Electrolysis Reaction Pathway for Lactic Acid in Subcritical Water

Abstract

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.

Structure–Reactivity Effects of Biomass‐based Hydroxyacids for Sustainable Electrochemical Hydrogen Production

“…The remaining products can be attributed to two HPLC signals observed at short retention times (Figure S11) that also increased with the reaction time. Further oxidation of LA might lead to acetaldehyde or acetate as reported previously, [27,29,31] but these products were not detected in the HPLC chromatograms as they would appear at longer retention times. Therefore, the unknown species obtained during this reaction are expected to be some kind of oligomerization products, as they can be formed under certain conditions [68,69] .…”

“…Yuksel et al. reported the electro‐oxidation of LA in alkaline media at subcritical conditions (10 MPa, 280 °C) using titanium electrodes [29] . Under these conditions, acetaldehyde was the main product (≈25 %), with an overall conversion of 55 %.…”

Tapping Renewables: A New Dawn for Organic Electrosynthesis in Aqueous Reaction Media

“…Reports on electrochemistry in aqueous reaction media at temperatures higher than 100 °C are scarce, as temperature and pressure‐resistant electrochemical cells, including electrodes, are needed. For instance it was reported that in subcritical water cellulose can be electrochemically converted to mixtures of furfurals and levulenic acid or lactic acid to mixtures of acrylic acid, acetic acid, and acetaldehyde . We believe that this can only be a starting point and hence, organic electrosynthesis in sub‐ and supercritical water can play an important role.…”

“…For instance it was reported that in subcritical water cellulose can be electrochemically converted to mixtures of furfurals and levulenic acid [12,13] or lactic acid to mixtures of acrylic acid, acetic acid, and acetaldehyde. [14] We believe that this can only be a starting point and hence, organic electrosynthesis in sub-and supercritical water can play an important role.…”

Glycerol in subcritical and supercritical solvents

“…A concurrent pathway led to a mixture of formic acid and glycolic acid. Lactic acid can be further decomposed, under subcritical water oxidation, into acryclic acid, acetaldehyde, and acetic acid …”