Catalysis Science & Technology
Paper
C–O BDE. Similar behaviour was observed for the β-O-4
model compounds, with the addition of methoxy and alcohol
functional groups increasing the complexity of the product
distribution. The catalytic oxidation of phenolic monomers
such as phenol and guaiacol resulted in the formation of
polyphenols/guaiacols. These species were found to inhibit
substrate conversion and reduce the mass balance of α-O-4
model compounds by incorporation of oxidation products
into polymeric species. In comparison to phenol and
guaiacol, catalytic oxidation of syringol yielded dipheno- and
benzoquinones, the selectivity for the formation of which was
dependent on the ratio of Co to substrate. However, the
diphenoquinone was susceptible to over-oxidation. Analysis
of the time-course data of syringol oxidation using the analyt-
ical solution to the rate equation of Neuenschwander and
Hermans17 demonstrated that the oxidation could usefully be
modelled to a first approximation by this simplified mecha-
nistic scheme. The successful application of this mechanistic
scheme will be explored for other aromatic substrates, such
as 4-tert-butyltoluene, and other lignin model compounds in
future investigations. The substitution of Br− for NHPI as the
radical promoting co-catalyst reduced the deleterious effects
of phenols for the conversion of a model α-O-4 compound,
improving the yield and selectivity of 3,4DMB-CHO and
guaiacol products. Preliminary application of NHPI to an ac-
tual lignin source afforded a modest improvement in the
yield of benzoquinone product, but, not unexpectedly, the to-
tal yield of aromatic aldehydes was lower. These results dem-
onstrate that NHPI can be a more selective radical promoter
for pure lignin model compounds than Br−, while further in-
vestigation is necessary to improve the oxidation of lignin
with Co/NHPI/H2O2.
edges the receipt of an Australian Postgraduate Award. This
work was supported by the Australian Research Council.
Notes and references
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Conflicts of interest
The authors declare no conflict of interest.
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