Visible light-enabled selective depolymerization of oxidized lignin by an organic photocatalyst†
The development of an economic, environmental-friendly and energy-saving process for the selective depolymerization of lignin is an outstanding challenge. Herein, a novel and efficient visible-light-induced photocatalytic process for the selective depolymerization of lignin model compounds and organosolv lignin was first developed by using perylene diimide (PDI) as a metal-free organocatalyst. Interestingly, it can completely decompose the oxidized lignin models to phenolic and ketone fragmentation molecules with very high selectivity at room temperature under visible light illumination. Furthermore, the use of a home-made photocatalytic continuous-flow reactor efficiently shortened the reaction time within an hour. Even for organosolv lignin, nearly 86% mass ratio of lignin was degraded to low-molecular-mass monoaromatic or diaromatic products. We found that superior performances were realized by single-electron transfer (SET) from the photoexcited strongly reducing PDI˙− anion to the ketone groups of the β-O-4 linkage in the lignin.