Lignin, the second largest component of biomass, is considered
as an important alternative source of fossil reserves for the production
of fuels and chemicals. Here, we developed a novel method to oxidatively
degrade organosolv lignin into value-added four-carbon esters, particularly
diethyl maleate (DEM), with the cooperative catalyst consisting of
1-(3-sulfobutyl) triethylammonium hydrogen sulfate ([BSTEA]HSO4) and 1-butyl-3-methylimidazolium ferric chloride ([BMIM]Fe2Cl7). Under optimized conditions (1.00 MPa initial
O2 pressure, 160 °C, 5 h), the lignin aromatic ring
was effectively cleaved by oxidation to form DEM with a yield of 15.85%
and a selectivity of 44.25% in the presence of the synergistic catalyst
of [BMIM]Fe2Cl7-[BSMIM]HSO4 (1/3,
mol/mol). The structure and composition analysis of lignin residues
and liquid products confirmed that the aromatic units in lignin were
effectively and selectively oxidized. Furthermore, the catalytic oxidation
of lignin model compounds was explored for obtaining a possible reaction
pathway of oxidative cleavage of lignin aromatic units to DEM. This
study provides a promising alternative method for the production of
traditional petroleum-based chemicals.