Selective production of p-hydroxybenzaldehyde in the oxidative depolymerization of alkali lignin catalyzed by copper-containing imidazolium-based ionic liquids

Abstract

The oxidative depolymerization of alkali lignin (AL) catalyzed by copper-containing imidazolium-based ionic liquids ([C4C1im]CuCl3) was investigated with CH3OH-H2O (9/1, v/v) as solvent under an initial oxygen pressure of 1.5 MPa at 150 °C for 3 h. Compared with other transition-metal chlorides and metal-containing ionic liquids (ILs), [C4C1im]CuCl3 presented a relatively remarkable catalytic capacity for AL oxidative depolymerization due to the cooperative effects of [C4C1im]+ and CuCl3−. [C4C1im]CuCl3 molar concentration (xMcIL) greatly influenced AL conversion and product selectivity. The highest AL conversion of 70.4% was achieved at xMcIL = 0.18 mol.%. For the obtained small-molecule products, the highest selectivity of p-hydroxybenzaldehyde reached 51.3% at xMcIL = 0.72mol.% (corresponding yield of 103.6 mg·g−1). The inter-unit linkages of lignin were effectively cleaved in the oxidative depolymerization catalyzed by [C4C1im]CuCl3, and the H unit in lignin was preferentially destroyed. The molecular interactions and microenvironment of [C4C1im]CuCl3–solvent (CH3OH-H2O) system were investigated with xMcIL varied from 0.05 mol.% to 5.12 mol.% by using attenuated total reflectance-infrared spectroscopy and molecular-dynamics method, respectively. The catalytic properties of [C4C1im]CuCl3 strongly depended on the interactions of [C4C1im]CuCl3 and solvent molecules with changes in microstructure organization. The optimum catalytic capacity of [C4C1im]CuCl3 was obtained when the ion pair formed through hydrogen bonding at xMcIL = 0.18–0.36 mol.%. The solvation of solvent molecules on CuCl3− and the formation of ionic cluster with the hydrogen-bonding self-association of [C4C1im]CuCl3 were unfavorable for the oxidative depolymerization of lignin.