Selective C-C bond cleavage of oxidized lignin in an aqueous phase under mild conditions

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/CeO 2 catalysts can selectively catalyze the C α -C β 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 C α -C β 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 CeO 2 support which facilitates active Pd 2+ 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 (&z.rad;O 2 − ) formation are the key steps for the selective C α -C β 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. Efficient and selective C-C cleavage offers the potential to depolymerize lignin under mild and green conditions to form valuable chemicals.