Valorization of lignin into carbon spheres, carbon dots, and graphene-like layers via hydrothermal conversion

Lignin is a renewable, aromatic-rich byproduct of biomass processing, and holds a significant potential as a precursor for sustainable carbon materials. This study presents a proof of concept for its conversion into diverse carbon-derived products via a two-stage hydrothermal process. In the first hydrothermal stage (250 °C, 60 min), lignin undergoes fragmentation and partial depolymerization, yielding a liquid phase rich in soluble aromatic fragments and a minor solid residue (5.2 %). In the second hydrothermal stage (300-350 °C, 15-60 min), aromatic precursors undergo conversion into carbon spheres (180-220 nm), carbon dots (<10 nm), and graphene-like layers. The carbon spheres form mainly via self-assembly and condensation, with a carbon content of 72.5 % and a mass yield of 42.3 %. Carbon dots, which confer photoluminescence, partially coalesce into larger structures, contributing to carbon sphere formation as a secondary pathway. Simultaneously, aromatic fragments undergo rebonding and planar fusion, reorganizing into graphene-like layers with localized crystalline domains. All carbon-derived products form within the first 15 min of the second stage, with longer times promoting minor structural rearrangements. Overall, the proposed chemical-free hydrothermal route represents a green strategy for valorizing lignin into functional carbon materials, serving as a basis for further investigation into their formation mechanisms and applications.