Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 146

d n 4 r 4 n g | 3

Scheme 5.2 Schematic pathway of the potential products from glycerol aerobic

oxidation with gold-based catalysis.

with respect to sodium glycerate at 50-60% conversion when gold was
supported on graphite in aqueous solution at 100 1C and in the presence of
NaOH. 87 They showed how to convert glycerol to lactate via preliminary re-
duction to 1,2-propanediol followed by oxidation of the primary alcohol
group to carboxylate. Some years later, they confirmed that a second metal
alloyed with gold can lead to boosted catalyst performance compared with
the monometallic version. 88 Gold-palladium supported on two different
materials, titania and carbon, prepared following wet impregnation and sol
immobilization, markedly improved the catalytic activity while retaining the
high selectivity towards lactate (up to 96% selectivity at 94% conversion).
The sol immobilization protocol led to catalysts allowing the highest activity
for lactate formation, whereas on comparing a series of C 3 alcohols with
glycerol the reactivity trend observed was glycerol 41,2-propanediol 41,3-
propanediol 41-propanol 42-propanol.
Parallel studies on the oxidation to glycerate carried out by Prati et al. were
aimed at evaluating gold nanoparticles, either unsupported or supported on
carbon and titania, in terms of selectivity and activity, the role of the base
added in the reacting mixture and also hydrogen peroxide formed during the
reaction. 89 It was found that the C-C bond fission of glycerol increased with
the reaction time and, partially, with the rate of degradation of the by-
product H 2 O 2 , thus lowering the selectivity to glycerate. The authors also
adopted the strategy of alloying gold with a second metal (Pt and Pd) in order
to overcome such limits. The catalytic activity could be enhanced by em-
ploying the bimetallic Au-Pt and Au-Pd systems, whereas the selectivity to

.