Device mimics photosynthesis to make chemical for plastic

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Researchers have developed a prototype device that mimics natural photosynthesis to produce ethylene gas using only sunlight, water, and carbon dioxide.

The novel method, which produces ethylene at room temperature and pressure using benign chemicals, could be scaled up to provide a more eco-friendly and sustainable alternative to the current method of ethylene production.

Ethylene, which is the building block of polyethylene, is an important chemical feedstock produced in large quantities for manufacturing plastics, rubber, and fibers. More than 170 million tons of ethylene were produced worldwide in 2015 alone, and the global demand is expected to exceed 220 million tons by 2020.

Current industrial production of ethylene employs steam cracking of fossil fuels at between 750°C to 950°C, which consumes a large amount of energy and poses a strain on natural fuel resources. The current method also leaves a significant carbon footprint, emitting about two tons of carbon dioxide for every ton of ethylene produced. As such, there is a growing demand for a cleaner and more sustainable way of producing ethylene.

“Our device not only employs a completely renewable energy source, but also converts carbon dioxide, a greenhouse gas into something useful.”

Recognizing the need for a more eco-friendly method, Jason Yeo Boon Siang, research lead and an assistant professor in the chemistry department at the National University of Singapore Faculty of Science and the Solar Energy Research Institute of Singapore (SERIS), and his team tapped into renewable energy to produce ethylene. The team first designed a copper catalyst in 2015 that could generate ethylene from readily available water and carbon dioxide when powered by electricity.

The researchers later introduced this copper catalyst into an artificial photosynthesis system to convert carbon dioxide and water into ethylene using only solar energy. The prototype device designed to carry out the reaction achieved a 30 percent faradaic efficiency of ethylene based on the number of electrons generated from solar energy. The overall energy efficiency of solar-to-ethylene is also comparable to the level of energy efficiency of natural photosynthesis by plants.

“Carbon capture is a key step in fighting human-driven climate change. There has been a steady increase in the atmospheric concentration of carbon dioxide, because the rate of carbon dioxide emissions exceeds that of carbon capture. This has been attributed as a major cause of global warming which leads to undesirable environmental changes,” Yeo says.

“Our device not only employs a completely renewable energy source, but also converts carbon dioxide, a greenhouse gas into something useful. This could potentially close the carbon cycle,” says Yeo.

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The team also incorporated a battery in the prototype device to attain stable and continuous production of ethylene, a key challenge in artificial photosynthesis systems. The battery stores excess solar energy collected in the day to power the device at night or under low light, ensuring that operations are not interrupted by varying amount of sunlight throughout the day.

The invention marks a milestone in the realization of a scalable artificial photosynthesis system for clean and sustainable production of important organic molecules like ethylene.

Moving forward, the team will continue to work on their device to scale up the production of ethylene as well as employ similar systems to produce liquid fuels such as ethanol and propanol.

Researchers report their findings in the journal ACS Sustainable Chemistry & Engineering.

Source: National University of Singapore

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