Lignin activation with selected ionic liquids based on kinetic and thermodynamic analyses

Herein, the potential of lignin activation with selected ionic liquids (ILs) was investigated to enhance the usefulness of lignin in materials science and electrochemical systems. The main objective was to increase the carbonyl content in lignin through selective oxidation, which would enable its use as a sustainable alternative, for example, in electrode materials and composite systems. Using ILs as activators, the modification process focused on maintaining the structural integrity of lignin while increasing its functional group profile. The research included the precise control of air supply as the oxidant and regulation of the process temperature to prevent lignin depolymerization. Advanced kinetic and thermodynamic analyses of thermal decomposition were performed using thermogravimetric analysis, differential thermogravimetric analysis, and differential thermal analysis, with kinetic modeling based on the Coats-Redfern method. These methodologies facilitated a detailed understanding of the thermal stability, degradation kinetics, and reactivity of the material. Results revealed that the activation of lignin with ILs significantly increases the carbonyl (quinone) group content, enhancing its potential as a reversible proton and electron acceptor in electrochemical applications. The study highlights the importance of balancing degradation kinetics and structural properties of lignin to optimize its reactivity and functional performance. Mechanisms such as F1 and D4 effectively describe the degradation process, with the activation energy (E_a) ranging from 66.691 to 309.389 kJ/mol. The enthalpy (ΔH) ranges from 62.488 to 302.950 kJ/mol, while the ΔS values, both positive and negative, reflect the heterogeneity of the reaction depending on the system and ionic liquid conditions.