Gold supported on a mesoporous CeO2 matrix as an efficient catalyst in the selective aerobic oxidation of aldehydes in the liquid phase

Article abstract of DOI:10.1039/b506685a

Gold supported on CeO₂ catalyses the selective aerobic oxidation of aliphatic and aromatic aldehydes better than other reported catalysts such as Pt/C/Bi materials; the activity is due to the nanometric particle size of Au and CeO₂. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506685a

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COMMUNICATION
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Gold supported on a mesoporous CeO2 matrix as an efficient catalyst in
the selective aerobic oxidation of aldehydes in the liquid phase{
Avelino Corma* and Marcelo E. Domine
Received (in Cambridge, UK) 12th May 2005, Accepted 23rd June 2005
First published as an Advance Article on the web 19th July 2005
DOI: 10.1039/b506685a
For comparison, Au/Fe2O3 [#02-03-WG-C] and Au/TiO2 [#02-
04-WG-C] samples from the World Gold Council as Au supported
European standard catalysts were tested.
Gold supported on CeO2 catalyses the selective aerobic
oxidation of aliphatic and aromatic aldehydes better than other
reported catalysts such as Pt/C/Bi materials; the activity is due
to the nanometric particle size of Au and CeO2.
All catalysts were analysed by available spectroscopic techniques
and by N2 adsorption and chemical methods. The physical and
textural properties of the solid catalysts used in this work are
summarized in Table 1.
A wide range of aliphatic and aromatic aldehydes can be oxidised
to carboxylic acids under mild reaction conditions using H2O2,
peracids, NaOCl, or O3 as oxidants.1,2 Molecular oxygen in
combination with Mn(II), Cu(II), Co(II) or Ni(II), in the form of
salts or in organic complexes, are efficient homogeneous systems
for the catalytic oxidation of aldehydes to acids at the laboratory3
and industrial scales.4
The homogeneous metal catalysts were: Co(II) chloride (Aldrich,
97%), Mn(II) acetate tetrahydrate (Panreac, puriss.), Ni(II) acetate
tetrahydrate (Aldrich, 98%), Co(II)-acetylacetonate (Aldrich, 97%),
Mn(II)-acetylacetonate (Aldrich, 97%), Ni(II)-acetylacetonate
(Aldrich, 95%).
The catalytic reactions were carried out in a glass flask batch
reactor equipped with a magnetic stirrer and a condenser, and
immersed in a silicon oil bath with temperature control. In a
typical experiment, 0.45 g (3.9 mmol) of n-heptanal, 3.5 g of
acetonitrile as solvent, and 0.035 g of n-decane as an internal
standard were mixed in the flask under stirring until the reaction
temperature (25 or 50 uC) was reached. At this time, 0.05 g of
the solid catalysts (0.01–0.015 g of homogeneous metallic
catalysts) were added (time zero). Then, a continuous flow of air
(1–2 ml min21 approx.) was bubbled into the reaction mixture at
atmospheric pressure. Small aliquots were taken at selected
reactions times. The liquids obtained were filtered off and analysed
by GC, GC-MS and HPLC-MS.
With heterogeneous catalysts, high yields of carboxylic acids
have already been reported in the liquid oxidation of aromatic
aldehydes with O2 (or air) at atmospheric pressure using an
organic solvent and with Pd or Pt supported on carbon.5,6 In this
case, the presence of Bi as co-catalyst enhances the activity of Pt or
Pd. However, leaching of the Bi occurs that, together with
environmental issues, results in an irreversible deactivation of the
catalyst.6
After the seminal work of Bond and Sermon,7 Hutchings8 and
Haruta et al.,9 gold has irrupted into the field of homogeneous and
heterogeneous catalysis, showing interesting properties for liquid
and gas phase oxidations,10–12 WGS reaction,13 decomposition
of NOx with hydrocarbons,14 selective hydrogenations,15 and also
C–C bond formation with boronic acid.16
Results from Fig. 1a show that the nanocrystalline CeO2 has
some activity for the oxidation of n-heptanal, but the selectivity to
the acid is slightly lower than for the thermal oxidation (Fig. 1b).
When gold was deposited on nanocrystalline CeO2, the initial rate
of the reaction increased by a factor of 3, with selectivity ¢95%.
When the gold catalyst was prepared on meso-structured CeO2,
Here we present that Au supported on CeO2 nanoparticles and
meso-structured CeO2 nanoparticles are highly active and selective
catalysts for the oxidation of aliphatic and aromatic aldehydes to
the corresponding carboxylic acids. Their activity and selectivity is
higher than with soluble transition metal catalysts, and better than
those reported using Pt/C/Bi catalysts for the industrially relevant
oxidation of 4-iso-propylbenzaldehyde into 4-iso-propylbenzoic
acid, which is an important intermediate for the production of
nateglinide, a chemical used for the treatment of diabetes.
Synthesis of regular-sized CeO2 was performed starting from
Ce(NO3)3 by following a well known procedure (see supplemen-
tary information{). Nanocrystalline CeO2 and meso-structured
CeO2 were prepared as in refs. 17 and 18. Deposition of the Au
particles on CeO2 supports was performed by a deposition–
precipitation method12 (see also supplementary information{).
Table 1 Physical and textural properties of different Au supported
catalysts
Surface area
Metal particle
Catalyst
wt.% Aub (BET)/m2 g21 size/nmc
CeO2
CeO2 nano
Meso CeO2 nano
—
—
—
170
215
225
—
5
—
Au/CeO2 precipitated 4.65
Au/CeO2 nano 2.30
Meso Au/CeO2 nano 5.00
145
163
152
4.5
,4
,4
a
Instituto de Tecnolog´ıa Qu´ımica (CSIC-UPV), Universidad Polite´cnica
de Valencia, Avda. Los Naranjos s/n, 46022, Valencia, Spain.
E-mail: acorma@itq.upv.es; Fax: +34 96 3877809; Tel: +34 96 3877800
{ Electronic supplementary information (ESI) available: Synthesis of
regular-sized CeO2, and deposition of Au on a CeO2 matrix. See http://
dx.doi.org/10.1039/b506685a
Au/Fe2O3
Au/TiO2
5.00
1.50
—
—
—
—
a
a
b
World Gold Council standard catalyst. Measured by X-ray
c
fluorescence spectroscopy (XRF). Calculated as the medium value
from transmission electron microscopy data (TEM).
4042 | Chem. Commun., 2005, 4042–4044
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and the initial rate was normalized with respect to the Au content
(See Table 1), it can be seen that catalytic activity is similar to that
of nanocrystalline Au/CeO2. Therefore, we do not see in this case
any benefit of potential reactant confinement within the regular
pores of the meso-structured catalyst. It has to be noticed (Fig. 1a)
that the oxidation activity of gold deposited on conventionally
precipitated CeO2 gives lower activity than on nanocrystalline
CeO2, despite the fact that the size of the Au particle is very similar
(#4 and 4.5 nm, respectively). The differences in catalytic activity
between these two Au/CeO2 catalysts can be based on the different
oxygen activated species formed on the nanocrystalline and
amorphous CeO2.16,19
TiO2, as well as with soluble Co, Mn, and Ni catalysts, on the basis
of the same amount of metals added (Figs. 2 and 3, respectively).
Results from Fig. 2 and 3 indicate that Au on a nanocrystalline
CeO2 support is the most active and selective catalyst. The
differences in catalytic activities were maintained when the
reaction temperature was dropped to 25 uC. At these reaction
conditions, the TON ratio calculated between the gold on ceria
and Co(II) acetylacetonate homogeneous catalyst was around
6 (TON ratio 5 240/41, from Table 2). Furthermore, after
reacting, filtering and washing the Au/CeO2 catalyst with a
mixture of acetonitrile–acetone (50 : 50), the conversion and
selectivity were maintained even after three consecutives re-uses of
From the point of view of selectivity, gold on both nanocrystal-
line and meso-structured nanocrystalline CeO2 give the highest
values (¢95% selectivity for #100% conversion). It has to be
taken into account that selectivity to heptanoic acid is generally
very high, even in the absence of catalyst. The competing reactions
observed are the formation of dimers, decarboxylation and
oxidation of secondary carbon atoms. In the case of gold on
nano and meso CeO2 the rate of oxidation of aldehyde is
enhanced, with respect to competing reactions, when compared
with the non catalyzed reactions. Thus, the selectivity to dimers,
decarboxylation and oxidation of secondary carbons decreases.
The much lower selectivity to acid achieved with gold on
precipitated CeO2 is remarkable. This is mainly due to the
formation of a large amount of dimers and trimers (selectivity
30%) on this catalyst.
Fig. 2 Comparative catalytic activity of Au metal supported on different
inorganic matrices in the air oxidation reaction of n-heptanal at 50 uC
during 3.5 h.
We have compared the conversion and selectivity of the best
Au/CeO2 catalysts with those of gold deposited on Fe2O3 and
Fig. 3 Comparative catalytic activity (yield of carboxylic acid, mol.%) of
Au/CeO2 and different metallic homogeneous catalysts for air oxidation of
n-heptanal at 50 uC during 3.5 h.
Table 2 Comparison of catalytic activity between Au/CeO2 (meso-
nano) material and Co(II)-acetylacetonate catalysts in the n-heptanal
air oxidation under different reaction conditions
Without
catalyst Au/CeO2 (meso-nano) Co(II)–acac.
Reaction
conditionsa
Yield Yield
( mol.%) ( mol.%)
Yield
( mol.%) TONb
TONb
50 uC
37.0
,12.0
1.8
93.6 (88.9)e 287 (273)e 81.8
54
41
30
57
25 uC
78.1
1.9
82.7
240
6
254
61.9
46.2
85.6
With BHTc
With AIBNd
a
40.0
Reaction conditions: 3.9 mmol of aldehyde, 3.5 g of acetonitrile,
Fig. 1 (a) Conversion of n-heptanal in the catalytic air oxidation reaction
over different heterogeneous metal catalysts at 50 uC during 6 h. (b)
Selectivity to n-heptanoic acid in the catalytic air oxidation reaction of
n-heptanal over different heterogeneous metal catalysts at 50 uC during 6 h.
b
0,05 g of catalyst, during 3.5 h. Calculated as mols of carboxylic
c
acid/mols of metal. With 6 mg BHT. With 8 mg of AIBN.
d
e
Values obtained after 3 consecutive re-uses of the catalyst.
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the solid, whereas negligible leaching of the metal to the liquids
was observed (Table 2). Particularly noteworthy is that the
Au/CeO2 catalyst shows excellent activity (#70% of conversion
and .73% of selectivity to acid, time 5 6 h) for aerobic oxidation
of aldehydes in the absence of acetonitrile as solvent. Under these
reaction conditions most gold supported materials failed to
catalyse oxidation.
Table 3 Selective aerobic oxidation of different type of aldehydes
over Au/CeO2 (meso-nano) catalyst at 65 uC during 3.5 ha
Conversion Selectivity to Yield
(mol.%)
Aldehyde
Acid (mol.%) (mol.%)
Dihydro cyclocitral
Cinammaldehyde
92.0
40.7
70.0
77.5
94.7
93.2
—
64.4
31.5
86.2
83.7
74–76
4-Iso-propyl-benzaldehyde 91.0
4-Iso-propyl-benzaldehydeb 89.9
In the case of aerobic oxidation reactions mediated by transition
metal homogeneous or heterogeneous catalysts, a radical or an
ionic reaction mechanism has been proposed when working with
organic or aqueous solvent, respectively.10 In the case of Au/CeO2
working in organic media, we have seen (Table 2) that the
introduction of a radical inhibitor (2,6-di-tert-butyl-4-methylphe-
nol, BHT) strongly diminishes the oxidation of n-heptanal. On the
other hand, the presence of a radical initiator, such as a,a9-azo-iso-
butyronitrile (AIBN) has no effect on conversion. Taking into
account that the amount of radical inhibitor is not enough to
totally inhibit the reaction on Au/CeO2 catalyst, it appears
reasonable to think that the oxidation of n-heptanal to n-heptanoic
acid by air takes place through a radical mechanism. Nevertheless,
these results are not conclusive and further research is needed to
confirm the oxidation pathway.
4-Iso-propyl-benzaldehydec
—
a
Reaction conditions: 3.9 mmol of aldehyde, 3.5 g of acetonitrile,
0.05 g of catalyst. After the 2nd re-use of the catalyst. Data from
b
c
ref. 5 with Pt (Bi)/C catalyst (55–75 uC).
Financial support by the Spanish MAT 2003-07945-C02-01 is
gratefully acknowledged. M. E. D. thanks the ITQ for its doctoral
fellowship.
Notes and references
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A2, 2003.
6 M. Besson and P. Gallezot, Catal. Today, 2000, 57, 127.
7 G. C. Bond and P. A. Sermon, Gold Bull., 1973, 6, 102.
8 G. J. Hutchings, Gold Bull., 1996, 29, 123.
9 M. Haruta, S. Tsubota, T. Kobayashi and S. Ijima, J. Catal., 1989, 115,
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12 S. Carretin, P. Concepcio´n, J. M. Lopez Nieto, V. F. Puntes and
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Different types of both saturated and unsaturated aliphatic
aldehydes of industrial interest can be converted to the
corresponding mono-carboxylic acids over our Au/CeO2 in the
presence of air. The results given in Table 3 show the versatility of
the gold catalyst, offering a high conversion level (.90%) for
aliphatic aldehydes. In the case of an unsaturated aldehyde, such
as cinammaldehyde, high selectivity to carboxylic acid (77.5%) at
moderated conversion was achieved.
We have applied the Au/CeO2 catalyst to carry out the
oxidation of 4-iso-propylbenzaldehyde to cumic acid (4-iso-
propylbenzoic acid), which is an intermediate for the manufactur-
ing of nateglinide. The reaction was carried out at similar reaction
conditions as reported in the patent literature5 when using Pt/C as
catalyst and Bi as co-catalyst. The results given in Table 3 show
that Au/CeO2 is able to perform the reaction with air, at 65 uC and
atmospheric pressure, giving higher conversion and selectivity than
the reported catalyst.
16 S. Carretin, J. Guzman, A. Corma and H. Garc´ıa, Angew. Chem., Int.
Ed., 2005, 44, 15, 2242; C. Gonza´lez-Arellano, A. Corma, M. Iglesias
and F. Sa´nchez, Chem. Commun., 2005, 15, 1990.
In conclusion, nanoparticulated Au on nanocrystalline or on
meso-structured nanocrystalline CeO2 supports represent an
alternative to actual metal soluble or heterogeneous catalysts for
the selective oxidation of aldehydes to carboxylic acids by air,
under mild reaction conditions.
17 A. Corma, J. Y. Chane-Ching, M. Airiau and C. Martinez, J. Catal.,
2004, 224, 2, 441.
18 J. Y. Chane-Ching, F. Cobo, D. Aubert, H. G. Harvey, M. Airiau and
A. Corma, Chem. Eur. J., 2005, 11, 3, 979.
19 J. Guzman, S. Carretin and A. Corma, J. Am. Chem. Soc., 2005, 127,
10, 3286.
4044 | Chem. Commun., 2005, 4042–4044
This journal is ß The Royal Society of Chemistry 2005