Electrochemical Molecular Conversion of α-Keto Acid to Amino Acid at a Low Overpotential Using a Nanoporous Gold Catalyst

Abstract

A nanoporous gold (NPG) electrode prepared through a facile anodization technique was employed in the electrochemical reductive amination of biomass-derivable α-keto acids in the presence of a nitrogen source to produce the corresponding amino acids. NPG showed a clear reductive current in the presence of α-keto acid and NH2OH, and the electrolysis experiments confirmed the production of L-amino acid. A reductive voltammetric signal at the NPG electrode appeared at a more positive potential by 0.18–0.79 V, compared with those at the planar-gold electrode without anodization and other previously reported electrode systems, indicating the high activity of the prepared nanostructure for the electrochemical reaction. Maximum Faradaic efficiencies (FEs) of 74–93% in the reductive molecular conversion to amino acids of Ala, Asp, Glu, Gly, and Leu were obtained under the optimized conditions. The FE values were strongly dependent on the applied potential in the electrolysis, suggesting that the hydrogen evolution reaction at the electrode surface was more significant as the applied potential became more negative. The effect of potential at the NPG was lower than that at the planar-gold electrode. These results indicate that nanostructurization decreases the overpotential for the electrochemical reductive amination, resulting in high FE.

Alanine Formation in a Zero‐Gap Flow Cell and the Role of TiO2/Ti Electrocatalysts

“…Faradaic efficiencies ( η ) up to 99 % in an H‐cell and up to 77 % in a flow cell have been reported [5] . The ERA of 5 α ‐keto acids using an anodized nanoporous gold electrode has been investigated, resulting in amino acid η of 74–93 % [18] . Ti based oxide electrocatalysts have also been used to obtain α ‐amino acids from α ‐hydroxyl acids, through combined oxidation and ERA [6,8] .…”

“…[5] The ERA of 5 α-keto acids using an anodized nanoporous gold electrode has been investigated, resulting in amino acid η of 74-93 %. [18] Ti based oxide electrocatalysts have also been used to obtain α-amino acids from αhydroxyl acids, through combined oxidation and ERA. [6,8] A common attribute of these methods is the use of NH 2 OH as the nitrogen source.…”

Amino Acid Synthesis through C–N Coupling between α-Ketoacids and Hydroxylamine from Nitrate Reduction

“…14,15 Many α-ketoacids are central components in biological processes and are widely supplied by lignocellulosic biomass. 3,15,16 Inspired by nature, we speculate that electrocatalytic C−N coupling with αketoacids as the C source and NO 3 − as the N source provides an opportunity for the artificial synthesis of amino acids. Recently, the production of α-amino acids has been achieved by NO x reduction and its further coupling with αketoacids 17−20 or oxalic acid.…”