Oxidative Biphasic Depolymerization (BPD) of Kraft Lignin at Low pH

Abstract

AbstractIn this work, we systematically investigated factors governing catalytic oxidation of Kraft lignin in a biphasic system at acidic conditions. The process is designated biphasic depolymerization (BPD) and is based on the concept of protective extraction of the target molecules from the reactive aqueous phase containing catalysts into an inert organic phase. The aim is to prevent over‐oxidation, one of the main causes of decreased yields of high‐value monomeric aromatics from depolymerization of lignin. Vanillin, a typical and commercially relevant product of lignin oxidation, was chosen as an indicator for understanding which factors impact the process. The conversion in water/octanol mixtures was shown to be more effective than in a monophasic system at corresponding conditions showing the proof of concept for BPD. A set of conversion conditions maximizing the yields of the target molecule was identified (T=170 °C, t=60 min, pO2=5 MPa). The presence of the oxidative catalyst (CuSO4) did not influence the conversion. The contributions of oxidation and acidolysis of lignin were assessed to be approx. 60 and 40%, respectively. Based on the results, a strategy for optimizing BPD, a promising technique for valorization of technical lignins in acidic media, is laid out.