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such supports well suited to commercialization. Pd nanoparticles confined
within such mesoporous silicas demonstrate good selectivity in crotyl and
cinnamyl alcohol selox to their respective aldehydes (470%) and excellent
TOFs of 7000 and 5000 h
1
for the respective alcohols. Similar activities have
been reported for secondary and tertiary allylic alcohols, highlighting the
versatility of silica-supported Pd nanoparticles.
24,38,39,85,86
The benefits of mesostructured supports are not limited to silica. Hackett
et al. showed that ultra-low loadings of palladium impregnated on a
surfactant template mesoporous alumina (350 m
2
g
1
) resulted in atomically
dispersed Pd
21
species.
40
Such single-site catalysts were 10 times more
active in crotonaldehyde and cinnamaldehyde production than comparable
materials employing conventional (100 m
2
g
1
) g-alumina, owing to the
preferential stabilization of higher concentrations of electron-deficient
palladium
37,40
due to either pinning at cation vacancies or metal
d
n
4
r
4
n
g
|
2
support
charge transfer.
87
These Pd/meso-Al
2
O
3
catalysts exhibited similar TOFs to
their silica counterparts (7080 and 4400 h
1
for crotyl and cinnamyl alcohol
selox, respectively),
40
consistent with a common active site and reaction
mechanism (Figure 4.9).
Mesoporous titania and ceria supports have attracted recent interest as
novel catalyst supports. The oxygen storage capacity of ceria-derived
materials is of particular interest due to their facile Ce
31
-
Ce
41
redox
chemistry.
77,88-92
Sacrificial reduction of the ceria supports by reactively
formed hydrogen liberated during the oxidative dehydrogenation of
alcohols could mitigate in situ reduction of oxidized palladium and
hence maintain selox activity and catalyst lifetime, with Ce
41
sites regener-
ated by dissociatively adsorbed gas-phase oxygen.
91,93,94
Owing to its high
density, conventional nanocrystalline cerias possess meagre surface areas
2
.
Figure 4.9
(Left) HAADF-STEM image of atomically dispersed Pd atoms on a
mesoporous Al
2
O
3
support; and (right) associated relationship between
Pd
21
content/dispersion and activity in crotyl alcohol selox over
Pd/alumina catalysts.
Adapted with permission from Ref. 40. Copyright 2007 Wiley-VCH Verlag
GmbH & Co. KGaA.
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