Lignin depolymerization and condensation during acidic organosolv delignification of Norway spruce (Picea abies)

When organosolv pulping is conducted under acidic conditions, two lignin reactions are most often associated with the rate determining step. They are alkyl aryl ether bond cleavage and condensation, both of which were investigated in the present study. Norway spruce (Picea abies) was selected as the only wood species because of its wide application in the pulping industry and its lignin having the reputation of being a typical softwood lignin. The presence of solvents in the pulping liquor improved delignification. The extent of delignification increased with an increase in the mole fraction of the solvents up to {dollar}{bsol}sim{dollar}0.50. Addition of an acid catalyst was necessary to obtain a significant delignification and a concentration of {dollar}{bsol}sim{dollar}0.01N H{dollar}{bsol}sb2{dollar}SO{dollar}{bsol}sb4{dollar} was close to optimum at 175{dollar}{bsol}sp{bsol}circ{dollar}C.; The phenolic hydroxyl contents of the residual lignins (in the pulp) were more than twice as high as that for the native lignin. These phenolic hydroxyl groups were generated by the cleavage of {dollar}{bsol}beta{dollar}-O-4 ethers, which was deemed to be the most important process. The concentration of {dollar}{bsol}beta{dollar}-O-4 dimers in the woodmeal and pulp lignins were estimated by dioxane/HCI acidolysis. Excellent correlation between the yield of acidolysis products and (a) lignin content and (b) % delignification were obtained for 15 different solvent/water formulations (6 different solvents). Each solvent mixture differed from others in terms of solubility parameter and acidity function.; There was a gradual decrease in the delignification rate as pulping proceeded. Permanganate oxidation was used to investigate whether the reduced reactivity of the residual lignin was due to an increase in condensed structures via condensation reactions, or selective removal of uncondensed structures. At 80% delignification, the uncondensed structures had decreased from {dollar}{bsol}sim{dollar}55% of the native lignin to {dollar}{bsol}sim{dollar}35%. Selective removal of uncondensed structures by cooking liquor was primarily responsible for accumulation of condensed structures in residual lignin. It was observed from both the pulp and model compound studies that condensation reactions were minor. As in kraft pulping, the cleavage of {dollar}{bsol}beta{dollar}-aryl ethers appeared to be rate determining.