Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 26

has been widespread in synthetic organic chemistry. 122-124 Owing to their
particular state of matter, between homogeneous and heterogeneous, these
species are often called 'semi-heterogeneous' catalysts. In general, the ad-
vantageous characteristics of nanoparticles are (i) high surface-to-volume
ratios, providing a considerable number of active sites per unit area com-
pared with their heterogeneous counterparts, 125 (ii) high zeta potential,
preventing the aggregation of nanoparticles in solution 126 and (iii) the
possibility of separation and recycle, making them cost-effective and min-
imizing the chance of contaminating the catalyst with the product. 127
Recently, nanometals and nanoclusters have been widely applied in some
important reactions, 128

d n 4 r 4 n g | 1

such as the carbon-carbon coupling, 129 carbon-

heteroatom bond formation 130 and hydrogenation. 131
A recent review by Garcia and co-workers 132 suggested that the intrinsic
activity of unsupported gold nanoparticles in some of the typical reactions
generally promoted by supported nanoparticles, such as aerobic alcohol
oxidation, is comparable to or even higher than those of conventional sup-
ported materials. As an example, monodisperse gold and palladium nano-
particles, stabilized by polyvinylpyrrolidone (PVP), were tested in the
oxidation of benzyl alcohols in water. 133 In these tests, Au nanoparticles
proved to be more active than Pd nanoparticles of similar size (about
1.5 nm). In addition, in kinetic experiments, the rate-determining step for
BA oxidation was reported to involve hydrogen abstraction by a superoxo-like
molecular oxygen (O 2 ) which was adsorbed on the catalyst. This specific
species can only be formed over small metal clusters, which might explain
the size-related catalytic activity in alcohol oxidation that was observed in
gold-based samples.
Dumeignil and co-workers 134 recently proposed a complex reaction
scheme for the liquid-phase oxidation of glycerol using a quasi-homogeneous
solution of Au nanoparticles. The highest conversion of glycerol obtained
with this system was 100% after 3 h of reaction at 100 1C and 6 bar oxygen
pressure, and the main products formed were glyceric, glycolic, formic, tar-
tronic and oxalic acid with selectivities of 28, 36, 25, 9 and 2%, respectively.
The mechanism reported indicated that the direct glycerol oxidation was
facilitated in the presence of Au nanoparticles, whereas high temperatures
and strongly basic conditions facilitated oxidative cleavage and straight
hydrolytic transformations, such as retro-aldol, dicarbonyl cleavage, oxidative
cleavage with carbon dioxide evolution and Cannizzaro cross-reactions.
So far, the role of the ligand generally used to stabilize nanometals from
grain growth and agglomeration during catalytic tests has not been suf-
ficiently investigated. In this regard, it has been commonly assumed that the
ligand plays a negative role, by significantly decreasing the catalytic activity
of nanoparticles. 135 Nevertheless, using PVP, it was observed that the
interaction of this polymer with metal nanocrystals is accompanied by
charge transfer from PVP to the nanometal. 136 Thus, in some cases, this
interaction can modify the structure of metal colloids in ways that are
sometimes beneficial to their catalytic performance. 137

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