Water-Accelerated Photooxidation and Degradation of Lignin Linkages Mediated by Plasmonic Catalysts

The selective oxidation and degradation of lignin are crucial for realizing its potential as a biofuel or petroleum substitute. Despite the importance of C–C bond cleavage for lignin valorization, this process is significantly challenging. Herein, we present plasmonic gold nanoparticles (Au NPs) as environmentally friendly and reusable photocatalysts for the chemoselective oxidation of the benzylic hydroxyl groups of lignin and subsequent lignin degradation. The oxidation process is driven by the generation of superoxide ions (O₂^•−), leading to proton release and initiating lignin photooxidation through a mechanism termed plasmon-driven hydrogen atom abstraction and degradation (p-HAADe). Our results demonstrate the significant suppression of lignin oxidation and degradation in acetonitrile-rich environments, while aqueous conditions notably enhance these processes. Furthermore, two distinct time-dependent regimes are identified, namely, the "oxidation dominant" regime, where lignin oxidation is predominant, and the "degradation dominant" regime, favoring C_α–C_β bond cleavage. These findings provide crucial insights into optimizing lignin conversion in biofuel applications, highlighting the potential of Au NPs for use in sustainable chemical processes.