Temperature-tuned hydrated DES/oxidation conversion of hemp stalks into carboxylated nanocellulose

Hemp stalks are abundant renewable agricultural residues and promising feedstocks for nanocellulose, yet green and controllable pathways for converting their cellulose fraction into stable nanoscale materials remain limited. Here, an agricultural-residue-oriented hydrated deep eutectic solvent (H-DES)/oxidation synergistic deconstruction strategy was developed for hemp stalks, focusing on the temperature-governed balance among component fractionation, cellulose retention, carboxylation, and nanofibrillation. The treatment removed non-cellulosic components while introducing surface carboxyl functionality mainly via oxalic-acid mediated esterification, affording a cellulose-rich solid containing 90.6% cellulose, 5.3% hemicellulose, and 3.0% lignin. Unlike conventional 2,2,6,6-Tetramethylpiperidinooxy (TEMPO) oxidation, which selectively converts C6 primary hydroxyl groups into carboxylates under alkaline conditions, this H-DES/oxidation route integrates fractionation and surface esterification in one medium and preserves crystalline cellulose domains more effectively. Consequently, the obtained nanocellulose showed high colloidal stability, with a zeta potential of -61.1 mV, an average width of 4.6 nm, stability for more than 7 days, and superior thermal stability. The nanocellulose was further assembled into robust films with a tensile strength of 145 MPa and lightweight aerogels with a compressive strength of 27.6 kPa at 90% strain and a thermal conductivity of 0.251 W·m^-1·K^-1. This study elucidates the relationship among processing conditions, structural evolution and material properties during the production of carboxylated nanocellulose from hemp stalks, thereby providing a practical foundation for the sustainable valorization of agricultural residues into high value cellulose materials.