Electrolysis Reaction Pathway for Lactic Acid in Subcritical Water

Electrolysis reactions of lactic acid were studied using a 500 mL continuous flow reactor made of SUS 316 stainless steel. In this system, a titanium wall acted as a cathode and a titanium plate-layered type electrode was used as an anode in a subcritical reaction medium. The reactor wall (stainless steel) and the cathode (titanium) were separated from each other by a cylindrical ceramic wall. This hydrothermal electrolysis process provides an environmentally friendly route that does not use any organic solvents or catalysts to produce value-added chemicals from wastewater treatment. Reactions were conducted with a 30 min residence time at a pressure of 10 MPa at 280 °C via application of various direct currents ranging from 0.5 to 2 A. In addition, to improve our understanding of the reaction mechanism, we investigated the effects of initial lactic acid and electrolyte (NaOH) concentrations on the degradation of lactic acid and the product yields using continuous flow hydrothermal electrolysis. Acrylic acid, acetic acid, and acetaldehyde were detected as the main reaction products using high-performance liquid chromatography. Increasing the applied current increased the conversion of lactic acid and product yields. With a current of 2 A, an electrolysis time of 30 min, and the addition of 50 mM NaOH, a 55% conversion was achieved. The acetaldehyde yield increased almost linearly with current, and at 2 A, 24.73% of the acetaldehyde was produced compared to a 2.25% yield of acetic acid under the same conditions. For acrylic acid, at higher currents (1.5 and 2.0 A), the rate of generation of acrylic acid decreased (values of 0.82, 0.65, and 0.49% at 1.0, 1.5, and 2.0 A, respectively). Increasing the pH of the feed solution resulted in a drastic decrease in the yields of acrylic acid and acetaldehyde.