Structural characterization of birch lignin isolated from a pressurized hot water extraction and mild alkali pulped biorefinery process

In this work we have characterized a mild soda pulped birch (betula pendula) lignin after a novel pressurized hot water extraction pilot process. In the process hemicelluloses are first isolated from woods chips with hot water at oxygen-starved conditions, followed by mild alkali pulping at oxygen-starved conditions to remove lignin from the cellulose. Lignin was precipitated, washed and fractionated from the black liquor and characterized by quantitative ¹³C NMR, ¹³C DEPT experiments, ³¹P NMR, 2D HSQC, FTIR, size exclusion chromatography (SEC), elemental analysis, pyrolysis-gas chromatography-mass spectrometry (py-GC–MS), methoxyl group quantification and residual carbohydrate analysis after methanolysis. The structure was compared to a MWL of birch obtained from the same wood chips and the differences were an increase in phenolic hydroxyl groups, carboxylic groups, polydispersity index, and reduction of aliphatic hydroxyl groups. The process resulted in β-O-4 cleavage and condensation of the structure. This lignin is therefore suitable to be used in its polymeric form, rather than for degradation of the structure to small building blocks.