O<inf>2</inf>-permeable membrane reactor for continuous oxidative depolymerization of lignin

Depolymerization of lignin into aromatic monomers is one of the highest priority targets for valorization of lignin obtained from biomass pretreatment. Oxidative lignin depolymerization proceeds rapidly under alkaline conditions at elevated temperature with O2; however, the aromatic products are susceptible to degradation under the same conditions, complicating practical application of these conditions. Here, we report a continuous-flow aerobic alkaline lignin depolymerization method using an O2-permeable membrane reactor. The flow reactor allows for continuous oxygen delivery to the alkaline lignin solution and precise control of the temperature and reaction time. Reaction time-course analysis provides direct insights into the rates of lignin depolymerization and monomer decomposition, enabling process optimization. Aromatic yields up to 43 wt % are observed with a residence time of less than 4 min. This process is applied to the depolymerization of multiple lignin materials derived from different biomass pretreatment methods and from both softwood and hardwood sources.