Electrochemical Depolymerization of Lignin in a Biomass-based Solvent

Abstract

Breaking down lignin into smaller units is the key to generate high value-added products. Nevertheless, dissolving this complex plant polyphenol in an environment-friendly way is often a challenge. Levulinic acid, which is formed during the hydrothermal processing of lignocellulosic biomass, has been shown to efficiently dissolve lignin. Herein, levulinic acid was evaluated as a medium for the reductive electrochemical depolymerization of the lignin macromolecule. Copper was chosen as the electrocatalyst due to the economic feasibility and low activity towards the hydrogen evolution reaction. After depolymerization, high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy revealed lignin-derived monomers and dimers. A predominance of aryl ether and phenolic groups was observed. Depolymerized lignin was further evaluated as an anti-corrosion coating, revealing enhancements on the electrochemical stability of the metal. Via a simple depolymerization process of biomass waste in a biomass-based solvent, a straightforward approach to produce high value-added compounds or tailored biobased materials was demonstrated.

Keywords: coating; depolymerization; electrocatalysis; levulinic acid; lignin.

Figure 1

A representation of the structure of levulinic acid and the insoluble fraction of Kraft lignin, proposed by Crestini et al. The typical interunit bonds are highlighted in blue.

Figure 2

(a) LSV (scan rate 10 mV s⁻¹) and (b) constant potential ECH at −1.7 V vs. Ag/AgCl (saturated KCl) of Kraft lignin in 5.17 m of levulinic acid.

Figure 3

Main products of kraft lignin depolymerization in levulinic acid identified by direct injection high‐resolution MS.

Figure 4

(a) ¹H and (b) ¹³C NMR spectra of reacted 5.17 m solution of levulinic acid and kraft lignin solution in levulinic acid before and after depolymerization.

Figure 5

Possible mechanisms of electron transfer in ECH reaction on Cu surface: electronation–protonation and electrocatalytic hydrogenation.

Figure 6

PDP curves of uncoated and coated Al samples recorded in 5 % NaCl at a 1 mV s⁻¹ scan rate. Tafel analysis of these curves showed a clear decrease in i _corr values for coated samples, demonstrating an inhibition of the corrosion process.

Figure 7

(a) Nyquist plots for uncoated and coated Al samples. (b) Circuit equivalent used for the data fitting of the uncoated (i) and coated samples (ii). The Nyquist plots show an increase of the diameter of the semicircle for coated samples, which is correlated to an increase of the charge transfer resistance (R _ct), and the corrosion inhibition.

Figure 8

SEM micrographs of aluminum plate (a, d) before and (b, c, e, f) after PDP scan. After scanning to potential higher than E _corr, uncoated sample (c, e) shows distinct pitting features due to corrosion; the coated sample Al20 (c, f) shows no such features and exhibits a smooth surface, demonstrating the ability of the coating to shield the surface from corrosion occurring during the PDP measurement. Circled in yellow (c) shows a defect area in the coating, created during sample preparation showing an underlying Al surface without corrosion features, further demonstrating the protecting ability of the coating.

Figure 9

SEM micrograph of Al20 on a defect area after corrosion measurement; the higher magnification allows to distinct the smooth Al surface. EDS analysis confirmed the organic nature of the coating and only showed Al signal for the underlying surface.