K.-i. Shimizu et al. / Journal of Catalysis 270 (2010) 86–94
93
Table 5
Initial rates for hydrogenation of nitrobenzene (VN) or styrene (VS).a
4. Conclusions
Catalyst
T (°C)
VN (mmol gÀ1 hÀ1
)
VS (mmol gÀ1 hÀ1
)
VN/VS
Silver clusters on h-Al2O3 catalyze highly chemoselective reduc-
tion of a nitro group for the reduction of substituted nitroaromat-
ics. Cooperation of the acid–base pair site on Al2O3 and the
coordinatively unsaturated Ag sites on the silver cluster is respon-
sible for the rate-limiting H2 dissociation to yield a H+/HÀ pair at
metal/support interface, while the basic site on Al2O3 acts as an
adsorption site of nitroaromatics. High chemoselectivity can be
attributed to a preferential transfer of H+/HÀ to the polar bonds
in the nitro group. These fundamental aspects are very close to
those for Al2O3-supported gold nanoparticle catalyst, which has
been shown to be effective for the title reaction in our previous
study [10]. Hence, the earlier mentioned concept will provide a
strategy to design d10 metal-based selective hydrogenation
catalysts.
Ag/Al2O3-l.l
Ag/MgO-3.0
Pt/Al2O3-1.3
120
160
40
47
7.6
27
3.6
1.4
77
13
5.4
0.35
a
Substrate (2 mmol), THF (15 mL), H2 (3 MPa), catalyst (0.1 mol%).
rene adsorbed on Ag/Al2O3-0.9 (Fig. 6) shows the bands at 1530
(m (ms(NO2)). The mas(NO2) band with simi-
as(NO2)) and 1350 cmÀ1
lar position was observed for TiO2 and Au/TiO2 (1532 cmÀ1) and
was assigned to the nitrostyrene interacting with the oxide surface
through the nitro group [13]. The mas(NO2) band for nitrobenzene
on h-Al2O3 appeared at 1530 cmÀ1 (result not shown). Hence, one
of the reason of the chemoselective hydrogenation of the nitro
group in nitrostyrene could be the preferential adsorption of nitro-
styrene through the nitro group on Ag/Al2O3.
Acknowledgments
A more important origin of the selectivity is the higher intrinsic
rate for the reduction of the nitro group than that of the olefinic
group. Table 5 compares the initial rates for hydrogenations of
nitrobenzene and styrene. For Ag/Al2O3-1.1, the rate for hydroge-
nation of nitrobenzene (VN) is 13 times higher than those for sty-
rene (VS). In contrast, Pt/Al2O3-1.3 exhibits higher activity for
styrene (VN/VS = 0.35). For the chemoselective hydrogenation of
polar bonds (C@O or C@N) with homogeneous metal–ligand
bifunctional catalysts, it is widely accepted that the reaction begins
with heterolytic cleavage of H2 to yield H+ in a OH or NH ligand and
This work was partly supported by a Grant-in-Aid for Scientific
Research B (20360361) from the Japan Society for the Promotion
Science. The X-ray absorption experiment was performed with
the approval of the Japan Synchrotron Radiation Research Institute
(Proposal No. 2008A1633).
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