(Table 1 entries 2, 3). One important role of the Pd in Au–Pd is
the catalytic reforming or dehydrogenation of methanol (step
(1) in Fig. 2a), which cannot happen on a monometallic Au
catalyst (Table 1, entry 7). This proposition can be further
confirmed by a density functional calculation of the adsorption
energy of methanol on pure Au (111), pure Pd (111), Au (111)
with a surface of Pd, and Pd (111) with a surface of Au
(Table 2). Methanol was found not to adsorb on Au (111) at
all. The in situ formed hydrogen from the reformation of
methanol will immediately reduce the nitroarenes adsorbed on
the Au–Pd particles (step (2) in Fig. 2a). It is well known that
Au has excellent selectivity for the catalytic hydrogenation of
nitroarenes into aryl amines.16 When Au was replaced with Fe
or Ru, the selectivity of imine significantly decreased. It should
be also pointed out that the Au and Pd particles alone without
the modification of each other are not active for the selective
reduction of nitroarenes using methanol as a hydrogen source
and the direct synthesis of imines from nitroarenes and
carbonyl compounds. Therefore, Pd and Au, respectively,
played a major role in steps (1) and (2). Pd also enhanced
the adsorption of nitrobenzene on the Au–Pd particles. Imine
was finally obtained from the condensation of aniline with
furfural at the Lewis acid site on Al2O3.
In order to extend the applicability of the current method,
the direct syntheses of imines from nitrobenzene and benzaldehyde
or cinnamaldehyde, 3-methylnitrobenzene and cinnamaldehyde,
or 4-methylnitrobenzene and cinnamaldehyde were also
investigated. As shown in Table 3, the formation rates of the
corresponding imines are 6.7, 7.5, 5.4 and 6.3 mmol gꢀ1 minꢀ1
,
respectively. These results are comparable to those in the
reaction of nitrobenzene with furfural.
In conclusion, the direct synthesis of imines from nitroarenes
and carbonyl compounds was realized over the Au–Pd/Al2O3
catalyst in the presence of methanol. The available limited in situ
hydrogen, from the aqueous-phase reformation of methanol
or hydrogen transfer using methanol as the donor, was the
vital factor controlling the selectivity of imines. Pd and Au,
respectively, played a major role in generating limited hydrogen
and in situ hydrogenation of nitroarenes. The produced aryl
amine from nitroarenes finally condensed with carbonyl
compounds to form imines at the Lewis acid sites on Al2O3.
This work was supported by National Nature Science
Foundation of China (NSFC-20976164).
Table 2 The adsorption energy of methanol, nitrobenzene and
furfural on Au (111), Pd (111), Au (111) with a surface of Pd, and
Pd (111) with a surface of Au (eV).
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Pd (111) Pd (111)–Au Au (111)–Pd Au(111)
Methanol
Nitrobenzene 1.36
Furfural 1.06
0.50
0.45
1.25
0.96
0.40
0.35
0.40
0.18
0.33
0.40
Table 3 Experimental results of the direct synthesis of imines from
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compounds
conversion (%)
Imines formation
rates/mmol gꢀ1 minꢀ1
and composition (%)
Nitroarenes
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R1–NO2
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a
Reactions were carried out at 408 K, 2.0 MPa (Ar pressure). The
load of Au–Pd/Al2O3 was 0.5 g. A mixture of 40 mL methanol, 10 mL
water, 1 mL nitroarenes, and 1 mL carbonyl compounds was fed at
0.1 mL minꢀ1
.
ꢁc
This journal is The Royal Society of Chemistry 2010
5920 | Chem. Commun., 2010, 46, 5918–5920