Valorizing Recalcitrant Cellulolytic Enzyme Lignin via Lignin Nanoparticles Fabrication in an Integrated Biorefinery

Conversion of condensed lignin into valueadded products in current lignocellulosic biorefineries has been challenging due to its structure recalcitrance. However, this work showed a technically feasible route to valorize recalcitrant cellulolytic enzyme lignin (CEL: lignin residue after enzymatic hydrolysis) via high-quality lignin nanoparticles (LNPs) fabrication. Three representative CELs obtained from hydrolysis of industrial relevant, steam-pretreated, agriculture reside corn stover, hardwood poplar, and softwood lodgepole pine were evaluated for their potential to produce LNPs through the prevalent dialysis method, which gave a LNPs yield of 81.8%, 90.9% and 41.0% with a corresponding average particle size of 218, 131, and 104 nm, respectively. The obtained high-quality LNPs were in sphere-like shapes, abundant with functional groups, and highly stable from pH 4 to 10, which showed tremendous promise for the applications in the emerging nanomaterial fields. When the substructures of these three LNPs were further characterized using prevalent 13C and 2D-HSQC NMR. techniques, they showed that their structure recalcitrance followed the order of lodgepole pine LNPs > poplar LNPs > corn stover LNPs. It was also apparent that the biomass lignin condensation occurring during steam pretreatment could be considered as a hydrophobic modification, which benefits the self-assembling of LNPs to small particle sizes and regular shapes.