Our studies demonstrated that lignin could be quantitatively isolated as a solid fraction from lignocellulose when cellulose and hemicellulose were hydrolyzed into monosugars in an inorganic ionic liquid (acidic lithium bromide trihydrate (ALBTH) system) under mild conditions (40 mM HCl at 110 °C), and the isolated ALBTH lignin had great potential for downstream valorization because of high purity, low molecular weight, and non-condensed nature. Based on the observations, a facile and fast method was developed for the quantitation of lignin in biomass, which gave comparable results with the existing Klason (sulfuric acid) method. The characterization of the isolated lignin indicated that the lignin was extensively depolymerized to low molecular weight (MW = 2,000 ~ 4,000) in solid state without solubilization, and the isolated lignin had abundant uncondensed moieties (i.e., Hibbert’s ketones and benzodioxanes), which had not been identified and detected in other isolated lignins before. Study using lignin model compounds verified that the lignin depolymerization was catalyzed by LiBr and acid synergistically in the ALBTH system via α-benzyl carbocation intermediate. The cleavage of the β–<i>O</i>–4-aryl ether bond led to the Hibbert’s ketone, while the ring formation between the α-benzyl carbocation and the methoxyl group on the adjacent benzene ring resulted in the benzodioxane structures. Condensation occurred when lignin model compounds were treated in ALBTH system, but the condensation of the real lignin in biomass was very limited in the ALBTH, because the mobility and accessibility of both the a-benzyl carbocation intermediates and the lignin fragments with electron-rich aromatic sites were limited or blocked in solid state, which prevented them from condensation.

Insights into the isolation and structural changes of lignin in inorganic ionic liquid

2019 Fall National Meeting

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