Alkaline aerobic oxidation of native softwood lignin in the presence of Na+-cyclic polyether complexes

• Published in: Journal of wood chemistry and technology Vol. 42; no. 1; pp. 1 - 14
• Main Authors: Hosoya, Takashi, Kawase, Kei, Hirano, Yuki, Ikeuchi, Mizuki, Miyafuji, Hisashi
• Format: Journal Article
• Language: English
• Published: Philadelphia Taylor & Francis 02.01.2022; Taylor & Francis Ltd
• Subjects: Cations; Crown ethers; Cryptomeria japonica; Depolymerization; Dimethyl ether; Hydroxybenzaldehydes; Lignin; Low molecular weights; mechanism; Molecular weight; organic cation; Oxidation; Phenols; Polyethers; Sodium hydroxide; Softwoods; Vanillin
• ISSN: 0277-3813; 1532-2319
• DOI: 10.1080/02773813.2021.1998127

Alkaline aerobic oxidation is a promising way to convert lignin to low molecular weight phenols, especially 4-hydroxybenzaldehydes. Our previous studies reported that oxidation of softwood lignin samples with a bulky cation, Bu 4 N + , facilitates selective production of vanillin (4-hydroxy-3-methoxybenzaldehyde). This study presents vanillin production from native softwood lignin in Japanese cedar (Cryptomeria japonica) in NaOH aq. in the presence of cyclic polyethers, with our expectation that Na + -polyether complexes exhibit effects similar to those of Bu 4 N + . Oxidation of wood flour (10 mg) in 4.0 M NaOH aq. (2.0 mL) at 120 °C under air gave vanillin with 6.2 wt% lignin-based yield, which was raised to 15.2 wt% by the addition of 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane). On the other hand, such effect was not observed with the addition of tetraethylene glycol dimethyl ether, a non-cyclic analog of 15-crown-5. Mechanistic study with a lignin model compound revealed that stabilization of a vanillin precursor by the complex cation was a reason for the increased vanillin yield exhibited by the crown ether. This is similar to the influence of Bu 4 N + reported previously, suggesting effective control of aerobic oxidation by large size cationic species.