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

Abstract

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.