Larisha Cisneros et al. / Chinese Journal of Catalysis 37 (2016) 1756–1763
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Deposition of platinum or palladium on gold catalyst was
Table 1
done from aqueous solutions of H2PtCl6·6H2O (Aldrich, > 37.5%
as Pt); or PdCl2 (Aldrich, 99%). Gold catalyst was impregnated
with the solution containing the desired amount of metal to
form a thick paste (incipient wetness impregnation). After a
perfect mixing of the wet solid, samples were dried at 373 K for
12 h. The resultant powders were reduced in fix bed reactor
using 50 mL/min of H2 at the selected temperature for 3 h
before reaction.
Deposition of platinum was done from aqueous solutions of
H2PtCl6. Supports (Active carbon Darco KB‐B, Aldrich) were
impregnated with the solution containing the desired amount
of metal to form a thick paste (incipient wetness impregnation).
As an example, 1.2 mL of an aqueous solution containing 10.62
mg of H2PtCl6·6H2O were contacted with 2 g of carbon Darco to
prepare the 0.2 wt% Pt/C catalyst. After a perfect mixing of the
wet solid, samples were dried at 373 K for 12 h. The resultant
powders were reduced in fix bed reactor using 50 mL/min of
H2 at the selected temperature for 3 h before reaction.
Effect of temperature on the selectivity to nitrone 3 formed from nitro‐
benzene 1, 5‐methylfurfural 2 and H2 (5 bar) using the different types
of catalysts [12].
4
O
N
O
NO2
OH
+
O
N
O
O
O
O
H
+
5-10 bar H2
+
N
6
0.2%-5% metal
313 K
3
2
1
5
Temperature
(K)
Conversion
Selectivity to
nitrone (%)
Catalyst
(%)
99
99
88
95
73
1 wt% Au/TiO2
1 wt% Au/TiO2
1 wt% Au/CeO2
0.2 wt% Ru/C
0.2 wt% Ru/C
393
373
333
308
0
15
12
84
73
333
reaction temperature, and the nature of solvent. More
particularly, higher reaction temperatures imply, in general,
lower selectivities to the nitrones, not only with the Au/TiO2
catalyst, but also with other metals (Table 1).
2.2. Catalytic performance
It was presented before that while Au/TiO2 was not selec‐
tive enough for producing nitrones from nitro aromatics com‐
pounds since this catalyst must operate at higher temperatures
to achieve practical reaction rates; with Pt on carbon the rate of
H2 dissociation could be enhanced and the catalyst can work at
lower temperature, showing very high selectivity to aromatic
substituted nitrones [12]. Unfortunately, Pt/C catalyst, used to
obtain nitrones in high yields from nitroaromatic compounds
and aldehydes, is less selective when nitroalkanes were react‐
ed, due to the concomitant formation of the corresponding
alcohol (Table 2). On the other hand, we realized that nitrones
coming from nitro aliphatic compounds were more stable to
reaction temperature and the Au/TiO2 catalyst (that requires
higher reaction temperatures) could be a suitable option.
With the objective to analyze the mechanism of interaction
between active centers and different functional groups with
Pt/C and Au/TiO2 catalysts, activation of nitrobenzene and
styrene was studied in a situation of competitive adsorption.
Fig. 1(a) shows presence of nitrobenzene avoiding double bond
reduction with Au/TiO2 and also with Pt/C catalysts; even
though with Pt/C catalyst a higher hydrogenation rate is ob‐
served. Competitive adsorption shows an important diminish
in styrene activity due to strong adsorption of nitro aromatic
compound on the surface avoiding styrene activation on active
centers, because of this, there is not an important reduction of
double bond until more than 90% of nitrobenzene has been
consumed.
Catalytic reactions were performed in a reinforced glass
reactor (2 mL) with temperature and pressure control, and
stirred magnetically. The feed composition was (mol): 90.5%
solvent, 6% nitro compound, 3% aldehyde, 0.5% o‐xylene
(internal standard). Typically, 100 mg of catalyst were used for
900 mg of feed. Reaction pressure and temperature are
indicated in the corresponding table of the main text.
Conversion and selectivities were determined using a gas
chromatograph (Varian 3900) equipped with an FID detector
and a 30‐m HP‐5 capillary column. The products were
identified by mass spectrometry using a GC/MS device (Agilent
MDS‐5973) equipped with a quadrupole electron‐impact
ionization detector.
3. Results and discussion
In this paper, we have designed a catalyst that affords
nitrones in higher yields than any preceding process, starting
from aliphatic nitro compounds, aldehydes (aromatic or
aliphatic) and H2 in a single step process, for doing that the
catalyst should hydrogenate selectively nitroaromatic
compounds, without causing reduction of other sensitive
groups such as carbonyls. This is a necessary condition for
obtaining nitrones via a cascade‐type reaction. The condition is,
however, not sufficient and previous attempts to accomplish
this reaction with gold catalysts were fruitless [12]. Despite the
tolerance of the carbonyl function, the Au/TiO2 catalyst
required higher temperatures to dissociate H2 and the catalyst
gives lower selectivity to the nitrone 3 when reacting with
5‐methylfurfural 2 (Table 1) since, the imine 4 (formed by
condensation of aniline and the 5‐methylfurfural 2) is obtained
in high yields (the corresponding imine forms according to the
sequence of reactions in Schemes 2 and 5).
An equivalent experiment with a nitro aliphatic compound,
like nitrobutane, in the presence of styrene (Fig. 1(b)) was done
and the results show a much higher rate of disappearance of
double bond with Pt/C catalyst, meanwhile good selectivity to
nitrobutane compound is preserved with Au/TiO2 catalyst. In
conclusion, reduction of double bond on Pt/C catalyst is much
faster when there is a nitro aliphatic compound in the reaction
media and the highest selectivity of Au/TiO2 catalyst showed in
this paper can be explained because the strong activation of
–NO2 group in the interface Me‐Ti [16].
A first series of experiments show that the selectivity to
nitrone compounds with Au/TiO2 is largely dependent on the