A High-Purity Ethylene Epoxide Stream Produced Using a Supported Electrocatalyst

Demand for ethylene oxide (EO)─a chemical building block for plastics─exceeds 30 Mt/year. Electrosynthesis of EO from ethylene has, in the highest-activity reports to date, relied on redox-mediated approaches; these lead to chlorinated organic byproducts and high product separation costs due to dilution in solvents as a result of homogeneous electrochemistry. Direct electro-epoxidation is outcompeted by the oxygen evolution reaction (OER) at higher overpotential, causing faradaic efficiencies for EO to drop below 50% above 10 mA/cm². We noted that the OER and ethylene oxidation share a common dependency on surface-adsorbed oxygen (M-O*), and that in Pt group metals, the stronger M-O* bonds enhance the kinetics of the OER relative to epoxidation. We therefore considered catalysts, such as Ag, having a less-strongly bound M-O* intermediate. Unfortunately, we found that Ag dissolved easily under anodic bias and that the oxidized Ag surface at a high oxidation state favors the overoxidation to CO₂. We then aimed to leverage metal-support interactions, with our goal being to render the Ag more stable while in its higher oxidation state. An Ag-ZrO₂ catalyst synthesized from a MOF template coupled with the MEA-PTFE system achieves 50% FE for ethylene oxide at 50 mA/cm², with a productivity of 460 μmol cm^-2 h^-1. The use of a hydrophilic PTFE separator minimizes system resistance, EO crossover, and hydrolysis, enabling an EO concentration of 48 wt % in the outlet stream at a full cell voltage of 2.1 V in electrolysis paired with HER.