Vanillin production from native softwood lignin in the presence of tetrabutylammonium ion

Vanillin is one of the industrially important compounds that can be produced from lignin. This study presents production of vanillin and vanillic acid (oxidized form of vanillin) through aerobic oxidation of Japanese cedar (Cryptomeria japonica) at 120 °C for 72 h in aqueous alkali solutions with several Bu₄N⁺ and OH^? concentrations (1.25, 2.50, and 3.75 mol/L), where Bu₄N⁺ is an enhancer of the vanillin formation reported in our previous study. The concentrations of Bu₄N⁺ and OH^? were adjusted by the additions of Bu₄NCl and solid NaOH into the base medium Bu₄NOH·30H₂O, which forms 1.25 mol/L aqueous solution of Bu₄NOH at the elevated temperature. Vanillin and vanillic acid were produced with the maximum yields of 21.0 and 1.7 wt% (lignin-base), respectively, at the 1.25 mol/L Bu₄N⁺ and 3.75 mol/L OH^? concentrations. This vanillin yield is close to that obtained by the alkaline nitrobenzene oxidation (26.5 wt%), indicating significantly high selectivity of our lignin degradation with Bu₄N⁺ toward vanillin formation. We also proposed a novel Bu₄NOH·30H₂O-free reaction medium, where Bu₄NOH·30H₂O as the base medium were substituted with an aqueous solution of Bu₄NCl and NaOH to avoid using expensive Bu₄NOH·30H₂O. The treatment of the Japanese cedar with this alternative medium exhibited the moderately decreased vanillin yield of 14.6 wt%, which is, however, much higher than the vanillin yield obtained with a simple 1.25 mol/L NaOH solution.