Investigation of functional group changes and reaction mechanisms during the thermochemical conversion of waste lignin

This study presents a multi-technique, real-time analysis that quantitatively correlates the pyrolysis stages of waste lignin with the sequential evolution of its key functional groups—a level of mechanistic insight beyond the macroscopic yield correlations of prior studies. The pyrolysis behavior and group evolution of lignin derived from agricultural and forestry residues were investigated under nitrogen (10 °C min−1, 30–900 °C) using thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and elemental analysis. The process was delineated into three distinct stages: (i) desorption of physically adsorbed water and light volatiles (<150 °C, ∼8 wt. % loss); (ii) cleavage of methoxy/ether linkages and partial decomposition of aromatic C=C and hydroxyl groups (150–500 °C, ∼60 wt. % loss), generating oxygenated volatiles; and (iii) condensation and aromatization of aliphatic structures (>500 °C), yielding ∼19 wt. % stable biochar. The functional group decomposition followed the sequence methoxy/ether > aromatic C=C > hydroxyl > aliphatic C–H, driving dominant pathways of dehydration, decarboxylation, decarbonylation, and condensation. These findings establish a quantitative functional-group-led framework for optimizing lignin conversion toward targeted products like high-quality bio-oil and high-value biochar.