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 O2in 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 ß-ether linkages in middle units of lignin polymer to form a glycerol end group, oxidation of the glycerol end group by O2to a HCa?O moiety, and release of vanillin from the HCa?O 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 CaHO end group) probably due to strong interaction between the cation and the HCa?O end group.