Monitoring technical lignin partition in aqueous/alcohol biphasic systems according to pH: influences of the molecular structure and solvent characteristics

This study deals with the comprehension of lignin partition in biphasic systems constituted of an alkaline aqueous phase containing dissolved lignin and a non-miscible alcohol phase. The main parameters studied are the botanical origin of lignins (softwood/hardwood/herbaceous), the solvent (n-butanol/n-pentanol/n-octanol) and the pH (12, 9.5, 7, 3.5). Lignin concentration is measured by UV spectroscopy to calculate partition coefficients. Lignin migration is driven by the protonation of hydroxyl phenolates at pH 12–9.5 and of carboxylates at pH 7–3.5. FTIR spectra revealed various functional groups in migrated fractions as well as their change in conformation. Notably, hydrogen bonding capacity within its chemical groups and with neighboring fragments and solvents are discussed. NMR results suggest a possibility to adjust separation conditions to tune the lignin composition including the G/S ratio and the content in specific structures (resinol, ferulate, and carbohydrate). GPC highlights a wide diversity of polymers of low molecular weight migrating at basic pH whereas increasing high molecular weight is correlated with aqueous phase acidification. The lignins seem to migrate according to similar structural features whatever the botanical origin, leading to very similar polymer distributions under given conditions. In general, butanol appears to be less selective and is able to solubilize a wide range of lignin polymers independent of the origin. This work provides valuable information regarding the separation of homogeneous lignin fractions enabling standardization of the chemical and physical properties of the lignins to increase their value and the numerous potential applications as a sustainable material.