Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 20

(mainly NaOH, KOH and K 2 CO 3 ) on the performance of metal-catalyzed
reactions were extensively reported. 66 It was hypothesized that the base
aids alcohol dehydrogenation by H abstraction, thus overcoming the rate-
limiting step in the oxidation, and helps in desorbing carboxylic acid formed
during the reaction, thus avoiding poisoning of the catalyst. Nevertheless,
the addition of homogeneous bases to the reaction solution may be negative
from environmental and economic standpoints, and a significant amount of
work has recently been devoted to research on alternative methods to
their use.
As an alternative to adding a homogeneous base, different groups have
investigated the use of solid bases (mainly MgO, but also more unusual
materials such as NiO) and hydrotalcite materials as supports for metal
catalysts. 77,78 Specifically, Ebitani and co-workers 78 demonstrated that
platinum/gold alloy nanoparticles supported over hydrotalcites were truly
effective catalysts for the selective aerobic oxidation of glycerol and 1,2-
propanediol in base-free aqueous solution when using molecular oxygen at
room temperature and atmospheric pressure. The high activity and select-
ivity of these catalysts were explained in terms of alterations of geometric
and electronic states of the catalytically active surface Pt sites by Au atoms
and starch ligands used for nanoparticle synthesis.
On the other hand, it appears that, using basic supports, the acid formed
in the oxidation reaction can either be absorbed on the solid surface or react
with the support, thus leading to catalyst deactivation or leaching of the
material into the reaction medium. Accordingly, basic supports seem to
serve more as a stoichiometric replacement for the homogeneous base ra-
ther than as catalytic materials.
A major topic in the aerobic oxidation of alcohols is the performance of
bimetallic catalysts. The alloying of different metals has indeed proven to
have the potential for preventing catalyst deactivation, enhancing reaction
rates and improving product selectivity. 79
As an example, the preparation of Au/Pd- and Au/Pt-supported materials
and their use in the oxidation of alcohols has attracted high levels of re-
search. 80,81 Supported Au/Pd nanoparticles proved to be highly effective as
catalysts for the solvent-free oxidation of BA. 82 Nevertheless, in some cases,
the coproduction of large amounts of toluene was observed, due to the
presence of a disproportionation reaction of benzyl alcohol. 70 This latter
reaction seems to be especially promoted by Pd. The use of supports such as
MgO or ZnO was reported to stop toluene formation completely, while the
thermal treatment of prepared catalysts at different temperatures led to
significant changes in the ratio between main products and by-products. 83
Recently, in order to elucidate the structure-activity relationships in the
reaction of BA oxidation, Hutchings and co-workers reported the catalytic
performance of Pt/Au and Pt/Pd nanoparticles supported over TiO 2 and ac-
tive carbon using tert-butyl hydroperoxide (TBHP) as the oxidant. The results
demonstrated that when Au/Pd catalysts were used, a notable amount of
toluene was identified, 84 whereas when Pt was used instead of Pd in the

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