Gas phase oxidation of alcohols to aldehydes or ketones catalysed by supported gold

Article abstract of DOI:10.1039/b210506c

Primary and secondary aliphatic alcohols are smoothly oxidised by air to the corresponding carbonyl derivatives with high selectivity using 1% Au on silica.

Full text of DOI:10.1039/b210506c

Gas phase oxidation of alcohols to aldehydes or ketones catalysed by
supported gold
Serena Biella and Michele Rossi*
Department of Inorganic, Metallorganic and Analytical Chemistry, Unit V of INCA Consortium, Centro
CNR, Via G, Venezian 21 20133, Italy. E-mail: michele.rossi@unimi.it
Received (in Cambridge, UK) 24th October 2002, Accepted 23rd December 2002
First published as an Advance Article on the web 13th January 2003
Primary and secondary aliphatic alcohols are smoothly
oxidised by air to the corresponding carbonyl derivatives
with high selectivity using 1% Au on silica.
been paid to the gas phase reaction and attempts at industrial
application failed owing to selectivity problems.11 Recently,
gold catalysts have received a growing interest for various
applications in organic and inorganic chemistry12 mainly
devoted to eco sustainable processes and, in particular, we and
other research groups have investigated the oxidation of
polyols13–16 and aminoalcohols17 to carboxylates, o-hydrox-
ybenzyl alcohol to aldehyde18 and the direct synthesis of
hydrogen peroxide.19
Oxidation of primary and secondary alcohols to the correspond-
ing aldehydes and ketones plays a fundamental role in organic
synthesis owing to the versatility of the carbonyl group as a
building block.1,2 Traditional methods based on the use of a
stoichiometric amount of reagents, mainly heavy metal salts in
high oxidation state, are useful tools for laboratory scale
preparation,3,4 but fail in industrial applications where serious
problems related to the disposal of undesired by-products
represent a strong drawback. Air, or pure oxygen, is a cheap and
clean reagent suitable for bulk scale oxidation. Therefore, the
aerobic oxidation of alcohols has, since the early experiments,
long been investigated 5 by using different transition metals as
catalysts, mainly belonging to the platinum group.6 In practice,
the oxidation of alcohols with air can be performed in the liquid
phase or in the gas phase, depending mainly on the thermal
stability and volatility of reagents and products. While many
recent papers deal with studies on the aerobic oxidation in
aqueous solution6–8 or in other solvents,9,10 less attention has
An application of gold catalysts to the gas phase oxidation of
ethanol has been reported in a preliminary study where an
unusual high selectivity to acetaldehyde has been observed.20
We have now extended this research on gold catalysts and, in
this paper, we show that several primary and secondary
aliphatic alcohols can be easily oxidised to the corresponding
carbonyl derivatives with high yields using air at atmospheric
pressure, thus demonstrating that this cheap and clean method-
ology could be of general interest for organic synthesis.
Gold catalyst (1 wt%) supported on silica (Aerosil 200, Grace
and Davison, surface area 200 m2 g21, pore volume 1.7 ml g21
)
was prepared in the following manner: 8.5 ml of an aqueous
solution of chloroauric acid containing 50 mg of Au were added
Table 1 Oxidation tests
Entry
1
Catalyst
Reagent
T (K)
Conversion (%)
Selectivity (%)
1% Au/SiO2
1-Propanol
523
573
27
44
100a
100a
2
3
4
1% Au/SiO2
1% Au/SiO2
1% Au/SiO2
1-Butanol
523
573
51
63
100a
94a
1-Pentanol
523
573
23
29
100a
100a
Phenylcarbinol
523
553
50
75
100a
98a
5
6
1% Au/SiO2
1% Au/SiO2
Prop-2-en-1-ol
2-Propanol
523
42
97a
373
423
69
100
100a
100a
7
8
1% Au/SiO2
1% Au/SiO2
1% Au/SiO2
1% Au/SiO2
2-Butanol
2-Pentanol
3-Pentanol
1-Propanal
393
423
34
64
100a
100a
393
423
72
85
87a
84a
9
393
423
85
97
100a
100a
10
523
573
14
16
4b
3b
11
12
SiO2
SiO2
1-Propanol
2-Propanol
523
413
20
4
4a
0a
a To carbonyl derivates. b To propanoic acid.
378
CHEM. COMMUN., 2003, 378–379
This journal is © The Royal Society of Chemistry 2003

to 5 g of silica while mixing with a glass rod for 10 minutes. 8
ml of aqueous NH3 (3%) was then added followed by 10 ml of
an aqueous solution of NaBH4 (60 mg per 10 ml), and the
suspension stirred at 333 K for 10 min. The solid product was
filtered, washed several times with water until the washings
contained no chloride. The presence of metallic gold in the
catalyst dried at 413 K for 5 h, was confirmed by XRPD analysis
(2q = 38.2).
The oxidation of alcohols was carried out in a fixed bed
vertical glass reactor fitted with a glass frit carrying the catalyst
(0.2 g) and provided with an electronically controlled furnace.
The air stream (1.2 mmol min21) was controlled by a mass flow
instrument and the liquid reagent (0.5 mmol min21) was
supplied through a syringe pump. Liquid vaporisation occurred
on the reactor wall prior to the catalytic bed. The condensable
reaction products were collected by bubbling the effluent into a
cold trap (273 K) containing ethanol and an appropriate internal
standard and analysed by GC ( HP Plot Q 30 m silica fused
capillary column ) using helium as a carrier gas. The selectivity
was calculated as mol of carbonyl product per mol of reacted
alcohol and the carbon mass balance was close to 100% in the
case of 100% selectivity. The estimated degree of confidence of
the reported figures is ±3%.
that the mean diameter of gold particles changed from 15 nm
(w0 = 0.52) to 25 nm ( w0 = 0.32) after 18 h and then remained
stable after 60 h. It is worth noting that the catalytic behaviour
here observed is due to relatively large gold particles.
In conclusion, 1% gold on silica represents an innovative
catalyst suitable for the preparation of carbonyl compounds by
clean oxidation of alcohols under mild conditions which could
be relevant also for industrial application. The comparison
between the gas phase oxidation reported here and the
previously reported liquid phase oxidation of the alcoholic
group17 outlines the versatility of supported gold which offers
the possibility of producing carbonyl derivatives in the first
case, and carboxylates in the second case.
Notes and references
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Table 1 reports the results obtained in the experiments and
shows a remarkable selectivity in the oxidation of aliphatic
primary alcohols to aldehydes and secondary alcohols to
ketones. The high chemoselectivity towards the alcoholic group
presented by the gold catalyst minimizes the oxidation of the C–
H and C–C–bonds and the further oxidation of the carbonyl
group. In fact, the conversion of 1-propanal under similar
experimental conditions is low and produces propanoic acid in
very low yields (entry 10). Moreover, the carbon–carbon double
bond in allyl alcohol was inactive to oxygen allowing its
transformation into acrolein with high selectivity (entry 5).
Comparing the reactivity of primary and secondary alcohols
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lower temperature. Different conversion values obtained for
homologous alcohols could be due to differences in the purity of
the reagents, particularly in the case of the 98% pure 1-butanol
where relatively low conversions have been observed.
In the case of primary alcohols, 1% gold on the support acts
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selectivity to aldehydes with respect to silica alone. In fact, the
acidic centres of the support address the reaction to a deeper
oxidation, as derived by the low yield and the excess of the
formed water (entry 11). In the oxidation of secondary alcohols,
gold acts also as a strong activator of the organic molecules
because the support is almost inactive at low temperature (413
K) (entry 12).
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During a prolonged oxidation test of 1-propanol at 573 K the
catalyst was stable over 60 h. XRPD analysis of gold showed
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