Correlation between pyrolysis behavior and the chemical structure of lignin nanoparticles (LNPs) isolated by deep eutectic solvent

Lignin nanoparticles (LNPs), characterized by their high aromaticity, reactivity, and high carbon content (approximately 60 %), are considered an excellent precursor for the fabrication of a variety of other carbon nanomaterials. Therefore, investigating the relationship between chemical structure and pyrolysis kinetics of LNPs is crucial for understanding their carbonization mechanisms. In this study, three representative LNPs were isolated from pine, poplar, and wheat straw biomass using deep eutectic solvent (DES), and their chemical properties and pyrolysis kinetics were investigated. Moreover, the evolution pathways of graphitic carbons that are derived from these three LNPs’ pyrolysis reactions under the high temperature of 1700 °C were elaborated. The findings reveal that wheat straw LNPs, due to their lower molecular weight, undergoes thermal degradation rapidly at lower temperatures (200–300 °C). Poplar LNPs, characterized by a higher content of S and G units and fewer ether bonds, are more susceptible to degradation at elevated temperatures (300–600 °C). During the carbonization stage (600–800 °C), pine LNPs pyrolysis produced higher amount of char, due to its predominant components of G units, which resulted in increased condensation via C₅ position. The results of high-temperature pyrolysis conducted at 1700 °C demonstrate that the lignin subunit significantly influences subsequent pyrolysis behaviors and carbon formation mechanisms, i.e., the carbon materials derived from pine LNPs exhibited a well-defined curled tubular morphology, while poplar LNPs-based carbon formed an ordered, multilayer sheet-like carbon structure. This research provides critical insights for the targeted regulation of lignin-based carbon nanomaterials using LNPs as precursors.