Researchers have developed a new catalytic method for upcycling abundant, seemingly low-value plastics (polyethylene) into high-quality liquid products, such as motor oils, lubricants, detergents and even cosmetics. The discovery also improves on current recycling methods that result in cheap, low-quality plastic products.

Northwestern University, Argonne National Laboratory and Ames Laboratory led the multi-institutional team. The open-access study was published in the journal ACS Central Science.

Northwestern’s Kenneth R. Poeppelmeier, the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern’s Weinberg College of Arts and Sciences, director of Northwestern’s Center for Catalysis and Surface Science and member of Northwestern’s Program on Plastics, Ecosystems and Public Health, co-led the work with Aaron D. Sadow, a scientist in the Division of Chemical and Biological Sciences at Ames Laboratory, and Massimiliano Delferro, group leader of Argonne National Laboratory’s catalysis program.

Each year, 380 million tons of plastic are created worldwide. As the plastics market continues to increase, many analysts predict production could quadruple by 2050. More than 75% of these plastic materials are discarded after one use. Many of them end up in oceans and waterways, harming wildlife and spreading toxins.

There are certainly things we can do as a society to reduce consumption of plastics in some cases. But there will always be instances where plastics are difficult to replace, so we really want to see what we can do to find value in the waste.

—Aaron Sadow

While plastics can be melted and reprocessed, this type of recycling yields lower-value materials that are not as structurally strong as the original material. Examples include down-cycling plastic bottles into a molded park bench.

When left in the wild or in landfills, plastics do not degrade because they have very strong carbon-carbon bonds. Instead, they break up into smaller plastics, known as microplastics. Whereas some people see these strong bonds as a problem, the Northwestern, Argonne National Laboratory and Ames Laboratory team saw this as an opportunity.

We sought to recoup the high energy that holds those bonds together by catalytically converting the polyethylene molecules into value-added commercial products.

—Massimiliano Delferro

The catalyst consists of platinum nanoparticles—two nanometers in size—deposited using atomic layer deposition onto a perovskite nanocubes, which are about 50-60 nanometers in size. The team chose perovskite because it is stable under the high temperatures and pressures and an exceptionally good material for energy conversion.

Electron micrograph of platinum nanoparticles deposited onto a perovskite nanotube. Credit: Argonne National Laboratory

Under moderate pressure and temperature, the catalyst cleaved plastic’s carbon-carbon bond to produce high-quality liquid hydrocarbons.

Pt/SrTiO₃ completely converts PE (Mn = 8000–158,000 Da) or a single-use plastic bag (Mn = 31,000 Da) into high-quality liquid products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H₂ and 300 °C under solvent-free conditions for reaction durations up to 96 h.

—Celik et al.

These liquids could be used in motor oil, lubricants or waxes or further processed to make ingredients for detergents and cosmetics. This contrasts commercially available catalysts, which generated lower quality products with many short hydrocarbons, limiting the products’ usefulness.

Too, the catalytic method produced far less waste in the process. Recycling methods that melt plastic or uses conventional catalysts generate greenhouse gases and toxic byproducts.

Resources

• Gokhan Celik, Robert M. Kennedy, Ryan A. Hackler, Magali Ferrandon, Akalanka Tennakoon, Smita Patnaik, Anne M. LaPointe, Salai C. Ammal, Andreas Heyden, Frédéric A. Perras, Marek Pruski, Susannah L. Scott, Kenneth R. Poeppelmeier, Aaron D. Sadow, and Massimiliano Delferro (2019) “Upcycling Single-Use Polyethylene into High-Quality Liquid Products” ACS Central Science doi: 10.1021/acscentsci.9b00722