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

Abstract

The performances of solvents, including γ-butyrolactone (GBL), γ-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 °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 β−O−4 linkage in lignin could be selectively cleaved in H2O–GBL cosolvent, whereas the β−O−4, α−O−4, and β−5 linkages were cleaved in H2O–EAC, H2O–THF, and H2O–2-MeTHF cosolvents. At 300 °C, the breakage of the β−γ bond prior to β−O−4 in H2O–GBL, H2O–THF, H2O–EAC, and H2O–2-MeTHF produced 4-ethylphenol and 4-ethylguaiacol selectively (accounting for ≈70 % of the total identified monophenols), whereas the α−1 bond was preferably broken in H2O–GVL to form guaiacol (accounting for ≈75 % of the total identified monophenols).