Production of Vanillin and Vanillic Acid by Aerobic Oxidation of Polyethylene Glycol (PEG)-modified Glycol Lignin in Tetrabutylammonium Hydroxide

Aerobic oxidation of lignin in reaction media containing Bu4NOH facilitates efficient production of vanillin (4-hydroxy-3-methoxybenzaldehyde) and vanillic acid (4-hydroxy-3-methoxybenzoic acid). This study presents production of these compounds from polyethylene glycol (PEG)-modified glycol lignin (PEG-lignin) from Japanese cedar (Cryptomeria japonica). A Bu4NOH-based reaction medium was prepared by the addition of NaOH(s) to 1.25 mol/L Bu4NOH aq. Degradation of the PEG-lignin (14 mg) in 2.0 mL of the medium at 120 ℃ under O2 in a sealed test tube produced vanillin and vanillic acid with 9.1 and 2.6 wt% yields, respectively, based on the Klason lignin amount of the PEG-lignin. The total yield of vanillin and vanillic acid reached 79.6 % of that in the corresponding alkaline nitrobenzene oxidation (14.7 wt%), indicating high performance of the medium. Bench-scale aerobic oxidation of the PEG-lignin (100 g /1.0 L) under O2 pressurized at 0.7 MPa gave the products with their maximum yields similar to those in the above lab-scale experiment. Further increase in the O2 pressure to 2.5 MPa significantly shortened the reaction time to achieve the maximum product yield. This pressure increase did not affect the vanillin yield, but suppressed the formation of O2-sensitive vanillic acid, by which selective vanillin production was achieved.