Biomimetic Fenton Reaction Facilitates Depolymerization of Organoslov Lignin to Aromatic Chemical Platforms

Lignin, as the second abundant natural polymer in plant cell wall, has great potential to produce aromatic chemical platforms or fuel precursors due to its unique structure as a cross-linked aromatic polymer comprised of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units. However, lignin depolymerization process is challenging because it requires harsh reaction conditions and is hindered by thermal re-condensation. It is known that, in a biomimetic Fenton reaction under mild condition, the transition metals (like Fe²⁺, Mn²⁺ and Cu²⁺ ions) can reduce H₂O₂ to generate free •OH to hydroxylate lignin. In this study, we propose that Fenton reaction could increase the conversion rate of lignin in subsequent thermal conversion through lignin demethoxylation. Therefore, in order to verify this hypothesis, organosolv lignin was modified with Fe²⁺/ H₂O₂ (Fenton catalyst) in different ratios prior to the pyrolysis GC/MS. The results showed that aromatic monomers were formed in high yield (36.7% of 4-hydroxybenzoic acid methyl ester) after pyrolysis at moderate temperature. The results also revealed that the Fenton reaction greatly depressed the lignin re-condensation, and reduced the char coke formation. The detailed structural features of treated lignin and untreated lignin were further characterized by ³¹P NMR, X-ray photoelectron spectroscopy, and GPC. The mechanism has been elucidated that Fenton reaction hydroxylates organosolv lignin to form a Fe²⁺ or Fe³⁺–lignin complex which greatly prohibits lignin re-condensation during thermal conversion.