Microwave-assisted acid–alkali pretreatment of oil palm empty fruit bunches: Sequence effects, delignification kinetics, and enzymatic saccharification

Oil palm empty fruit bunches (OPEFB), generated during oil palm (Elaeis guineensis Jacq.) processing, are promising lignocellulosic substrates for fermentable sugar production. However, untreated OPEFB exhibits poor saccharification because its lignin–hemicellulose framework restricts cellulase accessibility. This study investigated microwave-assisted OPEFB pretreatment using 1% H₂SO₄ and 1% NaOH under single-step and sequential acid–alkali schemes. Microwave power, duration, and reagent order were evaluated through compositional analysis, statistical analysis, SEM/FTIR/XRD characterization, enzymatic hydrolysis, and delignification kinetic modelling. The sequential dilute acid–alkali route gave the best result at 450 W and 100 °C for 30 min, producing a solid residue with 45.63% cellulose and 24.14% lignin, with 71.52% delignification. Enzymatic hydrolysis generated 13.66 g/L total reducing sugar (TRS) and an 85.35% TRS yield, representing a 4.2-fold increase in TRS concentration relative to untreated OPEFB. Delignification followed a third-order model, with R² values of 0,984 at 90 °C, 0.989 at 95 °C, and 0.986 at 100 °C. The activation energy was 81.9 kJ/mol, lower than reported values for related lignocellulosic delignification processes, including 100.71 and 98.5 kJ/mol, suggesting a lower energy requirement for lignin removal. SEM, FTIR, and XRD confirmed surface disruption, lignin/hemicellulose-related chemical changes, and crystallinity changes. Overall, microwave-assisted sequential dilute acid–alkali pretreatment improved OPEFB delignification and pretreatment efficiency under relatively mild conditions, while producing a reducing sugar-rich hydrolysate with potential as a fermentable carbon source for downstream bioconversion and integrated biomass valorization toward the production of biofuels and renewable chemicals.