Model-Designed Acid-Assisted Ethanol-Organosolv Fractionation of Coconut Shell Lignin: Structural Decoding and Biofunctional Insights for Green Material Prospects

Coconut shell is a lignin-rich agro-waste. In this study, lignin was extracted from coconut shell by ethanol organosolv catalyzed by hydrochloric acid (HCl). The effects of ethanol concentration (50 to 90%, v/v), time (15 to 90 min), temperature (100 to 190 °C) and HCl loading (0.4 to 4%, w/w) on yield were modeled with a Box–Behnken design. Temperature, time and HCl loading were sig-nificant (R² = 0.95, predicted R² = 0.70). The highest yield (18.6%) was obtained with 70% ethanol and 2.2% HCl at 190 °C for 90 min. This lignin (HCl-Ethanosolv-CSL) was compared with com-mercial alkaline lignin (AlkL). HCl-Ethanosolv-CSL contained 91.7% total lignin and 0.35% ash. FT-IR and NMR showed a G-rich GS-type lignin (S/G = 0.23) with p-hydroxybenzoate esters and ethoxyl groups introduced by HCl-catalyzed reaction with ethanol. Its total phenolic content was 785.9 mg gallic acid equivalent g−1, compared with 580.6 mg g−1 for AlkL. HCl-Ethanosolv-CSL scavenged radicals more effectively than AlkL (in-well IC50 11.6 vs 16.7 µg mL−1 for ABTS). This activity is attributed to free phenolic hydroxyl groups and to ortho-methoxyl groups that stabilize the phenoxyl radical. HCl-Ethanosolv-CSL showed weak, strain-dependent antibacterial activity, mainly against Bacillus species. GPC showed a large apparent molecular size, which indicates strong association of the lignin chains in solution.