Gold nanoparticles supported on ceria promote the selective oxidation of oximes into the corresponding carbonylic compounds

Article abstract of DOI:10.1016/j.jcat.2009.10.005

Gold supported on ceria (Au/CeO₂) is a highly active and selective catalyst for the aerobic oxidation of oximes to the corresponding carbonylic compounds. This reaction can be carried out in a mixture of ethanol- water by oxygen with a complete selectivity and very high conversion using Au/CeO₂ as a catalyst. This process appears to be general and aliphatic as well as oximes from aromatic ketones can be converted to the carbonylic compound in the absence of corrosive Broensted acids and without producing aqueous wastes. One example of particular industrial relevance is the transformation of carvone oxime into carvone.

Full text of DOI:10.1016/j.jcat.2009.10.005

Journal of Catalysis 268 (2009) 350–355
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Gold nanoparticles supported on ceria promote the selective oxidation
of oximes into the corresponding carbonylic compounds
*
*
Abdessamad Grirrane, Avelino Corma , Hermenegildo Garcia
Instituto Universitario de Tecnología Química CSIC-UPV, Universidad Politécnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold supported on ceria (Au/CeO2) is a highly active and selective catalyst for the aerobic oxidation of
oximes to the corresponding carbonylic compounds. This reaction can be carried out in a mixture of eth-
anol–water by oxygen with a complete selectivity and very high conversion using Au/CeO2 as a catalyst.
This process appears to be general and aliphatic as well as oximes from aromatic ketones can be con-
verted to the carbonylic compound in the absence of corrosive Brönsted acids and without producing
aqueous wastes. One example of particular industrial relevance is the transformation of carvone oxime
into carvone.
Received 31 July 2009
Revised 1 October 2009
Accepted 2 October 2009
Available online 4 November 2009
Keywords:
Heterogeneous catalysis
Gold nanoparticles as catalysts
Aerobic oxidation
Oximes
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
can render the corresponding carbonylic compound through the
intermediacy of the oxetane derivative [10,11].
Oximes are most generally obtained from carbonylic com-
pounds by condensation with hydroxylamine [1], but another
route through the hydrogenation of nitroalkanes has also been pro-
posed that avoids the use of hydroxylamine [2]. When oximes are
prepared by methods others than those requiring carbonyl com-
pounds as the starting materials, oximes can be used in turn as
synthetic precursors of the corresponding carbonyl compounds.
One particular case that exemplifies the above-mentioned route
of ketones from oximes is the commercial synthesis of carvone,
which is an essential oil used in fragrance industry [3]. D-Carvone
has a caraway odor and L-carvone has a sweet spearmint odor and
is the main constituent of spearmint [4]. Carvones can be synthe-
sized from carvone oxime that is prepared from limonene by reac-
tion with nitrosyl chloride [5,6]. Scheme 1 shows the synthetic
route for the preparation of carvone.
Generally, compounds having a C@N double bond undergo
hydrolysis to the corresponding carbonylic compounds [1]. How-
ever, compared to imines and other C@N double bond compounds,
oximes are typically stable and notably reluctant to undergo
hydrolysis. An alternative to the conventional hydrolysis of oximes
to carbonylic compounds under acid or basic conditions is the oxi-
dation of the C@N bond. It has been reported that metal oxides,
peracids, ozone, and other chemical oxidizing reagents can convert
oximes into the corresponding ketones through an oxidative
mechanism [7–9]. Also the photochemical oxidation of oximes
From the environmental point of view, the use of oxidizing re-
agents that do not generate wastes constitutes a considerable
advantage. This requires the shift from the use of stoichiometric
oxidizing reagents to catalytic oxidations [12–15]. In this context,
it has been reported that titanium silicalite TS-1 is a heterogeneous
catalyst that can promote the oxidation of oximes to carbonylic
compounds using hydrogen peroxide [16]. Although the by-prod-
uct of hydrogen peroxide is water, it is always more advantageous
to use oxygen instead of H2O2 as an oxidizing reagent. In addition
TS-1 is a medium pore size zeolite (ꢀ0.54 nm) and there is a limit
in the molecular dimensions of the substrates that can undergo
oxidation.
Herein we report that supported gold nanoparticles can be effi-
cient solid catalysts to promote the aerobic oxidation of oximes to
the corresponding carbonylic compounds in the absence of acids.
The use of gold nanoparticles as catalysts has attracted consid-
erable attention in the recent years [17–19]. It has been found that
gold is an excellent catalyst for the aerobic oxidation of alcohols
and amines among other compounds [20–22]. In the present work,
we expand the scope of gold nanoparticles as catalysts for aerobic
oxidations by showing that they are general catalysts to perform
the selective transformation of oximes into carbonylic compounds.
2. Experimental section
2.1. Catalyst preparation
Nanoparticulated ceria was obtained as reported [23] by the
hydrolysis of cerium nitrate aqueous solutions at pH 5 using
* Corresponding authors. Fax: +34 963877809 (H. Garcia).
E-mail address: hgarcia@qim.upv.es (H. Garcia).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.10.005

A. Grirrane et al. / Journal of Catalysis 268 (2009) 350–355
351
Cl
N
N
O
OH
OH
Carvoxime
(+)-Limonene
Carvone
Limonene Nitrosochloride
Scheme 1. Synthesis of carvone from limonene.
ammonia as a base. The colloid was purified by dialysis. Au was
deposited on the nanoparticulated cerium oxide by the following
procedure: a solution of 300 mg of HAuCl4ꢁ3H2O in 50 ml of deion-
ized water was brought to pH 10 by the addition of a solution of
NaOH 0.2 M. Once the pH value was stable the solution was added
to a slurry containing colloidal CeO2 (10 g) in H2O (80 ml). After
adjusting the pH with NaOH 0.2 M, the slurry was left under vigor-
ous stirring for 18 h at room temperature. The Au/CeO2 solid was
then filtered and exhaustively washed with distilled water until
no traces of chlorides were detected by the AgNO3 test. This is an
important treatment since traces of Clꢂ remain strongly bonded
to gold and are highly detrimental for the overall activity. The cat-
alyst was dried at room temperature under vacuum. Finally the Au/
CeO2 was activated by reduction by pouring the solid into boiling
1-phenylethanol for 2 h. After recovering, washing, and drying
the solid, it was ready to be used as a catalyst. The total Au content
of the final catalyst was 0.72% as determined by chemical analysis.
The average particle size of the gold nanoparticles as determined
by statistical analysis of a sufficiently large number of particles
was 3.97 nm. A second Au/CeO2 catalyst containing higher gold
loading (1.85 wt.%) was prepared following exactly the above-
mentioned procedure, but using 750 mg of HAuCl4. The average
particle size of this Au(1.85)/CeO2 catalyst was 5.45 nm.
673 K in air for 4 h. Following this procedure, 3.5 nm gold nanopar-
ticles supported on TiO2 are obtained.
Pt/TiO2 (5 wt.%) catalyst is prepared by the impregnation of 2 g
of TiO2 (Degussa P25, 10 g, SBET = 55 m2 gꢂ1) with a solution of
H2PtCl6ꢁ6H2O (Aldrich) in 7 ml of H2O (milliQ). The slurry was stir-
red for 2 h at room temperature, then all the liquid was evaporated
and the solid was dried at 373 K overnight and reduced with 1-
phenyletanol at 433 K for 2 h. The catalyst was then washed, fil-
tered, and dried at room temperature for 12 h. The final Pt content
was found to be 5 wt.% by atomic absorption analysis.
The Pd/TiO2 catalyst containing 1.81 wt.% of palladium was pre-
pared by the impregnation of 2 g of TiO2 (Degussa P25, 10 g,
SBET = 55 m2 gꢂ1) with a solution of 104 mg of PdCl2 (Aldrich, 60%
purity) in a mixture of 5 ml of H2O (milliQ) and 5 ml of acetone.
The slurry was stirred for 3 h at room temperature. Then, the liquid
phase was evaporated and the solid was then washed, filtered, and
dried at 373 K overnight. The catalyst was activated by reduction
under H2 flow (50 ml minꢂ1) at 473 K for 2 h. The final Pd content
on the solid was determined by quantitative atomic absorption
analysis.
The Au/C catalyst consists of 0.8 wt.% gold on active carbon and
was supplied by the World Gold Council (reference catalysts, Type
D).
The Au(0.72%)Pd(0.15%)/CeO2 sample was prepared by stirring
a suspension of 2 g of 0.72% Au/CeO2 in acetone (150 ml) contain-
ing PdCl2(PhCN)2 (2.5 ꢃ 10ꢂ4 M), the slurry was left under vigorous
stirring for 4 h at room temperature. The solid was then filtered,
exhaustively washed with distilled water, and dried at 373 K over-
night. Then Au(0.72%)Pd(0.15%)/CeO2 catalyst was activated by the
reducing the solid with 1-phenyletanol at 433 K for 2 h. The cata-
lyst was then washed, filtered, and dried at room temperature
for 12 h. The final Pd content was 0.15 wt.% as determined by
quantitative atomic absorption analysis.
Using also as support nanoparticulated ceria, two other sup-
ported noble metal catalysts were prepared. Pd/CeO2 and Pt/CeO2
catalysts containing 0.73 and 0.70 wt.% of Pd or Pt, respectively.
These catalysts were prepared by impregnation of 2 g of nanopar-
ticulated CeO2 with a H2O (5 ml)/acetone (5 ml) solution of 42 mg
of PdCl2 (Aldrich, 60% purity) or an aqueous solution (10 ml) of
45 mg of H2PtCl6ꢁ6H2O (Aldrich). The resulting slurry was stirred
for 3 h at room temperature. Then, the liquid phase was evapo-
rated and the solid was dried at 373 K overnight. The samples were
activated by reduction with excess of 1-phenyletanol at 433 K for
3 h. The catalyst was then washed, filtered, and dried at 373 K
for 12 h. The Pd or Pt content present on the solids was determined
by quantitative atomic absorption spectroscopy.
The Pt/C catalyst consists of 5 wt.% gold on active carbon and
was supplied from Sigma–Aldrich Company.
2.2. Catalytic experiments
Catalytic experiments were performed in reinforced glass semi
continuous reactors equipped with temperature and pressure con-
trollers. For each reaction, a 2-ml mixture of reactants and solvent
was placed into the reactor (3 ml capacity) together with appropri-
ate amount of catalyst. Using toluene as a solvent care has to be ta-
ken to avoid vapor mixtures within the explosion range. Working
under the experimental conditions 373 K and 5 bars of oxygen,
the composition of the vapor phase is outside the explosion mix-
ture range.
All the reactants used in this study, except carvoxime 5, are
commercially obtained from Sigma–Aldrich Company with purities
higher than 95%. Compound 5 was synthesized from carvone and
hydroxylamine as described in the literature [24], but using only
methanol as a solvent. Carvoxime 5 was obtained in 85% yield as
white crystals. Anal. Calc. for C10H15NO: C, 72.69; H, 9.15; N,
8.48. Found: C, 72.39; H, 9.48; N, 8.42. The 1H and 13C NMR spectral
data of compound 5 agree with those reported in the literature
[25,26].
The Au/TiO2 catalyst consists of 1.5 wt.% gold on TiO2 and was
supplied by the World Gold Council (reference catalysts, Type A).
It can also be prepared by depositing the gold from an aqueous
solution of HAuCl4 (Alfa Aesar) on a sample of TiO2 (P25 Degussa).
The deposition precipitation procedure is done at 343 K and pH 9
for 2 h, using (0.2 M) NaOH to maintain the pH constant. Under
these conditions, gold deposition occurs with 80% efficiency. The
catalyst is then recovered, filtered, washed with deionized water,
and dried at 373 K overnight. Finally, the powder is calcined at
In the cases in which no water was used as a solvent, conversion
and yields were estimated using dodecane as an internal standard.
After sealing the reactor, air was purged by filling the reactor with
oxygen (5 bar) and pumping out three times before final pressuri-
zation of the reactor with O2 at 5 bar. The reactor was deeply intro-
duced into a silicone bath that was preheated at the reaction
temperature. During the experiment, the pressure was maintained
constant and the reaction mixture was magnetically stirred at
1000 rpm. Aliquots were taken from the reactor at different reac-

352
A. Grirrane et al. / Journal of Catalysis 268 (2009) 350–355
tion times. Once the catalyst particles were removed from the solu-
tion by centrifuging at 12,000 rpm, the products were identified by
GC–MS and also by comparing the retention time with commercial
(Sigma–Aldrich Company) pure samples when available. When
mixtures of water–ethanol were used as a solvent, quantification
was performed adding a known weight of dodecane as an external
standard to aliquots of the reaction mixtures diluted in dichloro-
methane. Only experiments with mass balances P95% were
considered.
supported on nanoparticulated ceria. Using this Au (0.72 wt.%)/
CeO2 material as a catalyst in toluene, conversion and selectivity
increased substantially to high values (entry 6 in Table 1). Further-
more, since water has been found to be a suitable solvent for aer-
obic alcohol oxidation using ceria supported gold, and considering
that in the transformation of the oxime into ketone a hydrolysis
pathway could contribute to the formation of the target cyclohex-
anone, we also performed the reaction using Au (0.72 wt.%)/CeO2
as a catalyst in a mixture of ethanol and water. The results show
that in the presence of water the conversion was increased to very
high values with essentially complete selectivity toward cyclohex-
anone 2 at very short times (entry 7 in Table 1). Even more, the
reaction time can be substantially reduced if the reaction temper-
ature is increased to 130 °C (entry 8 in Table 1).
3. Results and discussion
In the first stage of our work we selected cyclohexanone oxime
(1) as a model compound and studied the aerobic oxidation of this
oxime 1 in the presence of the commercially available titania-sup-
ported gold nanoparticles [Au (1.5 wt.%)/TiO2] in various solvents.
The results presented in Table 1 show that although reactant 1 is
partially converted using acetonitrile or toluene as solvents (en-
tries 1 and 2 in Table 1), the conversion achieved at long reaction
time was low. Besides this it can be seen that after an initial period,
the reaction stops and the conversion measured at 3 and 23 h in
toluene using Au (1.5 wt.%)/TiO2 was identical (compare entries 2
and 3 in Table 1). This indicates deactivation of the catalyst. Other
commercial gold catalysts such as Au (0.8 wt.%)/C were more ac-
tive promoting the disappearance of the oxime 1, although the
selectivity to the ketone was very low (entry 4 in Table 1). Gold
was not the only noble metal exhibiting activity toward this aero-
bic oxidation. Platinum nanoparticles supported on active carbon,
working in toluene also exhibit a behavior similar to that of gold
(entry 5 in Table 1), i.e. they are able to convert the oxime 1, but
again with unsatisfactorily low selectivity toward cyclohexanone
(entry 5 in Table 1). The reason for the low selectivity toward
cyclohexanone 2 was the formation of other oxidation products
accompanying cyclohexanone (see footnote ‘‘a” in Table 1).
We tested the catalytic activity of the nanoparticulated ceria
used as support without gold. In related precedents [28], it has
been observed that CeO2 nanoparticles already exhibit some activ-
ity that is further promoted by the presence of supported gold
nanoparticles that increase activity and selectivity. In the present
case it was also observed that nanoparticulated CeO2 has a notable
activity toward oxidation (entry 9 in Table 1). However, the pres-
ence of gold more than doubles the activity and makes the reaction
totally selective toward the target cyclohexanone. This is attrib-
uted to the influence of gold nanoparticles increasing the oxygen
vacancies and structural defects already inherent to nanoparticu-
lated ceria [29].
Finally, other more elaborated catalysts, as those based on core–
shell alloys of gold (core) and palladium (shell) supported on
nanoparticulated ceria were also efficient even in toluene as a sol-
vent (entry 10 in Table 1), but they have the drawback of a more
complex preparation. Thus, from the results shown in Table 1 it
can be concluded that the use of Au (0.72 wt.%)/CeO2 as a catalyst
and of ethanol/water as a solvent are suitable conditions to per-
form the aerobic oxidation of cyclohexanone oxime (1) to the cor-
responding ketone 2.
The Au (0.72 wt.%)/CeO2 catalyst acts as a heterogeneous cata-
lyst as it can be deduced from the fact that the conversion stops
when this solid catalyst is filtered from the reaction medium. Also
the Au (0.72 wt.%)/CeO2 was reused three times without observing
any decay in activity or selectivity for the oxidation of 1 into 2.
We have expanded the results achieved for compound 1 to
show the general applicability of the oxidation process to obtain
not only cycloalkanones but also ketones and, more importantly,
In the aerobic oxidation of alcohols [20] and benzyl amines [27],
it has been found that the nature of the support plays an important
role influencing the catalytic activity of the supported Au nanopar-
ticles. For those transformations as well as for the low-temperature
CO oxidation and other reactions [17–19], a comparison among
different carriers has established that the use of cerium nanoparti-
cles as a gold support boosts the activity of gold nanoparticles or
vice versa. Therefore, an obvious catalyst to be tested will be gold
Table 1
Conversion and selectivity toward cyclohexanone (2) for the oxidation of cyclohexanone oxime (1) by oxygen in the presence of a series of supported gold catalysts. Reaction
conditions: compound 1: 1 mmol, solvent: 2 ml, oxygen pressure: 5 bar, Au/substrate ratio 1 mol%.
OH
N
O
O2
Au catalyst/solvent
1
2
Run
Catalyst
Time (h)
Solvent
T (°C)
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
10
Au (1.5 wt.%)/TiO2
Au (1.5 wt.%)/TiO2
Au (1.5 wt.%)/TiO2
Au (0.8 wt.%)/C
23
3
23
2
6
6.5
1
0.5
1.1
5.5
Acetonitrile
Toluene
Toluene
Acetonitrile
Toluene
Toluene
Ethanol/water 1/1
Ethanol/water 1/1
Ethanol/water 1/1
Toluene
100
100
100
100
100
100
100
130
130
110
23
41
41
71
76
90
99
100
95
99
95
99
99
27a
23a
100
100
100
70a
98
Pt (5 wt.%)/C
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
CeO2
Au (0.72 wt.%)–Pd (0.15 wt.%)/CeO2
a
Formation of other products reaction for example, 1,2-cyclohexanedione and 2-cyclohexenone oxime.

A. Grirrane et al. / Journal of Catalysis 268 (2009) 350–355
353
also aldehydes. Since efficient aerobic oxidation of aldehydes to the
corresponding carboxylic acids using supported gold catalysts has
been reported [30], it is of interest to establish whether or not it is
possible to stop selectively the aerobic oxidation of an aldoxime at
the stage of aldehyde or if consecutive cascade reactions occur
leading to a more complex reaction mixture.
The results obtained with the aerobic oxidation of keto- and
aldoximes are shown in Table 2. As it can be seen there, Au
(0.72 wt.%)/CeO2 in a mixture of ethanol water (1:1) or toluene is
also efficient to promote the aerobic oxidation of oximes 3a and
3b to acetophenone (4a) and benzaldehyde (4b). The aerobic oxi-
dation of compound 3b to render benzaldehyde was particularly
easy and can take place in very short times (5 min), even in tolu-
ene. For this transformation we checked that water, even in trace
amounts, is not needed to produce the formation of the aldehyde,
since the reaction takes also place with dried toluene in the ab-
sence of moisture. Moreover, as it can be seen there, water as sol-
vent exerts a negative influence since in this case concurrent over-
oxidation of benzaldehyde to benzoic acid is observed in a large ex-
tent. This is in agreement with the fact that water favors the oxida-
tion of aldehydes probably through the intermediacy of their gem-
diols [30].
Although catalysts containing higher gold loading on the
nanoparticulated ceria support and working to the same oxime/
Au mol ratio are also efficient to promote the aerobic oxidation,
no advantages derived from the use of higher Au loadings were
apparent.
Table 2
Conversion and selectivity toward carbonylic compounds determined for the oxidation of a series of oximes in the presence of Au/CeO2 catalyst at two different Au loadings (0.72
and 1.85 wt.%). Reaction conditions: substrate 0.5 mmol, solvent 1 ml, oxygen pressure 5 bar, Au/substrate mol ratio 1%.
OH
R2
O2
O
N
a: R1 = Ph; R2 = Me
b: R1 = Ph; R2 = H
R1
4a,b
R2
R1
Au catalyst
3a,b
Oxime
Catalyst
Time (h)
Solvent
T (°C)
Product
Conversion (%)
Selectivity (%)
3a
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (1.85 wt.%)/CeO2
4
15
2
0.1
0.25
EtOH/H2O 1:1
Toluene
130
100
130
130
130
4a
4a
4b
4b
4b
99
85
98
98
93
100
100
58a
98
3b
EtOH/H2O 1:1
Tolueneb
Tolueneb
96a
a
Formation of benzoic acid in 42% selectivity was also observed.
Anhydrous toluene.
b
Table 3
Conversion and selectivity toward carvone 6 determined for the oxidation of carvoxime 5 with supported noble metal catalysts. Reaction conditions: substrate 0.5 mmol, solvent
1 ml, oxygen pressure 5 bar, metal/substrate mol ratio 1%.
N
O
OH
O2
catalyst/solvent
(5)
(6)
Catalyst
Time (h)
Solvent
T (°C)
Conversion (%)
Selectivity (%)
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (0.72 wt.%)/CeO2
Au (1.85 wt.%)/CeO2
Pt (0.7 wt.%)/CeO2
Pt (0.7 wt.%)/CeO2
Pt (0.7 wt.%)/CeO2
Pt (0.7 wt.%)/CeO2
Pd (0.73 wt.%)/CeO2
Pd (0.73 wt.%)/CeO2
Pd (0.73 wt.%)/CeO2
Pd (0.73 wt.%)/CeO2
Pd (1.81 wt.%)/TiO2
CeO2
20
20
7.5
5
5
5
6
5
8
8
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH
100
110
120
130
130
130
130
130
100
110
120
130
100
110
120
130
130
130
130
130
130
130
78
97
99
99
45
80
75
75
81
92
96
99
51
18
27
35
32
85
42
98
30
93
100
100
100
100
12a
10a
13a
100
100
100
100
100
100
100
100
100
100
69
CH3C„N
Toluene
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
EtOH/H2O 1:1
5
3.5
22
8
8
5
5
5
5
3.5
5
Au (1.5 wt.%)/TiO2
Pt (5 wt.%)/TiO2
TiO2
100
100
66
Zeolite(HB-20) Si/Al(13/1)
5.5
10a
a
Formation of other unidentified products, particularly two with molecular weights 281 and 341 Da was also observed.

354
A. Grirrane et al. / Journal of Catalysis 268 (2009) 350–355
As commented earlier in the introduction, the efficient conver-
5 into ketone 6 was not complete after 23 h. In contrast, the reac-
tion goes to completion in less than 5 h when the reaction temper-
ature is increased at 130 °C (see Fig. 1).
sion of carvone oxime (5) into carvone (6) constitutes an example
of industrial relevance in which oximes act as synthetic precursors
of ketones. The structure of carvoxime contains two tri-substituted
C@C double bonds that can also undergo oxidation to epoxides and
other oxygen containing products if the oxidation process does not
exhibit sufficient chemoselectivity toward oxime oxidation. Thus,
it was of interest to apply the gold supported catalyst for the con-
version of oxime 5 into carvone 6. The results obtained are summa-
rized in Table 3.
As it can be seen there, Au (0.72 wt.%)/CeO2 was also efficient to
selectively catalyze the aerobic oxidation of oxime 5 into carvone.
Ethanol and toluene were not suitable solvents to attain high con-
versions. Even the selectivity toward carvone decreases when tol-
uene is present due to the occurrence of reaction involving the C@C
double bonds. On the other hand, water was also not an appropri-
ate solvent since it does not dissolve carvoxime 5 at room temper-
ature. A 1:1 mixture of ethanol and water was found to be the most
adequate. At 100 °C in 1:1 ethanol–water, the conversion of oxime
Higher gold loadings on ceria play an adverse role, decreasing
the conversion. Thus, the activity of Au (1.85 wt.%)/CeO2 was lower
than that of Au (0.72 wt.%)/CeO2 under the same conditions (see
Table 3). Related precedents on the negative influence of excessive
gold loading and the existence of an optimal gold loading can be
found in the literature [17]. Several possible reasons for this activ-
ity decrease upon gold loading increase such as increase in the
average size of the nanoparticles (see Section 2) and reduction
on the exposed support surface have been proposed [17].
As it was observed already for the oxidation of cyclohexanone
oxime 1, Au (0.72 wt.%)/CeO2 and Au (1.85 wt.%)/CeO2 were more
efficient than Au (1.5 wt.%)/TiO2. The latter sample was not able
to carry out the complete conversion of 5 under the optimal condi-
tions of the oxidation, once again exemplifying the influence of the
support on the efficiency of the gold catalyst. Fig. 2 shows the time-
yield plots for the oxidation of carvoxime 5 in the presence of
0.72 wt.% Au/CeO2 at different temperatures.
Also in line with the results commented for cyclohexanone
oxime, it was observed that besides gold catalysts, analogous plat-
inum and palladium catalysts supported on ceria at similar load-
ings as the most active Au (0.72 wt.%)/CeO2 also exhibit activity
for the aerobic oxidation of carvoxime 5 to carvone 6. It was found
that the activity of the platinum catalysts was similar to that of
gold and much higher than that of palladium (see Table 3). Table
4 shows the calculated turnover frequency (TOF) for the two Au/
CeO2 catalysts based on the estimated number of external gold
atoms present in the catalysts. As it can be seen in Table 4, the
inherent catalytic activity for gold in Au (0.72 wt.%)/CeO2 is about
1.5 times that of Au (1.85 wt.%)/CeO2 and the TOF absolute value
for carvoxime oxidation is somewhat in the lower range compared
to TOF values for alcohol oxidation [17].
100
75
50
25
0
0
1
2
3
4
5
Time (h)
Activation energies of the overall kinetics for Au (0.72 wt.%)/
CeO2, Pt (0.70 wt.%)/CeO2, and Pd (0.73 wt.%)/CeO2 were obtained
from the Arrhenius plot of the logarithm of the initial reaction rates
(r0) vs. the inverse of the absolute temperature (see Fig. 3). The r0
values were determined from the slope at zero time of the time-
yield plots as those shown in Fig. 2 for Au (0.72 wt.%)/CeO2.
The activation energies (Ea) obtained from the slope of the best
fitting of the experimental points to a straight line and applying
the Arrhenius equation [ln r0 = ꢂ(Ea/RT) + ln A] were 30.28, 15.23,
and 41.90 kcal molꢂ1 for Au (0.72 wt.%)/CeO2, Pt (0.70 wt.%)/CeO2,
and Pd (0.73 wt.%)/CeO2, respectively. These values indicate that
Pt is the metal for which the energy barrier is the lowest and about
one half that of gold.
Overall the data shown in Table 3 and Fig. 1 shows that gold
supported catalysts can effect the conversion of carvoxime into
carvone under neutral conditions avoiding highly corrosive Brön-
sted acids and without forming any waste or by-product.
As a summary of the previous data, it can be concluded that
gold on nanoparticulated ceria is a suitable and general catalyst
to effect the aerobic oxidation of oximes into carbonylic com-
pounds. The process is heterogeneous and the catalysts can be re-
used without decay. The reaction is quite sensible to the solvent, a
Fig. 1. Time conversion plot for the oxidation of carvoxime 5 into carvone 6 in the
presence of Au (0.72 wt.%)/CeO2. (ꢀ): conversion of 5; (j): yield of 6. Reaction
conditions: carvoxime, 0.5 mmol; ethanol/water (1/1), 1 ml; temperature, 403 K;
oxygen pressure, 5 bar; Au/substrate mol ratio 1%.
90
60
30
0
0
5
10
15
0
2
Time (h)
Fig. 2. Time-yield plots for the oxidation of carvoxime 5 in the presence of Au
(0.72 wt.%)/CeO2 at (100 °C) (ꢀ), (110 °C) (j), (120 °C) (N), (130 °C) (ꢃ). Reaction
conditions: carvoxime, 0.5 mmol; ethanol/water (1/1), 1 ml; oxygen pressure,
5 bar; Au/substrate mol ratio 1%.
Table 4
Number of external gold atoms present in the catalyst used to oxidize carvoxime 5 (0.5 mmol) into carvone 6 and the corresponding turnover frequency (TOF).a
Catalyst
hdi (nm)
NT (per nanoparticle)
m
Ns (per nanoparticle)
% Ns
Ns ꢃ 10ꢂ20a (in the reaction)
TOF (hꢂ1
)
Au (0.72 wt.%)/CeO2
Au (1.85 wt.%)/CeO2
3.97
5.45
1941
5022
8.83
11.95
616.18
1201.02
31.74
23.91
9.56
7.20
6800
4200
a
hdi: average gold nanoparticle size; NT: number of total gold atoms per particle; m: number of full shells in an average gold nanoparticle; Ns: number of surface gold atoms
per nanoparticles; % Ns: percentage of external gold atoms.

A. Grirrane et al. / Journal of Catalysis 268 (2009) 350–355
355
Acknowledgments
4
2
Financial support by the Spanish Ministry of Science and Inno-
vation (projects CTQ2006-5678) and Generalitat Valenciana (Pro-
meteo 2008) is gratefully acknowledged. A.G. thanks the CSIC for
a research associate JAE contract.
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mixture of ethanol/water being generally a convenient medium.
Other supports such as titania and active carbon are less suitable
than ceria that already exhibits an intrinsic activity for this oxida-
tion process. Low gold loading is optimal with respect to the activ-
ity for this aerobic oxidation. Palladium was much less efficient
than gold, but platinum exhibits a similar catalytic behavior as
gold, but platinum exhibits remarkably lower (about one half) acti-
vation energy than gold.
The process described here fulfills all the requirements of Green
Chemistry and can replace advantageously current processes that
use Brönsted acids that are notoriously unsatisfactory from the
environment point of view due to corrosion and the generation
of aqueous wastes. Furthermore, these results expand the list of
selective aerobic oxidations that are efficiently catalyzed by gold
nanoparticles.