Rapid fractionation of noncondensed lignin from birch via acidified LiCl molten salt hydrates

The conventional fractionation of lignocellulose is generally carried out at high temperatures, high acid or alkali, or using organic solvents due to the rigid matrix of lignocellulose and crystallinity of cellulose, leading to the condensation of lignin and environmental pollution, and hindering the further value-added utilization of lignin. In this work, a rapid fractionation method was proposed, in which hemicellulose and cellulose (holocellulose) were first dissolved and partially hydrolyzed in acidified LiCl molten salt hydrates, followed by a filtration process to isolate the lignin solid with a purity of 88.3%. It was found that the dissolution of holocellulose required the addition of a small amount of acid to break the cellulose chain into short chains. Due to the small amount of acid added and the rapid dissolution, the obtained lignin has as high as 89.5% of beta-O-4 bond content close to the theoretical value, suggesting that a promising native lignin structure was preserved. The isolated lignin can be selectively depolymerized into monophenol over Ru/C catalyst with a high yield of 24.2%. In addition, the dissolved holocellulose without containing lignin was selectively hydrolyzed into sugars in acidified LiCl molten salt hydrates, giving 63.1% of xylose, 43.9% of glucose, 29.5% of xylan oligomers, and 48.2% of glucan oligomers, respectively.