Acid-controlled lignin enrichment beyond native biomass level: High-lignin cellulose nanofibrils characteristics and...

This study proposed an acid-assisted lignin enrichment strategy for the preparation of high-lignin cellulose nanofibrils (HLCNF, defined here as lignin-containing CNFs with lignin contents exceeding that of the native feedstock). Through dilute sulfuric acid pretreatment of the applewood sample, hemicellulose and amorphous cellulose were preferentially removed, progressively increasing the relative lignin fraction from 23% in the raw material to ∼60%. The lignin-enriched substrate, together with the pretreatment-induced matrix loosening, facilitated fiber dissociation and enabled efficient mechanical fibrillation, resulting in nanofibrils with average diameters of 11.5-13.5 nm. Further characterization revealed that surface-associated lignin promoted markedly improved thermal stability, enhanced organic solvent compatibility, excellent ultraviolet absorption and hydrophobicity of the resulting HLCNF. As a performance demonstration, HLCNF was coated onto paper surfaces. The more hydrophobic and compact HLCNF coating enabled the paper substrate to achieve excellent barrier performance. Under optimized conditions (20 g/m² coating content with hot-pressing), the coated paper showed a Cobb value of 3.3 g/m², a WVTR of 12.1 g/(m²·d), and no detectable edible oil penetration. This work provided a framework for elevating lignin fractions beyond the native level while retaining a nanofibrillar network and contributing to the development of a sustainable bio-based material.