Iron-induced demethoxylation of lignin as an important source for methanol and its oxidation products in the environment

Lignin, the largest reservoir of aromatic carbon in plants and soils, undergoes well-characterized biological demethylation by fungi and bacteria, and is industrially processed under harsh conditions to generate bio-based aromatics. However, the abiotic fate of lignin-derived methoxy groups under environmentally relevant conditions remains unresolved. Here, we report an iron-mediated demethoxylation mechanism involving direct aromatic C-O bond cleavage under ambient conditions. Using Fe^II/Fe^III species and H₂O₂, we demonstrate the efficient release of entire methoxy groups from lignin monomers and methoxyphenols, forming methanol and subsequently formaldehyde. Isotopic labeling unequivocally shows that the complete -OCH₃ group is liberated intact. Density functional theory calculations support an exergonic demethoxylation pathway with low activation energy (43 kJ/mol) and favorable thermodynamics. Soil laboratory incubation experiments with lignin and diverse natural soils rich in iron and methoxylated organic matter confirm that this abiotic pathway operates under environmentally relevant wet-dry conditions. These findings establish iron-induced demethoxylation as a abiotic process, linking lignin turnover to microbial C₁ metabolism and soil carbon cycling, with implications for atmospheric trace gas fluxes.