Identifying at molecular scale the pyrolysis heavy components from two lignin monomers

4-hydroxy benzaldehyde (H) and vanillin (G) are typical primary pyrolysis products of beta-O-4 lignin dimers with key functional groups that affect the secondary reactions of lignin pyrolysis. In this study, the pyrolysis heavy components from these two lignin monomers were analyzed and identified at molecular scale for the first time with Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS) and compared with ligninderived results. The detected heavy components were typically phenolic oligomers distributed in the nominal mass range of 200-600 Da with 2-6 aromatic rings. They are assumed to be formed through the repolymerization of certain GC-MS-detected monomers during pyrolysis. In particular, the extra methoxy group in model compound G allows for more variations of monomer products, which serve as building blocks to form heavy components and char. Two-dimensional Kendrick mass defect (2D KMD) analysis was employed to reveal the evolution of different functional groups. Two evolution pathways were found to be dominant, namely the modification of phenol cores and methoxy groups. It was found that aldehyde groups promoted the evolution of heavy components with more aromatic rings (up to 6), which might serve as precursors for char. This influence was mitigated by the co-existence of methoxy group, which might compete for the linking positions on benzene rings.