In situ hydrogen from aqueous-methanol for nitroarene reduction and imine formation over an Au-Pd/Al2O3 catalyst

Article abstract of DOI:10.1039/c0cc00531b

In situ hydrogen from aqueous-methanol, instead of H₂ or CO, was used to synthesize imines with a high selectivity from nitroarenes and carbonyl compounds over an Au-Pd/Al₂O₃ catalyst.

Full text of DOI:10.1039/c0cc00531b

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In situ hydrogen from aqueous-methanol for nitroarene reduction and
imine formation over an Au–Pd/Al2O3 catalystw
Yizhi Xiang, Qiangqiang Meng, Xiaonian Li* and Jianguo Wang
Received 22nd March 2010, Accepted 7th June 2010
DOI: 10.1039/c0cc00531b
In situ hydrogen from aqueous-methanol, instead of H2 or CO,
was used to synthesize imines with a high selectivity from
nitroarenes and carbonyl compounds over an Au–Pd/Al2O3
catalyst.
ð5Þ
ð6Þ
Direct synthesis of complex molecules from simple starting
In this study, we report a new catalysis system for the direct
synthesis of imines from nitroarenes and carbonyl compounds
in the absence of H2 or CO over an Au–Pd/Al2O3 catalyst
(eqn (1)). In situ hydrogen, produced from aqueous-phase
reforming of methanol, or hydrogen transfer from methanol
as donor (in both routes CO2 was the only gas effluent
product, eqn (2), Fig. S1 and S2w), was used to reduce
nitroarenes into aryl amines (eqn (3)).14 The amines condensed
with carbonyl compounds and finally produced imines
(eqn (4)). High selectivity of the imine was reached because
only limited in situ hydrogen from aqueous-methanol reforming
could be used to selectively reduce nitroarenes (Table S1w)
without the occurrence of side reactions as illustrated in
eqn (5) and (6). Methanol in this process acted as both the
solvent and hydrogen source.
materials in a single reaction without separation of intermediates
is an important issue in chemistry.1 The direct synthesis of
N-containing compounds (e.g., amines, N-alkylamines, and
imines) from the reduction of nitroarenes followed by alkylation
or condensation of aryl amines is an important aspect of this
issue. Currently, the synthesis of N-alkylamine from nitroarenes
and aldehydes, ketones, or nitriles in the presence of reductants
(B10H14, H2, and metallic zinc or tin) has been extensively
investigated.2 The one-pot synthesis of N,N-dimethylaniline
and N-ethylaniline from nitrobenzene and methanol or ethanol
in an atmosphere of N2 or Ar over a RANEYs Ni catalyst has
also been reported.3 But there are only few reports on the
direct synthesis of imines.
Imines are bioactive N-containing compounds and often
used as intermediates for the synthesis of herbicides, pharma-
ceuticals, and other fine chemicals.4 Generally, imines are
produced from the condensation of primary amines with
carbonyl compounds,5 or from the oxidative dehydrogenation
of secondary amines using O2,6 iodosylbenzene,7 or persulfate8
as the oxidants. Imines were also synthesized from amines and
alcohols in the presence of O2.9 Recently, the direct synthesis
of imines from nitroarenes and carbonyl compounds was
Table 1 shows the results of imines synthesized from a
model system composed of ‘‘nitrobenzene (1 mL)/furfural
(1 mL)/methanol (40 mL)/water (10 mL)’’. As shown in entry 1,
the formation rate of imine was 14.1 mmol gꢀ1 minꢀ1 with the
conversion of nitrobenzene and furfural at 30.6 and 29.6%,
respectively. The selectivity for the imine based on furfural was
98.9% when the reaction was carried out over Au–Pd/Al2O3 at
408 K and 2.0 MPa Ar-pressure (Au–Pd/Al2O3 was not
deactivated, see Fig. S3w). H2 was not detected in the
gas effluent under this condition with a CO2 yield of
5.2 mmol gꢀ1 minꢀ1, indicating the nitrobenzene was reduced
by the in situ hydrogen from aqueous-methanol. When the
temperature increased from 408 K to 453 K, H2 was detected
with a yield of 1.3 mmol gꢀ1 minꢀ1 in the gas effluent.
Accordingly, the selectivity for the imine based on furfural
decreased to 87.4% with conversions of nitrobenzene and
furfural being 93.7 and 79.5%, respectively (the formation
rate of the imine was 33.5 mmol gꢀ1 minꢀ1), as shown in entry 2.
The use of H2 gas, instead of the in situ hydrogen from
aqueous-methanol, for the reduction of nitrobenzene showed
the formation rate and selectivity of imine were only
0.2 mmol gꢀ1 minꢀ1 and 0.4%, respectively while the conversions
of nitrobenzene and furfural were both 100% (entry 3). These
results implied that the utilization of excessive amounts of
hydrogen, either from H2 or from aqueous-phase reformation
of methanol at high temperature, was not favored in the synthesis
of imines.
10
11
realized using Au/TiO2 or Ni/SiO2 as catalyst in the
presence of H2, or using PdCl2(PPh3)2–SnCl2 as catalyst in
the presence of CO,12 or using Fe–HCl as both catalyst and
reductant.13 But these processes were not desirable due to the
use of H2, CO, PdCl2(PPh3)2–SnCl2, and Fe–HCl.
ð1Þ
(2)
CH3OH + H2O - 3H2 + CO2
R1–NO2 + 3H2 - R1–NH2 + 3H2O
(3)
ð4Þ
State Key Laboratory Breeding Base of Green Chemistry Synthesis
Technology, Industrial Catalysis Institute of Zhejiang University of
Technology, Hangzhou 310032, P.R. China. E-mail: xnli@zjut.edu.cn;
Fax: +86 571-88320409; Tel: +86 571-88320409
w Electronic supplementary information (ESI) available: Experimental
procedures, catalyst evaluation, DFT calculations. See DOI: 10.1039/
c0cc00531b
Entries 4–7 in Table 1 show the effects of Au–Pd on the
synthesis of imines. Obviously, both nitrobenzene and furfural
ꢁc
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5918 | Chem. Commun., 2010, 46, 5918–5920

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Table 1 Experimental results of the direct synthesis of imine from nitrobenzene and furfurala
x (%)
Products composition (%)c
Factors
affecting S (%)
Imine formation
rate/mmol gꢀ1 minꢀ1
Entry
Catalysts
S (%)b
NB
FF
Imine
AL + NMA
FA + THFA + AM
1
—
(1) Hydrogen
Au–Pd/Al2O3
Au–Pd/Al2O3
Au–Pd/Al2O3
Ru–Pd/Al2O3
Fe–Pd/Al2O3
Pd/Al2O3
Au/Al2O3
Au–Pd/MgO
Au–Pd/TiO2
14.1
33.5
0.2
10.1
7.8
4.2
0
3.2
30.6
93.7
100
26.2
19.5
9.7
—
27.3
27.8
29.6
79.5
100
23.3
18.1
9.4
—
6.7
25.9
98.9
87.4
0.4
90.2
89.7
92.9
—
89.4
76.2
0.3
74.6
76.8
79.4
—
9.6
1.0
11.0
69.2
8.1
18.3
7.0
—
2d
3e
4
12.8
30.5
17.3
4.9
13.6
—
(2) Active
components
5
6
7
8
(3) Support
100
100
72.6
81.0
27.4
19.0
—
—
9
12.5
a
Reactions were carried out at 408 K, 2.0 MPa (Ar pressure). The load of Au–Pd/Al2O3 was 0.5 g. A mixed solution (methanol 40 mL+ water
10 mL+nitroarenes 1 mL+furfural 1 mL) was fed at 0.1 mL minꢀ1
b
c
Selectivity of imine based on furfural. AL: aniline; NMA: N-methylaniline;
.
FA: furfural alcohol; THFA: tetrahydro furfural alcohol; AM: amines. Reaction temperature: 453 K. Molecular H2 was co-fed at 10 mL minꢀ1
with the liquid solution.
d
e
conversions and the formation rate of imine as well as the
selectivity were decreased in the absence of Au as a catalytically
active component. The formation rate of imine over Ru–Pd/
Al2O3, Fe–Pd/Al2O3 and Pd/Al2O3 catalysts were 10.1, 7.8
and 4.2 mmol gꢀ1 minꢀ1, respectively. The corresponding
selectivities for the imine based on furfural were 90.2, 89.7
and 92.9%, respectively, at 408 K and 2.0 MPa Ar-pressure
with in situ hydrogen from aqueous-methanol. Additionally, no
imine was produced from the monometallic Au/Al2O3 catalyst
(entry 7), but Au–Pd on MgO and TiO2 supports showed
relatively lower imine formation rates (entries 8, 9). These results
indicated that the bimetallic Au–Pd was the catalytically active
component and necessary for the formation of imines.
The X-ray photoelectron spectroscopy (XPS) analysis of
Au–Pd/Al2O3 revealed that the Au 4f7/2 and 4f5/2 core levels
showed binding energies of 83.5 and 87.1 eV respectively, and
the Pd 3d5/2 and 3d3/2 core levels showed binding energies of
335.1 and 339.8 eV, respectively. Representative HRTEM
images of Au–Pd/Al2O3 taken from several different particles
are shown in Fig. 1. The lattice spacings of most particles were
in the range of 0.227 to 0.229 nm and 0.233 to 0.236 nm, which
were close to the Pd (111) planes (0.225 nm) and the Au (111)
planes (0.235 nm), respectively. Some particles were multiply
twinned in a form of truncated decahedron (Fig. 1(d)). Similar
TEM results have also been reported by Wang et al.15
From the above analysis, the proposed mechanism for the
direct synthesis of imine from nitrobenzene and furfural is
shown in Fig. 2. The availability of limited hydrogen is the
vital factor to control the reaction selectivity. Too much
available hydrogen will result in a series of side reactions
The characteristics of the Au–Pd/Al2O3 catalyst were examined
to identify its catalytic mechanism. The IR spectra of CO
adsorbed on Au–Pd/Al2O3 and Pd/Al2O3 catalysts (Fig. S4w)
indicated the presence of Pd (111) on the catalyst surface.15
Fig. 2 The catalytic mechanism of the direct synthesis of imine
from nitrobenzene and furfural over the Au–Pd/Al2O3 catalyst with
(a) hydrogen from the aqueous-phase reforming of methanol,
(b) molecular H2 as a reducing agent.
Fig. 1 Representative HRTEM images of the Au–Pd/Al2O3 catalyst
obtained from several different particles.
ꢁc
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(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)
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Furfural 1.06
0.50
0.45
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Table 3 Experimental results of the direct synthesis of imines from
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compounds
conversion (%)
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rates/mmol gꢀ1 minꢀ1
and composition (%)
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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