High-Yield Production of Aromatic Aldehydes from Lignin via Oxidative Depolymerization

This study explores copper-catalyzed oxidative depolymerization (CCOD) of enzymatic hydrolysis lignins derived from aspen thermomechanical pulp (HW-EHL) and ball-milled softwood mechanical pulp (SW-EHL). Reaction conditions for HW-EHL depolymerization were optimized using a design of experiments (DOE) approach and subsequently applied to SW-EHL. Overall, the ideal conditions were temperatures of 160–180 °C, reaction durations of 0.75–1.25 h, and oxygen pressures of 135–240 kPa. Under these conditions, high monomer yields were achieved─up to 37% for HW-EHL and 23% for SW-EHL based on their respective lignin content─comparable to those obtained from milled wood, highlighting the strong potential of EHLs as feedstocks for aromatic monomer production. The process was successfully scaled up, and a protocol for the isolation of crystalline vanillin from the CCOD reaction mixture is provided. These findings underscore the value of integrating enzymatic hydrolysis with mechanical pretreatments for woody biomass valorization.