Researchers at Oak Ridge National Laboratory have developed a molten-salt process that converts common polyethylene waste into gasoline- and diesel-like fuels at temperatures below 200 degrees Celsius, according to a study published in the Journal of the American Chemical Society. The method bypasses the extreme heat and expensive precious-metal catalysts typically required to break down difficult consumer plastics.
Low-Temperature Polyethylene Conversion Using Molten Salts
Polyethylene—the material used to make everyday products like grocery bags and kitchen cutting boards—is notoriously difficult to recycle once discarded. Traditional pyrolysis methods typically require intense heat ranging from 450 to 500 degrees Celsius to break down the tough molecular bonds of the polymer, according to Oak Ridge National Laboratory data. By contrast, the new ORNL laboratory process operates at temperatures below 200 degrees Celsius (392 degrees Fahrenheit).
The technique relies on a simple mixture of commercially available inorganic salts containing aluminum chloride. In this system, the heated salt mixture functions simultaneously as the reaction environment and the catalyst driving the breakdown. According to ORNL staff scientist Zhenzhen Yang, the setup achieved a gasoline yield of about 60 percent during laboratory experiments.
“Unlike traditional techniques for converting polymer to fuel, the new process did not require noble-metal catalysts, organic solvents or external hydrogen,” Yang said. “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 temperature below 200 degrees Celsius.”
Did you know? Oak Ridge National Laboratory has studied molten-salt chemistry for decades, initially exploring the technology in the 1960s for nuclear reactors through the Molten Salt Reactor Experiment before adapting the materials for polymer recycling.
Aluminum Sites and Neutron Scattering Reveal Reaction Mechanics
To pinpoint how the molten salt breaks down long polymer chains, the research team tracked chemical changes using advanced atomic-level imaging and tracking. Polyethylene consists of long chains of carbon and hydrogen atoms. The team discovered that aluminum sites within the molten salt become strongly acidic, triggering reactions to snap those chains into smaller hydrocarbon molecules.
Researchers used deuterium—a heavier isotope of hydrogen—as a chemical marker to trace the reaction steps. According to ORNL Corporate Fellow Sheng Dai, the team utilized neutron scattering at ORNL’s Spallation Neutron Source because neutrons excel at discerning light elements like hydrogen and its isotopes. Additional soft X-ray analysis conducted with Min-Jae Kim and Jinhua Guo at Lawrence Berkeley National Laboratory’s Advanced Light Source revealed that aluminum edges shift to low-electron-density states, confirming that aromatic ring intermediates coordinate with the aluminum to drive the reaction.
“The polymer contains a lot of hydrogen,” Dai noted. “Neutrons are ideal at discerning light elements including hydrogen and its isotopes.” Measurements indicated that simpler polymer chains generated gasoline-like compounds, while more complex structures produced diesel-like products.
Overcoming Water Absorption Barriers for Industrial Scale-Up
Despite yielding positive laboratory results, the method faces a significant obstacle on the path to industrial adoption. The aluminum-based molten salt readily absorbs water, which makes the system less stable, according to the research findings. The team is currently exploring ways to contain and protect the molten salts using halogens or carbon-based materials to improve separation and processing durability.
If these containment hurdles are cleared, the process could offer a cost-effective route for industrial recycling plants to tackle consumer plastic waste. The researchers have already applied for a patent on the method.
“Polymer source material is abundantly available from consumer waste, and our catalyst system, aluminum molten salts, is very cheap,” said Liqi Qiu, a postdoctoral researcher at the University of Tennessee, Knoxville, who performed the experiments. “This advance may be promising for industry.”
Frequently Asked Questions
What temperature does the new ORNL molten-salt process require?
The process operates at temperatures below 200 degrees Celsius (392 degrees Fahrenheit), which is significantly lower than traditional pyrolysis methods that require 450 to 500 degrees Celsius.
What types of fuel does the process produce?
Laboratory experiments converted polyethylene waste into gasoline- and diesel-like liquid alkanes, yielding about 60 percent gasoline.
Why is water a problem for the method?
The aluminum-based molten salt absorbs moisture, which can destabilize the system. Researchers are working on shielding techniques to protect the salts for practical use.
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