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

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 L-DMSO 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 (M-w, 2050-2430 g/mol) to MWL. On the other hand, the fraction L-H20 prepared with EMIMAc/H2O contained relatively abundant carbohydrates (11.19%) and had a high M-w (4310 g/mol) than MWL. HSQC spectra analysis revealed that corncob lignin consisted of beta-O-4, beta-beta, beta-5, beta-1 linkages, and cinnamyl alcohol end groups. In addition, the amounts (Ar/100Ar) of beta-O-4 units increased in the order of L-H2O < L-DMSO < MWL. (C) 2012 Elsevier B.V. All rights reserved.