Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 98

decarbonylation reaction over clean, metallic palladium above 0 1Cto
liberate gas-phase propene and strongly bound CO and propylidene,
which may act as site blockers poisoning subsequent catalytic selox cycles.
Unexpectedly, pre-adsorbed atomic oxygen switched off undesired
decarbonylation chemistry, promoting facile crotonaldehyde desorption.

d n 4 r 4 n g | 2

4.2.5 Nature of the Active Site
The preceding observation that surface oxygen appears critical not only for the
removal of hydrogen adatoms but also to suppress decarbonylation of selox
products over metallic palladium is in excellent agreement with an in situ
attenuated total reflectance infrared (ATR-IR) spectroscopy study of cinnamyl
alcohol selox over Pd/Al 2 O 3 by Baiker and co-workers. 51 In an earlier body of
work employing aqueous electrochemical analysis, the same group postulated
that oxidative dehydrogenation of alcohols requires PGM catalysts in a
reduced state, with 'over-oxidation' responsible for deactivation of selox
catalysts. 32 Asubsequentoperando X-ray absorption spectroscopy (XAS) study
by Grunwaldt et al.,whichcloselymirroredthatpreviouslyreportedbyLee, 52
evidenced in situ reduction of oxidized palladium within an as-prepared
Pd/Al 2 O 3 catalyst during cinnamyl alcohol oxidation using a continuous flow
fixed-bed reactor. 53 Unfortunately, the reaction kinetics were not measured in
parallel with XAS to explore the impact of palladium reduction on catalysis;
however, a follow-up study of 1-phenylethanol selox employing the same re-
actor configuration (and oxygen-deficient conditions) demonstrated a strong
interplay between selox conversion/selectivity and palladium oxidation state. 54
The authors concluded that metallic Pd was the catalytically active species, an
assertion re-armed in follow-up in situ ATR-IR/XAS measurements of
benzyl 55-57 and cinnamyl alcohol 58 selox in toluene and under scCO 2 .Itis
interesting that in each of these studies the introduction of oxygen to the
reactant feed instantaneously improved alcohol conversion and aldehyde
production (Figure 4.5), observations which were attributed solely to hydrogen
abstraction from the catalyst surface 59,60 rather than to a change in palladium
oxidation state. In contrast to their liquid-phase experiments, high-pressure
X-ray absorption near edge structure (XANES) and extended X-ray absorption
fine structure (EXAFS) measurements on Pd/Al 2 O 3 -catalysed benzyl alcohol
selox in scCO 2 led Grunwaldt et al. to conclude that maximum activity arose
from particles mainly oxidized in the surface/shelf-edge. 61
In a parallel programme of research, Lee and co-workers set out to char-
acterize systematically the physicochemical properties of palladium nano-
particles as a function of size over non-reducible supports in order to
quantify structure-function relations in allylic alcohol selox. 36-40,62,63 The
combination of XPS and XAS measurements revealed that freshly prepared
alumina- 37,40 and silica-supported 38,62 nanoparticles are prone to oxidation
as their diameter falls below

.

4 nm, with the fraction of PdO proportional
to the support surface area and interconnectivity. Complementary kinetic
analyses uncovered a direct correlation between the surface PdO content and

B