Comparative Evaluation of p-Toluenesulfonic and Maleic Acid for Lignin Extraction from OPEFB via Microwave-Assisted Fractionation

The valorization of oil palm empty fruit bunches (OPEFB) through effective lignin extraction is essential for sustainable biomass utilization. In this study, a microwave-assisted acid hydrotropic fractionation approach was developed using p-toluenesulfonic acid (p-TsOH) and maleic acid (MA) to achieve rapid and selective delignification under mild conditions. Unlike conventional hydrothermal or solvent-based pretreatments, this method integrates microwave irradiation with organic acid hydrotropes, enabling enhanced energy transfer, reduced processing time, and minimized cellulose degradation. Lignin yield and quality were strongly influenced by acid type, concentration, and reaction time, with the optimal condition (P80t30T120 and MA80t60T120) yielding 85.27% and 63.69% lignin, respectively, surpassing most reported acid-based fractionation systems operating under similar or harsher conditions. Structural analyses (FTIR, ¹H NMR) confirmed the retention of syringyl and guaiacyl units, while thermal analysis revealed higher stability and glass transition temperature for p-TsOH-derived lignin. The p-TsOH-treated fiber (cellulose-rich residue) exhibited enhanced surface fibrillation and higher crystallinity (71.36%) compared to the MA-treated fiber (66.44%) and raw EFB (47.87%). These findings suggest that the microwave-assisted hydrotropic system is effective for achieving high lignin recovery while maintaining the cellulose-rich structure of the residual fibers, offering a green and efficient strategy for integrated biomass fractionation and the production of functional bio-based materials.