Oligomeric fragments distribution, structure and functionalities upon ruthenium-catalyzed technical lignin depolymerization

Abstract

The development of effective lignin depolymerization is essential for the extraction of aromatic building blocks from renewable resources. In this study, technical lignins (Soda and Kraft) were depolymerized in supercritical ethanol using Ru/C as a catalyst in an inert (N2) or a reducing (H2) atmosphere. The effects of lignin structure and atmosphere were studied on the depolymerization, and the products formed such as solid residue, oligomeric fragments and monomers. Reasonable yields of oligomers and monomers (32.7 wt% and 4.7 wt%, respectively) were obtained after Ru/C-assisted Soda lignin depolymerization under both tested atmospheres, while in the case of Kraft lignin, H2 and Ru/C were required to increase the yields of depolymerization products (30.9 wt% and 4.4 wt%, respectively). A comprehensive characterization of the reaction products (GC-MS, UV fluorescence and FTIR spectroscopy, 31P and 2D HSQC NMR) was performed to evaluate the monomer/oligomer distribution and to determine the structure and functionalities of the oligomers. The analytical results indicate the formation of more condensed oligomeric lignin structures with predominant 5-5′ inter-unit linkages and the prevalence of demethylation or demethoxylation reactions depending on the type of the technical lignin exposed to hydrodeoxygenation. The oligomeric fragments are catalytically modified and depolymerized lignin with a wide range of new applications (coatings, resins, antioxidant agents, adhesives) contributing to the replacement of the petroleum-derived phenolic building-blocks.