Lignin monomer production integrated into the γ-valerolactone sugar platform

Abstract

We demonstrate an experimental approach for upgrading lignin that has been isolated from corn stover via biomass fractionation using γ-valerolactone (GVL) as a solvent. This GVL-based approach can be used in parallel with lignin upgrading to produce soluble carbohydrates at high yields (≥70%) from biomass without the use of enzymes, ionic liquids, or concentrated acids. The lignin was isolated after an initial hydrolysis step in which corn stover was treated in a high-solids batch reactor at 393 K for 30 min in a solvent mixture consisting of 80 wt% GVL and 20 wt% water. Lignin was isolated by precipitation in water and characterized by 2D HSQC NMR, showing that the extracted lignin was similar to native lignin, which can be attributed to the low acid level and the low extraction temperatures that are achievable using GVL as a solvent. This lignin was upgraded using a two-stage hydrogenolysis process over a Ru/C catalyst. The isolated lignin was first dissolved to form a mixture of 10% lignin, 80% THF, 8.5% H₃PO₄ and 1.5% H₂O, and treated at 423 K under hydrogen. The THF was removed by evaporation and replaced with heptane, forming a biphasic mixture. This mixture was then treated at 523 K in the presence of Ru/C and H₂. The resulting heptane phase contained soluble lignin-derived monomers corresponding to 38% of the carbon in the original lignin. By adding 5% methanol during the second catalytic step, we produced additional monomers containing methyl esters and increased carbon yields to 48%. This increase in yield can be attributed to stabilization of carboxylic acid intermediates by esterification. The yield reported here is comparable to yields obtained with native lignin and is much higher than yields obtained with lignin isolated by other processes. These results suggest that GVL-based biomass fractionation could facilitate the integrated conversion of all three biomass fractions.