Manipulation of lignin condensation pathways to produce different bisphenols from biomass

Valorization of lignin is essential for establishing a profitable biorefining industry, yet its efficiency is critically constrained by lignin condensation. Understanding lignin condensation mechanisms and developing effective strategies to inhibit condensation have emerged as a global research imperative. Despite several successful practices in stabilizing the α carbon position of lignin under mild temperatures, a mechanistic understanding of lignin condensation under high temperatures and the corresponding strategy for preventing such condensation remain lacking. Herein, using monophenol as a probe molecule to capture reactive lignin fragments produced under high-temperature treatments, we unveil a novel condensation pathway involving the C_γ-position, which arises from the nucleophilic reaction of the C_γ-position and aromatic rings of lignin. We further demonstrated that phenol, serving as both a radical scavenger and a competitive nucleophilic reactant (vs. lignin aromatic rings), could suppress C_γ-condensation reactions among lignin fragments and, meanwhile, enable the production of novel lignin-derived bisphenols (1,3-diaryl propanes) with a molar yield of 35.2% (mass yield of 45.9 wt %) from eucalyptus feedstock. The mechanistic finding represents a significant advance in lignin chemistry and enables the manipulation of lignin condensation pathways in a wider temperature range for the production of valuable bisphenols (1,1-diaryl propane, 1,2-diaryl propane, and 1,3-diaryl propane), which can serve as precursors for synthesizing biobased polymers and sustainable aviation fuels.