Klara Hackenstrass: Modelling of lignin fundamentals in the context of kraft cooking

Abstract

Although the Kraft process has been widely employed to convert wood into pulp for decades, accurately modelling the delignification process remains challenging. The term “delignification” refers to the removal of lignin, one of the major components of wood. During Kraft cooking, lignin is fragmented and solubilized, enabling its transport out of the wood chip. However, the hierarchical structure of wood and the heterogeneous nature of lignin’s chemical structure complicate the understanding of the underlying mechanisms and material properties influencing delignification.

In this thesis, atomistic- and continuum-scale modelling are used to investigate how changes in lignin’s chemical structure influences its properties. At the atomistic level, inter-unit linkages, molecular weight, chemical functionality, and ionic charge are systematically varied in Molecular Dynamics simulations. The investigated properties include conformational flexibility, lignin-lignin and lignin-solvent interactions, as well as the thermodynamics of solubility. In addition, self-diffusion coefficients derived from the atomistic model are integrated into a Finite Element framework to simulate delignification on the continuum-scale.

Overall, the studies demonstrate that systematic variation of lignin’s chemical structure reveals which structural element have the strongest influence on specific properties, thereby  enabling extrapolation of atomistic insights to larger‑scale phenomena. In particular, the β-O4’ linkage plays a key role for conformational flexibility, allowing molecular reconfiguration. The chosen modelling approach provides insights into atomistic-level interactions, where ππ-interactions are quantified and shown to influence the molecules conformation rather than significantly contributing to aggregation. Thermodynamic analysis of solubility reveals enthalpy as the dominant driving force for the increased solubility of lignin in organic solvents compared to water. Regarding transport properties, molecular weight is identified as the most influential structural feature on the self-diffusion coefficient, enabling extrapolation of transport diffusion coefficients based on a single, experimentally accessible, characteristic. As a proof of concept, MD derived diffusion coefficients are integrated into a continuum modelling approach.