Exploring the Scope of Gas Diffusion Electrode-based Electro-Enzymatic Synthesis

Abstract

In the present work, gas diffusion electrode (GDE)-based electro-enzymatic syntheses were investigated using the enzymes chloroperoxidase from Caldariomyces fumago (CfuCPO) and an evolved version of the unspecific peroxygenase from Agrocybe aegerita (AaeUPO PaDaI). Both enzymes use H2O2 as the reduction equivalent which, however, leads to their irreversible inactivation at elevated concentrations. Electrochemical in situ H2O2 supply can minimize this inactivation. In the first part of this work, this effect was studied with the electro-enzymatic hydroxylation of ethylbenzene to 1-phenyl ethanol using AaeUPO PaDaI. As a first step, a suitable glass reactor was developed, which allows for easy handling at an as small as possible volume. Furthermore, after first tests, it was determined that in all further experiments a fresh GDE from pressed VulcanXC72 with 50 % PTFE should be used. Also, it was investigated which reaction media would offer the best compromise for the electrochemical as well as the enzymatic reaction. Further experiments were finally performed in 100 mM KPi with 3 % vol acetone at pH 7. Current densities between -5 and -30 mA cm-2 were tested, whereby the current efficiency relating to H2O2 reached 65 % to 78 %. At -10 mA cm-2 the highest total turnover number (TTN) of 401,916 molProduct molAaeUPO-1 at a corresponding space time yield (STY) of 13.2 g L-1 d-1 was reached. At -25 mA cm-2¬ the highest STY of 26 g L-1 d-1 at a corresponding TTN of 266,234 molProduct molAaeUPO-1 was reached. These values do not exceed the literature-reported benchmarks, nevertheless, they come very close. Also, the combination of high TTNs and STYs has not been reported in literature to date. In the second part of this work, a continuous electro-enzymatic system was investigated with the chlorination of monochlorodimedone (MCD) to dichlorodimedone (DCD) using CfuCPO. Investigations with a PET fabric-based CfuCPO immobilisate were not continued due to high enzyme leaching. In further experiments, enzyme retention using a dialysis tube was implemented. The continuous conversion was performed at a flow rate of 0.5 mL min-1 and current densities of -0.55 mA cm-2 and 1.1 mA cm-2. Under those conditions, a STY of 1.2 mg L-1 h-1 and 1.06 mg L-1 h-1 was reached, respectively. This yield is less than 1 % of a comparable literature-reported batch system.