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Selective cleavage of C–C bonds in the lignin structure is crucial for the depolymerization of lignin and producing value-added chemicals. In this study, we reported that Pd/CeO2 catalysts can selectively catalyze the Cα–Cβ bond cleavage of oxidized lignin model components and authentic lignin in an aqueous phase under mild conditions (50 °C, ambient pressure). A variety of β-O-4 and β-1 ketones with representative Cα–Cβ bonds were successfully cleaved yielding aromatic acids and phenols in 76.8–94.9% yields. Their unique catalytic properties are derived from the electron transfer between Pd and CeO2 support which facilitates active Pd2+ sites and reactive oxygen species formation. A mechanistic study elucidates that base-mediated C–H bond activation via enolization and in situ reactive oxygen species (˙O2−) formation are the key steps for the selective Cα–Cβ cleavage. Furthermore, the applicability of the system for the bond cleavage in an authentic lignin sample was demonstrated. Nearly 54.2 wt% of oxidized bamboo lignin was converted, generating aromatic monomer yields up to 35.9 wt%, with a high selectivity for aromatic aldehydes. [ABSTRACT FROM AUTHOR]

The conversion of lignin into aromatics as commodity chemicals and high-quality fuels is a highly desirable goal for biorefineries. However, the presence of robust inter-unit carbon–carbon (C–C) bonds in natural lignin seriously impedes this process. Herein, for the first time, we report the selective cleavage of C–C bonds in β-O-4 and β-1 linkages catalyzed by cheap copper and a base to yield aromatic acids and phenols in excellent yields in water at 30 °C under air without the need for additional complex ligands. Isotope-labeling experiments show that a base-mediated Cβ–H bond cleavage is the rate-determining step for Cα–Cβ bond cleavage. Density functional theory (DFT) calculations suggest that the oxidation of β-O-4 ketone to a key intermediate, i.e., a peroxide, by copper and O2 lowers the Cα–Cβ bond dissociation energy and facilitates its subsequent cleavage. In addition, the catalytic system could be successfully applied to the depolymerization of various authentic lignin feedstocks, affording excellent yields of aromatic compounds and high selectivity of a single monomer. This study offers the potential to economically produce aromatic chemicals from biomass. [ABSTRACT FROM AUTHOR]

The catalytic oxidation of lignin into phenolic monomers with aldehydes as the main products is very attractive but the yield of desired monomers is still limited. The aim of this work is to investigate the reaction pathway for the conversion of lignin to aldehydes and aldehydes stabilization in alkaline media. Studies on pure aldehydes stability revealed that over-oxidation to carboxylic acids and condensation are crucial factors of limited yield. A chemical protection method using aniline as the capping agent was applied to the depolymerization of real lignin and achieved the theoretical maximum monomer yield within 30 min at 160 °C. Aldehydes formation and over-oxidation and/or condensation during the alkaline oxidative depolymerization of native lignin. [Display omitted] • New insight into the mechanism of aldehydes formation based on intermediates experiments and DFT calculations. • Lignin β-O-4 linkage undergoes γ-hydroxyl oxidation followed by rapid retro-aldol C α –C β breakup. • Overoxidation to small carboxylic acids and condensation are crucial factors of limited aldehydes yield. • A novel chemical protection method was applied to stabilize aldehydes by using aniline as a capping agent. [ABSTRACT FROM AUTHOR]