Chemoselective synthesis of substituted imines, secondary amines, and β-amino carbonyl compounds from nitroaromatics through cascade reactions on gold catalysts

Article abstract of DOI:10.1002/chem.200900884

Substituted imines, α,β-unsaturated imines, substituted secondary amines, and β-amino carbonyl compounds have been synthesized by means of new cascade reactions with mono- or bifunctional gold-based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, α,β-unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO₂ system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three-step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO₂ can also be used as a bifunctional catalyst to obtain substituted β-amino carbonyl compounds from nitroaromatics and α,β-unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold-catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.

Full text of DOI:10.1002/chem.200900884

DOI: 10.1002/chem.200900884
Chemoselective Synthesis of Substituted Imines, Secondary Amines, and
b-Amino Carbonyl Compounds from Nitroaromatics through Cascade
AHCTUNGTERGRNNUN eactions on Gold Catalysts
Laura L. Santos, Pedro Serna, and Avelino Corma*[a]
Abstract: Substituted imines, a,b-un
ACHTUNGTRENNUNGurated imines, substituted secondary
sat-
of different substituted nitrogenated
compounds. For the preparation of
imines, substituted anilines are formed
and condensed in situ with aldehydes
to provide the final product through
two tandem reactions. High chemose-
lectivity is observed, for instance, when
double bonds or halides are present
within the reactants. In addition, we
show that the Au/TiO2 system is also
able to catalyze the chemoselective hy-
by starting from nitroaromatic com-
pounds and aldehydes. On the other
hand, Au/TiO2 can also be used as a bi-
functional catalyst to obtain substituted
b-amino carbonyl compounds from ni-
troaromatics and a,b-unsaturated car-
bonyl compounds. Whereas gold sites
promote the in situ formation of ani-
lines, the intrinsic acidity of Ti species
on the support surface accelerates the
subsequent Michael addition. Finally,
two gold-catalyzed reactions, that is,
the hydrogenation of nitro groups and
a hydroamination, have been coupled
to synthesize additional substituted
imines from nitroaromatic compounds
and alkynes.
amines, and b-amino carbonyl com-
pounds have been synthesized by
means of new cascade reactions with
mono- or bifunctional gold-based solid
catalysts under mild reaction condi-
tions. The related synthetic route in-
volves the hydrogenation of a nitroaro-
matic compound in the presence of a
second reactant such as an aldehyde,
a,b-unsaturated carbonyl compound,
or alkyne, which circumvents an ex situ
reduction process for producing the ar-
omatic amine. The process is shown to
be highly selective towards other com-
peting groups, such as double bonds,
carbonyls, halogens, nitriles, or cinna-
mates, and thereby allows the synthesis
drogenation of imines, so that sec
ary amines can be prepared directly
through a three-step cascade reaction
ACHTUNGTNERoNUNG nd-
AHCTUNGTRENNUNG
Keywords: cascade
reactions
·
chemoselectivity · gold · hydrogena-
tion · nitro compounds
Introduction
tion of polyfunctional nitrogenated compounds by means of
cascade reactions[6–11] involving substituted aromatic amines
that can now be generated in situ from nitroaromatic com-
pounds. This is of interest for the production of highly de-
manded chemicals through a process with superior atom ef-
ficiency and energy savings.[12–14]
We will show here that, by using the appropriate catalyst,
it is possible to couple the hydrogenation of nitroaromatic
compounds with consecutive reactions between the formed
amine and carbonyls, conjugated double bonds, or triple
bonds. The proposed approach is ahead of other alternatives
when it is taken into account that a) this synthetic route
saves an ex situ hydrogenation process to produce the aro-
matic amine and b) it is highly selective towards very sensi-
tive competing groups. Thanks to the chemoselectivity of
the Au/TiO2 system during the hydrogenation of nitroaro-
matics, as well as other catalytic properties for the activation
of a second functionality, different substituted imines, sec-
The recent discovery of gold catalysts for the chemoselective
reduction of substituted nitroaromatic compounds has intro-
duced new possibilities for the production of aniline deriva-
tives.[1–5] By means of rational design, it has been possible to
prepare catalysts that preferentially activate the nitro group
in the presence of other sensitive functionalities such as
double bonds, aldehydes, cyano groups, halides, etc. This in-
teresting property of Au catalysts opens the possibility of
planning efficient and clean synthetic routes for the prepara-
[a] Dr. L. L. Santos, Dr. P. Serna, Prof. A. Corma
Instituto de Tecnologꢀa Quꢀmica/UPV-CSIC
Universidad Politꢁcnica de Valencia
Avda. de los Naranjos, s/n, 46022, Valencia (Spain)
Fax : (+34)963-877-809
E-mail: acorma@itq.upv.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900884.
ACHUTNGERNoNUG ndHCATUNGTRNEaNUGN ry amines, and substituted b-amino carbonyl com-
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FULL PAPER
pounds have been successfully synthesized by means of the
present approach.
Results and Discussion
Synthesis of substituted imines and secondary amines from
nitroaromatic compounds and aldehydes: Imines are impor-
tant organic intermediates in the synthesis of secondary
amines[15–18] and propargylamines,[19–22] with applications for
fine chemicals, pharmacy, and agrochemistry.[23–33] The most
frequent routes to prepare imines include the condensation
of amines with carbonyl compounds[34,35] (Scheme 1a) or the
Scheme 2. One-pot synthesis of substituted aromatic imines by using Au/
TiO2 catalysts.
ported by coupling nitrobenzenes and aldehydes with Fe(0)
powder,[39] this process requires the presence of a strong in-
organic acid (HCl), and the global selectivity notably falls
down if olefinic groups are present.
In order to initially prove if the synthetic route given in
Scheme 2 could be catalyzed by gold-based materials, we
have first treated nitrobenzene with benzaldehyde. The re-
sults show that, at 4 bar of H2 and 1208C, N-benzylideneani-
line can be obtained with 93% selectivity at 94% conver-
sion after 2 hours of reaction by using Au/TiO2 as the cata-
lyst (Table 1, entry 1). The high yield of desired product ob-
Scheme 1. Most frequent reaction routes for the synthesis of imines.
direct oxidation of secondary
Table 1. Catalytic results during the synthesis of different substituted imines from nitroaromatics and alde-
hydes by using Au/TiO2 catalysts (see Table S1 in the Supporting Information for the corresponding feeding
compositions) .
amines[36–38] (Scheme 1b). In the
case of aryl imines, aniline and
aryl-substituted amines, which
are produced by the hydrogena-
tion of nitrobenzene and nitro-
benzene derivatives, respective-
ly, are generally used as starting
reactants. Whereas the hydro-
genation of nitrobenzene to
aniline can be easily carried out
with H2 to provide quantitative
yields of the desired product,
the complexity of the process
notably increases in the case of
nitroaromatic compounds sub-
stituted with double bonds, car-
bonyls, nitriles, or halogens.
Entry
Product
T
[8C]
PH
%Au[a]
t
Conversion
[%][b]
Selectivity
[%][c]
2
[bar]
[h]
1
120
120
4
0.24
2
6
94
91
93
93
1
2
3
8
0.25
0.25
2
3
120
8
9
92
91
It would be of interest to se-
lectively hydrogenate nitroaro-
matic compounds in the pres-
ence of a second molecule con-
4
5
120
130
4
0.22
0.97
2
7
96
91
90
87
4
5
taining
a carbonyl group, in
12
order to synthesize the corre-
sponding imine through a cas-
cade-type reaction, as shown in
Scheme 2. Although a similar
[a] Mol Au/mol nitroaromaticꢃ100. [b] Based on aniline after complete exhaustion (>95%) of the nitroaro-
matic. [c] Loss of selectivity due to further hydrogenation of the imine group to the corresponding secondary
approach has been recently re- amine.
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A. Corma et al.
tained shows that both the carbonyl group of benzaldehyde
and the C=N group of the product imine remain unaffected
during the reduction of nitrobenzene. Interestingly, this
result is much better in terms of conversion and selectivity
than those recently reported through the oxidation of N-
phenylbenzylamine,[36,37] in which, besides requiring at least
2 independent reaction steps, only 18% of the desired imine
is obtained. On the contrary, with the procedure presented
here, nitrobenzene is directly used as the starting reactant
and undergoes a hydrogenation process to form aniline,
which condenses in situ with the aldehyde to provide the
final imine through a cascade reaction.
The benefits of this catalytic process are more evident
when other easily reducible groups are present in the nitro-
aromatic compound or in the aldehyde. For instance, high
chemoselectivity is observed when double bonds or halide
functional groups are included within the reactants
(Table 1). In fact, the selective reduction of the nitro groups
is practically independent of the position of the substitutions
within the reactants, as can be inferred by comparing the
yields of imines when a vinyl group is bonded to the aromat-
ic ring of nitrobenzene (3-vinylnitrobenzene as a reactant:
Table 1, entry 2) or to the aromatic ring of benzaldehyde (3-
vinylbenzaldehyde as a reactant: Table 1, entry 3). In both
cases, the imines are obtained with selectivity over 90% and
at conversion levels above 90%, whereas the hydrogenation
of the carbonyl, the C=C, and the C=N groups are avoided.
Taking these results into account, we decided to apply the
methodology for coupling aniline (generated in situ from ni-
trobenzene) with an a,b-unsaturated aldehyde to produce
a,b-unsaturated imines through a cascade-type reaction
(Scheme 3). As confirmation of the feasibility of this route,
good yields of the desired product were obtained by using
Au/TiO2 as the catalyst (Table 1, entry 4).
strates is related to partial hydrogenation of the C=N bond,
which leads to formation of the corresponding secondary
amine. Nevertheless, global selectivity is still high (87%),
and no dehalogenation of the bromide group is observed.
Finally, we have considered the possibility of using the
proposed synthetic route for preparing substituted sec
ACHTUNGTRENNUNGond-
AHCTUNGTREGaNNNU ry amines, after taking into account the fact that gold is
able to catalyze the enantioselective hydrogenation of
imines.[17] Therefore, if the C=N group could be chemoselec-
À
tively reduced to C NH after complete exhaustion of the ni-
troaromatic compound, secondary amines could be prepared
by starting from nitroaromatics and aldehydes through a
three-step cascade reaction. During this process, several con-
secutive and parallel reactions can compete with the desired
transformation, as shown in Scheme 4 for a representative
case.
Scheme 4. Example of the one-pot synthesis of substituted secondary
amines from nitrobenzenes and aldehydes.
Scheme 3. One-pot synthesis of a,b-unsaturated aromatic imines by using
Au/TiO2 catalysts.
It should be considered that the selectivity of formation
of the imine may change depending on the electron-with-
drawing or electron-donating character of the groups at-
tached to the aromatic rings. Indeed, an increase in the rate
of hydrolysis of the resultant imine can be expected when
electron-withdrawing groups are present in the nitroaromat-
ic compound used as the initial reactant. In order to quanti-
fy this potential drawback, we treated 4-bromonitrobenzene
with benzaldehyde. However, 87% selectivity at 91% con-
version of the intermediate aniline (complete exhaustion of
the nitroaromatic starting material) was still achieved in this
case, which indicates that only a small fraction of the corre-
sponding imine is hydrolyzed by water (Table 1, entry 5). In
fact, the decrease in selectivity observed with these sub-
To check this, we have studied the reaction between nitro-
benzene and 3-vinylbenzaldehyde. It has been found that, if
the reaction is continued after complete exhaustion of the
nitroaromatic compound, the imine group can also be che-
moselectively hydrogenated by the Au/TiO2 catalyst without
reducing the vinyl group; this leads to the preferential for-
mation of N-(3-vinylbenzyl)aniline (6) with 90% selectivity.
The results displayed in Figure 1 show that the kinetic be-
havior of gold during this process allows the selective pro-
duction of either the substituted imine by stopping the hy-
drogenation just before complete exhaustion of the nitroaro-
matic compound or, on the contrary, the substituted sec
ary amine if the reduction reaction is allowed to proceed
further.
ACHTUNGTRENNUNGond-
AHCTUNGTRENNUNG
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Cascade Reactions on Gold Catalysts
FULL PAPER
In the present paper, we consider the possibility of using
Au/TiO2 as a heterogeneous catalyst to develop a new cata-
lytic route for synthesizing b-amino carbonyl compounds
through a simple, green, and efficient cascade reaction start-
ing directly from nitroaromatics and conjugated ketones
(see Scheme 6).
Scheme 6. One-pot synthesis of the b-amino carbonyl compound from ni-
trobenzene and 1-buten-3-one by using the Au/TiO2 catalyst.
To do this, we have first checked the possibility of using
TiO2 to catalyze the Michael addition between aromatic
amines and a,b-unsaturated ketones. When the uncatalyzed
reaction between aniline and 1-buten-3-one is carried out at
808C, and the conversion of the reactants was followed with
time, nucleophilic addition of the amine group to the conju-
gated double bond occurs, but at a low rate (Figure 2). On
Figure 1. Distribution of products as a function of the conversion level
during the three-component reaction between nitrobenzene, 3-vinylACHTGNUTRENNUNG
*
^
ACHTUNGTRENNUNGaldehyde, and H2 with the Au/TiO2 catalyst ( 3; & 6; ꢃ 12;
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
One-pot synthesis of substituted b-amino carbonyl com-
pounds from nitroaromatics and a,b-unsaturated ketones
with a bifunctional catalyst: For several decades, the Mi-
chael addition reaction between amines and a,b-unsaturated
ketones (see the example in Scheme 5) has been used as an
Scheme 5. Synthesis of b-amino carbonyl compounds through Michael
addition reactions between amines and ketones.
important tool for preparative organic synthesis. The ob-
tained b-amino carbonyl functionalities are ubiquitous
motifs in natural products such as alkaloids and poly-
ketides,[40–43] as well as important intermediates for the syn-
thesis of amino alcohols, diamines, and b-amino acid deriva-
tives or b-lactams,[44–51] many of which are used, among
other pharmaceutical applications, as powerful antibiotics.
Michael reactions are facilitated by protic acid, protic base,
or Lewis acid catalysts[40,48,52–57] and can proceed through
Markovnikov or anti-Markovnikov additions, which lead to
either branched or linear compounds, respectively. Together
with high levels of regioselectivity, high chemoselectivity
with regard to other sensitive functional groups is desired.
However, this may become problematic if strong acids/bases
or harsh reaction conditions are used, due to the generation
of different undesired byproducts.[57–61] Consequently, other
procedures, based on transition-metal catalysts in solution,
have been recently proposed.[62–69]
Figure 2. Catalytic results during the Michael addition of 1-buten-3-one
*
to aniline in the presence ( ) and absence (&) of TiO2. Reaction condi-
tions: 808C, air atmosphere, 50 mg TiO2/mL feeding. Feeding composi-
tion: 86 mol% toluene, 5 mol% aniline, 8 mol% 1-buten-3-one, 1 mol%
o-xylene.
the contrary, the reaction occurs approximately 20 times
faster in the presence of TiO2, to provide almost quantita-
tive yields of the desired product. This result opens up the
possibility of performing a cascade reaction by starting from
nitrobenzene and 1-buten-3-one with an Au/TiO2 catalyst.
The gold would initially produce the chemoselective hydro-
genation of the nitro group to an amine, then the mild acidi-
ty of TiO2 would promote the addition of the conjugated
double bond to the amine group.
Indeed, under H2 pressure, the corresponding b-amino
ketone is produced in excellent yields after one hour of re-
action (Table 2, entry 1). The practical absence of products
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A. Corma et al.
Table 2. Catalytic results during the synthesis of substituted b-amino carbonyl compounds from nitroaromatics
and 1-buten-3-one by using Au/TiO2 catalysts (see the Supporting Information for the corresponding feeding
compositions).
plex case in which two gold-cat-
alyzed reactions are carried out
in a one-pot process. It has
been reported that gold can ac-
tivate the coupling of triple
bonds with amines, which leads
to the formation of the corre-
sponding imines.[20,21] The per-
formance of this reaction
Entry
Product
T
[8C]
PH
[bar]
%Au[a]
t
Conversion
[%][b]
Selectivity
[%][c]
2
[h]
1
80
80
8
0.49
1
2
100
91
91
98
7
8
through
tion, by starting from a nitroar-
omatic compound, is real
a cascade-type reac-
2
3
10
0.58
0.55
a
challenge because the catalyst
has to reduce chemoselectively
the nitro group without hydro-
genating the triple bond, while
it also has to activate the triple-
bond function for the subse-
quent amine–alkyne coupling
(see Scheme 7).
120
8
5
92
91
9
[a] Mol Au/mol nitroaromaticꢃ100. [b] Based on aniline after complete exhaustion (>95%) of the nitroaro-
matic. [c] Loss of selectivity mainly due to the Michael addition of a second 1-buten-3-one molecule onto the
desired b-amino carbonyl compounds.
The results with nitrobenzene
and phenylacetylene as sub-
derived from reduction of the a,b-unsaturated ketone indi-
cates that the chemoselectivity of the Au/TiO2 catalyst for
the hydrogenation of nitroaromatic compounds is retained
when the double bond and the carbonyl function are conju-
gated. In this situation, the slight decrease in selectivity at
high conversion levels is due to the addition of a second 1-
butene-3-one molecule onto the obtained keto amine. On
the other hand, the short reaction times required to perform
the whole cascade process indicate effective cooperative be-
havior between the Au nanoparticles and the TiO2 support
to achieve the selective hydrogenation of the nitro group
and the consecutive Michael addition reaction, respectively.
Finally, to expand the variety of polyfunctional b-amino
carbonyl compounds that can be obtained through this
method, we have carried out analogous experiments with
different substituted nitroaromatic compounds, and the re-
sults are summarized in entries 2 and 3 of Table 2. The se-
lected reactants include sensitive functional groups such as
ethyl cinnamates or nitriles, which are very commonly used
for the synthesis of pharmaceuticals, fragrances, or bio-
cides.[70–72] In all cases, high yields of the corresponding keto
amines are obtained with Au/TiO2, a result indicating that
the proposed methodology is chemoselective with different
substituted nitrobenzenes.
Scheme 7. One-pot synthesis of substituted imines from nitrobenzenes
and alkynes by using Au/TiO2 catalysts.
strates (Table 3, entry 1) show that this synthetic route is
feasible with the Au/TiO2 catalyst and that it allows the cor-
responding imine to be obtained without producing other
byproducts such as styrene, ethylbenzene, or N-(1-phenyl-
AHCTUNGTREGeNNUN thyl)aniline (the hydrogenated imine). However, this reac-
tion is particularly complex due to other possible additional
side transformations, such as the hydrolysis of the imine to
provide aniline and acetophenone.[73] In contrast to the re-
sults with imines coming from anilines and benzaldehydes,
the presence of a methyl group adjacent to the nitrogen
atom has a strong effect on the reactivity of the N=C bond
and favors the attack of nucleophiles such as water (see
Scheme 8).
In conclusion, we have shown that the use of gold-based
bifunctional catalysts can open a new preparative route that
allows the chemoselective synthesis of substituted imines, as
well as the synthesis of the corresponding amines, and b-
amino carbonyl compounds, by starting directly from substi-
tuted nitroaromatics, through a cascade-type reaction.
As a consequence, the conversion decreases and so does
the selectivity, owing to the formation of acetophenone by
hydrolysis of the imine. Indeed, evidence for the formation
Synthesis of substituted imines involving nitroaromatic com-
pounds and alkynes: We have also considered a more com-
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Cascade Reactions on Gold Catalysts
FULL PAPER
Table 3. Catalytic results during the synthesis of substituted imines from nitroaromatics and alkynes by using
Au/TiO2 catalysts (see the Supporting Information for the corresponding feeding compositions).
to 83%. Improved selectivities
were also observed with other
substituted nitrobenzenes (for
example, 4-nitroanisole). This
unprecedented cascade-type re-
Entry
Product
T
[8C]
PH
%Au[a]
t
Conversion
[%][b]
Selectivity
[%][c]
2
[bar]
[h]
1
120
120
120
2
5
5
0.50
9
55
50
62
55
83
83
action represents
a powerful
synthetic route for the prepara-
tion of substituted imines that
combines the exceptional prop-
erties of gold for activating
nitro groups and triple bonds,
which allows the design of new
alternative routes for the syn-
thesis of substituted imines and
related derivatives.
10
10
11
2[d]
0.50
0.50
4.8
9
3[d]
[a] Mol Au/mol nitroaromaticꢃ100. [b] Based on aniline after complete exhaustion (>95%) of the nitroaro-
matic. [c] Loss of selectivity due to formation of acetophenone by hydrolysis of the imines with water. [d] Au/
TiO2 catalyst doped with Na.
Conclusion
Gold catalysts have been ap-
plied to design cascade-type reactions for the synthesis of
different nitrogen-based compounds. Firstly, substituted
imines have been successfully prepared from nitroaromatic
compounds and aldehydes, with results showing that the
method is efficient even for synthesizing a,b-unsaturated
imines. In addition, unknown properties of gold to reduce
selectively the imine group in the presence of sensitive func-
tional groups such as double bonds have been discovered,
which opens up the possibility of easily preparing substitut-
ed secondary aromatic amines through a cascade-type reac-
tion. We have also shown that the Au/TiO2 system can oper-
ate as a bifunctional catalyst for preparing substituted b-
amino carbonyl compounds from nitroaromatics and a,b-un-
saturated carbonyls. In this matter, while the gold sites pro-
mote the in situ formation of anilines by hydrogenation of
the nitroaromatic, the intrinsic acidity of the Ti species on
the support surface accelerates the subsequent Michael ad-
dition of the conjugated double bond. Finally, the gold-cata-
lyzed reaction between amines and alkynes to prepare
imines has been performed in a one-pot reaction by starting
directly from nitrobenzenes. The Au/TiO2 catalyst is shown
to be able to activate simultaneously the nitro group (hydro-
genation) and the triple bond (coupling reaction) without
losing selectivity due to the generation of other hydrogena-
tion byproducts. Moreover, we have found an appropriate
method to avoid other undesired side reactions, such as the
hydrolysis of the formed imine to acetophenone, by control-
ling the surface acidity of the support.
Scheme 8. Hydrolysis of imines in the presence of Lewis acids.
of acetophenone from this type of imines has been found, in
the presence of the Au/TiO2 catalyst, even when low con-
centrations of water are present.[73] This hydrolysis reaction
can be completely avoided in most cases by simply drying
the reactants, the solvent, and the catalyst before the reac-
tion. However, when the synthetic route in Scheme 7 is con-
sidered, water is formed as a byproduct during the nitroaro-
matic hydrogenation, which makes it much more difficult to
stabilize the N=C group. In this situation, only 55% conver-
sion of aniline (after complete exhaustion of nitrobenzene)
is obtained, and only 55% remains as the desired imine,
with acetophenone being the main byproduct, together with
small amounts of the hydrogenated imine.
Removal of the water formed during the reaction is not
an easy task under our reaction conditions, so we thought
about modifying the catalyst to avoid the hydrolysis reac-
tion. Thus, considering that hydrolysis reactions are usually
catalyzed by acids or bases,[35] we have attempted to reduce
the rate of hydrolysis by decreasing the acidity of the Au/
TiO2 system. To do that, the surface of the TiO2 has been
doped with a controlled amount of sodium acetate (see the
Experimental Section), and the resultant sample was tested
in the reaction between nitrobenzene and phenylacetylene
under H2 pressure. A significant increase of the selectivity
for the imine was observed, from an imine/ketone ratio of
ꢀ1 with the undoped Au/TiO2 sample to ꢀ5 for the
sodium-containing Au/TiO2 catalyst (Table 3, entry 2). Con-
sequently, selectivity for the desired imine was increased up
Experimental Section
Catalysts preparation: The gold catalyst used in the present work consists
of 1.5 wt% gold on TiO2, which can be supplied by the World Gold
Council (reference catalysts Type A) or, otherwise, could be prepared by
depositing the gold on TiO2 (P25 Degussa) from an aqueous solution of
HAuCl4. The deposition–precipitation procedure is done under stirring at
708C and with a pH value of 7, with NaOH used to maintain this pH
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A. Corma et al.
value for 2 h. Under these conditions, 80% of efficiency in the deposition
of gold is expected. The catalyst is then recovered from the solution by
filtering, washed with deionized water, and dried at 1008C overnight. Fi-
nally, the powder is calcined at 4008C in an air flow for 4 h. By using this
procedure, gold nanoparticles that present an average diameter of 3.5 nm
are deposited on the TiO2, as determined by TEM (see Figure S1 in the
Supporting Information).
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Received: April 3, 2009
Published online: July 16, 2009
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www.chemeurj.org
8203