Investigation of Dissolved Lignin in Autohydrolysis Liquor

Autohydrolysis (water, temperature, pressure) pretreatment of lignocellulosic biomass is a sustainable unit operation that effectively reduces biomass recalcitrance for downstream sugar production in a biorefinery.  The liquor stream produced is enriched with dissolved hemicellulose and lignin compounds with a distribution of molecular weights.  Valorization of this stream will increase the economic viability of the lignocellulosic biorefinery.  While dissolved hemicellulosic sugars are well-characterized in terms of carbohydrate structure and degree of polymerization, dissolved lignin has received less attention mainly due to the challenge in characterization.  The dissolved lignin is of particular interest since up to 35% of non-wood lignin and 19% of hardwood lignin can be made water soluble during autohydrolysis. Understanding the dissolved lignin will lend more knowledge toward valorization pathways for the autohydrolysis liquor.  Extractive-free non-wood sugarcane bagasse and hardwood sweetgum (L. styraciflua) biomasses were subjected to identical autohydrolysis pretreatments at 180 °C for 40 minutes and the liquor stream was collected through vacuum filtration.  Quantitative GC-MS analysis of the monomeric phenolics in autohydrolysis liquor showed that only ~1% of the dissolved lignin is monomeric, leaving the remaining dissolved lignin uncharacterized.  The predominant lignin monomers in non-wood liquor are para-coumaric acid and vanillin with a total monomer concentration of ~150 mg/L, while in hardwood liquors the dominant molecules were syringaldehyde and vanillin and a total monomer concentration of ~45 mg/L. The liquor was then dried to produce dissolved solids residue followed by solvent extraction to collect the lignin.  The experimental results from high-resolution MS for chemical structure, GPC for molecular weight, and NMR for inter-lignin bonding structures will be discussed.