Polyethylene is an essential material in modern commerce, widely utilized in everyday consumer goods such as single-use shopping bags and rigid white plastic cutting boards. Because of its durability and convenience, it is produced in staggering volumes globally. However, these same properties mean that vast quantities of polyethylene ultimately end up as stubborn, long-lasting waste in landfills and the natural environment. The new chemical breakthrough achieved by the ORNL research team offers a promising pathway to address this mounting environmental crisis by redirecting plastic waste into the energy sector.
The team’s novel method combines waste plastic with specialized molten salts containing aluminum chloride. In this chemical system, the molten salts play a dual role, functioning simultaneously as the reaction medium and as the active catalyst that drives the complex chemical conversion forward.
The researchers have already applied for a patent to protect the intellectual property behind the technology, and their detailed findings have been published in the prestigious Journal of the American Chemical Society, signaling the scientific community’s recognition of the breakthrough’s significance.
How Molten Salts Break Plastic Into Fuel
To fully understand why the process works so effectively, the scientists closely tracked the intricate chemical reactions that convert the high-molecular-weight polymer into simpler fuel molecules.
Using advanced analytical tools including soft X-ray spectroscopy and nuclear magnetic resonance, the research team discovered that charged aluminum atoms bind closely with three other atoms, creating highly concentrated and active acidic catalytic sites. These specialized sites possess the chemical power to attack the exceptionally long molecular chains that make up polyethylene, cleanly splitting them apart into smaller, more manageable hydrocarbon molecules.
Additional, highly targeted experiments using isotopic labeling and neutron scattering further illuminated how the structural characteristics of the starting polymer influence the composition of the resulting fuel. The researchers observed a distinct pattern: simpler polymer chains tended to produce lighter, gasoline-like compounds, while more structurally complex chains generated heavier, diesel-like fuels.
If this method can eventually be successfully scaled beyond laboratory bench experiments, the researchers believe it could contribute meaningfully to U.S. energy security while simultaneously strengthening industrial competitiveness by creating a domestic source of liquid fuels from waste.
"We developed an efficient and selective polyethylene-to-gasoline conversion," said Liqi Qiu, a postdoctoral researcher at the University of Tennessee, Knoxville, who performed the vast majority of the study’s experiments within the ORNL laboratory overseen by Sheng Dai. Dai serves as an ORNL Corporate Fellow and section head for separations and polymer chemistry at both ORNL and UTK, and is a co-corresponding author of the published research paper.
During laboratory testing, the experiments consistently achieved an impressive gasoline yield of about 60 percent under relatively mild reaction conditions.
Plastic-to-Gasoline Conversion Below 200 Degrees Celsius
One of the most notable and disruptive features of the new method is how little energy and auxiliary material it requires compared with more conventional, legacy plastic-to-fuel technologies.
"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 who also served as a co-corresponding author of the paper. "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."
Temperatures below 200 degrees Celsius are remarkably low in the context of polymer chemistry, roughly comparable to the gentle heat found inside a conventional kitchen oven. In contrast, earlier approaches for converting polyethylene into gasoline have typically relied on pyrolysis, an energy-intensive thermochemical process that uses intense heat to thermally break large polymer molecules into smaller hydrocarbons. Those traditional methods have routinely required severe operating temperatures ranging from roughly 450 to 500 degrees Celsius.
This dramatically lower operating temperature, combined with the complete absence of costly noble-metal catalysts, external hydrogen, organic solvents, and chemical initiators, could drastically simplify the industrial engineering required to build and operate conversion facilities.
"The ORNL system solves two fundamental issues," Dai explained. "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 breakthrough did not happen in a vacuum; it builds upon decades of dedicated scientific inquiry into molten salts at Oak Ridge National Laboratory. During the 1960s, the laboratory’s historic Molten Salt Reactor Experiment famously demonstrated that specialized mixtures of molten salts could function effectively as both nuclear fuel and reactor coolant simultaneously.
Building directly upon that long and storied history of institutional expertise, Dai proposed utilizing molten salts for an entirely different, forward-looking purpose: converting discarded, troublesome polymers into useful, high-value liquid fuels.
Molten salts are inorganic compounds distinguished by their ability to remain remarkably stable even under demanding and aggressive chemical reaction conditions. This inherent stability makes them ideal candidates for mediating difficult chemical transformations without degrading.
Tomonori Saito, who managed the project at ORNL and contributed deep expertise in polymer science, emphasized the broader implications of the work. "In this case we tackled polyethylene, a widely available commodity polymer, using molten salt," he said. "We’re trying to understand fundamental science that will lead to discoveries and new economic opportunities."
Unraveling the exact molecular choreography occurring during the reaction required a multidisciplinary team of scientists deploying a wide array of advanced analytical techniques.
Tracking the Chemistry Atom by Atom
At ORNL, Luke Daemen utilized neutron scattering instrumentation to help identify the specific hydrocarbon products created when different polymer chains reacted with the molten salt medium. Meanwhile, Felipe Polo-Garzon analyzed the resulting products using gas chromatography-mass spectrometry, a powerful analytical technique used to separate, identify, and quantify individual chemical compounds within a complex mixture.
When polyethylene interacted directly with an aluminum catalytic site, the reaction generated a transient, positively charged carbon ion. To track this elusive species, researchers Qiu, Yang, and Dai tagged that specific carbon ion with deuterium, a stable isotope of hydrogen. This isotopic tagging allowed them to accurately follow what happened to the carbon ion as the chemical reaction progressed through its various stages.
The research team also leveraged the advanced neutron capabilities at ORNL’s Spallation Neutron Source to monitor the behavior and movement of hydrogen atoms throughout the entire system.
"The polymer contains a lot of hydrogen," Dai noted. "Neutrons are ideal at discerning light elements including hydrogen and its isotopes, such as deuterium."
Furthermore, the researchers needed to determine precisely how the aluminum catalytic sites themselves changed and evolved during the course of the process.
To answer this question, Yang traveled to the Advanced Light Source at Lawrence Berkeley National Laboratory, where she collaborated closely with Min-Jae Kim and Jinhua Guo. Using soft X-ray spectroscopy, the researchers examined the subtle electronic interactions occurring between aluminum and polyethylene at both the atomic and electronic levels. Soft X-rays are especially valuable 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 explained. "We compared the findings with other techniques and confirmed an aromatic ring intermediate can coordinate with aluminum and cause a binding-energy change."
That distinct electronic shift provided definitive evidence that the aluminum sites were actively and directly participating in catalyzing the chemical reaction rather than acting as mere passive bystanders.
Simulations and Advanced Imaging Reveal the Mechanism
To complement the empirical data gathered in the laboratory, Bobby Sumpter of the Center for Nanophase Materials Sciences at ORNL employed sophisticated computer simulations to study the thermodynamic and kinetic energy changes taking place during the reaction. These simulations mapped out how stable carbon ions formed and how they were subsequently transferred into stable hydrocarbon products.
At the University of Tennessee, Knoxville, Michael Koehler used in situ X-ray diffraction to follow real-time changes in the phases of the reaction mixture as the chemistry unfolded before his instruments. Additionally, Carlos Alberto Steren utilized nuclear magnetic resonance spectroscopy to investigate the structural nature of the aluminum catalytic sites.
Other crucial contributions came from ORNL’s Tao Wang, who provided specialized expertise in molten salt chemistry, and Logan Kearney, who supplied high-density polymers and offered expert guidance regarding the most promising pathways for converting them into higher-value industrial products.
A Remaining Challenge: Keeping the Salts Stable
Despite its impressive advantages—including low operating temperatures, high selectivity, and the use of inexpensive materials—the aluminum-based catalytic system does possess one important technological limitation that must be addressed before commercialization. The catalytic material is hygroscopic by nature, meaning it readily and aggressively absorbs water from its surroundings. That unintended moisture absorption can significantly reduce the stability and longevity of the catalytic system.
To overcome this hurdle, the researchers are already planning their next steps. They intend to investigate various advanced methods to confine the molten salts, potentially utilizing specialized halogens or carbon-based hosting materials. Such confinement strategies could make the salts much easier to separate, handle, and process while simultaneously improving their operational stability over repeated cycles.
If successful, the ongoing research could ultimately expand the broader portfolio of chemical methods available for producing essential transportation and industrial fuels from abundant waste materials rather than virgin petroleum.
"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. Additional support for the gas chromatography-mass spectrometry work was provided by the Chemical Sciences, Geosciences and Biosciences Division under the Catalysis Science program. The research made extensive use of DOE Office of Science user facilities, including the Spallation Neutron Source and the Center for Nanophase Materials Sciences at ORNL, as well as the Advanced Light Source at Lawrence Berkeley National Laboratory.