Stabilization with Aldehydes for the High Yield Production of Targeted Monomer Mixtures from Lignin during Integrated Biomass Depolymerization

Lignin is the fraction of lignocellulosic biomass with the highest energy density and is one of the few natural large-scale sources of aromatic molecules. However, the lack of practical depolymerization processes that can produce a limited number of lignin monomers at high yields has limited the options for upgrading lignin. The development of these processes has been restricted by the presence of inter-unit carbon-carbon bonds within native lignin, and further by formation of such linkages during lignin extraction. We report that adding aldehydes and forming their associated protection group during biomass pretreatment produces a soluble lignin fraction that can be converted to guaiacyl and syringyl monomers at near-theoretical yields during subsequent hydrogenolysis. Up to a 48% yield can be achieved from wild type wood while yields as high as 78% can be achieved from high-syringyl transgenic poplar with low levels of carbon-carbon linked units. Furthermore, the nature of the aldehyde and upgrading conditions can be used to reduce the distribution of monomers. The resulting monomer yields were 3-7 times greater than those obtained without using aldehydes as protection groups, which prevented lignin condensation primarily by forming 1,3-dioxane structures with lignin side-chain hydroxyl groups. By depolymerizing cellulose, hemicelluloses, and lignin separately, overall monomer yields between 76 and 90% were achieved for these three major biomass fractions. Recovery of an isolated lignin fraction that is soluble in an organic solvent and can produce high yields of a limited number of monomers could facilitate continuous lignin processing and the development of heterogeneous catalysts for lignin conversion.