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

Valorization of lignin is gaining popularity due to its ability to make industrial processes more sustainable. However, the currently prevalent catalytic technologies use large amounts of energy and harsh reaction conditions. As a result, 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 assessed 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 cosolvent enhanced lignocellulosic fractionation (CELF) process. The CELF process has the ability 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 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. This ratio was chosen to mimic the electrolyte composition environment of the CELF pretreatment. The results from both ATR-IR and NMR characterization are consistent and show that β-O-4 interlinkage bonds were broken. Quantitative calculations from NMR show that during oxidative potential holds, the presence of aromatic compounds increased by 28.67% and aliphatic compounds decreased by 32.73%. On the other hand, during reductive potential holds, the presence of aromatic compounds decreased by 33.50% and aliphatic compounds decreased by 78.43%. These results indicate that electrooxidation and electroreduction may be used strategically to cleave interunit linkages. Thus, if electrochemical degradation of lignin is used as a secondary treatment in conjunction with the CELF process, it will greatly increase the possibility of transforming lignin into value-added products.