Selective production of bio-based aromatics by aerobic oxidation of native soft wood lignin in tetrabutylammonium hydroxide

Abstract

Aerobic oxidation of native soft wood lignin in an aqueous solution of Bu₄NOH facilitates efficient production of vanillin (4-hydroxy-3-methoxybenzaldehyde), which is one of the platform chemicals in industry. Oxidation of Japanese cedar (Cryptomeria japonica) wood flour at 120 °C for 4 h under O₂ in Bu₄NOH-based aqueous solutions produced vanillin in 23.2 wt% yield based on the Klason lignin content of the starting material. This yield was comparable to that in alkaline nitrobenzene oxidation of the same material (27.2%), which indicated that our aerobic oxidation exploited the full potential of the wood flour for vanillin production. Further mechanical investigation with lignin model compounds suggested that the vanillin formation occurred mainly through following successive reactions: alkaline-catalyzed degradation of β-ether linkages in middle units of lignin polymer to form a glycerol end group, oxidation of the glycerol end group by O₂ to a HC_αO moiety, and release of vanillin from the HC_αO end. One of the reasons for the high performance of Bu₄NOH for the vanillin production was explained by the general understanding in organic chemistry that Bu₄OH is a stronger base than simple alkali, e.g. NaOH. The other more fundamental mechanical aspect was that Bu₄N⁺ suppressed disproportionation of the vanillin precursor (the C_αHO end group) probably due to strong interaction between the cation and the HC_αO end group.