The electrosynthesis of ethylene oxide in a tandem recycle flow reactor system

Abstract

In this work an electrochemical tandem recycle reactor was developed for the selective electro-oxidation of ethylene into ethylene oxide with a concentrated product outflow, resulting in an more sustainable pathway towards EO as opposed to the partial oxidation process. The system consisted of (i) a cation exchange membrane equipped module operating at a constant setpoint of 133.3 mA/cm2 and (ii) an anion exchange membrane unit in sequence, with a variable setpoint to regulate the pH. An excess of ethylene was supplied to the porous anode of the cation exchange module, which subsequently reacted with evolved bromine into 2-bromoethanol. As a result the anolyte acidified, which in turn promoted a selective bromine production. The anion exchange module also contributed to the Br2 production, converted 2-bromoethanol into ethylene oxide and neutralized the anolyte. The system underwent equal charge experiments in varied anolyte concentrations with a (i) Ni gauze cathode and an IrO2 coated Ti anode in the cation exchange module and (ii) graphite electrodes in the anion exchange unit. For experiments of 6 up to 10 h, Faradaic efficiencies towards ethylene oxide of 80.2 ± 6% and 46.7 ± 4.5% were sustained in 0.5 M and 3 M KBr anolyte respectively. The reduced selectivity in augmented KBr concentrations was predominantly caused by organochemical thermodynamics, which provoked an increased 1,2-dibromoethane production. The highest ethylene oxide production was achieved in a 0.5 M KBr condition, i.e. 10 g/L, which is ten times higher than the maximum reported figure in literature. In terms of stability, pretreated anodes, i.e. baked at 450 °C in air prior to testing, showed no degradation before and after testing on SEM-EDS analyses, whereas untreated structures were susceptible to corrosion.