Molten aluminium chloride cracks polyethylene into gasoline at oven temperatures

The acid chemistry is what makes the low temperature possible. Soft X-ray spectroscopy and nuclear magnetic resonance showed charged aluminium atoms bound to three other atoms, forming highly acidic sites that attack polyethylene's long chains and split them into smaller hydrocarbons ^[8].

"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," said Zhenzhen Yang, an ORNL staff scientist and a co-corresponding author of the paper ^[9].

Pyrolysis, the route most earlier polyethylene-to-gasoline work used, needs roughly 450 to 500 degrees Celsius ^[5]. The ORNL reaction stayed below 200 ^[4]. The difference is 250 to 300 degrees ^[6], and the laboratory compares its own operating range to the inside of a conventional kitchen oven ^[10].

"We developed an efficient and selective polyethylene-to-gasoline conversion," said Liqi Qiu, a University of Tennessee, Knoxville postdoctoral researcher who performed most of the experiments in Sheng Dai's ORNL laboratory ^[11]. The yield figure behind that sentence comes from laboratory runs, and the announcement does not say whether it is measured by mass or by carbon, nor give numbers for salt consumption, reuse, throughput or cost ^[12].

The team's own feedstock work is the reason to hold the yield loosely. Isotopic labelling and neutron scattering showed the structure of the starting polymer setting the product, with simpler chains giving gasoline-like compounds and more complex chains giving diesel-like ones ^[7]. A single clean polymer grade and a stream of mixed waste should not be expected to give the same split.

Dai, an ORNL Corporate Fellow and section head for separations and polymer chemistry, put the claim on engineering ground rather than yield. "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," he said ^[13]. The researchers say that if the method can be scaled beyond laboratory experiments, it could contribute to U.S. energy security and industrial competitiveness ^[14].

Molten salts are old ground at the laboratory, which ran its Molten Salt Reactor Experiment in the 1960s ^[15].