Chemistry Reference
In-Depth Information
Although highly ordered, mesoporous alumina facilitates geraniol and
farnesol selox, the lack of macroporosity results in slower TOFs and lower
selectivities (
o
70%) than achieved with hierarchical macroporous-
mesoporous silica, further evidencing substrate mass transport as the
rate-limiting factor for the oxidation of bulky alcohols and indicating a
competing hydrogenation pathway. Pd-impregnated hydroxyapatites are
also versatile catalysts for the selox of primary, secondary and long-chain
alcohols with 480% selectivity to aldehydes and ketones, although they have
only been trialled at low substrate : catalyst ratios, hence their viability for
large-scale production remains to be proved.
111,121
A number of groups have attempted solvent-free aerobic selox, particularly
of benzyl alcohol, wherein the substrate necessarily is employed at molar
ratios far in excess of the catalyst. Palladium on silica, g-alumina and mixed
SiO
2
-Al
2
O
3
have been applied in this respect, with TOFs increasing
across this sequence of oxide supports. Pd/SiO
2
-Al
2
O
3
was also reported to
oxidize secondary alcohols, including phenylethanol, non-activated alcohols
such as cyclooctanol, 2-octanol and 1-octanol, and even the heterocyclic
2-thiophenemethanol, to their various aldehydes and ketones.
34
Support
functionalization, whether via modified pore architecture or surface acid-
base properties, is thus an effective means for optimizing catalyst activity
without sacrificing selectivity.
In summary, supported monometallic Pd catalysts can selectively oxidize
primary and secondary allylic and aromatic alcohols, limited heterocyclic
alcohols and (upon incorporation of macroporous networks) longer chain
alcohols including phytol, geraniol and farnesol. However, significant
selox activities have yet to be reported for linalool, a tertiary alcohol, and
non-allylic alcohols, leading to the postulate that the allyl bond is a
prerequisite for activation of the -OH bond, possibly by ensuring the correct
molecular orientation to bring the alcohol function close to the active site or
strengthening adsorption of the molecularly bound parent alcohols or a
reactive intermediate to facilitate thermal activation.
Palladium promotion by gold is advantageous in terms of both enhancing
activity and suppressing unwanted side reactions (hydrogenolysis and
decarbonylation).
47,64,130
Hutchings and co-workers demonstrated this
ecacy under solvent-free conditions.
121
Palladium surface enrichment of
Au catalysts increased the selectivity and TOFs towards benzyl alcohol, crotyl
alcohol, 1,4-butanediol and 1-phenylethanol with respect to those possible
over the individual metal components; product selectivities were reported to
exceed 90%, a marked increase on those reported for palladium. In some
instances, these impressive solvent-free performances were attained for
extremely high substrate : catalyst ratios, exceeding 180 000, improving the
green credentials of heterogeneously catalysed aldehyde/ketone production.
Unfortunately, these bimetallics were inert towards the selox of secondary
alcohols such as 2-octanol and 1,2-butanediol, attributed to a combination
of steric factors, strongly bound ketone adsorption and self-poisoning.
Au-Pd systems prepared via simple wetness impregnation were also
d
n
4
r
4
n
g
|
2
.
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