A. Abad et al. / Tetrahedron 62 (2006) 6666–6672
6671
between gold and ceria that at the end is also the result of
the nanometric size of the particles.
water, and dried at 110 ꢀC giving a solid with the stoichio-
metry of Ca10(PO4)6(OH)2 corresponding to hydroxyapatite.
4.2.3. Preparation of Au/CeO2 catalyst. Au was deposited
on the nanoparticulated ceria by the following procedure:
a solution of HAuCl4$3H2O (296 mg) in 60 mL of deionized
water was brought to pH 10 by addition of a solution of
NaOH 0.2 M. Once the pH value was stable the solution
was added to a gel containing of colloidal CeO2 (4.01 g) in
H2O (50 mL). After adjusting the pH of the slurry to a value
of 10 by addition of a solution of NaOH 0.2 M, the slurry
was left under vigorous stirring for 18 h at room tempera-
ture. The Au/CeO2 solid was then filtrated and exhaustively
washed with distilled water until no traces of chlorides were
detected by the AgNO3 test. The catalyst was dried at
vacuum at room temperature for 1 h. Then 3.5 g of the sup-
ported catalyst was added over 30 g of 1-phenylethanol at
160 ꢀC and the mixture was allowed for reduction during
20 min. The catalyst was filtered, washed, with acetone
and water, and dried under vacuum at room temperature.
The total Au content of the final catalyst Au/CeO2 was
1.54 wt % as determined by chemical analysis. This catalyst
3. Conclusions
Contrary to the general belief in Organic synthesis, homoge-
neous and heterogeneous catalysis have been developing
excellent systems to effect the aerobic oxidation of alcohols
using molecular oxygen as oxidant. This process has been
regarded by Organic chemists as being limited and specific
for large-scale industrial oxidation processes. But, currently
there are several noble metal catalysts that exhibit a general
activity to oxidize any hydroxyl group to the corresponding
carbonyl compound. Among this extremely active catalysts,
we have shown in the present work that commercially avail-
able, air- and moisture-stable Au/CeO2 shows a wide gener-
ality for the oxidation of primary and secondary, aliphatic
and benzylic hydroxy groups, thus, making aerobic oxida-
tion amenable to general Organic Chemistry. The current
work in this area is focused in further expanding the solvents
that can be used, determining compatibility with substrate
functional groups and the reusability of the gold catalyst.
4.2.4. Preparation of Pd supported catalysts. Colloidal
ceria or hydroxyapatite (4 g) was stirred at room temperature
for 3 h in 400 mL of acetone solution of PdCl2(PhCN)2
(1.5ꢃ10ꢁ3 M). The obtained mixture was filtered, washed
with acetone, and dried under vacuum at room temperature.
Then 3.5 g of the supported catalyst was added over 30 g of
1-phenylethanol at 160 ꢀC and the mixture was allowed for
reduction during 20 min. The catalysts were filtered,
washed, with acetone and water, and dried under vacuum
at room temperature. The total Pd content of the final cata-
lyst as determined by chemical analysis was 1.57 and
1.44% for Pd/CeO2 and Pd/apatite, respectively.
4. Experimental
4.1. General
HAuCl4, (NH4)2HPO4, Mg(CH3COO)2$4H2O, NH3 (25%),
Ca(NO3)2$4H2O, and Ce(NO3)4 were purchased from
Sigma–Aldrich Chemical Co. PdCl2(PhCN)2 was purchased
from ABCR GmbH Co. All reagents were used without fur-
ther purification. All solvents and acids used were reagent
grade, purchased from Sigma–Aldrich Chemical Co., and
used as received. All the experiments were performed using
mQ water.
4.2.5. Catalyst characterization. For crystal analysis and
indexation, the samples were examined by bright- and
dark-field electron microscopy in a Jeol 2200 HRTEM oper-
ated at an accelerating voltage of 200 kV. Dark field consists
on observing the image produced by diffracted electrons
corresponding to a determined lattice spacing leaving the
rest dark.
4.2. Catalysts
4.2.1. Synthesis of nanoparticulated ceria. A colloidal dis-
persion of CeO2 nanoparticles was prepared by thermolysis
of an acidified Ce(NO3)4 solution followed by re-dispersion.
The dispersion was purified and concentrated using an ultra-
filtration cell equipped with a 3KD membrane. The purifica-
tion was monitored by the residual acidity of the dispersion,
determined by an acid titration of the supernatant after ultra-
centrifugation at 50,000 rpm for 6 h. The resulting cerium
oxide has, owing to the small size of the nanoparticles,
a very high surface area (180 m2 gꢁ1) as determined by
isothermal nitrogen adsorption.
Chemical analyses of gold and palladium metals in the cat-
alysts were carried out after dissolving the solids by attack
with a 2:1 mixture of HNO3/HF on a Varian-10 Plus
Atomic Absorption Spectrometer or directly of the solids
using on a Philips MiniPal 25 fm Analytic X-ray apparatus
and a calibration plot. Analysis of reaction products was
carried out by GC on an HPꢁAgilent 5973 with a 6980N
mass selective detector.
4.2.2. Synthesis of hydroxyapatite. Hydroxyapatite was
prepared using a method as described in the literature. A
solution of (NH4)2HPO4 (40.0 mmol) in deionized water
(150 mL) was set at pH 11 with aqueous NH3 solution. This
solution was added dropwise over 30 min to a solution of
Ca(NO3)$4H2O (66.7 mmol) in deionized water (120 mL)
adjusted to pH 11 with aqueous NH3 solution. Vigorous stir-
ring at room temperature was maintained during the addition
process. The resulting milky solution was heated at 90 ꢀC for
10 min. The precipitate was filtered, washed with deionized
4.3. Typical procedure for the aerobic solventless
oxidation of alcohols
All alcohols provided by Aldrich were used without further
purification. The corresponding alcohol (12.5 mmol) was
added over Au/CeO2 catalyst (0.252 g), molecular oxygen
was bubbled continuously through the suspension
(35 mL mꢁ1). The resulting mixture was then heated at
120 ꢀC. After the reaction, acetone was added and the
Abad, Alberto
