Selective oxidation of glycerol to glyceric acid using a gold catalyst in
aqueous sodium hydroxide
a
a
b
b
a
Silvio Carrettin, Paul McMorn, Peter Johnston, Ken Griffin and Graham J. Hutchings*
a
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff, UK CF10 3TB.
E-mail: hutch@cardiff.ac.uk
Johnson Matthey, Orchard Road, Royston, Herts, UK SG8 5HE
b
Received (in Cambridge, UK) 29th January 2002, Accepted 19th February 2002
First published as an Advance Article on the web 5th March 2002
Glycerol is oxidised to glyceric acid with 100% selectivity
using either 1% Au/charcoal or 1% Au/graphite catalyst
under mild reaction conditions (60 °C, 3 h, water as
solvent).
catalysts can be selective for the oxidation of alcohols,
including diols, under relatively mild conditions. We have now
extended the study of gold catalysts and, in this communication,
we show that glycerol can be oxidised to glyceric acid with
1
00% selectivity at high conversion.
The identification of new catalysed reactions is of key
importance to the chemical industry and, in particular, with
respect to the production of fine chemicals. Selective oxidation
is a key reaction used in the activation of raw materials to form
1 wt% Gold catalysts supported on carbon and graphite were
prepared as follows. The carbon support (graphite, or activated
carbon, Johnson Matthey, 113.2 g) was stirred in deionised
water (1 l) for 15 min. An aqueous solution of chloroauric acid
1
useful products or chemical intermediates. Glycerol is a highly
2
(41.94% Au, Johnson Matthey, 2.38 g in 70 ml H O) was added
functionalised molecule that is readily available from biosus-
tainable sources, for example, it can be derived from rape seed
and sunflower crops. A large number of products can be formed
from glycerol oxidation (Scheme 1) and one of the key
problems concerns the selectivity by which the individual
products can be formed. However, if the products could be
formed in high selectivity, they are potentially valuable as
chemical intermediates in the fine chemicals industry. The
oxidation of glycerol using supported Pd and Pt catalysts has
slowly dropwise over a period of 30 min. The slurry was then
refluxed for 30 min, cooled and reduced with formaldehyde
over a period of 30 min. The slurry was refluxed for 30 min and,
following cooling, the catalyst was recovered by filtration and
washed with water until the washings contained no chloride.
The catalyst was dried for 16 h at 106 °C. This method was also
used to prepare 0.25 wt% Au/C and 0.5 wt% Au/C catalysts
using smaller amounts of chloroauric acid.
The oxidation of glycerol with oxygen was investigated using
2,3
10
been extensively studied by Kimura et al. and Gallezot and
coworkers.4 In general, Pd as a catalyst was found to be more
selective than Pt, and conditions were identified for which
the 1% Au/C catalysts in an autoclave and the results are given
,5
in Table 1. Water was used as solvent and sodium hydroxide
was added as basic conditions are essential to obtain selective
oxidation. Indeed, in the absence of sodium hydroxide, no
glycerol conversion is observed. In addition, the carbon support
in the absence of gold was also found to be inactive for glycerol
oxidation, under these conditions, even when NaOH is present.
3
relatively high selectivities for dihydroxyacetone (70–80%)
could be obtained, although it was found5 that dihydroxy-
acetone was readily converted to hydroxypyrruvic acid at high
conversions. Glyceraldehyde was also obtained in high selectiv-
ities4 (70–80%). However, it was noted that formic acid,
probably resulting from the formation of oxalic acid, was also
formed. In all these previous studies, mixtures of the possible
products for glycerol oxidation (Scheme 1) have been observed.
Recently, Prati and coworkers6 have shown that supported Au
11
The reaction products were analysed using HPLC. For all the
data presented in Table 1, the carbon mass balance was found to
2 1
be 100%, and no C or C by-products were observed to be
formed. In particular, it is apparent that the selectivity to
glyceric acid and the conversion of glycerol are very dependent
upon the glycerol/NaOH ratio. In general, with high concentra-
tion of NaOH, exceptionally high selectivities to glyceric acid
can be observed. However, decreasing the concentration of
glycerol, and increasing the mass of catalyst and the concentra-
tion of oxygen leads to the formation of some tartronic acid via
consecutive oxidation. It is apparent that, by careful control of
the reaction conditions, 100% selectivity to glyceric acid can be
readily achieved. For comparison, the data obtained for a 5% Pt/
activated carbon catalyst, similar to that used in previous
studies,1 are also shown in Table 1. It is apparent that
significant selectivities to glyceraldehyde or tartronic acid are
observed with the Pt catalyst. We consider that the oxidation of
glycerol to glyceric acid probably proceeds via initial formation
of glyceraldehyde. Previous studies using Pt catalysts have
confirmed that glyceraldehyde is rapidly oxidised to glyceric
acid and is typically not observed as a product.1
–9
–5
2,13
In a final set of experiments, the oxidation of glycerol and
propane-1,2-diol was investigated using 0.25 wt% Au/graphite
and 0.5 wt% Au/graphite catalysts, and the data are shown in
Table 2. It is apparent that, for glycerol oxidation, both the
selectivity to glyceric acid and the conversion of glycerol
increase with the Au concentration. This is not observed for the
oxidation of propane-1,2-diol, when the highest conversion is
observed for the 0.5 wt% Au/graphite catalyst, although the
selectivity to the monoacid does increase with increasing gold
6
–8
concentration. The previous studies by Prati and coworkers
also show that reaction selectivity for alcohol oxidation is
Scheme 1
6
96
CHEM. COMMUN., 2002, 696–697
This journal is © The Royal Society of Chemistry 2002