Illuminating single atoms for sustainable propylene production

More than 150 million metric tons of propylene are produced annually, making it one of the most widespread chemicals used in the chemical industry.

Propylene is the basis for polypropylene, a polymer used in everything from medical devices to packaging to household goods. But most propylene is produced through steam cracking, a high-energy process that uses heat to break down crude oil into smaller hydrocarbons. 

Now, Northwestern University chemists have found a way to create propylene using light. Their findings show that a nanoengineered photoactive catalyst can make propylene directly through a process called nonoxidative propane dehydrogenation (PDH).

The team found they could catalyze the reaction to create propylene and hydrogen from propane with a light-driven chemical process. If adopted at scale, the process could lead to lower emissions for the chemical industry, an important step toward industrial decarbonization.

The results were published today in the Journal of the American Chemical Society. First authors include graduate student Emma-Rose Newmeyer and postdoctoral scholar Yicheng Wang. 

“Propylene is a behemoth of the chemical industry, and now we show that we can make it through reactions that are milder than those typically used,” said Northwestern’s Dayne Swearer, who led the study. “This proves that we can leverage designer nanoparticles in our progress toward a more sustainable future. If industry was able to use a renewable energy source like light in these catalytic processes, it could reduce overall energy demands  even more.”

Swearer is an assistant professor of chemistry at Northwestern’s Weinberg College of Arts and Sciences and an assistant professor of chemical and biological engineering in the McCormick School of Engineering. He also is a faculty affiliate in the International Institute of Nanotechnology and co-chair of the Paula M. Trienen’s Institute for Sustainability and Energy’s Generate Pillar, which aims to develop a new class of solar energy production with high-efficiency materials. The Generate Pillar is part of the Trienens Institute’s Six Pillars of Decarbonization.

Making propylene by funneling light onto single atoms

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

Because of increased natural gas availability in the United States through shale resources, the idea of PDH as a propylene manufacturing process has become more popular. Not only could it potentially be cheaper, but it could also reduce the need for crude oil resources and help the transition to renewable energy.

Yet scientists have struggled to find the right catalysts for this process that would reduce environmental impact. 

For their study, Swearer and his team tested an unexplored idea: driving the reaction with a special type of nanoparticle that absorbs light but also has well-defined locations where a single atom catalyzes the reaction.  The team created an alloy of copper and platinum—a combination known to be a good thermal catalyst—and tested what happened when they shined light onto it. 

They found that when activated with a laser, the nanoparticles became excited and catalyzed the reaction to create propylene. 

The team experimented with different amounts of alloyed platinum, along with the color and intensity of the light. They found that when they included isolated platinum atoms within copper nanoparticles, the structure funneled the light down to the isolated platinum atoms, enabling the carbon-hydrogen bond to break more easily. 

“It’s this funneling of energy of the light onto single atoms that enables this reaction to take place,” Swearer said. 

And though they tested systems with more platinum, the structures that included only single platinum atoms worked best — meaning this process would only need a small amount of a precious metal “without sacrificing reactivity and selectivity,” Newmeyer said. 

An added bonus: the process also creates hydrogen at the same time, offering a secondary valuable byproduct.

A potential energy savings for industry

The team also found that they could reduce the overall temperature by 50 degrees Celsius (from standard operating temperatures) and get the same rate of conversion. That means that if this process were adopted by industry, it could result in major energy savings.

“It has the potential to heavily impact the emissions associated with chemical manufacturing by reducing the temperatures at which these industrial scale processes operate,” Newmeyer said. 

Next, the team hopes to continue developing this catalyst and testing it with other processes that are important for making the building blocks of the chemical industry. 

“There’s a lot of room to explore using these light-driven single atom alloys to drive different reactions,” Swearer said. 

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