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

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., beta - 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 C alpha-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.

Automated summary

An efficient phenol-assisted depolymerisation (PAD) process was developed to selectively cleave methylene linkages in condensed lignins. Phenol served as a solvent to dissolve lignin and also scavenged methylene linkages to facilitate the depolymerisation. The PAD process achieved higher yields of lignin monomers and bisphenols compared to existing depolymerisation methods, and the residual polyphenols could be used in adhesive synthesis for plywood applications.