Enabling Downstream Processing of Lignin Effluent from Cosolvent Enhanced Lignocellulosic Fractionation (CELF) Process through Controlled Electrocatalytic Oxidation and Reduction

Abstract

Lignin which is a complex organic polymer is abundant in nature. But due to its recalcitrant nature, it can't be converted to value added products easily. Previous studies have found that 40–60 million tons of lignin wastes are generated from the pulp and paper industry globally [1]. The sustainability of these industries can be ensured by converting these wastes into valuable products through environmentally friendly processes. Currently, there are many prevalent catalytic technologies for converting lignin to valuable fuels and chemicals. But these technologies use large amounts of energy and harsh reaction conditions. As a result, catalyst recovery and decomposition often become difficult under such harsh conditions. Besides, bonds between lignin precursors are cleaved unselectively and the selectivity of aromatic products is reduced. An electrocatalytic approach may allow improved control, and few studies have evaluated the electrocatalytic oxidation and reduction of lignin in organic solvents.Organic solvents like tetrahydrofuran (THF) are of interest for lignin electrocatalysis due to their use in the promising cosolvent enhanced lignocellulosic fractionation (CELF) process. Also, THF is a renewable solvent as it can be produced sustainably from the conversion of lignocellulosic biomass. In CELF process, high purity technical lignin can be produced at milder temperatures (1800C) from a mixture of THF and sulfuric acid. Thus, this process has the capability to overcome biomass recalcitrance by breaking β-O-4 aryl ether interunit linkages. Using electrocatalytic conversion processes, additional β-O-4, interunit linkages can be broken down selectively. As a result, if these two processes are integrated, a high amount of phenolic hydroxyl groups with low-content aryl ether linkages will be produced, making the product suitable for the development of biofuels and other chemicals. This study shows that by using controlled electrocatalytic oxidation and reduction in THF/aqueous acidic electrolytes in the ratio of 2:1, nonpolar β-O-4 linkages can be cleaved [2]. This ratio was chosen to mimic the electrolyte composition environment of the CELF pretreatment. The results from both attenuated total reflection-infrared spectroscopy (ATR-IR) and NMR characterization are consistent and show that β-O-4 interlinkage bonds were broken. Quantitative calculations from NMR show that during controlled oxidative potential holds (constant potential oxidation), the presence of aromatic structural components of the lignin polymer increased by 28.67% and aliphatic structural components decreased by 32.73%. On the other hand, during controlled reductive potential holds (constant potential reduction), the presence of aromatic structural compounds decreased by 33.50% and aliphatic structural compounds decreased by 78.43%. These results indicate that electrooxidation and electroreduction may be used strategically to cleave interunit linkages. Finally, it can be said, if electrochemical degradation of lignin is used as a secondary treatment along with the CELF process, it will substantially increase the possibility of transforming lignin into value-added products.References Collins, M. N., Nechifor, M., Tanasă, F., Zănoagă, M., McLoughlin, A., Stróżyk, M. A., Culebras,M., & Teacă, C. A. (2019). Valorization of lignin in polymer and composite systems for advanced engineering applications–a review. International journal of biological macromolecules, 131, 828-849.Hasan, M., Akbari, A., & Greenlee, L. F. (2023). Combined Electrocatalytic Oxidation and Reduction to Selectively Cleave β-O-4 Linkage of Lignin over Platinum Electrode in Organic Solvent: Secondary Treatment Opportunity for CELF Process. ACS Sustainable Chemistry & Engineering. Figure 1