Nature-Inspired Depolymerization of Soda Lignin: Light-Induced Free Radical Promoted Cleavage of β-O-4 Bonds

Abstract

The development of sustainable valorization methods for lignin is a challenging task because the vast majority of the reported studies involve either harsh conditions or expensive transition metal catalysts. Inspired by the sunlight degradation of lignin compounds, known as lignin yellowing, the use of a commercially available cheap organic photoinitiator, namely, phenacyl bromide (PAB) is reported here, for the efficient cleavage of lignin model compound 2-phenoxyacetophenone (2-PAP) and for the depolymerization of soda pulped lignin (SL) under UV-A irradiation and ambient conditions. Real-time NMR investigations of the photoreaction between 2-PAP and PAB shed light on the possible reaction mechanisms involving different radical species, HBr, and molecular oxygen. Interestingly, combined spectral, chromatographic, powder X-ray diffraction, and thermal studies of the photoreaction between PAB and SL indicate the formation of guaiacyl alcohol as the main product. The unprecedented performance of PAB is attributed to the excess generation of phenacyl radicals, the generation of photolabile brominated species, and HBr playing key roles in the cleavage of β-O-4 linkages. This work represents a new edge for sustainable lignin valorization under mild reaction conditions and offers the opportunity for large-scale production of valuable aromatics using technical lignins as feedstock.