Polyethylene is widely used in products such as shopping bags and white plastic cutting boards, and large amounts of it ultimately end up as waste. The ORNL team’s method combines the plastic with molten salts containing aluminum chloride. These salts perform two jobs at once, acting as both the reaction medium and the catalyst that drives the chemical conversion.

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

How Molten Salts Break Plastic Into Fuel

To understand why the process works, the scientists closely tracked the chemical reactions that convert the polymer into fuel molecules.

Using soft X-ray spectroscopy and nuclear magnetic resonance, the team found that charged aluminum atoms bind with three other atoms, creating highly acidic catalytic sites. These sites can attack the long molecular chains that make up polyethylene and split them into smaller hydrocarbon molecules.

Additional experiments using isotopic labeling and neutron scattering showed how the structure of the starting polymer influences the resulting fuel. Simpler polymer chains tended to produce gasoline-like compounds, while more complex chains generated diesel-like fuels.

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

“We developed an efficient and selective polyethylene-to-gasoline conversion,” said Liqi Qiu, a postdoctoral researcher at the University of Tennessee, Knoxville, who performed most of the study’s experiments in the ORNL laboratory of Sheng Dai, of ORNL and UTK. Dai, an ORNL Corporate Fellow and section head for separations and polymer chemistry, is a co-corresponding author of the paper.

The experiments achieved a gasoline yield of about 60 percent under relatively mild reaction conditions.

Plastic-to-Gasoline Conversion Below 200 Degrees Celsius

One of the most notable features of the method is how little it requires compared with more conventional 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 was also 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 comparable to those found inside a conventional kitchen oven. Earlier approaches for converting polyethylene into gasoline have typically relied on pyrolysis, a process that uses intense heat to break large polymer molecules into smaller hydrocarbons. Those methods have required temperatures of roughly 450 to 500 degrees Celsius.

The lower temperature, along with the absence of costly noble-metal catalysts, external hydrogen, organic solvents, and a chemical initiator, could simplify the process.

“The ORNL system solves two fundamental issues. 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.”

• Sheng Dai, ORNL Corporate Fellow and section head for separations and polymer chemistry

Decades of Molten Salt Research

ORNL has been studying molten salts for decades. During the 1960s, its Molten Salt Reactor Experiment demonstrated that mixtures of molten salts could function as both nuclear fuel and reactor coolant.

Building on that long history, Dai proposed using molten salts for an entirely different purpose: converting discarded polymers into useful fuels.

Molten salts are inorganic compounds that can remain stable even under demanding chemical reaction conditions.

“The ORNL system solves two fundamental issues,” Dai said. “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.”

ORNL’s Tomonori Saito managed the project and contributed expertise in polymer science.

“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.”

Understanding exactly what was happening during the reaction required researchers from several scientific disciplines and the use of multiple advanced analytical techniques.

Tracking the Chemistry Atom by Atom

At ORNL, Luke Daemen used neutron scattering to help identify the hydrocarbon products created when different polymer chains reacted. Felipe Polo-Garzon analyzed the products with gas chromatography-mass spectrometry, a technique used to separate and identify individual chemical compounds.

When polyethylene interacted with an aluminum catalytic site, the reaction produced a positively charged carbon ion. Qiu, Yang and Dai tagged that carbon ion with deuterium, an isotope of hydrogen, allowing them to follow what happened to it as the reaction progressed.

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

“The polymer contains a lot of hydrogen,” Dai said. “Neutrons are ideal at discerning light elements including hydrogen and its isotopes, such as deuterium.”

Researchers also needed to determine how the aluminum sites themselves changed during the process.

Yang traveled to the Advanced Light Source at Lawrence Berkeley National Laboratory, where she worked with Min-Jae Kim and Jinhua Guo. Using soft X-rays, the researchers examined interactions between aluminum and polyethylene at both the atomic and electronic levels. Soft X-rays are especially useful for studying relatively lightweight elements such as aluminum.

“The aluminum edge shifted to the low-electron-density edge, which means some electron-rich intermediates formed,” Yang said. “We compared the findings with other techniques and confirmed an aromatic ring intermediate can coordinate with aluminum and cause a binding-energy change.”

That shift provided evidence that the aluminum sites were actively catalyzing the chemical reaction.

Simulations and Advanced Imaging Reveal the Mechanism

Back at ORNL, Bobby Sumpter of the Center for Nanophase Materials Sciences used computer simulations to study the energy changes taking place during the reaction, including how stable carbon ions formed and were transferred into hydrocarbon products.

At UTK, Michael Koehler used in situ X-ray diffraction to follow changes in the phases of the reaction mixture as the chemistry unfolded. Carlos Alberto Steren used nuclear magnetic resonance to investigate the aluminum catalytic sites.

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

A Remaining Challenge: Keeping the Salts Stable

Although the aluminum-based catalytic system is inexpensive and chemically active, it has an important limitation. The material is hygroscopic, meaning it readily absorbs water. That moisture can reduce its stability.

The researchers now want to investigate ways to confine the molten salts, potentially using halogens or carbon-based materials, which could make the salts easier to separate and process while improving their stability.

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

“Polymer source material is abundantly available from consumer waste, and our catalyst system, aluminum molten salts, is very cheap,” Qiu said. “This advance may be promising for industry.”

The DOE Office of Science (Materials Sciences and Engineering Division) primarily supported the research as well as the gas chromatography-mass spectrometry work (Chemical Sciences, Geosciences and Biosciences Division, Catalysis Science program). The research employed DOE Office of Science user facilities at ORNL (the Spallation Neutron Source for neutron scattering at the VISION beamline and the Center for Nanophase Materials Sciences for quantum chemistry calculations) and Lawrence Berkeley National Laboratory (the Advanced Light Source for soft X-ray spectra).

Researchers from DTU, the University of Copenhagen, University College London, and other institutions have developed a long, needle-thin brain electrode equipped with microscopic channels. Known as the microfluidic Axialtrode (mAxialtrode), the device is designed to provide multiple functional points along the length of the implant. This allows researchers to record neural activity and deliver medication to specific locations across different parts of the brain.

The findings were published in the journal Advanced Science.

A Multifunction Tool for Brain Research

For now, the technology is primarily intended as a research tool. Scientists could use it to investigate how signals travel through different layers of the brain during processes involving epilepsy, memory, and decision-making.

Over the longer term, the researchers say the mAxialtrode could also have therapeutic applications. One possibility would be using the device to deliver drugs to precise locations while simultaneously applying electrical stimulation or light stimulation to selected areas of the brain.

Postdoc Kunyang Sui, who developed the mAxialtrode concept together with Associate Professor Christos Markos, says one of the main advantages is that several capabilities can be combined within a single implant. That could allow researchers to perform more precise experiments while reducing the need for multiple devices inserted into the brain.

“Most current brain implants are based on hard materials such as silicon, which can irritate the brain and trigger inflammatory reactions in the tissue. The new implant differs in that it is made of soft, plastic-like optical fibers and has a specially angled tip that makes it smaller and reduces the damage caused when it is placed in the brain,” says Kunyang Sui.

Sui cautions that the technology is still far from routine clinical use. Extensive testing, additional development, and regulatory approvals would be required before it could be used to treat patients.

Moving Beyond Conventional Optical Fibers

Brain researchers currently use flat-ended optical fibers in many experiments. These thin fibers, made from glass or plastic, can carry light into deep regions of the brain. They are frequently used in optogenetics, a technique in which specific nerve cells are activated using light.

Conventional fibers, however, have an important limitation. They typically interact with brain tissue only at the end of the fiber, meaning researchers can stimulate or monitor just one location at a time.

The outermost end is called the distal tip, or the “nose” of the fiber. Light is emitted and contact with brain tissue occurs only at this point. As a result, scientists may be limited to measuring or stimulating one brain layer at a time, even though many brain functions depend on communication among several layers and deeper structures.

How the New Brain Implant Works

The needle-thin mAxialtrode begins as a much larger polymer rod. Researchers heat the material and draw it into an extremely thin fiber, somewhat like producing a very fine strand of sugar, but with far greater precision.

A light-conducting core runs through the center of the fiber. Surrounding it are eight microscopic channels that can transport liquids. Those channels can also hold extremely thin metal wires used to measure electrical activity in the brain.

The finished fiber measures less than half a millimeter across. It is also highly flexible, allowing it to move along with brain tissue rather than pressing rigidly through it. This difference in stiffness could be important because harder implants can trigger inflammatory responses when they remain in the brain for long periods.

Tested in Living Mice

The researchers tested the system not only in the laboratory but also “in vivo,” meaning in living mice. The electrode was implanted in the animals’ brains and connected to light sources, recording equipment, and small pumps used to deliver fluids.

The experiments showed that the device could stimulate nerve cells using both blue and red light. At the same time, researchers were able to record electrical activity from shallow and deeper brain regions, including the cerebral cortex and hippocampus.

They were also able to inject different substances at separate depths, with delivery points spaced almost three millimeters apart. All of these measurements and forms of stimulation were carried out using a single lightweight fiber, which the mice were able to carry without any obvious signs of discomfort.

Potential Applications in Epilepsy and Neuroscience

The in vivo experiments and neurophysiological validation were performed in close collaboration with Associate Professor Rune W. Berg of the University of Copenhagen and Associate Professor Rob C. Wykes from University College London. Their contributions included expertise in analyzing neural circuits and models relevant to epilepsy.

The research team is now working to patent the technology behind the brain electrode. The scientists are also exploring what would be required to begin testing the device in patients within a clinical setting.