Separation and catalytic depolymerization of empty palm fruit bunch lignin

Abstract

Lignin is a heterogeneous and complex polymer and a potential renewable source of aromatic compounds as precursors to many industrially useful products. Microbial biotransformations for lignin valorization are reported but require not only its separation from biomass matrix but also breakdown into its monomeric components. Abundantly available empty palm fruit bunch was used as the lignin-rich biomass to process it to obtain a mixture of lignin-derived monomeric aliphatic and aromatic compounds. A two-step acid-alkali treatment was deployed to extract lignin and its breakdown into smaller fragments. Dilute nitric acid treatment could extract 80% of the lignin and the hydrolysate was fractionated through a series of nanofiltration membranes. The residual polymeric lignin obtained after monomer separation was catalytically depolymerized using Cu/Al2O3 under optimized conditions i.e., 180 °C, 7 MPa, 60 min, methanol: water ratio as 1:1 to give 80% conversion. Lignin mixture at every step was analyzed by GC-MS and gel permeation chromatography (GPC). The scheme resulted in 37% of the overall lignin being obtained as monomers.