Oak Ridge scientists turn plastic waste into gasoline and diesel fuel

Chemists at Oak Ridge National Laboratory have turned polyethylene, the plastic used in shopping bags and cutting boards, into gasoline- and diesel-range fuel by dissolving it in a bath of molten aluminum salt. The reaction ran at temperatures below 200 degrees Celsius, close to a kitchen oven, and still produced a gasoline yield of roughly 60 percent. That is a fraction of the 450 to 500 degrees Celsius that conventional pyrolysis needs to crack the same plastic apart.

The lab published the result out of Oak Ridge, Tennessee, describing a process built around a single molten salt that does the work normally split between a solvent, a catalyst, and an external hydrogen source. No noble-metal catalyst went into the mix, and the team added no outside hydrogen at all.

Aluminum Salts That Do Two Jobs at Once

The core of the method is an aluminum-based molten salt that acts simultaneously as the reaction medium the plastic dissolves in and the catalyst that breaks its carbon chains apart. Under those conditions the reaction achieved a gasoline yield of about 60 percent, converting long polyethylene chains into shorter, liquid hydrocarbons without the noble-metal catalysts, organic solvents, or added hydrogen that older upcycling routes have relied on. ORNL staff scientist Zhenzhen Yang, a co-corresponding author on the underlying study, said the process stands apart from prior chemistry because of what it leaves out. “Unlike traditional techniques for converting polymer to fuel, the new process did not require noble-metal catalysts, organic solvents or external hydrogen,” Yang said.

Liqi Qiu, a postdoctoral researcher at the University of Tennessee, Knoxville working inside the ORNL collaboration, ran the bulk of the experiments. A dozen additional scientists across ORNL, UTK, and Lawrence Berkeley National Laboratory contributed neutron scattering, X-ray diffraction, and nuclear magnetic resonance measurements to work out what the aluminum salt was actually doing to the polymer at a molecular level, and how it kept doing that job reaction after reaction without breaking down itself.

A Fraction of Pyrolysis’s Heat

Plastic-to-fuel pyrolysis is not new, but it is energy-hungry. Cracking polyethylene by heat alone typically demands 450 to 500 degrees Celsius, hot enough to require industrial furnace equipment and the energy input that comes with it. Running the same conversion below 200 degrees Celsius changes the economics of the process, since lower heat means cheaper reactors, less energy lost as waste heat, and a process that could plausibly run on a smaller, less specialized scale than a pyrolysis plant.

The lab’s own technology-transfer summary of the work describes the output not as gasoline alone but as the selective formation of liquid hydrocarbons across gasoline- and diesel-range fractions, meaning the reaction produces a spread of chain lengths rather than a single fuel product.

A Study Years in the Making, Announced Now

The chemistry behind the announcement was published in the Journal of the American Chemical Society, in a paper co-authored by Qiu, Yang, ORNL Corporate Fellow Sheng Dai, and colleagues describing the neat, hydrogen-source-free conversion of polyethylene into liquid alkanes.

Work on the project drew support from the Department of Energy’s Office of Science, through both its Materials Sciences and Engineering Division and its Chemical Sciences, Geosciences and Biosciences Division, plus access to two DOE user facilities — the Spallation Neutron Source at Oak Ridge and the Advanced Light Source at Berkeley — that let the team watch the molten salt interact with the plastic in real time, a detail confirmed in the Energy Department’s own record of the funded research.

Scaling Up Beyond the Lab Bench

Getting from a beaker of molten salt to an industrial process is its own challenge, and Dai framed the appeal of the chemistry in those terms. Because the salt itself does double duty as solvent and catalyst, he said, the process can be radically easier to scale up than routes that require separating a catalyst from the product afterward. Fewer components to isolate and recycle generally means fewer steps standing between a lab result and a working plant, and fewer places for a scaled-up version of the chemistry to go wrong on its way from a beaker to a reactor sized for an actual recycling facility.

The molten salt itself is not without drawbacks. Aluminum-based salts of this kind tend to absorb moisture from the air, a property that complicates storage and handling outside a controlled lab environment. ORNL’s team has said its next round of work will look at confining the molten salt inside halogen- or carbon-based materials, an approach aimed at stabilizing the chemistry and simplifying how the finished fuel gets separated out.

A patent application covering the process has been filed, though the lab has not set a timeline for moving the chemistry past bench scale. For now the demonstrated result is limited to polyethylene specifically, one polymer among the mix of plastics that end up in landfills and incinerators each year, leaving open how the same molten-salt approach performs on the other plastic types that pyrolysis plants currently have to handle instead.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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