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 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, 2050–2430 g/mol) to MWL. On the other hand, the fraction LH2OLH2O prepared with EMIMAc/H2O contained relatively abundant carbohydrates (11.19%) and had a high Mw (4310 g/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

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► A novel pretreatment with IL and H2O/organic solvents was used to isolate lignin.
► A maximum yield of 85.04% (based on the original lignin) was achieved for LDMSO.
► Lignins prepared with EMIMAc/organic solvents had the lowest content of sugars.
► The Mw of LH2OLH2O prepared with EMIMAc/H2O pretreatment was around 2-fold of MWL.
► The amounts (Ar/100Ar) of β-O-4   units increased in the order of LH2O