Three-step conversion of Indulin AT to muconic acid under mild conditions

The conversion of technical lignin into valuable chemicals is important for the complete utilization of lignocellulose. The kraft process dominates the pulp industry, and large amounts of kraft lignin are produced worldwide. However, the complexity of lignin limits its valorization. In this study, a kraft lignin, Indulin AT, was depolymerized under base conditions, and the effects of substrate loading, NaOH concentration, temperature, and hydrogen peroxide on depolymerization were investigated. The production of aromatic monomers is favored by a continuous flow process, even with a very short reaction time (2 min vs. 30 min in a batch reactor). Higher temperature and NaOH concentration led to a higher degree of depolymerization, and thus more guaiacol and vanillin. A guaiacol yield of 5.1% was obtained after depolymerization of 5 wt% of lignin and 2 wt% of NaOH at 200 °C with a residence time of 2 min. When taking the whole operation, including the cost and depolymerization efficiency, into consideration, the concentration, lignin loading of 4 wt% of lignin, NaOH concentration of 2 wt%, and the temperature of 210 °C were chosen as the optimal conditions for the production of a guaiacol-rich fraction. The permeate obtained by ultrafiltration of the depolymerized Indulin AT sample contained a higher amount of guaiacol, which was quantitatively converted to muconic acid using an engineered strain of Pseudomonas putida KT2440. The three-step process in this work involves base-catalyzed depolymerization, ultrafiltration, and biological conversion represent an efficient approach to convert Indulin AT lignin into muconic acid.