Vanillin Formation by Electrooxidation of Lignin on Stainless Steel Anode: Kinetics and By-Products

Abstract

Galvanostatic electrooxidation runs on stainless steel (SS-304) anode were carried out on soda lignins from wheat straw and bagasse obtained from acidification of respective soda black liquors. Vanillin concentration, produced during electrooxidation, is found to increase, attain a maximum, and then decline. Pseudo-first-order specific reaction rate constants for lignin degradation in electrooxidation over SS-304 anode were found to be 0.0308 and 0.0368 h⁻¹, respectively, for the two lignin samples. Degradation of vanillin during electrooxidation was found to be of second order with respect to vanillin concentration with specific reaction rate constant being 0.0009 L/(mg h). A kinetic model was proposed based on series-parallel multiple reaction scheme using a fourth-order Runge–Kutta iteration to fit the time-dependent vanillin concentration during electrooxidation. It was proposed that a vanillin precursor was formed from the parent lignin macromolecule with a pseudo-first-order specific reaction rate constant of 0.03 h⁻¹ for wheat straw soda lignin and 0.036 h⁻¹ for bagasse soda lignin. The vanillin precursor subsequently converted to vanillin with a pseudo-first-order specific reaction rate constant of 0.044 h⁻¹. In addition to vanillin, 4-hydroxy-3,5-dimethoxyphenyl ethanone, and 1,2-benzenedicarboxylic acid were detected as major by-products. Interestingly, some aliphatic and aromatic hydrocarbons were also formed.

KEYWORDS:

• Electrooxidation
• soda lignin
• vanillin
• kinetic model
• by-products

FUNDING

This work was supported by Extramural Research Grant, No. 02(0135)/13/EMR-II, from the Council of Scientific and Industrial Research, Government of India.