A new electrochemical process that makes ethylene oxide could help slash the carbon footprint of this important commodity chemical. The strategy is part of a burgeoning effort to green the chemical industry by developing alternative synthetic routes that run on renewable electricity.
The chemical industry makes about 20 million metric tons of ethylene oxide per year, which is used in plastics, detergents, and solvents. It is produced by uniting oxygen and ethylene over a silver catalyst at temperatures of 200–300 °C and pressures up to 3 MPa, and the reaction generates almost as much of the greenhouse gas CO2 as ethylene oxide. More than half of this CO2 comes from the overoxidation of ethylene, with the remainder emitted by the combustion of fossil fuels used to power the process.
An electrochemical approach could curb these emissions, but it faces two big challenges. The first is that ethylene is poorly soluble in water, the preferred electrolyte solvent used inside the electrochemical cell. Low solubility hampers ethylene’s interaction with the cell’s anode and reduces the efficiency of the oxidation process. Boosting the power of the cell might improve the rate of ethylene oxide production, but it also creates a second challenge: it can overoxidize ethylene, generating unwanted CO2.
Edward H. Sargent’s team at the University of Toronto has now solved this conundrum by using chloride ions in the electrolyte as a vehicle to carry charge between the cell’s anode and ethylene to improve their interaction (Science 2020, DOI: 10.1126/science.aaz8459). “It’s a kind of redox mediator,” says Wan Ru Leow, the team member who led the research.
As current flows through the anode of the electrochemical cell it converts chloride ions into chlorine (Cl2), which forms hypochlorous acid (HOCl) and hydrochloric acid (HCl). Hypochlorous acid then reacts with ethylene to form ethylene chlorohydrin (also known as 1-chloro-2-hydroxyethane). Meanwhile, the cathode splits water to release hydroxide anions and hydrogen gas, which can be captured as an additional product. The team’s cell contains a membrane that separates its catalytic electrodes and prevents the solutions in each side of the cell from mixing.
After the anode and cathode reactions occur, the researchers remove the two solutions from the cell and combine them, allowing ethylene chlorohydrin to react with hydroxide and make ethylene oxide. Leow says that in an industrial process, these operations could potentially be carried out in a continuous flow system to further improve efficiency.
Overall, around 70% of the electrical current supplied to the cell goes into making the product—a reasonably high efficiency—and it produces no CO2 emissions. About 97% of the ethylene that reacts is turned into the desired epoxide, and in principle unreacted ethylene could be recirculated through the cell, the team says. The cell also converts propylene to propylene oxide with similar efficiency.
The electrosynthesis operates at a high current density up to 1 A/cm2, which determines how much product can be made by a given electrode. “It’s quite exceptional,” says Karthish Manthiram at the Massachusetts Institute of Technology, who is also developing electrochemical methods for making epoxides and was not involved in the new work. “They’ve gone right up to the sorts of current densities that are needed for commercial operation.”
The researchers also carried out a technoeconomic analysis of their process and concluded that under optimum conditions the process could produce ethylene oxide at a cost of roughly $1,500 per ton ($1650 per metric ton), matching the conventional process.
Last year, Sargent’s team unveiled an electrochemical method for reducing CO2 to ethylene (Nature 2019, DOI: 10.1038/s41586-019-1782-2). By combining this earlier process with the new electrochemical system, the team also demonstrated it could convert CO2 all the way to ethylene oxide.
A pilot plant in Calgary running the team’s CO2-to-ethylene process can already produce 100 kg of ethylene per day, and Sargent says its might be possible to modify this equipment to test the new ethylene oxide process at a similar scale. He adds that there is growing commercial interest in these processes: “The global chemical industry is eager to see how it can decarbonize the sector.”