Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 93

4.2 Aspects of Heterogeneous Palladium Catalysts
in the Selective Oxidation of Alcohols

4.2.1 Introduction
The heterogeneously catalysed, aerobic selective oxidation (selox) of
alcohols, carbohydrates and related a,b-unsaturated substrates is particu-
larly demanding owing to the requirement to activate molecular oxygen
and C-O bonds in close proximity at a surface in a solid-liquid-gas
environment. 2,3 The past decade has seen significant progress in under-
standing the fundamental mode of action of palladium-derived hetero-
geneous catalysts for such transformations and the associated reaction
pathways and deactivation processes. 4 This insight has been aided by
advances in analytical methodologies, notably the development of in situ
or operando (under working conditions) spectroscopic 5-7 /microscopic 8-11
tools able to provide quantitative, spatio-temporal information on
structure-function relations of solid catalysts in the liquid and vapour
phase. Parallel improvements in inorganic synthetic protocols offer finer
control over preparative methods to direct the nanostructure (composition,
morphology, size, valence and support architecture) of palladium
catalysts 12-14 and thereby enhance activity, selectivity and lifetime in an
informed manner.

d n 4 r 4 n g | 2

4.2.2 Practical Considerations for Heterogeneous Selox
Catalysts
The scientific, technological and commercial importance of green
chemistry presents a significant challenge to traditional selox methods,
which employ hazardous and toxic stoichiometric oxidants, including
permanganates, chromates and peroxides, with concomitant poor atom
eciencies, and require energy-intensive separation steps to obtain the
desired carbonyl or acid product. Alternative heterogeneous catalysts
utilizing oxygen must not only offer high activity and selectivity in alcohol
selox, but also be scalable in terms of both catalyst synthesis and
implementation. For example, continuous flow microreactors have been
implemented in both homogeneous and heterogeneous oxidations
(Figure 4.1), and offer facile catalyst recovery from feed streams for the
latter, 15,16 but their scale-up/out requires complex manifolding and to
ensure adequate oxygen dissolution and uniform reactant mixing and
delivery. 17,18 Efforts to overcome mass-transport and solubility issues in-
herent to three-phase palladium-catalysed oxidations have centred around
the use of supercritical carbon dioxide (scCO 2 ) to facilitate rapid diffusion
of substrates to and products from the active catalyst site at modest
temperatures 19 affording enhanced turnover frequencies (TOFs), selectivity
and on-stream performance versus conventional batch operation in liquid
organic solvents. 20-24

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