From waste to fuel for less

Plastic waste can one day be upcycled into useful commodity chemicals instead of ending up in the environment. Credit: Image by Cortland Johnson Pacific Northwest National Laboratory

New technology could divert problematic plastics from landfills and convert them into fuel sources.

A plastic recycling innovation that increases the conversion of precious metal ruthenium into useful products while using less. It will be presented today (August 22, 2022) at the American Chemical Society fall meeting in Chicago.

“The main finding we report is very low metal loads,” said Pacific Northwest National Laboratory (PNNL) chemist Janos Szani, who led the research team. “This makes the catalyst much cheaper.”

The new technology more efficiently converts plastic into valuable commodity chemicals – known as “upcycling”. Furthermore, it produces much less methane, an unwanted greenhouse gas, as a byproduct, compared to other reported methods.

“It was very interesting to us that nothing showing this result had been published before,” said postdoctoral research scientist Linxiao Chen, who presented the research at ACS. “This research demonstrates the opportunity to develop efficient, selective, and versatile catalysts for plastic upcycling.”

What is plastic upcycling?

Plastic upcycling now offers a way to recycle waste carbon that clutters landfills and beaches. Credit: Animation by Sarah Levine | Pacific Northwest National Laboratory

Less metal is more in plastic upcycling

Petroleum-based plastic waste represents an untapped source of carbon-based chemicals that can serve as starting materials for useful durable materials and fuels. Despite an ample supply of recycling bins, very few plastics are currently recycled, mainly for economic and practical reasons. However, PNNL researchers are looking to change the dynamic by applying their expertise to effectively break chemical bonds.

It is well known that hydrogen – a reaction called hydrogenolysis – represents a promising strategy for converting plastic waste into value-added small hydrocarbons for difficult-to-recycle plastics such as polypropylene and polyethylene. But making this process economically feasible requires efficient and selective catalysts.

That’s where this latest PNNL-led research excels.

The team of scientists found that reducing the amount of precious metal ruthenium actually improved polymer upcycling efficiency and selectivity. In a recent study published in the journal Dr ACS catalystThey showed that efficiency improved when a lower ratio of metal to support structure shifted the structure from an ordered array of particles to disordered rafts of atoms.

Trapped atoms

PNNL’s track record of expertise in single-[{” attribute=””>atom catalysts helped the team understand why less is more. The researchers observed the transition to disorder on the molecular level and then used established theory to show that single atoms are actually more effective catalysts in this experimental work.

The work builds on research in atom trapping and single-atom catalysts by Yong Wang, a professor of chemical engineering at Washington State University, Pullman, and a PNNL Laboratory fellow.

“There has been a lot of effort from a material perspective to try to understand how single atoms or very small clusters can make effective catalysts,” said Gutiérrez.

At ACS, Chen also described new work that explores the role of the support material in improving the efficiency of the system.

“We have investigated cheaper and more easily available support materials to replace cerium oxide,” said Chen. “We found that a chemically modified titanium oxide may enable a more effective and selective pathway for polypropylene upcycling.”

Learning how to tolerate chlorine

To make the method practical for use with mixed plastic recycling streams, the researchers are now exploring how the presence of chlorine affects the efficiency of the chemical conversion.

“We are looking into more demanding extraction conditions,” said chemist Oliver Y. Gutiérrez, an expert in industrial applications for catalysis. “When you don’t have a clean plastic source, in an industrial upcycling process, you have chlorine from polyvinylchloride and other sources. Chlorine can contaminate the plastic upcycling reaction. We want to understand what effect chlorine has on our system.”

Now, that fundamental understanding may help convert waste plastic that would usually end up as pollution in the environment into useful products.

Reference: “Disordered, Sub-Nanometer Ru Structures on CeO2 are Highly Efficient and Selective Catalysts in Polymer Upcycling by Hydrogenolysis” by Linxiao Chen, Laura C. Meyer, Libor Kovarik, Debora Meira, Xavier I. Pereira-Hernandez, Honghong Shi, Konstantin Khivantsev, Oliver Y. Gutiérrez and János Szanyi, 5 April 2022, ACS Catalysis.
DOI: 10.1021/acscatal.2c00684

The research was supported by the Department of Energy, Office of Science. This research also used resources from the Advanced Photon Source, an Office of Science user facility operated for DOE by Argonne National Laboratory.

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