Lignin modification through steam explosion pretreatment: A comparison with direct organosolv pulping

This study investigates the effects of different delignification processes on oil palm frond (OPF) biomass, focusing on the structural characteristics and antioxidant properties of lignin isolated with organosolv pretreatment and a combined pretreatment of organosolv and steam explosion (SE). Functional groups in organosolv lignin (OL) and steam explosion organosolv lignin (SEOL) were analyzed and compared using multiple complementary analyses such as FTIR spectroscopy, NMR spectroscopy, GPC chromatography, CHN and thermal analyses, lignin antioxidant activity via reducing power assay and dissolution test. The findings reveal that SEOL achieved the highest solid recovery at 11.5 % yield among the samples, surpassing OL, which yielded 8.7 %. Both isolated lignin types predominantly feature non-condensed G-type and S-type units, with relatively fewer H-type units, consistent with existing research. Moreover, this study confirms that modification of lignin with SE pretreatment reduces its molecular weight (M_w) and generates smaller fragments, enhancing its water solubility relative to unmodified lignin (D_SEOL: 37.4 % > D_OL: 17.5 %). This demonstrates that SEOL exhibits enhanced antioxidant properties with superior reducing power compared to OL. Consequently, the modifications have improved the structural properties and antioxidant capabilities of lignin polymers, paving the way for innovative lignin-based applications.