Researchers at the Department of Energy's Oak Ridge National Laboratory developed a method to convert a commonly discarded hydrocarbon polymer into gasoline- and diesel-like fuels. The team has applied for a patent for the discovery, which treats polyethylene - the stuff of white cutting boards and shopping bags - with aluminum chloride-containing molten salts that serve as both solvent and catalyst. The results were published in the Journal of the American Chemical Society .
The scientists closely monitored the chemical reaction that turns the polymer into petrol to learn the secrets of its success. Soft X-ray spectroscopy and nuclear magnetic resonance showed that charged aluminum atoms each bind to three other atoms to create strongly acidic catalytic sites that break long polymer chains into shorter ones. Isotopic labeling and neutron scattering revealed how simpler polymer chains form gasoline-like fuels and more complex chains form diesel-like fuels.
If scaled beyond the laboratory, the process could strengthen U.S. energy security and 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 produced a gasoline yield of about 60 percent under mild conditions.
"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 temperature below 200 degrees Celsius."
That temperature is comparable to a conventional kitchen oven. Previously, converting polyethylene to gasoline required temperatures of 450 to 500 degrees Celsius through pyrolysis, a heat-driven process that breaks long polymer chains into smaller hydrocarbons.
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.
ORNL has pioneered molten salt research since the 1960s, when its Molten Salt Reactor Experiment showed that molten salt mixtures could serve as both fuel and coolant in a nuclear reactor.
Dai proposed using molten salts to turn polymer waste into fuel. Molten salts are inorganic compounds that remain stable under harsh 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 polymer expertise. "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."
Achieving that understanding required multidisciplinary expertise and advanced instruments.
At ORNL, to identify hydrocarbon products formed from reactions with various polymer chains, Luke Daemen employed neutron scattering, and Felipe Polo-Garzon used gas chromatography-mass spectrometry.
When the polymer interacted with an aluminum site, it created a positively charged ion of carbon. Qiu, Yang and Dai labeled that carbon ion with deuterium, an isotope of hydrogen, to track its behavior during the reactions. They also used neutrons at ORNL's Spallation Neutron Source to track hydrogen.
"The polymer contains a lot of hydrogen," Dai said. "Neutrons are ideal at discerning light elements including hydrogen and its isotopes, such as deuterium."
To probe structural changes to aluminum sites during the reaction, Yang used the Advanced Light Source at Lawrence Berkeley National Laboratory. Working with Min-Jae Kim and Jinhua Guo there, she used soft X-rays to examine how aluminum sites interacted with the polymer at atomic and electronic levels. Soft X-rays are ideal for imaging lightweight elements like 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 change indicated that the aluminum sites were catalytically active.
Back at ORNL, Bobby Sumpter of the Center for Nanophase Materials Sciences conducted simulations to examine the reaction's energy dynamics, such as formation and transfer of stable carbon ions to hydrocarbons.
At UTK, Michael Koehler used in situ X-ray diffraction to monitor phase changes in the reaction mixture, and Carlos Alberto Steren used nuclear magnetic resonance to examine aluminum sites.
ORNL's Tao Wang lent expertise in molten salts. ORNL's Logan Kearney provided high-density polymers and expert suggestions for their valorization paths.
Although the aluminum-site system is catalytically active and inexpensive, it is hygroscopic, meaning it absorbs water and loses stability. Next, the team hopes to explore ways to confine molten salts, maybe with halogens or carbons, to improve separation and processing.
The findings expand options for producing transportation and industrial fuels. "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).
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science . - Dawn Levy
