Electrocatalytic production of formaldehyde with formaldehyde dehydrogenase using a viologen redox mediator

Abstract

Methanol production based on the electrochemical reduction of CO₂ is one possible method for synthesizing electrofuels (e-fuels). A multi-electron system for reducing CO₂ to methanol using multi-biocatalysts and an artificial cofactor has attracted significant attention. For example, using formate, formaldehyde and alcohol dehydrogenase, CO₂ is electrochemically reduced to methanol via formate and formaldehyde intermediates, with the rapid reduction of formate to formaldehyde being a particularly important step. Here, we developed a system for the electrochemical production of formaldehyde from formate by formaldehyde dehydrogenase (FldDH; EC.1.2.1.46) using methylviologen (MV²⁺) as an artificial cofactor. Application of a single-electron reduction potential of MV²⁺ (−700 mV vs. Ag/AgCl electrode) to electrolyte containing formate and FldDH resulted in formaldehyde production, and this production improved as the pH of the electrolytic solution increased. The maximum conversion yield for formaldehyde production was 1.1%, achieved at pH 10.3 after 3 h continuous application of an external electrochemical bias. Thus, the electrochemical formaldehyde production efficiency catalyzed by FldDH with MV²⁺ as an electron mediator was determined for the first time. Furthermore, the interaction between the substrate binding site of FldDH and the cation radical of MV²⁺ was predicted by docking simulation, providing insights into the electrochemical formate reduction process. Since the reduction of formate catalyzed by FldDH to formaldehyde is a two-electron reduction process, two molecules of cation radical of MV²⁺ are required. There is insufficient space in the substrate binding pocket of FldDH for two molecules of the MV²⁺ radical cation and thus the re-reduction of MV²⁺ bound to FldDH is an important process. Our findings indicate that efficient electron injection from the electrode into MV²⁺ bound to FldDH is a key process in formate reduction, an important step towards the reduction of CO₂ to methanol.