Effect of cathode material on electrochemical reduction of levulinic acid to γ-valerolactone and valeric acid in aqueous and organic media

The electrochemical conversion of biobased levulinic acid (LA) into renewable chemicals and biofuel precursors represents an important and reasonable alternative to the high temperature conventional catalytic processes of great importance for the development of a sustainable and cost-effective biorefinery. The establishment of the mechanism of levulinic acid reduction is a promising strategy in choosing the optimal electrocatalyst for the redox-transformation of biobased substrates. Herein, we report a new approach to study an electrochemical reduction mechanism of levulinic acid using of proton-deficient non-aqueous reaction media. The electrochemical reduction of levulinic acid to γ-valerolactone (GVL) and valeric acid (VA) in aqueous and organic solutions on various electrodes (glassy carbon, graphite, Al, Pb) was studied. The mechanism of LA electrochemical reduction and major reaction products significantly was found to depend on the solvent, the presence of proton donors, the material of cathode, and the magnitude of the applied potential. In an aqueous solution the process proceeded with the formation of valeric acid on all the electrodes studied. In acetonitrile in the presence of protons, the electrochemical reduction of LA proceeded by various mechanisms, both with the participation of atomic hydrogen and the protonated form of LA, and led to the formation of GVL and/or VA. The difference (ΔE_1/2) between the reduction half-wave potential of protons and levulinic acid was found to play an important role in the reduction pathway of LA carbonyl group. At a large ΔE_1/2, as in the case of the GC electrode, the LA reduction resulted in the GVL formation. LA can be completely reduced to VA by transferring four electrons due to the close reduction potentials of protons and LA (a low ΔE_1/2), as on a Pb electrode. The pathway depends on the conditions of the reduction process and can be estimated based on electrochemical data obtained in the study of reaction products in organic media.