Researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) have successfully engineered a novel chemical pathway to transform polyethylene—one of the world’s most ubiquitous and problematic plastics—into high-value, gasoline- and diesel-like fuels. The breakthrough, which has been submitted for a patent and published in the Journal of the American Chemical Society, offers a potential new avenue for addressing global plastic waste while simultaneously contributing to domestic energy security and industrial competitiveness.
Polyethylene is deeply embedded in modern consumer life, utilized extensively in everyday products such as single-use shopping bags, packaging films, and white plastic kitchen cutting boards. Because of its durability and widespread consumption, massive volumes of polyethylene ultimately find their way into landfills, oceans, and natural ecosystems as persistent waste.
To tackle this challenge, the ORNL research team designed a streamlined method that combines the plastic feedstock with molten salts containing aluminum chloride. In this innovative setup, the molten salts fulfill a dual role, acting simultaneously as the reaction medium and as the active catalyst that drives the complex chemical conversion. Under these specialized conditions, the team achieved a remarkable gasoline yield of about 60 percent, all while operating under remarkably mild reaction parameters.
How Molten Salts Break Plastic Into Fuel
To fully comprehend the mechanics behind the transformation, the scientists closely tracked the intricate chemical reactions responsible for dismantling the stubborn polymer into usable fuel molecules.
By employing advanced analytical instruments, including soft X-ray spectroscopy and nuclear magnetic resonance, the research team discovered that charged aluminum atoms bind with three other atoms within the salt mixture. This coordination creates highly acidic catalytic sites. These specialized sites possess the chemical capability to attack the immensely long molecular chains that structurally define polyethylene, cleanly splitting them into smaller, lighter hydrocarbon molecules.
Further experimental validation using isotopic labeling and neutron scattering shed light on how the structural configuration of the starting polymer directly influences the resulting fuel output. The data revealed a clear pattern: simpler initial polymer chains tended to generate gasoline-like compounds, whereas structurally more complex chains yielded heavier, diesel-like fuels.
If this methodology can successfully bridge the gap between laboratory-scale experimentation and industrial application, the researchers believe it could significantly bolster U.S. energy security and provide new economic opportunities.
"We developed an efficient and selective polyethylene-to-gasoline conversion," said Liqi Qiu, a postdoctoral researcher at the University of Tennessee, Knoxville (UTK), who executed the majority of the study’s laboratory experiments. Qiu conducted the work within the ORNL laboratory of Sheng Dai, an ORNL Corporate Fellow and section head for separations and polymer chemistry at both ORNL and UTK, who serves as a co-corresponding author of the research paper.
Plastic-to-Gasoline Conversion Below 200 Degrees Celsius
One of the most scientifically notable and economically promising features of the newly developed method is how few external inputs it requires when contrasted with more conventional plastic-to-fuel technologies.
"He we converted polymer waste to value-added fuels by using commercially available inorganic salts as the reaction media to provide the catalytic sites," said Zhenzhen Yang, an ORNL staff scientist and co-corresponding author of the study. "Unlike traditional techniques for converting polymer to fuel, the new process did not require noble-metal catalysts, organic solvents or external hydrogen. This is the first time molten salts were used as media to produce high-value-added chemicals from waste without any catalytic initiator or solvent and at a temperature below 200 degrees Celsius."
Operating at temperatures below 200 degrees Celsius places the thermal requirement roughly on par with the environment found inside a standard residential kitchen oven. By contrast, earlier approaches aimed at breaking down polyethylene into gasoline have typically relied on energy-intensive pyrolysis. Pyrolysis is a thermal degradation process that utilizes severe heat—often ranging between 450 and 500 degrees Celsius—to fragment large polymer molecules into smaller hydrocarbons.
The substantially lower operating temperature, combined with the total absence of costly noble-metal catalysts, external hydrogen, organic solvents, and chemical initiators, drastically simplifies the chemical processing train.
"The ORNL system solves two fundamental issues," Dai noted. "One, for a stable system, the process can be radically easier to scale up. Two, the previous system needed an initiator to kick off catalytic reactions. However, the ORNL system does not need one."
Decades of Molten Salt Research
The foundation for this latest achievement rests on decades of institutional expertise in molten salt chemistry at Oak Ridge National Laboratory. During the 1960s, ORNL’s landmark Molten Salt Reactor Experiment famously demonstrated that specialized mixtures of molten salts could effectively function simultaneously as both nuclear fuel and reactor coolant.
Drawing inspiration from this long historical legacy, Dai proposed pivoting molten salt chemistry toward an entirely different application: breaking down discarded commercial polymers into useful, high-value fuels. Molten salts are inorganic compounds uniquely suited for demanding chemical environments because they remain physically and chemically stable even under harsh reaction conditions.
Tomonori Saito, who managed the project at ORNL, contributed vital expertise in polymer science to help guide the investigation. "In this case we tackled polyethylene, a widely available commodity polymer, using molten salt," Saito said. "We’re trying to understand fundamental science that will lead to discoveries and new economic opportunities."
Decoding the precise chemical choreography occurring during the reaction required a multidisciplinary team spanning several scientific domains and utilizing a suite of advanced analytical facilities.
Tracking the Chemistry Atom by Atom
At ORNL, researcher Luke Daemen utilized neutron scattering methodologies to help identify the specific hydrocarbon products generated when different polymer chains underwent the reaction. Meanwhile, Felipe Polo-Garzon analyzed the resulting compounds using gas chromatography-mass spectrometry, a powerful analytical technique designed to separate, identify, and quantify individual chemical components within a complex mixture.
When the polyethylene encountered an aluminum catalytic site, the reaction generated a positively charged carbon ion. To track the trajectory of this fleeting intermediate, Qiu, Yang, and Dai labeled the carbon ion with deuterium, a stable isotope of hydrogen. This isotopic tagging enabled the team to follow the ion’s transformation as the chemical process unfolded.
The researchers also leaned heavily on the unique capabilities of ORNL’s Spallation Neutron Source to monitor hydrogen behavior throughout the system.
"The polymer contains a lot of hydrogen," Dai explained. "Neutrons are ideal at discerning light elements including hydrogen and its isotopes, such as deuterium."
To complete the picture, the team needed to determine how the aluminum catalytic sites themselves evolved over the course of the reaction. Yang traveled to the Advanced Light Source at Lawrence Berkeley National Laboratory, collaborating with Min-Jae Kim and Jinhua Guo. By utilizing soft X-ray spectroscopy, the researchers examined the subtle electronic and atomic interactions taking place between the aluminum and the polyethylene. Soft X-rays are particularly well-suited for studying relatively lightweight elements like aluminum.
"The aluminum edge shifted to the low-electron-density edge, which means some electron-rich intermediates formed," Yang stated. "We compared the findings with other techniques and confirmed an aromatic ring intermediate can coordinate with aluminum and cause a binding-energy change."
This distinct energetic shift provided definitive physical evidence that the aluminum sites were actively driving the catalytic conversion.
Simulations and Advanced Imaging Reveal the Mechanism
Back at ORNL, Bobby Sumpter of the Center for Nanophase Materials Sciences utilized advanced computer simulations to model the thermodynamic and energy changes occurring throughout the reaction. These simulations mapped out how stable carbon ions formed and subsequently transferred into finished hydrocarbon products.
At the University of Tennessee, Knoxville, Michael Koehler employed in situ X-ray diffraction to monitor real-time phase transitions within the reaction mixture as the chemistry progressed. Additionally, Carlos Alberto Steren applied nuclear magnetic resonance spectroscopy to investigate the structural nature of the aluminum catalytic sites.
Further contributions came from ORNL’s Tao Wang, who provided expertise in molten salt chemistry, and Logan Kearney, who supplied high-density polymers and offered critical guidance on prospective pathways for converting waste plastics into higher-value commercial products.
A Remaining Challenge: Keeping the Salts Stable
Despite the clear economic and environmental advantages of using an inexpensive, highly active aluminum-based catalytic system, the technology still faces a notable chemical hurdle. The material is hygroscopic, meaning it has a strong natural tendency to absorb ambient moisture from the air, a factor that can compromise its long-term stability.
To overcome this limitation, the research team is currently investigating methods to effectively confine the molten salts—potentially utilizing halogens or carbon-based hosting materials. Such containment strategies could make the salts significantly easier to handle, separate, and process while simultaneously enhancing their operational stability.
Successfully addressing this challenge could eventually broaden the spectrum of viable industrial methods available for synthesizing transportation and industrial-grade fuels from abundant consumer waste streams.
"Polymer source material is abundantly available from consumer waste, and our catalyst system, aluminum molten salts, is very cheap," Qiu concluded. "This advance may be promising for industry."
The research was primarily supported by the Department of Energy Office of Science, specifically through the Materials Sciences and Engineering Division, with additional backing for gas chromatography-mass spectrometry work provided by the Chemical Sciences, Geosciences and Biosciences Division’s Catalysis Science program. The project also utilized DOE Office of Science user facilities, including the Spallation Neutron Source and the Center for Nanophase Materials Sciences at ORNL, alongside the Advanced Light Source at Lawrence Berkeley National Laboratory.