A novel
strategy for the oxidative cleavage of C–C and C–O
bonds in a series of model substrates including β-O-4 lignin
model compounds using CuCl2/polybenzoxazine composites
catalysts with H2O2 as oxidant at room temperature
showed good conversions (up to 88%) and an over 96% total selectivity
to aromatic monomers within 2 h. This approach then succeeded in application
to actual lignin depolymerization monitored by gel permeation chromatography
(GPC), 1H NMR, and 2D-NMR (HSQC). The results suggest that
lignin can be effectively degraded into an array of functionalized
dimer–trimeric aromatic acids, aldehydes, phenols, etc., obtained
from the selective cleavage of aliphatic C–C and C–O
bonds in the major linkages β-O-4′ aryl ethers, resinols,
and p-hydroxycinnamyl alcohols while leaving the
natural aromaticity intact. Furthermore, the mechanistic insights
into the catalytic reactions reveal a two-electron transfer process
involved with a phenoxy radical, and an oxidation-then-cleavage route
proposed for lignin depolymerization. The clean process, mild reaction
conditions, and high aromatics selectivity indicate that it is a promising
heterogeneous catalytic system for oxidative depolymerization of lignin
to valuable aromatic chemicals.
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