Illumination of a single atom for sustainable propylene production

Over 150 million tons of propylene are produced annually, making it one of the widest chemicals used in the chemical industry.

Propylene is the foundation of polypropylene, a polymer from medical devices to packaging and household products. However, most propylene is produced by steam cracking, a high energy process that uses heat to break down crude oil into small hydrocarbons.

Now, university chemists in the northwest have found a way to use light to create propylene. Their findings show that nano-engineered photoactive catalysts can directly make propylene through a process called non-oxidized propane dehydrogenation (PDH).

The team discovered that photo-driven chemical processes can catalyze the reactions that generate propylene and hydrogen from propane. If adopted on a large scale, this process could reduce emissions in the chemical industry. This is an important step towards decarbonisation in industrial use.

The results are published today in the Journal of the American Chemical Society. The first authors include graduate student Emma Rose Newmeyer and postdoctoral scholar Ishen Wang.

“Propylene is a giant in the chemical industry and now shows that it can be done through a milder reaction than is normally used,” said Northwestern’s Deine Swearerer, who led the study. “This proves that designer nanoparticles can be used in advances towards a more sustainable future. If the industry can use renewable energy sources like light in these catalytic processes, it will further reduce the overall energy demand.”

Propylene is produced by concentrating light on a single atom

To make propylene, scientists need to break down carbon hydrogen bonds at a molecular level. “It doesn’t sound very intense because there are carbon hydrogen bonds everywhere, but these are very stable bonds that are difficult to break,” Swearer said.

With the increased availability of natural gas in the US through shale resources, the idea of ​​PDH as a propylene manufacturing process has become more common. Not only is it potentially cheaper, it could also reduce the need for crude oil resources and help to move towards renewable energy.

However, scientists have struggled to find the right catalyst for this process to reduce environmental impact.

For their research, Swearer and his team tested untapped ideas. It drives the reaction with a special type of nanoparticle that absorbs light but has a distinct position where a single atom catalyzes the reaction. The team created a copper-platinum alloy, a combination known to be a great thermal catalyst, and tested what happened when it sparkled the light.

They found that when activated with a laser, nanoparticles are excited and catalyzed the reaction that produces propylene.

The team experimented with different amounts of alloy platinum, along with the color and intensity of the light. They found that inclusion of separated platinum atoms within copper nanoparticles causes the structure to leak light into isolated platinum atoms, allowing carbon hydrogen bonds to break more easily.

“It is this funnel of the energy of light to a single atom that allows this reaction to occur,” Swearer said.

And although they tested the system with more platinum, structures containing only a single platinum atom are the most effective. So the process requires only a small amount of precious metals “without sacrificing reactivity and selectivity,” Newmeyer said.

Additional bonus: This process simultaneously creates hydrogen and provides a valuable secondary by-product.

Industry’s potential energy savings

The team also found that the overall temperature can be reduced by 50 degrees (from the standard operating temperature) to get the same conversion speed. This means that if this process is adopted by the industry, it could save a lot of energy.

“Reducing the temperature at which these industrial scale processes operate can have a significant impact on emissions associated with chemical manufacturing,” Newmeyer said.

Next, the team hopes to continue developing this catalyst and test it in other processes that are important to create components of the chemical industry.

“There is a lot of room to use these light-driven single-atom alloys to promote a variety of reactions,” Swearer said.