Molten salt hydrates are concentrated aqueous solutions of inorganic salts, in which the molar ratio of water to salt is close to the coordination number of the salt cation. Because of their unique properties, in particular the ability to dissolve cellulose, they are promising solvents and reaction media for biomass processing. In our previous studies, LiBr molten salt hydrate (~61 wt% LiBr aqueous solution) was successfully used in hydrolysis of cellulose and lignocellulosic biomass, lignin quantitation, isomerization of glucose, furan-based chemicals from biomass, synthesis of oligosaccharides, and fabrication of films and aerogels from cellulose and biomass. It was also observed that lignin was extensively depolymerized when the biomass was treated in the LiBr system. This presentation is to report our recent investigations into the cleavage mechanisms of lignin ether bonds in the LiBr system. The results indicated that ether compounds except for diaryl ether could be cleaved in the LiBr system. The methoxyl group of lignin model compounds could be demethylated to a phenolic hydroxyl group. Real lignin samples from varying biomass species by different methods could be efficiently demethylated as well, which resulted in phenolic hydroxyl groups. The studies revealed that all the ether bonds of lignin in methoxyl, β-O-4, β-5, and β-β structures except for the 4-O-5 bond in diphenyl structure could be cleaved in the LiBr system. The ether bonds were cleaved via the SN2 substitution pathway except for β-O-4 bond. The latter was primarily cleaved via the benzyl cation and enol ether intermediates, leading to Hibbert’s ketones. Besides, some β-O-4 structures were transformed into benzodioxane structures, which were stable in the LiBr system. The studies provided insights into the cleavage of the lignin ether bonds in the LiBr system and demonstrated a new method for cleaving ether compounds and demethylating lignin.

Cleavage of lignin ether bonds in a concentrated aqueous solution of lithium bromide (a molten salt hydrate)

ACS Spring Meeting 2021

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