(717b) Quantitation, Depolymerization, and Demethylation of Lignin in Inorganic Ionic Liquid

It was found 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. 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 isolated ALBTH lignin had high purity, low molecular weight, and non-condensed 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 revealed that the lignin depolymerization was catalyzed by LiBr and acid synergistically via α-benzyl carbocation intermediate. The cleavage of the β–O–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 of lignin model compounds occurred when treated in ALBTH system, but the condensation of the real lignin in biomass was very limited in the ALBTH, because the limited and blocked mobility and accessibility of both the a-benzyl carbocation intermediates and the lignin fragments with electron-rich aromatic sites in solid state prevented them from condensation. Further investigation indicated that lignin could be selectively demethylated in the ALBTH system.