Mechanistic insights into lignin depolymerisation in acidic ionic liquids

Acidic anions of ionic liquids have been demonstrated as efficient catalysts for the cleavage of the beta-O-4 ether linkage prevalent in the lignin superstructure. Through the use of lignin model compounds with varying functionality and by monitoring reaction kinetics, a full mechanistic investigation into the hydrolysis of the beta-O-4 linkage in acidic ionic liquid solutions is reported. Hammett acidities are reported for different 1-butyl-3-methylimidazolium hydrogen sulfate [C(4)C(1)im][HSO4] ionic liquid systems with varying acid and water concentrations and were correlated to substrate reactivity. Results show that the rate of ether cleavage increases with an increase in acidity and the initial dehydration of the model compound is the rate-determining step of the reaction. The Eyring activation parameters of the reaction in hydrogen sulfate ionic liquids with a variety of cations are reported, indicating a consistent E1 dehydration mechanism. Hydrogen bonding in protic ionic liquids was shown to significantly influence anion-cation interactions, consequently altering the solvation of the protonated starting material and therefore the overall rate of reaction. Comparison of reaction rates in these ionic liquids with results within aqueous or aqueous/organic media indicate that the ionic liquids facilitate more rapid cleavage of the beta-O-4 ether linkage even under less acidic conditions. All the reported results give a complete overview of both the mechanistic and solvation effects of acidic ionic liquids on lignin model compounds and provide scope for the appropriate selection and design of ionic liquids for lignin processing.