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

Conversion of condensed lignin into value-added 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.