Performances of Several Solvents on the Cleavage of Inter- and Intramolecular Linkages of Lignin in Corncob Residue

The performances of solvents, including gamma-butyrolactone (GBL), gamma-valerolactone (GVL), tetrahydrofuran (THF), ethyl acetate (EAC), 2-methyltetrahydrofuran (2-MeTHF), and the corresponding mixtures with H2O, on the cleavage of inter- and intramolecular linkages of lignin in corncob residue were investigated. At 200 degrees C, miscible cosolvents (H2O-GBL, H2O-GVL, and H2O-THF) exhibited much better efficiency for lignin dissolution than that of both immiscible cosolvents (H2O-EAC and H2O-2-MeTHF) and pure solvents. The synergetic effect between H2O and organic solvent significantly promoted the breakage of intermolecular linkages between C6-O-H of amorphous cellulose and lignin. GBL and THF solvents preferentially dissolved lignin with H and G units, whereas GVL, EAC, and 2-MeTHF solvents exhibited high selectivity for the dissolution of lignin with S and G units. In addition to dissolution, the intramolecular beta-O-4 linkage in lignin could be selectively cleaved in H2O-GBL cosolvent, whereas the beta-O-4, alpha-O-4, and -5 linkages were cleaved in H2O-EAC, H2O-THF, and H2O-2-MeTHF cosolvents. At 300 degrees C, the breakage of the beta-gamma bond prior to beta-O-4 in H2O-GBL, H2O-THF, H2O-EAC, and H2O-2-MeTHF produced 4-ethylphenol and 4-ethylguaiacol selectively (accounting for approximate to 70% of the total identified monophenols), whereas the alpha-1 bond was preferably broken in H2O-GVL to form guaiacol (accounting for approximate to 75% of the total identified monophenols).