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

Abstract 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, Bu4N+, 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 Bu4N+. 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 Bu4N+ reported previously, suggesting effective control of aerobic oxidation by large size cationic species.