Electro-organic reactions. Part 60[1]. The electro-oxidative conversion at laboratory scale of a lignosulfonate into vanillin in an FM01 filter press flow reactor: preparative and mechanistic aspects

Abstract

The electrochemical conversion of a spruce lignosulfonate into vanillin, at nickel anodes, was explored in previously unobtainable detail. A flow reactor (FM01), in a rig that permitted considerable variation of electrolysis conditions, allowed up to 150 g to be electrolysed at up to12 A at a variety of electrode configurations. Samples taken during electrolysis gave detailed reaction profiles. The electrolyser operated at 145 °C/500 kPa/3 M NaOH and yields of vanillin were similar to those obtained industrially using chemical oxidants (about 5–7% w/w). Vanillin production was favoured by low current density and low initial concentration of lignosulfonate. Vanillin, alone, was consumed in a 2.7 F process under the above conditions. Historically, yields in chemical and electrochemical conversions of lignins into vanillin do not exceed 10%; the results herein explain this apparent limit as equilibrium between formation of vanillin, its concomitant oxidative destruction and further condensation of lignins.