Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 96

A strict particle size dependence was also reported for the catalytic
transformation of benzyl alcohol over Pd nanoparticles dispersed on alu-
mina, SiO 2 and NaX zeolite supports. 34,35 For Pd/NaX and Pd/SiO 2 -Al 2 O 3 ,
benzyl alcohol selox was maximized for particle sizes between 3 and 5 nm,
whereas geraniol and 2-octanol were insensitive to structure. Systematic
studies of particle size with cinnamyl and crotyl alcohols over amorphous
and mesostructured alumina and silica supports have likewise uncovered
pronounced size effects in both initial selox rates and TOFs, 36-39 which in-
crease monotonically with shrinking nanoparticle diameters even down to
single atoms. 40 High-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) measurements show that atomically dis-
persed palladium exhibits maximum rates towards benzyl, cinnamyl and
crotyl alcohols, with selectivities to their corresponding aldehydes of 470%.
We revisit the origin of such size effects later in this chapter. The application
of colloidal Pd nanoclusters for selox in aqueous media is rather limited 41-43
owing to the high anity for Pd aggregation and subsequent formation of Pd
black, which hinders catalytic performance. However, Lu and co-workers
successfully stabilized 3.6 nm Pd nanoclusters using Pluronic P123, an
amphiphilic, non-ionic triblock copolymer. In the selective oxidation of
benzyl alcohol, 100% aldehyde selectivity and high conversion rates were
obtained, even after 13 recycling reactions, indicating no decrease in activity
due to sintering and a route to overcome the stability problems which nor-
mally plague colloidal Pd nanoparticles. 44

d n 4 r 4 n g | 2

4.2.4 Surface Reaction Mechanism
The rational design and optimization of active and selective palladium selox
catalysts require a microscopic understanding of both the active catalytic
species responsible for alcohol and oxygen activation and the associated
reaction pathway to the aldehyde product and any competing processes.
A key characteristic of palladium is its ability to perform the desired selox
chemistry at temperatures typically between 60 and 160 1C and atmospheric
oxygen pressure, 2,45

.

via a long-accepted oxidative dehydrogenation route

(Figure 4.3). 2,30
Whether O-H or C-H scission of the a-carbon is the first oxidation step
remains a matter of debate, since the only fundamental studies over well-
defined Pd(111) surfaces to date employed temperature-programmed X-ray
photoelectron spectroscopy (XPS) 46 and metastable de-excitation spec-
troscopy (MDS) 47 with temporal resolutions on the second-minute time-
scale over which loss of both hydrogens appeared coincident. However,
temperature-programmed mass spectrometric 48 and vibrational 49 studies of
unsaturated C 1 -C 3 alcohols implicated O-H cleavage and attendant alkoxy
formation over Pd single-crystal surfaces. 45,50 It is generally held that the
resultant hydrogen adatoms react with dissociatively absorbed oxygen to
form water, which immediately desorbs at ambient temperature, thereby
shifting the

formation. 2,30

equilibrium to carbonyl In any event,