(50b) Lignin Depolymerization in a Molten Salt Hydrate (lithium bromide trihydrate)

A molten salt hydrates (MSH) is a concentrated aqueous solution of an inorganic salt, 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, some MSHs are promising solvents and reaction media for biomass processing. For example, LiBr molten salt hydrate (~61 wt% LiBr aqueous solution) has been 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 nanocrystals, films, and aerogels from cellulose and biomass in our previous studies. It was also observed that lignin was extensively depolymerized when biomass was treated in the LiBr system. This presentation is to report our recent investigations into the depolymerization mechanisms of lignin in the LiBr system. 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 S_N2 substitution pathway except for β-O-4 bond. The β-O-4 bond 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.