Enhanced ORR performance to electrochemical lignin valorization in a mixture of ionic liquid/organic solvent binary electrolytes

Abstract

Lignin is the largest renewable source of aromatic compounds, creating an opportunity for the chemical industry as a feedstock provided that the deconstruction and upgrading methods become economically feasible. Lignin valorization is performed using superoxide radicals (˙O₂⁻) as an oxidizing reagent, which are electro-generated in situ through the oxygen reduction reaction (ORR) in an aprotic ionic liquid [BMIM]BF₄. In this study, both the current efficiency and the percent conversion of lignin valorization are improved by simply reducing the viscosity of the electrolyte. The electrochemical analysis and molecular dynamics (MD) simulation results indicate that the presence of non-proton organic solvent CH₃CN in [BMIM]BF₄ can significantly improve the mass transfer process of O₂, thereby accelerating the generation rate of ˙O₂⁻. It is verified using p-benzylethoxyphenol (PBP) as a lignin model compound with a higher Faraday efficiency of 35.3% in [BMIM]BF₄/CH₃CN (7 : 3 v/v) than that of 15.5% in pure [BMIM]BF₄. Finally, an actual lignin sample in [BMIM]BF₄/CH₃CN exhibited a percent conversion of 50.3% to selectively obtain only 4 kinds of the main small molecule aromatic compounds.