Polyethylene is used in products such as shopping bags and white plastic cutting boards, but large amounts ultimately become waste. The ORNL team’s process combines the plastic with a molten salt containing aluminum chloride. The salt performs two functions at once: It acts as the reaction medium and provides catalytic sites that promote the chemical transformations.

The researchers have applied for a patent for the technology, and their findings were published in the Journal of the American Chemical Society.

How molten salt breaks down polyethylene into fuel

To determine why the process works, the scientists tracked the chemical reactions that transform polyethylene polymers into fuel molecules.

Using soft X-ray spectroscopy and nuclear magnetic resonance, the researchers found that a charged aluminum atom combines with three other atoms to create a highly acidic catalytic site. These sites attack polyethylene’s long molecular chains and split them into smaller hydrocarbon molecules.

Additional experiments using isotope labeling and neutron scattering revealed how the structure of the starting polymer affects the resulting fuel. Simpler polymer chains tend to produce gasoline-like compounds, while more complex chains produce diesel-like fuels.

If the method can eventually be expanded beyond laboratory experiments, the researchers say it could help strengthen U.S. energy security and industrial competitiveness.

“We have developed an efficient and selective conversion of polyethylene to gasoline,” said Liqi Qiu, a postdoctoral fellow at the University of Tennessee, Knoxville. Qiu performed most of the experiments for the study in the Sheng Dai lab at ORNL and UTK. Dai, an ORNL Corporate Fellow and Section Head of Separations and Polymer Chemistry, is a co-corresponding author of the paper.

In experiments, the team achieved gasoline yields of approximately 60% under relatively mild reaction conditions.

Plastic-to-fuel conversion below 200°C

One of the method’s most notable features is its relatively low operating temperature compared with traditional plastics-to-fuel technologies.

“We converted polymer waste into a value-added fuel by using commercially available inorganic salts as reaction media to provide catalytic sites,” said ORNL staff scientist Zhenzhen Yang, a co-corresponding author of the paper. “Unlike traditional techniques for converting polymers into fuels, the new process did not require precious metal catalysts, organic solvents, or external hydrogen.”

Yang said the work is the first reported use of molten salts as a medium to produce high-value chemicals from waste at temperatures below 200°C without catalytic initiators or solvents.

A temperature below 200°C is comparable to the temperature inside a traditional kitchen oven. By contrast, previous methods for converting polyethylene into gasoline typically relied on pyrolysis, which breaks down large polymer molecules at temperatures of approximately 450°C to 500°C.

In addition to operating at a lower temperature, the ORNL process may be simplified by eliminating expensive noble-metal catalysts, external hydrogen, organic solvents, and chemical initiators.

“The ORNL system solves two fundamental problems. First, the system is stable, which makes it much easier to scale up the process. Second, previous systems required an initiator to start the catalytic reaction,” said Sheng Dai, ORNL Corporate Fellow and Head of Separations and Polymer Chemistry.

Decades of molten-salt research at ORNL

ORNL has researched molten salts for decades. Experiments involving molten-salt reactors in the 1960s demonstrated that molten-salt mixtures could function as both nuclear fuel and reactor coolant.

Building on that history, Dai proposed using molten salts for a different purpose: converting waste polymers into useful fuels.

Molten salts are inorganic compounds that can remain stable under harsh chemical reaction conditions. In this study, the researchers used molten salt with polyethylene, a widely available commodity polymer.

“We are trying to understand the basic science that will lead to discoveries and new economic opportunities,” said Tomonori Saito of ORNL, who managed the project and contributed expertise in polymer science.

Advanced techniques reveal how the reaction works

Understanding the reaction required scientists from multiple disciplines and the use of several advanced analytical techniques.

At ORNL, Luke Daemen used neutron scattering to help identify the hydrocarbon products formed when different polymer chains reacted. Felipe Polo-Garzon analyzed the products with gas chromatography-mass spectrometry, which separates and identifies individual compounds.

When polyethylene interacts with the aluminum catalytic sites, the reaction produces positively charged carbon ions. Qiu, Yang, and Dai tagged the carbon ions with deuterium, a hydrogen isotope, to track how they changed as the reaction progressed.

The team also used neutrons at ORNL’s Spallation Neutron Source to monitor hydrogen in the system.

“Polymers contain large amounts of hydrogen,” Dai said. “Neutrons are ideal for identifying light elements, including isotopes such as hydrogen and deuterium.”

The researchers also examined how the aluminum components changed during the process.

Yang visited the Advanced Light Source at Lawrence Berkeley National Laboratory, where he worked with Kim Min-jae and Guo Jinhua. The team used soft X-rays to study interactions between aluminum and polyethylene at the atomic and electronic levels. Soft X-rays are particularly useful for examining relatively lightweight elements such as aluminum.

“The edge of the aluminum moved to the edge of low electron density, which means an electron-rich intermediate was formed,” Yang said. “We compared this finding with other techniques and confirmed that aromatic ring intermediates can coordinate with aluminum, causing a binding energy change.”

The change provided evidence that the aluminum site was actively catalyzing the chemical reaction.

Simulations and imaging clarify the catalytic mechanism

Back at ORNL, Bobby Sumpter of the Center for Nanophase Materials Science used computer simulations to study energy changes during the reaction, including how stable carbon ions form and transfer into hydrocarbon products.

At the University of Tennessee, Knoxville, Michael Koehler used in situ X-ray diffraction to track phase changes in reaction mixtures as the chemistry unfolded. Carlos Alberto Stellen used nuclear magnetic resonance to investigate catalytic sites in aluminum.

ORNL’s Tao Wang contributed expertise in molten-salt chemistry, while ORNL’s Logan Kearney supplied high-density polymers and provided guidance on possible routes for converting them into higher-value products.

Keeping the molten salt stable is the next challenge

Although aluminum-based catalyst systems are inexpensive and chemically active, they have important limitations. Aluminum chloride is hygroscopic, meaning it absorbs water quickly, and moisture can reduce the system’s stability.

The researchers plan to investigate halogen- and carbon-based materials that could trap the molten salts. These materials may make salt separation and processing easier while improving salt stability.

The research could ultimately expand the range of methods available for producing transportation and industrial fuels from plastic waste.

“Polymer feedstocks are abundantly available from consumer waste, and our catalyst system, aluminum molten salt, is very cheap,” Qiu said. “This progress could be promising for industry.”

The DOE Office of Science’s Materials Science and Engineering Division primarily supported the research, along with gas chromatography-mass spectrometry work through the Chemical Sciences, Earth Sciences, and Biological Sciences Division’s Catalysis Science Program. The study used DOE Office of Science user facilities at ORNL, including the Spallation Neutron Source for neutron scattering at the VISION Beamline and the Center for Nanophase Materials Science for quantum chemical computations, as well as the Advanced Light Source at Lawrence Berkeley National Laboratory for soft X-ray spectroscopy.