Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 145

oxidation of a series of C 3 -C 5 alcohols and benzyl alcohol carried out with
Au/SiO 2 . With the temperature ranging from 250 to 300 1C, the activity fol-
lowed the order C 4 4C 5 4C 3 , whereas secondary alcohols required milder
temperatures (100-150 1C). Regarding benzyl alcohol, a high selectivity
(499.5%) could be obtained only with a restricted range of conversion
(50-75%). 27 However, the need to produce benzaldehyde in high yields,
avoiding expensive and polluting purification techniques, prompted the
authors to extend this research. As a strategy, in order to stabilize the almost
total selectivity achieved with the gold on silica catalyst while ensuring the
complete conversion of the reagent, a series of mono- and bimetallic cata-
lysts were investigated. Among them, the gold-copper combination sup-
ported on silica was particularly effective. This study emphasizes the key-role
of gold while highlighting a synergistic effect with copper. Although gold
alone led to useful values of selectivity and conversion, the most promising
achievements were obtained with a bimetallic Cu-Au/SiO 2 catalyst, pro-
ducing over 99% selectivity at 98% conversion. 85
In the wide range of alcohols so far examined, glycerol is at present the
most intensively investigated, with numerous papers, patents and reviews
that definitely confirm supported gold to be a valid alternative to the tra-
ditional platinum- and palladium-based catalysts.
Another substrate of interest is allyl alcohol, being a potential starting
material for the synthesis of 3-hydroxypropionic acid, which occupies the
third position in the top-12 list of chemical intermediates to be converted
into a number of high-value materials. 86
The following sections are therefore focused on the oxidation of glycerol
(Section 5.3.1) and allyl alcohol (5.3.2).

d n 4 r 4 n g | 3

.

5.3.1 Glycerol Oxidation
The necessity to find eco-friendly routes for replacing petroleum derivatives
prompted both academia and industry to search for potential starting ma-
terials in biomass. The synthesis of biodiesel is probably the most promising
solution, being a fuel comprised of monoalkyl esters of long-chain fatty acids
produced by chemically reacting a vegetable oil or animal fat with an alcohol
such as methanol (transesterification reaction). However, the growing
overproduction of glycerol, the main by-product of biodiesel synthesis, has
meanwhile introduced another challenge for scientists, who are now called
upon to find effective routes for converting glycerol into valuable chemicals
(Scheme 5.2). 34,35
The catalytic approach employing gold supported on different materials
seems to meet expectations, according to two research teams particularly
involved in the application of gold catalysis to glycerol oxidation: Hutchings'
group at Cardiff University and Prati group at Milan University. Gold-based
catalysts are reported to guarantee high performance, especially in terms of
selectivity, which is so important in the entangled net of possible products.
Accordingly, in 2002, Hutchings and co-workers achieved 100% selectivity