Pathways for vanillin production through alkaline aerobic oxidation of a phenolic lignin model compound, guaiacylglycerol-β-guaiacyl ether, in concentrated aqueous alkali

Abstract

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is a crucial compound that can be produced through alkaline aerobic oxidation of softwood lignin. Our research group has been investigating an effective method for vanillin production from native lignin via alkaline aerobic oxidation of softwood in a highly concentrated alkaline medium (4.0 mol L⁻¹ NaOH aq.). This study reports on the analysis of vanillin production pathways from the phenolic β-O-4 structure, a major structural motif in lignin. The study focuses on the behavior of vanillin production from erythro-guaiacyl glycerol-β-guaiacyl ether (GG) under the previously proposed alkaline aerobic oxidation conditions (4.0 mol L⁻¹ NaOH aq./120 °C/O₂) for producing vanillin from native softwood lignin. Our experimental results confirm the existence of a pathway consistent with previous studies related to pulp bleaching. This involves the enol ether ((E/Z)-2-methoxy-4-[2-(2-methoxyphenoxy)ethenyl]phenol, EE) formed through the non-oxidative alkaline decomposition of GG. However, a detailed comparison of vanillin yields from GG and isolated EE suggests the presence of an undisclosed vanillin production pathway that does not involve EE as an intermediate. The existence of this unknown pathway is further supported by the significant difference in the deuterium incorporation rate (R_D/H) into the α-position of vanillin during alkaline aerobic oxidation in NaOD/D₂O, which markedly differs between GG (1.04) and EE (0.25) as starting materials.