Mechanistic Insights into Lignin-Monomer Repolymerization in Fast Pyrolysis via Aerosol Product Analysis

Lignin pyrolysis exhibits limited yield and selectivity of monophenols, primarily due to the formation of undesirable phenolic oligomers. Previous studies predominantly focused on oligomers derived from the incomplete depolymerization of lignin macromolecules, while the contribution of monomer repolymerization to oligomers remains poorly understood. To address this knowledge gap, this study introduces the first-time application of vacuum ultraviolet photoionization aerosol mass spectrometry (VUV-PI-AMS) for real-time tracking of the dimerization dynamics of lignin model monomers (guaiacol and syringol) during fast pyrolysis. By directly analyzing aerosol-phase products, our work reveals critical mechanistic insights. Results demonstrate that elevated pyrolysis temperatures and prolonged residence times significantly enhance both the diversity and the extent of dimer formation. Notably, syringol generates heavier and structurally more complex dimers than guaiacol, highlighting the role of methoxy groups in radical–radical coupling reactions. Complementary offline analyses (ESI-HRMS and GC-MS) further corroborate the prevalence of biphenyl-core structures linked via ether, ketone, or direct C–C bonds. These findings elucidate distinct radical-initiated pathways for monomer repolymerization in lignin pyrolysis, confirming its important role in phenolic oligomer formation. The mechanistic insights from this aerosol-focused study guide the development of future strategies to suppress oligomer formation and enhance monophenol yields.