Phenol-assisted depolymerisation of condensed lignins to mono-/poly-phenols and bisphenols

Abstract

Depolymerisation of lignin to well-identified aromatic chemicals has been recognized as one of the most promising ways to valorise lignin. Widely investigated hydrogenolysis and oxidative depolymerisation strategies selectively disrupt labile interunit C - O linkages (e.g., β − O − 4 linkages) but are ineffective on the disruption of robust interunit C - C linkages (e.g., methylene linkages) in condensed technical lignins. Herein, an efficient phenol-assisted depolymerisation (PAD) process was developed to selectively celave methylene linkages in condensed lignins. In the process, phenol not only served as a solvent to facilitate lignin dissolution but also scavenged methylene linkages from condensed lignin polymers to facilitate the depolymerisation. Results showed that scavenging Cα-induced methylene moieties resulted in monomers without a side chain whereas scavenging formaldehyde-induced methylene moieties led to monomers with side chains. Lignin monomers (or monophenols) yields up to 40 % and bisphenols yields up to 42 % could be achieved via the new PAD process, which were better than those resulting from existing depolymerisation methods. Besides, the residual polyphenols could be used to synthesize lignin-formaldehyde adhesives for plywood applications. We anticipate that this practical lignin valorisation method would create additional revenue for the pulp industry and enable economic biorefineries.