Structural Characterization of Pre-hydrolysis Liquor Lignin and Its Comparison with Other Technical Lignins

The kraft-based dissolving pulp production process can be a model for the forest biorefinery concept. The objective of this study is to characterize the dissolved lignin present in the pre-hydrolysis liquor (PHL) to facilitate its subsequent use following the forest biorefinery concept. In this work, lignin was isolated from PHL by acidification using dilute H2SO4, followed by purification through dissolution in a dioxane solution (9:1) and re-precipitation with diethyl ether. The characteristics of PHL lignin were compared with those of the dioxane lignin, acetic acid (AA) lignin and ethanosolv (EL) lignin isolated from the same mixed hardwood (maple, poplar and birch wood chips, in a ratio of 7:2:1), which is the raw material used for dissolving pulp production. The obtained lignin samples were characterized by UV, FTIR, 1H-NMR spectroscopy, molecular weight determination, elemental and methoxyl analyses. The results showed that the absorptivity of dioxane lignin at 276 nm was 10.0 l g-1cm-1, while that of PHL lignin was 17.2 l g-1cm-1. The presence of condensed structures in the PHL lignin was also observed in the FTIR spectrum (strong bands at 870 and 890 cm-1), which was also present in the AA and EL lignins. The lignin isolated from PHL had a lower molecular weight and methoxyl group per C9 unit, in comparison with other lignin samples. 1H-NMR analysis indicated a significant increase in the phenolic hydroxyl content in the PHL lignin, caused by cleavage of aryl-ether bonds during the prehydrolysis.