Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 27

Also with nanoparticles, owing to the limited reserves of noble metals,
one of the most interesting challenges is that of reducing their content in the
active phase. Both intermetallic noble-non-noble alloys and non-noble
metals have been demonstrated to be effective in reducing the use of
noble metals for catalytic applications. As with the case of supported
systems, in many cases the alloyed nanosystems not only combined the
properties of monometallic nanoparticles, but also showed a significant
improvement because of synergistic effects and different compositions. 81
Scott and co-workers 138 studied the oxidation of a,b-unsaturated alcohols
in water and an ionic liquid (tetraalkylphosphonium chloride) catalyzed by
monometallic and bimetallic Pd and Au nanoparticles. In water, bimetallic
Au-Pd nanoparticles were demonstrated to oxidize most of the unsaturated
alcohols; however, the particle size growth due to Ostwald ripening was
problematic. Conversely, the monometallic Pd system showed significant
catalytic activity when using tetraalkylphosphonium chloride as a solvent,
likely due to the easy oxidation of Pd in the high-chloride environment,
whereas gold was inactive.
If heterometals with magnetic properties are alloyed into active phases, it
is possible to overcome the drawback of nanoparticle separation and
reuse. 139 Thus, magnetically separable nanocatalysts may have potential
applications in catalysis.
Definite supports can also be chosen to enhance the recyclability
of nanoparticles. Microgels, for example, have been used for this
purpose. 140,141 For instance, Prati and co-workers synthesized microgel-
stabilized Au nanoparticles by using tailor-made soluble cross-linked
polymers as exotemplates and stabilizers. 141 The resulting stabilized system
could be conveniently isolated by precipitation, stored in the solid state and
re-dispersed in polar organic solvents and water. The prepared catalytic
system exhibited remarkable activity in the oxidation of benzylic acid and
polyols under mild conditions.
Looking at both the examples presented and the large number of papers
on catalysis by nanoparticles which have recently appeared in the literature,
we can conclude that significantly increased research in the area of un-
supported nanoparticles may be expected in the coming years. In fact, metal
colloids can actually be prepared following some accurate and reliable
procedures, 142 thus ensuring the synthesis of materials with controlled
characteristics and well-defined crystallographic planes, very close to theo-
retical models. Hence these tuned materials could be used to validate the
preferred reaction for specific plane orientation and also for testing specific
reaction mechanisms.

d n 4 r 4 n g | 1

.

1.5 Microwave-Assisted Reactions
The field of microwave (MW)-assisted chemistry is relatively young. Never-
theless, since the first published reports on the use of MW irradiation to
carry out chemical reaction in 1986, 143,144

the heating of the chemical