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.