Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 143

absence of a base and this has been a matter of study since alkali was found
to be compulsory in the oxidation of hydroxyl and carbonyl groups. The base
reasonably appears to be involved in the first step of the oxidative process for
the creation of the alkoxide anion. Hence the basic strength of the alcoholic
group is crucial. 81 Moreover, the presence of a base promotes the desorption
of the products from the surface, thus prolonging the catalyst life. Two ways
can be chosen for providing a reaction system with a basic environment:
continuous addition of alkali (pH-controlled conditions) or an excess of base
added at the beginning of the reaction. These conditions allow the formation
of an intermediate aldehyde, which is more easily oxidized to carboxylate
than the parent alcohol. In some cases, the aldehyde can be isolated in high
yield (90%) even in the presence of alkali, as in o-hydroxybenzyl oxidation
catalyzed by Au/Fe 2 O 3 at 50 1C, atmospheric pressure and with Na 2 CO 3 . 82
However, carrying out the reaction in the presence of a base represents a
limit when free carboxylic acids, and not the corresponding carboxylates, are
the desired products. The use of bimetallic catalysts seems to offer a valid
alternative, since most of the reactions carried out with gold alloyed with
palladium or platinum are not base assisted.
The reactions presented here were mostly performed in the presence of a
solvent and partly under solvent-free conditions. This is an important point
in catalysis: finding a proper solvent that improves catalytic performances
while respecting ecological restrictions is not always an easy task. The ideal
medium would be water, being eco-friendly, cheap and easy to handle, but
unfortunately its use depends on the hydrophobic/hydrophilic properties of
both the substrate and the catalytic system. Anyway, water represents the
solvent of choice for an wide range of substrates, such as polyhydroxylated
compounds, and solventless conditions - when possible - can provide a
valid solution for water-insoluble compounds. On the other hand, under
solventless conditions drawbacks can occur, principally related to the
physical state of the substrate and its thermal stability. However, solventless
conditions often emphasize the action of the base, so important in gold-
catalyzed alcohols oxidation. For instance, in the solvent-free oxidation of
benzyl alcohol conducted at moderate temperature (80 1C) with Au/TiO 2
catalysis and addition of K 2 CO 3 (0.2% with respect to the alcohol), a marked
effect of the base was evident as the TOF reached 7851 h 1 , much higher
than the ¼ 19 h 1 achieved without any alkaline promoter. On changing the
type of base, some differences were observed: sodium carbonate and acetate
improved the oxidation, whereas stronger bases did not have the same
prominent effect. 83 Gold-based catalysts are the ideal choice in water be-
cause, compared with more traditional catalysts such as palladium or plat-
inum, they do not suffer from deactivation when employed in the presence
of molecular oxygen; moreover, they are highly chemoselective. 5 Regarding
the support material, activated carbon and graphite are the most investi-
gated materials for aqueous-phase oxidation by gold due to both the high
performance under mild conditions and economic reasons: these materials
are, in fact, cheap and allow an easy recovery of the precious metal by

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.