Effect of ionic liquid/organic solvent pretreatment on the enzymatic hydrolysis of corncob for bioethanol production. Part 1: Structural characterization of the lignins

Abstract

Corncob was submitted to pretreatments with 1-ethyl-3-methylimadazolium acetate and water/organic solvents (DMSO, DMF, and DMAc) followed by alkaline extraction to isolate lignin. The lignin fractions obtained were comprehensively characterized by alkaline nitrobenzene oxidation, HPAEC, GPC, TGA, UV, FT-IR and HSQC spectra. The results showed that a maximum yield of 85.04% (based on the original lignin) was achieved for LDMSO prepared with the EMIMAc/DMSO pretreatment. The fractions prepared with EMIMAc/organic solvents contained lower amounts of carbohydrates (0.48–1.40%) than milled wood lignin (MWL, 8.73%) and had similar molecular weights (Mw, 2050–2430g/mol) to MWL. On the other hand, the fraction LH2O prepared with EMIMAc/H2O contained relatively abundant carbohydrates (11.19%) and had a high Mw (4310g/mol) than MWL. HSQC spectra analysis revealed that corncob lignin consisted of β-O-4, β-β, β-5, β-1 linkages, and cinnamyl alcohol end groups. In addition, the amounts (Ar/100Ar) of β-O-4 units increased in the order of LH2O<LDMSO<MWL.