Demethylation is one of the strategies to improve the reactivity and functionality of lignin for downstream applications. In our previous studies, it was found that a molten salt hydrate (lithium bromide trihydrate, ~61 wt% LiBr aqueous solution) can efficiently depolymerize lignin by cleaving β-<i>O</i>-4 linkage without causing condensation. Further studies revealed that other ether bonds of lignin in methoxyl, β-5, and β-β structures can also be cleaved in the LiBr system. This provided a new method for lignin demethylation. This presentation is to report our recent studies on the demethylation of kraft lignin and lignin-derived aromatics in the LiBr system. It was found that kraft lignin was extensively depolymerized and demethylated in the LiBr system, and the depolymerized and demethylated kraft lignin showed excellent antioxidant activity and Cr(VI)-scavenging capacity, compared with original kraft lignin. In addition, lignin-derived aromatic compounds, such as 4-propylguaicol, 4-propylsyringol, 4-propanolguaicol, and 4-propanolsyingol were efficiently demethylated under mild conditions, leading to the corresponding catechol and pyrogallol derivatives in high yield. The effects of reaction conditions and the substitutes on the aromatic ring on the demethylation of the aromatics were carefully investigated.

Demethylation of lignin and lignin-derived aromatics under moderate conditions in a molten salt hydrate

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