(600f) Cleavage of ?-O-4 Ether Bonds in Acidic Lithium Bromide Trihydrate for Lignin Depolymerization

Depolymerization of lignin is essential in pulping and biorefining processes. It was observed in our previous studies that lignin could be extensively depolymerized in acidic lithium bromide trihydrate media. In this study, the mechanism of lignin depolymerization in acidic lithium bromide trihydrate was investigated. First, it was demonstrated that the native lignin in lignocellulosic biomass was easily depolymerized under mild conditions (e.g. in 60 % LiBr with 40 mM HCl at 110 °C). Six types of biomass, including hardwood, softwood, and herbaceous biomass, were treated in acidic lithium bromide trihydrate, and the isolated lignin fractions were characterized with GPC and NMR. The result indicated that the β-O-4 ether linkages of the native lignin were selectively cleaved, leading to lignin depolymerization (Mw: 2000-4000). Second, lignin model compound (guaiacylglycerol-β-guaiacyl ether) was used to elucidate the cleavage mechanism of the β-O-4 ether bonds. It was found that the ether bonds were cleaved via the enol ether pathway initiated by the a-C catioinzation, and both LiBr and HCl catalyzed the cleavage. In addition, new C-C bonds were formed, resulting from the nucleophilic attack of the benzyl carbocations by the electron-rich lignin aromatic rings. Third, this lignin depolymerization strategy was applied to technical lignins isolated from pulping and biorefining processes. It was found that the residual β-O-4 ether linkages of the lignin could be selectively cleaved as well.