The bromine mediated electrosynthesis of ethylene oxide from ethylene in continuous flow-through operation

Abstract

In this work a reactor setup for the bromine mediated, electrochemical oxidation of ethylene oxide from ethylene was developed. This novel design featured a flow-through configuration for the ethylene feed to accommodate the in situ synthesis of the 2-bromoethanol intermediate, creating a low pH at the anolyte to selectively target the bromine evolution. The cell was characterized via chronopotentiometry in 0.5 M KBr electrolyte, using a (i) Pt or (ii) IrO2 coated gas diffusion anodes paired with a Ni foam cathode. Within a current density of 65 to 156 mA/cm2 for Pt and 65 to 133 mA/cm2 for IrO2, the productivity and selectivity of the reactor system was mapped. Throughout all conditions the reactor system retained a Faradaic efficiency of 80 to 90% towards ethylene oxide on both Pt and IrO2 coated gas diffusion anodes and exceeded an equivalent ethylene oxide production of 1 kg/hour/m2 at 156 mA/cm2. Furthermore, the reactor upheld identical selectivity and productivity for 4.5 h without any signs of fading performance. Analysis of the gaseous product phase at this highest current condition showed no CO2 or O2 side products, originating from hydrocarbon overoxidation or oxygen. Based on our findings, the bromine mediated pathway proves to be highly promising as the current best case of the chlorine mediated system achieves a 70% selectivity towards ethylene oxide. Additionally, the anodic catalyst stability was quantified by ICP-MS analysis and scanning electron microscopy coupled with energy dispersive X-ray analysis, which revealed IrO2 to be three orders of magnitude more dissolution resistant compared to Pt and confirm a homogeneous dispersion of the catalyst and binder material on the surface of the gas diffusion electrodes.