Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 24

Villa et al. 109 demonstrated that the modification of Au-based catalysts
with Pt or Pd metal also produced stable and recyclable catalysts. In par-
ticular, they reported that bimetallic Au 8 Pd 2 species supported over active
carbon - where Au and Pd metal are present in an 8 : 2 molar ratio - have the
highest activity and stability for the production of FDCA.
Another solution to the problem of catalyst durability was the addition of
bismuth to Pt-containing catalysts. 110 In this case, the oxidation of HMF
proceeded via the HMFCA and DFF intermediates; both of which were very
reactive and rapidly oxidized to 5-formylfurancarboxylic acid (FFCA). The
ex situ or in situ addition of a Bi promoter prevented the deactivation of the
Pt catalysts and accelerated the reaction. The highest activity was observed
for a Bi/Pt molar ratio of approximately 0.2.
Similar results were obtained by Villa et al. 111 when studying the modifi-
cation of Au-Pd catalysts with bismuth and using the prepared systems in
alcohol oxidation. Bi-containing materials were demonstrated to increase
selectivity by suppressing parallel reactions in both benzyl alcohol and gly-
cerol oxidation. However, the selectivity of the reactions notably varied only
when Bi was deposited on the surface of metal nanoparticles.

d n 4 r 4 n g | 1

1.3.3 Selective Oxidation of Alcohols in Non-Conventional
Media
One of the greatest challenges in liquid-phase oxidation catalysis is the
development of clean technologies that can operate in water; many of the
studies reported so far used this solvent, even though reactants and products
in alcohol oxidation are often insoluble in water, so other alternatives
to standard organic solvents have also been proposed, such as ionic
liquids 112,113 and supercritical fluids. 114-116
The properties of supercritical carbon dioxide (scCO 2 ) are a combination
of those associated with gases or liquids 117 and are particularly useful in the
reactions involving gaseous reagents such as hydrogenation with H 2 and
oxidation with O 2 . Moreover, scCO 2 is totally non-flammable and its prop-
erties can be manipulated by varying the pressure applied.
The selective aerobic oxidation of alcohols has been studied in depth in
scCO 2 using both batch and continuous-flow reactors. 114,118 The products
usually observed in the oxidation of primary alcohols with metal catalysts
and their formation routes are depicted in Scheme 1.20.
Since scCO 2 can dry wet material by eliminating the water formed during
alcohol oxidation, its application is convenient for achieving high selectivity
to aldehyde, while suppressing the formation of carboxylic acid via the
favored hydration of aldehyde. 119
Apart from reaction selectivity, a strong dependence of the reaction rate on
pressure has also been found in some cases where CO 2 has been used as the
solvent. As an example, Caravati et al. 120 demonstrated that the conversion
of benzyl alcohol to benzaldehyde in CO 2 increased from 25 to 75% when the
pressure was increased from 140 to 150 bar. Moreover, the same group 121

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