A collaborative effect between gold and a support induces the selective oxidation of alcohols

Article abstract of DOI:10.1002/anie.200500382

Ceria nanoparticles as a support stabilize positive gold species and provide oxygen vacancies. The resulting solid exhibits an exceedingly high efficiency for the solventless aerobic oxidation of primary and secondary alcohols to carbonyl compounds (see picture). (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200500382

Communications
to oxidize alcohols at atmospheric oxygen pressure even in
the absence of solvents. Mizuno et al.[13,14] have described a
ruthenium-containing solid catalyst that is able to selectively
produce carbonyl compounds from alcohols in the presence of
oxygen at atmospheric pressure. Other solid catalysts based
on hydrotalcites,[15] apatites and mixed oxides,[16] or supported
Pt and Pd[17,18] have also been studied.
Small-crystal-size gold supported on inorganic oxides or
active carbon has recently attracted considerable attention
since these catalysts are able to promote the selective
oxidation of alcohols.[19–25] Taking into account that inorganic
oxides contain some sites that are able to oxidize alcohols
stoichiometrically, it appeared to us that a new concept of
catalyst could be put forward if these stoichiometric sites can
be converted into catalytic sites by introducing a solid co-
catalyst that facilitates the reoxidation of the intermediate
metal hydrides to water and the original inorganic oxide. To
prove this concept we selected cerium oxide, which contains
stoichiometric oxidation sites of alcohols, and cationic gold,
which is a metal that transfers hydrides reversibly.
In particular, the combination[26] of small-crystal-size gold
(2–5 nm) and nanocrystalline ceria ( ꢀ 5 nm) turned out to
give a highly active, selective, and recyclable catalyst for the
oxidation of alcohols into aldehydes and ketones with high
turnover numbers (TON) and frequencies (TOF) using
oxygen at atmospheric pressure as oxidant in the absence of
solvent and base.
Pure, nanocrystalline cerium oxide contains sites that are
able to perform the oxidation of alcohols in a stoichiometric
manner. Indeed, when IR experiments were carried out in situ
by adsorbing 2-propanol on nanocrystalline cerium oxide (see
Supporting Information), two IR bands associated with
cerium alkoxide appeared; the growth of a band attributable
to cerium hydride, was also observed. The appearance of a
carbonyl band indicates that acetone was formed at the same
time as the cerium hydride. Upon subsequent introduction of
O2 into the IR cell at room temperature, the band due to
cerium hydride remained unaltered and the formation of
water was not observed. These experiments show that cerium
oxide alone is able to perform one reaction cycle, but the
reduced cerium is stable in the presence of physisorbed O2
and the catalytic cycle is not closed.
When gold nanoparticles were deposited onto the nano-
crystalline cerium oxide (see Supporting Information for the
detailed preparation procedure), the XPS spectrum of the
Au4f7/2 core level shows three bands (see Supporting
Information), which correspond to Au3+, Au+, and Au0.[27]
The presence of cationic gold was also confirmed by the IR
band of adsorbed CO (see Supporting Information).[28] These
results can be rationalized by assuming that the gold nano-
particles formed on the AuꢁCeO2 catalyst (see HRTEM
images in the Supporting Information) interact with the
nanometric ceria surface, which stabilizes the positive oxida-
tion states of gold[26,29,30] by creating Ce3+ and oxygen-
deficient sites in the ceria.
Supported Catalysts
A Collaborative Effect between Gold and a
Support Induces the Selective Oxidation of
Alcohols**
Alberto Abad, Patricia Concepciꢀn, Avelino Corma,*
and Hermenegildo Garcꢁa
The selective oxidation of alcohols is one of the most
challenging reactions in green chemistry.[1–4] An ideal green
oxidation process should involve a highly active and selective
recyclable catalyst that is able to work at atmospheric
pressure in the presence of oxygen and the absence of
solvents and bases.[5] Stoichiometric oxidations using transi-
tion metal compounds or sulfoxides (Swern oxidation) are
still commonly used, despite the formation of a large amount
of undesirable products. Several homogeneous Pd-, Cu-, or
Ru-based catalysts are able to perform the selective oxidation
of alcohols, but they require the use of organic solvents or
high oxygen pressure.[6–11]
From the point of view of heterogeneous catalysis,
probably the best system has been reported recently by
Kaneda et al.,[12] where a palladium-containing apatite is able
[*] A. Abad, Dr. P. Concepciꢀn, Prof. A. Corma, Prof. H. Garcꢁa
Instituto de Tecnologꢁa Quꢁmica (CSIC-UPV)
Universidad Politꢂcnica de Valencia
Avda. de los Naranjos s/n. 46022 Valencia (Spain)
Fax: (+34)96-387-7809
E-mail: acorma@itq.upv.es
When the anaerobic oxidation of 2-propanol on
AuꢁCeO2 was monitored in situ by IR spectroscopy (see
Supporting Information) in the same way as with pure
nanocrystalline CeO2, the IR bands of the cerium alkoxide
[**] The authors thank CICYT (MAT 2003-07945-C02-01) and AURICAT
(HPRN-CT-2002-00174) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4066 ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200500382
Angew. Chem. Int. Ed. 2005, 44, 4066 –4069

Angewandte
Chemie
and hydride were again observed, and acetone was also
formed. However, when O2 was subsequently introduced the
metal hydride disappeared and water was formed, contrary to
what occurred with CeO2. At this point the cerium oxide is
therefore ready to perform another catalytic cycle.
After these spectroscopic studies, AuꢁCeO2 was tested as
a solid catalyst for the selective aerobic oxidation of a large
variety of alcohols. The reactions were performed in a
magnetically stirred, glass batch reactor in the absence of
solvent and base at 808C, with O2 at atmospheric pressure. In
other cases the oxidations were also performed in basic water
(see Supporting Information for experimental details). High
conversions and selectivities were obtained with short reac-
tion times (Table 1).
Figure 1. Turnover frequencies for the oxidation of 3-octanol, given as
the ratio of moles of 3-octanone per mole of Au per hour, measured at
t=10 min (see Table 1 for reaction conditions).
The benefit of using nanoparticles of CeO2 as a support
for the gold becomes obvious when comparing the activity
(TOF) of AuꢁCeO2 with the activities of Au supported on
conventional CeO2 (Au/CeO2), Au/carbon (Au/C) (see Sup-
porting Information for experimental details of the prepara-
tion), Au/TiO2, and Au/Fe2O3 (these two last catalysts were
supplied by the International Gold Council; Figure 1).[31]
Note that AuꢁCeO2 is active for the oxidation of alcohols
in the absence of solvent and base, conditions under which
most other supported gold catalysts are inefficient.[5] For
instance, Au/C does not catalyze oxidation reactions in the
absence of base and water.
as indicated in Scheme 1. Indeed, we have seen that the
introduction of trimethyl orthoformate, which acts as a
scavenger for aldehydes, into the reaction mixture gives the
dimethyl acetal of the aldehyde with very high selectivity and
completely inhibits the formation of the ester. Note that
AuꢁCeO2 is also very active for the oxidation of alcohols in
basic aqueous solution. In this case pH plays an important
role, with the TOF decreasing upon lowering the pH. In basic
aqueous solution, the oxidation of primary alcohols gives the
carboxylic acids rather than the esters.
Secondary alcohols can be oxidized with essen-
tially complete conversion and high selectivities on
AuꢁCeO2 in the absence of solvent, and benzylic or
allylic alcohols are selectively oxidized to the corre-
sponding unsaturated aldehydes, although longer
reaction times are required. Aliphatic primary alco-
hols are more reluctant to undergo oxidation in the
absence of solvent. Notably, they give predominantly
the corresponding ester with high selectivity, accom-
panied by lesser amounts of the aldehyde. We have
found that esters are directly formed via the hemi-
Scheme 1. Reaction route proposed for the formation of esters in the oxidation of primary
alcohols.
acetal, which is detectable by 1H NMR spectroscopy,
Table 1: Aerobic alcohol oxidation catalyzed by AuꢁCeO2.[a]
Entry
Substrate
t [h]
Conversion [%]
Product
Selectivity [%]
1[b]
2[c]
3-octanol
3-octanol
2-phenylethanol
2,6-dimethylcyclohexanol
1-octen-3-ol
cinnamyl alcohol
3,4-dimethoxybenzyl alcohol
3-phenyl-1-propanol
vanillin alcohol
3.5
2.5
2.5
2.5
3.5
7
7
6
2
97
89
92
78
80
66
73
70
96
3-octanone
3-octanone
acetophenone
2,6-dimethylcyclohexanone
1-octen-3-one
>99
96
97
94
>99
73
83
98
98
3[b]
4[b]
5[b]
6[b]
cinnamaldehyde
7[b]
3,4-dimethoxybenzaldehyde
3-phenylpropyl 3-phenylpropanoate
vanillin
8[b]
9[d]
10[d]
11[d]
12[d]
13[e]
14[d]
2-hydroxybenzyl alcohol
3,4-dimethoxybenzyl alcohol
cinnamyl alcohol
n-hexanol
2
2
3
10
5
>99
>99
>99
>99
>99
2-hydroxybenzaldehyde
3,4-dimethoxybenzylic acid
cinnamylic acid
hexanoic acid
acetophenone
87
>99
98
>99
51
2-phenylethanol
[a] Conversion and selectivity were determined by GC using nonane and nitrobenzene as external standards. [b] Substrate (4.85 mmol), AuꢁCeO2
(0.5 mol%), 353 K, p=1 atm O2 (flow: 25 mLminÀ1). [c] As [b], but TEMPO (2,2,6,6-tetramethylpiperidine oxide; 4 equiv with respect to Au) was
added. [d] Substrate (0.4 mmol), AuꢁCeO2 (0.66 mol%), H2O (5 mL), Na2CO3 (0.3 g), 323 K, p=1 atm O2 (flow: 25 mLmÀ1). [e] Substrate (1 mmol),
AuꢁCeO2 (0.25 mol%), H2O (5 mL), Na2CO3 (0.71 g), 373 K, p=25 atm (synthetic air).
Angew. Chem. Int. Ed. 2005, 44, 4066 –4069
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On the basis of the AuꢁCeO2 characterization and IR
conversion of 1-phenylethanol into acetophenone at 1608C,
with greater than 99% selectivity for the desired product. The
catalyst is fully recyclable after filtering and washing (NaOH
0.5m), with a TON of 250000 after three recycles. This shows
that gold, which was previously believed to be of
little catalytic interest, can become an interesting
oxidation catalyst when combined with the right
support.
study, a reasonable mechanism for the catalytic reaction is
depicted in Scheme 2. According to this mechanism, the
interaction between gold and ceria will give rise to an
In conclusion, we have shown that gold nano-
particles transform nanocrystalline cerium oxide
from a stoichiometric oxidant into a catalytic
material for the selective oxidation of primary and
secondary alcohols to aldehydes and ketones in
the presence of oxygen at atmospheric pressure,
with high TOFs and selectivities observed. This
catalyst makes the process interesting from both
an economic and environmental point of view.
Received: February 1, 2005
Published online: May 25, 2005
Keywords: alcohols · gold · nanoparticles ·
.
oxidation · supported catalysts
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