Electrocatalytic cleavage of aryl ether C-O linkages in lignin model dimers

Electrocatalytic hydrodeoxygenation (ECH) is a promising strategy for upgrading lignin-derived compounds to hydrodeoxygenation products under mild conditions. Here, we investigated the C − O bonds cleavage and oxygen-containing intermediates hydrogenation of lignin-derived aryl ether dimers, including Diphenyl ether (4-O-5 linkage), Benzyl phenyl ether (α-O-4 linkage), and Phenethoxybenzene (β-O-4 linkage), in a Phosphotungstic acid-Pt/C based fluidized ECH system. The ECH of three kinds of aryl ether dimers all afforded >90% of the corresponding hydrodeoxygenation products under the optimized conditions (80 °C and a current density of 25 mA cm−2 within 90 min). It showed some difference in the reactivity of α-O-4, 4-O-5, and β-O-4. The reaction rate of the CO bond decreased in the order of α-O-4 > 4-O-5 > β-O-4. The CO bond of the α-O-4 and β-O-4 linkages was mainly cleaved by hydrogenolysis and reductive hydrolysis, respectively. In comparison, the CO bonds of 4-O-5 linkage were cleaved by parallel hydrogenolysis and reductive hydrolysis. This difference was mainly attributed to the reaction rate of the further hydrogenolysis of the Caliphatic-OH fragment (produced by the hydrolysis of the α-O-4 and β-O-4 linkages). While phenol (produced by hydrolysis of the 4-O-5 linkage) mainly generated cyclohexanol by hydrogenation pathway.