Freeze-thaw assisted maleic acid pretreatment of eucalyptus to prepare cellulose nanocrystals and degraded lignin

[Display omitted] •Eucalyptus was separated by freeze–thaw assisted maleic acid treatment.•Freeze-thaw conditions were −18 ℃, 12 h, and thawing at 25 ℃.•Removal of cellulose and hemicellulose up to 74.5% and 95.2%.•CNCs had uniform size, excellent thermal stability, and high crystallinity.•Lignin had high phenolic hydroxyl content and excellent homogeneity (PDI ≤ 1.86). A green and effective method is proposed for the pretreatment of eucalyptus by freeze–thaw assisted maleic acid tactic, wherein the effects of freeze–thaw, maleic acid concentration, reaction time, and temperature on the fractionation were examined. Results showed that under optimal conditions (60% maleic acid, 120 °C, and 2 h), a remarkable removal of 74.5% lignin and 95.2% hemicellulose was achieved after freeze–thaw treatment. The resulting cellulose-rich solid residues were further processed with maleic acid to prepare cellulose nanocrystals, which displayed uniform sized rod-like structures and high crystallinity (62.51%). Moreover, maleic acid pretreatment resulted in lignin with low molecular weight (2110–2530) and excellent homogeneity (PDI ≤ 1.86), while maintaining a relatively intact structure. The lignin had high β-O-4 aryl ether bond contents (≥77.5%) and abundant phenolic hydroxyl contents (2.33–3.63 mmol/g). Overall, the process exhibits notable benefits in effectively separating lignocellulose for high valorization.