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. In this work, the lignin depolymerization process under hydrothermal conditions ( T R = 300 – 350 °C, t R = 0 – 30 min, p = 200 bar) was studied, 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 main 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. According to 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 can be used for the HTL of softwood black liquor with minor adjustments, as the dissolved salts presumably have a significantly greater influence than the type of wood. • Bio-based aromatics can be produced from lignin dissolved in black liquor via HTL • Catechol and its derivates are the main compounds under the investigated conditions • A kinetic model is developed based on experimental data • Good alignment between kinetic model and literature data • Additional data shows flexibility of the model for other black liquors