Producing Ethylene Using Solar Energy

Scientists in Singapore have developed an artificial photosynthesis device for greener ethylene production.

A team of scientists from the National University of Singapore (NUS) has developed a prototype device that mimics natural photosynthesis to produce ethylene gas using only sunlight, water and carbon dioxide. They published their findings in ACS Sustainable Chemistry & Engineering.

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 was 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 steam cracking 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.

Recognizing the need for a more eco-friendly method, a team of researchers led by Assistant Professor Jason Yeo Boon Siang from the Department of Chemistry at the NUS Faculty of Science and the Solar Energy Research Institute of Singapore 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.

This copper catalyst was subsequently introduced 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, which refers to the efficiency of electron transfer in a system facilitating an electrochemical reaction. 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 change,” said Yeo.

“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.”

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.


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