Valorizing Kraft Lignin by a Catalytic Reductive Depolymerization in Ethanol/Water with Formic Acid as a Supplementary H2 Donor

Lignin is a promising biopolymer to serve as a sustainable resource for a multitude of applications (e.g., thermoset materials, production of bulk chemicals) thereby substituting fossil-based carbon sources. In this work, the reductive depolymerization of Kraft lignin (KL) was studied in ethanol/water aided by formic acid (FA) as the way forward to valorize lignin. The effect of various process conditions was elucidated using P-31-NMR, pH, GPC, CHNSO, GC-MS and 2D-LC analyses. It is found that the addition of a small amount of FA (3.6 vol%) is beneficial for obtaining smaller lignin fragments with more phenolic OH (PhOH) functionalities upon depolymerization at 250 degrees C for 8 h. Besides, a higher FA concentration causes acid catalyzed lignin repolymerization and a longer reaction time results in only a limited reduction in molecular weight of the obtained lignin fragments. The addition of a supported Pd catalyst leads to a more pronounced depolymerization (smaller lignin fragments with more PhOH functionalities) as well as a stronger decrease in oxygen and sulfur content. Furthermore, several experiments and multiple analysis techniques support the hypothesis that FA acts as H-2 donor under the investigated conditions in this study. In conclusion, KL (M-W similar to 16,436 g mol(-1) and 3.31 mmol g(-1) PhOH) was successfully depolymerized into a low molecular weight lignin (M-W similar to 3250 g mol(-1)) with more PhOH functionalities (5.29 mmol g(-1)).