On Laccase-Catalyzed Polymerization of Biorefinery Lignin Fractions and Alignment of Lignin Nanoparticles on the Nanocellulose Surface via One-Pot Water-Phase Synthesis

Two series of well-defined lignin fractions derived from birch and spruce alkaline lignin (AL) by sequential solvent fractionation (i-PrOH-EtOH-MeOH) were engaged in a structure-property-application relationship study. The bacterial-derived alkaliphilic laccase (MetZyme) extensively catalyzed the oxidation and polymerization of AL fractions in an aqueous alkaline solution (pH 10). Lignin fractions with low molar mass reached a higher polymerization degree due to more phenolic-OH groups serving as reactive sites of oxidation and better lignin-laccase accessibility arose from a lower lignin condensation degree than the high molar mass ones. In comparison, AL fractions from spruce were found to be less reactive toward the laccase-catalyzed polymerization than those from birch, which was attributed to the much pronounced aryl-vinyl moieties' oxidation. Furthermore, in situ polymerization of birch AL fractions using microfibrillated cellulose as a structural template was conducted in an aqueous medium and a dispersion of nanocellulose with its fiber network evenly coated by aligned lignin nanoparticles was obtained. The present study not only provides fundamental insights on the laccase-assisted oxidation and polymerization of lignin but also presents a new perspective for valorizing lignin in biobased fiber products through green processing of solvent fractionation and enzymatic treatment.

Automated summary

This study investigated the structure-property-application relationship of lignin fractions derived from birch and spruce alkaline lignin after sequential solvent fractionation. It was found that lignin fractions with low molar mass exhibited higher polymerization degree due to more reactive phenolic-OH groups and better accessibility of lignin-laccase, compared to high molar mass fractions. Additionally, in situ polymerization of birch lignin fractions using microfibrillated cellulose as a template resulted in a dispersion of nanocellulose with lignin nanoparticles coated on its fiber network.