(717d) Quantum Mechanical Studies of the Depolymerization of Lignin By the ZnCl2:Ethylene Glycol Deep Eutectic Solvent

Lignin makes between 15-35 % of the worlds dry biomass. It consists of phenolics connected to each other through various linkages. In order to convert lignin into value added chemicals, the phenolic-phenolic linkages must be broken, a process known as depolymerization. Several methods, such as pyrolysis, catalytic reductive or oxidative, and the use of ionic liquids (IL), have been reasonably effective, but are fraught with undesired toxic reagents, high costs, or modest yields. Compared to ILs, Deep Eutectic Solvents (DES) are lower in cost, easily prepared, and efficiently recycled. DESs have found utility in several areas, but in the realm of lignin chemistry, have been shown to solvate lignin. In addition, several metal chlorides (MCl), such as ZnCl₂, and AlCl₃, have been shown to aid in the depolymerization of lignin. In the combination of a metal chloride (MCl), a DES can be formed which both solvates lignin, and also aids in its depolymerization. Indeed, one such DES consisting of ethylene glycol, and ZnCl₂ in a 4 to 1 molar ratio has been developed and shown to solvate lignin in concentrations of at least 10 Mol %. One area, which still needs to be understood, is the mechanism by which ZnCl₂ aids depolymerization. To that end, Quantum Mechanics calculations were carried out on a series of tri-lignol + ZnCl₂ Potential Energy Surfaces (PES). Tri-lignols are amenable to high level Quantum Mechanics calculations, yet are complex enough to shed insight into the depolymerization process. Our results show that there are several places on lignin which ZnCl₂ can bind to. Of these, the strongest attractions occur between zinc and aromatic carbons, as well as to the oxygen atom of a β-O4 linkage. Our results add to the knowledge of catalysis in lignin chemistry, and also serve as a foundation for the development of future lignin fractionation strategies.