Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 140

systems were tested in the selective oxidation of glycerol under basic con-
ditions, for evaluating the effect of the preparation technique on the activity
and selectivity. As expected, the catalytic performances depended markedly
on the gold crystallite size. The sol immobilization method was found to be
the most suitable technique to prepare gold supported on CNTs. In contrast
to what was observed for the catalytic activity, the diverse preparation
methods did not significantly affect the selectivity. The use of MWCNTs as a
supporting material for Au nanoparticles resulted in the oxidation of the
secondary hydroxyl group and a notable high dihydroxyacetone selectivity of
about 60% was attained independently of the preparation protocol. It was
also concluded that dihydroxyacetone in the final mixture can be stabilized
by lowering the pH to about 3.
A bifunctional, ecient and recyclable heterogeneous catalyst consisting
of gold nanoparticles supported on supramolecular ionic liquid-grafted
graphene was recently presented by Shaabani et al. 54 This catalytic system
was employed for the aerobic oxidation of various primary and secondary
aliphatic and aromatic alcohols to the corresponding aldehyde and ketone
derivatives in water as a green solvent and at room temperature. The in-
novative technique of grafting a supramolecular ionic liquid to graphene led
to a suitable support for the stabilization and encapsulation of gold nano-
particles able to control their size and uniform distribution on the support.
Moreover, this heterogeneous graphene-based catalyst displays good dis-
persity in water and removes the need for an added base due to the presence
of basic centers in its structure guaranteeing an environmentally friendly
process with excellent yield and selectivity. Also, the catalyst is stable in the
reaction media, as proved by investigations on recyclability.
Probably the first example of full conversion of alcohols by aerobic oxi-
dation was the recent study of Kobayashi et al., who engineered an in-
novative gold-immobilized microchannel flow reactor. The oxidation of
various alcohols to the corresponding aldehydes and ketones with good to
excellent yields was always achieved, with no gold leaching and steady
catalytic activity for 4 days. 80
It has already been mentioned that the catalytic performance of gold can
be boosted when it is alloyed to other metals, mainly palladium and plat-
inum. Hutchings and co-workers are particularly active in this field, re-
sulting in several papers reporting different methods of preparation of
bimetallic catalysts and their application in the selective oxidation of various
substrates, including alkanes and alcohols. They use benzyl alcohol oxi-
dation as a model reaction aiming at gaining insight into the synergistic
effect between gold and the second metal. One of their most recent studies
involved Au-Pd nanoalloys supported on Mg-Al mixed metal oxides (MgAl-
MMO) prepared using sol-immobilization and tested in the solvent-free
oxidation of benzyl alcohol with molecular oxygen at low pressure and
temperature. 53 When using this support, alloying Pd with Au resulted in an
increase in both activity and selectivity towards benzaldehyde and also an
improved resistance to catalyst deactivation in comparison with the

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.