Spectro-electrochemistry of guaiacol oxidation: tracking intermediates in a membrane-separated cell with in situ attenuated total reflectance-infrared spectroscopy

Abstract

Lignin, a structurally complex biopolymer, represents a promising renewable feedstock for the production of platform chemicals, including functionalized aromatic molecules. However, efficient lignin valorization remains a major challenge due to its chemical stability, structural heterogeneity, and the propensity of reactive intermediates to undergo recondensation. To overcome these barriers and gain mechanistic insight into lignin oxidation pathways, we have developed a membrane-separated, two-compartment attenuated total reflectance infrared (ATR-IR) spectro-electrochemical cell for the in situ monitoring of the electrochemical oxidation of lignin model compounds. Using guaiacol as a representative model compound of the β-O-4 linkage monomer, we tracked real-time spectral changes during electrochemical oxidation. Characteristic vibrational signatures revealed the depletion of guaiacol and the formation of oxidized species, including quinones, catechols, and dimers and oligomers. In contrast, control experiments conducted without membrane separation exhibited additional spectral features, suggesting the occurrence of competing side reactions under conditions of unrestricted mass transport. These results highlight the importance of proper cell design for providing mechanistic insights and demonstrate the value of in situ ATR-IR spectroscopy in tracking the complex electrochemical transformation of lignin-derived molecules, to offer insights critical for advancing lignin valorization strategies under mild and tunable reaction conditions.