Nickel-Driven Electrochemical Upgrading of Kraft Lignin to Value-Added Aliphatic and Phenolic Products

Abstract

The depolymerization of lignin represents a promising strategy for its efficient utilization as a precursor for industrial raw materials. However, achieving both high efficiency and environmental sustainability remains a significant challenge. In this study, we present an aqueous electrochemical approach employing nickel as an electrocatalyst, enabling both depolymerization and partial de-aromatization of Kraft lignin under mild reaction conditions. Using an aqueous sodium carbonate medium, room temperature and ambient pressure, we achieved lignin depolymerization over reaction times ranging from 5 to 20 h. Characterization by nuclear magnetic resonance (NMR) spectroscopy confirmed the formation of aliphatic products such as acetate and formate, while high-resolution mass spectrometry (HRMS) confirmed the formation of a wide range of phenolic compounds. The conversion of lignin into valuable aromatic and aliphatic compounds offers a promising pathway for the synthesis of a wide range of organic chemicals and their subsequent industrial utilization, thereby supporting the development of a more sustainable economy.

Keywords: Green Chemistry; depolymerization; electrocatalysis; lignin; nickel.

Figure 1

Illustration of the nickel-catalyzed electrocatalytic depolymerization and partial de-aromatization of Kraft lignin in an aqueous sodium carbonate solution. The process involves the application of a constant electrical current (−175 mA), driving the electrochemical reduction of lignin at room temperature and ambient pressure. The cleavage of lignin results in the formation of both aliphatic compounds, such as formate and acetate, and aromatic species. The figure emphasizes the role of nickel as an electrocatalyst in improving product distribution compared to previous systems and shows the distribution of products for different work-up methods.

Figure 2

Electrochemical depolymerization of Kraft lignin in an aqueous sodium carbonate solution at −175 mA reacting for 0 h, 5 h, 10 h, 15 h, and 20 h, respectively. During depolymerization, the color of the solution changes over the reaction time and becomes clearer and more transparent as the reaction time increases. This observation can indirectly reveal the degree of depolymerization.

Figure 3

(a) ¹H NMR spectra (CD₃OD, 600.13 MHz) of depolymerized Kraft lignin for different reaction times: 0 h, 5 h, 10 h, 15 h, and 20 h obtained from Work-up A. Formate (green) and acetate (orange) were identified as aliphatic products [6,7]. (b) Enlarged section from 6.0 to 8.5 ppm of ¹H NMR spectra (CD₃OD, 600.13 MHz) of depolymerized Kraft lignin at different reaction times.

Figure 4

(a) FTIR spectra of Kraft lignin and depolymerized Kraft lignin from Work-up A at different depolymerization times. (b) FTIR spectra of Kraft lignin and depolymerized Kraft lignin obtained from Work-up B at different depolymerization times.

Figure 5

Comparison between the lignin depolymerization products obtained in this study and those reported in previous work involving the electrochemical reductive depolymerization of Soda and Kraft lignin under varying conditions [8,9,10]. The Figure illustrates the influence of different electrocatalysts (nickel, copper, carbon, and silver) and reaction parameters (current, reaction time, and electrolyte composition) on product distribution. The results underscore the critical role of electrocatalyst selection in determining the selectivity toward aliphatic or aromatic compounds.