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

The oxidative depolymerization of alkali lignin (AL) catalyzed by copper-containing imidazolium-based ionic liquids ([C 4 C 1 im]CuCl 3 ) was investigated with CH 3 OH-H 2 O (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), [C 4 C 1 im]CuCl 3 presented a relatively remarkable catalytic capacity for AL oxidative depolymerization due to the cooperative effects of [C 4 C 1 im] + and CuCl 3 − . [C 4 C 1 im]CuCl 3 molar concentration ( x McIL ) greatly influenced AL conversion and product selectivity. The highest AL conversion of 70.4% was achieved at x McIL  = 0.18 mol.%. For the obtained small-molecule products, the highest selectivity of p -hydroxybenzaldehyde reached 51.3% at x McIL  = 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 [C 4 C 1 im]CuCl 3 , and the H unit in lignin was preferentially destroyed. The molecular interactions and microenvironment of [C 4 C 1 im]CuCl 3 –solvent (CH 3 OH-H 2 O) system were investigated with x McIL 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 [C 4 C 1 im]CuCl 3 strongly depended on the interactions of [C 4 C 1 im]CuCl 3 and solvent molecules with changes in microstructure organization. The optimum catalytic capacity of [C 4 C 1 im]CuCl 3 was obtained when the ion pair formed through hydrogen bonding at x McIL  = 0.18–0.36 mol.%. The solvation of solvent molecules on CuCl 3 − and the formation of ionic cluster with the hydrogen-bonding self-association of [C 4 C 1 im]CuCl 3 were unfavorable for the oxidative depolymerization of lignin.