Investigation on the structure and antioxidant properties of modified lignin obtained by different combinative processes of oil palm fronds (OPF) biomass

• Different combinative pretreatments were used to isolate lignin from OPF.
• Combined autohydrolysis pretreatment gave smaller lignin fragment with higher phenolic-OH content.
• Repolymerization of lignin during dilute sulphuric acid treatment affects its structure and antioxidant activity.
• 2-Naphthol has been confirmed to reduce lignin condensation reaction.
• Higher phenolic-OH content and lower Mw of lignin may affect its antioxidant properties.

The present work reports on the structural characteristic and antioxidant activity of the ethanol organosolv lignin obtained from oil palm fronds (OPF) via different pretreatment combinative processes. Physicochemical analyses of pretreated lignins have shown that autohydrolysis method prior to organosolv pulping gave smaller fragments of lignin with higher phenolic hydroxyl content compared to dilute sulphuric acid method. Indeed, repolymerization tends to occur during dilute sulphuric acid pretreatment, affecting its lignin structure and antioxidant activity. Chemical modification of lignin by utilizing organic scavenger (2-naphthol) seems to improve the properties of lignin by reducing the possibility of condensation reaction. Oxygen uptake measurement has shown that the antioxidant activity of the different pretreated organosolv lignins (autohydrolyzed lignin, AH EOL: 82% > autohydrolyzed + 2-naphthol lignin, AHN EOL: 78% > dilute sulphuric acid treated lignin, DAP EOL: 75%) were closely related to its average molecular weight and phenolic hydroxyl content.