Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 95

Ultimately, heterogeneous palladium catalysts may offer significant
advantages over homogeneous analogues in respect of initial catalyst cost,
product separation and metal recovery and recyclability. 25 Catalyst devel-
opment can therefore no longer be considered simply a matter of reaction
kinetics, but as a clean technology where all aspects of process design, such
as solvent selection, batch/flow operation, catalyst recovery and waste
production and disposal are taken into account. 26 The ecacy of platinum-
group metal (PGM) surfaces towards the liquid-phase oxidation of alcohols
has been known for over 50 years, 27 and the development of heterogeneous
platinum selox catalysts (and more recently coinage metals such as gold) 28,29
are the subject of several reviews. 2,30-32 Hence here we focus on palladium
selox catalysis.

d n 4 r 4 n g | 2

4.2.3 Particle Size Effects
In the field of nanocatalysis, it is well known that particle size is an
important parameter influencing both activity and selectivity. This reflects
the combination of quantum and geometric effects associated with the
respective evolution of electronic properties from atomistic to condensed
matter band structure and transition from low to high coordination surface
atoms, with increasing nanoparticle size and dimensionality. Kaneda and
co-workers hypothesized that the unique reactivity of Pd 2060 clusters sup-
ported on titania towards aromatic alcohol selox arose from a distribution of
Pd 0 ,Pd 1 and Pd 21 surfaces sites, with p-bonding interactions between the
phenyl group and Pd 21 species facilitating subsequent oxidative addition of
the O-H bond by neighbouring Pd 0 and eventual b-hydride elimination. 33
Surface palladium hydride was then hypothesized to react with oxygen from
a neighbouring Pd 2 O centre forming H 2 O and regenerating the metal site
(Figure 4.2). Optimal activity for cinnamyl alcohol selox to cinnamaldehyde
coincided with clusters possessing the maximum fraction of Pd 1 character.

.

Figure 4.2 HRTEM image of Pd/TiO 2 selox catalyst and the suggested Pd nanopar-
ticle model predicting the presence of different oxidation states and
surface environments.
Adapted from Ref. 33 with permission from The Royal Society of
Chemistry.