The influence of interunit carbon–carbon linkages during lignin upgrading

The cleavage of β-O-4 linkages in lignin can generate monomers with a phenyl propane structure that can easily be upgraded into valuable hydrocarbon biofuels and renewable aromatic chemicals. High-yield lignin monomer production from extracted (or technical) lignin that is produced in a practical way could facilitate the productivity and profitability of biomass conversion processes. However, interunit carbon–carbon (C–C) linkages present in native lignin or formed during lignin condensation in biomass pretreatments dramatically reduce lignin monomer yields. Here, we present a perspective on biological and chemical strategies that have been successfully used to reduce the formation of C–C linkages in native or technical lignin. We analyze the mechanisms involved in these strategies and offer our views on improving the quality of technical lignin resulting from biomass conversion in order to achieve high-yield lignin monomer production.