From pulp to aromatic products – Kinetic modeling of hydrothermal lignin depolymerization

Lignin is a promising renewable feedstock for producing aromatic platform chemicals due to its high content of aromatic structures linked by various chemical bonds. In the pulp industry, lignin occurs in large quantities dissolved in the black liquor providing a possible feedstock for depolymerization into interesting aromatic monomers. This work studied the lignin depolymerization process under hydrothermal conditions (TR = 300 – 350 °C, tR = 0 – 30 min, p = 200 bar), using a black liquor based on eucalyptus wood as feedstock. The aim was to understand the main reaction pathways of the lignin depolymerization. Experiments were performed in batch micro autoclaves and the product phases were separated and characterized. The work focused on the principal aromatic monomers, mainly different forms of catechol. Based on the experimental results a reaction scheme was set up consisting of the aromatic monomers, lumped groups like oligomers, and gaseous compounds. A kinetic model was developed, parametrized, and trained, providing adequate results. The calculated kinetic parameters and the developed reaction scheme were compared with data from literature. ... mehrIn accordance with our model, the main pathway from lignin to methylated catechols is via 3-methoxycatechol and catechol. Finally, experimental results of hydrothermal liquefaction (HTL) with softwood black liquor were compared with the calculated yield curves and showed limited deviations. Hence, this implies that the model, with minor adjustments, can be used for the HTL of softwood black liquor as the dissolved salts presumably have a significantly greater influence than the type of wood.