Cleavage of the -O-4 bond in a lignin model compound using the acidic ionic liquid 1-H-3-methylimidazolium chloride as catalyst: a DFT mechanistic study

Understanding the mechanism for the catalyzed cleavage of the -O-4 ether linkage in lignin is crucial to developing efficient strategies for depolymerizing lignin. In this work, veratrylglycerol--guaiacyl ether (VG) was used as a lignin model compound in a theoretical investigation of the mechanism for the cleavage of the -O-4 bond as catalyzed by the acidic ionic liquid (IL) 1-H-3-methylimidazolium chloride ([HMIM]Cl). The reaction was found to involve two processesdehydration and hydrolysisin which the cation functions as a BrOnsted acid (donating a proton) and the anion acts as a nucleophile (promoting dehydration) or interacts with the substrate through hydrogen bonding, stabilizing the intermediate. These roles of the anion and cation of [HMIM]Cl explain why the [HMIM]Cl medium catalyzes the depolymerization of lignin. In addition, calculations predict that adding formaldehyde during the depolymerization of VG prevents the condensation of VG without significantly altering the mechanism of depolymerization, thus suggesting a method for potentially improving the efficiency of lignin depolymerization.