Isolation and Characterization of Organosolv and Alkaline Lignins from Hardwood and Softwood Biomass

Isolation of lignins from hardwood and softwood biomass samples, containing 26.1% and 28.1% lignin, respectively, has been performed with the use of alkaline and organosolv pretreatment methods. The effect of catalyst loading, ethanol content, particle size, and pretreatment time on the yields and properties of the isolated lignins were investigated. Alkaline lignins had higher carbohydrate content - up to 30% - and exhibited higher molecular weights in the range of 3000 Da, with a maximum phenolic hydroxyl content of 1 mmol g−1 for birch and 2 mmol g−1 for spruce. Organosolv lignins, on the other hand, showed high purity - 93% or higher - despite the more extensive biomass dissolution into the pretreatment medium; they also exhibited a lower range of molecular weights between 600 and 1600 Da depending on the source and pretreatment conditions. Due to the lower molecular weight, phenolic hydroxyl content was also increased, reaching as high as 4 mmol g−1 with a simultaneous decrease in aliphatic hydroxyl content as low as 0.6 mmol g−1. Efficient lignin dissolution of 62% for spruce and 69% for birch, achieved at optimal pretreatment conditions, was combined with extensive hemicellulose removal.