Mechanistic insight into β‒O‒4 linkage cleavage of lignin model compound catalyzed by a SO3H-functionalized imidazolium ionic liquid: An unconventional E1 elimination

Abstract

Efficient liquefaction of lignin has been realized by using 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogensulfate [C3SO3HMIM][HSO4] ionic liquid (IL) as catalyst (Green Chem. 18 (2016) 4098–4108). To gain insight into the mechanism of the catalytic cleavage of the dominant linkage (β−O−4 linkage) in lignin, a DFT study has been performed on the β‒O‒4 linkage cleavage of the lignin model compound, veratrylglycerol-β-guaiacyl ether (VG). The calculated energetically viable pathway consists of five stages: (1) protonation-dehydration, (2) β-H elimination, (3) protonation, (4) hydroxylation, and (5) β−O−4 bond cleavage. The initial deprotonation-dehydration process with a free energy barrier of 29.7 kcal/mol is identified as the bottleneck step. Throughout all elementary steps, both the cation and anion of the IL are found to play substantial roles. The cation acts as a Brønsted acid, while the counteranion plays a dual role of Brønsted base and proton shuttle. The identified mechanism refers to an unconventional E1 elimination, which preferably leads to a (Z)-olefin derivative, i.e. (Z)-enol-ether, rather than its (E)-isomer. Distortion/interaction analysis demonstrated that the deformation of the VG moiety, which is induced by the orbital interaction between the developing sp2 hybridized Cβ atom and the β-O atom, controls the stereoselectivity of the E1 elimination. Our study provides detailed mechanistic insights for the cleavage of β−O−4 linkage in lignin promoted by [C3SO3HMIM][HSO4] IL, and guides the design of efficient functionalized imidazolium IL catalysts for lignin depolymerization.