Molten Salt Turns Hard-to-Recycle Plastic Into Fuel at Low Heat

Researchers at Oak Ridge National Laboratory (ORNL) have found a way to convert polyethylene, one of the world's most widely used and hardest-to-recycle plastics, into gasoline-like and diesel-like fuels using a mixture of heated salts. In laboratory tests, the process worked at temperatures below 200 degrees Celsius (392 Fahrenheit) — far cooler than conventional methods — and produced a gasoline yield of about 60%.

The common industrial method, pyrolysis, breaks plastics apart using intense heat, typically around 450 to 500 degrees Celsius (842 to 932 Fahrenheit). The new process instead relies on molten salts containing aluminum chloride, which serve as both the reaction environment and the catalyst, and avoids ingredients such as precious-metal catalysts, organic solvents or an external hydrogen supply that conventional techniques usually require. "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," said Zhenzhen Yang, an ORNL staff scientist and co-corresponding author of the study.

Researchers found that acidic aluminum sites within the molten salt trigger reactions that break polyethylene's long carbon-and-hydrogen chains into the smaller hydrocarbon molecules found in fuels. To trace the reaction, they used deuterium, a heavier form of hydrogen, as a chemical marker, and studied it with neutron scattering at ORNL's Spallation Neutron Source, a technique well suited to tracking hydrogen and its isotopes. The measurements showed that simpler polymer chains tended to produce gasoline-like compounds, while more complex chains generated diesel-like products.

At Lawrence Berkeley National Laboratory's Advanced Light Source, the team also used soft X-rays to examine how the aluminum catalyst itself changed during the reaction, alongside nuclear magnetic resonance, X-ray diffraction, gas chromatography-mass spectrometry and computer simulations to confirm their findings.

The work builds on decades of molten salt research at Oak Ridge, dating back to the 1960s, when the laboratory's Molten Salt Reactor Experiment explored the same materials for use in nuclear reactors.