rapid desorption of the hydrogenation product on the surface
of the catalyst. As a result, the hydrogenation rate of CNB is greatly
improved and the dehalogenation was difficult to occur. Of course,
an intermediate p-chloronitrosobenzene was detected in the pres-
ence of water under the condition of a short reaction time. The re-
sult indicated that the hydrogenation in this system is still finished
step by step as the reported results [32].
be further hydrogenated to the desired product at the complete
hydrogenation of p-chloronitrobenzene. However, we used com-
mercial ZrO
2
to prepare catalyst Ir/ZrO
2
with impregnation meth-
ÁxH O. Ir/
(entry 8 in Table 1) showed a much lower activity than Ir/
ÁxH O (entry 4 in Table 1). Replacing a part of ethanol with
(entry
in Table 1) increased only from 15.4% to 35.3%, but the selectivity
od, which has the same iridium loading with Ir/ZrO
2
2
ZrO
ZrO
2
2
2
water, the conversion of p-chloronitrobenzene over Ir/ZrO
9
2
to p-CAN decreased from 93.0% to 86.1% due to the formation of
intermediate product p-chloronitrosobenzene (p-CNSB). It can be
seen that water plays a great promotion role and the role is espe-
cially obvious in the presence of Ir/ZrO
2
ÁxH
2
O. To our knowledge,
this is the first report on that the introduction of water can greatly
promote the hydrogenation of haloaromatic nitro compounds.
Compared with the reported catalytic systems, which are highly
selective to hydrogenation of haloaromatic nitro compounds (Ta-
ble 2), the catalytic performance of this catalytic system consisted
of Ir/ZrO
best one. In spite of Ag/SiO
lysts being of almost the same selectivity with Ir/ZrO
hydrogenation of various haloaromatic nitro compounds, their
hydrogenation activities at the high reaction temperature of
2
ÁxH
2
O and the mixture solvent of ethanol and water is the
2
[19], Au/SiO
2
[20], Au/ZrO
2
[21] cata-
O for the
2
ÁxH
2
1
40 °C and hydrogen pressure of 4 MPa are obviously lower than
Ir/ZrO O at room temperature and atmospheric pressure. Espe-
2
ÁxH
2
cially, in order to obtain a good hydrogenation result, in many sys-
tems, an additive or promoter must be introduced [24]. But this
system does not need any additive and it can give this excellent
hydrogenation result. To the best of our knowledge, the catalytic
activity and selectivity are unprecedented for the hydrogenation
of haloaromatic nitro compounds.
For the dehalogenation mechanism in the hydrogenation of
haloaromatic nitro compound, some researchers suggested that it
is caused by the electrophilic attack of the activated hydrogen on
the absorbed aromatic halides [28–31] and the second metal or
some additives need to be introduced in order to suppress the
dehalogenation [31,32]. In this case, the dehalogenation over Ir/
Table 2
Hydrogenation of halonitroaromatic compounds over different catalysts.
À1
Catalysts
Ru/SnO
Ag/SiO
Au/SiO
Au/ZrO
Ir/ZrO
Substrate
S/C
Conditions
Product (yield)
TOF (h
)
2
[15]
[19]
[20]
[21]
o-CNB
o,p-CNB
x-CNB
p-CNB
x-CNB
155
86
627
415
758
60 °C, 4.0 MPa, methanol
140 °C, 2.0 MPa, ethanol
140 °C, 4.0 MPa, ethanol
150 °C, 1.0 MPa, ethanol
x-CAN (>99.9)
o-CAN (100)
x-CAN (100)
p-CAN (99.4)
x-CAN (>99.9)
155
29
209
83
2
2
2
2
2
ÁxH O
r.t., l atm, ethanol/H
2
O
253
Reaction conditions: ethanol, 5.0 ml; H
2
O, 2.0 ml; p-CNB, 1.0 mmol; catalyst, 25 mg, reaction temperature, 25 °C; H
2
pressure, 1 atm; reaction time, 2.5 h.