From waste to fuel for less

Plastic waste could one day be recycled into useful 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 turn them into fuel sources.

A plastics recycling innovation that does more with less at the same time increases conversion to useful products by using less precious metal ruthenium. It will be presented today (August 22, 2022) at the fall meeting of the American Chemical Society in Chicago.

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

The new technique more efficiently converts plastic into valuable commodity chemicals—a process called “upcycling.” Additionally, it produces far less methane, an unwanted greenhouse gas, as a byproduct compared to other reported methods.

“It was very interesting to us that there was nothing previously published showing this result,” said postdoctoral research scientist Linxiao Chen, who presented the research at ACS. “This research demonstrates the possibility to develop effective, selective and versatile catalysts for plastics recycling.”

What is plastic recycling?

Plastic recycling offers a way to reuse the carbon waste that now clutters landfills and beaches. Credit: Animation by Sara Levine | Pacific Northwest National Laboratory

Less metal is more in plastic recycling

Petroleum-based plastic waste represents an untapped source of carbon-based chemicals that can serve as starting material for useful sustainable materials and fuels. Despite abundant supplies in recycling bins, very little plastic is currently recycled, mainly for economic and practical reasons. However, PNNL researchers are trying to change the dynamic by applying their expertise in efficiently breaking chemical bonds.

It is well known that the addition of hydrogen – a reaction known as hydrogenolysis – to hard-to-recycle plastics such as polypropylene and polyethylene represents a promising strategy to convert plastic waste into value-added small hydrocarbons. But this process requires efficient and selective catalysts to make it economically feasible.

This is where this latest PNNL-led research shined.

The team of scientists found that reducing the amount of the precious metal ruthenium actually improved the recycling efficiency and selectivity of the polymer. In a study recently published in the journal ACS catalysisthey showed that the efficiency improvement occurred when the low metal-to-support structure ratio caused the structure to shift from an orderly array of particles to disordered arrays of atoms.

Trapped atoms

A history of PNNL expertise in one[{” 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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