Chemical modification of kraft lignin using black liquor heat treatment

Abstract Utilizing kraft technical lignin to produce value-added products is one of the obstacles associated with kraft mill work as an operational biorefinery. The objective of this work was to evaluate chemical modification in eucalypt kraft lignin after heat treatment of black liquor. To prove possible structural changes, advanced analytical tools were used, such as pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC/MS), Fourier transform infrared spectroscopy (FTIR), and two-dimensional nuclear magnetic resonance spectroscopy (2D NMR). Kraft lignins extracted from heat-treated liquors compared with the untreated liquors showed a high total lignin content, varying between 96.4 and 98.3 %, and a lower content of sugars and ash. Py-GC/MS, FTIR and 2D NMR analyses showed interesting modification on the lignin structure, such as a reduction in S-type and G-type lignin and an increase in H-type and Ca-type lignin with heat treatment of the black liquor. The heat treatment of the black liquor proved to be efficient in the chemical modification of the extracted kraft lignins. Demethylation and demethoxylation reactions occurred and they generated kraft lignin with free phenolic groups and that were rich in catechol groups.