3-43: Fractionation and Recovery of Lignin and Xylan from Alkali Liquors – Characterization for Biorefinery Processes

Understanding the behavior of plant cell wall biopolymers during alkaline biorefining processes and how their properties are affected by the processing conditions is an important analysis for maximizing value from woody lignocellulosic feedstocks. In this work, the fractionation of alkali pulping liquors was investigated to characterize the extractability, recoverability, and properties of the solubilized hemicelluloses and lignin during the alkali impregnation and heat-up stages of alkali organosolv pulping or soda pulping for three hardwood species: sugar maple (Acer saccharum), silver birch (Betula pendula), and a hybrid poplar (Populus nigra x maximowiczii cv. NM6).  Recoverability of the solubilized biopolymers were determined using a variety of methods including aggregation and precipitation by acidification.  The recovered materials were analyzed for composition, while the xylans were analyzed for uronic acid content and molecular weight distributions by gel-permeation chromatography (GPC) and a novel reducing end technique.  Lignins were characterized by GPC, gravimetrically by solubility-based partitioning in different organic solvents, and by ¹H, ¹³C, and ³¹P NMR.  Under the conditions tested, only 10-20% of the total cell wall xylan was present in the liquor at any time while up to 50% of the lignin was solubilized.  The xylan showed a decreasing degree of polymerization (DP) with increasing temperature, which stabilized at a DP between 10-30.  The recovered lignins exhibited differences in their solvent partitioning behavior and these results were found to be correlated to the quantified properties of the lignin.