In the hopes of slowing climate change, researchers are seeking ways to get rid of planet-warming carbon dioxide. Making something valuable from it in the process, such as commodity chemicals, is a double win. Researchers at the University of Toronto and the California Institute of Technology now report they have carried off such a trick by improving the efficiency of a process to make the plastics precursor ethylene from CO2 electrochemically (Nature 2019, DOI: 10.1038/s41586-019-1782-2).
There’s a huge existing market for ethylene, says Edward Sargent, an electrical engineer at the University of Toronto. But the compound also has a big CO2 footprint because its manufacture is fossil fuel based. “So if we could instead make renewable ethylene,” Sargent says, “we could displace some of the use of fossil fuel–derived ethylene, and we could consume rather than emit CO2 while we’re doing it.”
For this work, Sargent and coworkers teamed up with Jonas Peters and Theodor Agapie at Caltech. They improved their electrochemical system by adding a new coating to the surface of their copper catalyst. The coating helps the carbon compounds stick to its surface, thereby encouraging the coupling reactions that yield ethylene. To encourage this pairing, they needed a matchmaker molecule. The researchers dimerized a series of arylpyridiniums on the Cu surface, enabling the C atoms to get close enough to pair and then undergo an electrochemical reduction to form ethylene.
The resulting multistep reaction involves a lot of electron transfers, Sargent says. The team flows CO2 and electrolyte into the system. CO2 adsorbs onto the Cu catalyst, whose electrons reduce it to carbon monoxide, which then binds to the surface. The dimerized arylpyridinium molecules direct the CO into just the right position, setting it up for C–C coupling. After the addition of hydrogens from water, the system spits out ethylene.
The system is efficient compared with previous attempts, converting 72% of the energy input into ethylene product. The team also ran the electrochemical synthesis at a high enough current to appeal to industry. The key, Sargent says, was finding the right arylpyridinium compound to maximize ethylene production. “We screened a rather wide library of molecules,” he says, and the researchers found that by paying careful attention to how the different coating molecules held charge, they could increase the amount of ethylene they made.
Ideally, Sargent says, the reaction would be powered by renewable electricity, and the CO2 would come from an industrial flue. “Instead of just going into the atmosphere, the CO2 would become a chemical feedstock for the synthesis of something more valuable,” he says.
Feng Jiao, a chemical engineer at the University of Delaware, says the work shows how reaction selectivity can be tuned by functionalizing a Cu catalyst surface. He sees room for improvement in the system’s efficiency but says it’s encouraging to see this reaction being done in neutral solutions—an improvement over an earlier version of the system that worked in harsh alkaline conditions (Science 2018, DOI: 10.1126/science.aas9100).