Depolymerization of Lignin to Aromatics by Selectively Oxidizing Cleavage of C–C and C–O Bonds Using CuCl2/Polybenzoxazine Catalysts at Room Temperature

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.