Oak Ridge Lab Converts Plastic Waste to Fuel with Molten
Scientists at Oak Ridge National Laboratory have developed a method using aluminum-based molten salts to convert polyethylene plastic into gasoline and
Researchers at Oak Ridge National Laboratory have transformed polyethylene plastic into gasoline and diesel-like fuels. The method uses inexpensive aluminum chloride molten salts as both a reaction medium and a catalyst under mild conditions.
According to a study published in the Journal of the American Chemical Society, the process achieved a gasoline yield of about 60 percent. The team, led by scientists from ORNL and the University of Tennessee, Knoxville, has applied for a patent on the technology.
A Simpler, Cooler Conversion Process
One key advantage is the lower energy requirement. Zhenzhen Yang, an ORNL staff scientist and co-corresponding author, stated the process operates "at a temperature below 200 degrees Celsius." This is far cooler than conventional pyrolysis methods, which typically need 450 to 500 degrees Celsius.
The system also eliminates several costly components. It requires no noble-metal catalysts, organic solvents, external hydrogen, or a chemical initiator to start the reaction. "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," Yang said.
Sheng Dai, an ORNL Corporate Fellow and co-corresponding author, explained the system's stability. For a stable system, the process can be radically easier to scale up, Dai said. The previous system needed an initiator, but the ORNL system does not.
Probing the Chemical Mechanism
Understanding the reaction required a suite of advanced techniques. Using soft X-ray spectroscopy and nuclear magnetic resonance, the team found that charged aluminum atoms create highly acidic sites. These sites attack and break the long polymer chains into smaller fuel hydrocarbons.
Isotopic labeling and neutron scattering revealed how the polymer's structure dictates the fuel output. Simpler polyethylene chains yielded gasoline-like compounds, while more complex chains produced diesel-like fuels. Luke Daemen used neutron scattering at ORNL's Spallation Neutron Source to help identify these products.
To track the reaction's progress, researchers tagged a key carbon ion with deuterium, an isotope of hydrogen. Neutrons are ideal at discerning light elements including hydrogen and its isotopes, such as deuterium, Dai noted.
Confirming the Catalyst's Role
Evidence that the aluminum sites were actively catalyzing the reaction came from soft X-ray studies. Yang traveled to Lawrence Berkeley National Laboratory's Advanced Light Source with colleagues Min-Jae Kim and Jinhua Guo. They examined interactions between aluminum and polyethylene at atomic and electronic levels.
The aluminum edge shifted to the low-electron-density edge, which means some electron-rich intermediates formed, Yang explained. Comparisons with other techniques confirmed an aromatic ring intermediate coordinates with aluminum, causing a binding-energy change.
Back at ORNL, computer simulations by Bobby Sumpter studied the energy changes during the reaction. In situ X-ray diffraction and further nuclear magnetic resonance investigations provided additional details on the reaction phases and catalytic sites.
Challenges and Future Potential
A significant hurdle remains. The aluminum-based salts are hygroscopic, meaning they readily absorb water, which can reduce their stability. The researchers now aim to investigate confining the molten salts using halogens or carbon-based materials to improve stability and ease of processing.
Despite this, the team sees industrial promise. Polymer source material is abundantly available from consumer waste, and our catalyst system, aluminum molten salts, is very cheap, said Liqi Qiu, the postdoctoral researcher who performed most of the experiments. The advance builds on ORNL's decades of molten salt research, which included the 1960s Molten Salt Reactor Experiment.
Tomonori Saito, who managed the project, framed the work as fundamental science. We're trying to understand fundamental science that will lead to discoveries and new economic opportunities, he said. The research was primarily supported by the DOE Office of Science.
