(144a) Solid-State Depolymerization and Isolation of Lignin from Lignocellulosic Biomass

This study demonstrated that lignin could be efficiently depolymerized in the solid state, with minimal condensation, and separated from dissolved and hydrolyzed cellulose and hemicelluloses, in an inorganic ionic liquid (acidic lithium bromide trihydrate (ALBTH) system) under mild conditions (with 40 mM HCl at 110 °C). The resultant ALBTH lignin contained abundant uncondensed moieties (i.e., Hibbert’s ketones and benzodioxanes), from partial depolymerization, and could readily be isolated from biomass. Reactions using lignin model compounds confirmed the formation of the uncondensed moieties and revealed the synergy between LiBr and acid in inducing the crucial intermediate α-benzyl carbocations, which then led to cleavage of the β–O–4-aryl ether bonds to produce Hibbert’s ketones, demethylation to produce benzodioxanes, and condensation reactions. Unlike lignin model compounds, the condensation of the real lignin in biomass under ALBTH conditions was greatly diminished, possibly due to lignin’s remaining in the solid state, limiting its mobility and accessibility of the a-benzyl carbocation to the electron-rich aromatic sites for condensation. This study provided a new approach to effectively isolate depolymerized lignin from lignocellulose in a less condensed form for boosting its downstream valorization.