Ethylene – an important raw material for plastics production – is one of the five best-selling compounds in the chemical industry. Now two teams have independently discovered new electrocatalytic processes to selectively and efficiently produce pure ethylene. It is important that these reactions work under ambient conditions with abundant copper catalysts and are a more environmentally friendly alternative to traditional solutions.
Most of the ethylene comes from steam cracking – a process in which hydrocarbons are broken down into smaller molecules – and then separated by distillation. However, ethylene streams contain small amounts of acetylene, an undesirable by-product. “Acetylene impurities quickly poison Ziegler-Natta catalysts during the polymerization of olefins,” explains Jian Zhang from Northwestern Polytechnical University in Xian, China, lead author of one of the papers.1 “This greatly degrades the quality of the target polymers … polymer-grade impurities in ethylene must be reduced to less than 5ppm.” Their novel electrocatalytic approach converts acetylene impurities into ethylene using water as the source of hydrogen.
“This process is much cheaper and more environmentally friendly,” adds Zhang. “We have achieved continuous production of polymer grade ethylene streams at high space velocity” [and] under ambient temperature and pressure. ‘
Conventionally, acetylene is removed using a thermal catalytic hydrogenation reaction that requires high temperatures and expensive palladium catalysts. “In addition, thermal catalysis requires an excess of hydrogen, which is generally produced during reforming,” says Run Shi from the Technical Institute of Physics and Chemistry in Beijing, China, co-first author of the second article.2This dependence on fossil hydrogen makes the cleaning process particularly harmful to the environment as it generates large amounts of carbon dioxide. ‘[Our] electrochemical process shows particular advantages in the energy and nuclear industries, ”adds Shi.
In addition, these new electrocatalytic solutions also improve selectivity and efficiency. “Since thermal processes often rely on excess amounts of hydrogen, ethylene is often further hydrogenated to ethane,” explains Gastón Larrazábal, Expert in catalysis and electrochemistry at the Technical University of Denmark. “This approach avoids this classic limitation of thermal catalysis and successfully removes the acetylene contamination.”
The most important breakthrough for Larrazábal is the introduction of innovative gas diffusion electrodes. “They give acetylene better access to the reactive centers of the catalyst – an efficient electrochemistry with reagents in the gas phase,” he explains. Both teams agree: the new gas diffusion approach offers a robust alternative to cleaning ethylene. “We are opening a new area of research in ethylene-related electrochemistry,” says Shi.
The researchers are targeting copper as it is an abundant, sustainable alternative to palladium. “As far as we know, copper is the best catalyst,” says Zhang. His team conducted both experimental and theoretical research to confirm this hypothesis, comparing copper to palladium, silver, gold, and nickel. “Copper in abundance on earth shows the best activity and selectivity for electrocatalytic acetylene half-hydrogenation,” adds Zhang. Nevertheless, there will certainly be further developments in the materials soon. “The composition of the catalyst, the stability of gas diffusion electrodes and the structure of alkaline solid-state electrolytes will be the next breakthroughs,” explains Shi.
Despite their novelty, these devices should be easily scalable because “the equipment is similar to fuel cells,” says Shi. “This technology enables a high mass transfer between the gas phase and the catalyst and thus higher reaction speeds – the potential for industrial implementation is great,” says Larrazábal.
Zhang and his co-workers are already expanding their process and moving to electrodes that are 20 times larger. “This will increase the maximum space velocity,” he says. “Then we will connect several devices in parallel to drive industrialization forward,” explains Zhang. “We are making great efforts to optimize your stability … to more than 1000 hours.”
“This is a novel, clean way to purify ethylene for polymer production,” says Larrazábal, who also notes that the carbon source is still not renewable. “All hydrogen comes from water, which kills two birds with one stone: avoiding an extra step when using a green hydrogen source,” he adds. “And it’s really cool to see researchers explore new frontiers in electrocatalysis, even via CO. out2 reduction.’